鼓泡床内气泡直径的分布

气泡的直径分布是决定鼓泡床反应器气液接触面积的重要参数，其轴向分布对鼓泡床反应器的设计和性能优化具有重要意义。喷嘴作为气体分布器的最小单元，对气体分布器的分布性能起决定性因素。本文考察了不同孔径喷嘴对鼓泡床内气泡分散的影响，结果表明，对于在孔口产生的小于最大稳态气泡直径ds的气泡，将稳定存在于床层内，而直径大于ds的气泡，将迅速破碎，最后形成近乎一致的气泡粒度分布，喷嘴的影响距离在21cm左右。     

鼓泡床中的气泡尺寸分布

根据气泡聚并和破裂速率模型,提出一个描述鼓泡床中气泡尺寸分布的总体平衡(Population Balance)模型。该模型可预计鼓泡床中气泡尺寸分布随床高的变化。对空气-水体系的模拟结果与实测结果吻合较好。     

鼓泡床内气体分布器设计及其对水力学影响

本文比较了目前常用的几种分布器，通过照像法观察了三种分布器（单孔板、多孔板和烧结金属板）上的气泡形成过程，然后测定了这三种分布器的干板压降和湿板压降，并就它们对水力学条件的影响进行了考察。其结果对鼓泡床内分布器的设计具有一定的参考价值。     

二维鼓泡床内气泡尺寸分布的实验与CFD模拟

在有机玻璃制成的二维鼓泡床(0.20m×0.02m×2.00m)内,采用摄像法对空气－自来水的气液两相体系的气泡尺寸分布进行了考察。以商业计算流体力学软件ANSYS CFX 10.0为平台,在双流体模型的基础上,采用k-ε湍流模型和GRACE曳力模型对气液鼓泡床内流体动力学行为进行了多相流CFD数值模拟。结果表明 MUSIG(Multiple Size Group)模型实现了对多气泡体系内气泡尺寸分布特性的考察,气泡尺寸分布的模拟结果与实验结果吻合得较好,从而说明了考虑了气泡聚并破碎的MUSIG模型能很好地反映出鼓泡床内气泡尺寸分布特性。     

加压大型鼓泡床反应器内大小气泡气含率研究

在内径0.3m、高6.6m的加压鼓泡床反应器内采用床层塌落技术测量床层内的大小气泡气含率实验。由于大小气泡上升速度不同,床层塌落曲线存在一水平段,在此基础上,详细考察了表面张力、粘度、系统压力、表观气速对大小气泡气含率的影响,得出大气泡气含率随粘度和表面张力升高而升高,随压力升高而降低;小气泡气含率随粘度和表面张力升高而降低,随压力升高而升高;并根据质量守恒定理,进行了解释。     

气液鼓泡床内的液体流速分布

引言 鼓泡床是一种重要的气液或气液固多相反应器.液体循环流动是鼓泡床的一个重要流体力学特征,从20世纪50年代人们就开始对此进行了比较系统的实验研究[1-6].这个特征对鼓泡床的流体返混行为、气含率、气液界面积以及传热传质系数都有很大影响,特别是液体返混行为可以由液体循环特性直接决定.如何准确地描述和预测鼓泡床中的液体流速沿径向的分布,关系到鼓泡床反应器的设计、放大和优化.因此,许多年来它一直是人们致力探讨的重要课题之一[7-8].     

鼓泡床内气体分布器设计及其对水力学的影响

本文比较了目前常用的几种分布器，通过照像法观察了三种分布器（单孔板、多孔板和烧结金属板）上的气泡形成过程，然后测定了这三种分布器的于板压降和湿板压降，并就它们对水力学条件的影响进行了考察。其结果对鼓泡床内分布器的设计具有一定的参考价值。     

二维鼓泡床内气液流动特性实验与数值模拟

采用高速摄像法测量了0.20 m×0.02 m×2.00 m拟二维床内气泡尺寸分布和流型等变化规律,结果表明,随着表观气速的增大,鼓泡床内依次呈现均匀鼓泡区、过渡区和湍动区3种形式,以气泡个数概率表示的气泡尺寸分布呈对数正态分布。以计算流体力学软件ANSYS CFX 10.0为平台,采用k-ε湍流模型和GRACE曳力模型对气液鼓泡床内流体动力学行为展开了数值模拟,其结果与实验值比较吻合。研究表明,从多相流理论出发的计算流体力学模拟方法可以用来预报鼓泡床内流型过渡等流体动力学特性。     

浆态鼓泡床反应器气液传质特性的研究进展

本文总结了有关浆态鼓泡床反应器气液传质特性的研究成果。详细地阐述了主要影响因素如系统压力、温度、气体表观气速、液体性质,固体浓度及其物性等对传质特性的影响,并对浆态鼓泡床传质模型进行了归纳介绍,最后对反应器未来的研究方向进行了预测。     

高温高压下浆态鼓泡床气液传质系数的测定

采用气体动态吸收法,对高温高压下浆态鼓泡床中气液传质系数进行了测定,考察了系统压力、温度、表观气速及固含量等操作条件对传质系数的影响. 结果表明,在温度298~473 K、气体压力1.0~3.0 MPa、表观气速0.03~0.10 m/s、固含量0%~20%(w)的实验条件下,体积传质系数随着压力、温度及表观气速的增加而增大,随固含量的增加而减小. 同时对不同塔径的传质系数进行了比较,表明在相同的操作条件下,大塔的传质系数略高于小塔.     

Flow regimes and mass transfer in counter-current bubble columns

Counter current bubble columns have the feature that specific gas-liquid interfacial area and gas holdup are larger than those for standard and cocurrent bubble columns. In this study, three different flow regimes, churn-turbulent flow, bubble flow and bubble down-flow, have been observed in a counter-current bubble column and correlations of gas holdup and volumetric liquid-phase mass transfer coefficient have been proposed as functions of operating variables such as the superficial velocities of gas and liquid, the gas-liquid slip velocity and the liquid properties.     

Determining equivalent pore diameter for rigid porous spargers

Prediction of the properties of gas dispersions (e.g., bubble size) generated by porous spargers requires a knowledge of the pore diameter. Three methodologies for determination of an equivalent pore diameter D_e of a rigid SS sparger of the type used in column flotation have been developed. In the first, D_e is determined by fitting the bubble diameter predicted by the Kumar/Kuloor model to that measured. In the second, an estimation of bubble diameter is substituted. Estimation was tested using drift flux analysis and Molerus' uniform bubbling model: the former gave acceptable agreement with the measured. Fitting to the estimated bubble diameter offers a simplification over using the measured bubble diameter. The third approach was a further simplification. By combining the Kumar/Kuloor model with drift flux analysis, D_e is estimated by fitting predicted to measured gas holdup. The methodologies are evaluated on a set of SS spargers using water with a flotation frother, Dowforth 250C. All three methodologies give similiar estimates of D_e.     

Gas hold-up and volumetric mass transfer coefficient in bubble columns under foam control

Experimental measurements of the gas hold-up and volumetric mass transfer coefficient have been made for baffle columns (BCs) reacting various foaming liquids under mechanical and chemical foam control. The gas hold-up and the volumetric mass transfer coefficient in a mechanical foam-control system (BCs with rating-disk mechanical foam-breakers) were larger than those in a chemical foam-control system (BCs with an antifoam agent added). Correlations for the gas hold-up and the volumetric mass transfer coefficient in BCs under foam control are presented. Comparison of the volumetric mass transfer coefficient between the mechanical foam-control system and the chemical foam-control system in terms of the specific power input also demonstrated higher mass transfer performance and saving power requirements for the mechanical foam-control system.     

Effects of sparger height and orifice orientation on solids dispersion in a slurry bubble column

The effects of gas distributor height and the orientation of its orifices are investigated on solids dispersion and gas holdup profiles in a three-phase slurry bubble column. The height of the distributor was varied to cover locations from near column bottom to above the settled solids bed height. The orifice orientations were changed from upward facing to downwards facing directions. The measurements were conducted in a Plexiglas column of 0.15 m ID and 2.5 m height. The gas phase was oil-free compressed air while tap water was used as liquid phase. Glass beads with an average particle diameter of 35 μm and density of 2450 kg/m³ constituted the solid phase. The settled bed height was about 0.4 m which provided an average slurry concentration of about 15% (v/v) when all solids were dispersed. Both axial and column average phase holdups were measured. Effects of sparger location, gas jets formation and liquid circulation patterns on gas holdups and solids dispersion are analyzed. Empirical correlations are developed to relate sparger location to solids dispersion as a function of gas velocity. Optimum sparger height and orifice orientation is proposed based on the measurement of this study.     

Diluted alcohol solutions in bubble columns and draft tube airlift reactors with a single orifice sparger: experiments and simple correlations

BACKGROUND: Bubble columns (BCs) and airlift reactors (ALRs) have important applications as bioreactors, chemical reactors and as contactors in waste‐water treatment. The liquid phase properties in these reactors significantly influence the main hydrodynamic and mass transfer characteristics. Dilute alcohol solutions can be used to simulate real industrial systems in bioreactors. However, only a few research studies have considered such systems. The aim of this paper is to broaden the existing experimental data related to the influence of alcohol addition on the main characteristics of draft tube airlift reactors (DT‐ALRs), and to propose simple correlations for their prediction. RESULTS: New experiments were conducted in a DT‐ALR with a single orifice sparger, and with dilute aliphatic alcohol solutions from methanol to n‐octanol. Also, simple correlations were developed to predict the gas hold‐up and volumetric mass transfer coefficients in BCs and DT‐ALRs, but also the downcomer liquid velocity and liquid circulation time DT‐ALRs with single orifice sparger and dilute alcohol solutions. The proposed correlations included, in addition to the superficial gas velocity, the surface tension gradient as the only factor to characterize the liquid phase. CONCLUSIONS: General conclusion can be made that the gas holdup increased, but the downcomer liquid velocity decreased in a DT‐ALR, with increase in surface tension gradient of the alcohol solutions. Also, very good agreement was achieved between experimental and calculated data, by applying the developed correlations, with relative average errors less than 5%, except for gas hold‐up, where it was in the range 8–32%. Copyright © 2009 Society of Chemical Industry     

Hydrodynamic studies with foaming and non-newtonian solutions in bubble columns

Experiments were conducted in 0.05 m ID and 0.23 m ID by 3 m tall bubble columns to study the effect of surfactants and viscosity of liquid medium on gas hold-up and Sauter mean bubble diameter. The addition of n-butanol (0.5 and 1 wt.%) to water leads to the formation of foam and consequently produces higher gas hold-ups. The foam could be eliminated completely with the addition of a sufficient quantity (0.5 wt.%) of carboxymethyl cellulose to the aqueous alcohol solution. In the absence of foam, gas hold-ups were similar to those obtained with pure liquids. Sauter mean bubble diameters, obtained using the dynamic gas disengagement technique, increase with viscosity of liquid medium.     

Semi‐theoretical prediction of volumetric mass transfer coefficients in bubble columns with organic liquids at ambient and elevated temperatures

A semi‐theoretical approach for predicting k_La values (referred to liquid volume) in 18 organic liquids [acetone, aniline, 1‐butanol, benzene, cyclohexane, decalin, 1,2‐dichloroethane, 1,4‐dioxane, ethanol (96%), ethylacetate, ethylbenzene, ligroin, methanol, nitrobenzene, 2‐propanol, tetralin, toluene, and xylene] at various operating conditions (including elevated temperatures and pressures) was developed. It was found that the approach is applicable regardless of the hydrodynamic regime (at u_G ≤ 0.1 m/s). Temperatures up to 353 K and pressures up to 0.5 MPa were tested. Two different distributors (multiple‐hole and single‐hole type) were employed. The liquid‐phase mass transfer coefficient k_L was calculated theoretically from the penetration theory on the basis of original definition of gas–liquid contact time. The interfacial area a was defined with respect to the liquid volume. It was found that their product k_La must be multiplied by some correction factor in order to take account of the non‐spherical (ellipsoidal) shape of the bubbles. When the correction term is correlated to both the Eötvös number (Eo) and the dimensionless temperature ratio, 198 experimental k_La values can be fitted reasonably well (average relative error 9.3%).     

Frictional loss in multiplate bubble columns

Gas holdup and pressure loss were measured for various gas-liquid systems in a batch, multistage bubble column. Experimental results show that the use of screen plates (α = 0.64) considerably increases the gas holdup but introduces a significant pressure loss for the two-phase mixture. The pressure loss was found to be independent of physical properties of the mixture, and predictable either from the modified separated flow model (Chen et al., 1986) or from the kinetic energy loss based on the liquid circulation velocity.     

Gas hold-up and mass transfer in a bubble column with viscoelastic fluids

The influence of viscoelasticity on gas hold-up and the volumetric mass transfer coefficient in a bubble column is discussed and examined experimentally. It was found that the gas hold-up increased due to an increase in the number of entrapped very small bubbles formed because of the elasticity of the liquid. On the other hand, a decrease in volumetric mass transfer coefficient due to the fluid elasticity was observed. Qualitative effects of an antifoam agent and a draft tube on the performance of the bubble column with viscoelastic fluids were also examined.