Holdup and backmixing investigations in cocurrent and countercurrent bubble columns

The effect of some liquid physical properties on gas-holdup was investigated in a glass column of 15·9 cm dia. and 390 cm high. Oil emulsions were also employed. Data were obtained for the variation of the effective axial dispersion coefficient with gas and liquid velocities. The steady-state tracer injection technique and the axial-dispersed-plug-flow model were used. Results were compared with literature data for both smaller and larger columns.     

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.     

Mass transfer to planar and expanded metal electrodes in bubble columns

Mass transfer rates at planar electrodes and electrodes of expanded metal placed in the centre of a bubble column were measured. The gas velocity and the physical properties of the electrolytic solutions were varied and different types of expanded metal were investigated. In some cases increases in the mass transfer coefficient over the planar electrode value of more than 100% were obtained. Dimensionless correlations are presented for the different systems.Nomenclature A mean mesh aperture - D diffusivity - D _c column diameter - g acceleration due to gravity - Ga Galileo number =gL ³/v ² - Gr Grashof number =gL ³/v ² - k mass transfer coefficient - L electrode height - r radial position - R column radius - Re Reynolds number =R _h V _s/ - R _h hydraulic radius = / - Sc Schmidt number = /D - Sh Sherwood number =kL/D - V_s superficial velocity - gas void fraction - _M porosity of expanded metal - kinematic viscosity - density - electrode area per unit volume - electrode area per unit net area     

A gas holdup model for cocurrent air-water-fiber bubble columns

A gas holdup model is developed for cocurrent air-water-fiber bubble column flows using the drift-flux model. The model coefficients are estimated using a nonlinear least square method and systematically acquired experimental data. The model correlates gas holdup with superficial gas and liquid velocity, and fiber type and mass fraction. The model reproduces most experimental data within ±10% error and all but 3 of the 3839 experimental data points within ±15% error. It also accurately predicts air-water bubble column gas holdup data; these data were not used in estimating the model coefficients. The physical implications of the model coefficients are also discussed.     

Mass transfer characteristics of low H/D bubble columns

The mass transfer characteristics of 0.2, 0.6 and 1.0 m diameter bubble columns having a low height to diameter ratio (0.6 < H/D < 4) and operated at low superficial gas velocities (0.01 < V_G < 0.08 m/s) were investigated. Different types of spargers were used to study their effect on the column performance. The values of effective interfacial area, a , and volumetric mass transfer coefficient, k_L a , were measured by using chemical methods. The values of a and k_L a were found to vary from 40 to 420 m²/m³ of clear liquid volume and from 0.01 to 0.16 s^?1, respectively, in the range of V_G, and V_L covered in this investigation. The value of the liquid-side mass transfer coefficient, k_L, was found to vary from 3 × 10^?4 to 7 × 10⁴ m/s. The effect of the physical properties of the system on the values of a was also investigated. The height to diameter ratio and the column diameter did not have significant effect on the values of gas holdup, a and k_L a . It was found that the sparger design is not of critical importance, provided multipoint/multiorifice gas spargers are used. The comparative performance of bubble columns having low H/D with horizontal sparged contactors and tall bubble columns has been considered.     

Mass transfer from bubbles in cocurrent flow

An experimental study has been made of liquid phase mass transfer from bubbles travelling cocurrently with liquid in a horizontal pipe. A simple model of the process based on streams of spherical bubbles has been developed and solved in the form of fractional absorption as a function of dimensionless groups. The derivation of and results obtained from this model are presented and explained. Experimental absorption rates measured in turbulent liquid flow over very wide ranges of bubble frequencies support the predictions of this model. A very effective bubble-frequency counter has also been developed capable of counts as high as 1600/min.     

Oxygen mass transfer in bubble columns

The mass transfer of oxygen between air and water has been studied in a bubble column over wide ranges of liquid and gas velocity. An oxygen probe was used to map the steady-state liquid phase concentration of oxygen throughout the column.At any given point in the column, the oxygen concentration increased with gas velocity. Minima were observed in plots of concentration against liquid velocity.Two distinct absorption regions were observed. Close to the distributor the concentration decreased rapidly with height and volumetric mass transfer coefficients ranged from about 0.2 to 2.1 s^?1. These high values were attributed to enhanced mass transfer due to turbulence induced by the liquid and gas jets in the grid region. In the bulk of the column, axial concentration gradients were much smaller and the mass transfer coefficients were up to two orders of magnitude lower than in the grid region.     

Mass transfer between liquids in cocurrent pipeline flow

Mass transfer between n-butanol and water has been studied in cocurrent pipeline flow. Phase NTU's correlated with individual phase velocities, and also depended on the input ratio of the two phases and the contactor length. Mass transfer stage efficiencies varied from near zero to essentially 100%. End effects were large. Pressure drop data agreed with the correlation of Charles and Lilleleht. The pipeline contactor showed contactor effectiveness and energy requirements similar to those of other experimental liquid-liquid extraction devices.     

Penetration theory solution for gas absorption and chemical reaction in cocurrent and countercurrent flow wetted-wall columns

Penetration theory solutions are provided for gas absorption with or without a (pseudo) first-order irreversible chemical reaction in cocurrent and countercurrent flow wetted-wall columns, taking into account a constant gas-film resistance as well as the axial decrease in gas composition due to absorption, while assuming non-rippling laminar flow for the liquid phase and plug flow for the gas phase. Limiting solutions are also obtained for situations when either the gas phase resistance or the axial variation of gas composition is negligible. The results are suitable for ϕ ≥ 3 and can be used for both Newtonian and non-Newtonian liquids.     

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.     

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.     

Interfacial area and mass transfer in gas-liquid cocurrent upflow and countercurrent flow in reciprocating plate column

The volumetric mass transfer coefficient and the interfacial area were measured for carbon dioxide absorption into water using a reciprocating plate column of plate geometry different from a Karr column. The specific interfacial area was governed by a change in bubble size at low agitation rates and by a variation in gas holdup at high agitation rates. The liquid phase mass transfer coefficient was strongly influenced by the agitation rate, the phase velocities and the plate geometry.     

A comprehensive study on co2-interphase mass transfer in vertical cocurrent and countercurrent gas-liquid flow

Cocurrent and countercurrent absorption and desorption of CO₂ in water was investigated in tall bubble columns (length 440 and 720 cm, diameter 15 and 20 cm, respectively). Operating conditions were applied which provided for high interphase mass transfer rates. Under these circumstances the relative gas holdup varies considerably with axial position whereas the mean bubble diameter measured at two points was found to be approximately constant. The measured data permit the calculation of local values of interfacial areas, superficial gas velocities, and frequency factors for bubble coalescence and break up. A dispersion model which takes into account the hydrostatic head and a variable gas velocity was applied to describe the measured concentration profiles in both phases. If increased mass transfer coefficients at the column bottom and measured local values of the hold up were used a striking agreement between experimental and predicted profiles could be obtained. The findings lead to a more sophisticated picture of the complex behaviour of gas-liquid dispersions at high interphase mass transfer rates.     

Hydrodynamic and mass transfer characteristics of bubble and packed bubble columns with downcomer

The hydrodynamic and mass transfer characteristics of bubble and packed bubble columns with downcomer were investigated. The contactor consisted of two concentric columns of 0.11 and 0.2 m i.d., with the annulus acting as the downcomer. The packing used in this investigation was standard 16 mm stainless steel Pall rings. The superficial gas and liquid velocities, V_G and V_L, were varied from 0.01 to 0.09 and 1 × 10^?3 to 8.8 × 10^?3 m s^?1 respectively. Two flow patterns, namely the bubble and pulse flows were observed in the packed bubble column with downcomer, as shown by a flow map. The liquid circulation velocity in both the contactors was observed to be constant throughout the ranges of V_G and V_L covered in this work. The effect of liquid viscosity (0.8 to 9.5 mPa ? s) and surface tension (45 to 72 mN m^?1) on the flow pattern, liquid circulation, gas hold-up and pressure drop was investigated. The pressure drop characteristics across the two contactors have been compared with those across a bubble column. Values of the effective interfacial area, a, and the volumetric mass transfer coefficient, k_L a, were measured by using chemical methods. Values of a as high as 180 and 700 m^?1 and k_L a as high as 0.075 and 0.22 s^?1, in the bubble and packed bubble columns with downcomer, respectively, were obtained. The values of true liquid-side mass transfer coefficient, k_L, were found to be independent of V_G and were of the order of 5.5 × 10^?4 and 3.5 × 10^?4 m s^?1, respectively, in the two contactors.     

Mass transfer in a recirculating bubble column

The mass transfer of oxygen between air and water in a recirculating bubble column has been studied, at gas and water superficial velocities of up to 0.23 m s^-1 and 0.68 m s^-1 respectively. Experiments show that the assumption of plug flow for the gas phase is reasonable, eliminating a possible source of error identified by other workers in calculating the mass transfer rates. The results obtained are consistent with other published work. It is also shown that for the air-water system breaking up the gas bubbles to increase the mass transfer does not have a large effect, due to rapid recoalescence of the bubbles.     

Dispersed phase holdup and bubble size distributions in gas–liquid cocurrent upflow and countercurrent flow in reciprocating plate column

Dispersed phase holdup and the bubble size distribution were measured in a reciprocating plate column under cocurrent upflow and countercurrent flow of gas and liquid phases. The response of the system to a variation in design and operating conditions was found similar to that for liquid–liquid contacting; the magnitude of response, however, differed significantly between them. Taking into consideration the dominant forces encountered in gas–liquid dispersions, the experimental data are satis–factorily correlated in terms of Froude, Weber and Gallileo numbers.     

Mass transfer and pressure drop in a cocurrent reciprocating plate extraction column

A 5-cm-diameter reciprocating plate extraction column has been operated in cocurrent flow. The pressure drop for water flow, and the local and average mass transfer products for the system acetic acid/kerosene/aqueous sodium hydroxide have been measured. Under well-agitated conditions, the mass average transfer product is predictable by a model, which is also applicable to data reported earlier by Karr for a 2.54-cm-diameter column.     

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.     

Gas absorption in horizontal cocurrent bubble flow

Cocurrent flow pipeline reactors would have several advantages for gas-liquid reacting systems. However, design data are lacking, particularly for absorption rates of gas. The present study was purely exploratory, for the purpose of obtaining the first data on liquid-phase controlled absorption of gases in horizontal cocurrent two phase flow. CO₂ and helium were used as gases and water, ethanol and ethylene glycol as liquids over a wide range of operating conditions. The data in the bubble and plug flow regions have been correlated by an equation based on qualitative observation and analogy to the case of a single flowing fluid. The resulting expression, reproduces most bubble flow data to ± 15%, and includes the effects of gas and liquid flow rates, pipe diameter, surface tension, liquid viscosity and diffusivity.