Process for obtaining C.sub.2+ or C.sub.3+ hydrocarbons

For obtaining C.sub.2+ or C.sub.3+ hydrocarbons from gas mixtures containing essentially light hydrocarbons and optionally, hydrogen or nitrogen, the gs mixture is first cooled and subjected to a phase separation, and the thus condensed components are fractionated by rectification. The uncondensed portion of the gas mixture along with the overhead product of the rectification after it has been partially condensed, is subjected to treatment in a recontacting column wherein mass transfer and heat transfer occur, and wherein C.sub.2+ or C.sub.3+ hydrocarbons are transferred from the gas phase to the liquid phase. The liquid phase thus collected is fed into the rectification column as external reflux while the remaining gas is removed, after heating, as a residual gas.

CROSS REFERENCE TO RELATED APPLICATIONS 
This application is related to concurrently filed, and commonly assigned, 
applications entitled "Separation of C.sub.3+ Hydrocarbons by Absorption 
and Rectification", Sapper, Ser. No. 809,953; "Process for Separation of 
C.sub.2+, C.sub.3+ or C.sub.4+ Hydrocarbons", Bauer, Ser. No. 809,956; and 
"Process for the Separation of C.sub.2+ or C.sub.3+ Hydrocarbons from a 
Pressurized Hydrocarbon Stream", Bauer, Ser. No. 809,957, all incorporated 
by reference herein. 
BACKGROUND OF THE INVENTION 
This invention relates to a process and apparatus for the separation of 
C.sub.2+ or C.sub.3+ hydrocarbons from a gas stream containing light 
hydrocarbons, wherein the gas stream, under super atmospheric pressure, is 
cooled, partially condensed and separated into a liquid and a gaseous 
fraction, and wherein the liquid fraction is subjected to rectification to 
obtain a product stream containing essentially C.sub.2+ or C.sub.3+ 
hydrocarbons and a residual gas stream containing predominantly lower 
boiling components. By light hydrocarbons is generally meant aliphatic 
hydrocarbons containing 1-5 carbon atoms. 
Such processes are used mainly to separate ethane or propane from natural 
gases or other gases, for example refinery tail gases. In addition, these 
processes are suitable for the separation of analogous unsaturated 
hydrocarbons, for example ethylene or propylene, from a gas stream 
containing these components, for example, refinery tail gases. 
Reprocessing of refinery tail gases has recently become economically 
attractive since the market prices for LPG (C.sub.3 /C.sub.4 hydrocarbon 
mixtures) have increased while, in contrast, the demand for vacuum 
residues such as heavy oil has decreased. For this reason, heavy fractions 
are often burned to cover the internal fuel needs of a refinery whereas 
the C.sub.2+ or C.sub.3+ hydrocarbons which collect in large amounts, 
especially in the processing of light crude oil components into gasoline, 
are separated from tail gases. 
In an earlier German patent application P 34 08 760.5 filed Mar. 9, 1985, 
having a common assignee and corresponding substantially to U.S. 
application Ser. No. 709,742 filed Mar. 8, 1985 by Bauer et al, 
incorporated by reference herein, a process of this type is disclosed 
which relates to the separation of C.sub.3+ hydrocarbons. An important 
feature of this earlier application resides in the fact that the C.sub.3+ 
hydrocarbons to be separated condense out during the partial condensation 
to such an extent that only the condensate needs to be fed into the 
rectification, while the uncondensed portions contain so little C.sub.3+ 
hydrocarbons that further condensation can be dispensed with. Because of 
this, the uncondensed portions can be immediately heated again, preferably 
in indirect exchange with feed gas, and thereafter removed as a residual 
gas without first having to go through rectification. That leads to more 
advantageous rectification conditions, particularly since a higher 
overhead temperature can be used. 
SUMMARY OF THE INVENTION 
An object of one aspect of this invention is to provide an improved process 
of the above-mentioned type, especially wherein the yield of C.sub.2+ or 
C.sub.3+ hydrocarbons is increased. 
Another object is to provide an installation or apparatus for conducting 
the improved process. 
Upon further study of the specification and appended claims, further 
objects and advantages of this invention will become apparent to those 
skilled in the art. 
According to the process aspect of this invention, the gaseous fraction 
separated after partial condensation is fed into a recontacting column in 
which C.sub.2+ or C.sub.3+ hydrocarbons are scrubbed from the gaseous 
fraction with condensed residual gas obtained from the rectification and 
the liquid fraction which collects in the bottom of the recontacting 
column is fed into the rectification. The recontacting column of this 
invention, hereinafter referred to as the R-C column, is characterized by 
a number 1 to 10, preferably 2 to 5 theoretical plates. 
By this invention, an extensive separation of C.sub.2+ or C.sub.3+ 
hydrocarbons is achieved in the R-C column. This is especially surprising 
because the condensation of these heavy components still contained in the 
gaseous fraction is achieved by bringing them into contact with a lighter 
fraction. Usually, in contrast, to scrub specific hydrocarbons from a gas, 
hydrocarbons which are heavier than the components to be scrubbed are used 
as scrubbing agents. 
Without being bound by an explanation of the mechanism of this invention, 
the high yield attainable by the process according to the invention is 
believed to be attributed to the favorable interaction of several effects. 
Accordingly, the partially condensed residual rectification gas acts as a 
cooling agent since, on entering the R-C column, it is expanded from a 
relatively high partial pressure, under which the condensate was formed, 
to a low partial pressure so that a part of the condensate reevaporates 
with the simultaneous release of cold. This cooling leads to a temperature 
which clearly is below, e.g., by at least 3.degree. K., preferably at 
least 10.degree. K., the lowest prevailing temperature realized in the 
partial condensation of the feed gas stream, as a result of which there 
are condensed further heavy components still remaining in the gaseous 
fraction from the feed gas partial condensation step. Since, on the other 
hand, the achievable degree of separation in the R-C column is clearly 
higher than what might be expected based solely on additional cooling, 
still other effects, especially solubility effects, must also contribute 
favorably. 
In an advantageous improvement of the invention, the liquid fraction which 
collects at the bottom of the R-C column is fed into the rectification 
column as reflux liquid. This eliminates the necessity of producing reflux 
liquid for the rectification separately, for instance by using an external 
source of refrigeration. 
In a further advantageous improvement of the invention, the overhead 
product of the R-C column, which generally is delivered as a residual gas 
after reheating to ambient temperature, is first expanded and then used to 
cool the head of the R-C column. The additional head cooling of the R-C 
column leads to a further, increased cooling and thus to a further 
increase in the yield of the process. The expansion can occur by simply 
constricting the gas stream in a valve, but with sufficiently large gas 
streams or with a large portion of light components, for instance with a 
hydrogen portion higher than 20%, the use of an expansion turbine is also 
contemplated. 
In a further especially advantageous modification of the invention, the 
gaseous fraction separated after the partial condensation is first cooled 
to a lower temperature through further indirect heat exchange, before it 
is fed into the R-C column. The components which thus condense, which 
contain a relatively high proportion of the C.sub.2+ or C.sub.3+ 
hydrocarbons remaining in the gaseous fraction, are separated and also fed 
into the rectification, while only the uncondensed portion of the gaseous 
fraction is fed into the R-C column. This procedure offers advantages 
especially when the bottom fraction obtained in the R-C column is used as 
a reflux liquid in the rectification column. The condensed heavier 
components, e.g., mostly C.sub.2+ or C.sub.3+ the gaseous fraction are 
then separated to a large extent, e.g., at least 30% before the formation 
of the reflux liquid and can be fed into the rectification at a lower 
point than the reflux liquid column, thus improving rectification 
efficiency. In transferring the gaseous fraction from the phase separator 
to the recontacting column the gaseous fraction is not subjected to any 
external pressure-reduction stages. Other than normal pressure losses from 
friction in pipe flow and partial condensation from heat exchange, the 
pressure of the gaseous fraction is not reduced between the phase 
separator and the recontacting column. 
In an advantageous further development of this modification of the process, 
the components which condense from the gaseous fraction by indirect heat 
exchange are fed to the rectification column in the same feed pipe with 
the liquid fraction obtained by the partial condensation. Of course, 
separate piping and feeding of both condensate fractions are possible, but 
the advantages attainable in this way are often so limited as to be not 
worth the higher equipment expenses. The processing of combined condensate 
fractions can, on the other hand, be particularly simply designed since 
the further cooling and partial condensation of the gaseous fraction is 
conducted simply inside a phase separator which is provided in any case 
for separating the liquid from the gaseous fraction after the first 
partial condensation. By placing a heat exchanger in the upper region of 
the phase separator, a mixing of both condensate fractions is accomplished 
without any other design measures. 
The cooling of the residual gas from the rectification column is conducted 
at a sufficiently low temperature so that 50 to 99%, especially 70 to 95%, 
for instance 90% of the residual gas is condensed. With a C.sub.3+ bottoms 
separation, the residual gas typically contains small amounts of hydrogen 
(to the extent that the gas stream contains hydrogen), methane, C.sub.2 
hydrocarbons as main components and small portions of C.sub.3 
hydrocarbons, but each individual case depends on the particular 
composition of the gas stream to be fractionated and the actually employed 
process conditions. In the case of a C.sub.2 separation in the 
rectification bottoms, the component spectrum shifts to methane as the 
main component in the residual gas C.sub.2 being present only in small 
amounts and C.sub.3 practically not at all. To effect partial 
condensation, the residual gas is cooled to at least the temperature to 
which the gas stream, in the framework of its partial condensation, is 
cooled. For this purpose, the residual gas stream can be advantageously 
fed through a separate cross section of the heat exchanger used for 
cooling the crude gas, although cooling in a separate heat exchanger is 
also possible. 
At the head of the R-C column, a markedly lower temperature appears than 
that of the gas after partial condensation, for instance a temperature 
10.degree. to 20.degree. C. lower. To the extent that the gaseous fraction 
which comes from the head of the R-C column and which is heated as a 
residual gas and removed from the installation, is fed directly through 
the heat exchanger used for cooling the crude gas, relatively high 
temperature differences result at the cold end of this heat exchanger, 
resulting in relatively high heat losses. To avoid this inefficiency, it 
is contemplated in another design modification of the invention that at 
least part of the overhead product of the R-C column first enter into a 
heat exchange with previously partially cooled or condensed residual gas 
from the rectification. In this way, not only can large temperature 
differences at the cold end of the heat exchanger used for cooling the 
crude gas be avoided, but in doing so a further cooling or condensation of 
the residual gas of the rectification results as an additional effect. 
In a further advantageous modification of the invention, the liquid 
fraction separated from the crude gas after partial condensation is at 
least partially heated, before rectification, against the gas stream to be 
cooled, and the resultant liquid-gas mixture is fed to an appropriate feed 
point in the rectification column. 
In processing gas streams rich in components boiling lower than methane, 
there is another modification of the invention wherein these components 
are enriched while C.sub.1 and C.sub.2 hydrocarbons are separated by 
partial condensation from the overhead product of the R-C column. This 
procedure can, for instance, be applied in separating C.sub.2+ or C.sub.3+ 
hydrocarbons and nitrogen from nitrogen-rich natural gas or especially for 
obtaining said heavy hydrocarbons and hydrogen from hydrogen-rich refinery 
gases. This type of separation is advantageous, especially when the 
feedstock stream contains a relatively high amount of low-boiling 
components, for instance a hydrogen content on the order of magnitude of 
50 to 90 mol-%. Such a hydrogen amount is in fact sufficient to produce, 
by expansion, the cold required for the additional separation without it 
being necessary to use an additional external source of energy. 
In many applications, a further fractionation of the C.sub.2+ or C.sub.3+ 
hydrocarbon product, especially separation of a C.sub.3 /C.sub.4 
hydrocarbon mixture and C.sub.5+ hydrocarbons, is desirable. For this 
purpose, according to a preferred design of the process according to the 
invention, before the formation of the liquid and gaseous fractions the 
majority of the C.sub.5+ hydrocarbons is separated from the gas stream, if 
the concentration of these components is high enough, e.g., at least to 1 
to 10 mol-% to make such a separation worthwhile. 
The C.sub.5+ separation is conducted in practice by partial condensation at 
a temperature higher than that at which the above-mentioned liquid and 
gaseous fractions are formed. By means of preliminary separation of the 
heavy components, the mixture fed into the rectification column is nearly 
free of C.sub.5+ hydrocarbons, so that there is obtained from a subsequent 
C.sub.3+ rectification of the liquid fraction, a product stream which is a 
conventional commercial LPG fraction. 
To increase the yield of C.sub.3 and C.sub.4 hydrocarbons, the separated 
heavy hydrocarbons are also fed into the rectification column, wherein the 
introduction of the C.sub.5+ fraction into the column occurs, according to 
the equilibrium conditions in the column, below the feed point of the 
liquid fraction formed by partial condensation and wherein it is 
furthermore provided that a stream containing essentially C.sub.3 and 
C.sub.4 hydrocarbons is removed between the two feeds. By the additional 
rectification of the C.sub.5+ fraction, C.sub.3 /C.sub.4 hydrocarbons 
condensed or absorbed during the condensation of the C.sub.5+ fraction 
are recovered as a product. Between the two feed points, a region of 
maximal C.sub.3 /C.sub.4 concentration is formed within the rectification 
column where the C.sub.3 /C.sub.4 product stream is advantageously 
removed. 
An installation or apparatus for conducting the process according to the 
invention includes, as essential parts, at least one heat exchanger for 
cooling and partially condensing the gas stream, a phase separator for 
separating the partially condensed portion of the gas stream, a 
rectification column for fractionating the partially condensed portion of 
the gas stream, and an R-C column the lower region of which is connected 
to the vapor chamber of the separator and whose upper region is connected 
to the head of the rectification column, with a heat exchanger is located 
between the head of the rectification column and the upper region of the 
R-C column. 
In an especially advantageous structural embodiment, the phase separator 
and the R-C column have a common housing. Preferably the R-C column is 
placed above the separator and separated from it by means of a column 
plate, so that the gaseous fraction leaving the separator can enter the 
lower region of the R-C column via a riser in a bubble-cap or the like. In 
another structural embodiment, heat exchange pipes are placed in the upper 
region of the separator through which cold process streams or other cold 
fluids can be conducted to condense out the heavy constituents from the 
gaseous fraction before it is fed into the R-C column.

DETAILED DESCRIPTION OF THE DRAWINGS 
In the embodiment shown in FIG. 1, the gas stream feed to be fractionated 
is fed by pipe 1 to heat exchanger 2 under elevated pressure and at 
approximately ambient temperature where it is cooled to the extent that 
most of the hydrocarbons to be separated, that is to say the C.sub.2+ or 
C.sub.3+ hydrocarbons, condense. The partially condensed gas stream is 
then subjected to a phase separation in a phase separator 3, and the 
resultant condensate is first passed in conduit 4 to heat exchanger 2 
where it is partially vaporized, and the resultant fluid mixture is then 
fed into a rectification column 5. In rectification column 5, the 
condensate is fractionated into: (i) a C.sub.2+ or C.sub.3+ fraction 
removed as a product stream by conduit 6 from the bottom of the column, 
and (ii) a residual gas stream 7 containing lower boiling components. The 
rectification is conducted using an external reflux introduced by pipe 8 
and reboiler which, for instance, is run on low-pressure steam or hot 
water. 
The overhead product from the rectification column, removed by pipe 7, 
which consists essentially of components boiling lower than the product 
fraction removed by pipe 6, is fed into heat exchanger 2 and again cooled, 
whereby higher boiling components still remaining in this gas partially 
condense. The condensate thus formed occurs in an amount which is greater 
than the amount of reflux needed for rectification. This partially 
condensed residual gas is fed into the upper region of a recontacting 
column 10 in which it is brought into countercurrent contact with the 
gaseous fraction which was obtained from phase separator 3 via pipe 11. 
The liquid collecting at the bottom of column 10 is removed by pipe 12 and 
fed, by pump 13 through pipe 8, into rectification column 5 as external 
reflux. At the head of the R-C column, a gaseous fraction almost 
completely devoid of the C.sub.2+ or C.sub.3+ hydrocarbons to be 
separated, is removed by pipe 14. This gas stream is expanded in valve 15 
to a desired residual gas pressure, and the cold thus obtained is 
transferred in a cold trap 16 to the gaseous fraction in R-C column 10. 
Subsequently, the residual gas is heated to ambient temperature in heat 
exchanger 2 and finally removed by pipe 17. 
The cold trap 16 is an indirect heat exchange means for transferring the 
cold values obtained from the expansion step. 
In a specific example, according to FIG. 1, a crude gas is introduced by 
pipe 1 at a temperature of 313.degree. K. at 20 bar pressure. It contains 
15% hydrogen (percentages below always in mol-%), 3% nitrogen, 37% 
methane, 26% ethane, 14% propane, 14% butane and 1% pentane. After being 
cooled in heat exchanger 2 to 237.degree. K. a condensate is separated in 
phase separator 3 which contains 0.4% hydrogen, 0.2% nitrogen, 9.4% 
methane, 38.5% ethane, 36.3% propane, 12.1% butane and 3.1% pentane. The 
remaining gaseous portion, about 68% of the crude gas, is brought into 
contact, in R-C column 10, with the residual gas from the rectification, 
also cooled to 237.degree. K. This residual gas from pipe 7 contains 0.5% 
hydrogen, 0.3% nitrogen, 15.0% methane, 80.8% ethane and 3.4% propane. The 
overhead product of the R-C column is removed by pipe 14 at a temperature 
of 221.degree. K., expanded in valve 15 to the pressure of the residual 
gas and simultaneously cooled to 210.degree. K., heated is cold trap 16 to 
218.degree. K. and subsequently heated again in heat exchanger 2 to 
310.degree. K. before it is delivered by pipe 17 as residual gas at a 
pressure of 5 bar. This residual gas contains 18.6% hydrogen, 3.7% 
nitrogen, 45.8% methane, 31.7% ethane and only 0.2% propane. 
The liquid product removed by pipe 12 from the bottom of R-C column 10 
consists of 0.3% hydrogen, 0.2% nitrogen, 10.5% methane, 74.0% ethane, 
14.2% propane, and 0.8% butane and pentane. It is fed into the head of 
rectification column 5 operated at 18 bar. In the bottom of the 
rectification column, a C.sub.3+ product stream is withdrawn via pipe 6, 
said product stream containing 2.0% ethane, 71.9% propane, 20.9% butane 
and 5.2% pentane. The yield of C.sub.3+ in this process is about 98.9%. 
The embodiment represented in FIG. 2 is a variant of the process according 
to the invention wherein a C.sub.5+ separation from the gas mixture is 
carried out in a first step of the process. For this purpose, feed stream 
1 is first cooled in heat exchanger 2 only sufficiently to condense out 
most (e.g., at least 70%) of the C.sub.5+ components. The partially cooled 
mixture from heat exchanger 2 at an intermediate temperature is passed to 
phase separator 20, wherein the condensed fraction is withdrawn via pipe 
21, and after partial heating in heat exchanger 2 is passed by conduit 22 
to a rectification column 25. The remaining gaseous fraction from phase 
separator 20 is cooled via pipe 23 in heat exchanger 2 and ultimately fed 
into phase separator 24, which corresponds to phase separator 3 of the 
previously described embodiment of FIG. 1. 
The rectification of the condensates separated in phase separators 20 and 
24 occurs in a rectification column 25 which, in contrast to the 
rectification column used in the previous example, exhibits a larger 
number of plates, e.g., about 20 to 50 plates as compared to 10 to 30, or 
about 50 to 100% more plates. Between the two feed pipes 4 and 22, a 
discharge pipe 26 is located in the column at the point where the highest 
C.sub.3 /C.sub.4 concentration is found. In the bottom of column 25 a 
liquid collects which contains essentially C.sub.5+ hydrocarbons and which 
is removed as a product stream by pipe 27. A light fraction containing 
essentially C.sub.1 and C.sub.2 hydrocarbons is removed from the head of 
column 25 by pipe 7 as in the previous example. 
In this process the heavy portions which have been separated in phase 
separator 20 are also fed into the rectification. In this way, with 
relatively low cost, a very high yield of C.sub.3 and C.sub.4 hydrocarbons 
can be achieved. 
In the embodiment shown in FIG. 3, a crude gas is introduced by pipe 28 at 
a temperature of 305.degree. K. and a pressure of 28.9 bar. It contains 
67.5% hydrogen, 11.8% methane, 8.8% C.sub.2, 7.8%, C.sub.3, 3.3% C.sub.4 
and 0.8% C.sub.5+ hydrocarbons. After being cooled in heat exchanger 29 to 
a temperature of 230.degree. K., by indirect heat exchange with process 
streams to be heated and by an external cooling cycle indicated 
diagrammatically by pipe 30, the partially condensed mixture is fed by 
pipe 31 into a phase separator placed in the lower part of a vessel 32 
also housing an R-S column in the upper part thereof. Liquid removed from 
the phase separator by pipe 33 contains 1.4% hydrogen, 3.5% methane, 23.0% 
C.sub.2, 45.1% C.sub.3, 21.5% C.sub.4 and 5.5% C.sub.5+ hydrocarbons. By 
pump 34, the bottom liquid is fed first to heat exchanger 29 at a pressure 
of 30 bar and, after partial heating, is fed into the rectification 
column 35. 
In rectification column 35, the phase separated liquid is fractionated into 
a C.sub.3+ bottom fraction and a C.sub.2- fraction. The C.sub.3+ 
hydrocarbons are removed by pipe 36 as a bottoms product stream. A partial 
stream thereof is branched off by pipe 37, heated in reboiler 38 and fed 
back into the lower region of column 35 to heat the bottom. The product in 
pipe 36 collects at a temperature of 362.degree. K. at a pressure of 29 
bar and consists of 2.0% C.sub.2, 63.6% C.sub.3, 27.5% C.sub.4 and 6.9% 
C.sub.5+ hydrocarbons. At the head of rectification column 35, a fraction 
collects which contains 2.8% hydrogen, 8.7% methane, 84.4% C.sub.2 and 
4.1% C.sub.3 hydrocarbons. This fraction is withdrawn via pipe 39 and 
cooled before being fed into the R-C column. The cooling occurs first in 
heat exchanger 29 to a temperature of 202.degree. K. The resultant mostly 
condensed fraction is fed by pipe 41 into the upper part of vessel 32 at 
the head of the R-C column. A liquid collects in the lower region of the 
R-C column on column plate 42, which separates the R-C column from the 
phase separator underneath it. This liquid on plate 40 contains on the one 
hand, the overhead product from rectification column 35 to the extent that 
it is condensed in heat exchangers 29, 40 and not reevaporated in the R-C 
column and, on the other hand, the heavy components scrubbed from the 
gaseous fraction passed upwardly through plate 42 into the R-C column. The 
liquid which collects on column plate 42 is removed by pipe 43 and is 
introduced, by pump 44, into the head of rectification column 35 as a 
reflux liquid. This reflux liquid collects at a temperature of 200.degree. 
K. and consists of 1.5% hydrogen, 6.0% methane, 78.6% C.sub.2, 13.6% 
C.sub.3 and 0.3% C.sub.4 hydrocarbons. 
In the R-C column, a residual gas is withdrawn via pipe 45, and then 
divided into two partial streams. Via pipe 46 a first partial stream 
reaches heat exchanger 40, where it is used to cool the overhead fraction 
from the rectification column from about 230.degree. K. to 202.degree. K., 
whereas the other partial stream is passed through the upper region of the 
phase separator by pipe 47 to transfer its peak cold by indirect heat 
exchange with the gaseous fraction in the phase separator. The partial 
stream in pipes 46 and 47 are then reunited at 48 and engine expanded in a 
turbine 49 to such an extent that the delivery pressure to be maintained 
for the residual gas is substantially maintained at the outlet side of the 
turbine. The residual gas cooled during the engine expansion to 
190.degree. K. is first passed via pipe 50 through the vapor chamber of 
the phase separator, and then is passed by pipe 51 to heat exchanger 29 
where it is heated against process streams to be cooled, to a temperature 
of 302.degree. K. before it is withdrawn via pipe 52 at a pressure of 17 
bar. This residual gas contains 76.8% hydrogen, 13.3% methane, 9.8% 
C.sub.2 and 0.1% C.sub.3 hydrocarbons. 
With this process, 99.5% of the C.sub.3+ hydrocarbons contained in the 
crude gas collects in the bottom of the rectification column 35 and is 
removed via pipe 36 as a product. 
The process shown in FIG. 4 represents a variation in the process of FIG. 
3; therefore, primarily the differences between the two processes will be 
described. According to FIG. 4, the overhead product from rectification 
column 35 is fed to a heat exchanger 53 by pipe 39, in which it is cooled 
first against the bottoms product from the phase separator (said bottoms 
being heated before being fed into rectification column 35) and then 
against the overhead product from the R-C column branched off conduit 45 
via pipe 46. After being withdrawn from heat exchanger 53, partial stream 
46 of the overhead product from the R-C column is fed directly, by pipe 
54, to heat exchanger 29 where it is heated to ambient temperature, and 
removed by pipe 55 substantially at the pressure of the R-C column 
(diminished in pressure only because of unavoidable pressure losses in the 
pipes and heat exchangers). Another partial stream of the overhead product 
from the R-C column is branched from conduit 45 via pipe 47, e.g., at 
about 200.degree. K., reheated in the upper region of the phase separator 
to about 220.degree. K. and then fed, by pipe 56 to a first expansion 
turbine 57 and expanded to an intermediate pressure and a temperature of 
e.g., about 180.degree. K. The cold thus obtained is also transferred to 
the gaseous fraction in the phase separator by passing the turbine output 
via pipe 58, through the upper region of the phase separator via a heat 
exchanger disposed therein. The resultant heated turbine effluent is fed 
by pipe 49 to a second expansion turbine and again cooled by substantially 
isentropic expansion to a temperature of about, e.g., 180.degree. K. The 
resultant cold residual gas is again fed into the upper region of the 
phase separator and again transfers its peak cold to the gaseous fraction 
therein before it is ultimately fed to heat exchanger 29 via pipe 62 and 
removed as a low-pressure residual gas by pipe 63. 
The rectification conditions, especially pressure and temperature, in 
obtainin C.sub.2+ or C.sub.3+ hydrocarbons are usually adjusted on the 
basis of the usual parameters, especially with regard to the composition 
of the mixture to be rectified. Furthermore, the gas mixture to be 
fractionated can also be available under differing conditions, especially 
at varying degrees of high pressure. In individual cases, therefor, the 
separation process can be conducted under optimal conditions in such a way 
that the pressure in the rectification is higher or lower than the 
pressure of the partially condensed gas stream. In the embodiments 
according to FIGS. 1 to 4 it is assumed that no significant pressure 
differences exist. If they should exist in an individual case, the process 
described can easily be adapted to the altered conditions, for example in 
the case of rectification under higher pressure, by placing a pump in pipe 
4, replacing feed pump 13 by a pump with a correspondingly higher 
compression ratio and by expanding the partially condensed residual gas 
into R-C column 10 (FIGS. 1 and 2). 
The preceding examples can be repeated with similar success by substituting 
the generically or specifically described reactants and/or operating 
conditions of this invention for those used in the preceding examples. 
From the foregoing description, one skilled in the art can easily ascertain 
the essential characteristics of this invention, and without departing 
from the spirit and scope thereof, can make various changes and 
modifications of the invention to adapt it to various usages and 
conditions.