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。     

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.     

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.     

Effect of polymer additives on hydrodynamics and oxygen transfer in a bubble column bioreactor

The influence of polymer additives (polyethylene oxide and polyacrylamide) on the hydrodynamics and oxygen transfer in a bubble column bioreactor was examined. The addition of small amounts of these polymers has been known to cause significant drag reduction in turbulent flow circumstances. The gas hold-up was slightly decreased and the liquid-phase mixing was somewhat enhanced due to the addition of the polymers. The addition of polymer additives brought about a reduction of the volumetric oxygen transfer coefficient by about 40%. In dilute polymer solutions, large bubbles formed by bubble coalescence moved with high rise velocities in the presence of many small bubbles and the bubble size distributions were less uniform compared with those in water. The complicated changes in bubble hydrodynamic characteristics were examined to give possible explanations for oxygen transfer reduction.     

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     

Effect of ultrasonic pretreatment on the composition of lignocellulosic material in biotechnological processes

The ultrasonic impact and possibilities of using its damaging effect on lignocellulosic material in order to increase the reactivity of plant biomass are studied. The main aim of pretreatment is the disruption of the crystalline, highly ordered structures of cellulose and lignin, or the removal of the latter. At the selected optimum parameters of ultrasonic pretreatment (frequency, 30 kHz) of the raw material, degradation of cellulose reaches 16% of a.d.s. (absolutely dry substance) while that of lignin is 11.4 % of a.d.s. Pretreatment of a substrate with ultrasound changes the rate and depth of the destruction of the initial material in the course of cultivation and can be used to accelerate the bioconversion of lignocellulosic substrates in biodiesel fuel production.     

Effect of surfactants on the rate of solid-liquid mass transfer in packed bubble columns

The effect of sodium lauryl sulphate (anionic) and Triton X-100 (nonionic) on the solid-liquid mass transfer at a gas-sparged fixed bed of copper Raschig rings was studied by measuring the diffusion-controlled dissolution of copper rings in acidified chromate solution. The variables studied were the nitrogen flow rate, the type of surfactant, and the surfactant concentration. It was found that an increase occurs in the solid-liquid mass transfer coefficient with increasing the nitrogen flow rate. Increasing the surfactant concentration was found to decrease the mass transfer coefficient. For a given surfactant concentration, it was found that Triton X-100 reduces the mass transfer coefficient more than sodium lauryl sulphate.     

Multiple effects of operating variables on the bubble properties in three-phase slurry bubble columns

Flow properties of gas phase reactants such as size, rising velocity and frequency were investigated in simulated three-phase slurry bubble column reactors. Effects of gas velocity, reactor pressure, liquid viscosity, solid content in the slurry phase and column diameter on the flow properties of a gas reactant were determined. The multiple effects of operating variables on the bubble properties were well visualized by means of contour maps. The effects of operating variables on the flow properties of bubbles changed with changing column diameter of the reactor. The size, rising velocity and frequency of reactant gas bubbles were well correlated in terms of operating variables including column diameter of the reactor. This work was presented at the 7 ^th China-Korea Workshop on Clean Energy Technology held at Taiyuan, Shanxi, China, June 26–28, 2008.     

Effect of a dispersed immiscible liquid phase on the hydrodynamics of a bubble column and ebullated bed

Secondary undesired reactions in ebullated bed resid hydroprocessors can generate an additional dispersed liquid phase, referred as mesophase, which is denser and more viscous than the continuous liquid phase and affects the operation and transport phenomena of the fluidized bed. This study investigates the effect of a dispersed immiscible liquid phase on the overall phase holdups, bubble properties, and fluidization behavior in a bubble column and ebullated bed. The experimental system consisted of biodiesel as the continuous liquid phase, glycerol as the dispersed liquid phase, 1.3 mm diameter glass beads, and nitrogen. The addition of dispersed glycerol reduced the gas holdups in the bubble column for the studied gas and liquid superficial velocities. Dynamic gas disengagement profiles reveal a rise in the large bubble population and reductions to the small and micro bubble holdups when increasing the glycerol concentration. Liquid–liquid–solid bed expansions at various liquid flowrates confirm particle agglomeration in the presence of a more viscous dispersed liquid phase. Overall phase holdups in a gas–liquid–liquid–solid ebullated bed were obtained while varying the gas and liquid flowrates as well as the glycerol concentration. A coalesced bubble flow regime was observed in the bed region without glycerol whereas the addition of glycerol resulted in the dispersed bubble flow regime due to particle clustering and a greater apparent particle size. The resulting bubble flow regime increased the bed and freeboard region gas holdups due to enhanced bubble break-up. Observations of the fluidized bed behavior following the addition of the dispersed glycerol are also discussed.     

Simulating the effects of liquid circulation in bubble columns with internals

Liquid circulation and mixing patterns in bubble columns may be significantly altered by the presence of bundles of vertical heat-exchange tubes. Unlike hollow vessels, the lack of unified geometrical standards for bubble columns equipped with internals complicates the deciphering efforts of fundamental studies. To unveil some of the refined features associated with internals-containing bubble columns, use was made in this work of two-fluid Euler continuum transient 3-D simulations to simulate five pilot-scale configurations: vessels of uniform filling (dense and sparse), vessels of non-uniform filling with large core and wall clearances, and equal cross-sectional hollow vessels. The study revealed that liquid circulation pattern was affected in various complex manners depending on the arrangement of implanted tubes and prevailing bubble sizes. The inter-tube gap represented a length-scale bottleneck which was shown to impede the growth of the larger structures in the flow at the expense of promoting smaller ones. Such forcing mesoscale length gave rise to 3-D effects on the time-averaged flow variables even in the fully developed region. Furthermore, a sharp decrease of the liquid kinetic turbulent energy was obtained upon insertion of the heat-exchange tubes even for low enclosure occlusions. The liquid gross flow structure was more or less of a core-annulus type for uniformly implanted heat-exchange tubes, whereas non-uniform, although geometrically regular arrangements yielded complex flow patterns with sometimes core liquid downward circulations.     

Verification of the mathematical modeling of bio-physico chemical processes in activated carbon columns

Plug-flow stationary solid phase upflow and completely mixed recycle fluidized adsorber experiments were conducted to test the predictive models developed earlier. Model parameters were determined independently from adsorption equilibrium, kinetic investigations, biokinetic experiments and correlation techniques. Two ideally adsorbable and biodegradable compounds, glucose and sucrose, as well as two actual wastewaters, a dairy waste and a landfill leachate, were used to compare the model profiles and experimental data of non-bioactive and bioactive adsorber systems. The performance prediction by the models satisfactorily described the experimental data.     

Explorations on the multi-scale flow structure and stability condition in bubble columns

Physical understanding of heterogeneous flow structure is of crucial importance for modelling and simulation of gas-liquid systems. This article presents a review and report of recent progress in our group on exploratory application of the variational (analytical) multi-scale approach to gas-liquid systems. The work features the closure of a hydrodynamic model with the incorporation of a stability condition reflecting the compromise between the dominant mechanisms in the system. A dual-bubble-size (DBS) model is proposed to approximate the heterogeneous structure of gas-liquid systems based on a single-bubble-size (SBS) model previously established. Reasonable variation of the gas holdup and the composition of the two bubble species with operating conditions have been calculated and the regime transition can therefore be reasonably predicted for air-water system, suggesting that stability condition may provide an insightful concept to explain the general tendencies in gas-liquid systems out of their hydrodynamic complexity, and to give simple models of their overall behaviors. Of course, the diversity of the correlations for drag force and minimum bubble size and the sensitivity of the model predictions to these correlations may suggest the necessity to clarify further the essential and robust results in the current model and to reduce the uncertainties involved.     

Effect of organic surfactant additives on gas holdup in the pseudo-homogeneous regime in bubble columns equipped with fine pore sparger

New experimental data concerning the gas holdup in bubble columns equipped with porous sparger were acquired. The effect of surfactant additives on gas holdup in the pseudo-homogeneous regime has been studied. Three different commercial surfactants (Triton X-100^®, SDS^®, CTAB^®) were used and four aqueous solutions of each one were employed, in order to study the effect of the surfactant concentration and type (i.e., non-ionic, anionic, cationic). A general correlation, which includes dimensionless numbers (i.e., Froude, Archimedes and Bond) as well as the geometric characteristics of the column and the sparger, can predict the gas holdup in various systems (i.e., pure substances, ionic surfactants, non-ionic surfactants) with reasonable accuracy.