US2978876A

US2978876A - Reliquefaction system for liquefied gases

Info

Publication number: US2978876A
Application number: US709290A
Authority: US
Inventors: Pastuhov Alexis
Original assignee: Conch International Methane Ltd
Current assignee: Conch International Methane Ltd
Priority date: 1958-01-16
Filing date: 1958-01-16
Publication date: 1961-04-11
Anticipated expiration: 1978-04-11
Also published as: DE1086255B; FR1220997A; BE574768A; GB855046A

Description

April 11, 1961 A. PASTUHOV RELIQUEFACTION SYSTEM FOR LIQUEFIED GASES IN V EN TOR. Alex/s Pasyu/vov Filed Jan. 16, 1958 om f f 0% ATTORNEY U ited State atent RELIQUEFACTION SYSTEM FOR LIQUEFIED GASES Alexis Pastuhov, Harvard, Mass., assignor, by mesne"' assignments, to Conch International Methane Limited, Nassau, Bahamas, a corporationyof the Bahamas Filed Jan. 16, 1958, Ser. No. 709,290

3 Claims. (Cl. 62-54) 'This invention relates to the storage or transportation of low boiling liquefied gases and it relates more particularly to a means employed in combination with such tanks as are used in the storage or transportation of a low boiling liquefied gas for the recovery of the vapors released by such liquids in storage or transportation and for the return of the vapors in a liquefied form to the respective tank from which it was released.

Although extensive work has been done in the fabrication of an insulated tank to minimize heat leaks into the tank to cause vaporization of the cold boiling liquefied gas contained therein, absolute elimination of heat leaks has not been achieved and may not be capable of achievement. Thus provision must be made for the handling of the vapors that are released by the liquid housed in the storage container. It is well known that as such vapors are released by the container, pressure, builds up in the container and that means must be pro-' vided for the relief of pressure to prevent build-up to dangerous levels.

If the vapor is of little economical valve, the "most expedient means for the relief of built-up pressure isto employ a pressure relief valve for each tank which will operate to vent off the vapors into the atmosphere when a predetermined pressure level is reached within the tank. If, on the other hand, the vapors are of economical importance, or have a commercial value which exceeds their cost of recovery or otherwise cannot safely be released to the atmosphere, it is desirable to recover the vapor as a liquefied material for return to the storage tank. Hence it is desirable to have a relatively simple and inexpensive system for recovering and recondensing the vapor-so that it can be returned to the liquid storage tank.

Insuch cases as shipboard installations, where it may be desirable to havea number of insulated tanks suitable for carrying liquefied gases, as in the system described in the copending Henry application Ser. No. 582,965 for the transportation of liquefied natural gas in large volume in storage tanks of large capacity mounted within the hold of the. ship, itwould be inconvenient and expensive toprovide each tank with its own individual recovery system. r 1

Further, the limited space available on theship-would render suchmultiple units impractical from the stand point of availability of space. On the other hand, if only a. single system were. to be employed for the recovery and reliquefaction of the vapors escaping from all of the tanks, it would be difficult to proportion the reliquefied gas for returnto thetanks in the amount'vaporized since somettanks would be characterizedvby greater heat leaks'thariothers, depending upon itsilocation in the ship and depending upbn the characteristicsof the tank and its insulatio Illius there would b e apossibility of overfor the recovery of released vapors, there is a need for a simple, eflicient and safe system for the recovery of of vapors released so as to,provide for the characteristics of an individual recovery system for each tank without theexpense and other inconveniences of a multiple recovery system thereby to enable vapors of each tank to remain divorced from the vapors of another tank so that the vapors of each tank will be capable of recovery and reliquetaction for return to the tank from which they were released. a

It is therefore an object of this invention to provide a system which permits the storage or shipment of large quantities of liquefied gases over an extended period of time.

It is another object to provide such a system which permits a simple and economical recovery of vapors released during extended storage or shipment.

It is a further object to provide a recovery and recondensing system which will serve one or more liquefied gas tanks without introducing the problem of metering the recondensed vapors back to the liquid containing tanks.

It is stillanother object to provide a recovery and recondensing system which may be capable of application to the storage and handling of more than one liquefied gas.

These and other objects and advantages of this invention will hereinafter appear and for purposes of illustration, but not of limitation, an embodiment of this invention is shown in the accompanying drawing in which Figure 1 is a how diagram of a typical cycle of a system embodying the features of this: invention.

By the process of this invention, there is provided a recovery system consisting of a bath of a liquid refrigerant in which is submerged a condensing coil for each tank bringing, in a closed system, vaporized gases from the storage tank and returning the condensed and liquefied gases to the same tank. Such a system as disclosed by this'invention permits one recovery system to serve on or more tanks using a relatively simple thermo-dynamic cycle. The operation of the process of this invention may be more fully explained with reference to Figure l in which i a cycle is shown wherein gas is recovered and reconwith a filling line 33 having a valve 32. for the introduc of liquefied gas from the tank. While line 351} is shown tion of the liquefied gas and which is. equipped with a draw-01f line 36 incorporating a valve 31 for the removal as an outlet at the bottom, it is often preferred to have the line extend upwardly from the bottom through the top of the tank. Generally, it will be impractical compietely to fill the tank with liquefied gas. Thus a space 13 will remain at the top of the tank which will be filled by vapor released from the liquid. As heat leaks into the tank, additional liquid. will be vaporized and will operate to build up the pressure in the tank unless re-.

lieved While the tanks are constructed to take some" pressure, itis .undesirableand economically impractical many instjanees to construct tanks of large capacity to i take highpressrifes. To maintain pressure inl area 13 at a safe leviil, the excessgas-tormedby vaporizatioii PatentedApr. 11, 1961 within an insulation 28. The tank is partially filled with a liquid 21 selected to be at a temperature sufficiently low enough to recondense the gases entering through line 14 into the condensing coils 17 which are immersed in the liquid 21. The temperature of the liquid should not be so low as to cause solidification of the liquid condensed in the coils 17. Thus the liquid 21 should be characterized by a temperature below the condensation temperature for the gas but above its solidification temperature. 'The liquid 21 does not fill the tank 19 so as to' leave an area 22,.much like the area 13 in the storage tank, to contain vapor formed by the vaporization of the liquid 21 upon the absorption of heat from the condensing vapors in the condensing coils 17.

The liquefied gases 12 (in the storage tank) and 21 (in the condensing tank) may be the same. In such instance, sufiicient differential in temperatures to efiect condensation of the gas from the storage tank may be achieved by maintaining a predetermined pressure in area 13 of the storage tank and a somewhat lower pressure in the area 22 of the recondensing tank. The pressure difference in these two areas can be so adjusted as to fix the boiling points of the liquefied gases and thereby control the temperatures within the relatively small limits required. For example, if liquid methane werev being stored in the tank 10, then the area 13 could be maintained under a pressure of about 16 p.s.i.a. while the pressure in the tank 19 could be maintained at about p.s.i.a. This would provide for a temperature differential of about 1:2 R, a differential sufiicient to recondense methane gas entering condensing coils 17 and to return it via return line 13 as a liquid to the liquid storage tank.

Alternately, the liquid 21 in which the condensing coil 17 is immersed may be a liquid different from the liquid 12 which is stored or shipped. Because of the low temperature requirement, cooling liquid 21 is conveniently a liquefied gas, such as liquid nitrog n, liquid air, liquid helium or the like. The choice of the liquid to be used will of course depend upon the liquefied gas 12 which is being stored. Normally, if the liquid 21 is not thesame as liquid 12, then liquid 21 should have a boiling point a little below the boilingpoint of the liquid 12, unless, as in the case where the liquids are identical, a sufiicient temperature differential can be achieved by proper control of pressures in the

areas

13 and 22.

Alternate methods for returning the liquefied gas from the recondensing system to the tanks 10 may be employed instead of the blower 16 used to force the gas through the coil 17 and into the return line 18. A gravity feed system may be provided if the recondensing system is elevated safely above the tank 10 and. the coils 17 and so oriented as to permit the liquid to drain out as it is a formed. it is also possible to make use of a liquid pump in thereturn line 18 to direct the liquefied gas to the tank 19.

inorder to maintain a sufficient liquid level in the drawing cit the gas through an outlet line 23 and comas a Joule-Thomson valve 28, to expand the compressed gas to the pressure existing in the tank 19. Some or all of the gas from the liquid 21 may be condensed when pressing the gas by means of-a compressor 29. after which with'acooling element '24- which maybe providedfwitli V Iany suitable refrigeration system, such for example asby"- uid r par :in-coiLZ S and by liquid v'ethylen e'; i con 7 After leaving the cooling system,f.the compressed .gas

. i lsconduc ted lthrough lineftflv to a lsuit abl'e 'jexpander, suchreduced in temperature by the refrigeration in

lines

25 and 26. If condensation has taken place, then the valve 28 will operate to drop the pressure of the liquid to the,

pressure conditions existing in the tank 19 while a small part of the liquid will be flashed. If no condensation has taken place or if the compressed vapors are only partially condensed in the heat exchanger, the expansion of the gas to the pressures existing in the tank 1-9- will be achieved with a concurrent drop in temperature and some condensation to provide a wet gas which is introduced into the tank. The condensed portion will join the liquid surrounding the condensing coils to make up for the amount vaporized while the gaseous portion will be recycled with the released vapors through the described condensing system thereby to. maintain the desiredlevel V of liquid in the tank 19.

The recondensing system, as illustrated in Figure 1, may serve to recondense the gases from a single storage tank or from a plurality of storage tanks. The recondensing system of this invention will also serve a plurality of tanks containing dilferent liquefied gases; but if the gases to be stored or shipped are different, their boiling points at the pressures at which they aremaintained should not vary so widely that the liquid 21 in the tank 19 would solidify any one or fail to liquefy another. It will be apparent that when such widely varying' conditions exist, use can be made of the cooling ele ment 24 as a recondensing system for gases which have considerably higher boiling points than those stored in tank 10 and which are to be shipped or stored over a" period of time. In such an arrangement, a system embodying condensing coils and valves similar to that shown leading from tank 10 may be used to circulate the gas though the cooling element 24.

The system of this inventiorn'a's illustrated in Figure 1, may be varied in many ways which will occur to those skilled in the art without departing from the scope of the invention. The number of storage tanks served by one recondensing system may be any convenient number and the storage tanks may be arranged in various ways in relation to the recondensing system. The storage and recondensing tanks along with their insulations may be-of any suitable metal, suchas stainless steel, aluminum, or of other material, for storing liquefied gas atlow temperatures, -white' such additional equipment as compressors, expansion valves,- auxiliary cooling systems, etc. will be chosen with regard tothe gases'to be handled and the temperatures involved. The lines leading from the various parts of the system may desirably be insulated. in any proper manner to minimize thermallosses and heat leaks.

The systemembodying: the features of this invention, along with-its possible variations, affords 'a' means for minimizing losses of liquefiedgases which are normally encountered in storage-or in the shipment of such liquefied gases over'an extended. period of time or under conditions where heat. leaks into the receptacles are relatively large. These losses are minimized by the used a fairly simple but. efiicient system for recondensingthe gases released by vaporization of the liquefiedigas during such shipment or'storage. system which can serve a numbcr of storage .or shipping receptacles and which, at the. same time, permits each receptacle to remain ina closed cycle, the problem of metering'reliquefied gases to. the proper receptacle is eliminated and 'itlbecornes possible l'urther'tofuse asingle system for theIreliquefact-ion of. yapors rele aseduby. tanks containing difierentmeliqu efied' g'asesx' itwill be'understoodithat changes ay be rnade in the. H details of constructi'on; arrangement {and operatiQIk-as previously described:withoutldepartingv fro mythe pirit 051t v Q P Y" h s lin'th-flfo t w cl im 3 i 1 "1 By providing a recondensingf I claim:

1. Method of recovering gas boiled ofi? from a plurality of tanks containing liquefied gas at about atmospheric pressure and for return of the liquid to each of said tanks in about the amount boiled off to maintain balance which comprises conducting said gas from each of said tanks through closed circuits passing through a liquid bath in a recondensing system at a temperature below the boiling point and above the solidifying point of said liquefied gas, thereby causing said gas to liquefy, returning the resulting liquefied gas to said tanks, compressign, cooling, and expanding the vapor boiled off from said liquid bath, and returning the resulting liquefied vapor to said liquid bath.

2. Method in accordance with claim 1 wherein said liquid bath consists of a quantity of said liquefied gas and said liquid bath is maintained at the required temperature by maintaining its pressure below the pressure of said liquefied gas.

3. Apparatus for recovering boil-01f gas comprising a plurality of tanks containing a liquefied gas which boils at a temperature far below ambient temperature, a heat exchanger separate and apart from the tanks, means for circulating a liquid refrigerant through said heat exchanger wherein the liquid refrigerant is maintained at a temperature below the boiling point temperature of the liquefied gas but above the solidification temperature of said gas, separate closed circuit means including means separate for each of the tanks for drawing off gas that has been vaporized, means for passing said draw-off gas through said common heat exchanger in heat exchange relationship with the refrigerant, and means for returning the condensed gas from the heat exchanger to the tank from which the gas was removed.

References Cited in the file of this patent UNITED STATES PATENTS 1,490,782 Milligan Apr. 15, 1924 2,059,942 Gibson Nov. 3, 1936 2,142,828 Smith Ian. 3, 1939 2,379,215 Brinkmann June 26, 1945 2,753,691 Wissmiller July 10, 1956 2,783,624 Morrison Mar. 5, 1957