Control of Fluid Bed Drying of Tea

Abstract

In the manufacture of blade tea, shoots of three leaves and a bud are withered to approximately 72% MC WB. They are men further processed and dried when the moisture content is reduced to about 3%. A continuous fluid bed dryer achieves this in about 20 minutes. Manual control of this operation is difficult because of the time taken to reach stability. Fluidization requirements set the air flow rates possible in the dryer. For a given moisture content there is a very narrow range of superficial velocities to produce good fluidization with minimal elution of the light fraction. Temperature control is the only control variable remaining. A study of the equilibrium moisture content of tea and thin layer drying provides data for modelling. A MATLAB model simulates the dryer as a whole. This allows various control strategies to be studied before being implemented in practice.