Method for cooling a gas-phase reactor for the polymerization of olefins

A method for controlling the temperature in a polymerization reactor equipped with a cycle gas line for withdrawing reactor gas from the reactor, leading the reactor gas through a heat-exchanger, which is cooled by a cooling medium, which is conveyed in a cooling system through the heat-exchanger, and feeding the reactor gas back to the reactor by adjusting the temperature of the cooling medium entering the heat exchanger, wherein the temperature of the cooling medium entering the heat exchanger is controlled by adjusting the flow rate of the cooling medium in a part of the cooling system by a flow control system comprising two flow control devices of different size, which are connected in parallel, a process for polymerizing olefins comprising feeding at least one olefin and at least one polymerization catalyst to a polymerization reactor equipped with a cycle gas line for withdrawing reactor gas from the reactor, leading it through a heat-exchanger for cooling and feeding it back to the reactor, polymerizing the olefins in the presence of the polymerization catalyst and withdrawing the obtained polyolefin from the polymerization reactor, wherein the temperature in the polymerization reactor is controlled by such a method, and a process for controlling the flow rate of a fluid medium by a flow control system comprising two flow control devices of different size, which are connected in parallel, wherein the flow control system is controlled by a controller combination comprising a controller controlling the smaller flow control device and a controller controlling the larger flow control device, in which the output of the controller controlling the smaller flow control device is not only fed to the smaller flow control device but also twice to the controller controlling the larger flow control device, once directly as actual value of the controller controlling the larger flow control device and once, after having passed a limiting device, which limits a signal to a centered range, as setpoint of the controller controlling the larger flow control device.