Two‐Phase Sequential Simulation of a Fluidized Bed Reformer

A two‐phase flow model is adapted in order to predict the performance of a fluidized bed reformer using the sequential modular simulator. Since there are physical and chemical phenomena interacting in the reformer, two sub‐models appear to be necessary to describe the overall model. These are the hydrodynamic and reaction sub‐models. The hydrodynamic sub‐model is based on the dynamic two‐phase model and the reaction sub‐model is derived from the literature. In the overall model, the bed is divided into several sections. At each section, the flow of the gas is considered as plug flow through the bubble phase and to be perfectly mixed through the emulsion phase. Two sets of experimental data from the literature at different hydrodynamic regimes were used in order to validate the proposed model. A close agreement was observed between the model predictions and the experimental data. The model proposed in this work may be used as a framework for the development of sophisticated models for non‐ideal reactors inside process simulators.     

Flue Gas Desulfurization by a Powder‐Particle Spouted Bed

The powder‐particle spouted bed process is one of the semi‐dry processes that have been developed for flue gas desulfurization. In this study, which is designed for SO₂ removal by a powder‐particle spouted bed, the reaction term is included in one‐dimensional and streamtube models that were presented previously for spouted beds. Hydrated lime is used as the sorbent in this process. The predictions of the models are compared with some published experimental data and it is found that the developed models are valid. The results of two models are compared with each other and their various properties are evaluated. The effects of different operating conditions on SO₂ removal efficiency are also investigated and preferred operating conditions are discussed.     

Behaviour of Cohesive Powders in a Powder‐Particle Spouted Bed

Behaviour of cohesive powders in a powder‐particle spouted Bed (PPSB) has been investigated under several operating variables and solids properties. The elutriation rate constant based on the hold‐up of fine powders in the bed decreased with a decrease in the size of fine powders, and with an increase in the size of coarse particles under a constant superficial gas velocity. This finding is quite different from the elutriation phenomena of particles more than 100 µm in size. Moreover, the mean residence time of fine powders increased with a decrease in the superficial gas velocity and the size of fine powders, and decreased with a decrease in the size of coarse particles.     

Numerical Investigation of the Gas‐Solid Flow Characteristics in a Three‐Dimensional Spouted Bed with a Draft Tube

Gas‐solid motions in a three‐dimensional conical spouted bed with a draft tube are investigated based on a simulation carried out by the coupling approach of computational fluid dynamics combined with the discrete‐element method. The distribution properties of the velocity, the concentration, and the flux of the solid phase are discussed. The vertical solid velocity in the central region initially increases, diminishes gradually, and finally decreases sharply in the region above the draft tube. Vigorous lateral solid motion occurs in the periphery of the fountain and the spout‐annulus interface. In addition, the vertical solid flux shows a large value in the spout. A larger vertical velocity but a more dilute solid concentration can be detected along the axial direction when enlarging the gas flow rate.     

Computational Fluid Dynamics Modeling of Gas‐Liquid Two‐Phase Flow around a Spherical Particle

Microscale studies, which can provide basic information for meso‐ and macroscale studies, are essential for the realization of flow characteristics of a packed bed. In the present study, the effects of gas velocity, liquid velocity, liquid‐solid contact angle, and liquid viscosity on the flow behavior were parametrically investigated for gas‐liquid two‐phase flow around a spherical particle, using computational fluid dynamics (CFD) methodology in combination with the volume‐of‐fluid (VOF) model. The VOF model was first validated and proved to be in good agreement with the experimental data. The simulation results show that the film thickness decreases with increasing gas velocity. This trend is more obvious with increasing operating pressure. With increasing liquid velocity, the film thickness tends to be uniform on the particle surface. The flow regime can change from film flow to transition flow to bubble flow with increasing contact angle. In addition, only at relatively high values does the liquid viscosity affect the residence time of the liquid on the particle surface.     

Numerical Simulation of a Spouted Bed Using Computational Fluid Dynamics (CFD)

In this article, computational fluid dynamics (CFD) technology is used to model a spouted bed(SB). The multifluid Eulerian-Eulerian approach based on kinetic theory of granular flows and Gidaspow's drag model for the interaction between gas and particles are applied in the modeling. The effects of the SB properties—that is, cone angle, particle size, cylinder diameter, and static bed height of particles—on its dynamics performance are investigated. The simulated results—that is, flow pattern of particles, fountain height, voidage, and particle velocity of the spout zone—are presented. It is shown that periodic fluctuation of spouting appears in an SB with conical angle of 30° and inlet velocity at 16.6 m/s. When the SB cylinder diameter becomes 0.52 m, periodic fluctuation appears, too. The stable spouting of the SB with a 90° cone angle could be obtained at an inlet air velocity of 24.3 m/s. The fountain height of particles decreased with an increase in particle size and the static bed height of particles. It is kept at about 0.19 m when different SB cylinder diameters in the range of 0.36 to 0.48 m are used. In the spouting region, the voidage decreased with static particle height in bed, but the particle velocity increased. For a certain particle size, the voidage decreased with an increase in particle height, but the velocity of the particles increased. It was also found that the cylinder diameter did not affect the volume fraction of particles except for the cylinder diameter 0.52 m and the change in particle velocity was minimal in the spout zone. With the different static bed height of particles used, the voidage and particle velocity did not change much at the same level of spout zone.     

Fluid Dynamics of Spouted Bed Apparatus with Two Fluid Inlets: An Experimental Study

Spouted bed apparatuses are already used in some technical areas for a variety of chemical and metallurgical operations. One of the advantages of spouted beds compared to the fluidized beds is the possibility of achieving better conditions for an intense heat and mass transfer. Spouted beds are characterized by a relatively simple construction and an easy design, which allows the scale‐up of the spouted bed apparatuses and the creation of industrial equipments.     

Analysis of Fluid Dynamics Behavior of Conical Spouted Bed in Presence of Pastes

Abstract

A conical spouted bed (CSB) was employed to obtain the drying and fluid dynamics data in the presence of egg paste and glycerol for a range of operating conditions. In this case, egg paste was added continuously into the bed at several feed rates, whereas glycerol was fed in batch process. Under these conditions, minimum spouting velocity, pressure drop, fountain height, paste concentration, and film moisture content were evaluated. The results showed that ΔP_s and U_ms decreases with increasing glycerol concentration; however, U_ms presented an opposite behavior for egg paste. Fountain height increases with increasing glycerol into the bed. Film moisture content and homogenized egg paste concentration were maintained uniformly scattered at all points of the bed. In addition, the methodology proposed by Schneider and Bridgwater^{[ 1} Schneider , T. ; Bridgwater , J. The stability of wet spouted beds . Drying Technology 1993 , 11 ( 2 ), 277 – 301 .[Taylor & Francis Online], [Web of Science ®] , [Google Scholar] ^] showed itself to be appropriate for predicting stable spouting regimes. Furthermore, most of the experimental data of this study was compared to other results and to some empirical and mechanistic models reported in literature. Thus, our experimental data could be well described by an empirical model like Passos and Mujumdar's^{[ 2} Passos , M.L. ; Mujumdar , A.S. Effect of cohesive forces on fluidized and spouted beds of wet particles . Powder Technology 2000 , 110 , 222 – 238 . [CROSSREF] [Crossref], [Web of Science ®] , [Google Scholar] ^] model. On the other hand, the mechanistic model proposed by Cunha^{[ 3} Cunha , F.O. Contributions to the analyses of drying of pastes in spouted bed with inert particles . M.Sc. Thesis , Federal University of Sao Carlos , Brazil . 1999 . ( In Portuguese ). [Google Scholar] ^] overestimated the theoretical minimum spouting velocity at a range of values of 10 to 25%.     

Hydrodynamic Characteristics of a Powder‐Particle Spouted Bed with Powder Entrained in Spouting Gas

Spouting of 3.7 mm polyvinyl chloride particles in a cone‐based cylindrical column is subjected to entrainment of FCC powder in the spouting air. It is found that the powder entrainment reduces the minimum spouting velocity, increases the bed pressure drop and reduces the maximum spoutable bed height. At any given bed height and value of U/U_ms, there is a critical value of powder loading ratio above which spouting gives way to slugging.     

Modeling of Gas‐Particle Turbulent Flow in Spout‐Fluid Bed by Computational Fluid Dynamics with Discrete Element Method

Gas and solid turbulent flow in a cylindrical spout‐fluid bed with conical base were investigated by incorporating various gas‐particle interaction models for two‐way coupling simulation of discrete particle dynamics. The gas flow field was computed by a k‐ϵ two‐equation turbulent model, the motion of solid particles was modeled by the discrete element method. Drag force, contact force, Saffman lift force, Magnus lift force and gravitational force acting on individual particles were considered in the mathematical models. Calculations on the cylindrical spout‐fluid bed with an inside diameter of 152 mm, a height of 700 mm, a conical base of 60° and the ratio of void area of 3.2 % were carried out. Based on the simulation, the gas‐solid flow patterns at various spouting gas velocities are presented. Besides, the changes in particle velocity, particle concentration, collision energy, particle and gas turbulent intensities at different proportions of fluidizing gas to total gas flow are discussed.     

Influence of Operating Parameters on the Average Spout Width in Two‐Dimensional Spouted Beds

In this work a set of experiments were performed in a two‐dimensional spouted bed to study the effect of different operating conditions on the spout cavity width. The measurement of spout‐annulus interface along the bed level was made by visual observations on a flat face column. A numerical integration formula was used to calculate the average spout width. The experimental runs were performed with different materials, bed dimensions, static bed heights, fluid velocities and gas inlet orifice width. The influences of these variables on average spout width were verified through a statistical analysis. An empirical correlation is proposed to predict the average spout width. The empirical model parameters were found through a statistical analysis.     

Two‐Dimensional Model for Oxidative Coupling of Methane in a Packed‐Bed Membrane Reactor

A two‐dimensional (2D) model of a packed‐bed membrane reactor was developed to describe ethylene production by oxidative coupling of methane (OCM). The model covers all relevant energy and mass transport processes in the reactor and allows a more precise prediction of the temperature and conversion patterns. It is demonstrated that the fast OCM reaction leads to oxygen depletion in the vicinity of the membrane, causing strong radial concentration gradients in the bed. Further results indicate that the detailed 2D model can provide more accurate predictions of experimental data than the simplified one.     

Voidage Profiles in a Slot‐Rectangular Spouted Bed

Local voidages were determined using optical fibre probes for 1.4 and 2.4 mm glass beads in slot‐rectangular spouted beds of width 150 mm. The effects of air inlet flow rate, slot width and bed thickness were investigated. Spout widths were determined from standard deviations of local voidage fluctuations. Both spout and annulus voidages increased with increasing air flow. Annulus voidages were usually higher than corresponding loose‐packed voidages and decreased with increasing height. Annulus and spout voidages were very sensitive to the air entry slot width and particle diameter. With increasing height, spouts became increasingly circular (three‐dimensional), despite the initial rectangular geometry.     

Effect of a Draft Tube on the Fluid Dynamics of a Spouted Bed: Experimental and CFD Studies

Knowledge about the gas and particle dynamics in spouted beds is important in the evaluation of particle circulation rates and the efficiency of gas-solid contacts. In this work, the mechanism of transition from a static bed to a spouted bed was numerically simulated using a Eulerian multiphase model. This model was applied to two distinct spouted bed geometries: a conventional device and a spouted bed with draft tube. The radial voidage and particle velocity profiles along the longitudinal position in the annular and spout regions were simulated for the geometries under study. The characteristic simulated curves were congruous with the experimental data.     

Effects of Draft Tubes on Particle Velocity Profiles in Spouted Beds

The vertical particle velocity profiles in a full‐column cylindrical conical spouted bed, with or without a draft tube, are measured using a fibre optic probe system. The profiles have different characteristics for a draft tube spouted bed (DTSB) than for a conventional spouted bed (CSB). The spout of a CSB consists of a central flow where particle velocities fit exponential distributions, and a boundary layer where particle velocities are nearly uniform. The spout of a DTSB has no boundary layer and its radial particle velocity profiles are approximately linear. The particle velocities in the spout of a DTSB increase when superficial gas velocity increases, draft tube diameter decreases, or when entrainment height decreases. A kinematic model has been used to simulate the granular flow in the annulus of a CSB and DTSB, and they are compared with the experiments. The particle velocities in the annulus of a DTSB are much lower than that of a CSB. Their radial profiles are also different with a CSB. The dependence of particle velocities in the annulus of a DTSB on superficial gas velocity, draft tube diameter, and entrainment height are also discussed. One concludes that the draft tube diameter and entrainment height are two key factors for the solid circulation rate of a DTSB.     

Three‐Dimensional CFD Simulation of Stratified Two‐Fluid Taylor‐Couette Flow

Two‐fluid Taylor‐Couette flow, with either one or both of the co‐axial cylinders rotating, has potential advantages over the conventional process equipment in chemical and bio‐process industries. This flow has been investigated using three‐dimensional CFD simulations. The occurrence of radial stratification, the subsequent onset of centrifugal instability in each phase, the cell formation and the dependency on various parameters have been analyzed and discussed. The criteria for the stratification, Taylor cell formation in each phase have been established. It can be stated that the analysis of single‐phase flow acts as the base state for the understanding of radial stratification of the two‐fluid flows. The extent of interface deformation also has been discussed. A complete energy balance has been established and a very good agreement was found between dissipation rate by CFD predictions and the energy input rate through the cylinder/s rotation.     

Effect of a Draft Tube on the Fluid Dynamics of a Spouted Bed: Experimental and CFD Studies

Knowledge about the gas and particle dynamics in spouted beds is important in the evaluation of particle circulation rates and the efficiency of gas-solid contacts. In this work, the mechanism of transition from a static bed to a spouted bed was numerically simulated using a Eulerian multiphase model. This model was applied to two distinct spouted bed geometries: a conventional device and a spouted bed with draft tube. The radial voidage and particle velocity profiles along the longitudinal position in the annular and spout regions were simulated for the geometries under study. The characteristic simulated curves were congruous with the experimental data.     

Removal of SO2 in Semi‐Dry Flue Gas Desulfurization Process with a Powder‐Particle Spouted Bed

Semi‐dry flue gas desulfurization was investigated with several kinds of SO₂ sorbents, such as slaked lime, limestone, Mg(OH)₂ and concrete pile sludge, in a powder‐particle spouted bed. Slurry droplets including sorbent fine particles were fed to a spouted bed of coarse inert particles spouted with hot gas containing SO₂. SO₂ removal efficiency was strongly affected by the approach to saturation temperature, Ca/S molar ratio and particle size of sorbent. Slaked lime showed the highest desulfurization efficiency. In this process, despite very short gas residence time, more than 90% SO₂ removal was easily achieved by choosing appropriate conditions.     

Comparison of Two‐Fluid and Discrete Particle Modeling of Dense Gas‐Particle Flows in Gas‐Fluidized Beds

Both two‐fluid models embedding the kinetic theory of granular flow for particulate phase stress (TFM) and discrete particle models (DPM) are widely used for the numerical simulation of gas fluidization. In this study, a detailed comparison between results obtained from both TFM and DPM is reported, including axial and radial solid concentration profiles, solids circulation patterns, pressure drop and its standard deviation and granular temperature. It was shown that good agreement can be obtained even in cases of low restitution coefficient, which suggests the possible applicability of kinetic theory of granular flow beyond its nominal range of validity and clearly indicates that the continuum treatment of the solids phase in TFM provides a good approximation of its discrete nature.     

Computational Fluid Dynamics Simulation of Pilot‐Plant‐Scale Two‐Phase Separators

Two computational fluid dynamics (CFD) modeling approaches, the discrete phase model (DPM) and the combination of volume of fluid (VOF) and DPM, are developed to simulate the phase separation phenomenon in four pilot‐plant‐scale separators. The incipient vapor phase velocity, at which liquid droplet carryover occurs, and separation efficiency plots are used as criteria for evaluating the developed CFD models. The simulation results indicate that the VOF‐DPM approach is a substantial modification to the DPM approach in terms of the predicted separation efficiency data and diagrams. CFD simulation profiles demonstrate that all the separators are essentially operating at a constant pressure. The CFD results also show that mist eliminators may operate more efficiently in horizontal separators than in vertical separators.