A FLUIDIZED BED DRIER FOR SUSPENSIONS AND PASTES

Abstract

A fluid bed dryer with inert particles was used for drying of suspensions. The effects of the operating conditions on dryer throughput and product quality were investigated. Experiments were performed in a cylindrical column 215 mm in diameter and 800 mm in height with glass spheres 0.925 mm in diameter as fluidizing media. The fungicides and pesticides (Cineb, Ciram, Propineb, Mangoceb, Copper-hydroxide, Copper oxy-chloride, Copper oxy-sulphate), other inorganic compounds (Calcium carbonate, Calcium sulphate, Cobalt carbonate) and complex compound (Organo-bentonite) and pure water were used as feeding materials. With respect to the main efficiency criteria, i.e. specific water evaporation rate, specific heat consumption and specific air consumption the fluid bed dryer with inert particles represents a very attractive alternative to the other drying technologies. High drying efficiency results from the large contact area and from the large temperature difference between inlet and exit air. The rapid mixing of particles, due to the aggregative fluidization, leads to the nearly isothermal conditions throughout the bed. In our experiments were successfully treated suspensions and very dense pastes. Suspension and product holdup in the bed varies between 6 and 8% by mass and the product with the same particle size as raw material is obtained. INTRODUCTION Many processes in the chemical, pharmaceutical and food processing industries involve drying of solutions, suspensions and pastes. This unit operation is most energy-consuming industrial operation. Although there are hundreds of variants actually used in drying, the research efforts over the world are associated with the development of more sophisticated systems. In general, the trends in drying technology are associated with higher energy efficiency, enhanced drying rates and development of more compact dryers, better control for enhanced quality and optimal capacity, developments of multi-processing units (for example filter-dryer), etc. Mujumdar (Mujumdar, 1990) pointed out that numerous new or improved drying technologies are currently at various stages of development. Particularly, with respect to the drying of solutions, suspensions and pastes, the use of fluidized, spouted, vibrofluidized and agitated beds of inert particles will be very important alternative to the classic drying technologies. An efficient drying system should meet several conditions: high values of heat and mass transfer coefficients, high contact area, high input of heat carrier gas, uniform temperature distribution over the drying chamber, the use of concentrated suspensions (as high as possible) in order to minimize water amount which should be evaporated and the use high inlet air temperature, as much as possible. However, many of these conditions are conflict each to other. The only drying concept that meets majority of the mentioned conditions is drying in an agitated bed of inert particles. Several systems (fluidized, spouted, spout-fluidized and different modifications) are at various stages of development (Mujumdar, 1990; Povrenovic et al., 1992). DRYING OF SUSPENSIONS IN FLUIDIZED BED OF INERT PARTICLES Fig.1. gives the schematic diagram of the proposed drying system. The feed material is directly supplied into the column where the inert particles are fluidized by hot air. The product is separated from the exhaust air by the cyclone and bag filters. The drying mechanism is consisted from three steps, which simultaneously occur in the different region of the bed, as