US2975607A - Revaporization of liquefied gases - Google Patents

Description

March 21, 1961 w. w. BODLE 2,975,607

REVAPORIZATION 0F LIQUEFIED GASES Filed June 11, 1958 3 Sheets-Sheet 1 E- I L 55 FUEL T0 V ii GENERATORS Z j LEA/(AGE ETC. E

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A TTOP/VE Y March 21, 1961 w. w. BODLE 2,975,607

REVAPORIZATION OF LIQUEFIED GASES Filed June 11, 1958 5 Sheets-Sheet 2 72 b \J F 1'5. 2

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PM. a m/mm w 664/! BY am I WLUM a WM ATTOPNEY March 21, 1961 w. w. BODLE REVAPORIZATION OF LIQUEFIED GASES 3 Sheets-Sheet 3 Filed June 11, 1958 INVENTOR. 0M Ha f/ WM/m/n BY A TI'OENE-X United States Patent REVAPORIZATION or LIQUEFIED GASES William W. Bodle, Deerfield, Ill., assignor, by mesne assignxnents, to Conch International Methane Limited,

Nassau, Bahamas, a corporation of the Bahamas Filed June 11, 1958, Ser. No. 741,336

9 Claims. (CI. 62-52) This invention relates generally to improvements in the art of preparing a liquefied natural gas for use as a fuel, and more particularly, but not by Way of limitation,

to an improved method of revaporizing a liquefied gas. Natural gas is available in certain localities in amounts considerably greater than demanded in those localities, While in other localities a marked deficiency exists in the amount of natural gas available for use. For the most part, where the source of plentiful supply is joined by land with the areas where a deficiency exists, transfer can be economically achieved by means of pipeline and the like wherein transfer is effected while the gas remains in the gaseous state. Where the area having a deficiency is somewhat isolated, or where the source of supply and the area where a deficiency exists are separated by a large body of water, transfer by pipeline becomes impractical. In the latter instance, an industry is in the stage of development wherein the natural gas is liquefied at the source of supply and transported in the liquefied state to the area wherein a deficiency exists, and the liquefied natural gas is revaporized at that point for use. By conversion of the natural gas from the gaseous state to the liquefied state, it becomes possible to carry as much as 600 times more gas in a given space, thereby making transportation practical.

Transportation is effected with the liquefied natural gas housed in large insulated containers at about atmos pheric pressure or slightly above, and vw'th the natural gas at a temperature as low as -258 F. The latter temperature represents the boiling point temperature for methane at atmospheric pressure. However, since natural gas has small amounts of heavier and higher boiling hydrocarbons, such as ethane, propane, butane and the like, the liquefied gas will be characterized by a some- What higher boiling temperature, usually ranging from 240 to 258 F., depending upon the amount of the heavier hydrocarbons.

At the point of use, the liquefied natural gas must, in all cases, be vaporized before being used as a fuel. However, a natural gas containing a substantial portion of methane will, in many countries, have a heating value far above the specifications for a gas which may be used in existing equipment, and variation or adjustment might also be required in its specific gravity. In these countries it is therefore required that the liquefied gas not only be revaporised, but also reformed to a lower heating value and to adjust the specific gravity. Such reforming operations may be carried out in the locality where the liquefied gas is revaporized. In many instances, the actual point of use of the gas may be located a substantial distance from the point where the liquefied gas becomes available, as when the point of use is inland and the liquefied gas is transported by ship. In such instances, it is desirable that the liquefied gas be revaporized and pressurized at the point where the liquefied gas is made available, but that the gas be reformed nearer the point of use. This is particularly desirable where the reformed 2,975,667 Patented Mar. 21, 1961 gas will have a volume substantially greater than that of the revaporized natural gas.

As previously noted, a liquefied natural gas at about atmospheric pressure will have a temperature of about 240 to 258 F. Such a liquefied gas may be revapon'zed in accordance with present vaporization practices by passing the same in heat exchange relation with a readily available heat source, such as air or sea water. However, when such a cold material is passed in heat exchange relation with water, the tubes of the heat exchanger will be at a temperature far below the freezing temperature of the water. The tubes will rapidly become coated with ice to reduce the heat transfer efiiciency, either resulting in complete stoppage of water flow through the heat exchanger, or requiring an over design of the heat exchanger to accommodate the inherent ice formation. When using air instead of water, hydrates will form and precipitate out of the air onto the tubes of the heat exchanger, resulting in substantially the same problem as when using water.

The present invention contemplates a novel method of revaporizing a liquefied gas by use of a heat transfer medium passing in heat exchange relationship with a readily available and cheap heat source, such as sea water or air, and, alternately, the liquefied gas. The primary requirement of the heat transfer medium is that its freezing temperature be below the temperature of the liquefied gas, such that no solids will form on the tubes of the heat exchanger through which the liquefied gas and the heat transfer medium are passed. Also, the heat transfer medium is used in such a quantity that the temperature thereof when passing in heat exchange relation with the heat source is higher than the freezing temperature of any component of the heat source, but [lower than the temperature of the heat source. In a preferred embodiment of this invention, the heat transfer medium is a liquid which is vaporized by heat exchange with the heat source and condensed by heat exchange with the liquefied gas, such that the latent heat of the heat transfer medium will be the principal factor in vaporizing the liquefied gas.

This invention further contemplates reducing the pressure of the vaporized heat transfer medium prior to passage thereof in heat exchange relation with the liquefied gas to obtain work from the heat transfer medium. After the heat transfer medium is condensed by the liquefied gas, it is again increased in pressure before being revaporized by the heat source. The difference in the work obtained by a decrease in the pressure of the vapor, and the work required to increase the pressure of the condensed heat transfer medium, may be utilized as an auxiliary power supply in a system involving practice of the invention.

An important object of this invention is to facilitate the preparation of a liquefied natural gas for use as a fuel.

Another object of this invention is to efficiently and economically revaporize a liquefied gas either for use as a fuel or for transportation through a pipeline or the like to a reforming plant.

Another object of this invention is to utilize heat from a readily available and cheap heat source to revaporize a liquefied gas having a boiling temperature far below the freezing temperature of some component of the heat source, without the formation of solids on the tubes of the heat exchanger used for vaporizing the liquefied gas. A further object of this invention is to utilize a readily available heat transfer medium for transferring the heat from a cheap heat source to a liquefied gas for revapon'zing the liquefied gas.

Another object of this invention is to provide a method evident from the following detailed description, when read in conjunction with the accompanying drawings which illustrate this invention.

In the drawings:

Figure lis a flow diagram illustrating a practice of this invention.

Figure 2 is a flow diagram illustrating a modified practice of this invention.

Figure 3 is a modification of Fig. 2 illustrating still anot-her practice of this invention.

Figure 4 is a flow diagram of a typical commercial installation illustrating a practice of the present invention.

Referring to the drawings in detail, and particularly Fig. 1, reference character 6 designates a line for feeding liquefied natural gas to a stationary, insulated storage tank 8. The line 6 extends from a container (not shown) used for transporting the liquefied natural gas which, as previously noted, will ordinarily be aboard a ship. The liquefied natural gas in the tank 8 will normally be at about atmospheric pressure, or slightly above, and have a temperature of about 240 to -258 F.

Although a portion of the liquefied natural gas in the tank 8 will boil off as a vapor during storage, as will be hereinafter more fully described, the major portion of the liquefied natural gas in the tank 8 is fed through a line 10 to a suitable pump 12. The pump 12 increases the pressure of the liquefied natural gas to the pressure at which it is desired to either immediately reform the gas, used the vaporized gas as fuel, or transport the gas through a pipe line to a distant reforming plant, as previously indicated. The pressure of the liquefied natural gas discharging from the pump 12 may therefore be anywhere from slightly above atmospheric pressure to about 600 pounds per square inch, but is usually from about 50 to about 200 pounds per square inch.

The liquefied natural gas discharged through the line 14 from the pump 12 is directed through a vaporizer 16 where the liquefied gas is revaporized by a heat transfer medium circulated in a closed cycle, as will be described in detail below. The revaporized natural gas is then directed through a line 18 to a separator 20 for removing any condensate which may exist after passage of the stream through the vaporizer 16. The condensate removed in the separator 20 is returned through a line 22 to the intake of the pump 12 where it may be re-circulated to the vaporizer 16. The overhead from the separator 20 consists solely of revaporized natural gas and is discharged through a line 24 to either a fuel gathering system or a reforming plant, as previously indicated.

The vapor boil-off or over head from the storage tank 3 is directed through a line 26, partially to a compressor 28 and partially to he used as a fuel in an engine 34} operating the compressor 28. The vapor passing through the compressor 28 is increased in pressure to the pressure of the liquefied natural gas in the line 14 and is discharged through a line 32 to be combined with the vapor in the line 24 discharging from the separator 20. It will also be noted that a by-pass line 34 may be run from the line 24 back to the engine 30 for supplying additional fuel if desired.

The heat transfer medium previously mentioned is circulated from-the vaporizer 16 through a line 36 to another vaporizer 38, and then through a line 44} back to the vaporizer 16. This heat transfer medium, 'as will be described, provides a transfer of heat from a readily available and cheap heat source circulated through the vaporizer 38 ization progresses.

to the natural gas circulated through the vaporizer 16. The heat source for the vaporizer 38 must have a temperature above the boiling temperature of the liquefied gas being vaporized and may take any desired form, but is preferably a material which is readily available and cheap, such as sea water or air. Sea water is the preferred heat source. The water is directed through a line 42 from a source of supply (not shown) and is pumped by a suitable pump 44 through a line 46 to the vaporizer 38. In the vaporizer 38, the water is passed in heat exchange relation with the heat transfer medium to supply an amount of heat to the heat transfer medium at least equal to the amount of heat dissipated from the heat transfer medium in the vaporizer 16, as previously indicated. After passage through the vaporizer 38, the water is discharged through a line 48 to a suitable disposal point.

The heat transfer medium may be any fluid having a freezing point below the boiling temperature of the liquetied natural gas, to prevent the deposition of solids in the vaporizer 16, and which, in passage through the vaporizer 38, has a temperature above the freezing temperature of the heat source but below the actual temperature of the heat source. The heat transfer medium may therefore be in liquid form during its circulation through both of the vaporizers 16 and 38 to provide a transfer of sensible heat alternately to and from the heat transfer medium. When the heat transfer medium is in continuous liquid form, however, a large volume of heat transfer medium must be circulated through the system, since the temperature reduction thereof by passage through the vaporizer 16 is necessarily limited to such an extent as to retain the temperature of the heat transfer medium returning to the vaporizer 38 at a temperature higher than the freezing temperature of the heat source. It is therefore preferred that a heat transfer medium be used which goes through phase changes during circulation through the vaporizers 16 and 38, with a resulting transfer of latent heat.

The preferred heat transfer medium has a moderate vapor pressure at a temperature between the actual temperature of the heat source and the freezing temperature of the heat source to provide a vaporization of the heat transfer medium during passage thereof through the vaporizer 38. Also, the transfer medium, in order to have a phase change, must be liquefiable at a temperature above boiling temperature of the liquefied natural gas, such that the heat transfer medium will be condensed during passage through vaporizer 16. As before, the freezing temperature of the heat transfer medium must still be below the boiling temperature of the liquefied natural gas. It should also be noted that when the liquefied gas is a pure, or substantially pure, compound, it will boil at a constant temperature and absorb latent heat at that temperature. If the liquefied gas is a mixture of compounds, it will, in most cases, boil over a temperature range, the temperature increasing as Vapor- In this case, it will be desirable to make the heat transfer medium av mixture of compounds .of such composition that the heat transfer medium will condense over a range of temperatures some what above the vaporizing temperature range of the liquefied gas, thereby making it possible to recover all of the latent heat in the liquefied gas by condensing the heat transfer medium.

Although some commercial refrigerants may be used as heat transfer mediums in the practice of this invention, propane and ethane and preferred heat transfer mediums, particularly in View of the fact that they are normally present in at least minor amounts in natural gas and therefore readily available. It should be noted in passing that neither methane nor butane will Work as a heat transfer medium in the practice of this embodh ment of the invention, since they do not possess the required characteristics. r

By using a heat transfer medium which goes through phase changes in circulating through the Vaporizers 16 and 38, two important advantages are obtained. Firstly, the required quantity of heat transfer medium is reduced to a minimum, since mostly (or only) latent heat changes in the heat transfer medium are utilized, rather than only sensible heat changes. Secondly, the heat transfer medium may be circulated through the Vaporizers 16 and 38 by gravity. By locating the vaporizer 16 physically above the vaporizer 38, the heat transfer medium condensing in the vaporizer 16 will flow by gravity into the lower vaporizer 38. On the other hand, the heat transfer medium being vaporized in the vaporizer 38 will rise through the line 40 into the vaporizer 16, thereby reducing the energy required to vaporize the liquefied natural gas.

In a commercial system, a portion of the revaporized natural gas discharging through the line 24 will be diverted through a line 50'for such purposes as heat for buildings used in conjunction with the practice of the invention, and as a fuel for generators and the like, as indicated by the blocks in the lower portion of the flow diagram. Also, a block has been included to indicate that a certain portion of the natural gas is inherently lost by venting and leakage.

A modified system is illustrated in Fig. 2, wherein reference character 52 designates a heat exchanger functioning as a boiler, and reference character 54- designates another heat exchanger functioning as a condenser for a heat transfer medium. In this embodiment, the heat source is again preferably sea water which is circulated through a line 56, the tubes of the heat exchanger 52, and then discharged through another line 58 to a suitable disposal point. The liquefied gas being revaporized is directed to the tubes of the other heat exchanger 54 by a line 60 and discharged from the exchangr 54 as a vapor through line 62 leading to any desired point of use. It will be assumed that the liquefied gas passing through the line 60 has been pressurized to the desired pressure as described in connection with Fig. 1.

The heat transfer medium is in the form of a material which undergoes phase changes during passage through the heat exchangers 52 and 54 substantially in the same manner as previously described. The vaporized heat transfer medium is discharged from the heat exchanger 52 through a line 64 to the inlet of a suitable device 66 forming a work-producing zone. The device 66 is preferably a'turbine, but may be any other form of engine which may be operated by expansion of the vaporized heat transfer medium. The heat transfer medium is reduced in pressure by passage through the work-producing device 66, and the resulting energy may be recovered in any desired form, such as by rotation of the shaft of a turbine. The heat transfer medium discharging from the work-producing device 66 is still at least mostly in the form of a vapor, but at reduced pressure. This reduced pressure heat transfer medium is directed through another line 68 to the heat exchanger 54 wherein the heat transfer medium is condensed and the liquefied gas is vaporized by a transfer of heat from the heat transfer medium to the liquefied gas. The major portion of this heat is preferably derived as latent heat, such that the temperature of the heat transfer medium will not be sub stantially reduced by passage through the heat exchanger 54. The condensed heat transfer medium is discharged from the heat exchanger 54 through a line 70 to a pump 72, whereby the pressure of the condensed heat transfer medium is substantially increased. The pressurized and condensed heat transfer medium is returned through a line 74 back to the heat exchanger 52.

. The maximum power recovery possible in a cycle as disclosed in Fig. 2 and described above, is related to the heat rejection by the heat transfer medium in the heat exchanger 54 and the temperatures of the heat source and the liquefied gas passing through the heat exchanger 54 in the following manner:

where:

W=maximum work, B.t.u./hr.

Q =heat rejection, B.t.u./hr. T =temperature of heat source, R. T =temperature of liquefied natural gas, R.

Consider the case of a cycle working between 50 F. and -2.40 F. Substitution in the above formula gives:

It is theoretically possible, therefore, to recover 1.32 B.t.u./hr. of work equivalent for each B.t.u./hr. of heat removed in the heat exchanger 54 (the condenser) when working between these temperature levels. The actual amount of power recovery will, of course, be less, since the various apparatii do not operate at 100 percent efficiency. However, the power required to operate the pump 72 and provide an increase in the pressure of the condensed heat transfer medium equal to the pressure drop through the device 66 is small, compared with the power recovered through the device 66, such that the net power recovery may be profitably used to operate auxiliary equipment in a system practicing the present invention.

In the event a portion of the heat transfer medium condenses in the device 66, a superheater 76 may be interposed in the line 64 leading from the heat exchanger 52 to the device 66 as shown in Fig. 3. The superheater 76 may be heated by any suitable means, such as steam circulated to the superheater through a line 78 and discharged through line 80.

In the event the temperature of the condensed heat transfer medium gets below the freezing temperature of the heat source, or Whenever desired, a portion of the vaporized heat transfer medium may be by-passed to preheat the heat transfer medium entering the heat exchanger 52. This may be accomplished by extending a line 82 from line 64 downstream of the heat exchanger 52 and upstream of the device 66 back to the line 74 downstream from the pump 72, as also illustrated in Fig. 3. However, a pump 84 must be interposed in the line 74 downstream of the connection of the by-pass line 82' to feed the mixed heat transfer medium to the heat exchanger 52.

Where continuous revaporization is required, it is preferred to provide a plurality of systems in parallel, such as the three systems generally indicated by :reference character 86 in Fig. 4. Each of the three systems 86 may be easily designed to handle 50 percent of the required revaporization capacity, such that one of the systems may be out of operation at any one time for inspection and repairs. It will be understood that when only two systerns are provided, each system should have a capacity equal to the total revaporization capacity in order that either of the systems may be taken out of operation for inspection or repair while the other system continues the revaporization process.

The liquefied natural gas is fed to an installation such as illustrated in Fig. 4, through a line 88 from a suitable liquefied natural gas storage tank, such as the tank 8 illustrated in Fig. 1 and previously described. The liquefied natural gas in the line 88 will ordinarily consist of a major portion of methane and minor portions of heavier hydrocarbons, such as butane and propane, and will be at about atmospheric pressure and at a temperature from 240 to 258 F., depending upon the composition of the stream. This liquefied natural gas in the line 88 is fed to three separate pumps 90 for increasing the pressure of the liquefied natural gas and feeding the liquefied natural gas to a header 92 communicating with the outlet of each of the pumps 98. It will be observed that any of the pumps 90 may be isolated, such that the liquefied natural gas from the line 83 will be directed through the remaining pumps 90 to the header 92. It is also desirable to provide a by-pass line 94, at the outlet of each pump 96 to selectively direct a portion of the high pressure liquefied natural gas into a header 96. The high pressure liquefied natural gas in the header 96 may be used to prime any of the pumps 90 when such pumps are being placed in operation.

The high pressure liquefied natural gas in the header 92 is selectively directed through feed lines 98, 100 and 102 to the separate revaporization systems 86. Since the systems 86 are of the same design, it is believed necessary only to describe one in detail, such as the system shown at the left end of Fig, 4. The high pressure liquefied natural gas flowing through the line 98 is directed through the tubes of a heat exchanger 104 acting as a vaporizer for the natural gas, such that the natural gas flowing through the discharge line 1% from the heat exchanger 164 will be substantially in the form of a vapor. The gas discharged from the heat exchanger 104 is directed into a separator 108 for removing condensate from the stream. A portion of the condensate from the separator 8 is directed back to the heat exchanger 104- through a line 1111 for reheating thereof. This portion of the condensate in the separator 1118 may be directed through the heat exchanger 104 by a gravity process, such that when the condensate level in the separator 108 tends to exceed the liquid level in the exchanger 104, an additional amount of condensate will be directed back through the exchanger 1% for reheating.

As previously indicated, a liquefied natural gas will ordinarily contain at least a minor percentage of heavy ends, such as butane and propane, which will ordinarily collect in the lower portion of the separator 108 as a condensate. Therefore, a portion of the condensate from the separator 108 may be directed into a header 1121eading to a suitable storage vessel 114 where the heavy ends may be collected and selectively discharged through a line 116. It will also be observed that condensate from the separators 1118 of the remaining systems is also directed into the header 112i for storing additional heavy ends in the vessel 11.4. The fiow of condensate into the header 112 from each separator 108 is regulated by a valve 118 in turn controlled by a liquid level controller 120 mounted on the respective separator.

The overhead from the separator 108' will be in the form of a vapor and is discharged through a line 122. A flow controller 123 is connected to the line 122 and a valve 124' in the liquefied gas feed line 98 to correlate the feed of liquefied gas with the amount of vapor produced by the heat exchanger 104. For example, if the composition of the liquefied gas feed changes, the amount of vapor produced will vary, and the quantity of liquefied gas fed to the respective system must be varied accordingly to prevent an over or under supply of liquefied gas to the exchanger 104- and separator 1118.

The natural gas revaporized by each of the systems 86 is directed into a header 126 leading to another separator 12% for removal of any condensate which may have formed in the lines 122 or the header 126. The condensate in the separator 128 is discharged through a line 130 into the heavy ends storage vessel 114. It will also be noted that the flow through the discharge line 130 is controlled by a liquid level controller 132 mounted on a side of the separator 128. The vapor overhead from the separator 123 is directed through a discharge line 134 for use either as a fuel or for use in a subsequent reforming operation in the manner previously described. The natural gas in the line 134 will be in gaseous form and at an elevated pressure for ease of transporation or subsequent use as a fuel.

The heat exchanger 104 is preferably heated by a closed propane cycle, wherein propane is condensed in the exchanger 104 and then falls by gravity through a line. 136 to a lower'heat exchanger 13?. As before, the condensed propane is revaporized in the heat exchanger 138, such that the propane vapors will rise through a line 140 from the heat exchanger 138 back into the heat exchanger 104; The usual heat source for vaporizing the propane in the heat exchanger 138 is in the form of sea Water constantly available at an inlet line 142. Sea water from the inlet line 142 is pumped through line 144 to the tubes ofthe heat exchanger 138, as well as to the heat-exchangers 138 of the remaining systems 86. The sea water discharging from the heat exchanger 138 is directed back through a line 146 to a header 148 for convenient disposal of the used water. The amount of sea water directed throughthe heat exchanger 138 is regulated by a valve 149 which in turn is controlled by the temperature of the condensed propane through use of a temperature controller 150 con-. nected to line 136, such that the amount of heat available. to the heat exchanger 138 may be controlled as desired.

In the event sufiicient sea water is not available, or is not available at a suificiently high temperature to provide an effective heat source for vaporizing the propane, or whenever desired, a furnace 151 may be used. A furnace 151 is preferably installed for each of the systems 86 and is heated by a suitable fuel from a fuel line 152. This fuel may be easily obtained by bleeding off a portion of. the natural gas in the natural gas discharge line 134 as illustrated at the right hand end of the flow diagram. The condensed propane may be directed from the line 136 through a by-pass line 154 for passage through the respective furnace 151 where the propane will be revaporized; The propane vapors will in turn rise through a line 156 which is joined with the previously describedpropane vapor line 140 for flow into the upper heat ex changer 1194. The amount of fuel fed to the furnace 151,

and hence the temperature of the furnace 151, is com trolled by a valve 158 in turn controlled by the temperature of the condensed propane in the line 136, in the same manner as the temperature of the exchanger 138 was controlled, as previously described.

Although the propane cycle for each of the systems '86 is a separate closed system, the amount of propane in each system will invariably change. make-up line 160 is extended 138 to :a propane supply and in turn communicates with a from the heat exchanger discharge line 162, which requires additional propane, the additional be directed through the respective line 160 with the propane in the respective system.

hand, excessive propane in discharged through the line On the other As previously noted, the condensate from each of the separators 108 is directed through the header 112 tothe heavy ends storage vessel 114, and these heavy ends are in turn discharged through line 116. However, a portion of the condensate in the Therefore, this boil-01f or a line 164 into the fuel gas header 152 to combine with fuel supplied to the header 152 from the natural gas supply and by use of a cheap and readily availableheatI source, without the formation of any solids on the heat Therefore, a propanesuitable propane storage- (not shown). In the event either of the propane systems propane mayand combined one of the systems may be vessel 114 will vaporize and may be readily used as a fuel for firing the furnaces 151. overhead is directedthroughr provides a simple and 9 exchangers used for the revaporization process. Furthermore, the present invention provides for a power recovery in a revaporization of liqueued natural gas, such that the net power required for a system will be reduced to a minimum.

Changes may be made in the combination and arrangement of steps and procedures, as well as the various items of equipment and apparatus, heretofore set forth in the specification and shown in the drawings, it being understood that changes may be made in the precise embodiments disclosed without departing from the spirit and scope of the invention as defined in the following claims.

I claim:

1. A method of vaporizing a liquefied gas having a boiling temperature range below the temperature of a heat source, comprising the steps of:

(a) vaporizing a liquid heat transfer medium with the heat source,

(b) reducing the pressure of the vaporized heat transfer medium in a work-producing Zone,

() condensing the heat transfer medium with the liquefied gas while simultaneously vaporizing the liquefied (d) pressurizing the condensed heat transfer medium, and

(e) recycling the heat transfer medium through steps and 2. A method as defined in claim 1 characterized further in superheating the vaporized heat transfer medium prior to reducing the pressure thereof in the work-producing zone.

3. A method as defined in claim 1 characterized further in combining a portion of the vaporized heat transfer medium with the pressurized and condensed heat transfer medium to preheat the heat transfer medium prior to revaporization thereof, and pressurizing the combined vaporized and condensed heat transfer medium prior to revaporiz-ation thereof.

4. A method as defined in claim 1 characterized further in that the heat source is sea water.

5. A method as defined in claim 1 characterized further in that heat source is 6. A method as defined in claim 4 characterized further in that the heat transfer medium is propane.

7. A method of utilizing heat from a readily available heat source for vaporizing a liquefied gas having a boiling temperature below the freezing temperature of at least one component of the heat source comprising passing the liquefied gas in heat exchange relationship with a vaporized heat transfer medium having a condensation temperature above the boiling point temperature of the liquefied gas and a freezing point temperature below the boiling point temperature of the liquefied gas and at a rate to vaporize the liquefied gas and condense the heat transfer medium, passing the condensed heat transfer medium in heat exchange relationship with the heat source '5. having a temperature above the boiling point temperature of the heat transfer medium and in which no component of the heat source has a freezing point temperature above the temperature of the heat transfer medium and at a flow rate to vaporize the heat transfer medium, and repeating the cycle.

8. A method of utilizing heat from a readily available heat source for vaporizing a liquefied gas having a boiling temperature below the freezing temperature of at least one component of the heat source comprising passing the heat transfer medium in heat exchange relationship with the liquefied gas and alternately the heat source, said heat transfer medium having a frezmg temperature below the boiling temperature of the liquefied gas and having a temperature between the temperature of the heat source and the freezing temperature of any component of the heat source while passing in heat exchange relationship with the heat source, passing the vaporized heat transfer medium through a Work producing zone with a reduction in pressure before passage thereof in heat exchange relationship with the liquefied gas, pressurizing the condensed heat transfer medium before passage thereof in heat exchange relationship with the heat source and lay-passing a portion of the heat transfer medium from upstream of the work producing zone to the pressurized and condensed heat transfer medium to preheat to heat transfer medium before passage thereof in heat exchange relationship with the heat source.

9. A method of utilizing heat from a readily available heat source for vaporizing a liquefied gas having a boiling temperature below the freezing temperature of at least one component of the heat source, comprising passing a heat transfer medium in heat exchange relation with the liquefied gas, and, alternately, the heat source, said heat transfer medium having a freezing temperature below the boiling temperature of the liquefied gas and having a temperature between the temperature of the heat source and the freezing temperature of any component of the heat source while passing in heat exchange relation with the heat source, and characterized further in that the heat transfer medium is a liquid having a moderate vapor pressure at a temperature between the temperature of the heat source and the freezing temperature of any component of the heat source for vaporization of the heat transfer medium upon passage in heat exchange relation with the heat source.

References Cited in the file of this patent UNITED STATES PATENTS 2,111,618 Erback Mar. 22, 1938 2,484,875 Cooper Oct. 15, 1949 2,495,549 Roberts Jan. 24, 1950 2,799,997 Morrison July 23, 1957 FOREIGN PATENTS 736,736 France Sept. 26, 1932

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