US2595284A

US2595284A - Method and apparatus for treatment of gaseous hydrocarbon mixtures

Info

Publication number: US2595284A
Application number: US68445A
Authority: US
Inventors: Paul V Mullins
Original assignee: US Department of the Interior
Current assignee: US Department of the Interior
Priority date: 1948-12-31
Filing date: 1948-12-31
Publication date: 1952-05-06
Anticipated expiration: 1969-05-06

Description

y 1952 P. v. MULLINS METHOD AND APPARATUS FOR TREATMENT OF GASEOUS HYDROCARBON MIXTURES Filed Dec. 31, 1948 INVENTOR PAUL v. MULLINS BY W ATTORNEY Patented May 6, 1952 METHOD AND APPARATUS FOR TREATMENT OF GASEOUS HYDROCARBON MIXTURES Paul V. Mullins, Amarillo, Tex., assignor to the United States of America as represented by the Secretary of the Interior Application December 31, 1948, Serial No. 68,445

7 Claims. (Cl. 62-122) (Granted under the act of March 3, 1883, as amended April 30, 1928; 370 O. G. 757) The invention herein described and claimed may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

This invention relates to improvements in the art of separating mixed gases by liquefaction and rectification, being particularly useful in the separation of the constituents of natural gas, whereby at least one of the constituents of the natural gas is separated and the remaining gaseous mixture is enriched in the other constituents, thereby enhancing the value of the gas.

It is an object of the invention to provide a practical and efficient method and apparatus for the separation, removal and recovery of diluent nitrogen from certain natural gas compositions of high nitrogen content by liquefaction and I fractionation.

It is a further object of the invention to treat gaseous mixtures composed essentially ofmethane and nitrogen by compression and expansion of the mixture, combined with utilization of the refrigerative capacity of the several fractions separated in the operation as to effect substantial separation of the nitrogen and the methane, after a preliminary separation of the heavier hvdrocarbons, propane, butane and pentane (with some ethane) and helium, if desired.

It is a further object of my invention to effect a substantially complete removal of nitrogen from such natural gas compositions by passing compressed natural gas of high nitrogen content in counter-current indirect heat exchange with cold effluent nitrogen and methane, in a plurality of separate zones, with intermediate separation and withdrawal of the heavier hydrocarbon gases, propane, butane and pentane, liquefying the resulting cooled and stripped gas by adiabatic expansion thereof, rectifying the cold liquid mixture of nitrogen and methane to separate the nitrogen as an overhead vapor fraction while maintaining the substantially nitrogen-free methane as a liquid fraction and utilizing the refrigerative capacitv of t ese fractions to cool compressed natural gas feed to the process.

These and other objects will be apparent from the following description of my invention.

The method is particularly applicable to nat-- ural gas and similar mixtures of gases containing low molecular weight hydrocarbons and appreciable amounts of nitrogen as a diluent. In some natural gas fields the nitrogen content is so high that exploitation thereof has been ser1- ously retarded or prevented altogether. By re moval of the nitrogen from the natural gas and/or similar gaseous mixtures, the concentration of the remaining components is increased, thereby raising the fuel value, lowering the viscosity and specific gravity, and other improvements in the physical properties.

Separation of nitrogen from gaseous mixtures of the type mentioned will provide the following benefits:

1. Increase the heating value, with consequent increased marketability, of the gasfrom certain fields.

2. Make available for use as a chemical raw material, natural gas from certain fields not now utilized because of the high nitrogen content.

3. Increase the effective capacity of long distance pipe lines by eliminating, in the field, the nitrogen content of the gas now being transported through these lines.

4. Increase the efficiency of pipe line transportation through improvement of the flow char-- acteristics of the natural gas supply.

Broadly such a process is carried out by passing the gaseous mixture counter-current to cold separated constituents in order to bring about liquefaction of the major constituents, and separation of practically pure nitrogen followed by evaporation of the residual liquid and recovery of the gases, with an enhanced value, by countercurrent heat exchange with relatively warm gas feed to the process. If helium is present in the natural gas, it may also be recovered before separation of nitrogen and methane.

In the attached dra in the single figure shows more or less digrammatically a suitable system for applying the invention to the separation of nitro en (and helium) from natural gas. A, AA and B are heatinterchangers in which the entering compressed gaseous mixture is cooled counter-currently by returning enriched gas and separated nitrogen. In interchan er C the gases mav be further cooled and partly liquefied by heat exchange with an auxiliary refrigerant supplied by the unit L.

In separator D heavier (molecular weight above ethane) hydrocarbons, which have liquefied are removed from the cooled gaseous mixture.

In interchanger E the stripped gaseous mixture is further cooled by counter-current heat exchange with enriched gas. The chilled mixture is expanded throu h throttling valve 6 in line [4, into separator F for a preliminary separation of gaseous helium and nitrogen from the liquefied fractionator H.

methane. From separator F the resulting vapor and liquid products pass into heat exchanger G, and are further cooled by separated nitrogen from fractionator H. Heat exchanger G serves as a fractional condenser so that the major portion of the nitrogen in the vapor fraction is condensed with the liquefied methane. Effluent gaseous nitrogen from condenser G is expanded in engine 1V, and then is passedthrough exchanger AA, before leaving the system, as practically pure nitrogen. Engine M may beused-to drive refrigerating unit L for compressing the auxiliary refrigerant which is passed through interchanger C.

The mixture of liquefied nitrogen andmethane is passed to fractionator H for separation of the nitrogen as overhead vapors and methane in liquid form. Pump K removes liquid mixture accumulating in the lower portion of fractionator or rectifier H and returns it to interchanger' E for cooling the incoming gas, whence it passes .through-interchanger'B to yield the desired enhanced mixture of hydrocarbon'gases.

The conditions of operation on a high nitrogen content naturalgasfeed are as follows:

The rawv gas feed compressed'to 300-600 p. s. i., depending upon gas compositionand conditions of operation, and suitably treated to remove watervapor and all acidic gases such as CO2 and 'Hzs'enters at i. If no H2S is present in the raw gas feed, the CO2 and water vapor maybe removed by having aduplicate 'set of interchangers *A, AA and B, which would 'permit'intermittent use .of first one set and then the other of the interchangers'as so-called cold accumu ators to 'remove'Coz and'water and to be alternately warmediandthawed bypassage therethrough of ra wrgas feed. :However,"th arrangement shown fandspeoifiedis preferred. The incoming raw gas feed'stream is divided at'j2 and again at 2A and passed through interchangers A, AA and B, where 'the' divided streams are cooled by interchange of heat tocold outgoing products and then rejoin at 4 and 4A to again'form'a single stream. The majorportion' of the incoming raw gas feed would normallypass throughinterchanger B and only 'thosequantities necessary to'interchange heat with'the'smaller fractions of outgoing separated products pass through exchangers A and AA. Theamounts of raw gas feed passing through each of the interchangers A and'AA (and B) are controlled by valves 3 and 3A respectively, which can be ofanautomatic thermal typeeliminating 'the necessity for manual manipulation.

The partly-liquefied raw gas feed enters interchanger C, where it is further cooled and liquefied by interchange of heat toa cold circulating refrigerant supplied by the auxiliary refrigerator unit L. The two-phase mixture then enters separator D, where the gas and liquid phases are separated and the latter is withdrawn for utilization'or further processing elsewhere. This fraction constitutes the more readily liquefiable or heavier hydrocarbon fraction which in a natural gas is composed of propane, butane and heavier hydrocarbons, with perhaps a small fraction of ethane. However, a larger amount or fraction may be withdrawn, if convenient and practical,

and thus reduce correspondingly the amount of liquids that must be handled in separator G and Any portion of these hydrocarbons which it-is desired to return to the cycle and which possess some recoverable refrigeration can be injected into the return processed gas stream-at some appropriate point of low pressure,

suchas the upstream return side of interchanger A. The interohanger surfaces provided in interchangers B, C and E are selected to establish and maintain predetermined temperature and phase equilibrium conditions for withdrawal: of the desired (heavy) hydrocarbon: fraction. from separator D. The process cycle may be made to operate satisfactorily without auxiliary refrigeration unit L or intcrchanger C by utilizing refrigeration otherwiseavailable as mentioned herein and by soproporticning the heat exchange between .interchangers B andE as to provide the desired liquid fraction in separator D. However, the

1arrangement-shown is preferred as a means of pandedby throttling at :valve 6 :to 'a-rsomewhat lower pressure, :depending upon the-composition of the raw gas.andithegzconditionswjcf :operation, and passes rintozseparator :Frwhere thegaS-and liquid phases: are .separated the, former entering the condenser Gat suitable anoint} onthe side of the vesseliwh'ilestheilatter-entersthe same vessel :at" 8; near. the rbottom. .iacilitates disendgementrof the'rgaszfrom ztherliquidsph s u sequently completedinzc ndenser (3- ;Also; i nn er .'rWhich .preierablydspf sh ll and t b construction; the gas andliquidvphasesare'furth cooled'by interchang of heat to co ld: outgoing products and :the'; former, :consisting-principally of heliufil and nitrogen when processing natural gas,

isrrectified;-,v. ith:someincre in h helium co centrationzor,puritybeforewithdrawal :at 9 at the top. of. separator G. Thewrefrigerative capacity of '1 the cold :helium-nitrogem mixture of crude helium, ;if gslifflcient inrvolume may he recovered in. anyconvenientrmanner if desired. ,Suchheat interchange; as well as-anysubsequent treatment and/ or purificationaofseparated helium forms no part of the present invention.

If little or, no-helium is'present' in: the raw gas feed, separatorFand condenser: Gmay be elimi nated from-the process cycle. .However, eon-- denser G rserves asa temperaturestabilizing chamber for the coldliquids-:thereinand thus permits greaterstability of subsequent fractionation ofthe liquid and therefore use of condenser G is preferred. While'maintai-ning-a liquid-seal in "the bottom of condenser G,.-li quid is withdrawn, its flow controlled by-valve l0,- and passed without substantial pressure reduction to rectifier Hat point H-aboutwmidway of theheight .ofthe column.

'RectifierjI-Iaisa. conventional paclged or bubble plate columninwhich the liquefied: gas-mixture is fractionated to separate virtually pure nitrogen as an overhead vapor while withdrawing from the bottom thereof, virtually nitrogenerfree hydrocarbon "liquid, principally methane- :when processing-natural .gas. Heat required for .the fractionation is supplied-'inthebase of the fractionatorH through coil l2 'by aside'stream-uof warmer raw feed gas: taken from thecycleat some convenient point suchas l3. and returned at aconvenient point of lower pressurersuchas M with control of the flow by valve I5. Liquid nitrogen as reflux is added to the top of the column at 24 from a source described later herein, while maintaining a liquid seal in the bottom of fractionator H, liquid is withdrawn by cold liquid pump K which restores a part of the pressure originally on the raw gas feed while passing the cold liquids through interchangers E and B to cool incoming raw gas feed and to discharge at IT at near atmospheric temperature a processed gas substantially free of nitrogen, helium, and certain heavy hydrocarbon constituents. The cold liquid pump can be omitted and the cold liquid fraction allowed to pass through interchangers E and B at or near the prevailing pressure with resulting warming and regassification and then be recompressed as desired 101- lowing discharge at IT at near atmospheric temperature. However the arrangement shown, using the cold liquid pump is preferred as it results is reduced overall power consumption when the processed gas is to be compressed to a higher pressure. Cold nitrogen vapor leaves the top of fractionator H at l8, a portion going to one pass of liquefier J (at without substantial reduction in pressure while the balance passes through interchanger-type separator G where it cools the products therein as previously described and emerges therefrom at 2| warmed somewhat in temperature and is then expanded through a reciprocating engine or turbine M to near atmospheric pressure which results in the performance of work and cooling of the nitrogen which is then utilized as refrigerant to cool a portion of incoming raw gas feed in interchanger AAfoL lowed by discharge at near atmospheric temperature at 22. Expansion unit M is shown as a source of power for auxiliary refrigeration unit L, but, depending upon the condition of operation, might be used to drive cool liquid pump K or reflux pump N. The expansion unit M could be omitted and the outgoing cold nitrogen throttled to near atmospheric pressure after leaving fractionator H and before entering separator G and thus afford a similar arrangement though somewhat less cooling for refrigeration purposes. However, the arrangement shown in Figure I is preferred as providing greater refrigeration and permitting a reduction in power otherwise required for the process.

A part of the hydrocarbon liquids in the bottom of fractionator H is throttled through valve 15 .to near atmospheric pressure and is passed through liquefier J where it provides refrigeration to liquefy nitrogen under pressure in the other circuit of the interchanger. The hydrocarbon mixture emerging from the top of the liquefier J is passed to interchanger A to cool a portion of the incoming raw gas feed and is finally discharged at near atmospheric temperature at for use as low pressure fuel gas or to be recompressed and commingled with the main stream of processed gas out at H. Where the quantity of hydrocarbon liquid utilized in liquefier J is appreciably in excess of the quantity of nitrogen removed in the process, it is preferred to pass hydrocarbon vapor instead of the nitrogen vapor through separator G for rectification of the gas phase and cooling of the liquid phase therein. Or both hydrocarbon and nitrogen vapors may be passed through separator G by provision of separate circuits therethrough. In either case, passage of vapors through G Would be followed by passage through interchangers A and AA as indicated and previously described.

hydrocarbons in the gas.

Liquid nitrogen is withdrawn from liquefler J at substantially a uniform rate by reflux pump N and discharged into the top of fractionator H at 24 as reflux for the fractionating process. Reflux pump N may be eliminated by placing liquefier- J at a higher elevation than the top of fractionator H and utilizing gravity flow of liquid nitrogen reflux from J to H. However, use of reflux pump N permits better control of the quantity of reflux added to H and tends to improve the operating stability thereof. Thus use of the reflux pump is a preferred arrangement.

Typical of the results that may be obtained with natural gas are the data given in the following examples:

Example I .-Gas compositions (gas "41) Before removal After removal of nitrogen g sg g} of nitrogen and heavy mtm en and heavy hydrocarbons g hydrocarbons Nitrogen 9. 7 Methane 79. 4 87. 9 93. 1 Etl1ane.. 5.9 6.5 6.9 Propane.. 3.3 3.7 Butane,plus. 1.7 1.9

Example II .-Gas compositions (gas "3) Before removal tel, After removal of nitrogen inoval of of nitrogen and heavy mm) en and heavy hydrocarbons, g hydrocarbons Nitrogen 27. 3 Methane (i5. 9 9-). 6 94. 7 Ethane 3. 7 5.1 6. 3 Propane l. 8 2. 5 1.3 1. 8

Butane, plus Example III.Gas c mpositions (gas *C) Before removal After removal After reof nitrogen of nitrogen and heavy gs g and heavy hydrocarbons n hydrocarbons Nitrogen l5. 0 0 0 Methane 73. 0 85. 9 g 92. 4 Ethane... 6.0 7.0 7.6 Propan c 4. (l i. 7 0 Butane, plus 2.0 2. 4 0

By calculation it can be shown that in Example III the original heating value of the gas (1018 B. t. u./cu. ft.) was raised 17.7 percent (up to 1198 B. t. u./cu. ft.) after the nitrogen removal, and 5.3 percent (up to 1072 B. t. u. /cu. ft.) after nitrogen and heavy hydrocarbon removal.

The results indicate that to improve the heating value alone, it is preferable to leave the heavy However, removal of them so improves the handling properties of the resulting gas, in pipe lines, etc., that in our preferred process, they are always removed, prior to separation of the remaining gases from the nitrogen.

While any appreciable amount of helium can be recovered, if this is of economic importance, the actual recovery of the small helium content (0.2 to 2.0% by volume) in some natural gas is not an essential part of this invention.

As used herein and in the appended claims, the term methane is not limited to substantially pure methane alone, but also to include methaneethane mixtures in the proportions customarily found in natural gas compositions.

While I have, in accordance with the patent laws, described and shown my invention in a pre- "-r erred. iorm,.;=it should-h un tood that t e .prcaess and.apuaratusareznot. m ted tothezsp c ific description herein, but iscapabla of various modifications without departing from the spirit :the;inve ntion-, -and that all such modifications as fall-within the scope of the appended-claims .are intendedto be included therein.

.-I-;clai m: ;,1. ;A process for. the treatment of 1 gaseou hydrocarbon-mixtures containing asubstantial pro- ;portion of; nitrogen as a diluent comprising re- :portion of the nitrogen vapors, thereby producing ;liquid nitrogen for reflux in the rectifying zone, and utilizing the remaining vapor and liquid fractions for cooling the gas being treated in said second and first heat exchange zones.

2. A process according to claim 1 in which additional refrigeration is supplied to the process by expansion of the nitrogen from the second heat exchange zone.

'3.--Apparatus for the separation of gaseous mixtures by liquefaction and rectification com- --prising, in combination, a first heat exchanger, means for supplying a gaseous mixture under 7.

pressure to said first heat exchangenasecond heat exchanger in series with said first heat exchanger,..means for withdrawing the relatively cold fluid mixture from said second heat eX- changer, throttling means for controlledly reducing the pressure of said fluid mixture as it is withdrawn from said second heat exchanger, a condenser including a cooling coil for receiving said fluid mixture under reduced pressure, a rectifier, means for transferring to said rectifier liquid accumulating in said condenser, means in said rectifier for withdrawing a vaporous overhead fraction, means in said rectifier for withdrawing a liquefied bottom fraction, meansfor supplying vapors from said, rectifier to'the coolin coil of saidcondensenimeans for, supplying at least aportion of the liquefied bottom fraction from said rectifier. to saidsecond heat exchanger, means on said first heat exchangerforwithdrawingthe thereby warmed and vaporized bottom fraction, and means'connected with the outlet of the cooling coil of ,said'condenser for withdrawing the efliuent overhead fraction.

,4. A process for the treatment of naturalgas of high nitrogen content comprising precooling the natural. gas under superatmospheric I pres- ,sure in a first heat exchange zonewith cold .methane gas derived from, the process. to. a.,.tem-

perature; at which hydrocarbons heavier than ethane are co ndensed, separating. the. resulting liquid. hydrocarbon fraction, passing the, stripped natural gas in indirect heat exchange with effluent liquid methane in asecond heat exchange zone-thereby vaporizing the liquid methane and further cooling the stripped natural gas, adiabatically expanding the strippedand cooled natural gas then passing the resulting vaporsthrougha third heat exchange zone in indirect heat. ex- ,changeMi-th colder efiluentnitrogen. vapors, rectifying the resulting liquid; and ,vapor fractions with liquid nitrogen, thereby separating the .methaneas aliquidfr ction and the ni o en a anoverhead vapor fraction, andutilizing atleast a -portion,of; these fractions as the coolantinthe said; second ,andthird heat-exchange 20 65,. :re- ,spectively.

5.. Alprocess as in claim 4 in which a 7 portion .ofthe liquid methane fraction is vaporized under reduced pressure inindirect heat exchange with a portion of the nitrogen vapors, thereby producing-the liquid nitrogen ferrectifying theliquid and vapor fractions from the third heat exchange zone.

GPA process for theimprovement-of a hydrocarbon g-as of high nitrogen content :which comprises cooling said gas under superatmospheric pressure by indirect heat exchange in at least :one heatexchangezone with vaporizing liquid methane derived from the process'further cooling said gasby adiabatic expansion thereof, rectifying ,the resulting liquid and vapor' fractions with a liquid-nitrogen reflux, therebyseparating methane as a liquidfraction and nitrogen as aanoverhBad vapor fraction, utilizing the liquified methane to cool incoming warm gas by vaporization thereof inindirect heat exchange withgsaid warm gas, and utilizing the refrigerative capacity ofa portionof said liquid methane for ,cooling a portion of saidnitrogen vapors, thereby producing liquid nitrogen for reflux in said rectifying zone.

7. A process according to claim 6 in which the gas fed-t0 said processis under a pressure-of from 300 to 600 lbs/sq. in.

PAUL V. MULLINS.

*CITED The following references are of record in the fiieof this patent:

UNITED STATES PATENTS Number Name Date 1,773,012 Schuftan Aug. 12, 1930 2,475,957 Gilmore July 12, 1949 2,509,118 Cooper Mar. 7, 1950

Claims

1. A PROCESS FOR TREATMENT OF GASEOUS HYDROCARBON MIXTURES CONTAINING A SUBSTANTIAL PROPORTION OF NITROGEN AS A DILUENT COMPRISING REFRIGERATING A STREAM OF THE GAS UNDER SUPERATMOSPHERIC PRESSURE WITH EFFLUENT METHANE IN A FIRST HEAT EXCHANGE ZONE, LIQUEFYING THE NITROGEN AND METHANE OF THE GAS BY ADIABATIC EXPANSION THEREOF FOLLOWED BY HEAT EXCHANGE WITH COLDER NITROGEN VAPORS IN A SECOND HEAT EXCHANGE ZONE, REFLUXING THE RESULTING VAPOR AND LIQUID FRACTIONS WITH LIQUID NITROGEN IN A RECTIFYING ZONE TO SEPARATE THE NITROGEN AS A VAPOR FRACTION AND THE METHANE AS A LIQUID FRACTION, VAPORIZING A PORTION OF THE LIQUID FRACTION IN HEAT EXCHANGE WITH A PORTION OF THE NITROGEN VAPORS, THEREBY PRODUCING LIQUID NITROGEN FOR REFLUX IN THE RECTIFYING ZONE, AND UTILIZING THE REMAINING VAPOR AND LIQUID FRACTIONS FOR COOLING THE GAS BEING TREATED IN SAID SECOND AND FIRST HEAT EXCHANGE ZONES.