Abstract: The invention relates to a method and to a device for producing a thermoplastic polyester, having the following steps: a) producing polyester pre-polymer particles; b) crystallizing the polyester pre-polymer particles for producing partially crystalline polyester pre-polymer particles; c) heating the partially crystalline polyester pre-polymer particles to a suitable reaction temperature for producing heated polyester pre-polymer particles; d) reacting the heated polyester pre-polymer particles for producing polyester polymer particles having an intrinsic viscosity between 0.70 and 0.95 dl/g. The reaction in step d) takes place in at least one reactor through which the particles flow by means of gravity. The dwell time in the reactor equals between 6 and 30 hours. The particles are supplied at least to step d) at a mass flow of between 40 and 100 t/h. The present invention is characterized in that a settling rate of the particles in the reactor equals between 2 and 6 m/h.