Liquefaction of nitrogen using mixed refrigerant processes
G. Venkatarathnam∗
Refrigeration and Airconditioning Laboratory
Department of Mechanical Engineering
Indian Institute of Technology Madras
Chennai 600036
Abstract
Mixed refrigerant cycle refrigerators and liquefaction systems are under development
in our laboratory. The main advantage of these processes is the requirement of low
working pressures, typically less than 20 bar. The cost of the system is quite low since
single stage traditional refrigeration (R22) compressors can be used in these systems.
In case of nitrogen liquefiers, nitrogen pressures of 5 to 6 bar is adequate in many
processes. In this paper, we present some results obtained with our mixed refrigerant
refrigerator prototypes.
Introduction
The use of refrigerant mixtures for cryogenic refrigeration was first proposed by
Podbielniak in a U.S. patent in 1936 [1]. Kleemenko demonstrated the liquefaction of
natural gas using a mixed refrigerant process in 1959 [2]. Most large base load natural
gas liquefaction plants operate on processes derived from the basic Kleemenko
process, the most popular process being the C3-MR process pioneered by Air
Products and Chemicals, USA. Most of the early research on the use of refrigerant
mixtures for cryogenic refrigerator and liquefier applications appears to have been
done in the former Soviet Union under classified programs. In the mid 90s, DARPA,
USA initiated a program for the development of a $1000 cryocooler [3]. Two
commercial refrigerators, one by APD Cryogenics (now Polycold) and MMR Inc.
resulted from this funding in the 90’s.
Praxair has been granted several patents during the last four years on liquefaction of
gases and air separation using mixed refrigerant cycles [4-5]. These cycles employ
non flammable mixtures. Air Products has patented a hybrid mixed refrigerant and
turbine process for liquefaction of nitrogen. Their mixture consists of nitrogen and
other hydrocarbon refrigerants [6].
Mixed refrigerant cycle refrigerators have been under development in our laboratory
for more than five years. Several undergraduate, postgraduate and doctoral theses
have been completed on different aspects of mixed refrigerant cycle cryogenic
refrigerators. In this paper we describe the fundamental aspects of mixed refrigerant
cycles and the work being done at IIT Madras.
∗
http://mech.iitm.ac.in/faculty/vg.html, Email: gvenkat@iitm.ac.in, phone: 2257-8552, 2257-8581
MIXED REFRIGERANT CYCLE REFRIGERATORS
The simplest of cryogenic refrigeration processes is the Linde-Hampson process
(Fig. 1). Linde-Hampson refrigerators are also known in literature as Joule Thomson
coolers, and are used in a variety of applications including cooling of infra red
detectors in missiles etc. In most cases nitrogen is used as the working fluid in these
refrigerators. The main drawback of the Linde-Hampson refrigerator is the
requirement of a very high operating pressure, typically above 200 bar. The
requirement of high pressure limits the use of Linde-Hampson refrigerator to a small
number of applications in which the high pressure gas is supplied in a gas bottle. The
main advantages of the cycle are its simplicity, absence of any moving components,
and fast cool down. Refrigerators such as Stirling, Gifford-McMahon and Pulse-tube
refrigerators are not widely used in civilian applications because of the high cost
associated with the specialized hardware required in these systems. Stirling
refrigerators are used in our country in a number of rural veterinary hospitals and
universities for liquefaction of nitrogen. However, their initial cost is quite
prohibitive.
The working pressure of the Linde-Hampson refrigerator can be reduced to less than
20 bar if the pure working fluid such as nitrogen is replaced by a mixture of gases
such as nitrogen, methane, ethane and propane. The small working pressures allow us
to use conventional refrigeration compressors (R22), which are built in large numbers.
The low pressures also allow us to use regular refrigeration materials such as copper
tubes in heat exchangers, and conventional refrigeration compressors. The cost of a
Linde-Hampson refrigerator will therefore be only a small fraction of the cost of
Stirling, G-M or Pulse-tube refrigerators. The main problem in using conventional oil
lubricated refrigeration compressors is that the lubricating oil carried over needs to be
removed completely before the refrigerant enters the cold box (cryostat).
5
6
2a
1
.
- Wc
Co mpressor
Evaporator
.
2 (m)
.
Qo
Af tercoo ler
3
He at
exchanger
.
Q
4
Expansio n
valve
450
2
1
6
Temperature
200 bar
1 bar
3
2a
Mixture
20 bar
2 bar
350
Temperature (K
)
Nitrogen
400
300
Ambient temperature
2
6
1
250
200
150
45
f
Entropy
100
3
4 5
-300 -280 -260 -240 -220 -200 -180 -160 -140 -120 -100
Entropy (J/mol.K)
Figure 1 : Linde-Hampson cycle refrigerator working with
pure nitrogen and mixtures
REFRIGERANT MIXTURES
Our own research in mixed refrigerant cycles has shown that nitrogen-neonhydrocarbon mixtures such as nitrogen/neon/methane/ethane/propane are most
suitable for obtaining temperatures from 70 K to 120 K. Nitrogen-hydrocarbon
mixtures such as nitrogen/methane/ethane/propane are excellent for temperatures in
the range of 90-120 K, where as Argon-hydrocarbon mixtures such as
argon/methane/ethylene/propane are excellent for temperatures in the range of
120-150 K. We’ve also tested mixtures such as argon-nitrogen-helium-methaneethane-propane mixtures, which have been found to be excellent for operation in the
80-120 K temperature region. The composition to be used depends on a variety of
factors including the type and size of the heat exchanger used, type of compressor
used, the desired amount of refrigeration etc. There’re no known rules for choosing
the mixture composition. Dozens of patents exist on the “best” composition to be
used. Several patent applications have also been filed by us during the last one year,
and one patent has already been granted.
60
Heat load (Watts)
50
40
30
b
20
a
N2-Ar-He-HC mixture
b
b
10
a
a
N2-Ar-HC mixture
Ar-HC mixture
0
75
80
85
90
95 100 105 110 115 120 125 130 135 140 145 150
Refrigerant temperature at evaporator outlet (K)
Figure 2: Heat load characteristics with different refrigerant mixtures
with a power input of 900 to 1100 Watts
45
Heat load (Watts)
40
35
30
25
20
mixture 1
mixture 2
mixture 3
15
10
5
0
90
100
110
120
130
140
Refrige rant temperature at evaporator exit (K)
Figure 3: Heat load characteristics of our most recent prototype with power input of
600-750 Watts
Figure 2 shows the performance of our third generation refrigerator with a power
input of 900 to 1100 Watts. Figure 3 shows the performance of our most recent
prototype with a power input of 600 to 750 Watts using nitrogen hydrocarbon
mixtures. The exergy efficiency of our refrigerator is close to that of commercial
Gifford-McMahon cryogenic refrigerators at 100 K. The main advantage with our
refrigerator is the low cost, because of the use of refrigerant compressors used in
domestic air conditioners and off the shelf components.
Warm end temperature difference (K)
5.0
mixture-1
mixture-2
mixture-3
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
0
10
20
30
Load (Watts)
40
50
Figure 4: Warm end temperature difference in the heat exchanger in our most recent
prototype with different nitrogen hydrocarbon mixtures
The excellent performance of our system is largely due to the very small temperature
approaches or high effectiveness of the heat exchanger in our refrigerator, and the
mixtures tested. It can be seen from Fig. 4 that the warm end temperature approach
between the streams is only 1 to 4.5 K, corresponding to a temperature change of
about 180 to 210 K for the cold stream.
A single refrigerator prototype costs us less than Rs. 50,000 to build (with out the
cryostat), with the cost of the imported oil filters being about half that price. Figure 5
shows the pictorial view of our refrigerator.
Compressor, aftercooler
Cold box
Figure 5 : Pictorial view of our cryogenic refrigerator showing the compressor and
cold box. A 1.0 TR R22 Window air conditioner compressor is used in the system.
The system has been tested in the temperature range of 80-120 K with working
pressures of 15-20 bar.
LIQUEFACTION OF GASES USING MIXED REFRIGERANT CYCLES
Mixed refrigerant cycles are used in very large natural gas liquefaction plants
(up to 5 MMTY). However, there is considerable interest worldwide in using them for
liquefaction of other gases such as nitrogen, air etc. The simplest cycle that can be
used for the liquefaction of gases is shown in Fig. 6. In this cycle a mixed refrigerant
is pressurized to about 15-20 bar in the refrigeration system. The refrigeration system
provides all the refrigeration necessary for cooling nitrogen at 5 to 6 bar pressure to
saturated or even subcooled liquid. In large systems the refrigeration available with
the flash gas can also be used to improve the liquid yield. The system has high
efficiency and can compete with turbine based plants. The cost, however, would be
much smaller because of the use of low pressure compressors and absence of turbine.
Air products and chemicals have recently patented a hybrid mixed refrigerant-turbine
cycle for the liquefaction of nitrogen.
LN2
Mixed
refrigerant
cycle
nitrogen
in
Figure 6 : Simple mixed refrigerant cycle nitrogen liquefaction cycle
Small nitrogen liquefaction cycles based on mixed refrigerant processes are very ideal
for our country and can compete effectively with small Stirling cycle liquefiers. A
small nitrogen liquefier prototype is currently under development in our laboratory,
the results of which will be shared with this august audience in the future AIIGMA
meetings.
References
[1] W.J. Podbielniak, Art of Refrigeration,U.S. Patent 2,041,725 (1936)
[2] A. P. Kleemenko, One flow cascade cycle, Proceedings of the 10th international
congress of refrigeration, Vol. 1, 34-39 (1959)
[3] M. Nisenoff, F. Patten and S.A. Wolf, ...And What about Cryogenic Refrigeration,
Proc. of International Cryocooler Conference, June 25-27, 1996, Waterville, USA
[4] A. Bayram, W. J. Olszewski, J. A. Weber, D. P. Bonaquist. A. Acharya and
J. H. Royal, Multicomponent refrigerant cooling with internal recycle, US Patent
6,065,305 (2000)
[5] B. Arman, D. P. Bonaquist, J. A. Weber, J. H. Ziemer, A. Acharya and
M. A. Rashad, Cryogenic rectification method for producing nitrogen gas and liquid
nitrogen, US Patent 6,125,656 (2000)
[6] A. A. Brostow, R. Agrawal, D. M. Herron, and Mark Julian Roberts, Process for
nitrogen liquefaction, US Patent 6,298,688 (2001)