Process and apparatus for C3 recovery

An improved process for separating a hydrocarbon bearing feed gas containing methane and lighter, C.sub.2 (ethylene and/or ethane), C.sub.3 (propylene and/or propane) and heavier components into a fraction containing predominantly C.sub.2 and lighter components and a fraction containing predominantly C.sub.3 and heavier hydrocarbon components including the steps of cooling and partially condensing and delivering the feed stream to a separator to provide a first residue vapor and a C.sub.3 containing liquid, directing a portion of the C.sub.3 containing liquid into a heavy-ends fractionation column wherein the liquid is separated into a second hydrocarbon bearing vapor residue, a second C.sub.2 containing liquid stream, and a C.sub.3 containing product. Directing part of the second liquid into the lights-ends fractionation column, which liquid provides additional liquefied C.sub.2 's which act as a direct contact refrigerant to thereby condense C.sub.3 's and heavier components while the C.sub.2 's are evaporated in the light-ends fractionation column to thereby obtain third residue vapors and liquids, supplying the liquids recovered from the light-ends fractionating column to the heavy-ends fractionation column as a feed thereto.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 This invention relates to an improved process for separating a
 hydrocarbon-bearing feed gas which contains methane and lighter
 components, (not necessarily all hydrocarbon components), C.sub.2
 (ethylene and ethane), C.sub.3 (propylene and propane), and heavier
 hydrocarbon components into two fractions. The first fraction contains
 predominantly C.sub.2 's and lighter components and the second fraction
 contains the recovered desirable C.sub.3 and heavier components. More
 particularly, this invention relates to a process and apparatus wherein
 the yield of C3's is increased.
 2. The Prior Art
 Hydrocarbon-bearing gas may contain lighter components (e.g., hydrogen,
 nitrogen, etc.) methane, ethane, and/or ethylene, and a substantial
 quantity of hydrocarbons of higher molecular weight, for example, propane,
 butane, pentane and often their unsaturated analogs. Recent changes in
 propylene/propane demand have created increased markets for
 propylene/propane and have created a need for more efficient processes
 which yield higher recovery levels of this product. In more recent times,
 the use of cryogenic processes utilizing the principle of gas expansion
 through a mechanical device to produce power while simultaneously
 extracting heat from the system have been employed. The use of such
 equipment depends upon the pressure of the gas source, the composition of
 the gas and the desired end results. In the typical cryogenic
 expansion-type recovery processes used in the prior art, a gas stream
 under pressure is cooled by heat exchange with other streams of the
 process and/or external sources of cooling are employed such as
 refrigeration systems. As the gas is cooled, liquids are condensed and are
 collected and separated so as to thereby obtain desired hydrocarbons. The
 high pressure liquid feed is typically transferred to a deethanizer column
 after the pressure is adjusted to the operating pressure of the
 deethanizer. In such fractionating column the liquid feed is fractionated
 to separate the residual ethylene/ethane and lighter components from the
 desired products of propylene/propane and heavier hydrocarbon components.
 In the ideal operation of such separation processes, the vapors leaving
 the process contain substantially all of the ethylenelethane and lighter
 components found in the feed gas and substantially no propylene/propane or
 heavier hydrocarbon components remain. The bottom fraction leaving the
 deethanizer typically contains substantially all of the propylene/propane
 and heavier hydrocarbon components with very little ethylenelethane or
 lighter components which is discharged in the fluid gas outlet from the
 deethanizer.
 A patentability search was conducted on the present invention and the
 following references were uncovered.

Inventor U.S. Pat. No. Issue Date
 Harandi 4,664,784 5/12/1987
 Buck et al 4,895,584 1/23/1990
 Campbell et al 5,771,712 9/01/1998
 Wilkinson et al 5,699,507 6/30/1998
 U.S. Pat. No. 4,664,784--Issued May. 12, 1987
 M. N. Harandi to Mobil Oil Corporation
 In a reference directed to fractionation of hydrocarbon mixtures, teachings
 are found on column 4, line 32 et sequitur re: a zone (81) wherein a
 descending liquid heavy-ends portion contacts an ascending vaporous
 light-ends portion so as " . . . to aid in heat transfer between vapor and
 liquid." (column 4, line 44).
 U.S. Pat. No. 4,895,584--Issued Jan. 23, 1990
 L. L. Buck et al to Pro-Quip Corporation
 A reference that claims an improved process for hydrocarbon separation and
 teaches supplying of the liquids recovered from the light-ends
 fractionating column to the heavy ends fractionating column and directing
 part of the (C2 containing) liquid from a first step into intimate contact
 with a second residue, which liquid provides additional liquefied methane
 which acts with the partially condensed second residue as a direct contact
 refrigerant to thereby condense C2 and heavier comprising hydrocarbons
 while methane itself is evaporated in the light-ends fractionating column.
 On column 1, lines 56-67 the following teachings are found: " . . . feed
 gas is first cooled and partially condensed and delivered to a separator
 to provide a first residue vapor and a C2 containing liquid . . . Part of
 the C2 containing liquid from the separator may be directed into a heavy
 ends fractionating column wherein the liquid is separated into a second
 residue containing lighter hydrocarbons and C2 containing products. A part
 of the first residue vapors with at least part of the partially condensed
 second residue are counter currently contacted and co-mingled in a light
 ends fractionating column (emphasis added) . . . "
 On column 2, lines 1-10 the following teachings are found: "The liquids
 recovered from the light-ends fractionating column are then fed to the
 heavy-ends fractionating column as a liquid feed. A portion of the C2
 containing liquids from the separator is fed into intimate contact with
 the second residue prior to discharging the co-mingled liquids and gases
 into the light-ends fractionating column to thereby achieve mass and heat
 transfer (emphasis added) to thereby liquefy a higher percent of the C2
 and heavier hydrocarbon components while the methane is vaporized (column
 2, lines 1-10).
 The following Elcor Corporation references describe the recovery of C3 and
 heavier hydrocarbons via processes wherein counter-current contact of a
 stream drawn from a deethanizer with a stream in a separator/absorber
 takes place:
 U.S. Pat. No. 5,799,507--Issued Sep. 1, 1998
 J. D. Wilkinson et al to Elcor Corporation
 See column 4, line 2 re: ". . . liquid portion of expanded stream comingles
 with liquids falling downward from the absorbing section . . . " l.o.w.,
 the stream (36) from the deethanizer (17) flows through heat exchanger
 (20) to become stream (36a) which flows into the upper section of
 separator (15) where it . . . contacts the vapors rising upward through
 the absorption section" (column 5, lines 3-4).
 U.S. Pat. No. 5,771,712--Issued Jun. 30, 1998
 R. E. Campbell et al to Elcor Corporation
 This reference teaches essentially the same as Wilkinson et al.
 None of the foregoing patents discussed above embody the present invention.
 SUMMARY OF THE INVENTION
 The present invention provides processes for increasing the propylene and
 propane component of the liquid discharge from the process unit at reduced
 energy consumption than the prior art. The foregoing advantage is achieved
 in the present invention by a process in which the feed gas is first
 cooled and partially condensed and delivered to a separator to provide a
 first residue vapor and a C.sub.3 containing liquid which liquid also
 contains lighter hydrocarbons. Part of the C.sub.3 containing liquid from
 the separator may be directed into a heavy-ends fractionation column,
 wherein the liquid is separated into a second residue containing lighter
 hydrocarbons, and a C.sub.3 containing product. The second residue is
 partially or fully condensed and delivered to a separator where a C.sub.2
 containing liquid is discharged. Where the second residue is only
 partially condensed, a fourth residue will be discharged also. A part of
 the C.sub.2 containing liquid is further cooled before being sent to the
 light-ends fractionating column. The portion of the C.sub.2 containing
 liquid that is not cooled and sent to the light-ends fractionating column
 is recycled back to the heavy-ends fractionating column as reflux. A part
 of the first residue vapors is counter-currently contacted and co-mingled
 with the cooled part of the C.sub.2 containing liquid in a light-ends
 fractionating column to thereby provide third residue vapors and liquids
 which are separately discharged. The liquids recovered from the light-ends
 fractionating column are then fed to the heavy-ends fractionation column
 as a liquid feed. A cooled portion of the C.sub.2 containing liquids is
 fed into the light-ends fractionating column to thereby achieve mass and
 heat transfer and to thereby liquefy a higher percent of the C.sub.3 and
 heavier hydrocarbon components while the ethylene/ethane is vaporized. In
 this manner a higher proportion of the C.sub.3 and heavier hydrocarbon
 components are recovered.
 A better understanding of the invention will be had with reference to the
 following description and claims, taken in conjunction with the attached
 drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS
 The improved processes of the present disclosure include the steps of
 cooling a gaseous hydrocarbon-containing feed stream to form a vapor
 stream and a liquid stream. The liquid stream may be partially transferred
 to a heavy-ends fractionation column while the vapor stream is expanded
 and transferred to the bottom of a light-ends fractionation column. The
 heavy-ends fractionation column overhead, which consists mainly of
 methane, ethylene, and/or ethane, is cooled to produce a liquid. A portion
 of the produced liquid is used to reflux the heavy-ends fractionation
 column whereas the other portion is fed to the upper portion of the
 light-ends fractionation column. The liquid flows downwardly within the
 light-ends fractionation column and contacts gaseous propylene and/or
 propane and heavier hydrocarbons that flow upwardly. The methane and
 ethylene/ethane portion of the liquid stream is vaporized by absorbing
 heat from the gaseous propylene/propane and heavier hydrocarbons which
 causes the propylene/propane and heavier hydrocarbons to condense and exit
 at the bottom of the light-ends fractionation column. The gaseous ethylene
 and/or ethane and lighter components within the light-ends fractionation
 column are removed from the overhead as a product of the process. The
 liquid at the bottom of the light-ends fractionation column is removed and
 fed to the upper portion of the heavy-ends fractionation column. The
 liquid at the bottom of the heavy-ends fractionation column is removed as
 a product of the process.
 The improved process of this invention is illustrated in a first embodiment
 in FIG. 1. The incoming gas stream 1 is first fed to a booster compressor
 33. The compressed gas is cooled in heat exchanger 36 and the output
 thereof to a second booster compressor 32.
 The stream then flows through a heat exchanger 37. The gas exits from the
 heat exchanger 37 at a temperature of 102.degree. F., but the pressure
 thereof has been raised to 305 psia. The gas then passes through heat
 exchanger 38, so that the temperature thereof is reduced to about
 -30.degree. F. Pressure is reduced as the gas flows through the heat
 exchangers resulting in a pressure of 300 psia at -30.degree. F. at which
 the raw gas is delivered into a separator 44. Within separator 44 the
 cooled gas stream is separated into a first residue vapor which is passed
 through a turbo expander 46. The shaft of turbo expander 46 is connected
 directly to the shaft of the booster compressor 32. From the turbo
 expander, the first residue gas having a temperature of about -79.degree.
 F. at 125 psia passes by way of stream 5 into a light ends fractionating
 column 52.
 From the separator 44, by way of stream 4, the C.sub.3 containing liquid is
 channeled through heat exchanger 38 where its temperature is increased to
 60.degree. F. and exits heat exchanger 38 as stream 8. The liquid, by way
 of stream 8, is then conducted into a heavy ends fractionation column 56.
 The off-gas from heavy ends fractionation column 56, having a temperature
 of about 21.degree. F., is fed by stream 14 through heat exchanger 38 and
 by way of stream 16 into the reflux separator 57. At least a portion of
 the liquid residue from the reflux separator 57 is routed by stream 23
 through heat exchanger 38 where its temperature is reduced to -100.degree.
 F. at a pressure of 290 psia. This liquid stream is then passed by the
 stream 23A into the light ends fractionating column 52. The liquid from
 stream 23A passes downwardly through the light ends fractionating column
 52 and encounters the rising off gas from stream 5 so that mass and latent
 heat transfer occur.
 The light ends fractionating column 52 functions as a combination heat and
 mass transfer device. The column has two feed streams; that is, streams 5
 and 23A, and two product streams; that is, streams 10 and 9. The light
 ends fractionating column 52 consists of at least one, and preferably
 more, liquid-vapor equilibrium stages.
 The methane, ethylene, ethane and lighter constituents and un-recovered
 propylene and propane, exit as a dew point vapor from the top tray or
 separation stage of the light ends fractionating column 52. Vapors enter
 by way of stream 5 as a bottom feed while the top feed is by way of stream
 23A which is a liquid enriched by a condensed ethylene and ethane.
 The top feed through stream 23A into the light ends fractionating column
 52, and particularly the ethylene/ethane content thereof serves as a
 reflux in the column. In flowing from stage to stage within column 52, the
 liquid ethylenelethane is vaporized and in turn the liquid is
 progressively enriched in propylene and propane condensed from the
 upflowing bottom feed vapors from stream 5.
 The liquid discharge from the lower end of the heavy ends fractionation
 column 56 is passed by way of stream 15 to product discharge.
 The off-gas discharged from light ends fractionation column 52, combine
 with the vapors in stream 18 exiting the reflux separator 57. The combined
 vapors pass through heat exchanger 38 for discharge from the system. At
 this stage, the off-gas in stream 21 has a temperature of 97.degree. F.
 and a pressure of 110 psia. If it is desired to return the discharge gas
 to the same system from which the raw gas was taken, such as for further
 transportation of the gas, the pressure will need to be raised back up to
 that substantially equal to the incoming pressure of 165 psia in stream 1.
 A simulation of the process of FIG. 1 is set forth in Table 1 wherein the
 moles per hour of various constituents of the streams set forth. The
 process achieves a recovery of about 96.30 percent of the C.sub.3 content
 of the feed gas in addition to substantially complete recovery of the
 C.sub.4 and heavier hydrocarbon components of the feed gas stream.
 FIG. 2 shows an alternate embodiment of the invention. The components of
 the process of FIG. 2 having the same basic structure and function of
 those of the system of FIG. 1 are given like numbers. The process is as
 described with reference to FIG. 1, except for the addition of a second
 separator 48, turbo expander 50, booster compressor 34 and the treatment
 of the off-gas from the heavy ends fractionation column 66. In the
 arrangement of FIG. 2, the off-gas, flowing through stream 14, is passed
 first through a heat exchanger 42 and then to the reflux separator 57. The
 offgas from the light ends fractionating column 52 and liquids from the
 light ends fractionating column pass through exchanger 42 thus providing
 cooling to stream 14 and stream 23. The heavy ends fractionating column
 off-gases exit exchanger 42 as stream 16 and flow into the reflux
 separator 57. The liquid stream 23A passes through heat exchanger 42 and
 valve 62 before discharge into the upper portion of light ends
 fractionating column 52. Within such column the recycled liquid portion of
 the effluent functions in heat exchange action with the hydrocarbon
 containing components of the gas passing upwardly in the fractionating
 column to condense and absorb at least a substantial portion of the
 C.sub.3 and heavier hydrocarbon components. The arrangement of the FIG. 2
 embodiment of the system compared to that of FIG. 1 provides an alternate
 method of ethylene/ethane condensation in the combined gas and effluent
 inserted into the upper end of the light ends fractionating column.
 Table 2, which shows the moles per hour calculations of a simulation of the
 system of FIG. 2 provides a comparison of the contents of the various
 streams of this embodiment of the process compared to that of the process
 of FIG. 1. The process achieves a recovery of about 96.63 percent of the
 C.sub.3 content of the feed gas in addition to substantially complete
 recovery of the C.sub.4 and heavier hydrocarbon components of the feed gas
 stream.
 FIG. 3 illustrates another alternate embodiment of the process. In this
 embodiment, the off-gas from the heavy ends fractionation column is passed
 through a heat exchanger 43 and into reflux separator 57. The resulting
 condensed liquids are then separated into heavy ends fractionation column
 reflux and stream 23. Stream 23A is sub-cooled in heat exchanger 42 and
 flows via value 62 into the upper end of the light ends fractionating
 column 52.
 Table 3 provides the molar rates of the various streams in the process of
 the embodiment of FlG. 3 showing the percentage recoveries of propylene
 and propane. The process achieves a recovery of about 97.65 percent of the
 C.sub.3 content of the feed gas in addition to substantially complete
 recovery of the C.sub.4 and heavier hydrocarbon components of the feed gas
 stream.
 FIG. 4 shows an embodiment of the process similar to FIG. 2. This
 embodiment features a single turbo expander and reduced heat integration
 of the heavy ends fractionation column 56 with the feed heat exchanger 38.
 Table 4 shows the calculated moles per hour of the various streams in the
 embodiment of FIG. 4 and the pressure and temperature of the streams. The
 process achieves a recovery of about 99.08 percent of the C.sub.3 content
 of the feed gas in addition to substantially complete recovery of the
 C.sub.4 and heavier hydrocarbon components of the feed gas stream. The
 feed stream 1 in Table 4 does not contain propylene and as such reflects a
 composition representative of a natural gas stream.
 The process has been illustrated using various standard components employed
 for the sequence of treating steps with it being understood that the
 process may be practiced utilizing different physical apparatus. For
 instance, the turbo expanders 46 and 50 can, in many instances, be
 eliminated or replaced by Joule-Thomson isenthalpic control valves. The
 difference is that where the expander is eliminated or where the
 JouleThomson valves are substituted for the turbo expanders, normally
 greater inlet and refrigeration compression duties are required.
 Various arrangements have been shown in the alternate embodiments for
 cooling the second residue effluent and, in some instances, the combined
 residue effluent and heavy-ends fractionation column off-gas; however, it
 has been determined that the resultant C.sub.3 recovery is essentially
 identical provided an equal amount of heat is removed in any of the
 various embodiments of the process which have been described.
 Some of the illustrated processes in each instance use two turbo expanders
 as shown in FIG. 2. The desirability of the use of multiple turbo
 expanders is predicated primarily upon the amount of hydrogen content of
 the inlet gas in stream 1. It is understood that, according to the inlet
 gas content, only single turbo expanders may be employed in practicing the
 process; or, in some instances as previously indicated, turbo expanders
 may be eliminated completely or substituted by one or more Joule-Thomson
 isenthalpic expansion valves.
 An important element of the process is employment of the light-ends
 fractionating column 52 which functions as a combination heat and mass
 transfer device. The use of the reflux in the top stage means that the
 liquid methane, ethylene, and ethane of the reflux is vaporized; and in
 turn the liquid is progressively enriched in propylene and propane
 condensed from the upflowing bottom feed vapors to thereby recover a
 higher percent of the C.sub.3 components.
 For a given propylene/propane recovery, the process allows reducing the
 pressures that were required in the first cold separator; thus reducing
 the inlet compression capital and operating costs compared to the prior
 art. The improved process of this invention as exemplified in the
 embodiments of FIGS. 1 through 4 achieves a given propylenelpropane
 recovery while requiring relatively lower inlet gas pressure and
 refrigeration compression horse power than with the known prior art
 process. Further, in the processes exemplified in FIGS. 1 through 4, the
 propylene/propane recovery is achieved in a manner wherein the process is
 operated at a higher temperature level than in the previously known
 processes.
 The claims and the specification describe the invention presented and the
 terms that are employed in the claims draw their meaning from the use of
 such terms in the specification. The same terms employed in the prior art
 may be broader in meaning than specifically employed herein. Whenever
 there is a question between the broader definition of such terms used in
 the prior art and the more specific use of the terms herein, the more
 specific meaning is meant.
 While the invention has been described with a certain degree of
 particularity, it is manifest that many changes may be made in the details
 of construction and the arrangement of components without departing from
 the spirit and scope of this disclosure. It is understood that the
 invention is not limited to the embodiments set forth herein for purposes
 of exemplification, but is to be limited only by the scope of the attached
 claim or claims, including the full range of equivalency to which each
 element thereof is entitled.
 TABLE 1