Contact drying combined with membrane separation: Dewatering rates of porous spheres wetted with four different liquid mixtures

Selective thermal dewatering of a mixture-wetted porous material can be achieved by combining contact drying and membrane separation. The evaporated vapour is circulated between the dryer and the membrane module in a closed circuit. In the membrane module, a hydrophilic membrane with a high selectivity to water promotes dewatering of the vapour mixture. This study presents measured dewatering rates for porous spheres wetted with four different binary and ternary mixtures of water and volatile organic compounds. A mathematical model of the combined process was developed. The model combines descriptions of the following transport phenomena: Heat transport from the heated plate to the stirred material, 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 process with drying rates in conventional vacuum drying. It is found that the dewatering rates in the combined process are three to six times lower.