US2896414A - Methane liquefaction cycle - Google Patents

Description

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SHAO E. TUNG METHANE LIQUEFACTION CYCLE Jury 28, 1959 Filed Sept. 12, 1955 Qn 2e@ m 5.308 RE SS Mbbwm Sr u H 2G @Cw L:

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coMPlzEsseo Fon FunrHsnuQI/I I-ncv-l 0N.) ATTORNEYS United States Patent() 2,896,414 METHANE LIQUEFACTION CYCUE Shao E. Tung, Ponca City, Okla., assignor to Constock Liquid Methane Corporation, New York, N.Y., a 'corporation of Delaware Application September 12, 195'5, SerialNo. 533,713 1s claims. (C1. sz-12) This invention relates as indicated to the liquefaction of natural 4gas and more especially to a process by which such results can be accomplished ata maximum of etliciency.

Other objects of the invention will appear as the description proceeds.

To the accomplishment of the foregoing and related ends, the invention, then, comprises the features hereinafter fully described and particularly pointed out in the claims, the following description and the annexed drawings setting forth in detail certain illustrative embodiments of the invention, these being indicative, however, of but a few of .the various ways in which the principle of the invention may be employed.

In said annexed drawings:

Fig. 1 is a flow sheet showing the first stage and half of the intermediate stage of the liquefaction process;

Fig. 1A is a continuation at the bottom of the ilow sheet of Fig. 1 and comprises the other half of said intermediate stage and the nal stage of the liquefaction process; and

Fig. 1B is a modification of that portion of the liquefaction process shown in Fig. 1A.

Broadly stated, this invention comprises the method of continuously liquefying a stream of natural gas which is available at a pressure substantially in excess of the critical pressure thereof, which comprises a multistage temperature reduction of such stream including the following:

(a) an initial stage within which such temperature reduction is effected by having the stream in the gaseous state and by a reduction of its pressure, perform work;

(b) a final stage within which the stream substantially entirely in liquid form and at superatmospheric pressure, by an evaporation of a portion thereof, has the pressure thereof reduced to :substantially atmospheric and the temperature of the liquid residue reduced to its boiling point at such pressure; and

(c) an intermediate stage within which the stream, between said initial and nal stages, is substantially liqueed without substantial reduction in pressure thereof, by the combined refrigeration effect of external refrigeration and that portion of the stream vaporized in said nal stage. f

In the initial stage of the process, referring to Fig. l, well gas is taken at well head pressures, cooled to 90 F. by the water cooler 1, and then sent through the turboexpander 2 to do external work while reducing its pressure to 550 p.s.i.a. At this point the acid gas (carbon dioxide and hydrodisuliide) is taken out of the stream in a gas cleaner A3.. By only reducing the Ygas pressure to 550 p.s.i.a. ythere is no danger of the carbon dioxide solidifying in the turbo-expander, and the process for the extraction of the acid gas at this pressure level lower than 1000 p.s.i.a. has been commercially established.

The intermediate stage of the process begins essentially after the acid `gas cleaner. From here the natural gas ;llows in separate heat exchangers, rst with a propane ice cycle in exchanger B and secondly with an ethylene cycle in exchanger C. Also, taking heat from lthe natural gas in these exchangers is cold natural gas on its way to be recycled entering the stream after the initial stage. By the cold supplied by the natural gas to be recycled and by the two other gases used in the heat exchange cascade,

I am able to liquefy all the natural gas at a temperature of -140 F. and a pressure of about 550 p.s.i.a. in exchanger C, except for a slight pressure drop due to flow, no appreciable decrease in pressure having taken place.

Referring now to Fig. 1A, the liquid is sub-cooled to a temperature of about F. in exchanger D in heat exchange with only cold natural gas on its way to be recycled.

In the final stage, the liquid is passed through throttle valves 4, 5, and 6, each of which vaporizes some of the liquid and thus reduces the temperature and pressure of the stream including that o-f the remaining liquid in each of the liquid storage tanks P, Q, and R which lie in succession between the throttle valves. The temperature and pressure in the last liquid storage tank R. are -242 F. and` 30 p.s.i.a., respectively. This leaves the liquid under enough pressure to send it to a stationary or movable liquid storage tank at atmospheric pressure from which ,the #gas generated in final storage can, after yielding its available cold to the incoming high pressure stream, either be -used as fuel or recompressed for further liquefaction. The gas in storage tanks P, Q, and R which was generated in lvaporization by the throttle valves 4l, 5, and 6 is passed back into heat exchangers D, C, B, and A in the second stage to be used as a coolant as it warms up. After passing through the heat exchangers, referring again to Fig. l, 4the 'gas in each of the lines 7, 8, and 9 is still substantially at the respective pressure of the tank from which it carne, 30 p.s.i.a., 105 p.s.i.a., land 330 p.s.i.a. respectively, since the pressure drops across the heat exchangers are relatively `low Any inert gas included in the well gas will not liquefy in the heat exchanger D but will appear, except for those dissolved in liquid methane, in the gas phase of the storage P. In the ideal case, the re-evaporation of methane in passing through throttle valve 4 should be kept at the lowest possible value. Some re-evaporation, however, is here allowed to insure smooth operation. 'Ihe methane so re-evaporated is used as fuel. The pressure energy of Vthis fuel stream 9 may be recovered if desired.

The gas in lines 7 and 8 has to be recompressed in order to be recycled. In compressor 10, the gas in line 7 is brought to the pressures of the gas in line 3. Then all of the gas from both lines is recompressed in compresser 11 to the pressure of the natural gas that is leaving the initial stage. After compression, the returning gas is cooled in the water cooler 12 and then refrigerated in exchanger A. From there it enters the original stream following the initial stage.

Fig. 1B is a modification of Fig. 1A in that heat exchanger D is divided into three separate heat exchangers E, F, and G. In this design, the coldest gas to be recycled is first warmed to the temperature of the second coldest gas by means of a separate heat exchanger G, and then these two gases are warmed to the temperature of the warmest gas in another separate heat exchanger F. Finally, all three are warmed at once in the heat exchanger E, the rest of the process being the same.

In Fig. l, both gas cycles of the refrigerating cascade are shown, and they are so arranged to achieve the greatest possible efliciency. In the first of the cycles, propane is compressed in compressor 13 and then cooled and condensed in the water color 14. Then, before it is sent through the throttle valve 16, it is :sub-cooled by heat `exchange with the ethylene cycle in the propane coolerwlS. After passing through the throttle Valve 16,

.ethylene'condensor 17vfrom which it too is returned to thecompressor 13 and recycled.

In the ethylene cycle the ethylene is compressed at compressor 18 and then cooled in the water cooler 19 before being condensed by heat exchange with the propane cycle in the ethylene condensor 17. Following the conde nsor, the ethylene is sent through the throttle valve 20 which lreduces its pressure to a pressure slightly higher than atmospheric and its temperature to about -l52 F. The ethylene then refrigerates the natural gas in the heat exchanger C from which it goes through the propane cooler 15 and is then recompressed and recycled. Ethylene'is used because of its low boiling point characteristic. Many other refrigerants such as ammonia, propylene, Freon 22, Freon l, 2, etc. may be used in place of propane. Ethane may be used, although with some sacrifice in cycle efliciency, in place of ethylene.

Pressures slightly Vhigher than atmospheric are used here in both propane and ethylene cycles after the throttle valves 16 and 20, respectively. Subatmospheric pressures may be used if desired.

There is a desirable pressure range to which the natural gas component of the stream should be reduced in the initial stage. In order to be able to liquefy the natural gas at heat exchanger C by liquid ethylene at pressure slightly higher than atmospheric, the pressure after the initial stage cannot be lower than 450 p.s.i.a.; and, in order to have the condensation taking place in the second exchanger which is helpful from heat transfer standpoint, it is beneficial to keep the pressure after the initial stage below the critical pressure of the gas which is 678 p.s.i.a. for pure methane. However, operation above the critical point of the gas is not excluded. The

desired pressure of the natural gas component of 550 p.s.i.a. is here selected. But this means that the .total pressure of the gas following the turbo-expander 2 and the compressor 11, will vary somewhat above 550 p.s.i.a. depending on the percentage content of inert gas in the natural gas stream.

I claim: l

l. The method of continuously liquefying a stream of natural gas formed mostly of methane and which is available at a pressure substantially in excess of the critical pressure thereof, which comprises a multi-stage temperature reduction of such stream including the following: (a) in an initial stage, expanding the process stream with work to an intermediate pressure with corresponding reduction in temperature; (b) in an intermediate stage within which the process stream is substantially liquefied, refrigerating the cooled and expanded process stream from the initial stage to liquefy substantially all of the gas in the process stream without substantial reduction in pressure; and (c) in a nal stage within which the process stream is substantially entirely in liquid form at super-atmospheric pressure, comprising reducing the pressure of the liquefied gas to substantially atmospheric pressure with consequent reduction of the temperature of the liquid to its boiling point at such the critical pressure of the principal gaseous component of the stream or slightly above.

4. The method of claim l characterized further in that a substantial portion of the gas boiled off in said iinal stage is recycled to enter the stream following said initial stage. l a

5. The method of claim 1 characterizedpfurther in that in said nal stage the pressure on said stream isl reduced stepwise with each such pressure reduction accompanied by a vaporization of a portion of said stream, the vaporized portion in the iirst such step after being used as a refrigerant in said intermediate stage, being discharged from the cycle.

6. The method of claim 1 characterized further in that acid gas is removed from said stream following said initial stage, and in said final stage the pressure on said stream is reducedstepwise with each such pressure reduction accompanied by a vaporization of a portion of said stream,

the vaporized portion in the first such step after being used as a refrigerant in said intermediate stage being discharged from the cycle.

7. The method of claim 1 characterized further in that in said initial stage the pressure on said stream is reduced to within the range of from about 450 p.s.i.a. to the critical pressure of the principal gaseous component o'f the stream or slightly above and in said nal stage the pressure on said stream is reduced incrementally.

8. The method of claim l characterized further in that in said nal stage the pressure on said stream is reduced incrementally, and a substantial portion of the gas boiled off in said final stage is recycled to enter the stream following said initial stage.

9. The method of claim l characterized further in that in said initial stage the pressure on said stream is reduced to within the range of from about 45 0 p.s.i.a. to the critical pressure of the principal gaseous component of the stream or slightly above, and a substantial portion of the gas boiled off in said iinal stage is recycled to enter the stream following said initial stage.

l0. The method of claim l characterized further in that in said initial stage the pressure on said stream is reduced to within the range of from about 450 p.s.i.a. to the critical pressure of the principal gaseous component of the stream or slightly above, and in said final stage the pressure on said stream is reduced stepwise with each such pressure reduction accompanied by a vaporization of a portion of said stream, the vaporized portion in the first such step after being used as a refrigerant in said intermediate stage Vbeing discharged from kthe cycle.

11. The method of claim l characterized further in that in said nal stage the pressure on said stream is rei duced stepwise with each such pressure reduction accompanied by a vaporization of a portion of said stream, the

vaporized portion in the rst such step after being used as a refrigerant in said intermediate step being discharged from the cycle, the remainder of the vaporized gas being recycled to enter the stream following said initial stage.

pressure and with vaporization of Va portion thereof as 3. The method of claiml characterized further in that inrsaid initial stage the pressure on said stream'is reduced to within the range of from about 450 p.s.i.a. to

l2. The method of claim l characterized further in that in said initial stage the pressure on said stream is reduced to within the range of from about 450 p.s.i.a. to the critical pressure of the principal gaseous component of Ythe stream or slightly above, in said final stage the pressure on said stream is reduced incrementally, and a substantial portion of the gas boiled off in said iinal stage is recycled to enter the stream following said initial stage.

13. The method of claim l characterized further in that in said initial stage the pressure on said stream is reduced to Within the range of from about 450 p.s.i.a. to the critical pressure of the principal gaseous component of the stream or. slightly above, and in said final stage the pressure on said stream is reduced stepwise with each such pressure reduction accompaniedkby a vaporization of a portion of said stream, the vaporized portion in the rst such step after being used as a refrigerant in said intermediate stage being discharged from the cycle.

14. The method of claim l characterized further in that in said initial stage the pressure on said stream is reduced to within the range of from about 450 p.s.i.a. to the critical pressure of the principal gaseous component of the stream or slightly above, and in said final stage the pressure on said stream is reduced stepwise with each such pressure reduction accompanied by a vaporization of a portion of said stream, the vaporized portion in the first such step after being used as a refrigerant in said intermediate step being discharged from the cycle, the remainder of the vaporized gas being recycled to enter the stream following said initial stage.

15. The method of claim 1 characterized further in that in said initial stage the pressure on said stream is reduced to within the range of from about 450 p.s.i.a. to the critical pressure of the principal gaseous component of the stream or slightly above, acid gas is removed from said stream following said initial stage, and in said final stage the pressure on said stream is reduced stepwise with each such pressure reduction accompanied by a vaporization of a portion of said stream, the vaporized portion in the rst such step after being used as a refrigerant in said intermediate stage being discharged from the cycle the remainder of the vaporized gas being recycled to enter the stream following said initial stage.

16. The method of claim l characterized further in that acid gas is removed from said stream following said initial stage, and in said final stage the pressure on said stream is reduced stepwise with each such pressure reduction accompanied by a vaporization of a portion of said stream, the vaporized portion in the first such step after being used as a refrigerant in said intermediate step being discharged from the cycle, the remainder of the vaporized gas being recycled to enter the stream following said initial stage.

17. The method of continuously liquefying a stream of natural gas composed mostly of C1 hydrocarbons and including some lower boiling nonhydrocarbon gases which comprises the following stages:

(a) progressively reducting the temperature of said stream of natural gas to reduce the stream including the methane to a liquid state at superatmospheric pressure;

(b) then progressively reducing the liquefied stream of natural gas to substantially atmospheric pressure with consequent flashing of a portion of the liquefied gas to vapor;

(c) venting from the cycle the gas vaporized in the first stage of pressure reduction of said liquefied stream of natural gas containing most of the lower boiling non hydrocarbons; l

(d) recycling all of the remaining gas vaporized from said liquefied stream as its pressure is reduced; and

(e) passing said remaining vaporized gas in indirect heat exchange relation with said stream of natural gas in stage (a) for reducing the temperature of said stream.

18. The method of continuously liquefying a stream of natural gas composed mostly of C1 hydrocarbons and iucluding some lower boiling non-hydrocarbon gases which comprises the following stages:

(a) progressively reducing the temperature of said stream of natural gas to reduce the stream including the methane to a liquid state at superatmospheric pressure;

(b) then progressively reducing the liquefied stream of natural gas to substantially atmospheric pressure with consequent dashing of a portion of the liquefied gas to vapor;

(c) venting from the cycle the gas vaporized in the r rst stage of pressure reduction to remove most of the lower boiling non-hydrocarbon gases from the stream after passing said vaporized gas in indirect heat exchange with said stream of natural gas in stage (a).

References Cited in the file of this patent UNITED STATES PATENTS 2,258,015 Keith et al. Oct. 7, 1941 2,265,527 Hill Dec. 9, 1941 2,309,075 Hill Jan. 19, 1943'y

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