Fluidized Bed Dryers — Recent Advances

Abstract

Although industrial fluidized bed dryers have been used successfully for the drying of wet solid particles for many years, the development of industrial fluidized bed dryers for any particular application is fraught with difficulties such as scaling-up, poor fluidization and non-uniform product quality. Scaling-up is the major problem and there are very few good, reliable theoretical models that can replace the expensive laboratory work and pilot-plant trials. This problem is mainly due to the different behavior of bubbles and mixing regimes in fluidized bed dryers of different size. Simple transformation of laboratory batch drying data to continuous back-mixed dryers using the residence time distribution of the solids is insufficient to account for the complex flow and heat and mass transfer phenomena occurring in the bed. Although time scaling using temperature driving forces and solids mass flux for the same change in moisture content in the batch and continuous dryers has been successful in predicting moisture content profiles in the continuous dryer at the constant rate period, it does not take into account solid mixing. Two-phase Davidson-Harrison models have been used in modeling of the continuous back-mixed dryer with various degrees of success. On the other hand, the three-phase Kunii-Levenspiel model is seldom used in modeling fluidized bed dryers because it is too complex to handle. A combination of multi-phase models and residence time distribution could improve predicting power for back-mixed dryers because this combination takes into account both the bubbles and solid mixing phenomena. Incremental models were widely used to model continuous plug flow fluidized bed dryers, but the cross-flow of drying medium has not been sufficiently modeled except by the author. In some incremental models, axial dispersion is modeled using the Peclet number, Pe. A combination of an incremental model with an axial dispersion and cross-flow model of drying medium would improve predicting power. Poor fluidization of Geldart group C particles could be improved by the assistance of external means such as vibration, agitation, rotation and centrifugation. Both vibrated and agitated fluidized bed dryers have been successfully used in industry, but rotating or centrifugal fluidized bed dryers are still not available for industrial use. Nomenclature A bed area (m 2 ) E(t) RTD curve (-) ε bed porosity (-) F dry basis solid flow rate (kg/h) f falling rate drying factor ( • C) G dry basis gas flow rate (kg/h) h bed height (m) L total length of dryer (m) m B mass of bed (kg) N V local solids dying rate (kg/h/m 2 ) NTU number of transfer units (-) Nu nusselt number (-) Pe Peclet number (-)Pr Prandtl number (-) Re p particle Reynolds number (-) T G local gas temperature ( • C) T GI initial gas temperature ( • C) T i inlet gas tenmperature ( • C) T o exhaust gas temperature ( • C) T S local solids temperature ( • C) T wb wet bulb temperature ( • C) t time (s) τ residence time (s) X dry basis solids moisture content (kg/kg) X i inle...

Modeling the Drying Kinetics of Green Bell Pepper in a Heat Pump Assisted Fluidized Bed Dryer

“…Fluidized bed drying is a novel and efficient method which makes it feasible to dry food products in continuous and moderate conditions. High rate of heat transfer makes drying a thrifty process; on the other hand, no need for various mechanical parts for drying reduces maintaining costs (Wan Daud ). It is very difficult to develop a complete mathematical model for predicting dependent variables of a process considering all independent variables.…”

Development of a Fluidized Bed Dryer for Drying of a Sago Bagasse

“…Thus, higher inlet temperatures of drying air should be met, which leads to shorter drying times. However, the product quality considerations limit the applicable rise to the air temperature (Mujumdar, 1995;Daud, 2008). Excessive hot air can almost completely dehydrate the sago bagasse surface, making its pores shrink and almost close, leading to crust formation or "case hardening", which is usually undesirable.…”

Investigation of the Convection Drying Process in a Multistage Apparatus with a Differential Thermal Regime

“…Various energy-consuming methods are applied for the disperse material drying [2]. Even though energy-consumption in convection drying is not the lowest, the convection dryers are often used for the disperse materials due to numerous advantages [3,4]. An active hydrodynamic regime used in such dryers contributes to the intensification of the process without reducing its cost efficiency, and it has the following advantages [5,6]: -the hydrodynamic sustainability of the process; -an increase in the relative velocity of the interacting phases motion; -a developed surface of the contacting phases interaction; -the approximation of a hydrodynamic model of flows in the apparatus to an ideal displacement model; -the reduced energy-consumption of the process and a lower specific amount of metal in apparatuses.…”