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) 0.305 m diameter with a bed of polypropylene beads and (2) spout-fluid bed 0.143 m diameter with draft tube submerged in a bed of FEP® pellets (S-FBDT). RTD, mean residence time tm, and spread of the distribution σ2, were determined at different drying temperatures, spouting velocities, bed depths, spraying pressures, and feed concentrations. Average values of tm and σ2 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. © 2011 Canadian Society for Chemical Engineering

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