US3609984A

US3609984A - Process for producing liquefied hydrogen,helium and neon

Info

Publication number: US3609984A
Application number: US819242A
Authority: US
Inventors: Leo Garwin
Original assignee: Individual
Current assignee: Individual
Priority date: 1969-04-25
Filing date: 1969-04-25
Publication date: 1971-10-05
Anticipated expiration: 1988-10-05

Abstract

A PROCESS FOR LIQUEFYING HYDROGEN, HELIUM AND NEON MORE EFFICIENTLY AND ECONOMICALLY THAN BY METHODS PREVIOUSLY PRACTICED, WHICH PROCESS INCLUDES THE STEPS OF COMPRESSING THE GAS (HYDROGEN, HELIUM OR NEON) TO A PRESSURE SUCH THAT, UPON ISOBARICALLY COOLING THE THUS COMPRESSED GAS, A TEMPERATURE ABOVE THE CRTITCAL TEMPERATURE OF THE GAS IS REACHED AT WHICH THE GAS CAN BE ISENTRIOPICALLY EXPANDED TO YIELD SUBSTANTIALLY A SINGLE LIQUID PHASE AT ATMOSPHERIC PRESSURE; THEN ISOBARICALLY COOLING THE GAS AT THIS PRESSURE TO SUCH TEMPERATURE; AND FINALLY ISENTROPICALLY EXPANDING THE GAS TO SUBSTANTIALLY ATMOSPHERIC PRESSURE THROUGH A WORK ENGINE.

Description

L. GARWIN Oct. 5, 1971 PROCESS FOR PRODUGING LIQUEFIED HYDROGEN, HELIUM AND NEON 2 Sheets-Sheet l Filed April 25, 1969 TMOSPHEPS w m v m m Q 6A SEOUS A//TPOG/V L. GARWIN Oct. 5, 1971 PROCESS FOR PRODUGING LIQUEFIED HYDROGEN, HELIUM AND NEON Filed April 25, 1969 2 Sheets-Shea?l 2 TMOWHEQES INVENTOR. 50 M/)QAQw//v United States Patent Office 3,609,984 Patented Oct. 5, 1971 U.S. Cl. 62-22 9 Claims ABSTRACT OF THE DISCLOSURE A process for liquefying hydrogen, helium and neon more efficiently and economically than by methods previously practiced, which process includes the steps of compressing the gas (hydrogen, helium or neon) to a pressure such that, upon isobarically cooling the thus compressed gas, a temperature above the critical temperature of the gas is reached at which the gas can be isentropically expanded to yield substantially a single liquid phase at atmospheric pressure; then isobarically cooling the gas at this pressure to such temperature; and finally isentropically expanding the gas to substantially atmospheric pressure through a work engine.

BACKGROUND OF THE INVENTION Field of the invention This invention relates to a method for liquefying certain low molecular weight gases, and more particularly, to a method for more economically liquefying hydrogen, helium and neon.

Brief description of the prior art Helium and hydrogen are presently often industrially liquefied by initially cooling them by heat exchange or other means, such as expansion engines, until they are brought below their inversion temperature. They are then usually further cooled and liquefied by a Joule-Thomson expansion. The Joule-Thomson expansion is a highly irreversible thermodynamic process, wasteful of process energy and therefore economically inefiicient. Its advantage is that the equipment used is simple and reliable. It is well recognized that expansion of the cooled gas in an engine which does outside work is a more efficient and economical process, but such method of liquefaction by expansion has not been used except in conjunction with a subsequent Joule-Thomson expansion due to the serious and frequently disastrous mechanical effects which occur when the expansion of the fluid through the turbine or engine produces both liquid and gaseous phases in this equipment.

Quite recently it has been proposed to liquefy gases by a method which entails expanding cooled compressed gas through a work engine under conditions where no liquid or gas phases concurrently exist. This method is described in U.S. Pat. 3,383,873 to Becker. In the Becker process, a gas is first compressed to a pressure above the critical pressure of the gas. This compressed gas is then isobarically cooled to a temperature below the critical temperature. The cooled fiuid is then expanded through a turbine or other work engine under conditions wherein only liquelfied gas is present. 'Ihe engine-expanded cooled fluid is then isobarically further cooled and this fluid is finally throttled to atmospheric pressure.

The process of Becker prescribes the isobaric cooling of the initially compressed gas to a temperature which is at, or below, the critical temperature of the gas in order to then isentropically expand the cooled gas through a work engine to produce only liquefied gas in the engine. Even then, however, with methane and the other types of gases contemplated by Becker, unless cooling is carried out to a temperature lower than the critical temperature, the isentropic expansion in the work engine Imust be carefully controlled to expand the cooled fluid to a pressure substantially above atmospheric pressure, since expansion to near atmospheric pressure will result in the production of both liquid and undesirable quantities of gas in the engine. The above atmospheric pressure and temperature liquid produced by the method of Becker under the controlled pressure condition by engine expansion is then removed from the engine and isobarically cooled further, followed by a final expansion through a throttle valve to atmospheric pressure. This final expansion results in the production of a smaller amount of gas.

BRIEF DESCRIPTION OF THE PRESENT INVENTION The present invention provides a process for improving the economy of liquefaction of certain low molecular weight gases which are commonly characterized in having the one atmosphere point on the liquidus curve located not more than 20 K. below the critical temperature. More specifically, the process of the invention is broadly applicable ot the liquefaction of hydrogen, helium and neon, and has marked specific advantages when used in the liquefaction of helium.

My discovery of the process of the present invention is based on a study of the older published entropy-enthalpy characteristics of helium, a further development of new data to establish the entropy-temperature characteristics of helium at higher pressures (above 100 atmospheres) than those previously studied in this respect, and the perception of a more economical and efficient method for liquefying helium than those which have previously been utilized in practice for this purpose, as well as an improvement with respect to the Becker method. The method, in its broader aspects, was then perceived lto be applicable also to the liquefaction of hydrogen and neon, which have temperature-entropy characteristics rendering them susceptible to the same basic principles utilized in the helium liquefaction procedure.

Broadly described, the method of the present invention comprises compressing the gas at a temperature above the critical temperature to a pressure such that the gas may be isobarically cooled to a temperature above the critical temperature from which the gas may be isentropically expanded through a work engine to produce, at atmospheric pressure, a single liquid phase by conversion of substantially all of the gas to liquid. After compressing the gas in the manner described, it is isobarically cooled to a temperature such that the following substantially isentropic expansion will result in the conversion of substantially all the gas to a liquid at atmospheric pressure. The isentropic expansion through a turbine or other work engine then completes the process.

The methods, as thus broadly described, is preferably practiced, in the case of helium gas, in combination with certain other steps now utilized in the production of helium from naturally occurring sources, where such other steps yield a stream of helium at pressures of 20-200 atmospheres and temperatures of from about 65 K. to about 300 K., and most preferably from about 150 atmospheres to about 200 atmospheres. In such practice, the latter stream is used as a feed stream to the apparatus utilized in carrying out the broadly described steps characteristic of the present invention in its most general applicability.

It is an object of the present invention to provide a method for economically liquefying helium, hydrogen and neon by a final expansion through a Work engine.

Another object of the invention is to provide a method BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a temperature-entropy diagram illustrating i certain of the thermodynamic properties of helium.

FIG. 2 is a temperature-entropy diagram illustrating certain of the thermodynamic properties of hydrogen.

FIG. 3 is a schematic flow sheet illustrating a preferred process of the present invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION It is common engineering knowledge that less work is required to liquefy a gas by compression at ordinary temperatures (assuming the gas is below its crtical ternperature) than to liquefy it at ordinary pressures by removal of the sensible heat and the latent heat of vaporization by heat exchange. To the extent that liquefaction of any gas can take place at higher temperatures, through pressurization, as contrasted with lower temperatures by refrigeration, the process of liquefaction is more efficient in terms of energy requirement. It is also well recognized that expansion of the gas through a work engine which does external work is a much more efiicient and economical method of cooling the gas, whether with or without liquefaction, than a Joule-Thomson expansion through an expansion valve. Where partial liquefaction occurs in the work engine, however, mechanical problems are encountered Which make this approach to liquefaction of gases extremely difficult, and in many instances, infeasible.

By a study of the thermodynamic properties of helium and hydrogen, I have now perceived that these gaseous materials lend themselves in a unique way to liquefaction through optimum use of compression and subsequent isentropic expansion through a work engine, with accompanying minimization of the extent of cooling of the gases by refrigeration heat exchange required in the liquefaction process. The economy with which these gases can be liquefied is thus vastly improved over processes which effect the final cooling of the gases by a Joule-Thomson expansion through a valve. Moreover, the thermodynamic properties of neon are sufficiently analogous to those of helium and hydrogen that this gas can also be liquefied by the process of the invention.

In referring to FIG. 1 of the drawings, it will be noted that the liquidus (saturated liquid) curve of the temperature-entropy diagram for helium is relatively fiat, and that the one atmosphere liquid line is located only about 1 K. below the critical temperature. As contrasted with this thermodynamic property, the one atmospheric liquid line for methane is located about 79 K. below the critical temperature of this gas in the temperature-entropy diagram, a general illustration of which appears in the Becker patent to which reference has been made. In comparing the temperature-entropy diagrams of methane and helium, it will be perceived that, for different gases, as the one atmosphere liquid line becomes further removed from the critical temperature, a gas compressed above its critical pressure must be isobarically cooled to a greater degree (to a relatively lower temperature) before it can be isentropically expanded to yield saturated one atmosphere liquid. It is also apparent that the critical pressure of helium (2.26 atmospheres) is relatively low in comparison to that of methane (45.8 atmospheres). Thus, at a relatively low pressure, say, 25 atmospheres, the helium gas need be cooled only to about 6 K., a temperature above its critical temperature, in order to then isentropically expand the gas to yield a saturated liquid at one atmosphere pressure. If allowance is made for some irreversibility and energy losses in an expansion engine, then perhaps the 25 atmosphere helium would need to be cooled to about the critical temperature in order to avoid formation of gas in approximating isentropic expansion by exhausting the gas through such an engine. From the helium temperature-entropy diagram it can further be perceived that if the helium is compressed to, say, 200 atmospheres, it then need only be cooled to about 10 K. in order to yield one atmosphere saturated liquid upon isentropic expansion. As will be appreciated by those skilled in the cryogenics art, a large amount of work is required to abstract from a given quantity of helium gas sufficient heat to lower its temperature from about 10 K. to about 6 K.; much less work is required to compress the gas from 25 atmospheres to about 200 atmospheres than is required to remove the amount of heat necessary to achieve this drop in temperature. From my development of the temperature-entropy curves for helium in the range of 100 to 400 atmospheres, I have determined that there is no problem with the formation of solid helium during the compression of helium to several hundred atmospheres, followed by the reduction of its temperature to around from 6 K. to 12 K. Thus, helium lends itself Well to high compression for the purpose of minimizing the amount of isobaric cooling subsequently required in order to achieve, in accordance with the present invention, a final objective of isentropically expanding the thus compressed, cooled helium through a work engine in order to yield saturated liquid helium at atmospheric pressure.

Having perceived from the temperature-entropy diagram of FIG. l as I haw/e developed it in the above-100 atmosphere range for helium, the possibility of highly compressing the helium in order to increase the temperature from which it may be substantially isentropically expanded in a work engine to produce one atmosphere liquid, a comparison with the Becker method heretofore known further disclosed that this method of liquefying helium possessed marked advantages over the Becker method. Moreover, the method of liquefying helium by initially compressing it to above 100 atmospheres, followed by a minimal amount of isobaric cooling, provides a method of liquefaction which can be used quite well in combination with certain procedures now frequently employed in the production of helium which yield high pressure streams of relatively pure helium in the course of the process. For example, procedures employed by the U.S. Bureau of Mines, which produces a majority of the high purity helium produced in the United States, yield product streams of helium at pressures of from about 150 atmospheres to about 200 atmospheres, and at final purification temperatures of from about 65 K. to about K., before being brought back up to about 300 K. At these relatively high pressures, the temperature can be economically isobarically reduced until the entropy of the gas allows isentropic expansion of the gas to one atmosphere saturated liquid in an engine doing wonk. Not only does the high pressure of the plant stream avoid the necessity of lowering the temperature to, or below, the difficulty attainable critical temperature of helium, but the engine expansion to the saturated one atmosphere liquid yields a percent liquid product at atmospheric pressure ready for storage. In helium liquefaction as previously practiced, final liquefaction of the compressed gas was obtained by a Joule-Thomson expansion in which nok work was performed and only about one-third of the expandable gas liquefied.

A typical process ow diagram for the liquefaction of helium by the process of the present invention, and using a high pressure stream (about 2940 p.s.i.a. or about 200 atmospheres) of the type described is shown in FIG. 3. Reference to this figure in conjunction with an example will senve to illustrate the advantages afforded by the process of the present invention as compared to a helium liquefaction procedure currently in widespread usage.

Feed helium gas in the amount of 5000 standard cubic feet per hour, at a pressure of 200 atm. and at ambient temperature, enters heat exchanger 1 in which it is cooled to about 80 K. by countercurrent heat exchange with refrigerant helium return gas and with refrigerant nitrogen, the latter supplied by nitrogen liquefaction system .11, to which the nitrogen effluent from exchanger 1 returns. Thereupon, the feed helium gas continues into heat exchanger 2, in which it is cooled by refrigerant helium return gas to a temperature of about 40A K. Similarly, it proceeds to heat exchanger 3, in which its temperature is lowered to about 24 K., then to heat exchanger 4, from which it leaves ata temperature of about 18 K., and finally to heat exchanger 5, which lowers its temperature to about 8 K. 'Ihe feed gas, still at essentially 200 atm. (less a small pressure drop in exchangers 1 through is essentially isentropically expanded in expansion engine 6 to produce one atmosphere liquid with little or no gas, in the amount of about 200 liters per hour. The liquid is collected in receiver "I, from which it may be withdrawn as needed.

The refrigeration required for cooling the helium feed stream is provided by a conventional helium closed cycle refrigerator system, operating between approximately 18 atm. and 1 atm. The refrigerant helium at 18 atm. from helium compressor 8 flows through

exchangers

1 and 2 and then splits, roughly 45 percent continuing through heat exchangers 3l and 4, while the remainder passes through high temperature refrigerant helium expander 9, in which it expands to l atm. and enters the refrigerant helium return gas channel at the low temperature side of exchanger 3, passing through

exchangers

3, 2 and 1 in turn, and entering the suction side of refrigerant helium compressor 8 for compression and recycling. The high pressure helium refrigerant stream leaving exchanger 4 is expanded to 1 atm. in low temperature refrigerant helium expander 10, flows back through exchanger 5 (in which it is the sole coolant for the feed 'helium stream), thence through exchanger 4, after which it combines with the eflluent from high temperature refrigerant helium expander 9, and the combined stream flows back to compressor 8 through

exchangers

3, 2 and 1, in turn.

The power heretofore typically required for the liquefaction of one atmosphere helium at the rate of 5000 standard cubic feet per hour is about 9 kw.h. per 100 s.c.f., or about 450 kw. By the method of the present invention, as illustrated in the foregoing example, the power requirements for liquefaction in a plant of the same capacity, starting with 1 atmosphere gas, are only about 6.6 kw.h. per 100 s.c.f. If the gas is already available, because of prior processing, at a pressure of 200 atm., as actually shown in the example, the liquefaction power requirements are reduced to about 6.0 kw.h. per 100 s.c.f. The availability of feed gas at 200 atm. instead of at Il atm. is of little benefit, power-wise, to the cost of liquefaction by conventional means. In some instances, the 200 atmosphere helium stream may be available at a temperature of 80 K. Where this is the case, the cooled, pressurized helium stream may be introduced to the system depicted in FIG. 3 in the manner shown by the dashed line. Thus, the heat exchanger 1 is partially by-passed, and there is a further decrease in the power requirement.

The similarity between the thermodynamic properties of hydrogen and helium can be seen in comparing the temperature-entropy diagrams of FIGS. l and 2. It will be noted that about 13 K. separates the critical temperature from the one atmosphere saturated liquid in the case of hydrogen, thus making this material much more analogous to helium in this respect than to methane to which reference has been previously made. Moreover, by compressing the hydrogen to about y600 atmospheres, it can then be isobarically cooled to a temperature of about 34 K. (above its critical temperature) without formation of a solid phase, and at this temperature and pressure it may be engine-expanded to yield saturated one atmosphere liquid.

Examination of other cryogens shows that only neon, having a onev atmosphere saturated liquid point on the liquidus curve which is about 17 K. below its critical temperature, compares relatively closely in this respect to helium and hydrogen. Thus, neon can be included in the group of low molecular weight gases (having a molecular weight below about 20) which can be liquefied by the process of the present invention with a realization of the described advantages.

It should be pointed out that in carrying out the process of the invention using compression and expansion apparatus currently available, care should preferably be exercised to avoid compression and cooling of the fluids in a way such that solid material is formed (note the location of the saturated liquid line on the entropy temperature diagram of FIG. l). It is envisioned that in some types of heat exchangers and work engines which may be developed in the future, the presence of solid particles of the compressed material will not be detrimental, yand may even be beneficial, but such is not presently the case.

Although certain preferred embodiments of the invention have been herein described in order to enable those skilled in the art to practice the process, it will be appreciated that various modifications and changes in the apparatus employed, and in the process parameters utilized, can be effected without departure from the basic principles of the invention. All changes and innovations of this type are therefore considered to be encompassed by the spirit and scope of the invention.

What is claimed is:

1. A process for liquefying low molecular weight fluids wherein the fluid is helium, hydrogen, neon or mixtures thereof comprising:

compressing and cooling the fluid to a pressure above the critical pressure and a temperature above the critical temperature so that the entropy of the compressed and cooled fluid is no greater than the entropy of the saturated liquid phase at one atmosphere pressure; then substantially isentropically expanding the compressed and cooled fluid through :an engine to accomplish work While converting the fluid to a single phase of saturated liquid at one atmosphere pressure. 2. A process as defined in claim 1 wherein said fluid is Initially compressed to a pressure exceeding the critical pressure by an amount such that the fluid can then be isobarically cooled to said temperature above the critical temperature.

3. A process as defined in claim 2 wherein said fluid is helium and is compressed to a pressure exceeding about 25 atmospheres before commencing the substantially isentropic expansion thereof.

4. A process as defined in claim 3 wherein the helium is compressed to a pressure exceeding about atmospheres before commencing the substantially isentropic expansion.

5. A process for liquefying helium comprising: compressing helium gas to a pressure of from about to about 200 atmospheres at a temperature of from about 65 K. to about 300 K.;

isobarically cooling the compressed helium to a temperature above the critical temperature of helium and to an entropy value of less than about 0.8 calorie per gram K. without the concurrent production of solid helium; then expanding the compressed helium to atmospheric pressure through a device operated by such expansion to do external work while producing a single liquid phase.

6. A process as defined in claim 5 wherein after compressing the helium it is isobarically cooled to a temperature not exceeding about 9 K.

7. A process as defined in claim 5 wherein the expansion of the compressed helium is effected substantially isentropically. v

8. A process for liquefying helium comprising:

compressing the helium to a pressure exceeding 150 atmospheres;

cooling the compressed helium isobarically to the highest temperature at which the cooled, compressed helium may then be expanded through an engine doing external work to yield a single saturated liquid phase at one atmosphere pressure; then expanding the cooled, compressed helium through said engine to convert substantially all of the heliumto a liquid at one atmosphere pressure.

9. A process for liquefying helium comprising:

compressing the helium to a pressure above its critical pressure;

cooling the helium until its entropy is at least as low as the entropy of saturated liquid helium at one atmosphere pressure; then expanding the helium through a Work producing engine under conditions such that no gas is produced, and the liquid produced is at atmospheric pressure.

, 8 References Cited UNITED STATES Y PATENTS 'y OTHER REFERENCES Scott, R. Bi: Cryogenic Engineering, Van Nostrand,

N.Y., 1960, pp. 67-73.

Wylen et al.: Cryogenic Engineering Fundamentals;

Dept. of Mech. Eng., Univ. of Michigan, Ann Arbor, Mich., 1962, pp. 98, 100, 101.

NORMAN YUDKOFF, Primary Examiner A. F. PURCELL, Assistant Examiner Us. c1. XR.