oa South African Journal of Chemical Engineering - Dehydration of zinc sulphate in a fluidised bed drier
The rate of drying of zinc sulphate hexahydrate was measured in a 300 mm i.d. fluidised bed dryer at air temperatures of 20-200°C. The final product at these conditions is zinc sulphate monohydrate. The effect of particle size and content of crystallization water on minimum fluidisation velocity, and the rate of dehydration on the air temperature were investigated for zinc sulphate heptahydrate. A simple model has been developed to correlate the drying rate with the conditions of fluidisation. This model was used for a design of a pilot plant vibro-fluidised dryer. The dryer was 4 m long and 0.5 m wide with plenum consisting of four chambers. Each chamber had its own supply of air with temperature control to ensure the right regime of fluidization and drying rate for the required product quality. The rate of dehydration was governed by two process variables: the driving force, and the excess gas velocity U-Umf, which can be taken as the approximate bubble flow rate through the bed. On the basis of laboratory data the flow rate of air into individual chambers was adjusted to keep the excess gas velocity equal. Temperatures in individual chambers of the vibro-fluidized dryer were adjusted to prevent dissolution of crystals in the first chamber by the liberated water. The capacity of the dryer was 280 kg/h of zinc sulphate monohydrate.
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