US3740962A

US3740962A - Process of and apparatus for the recovery of helium from a natural gas stream

Info

Publication number: US3740962A
Application number: US00073300A
Authority: US
Inventors: S Fan
Original assignee: COMMW ASS Inc
Current assignee: COMMONWEALTH ASS INC US; COMMW ASS Inc
Priority date: 1970-09-18
Filing date: 1970-09-18
Publication date: 1973-06-26
Anticipated expiration: 1990-06-26

Abstract

A process for recovery of helium from a natural gas stream containing components such as acid gases, moisture, hydrocarbons, nitrogen and helium. The process includes steps of feed gas preparation, refrigeration cooling and liquefaction, and separation of helium from the hydrocarbons and then nitrogen. Apparatus for carrying out the process is disclosed.

Description

United States Patent [1 1 Fan [ June 26, 1973 PROCESS OF AND APPARATUS FOR THE RECOVERY OF HELIUM FROM A NATURAL GAS STREAM [75] Inventor: Sin Chou Fan, East Lansing, Mich.

[731 Assignee: Commonwealth Associates Inc.,

Jackson, Mich.

22 Filed: Sept. 18,1970 211 Appl.No.:73 ,300

s21 U.S.Cl ..62/29,62/22,62/17, 62/30,62/23 [51] lnt.Cl. F25j 1/00, F25j 3/00, F25j 3/02 [58] Field oISearch 62/22, 23, 24, 27, 62/28, 29

[56] References Cited UNITED STATES PATENTS 3,057,168 10/1962 Becker..... 62/29 3,148,966 9/1964 Kitchen.... 3,359,744 12/1967 Bolez 62/23 3,355,902 12/1967 Crawford 62/28 2,765,637 10/1956 Etienne 62/27 3,293,863 12/1966 Karbosky.. 62/23 3,512,363 5/1970 Harper 62/27 3,037,359 6/1962 Knapp 62/22 OTHER PUBLICATIONS Garwin, Leo; A look at Helium Hydrocarbon Processing, April 1969 pgs. 97-100 Primary Examiner-Norman Yudkoff Assistant Examiner-Arthur F. Purcell Attorney-Olsen and Stephenson [57] ABSTRACT A process for recovery of helium from a natural gas stream containing components such as acid gases, moisture, hydrocarbons, nitrogen and helium. The process includes steps of feed gas preparation, refrigeration cooling and liquefaction, and separation of helium from the hydrocarbons and then nitrogen. Apparatus for carrying out the process is disclosed.

3 Claims, 1 Drawing Figure NVzl nOHQ NIS HOLNHANI PATENTEB JUN 26 I975 Q & Q

k INVENTOR SIN CHOU FAN NUQWOWWQ ATTORNEYS PROCESS OF AND APPARATUS FOR THE RECOVERY OF HELIUM FROM A NATURAL GAS STREAM BACKGROUND OF THE INVENTION The present invention relates to the recovery of helium from natural gas streams.

Natural gas is the commercial source of helium. The helium contents vary from a few parts per million to several percent. Presently natural gas streams having a helium content of approximately 0.4 percent and up are being processed for helium. In the future, the helium processors, however, may have to extract helium from gas streams with very low helium contents so that more effective procedures for helium recovery will be required than are now employed. For example, gas with a helium content of 0.05-0.07 percent is being processed in Russia now.

Natural gas is a complex gas mixture and contains variable quantity of methane and heavier hydrocarbons. There are also present some inert gases such as nitrogen and acid gases such as CO and H 8. In addition, there are added during production and transmission impurities which can be anything from dusty solids to lube oil. All the heavy hydrocarbons, inert and acid gases will affect the process of liquefaction to some extent and must be reduced to a tolerable level. Also, all the added impurities should be removed completely from the gas streams.

SUMMARY OF THE INVENTION The present invention is directed to a helium recovery plant wherein the feed or natural gas is taken from a high pressure pipe line and the tail gas is recompressed back to the same pipe line. The extraction process for the helium recovery can be divided into the steps of feed gas preparation, cooling and partial lique faction, and the separation of the helium from hydrocarbons and nitrogen.

One of the improvements of this process is the operation at a higher pressure level, as compared to the existing processes, which will result in refrigeration savings. Another is taking the advantage of a double distillation column for the separation of nitrogen and helium from other hydrocarbons. With the use of such a double column, the upper section is operated at a lower pressure and the lower section at a higher pressure, which carries the major separation. By so doing, the refrigeration requirement for reflux condensation in the lower section can be properly met by utilizing part of the bottoms in the upper section. This will result in additional savings in refrigeration. The bottom stream from the upper section will be used as purge gas for effective regeneration of the desiccant dryers.

The bottom stream from the lower section will be first utilized as a refrigerant to cool the incoming gas and then compressed back to the pipe line. The higher the pressure level of this stream, the less the power requirement will be. This will result in savings in fixed and operation costs.

According to one form of the present invention, a process is provided for the separation of helium from a natural gas stream containing acid gases, moisture, hydrocarbons, nitrogen and helium comprising the steps of purifying the atream by removing acid gases and moisture at a relatively high pressure of about 700 psia, gradually cooling the gas stream to a temperature sufficient to partially liquefy the purified stream to provide a liquid portion of relatively heavier hydrocarbons and a gaseous portion at about 600 psia and 126" F enriched in helium, nitrogen and relatively lighter hydrocarbons, further cooling the enriched gaseous portion to about 130 F and expanding the same into the lower section of a double distillation column at a pressure of about 500 psia to produce a lower liquid stream and a top stream, expanding the top stream to a pressure of 300 psia into the upper section of the same column in heat transfer relation above said first lower section to produce a top stream containing mainly, nitrogen and helium and a lower liquid stream in the bottom of the upper section in cooling relation to the top stream of the lower section, cooling the top stream in said upper section to about 255 F and then expanding the top stream of nitrogen and helium into a succeeding distillation column at a pressure of about psia to produce a lower liquid stream of mainly nitrogen at about 290 F and an upper stream of crude helium, and passing said lower liquid stream of nitrogen in heat transfer relationship to the top stream in said upper section of the double distillation column for cooling the top stream to 255 F. The lower liquid stream from the lower section of the double column represents high pressure tail gas at about 500 psia which can be passed in heat transfer relation to said feed gas stream for cooling the same and which is then vaporized and recompressed to about 700 psia for discharge into the source of the natural gas stream. The lower liquid stream from the upper section of the double column is passed in heat transfer relation to the gas stream for cooling the same and is then utilized as purge gas for the regeneration of the desiccant dryers.

Thus, operating pressures of about 600 psia are utilized initially in separating the gaseous portion en riched in helium, nitrogen and relatively lighter hydrocarbons, and thereafter, pressures of about 500 psia are utilized in the lower section of the double column. By virtue of these operating procedures, a less amount of power is required in order to recompress the tail gas and return it to the original source or stream of natural gas. Also, the effective use that is made of the tail gases for cooling purposes reduces the extent of refrigeration required during cooling and liquefaction.

The process is carried out by apparatus which is constructed and arranged to permit recovery of the helium through a series of steps at the pressures and temperatures set forth. In part, this apparatus includes unique double column separation unit for separating the nitrogen and helium from the gas stream.

Accordingly, it is among the objects of the present invention to provide improved method and apparatus for extracting helium from a natural gas stream characterized by the effective and economical results realized.

More specifically, the primary objects of this invention is to provide an improved process for effectively recovering helium from natural gases and to accomplish this recovery with a minimum requirement of external power for compression and refrigeration. An additional object is to recover helium with a method which minimizes the pressure differences between the feed gas and the tail gas.

Other objects of this invention will appear in the following description and appended claims, reference being had to the accompanying drawing forming a part of this specification.

BRIEF DESCRIPTION OF THE DRAWING The drawing illustrates schematically one embodiment of apparatus for carrying out the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Before explaining the present invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and arrangement of parts illustrated in the accompanying drawing, since the invention is capable of other embodiments and of being practiced or carried out in various ways. Also, it is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation.

Referring now to the drawing and to the follwing Table I, the invention will be described in greater detail. It should be pointed out that the information listed in Table I is only one example. In case the composition of the feed gas shown on Column A of Table I varies, the results in other columns will vary also. In Table I, the data set forth in the columns under the letters A-U, inclusive, correspond respectively to locations in the drawing identified with these letters.

The feed gas is taken from the pipe line through the conduit 12 to a gas filter 14 where impurities such as dust solids, lube oil, and the like will be filtered out and can be removed at 16 and the purified gas flows through the conduit 18 to the absorber 20.

Carbon dioxide and water vapor are the most troublesome impurities due to freezing characteristics at low temperature. The ice formation on the surfaces of heat exchanger fins and tubes will not only add resistance to heat transfer, but also in the extreme case, block the gas flow. Systems for carbon dioxide removal are known. These include amine treating, hot potasium carbonate scrubbing, water washing, etc. The absorber 20 is part of an amine treating system for removal of carbon dioxide. As shown in column A of Table I, the natural gas stream flowing through the conduit 12 contains carbon dioxide, which can readily be removed in the absorber 20 together with any other acid gases which may be present. As shown in the drawing, the gas umn. The gas from which the acid gases have been removed leaves the absorber 20 at the top via the conduit 24 with the gas composition being that shown in column B of Table I.

the MEA solution. The regenerated MEA solution flows out of the stripper 32 through conduit 34 and via the heat exchanger 30 to the amine pump 36 for circulation back to the absorber 20 by way of the conduit 22. For regeneration purposes, the stripper is provided with steam coils 37 at the bottom and cooling coils 38 at the top. The stripped gases are discharged from the top of the stripper through conduit 40. Any make-up MEA solution will be added into the system through the conduit 41.

The natural gas stream next flows through the heat exchanger 42 where the temperature of the stream is reduced to about F. Due to the lowering in temperature, there will be water condensation. The moisture content in the gas stream is thus reduced, about 30 percent lower than the original, and the condensed water being collected and removed in the separator 44. The natural gas stream then flows through the conduit 46 to the two desiccant dryers 48 and 50 which are piped in parallel as shown, one being in service while the other is in regeneration. Such dryers are filled with desiccant materials such as molecular sieves. The gas to be dried has the composition shown in column C of Table I and enters into the appropriate dryer at the top and flows out the bottom through the conduit 52.

In special circumstances the procedure of operation ofthe apparatus described above may vary. For instance, the removal of acid gases and drying may be carried out simultaneously in columns 48 and 50 if the content of such acid gases in the feed gas is very low. That is, the feed gas will be lead directly to columns 48 and 50 and the

columns

20 and 32 and their auxiliaries will be eliminated from use.

The desiccant dryers, when saturated with moisture, are subjected to regeneration. The valves are so arranged and the regeneration may be carried out in the following three steps: (a) heating of the dryer to approximately 600 F. by means of the gas heater 54 which is arranged to heat return low pressure tail gas flowing in conduit 56 to this temperature; (b) stripping with the return low pressure tail gas from the conduit 56 for discharge through the conduit 58; and (c) by means of the return tail gas, cooling to the operating temperature of approximately 80 F. The low pressure tail gas flowing from the conduit 56 and either through the gas heater 54 and one of the dryers 48, 50 for heating the same or directly through such dryer for cooling the same then is discharged via conduit 58 to the power plant of the system. The fuel gas for the heater 54 may be obtained from the conduit 58, as shown.

The purified natural gas stream having the composition shown in Column D now flows via the conduit 52 'lAllLIC l. MA'II'IIHAL liAliANtllC A" 15* l) l' I (i II" I .l l\' l M N (l I Q It S I ll ll('. 0. 4 0. 4 0. 4 0. 4 0. l 0. -l 0. 4 0. 4 'llnt'fl 0. 30 0. 0l 0. 3R 0. 0| N2. 3. 4 3. 4 I. 4 I. 4 3. l 3. 'l 3. -l 3. 0 0. -l h 0. .5 0. 2 2. ti (,()7 3.5 Nll lI S (1H K0. 8 XII. X 80. X 80. X 80. H X5. X5 85. H5 20. Ii 0."). 25 0. 3 20. 3 0. 3 0. 05 (.glIn 0.5 0.5 0.5 0.5 0.5 0.!Jl 0.0l 0.0l 5.5!! Calla, 2.5 ".5 2.5 2.5 2.5 2.5 (lnlllll. 0. l 0.4 0. 1 0. 4 0. 4 0.1 Moles/hi I02. 5 I00 I00 I00 00. 50 21 lit 50 3. 10 20. 5| 0. 5H 3.01 l). -'l-'l llqlA 700 (300 500 500 300 300 300 75 75 75 000 200 .500 F I00 100 80 -35 -l00 l2ti I30 I50 -25?) l00 -3l l 200 15 Dry lmsis.

" Partially liquiliml state.

Operating comlililms given are :tpplnxinmli'.

The absorber 20 has an outlet 26 at the bottom through which a rich solution of the acid gases flows first through a liquid filter 28 and then via the amine heat exchanger 30 to the stripper 32 for regenerating to heat exchanger 60 equipped with a propane refrigeration system 62. As shown in Column E, the exit temperature is 35 F at conduit 64. The heat exchanger 60 is a four stream design, one for the gas stream to be cooled, one for the propane refrigeration, one for high pressure tail gas which will flow through the heat exchanger for cooling purposes from the conduit 66 to the conduit 68, and one for low pressure tail gas, also for cooling purposes, which will enter from the conduit 70 and will be discharged through the conduit 56.

The main gas stream is further cooled to -100 F by passing through heat exchanger 74 and being discharged to the conduit 76. The heat exchanger 74 is also a four stream design; one for the main gas stream, the second for the stream from the ethylene refrigeration system 78, the third for the high pressure tail gas entering from the conduit 80 and discharging via the conduit 66, and the fourth for the low pressure tail gas bon separation column 86 where the heavier hydrocarbons will be condensed and separated from the gas stream. The gas and liquid stream compositions from the separation column 86 are shown in Columns G and S, respectively, of Table I.

Approximately 90 percent of the heavier hydrocarbons are removed from the feed gas in separation column 86. The heavier hydrocarbons from the bottom will be removed at 88 where they can be further processed into ethane and propane-plus, if this should be necessary. This can be accomplished in the separation column 90 wherein ethane can be removed at the top via the conduit 92 and propane-plus can be removed at the bottom via the conduit 94.

In the separation column 86 cooling is provided at the top of the separation column with the coils 96 by vaporizing part of the return liquid via conduit 98. Heat is provided by the higher temperature gas via conduit 99 at the bottom of the separation column.

The gas stream from the top of the separation column 86 contains mainly methane, nitrogen and helium, and this gas mixture is further cooled to a temperature of approximately 130 F in the heat exchanger and vaporizer 100. The latter is a four stream design; one is for the gas stream flowing through the conduit 102 to the outlet conduit 104, one is for the high pressure liquid flowing through the conduit 98 vaporized in the vaporizer 100 and discharging through the conduit 80, and the third is for the low pressure tail gas flowing from the conduit 106 and discharging through the conduit 82, and the fourth is for the ethylene refrigeration system 78. The partially liquefied gas stream in conduit 104 next flows to the separation or fractional distillation column 108 which is divided into two sections, the lower section 110 being operated at a pressure of 500 psia, and the upper section 112 being operated at a pressure of 300 psia. The conduit 104 passes through the lower portion of the lower section 110 to provide the necessary heat requirement in the reboiler and then through the expansion valve 114 whereby the gas is expanded to a pressure of 500 psia and discharged into an intermediate portion of the lower section as shown. The top streams from the lower section 110-will be fed to the upper section via the conduits 115 and 116 through the expansion valves 117 and 118 so that the pressure is reduced to 300 psia in the upper section 112. The feed to the top section is designed to be approximately 25 percent of the feed gas to the plant in the disclosed form of the invention. The gas stream or low pressure tail gas discharged from the lower portion of the upper section 112 will be used later for purging of columns 48 or 50 and then as plant fuel. The liquid stream from the bottom of the lower section will have the composition shown in Column K of Table I, and the average composition of the top streams will have the composition shown in Column J of the table. Likewise, the vapor taken from the bottom of the upper section will have the composition shown in Column N of the Table I. As previously explained, the liquid from the bottom of the lower section 110 passes back through the heat exchangers via the conduit 98 for cooling purposes, and vaporized, and there it is recompressed to 700 psia by the compressor 111 and is discharged into the supply pipe line 10.

The gas stream from the top of the upper section 112 contains mainly nitrogen and helium as is shown in Column M of Table I. This gas which is at a pressure of 300 psia and a temperature of about 250 F next flows to the helium separation or fractional distillation column 119 via the conduit 120 and after passing through the expansion valve 122 is discharged into the column at a pressure of 75 psia. The temperature at the top of this column is 314 F and at the bottom is 290 F. In order to achieve the temperature at the top, it is necessary that liquid nitrogen refrigeration be used at 124. The liquid nitrogen refrigeration system is conventional. The helium product which is extracted via the conduit 126 has the composition shown in Column Q. This crude product can be compressed and stored in gas form in conventional storage bottles. The liquid nitrogen is extracted at the bottom of the separation column 119 and has the composition shown in Column R. This liquid nitrogen can be passed via the conduit 128 through the cooling coils at the top of the section 112 of column 108 to provide the cooling requirement for reflux condensation. Additional liquid nitrogen will be added via conduit 127, if necessary.

From the foregoing description it will be recognized that the disclosed helium recovery plant is designed to operate so that the pressure drop through the system is kept small. By so doing, there are savings in refrigeration as well as in power which is required to recompress the tail gas to the inlet pressure. Thus, the process is more economical by virtue of the operating pressures for the separation of the light hydrocarbons, nitrogen and helium from the gas stream being maintained at a high level. Also, maximum refrigeration economy is realized by the flow circuits disclosed. Of special significance in this respect is the construction and arrangement of the separation column 108.

I claim:

I. A process for the separation of helium from a natural gas stream containing carbon dioxide, moisture, relatively heavier and lighter hydrocarbons, nitrogen and helium comprising the steps of purifying said stream by removing carbon dioxide and moisture from said gas stream at a relatively high pressure of about 700 psia, gradually cooling the purified gas stream to a temperature sufficient to partially liquefy the purified stream to produce a liquid portion of relatively heavier hydrocarbons and a gaseous portion at about 600 psia and 1 26 F enriched in helium, nitrogen and relatively lighter hydrocarbons, further cooling the enriched gaseous portion to about l F and expanding the same into a lower section of a double section, fractional distillation column at a pressure of about 500 psia to produce a lower liquid stream and a top stream, expanding the top stream to a pressure of about 300 psia in the upper section of the fractional distillation column in heat transfer relation above said lower section to produce a top stream of nitrogen and helium and a lower liquid stream in the bottom of the upper section in cooling relation to the top stream of the lower section, the top stream of said lower section being condensed and used as reflux in said lower section, cooling the top stream in said upper section to about 255 F and then expanding the top stream of nitrogen and helium into a succeeding fractional distillation column at a pressure of about 75 psia to produce a lower liquid stream of nitrogen at about 290 F and an upper stream of crude helium, and passing said lower liquid stream of nitrogen in heat transfer relation to the top stream in said upper section for cooling the top stream to 255 F.

2. The process that is defined in claim 1, wherein the lower liquid stream from said lower section is passed in' heat transfer relation to said gas stream for cooling the same and is then recompressed as a gas to 700 psia for discharge into the source of the natural gas stream.

3. The process that is defined in claim 1, wherein the lower liquid stream from said upper section is passed in heat transfer relation to said incoming gas stream for cooling the same and is then utilized as purge gas in conjunction with the removal of moisture from the natural gas stream and thereafter as plant fuel gas.

Claims

2. The process that is defined in claim 1, wherein the lower liquid stream from said lower section is passed in heat transfer relation to said gas stream for cooling the same and is then recompressed as a gas to 700 psia for discharge into the source of the natural gas stream.
3. The process that is defined in claim 1, wherein the lower liquid stream from said upper section is passed in heat transfer relation to said incoming gas stream for cooling the same and is then utilized as purge gas in conjunction with the removal of moisture from the natural gas stream and thereafter as plant fuel gas.