Convective drying of particulate solids – Packed vs. fluid bed operation Milan Stakic ´ ⇑ , Predrag Stefanovic ´ , Dejan Cvetinovic ´ , Predrag Škobalj Laboratory for Thermal Engineering and Energy, University of Belgrade – Institute of Nuclear Sciences ‘‘Vinc ˇa’’, Belgrade, Republic of Serbia article info Article history: Received 25 September 2012 Accepted 27 November 2012 Available online 2 January 2013 Keywords: Fine-grained hygroscopic materials Heat and mass transfer Drying kinetics Drying coefficient Modeling abstract The paper addresses results for the case of convective drying of particulate solids in a packed and in a fluid bed, analyzing agreement between the numerical results and the results of corresponding experi- mental investigation, as well as the differences between packed and fluid bed operation. In the fluid bed simulation model of unsteady simultaneous one-dimensional heat and mass transfer between solids, gas phase and bubble phase during drying process, based on two-phase bubbling model, it is assumed that the gas–solid interface is at thermodynamic equilibrium. The basic idea is to calculate heat and mass transfer between gas and particles (i.e., the drying process) in suspension phase as for a packed bed of particles, where the drying rate (evaporated moisture flux) of the specific product is calculated by apply- ing the concept of a ‘‘drying coefficient’’. Mixing of the particles (i.e., the impact onto the heat and mass transfer coefficients) in the case of fluid bed is taken into account by means of the diffusion term in the differential equations, using an effective particle diffusion coefficient. Model validation was done on the basis of the experimental data obtained with narrow fraction of poppy seeds characterized by mean equivalent particle diameter (d S,d = 0.75 mm), re-wetted with required (calculated) amount of water up to the initial moisture content (X 0 = 0.54) for all experiments. Comparison of the drying kinetics, both experimental and numerical, has shown that higher gas (drying agent) temperatures, as well as velocities (flow-rates), induce faster drying. This effect is more pronounced for deeper beds, because of the larger amount of wet material to be dried using the same drying agent capacity. Bed temperature differences along the bed height are significant inside the packed bed, while in the fluid bed, for the same drying con- ditions, are almost negligible due to mixing of particles. Residence time is shorter in the case of a fluid bed drying compared to a packed bed drying. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Drying is an energy-intensive operation. Additionally, conven- tional dryers often operate at low thermal efficiency, typically be- tween 25% and 50%, but it may be as low as 10% [1,2]. The increase in energy costs, as well as the adoption of more strict safety and envi- ronmental regulations, initiated an increasing interest in designing energy-saving systems all over the conventional chemical industry. Furthermore, in the case of industrial dryers, the wide variety of products increases the concern to meet high quality specifications. Therefore, the need for optimal management of energy during dry- ing, with the demand for high quality products, leads to the develop- ment of control strategies for the drying plants studied. Fluid bed processing of biological materials and food involves drying, cooling, agglomeration, granulation, and coating of particu- late materials. Fluid bed dryer is used because of the shorter drying time required and simple maintenance and operation. It is ideal for a wide range of both heat sensitive and non-heat sensitive prod- ucts. Uniform processing conditions are achieved by passing a gas (usually air) through a product layer under controlled velocity conditions to create a fluidized state. Heat is supplied by the fluid- ization gas, but it can be also effectively introduced by heating sur- faces (panels or tubes) immersed in the fluidized layer. Fluid bed drying offers important advantages over other methods of drying particulate materials such as easy material transport, high rates of heat exchange at high thermal efficiency while preventing over- heating of individual solids. The properties of a given product are determined from drying kinetics data, i.e., moisture content and temperature changes with time under controlled conditions. Other important properties are dimension, shape and density of the solids, fluidization gas veloc- ity, fluidization point (minimum fluidization velocity), equilibrium moisture content (desorption isotherms), and heat and mass trans- fer coefficients. These and other data are applied in a computa- tional model of fluid bed processing, thus enabling dimensioning of industrial drying systems. Much work has been done to model and analyze both continuous and batch fluid bed dryers [3–12]. Each of the models has its own 0017-9310/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijheatmasstransfer.2012.11.078 ⇑ Corresponding author. Address: Institute of Nuclear Sciences Vinc ˇa, P.O. Box 522, 11001 Belgrade, Republic of Serbia. Tel.: +381 11 340 836. E-mail addresses: mstakic@vinca.rs (M. Stakic ´), pstefan@vinca.rs (P. Stefanovic ´), deki@vinca.rs (D. Cvetinovic ´), p.skobalj@vinca.rs (P. Škobalj). International Journal of Heat and Mass Transfer 59 (2013) 66–74 Contents lists available at SciVerse ScienceDirect International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt