Residence time distribution in spouted bed drying of maltodextrin solutions on a bed of inert particles

Residence time distributions (RTD) for aqueous maltodextrin solutions were determined in two kinds of spouted bed dryers: (1) conventional spouted bed (CSB) 0.305m diameter with a bed of polypropylene beads and (2) spout-fluid bed 0.143m diameter with draft tube submerged in a bed of FEP r pellets (S-FBDT). RTD, mean residence time tm, and spread of the distribution 2, were determined at different drying temperatures, spouting velocities, bed depths, spraying pressures, and feed concentrations. Average values of tm and 2 were 6.5 min and 26.6 min2 for the CSB and 6.9 min and 36 min2 for the S-FBDT, respectively, for all operating conditions except spraying pressure. RTD curves were well represented by the response of an ideal stirred tank with a superimposed bypass of 15% on average for the CSB and 7% on average for the S-FBDT dryer for all operating conditions. Increase in spraying pressure produced a reduction of tm and an increase in the bypass fraction of the product in both dryers.