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.     

Hydrodynamics and mass transfer characteristics of co-current downflow packed tube columns

Hydrodynamics and effective interfacial area in a 25 mm i.d. packed tube column were studied over a wide range of operating conditions for demister pad packings (DPP). Flow maps have been prepared. Values of effective interfacial area as high as 1880 m^?1 in the spray flow regime were obtained. Data on pressure drop and effective interfacial area have been correlated for different flow regimes. Values of liquid side volumetric mass transfer coefficient, k_L a , were measured by absorption and desorption of oxygen in different packed tube columns containing Pall rings (standard and low height to dia. ratio), multifilament wire gauge packings (MFWGP) and DPP. kL a was found to vary from 0.017 to 0.34 s^?1 for DPP. Values of wall side solid-liquid mass transfer coefficient, k_SL, were obtained in a 25 mm i.d. copper tube column packed with MFWGP by the dissolution of copper in acidic dichromate solutions. Values of wall side heat transfer coefficient could be obtained by analogy.     

Predicting mass transfer in packed columns

Countercurrent-flow columns are widely used in production processes in the chemical industry and their application in ecological engineering is of increasing importance. A theoretical model is presented here that allows mass transfer to be described in terms of packing geometry and physical properties which influence the gas-liquid or vapour-liquid systems in absorption, desorption and rectification columns. The relationships derived from the model can be applied to all countercurrent-flow columns, regardless of whether the packing has been dumped at random or arranged in a geometric pattern.     

空分填料塔设计中的流体力学特性

空分精馏主要在-200℃左右的深冷条件下进行,填料塔内流体力学特性参数难以由试验直接获取,如何将常压常温状态(简称常态)下空气-水物系的流体力学测试结果转换适用于空分设计变得尤为关键。通过Bain-Hougen公式及SRP(Ⅱ)模型结合Sp iegel持液量计算推导出泛点及压降关联式以预测填料的流体力学特性。对空分中常用的750Y型金属孔板波纹填料进行常态下流体力学特性测试,结果表明:采用的方法可用于填料流体力学特性的预测。对空分状态下的泛点与压降结果预测表明,空分设计中采用常态下的流体力学数据进行设计偏于保守。     

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.     

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.     

Hydrodynamic and mass transfer characteristics of external-loop airlift reactors without an extension tube above the downcomer

The effects of the horizontal connection length (0.1≤L_c≤0.5 m), the downcomer-to-riser cross-sectional area ratio (0.11≤A_d/A_r≤0.53) and the superficial gas velocity (0.02≤U_G≤0.18 ms^-1) on gas holdups in the riser and downcomer, the circulation liquid velocity, the mixing time, and the overall volumetric mass transfer coefficient were determined in external-loop airlift reactors without an extension tube above the downcomer [configuration (a)]. For otherwise fixed conditions, the absence of the extension tube strongly affected the hydrodynamic and mass transfer characteristics of external-loop airlift reactors. In contrast with the external-loop airlift reactor with the extension tube [configuration (b)], a large air pocket formed in the top horizontal connection and the surface aeration took place in the external-loop airlift reactor without the extension tube [configuration (a)]. As a result, the riser circulation liquid velocity in configuration (a) was slower than that in configuration (b). The riser and downcomer gas holdups, the mixing time and the overall volumetric mass transfer coefficient in configuration (a) were larger than those in configuration (b), respectively.     

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.     

Absorption in packed bubble columns

The mass transfer characteristics of packed bubble columns were studied by employing various packings of different sizes and shapes in 10–38·5 cm i.d. columns. The theory of absorption accompanied by pseudo-mth order reaction was used to obtain the values of effective interfacial area. The values of liquid side mass transfer coefficient were obtained by using the theory of absorption accompanied by slow chemical reaction. The superficial gas velocity was varied from 5 to 25 cm/sec. The packed bubble columns showed a considerable improvement in the performance over empty bubble columns. A criterion for the scale-up of these columns has been suggested.     

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.     

Heat,mass and momentum transfer in packed bed distillation columns

This contribution presents the basic equations for heat, mass and momentum transfer in multicomponent packed bed distillation processes. In some situations, the use of strongly simplified models is justified, but when approaching more difficult and, at the same time, economically more interesting regions of operation where non-linear effects are significant, these models are likely to fail. Consequently, a more rigorous vapour-liquid equilibrium model should be employed since the pressure drop in the column will not be negligible in those regions. Furthermore, neither constant parameter hold-ups nor heat and mass transfer coefficients are assumed. Simulations demonstrate some interesting process properties. The impact of the surroundings on the process is discussed and a three-dimensional model extension is outlined.     

Hydrodynamic studies with foaming and non-newtonian solutions in bubble columns

Experiments were conducted in 0.05 m ID and 0.23 m ID by 3 m tall bubble columns to study the effect of surfactants and viscosity of liquid medium on gas hold-up and Sauter mean bubble diameter. The addition of n-butanol (0.5 and 1 wt.%) to water leads to the formation of foam and consequently produces higher gas hold-ups. The foam could be eliminated completely with the addition of a sufficient quantity (0.5 wt.%) of carboxymethyl cellulose to the aqueous alcohol solution. In the absence of foam, gas hold-ups were similar to those obtained with pure liquids. Sauter mean bubble diameters, obtained using the dynamic gas disengagement technique, increase with viscosity of liquid medium.     

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.     

Exclusion of gas sparger influence on mass transfer in bubble columns

Gas phase CO₂ concentration profiles were measured in two sizes of bubble columns with different gas spargers and with the liquid phase (tap water) entrance or exit (cocurrent or countercurrent flow) at a certain height above the gas distributor. The region of high turbulence intensity near the sparger was locally separated from the region of high mass transfer rates in such columns. A modified back flow cell model was applied to describe the experimental data. The k_L-values obtained from fitting the profiles agreed for both columns and, in addition, did not differ for cocurrent and countercurrent flow. This is in remarkable contrast to previous findings(10,11). The large influence of the gas sparger on the k_L-values even in tall bubble columns was thus demonstrated. It is thought that this may probably be one of the reasons why correlations for prediction of k_L differ so significantly.     

Simultaneous measurement of flow pattern and mass transfer coefficient in bubble columns

Mass transfer studies were carried out in a bubble column using the chemical method. Catalytic oxidation of sodium sulfite was chosen for the studies and the corresponding specific rates of oxidation were obtained using a stirred cell. Laser Doppler anemometer (LDA) was used to measure the instantaneous velocities in the same stirred cell as well as in bubble columns (100 and i.d.). An efficient algorithm based on the multiresolution analysis of the velocity-time data using wavelets was used for the isolation of data belonging to the gas and liquid phases. Eddy isolation model was used for the characterization of the eddy motion including the estimation of the energy dissipation rate. Using the knowledge of eddy motion, a methodology was developed for the prediction of true mass transfer coefficient (k_L) in a stirred cell as well as in bubble columns. The predicted values of k_L have been compared with the experimental values obtained by the chemical method.     

Gas-liquid mass transfer in bubble columns with a T-junction nozzle for gas dispersion

Bubble break-up, gas holdup, and the gas-liquid volumetric mass transfer coefficient are studied in a bubble column reactor with simultaneous injection of a gas and liquid through a T-junction nozzle. The theoretical dependence of bubble break-up and the volumetric mass transfer coefficient on liquid velocity in the nozzle is developed on the basis of isotropic turbulence theory. It is shown that correlations which are developed based on liquid jet kinetic power per nozzle volume explain average gas holdup and the volumetric mass transfer coefficient within an error of 15% for all gas and liquid flow rates and nozzle diameters used. Experiments with a larger scale column, height 4.64 m and diameter 0.98 m, show a transition from homogeneous to heterogeneous flow at a certain liquid flow rate through the nozzle. Liquid composition was found to have a significant effect on gas-liquid mass transfer. A phenol concentration of 10–30 mg/l in water increases the volumetric mass transfer coefficient of oxygen by 100%. This phenomenon may have significance in the chemical oxidation of wastewater.     

Modelling of mass transfer phenomena of associated systems in packed distillation columns

Mass transfer between liquid and vapour phases in packed distillation columns has been investigated by the two-film theory for binary associated systems. The effects of association reaction in systems with the association of one component in both phases, taking into account a concentration-dependent liquid-phase association constant, were examined. The mass transfer coefficients are modelled, taking into account the association constant. The experiments were carried out at atmospheric pressure with acetic acid-benzene and acetic acid-toluene systems in a column packed with Rasching rings.     

Modeling liquid-phase chemical reaction and interphase mass transfer in churn-turbulent bubble columns

The slug and cell model of liquid-phase mixing has been extended to consider the effects of interphase mass transfer and liquid-phase chemical reaction in isothermal churn-turbulent bubble columns. Inclusion of the effects of the gas phase is straightforward since the gas phase has previously been considered as it strongly affects the mixing of the liquid phase. The computational simplicity of the slug and cell model has been retained with the species conservation equations being reduced to a system of algebraic equations. The behavior of the model has been examined for two cases: (1) liquid-phase chemical reaction in the absence of interphase mass transfer and (2) in the presence of interphase mass transfer without liquid-phase chemical reaction. In the second case the model behavior has been compared to available literature data for the absorption of oxygen into water.