US2940271A

US2940271A - Low temperature fractionation of natural gas components

Info

Publication number: US2940271A
Application number: US801615A
Authority: US
Inventors: Steven B Jackson
Original assignee: Fluor Corp
Current assignee: Fluor Corp
Priority date: 1959-03-24
Filing date: 1959-03-24
Publication date: 1960-06-14
Anticipated expiration: 1977-06-14

Description

June 14, 1960 s. B. JACKSON 2,940,271

LOW TEMPERATURE FRACTIONATION OF NATURAL GAS COMPONENTS Filed March 24. 1959 JNVENTOR. 37m/EN JMA/50N Steven B. Jackson, Fullerton, Calif., assignor to The Fluor Corporation, Ltd., Los Angeles, Calif., a corporation of California Filed Mar. 24, 1959, Ser. No. 801,615

16 Claims. (Cl. 62-31) This invention relates generally to low temperature fractionation or separation of natural gas into at least some of its components, and it is directed particularly to an improved processfor the separation of nitrogen, or both nitrogen and helium, by methods characterized by their greater practicability and economy than those customarily employed or previously proposed.

Generally contemplated lfor fractionation in laccordance with the invention, are natural gases containing C2 (ethane) and heavier fractions recoverable as LRG., as well as natural gasoline components, together with methane, nitrogen and helium. One overall primary object of the invention is to provide for the eicient removal of nitrogen from the other components, regardless of for what purpose they may be used, as where the major consideration may be the separation from fuel or synthesis stock components of inert and useless nitrogen; or where helium recovery is desired and is justified-by the helium content ofthe natural gas, the major object may be helium recovery together with nitrogen separation.

The methane content of the gas is employed in novel and signiicant relation to accomplishment of the invention, not merelyl because of the contemplation of separation and use of methane vas a product component, but also by tolerationandutilization of methane in the fractionation and final separation stages in a manner contributing both uniqueness and practical economy in the nitrogen separation. In this respect and as will later appear, the invention achieves the Vend result of nitrogen separation to the desired; completenesmbut without the expensive undertaking of precise or complete single stage methane and nitrogen separation, by so conducting and controlling the fractionation process as to recover methane usable for the plant Vfuel requirements from a deliberately incomplete separation of methane and nitrogen, while employing separated methane as a refrigerant in the course of achieving the low temperature nitrogen fractionation.

ln terms of the general process contemplated, the feed gas passes successively'through a series of refrigeration zones which may comprise heat exchangers of appropriate construction through which components or fractions being separated at successive stages in theprocess, are passed in cumulative and generally counterflow relation to the feed gas as it becomes progressively depleted of separated components. Thus an-initial separation'may be the removal of LRG. components to deplete the gas of CS and heavier hydrocarbons, and thus render the gas properly composed for a second stage separation of nitrofrom methane and C2 hydrocarbons present. The invention is concerned primarily with this second stage nitrogen separation and related in uences, and is characterized by so controlling the fractionation as to provide in the separated nitrogen stream a substantial amount of methane. The remaining methane condensate is employed in the manner later described as refrigerant in the nitrogen fractionation stage, and is recoverable as nited States Patent- O 2,940,271 Patented vJ une 14, 1 960 ice a separated methane plus C2 fraction. Thus the nitrogen separation is permitted to occur without excessive rerigeration or expensive physical equipment otherwise, which would be required for more precise methane-nitrogen separation, all to the advantages of lower initial and operating costs of the process.

T he methane-containing nitrogen mixture then is subject to further cooling and straight forward separation of all the methane, by simple equipment and procedures consistent with the overall economy of the process. The condensate derived from the final nitrogen-hydrocarbon separation has suicient methane'content as fuel in the plant boilers, fired heaters, gas turbines and the like.

The invention is particularly concerned with the fractionation of helium-containing natural gas, with the end objective of separating out the bulk of the nitrogen as waste in the fuel gas and as waste, and recovering a helium rich product which may be predominately helium and capable of separation as by known low temperature methods, into substantially pure helium and wasteresidual nitrogen. As employed primarily for helium recovery, the invention possesses all the operational and low-cost advantages mentioned in the foregoing, since throughout the fractionation and separation stages, up to the final nitrogen and helium-rich residue separation, the helium remains uncondensed along with the nitrogen content of the gas.

All the features and objects of the invention, as Well l as the details of a typical and preferred embodiment, will be understood from the following detailed description of `the accompanying drawing which illustrates the process in flow sheet form. Y

The `system will be described with reference to the fractionation of natural gas of the general type and composition produced in regions of Southwestern United States, typically the Texas Panhandle-area. Such gas, fed to the system through line 10, may contain from about 0.4 to 3% helium, 5 -to 30% nitrogen, 3 to 15% C2 and heavier hydrocarbons, and the balance methane. It Will be understood that the feed gas may have been pre-treated for such purposes as moisture and acid gas removal, to satisfactorily condition it for component separation or fractionation in accordance with the invention. Typically,

the gas may be fed to the system at a pressure in the range of about 250 to 1000 p.s.i.g. and at a tempera-ture that may range from around 40 F. up to higher temperatures in Vthe neighborhood of atmospheric temperature, particularly where the gas contains no consequential LRG-components. p

Thecolumn or shell 11 may be regarded as illustrative of any suitable chamber through which the feed gas is passed to be cooled by indirect heat exchange with the later described low temperature streams passingv through coils contained in the chamber. The feed gas undergoes partial condensation in the chamber 11 and the gas and condensate mixture passes through line 12 to separator 13 from which the condensate composed essentially of C3 and heavier hydrocarbons going to make up L.P.G. and natural gasoline, are withdrawn through line 14 to be passed through coils 15 in chamber 11, to L.P.G.- natural gasoline separation, purication and storage. The condensate may be recovered at a pressure from 20 to p.s.i. and at a temperature in the range of about -80 column. The latter is operated and controlled under particularly contemplated conditions providing forsubstantial-methane content in the .overhead gas stream leaving Ythecolurnn throughV line 21.V Temperature and pressure conditions in the column are controlled so that the'gas mixture going to line 21 will contain the helium and nitrogen content of the feed, possible minor quantities of Cz hydrocarbons, and from y to 50 percent` methane, the

upper methane limit in this range being dependent upon the quantity of methane usable as fuel in the plant, i.e. for operation of the later-mentioned compressor Z2, and

Y possibly also for heating purposes in Vequipment serving to pre-treat the gas before its feed Yto exchanger 11. The column t19 may contain suitable packing or trays 23v as Vrequired for the fractionation, and also an upper exchanger lsection 24V (which may be inside or outside the column) comprising tubes through which the overhead flows to the outlet line 21. Depending upon `such considerations as the gas composition and the percentageV of methane to be retained in the overhead, the column 19 vmay be operated in the pressure range of about 240 to 450 p.s.i.g. and at a top temperature, above the exchanger section 24, of about -l70 to -240 F.

Y A portion of the condensate bottoms inrcolumn 19, and consisting for the most part of pure methane, minor percentages of C2 fractions and possibly a small concentration (e.g. 2 to 5 percent) of nitrogen, is withdrawn from the base of the column at a temperature in the Vrange of'say 130 to '-180" F., through line 24 and throttled at valve 241 to pass through exchanger tubes Y 25 and in heat exchange with the fractionatingcolumn v'feed stream being cooled in chamber 17, thence `through line 26V and exchanger tubes 2'7V in chamberfll, and

inally to suitable disposition through line 28,.v Most gen- `erally, throttling willl'be accomplished by valve 241 instead of at valve 20. A second portion of the fractionating column-bottoms vis withdrawn through line-29 o toow Ythrough tubes 30 in exchanger chamber- 31, AVand thence through line 32 past valve 33 intothe exchanger i section 24' of column 19. I'he pressure reduction at valve 33 will be 'controlled toexternallyQ-cool the tubes 24 and v maintain theproper column top temperature. The reffrigcrant stream, consisting` mainlyfof -rnethaneY leavesV the exchanger section through line 3,21 to flow Ythrough 31, and thence is introduced through line 38 to the Alower Vportion of Vcolumn 19 to serve as stripping gas. Thus,

that portion of the fractionating column bottoms withdrawn through line 29 serves the dual purposes of a reis Vpassedrrthrough exchanger chamber 40, ,wherein the Vmethane is condensed and withdrawn through line 41, Y with orwithout pressure reduction atY valve 42, to sepa- .f rator 43 maintainedtypically at a pressure in the range of about 190 -to 240 p.s.i. and at a temperature of about "-7265 vto 5290 F.V Y.The uncondensed -gasescon'sisting {essentiallyrof helium and nitrogen, low from the sepa-V in chamber 17, line 53, coil v54 in exchanger 11, and thence to disposal through line V5,5. Y

The gas remaining uncondensed in exchanger 45 and consisting of helium together with a lesser quantity of nitrogen, saywithin the range of 10 to 40 percent nitrogen, is withdrawn through section 46 through line 56 andV passed successively through coil 57 inexchanger 45, line 58, coil 5,9in exchanger line 60, coil 61 in exchanger 17, line 62j, coil 63 ii i"e xclianlgerf11,*aiidnally to storage orvfurther disposition through line 64.Y As will be understood, this product helitun-nitrogen gas-may be subjected to further puricationlbyany of the,V known methods for recovery of helium at the purity required.

. Referring back tolthesep'aratsr.; 43 themethane-nitrvgen condensatewithdrawn therefromV through line 65, passes thence through coil 66' exchanger 45, line 67, coil 68 in exchanger 59, line 69, coil 70. in exchanger 17, line 71, coil 72' in exchanger 1,1 and then through line .'73 for use as fuel gas'iri the plant.

As previously observed, the compressor (22) operation and consequent refrigeration cost, are governed primarily by the methane puri-ty desired `in, the fractionator column bottoms withdrawn through

lines

24 and 29, and the methane composition of the fractionating column overhead stream in liner 2,1 i's governed by the quantity of t methane usable as fuel in the plant or other local facilities.

To cite typical specic'operating temperature and pressure conditions, where the rfeed gas may contain about 65 percent methane, 26 percent nitrogen', 2 percent helium and 7 percent C2 and heavier hydrocarbons, the feed stream may'enter exchanger .1'1 at atemperature of about 30 F. to be cooled in theseparator 13 at around -120 F. The stream leaving exchanger 17 enters column 19 at a temperature of around-190 F. and the conditions of fractionation under pressre'of about 400 -p.s.il Yare such that the overheadgas leaves the column at about -'225 P. with `thefliquid bottoms at about V-l50 F. V,ExchangerY 40 operates to further cool the gas to a temperature in separator 43 of about 280 F.

iat"200 p.s.i. In chamber 45 the liquid nitrogen and gaseous impure helium separate at about -300 F. and at a "pressure in the-neighborhood of 190 p.s.-i. The product helium-nitrogenv stream recovered .through'line 64 will Vcontain about 8O Vtoy 90v percent helium;l f

condensate returned for feedexchange fromrthefractionator 19 through Y chamber 31 to one or moreappropriatecompressors, diagrammaticallyV indicated at 2,2, which operates to compress and discharge the predominately methane stream through Y vline 34 to cooler 35. Fromthe cooler the methane stream flows through line 36 and exchanger tubes 37 in chamber Y rator through line 44 to pass through the low temperature' l denswate consisting essentially Vo'f'mtrogen.il TheV liquid nitrogen is withdrawn through lin`ef47Y and passed successively through exchanger coil 48 in chamber'45, line f75 4 9, een lso it chamber 40,1111@ s1, ,erhanger een s2 VYline -241to the secondstrearnftaken for refrigeration through line 29 will depend'pon the nitrogenV 'content of Y the feed gas. YUnder-the `typical operating conditions just cited, .the quantity ratiorfof 'the condensate'streams in vlines 24 and-'29 may be'fabjout 2. to l. At lower nitrogen contents, say intheyrieighboirhoodY of V13 percent, the corresponding ratio may be about'4 to ,1, and in the frac- Y.tionation of residue or refinery gases having still lower nitrogen content, the same ratio mayv range upwardly Y gen into alvertically extendedffractionatingzone partially condensingthe methane @l1-tent of the XDUTIIS. COD

:lensing zone at thetop Lof* saidizone and removing therefrom an overhead'.nitroggen-nrethaneV streanr` containing between l0.-50%rnethane,pas`sin'g an essentially methane stream from the bottom of'said fractionating 'zone through one Vpassage in.. a lheatY .exchangel zone, `expanding the Y methane'stream yand passing it as coolant through said i condensing zone and thence through a second passage in said heat exchange zone'in heat (transfer-relation with the methane said tirst'pa'ss'ag'ef thereby precoolingthe methane stream in advance of; passage 'through said condensing'zone then compressing the methane from said second passage, and cooling the compressed methane and returning it to said ractionating Zone.

2. The process according to claim 1, in which the compressed and cooled methane -is passed through said heat exchange zone in indirect heat transfer relation with methane in said second passage.

3. The process according to claim l, in which methane is Withdrawn from the base oi said ractionating zone and is passed in indirect heat exchange relation with the methane-nitrogen feed to said zone.

4. The process according to claim 1, in Which the methane-nitrogen mixture is fed to said fractionating zone at a temperature between about -140 and 200 F.

5. The process according to claim 4, in which said overhead stream is removed from the fractionating and condensing zones at a temperature between about -170 to 240 F.

5. The process according to claim 5, in which said overhead stream is passed through a condensing zone maintained at a temperature suiciently low to condense out substantially all of the methane.

7. The process according to claim 1, in which the natural gas contains also LPG. hydrocarbons, and in which said hydrocarbons are condensed out of the gas in advance of the feed of residual methane and nitrogen to the fractionating zone.

8. The process according to claim 1, in which the natural gas contains also LRG. hydrocarbons, and in which the gas is cooled by indirect heat exchange with methane from said ractionating zone to condense said hydrocarbons out of the gas in advance of the feed of residual methane and nitrogen to the fractionating zone.

9. The process according to claim 1, in which the gas contains helium which is carried through the ractionating Zone into said overhead stream, the process including the further step of condensing and separating methane and nitrogen from the overhead stream to leave a helium rich residual gas.

10. The process according to claim 9, in which said overhead stream is cooled to condense said methane and nitrogen therefrom by adiabatic pressure reduction and expansion of components of said stream and heat exchange between resultant liquid and vapor phases.

11. The process according to claim 9, in which methane separated from said overhead stream is passed in indirect heat exchange with said overhead stream to produce cooling from the methane condensation. i

12. The process according to claim 9, in which the methane and nitrogen are sequentially condensed om said overhead stream, condensed nitrogen and said residual gas are individually passed in indirect heat exchange With said overhead stream to produce cooling for the methane condensation.

13. The process according to claim 12, in which streams of said condensed methane, nitrogen and said residual gases after said heat exchange with the overhead stream are heat exchanged with the nitrogen-methane feed to said fractionating zone.

14. The process according to claim 13 in which the natural gas fed to the system contains LPG. hydrocarbons, and said streams of methane, nitrogen and residual gases after heat exchange with the fractionating zone feed are heat exchanged with gas fed to the system containing said L.P.G. hydrocarbons to produce LRG. condensate.

15. The process for low temperature fractionation of natural gas containing methane, nitrogen, and helium, that comprises cooling the gas to a low sub-zero temperature, feeding the cooled gas with its contained methane, nitrogen, and helium into a vertically extended fractionating zone, partially condensing the methane content of the mixture in a condensing Zone at the upper interior of said zone and removing therefrom an overhead nitrogenmethane-helium stream containing between 10-5 0% methane, circulating a normally liquid coolant uid stream through said condensing zone, cooling said overhead stream `to condense and separate methane and nitrogen therefrom to leave a helium rich residual gas by expanding said coolant fluid at least partially to vapor into said condensing Zone, and thereafter compressing and cooling the expanded uid to convert it to liquid phase.

16. The process according to claim 15, in which the methane-nitrogen-helium mixture is fed to said fractionating zone at a temperature between about and 200 F., and said overhead stream is removed from the fractionating and condensing zones at a temperature between about to 240 F.

References Cited in the Jdie of this patent UNITED STATES PATENTS 1,073,843 Blau Sept. 23, 1913 1,773,012 Schuftan Aug. 12, 1930 1,913,805 Hausen June 13, 1933 2,122,238 Pollitzer dune 28, 1938 2,327,643 Houghland Aug. 24, 1943 2,600,110 Hachrnuth June 10, 1952 2,619,814 Kniel Dec. 2, 1952 2,765,637 Etienne Oct. 9, 1956 2,849,867 Haringhuizen Sept. 2, 1958