Method for producing vinyl chloride monomer from acetylene

Abstract

This specification describes a method for producing vinyl chloride monomer (VCM) from acetylene and HCl. This method involves a primary reactor and a secondary reactor, each containing a hydrogenation catalyst. The feed from the primary reactor is sent to a knockout drum. The feed from the secondary reactor is split, with a portion combined with the feed from the primary reactor, while the other portion is sent to a vent recovery unit. The vent recovery unit includes one or more stages where a liquid containing VCM is separated and sent directly to the KO drum. Low-temperature vapor is generated from the KO drum, sent to a compressor, and subsequently sent to a light component separation unit and a light component column.

Description

Technical Field

[0001] The present invention relates to a method for converting acetylene into vinyl chloride monomer (VCM).

Background Art

[0002] The hydrochlorination of acetylene to produce VCM as a precursor to polyvinyl chloride (PVC) is currently a large-scale industrial process through the route from natural gas to acetylene, especially in regions rich in coal such as China and regions rich in natural gas. More than 20 million tons of VCM are produced annually by acetylene hydrochlorination, and most of them utilize a mercuric chloride (HgCl 2 ) catalyst supported on activated carbon.

[0003] U.S. Patent No. 3,268,299 (Crawford & Russell Inc) describes an apparatus for performing a catalytic reaction in a fixed-bed reactor and can be used for reactions including the reaction between acetylene and hydrogen chloride to produce vinyl chloride. This apparatus reduces the formation of hot spots during the highly exothermic hydrochlorination reaction. FIG. 8 shows a typical setup process.

[0004] Chinese Patent Application Publication No. 1,884,241 (A) (Haiji) 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 HgCl 2 / C catalyst to produce a crude vinyl chloride mixture. The crude mixture is cooled to about 40 °C, compressed, cooled, condensed to remove VCM, the uncondensed gas is compressed, and reacted together in a second-stage reactor at a temperature of 100 - 180 °C and a pressure of 270 kPa in the presence of HgCl 2 / C. The non-condensable substances from the second-stage reactor are combined with the products from the first-stage reactor before cooling.

[0005] The problem with this configuration is that the compressor and the compressor outlet cooler are prone to fouling by heavy substances generated when the VCM is exposed to temperatures exceeding 100°C. The heat (Q) exchanged in the outlet cooler is proportional to the area (A), the temperature difference (ΔT), and the heat transfer coefficient (U), i.e., Q = U·A·ΔT. As the surface becomes fouled, U decreases and Q also decreases. Therefore, the cooler must be sized to have a sufficiently large area to cope with fouling, making the cooler more expensive. If the fouling becomes too severe, it is necessary to stop the process to remove the fouling, which is costly.

[0006] It would be advantageous if the process could be adapted to minimize fouling in the compressor and the compressor outlet cooler, thereby avoiding downtime and the need to oversize the compressor cooler. The present invention addresses this problem.

Summary of the Invention

[0007] In the present invention, a knockout (KO) drum is introduced in front of the compressor. The cooled product from the primary reactor is fed to the KO drum. Vapors (including acetylene, HCl, and residual VCM) are generated in the KO drum by evaporating the liquid VCM in the KO drum that cools the contents of the drum. The KO drum is also supplied with a VCM-rich liquid separated from the vent recovery unit downstream of the secondary reactor. The vent recovery unit is operated in one or more stages, and the liquid from each stage is sent directly to the KO drum. The vent recovery and KO drum arrangement means that the feed to the compressor can be cooled to a lower temperature than in conventional processes without requiring uneconomical amounts of cooling. The vapor is at a low temperature, typically -10°C, which is much lower than the temperature of the feed to the compressor (10 - 40°C) in Chinese Patent Application Publication No. 1884241(A), meaning that the compressor has to cope with a smaller volumetric throughput, which can be made smaller, and the temperature at the compressor outlet is lower, which means that fewer fouling products are generated and the compressor and the compressor outlet cooler are less prone to fouling.

[0008] The present invention relates to a method for producing vinyl chloride monomer (VCM), comprising: supplying a feed stream containing acetylene and HCl to a primary reactor, and performing hydrochlorination in the primary reactor in the presence of a first hydrochlorination catalyst to produce a primary reactor product stream containing vinyl chloride monomer together with unreacted acetylene and HCl; cooling the primary reactor product stream and supplying the obtained cooled primary reactor product stream to a knockout (KO) drum; evaporating VCM in the KO drum to generate a vapor having a temperature of -20°C to +10°C; supplying the vapor from the KO drum to a compressor to generate a compressed stream; supplying the compressed stream to a light component separation unit and separating the compressed stream into a liquid fraction and a top fraction containing unreacted acetylene, HCl and residual VCM; sending the liquid fraction from the light component separation unit to a light component column and separating the liquid fraction into a bottom fraction containing VCM and a top fraction returned to the light component separation unit; sending the top fraction from the light component separation unit to a secondary reactor and performing hydrochlorination in the secondary reactor in the presence of a second hydrochlorination catalyst to produce a secondary reactor product stream containing vinyl chloride monomer together with unreacted acetylene and HCl; dividing the secondary reactor product stream into a first portion sent to a vent recovery unit and a second portion combined with the primary reactor product stream; condensing the first portion into a liquid in the vent recovery unit in one or more stages and directly sending the liquid from each stage to the KO drum. BRIEF DESCRIPTION OF THE DRAWINGS

[0009]

Figure 1

Figure 2

Figure 3

Figure 4

Mode for Carrying Out the Invention

[0010] When the temperature is reported as “approximately [] °C”, the value can vary by ±5 °C. For example, a temperature of “about -10 °C” should be understood to mean any temperature between -15 °C and -5 °C.

[0011] Primary reactor The primary reactor contains a first hydrogenation catalyst that is active for the conversion of acetylene to VCM. Any suitable hydrogenation catalyst may be used. In a preferred embodiment, the hydrogenation catalyst contains gold. Gold catalysts have been well studied as hydrogenation catalysts. Preferred catalysts include those described in WO 2013 / 008004 and WO 2020 / 254817 (Johnson Matthey), the contents of which are incorporated herein by reference. A particularly preferred catalyst contains a complex of gold and a thiosulfate ligand on a carbon support.

[0012] In a preferred embodiment, the primary reactor is a shell and tube reactor.

[0013] The feed to the primary reactor is typically a mixture of acetylene and HCl in a molar ratio of about 50:50. Preferably, there is a slight excess of HCl to ensure that the catalyst remains in the active state. The feed to the primary reactor typically has a pressure of 0 - 4 barg (i.e., 1 - 5 bara), preferably 0 - 1 barg (i.e., 1 - 2 bara), for example 0 - 0.8 barg or 0.2 - 0.6 barg. Higher pressures, for example on the order of 20 barg, are also possible.

[0014] The acetylene hydrochlorination is exothermic, and the product stream from the primary reactor is a hot gas containing a mixture of VCM, acetylene, and HCl. The primary reactor product stream is preferably cooled using cooling water before being sent to the KO drum. Cooling at this stage reduces the load on the KO drum. Typically, the cooling water is supplied at 30 °C and the secondary reactor product stream is cooled to 40 °C. The cooled stream is referred to as the cooled primary reactor product stream. The cooled primary reactor stream is preferably passed through a filter before being fed to the KO drum.

[0015] As will be described in more detail under the heading "Secondary Reactor" below, the product stream from the secondary reactor is split into a first portion that is sent to the vent recovery unit and a second portion that is combined with the product stream from the primary reactor. The secondary reactor product stream is preferably cooled before being split. This is preferred because the product stream from the secondary reactor is at a higher pressure than the product stream from the primary reactor, and cooling of the secondary reactor product stream can be achieved in a smaller and more efficient cooling unit. Therefore, it is preferred to cool the secondary reactor product stream to produce a cooled secondary reactor product stream and then split it into a first portion that is sent to vent recovery and a second portion that is combined with the cooled primary reactor product stream. It is possible to split the secondary reactor product stream before any cooling and combine it with the primary reactor product stream, but this is a less preferred option.

[0016] KO drum The role of the KO drum is typically to generate low-temperature steam at a temperature of about -10 °C, and the cold steam is supplied to the compressor. The KO drum receives the cooled primary reactor stream and the liquid fraction from the vent recovery, which is described in detail under the heading "Vent Recovery". The steam is created by evaporating the liquid fraction from the vent recovery, which contains VCM as the main component and cools the entire contents of the KO drum. Typically, the liquid fraction from the vent recovery is sprayed into the KO drum to create a high surface area for evaporation. For example, if the amount of VCM from the vent recovery is insufficient to cool the steam to the appropriate temperature, additional liquid VCM, such as a portion of the reflux from the light ends separation unit or the light ends column, can be sent to the KO drum to achieve additional cooling.

[0017] Compressor The secondary reactor operates at a higher pressure than the primary reactor, which means that the primary reactor product stream must be compressed. The compressor is located downstream of the KO drum and increases the pressure of the steam before the light ends separation unit and the light ends column. Any suitable compressor can be used, but the preferred type is a screw compressor because screw compressors are generally less sensitive to fouling and are less expensive than centrifugal compressors. An oil-free screw compressor is most preferred.

[0018] The pressure at the outlet of the compressor depends on the desired pressure inside the secondary reactor. The pressure inside the secondary reactor can be increased to about 20 barg, but more typically, the pressure inside the secondary reactor is 2 - 5 barg, for example 3 - 5 barg. Therefore, the steam is compressed to 2 - 5 barg, for example 3 - 5 barg. To allow for pressure losses between the compressor and the secondary reactor (e.g., via the light ends separation unit and the light ends column), the compressor typically compresses the steam to a pressure about 0.5 bar higher than the pressure of the light ends column. For example, if the light ends column operates at 3.5 barg, the compressor outlet should be 4 barg.

[0019] In the present invention, the vapor entering the compressor has a temperature of about -10°C, and the vapor exiting the compressor has a temperature of about 90°C. This is in contrast to the situation of Chinese Patent Application Publication No. 1884241(A), where the temperature of the crude vinyl chloride stream supplied to the compressor is 40°C and it is compressed to 370 - 400 kPa(g). Assuming a similar temperature rise across the compressor, the temperature of the compressed vapor is expected to be about 140°C, which surely exceeds 100°C, the temperature at which fouling begins. The lower outlet temperature from the compressor in this method avoids fouling in the compressor and the compressor outlet cooler.

[0020] The compressed stream is preferably cooled before being sent to the light component separation unit. The purpose of this cooling is to lower the temperature of the feed to the light component separation unit without separating the liquid stream. Water is particularly suitable for such cooling.

[0021] Light component separation unit The role of the light component separation unit is to separate the compressed stream into a liquid fraction containing VCM as the main product and a top fraction containing unreacted acetylene, HCl, and residual VCM, and the top fraction is sent to the secondary reactor.

[0022] One or more liquid fractions produced by the light component separation unit are preferably collected in a receiver drum before being sent to the light column. To maintain the vapor from the KO drum at a sufficiently low temperature, a portion of the liquid from the receiver drum can also be sent to the KO drum. Any non - condensable substances from the receiver drum can be sent to the secondary reactor.

[0023] The light fraction separation unit includes at least one cooling stage where the liquid fraction is separated. The light fraction separation unit preferably includes a series of cooling stages carried out at decreasing temperatures (i.e., using a coolant at decreasing temperatures), where the liquid fraction is separated at each stage and the top fraction is sent to the next cooling stage. As described above, the liquid fraction is preferably collected in a receiver drum before being sent to the light fraction column. These fractions may be combined or each may be sent separately to the receiver drum. The top fraction from the final cooling stage is fed to the secondary reactor.

[0024] In a particularly preferred embodiment, the light fraction separation unit includes a first cooling stage using a coolant at a temperature of about +3°C, a second cooling stage using a coolant at a temperature of about -10°C, and a third cooling stage using a coolant at a temperature of about -25°C. In each case, the liquid fraction obtained on the process side is about 5°C higher than the temperature of the respective coolant. The top fraction from the first cooling stage is fed to the second cooling stage. The top fraction from the second cooling stage is fed to the third cooling stage, and the top fraction from the third cooling stage is fed to the secondary reactor.

[0025] As far as the inventors are aware, the previously described VCM plant can cool the compressed gas in a series of stages, but it has not been described that the liquid fraction is separated at each stage. As shown in Figure 1 of Chinese Patent Application Publication No. 1884241(A), there is only a single condensation unit, which means that all of the compressed stream is cooled to the lowest temperature of the refrigeration unit. As a result, there is a large load on the condenser and the VCM distillation column that have to reboil the cold liquid. Therefore, separating the liquid fraction at each stage is more efficient and reduces the load on the light fraction separation unit and the light fraction column.

[0026] Light fraction column The role of the light ends column is to separate the liquid fraction from the light ends separation unit into a bottoms fraction containing VCM and a tops fraction. This can be achieved by conventional distillation using a multi-stage distillation column. The tops fraction from the light ends column is returned to the light ends separation unit. The bottoms fraction contains VCM and heavy components and is preferably sent for further purification to remove the heavy components. A part of the bottoms fraction can also be sent to the KO drum as described under the heading "KO drum", but it is preferred to use the liquid from the receiver drum for this purpose.

[0027] A preferred flow diagram of the light ends separation unit and the light ends column is shown in Figure 3. The compressed stream from the compressor, which is usually cooled with, for example, water (not shown), enters the first cooling stage within the light ends separation unit. The liquid fraction from the first cooling stage is collected in a drum and the non-condensable fraction is sent to a second cooling stage operating at a lower temperature than the first cooling stage. This process is repeated such that the non-condensable material from the second cooling stage is sent to a third cooling stage operating at a lower temperature. The non-condensable material from the third cooling stage is sent to the secondary reactor. The combined liquid fractions are collected in the receiver drum and then sent to the light ends column where they are separated into a bottoms fraction which is sent for heavy component separation. The non-condensable material is returned to the light ends separation unit.

[0028] Secondary reactor The tops fraction from the light ends separation unit is sent to the secondary reactor. For example, when using a thiosulfate gold complex catalyst described in WO 2013 / 008004 and available from Johnson Matthey under the trademark PRICAT™ MFC, the conversion in the primary reactor is typically good and conversions in excess of 85% can be achieved. Thus, the throughput to the secondary reactor is much less than that of the primary reactor and the capacity of the secondary reactor can be made much smaller than that of the primary reactor. The secondary reactor needs to be designed to handle a feed at a higher pressure. The pressure of the feed may be as high as about 20 barg, but is typically at a pressure of 2 - 3 barg.

[0029] The secondary reactor contains a second hydrogenation catalyst. Any suitable hydrogenation catalyst can be used. In a preferred embodiment, the hydrogenation catalyst contains gold. Gold catalysts have been well studied as hydrogenation catalysts. Preferred catalysts include those described in WO 2013 / 008004 and WO 2020 / 254817 (Johnson Matthey), the contents of which are incorporated herein by reference. To avoid misunderstanding, the second hydrogenation catalyst may be the same as or different from the first hydrogenation catalyst. A particularly preferred catalyst comprises a complex of gold and a thiosulfate ligand on a carbon support.

[0030] In a preferred embodiment, the secondary reactor is a shell and tube reactor.

[0031] As described under the heading "primary reactor", the secondary reactor product stream is split. The first portion is sent to a vent recovery unit and the second portion is combined with the primary reactor product stream. The relative ratio sent to the vent recovery unit depends on the amount of inert substances in the system. The greater the amount of inert substances, the greater the proportion of the secondary reactor product sent to the vent recovery unit.

[0032] The secondary reactor product stream is preferably cooled, preferably using cooling water, before being split. Typically, the cooling water is supplied at 30 °C and the secondary reactor product stream is cooled to 40 °C. The cooled stream is referred to as the cooled secondary reactor product stream. The secondary reactor stream is preferably passed through a filter either before or after cooling.

[0033] Vent recovery unit The overhead product from the light ends column is returned to the light ends separation unit. This is any inert substances present in the feed to the primary reactor (e.g., N 2, such as Ar, can accumulate within the system. To avoid this, a vent recovery unit is included downstream from the secondary reactor. The role of the vent recovery unit is to separate the inert substances from the first part from the secondary reactor product stream. The vent recovery unit also plays an important role in generating the liquid VCM that is fed to the KO drum.

[0034] The vent recovery unit condenses the first part into a liquid in one or more cooling stages. At each stage, a liquid is produced that mainly contains VCM, unreacted acetylene, and HCl. Typically, the liquid contains about 90 mol% VCM, and the remainder is acetylene and HCl. The liquid from each stage is sent directly to the KO drum. As used herein, "directly" means that there is no combination of the liquids from each stage to the KO drum. This is important because the VCM that evaporates within the KO drum is used to keep the temperature of the vapor to the compressor low. If the VCM fractions from the cooling stages were combined, they could evaporate prematurely before the KO drum, which would be wasted as it does not contribute to the cooling of the products from the primary and secondary reactors.

[0035] In a preferred embodiment, the vent recovery unit includes a series of refrigeration stages that operate at decreasing temperatures. In a particularly preferred embodiment, the vent recovery unit includes a first cooling stage that uses a coolant at about -10°C and a second cooling stage that uses a coolant at about -25°C. In an even more preferred embodiment, the vent recovery unit includes a third cooling stage that uses a coolant at about -65°C. In each case, the liquid fraction obtained on the process side is about 5°C higher than the temperature of the respective coolant.

[0036] Suitable coolants are well known to those skilled in the art. Ethane is particularly suitable for producing a coolant at -65°C. Propane is particularly suitable for producing coolants at -25°C, -10°C, and +3°C.

[0037] The flow diagram of a preferred vent recovery unit is shown in Figure 4. The cooled secondary reactor product enters the first cooling stage within the vent recovery unit. The liquid fraction from the first cooling stage is sent to the KO drum, while the non-condensable fraction is sent to a second cooling stage operating at a lower temperature than the first cooling stage. This process is repeated such that the liquid fraction from the second cooling stage is sent to the KO drum and the non-condensable material from the second cooling stage is sent to a third cooling stage operating at a lower temperature. The non-condensable material (primarily inert material) from the third cooling stage is discharged to avoid accumulation within the system. VCM evaporates within the KO drum, cooling the products from the primary and secondary reactors, which are also fed to the KO drum. This creates the cold vapor that is sent to the compressor.

Claims

1. A method for producing vinyl chloride monomer (VCM), The steps include supplying a feed stream containing acetylene and HCl to a primary reactor (21), performing hydrogenation in the primary reactor in the presence of a first hydrogenation catalyst to generate a primary reactor product stream containing vinyl chloride monomer together with unreacted acetylene and HCl, The steps include cooling the primary reactor product stream and supplying the resulting cooled primary reactor product stream to a knockout (KO) drum (23), The steps include: evaporating VCM in the KO drum to generate steam having a temperature of -20°C to +10°C; The steps include supplying steam from the KO drum to the compressor (24) to generate a compressed flow, The compressed flow is supplied to a light material separation unit (25), and the compressed flow is separated into a liquid fraction and a top fraction containing unreacted acetylene, HCl and residual VCM. The steps include sending the liquid fraction from the light material separation unit to the light material column (26), separating the liquid fraction into a bottom fraction (27) containing VCM and a top fraction (28) that is returned to the light material separation unit, The steps include sending the top fraction (29) from the light material separation unit to a secondary reactor (210), carrying out hydrogenation in the secondary reactor in the presence of a second hydrogenation catalyst to generate a secondary reactor product stream containing vinyl chloride monomer along with unreacted acetylene and HCl, The steps include dividing the secondary reactor product flow into a first portion (212a) that is sent to the vent recovery unit (213) and a second portion (212b) that is combined with the primary reactor product flow, A method comprising the steps of condensing the first portion into a liquid in the vent recovery unit in one or more stages, and sending the liquid from each stage directly to the KO drum.

2. The method according to claim 1, wherein the primary reactor product flow is cooled using cooling water.

3. The method according to claim 1 or 2, wherein the steam is compressed to a pressure of 2 to 5 barg.

4. The method according to claim 1 or 2, wherein the compressed flow is cooled before being sent to the light material separation unit.

5. The method according to claim 1 or 2, wherein the compressed flow is cooled with water before being sent to the light material separation unit.

6. The method according to claim 1 or 2, wherein the light material separation unit includes a series of cooling steps (35a, 35b, 35c) carried out at decreasing temperatures, wherein a liquid fraction is separated at each step and the top fraction is sent to the next cooling step.

7. The light material separation unit, A first cooling step (35a) using a coolant at a temperature of approximately +3°C, A second cooling step (35b) using a coolant at a temperature of approximately -10°C, The method according to claim 6, comprising a third cooling step (35c) using a coolant at a temperature of approximately -25°C.

8. The method according to claim 1 or 2, wherein the vent recovery unit includes a series of cooling steps (413a, 413b, 413c) performed at a decreasing temperature.

9. The vent recovery unit, A first cooling step (413a) using a coolant at a temperature of approximately -10°C, A second cooling step (413b) using a coolant at a temperature of approximately -25°C, The method according to claim 8, comprising a third cooling step (413c) using a coolant at a temperature of approximately -65°C.

10. The method according to claim 1 or 2, wherein the liquid fraction produced by the light material separation unit is collected in a receiving drum (316) before being sent to the light material column.

11. The method according to claim 1 or 2, wherein the secondary reactor product stream is cooled before it is divided.

12. The method according to claim 11, wherein the secondary reactor product stream is cooled to a temperature of about 40°C using cooling water.

13. The method according to claim 1 or 2, wherein the first hydrochloride catalyst and / or the second hydrochloride catalyst is a catalyst comprising a complex of gold and a thiosulfate ligand on a carbon support.

14. The method according to claim 1 or 2, wherein the primary reactor and / or secondary reactor is a shell-and-tube reactor.

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