The process for polymerizing olefins uses a plurality of series-connected
polymerization reactors in which at least one of second and subsequent
reactors is a vapor-phase polymerization reactor containing a
multicomponent gas. The process comprises the steps of: taking the
multicomponent gas out of the vapor-phase polymerization reactor; mixing
the multicomponent gas with an inert gas which is lighter than at least
one component of the multicomponent gas, thereby obtaining a mixed gas;
compressing and/or cooling the mixed gas to liquefy a part of the
multicomponent gas; discharging at least a part of a gaseous mixture
comprising the inert gas and an unliquefied multicomponent gas out of a
reaction system; and returning a fluid comprising the remainder of the
gaseous mixture and the liquefied multicomponent gas to the vapor-phase
polymerization reactor. By mixing the multicomponent gas taken out of the
vapor-phase polymerization reactor with the inert gas, the composition of
gas in the vapor-phase polymerization reactor is effectively controlled,
thereby ensuring a stable production of olefin polymers of a desired
molecular weight distribution and composition.
