Evaporation due to infrared heating and natural convection

The results obtained from a drying experiment using infrared heating are compared to those obtained from an analytical model based on correlations for the mass transfer coefficients, which are commonly used to obtain evaporation rates for drying applications. It was shown that the mass transfer coefficients calculated by natural convection correlations are approximately ten times lower than those measured under infrared (IR) exposure. In an attempt to explain this discrepancy, which has been also observed in other studies and explained on the basis of thermal radiation effects, optical properties of the solvent are presented and the implications of the IR and near infrared (NIR) spectrum on the thermal radiation effects and ensuing evaporation rates are discussed. The results show that the correlation for mass transfer coefficient has to take into account the IR radiation temperature and the surface emissivity. This study provides important evaporation data on mass flux and mass transfer coefficients, which cannot be predicted using traditional correlations, for the design of IR assisted drying systems.