Studies of bed voidage and heat transfer in solid-liquid fluidized bed heat exchangers.

In recent years, there has been a considerable interest in the application of solid-liquid fluidized bed heat exchangers in various process industries. The importance of these heat exchangers arises from their ability to remove large quantities of heat per unit time and area from a hot surface. The value of the heat transfer coefficient in solid-liquid fluidized beds can be up to 8 times higher than for single phase forced convection, due to the increase in turbulence level. In addition, in cases where severe fouling is expected, any deposits that may form on the heat transfer surfaces are immediately removed by the abrasive action of the particles. Accurate prediction of hydrodynamic design parameters, such as particle terminal velocity, bed voidage, minimum fluidization velocity and its corresponding bed voidage, and heat transfer coefficient is essential for sizing solid-liquid fluidized bed heat exchangers. In recent years, solid-liquid fluidized beds are also finding application in the chemical, petrochemical and mineral process industries in which the liquid phase is viscous with non-Newtonian behavior. In this investigation, after an extensive literature review of the subjects, a complete experimental set-up for measuring particle terminal velocity, bed voidage and heat transfer was constructed. Then, using Newtonian and non-Newtonian liquids experimental results on particle terminal velocity, bed voidage, minimum fluidization velocity and heat transfer were obtained. It is necessary to mention that the term of non-Newtonian in this investigation implies shear-thinning power law liquids. New experimental data were obtained where published results were insufficient. The results were systematically analyzed and compared with previously published models. Particle terminal falling velocity was measured for different spherical and cylindrical particles with Newtonian and non-Newtonian liquids. The results show that for predicting the particle terminal velocity with Newtonian liquids, the correlation suggested by Hartman et al. (1992) has the best accuracy and is suitable. Few attempts have been made to establish the functional dependence on the particle terminal Reynolds number for solutions with non-Newtonian behavior. Therefore, by analyzing the measured data, a correlation for predicting the particle terminal velocity in non-Newtonian liquids was developed that predicts the experimental results with very good accuracy. The minimum fluidization velocity point represents the transition between the fixed and fluidized states. In this study, the minimum fluidization velocity was obtained by plotting pressure drop gradient versus liquid phase Reynolds number for fixed bed and fluidized regime. The results of the present investigation show that the correlation proposed by Wen and Yu (1966) is the best correlation for predicting the minimum fluidization velocity for Newtonian and non-Newtonian solutions if the apparent viscosity is used for Remf and Ar in fluidization with non-Newtonian liquids. Bed voidage was studied for fluidization with Newtonian and non-Newtonian liquids. The different effects of particle and liquid properties such as particle size and density, and liquid viscosity on bed voidage were studied. A new semi-theoretical model was developed which predicts the bed voidage for fluidized beds with Newtonian and non-Newtonian liquids with very good accuracy. Convective heat transfer coefficients were measured for fluidization of various particles with Newtonian and non-Newtonian liquids under constant heat flux. Different effects of operational parameters such as particle size and density, bulk temperature, liquid viscosity and bed voidage were studied. A new model that is based on the findings of this study and on previous investigations was developed for predicting the heat transfer coefficient in solid- liquid fluidized beds with Newtonian and non-Newtonian liquids. This model predicts the experimental data with good accuracy. A data bank of published fluidized bed heat transfer data was obtained and completely revised. The measured heat transfer coefficients in this study and all results in the refined data bank were compared with the predictions of 39 published correlation and also with the present model. For both Newtonian and non- Newtonian liquids, the present model out-performs all others correlations