Process for the gas-phase polymerization of olefins in a fluidized-bed reactor

A process for the gas phase polymerisation of one or more olefins containing from 2 to 10 carbon atoms, in a fluidised-bed reactor in the presence of a catalyst system of the Ziegler-Natta type or a catalyst based on chromium oxide, introduced into the reactor continuously or intermittently, the polymer being produced in the fluidised bed at a temperature T1 which is below the melting point of the polymer and drawn off from the reactor continuously or intermittently, the solid particles of the bed being kept in the fluidised state by means of a gaseous reaction mixture comprising the olefin or olefins to be polymerised, which gaseous reaction mixture passes through the reactor in a rising stream, leaves through the top of the reactor with fine particles entrained and directly returns into the bottom part of the reactor by means of a gas recycling line external to the reactor which line includes in succession in the direction of flow of the gaseous reaction mixture, a first heat transfer cooling means, a compressor, and a second heat transfer cooling means, characterised in that:

• a readily volatile liquid hydrocarbon is introduced either into the inlet of the first heat transfer means or into the recycling line, upstream and in the vicinity of the first heat transfer means, such that the mixture entering the first heat transfer means contains readily volatile hydrocarbon in the liquid state, and is cooled by the first heat transfer means to a temperature T2, which is below T1 and is such that no gaseous constituent of the said mixture condenses and such that the readily volatile liquid hydrocarbon volatilizes in the first heat transfer means so the mixture leaves the first heat transfer means totally in the gaseous state to achieve a state of cleanliness in the heat transfer means, and the gaseous reaction mixture to which the readily volatile liquid hydrocarbon has been added is cooled by the second heat transfer means to a temperature T3, which is below T2 and is such that the temperature of the fluidised bed is maintained at the desired temperature T1.