US3884044A

US3884044A - Mixed refrigerant cycle

Info

Publication number: US3884044A
Application number: US304276A
Authority: US
Inventors: Sheldon Bodnick; Thomas M Stark
Original assignee: Exxon Research and Engineering Co
Current assignee: ExxonMobil Technology and Engineering Co
Priority date: 1970-02-09
Filing date: 1972-11-06
Publication date: 1975-05-20
Anticipated expiration: 1992-05-20

Abstract

A process for cooling and condensing a first mixture by means of a second multicomponent mixture. The second mixture is cycled through a plurality of heat exchange stages in convective heat transfer relationship to the first mixture. The second mixture is withdrawn from an intermediate heat exchange stage and compressed at low temperatures. The refrigeration supplied to warmer exchange stages in the prior art by the second mixture is supplied according to the new process by a separate refrigeration system or by a subsidiary refrigeration loop within the main second mixture refrigeration system.

Description

Bodnick et al.

1 1 May 20, 1975 [54] MIXED REFRIGERANT CYCLE 3,581,511 6/1971 Peck 62/11 3,593,535 7/1971 Gaumer [75] mentors? Bmimck, Rocklfway; 3,702,063 11 1972 Etzbach 62/40 Thomas M. Stark, Morr1stown, both of NJ; FOREIGN PATENTS OR APPLICATIONS [73 Assigneez Exxon Research & Engineering 895,094 /l962 United Kingdom 62/40 Company, Linden, NJ. Primary Examiner-A. Louis Monacell [22] 1972 Assistant Examiner-Hiram H. Bernstein [21 Appl. No.: 304,276 Attorney, Agent. or Firm-Harold N. Wells Related U.S. Application Data [63] Continuation-impart of Ser. No. 9.499, Feb. 9, 1970, [57] ABSTRACT abandoned A process for cooling and condensing a first mixture by means of a second multicomponent mixture. The [52] Cl. 62/9; 62/40; Second mixture is cycled through a plurality of heat [51] II}!- Cl F251 3/00 exchange Stages in convective heat transfer relation [58] held of Search 62/9 1 ship to the first mixture. The second mixture is with- 62/27 drawn from an intermediate heat exchange stage and compressed at low temperatures. The refrigeration [56] References cued supplied to warmer exchange stages in the prior art by UNITED STATE PA the second mixture is supplied according to the new 3,274,787 9/1966 Grenier 62/28 process by a separate refrigeration system or by a sub- 3.364,685 1/1968 Perret 62/9 sidiary refrigeration loop within the main second mix- 3 418,819 12/1968 Grunberg..... 1. 62/1 1 lur refrigeration system, 3,578,073 5/1971 Bosquain 62/40 3581510 6/1971 Huches 62/40 8 Claims, 4 Drawing Figures m0 m2 104 I06 MIXED REFRIGERANT CYCLE CROSS REFERENCE TO RELATED APPLICATIONS This application is a continuation-in-part of application Ser. No. 9499 and now abandoned.

BACKGROUND OF THE INVENTION In the prior art several methods are employed for the liquefaction of natural gas feeds. Among these is a multicomponent refrigeration cycle in which natural gas feed at ambient temperature, normally about 100F., is successively cooled and liquefied in a plurality of heat exchange stages, resulting in a liquefied natural gas product at a temperature of about 260F. The cooling medium is a multicomponent refrigerant.

In the prior art method this multicomponent refrigerant having given up its residual cold in a first heat exchange stage, enters a compressor in the vapor phase, at a pressure of about 1-5 atmospheres and a temperature essentially ambient. By ambient temperature is meant the averagetemperature of the surrounding environment and thus, as applied to process streams, it is the temperature which can be closely approached by contacting those streams with the air, water, etc. The refrigerant is pressurized and cooled by heat exchange with water or air to form a two-phase mixture at a temperature slightly above ambient. This two-phase mixture is cycled to a separation drum upstream of the first heat exchange stage. The two phases are separated in this drum and both vapor and liquid phases enter the first heat exchange stage along with the natural gas feed. All of the above three streams are cooled by the recycled multicomponent refrigerant stream. This recycled refrigerant stream enters the first heat exchange stage as a two-phase mixture countercurrent to the above three streams, generally at F. or below. In the course of cooling the three streams, the recycled refrigeration stream is warmed, exiting as a gas as discussed above at a temperature about ambient. Typical of the prior art method is US. Pat. No. 3,593,535 to Gaumer et al.

The disadvantages of this prior art method lie first in the high power requirements of the refrigeration compression step. A second disadvantage is the high capital cost of the heat exchange stages. All the heat exchange stages in the prior art method are constructed of high cost alloy materials. This is necessary in order to insure that all heat exchanges and heat exchange stages operate without danger of rupture in the low temperature environment that they are sub ected to in the prior art process.

SUMMARY OF THE INVENTION The method of the instant invention is directed to a process for cooling and condensing a gaseous mixture ant stream is less than the work saved by lowering the compressor section temperature, the process of the instant invention results in considerable power savings.

The instant invention is also directed to a process in which the capital costs are lower than the equivalent prior art process. Thus, in the instant invention the first heat exchange stage is constructed of carbon steel rather than high cost alloy materials. This is due to the method of the instant invention wherein the first stage heat exchanger no longer contacts a refrigerant stream so cold that carbon steel is not suitable.

In accordance with the instant invention a first mixture, i.e., the natural gas feed, is cooled and condensed in a series of heat exchange stages by means of a'second mixture which acts as a refrigerant. The first mixture and the second mixture are both cooled in a first heat exchange stage, both entering at essentially ambient temperature and leaving said stage below ambient temperature. The second mixture is then separated into liquid and gaseous phases after leaving the first heat exchange stage. Thereafter, the first mixture and the liquid and gaseous phases of the second mixture are further cooled in a second heat exchange stage by means of a cold recycle of the second mixture. The gaseous phase of the second mixture which is cooled in the second heat exchanger is again separated into gaseous and liquid phases. The steps of cooling the first mixture and the liquid and gaseous phases of the second mixture by means of a cold recycle of the second mixture and then separating the cooled gaseous phase of the second mixture into liquid and gaseous phases are repeated until the first mixture is cooled and condensed to the desired temperature. The cold recycle of the second mixture after exiting the second heat exchanger is compressed while still below ambient temperature and cooled and thereafter cycled back into the first heat exchange stage thus completing the cycle. The refrigeration value lost by recycling the cold second mixture is supplied by one of several alternative methods.

In a preferred embodiment the first and second mixtures are cooled in the first heat exchange stage by means of a separate refrigeration cycle using, instead of a multicomponent mixture, an essentially single component refrigerant.

In another preferred embodiment the cold recycle exiting the second heat exchange stage is compressed in two stages, A portion of the second mixture is compressed to an intermediate pressure, cooled to near ambient temperature, and then recycled to the first heat exchange stage, thereby providing the cooling for the remainder of the second mixture and all of the first mixture.

In still another preferred embodiment a portion of the second mixture after leaving the first heat exchange stage is recycled and flashed back through the first heat exchange stage thus providing a third means of cooling the first and second mixtures in the first heat exchange stage.

The method of the instant invention requires lower power requirements since the recycle is compressed after leaving the second heat exchange stage at a much lower temperature rather than at the ambient temperature typical in the prior art in which the recycle continued on through the first heat exchange stage. Additionally, since the cold recycle is sent to the compressor after leaving the second heat exchange stage, the first heat exchange stage is designed with a separate cooling system which is independent of the colder temperatures existing downstream in the higher heat exchange stages. Hence, the first heat exchange stage may be constructed of carbon steel since the temperature in the first heat exchange stage can be designed to never encounter temperatures below F.

BRIEF DESCRIPTION OF THE DRAWINGS The invention may be better understood by reference to the accompanying drawings of which:

FIG. 1 is a flow diagram of a multicomponent refrigeration cycle with an external high level refrigerant cooling the first heat exchange stage;

FIG. 2 is a flow diagram of a multicomponent refrigeration cycle with cooling in the first heat exchange stage provided by expansion of the intermediate pres sure second mixture liquid;

FIG. 3 is an expanded view of a portion of FIG. 2 showing an alternate method of heat transfer in the first heat exchange stage;

FIG. 1 as a countercurrent stream) to the streams that are cooled therein, at a temperature in the range of 30 to 0F. This refrigeration stream exits stage 100 through conduit 76 as a vapor near or slightly below ambient temperature. It passes to the remainder of the refrigeration cycle, which is not shown in the drawing, and returns after heat has been rejected to the first heat exchange stage 100 as a liquid through conduit 75.

The first mixture flows from the first heat exchange stage 100 into a second heat exchange stage 102 by means of a conduit 4. It is cooled in exchanger 102 to about 100F. The first mixture leaves exchanger 102 through conduit 8.

The second mixture stream which exits the first heat exchange stage l00 through conduit 26 as a two-phase mixture is separated into liquid and gaseous phases in FIG. 4 is a flow diagram of a multicomponent refrig- 1 eration cycle with cooling in the first heat exchange stage provided by an expansion and recycle of the second mixture liquid exiting the first heat exchange stage.

DETAILED DESCRIPTION Referring to FIG. 1, in detail, reference numeral 2 denotes a conduit supplying a first mixture, which in a preferred embodiment is a natural gas multicomponent feed stock at about ambient temperature, typically about 100F., into a first heat exchange stage 100. If the gas is not already at near ambient temperature, it will be precooled by a indirect heat exchange against environmental streams, usually air or water, since this is more economical than cooling by refrigeration. The first mixture is cooled to a temperature of about 0F. and exits through conduit 4. A second mixture which comprises a multicomponent mixture of nitrogen, methane, ethane, propane, butane and heavier hydrocarbons leaves compressor 130 through conduit 22 as a gaseous mixture at a pressure of about 350 to 500 pounds per square inch. The high pressure refrigerant stream thereafter is cooled to near ambient temperature and partially condensed against air or water in an after-cooler 132. This second mixture then flows through conduit 24 into the first heat exchange stage 100, cocurrent with the natural gas feed stream. Along with the natural gas multicomponent mixture, this second multicomponent mixture is cooled further in stage 100. The two-phase stream exits stage 100 at a temperature of about 0F. through conduit 26, thereafter entering a separation drum 110.

The two above-described streams entering stage 100 through conduit 2 and conduit 24, respectively, are cooled by convective heat transfer. In a preferred embodiment, a plurality of streams are used to cool the first and second mixtures. Each stream comprises the same single component refrigerant but each operates at a different pressure from the others so that a plurality of boiling temperatures for the same refrigerant is attained. The separate refrigeration cycle removes heat from the ambient temperature incoming streams and rejects it to the environment. This external cycle may operate with any single component refrigerant, but the temperature range suggests that propane would be a preferred refrigerant. The external refrigerant enters stage 100 either cocurrent or countercurrent (shown in a separation drum 110. The vapor phase of the multicomponent second mixture is conveyed from the drum 1 10 to heat exchange stage 102 through conduit 28, entering stage 102 cocurrent with the natural gas feed. It is therein cooled to a temperature of about F. exiting said stage 102 as a two-phase mixture through conduit 34. Conduit 34 is in communication with a separation drum 112 into which the two-phase mixture is discharged. The liquid and gas phases are separated therein.

The liquid phase constituent of drum is conveyed therefrom to heat exchange stage 102 through conduit 30. Conduit 30 feeds this liquid stream cocurrently into stage 102 with the natural gas stream and the vapor phase of the refrigerant to stage 102. The liquid is chilled therein to a temperature of about 100F.

The cooling medium for these three above-described cocurrent streams comprises in part the liquid phase of the second mixture. This stream exits the second stage 102 through conduit 36 and is flashed in an expansion valve 120. The resultant flashed, two-phase mixture, at 1 a temperature of about 100 to 130F. and pressure of from I to 5 atm, flows from the expansion valve into a conduit 38. This two-phase multicomponent stream thereupon enters a conduit 74, where it mixes with the exiting multicomponent refrigeration stream of the second mixture from a downstream heat exchange stage 104 at approximately the same temperature. The combined stream flows through conduit 74 into the stage 102. This cooling stream is heated in the stage 102 thereby vaporizing the mixture. The recycle stream leaves stage 102 through conduit 20 at a temperature of about 20F. to 10F. and a pressure of 1 to 5 atm. The stream flows through conduit 20 back to the compressor 130. As will be described hereinafter, the second mixture recycle stream includes second mixture streams recycled from downstream heat exchange stages. Thus, it is apparent from a simple material balance that the second mixture vapor stream in conduit 20 includes all of the second mixture that enters into the second heat exchange stage through

conduits

28 and 30.

In another preferred embodiment the liquid phase of the second mixture exiting separation drum 110 is immediately flashed, combined with the cold essentially vapor recycle portion of the second mixture, and passed through heat exchange stage 102 countercurrent to the first mixture stream and the vapor phase of the second mixture entering stage 102 through

conduits

4 and 28, respectively. It should be appreciated that this alternate method may be applied to any or all of the heat exchange stages in which the liquid phase of the second mixture is flashed and passed countercurrently to the first mixture. This alternate method is applicable not only to FIG. 1 but to the embodiments disclosed in FIGS. 2, 3 and 4 hereinafter.

The first mixture in conduit 8 next enters into the third heat exchange stage 104 at a temperature of about lF. A second cocurrent stream enters heat exchange stage 104 through conduit 40. It represents the overhead vapor phase of the second mixture contained in separation drum 112. A third inlet stream into stage 104 is the liquid phase of the second mixture contained within drum 112. It enters stage 104 through conduit 42. The three streams are cooled to a temperature in the range of 1 60 to -2l0F. by a flashed recycle stream of the second multicomponent mixture which flows countercurrently in stage 104, to the above-described streams. The liquid phase stream exits stage 104 into a conduit 46 which leads the liquid stream into an expansion valve 122 wherein the liquid is flashed resulting in a two-phase mixture at a temperature range of about 160 to 230F. and a pressure of latm. This two-phase stream leaves valve 122 through conduit 48 and combines with a cooling stream exiting a downstream heat exchange stage 106. The combined two-phase multicomponent second mixture stream enters heat exchange stage 104 through a conduit 70 countercurrent to the above-described warmer streams, wherein the combined stream is heated to a temperature in the range of 100 to 130F. This combined stream exits as a vapor or a two-phase mixture through conduit 72. It is then combined, with the exiting flashed stream from conduit 38, in conduit 74 to form the inlet refrigerant to exchange stage 102.

The cooled first mixture stream leaves stage 104 by way of a conduit 12. The cooled gaseous phase stream entering through conduit 40 is cooled and substantially condensed exiting stage 104 through conduit 54.

Since the overhead vapor phase is substantially condensed in the stage 104, there are only two streams cooled in a fourth heat exchange stage 106. They are, respectively, the natural gas multicomponent stream (first mixture) which enters stage 106 through conduit 12 and the multicomponent refrigerant stream (second mixture) which enters the fourth stage 106 through conduit 54. Both streams are cooled to about 260F. by means of the exiting chilled liquid phase of the second mixture which exits exchange stage 106 through conduit 60. Again, the liquid contained within conduit 60 is flashed to a lower temperature, in an expansion valve 126, exiting said valve 126 as a two-phase mixture through a conduit 62. The refrigerant in conduit 62 is passed countercurrent to the above two streams exiting as a vapor or two-phase multicomponent stream by way of a conduit 64 at a temperature of about -1 60 to 230F. It is thereafter combined with the flashed stream exiting conduit 48 to providethe coolant to heat exchange stage 104, said coolant stream entering said stage 104 through conduit 70.

The first mixture stream which entered stage 106 through conduit 12 exits said stage 106 through conduit 14 as a liquid at a temperature of approximately -260F. This comprises the final liquefied natural gas product of the above-described method.

It should be appreciated that in the above disclosure and in FIG. 1, convective heat transfer occurs in heat exchange stages. It should not be inferred that a heat exchange stage is equivalent to a single heat exchanger. On the contrary, a heat exchange stage should be understood to include one or more heat exchangers of various kinds which may be disposed in parallel and/or series configurations. This interpretation should be given also to the disclosure which follows.

Turning now to FIG. 2 in detail, in another preferred embodiment of this invention, a first mixture which in a preferred embodiment is a natural gas feed enters a first heat exchange stage 200 through a conduit 2 at near ambient temperature, typically about lOOF. A multicomponent second mixture which acts as a refrigerant enters into stage 200 through a series of steps starting at a first compressor 230. The multicomponent refrigerant leaves the compressor 230 through conduit 252 as a gas at a pressure of about 100 to 200 psia. This stream thereupon is cooled in an intercooler 232 wherein the temperature is reduced to approximately 100F. by indirect heat exchange against air or water at ambient temperature thereby condensing the higher boiling point components of the multicomponent refrigerant. Thus, a two-phase multicomponent mixture flows out of intercooler 232 into conduit 254. This multicomponent mixture thereupon enters into a stage knockout drum 234 wherein the phases are separated. The vapor phase is further pressurized in a second stage compressor 240. The vapor phase enters the compressor 240 from the drum 234 by way of conduit 256 at an approximate temperature of l0OF., and pressure of 100 to 200 psia. Upon leaving compressor 240, the vapor stream is pressurized to about 350 to 500 psia. The stream is then cooled in an aftercooler 242 by indirect heat exchange against air or water at ambient temperature, said stream entering said aftercooler 242 by way of conduit 258. Part of the multicomponent mixture may be condensed in said aftercooler 242 resulting in a two-phase mixture at a temperature of about 100F. while maintaining essentially constant pressure. The two-phase mixture exits aftercooler 242 by way of a conduit 260. Conduit 260 is in communication with the first heat exchange stage 200. Thus, the two-phase stream flows into stage 200 in a direction cocurrent with the natural gas feed which enters the first stage 200 through conduit 2.

Since the main refrigerant stream returns to compressor 230 from second stage 102, the cooling stream required for cooling of the two above-mentioned streams is provided by means of the second mixture liquid phase of the stage knockout drum 234. The liquid phase at a temperature of about 100F. and a pressure of 100 to 200 psia is conducted by means of conduit 270 into an expansion valve 280. The stream is flashed in said valve 280 resulting in a two-phase mixture at a temperature in the range of 30 to 0F. and a pressure of l to 5 atm. A conduit 274 leads the two-phase stream from the valve 280 into stage 200 in a direction countercurrent to the streams entering through constead of conducting the liquid phase of the second mixture from knockout drum 234 into an expansion valve 280 and then passing the resultant twophase mixture through the first heat exchange stage 200 countercurrent to the warm streams, the liquid phase may alternately be conveyed cocurre'nt to the warm streams entering through conduits 2 and 260. This is shown in detail in FIG. 3. The liquid phase in drum 234 enters the first heat exchange stage 200 through conduit 271 at the same temperature and pressure previously disclosed for the stream in conduit 270 in FIG. 2. The liquid stream is further chilled to a temperature of about F. exiting stage 200 through conduit 272. It is then flashed in expansion valve 280 exiting as a two-phase mixture at a temperature of about 0 to 30F. and a pressure of 1 to atm. This resultant two-phase stream is conducted by way of conduit 273 through the heat exchange stage 200 countercurrently to the three above-described streams exiting as a gas approaching ambient temperature (slightly below 100F). The gaseous stream leaves said stage 200 by way of conduit 275 and joins the main refrigerant stream of line (not shown).

Returning now to FIG. 2, the exiting second mixture stream from conduit 276, or in the aternative, conduit 275, enters a conduit 250 thus merging with the bypass stream exiting heat exchanger stage 102 by way of conduit 20. The gaseous second mixture stream exiting through conduit 20 has been previously thermodynamically defined in the description of the embodiment illustrated in FIG. 1. The combined stream in conduit 250 is a gas at a pressure of l to 5 atm. This gaseous stream returns now to the first stage compressor 230 and is compressed thereby completing the cycle.

The natural gas feed leaves the first heat exchange stage 200 through conduit 4 at the same conditions as described in FIG. 1. The two-phase second mixture stream entering the stage 200 from conduit 260 is cooled therein to a temperature of about 0F. exiting the heat exchange stage 200 through a conduit 30 as a two-phase mixture. The second mixture stream enters a separation drum 210. The vapor and liquid phases are separated therein and are then passed into heat exchanger stage 102 at a temperature of about 0F.

The process previously described in the FIG. 1 embodiment is applicable to the instant embodiment (FIGS. 2 and 3) insofar as FIG. 1 refers to flow streams downstream of the first heat exchange stage. Thus, the reference numerals of the conduits, heat exchange stages and the like have not been changed in FIGS. 2 and 3. This is not to say the conditions of all streams with the same number are exactly the same as the same numbered stream in FIG. 1. It is to say that conditions are approximately the same so that the use of the same numbers are justified in view of the insignificant changes in thermodynamic properties in the same numbered streams. It should further be appreciated that those conduits, separators and the like upstream of the secoond heat exchange stage which are numbered with the same numbers in FIGS. 2 and 3 as in FIG. 1 are subject to the same interpretation.

In a third preferred embodiment, illustrated in FIG. 4, a first mixture, again a gaseous feed stream exactly the same as the feed stream previously described in the previous embodiments, enters a first heat exchange stage 300 through conduit 2. It is cooled in said first stage 300 and exits through conduit 4. It should be understood that the temperature, pressure and the other properties of the first multicomponent product stream are the same as previously disclosed in the embodiment shown in FIGS. 1 and 2.

A second cocurrent stream enters the first heat exchange stage 300 through a conduit 342. This stream is the second mixture which enters as a two-phase mixture at approximately F. and 350-500 psia. The two-phase multicomponent second mixture is further cooled in stage 300 and exits said stage 300 still as a two-phase mixture, at a temperature of about 0F. The exiting second mixture stream flows through a conduit 344 into a separation drum 310. The liquid phase contained within said drum 310 exits the drum 310 through a conduit 346 which branches into two additional conduits. One conduit 348 leads a portion of the liquid into an expansion valve 380. The liquid stream is therein flashed exiting as a two-phase mixture at a temperature of about 0 to 30F. and an approximate pressure of 100-200 psia. This second mixture, two-phase stream thereafter is recycled back into the first stage 300 countercurrent to the streams entering through conduits 2 and 342. It cools these streams, thereby absorbing enough heat to exit stage 300 as a gas approaching ambient temperature (slightly below 100F). This gaseous stream flows through a conduit 376 from stage 300 to a downstream stage of a refrigerant compressor 330. It should be understood that this gaseous stream enters a stage downstream of the inlet of compressor 330 since it is at a higher pressure than the stream entering the inlet of compressor 330. Thus, it requires a lesser compression step to bring it up to the required high pressure. It should be further undertsood that alternately the two-phase gaseous stream of the second mixture may alternately enter the inlet of a second compressor as will be described hereinafter.

At the same time that the stream in conduit 376 enters a downstream stage of compressor 330, the cold gaseous recycle stream of the second mixture in conduit 20 is recycled back to the upstream end of the compressor 330. It should be understood that the stream in conduit 20 has similar thermodynamic properties, as were previously described, to the stream exiting through conduit 20 in FIG. 1. The stream exiting conduit 20 into the upstream end of compressor 330 is of course at a much lower pressure than that gaseous stream entering'a downstream stage of the compressor 330 through conduit 376. The combined second mixture gaseous streams exit said compressor 330 as a pressurized gas at a pressure of 350 to 500 psia through conduit 340. The gaseous multicomponent second mixture stream thereupon enters an aftercooler 332 wherein it is cooled by indirect heat exchange against air or water and is partially condensed. The two-phase stream leaves said aftercooler 332 at a' temperature of about 100F. This two-phase stream flows through conduit 342 back into the first heat exchange stage 300 thereby completing the cycle.

In an alternate embodiment the second mixture stream in conduit 20 enters a first compressor and is compressed to a pressure equal to that in the second mixture gaseous stream in conduit 376. Thence, the stream exiting the first compressor is combined with the stream in conduit 376 and together they enter the second compressor. Other embodiments employing more than two compressors may also be employed. The

alternate embodiment described herein is given by way of illustration and is not inclusive.

The rest of the process steps are similar to those previously described in the previous embodiments. Therefore, the numbers assigned to the various streams, apparatus and the like in FIG. 4 are the same as those used in FIGS. 1, 2 and 3. The temperatures, pressures, and phases ascribed to similarly numbered streams, apparatus and the like are applicable to'this embodiment, subject to the comments made previously as to their approximate applicability. The one exception is the liquid phase second mixture stream entering the second heat exchange stage 102. Unlike the embodiment disclosed in FIG. 1 only a fraction of the liquid phase of the second mixture in separation 310 flows intoheat exchange stage 102. The fraction which is approximately 60 to 90 percent of the liquid phase content of separator 310 flows into the second heat exchange stage 102 from the separator 310 by way of a conduit 350. The remaining to 40% of the liquid contents of separator 310 is flashed and recycled into the first heat exchange stage 300 by way of conduit 348.

it should be understood that the above preferred embodiments may be modified without departing from the scope and spirit of the invention. Thus, other temperatures, pressures and phase conditions may be employed to achieve optimum operating conditions depending upon the particular circumstances under which the natural gas is to be liquefied. Furthermore, the number of heat exchange stages employed in the preferred embodiments is illustrative and not limiting. Hence, more or less than four heat exchange stages may be used without departing from the scope of the invention.

We claim:

1. A method of cooling and liquefying natural gas in a warm stage by a first refrigeration system employing a first refrigerant and thereafter further cooling andliquefying said natural gas in a cold second stage by a second refrigeration system employing a multicomponent second refrigerant, each of said first and second refrigeration systems discharging heatreceived from said cooling and liquefying of natural gas in said associated first and second stages by compressing said refrigerants and condensing them by indirect heat exchange against environmental cooling streams comprising the steps of: a. precooling said natural gas to essentially ambient temperature by indirect heat exchange against at least one of the group of environmental cooling streams consisting of air and water and thereafter;

b. cooling below ambient temperature in saidwarm first stage by indirect heat exchange said natural gas and independent therefrom said multicomponent second refrigerant by said first refrigerant which exits said first stage after being warmed therein and after being compressed is cooled by one of the group of environmentalstreams consist-. ing of air and water, thereby rejecting heat received from the natural gas and the second refrigerant in said first stage to the environment and is expanded and returns to said warm first stage as refrigerant;

c. further cooling and liquefying said natural gas in said cold second stage by indirect heat exchange with said multicomponent second refrigerant which thereafter exits said cold second stage after absorbing heat therein at a temperature below that of said natural gas entering said cold stage and enters the second refrigeration system compressor suction at substantially its exit temperature and pressure and thereafter is compressed and cooled to near ambient temperature; thereby rejecting heat absorbed in said second stage by indirect heat exchange against at least one of the group of environmental streams consisting of air and water and thereafter is cooled below ambient temperature in said warm first stage of (b) and thereafter separated into liquid and vapor phases and is returned to said cold second stage wherein said liquid phase is cooled and thereafter reduced in pressure, and returned through said second stage as a portion of said second refrigerant and said vapor phase is cooled. 2. The method of claim 1 wherein said first refrigerant substantially comprises propane.

3. A method of cooling and liquefying natural gas in a warm first stage by a first multicomponent refrigerant and thereafter further cooling and liquefying said natural gas in a cold second stage by a second multicomponent refrigerant, each of said first and second refrigerants discharging heat received from said cooling and liquefying of natural gas in said first and second stages by indirect heat exchange against environmental cooling streams comprising the steps of:

a. precooling said natural gas to essentially ambient temperature by indirect heat exchange against at least one of the group of environmental cooling streams consisting of air and water and thereafter;

b. cooling below ambient temperature in said warm first stage by indirect countercurrent heat exchange said natural gas and independent therefrom said second multicomponent refrigerant against said first mutlicomponent refrigerant which exits said first stage after being warmed therein and while at substantially its exit temperature joins said second refrigerant leaving said second stage and is compressed together with said second refrigerant and thereafter condensed and cooled to near ambient temperature by indirect heat exchange against at least one of the group of enviromental streams consisting of air and water and thereafter separated as a liquid from the vaporized second refrigerant and thereafter expanded to lower pressure and returned to said warm first stage as refrigerant;

c. further cooling and liquefying said natural gas and said second refrigerant in said cold second stage by indirect countercurrent heat exchange against said second refrigerant which after absorbing heat therein exits from said second stage at a temperature below that of said natural gas entering said cold stage and thereafter without significant intervening temperature change joins said first refrigerant and is compressed jointly with said frist refrigerant to a pressure at which said first refrigerant is fully condensed and cooled to near ambient temperature by indirect heat exchange against at least one of the group of environmental streams consisting of air and water, said first and second refrigerants thereafter being separated and said second refrigerant being further compressed and cooled to near ambient temperature by indirect heat exchange against at least one of the group of environmental streams consisting of air and water and thereafter is cooled below ambient temperature in said wann first stage of (b) and then separated into liquid and vapor streams and is returned to said cold second stage.

4. The method of claim 3 wherein said first and second multicomponent refrigerants comprise mixtures of nitrogen, methane, ethane, propane, butane and heavier hydrocarbons, with the compositions of said refrigerants being selected to provide suitable temperatures for cooling said first and second stages respectively.

5. A method of cooling and liquefying natural gas in a warm first stage by a first multicomponent refrigerant and thereafter further cooling and liquefying said natural gas in a cold second stage by a second multicomponent refrigerant, the heat removed by said refrigerants in said first and second stages being discharged by cornpressing said refrigerants and condensing them by indirect heat exchange against environmental cooling streams comprising the steps of:

b. cooling below ambient temperature in said warm first stage said natural gas and the combined first and second refrigerants by indirect countercurrent heat exchange against said first refrigerant which exits said first stage after being warmed therein;

c. separating the combined first and second refrigerants into a vapor and a liquid stream and thereafter separating a portion of said liquid stream and expanding and returning said portion to said first stage as said first refrigerant;

d. further cooling and liquefying said natural gas and the second refrigerant consisting of the vapor stream of (c) and the remainder of the liquid stream of (c) in said cold second stage by indirect countercurrent heat exchange against said second refrigerant which therafter exits said second stage after absorbing heat therein at a temperature below that of said natural gas entering said cold stage and without significant intervening temperature change is compressed and combined with the warmed first refrigerant from said first stage and thereafter said combined refrigerants are further compressed and cooled to near ambient temperature by indirect heat exchange against at least one of the group of environmental streams consisting of air and water, said combined refrigerants thereafter passing to said warm first stage.

6. A method of cooling and liquefying natural gas in a warm first stage by a first refrigeration system employing a first refrigerant and therafter further cooling and liquefying said natural gas in more than one cold second stage by a second refrigeration system employing a multi-component second refrigerant comprising the steps of:

a. precooling said natural gas to essentially ambient temperature by indirect heat exchange against at least one of the group of environmental cooling streams consisting of air and water;

b. compressing and cooling said second refrigerant to essentially ambient temperature by indirect heat exchange against at least one of the group of environmental streams consisting of air and water;

c. further cooling below ambient temperature the precooled natural gas of (a) and the cooled second refrigerant of (b) by indirect countercurrent heat exchange in said first stage against said first refrigerant, whereby said first refrigerant absorbs heat from said natural gas and said second refrigerant and is vaporized and thereafter is compressed and condensed by indirect heat exchange against environmental streams to reject the said absorbed heat to the environment, and thereafter is expanded and returned to said first stage to absorb heat to complete a refrigeration cycle;

d. separating the second refrigerant of (c) after exiting said first stage into a liquid stream and a vapor stream;

e. further cooling said natural gas and said separated liquid and vapor streams of (d) in said second stage by indirect countercurrent heat exchange against said second refrigerant, said second refrigerant absorbing heat from said natural gas and said liquid and vapor streams and being vaporized leaves said second stage below ambient temperature and thereafter is compressed at substantially said exit temperature in step (b) thereby completing the second refrigeration cycle;

f. further cooling said natural gas and the vapor stream of (d) in a third stage, said vapor stream being flashed into liquid and vapor portions before cooling in said third stage, said third stage being cooled by said second refrigerant depleted of said liqiud stream of (d) which is flashed after passing through said second stage and joins said depleted second refrigerant before entering said second stage as said second refrigerant.

7. The method of claim 6 wherein said first refrigerant substantially comprises propane.

8. A method of cooling and liquefying natural gas in warm first stage by a first multicomponent refrigerant and therafter further cooling and liquefying said natural gas in a cold second stage by a second multicomponent refrigerant, each of said first and second refrigerants discharging heat received from said cooling and liquefying of natural gas in said first and second stages by indirect heat exchange against environmental cooling streams comprising the steps of:

b. cooling below ambient temperature in said warm first stage by indirect countercurrent heat exchange said natural gas and independent therefrom said second multicomponent refrigerant against said first multicomponent refrigerant which exits said first stage after being warmed therein and while at substantially its exit temperature joins said second refrigerant leaving said second stage and is compressed together with said second refrigerant and thereafter condensed and cooled to near ambient temperature by indirect heat exchange against at least one of the group of environmental streams consisting of air and water and thereafter separated as a liquid from the vaporized second refrigerant and returned to said warm first stage and cooled therein and thereafter expanded and returned to said first stage as refrigerant;

. further cooling and liquefying said natural gas and said second refrigerant in said cold second stage by indirect countercurrent heat exchange against said second refrigerant which after absorbing heat therein exits from said cold second stage at a temfrigerants thereafter being separated and said second refrigerant being further compressed and cooled to near ambient temperature by indirect heat exchange against at least one of the group of environmental streams consisting of air and water and thereafter is cooled below ambient temperature in said warm firststage of (b) andthen separated into liquid and vapor streams and is returned to said cold second stage.

Claims

1. A method of cooling and liquefying natural gas in a warm stage by a first refrigeration system employing a first refrigerant and thereafter further cooling and liquefying said natural gas in a cold second stage by a second refrigeration system employing a multicomponent second refrigerant, each of said first and second refrigeration systems discharging heat received from said cooling and liquefying of natural gas in said associated first and second stages by compressing said refrigerants and condensing them by indirect heat exchange against environmental cooling streams comprising the steps of: a. precooling said natUral gas to essentially ambient temperature by indirect heat exchange against at least one of the group of environmental cooling streams consisting of air and water and thereafter; b. cooling below ambient temperature in said warm first stage by indirect heat exchange said natural gas and independent therefrom said multicomponent second refrigerant by said first refrigerant which exits said first stage after being warmed therein and after being compressed is cooled by one of the group of environmental streams consisting of air and water, thereby rejecting heat received from the natural gas and the second refrigerant in said first stage to the environment and is expanded and returns to said warm first stage as refrigerant; c. further cooling and liquefying said natural gas in said cold second stage by indirect heat exchange with said multicomponent second refrigerant which thereafter exits said cold second stage after absorbing heat therein at a temperature below that of said natural gas entering said cold stage and enters the second refrigeration system compressor suction at substantially its exit temperature and pressure and thereafter is compressed and cooled to near ambient temperature; thereby rejecting heat absorbed in said second stage by indirect heat exchange against at least one of the group of environmental streams consisting of air and water and thereafter is cooled below ambient temperature in said warm first stage of (b) and thereafter separated into liquid and vapor phases and is returned to said cold second stage wherein said liquid phase is cooled and thereafter reduced in pressure, and returned through said second stage as a portion of said second refrigerant and said vapor phase is cooled.

2. The method of claim 1 wherein said first refrigerant substantially comprises propane.

3. A method of cooling and liquefying natural gas in a warm first stage by a first multicomponent refrigerant and thereafter further cooling and liquefying said natural gas in a cold second stage by a second multicomponent refrigerant, each of said first and second refrigerants discharging heat received from said cooling and liquefying of natural gas in said first and second stages by indirect heat exchange against environmental cooling streams comprising the steps of: a. precooling said natural gas to essentially ambient temperature by indirect heat exchange against at least one of the group of environmental cooling streams consisting of air and water and thereafter; b. cooling below ambient temperature in said warm first stage by indirect countercurrent heat exchange said natural gas and independent therefrom said second multicomponent refrigerant against said first mutlicomponent refrigerant which exits said first stage after being warmed therein and while at substantially its exit temperature joins said second refrigerant leaving said second stage and is compressed together with said second refrigerant and thereafter condensed and cooled to near ambient temperature by indirect heat exchange against at least one of the group of enviromental streams consisting of air and water and thereafter separated as a liquid from the vaporized second refrigerant and thereafter expanded to lower pressure and returned to said warm first stage as refrigerant; c. further cooling and liquefying said natural gas and said second refrigerant in said cold second stage by indirect countercurrent heat exchange against said second refrigerant which after absorbing heat therein exits from said second stage at a temperature below that of said natural gas entering said cold stage and thereafter without significant intervening temperature change joins said first refrigerant and is compressed jointly with said frist refrigerant to a pressure at which said first refrigerant is fully condensed and cooled to near ambient temperature by indirect heat exchange against at least one of the group of environmental streams consisting of air and water, said first and second refrigeranTs thereafter being separated and said second refrigerant being further compressed and cooled to near ambient temperature by indirect heat exchange against at least one of the group of environmental streams consisting of air and water and thereafter is cooled below ambient temperature in said warm first stage of (b) and then separated into liquid and vapor streams and is returned to said cold second stage.

5. A method of cooling and liquefying natural gas in a warm first stage by a first multicomponent refrigerant and thereafter further cooling and liquefying said natural gas in a cold second stage by a second multicomponent refrigerant, the heat removed by said refrigerants in said first and second stages being discharged by compressing said refrigerants and condensing them by indirect heat exchange against environmental cooling streams comprising the steps of: a. precooling said natural gas to essentially ambient temperature by indirect heat exchange against at least one of the group of environmental cooling streams consisting of air and water and thereafter; b. cooling below ambient temperature in said warm first stage said natural gas and the combined first and second refrigerants by indirect countercurrent heat exchange against said first refrigerant which exits said first stage after being warmed therein; c. separating the combined first and second refrigerants into a vapor and a liquid stream and thereafter separating a portion of said liquid stream and expanding and returning said portion to said first stage as said first refrigerant; d. further cooling and liquefying said natural gas and the second refrigerant consisting of the vapor stream of (c) and the remainder of the liquid stream of (c) in said cold second stage by indirect countercurrent heat exchange against said second refrigerant which therafter exits said second stage after absorbing heat therein at a temperature below that of said natural gas entering said cold stage and without significant intervening temperature change is compressed and combined with the warmed first refrigerant from said first stage and thereafter said combined refrigerants are further compressed and cooled to near ambient temperature by indirect heat exchange against at least one of the group of environmental streams consisting of air and water, said combined refrigerants thereafter passing to said warm first stage.

6. A method of cooling and liquefying natural gas in a warm first stage by a first refrigeration system employing a first refrigerant and therafter further cooling and liquefying said natural gas in more than one cold second stage by a second refrigeration system employing a multi-component second refrigerant comprising the steps of: a. precooling said natural gas to essentially ambient temperature by indirect heat exchange against at least one of the group of environmental cooling streams consisting of air and water; b. compressing and cooling said second refrigerant to essentially ambient temperature by indirect heat exchange against at least one of the group of environmental streams consisting of air and water; c. further cooling below ambient temperature the precooled natural gas of (a) and the cooled second refrigerant of (b) by indirect countercurrent heat exchange in said first stage against said first refrigerant, whereby said first refrigerant absorbs heat from said natural gas and said second refrigerant and is vaporized and thereafter is compressed and condensed by indirect heat exchange against environmental streams to reject the said absorbed heat to the environment, and thereafter is expanded and returned to said first stage to absorb heat to complete A refrigeration cycle; d. separating the second refrigerant of (c) after exiting said first stage into a liquid stream and a vapor stream; e. further cooling said natural gas and said separated liquid and vapor streams of (d) in said second stage by indirect countercurrent heat exchange against said second refrigerant, said second refrigerant absorbing heat from said natural gas and said liquid and vapor streams and being vaporized leaves said second stage below ambient temperature and thereafter is compressed at substantially said exit temperature in step (b) thereby completing the second refrigeration cycle; f. further cooling said natural gas and the vapor stream of (d) in a third stage, said vapor stream being flashed into liquid and vapor portions before cooling in said third stage, said third stage being cooled by said second refrigerant depleted of said liqiud stream of (d) which is flashed after passing through said second stage and joins said depleted second refrigerant before entering said second stage as said second refrigerant.

8. A method of cooling and liquefying natural gas in warm first stage by a first multicomponent refrigerant and therafter further cooling and liquefying said natural gas in a cold second stage by a second multicomponent refrigerant, each of said first and second refrigerants discharging heat received from said cooling and liquefying of natural gas in said first and second stages by indirect heat exchange against environmental cooling streams comprising the steps of: a. precooling said natural gas to essentially ambient temperature by indirect heat exchange against at least one of the group of environmental cooling streams consisting of air and water and thereafter; b. cooling below ambient temperature in said warm first stage by indirect countercurrent heat exchange said natural gas and independent therefrom said second multicomponent refrigerant against said first multicomponent refrigerant which exits said first stage after being warmed therein and while at substantially its exit temperature joins said second refrigerant leaving said second stage and is compressed together with said second refrigerant and thereafter condensed and cooled to near ambient temperature by indirect heat exchange against at least one of the group of environmental streams consisting of air and water and thereafter separated as a liquid from the vaporized second refrigerant and returned to said warm first stage and cooled therein and thereafter expanded and returned to said first stage as refrigerant; c. further cooling and liquefying said natural gas and said second refrigerant in said cold second stage by indirect countercurrent heat exchange against said second refrigerant which after absorbing heat therein exits from said cold second stage at a temperature below that of said natural gas entering said cold stage and thereafter without significant intervening temperature change joins said first refrigerant and is compressed jointly with said first refrigerant to a pressure at which said first refrigerant is fully condensed and cooled to near ambient temperature by indirect heat exchange against at least one of the group of environmental streams consisting of air and water, said first and second refrigerants thereafter being separated and said second refrigerant being further compressed and cooled to near ambient temperature by indirect heat exchange against at least one of the group of environmental streams consisting of air and water and thereafter is cooled below ambient temperature in said warm first stage of (b) and then separated into liquid and vapor streams and is returned to said cold second stage.