Polymerization reactor and method for producing water absorbent resin

A method for producing a water absorbent resin by subjecting a water-soluble ethylenically unsaturated monomer to polymerization in a dispersion medium, wherein the polymerization is performed in a polymerization reactor that comprises a container body, and a jacket covering an outer surface of the container body and defining a passage for passing a cooling/heating medium between itself and the outer surface of the container body, wherein the container body is made of a clad metal plate comprising a support metal layer having an inner surface at an inner side of the container body and an outer surface at an outer side of the container body, and an inner corrosion-resistant metal skin layer bonded to the inner surface of the support metal layer and being smaller in thickness than the support metal layer, the jacket including an inlet for introducing the cooling/heating medium into the passage and an outlet for discharging the cooling/heating medium from the passage, the outlet including a branched pipe connected to an inert gas supplier for supplying inert gas into the passage through the branched pipe;

• the method comprising:
• a first-stage polymerization comprising adding a first-stage solution of water-soluble ethylenically unsaturated monomer to the dispersion medium in the container body together with a polymerization initiator, heating the container body to a first predetermined polymerization temperature by passing a heating medium through the jacket, and cooling the container body to a second predetermined temperature lower than the first predetermined polymerization temperature for obtaining a first-stage reaction mixture by passing a cooling medium through the jacket;
• a second-stage polymerization comprising adding a second-stage solution of water-soluble ethylenically unsaturated monomer to the first-stage reaction mixture in the container body together with a polymerization initiator, heating the container body to a third predetermined polymerization temperature by passing a heating medium through the jacket for obtaining a second-stage reaction mixture;
• and
• causing the inert gas supplier to supply the inert gas into the passage through the branched pipe upon determining that a level of the cooling medium or the heating medium in the jacket is lowered during the polymerization.