Hydrodynamics of a novel multi-stage external loop airlift reactor

In the present investigation a novel multi-stage external loop airlift reactor with hydro-dynamically induced continuous bubble generation, breakup and regeneration has been proposed. The system has been designed to operate with relatively large sized bubbles, so that interfacial circulation can be induced in the liquid-bubble interfaces and faster transfer of components can take place by turbulent diffusion through the interface of the bubbles and also due to the physical rupture and reformation of the bubbles. The system was also designed to operate in three stages operating in series so that in each stage completely deaerated liquid could be brought in contact with freshly generated bubbles. Detailed studies on the gas holdup and liquid circulation velocity have been carried out with respect to various values of superficial gas as well as liquid velocities. The gas holdup of the proposed multi-stage system is 45% higher than the single stage system, which results in better mass transfer characteristics. Empirical correlations describing the performance of the proposed reactor have been presented in this paper.     

气升式反应器的优化

从强化传质与节能方面概述了近年来国内外在气升式反应器结构参数优化、操作参数的优化及溶液性质等方面的研究工作 ,提出了今后的研究方向     

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。     

Liquid circulation hydrodynamics in an external loop airlift reactor

The radial profiles of axial liquid velocity and gas hold‐ups are investigated in the riser of a pilot plant scale external loop airlift reactor (ELAR) using a modified Pavlov tube and differential pressure technique. The experimental investigation reveals that there exist two different kinds of liquid circulation structures in an ELAR, which has rarely been reported in the literature, namely internal liquid circulation, which exists only in the riser and external liquid circulation, which circulates through the downcomer. A power–law relationship is used to correlate the gas hold‐up and superficial gas velocity, which gives good agreement with experimental data. Experiments for axial liquid velocity profiles are analysed in analogy to a model described for a conventional bubble column. The results predicted by the model are in excellent agreement with the experimental data obtained under various operating conditions. © 2011 Canadian Society for Chemical Engineering     

Hydrodynamics and bubble behaviour in a three-phase two-stage internal loop airlift reactor

Local hydrodynamics of a gas–liquid–solid system,such as bubble circulation regime,gas holdup,liquid velocity and axial profile of solid concentration,are studied in a two-stage internal loop airlift reactor.Empirical correlations for gas holdup and liquid velocity are proposed to ease the reactor design and scale-up.Different bubble circulation regimes were displayed in the first(lower) and second(upper) stages.Increasing superficial gas velocity and solid loading can promote regime transition of the second stage,and the gas holdup of the second stage is higher than that of the lower stage.In addition,the effects of solid loading on bubble behaviour are experimentally investigated for each stage.It is found that bubble size in the downcomer decreases with the presence of solid particles,and bubble size distribution widens under higher superficial gas velocity and lower solid loading.     

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.     

Gas holdup, liquid circulating velocity and mass transfer properties in a mini-scale external loop airlift bubble column

The external loop airlift bubble column has been regarded as a promising type of gas-liquid or gas-liquid-solid biooreactor because of the liquid circulating flow between the riser and downcomer. A mini-scale column is useful and efficient in the process research and development for highly specialized materials such as fine chemicals, advanced bioproducts and biocatalysts utilized in two or three phase system. In this work, a mini-scale glass column of in volume was designed and characterized. The gas holdup ε_G in the riser was obtained by measuring the volume expansion through photographs taken with a digital camera. The liquid circulating velocity U_L was measured by observing the time required for a tracer particle to travel a fixed distance in the downcomer through analysis of the images taken by a video camera. The gas-liquid volumetric oxygen transfer coefficient k_La and liquid-solid oxygen transfer coefficient k_S were determined by our previous method in which the air oxidation of glucose was catalysed by the immobilized glucose oxidase gel beads suspended in the column to obtain a pseudo steady state concentration of the dissolved oxygen and the corresponding constant rate of glucose consumption. It was shown that even such a mini-scale external loop bubble column could be characterized in terms of gas holdup, liquid circulating velocity and mass transfer properties according to our previous correlations proposed for the bench to pilot scale column.     

级间隙高度和表观气速对多级环流反应器混合和传质的影响

针对应用广泛的简单多级环流反应器,研究了级间隙高度和表观气速对其混合和传质的影响规律。发现简单多级环流反应器的各级存在着非正常流动、过渡及正常流动三个典型流动状态,且流动状态的转变存在着受级间隙高度影响的两个临界表观气速,并提出了相应的预测模型。研究结果表明：级间隙高度越大,多级环流反应器内形成正常流型所需的表观气速越大;各级上升管和降液管的气含率会增高,且相同条件下第三级气含率最大,第二级次之,第一级气含率最小;各级的循环液速会增大,且第一级循环液速最大,第二级次之,第三级最小;混合时间会缩短,而传质系数会增大。本研究可为工业多级环流反应器的科学设计、放大和操作提供重要指导。     

Impacts of solid physical properties on the performances of a slurry external airlift loop reactor integrating mixing and separation

Solid physical properties are vital for the design, optimization, and scale-up of gas–liquid–solid multiphase reactors. The complex and interactional effects of the solid physical properties, including particle diameter, density, wettability, and sphericity, on the hydrodynamic behaviors in a new external airlift loop reactor(EALR) integrating mixing and separation are decoupled in this work. Two semi-empirical equations are proposed and validated to predict the overall gas holdup and liquid cir...     

超声波气升式内环流反应器合成乙酸苄酯

以乙酸、苯甲醇为原料,浓硫酸为催化剂,在常温下用超声合成乙酸苄酯。实验考察有无超声、超声声强、不同通气量、不同气体性质及不同反应器等参数对催化合成乙酸苄酯的影响。实验结果表明:在超声频率为10 kHz条件下,超声声强为1.0 W/cm2,反应体系中空气流速为0.3 L/m in时,合成乙酸苄酯的酯化率最佳,酯化率达到65.8%。实验中采用的超声波气升式内环流反应器较普通的气升式内环流反应器具有较好的优越性,合成方法反应条件温和,操作简单且反应速度很快。     

Modeling of modified airlift loop reactor with a concentric double-draft tube

In this work, mathematical modeling based on the tanks-in-series model was employed to characterize the performance of a novel modified airlift loop reactor. Liquid mixing characteristics, i.e., mixing time and circulation time, were employed to describe the performance of the proposed reactor. These values were determined by using the classical tracer response technique in a column 18.5 cm in diameter and 90 cm in height, and two concentric draft tubes 14 and 8.5 cm in diameter and 80 cm in height. Matlab 7.1 software was used to solve the model equations in the Laplace domain and determine the model parameter. A comparison between the numerical solution and experimental data showed that the applied model can accurately describe the behavior of a modified airlift reactor. Experimental results in the homogeneous bubble flow regime showed that the proposed airlift reactor configuration with a double-draft tube significantly improves the liquid mixing quality compared to a conventional concentric-tube airlift reactor with an identical operating volume. By using this high-performance reactor, the mixing and circulation times can be reduced by up to 48.3% and 35.5%, respectively.     

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     

底部间隙对多室环流反应器流动特性的影响

在空气-水-石英砂三相多室气升式环流反应器(MALR)中,调节底部转角连接处间隙高度分别为18,28,38mm,在表观气速1.2～4.2cm/s范围内,实验研究了底部阻力系数、相含率、循环液速随反应器底部间隙的变化规律。结果表明,随着底部间隙的增大,底部转角处的局部阻力系数减小,循环液速增大,流体夹带进入下降室的气泡和固体颗粒均增多,下降室的气含率和固含率均增大。     

多室环流反应器气含率的三维数值模拟与分析

在常温常压条件下,以空气为气相、水为液相,对多室气升式环流反应器内气液两相流动状态进行了数值模拟,采用Euler两相流模型模拟了不同表观气速条件下各室气含率的分布情况。数值模拟计算结果表明,上升室气含率随表观气速的增大近似呈线性增长;各上升室气含率受自身表观气速的影响较大,而受另外一上升室表观气速的影响较小。     

Production of the fungal exopolysaccharide scleroglucan by cultivation of Sclerotium glucanicum in an airlift reactor with an external loop

Sclerotium glucanicum NRRL 3006 was cultivated in a 120 dm³ working volume airlift reactor with external recirculation loop for the production of the exopolysaccharide (EPS), scleroglucan. When culture pH was not controlled EPS production, at a range of air flow rates, was poor (<2 kg m^?3). Biomass formation generally increased with increasing aeration rate. When culture pH was controlled at 4·5 ± 0·2 EPS production was maximal at an air flow rate of 100 dm³ min^?1 and fell as air flow rate decreased. Operation of the reactor at a high (100 dm³ min^?1) air flow rate for the early part of the process (up to 96 h), followed by a low air flow rate (20 dm³ min^?1), led to reduced biomass and oxalate formation, and a slight increase in EPS concentration relative to operation at a constant air flow rate of 100 dm³ min^?1. EPS production in this pneumatically-agitated reactor was equal to the highest levels reported in small-scale stirred tank reactors.     

气升式发酵罐生产核酸酶P_1发酵过程的动力学

利用15L外循环气升式反应器,初步研究了桔青霉分批式液体深层发酵生产核酸酶P1的发酵动力学过程。Logistic模型可以较好地反映菌体的生长动力学规律,Luedeking-Piret则比较好地描述了发酵过程中的核酸酶P1的活力与菌体生长的关系,并利用此模型可以判断桔青霉发酵生产核酸酶P1的过程中,核酸酶P1的产出与菌体的生长是属于部分耦联的,其发酵过程属于部分耦联型发酵。     

固体颗粒对三相气升式环流反应器流动特性的影响

将液相视为连续相,气相和固相视为分散相,同时考虑各相之间的相互作用,结合气液固三相流体的动力学理论,建立气液固三相环流反应器三流体湍流流动的Eulerian模型.采用计算模拟软件Fluent对三相环流反应器的流动状况进行模拟,考察表观气速、固含率、颗粒大小对反应器的气含率以及液体流动速度的影响.模拟结果较好地解释了气升式环流反应器内的三相流体行为,模型与实验结果较好地吻合,表明了模型的可行性.     

Influence of configuration on the performance of external loop airlift contactors

Various types of external loop airlift contactors (ELALCs) were examined for their hydrodynamic and mass transfer behavior. The investigation covered a variety of design parameters including the length of connection tubes (L_c), height of riser and downcomer (L_h), and the airlift configurations while maintaining the ratio between downcomer and riser cross sectional area constant at 0.269. The results demonstrated that the behavior of the external loop airlift could be modified by adjusting the design and operating variables. In general, a faster liquid velocity led to a presence of lower gas holdup and gas–liquid mass transfer rate. Increasing L_c and L_h seemed to increase liquid velocity while decreasing the overall gas holdup and the overall volumetric mass transfer coefficient. Empirical correlations for the estimation of the system behavior were finally formulated.     

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。