Slurry loop reactors

An apparatus defined as a set of loop reactors suitable for the polymerization process of a monomer, preferably ethylene and optionally an olefin comonomer, comprising for each of said reactors : a plurality of interconnected pipes P defining a flow path for a polymer slurry, said slurry consisting essentially of ethylene, optionally a comonomer, a polymerization catalyst, liquid diluent and solid olefin polymer particles, means for feeding monomer, optionally a co-monomer, diluent and optionally hydrogen in the reactor, means for feeding a polymerization catalyst in the reactor, a pump suitable for maintaining the polymer slurry in circulation in such reactor, one or more settling legs connected to the pipes P of such reactor for settling of polymer slurry, one or more control valves connected to the outlet of such settling legs, and one or more flash lines for discharging settled polymer slurry out of the reactor characterized in that each of said loop reactors comprises a three-or-more-way valve defining three-or-more ends, one end A being positioned at the outlet of said one or more control valves. A method to switch from series to parallel configuration a set of loop reactors. The use of the apparatus to switch polymerization reactors from parallel to series configuration and vice-versa.