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.     

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.     

Radial Profiles of Gas Bubble Behavior in a Gas‐Liquid Bubble Column Reactor under Elevated Pressures

The effects of operating conditions on radial variation of gas holdups, bubble swarm rising velocity, and Sauter mean diameter were investigated in a bubble column reactor under elevated pressures using a conductivity probe method. Air served as gas phase and tap water as liquid phase with varying gas velocity and pressure. All three parameters increased constantly with higher superficial gas velocity. Maximum holdup, velocity, and Sauter mean diameter were found at the center of the cross section. Two different cases for Sauter mean diameter distribution were observed. The gas holdups increase continuously with higher system pressure, but decrease for bubble swarm rising velocity and Sauter mean diameter. According to experimental results, an empirical correlation of the gas holdup profiles is presented.     

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.     

Large Bubble Sizes and Rise Velocities in a Bubble Column Slurry Reactor

The results are reported of an experimental study of the gas holdup, ?_G, large bubble diameter, d_Lb, and large bubble rise velocity, V_Lb, in a 0.1 m wide, 0.02 m deep and 0.95 m high rectangular slurry bubble column operated at ambient temperature and pressure conditions. The superficial gas velocity U was varied in the range of 0–0.2 m/s, spanning both the homogeneous and heterogeneous flow regimes. Air was used as the gas phase. The liquid phase used was C₉‐C₁₁ paraffin oil containing varying volume fractions (?_S = 0, 0.05, 0.10, 0.15, 0.20 and 0.25) of porous catalyst (alumina catalyst support, 10 % < 10 μm; 50 % < 16 μm; 90 % < 39 μm). With increasing slurry concentrations, ?_G is significantly reduced due to enhanced bubble coalescence and for high slurry concentrations the “small” bubbles are significantly reduced in number. By the use of video imaging techniques, it was shown that the large bubble diameter is practically independent of the gas velocity for ?_S > 0.05 and U > 0.1 m/s. The measured large bubble rise velocity V_Lb agrees with the predictions of a modified Davis‐Taylor relationship.     

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.     

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.     

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.     

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.     

Energy Analysis and Air Entrainment in an Ejector Induced Downflow Bubble Column with Non‐Newtonian Motive Fluid

In an ejector induced downflow bubble column energy supplied as a high velocity liquid jet is utilized in different sections of the ejector‐contactor system, which leads to air entrainment at the secondary entrance of the ejector. The energy losses in the different sections, viz. ejector, mixing zone and gas‐liquid bubbly flow zone have been evaluated theoretically. Experimental results show that the total energy losses calculated on the basis of theoretical expression are almost the same as energy supplied by the liquid jet. A simple correlation was developed for the air entrainment rate in terms of operating and design parameters of the system.     

CFD Simulations of a Bubble Column Operating in the Homogeneous and Heterogeneous Flow Regimes

Bubble columns are operated either in the homogeneous or heterogeneous flow regime. In the homogeneous flow regime, the bubbles are nearly uniform in size and shape. In the heterogeneous flow regime, a distribution of bubble sizes exists. In this paper, a CFD model is developed to describe the hydrodynamics of bubble columns operating in either of the two flow regimes. The heterogeneous flow regime is assumed to consist of two bubble classes: “small” and “large” bubbles. For the air‐water system, appropriate drag relations are suggested for these two bubble classes. Interactions between both bubble populations and the liquid are taken into account in terms of momentum exchange, or drag‐, coefficients, which differ for the “small” and “large” bubbles. Direct interactions between the large and small bubble phases are ignored. The turbulence in the liquid phase is described using the k‐ϵ model. For a 0.1 m diameter column operating with the air‐water system, CFD simulations have been carried out for superficial gas velocities, U, in the range 0.006–0.08 m/s, spanning both regimes. These simulations reveal some of the characteristic features of homogeneous and heterogeneous flow regimes, and of regime transition.     

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.     

Simultaneous Measurement of Mean Bubble Diameter and Local Gas Holdup Using Ultrasonic Method with Neural Network

One of the greatest challenges in the characterization of bubbles in a bubble column has been the prediction of the bubble diameter and the gas holdup. In this study a novel technique for predicting the mean bubble diameter and the local gas holdup using a non‐invasive ultrasonic method with neural network was investigated. The measurement parameters of the energy attenuation and the transmission time difference of ultrasound are used to obtain the mean bubble diameter and the local gas holdup in an air‐water dispersion system using neural network reconstruction. Bubble size distributions in a 2‐D bubble column are obtained experimentally by using a photographic method. An adequate selection of the neural network structure has been carried out to represent the training data. The representative results using the present structure show good agreement with the measured data.     

Experimental Study of Oxygen Mass Transfer Coefficient in Bubble Column with High Temperature and High Pressure

The volumetric gas‐liquid mass transfer coefficient, k_Lα, for oxygen was studied by using the dynamic method in slurry bubble column reactors with high temperature and high pressure. The effects of temperature, pressure, superficial gas velocity and solids concentration on the mass transfer coefficient are systemically discussed. Experimental results show that the gas‐liquid mass transfer coefficient increases with the increase in pressure, temperature, and superficial gas velocity, and decreases with the increase in solids concentration. Moreover, k_Lα values in a large bubble column are slightly higher than those in a small one at certain operating conditions. According to the analysis of experimental data, an empirical correlation is obtained to calculate the values of the oxygen volumetric mass transfer coefficient for a water‐quartz sand system in two bubble columns with different diameter at high temperature and high pressure.     

Experimental Study on Bubble Rising and Descending Velocity Distribution in a Slurry Bubble Column Reactor

To determine bubble rising and descending velocity simultaneously, a BVW‐2 four‐channel conductivity probe bubble parameters apparatus and its analysis are used in gas‐liquid and gas‐liquid‐solid bubble columns. The column is 100 mm in internal diameter and 1500 mm in height. The solid particles used are glass beads with an average diameter of 17.82 μm, representing typical particle size for catalytic slurry reactors. The effects of superficial gas velocity (1.0 cm/s ≤ U_g ≤ 6.4 cm/s), solid holdup (0 % ≤ ?_s ≤ 30 %), and radial location (r/R = 0, 0.4, and 0.7) on bubble velocity distributions are determined. It is found that increasing U_g can increase the velocity of bubbles but do not exert much influence on bubble velocity distribution. Solid holdup mainly affects the distribution of bubble velocity while the radial direction affects bubble velocity distribution only slightly. The ratio of descending bubbles to rising bubbles increases from the bubble column center to the wall. It can be proved experimentally that large bubbles do not always rise faster than small bubbles at higher U_g (for example 6.4 cm/s).     

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.     

Steady‐State Multiplicity in High‐Pressure Bubble Column Reactors

The effect of pressure on the possible existence of multiple steady states in bubble column reactors is investigated. A mathematical model involving fast pseudo first‐order kinetics is employed to describe the performance of non‐isobaric, non‐isothermal reactors. The numerical analysis reveals that the existence of multiplicity is sensitive to pressure variation, yet high‐pressure operating conditions do not necessarily lead to a higher likelihood of multiplicity in these contactors.     

Gas-Liquid Mass Transfer Characteristics in a Gas-Liquid-Solid Bubble Column under Elevated Pressure and Temperature

abstract The volumetric mass transfer coefficient kLa of gases (H2, CO, CO2) and mass transfer coefficient kL on liquid par-affin side were studied using the dynamic absorption method in slurry bubble ...