The Mixing Of The Fluidized Bed Dryer Hydrodynamics And Heat Transfer Performance

The term "agitated fluidized bed (AFB)" generally refers to a type of fluidized bed equipped with mechanical agitation. In this paper, results of an experimental study on the aerodynamic and heat transfer characteristics of an agitated fluidized bed dryer (AFBD) is presented and discussed.Firstly, aerodynamic behavior in AFB of powders in group A, B and D of Geldart's powder groups is discussed. For each powder group, both sticky and non-sticky materials are used. It can be seen that when an agitation is used, the pressure drop AP is reduced and Umf is increased in the fluidization of powders of group B or group D. Agitation can help reduce channeling in the fluidized bed for most cases. The use of slow speed of stirring can help to initiate fluidization of powders of group A, which otherwise can hardly be fluidized. Variables such as agitator rotational speed N, height of the bed H, particle size dp and particle density pp are discussed. According to the results, and increase in N results in a reduction of AP and an increase in Umf. The effect of agitator rotational speed N on AP becomes insignificant when the height of the bed H is more than some certain quantity. The notion of "effective area of the agitator" is proposed in order to explain this phenomenon. It is found that for a certain fluidized bed, the effective area of the agitator depends on form and size of the impeller, height of the bed H and characteristics of the particulate, etc. A correlation of Umf is proposed as follows:Remf = 0.048Ar0.783Fr0.0803Secondly, drying characteristics of AFBD in the constant drying period is studied. It is found that drying rate in the constant drying period is higher in an AFBD than in a fluidized bed without agitation when the agitator rotational speed is slow. The effect of variables such as air temperature, air velocity, agitator rotational speed and particle size on the heat transfer coefficient between air and particles are investigated experimentally, it is seen that the air temperature has no evident effect on the heat transfer coefficient, but the heat transfer coefficient increases with the air velocity almost linearly. The results also indicate the possible existence of an optimum agitator rotational speed, which would give the largest heat transfer coefficient. A correlation of heat transfer coefficient is proposed as follows:Nu = 0.0337Re0.6317 Fr-0.1251(H/dp_-1.1345The calculated results agree well with the experimental ones.