The use of Rotary Kiln or cement kiln reactors for thermally treating plastic pellets, or chips, in particular granules, pellets or chips of crystallizable polyesters containing at least 75% of their acid units derived from terephthalic acid, orthophthalic acid, 2,6 naphthalate dicarboxylic acid or their respective diesters has been described previously in the patent application WO 2004/018541. An essential feature of WO 2004/018541 is the use of plug flow like behaviour to achieve uniformity of the properties of the granules at the exit. While WO 2004/018541 contemplates the use of baffles or internals it does nothing to teach the design of such baffles necessary to increase the mixing in the radial turning direction and yet maintain plug flow like behaviour.
U.S. Pat. No. 3,767,601 describes a rotary kiln reactor for polyester flake with internals for good mixing. U.S. Pat. No. 3,767,601 discloses kilns with internals for both batch and continuous processes. A batch process by definition cannot have plug flow, and nothing in U.S. Pat. No. 3,767,601 indicates that the internals are specially configured for mixing while maintaining plug flow like behaviour.