MEASUREMENT AND SIMULATION OF THE VACUUM CONTACT DRYING OF PASTES IN A LIST-TYPE KNEADER DRIER

The drying of paste-like products is a common industrial task. Usually a batch vacuum contact drying process is used and the process is carr ied out in a kneader dryer with a low stirrer speed due to very high t orques appearing during the drying process. The process can be divided into two parts, the paste and the particulate regime. Both regimes ar e separated by a sharp maximum in the torque curve. The position of th e maximum depends on the type of product. In the paste regime the dryi ng rate decreases with decreasing moisture content of the product, whi le torque increases and the product temperature remains nearly constan t. In the particulate regime the drying rate decreases further with de creasing product moisture content. At the same time product comminutio n 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 th e main drying parameters on the drying process is investigated experim entally and theoretically. The measured drying rate curves are compare d to calculations based on the penetration model for contact drying. N eglecting mass transfer effects and assuming that the particle size di stribution of the product remains constant, the penetration model is a ble to describe both the paste regime and the particulate regime of th e drying process over the whole moisture range. Measured and calculate d drying rate curves are in good agreement for a variety of drying par ameters. (C) 1997 Elsevier Science S.A.