Process for producing vinyl chloride monomer from acetylene

Abstract

A process for producing vinyl chloride monomer (VCM) from acetylene and hydrogen chloride (HCl), wherein the reaction product is subjected to sequential cooling an compression, wherein the compressed

Description

Field of the Invention The present invention relates to a process for the conversion of acetylene to vinyl chloride monomer (VCM). 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, separation of VCM from incondensables in a condenser, further separation of VCM from incondensables in a lights column. The incondensables from the condenser and / or lights column are usually fed to a purification section (typically a pressure-swing absorption unit) for recovery of acetylene. If a mercury catalyst is used in the reactor then conversion in the reactor is typically 97+%. An example of this arrangement is shown in Figure 1 of CN117018826A. One problem with this arrangement is that if conversion in the reactor is reduced, which is often the case when using mercury-free catalysts, the combined volume of incondensables in the condenser and lights column overheads is increased, as is the duty on the purification section. This reguires a larger PSA which adds to opex and capex costs. A further problem with this arrangement is that, if the crude product stream from the reactor contains significant amounts of H2, then the separation of VCM from incondensables in the condenser and lights column is less efficient. The HCI used in an acetylene-based VCM process is normally prepared by the reaction between H2 and CI2. Residual CI2 in the HCI feed needs to be avoided because of the potentially explosive reaction when mixed with acetylene. Forthat reason a slight excess of H2 is used in the reaction between H2 and CI2, which results in the HCI feed having residual H2 in an amount of ~5 vol%. Significant amounts of H2 in the condenser and lights column overheads add to the duty on the PSA, which adds to opex and capex costs. One way of avoiding the presence of H2 in the feed to the condenser and lights column is to remove H2 upstream of the hydrochlorination reactor. This is practiced in the flowsheet shown in Figure 1 of CN117018826A. Another way of removing H2 is to scrub the HCI using firstly a weak hydrochloric acid solution to produce a concentrated hydrochloric acid solution; HCI is then stripped from the concentrated hydrochloric acid solution to produce HCI gas free of H2. However, the generation of a H2-free stream of HCI involves significant additional capex and opex costs. There is a need for an acetylene-based VCM process which can (i) tolerate low single pass conversion; and / or (ii) does not require the HCI stream to be pre-treated to remove H2 upstream of the reactor; without increasing opex of the process. Summary of the Invention The solution proposed by the present inventors differs from conventional arrangements in that it includes a condensation section, vent recovery section and lights column with a specific arrangement as shown in Figure 1a and further described as follows. A cooled reactor product stream is fed to a compressor and compressed to generate a compressed stream. The compressed stream is fed to a condensation section where one or more steps of condensation are carried out. The condensation section separates a condensation overheads stream which is sent to the vent recovery section, and a condenser bottoms stream which is sent to the lights column. The lights column separates the condensation bottoms stream into a lights column bottoms stream which is rich in VCM, and a lights column overheads stream. Depending on the amount of acetylene and VCM remaining in the vent recovery overheads stream and the lights column overheads stream, one or both of these streams may be sent to a purification section for further separation of acetylene and VCM. An advantage of the present invention is that, by including a vent recovery section, the duty on the purification section is reduced, or the requirement for a purification section may be removed entirely. This makes it possible to operate with low single pass conversion in the reactor and / or with significant amounts of H2 in the reactor product stream, without requiring a large purification section. In some arrangements, where there is a high degree of separation of VCM from incondensables in the vent recovery section, it is possible to avoid the need for a purification section entirely. The invention provides a process for the production of vinyl chloride monomer (VCM), comprising the steps of: (i) generating a reactor feed stream (101) by combining an acetylene feed stream (103) and an HCI feed stream (105); (ii) feeding the reactor feed stream to a reaction section (107) comprising one or more reactors, and carrying out hydrochlorination in said reaction section in the presence of a hydrochlorination catalyst to produce a reactor product stream (109) comprising vinyl chloride monomer along with unreacted acetylene and HCI; (iii) cooling the reactor product stream in one or more cooling stages (111) to generate a cooled reactor product stream (113); (iv) feeding the cooled reactor product stream to a compressor (123) to generate a compressed vapour stream (125); (v) condensing the compressed vapour stream in one or more condensation stages (127) to generate a condensation overheads stream (133) and a condensation bottoms stream (135); (vi) feeding the condensation overheads stream to a vent recovery section (117) comprising one or more condensation stages, and separating the condensation bottoms stream into a vent recovery bottoms stream (115) and a vent recovery overheads stream (137); (vii) feeding the condensation bottoms stream to a lights column (139) and separating the condensation bottoms stream into a lights column overhead stream (141) and a lights column bottoms stream (143). Many operational acetylene-based VCM plants, particularly in China, already have many of the unit operations of the process, namely the reaction section, compressor, condensation section, lights column and purification section. An advantage of this process is that existing plants may be retrofitted to operate the present process by installing a vent recovery section and associated piping, KO drums etc., at relatively low additional capex. The invention is also applicable to a grassroots VCM plant. Description of the figures Figure 1a illustrates the process. A reactor feed stream (101) is generated by combining an acetylene feed stream (103) and an HCI feed stream (105). The reactor feed stream is fed to a reaction section (107) comprising one or more reactors, to produce a reactor product stream (109) comprising vinyl chloride monomer along with unreacted acetylene and HCI. The reactor product stream is cooled in one or more cooling stages (111) to generate a cooled reactor product stream (113), which is fed to a compressor (123) to generate a compressed vapour stream (125). The compressed vapour stream is cooled in one or more condensation stages (127) to generate a condensation overheads stream (133) and a condensation bottoms stream (135). The condensation overheads stream is fed to a vent recovery section (117), comprising one or more condensation stages, and separated into a vent recovery bottoms stream (115) and a vent recovery overheads stream (137). The condensation bottoms stream is fed to a lights column (139) where it is separated into a lights column overhead stream (141) and a lights column bottoms stream (143). Figure 1b illustrates a preferred arrangement of the process shown in Figure 1a. Instead of feeding the cooled reactor product stream (113) directly from the cooling stages (111) to the compressor (123), the cooled reactor product stream is fed to a compressor feed knockout drum (119). A vapour stream (121) generated by the compressor feed knockout drum is sent to compressor (123) to generate a compressed vapour stream (125). This arrangement avoids damage to the compressor in a situation where the cooling stages cause condensation of liquid VCM. Figure 1c illustrates a preferred arrangement of the process shown in Figure 1b. Both the cooled reactor product stream (113) and the vent recovery bottoms stream (115) are fed to the compressor feed knockout drum (119). Figure 1d illustrates a preferred arrangement of the process shown in Figure 1c. The process is as shown in Figure 1c, except that the vent recovery overheads stream (137) and the lights column overhead stream (141) are fed to a purification section (145), which separates an acetylene-rich stream (147) and an off-gas stream (149). The acetylene-rich stream is recycled upstream of the reaction section (107). Figure 1e illustrates a preferred arrangement of the process shown in Figure 1c. The process is as shown in Figure 1d, except that this arrangement the lights column overheads stream (141) bypasses the purification section and is recycled upstream of the reaction section. Figure 2a illustrates the flowsheet described in Example A. Figure 2b illustrates the flowsheet described in Example B. Figure 2c illustrates the flowsheet described in Example C. Figure 2d illustrates the flowsheet described in Example D. Figure 3a illustrates the flowsheet described in Example E. Figure 3b illustrates the flowsheet described in Example F. Figure 4a illustrates the flowsheet described in Example G. Figure 4b illustrates the flowsheet described in Example H. 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. Reactor feed stream The reactor feed stream is generated by combining an acetylene feed stream and an HCI feed stream. An HCI stream generated by the reaction between H2 and Ch typically comprises about 5 vol% H2 because H2 is used in excess on safety grounds as was explained previously. Whilst HCI feeds having already been purified to remove H2 may be used in the present invention, it is an advantage that the present invention that high levels of H2 in the HCI feed can be tolerated. Therefore, in some embodiments the HCI feed stream comprises 1-10 vol% H2, preferably 3-7 vol% H2, preferably as 4-6 vol% H2. It is preferred that there is no step of removing H2 from the HCI feed stream upstream of the reactor. The reactor feed stream typically and preferably comprises acetylene and HCI at a molar ratio of approximately 50:50. There is preferably a slight excess of HCI to ensure that the catalyst remains in the active state. The acetylene : HCI molar ratio in the reactor feed stream is preferably from 50 : 50 to 48 : 52. The reactor feed stream typically and preferably has a pressure of 0 to 1 barg. One or more of: the acetylene-rich stream generated by the purification section; the vent recovery overheads stream; and / or the lights column overheads stream, may recycled and used to produce the reactor feed stream. The reactor feed stream is preferably pre-heated upstream of the reaction section. Reaction section The reaction section includes at least one reactor. The reaction section may include a single reactor, or may comprise two or more reactors in series, parallel, or series-parallel. Each reactor within the reaction section is cooled using a coolant, which is typically either heptane (used extensively in existing VCM plants) or water. In a preferred embodiment the reactor, or each reactor, is a shell-and-tube reactor. The reactor, or each reactor, includes a hydrochlorination catalyst which is active for the conversion of acetylene to VCM. Any effective hydrochlorination catalyst may be used, but it is preferred that the hydrochlorination catalyst is mercury-free because of the known toxicity issues with mercury catalysts. For reasons explained previously, the process of the invention makes it more economically feasible to operate with lower conversion in the reaction section. Therefore, in some embodiments the degree of conversion of acetylene in the reaction section is <95%. Gold catalysts have been well studied as hydrochlorination catalysts and examples are described in WO2013 / 008004A2, WO2020 / 254817A1 and WO2023 / 111537A1. In a preferred embodiment the hydrochlorination catalysts comprises gold or a complex of gold, in each case preferably on a carbon support. Cooling stages The reaction section generates a reactor product stream which comprises VCM along with unreacted acetylene and / or HCI. The hydrochlorination reaction is exothermic and the reactor product stream requires cooling upstream of the compressor, in order to reduce the volume of gas fed to the compressor. It is preferred that the cooling stages includes a first step of cooling using cooling water. This stage cools the vapour and may also condense VCM. The cooling stages may include, instead of or in addition to a first stage of cooling using cooling water, a step of scrubbing the reactor product stream to remove residual HCI. In addition to scrubbing HCI from the reactor product stream, the scrubbing provides a cooling effect. Scrubbing is preferably carried out using aqueous solution, preferably aqueous alkaline solution. An advantage of this arrangement is that it simplifies the design of equipment downstream of the reactor because the equipment does not have to be designed to be compatible with HCI. In addition, separation of VCM from incondensables in the column is more efficient if HCI has been scrubbed out of the reactor product stream. Alternatively, there is no step of scrubbing the reactor product stream to remove residual HCI. In this arrangement HCI is removed together with acetylene in the overhead fraction from the condensation stages. In this arrangement the equipment used in the stages downstream of cooling need to be designed to be compatible with HCI. The cooled reactor product stream from the cooling stages may be fed directly to the compressor. However, depending on the extent of cooling during step (iii), some VCM may condense out of the reactor product stream. The presence of significant amounts of liquid in the compressor feed may cause damage to the compressor. If there is condensation of liquid during step (iii), which is preferred, then it is preferred that the cooled reactor product stream is fed to a compressor feed KO drum located upstream of the compressor. Compressor feed KO drum When present, the compressor feed KO drum receives a cooled reactor product stream from the cooling stages. The compressor feed KO drum preferably also receives the vent recovery bottoms stream from the vent recovery section. If the vent recovery section includes two or more condensation stages then a bottoms stream from at least one condensation stage is preferably sent to the compressor feed KO drum. The compressor feed KO drum generates a vapour stream which is sent to the compressor. Compression and condensation stage(s) The role of the compressor is to increase the pressure of the vapor before the one or more condensation stages, vent recovery section and lights column, 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 at the outlet of the compressor will depend on the desired pressure in the condenser. The vapour is preferably compressed to a pressure of 2 to 15 bara, preferably 3 to 12 bara, preferably 3 to 8 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 vapour stream into a VCM-rich condensation bottoms stream and a VCM-lean condensation overheads stream. Whilst described as “VCM-lean” the overheads will typically include a significant amount of VCM which requires separation by the vent recovery section. The condensation stage(s) include at least one stage in which the compressed vapour stream is condensed using a coolant. It is preferred that the condensation stages include two or more condensation stages using a coolant of decreasing temperature in each stage. The condensation stages are preferably preceded by a cooling 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. A preferred arrangement involves a first stage of cooling using cooling water followed by a second stage of condensation using a coolant having a temperature of 5 to 15 °C. A more preferred arrangement involves a first stage of cooling using cooling water followed by a second stage of condensation using a coolant having a temperature of 5 to 15 °C, followed by a third stage of condensation using a coolant having a temperature of-20 to -10 °C. The vapour phase following the condensation stage (in the case of a single stage), or from the last condensation stage (in the case of multiple condensation stages), is referred to herein as the “condensation overheads stream”. The condensate from the condensation stage(s) is referred to as the condensation bottoms stream. Vent recovery section The role of the vent recovery section is to carry out further recovery of VCM from the condensation overheads stream. The vent recovery sections separates out one or more vent recovery bottoms stream(s) which are VCM-rich, and a vent recovery overheads stream which is VCM-lean. In a preferred arrangement the vent recovery section comprises sequentially a first compressor, one or more condensation stages, and a first knockout drum, wherein the first knockout drum separates the vent recovery bottoms stream and the vent recovery overheads stream. This arrangement has a single compression / condensation / separation stage. In another preferred arrangement the vent recovery section comprises sequentially: a first compressor, one or more condensation stages, and a first knockout drum, wherein the first knockout drum separates the vent recovery bottoms stream; and a second compressor, one or more condensation stages, and a second knockout drum, wherein the second knockout drum separates the vent recovery overheads stream and a further bottoms stream. This arrangement has two compression / condensation / separation stages. In another preferred arrangement the vent recovery section comprises sequentially: a first compressor, one or more condensation stages, and a first knockout drum, wherein the first knockout drum separates the vent recovery bottoms stream; a second compressor, one or more condensation stages, and a second knockout drum, wherein the second knockout drum separates a further bottoms stream; and a third compressor, one or more condensation stages, and a third knockout drum, wherein the third knockout drum separates the vent recovery overheads stream and a further bottoms stream. This arrangement has three compression / condensation / separation stages. In each of the embodiments with one, two, three or more compression / condensation / separation stages, it is preferred that the condensation stages include two or more stages using a coolant of decreasing temperature in each stage. A preferred arrangement involves a first stage using cooling water followed by a second stage using a coolant having a temperature of-20 to -10 °C. In each of these embodiments the first compressor, second compressor and third compressor may be a single compressor or multiple compressors. The major component of the further bottoms stream(s) is acetylene and therefore these are preferably recycled upstream of the process. In embodiments with two, three or more, compression / condensation / separation stages, particularly with three or more compression / condensation / separation stages, the residual content of acetylene and VCM in the vent recovery overheads stream is low. The content of these streams is primarily inerts and therefore it is preferred that this stream is vented rather than being sent to the purification section. The vapour phase following the compression / condensation / separation stage (in the case of a single stage), or from the last compression / condensation / separation stage (in the case of multiple stages), is referred to herein as the “vent recovery overheads stream”. Purification section The purification section is optional. When present, the role of the purification section is to accept the vent recovery overheads stream and / or the lights column overheads stream, and separate these stream(s) into an acetylene-rich stream and an off-gas stream. The off-gas includes incondensables (such as N2, O2, Ar etc...) which enter the process with the acetylene and / or HCI feed. The off-gas is desirably separated out to prevent inerts from excessively accumulating in the process. The purification section may optionally also separate a VCM-rich stream. It is preferred that the purification section comprises or consists of a pressure-swing adsorption unit. In some embodiments the acetylene-rich stream is fed to a secondary reactor. VCM production processes using a primary reactor and a secondary reactor are known and are described for example in CN1884241A and WO2023 / 237854A1. However, this arrangement is less preferred because of the additional capex and opex associated with the secondary reactor. It is preferred that the acetylene-rich stream is recycled upstream of the reactor, i.e. it is used to produce the reactor feed stream. Lights column The role of the lights column is to separate the condensation bottoms stream into a lights column bottoms stream which is VCM-rich, and a lights column overheads stream which is VCM-lean. Separation in the lights column may be achieved by conventional distillation, e.g. by heating the contents of the lights column using a reboiler. When a purification section is present, the lights column overheads stream, or a portion thereof, may be fed to the purification section. However, this adds to the duty of the purification section and may require the purification section to be larger. It is therefore preferred that the lights column overheads stream, or a portion thereof, is recycled upstream of the reactor, i.e. bypassing the purification section. The lights column bottoms fraction is preferably fed to a heavies column for removal of heavy fractions. Examples The flowsheets shown in Examples A-H were modelled using AVEVA™ Process Simulation software version 2024.1. The flowsheets include the following utilities: steam (150 °C), cooling water (40 °C), refrigerant 1 (10 °C), and refrigerant 2 (-15 °C). Unless indicated otherwise, conversion of acetylene in the reaction section was set to 97%, which is typical of mercury-based VCM plants. HCI scrubbing Examples A, D, F and H include a scrubber to remove HCI from the reactor product stream. As well as removing HCI from the reactor product stream the scrubber also cools the stream and is therefore taken to be part of the cooling stages (cf. step (iii)). An HCI scrubbing section, comprising a single scrubbing step followed by stripper with a steam reboiler, was simulated. The crude HCI stream was scrubbed with dilute hydrochloric acid solution in a scrubber, a bottoms stream from the scrubber was pre-heated by heat exchange with a bottoms stream from the stripper, and then sent to the stripper where HCI is removed in the overheads. The bottoms from the stripper were cooled by heat exchange with the hydrochloric acid solution, and then cooled in a series of cooling stages using refrigerant 1 (10 °C) to regenerate the dilute hydrochloric acid solution. A simple model estimated the utility costs of stripping to be: Steam duty: 237,971 kJ / kmol HCI Refrigerant 1 duty (10 C): 113,533 kJ / kmol HCI Total: 351,504 kJ / kmol HCI Other opex associated with HCI regeneration were minor and were ignored. The above figures were used in the following simulations. PSA unit The PSA is simulated as a block unit imposing the desired results of 90% acetylene recovery and 96% VCM recovery. In each case the PSA also separated a VCM-rich stream (not shown). The size of the PSA is assumed to scale proportional to the acetylene flow feeding into it and inversely proportional to the pressure drop available. Example A The flowsheet is shown in Figure 2a. The arrangement resembles the flowsheet shown in Figure 1d with the following additions / differences. The reactor product stream is heated upstream of the reactor in heat exchanger (206) to a temperature 120 °C using steam of 150 °C. The reactor product stream (209) is cooled in cooling stages (211), which comprise a first stage of cooling in heat exchanger (211a) using cooling water, followed by scrubbing in scrubber (211b). Condensation of VCM from the compressed vapour stream (225) is achieved in condensation stages (227) which comprise heat exchangers (227a, 227b, 227c). Cooling water is used in heat exchanger (227a), refrigerant 1 is used in heat exchanger (227b) and refrigerant 2 is used in heat exchanger (227c). The condensation stages are followed a gas / liquid separator (231). The vent recovery section (217) comprises a first step of compression in compressor (217a) where the condensation overheads stream (233) is compressed, followed by cooling / condensation in heat exchangers (217b, 217c); cooling water is used in heat exchanger (217b) and refrigerant 2 is used in heat exchanger (217c). The cooled stream from heat exchanger (217c) is sent to a KO drum (217d) within the vent recovery section, where it is separated into a vent recovery bottoms stream (215) and a vent recovery overheads stream (237). The condensation bottoms stream is fed to a lights column (239) which is heated by a reboiler using steam. Example B The flowsheet is shown in Figure 2b. The flowsheet is as for Example A except that after the reactor product stream (209) is cooled in heat exchanger (211a), the cooled stream is sent directly to KO drum (219) without HCI scrubbing. 5 Example C The flowsheet is shown in Figure 2c. The flowsheet is as for Example B except that the lights column overheads (241) are not send to the PSA. Instead, the lights column overheads bypass the PSA and are combined with the acetylene-rich stream (247) and the resulting stream is recycled upstream of 10 the reactor. Example D The flowsheet is shown in Figure 2d. The flowsheet is as for Example C except that HCI scrubbing 15 was introduced between heat exchanger (211 a) and KO drum (219). Example E The flowsheet is shown in Figure 3a. The flowsheet is as for Example C except that the vent recovery 2 0 section (317) is arranged with two stages of compression / condensation / separation. The vent recovery section includes compressors (317a, 317e) which compress the stream to 15 bara and 36 respectively. Following each compressor there is a first stage of cooling using cooling water in heat exchangers (417b, 417f) and a second stage of cooling / condensation using coolant with temperature of-15 °C in heat exchangers (417c, 417g). Bottoms from the second heat exchanger (417g) are 2 5 recycled upstream of the reactor. Example F The flowsheet is shown in Figure 3b. The flowsheet is as for Example E except that HCI scrubbing 3 0 was introduced between heat exchanger (311a) and KO drum (319). Example G The flowsheet is shown in Figure 4a. The flowsheet is as for Example E except that the vent recovery 35 section (417) is arranged with three stages of compression / condensation / separation. The vent recovery section includes compressors (417a, 417e, 417i) which compress the stream to 15 bara, 36 bara and 108 bara respectively. Following each compressor there is a first stage of cooling using cooling water in heat exchangers (417b, 417f, 417j) and a second stage of condensation using coolant with temperature of-15 °C in heat exchangers (417c, 417g, 417k). Bottoms from the second and third heat exchangers (417g, 417k) are recycled upstream of the reactor. In this arrangement the recovery of VCM is much more complete and therefore it is unnecessary to sent the vent recovery overheads (437) for further purification; the purification section is omitted in this arrangement. The lights column overheads (241) are recycled upstream of the reactor rather than being sent to a purification section. Example H The flowsheet is shown in Figure 4b. The flowsheet is as for Example G except that that HCI scrubbing was introduced between heat exchanger (411a) and KO drum (419). The features of Examples A-H are summarised in Table 1. Example HCI scrubbing Compression / condensation / separation stages in vent recovery section Vent recovery overheads to PSA Lights column overheads to PSA A z 1 z z B X 1 z z C X 1 z X D z 1 z X E X 2 z X F z 2 z X G X 3 x (no PSA) x (no PSA) H 3 x (no PSA) x (no PSA) Table 1. Impact of compressor (223) pressure at low vent recovery and lights column pressure The flowsheet of Example A was modelled. The pressure increase across compressor (217a) was set to be 0 which is equivalent to simulating the flowsheet without compressor (217a) being present. Example Compressor (223) pressure Compressor (217a) pressure Lights column (239) pressure A-1 (Reference) 4 bara 4 bara 6 bara A-2 5 bara 5 bara 6 bara A-3 6 bara 6 bara 6 bara The results relative to the reference case (A-1) are shown in Figure 5. These results show that the size of PSA (245) scales inversely with the duty on compressor (223), as expected. Impact of pressure within vent recovery section The flowsheet of Example A was modelled. The pressure of compressor (217a) within the vent recovery section was varied. The lights column pressure was set to be the same as the compressor (217a) cases in all examples apart from A-1. Example Compressor (223) pressure Compressor (217a) pressure Lights column (239) pressure A-1 (Reference) 4 bara 4 bara 6 bara A-4 4 bara 6 bara 6 bara A-5 4 bara 8 bara 8 bara A-6 4 bara 10 bara 10 bara A-7 4 bara 12 bara 12 bara The results relative to the reference case (A-1) are shown in Figure 6. These results show that the required size of the PSA (245) scales inversely with the duty on compressor (217a). Steam usage by the lights column (231) increases slightly with increasing duty on compressor (217a), which may be due to a narrowing of the vapour liquid equilibrium in the lights column as pressure increases. The yield of VCM also increases with increasing pressure in the vent recovery section; this is due to the fact that the VCM separation by the vent recovery section is more complete as pressure increases, so less VCM is sent to the PSA and therefore losses from the PSA are reduced. Impact of compressor (223) pressure at high vent recovery and lights column pressure The flowsheet of Example A was modelled. The pressure of compressor (217a) was varied, this time using a higher pressure (12 bara) in both the compressor (217a) and lights column (239). Example Compressor (223) pressure Compressor (217a) pressure Lights column (239) pressure A-1 (Reference) 4 bara 4 bara 6 bara A-8 4 bara 12 bara 12 bara A-9 6 bara 12 bara 12 bara A-10 8 bara 12 bara 12 bara A-11 10 bara 12 bara 12 bara A-12 12 bara 12 bara 12 bara The results relative to the reference case are shown in Figure 7. Whilst the duty on refrigerant 1 declines with increasing pressure of stream (225), the duty on both the compressor (223) and the required duty on refrigerant 2 increases. Because refrigerant 2 is at a lower temperature than refrigerant 1, the energy demands increase on increasing the pressure of stream (225). Impact of lights column overheads bypassing purification section The flowsheet of Example D was modelled. The pressure of compressor (217a) was varied, this time using a higher pressure (12 bara) in both the compressor (217a) and lights column (239). Example Compressor (223) / (323) pressure Compressor (217a) / (317a) pressure Lights column (239) / (339) pressure A-4 (Reference) 4 bara 6 bara 6 bara D-1 4 bara 6 bara 6 bara D-2 4 bara 8 bara 6 bara D-3 4 bara 10 bara 6 bara D-4 4 bara 12 bara 6 bara The results relative to the reference case are shown in Figure 8. The reduction in PSA size is very large, which is because the lights column overheads include a large amount of acetylene. Recycling the lights column overheads, bypassing the purification section, therefore allows a large reduction in PSA size compared to an arrangement where the lights column overheads are fed to the purification section. Impact of HCI scrubbing The flowsheet of Example C (no HCI scrubbing) was modelled. Example Compressor (323) pressure Compressor (317a) pressure Lights column (339) pressure C-1 4 bara 6 bara 6 bara C-2 4 bara 8 bara 6 bara C-3 4 bara 10 bara 6 bara C-4 4 bara 12 bara 6 bara Figure 9 compares each example relative to the equivalent example with HCI scrubbing, i.e. C-1 vs D-1, C-2 vs D-2, C-3 vs D-3, C-4 vs D-4. The changes are practically the same throughout. Without HCI scrubbing the refrigerant 1 and steam duties are reduced, but the refrigerant 2 and compressor duties are increased. Without HCI scrubbing the separation of VCM from overheads in the condenser is less complete, which puts a larger duty onto the vent recovery section. In effect by removing the HCI scrubbing, the duty originally spent on HCI scrubbing are moved to additional duty on the vent recovery section. Impact of conversion in reactor (with HCI scrubbing) Allowing the lights column overheads to bypass the PSA (Examples C, D) proved to be very effective for reducing the size of the PSA. The flowsheet of Example D was modelled with varying degrees of conversion in the reactor. Example Conversion Compressor (323) pressure Compressor (317a) pressure Lights column (339) pressure D-5 97% 4 bara 12 bara 12 bara D-6 95% 4 bara 12 bara 12 bara D-7 93% 4 bara 12 bara 12 bara D-8 91% 4 bara 12 bara 12 bara D-9 89% 4 bara 12 bara 12 bara D-10 87% 4 bara 12 bara 12 bara D-11 85% 4 bara 12 bara 12 bara D-12 83% 4 bara 12 bara 12 bara D-13 81% 4 bara 12 bara 12 bara Figure 10 compares each example relative to the reference case D-5. The most notable change is the increase in PSA size required to achieve the desired separation. As the conversion reduces, more acetylene is passed to the PSA. The duty on the HCI scrubber increases and thus the steam duty of the HCI stripper increases. Impact of conversion in reactor (without HCI scrubbing) The flowsheet of Example C was modelled with varying degrees of conversion in the reactor. Example Conversion Compressor (323) pressure Compressor (317a) pressure Lights column (339) pressure C-5 97% 4 bara 12 bara 12 bara C-6 95% 4 bara 12 bara 12 bara C-7 93% 4 bara 12 bara 12 bara C-8 91% 4 bara 12 bara 12 bara C-9 89% 4 bara 12 bara 12 bara C-10 87% 4 bara 12 bara 12 bara C-11 85% 4 bara 12 bara 12 bara C-12 83% 4 bara 12 bara 12 bara C-13 81% 4 bara 12 bara 12 bara Figure 11 compares each example relative to the reference case C-5. With decreasing conversion, the necessary PSA size increases, even more so than compared to when HCI scrubbing is included. This is because HCI is present in the column overheads and makes the flow to the PSA larger. The refrigerant 2 duty increases because of the larger demands on the vent recovery section. Figure 12 shows the impact of removing HCI scrubbing at different conversions. D-5 is taken as the base case for C-5, D-6 is taken as the base case for C-6 etc...The results show that the steam and refrigerant 1 duty decrease because of the absence of HCI scrubbing, but the refrigerant 2, compressor (323) and PSA duties increase because of the larger volume of column overheads. Impact of additional compression / condensation / separation stages The flowsheets of D, F and H were modelled. In flowsheet D the pressure of compressor (217a) was 12 bara. In flowsheet F the pressure of compressor (317a) was 12 bara and the pressure of compressor (317e) was 36 bar. In flowsheet D the pressure of compressor (217a) was 12 bara. In flowsheet F the pressure of compressor (317a) was 12 bara, the pressure of compressor (317e) was 36 bar, and the pressure of compressor (317i) was 108 bara. Figure 13 compares each example with the reference case of flowsheet D. The addition of a second stage reduces the required PSA size by 80%. The addition of a third stage and removal of the PSA completely did lead to a slight loss in VCM yield because residual VCM in the vent recovery overheads is not recovered, but the reduced capex and opex achieved by not including a PSA 5 compensates for that.

Claims

1. A process for the production of vinyl chloride monomer (VCM), comprising the steps of:(i) generating a reactor feed stream (101) by combining an acetylene feed stream (103) and an HCI feed stream (105);(ii) feeding the reactor feed stream to a reaction section (107) comprising one or more reactors, and carrying out hydrochlorination in said reaction section in the presence of a hydrochlorination catalyst to produce a reactor product stream (109) comprising vinyl chloride monomer along with unreacted acetylene and HCI;(iii) cooling the reactor product stream in one or more cooling stages (111) to generate a cooled reactor product stream (113);(iv) feeding the cooled reactor product stream to a compressor (123) to generate a compressed vapour stream (125);(v) condensing the compressed vapour stream in one or more condensation stages (127) to generate a condensation overheads stream (133) and a condensation bottoms stream (135);(vi) feeding the condensation overheads stream to a vent recovery section (117) comprising one or more condensation stages, and separating the condensation bottoms stream into a vent recovery bottoms stream (115) and a vent recovery overheads stream (137);(vii) feeding the condensation bottoms stream to a lights column (139) and separating the condensation bottoms stream into a lights column overhead stream (141) and a lights column bottoms stream (143).

2. A process according to claim 1, comprising a step (viii) of feeding the vent recovery overheads stream and / or the lights column overheads stream to a purification section (145), separating an acetylene-rich stream (147) and an off-gas stream (149) from the purification section, and recycling the acetylene-rich stream upstream of the reactor.

3. A process according to claim 1 or claim 2, wherein the cooled reactor product stream is fed to a compressor feed knockout drum (119), and a vapour stream (121) generated by the compressor feed knockout drum is fed to the compressor.

4. A process according to claim 3, wherein the cooled reactor product stream and the vent recovery bottoms stream are fed to the compressor feed knockout drum.

5. A process according to any of claims 1 to 4, wherein there is no step of removing H2 from the HCI feed stream upstream of the reactor.

6. A process according to any of claims 1 to 5, wherein the HCI feed stream comprises 1-10vol% H2.

7. A process according to any of claims 1 to 6, wherein the hydrochlorination catalyst ismercury-free.

8. A process according to any of claims 1 to 7, wherein the degree of conversion of acetylenein the reaction section is <95%.

9. A process according to any of claims 1 to 8, wherein step (iii) includes a step of coolingusing cooling water.

10. A process according to any of claims 1 to 9, wherein step (iii) includes a step of scrubbingthe reactor product stream to remove residual HCI.

11. A process according to any of claims 1 to 9, wherein step (iii) does not include a step ofscrubbing the reactor product stream to remove residual HCI.12.A process according to any of claims 1 to 11,whereinthecompressedvapourstreamgenerated in step (iv) has a pressure of 2 to 12 bara.13.A process according to any of claims 1 to 11,whereinthecompressedvapourstreamgenerated in step (iv) has a pressure of 3 to 12 bara.14.A process according to any of claims 1 to 11,whereinthecompressedvapourstreamgenerated in step (iv) has a pressure of 3 to 8 bara.

15. A process according to any of claims 1 to 14, wherein step (v) includes two or more condensation stages using a coolant of decreasing temperature in each stage.

16. A process according to any of claims 1 to 15, wherein the vent recovery section comprises sequentially: a first compressor (217a), one or more condensation stages (217b, 217c), and a first knockout drum (217d), wherein the first knockout drum separates the vent recovery bottoms stream and the vent recovery overheads stream.

17. A process according to any of claims 1 to 15, wherein the vent recovery section comprisessequentially:a first compressor (317a), one or more condensation stages (317b, 317c), and a first knockout drum (317d), wherein the first knockout drum separates the vent recovery bottoms stream; anda second compressor (317e), one or more condensation stages (317f, 317g), and a second knockout drum (317h), wherein the second knockout drum separates the vent recovery overheads stream (337) and a further bottoms stream.

18. A process according to any of claims 1 to 15, wherein the vent recovery section comprises sequentially:a first compressor (417a), one or more condensation stages (417b, 417c), and a first knockout drum (417d), wherein the first knockout drum separates the vent recovery bottoms stream;a second compressor (417e), one or more condensation stages (417f, 417g), and a second knockout drum (417h), wherein the second knockout drum separates a further bottoms stream; anda third compressor (417i), one or more condensation stages (417j, 417k), and a third knockout drum (4171), wherein the third knockout drum separates the vent recovery overheads stream (437) and a further bottoms stream.

19. A process according to claim 17 or claim 18, wherein the vent recovery overheads stream is vented.

20. A process according to any of claims 17 to 19, wherein the further bottoms stream(s) are recycled upstream of the reaction section.

21. A process according to any of claims 2 to 20, wherein the purification section comprises or consists of a pressure-swing adsorption unit.

22. A process according to any of claims 1 to 21, wherein the lights column overhead stream, or a portion thereof, is recycled upstream of the reaction section.

23. A process according to any of claims 2 to 22, wherein the lights column overhead stream, or a portion thereof, is fed to the purification section.s

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