Mathematical modeling of impingement drying of corn tortillas

Abstract

A mathematical model of impingement drying of corn tortillas was developed based on governing equations for heat and mass transfer during the drying process. Mass transfer within the product was modeled as diffusion-driven mass flux. Heat transfer was driven according to Fourier's Law of conduction. Boundary conditions for drying in both air and superheated steam were developed for incorporation into the model. Convective heat transfer accounted for heat flow into the product at the surface. When drying in air, convective mass transfer prevailed; in superheated steam, differences in vapor pressure between the drying medium and the product surface accounted for mass transfer. The explicit finite difference scheme was employed for model solution. A simulation program was written to predict temperature at the center of the product and average moisture content during drying. Results were validated with data collected in previous studies at varying drying temperatures and convective heat transfer coefficients. Temperature predictions followed the trends observed in previous experiments with both air and steam drying at temperatures ranging from 115 to 145° C. Temperatures at the product's center initially rose to the boiling point, remained at this temperature during a phase of latent heating, and finally rose to the temperature of the drying medium. Moisture content predictions in air drying also followed the experimental trends; however, steam condensation unaccounted for by the model resulted in under predictions in the moisture content in steam drying at low drying medium temperatures. At high drying medium temperatures, drying in superheated steam was faster than drying in air. Sensitivity analyses indicated that product thickness and drying medium temperature had a significant effect on moisture content and temperature profile over time while convective heat transfer coefficient and initial moisture content had less impact on drying within the ranges analyzed.