US2583090A

US2583090A - Separation of natural gas mixtures

Info

Publication number: US2583090A
Application number: US203412A
Authority: US
Inventors: Joe L Cost
Original assignee: Elliott Co
Current assignee: Elliott Co
Priority date: 1950-12-29
Filing date: 1950-12-29
Publication date: 1952-01-22
Anticipated expiration: 1969-01-22

Description

Jan. 22, 1952 I J 051- 2,583,090

SEPARATION OF NATURAL GAS MIXTURES Filed Dec. 29, 1950 ,5 Sheets-Sheet 1 IN VEN TOR.

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Jan. 22, 1952 J. L. COST 2,583,090

' SEPARATION OF NATURAL GAS MIXTURES Filed Dec. 29, 1950 5 Sheets-Sheet 2 IN VEN TOR.

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SEPARATION OF NATURAL GAS MIXTURES Filed Dec. 29, 1950 s Sheets-Sheet 3 Patented Jan. 22, 1952 SEPARATION OF NATURAL GAS MIXTURES Joe L. Cost, Madeira, Ohio, assignor to Elliott Company, Jeannette, Pa., a corporation of Pennsylvania Application December 29, 1950, Serial No. 203,412

9 Claims. 1

This invention relates to the separation of natural gas mixtures by rectification at low temperature, and more particularly, to the separation of mixtures composed substantially of nitrogen and methane to recover the latter as a gas.

It is among the objects of this invention to provide a method and apparatus for effecting the separation above described, in which there will be available suilicient refrigeration at low temperatures to assure a high yield of methane product with a minimum expenditure of power in producing that refrigeration and with a minimum amount of equipment to utilize it.

It is a further object of the invention to separate nitrogen-methane mixtures and recover a gaseous methane product therefrom at a substantial pressure for delivery to gas distribution mains, or to storage, with little or no additional compression of that product.

In accordance with this invention, natural gas of the indicated composition is compressed to a very high pressure; and it is then successively cooled to a low temperature, expanded to an intermediate pressure, and rectified at that pressure to separate its contained nitrogen and methane. The cold nitrogen so obtained is cooled and partially liquefied, the liquefied portion providing the liquid reflux necessary to assure a high degree of rectification and, thereby, a high yield of separated methane, while the unliquefied portion provides the low temperature refrigeration necessary for such partial liquefaction. That refrigeration is made available by expanding the unliquefied portion of the separated nitrogen in a plurality of stages to substantially atmospheric pressure with the performance of external work, and using the cooled nitrogen resulting from each expansion to absorb heat from the unexpanded nitrogen.

The invention is illustrated in the accompanying drawings, in which Fig. 1 shows diagrammatically one embodiment thereof Fig. 2, a modification, in which the unliquefied portion of the separated nitrogen is compressed between successive expansions; and Fig. 3, a modification of Fig. 2.

Referring to Fig. 1, natural gas containing, for example, about forty per cent nitrogen and sixty per cent methane is compressed in a multi-stage compressor I, provided with the usual aftercoolers, to a pressure of around 200 atmospheres. The compressed mixture at a temperature 01' about 100 F. is passed through a countercurrent heat exchanger 2 and is there cooled by the outgoing cold separation products to a temperature of about -125 F. It is further cooled to about -l85 F. and partially liquefied in a reboiler 3 at the bottom of a rectifying column 4, where the mixture gives up heat to liquid methane surrounding the reboiler. The mixture is further liquefied and subcooled in a second heat exchanger 6 by giving up heat to cold separated nitrogen, as hereinafter described. After leaving exchanger 6, the mixture passes through a throttle valve 1, where it is expanded to an intermediate pressure of about 130 p. s. i. a. and is thereby cooled to a still lower temperature. It is then introduced into the rectifying column 4 at an intermediate level and there rectified in the usual manner.

The cold gaseous nitrogen that has been separated from the mixture by rectification leaves the top of the column through a pipe 8 at a temperature of about -245 F. and at the intermediate pressure prevailing in the column. This cold nitrogen passes through a liquefier 9, where it is cooled and partially liquefied, as hereinafter described. The resulting liquid is separated from the unliquefied portion in a separator ll, withdrawn from the separator through a pipe I2, and introduced into the top of the column as liquid reflux at a temperature of about 253 F. The unliquefied portion is, in turn, withdrawn from separator l l by a pipe l4 and passes through exchanger 6, where it is warmed to about 210 F. in further cooling the incoming compressed mixture. Part of the latent refrigeration in this warmed nitrogen is made available by passing the gas through an expander l6, where it is expanded with performance of external work to a lower intermediate pressure of about 45' p. s. i. a. and is thereby cooled to a lower temperature than before. The partially expanded nitrogen is then conducted back to liquefier 9, where it acts as a cooling agent in effecting the partial liquefaction of the nitrogen stream from the rectifier, and is thereby rewarmed. Since the rewarmed nitrogen is still under pressure, its remaining latent refrigeration is made available by passing it through a second expander ll, where it is expanded to substantially atmospheric pressure with performance of external work, and thereby once more recooled to a low temperature. The recooled nitrogen is conducted back to liquefier 9 as a second cooling agent therein. After being rewarmed in the liquefier, the temperature of this low pressure nitrogen is still sufficiently low to permit its being used as a cooling agent in exchanger 6, and it is conducted to that exchanger by a pipe I8 and 3 there further warmed to a temperature of about -189 F. The warmed nitrogen is then conducted to exchanger 2 to effect part of the initial cooling of the entering mixture, after which it is discharged as waste through a pipe l9 at a temperature of about 46 F.

In rectifying the mixture, some of the methane that is separated accumulates as a liquid at the bottom of the column around the reboiler 3. The warmer mixture entering the reboiler from exchanger 2 causes this liquid methane to vaporize, forming the vapor reflux necessary for rectification. The gaseous methane product that is sepa-- rated from the mixture is withdrawn from the column through a pipe 2| at a temperature of around l9'7 F. and is conducted to exchanger 2, where it helps to cool the incoming mixture. At the warm end of that exchanger, the methane product is discharged through a pipe 22 at a temperature of about 46 F. and at a pressure of about 125 p. s. i. a. and may then be delivered to gas mains or to storage, as desired, with little or no additional compression.

The embodiment of this invention shown in Fig. 1 ismost suitable where the natural gas to be separated has a fairly high proportion of nitrogen, on the order of 40 per cent or higher. If the nitrogen content is lower, for example, around 30 per cent, there will not be sufficient nitrogen available in the open type cycle just described to produce the refrigeration and liquid reflux necessary for rigorous rectification of the mixture, so that a substantial proportion of the methane in the mixture would be carried off by the waste nitrogen. This difficulty is avoided by using the modified apparatus shown in Figs. 2 and 3, in which some of the separated nitrogen is recycled to provide, in effect, a closed external refrigeration cycle.

Referring to Fig. 2, in which apparatus similar to that in Fig. 1 is similarly numbered, the entering mixture, here assumed to consist of 30 per cent nitrogen and '10 per cent methane, is compressed, cooled, throttled and rectified in the same apparatus and in the same sequence as in Fig. l. The rectification, however, is preferably carried out at a slightly higher pressure (about 160 p. s. i. a.), and at a correspondingly higher temperature, than in Fig. 1. Again the separated nitrogen is cooled and partially liquefied by expanding the unliquefied portion in two stages and utilizing the refrigeration obtained from each expansion. The supply of cold nitrogen available for this purpose is here not limited (as it is in Fig. 1) to the vapor withdrawn from separator II, but may be augmented as desired by recycling a portion of the nitrogen leaving the warm end of exchanger 6. The recycled nitrogen is conducted through a pipe 3| communicating with the warm nitrogen outlet of that exchanger to a heat exchanger 32, where it is warmed. It is then compressed in a compressor 33 to a pressure higher than the rectification pressure in column 4, and recooled in exchanger 32 by thenitrogen from pipe 3|. It is further cooled to the temperature of the unliquefied nitrogen withdrawn from separator H by expanding it with performance of external work in an expander 34 to a pressure of about 160 p. s. i. a. This expanded recycled nitrogen is then added by a pipe 36 to the supply of cold nitrogen at substantially the same temperature and pressure withdrawn from the separator by pipe 31. As a result of this increased supply of cold nitrogen under pressure, the available refrigeration for producing liquid reflux by successive expansions of nitrogen in expanders l8 and I1 is increased sufficiently to assure eflicient rectification of the mixture to be separated.

In Fig. 3, the apparatus is designed, like that in Fig. 2, to provide sufficient liquid reflux for the separation of natural gas mixtures containing relatively low proportions of nitrogen, for example, around thirty per cent or lower. The arrangement differs from that shown in Fig. 2 in that the nitrogen that is expanded in expander l3 and then warmed in liquefier 9 is still further warmed before it is again expanded and reintroduced into the liquefier. Another difference is that the nitrogen to be recycled is withdrawn from the system at a lower temperature than is the case in Fig. 2, so that it need not be expanded before it is added to the cold nitrogen withdrawn from the separator and, accordingly, need not be compressed to as high a pressure as in Fig. 2.

Referring to Fig. 3 in detail, the entering mixture is cooled, throttled, and rectified as previously described in conection with Fig. 2. The separated nitrogen, at a pressure of about 160 p. s. i. a., is withdrawn from the column, cooled, and partially liquefied, as before. The unliquefled portion is then partly warmed in exchanger 6 and expanded to a lower intermediate pressure in expander l6, and the resulting refrigeration given up in liquefier 9, as before. This nitrogen after being warmed in that liquefier, instead of being again immediately expanded as in Fig. 2, is first led by a pipe 4| back to exchanger 6, where it is further warmed; and it is then expanded in an expander 42 (equivalent to expander 11 in Fig. 2) to substantially atmospheric pressure to recool it to a low temperature. Part of this twiceexpanded cold nitrogen is successively used as a cooling agent in liquefier 9, exchanger 6, and exchanger 2. The remaining nitrogen from expander 42 is led by a pipe 43 to exchanger 32, where it is warmed. It is then compressed in compressor 33 to about p. s. i. a. and cooled in exchanger 32 to substantially the same temperature as the unliquefied nitrogen withdrawn from separator Those two nitrogen streams at substantially the same pressure and temperature are then commingled in pipe 44 to augment the supply of cold nitrogen in the system.

It is a feature of this invention that the nitrogen that is separated from the mixture under pressure is expanded in a plurality of successive stages to obtain the refrigeration over a specific low temperature range that is required to condense the needed liquid reflux for efficient rectification. By first expanding the nitrogen over only a portion of the total pressure drop available and using the resultinglow temperature refrigeration up to a certain temperature level and then further expanding the nitrogen down to substantially atmospheric pressure, it is possible to obtain refrigeration within the desired low temperature range. These successive expansions may ta e place in more than two stages, with a resultant increase in the low temperature range refrigeration; but two expansions, as herein described, are likely to be the maximum number that will be economically justified in most cases.

Another advantage of this invention is the large temperature difference prevailing at the warm end of exchanger 2 between the entering mixture and the two separation products, reflecting an excess of refrigeration at the higher temperature ranges prevailing at the warm end of the system. That excess refrigeration can be utilized to repermit the use of a smaller exchanger 2, with less heat exchange surface, than would be possible if the temperature differences at the warm end of that exchanger were smaller.

It is a further feature of this invention that the gaseous methane product recovered at the warm end of exchanger 2 is under a substantial pressure of around 135 to 155 p. s. i a., so that it can be introduced into delivery mains or storage facilities (where comparable or higher pressures prevail) with little or no additional compression. The methane that is recovered is approximately 9! percent pure and the yield of methane israbout 90 percent, a high figure in view of the simplicity of the apparatus used in practicing the invention.

According to the provisions of the patent statues. I have explained the principle of my invention and. have illustrated and described what I now consider to represent its best embodiment.

However, I desire to have it understood that,

liquefying the nitrogen resulting from said recti-v fication to form liquid reflux for use in rectifying the mixture, partially warming the unliquefied portion of the cooled nitrogen by using it as a cooling agent in cooling the compressed mixture, expanding the nitrogen so warmed to a lower intermediate pressure with performance of external work to recool it to a lower temperature than before, using the nitrogen so expanded as a cooling agent in effecting said cooling and partial liquefaction of nitrogen, again expanding the once-expanded nitrogen to substantially atmospheric pressure with performance of external work to recoil it to a low temperature, using the twice-expanded nitrogen so recooled as a further cooling agent in effecting said cooling and partial liquefaction of nitrogen, thereafter using said twice-expanded nitrogento cool the compressed mixture, and utilizing the separated methane at the intermediate rectification pressure to cool the compressed mixture.

2. Apparatus for separating natural gas mix tures composed substantially of nitrogen and methane that includes the following elements: a compressor for compressing the mixture to a high pressure, a first heat exchanger for initially cooling the compressed mixture, a second heat exchanger for further cooling the compressed mixture; a throttle valve for expanding the cooled compressed mixture to an intermediate pressure and thereby cooling it still further, a

"rectifying column for receiving the throttled mixture and rectifying it at said intermediate pressure to separate its contained nitrogen and methane, a liquefler for cooling and partially liquefylng the nitrogen resulting from said rectiflcation to form liquid reflux for use in rectifying the mixture, a conduit for conducting the unliquefied portion of the cooled nitrogen to the second heat exchanger as a cooling agent therein, a first expander for receiving the unliquefied nitrogen warmed in the second exchanger and for expanding it with performance of external work to a lower intermediate pressure to recool it to a lower temperature than before, a conduit for conducting the nitrogen so expanded to the liquefler as a cooling agent therein, a second expander for receiving the once-expanded nitrogen warmed in the ilquefier and further expanding it to substantially atmospheric pressure with performance of external work to recool it to a low temperature, a conduit for conducting the twice-expanded nitrogen from the secondexpander to the liquefler as a cooling agent therein, conduits for successively conducting the twiceexpanded nitrogen warmed in the liquefier to the second exchanger and then to the first exchanger as a cooling agent therein, and a conduitfor conducting the separated gaseous methane at an intermediate pressure from the column to the first heat exchanger as a cooling agent therein.

- 3. Apparatus according to claim 2 that includes the following additional elements: a third heat exchanger for receiving and further warming a portion of the twice-expanded nitrogen warmed in the second exchanger, a compressor for compressing the nitrogen so warmed, a conduit for conductin the compressed nitrogen back to the third heat exchanger where it is cooled by giving up heat to and warming the twice-expanded nitrogen therein, a third expander for receiving the cooled compressed nitrogen and for expanding it to an intermediate pressure with performance of external work to cool it further, and a conduit for adding the nitrogen s0 expanded to the unliquefied portion of cooled nitrogen to augment the supply of the latter.

4. Apparatus for separating natural gas mixtures composed substantially of nitrogen and methane that includes the following elements: a compressor for compressing the mixture to a high pressure, a first heat exchanger for initially cooling the compressed mixture, a second heat exchanger for further coolin the compres ed mixture, a throttle valve for expanding the cooled compressed mixture to an intermediate pressure and thereby cooling it still further, a rectifying column for receiving the throttled mixture and rectifying it at said intermediate pressure to separate its contained nitrogen and methane, a liquefier for cooling and partially liquefying the 4 nitrogen resultin from said rectification to form liquid reflux for use in rectifying the mixture, a conduit for conducting the unliquefied portion of the cooled nitrogen to the second heat exchanger as a cooling agent therein, a first expander for receiving said unliquefied nitrogen warmed in the second exchanger and for expandin it with performance of external work to a lower intermediate pressure to recool it to a lower temperature than before, a conduit for conducting the expanded nitrogen from the first expander to the liquefier as a cooling agent therein, a conduit for conducting the expanded nitrogen warmed in the liquefier back to the second exchanger as a cooling agent therein, a second expander for receiving the once-expanded nitrogen warmed in the second exchanger and for expanding it further to substantially atmospheric pres- 7 sure with performance of external work to recool it to a low temperature, a conduit for conducting a portion of the low pressure nitrogen leaving the secondexpander to a third heat exchanger as a coolin agent therein, a compressor for receiving and compressing the low pressure nitrogen warmed in the third exchanger, a conduit for conducting the compressed nitrogen back to the third exchanger where it is cooled by the cooling agent therein, a conduit for addin the compressed nitrogen so cooled to the unliquefied portion of cooled nitrogen to augment the supply of the latter, a conduit for'conducting the remainder of the low pressure nitrogen leaving the second expander to the liquefier as a cooling agent therein, conduits for successively conductin the low pressure nitrogen warmed in the liquefier to the second heat exchanger and then to the first heat exchanger as a cooling agent therein, and a conduit for conducting separated gaseous methane at an intermediate pressure from the column to the first heat exchanger as a coolin agent therein.

5. The method of separating as mixtures composed substantially of nitrogen and methane that includes the following steps: compressing the mixture to a high pressure, cooling the compressed. mixture to a low temperature, throttling the mixture to an intermediate pressure to cool it further; rectifying the throttled mixture at that intermediate pressure to separate its contained nitrogen and methane, cooling and partially liquefying nitrogen resulting from said rectification to form liquid refiux' for use in rectifying the mixture, expanding the cooled nitrogen that-isnot liquefied in a plurality of stages to substantially atmospheric pressure with the performance of external work to recool it to a lower temperature than before, using expanded nitrogen after each of said expansions to effect said cooling and partial liquefaction of the nitrogen resulting from said rectification, compressing some of the nitrogen that has been expanded to substantially atmospheric pressure, cooling the nitrogen so compressed by heat exchange with some of the cold expand ed nitrogen, and addin the cooled compressed nitrogen to said unliquefied portion of the cooled nitrogen before the latter is first expanded thereb to augment the supply of cold nitrogen in the system.

6. A method according to claim 5, in which said cooled compressed nitrogen is expanded to said intermediate pressure before it is added to said unliquefied portion of the cooled nitrogen.

7. In the separation of natural gas mixtures composed substantially of nitrogen and methane by rectification under pressure at low temperature, the method of obtaining suflicient liquid reflux to assure efilcient rectification that includes the following steps: cooling and particularly liquifying the nitrogen fraction resulting from said rectification, separating the resulting liquid nitrogen from the unliquefied nitrogen, expanding the unliquefied nitrogen in a plurality of stages to substantially atmospheric pressure with performance of external work to recool it to a lower temperature than before, using recooled nitrogen resulting from each of said expansions to effect said cooling and partial liquefaction of the nitrogen resulting from said rectification, compressing some of the low pressure nitrogen resulting from the last of said expansions, cooling said compressed nitrogen by heat exchange with said low pressure nitrogen before the latter is compressed, adding said cooled compressed nitrogen to the supply of said unliquefied nitrogen before the latter is first expanded, and using the separated liquid nitrogen resulting from said partial liquefaction as liquid reflux in the rectification of the mixture.

8. Apparatus for producing liquid nitrogen reflux for separating natural gas mixtures composed substantially of notrogen and methane that includes the following elements: a rectifying column for receiving the mixture after it has been cooled to a. low temperature and for rectify- .ing it at an intermediate pressure to separate its contained nitrogen and methane, a liquefier for cooling and partially liquefying the nitrogen resulting from said rectification to form liquid refiux for use in retifying the mixture, a first ex- ,pander for receiving the unliquefied portion of the cooled nitrogen and for expanding it to a lower intermediate pressure with performance of external work to cool it to a lower temperature than before, a conduit for conducting the expanded nitrogen to the liquefier as a cooling agent therein, a second expander for receiving the onceexpanded nitrogen from the liquefier and expanding it further to substantially atmospheric pressure with performance of external work to recool it to a low temperature, a conduit for conducting the expanded low pressure nitrogen from the second expander to the liquefier as a cooling agent therein, a heat exchanger for receiving and further warming 9. portion of the twice-expanded low pressure nitrogen that has been used as a cooling agent in the liquefier, a compressor for compressing the nitrogen so warmed, a conduit for conducting the compressed nitrogen back to the heat exchanger where it is cooled by giving up heat to and warming said low pressure nitrogen fiowing through that exchanger, and a conduit for withdrawing the cooled compressed nitrogen from the exchanger and adding it to said unliquefied port-ion of cooled nitrogen admitted to the first expander thereby to augment the supply of cold nitrogen available for expansion.

9. Apparatus according to claim 8, that includes the following additional element: a third expander for receiving the cooled compressed nitrogen and for expanding it to said intermediate pressure to cool it further before it is added to the unliquefied portion of cooled nitrogen.

JOE L. COST.

REFERENCES CITED The following references are of record in the file of this patent:

4 UNITED STATES PATENTS Etienne Dec. 19, 1950

Claims

1. THE METHOD OF SEPARATING NATURAL GAS MIXTURES COMPOSED SUBSTANTIALLY OF NITROGEN AND METHANE AND OF RECOVERING GASEOUS METHANE THEREFROM UNDER SUBSTANTIAL PRESSURE THAT INCLUDES THE FOLLOWING STEPS: COMPRESSING THE MIXTURE TO A HIGH PRESSURE, COOLING THE COMPRESSED MIXTURE TO A LOW TEMPERATURE, THROTTLING THE MIXTURE TO AN INTERMEDIATE PRESSURE TO COOL IT FURTHER, RECTIFYING THE THROTTLED MIXTURE AT THAT INTERMEDIATE PRESSURE TO SEPARATE ITS CONTAINED NITROGEN AND METHANE, COOLING AND PARTIALLY LIQUEFYING THE NITROGEN RESULTING FROM SAID RECTIFICATION TO FORM LIQUID REFLUX FOR USE IN RECTIFYING THE MIXTURE, PARTIALLY WARMING THE UNLIQUEFIED PORTION OF THE COOLED NITROGEN BY USING IT AS A COOLING AGENT IN COOLING THE COMPRESSED MIXTURE, EXPANDING THE NITROGEN SO WARMED TO A LOWER INTERMEDIATE PRESSURE WITH PERFORMANCE OF EXTERNAL WORK TO RECOOL IT TO A LOWER TEMPERATURE THAN BEFORE, USING THE NITROGEN SO EXPANDED AS A COOLING AGENT IN EFFECTING SAID COOLING AND PARTIAL LIQUEFACTION OF NITROGEN, AGAIN EXPANDING THE ONCE-EXPANDED NITROGEN TO SUBSTANTIALLY ATMOSPHERIC PRESSURE WITH PERFORMANCE OF EXTERNAL WORK TO RECOIL IT TO A LOW TEMPERATURE, USING THE TWICE-EXPANDED NITROGEN SO RECOOLED AS A FURTHER COOLING AGENT IN EFFECTING SAID COOLING AND PARTIAL LIQUEFACTION OF NITROGEN, THEREAFTER USING SAID TWICE-EXPANDED NITROGEN TO COOL THE COMPRESSED MIXTURE, AND UTILIZING THE SEPARATED METHANE AT THE INTERMEDIATE RECTIFICATION PRESSURE TO COOL THE COMPRESSED MIXTURE.