Method and apparatus for controlling the recovery of solid polyolefin from a continuous reaction zone

A Polymerisation process and a loop reactor comprising polymerising olefins in a liquid diluent to produce a liquid slurry containing particles of normally solid polymer within the loop reactor, allowing the polymer to settle in a settling leg, periodically opening a 180° rotating product take-off valve located at the end of the settling leg to allow a charge of particles to flow out of the settling leg, the product take-off valve being operated by a pneumatically driven double-acting actuator, the pneumatic system being regulated by a system comprising pneumatic control valves, the improvement which consists in using automatic control valves, which are automatic v-ball valves.