PROCESS FOR INCREASING ETHYLENE AND PROPYLENE YIELD FROM A PROPYLENE PLANT

Abstract

The disclosure provides a process for recovery of C.sub.2 and C.sub.3 components in an on-purpose propylene production system 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 C.sub.2 and C.sub.3 components via an on-purpose propylene production system comprising: cooling a combined de-ethanizer overhead light vapor and cracked gas vapor to a temperature at which at least 80 wt % of the vapor 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 % 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; 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 % C.sub.2 components and a first residual liquid which comprises from 75 to 95% C.sub.2 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; optionally, using the second residual vapor as a fuel gas and/or as a cooling medium in the one or more heat exchanger stages; 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 passing a second portion of the second 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 % CI 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 the trays are one or more selected from the group consisting of bubble-cap trays, sieve trays, and valve trays.

Description

[0026] 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.

[0027] 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.

[0028] 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.

[0029] 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.

[0030] 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.

[0031] 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.

[0032] As used herein, the term “C.sub.1 components” means methane as well as lighter gasses such as hydrogen and nitrogen.

[0033] As used herein, the term “C.sub.2 components” means ethane and ethylene.

[0034] As used herein, the term “C.sub.3 components” means propane and propylene.

[0035] 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.

[0036] 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.

[0037] 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 %.

[0038] 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.

[0039] 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 %.

[0040] 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 %.

[0041] 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.

[0042] In one aspect, a fourth step in the process is passing the rectifier liquid bottom stream to the de-ethanizer. In a particular aspect, the rectifier liquid bottom stream is used in the de-ethanizer as a reflux stream.

[0043] 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 %.

[0044] 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.

[0045] 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.

[0046] 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.

[0047] 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.

[0048] 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.

[0049] 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 %.

[0050] 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.

[0051] 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.

[0052] 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.

[0053] 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.

[0054] 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.