The development and validation of a system model for a countercurrent cascading rotary dryer

An overall system model for a countercurrent rotary dryer has been develope d with the ultimate aim of assessing controller pairings in these dryers. T his model is based on heat acid mass balances within dryer regions combined with two subsidiary models, one describing the equipment (which determines particle transport and heat transfer) and the other describing the behavio ur of the material (the drying kinetics). Six partial differential equation s have been set up to evaluate six state variables: solids moisture content , solids temperature, gas humidity, gas temperature, solids holdup and gas holdup as functions of time and rotary dryer length. A control-volume metho d has been used to reduce the six partial differential equations with respe ct to time and the length of the rotary dryer to six ordinary differential equations in time. The drying model has been implemented in the SPEEDUP flo wsheeting package (with FORTRAN subroutines). The model has been validated by fifteen experiments in a pilot scale countercurrent-flow rotary dryer (0 .2m in diameter and 2m in length) using sorghum grain. The average discrepa ncy for the outlet solids moisture content between the simulation and the e xperiment is 6.4%. Also, the drying model predicts the outlet solids temper ature with an average discrepancy of-5.2%.