CFD Modeling of Bubble Column Reactor Including the Influence of Gas Contraction

In this paper a CFD model for a bubble column reactor undergoing a first order reaction A → B is developed. The reactor operates in the homogeneous bubbly regime and has a diameter D_T = 1 m and height H_T = 5 m. The incoming gas stream contains inerts, varying in proportion from 10 % to 90 %. Three‐dimensional transient Eulerian simulations were carried out for an inlet superficial gas velocity U_G = 0.04 m/s. Due to the consumption of A, the gas phase suffers contraction along the height of the reactor and as a consequence there is a significant change in the gas velocity along the column height; this variation in gas velocity is stronger when the incoming gas contains a smaller proportion of inerts. The CFD simulations show that there is a considerable influence of gas contraction on both the bubble column hydrodynamics and on the reactor conversion. None of the conventionally used reactor models is capable of describing the reactor performance in the case of high gas phase contraction.     

CFD Modeling of a Bubble Column Reactor Carrying out a Consecutive A → B → C Reaction

In this paper, we develop a CFD model for describing a bubble column reactor for carrying out a consecutive first‐order reaction sequence A → B → C. Three reactor configurations, all operating in the homogeneous bubbly regime, were investigated: (I) column diameter D_T = 0.1 m, column height H_T = 1.1 m, (II) D_T = 0.1 m, H_T = 2 m, and (III) D_T = 1 m, H_T = 5 m. Eulerian simulations were carried out for superficial gas velocities U_G in the range of 0.005–0.04 m/s, assuming cylindrical axisymmetry. Additionally, for configurations I and III fully three‐dimensional transient simulations were carried out for checking the assumption of cylindrical axisymmetry. For the 0.1 m diameter column (configuration I), 2‐D axisymmetric and 3‐D transient simulations yield nearly the same results for gas holdup ?_G, centerline liquid velocity V_L(0), conversion of A, χ_A, and selectivity to B, S_B. In sharp contrast, for the 1 m diameter column (configuration III), there are significant differences in the CFD predictions of ?_G, V_L(0), χ_A, and S_B using 2‐D and 3‐D simulations; the 2‐D strategies tend to exaggerate V_L(0), and underpredict ?_G, χ_A, and S_B. The transient 3‐D simulation results appear to be more realistic. The CFD simulation results for χ_A and S_B are also compared with a simple analytic model, often employed in practice, in which the gas phase is assumed to be in plug flow and the liquid phase is well mixed. For the smaller diameter columns (configurations I and II) the CFD simulation results for χ_A are in excellent agreement with the analytic model, but for the larger diameter column the analytic model is somewhat optimistic. There are two reasons for this deviation. Firstly, the gas phase is not in perfect plug flow and secondly, the liquid phase is not perfectly mixed. The computational results obtained in this paper demonstrate the power of CFD for predicting the performance of bubble column reactors. Of particular use is the ability of CFD to describe scale effects.     

Two‐Phase Flow with Mass Transfer in Bubble Columns

Bubble columns are widely used in the chemical and biochemical industries. In these reactors a gaseous phase is dispersed into a continuous liquid phase thus the rising bubble swarm induces a circulating flow field. For the dimension of these reactors the local interfacial area and the residence time of the liquid and the gaseous phase are key parameters. In this paper an Euler‐Euler approach is used to calculate the flow field in bubble columns numerically. Therefore a transport equation for the mean bubble volume based on a population balance equation approach is coupled with the balance equations for mass and momentum. The calculations are performed for three‐dimensional, instationary flow fields in cylindrical bubble columns considering the homogeneous and the heterogeneous flow regime. For the interphase mass transfer the physical absorption of the gaseous phase into the liquid is assumed. The back mixing in the gaseous and liquid phase is calculated from the local and time dependent concentration of a tracer.     

Bubble Behavior of a Large‐Scale Bubble Column with Elevated Pressure

Gas holdups and the rising velocity of large and small bubbles are measured using the dynamic gas disengagement approach in a pressured bubble column of 0.3 m in diameter and 6.6 m in height. The effects of superficial gas velocity, liquid surface tension, liquid viscosity, and system pressure on the gas holdups and the rising velocity of small and large bubbles are investigated. The holdup of large bubbles and the rising velocity of small bubbles increase with increasing liquid viscosity and liquid surface tension. Meanwhile, the holdup of small bubbles and the rising velocity of a swarm of large bubbles decrease. Moreover, the holdup of large bubbles and the rising velocity of a swarm of small bubbles decrease with increasing system pressure. A correlation for the holdup of small bubbles is obtained from experimental data.     

Simulations of Gas Distributors in the Design of Shallow Bubble Column Reactors

Computational fluid dynamics (CFD) was used to simulate the effect of sparger construction in gas holdup and liquid axial velocity in a shallow bubble column reactor for the air‐water system. Model parameters were evaluated in 2‐ and 3‐D simulations by using a two‐fluid model and the standard k‐? turbulence model. The Eulerian‐Eulerian approach was employed to predict the height of column that is affected by the sparger. It was found that increasing the number of orifices in the sparger increases the total gas holdup. Moreover, each orifice causes an increase in the circulation and mixing of liquid in the column. The results of the simulations follow the trends observed in the findings of Dhotre and Joshi [1].     

Studies of Gas Holdup in Bubble Column Reactors

Gas holdup in bubble columns has been investigated over a wide range of operational and geometrical parameters. A criterion has been developed for the prediction of the transitional velocity from the homogeneous to heterogeneous flow regime. Correlations for gas holdup in both regimes are developed and verified against experimental data.     

Measurement of Local Phase Holdups in a Two‐ and Three‐Phase Bubble Column

A new invasive sensing probe for the measurement of local phase holdups in two‐ and three‐phase reactors is described. The local gas and solids holdups in a bubble column with a volume of V = 2 m³ at varying operating conditions (gas velocity, sparger design, solids content and density) are measured by means of differential pressure measurement in combination with either time domain reflectometry or electrical conductivity measurement. The phase distribution profiles at two‐ and three‐phase operating conditions are described. The influence of the sparger design on the shape of these profiles, the influence of the solid phase on the gas distribution, the solids distribution and the gas‐stow effect above the sparger because of a dense particle layer are capable of experimental proof for the first time.     

Gas Holdup and Volumetric Mass Transfer Coefficient in a Slurry Bubble Column

The gas holdup, ?, and volumetric mass transfer coefficient, k_La, were measured in a 0.051 m diameter glass column with ethanol as the liquid phase and cobalt catalyst as the solid phase in concentrations of 1.0 and 3.8 vol.‐%. The superficial gas velocity U was varied in the range from 0 to 0.11 m/s, spanning both the homogeneous and heterogeneous flow regimes. Experimental results show that increasing catalyst concentration decreases the gas holdup to a significant extent. The volumetric mass transfer coefficient, k_La, closely follows the trend in gas holdup. Above a superficial gas velocity of 0.04 m/s the value of k_La/? was found to be practically independent of slurry concentration and the gas velocity U; the value of this parameter is found to be about 0.45 s^–1. Our studies provide a simple method for the estimation of k_La in industrial‐size bubble column slurry reactors.     

Comparison of Hydrodynamics and Mass Transfer in Airlift and Bubble Column Reactors Using CFD

Computational Fluid Dynamics (CFD) is used to compare the hydrodynamics and mass transfer of an internal airlift reactor with that of a bubble column reactor, operating with an air/water system in the homogeneous bubble flow regime. The liquid circulation velocities are significantly higher in the airlift configuration than in bubble columns, leading to significantly lower gas holdups. Within the riser of the airlift, the gas and liquid phases are virtually in plug flow, whereas in bubble columns the gas and liquid phases follow parabolic velocity distributions. When compared at the same superficial gas velocity, the volumetric mass transfer coefficient, k_La, for an airlift is significantly lower than that for a bubble column. However, when the results are compared at the same values of gas holdup, the values of k_La are practically identical.     

Development of a CFD Model of Bubble Column Bioreactors: Part Two – Comparison of Experimental Data and CFD Predictions

The application of computational fluid dynamics (CFD) as a tool to simulate bubble column bioreactors is investigated. A three‐dimensional model utilizing the Euler‐Euler approach is evaluated. The role of various terms, i.e., lift, drag, bubble‐induced turbulence, and volume fraction correction terms for drag, is determined. Good agreement between experimental data and simulation results was obtained by means of a single‐bubble size model provided that bubble‐induced turbulence and the reduction in drag due to the presence of other bubbles were taken into account.     

气液鼓泡床泡罩式气体分布器流场的CFD模拟

采用ANSYS FLUENT商用CFD软件对气液鼓泡床内泡罩式气体分布器单体进行了数值模拟。以Ug=12.34m/s的入口速度对计算模型进行单相三维稳态模拟。研究了泡罩内径、泡罩齿片和入口管开孔分别对塔内不均度和泡罩式气体分布器的压降影响规律。研究表明,随着泡罩内径的增加,塔内不均度和分布器压降减小;泡罩齿片加宽,齿片对气体的阻力增加;齿片间距增加,齿片对气体的阻力减小;入口管开孔数对塔内不均度没有任何影响。开孔数很少时,开孔数量的增加使分布器的压降减小,然而开孔数增加到一定数量时,开孔数对分布器的压降影响甚微。对泡罩式气体分布器进行了结构优化。     

Multi‐Phase‐Multi‐Size‐Group Model for the Inclusion of Population Balances into the CFD Simulation of Gas‐Liquid Bubbly Flows

A CFD model for the simulation of gas‐liquid bubbly flow is developed. In the model, the multi‐phase flow is simulated by an Eulerian‐Eulerian approach using several phase definitions (from 3 to 10). The bubble size distribution is simulated by a solution of the discretized population balance equation with coalescence and break‐up of bubbles. The number of the discretized population balance equations in the model is larger than the number of the phases used in the flow field simulation. A desired accuracy in the simulation can be achieved by choosing a suitable number of phases as a compromise between accuracy and computational cost. With this model, more detailed flow hydrodynamics and bubble size distribution can be obtained. The model was tested with different operating conditions and for different numbers of dispersed phases in a bubble column, and was verified with a bubble size distribution obtained experimentally.     

Gas‐Liquid Mass Transfer in a Slurry Bubble Column at High Slurry Concentrations and High Gas Velocities

The volumetric mass transfer coefficient k_La in a 0.1 m‐diameter bubble column was studied for an air‐slurry system. A C₉‐C₁₁ n‐paraffin oil was employed as the liquid phase with fine alumina catalyst carrier particles used as the solid phase. The n‐paraffin oil had properties similar to those of the liquid phase in a commercial Fischer‐Tropsch reactor under reaction conditions. The superficial gas velocity U_G was varied in the range of 0.01 to 0.8 m/s, spanning both the homogeneous and heterogeneous flow regimes. The slurry concentration ?_S ranged from 0 to 0.5. The experimental results obtained show that the gas hold‐up ?_G decreases with an increase in slurry concentration, with this decrease being most significant when ?_S < 0.2. k_La/?_G was found to be practically independent of the superficial gas velocity when U_G > 0.1 m/s is taking on values predominantly between 0.4 and 0.6 s^–1 when ?_S = 0.1 to 0.4, and 0.29 s^–1, when ?_S = 0.5. This study provides a practical means for estimating the volumetric mass transfer coefficient k_La in an industrial‐size bubble column slurry reactor, with a particular focus on the Fischer‐Tropsch process as well as high gas velocities and high slurry concentrations.     

Studies on the Hydrodynamics of Chaotic Bubbling in a Gas‐Liquid Bubble Column with a Single Nozzle

In this work, the chaotic bubbling mechanism in a gas‐liquid bubble column with a single nozzle was investigated. The signal for the analysis was the time series of pressure fluctuations measured from a pressure transducer probe placed in the bubble column close to the nozzle. In order to study the bubbling process, statistical analysis, qualitative and quantitative nonlinear analyses were carried out for the pressure fluctuations. Power spectra used as standard statistical measures provided preliminary evidence that bubbling in the middle values of gas flow rates may be chaotic in nature. Phase plots provided a qualitative means of analyzing the fine geometry structure of the attractor reconstructed from the bubbling time signal. Positive finite estimates of the Kolmogorov entropy provided a quantitative evidence of behavior consistent with chaos. Besides previous diagnostic tools, the local nonlinear short‐term prediction was also used as a supplement method. It was found that the bubbling process exhibits a deterministic chaotic behavior in a certain range of the gas flow rate. When increasing the gas flow rate, the sequence of periodic bubbling, primary and advanced chaotic bubbling, and jetting or random bubbling were successively observed. However, no clear period doubling sequence leading to chaotic behavior was observed. The sharp loss of the ability to predict the pressure signal successfully with the nonlinear prediction method provides the strongest evidence of the presence of the chaotic bubbling. The variations of the nonlinear invariants, such as the Kolmogorov entropy and the correlation dimension together with the plot of the correlation integral with the operation conditions, might be developed as potential and effective quantitative tools for flow regime identification of the bubbling process.     

Use of Neural Network–Ultrasonic Technique for Measuring Gas and Solid Hold‐ups in a Slurry Bubble Column Bubble Colum

A novel technique for measuring simultaneously the gas and solid hold‐ups in a slurry bubble column using a combination of neural network–ultrasonic method was investigated in this study. A one‐dimensional model using the basic parameters of ultrasound (the energy attenuation and the velocity change in terms of the transmission time difference) for measuring the gas and the solid hold‐ups has been proposed to show the complexity of the system. The three layers feed‐forward neural network (3‐FFNN) structure has been used to try and solve the nonlinear relationship between parameter sensing and measurement purpose. An adequate selection of the neural network structure has been chosen to perform the relationship between the measurement sensing (input of the network) and the measurement purpose (output of the network). Preliminary representation results of the gas and the solid hold‐ups using the proposed method compare relatively well with measured data.     

Flow and Heat Transfer Prediction in a Corrugated Plate Heat Exchanger using a CFD Code

The purpose of this study is to explore the potential of using a general purpose CFD code to compute the characteristics of the flow field, and of the heat transfer augmentation in conduits with corrugated walls, encountered in commercial plate heat exchangers (PHE). The CFD code is used to simulate the performance of a PHE model comprised of stainless steel plates, following a herringbone design and assembled for single‐pass countercurrent flow. The code is validated by comparing the numerical results with experimental data on pressure drop and overall temperature differences acquired for the countercurrent flow of water at both sides of the model PHE. The limited data published in the literature are also in fairly good agreement with the results of the present study. It is shown that the CFD code is an effective and reliable tool for studying the effect of various geometrical configurations on the optimum design of a PHE.