PROCESS DESIGN OF TURBOEXPANDER BASED NITROGEN LIQUEFIER Department of Mechanical Engineering

PROCESS DESIGN OF TURBOEXPANDER BASED NITROGEN LIQUEFIER  Department of Mechanical Engineering
PROCESS DESIGN OF TURBOEXPANDER
BASED NITROGEN LIQUEFIER
A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE
REQUIREMENTS FOR THE DEGREE OF
Master of Technology
in
Mechanical Engineering
By
Balaji Kumar Choudhury
Department of Mechanical Engineering
National Institute of Technology
Rourkela
2009
PROCESS DESIGN OF TURBOEXPANDER
BASED NITROGEN LIQUEFIER
A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE
REQUIREMENTS FOR THE DEGREE OF
Master of Technology
in
Mechanical Engineering
By
Balaji Kumar Choudhury
Under the guidance of
Prof. Ranjit Kumar Sahoo
Department of Mechanical Engineering
National Institute of Technology
Rourkela
2009
National Institute of Technology
Rourkela
CERTIFICATE
This is to certify that the thesis entitled, “PROCESS DESIGN OF
TURBOEXPANDER BASED NITROGEN LIQUEFIER” submitted by Mr. Balaji
Kumar Choudhury in partial fulfillment of the requirements for the award of Master
of Technology Degree in Mechanical Engineering with specialization in Thermal
Engineering at the National Institute of Technology, Rourkela (Deemed University)
is an authentic work carried out by him under my supervision and guidance.
To the best of my knowledge, the matter embodied in the thesis has not been
submitted to any other University/ Institute for the award of any degree or diploma.
Date:
Prof. RANJIT KUMAR SAHOO
Department of Mechanical Engineering
National Institute of Technology
Rourkela – 769008
CONTENTS
CERTIFICATE
i
CONTENTS
ii
ACKNOWLEDGEMENT
iv
ABSTRACT
v
LIST OF FIGURES
vi
LIST OF TABLES
vii
NOMENCLATURE
viii
CHAPTER 1
1
1. INTRODUCTION
2
1.1 Principle of liquefaction
2
1.2 Requirement of nitrogen liquefier
3
1.3 Production of liquid nitrogen
3
1.4 Objectives of the work
4
CHAPTER 2
5
2. Literature review
5
2.1 History of liquefaction
5
CHAPTER 3
13
3. Process design
14
3.1 Modified Claude cycle for Nitrogen Liquefier
14
3.2 Steps of the process design calculations
15
3.3 Process design calculation using Microsoft Excel
20
3.4 Process design using Aspen Hysys
22
3.4.1 Introduction to Aspen Hysys
22
3.4.2 Procedure of Process Design in Aspen Hysys
24
3.4.3 Input values in Aspen Hysys
26
3.4.4 Results in Aspen Hysys
27
ii
CHAPTER 4
29
4. RESULTS AND DISCUSSION
29
4.1 Performance Analysis
30
4.1.1 Effect of Variation of expander flow ratio, α
30
4.1.2 Effect of Variation of effectiveness of HX1, ε1
32
4.1.3 Effect of Variation of pinch point of second heat exchanger
33
4.1.4 Effect of Variation of turbo expander efficiency η
34
4.2 Variable Specific Heat Analysis of Heat Exchangers
35
4.2.1 Analysis of Heat exchanger-1
36
4.2.2 Analysis of Heat exchanger-2
37
4.3 Cumulative Enthalpy Analysis of Heat Exchangers
38
4.3.1 Analysis of Heat exchanger-1
38
4.3.2 Analysis of Heat exchanger-2
40
CHAPTER 5
42
5. CONCLUSIONS
43
6. BIBLIOGRAPHY
44
iii
ACKNOWLEDGEMENT
I am extremely fortunate to be involved in an exciting and challenging research project like
“process design of turboexpander based nitrogen liquefier”. It has enriched my life, giving
me an opportunity to work in a new environment of Aspen Hysys. This project increased my
thinking and understanding capability as I started the project from scratch.
I would like to express my greatest gratitude and respect to my supervisor Prof. Ranjit Kumar
Sahoo, for his excellent guidance, valuable suggestions and endless support. He has not only
been a wonderful supervisor but also a genuine person. I consider myself extremely lucky to
be able to work under guidance of such a dynamic personality. Actually he is one of such
genuine person for whom my words will not be enough to express.
I would like to express my sincere thanks to Prof. Sunil Kumar Sarangi for his precious
suggestions and encouragement to perform the project work. He was very patient to hear my
problems that I am facing during the project work and finding the solutions. I am very much
thankful to him for giving his valuable time for me.
I would like to express my thanks to all my classmates, all staffs and faculty members of
mechanical engineering department for making my stay in N.I.T. Rourkela a pleasant and
memorable experience and also giving me absolute working environment where I unlashed ,y
potential .
I want to convey my heartiest gratitude to my parents for their unfathomable encouragement.
I would also like express heartiest feelings to my brothers and sister in law for providing me
encouragement and financial support for higher education. The sacrifice they made to make
my dream come true beyond any words I can write.
Date:
Balaji Kumar Choudhury
Roll. No. 207ME305
M.tech. (Thermal)
iv
Abstract
Our country is still dependent on imports for most of its needs in cryogenic
refrigerators and liquefiers. These products are proprietary in nature which makes it very
expensive for its cost and maintenance. Also as a strategy of promoting the sale, the company
changes their model by limiting the spare parts of the earlier model to enforce on the
customers to buy the new product. So it is imperative that our country develops an indigenous
nitrogen liquefier to meet the need of liquid nitrogen. With support from the Department of
Atomic Energy, our institute has initiated a program on development and study of a nitrogen
liquefier of intermediate capacity in the range of 10-50 litres/hr by using technologies already
developed in our country.
The process is based on a suitable modified Claude cycle which minimizes the
number of heat exchangers and also takes care to accommodate the in house developed turbo
expander. The process design is carried out using the standard calculation procedure and is
validated by using process simulation software, Aspen Hysys.
Parametric analysis is carried out to access the role of different component
efficiencies in predicting overall system efficiency at the design and off design conditions. In
this analysis, the available turbo expander efficiency is considered to evaluate the feasible
heat exchanger efficiency in order to optimize the plant efficiency. The thermodynamic
parameters (temperature, pressure, pinch point temperature) are evaluated to obtain the
optimum mass fraction through turbo expander for maximum liquid yield. This investigation
not only gives the analysis of nitrogen liquefier, but also it will act as a basic frame work for
any liquefier and helium liquefier in particular as a future mission.
v
List of figures
Figure No.
Title
Page No.
CHAPTER 2
2.1
Cailletet’s gas compressor and liquefaction apparatus
6
2.2
Pictet's cascade refrigeration and liquefaction system
7
2.3
Linde air liquefaction system
8
2.4
T-S Diagram of Linde cycle
8
2.5
Tripler's laboratory showing 175 kW steam driven multistage air compressor
and 25 dm h- air liquefier
9
2.6
Claude air liquefaction system
10
2.7
T-S Diagram of Claude Cycle
10
2.8
Kapitza system
11
2.9
Helandt system
12
CHAPTER 3
3.1
Process Diagram Nitrogen Liquefier
15
3.2
Pinch point of HX2
17
3.3
Expansion in Turbo Expander
18
3.4
T-S Diagram of Nitrogen Liquefier
21
3.5
Process Flow Diagram of Nitrogen Liquefier in Aspen Hysys
27
CHAPTER 4
4.1
Variation of yield with expander flow ratio
31
4.2
Variation of compressor work per kg of liquid with expander flow ratio
31
4.3
Variation yield with effectiveness of HX-1
32
4.4
Variation yield with Pinch temperature of HX-2
33
4.5
Variation yield with Turboexpander efficiency
34
4.6
Heat exchanger split into parts
35
4.7
Operating Temperature Line for Heat exchanger 1
37
4.8
Operating Temperature Line for Heat exchanger 2
38
4.9
Cumulative enthalpy- Temp Diagram for Heat Exchanger 1
39
4.10
Cumulative enthalpy- Temp Diagram for Heat Exchanger 2
41
vi
List of Tables
Table No.
Title
Page No.
CHAPTER 3
3.1
Material stream properties in Aspen Hysys
27
CHAPTER 4
4.1
Effect of variation of expander flow ratio
30
4.2
Effect of Variation of effectiveness HX1
32
4.3
Effect of Variation of pinch point of HX-2
33
4.4
Effect of Variation of turbo expander efficiency
34
4.5
Calculation of UA of HX1 considering variable specific heat
36
4.6
Calculation of UA of HX2 considering variable specific heat
37
4.7
Cumulative enthalpy in L.P. Side of HX-1
38
4.8
Cumulative enthalpy in H.P. Side of HX-1
39
4.9
Cumulative enthalpy in L.P. Side of HX-2
40
4.10
Cumulative enthalpy in H.P. Side of HX-2
40
vii
Nomenclature
HX1 = Heat exchanger 1
HX 2a = Heat exchanger 2 (non-condensing part)
HX 2b= Heat exchanger 2(condensation part)
Tex = Turboexpander
T = Temperature
P=Pressure
h = specific enthalpy
s= specific entropy
x= dryness fraction
1= Compressor inlet
2= Compressor outlet & HX 1 inlet (hot inlet, H.P side)
3 = hot outlet of HX 1 & Turboexpander inlet
4 = hot outlet of HX 2 & J.T Valve inlet
p= Pinch point in low pressure stream, inside HX2
5 = J.T Valve out & Phase separator in
6 = Turboexpander exit (actual state)
6' = Turboexpander exit (isentropic state)
7 = Exit of mixer 2 & cold inlet for HX 2
8 = cold outlet of HX 2 & cold in for HX 1
9 = cold out of HX 1 & mixer 1 inlet
f, g indicate saturated liquid and saturated vapor states respectively
η = efficiency of turboexpander
α = mass fraction of nitrogen diverted through turboexpander, mt/m
ε1 = Effectiveness of HX 1
ε2 = Effectiveness of HX 2
y = yield, liquid nitrogen separated per mass of gas compressed in the separator, mf/m
mf = mass of liquid nitrogen separated in the phase separator
m=mass of nitrogen delivered from compressor, kg/sec
mt=mass of nitrogen gas diverted through turboexpander, kg/s
viii
Chapter 1
Introduction
1. INTRODUCTION
1.1 Principle of Liquefaction
Liquefaction of gases is always accomplished by refrigerating the gas to some
temperature below its critical temperature so that liquid can be formed at some suitable
pressure below the critical pressure. Thus gas liquefaction is a special case of gas
refrigeration and cannot be separated from it. In both cases, the gas is first compressed to an
elevated pressure in an ambient temperature compressor. This high-pressure gas is passed
through a countercurrent recuperative heat exchanger to a throttling valve or expansion
engine. Upon expanding to the lower pressure, cooling may take place, and some liquid may
be formed. The cool, low-pressure gas returns to the compressor inlet to repeat the cycle. The
purpose of the countercurrent heat exchanger is to warm the low-pressure gas prior to
recompression and simultaneously to cool the high-pressure gas to the lowest temperature
possible prior to expansion. Both refrigerators and liquefiers operate on this basic principle.
In a continuous refrigeration process, there is no accumulation of refrigerant in any
part of the system. This contrasts with a gas liquefying system, where liquid accumulates and
is withdrawn. Thus, in a liquefying system, the total mass of gas that is warmed in the
countercurrent heat exchanger is less than that of the gas to be cooled by the amount
liquefied, creating an imbalance mass flow in the heat exchanger. In a refrigerator the warm
and cool gas flows are equal in the heat exchanger. This results in what is usually referred to
as a "balanced flow condition" in a refrigerator heat exchanger. The thermodynamic
principles of refrigeration and liquefaction are identical. However the analysis and design of
the two systems are quite different because of the condition of balanced flow in the
refrigerator and unbalanced flow in liquefier systems.
The Joule-Thomson coefficient is a property of each specific gas. It is a function of
temperature and pressure, and may he positive, negative, or zero. For instance, hydrogen,
helium, and neon have negative J-T coefficients at ambient temperature. Consequently, to be
used as refrigerants in a throttling process they must first be cooled either by a separate pre
coolant liquid. Only then will throttling cause a further cooling rather than a heating of these
gases.
Another method of producing low temperatures 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 therefore isentropic. In this case, we can
2
define the isentropic expansion coefficient which expresses the temperature change due to a
pressure change at constant entropy. An isentropic expansion through an expander always
results in a temperature decrease. Whereas an expansion through an expansion valve may or
may not result in a temperature decrease. The isentropic expansion process removes energy
from the gas in the form of external work, so this method of low-temperature production is
sometimes called the external work method.
1.2 Requirement of nitrogen liquefier
Nitrogen liquefier used to produce liquid nitrogen. Because of its low production cost and
relatively higher levels of safety is the most common cooling medium in the cryogenic
temperature range above 77 K. The application covers such diverse areas as:
•
Pre coolant in production of liquid helium and low temperature refrigerators
•
Cryotreatment of critical metallic components such as hubs, milling cutters, knives,
rollers, needles, dies and punches, bearings and precision measuring equipment,
•
Preservation of live biological material as blood, animal and human sperms, embryos,
bacterial cultures etc
•
Cold trap in vacuum systems and in adsorption pumps, and
•
Miscellaneous laboratory and industrial applications.
1.3 Production of Liquid Nitrogen
In some parts of our country, it is possible to buy liquid nitrogen from bulk suppliers
at low cost. But in most cases, including some major metropolitan areas, a laboratory needs to
operate its own liquid nitrogen generator. There are three major international suppliers of
nitrogen liquefiers in our country:
•
Stirling Cryogenics of Netherlands,
•
Linde AG, Germany, and
•
Consolidated Pacific Industries, USA.
The liquefier from Stirling Cryogenics is based on the integral Philips-Stirling Cycle,
while the latter two use turbine for cold production. The Linde turbine uses gas bearings,
while the CPI machine uses antifriction bearings. The plants are enormously expensive to buy
and to maintain and owners are often forced to buy new plants due to non-availability of
proprietary spares. It is imperative that our country develops an indigenous nitrogen liquefier
of capacity in the range 10 to 50 l/hour. Hence a nitrogen liquefier is to be designed.
3
1.4 Objectives of the work
Prior to the making of the turboexpander based nitrogen liquefier, the thermodynamic
processes is to be designed and each equipment specifications are to be determined. A system
runs continuously when it follows definite processes in a cyclic path. Process design means,
determination of the type of thermodynamic processes included in the thermodynamic cycle
and fixing the points i.e. pressure and temperature. While designing the process, equipment
availability, constraints and cost should be kept in mind. Process design also includes the
setting the parameters up to the optimum condition that maximum amount of liquid will be
obtained.
4
Chapter 2
Literature Review
2. LITERATURE REVIEW
2.1 History of Liquefaction
Before 1877, a number of workers had discovered by visual observation in thick-walled glass
tubes that the permanent gases, including hydrogen, nitrogen, oxygen and carbon monoxide,
could not be liquefied at pressures as high as 400 atm. At first in 1877 oxygen gas is liquefied
by Cailletet and Pictet. It is the first permanent gases to be liquefied. The term 'permanent'
arose from the experimentally determined fact that such 'permanent' gases could not be
liquefied by pressure alone at ambient temperature, in contrast to the non-permanent or
condensable gases like chlorine, nitrous oxide and carbon dioxide, which could be liquefied
at quite modest pressures of 30-50 atm.
Fig. 2.1 Cailletet’s gas compressor and liquefaction apparatus
Figures 2.1 show the apparatus which Cailletet used to produce a momentary fog of oxygen
droplets in a thick walled glass tube. The oxygen gas was compressed using the crude
Natterer compressor in which pressures up to 200 atm. were generated by a hand-operated
screw jack. The pressure was transmitted to the oxygen gas in the glass tube by hydraulic
transmission using water and mercury. The gas was cooled to -110°C by enclosing the glass
tube with liquid ethylene, and was then expanded suddenly by releasing the pressure via the
hand wheel. A momentary fog was seen, and the procedure could then be repeated for other
observers to see the phenomenon.
6
Simultaneously at the first liquefaction of oxygen by Cailletet, Pictet also liquefied oxygen in
the same year 1877.
Fig 2.2 Pictet's cascade refrigeration and liquefaction system
Figure 2.2 shows the cascade refrigeration system of Pictet, in which oxygen was first cooled
by sulphur dioxide and then by liquid carbon dioxide in heat exchangers, before being
expanded into the atmosphere by opening a valve. The expansion yielded a transitory jet of
liquid oxygen, but no liquid could be collected from the high velocity jet. The figure shows
how Pictet used pairs of compressors to drive the SO2 (-20°C) and CO2 (-60°C) refrigerant
cycles on a continuous basis, and this is probably the first example of a cascade refrigeration
system operating at more than one temperature level. His use of the cascade system inspired
others like Kamerlingh Onnes and Dewar.
In 1883, the Polish scientists Olzewski and Wroblewski, at Cracow, had improved Cailletet's
apparatus by:
1. Adding an inverted U to the glass tube; and
2. Reducing the ethylene temperature to -136°C by pumping it below atmospheric
pressure.
These improvements enabled them to produce small quantities of liquid oxygen in the U tube
and to liquefy carbon monoxide and nitrogen for a few seconds.
7
From first liquefaction of oxygen to 1895, there was little progress in the developments of
liquefiers. Then in 1895, Hampson in London and Linde in Munich simultaneously patented
compact and efficient air liquefiers which used self-intensive or regenerative cooling of the
high pressure air by the colder low pressure expanded air in long lengths of coiled heat
exchanger. In this simple way, the complications of cascade precoolers employing liquid
ethylene and other liquid cryogens were removed. A further advantage of this simple liquefier
was the absence of moving parts at low temperature, the cooling being produced by JouleThomson expansion through a nozzle or valve. Carl von Linde made rapid progress in
developing this technological breakthrough. He was a professor and research worker at the
University of Munich, and he had his own company constructing refrigeration plant.
The Linde-Hampson is the simplest of all the liquefaction systems. A schematic of the
Linde-Hampson system is shown in Fig. 2.3 and the cycle is shown on the T-s plane in Fig.
2.4.
Fig. 2.3 Linde air liquefaction system
Fig. 2.4 T-S Diagram of Linde cycle
Process 1 to.2 would actually be two processes: an irreversible, adiabatic or polytropic
compression followed by an after cooling 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 stream to
point 3. From point 3 to point 4, the gas expands through an expansion valve to P4. At point
4, some of the gas stream is in the liquid state and is withdrawn at condition f (saturatedliquid 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
8
energy at constant pressure (ideally) from the incoming high-pressure stream. The liquid air
produced is very less.
By 1898, Charles Tripler, an engineer in New York, had constructed a similar but
much larger air liquefier, driven by a 75 kW steam engine, which produced literally gallons
of liquid air per hour. Tripler discovered a market for liquid air as a medium for driving air
expansion engines (the internal combustion engine was still unreliable at that time) and
succeeded to launch his Liquid Air Company.
Fig. 2.5 Tripler's laboratory showing 175 kW steam driven
multistage air compressor and 25 dm h- air liquefier
In the year, 1902, a young French innovative engineer Georges Claude, with wide
connections in the scientific world of Paris, had succeeded in producing a piston expansion
engine working at the low temperatures required for the liquefaction of air. The increase in
cooling effect over the Joule-Thomson nozzle expansion of the Linde, Tripler, and Hampson
designs was so large as to constitute a second technological breakthrough. Claude developed
air liquefiers with piston expanders.
9
Fig. 2.6 Claude air liquefaction system
Fig. 2.7 T-S Diagram of Claude Cycle
The expansion through an expansion valve is an irreversible process. Thus if we wish
to approach closer to the ideal performance, we must seek a better process to produce low
temperatures. In the Claude system, shown in Fig. 2.6, energy is removed from the gas stream
by allowing it to do some work in an expansion engine or expander.
The Claude cycle is shown on the T-s plane in Fig. 2.7. If the expansion engine is
reversible and adiabatic, the expansion process is isentropic, and a much lower temperature is
attained than for an isenthalpic expansion, In the Claude system, the gas is first compressed
to pressures on the order of 4 MPa (40 atm or 590 psia) and then passed through the first heat
exchanger. Between 60 and 80 percent of the gas is then diverted from the mainstream,
expanded through an expander, and reunited with the return stream below the second heat
exchanger. The stream to be liquefied continues through the second and third heat exchangers
and is finally expanded through an expansion valve to the liquid receiver. The cold vapor
from the liquid receiver is returned through the heat exchangers to cool the incoming gas.
In 1882, Kamerlingh Onnes set up a cryogenic laboratory at the University of Leiden
in the Netherlands. In 1866, Van der Waals had published his first paper on 'the continuity of
liquid and gaseous states' from which the physical understanding of the critical state and of
liquefaction and evaporation was to grow. This information inspired Kamerlingh Onnes and
for the first time in 1908, he was able to liquefy helium. He had only 360 liters of gaseous
helium obtained by heating monazite sand from India. More than 60 cm2 of liquid helium was
produced by ones in his first attempt.
10
Fig. 2.8 Kapitza system
Kapitza (1939) modified the basic Claude system by eliminating the third or low
temperature heat exchanger, as shown in Fig. 2.8. Several notable practical modifications
were also introduced in this system. A rotary expansion engine was used instead of a
reciprocating expander. The first or high-temperature heat exchanger in the Kapitza system
was actually a set of valved regenerators, which combined the cooling process with the
purification process. The incoming warm gas was cooled in one unit and impurities were
deposited there, while the outgoing stream warmed up in the other unit and flushed out the
frozen impurities deposited in it. After a few minutes, a valve was operated to cause the highand low-pressure streams to switch from one unit to the other. The Kapitza system usually
operated at relatively low pressures-on the order of 700 kPa (7 atm or 100 psia).
Around 1942 Samuel C. Collins of the department of mechanical Engineering at
Massachusetts Institute of technology developed an efficient liquid helium laboratory facility.
He developed Collins helium cryostat results economical and safe production of liquid
helium.
11
Fig. 2.9 Helandt system
Helandt (Davies 1949) noted that for a high pressure of approximately 20 MPa (200
atm) and an expansion-engine flow-rate ratio of approximately 0.60, the optimum value of
temperature before expansion through the expander was near ambient temperature. Thus, one
could eliminate the first heat exchanger in the Claude system by compressing the gas to 20
MPa. Such a modified Claude system is called the Heylandt system, after its originator, and is
used extensively in high-pressure liquefaction plants for air. The system is shown
schematically in Fig. 2.9. The advantage of the Heylandt system is that the lubrication
problems in the expander are not difficult to overcome. In the air-liquefaction system, the gas
enters the expander at ambient temperature and leaves the expander at approximately 150 K
(-190°F), So that light lubricants can be used.
From time to time a lot of modifications had been made in all these cycles to optimize
the results. The efficiency and performance of the components are increased, and so little
modification in those cycles can reach up to lowest temperature and produce liquid for longer
period.
12
Chapter 3
Process Design
3. PROCESS DESIGN
3.1 Modified Claude Cycle for Nitrogen Liquefier
A modified Claude cycle is taken into consideration to design nitrogen liquefier to
take the advantage of both the turboexpander and JT valve. Instead of three heat exchangers
as in the Claude cycle, two numbers of heat exchangers are used in this liquefier. Last two
heat exchangers of the Claude cycle are combined to a single heat exchanger to reduce the
cost of the liquefier.
A turbo expander based nitrogen liquefier consists of following parts:
•
Compressor
•
Heat exchangers
•
Turboexpander
•
JT Valve
•
Phase separator
•
Cold box
•
Piping
•
Instrumentation
A screw compressor will be installed to provide the compressed nitrogen gas. Heat
exchangers are vital components of any cryogenic refrigerator. To exchange high heat in
small area plate fin compact heat exchanger are used. The turboexpander is the heart of the
liquefier and it can used lowering the temperature to expectable amount adiabatically. JT
valve is used for isenthalpic expansion. Phase separator is used to separate liquid and gas.
Piping and other instrumentations are required to connect and control the systems. Whole
thing is kept inside the cold box.
Fig.3.1 shows the process diagram of the nitrogen liquefier. At atmospheric
temperature and pressure at 1.1 bar the pure nitrogen gas is feed into the screw compressor
and compressed up to 8 bars. The compressed gas is passed through the first heat exchanger
i.e. HX1. Then some mass is diverted through the turboexpander and remain passes through
the second heat exchanger i.e.HX2 for liquefaction. For easy calculation HX2 split into two
parts i.e. HX2a and HX2b. From the HX2, isenthalpic expansion takes place by using JT
valve which results liquid nitrogen. Liquid nitrogen taken out and remain vapor nitrogen meet
with the isentropic expanded nitrogen by the turboexpander and feed again to the compressor
by passing through the HX2 and HX1.
14
Fig. 3.1 Process Diagram Nitrogen Liquefier
3.2. Steps of the process design calculations
A. Known values
Pure nitrogen feed to the screw compressor at temperature, 300 K and pressure 1.1 bars.
Isothermal compression is considered but in real case, temperature is increased. Let it is
increased to the temperature 310 K and pressure is 8 bars.
Generally maximum pressure drop in both the heat exchangers is taken as 0.05 bars. Hence
the pressure of high pressure stream after the HX1 is 7.95 bars. Similarly after HX2, the
pressure is 7.9 bars. Passing through the HX2 Nitrogen in the high pressure stream comes to
two phase state. The saturated temperature at 7.9 bars is 100.13 K.
The pressure inside the phase separator should be just higher than the atmospheric, so that the
liquid will come out of the cold box. So phase separator pressure is fixed to 1.2 bars.
Saturated temperature of nitrogen at 1.2 bars inside phase separator is 78.8 K.
Between HX2 and phase separator, JT Valve is placed for isenthalpic expansion of nitrogen
from 7.9 bars to 1.2 bars.
The fraction of mass flow through the turboexpander expanded to the pressure 1.3 bars to
maintain the pressure ratio of 6. So that mach No. should not exceed 1.
P3
=6
P6
15
Then from mixture comes out at 1.2 bars and return to compressor at 1.1 bars by pressure
drop of 0.05 bars at each heat exchanger. Pressure difference is maintained at all equipments
so flow will occur cyclically.
B. Parameters:
The parameters, by changing which, the amount of liquid nitrogen effected or by
controlling which we optimize the output are:
•
Effectiveness of heat exchanger 1,ε1
•
Pinch point for heat exchanger 2, p
•
Efficiency of turbo expander,η
•
Mass flow ratio diverted through Turbo expander, α
C. Unknown Variables:
Following are the unknown variables which value are to be determined.
•
Enthalpy at the exit of first heat exchanger, h3
•
Enthalpy at the exit of second heat exchanger, h4
•
Enthalpy at the exit of JT valve, h5
•
Enthalpy at the exit of turboexpander, h6
•
Enthalpy at the exit of turboexpander (Isentropic expansion), h6s
•
Enthalpy at after mixing from phase separator and exit of turboexpander, h7
•
Enthalpy at the inlet to the second heat exchanger, h8
•
Enthalpy at the inlet to the first heat exchanger, h9
•
Dryness fraction in the phase separator, x5
•
Ratio of mass of liquid produced to mass of gas compressed, yield, y
•
Enthalpy at the pinch point temperature of low pressure stream of second heat
exchanger, hp
D. Component Analysis
Some initial value of Yield, y and low pressure stream outlet enthalpy from
HX1, h9 = ε 1h2' should be taken.
i. Pinch point specification of Heat exchanger-2
Splitting the HX2 into two parts, First heat exchanger being the one where the hot
nitrogen gas is cooled up to the saturation temperature of 100.13 K & the second part being
16
the condensing part. The minimum temperature difference occurs at the point where the
condensation begins and is called as pinch point.
Fig. 3.2 Pinch point of HX2
For the specified pinch value p, for HX2, we have
T4 g − T p = p,
T p = T4 g − p
(3.1)
We can get enthalpy hp, at that pinch temperature and pressure.
ii. Heat Exchanger-1
For the specified value of effectiveness of heat exchanger 1 and the pinch point
specification for HX2, h8, h3 and h9 calculated from the effectiveness definition and energy
balance between hot and cold fluids for HX1 and HX2a.
Assume
h8 =
h3 =
[h9 (1 − y )(1 − α ) − h2 (1 − α ) + h4 g (1 − α ) − h p (1 − y )]
[(−α )(1 − y )]
[h4 g (1 − α ) + (1 − y )(h8 − h p )]
(3.2)
(3.3)
(1 − α )
h9 = ε 1h2' + (1 − ε 1)h8
(3.4)
The updated value of h9 is calculated. It should be checked with the previous value. This
iteration should be done until both are equal.
iii. Turbo-expander
From the Fig. 3.2, it is clear that 3-6s is the isentropic expansion and 3-6 is the actual
expansion.
17
From property table, entropy at 3, i.e. s3, can be found out at h3 and p3.
The enthalpy at the end of expansion is found out as
s6 s = s3
(3.5)
h6s can be get from p6s and s6s.
h6 = h3 − η (h3 − h6 s ) (3.6)
(6)
3
6
6s
Fig. 3.3 Expansion in Turbo Expander
vi. Mixer
Applying energy balance equation for the mixer, enthalpy at outlet of mixer is
(1 − α − y )h5 g + αh6 = (1 − y )h7
h7 =
[αh6 + (1 − α − y )h5 g ]
(3.7)
(1 − y )
v. Heat Exchanger 2
Enthalpy at outlet of hot fluid is found out by energy balance between hot and cold fluids as
h4 =
[h3 (1 − α ) − (1 − y )(h8 − h7 )]
(1 − α )
(3.8)
vi. Throttle valve:
Throttling is an isenthalpic process. Equating the enthalpies before and after throttling
h5 = h4
(3.9)
18
x5 =
(h5 − h f 5 )
(3.10)
( hg 5 − h f 5 )
vii. Yield:
The liquid yield obtained per kg of gas passing through the throttling valve is therefore
(1-x5).
For (1 − α ) kg of gas passing through the throttling valve is
y = (1 − α )(1 − x5 )
(3.11)
Again check the calculated value of y with the assumed one and calculated h9 with
assumed h9. Replace it with the new value of h9 and y and calculate again till both the
assumed values match with calculated value
19
3.3 Process Design Calculation Using Microsoft Excel
Thermodynamic properties like pressure, temperature, enthalpy, entropy all are
calculated by using the above equations and following the above procedure. Below there is
table showing all the thermodynamic property values and liquid yield produced.
Working Fluid
:
Nitrogen
Asumed Yeild , Y
=
0.043262
Flow Through Turbo Expander,α
=
0.94
Effectiveness of HX1
=
0.98
Efficiency of Expander
=
0.5
Pinch temperature HX2
=
1
Mass Flow Rate
=
296 kg/hr
=
0.08222 kg/s
Assumed Enthalpy at
9
=
317.461
Points
Presssure
Temperature
Enthalpy
Entropy
Quality
(bar)
(K)
kJ/kg
kJ/kg/K
1
1.1
300.00
311.44
6.8194
2
8
310.00
320.44
6.2602
2*
1.1
310.00
321.85
6.8535
3
7.95
120.52
114.76
5.2269
4
7.9
100.13
-64.24
3.4653 0.0541
4f
7.9
100.13
-72.91
3.3788 0.0000
4g
7.9
100.13
87.39
4.9786 1.0000
5
1.2
78.78
-64.24
3.5678 0.2790
5f
1.2
78.78
-119.19
2.8708 0.0000
5g
1.2
78.78
77.78
5.3693 1.0000
6s
1.3
79.49
68.24
5.2269 0.9487
6sf
1.3
79.49
-117.72
2.8892 0.0000
6sg
1.3
79.49
78.29
5.3533 1.0000
6
1.3
90.81
91.496
5.5090
>1
7
1.2
90.40
91.26
5.5293
P
1.15
99.13
100.77
5.6420
8
1.15
100.73
102.48
5.6592
9
1.1
305.78
317.461
6.8393
Calculated Yeild , Y in Phase Separator
=
0.043262
Energy Balance
=
0.043261
Temperature Drop in Turbo Expander
=
29.71
Cumulative UA for Hx 1
=
1.8787 kW/K
Cumulative UA for Hx 2
=
0.2032 kW/K
Liquid Nitrogen Produced
12.805206 kg/hr
* Note :Text written inside double lined border are the input values
20
Mass Flow
Rate kg/s
0.08222
0.08222
0.07728
0.00493
0.00493
0.00355
0.00137
0.07728
0.07866
0.07866
0.07866
Fig. 3.4 T-S Diagram of Nitrogen Liquefier
21
3.4 Process Design Using Aspen Hysys
3.4.1 Introduction to Aspen Hysys
Aspen Hysys is a process simulation environment designed to serve many processing
industries especially Oil & Gas and Refining. With Aspen Hysys one can create rigorous
steady state and dynamic models for plant design, performance monitoring, troubleshooting,
operational improvement, business planning, and asset management. Through the completely
interactive Aspen Hysys interface, one can easily manipulate process variables and unit
operation topology, as well as fully customize your simulation using its customization and
extensibility capabilities. The process simulation capabilities of Aspen Hysys enables
engineers to predict the behavior of a process using basic engineering relationships such as
mass and energy balances, phase and chemical equilibrium, and reaction kinetics. With
reliable thermodynamic data, realistic operating conditions and the rigorous Aspen Hysys
equipment models, they can simulate actual plant behavior. Some of the important Aspen
Hysys features are listed below:
¾ Windows® Interoperability : Interface contains a process flow sheet view for
graphical layout, data browser view for entering data, the patented Next expert
guidance system to guide the user through a complete and consistent definition of the
process flow sheet.
¾ Plot Wizard: Hysys enables the user to easily create plots of simulation results.
¾ Flowsheet Hierarchy and Templates: Collaborative engineering is supported through
hierarchy blocks that allow sub-flowsheets of greater detail to be encapsulated in a
single high-level block. These hierarchy blocks can be saved as flowsheet templates
in libraries.
¾ Equation-Oriented Modeling: Advanced specification management for equation
oriented model configuration and sensitivity analysis of the whole simulation or
specific parts of it. The unique combination of Sequential Modular and Equation
Oriented solution technology allows the user to simulate highly nested processes
encountered typically in the chemical industry.
22
¾ Thermo physical Properties: Physical property models and data are key to generating
accurate simulation results that can be used with confidence. Aspen Hysys uses the
extensive and proven physical property models, data and estimation methods
available in Aspen Properties™, which covers a wide range of processes from simple
ideal behavior to strongly non-ideal mixtures and electrolytes. The built-in database
contains parameters for more than 8,500 components, covering organic, inorganic,
aqueous, and salt species and more than 37,000 sets of binary interaction parameters
for 4,000 binary mixtures.
¾ Convergence Analysis: to automatically analyze and suggest optimal tear streams,
flowsheet convergence method and solution sequence for even the largest flowsheets
with multiple stream and information recycles.
¾ Sensitivity Analysis: to conveniently generate tables and plots showing how process
performance varies with changes to selected equipment specifications and operating
conditions.
¾ Design Specification: capabilities to automatically calculate operating conditions or
equipment parameters to meet specified performance targets.
¾ Data-Fit: to fit process model to actual plant data and ensure an accurate, validated
representation of the actual plant.
¾ Determine Plant Operating Conditions that will maximize any objective function
specified, including process yields, energy usage, stream purities and process
economics.
¾
Simulation Basic Manager: This feature available in Aspen Hysys for using different
fluids like nitrogen, air, acetylene as per requirement. Also several fluid packages like
BWRS, MWRS, and ASME are provided to calculate properties at different states.
23
3.4.2 Procedure of Process Design in Aspen Hysys
To create a new case, From the File menu, select New. In the sub-menu, select Case.
The Simulation Basis Manager window will appear.
The Simulation Basis Manager is the main property view of the Simulation
environment. One of the important concepts that HYSYS is based upon is Environments. The
Simulation Basis environment allows you to input or access information within the
Simulation Basis manager while the other areas of HYSYS are put on hold avoiding
unnecessary Flowsheet calculations. Once you enter the Simulation environment, all changes
that were made in the Simulation Basis environment will take effect at the same time.
Conversely, all thermodynamic data is fixed and will not be changed as manipulations to the
Flowsheet take place in the Simulation environment. The minimum information required
before leaving the Simulation Basis manager is atleast one installed Fluid Package with an
attached Property Package and At least one component in the Fluid Package.
The Components Manager is located on the Components tab of the Simulation Basis
Manager. This tab provides a location where sets of chemical components being modeled
may be retrieved and manipulated. These component sets are stored in the form of
Component Lists that may be a collection of library pure components or hypothetical
components.The Components Manager always contains a Master Component List that cannot
be deleted. This master list contains every component available from "all" component lists. If
you add components to any other component list, they automatically get added to the Master
Component List. Also, if you delete a component from the master, it also gets deleted from
any other component list that is using that component.
In HYSYS, all necessary information pertaining to pure component flash and physical
property calculations is contained within the Fluid Package. This approach allows you to
define all the required information inside a single entity. There are four key advantages to this
approach:
¾ All associated information is defined in a single location, allowing for easy creation
and modification of the information.
¾ Fluid Packages can be exported and imported as completely defined packages for use
in any simulation.
24
¾ Fluid Packages can be cloned, which simplifies the task of making small changes to a
complex Fluid Package.
¾ Multiple Fluid Packages can be used in the same simulation.
The Fluid Package Manager is located on the Fluid Pkgs tab of the Simulation Basis
Manager. This tab provides a location where multiple fluid packages can be created and
manipulated. Each fluid package available to your simulation is listed in the Current Fluid
packages group with the following information: name, number of components attached to the
fluid package, and property package attached to the fluid package. From the Fluid Pkgs tab of
the Simulation Basis Manager click either the View or Add button to open the Fluid Package
property view. Make sure you select the proper fluid package when using the view option.
Click on the Set Up tab. From the Component List Selection drop-down list, select the
components you want to use in your fluid package.
Here Benedict-Webb-Rubin-Starling (BWRS) fluid package was used. This model is
commonly used for compression applications and studies. It is specifically used for gas phase
components that handle the complex thermodynamics that occur during compression, and is
useful in both upstream and downstream industries.
After selecting fluid packages and components , a process flowsheet window will
apear on which the unit opearations can be installed. There are a number of ways to install
unit operations into your flowsheet. Many unit operations are available in the flowsheet
palette. All information concerning a unit operation can be found on the tabs and pages of its
property view. Each tab in the property view contains pages, which pertain to a certain aspect
of the operation, such as its stream connections, physical parameters (for example, pressure
drop and energy input), or dynamic parameters such as vessel rating and valve information.
In steady state analysis recycler unit operations can be used to calculate the unknown
parameters in the process flow diagram.
The process flow diagram (PFD) provides the best representation of the flowsheet as a
whole. Using the PFD gives you immediate reference to the progress of the simulation
currently being built, such as what streams and operations are installed, flowsheet
connectivity, and the status of objects. In addition to graphical representation, you can build
your flowsheet within the PFD using the mouse to install and connect objects. A full set of
manipulation tools is available so you can reposition streams and operations, resize icons, or
reroute streams. All of these tools are designed to simplify the development of a clear and
25
concise graphical process representation. The PFD also possesses analytical capabilities. You
can access property views for streams or operations directly from the PFD, or install custom
Material Balance Tables for any or all objects. Complete Workbook pages can also be
displayed on the PFD and information is automatically updated when changes are made to the
process.
3.4.3 Input values in hysys
From simulation basis manager in the component pure nitrogen is taken as material
stream and BWRS as fluid packages. Then enter into the simulation environment. There all
unit operations are arranged in order and linked by material streams. For each unit operations
follwing input values are entered.
1. Compressor
Mass flow rate =296 kg/hr
Inlet temperature = 300 K
Inlet pressure
= 1.1 bar
Outlet pressure =8 bar
2. Cooler
Outlet temperature = 310 K
Outlet pressure
=8 bar
3. Heat exchanger 1
Minimum Approach = 4.15 K
Pressure drop in both streams =0.05 bar
4. Tee
Flow ratio through turbo expander = 0.94
5. Turbo expander
Efficiency of turbo expander = 50 %
Outlet pressure
=1.3 bar
6. Heat exchanger 2
Minimum Approach
=1K
Pressure drop in both streams =0.05 bar
7. JT Valve
Outlet pressure = 1.2 bar
26
3.4.4 Results in Aspen Hysys
Amount of liquid yeild can be seen in the liquid stream of the phase separator. It comes
12.558 kg/hr.
Fig. 3.5 Process Flow Diagram of Nitrogen Liquefier in Aspen Hysys
Figure 3.5 shows the process flow diagram that drawn in Hysys. Table 3.1 shows the state
and properties of all the streams in the process flow diagram mass flow at 5f , gives the liquid
nitrogen that produced.
Table 3.1 Material stream properties in Aspen Hysys
Material Streams
1
Vapour Fraction
2a
2
8
3tex
3hx
1.0000
1.0000
1.0000
1.0000
1.0000
1.0000
Temperature
K
300.00
595.78
310.00
100.98
120.94
120.94
Pressure
bar
1.100
8.000
8.000
1.150
7.950
7.950
Molar Flow
kgmole/h
10.57
10.57
10.57
10.12
9.933
0.6340
Mass Flow
Liquid Volume
Flow
kg/h
296.00
296.00
296.00
283.45
278.24
17.760
m3/h
0.3671
0.3671
0.3671
0.3515
0.3451
2.202e-002
kW
0.1381
26.12
0.8789
-16.00
-14.79
-0.9442
5
5g
5f
6
Heat Flow
4
Vapour Fraction
7
0.0086
1.0000
0.2929
1.0000
0.0000
1.0000
Temperature
K
100.35
90.032
79.161
79.161
79.161
90.441
Pressure
bar
7.900
1.200
1.200
1.200
1.200
1.300
Molar Flow
kgmole/h
0.6340
10.12
0.6340
0.1857
0.4483
9.933
Mass Flow
Liquid Volume
Flow
kg/h
17.760
283.45
17.760
5.2020
12.558
278.24
m3/h
2.202e-002
0.3515
2.202e-002
6.451e-003
1.557e-002
0.3451
-1.855
-16.92
-1.855
-0.3274
-1.528
-16.59
Heat Flow
kW
27
5g1
Vapour Fraction
3
10
9
11
12
1.0000
1.0000
1.0000
1.0000
1.0000
1.0000
Temperature
K
79.160
120.94
120.94
305.85
300.00
300.00
Pressure
bar
1.200
7.950
7.950
1.100
1.100
1.100
Molar Flow
kgmole/h
0.1858
10.57
10.57
10.12
10.12
0.4482
Mass Flow
kg/h
5.2056
296.00
296.00
283.45
283.45
12.554
Liquid Volume
Flow
m3/h
6.456e-003
0.3671
0.3671
0.3515
0.3515
1.557e-002
Heat Flow
kW
-0.3276
-15.74
-15.74
0.6125
0.1322
5.857e-003
28
Chapter 4
Results and Discussion
4. RESULTS AND DISCUSSION
4.1 Performance Analysis
The effect of parametric variation is done on the liquefaction system gives the
optimum performance. This analysis also depicts the off design performance analysis. The
parameters are
¾ Effectiveness of heat exchanger 1,ε1
¾ Pinch point for heat exchanger 2, p
¾ Efficiency of turbo expander,ηt
¾ Mass flow ratio diverted through Turbo expander, α
4.1.1 Effect of Variation of expander flow ratio, α
The value of turbine efficiency, effectiveness of HX1, and pinch point of HX2 are
kept constant. The effect of yield with the variation of mass flow ratio through turboexpander
is studied as shown in figure 4.1. Yield increases with the increase in mass fraction through
turboexpander. But after a mass fraction liquid does not produced. This indicates that there is
an optimum yield at a particular mass flow ratio through the turboexpander.
Table 4.1 Effect of variation of expander flow ratio
Turbo expander
flow ratio ,α
Yield (Excel)
0.94
0.90
0.85
0.80
0.70
0.60
0.50
0.40
0.30
0.20
0.04278
0.04096
0.03870
0.03644
0.03197
0.02756
0.02327
0.01917
0.01543
0.01251
Compressor work Compressor work
per kg of liquid
per kg of liquid
Yield (Hysys)
produced
produced
in kJ/kg (Excel)
in kJ/kg (Hysys)
0.04211
210.36
213.75
0.04030
219.71
223.32
0.03804
232.58
236.59
0.03585
246.98
251.08
0.03128
281.55
287.77
0.02704
326.52
332.86
0.02249
386.76
400.16
0.01827
469.53
492.70
0.01459
583.39
616.91
0.01091
719.25
824.59
30
Fig. 4.1 Variation of yield with expander flow ratio
Fig. 4.2 Variation of compressor work per kg of liquid with expander flow ratio
31
4.1.2 Effect of Variation of effectiveness of HX1, ε1
The value of turbine efficiency, mass fraction flow through turboexpander, and pinch
point of heat exchanger-2 are kept constant. The effect of yield with the variation of
effectiveness of heat exchanger-1 is studied as shown in figure. There is a lower limitation of
effectiveness of HX1 below which liquid does not come. Figure shows that yield increases
with increase in effectiveness of HX1. But there is manufacturing limitations of effectiveness.
Table 4.2 Effect of Variation of effectiveness HX1
Effectiveness of
Yield (Excel) Yield (Hysys)
Heat exchanger 1,ε1
0.98
0.04278
0.04211
0.97
0.03838
0.03784
0.96
0.03395
0.03365
0.95
0.02948
0.02937
0.94
0.02499
0.02503
0.92
0.01591
0.01628
0.90
0.00671
0.00756
0.89
0.00206
0.00304
Fig. 4.3 Variation yield with effectiveness of HX-1
32
4.1.3 Effect of Variation of pinch point of second heat exchanger
The value of turbine efficiency, mass fraction flow through turboexpander, and
effectiveness of first heat exchanger are kept constant. The effect of yield with the variation
of pinch point of second heat exchanger is studied as shown in figure. Pinch point does not
affect much on yield but decreasing the pinch temperature performance of HX2 will be
improved.
Table 4.3 Effect of Variation of pinch point of HX-2
Pinch Temperature
Yield (Excel) Yield (Hysys)
Of heat exchanger 2
1.00
0.04326
0.04258
2.00
0.04278
0.04211
3.00
0.04231
0.04162
4.00
0.04185
0.04123
8.00
0.04005
0.03944
12.00
0.03836
0.03783
Fig. 4.4 Variation yield with Pinch temperature of HX-2
33
4.1.4 Effect of Variation of turbo expander efficiency η
The value of mass ratio through turboexpander, effectiveness of first heat exchanger
and pinch temperature of HX2 are kept constant. The effect of yield with the variation of of
turbo expander efficiency is studied as shown in figure. Yield increases with the increase in
the efficiency of turboexpander. But due to availability of the turboexpander, it is limited to
50%.
Table 4.4 Effect of Variation of turbo expander efficiency
Turbo
expander
efficiency,
Yield
(Excel)
Yield
(Hysys)
0.03215
0.04278
0.05385
0.03172
0.04211
0.05186
η
0.40
0.50
0.60
Fig. 4.5 Variation yield with Turboexpander efficiency
34
4.2 Variable Specific Heat Analysis of Heat Exchangers
NTU-effectiveness relationships, was integrated with the restriction that the specific
heats of the fluids was constant. The fluid properties do vary considerably in the near critical
region, and cryogenic heat exchangers may operate in this regime. The specific heat also
varies significantly in a condenser in which the fluid enters as a superheated vapor.Chowdhry
and Sarangi (1984b) examined the effect of variable specific heats on the performance of
hydrogen heat exchangers. Oonk and Hustevedt (1986) examined the same effect for helium
heat exchangers. Soyars (1991) examined the effect of variable fluid properties on the
accuracy of analysis of helium heat exchanger performance in the temperature range below
15 K .Their results indicated that noticeable errors result for helium heat exchangers in
refrigeration systems if the helium specific heat was treated as a constant below 15 K.
Fig.4.6 Heat exchanger split into parts
A technique similar to the finite element approach will be used; the heat exchanger
will be subdivided into small elements, in which the specific heat variation is relatively small
and may be treated as constant. In the heat exchanger 2, Temperature of the hot fluid was
divided into five parts and their enthalpies found out by using Allprops. The number of
Transfer units for each element is found out as follows:
Enthalpy of cold fluid at inlet of element 1 is found out from energy balance between hot and
cold fluids as
mh (ih1 − ih 2 ) = mc (ic 2 − ic1 )
ic1 =
[ic 2 mc + mh (ih 2 − ih1 )]
mc
(1)
Heat capacities of cold and hot streams are found as
Ch =
(ih1 − ih 2 )
(Th1 − Th 2 )
(2)
Cc =
(ic 2 − ic1 )
(T c 2−Tc1 )
(3)
35
CR =
Cmin
Cmax
The effectiveness of Heat exchanger 1 is found out as
ε=
(Tc 2 − Tc1 )
(Th1 − Tc1 )
if C c < C h
(4)
Or else,
ε=
(Th1 − Th 2 )
(Th1 − Tc1 )
(5)
The number of transfer units for each element is found out as
Ntu =
log e {(1 − C R ε ) /(1 − ε )}
(1 − C R )
(6)
UA = Ntu × C min
(7)
4.2.1 Analysis of Heat exchanger-1
Heat exchanger-1 is divided into ten elements. Heat transfer area required is
calculated considering the specific heat variation. Fig 4.7 shows the operating temperature
line of HX1. The distance of the operating temperature line from equilibrium line gives the
minimum temperature difference or pinch point temperature. In HX1 temperature pinch
occurs at hot end of the heat exchanger.
Table 4.5: Calculation of UA of HX1 considering variable specific heat
Element
No.
from
hot end
Temp.
in
Cold
stream
(K)
Enthalpy
in Cold
stream
(kJ/kg)
Enthalpy
in Hot
stream
(kJ/kg)
Temp.
in Hot
stream
(K)
0
305.78
317.45
320.44
310.00
1
285.28
296.11
300.02
2
264.77
274.76
3
244.27
4
223.76
5
Heat
Capacit
y of cold
stream
(kJ)
Heat
Capacit
y of cold
stream
(kJ)
Heat
capacity
ratio (Cr)
Effectiv
-eness
Ntu
UA
in
kW/K
290.59
0.0819
0.0865
0.9468
0.8294
4.3239
0.3541
279.60
271.21
0.0819
0.0867
0.9446
0.7943
3.4992
0.2866
253.40
259.15
251.88
0.082
0.0869
0.9436
0.7611
2.9299
0.2402
232.00
238.68
232.60
0.0821
0.0873
0.9404
0.7294
2.4997
0.2052
203.26
210.57
218.18
213.40
0.0822
0.0878
0.9362
0.6989
2.1646
0.1779
6
182.75
189.10
197.64
194.31
0.0824
0.0885
0.9311
0.6691
1.8931
0.156
7
162.25
167.58
177.05
175.37
0.0826
0.0894
0.9239
0.6396
1.6647
0.1375
8
141.74
145.99
156.39
156.66
0.0828
0.0908
0.9119
0.6098
1.4642
0.1212
9
121.24
124.31
135.65
138.32
0.0832
0.093
0.8946
0.5789
1.2838
0.1068
10
100.73
102.48
114.77
120.59
0.0837
0.0969
0.8638
0.5456
1.1121
0.0931
0
0
Cum. UA =
1.8787
Avg Cc =
0.0825
Avg Ch =
0.0893
NTU=
36
22.782
Fig. 4.7 Operating Temperature Line for Heat exchanger 1
4.2.2 Analysis of Heat exchanger-2
HX2 is divided into six elements. From element 0 to 5 are in single phase while 5 to 6
is in two phase. Fig 4.8 shows the operating temperature line of HX2. The distance of the
operating temperature line from equilibrium line gives the minimum temperature difference
or pinch point temperature. In HX2 temperature pinch occurs at the time of change of phase.
Table 4.6: Calculation of UA of HX2 considering variable specific heat
Element
No.
from
hot end
Temp
in Hot
stream
(Th)
Enthalpy
in Hot
stream
(hh)
Enthalpy
in Cold
stream
(hc)
Temp
in Cold
stream
(Tc)
0
1
2
3
4
5
6
120.52
116.44
112.36
108.28
104.20
100.13
100.13
114.76
109.75
104.61
99.28
93.61
87.39
-64.24
102.48
102.17
101.85
101.52
101.16
100.77
91.26
100.73
100.44
100.14
99.83
99.50
99.13
90.31
Heat
Capacity
of cold
stream
(Cc)
Heat
Capacity
of hot
stream
(Ch)
0.0842
0.0842
0.0843
0.0843
0.0843
0.0848
1.1327
1.1620
1.2057
1.2816
1.4106
Heat
capacity
ratio
(Cr)
Effectiv
eness
Ntu
UA
in
(kW/K)
0.0743
0.0725
0.0699
0.0658
0.0598
0.0000
0.0145
0.0183
0.0246
0.0383
0.0718
0.8985
0.0146
0.0185
0.0249
0.0391
0.0747
2.2872
0.0012
0.0016
0.0021
0.0033
0.0063
0.1940
Cum. UA=
37
0.2084
Fig. 4.8 Operating Temperature Line for Heat exchanger 2
4.3 Cumulative Enthalpy Analysis of Heat Exchangers
Cumulative enthalpy analysis is done to find the variation of temperature along the
length of the heat exchanger. It will also shows the whether there is any temperature cross.
4.3.1 Analysis of Heat exchanger-1
Table 4.7 Cumulative enthalpy in L.P. Side of HX-1
Temp.
(k)
L P side
(1.1 bar )
Cp
Change in
Temp
Average
Cp
Mass
Flow Rate
Change in
Enthalpy
Cum
Enthalpy
305.78
280
260
240
220
200
190
180
170
160
150
145
140
135
130
125
120
115
110
105
100.73
1.0404
1.0409
1.0417
1.0428
1.0443
1.0461
1.0471
1.0483
1.0496
1.0510
1.0526
1.0535
1.0545
1.0556
1.0568
1.0582
1.0597
1.0615
1.0636
1.0662
1.0690
0.00
25.78
20.00
20.00
20.00
20.00
10.00
10.00
10.00
10.00
10.00
5.00
5.00
5.00
5.00
5.00
5.00
5.00
5.00
5.00
4.27
1.04036
1.0406
1.0413
1.0423
1.0436
1.0452
1.0466
1.0477
1.0489
1.0503
1.0518
1.0531
1.0540
1.0551
1.0562
1.0575
1.0590
1.0606
1.0626
1.0649
1.0676
0.07866
0.07866
0.07866
0.07866
0.07866
0.07866
0.07866
0.07866
0.07866
0.07866
0.07866
0.07866
0.07866
0.07866
0.07866
0.07866
0.07866
0.07866
0.07866
0.07866
0.07866
0.000
2.110
1.638
1.640
1.642
1.644
0.823
0.824
0.825
0.826
0.827
0.414
0.415
0.415
0.415
0.416
0.416
0.417
0.418
0.419
0.369
0.000
2.110
3.748
5.388
7.030
8.674
9.497
10.321
11.146
11.972
12.799
13.213
13.628
14.043
14.458
14.874
15.290
15.707
16.125
16.544
16.913
38
Table 4.8 Cumulative enthalpy in H.P. Side of HX-1
Temp.
(k)
H P side
( 8 bar ) Cp
Change in
Temp
Average
Cp
Mass
Flow Rate
Change in
Enthalpy
Cum
Enthalpy
310.00
300.00
280.00
260.00
240.00
220.00
200.00
190.00
180.00
170.00
160.00
150.00
145.00
140.00
135.00
130.00
125.00
120.52
1.0507
1.0516
1.0541
1.0577
1.0624
1.0688
1.0778
1.0836
1.0907
1.0995
1.1107
1.1254
1.1345
1.1453
1.1582
1.1740
1.1937
1.2163
0.00
10.00
20.00
20.00
20.00
20.00
20.00
10.00
10.00
10.00
10.00
10.00
5.00
5.00
5.00
5.00
5.00
4.48
1.0507
1.0512
1.0529
1.0559
1.0600
1.0656
1.0733
1.0807
1.0871
1.0951
1.1051
1.1181
1.1300
1.1399
1.1518
1.1661
1.1838
1.2050
0.08222
0.08222
0.08222
0.08222
0.08222
0.08222
0.08222
0.08222
0.08222
0.08222
0.08222
0.08222
0.08222
0.08222
0.08222
0.08222
0.08222
0.08222
0.000
0.864
1.731
1.736
1.743
1.752
1.765
0.889
0.894
0.900
0.909
0.919
0.465
0.469
0.474
0.479
0.487
0.444
0.000
0.864
2.595
4.331
6.074
7.826
9.591
10.480
11.374
12.274
13.183
14.102
14.567
15.036
15.510
15.989
16.476
16.920
Fig. 4.9 Cumulative enthalpy- Temp Diagram for Heat Exchanger 1
39
4.3.2 Analysis of Heat exchanger-2
Table 4.9 Cumulative enthalpy in L.P. Side of HX-2
Temp.
(K)
L P side
1.15 bar Cp
Change in
Temp
Average
Cp
100.73
99.00
98.00
97.00
95.00
93.00
90.40
1.0705
1.0720
1.0730
1.0741
1.0766
1.0800
1.0865
0.00
1.73
1.00
1.00
2.00
2.00
2.60
1.0705
1.0713
1.0725
1.0735
1.0754
1.0783
1.0833
Mass
Flow
Rate
0.0787
0.0787
0.0787
0.0787
0.0787
0.0787
0.0787
Change in
Enthalpy
Cum
Enthalpy
0.000
0.146
0.084
0.084
0.169
0.170
0.221
0.000
0.146
0.230
0.315
0.484
0.653
0.874
Table 4.10 Cumulative enthalpy in H.P. Side of HX-2
Temp.
(K)
120.52
118.00
116.00
112.00
110.00
108.00
106.00
104.00
101.00
100.13
100.13
H P side
Change in Average
7.95 bar
Temp
Cp
Cp
1.0425
0.00
1.0425
1.0423
2.52
1.0424
1.0422
2.00
1.0422
1.0419
4.00
1.0420
1.0417
2.00
1.0418
1.0415
2.00
1.0416
1.0413
2.00
1.0414
1.0412
2.00
1.0412
1.0408
3.00
1.0410
1.0407
0.87
1.0408
PHASE CHANGE
40
Mass
Flow Rate
Change in
Enthalpy
Cum
Enthalpy
0.00493
0.00493
0.00493
0.00493
0.00493
0.00493
0.00493
0.00493
0.00493
0.00493
0.00493
0.000
0.013
0.010
0.021
0.010
0.010
0.010
0.010
0.015
0.004
0.748
0.000
0.013
0.023
0.044
0.054
0.064
0.075
0.085
0.100
0.105
0.873
Fig. 4.10 Cumulative enthalpy- Temp Diagram for Heat Exchanger 2
41
Chapter 5
Conclusions
CONCLUSIONS
The process design is carried out using the standard calculation procedure and is
validated by using process simulation software, Aspen Hysys. The preliminary data required in
terms of mass flow rate, pressure and temperatures across heat exchanger, turbo expander and
other components of the nitrogen liquefier are found out. Parametric study is carried out to study
the role of the different component efficiencies in deciding overall system efficiency. It is found
that the liquid yield is directly proportional to the effectiveness of heat exchanger, efficiency of
turbo expander and mass fraction diverted through turbo expander. However, on the limit, no
yield condition prevails if effectiveness of HX-1 is less than 0.88 and mass fraction through the
turbo expander exceeds 0.95.The effect of increase in pinch point deteriorates the yield. Though
higher turbine efficiency favors the yield, the turbine available for the purpose is limited to 50%
efficiency and it limits the yield to only about 4%. The simulation done and the analysis carried
out can serve as guide lines for the development of nitrogen liquefier in our nation and for the
helium liquefier in particular as a future mission.
43
Bibliography
Bibliography
1.
Barron,R.F. Cryogenic systems, Oxford university Press(1985)
2.
Flynn,T.M.Cryogenic Engineering,Marcel-Dekker Inc.(1996)
3.
Timmerhaus,K.D. and Flynn T.M,Cryogenic Process Engineering Plenum
Press(1989)
4.
M.D.Attrey,Thermodynamic analysis of Collin’s helium liquefaction cycle,
Cryogenics 38(1998) ,1199-1206
5.
Stoecker W.F, ‘Design of Thermal systems’, Toronto, Tata McGraw Hill, 1986.
6.
Aspen Tutorial #1: Aspen Basic
7.
Aspen Tutorial #4: Thermodynamic Method.
8.
Aspen simulation by Chuen Chan, Dept of Chemical Engineering.
9.
Design Modeling and simulation of air liquefaction by John E. Crowley, School of
Aerospace Engineering Space Systems Design Lab Georgia Institute of
Technology.
10.
Herbert sixsmith, Javier Valenauela and Walter Lswift, small turbo-brayton
cryocoolers, Advances in cryogenics,33(1988),pp 827-836.
45
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