Hydrodynamics of the flow transition from a bubble column to an airlift-loop reactor

The hydrodynamics of an airlift-loop reactor (ALR) and a bubble column (BC) were studied in the same reactor unit. When the liquid circulation in the ALR was impeded gradually in order to obtain a BC mode of operation, there was a transition flow regime inbetween that of the ALR-type flow and the BC-type flow. In the BC the heterogeneous flow was represented by an instationary circulatory flow pattern and characterized by a liquid circulation velocity. The liquid flow in the ALR was represented by a drift-flux model. In the transition flow regime, hydrodynamic calculations based on the plug-flow behavior of an ALR appeared to be valid up to a certain defined value of the total gas-liquid flow rate. To distinguish between BC and ALR flow characteristics, a simple criterion is proposed, qualifying that the distinction between both flow patterns is determined by the superficial liquid velocity and the liquid circulation velocity. If the latter velocity exceeds the superficial liquid velocity a hydrodynamic transition will occur from a uniform ALR type of flow to a heterogeneous BC type of flow. The criterion coincides with an empirical power law function in which the liquid velocity is given as a function of the gas velocity. The values of the power-law coefficients depended on the characteristics of the two-phase flow. The change in value cohered with the onset of a change in the flow pattern.     

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.     

Hydrodynamic Modelling of Internal Loop Airlift Reactor Applying Drift‐Flux Model in Bubbly Flow Regime

A new model for the liquid circulation rates in airlift reactor (ALR) is presented. The model is based on the energy balance for the flow loop (riser, turn riser‐downcomer, downcomer, and turn downcomer‐riser) coupled with a drift flux theory of two‐phase flow gas‐liquid system, considering a bubbly flow regime. The predicted values of the liquid circulation rates by the developed model are compared with experimental results performed in a 22 dm³ internal loop airlift reactor and with the results obtained in the literatures. The proposed model predicted the experimental results very well. Slip velocity relationship based on the drift flux model was proposed; including the gas holdup, bubble size and the liquid physical properties. The predicted slip velocity was similar to that obtained from the literature. The study revealed that appropriate arrangements of internal bioreactor parts can positively influence the liquid circulation velocity at the same energy consumption. The proposed models are useful in the design; scale up and characterization of the internal loop airlift reactors, and provides a direct method of predicting hydrodynamic behaviour in gas‐liquid airlift reactors.     

Effects of viscosity and relaxation time on the hydrodynamics of gas–liquid systems

Effects of liquid properties on the hydrodynamics of gas–liquid systems were investigated in lab-scale bubble column (BC) and internal loop airlift (ILA). Alginate solutions, a glycerol solution and a Boger fluid were adopted to separately address the effects of viscosity and of surface tension for Newtonian fluids, and the effects of relaxation time for non-Newtonian fluid characterized by approximately constant viscosity (low shear thinning). Hydrodynamic regimes were characterized in terms of overall gas holdup, gas–liquid mass transfer coefficient, drift-flux and liquid circulation velocity. The superficial gas velocities at the transition between hydrodynamic regimes (homogenous regime–vortical-spiral regime–heterogeneous regime) as a function the liquid viscosity was characterized by a maximum. The same behavior was observed for the maximum stable gas holdup and gas–liquid mass transfer coefficient in BC. Viscosity enhances homogeneous regime stability for μ<4.25 mPa s, in BC, and μ<7.68 mPa s, in ILA. For non-Newtonian fluids the transition velocity increases with liquid elasticity. The stabilization mechanism related to the relaxation time of Boger fluids has been discussed.     

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.     

大颗粒三相环隙气升式环流反应器流体力学行为

研究了大颗粒体系气升式环流反应器的流体力学行为,考察了表观气速和颗粒质量分数对床层膨胀高度、循环液速和固含率分布的影响。实验结果表明,按颗粒的运动状态不同可以将反应器内的流动分为3个区域,即固定床区域、膨胀床区域和循环床区域,各流动区域内的流动行为存在显著差异。随着颗粒质量浓度的增大,起始流化气速和最小循环气速均显著增大。基于三相流化床的流化模型和环流反应器的特点建立了相应的数学模型,对大颗粒三相气升式环流反应器的起始流化气速和最小循环气速进行了预测,模型预测值与实验测量值吻合良好。     

Studies on Regime Transition,Operating Range and System Stability in a Liquid‐Solid Circulating Fluidized Bed

In the present work, the variations in the solids circulation rate and solids holdup were analyzed to study the behavior of a liquid‐solid circulating fluidized‐bed (LSCFB) regime. The results confirm the existence of two regions in the regime of LSCFB. A new concept of critical liquid velocity, j_lc, is proposed in the present work for demarcation between region 1 and region 2, which is found to be a constant value of about 1.3 u_t for all particles considered. The operating range of the LSCFB regime is obtained for the various particles and a correlation is developed from the data to estimate the maximum total liquid velocity. The predicted maximum liquid velocity was compared with the experimental values and found to be in good agreement within ±9 %. The effects of total liquid velocity, particle size and density on the stable operating range are discussed. Analysis of the experimental results shows that stable operation prevails both in region 1 and region 2.     

Structure of trickle-to-pulse flow regime transition and pulse dynamics at elevated temperature in slow-mode cyclic operation

The effects of temperature and pressure on the structure of the trickle-to-pulse flow regime transition in slow-mode cyclic operation in trickle-bed reactors were reported. The relationship between liquid holdup and liquid velocities at the trickle-to-pulse flow transition in cyclic operation, the shock wave behavior as a function of bed depth, as well as the pulsing flow regime properties were investigated for Newtonian (air-water) and non-Newtonian (air-0.25% carboxymethylcellulose (CMC)) liquids. At a given temperature, the breakthrough, plateau and decay times of the shock wave were found to decrease with bed depth. The pulse velocity and pulse frequency for pulsing flow regime both in cyclic operation and in natural pulsing (constant-throughput operation) were observed to increase with temperature. However, increasing the reactor pressure led to increased pulse frequency and decreased pulse velocity. Analysis of the transition liquid holdups for natural pulse flow and cyclic operation revealed that the liquid holdup decreased with temperature and pressure. The transition liquid holdups and superficial liquid pulse velocities in symmetric peak-base cyclic operation surpassed those in constant-throughput operation for given temperature, pressure and gas velocity, giving rise to wider trickle flow regime area in cyclic operation. The behavior of both Newtonian and power-law non-Newtonian fluids was similar regarding the effect of temperature, pressure and gas velocity.     

Impact of gravity on the bubble‐to‐pulse transition in packed beds

A model based on two‐phase volume‐averaged equations of motion is proposed to examine the gravity dependence of the bubble‐to‐pulse transition in gas‐liquid cocurrent down‐flow through packed beds. As input, the model uses experimental correlations for the frictional pressure drop under both normal gravity conditions and in the limit of vanishing gravity, as well as correlations for the liquid‐gas interfacial area per unit volume of bed in normal gravity. In accordance with experimental observations, the model shows that, for a given liquid flow, the transition to the pulse regime occurs at lower gas‐flow rates as the gravity level or the Bond number is decreased. Predicted transition boundaries agree reasonably well with observations under both reduced and normal gravity. The model also predicts a decrease in frictional pressure drop and an increase in total liquid holdup with decreasing gravity levels. © 2013 American Institute of Chemical Engineers AIChE J 60: 778–793, 2014     

Experimental investigation on multiscale hydrodynamics in a novel gas–Liquid–Solid three phase jet-Loop reactor

Multiphase flow hydrodynamics in a novel gas–liquid–solid jet-loop reactor (JLR) were experimentally investigated at the macroscales and mesoscales. The chord length distribution was measured by an optical fiber probe and transformed for bubble size distribution through the maximum entropy method. The impacts of key operating conditions (superficial gas and liquid velocity, solid loading) on hydrodynamics at different axial and radial locations were comprehensively investigated. JLR was found to have good solid suspension ability owing to the internal circulation of bubbles and liquid flow. The gas holdup, axial liquid velocity, and bubble velocity increase with gas velocity, while liquid velocity has little influence on them. Compared with the gas–liquid JLRs, solids decrease the gas holdup and liquid circulation, reduces the bubble velocity and delays the flow development due to the enhanced interaction between bubbles and particles (Stokes number >1). This work also provides a benchmark data for computational fluid dynamics (CFD) model validation. © 2019 American Institute of Chemical Engineers AIChE J, 65: e16537, 2019     

Prediction of The Kolmogorov Entropy Derived from CT Data in a Bubble Column Operated under The Transition Regime and Ambient Pressure

The Kolmogorov entropy (KE) algorithm was successfully applied to single source γ‐ray Computed Tomography (CT) data measured by three scintillation detectors in a 0.162 m‐ID bubble column equipped with a perforated plate distributor (163 holes × ?? 1.32 · 10^–3 m). The aerated liquid height was set at 1.8 m. Dried air was used as a gas phase, while Therminol LT (ρ_L = 886 kg m^–3, μ_L = 0.88 · 10^–3 Pa s, σ = 17 · 10^–3 N m^–1) was used as a liquid phase. At ambient pressure, the superficial gas velocity, u_G, was increased stepwise with an increment of 0.01 m s^–1 up to 0.2 m s^–1. Based on the sudden changes in the KE values, the boundaries of the following five regimes were successfully identified: dispersed bubble regime (u_G < 0.02 m s^–1), first transition regime (0.02 ≤ u_G < 0.08 m s^–1), second transition regime (0.08 ≤ u_G < 0.1 m s^–1), coalesced bubble regime consisting of four regions (called 4‐region flow; 0.1 ≤ u_G < 0.12 m s^–1), and coalesced bubble regime consisting of three regions (called 3‐region flow; u_G > 0.12 m s^–1). The KE values derived from three scintillation detectors in the first transition regime were successfully correlated to both bubble frequency and bubble impact. The latter was found to be inversely proportional to the bubble Froude number. The KE model implies that the bubble size in this particular flow regime is a weak function of the orifice Reynolds number (d_b = 7.1 · 10^–3Re₀^–0.05).     

Two regime transitions to pseudo-homogeneous and heterogeneous bubble flow for various liquid viscosities

The gas hold-up variation and regime transition were investigated with different liquid viscosities ranging from 1.0 mPa s to 31.5 mPa s using a 0.15-m-in-diameter bubble column. In contrast to common observations, the gas hold-up graph with the superficial gas velocity could be categorized into three flow regimes: homogeneous, pseudo-homogeneous and heterogeneous flow regimes. The formation of large bubbles caused a transition from the homogeneous to the pseudo-homogenous flow regime, in which large bubbles rose vertically without oscillatory turbulence. According to the results from the dynamic gas disengagement (DGD) technique, large bubbles began to form at the transition superficial gas velocity to the pseudo-homogeneous flow regime. The transition to a heterogeneous flow regime was initiated by the turbulent movement of large bubbles. The transition superficial velocities to pseudo-homogeneous and heterogeneous flow regimes, u_t1 and u_t2, decreased with increasing liquid viscosity below a critical viscosity and converged to a certain value above that viscosity. However, the correlations from the literatures could not make a reasonable estimation of the transition superficial velocities because they did not consider the possible transition to a pseudo-homogeneous flow regime. Therefore, the two transition points should be predicted separately.     

新型流化床内流体重构的流体力学特性变化及其数值模拟

针对前期研究开发的新型三重环流与多重环流流化床,构建气液两相流的二维数值模型,分析微观流场、液体运动速度和气相含率,剖析环流数对流化床流体力学特性的影响和对内循环过程的改善,寻找反应器运行过程节能的结构与优化的操作条件。通过数值模拟发现:基于单重环流,多重环流作用主要改变流体在上升区和下降区之间的相互混合和交汇,增大环流数可缩短流体运动的循环路程和时间,有利于加强相际之间的混合,但床内整体流态仍接近推流,微循环的存在所占比例较小;多重环流流化床的流体力学性能优于三重环流,随着环流次数的增多,液速在径向和轴向上分布更均匀,气泡逆流进入下降区,有利于反应器整体氧传质效率的提高。通过流化床结构改变实现的流体重构支持流体力学性能的改善,是高负荷有机废水好氧生物处理期待选择的开发方向。     

Gas holdup,bubble size distribution,and mass transfer in an airlift reactor with ceramic membrane and perforated plate distributor

The airlift reactor is one of the most commonly used gas–liquid two-phase reactors in chemical and biological processes. The objective of this study is to generate different-sized bubbles in an internal loop airlift reactor and characterize the behaviours of the bubbly flows. The bubble size, gas holdup, liquid circulation velocity, and the volumetric mass transfer coefficient of gas–liquid two-phase co-current flow in an internal loop airlift reactor equipped with a ceramic membrane module (CMM) and a perforated-plate distributor (PPD) are measured. Experimental results show that CMM can generate small bubbles with Sauter mean diameter d₃₂ less than 2.5 mm. As the liquid inlet velocity increases, the bubble size decreases and the gas holdup increases. In contrast, PPD can generate large bubbles with 4 mm < d₃₂ < 10 mm. The bubble size and liquid circulation velocity increase as the superficial gas velocity increases. Multiscale bubbles with 0.5 mm < d₃₂ < 10 mm can be generated by the CMM and PPD together. The volumetric mass transfer coefficient k_La of the multiscale bubbles is 0.033–0.062 s⁻¹, while that of small bubbles is 0.011–0.057 s⁻¹. Under the same flow rate of oxygen, the k_La of the multiscale bubbles increases by up to 160% in comparison to that of the small bubbles. Finally, empirical correlations for k_La are obtained.     

Gas hold-up and liquid velocity in a two-dimensional air lift reactor

Gas hold-up and liquid circulation velocity were studied in a two-dimensional air lift reactor. A comparison was made between bubble column and air lift operation.Four different gas spargers were studied, and several different gases: He, CO₂, Ar, air and Freon 114. No influence on the measured variables could be attributed to gas distribution. The nature of the gas had a strong influence in the case of bubble column operation. Helium does not exhibit the maximum in the hold-up which is characteristic of the “transition regime”, and in general gives lower values.In air lift operation, the differences due to the nature of the gases are much smaller, and can be neglected.The liquid recirculation velocity depends on the gas superficial velocity, J_G, raised to the power 0.4. This concurs with published data obtained in large air lift devices. It is concluded that a two-dimensional air lift can be used for research and development, since it present the same basic fluid—dynamic characteristics as large air lift reactors. In addition, the two-dimensional device allows the examination of flow patterns and configuration, which are decisive in design and scale-up of this type of reactor.     

底隙设置挡板内循环流化床水力特性分析

针对流化床反应器中多相流体混合碰撞的复杂性、突变性的问题,以底隙设置十字形挡板反应器的水力特性改变为研究对象,采用电导法测定反应器内液体循环速度和混合时间等动力学数据及其变化,分析新型内构件的强化作用原理。研究结果发现,两相条件下,十字形挡板的设置使反应器的升流区及降流区的液体循环速度分别提高9.5%±1.0%和11.8%±1.0%,低流速时,液相混合时间变长,高流速时,混合时间反而短缩,变化范围在±5%,计算的摩擦阻力系数由4.13降低为2.75,证明了流体在反应器底部碰撞能量消耗的下降。通过全环路能量衡算得到液体循环速度模型计算参数的实验关联式,应用于三相条件,计算值与实验值误差在8%以内。表明在急剧湍流内循环流化床底部设置挡板所实现的流态有序、矢量归一的目标能有效改善反应器中流体的水力特性而使流体稳定,并进一步实现稳态运行条件下的节能。     

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.     

Experimentally‐based constitutive relations for co‐current gas‐liquid flow in randomly packed beds

Experimental Observations on average pulse velocity and frequency in concurrent gas‐liquid (down) flow through randomly packed beds are used to extract constitutive relations for the gas‐liquid interaction and mean curvature terms that appear in a recently proposed volume‐averaged two‐fluid model for bubbly flow. The proposed closures lead to a reasonably quantitative prediction of the average pressure drop and liquid saturation under bubbly flow conditions and in the near pulse regime. In addition, the proposed closures provide realistic estimates for the location of the bubble‐to‐pulse transition in microgravity and in 1g down‐flow and predict the disappearance of the bubbly flow pattern at low liquid fluxes in 1g down‐flow. © 2016 American Institute of Chemical Engineers AIChE J, 63: 812–822, 2017     

PDA measurements in a three‐phase bubble column

Phase Doppler anemometry was used to quantify the flow characteristic of a three phases (liquid, solid, and bubbles) cylindrical bubble column driven by a point air source made of a 30‐mm diameter perforated air stone centrally mounted at the bottom. The cylindrical bubble column had an inner diameter of 152 mm and was filled with liquid up to 1 m above the point source. Acrylic beads with a nominal diameter of 3 mm were used as the solid phase. To match the density of the solid phase which was 1.05 kg/m³, the liquid density was raised to about 1.0485 kg/m3 by added salt. The bubble diameters generated were within the range of 600–2400 µm. The detailed turbulent characteristics of the liquid‐phase velocity, bubble diameter, bubble velocity, and solid velocity were measured at three different air rates, namely 0.4, 0.8, and 1.2 L/min (corresponding to average gas volume fraction of 0.0084, 0.0168, and 0.0258, respectively) for the homogeneous bubble column regime. With the addition of the solid phase, the flow field was found to be relatively steady compared to the two‐phase column referencing the probability density functions for both the liquid and bubble velocities. An analysis based on the determination of the drag forces and transversal lift forces was performed to examine the flow stability in the three‐phase bubble column. The analysis illustrated that how the added solid phase effectively stabilized the flow field to achieve a steady circulation in the bubble column and a generalized criterion for the flow stability in the three‐phase bubble column was derived. Further investigation for the transition and the heterogeneous bubble column regime with air rates at 2.0 and 4.0 L/min shown that this criterion can also be used as a general prediction of flow stability in this three‐phase bubble column. © 2013 American Institute of Chemical Engineers AIChE J, 59: 2286–2307, 2013