Polymer degassing process control
Process for operating a vertical fixed bed degasser which is continuously fed with an active polymer powder produced by a catalytic olefin polymerization process in a polymerization reactor. The polymerization conditions in the degasser are such that there is produced an additional amount of polymer corresponding to between 0.001 and 0.015 times the amount which entered the degasser. The degassing process includes introducing into the degasser a stripping gas containing principal monomer, with the temperature inside the degasser being the polymerization reactor temperature plus or minus 20° C.
1. Process for operating a vertical fixed bed degasser which is continuously fed with an active polymer powder produced by a catalytic olefin polymerisation process in a polymerisation reactor wherein
the polymerisation conditions in the degasser are such that there is produced an additional amount of polymer corresponding to between 0.001 and 0.015 times the amount which entered the degasser,
the degassing process comprises the introduction into said degasser of a stripping gas comprising from 5 to 60% by volume of principal monomer, and
the degassing conditions are selected such that the temperature inside the degasser is maintained within a range of temperatures comprised between the polymerisation reactor temperature plus or minus 20 degrees centigrade, and further wherein at least one of the following applies:
a) the degassing process comprises the introduction into said degasser of at least one polymerization deactivating agent and the selection of the degassing conditions comprises the control of the content of the said deactivating agent, or
b) the gas exiting the degasser is introduced in to a vent recovery unit which comprises refrigeration, compression and purification stages which allow separating the gas exiting the degasser into a gas/liquid stream which can be recycled in the upstream operations and the stripping gas stream which can then be used in the degasser, the stripping gas stream consisting essentially of a mixture of inert gas and principal monomer gas.
2. Process according to claim 1 wherein the degassing process comprises the introduction into said degasser of at least one polymerization deactivating agent and wherein the deactivating agent is selected from oxygen, carbon monoxide, carbon dioxide, water, or any mixture thereof.
3. Process according to claim 2 wherein the deactivating agent is water.
4. Process according to claim 1 wherein the degassing process comprises a control based on one or more of the following:
i) the principal monomer content in the degassing vessel,
ii) the total pressure at the top of the degassing vessel, and
iii) the polymer residence time inside the degassing vessel.
5. Process according to claim 2 wherein the degassing process comprises a control based on one or more of the following:
i) the principal monomer content in the degassing vessel,
ii) the total pressure at the top of the degassing vessel, and
iii) the polymer residence time inside the degassing vessel.
6. Process according to claim 1 wherein the catalyst used in the upstream polymerisation reactor process is a metalloncene catalyst, a Ziegler-Natta catalyst, or a chromium oxide catalyst.
7. Process according to claim 6 wherein the catalyst is a chromium oxide catalyst.
8. Process according to claim 6 wherein the catalyst is a Ziegler-Natta catalyst.
9. Process according to claim 1 wherein the gas exiting the degasser is depleted in principal monomer before its recycling as stripping gas.
10. Process according to claim 1 wherein the principal monomer is ethylene or propylene.
11. Process according to claim 10 wherein the principal monomer is ethylene.