A PROCESS FOR THE TERMINATION AND START-UP OF POLYMERIZATION
Publication Date: 2003-Sep-16
The IP.com Prior Art Database
A process for the termination and start-up of polymerization comprising (i) contacting one or more alpha-olefins, in the gas phase, under polymerization conditions, with a chromium, vanadium or a titanium based catalyst system; (ii) introducing an ether in a sufficient amount to essentially terminate the polymerization; and (iii) optionally, thereafter, introducing an aluminum alkyl in an amount sufficient to start up the terminated polymerization.
A PROCESS FOR THE TERMINATION AND
This invention relates to a process for the termination and start-up of polymerization through the use of a reversible kill agent.
Titanium and vanadium based Ziegler-Natta catalyst systems can be used to provide homopolymers and copolymers of alpha-olefins in one or more reactors. Chromium based catalysts, particularly chromium oxide based catalysts, are also used as catalysts for these polymerizations. The high activity at temperatures in the range of about 50°C to about 110°C, which makes these catalyst systems so attractive, is also responsible for a negative characteristic. This deficiency exhibits itself in the form of a kinetic profile, which can be described as "autoacceleration", and leads to processes, which are, in effect, out of control; processes in which the reactants are "over reactive"; agglomeration of particular product; and other various undesirable results such as hot spotting, chunking, and sheeting.
Autoacceleration can be defined as an abrupt increase in the reaction rate of a process to an undesirable level due to a sudden rise in temperature. In effect, the system is unable to remove the heat as fast as it is generated. Control of this behavior is essential for the smooth operation of the process in the reactor(s). In some cases, the autoacceleration only affects one of the reactors in a series of two or more reactors. In any case, termination of polymerization is called for in order to rectify the situation.
In the event of downstream process upsets, e.g., purge bin pluggage or extruder failure, or other upsets, which are caused by forces other than autoacceleration, it is also desirable to terminate polymerization to keep the reactor(s) or purge bin from overfilling with resin.
Termination of polymerization can be accomplished by introducing a kill agent such as carbon monoxide into the polymerization reactor; however, carbon monoxide and similar kill agents require the purging of the reactor gases before the polymerization process can be restarted. The loss of monomer(s) through the purge, which can amount to thousands of pounds in a commercial operation, is very expensive. Further, it is found that these kill agents change the character of the catalyst such that a "different" polymer is formed if and when the catalyst can be reactivated.
It would be advantageous to provide a kill agent, which avoids the necessity of purging; terminates polymerization essentially completely with a one shot application; is quickly reversible; and, during termination and start-up, does not change the character of the catalyst. A reversible kill agent can be defined as a chemical which, when added to the polymerization reactor, results in a dramatic decrease in propagation rate (slow down in the rate of polymerization) without affecting the catalyst decay rate, with the stipulation that the polymerization can be restarted rapidly and ...