Polymerization reactor system comprising at least one withdrawal valve

Described herein is a polymerization reactor system comprising at least one loop reactor and/or at least one transfer line, and further comprising at least one withdrawal valve, wherein the at least one withdrawal valve is mounted to a wall of a lower horizontal segment of the loop reactor and/or to a wall of the transfer line, at an angle a of more than 0° and equal to or less than 85°, determined from perpendicular to a tangent of the wall at the mounting position in flow direction of a slurry in the loop reactor and/or in the transfer line. The valve piston of the at least one withdrawal valve comprises a valve plate at an end directed to the at least one loop reactor and/or at an end directed to the at least one transfer line, the valve plate being shaped according to an inner wall of the at least one loop reactor and/or according to an inner wall of the at least one transfer line such that the valve piston is flush with the inner wall of the at least one loop reactor and/or with the inner wall of the at least one transfer line in a closed position of the withdrawal valve. By using such a withdrawal valve, a limitation of the effective withdrawal area can be avoided or at least be reduced such that the liquid slurry can efficiently be withdrawn and the risk of plugging is reduced. Further disclosed is a method for producing an olefin polymer in the inventive polymerization reactor system.