high molecular weight polyethylene terephthalate is obtained by solid state polycondensation of low molecular weight polymer granulate or chips . the present invention relates to a moving bed reactor wherein the crystallized , low molecular weight polymer granulate or chips move by gravity flow from the top to the bottom of a cylindrical , vertical reactor . by means of an automatic sampling system , samples of the polymer chips are taken from different points in the moving bed and analyzed to continuously establish the intrinsic viscosity , i . e . molecular weight , of the chips . a countercurrent flow of cold and / or heated gas and the chip residence time are then adjusted in response to the viscosity profile in the reactor in order to control and achieve a desired chip quality .