Bubble swarm behaviour and gas absorption in non-newtonian fluids in sparged columns

Mean relative gas hold up, slip velocity, bubble size distribution, and volumetric mass transfer coefficient of oxygen were measured in sparged columns of highly viscous non-Newtonian fluids (CMC solutions) as a function of the gas flow rate, and CMC concentration (fluid consistency index k, and flow behaviour index n).By comparison of the measured bubble swarm velocities with those calculated by relations for single bubbles the bubble swarm behaviour was investigated. It could be shown that small bubbles in swarm have higher rising velocities than single bubbles, expecially in highly viscous media. Large single bubbles rise with high velocity due to the change of their shape caused by the swarm of the smaller bubbles. No large bubbles with spherical cap shape could be observed. The volumetric mass transfer coefficient decreases rapidly with increasing CMC-concentration.A comparison of the volumetric mass transfer coefficients with those measured in mechanically agitated vessels indicates, that the performance of sparged columns is comparable with the one of agitated vessels. Because of their lower energy requirement sparged columns are more economical than mechanically agitated vessels. It is possible to improve the performance of sparged columns by the redispersion of large bubbles in a multistage equipment.     

Effect of Surface Tension on Hydrodynamics and Mass Transfer Coefficient in Airlift Reactors

The hydrodynamics and volumetric mass transfer coefficient play important roles in the design and scale-up of airlift reactors. The effect of surface tension on hydrodynamics and volumetric mass transfer coefficient in internal loop airlift reactors was investigated. With reduction of the surface tension of the fluid, the hydrodynamic parameters raised, namely, gas phase holdup, flow regime transition point, and interfacial area, whereas the bubble diameter as well as the liquid velocity decreased and the volumetric mass transfer coefficient increased. Empirical correlations are proposed for gas-phase holdup and volumetric mass transfer coefficient in terms of dimensionless numbers and can be applied in the design of airlift reactors.     

Erstes Design und Einsatz 3D‐gedruckter Strukturen in Blasensäulenreaktoren

A new method is described, in which 3D‐printed structures are implemented in bubble column reactors to dissolve the macroscopic and the microscopic influences. The effect of these structuring method on the fluid dynamic behavior, the gas distribution, mass transfer, and fluid velocity inside bubble columns is shown in this contribution.     

Flow regimes and mass transfer in counter-current bubble columns

Counter current bubble columns have the feature that specific gas-liquid interfacial area and gas holdup are larger than those for standard and cocurrent bubble columns. In this study, three different flow regimes, churn-turbulent flow, bubble flow and bubble down-flow, have been observed in a counter-current bubble column and correlations of gas holdup and volumetric liquid-phase mass transfer coefficient have been proposed as functions of operating variables such as the superficial velocities of gas and liquid, the gas-liquid slip velocity and the liquid properties.     

Hydrodynamics and mass transfer in a bubble column with an organic liquid

Hydrodynamic and mass transfer data in Soltrol-130 (a mixture of C₉+ iso-paraffins) were measured in a 0.305 m diameter bubble column. The gas holdup structure (i.e., the contributions to holdup from the small and large bubble fractions of the dispersion) for this hydrocarbon liquid in the churn turbulent flow regime was analyzed using the dynamic gas disengagement technique. The validity of the assumption of axially uniform gas holdup structure was checked. Literature correlations were found inadequate to explain the observed gas holdup and the volumetric mass transfer coefficients for Soltrol-130. The volumetric mass transfer coefficient per unit volume of large bubbles is shown to be independent of superficial gas velocity for the fully developed churn turbulent regime. The present hydrodynamic and mass transfer data in the churn turbulent regime should be useful in the design and scale-up of bubble columns used in organic process industries.     

鼓泡塔细胞反应器流体力学与传质特性

以超大规模细胞培养为目的,构建了与细胞培养体系十分接近的冷模实验体系,系统地研究了微载体(Cytodex I)、细胞保护剂(Pluronic F68)和消泡剂(Antifoam C)对鼓泡塔反应器中气、固、液三相流流体力学和氧传质特性的影响。在0.04～0.17cm/s 表观气速范围内采用 50 μm 孔径的烧结金属滤芯曝气时,在含有 0.5 和 1.0 g/L 的 Pluronic F68 的磷酸盐缓冲溶液冷模体系中,气含率与表观气速成线性增加关系,而气泡直径受表观气速影响较小;相同气速下的冷模体系与空气-水体系相比,气含率显著提高,气泡直径明显减小。在所研究的表观气速范围内微载体均可全悬浮,对气含率有一定增强,但微载体浓度为14%～20% 时对气泡大小几乎无影响。消泡剂用量在 1.60×10^-4时,可以有效抑制泡沫的形成。添加剂对液膜传质系数 k_L有较大负面作用,抵消了小气泡带来的传质面积增加,总的体积传质系数k_La 变化不大。Euler-Euler 多相流计算流体力学模型与拟稳态实验数据吻合较好,可用于指导反应器放...     

鼓泡反应器中幂律型流体流动与传质特性

很多废水处理装置涉及非牛顿型流体中的多相流动和传质问题,研究其中的气液传质过程有助于实现装置的优化设计和高效节能运行。以鼓泡反应器内清水和不同质量分数的羧甲基纤维素钠（CMC）水溶液为实验对象,分别研究气相表观气速和液相流变特性对气泡尺寸分布、全局气含率和体积氧传质系数的影响。实验结果表明,液相的流变特性对其传质特性参数均有较大影响。与清水相比,CMC水溶液中气泡平均直径和分布范围更大;清水和CMC水溶液的全局气含率均随表观气速的增加而增大;CMC水溶液的体积氧传质系数随CMC水溶液质量分数的增加而减小。基于实验研究,得出修正的体积氧传质系数公式和适用于幂律型非牛顿流体流动体系氧传递过程的无量纲关联式,可很好地实现非牛顿流体流动的废水处理装置中气液传质参数的计算。     

Gepackte Aufstrom-Blasensäulen

Packed upflow bubble columns . Packed upflow bubble columns are used in the chemical industry, in biotechnology, and in waste-water purification. They are usually operated in the co-current mode and have various advantages, but also disadvantages, compared with empty bubble columns. This survey reports the current state-of-the-art in fluid dynamics (flow states, pressure drop, holdup and dispersion, bubble size and bubble rise velocity, interfacial area), mass and heat transfer (mass transfer coefficients at the gas/liquid and liquid/solids interface, heat transfer coefficients for fluid/solids and fluid/wall interfaces, effective thermal conductivity of the bulk solids through which flow occurs), flow models (continuum models, stage models, zone models), and other aspects of this type of multiphase reactor; gaps in our knowledge are also indicated.     

Fluid dynamics and mass transfer in a bubble column with suspended particles

Slurry bubble columns are widely used in biotechnology. Therefore, the effects of solid particles on fluidization characteristics, gas hold-up and volumetric liquid-side mass transfer coefficient were measured in a slurry bubble column (i.d. 0.14 m). The density and diameter of the suspended particles were similar to those applied in biotechnology with immobilized bacteria. Based on models of turbulence and of liquid circulation induced by rising gas bubbles, equations for critical gas velocity, gas hold-up and volumetric liquid-side mass transfer coefficient were obtained by dimensional analysis.     

Gas holdup in homogeneous and heterogeneous gas—liquid bubble column reactors

The velocity‐holdup relationship is the most important design parameter for gas—liquid bubble column reactors, providing the basis for the prediction of heat and mass transfer coefficients and information on hydrodynamic conditions. A summary of the literature on gas holdup in bubble columns is supplemented by new experimental results which extend the data range. A criterion for the gas velocity leading to the transition between homogeneous and heterogeneous regimes for perforated plate gas distributors has been developed. Correlations for gas holdup in both regimes are developed and verified against both new and existing data.     

Hydrogenation of 2‐ethylanthraquinone under Taylor flow in single square channel monolith reactors

The hydrogenation of 2‐ethylanthraquinone (EAQ) to 2‐ethylanthrahydroquinone (EAHQ) was carried out under Taylor flow in single square channel monolith reactors. The two opening ends of opaque reaction channel were connected with two circular transparent quartz‐glass capillaries, where Taylor flow hydrodynamics parameters were measured and further used to obtain practical flow state of reactants in square reaction channels. A carefully designed gas‐liquid inlet mixer was used to supply steady gas bubbles and liquid slugs with desired length. The effects of various operating parameters, involving superficial gas velocity, superficial liquid velocity, gas bubble length, liquid slug length, two‐phase velocity and temperature, on EAQ conversion were systematically researched. Based on EAQ conversion, experimental overall volumetric mass transfer coefficients were calculated, and also studied as functions of various parameters as mentioned earlier. The film model, penetration model, and existing semi‐empirical formula were used to predict gas‐solid, gas‐liquid, and liquid‐solid volumetric mass transfer coefficients in Taylor flow, respectively. The predicted overall volumetric mass transfer coefficients agreed well with the experimental ones. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

搅拌反应器内三种桨型的气、液分散与相际传质特性研究

本文以改进搅拌发酵罐的桨型为目的,对空气-水、空气-亚硫酸钠溶液系统就六平叶、弯叶、箭叶三种圆盘透平桨产生的气泡平均直径、气含率及容量传质系数的变化规律作了较为系统的研究。实验发现:相同单位体积功率、表观气速条件下,三种桨型各自产生的气泡平均直径相差不大;箭叶桨的气含率较低;六平叶圆盘透平桨具有最大的容量传质系数。     

高温高压浆态鼓泡床反应器中的气-液传质

The gas-liquid mass transfer of H2 and CO in a high temperature and high-pressure three-phase slurry bubble column reactor is studied. The gas-liquid volumetric mass transfer coefficients kLa are obtained by measuring the dissolution rate of H2 and CO. The influences of the main operation conditions, such as temperature, pressure,superficial gas velocity and solid concentration, are studied systematically. Two empirical correlations are proposed to predict kLa values for H2 and CO in liquid paraffln/solid particles slurry bubble column reactors.     

Bubble column research in Japan

Many experimental studies on the bubble column have been reported by Japanese researchers since around 1960. They include studies of bubble behaviour, bubble size distribution, transition from the homogeneous bubbly flow regime to the heterogeneous liquid circulation regime, liquid velocity distribution, longitudinal liquid mixing, hydrodynamic modelling, the gas holdup, and the volumetric coefficient of gas-liquid mass tranfer k_La. Studies covered various modified bubble columns, such as the airlift reactor with an external or internal loop, the packed bubble column, and others. Performance of three-phase bubble columns, which deal with suspensions or emulsions, and their use as bioreactors or chemical reactors were also studied.     

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.     

三重环流生物流化床的流体力学与传质特性

从气相含率,液体循环速度和体积氧传质系数方面研究自行设计新型结构的三重环流生物流化床的流体力学与传质特性。流化床反应器的有效体积２３Ｌ,实验条件是以空气为气相,水为液相,树脂为固相,固含率分别为０％,３％,６％和９％。实验结果表明,气相含率是影响反应器流体力学和传质特性的主要因素,气相含率增大可降低液体循环速度,增大体积氧传质系数。     

Local gas–liquid slip velocity distribution in bubble columns and its relationship with heat transfer

The knowledge of the local gas–liquid slip velocity distribution can offer a better understanding for the complex transport phenomena in bubble columns. In this work, CFD–PBM simulations are carried out to investigate the effect of superficial gas velocities, axial positions, and scale of bubble columns on the time-averaged radial profiles of gas–liquid slip velocities. Furthermore, the relationship between local slip velocities and local heat transfer coefficients in pilot-scale bubble columns at superficial gas velocities of 0.05 m/s, 0.20 m/s, and 0.35 m/s is studied. The results indicate that the slip velocities decrease with the increase of r/R (r-radial position, R-column radius), while increase with increasing superficial gas velocities in general. In the fully developed region, the axial positions have small impact on the local slip velocities. A strong linear relation between heat transfer coefficients and slip velocities in the fully flow developed region is observed.     

COMPREHENSIVE STUDY OF THE GAS HOLD UP AND BUBBLE SIZE DISTRIBUTION IN HIGHLY VISCOUS LIQUIDS II. CMC SOLUTIONS

Following properties of short bubble columns employing CMC solutions (1.0, 1.4 and 2.0%) and perforated plates (0.5, 1.0 and 3.0 mm hole diameters) were determined: relative mean gas hold up, E_G, bubble size distribution, Sauter bubble diameter, d_s, and the specific geometrical bubble surface areas, a' and a' due to the “intermediate to large bubbles.” The a' values were compared with the corresponding volumetric mass transfer coefficients, k_La's and the mass transfer coefficients, k_L, were estimated. The properties of these systems were investigated as function of the superficial gas velocity, W_SG, CMC concentration and aerator type.     

Pressure Drop and Mass Transfer Study in Static Mixers with Gas Continuous Phase

The aim of this work was to characterize both the influence of the gas and the liquid flow rates on the pressure drop generated by static mixers type Sülzer, and on the mass transfer performances of these gas‐liquid contactors. The originality of this work rests on the use of the static mixers with a gas continuous phase. Several configurations were studied: vertical upward flow, vertical downward flow, and horizontal positions, with one to five mixing elements. It was concluded that the pressure drop is mainly generated by the gas phase, whatever the configuration chosen. Moreover, the volumetric mass transfer coefficients k_La found were lower than those obtained with the same static mixers used in classic conditions (that is with a liquid continuous phase), but greater than values obtained with classic reactors like bubble columns or packed columns. The efficiency of these gas‐liquid reactors was found high, as well as the energy dissipated, unfortunately.     

Effect of aeration mode on the performance of center‐ and annulus‐rising internal‐loop airlift bioreactors

Extensive experimental studies on internal‐loop airlift reactors, including center‐rising (CR‐ALR) and annulus‐rising airlift reactors (AR‐ALR), have been reported in the literature. However, to the best of the authors’ knowledge, the effects of the aeration mode on the local hydrodynamics remain an under‐investigated area, especially for complex culture media. At present, it is difficult to select the best aeration mode for ALRs due to limited understanding of the pros and cons of the different modes. This study presents a detailed quantitative investigation of the overall gas holdup, local liquid velocity, liquid circulation time, shear rate distribution, and volumetric mass transfer coefficient in center‐ and annulus‐rising airlift bioreactors to better understand the effect of aeration mode on airlift bioreactor performance. Particle image velocimetry is employed to conduct local measurements. The results show that the overall gas holdup, liquid circulation time, and volumetric mass transfer coefficient are larger in the AR‐ALR than in the CR‐ALR. The local liquid velocity circulating into the downcomer of the AR‐ALR, which contributes to bubble entrainment and therefore to overall gas holdup, is higher than in the CR‐ALR. It was observed that a large circulation loop formed in the CR‐ALR, whereas two counter‐looping circulation cells appeared in the AR‐ALR. It was also found that the shear rate field was more uniform in the AR‐ALR than the CR‐ALR although the shear rates were similar in magnitude.