CA1048398A - Hydrocarbon gas processing - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A process for separating hydrocarbon gases is described for the recovery of gases such as ethane and heavier hydrocarbons from natural gas streams or similar refinery or process streams. In the process described, the gas to be separated is cooled at a high pressure to produce partial condensation, and the vapor and liquid portions are separated.
Liquid from the partial condensation is further cooled and then expanded to a lower pressure. At the lower pressure, the liquid is supplied to a distillation column, wherein it is separated into fractions. The vapor portion is work-expanded to the operating pressure of the distillation column and supplied to the distillation column below the feed point of the expanded liquid portion. The operating efficiency of the process is improved by turning back at least part of the vapor portion and warming it by heat exchange against incom-ing feed before it is work-expanded. By thus warming the vapor, more work and refrigeration can he recovered in the work expansion machine.

Description

. J 1 4 ~

483~

This inventlon relates to the processing of gas sereams containing hydrocarbons and other gases of similar volatility to remove desired condensible fractions. In par- ~
~ . . ..
ticular, the invention is concerned with processing of gas streams such as natural gas, synthetic gas and refinery gas streams to recover most of the propane and a major portion of `i;
the ethane content thereof, together with substantially all of the heavier hydrocarbon content of the gas.
Gas streams containing hydrocarbons and other gases of similar volatility which may be processed according to the present invention include natural gas, synthetic gas streams obtained from other hydrocarbon materials such as coal, crude oil, naphtha, oil shale, tar sands, and lignite. ~atural -gas typically has a major proportion of methane and ethane (i.e., the combined Cl and C2 fractions constitute at least 50% of the gas on a molar bases). There may also be lesser amounts of the relatively heavier hydrocarbons such as pro~
pane, butanes, pentanes, and the like as well as H2, N2, C2 and other gases. A typical analysis of a natural gas stream to be processed in accordance with the invention would be, in approximate mol %, 80% methane, 10% ethane, 5%
propane, 0.5% iso-butane, 1.5% normal butane, 0.25% iso-pentane, 0.25% normal pentane, 0.5~ hexane plus, with the balance ~ade up of nitrogen and carbon dioxide. Sulfur-25 containing gases are also often found in natural gas. ; ~ ;~

Recent substantial increases in the market for the `~
ethane and propane components of natural gas has provided demand for processes yielding higher recovery levels of ~hese ~ ~, ~: -: .: . , --1~4839~
products. Available processes for separating these materials include those based upon cooling and refrigeration of gas, oil absorption, refrigerated oil absorption, and the more recent cryogenic processes utilizing the principle of gas expansion through a mechanical device to produce power while simultaneously extracting heat from the system. ~epending upon the pressure of the gas source, ehe richness (ethane and heavier hydrocarbons content) of the gas and the desired end products, each of these prior art processes or a combina-tion thereof may he employed.
The cryogenic expansion type recovery process is now generallv preferred for ethane recovery because it provides maximum simplicitv with ease of start up, operating flexibility, good effici~ncv. safety, and good reliability. U.S. Patents Nos. 3,360,944, 3,292,380, and 3,292,38l describe relevant processes.
In a typical cryogenic expansion tvpe recovery pro-cess a feed gas stream under pressure is cooled by heat ex-change with other streams of the process and/or external sources of cooling such as a propane compression-refrigeration system. ;~
As the gas i9 cooled, liquids are condensed and are collected in one or more separators as a high-prexsure liquid feed con-taining most of the desired C2+ components. The high pressure liquid feed is then expanded to a lower pressure. The vaporiza-tion occurring during expansion of the liquid results in fur-ther cooling of the remaining portion of the liquid. The cooled stream comprising a mixture of liqllid and vapor is demethanized in a demethanizer column. The demethanizer i~s a fractionating column in which the expansion-cooled stream is fractionated to separate residual methane, nitrogen and :, . , , :
:' :. :. ' , ~ : ' ~q~4~398 :
other volatile gases as overhead vapor from the desired pro-ducts of ethane, propane and heavier component~s as bottom product.
If the feed ~stream is not totally condensed, typlcally it is not, the vapor remaining from this partial condensation is expanded to a lower pressure. Additional llquids are con-densed as a result of the further cooling of the stream during expansion. The pressure after the expansion is usually the same pressure at which the demethanizer is operated. Liquids thus obtained are also supplied as a feed to the demethanizer.
Typically, remaining vapor and the demethanizer overhead vapor are combined as the residual methane product gas.
~:
In the ideal operation of such a separation process ;
the vapors leaving the process will contain substantially all of the methane found in the feed gas to the recovery plant, ; and substantially no hydrocarbons equivalent to ethane or heavier components. Tlle bottoms fraction leaving the demetha-nizer will contain substantially all of the heavier components ~ and essentially no methane. In practice, however, this ideal ;~ 20 situation is not obtained for the reason that the conventional demethanizer is operated largely as a stripping column. The methane product in the process, therefore, typically comprises vapors leaving the top fractionation stage of the column to-gether with vapors not subjected to any rectification step. ~ :~
Substantial los~ses of ethane occur because the vapors dis- ~:
charged from the low temperature separation steps contain ;
;~ ethane and heavier components which could be recovered if those vapors could be brought to lower temperatures or if they were brought in cor.tact with a significant quantity of relatively heavy hydrocarbons, for example C3 and heavier, 1~4~3 :

capable of absorbing the ethane.
As described in co-pending application,Serial No.
271,357 ~iled February 8, 1977 of Campbell, Wilkinson and Rambo, improved ethane recovery is achieved by pre-cooling the con-densed high-pressure liquid prior to expansion. Such pre-cooling will reduce the te~perature of the flash-expanded ~-liquid feed supplied to the demethanizer and thus imp.rove ethane recovery. Moreover, as described in said applica-tions, by pre-cooling the high pressure liquid feed, the ~ :
temperature of the expanded liquid may be sufficiently reduced that it can be used as top column feed in the de-methanizer, while the expanded vapor is supplied to the de~
methanizer at a feed .point intermediate the top feed and column bottom. This variation permits recoverY of ethane ~ :~
,::.~: ,, ~.
15 contained in ~he expanded vapor which would otherwise be lost. ;~
It will be obvious that to supply external refrigera-tion at this stage of the process is difficult because of the extremely low temperatures encountered. In typical demethanizer operations the expanded liquid and vapor feeds are typically at temperatures in the order of -120~F. to -190F. Accordingly, .~ `~
pre-cooling of the condensed high pressure liquid stream feed can best be achieved by heat exchange of the condensed high pressure liquid stream feed with streams derived within the proc2ss as descr$bed in co-pending applications Serial NoO
271,357.
As already indicated, in modern gas processing plants the vapors remaining from partial condensation of _ r~

~ 8398 the feed gas are usually expanded to the operating .
pressure of the demethanizer column.in a turbo-expander and, prior to the invention disclosed in co-pending application Serial No. 271,357 supplied to the demethanizer as the top feed. A turbo-expander is a machine which ex~
tracts useful work from the gas during expansion by expanding that gas in a substantially isentropic fashion. Such a work expansion has two advantages. First, it perm~ts cooling t~e vapor portion to the coldest practicable temperature. At-10 tainment of such cold te~peratures is-important in the top -feed to the demethanizer to provid~ the most complete re~
covery of C2+ components from the incoming feed gas. ~Second, the useful work recovered by isentropic expanslon can be used to supply a portion of the compression requirements 15 ordinarily required in the process. ;;~;
As explained in the co-pending applications of~

Campbell, Wilkinson and Rambo, Serial No. 271,357, the liquids recovered from partial condensation of the feed gas may be cooled below their bubble point, and i this is done, upon flash expansion it is poæsible to achieve flash-expanded temperatures of that sub-cooled liquid even below the temperature achieved by work-expansion of the . :
vapors fro~ partial condensation. Where such low tempera- ;-~
tures are achieved in the flash-expanded liquids, it is then usually preferable to supply that flash-expanded liquid as the column feed at a point above the feed point of the work-' ~ 6- ~ ~:

- expanded vapor recovered from partial condensation, The flash-expanded temperature of the sub-cooled liquid may be further reduced by comblning the liquid with a process gas stream which reduced the bubble point of the sub-cooled liquid as explained in Applicant's co-pending application, Serial No. 271,343 filed February 8, 1977.
In accordance with the present invention, it has now been discovered that when an alternate process stream is available to maintain top column condition, some (or all) of the vapors separated upon partial condensation may be turned back and reheated prior to expansion. The r~heating of the turn-back vapor stream can provide refrigeration to an earlier process stage.
For example, when the liquid portion of the partially condensed feed gas is subcooled and employed as the top column ~;
` feed, as explained in the aforementioned co-pending application, Serial No. 271,357, the expanded, cooled liquid may be able to maintain a cold top column temperature, and the vapors from work expansion of the partially condensed feed gas can be employed as a feed to the demethanizing column at an inter~
mediate position~ In this event~ it is advantageous to turn back some or all of those vapors and warm them prior to work expansion. The mechanical work recovèred and refrigeration developed in the expansion machine is greater as a result of expansion beginning at a warmer temperature. Because of the importance of heat economy in natural gas processing, it is generally preferable to turn back the vapors from partial con-densation in accordance with the present invention and employ those vapors in a heat exchange relation ~1~483~
~ with all or a portion of the incoming feed stream. This provides the desired warming of the turned-~ack vapor portion. In this manner vapor turn-~ack can reduce the need for external refrigera- ~-tion which might otherwise be required, or alternately could be used to increase recovery of liquid products.
In one aspect of this invention there is provided in a process for separation of a feed gas into a volatile residue gas and a relatively less volatile fraction, said feed gas containing hydrocarbons, methane and ethane together comprising a major portion of the feed gas, wherein ~ . .
(1) said feed gas under pressure is cooled to partially condense said gas and form thereby a liquid portion and a feed ;
gas vapor;

(2) at least some of the liquid portion thereby obtained is cooled to a temperature below its bubble point;

(3) the cooled liquid portion is expanded in an expansion means to a lower pressure whereby a first part of said liquid portion vaporizes t~ cool the expanded liquid portion;

(4) at least part of said expanded liquid portion is 20 -thereafter supplied to a fractionation column at a first feed -~
point wherein said relatively less volatile fraction is separated;

(5) said feed gas vapor is expanded to said lower pressure ;-in a work-expansion machine, wherein work is extracted therefrom;
and

(6) at least part of the expanded feed gas vapor is supplied to said ~ractionation column, -the improvement comprising (a) reheating at least some of said feed gas vapor prior to expansion thereof; and ~ ;~
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: ~ ' $ --1q~4839~
(b) thereafter expanding said reheated portion in a work expansion machine and supplying said expanded reheated portion at a second feed point in said fractionation column below said first feed point.
In another aspect of this invention there is provided in a process for separation of a feed gas into a volatile residue .
gas and a relatively less volatile fraction, said feed gas con-taining hydrocarbons, methane and ethane together comprising a major portion of the feed gas, wherein~
(1) said feed gas under pressure is cooled to partially condense said feed gas and form thereby a liquid portion and a feed gas vapor;
(2) said liquid portion is expanded to a lower pressure;
t3) at least a part-of the expanded liquid portion is supplied as a feed to a fractionation column wherein said relatively less volatile fraction is separated; and ~, ~
(43 said feed gas vapor is expanded to said lower pressure, and at least a part of the expanded feed gas vapor is supplied as a feed to said fractionation column, the improvement wherein :~
(a) a first part of said feed gas vapor is expanded to said ;`
lower pressure, whereby said first part is cooled;
(b) at least a portion of said expanded first part is :;
supplied to the fractionation column as column top feed;
(c) a second part of said feed gas vapor is reheated by :~
directing said second part into heat exchange relation with at -: .
least a portion of said feed gas under pressure; ;:
(d) said reheated second part is expanded in a work expansion machine to said lower pressure, whereby it is cooled;
.-. ~, . .
30 and; ;.......... .

- 8(a) - ~ :

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, .. _ ........................................ . - -: ~ ; s ~4~39~3 (e) said expanded second part is thereafter supplied to said fractionation column at a feed point beIow the top column feed.
In a further aspect of this invention there i5 provided in an apparatus for the separation of a feed gas into ;`~
a volatile residue gas and a relatively less volatile fraction, said feed gas containing hydrocarbons, methane and ethane together comprising a major portion of the feed gas, said `~
apparatus having :~
(a) cooling means for cooling said feed gas under pressure to partially condense said gas sufficiently to fonm a liquid ~ ~`
portion and a feed gas vapor;
(b) sub-cooling means connected to said cooling means (a) ~ :
to receive at least some of said liquid portion and to sub-cool it to a temperature below its bubble point;
(c) a first expansion means connectad to said sub-cooling -means to receive the sub-cooled liquid portion and to expand it ..
to a lower pressure, whereby a part of said liquid portion vaporizes to cool the expanded sub-cooled liquid portion;
(d) a fractionation column connected to said first expansion means to receive at least part of the expanded sub-cooled liquid portion at a first feed point and to separate said relatively less volatile fraction; .
(e) a serond expansion means connected to said cooling means (a) to receive feed gas vapor therefrom and expand it to .
said lower pressure in a work-expansion engine, wherein work is extracted therefrom, said second expansion means being further `. :
connected to said fractionation column to supply at least part -~
of the expanded feed gas vapor to said fractionation column, b) ~
,., . :

~L09~839~3 ~ the improvement comprising means for turning back at least a portion of said feed gas vapor prior to expansion thereof, said turn-back means comprising (i) heat exchange means connected to said cooling means (a) to receive at least a portion of said feed gas vapor, said heat exchange means being connected to reheat said port.ion of said feed gas vapor;
(ii) a third expansion means connected to said heat exchange means (i) to receive said reheated portion of said feed gas vapor and to expand said reheated portion to said ~ ~
lower pressure while extracting work therefrom; and ~: :
(iii) means connecting said third expansion means (ii) to said fractionation column (d) to supply the expanded reheated feed gas thereto at a second feed point, said second feed point being at a lower position on said fractionation column than said first feed point.
In a still further aspect of this invention there is provided in an apparatus for the separation of a feed gas into a volatile residue gas and a relatively less volatile fraction, :~-said feed.gas containing hydrocarbons-, methane and ethane together comprising a major portion of the feed gas, said apparatus having ;~
(a) cooling means for cooling said feed gas under pressure to partially condense it and to form thereby a liquid portion ~ :~
and a feed gas vapor;
(b) sub-cooling means connected to said cooling means to receive at least some of said liquid portion, and to sub~
cool it to a temperature below its bubble point;
(cj expansion means connected to said sub-cooling means (a) to receive the sub-cooled liquid portion and to expand it to a lower pre~sure;

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.

: . . . . . .. . . . .
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iL09~398 ., ~ (d) a fractionation column connected to receive at least a portion of the expanded suh-cooled liquid portion at a first feed point and to separate said relatively less volatile fraction;
(e) a second expansion means connected to said cooling means (a) to receive said feed gas vapor and to expand it to said lower pressure, said second expansion means being further connected to said fractionation column to supply at least a ~:
portion of the expanded feed gas vapor thereto as a feed, ` ~-. .:
the improvement comprising (i) dividing means connected to said cooling means (a) to receive said feed gas vapor and to divide it into a first ~ ~ -part and a second part;
(ii) means connecting said dividing means (i) to receive said first part of said feed gas vapor and supply it to said second expansion means (4) wherein said first part is expanded and supplied to said fractionation column;
(iii) heat exchange means connected to said dividing means (i) to reseive said second part of said feed gas vapor, : 20 said heat exchange means being further connected to receive a portion of said feed gas under pressure, thereby to direct said second part of said feed gas vapor into heat exchange relation with said feed gas under pressure to reheat said feed gas vapor;
(iv) expansion means connected to said heat exchange means (iii) to receive said reheated second part of said feed . ~.
gas vapor and to expand i* to said lower pressure while extract~
ing work therefrom; and ~ v) means connecting said expansion means tiV) to said fractionation column at a second feed point to supply said expanded second part to said fractionation column at said second ~ :
..; -': ., ::

- 8(d) - ~ ~

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~L~483~8 ~ :feed point, said second feed point ~eing at a lower column position than said first feed point.
The present invention will be better understood by reference to the following drawings and examples, in which:
Figure 1 is a flow diagram of a single-stage cryo-genic expander natural gas processing plant of the prior art incorporating a set of conditions for a typically rich natural gas stream;
Figure 2 is a flow diagram of a single-stage cryo-genic expander natural gas processing plant of the prior art incorporating a set of conditions for a typically lean natural ~ ;~
gas stream;
Figure 3 is a flow diagram of a gas processing plantembodying the invention forming the subject matter of Applicant's Canadian patent application Serial Nos. 271,357 and 271,343, both filed February 8, 1977, which is employed as a base case;
Figure 4 is a flow diagram of a gas proFessing plant in accordance with the present invention. ~ ;
Figure 5 is a variation of the present invention in ~20 which a portion of the condensed high-pressure liquid feed is sub-cooled and supplied as an intermediate column feed.
Figure 6 is a varlation o~ the present invention in which a portion of the high-pressure vapor is used as vapor turn-back and a portion is expanded directly to the demethanizer, Figures 7A and 7B are graphs showing carbon dioxide - 8~e) ~

'; ' ' - :

~L~4839~3 as a function of temperature for one embodiment of this invention compared to the prior art.
In the following explanation of the above ~igures, tables are provided summarizing fln~ rates calculated for repre-senta~ive process conditions, Tn the tables appearing herein, the values for flow rates (in pound moles per hour) have been rounded to the nearesr whole numher, fnr convenience. The tntal stream flow rates shown in the tables include a]l nnn-hydrocarbon components and hence are generally larger than the sum of tllc stream flow rates for the hydrocarbon components. Temperatures indicated are approximate values, rounded to the nearest degree.
Referring tn Figure l, for a fuller description of a typical conventional ethane recovery process,~plant inlet gas from which carhon dioxide and sulfur compollnds have heen removed (if the concentrati~n nf these cumpounds in the plant inle~ gas would cause the product stream not to meet specificatinns, or cause icing in the equip~ent), and which has been dehydrated enters the ,process at l20F. and 9ln psia as stream 23. Tt is ~-divided into two parallel streams and cooled to 45F. by heat exchange with cool residue gas at 5F. in exchanger 10; with product liquids (stream 26) at 82F. in exchang-er 11; and with demethanizer liquid at 53F. in demeth-anizer reboiler 12. From these exchangers, the streams `
recombine and enter the gas chiller, exchanger 13, where ~; 25 the combined stream is cooled to 10F. with propane refrig-erant at 5F. The cooled stream is again divided into two ~ -parallel streams and further chilled by heat exchange with : cold residue gas (gtream 29i at -107~F. in excilanger 14, ~; and with demethanizer liquids at -80F. in demethanizer side reboiler 15. The streams are recombined and enter a ~ ':
' .

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~ 13398 high-pressure separator 16 at -45F. and 9C0 psia as stream 23a. The condensed liquid (stream 24~ is separated and fed ~' ~ to the demethanizer l9 through expansion valve 30. An ex-pansion engine may be used in place of the expansion valve 30 if desired.
The cooled gas from the high pressure separator 16 flows through expander 17 where it is work expanded from 900 psia to 290 psia. The work expansion chills the ga~ to -125F.
Expander 17 is preferably a turbo-expander, having a compressor 21 mounted on the expander shaft. For convenience. expander 17 is sometimes hereinafter referred to as the expansion means.
In certain prior art embodimen'ts, expander ]7 is replaced by a conventional expansion valve. ~
Liquid condensed during expansion is separated in , ,, low pressure separator 18. The llquid i5 fed on level control through line 25 to the demethanizer column 19 at the top and flows from a chimney tray (not shown) as top feed to the column l9. ,~
,It should he noted that in certain embodiments lou pressure separator 18 may be included as part of demethanizer l9, occupying the top section of the column. In this case, the expander outlet stream enters above a chimney tray at the bottom of ~he ~separator section, located at the top of ~;

the column. The liquid then flvws from the chimney tray as top feed to the~demethanizing section of the column.
As liquid fed to demethanizer 19 flows down the column, it is contacted by vapors which strip the methane ~' from the liquid to produce a demethanized liquid product at '~
the bottom. The heat required to generate stripping vapors -i~48~g~
is provided ~y heat e~changers 12 and 15.
The vapors stripped from the condensed liquid in demethanizer 19 exit ~hrough line 27 to join the cold outlet gas from separator 18 via line 28. The combined vapor stream then flow through line 29 back through heat exchangers 14 and 10. Following these exchangers, the gas flows through compressor 21 drivan by expander 17 and directly~coupled thereto. Compressor 21 compresses the gas to a discharge pressure of about 305 psia. The gas then enters a compressor ~ -22 and is compressed ~o a final discharge pressure of 900 psia.
Inlet and llquid component flow rates/ outlet liquid recoveries and compression requirements for~this prior art process shown in Figure 1 are given in the following table:

TABLELI
(Flg. 1) St~eam Flow Rate Su~mary - Lb. Moles/Hr.
STREAMMETHANE ETHANEPROPANE BUTANES+ TOTAL ~ ' lLOO22~Z~ 163 130 1647 1 32 ~
26 3 162 I57 130 453 , , ;
RECOVERIES `
Ethane72.9%29~296 GAL/DAY
Propane96.2%39,270 GAL/DAY
CO PRESSION HORSEPOWER ~;
Re~rlgeration 256 BHP
Recompression 892 B~IP
Total 1148 BHP
In Figure 2 a typical lean natural gas stream is processed and cooled using a prior art process similar to . `, .
- 1 1- ". :: ~

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.
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~04839~3 `
that shown in Figure 1. The inlet gas stream 33 is cooled to -67 F. and flows to high pressure separator 16 as stream 33a where the liquid contained therein is separated and fed on level control through line 34 and expansion valve 30 to demethanizer 19 i~ the middle of the column.
Cold gas from separator 16 flows through expander 17 where because of work e~pansion from 900 psia to 250 psia, the gas is chilled to -153F. The liquid condensed during expan-sion is separated in low pressure separator 18 and is fed on level control through line 35 to the demethanizer 19 as top feed to the column.
The data for this case are given in the following table:

TABLE II
(Flg. 2 Stream Flow ~ate Summ~y - Lb. Moles/Hr.
STREAN riMETHANE I ETHANE PROPANE BUTANES+ TOTAL

34 280 42 25 39 391 ;~

RECOVERIES
Ethane 79~0/O 17,355 GAL/DAY
Propane 98.2% 8,935 GAL/DAY
COMPRESSION HORSEPOWER
- : .
Refrigeration0 BHP
Recompression1180 BHP ~ ~
Total1180 BHP ;
In the prlor art cases discussed with respect to Figure 1 and Figure 2 above, recoveries of ethane are 73% ~ ;

_12-: - - , , ., , :, ~: ,i , . - .

104839~
for the case of the rich gas feed and 79% for the lean gas feed. It is recognized that some improvement in yield may result by adding one or more~.cooling steps followed by one or more separation steps9 or by altering the temperature of separator 16 or the pressure in separator 18. Recoveries of ethane and propane obtained in this manner, while possibly improv0d over the cases illustrated by Figure 1 and Figure 2, are significantly less than yields which can be obtained in ~ :
accordance with the process of the present invention. ~::
For purposes of further comparison, a base case B
has been calculated following the same flow diagram as in ;.
Figure 3 but at a somewhat lower column pressure. Under the s;~
conditions of base case B~ more refrigeration could be ex- . .
~racted.~ifrom residue gas streams 43, 43a and 43b, and the demethani~er reboiler,~lmaking it possible to eliminate ex~
ternal refrigeration i~ heat exchanger 13. This reduced the ~.
horsepower required by the process but also reduced the ethane ..
and propane recoveries.
A summary of the process conditions of the princi- .
pal streams for base case B is set forth below in Table III ;~
and a stream flow rate su~mary for base case B is set forth . .
below in Table V. ~ ~

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~4~398 ~: ~
TABI,E III
; (ri~. 3 STREA!~ CONDITIONS
Stream Base Case A Base_Case B
33 120F.; sln psia 120F.; 910 psia 33a, 34, 41, 42 -67F.; 900 psla -67F.; 900 psia 41a -145F.; 290 psia -148F.; 275 psia 43 -154F. -154F.
43a _75F. -112F.
. '.:.
43b -27F. 25F.
43c - 9 8 F . 115F. ` ;~
44 46~F. 44F-47 -146F.; 900 psia -145F.; 900 psia ' O' ' ' .
47a -]55~F.; 290 psia -155 F.; 275 psia As indicat~d above, the present inventi~,n may be used as an impr~vement in the ga~ re~very process 8S set forth in ~. ... .
~` said co-pending application, Serial No. 271,357 filed February 8, 1977. Figure 3 illustrates a gas recovery . :
facility employing the invention described in these applications 20 and will be employed as a base case for purposes of explaining ;~ ~ -the present invention. In addition, in the flow plan of .
Figure 3, the subcooled liquid is combined with a portion of ~ the vapors from partial condensation. Such a further step `~
:~ reduces the bubble point of the subcooled liquid as explained ,~. .~-. .
'~25 in Applicant's co-pending application Serial No. 271,343 filed Febxuary 8, 1977. With respect to Figure 3, the process .
.. : flow conditions discussed below and flow rates set forth in Table III have been calculated on the ~ 30 -~

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basi~ of a lean feed gas composition as see forth in Table II
~8 stream 33.
Referring eo Figure 3, p]ant inlet gas 33 from which carbon dioxide and sulur compounds have been removed .
ant which has been dehydrated enters the process at 120F.
and 910 psla. It is div~ded into two parallel streams and cooled to -3F. by ileat exchange with cool residue gas 43b at -27F. in heat exchanger 10; wlth liquid product (stream ~;
44) st 46F. in heat exchanger 11; and with demethanizer liquid at 4~'F. in demethanizer reboiler 12. After recom~
bining the combined stream at -3F. is further cooled to - -21F. by external refrigeration u-ch as~a propane refrig~
- erant at -27F. The stream is again divided into two paral-lel streams and is further cooled by heat exchange with cold residue gas stream 43a at -75F. in heat exchanger 14 and with demethanizer liquids at -139~F. in demethanizer side .
reboiler 15. The streams are c~mhined and supplled 3S stream 33a to high' pressure separator 16 at -67F. and 900 psia ~ ~
where the condensed liquid is separated. The liquid from . ;
separator 16 (stream 34) is comblned with a portion of the vapor from separator 16 (stream 42). The combined stream then passes through heat exchanger 45 in heat exchange rela- -tion with overhead vapor s~ream 43 f~rom the demethanizer.
This cools and condenses ehé combined stream. The cooled and condensed stream at -146~. is thén~ expanded through an 8ppropriate expansion device such as expansion valve 46 to a pressure of about 290 psia. During expansion. a portion of the feed will vaporize, resulting in coolin~ of the remain-lng portion. In the process illustrated in this case, expanded 30 seream 47a leaving expansion valve 46 reaches a temperature ~ -15-.. . - . - , . . ..

: : , . .
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... . . .

~134839B

o -155JF. and ls supplied to the demethanizer 19 as the top feed.
The remainin& vapor from separator 16 (stream 41) enters a work expansion en~ine ~n which mechanical energy is e~tracted from this portion of the high pressure vapor. As the vapor is expanded from a pressure of about 900 psia to about 290 psia, work expansion cools the expanded vapor 4~a to a temperature of approxi~ately -145~F. The expanded and partially condensed vapor 41a is supplied to the demethanizer 10 19 at an intermediate point. - ~ `
The temperature and pressure condltions of some of the princ$pal streams are summarized in ~able III below as base case A, and a stream flow summary for base case A is ~ -set forth in Table IV below.

Base Case ~ Base Case B
Ethane 92.56%; 20,323 GALIDAY 90.52%; 19,876 GAL/DAY
. : -Propa~e 97.89%; 8,910 GAL/DAY 97.56X; 8,881 GALiDAY

HORSEPOWER REQIIIREME~TS
, . .
` 20 . Base Case A Base Case B
Refrigeration 118 BHP O BNP
Recompression 1045 BHP 1116 BHP
Total 1163 BHP 1116 BHP
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TABLE IV
(Fig. 3) Stream Flow Rate Summarv, Base Case A - Lb. Moles/Hr.
STREA:~ ~IETHANEETHA:lEPR(lPANE BUTANES+ TOTAL :
33 ~1447 90 36 43 1647 ~ ,:
34 280 42 25 39 391 .

~2 311 12 3 1 335 `
43 1446 6 0 . O 1475 .
44 1 84 36 43. 172 ~ ~ .

rABLE \T .
(Fig. 3) -Stream Flow Rate Sllmmarv, Base Case B - Lb. .~loles/Hr.
STREA~ ~IETHANE ETHANE PROPANE B~TANES+ TOTAL
33 1447 90 36 - 43 1647 ~
.34 280 h2 ~5 49 391 . .
41 1078 45 10 ~ 1160 42 89 3 1~ ~ . 0 96 43 - 1445 8 0 0 . 1479 : ~ :
~ 20 44 2 82 36 43 168 :, . ' ~ ' The present invention is illustrated by the follow~
ing examples:
:
Example 1 -Figure 4 sets forth a process diagram for a typical natural gas plant in accordance with the present invention.
The flow plan is sintilar to the flow plan of Figure 3 except for the provision for vapor turnback. In Figure 4, ~nlet gas is cooled and partlallv condensed t.hrough heat ex-changers 10~ Il, 12, 14 and 15 generally as described in con-: ~ :
-, . ~ , -17- .
,: ~ .

~8398 nection with Figure 3. It will be noted, however, that it was not found necessary in Figure 4 to make provision for external refrigeration (e.g., heat exchanger 13 of Flgure 3). Moreover, in Flgure 4, it will also be noted that in the second set of feed gas coolers, the feed is diviAed into three portions rather S than two. A portion of the feed is cooled in heat exchanger 57, as wlll be further explained below; another portion is cooled in heat exchanger 14 by heat exchange with cool residue gas stream 52a; and the third portion is cooled in heat exchanger lS by heat exchange with demethanizer liquid in demethanizer side rehoiler 15. The cooled and partially condensed feed gas 33a is supplled to separator 16 at -67F. and 900 psia.
Following first the liquid from separator 16, strea~
.~ 34 ~s combined with a portton 50 of the vapor f rom separator -~ 16. The combined stream then passes through heat exchanger lS 54 in heat exchange relation with the overhead vapor product ~stream 52) from demethanizer 19, resultin~ in cooling and i condensati~n of the combined stream. The cooled stream 55 is then expanded through an appropriate expansion device, such as~
expansion valve 56, to a pressure of about 290 psia. During ~ ~ -expansion, a portion of the feed will vaporize, resulting ln ~ ..
coollng of the remaining part. In th~ process of Figure 4, ~ ~
. ~
,~ ~ the expanded s~ream 55a leaving expansion valve 56 reaches a ~; temperature of -1;5F., and is supplied to demethanizer 19 as top feed.
~ The remalning vapor irom separator 16 (stream 51~
becomes ehe turn-back stream. The vapor turn-back 51 flows -~i ;
through heat exc~hanger 57 in lleat exchange relatinn with part of the plant inlet feed, In ~he process~of Figure 4, the turn-hack vapor 51a from exchanger 57 is~at about 5F. and .,~ ' :

: :

., -, . . . .

~839~
flows through expander 17 where because oE work expansion from about 895 psia to 290 psia, the gas 51b is chilled to -99 F. The chilled stream 51b from expander 17 flows to de~
methani7er 19 at an intermediate point.
A summ~ry of the principal streams in this example of the present case is set forth below in Table VI.
As will be seen from Table VI below, in this example of the present invention 92.53% of the ethane and 97.88% of -the propane were recovered. 1005 brake horsepower of recom~
pression were required to operate the process. By comparison with base case A above, it will be seen that for substantially the same recovery, the present invention reduces the horsepower requirements for process operation and in the case of this exmmple eliminated the need for external refrigeration.

;
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83Y1~3 TABl.E Vl ~Fig. 4j Stream summary, Exan~le l.
S~REA~ ~ETHA~E ETHA~E PROPANE BUTANES+ TOTAL CONDITIONS
5 33 1447 90 36 43 1647120F, 910 psia 33a 1447 90 36 43 1647-67F, 900 psia 10 51a 856 36 8 3 9215~F., 895 psla ~ .
51b 856 36 8 3 921-99'~F., 290 p~ta 52 1445 h 0 0. 1475 -154F... 290 psia ~2a 1445 6 0 0 -1475 -75F
52b 1445 6 O 0 1475 8F
~15 52c 1445 6 0 0 1475 110F
53 2 84 36. 43 172 45F
591 54 ~8 4n J26-146F, 900 psia 55a 591 54 28 40 726-155F, 290 psia -~
RECOVERIES ~ ' ~20 Ethane92.53~20,318 GAL/DAY
: Propane97.88~8,910 GAL/DAY
HORSEPOWER REQUIRE~E~TS
Refrtgeration 0 BHP ~
Recompression1005 BHP ~; :
Total1005 BHP

:-, ~

,:

' " ' ,' . ' '~ ' ' ' ~ ' ' 39~3 ~XAMPLE 2 Ex~mple 2 (Figure 5~ ~s another ~llustration of the present invention. In Example 2, a portion of the high-pressure liquid condensate was sub-cooled by residue gas from the demethanizer and flashed directly into the demethani~er at an intermediate feed position in the column.
Referring to Figure 5, the inlet gas is processed and cooled in a manner similar to that of Figure 4 in heat exchangers 10, 11, 1~, 14, 15 and 57 to provide a partly condensed feed gas 33a at -67CF. at -900 psia. The cooled inlet stream 33a then enters high-pressure separator 16 where the condensed liquid is separated.
The vapor from high-pressure separator 16 is divided into two portions. The first portion 60 is combined with a portion 64 of the liquid 34 stream from exchanger 62 wherein liquid 31 from separator 16 is sub-cooled. The remaining -~
portion of vapor from separator 16 enters heat exchanger 57 ~here it is used to cool a portion of plant inlet feed gas.
~r~m exchanger 57 the vapor stream 61a enters expander 17 ~here, because of work expansion from 895 psia to 250 psia, t~e gas is chilled to -108F. From expander 17 the stream 61b flows to demethanizer 19 at its lowest point.
The cooled liquid 34 from high pressure separator 16 enters heat exchanger 62 where it is sub-cooled to about -150F.
by heat exchange with a portion of cold residue gas 70. Following ; exchanger 62 the sub-cooled liquid is divided into two portions.
The first portion 63 flows through expansion valve 65 where it `;
undergoes expansion and flash vaporization and is cooled to rl58F. ~rom expansion valve 65 the stream 63a enters ~ 30 demethanizer 19 at its m~ddle feed point. The remaining liquid ! -21-, ~ ,, .. , . . -, .. ...

:', , ~04~339E~
portion 64 is combined with a portion fiO of the high pressure separator vapor. The combined stream then flows through heat exchanger 66 where it is cooled to -153F. by heat exchange , with a portion of the cold residue gas stream 70. From ex-changer 66 the subcooled stream 67 enters expanslon valve 68 and undergoes flash vaporization as the pressure is reduced to about 250 psla. From valve 68, the stream 67a now at -163F.
. flows to demethanizer 19 at its top feed point.
The vapors stripped from the condensed liquid in demethanizer 19 exit a.~ residlle gas 70. As already indicated, the residue gas 70 is divided and used as the refrigerant in exchangers 62 and 6~. The residue gas from these exchangers ~: is recombined and flows throl]gh the ha].an~e of the system to exchangers ]4 and 10 where it is used to cool and partially ~;
~` 15 condense the feed gas 33.
~` A summary of the condition of some of the principal streams is set forth in Table VII. .

~.~. . ' .

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:i, ' ~' , ~ 22-8~98 TABLE VII
~Fignre 5) Stream Conditions and Flow Rates STREAM METHANE ETHANE PROPANE BUTANES+ TOTAL CONDITION
331447 90 36 93 1647 120F., 910 psia 33a1447 90 36 43 1647 -67F., 900 psia 34280 42 25 39 391 -67 F.
60164 h 2 1 176 -67F.

61a1003 42 9 3 1080 5F., 900 psia 61b1003 42 9 3 1080 -108F., 250 p6ia 63140 21 12 19 195 -150F., 900 psia 63a140 21 12 19 195 -158F., 250 psia 64140 21 12 19 195 -150F., 900 pSiA
67304 27 14 20 272 -153F., 900 psia 67a304 27 14 20 272 -163F., 250 psia 701444 6 0 0 1479 -161F., 250 psia 71 3 84 36 43 168 39F.
` RECOVERIES
Ethane 93.02% 20,426 GAL/DAY
Propane 98.57% 8,972 GAL/DAY
COMPRESSION HORSEPOWER
Refrigeration O BHP
Recompression llll BHP
25Total llll BHP
'' ~
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, ~ ~

~ -23-The foregoing invention of turning back some (or all) ~ -of the high pressure feed gas vapors separated upon partial condensation is generally applicable in process flow plans where an alternate stream is available to maintain the demethanizer column at the desired overhead operating temperature.
Cooling of high-pressure condensate prior to expansion, and supplying the cooled expanded condensate at upper feed point in the column is a particularly pre~erred means of maintaining ; column overhead temperature. As indicated above, in co-pending application Serial No. 271,357 of Campbell, Wilkinson and Rambo, a variety of processes are disclosed for cooling of the high-pressure condensate recovered from the feed gas before expand~
ing that condensate to the demethanizer operating pressure. ~ ;~
The advantages of cooling the high pressure condensate before ~-~
expansion can be enhanced in accordance with the co-pending application No. 271,343 by combining that condensate with a portion of the vapor from the high-pressure separator in order to lower the temperature which would be obtained upon expansion of the condensate.
Variations of the invention of this application include the following:
~; (1) Some or all of the high-pressure condensate may be cooled by auto-refrigeration. In such a procedure, a cooled portion of the high-pressure condensate i5 divided into two !`,~`~ portions. One portion is expanded to the column operating pres-sure which causes a portion of it to vaporize and to cool the expanded stream. The expanded portion is then directed into heat exchange relation with the high-pressure condensate to ` 3 , . ! '., ....
' ` ` ' . ' ' ' ~ . :. .

;, '` ' ' '', ; ~ '~ . ' ~

' . .

~()41~39~ ~ `
obeain the cooled c~ndensate prior to e~pansion, The second portion of the cooled stream is expanded to a low temperature snd supplied to the demethanizer as the column top feed. In ; this embodiment the cooled high-pressure condensate may be divided into two portions and each portion separately expanded.
If more convenient, the entire cooled condensate stream can be expanded to the demethanizer pressure, and the expanded stream resulting then divided into the two portions discussed above.
(2) In another variation, all or a portion of the high-pressure condensate supplied as the top column feed may be heat exchanged prior to expansion with liquid in the de- ~ `
$~ methanizer column in one or more side stream reboilers.
t3) In either variation (1) or (2?, the amount of cooling obtained by expansion of the cooled high-pressure liquid can be enhanced by combining the high-pressure condensate with a portion of the high-pressure vapor as explained in Applicant's co-pending application, Serial No. 271,343. This variation is also applicable, as shown in Figures 4 and 5, to cases where the high-pressure condensate is cooled by residue gas.
This variation is particularly valuable in the treatment of ; lean feed gases, where there is sometimes a limited amount .
~ of high-pressure condensate available.

:~ (4) When employing turn-back of the high-pressure vapors, particularly in lean gas cases, very substantlal amounts - ~`
of high-pressure vapor are available and, if heated to too great . an extent in the turn-back heat exchanger (e.g., exchanger 57 ~ . . . . ..
`t f Figures 4 and 5~, the temperature reached by the turn-back i; gases after expansion will tend to overheat the demethanizer column and thus raise the column overhead tempera~ure.

~~ 30 v : - 25 -~: : . :: -,: . ,: .

A number of expedients are available ln such a ~ltu-ation: Only a portion of the hlgh pressure vapors may be turned back through exchanger S7, and the balsnce of the high-pressure vanors supplied directly to the demethanizer to maintain column overhead temperature. The balance thus supplied directly to the demethanizer may be expanded in a turbo-expander~ may be cooled (or partially condensed) by heat exchange against column overhead vapors and expanded into the demethanizer column or it may be used to enrich all or a portion oE the hlgh-pressure liquid condensate as explained above. S~ill another alternate would be to cool the expanded turn-back vapors if a cooling stream is available at an appropriate temperature wlthin the process. In ; cases where large amounts of vapors are available for use as a turn-back stream, it may be necessary, to avoid overheating the 15 demethanizer, to limit the temperature to which the turned-back ~::
vapors are warmed in exchanger 57. - -(5) In the ill~lstrations o~ ~he presen~ inve~tion set forth in the above examples, the turn-back vapors have been used to cool a portion of ehe incoming feed gas in ~he secood~
set of heat exchangers (i.e., in parallel with exchangers 14 aod 15 of Figures 4 and 5). It will be appreciated. however, ~hat in a gas treatment process as generally illustrated in Figures 1 through 5 there may be a variety of alternate needs for a cold gas stream, such as is available from ehe hlgh-; ~ .
pressure separator, where the refrigeration therein may beused even more effectively than indicated in examples l and 2.
By way of illustration, the turned-back vapors may be used in~
lieu of propane refrigeration in a heat exchanger located inter- ~
mediate between the two sets of feed gas precoolers such as heat ~ ;
.

: . . : ~ : . : . ::

: ,. ~ : .. : . .
~. - : .. . ~ ; :.

- ` ~0~398 exchanger 13 of Figure 1. Still another varlant, the turned-back vapors may be uqed to cool all or a portlon of the in-eoming feed gas at the initial condicion of 120F. such as through exchangers lO, 11, nr 12 of Figures 1 and 2. Still another variation, where propane refrigeration i9 emplayed, 1~ to use the turned-back vapors to subcool the condensed propane refrigerant prinr to employing the refrigerant in the process operatton.
(6) In still another variation of the presen~ in-vention, the feed gas vapor from separator 16 may be dlvided : into two portions, the first of which is used as the vapor turn-back and the second of which is used ~o c~ntrol the column overhead. In this embodiment, the second stream would be heat exchanged against cold residue gas from the demethani-~e. overhead. This may result in substantial condensation of the coole.d feed gas vapor if the vapor is below its critical ~r pressure. If the-stream is ahove the critical pressure, it wlll remaln single phase through the cooling. The second por-tion would then be expanded and supplied as the top column feed.
, 20 The vapor turn-back portion would be reheated as previously described, work expanded, and supplied as a lower column feed.
. In these variations the expanded turn-back vapors may also be - heat exchanged with residue gas.
.~ (7) The process flow plans and examples of the pre-2S sent invention have.heen d~scribed for convenience using shell and tube heae exchangers. In cryogenic opera~ions,~it is usually : .
preferred to use specially designed heat exchangers such as plate-fin heat exchangers. Such special heat exchangers have imprnved heat transfer characteristics which may permit closer ;, - , ~ 30 temperature approaches in the heat exchangers, lnwer cost, and also permit flow arrangements to accommodate heat exchange of several streams cnncurrent~

!~
2?

,;, , . , ' ,,~ ', , . ' ' ' .
/

33~8 Example_ 3 Example 3, as illustrated in Figure 6, is an example of the present lnvention in which a portion of the high-pres-sure feed gas obtained from partial condensation is employed as turn-back vapor and another portion is expanded directly to column pressure through a work expansion engine.
Referring to Figure 6, a lean feed gas is supplied to the process at a temperature of 120F. and a pressure of 910 psia at stream 33. Lean feed gas 33 i5 of the same com-position referred to above in connection with Figure 2. The feed gas is cooled to a temperature of -67F. and a pressure J of 900 psia through heat exchanges 10, 11, 12, 14, 15, and 57, generally as described above in connectinr, with Figures 4 and 5. The partially condensed feed 33a is supplied to separator 16 wherein the liquid and vapor is separated. Liquid portion 34 is drawn off frnm separator 1~, cnoled in heat exchanger 75 to a temperature nf -15n~F. (stream 34a), and then passed through expansinn~valve 76. The expanded stream 34b at -158F. is sup-plied to demethanizer 19 as a top column feed.
The vapors drawn nff from separator 16 are separated into two portions, 77 and 78. Portions 77 is expanded in work expansion engine 79. The expanded stream 77a achieves a tem-perature of -153F. and is supplied to the demethanizer as an intermediate column feed. Work extracted from stream 77 in ?~ 25 expander 79 is in part employed to recompress residue gas by means o~ associaeed compressor 80.

; ,:

~, -2~-,... . . . .. .. .. . . . .

T~lrn-back vapors 78 from separator 16 are directed : through heat exchanger 57 to precool a portion of liquid feed 33. The warmed turn-back vapors 78a leave exchanger 57 at a temperature of 50F. The warmed vapors are then expanded in expansion engine 81 and supplied to the demethanizer column as a second intermediate feed at a feed point below feed 77a at -77 F. Expander 81 is connected to an associated compressor Residue gas in the process illustrated in Figure 6 is obtained as a demethanizer overhead 83~ Demethanizer over- -head is employed to provide a part of the refrigeration required in the process by cooling (i) liquid 34 in exchanger 75, (ii) partly cooled feed gas in exchanger 14, and (iii~ hot feed gas in exchanger 10. Thereafter, the residue gas is recompressed to line pressure first in compressor 80 driven by work engine 79; second3 in compressor 82 driven by work engine 81; and fin- `~
ally, in supplementary compressor 84.
A summary of the principal stream flow rates and con-ditions is set forth below in Table VIII. As can be seen, in the process illustrated in Figure 6, an ethane recovery of 89.16%
and propane recovery of 97~73% at a total horsepower requirement of 1057 BHPé

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., . , , : - ,. ~ , .' ' ' :' ` ' . .' - ' ' ' ~ , ;

~483~8 TABLE IX
(Fig. fi) Process Stream Summar~
STREAM METHANE ETHA~E PROPANE BUTANES+ TOTAL CONDITIONS
331447 90 36 43 1647120F., 910 psia 33a1447 90 36 43 1647-67F., 900 psia 34 280 42 25 39 391-67F.
34a280 42 25 39 391-150 F., 900 psia 34b280 42 25 39 391-158F., 250 psia 77 584 24 5 2 628-67F., 900 psia 77a584 24 5 2 628-153F., 250 psia 78 584 24 5 2 628-67 F., 900 psia 78a584 24 5 2 62850F., 895 psia 78b584 24 5 2 628-77F., 250 psia . ~
831445 10 1 0 1483-156F., 250 psia 83a1445 10 1 0 1483-124F.
83b1445 10 1 0 148370F. ,~
83c1445 10 1 0 148384F.
` ~ 85 2 81 36 43 16442F.
RECOVERIES
.
, Ethane 89.16% 19,578 GAL/DAY
Propane 97.73X 8,896 GAL/DAY
.~, .
~,~ COMPRESSION HORSEPOWER
Refrigeration 0 BHP
Recompression 10_7 BHP

'. ~

,~ ~
'~, :

~ -30-~83~8 As is well known, natural gas streams usually con-tain carbon dioxide, sometimes in substantial amounts. The presence of carbon dioxide in the demethanizer can lead to icing of the column internals under cryogenic conditlons.
Even when feed gas contains less than 1% carbon dioxide it fractionates in the demethanlzer, and can build up to concen-trations of as much as 5% to 10% or greater. At such concen-trations, carbon dioxide can freeze out depending on tempera-ture, pressure, whether the carbon dioxide ls in the liquid or vapor phase, and the solubility of carbon dioxide in the liquid phase.
In the present invention, it has been found ehat when the vapor from the high pressure separator is expanded ; and supplied to the demethanizer below the top column feed position, the problem of carbon dioxide icing can be substan-tially mitigated. The high-pressure separator gas typically contains a large amount of methane relative to the amount of ethane and,carbon dioxide. When supplied as a mid-column ; feed, therefor, the high-pressure separator gas tends to di-lute the carbon dioxide concentration and to prevent it from increasing to icing levels.
The advantage of the present invention can be readily seen by plotting carbon dioxide concentration and temperature for various trays of the demethanizer when practicing the present invention and when following the prior art. A chart thus constructed for processing the gas as described above in Example 1 (see Figure 4 and Table IV) and containing 0.72% ~ `
carbon dioxide, can be compared with a similar chart constructed for the process of Figure 2 (prior art) applied to the same gas,(see Figures 7-A and 7-B). These charts also include ` equilibria for vapor-solid and liquid-solid conditions. The equilibrium data given in Figures 7A and 7B are for the methane-~:

!

~8~9~
car~on dioxide system. These data are generally considered representative for the methane and ethane systems. If the C2 concentration at a particular point in the column is at or above the equilibrium level for that temperature, icing can be expected. For practical design purposes, the engineer usually requires a margin of safety, i.e., the actual concen-tration be less than the "icing" concentration by a suitable safety factor.
As is evident, when following the prior art process of Figure 2 (per Figure 7-A), the vapor conditions at point A touches the line representing solid vapor phase equilibria.
By contrast, in Figure 7-B, neither the liquid nor vapor con-ditions reach or exceed their related equilibria condition.
Hence, icing risks are materially reduced.
It should be noted in connection with the foregoing that when designing demethanizer columns for use in the present invention the designer will routinely verify that ~; icing in tHe column will not occur. Even when vapor is fed at a mid column position it is possible that icing may occur if the process is designed for the highest possible ethane recovery. Such designs normally call for the coldest prac-' tical temperature at the top of the column. This will result in the carbon dioxide concentrations shifting to the right ; on the plots of Figures 7-A and 7-B. Depending on the par-ticular application, the result can be an objectionably high concentration of carbon dioxide near the top of the column.
For such a circumstance, it may be necessary to accept a some-what lower ethane recovery to avoid column icing or to pre-- treat the feed gas to reduce carbon dioxide levels to the point where they can be tolerated in the demethanizer. In ., .

:, ' ' .
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~1483~8 the alternative, it may be possible to avoid icing in such a circumstance by other alterations in the process conditions.
For instance, it may be possible to operate the high pressure separator at a dlfEerent temperature, to change the amount of re-heat, or to increase the quantity of vapor directed through the re-heater. If such alterations can be made within the limitations of the process heat balance, icing may be avoided without significant loss of ethane recovery.
In connection with the foregoing description of our invention, it should be noted that where the feed to the top of the demethanizer is a liquid which is expanded from a high pressure to a lower column operating pressure (as in Figures 4, 5, and 6), liquid may be auto cooled before expansion.
~i~ Such auto cooling will involve splitting the top liquid feed into two streams either before or after expansion, and directing one of the two streams thus obtained after expan-sion into heat exchange relation with the top column liquid feed before expansion.

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Claims (24)

WE CLAIM:

1. In a process for separation of a feed gas into a volatile residue gas and a relatively less volatile fraction, said feed gas containing hydrocarbons, methane and ethane together comprising a major portion of the feed gas, wherein (1) said feed gas under pressure is cooled to par-tially condense said gas and form thereby a liquid portion and a feed gas vapor;
(2) at least some of the liquid portion thereby obtained is cooled to a temperature below its bubble point;
(3) the cooled liquid portion is expanded in an ex-pansion means to a lower pressure whereby a first part of said liquid portion vaporizes to cool the expanded liquid portion;
(4) at least part of said expanded liquid portion is thereafter supplied to a fractionation column at a first feed point wherein said relatively less volatile fraction is sep-arated;
(5) said feed gas vapor is expanded to said lower pressure in a work-expansion machine, wherein work is extracted therefrom; and (6) at least part of the expanded feed gas vapor is supplied to said fractionation column, the improvement comprising (a) reheating at least some of said feed gas vapor prior to expansion thereof; and (b) thereafter expanding said reheated portion in a work expansion machine and supplying said expanded reheated portion at a second feed point in said fractionation column be-low said first feed point.

2. The improvement according to claim 1 wherein said feed gas vapor portion (a) is directed into heat exchange re-lation with at least a portion of said feed gas, thereby to provide part of the cooling required to partially condense said feed gas in step (1).

3. The improvement according to claim 1 wherein said cooled liquid (2) is obtained by directing a portion of said, liquid portion (1) into heat exchange relation with residue gas.

4. The improvement according to claim 1 wherein said cooled liquid (2) is obtained by directing at least some of said liquid portion (1) into heat exchange relation with liquid in said demethanizer column in a side reboiler.

5. The improvement according to claim 1 wherein:
(i) said cooled liquid (2) is divided into a first part and a remaining part;
(ii) said first part is expanded to said lower pressure, whereby a portion thereof vaporizes to cool the ex-panded first part;
(iii) said expanded first part is directed into heat exchange relation with at least some of said liquid portion (1) whereby said cooled liquid (2) is obtained; and (iv) said remaining part is expanded to said lower pressure, and at least some of the expanded remaining part is supplied to the demethanizer at said first feed point.

6. The improvement according to claim 1 wherein:

(i) a part of said feed gas vapor (1) is combined with said liquid portion (2) prior to expansion:

(ii) while the combined stream is at a temperature below the bubble point of said liquid portion (1), said com-bined stream is supplied to said expansion means, and then ex-panded to said lower pressure; and (iii.) at least some of the expanded combined stream is thereafter supplied to the fractionation column at said first feed point.

7. The improvement according to claim 1 wherein:
(i) said feed gas vapor (1) is divided into at least a first part and a second part;
(ii) said first part is reheated prior to expansion and thereafter expanded to said lower pressure and supplied to the fractionation column at said second feed point;
(iii) said second part without heating thereof is expanded to said lover pressure; and (iv) the expanded second part is supplied to the fractionation column at a feed point above said second feed point.

8. The improvement according to claim 1 wherein:
(i) said feed gas vapor (1) is divided into at least a first part and a second part;
(ii) said first part is reheated prior to expansion, and thereafter expanded to said lower pressure and supplied to the fractionation column at said second feed point;
(iii) said second part is cooled prior to expansion, and then expanded to said lower pressure; and (iv) the expanded second part is supplied to said fractionation column at a feed point above said. second feed point.

9. The improvement according to claim 1 wherein said expanded reheated portion is directed into heat exchange relation with at least some residue gas whereby said expanded reheated portion is cooled, and thereafter said expanded vapor feed gas is supplied to the fractionation column at said second feed point.

10. In a process for separation of a feed gas in-to a volatile residue gas and a relatively less volatile frac-tion, said feed gas containing hydrocarbons, methane and ethane together comprising a major portion of the feed gas, wherein:
(1) said feed gas under pressure is cooled to partially condense said feed gas and form thereby a liquid portion and a feed gas vapor;
(2) said liquid portion is expanded to a lower pressure;
(3) at least a part of the expanded liquid portion is supplied as a feed to a fractionation column wherein said relatively less volatile fraction is separated; and (4) said feed gas vapor is expanded to said lower pressure, and at least a part of the expanded feed gas vapor is supplied as a feed to said fractionation column;
the improvement wherein (a) a first part of said feed gas vapor is expanded to said lower pressure, whereby said first part is cooled;
(b) at least a portion of said expanded first part is supplied to the fractionation column as column top feed;
(c) a second part of said feed gas vapor is re-heated by directing said second part into heat exchange re-lation with at least a portion of said feed gas under pressure;

(d) said reheated second part is expanded in a work expansion machine to said lower pressure, whereby it is cooled;
and;
(e) said expanded second part is thereafter supplied to said fractionation column at a feed point below the top col-umn feed.

11. The improvement according to claim 10 wherein said first part is cooled prior to expansion thereof by heat exchange with at least a portion of said residue gas.

12. The improvement according to claim 10 wherein said expanded second part is further cooled by directing it into heat exchange relation to at least a portion of the residue gas.

13. In an apparatus for the separation of a feed gas into a volatile residue gas and a relatively less volatile fractions said feed gas containing hydrocarbons, methane and ethane together comprising a major portion of the feed gas, said apparatus having (a) cooling means for cooling said feed gas under pressure to partially condense said gas sufficiently to form a liquid portion and a feed gas vapor;
(b) sub-cooling means connected to said cooling means (a) to receive at least some of said liquid portion and to sub-cool it to a temperature below its bubble point;
(c) a first expansion means connected to said sub-cooling means to receive the sub-cooled liquid portion and to expand it to a lower pressure, whereby a part of said liquid portion vaporizes to cool the expanded sub-cooled liquid portion;
(d) a fractionation column connected to said first expansion means to receive at least part of the expanded sub-cooled liquid portion at a first feed point and to separate said relatively less volatile fraction;
(e) a second expansion means connected to said cooling means (a) to receive feed gas vapor therefrom and expand it to said lower pressure in a work-expansion engine, wherein work is extracted therefrom, said second expansion means being further connected to said fractionation column to supply at least part of the expanded feed gas vapor to said fractionation column, the improvement comprising means for turning back at least a portion of said feed gas vapor prior to expansion thereof, said turn-back means comprising (i) heat exchange means connected to said cooling means (a) to receive at least a portion of said feed gas vapor, said heat exchange means being connected to reheat said portion of said feed gas vapor;
(ii) a third expansion means connected to said heat exchange means (i) to receive said reheated portion of said feed gas vapor and to expand said reheated portion to said lower pressure while extracting work therefrom; and (iii) means connecting said third expansion means (ii) to said fractionation column (d) to supply the expanded reheated feed gas thereto at a second feed point, said-second feed point being at a lower position on said fractionation column than said first feed point.

14. The improvement according to claim 13 where-in said heat exchange means (i) is connected to direct said feed gas vapor portion into heat exchange relation with a portion of said feed gas under pressure, whereby said heat exchange means (i) provides a portion of the cooling required to partially condense said feed gas.

15. The improvement according to claim 13 wherein said sub-cooling means (b) is a heat exchanger connected to direct said liquid portion to be sub-cooled into heat exchange relation with cold residue gas, whereby said liquid portion is sub-cooled and said cold residue gas is warmed.

16. The improvement according to claim 13 wherein sub-cooling means (b) comprises means to direct said liquid portion to be sub-cooled into heat exchange relation with liquid in said fractionation column in a side reboiler.

17. The improvement according to claim 13 having the further improvement wherein (1) dividing means are connected to receive at least part of sub-cooled liquid portion from said sub-cooling means (b) and to divide it into a first part and a remaining part;
(2) means connected to said dividing means (1) to receive said first part and to expand said first part to said lower pressure, whereby a portion thereof vaporizes to cool the expanded first part;
(3) means connected to said means (2) to receive expanded first part and to direct said first expanded part to said sub-cooling means (b), wherein it passes in heat ex-change relation with the liquid portion to be sub-cooled;
(4) means connected to said dividing means (1) to receive said remaining part and to supply it to said first expansion means (c) wherein it is cooled, and from which at least a portion thereof is supplied to said fraction-ation column at said first feed point.

18. In the improvement according to claim 13, the further improvement wherein (1) combining means are connected to said cooling means (a) to combine a second portion of the feed gas vapor with said liquid portion to be sub-cooled prior to expansion thereof, to form thereby a combined stream;
(2) said sub-cooling means are connected to cool at least one of the second portion of the feed gas vapor, the liquid portion to be sub-cooled and said combined stream, whereby said combined stream is cooled to a temperature below the bubble point of said liquid portion; and (3) means are provided to supply the expanded com-bined stream at a temperature below the bubble point of said liquid portion to said first expansion means (c).

19. The improvement according to claim 13 includ-ing (1) means connected to said cooling means (a) to receive said feed gas vapor and to divide said feed gas vapor into at least a first part and a second part;
(2) means connecting said dividing means (1) to said heat exchange means (i), whereby said first part is directed to said heat exchange means (i) to be reheated and thereafter directed to said third expansion means (ii) to be expanded to said lower pressure and supplied to said fraction-ation column at said second feed point;
(3) expansion means connected to said dividing means (1) to receive said second part of said feed gas vapor and to expand said second part to said lower pressure;
and (4) means connecting said expansion means (3) to said fractionation column to supply the expanded second part of said feed gas vapor to said fractionation column at a feed point above said second feed point.

20. The improvement according to claim 13 includ-ing (1) dividing means connected to said cooling means (a) to receive said feed gas vapor and to divide it into at least a first part and a second part;
(2) means connecting said dividing means (1) to dais heat exchange means (i), to direct said first part to said heat exchange means (i) to be reheated prior to expan-sion, and thereafter directed to said expansion means (ii) to be expanded to said lower pressure and supplied to said fractionation column at said second feed point;
(3) heat exchange means connected to said dividing means (1) to receive said second part and to cool said second part;
(4) expansion means connected to said heat exchange means (3) to receive said cooled second part and to expand said cooled second part to said lower pressure; and (5) means connecting said expansion means (4) to said fractionation column to supply said expanded second part to said fractionation column at a feed point above said second feed point.

21. The improvement according to claim 13 includ-ing (1) heat exchange means connected between said expansion means (ii) and said fractionation column (4) to receive said expanded reheated portion;
(2) means further connecting said heat exchange means to receive at least a portion of residue gas and to direct said residue gas into heat exchange relation with said expanded reheated portion, whereby said expanded reheated portion is cooled; and (3) means connecting said heat exchange means to said fractionation column to supply said cooled expanded re-heated portion to said fractionation column at said second feed point.

22. In an apparatus for the separation of a feed gas into a volatile residue gas and a relatively less volatile fraction, said feed gas containing hydrocarbons, methane and ethane together comprising a major portion of the feed gas, said apparatus having (a) cooling means for cooling said feed gas under pressure to partially condense it and to form thereby a liquid portion and a feed gas vapor;
(b) sub-cooling means connected to said cooling means to receive at least some of said liquid portion, and to sub-cool it to a temperature below its bubble point;
(c) expansion means connected to said sub-cooling means (a) to receive the sub-cooled liquid portion and to ex-pand it to a lower pressure;
(d) a fractionation column connected to receive at least a portion of the expanded sub-cooled liquid portion at a first feed point and to separate said relatively less volatile fraction;

(e) a second expansion means connected to said cooling means (a) to receive said feed gas vapor and to ex-pand it to said lower pressure, said second expansion means being further connected to said fractionation column to supply at least a portion of the expanded feed gas vapor thereto as a feed, the improvement comprising (i) dividing means connected to said cooling means (a) to receive said feed gas vapor and to divide it into a first part and a second part;
(ii) means connecting said dividing means (i) to receive said first part of said feed gas vapor and supply it to said second expansion means (4) wherein said first part is expanded and supplied to said fractionation column;
(iii) heat exchange means connected to said dividing means (i) to receive said second part of said feed gas vapor, said heat exchange means being further connected to receive a portion of said feed gas under pressure, thereby to direct said second part of said feed gas vapor into heat exchange relation with said feed gas under pressure to reheat said feed gas vapor;
(iv) expansion means connected to said heat exchange means (iii) to receive said reheated second part of said feed gas vapor and to expand it to said lower pressure while ex-tracting work therefrom; and (v) means connecting said expansion means (iv) to said fractionation column at a second feed point to supply said expanded second part to said fractionation column at said second feed point, said second feed point being at a lower column position than said first feed point.

23. The improvement according to claim 22 wherein said sub-cooling means (b) includes means to direct the liquid portion to be sub-cooled into heat exchange relation with at least a portion of said residue gas, thereby to cool said first part prior to expansion thereof.

24. The improvement according to claim 22 includ-ing a further heat exchange means connected between said ex-pansion means (iv) and said fractionation column to receive expanded second part, said further heat exchange means being further connected to direct said expanded second part into heat exchange relation with at least a portion of said residue gas, thereby to further cool said expanded second part prior to supplying it to the fractionation column at said second feed point.