Absorption and reaction of isobutene in sulfuric acid—III: Considerations on the scale up of bubble columns

In spite of their simple construction the scale up of bubble columns of industrial size demands application of models which account for dispersion effects and variations of pressure and gas flow rate. However, using such models and parameter values obtained from other studies it was not possible to describe successfully measured conversions of the absorption of isobutene in a 7 m bubble column though the interfacial area was determined separately. The measurements were carried out under such conditions at which the absorption takes place in the slow reaction regime of mass transfer. A sufficient agreement between experimental and predicted conversions could be obtained merely if a lower value of k_L was used. A more detailed analysis of bubble size distributions indicated that the decrease of k_L may be apparently only since the interfacial areas determined photographically must not necessarily be the area which is effective to mass transfer. k_La-values in larger bubble columns with gas spargers which are common in industry are considerably lower than k_La-data found in smaller columns with porous gas distributors.     

Improving Gas‐Liquid Mass Transfer in Bubble Columns by Applying Low‐Frequency Vibrations

We show that application of low‐frequency vibrations, in the 50–200 Hz range, to the liquid phase of an air‐water bubble column causes significantly smaller bubbles to be generated at the distributor plate. For bubble column operation in the homogeneous flow regime, measurements of the volumetric mass transfer coefficient using the oxygen absorption technique show that the increase in the k_La values ranges from 50–100 % depending on the flow rate. It is concluded that application of low‐frequency vibration has the potential of improving the performance of bubble columns.     

Comparison of Hydrodynamics and Mass Transfer in Airlift and Bubble Column Reactors Using CFD

Computational Fluid Dynamics (CFD) is used to compare the hydrodynamics and mass transfer of an internal airlift reactor with that of a bubble column reactor, operating with an air/water system in the homogeneous bubble flow regime. The liquid circulation velocities are significantly higher in the airlift configuration than in bubble columns, leading to significantly lower gas holdups. Within the riser of the airlift, the gas and liquid phases are virtually in plug flow, whereas in bubble columns the gas and liquid phases follow parabolic velocity distributions. When compared at the same superficial gas velocity, the volumetric mass transfer coefficient, k_La, for an airlift is significantly lower than that for a bubble column. However, when the results are compared at the same values of gas holdup, the values of k_La are practically identical.     

Mass transfer in bubble columns packed with motionless mixers

The cocurrent upward mode was employed to absorb pure oxygen into water in bubble columns packed with Koch (Sulzer) motionless mixers. The liquid-side volumetric mass transfer coefficient, K_La, in the packed bubble column was found to be always larger than that in the unpacked bubble column. In the range of liquid velocities from 6.7 cm/sec to 39.9 cm/sec, the value of K_La in the packed bubble column increased with the increasing liquid velocity while that in the unpacked bubble column was almost independent of the liquid velocity. The equation of the formK_La= mν_l^β? was successfully adopted to correlate the K_La data.     

Using image analysis in the study of multiphase gas absorption

For the air-water-calcium alginate beads system, the effect of the presence of solids on the mass transfer characteristics in a bubble column was experimentally studied.Volumetric liquid side mass transfer coefficient, k_La, specific interfacial area, a, and hence liquid side mass transfer coefficient, k_L, were determined under different solid concentrations (0, 5, and 10 vol%), superficial gas velocities (up to 0.27 cm/s) and solid sizes (1.2 and 2.1 mm diameter). The bubble characteristics, namely the interfacial area, were obtained using an image analysis technique.This technique proved to be a suitable and practical method to characterize mass transfer phenomena in bubble columns for the range of operating conditions used. The solids affect negatively k_La, decreasing both a and k_L, the effect being more pronounced for the smaller particles. For these particles the variation of k_La is due to the variation of its two components, while for larger particles k_La variation is due, essentially, to changes in k_L as no significant differences in a were observed.     

Effect of contaminants on mass transfer coefficients in bubble column and airlift contactors

In this work, the effects of surface-active contaminants on mass transfer coefficients k_La and k_L were studied in two different bubble contactors. The oxygen transfer coefficient, k_L, was obtained from the volumetric oxygen transfer coefficient, k_La, since the specific interfacial area, a, could be determined from the fractional gas holdup, ε, and the average bubble diameter, d₃₂. Water at different heights and antifoam solutions of 0.5- were used as working media, under varying gas sparging conditions, in small-scale bubble column and rectangular airlift contactors of 6.7 and capacity, respectively. Both the antifoam concentration and the bubble residence time were shown to control k_La and k_L values over a span of almost 400%. A theoretical interpretation is proposed based on modelling the kinetics of single bubble contamination, followed by sudden surface transition from mobile to rigid condition, in accordance with the stagnant cap model. Model results match experimental k_L data within ±30%.     

Mixing and mass transfer in tall bubble columns

In cocurrent bubble columns (15 and 20 cm diameter, 440 and 723 cm high) with different gas distributors measurements were carried out with tap water and solutions of salts and molasses. A stationary and a transient method were applied to determine the dispersion coefficients. Absorption and desorption of oxygen was studied by measuring the concentration profiles of oxygen in the liquid phase. Liquid phase mass transfer rates k_La were obtained adjusting the experimental profiles with the predictions of the axial dispersed plug flow model. Owing to the different gas spargers the k_La values of both columns differ by a factor of about two. Correlations are proposed for the k_La data of the various liquid phases which only depend on the gas velocity.     

Bubble column research in Japan

Many experimental studies on the bubble column have been reported by Japanese researchers since around 1960. They include studies of bubble behaviour, bubble size distribution, transition from the homogeneous bubbly flow regime to the heterogeneous liquid circulation regime, liquid velocity distribution, longitudinal liquid mixing, hydrodynamic modelling, the gas holdup, and the volumetric coefficient of gas-liquid mass tranfer k_La. Studies covered various modified bubble columns, such as the airlift reactor with an external or internal loop, the packed bubble column, and others. Performance of three-phase bubble columns, which deal with suspensions or emulsions, and their use as bioreactors or chemical reactors were also studied.     

A unified approach to the hydraulics and mass transfer in randomly packed towers

New robust correlations and mechanistic model of macroscopic fluid dynamic and gas–liquid mass transfer characteristics for randomly packed towers were developed based on first principles, neural network computing and dimensional analysis (artificial neural network and dimensional analysis, ANN–DA). These tools concerned the loading and flooding capacities, the total liquid hold-up, the irrigated pressure drop, the local volumetric liquid-side, k_La, and gas-side, k_Ga, mass transfer coefficients, the overall volumetric, K_La and K_Ga, mass transfer coefficients, and the packing fractional wetted area. Validation of these tools was performed by interrogating a broad experimental database including over 10,750 measurements published in the literature over the past seven decades. The fully-predictive mechanistic model proved powerful in forecasting the tower hydraulics below the loading point without requiring any adjustable parameter. On the other hand, the ANN–DA correlations proved highly powerful in correlating the tower fluid dynamics and gas–liquid mass transfer regardless of the operating flow regime. These approaches were also benchmarked with respect to the comprehensive Billet and Schultes (Trans. Industr. Chem. Eng. 77 (1999) 498) phenomenological approach and the classical Onda et al. (J. Chem. Eng. Japan 1 (1968) 56) mass transfer correlations.     

Absorption of oxygen in countercurrent multistage bubble columns—III Viscoelastic liquids. Comparison of systems with high viscosity

The volumetric mass transfer coefficients, k_La, were measured by a steady state method in the first stage of a multistage countercurrent column 20 cm in diameter. Perforated plate trays with different bore hole diameters and free cross sectional area were employed. The height of the stage was varied.Viscoelastic fluids (PAA) solutions) were used as media. In general k_La increases with increasing superficial gas velocity, with decreasing bore hole diameter and free cross sectional area of the perforated plate trays as well as with decreasing concentration and viscosity.With increasing height of the bubbling layer k_La diminishes. This effect is lower with an increase of PAA concentration. A qualitative comparison of the rheologically corresponding media indicates that under similar conditions PAA solutions yield the highest, and the glycerol solution the lowest k_La values. With increasing specific power input k_La becomes larger. At constant power input the highest k_La is attained with the smallest bore hole diameter of the trays for CMC and PAA solutions. For glycerol no such effect was found.     

Mass transfer in bubble column for industrial conditions—effects of organic medium, gas and liquid flow rates and column design

Most of available gas-liquid mass transfer data in bubble column have been obtained in aqueous media and in liquid batch conditions, contrary to industrial chemical reactor conditions. This work provides new data more relevant for industrial conditions, including comparison of water and organic media, effects of large liquid and gas velocities, perforated plates and sparger hole diameter.The usual dynamic O₂ methods for mass transfer investigation were not convenient in this work (cyclohexane, liquid circulation). Steady-state mass transfer of CO₂ in an absorption-desorption loop has been quantified by IR spectrometry. Using a simple RTD characterization, mass transfer efficiency and k_La have been calculated in a wide range of experimental conditions.Due to large column height and gas velocity, mass transfer efficiency is high, ranging between 40% and 90%. k_La values stand between 0.015 and and depend mainly on superficial gas velocity. No significant effects of column design and media have been shown. At last, using both global and local hydrodynamics data, mass transfer connection with hydrodynamics has been investigated through k_La/ε_G and k_La/a.     

HYDRODYNAMICS AND MASS TRANSFER IN DOWNFLOW BUBBLE COLUMN

Gas holdup, effective interfacial area and volumetric mass transfer coefficient were measured in two and three phase downflow bubble columns. The mass transfer data were obtained using the chemical method of sulfite oxidation, and the gas holdup was measured using the hydrostatic technique. Glass beads and Triton 114 were used to study the effects of solids and liquid surface tension on the gas holdup and the mass transfer parameters a and k_L a . The gas holdup in three phase systems was measured for non-wettable (glass bead) and wettable (coal and shale particles) solids.

The mass transfer data obtained in the downflow bubble column were compared with the values published for upflow bubble columns. The results indicate that in the range of superficial gas velocities (0.002-0.025) m/s investigated, the values of the mass transfer coefficient were of the same order of magnitude as those observed in upflow systems, but the values of interfacial area were at least two fold greater. Also, the results showed that the operating variables and the physical properties had different influences on a and k_L a in the downflow bubble column. Correlations for a and k_L a for the downflow bubble column are proposed which predict the data with adequate accuracy in the range of operating conditions investigated.     

Bubble breakage phenomena,phase holdups and mass transfer in three-phase fluidized beds with floating bubble breakers

The individual phase holdups and mass transfer characteristics in three-phase fluidized beds with different floating bubble breakers have been determined in a 2.0 m high Plexiglas column of inner diameter 0.142 m. The bubble breaking phenomena by the breakers have been studied via a photographic method in a two-dimensional Plexiglas column. The volumetric mass transfer coefficient k_La in three-phase fluidized beds with hexagonal-shaped breakers is up to 40% greater than that in beds without floating bubble breakers. The bed porosity ε_L + ε_g, gas-phase holdup ε_g, and volumetric mass transfer coefficient k_La increase with an increase in the volume ratio of floating bubble breakers to solid particles, V_f/V_s, up to around 0.15, and thereafter decrease with V_f/V_s in three-phase fluidized beds with floating bubble breakers. Also, k_La increases with increasing breaker density, projected area and contact angle between the floating bubble breakers and the water. The volumetric mass transfer coefficients in terms of the Sherwood number in three-phase fluidized beds with the various floating bubble breakers have been correlated with the volume ratio of floating bubble breakers to solid particles, the particle Reynolds number based on the local isotropic turbulence theory and the modified Weber number.     

Predicting inter-phase mass transfer for idealized Taylor flow: A comparison of numerical frameworks

Four numerical frameworks were derived to investigate the impact of underlying assumptions and numerical complexity on the predicted mass transfer between a Taylor bubble and liquid slug in circular capillaries. The separate influences of bubble velocity and film length, slug length, and bubble film thickness on k_La were compared to empirical and CFD-based predictions from existing literature. Reasonable agreement was obtained using a Slug Film model, which accounted for diffusion-limited mass transfer between the slug film and circulating bulk without the need for an iterative numerical solution. Subsequent investigation of the relative contributions of film and cap mass transport for industrially relevant conditions suggests that both mechanisms need to be accounted for during the prediction of k_La.     

Effect of surfactants on liquid-side mass transfer coefficients

In the present paper, the effect of liquid properties (surfactants) on bubble generation phenomenon, interfacial area and liquid-side mass transfer coefficient was investigated. The measurements of surface tension (static and dynamic methods), of critical micelle concentration (CMC) and of characteristic adsorption parameters such as the surface coverage ratio at equilibrium (s_e) were performed to understand the effects of surfactants on the mass transfer efficiency. Tap water and aqueous solutions with surfactants (cationic and anionic) were used as liquid phases and an elastic membrane with a single orifice as gas sparger. The bubbles were generated into a small-scale bubble column. The local liquid-side mass transfer coefficient (k_L) was obtained from the volumetric mass transfer coefficient (k_La) and the interfacial area (a) was deduced from the bubble diameter (D_B), the bubble frequency (f_B) and the terminal bubble rising velocity (U_B). Only the dynamic bubble regime was considered in this work (Re_OR=150-1000 and We=0.002-4).This study has clearly shown that the presence of surfactants affects the bubble generation phenomenon and thus the interfacial area (a) and the different mass transfer parameters, such as the volumetric mass transfer coefficient (k_La) and the liquid-side mass transfer coefficient (k_L). Whatever the operating conditions, the new k_La determination method has provided good accuracy without assuming that the liquid phase is perfectly mixed as in the classical method. The surface coverage ratio (s_e) proves to be crucial for predicting the changes of k_L in aqueous solutions with surfactants.     

Evolution axiale du coefficient de transfert de matière en colonne à bulles et en lit fluidisé triphasique

Liquid phase volumetric mass transfer coefficients for oxygen are determined in a three-phase fluidized bed and in a bubble column. The concept of exponential decreasing axial variation of volumetric mass transfer coefficient leads to a better representation of oxygen concentration profiles inside the column. Compared to the bubble column, k_la axial variations are more important in the lower part of the fluidized bed column, where solid particles increase the coalescence phenomenum, particularly with viscous liquids.     

Mass transfer properties of bubble columns suspending immobilized glucose oxidase gel beads for gluconic acid production

The volumetric gas-liquid oxygen transfer coefficient, k_L a, and the liquid–solid coefficient, k_S, were measured in a 6.7 L external loop airlift bubble column (ELBC), a 2.5 L internal loop airlift (ILBC) and a 2.5 L normal bubble column (NBC) by the steady state method proposed previously using the oxidation of glucose with air catalyzed by glucose oxidase, GO. For an improved and simultaneous determination of k_L a and k_S, GO was entrapped in calcium alginate gel beads together with fine palladium particles instead of catalase to decompose the hydrogen peroxide produced. The gas holdup, ?_G, in each type of bubble column and the liquid circulation velocity, u_L, governing ?_G in the ELBC were also measured to correlate the data on k_La according to the previous correlations proposed for a larger scale of the ELBC, ILBC and NBC. The data on k_L a, k_S, ?_G and u_L (only for the ELBC) in the reaction system were compared to each other for the three types of bubble columns. The results are well predicted by the previous correlations.     

Absorption of oxygen in countercurrent multistage bubble columns—II: Aqueous solutions with high viscosity

The mean relative gas hold up, ?_g, and the volumetric mass transfer coefficients, k_La, were measured by steady state method in the first stage of a multistage countercurrent column 20 cm in diameter. Perforated plate trays with different bore hole diameters and free cross sectional area were applied. The height of the stage was varied. Newtonian (glycerol) and pseudo plastic (CMC) solutions were used as media.In general with increasing superficial gas velocity, with decreasing bore hole diameter and free cross sectional area of the perforated platé trays as well as with decreasing concentration and viscosity k_La increases.With increasing height of the bubbling layer k_La diminishes except for glycerol, for which no or only slight influence of the bubble layer height prevails.This effect for CMC diminishes with the increase of their concentration. With increasing specific power input k_La becomes larger. At constant power input the highest k_La is attained with the smallest bore hole diameter of the trays for CMC solutions. For glycerol no such an effect was found.     

Bubble size and mass transfer characteristics of sparged downwards two-phase flow

When gas is continuously fed through a sparger into a downflowing liquid in a pipe a ventilated cavity is often formed. The cavity remains attached to the sparger even in the presence of high liquid flow rates that would wash away a free slug bubble. Small bubbles are shed from the base of this cavity by the falling liquid film at the wall of the pipe and these bubbles are swept downwards forming a bubbly flow that is highly effective for mass transfer. The ventilated cavity is undesirable since it reduces the driving force for liquid circulation when the pipe is the downcomer of an external air loop fermenter or analogous gas/liquid reactors. The cavity also reduces the available interfacial area for mass transfer. It has been shown [Thorpe et al., 1997. Proceedings of the Fourth International Conference on Bioreactor and Bioprocess Fluid Dynamics; Lee, 1998. Ph.D Thesis, University of Cambridge, UK], that the length of the cavity can be reduced by replacing the common industrial design of a horizontal sparger (HS) with two novel spargers; a peripheral sparger (PS) and a plunging jet sparger (PJS) (Fig. 3). In this paper we investigate the effect of PS and PJS on mass transfer and the resulting bubble size.Experiments were carried out with air and water in a large circulating rig with a 0.105 m diameter test section. The local average bubble size in the bulk two-phase flow region below the ventilated cavity was determined using photography for three combinations of liquid and gas volumetric flow rates. The average bubble size was essentially the same (differences within 10%) for the PS, central spranger (CS) and HS. The PS created the largest bubble in all cases examined. The PJS created smaller bubbles than all the other spargers and did not allow the formation of cavities, which suggests that it has the superior performance. The estimated increase in k_La due to the smaller bubble size for the PJS was by a factor of 1.3.In order to check this result, the effects of sparger type on the volumetric mass transfer coefficient (k_La) were also measured. The k_La was determined with a dynamic method, by using unsteady state absorption of oxygen. The results confirmed the apparent superiority of PJS over the other spargers. An average increase of 19% in the k_La was observed when the PJS was used instead of the industrial design (HS). The CS and PS showed similar k_La values again within 10% of the HS.However the power consumption is larger when the PJS is used instead of the industrial design HS. Hence an attempt was made to adjust the bubble size and mass transfer coefficients of the PJS to account for the differences in energy consumption. When this is done the PJS and HS produce roughly the same bubble size and have the same mass transfer performance. Still the PJS had the important operational advantages of producing shorter cavities and having the greater resistance to stall at low liquid flow rates.     

Experimental Study of Oxygen Mass Transfer Coefficient in Bubble Column with High Temperature and High Pressure

The volumetric gas‐liquid mass transfer coefficient, k_Lα, for oxygen was studied by using the dynamic method in slurry bubble column reactors with high temperature and high pressure. The effects of temperature, pressure, superficial gas velocity and solids concentration on the mass transfer coefficient are systemically discussed. Experimental results show that the gas‐liquid mass transfer coefficient increases with the increase in pressure, temperature, and superficial gas velocity, and decreases with the increase in solids concentration. Moreover, k_Lα values in a large bubble column are slightly higher than those in a small one at certain operating conditions. According to the analysis of experimental data, an empirical correlation is obtained to calculate the values of the oxygen volumetric mass transfer coefficient for a water‐quartz sand system in two bubble columns with different diameter at high temperature and high pressure.