Effect of solids on homogeneous-heterogeneous flow regime transition in bubble columns

Experiments were conducted to study the effect of the presence of the solid phase on the homogeneous-heterogeneous flow regime transition in a bubble column 0.14 m diameter. Air, distilled water and calcium alginate beads (2.1 mm, ) at concentrations c=0-30% (vol.) were the phases. The basic data were the voidage-gas flow rate dependences. The critical point, where the homogeneous regime loses stability and the transition begins, was evaluated by the drift flux model. The critical values of voidage and gas flow rate were the quantitative measures of the homogeneous regime stability. These were plotted against the solid phase concentration. It was found, that both the voidage and the critical values increased with the solid content at low solid loading, approx. c=0-3%, and decreased at higher loading, c>3%. The homogeneous regime was thus first stabilized and then destabilized. To explain this dual effect, possible physical mechanisms of the solid phase influence on the uniform bubble bed were discussed.     

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。     

Analogous description of the hydrodynamics of gas-solid fluidized beds and bubble columns

In this paper we stress analogies in the hydrodynamic behaviour of gassolid fluidized beds and bubble columns. Using published experimental data, it is demonstrated that the analogous hydrodynamic-behaviour is not only qualitative but also quantitative in nature. Specifically, we show the following.(1) The gas holdup in the homogeneous regimes of bubble columns and fluidized beds can be modelled in a unified way using V_slip = υ_∞(1 − ϵ_d)ⁿ⁻¹, where V_slip refers to the slip velocity between the dispersed (bubbles or particles) and continuous phases and ϵ_d the dispersed phase holdup. The Richardson-Zaki exponent n decreases with increasing gas density.(2) The transition from homogeneous to heterogeneous flow regimes in gasliquid bubble columns and gassolid fluid beds is delayed by increasing system pressure. Extrapolation of the influence of increased gas density allows us to consider liquidliquid dispersions and liquidsolid fluid beds as limiting cases.(3) In the heterogeneous flow regime of operation the classic two-phase theory of fluidized beds can be applied with profit to also describe the hydrodynamics of gasliquid bubble columns provided that the “dilute” phase is identified with the fast-rising large bubbles and the “dense” phase is identified with the liquid phase containing entrained “small” bubbles. Tentative analogies can also be drawn for the interphase mass transfer processes.(4) The “dense” phase backmixing can be modelled in a unified manner.(5) The two-phase theory can be extended to describe slurry reactors.It is argued that, because of cross-fertilization of concepts and information, appreciation of analogies can be invaluable tool in scaling up.     

Experimental and numerical investigations of scale-up effects on the hydrodynamics of slurry bubble columns

Experiments and simulations were conducted for bubble columns with diameter of 0.2 m(180 mm i.d.), 0.5 m(476 mm i.d.) and 0.8 m(760 mm i.d.) at high superficial gas velocities(0.12–0.62 m·s-1) and high solid concentrations(0–30 vol%). Radial profiles of time-averaged gas holdup, axial liquid velocity, and turbulent kinetic energy were measured by using in-house developed conductivity probes and Pavlov tubes. Effects of column diameter, superficial gas velocity, and solid concentration were investigated in a wide range of operating conditions. Experimental results indicated that the average gas holdup remarkably increases with superficial gas velocity, and the radial profiles of investigated flow properties become steeper at high superficial gas velocities. The axial liquid velocities significantly increase with the growth of the column size, whereas the gas holdup was slightly affected. The presence of solid in bubble columns would inhibit the breakage of bubbles, which results in an increase in bubble rise velocity and a decrease in gas holdup, but time-averaged axial liquid velocities remain almost the same as that of the hollow column. Furthermore, a 2-D axisymmetric k–ε model was used to simulate heterogeneous bubbly flow using commercial code FLUENT 6.2. The lateral lift force and the turbulent diffusion force were introduced for the determination of gas holdup profiles and the effects of solid concentration were considered as the variation of average bubble diameter in the model. Results predicted by the CFD simulation showed good agreement with experimental data.     

Effect of bubble deformation on stability and mixing in bubble columns

The paper deals with hydrodynamics in bubble columns. The objective of the paper is to study stability and mixing in a bubble column. The modeling of parameters such as stationary drag and added mass is addressed. In addition, the effect of bubble deformation in terms of eccentricity is highlighted. In a previous paper, the transition between homogeneous and heterogeneous regimes in bubble column without liquid flow has been shown to be driven by the deformation of the bubbles associated to drag and added mass. In the present paper, this work is generalized to bubble column with liquid flow and to the transition from bubble flow to slug flow in a vertical pipe. Numerical simulations of gas-liquid reactors are presented. The numerical simulations are validated in the case of gas plume after the Becker et al. data (Becker, S., Sokolichin, A., & Eigenberg, G. (1994) Gas-liquid flow in bubble columns and loop reactors: Part II. Comparison of detailed experiments and flow simulations. Chemical Engineering Science, 49 (24B), 5747-5762. The numerical simulations are finally applied to a bubble column. The simulations of residence time distribution coupled to transient hydrodynamics are shown to be very sensitive to the modeling of interfacial transfer of momentum from the bubbles to the liquid in terms of drag and added mass, including the effect of bubble deformation.     

Modelling of the coalescence/redispersion processes in bubble columns

In order to determine physically formed relations between bubble characteristics and physicochemical properties of the liquid employed, bubble size distributions and average bubble diameters were calculated using a simplified version of the theoretical model developed by Prince and Blanch (A.I.Ch.E. Journal, 36 (10) (1990) 1485–1499). The results of calculations were compared with experimental data, obtained using two different columns: a laboratory column and a pilot plant column. Good agreement was found between calculated and experimental values.     

Mechanical stress on suspended particles in two- and three-phase airlift loop reactors and bubble columns

The effect of power input, fluid phase viscosity and solids loading on the mechanical stress on suspended particles was examined. Experiments were carried out in an airlift loop reactor and a bubble column operated in two- and three-phase mode. The disintegration of a shear sensitive floc system was observed with an optical in-line particle system analyser and information about the mechanical stress was obtained by means of mathematical analysis of the raw data. The volumetric power input has been derived to be the governing factor and a linear dependence between volumetric power input and resulting mechanical stress was observed. The addition of a solid phase leads to a drastical change of mechanical stress on the particles with a dominating increase of the stress at high solids loadings. Likewise higher fluid phase viscosity leads to higher mechanical stress on the floc system.     

The influence of suspended solids on oxygen transfer to organic liquids in a bubble column

Volumetric mass transfer coefficients (k_La), and gas holdups (ε_G) were studied for the oxygen transfer into organic slurries. Aluminium oxide, polyvinylchloride and polyethylene were used as solid particles in ligroin and tetralin. It was found that the presence of particles can affect the mass transfer remarkably. Solids with high densities can increase k_La in small concentrations for liquids with low density and viscosity. When the density difference is small or the liquid viscosity is high, the slurry behaves as a pseudohomogeneous phase. Successful correlations were proposed for k_La and ε_G as a function of the effective suspension viscosity.     

Effect of gas distributor on hydrodynamics in shallow bubble column reactors

The effects of gas distributor on hydrodynamics in an air–water shallow bubble column reactor are investigated. Three types of distributors, namely, single nozzle, perforated plate and porous plate are being studied. The overall gas holdup, bubble size distributions and bubble rise velocity distributions are studied over a range of superficial gas velocities. The results show that single nozzle is not suitable for shallow bed operation. While perforated plate and porous plate distributors have comparable behaviour in the absence of solids, the addition of solids particles causes the two distributors to behaviour differently. The presence of solids promotes bubble coalescence for perforated plate distributor while the same inhibits bubble coalescence for porous plate distributor.     

Gas holdup and mass transfer in solid suspended bubble columns in presence of structured packings

Electrochemical gas absorption or biotechnical purification processes using structured packing as electrode or as biological support, respectively, may operate in bubble columns in presence of suspended solids. In both systems the knowledge of mass transfer rates from the liquid to the packing is important for the design of equipment. In the present investigation, the fluid dynamic behavior of a simple bubble column and a bubble column containing small size particles, both in presence of structured packing, was studied. Furthermore, mass transfer coefficients between the liquid and the structured packing were obtained by the electrochemical method. The influence of physical properties of the liquid phase, gas flow rate, kind and concentration of the suspended particles on both gas holdup and mass transfer was investigated. Correlations of the experimental data of mass transfer using dimensionless groups were derived and compared to previous correlations. Similarity with a heat transfer expression already used in two-phase systems was found.     

Effect of solid particles on gas hold-up in bubble columns

The gas hold-up in bubble columns containing fluidised plastic particles as solid phase was measured as a function of superficial gas velocity and solids concentration. The effects of particle size, density, wettability and concentration on gas hold-up and bubble coalescence were studied. It was found that the addition of non-wettable solids to the air/water mixture promotes bubble coalescence and, therefore, reduces the gas hold-up, while the addition of wettable solids suppresses bubble coalescence and increases the gas hold-up.     

Effect of antifoam agents on bubble characteristics in bubble columns based on acoustic sound measurements

In this paper, the effect of antifoam agents on bubble characteristics in bubble columns is studied. Specifically, the bubble characteristics of air in tap water are compared to those of air in 5% and 10% antifoam solutions. Bubble characteristics such as gas holdup, bubble diameter, bubble-size distribution, and damping ratio were investigated at various superficial gas velocities. These properties were deduced from the acoustic sound measurement. The study revealed that the addition of antifoam chemicals reduces the overall gas holdup and increases the average bubble diameter. The bubble-size distribution in tap water is found to be homogeneous while in antifoam solutions to be heterogeneous. It is also found that at low gas velocities the damping ratio for antifoam solutions is higher than that for tap water, while at high gas velocities the damping ratio is not affected. The results affirm that acoustic probes are excellent measuring tools over classical tools at moderate gas velocities.     

静电效应对有无埋管气固鼓泡床内气泡特性的影响分析

采用基于双流体模型(TFM)耦合静电模型的方法,研究颗粒的静电对有无埋管气固鼓泡床内气固流动特性和气泡特性的影响。首先在无静电场存在的条件下,利用双流体模型对自由鼓泡床和埋管鼓泡床内的流动情况进行模拟并与实验结果进行对比;进一步耦合静电模型,考察静电对自由鼓泡床和埋管鼓泡床内床层的整体性质和气泡特性的影响。研究结果表明,在无静电场条件下采用双流体模型能较好地预测自由鼓泡床和埋管鼓泡床内的气固流动状况以及气泡的平均直径和气泡的上升速度。埋管的存在使鼓泡床内气固流动发生强烈扰动,并使气泡的平均直径和气泡的上升速度均呈振荡分布。静电的存在对自由鼓泡床和埋管鼓泡床内床层的平均固含率影响不大,但对气泡分布规律影响较大,使得自由鼓泡床内气泡数目减少,而埋管鼓泡床下部区域的气泡分布比较集中,上部有大气泡出现。     

Modeling of the evolution with length of bubble size distributions in bubble columns

Many of the existing methods, for the determination of the specific interfacial area in bubble columns, consider the column in a dynamic equilibrium between bubble coalescence and breaking-up. The aim of this work is to study if this consideration can be considered true for low superficial gas velocities. Two existing models have been chosen from literature in order to predict the break-up [Wang, T., Wang, J., Jin, Y., 2003. A novel theoretical breakup kernel function for bubbles/droplets in a turbulent flow. Chemical Engineering Science 58, 4629-4637] and the coalescence [Lehr, F., Millies, M., Mewes, D., 2002. Bubble size distributions and flow fields in bubble columns. A.I.Ch.E. Journal 48, 2426] rates. In order to confirm the validity of the models, predictions were compared with experimental results obtained by image analysis. Several simulations were performed for different superficial gas velocities and initial bubble size distributions. The column length needed to reach dynamic equilibrium was calculated for each simulation. The results show that the necessary length to reach the dynamic equilibrium does not depend on the shape of the initial distribution, but essentially on its Sauter mean diameter. The necessary length to reach the dynamic equilibrium is very important for low superficial gas velocities. The assumption that the entire column is in dynamic equilibrium is in general not valid. Therefore, the initial Sauter mean diameter and the total column length are important parameters for the determination of the specific interfacial area.     

Influence of elevated pressure and particle lyophobicity on hydrodynamics and gas–liquid mass transfer in slurry bubble columns

This article reports on the influence of elevated pressure and catalyst particle lyophobicity at particle concentrations up to 3 vol % on the hydrodynamics and the gas‐to‐liquid mass transfer in a slurry bubble column. The study was done with demineralized water (aqueous phase) and Isopar‐M oil (organic phase) slurries in a 0.15 m internal diameter bubble column operated at pressures ranging from 0.1 to 1.3 MPa. The overall gas hold‐up, the flow regime transition point, the average large bubble diameter, and the centerline liquid velocity were measured along with the gas–liquid mass transfer coefficient. The gas hold‐up and the flow regime transition point are not influenced by the presence of lyophilic particles. Lyophobic particles shift the regime transition to a higher gas velocity and cause foam formation. Increasing operating pressure significantly increases the gas hold‐up and the regime transition velocity, irrespective of the particle lyophobicity. The gas–liquid mass transfer coefficient is proportional to the gas hold‐up for all investigated slurries and is not affected by the particle lyophobicity, the particle concentration, and the operating pressure. A correlation is presented to estimate the gas–liquid mass transfer coefficient as a function of the measured gas hold‐up: $k_{\rm l}a_{\rm l}/\varepsilon_{\rm g} = 3.0 \sqrt{Du_{\rm b}/d_{\rm b}^3}\;{\rm s}^{-1}$ . © 2009 American Institute of Chemical Engineers AIChE J, 2010