A Computational Study of Surface to Bed Heat Transfer and Reactive Flows in Gas Fluidized Beds
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This thesis presents the study of surface to bed heat transfer and reactive flows in dense phase gas-fluidized beds. The bulk of the thesis comprises of computational studies although some experimental work is also done in order to validate the model predictions. The computations are performed on the three dimensional, finite volume in-house code FLOTRACS-MP-3D in a Cartesian coordinate system. The in-house code is based on the Eulerian-Eulerian approach which treats the gas and solid phases as interpenetrating continua. The in-house code has evolved as a result of previous hydrodynamic studies conducted at Telemark University College on dilute gas-particle flows that are observed in circulating fluidized beds. Since the main focus of this work is heat transfer and reactive flows in gas fluidized beds, thermal energy and species balance equations are added to the in-house code in order to enable heat transfer and reactant conversion calculations. A frictional stress model is also added to the inhouse code in order to handle dense solid regions encountered in bubbling beds where sustained contacts between the particles give rise to tangential and normal frictional stresses. The main part of the thesis comprises of three original papers. A fourth paper which reviews the empirical and mechanistic models for convective heat and mass transfer in gas-fluidized beds is presented as an appendix. In paper I, the Eulerian-Eulerian approach is used to predict wall to bed heat transfer coefficient in a gas-solid fluidized bed with a jet by a heated wall. The constant viscosity model (CVM) and kinetic theory of granular flows (KTGF) are used to describe the solid phase rheology. In order to validate the model predictions a pseudo two dimensional fluidized bed setup is constructed. A solid phase molecular thermal conductivity model specifically developed for the near wall region is used in the present work since wall to bed heat transfer occurs through the particle layer in contact with the wall. The predicted heat transfer coefficient with the inclusion of this thermal conductivity model into the Eulerian-Eulerian fluid dynamic model shows better agreement with the measured values as compared to previous works. A comparison of the predicted and measured heat transfer coefficient is presented for different jet velocities, particle sizes and particle types and good agreement is observed between the predicted and measured values. It is observed that the predicted heat transfer coefficient is not affected significantly by the drag model or solid phase rheology model (CVM or KTGF) provided all other model parameters and operating conditions are same. Additionally for KTGF, over-prediction of heat transfer coefficient is observed in the case where solid phase thermal conductivity is expressed in terms of granular temperature rather than molecular conduction. Inclusion of particle rotation in the KTGF model reduces this over-prediction by around 17% due to lower granular temperature as energy dissipation increases due to frictional losses not accounted for in the original KTGF model without rotation. Paper II presents a two dimensional Eulerian-Eulerian simulation of tube-to-bed heat transfer for a cold gas-fluidized bed with immersed horizontal tubes. The horizontal tubes are modelled as obstacles with square cross section in the numerical model. Simulations are performed for two gas velocities exceeding the minimum fluidization velocity by 0.2 m/s and 0.6 m/s and two operating pressures of 0.1 MPa and 1.6 MPa. Local instantaneous and time averaged heat transfer coefficients are monitored at four different positions around the tube and compared against experimental data reported in literature. The role of constitutive equations for the solid phase thermal conductivity on heat transfer is investigated and a fundamental approach to model the solid phase thermal conductivity is implemented in the present work. Significant improvement in the agreement of the predicted and the measured local instantaneous heat transfer coefficient is observed in the present study when compared to the previous works which over predicted the local instantaneous heat transfer coefficient. At the atmospheric pressure, the local instantaneous heat transfer coefficient shows good agreement with the measured values both qualitatively and quantitatively. At the higher pressure, similar to the measurements, the predicted local instantaneous heat transfer coefficient shows an increase with increasing pressure. However, the agreement is more qualitative and quantitatively the maxima and minima of the local instantaneous heat transfer coefficient are under predicted. The local time averaged heat transfer coefficients are within 20 % of the measured values at the atmospheric pressure. In contrast, under prediction of the time averaged heat transfer coefficient is observed at the higher pressure. Paper III presents a computational study of flow behaviour and conversion in a freely bubbling bed of porous cracking catalyst particles fluidized by a mixture of ethylene and hydrogen on the in-house code FLOTRACS-MP-3D. The solid phase viscosity and pressure are modelled on the basis of kinetic theory of granular flows (KTGF). An effective solid density is calculated to account for the inherent porosity of particles. The cohesive interparticle forces are incorporated into the CFD model by using an empirical approach proposed in literature. Qualitatively, the CFD model captures the flow behaviour and heat transfer in the bed quite well. On the quantitative front, the variation of conversion with gas velocity is predicted fairly well with the deviation between the predicted and measured conversion remaining within 20 %.