MATHEMATICAL-MODELING OF DRYING OF LIQUID-SOLID SLURRIES IN STEADY-STATE ONE-DIMENSIONAL FLOW

A mathematical model of simultaneous mass, heat apd momentum transfer for two-phase flow of a gas and a solid/liquid slurry was developed. T he model was applied to calculation of the drying process of coal-wate r slurry droplets in a gas medium in a steady one-dimensional flow. Th e model was based on the well-known two-stage drying process for slurr y droplets. After the first period of drying, in which the evaporation rate is controlled by the gas phase resistance, the evaporating liqui d diffuses through the porous shell (crust) and then, by convection, i nto the gas medium. Inside the dry external crust of the drop, a wet c entral core forms, which shrinks as evaporation proceeds. The temperat ure of the slurry droplet rises. The process ends when the temperature of the dry outer crust reaches the coal ignition temperature in the c ase of combustion or when the moisture of the particle reaches the fin al required moisture. The developed model was based on one-dimensional balance equations of mass, energy and momentum for the liquid/solid a nd gas phases. The system of governing equations was represented by fi rst-order differential equations and solved simultaneously. The numeri cal solution of the governing equations was obtained using Gear's meth od. The model permitted calculation of the mass transfer ratio, the ch ange of the slurry droplet diameter, and the change of the temperature of the slurry droplet surface and crust. The conditions of break-up o f the droplet shell were analyzed. The results obtained from the numer ical model were compared with experimental results.