PROCESS FOR PRODUCING VINYL CHLORIDE MONOMER FROM ACETYLENE
The specification describes a process for the production of vinyl chloride monomer (VCM) from acetylene and HCl. The process involves a primary reactor and a secondary reactor each comprising a hydrochlorination catalyst. The feed from the primary reactor is sent to a knockout drum. The feed from the secondary reactor is split, part is combined with the feed from the primary reactor while the other part is sent to a vent recovery unit. The vent recovery unit includes one or more stages in which liquid containing VCM is separated and sent directly to the KO drum. A cold vapor is generated from the KO drum which is sent to a compressor, followed by a lights separation unit and a lights column.
1 . A process for the production of vinyl chloride monomer (VCM), comprising the steps of:
feeding a feed stream comprising acetylene and HCl to a primary reactor and carrying out hydrochlorination in said primary reactor in the presence of a first hydrochlorination catalyst to produce a primary reactor product stream comprising vinyl chloride monomer along with unreacted acetylene and HCl;
cooling the primary reactor product stream and feeding the resulting cooled primary reactor product stream to a knockout (KO) drum;
generating vapor having a temperature of −20° C. to +10° C. by evaporating VCM in the KO drum;
feeding vapor from the KO drum to a compressor to produce a compressed stream;
feeding the compressed stream to a lights separation unit and separating the compressed stream into a liquid fraction and an overhead fraction containing unreacted acetylene, HCl and residual VCM;
sending the liquid fraction from the lights separation unit to a lights column and separating the liquid fraction into a bottom fraction containing VCM and an overhead fraction which is returned to the lights separation unit;
sending an overhead fraction from the lights separation unit to a secondary reactor and carrying out hydrochlorination in said secondary reactor in the presence of a second hydrochlorination catalyst to produce a secondary reactor product stream comprising vinyl chloride monomer along with unreacted acetylene and HCl;
splitting the secondary reactor product stream into a first portion which is sent to a vent recovery unit and a second portion which is 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 sending liquid from each stage directly to the KO drum.
2 . A process according to claim 1 , wherein the primary reactor product stream is cooled using cooling water.
3 . A process according to claim 1 , wherein the vapor is compressed to a pressure of 2 to 5 barg.
4 . A process according to claim 1 , wherein the compressed stream is cooled before being sent to the lights separation unit.
5 . A process according to claim 1 , wherein the compressed stream is cooled using water before being sent to the lights separation unit.
6 . A process according to claim 1 , wherein the lights separation unit comprises a series of cooling stages carried out at decreasing temperatures with a liquid fraction being separated at each stage and the overheads being sent to the next cooling stage.
7 . A process according to claim 6 , wherein the lights separation unit comprises:
a first cooling stage using a coolant at a temperature of approximately +3° C.;
a second cooling stage using a coolant at a temperature of approximately −10° C.;
a third cooling stage using a coolant at a temperature of approximately −25° C.
8 . A process according to claim 1 , wherein the vent recovery unit comprises a series of cooling stages carried out at decreasing temperatures.
9 . A process according to claim 8 , wherein the vent recovery unit comprises:
a first cooling stage using a coolant at a temperature of approximately −10° C.;
a second cooling stage using a coolant at a temperature of approximately −25° C.;
a third cooling stage using a coolant at a temperature of approximately −65° C.
10 . A process according to claim 1 , wherein the liquid fraction(s) produced by the lights separation unit are collected in a receiver drum before being sent to the lights column.
11 . A process according to claim 1 , wherein the secondary reactor product stream is cooled before being split.
12 . A process according to claim 11 , wherein the secondary reactor product stream is cooled using cooling water to a temperature of approximately 40° C.
13 . A process according to claim 1 , wherein the first hydrochlorination catalyst and/or the second hydrochlorination catalyst is a catalyst comprising a complex of gold with a thiosulphate ligand on a carbon support.
14 . A process according to claim 1 , wherein the primary reactor and/or secondary reactor is a shell-and-tube reactor.