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significant number of researchers have devoted their interest to fluidized bed drying and a good number of papers have been A published in the area, covering theoretical as well as experimental aspects. In this type of process three different tasks have to be accomplished by gas fluidization: establish proper dynamic fluidization conditions, bring enough heat, and remove all evaporated moisture without saturation. A good contact between particles and fluid provides very high rates of heat and mass transfer in the bed. Also, through a very good level of solids mixing a practically homogeneous bed temperature is achieved to facilitate temperature control. Due to these qualities, fluidization is considered one of the most appealing processes in the chemical industry. A particular type of fluidization, spouted bed, has also been used for drying grain over the last 40 years.However, the use of the fluidized bed in the drying process has been abandoned in some fields, such as in the fine chemicals and the pharmaceutical industries, because of complications with thermal degradation and safety. Thermolabile products can be subject to the partial degradation in drying processes at relatively high temperatures or long residence times. At the same time, in the pharmaceutical industry the process is oftenly involved with evaporation of an organic solvent; as a result, a mixture within the flammability limits but with a high risk of explosion can be formed. Operating a fluidized bed under vacuum conditions offers the possibility of eliminating these problems. The degree of thermal degradation can be reduced by lowering operating temperatures provided under vacuum conditions. At the same time, a safer process outside of flammability limits can be achieved through low-pressure operation.Nevertheless, relatively few papers have been published on fluidization under vacuum conditions. In one of the first studies on the hydrodynamics of fluidized beds at reduced pressure, a bed behaviour similar to that found at atmospheric pressure was observed by Kawamura and Suezawa (1961). Cermain and Claude1 ( 1 976) detected the coexistence of an upper fluidized layer and a zone of fixed bed. The existence of a fluidized upper layer in the bed was also observed by Kusakabe et al. (1989), at the point when the bed pressure drop and the absolute operating pressure basically have the same order of magnitude. In their low pressure experiments, Wraith and Harris (1 992) measured the same type of pressure drop characteristics and observed a fluidization front progressing downwards with increasing gas flow. In a later study, Llop e t al. (1996) proposed an equation to calculate the minimum *Author to whom correspondence may be addressed. E-mail address: bulent@mail. udlap. my A drying process in a fluidized bed under vacuum conditions is experimentally and theoretically studied. A fluidized bed operating in reduced pressure presents a possibility for a better quality of production, with safer process conditions. A mathematical model is developed for...

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