GB2641542A - Heat integration in an acetylene-based process for producing vinyl chloride monomer - Google Patents

Abstract

A process for the production of vinyl chloride monomer (VCM) from acetylene and HCl in the presence of a hydrochlorination catalyst. The process involves a specific arrangement between the lights column reboiler and the refrigeration service wherein a portion of the hot refrigerant vapour generated by the refrigeration service is used for the heating duty in the lights column. This arrangement is disclosed to reduce the amount of cooling water required by the refrigeration service and avoids the need to generate steam or hot water for the lights column reboiler. The hydrochlorination catalyst may be mercury-free. The reactor product stream may be subjected to a scrubbing step to remove residual HCl prior to the cooling stages. Th reactor product stream may be cooled indirectly or directly via heat exchange with the liquid refrigerant stream, wherein if indirect heat exchange is used an intermediate refrigerant stream is used, which may be brine. The heating duty for the lights column reboiler may be provided by the hot refrigerant vapour.

Description

[0001] Heat integration in an acetylene-based process for producing vinyl chloride monomer

[0002] Field of the Invention

[0003] The present invention relates to a process for the conversion of acetylene to vinyl chloride monomer. 5 Background The hydrochlorination of acetylene to produce vinyl chloride monomer (VCM) as the precursor to polyvinylchloride (PVC) is currently 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 acetylene hydrochlorination with the vast majority utilising mercuric chloride (HgC12) catalysts supported on activated carbon.

[0004] CN1884241A describes a process in which acetylene and hydrogen chloride are reacted together in a first stage reactor at a temperature of 100-180 °C in the presence of a HgC12/C catalyst to produce a crude vinyl chloride mixture. The crude mixture is cooled to around 40 °C, compressed, cooled, and then enters a two-stage condenser for cooling and condensing with a refrigerant. Non- 1 5 condensable gases are then sent to a second stage reactor.

[0005] W02023/237854A1 also describes a process in which acetylene and hydrogen chloride are reacted together in a primary reactor and a secondary reactor. The product stream from the primary reactor is cooled and fed to a knockout (KO) drum. A vapor is generated by evaporating VCM in the KO drum; the vapour is compressed, fed to a lights separation unit where it is separated into a liquid 2 0 fraction containing VCM and an overhead fraction containing unreacted acetylene, HCI and residual VCM. The liquid fraction is sent to a lights column where it is further separated into a bottom fraction and an overhead fraction which is returned to the lights separation unit, and a bottom fraction containing VCM. The overhead fraction from the lights separation unit is sent to a secondary reactor.

[0006] CN117018826A describes a process for producing VCM in a two-stage reaction using a mercury- 2 5 free catalyst. In the arrangement shown in Figure 1 of this reference, the crude VCM product is passed sequentially through a water washing tower followed by an alkali washing tower, then compressed and sent to a condenser. A bottoms fraction is withdrawn from the bottom of the condenser, split and a portion sent to a thermosyphon reboiler which is heated by direct injection of flash steam.

[0007] 3 0 The hydrochlorination of acetylene is exothermic and the product stream needs to be cooled before separating unreacted acetylene and HCI from the VCM product. Separation of unreacted acetylene and HCI from the VCM product can be achieved in a lights column by withdrawing a bottoms fraction, splitting a portion of the bottoms fraction and sending this portion to a lights column reboiler where the fraction is heated, and then reintroducing this fraction into the lights column, for example as is shown in the arrangement in CN117018826A. The heating duty in the lights column reboiler is normally provided by steam or hot water, which contributes to the operational costs of the plant.

[0008] The refrigeration service within a VCM production process generates hot refrigerant which requires cooling. In practice this is normally achieved by cooling using cooling water, which also contributes to the operational costs of the plant.

[0009] There is a need for alternative arrangements having lower operational costs. Summary of the Invention A refrigeration service used in a VCM plant generates a compressed refrigerant stream as part of the cooling cycle. The inventors have realised that the amount of waste heat in the compressed refrigerant stream is in excess of what is required by the lights column reboiler in a VCM plant. A portion of the compressed refrigerant can therefore conveniently be used, either as a partial or total replacement, for the heating duty in the lights column reboiler instead of steam or condensate, thus reducing the steam import. The remaining portion of the hot refrigerant is condensed and cooled using cooling water, as in a conventional arrangement. A further advantage of this arrangement is that, because some of the compressed refrigerant is sent to the lights column reboiler, the total amount of cooling water required is reduced compared to conventional arrangements. This arrangement therefore reduces overall cooling water usage, and reduces or eliminates the steam 2 0 required by the lights column reboiler.

[0010] The invention relates to process for the production of vinyl chloride monomer (VCM), comprising the steps of: (i) feeding a reactor feed stream (201) comprising acetylene and HCI to a reactor (202) and carrying out hydrochlorination in said reactor in the presence of a hydrochlorination catalyst to produce a reactor product stream (203) comprising vinyl chloride monomer along with unreacted acetylene and HCI; (ii) cooling the reactor product stream in one or more cooling stages (204) by direct or indirect heat exchange with a liquid refrigerant stream (205a) provided by a refrigeration service (206) to generate a cooled reactor product stream (207) and a vapour refrigerant stream (205b) which is retumed to the refrigeration service; (iii) feeding the vapour refrigerant stream to a compressor (208) to generate a compressed vapour refrigerant stream (209) and splitting the compressed vapour refrigerant stream into a first compressed vapour refrigerant stream (209a) and a second compressed vapour refrigerant stream (209b); (iv) feeding the cooled reactor product stream to a lights column (210) where it is separated into a bottoms fraction (211) and an overhead fraction (212); (iv) splitting a portion of the bottoms fraction (211a) and feeding said portion to a lights column reboiler (213), where the portion is heated by heat exchange with the first compressed vapour refrigerant stream (209a) to generate a heated bottoms fraction (211 b) which is returned to the lights column, and a first liquid refrigerant stream (214) which is returned to the refrigeration service; (v) condensing the second compressed refrigerant stream (209b) by heat exchange with a cooling water stream (215a) in heat exchanger (216) to generate a second liquid refrigerant stream (217) and a used cooling water stream (215b); (vi) combining the first liquid refrigerant steam (214) and second liquid refrigerant stream (217) to generate liquid refrigerant stream (205a).

[0011] Description of the figures

[0012] Figure 1 shows a process according to the prior art. A reactorfeed stream (101) comprising acetylene and HCI is fed to a reactor (102) and hydrochlorination is carried out in the presence of a hydrochlorination catalyst to produce a reactor product stream (103) comprising vinyl chloride monomer along with unreacted acetylene and HCI. The reactor product stream in cooled in one or more cooling stages (104) by heat exchange with a liquid refrigerant stream (105a) provided by a refrigeration service (106), to generate a cooled reactor product stream (107) and a vapour refrigerant stream (105b) which is returned to the refrigeration service. The vapour refrigerant stream is compressed by compressor (108) to generate a compressed vapour refrigerant stream (109) which is then cooled and condensed in heat exchanger (116) using cooling water (115a) to produce liquid refrigerant stream (105a) and used cooling water stream (115b). The cooled reactor product stream is fed to a lights column (110) where it is separated into a bottoms fraction (111) and an overhead fraction (112). The bottoms fraction is split and a portion of the bottoms fraction (111a) is fed to a lights column reboiler (113), where the portion is heated by heat exchange with steam or hot water (118a) to generate a heated bottoms fraction (111 b) and used steam or used hot water (118b).

[0013] Figure 2 shows a process according to claim 1. Instead of sending all of the compressed vapour stream (209) to heat exchanger (216), this stream is split into a first portion (209a) which is sent to the lights column reboiler (213) in place of the steam or hot water, and a second portion (209b) which is sent to the heat exchanger (216) within the refrigeration service. The lights column reboiler (213) generates a first liquid refrigerant stream (214) which is returned to the refrigeration service. The second portion (209b) is cooled in condenser (216) using cooling water (215a) to generate a second liquid refrigerant stream (217). The first and second liquid refrigerant streams are combined to generate the liquid refrigerant stream (205a).

[0014] Figure 3 shows another embodiment according to the present invention. The process corresponds to that shown in Figure 2, but instead of cooling the reactor product stream (303) by direct heat exchange with the liquid refrigerant stream (305a), the reactor product stream is cooled using an intermediate refrigerant stream (319a) to generate a cooled reactor product stream (307) and a used intermediate refrigerant stream (319b). Heat exchange between the used intermediate refrigerant stream (319b) and liquid refrigerant stream (305a) in heat exchanger (318) regenerates the intermediate refrigerant stream (319a) and generates the vapour refrigerant stream (305b).

[0015] Detailed description of the invention

[0016] Sub-headings are included for convenience only and are not intended to limit the invention.

[0017] Reactor The 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 given the known toxicity issues with mercury catalysts. Gold catalysts have been well studied as hydrochlorination catalysts and examples are described in W02013/008004A2, W02020/254817A1 and W02023/111537A1. It is preferred that the hydrochlorination catalyst comprises gold. The reactor may be a single reactor, or may comprise two or more reactors in series, parallel, or series-parallel.

[0018] The reactor generates a reactor product stream which comprises VCM along with unreacted acetylene and HCI. Depending on the design philosophy of the VCM plant, the process may include a step of scrubbing the reactor product stream to remove residual HCI upstream of the cooling stages, for example as described in CN117018826A. Alternatively, no step of scrubbing occurs upstream of the cooling stages, and the HCI is removed together with acetylene in the overhead fraction from the lights column.

[0019] The reactor product stream, having optionally first undergone scrubbing, is preferably compressed upstream of the cooling stages. Compression makes the cooling stages more efficient and makes it possible to operate with a smaller lights column. The reactor product stream is preferably compressed to a pressure of 2.0 to 15.0 barg, preferably 2.0 to 6.0 barg, preferably 2.5 to 4.0 barg.

[0020] It is preferred that, prior to the cooling stage(s) (step (ii)), there is a step of cooling the reactor product stream using cooling water. This is a preferred way of taking the majority of the residual heat out of the reactor product stream. The cooling water preferably has a temperature of 20-40 °C.

[0021] Cooling stages The reactor product stream, optionally having first undergone scrubbing and/or compression and/or cooling using cooling water, is cooled by direct or indirect heat exchange using a liquid refrigerant stream generated by a refrigeration service.

[0022] As used herein, "direct heat exchange" refers to an arrangement where a liquid refrigerant stream generated by the refrigeration service is used for heat exchange with the reactor product stream. This arrangement is illustrated in Figure 1.

[0023] As used herein, "indirect heat exchange" refers to an arrangement where the liquid refrigerant stream is used for heat exchange with an intermediate refrigerant stream. The liquid refrigerant stream generated by the refrigeration service is sent to a heat exchanger along with a used intermediate refrigerant stream returning from the cooling stages, to generate an intermediate refrigerant stream and a vapour refrigerant stream. The intermediate refrigerant stream is then used for cooling of the reactor product stream. This arrangement is illustrated in Figure 3. A preferred intermediate refrigerant is brine.

[0024] The liquid refrigerant stream is expanded, during which it cools, and is then used to cool either the 2 0 reactor product stream (direct heat exchange) or used intermediate refrigerant stream (indirect heat exchange), during which the liquid refrigerant evaporates and generates a vapour refrigerant stream. Heat is removed from the reactor product stream, or used intermediate refrigerant stream, as latent heat taken on by the vapour refrigerant stream, thereby cooling the reactor product stream. The vapour refrigerant stream is returned to the refrigeration service and the cooled reactor stream is fed to the lights column.

[0025] In some embodiments step (ii) comprise two or more cooling stages using a liquid refrigerant stream of decreasing temperature in each successive cooling stage. The liquid refrigerant used in each successive cooling stage may be the same or different.

[0026] Refrigeration service The role of the refrigeration service is to generate the liquid refrigerant stream used by the cooling stages, and to generate the compressed vapour refrigerant stream used by the lights column reboiler for heating duty. The skilled person will be familiar with the design of refrigeration systems. A typical refrigeration cycle used to provide cooling duties comprises a cycle of compression, condensation, expansion and evaporation.

[0027] A typical VCM plant requires a total liquid refrigerant volume of several cubic meters. Ideally the refrigerant should be relatively inexpensive and should be capable of generating a vapour having a temperature of -25 °C, preferably at a pressure of 1 atm or above. Propane is a particularly preferred refrigerant.

[0028] The cooling stages generate a vapour refrigerant stream which is returned to the refrigeration service. The vapour refrigerant stream is compressed in a compressor to generate a compressed vapour refrigerant stream. The pressure to which the refrigerant is compressed will depend on the temperature of portion of the bottoms fraction sent to the lights column reboiler and the choice of refrigerant. Ideally the refrigerant should condense at a temperature which is 10-20 °C above the process side (i.e. bottoms fraction) temperature.. The compressed vapour refrigerant stream is then split into a first compressed vapour refrigerant stream and a second compressed vapour refrigerant stream.

[0029] The first stream is to provide the heating duty in the lights column reboiler.

[0030] The second stream is sent to a heat exchanger where it is condensed using cooling water, preferably cooling water with a temperature of 20-40 °C, generating used cooling water. Some cooling of the second stream may also take place during this step.

[0031] The first liquid refrigerant steam, generated by the lights column reboiler, and the second liquid refrigerant stream generated by the water cooled heat exchanger within the refrigeration service are combined to generate liquid refrigerant stream which is used in the cooling stages as described previously.

[0032] Lights column and lights column reboiler The role of the lights column is to separate the cooled reactor product stream into a bottoms fraction which is rich in VCM, and an overhead fraction which comprises acetylene (and HCI if this has not 3 0 be scrubbed earlier).

[0033] The bottoms fraction is withdrawn from the bottom of the lights column. The bottoms fraction is split and a first portion which is sent to lights column reboiler. The remaining portion may be sent for further purification, if required. The first portion is heated by heat exchange with the first compressed vapour refrigerant stream generated by the refrigeration service, to generate a heated bottoms fraction which is returned to the lights column and a first liquid refrigerant stream which is returned to the refrigeration service.

[0034] It is preferred that the heating duty for the lights column reboiler is provided entirely by the first compressed vapour refrigerant stream. This can be achieved by splitting an appropriately sized amount of the compressed vapour refrigerant stream as the first compressed vapour refrigerant stream sent to the lights column reboiler.

[0035] The overhead fraction may be recycled and used to produce the reactor feed stream, or may be sent to a secondary reactor for further hydrochlorination.

Claims (15)

1. Claims 1. A process for the production of vinyl chloride monomer (VCM), comprising the steps of: (i) feeding a reactor feed stream (201) comprising acetylene and HCI to a reactor (202) and carrying out hydrochlorination in said reactor in the presence of a hydrochlorination catalyst to produce a reactor product stream (203) comprising vinyl chloride monomer along with unreacted acetylene and HCI; (ii) cooling the reactor product stream in one or more cooling stages (204) by direct or indirect heat exchange with a liquid refrigerant stream (205a) provided by a refrigeration service (206) to generate a cooled reactor product stream (207) and a vapour refrigerant stream (205b) which is returned to the refrigeration service; (iii) feeding the vapour refrigerant stream to a compressor (208) to generate a compressed vapour refrigerant stream (209) and splitting the compressed vapour refrigerant stream into a first compressed vapour refrigerant stream (209a) and a second compressed vapour refrigerant stream 15 (209b); (iv) feeding the cooled reactor product stream to a lights column (210) where it is separated into a bottoms fraction (211) and an overhead fraction (212); (iv) splitting a portion of the bottoms fraction (211a) and feeding said portion to a lights column reboiler (213), where the portion is heated by heat exchange with the first compressed vapour 2 0 refrigerant stream (209a) to generate a heated bottoms fraction (211 b) which is returned to the lights column, and a first liquid refrigerant stream (214) which is returned to the refrigeration service; (v) condensing the second compressed refrigerant stream (209b) by heat exchange with a cooling water stream (215a) in heat exchanger (216) to generate a second liquid refrigerant stream (217) and a used cooling water stream (215b); (vi) combining the first liquid refrigerant steam (214) and second liquid refrigerant stream (217) to generate liquid refrigerant stream (205a).

2. A process according to claim 1, wherein the hydrochlorination catalyst is mercury-free.

3. 3 0 3. A process according to claim 1 or claim 2, wherein the process includes a step of scrubbing the reactor product stream to remove residual HCI upstream of the cooling stages.

4. A process according to any of claims 1 to 3, wherein the reactor product stream is compressed upstream of the cooling stages.

5. A process according to claim 4, wherein the reactor product stream is compressed to a pressure of 2.0 to 15.0 barg.

6. A process according claim 4, wherein the reactor product stream is compressed to a pressure of 2.0 to 6.0 barg.

7. A process according to any of claims 1 to 6, wherein prior to step (ii) there is a step of cooling the reactor product stream using cooling water.

8. A process according to any of claims 1 to 7, wherein step (ii) comprise two or more cooling stages using a liquid refrigerant stream of decreasing temperature in each successive cooling stage.

9. A process according to any of claims 1 to 8, wherein the refrigerant is propane.

10. A process according to any of claims 1 to 9, wherein the cooling water stream has a temperature of 20-40 °C.

11. A process according to any of claims 1 to 10, wherein in step (H) the reactor product stream in cooled by direct heat exchange with the liquid refrigerant stream.

12. A process according to any of claims 1 to 10, wherein in step (H) the reactor product stream in cooled by indirect heat exchange with the liquid refrigerant stream, using an intermediate 2 0 refrigerant stream (319a).

13. A process according to claim 12, wherein intermediate refrigerant is brine.

14. A process according to any of claims 1 to 13, wherein the heating duty for the lights column 2 5 reboiler is provided entirely by the first compressed vapour refrigerant stream.

15. A process according to any of claims 1 to 14, wherein the overhead fraction from the lights column is sent to a secondary reactor.