A process and apparatus for the solid phase polymerization of polymers wherein cold amorphous polymer is introduced to a crystallizer. Hot heat transfer liquid is employed in the crystallizer for heating the polymer through indirect heat exchange to crystallize the polymer. The crystallized polymer is discharged to a reactor for solid phase polymerization, and then passed from the reactor to a cooler. Cold heat transfer liquid is employed in the cooler to cool the polymer through indirect heat exchange. A forced recirculation closed-loop system for the heat-transfer liquid is provided between the crystallizer and the cooler whereby the heat recovery process is achieved.