Energy and exergy analysis of fluidized bed dryer based on two-fluid modeling M.R. Assari a, * , H. Basirat Tabrizi b , E. Najafpour c a University of Jundi Shapor, Dezful, Iran b Amirkabir University of Technology, Mech. Eng. Dept., Tehran, Iran c Mech. Eng. Dept., Dezful Branch, Islamic Azad University, Dezful, Iran article info Article history: Received 24 July 2010 Received in revised form 11 November 2011 Accepted 30 August 2012 Available online 12 October 2012 Keywords: Batch fluidized bed dryer Gasesolid flow Two-fluid model Exergy abstract Energy and exergy analysis for batch fluidized bed dryer based on the Eulerian two-fluid model (TFM) is performed to optimize the input and output and keep the quality of products in good condition. The two- fluid model is used based on a continuum assumption of each phase. Two sets of conservation equations are applied for gasesolid phases and are considered as interpenetrating continuum. Further this study considers the two-dimensional, axis-symmetrical cylindrical energy and exergy equations for both phases and numerical simulation is preformed. The governing equations are discretized using a finite volume method with local grid refinement near the wall and inlet. The effects of parameters such as: the inlet gas velocity, inlet gas temperature and the particle size diameter on the energy, exergy efficiencies and the availability of gas are sought. Two-fluid model prediction indicates good agreement between the available experimental results and reported non-dimensional correlations and other model predictions. It is illustrated that at the beginning of the drying process, the energy efficiency is higher than the exergy efficiency for a very short time. However two efficiencies come closer to each other at the final stage of the drying. Increasing particle size will decrease both efficiencies and the gas availability at the starting process. Ó 2012 Elsevier Masson SAS. All rights reserved. 1. Introduction Particle drying is an important process in food, pharmaceutical and chemical industries, which consume significant amount of energy. A large number of independent variables such as particle density, size, shape, permeability, and hygroscopicity can influence drying behavior. Fluidized bed drying is one of the most successful methods. In fluidized bed dryer most particles are suspended in a hot air or stream. Fluidized bed drying, compared with other drying techniques, offers many advantages such as higher heat and mass transfer rates due to better contact between particles and gas, uniform bed temperature due to intensive solid mixing and ease in a control of the bed temperature and operation. Collectively, their advantages result in higher drying rates. In fluidization phenom- enon, particles flow in a fluid and particle-fluid will influence each other, and cause a complex phenomenon. Most of this process occurs in conjunction with other processes such as heat transfer, mass transfer or heat and mass transfer together. The fluidized bed models can be classified into two broad groups: engineering models such as two-phase, three-phase models [1,2] and CFD based models that are based on a continuum assumption of phases [3,4]. The engineering models comprise a bubble phase without solids and a dense phase consisting of gas and solid particles. The dense phase is assumed to be well mixed; the modeling is applied to each phase separately and incorporates experimental findings by others (see Ref. [4]). Two methods have been typically used for CFD modeling of gasesolid flows, namely "EulerianeLagrangian" method and "EulerianeEulerian" approach. In the "EulerianeLagrangian" approach, the Lagrangian trajectory for the study of motion of individual particles is coupled with the Eulerian formulation for gas. The "EulerianeEulerian" or two-fluid used in the current study provides a field description of the dynamics of each phase. Researchers have conducted several numerical studies to describe fluidized bed drying process. Palancz [5] proposed a mathematical model for continuous fluidized bed drying based on the two-phase fluidization. According to this model, the fluidized bed was divided in two phases involving a bubble and an emulsion phase. Lai and Chen [6] proposed an improvement for the Palancz’s model. Hajidavalloo and Hamdullahpur [7,8] proposed a mathe- matical model of simultaneous heat and mass transfer in fluidized bed drying of large particles. They employed a set of coupled nonlinear partial differential equations based on three-phase model representing a bubble, interstitial gas and solid phase that * Corresponding author. E-mail address: mr_assari@yahoo.com (M.R. Assari). Contents lists available at SciVerse ScienceDirect International Journal of Thermal Sciences journal homepage: www.elsevier.com/locate/ijts 1290-0729/$ e see front matter Ó 2012 Elsevier Masson SAS. All rights reserved. http://dx.doi.org/10.1016/j.ijthermalsci.2012.08.020 International Journal of Thermal Sciences 64 (2013) 213e219