US3413816A

US3413816A - Liquefaction of natural gas

Info

Publication number: US3413816A
Application number: US578114A
Authority: US
Inventors: Marco Salvador S De
Original assignee: Phillips Petroleum Co
Current assignee: Phillips Petroleum Co
Priority date: 1966-09-07
Filing date: 1966-09-07
Publication date: 1968-12-03
Anticipated expiration: 1985-12-03

Description

Dec. 3, 1968 s. 5. DE MARCO 3,413,816

LIQUEFACTION OF NATURAL GAS Filed Sept. 7, 1966 2 Sheets-Sheet 1 N 2 Lu U w L it;

LL] 3 U U LLI Z a O O. [I O.

INVENTOR i s. 5. DE MARCO O: D :2 BY wJ Q w 2 O h ATTORNEYS Dec. 3, 1968 s. 5. DE MARCO LIQUEFACTION OF NATURAL GAS 2 Sheets-Sheet 2 iled Septv A 7' TOR/VEVS lNl ENTOR S. S. DE MARCO JmDm O M336 wziimz l nited States Patent G 3,413,816 LIQUEFACTION OF NATURAL GAS Salvador S. De Marco, Bartlesville, ()lrla., assignor to Phillips Petroleum Company, a corporation of Delaware Filed Sept. 7, 1966, Ser. No. 578,114 6 Claims. (CI. 62-21) ABSTRACT OF THE DISCLOSURE Natural gas is liquified by heat exchange with a series of separate progressively decreasing temperature level refrigerants. Each of the cold liquid refrigerants is expanded to a vapor-liquid condition, then passed in heat exchange with the natural gas and then with the cold unexpanded liquid refrigerant causing the liquid portion of the refrigerant to vaporize. The vaporized refrigerant is recompressed and condensed to complete the refrigeration cycle. Expansion of cold refrigerant is controlled responsive to the temperature of the cold refrigerant upstream of the expansion means and downstream of the heat exchanger where liquid refrigerant is cooled prior to expansion.

This invention relates to the liquefaction of a gas. In one aspect, it relates to a method and apparatus for the liquefaction of natural gas which is composed mostly of methane but which may contain heavier hydrocarbons such as ethane, propane, butane and the like, small amounts of aromatic hydrocarbons and variable amounts of non-hydrocarbons such as nitrogen, helium, carbon dioxide, hydrogen sulfide and the like. Illustration of this invention is made With reference to the liquefaction of natural gas but it will be understood that the concepts employed are capable also of application to other low boiling liquefiable gases such as nitrogen, helium, air, oxygen, and the like.

For many practical and economical reasons, the oil and gas industry is interested in reducing natural gas to a liquefied state. It is more practical to store and transport natural gas in its liquefied state rather than its gaseous state because it occupies less space.

There are many patents and publications which deal with the problem of reducing natural gas to its liquefied state. For example, it is known to employ a refrigeration cycle for reducing the compressed natural gas to a liquefied state using a modified cascade system in combination with a modified expansion system. The refrigeration system can be divided into a propane cycle, an ethylene cycle and a methane cycle, each adapted to achieve a reduction in temperature of the compressed gas in the area where each of the cycle subdivisions can be operated with greatest efiiciency from the standpoint of horsepower of refrigeration required to achieve the desired refrigeration.

The subject invention to be described in greater detail hereinbelow uses the vapors from the refrigerant employed to cool the natural gas stream and to cool the liquid refrigerant prior to its being passed in heat exchange with the natural gas stream. The applicant has surprisingly discovered that if the refrigerant is maintained simultaneously in its liquid and vaporous state, better control of the liquefaction of the gas is obtained without the necessity of maintaining specified liquid levels in the heat exchangers in order to efiect the required vaporization.

One object of this invention is to provide an apparatus and method for the economical and efficient conversion of a gas, especially natural gas, from a gaseous state to a liquefied state for storage and transportation.

Another object of this invention is to provide a method for heat exchanging the gas with a uniform concentration of liquid and vapor which are passed in heat exchange therewith.

Yet another object of this invention is to provide a process for the liquefaction of natural gas which makes use of a minimum amount of horsepower refrigeration, which makes use of a minimum amount of heat exchange surface or equipment, which is capable of being carried out in a continuous operation with a minimum amount of labor and materials which is relatively free of the necessity of shut-down for the repair and replacement of equipment, which supplies a relatively pure liquefied gas at about atmospheric pressure; which is safe in operation and which is flexible in operation to enable adjustments to meet existing conditions from the standpoint of gas composition or increase in capacity due to shut-downs in adjacent lines and the like.

Other objects and many of the attendant advantages of this invention will be readily appreciated as the same becomes better understood by reference to the following detailed description, which is considered in connection with the accompanying drawing wherein:

FIGURE 1A is a flow diagram of a process for the liquefaction of natural gas in accordance with the'teachings of this invention;

FIGURE 1B is a continuation of FIGURE 1A describing the methane cycle; and

FIGURE 2 is a diagrammatic sketch of a modification of the flow diagram of FIGURE 1.

Briefly, the subject invention relates to a method and apparatus for liquefying a process gas, which is maintained at an elevated pressure, through heat exchanging it with suitable refrigerants in a closed cascade refrigeration system, the improvements comprising temperature control means for maintaining the refrigerant at a predetermined temperature sufficient to maintain the refrigerant in a vaporizing state as it is heat exchanged with the process stream at each of the various pressure levels.

The process will hereinafter be described in detail with reference to the liquefaction of natural gas using a specific set of temperature and pressure conditions. It is to be understood, however, that the conditions set forth are merely illustrative and may easily and properly be varied in accordance with design and capacity of the system, the character of the gas from the standpoint of composition, temperature and pressure, and the conditions under which the liquefaction is carried out as influenced by the volume of material, types of refrigerants and the like. Such variations are within the skill of the art and are all considered within the scope of the invention.

The refrigeration cycle for reducing the compressed gas to a liquefied state by a modified cascade system and a modified expansion system will be divided into a sequence of refrigeration steps including a propane refrigeration cycle (P), an ethylene refrigeration cycle (E), and a methane refrigeration cycle (M), each of which is adapted to achieve a reduction of temperature of the compressed gas in the area where each of the cycle subdivisions can be operated with greatest efficiency from the standpoint of horsepower refrigeration required to achieve the desired refrigeration.

The propane refrigeration cycle (P) can in turn be subdivided into a high level propane refrigeration stage (P and low level propane refrigeration stage (P and an ethylene condensing stage (P Natural gas is passed from a suitable source not shown through conduit 1 to be cooled in the propane cycle of the liquefaction system at elevated pressure and temperature, such as 700 p.s.i. pressure and 90 F, temperature.

Propane refrigeration cycle In the high level propane refrigeration stage (P liquid propane refrigerant is maintained in a receiver 3 at about Patented Dec. 3, 11368 175 p.s.i. pressure and 90 F. This liquid propane is fed via conduit 5 through heat exchanger 7 wherein the temperature is lowered to about 25 F. The propane is then passed via conduit 9 and is split into two streams one of which passes via conduit 4 and the other of which passes via conduit 13. The flow of liquid propane in conduit 4 is controlled by an expansion valve wherein the valve is operatively connected to a temperature indicator controller 17. This controller 17 is operatively connected to a suitable thermocouple .19 located in conduit 9 which measures the temperature of the liquid propane flowing in conduit 9. In this example, the temperature is maintained at 25 F. and valve 15 is operatively controlled by the temperature indicator controller 17 to permit suflicient flow of liquid propane in conduit 4 to maintain the temperature in conduit 9 at about 25 F. Upon passage through valve 15, the pressure on the propane is reduced to about 55 p.s.i. whereupon part of the propane flashes resulting in a two-phase fluid being passed into heat exchanger 21.

In order to insure maximum efliciency when the heat exchange medium is a two-phase fluid, heat exchanger 21 is of a special construction which has previously been described in a Patent, 3,212,277, issued on Oct. 19, 1965, to E. A. Harper et al. and assigned to Phillips Petroleum Company.

The refrigerant exits heat exchanger 21 via conduit 24, passes through heat exchanger 7, compressor 23, cooler wherein it is returned to receiver 3 at a pressure and temperature from which it is started to repeat the high level propane cycle.

When the process stream is continuously advanced via conduit 1 through heat exchanger 21, heat will be extracted from the natural gas in amounts sufficient to reduce the temperature of the process stream from 90 F. to about 25 F. If the process stream were to be supplied at a temperature considerably below 90 F the described high level propane refrigeration step can be omitted in favor of a single heat exchange step through the low level propane refrigeration cycle.

In the low level refrigeration stage (P the liquid propane in conduit 13 passes through an economy heat exchanger 25, wherein the temperature of the stream is reduced from 25 F. to approximately 7 F. The liquid propane leaving heat exchanger 25 via conduit 27 is again subdivided such that a portion passes via conduit 33 through an expansion valve 29 wherein the pressure is let down to slightly above atmospheric pressure, such as 21 pounds p.s.i. for introduction into the low level propane heat exchanger 31 and a portion passes via conduit 35.

The construction of heat exchanger 31 is similar to that described hereinabove with relation to heat exchanger 21. At this pressure, the propane will be maintained at a temperature of about 28 F. and will be in a mixed gas-liquid phase as it passes through exchanger 31.

Again the flow of propane in conduit 33 is maintained by a temperature indicator controller 41 which measures the temperature in conduit 33 via thermocouple 34 and controls the flow through expansion valve 29 to maintain a predetermined temperature in conduit 33 which is about 7 F.

The remainder of the propane from line 27 is passed via conduit 35 through an expansion valve 37 wherein the pressure is let down to about the same pressure as that of the propane in conduit 33. This propane is passed through the heat exchanger 39 wherein recompressed vapors of ethylene in the high level ethylene refrigeration stage are liquefied for use in the ethylene refrigeration cycle.

A temperature controller 43 controls the operation of valve 37 in accordance with the temperature in conduit 45 as measured by thermocouple 44. This propane passes via conduit 45 and is combined with the propane refrigerant which exits from heat exchanger 31 via conduit 47.

The two phase propane fluid in line 47 is completely vaporized in heat exchangers 25 and 7 and is compressed in the low stage propane compressor 49 to raise the pressure to a level which corresponds to the pressure of the propane released from heat exchanger 21 (about p.s.i.) so that the two streams of propane vapor can be joined for subsequent processing through high stage compressor 23.

The high stage compressor raises the pressure of the propane vapors to a pressure level at which they can be readily condensed. The propane vapor is raised to about 175 pounds psi. for subsequent liquefaction and storage in receiver 3. Heat of compression in compressor 49 raises the temperature of the propane fluid in line 47 and it is passed through cooler 51 in order to bring its temperature down to about 90 F. wherein it can be joined with the propane fluid in line 24.

When the process stream is continuously advanced via conduit 1 through heat exchanger 31 heat will be extracted from the gas in amounts suflicient to reduce its temperature from 25 F. to about 23 F.

Ethylene refrigeration cycle The ethylene refrigeration cycle (E) is, in a manner similar to the propane cycle except that it is subdivided into a high level ethylene refrigeration stage (E an intermediate level ethylene, refrigeration stage (E and a low level ethylene refrigeration stage (B In the high level ethylene stage (E liquid ethylene maintained in a receiver 53, at 280 p.s.i.g. and 22 F. is passed via conduit 55 through a heat exchanger 57 wherein the temperature is further reduced to -48 F. The ethylene at this temperature exits from heat exchanger 57 via conduit 59 wherein it is subdivided into conduits 60 and 77.

The portion in conduit 60 passes through expansion valve 61 wherein the pressure is let down to about 107 pounds for introduction into the low level ethylene heat exchanger 63 which is similar in construction to

heat exchangers

21 and 31. At this pressure, the ethylene is a dual phase fluid as it enters heat exchanger 63.

Just as in the propane cycle, the flow rate of ethylene via conduit 60, is regulated by valve 61 whose operation is controlled by temperature controller 81. Temperature controller 81 receives a signal from thermocouple 83 indicative of the temperature in conduit 59. The flow rate via conduit 60 is kept at a rate suflicient to maintain the temperature in conduit 59 at 48" F. for this illustrative example.

The ethylene two phase fluid refrigerant exits from heat exchanger 63 via conduit 65 and passes through heat exchanger 57 wherein its temperature is increased to 27 F. At this point it is in the vapor phase. The ethylene refrigerant passes via conduit 65 through heat exchanger 67 wherein its temperature is further increased to F.

This ethylene vapor is compressed in compressor 69, passed via conduit 73 through a cooler 71 wherein the temperature is reduced to F., through heat exchanger 67 wherein the temperature is reduced to 2 F., through the propane heat exchanger 39 wherein the ethylene is liquefied by heat exchanging it with the propane in heat exchanger 39 from which it exits via conduit 75 to the ethylene receiver 53.

The process stream enters, via conduit 1, the low level ethylene heat exchanger 63 at about 23 F. and will leave at about --70 F. While still at about 700 p.s.i.

pressure.

The other portion of ethylene refrigerant in conduit 77 is passed through heat exchanger 79 wherein its temperature is dropped to 66 F.

The ethylene refrigerant exits heat exchanger 79 via conduit 85 for subdivision wherein a portion is fed into conduit 87 for passage to the intermediate ethylene refrigeration heat exchanger and the remaining portion is fed into conduit 103 from whence a portion is passed into the low level ethylene heat exchanger 113.

Temperature controller 91 in cooperation with thermocouple 93 controls expansion valve 89 in the same manner as described hereinabove with relation to the other cycles so as to maintain a temperature in conduit at 66 F. This temperature will permit the ethylene to enter heat exchanger as a two phase fluid at a pressure of 46 p.s.i.

The construction of this heat exchanger is also similar to the heat exchangers previously described hereinabove in order to insure maximum efiiciency which can be obtained from the two phase fluid.

The ethylene refrigerant exits from heat exchanger 95 via conduit 97 at a pressure of approximately 46 p.s.i. The ethylene refrigerant is passed via conduit 97 through heat exchanger 79 wherein its temperature is raised to 79 F., through heat exchanger 57 wherein its temperature is raised to 27 F. through heat exchanger 67 wherein its temperature is raised to 80 F. It is then passed to an intermediate level compressor 99 to raise the pressure to a level which corresponds to the pressure of the ethylene vapor released from the heat exchanger 63.

Upon leaving the compressor 99 the ethylene refrigerant is cooled in water cooler 101 in order to lower its temperature to that of the ethylene coming from heat exchanger 63 with which it is combined in conduit 65 for passage through compressor 69. When the natural gas is passed continuously through the intermediate ethylene heat exchanger 95, its temperature will be reduced from 70 F. to lll F. while still at about 700 p.s.i. pressure.

The other portion of the ethylene refrigerant in conduit 103 is passed to a heat exchanger 105 wherein its temperature is reduced to approximately -77 F.

The liquefied ethylene refrigerant leaves heat exchanger 105 via conduit 107 for subdivision wherein a portion is fed via conduit 109 to an expansion valve 111 wherein the pressure is let down to slightly above atmospheric pressure, such as 22 pounds p.s.i. whereby the ethylene is introduced into the low level ethylene heat exchanger 113 as a two phase fluid. This heat exchanger is of the same construction as described hereinabove.

Just as stated hereinabove, the flow in conduit 109 is controlled by a temperature controller 115 which is operatively connected to a thermocouple 117 positioned in conduit 107 for measuring the temperature therein. The temperature controller is regulated to maintain a predetermined temperature of the ethylene refrigerant in conduit 107 77 F.) so that when it passes through valve 111 it will only be partially vaporized.

The ethylene refrigerant exits from the low level ethylone heat exchanger 113 via conduit 119 and passes through heat exchanger 105 wherein its temperature is raised from 143 F. to 116" B, through heat exchanger 79 wherein its temperature is raised to 79 F., through heat exchanger 57 wherein its temperature is raised to --27 F. and finally through heat exchanger 67 wherein its temperature is raised to 80 F. The vaporized ethylene is then passed through low level compressor 121 to raise the pressure to a level which corresponds to the pressure of the ethylene refrigerant at the intermediate level.

The ethylene refrigerant exits from compressor 121 and passes through water cooler 123 in order to lower its temperature to approximately that of the ethylene which exited from intermediate heat exchanger 95 with which it is combined for passage into intermediate level compressor 99.

The ethylene refrigerant in conduit 107 also passes via conduit 125 wherein it passes through an expansion valve 127 and then through a methaneliquefaction heat exchanger 129 (shown in FIGURE lb).

Expansion valve 127 is operatively controlled by temperature regulator 133 and thermocouple 131 located in conduit 125. The flow of ethylene through valve 127 is maintained at a rate sufiicient to keep the temperature of the ethylene in conduit 125 the same as that entering heat exchange 113 via conduit 109. This temperature is 143 F.

The ethylene is passed via conduit 125 and combined with the ethylene in conduit 119.

The process stream advancing continuously via conduit 1 enters heat exchanger 113 at a temperature of -111 F. and leaves at a temperature of l38 F. for entry into the methane refrigeration cycles.

Methane refrigeration cycle FIGURE 1B In the methane refrigeration cycle, which is subdivided into a high level methane refrigeration stage (M an intermediate level methane refrigeration stage (M and a low level methane refrigeration stage (M the methane is stored in its liquefied stage in receiver 135 at a temperature of 138 F. and a pressure of 485 p.s.i. The liquefied methane passes from receiver 135 via conduit 137 through the heat exchanger 139 and exits therefrom via conduit 141 at a temperature of -F. wherein a portion passes into conduit 143 for passage into the low level methane heat exchanger 147 and the remaining portion passes into conduit 159.

Heat exchanger 147 is of the same construction as the heat exchanger used in the propane and ethylene cycles.

The flow rate of methane in conduit 143 is regulated by temperature controller 115 in accordance with a signal received from thermocouple 157 located in conduit 141. This temperature is maintained at a level suthcient to produce a two phase fluid in conduit 143 after the pressure has been dropped in valve 145. This temperature is 155 F. and the pressure is 200 p.s.i.

The methane refrigerant exits from the high level methane heat exchanger 147 via conduit 149 wherein it is passed through heat exchanger 139, wherein its temperature is raised from 179 F. to 143 F. and heat exchanger 147, wherein its temperature is raised to approximately 80 F. It then goes through compressor 150, wherein the pressure is raised to 485 p.s.i., and exits via conduit 153. The methane is passed through water cooler 151 to remove heat of compression from the methane.

The methane is passed via conduit 153, through heat exchanger 147 wherein its temperature is lowered to --139 F. and is then passed in heat exchange with the ethylene in the ethylene cycle in heat exchanger 129 to liquify the methane for refrigeration at 130 F. and 485 p.s.i.

The natural gas in conduit 1 enters the high level methane heat exchanger 147 at a temperature of 138 F. and leaves at a temperature of 174 F.

The liquid methane in conduit 159 passes through heat exchanger 161 wherein its temperature is lowered to 175 F. The liquid methane exits from heat exchanger 161 via conduit 163. A portion of the methane is passed via conduit 165 to the intermediate level methane heat exchanger 169, and the remaining liquid methane is passed to conduit 183.

The methane in conduit 165 passes through expansion valve 167, wherein the pressure is dropped to 81 p.s.i. and a portion of the methane is flashed, to form a two phase fluid which enters the intermediate level methane heat exchanger 169 which is constructed like the previous heat exchanger.

Temperature controller 179 is operatively connected to valve 167 in order to regulate it in response to a signal received from thermocouple 181 located in conduit 163. The flow rate is maintained sufiicient in conduit 165 to keep a temperature of F. in conduit 163.

The methane refrigerant exits from heat exchanger 169 via conduit 171 for passage through heat exchanger 161, wherein the temperature is raised from 213 F. to l79 F., heat exchanger 139, wherein the temperature is raised from 179 F. to 143 F., and heat exchanger 147, wherein the temperature is raised from 143" F. up to 80 F. The methane is then passed through compressor 173 from the heat exchanger 147. The methane is then passed through water cooler 172 wherein it is combined with the methane in conduit .149 for passage into compressor 150.

The natural gas enters heat exchanger 169 at a temperature of l74 F. and exits at a temperature of 208 F.

The methane in conduit 183 passes through a heat exchanger 185 wherein its temperature is reduced to -187 F. The methane exits from heat exchanger 185 via conduit 187 and is fed through an expansion valve 189 wherein the pressure is let down to about 36 psi. for entry into the low level methane heat exchanger 191.

Expansion valve 189 is operatively connected to temperature controller 201 which regulates the flow of methane in conduit 187 in accordance with a signal received from thermocouple 203 located in conduit 187. The predetermined temperature desired in the example is 187 F.

The methane enters heat exchanger 191 as a two phase fluid and the construction of this heat exchanger is identical to those described hereinabove.

The methane exists from the heat exchanger 191 via conduit 193, passes through heat exchanger 185, wherein its temperature is raised to -2l3 F., heat exchanger 161 wherein its temperature is raised to -179 F., heat exchanger 139, wherein its temperature is raised to 143 F. and heat exchanger 147 wherein its temperature is raised to 80 F. This low level methane is then fed through compressor 195 wherein it is compressed to increase its pressure to that of the methane exiting the intermediate methane heat exchanger in conduit 171. The methane exits from compressor 195 via conduit 197, passes through water cooler 199, and is reunited with the methane from the heat exchanger 169 in conduit 171.

The process stream flowing in conduit 1 enters heat exchanger 191 at a temperature of 208 F. and exits at a temperature of -230 F. The natural gas passes through valve 203 into a flash tank 205 wherein any uncondensed light material flashes off via conduit 207 and passes through heat exchanger 147 wherein the temperature of this light material is raised to 80 F. for passage through compressor 209. The compressed gas is then burned as fuel.

The liquefied natural gas is removed from flash tank 205 and sent to storage via conduit 211.

FIGURE 2 describes another embodiment of the control system for operating the high level propane refrigeration portion of the cascade system. In the interest of brevity only this one cycle is shown; however, this aspect of the control system can be employed to each or all of the other refrigeration subdivisions. In this concept temperature controller 17 of FIGURE 1 is connected to expansion valve where it is positioned in a bypass line 217, which bypasses heat exchanger 21 and connects with exit conduit 24. The flow rate through bypass line 217 is maintained at a rate suflicient to maintain a predetermined temperature in line 9 just as was described in regard to FIGURE 1. A second expansion valve, positioned upstream from the bypass line 217 is operatively connected to a second temperature regulator 212 which is responsive to a signal received from thermocouple 214 located in process line 1 downstream from heat exchanger 21.

In this embodiment the flow of propane through the heat exchanger is controlled by the degree of cooling desired to be accomplished on the natural gas in the heat exchanger. Temperature recorder maintains a flow rate of propane through bypass line 217 suflicient to maintain a predetermined temperature in conduit 9.

It will be understood that changes may be made in the details of construction, arrangement and operation without departing from the spirit of the invention, as defined in the appended claims.

What is claimed is:

1. The method of cooling a gas by indirect heat exchange with a cold refrigerant which comprises expanding the cold refrigerant to provide a mixed vapor-liquid refrigerant stream, passing said mixed vapor-liquid refrigerant stream in indirect heat exchange relation with the gas to cool said gas, passing the heat exchanged refrigerant in further indirect heat exchange relation with compressed and condensed refrigerant to cool said compressed and condensed refrigerant and thereby provide said cold refrigerant and thereby vaporizing the liquid portion of said heat exchanged refrigerant, thereafter compressing and condensing said heat exchanged refrigerant to provide said compressed and condensed retrigerant, and regulating the expanding. of said cold refrigerant in response to the temperature of said cold refrigerant prior to the expansion thereof and subsequent to the cooling of said compressed and condensedrefrigerant in order that said expanding results in a mixed vapor-liquid state of said cold refrigerant.

2. A method according to claim 1 further including separating a portion of the cold refrigerant prior to the expansion thereof to provide a second cold refrigerant stream, expanding said second cold refrigerant stream to provide a second mixed vapor-liquid stream, passing said second mixed vapor-liquid stream in indirect heat exchange with the cooled gas to further cool said gas, passing the second heat exchanged refrigerant stream in further indirect heat exchange with said second cold refrigerant stream to further cool said second cold refrigerant stream prior to the expansion thereof, thereafter passing the second heat exchanged stream in indirect heat exchange with said compressed and condensed refrigerant to provide said cold refrigerant thereby vaporizing the liquid portion of said second heat exchanged stream, thereafter compressing and condensing said second heat exchanged stream to provide said compressed and condensed refrigerant; and regulating the expanding of said second cold refrigerant stream at a preselected value in response to the temperature of said second cold refrigerant stream prior to the expansions thereof and subsequent to the further heat exchange of said second heat exchanged refrigerant stream with said second cold refrigerant stream in order that said expanding results in a mixed vapor-liquid state of second cold refrigerant stream.

3. A method according to claim 2 further including the incremental reduction in the temperature of said gas utilizing a series of separate progressively decreasing temperature level refrigerants, each of which is utilized as said refrigerant according to claim 2, and wherein said gas is cooled to a temperature sufficient to transport said gas in a liquid state at atmospheric pressure.

4. A method according to claim 3 wherein said refrigerants are propane, ethylene, and methane, and said gas is natural gas.

5. The method of cooling a gas by indirect heat exchange with a cold refrigerant which comprises expanding the cold refrigerant to provide a mixed vapor-liquid refrigerant stream, passing said mixed vapor-liquid refrigerant stream in indirect heat exchange relation with the gas to cool said gas, passing the heat exchanged refrigerant in further indirect heat exchange relation with compressed and condensed refrigerant to cool said compressed and condensed refrigerant and thereby provide said cold refrigerant and thereby vaporizing the liquid portion of said further heat exchanged refrigerant, thereafter compressing and condensing said further heat exchanged refrigerant to provide said compressed and condensed refrigerant, and regulating the temperature of said cold refrigerant by passing a portion of the cold refrigerant prior to expansion to the corresponding heat exchanged refrigerant thereby bypassing a portion of the said cold refrigerant around the indirect heat exchange relation with said gas, and regulating the flow in the bypass in response to the temperature of said cold refrigerant prior to the expansion thereof and subsequent to the cooling of said compressed and condensed refrigerant in order that the said expanding results in a mixed vapor-liquid state of the cold refrigerant.

6. The method of cooling a gas by indirect heat exchange wherein the incremental reduction in the temperature of said gas is accomplished by a series of separately progressively decreasing temperature level cold refrigerants, wherein each of said refrigerants is utilized to reduce the temperature of said gas by the steps of expanding the cold refrigerant to provide a mixed vaporliquid refrigerant stream, passing said mixed vapor-liquid refrigerant stream in indirect heat exchange relation with the gas to cool said gas, passing the heat exchanged refrigerant in further indirect heat exchange relation with compressed and condensed refrigerant to cool said compressed and condensed refrigerant and thereby provide said cold refrigerant and thereby vaporizing a liquid portion of said heat exchanged refrigerant, thereafter compressing and condensing said heat exchanged refrigerant to provide said compressed and condensed refrigerant, and regulating the expanding of said cold refrigerant in response to the temperature of said cold refrigerant prior to the expansion thereof and subsequent to the cooling of said compressed and condensed refrigerant in order that said expanding results in a mixed vapor-liquid state of said cold refrigerant.

References Cited UNITED STATES PATENTS 2,960,837 11/1960 Swenson et a1. 6240 XR 3,020,723 2/1962 DeLury et al 62-40 XR 3,066,492 12/1962 Grumberg et al. 62-9 3,228,860 1/1966 Larson 62-21 XR 3,254,495 6/ 1966 Jackson et al 62-40 XR 3,315,477 4/1967 Carr 6240 XR 3,318,102 5/1967 Henderson 6240 XR 3,237,416 3/1966 Seddon 6240 XR 3,264,837 8/1966 Hamish 62-513 XR 3,350,896 11/1967 Harnish 62200 NORMAN YUDKOFF, Primary Examiner.

W. PRETKA, Assistant Examiner.