Patent ID: 8198383

Claim:
A process which comprises polymerizing α-olefins CH 2 ═CHR, where R is hydrogen or a hydrocarbon radical having 1-12 carbon atoms, in the gas phase in two interconnected polymerization zones, a riser and a downcomer, wherein the growing polymer particles flow through the riser under fast fluidization conditions, leave said riser and enter the downcomer through which they flow downward in a densified form, leave said downcomer and are reintroduced into said riser, thus establishing a circulation of polymer between the riser and the downcomer, wherein: (a) the gas mixture present in the riser is totally or partially prevented from entering the downcomer by introducing into the upper part of said downcomer a liquid barrier L B wherein said liquid barrier LB comprises condensable inert compounds selected from C 2 -C 8 alkanes, and having a composition different from the gaseous mixture present in the riser; and (b) the ratio R between the flow rate F p of polymer circulated between said downcomer and said riser and the flow rate L B of said liquid barrier is adjusted in a range from 10 to 50.