US3395546A - Process for making semisolid cryogens - Google Patents

Description

6, 1968 H. B. SHERLOCK ET AL 3,395,546

PROCESS FOR MAKING SEMISOLID CRYOGENS Filed July 31, 1964 2 Sheets-Sheet 1 TfMPERA 7UR E/VTROPY (5) FIG. 4.

T E MPERA TUBE 090/0 -V/IPOR E/WAOP) (S) INVENTOR5 H. BENTLEY SHERLOCK NEWMAN E. STANLEY g- 5, 1968 H. B. SHERLOCK ET AL 3,395,546

PROCESS FOR MAKING SEMISOLID CRYOGENS 2 Sheets-Sheet 2 Filed July 31, 1964 m. 4 1 v V Ha l l 19 TTORNEKS United States Patent 3,395,546 PROCESS FOR MAKING SEMISOLID CRYOGENS Harrison Bentley Sherlock and Newman E. Stanley, St. Louis County, Mo., assignors to McDonnell Aircraft Corporation, St. Louis County, Mo., a corporation of Maryland Filed July 31, 1964, Ser. No. 386,618 11 Claims. (Cl. 62-10) The present invention relates to the process of making semi-solid refrigerants and more particularly to a process for preparing semi-solid cryogens which have become commonly known as slush cryogens.

In recent times both advanced aircraft and spacecraft are using cryogenics in more and more applications. Typically, cryogenics are being used for breathing oxygen, prime moving energy source, pressurant gas, fuel for fuel cells, and as energy sources for secondary power systems. The most crucial problems associated with the handling of cryogenics concerns the loading of high quality, single phase liquid on board the craft during preflight servicing. At best, due to heat flux, friction pressure drop and various other heat imparting operations to the cryogenics, such operations are sufiicient to initiate two phase conditions and consequent degradation of the cryogenics liquid quality. Hence, instead of a pure liquid phase, much gas entrainment occurs within the liquid causing reduced quantities of cryogenic liquid on board the craft after servicing. Moreover, the problem of cryogenic liquids developing more of a gas phase continues after the initial servicing cool down and filling operation because of heat losses through the vehicle storage tanks, and likewise such increasing gas phase prevents retention of good quality liquid cryogens on board the craft.

Furthermore, some cryogenics, having low density and low boiling point, are undesired because of the current difficulty in providing such liquid cryogen on board the craft. Hydrogen is typical low density and boiling point cryogen.

Heretofore the problems of cryogenic liquids having a gas phase when loaded on board the aircraft or spacecraft were avoided by sub-cooling the cryogen. Sub-cooling was accomplished through heat exchanges submerged in a different cryogen whose normal boiling point is substantially lower than the cryogen being cooled. Autogenous cooling by low pressure boil off that pulls a vacuum over the liquid is another popular method of sub cooling cryogens. Still a third technique in sub-cooling cryogens is the lowering of the vapor pressure by injection of a non-condensible gas. These methods are not without difliculties. For example, inexpensive cryogenic heat sinks are not available and an airborne Dewar flask full of saturated cryogen would overflow under gas injection.

The present invention avoids the difficulties involved in providing suitable cryogens on board aircraft and spacecraft by providing cryogens as a semi-solid or slush cryogen. The process appertaining to the invention herein modifies the Brayton cycle to an open cycle which can be adapted to batch or continuous operation for producing slush cryogens. Briefly, the process of the invention advantageously employs constant entropy compression of the cryogen to a higher pressure, cooling the high pressure of cryogen by transferral of the cryogen heat content into a heat sink of the same cryogen maintained at the normal boiling point, allowing the cryogen at high pressure to solidify at least partly, and thereafter expanding the frozen cryogen at constant entropy to the initial pressure. Typically, the modified open Brayton cycle for hydrogen operates between normal atmospheric pressure and 5000 p.s.i.a.

It will be understood that by providing a high pres- 3,395,546 Patented Aug. 6, 1968 sure pump the liquid cryogen can be compressed isentropically continuously at one end of a transfer line and continuously transferred at the high pressure through the line While undergoing solidification. As the distance transferred increases, the amount of freezing increases, then the frozen cryogen is passed continuously through an expansion engine having a high pressure inlet and being discharged as slush cryogen into appropriate containers in servicing areas for aircraft or spacecraft. It will be appreciated for the continuous process and transfer system appertaining to the invention that the further distance the cryogen is transferred the more freezing occurs. Such increased cooling with increase trans-fer distance is not possible in usual transfer systems.

It is therefore an object of the invention to provide a process for making semi-solid cryogen which is simple and easy to perform and may be operated as a batch process or a continuous process.

It is another object of the invention to provide a modified open Brayton cycle for the manufacture of slush cryogen.

-It is another object of the invention to provide a method of making semi-solid refrigerants by a refrigeration cycle utilizing constant entropy compression, cooling of the compressed refrigerant and constant entropy expansion of the refrigerant, such cycle forming semi-solid refrigerant.

It is another object of the invention to provide a method of making semi-solid refrigerant utilizing a constant entropy compression and expansion process in which the refrigerant is transferred through a heat exchanger of the same refrigerant material maintained at its normal boiling point intermediate between the constant entropy compression machine and constant energy expansion machine.

Still another object of the invention is to provide a method of making slush hydrogen from saturated liquid hydrogen at atmospheric pressure by subjecting the liquid hydrogen to constant entropy compression to a high pressure, cooling the pressurized liquid hydrogen in a bath of liquid hydrogen maintained at the normal boiling point to freeze the high pressure liquid hydrogen, and thereafter expanding at constant entropy the solid high pressure hydrogen generating semi-solid hydrogen.

Still another object of the invention is to provide a continuous process for preparing and transferring semisolid hydrogen by compressing liquid hydrogen at constant entropy to a high pressure and forcing the liquid through a transfer line and passing liquid hydrogen at its boiling point in an annular passage of the transfer line, the high pressure hydrogen freezing by the heat absorption of the liquid hydrogen vaporizing, discharging the high pressure frozen hydrogen through an expansion engine to reduce the hydrogen to semi-solid state at atmospheric pressure.

A further object of the invention is to provide a method for generating liquid hydrogen containing about 68 percent soild hydrogen comprising compressing isentropically saturated liquid hydrogen at its boiling point and atmospheric pressure, to about 5000 p.s.i.a. and exposing the high pressure liquid hydrogen to a boiling hydrogen bath until the high pressure liquid hydrogen freezes throughout and then isentropically expanding the solid hydrogen to atmospheric pressure.

These and other objects and advantages of the invention will become apparent from the detailed description and the appended claims in conjunction with the drawings wherein:

FIG. 1 is a temperature-entropy diagram of the open Brayton cycle appertaining to the invention;

FIG. 2 is a schematic diagram of batch process for making semi-solid refrigerant;

FIG. 3 is a schematic diagram of apparatus for the continuous process of making semi-solid refrigerant; and

FIG. 4 is a temperature-entropy diagram depicting the process for continuous preparation and transfer of semisolid refrigerant.

Referring specifically to FIG. 1, the modified open Brayton cycle appertaining to the invention will be described. The curve L has a saturated liquid line for the cryogen under study. The condition of liquid to the right of line -L is part liquid and part vapor. Curve F is the fusion line for liquid cryogen at which entropy it begins to turn to solid. Curve M is the solidus line for the refrigerant. Between curves M and F both solid and liquid refrigerant occurs. Thus by producing a material with the entropy between lines M and F we have semi-solid or slush refrigerant. Assuming a given liquid at point A is at atmospheric pressure and has an entropy S then after being compressed isentropically, the liquid will have the same entropy S but will be at a temperature of point B. Next, the compressed liquid at point B is cooled thus decreasing in entropy, and at point C where it has cooled to the fusion temperature, the liquid has a new entropy S When maintained at the fusion temperature for a varying period of time the entropy of the liquid changes from S and approaches the solidus line at point D with an entropy of S Point D represents fusion of all of the high pressure liquid refrigerant to a solid. Any further decrease in entropy from S will be accomplished by a decrease in temperature of the solid. At point D solid refrigerant exists at a high pressure and the fusion temperature with an entropy S On constant entropy or isentropic expansion of the solid at point D to atmospheric pressure, the refrigerant material will be at point E with an entropy S which is identical with the entropy S However, refrigerant at point B and atmospheric pressure has an entropy higher than that of the solid refrigerant represented by atmospheric pressure. Therefore, after isentiropically expanding the refrigerant from its solidus temperature at point D, part of the solid has melted and consequently point B represents the semi-solid refrigerant or slush refrigerant.

Referring now to FIG. 2 a schematic representation of the batch process will be described. The batch system 10 includes a central cylinder 1 surrounded by an outer cylinder 2. At one end of cylinder or pipe 1 is a piston 3 and at the same end nipple 4 is attached to pipe 1. The nipple 4 has a three Way valve V which couples nipple 4 to a source of high pressure helium, or alternatively, to a vent. The opposite end of pipe 1 has a valve V closing tube 1. Outer cylinder 2 has a liquid hydrogen inlet 5 and a hydrogen vapor outlet 6. The entire surface of outer cylinder 2 is surrounded by insulation 7 to reduce heat transfer between the outer cylinder 2 and the surrounding environment to a minimum. A liquid hydrogen inlet, line 8, including valve V controls the liquid hydrogen flow into inner pipe 1.

Referring to FIGS. 1 and 2, the operation of the batch process will now be described. Utilizing hydrogen as a refrigerant or cryogen and the conditions set forth as points A, B, C, D and E, the manufacture of slush hydrogen will be noted. Saturated liquid hydrogen at atmospheric pressure and 20.4 K., represented by point A in FIG. 1 is supplied. With valve 1 closed, valve 2 open and valve 3 closed, the liquid hydrogen is then introduced into inner cylinder 1, conditions represented by point A. Next valve V after filling inner cylinder 1 and outer cylinder 2, is closed. Valve V is open to apply pressure against piston 3. With helium at 5000 p.s.i., piston 3 compresses the hydrogen in pipe 1 against valve V Thus the liquid hydrogen is at the condition represented by point B as seen in FIG. 1. Next, the liquid hydrogen in outer cylinder 2 begins absorbing heat from the compressed hydrogen in inner cylinder 1 at point B. After the work of compression is rejected into the liquid hydrogen in outer cylinder 2 the temperature of the liquid hydrogen in cylinder 1 is reduced at constant pressure to the fusion temperature represented at point C of FIG. 1. As the hydogen compressed in pipe 1 further rejects heat to the boiling liquid hydrogen in outer cylinder 2, the liquid in cylinder 1 approaches the solidus point D in FIG. 1. Thus, it will be noted that from point B to point D the hydrogen has decreased in entropy from S to a value S After isentropic expansion by opening valve V to vent, open valve V to discharge the slush hydrogen in inner cylinder 1, the slush hydrogen is at point B of FIG. 1.

Referring now to FIG. 3, a schematic representation of the continuous process apparatus 20 for generating and transferring slush cryogen will be described. In the description hydrogen will be used to indicate a suitable cryogen. The system includes a liquid hydrogen source 21 connected by a valve 22 to a high pressure pump designated generally by the numeral 30. Liquid line 23 connnects valve 22 into a by-pass line 24 having an intermediate valve 25 into the outer jacket 26 of transfer line 40. The entire line 40 and other portions of the system are surrounded by insulation 43. Line 23 opens into a high pressure pump 30 which is depicted symbolically and may be a rotary pump or other suitable type with high pressure discharged into the inner channel 41 of transfer line 40 at pressure up to 5000 p.s.i.g. Outer jacket 26 has a hydrogen vapor discharge line 42 near expansion engine which couples channel 41 through rotary expansion wheel 51 to atmospheric discharge line 52. The engine 50 is a roller supported vane expander.

Referring now to FIG. 4 the entropy diagram of the continuous process schematically represented in FIG. 3 is illustrated. It will be noted that the entropy diagram of FIG. 4 is substantially identical to the entropy diagram of FIG. 1 with the exception that the entropy change from the saturated liquid fusion line at point C to the saturated solid line at point D has intermediate points D D D and D D represents a point at which transfer line 40 is relatively short and consequently the saturated liquid of point C decreases in entropy to D; at which point the liquid including solid has reached the expansion engine 50 and undergoes constant entropy expansion from point D; to point E Similarly in the long system the saturated liquid at point C will decrease in entropy and freeze at point D. Thus, it will be observed that the long system is preferably a length at which the entropy just reaches point D before transfer through the expansion engine at constant entropy to point B. Intermediate points D and D D represent intermediate lengths for the system between a short system and a long system and in each case constant entropy expansion to points E E and E respectively, would be accomplished in the expansion engine 50.

From the above description of the continuous process and transfer of slush cryogens it will be noted that the entropy of the cryogen discharged from the expansion engine will be much lower for the long system as compared to the short system. This is a unique advantage afforded by the invention hereof because the system avoids the problem of cryogens for aircraft and spacecraft gaining entropy during transfer from the place of manufacture or storage to the servicing area, loading area, or using area.

It will be appreciated that various embodiments have been utilized to describe the invention and many changes and modifications will be readily apparent and will occur to those skilled in the art which do not depart from the spirit and scope of the invention; and such changes as these are deemed to be within the scope of the present invention which is limited solely by the scope of the appended claims.

What is claimed is:

1. A method of making semi-solid refrigerant without a vacuum comprising the steps of isentropically compressing a liquid refrigerant from an initial to a higher pressure, both pressures within the range from atmospheric to 5000 p.s.i., cooling the compressed liquid refrigerant at substantially constant pressure to a point having an entropy less then the triple point entropy by transfer of the heat of compression to a boiling liquid body of the refrigerant, allowing the compressed and cooled liquid refrigerant to freeze, and isentropically expanding the frozen refrigerant to the initial pressure thereby permitting part of the frozen refrigerant to melt.

2. A method of making semi-solid cryogen Without a vacuum comprising the steps of isentropically compressing liquid cryogen to a higher pressure, cooling the compressed liquid cryogen at substantially constant pressure to a point having an enropy less then the triple point entropy by transfer of heat therefrom to a vaporizing body of the cryogen, continuing heat removal from the compressed cryogen until partly solidified, and isentropically expanding the partly solidified cryogen thereby creating semi-solid cryogen.

3. The method of making semi-solid cryogen in claim 2, wherein the cryogen is selected from the group consisting of hydrogen, nitrogen, fluorine, argon and neon.

4. The method of making a semi-solid cryogen comprising the steps of forming a saturated liquid body of cryogen at atmospheric pressure, compressing the cryogen to a substantially higher pressure, cooling the cryogen at substantially constant pressure to the fusion temperature by heat transfer exposure to a boiling liquid body of the same cryogen, removing heat of fusion from said liquid cryogen at substantially the same pressure to a point having an entropy less than the triple point entropy, and insentropically expanding the solid cryogen to atmospheric pressure thereby melting part of the solidified material.

5. The method of making semi-solid hydrogen comprising the steps of compressing hydrogen liquid isentropically from atmospheric pressure, rejecting the heat of compression at substantially constant pressure into a body of liquid hydrogen maintained at 37 R., decreasing the entropy of the hydrogen liquid at substantially the same pressure to a point having an entropy less than the triple point entropy to cause solidification thereof, and releasing the pressure on the solidified hydrogen liquid to allow isentropic expansion into atmospheric pressure to form a semi-solid hydrogen.

6. The method of continuously making semi-solid refrigerant comprising the steps of continuosly isentropically compressing a flowing liquid cryogen to a high pressure, passing the continuous flowing liquid refrigerant at substantially constant pressure through a heat removal path provided by a vaporizing liquid body of the refrigerant for a period of time sufficient to solidify at least some of the refrigerant at a point having an entropy less than the triple point entropy, passing the high pressure at least partly solidified refrigerant through an expansion machine which isentropically expands the refrigerant, discharging at least partly solidified refrigerant.

7. Method of continuously making and transferring semi-solid cryogen comprising the steps of continuously introducing liquid cryogen into an isentropical compressor stage, discharging the liquid cryogen from the compressor at a high pressure into a transfer line, said transfer line surrounded by a body of the liquid cryogen at its vaporization point, flowing the high pressure cryogen at substantially constant pressure through said transfer line for a distance sutficient to cause at least some solidification of the high pressure liquid cryogen at a point having an entropy less than the triple point entropy, and introducing the semi-solid cryogen into an isentropical expansion engine that discharges the semi-solid cryogen at a pressure equal to the inlet pressure of the isentropical compression stage.

8. The method of continuously making semi-solid cryogen comprising the steps of continuously compressing a stream of liquid cryogen at a constant entropy, super cooling the cryogen at substantially constant pressure to an entropy state below the entropy of the atmospheric pressure fusion point and below the triple point entropy, and continuously isentropically expanding the liquid cryogen containing solid cryogen to atmospheric pressure to form a semi-solid cryogen whose entropy is less than the entropy at the fusion point without a vacuum.

9. A method of making semi-solid cryogen without using vacuum comprising isentropically compressing a liquid cryogen from atmospheric pressure to a higher pressure and increasing the temperature of the liquid; cooling the liquid at said higher pressure and reducing the entropy until the fusion line is reached; freezing the liquid at said pressure until at least a portion of solids are formed and the entropy of the fluid is below the triple point; isentropically expanding the material to atmospheric pressure to melt some of the solids, and recovering a semi-solid slush at the said entropy.

10. The process of claim 9 wherein the cryogen is hydrogen.

11. The process of claim 9 wherein the liquid is frozen substantially completely solid before being expanded isentropically.

References Cited UNITED STATES PATENTS 3/1966 Null et a1 6210 X OTHER REFERENCES Hydrogen Subcooling for Aerospace Vehicles, Elrod,

IEEE Transactions on Aerospace, vol. AS-l, No. 2,

NORMAN YUDKOFF, Primary Examiner. W. PRETKA, Assistant Examiner.

Claims (1)

1. A METHOD OF MAKING SEMI-SOLID REFRIGERANT WITHOUT A VACUUM COMPRISING THE STEPS OF ISENTROPICALLY COMPRESSING A LIQUID REFRIGERANT FROM AN INITIAL TO A HIGHER PRESSURE, BOTH PRESSURES WITHIN THE RANGE FROM ATMOSPHERIC TO 5000 P.S.I., COOLING THE COMPRESSED LIQUID REFRIGERANT AT SUBSTANTIALLY CONSTANT PRESSURE TO A POINT HAVING AN ENTROPY LESS THEN THE TRIPLE POINT ENTROPY BY TRANSFER OF THE HEAT OF COMPRESSION TO A BOILING LIQUID BODY OF THE REFRIGERANT, ALLOWING THE COMPRESSED AND COOLED LIQUID REFRIGERANT TO FREEZE, AND ISENTROPICALLY EXPANDING THE FROZEN REFRIGERANT TO THE INITIAL PRESSURE THEREBY PERMITTING PART OF THE FROZEN REFRIGERANT TO MELT.

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