US3377811A - Liquefaction process employing expanded feed as refrigerant - Google Patents

Description

United States Patent 3,377,811 LIQUEFACTION PROCESS EMPLOYIN G EX- PANDED FEED AS REFRIGERANT George W. Siegrist, Coopersburg, and Kenneth Zeitz,

Allentown, Pa., assignors to Air Products and Chemicals, Inc., Allentown, Pa., a corporation of Delaware Filed Dec. 28, 1965, Ser. No. 516,887 2 Claims. (Cl. 6211) ABSTRACT OF THE DISCLOSURE Low temperature refrigeration process in which gaseous material is compressed in a multi-st-age compressor to provide intermediate pressure gaseous material and high pressure gaseous material. The intermediate pressure ma terial is cooled, expanded with work and then valve expanded to effect its partial liquefaction, the unliquefied part being used to cool the intermediate pressure material and then returned to the inlet of the compressor. The high pressure material is expanded with work and used to cool the intermediate pressure material prior to expansion and then return to the inlet of an intermediate stage of the compressor.

This invention relates to refrigertaion and more particularly to a process for producing refrigeration at cryogenic temperatures.

It is an object of the present invention to provide an improved process for producing refrigeration at cryogenic temperatures.

Another object is to provide a novel low temperature refrigeration process which may be used to refrigerate a device to be cooled, to effect liquefaction of gaseous material, or both.

A further object is to provide an improved low temperature refn'geration process of the foregoing type which is capable by relatively simple adjustments to meet changes in refrigeration and liquefaction loads.

Still another object of the present invention is to provide a novel low temperature refrigeration process which obtains improved efiiciency by a novel use of Joule-Thomson expansion and expansion with production of external work.

Other objects and features of the present invention will become apparent from consideration of the following detailed description in connection with the accompanying drawing, which is a diagrammatic showing of a low temperature refrigeration process according to a preferred embodiment of the invention.

Referring to the drawing in greater detail, gaseous material enters the process through conduit 10, having a valve 11, from a storage vessel or other sutiable source, not shown, and is conducted to first stage 12 of a multistage compressor, indicated generally at 14, including second stage 15 and third stage 17. The compression stages may be of the reciprocating, centrifugal or other suitable type and provided with conventional inter and after coolers, not shown. From the first stage 12, gaseous material and recycle gas, to be more fully described later, is conducted by conduit 13 to the second stage 15 and by conduit 16 from the second stage to the third stage 17. The compressed gaseous material from the third compression stage is divided at point 18 with a portion being conducted 3,377,811 Patented Apr. 16, 1968 by conduit 20 for flow through passageway 21 of heat exchange device 22 in countercurrent heat interchange with relatively cold fluids described below. The compressed gaseous material leaves the cold end of the heat exchange device 22 at a lower temperature and is passed by conduit 23 for flow through coil 24 in heat interchange with a pool 78 of boiling liquid refrigerant, such as nitrogen, contained in vesel 25 to effect further cooling of the compressed gaseous material to a temperature close to the temperature of the boiling refrigerant. The cool compressed gaseous material is then conducted by conduit 26 for flow through passageway 27 of heatexchange device 28 in countercurrent heat interchange with relatively cold fluids, described below, to effect further cooling of the gaseous mixture which leaves the cold end of the heat exchange device 28 by conduit 29.

The relatively cold fluids which effect cooling of the compresed gaseous material to the low temperature existing in conduit 29 are derived from the liquid refrigerant in vessel 25, an independent refrigeration cycle and relaively cold vapor of the compressed gaseous material, derived in a manner described below, which enters the shell space of heat exchange device 28 through conduit 49. Such vapor flows through the shell space 50 in countercurrent heat interchange with the compressed gaseous material in passageway 27 and is then conducted by conduit 51 for flow through the shell space 52 of the heat exchange device 22 for further countercurrent heat interchange with the compressed gaseous material; the vapor leaves the warm end of the heat exchange device 22 at about ambient temperature and may be merged with the gaseous material entering the process through conduit 10.

The refrigeration cycle may be a self-contained, independent system employing a refrigerant the same as or different from the gaseous material fed to conduit 10, or may embody the compression arrangement decribed above which is the preferred arrangement when the re frigerant is the same as the gaseous material. As shown, a part of the compressed gaseous material from the third compression stage 17 is used as a refrigerant and fed by conduit 54 to a compressor 55 which may be considered the fourth stage of the compressor 14. High pressure refrigerant from compressor 55 is conducted by conduit 56 for flow through pasageway 57 of heat exchange device 58 to cool the high pressure refrigerant upon countercurrent heat interchange with relatively cold fluids, described below. Cooled high pressure refrigerant leaves the cold end of heat exchange device 58 by conduit 59 and is conducted thereby for flow through coil 60 in heat exchange relation with the pool 78 of boiling refrigerant contained in the vessel 25. The high pressure refrigerant leaves the coil 60 at a temperature close to the temperature of the boiling refrigerant and is passed by conduit 61 for flow through passageway 62 of heat exchange de- Vice 63 to effect further cooling of the high pressure refrigerant upon counter-current heat interchange with relatively cold low pressure refrigerant described below. From the heat exchange device 63, the cold high pressure refrigerant is conducted by conduit 64 to the inlet of expansion engine 65, of the turbine or reciprocating type, which functions to expand the high pressure refrigerant to a relatively low superatmospheric pressure with production of external work. The expansion engine 65 is preferably operated to maintain the efliuent within the vapor phase region but close to saturation temperature at the relatively low dischar e pressure. The low pressure refrigerant discharged from the expansion engine at low temperature is passed by conduit 66 for flow through passageway 67 of heat exchange device 28 in countercurrent heat interchange with relatively warm compressed gaseous mixture flowing through the pasageway 27 of the heat exchange device as described above. The low pressure refrigerant is then employedto cool the high pressure refrigerant prior t expansion in the engine 65. As shown, the low pressure refrigerant is conducted by conduit 68 for flow through the shell space 69 of heat exchange device 63 and then by conduit 70 for flow through the shell space '71 of heat exchange device 58. The low pressure refrigerant leaves the heat exchange device 58 at about ambient temperature and is conducted by conduit 72 to the inlet of compressor stage 15. As mentioned above, the refrigeration cycle may comprise an independent closed system, in which case the conduit 72 would be connected to the inlet of a refrigeration cycle compressor such as the compressor 55, which may be the multi-stage type, if desired.

The pool 78 of liquid refrigerant, such as nitrogen, colecting in the vessel from the supply conduit 77, is vaporized upon cooling the compressed gaseous material in coil 24 and the high pressure refrigerant in coil 60, and cold nitrogen vapor is withdrawn from the vessel 25 through conduit 79 and utilized to precool the compressed gaseous mixture and the high pressure refrigerant. One part of the cold nitrogen vapor is flowed through passageway S2 of heat exchange device 22 by way of conduit 80 and another part is passed by conduit 31 for flow through passageway 83 of heat exchange device 58. The nitrogen vapor leaves the heat exchange devices 22 and 58 at substantially ambient temperature through conduits 84 and 85, respectively, and is merged in conduit 86 and withdrawn from the process.

As mentioned above, one of the objects of the present invention is to provide a low temperature refrigeration process of improved efficiency by employing a novel combination of Joule-Thomson expansion and expansion with production of external work. It has been discovered that a substantial increase in refrigeration produced per unit of input energy is obtained by expanding compressed and cooled gaseous mixture in two stages, first, to a lower, intermediate pressure which may be superatmospheric pressure or below, by expansion with production of external work and, second, by Joule-Thomson expansion of the gaseous mixture from the intermediate pressure to a lower pressure, while maintaining the gaseous material fed to the expansion valve at a temperature as close as possible to the temperature of the gaseous material discharged from the expansion engine. The expansion engine 33 is operated under such pressure and temperature conditions to prevent liquefaction of the gaseous material. This may be accomplished by maintaining the temperature of the gaseous mixture fed to the expansion engine above a predetermined value; however, it is preferred, when practicing the present invention, to operate the expansion engine under an exhaust pressure slightly greater than the critical pressure of the gaseous mixture to permit cooling of the gaseous mixture fed to the expansion engine to the lowest possible temperature obtainable from the process. Operation of an expansion engine under a discharge pressure greater than the critical pressure of the material being expanded ordinarily does not obtain the refrigeration that, under certain circumstances, would be produced under lower discharge pressures. However, it has been discovered that the "feature of Joule-Thomson expansion of eflluent of an expansion engine, although at a pressure greater than the critical pressure of the gaseous material being expanded, makes it possible to obtain a greater quantity of refrigeration per unit of input energy.

With reference to the drawing, cold compressed gaseous mixture from the heat exchange device 28 is conducted by conduit 29 for flow through passageway 30 of heat exchange device 31 and thereby further cooled by counten current heat interchange with relatively cold vapor of the gaseous material, described below. From the heat exchange device 31, the cold compressed gaseous mixture is fed by conduit 32 to the inlet of expansion engine 33 which may be of the reciprocating or turbine type. In the expansion engine 33, the gaseous mixture is reduced in pressure to a lower, intermediate pressure with a concomitant reduction in temperature. Preferably, for reasons discussed above, the discharge pressure of the expansion engine is maintained at a superatmospheric pressure slightly greater than the critical pressure of the gaseous material. The gaseous mixture discharged from the expansion engine is fed by conduit 34 directly, i.e., without an intervening heat interchange step, to an expansion valve 35 wherein the gaseous mixture, at a temperature substantially corresponding to the temperature of the expander efliuent, undergoes Joule-Thomson expansion to a lower pressure with a concomitant reduction of temperature to within the liquefaction region to thereby effect partial liquefaction of the gaseous mixture. The partially liquefied gaseous mixture is fed by conduit 36 to phase separator 37 which, in one mode of operation, functions to part the vapor portion from the liquid portion, the liquid portion being withdrawn from the phase separator through a valved discharge conduit 38 and the vapor portion being withdrawn from the top of the phase separator. The vapor portion is passed through valve 39 and conducted by conduit 47 for flow through shell space 48 of heat exchange device 31 to cool the compressed gaseous mixture to the low temperature prior to expansion in the engine 33. Thereafter, the vapor portion is conducted by conduit 49 for further countercurrent heat interchange with the compressed gaseous mixture as described above.

In the process shown in the drawing, the components operating under a temperature below the boiling point of the liquid refrigerant in the vessel 25, including expansion engines 33 and 65, phase separator 37, heat exchangers 28, 31 and 63, are located within a chamber defined by a Dewar vessel 73 having a vacuum space 74. Heat leakage is minimized by a liquid nitrogen cooled radiation shield shown as a conduit 75 disposed in the vacuum space 74. The liquid nitrogen enters the conduit 75 through valve 76 from a suitable source, flows through the conduit 75 and exits through conduit 77 which feeds liquid nitrogen to the vessel 25.

As mentioned above, one of the objects of the present invention is to provide a low temperature process that may be used to refrigerate a device to be cooled, to effect liquefaction of gaseous material, or simultaneously perform both functions. Liquefied gaseous material may be withdrawn as product from the phase separator 37 through the discharge conduit 38, as described above. Also, vapor withdrawn from the phase separator 37 or the liquid-vapor mixture fed to the phase separator may be used to refrigerate a device to be cooled such as device 43 including a vacuum chamber provided with an internal cool wall represented by a cooling coil 44. One end of the cooling coil 44 is connected to the vapor outlet of the phase separator by conduit 42, having a control valve 40, and the other end of the coil 44 is connected to the conduit 47 by a conduit 46, the latter conduit having a control valve 41 and the valve 33 being connected between the conduits 42 and 46.

When the process is operated to produce liquid product, the valves 4%) and 41 are closed, valve 39 is open and liquefied gaseous material is withdrawn from the process through conduit 38. In this mode of operation, the total unliquified portion of the gaseous mixture entering the phase separator 37 is conducted by conduit 47 for countercurrent heat interchange with the compressed gaseous mixture. In operation of the process to utilize the maximum available refrigeration to cool the device 43, the

valve 39 is closed, the valved conduit 38 is closed, and valves 40 and 41 are open. With such arrangement, the total liquid-vapor mixture from the expansion valve 35 flows through the cooling coil 44 where the liquefied portion is vaporized and, when it is desired to maintain the device 43 at the lowerest possible temperature, the vapor leaves the cooling coil at saturation temperature and thereafter flows by way of conduits 46 and 47 for countercurrent heat interchange with the compressed gaseous mixture. The process is also operable simultaneously to produce liquefied product and cool the device 43. In such operation, the valve 39 ordinarily is closed, the valves 40 and 41 are open and liquefied gaseous material is withdrawn through valved conduit 38. The flow of saturated vapor to the cooling coil 44 through conduit 42 makes it possible to maintain the device 43 at a low temperature above the lowest temperature obtained by the process; however, when it is desired to maintain the de vice 43 at the lowest possible temperature and simultaneously produce liquefied gaseous material as product, the phase separator 37 may be maintained under a slightly elevated pressure relative to the pressure of the fluid in the cooling coil 44 by inserting pressure regulating valves in the conduits 38 and 42 and, downstream of such pressure regulating valves, a controlled quantity of liquefied gaseous material may be transferred from conduit 38 to conduit 42 so that saturated vapor leaves the cooling coil 44. Inasmuch as the production of liquid product withdraws refrigeration from the process, the relative mass of compressed gaseous material in conduit 20 and high pressure refrigerant in conduit 56 is adjusted by means of control valves 90 and 91, respectively, in accordance with the quantity of liquefied gaseous material withdrawn from the process. When the process is employed solely to cool the device 43, a maximum mass of the compressed gaseous material flows through conduit 20 and minimum refrigeration is required by the refrigeration cycle whereas, when the process is used solely to produce liquefied gaseous mixture as product, the refrigeration cycle produces maximum refrigeration and a minimum mass of compressed gaseous material flows to the expansion engine 33 to obtain the quantity of refrigeration necessary to sustain the process. When the process is employed simultaneously to produce liquid product and cool the device 43, the relative mass of the compressed gaseous mixture and the high pressure refrigerant is adjusted between the maximum and minimum limits depending upon the quantity of liquefied gaseous material withdrawn as product. Although maximum efliciency of components of the process, for example, the expansion engines 33 and 65, is obtained when designed for a specific mass flow, the process is so characterized that high efiiciency is obtained irrespective of the mode of operation by designing the components for operation on the basis of a mean mass flow. Of course, when the process is intended primarily for a particular mode of operation, the components may be designed to obtain greatest efiiciency when operating according to that mode.

As a specific example, the process provided by the present invention was performed employing US. Bureau of Mines Grade A helium as the gaseous material and as the refrigeration medium. About 62% of the helium discharged from compressor stage 17 at about 192 p.s.i.a. and 316 K. was flowed through the heat exchange device 22 and then the coil 24 in heat interchange with liquid nitrogen in vessel 25 boiling under substantially atmospheric pressure to cool the thereby compressed helium to about 80 K. The compressed helium was further cooled to about 13 K. upon flowing through heat exchange device 28 and then to about 5.5 K. upon flow through the heat exchange device 31, the pressure at the latter point in the process having dropped to about 182 p.s.i.a. In the expansion engine 33, the cold helium was reduced in pressure to about 37 p.s.i.a. with production of external work and with concomitant cooling to about 5 K. Helium gas at substantially the temperature of 5 K. was expanded in valve 35 from about 37 p.s.i.a. to about 17 p.s.i.a with a concomitant reduction in temperature to about 4.5 K. to form a liquid-vapor mixture of about 70% liquid and 30% vapor. Such liquid-vapor mixture was passed through conduit 42 to coil 44 to cool the chamber to about 4.5 K., the chamber being under a vacuum of about 5 10- torr. The liquefied portion was substantially vaporized in the coil 44 and a saturated vapor at about 4.5 K. and 16.5 p.s.i.a. was returned to the shell side 48 of heat exchange device 31 wherein it was warmed to about 12 K., further warmed to about 78 K. in heat exchange device 28, then discharged from heat exchange device 22 at about 305 K. and recycled with the incoming helium feed. The remaining portion of the helium from the compressor stage 17 was increased to about 450 p.s.i.a. by compressor 55 and, after flowing through heat exchange device 58 and coil 60, was cooled to about 80 K. The helium refrigerant was further cooled to about 21 K. in heat exchange device 63, and in the expansion engine 65, the helium refrigerant was reduced in pressure to about 37 p.s.i.a. with concomitant cooling to about 12 K. Cold effluent of the expansion engine 65 was warmed to about 18 K. upon flowing through heat exchange device 28, was further warmed to about 78 K. after flow through the heat exchange device 63 and flowed from the heat exchange device 58 at about 305 K. and then fed to the inlet of compression stage 15. When the process was operated as a liquefier, the pressure and temperature conditions corresponded substantially to those given in the foregoing example; however, about 35% of the compressed helium from the compression stage 17 was flowed through the process to the expansion engine 33 and the expansion valve 35 with the remainder flowing through the refrigeration cycle.

Although the invention has been described in connection with preferred embodiments, it is to be expressly understood that various changes and substitutions may be made therein without departing from the spirit of the invention as well understood by those skilled in the art. Reference therefore will be had to the appended claims for a definition of the limits of the invention.

What is claimed is:

1. A low temperature refrigeration process comprising the steps of compressing gaseous material successively in a first compression zone, an intermediate compression zone to provide gaseous material at a first superatmospheric pressure above the critical pressure of the gaseous material and a final compression zone to provide high pressure refrigerant,

passing gaseous material at the first superatmospheric pressure in heat exchange with relatively cold fluid to cool the gaseous material at said first superatmospheric pressure,

expanding the cool gaseous material at said first superatmospheric pressure with production of external work to a second pressure lower than said first superatmospheric pressure with concomitant cooling of the gaseous material to a temperature not lower than saturation temperature of the gaseous material at the second pressure,

further expanding the gaseous material by Joule-Thomson expansion to further cool and at least partially liquefy the gaseous material,

utiizing the unliquefied part of the gaseous material to cool the compressed gaseous material at said first superatmospheric pressure and thereafter feeding the unliquefied part to the inlet of the first compression zone,

cooling the high pressure refrigerant and expanding the cooled high pressure refrigerant with production of external work,

passing the work expanded refrigerant in heat interchange with the compressed gaseous material at said first superatmospheric pressure to aid in cooling the compressed gaseous material prior to its expansion, and then introducing the refrigerant into the inlet of the intermediate compression zone. 2. A low temperature refrigeration process as defined in claim 1 comprising the further step of controllably adjusting the relative mass of the compressed gaseous material and the refrigerant.

References Cited UNITED STATES PATENTS 2,909,903 10/1959 Zimmermann.

8 4/1960 Mordhorst et a1. 62-9 10/1960 Simonet 62-38 X 5/1963 Becker 62-38 X 5/1966 Davis 62-38 X FOREIGN PATENTS 8/ 1965 Great Britain. 5/1962 U.S.S.R.

10 NORMAN KUDKOFF, Primary Examiner.

V. W. PRETKA, Assistant Examiner.

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