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.     

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.     

Liquid/Solid Mass Transfer in Fixed Beds

The liquid/solid mass transfer behavior of a fixed bed of cylinders was studied using the diffusion‐controlled dissolution of copper in acidified potassium dichromate. Variables studied were solution velocity and cylinder diameter. For a steady flow, the data were correlated for the conditions 25 < Re < 600 and Sc = 990 by the equation Sh = 2.65 Sc^0.33 Re^0.5. A comparison between the present data and previously obtained data for a fixed bed of Raschig rings has revealed that cylinder packing produces higher rates of mass transfer than Raschig rings.     

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.     

Solid‐Liquid Mass Transfer at Gas Sparged Tube Bundles

The solid‐liquid mass transfer characteristics of an in‐line tube bank immersed in a gas‐liquid bubble column were investigated by measuring the rate of diffusion‐controlled dissolution of copper surface in acidified dichromate solution. Variables studied were the number of rows in the tube bank, physical properties of the solution, and nitrogen flow rate. The mass transfer coefficient was found to increase with increasing nitrogen flow rate. Increasing the number of rows in the tube bank was found to decrease the mass transfer coefficient. The data were correlated for the following conditions: 0.0021 < Fr.Re < 0.1603, 1 < N_r < 5 and 850 < Sc < 1370 by the equation J = 0.15 ( Fr.Re )^–0.15. Comparison was made between the present mass transfer data and previous heat and mass transfer studies conducted in packed and empty bubble columns.     

Volumetric Mass Transfer Coefficient of Oxygen in An Internal Loop Airlift Reactor with a Convergence‐Divergence Draft Tube

Experimental measurements of hydrodynamics and the volumetric mass transfer coefficient of oxygen (VMTCO) in an internal loop airlift reactor with different types of draft tubes are reported for the two‐phase systems, air/water and air/carboxyl methyl cellulose (CMC) solution, and a three‐phase system, air/water/resin particle, respectively. The properties of convergence‐divergence draft tubes with three different structural parameters are compared with those of the conventional column draft tube. The results indicate that gas holdups in convergence‐divergence draft tubes are higher than those in conventional draft tubes, the volumetric mass transfer coefficient of oxygen increases with increasing superficial air flow rates. The convergence‐divergence draft tubes all show higher mass transfer capacity than the traditional ones. A 10% higher mass transfer coefficient is observed for the three structural parameters. In the air/CMC system, the volumetric mass transfer coefficient of oxygen decreases with increasing bulk viscosity, while in the three‐phase system VMTCO increases with the resin particle loading. The correlation equation of the volumetric mass transfer coefficient with the operating conditions and structural parameters is established.     

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 ...     

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.     

滴流床小颗粒催化剂滴流区和脉动流区传质的研究

本文研究了滴流床小颗粒催化剂在滴流区和脉动流区的气相容积传质系数k_Gα,气液接触有效界面积“α”和脉动流形成时分散气泡的平均直径d_b。实验表明:不同流区的传质与流型和流动介质的能量损失有关。强化传质会增加流动介质的能量消耗及减小催化剂粒度。     

Impact of Surfactant Addition on Oxygen Mass Transfer in a Bubble Column

The impact of sparger design and surfactant addition on the oxygen transfer rate in a bubble column was examined. Additionally, measurements were also made of the holdup and bubble size distribution, allowing both the interfacial area for mass transfer and the liquid film mass transfer coefficient to be determined for a range of industrially relevant superficial velocities. It was found that for the velocity range examined changes in the superficial velocity had a minimal impact on the observed value of liquid film mass transfer coefficient. In contrast, addition of both hydrophilic and hydrophobic surface‐active compounds led to an approximately threefold reduction in liquid film mass transfer coefficient.     

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.     

并流降膜式多管湿壁塔的质量传递特性

在内径为0.1 m,由38根内径为8 mm,外径为10 mm,高120 mm陶瓷管组成的多管湿壁塔中,采用SO2/NaOH和CO_2/H_2O系统,研究了20℃的气液传质速率。实验结果表明,液相雷诺数在Re_L在55~165之间,气相雷诺数Re_G在360~2 000之间,液相舍伍德数分别与Re_G的0.662次方和Re_L的0.851次方成正比;气相舍伍德数分别与Re_G的1.103Re_L的0.226次方成正比。     

Improving Gas‐Liquid Mass Transfer in Bubble Columns by Applying Low‐Frequency Vibrations

We show that application of low‐frequency vibrations, in the 50–200 Hz range, to the liquid phase of an air‐water bubble column causes significantly smaller bubbles to be generated at the distributor plate. For bubble column operation in the homogeneous flow regime, measurements of the volumetric mass transfer coefficient using the oxygen absorption technique show that the increase in the k_La values ranges from 50–100 % depending on the flow rate. It is concluded that application of low‐frequency vibration has the potential of improving the performance of bubble columns.     

鼓泡塔中非牛顿流体体系的传质研究

本文在直径0.10 m、高1.05 m的鼓泡塔中,以羧甲基纤维素钠作为模拟介质,采用单孔喷嘴布气、孔径d_o=0.01 m,测定了该类反应器的比相界面积和容积传质系数,提出了比相界面积和传质系数的关联式。     

Mass transfer to clean bubbles at low turbulent energy dissipation

Experimental gas-liquid mass transfer data from a system where single bubbles are kept stationary by a low turbulence downward liquid flow are compared with predictions from theoretical models which take turbulence into account. The data clearly show that, for the turbulence intensities at play, turbulence does not affect mass transfer. Higbie's equation with contact time t_e=d/u may be used for the calculation of the gas-liquid mass transfer coefficient of clean bubbles in systems with levels of turbulent kinetic energy dissipation of, at least, up to .     

A Simple Approach to Measuring the Gas Phase Heat and Mass Transfer Coefficients in a Bubble Column

A simple experimental approach was developed to measure the gas phase volumetric heat and mass transfer coefficients in a bubble column and a slurry bubble column employing a single gas nozzle. The experimental technique was based on a transfer model that simulates humidification and direct contact evaporation models in the case of a gas bubble rising in a liquid of uniform temperature. The temperature and relative humidity of the inlet and outlet gas in the column are the only measurements required in this technique. Experiments were carried out in a 0.15 m inner diameter column using water as the liquid phase, air as the gas phase, and cation resins of 0.1 mm diameter and a specific gravity of 1.2, as the solid phase. The results showed that, when using solid concentrations in the range of 7–10 wt %, both the volumetric gas‐phase heat and mass transfer coefficients increased with an increase in the gas superficial velocity and were further enhanced by increasing the solid load after a certain minimum superficial velocity had been reached in the column (0.044 m/s in the system used). Increasing the solid load beyond 10 wt %, did not contribute to a further increase in these coefficients. Furthermore, the gas holdup in the column increased with the superficial gas velocity and was further enhanced when the solid‐phase load was in the range of 7–10 wt %. These observations agree well with previously reported findings by other investigators.     

Ozone Mass Transfer in Stirred Vessel

A method is proposed and a model is developed which are capable of providing a correlation of the mass transfer coefficient k_La, with stirrer speed and gas superficial velocity. The method can be adopted for deriving a correlation which can be profitably used for ozone gas–liquid reaction both for assessing the absorption regime and for the simulation of oxidation processes which evolve according to slow reaction regime.     

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.