HYDRODYNAMICS, MIXING AND MASS TRANSFER IN A LARGE DIAMETER JET BUBBLE COLUMN

Experimental measurements for the axial and radial variations in gas holdup, axial and radial dispersion coefficients, volumetric gas-liquid mass transfer coefficient and liquid phase circulation velocity in a cone of a large diameter (122 cm) jet bubble column are presented. Two diameters of the inlet nozzle, namely 10.16 cm and 15.24 cm, three superficial gas velocities (based on cylinder diameter), 3 cm/sec, 6 cm/sec and 8 cm/sec and two superficial liquid velocities, 0.3 cm/sec and 0.6 cm/sec, are examined. The experimental data are obtained for two different bed heights.

The experimental data showed significant axial and radial variations in the gas holdup. The volumetric average gas holdup was higher at higher gas velocity and larger nozzle diameter and somewhat higher at lower liquid velocity. The axial dispersion was high while the radial dispersion was low. The volumetric gas-liquid mass transfer coefficient was larger at higher gas velocity and larger nozzle diameter. The liquid recirculation begins only at the upper end of the cone. In general, experimental data indicate that a jet bubble column provides a high degree of mixing and transport rates.     

Hydrodynamics and mass transfer in an upward venturi/bubble column combination

The hydrodynamics and mass transfer characteristics of a venturi/bubble column combination were studied at high liquid superficial velocities of up to 0.35 m/s. The gas hold-up was increased by 50% to 150% and the overall volumetric mass transfer coefficient was tripled when the venturi was used as “gas distributor” instead of a porous distributor. A correlation of the overall volumetric mass transfer coefficient (K_La) with the gas hold-up, valid for gas hold-ups as high as 0.3, was proposed for the cylindrical bubble column section. The energy consumption per mole of oxygen transferred was lower than with most distributors and the oxygen transfer rate per unit of reactor volume was higher than in a bubble column with a porous distributor. The venturi/bubble column combination is a compact and efficient system which does not have the operating problems of systems which require internals.     

Oxygen mass transfer coefficient in bubble column slurry reactor with ultrafine suspended particles and neural network prediction

The gas–liquid volumetric mass transfer coefficient was determined by the dynamic oxygen absorption technique using a polarographic dissolved oxygen probe and the gas–liquid interfacial area was measured using dual‐tip conductivity probes in a bubble column slurry reactor at ambient temperature and normal pressure. The solid particles used were ultrafine hollow glass microspheres with a mean diameter of 8.624 µm. The effects of various axial locations (height–diameter ratio = 1–12), superficial gas velocity (u_G = 0.011–0.085 m/s) and solid concentration (ε_S = 0–30 wt.%) on the gas–liquid volumetric mass transfer coefficient k_La_L and liquid‐side mass transfer coefficient k_L were discussed in detail in the range of operating variables investigated. Empirical correlations by dimensional analysis were obtained and feed‐forward back propagation neural network models were employed to predict the gas–liquid volumetric mass transfer coefficient and liquid‐side mass transfer coefficient for an air–water–hollow glass microspheres system in a commercial‐scale bubble column slurry reactor. © 2012 Canadian Society for Chemical Engineering     

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.     

Gas-liquid mass transfer in bubble columns with a T-junction nozzle for gas dispersion

Bubble break-up, gas holdup, and the gas-liquid volumetric mass transfer coefficient are studied in a bubble column reactor with simultaneous injection of a gas and liquid through a T-junction nozzle. The theoretical dependence of bubble break-up and the volumetric mass transfer coefficient on liquid velocity in the nozzle is developed on the basis of isotropic turbulence theory. It is shown that correlations which are developed based on liquid jet kinetic power per nozzle volume explain average gas holdup and the volumetric mass transfer coefficient within an error of 15% for all gas and liquid flow rates and nozzle diameters used. Experiments with a larger scale column, height 4.64 m and diameter 0.98 m, show a transition from homogeneous to heterogeneous flow at a certain liquid flow rate through the nozzle. Liquid composition was found to have a significant effect on gas-liquid mass transfer. A phenol concentration of 10–30 mg/l in water increases the volumetric mass transfer coefficient of oxygen by 100%. This phenomenon may have significance in the chemical oxidation of wastewater.     

PHASE DISTRIBUTION AND MIXING IN A JET BUBBLE COLUMN

This paper describes the results of an experimental study to evaluate phase holdups and RTD for a jet bubble column. The experimental data were obtained in a 61 cm diameter jet bubble column with a conical inlet. Air and water were used as a two-phase system. The ranges of gas and liquid velocities examined were 0 to 9 cm/sec and 0 to 0·6 cm/sec respectively, both based on the cylinder diameter. The experimental data indicate that in the conical section of the column, the gas holdup first decreases with an increase in distance away from the cone inlet, achieves a minimum and then increases until it reaches a somewhat constant value within the cylinder. Gas holdup varies radially with the maximum at the center and the minimum near the wall. Radially-averaged gas holdup increased with gas velocity and remained essentially unchanged with liquid velocity. The RTD measurements were correlated by a two-dimensional dispersion model. The axial dispersion coefficient increased linearly from the cone inlet to the cylinder. It also increased with the gas velocity. The radial dispersion coefficients were considerably smaller than the axial dispersion coefficients.     

Statistical analysis of effects of experimental variables on mass transfer coefficient in a novel hybrid bubble column using Box–Behnken design

Statistical analysis of effects of experimental variables on volumetric mass transfer coefficient in a novel hybrid rotating and reciprocating perforated plate bubble column is studied. The novel bubble column is designed indigenously using bevel gear arrangement. Agitation level, superficial gas velocity, superficial liquid velocity, perforation diameter and plate spacing are the experimental variables. Air–sodium sulphite solution system is used in this investigation. The Box–Behnken design in response surface methodology is employed for statistical analysis. The relationship between experimental variables and the desired response of volumetric mass transfer coefficient is established for this novel hybrid column. The linear, quadratic and interactive effects of experimental variables are found to be significant on the desired response of volumetric mass transfer coefficient. Results show that the data adequately fit into the second‐order polynomial model. An F‐test and P‐value show the significance of parameters on volumetric mass transfer coefficient. © 2012 Canadian Society for Chemical Engineering     

Gas hold-up and mass transfer in a bubble column with viscoelastic fluids

The influence of viscoelasticity on gas hold-up and the volumetric mass transfer coefficient in a bubble column is discussed and examined experimentally. It was found that the gas hold-up increased due to an increase in the number of entrapped very small bubbles formed because of the elasticity of the liquid. On the other hand, a decrease in volumetric mass transfer coefficient due to the fluid elasticity was observed. Qualitative effects of an antifoam agent and a draft tube on the performance of the bubble column with viscoelastic fluids were also examined.     

Effect of particles on hydrodynamics and mass transfer in a slurry bubble column: Correlation of experimental data

The effects of particle concentration and size on hydrodynamics and mass transport in an air–water slurry bubble column were experimentally studied. When the particle concentration α_s increased from 0% to 20%, the averaged gas holdup decreased by ~30%, gas holdup of small bubbles and gas–liquid volumetric mass transfer coefficient decreased by up to 50%, while the gas holdup of large bubbles increased slightly. The overall effect of particle size was insignificant. A liquid turbulence attenuation model which could quantitatively describe the effects of particle concentration and size was first proposed. Semi-empirical correlations were obtained based on extensive experimental data in a wide range of operating conditions and corrected liquid properties. The gas holdup and mass transfer coefficient calculated by the correlations agreed with the experimental data from both two-phase and three-phase bubble columns, with a maximum error <25%.     

Development of airlift bubble column dispersed with micro‐bubbles

Micro‐bubbles were dispersed in the bubble column with draft tube, and the length and diameter of draft tube were changed. The flow characteristics in air–water system were measured. Ozone gas and methylene‐blue aqueous solution were used, and the decomposition performance was examined. With increasing draft tube length, both the gas holdup and liquid velocity in the annular section increased. When the diameter ratio of draft tube to column was about 0.5, both the gas holdup and liquid circulation flow rate had maxima. For the decomposition by using ozone, the installation of draft tube enhanced the mass transfer and decomposition performance.     

Influence of elevated pressure and particle lyophobicity on hydrodynamics and gas–liquid mass transfer in slurry bubble columns

This article reports on the influence of elevated pressure and catalyst particle lyophobicity at particle concentrations up to 3 vol % on the hydrodynamics and the gas‐to‐liquid mass transfer in a slurry bubble column. The study was done with demineralized water (aqueous phase) and Isopar‐M oil (organic phase) slurries in a 0.15 m internal diameter bubble column operated at pressures ranging from 0.1 to 1.3 MPa. The overall gas hold‐up, the flow regime transition point, the average large bubble diameter, and the centerline liquid velocity were measured along with the gas–liquid mass transfer coefficient. The gas hold‐up and the flow regime transition point are not influenced by the presence of lyophilic particles. Lyophobic particles shift the regime transition to a higher gas velocity and cause foam formation. Increasing operating pressure significantly increases the gas hold‐up and the regime transition velocity, irrespective of the particle lyophobicity. The gas–liquid mass transfer coefficient is proportional to the gas hold‐up for all investigated slurries and is not affected by the particle lyophobicity, the particle concentration, and the operating pressure. A correlation is presented to estimate the gas–liquid mass transfer coefficient as a function of the measured gas hold‐up: $k_{\rm l}a_{\rm l}/\varepsilon_{\rm g} = 3.0 \sqrt{Du_{\rm b}/d_{\rm b}^3}\;{\rm s}^{-1}$ . © 2009 American Institute of Chemical Engineers AIChE J, 2010     

射流鼓泡反应器中液相体积传质系数的测定

羰基合成反应一般采用射流鼓泡反应器,该类反应器气液混合的方式采用射流而非机械搅拌,其主要优点是结构简单、制作简便、维护费用低。研究该类型反应器的传质系数对于其设计、优化及放大操作均具有重要意义。本研究采用缩颈式圆形喷嘴,以动态溶氧法对射流鼓泡反应器内的液相体积传质系数进行测定,考察了表观气速、射流雷诺数对液相体积传质系数的影响。研究发现,随气速增大液相体积传质系数的变化规律为先增大而后保持不变。维持表观气速不变,随雷诺数增加液相体积传质系数增大,但当表观气速小于0.0012 m/s时,雷诺数对传质改善较小。建立了液相体积传质系数的经验关联式,当气体输入功率占总功率56%时,液相体积传质系数最大,气体鼓泡和液体射流的协同作用最强。     

CIRCULATION IN LARGE SCALE JET BUBBLE COLUMNS

A hydrodynamic study in a gas-liquid jet bubble column was undertaken in a column with a 122 cm diameter cylindrical section and a conical bottom section approximately 180cm in height. Due to the jetting action in the cone, the circulation patterns are different from those in cylindrical bubble columns. In order to examine this difference in flow pattern, circulation velocity measurements were undertaken (for V₅ = 0.39-2.73 cm/s and V, = 0-0.044 cm/s) using the Pavlov tube technique. These measurements should be helpful in understanding other design parameters (such as mixing, phase distribution, transport coefficients, etc.). Pressure drops were also measured at nine axial taps and using these values, sectional average gas holdup values were calculated. The study was centered on the conical section; the cylindrical bubble column was assumed to be the limiting case for the interpretation of the data. Using the experimental observations, the height of the jet effective region was approximated by two different methods. A simple empirical correlation to find the centerline velocity is proposed.     

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.     

垂直管内弹状气泡上升中壁面传递的实验研究

垂直管内弹状流壁面传递是诸多工业应用中需研究的重要问题之一。今用极限扩散电流技术,对弹状气泡上升时瞬时壁面剪应力和传质系数进行了测定,结果显示:当基于表观气速的Froude数FrG < 0.74时,壁面剪应力随弹状气泡和液塞的到来呈现方向相反的交替变化,壁面传质系数亦相应变化;而当FrG > 0.74时,剪应力方向一直向下,说明液膜向下流动,且弹状气泡和液塞的到来对壁面传质系数的影响很小。这说明下落液膜射流穿透了液塞段,控制了整个壁面传递过程。研究还对下落液膜区、尾迹区及液塞段的不同传递特征及机理进行了分析, 并结合气泡塔熔融结晶器中弹状气泡上升时的传热,对结晶操作条件的合理选择进行了讨论。     

鼓泡床反应器内流动与传质行为的研究进展

总结了有关浆态鼓泡床反应器内流动、混合用气液传质特性的研究成果,详细地介绍了鼓泡床反应器内气含率、液速、液体轴向扩散系数、传质系数的测量方法,阐述了鼓泡床反应器性能的主要影响因素,如系统压力、温度、气体表观气速、液体性质及固含率等对流动、液相混合和传质特性的影响,并对鼓泡床反应器的应用前景进行了详述.     

Mathematical modeling of gas-liquid mass transfer rate in bubble columns operated in the heterogeneous regime

In this paper, a multi-scale approach is followed to study gas-liquid mass transfer in bubble columns. First, a single bubble of equivalent diameter d is considered. Its morphology and its gas to liquid relative velocity are related to the bubble diameter through the use of known correlations. Then, the gas-liquid mass transfer between the bubble and the surrounding liquid is studied theoretically. An equation describing the transport of the transferred species in the viscous boundary layer around the bubble is solved. In a second step, a bubble column of 6-10 m height is studied experimentally. The gas phase in the column is characterized experimentally by means of a gammametric technique. Finally, the two studies are linked, yielding a 1D mathematical model able to predict the gas-liquid mass transfer rate in a bubble column operated in the heterogeneous regime.     

新型气液逆流撞击洗涤喷嘴的优化

针对一种新型气液逆流撞击式洗涤喷嘴,通过冷模实验,采用溶氧法考察了不同结构喷嘴的气液两相传质性能。结合解析率及流型变化,考察了喷嘴出口直径、切向进液口倾角、旋流室收敛段锥角、切向进液口直径、喷口长度5个参数对传质的影响,确定了优选喷嘴的结构尺寸,分析了该优选喷嘴在不同操作条件下(气速、表观液气比和轴切比)的传质效果。结果表明,优选喷嘴在轴切比为0.4~0.6且气速较高时传质效果较好。     

An optimization method for enhancement of gas–liquid mass transfer in a bubble column reactor based on the entropy generation extremum principle

In this study, an optimization method is proposed to enhance the gas–liquid mass transfer in bubble column reactor based on the entropy generation extremum principle. The mass transfer–induced entropy generation can be maximized with the increase of mass transfer rate, based on which the velocity field can be optimized. The oxygen gas–liquid mass transfer is the major rate–limiting step of the toluene emissions biodegradation process in bubble column reactor, so the entropy generation due to oxy...     

Impact of Cellulose and Surfactants on Mass Transfer of Bubble Columns

The effects of cellulose, surfactants, and their combination on the hydrodynamic behavior and the liquid‐side mass transfer coefficient of a bubble column were evaluated. For that purpose, different aqueous solutions containing surfactants (sodium dodecyl sulfate) and cellulose (microcrystalline cellulose, MCC) were investigated. The interfacial areas were calculated from the bubble diameters, the bubble frequencies, and the terminal bubble rising velocities. The liquid‐side mass transfer coefficients were determined from the volumetric mass transfer coefficients measured by the dynamic method. In the concentration range under test, the experimental results proved that the addition of MCC to the studied liquid phases did not affect the mass transfer coefficient.