Patent ID: 7723540

Claim:
A method for producing a polyhydroxy carboxylic acid which comprises producing a polycondensate by polycondensation reaction by heating a hydroxy carboxylic acid or a polycondensate thereof under reduced pressure by a polycondensation device, introducing the polycondensate produced by the polycondensation device into a depolymerization device from a feed opening for the polycondensate provided at a casing of the depolymerization device, forming a liquid film by a rotating blade rotating inside the casing about a rotary shaft, heating the liquid film by a heating part to depolymerize it into an annular dimer, discharging the annular dimer from an annular dimer discharge opening formed at the casing, introducing the annular dimer vaporized in the depolymerization device into a lower distillation column provided between the depolymerization device and a condenser for condensing the annular dimer vaporized by the depolymerization device, separating the polycondensate from the annular dimer and refluxing the polycondensate, wherein the shape of the blade is formed so that an outer diameter of the rotating blade equivalent to the diameter of rotation of the rotating blade as measured perpendicular to the rotary shaft gradually decreases from a larger outer diameter of the blade to a smaller outer diameter of the blade relative to the larger outer diameter, and the casing is formed so as to have an inner wall parallel with the outer shape of the blade, and wherein a feed opening is formed at the casing on the side of the larger outer diameter of the rotating blade, a discharge opening through which the annular dimer is discharged is formed at a first part of the casing on the side of the smaller outer diameter of the rotating blade, and a drain opening is formed at a second part of the casing on the side of the smaller outer diameter of the rotating blade and opposed to the first part of the casing with respect to the rotary shaft.