FLUIDIZED BED DRYING OF LARGE PARTICLES D. G. DiMattia, P. R. Amyotte, F. HamduUahpur ABSTRACT. Batch drying of red spring wheat in a fluidized bed was performed. Effects of bed height, gas velocity, initial moisture content and air temperature on drying rate were investigated. Wheat was conditioned to simulate high moisture contents just after harvest. Heated air was the medium for moisture removal with thermocouples located along the height of the freeboard recording bed temperatures. Samples were removed at regular intervals during drying and their moisture contents determined. It was found that the rate of moisture removal was controlled by the internal rate of diffusion and hence slugging had no effect on the drying rate. The temperature distribution within the bed was uniform due to the high degree of mixing that occurs as wall slugs are formed with wheat. At lower bed temperatures the rate of drying slowed considerably. Drying times were considerably shorter than other drying apparatus. Keywords, Fluidized beds, Drying, Wheat, Crop drying, Heat transfer A pproximately 10% of the grain grown never reaches the market due to losses during harvesting and farm storage. Many of these losses occur in the field and can be eliminated by harvesting at the proper stage of maturity followed by drying. It has been estimated that the annual cost of this loss, in the U.S., during harvesting and storage of cereal crops and hay is over $1.25 billion (Hall, 1980). The nutritional changes that occur during growth in the field, drying and storage give important guides for proper drying and handling. For example, more than five times as much carotene in alfalfa is preserved with artificial drying as compared to sun drying. By early and frequent clipping of forages, from 40 to 60% more protein can be obtained due to the extra foliage produced. Early harvest and frequent clipping combined with drying provide a very good method of preserving this protein. Also, in order to conserve a maximum amount of dry matter in hay, it is necessary to begin preservation as soon as possible after the plant is cut. The quality of gas-particle interaction and temperature distribution in a fluidized bed has significant importance in a unit operation such as the fluidized bed drying of agricultural crops. This importance is reflected in the increased efficiency of energy utilization and reduced energy consumption and hence increased economic feasibility of the unit. An example would be the drying of wheat in a fluidized bed before it is sold to the retailer. It is in the best interest of the wheat grower to dry the crop in the most efficient manner possible to maximize profits. The Article was reviewed and approved for publication by the Food and Process Engineering Inst, of ASAE. The authors are Dino G. DiMattia, Graduate Student, Paul R. Amyotte, Professor, and Feridun HamduUahpur, Professor, Technical University of Nova Scotia, Halifax. Corresponding author: Feridun HamduUahpur, Dept. of Mechanical Engineering, Technical University of Nova Scotia, PC. Box 1000, Halifax, Nova Scotia, Canada B3J 2X4; telephone: (902) 420-7660; fax: (902) 423-6711; e-mail: <hamdullf@tuns.ca>. main objective of the present study is to investigate fluidization in the slugging regime while drying wheat. Slugging can be a common occurrence in a fluidized bed utihzing large particles, and involves piston-like movement of the bed material. FLUIDIZED BED DRYING If the particles containing bound moisture are placed in a fluidizing medium of specific temperature and humidity, they will lose or gain moisture until the vapor pressure of the moisture within the particles becomes equal to its partial pressure in the medium. This moisture content, which is dependent on the humidity and temperature of the medium, is called the equilibrium moisture content. The moisture that can be removed from the particles, by the fluidizing medium, is termed free moisture. High intensity drying, uniform and closely controllable temperature throughout the bed of material, high thermal efficiencies (reduced energy costs), and lower drying times make fluidized bed dryers an attractive choice over several conventional techniques. In a fluidized bed, drying can take place in the dense phase, in the dilute phase, or when particles are entrained by the gas stream as in pneumatic drying. The particles are fed to the dryer through an entry port while the drying medium is introduced through the plenum to a distributor plate which allows the gas to be distributed evenly over the cross-section of the dryer. On leaving the freeboard (or top of the dryer) the gas may carry with it the finer fraction of the material. These fines are either produced during the drying run or are already present in the original charge. The coarser part of the bed material is removed from the bed directly, while the fines can be collected in a device such as a cyclone. Fluidized bed drying may be carried out as either a batch or continuous process. This work deals solely with a batch fluidized bed system. In batch drying the bed is filled with wet material, the flow of the drying medium is begun, and when the particles have reached the appropriate moisture content they are removed from the dryer. In a VOL. 39(5): 1745-1750 Transactions of the ASAE © 1996 American Society of Agricultural Engineers 0001-2351 / 96 / 3905-1745 1745