The drying of paste-like products is a common industrial task. Usually a batch vacuum contact drying process is used and the process is carried out in a kneader dryer with a low stirrer speed due to very high torques appearing during the drying process. The process can be divided into two parts, the paste and the particulate regime. Both regimes are separated by a sharp maximum in the torque curve. The position of the maximum depends on the type of product. In the paste regime the drying rate decreases with decreasing moisture content of the product, while torque increases and the product temperature remains nearly constant. In the particulate regime the drying rate decreases further with decreasing product moisture content. At the same time product comminution takes place, the torque decreases and the product temperature rises. Drying rate curves are measured for aqueous suspensions of china clay using a batch kneader drier at pilot plant scale. The influence of the main drying parameters on the drying process is investigated experimentally and theoretically. The measured drying rate curves are compared to calculations based on the penetration model for contact drying. Neglecting mass transfer effects and assuming that the particle size distribution of the product remains constant, the penetration model is able to describe both the paste regime and the particulate regime of the drying process over the whole moisture range. Measured and calculated drying rate curves are in good agreement for a variety of drying parameters.
Measurement and simulation of the vacuum contact drying of pastes in a LIST-type kneader drier
A. Dittler. T. Bamberger, D. Gehrmann, E.-U. Schlünder
Chemical Engineering and Processing, 1997, 36, 301-308