Process for increasing ethylene and propylene yield from a propylene plant

Abstract

A process for recovery of C.sub.2 and C.sub.3 components in an on-purpose propylene production system includes utilizing a packed rectifier with a countercurrent stream to strip C.sub.2 and C.sub.3 components from a combined de-ethanizer overhead lights vapor and cracked gas vapor stream.

Claims

1. A process for recovery of C2 and C3 components via an on-purpose propylene production system comprising: cooling a combined vapor stream of a de-ethanizer overhead light vapor from a de-ethanizer and a cracked gas vapor to a temperature at which at least 80 wt % of the combined vapor stream is condensed to form a first partial condensate and a first residual vapor; passing the first partial condensate to a bottom portion of a rectifier; passing the first residual vapor up through packing of the rectifier while contacting the first residual vapor with a countercurrent stream which comprises from 75 to 95 wt % C2 components, from 5 to 25 wt % Ci components and from 0 to 5 wt % C3 components thereby producing a rectifier overhead stream which comprises from 20 to 45 wt % Ci components, from 40 to 60 wt % C2 components and from 0 to 5 wt % C3 components and a rectifier liquid bottom stream; passing the rectifier liquid bottom stream to the de-ethanizer; partially condensing the rectifier overhead stream in one or more heat exchanger stages to cool the rectifier overhead stream to produce a second partial condensate with a second residual vapor which comprises less than 5 wt % C2 components and a first residual liquid which comprises from 75 to 95% C2 components; passing the combined second residual vapor and first residual liquid to a knockout drum to separate the second residual vapor from the first residual liquid; passing a first portion of the first residual liquid to a top portion of the packing in the rectifier to serve as the countercurrent stream; and passing a second portion of the first residual liquid to a product recovery system.

2. The process for recovery of C.sub.2 and C.sub.3 components according to claim 1, wherein the one or more heat exchanger stages cool the rectifier overhead stream to a temperature less than or equal to 100 C.

3. The process for recovery of C.sub.2 and C.sub.3 components according to claim 1, wherein the second residual vapor comprises at least 95 wt % C.sub.1 components.

4. The process for recovery of C.sub.2 and C.sub.3 components according to claim 1, wherein the second residual vapor is first used as a cooling medium in the one or more heat exchanger stages and is warmed to a temperature of from 50 to 90 C., then subsequently expanded and cooled to a temperature from 110 to 180 C., then used again as a cooling medium in one or more heat exchanger stages.

5. The process for recovery of C.sub.2 and C.sub.3 components according to claim 1, wherein the rectifier is a tray type column having a plurality of trays wherein individual trays of the plurality of trays are selected from the group consisting of bubble-cap trays, sieve trays, and valve trays.

6. The process for recovery of C.sub.2 and C.sub.3 components according to claim 1, using the second residual vapor as at least one of a fuel gas and a cooling medium in the one or more heat exchanger stages.

7. The process for recovery of C.sub.2 and C.sub.3 components according to claim 1, wherein partially condensing the rectifier overhead stream in the one or more heat exchanger stages to cool the rectifier overhead stream comprises cooling the rectifier overhead stream to a temperature of less than or equal to 140 C.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

(1) The following detailed description of specific embodiments of the present disclosure can be best understood when read in conjunction with the following drawings, in which:

(2) FIG. 1 is a schematic view of a process for upgrading a hydrocarbon-based composition;

(3) FIG. 2 is a schematic view of a process for upgrading a hydrocarbon-based composition;

(4) FIG. 3 is a schematic view of a process for upgrading a hydrocarbom-based composition; and

(5) FIG. 4 is a schematic view of a process for upgrading a hydrocarbon-based composition, according to the present embodiments.

(6) FIG. 1 herein reproduces FIG. 4 from Cambell et al (U.S. Pat. No. 5,771,712). In the process shown in FIG. 1, feed gas enters at 80 F. and a pressure of 580 psia as stream 31. The feed stream 31 is cooled in exchanger 10 by heat exchange with cool residue gas at 88 F. (stream 34a), with separator liquids at 92< F. (stream 33a), and with separator/absorber liquids at 107 F. (stream 35a). The cooled stream 31a enters separator 11 at 78 F. and 570 psia where the vapor (stream 32) is separated from the condensed liquid (stream 33).

(7) The vapor (stream 32) from separator 11 enters a work expansion machine 13 in rhich mechanical energy is extracted from this portion of the high pressure feed. The machine 13 expands the vapor substantially isentrupically from a pressure of about 570 psia to a pressure of about 396 psia (the operating pressure of separator/absorber 15), with the work expansion cooling the expanded stream 32a to a temperature of approximately 107 F. The expanded and partially condensed stream 32a enters the lower section of separator/absorber 15. The liquid portion of the expanded stream commingles with liquids falling downward from the absorbing section and the combined liquid stream 35 exits the bottom of separator/absorber 15 at 108 F. The vapor portion of the expanded stream rises upward through the absorbing section and is contacted with cold liquid falling downward to condense and absorb the propane and heavier components.

(8) The combined liquid stream 35 from the bottom of the separator/absorber 15 is routed to heat exchanger 10 by pump 16 where it (stream 35a) is heated as it provides cooling of the incoming feed gas as described earlier. The combined liquid stream is heated to 46 F., partially vaporizing stream 35b before it is supplied as a mid-column feed to deethanizer 17. The separator liquid (stream 33) is flash expanded to slightly above the 411 psia operating pressure of deethanizer 17 by expansion valve 12, cooling stream 33 to 92 F. (stream 33a) before it provides cooling to the incoming feed gas as described earlier. Stream 33b, now at 70 F., then enters deethanizer 17 at a lower mid-column feed point. In the deethanizer, streams 35b and 33b are stripped of their methane and C.sub.2 components. The resulting liquid product stream 37 exits the bottom of the deethanizer at 198 F. and is cooled to 110 F. (stream 37a) in heat exchanger 19 before flowing to storage.

(9) The operating pressure in deethanizer 17 is maintained slightly above the operating pressure of separator/absorber 15. This allows the deethanizer overhead vapor (stream 36) to pressure flow through heat exchanger 20 and thence into the upper section of separator/absorber 15. In heat exchanger 20, the deethanizer overhead at 25 F. is directed in heat exchange relation with the overhead (stream 34) from separator/absorber 15, cooling the stream to 112 F. (stream 36a) and partially condensing it. The partially condensed stream is then supplied to the separator section in separator/absorber tower 15 where the condensed liquid is separated from the uncondensed vapor. The uncondensed vapor combines with the vapor rising from the lower absorbing section to form the cold distillation stream 34 leaving the upper region of separator/absorber 15. The condensed liquid is divided into two portions. One portion, stream 40, is routed to the lower absorbing section of separator/absorber 15 as the cold liquid that contacts the vapors rising upward through the absorbing section. The other portion, stream 39, is supplied to deethanizer 17 as reflux by pump 21, with reflux stream 39a flowing to a top feed point on deethanizer 17 at 112 F.

(10) The distillation stream leaving the top of separator/absorber 15 at 113 F. is the cold residue gas stream 34. The residue gas stream passes countercurrently to deethanizer overhead stream 36 in heat exchanger 20 and is warmed to 88 F. (stream 34a) as it provides cooling and partial condensation of the deethanizer overhead stream. The residue gas is further warmed to 75 F. (stream 34b) as it passes countercurrently to the incoming feed gas in heat exchanger 10. The residue gas is then re-compressed in two stages. The first stage is compressor 14 driven by expansion machine 13. The second stage is compressor 22 driven by a supplemental power source which compresses the residue gas (stream 34d) to sales line pressure. After cooling in discharge cooler 23, the residue gas product (stream 34e) flows to the sales gas pipeline at 110 F. and 613 psia.

(11) U.S. Pat. No. 5,771,712 teaches a process for recovery of C3+ components from a feed hydrocarbon gas and creates a cooled C3 containing liquid stream. U.S. Pat. No. 5,771,712 teaches feeding an inlet gas, only two products, only a residual gas is created from the overhead of the deethanizer. U.S. Pat. No. 5,771,712 teaches cooling and partially condensing the inlet gas and feeding the partially condensed stream to an absorption column where the vapor passes upward through the column contacting a cold downcoming liquid. However, U.S. Pat. No. 5,771,712 teaches using the same liquid composition (drawn from the same stream) for the reflux to the top of the de-ethanizer as the cold liquid that is used in the absorption column.

(12) U.S. Pat. No. 5,890,378 (Rambo et al.) relates to a process for recovery of ethane, ethylene, propane, propylene and heavier hydrocarbon components from a hydrocarbon gas stream. The process begins by dividing the hydrocarbon gas stream into a first stream and a second stream, with the latter being cooled and expanded to a lower pressure and supplied to a contacting device. The first stream is cooled to condense substantially all of it, expanded to the lower pressure and then used to cool a warmer distillation stream from a distillation column to at least partially condense the distillation stream. At least a portion of the partially condensed distillation stream is directed to the contacting device to intimately contact the expanded second stream and yield a combination of vapors and liquids that are separated from the contacting device, with the liquids being supplied to the distillation column.

(13) U.S. Pat. No. 5,890,378 teaches a process for recovery of C2 and heavier hydrocarbons into a single stream from a hydrocarbon gas stream. This reference only teaches creating two product streams from an inlet gas stream as opposed to three in the present invention. This patent teaches cooling and partial condensation of the overhead de-ethanizer stream but directs this partially condensed de-ethanizer overhead to the absorber column and heating the overhead vapor from the absorber. This reference teaches partially condensing the de-ethanizer overhead and routing the partially condensed liquid to the upper sections of an absorber.

(14) U.S. Pat. No. 6,560,989 (Roberts et al.) relates to a method for the recovery of hydrogen and one or more hydrocarbons having one or more carbon atoms from a feed gas containing hydrogen and the one or more hydrocarbons, which process comprises cooling and partially condensing the feed gas to provide a partially condensed feed; separating the partially condensed feed to provide a first liquid stream enriched in the one or more hydrocarbons and a first vapor stream enriched in hydrogen; further cooling and partially condensing the first vapor stream to provide an intermediate two-phase stream; and separating the intermediate two-phase stream to yield a further-enriched hydrogen stream and a hydrogen-depleted residual hydrocarbon stream. Some or all of the cooling is provided by indirect heat exchange with cold gas refrigerant generated in a closed-loop gas expander refrigeration cycle.

(15) FIG. 2 herein reproduces FIG. 1 from Roberts et al. Pretreated feed gas is provided in line 101, typically at a pressure of 100 to 1000 psia and at ambient temperature, and contains hydrogen, one or more light hydrocarbons selected from methane, ethane, ethylene, propane, propylene, and optionally carbon monoxide, nitrogen, and/or C.sub.4.sup.+ hydrocarbons. The feed gas is pretreated in an upstream pretreatment step (not shown) to remove water and other components which can freeze out in the downstream processing. The feed gas is cooled and partially condensed in feed cooler or first heat exchange zone 103 by indirect heat exchange with several cold process streams (described later) to yield partially condensed feed in line 105. The partially condensed feed is separated in stripping column feed drum 107 to provide a first liquid stream enriched in hydrocarbons in line 109 and a first vapor stream enriched in hydrogen in line 111.

(16) The first liquid stream may be reduced in pressure across valve 113 and introduced into optional stripping column 115, in which lighter hydrocarbons, residual hydrogen, and other light gases are stripped and withdrawn in overhead line 117. The heavier hydrocarbon fraction is withdrawn as liquid from the bottom of the stripping column via line 119 and contains C.sub.2.sup.+ product components such as ethylene and/or propylene. A portion of the stripping column bottoms stream is vaporized in heat exchanger 121 and returned to the column as boilup or stripping vapor. The stripping column typically operates at 50 to 400 psia in the temperature range of 200 F. to +50 F.

(17) The first vapor stream enriched in hydrogen in line 111 is cooled and partially condensed in hydrogen recovery heat exchanger or second heat exchange zone 123 by indirect heat exchange with cold process streams (defined later) to yield a partially condensed stream typically at 200 F. to 300 F. in line 125. The partially condensed stream is separated in hydrogen recovery drum 127 to yield a hydrogen product stream in line 129 and a hydrogen-depleted residual hydrocarbon stream in line 131. The hydrogen product stream in line 129 is warmed in second heat exchange zone 123 by indirect heat exchange to provide a portion of the cooling for the stream entering in line 111, and then is further warmed in first heat exchange zone 103 by indirect heat exchange to provide a portion of the cooling for the feed gas entering in line 101. The final hydrogen product in line 130 is typically at ambient temperature and 15 to 30 psi below the feed gas pressure in line 101. This hydrogen product stream, which typically contains 80 to 97 mole % hydrogen, may be further purified by a pressure swing adsorption or membrane process if desired.

(18) The hydrogen-depleted residual hydrocarbon stream in line 131 may be reduced in pressure across valve 133 and is warmed in second heat exchange zone 123 by indirect heat exchange to provide an additional portion of the cooling for the stream entering in line 111. The warmed stream in line 135 may be combined with the stripped gas stream in line 117, in which case the combined stream in line 137 is further warmed in first heat exchange zone 103 by indirect heat exchange to provide a portion of the cooling for the feed gas entering in line 101. The warmed residual hydrogen-hydrocarbon stream is withdrawn via line 138 and can be used as fuel.

(19) A major portion of the refrigeration for this embodiment is provided by a closed-loop gas expander refrigeration system, refrigerant gas, for example nitrogen, is withdrawn in line 139 from first heat exchange zone 103 and compressed to 600 to 1500 psia in refrigerant compressor 141. Other refrigerants may be used such as, for example, methane, a mixture of nitrogen and methane, or air. The compressed refrigerant gas is cooled in passage 144 of first heat exchange zone 103 to provide a cooled compressed refrigerant gas, which is divided into a first refrigerant gas stream withdrawn in line 145 and a second refrigerant gas stream in heat exchanger passage 147. The second refrigerant gas stream is further cooled in heat exchanger passage 147 to provide cooled refrigerant gas in line 149.

(20) The first refrigerant gas stream in line 145 is work expanded in warm expander 150 to provided a cooled work-expanded refrigerant gas stream in line 151. The further cooled refrigerant gas in line 149 is work expanded in cold expander 153 to provide a cooled reduced-pressure refrigerant gas stream in line 155. Alternatively, instead of work expansion, the gas in line 149 can be reduced in pressure and cooled by Joule-Thomson expansion across a throttling valve (not shown). The cooled reduced-pressure refrigerant gas stream in line 155 is warmed in second heat exchange zone 123 to provide at least a portion of the cooling of the stream entering in line 111, thereby providing a warmed reduced-pressure refrigerant gas stream in line 157. The warmed reduced-pressure refrigerant gas stream in line 157 and the warmed work-expanded refrigerant gas stream in line 151 may be combined, in which case the combined stream in line 159 is warmed in first heat exchange zone 103 by indirect heat exchange to provide a portion of the cooling for the feed gas entering via, line 101 and for the refrigerant flowing through passages 144 and 147. This provides a warmed reduced-pressure refrigerant gas stream in line 139 which is the refrigerant gas described above.

(21) In stripping column 115, the first liquid stream in line 109 is separated to produce a light overhead gas stream in line 117 and a C.sub.2.sup.+- or C.sub.3.sup.+-enriched hydrocarbon product stream line 119 that can be further separated and purified in additional columns if desired. The light overhead gas stream in line 117 from the stripping column can be recovered separately or combined with the hydrogen-depleted residual hydrocarbon stream in line 135 from the hydrogen recover heat exchanger 123 and rewarmed in feed cooler 103 to be recovered as a fuel stream in line 138. Optionally, a refluxed de-methanizer or de-ethanizer column can be utilized in place of stripping column 115 to increase recovery of the desired hydrocarbon products. Alternatively, the first liquid feed stream in line 109 can be recovered directly from feed drum 107 without stripping or distillation, either as a liquid or vapor product that can be rewarmed in feed cooler 103 if desired to recover refrigeration.

(22) Multiple partial condensation stages can be utilized to provide multiple feed streams to the column or to produce separate hydrocarbon products. For example, a C.sub.3-rich hydrocarbon product could be produced from a warmer partial condensation stage and a C.sub.2-rich hydrocarbon produced from a colder partial condensation stage. Stripping columns or refluxed distillation columns could be added to remove lighter impurities from one or both hydrocarbon products.

(23) Alternatively, if the feed gas is lean in C.sub.2 and heavier hydrocarbons, or if no C.sub.2.sup.+ hydrocarbon product is desired, only methane and upgraded methane-rich fuel gas would be recovered. Referring to FIG. 1, the upgraded methane-rich fuel gas product could be the hydrogen-depleted hydrocarbon stream in line 131, or the stripped gas stream in line 117 if stripping column 115 is utilized, or a combination of both, as in line 137. If the stripping column is utilized, the bottom liquid stream in line 119 could be vaporized in feed cooler 103 to provide refrigeration therein.

(24) U.S. Pat. No. 6,560,989 teaches a process for the recovery of hydrogen and one or more hydrocarbon streams from a hydrocarbon feed gas containing hydrogen, methane, C2's, C3's, and optionally carbon monoxide, nitrogen, and C4+ hydrocarbons. This reference teaches partially condensing the feed gas into a first residual vapor and a first residual liquid, separating the liquid, and feeding it to the top of a stripping column. U.S. Pat. No. 6,560,989 teaches the direction of the overhead gas from the stripping column to a product fuel gas stream and does not teach cooling and partially condensing this stream to create a liquid reflux from its own partial condensate as in the present invention. This reference also teaches directing the first residual vapor to further cooling and partial condensation and then passing this to a subsequent hydrogen recovery drum creating a second residual liquid and a second residual vapor. The two products from this step, a fuel gas and a hydrogen stream, are directed to product recovery, and the liquid is not used as reflux to a rectifier column.

(25) As used herein, the term C.sub.1 components means methane as well as lighter gasses such as hydrogen and nitrogen.

(26) As used herein, the term C.sub.2 components means ethane and ethylene.

(27) As used herein, the term C.sub.3 components means propane and propylene.

(28) In some aspects, this invention is a process for recovery of C.sub.2 and C.sub.3 components from an on-purpose propylene plant, the process comprising: (a) cooling a de-ethanizer overhead lights and cracked gas vapor to a temperature at which at least 80 wt % of the vapor is condensed to form a first partial condensate with a first residual vapor; (b) passing the first partial condensate to a bottom portion of a rectifier; (c) passing the first residual vapor up through a packing of the rectifier while contacting the first residual vapor with a countercurrent stream which comprises from 75 to 95 wt % C.sub.2 components, from 5 to 25 wt % C.sub.1 components and from 0 to 5 wt % C.sub.3 components thereby producing a rectifier overhead stream which comprises from 20 to 45 wt % C.sub.1 components, from 40 to 60 wt % C.sub.2 components and from 0 to 5 wt % C.sub.3 components and a rectifier liquid bottom stream; (d) passing the rectifier liquid bottom stream to the de-ethanizer; (e) partially condensing the rectifier overhead stream in one or more heat exchanger stages to cool the rectifier overhead stream to produce a second partial condensate with a second residual vapor which comprises less than 5 wt % C.sub.2 components and a first residual liquid which comprises from 75 to 95 wt % C.sub.2 components; (f) passing the combined second residual vapor and first residual liquid to a knockout drum to separate the second residual vapor from the first residual liquid; (g) optionally, using the second residual vapor as a fuel gas and/or as a cooling liquid in the one or more heat exchanger stages; (h) passing a first portion of the second residual liquid to a top portion of the packing in the rectifier to serve as the countercurrent stream; and (i) passing a second portion of the second residual liquid to a product recovery system.

(29) In one embodiment of an on purpose propylene plant, the temperature of the de-ethanizer overhead vapor is from 30 to 50 C. To condense at least 80 percent by weight (wt %) of such de-ethanizer overhead light vapor and cracked gas vapor, the vapor is cooled to from 40 to 60 C. in a first step. In another aspect, the de-ethanizer overhead vapor is cooled to 48 C. The de-ethanizer overhead lights vapor and cracked gas vapor is preferably cooled by passing it through three heat exchange stages.

(30) In one aspect, the first step of the process includes cooling the combined de-ethanizer overhead lights vapor and cracked gas vapor to a temperature at which at least 80 percent by weight (wt %) of the de-ethanizer overhead lights vapor and cracked gas vapor is condensed to form a partial condensate with a first residual vapor. All individual values and subranges from at least 80 wt % are included and disclosed herein. For example, the portion of the de-ethanizer overhead lights vapor and cracked gas vapor which is condensed can be at least 80 wt %, or in the alternative, at least 84 wt %, or in the alternative, at least 88 wt %, or in the alternative, at least 90 wt %, or in the alternative, at least 92 wt %, or in the alternative, at least 94 wt %, or in the alternative, at least 96 wt %.

(31) In one aspect, the second step in the process is passing the partial condensate to a bottom portion of a rectifier. The rectifier useful in the disclosed process is a packed bed rectifier. Packed bed rectifiers are known in the art and any such packed bed rectifier may be used. The rectifier may be packed with any typical packed column material, such as rings, saddles, or structured packing. Alternatively, the rectifier could be a tray type column with bubble-cap, sieve, or valve trays 352.

(32) In one aspect, the third step of the process is passing the first residual vapor up through the packing of the rectifier while contacting the first residual vapor with a countercurrent stream which comprises from 75 to 95 wt % C.sub.2 components, from 5 to 25 wt % C.sub.1 components and from 0 to 5 wt % C.sub.3 components. All individual values and subranges from 75 to 95 wt % C.sub.2 components in the countercurrent stream are disclosed and included herein. For example, the countercurrent stream may comprise from 75 to 95 wt % C.sub.2 components, or in the alternative, from 75 to 90 wt % C.sub.2 components, or in the alternative, from 80 to 95 wt % C.sub.2 components. All individual values and subranges from 5 to 25 wt % C.sub.1 components in the countercurrent stream are disclosed and included herein. For example, the amount of C.sub.1 components can be from 5 to 25 wt %, or in the alternative, from 5 to 20 wt %, or in the alternative, from 10 to 25 wt %. All individual values and subranges from 0 to 5 wt % C.sub.3 components in the countercurrent stream are disclosed and included herein. For example, the amount of C.sub.3 components in the countercurrent stream can be from 0 to 5 wt %, or in the alternative, from 0 to 3 wt %.

(33) Upon passing the first residual vapor up through the packing of the rectifier while contacting the first residual vapor with a countercurrent stream a rectifier overhead stream and a rectifier liquid bottom stream are produced. The rectifier overhead stream comprises from 20 to 45 wt % C.sub.1 components, from 40 to 60 wt % C.sub.2 components and from 0 to 5 wt % C.sub.3 components. All individual values and subranges from 20 to 45 wt % C.sub.1 components are included and disclosed herein. For example the amount of C.sub.1 components in the rectifier overhead stream can be from 20 to 45 wt %, or in the alternative from 25 to 45 wt %, or in the alternative, from 30 to 45 wt %. All individual values and subranges from 40 to 60 wt % C.sub.2 components are included and disclosed herein. For example, the amount of C.sub.2 components can be from 40 to 60 wt %, or in the alternative, from 45 to 60 wt %, or in the alternative, from 50 to 60 wt %.

(34) It will be readily understood that the second and third steps are generally practiced simultaneously in the on purpose propylene plant. However, if the second and third steps are not completed simultaneously, the order in which they are accomplished is not an integral aspect of the disclosed process. That is, the second step may be operated prior to the third step or vice versa.

(35) In one aspect as shown in FIG. 4, a schematic, a fourth step in the process is passing the rectifier liquid bottom stream 902 to the de-ethanizer 904. In a particular aspect, the rectifier liquid bottom stream 902 is used in the de-ethanizer 904 as a reflux stream.

(36) In one aspect, a fifth step in the process is partially condensing the rectifier overhead stream in one or more heat exchanger stages to cool the rectifier overhead stream to produce a partial condensate with a second residual vapor which comprises less than 5 wt % C.sub.2 components and a first residual liquid which comprises from 75 to 95% C.sub.2 components. All individual values and subranges from less than 5 wt % C.sub.2 components in the second residual vapor are included and disclosed herein. For example, the upper limit of the C.sub.2 components in the second residual vapor can be 5 wt %, or in the alternative, 4 wt %, or in the alternative, 3 wt %, or in the alternative 2 wt %, or in the alternative 1 wt %. All individual values and subranges from 75 to 95% C.sub.2 components in the first residual liquid herein. For example, the amount of C.sub.2 components in the first residual liquid can be from 75 to 95 wt %, or in the alternative, from 80 to 95 wt %.

(37) In one aspect, the one or more heat exchanger stages cool the rectifier overhead stream to a temperature less than or equal to 140 C. All individual values and subrangges of less than or equal to 140 C. are included and disclosed herein. For example, the rectifier overhead stream may be cooled to a temperature less than or equal to 140 C., or in the alternative, less than or equal to 150 C., or in the alternative, less than or equal to 160 C.

(38) It will be readily understood that the fourth and fifth steps are generally practiced simultaneously in the on purpose propylene plant. However, if the fourth and fifth steps are not completed simultaneously, the order in which they are accomplished is not an integral aspect of the disclosed process. That is, the fourth step may be operated prior to the fifth step or vice versa.

(39) In one aspect, a sixth step in the process is passing the combined second residual vapor and first residual liquid to a knockout drum to separate the second residual vapor from the first residual liquid. In a particular aspect, the second residual vapor, following separation, is used as a fuel gas and/or as a cooling liquid in the one or more heat exchanger stages.

(40) In one aspect, a seventh step of the process is passing a first portion of the first residual liquid to a top portion of the packing in the packed bed rectifier to serve as the countercurrent stream.

(41) In one aspect, an eighth step of the process is passing a second portion of the second residual liquid to a product recovery system. In a particular aspect, the second portion of the second residual liquid is passed into a product recovery system of an on-purpose ethylene plant. The on-purpose ethylene plant may, in one aspect, be located proximate to the on purpose propylene production system.

(42) In one aspect, the second residual vapor comprises at least 95 wt % C.sub.1 components. All individual values and subranges of at least 95 wt % C.sub.1 components are included and disclosed herein. For example, the amount of C.sub.1 components in the second residual vapor can be at least 95 wt %, or in the alternative, at least 97 wt %, or in the alternative, at least 99 wt %.

(43) In one aspect, the second residual vapor is first used as a cooling fluid in the one or more heat exchanger stages and is warmed to a temperature of from 50 to 90 C., then subsequently expanded and cooled to a temperature from 110 to 180 C., then used again as a cooling liquid in one or more heat exchanger stages. Methods for expansion and cooling are known, such as by the use of turbo-expanders or Joule-Thompson valves. All individual values and subranges from 50 to 90 C. as the temperature to which the second residual vapor is warmed are included and disclosed herein. For example, the temperature to which the second residual vapor is warmed can be from a lower limit of 90, 80, 70, or 60 C. to an upper limit of 85, 75, 65, 55 or 50 C. For example, the temperature to which the second residual vapor is warmed can be from 50 to 90 C., or in the alternative, from 50 to 75 C., or in the alternative, from 75 to 90 C., or in the alternative, from 60 to 80 C., or in the alternative, from 65 to 85 C. All individual values and subranges from 110 to 180 C. as the temperature to which the second residual vapor is subsequently cooled are included and disclosed herein. For example, the temperature to which the second residual vapor is subsequently cooled can be from a lower limit of 180, 170, 160, 150, 140, 130 or 120 C. to an upper limit of 110, 120, 130, 140, 150, 160, 170 or 175 C. For example, the temperature to which the second residual vapor is subsequently cooled can be from 110 to 180 C., or in the alternative, from 110 to 150 C., or in the alternative, from 150 to 180 C., or in the alternative from 130 to 170 C., or in the alternative, from 140 to 170 C.

(44) FIG. 3 is a schematic illustrating a first embodiment of the process for recovery of C.sub.2 and C.sub.3 components from an on-purpose propylene plant, as generally disclosed herein. As shown in FIG. 3, the de-ethanizer overhead and the cracked gas vapor streams 300 are fed into knock-out drum 305 as vapor. While shown as a single stream, the de-ethanizer overhead and dried cracked gas may be fed either jointly or separately. A bottoms stream 320 from the knock-out drum 305 is sent back to the de-ethanizer (not shown). The stripped stream 325 is then routed to one or more stages of heat exchanger 330 which cool the stripped stream. The cooled stream 335 is mostly condensed liquid (first partial condensate) but also contains a small vapor fraction (first residual vapor, generally less than or equal to 6 wt %). The cooled stream 335, which is a liquid/vapor stream, is routed to the bottom of a packed bed rectifier 350. The vapor fraction passes up through the packed bed rectifier 350 and is rectified wherein most of the C3's are removed to a bottom liquid stream 355. The liquid stream 355 is pumped back to the de-ethanizer (not shown) as reflux.

(45) As the first residual vapor passes up through the rectifier 350, it is contacted by a countercurrent flow of a C.sub.2's liquid stream 360 at about 145 C. The C.sub.2's liquid stream 360 rectifies the rising first residual vapor removing the heavier components into the bottom liquids stream 355 and therefore concentrating the lights in the vapor stream. This acts to produce a rectifier overhead stream 365.

(46) The rectifier overhead stream 365 is then routed to one or more stages of heat exchanger 330 where it is cooled and partially condensed to about a 39% vapor fraction 375, (forming a second partial condensate). The second partial condensate and vapor stream 375 is then routed to a second knock-out drum 380. About half 360 the liquid bottoms stream 385 from the second knock-out drum 380 is routed to the top of the rectifier 350 and the other half 405 is heated as it is routed to one or more stages of heat exchanger 330. Following heating in the one or more heat exchanger stages, the stream 405 is passed to a C.sub.2's recovery system.

(47) The overhead vapor 415 from the second knock-out drum 380 contains 99% C1's and lighter and is first routed to one or more stages of heat exchanger 330 where it is warmed to about 70 C. After warming in one or more stages of heat exchanger 330, the overhead vapor 415 is expanded in the turbo-expander/compressor 390 from about 400 psia to about 60 psia and cooled to about 150 C. The resulting cold stream 425 from the turbocompressor/expander 390 is then routed to one or more stages of heat exchanger 330, where it is heated to about 40 C. and then used as fuel gas.