The present invention relates to a continuous process for heat treatment of a particulate thermoplastic material comprising a thermoplastic semi-crystalline polymer in the solid state in a single flow-through installation comprising one or more heating zones, wherein each heating zone comprises a contact heater consisting of an array of heating plates, the granulate material is transported as a moving packed bed in the flow-through installation, the process comprises one or more heating steps and in each heating step the particulate material is heated through contact heating via the contact heater, or one of the contact heaters, and optionally a flow of inert gas is applied in one or more heating zones with a mass ratio G/M of at most 2, wherein G is the inert gas flow in kg/hour and M is the particulate material flow in kg/hour.