This invention relates to the efficient liquefaction of gases such as nitrogen, argon, oxygen, among others, on a small scale in a batch process.
Liquefiers operating with mixed refrigeranta are ideal for liquefaction of fluids, such as, nitrogen on a small scale. The prior art uses a throttling device which operates at constant enthalpy for the reduction of nitrogen pressure at a point close to the delivery of nitrogen which is efficient for a continuous delivery of the liquefied product.
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This invention, however, usea isentropic expansion while operating in a batch mode, but without the use of a turbine expander, resulting in an improved performance of small scale liquefiers.
Liquefied gases such as liquid nitrogen, liquid oxygen, liquid argon, liquid helium, liquid hydrogen, among others, find wide and varied industrial, scientific and other uses.
Atmospheric gases such as hquid nitrogen, liquid oxygen or liquid argon are produced on a large scale (typically several hundred tons per day) in air separation plants which operate on turbine based cycles such as the Kapitza cycle. All the cycles used in large plants operate on steady processes in which the pressure and flow rate are steady and do not vary with time. Nitrogen is liquefied on a small scale jtypically between 50 and 500 kg/day) using liquefiers based cm the Stirling cycle or the Gifford-McMahon cycle. The pressure and flow rate vary < periodically in these cycles, and the flow is bi-directional.
Cycles similar to Kaptiza cycle cantiot be used in small sizes because of the difficulty in rniniaturizing expansion turbines, the problems associated with operating small turbines at very high speeds (typically 100,000 to 1,000,000 rpmj, and the cost.
Cycles similar to Stirling and Gifford-McMahon cannot be scaled up due to difficulties in making highly effective regenerators used in these systems to large sizes.

DC Cycles such as the Joule-Thorns on, also known as the Linde or Linde-Hampson cycle do not use any turbines and are therefore
excellent from the point of miniaturization. However, they require operating pressures of 15Q to 300 bar. The difficulties in making multi-stage small compressors, the high capital cost, as well as their poor efficiency make it uneconomical to use Joule-Thomson cycles in practical liquefiers.
There is another class of DC cycles known as the mixed refrigerant liquefaction eyeless: Originally proposed by Kleemenko in 1959 for the liquefaction of natural gas, these cycles belong to the class of vapour compression cycles operating with refrigerant mixtures. No turbine is used in these cycles, and the operating-pressures can range from 15 to 60 bar, depending on the type of fluid being liquefied, the desired efficiency of the cycle and so on.
It is also possible to design a cycle that operates with refrigerant mixtures and operates at pressures between 15 and 20 bar using a single stage compressor. Such cycles have been studied by Potapov, and Golubev. Because of the low pressures and the -absence of a turbine, mixed refrigerant cycle liquefiers are also ideal for miniature liquefaction systems for the liquefaction of nitrogen, argon, oxygen etc. A large number of mixed refrigerant cycles for the liquefaction of natural gas as well as other fluids have, been studied. The particular advantage with mixed refrigerant cycle nitrogen liquefiers is that off-the-shelf equipment such as compressors used in home refrigerators and air conditioners can be used, resulting in low cost. The cost of such nitrogen liquefiers can be a fraction of that of other competing cycles such as the Stirling liqueiier. This invention deals with a method of decreasing the operating cost of liquid nitrogen produced using very small mixed refrigerant cycle liquefiers.
Known to the art is US Patent 7,165,422 which teaches a small nitrogen UqueFier that operates with refrigerant mixtures. The feed nitrogen is cooled to a low temperature and expanded from a high pressure to atmospheric pressure and a portion of the nitrogen gas evaporates, cooling a fraction of the remaining gas to the point where it is a liquid at atmospheric pressure- A. mixed refrigerant process or a Pulse-Tube refrigerator is used to cool the

nitrogen gas along with the evaporated nitrogen to low temperature, The expansion of nitrogen from a high pressure to a low pressure in a throttle is one of the features.
Another system known to the art teaches a method for cooling/liquefying an industrial gas such as nitrogen using a two stage process, with an azeotropic refrigerant in the first stage, and a zeotropic refrigerant in the second stage. In the example cited, feed nitrogen at a pressure of 70.5 psi is liquefied and subcooled using the two stage mixed refrigerant process.
R.F. Barron, in Cryogenic Systems {19S3), a standard text book on cryogenic engineering, describes a Simon helium Uquener. In this liquefier, helium gas at very high pressure, typically several hundred bar, is collected in a container, cooled to a temperature of 20 K, first with liquid nitrogen circulating outside the container, and then with liquid hydrogen, circulating outside the container. The pressure of the helium gas in the container is released from a very high pressure to a low pressure, resulting in part of the gas liquefying in the system. The process can be considered an isentropic process when the walls are made adiabatic. It is well known to those in die art that an isentropic expansion is much more efficient than an isenthalpic expansion. The advantage of the Simon process is that no turbine is required in this process. However, the process is a baton process. In processes such as the Kapitza process, a turbine expander is used to expand the gas to be cooled isentropically. The requirement of high pressures essentially makes the system uneconomical for use with nitrogen. There are also other thermodynamic disadvantages in expanding nitrogen gas from a very high pressure, typically a few hundred bar, to atmospheric pressure, as explained by Barron. Other processes such as the Kapitza process, which requires Vow pressures, typically leas than 50 bar, and more typically less than 10 bar, are normally used for the liquefaction of nitrogen on a large scale.
Liquid nitrogen is collected in large tanks from the liquefaction plant continuously, and the liquid nitrogen is dispensed to customers from these tanks as required. Some liquid nitrogen is always lost during the dispensing process to cool the pipe lines that connect the tank to the customers' own tanks (dewars^, as well as cool the customers tanks to liquid nitrogen temperature.

Those in the art know that when the dispensing pipe lines are cooled from ambient temperature to liquid nitrogen (77 K) temperature, the liquid nitrogen in the pipe line evaporates and provides the desired cooling. The evaporation process will first be characterised by flow film boiling, followed by convective flow boiling regimes. The flow film boiling regime is characterized by a large temperature difference between the pipes and the liquid nitrogen, large flow oscillations, and somewhat discontinous flow of liquid through the pipe line. The convective flow boiling, on the other hand is characterised by small temperature difference between the pipe line and liquid nitrogen, small or negligible flow oscillations and a continuous' flow of liquid. The cooling of pipe line process described above is same whether it is a large or a small pipe dispensing line.
The amount of liquid nitrogen required to cool the dispensing pipe lines is large compared to the amount of liquid nitrogen dispensed in the case of small liquefiers compared to large liquefiers. It is necessary to optimize this loss to make any system efficient, particularly in small liquefiers. The amount of heat gained by the dispensing pipe lines from supports is also relatively large in the case of small pipe lines, used in small ■ liquefiers.
It is well known to those in the art that the efficiency of mixed refrigerant cycle systems reduces when operated below 100 K, and more substantially when operated below 90 K. The boiling point of liquid nitrogen is 77A K at atmospheric pressure. This problem is overcome, by liquefying nitrogen at a pressure well above 90 K, and more preferably above 100 K. The efficient of the refrigerator at 100 K will be much higher than at 77 K, and will therefore result in low operating costs. Liquid nitrogen, however, needs to be stored in open containers because of tine possibility of explosion when the lid is closed due to heat in leak. Liquid nitrogen which is liquefied in the refrigerator therefore needs to be further reduced to atmospheric pressure. The prior art uses an isenthalpic expansion while the present invention uses an isentropic expansion, which is thermodynamically superior. More importantly, the nitrogen gas that is produced on expansion is used beneficially in cooling the transfer lines in this invention in a small liquefier.

This invention deals with a novel method for the liquefaction of gases on a small scale that operates on a batch mode for the delivery of liquid nitrogen.
Referring to the accompanying drawings
Figure 1 is a schematic of a device for the liquefaction of nitrogen according to an embodiment of this invention
Figure 2 is a schematic of the thermodynamic process followed in this invention
Figure 3 is a schematic of the thermodynamic process known to the art
& schematic of the device for the liquefaction of nitrogen according to an embodiment of this invention is shown in Fig. 1. A/though the description given here is for the liquefaction of nitrogen, the device can also be used for the liquefaction of other gases such as oxj'gen, argon, volatile organic compounds and therefore this specification,, including the statement of Claims therein, should be construed accordingly.
Referring to Fig. 1 nitrogen gas is supplied from either a gas bottle, or a nitrogen supply system through line hi. The nitrogen supply system can consist of one ox more methods for the separation, of air such as pressure swing adsorption, membrane separation, among others. The nitrogen is normally purified and removed of contaminants such as water vapour, carbon dioxide among others, in a purifier P. There will be no need for the purifier if the nitrogen is supplied from a gas bottle, and is free of impurities that can freeze at low temperatures. The nitrogen gas is usually supplied at pressures, typically between 6 and 25 bar, and more preferably between 8 and 12 bar. The nitrogen gas leaving the purifier P enters a refrigerator R, through line L2, where it cools from ambient temperature to the point where it is completely liquid. The liquid is preferably subcooled by a few Kelvin, The liquid nitrogen leaving the refrigerator through line h3 is still at a pressure close to that entering the system through line LI. The hquid nitrogen then enters a storage vessel S where

it is stored at high pressure. The liquid nitrogen is preferably well below its boiling point in a subcooled condition. The outlet valve of the storage vessel S is normally in closed condition.
When the liquid nitrogen storage vessel S is full, the nitrogen supply to the system is stopped and the outlet valve V is opened to atmosphere. This results in a sudden drop in pressure in the storage vessel from its operating pressure, preferably between o and 12 bar to a value close to the atmospheric pressure, typically 1.5 to 2 bar, Thermo dynamically such an expansion is known, as the isentropic expansion since no heat transfer takes place between the storage vessel and the surroundings in the short time between the opening of the valve V and the reduction of pressure to a low pressure. The sudden drop in pressure results in a conversion of part of the liquid into gaseous nitrogen. This gaseous nitrogen travels through the transfer line L5 between the storage vessel and a dewar used for the external storage of liquid nitrogen, and coois it to the same temperature as the liquid nitrogen in the storage vessel Liquid nitrogen then flows through the pipe line.
Those in the art 'know that cooling of large pipe lines in industry and space applications, is first carried out using a gaseous cryogen. Liquid is passed through a pipe line only when the pipe line is cold enough to prevent large scale oscillations that result from flow film boiling of liquid nitrogen. Colder the pipe, smaller is the chance of such large, oscillations. The amount of cryogen used to cool a pipe line is saved substantially by first using gaseous nitrogen to cool the pipe line befc-Te sending liquid. A similar procedure is used in this invention, with the nitrogen gas generated by expansion of liquid used to cool the pipe line before substantial amount of liquid enters the pipe line. On the other hand, in the prior art, the storage vessel contains only liquid nitrogen at atmospheric conditions. In most cases, warm gaseous nitrogen is used to deliver the liquid nitrogen from the storage vessel to an external dewar. Precious liquid nitrogen is thus wasted for cooling the pipe line in the prior art, where as it is used for cooling the pipe lines in the present invention.
The difference between the transfer of a large amount of liquid nitrogen as in a commercial nitrogen liquefaction plant and a small nitrogen liquefier lies in the size of the pipe lines used to

transfer liquid nitrogen from the plant to the external storage devices. Large plants use super insulated transfer lines that minimize the tosses. Liquid nitrogen flows through the pipelines continuously, thus keeping the transfer lines cold ail the time. On the other hand, in a small nitrogen liqueRer, liquid nitrogen is transferred from the storage vessel to external dewar only when liquid nitrogen is required. The transfer lines are therefore near ambient temperature before the transfer of liquid nitrogen is initiated.
A substantial amount of liquid nitrogen is lost in cooling the transfer lines from near ambient temperature to its operating temperature, the boiling point of nitrogen in the case of small nitrogen liquefiers. The amount of liquid nitrogen lost this way is sometimes as high as 25 to 50% of the total liquid nitrogen produced in the system in small liquefiers. The present invention reduces this substantially by using the gaseous vapour generated on expansion for cooling the gas.
Those in the art know that a large amount of liquid nitrogen is wasted when the external storage dewars at room temperature are used for collecting liquid nitrogen from the plant. The . external storage dewar should itself cool down to liquid nitrogen temperature, and this results in a substantial evaporation of liquid nitrogen. In the present invention, the nitrogen gas leaving the transfer line can be used to part cool the external storage device too.
The present invention is not only economical over prior art in using the nitrogen gas generated on expansion, it is also thermodynamically more efficient than that known to the prior
art.
tn the prior art, the gaseous nitrogen is converted into liquid nitrogen using a refrigerator, and the liquid nitrogen so produced at high pressure is expanded to near atmospheric pressure in the storage vessel. The storage vessel is always at a pressure close to atmospheric pressure in the prior art, compared to a high pressure, very close to that of nitrogen supply pressure in this invention. The expansion of high pressure liquid nitrogen to low pressure liquid nitrogen is carried out in a throttling device such as a capillary tube or an expansion valve. The outlet valve of the

storage vessel in the prior art is always open. The nitrogen gas is generated continuously, resulting in a continuous flow of nitrogen gas from the system.
Figure 2 shews the schematic of the expansion process undergone by the liquid nitrogen in the storage container as the pressure is reduced in this invention. Figure 3 shows the thermodynamic process in prior art systems, where a throrrie is used. Point 1 shows the condition of nitrogen before expansion and point 2 that after expansion. The ratio of the lines 2-f and g-f indicates the amount of liquid nitrogen that has been converted to a gas on expansion. A comparison of Figs. 2. and 3 shows that a lot more of liquid nitrogen is converted into gaseous nitrogen on expansion in prior art because of use of a constant enthalpy expansion, as compared to constant entropy expansion iti tin's invention. A constant enthalpy expansion used in prior art is also known as irreversible adiabatic process, while the constant expansion process used in this invention is also known as TeveisiVAe adiabatic process. The irreversibility dAie to the isenthaipic expansion results in a lower efficiency compared to the present invention.
In one embodiment of the invention showed in Fig. 1 a mixed refrigerant cycle nitrogen liquefier is used for the liquefaction of nitrogen. In other embodiments, a pulse-tube, Gifford-McMahon or any other type of refrigerators/UqueSers known in art can he used.

We Claim:
1.A method of liquefaction of nitrogen on a small scale comprising the steps of supplying the nitrogen gas at ambient temperature and high pressure of 6 to 25 bar from a source to a refrigerator through a supply line; cooling the gas from ambient temperature to the point where it is completely liquid; allowing the liquid nitrogen to leave the refrigerator through a line, at a pressure near that of its supply pressure, to a. storage vessel where it is stored at near the same pressure, the said refrigerator and storage vessel being maintained in vacuum of 10~B bar, the outlet valve of the storage vessel being normally in closed condition; stopping the nitrogen supply from the source when the storage vessel is full; suddenly opening the outlet valve of the storage vessel to atmosphere thereby causing a sudden drop in pressure in the storage vessel from its operating pressure, between 8 and 12 bar to a value near the atmospheric pressure thereby creating an isentropic expansion, resulting in conversion of a part of the liquid nitrogen into gaseous form; sending said gaseous nitrogen through a transfer line between the storage vessel and a dewar used for the external storage of liquid nitrogen, thus cooling the said transfer line as well as the dewar to nearly the same temperature as that of the liquid nitrogen in the storage vessel.
2. A method as claimed in Claim 1 wherein the nitrogen is supplied from a nitrogen supply source to the refrigerator.
3. A method as claimed in Claim 1 wherein the nitrogen is supplied from a. gas. bottle.
4. A method as claimed in Claim 2 wherein before entering the refrigerator the nitrogen is sent to a purifier to Temove contaminants,
5. A method as claimed in any one of the preceding Claims
wherein the nitrogen is supplied from the said source at a high
pressure between 6 and 25 bar.
6. A method as claimed in any one of the preceding Claims
wherein the liquid nitrogen is subcooled in the refrigerator-

7.A method as claimed in any one of the preceding Claims wherein the refrigerator is one operating with a mixture of refrigerants
8. A method as claimed in any one of the preceding Claims 1 to 6 wherein the refrigerator is a pulse-tube refrigerator.
9. A. method as claimed in any one of the preceding Claims 1 to 6 wherein the refrigerator is a Gifford-McMahon refrigerator.
10 A method for the liquefaction of nitrogen on a small scale substantially as herein described with reference to, and as illustrated in, the accompanying drawings.