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.     

Liquid phase mixing and gas hold-up in a multistage-agitated contactor with co-current upflow of air/viscous fluids

A hydrodynamic study is carried out in a multistage-agitated contactor with cocurrent upflow of air/viscous fluids. The liquid phase backmixing is characterized through a residence time distribution study in various viscous fluids (up to 20 mPa s). The liquid Reynolds number with respect to the impeller applied in the investigation ranges from 1000 to 50 000. A cascade of stirred tanks with back flow model satisfactorily fits all experimental residence time distribution curves in the present study. It is found that the increase in the liquid viscosity reduces the backmixing in the liquid phase due to its dampening effect on turbulence. The presence of the gas phase helps in reducing the backmixing by the straightening effect and entrainment effect in a co-current operation manner. The gas hold-up measured in the present study is comparable to the literature data. A new type of correlation taking into account the influence of the gas phase is proposed to predict the backmixing in a multistage-agitated contactor.     

Bubble column with internals: Effects on hydrodynamics and local heat transfer

Local heat transfer and column hydrodynamics are investigated in a bubble column in presence of internals of different configurations. The liquid phase used is tap water and the gas phase is oil-free compressed air. The gas velocity is varied over a wide range of 0.03–0.35 m/s. The heat transfer variations are measured with a fast response probe capable of capturing bubble dynamics as well as detect local flow direction and deduce local liquid velocity. Measurements obtained in presence of internals are compared with those without internals to elucidate the effects of different internals design. Comparisons are based on average values and temporal variations obtained with the fast response probe. The average gas holdup, local liquid velocity and bubble fractions holdups obtained with and without internals are also compared to further point out the differences. The observed differences are discussed based on the insights provided by these comparisons. The results obtained show influence of internals design on column hydrodynamics which need to be considered for their proper design and modeling.     

Modulation of liquid holdup along a trickle bed reactor with periodic operation

Temporal variations of the liquid holdup in a mini-pilot scale trickle bed reactor cold-mockup, induced by an ON-OFF liquid flow modulation strategy of operation, are explored at different axial positions. The reactor is packed with porous beads of γ-Al₂O₃ and the liquid holdup is approximately estimated with a conductimetric technique, using probes that mimic the packing. The effects of the liquid and gas superficial velocities, the bed depth and the cycling parameters, cycle period and split, on the liquid holdup modulation are examined for a wide range of conditions. For slow and intermediate cycle periods, the liquid holdup time dependence observed during the dry period is represented by an exponential function. The characteristic value of the decay is correlated to the examined variables. The correlation allows reconstruction of the liquid holdup time dependence along the column.     

两级气液内环流反应器内气含率和循环液速

基于多釜串联可以有效减小返混的原理,通过引入特殊设计的级间构件构建了一种新型的两级内环流反应器。实验研究了级间构件形式、表观气速、表观液速和气液分离器对每一级内气含率和循环液速的影响。实验结果表明,表观气速对反应器二级(上一级)中上升管与下降管气含率之差和循环液速影响较大,而对一级(下一级)的影响较小;各级内上升管和下降管的气含率均随表观液速的增大而减小,但影响程度较小。基于推动力和阻力平衡建立了预测反应器中每一级的气含率和循环液速的流体力学模型,模型预测值与实验结果吻合较好。     

CFD simulation and experimental measurement of gas holdup and liquid interstitial velocity in internal loop airlift reactor

This paper documents experiments and CFD simulations of the hydrodynamics of our two-phase (water, air) laboratory internal loop airlift reactor (40 l). The experiments and simulations were aimed at obtaining global flow characteristics (gas holdup and liquid interstitial velocity in the riser and in the downcomer) in our particular airlift configurations. The experiments and simulations were done for three different riser tubes with variable length and diameter. Gas (air) superficial velocities in riser were in range from 1 to 7.5 cm/s. Up to three circulation regimes were experimentally observed (no bubbles in downcomer, bubbles in downcomer but not circulating, and finally the circulating regime). The primary goal was to test our CFD simulation setup using only standard closures for interphase forces and turbulence, and assuming constant bubble size is able to capture global characteristics of the flow for our experimental airlift configurations for the three circulation regimes, and if the simulation setup could be later used for obtaining the global characteristic for modified geometries of our original airlift design or for different fluids. The CFD simulations were done in commercial code Fluent 6.3 using algebraic slip mixture multiphase model. The secondary goal was to test the sensitivity of the simulation results to different closures for the drag coefficient and the resulting bubble slip velocity and also for the turbulence. In addition to the simulations done in Fluent, simulation results using different code (CFX 12.1) and different model (full Euler–Euler) are also presented in this paper. The experimental measurements of liquid interstitial velocity in the riser and in the downcomer were done by evaluating the response to the injection of a sulphuric acid solution measured with pH probes. The gas holdup in the riser and downcomer was measured with the U-tube manometer. The results showed that the simulation setup works quite well when there are no bubbles present in the downcomer, and that the sensitivity to the drag closure is rather low in this case. The agreement was getting worse with the increase of gas holdup in the downcomer. The use of different multiphase model in the different code (CFX) gave almost the same results as the Fluent simulations.     

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％。     

固体装载率对局部相含率与液体速率的影响

在高径比(H/D=22.2)的气升式内环流反应器中,以空气-水-石英砂为物系,研究了固体装载率和表观气速对导流筒内局部固含率、下降区固含率、下降区液体速率的影响,以及导流筒内局部气含率、固含率随轴向高度的分布规律。结果表明,固体体积分数小于或等于1.0%时,导流筒内局部固含率和下降区固含率与表观气速无关;下降区液体速率随表观气速和固体装载率的增加而下降;固定表观气速,在固体体积分数小于或等于2.0%时,导流筒内气含率在轴向是均匀分布的。     

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...     

旋流气升式环流反应器的局部气、固含率研究

新开发的有机玻璃制成的旋流气升式环流反应器高为2m、内径75mm,内部的导流筒(材料为PVC)高1.5m、内径30mm、壁厚1mm且装有10组扇形翅片。以空气-水为两相物系、空气-水-K树脂为三相物系,常温常压下,利用直接取样法和压差法测量了上升区局部的气含率、固含率,研究了不同底部间隙、不同固体装填体积分数下表观气速和轴向高度对上升区气含率、固含率的影响规律。结果表明:在均匀鼓泡流时上升区气含率随着表观气速的增大而增大,随着固体装载量的增加而下降;在非均匀鼓泡流时,三相物系的气含率高于两相物系的气含率。随着上升区轴向高度的增大,上升区局部气含率变化不大;底部间隙越大,气含率越小。在轴向高度较低时上升区固含率随着轴向高度的增大而减小,轴向高度较高时上升区固含率基本保持不变。     

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.     

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.     

Evaluation of pseudohomogeneous models for heat transfer in packed beds with gas flow and gas-liquid cocurrent downflow and upflow

Several pseudohomogeneous models are used by researchers in the study of heat transfer in packed beds. In this work, five of the most used pseudohomogeneous models (to one, two and three parameters) are analyzed, for gas and gas-liquid flow configurations. The models were evaluated concerning the following aspects: (a) the fitting between calculated and measured temperatures, (b) the values of thermal parameters, (c) their confidence intervals, (d) the quality of the estimation of the thermal parameters by analysis of their Box biases, and (e) the nonlinear dependence of the calculated temperatures on the thermal parameters (using the curvature measures of Bates and Watts). It was observed, particularly in gas-liquid flow, that the fittings between calculated and measured temperature profiles are better for models in which a wall heat transfer coefficient is incorporated to consider the convective resistance at the bed wall. It was also noted that the values of the thermal parameters fitted from the pseudohomogeneous models may be very different at identical operational conditions. The effective axial thermal conductivity may be neglected in the modeling because its estimation does not affect the residual functions. Besides, the estimation of k_a is tricky because it depends on the initial guess and also because the parameter is extremely sensitive to changes in the operational conditions. The confidence intervals for the parameters depend on the model and are also affected by the experimental conditions. The estimation of the parameters was adequate for the k_r-h_W and k_r-k_a models and the curvatures measures were satisfactory only for models in which h_W was not incorporated.     

X-ray computed tomography of a gas-sparged stirred-tank reactor

X-ray computed tomography (CT) is used to explore the differences in gas dispersion in a stirred-tank reactor (STR) for different operating conditions by varying the impeller speed and gas flow rate. X-ray imaging has been carried out in a 0.21 m ID acrylic STR equipped with a nylon Rushton-type impeller. From the CT images, major differences in local gas holdup are observed for different operating conditions. Completely dispersed conditions have a relatively uniform holdup profile while flooded conditions show a high gas holdup near the impeller shaft. The high resolution of the X-ray system allowed fine details such as recirculation regions behind the baffles to be visualized.     

分布器结构对环流反应器气含率分布的影响

采用κ-ε二方程模型和欧拉多相流模型,对一种单筒单级气升式气液环流反应器内的湍流气液两相流进行了全尺寸的数值模拟研究,考察了采用3种不同气体分布器时反应器内气含率和流速分布的细节.模拟结果表明不同结构的分布器对总体气含率和内筒中的两相速度分布有很大影响,因而对气含率分布和气液两相接触效果有较大影响,从而对反应过程产生影响.单环分布器产生的气液两相接触效果较差,对于反应过程很不利.对于大直径的环流反应器推荐使用多环分布器.计算所得的整体气含率与实测的整体气含率进行了对比,吻合较好.     

Simulating a multiphase reactor with continuous phase transition

A novel multiphase fixed-bed reactor with continuous phase transition for benzene hydrogenation to cyclohexane has been presented by Cheng et al. (A.I.Ch.E. J. 47 (2001a) 1185; Chem. Eng. Sci. 56 (2001b) 6025; 57 (2002) 3407; A.I.Ch.E. J. 49 (2003) 225) which makes use of evaporation of volatile components to remove the reaction heat. This paper investigates issues related to the numerical simulation of the model for this novel fixed-bed reactor. The reactor is supposed to be divided into two parts subject to the phase transition point on the bed level, that is, the liquid-contacted zone at the lower part and the gas-contacted zone at the upper part. A steady-state mathematical model for the reactor is developed. All external mass and heat transfer resistances can be neglected in the case investigated, as well as temperature differences in the particles. The effects of feed inlet temperature, inlet benzene concentration, gas flow rate and liquid flow rate on the reactor performance are theoretically studied. Reliability of the model is verified by bench-scale experiments. The results show that dryout phenomena will occur in the reactor both on the bed-scale and on the pellet-scale under certain conditions.     

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   

Gas holdup and overall volumetric mass transfer coefficient in a modified reversed flow jet loop reactor

Experiments were conducted in a modified reversed flow jet loop reactor having the liquid outlet at the top of the reactor to determine the gas holdup and overall volumetric mass transfer coefficient in the air-water system. The influence of gas and liquid flow rates, and the draft tube to reactor diameter ratio were studied. It was observed that both gas holdup and volumetric mass transfer coefficient increased with increased gas and liquid flow rates and were found to be significantly higher in the modified reactor compared to the conventional one. The optimum draft tube to reactor diameter ratio was found to be in the range of 0.4 to 0.5. Empirical correlations are presented to predict gas holdup and overall volumetric mass transfer coefficient in terms of operational and geometrical variables.     

Hydrodynamics of trickle bed reactors at high pressure: Two-phase flow model for pressure drop and liquid holdup, formulation and experimental validation

A large experimental database has been established at IFP on the same experimental setup to measure simultaneously pressure drop and liquid holdup in packed bed reactor operated in trickle for a large range of operating conditions. The varying parameters are liquid viscosity and density, gas density, bed particle shape and size. The range for gas density range is particularly large (from 1.3 to ), thanks to the use of dense gas to simulate very high pressure conditions. This data bank has been first used to compare the prediction accuracy of the different models from the literature. Finally, the mechanistic model proposed by Attou et al. [1999. Modelling of the hydrodynamics of the cocurrent gas-liquid trickle flow through a trickle-bed reactor. Chemical Engineering Science 54, 785-802] has been improved by adding a new formulation for liquid film tortuosity in two-phase flow conditions. This model has been validated over the whole data range and the accuracy has been checked with data external to the data bank. The prediction accuracy is significantly increased when compared with the best available models for pressure drop and liquid retention in trickle flow reactors.