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Liquefaction of nitrogen using mixed refrigerant processes

G. Venkatarathnam

Refrigeration and Airconditioning LaboratoryDepartment of Mechanical Engineering

Indian Institute of Technology MadrasChennai 600036

Abstract

Mixed refrigerant cycle refrigerators and liquefaction systems are under developmentin our laboratory. The main advantage of these processes is the requirement of lowworking pressures, typically less than 20 bar. The cost of the system is quite low sincesingle 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 manyprocesses. 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 byPodbielniak in a U.S. patent in 1936 [1]. Kleemenko demonstrated the liquefaction ofnatural gas using a mixed refrigerant process in 1959 [2]. Most large base load naturalgas liquefaction plants operate on processes derived from the basic Kleemenkoprocess, the most popular process being the C3-MR process pioneered by AirProducts and Chemicals, USA. Most of the early research on the use of refrigerantmixtures for cryogenic refrigerator and liquefier applications appears to have beendone 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]. Twocommercial refrigerators, one by APD Cryogenics (now Polycold) and MMR Inc.resulted from this funding in the 90s.

Praxair has been granted several patents during the last four years on liquefaction ofgases and air separation using mixed refrigerant cycles [4-5]. These cycles employnon flammable mixtures. Air Products has patented a hybrid mixed refrigerant andturbine process for liquefaction of nitrogen. Their mixture consists of nitrogen andother hydrocarbon refrigerants [6].

Mixed refrigerant cycle refrigerators have been under development in our laboratory

for more than five years. Several undergraduate, postgraduate and doctoral theseshave been completed on different aspects of mixed refrigerant cycle cryogenicrefrigerators. In this paper we describe the fundamental aspects of mixed refrigerantcycles and the work being done at IIT Madras.

http://mech.iitm.ac.in/faculty/vg.html, Email: [email protected], phone: 2257-8552, 2257-8581


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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 Thomsoncoolers, and are used in a variety of applications including cooling of infra reddetectors 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 therequirement of a very high operating pressure, typically above 200 bar. Therequirement of high pressure limits the use of Linde-Hampson refrigerator to a smallnumber of applications in which the high pressure gas is supplied in a gas bottle. Themain advantages of the cycle are its simplicity, absence of any moving components,and fast cool down. Refrigerators such as Stirling, Gifford-McMahon and Pulse-tuberefrigerators are not widely used in civilian applications because of the high costassociated with the specialized hardware required in these systems. Stirlingrefrigerators are used in our country in a number of rural veterinary hospitals anduniversities for liquefaction of nitrogen. However, their initial cost is quiteprohibitive.

The working pressure of the Linde-Hampson refrigerator can be reduced to less than20 bar if the pure working fluid such as nitrogen is replaced by a mixture of gasessuch as nitrogen, methane, ethane and propane. The small working pressures allow usto use conventional refrigeration compressors (R22), which are built in large numbers.The low pressures also allow us to use regular refrigeration materials such as coppertubes in heat exchangers, and conventional refrigeration compressors. The cost of aLinde-Hampson refrigerator will therefore be only a small fraction of the cost ofStirling, G-M or Pulse-tube refrigerators. The main problem in using conventional oillubricated refrigeration compressors is that the lubricating oil carried over needs to beremoved completely before the refrigerant enters the cold box (cryostat).

Compressor

Hea t E xpans ionvalve

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exchanger

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Figure 1 : Linde-Hampson cycle refrigerator working withpure nitrogen and mixtures


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REFRIGERANT MIXTURES

Our own research in mixed refrigerant cycles has shown that nitrogen-neon-hydrocarbon mixtures such as nitrogen/neon/methane/ethane/propane are mostsuitable for obtaining temperatures from 70 K to 120 K. Nitrogen-hydrocarbonmixtures such as nitrogen/methane/ethane/propane are excellent for temperatures in

the range of 90-120 K, where as Argon-hydrocarbon mixtures such asargon/methane/ethylene/propane are excellent for temperatures in the range of120-150 K. Weve also tested mixtures such as argon-nitrogen-helium-methane-ethane-propane mixtures, which have been found to be excellent for operation in the80-120 K temperature region. The composition to be used depends on a variety offactors including the type and size of the heat exchanger used, type of compressorused, the desired amount of refrigeration etc. Therere no known rules for choosingthe mixture composition. Dozens of patents exist on the best composition to beused. Several patent applications have also been filed by us during the last one year,and one patent has already been granted.

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Ar-HC mixtureN2-Ar-HC mixture

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N2-Ar-He-HC mixtureHeatload(Watts)

Figure 2: Heat load characteristics with different refrigerant mixtures

with a power input of 900 to 1100 Watts

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 powerinput of 900 to 1100 Watts. Figure 3 shows the performance of our most recentprototype with a power input of 600 to 750 Watts using nitrogen hydrocarbonmixtures. The exergy efficiency of our refrigerator is close to that of commercialGifford-McMahon cryogenic refrigerators at 100 K. The main advantage with ourrefrigerator is the low cost, because of the use of refrigerant compressors used indomestic air conditioners and off the shelf components.


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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 temperatureapproaches or high effectiveness of the heat exchanger in our refrigerator, and themixtures 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 ofabout 180 to 210 K for the cold stream.

A single refrigerator prototype costs us less than Rs. 50,000 to build (with out thecryostat), with the cost of the imported oil filters being about half that price. Figure 5shows the pictorial view of our refrigerator.

Compressor, aftercooler Cold box

Figure 5 : Pictorial view of our cryogenic refrigerator showing the compressor andcold 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 workingpressures of 15-20 bar.


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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 forliquefaction of other gases such as nitrogen, air etc. The simplest cycle that can beused 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 systemprovides all the refrigeration necessary for cooling nitrogen at 5 to 6 bar pressure tosaturated or even subcooled liquid. In large systems the refrigeration available withthe flash gas can also be used to improve the liquid yield. The system has highefficiency and can compete with turbine based plants. The cost, however, would bemuch smaller because of the use of low pressure compressors and absence of turbine.Air products and chemicals have recently patented a hybrid mixed refrigerant-turbinecycle for the liquefaction of nitrogen.

LN2

Mixedrefrigerantcycle

nitrogenin

Figure 6 : Simple mixed refrigerant cycle nitrogen liquefaction cycle

Small nitrogen liquefaction cycles based on mixed refrigerant processes are very idealfor our country and can compete effectively with small Stirling cycle liquefiers. Asmall nitrogen liquefier prototype is currently under development in our laboratory,the results of which will be shared with this august audience in the future AIIGMAmeetings.

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 internationalcongress 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 andJ. H. Royal, Multicomponent refrigerant cooling with internal recycle, US Patent6,065,305 (2000)[5] B. Arman, D. P. Bonaquist, J. A. Weber, J. H. Ziemer, A. Acharya andM. A. Rashad, Cryogenic rectification method for producing nitrogen gas and liquidnitrogen, US Patent 6,125,656 (2000)[6] A. A. Brostow, R. Agrawal, D. M. Herron, and Mark Julian Roberts, Process fornitrogen liquefaction, US Patent 6,298,688 (2001)

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