Polymerization process in which inert gas supply is controlled according to ranking

1. A polymerisation process comprising a plurality of processing steps in which an inert gas is used or is provided as a back-up, each of said processing steps being assigned a ranking based on the requirement for an inert gas flow in the processing step,

said polymerisation process in the event of a process upset, comprising controlling with a control system the supply of available inert gas based on said rankings.

2. A process according to claim 1 wherein processing steps where the consequence of a loss of the inert gas flow in said step is an unsafe condition are relatively highly ranked.

3. A process according to claim 1 wherein in the event of a process upset the control system allocates the available inert gas to the processing steps of higher ranking at the expense of those of lower ranking.

4. A process according to claim 3 where the processing steps of lower ranking include the purging of seals or valves and/or the use of inert gas in catalyst preparations.

5. A process according to claim 4 where the processing steps of lower ranking include the use of inert gas in catalyst activation vessels.

6. A process according to claim 1 wherein flow of inert gas is reduced in certain processing steps to free up inert gas capacity for use elsewhere.

7. A process according to claim 6 where the flow of inert gas is reduced to storage silos.

8. A process according to claim 1 wherein the ranking, or relative ranking, of one or more of the processing steps changes over time.

9. A process according to claim 8 wherein a processing step has a high ranking initially, but the ranking is reduced subsequently.

10. A process according to claim 1 wherein a nitrogen capacity is installed which is reduced to less than 80% of the nominal inert gas capacity.

11. A process according to claim 10 wherein the nitrogen capacity which is installed is reduced to less than 70% of the nominal inert gas capacity.

12. A process according to claim 10 wherein the nitrogen capacity which is installed is reduced to less than 60% of the nominal inert gas capacity.

13. A process according to claim 10 wherein the nitrogen capacity which is installed is reduced to less than 50% of the nominal inert gas capacity.

14. A process according to claim 1 wherein there are separate inert gas systems for supply of different types of inert gas to parts of the polymerisation process, and, at least in an emergency situation, one or more of any different types of inert gas may be available for use by another inert gas system.

15. A process for controlling the supply of inert gas to one or more processing steps of a polymerisation process in the event of a process upset, said process comprising controlling the supply of available inert gas to the processing steps based on the requirement for an inert gas flow in each processing step.

16. A process according to claim 15 wherein each of said processing steps is assigned a ranking based on the requirement for an inert gas flow in the processing step in the event of a process upset, and the supply of available inert gas is controlled based on said rankings.

17. A process control system for a polymerisation process, said polymerisation process comprising a plurality of processing steps in which an inert gas is used or is provided as a back-up, said process control system being comprised such that each of said processing steps is assigned a ranking based on the requirement for an inert gas flow in the processing step in the event of a process upset, and the process control system controls the supply of the available inert gas to the processing steps based on said rankings.