Gas phase hydrodynamics of a gas-solid turbulent fluidized bed reactor (original) (raw)
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Two-phase model for a catalytic turbulent fluidized-bed reactor: Application to ethylene synthesis
Chemical Engineering Science, 1999
A turbulent #uidized-bed (TFB) reactor for the ethylene synthesis by catalytic oxidation of natural gas was simulated employing a two-phase model, and the hydrodynamic structure of the TFB was characterized for the MgO catalyst particles. The overall gas phase distribution in bubbles and emulsion phases was estimated by using the probability distribution function of local voidage #uctuations in the bed. The mean voidage corresponding at the bubble phase increased with gas super"cial velocity according to a "rst-order model, and the mean voidage in emulsion phase rose proportionally with gas super"cial velocity. Moreover, the emulsion/bubble voidage ratio at the center of the bed was almost constant in the turbulent regime, and radial pro"les of the phase distribution ratio were observed in the turbulent bed. The two-phase model developed predicted satisfactorily the experimental data and can be used to quantify the in#uence of homogeneous and catalytic reaction in the TFB for the oxidative coupling of methane.
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.
Performance of the wide-ranging models for fluidized bed reactors
Advanced Powder …, 2004
Modeling of a bubbling/turbulent fluidized bed reactor has been studied for catalytic reactions in the presence of different catalysts. The performance of the fluidized reactor has been investigated by three different hydrodynamic models: (i) simple two-phase model, (ii) dynamic twophase model and (iii) generalized bubbling/turbulent model. It has been shown that the simple two-phase model is suitable only for slow reaction rates. The dynamic two-phase model and generalized bubbling/turbulent model are able to predict the performance of the fluidized bed reactor satisfactorily over a wide range of gas superficial velocities. The performance of the dynamic two-phase model could be further improved by choosing the proper constants. In the case of the generalized bubbling/turbulent model, a more realistic distribution function for the probability of being in the turbulent regime of fluidization would further enhance the performance of this model.
Fundamental hydrodynamics related to pressurized fluidized bed combustion
Progress in Energy and Combustion Science, 1995
Pressurized fluidized bed combustion (PFBC) is an emerging energy conversion technology for solid fuels, well suited to low-grade fuels and waste materials. The chemical and thermal behaviour of fluidized bed combustors is strongly influenced by the hydrodynamics. The fundamental hydrodynamics of fluidization are the subject of this article. Most PFBC units operate in the bubbling regime, so we concentrate on bubbling fluidization, but also include information on behaviour outside current operating practice. In addition to fundamental hydrodynamics, mixing, heat and mass transfer, elutriation and entrainment, and reactor models are treated. CONTENTS Notation I. Introduction 2. The Phenomenon of Fluidization 3. Bubbles and Bed Expansion 3.1. Properties of single bubbles 3.2. The Davidson model 3.3. Bubble formation 3.4. Bubble growth and coalescence 3.5. Two-phase theory 3.6. Bed expansion 3.7. Slugging 4. Particle Classification, Regimes of Fluidization and Scaling 4. I. Particle classification 4.2. Regimes of gasssolid flow 4.3. Dimensionless parameters and scaling 5. Mixing of Gas and Particles 5. I. Interphase transfer 5.2. Particle mixing processes 5.3. Gas mixing processes 5.4. Mass transfer between gas and particles 6. Elutriation and Entrainment 6. I. Observations 6.2. Particle flux in the freeboard 6.3. Exit flux 7. Internal Surfaces in Fluidized Beds 7. I Hydrodynamic and design considerations: horizontal tubes 7.2. Hydrodynamic and design considerations: vertical tubes 8. Heat Transfer 9. Reactor models for fluid beds 9. I. Two-phase models 9.2. Bed models 9.3. Grid models 9.4. Freeboard models IO. Closure 419 421 43s 440 441 444 446 447 448 448 44x 44x NOTATION '1 A Interfacial area per unit total reactor volume; A, Cross-sectional area of bubble or decay constant for particle flux in the A,, ,423 A, Consolidated parameters in solution of plugfreeboard plug model Cross-sectional area of the bed, or index of B,, B2 Consolidated parameters in solution of plugchemical species mixed models
CFD Modeling of Binary Mixture Hydrodynamics in Gas-Solid Particle Fluidized Bed Reactor System
Jurnal Teknologi
The objective of this research was to compare the effect of a binary mixture between coal, including 500, 700 and 1000-micron size, and sand, 180-micron size, on the mixing behavior in a fluidized bed system. In addition, suitable computational fluid dynamics drag models were explored, including an EMMS model, Gidaspow model and Wen & Yu model. The simulation results were compared for correctness with real plant information. The EMMS model matched well with the obtained data, This is because the employed model considers the particle cluster effect. The EMMS drag model was then used for further computational fluid dynamics simulation. The levels of mixing between sand and coal were predicted by turbulent dispersion coefficient. These coefficients of coal particle were exhibited in axial and radial direction. The highest turbulent dispersion coefficients were found in the mixture with 500 and 1000 micron coal size for radial and axial directions, respectively. The low axial turbulent ...
Simulation of a catalytic turbulent fluidized bed reactor using the sequential modular approach
Fuel processing technology, 2004
Combustion of natural gas in fluidized bed reactors is considered as an economical way for producing energy and food-grade CO 2 largely needed in food and chemical industries. Therefore, their simulation and modeling could be of great industrial importance. In this study, a model is developed based on the sequential modular approach for combustion of natural gas in a catalytic turbulent fluidized bed (TFB) reactor. The proposed model integrates hydrodynamic parameters, reaction model and kinetic data necessary to simulate the combustion of natural gas in the catalytic turbulent fluidized bed reactor. For the purpose of this study and based on hydrodynamic considerations, a number of ideal reactors have been considered to simulate the overall performance of the reactor. The validity of the proposed model was demonstrated using the pilot plant experimental data from the literature. The agreement between the simulation results and the experimental data was found to be satisfactory. D
Liquid mixing and gas–liquid mass transfer in a three-phase inverse turbulent bed reactor
Chemical Engineering Journal, 2005
In this research work, hydrodynamic characteristics and gas-liquid mass transfer in a laboratory scale inverse turbulent bed reactor were studied. In order to characterize internal flow in the reactor, the residence time distribution (RTD) was obtained by the stimulus-response technique using potassium chloride as a tracer. Different solid hold-up (0-0.37) and air superficial velocity (2.7-6.5 mm s −1 ) values were assayed in RTD experiments. The parameters that characterize the RTD curve, mean residence time and variance were independent of the solid hold-up, thus the solid particle concentration did not influence liquid mixing in the reactor. The hydrodynamic of the inverse turbulent bed was well represented by a model that considers the reactor as two-mixed tank of different volumes in series. The value of the volumetric gas-liquid mass transfer coefficient (k L a) was independent of the solid hold-up. This result enhances a previously suggested hypothesis, which considers that the solid and liquid form a pseudo-fluid in the inverse turbulent bed reactor.
Powder Technology, 2017
Gas-flow distribution plays a critical role in the performance of fluidized beds because it directly affects gas residence-time and solids mixing. However, measuring it accurately in the harsh conditions of larger reactors is not possible. Therefore, this study is focused on the development of a rigorous computational framework for quantifying gas-flow distribution during fluidization. To this end, fine-grid simulations are conducted for the bubbling fluidization of two distinct Geldart B particles-1.15 mm LLDPE and 0.50 mm glass particles, at superficial gas velocities U/U mf =2 and 3 in a 50 cm diameter bed. The Two-Fluid Model (TFM) is employed to describe the solids motion efficiently and in-house developed tool MS3DATA (Multiphase-flow Statistics using 3D Detection and Tracking Algorithm) to compute detailed bubble statistics. The overall gas flow is divided into three phases: (a) dense flow in areas relatively rich is solids concentration (b) "visible" bubble flow associated with rising bubbles and (c) throughflow accounting for the gas flow which mostly bypasses through bubbles. It is found that conditions within the dense-phase depend largely on the particle properties while bubbling dynamics are significantly affected by superficial gas velocity. Calculations show that the throughflow increases in areas frequented by bubbles because the voidage distribution around bubbles increases the local dense-phase permeability. Throughflow may account for up to 40% of the overall gas flow, especially in the fluidization of large particles. This is not desirable because its residence-time is almost 2× shorter (as compared to the dense flow) and contributes minimally to solids mixing. Finally, it is shown that in comparison to lab-scales, larger beds exhibit more homogeneous gas mixing. Insights from this study and the methodology developed will be useful in investigating gas flow distribution in complex fuel conversion systems.
Aanalysis of Lateral Fuel Mixing in a Fluiddynamically Down-Scaled Bubbling Fluidized Bed
A method to evaluate the lateral mixing process of fuel particles in bubbling fluidized beds is proposed. The method determines the lateral dispersion coefficient of the fuel particles by means of digital image analysis to video recordings of tracer particle measurements in a fluid-dynamically downscaled 3-dimensional cold-flow model. The work applies direct measurements of tracer particles coated with fluorescent paint, which are irradiated with an ultraviolet light emitting lamp, mounted above the bed. The cold-flow model has cross-sectional dimensions of 0.3 m x 0.3 m and can be operated with bed heights up to 0.16 m. According to the scaling laws this setup is assumed to fluid-dynamically resemble a bubbling fluidized bed operated at 900°C with crosssectional dimensions of 1.5 m x 1.5 m and bed heights up to 0.8 m. The measurements were made for fluidization velocities ranging from 0.17 to 0.83 m/s (up-scaled). The lateral fuel dispersion coefficient is found to increase with superficial velocity over the entire range of velocities investigated. The results show up-scaled (i.e. at hot conditions) dispersion coefficients ranging from 10-2 to 10-1 m 2 /s, which is similar to values obtained previously in large-scale bubbling beds under hot conditions.