Numerical and experimental investigations of gas-liquid dispersion in an ejector

The effects of the ejector geometry (nozzle diameter and mixing chamber) and the operating conditions (liquid circulating rate, liquid level in column) on the hydraulic characteristics in a rectangular bubble column with a horizontal flow ejector were determined. The gas phase holdup increases with increasing liquid circulating rate but decreases with increasing liquid level in the column. In the multiphase CFD simulation with the mixture model and the experiments, the gas suction rate increases with increasing liquid circulating rate. However, the gas suction rate decreases with increasing the liquid level in the column and nozzle diameter. The predicted values from the CFD simulation are well accord to the experimental data.     

Studies on the hydrodynamic behavior and mass transfer in a down‐flow jet loop reactor with a coaxial draft tube

The effects of certain pertinent parameters such as gas and liquid flow rates and nozzle position on the behavior of a down‐flow jet loop reactor (DJR) have been studied. The mean residence times of gas and liquid phases and the gas holdup within the reactor have been measured. In addition, the overall volumetric mass transfer coefficient, and the influence of the gas flow rate and the position of the nozzle inside the draft tube on the latter has been determined. Correlations have been presented for the gas holdup and k_La which take into account the length of the draft tube and the nozzle immersion height. The k_La values obtained at different power per unit volume (P/V) values in the DJR used in the present study compare favorably with data presented for stirred tanks and bubble columns in the literature. The liquid residence time distribution (RTD) within the reactor has been studied by tracer analysis for various operating conditions and nozzle immersion height and the results are indicative of the high mixing intensities that can be obtained in such reactions. © 2001 Society of Chemical Industry     

Gas holdup in a two‐phase reversed flow jet loop reactor

Experimental investigations have been carried out in Reversed Flow Jet Loop Reactor (RFJLR) to study the influence of liquid flow rate, gas flow rate, immersion height of two‐fluid nozzle in reactor and nozzle diameter on gas holdup without circulation, that is, gas–liquid mixture in draft tube only (E_gd) and gas holdup with circulation loop (E_g). Also critical liquid flow rate required for transition from draft tube to circulation loop has been determined. Gas holdup was measured by isolation valve technique. Gas holdup in draft tube and circulation loop increased with increase in liquid flow rate and gas flow rate. It is observed that the increased flow rate is required for achieving a particular value of gas holdup with larger nozzle diameter. Nozzle at the top edge of draft tube have higher gas holdup as compared to other positions. It has been noted that, no significant recirculation of gas bubbles into the top of draft tube from annulus section has been observed till a particular liquid flow rate is reached. A plot of gas holdup with no circulation and with circulation mode determines minimum liquid flow rate required to achieve complete circulation loop. Critical liquid flow rate required to achieve complete circulation loop increases with increase in gas flow rate and is minimum at lowest immersion height of two‐fluid nozzle.     

OXYGEN TRANSFER IN THE WATER-JET VESSEL

This is an investigation of the gas holdup and the volumetric mass transfer coefficient of a plunging water jet in an air-water system. We sound k_La to be directly proportional to gas holdup in two regions. For the first time, this has been clarified in the plunging liquid jet system. The volumetric mass transfer coefficient and the gas holdup have been well correlated in terms of the Froude number, liquid jet length, nozzle diameter and vessel diameter.     

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.     

Hydrodynamics of a spouted bed with a porous draft tube containing a small amount of finer particles

A small-size spouted bed with a porous draft tube was employed to obtain hydrodynamic data of binary mixtures of glass beads for a range of operating conditions and design factors. In this case, a small amount of finer particles was added mostly to the large majority of coarser particles. Under this condition, minimum spouting velocity, bed pressure drop, hold-up of solid particles within a draft tube, gas flow rate through the annulus and solids circulation rate were determined by changing the total gas flow rate and the mass fraction of finer particles as operating parameters, and by changing the height of the entrainment zone and the draft tube diameter as geometric parameters. The results show that the gas flow rate through the annulus increases by increasing the distance between the gas inlet nozzle and the bottom of the draft tube, that is, the height of the entrainment zone, but decreases with increasing draft tube diameter and mass fraction of finer particles. The porous draft tube shows a higher gas flow rate through the annulus than the non-porous draft tube, particularly in the case of the low height of the entrainment zone. The solids circulation rate increases with increasing gas velocity, the height of the entrainment zone and the porous draft tube diameter. Moreover, the porous draft tube leads to a higher solids circulation rate than the non-porous draft tube.     

三相下喷式环流反应器的传质性能

在三相非牛顿型流体体系中，对下喷式环流反应器传质特性进行了实验研究。讨论了表观气速、能量耗散速率、导流筒直径与反应器直径比、喷嘴直径、导流筒下端距反应器底部的距离、固体装填量、羧甲基纤维素钠（ＣＭＣ）溶液浓度及其流变特性对它的影响。实验结果表明，容积传质系数随表观气速和能量耗散速率的增加有所增加，在实验条件下，发现最优的导流筒直径与反应器直径比在０．４～０．４５这一范围、固体装填量大约为３％（体积百分比）、导流筒下端距反应器底部的距离为０．０８ｍ左右。同时提出了容积传质系数的经验关联式。     

内循环流化床结构参数及其反应器性能的相关性

内循环三相流化床在化学工程、生物工程及废水处理等领域中均有广泛的应用前景,目前影响这类反应器成功应用于实际废水生物处理的主要原因是缺乏系统的反应器结构参数与反应器性能的相关性分析与量化关系模型,无法实现不同规模与不同应用目标的系统优化设计。基于此,设计了不同尺寸和内构件的反应器,系统研究主要结构参数包括升流管与反应器直径之比、反应器高径比、升流管上方液面高度、底隙高度以及操作参数如固相负荷及通气量等因素对内循环流化床反应器流体力学与传质特性的影响规律;实验分析主要过程参数如气含率、液体循环速度、氧传递系数及混合时间与结构参数、操作参数的相关性。通过总结归纳,分别给出了定量、半定量或定性方面的结论,为反应器的结构放大和性能变化预测提供理论基础.     

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.     

双喷嘴矩形导流管喷动床喷动压降

引言 传统的柱锥形喷动床(CSBs)的应用受到一些因素的限制,例如处理物料能力、起始压力大及难以放大等,为此,Romankov等[1]最早提出了长方形截面的矩形喷动床结构,来克服CSBs的缺点.此后,Rocha等[2]用矩形喷动床进行了片剂包衣的研究.     

Experimental determination of gas holdup and volumetric mass transfer coefficient in a jet bubbling reactor

The hydrodynamics and mass transfer characteristics of a lab-scale jet bubbling reactor(JBR) including the gas holdup, volumetric mass transfer coefficient and specific interfacial area were assessed experimentally investigating the influence of temperature, p H and superficial gas velocity. The reactor diameter and height were 11 and 30 cm,respectively. It was equipped with a single sparger, operating at atmospheric pressure, 20 and 40℃, and two p H values of 3 and 6. The height of the liquid was 23 cm, while the superficial gas velocity changed within 0.010–0.040 m·s~(-1) range. Experiments were conducted with pure oxygen as the gas phase and saturated lime solution as the liquid phase. The liquid-side volumetric mass transfer coefficient was determined under unsteady-state oxygen absorption in a saturated lime solution. The gas holdup was calculated based on the liquid height change, while the specific interfacial area was obtained by a physical method based on the bubble size distribution(BSD) in different superficial gas velocities. The results indicated that at the same temperature but different p H, the gas holdup variation was negligible, while the liquid-side volumetric mass transfer coefficient at the p H value of 6 was higher than that at the p H = 3. At a constant p H but different temperatures, the gas holdup and the liquid-side volumetric mass transfer coefficients at 40℃ were higher than that of the same at 20℃. A reasonable and appropriate estimation of the liquid-side volumetric mass transfer coefficient(kla) in a pilot-scale JBR was provided which can be applied to the design and scale-up of JBRs.     

Experimental study on gas–liquid dispersion and mass transfer in shear-thinning system with coaxial mixer

The effects of impeller type, stirring power, gas flow rate, and liquid concentration on the gas–liquid mixing in a shear-thinning system with a coaxial mixer were investigated by experiment, and the overall gas holdup, relative power demand, and volumetric mass transfer coefficient under different conditions were compared. The results show that, the increasing stirring power or gas flow rate is beneficial in promoting the overall gas holdup and volumetric mass transfer coefficient, while the increasing system viscosity weakens the mass transfer in a shearing–thinning system. Among the three turbines, the six curved-blade disc turbine (BDT-6) exhibits the best gas pumping capacity; the six 45° pitched-blade disc turbine (PBDT-6) has the highest volumetric mass transfer coefficient at the same unit volume power.     

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.     

Effect of geometrical parameters of draft tubes and clear liquid height on gas holdup in a bubble column for gas dispersion into tubes

The effects of the geometrical parameters of draft tubes and the clear liquid height on the average gas holdup E_G in a 0.16 m I.D. bubble column for gas dispersion into the tubes were experimentally studied in an airtap water system. The gas holdup depended on the superficial gas velocity U(ING), the kinds of gas spargers, the diameter and length of the draft tubes, clearance C_b between the lower end of the draft tube and the bottom of the bubble column, and the clear liquid height H_L. E_G increased with decreasing hole diameter of the gas sparger at a small gas velocity U_G, but did not depend on the kinds of gas spargers at a large U_G. E_G decreased with increasing clear liquid height H_L. The effect of H_L on E_G was well expressed by the modified three-region model. The experimental data of E_G were correlated.     

Hydrodynamics in down flow jet loop reactor

Gas holdup and liquid circulation time were measured in a down flow jet loop reactor for air–water system. It was observed that the circulation time decreases with an increase in nozzle diameter, draft tube to column diameter ratio and liquid velocity. Th gas holdup increases with an increase in gas and liquid velocities. The optimum draft tube to column diameter ratio was found to be 0.438. Correlations for gas holdup and circulation time involving operational and geometrical variables are presented.     

连续内环流三相反应器局部流动特性

在φ200 mm×2500 mm连续内环流三相反应器内,考察了空气 水 玻璃珠体系反应器内局部流动参数随操作条件的变化规律。结果表明,导流筒内截面平均气含率随表观气速的增大而增大,较之气液两相流,在低固含率时,加入固体对气含率影响不明显,而在较高固含率下,气含率有明显降低,但固体再增加时对气含率变化影响不大。在较低表观气速下,进料浆速对导流筒内气含率轴向分布趋势有一定的影响,但在较高表观气速下影响不大,导流筒内的气含率大于环隙内的气含率且随气速增大差别更加明显,浆相连续有利于气相分散并增大环隙内的气含率。导流筒内循环浆速径向分布呈抛物状,中心高、近壁处低,受进料浆速和入口固含率影响都不大。浆相循环强度最低为20,高可达180。固含率轴、径向分布受表观气速和进料浆速的影响,固含率轴、径向分布基本均匀,随进料浆速增加,反应器内固含率降低。     

气升式内循环反应器的数值模拟和结构参数

采用Euler-Euler双流体模型对内循环反应器(高1240 mm,直径165 mm,导流筒高590 mm)进行数值模拟,考察了表观气速、导流筒结构(导流筒内径比Dt/D,底隙高度)对反应器内上升区、下降区流体力学参数(气含率、液体速度)的影响. 结果显示,表观气速、导流筒内径、底隙高度对反应器气含率、液体速度有很大影响,随表观气速增加,反应器上升区、下降区气含率都增加,导流筒内径比为0.58时更易实现气液循环,底隙高度为30 mm时反应器内下降区气含率、气液速度都最小;气液分离器角度越大,进入下降区的气体越多,当气液分离器角度为45o时,能更好地实现气液循环.     

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.     

Gas holdup and overall volumetric mass transfer coefficient in a modified reversed flow jet loop reactor

Experiments were conducted in a modified reversed flow jet loop reactor having the liquid outlet at the top of the reactor to determine the gas holdup and overall volumetric mass transfer coefficient in the air-water system. The influence of gas and liquid flow rates, and the draft tube to reactor diameter ratio were studied. It was observed that both gas holdup and volumetric mass transfer coefficient increased with increased gas and liquid flow rates and were found to be significantly higher in the modified reactor compared to the conventional one. The optimum draft tube to reactor diameter ratio was found to be in the range of 0.4 to 0.5. Empirical correlations are presented to predict gas holdup and overall volumetric mass transfer coefficient in terms of operational and geometrical variables.     

Prediction of Pressure Drop and Holdup for Homogeneous Ternary Mixtures of Spherical Glass Bead Particles in a Three-Phase Fluidized Bed

Hydrodynamic characteristics, viz. bed pressure drop and gas holdup, have been studied for ternary mixtures of homogeneous regular particles in a co-current three-phase fluidized bed. For this, a series of experiments have been carried out in a 5-cm diameter column with air as the gas phase, water as the liquid phase, and ternary mixtures of glass beads (1.54, 1.3, and 1.1 mm) as the solid phase. The dependence of bed pressure drop on the average particle diameter, superficial gas velocity, and initial static bed height has been discussed. Based on the dimensional and statistical analyses, correlations have been developed with the system parameters, for both bed pressure drop and gas holdup. Experimental values of bed pressure drop and gas holdup have been found to agree well with those calculated from developed correlations.