Control of olefin polymerization

A gas-phase olefin polymerisation system including a gas-phase polymerisation reactor, means to detect, within the polymerisation reactor, an undesirable, or a potentially undesirable, reaction condition, a source of gaseous carbon dioxide, means connecting said source of carbon dioxide to the polymerisation reactor, means for maintaining said source of carbon dioxide disconnected from the reactor during normal operation of the polymerisation reaction and means operative by said detecting means to connect said carbon dioxide source to said reactor to automatically introduce carbon dioxide into the reactor only upon detecting said undesirable, or potentially undesirable, reaction condition and to thereby at least reduce the rate of polymerisation, the polymerisation reactor being provided with a stirrer to effect stirring of powder within the polymerisation reactor and said detecting means including means to detect failure of rotation of the stirrer as an indication of the undesirable, or potentially undesirable, reaction condition.