An investigation of percolation kinetics

The kinetic mechanisms of adsorptive drying were studied. The gas temperature and concentration expiration curves were determined for moist air and the two desiccants, activated alumina (Alcoa F-1) and Linde molecular sieve 4-A under adiabatic fixed bed conditions. Three different granule sizes, two different flow rates, one bed diameter, two different bed lengths, and two different feed gas concentrations were studied.

Models of kinetic mechanisms were devised and incorporated into a computerized numerical analysis program along with previously determined sorption therms, diffusivity relations, and heat transfer coefficient correlations. The models included the bipore concept and surface/ internal release of heat of adsorption among others. The program was repeatedly executed with parameter adjustments in between until coincidence was obtained between theorectical and experimental results.

During the course of parameter adjustments, it was found that the total diffusivity, D, the heat of adsorption, λ, the mass transfer film coefficient, k_G, and the coefficient of heat transfer by radiation, h_R, had to be multiplied by 2.4, 1.27, 3.0, and 0.5 for Linde molecular sieve 4-A, and 3.0, 1.4, 3.0, and 0.5 for activated alumina, respectively. These multiplications are justified in the thesis.

In the computer program, the diffusion equations were expressed in terms of partial pressure and temperature instead of concentration and temperature which gave a remarkable advantage in the finite difference

calculation of mass transfer at the interface. The finite difference scheme of Crank and Nicolson was used due to the shortcomings of the Douglas method discussed in the thesis. A variable increment size was used in the granule to increase the speed of calculation. The explicit nature speeded calculations because iterative loops were eliminated.