Drying of wet solid particles in a steady-state one-dimensional flow

A mathematical model for mass, momentum and heat transfer in a one-dimensional two-phase flow is presented. This model was applied to the drying process of wet coal particles in a gas flow. The coal particles were assumed to have a wet core and a dry outer crust. The evaporation process of liquid from a particle was assumed to be governed by diffusion through the particle crust and convection into the gas medium. As evaporation proceeds, the wet core shrinks as the particle dries. The drying process is assumed to stop when: the moisture content of a particle falls to a predefined value; or the crust temperature reaches the ignition temperature; or break-up of the particle, caused by a pressure rise in the wet core, has occurred. The model was based on the one-dimensional balance equations for mass, momentum and energy of the gas and the dispersed phases. The system of the governing equations was represented by first-order differential equations and solved simultaneously by a numerical method. The model permitted calculation of the mass transfer ratio, change of the core diameter, and change of the average temperatures of the core and the crust. Four operating conditions were simulated using the model: isothermal; adiabatic, fixed wall temperature; and known heat Aux. The model is also capable of simulating a dispersed gas-solids flow, without mass or heat transfer, in a one-dimensional transport system. The prediction of the numerical simulation, for the last case, was compared with experimental results of coal-nitrogen conveying in high pressure systems. (C) 1998 Elsevier Science S.A.