SIMULATION OF AIR LIQUEFACTION USING ASPEN PLUS

SIMULATION  OF  AIR LIQUEFACTION  USING ASPEN PLUS
SIMULATION OF AIR
LIQUEFACTION USING ASPEN
PLUS
THIS REPORT IS SUBMITTED IN THE PARTIAL
FULFILLMENT OF THE REQUIREMENT FOR THE
DEGREE OF
BACHELOR OF TECHNOLOGY
IN
CHEMICAL ENGINEERING
by
ARUNIM BOSE
(Roll No.: 108CH051)
Under the guidance of
Prof. (Mrs.) Madhusree Kundu
DEPARTMENT OF CHEMICAL ENGINEERING
NATIONAL INSTITUTE OF TECHNOLOGY
ROURKELA
This
is
to
certify
LIQUEFACTION
that
USING
“SIMULATION
the
report
entitled
ASPEN
PLUS”
submitted by ARUNIM BOSE
OF
AIR
(ROLL NO: 108CH051) in the partial fulfillment of the requirement for the
degree of the B.Tech in Chemical Engineering, National Institute Of
Technology, Rourkela is an authentic work carried out by him under my
super vision. To the best of my knowledge the matter embodied in the
report has not been submitted to any other university/institute for any
degree.
Prof. (Mrs.) Madhusree Kundu
Department Of Chemical Engineering
National Institute Of Technology
Rourkela
i
ACKNOWLEDGEMENT
I would like to make my deepest gratitude to Prof. Madhusree Kundu, Associate
Professor in the Department of Chemical Engineering, NIT Rourkela for giving me
the opportunity to work under her and lending every support at every stage of this
project work. I would also like to convey my sincerest gratitude and indebtness to
all the faculty members, friends and staff of the Department of Chemical
Engineering, NIT Rourkela, for their invaluable support and encouragement.
Lastly I would like to thank my parents for their constant support, encouragement
and good wishes, without which working on this project would not have been
possible.
Arunim Bose
Department of Chemical Engineering
National Institute of Technology
Rourkela
ii
ABSTRACT
Air is a mixture of various gases. Each of these gases have got some kind of uses. As
such, separation of air into its constituent gases becomes very necessary from commercial
point of view. For separation of air into its constituent gases, it is necessary to liquefy
the same. In this project, simulation of liquefaction of air, followed by its separation was
done using Aspen Plus simulating tool. The model under consideration was Linde singlecolumn system. Also, the effect of various process conditions on the yield and purity of
final product were analyzed. It was found that by using Linde single-column system,
oxygen of almost 98-99% purity could be obtained. However, the purity of nitrogen
obtained was only about 90%.
Keywords: Aspen Plus, Linde single-column system.
iii
CONTENTS
CHAPTER
PAGE NO.
CERTIFICATE
i
ACKNOWLEDGEMENT
ii
ABSTRACT
iii
CONTENTS
iv
LIST OF FIGURES
vii
LIST OF TABLES
viii
1. INTRODUCTION AND LITERATURE REVIEW
1
1.1 AIR LIQUEFACTION AND SEPARATION
2
1.2 CRYOGENIC GAS LIQUEFACTION SYSTEMS
2
1.2.1 PRODUCTION OF LOW TEMPARATURES
3
1.2.1.1 JOULE-THOMSON EFFECT
3
1.2.1.2 ADIABATIC EXPANSION
3
1.3 CRYOGENIC SEPARATION SYSTEMS
4
1.3.1 PRINCIPLES OF GAS SEPARATION
4
1.3.1.1 SIMPLE CONDENSATION OR EVAPORATION
4
1.3.1.2 RECTIFICATION
5
2. THERMODYNAMIC ASPECT
6
2.1 CRYOGENIC GAS LIQUEFACTION SYSTEMS
7
2.1.1 IDEAL LIQUEFACTION SYSTEM
7
2.1.1.1 WORKING PRINCIPLE
8
2.1.2 SIMPLE LINDE-HAMPSON SYSTEM
8
2.1.2.1 WORKING PRINCIPLE
9
iv
2.2 CRYOGENIC SEPARATION SYSTEMS
9
2.2.1 IDEAL SEPARATION SYSTEM
9
2.2.1.1 WORKING PRINCIPLE
10
2.2.2 LINDE SINGLE-COLUMN SYSTEM
11
2.2.2.1 WORKING PRINCIPLE
11
3. DESCRIPTION OF SIMULATING TOOL
13
3.1 ASPEN TECHNOLOGY
14
3.1.1 INTRODUCTION
14
3.1.2 ASPEN ONE
14
3.1.2.1 ASPENONE ENGINEERING
14
3.2 ASPEN PLUS
15
3.2.1 INTRODUCTION
15
3.2.2 FEATURES
15
3.2.3 SIMULATING ENVIRONMENT
16
3.2.4 EQUATION OF STATE USED FOR SIMULATION
17
4. RESULTS AND DISCUSSIONS
18
4.1 SIMULATION OF AIR LIQUEFACTION AND SEPARATION
19
4.2 EFFECT OF CHANGE IN VARIOUS PROCESS CONDITIONS
22
4.2.1 VARIATION OF LIQUID FRACTION WITH
PRESSURE OF COMPRESSION
22
4.2.2 VARIATION OF LIQUID FRACTION WITH
MOLAR FLOW RATE
23
4.2.3 VARIATION OF LIQUID FRACTION WITH
EXTENT OF COOLING IN KETTLE SECTION
OF DISTILLATION COLUMN
23
v
4.2.4 VARIATION OF HOT STREAM AND COLD
STREAM OUTLET TEMPERATURE WITH
APPROACH OF HEAT EXCHANGER
24
4.2.5 VARIATION OF PURITY OF BOTTOM AND TOP
PRODUCTS WITH CHANGE IN NUMBER OF
STAGES IN DISTILLATION COLUMN
25
5. CONCLUSIONS AND FUTURE ASPECTS
27
6. REFERENCES
29
vi
LIST OF FIGURES
FIGURE NO.
NAME OF FIGURE
PAGE NO.
1.1
Isenthalpic expansion of a real gas
3
2.1
The thermodynamically ideal liquefaction system
7
2.2
Linde-Hampson liquefaction system
8
2.3
Thermodynamically ideal gas-separation system
10
2.4
Linde single-column gas-separation system
11
3.1
aspenONE engineering classification
14
3.2
Simulation Environment of Aspen Plus
16
4.1
Process Flow Diagram for Linde single-column
gas separation system
19
4.2
Success report of simulation
20
4.3
Plot for liquid fraction vs pressure of compression
22
4.4
Plot for liquid fraction vs molar flow rate
23
4.5
Plot for liquid fraction vs temperature of stream
before entering expansion valve
24
Plot for % purity of nitogen in top vs number of
stages in distillation column
25
Plot for % purity of oxygen in bottom vs number
of stages in distillation column
26
4.6
4.7
vii
LIST OF TABLES
TABLE NO.
4.1
4.2
4.3
4.4
4.5
4.6
NAME OF TABLE
PAGE NO.
Stream Table for simulation of Linde
single column gas separation system
21
Table for variation of liquid fraction with
pressure of compression
22
Table for variation of liquid fraction with
molar flow rate
23
Table for variation of liquid fraction with
extent of cooling in kettle section of
distillation column
23
Table for variation of hot stream and cold
stream outlet temperature with approach
of heat exchanger
24
Table for variation of purity of bottom and
top products with change in number of
stages in distillation column
25
viii
CHAPTER 1
INTRODUCTION AND
LITERATURE REVIEW
1
1. INTRODUCTION AND LITERATURE REVIEW
1.1 AIR LIQUEFACTION AND SEPARATION
Air is a mixture of various types of gases. Atmospheric air has 78.084% nitrogen,
20.946% oxygen, 0.934% argon 0.03768% other gases by volume (on dry basis) [1].
Separation of atmospheric air is done in an air separation plant basically to obtain
nitrogen and oxygen, and sometimes also argon and rarely other inert gases. There are
various technologies for separation process, the most common being cryogenic distillation.
In addition to the cryogenic method, there are other methods such as Membrane,
Pressure Swing Adsorption (PSA) and Vacuum Pressure Swing Adsorption (VPSA),
which are typically used to separate a single component.
1.2 CRYOGENIC GAS LIQUEFACTION SYSTEMS
Of the various gas liquefaction techniques developed by various cryogenic experts
some of them are listed below:-
[2]-[5]
,
1: Simple Linde Hampson system
2: Precooled Linde Hampson system
3: Linde dual- pressure system
4: Cascade system
5: Claude system
6: Kaptiza system
Liquefaction of gases is used to convert a gas into a liquid state. Many gases can be
put into a liquid state at normal atmospheric pressure by simple cooling; a few, such as
carbon dioxide, require pressurization as well.
Liquid oxygen is provided to hospitals for conversion to gas for patients suffering from
breathing problems, and liquid nitrogen is used in the medical field for cryosurgery, and
by inseminators to freeze semen [6].
Liquid air is air that has been cooled to very low temperatures so that it has condensed
to a pale blue mobile liquid. It is often used for condensing other substances into liquid
and/or solidifying them, and as an industrial source of nitrogen, oxygen, argon, and other
inert gases. Nowadays, liquid air is also replacing liquid nitrogen for theatrical smoke
and fog effects [7].
2
1.2.1 PRODUCTION OF LOW TEMPERATURES
1.2.1.1 Joule-Thomson Effect
Most of the practical liquefaction systems utilize an expansion valve or a Joule Thomson
valve to produce low temperatures. Although the flow within the valve is irreversible
and is not an isenthalpic process, the inlet and the outlet do lie on the same enthalpy
curve. We note that there is a region in which an expansion through the valve produces
an increase in temperature, while in another region the expansion results in a decrease in
temperature. It is made sure that the expansion valve is operated in the region where a
net decrease in temperature occurs. The curve that separates two regions is called the
inversion curve [2].
Figure 1.1 Isenthalpic expansion of a real gas.
1.2.1.2 Adiabatic Expansion
The second method of lowering temperature is the adiabatic expansion of the gas
through a work producing device, such as an expansion engine. In the ideal case, the
expansion would be reversible and adiabatic and hence isentropic [2].
3
1.3 CRYOGENIC SEPARATION SYSTEMS
In many cases, the separation of mixtures at cryogenic temperatures is the most
economical method of all [2]. Almost all the commercially produced oxygen, nitrogen,
argon, neon, krypton, and xenon are obtained through the rectification of liquid air. The
effectiveness of other separation methods, such as physical adsorption and refrigeration
purification, are increased as the temperature is lowered into the cryogenic range.
Some common air separation systems developed by cryogenic engineers
below:
[2]-[5]
are listed
1: Linde single-column system
2: Linde double-column system
3: Linde-Frankl system
4: Heylandt system
5: Linde-Bronn system
6: I’Air liquide system
1.3.1 PRINCIPLES OF GAS SEPARATION
1.3.1.1 Simple Condensation or Evaporation
Separation of some mixtures can be attained by partial condensation. Mixtures with
widely different boiling points usually can be separated effectively by partial
condensation, whereas the separation is usually less effective for substances having
boiling points close together [2].
It is not possible to separate air into practically pure components by a single
condensation. Some separation can be achieved, but not enough for most practical
purposes. On the other hand, nitrogen-helium mixtures, ammonia-hydrogen mixtures, and
so on, can be separated fairly effectively by a single partial condensation.
4
1.3.1.2 Rectification
Rectification is the cascading of several evaporations and condensations carried out in
counterflow. As the mixture to be purified is heated, its vapors rise to the fractionating
column. As it rises, it cools, condensing on the condenser walls and the surfaces of the
packing material. Here, the condensate continues to be heated by the rising hot vapors; it
vaporizes once more. However, the composition of the fresh vapors is determined once
again by Raoult's law. Each vaporization-condensation cycle (called a theoretical plate)
will yield a purer solution of the more volatile component. This method has been used
successfully for separation of air mixture into its various constituents like nitrogen,
oxygen, argon, neon etc.
5
CHAPTER 2
THERMODYNAMIC
ASPECT
6
2. THERMODYNAMIC ASPECT
2.1 CRYOGENIC GAS LIQUEFACTION SYSTEMS
2.1.1 IDEAL LIQUEFACTION SYSTEM
This system is ideal in the thermodynamic sense, but it is not ideal as far as practical
system is concerned. The perfect cycle in thermodynamics is the Carnot cycle.
Liquefaction is essentially an open system process, therefore for an ideal liquefaction, the
first two processes in the Carnot cycle are chosen; a reversible isothermal compression
followed by a reversible isentropic expansion [2].
(a)
(b)
Figure 2.1 The thermodynamically ideal liquefaction system
(a) Thermodynamic cycle T-S plane (b) Apparatus setup
7
2.1.1.1 Working Principle
The gas to be liquefied is compressed reversibly and isothermally from ambient
conditions to some high pressure. This high pressure is selected so that gas will become
saturated liquid upon reversible isentropic expansion through the expander. The final
condition is taken as the same pressure as the initial pressure. The pressure attained at
the end of isothermal compression is extremely high in the order of 70Gpa or 80GPa
(107 psia) for nitrogen [2]. It is highly impracticable to attain this pressure in a
liquefaction system, which is the reason it is not an ideal process for a practical system.
2.1.2 SIMPLE LINDE-HAMPSON SYSTEM
The Linde-Hampson system was the second used to liquefy gases (the cascade system
was the first), although it is the simplest of all the liquefaction systems. A basic
differentiation between the various refrigeration cycles lies in the expansion device. The
expansion engine approaches an isentropic process and the valve an isenthalpic process.
Isentropic expansion implies an adiabatic reversible process while isenthalpic expansions
are irreversible. In the Linde system, the basic principle of isenthalpic expansion is also
incorporated [2].
(a)
(b)
Figure 2.2 Linde-Hampson liquefaction system
(a) Linde-Hampson cycle (b) Apparatus setup
8
2.1.2.1 Working Principle
The gas is first compressed from ambient conditions at point 1 reversibly and
isothermally to point 2. In a real system, process 1 to 2 would actually be two
processes: an irreversible adiabatic or polytropic compression followed by an aftercooling
to lower the gas temperature back to within a few degrees of ambient temperature. The
gas next passes through a constant-pressure heat exchanger (ideally) in which it
exchanges energy with the outgoing low-pressure steam to point 3. From point 3 to
point 4, the gas expands through an expansion valve to p4=p1. At point 4, some of the
gas stream is in the liquid state and is withdrawn at condition f (saturated-liquid
condition), and the rest of the gas leaves the liquid receiver at condition g (saturatedvapor condition). This cold gas is finally warmed to the initial temperature by absorbing
energy at constant pressure (ideally) from the incoming high-pressure stream [2].
2.2 CRYOGENIC SEPARATION SYSTEMS
2.2.1 IDEAL SEPARATION SYSTEM
An ideal system would be the one in which all the processes are reversible. Mixing two
gases together is ordinarily an irreversible process because the gases would not unmix
themselves without some external assistance. There is one method, however, we could
imagine that allows reversible mixing and separation. This method employs
semipermeable membranes- that is, membranes that allow the complete free passage of
one gas but completely block the passage of all other gases. Using these devices, the
gases could be mixed while work is obtained, and the same amount of work would be
expended to unmix the gases. Hence, the process may be imagined to be reversible [2].
The idea of semipermeable membranes is not a completely theoretical concept because
materials do exist in nature that are permeable to certain gases. For example, a thin
sheet of palladium allows molecules of hydrogen to pass through it, but does not allow
other gases to pass through. Since we are concerned with an idealized reversible system,
it does not matter whether such membranes may or may not physically exist [2].
9
Figure 2.3 Thermodynamically ideal gas-separation system
2.2.1.1 Working Principle
Let us consider the separation of a mixture of two gases in the double-cylinder
arrangement shown in Fig. 2.3. The left-hand piston is permeable to gas A but not to
gas B, and the right-hand piston is permeable to gas B but not to gas A. By moving
the two pistons together, the mixture of A and B can be separated reversibly into pure
A and pure B [2].
10
2.2.2 LINDE SINGLE-COLUMN SYSTEM
One of the most simple air-separation systems is the Linde single-column system shown
in Fig. 2.4. This system was first operated by Linde in 1902 [2].
Figure 2.4 Linde single-column gas-separation system
2.2.2.1 Working Principle
The entering gas is compressed, the water vapor and carbon dioxide are removed, and
the air is passed through a heat exchanger in which the incoming gas is cooled. If the
oxygen is desired as a liquid, the heat exchanger is a two-channel type, as shown in the
figure. The cold oxygen gas is used to help cool down the incoming air. The liquid
oxygen is then withdrawn from the lower section of the column (called the kettle, in the
terminology of the petroleum industry).
The cool air is further cooled and partially condensed by exchanging energy with the
liquid in the kettle. This accomplishes two purposes:
1. The incoming air is further cooled, which allows a greater liquid yield, and
11
2. The heat is made available to boil some of the liquid in the kettle to furnish the
upward-flowing vapor in the column without the addition of heat from ambient
temperatures.
The cold air is expanded through a Joule-Thomson valve, after which some liquid is
formed. This liquid is then passed to the top of the column, where it begins to travel
down the column across the plates. As the liquid flows down the column over the
plates, it is enriched in oxygen by exchange with the upward-flowing vapor. When the
liquid reaches the bottom of the column, a portion is evaporated by the incoming air.
This vapor flows up the column, bubbling through the liquid layers and picking up
nitrogen on its way. The gas at the top of the column is removed and passed through
the heat exchanger to help cool the incoming air [2].
When the oxygen is desired as a gas, the air need to be compressed only to pressure on
the order of 3 MPa to 6 MPa (30 atm to 60 atm); however, if liquid oxygen is desired,
the air must be compressed to approximately 20 MPa (200 atm) for effective operation.
The pressure within the column itself is usually on the order of 130 kPa to 200 kPa (1.3
atm to 2 atm) [2].
12
CHAPTER 3
DESCRIPTION OF
SIMULATING TOOL
13
3. DESCRIPTION OF SIMULATING TOOL
3.1 ASPEN TECHNOLOGY
3.1.1 INTRODUCTION
Aspen Technology, Inc. is a provider of software and services for the process industries.
Headquartered in Burlington, Massachusetts, USA, Aspentech is having 34 offices in 27
countries, spanning 6 continents.
Founded in 1981, Aspentech was born out as a joint research project between the
Massachusetts Institute of Technology (MIT) and US Department of Energy—an Advanced
System for Process Engineering (ASPEN ) Project [8].
3.1.2 ASPEN ONE
aspenONE is AspenTech’s comprehensive set of software solutions and professional
services designed for helping process companies to achieve their operational excellence
objectives. It leverages the value of simulation models to help process companies
increase operational efficiency and profitability across their global enterprise. Aspen-one
covers four major fields: Chemical, Energy, Polymer and Pharmaceuticals [9].
3.1.2.1 AspenONE Engineering
Figure 3.1 aspenONE engineering classification
14
3.2 ASPEN PLUS
3.2.1 INTRODUCTION
Aspen Plus is a process modeling tool for conceptual design, optimization, and
performance monitoring for the chemical, polymer, specialty chemical, metals and
minerals, and coal power industries. Aspen Plus is one of the core elements of
AspenTech’s aspenONE® Engineering applications [10].
3.2.2 FEATURES
[10]
1. Best-in-class physical properties methods and data. Aspen Plus includes the
world’s largest database of pure component and phase equilibrium data for
conventional chemicals, electrolytes, solids, and polymers. Regularly updated data
from the U.S. National Institute of Standards and Technology (NIST) ensures easy
access to the best available experimental property data, enabling process engineers
to save months of effort when developing chemical process models.
2. Aspen Properties Mobile provides access to rigorous physical property calculations
and data when and where you need it – at a customer site, in a conference room,
from home, on the road – anywhere you can remotely access your corporate
network.
3. Improved conceptual design workflow. Aspen Plus has been tightly integrated with
AspenTech’s industry-leading cost analysis software and heat exchanger design
software. Process engineers can rapidly estimate the relative costs of proposed
designs and make decisions based on capital and operating cost estimates using
proven cost modeling technology. Key equipment such as heat exchangers and
distillation columns can be rigorously sized or rated from within the simulation
environment. The tight integration of design and costing software with process
simulation eliminates costly manual iterations and promotes more optimal designs
based on rigorous cost estimates.
4. Scalability for large and complex processes. Aspen Plus’s unique Equation
Oriented (EO) modeling capability and hierarchical flow sheeting lets you simulate
even the most large scale and complex processes; even highly integrated processes
with multiple recycles. Customers can build models spanning entire sites to find
the globally optimal operating conditions.
5. State of the art column internals calculations for flooding and pressure drop.
Aspen Plus includes a library of more than 300 packings and 5 different tray
types with parameters fitted for calculation of tray internals.
6. Aspen Plus provides users with rate-based distillation technology which enables
distillation calculations while accounting for rigorous mass transfer on trays or on
packings.
15
7. Greenhouse gas emissions report of carbon equivalents generated in the process or
by consumption of process utilities. Annual carbon tax can also be calculated.
8. Aspen Plus also enables users to model batch distillation in a rigorous manner.
9. Additionally, Aspen Plus also enables users to identify azeotropes and residue
curves which can help users to design and synthesize non-ideal distillation
columns.
10. Aspen Plus also provides users with a complete set of polymer thermodynamics
methods and data, rate-based polymerization reaction models, and a library of
industrial process models.
11. Online deployment of models as part of an open-loop operator advisory system or
in closed-loop, real-time optimization/advanced process control applications when
used with Aspen Online Deployment™ and Aspen Simulation Workbook™.
12. Workflow automation. Aspen Plus models can be linked to Microsoft Excel®
using Aspen Simulation Workbook or Visual Basic® and used to automate the
engineering workflow and deploy the model to a wider range of end users in the
field.
13. Aspen Plus Dynamics® is used for safety and controllability studies, sizing relief
valves, optimizing transition, startup, and shutdown policies.
14. Links to third-party tools. Aspen Plus includes links to several well-known tools
including the OLI’s electrolyte package and Technip’s SPYRO ethylene cracker
models.
3.2.3 SIMULATION ENVIRONMENT
The Simulation environment contains the main flow sheet area where the majority of the
work (installing and defining streams, unit operations, columns and sub flow sheets) is
done.
Figure 3.2 Simulation Environment of Aspen Plus
16
The flow sheet area in Aspen Plus shows the various components needed to draw a flowsheet
and henceforth, simulate the process.
3.2.4 EQUATION OF STATE USED FOR SIMULATION
An equation of state is a relation between state variables. More specifically, an equation
of state is basically a thermodynamic equation describing the state of matter under a
given set of physical conditions.
For simulation of air liquefaction, Peng-Robinson equation of state was used. PengRobinson is a Cubic equation of state [9] and is stated as follows:
p=((RT)/(Vm-b))-((aα)/(Vm2+2bVm-b2))
a=0.457235R2Tc2/pc
b=0.077796RTc/pc
α=(1+(0.37464+1.54226ω-0.26992ω2)(1-Tr0.5))2
Tr=T/Tc
In polynomial form,
A=(aαp)/(R2T2)
B=(bp)/(RT)
Z3-(1-B)Z2+(A-2B-3B2)Z-(AB-B2-B3)=0
where, ω is the acentric factor of the species, R is the universal gas constant and
Z=PV/(RT) is compressibility factor.
17
CHAPTER 4
RESULTS AND
DISCUSSIONS
18
4. RESULTS AND DISCUSSIONS
4.1 SIMULATION OF AIR LIQUEFACTION AND SEPARATION
Given Conditions:
Ambient Temperature = 298K, Ambient Pressure = 1atm, Maximum Pressure for
Compression = 200atm, Molar Flow Rate = 200moles/sec, Approach of Heat Exchanger
= 5K, Temperature of Stream after passing through kettle section = 91K, Pressure within
Distillation Column = 1atm, Reflux Ratio = 7.502, Distillate to feed mole ratio = 0.875,
Number of stages = 7 (100% efficient).
Assumptions:
1. Air is a mixture of nitrogen and oxygen only, with nitrogen accounting for 79% and
oxygen accounting for 21% by volume (on dry basis).
2. There is no pressure drop across the compressor, cooler and heat exchanger.
Figure 4.1: Process Flow Diagram for Linde single column gas separation system.
19
Explanation:
The first step in Linde single column gas separation system is isothermal compression.
Block COMPRESS takes care of the compression. In order to make sure that we have
an isothermal compression, a cooler is added after block COMPRESS and is named as
block COOLER1. After that, cooling of the stream is done by using heat exchanger,
which is taken care by the block HTEXCH. After that, further cooling takes place in the
kettle section of the distillation column, which is taken care of by the block COOLER2.
After that, the expansion valve (block J-TVALVE) releases the pressure to obtain liquid
air. Separation of the liquid air into nitrogen and oxygen occurs in block DSTCOL.
Simulation Result:
The simulation was performed successfully and results were obtained without any error.
Figure 4.2: Success report of simulation
20
Table 4.1: Stream Table for simulation of Linde single column gas
separation system
From the generated stream table, it is seen that the liquid fraction in LIQAIR stream is
nearly 0.1637, which implies that the liquefaction of air has taken place under the given
conditions.
Also, the BOTTOM stream has 0.985 mole fraction of oxygen and 0.015 mole fraction
of nitrogen. The DISTILL stream has 0.901 mole fraction of nitrogen and 0.099 mole
fraction of oxygen. Hence, the separation of liquefied air into oxygen and nitrogen has
been achieved and the purity of oxygen and nitogen obtained from bottom and top of
distillation column are nearly 98% and 90%, respectively.
21
4.2 EFFECT OF CHANGE IN VARIOUS PROCESS CONDITIONS
4.2.1 VARIATION OF LIQUID FRACTION WITH PRESSURE OF
COMPRESSION
Liquid Fraction
100
150
200
250
300
350
400
Pressure
(atm)
0.1617945 0.1630618 0.1636859 0.1640575 0.1643040 0.1644795 0.1646108
Liquid
Fraction
450
500
550
600
650
700
Pressure
(atm)
0.1647128 0.1647942 0.1648607 0.1649162 0.1649630 0.1650031
Liquid
Fraction
Table 4.2: Table for variation of liquid fraction with pressure of compression
0.1655
0.165
0.1645
0.164
0.1635
0.163
0.1625
0.162
0.1615
0
200
400
Pressure (atm)
600
800
Figure 4.3: Plot of liquid fraction vs pressure of compression.
The above figure shows that the liquid fraction obtained increases with increase in
pressure of compression. However, for increase of compression pressure, the work
requirement increases, thereby increasing the overall cost of production. For most
industrial situations, a compression pressure of 200atm is used, when oxygen is the
desirable product in liquid form [2].
22
4.2.2 VARIATION OF LIQUID FRACTION WITH MOLAR FLOW RATE
Liquid Fraction
150
200
250
300
350
Molar Flow
Rate(moles/sec)
0.1636859
0.1636859
0.1636859
0.1636859
0.1636859
Liquid
Fraction
Table 4.3: Table for variation of liquid fraction with molar flow rate
0.18
0.16
0.14
0.12
0.1
0.08
0.06
0.04
0.02
0
0
100
200
300
Molar Flow Rate (moles/sec)
400
Figure 4.4: Plot for liquid fraction vs molar flow rate.
The above plot shows that the liquid fraction is independent of the molar flow rate of
air. Hence, the flow rate has no contribution in determining the fraction of air liquefied.
The current project considers the simulation of a plant with a molar flow rate of
200moles/sec, which corresponds to nearly 498 tonnes/day.
4.2.3 VARIATION OF LIQUID FRACTION WITH THE EXTENT OF COOLING
IN KETTLE SECTION OF DISTILLATION COLUMN
91
95
100
105
110
Temperature
before
entering
expansion
valve (K)
0.1636859 0.1581833
0.1522229
0.1474877
0.1442894
Liquid Fraction
Table 4.4: Table for variation of liquid fraction with the extent of cooling in kettle
section of distillation column.
23
0.165
Liquid Fraction
0.16
0.155
0.15
0.145
0.14
0
50
100
150
Temperature before entering expansion valve (K)
Figure 4.5: Plot for liquid fraction vs temperature of stream before entering expansion
valve.
The above figure shows that as the temperature of the stream before entering the
expansion valve decreases, the fraction of liquid obtained increases. Hence, more the
cooling in the kettle section of the distillation column, more is the yield of the liquid.
For industrial purposes, usually it is taken care that the temperature of the stream before
entering the expansion valve lies in the range of 90K to 100K.
4.2.4 VARIATION OF HOT STREAM AND COLD STREAM
TEMPERATURE WITH APPROACH OF HEAT EXCHANGER
OUTLET
1
3
5
7
9
11
13
15
Temperature
Approach (K)
152
152
153
154
154
155
156
156
Hot
Stream
Outlet
Temperature(K)
151
149
148
147
145
144
142
141
Cold
Stream
Outlet
Temperature(K)
Table 4.5: Table for variation of hot stream and cold stream outlet temperature with
approach of heat exchanger.
The table shows that the effect of temperature approach on hot stream outlet temperature
is less as compared to its effect on cold stream outlet temperature. When the
temperature approach of heat exchanger changes from 1K to 15K, the cold stream outlet
temperature changes from 151K to 141K, whereas, the hot stream outlet temperature
changes only from 152K to 156K.
24
4.2.5 VARIATION OF PURITY OF BOTTOM AND TOP PRODUCTS WITH
CHANGE IN NUMBER OF STAGES IN DISTILLATION COLUMN
7
10
15
20
25
30
Number
of stages
90.15386
90.17717
90.18547
90.18953
90.19185
% purity 90.07161
of nitrogen
in top
99.07744
99.24645
99.30468
99.33285
99.34905
% purity 98.50202
of oxygen
in bottom
Table 4.6: Table for variation of purity of bottom and top products with change in
number of stages in distillation column.
% purity of nitrogen in top
90.2
90.18
90.16
90.14
90.12
90.1
90.08
90.06
0
10
20
Number of stages
30
40
Figure 4.6: Plot for % purity of nitrogen in top vs number of stages in distillation
column.
25
% purity of oxygen in bottom
99.4
99.3
99.2
99.1
99
98.9
98.8
98.7
98.6
98.5
98.4
0
10
20
Number of stages
30
40
Figure 4.7: Plot for % purity of oxygen in bottom vs number of stages in distillation
column.
The table 4.6 shows that with increase in number of stages, the purity of both nitrogen
and oxygen in top and bottom, respectively increases, but the increase is marginal. So,
this single column method is widely used where oxygen is desirable product since
oxygen has almost 99% purity. But, when nitrogen is desirable product, this method is
not preferred as its purity is only 90% and also huge amount of oxygen is wasted from
the top stream of the distillation column.
26
CHAPTER 5
CONCLUSIONS AND
FUTURE ASPECTS
27
5. CONCLUSIONS AND FUTURE ASPECTS
Simulation of air liquefaction and its separation was done successfully using Aspen Plus
simulating tool. The model under consideration was Linde single-column system. Also,
the effect of variation of various process conditions on yield, purity of final product, and
temperature were analyzed. Results obtained showed that by using Linde single-column
system, oxygen of almost 98-99% purity could be obtained. However, the purity of
nitrogen obtained was only about 90%. As such, Linde single-column system can be
used when oxygen is the desirable product.
In the project, cost analysis of the processes were not considered. So, as a future
recommendation, cost analysis can be done, leading to optimization of the entire process.
Also, simulation of much complex liquefaction systems can be considered which
produces argon, along with oxygen and nitrogen.
28
CHAPTER 6
REFERENCES
29
6. REFERENCES
[1]
Composition
of
c.com/charts/atmosph.htm
[2]
the
Atmosphere-
Relative
Abundance:
http://www.c-f-
Barron Randall F., Cryogenic Systems. New York, Oxford University Press, 1985
[3]
Trougott, H.K., Yuan, S.W.K., Cryogenics-Low Temperature Engineering and Applied
Science , 1986
[4]
Ventura, G., Risegari, L., The Art of Cryogenics, Low-temperature experimental Elsevier,
2008
[5]
Richard, T. Jacobsen, Steven, G.Penoncello and Eric, W.Lemmon, Thermodynamic
Properties of Cryogenic Fluid Plenum Press
[6]
The Encyclopaedia Wikipedia, Liquefaction of gas
http://en.wikipedia.org/wiki/Liquid_air
[7]
The Encyclopaedia Wikipedia, Liquid air
http://en.wikipedia.org/wiki/Liquid_air
[8]
Aspentech Corporate Overview: http://www.aspentech.com/corporate/press/media_kit.cfm
[9]
Prasad, S., Simulation of nitrogen liquefaction systems NIT Rourkela, 2009
[10]
Aspentech Aspen Plus: http://www.aspentech.com/core/aspen-plus.aspx
30
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