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.     

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.     

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.     

Hydrodynamic behaviour of a draft tube gas-liquid-solid spouted bed

Experiments were conducted using various types of solid particles to investigate the hydrodynamic properties of a gas-liquid-solid spouted bed with a draft tube. The hydrodynamic properties under study include flow modes, pressure profile and pressure drop, bubble penetration depth, overall gas holdup, apparent liquid circulation rate and bubble size distribution. Three flow modes were classified: a packed bed mode, a fluidized bed mode and a circulated bed mode. It was found that the friction factor accounting for the friction loss in the bed varies linearly on a logarithmic scale with the Reynolds number defined based on the apparent liquid circulation rate. The bubble penetration depth in the annular region, overall gas holdup and apparent liquid circulation rate increase with an increase in gas or liquid velocity. At high gas flow conditions an optimal solids loading exists which yields a maximum apparent liquid circulation rate. A model was proposed to describe the liquid circulation behaviour in the draft tube three-phas spouted bed. The average bubble size in the draft tube region is higher than that in the annular region for both the dispersed bubble regime and the coalesced bubble regime in the draft tube region.     

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     

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 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.     

Effect of liquid viscosity on gas holdups in a reversed flow jet loop reactor

The effect of draft tube diameter and liquid viscosity on overall and annulus gas holdups were studied in a reversed flow jet loop reactor. The draft tube diameter to reader diameter ratio (D_d / D) and liquid viscosity were varied in the ranges 0.34-0.67 and 1.5-43 mPa. s, respectively. The maximum gas holdup was obtained when the D_d / D value ranged btween 0.47 and 0.61. The gas holdup decreased with increasing viscosity. Empirical correlations are presented to predict the gas holdups.     

HYDRODYNAMIC CHARACTERISTICS AND GAS-LIQUID MASS TRANSFER IN A DRAFT TUBE SLURRY REACTOR

Fundamental characteristics of hydrodynamics and mass transfer have been measured in an air lift slurry reactor with a draft tube. The solid suspension capacity, i.e., the critical solid holdup, the gas holdup and the volumetric gas-liquid mass transfer coefficient were measured in the two draft tube columns of 0.1485 and 0.10?m in diameter. Four activated carbon beads ranging in size from 0.25 to 2.19?mm in average diameter were utilized as suspended solids in the experiments.

The critical solid holdup in the draft tube slurry column is found to be much greater than that in the conventional bubble column. An empirical correlation is developed to account for the critical solid holdup behavior in the draft tube column. The gas holdup in the draft tube column agrees well with that in the bubble column. The overall gas-liquid mass transfer coefficient, k₁awas measured by the oxygen probe method. The effect of solid holdup on k₁a is found to be negligible in the present system. The empirical equation is developed to correlate k₁a in the draft tube slurry reactor.     

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。     

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。     

内循环流化床结构参数及其反应器性能的相关性

内循环三相流化床在化学工程、生物工程及废水处理等领域中均有广泛的应用前景,目前影响这类反应器成功应用于实际废水生物处理的主要原因是缺乏系统的反应器结构参数与反应器性能的相关性分析与量化关系模型,无法实现不同规模与不同应用目标的系统优化设计。基于此,设计了不同尺寸和内构件的反应器,系统研究主要结构参数包括升流管与反应器直径之比、反应器高径比、升流管上方液面高度、底隙高度以及操作参数如固相负荷及通气量等因素对内循环流化床反应器流体力学与传质特性的影响规律;实验分析主要过程参数如气含率、液体循环速度、氧传递系数及混合时间与结构参数、操作参数的相关性。通过总结归纳,分别给出了定量、半定量或定性方面的结论,为反应器的结构放大和性能变化预测提供理论基础.     

Effect of geometrical parameters of draft tubes and clear liquid height on gas holdup in a bubble column for gas dispersion into tubes

The effects of the geometrical parameters of draft tubes and the clear liquid height on the average gas holdup E_G in a 0.16 m I.D. bubble column for gas dispersion into the tubes were experimentally studied in an airtap water system. The gas holdup depended on the superficial gas velocity U(ING), the kinds of gas spargers, the diameter and length of the draft tubes, clearance C_b between the lower end of the draft tube and the bottom of the bubble column, and the clear liquid height H_L. E_G increased with decreasing hole diameter of the gas sparger at a small gas velocity U_G, but did not depend on the kinds of gas spargers at a large U_G. E_G decreased with increasing clear liquid height H_L. The effect of H_L on E_G was well expressed by the modified three-region model. The experimental data of E_G were correlated.     

78.5升气升式环流反应器内构件优化

研究了78.5L气升式环流反应器内部结构对流动性能的影响规律, 并给出最佳区间来为工业装置提供理论指导。利用Fluent软件建立数学模型与实验装置作对比, 模拟了不同气液分离区高度与外筒高度比、导流筒长度与外筒长度比和筒内外直径比对流动行为的影响规律。结果表明:数学模型和实验结果误差较小, 可以用来预测气升式环流反应器流动行为。气液分离区高度与外筒高度比值过大会导致环流阻力增大, 从而不利于流动, 比值为0.34~0.36时流动性能最佳;导流筒长度与外筒长度的比值增大可增加气含率和环流液速, 但是比值过大会引起气泡的聚合, 从而影响流动性能, 当比值为0.60~0.62时流动性能最佳;在一定范围内增加内外筒直径比会改善流动效果, 但环隙面积过小会增加环形阻力, 内外筒直径比为0.73~0.77时流动效果比较理想。     

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.     

Local gas holdup in a draft tube airlift bioreactor

Airlift column bioreactors are gas–liquid contact devices characterized by a rising channel and a down flow channel due to gas holdup differences in these two channels. Local gas holdup distribution strongly affects the overall gas–liquid flow dynamics in airlift columns. In this work, local gas holdup distributions in a draft tube airlift column covering both bubbly flow and churn–turbulent flow regimes have been studied using computed tomography (CT) technique as well as conventional techniques. The radial and axial evolutions of the gas holdup distribution will be discussed, together with the effects of superficial gas velocity and geometry parameters. The obtained gas holdup results will also be used to verify various empirical and semi-empirical correlations in the literature. Moreover, the obtained gas holdup information, combined with liquid flow dynamic information reported in Luo and Al-Dahhan, 2008a, Luo and Al-Dahhan, 2008b, forms a benchmark database for the design and scale-up of airlift column bioreactors and for computational fluid dynamic (CFD) modeling validations.     

Solid circulation and gas bypassing in an internally circulating fluidized bed with an orifice-type draft Tube

The effects of orifice diameter in the draft tube, particle size, gas velocities and bed height on the circulation rate of solids and gas bypassing between the draft tube and annulus have been determined in an internally circulating fluidized bed (i.d., 0.3 m ; height, 2.5 m) with an orifice-type draft tube. A conical shape gas separator has been employed above the draft tube to facilitate the separation of gases from the two beds. The circulation rate of solids and the quantity of gas bypass from the annulus to draft tube show their minimums when the static bed height is around the bottom of the separator. The circulation rate of solids increases with an increase in orifice diameter in the draft tube. At fixed aeration to the annulus, gas bypassing from the draft tube to annulus sections decreases, whereas reverse gas bypassing from the annulus to the draft tube increases with increasing the inlet gas velocity to the draft tube. The obtained solids circulation rate has been correlated by a relationship developed for the cocurrent flow of gas and solid through the orifice.     

洗涤冷却室气液两相流的模拟及结构优化

洗涤冷却室是激冷式煤气化炉的重要组成部分,对气化室内生成的高温合成气进行降温及洗涤。目前的洗涤冷却室在工业上依然存在诸多问题,如液位及压力波动较大、装置运行不稳定、洗涤效率不够高等,需要对其结构进行优化改进。采用三维欧拉-欧拉模型对三种不同结构的洗涤冷却室进行了CFD模拟。三种洗涤冷却室的结构分别为无内构件、含有支管内构件及新型耦合支管与导流筒内构件。与文献实验数据的对比表明,模型能较准确地预测洗涤冷却室内的气含率分布。支管的加入分流了部分气量,使部分气体能够从鼓泡区域中心上升,促进了气体的径向扩散,有助于维持装置的稳定运行;而导流筒内构件的加入增大了支管气体分流的比例,提升了洗涤冷却室液面下的全局气含率,增强了气液回流,从而进一步强化洗涤效果。     

Bubble size in a forced circulation loop reactor

BACKGROUND: The bubble size distribution in gas‐liquid reactors influences gas holdup, residence time distribution, and gas‐liquid interfacial area for mass transfer. This work reports on the effects of independently varied gas and liquid flow rates on steady‐state bubble size distributions in a new design of forced circulation loop reactor operated with an air–water system. The reactor consisted of a cylindrical vessel (～26 L nominal volume, gas‐free aspect ratio ≈ 6, downcomer‐to‐riser cross‐sectional area ratio of 0.493) with a concentric draft tube and an annular riser zone. Both gas and liquid were in forced flow through a sparger that had been designed for minimizing the bubble size. RESULTS: Photographically measured bubble size distributions in the riser zone could be approximated as normal distributions for the combinations of gas and liquid flow rates used. This contrasted with other kinds of size distributions (e.g. bimodal, Gaussian) that have been reported for other types of gas‐liquid reactors. Most of the bubbles were in the 3 to 5 mm diameter range. At any fixed low value of aeration rate (≤1.8 × 10^?4 m³s^?1), increase in the liquid flow rate caused earlier detachment of bubbles from the sparger holes to reduce the Sauter mean bubble size in the riser region. CONCLUSION: Unlike in conventional bubble columns where bimodal and Gaussian bubble size distributions have been reported, a normal bubble size distribution is attained in forced circulation loop reactors with an air–water system over the entire range of operation. Copyright © 2007 Society of Chemical Industry     

LIQUID PHASE MIXING IN A SLURRY BUBBLE COLUMN WITH DRAFT TUBE

In this work we characterize liquid phase mixing in a 150 L bubble column with a draft tube (internal airlift configuration) for a water-air-sand system at high solid concentrations. Liquid mixing is assessed by measuring the evolution of sodium chloride concentration after a pulse of concentrated NaCl solution is injected. Tracer concentrations were measured by means of electrical conductivity probes. The experimental set up consists of a 0.29 m internal diameter, 3 m length Plexiglas column with a conical bottom (cone apex angle of 60°) and a concentric draft tube with 0.14 m internal diameter and 2 m length. The gas superficial velocity based on the cross section of the column vaired from 0.057 to 0.22 m/s. Sand particles of 280 μm in average size were used, with slurry concentrations ranging from 120 to 500 kg/m³. From the tracer outputs, circulation time (time between peaks of the response curve) and mixing time (time required to achieve a 95% homogeneous solution) were determined after the pulse had been injected. The experimental data were analyzed by means of a tank in series model with recirculation. The circulation and mixing times were found to increase with solids concentration, and to decrease as the riser gas velocity was increased. The circulation limes were larger in continuous operation than in semibatch mode. The results show that the system has equivalent dispersion coefficients that are one order of magnitude lower than those found in a conventional bubble column.