Effects of surfactants on hydrodynamics and mass transfer in a split‐cylinder airlift reactor

Effects of various concentrations (0–5 ppm) of anionic (sodium dodecyl sulfate, SDS) and non‐ionic (Tween‐80 and Triton X‐405) surfactants on gas hold‐up and gas–liquid mass transfer in a split‐cylinder airlift reactor are reported for air–water. Surfactants were found to strongly enhance gas hold‐up. Non‐ionic surfactants were more effective in enhancing gas hold‐up compared to the anionic surfactant SDS. An enhanced gas hold‐up and a visually reduced bubble size in the presence of surfactants implied an enhanced gas–liquid interfacial area for mass transfer. Nevertheless, the overall gas–liquid volumetric mass transfer coefficient was reduced in the presence of surfactants, suggesting that surfactants greatly reduced the true liquid film mass transfer coefficient and this reduction outweighed the interfacial area enhancing effect. Presence of surfactants did not substantially affect the induced liquid circulation rate in the airlift vessel.     

气升式反应器研究进展

对近年来有关气升式反应器的研究成果进行了总结。介绍了其结构上的改进,包括基本结构、气体分布器和各种内部构件等方面。阐述了操作条件对气升式反应器的流体力学和传质性能的影响规律,包括表观气速、液相性质及液位高度、固相性质及固含率以及其他因素,如电解液和磁场等。     

Characterization of an airlift reactor with helical flow promoters

The helical flow promoter (HFP), inserted in the downcomer of an airlift reactor (ALR), generates a helical flow pattern in the circulating gas–liquid (solid) mixture. Data on the fluidization capacity, gas holdup, liquid velocity and mass transfer rate for two- and three-phase systems with two different carboxymethylcellulose solutions collected in a 58 L ALR-HFP are presented and compared with those of common pneumatic reactors. Generally, an increasing solid concentration led to a slight decrease in gas holdup and liquid velocity but to a considerable decrease in mass transfer rates. Insertion of HFPs produced a significantly enhanced fluidization capacity of solid particles compared to the common systems.     

Influence of impeller type on hydrodynamics and gas‐liquid mass‐transfer in stirred airlift bioreactor

The influence of impeller type in a mechanically stirred airlift bioreactor was analyzed in relation to the non‐Newtonian viscous fluids. The agitation was carried out through a marine impeller (axial impeller) and a paddle impeller (radial impeller) located along with the gas sparger in the region comprised by the riser. The bioreactor was sparged with air under different velocities (0.036–0.060 m s^?1). Carboxymethylcellulose 1.94% and xanthan 1.80% were used as a fluid model. The gas holdup and volumetric mass‐transfer coefficient increased in up to five and three times, respectively, when compared to a conventional airlift bioreactor; however, better results were obtained when the straight paddle impeller type was used. The results suggest that the studied bioreactor can be used successfully in viscous fluid, and it can be more efficient than conventional airlift bioreactors. The results obtained suggest the use of radial impellers. © 2015 American Institute of Chemical Engineers AIChE J, 61: 3159–3171, 2015     

漏斗型导流内构件对内循环三相流化床流体力学与传质特性的影响

通过增加新型内构件来改善内循环三相流化床的流体力学与传质特性,以实现化工、环保领导中追求高氧利用率的过程。针对此过程设计了3种不同结构参数的漏斗型导流内件并设置于导流筒顶端,分别测定反应器内气含率、液相混合时间、液体循环速度、体积氧传质系数的数据并分析其变化规律,以解析内件的作用机制。实验在有效体积39L,以空气为气相、水为液相、多孔泡沫颗粒为固相的反应器中进行,研究发现：漏斗型导流内件的设置使升流区气含率平均增大10%,体积氧传质系数kLa提高了15%,液相混合时间下降10%-25%;内件的设置可以改变液体循环速度,当表观气速<0.5cm/s时,液体循环速度加快,当表观气速>0.5 cm/s时,液体循环速度下降;此外,漏斗型导流内件的结构参数变化对流化床流体力学与传质特性有较大影响。结果表明,流化床内增加新型内构件并合理设置能够实现反应器效能的提高。     

Influence of the gas—liquid separator design on hydrodynamic and mass transfer performance of split-channel airlift reactors

Three geometric configurations of gas–liquid separators were used in split-channel airlift reactors (0·1 m³ liquid volume; riser-to-downcomer cross-sectional area ratio = 0·7; aspect ratio = 3.6) to test the effect of geometry on hydrodynamic performance and oxygen transfer behaviour. For otherwise fixed conditions, the design of gas–liquid separators affected the induced liquid circulation rate, the depth of penetration of the bubble layer in the downcomer, the gas holdup in the downcomer, the mixing time and the overall volumetric gas–liquid oxygen transfer coefficient. The gas holdup in the riser was only marginally affected by the design of the separator. The impact of the various separator designs on hydrodynamic behaviour could be explained as emanating from a combination of the gas–liquid separating ability of the design and its hydraulic resistance.     

内循环气升式环流反应器生物降解苯酚废水过程的计算传质学模拟研究

利用计算传质学方法对内循环气升式环流反应器（ILALR）内生物降解苯酚废水过程进行了研究。采用欧拉多相流模型结合RNG k-ε湍流模型对ILALR中气-液两相流动过程进行模拟,采用气泡群平衡模型（PBM）对反应器内气泡的尺寸分布进行预测。利用近年来提出的计算传质学\overline{c^{\mathrm{2}}}-ε_c模型对湍流扩散系数进行计算,从而摆脱了传统需要预估湍流Schmidt数的经验方法。模拟得到的液相苯酚和菌体浓度与实验测量值吻合良好,从而证明了所建立模型的有效性。研究结果表明ILALR内湍流传质过程中湍流扩散系数及湍流Schmidt数并非常数,因此基于传递相似性假设得到的湍流Schmidt数经验模型不适用于ILALR内湍流传质过程的模拟。模拟得到的ILALR中液相剪应力随表观气速的增大而增大,局部最大值出现在导流筒的上部。     

气升式外环流反应器的体积传质系数

以Higbie的渗透理论和Kolmogoroff的湍流理论为基础,提出了计算液体旋涡在气液相界面暴露时间的方法,并建立了预测体积传质系数的模型方程。在不同管径比下的外环流反应器中,对空气 水体系测定了操作气速对体积传质系数、循环液速和气含率的影响。将体积传质系数与表观气速和下降管与上升管的面积比按幂函数进行关联,其预测值和试验值符合较好。     

Simulation and experimentation on the gas holdup characteristics of a novel oscillating airlift loop reactor

BACKGROUND: The classical airlift loop reactor (ALR) has been widely used in petrochemical, biochemical, energy and environmental processes due to such advantages as simple structure, without motional mechanism, easy sealing and low energy consumption. A novel ALR has been designed using forced periodic reversible ventilation, termed an airlift reversible loop reactor (ARLR). RESULTS: Using computational fluid dynamics (CFD) simulation and experimental validation, the mass transfer characteristics of the ARLR were studied. The simulation results predicted the experimental data well, especially at low ventilation capacity. The oscillation period had significant effects on gas holdup and the mass transfer coefficient of the ARLR. When ventilation capacity was increased from 0.22 to 3.49 vvm, compared with central airlift and annular airlift reactors, ARLR increased the mass transfer coefficient by 9–31% and 10–58%, respectively, according to simulation results, and by 11–25% and 14–58%, respectively, according to experimental data. CONCLUSION: The results showed that the ARLR could significantly enhance gas holdup and mass transfer coefficient compared with traditional central airlift and annular airlift reactors. Results indicated that the optimum oscillation period decreased with increase of ventilation capacity. ARLR has the potential for application in aerobic fermentation. © 2012 Society of Chemical Industry     

Effect of liquid volume in the gas‐separator on the hydrodynamics of airlift reactors

Gas hold‐up and liquid circulation velocity measurements were made using a 167 dm³ external loop airlift reactor. The gas‐separator was of the open channel configuration. The reactor height was 2.5 m with riser and downcomer diameters of 0.19 m and 0.14 m respectively. The systems investigated were Newtonian air–water and air–glycerol with the superficial air velocity varying between 0.02 and 0.12 m s⁻¹. The ratio of the liquid volume in the gas‐separator to the liquid volume in the reactor (volume‐ratio) was varied from 0.0% to 37%, to find its minimum critical value for optimum operation of the airlift reactor. For the air–water system, discernible effects of the volume‐ratio on riser and, downcomer gas hold‐ups and liquid circulation velocity were observed at volume ratios ≤7%. Beyond this value, the volume‐ratio had no effect. For a viscous and foaming air–glycerol system the critical volume‐ratio was increased to 19%. New and simple correlations for predicting gas hold‐up in the riser, gas hold‐up in the downcomer, and liquid circulation velocity were developed with reasonable accuracy. © 1999 Society of Chemical Industry     

Influence of dissolved hydrocarbons on volumetric oxygen mass transfer coefficient in a novel airlift contactor

Hydrocarbon compounds are sparsely soluble in aqueous systems but, nonetheless, their presence can influence significantly mass transfer behavior in gas-liquid systems. water-p-xylene and water-p-xylene-naphthalene mixtures were employed in order to determine the influence of dissolved hydrocarbons on mass transfer of oxygen from air bubbles to water. The surface renewal-stretch model has been modified for predicting the volumetric mass transfer coefficient, (K_La)_h, in the presence of surface contaminant molecules, including hydrocarbon compounds and surfactants. Theory and experimental oxygen transfer results were found to be in satisfactory agreement with average absolute deviation of 15%. Pendant drop and contact angle measurements by axisymmetric drop shape analysis were carried out to determine the reduction in surface tension of water due to the addition of p-xylene and naphthalene. Molecular orientation caused by instantaneous attraction of the polar moieties of the organic compounds toward the water interface has been found to be the main cause of reduction in surface tension. It was predicated that changes in gas-liquid mass transfer behavior resulted from surface contamination and that the significant parameter was the reduction in surface tension.     

Volumetric mass transfer coefficients in an internal loop airlift reactor with low‐density particles

The influence of organic additives (propanol, benzoic acid, isoamyl alcohol and carboxymethylcellulose) on the volumetric mass transfer coefficient, k_La, in an internal loop airlift reactor with low‐density particles (nylon‐6 and polystyrene) was investigated. The k_La values increased with increase in superficial gas velocity, U_sg, and decreased with increase in solid loading. A draft tube to reactor diameter ratio, D_R/D, of 0.4 gave maximum k_La values. The addition of benzoic acid and propanol increased the k_La values owing to their coalescence inhibiting characteristics. The addition of isoamyl alcohol decreased k_La, owing to the formation of rigid bubbles and recirculation of small bubbles having a low oxygen content. The k_La values decreased with increase in the concentration of the non‐Newtonian fluid carboxymethylcellulose (CMC). The proposed correlations predicted the experimental data well. Copyright © 2006 Society of Chemical Industry     

Solid effects on hydrodynamics and heat transfer in an external loop airlift reactor

The effect of a solid presence on global hydrodynamic parameters and heat transfer in an external loop airlift reactor has been experimentally investigated. Results obtained in both two- and three-phase flow are presented in this study. Two different external loop airlift reactor sizes have been used and local hydrodynamic characteristics including local gas hold-up and bubble velocity have been obtained in two-phase flow. Optical and ultrasound probes have been used to obtain this information, respectively. It was found that an increase of solid hold-up leads to a decrease of liquid velocity and heat transfer coefficient. Measured in a two- and three-phase reactor using a horizontal-heating probe, a correlation of the average gas hold-up and heat transfer coefficient is proposed. Correlation parameters are identified in homogeneous and heterogeneous flow regimes, which have been derived from the gas slip velocity concept. The experimental liquid velocity and gas hold-up in the riser have been represented in a satisfactory way by a hydrodynamic model, either in the absence or in the presence of solid particles.     

环流反应器的流动、混合与传递特性

由于环流反应器内存在定向循环,与鼓泡塔反应器相比,其混合性能大幅提升,已广泛应用于许多工业过程,如发酵、反应器结晶等工业过程,近年来成为国内外学者研究的热点。针对环流反应器内操作条件下的流动形态、流体力学（包括相含率分布、循环液速、混合时间以及离集指数等）及传质/传热特性,总结了其最新研究进展,分析了相含率尤其是固含率变化对反应器中关键参数如循环液速和化学反应速率的影响,展望了从机理上研究相互耦合的多相流动、传质/传热和化学反应规律,为进一步推动其工业应用提供参考。     

Local measurements for the study of external loop airlift hydrodynamics

Hot-film anemometry and an optical biprobe are used to measure local flow characteristics in the riser of an external loop airlift reactor. Important flow asymmetries are observed above the sparger and developing flow persists through a large part of the riser. As gas flow rate increases, radial gas hold-up profiles change from relatively flat to parabolic while the shape of radial liquid velocity profiles remains constant and Sauter bubble diameter increases. At large gas superficial velocities, slip velocity is found to deviate considerably from the frequently used value of 0.25 m/s. Local measurements allow a better understanding of two-phase flow in airlift reactors and can be used for CFD-modeling development and validation.     

Influence of channel geometry on hydrodynamics and mass transfer in the monolith film flow reactor

The hydrodynamics and the gas–liquid mass transfer as a function of the channel geometry have been investigated for the monolith film flow reactor. For the hydrodynamic studies, the liquid distribution and the flooding boundaries have been experimentally determined. The liquid distribution improved with increasing liquid flow rate. The flooding limits are in the range of other commercial structured packings and allow operation under industrially relevant conditions. Larger channel sizes and lower surface tension expand the operating window, while viscosity seems to have a minor impact. The gas–liquid mass transfer is a strong function of the surface to volume ratio defined by the channel dimensions. Co- and counter-current flow operation result in similar performance. Furthermore, shorter monoliths, with larger contribution of the inlet section have significant higher mass transfer due to the development of the concentration profile. The obtained k_GLa_V values of around 0.01 s⁻¹ are in the range of other commercial packings in counter-current flow operation. A three-dimensional single channel model describing the hydrodynamic and diffusion phenomena in the monolith is in good agreement with the experimental results. The flexibility in channel size and dimension allows tailoring the monolith reactor to the specific needs of the individual application.     

Region-dependent mass transfer behavior in a forced circulation airlift loop reactor

The contribution of local regions to global mass transfer holds the key to optimization and scale-up of a reactor. Extensive study has been conducted to investigate gas-liquid mass transfer occurring in the internal airlift loop reactor, but mostly restricted to global mass transfer performance. A cold model forced circulation internal airlift loop reactor was employed and divided into six regions in which dissolved oxygen concentration in slurry and mass transfer interfacial area were measured respectively. Different models were utilized to calculate volumetric mass transfer coefficient. Contributions of individual region to global mass transfer performance were calculated and compared. It was found that mass transfer coefficient and mass transfer interfacial area of individual region increases with increasing superficial gas velocity and slurry feed flowrate. The feed affected region has the greatest mass transfer coefficient and volumetric mass transfer coefficient, contributing more than 30% to global mass transfer in most operating condition. Mass transfer interfacial area is close in the gas distributor region, feed affected region and the gas-slurry separator region. In the present work, circulating bubbles are rare and contribute negligibly to the global mass transfer. Global volumetric mass transfer coefficient is close to that of the gas-slurry separator region, ranging from 0.02 to 0.1 1/s. Comparison of k_La is made between this work and literatures, suggesting a great improvement of mass transfer due to external liquid circulation.     

组合环单体填料的流体力学和传质性能

本文介绍了新近研制的组合环单体填料对于汽液接触过程的流体力学与传质性能。实验表明,这种填料具有比矩鞍环填料阻力小、传质效率高的优点,可以代替鲍尔环、矩鞍环等常用填料,以取得良好经济效益。     

Motion of a magnetic flow follower in two‐phase flow — application to the study of airlift reactor hydrodynamics

A low‐cost and simple magnetic particle tracer method was adapted to characterize the hydrodynamic behavior of an internal‐ and an external‐loop airlift reactor (ALR). The residence time distribution of three magnetic particles differing in diameter (5.5, 11.0 and 21.2 mm) and with a density very close to that of water was measured in individual reactor sections. The measured data were analyzed and used to determine the velocity of the liquid phase. Validation of the experimental results for liquid velocity was done by means of the data obtained by an independent reference method. Furthermore, analysis of the differences found in the settling velocity of the particle in single‐liquid and gas‐liquid phases was carried out, using a simplified 3D momentum transfer model. The model considering particle‐bubble interaction forces resulting from changes in the liquid velocity field due to bubble motion was able to predict satisfactorily the increase in the particle settling velocity in the homogeneous bubbly regime. The effective drag coefficient in two‐phase flow was found to be directly dependent on particle Reynolds number to the power of ? 2 but independent of gas flow‐rate for all particle diameters studied. Based on the experimental and theoretical investigations, the valid exact formulation of the effective buoyancy force necessary for the calculation of the correct particle settling velocity in two‐phase flow was done. In addition, recommendations concerning the use of flow‐following particles in internal‐loop ALRs for liquid velocity measurements are presented. Copyright © 2006 Society of Chemical Industry