FLUIDIZED-BED DRYING OF MATERIALS WETTED WITH A BINARY-LIQUID MIXTURE

In an experimental and theoretical study, the drying of porous particles wetted with a binary liquid mixture in a gas fluidized bed was examined. The theoretical approach reveals, that the process can be interpreted as a transition from an initial state of controlled equilibrium into a phase of diffusion controlled mass transfer, which is characterized by increasing stagnation of the moisture composition. The main parameters of influence are the Biot number(s) and/or a reciprocal Fourier number, which are both proportional to the square of the particle diameter and the gas velocity and are reciprocal to the bed height. The transition into the diffusion controlled phase takes place at an early stage with large Biot numbers, while drying almost remains in the equilibrium phase with small Biot numbers. A higher gas inlet temperature accelerates the process; the composition curve, however, is not affected much, since the diffusivities are also increased. Drying curves predicted by the model are in good agreement with the experimental results.