Volumetric Mass Transfer Coefficient of Oxygen in An Internal Loop Airlift Reactor with a Convergence‐Divergence Draft Tube

Experimental measurements of hydrodynamics and the volumetric mass transfer coefficient of oxygen (VMTCO) in an internal loop airlift reactor with different types of draft tubes are reported for the two‐phase systems, air/water and air/carboxyl methyl cellulose (CMC) solution, and a three‐phase system, air/water/resin particle, respectively. The properties of convergence‐divergence draft tubes with three different structural parameters are compared with those of the conventional column draft tube. The results indicate that gas holdups in convergence‐divergence draft tubes are higher than those in conventional draft tubes, the volumetric mass transfer coefficient of oxygen increases with increasing superficial air flow rates. The convergence‐divergence draft tubes all show higher mass transfer capacity than the traditional ones. A 10% higher mass transfer coefficient is observed for the three structural parameters. In the air/CMC system, the volumetric mass transfer coefficient of oxygen decreases with increasing bulk viscosity, while in the three‐phase system VMTCO increases with the resin particle loading. The correlation equation of the volumetric mass transfer coefficient with the operating conditions and structural parameters is established.     

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.     

连续内环流三相反应器局部流动特性

在φ200 mm×2500 mm连续内环流三相反应器内,考察了空气 水 玻璃珠体系反应器内局部流动参数随操作条件的变化规律。结果表明,导流筒内截面平均气含率随表观气速的增大而增大,较之气液两相流,在低固含率时,加入固体对气含率影响不明显,而在较高固含率下,气含率有明显降低,但固体再增加时对气含率变化影响不大。在较低表观气速下,进料浆速对导流筒内气含率轴向分布趋势有一定的影响,但在较高表观气速下影响不大,导流筒内的气含率大于环隙内的气含率且随气速增大差别更加明显,浆相连续有利于气相分散并增大环隙内的气含率。导流筒内循环浆速径向分布呈抛物状,中心高、近壁处低,受进料浆速和入口固含率影响都不大。浆相循环强度最低为20,高可达180。固含率轴、径向分布受表观气速和进料浆速的影响,固含率轴、径向分布基本均匀,随进料浆速增加,反应器内固含率降低。     

缩放型导流筒气升内环流反应器液体循环速度的研究

研究气升式内环流生物反应器液体循环速度,分别采用一种传统圆柱型导流筒和三种不同结构参数缩放型导流筒,试验条件分别为空气－水和空气－ＣＭＣ两相系统以及空气－水－树脂三相系统,试验结果表明,对于空气－水和空气－ＣＭＣ溶液两相系统以及空气－水－树脂三相系统,液体循环速度随能气速度提高而增大;     

缩放型导流筒气升式内环流生物反应器流体力学与传质特性

从气相含率、液体循环速度和体积氧传质系数方面研究缩放型导流筒气升式内环流生物反应器内的流体力学与传质特性。实验结果表明,与传统圆柱形导流筒相比较,缩放型导流筒气相含率和体积氧传质系数分别提高８％和１０％以上。气相含率和体积氧传质系数随固含率的增加而提高,液体循环速度随固含率的增加而减小;同一内管反应器随介质粘度的增加,体积氧传质系数减小。此外还在Ｈｉｇｂｉｅ穿透理论和Ｋｏｌｏｍｏｇｏｒｏｆｆ各向同     

Hydrodynamics in down flow jet loop reactor

Gas holdup and liquid circulation time were measured in a down flow jet loop reactor for air–water system. It was observed that the circulation time decreases with an increase in nozzle diameter, draft tube to column diameter ratio and liquid velocity. Th gas holdup increases with an increase in gas and liquid velocities. The optimum draft tube to column diameter ratio was found to be 0.438. Correlations for gas holdup and circulation time involving operational and geometrical variables are presented.     

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.     

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.     

Development of airlift bubble column dispersed with micro‐bubbles

Micro‐bubbles were dispersed in the bubble column with draft tube, and the length and diameter of draft tube were changed. The flow characteristics in air–water system were measured. Ozone gas and methylene‐blue aqueous solution were used, and the decomposition performance was examined. With increasing draft tube length, both the gas holdup and liquid velocity in the annular section increased. When the diameter ratio of draft tube to column was about 0.5, both the gas holdup and liquid circulation flow rate had maxima. For the decomposition by using ozone, the installation of draft tube enhanced the mass transfer and decomposition performance.     

Interrelation among Liquid Velocity,Impeller Speed and Gas Flow Rate in the Stirred Airlift Reactor

We investigate the liquid circulation velocity in the draft‐tube airlift reactor, with mechanical agitation in the internal column as a riser, at the impeller speed of up to 40 s^–1. An influence of the gas flow rate and the stirrer speed on the riser and downcomer gas hold‐up difference and on the liquid circulation velocity is also investigated. The character of the liquid circulation velocity changes depends on the relation between the gas flow rate and the impeller speed. A monotonic increase of the liquid circulation velocity with an increase of the gas velocity is observed at the impeller speed of lower than 15 s^–1. A distinct decrease of the liquid velocity is found at higher impeller speeds and at low gas velocities. The decrease is larger for higher impeller speeds. We observe the minimum on the curve of the liquid velocity dependence on the gas velocity followed by the monotonic increase of the liquid velocity with an increase of the gas flow velocity. The minimum of the liquid circulation velocity appears if the ratio of the gas flow number to the impeller speed is about 0.0006. The minima are shifted towards the higher gas velocities at higher impeller speeds. An experimental equation for the prediction of the liquid circulation velocity in the stirred airlift reactor is presented.     

气升式环流反应器在不同体系下的循环液速和局部气含率

在气升式环流反应器中,分别研究了空气—水、空气-0．1％乙醇水溶液和空气-水-活性污泥体系中的循环液速以及气含率随操作条件的变化规律。实验结果表明,循环液速随着表观气速的增加而增大,不同体系中的循环液速差别不大;导流筒内、外不同高度处的局部气含率均呈自下向上增大的趋势,且导流筒内部气含率高于导流筒外部的气含率。体系的聚并特性对平均气含率和局部气含率有较大影响。     

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     

气升式内循环反应器的数值模拟和结构参数

采用Euler-Euler双流体模型对内循环反应器(高1240 mm,直径165 mm,导流筒高590 mm)进行数值模拟,考察了表观气速、导流筒结构(导流筒内径比Dt/D,底隙高度)对反应器内上升区、下降区流体力学参数(气含率、液体速度)的影响. 结果显示,表观气速、导流筒内径、底隙高度对反应器气含率、液体速度有很大影响,随表观气速增加,反应器上升区、下降区气含率都增加,导流筒内径比为0.58时更易实现气液循环,底隙高度为30 mm时反应器内下降区气含率、气液速度都最小;气液分离器角度越大,进入下降区的气体越多,当气液分离器角度为45o时,能更好地实现气液循环.     

导流管结构对三相环流反应器相含率的影响

本文阐述了分别在三套内环流反应器内,采用空气—水—石英砂三相物系,导流管外径依次取为75mm和60mm,导流管底隙高度为17mm和30mm.研究了在一定的固相质量分数及初始静液高度下,导流管内局部相含率与表观气速的随动关系以及在固定操作气速下相含率在导流管内的轴向分布情况.经装置间平行比对得出:采用75mm外径的导流管...     

下喷式环流反应器气含率研究

本文从奈维—斯托克斯运动方程出发,结合体系特性及反应器内流体流动状况,建立了计算下喷式环流反应器气含率的模型方程式,并依实验结果确立了模型参数.该模型方程式适用于牛顿型与非牛顿型流体体系.     

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.     

Effect of a gas–liquid separator on the hydrodynamics and circulation flow regimes in internal‐loop airlift reactors

The role of the gas–liquid separator on hydrodynamic characteristics in an internal‐loop airlift reactor (ALR) was investigated. Both gas holdup and liquid velocity were measured in a 30 dm³ airlift reactor with two different head configurations: with and without an enlarged separator. A magnetic tracer method using a neutrally buoyant magnetic particle as flowfollower was used to measure the liquid velocity in all sections of the internal‐loop airlift reactor. Average liquid circulation velocities in the main parts of the ALR were compared for both reactor configurations. At low air flow rates the separator had no influence on gas holdup, circulation velocity and intensity of turbulence in the downcomer and separator. At higher superficial air velocities, however, the separator design had a decisive effect on the hydrodynamic parameters in the downcomer and the separator. On the other hand, the gas holdup in the riser was only slightly influenced by the separator configuration in the whole range of air flow. Circulation flow regimes, characterising the behaviour of bubbles in the downcomer, were identified and the effect of the separator on these regimes was assessed. © 2001 Society of Chemical Industry     

Experimental Investigation on the Holdup Distribution of Oil‐Water Two‐Phase Flow in Horizontal Parallel Tubes

An experimental investigation was conducted to study the holdup distribution of oil and water two‐phase flow in two parallel tubes with unequal tube diameter. Tests were performed using white oil (of viscosity 52 mPa s and density 860 kg/m³) and tap water as liquid phases at room temperature and atmospheric outlet pressure. Measurements were taken of water flow rates from 0.5 to 12.5 m³/h and input oil volume fractions from 3 to 94 %. Results showed that there were different flow pattern maps between the run and bypass tubes when oil‐water two‐phase flow is found in the parallel tubes. At low input fluid flow rates, a large deviation could be found on the average oil holdup between the bypass and the run tubes. However, with increased input oil fraction at constant water flow rate, the holdup at the bypass tube became close to that at the run tube. Furthermore, experimental data showed that there was no significant variation in flow pattern and holdup between the run and main tubes. In order to calculate the holdup in the form of segregated flow, the drift flux model has been used here.     

硅油光-热老化气升式反应器数值模拟

为进行液浸聚光光伏系统中液浸液体即二甲基硅油在光热条件下的老化实验研究,设计制作了气升式外环流光催化反应器。采用Fluent软件欧拉多相流模型和RNG k-ε湍流模型对反应器进行了三维全尺寸数值模拟。分析了结构参数、操作参数和硅油物性参数对反应器中气液两相流动的影响,得到反应器内循环液速和气含率等参数的分布。模拟结果与实验结果有较好的一致性。模拟结果表明:硅油黏度越小,循环液速越大,气含率越小;反应器适宜操作气速不应大于0.02 m/s;反应器最适宜高径比和环隙面积比分别为47.6和0.31~0.41。     

三相下喷式环流反应器的传质性能

在三相非牛顿型流体体系中，对下喷式环流反应器传质特性进行了实验研究。讨论了表观气速、能量耗散速率、导流筒直径与反应器直径比、喷嘴直径、导流筒下端距反应器底部的距离、固体装填量、羧甲基纤维素钠（ＣＭＣ）溶液浓度及其流变特性对它的影响。实验结果表明，容积传质系数随表观气速和能量耗散速率的增加有所增加，在实验条件下，发现最优的导流筒直径与反应器直径比在０．４～０．４５这一范围、固体装填量大约为３％（体积百分比）、导流筒下端距反应器底部的距离为０．０８ｍ左右。同时提出了容积传质系数的经验关联式。