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Generic Closed Control Loop of a High Efficiency Low
Volume Bioethanol Distillery
A thesis presented in partial fulfilment of the
requirements of the degree of
Masters of Engineering
in
Mechatronics
At
Massey University
Auckland, New Zealand
Andrew Pearson 03286614
2012
1
Foreword
To my adoring mother who always believed that I could obtain anything
that I wanted, and who supported me throughout my University Carreer
and to Johan, thank you for teaching me over the last 8 years, you have
been a great role model for me and many other students.
2
Table of Contents
1.0 Abstract.................................................................................................................... 6
2.0 Introduction............................................................................................................. 7
2.1 Background........................................................................................................... 7
2.2 Problem................................................................................................................. 9
2.3 Solution............................................................................................................... 10
2.4 Aim....................................................................................................................... 12
2.5 Summary............................................................................................................. 12
3.0 Literature Review:................................................................................................ 13
3.1 Fermentation..................................................................................................... 13
3.1.1 Ethanol........................................................................................................ 14
3.1.2 Fermentation.............................................................................................. 19
3.2 Distillation.......................................................................................................... 22
3.3 Control System.................................................................................................. 27
3.3.1 Programmable Logic Controller...............................................................28
3.3.2 PID Control Algorithm...............................................................................34
3.3.3 PID Tuning................................................................................................... 39
3.3 Research Related Literature Review ............................................................42
3.3.0 Distillation Research..................................................................................44
3.3.1 Ethanol Fuel Research...............................................................................47
4.0 Process.................................................................................................................... 53
4.1 Brewing Ethanol................................................................................................ 53
4.1.1 Brewing Temperature................................................................................56
4.1.2 Air Lock........................................................................................................ 57
4.1.3 Measuring Fermentation...........................................................................58
4.2 Distillation.......................................................................................................... 60
4.3 Program design:................................................................................................ 65
4.4 Electrical design:............................................................................................... 74
4.4.1 PLC Commissioning....................................................................................74
3
4.4.2 Solid State Relays.......................................................................................77
4.4.3 Analogue Input Module.............................................................................80
4.4.4 Thermocouple.............................................................................................82
5.0 Discussion & Results............................................................................................ 84
5.1 Design.................................................................................................................. 84
5.1.1 Still Interface.............................................................................................. 85
5.1.2 Temperature Measurements....................................................................88
5.1.3 Cooling Circuit............................................................................................ 89
5.2 PID Tuning.......................................................................................................... 92
5.3 Results................................................................................................................. 94
5.3.1 Energy Consumption..................................................................................98
5.3 Further Enhancements..................................................................................... 99
6.0 Conclusion............................................................................................................ 101
7.0 References........................................................................................................... 103
8.0 Bibliography........................................................................................................ 105
4
Table of Figures
3.0 Ethanol flame
............................................................................................14
3.1 Koenigsegg biofuel super car
....................................................................17
3.2 Yeast cell budding
............................................................................................20
3.3 Fractional still
............................................................................................23
3.4 Fractional still trays ............................................................................................24
3.5 Phase change diagram
................................................................................25
3.6 Packed column, binary distillation ....................................................................26
3.7 PLC block diagram
............................................................................................29
3.8 PLC and expansion module dimensions ........................................................30
3.9 EM231
........................................................................................................31
3.10 PLC Scan Cycle
............................................................................................33
3.11 PID Control Loop
............................................................................................34
3.12 Effect of P Variable on PID Loop
....................................................................36
3.13 Effect of I Variable on PID Loop
...................................................................36
3.14 Effect of D Variable on PID Loop
...................................................................36
3.15 Mathematical Representation of PID Loop
...........................................37
3.16 PID Equation for PLC ...........................................................................................37
3.17 Quarter Amplitude Decay
3.18 PID Tuning Table
...........................................................................................41
3.19 Ziegler-Nichols Method
3.20 Auto-tune
...............................................................................40
...............................................................................41
.......................................................................................................42
3.21 BSE Still Diagram
...........................................................................................46
3.22 SEW Exceedance days
...............................................................................48
3.23 Neat Alcohol VW Turbo Specifications
4.0 Turbo Yeast Packet
...........................................51
...............................................................................55
4.1 Air Lock
…...................................................................................................58
4.2 Hydrometer
.......................................................................................................59
4.3 Euro Still
.......................................................................................................61
4.4 Boiling Point Equation for Water/Ethanol Mix
5
..........................................63
4.5 Siemens WinCC IDE …........................................................................................64
4.6 PID Tuning Wizzard ….......................................................................................64
4.7 PLC Motor Stop/Start Example
....................................................................65
4.8 IDE Explained ........................................................................................................66
4.9 S7-200 …................................................................................................................75
4.10 S7 Series Mounting ............................................................................................76
4.11 Wiring Diagram
............................................................................................77
4.12 Siemens Suppression Circuit
…................................................................78
4.13 SSR Diagram …....................................................................................................78
4.14 SSR Mounted in my System
…................................................................79
4.15 SSR Maximum Load v Ambient Temperature
4.16 S7-200 Analogue to Digital Converter
…........................................80
…....................................................82
4.17 Thermocouple Measuring Circuit …................................................................83
4.18 Thermocouple Specifications
5.0 Distillation Apparatus
5.1 Tank Sensor
…................................................................83
…............................................................................86
…....................................................................................................87
5.2 Column Sensor
…........................................................................................88
5.3 PID Tuning IDE
…........................................................................................90
5.4 Condenser Temperature
…............................................................................92
5.5 Results …................................................................................................................97
6
1.0 Abstract
Bioethanol is a type of biofuel that is created by fermenting organic
materials into a solution called mash. This mash contains water, dead
yeast cells, feed stock solids, and 10% - 15% bioethanol (alcohol). The
bioethanol is extracted by heating the mash above the boiling point of
ethanol to create a vapour which is then condensed in to a liquid that is
greater than 93% bioethanol in a process called distillation.
bioethanol is a viable replacement for petrol, however comparisons
between the two fuel types show that with the current processes used
petrol has a higher net energy yield. Bioethanol contains 30% less energy
than petrol, so to compete with petrol bioethanol must be created in a
way that greatly reduces its total energy cost. The most energy intensive
process in the production of bioethanol is distillation, an Advanced
Process Control algorithm (APC) must be implemented to make this
process more efficient.
My project is based on the implementation of an APC to increase the
efficiency of a bioethanol still. By using a Siemens PLC (Programmable
Logic Controller), combined with their PID (Proportional, Integral, and
Differential) control algorithms I intend to monitor and control the
distillation of a mash containing 14% bioethanol.
With this approach I have been able to manufacture a low volume still
that can produce high quality bioethanol consistently. This approach
increased the total bioethanol yield by 10%, also producing a solution
that is consistently above 93% ethanol which can be fed straight into a
molecular sieve for dehydration, producing 100% bioethanol that can be
used as a biofuel.
7
2.0 Introduction
With the increase in fuel prices and the imminent end of the worlds oil
reserves, alternative technologies are being considered to fuel our
transportation needs. Technology for both fast charging electric cars and
hydrogen fuel cells are becoming more socially acceptable and financially
viable. A transition phase from combustion engines to electric motors
will be necessary and with it the possibility that oil may become a
financially un-viable energy source before this transition is complete.
Ethanol is the most logical bridge for this transition phase as most states
in America already use petrol containing 10% bioethanol (Hulsey &
Coleman, 2006), with both financial and environmental benefits. It is
cheaper than petrol and reduces emissions from standard petrol power
plants by up to 16%(Hulsey, Coleman, 2006). Auto mobile companies are
already manufacturing production models that can run on 100% ethanol,
a mix of between 85% to 15% ethanol, and even regular petrol if biofuel
is not available (Flexifuel Vehicles).
2.1 Background
Early combustion engines and cars were originally configured to run on
alcohol, however auto mobile companies opted to manufacture cars
configured for the cheaper alternative, which in the 1900s was petrol.
Combustion engines are designed to burn nearly any liquid fuel and
ethanol was originally chosen, not only for its renew-ability but because
it was a visibly cleaner burning fuel compared to petrol. In 1917
Alexander Graham Bell said "Alcohol makes a beautiful, clean and efficient
fuel… Alcohol can be manufactured from corn stalks, and in fact from
almost any vegetable matter capable of fermentation… We need never
fear the exhaustion of our present fuel supplies so long as we can produce
8
an annual crop of alcohol to any extent desired.” (“Timeline of alcohol
fuel”, 2012).
The production of bioethanol is achieved through a biological process
called fermentation. Yeast cells are introduced in to a sugar-rich
environment, in which the yeast converts the sugar into ethanol and
carbon dioxide. This process creates a mixture of ethanol, water, and
feed stock solids (un-fermentable organic components and dead yeast
cells) called mash.
The ethanol is then extracted from the mash by a process called
distillation. Distillation is a process in which the mash is boiled creating a
vapour that is then condensed into a liquid of greater purity than the
mash. The boiling point changes as the percentage alcohol in the mash
decreases, control of this boiling point is necessary to ensure the vapour,
and distillate stream are greater than 93% ethanol.
The ethanol can not be distilled above a purity of 95% as at this purity
the vapour creates an azeotrope (a mixture of 2 components has a lower
boiling point than either component). This azeotrope of 95% ethanol and
5% water has a lower boiling point (approximately 78.5 oC) than that of
100% ethanol (approximately 80oC) so it can not be distilled further. For
use as a fuel or fuel additive the bioethanol must be passed through a
molecular sieve which allows the ethanol molecules to pass through,
while stopping the larger water molecules to produce a solution that is
100% ethanol.
bioethanol is a more efficient fuel than petrol as it is a more complete
burning fuel that has more favourable combustion characteristics. These
characteristics allow more bioethanol to be combusted than petrol in a
fixed volume. Since more bioethanol can be combusted, more energy can
9
be released allowing a engine running on bioethanol to produce more
power than petrol from a combustion engine of the same size.
For example; a family sedan with a 2L petrol engine has a power output
that is restricted by many factors. These factors will include noise, fuel
efficiency, and durability, all of which are dependent on the fuel type the
engine is optimised for. The efficiency at which bioethanol burns results
in bioethanol engines being able to produce more power from a 2L
engine than petrol without seriously effecting durability or the driveability of the vehicle.
This increased fuel combustion will result in poor fuel efficiency. A
smaller engine that took full advantage of bioethanols favourable
combustion characteristics could be used. These characteristics will allow
the engine to be optimised in such away that the smaller engine could
produce the same power as a 2L petrol engine with a comparable fuel
efficiency.
2.2 Problem
Crude oil refining has evolved other the past 100 years with research
focusing on higher yields and lowering costs per litre. Although ethanol
distillation is not a new process, its production has evolved for human
consumption, with research focused towards high quality alcohol and
post distillation treatment for flavour enhancement. Production of
ethanol for use as fuel has been steadily increasing, followed by a
increase in research focused on producing more alcohol per litre of mash
and also reducing the cost per litre.
The main problem with bioethanol production is that energy balance
equations used to calculate a fuels perceived inputs and outputs show
10
bioethanol costs energy rather than creates energy. As bioethanol is
constantly competing with petrol for world wide adoption, bioethanol
needs to be created in a way that is more energy efficient. By being more
energy efficient biofuel production will be kinder to the environment
and change the energy balance equation to prove that bioethanol is a
viable petrol alternative.
There are two distinct areas of research for the production of
bioethanol, 1) fermentation of alcohol and 2) the distillation of ethanol.
Fermentation research uses both chemical and genetic engineering to
increase the yield from the fermentation process. Distillation research
uses mechatronics engineering to increase the yield from the distillation
process while reducing the energy needed for this process. The area in
which I will be studying is the distillation process, mainly around the
reduction of energy consumption through the use of closed loop control.
2.3 Solution
Bioethanol distillation uses large amounts of energy to separate the
bioethanol from the mash. Optimisation of this process is required to
increase the total energy yield from the bioethanol. Reducing the
amount of energy needed to extract the bioethanol will not only
increase the energy yield from the production process, but also reduce
the carbon footprint, making bioethanol a much cleaner fuel supply.
Control of the distillation process will require the implementation of a
APC algorithm. A PID algorithm has been chosen as this type of
algorithm is implemented on top of a closed loop control system. PID
algorithms are generic closed loop control algorithms used to monitor
and control the amount of energy that is introduced in to the still.
11
A similar project at Badger State Ethanol (BSE) used a APC algorithm to
increase production of corn based bioethanol. BSE used a non-linear
predictive model to increase overall production by 10.24%, and reduced
steam consumption by 9.9%(Rueda & Duke, 2008). This algorithm used a
mathematical model of the entire plant and process to calculate the
amount of input needed to get a desired change in output.
PID control loops gave a similar improvement without the expense of
modelling the plant and process. PID control of a bioethanol plant was
not only a more financially viable choice, it also allowed for greater
scalability compared to fixed model solution. A generic control algorithm
can be used on many differed plants, the model produced for BSE is only
applicable to their plant, or very similar plants which could use a
modified model.
To control the system I am using a Siemens S7-200 PLC to automate the
distillation process, which will allow me to control, monitor, and easily
reprogram the system throughout the design process. The Siemens PLC
programming IDE (Integrated Development Environment) has tools for
tuning the PID control loops, greatly reducing the development time.
The IDE will also allow me to monitor crucial information about the
distillation process during both automated and manual distilling, which
will give me access to data needed for analytical comparison of the two
distillation processes.
In my project I will be using a “Home Brew” distillation unit and spirit
distilling tools with the goal of creating an aqueous solution that I can
distill both manually and automatically. Using “Home Brewing”
equipment will allow me to create a mash that will consistently yield 14%
alcohol. This will allow me to use these results for scientific analysis to
determine both the efficiency and effectiveness of the still.
12
2.4 Aim
The aim of this thesis is to produce evidence that closed loop control can
both reduce the amount of energy needed to produce bioethanol, and
that this type of control will remove more bioethanol per litre of mash.
When ethanol is removed from a mash manually, the quality of ethanol
produced reduces as time increases. The solution obtained at the end of
this process will be of a low quality requiring further processing and
energy to produce a solution that is 100% bioethanol.
Objectives:
• To create a control system that will automate the distillation process
that is more cost effective than other distillation methods.
• Create a control algorithm that will produce ethanol that is of greater
purity than manual stilling by keeping tighter controls on the system,
ensuring that only ethanol of 93% and above is being produced by the
still.
2.5 Summary
The research that follows is based on the manufacturing of a low volume,
high efficiency bioethanol still, and the comparison of this still against
manual distillation. The still I am using for this project is a 60L still
producing ethanol at a rate of 1L/hr, and at a proposed quality of greater
than 93%. This can only be achieved with exceptional control of both the
distillation tank and the column and closed loop control.
I will be using a Siemens PLC to implement a PID control loop to monitor
and adjust the inputs to the distillation process. With this control system
I hope to increase the amount of total ethanol extracted while
decreasing the energy used. This is with the aim of producing bioethanol
in a more efficient way that is financially and environmentally friendly.
13
3.0 Literature Review:
The literature review contains two types of information:
1) Information directly related to this thesis,
2) Information from other studies carried out on biofuel.
Information directly related to this thesis is information needed to
understand the thesis, including information about how ethanol is
created, the process of distillation, the control system and its
implementation. Other studies in the biofuel area are included to show
the current state of technology and other research being carried out
around the production of bioethanol, most applicable being that in the
area of optimisation of road car engines to run on neat alcohol.
The information directly related to my research is separated into 3
sections:
• Fermentation (bioethanol and its creation)
• Distillation (separating bioethanol from the mash)
• The algorithm and control systems implemented to control the
distillation process.
3.1 Fermentation
In this section I will discuss ethanol, how it is created (a brief description
of the chemical reaction that takes place), and how it fits in to the
transportation industry. Knowledge of both the product and process are
necessary as fermentation is a key part of my project and it must be
carried in accordance with scientific method in order to produce
standardised results. This is to ensure results found reflect the
distillation process and not the fermentation process. Although
fermentation is one of the oldest biological processes known to man,
14
precautions must be taken to ensure each fermentation is carried out in
such a fashion so as not to artificially reduce or increase the amount of
ethanol produced between production runs.
3.1.1 Ethanol
Ethanol is a clear, colourless liquid that has many different industrial
uses most notably as a psychoactive drug found in alcoholic beverages.
The same alcohol can be used as a solvent, temperature sensor in
thermometers, fuel additive for petrol auto mobiles, and as a fuel for
specially modified vehicles. The molecular formula for Ethanol is
C2H5OH, which is a straight chain alcohol that burns with a smokeless
blue flame (Figure 3.0).
Figure 3.0 The blue smokeless flame of ethanol combusting
15
Ethanol burns cleanly because it has a high oxygen content, thus requires
less added oxygen to be fully combusted. When added to normal petrol
it can increase the octane rating and reduce harmful emissions from a
standard petrol engine. The added ethanol reduces harmful emissions by
burning the fuel more completely, this is due to the extra oxygen
supplied by the ethanol.
Ethanol is already used around the world as a fuel source, most notably
in Brazil and more recently in America. The automotive industry already
produces standard productions models with Flexifuel power plants that
enable the driver to select between petrol and E85 (85% bioethanol and
15% petrol) fuel. Flexifuel cars do not take advantage of ethanols
favourable combustion properties, which allow racing cars to produce
performance figures an equivalent petrol engine could not produce.
As a result from 2007 – 2008 ethanol in global gasoline type fuel
increased from 3.7% to 5.7%, with Brazil and America accounting for
88% of global ethanol production (“Ethanol fuel in Brazil”, 2012).
Currently Flexifuel vehicles make up 22% of all light vehicles registered
in Brazil and 4.0% in America (“Ethanol fuel in Brazil”, 2012) removing
approx 30 million petrol dependent vehicles from the roads. The biggest
barrier for use of E85 fuel has been determined as education- 68% of
American Flexifuel vehicle owners did not know they owned Flexifuel
cars that could run on E85 (“Ethanol fuel in Brazil”, 2012).
Brazil is the world second largest producer of bioethanol fuel and is
considered the worlds first sustainable biofuel economy and industry
leader. Since 1976 the government made it mandatory to blend ethanol
with gasoline (“Ethanol fuel in Brazil”, 2012). At present the lowest blend
allowed to be sold is E18 although E100 is available at all petrol stations.
Today there are no light vehicles that run on pure petrol and all vehicles
16
in Brazil can run on a mixture of up to 25% ethanol. This requires slight
modifications to standard petrol engines, but all vehicles manufactured
or sold within Brazil are modified to use such fuel blends (“Ethanol fuel
in Brazil”, 2012).
When combusted, bioethanol produces less energy than petrol, however
bioethanol is burned more completely due to its higher oxygen content,
reducing pollution. When combusted, bioethanol produces 22.3 MJ of
energy per litre, while petrol produces 32 MJ of energy per litre , which
is 30% less energy. Petrol has more energy per litre, however bioethanol
has more oxygen per litre so requires less added oxygen to combust, and
also a lower latent heat, and better ignition authority than petrol
(Alcohol fuel, 2012).
The fuel contains the energy and the combustion chamber releases and
converts this energy into rotation. The more fuel a combustion chamber
can burn, the higher the energy output. An engine cylinder has a fixed
volume, which limits the amount of fuel that can be burnt in the
combustion chamber by limiting the amount of oxygen it can hold.
Bioethanol can combust with less oxygen than petrol, meaning more
ethanol can be combusted in the same sized combustion chamber,
conceivably doubling the power output without increasing the engine
size and with no effects on durability.
The fuel to air ratio is important when discussing internal combustion
engines, as an incorrect ratio can lead to problems with the engine. Too
much fuel in the combustion chamber means the engine will not perform
properly, slow throttle response, and lower power as not all the fuel is
burnt. Too little fuel and the engine will detonate before the piston is at
the top of its stroke and cause damage to the engine. The power output
of a combustion engine can only be increased by increasing the amount
17
of fuel it can burn, this is achieved by increasing the amount of oxygen
the combustion chamber can consume (“Internal combustion engine”,
2012).
Petrol engines are further limited by the physical properties of petrol as
it releases large quantities of excess heat. When increasing the
performance of a combustion engine there will be a performance ceiling,
at which point the engine will be unable to run as the heat produced will
either weld the pistons to the cylinder, or poor spark authority will
detonate and destroy the engine block. To get further performance
increases, one must increase the volume of the engine. Bioethanol is
limited, but the heat threshold is far higher than petrol, allowing for
great performance figures from an engine smaller than its petrol
performance equivalent.
Figure 3.1 Biofuel powered super car the Koenigsegg
Swedish super car manufacturer Koenigsegg have developed a midengine super car that is optimised to run on E100 and E85 (Figure 3.1).
This car is able to produce more power using bioethanol than if it ran on
petrol, due to bioethanols cooling properties and better spark authority,
18
allowing them to use large amounts of supercharger boost. This
configuration enables the 4.7L V8 engine to produce 1,064 hp. In
comparison the Bugatti Veyron uses a 8.0L W16, quad turbo engine
optimised to run on petrol produces 1,001hp which is nearly double the
size of the bioethanol optimised engine with similar performance.
Top fuel drag cars are able to produce thousands of horsepower by using
Nitrogen-Methane, an alcohol that is 90% Nitro-glycerine and 10%
Methanol. Similar to ethanol, this fuel has less energy per litre than
petrol but higher oxygen content per litre meaning 8.7 times the amount
of nitrogen methane can be burnt in the combustion chamber. This
means it can produce four times as much power as the petrol engine,
which is limited by the volume of air it can fit in the cylinder, and the
physical properties of petrol. Top fuel drag cars do not need a radiator as
alcohol is burnt so efficiently that minimal heat energy is released. This
further reduces the weight of an alcohol powered vehicle, again adding
to its efficiency (“Top Fuel”, 2012).
Flexifuel cars can run on both petrol or bioethanol depending on its
availability, as bioethanol is not yet available at every petrol station
across America. At this point in time most vehicles that run on biofuel are
Flexifuel cars, these use modified petrol engines. A Flexi fuel vehicle can
run on both petrol and bioethanol by adding more bioethanol via a
modified ECU map (Engine Control Unit) (“Flexible-fuel vehicle”, 2012).
These engines are not able to take full advantage of the favourable
combustion characteristics of bioethanol as they must still be able to
operate with petrol as well. This reduces the efficiency of the bioethanol
fuel, resulting in poor performance compared to petrol. A similar
example would be running petrol in a diesel engine; it is possible but the
result will be poor compared to diesel fuel (“E85”, 2012).
19
The lower energy content of ethanol is cited as one of the main reasons
it is an unsuitable replacement for petrol. Application of the above
properties and further research into neat alcohol engines could lead to a
change in the engine configurations of biofuel models that car
manufacturers release. Research shows that engines configured to run
on pure bioethanol can achieve greater than 40% brake thermal
efficiency resulting in engine configurations that can a achieve better
economy than a similar sized diesel engine (Brusstar & Bakenhus, 2002).
3.1.2 Fermentation
There are two methods which can be used to obtain ethanol - Synthesis,
which requires petroleum substrates so will not be considered,
or
fermentation, which is a biological process where yeast saccharomyces
consume sugars (fructose, sucrose and glucose) excreting CO 2 and
ethanol. In my project an understanding of this type of ethanol
production is necessary as the organic materials needed to feed this
reaction are considered a renewable energy source. Bioethanol is a
renewable energy source meaning that we are able to reproduce the
materials that are needed for its creation.
Yeast cells are Chemo-organotrophs, meaning they use organic material
as a source of energy for reproduction and respiration and do not
require sunlight to grow. The life-cycle of a yeast cell begins with
reproduction, consuming excess oxygen and sugar to bud (reproduce)
then in a low oxygen phase the cells ferment the available sugar into
ethanol and CO2 until it dies from excessive reproduction scars. This
attribute allows the single celled fungi to ferment plant material into
ethanol.
Figure 3.2 shows a yeast cell budding to create a daughter cell, also
20
behind this is a cell with multiple budding scars. Once a cell has
reproduced to a point where its cell wall is fully covered in scars it can no
longer reproduce. A colony that is badly scarred is unable to reproduce
and can no longer metabolise sugar or reproduce and should be
discarded.
Figure 3.2 Yeast cells budding
When introduced to a environment that is optimal for fermentation,
yeast cells begin by reproducing (budding) to create a high concentration
of yeast cells. These yeast cells use excess oxygen in an aerobic phase to
asexually reproduce, then in a low oxygen anaerobic phase the yeast
cells will only ferment the remaining sugar. In this phase the yeast
consumes sugar and
excrete ethanol and CO 2, if oxygen is present
throughout the fermentation the yeast will ferment the sugar right
down to CO2 and water.
Oxygen is introduced early in the mash creation process by agitating the
mash, this is then sealed usually with some form of airlock allowing the
CO2 to escape. The air lock not only stops unwanted oxygen and micro21
organisms entering the fermentation but allows the brewer to visually
monitor the CO2 production. CO2 is the best indicator for the health of a
fermentation; if CO2 is not being produced the yeast have stopped
fermenting.
The feedstock used to fuel a fermentation comes in the form of sugar
(sucrose, fructose, glucose, and maltose). Yeast use the energy stored in
the sugar for reproduction and respiration. The sugar can come from
sugar crops such as cane sugar, sugar beats or other plants with a high
sugar content. A cheaper source of sugar that can be used is starchy food
such as grain, corn, and potato, which require pre-processing to release
the sugar from the starch enzymes.
Starch based plants can also be grown in more tepid climates making
them more abundant and a better feedstock for biofuel. America has
built a healthy biofuel industry around starch based feedstock, most
notably corn. Starch based feedstock is cheaper to purchase than sugar
as they are more abundant and need pre-processing to release the
available sugars.
In Brazil, sugar cane and its associated by products are used in
conjunction with other sugar crops and by-products as feedstock for a
healthy and mature biofuel industry. Sugar crops specifically grown for
the transport industry are supplemented with products form other
farming sectors which may not be fit for human consumption. These
feedstocks are readily available in Brazil due to its climate and geological
location, Brazil has the most sustainable biofuel industry and economy in
the world (“Ethanol fuel in Brazil”, 2012).
For my experiment I will be using Chelsea grade 1A sugar as the feed
stock, which is sucrose (table sugar, formula is C 12H22O11). Yeast can only
22
ferment glucose, so the sucrose must first be broken down in to glucose.
When added to water the sucrose is broken down into two parts glucose
by a process called hydrolysis, catalysed by the enzyme invertase.
C12H22O11 +H2O + invertase →2 C6H12O6
After the sucrose is broken down into glucose the yeast use an enzyme,
zymase, to catalyse the chemical reaction that converts the glucose into
ethanol. One mole of glucose is converted into two parts ethanol and
two parts carbon dioxide.
C6H12O6 + Zymase → 2C2H5OH + 2CO2
The conversion of sucrose to ethanol is dependent on the temperature
of the fermenting environment, and the presence of trace nutritional
elements such as nitrogen needed for yeast to reproduce and ferment.
Organic material (corn, fruit or organic waste) contain these required
nutrients, sucrose fermentation requires the addition of these special
nutrients.
3.2 Distillation
Distillation is a physical process in which an aqueous solution made up of
multiple liquid components is separated into its individual components.
This process works on the basis that different solutions have differing
boiling points because of the different atomic structures of the
individual solutions. By boiling the solution, the component with the
lower boiling point will turn into steam first, the steam is then captured
and condensed into a liquid.
One of the most common types of industrial distillation is fractional
distillation, which enables the column to extract different compounds at
23
different temperatures up the still column. A fractional still (figure 3.3) is
made up of a vertical column with a heated mixture at the bottom and a
cool reservoir at the top of the column. The column is subject to a
temperature gradient along its length, and as the heat transfers up the
column, this temperature gradient allows for extraction of substances
with specific boiling points at specific points up the column.
Figure 3.3 Basic distillation apparatus
Along the column are plates or reservoirs that allow the vapor to
condense and then evaporate at an increased purity compared to the
plates below. This process uses the heat energy from the rising steam to
24
phase change from liquid to vaporous. This cycle repeats up the column
on each plate, re-distilling the vapour multiple times producing a highly
concentrated vapour at the top of the still column to be extracted as a
high purity liquid (greater than 90%).
Figure 3.4 is a diagram of a how the fractional still works as vapour rises
up the column collecting on plates for further distillation. Vapour is
passed through the caps and is cooled by the collected liquid on the tray,
this transfers energy into the liquid allowing the more volatile
component to phase change into vapour and continue up the still. As this
happens the less volatile liquid is allowed to collect in the tray until it
overflows back down to the tray below until eventually all the volatile
liquid has passed up the column and the less volatile liquid is left in the
still tank.
Figure 3.4 Fractional distillation column with bubble caps
25
The phase diagram in figure 3.5 is an example of a binary distillation of
hexane and pentane carried out on a fractional column with 3 plates. This
example is used as it has similar physical characteristics to water/ethanol
distillation. The vapor point (The point at which a solution turns into
vapor) of a one to one mix of pentane and hexane is at point L1, this
vapor condensates at point V1 at a quality of less than 20% hexane. The
solution vaporises at point L2 condensing at point V2, and finally
condensing at point L3 which is pure pentane. This distillation is using
idealised liquids so we would not be able to obtain complete separation
in three phase changes.
Figure 3.5 Phase change diagram of Hexane and Pentane
Figure 3.5 is the basis for a fractional distillation, each plate allows the
vapor to condensate then evaporate at a different purity. This process
will be repeated many times up the column allowing a highly purified
liquid to be collected in a real world situation. The purity and volume of
distillate to be processed determines the number of plates and length of
the column (Halvorsen & Skogestad, 1999).
A fractional still not only increases the purity at which a component can
26
be distilled but also allows for the separation of a multi-component
mixture into its individual components. The temperature at a specific
point up the column can be used to collect a component from the
solution with a similar boiling point. This component will not be able to
travel further up the column so will collect at this point while a
component with a lower boiling point will continue up the still to a point
where it can be collected.
As I am only separating ethanol from water (binary distillation) and
collecting it from the column, I do not need the added functionality of a
fractional still. A packed column is similar in principle to a fractional still
but uses a packing medium (Figure 3.6) instead of column trays. The
packing medium functions as a fractional column allowing the vapor
mixture to condense and evaporate on the packing medium also
ensuring a constant temperature gradient along the column.
Figure 3.6 Fractional still using a packing medium
27
A binary distillation is made up of components A and B, A representing
alcohol and B representing water. As the mash is heated up the
molecules begin to separate off and travel up the column as a vapor
mixture of A and B with a higher concentration of A than in the mash. As
this mixture condenses on the packing medium some of the B molecules
will fall back down the column while the A molecules are left to
evaporate. This evaporation will be at and even higher purity than the
first vapor to collect on the packing medium and this process will
continues up the entire column.
Producing high quality ethanol (>90%) from the still requires the
temperature control of two separate areas of the still that must be
maintained at a steady temperature. Maintaining these temperatures
requires closed loop control to ensure only the required amount of
energy is entered into the system. By monitoring these temperatures
and applying only energy needed to maintain these temperatures, my
still will be able to increase the purity the alcohol extracted, thus
increasing the the total yield while reducing the amount of energy
needed to distill alcohol.
3.3 Control System
While theory of both fermentation and distillation are important to
understand my studies and required to execute the experiments
effectively, the control system is where most of the research and
resources were used. The main resource was the PLC used to create a
control system that would run the still consistently without failure and
control the plant in such a way as to produce high quality ethanol.
Background research was used to generate initial solutions to the
problem but these were refined throughout the experimentation
process to create the system used to obtain data displayed in this thesis.
28
3.3.1 Programmable Logic Controller
A computer program is a list of instructions a computer carries out in a
specified sequence. This sequence can be changed in response to inputs,
which will lead to manipulation of the computer program and changes to
the outputs augmenting the system. Programable Logic Controllers
(PLCs) are a special type of computer called a hard real-time systems
allowing the programmer/engineer to control a system in which specific
inputs generate known outputs.
A hard real-time system is a system that reacts to a change in the system
within strict time constraints. If this deadline is missed the whole system
has failed. This type of system is used in situations where a failure to
react to a signal will lead to physical damage of the system, its
surroundings or humans (directly or indirectly). An example is a nuclear
reactor, failure to respond to a signal within a set time constraint can
lead to destabilising of the reaction resulting in a reactor meltdown.
PLCs were invented in 1968 at the request of the American automotive
industry for use in industrial control to reduce production line down time
(Mastilovich, 2010). Large costs were incurred when production lines
were being changed from one job to the next requiring large, highly
skilled teams to rewire large relay boards and reset hundreds of
machines and cams. These cost were to be reduced by replacing these
production line changes with software revisions instead.
A PLC is more suited to controlling industrial plant operations than micro
controllers or PC based control systems. These controllers are built
specifically to meet many industry standards and are made by leading
electrical manufacturers, therefore are easily interfaced with electrical
devices and sensors. PLCs are also designed to operate in harsh industrial
environments
(excessive
electric
29
noise,
vibration,
and
dusty
environments) making them ideal for my project.
The modular design of a PLC means everything is built into the system so
that it can operate when power is applied. The internal design of a PLC is
shown in figure 3.7, the CPU, memory and communications are all built in
to the module. These features of the PLC must be considered when
selecting a PLC for your specific application as they will determine how
large your computer program can be (memory constraints), and also the
complexity of your computer program (processing constraints).
Figure 3.7 PLC hardware architecture
Figure 3.7 shows how the PLC communicates with the inputs and outputs
of the system through its digital inputs and outputs that are buffered
from the CPU by processing circuitry. This buffer allows the programmer
30
to specify situations or events that allow the inputs and outputs to use
CPU hardware efficiently. An example is a hardware interrupt that can be
triggered by a user defined input, a emergency stop button. This must
interrupt the current state of the system and stop the process
immediately or the system could physically damage its environment.
The modular design specific to the Siemens S7-200 series PLC is an
exceptional feature of the Siemens range of PLCs (Figure 3.8). The design
allows an engineer to easily expand the PLC with off the shelf products.
These modules can be used to increase the number of inputs, outputs,
type of inputs and outputs, and also the CPUs capability. An advanced
control system such as the distillation apparatus needs to constantly
monitor the temperature of the system.
Figure 3.8 S7-200 Mounting instructions
The S7-200 CPU222 Unit is supplied with 8 digital inputs and 6 digital
outputs. To read the temperature of the still column and condenser tube
31
I have used K type thermocouples to monitor these temperature from
the still apparatus. This type of electronic temperature measurement
outputs an analog signal that needs to be conditioned before it can be
used by the PLC CPU. I have used a EM231 (Analogue input Expansion
Module) and configured it to work with K type thermocouples.
Figure 3.9 S7-200 PLC Layout
Figure 3.9 is a diagram of the EM231 which uses the configuration
switches in the bottom right corner of the module to determine the
operation parameters. These switches are configured to each application
and allow the engineer to determine their own preferences for
thermocouples, temperature, accuracy, and error signals.
• Switches 1-3 are used to determine which type of thermocouple the
module is reading form, for a K-type thermocouple the configuration is
001.
32
• Switch 4 must always be off as stated in the data sheet.
• Switch 5 is used to determine whether the open wire detection is for
positive or negative temperatures it is 0 for my configuration which
configures it for positive numbers,
• Switch 6 is used to enable open wire detection which sends an error
message if the thermocouples break or come loose, this is enabled and
set to 0 for the still.
• Switch 7 is used to select between Celsius (0) and Fahrenheit (1). This is
set to 0 for my still as I am measuring in oC.
• Switch 8 is used to disable cold junction compensation for using RTD
(Resistive
Temperature
Device),
this
must
be
enabled
for
thermocouples so is set to 0.
For my specific application I have configured the DIP switches to
00100000. This tells the module to measure and condition the output for
degrees Centigrade from a K-type thermocouple with open wire
detection enabled for positive readings.
The critical nature of industrial control and the integration of machines
into the human workspace increases the impact a system failure has on
its environment. The hard real-time system model is used by most PLC
manufacturers to allow their hardware to integrate safely in to the
human-machine workspace. This ensures a PLC scan cycle is executed
within a defined time (usually in mSec), a feature that must be taken into
consideration when writing a PLC program.
A PLC does not read or write to individual inputs and outputs, instead
the PLC reads in all inputs and writes to all outputs every scan cycle. This
scan cycle determines how quickly the system reacts to a signal and can
be impacted by some complex algorithms. Figure 3.10 illustrates this
concept .
33
Figure 3.10 PLC scan cycle
A PLC runs on a four step cycle as shown in figure 3.10:
1)
Scan Input Circuit
•
If a input has been given a special function (Interrupt), jump to
specified code
•
Inputs are updated in the input register for use by the PLC program.
2)
Execute program cycle
•
Use Input register to change state of the program and evaluate any
algorithms.
3)
CPU requests and house keeping
•
Any specialized CPU requests are processed such as communication
(Internet monitoring and/or SCADA).
•
CPU clean up of temporary variables.
4)
Scan Output Cycle
•
Output register is updated by the PLC changing the plants state.
34
3.3.2 PID Control Algorithm
A control algorithm is a series of tasks carried out in sequence to control
a system. Different algorithms have different tasks ordered in different
sequences. A control algorithms main task is to control a systems output
in such a way that minimizes the error (the difference between the setpoint, and the process variable). A PID loop is an example of a control
algorithm Figure 3.11 shows the control loop for a PID algorithm this is a
closed loop control algorithm because it requires the monitoring of the
output, which is then fed back into the system and multiplied by 3
different constants which are then summed to determine the output.
Figure 3.11 PID loop algorithms
The PID algorithm is expressed in figure 3.11 and shows the output
subtracted from the set-point to calculate the systems error. The error is
a measure of how far away the system is from the setpoint. The error is
then fed into the PID algorithm which calculates how the systems output
should respond. A larger error requires a larger response with the goal of
finding a steady state for the system with minimal oscillation (+/- 0.1% of
the setpoint, pending tolerances) (“PID controller”, 2012).
35
The PID control algorithm is a generic closed loop control algorithm used
extensively throughout industrial control systems. One of the main
advantages PID algorithms have over other control algorithms is the fact
that an in depth knowledge of the system is not needed, (you do not
need to model the system). Extremely complicated or financially unviable system models need not be created to gain outstanding control of
a system quickly and efficiently.
These systems are very dynamic and can make adjustments to
compensate for loss of control medium. The PID algorithm subtracts the
error from the set point and calculates an output for the system. The
algorithms tries to reduce the error through each iteration to get to a
steady state that oscillates around the set-point by +/- 0.001, or a value
that is negligible to the system.
Adjustments to the algorithm is made by adjusting the gain (P), the
integral term (I) and the differential term (D). Each variable adjusts the
systems response characteristics and determines how aggressively the
system responds to a change in the output. An engineer must adjust
each variable to suit their specific system with a desirable system
response.
Figures 3.11.0 – 3.11.2 show how each variable effects the output
response of the system independently. The gain P (Figure 3.11.0) adjusts
the sensitivity of the system to the output. The I (Figure 3.11.1) variable
adjusts the size of the error or the amount of fluctuation the system
experiences in early stages of initialization. And the D (Figure 3.11.2)
variable adjusts the length of the error or how long the system will take
to settle to a steady state.
36
Figure 3.12 PID adjusting P value
Figure 3.13 PID adjusting I value
Figure 3.14 PID adjusting D value
37
Figure 3.11 shows the PID loop algorithm, to use this on a PLC it needs to
be reduced down to a mathematical formula that can be turned into a
computer program. Figure 3.15 is a mathematical representation of
Figure 3.11 and expresses the output M(t) as the function of
a
proportional term, an integral term and a differential term.
Figure 3.15 PID calculations separated in to individual terms
M(t)
Loop output as a function of time
Kc
The Loop gain
e
The Loop error
Mintial The initial value of the loop output.
In order to calculate M(t) the continuous functions must be quantized
(sample the continuous function so that is becomes a discreet data set)
into periodic samples. Figure 3.15 can be further simplified for use by the
S7-200 CPU to figure 3.16.
Figure 3.16 Quantized PID algorithm
Mn
The calculated value of the loop output at time n
MPn The proportional term of the of the loop at time n
MIn
The Integral term of the loop at time n
MDn The Differential term of the loop at time n.
38
The proportional term is the product of the gain (Kc) and the error:
MPn = Kc × ( SPn − PVn) (1)
MPn is the value of the proportional term of the loop output at
sample time n
Kc
is the loop gain
SPn
is the value of the setpoint at sample time n
PVn
is the value of the process variable at sample time n.
The integral term MIn is proportional to the sum of the error over time:
MIn =
MIn
Kc × Ts
(2)
Ti × (SPn − PVn) + MX
is the value of the integral term of the loop output at
sample time n
Kc
is the loop gain
Ts
is the loop sample time
Ti
is the integral time (also called the integral time or reset)
SPn
is the value of the setpoint at sample time n
PVn
is the value of the process variable at sample time n
MX
is the value of the integral term at sample time n-1 (integral
sum or the bias).
MX is updated after each loop calculation with the initial value set to
Intimal just prior to the first loop output.
The differential term MD is proportional to the change in the error:
39
MDn =
Kc × TD
(3)
TS × (PVn−1 − PVn)
MDn is the value of the differential term of the loop output at
sample time n
Kc
is the loop gain Ts is the loop sample time
TD
is the differentiation period of the loop (also called the
derivative time or rate)
SPn
is the value of the set-point at sample time n
SPn-1 is the value of the set-point at sample time n – 1
PVn
is the value of the process variable at sample time n – 1
PVn-1 is the value of the process variable at sample time n – 1
3.3.3 PID Tuning
Tuning a PID control loop requires adjustment of the control parameters
to obtain optimum values that give an acceptable system response to
variable inputs. As PID is a generic control algorithm, and “acceptable
response” will mean different things to different systems. An overdamped system figure 3.12.1, Ki = 0.5 (one in which the process variable
does not go over the set-point) may be perfect for an oven where a
substance cannot be exposed to heat above the desired set-point,
however the response time maybe unacceptable for another application.
A PID loop can be used on many different systems without and in depth
knowledge of the system. Values of the P, I and D variables must be
determined for each implementation, with no two implementations
having correlated P,I and D variables. To obtain these values the system
must be on-line and operating for an engineer to obtain and tunes these
40
variables, even if the engineer is using one of the mathematical tuning
algorithms. There are 3 main tuning algorithms: manual tuning, ZieglerNicholas method, and Auto-tune. All 3 have differing advantages but
both Ziegler-Nicolas and Auto-tune require some degree of manual fine
tuning.
Manual tuning requires the engineer to have past experience with similar
systems so that they maybe able to approximate variables to allow the
system to start operating. One documented technique for manual tuning
is to set the I and D variables to zero, then increase the gain (P) to a point
where the output expresses a sinusoidal oscillation. At this point the
gain should be decreased by approximately half, to a value that drives
the output oscillation with a quarter amplitude decay. The second peak
where the PV crosses over the SP is ¼ the amplitude of the first peak
( Figure 3.17) .
Figure 3.17 PV ¼ amplitude decay
Once the system is oscillating with a decay the I variable should be
incrementally increased, decreasing the time the system needs to settle
on the set-point. The D variable should only be used to decrease the
response time of the system if it is unsatisfactory, as too much D or I can
lead to a unstable system. Manual tuning is greatly aided by the use of
41
the table in figure 3.18 as this helps beginners and experts to see how
the response of a system will be affected by increasing the
corresponding variables.
Figure 3.18 PID manual tuning table
Ziegler Nichols method is similar to manual tuning and requires the
system to be on-line and operational. Initially the gain is increased until it
oscillates around the set-point, this gain value is determined as Ku, and
the oscillation period PU is determined as the time between one
complete cycle. These are substituted into the table Figure 3.19 to
determine the corresponding P, I, and D variables. After this tuning takes
place some minor tweaking is required, using the table in figure 3.18 to
obtain an acceptable control of the system, with acceptable response
time and error.
Figure 3.19 Ziegler-Nicholas tunning table
42
The auto tuning algorithm used in the S7-200 is based upon a technique
called relay feedback in which a small oscillation is produced and
sustained in the process. By setting numerical bounds for the process
variable (PV) that are within an acceptable range for the desired system
the auto tune algorithm can drive the system to the upper limit of the PV
and then back to the lower bound. After each iteration (driving the PV
from the upper limit to the lower limit) the algorithm determines new
values which are then used to evaluate the next iteration. This process is
carried out until the PV oscillates around the set point with the desired
amount of amplitude determined by the type of response needed for
the system Figure 3.20.
Figure 3.20 Siemens PLC IDE auto-tuning screen
3.3 Research Related Literature Review
The literature review is a overview of the work that has been undertaken
in the field that best relates the specific thesis. Simai Haji Mati has
43
gracefully summed up the purpose of a literature in a quote that states
“a literature review should be referred to as reviewing and analyzing the
work of literature in relation to the specified topic in research”.
The aim of my literature review is to summarise material from two
different subcategories of bioethanol research: optimization and control
of bioethanol distillation, and the use of bioethanol in the transport
sector. Most studies carried out on biofuel focus on biofuel and its
proposed cost benefit ratio, however this is not reviewed on its own as
this takes into account growing costs, transportation, and other inputs
which I have not used or controlled in my research. The energy cost was a
topic of research for my project but only on the distillation side as this is
the most energy intensive process in the production of bioethanol
consuming up to 50% of the overall energy cost, proposed by C. Black
(cited in Collura & Luyben, 1988).
The drive for an alternate transport fuel has two main factors:
1)
the need to replace oil with a renewable alternative as oil reserves
are running lower each day.
2)
the gases released from burning oil based products are filled with
chemicals that pollute the air, harming all living organisms.
Oil based fuels (diesel and petroleum) are becoming more and more
expensive as the demand for oil increases while the supply (oil, at
present can not be produced synthetically) is decreasing. With as little as
50 years of oil left in reserve, the supply versus demand is driving the
price of a barrel of oil to record highs. Bioethanol is both a renewable
energy source and a cleaner burning energy source reducing, emissions
from both neat alcohol power plants and blended fuel power plants.
44
3.3.0 Distillation Research
Distillation was discovered around the 8th century with water distillation
being used in the 1 st century Alexandria, and the principle has not
changed much since then (“Distillation”, 2012). Distillation requires the
use of an energy source to change a liquid into steam then a cooling
source to convert the steam back to a liquid.
Although process equipment has become bigger and more efficient in
separating the base elements, the general method has not changed.
Most of the advancements in modern alcohol distillation processes have
been in the pre and post distillation phases. These advances are mainly
based around the fermentation (the creation of ethanol), aging and
flavor enhancements in post production for alcoholic beverages.
Studies into the distillation process for chemical engineering are aimed
at reducing the amount of energy a distillation run consumes, while
increasing the amount of distillate collected. A paper by Lina Rueda and
Jacob Duke (2008) states “Distillation is the top energy consuming
process in the chemical engineering industry. Increasing it production
while reducing energy usage requires continuous optimization to drive
the equipment to peak performance”. This paper stated the use of a
advanced process control (APC) with the use of a non-linear model
predictive control algorithm (MPC) which used the model as a reference
to adjust plant outputs for known inputs.
In a paper by Michael A Collura and Wiliam Luyben written in 1988 based
on energy saving distillation designs in alcohol production it was
concluded “To produce a distillate concentration above 95% the column
must be operated at sub atmospheric pressure”(Collura, Luyben, 1988).
In theory, reducing the pressure in the distillation tank reduces the
boiling point of ethanol, allowing for a reduction of input energy. The
45
consequence is that the energy required to place a large volume under a
sub-atmospheric pressure is unrealistic.
The same quality of bioethanol can be achieved by double distilling the
ethanol collected. This is not ideal as the energy input to distill the
fermented alcohol is doubled. A hybrid solution in which two distillation
columns are used will produce similar results. The mash is preheated or
roughly distilled at a high temperature in the first column producing a
low purity ethanol steam (approximately 60%). This is then fed into the
distillation column, which acts like another distillation process using only
energy from the first column to excite the ethanol molecules with and
output stream over 93% ethanol.
MPC at Badger State Ethanol (BSE) used a mathematical model to
describe the plant, from beer column through to the 100% bioethanol
produced out of the molecular sieve (Rueda, Duke 2008). The MPC
algorithm is based on a dynamic model produced by Pavilion
Technologies for this specific plant with the critical component in the
algorithm being this model (Rueda, Duke 2008). Emperical methods and
first order principles were used to model the plant “Empirical modeling
alone is often used in MPC because solving a set of complex differential
equations of first order principles models for the calculation of the
optimal sequences at each control interval may not be feasible”(Rueda,
Duke 2008).
Figure 3.21 is a diagram of the Badger State Ethanol distillation plant.
The 95% (190 proof) ethanol is feeds in to a series of molecular sieves
which use recycled product and energy to regenerate spent sieves. BSE
sends mash in to a beer column which only partly distills the mash into a
solution of approximately 60%. The low purity steam is then fed in to the
rectifier column which further separates the the water from the solution
46
to send 95% ethanol vapour to the molecular sieves.
Figure 3.21 BSE
Once the steam is passed into the rectifier column, the high purity
ethanol travels up and out of this column. Low quality ethanol and water
will fall to the bottom of the rectifier column. This liquid is collected in
the side stripper which allows high purity ethanol to escape, while
feeding water and low quality ethanol back into the beer column to
ensure no fermented ethanol is lost during the distillation process.
Badger State Ethanol contracted Pavilion Technologies to implement an
APC system on this established bioethanol plant to increase the output
47
of bioethanol collected while reducing the the total energy consumed
(Rueda, Duke 2008). Badger State Ethanol uses a multi-column still to
produce 95% ethanol, which is fed through a molecular sieve that
removes the last 5% water to produce 100% ethanol. The MPC algorithm
that Pavilion technologies implemented reduced the amount of steam
usage for the plant by 9.95% while increasing the plants bioethanol
production by 10.24% (Rueda, Duke 2008).
3.3.1 Ethanol Fuel Research
With the increased production capacity of bioethanol and the reduction
in cost per litre research into the best way to use this new fuel source is
steadily maturing. A future in which cars run on neat bioethanol (100%
ethanol as their primary source of energy) is the goal which will totally
remove the need to consume oil. However the oil industry has spent
hundreds of billions of dollars on infrastructure and supply chains to
ensure a constant supply of oil based products in every country.
The adoption of bioethanol has already begun with America running
most petrol powered vehicles on E10 (10% bioethanol and 90% petrol)
with the next logical step being E30, followed by E50, eventually running
all spark injection power plants on E100. This staged approach to biofuel
adoption is not only the most logical approach to the oil crisis it is also an
environmentally sensitive choice. Bioethanols higher oxygen content
make it a great fuel additive as it aids fuel combustion by supplying extra
oxygen. Significant pollution reduction can be achieved from blended
fuel types, reducing overall pollution by up to 16%, even in blends as low
as E10(Hulsey, Coleman, 2006).
Some research into the use of bioethanol in auto mobiles concludes that
it causes an increase in ozone potential chemicals (Hulsey, Coleman,
48
2006). These studies are however based on computer models that do not
relate to analytical results. Analytical experiments carried out in
Wisconsin, California, and New York show a consistent reduction in air
pollution that corresponds to the adoption of E10 fuel in the associated
cities. Clearing the Air with Ethanol by Brett Hulsey and Brooke Coleman
(2006), reviewed data about air pollution and the associated emissions
benefit from the use of E10. This paper concluded that “Ethanol reduces
carbon monoxide (CO) and soot particulate matter (PM) emissions by at
least one-third” and can increase overall air quality up to 16%(Hulsey,
Coleman, 2006).
A study in South Eastern Wisconsin (SEW) compared the areas ozone
excedence days before and after adopting the use of E10 for all petrol
vehicles. This study showed a dramatic reduction in ozone excedance
days after 1994, the year that Wisconsin introduced E10. “Before 1994
the average was 630, after 1994 the average was 539 a reduction of 16%
in ozone excedance days”(Hulsey, Coleman, 2006).
Figure 3.22 Graph of SEW Ozone exceedance days
49
While it is proven E100 would greatly reduce both on-road and off-road
carbon monoxide emissions, the infrastructure needed to supply this fuel
is cost prohibitive and irresponsible with the current global economic
state. Lower blend fuels (E10 and E30) not only reduce emissions from
standard petrol engines but they are also a catalyst for bio-industrial
growth and fuel diversification. This will allow the biofuel industry to
grow using private money. With 80% of the American emission inventory
for carbon monoxide being accounted for by on- and non-road vehicles,
a reduction of 20% emissions will have a great impact. A 36% decrease in
soot particles is predicted to dramatically reduce health side effects
associated to petrol powered vehicles and their emissions(Hulsey,
Coleman, 2006).
Comparisons of petrol engines to petrol engines running on E85 are not
a good representation of the potential biofuel has as a fuel source for
the future. Flexifuel vehicles can run on E85 but they are not optimized
to, and the need to run on petrol restricts the extent to which these
combustion engines can be reconfigured. The comparison is similar to a
diesel engine running on petrol, it is theoretically possible but the
engine will perform poorly compared to its diesel equivalent.
Flexifuel vehicles which can run using either petrol or blended fuels are
petrol power plants that have been modified to enable their petrol
engine to run on neat bioethanol. When using neat bioethanol these
engines produce less power and consume more fuel than running on
petrol. When running on alcohol the engine does not change but the
ECU (Electronic Control Unit) uses a different fuel map, increasing the
amount of fuel supplied, which negatively impacts the fuel consumption.
The combustion characteristic are very different for petrol and
bioethanol, which is due to their chemical structure (similar to petrol and
50
diesel). While bioethanol can be combusted in a environment optimised
for petrol combustion, petrol can not be used to fuel a power plant
optimised for bioethanol. Flexifuel vehicles do not utilise the
advantageous characteristics of bioethanol, most notably the reduced
amount of oxygen it needs to combust, resulting in smaller engines that
can produce similar power figures to a much larger petrol engine.
“The benefits and challenges of neat alcohol fuels in PFI (Port Fuel
Injection) applications have been demonstrated in numerous earlier works.
Benefits such as higher efficiency and specific power and lower emissions
may be realized with alcohols: their high octane number gives the ability to
operate at higher compression ratio without preignition [5]; their greater
latent heat of vaporization gives a higher charge density [1-3, 6]; and their
higher laminar flame speed allows them to be run with leaner, or more
dilute, air/fuel mixtures [7]. In addition, alcohols generally give lower fuel
heat release rates, resulting in lower NOx emissions and reduced
combustion noise [2]. The engine described in the present work uses these
inherent advantages of alcohol fuels as the basis for its design and control,
thereby enabling attainment of efficiency levels exceeding that of the
diesel, with low emissions.” (Brusstar, Stuhldreher, swain, and Pidgeon,
2002).
The paragraph above was from a research paper High Efficiency and Low
Emissions from a Port-Injected Engine with Neat Alcohol Fuels by the U. S.
Environmental Protection Agency, in which they state “Alternative fuels,
especially alcohol fuels, offer potential to mitigate national security and
economic concerns over fuel supplies as well as environmental concerns
over tailpipe emissions and resource sustainability” (Brusstar, Stuhldreher,
swain, and Pidgeon, 2002). In this paper a heavily modified VW Turbo
charged diesel engine is used as a test bed for neat alcohol combustion.
A diesel engine was used as the compression ratio at factory is quite high
51
so only slight modification were made to the compression ratio. Spark
ignition was fitted with direct port inject to increase spark authority and
increase efficiency, Figure 3.23 shows the test engines specifications.
Figure 3.23 Bio-fuel engine specifications
This study concluded that engines optimized to run on bioethanol can
produce better than 40% brake thermal efficiency, better than a
comparable diesel plant and with extremely lower emissions(Brusstar,
Stuhldreher, swain, and Pidgeon, 2002). This configuration has one last
barrier which is cold start emissions but further research will reduce
these, “The present engine, optimized for alcohol fuels, exceeds the
performance of current conventional- fueled engines, and has potential as
a lower-cost alternative to the diesel”.(Brusstar, Stuhldreher, swain, and
52
Pidgeon, 2002)
“An important step toward increasing alcohol fuel demand, then, may lie in
providing economical engine technology options that utilize such fuels
more efficiently, to compensate for the lower fuel energy density. The FFVs
produced today, however, use fairly typical gasoline engines, which,
because they must retain dual-fuel capability, are not able to take full
advantage of the favorable combustion characteristics of alcohols”
(Brusstar, Stuhldreher, swain, and Pidgeon, 2002)
53
4.0 Process
The chapter below describes the processes used to create the mash,
assemble the distillation apparatus, and how the results were measured.
This is with the aim of displaying my understanding of how ethanol is
created and the control algorithms and hardware that are used to
control this process. I intend to convey this understanding though
thorough explanations of the mash creation process and how I used the
Siemens hardware and IDE to control and measure the distillation
process.
This chapter will also be useful reference material for students wishing
to further the study of energy efficient bioethanol distillation. Supplying
information which will enable a person to create and distill bioethanol
quickly, allowing them to focus their resources on their desired subtopic.
Both of these goals will be achieved by describing the procedures used
step by step with in-depth explanations accompanied by photos from the
actual process used in my experiments, supplemented with research
media.
4.1 Brewing Ethanol
In section 3.1.2 (Fermentation) it is explained that yeast can ferment
nearly any sugar source to create bioethanol. When using raw organic
material (fruits, vegetables, and organic waste material) the sugar
content is different between each type and batch of feedstock used. For
a viable fermentation (greater than 10% bioethanol is present in the
mash) to occur the sugar content will need to be measured and the
amount of feedstock adjusted to create a viable mash.
The feedstock used will determine the amount and type of nutrients
54
supplied to the mash, and may also have an impact on the pH level. Even
when using the same feedstock between batches these values may
fluctuate
due
to
differing
agricultural
environments.
Nutrient
corrections need to be carried out to ensure the mash is an optimum
fermentation environment, ensuring complete fermentation of the
available sugar. For this type of fermentation these inputs cannot be
fixed and the results may not relate to the distillation process alone.
Table sugar (Sucrose) was chosen as the feedstock for my experiments as
it can be assumed that the quantity of glucose is the same between
batches. This means the amount of feedstock required for each
fermentation is fixed, resulting in a known quantity of bioethanol from
each batch. As the amount of feedstock is fixed between batches the
nutrient and pH adjustments can also be fixed. Fixing these inputs is
important as the aim on my thesis is to maximise the bioethanol yield
from the distillation process, not the yield from the entire process.
When using sucrose as a feedstock more nutrient corrections are needed
than a organic material feedstock. This is because the sugar does not
contain the trace mineral elements needed for fermentation. Organic
materials contain these trace elements, most notably nitrogen. These
minerals and a pH of 4.5 are necessary for the reproduction of yeast cells
and the fermentation of sucrose to bioethanol and carbon dioxide.
In the introduction I explained that I will be using “off the shelf” brewing
equipment and products to not only ferment the alcohol but also to
separate the alcohol from the mash. Off the shelf or hobbyist brewing
materials simplify the process of brewing ethanol and are available in
convenient quantities. These products are also designed with manual
distillation in mind, allowing me to use one distillation apparatus for
both manual and automatic distillations.
55
These products are designed for hobbyist brewers, removing the need
for a laboratory to culture yeast and measure the nutrient corrections
needed for the mash. The simplicity of these products reduces the
possibility of human error by providing pre-packaged quantities of yeast,
nutrients and pH correction that are guaranteed to ferment. This allows
me to create a test scenario in which all inputs are fixed between
experiments ensuring standardised results.
Turbo yeast fermentation packs (Figure 4.0), which can be brought off
the shelf, are designed to yield 14% ethanol consistently in home
brewing situations. These packs contain the correct amount of yeast,
nutrients and pH corrections for a 25 litre mash with 6 kilograms of
sugar. These packs are stacked (2 packs used in the same fermentation)
in my experiments with 12 kilograms of sugar to produce a 50 litre mash
at 14% alcohol content, approximately 7 litres of bioethanol.
Figure 4.0 Turbo yeast packet
56
These packets come with a specific procedure for the creation of the
mash. This
procedure is to ensure the right temperature for
fermentation is achieved and that there is full fermentation of the
available sugar. Sanitation is extremely important to ensure that no
unwanted organisms can grow in the mash. This can cause the
fermentation to stall as the organisms consume all the sugar before the
yeast can ferment it.
1.
The fermentation tank and all brewing equipment are washed
using bottle wash to ensure no micro-organisms are present
2.
12Kg of sugar is added to a 60L fermentation tank
3.
Boiling water (approximately 15L) is added to the sugar
4.
The syrup is mixed until all the sugar is dissolved
5.
Tepid water is added to create a solution of 50L at 30 oC
6.
Add 2 packets of Turbo yeast 48Hr to the solution immediately
7.
Mix solution vigorously for 1 minute to introduce excess oxygen
and stimulate yeast reproduction.
8.
A Pre-fermentation specific gravity reading is taken and recorded.
4.1.1 Brewing Temperature
Once the yeast has been pitched (added and mixed to the sugar syrup)
and the fermentation begins, the temperature of the mash must be
closely monitored. Yeast can only ferment in a small temperature
window between 20oC and 40oC. Below 20oC the yeast begin to hibernate
and cannot ferment, conversely, over 40 oC the yeast will die. Outside of
these conditions the number of yeast cells available to convert sugar into
ethanol is reduced, resulting in slower fermentations and reduced
ethanol productions through inefficient sugar conversion.
Temperature control of the fermentation tank was not used in my
57
experiment as it was unnecessary for the size of the tank. Precautions
were taken to ensure that the environment in which the mash was
fermented did not fall below 20oC as it can be assumed ambient
temperatures in New Zealand do not go over 40 oC. The best time for
fermentation was summer as the overnight temperatures indoors did
not fall below 20oC.
I tried to avoid fermentation during winter but it was necessary and the
mash was fermented next to a heater. This was to avoid the tank
temperature falling below 20 oC, and the mash was monitored every 6
hours to ensure it was fermenting. This worked well for a fermentation
tank of 50 litres, however anything larger and this solution would not
work as there is not enough surface area to keep the mash warm.
4.1.2 Air Lock
Yeast cells are living organisms and the biological process where yeast
cells convert sugars into ethanol and carbon dioxide (fermentation) goes
through two phases. In the first phase, reproduction, the yeast consumes
the excess oxygen in the fermentation tank and uses it with the glucose
as an energy source for reproduction budding. In the second phase,
respiration, the yeast consume the glucose without oxygen and excrete
ethanol and carbon dioxide.
The use of an air lock (Figure 4.1) in the production process is necessary
even if it is only applied after the first 24 hours. This will allow the rapid
production of carbon dioxide to take place while supplying oxygen to the
yeast to ensure good reproduction is achieved. After the colony of yeast
has fully reproduced (approximately 24 hours) the air lock should be
applied to ensure no oxygen is present for fermentation, while allowing
the carbon dioxide produced to escape ensuring a good conversion of
58
glucose to ethanol.
Figure 4.1
Fermentation air lock
Both oxygen and sugar are introduced into the mash in the early stage of
mash creation. First the sugar is mixed with water vigorously, serving
two purposes: it aerates the mash introducing excess molecular oxygen
for yeast reproduction, and it mixes all the sugar into the solution
ensuring full fermentation of all available sugar. Both sugar and oxygen
are vital but without the proper nutrients available and a slightly acidic
mash,
proper
fermentation
will
not
occur.
Slow/low
yielding
fermentation may occur but there are no guarantees.
4.1.3 Measuring Fermentation
A fermentation is complete when all the sugar in the mash has been
consumed by the yeast, which is indicated by the specific gravity of the
59
mash. The difference between the pre-fermentation specific gravity and
the current specific gravity reading of the mash can be used to
determine how much ethanol has been produced. For a sucrose
feedstock fermentation the final specific gravity reading should be
below 0.992, from an original gravity of 1.100 this would indicate a mash
of 14% Alcohol, approximately 3.5 litres from 25 litres of mash. Equation
(1) is used to determine expected yield, equation (2) is readings from my
experiments.
((1.05 × (OG − FG)) / FG ) / 0.79 × 100
(1)
((1.05 × (1.100 − 0.992)) / 0.992) / 0.79 × 100 = 14.47
(2)
Specific gravity is the ratio of the density of the liquid measured to the
density of water. To measure the specific gravity of a liquid, an
instrument called a hydrometer is used. A hydrometer is usually made
from glass with a cylindrical stem and a bulb weighted at the bottom to
make it float vertically. The hydrometer is placed into the liquid allowing
it to float freely, the point at which the surface of the liquid touches the
stem of the hydrometer is noted.
Figure 4.2 Hydrometer
60
Hydrometers usually contain a scale inside the stem so that the specific
gravity can be read directly, a variety of scales exist and are used
differently depending on the context. The hydrometer I used was
designed to determine the specific gravity of a fermenting mash (Figure
4.2). The readings from the hydrometer scale and equation (1) are used
to determine alcohol potential. When sugar is mixed into the mash it
increases the density of the mash, as the yeast consumes the sugar and
excretes alcohol the density of the mash decreases.
By taking readings on a regular basis and recording them the brewer can
then use this information to determine how much fermentation has
occurred and calculate the rate of fermentation. The rate at which the
fermentation is occurring can be used to determine the the health of the
fermentation batch, and may signal to a problem before it occurs. A
stalled fermentation is one in which the decrease in specific gravity stops
before it has used all of the available feedstock, resulting in wasted
feedstock.
4.2 Distillation
Once fermentation has been completed, the mash must be moved into
the still tank (Figure 4.3 (1)). Once in the tank the still head (column) is
fitted to the top of the tank (Figure 4.3 (2)) by slotting the rubber seal on
to the tank flange. After fitting the column the water lines are connected
to the water mains and the drain is routed outside into the fermentation
tank. Thermocouples 0 and 1 are connected to the head and the
condenser respectively (Figure 4.3 (3)) , and the program started.
The still used in my experiments is a 60 litre Euro still with a 1.5 kilowatt
main coil and a 2 kilowatt booster coil. The distillation head is packed
with steel wool and has six cold water injection points, the longest being
61
down the condenser shaft. The column and the condenser are two areas
that must be controlled independently to obtain high quality bioethanol.
Figure 4.3 50 Litre Euro-still
The still separates the solution by introducing heat energy to change the
ethanol from a liquid phase into a gaseous phase. This gas then rises up
the distilling head, condensing and evaporating over the entire length of
the column until the vapour enters the condenser. The column has cold
water introduced at five locations to remove some of the heat energy
from the vapour before it enters the condensing chamber to be cooled
below 70oC. Vapour entering the condenser is flowing in the opposite
direction from the column to ensure the ethanol vapour is pure.
Temperature control of the mash is important as this will determine both
the purity and the efficiency of the distillation. Oscillation around the
boiling point is not ideal as this will both increase the amount of water in
the receiver tank lowering the quality of the biofuel, and increase the
time of the distillation. A longer distillation increases the total energy
62
cost of the batch, decreasing the efficiency of the still . Low quality
bioethanol also increases the total energy cost, as further processing is
needed if the total batch quality is below 93%.
To manually conduct a distillation the tank must be heated to
approximately 80oC, at which point the booster coil is turned off. Once
steam rises up the head to the condenser, water must be passed through
the cooling circuit and adjusted to keep a steady stream of ethanol
flowing out of the condenser. This process must be watched closely
because as the temperature increases in the column, the amount of
water must to be increased accordingly. For manual distillation there is
limited control over the heat energy entering the tank so the percentage
bioethanol steadily decreases as the distillation progresses.
The key to a good distillation is determining how much energy is needed
to convert the ethanol in the solution to a vapour. This is the Set Point
(SP) of the distillation and the control system must maintain the Process
Variable (PV) as close as possible to the SP. Controlling the amount of
energy used to separate the bioethanol from the mash is important, as
this will control the purity of the distillate collected and also the
efficiency at which it is collected.
The amount of energy needed to distill 95% ethanol must be determined
as to remove the bioethanol and no water. Water has a boiling point of
100oC and ethanol 80oC. An aqueous solution of these two chemicals has
a boiling point in relation to the molar volumes of each chemical in the
solution. Figure 4.4 is the equation that is used to determine the vapour
pressure (Yethanol ) at which the ethanol will vaporise off the solution made
up of PoEthanol and PoWater where Po is molar percentage.
63
Figure 4.4 Ethanol vapour pressure calculations
Using figure 4.4 to calculate the SP of the distillation tank shows that as
the ethanol evaporates the boiling point increases. This was confirmed
through experimentation, as the SP of PID loop 0 had to be increased
every hour or so to generate a bioethanol stream out of the still. This
was solved by moving the thermocouple to the top of the column and
changing the SP to 78.5 oC (temperature of the azeotrope of 95%
ethanol and 5% water) which produced an constant stream of bioethanol
at over 93% purity.
To control the still autonomously I programmed the Siemens S7-200 with
two PID control loops to control two areas of the still and maintain
constant temperatures of these areas independently. These control
loops were set up from the MicroWin PLC programming IDE (Figure 4.4)
with the PID tuning tools supplied in the IDE. The MicroWin IDE was used
to program the PLC, tune the PID loops and used to debug and monitor
the distillation process. Figure 4.5 is the IDE being used to monitor and
debug the PLC, blue networks are active and grey networks are inactive.
The PID tuning wizard (Figure 4.6) enables the programmer to directly
input variables for their PID loop. This can also be used to monitor the
PID loop in real time. The blue line is the output, red line is the input and
the green is the set-point. This screen allows you to view the PID loop in
real time and adjust the variables so that you can see how this effects
the PID algorithm immediately.
64
Figure 4.5 Siemens PLC IDE programmer
Figure 4.6 Siemens PLC IDE PID wizzard
65
4.3 Program design:
The S7-200 PLC continuously loops through the program logic until it is
either manually stopped or a user defined state is reached stopping the
process. The PLC cycle begins by reading the status of the inputs, which
are then read by the control logic (computer program) and used to
evaluate the current state of the system. Once the program has cycled
through, the PLC writes the new control logic data to the outputs. Figure
4.7 shows a basic PLC application where a Stop/Start switch is used to
turn on or off a motor, when the Start_PB is pressed the logic evaluates
the current state and turns M_Starter on starting the motor.
Figure 4.7 Basic PLC program
In figure 4.7 the first two logic operations start and run the motor,
Start_PB is activated which activates M_Starter, this memory location
then keep the network live until E_Stop is activated. The first two logic
operations are normally open, meaning they must be activated to close
66
the circuit, E_Stop is normally closed which will break the circuit if the
E_Stop button is depressed. These logic operations are the computer
program, and are scanned every cycle of the PLC ensuring the control
system is always monitoring the state of the physical system.
Figure 4.8 shows the MicroWin IDE used to program the PLC. There are
three main areas of the IDE: the navigation bar, the instruction tree and
the program editor. The navigation bar allows the IDE to quickly swap
between screens containing vital information about the PLC, the
instruction tree displays all the available instructions the PLC can use,
and the Program editor is used to write and debug PLC programs.
Figure 4.8 Siemens PLC IDE
67
To control the still in a efficient and safe manner the program was
separated in to 4 areas that are independent from each other. The
interrupt routine, recording the highest temperature, and the two PID
loops are all independent and are controlled as such. Each of these
sections was evaluated to ensure that a failure of a component or the
PLC did not cause the plant to become unsafe. An example is the
emergency stop button - a failure in the wiring or the button would not
be apparent until it was needed. Holding the input high ensures an
emergency stop would occur and the plant would not work if a failure
occurred in the circuit.
Interrupt routine: The interrupt routine is used to stop the system if the
Emergency stop button is activated. By using the input hardware buffer
the PLC can allocate one of its inputs to interrupt the CPU at any stage of
the scan cycle and execute a user defined routine. This routine turns all
the outputs off and holds the system until the Emergency stop button is
activated, as the interrupt routine is activated by the falling edge of the
Emergency stop input.
Recording temperatures: The PID loop used to control the temperature
of the column and condenser can become unstable if the P, I and D
variables are not correct. This instability can be hard to detect as it may
only occur after long operation runs. By recording the highest
temperature read by the thermocouples one can identify this problem
and make adjustments to increase the systems stability.
PID loop 0 and 1: The column and the condenser are two areas which
are controlled independently. This is so different PID loops can be used
to control the different systems. Although these areas are attached to
the same system they need independent control as they have differing
operational characteristics.
68
The above sections were translated to the actual program below on
pages 71, 72, and 73 that was used to run a distillation autonomously,
achieving over 93% ethanol throughout the entire distillation. Each
network (line of code) is explained in-depth in the following pages.
Network 1: SM0.0 (this bit is a Special Memory bit that is always on) to
enable interrupts and assign this interrupt to an input. INT_0 assigns the
interrupt subroutine activation to be attached to I0.3, which is my
emergency stop button. Attaching I0.3 to the interrupt switches the
hardware buffer to activate a CPU interrupt if I0.3 is triggered. EVNT = 4
assigns this hardware interrupt to activate subroutine INT_0 on the
falling edge of I0.3. This is a safety feature as the emergency button is
always on so when it is pressed the input I0.3 will fall, triggering the
interrupt routine which resets all outputs and shuts down all the coils
immediately. This case is also true if there is a failure with the emergency
stop circuit, as a failure of this circuitry would cause I0.3 to fall stopping
the system.
Network 2: recording the maximum temperature at the condenser
throughout operation, to indicate if the PID algorithm is functioning and
is not becoming unstable during operation. This is useful for problem
solving, if the feed purity suddenly drops or the overall production is of
poor quality one can quickly determine weather it is a still problem or a
fermentation problem. The CondTemp is read from AIW0 and compared
to VW12 which is a memory location storing a word at position 12 in
Variable memory which is the previous Maximum temperature. Each
cycle the comparison determines whether the current value is greater
than the previous maximum temperature, if it is the reading is moved
into the CondMax variables (VW12), if not the program moves on and
compares values next cycle.
69
Network 3: record the maximum temperature at the tower (distillation
column) throughout operation, to indicate if the PID algorithm is
functioning properly. The TowrTemp is read from AIW2 and compared to
VW10 which is a memory location storing a word at position 10 of
variable memory which is the previous Maximum temperature. Each
cycle the comparison determines whether the current value is greater
than the previous maximum temperature, if it is the reading is moved
into the TowrMax variables (VW10), if not the program moves on and
compares values next cycle.
Network 4: PID_0 loop is used to control the 1.5 kilowatt coil, this block
requires SM0.0 to keep it running at all times and drives bit V8.0 which is
a variable I have assigned TowrDrv. The PID loop compares the
TowrTemp to VD8 which is a Double variable at variable memory position
8 this contains the set-point of the tower. This network then drives
network 7 and 8, network 7 turns on the coils when V8.0 is on and
network 8 turns the coil off when V8.0 is off.
Network 5: This network is used to control the larger 2 kilowatt coil as
this coil is only used to quickly bring the mash up to temperature. The 2
kilowatt is turned off once the tower is over 70 oC Network 5 compare
AIW2 to a fixed value of 700 this equates to 70.0 oC. If the comparison is
true (the tower is over 70 oC) the Network resets the Q0.1 output turning
the 2 kilowatt coil off.
Network 6: During the early phases of distillation a booster coil is used
to increase the temperature of the mash quickly. Network 5 turns the
coil off, Network 6 is used to turn the coil on by comparing the AIW2 to a
fixed value of 700 if its lower than 700 the coil is switched on.
Network 7/8: Network 7 and 8 are used to drive the 1.5 kilowatt coil
70
controlled by the PID loop. In the Siemens IDE the PID loop can not
directly drive the outputs of the PLC so Networks 7 and 8 switch the 1.5
kilowatt coil on or off controlled by the PID loop.
Network 9: PID_1 loop is used to control the water valve, this block
requires SM0.0 to keep it running at all times and drives bit V8.1 which is
a variable I have assigned CondDrv. The PID loop compares the
CondTemp to VD4 which is a Double variable at variable memory
position 4 this contains the set-point of the condenser. This network then
drives Network 10 and 11, network 10 turns on the coils when V8.1 is on
and network 11 turns the coil off when V8.1 is off.
Network 10/11: Network 10 and 11 are used to drive the coil controlling
the water valve which introduces the water needed to cool the
condenser. These networks are the same as Network 7 and 8, interfacing
between the PID loop and the physical coils it is driving.
71
72
73
74
4.4 Electrical design:
The electrical part of the system was separated into two areas: the
control system and the plant (distillery). The control system utilises
industrial control hardware to operate the still to enable the systems to
be scaled up for further research. Industrial distillation apparatus were
not used due to the associated operational costs. The second part of the
electrical system containing the still elements and water valve were not
industrial quality but did not fail during operation. The water control
valve was from an automated garden watering system, and the hot water
elements are no different to those found in most household kettles.
The scalability of the PLC had was a major factor when choosing a control
system but also the hard, real-time nature of a PLC was far more
desirable than cost benefits for a micro controller. When operating a still,
dangers are present in the form of high temperature liquids, high
pressure steam, and toxic gas (95% ethanol vapour is harmful to
humans). The loss of control of such a system can cause serious damage
to the systems environment and its human operators. For a small scale
system like my still a micro controller would be sufficient but constant
monitoring is needed to ensure the system does not get out of control.
With the PLC after the first few operations it was observed that with the
system stability one could safely operate the system remotely.
4.4.1 PLC Commissioning
Siemens offer a large number of PLC configurations to enable control of
a wide variety of industrial systems. When selecting a PLC one must
allow for scaling to occur as the machine may grow in capabilities, the S7
PLC series allows the engineer to use the same PLC for many machine
iterations by increasing the functionality of the PLC with expansion
modules. Figure 4.9 is the S7-200 used in my project. It has eight digital
inputs and six digital outputs, with a processor capable of evaluating
75
Boolean instructions in 0.22 milliseconds.
Figure 4.9 S7-200 PLC
To ensure that no live inputs or outputs become loose and short during
plant operation, the PLC must be rigidly fixed to some form of common
mounting board to which all electrical devices are attached. In industrial
situations an enclosure should be used to ensure minimal dust, water or
other foreign objects do not interfere with normal operation. Enclosures
also ensure humans do not inadvertently come in contact with high
voltage terminals or allow them to interrupt or override normal scan
cycles, causing damage to machines.
When mounting the PLC, cable guides need to be used to ensure initial
commissioning and set-up are able to be commenced as quickly as
possible. This also means that time spent fault finding is dramatically
reduced. These guides should run in a square enclosing the mounting
surface with horizontal rungs placed like a ladder from the top to the
bottom of the surface, allowing ample space for din rail and hardware
attached to the din rail. As the PLC could be running non stop processing
76
complex algorithms, a 20mm gap above a below the PLC must be allowed
for to ensure ample convective cooling can take place.
Figure 4.10 S7-200 Mounting instructions
The PLC was mounted to a prototyping board along with a 240v/12v
power supply used to power the PLC and other electrical devices in the
system. The analogue input module was connected to the PLC via the
expansion port connected through a Siemens specific bus connector.
Figure 4.10 shows the mounting positions for all its S7-200 PLC series
and all the applicable expansions modules. This guide was used to mount
both modules ensuring good air flow for the convective cooling
employed by these modules
There are three inputs: the emergency stop (Digital) and the two
77
thermocouples (Analogue), and three outputs: the 1.5 kilowatt and 2
kilowatt coils and a water valve, all of which are digital. The emergency
stop button is connected straight to one of the S7-200s inputs while
both thermocouples are attached directly to the EM231 module. All of
the outputs use an optically isolated Solid State Relay to interface the
PLC digital outputs and the coils in the devices they are driving. Figure
4.11 is a wiring schematic used for my project.
Figure 4.11 Still wiring diagram
4.4.2 Solid State Relays
Back EMF generated when power is removed from a induction coil can
generally be disregarded, however the frequency at which the induction
coils are driven can generate large amounts of back EMF and will cause
reverse breakdown of the silicon in the PLC output circuitry. The output
circuitry has adequate internal protection for most applications relays
78
and other coils need external protection, the S7-200 manual suggests a
suppression circuit Figure 4.12. The Diode A is used to ensure current can
only flow from the PLC output and not into its circuitry, negating the
back EMF. Additionally, diode B should be used for high frequency
applications as it reduces the time it takes to dissipate the back EMF.
Figure 4.12 Inductive load protection circuit
A more robust solution is to use a optically isolated Solid State Relays
(SSR). These relays separate the driving circuit and load circuit using a
LED and LRD to activate and close the load circuit. Figure 4.13 is a
diagram of the internals of the SSR used in my project. Points 3 and 4 are
connected to the driving circuit (the PLC) used to activate the load circuit
(the heating elements and the water valve) connected through points 1
and 2.
Figure 4.13 optical isolating circuit
79
The physical size of the relay is due its oversized driver chip used to
activate the load circuit. This allows the SSR to be used in conditions
where high back EMF is expected and high frequency drive occurs,
ensuring the long life expectancy of such a device. This was the reason
for using such a device to ensure that long run cycles (10hrs+) would not
break or destroy the circuit, resulting in poor or unexplainable operation
of the plant.
The solid state relays (SSR) need to be connected to a heat sink to ensure
proper operation of the SSR is not impeded by heat soak at full load. The
SSR is attached to the large aluminium prototyping board that holds the
entire electrical system used by my still as seen in figure 4.14. This is not
the heat sink specified by the manufacturing company but can be
assumed to be sufficient as the SSR are mounted directly on to the
aluminium, giving them good contact between the SSR and the heat sink.
The effects of the heat sink is apparent from figure 4.15, reducing the
SSR max load current by 60%.
Figure 4.14 Optical isolators
80
Figure 4.15 shows a graph of the maximum load current that can be used
on the SSR versus ambient temperature, and clearly shows the
importance of a heat sink for this device. From the graph it can be seen
that a load current on the SSR should not exceed 4 amps (RMS), when
using a heat sink a load of 10 amps(RMS) can be used. It can also be seen
that active cooling needs to be implemented in applications in which this
device will be operated in temperatures over 40 oC . As it can be assumed
at this point in time these ambient temperatures are unlikely in New
Zealand, only passive cooling was used with these SSRs.
Figure 4.15 Load vs Ambient Temperature for Optical
Isolators
4.4.3 Analogue Input Module
The EM 231 is a low cost analogue input module with a high speed 12 bit
analogue to digital converter, allowing it to convert an analogue signal to
a corresponding digital value in 149 milliseconds. The analogue
conversion executes each time the program accesses the memory
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location corresponding to the analogue input. This time is added to the
basic execution time of the instruction and should be considered if
execution time restraints are important to your system. The EM 231
module sends the PLC an unprocessed digital representation of the
supplied analogue voltage, that can follow rapid changes in signal
including noise. For a slow changing process value (PV) like the still the
inputs need to averaged to minimise reading to reading variations.
The analogue input module supplied by Siemens for use with the S7
series PLC can be configured for all types of thermocouples or resistive
temperature detectors (RTD) out of the box. This allowed me to attach
two K-type thermocouples directly to the AI module with out any
calibration. After attaching the module to the PLC and the two
thermocouples the system could read temperatures in degrees celcius
within +/-0.1oC allowing for precise measurements without the need to
calibrate the system.
The EM 231 and all devices that use thermocouples to measure
temperature are actually reading the voltage created by the
thermocouple. This analogue voltage is then converted into a digital
word that represents a number, figure 4.16 shows how the EM 231
system actually works and how the information is transferred to the PLC.
The PLC program calls either A,B,C, or D which latches the current
reading into the analogue to digital converter circuitry, this value is then
stored in the appropriate memory location and used accordingly.
When a reading is called from the PLC the appropriate circuit is
energised and the voltage across the cold junction is measured and fed
into the Op-amp which magnifies the signal from a few millivolts to a
voltage comparable to the reference voltage (12V). The output from the
Op-amp is passed through a gain filter to allow for calibration if
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excessive noise is present in the circuit, this is fed into a buffer to allow
the A/D (analogue/digital) converter to take synchronised readings
without data colliding on the actual A/D converter.
Figure 4.16 S7-200 Analog to Digital converter
4.4.4 Thermocouple
A thermocouple is a device that uses this principle of voltage creation in
relation to temperature difference, and are a widely used temperature
sensor in industrial processes. Any junction of two dissimilar metals will
produce a small electrical voltage that is related to the temperature
difference experienced by this junction. Different alloys are used for
different temperature ranges, all of which produce predictable and
repeatable relationships. Figure 4.17 shows how a thermocouple is used
in most applications, with two dissimilar metals attached at the “Hot
junction”, and the “Cold junction” used to measure the voltage produced
by the temperature difference between these two junctions.
The K type thermocouple was chosen after reference to figure 4.18, as it
has a continuous working range of 0 – 1100 (the junction will not fail in
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these ranges), is cheap, widely available and was concluded to be the
best thermocouple for this application. Another thermocouple that
could replace the K-type if unavailable is the T-type, which has a much
smaller range than the K-type, resulting in better accuracy across this
range.
Figure 4.17 Thermocouple circuit
Figure 4.18 Thermocouple colour guide
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5.0 Discussion & Results
In this chapter the results are compared from both manual and
automated distillation, and statistical analysis is used to provide
evidence in favour of PID control system implementation on distillation
apparatus. These result show both the direct benefits to the distillation
process and the benefit this control system has on the process of
bioethanol production. The results provide evidence that shows a 28%
increase in total energy gained from the PID controlled still, which
positively impacts the energy balance equation for bioethanol.
The design process of the still from early concept to functioning plant
and potential improvements to the still are documented. The still that
was used to generate data for this thesis was designed through
experimentation, only finalising the still once satisfactory results were
obtained. Further improvements could not be made due to resource
restraints but these improvements are documented as I believe these
modifications could be used to further increase the amount of energy
gained from the still.
5.1 Design
Although the still was an “off the shelf” unit, modifications were
required to allow the still to interface with the control system (Figure
5.0). Additional modifications were made to the still hardware to achieve
a greater quality of ethanol. These modifications were identified
throughout the experimentation process and were implemented to
reduce the stress on the control system and the still.
The final iteration of the bioethanol still was created with features
selected both from basic assumptions about distilling alcohol and
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research into energy efficient distillation apparatus. These features were
tested during early experimentation with the final design being used to
run six distillations (three manual and three automated) which were
recorded for use in this thesis. Features that impacted positively on the
distillation process, both for manual and automated distillation were
adopted to ensure the experimentations were fair and did not skew the
results to either distillation process.
Figure 5.0 Distillation test rig
5.1.1 Still Interface
Initially it was assumed that the mash could be brought up to 80 oC and
the ethanol would separate out from the mash. It was quickly discovered
that this was not the case, and in fact the lower the percentage ethanol
present in the mash the higher the boiling point of the mash. The mash
needed to be heated above the boiling point to excite the ethanol
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molecules enough to be released as steam, this steam would then pass
up the column for collection out of the condenser.
On the first iteration of my still, the first thermocouple was located at
the top of the boiler tank (Figure 5.1). This was used as the process
variable for PID loop 0 to control the temperature in the distillation tank.
The original mounting position at the top of the tank was not actually in
contact with the mash, it read the temperature of the steam above the
mash and had little correlation to the actual purity of the steam being
collected at the top of the column.
Figure 5.1 Mash temperature sensor location
This was a problem because the set-point of 80 oC would not produce
ethanol as there was not enough energy to move the steam up the
column. The set-point of the tank had to be calculated to produce a
steam temperature of 78.5oC at the top of the column. The temperature
of the tank and the rate at which this temperature would need to
increase as the percentage ethanol in the mash deceased throughout the
distillation process had to be calculated.
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It was later discovered that better control of the distillation tank could
be gained by moving this thermocouple to the top of the distillation
column (Figure 5.2). Measuring this temperature directly correlated to
the temperature of the steam that had passed up the column to be
condensed to a liquid. This gave better control over the quality of
bioethanol extracted from the mash because at 78.5 oC the steam would
form an azeotrope consisting of 95% alcohol and only 5% water.
Figure 5.2 Column Temperature sensor location
Controlling the temperature of the steam at the top of the column
reduced the complexity of the control system. This required the top of
the column to be kept constant at a temperature of 78.5 oC. Controlling
the temperature of boiler tank required the set-point temperature to
increase as the percentage bioethanol in the mash decreased during the
distillation process.
Calculating the increasing set-point would require the the system to
measure the amount of Bio-ethnaol left in the mash. Measuring how
much bioethanol was left in the mash would increase the complexity of
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the physical system as well as the algorithm used to run the still.
Conversely a control system that incremented the temperature every
hour would not adhere to the Advanced Process Control algorithm and
could cause unforeseen problems.
5.1.2 Temperature Measurements
Temperature control of the column was done using a PID control loop to
maintain a constant rate of separation of bioethanol from the mash.
While the thermocouple was moved to the top of the column the PID
loop remained relatively the same. The response time of the loop
needed to be increased, as the steam temperature in the column
changes much faster than the mash temperature.
The PID loop was adjusted to respond quicker to these changes in
temperature. By increasing the Integral (I) variable and the Derivative (D)
variable the system was able to respond in an acceptable manner to
temperature changes, maintaining the top of the column at 78.5 oC. The I
variable was used to decrease the response time of the control system,
with the D variable used to bring stability back in to the output.
The adjustments were made to the system through the WinCC IDE
during a distillation run, meaning the system was on-line. While
monitoring the Process Variable (PV), the set-point, and the output
through the IDE I was able to incrementally increase the variables and in
real time view how the changes had effected the systems response.
Figure 5.3 shows the PID wizard which is used to tune the PID loop for
my individual application, with the P,I, and D variable inside the red box.
Auto-tuning was available but I felt that the adjustments made produced
an adequate system response.
89
Figure 5.3 PID manual control value adjustment
5.1.3 Cooling Circuit
It was initially assumed that a PID loop used to control the rate of
separation would achieve a quality of greater than 93% bioethanol.
Controlling the condenser with a on/off type control loop was assumed
to be sufficient as this control loop would maintain the temperature of
the condenser circuit within +/- 2oC. This did not work and it was
discovered that the condenser is of similar importance if not greater
importance than just the rate of separation.
The condensing unit was initially thought to contain the column and the
condensing shaft. This entire unit was kept below the boiling point of
ethanol to allow the ethanol vapour to re-condense into a solution of
95% ethanol. After the first experiment it was discovered that this
theory stopped the ethanol from travelling up the entire length of the
column and down the condenser shaft.
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With this column configuration the ethanol would collect at the top of
the column (which was maintained at approximately 70 oC ) and at
random ethanol would spurt out of the still at a lower percentage than
required. It was concluded that this happened because the column head
would flood and low quality ethanol would be pushed out the condenser
shaft. At this point it was concluded that the still was made up of three
sections: the tank, the column and the condenser.
After these initial experiments I moved the second thermocouple half
way down the condensing shaft. This allowed the ethanol vapour to
travel up the entire length of the column allowing the distillate to take
full advantage of the packed column. This ensured that the vapour was
of the highest quality possible also ensuring that the vapour would not
collect but instead flow out of the condenser as a steady stream.
This adjustment greatly improved not only the quality of bioethanol
produced, but also the production rate as the still was no longer
impeded. By controlling the temperature of the column and vapour, the
purity of the ethanol to be collected could be controlled by ensuring the
temperature was constantly 78.5oC. Treating the condenser shaft as a
separate section of the still allowed the high quality vapour to be
condensed into a high quality solution.
The cooling circuit used a solenoid activated water valve attached to a
mains water tap. By adjusting the length of time the valve was opened
the PLC could determine how much cooling was applied. This time was
determined by measuring the temperature of the condenser shaft which
is then used in PID loop 1 and adjusted by the PLC using PWM (pulse
width modulation) to vary the solenoids operation.
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This cooling circuit configuration did enable the still to produce overall
great results, allowing the still to produce a constant stream of ethanol
at greater than 93% purity. The cooling system however did not produce
good temperature control over the condenser column, as in Figure 5.4 it
can be seen to fluctuate +/- 20 oC which is not acceptable. These
fluctuations are caused by the delay introduced by the physical layout of
the cooling circuit.
Figure 5.4 Cooling system output
The length of the cooling circuit and the pressure at the mains tap are
believed to be the cause of the delay experienced by the cooling circuit.
The distance between the mains tap introduces a delay between the
system being activated and water passing through the system. Adjusting
the pressure at the mains tap yielded little benefit as it would decrease
over the distillation process.
Although the control system could be adjusted to drive the water valve
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at a slower duty cycle the pressure drop cannot be solved so easily. The
current duty cycle activates the solenoid but no water flows through the
system. Slowing the duty cycle will allow the activation to pass water
through the system. It was concluded that controlling the flow rate of
the water through the system would achieve greater control over the
condenser temperature.
When using the current cooling system, 50 litres of water is used to cool
the column for each distillation run. This water is re-used to clean
fermentation tanks and fill them with warm water for mash creation, but
ultimately it is considered waste water. A better solution to the current
configuration would be to use a 50 litre reservoir that feeds a sealed
cooling circuit that circulated water through it utilising a radiator to
dissipate heat.
This system would use a pump that feeds the cooling circuit and be
controlled by the PLC with is operation speed determined by the PID
loop. This would reduce the fluctuation experienced in the condenser
temperature by adjusting the flow rate of the water, not just the supply
of the water. This will ensure the cooling system is constantly filled with
water which will terminate the delay experienced by the current
solution. By giving the system continuous control over the flow, it will
allow for a faster response and will also reduce the amount of waste
water produced by the system.
5.2 PID Tuning
The PID control algorithm is used to gain exceptional control of two
regions in my distillation apparatus. These regions need to be kept at a
constant temperature to ensure energy efficient distillation of the mash,
while producing high quality ethanol. These control loops are used to
93
monitor and control the amount of energy going into the system,
ensuring the still is operated in the most energy efficient way possible.
When tuning PID variables one usually wants a system that overshoots
slightly, to obtain a system with a quick response time that will also
settles to a steady state quickly. When initial trials were done it became
apparent that both overshoot and undershoot were not ideal for the
creation of high quality bioethanol, as both states produced
unacceptable output. Overshoot created bioethanol that was less than
90%, and undershoot produced no ethanol. It was concluded that an
over-damped system would produce the best results.
The over-damped system ensured that energy was constantly applied to
the mash at a rate that allowed the ethanol to separate without ever
going over the set-point. This approach for ethanol production did not
allow the system to produce low quality ethanol, while producing a
constant stream of high quality bioethanol. By removing the overshoot
characteristics of the PID loops I was able to eliminate the production of
low quality ethanol and the undershoot.
PID loop 0, used to control the temperature of the distillation tank could
not be auto tuned in the stills initial configuration, where the
temperature sensor was in the distillation tank. The auto tuning
algorithm needs 100 samples to compute the appropriate P, I, and D
values but within a time limit of 20 seconds. The system was unable to
determine 100 samples in this time because the mash temperature could
not oscillate at this frequency. The time between the system
overshooting the set point (heating up) and then undershooting (cooling
down) was too long for the auto-tuning algorithm to compute the
required values.
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Since auto-tune was not working and I had not yet moved the
temperature sensor to its final position, the Nicola-Kieger method was
used to get rough P, I, and D variables. This brought the system into a
stable state that was then manually tuned for optimal operation of
ethanol productions. Manual tuning was done straight from the PID
tuning screen which showed a real time graph of the input, set point, and
output versus time allowing me to view how my changes affected the
PID loop.
Manual optimisation tuning was done with aid from the table in Figure
3.14. Optimal control of the system was determined as production of
ethanol that was greater than 93% at a constant flow of approx 1 litre
per hour. After initial trials it was determined that optimal control of the
still could not be obtained by controlling the tank so the thermocouple
was moved to the top of the still column.
After moving the thermocouple up the still column I was able to use the
IDE to adjust the rough PID variables to gain exceptional control of the
still. Using an over-damped PID loop still proved to be the best way to
obtain ethanol although a slightly faster response was needed as the
column reacts faster to energy input. I increased the I variable to
increase the response time of the system. This made the system
unstable so a slight increase was made to the D variable to bring the
system back in to optimal production that was extremely stable.
The cooling circuit was found to be best controlled with a PID loop, as an
on/off algorithm was not cooling the condenser sufficiently. A second
PID loop was implemented (PID loop1) to control the temperature of the
condenser. The cooling circuit thermocouple was moved down the
condenser to read the temperature of the distillate that was to be
collected. The PID variables were obtained through the use of the Auto95
tuning algorithm to obtain a control system that enabled the condenser
to produce bioethanol greater than 93% purity.
After the PID loop was used to control the condenser circuit it was
discovered that the cooling circuit hardware was insufficient to cool the
ethanol at the current rate of production. To gain exceptional
temperature control of the condenser, modification to the cooling circuit
need to be made.
5.3 Results
After experimenting with PID variables and control systems I was able to
create a control system that fully automated the operation of the still.
The control system monitored the temperatures of both the column and
the condenser, then calculated the appropriate inputs needed to keep
these areas at their specific set-points. This allowed the still to produce
ethanol at a purity of greater that 93% without any input from the user,
and by producing more ethanol with less energy I was able to increase
total energy gain by 28%.
By controlling the temperature of the still and only applying the
necessary energy to maintain this temperature I was able to reduce the
energy needed to distill high purity alcohol. The amount of power used
for each distillation was measured to compare the total energy used by
the still and also to compare gross energy yielded (amount of potential
energy stored in the ethanol). This was achieved using an off the shelf
power meter to measure the total power used by the still over the 7 hour
period, and from this the total net energy used was calculated.
To compare the manual still to the automated still 50 litres of mash at
14% alcohol was placed inside the distillation tank. A distillation was
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carried out over 7 hours with an expected yield of 7 litres of alcohol, at
which point the experiment was stopped. Measurements of the alcohol
purity were recorded at each hour using a hydrometer taking a sample
directly from the condenser stream.
The final concentration of the bioethanol collected from each distillation
was measured and compared to the result from purity measurements to
confirm these measurements were correct. This measurement allowed
me to calculate the total ethanol collected from each distillation. With
the total amount of 100% bioethanol extracted form the mash I could
calculate the total potential energy extracted from the system by
multiplying the total bioethanol collected by the 21.2 Mj/L.
100.00
90.00
80.00
70.00
60.00
50.00
40.00
30.00
20.00
10.00
0.00
%
%
%
%
%
%
1
2
3
4
5
6
Alcohol Man 1
Alcohol Man 2
Alcohol Man 3
Alcohol PID 1
Alcohol PID 2
Alcohol PID 3
7
Figure 5.5 Manual vs Automated distillation Alcohol %
Figure 5.5 is a graph with both manual and automated distillations using
purity against time, and shows that the automated distillations were able
to produce ethanol that was consistently greater that 93% purity. The
manual distillation was rather erratic and this was further qualified by
the overall percentage bioethanol of the manual batches. All PID
controlled batch were able to produce a batch of ethanol that was over
97
93% ethanol, the average quality of the manual controlled distillations
was averaged out at 83%. This increase in quality enables the PID
controlled still to produce on average 10% more pure bioethanol from
each batch of mash processed through the same distillation apparatus.
The manual distillations all seem to follow a similar distillation cycle, in
which they begin at 95% then slowly tapper down to 73%, this then rises
back up to ~90%. After the distillation has risen to approximately 90%
the manual distillation will taper off for the last 2 hours of operation.
One could assume that this purity oscillation was due to operator error,
however this cycle is repeated in each manual distillation removing this
hypothesis from possible conclusions.
These results were obtained by taking samples from the still, each
subsequent hour after starting the distillation. The first part of these are
what would be expected of a manual distillation, starting well and slowly
tapering off. No conclusion can be made as to why these distillations
increase in purity near the end of their distillation run and are not what
one would expect. These results were cross checked and confirmed by
comparing the average purity of the measurements to the final purity of
the batch.
The height, diameter ratio of the distillation column has a major effect
on the quality of ethanol that the still can produce. The Euro still I was
using had a short column, as a consequence the still could only achieve a
quality of 93%. To obtain 95% purity requires the height of the column
increase to produce a ratio of 25:1 (height:diameter) that would allow for
a longer temperature variation. This increased variation allows the
ethanol vapour to distill many more times increasing its final purity, also
allowing the distillate to dissipate any excess heat energy reducing the
stress on the condenser.
98
While a larger column would be beneficial, a better solution could be to
use a second distillation column. This would allow the distillation run to
double distill a batch of mash and could achieve 95% ethanol with little
extra energy needed. This column would be of similar size to the first
column but would allow the vapour to collect in the bottom as a liquid of
approx 60% - 80% ethanol. This would be better, as the short column
would ensure a short run time, a larger column will increase run time,
also increasing energy used.
The second column would be fed steam from the first distillation column,
passing through a solution of approximately 70% ethanol which would
have a much lower boiling point than that of the 14% mash. This lower
boiling point would allow the second distillation to use only the steam
energy from the first distillation to raise the solution temperature to
remove ethanol at a higher percentage, allowing the steam to go
through another set of phase changes.
This type of distillation is used at BSE and most other bioethanol plants
as the lower the concentration of ethanol the harder separation is, as the
boiling point is closer to that of water. The first column is referred to as a
beer column, in which the mash containing <10% ethanol is processed
removing the ethanol as a solution of >60% ethanol. This has a much
lower boiling point than the mash, this is then processed in the
distillation column and removed at 95%.
5.3.1 Energy Consumption
The energy consumption of the still running manually can be calculated
by multiplying 1.5 kw by 8 hours and adding 3Kwh to account for the the
2Kw coil being on for first 1.5 hours of operation. Measuring how much
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energy the still consumed was done using a Kwh meter and the
measurements showed a decrease in energy consumption of 10%. This
was expected as during manual stilling the coil is left on until all the
ethanol is removed, steadily decreasing the quality of ethanol being
extracted. With automated control of the still, only the energy that is
needed is introduced so not only does the quality stay high but the
energy consumed reduces.
Insulation of the tank could also decrease power consumption as energy
escaping through the large tank walls would be decreased. This would
also heat the mash up to temperature faster decreasing the time the
booster coil would need to active. This could be achieved wither through
building a double walled tank or covering with a insulation layer.
The basic principle of distillation is to heat one area then remove this
heat in another area. When removing heat energy from my still the heat
is wasted as it is ejected through the water system. Recycling this water
through a cooling circuit can introduce many areas in which this energy
can be reused. Running this circuit through the fermenting mash could
be used as a heat source for temperature control, this could also be used
to raise the mash temperature for pitching yeast.
5.3 Further Enhancements
Efficient control of any system must implement some form of APC.
Simple on/off control will not work especially when controlling large
volumes of liquid as they tend to work as large thermal reservoirs,
introducing a oscillation of temperature around the set-point in a
sinusoidal pattern. The use of PID loops was a necessity to achieve the
results I did from this distillation apparatus and its current configuration.
Further improvements in distillation and energy yield could only be
100
achieved by modifying the still:
1)
Insulation of the tank and column.
2)
Adding a 2nd column to the still.
At present the still tank is exposed to its surroundings, allowing for
thermal conduction between the mash and the stills environment
through convection. Distillation is a process that uses energy to change
the phase of a chemical with a lower boiling point than another
substance, and reducing the loss of this energy will reduce the amount
of energy needed. A booster coil will still be needed but the amount of
time it is required will be reduced. Additionally, the coil that is used to
power the distillation could be reduced, possibly even below 1 kilowatt.
The challenge with bioethanol distillation is the small quantity of ethanol
present in the mash. In perfect conditions yeast can only ferment up to
20% ethanol but generally a mash will contain only 10-14% alcohol. As
the bioethanol is removed from the mash the boiling point of the mash
increases, getting closer to that of water. As the boiling point
approaches that of water the possibility of more water particles being
present in the steam increases, making the system work harder to
maintain a high quality of bioethanol.
When this happens in the alcoholic beverages industry the excess water
can contain impurities which are introduced in the alcohol. These
impurities can lead to bad tastes or odours in the final product which is
not ideal, so these impurities must be removed from the alcohol before
it can be bottled. The solution used is to double distill the alcohol or
even triple distill in some high end liqueurs as each distillation removes
more impurities.
101
A second column would act much the same as double distilling the
bioethanol without significantly increasing energy expenditure, as the
column can be powered by steam from the tank. The column will act like
a second distillation tank, allowing distillate to collect in the bottom of
the column at a higher purity than the first tank (approximately greater
than 80%). This distillate will then be heated up to a point where the
ethanol will boil off at an even higher purity. This requires less energy, as
the purity is so high and the volume is quite small in this reservoir that it
could use the energy released from the first tank to operate.
102
6.0 Conclusion
Data retrieved from my experiments showed the PID control system
produced more pure alcohol (100% bioethanol). Both distillations
methods produced 7 litres of ethanol but the PID controlled system
constantly produced bioethanol that was above 90%, while manual
distillation would fluctuate and go as low as 70%. This consistency
allowed the PID system to increase pure alcohol yield from the individual
batches.
The PID controlled distillations increased pure alcohol yield from
individual batches by 13%. Manual distillation produced an average of
5.671 litres of 100% bioethanol, while the PID system created 6.51 litres
of bioethanol. Equation (1) and (2) represent the gross energy yield from
the distillation process, (1) is for manual distillation and (2) is for the PID
controlled distillation.
5.67L × 21.20MJ = 120.20MJ
(1)
6.51L × 21.20MJ = 138.01MJ
(2)
The increased quality of bioethanol production is attributed to the
control the PID system exerts on the still. The control algorithm limits
the amount of energy introduced in to the still ensuring the bioethanol
produced is above 90%. This attribute decreases the amount of energy
the PID system uses to produce 7 litres of bioethanol, increasing the net
energy yield.
120.02MJ − (14.00kWh × 3.60MJ ) = 69.83MJ
(3)
138.01MJ − (11.20kWh × 3.60MJ ) = 97.70MJ
(4 )
The net energy (gross energy yield, minus energy cost) for manual (3)
compared to PID controlled (4) distillation shows a 28.5% increase in net
103
energy yield. Manual distillation uses 14kWh of energy while PID
distillation uses only 11.2kWh of energy, a 20% decrease in energy
consumption. The decrease in energy consumption coupled with the
increase in material yield leads to an increase in production efficiency of
28.5%.
This consistent quality of ethanol produced not only increases the overall
energy yield from the PID controlled still but allows the output to be
directly fed into a molecular sieve. The only way to remove the last 5%7% of water left after distillation is through physically removing the
water molecules with a molecular sieve. This process requires an input
stream of between 93% - 95% ethanol to remove the water producing
100% ethanol which can be used as a biofuel. The quality at which the
PID controlled system produces allows the addition of a molecular sieve
into the system.
This increase in total energy gained can be directly attributed to the use
of the PID algorithm. This control algorithm and automation system
ensured the production of high quality bioethanol also limiting the
energy input to reduce the amount of energy that may otherwise have
been wasted. These results can be further improved by making physical
changes to the distillation apparatus.
104
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105
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8.0 Bibliography
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