THE EFFECT OF SOLUTES ON BUBBLE SIZE IN AIR-WATER DISPERSIONS

Bubble size distributions in a bubble column of 200 mm diameter were measured by means of a photoelectric probe. The gas-liquid dispersion was generated by distributing air with a porous plate into aqueous solutions of electrolytes and of organic compounds (alcohols, glycols, ketones, carboxylic acids, saccharose, carboxymethyl-cellulose, detergents). For all these solutes there exists a rather narrow concentration range in which the change from quick coalescence in pure water to coalescence suppression takes place. Though rough relationships between concentrations for coalescence suppression and molecular properties can be found (ionic strength for electrolytes, number of carbon atoms in a homologous series of organic compounds), exact equations based on a theory of coalescence cannot be given.

The bubble size distributions obtained in the experiments have also been used to show the impact of coalescence phenomena on gas-liquid mass transfer. For this purpose, surface areas were calculated from measured gas hold-up and mean bubble diameters (Sauter diameters). A comparison of these data with the trends for volumetric mass transfer coefficients reported in the literature for n-alcohols (C₁-C₂i) showed good agreement, despite the fact that the mass transfer data had been obtained in a quite different gas-liquid contacting device (stirred vessel).     

Numerical simulations of gas-liquid mass transfer in bubble columns with a CFD-PBM coupled model

Gas-liquid mass transfer in a bubble column in both the homogeneous and heterogeneous flow regimes was studied by numerical simulations with a CFD-PBM (computation fluid dynamics-population balance model) coupled model and a gas-liquid mass transfer model. In the CFD-PBM coupled model, the gas-liquid interfacial area a is calculated from the gas holdup and bubble size distribution. In this work, multiple mechanisms for bubble coalescence, including coalescence due to turbulent eddies, different bubble rise velocities and bubble wake entrainment, and for bubble breakup due to eddy collision and instability of large bubbles were considered. Previous studies show that these considerations are crucial for proper predictions of both the homogenous and the heterogeneous flow regimes. Many parameters may affect the mass transfer coefficient, including the bubble size distribution, bubble slip velocity, turbulent energy dissipation rate and bubble coalescence and breakup. These complex factors were quantitatively counted in the CFD-PBM coupled model. For the mass transfer coefficient k_l, two typical models were compared, namely the eddy cell model in which k_l depends on the turbulent energy dissipation rate, and the slip penetration model in which k_l depends on the bubble size and bubble slip velocity. Reasonable predictions of k_la were obtained with both models in a wide range of superficial gas velocity, with only a slight modification of the model constants. The simulation results show that CFD-PBM coupled model is an efficient method for predicting the hydrodynamics, bubble size distribution, interfacial area and gas-liquid mass transfer rate in a bubble column.     

INFLUENCE OF LIQUID PROPERTIES ON THE DETERMINATION OF THE VOLUMETRIC MASS TRANSFER COEFFICIENT WITH THE HYDRAZINE METHOD

The oxidation of hydrazine in aqueous solution by atmospheric oxygen in presence of a homogeneous catalyst (copper tetrasulphophthalocyanine) is well suited for the determination of volumetric mass transfer coefficients. In contrast to other chemical methods the hydrazine oxidation permits to vary coalescence behaviour of the liquid phase by adding electrolytes or organic compounds, because without these solutes the reaction liquid does not inhibit bubble coalescence. With appropriate high molecular additives the hydrazine method can also be used for mass transfer measurements in liquids of high viscosity. Compared to the dynamic method the hydrazine method as a steady state method is far less influenced by systematic errors originating from the evaluation model. For this reason the application of the hydrazine method for testing semi-industrial and industrial gas-liquid contacting devices should be especially attractive.     

HYDRODYNAMICS AND MASS TRANSFER IN BUBBLE COLUMNS: EFFECT OF SOLIDS

The hydrodynamics and mass transfer in a large diameter bubble column (D_c 0.305 m), specifically, the effects of gas velocity and the presence of solids on the gas holdup structure, gas-liquid interfacial area, and volumetric mass transfer coefficients in viscous as well as low viscosity solutions are studied. The sulfite oxidation technique was employed to measure the gas-liquid interfacial area. Volumetric mass transfer coefficients were measured using a chemical method (sulfite oxidation) as well as physical absorption of oxygen from air, and the overall gas holdups were measured using the hydrostatic head technique. The effect of solids on the gas holdup structure was examined using the dynamic gas disengagement method. With the addition of polystyrene particles, the gas-liquid interfacial area decreased for low viscosity systems, whereas it increased for viscous systems. This was shown to be due to the effect of solids on bubble coalescence. The wettability characteristics of solid surfaces in the presence of different liquids have been suggested as the reason for the effect of solids on coalescence. Oil shale slurries presented a special case because of the mineral dissolution effect.     

Bubble coalescence at sieve plates: II. Effect of coalescence on mass transfer. Superficial area versus bubble oscillations

Bubble columns are among the most used equipments for gas-liquid mass transfer processes. This equipment's aim is to generate gas dispersions into a liquid phase in order to improve the contact between phases. Bubble coalescence has always been one of their greatest problems, since it reduces the superficial gas-liquid contact area. However, bigger bubbles can oscillate, and these oscillations increase the mass transfer rate by means of modifying the contact time as well as the concentration profiles surrounding the bubble. In the present work, the coupled effect has been studied by means of two-holed sieve plates with diameters of 1.5, 2 and 2.5 mm each, close enough to allow the coalescence and separated enough to avoid it. The results show that although coalescence decreases mass transfer rate from bubbles the deformable bubble generated can, in certain cases, balance the decrease in mass transfer rate due to the reduction in superficial area. This fact can then be used to avoid the harmful effect of coalescence on the mass transfer rate. Empirical and theoretical equations have also been used to explain the phenomena.     

单孔及微孔曝气低气速鼓泡床内气泡行为比较

引言鼓泡床反应器被广泛应用于吸收、液相氧化、好氧生化等气液反应过程,气体在液相中的分散情况对鼓泡床的反应和传质特性都有很大影响.为了提高气液传质效率,增加生产强度,工业反应器很多都是在高气速下操作(Ug>0·05m·s-1),很多研究都集中在高气速湍动鼓泡区[1~3].但对有机     

ILAR中醇及电解质对非牛顿流体中气-液传质的影响

在-气升式内环流反应器中试验考察了非牛顿流体羧甲基纤维素钠(CMC)中的气泡聚并现象以及表面活性物质对液相体积传质系数的影响。结果表明,非牛顿流体中气-液传质效率随黏度的增加而降低,其原因是黏度增加使Taylor泡的尾流趋于稳定,降低了液相扰动,气泡间易聚并,从而气-液传质效率低。向非牛顿流体中添加醇类物质会影响气-液传质行为,对于聚合物含量低的流体,添加微量醇可以促进气-液传质,聚合物含量高的非牛顿流体,微量醇的加入反而不利于气-液间传质过程。非牛顿流体在ILAR上升管中的气含率随着黏度的增加变化不大,而下降管中的气含率有所提高。     

Gas-liquid mass transfer studies: The influence of single- and double-impeller configurations in stirred tanks

The influence of impeller structure on the mass transfer characteristics was studied with the steady-state method for gas-liquid volumetric mass transfer coefficient (k _L a). The single-impeller configurations included eight impeller types (three radial flow impellers, four axial flow impellers and one mixed flow impeller), and the doubleimpeller included three configurations (RT+RT, RT+WH _D , WH _D +WH _D ). For single-impeller, the gas-liquid mass transfer rates of radial flow impellers were better than those of axial flow impellers under the same rotation speed and gas flow rate. The mass transfer performance (defined as the volumetric mass transfer coefficient per unit power input) of radial flow impellers were also better than that of axial flow impellers. With the same kLa value under a certain gas flow rate, the local bubble size distribution between radial flow impeller and axial flow impeller was similar. As for double impellers, RT+RT provided the highest mass transfer rate under certain rotation speed and gas flow rate, while WH _D +WH _D gave the highest values of gas-liquid mass transfer coefficient with the same power consumption.     

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.     

Mass transfer in sparged and stirred reactors: influence of carbon particles and electrolyte

Mass transfer in multiphase systems is one of the most studied topics in chemical engineering. However, in three-phase systems containing small particles, the mechanisms playing a role in the increased rate of mass transfer compared to two-phase systems without particles, are still not clear. Therefore, mass transfer measurements were carried out in a 2D slurry bubble column reactor , a stirred tank reactor with a flat gas-liquid interface, and in a stirred tank reactor with a gas inducing impeller. The rate of mass transfer in these reactors was investigated with various concentrations of active carbon particles (average particle size of ), with electrolyte (sodium gluconate), and with combinations of these. In the bubble column, high-speed video recordings were captured from which the bubble size distribution and the specific bubble area were determined. In this way, the specific mass transfer area a_gl was determined separately from the mass transfer coefficient k_l. Mechanisms proposed in literature to describe mass transfer and mass transfer enhancement in stirred tank reactors and bubble columns are compared. It is shown that the increased rates of mass transfer in the 2D bubble column and in the stirred tank reactor with the gas inducing impeller are completely caused by an increased gas-liquid interfacial area upon addition of carbon particles and electrolyte. It is suggested that an increased level of turbulence at the gas-liquid interface caused by carbon particles accounts for a smaller effective boundary layer thickness and an enhancement of mass transfer in the flat gas-liquid surface stirred tank reactor. However, for the carbon particles used in this study, it is rather unlikely that mass transfer enhancement takes place due to the well-known shuttle or grazing effect.     

Inhibition of bubble coalescence by the electrolytes

Coalescence frequency and coalescence time were measured in electrolyte solutions. Marrucci model was acceptable to predict the coalescence time of one pair bubble in the dilute solution of electrolytes. Transition concentration decreased with increase of bubble forming frequency. This tendency was same as the results in low molecular alcohol solution. For a bubble column study, the effects of electrolyte on the gas holdup and bubble characteristics were invstigated. The inhibition effect of bubble coalescence of the K₂SO₄ was slightly higher than that of the KCI at a same ionic strength. In this work, transition concentration was 0.36 kmol/m³, which is larger than the value predicted in pair bubble study.     

Hydrodynamics and mass transfer behavior in multiple draft tube airlift contactors

The draft tube configuration significantly affected the performance of an airlift contactor. The multiple draft tube configuration was demonstrated to give a better gas-liquid mass transfer when compared with a conventional one-draft-tube system. The airlift with a larger number of draft tubes allowed a higher level of bubble entrainment, which rendered a high downcomer gas holdup. This resulted in a higher overall gas holdup in the contactor. Liquid velocity was also enhanced by increasing the number of draft tubes. The ratio between downcomer and riser cross sectional areas, A _d /A _r , had a great effect on the system performance, where a larger A _d /A _r led to a lower downcomer liquid velocity and smaller quantity of gas bubbles being dragged into the downcomer. This resulted in low gas holdup, and consequently, low gas-liquid interfacial mass transfer area, which led to a reduction in the overall volumetric mass transfer coefficient. The presence of salinity in the system drastically reduced the bubble size and subsequently led to an enhancement of gas entrainment within the system. As a result, higher gas holdups and gas-liquid interfacial area were observed, and hence, a higher rate of gas-liquid mass transfer was obtained.     

Modelling mass transfer in an aerated 0.2 m vessel agitated by Rushton, Phasejet and Combijet impellers

We assembled a set of models that allows investigation of local variables that are difficult to measure, validation of mechanistic physical models, and comparison of different numerical solutions. Population balances (PB) for bubbles were combined with local flow modelling in order to investigate G–L mass transfer in an air–water system. Performance of three different impeller geometries was investigated: Rushton (RT), Phasejet (PJ) and Combijet (CJ). Simulations were compared against experimental mixing intensity, gas hold-up, vessel-averaged volumetric mass transfer rates (k_La), and local bubble size distributions (BSDs).The simulations qualitatively predict k_La's with different impellers at the fully dispersed flow region and gave new insight on how k_La is formed and distributed in the stirred vessels. The used bubble breakage and coalescence models are able to describe both air–water and viscous non-Newtonian G–L mass transfer. Difference between experimental mass transfer rates of the three impellers was within experimental error, even trough the flow patterns, gas distribution, and local BSDs differ considerably. The population balance for bubbles was modelled in two different ways, with multiple size groups (MUSIGs) and with the bubble number density (BND) approach. MUSIG calculations took over twice as much computational time than BND, but there was little difference in the results. The Rushton turbine k_La was described with best accuracy, which is not surprising since most phenomenological models are fitted based on RT experiments. We suggest that these models should be validated over a wider range of vessel geometries and operating conditions.     

Theoretical modelling of the effect of surface active species on the mass transfer rates in bubble column reactors

The complex composition of the liquid media in bubble column reactors makes their understanding and theoretical modelling challenging. In this work we have studied the effect of surface tension and contaminants, salts, on the mass transfer rates from a theoretical point of view, looking for a deeper understanding on the effect of surface active species which usually reduce surface tension and modify bubble surface behaviour. The specific contact area is obtained using a population balance where the effect of the presence of contaminants is addressed by the proper theoretical closures for bubble coalescence efficiency, for partially and fully immobile surfaces, and bubble break-up. Meanwhile, the contribution of contaminants to the liquid-film resistance is implemented as function of the coverage of the surface of the bubbles. It was found that the degree of bubble surface coverage not only affects bubble coalescence but also their break-up. The ion strength defines bubbles stability and the critical Weber number can be predicted as function of ion strength. Furthermore, the mass transfer rates are function of the surface coverage by the electrolytes. The model was able to predict k_La taking into account the fact that the concentration profiles surrounding individual bubbles are not completely developed due to the presence of other bubbles, in agreement with previous results from the literature.     

Blasensäulen-Reaktoren mit suspendiertem Feststoff: Grundlagen — Auslegung — Anwendungen

Bubble column reactors with suspended solids – fundamentals, design, and uses. The article describes the determination of hydrodynamic functions such as bubble size frequency distributions, relative gas contents, Sauter diameters, and specific interfacial areas in various liquids and in binary and ternary mixtures of organic liquids. The article also covers the local variation of these quantities as a result of coalescence, and axial and radial mixing coefficients. Formal Reynolds' analogies could be set up for heat and mass transfer – both across the gas/liquid interface and at the solid wall/bubble column interface without or with suspended solids. A brief survey of industrial processes in three-phase reactors, especially in petrochemistry and biotechnology, closes with an indication of the expected growth in importance of such reactors as a consequence of the present trend towards heterogenization in homogeneous catalysis.     

鼓泡反应器中幂律型流体流动与传质特性

很多废水处理装置涉及非牛顿型流体中的多相流动和传质问题,研究其中的气液传质过程有助于实现装置的优化设计和高效节能运行。以鼓泡反应器内清水和不同质量分数的羧甲基纤维素钠（CMC）水溶液为实验对象,分别研究气相表观气速和液相流变特性对气泡尺寸分布、全局气含率和体积氧传质系数的影响。实验结果表明,液相的流变特性对其传质特性参数均有较大影响。与清水相比,CMC水溶液中气泡平均直径和分布范围更大;清水和CMC水溶液的全局气含率均随表观气速的增加而增大;CMC水溶液的体积氧传质系数随CMC水溶液质量分数的增加而减小。基于实验研究,得出修正的体积氧传质系数公式和适用于幂律型非牛顿流体流动体系氧传递过程的无量纲关联式,可很好地实现非牛顿流体流动的废水处理装置中气液传质参数的计算。     

EFFECT OF STATIC LIQUID HEIGHT ON GAS-LIQUID MASS TRANSFER IN A DRAFT-TUBE BUBBLE COLUMN AND THREE-PHASE FLUIDIZED BED

Experiments are performed under batch-liquid operating conditions to investigate the effect of static liquid height on the gas-liquid mass transfer coefficient (K_La) in a draft-tube bubble column (DTBC) and a draft-tube three-phase fluidized bed (DTFB). In addition, the effects of column diameter, gas-distributor, and draft-tube diameter are studied. The results indicate that for a given system with a porous plate gas-distributor at low superficial gas velocities (<70 m/hr), increasing static liquid height decreases K_La. At high gas velocities, K_La is independent of the static liquid height. For systems with a perforated gas-distributor, there is no effect of static liquid height on K_La. The formation of small dispersed bubbles at low gas velocities in the porous plate distributor system accounts for the considerably high K_La values and the observed effect of liquid height. On the other hand, the formation of large spherical-cap bubbles and the bubble coalescence at high gas velocities reduce the performance of the porous plate distributor system to that of the perforated one.     

Effect of electrolytes in aqueous solutions on oxygen transfer in gas–liquid bubble columns

The effects of inorganic electrolytes (NaCl, MgCl₂, CaCl₂) in aqueous solutions on oxygen transfer in a bubble column were studied. Electrolyte concentrations (c) below and above the critical concentrations for bubble coalescence (c_tc), and six superficial gas velocities (v_sg), were evaluated. The volumetric mass transfer (k_La) and the mass transfer (k_L) coefficients were experimentally determined. It was found that the concentration of electrolytes reduced the k_L, but the interfacial area (a) increased enough to result in a net increase of k_La. Using as independent variable a normalizing variable (c_r = c/c_tc), and maintaining fixed v_sg, similar values of k_La were observed regardless the kind of electrolyte; the same happened for k_L. This suggests that c_r quantifies the structural effects that these solutes exert on mass transfer. Also, once c_r = 1 was reached, no significant variations were found in k_La and k_L for constant v_sg. It is concluded that the gradual inhibition of bubble coalescence (c_r < 1) governs the significant changes in hydrodynamics and mass transfer via the reduction of bubble size and the consequent increment of a and gas holdup (?_g). Finally, regarding the effects of v_sg on mass transfer, transition behaviors between those expected for isolated bubbles and bubble swarms were observed.     

Hydrodynamik in Blasensäulen – Messung von relativem Gasgehalt und Blasengröße

In bubble column reactors mass transfer kinetics are determined by local hydrodynamic processes (bubble formation, velocity, coalescence, break‐up, etc.). For a better understanding of the influence of pressure and particle load on these processes, a measurement system for detection of local bubble size distributions in opaque bubbly flows is needed. Therefore, in this work an optical needle probe was applied in a bubbly flow. In a first step, the needle probe was validated by means of single‐bubble measurements. The measurement technique shows high potential for application in more complex and opaque systems.