CONTACT DRYING COMBINED WITH MEMBRANE SEPARATION - DEWATERING RATES OF POROUS SPHERES WETTED WITH 4 DIFFERENT LIQUID-MIXTURES

Selective thermal dewatering of a mixture-wetted porous material can b e achieved by combining contact drying and membrane separation. The ev aporated vapour is circulated between the dryer and the membrane modul e in a closed circuit. In the membrane module, a hydrophilic membrane with a high selectivity to water promotes dewatering of the vapour mix ture. This study presents measured dewatering rates for porous spheres wetted with four different binary and ternary mixtures of water and v olatile organic compounds. A mathematical model of the combined proces s was developed. The model combines descriptions of the following tran sport phenomena: Heat transport from the heated plate to the stirred m aterial, water diffusion from the material surface to the circulating vapour and water permeation through the membrane. The model describes the dewatering of all four systems very well. The study also presents a comparison between the dewatering rates reached in the combined proc ess with drying rates in conventional vacuum drying. It is found that the dewatering rates in the combined process are three to six times lo wer. (C) 1998 Elsevier Science S.A. All rights reserved.