Hydrodynamics in an Internal Loop Airlift Reactor with a Convergence‐Divergence Draft Tube

Gas holdup and liquid circulation of one conventional draft tube and three different convergence‐divergence draft tubes in an internal loop airlift reactor were investigated. Experiments were carried out in two‐phase systems with air‐water and air‐CMC (carboxyl methyl cellulose) solution and three‐phase system with air‐water‐resin particles. The two‐phase drift‐flux model was used to estimate gas holdup for three‐phase Newtonian and two‐phase non‐Newtonian systems. It is shown that gas holdup in convergence‐divergence draft tubes is higher than that in a conventional draft tube and increases with superficial gas velocity. Variation of the structural parameters of convergence‐divergence draft tubes has little effect on gas holdup in the two‐phase and three‐phase system. The mathematical model, which is based on a drift‐flux model, was developed to describe the liquid circulation velocity in the reactor satisfactorily.     

Hydrodynamics of a slurry airlift reactor at high solid concentrations

The hydrodynamics of a slurry airlift reactor at high solid concentrations were experimentally studied. The influences of the average solid concentration, superficial gas velocity and particle size on the radial and axial profiles of the solid holdup, average gas holdup and liquid circulation velocity were investigated. The local solid holdup was measured with an electrical conductivity probe. At low solid concentrations or high superficial gas velocities, the radial profile of the solid holdup was uniform. At high solid concentrations, the radial profile of the solid holdup was nonuniform, with higher values near the wall. This radial nonuniformity increased with decreased superficial gas velocity or increased average solid concentration. The axial profile of the cross-sectional average solid holdup was uniform at all conditions in this work, even at high solid concentrations. The average gas holdup and liquid circulation velocity increased with the superficial gas velocity but decreased with the average solid concentration. A mathematical model based on the balance of the transverse lift force and turbulent dispersion force was proposed to predict the radial profile of the solid holdup. Reasonable predictions were obtained from this model with an adjustable model parameter.     

CFD analysis of two‐phase turbulent flow in internal airlift reactors

The hydrodynamic performance of three internal airlift reactor configurations was studied by the Eulerian–Eulerian k–ε model for a two‐phase turbulent flow. Comparative evaluation of different drag and lift force coefficient models in terms of liquid velocity in the riser and downcomer and gas holdup in the riser was highlighted. Drag correlations as a function of Eötvös number performed better results in comparison to the drag expressions related to Reynolds number. However, the drag correlation as a function of both Reynolds and Eötvös numbers fitted well with experimental results for the riser gas holdup and downcomer liquid velocity in configurations I and II. Positive lift coefficients increase the liquid velocity and decrease the riser gas holdup, while opposite results were obtained for negative values. By studying the effects of bubble size and their shape, the smaller bubbles provide a lower liquid velocity and a gas holdup. The effects of bubble‐induced turbulence and other non‐drag closure models such as turbulent dispersion and added mass forces were analysed. The gas velocity and gas holdup distributions, liquid velocity in the riser and downcomer, vectors of velocity magnitude and streamlines for liquid phase, the dynamics of gas holdup distribution and turbulent viscosity at different superficial gas velocities for different reactor configurations were computed. The effects of various geometrical parameters such as the draft tube clearance and the ratio of the riser to the downcomer cross‐sectional area on liquid velocities in the riser and the downcomer, the gas velocity and the gas holdup were explored. © 2011 Canadian Society for Chemical Engineering     

高长径比三相内环流反应器中细颗粒相含率和循环液速的分布研究

在高径比为22的三相内环流反应器中,在常温常压下以空气-水-石英砂为物系,研究了在不同粒径下上升区固含率、下降区固含率和上升区循环液速随表观气速的变化规律和不同粒径下轴向固含率的分布情况,以及在固体体积分数不同的条件下,平均气含率和上升区气含率随表观气速的变化情况。结果表明:当粒径(ds)≤0.3 mm时,上升区固含率随表观气速的增加呈平缓变化趋势,下降区固含率随表观气速的增加而增加;当0.3 mm相似文献   

三相环流反应器中的局部相含率

将压差法与气-液相间滑移速度相结合,提出了利用压差法测量三相区局部相含率的新方法,将测得的局部固含率进行轴向平均并与由颗粒装填量计算所得的固含率进行比较,证明了此方法的可靠性. 利用所提出的测量方法,考察了三相环流反应器中气含率和固含率随操作条件的变化规律. 结果表明,气含率随表观气速的升高而增大,且随轴向位置的升高而增大;大颗粒具有破碎气泡的作用,能够增大反应器内的气含率;固含率随表观气速的增加而降低,且沿轴向变化较大. 表观气速较高时,固含率沿轴向近似呈S形分布.     

Time-dependent gas holdup variation in an air-water bubble column

Time-dependent gas holdup variation in a two-phase bubble column is reported with air and tap water as the working fluids. The results indicate that time-dependent gas holdup is closely related to the water, whose quality is unsteady and changes, not only during the two-phase flow, but also during idle periods. The significance and characteristics of the time-dependent gas holdup variation are influenced by the bubble column operation mode (cocurrent or semi-batch), the sparger orientation, the superficial gas velocity, and the superficial liquid velocity. It is proposed that a volatile substance (VS), which exists in the water in very small concentrations and inhibits bubble coalescence, evaporates during column operation and results in a time-dependent gas holdup. The influence of bubble column operation mode, sparger orientation, superficial gas velocity, and superficial liquid velocity on the time-dependent gas holdup variation are explained based on their effects on bubble size, bubble contacting frequency and mixing intensity. This work reveals that regular tap water may cause significant reproducibility problems in experimental studies of air-water two-phase flows.     

3‐D Simulations of an Internal Airlift Loop Reactor using a Steady Two‐Fluid Model

Three‐dimensional (3‐D) simulations of an internal airlift loop reactor in a cylindrical reference frame are presented, which are based on a two‐fluid model with a revised k‐? turbulence model for two‐phase bubbly flow. A steady state formulation is used with the purpose of time saving for cases with superficial gas velocity values as high as 0.12 m/s. Special 3‐D treatment of the boundary conditions at the axis is undertaken to allow asymmetric gas‐liquid flow. The simulation results are compared to the experimental data on average gas holdup, average liquid velocity in the riser and the downcomer, and good agreement is observed. The turbulent dispersion in the present two‐fluid model has a strong effect on the gas holdup distribution and wall‐peaking behavior is predicted. The CFD code developed has the potential to be applied as a tool for scaling up loop reactors.     

Hydrodynamics of a Multi‐Stage Internal Loop Airlift Reactor

The local hydrodynamic properties in a multi‐stage internal loop airlift reactor were investigated in this study. The gas‐liquid two‐phase flow hydrodynamic properties, including gas holdup, bubble velocity, bubble diameter, and liquid circulation velocity at various stages were measured by dual electrical resistivity probes and conductivity cells. Detailed studies on the gas holdup, bubble velocity, bubble diameter, and liquid circulation velocity were conducted with respect to various values of superficial gas. The Zuber and Findlay drift flux model was used to represent the variation of slip velocity with total gas‐liquid velocity at various stages and the model fits the data well.     

多室气升式环流反应器流动特性的数值模拟

在空气-水两相多室气升式环流反应器（MALR）中,采用欧拉欧拉两相流模型对扇形反应室内气液两相流动过程进行了数值模拟研究,考察了上升室的气含率、液体速度随表观气速的变化,最后用实验数据对模拟结果进行了验证.结果表明,某一上升室气含率受该室表观气速的影响较大,与另一上升室表观气速的影响较小;循环液体与上升室流体流动型式有关;气含率和循环液速的模拟值与实验值的平均相对误差分别为5.36％和8.28％;说明了应用数值模拟方法研究MALR流动特性的可行性.     

湍动浆态床流体力学研究（Ⅰ）气含率及其分布规律

对高气速、高固含率、大塔径条件下的湍动浆态床平均气含率和气含率径向分布进行了实验测定,结合工业实验数据,归纳出可用于工业条件的气含率计算关联式,给出了简化的流体力学模型用于气含率分布的模拟。结果表明,浆态床气含率将随塔径增加而降低,固含率与塔径之间存在交互影响;同时,气含率的径向分布也随气速和塔径的增大而改变,存在明显的放大效应,简化模型能够较好地模拟实验结果。     

气液外环流反应器中气泡行为的实验研究

详细考察了气液外环流反应器中上升管、下降管的气泡行为随轴向、径向的变化规律. 由于外环流反应器的结构特点,发现在上升管底部存在偏流,并对分布板区气泡行为随角向的变化规律进行了研究. 分析实验结果得出,气含率和气泡速度均随表观气速的升高而升高;在上升管内,气含率和气泡速度自中心向边壁逐渐降低,而沿轴向变化很小;在分布板区,由于受分布器及下降管的影响,使气含率和气泡速度在不同角向存在不同的径向分布. 在下降管中,气含率自中心向边壁逐渐降低,而气泡速度则基本不变;且下降管中的气体循环率随表观气速的升高而升高.     

三相多室气升式环流反应器气含率的研究

在1个4流道的三相多室气升式环流反应器中,以空气-水-K树脂为体系,采用压差法测量气含率,考察了上升室气体表观速率、固体装载量对上升室与下降室气含率的影响.结果表明:上升室的气含率随着该室气体表观速率增加而增加,而随着另一上升室气体表观速率增加而略有降低;下降室的气含率随着该室气体表观速率增加呈现3种变化趋势,而随着另...     

Effects of Gas‐Agitation and Packing on Hydrodynamics and Mass Transfer of Extraction Column

The effects of gas‐agitation and packing on hydrodynamics and mass transfer were investigated through experiments with air‐kerosene (benzoic acid)‐water system and corrugated‐packing of calendering plate with hole. The holdup of gas, holdup of dispersed liquid phase and mass transfer coefficient increase and the flooding velocity decrease with the increase in superficial gas velocity. Over‐agitation of gas causes over‐dispersion and emulsification of dispersed liquid phase, reduction of mass transfer performance and even flooding. The mass transfer performance of a packed column is far better than that of an unpacked column.     

高长径比三相内环流反应器中相含率的分布研究

在长径比为22的三相内环流反应器中,常温常压下,以空气-水-石英砂为物系,根据无因次准数建立了气含率、固含率的预测模型,考察了在不同粒径下上升区气含率、下降区气含率和上升区固含率、下降区固含率随表观气速的变化规律和不同固体体积分数下轴向固含率的分布情况。结果表明：不同粒径下上升区和下降区气含率均随表观气速的增大而增大;当粒径（ds）≤0．3mm时,上升区固含率随表观气速的增加呈平缓趋势,下降区固含率随表观气速的增加而增加,当0．3mm〈d。≤1．2mm时,上升区固含率随表观气速的增加而呈先下降后增加的趋势,下降区固含率随表观气速的增加而下降;不同固体体积分数下的固体颗粒的固含率随着轴向高度的增大而变化平缓,能够均匀的分布在反应器中;气含率和固含率的计算值和实验值吻合较好,其平均相对误差分别为6．32％、4．56％。     

采用压力传感技术测量鼓泡床中流体力学参数

引　言鼓泡床以其良好的传热、传质、相间充分接触和高效的可连续操作特性在许多领域得到了广泛应用 .在过去的 4 0多年里 ,人们采用许多测量方法(光导纤维、多普勒测速仪、电导法、压力传感技术 )对鼓泡床中的各种流动行为进行了大量研究 ,由于床层内流动的复杂性以及各测量手段间的差别 ,得出的结论不尽相同[1] ,而且工业反应多数在高温、高压、非透明体系下进行 ,这限制了许多测量技术的应用 .压力传感技术以其适用范围广、所需仪器便宜、耐用、测量结果准确的特点在鼓泡床流体力学参数测量中得到了广泛应用床层塌落法是压力传感技术在鼓泡床流体力学参数测量中的一个重要应用 .Sriram和Mann[2 ] 较早地将其应用于测量鼓泡床中的气含率 ;Fan等[3] 也曾利用此方法测量鼓泡床内的固含率 .前人大都采用压力传感技术测量床层内的平均相含率 ,而采用此方法测量大小气泡分数和气泡上升速度的报道很少 .本文根据前人在此方面的研究成果并结合本实验的特点进行了这方面的研究1　实验装置本实验结合工业对二甲苯氧化反应器的特点设计并建立了其流体力学冷模实验装置 ,如图 1所示 .鼓泡床高 6 6m ,内径 0 3m .在鼓泡床一侧自...     

Local hydrodynamics of gas–liquid-nanoparticles three-phase fluidization

The laser Doppler anemometer (LDA) and conductivity probes were used for measuring the local hydrodynamic performances such as gas holdup and liquid velocity in a lab-scale gas–liquid–TiO₂ nanoparticles three-phase bubble column. Effects of operating parameters on the local gas holdup and liquid velocity were investigated systematically. Experimental results showed that local averaged axial liquid velocity and local averaged gas holdup increased with increasing superficial gas velocity but decreased with increasing TiO₂ nanoparticles loading and the axial distance from the bottom of the bubble column. A three-dimensional computational fluid dynamic (CFD) model was developed in this paper to simulate the structure of gas–liquid–TiO₂ nanoparticles three-phase flow in the bubble column. The time-averaged and time-dependent predictions were compared with experimental data for model validation. A successful prediction of instantaneous local gas holdup, gas velocity, and liquid velocity were also presented.     

Bubble Behavior of a Large‐Scale Bubble Column with Elevated Pressure

Gas holdups and the rising velocity of large and small bubbles are measured using the dynamic gas disengagement approach in a pressured bubble column of 0.3 m in diameter and 6.6 m in height. The effects of superficial gas velocity, liquid surface tension, liquid viscosity, and system pressure on the gas holdups and the rising velocity of small and large bubbles are investigated. The holdup of large bubbles and the rising velocity of small bubbles increase with increasing liquid viscosity and liquid surface tension. Meanwhile, the holdup of small bubbles and the rising velocity of a swarm of large bubbles decrease. Moreover, the holdup of large bubbles and the rising velocity of a swarm of small bubbles decrease with increasing system pressure. A correlation for the holdup of small bubbles is obtained from experimental data.     

加长循环流化床提升管中加速段长度的实验研究

Systematic experimental work was conducted to investigate the solid acceleration length in a 16 m long circulating fluidized bed riser with fluid cracking catalyst particles over a wide range of operating conditions. A more feasible method is proposed to determine the acceleration length from the measured axial profiles of pressure gradient (or apparent solid holdup). With this new method and large amount of experimental data, a clear picture on the variation of the acceleration length with both solid circulating rate and superficial gas velocity is obtained.It is found that the acceleration length increases generally with increasing solid flow rate and/or decreasing gasvelocity. However, the trend in variation of the acceleration length with operating conditions are quite different in different operation ranges. Reasonable explanations are suggested for the observed variation patterns of acceleration length.     

The Effect of Solids Inventory and Other Factors on the Axial Solids Holdup Profiles in a Twin‐Riser System

The axial profiles of cross‐sectional average solids holdup profiles were studied in two 10 m long, 76 mm i. d. and 203 mm i. d. risers with the solids circulation rate up to 550 kg/m²s, superficial gas velocities up to 10.0 m/s, and solids inventory up to 410 kg in the 320 mm i. d. storage tank. The shape of the axial profiles of the cross‐sectional average solids holdup changes with solids fluxes. Under high‐flux conditions, the shapes of the profiles are quite different from those under low‐flux conditions. It is clear that solids holdup increases with the increase of solids fluxes. While the superficial gas velocities have no obvious influence on the shape of the axial profiles of the cross‐sectional average solids holdup under given solids flux for both risers, solids holdup decreases with the increase of superficial gas velocity under all solids inventories. The solids inventory and riser diameter have little influence on the shape of the profile, but the solids holdup is higher with larger bed diameter and/or larger solids inventory. Meanwhile, the solids inventory and riser diameter have important influence on the maximum value of operating solids fluxes of the system. The maximum of solids fluxes increases with the increase of solids inventory, and decreases with the increase of riser diameter.     

LOCAL HYDRODYNAMICS IN AN EXTERNAL LOOP AIRLIFT SLURRY REACTOR WITH AND WITHOUT A RESISTANCE-REGULATING ELEMENT

The hydrodynamic behavior of an external loop airlift slurry reactor (ALSR) with and without a resistance-regulating element was studied with a fiber optic probe and ultrasound Doppler velocimetry. The influences of the superficial gas velocity and solid holdup on the global gas holdup and radial profiles of the suspension circulation velocity in the downer and of gas holdup, bubble size, and bubble rise velocity in the riser were studied. Local measurements allow a better understanding of the flow behavior in the reactor and can be used for CFD modeling and validation. Experimental results show that the resistance-regulating element increases the gas holdup and decreases the suspension circulation velocity, indicating that an optimum design of the flow resistance is needed to obtain the maximum gas-liquid volumetric mass transfer coefficient for a specific superficial gas velocity. A high superficial gas velocity and low solid holdup are favorable for increased uniformity of the radial profile of the gas holdup and bubble rise velocity. Hydrodynamic models that predict the gas holdup and suspension circulation velocity were developed for an ALSR with and without a resistance-regulating element. Good agreement was obtained between measured and predicted values.