Modeling of vanillin production in a structured bubble column reactor

Vanillin is an important flavour. Semi-synthetic vanillin can be produced by the oxidation of lignin. Experimental studies leading to vanillin production in a batch reactor and a structured bubble column reactor (SBCR) lead us to the conclusion that the SBCR could have non-idealities such as dispersion. The radial and axial liquid-phase dispersion within the packed criss-crossing sandwich structures of Mellapak-750Y had been studied. A 2D model accounting for axial and radial velocities and dispersion was formulated and solved. The model predictions were compared with that of an experimental residence time distribution curve. The axial dispersion coefficient of the liquid phase is of the same order of magnitude as the radial dispersion coefficient. The reaction kinetics available in literature is adopted for the present study. Model for the SBCR was formulated and simulated using commercial modeling software. Simulation experiments were conducted in a SBCR. The effect of the following parameters on the yield of vanillin is studied: lignin concentration, lignin molecular weight, oxygen partial pressure and reaction temperature. It can be said that lignin molecular weight is a crucial parameter in vanillin production.     

Influence of structured packing on gas holdup in a three-phase bubble column

In this work, the influence of structured packing on gas holdup in gas-liquid-solid dispersions has been studied. The experiments were carried out in an empty column and in column containing structured packing operating under identical conditions. Glass beads and silicon carbide particles were used as the solid material and the volumetric fraction of solids was varied from 0% to around 10%. The liquid viscosity was strongly modified using water, CMC solution and glycerol. The experimental results obtained with both columns were compared with previous results obtained in two-phase bubble columns. The influence of structured packing on the total gas holdup for different superficial gas velocities was found to be similar with and without suspended solids. Therefore, the results obtained in this work were analysed on the basis of correlations derived earlier for gas-liquid dispersions. Excepting the results obtained with glycerol, these correlations can predict the gas holdup of three-phase bubble columns with reasonable accuracy.     

Catalyst mixing in bubble column slurry reactors

The reported experimental data of Pandit and Joshi (1984) on axial and radial steady-state catalyst concentration in a semibatch bubble column slurry reactor is interpreted by the dispersion model. The elliptic partial differential equation with its associated boundary conditions is solved analytically for catalyst concentration by the method of separation of variables. The proposed model adequately fits the experimental data.     

Hydrokinetic optimization of commercial scale slurry bubble column reactor

An in‐depth numerical study has been carried out to investigate a high‐pressure commercial scale (2–8 m diameter, 30–40 m in height) slurry bubble column reactor. Typical superficial gas velocities are in the range of 0.5–3 m/s, and overall vapor holdups are in the range of 0.45–0.85. The study revealed that steady compartmental reaction models do not match plant data when reaction time constants are fast. Also, off‐the‐shelf commercial computational fluid dynamics codes do not produce useful information about a reactive column of this scale without first validating the model using data “anchors” from full‐scale operational columns. Important measures include both transient and time‐averaged profiles, integrals, and extrema of vapor holdup and reactants. Reactor designs based on this study show both improved productivity and product quality, allowing record production from existing plants along with substantial capital scope reduction for new plants. © 2011 American Institute of Chemical Engineers AIChE J, 2012     

Determination of interfacial area in a tapered bubble column

BACKGROUND: Investigations of hold up (ε_g) and interfacial area (a) in cylindrical bubble columns have been reported extensively but reported similar investigations in tapered bubble columns are sparse in the existing literature. Thus the current article reports the experimental determination of ε_g and a using a tapered bubble column. RESULTS: The present system generated ε_g (0.556 to 0.641) in a CO₂? NaOH system 20% higher than in an air–water system (0.466 to 0.534) and values were higher than in existing systems. Also, the values of ε_g in the air–water system were higher than reported for a column with shorter tapered angle. Values of ε_g fitted very well with the well‐known Akita and Yoshida correlation. The observed values of a (235 and 700 m² m^?3) were higher than values obtained (2 to 600 m² m^?3) in existing systems. The energy dissipation was 203 to 335 W m^?3, which was lower than that (100 to 1200 W/m³) in existing systems. A correlation developed to predict the pressure drop in terms of Euler number was statistically highly significant. CONCLUSION: The present research a chieved higher values of hold up and interfacial area, and lower values of energy dissipation per unit volume of dispersion compared with existing systems. Findings of the present study coupled with previous studies indicate that the tapered bubble column developed could find potential application not only in air pollution control but also in gas‐liquid mass transfer operations. Copyright © 2011 Society of Chemical Industry     

Liquid phase mixing in trayed bubble column reactors

The compartmentalization of conventional bubble columns by perforated trays constitutes a very effective method to reduce the liquid backmixing. The effect of tray design and operating conditions on the overall liquid mixing was studied in a bench-scale trayed bubble column. The extent of liquid backmixing in the column was investigated in light of liquid-phase tracer response experiments. In average, a three fold reduction in the liquid backmixing was achieved in the trayed column as compared to the column without the trays. Moreover, the tray open area and the superficial liquid velocity were found to have the strongest effects on the liquid backmixing. The N-CSTR with Backmixing Model was found to match the experimental tracer response curves better than the Axial Dispersion Model.     

A scale up strategy for bubble column slurry reactors

The hydrodynamics of bubble column slurry reactors are strongly influenced by the scale of operation. We suggest a strategy for scaling up reactors from laboratory scale to commercial size that relies on a fundamental understanding of bubble hydrodynamics, which is incorporated into a computational fluid dynamics (CFD) model.     

斜插穿流式挡板强化鼓泡塔内流体混合的研究

乙炔二聚反应制备乙烯基乙炔是氯丁橡胶生产等工艺中的重要操作单元,通常在全混式鼓泡塔反应器中进行。但塔内流体混合模式单一、流场对称性强、液相轴向返混较高,乙烯基乙炔容易滞留在催化剂中,并与乙炔进一步反应生成二乙烯基乙炔和高聚物。强化乙炔二聚鼓泡塔内的反应,提高乙炔单程转化率和选择性,有助于氯丁橡胶生产实现节能减排。应用FLUENT软件模拟,计算了鼓泡塔内流场结构及变化规律,发现斜插穿流式挡板可有效地调控乙炔二聚鼓泡塔内流场结构,减小漩涡尺寸,降低液相湍流流速,减小液相返混区域,强化鼓泡塔内乙炔二聚反应。     

Axial dispersion and oxygen transfer in the transition from bubble column to airlift-loop reactor

Axial dispersion and oxygen transfer were investigated in a bubble column (BC) with a circulation loop. A butterfly valve, situated at the bottom of the loop enabled the above-mentioned physical characteristics in the transition regime between typical airlift-loop-reactor (ALR) flow and BC flow to be studied. The Bodenstein number was found to decrease when the liquid velocity was reduced, implicating a less established plug-flow character. The number of circulations required to achieve complete mixing in the reactor was diminished if the liquid circulation was hampered and appeared to be proportional to the Bodenstein number. The volumetric oxygen-transfer coefficient was estimated by an ideally stirred-tank-reactor (STR) model and a plug-flow model. The STR model yielded reliable results for the whole range of operation while the plug-flow model only appeared to be appropriate for the ALR operation mode. The k_La values obtained were included in a generalized correlation for the transition flow regime and were found to increase gradually when the circulation velocity was reduced.     

加压大型鼓泡床反应器内大小气泡气含率研究

在内径0.3m、高6.6m的加压鼓泡床反应器内采用床层塌落技术测量床层内的大小气泡气含率实验。由于大小气泡上升速度不同,床层塌落曲线存在一水平段,在此基础上,详细考察了表面张力、粘度、系统压力、表观气速对大小气泡气含率的影响,得出大气泡气含率随粘度和表面张力升高而升高,随压力升高而降低;小气泡气含率随粘度和表面张力升高而降低,随压力升高而升高;并根据质量守恒定理,进行了解释。     

Effects of floating contactors on oxygen transfer and hydrodynamics in a bubble column

This study investigates the adoption of floating contactors to promote the rate of oxygen transfer from non-uniform air bubbles to liquid in a bubble column with continuous operation. The volumetric oxygen transfer coefficient and axial dispersion coefficient of a liquid phase have been analyzed based on the axial dispersion model. Attention was focused on the effects of the volume fraction of the floating contactors on the volumetric oxygen transfer coefficient, axial dispersion coefficient of a liquid phase, and gas phase hold-up in the bubble column. The results have shown that the volumetric oxygen transfer coefficient and gas phase hold-up can increase by up to 25% and 13%, respectively, while the axial dispersion coefficient of a liquid phase decreases by up to 30% by adding floating contactors in the column.     

Design and optimisation of a multi-stage bubble column slurry reactor for Fischer–Tropsch synthesis

For carrying out the Fischer–Tropsch synthesis of heavy paraffins starting from syngas (CO+H₂), a multi-stage bubble column slurry reactor design is carried out. The advantages of this reactor configuration with respect to a conventional slurry reactor design, consisting of one well-mixed stage, are (a) increased syngas conversion, and (b) increased reactor productivity. The multi-stage bubble column construction requires installation of additional cooling surface area in order to keep the exothermic reaction within the desired temperature limits.     

Liquid circulation and mixing in an inclined bubble column

Gas and liquid velocities were measured in an inclined rectangular column, at inclinations in the range 0?45° and superficial gas velocities of 0.002 to 0.12 cm/s. Liquid circulation was also characterized by measuring the time to mix a pulse of salt solution in the column. Inclining the column by up to 10° from the vertical induced a drastic change in behavior, from a bubble column with evenly distributed gas bubbles to a segregated system with rapid liquid circulation pumped by rising gas bubbles. The liquid velocities in the inclined column ranged from 10 to 50 times the superficial gas velocity. Further inclination of the column in the range of 10?45° had little effect on liquid velocity or mixing time. A compartmental model for the recirculating liquid flow matched the observed mixing times, but predicted oscillations in tracer concentration which were not detected in experiments.     

Gas phase flow in bubble columns: A convective phenomenon

The axial dispersion model has been commonly used to describe gas phase flow in bubble columns. Scatter in dispersion coefficients reported to date may be a result of the misuse of the axial dispersion model when a convective model would be more appropriate. Using simple tests with radioactive tracer response curve moments, convective and dispersive behaviours are differentiated. A convective model is presented. The model fits both tracer response curves and mean gas velocity well in both the bubbly and churn turbulent flow regimes, and may be used as a technique to calculate bubble rise velocity distributions.     

A model for two-phase turbulent mixing in a jet bubble column

Mixing behavior of the two phase air-water turbulent flow in a jet bubble column is examined. The time evolution of the mixing behavior of a liquid tracer in a turbulent air-water flow within a jet bubble column is predicted using a model based on the fundamental governing equations of fluid motion. The predictions of the model are compared with experimental measurements. Measured residence time distributions (RTD) of the liquid tracer within the cone agree well with the predicted values given by the model. For the range of parameters considered in the study, lack of radial mixing and large axial mixing are evident within the cone of the jet bubble column. Use of fundamental mathematical models for the study of hydrodynamics in a two-phase conventional bubble column has been reported earlier (Torvik, 1990; Jakobsen et al., 1993). The present paper extends the use of such models to predict the mixing characteristics in a jet bubble column.     

二维鼓泡床内气液流动特性实验与数值模拟

采用高速摄像法测量了0.20 m×0.02 m×2.00 m拟二维床内气泡尺寸分布和流型等变化规律,结果表明,随着表观气速的增大,鼓泡床内依次呈现均匀鼓泡区、过渡区和湍动区3种形式,以气泡个数概率表示的气泡尺寸分布呈对数正态分布。以计算流体力学软件ANSYS CFX 10.0为平台,采用k-ε湍流模型和GRACE曳力模型对气液鼓泡床内流体动力学行为展开了数值模拟,其结果与实验值比较吻合。研究表明,从多相流理论出发的计算流体力学模拟方法可以用来预报鼓泡床内流型过渡等流体动力学特性。     

Prediction of gas hold‐up in a bubble column filled with pure and binary liquids

A dimensionless correlation for gas hold‐up in a bubble column filled with pure liquids and binary mixtures has been developed. The criteria of bubble coalescence in pure liquids and foamability of binary mixtures have been included successfully in the proposed correlation. For the first time, comprehensive data of gas hold‐up with respect to concentration in four binary systems are presented. The often‐reported enhancement of gas hold‐up in aqueous alcohol mixtures has been observed experimentally. The proposed correlation is able to predict successfully the trend of gas hold‐up enhancement with respect to concentration. The estimated values are found to be within 7% of the measured values.     

Bubble size and radial gas hold‐up distributions in a slurry bubble column using ultrafast electron beam X‐ray tomography

Gas hold‐up and bubble size distribution in a slurry bubble column (SBC) were measured using the advanced noninvasive ultrafast electron beam X‐ray tomography technique. Experiments have been performed in a cylindrical column (D_T = 0.07 m) with air and water as the gas and liquid phase and spherical glass particles (d_P = 100 μm) as solids. The effects of solid concentration (0 ≤ C_s ≤ 0.36) and superficial gas velocity (0.02 ≤ U_G ≤ 0.05 m/s) on the flow structure, radial gas hold‐up profile and approximate bubble size distribution at different column heights in a SBC were studied. Bubble coalescence regime was observed with addition of solid particles; however, at higher solid concentrations, larger bubble slugs were found to break‐up. The approximate bubble size distribution and radial gas hold‐up was found to be dependent on U_G and C_s. The average bubble diameter calculated from the approximate bubble size distribution was increasing with increase of U_G. The average gas hold‐up was calculated as a function of U_G and agrees satisfactorily with previously published findings. The average gas hold‐up was also predicted as a function of C_s and agrees well for low C_s and disagrees for high C_s with findings of previous literature. © 2012 American Institute of Chemical Engineers AIChE J, 59: 1709–1722, 2013     

Influence of bubble column diameter on local heat transfer and related hydrodynamics

Heat transfer coefficients measured in a 0.15 m ID bubble column are compared with similar studies in larger diameter columns to identify influence of column diameter. Gas phase used is oil free compressed air and its flow rate is varied from 0.03 to 0.35 m/s. Tap water is the liquid phase and the solid particles used are 49 μm glass beads and their concentration is varied up to 20 vol%. The observed increase in heat transfer coefficients can be related to increase in liquid circulation velocity with column diameter which in turn is related to increase in large bubbles rise velocity. A simplified scale-up procedure is presented based on available data and suitably modified literature correlations for heat transfer coefficient.