CFD simulation of hydrodynamics of gas–liquid–solid fluidised bed reactor (original) (raw)
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Processes, 2019
In this work, we employed a computational fluid dynamics (CFD)-based model with a Eulerian multiphase approach to simulate the fluidization hydrodynamics in biomass gasification processes. Air was used as the gasifying/fluidizing agent and entered the gasifier at the bottom which subsequently fluidized the solid particles inside the reactor column. The momentum exchange related to the gas-phase was simulated by considering various viscous models (i.e., laminar and turbulence models of the re-normalisation group (RNG), k-ε and k-ω). The pressure drop gradient obtained by employing each viscous model was plotted for different superficial velocities and compared with the experimental data for validation. The turbulent model of RNG k-ε was found to best represent the actual process. We also studied the effect of air distributor plates with different pore diameters (2, 3 and 5 mm) on the momentum of the fluidizing fluid. The plate with 3-mm pores showed larger turbulent viscosities above the surface. The effects of drag models (Syamlal-O'Brien, Gidaspow and energy minimum multi-scale method (EMMS) on the bed's pressure drop as well as on the volume fractions of the solid particles were investigated. The Syamlal-O'Brien model was found to forecast bed pressure drops most consistently, with the pressure drops recorded throughout the experimental process. The formation of bubbles and their motion along the gasifier height in the presence of the turbulent flow was seen to follow a different pattern from with the laminar flow.
CFD Modeling of Bubble Rise Velocity in Fluidized Bed Reactor
In this research the ability of computational fluid dynamics to predict bubble diameter and bubble rise velocity with different particle diameters in a gas-solid fluidized bed reactor has been investigated. A multi fluid Eulerian model incorporating with the KTGF has been applied to simulate the unsteady-state behavior of this reactor. Results have been compared with some correlations. Pressure drop distributions predicated by CFD at different particle diameter were in a good agreement with correlations. The CFD results reveal that smaller bubbles are produced at the bottom of the bed. It has been found that the bubble diameter and bubble rise velocity are sensitive to particle diameter.
Hydrodynamic Studies of Three-Phase Fluidized Bed by Experiment and CFD Analysis
2013
The complex hydrodynamics of three-phase (gas-liquid-solid) fluidized beds are not well understood due to complicated phenomena such as particle-particle, liquid-particle and particle-bubble interactions. In the present work both experimental and computational studies have been carried out on two and three dimensional fluidized beds to characterize there hydrodynamic behavior. Air, water and low density solid particles have been used as the gas, liquid and solid phase to analyze the system behaviors. Eulerian multi-phase model has been used to simulate the system by using the commercial CFD code ANSYS Fluent 13.0. Gidaspow and Schiller-Neumann drag models have been used to calculate inter-phase drag force. Two-equation standard k-e model has been used to describe the turbulent quantities. CFD simulation of three-phase fluidized bed systems with a distributor plate is not seen in literature. In the present work fluidized bed with distributor having orifice diameter 0.002 m has been s...
Hydrodynamic Model for the Gas Flow in Circulating Fluidized Bed Reactors
Industrial & Engineering Chemistry Research, 2002
The circulating fluidized bed (CFB) riser may be regarded as composed of two hydrodynamically distinct regions: in the central part, the "core", particles, and gas are transported upward in a cocurrent dilute flow with a fairly high voidage. In the wall zone or "annulus", particles are, on average, transported downward, countercurrently to the gas flow: the voidage in this region is significantly lower than that in the central region. The hydrodynamics of the gas phase in CFBs are of dominant importance for predicting the conversion of CFB chemical reactions. Both the core plug-flow gas and the annulus backmixing determine the average residence time, the residence time distribution, and hence the reaction yield. The scope of this paper is to develop a model to predict the residence time and the residence time distribution of the gas phase. Values of underlying parameters are selected from fitting experimental data and model predictions. The model contains two dominant parameters: the exchange coefficient for the gas phase, k g , and the exchange coefficient for the solids between the annulus and core, K AfC. The model is evaluated for different values of these coefficients so that the predicted response curve for the concentration in the gas phase is in best possible agreement with the measured one. The values of both parameters are given and compared with literature data. To complete the assessment, a sensitivity analysis was performed.
Asia-Pacific Journal of Chemical Engineering, 2018
The riser of conventional and stage circulating fluidized bed reactors were simulated using computational fluid dynamics. The objective was to examine the design parameter effects using a 2 3 statistical experimental design method. The results showed that number of reactor stages had the highest effect on the standard deviation of the solid volume fraction (SVF), the average SVF, and standard deviation of gas temperature in a horizontal direction. In addition, the reactor wall temperature had the highest effect on the average holding time of catalysts. Increasing the number of reactor stages could reduce the back-mixing and increase the system turbulence. The average vertical and horizontal solid particle velocities in the reactor stage region were higher than in the other regions. At the reactor stage region, peaks in the gas temperature were observed. The advantages of a stage reactor were confirmed to be applied in all the operating condition cases, independent of the solid particle mass flux.
Gas phase hydrodynamics of a gas-solid turbulent fluidized bed reactor
Chemical Engineering Science, 1996
Transient gas mixing tests in a gas--solid fluidized bed are analyzed with a dispersive plug flow model from the bubbling to the turbulent regime, and with the two-phase model of van Deemter. It is shown that dispersion fluctuations can also be used besides pressure fluctuations or capacitance probes to characterize the transition between the bubblir~g and the turbulent regime. Furthermore, the gas Peclet number in the turbulent regime is correlated in terms of the operating parameters, the bed diameter and the particles and gas properties as:
Chemical Engineering Research and Design, 2022
Deep understanding of the complex relationship between the complex hydrodynamics and reactor performance in a reactive gas-fluidized bed is crucial to optimization of engineering design and industrial operation. In this work, multiphase particle-in-cell (MP-PIC) simulations coupled with reaction kinetics models are conducted to investigate the effect of gas distribution on reactor performance. A batch fluidized bed reactor with different distributors for regenerating spent FCC catalyst is simulated under different superficial gas velocities. In all simulation cases, the air flowrate and the spent catalyst inventory are kept constant. The results show an indiscernible effect of superficial gas velocity, but the induced-maldistribution conditions generally deteriorate the reactor performance. In addition to wall-slug formation, solids holdup, and slip velocities also decrease significantly in the plugged-gas distribution cases. Strong linear relationship between hydrodynamics and reactor performance are established. Present study sheds light on the importance of uniform gas distribution in industrial fluidized bed reactors.
2 D Simulations of Bubbling Gas-Solid Fluidized Bed Reactors
2005
This paper discusses the simulation of bubbling gas-solid flows by using an Eulerian modeling approach. Two closure models, the constant particle viscosity model (CPV) and a model based on kinetic theory of granular flow (KTGF), are compared in performance to describe the time-averaged velocity fields and bed expansion in both a circular and rectangular column. The time averaged velocity fields and bed expansion obtained in the simulations are compared with experimental data obtained by Lin et al. (1985) for validation. In this work an in-house code has been developed based on the finite volume and the fractional step approach using a staggered grid arrangement. The velocities in both phases are obtained by solving the two-dimensional Reynolds-averaged Navier/Stokes equations using a partial elimination algorithm (PEA) and a coupled solver. The k-epsilon turbulence model is used for the continuous phase. Axi-symmetric simulations using the KTGF model gives results in fairly well agr...
International Journal of Thermal Sciences, 2018
In this article, the effects of operating temperature on the hydrodynamics of dense gas-solid flow inside the fluidized bed reactor are investigated systematically. To this end, 3D simulations have been carried out by incorporating the appropriate model parameters and using the well-known Euler-Euler two fluid methodology in ANSYS FLUENT. The methodology is validated against the experimental results available in the literature. Temperature, air velocity and particle sizes are varied systematically. Results show that the variation of minimum fluidization velocity with temperature depends upon the particle size. For small particles (Geldart's group B), the minimum fluidization velocity decreases with an increase in temperature and the trend is reversed when large particles (Geldart's group D) are fluidized. This behavior is explained by analyzing the modification in inter-phase momentum exchange coefficient due to temperature variation. Voidage profiles, particle velocity and bubble characteristics are also seen to be influenced by the thermal conditions of the fluidized bed reactor. Particle axial velocity tends to decrease with an increase in the operating temperature. For group B particles, temperature has negligible effect on bubble size and expanded bed height. On the other hand, bubble size and expanded bed height decrease with an increase in temperature for group D particles.
Computer Simulation of Flow Processes in Fluidized Bed Reactors
Kona Powder and Particle Journal, 1992
The paper outlines and exemplifies a multi-dimensional multi-phase Computational Fluid Dynamics (CFD) model for the various processes that occur in fluidized bed reactors. The model is based on the Eulerian description of the two phases: gas and particles. This means that separate conservation equations are set up for these phases. Calculations are shown for some examples that include flow patterns in circulating beds.