Combined Heat and Mass Transfer During Drying of Molecular Sieve Pellets

Abstract

For fundamental studies on sorption kinetics, gravimetrical experiments were carried out with spherical single pellets in a flow system to determine the rate limiting step (external mass transfer, macro-or micropore diffusion or heat transfer). By this way it is possible to investigate separately the influence of different parameters as pellet size, temperature, gas velocity and partial pressure jump. Good agreement with experimental results was obtained with a numerical calculated macropore diffusion-model taking into account the isotherm shape, which is favourable for adsorption but unfavourable for desorption. Mass transfer resistances in the boundary layer influenced only slightly the sorption kinetics. Here, heat effects are only of secondary influence as shown with numerical calculated and measured (with very thin thermocouples) temperature profiles versus time.

With regard to technical plants experiments were carried out in adiabatic fixed beds. At different heights of the column the temperature profiles (axial and radial) and concentration profiles were measured. Results for adsorption of H₂O and CO₂ from air and thermal desorption as well as purge gas desorption are presented. With high feed-concentrations a first mass transfer and second heat controlled zone were obtained. Here, for numerical calculation mass and energy balance as well as transfer equations must be solved simultaneously. With thermal desorption, cycle time, heat and air requirements for drying of ms-fixed beds can be calculated approximately with a heat balance.