Convective Drying of Porous Materials Containing Binary Mixtures

Abstract

The estimation of the size of a dryer requires the knowledge of the drying rate as a function of the moisture content, called drying curve. In addition to that, for the drying of materials containing mixtures it is important to know the composition of the moisture as a function of the moisture content, called composition curve.

In the theoretical part of this work the drying curve and the composition curve for the drying of materials containing binary mixtures are defined. The influence of thermodynamic equilibrium, gas and liquid side mass transfer on the composition curve are estimated by solving the differential equation for the unsteady state diffusion equation in the liquid phase.

In the experimental part samples wetted with a mixture of isopropyl alcohol and water were dried in a drying channel. The drying curve As obtained by weighing the sample while the composition curve was determined by analyzing the outlet air with an infrared gas analyzer.

The investigation showed that the drying curves of materials containing binary mixtures have the same characteristical shape as the drying curves of pure compounds. Further it was found that the composition curve is influenced by thermodynamic equilibrium, gas and liquid side mass transfer depending on the drying conditions. If the dimensions of the sample are large the selectivity is mainly influenced by the liquid side mass transfer, thus the drying is mainly nonselective. Only at the beginning of the drying a selectivity can be achieved for a few minutes. If the dimensions of the sample are small the selectivity is mainly influenced by the thermodynamic equilibrium as well as the gas side mass transfer.