Process for separating vinyl chloride monomer from incondensables

Abstract

A process for separating vinyl chloride monomer (VCM) from incondensables. A stream comprising VCM and incondensables 201 is compressed in compressor 203. VCM is condensed from compressed stream 205 i

Description

Field of the Invention The present invention relates to a process for separating vinyl chloride monomer from incondensables, such as is required during the production of vinyl chloride monomer by the hydrochlorination of acetylene. Background The hydrochlorination of acetylene to produce VCM as the precursor to polyvinyl chloride (PVC) is a large scale industrial process, particularly in coal rich areas such as China and in areas rich in natural gas through natural gas to acetylene routes. Over 20 million tonnes of VCM are produced annually through hydrochlorination of acetylene (“acetylene-based VCM process”) with the vast majority utilising mercuric chloride (HgCh) catalysts supported on activated carbon. A typical acetylene-based VCM process includes the following stages: catalytic hydrochlorination of acetylene in a reactor, cooling, compression, followed by separation of VCM from incondensables. The separation of VCM from incondensables is normally achieved by one or more cooling / condensation stages, followed by further separation in a lights column. One arrangement of separating VCM from incondensables is shown in Figure 1 of CN117018826A. A simplified version of this flowsheet is shown in Figure 1a of the present application; for ease of discussion the reference numerals from CN117018826A have been used. A stream comprising VCM and incondensables is compressed in compressor (33), condensed in “total condenser” (34) and sent to low boiling tower (41). The liquid in the low boiling tower is heated via reboiler (411) using flash steam. The overheads from the total condenser and low boiling tower are combined, condensed in secondary condenser (38) and further condensed in tail gas condenser (51). Hereafter, for consistency with the terminology used in later sections, the “low boiling tower” will be referred to as the “lights column” and the pressure of the stream immediately following the compressor is referred to as the “compressor discharge pressure”. Figure 1a shows the vent of the total condenser (34) being fed to condenser (38). This implies that condenser (38) operates at a pressure equal or less than the pressure in condenser (34), and therefore the pressure in condenser (38) is less or equal to the compressor discharge pressure. It would be normal engineering practice to pump the liquid stream from the total condenser (34) to the lights column (41), in which case the pressure in the lights column is limited by the pump discharge pressure. The pressure of the overheads in the lights column would depend on details, notably pressure control system, of the configuration which are not discussed in CN117018826A. In one possible configuration, shown in Figure 1b, the pressure of the lights column overheads is higher than the compressor discharge pressure; this would require a valve (42) to be present before combining the lights column overhead with the total condenser overheads. In an alternative configuration, shown in Figure 1c, the pressure of the lights column overheads is equal to the total condenser overheads. In this arrangement the pressure is controlled by including a valve downstream of condenser (38). In the arrangement shown in Figure 1 c this valve is located immediately after condenser (38) but it would also be possible to locate the valve downstream of condenser (51) ordownstream of PSA (52). One drawback with the arrangements shown in Figures 1a, 1b and 1c is that the pressure in condenser (38) is limited to a maximum of the compressor discharge pressure. This limits the efficiency of the condensation that can be achieved in condensers (38) and (51). Another drawback is that the lights column overheads, which can be expected to be richer in VCM compared to the total condenser overheads, are diluted by being combined with the total condenser overheads before condensation. As a result, the refrigerants used in condensers (38) and (51) have to be at a suitably low temperature to condense VCM from the mixed stream, which adds duty onto the refrigeration service. There is a need for an alternative arrangement which achieves more efficient separation of VCM from incondensables. As used herein, the term “incondensables” should be understood to mean a mixture comprising one or more of acetylene, HCI, H2, N2, O2 and / or Ar. Summary of the Invention The solution proposed by the present inventors is to provide a dedicated lights column condenser to separate VCM from the lights column overheads. A VCM-depleted lights column overheads stream generated by the lights column condenser is then depressurised before being combined with an incondensables stream generated by condensation stages following the compressor. In this arrangement the lights column overheads are not diluted with incondensables prior to condensation and therefore the concentration of VCM in the overheads stream sent to the lights column condenser is generally higher than the VCM content of the combined stream fed to the condenser in the prior art arrangement (i.e. the stream fed to condenser (38) in Figure 1a). This makes the condensation of VCM more efficient compared to the prior art arrangement. In addition, this arrangement also makes it possible to operate the lights column condenser above the compressor discharge pressure, and therefore a more thorough separation of VCM from incondensables can be achieved in the lights column condenser. Alternatively, comparable separation can be achieved to existing arrangements, but using a coolant of higher temperature in the lights column condenser. The possibility of using a coolant of higher temperature is advantageous because it reduces the energy used by the refrigeration service. In a first aspect the invention relates to process for separating vinyl chloride monomer (VCM) from incondensables, comprising the steps of: (i) compressing a stream comprising VCM and incondensables in a compressor to produce a compressed stream; (ii) condensing VCM from the compressed stream in one or more condensation stages (207) to produce a condensed VCM stream and an incondensables stream; (iii) pumping the condensed VCM steam using a pump to a lights column heated via a reboiler; (iv) separating the condensed VCM stream in the lights column into a lights column bottoms stream (219) and a lights column overheads stream; (v) separating the lights column overheads stream into a recovered VCM stream and a VCM-depleted lights column overheads stream in a lights column condenser, and returning the recovered VCM stream to the lights column; (vi) reducing the pressure of the VCM-depleted lights column overheads stream by a valve to produce a depressurised stream; and (vii) combining the depressurised stream with the incondensables stream to generate a combined incondensables stream. Many operational acetylene-based VCM plants, particularly in China, already have many of the unit operations of the process, namely the compressor, condensation stages and lights column. An advantage of this process is that existing plants may be retrofitted to operate the present process by installing a lights column condenser and associated piping, etc. at relatively low additional capex. The invention is also applicable to a grassroots VCM plant. Description of the figures Figure 1a illustrates a portion of the flowsheet described in CN117018826A. Figure 1b illustrates a possible arrangement in which the lights column overheads are at a pressure above the total condenser overheads pressure. Figure 1c illustrates a possible arrangement in which the lights column overheads and the total condenser overheads are at the same pressure. Figure 2 illustrates the process of the present invention. A stream comprising VCM and incondensables (201) in fed to a compressor (203) and compressed to produce a compressed stream (205) having pressure Pcs. VCM is condensed from the compressed stream in a condensation stage (207) to produce a condensed VCM stream (209) and an incondensables stream (211). Although a single condensation stage is shown in Figure 2, multiple condensation stages could be used. The condensed VCM steam is pumped using a pump (213) to a lights column (215) which is heated via a reboiler (217). The lights column separates the condensed VCM stream into a lights column bottoms stream (219) and a lights column overheads stream (221) having a pressure Plc. Further separation of VCM from incondensables is achieved in a lights column condenser (223) to produce a recovered VCM stream (225) and a VCM-depleted lights column overheads stream (227). The VCM stream is returned to the lights column. The VCM-depleted lights column overheads stream is depressurised by a valve (229) to produce a depressurised stream (231). In the arrangement shown the pressure of the lights column overheads stream is measured at a location downstream from the lights column condenser and the valve is opened as required to maintain the desired pressure. The depressurised stream is combined with the incondensables stream to generate a combined incondensables stream (233). Figure 3 illustrates a preferred arrangement of the process. This arrangement resembles Figure 2 but the condensation stage (307) comprises a heat exchanger (335) followed by a gas / liquid separator (337). The gas / liquid mixture from the heat exchanger is fed to the gas / liquid separator where it is separated into the condensed VCM stream (309) and the incondensables stream (311). Figure 4 illustrates another preferred arrangement of the present invention. The arrangement is similar to that shown in Figure 3 but the lights column condenser (423) comprises a heat exchanger (439) and a gas / liquid separator (441) in which the VCM stream (425) and VCM-depleted lights column overheads stream (427) are separated. The VCM stream is returned to the lights column via pump (443). Detailed description of the invention As used herein, the term “cooling water” refers to water at ambient or slightly above ambient temperature, which is used for cooling duties. Wherever used, cooling water preferably has a temperature of 20 to 50 °C, preferably 30 to 50 °C, more preferably 35 to 45 °C. As used herein, the term “cooling stage” is used to refer to a step in which the temperature of the vapour is reduced. A cooling stage may or may not be accompanied by condensation of liquid from the vapour. As used herein, the term “condensation stage” is used to refer to a step in which there is condensation of liquid from the vapour. Sub-headings are included for convenience only and are not intended to limit the disclosure. Compression and condensation stage(s) The compressor is fed with a vapor stream comprising VCM and incondensables. Such streams may be produced for instance by the catalytic hydrochlorination of acetylene or by the thermal or catalytic dehydrochlorination of acetylene. The stream may optionally have been pre-treated (e.g. to remove residual HCI and / or cooled). The role of the compressor is to increase the pressure of the vapor before the one or more condensation stages, so as to reduce the size of these units and to increase the efficiency of VCM separation in these units. Any suitable compressor may be used, but a preferred type is a screw compressor because these are generally less sensitive to fouling and cheaper than centrifugal compressors. Oil-free screw compressors are most preferred. The pressure chosen for the compressed stream immediately after the compressor (Pcs) will depend in part on the design of the condensation stage(s) and temperature of the coolant used in the condensation stage(s). Condensation becomes more efficient with increasing pressure, and the use of higher pressures may allow for a coolant of higher temperature to be used in the condensation stages. There is therefore a trade-off between the duty on the compressor and the duty on the refrigeration service (higher duty as coolant temperature decreases). A pressure Pcs of 2 to 26 bara provides a good balance between the duty on the compressor and the duty on the refrigeration service. A preferred pressure Pcs is 3 to 16 bara, preferably 3 to 8 bara, more preferably 4-6 bara. The compressed vapour stream is then condensed in one or more condensation stages, hereafter “condensation stage(s)”. The role of the condensation stage(s) is to separate the compressed stream into a condensed VCM stream (which is VCM-rich compared to the compressed stream) and an incondensables stream which is VCM-lean compressed to the compressed stream). Depending on the design of equipment used in each condensation stage, each individual condensation stage may either produce a single process stream comprising a gas / liquid mixture (e.g. when using a heat exchanger) or may produce separate vapour and liquid streams. It is necessary to ensure that liquid and vapour are separated prior to the pump because the presence of bubbles in the liquid could damage the pump. It is therefore preferred that a gas / liquid separator is included upstream of the pump. Where present, the gas / liquid separator produces the condensed VCM stream and the incondensables stream. In a preferred embodiment step (ii) is carried out in one or more heat exchangers followed by a gas / liquid separator. Where there are two or more heat exchangers, it is preferred that a coolant of decreasing temperature is used in each heat exchanger, in order to achieve further condensation of VCM in each step. The coolants are provided by a refrigeration service. The coolants used in each stage may be the same (albeit at different temperatures) or may be different, but are preferably the same in order to reduce the complexity of the refrigeration service. In a particularly preferred embodiment step (ii) includes a first heat exchanger using a coolant having a temperature of-10 to 0 °C, followed by a second heat exchanger using a coolant having a temperature of-20 to -10 °C. It is preferred that one or more cooling stages are carried out on the compressed stream prior to the one or more condensation stages. The term “cooling stage” means a stage in which the temperature of the compressed vapour stream is reduced but without condensing VCM. This is preferred in order to reduce the coolant duty associated with the condensation stages. The cooling stage(s) preferably use cooling water as the coolant. The condensed VCM stream is pumped via a pump to the lights column. The pump discharge pressure sets the pressure of the liquid entering the lights column. Lights column The role of the lights column is to separate the condensed VCM stream into a lights column bottoms stream (which is VCM-rich compared to the condensed VCM stream), and a lights column overheads stream (which is VCM-lean compared to the condensed VCM stream). The lights column bottoms stream is split, a portion is heated by a reboiler then returned to the lights column. The remaining portion of the lights column bottoms fraction is preferably fed to a heavies column for removal of heavy fractions. The pressure of the lights column overheads stream (Plc) may be less than, equal to, or greater than Pcs. It is preferred that the pressure of the lights column overheads stream (Plc) is greater than Pcs, in order that further separation of VCM can be achieved in the lights column condenser without adding too much duty to the refrigeration service. The inclusion of a dedicated condenser for the lights column means that Plc is not limited to at most Pcs, unlike in prior art arrangements. The pressure difference (Alc cs = Plc - Pcs) is preferably at least 0.1 bar, preferably 0.1-10 bar. The lights column overheads fraction is separated into a recovered VCM stream and a VCM-depleted lights column overheads stream by a lights column condenser. The lights column condenser preferably comprises one or more heat exchangers followed by a gas / liquid separator. The recovered VCM stream is preferably returned to the lights column. The VCM-depleted lights column overheads stream is depressurised by a valve downstream of the lights column condenser. The valve may be controlled by including a pressure sensor downstream of the valve, and opening the valve once the pressure reaches a pre-set threshold. The depressurised stream is then combined with the incondensables stream (from the condensation stages in step (ii)) to generate a combined incondensables stream. Optionally, one or more further condensation steps may be carried out on the combined incondensables stream. However, this is not preferred because it would require a coolant of lower temperature than that used in the lights column condenser. Treatment of the combined incondensables stream In some embodiments a hydrochlorination reactor is located upstream of the compressor. In these embodiments it is preferred that the combined incondensables stream is sent to a pressure-swing absorber (PSA). The PSA separates an acetylene-containing stream which is used as part of the feed for the hydrochlorination reactor. By “part of the feed” it will be appreciated that additional acetylene and HCI will be required to make up the feed for the hydrochlorination reactor. In some embodiments the process is part of a VCM plant comprising a primary hydrochlorination reactor and a secondary hydrochlorination reactor. Processes using a primary and secondary hydrochlorination reactors are known and are described for instance in WO2023 / 237854A1 and CN1884241A. In this embodiment a primary hydrochlorination reactor is located upstream of the compressor and a secondary hydrochlorination reactor is located downstream of the lights column; the combined incondensables stream is used as the feed for the secondary hydrochlorination reactor. Typically the composition of the combined incondensables stream will require adjustment before the secondary hydrochlorination reactor, e.g. adjustment of the acetylene : HCI ratio by addition of acetylene and / or HCI.

Claims

1. A process for separating vinyl chloride monomer (VCM) from incondensables, comprising the steps of:(i) compressing a stream comprising VCM and incondensables (201) in a compressor (203) to produce a compressed stream (205);(ii) condensing VCM from the compressed stream in one or more condensation stages (207) to produce a condensed VCM stream (209) and an incondensables stream (211);(iii) pumping the condensed VCM steam using a pump (213) to a lights column (215) heated via a reboiler (217);(iv) separating the condensed VCM stream in the lights column into a lights column bottoms stream (219) and a lights column overheads stream (221);(v) separating the lights column overheads stream into a recovered VCM stream (225) and a VCM-depleted lights column overheads stream (227) in a lights column condenser (223), and returning the recovered VCM stream to the lights column;(vi) reducing the pressure of the VCM-depleted lights column overheads stream by a valve (229) to produce a depressurised stream (231); and(vii) combining the depressurised stream with the incondensables stream to generate a combined incondensables stream (233).

2. A process according to claims 1, wherein the pressure of the compressed stream immediately after the compressor (Pcs) is 3 to 16 bara.

3. A process according to claim 2, wherein Pcs is 3 to 8 bara.

4. A process according to claim 2, wherein Pcs is 4 to 6 bara.

5. A process according to any of claims 1 to 4, wherein one or more cooling stages are carriedout on the compressed stream prior to the one or more condensation stages.

6. A process according to claim 5, wherein cooling water is used as the coolant in the one or more cooling stages.

7. A process according to any of claims 1 to 6, wherein step (ii) is carried out in one or more heat exchangers (307) followed by a gas / liquid separator (335).

8. A process according to claim 7, wherein step (ii) includes two or more heat exchangers using a coolant of decreasing temperature in each heat exchanger.

9. A process according to claim 8, wherein step (ii) includes a first heat exchanger using a coolant having a temperature of-10 to 0 °C, followed by a second heat exchanger using a coolant having a temperature of-20 to -10 °C.

10. A process according to any of claims 1 to 9, wherein the pressure difference (Alc-cs)between the pressure of the lights column overheads stream (Plc) and the pressure of thecompressed stream immediately after the compressor (Pcs) is at least 0.1 bar.

11. A process according to claim 10, wherein Alc-cs is 0.1-10 bar.

12. A process according to any of claims 1 to 11, wherein the lights column condenser (423)comprises one or more heat exchangers (439) followed by a gas / liquid separator (441).

13. A process according to any of claims 1 to 12, comprising a hydrochlorination reactor located upstream of the compressor, wherein the combined incondensables stream is sent to a pressureswing absorber, and an acetylene-containing stream generated by the pressure-swing absorber is used as part of the feed for the hydrochlorination reactor.

14. A process according to any of claims 1 to 13, comprising a primary hydrochlorination reactor located upstream of the compressor, and a secondary hydrochlorination reactor located downstream of the lights column, wherein the combined incondensables stream is used as part of the feed for the secondary hydrochlorination reactor.AMENDMENTS TO THE CLAIMS HAVE BEEN FILED AS FOLLOWS1018 06 25Claims1. A process for separating vinyl chloride monomer (VCM) from incondensables, comprising the steps of:5 (i) compressing a stream comprising VCM and incondensables (201) in a compressor (203)to produce a compressed stream (205);(ii) condensing VCM from the compressed stream in one or more condensation stages (207) to produce a condensed VCM stream (209) and an incondensables stream (211);(iii) pumping the condensed VCM stream using a pump (213) to a lights column (215) 10 heated via a reboiler (217);(iv) separating the condensed VCM stream in the lights column into a lights column bottoms stream (219) and a lights column overheads stream (221);(v) separating the lights column overheads stream into a recovered VCM stream (225) and a VCM-depleted lights column overheads stream (227) in a lights column condenser (223), and 15 returning the recovered VCM stream to the lights column;(vi) reducing the pressure of the VCM-depleted lights column overheads stream by a valve (229) to produce a depressurised stream (231); and(vii) combining the depressurised stream with the incondensables stream to generate a combined incondensables stream (233).

202. A process according to claims 1, wherein the pressure of the compressed stream immediately after the compressor (Pcs) is 3 to 16 bara.

3. A process according to claim 2, wherein Pcs is 3 to 8 bara.

254. A process according to claim 2, wherein Pcs is 4 to 6 bara.

5. A process according to any of claims 1 to 4, wherein one or more cooling stages are carriedout on the compressed stream prior to the one or more condensation stages.

306. A process according to claim 5, wherein cooling water is used as the coolant in the one or more cooling stages.

7. A process according to any of claims 1 to 6, wherein step (ii) is carried out in one or more 3 5 heat exchangers (307) followed by a gas / liquid separator (335).11

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