Estimation of kLa Values in Bench‐Scale Stirred Tank Reactors with Self‐Inducing Impeller by Multiphase CFD Simulations

A multiphase computational fluid dynamics (CFD) simulation methodology is developed and proposed for the estimation of the spatial distribution of k_La values in a bench‐scale reactor equipped with a self‐inducing impeller. The importance of estimating an apparent drag coefficient, which considers the effect of turbulence on the gas bubble rising velocity, is also tackled by applying different correlations available in literature, namely, Brucato, modified Brucato, and Pinelli correlations. The spatial distribution of k_La values in the agitated vessel is found from the CFD results using Danckwert's surface renewal model. An analysis of the gas volume fraction distribution obtained from the simulations is performed in order to choose the most suitable drag model. The modified Brucato correction correlation for the drag force exhibits the best agreement with experimental data.     

The Influence of Interfacial Areas for Gas Absorption in the Presence of Solid Particles

The mass transfer is investigated for physical absorption of oxygen in water for varying interfacial areas as well as the influence of suspended glass beads and activated carbon. Under higher rotational speed, the volumetric mass transfer coefficient as well as the mass transfer coefficient values increase for all specific interfacial areas due to the changes in hydrodynamics. The configuration of the free gas‐liquid interface is of minor relevance. In the presence of glass beads, the mass transfer is reduced compared to physical adsorption, whereas an enhancement of the mass transfer can be observed in the presence of activated carbon. This indicates that the mobility of the gas‐liquid interface is the determining factor. The renewal of the fluid elements is increased by adsorption of surfactants on the surface of activated carbon, leading to an improved mass transport.     

Correlation and Prediction of Carbon Dioxide Solubility in n‐Paraffin Systems

This work discusses high‐pressure gas solubility. Reference state is the Henry's law and a two‐parameter corresponding states model is used for the liquid phase. Pure gas is used as the vapor phase for the high‐molecular paraffin's gas‐liquid equilibrium. The systems under study are mixtures of carbon dioxide and normal paraffin up to a chain length of 44 carbon atoms. Correlated liquid compositions are in the 1 % to 3 % average absolute relative deviation range, with a maximum error of about 7 %. Using a known system as reference, the model could be used to predict solubility. In this case average absolute relative deviations are below 6 % in most cases.     

Effect of Dual Impeller‐Sparger Geometry on the Hydrodynamics and Mass Transfer in Stirred Vessels

The understanding of the effect of impeller‐sparger configurations on gas dispersion and mass transfer is very important to improve the performance of gas/liquid contactor systems. The influence of the impeller positions, the upper turbine diameter, the sparger ring diameter and its location in regard to the lower impeller on the power consumption, the volumetric mass‐transfer coefficient and the overall oxygen transfer efficiency were studied in a nonstandard curved bottomed reactor with an agitated system with dual disk style turbines. In the range of the gas flow rates studied, the most efficient impeller‐sparger arrangement for the oxygen transfer is the impeller system with turbines of different diameters located at C = 0.25 and IC = 0.5, and with the sparger of smaller diameter than the lower impeller settled below the impeller. A new model to estimate the k_La with an average relative error of 8 %, which takes the reactor operation conditions and the influence of the impeller‐sparger geometry into account, was also proposed.     

Liquid‐Solid Mass Transfer Behavior of a New Stirred‐Tank Reactor with a Packed Bed Fixed to its Wall

Rates of liquid‐solid mass transfer at a packed bed of Raschig rings fixed to the wall of a stirred tank were measured by a technique which involves the diffusion‐controlled dissolution of copper in acidified dichromate. Variables studied were impeller rotation speed, impeller geometry, Raschig ring diameter, bed thickness, presence of baffles, physical properties of the solution, and effect of superimposed flow. Mass transfer data for the batch reactor were correlated by a dimensionless equation. For a given set of conditions, the radial‐flow impeller was found to produce higher rates of mass transfer than the axial‐flow impeller. The presence of baffles increased the rate of mass transfer inside the bed. Applications of the suggested reactor in conducting different diffusion‐controlled liquid‐solid reactions were evaluated.     

Numerical Simulation of Gas‐Liquid Flow in a Stirred Tank with Swirl Modification

Although the standard k‐? model is most frequently used for turbulence modeling, it often leads to poor results for strongly swirling flows involved in stirred tanks and other processing devices. In this work, a swirling number, R_S, is introduced to modify the standard k‐? model. A Eulerian‐Eulerian model is employed to describe the gas‐liquid, two‐phase flow in a baffled stirred tank with a Rushton impeller. The momentum and the continuity equations are discretized using the finite difference method and solved by the SIMPLE algorithm. The inner‐outer iterative algorithm is used to account for the interaction between the rotating impeller and the static baffles. The predictions, both with and without R_S corrections, are compared with the literature data, which illustrates that the swirling modification could improve the numerical simulation of gas‐liquid turbulent flow in stirred tanks.     

Theoretical prediction of gas-liquid mass transfer coefficient, specific area and hold-up in sparged stirred tanks

A prediction method for calculating the volumetric mass transfer coefficient, k_La, in gas-liquid sparged stirred tanks is proposed. A theoretical equation based on Hibie's penetration theory and the isotropic turbulence theory of Kolmogoroff is used for k_L determination. The values of the interfacial area have been calculated from a hold-up theoretical equation and the mean size of the gas bubble. Both Ostwald-De Waele and Casson models are used to describe the rheological properties of the fluid. The model predicts the mass transfer coefficient and the interfacial area values in stirred tank reactors, analysing the influence of different variables. The values of the volumetric mass transfer coefficient can be calculated for different geometries of the reactor, different physicochemical properties of the liquid and under different operational conditions. The capability of prediction has been examined using experimental data available in the literature for Newtonian and non-Newtonian fluids, for very different vessel sizes, different numbers and types of stirrers and a wide range of operational conditions, with very good results.     

Kinetic Study of Heterogeneously Catalyzed Glucose Oxidation in a Stirred Cell

Liquid phase oxidation of glucose to gluconic acid using Bi promoted Pd catalysts was studied in a stirred cell. Gas‐liquid mass transfer limitations are observed at lower but not at higher rotational speeds. The conversion and the deactivation of the Pd catalyst depend on the O₂ concentration in the liquid phase. With decreasing glucose concentration, the reaction rate decreases leading to higher oxygen concentration in the liquid, which deactivates the catalyst due to over‐oxidation. Severe mass transfer limitations even at low Pd loadings could be attributed to intraparticle or liquid‐to‐solid mass transfer.     

Effect of Tween 80 on Bubble Size and Mass Transfer in a Bubble Contactor

Gas absorption in aqueous solutions with Tween 80 and absorption processes based on hydrodynamics and mass transfer is determined. The impact of surfactant concentration on gas holdup and gas‐liquid interfacial area is analyzed, observing an increase of these parameters with surfactant concentration. The influence of liquid‐phase contamination on the absorption process is investigated on the basis of the liquid‐film mass transfer coefficient, removing the effect caused by the presence of a surfactant and the gas flow rate on the interfacial area and, thereby, on the volumetric mass transfer coefficient. The opposite effect on the mass transfer coefficient can be observed which decreases in the presence of the surfactant.     

Surfactant adsorption rather than “shuttle effect”?

In the absence of chemical reaction, mass transfer enhancement by suspended particles (mostly activated carbon) at the gas/liquid interface has been frequently reported and is usually explained by a “shuttle mechanism” exerted by particles with a high adsorption capacity for the transfer component. A major problem of this model is that unrealistic enrichment of the solids at the interface as compared to the bulk concentration has to be assumed. A comprehensive study has been carried out in a stirred tank in a wide range of the stirring speed (0-) with 9 different powdered solids suspended in water. With a flat gas/liquid interface, moderately hydrophobic solids significantly increased the mass transfer rates at low solid loadings (0.1-). However, the effect is not limited to particles with a high adsorption capacity for the gas (e.g. activated carbon) but it is observed also for non-porous particles (e.g. graphite or sulphur). When the particles are removed by rinsing, the absorption rates remain high. When the system is kept very clean (surfactant free), the enhancement effect is not observed. Based on these findings, it is concluded that adsorption of surfactants on hydrophobic solids cleans the interface resulting in higher mass transfer coefficients k_L.     

Performance Evaluation of Two High‐Shear Impellers in an Unbaffled Stirred Tank

High‐shear impellers (HSIs) are mixers used in industrial stirred tanks to incorporate powders into liquids and break down particle agglomerates. A detailed numerical study of two commercial ring‐style HSIs of laboratory scale was carried out and their performance was compared with the Rushton turbine (RT). It was found that power and pumping numbers or their ratio cannot be simply connected for properly selecting an impeller in applications where highly localized viscous dissipation is desirable. The ratio of the average viscous dissipation in the impeller swept volume to the mean in the entire volume at two constant values of power input turned out to be lower for HSIs compared to that evaluated for RT. However, at higher power input, the dimensionless average viscous dissipation in the blade swept volume was found to be similar for the HSI of two rings and the RT, corroborating the high local viscous dissipation of this HSI when operated at higher speeds.     

Surfactant Effects on Aeration Performance of Stirred Tank Reactors

The effect of surfactants on aeration performance in stirred tank reactors (STR) at high rates of foaming is studied. The volumetric oxygen transfer coefficient (k_La) and foaming activity estimated as foaming height (H_f) were determined. Biotechnology of lipopeptide biosurfactants from aerobic organisms, e.g., Bacillus subtilis were addressed. Using model solutions of known foam‐generating properties, high‐molecular weight surfactin and low‐molecular weight sodium dodecyl sulphate (SDS), as well as impellers of different types, with flat and fluid‐foil blades, clues on the concentration dependence of STR oxygen transfer and foaming as well as options for foam reduction in the presence of biosurfactant were sought. In response to a two‐fold decrease of surface tension by surfactin, k_La values decreased up to 30 % but remained within the range expected for the mixing system in water; the experiments with SDS showing stronger dependence on surfactant concentration and surface tension. Mixing of surfactant media by a standard six‐blade disc turbine (RT) imposed rate limitations on gassing. A low‐shear impeller Narcissus (NS) could be used to avoid bulk foam outflow, while preserving k_La values that remained unchanged. The ‘power per unit volume' correlation of k_La in stirred tanks is tested in the presence of surfactin.     

Effects of Contaminants on the Mass‐Transfer Characteristics of a Two‐Impinging‐Streams Gas‐Liquid Reactor

The mass‐transfer characteristics of a new type of two‐impinging‐streams reactor (TISR) was studied by means of sodium sulfite solution as the liquid phase and air as the gas phase, in the presence and absence of various types of surface‐active agents (SAAs). The influences of anionic, cationic, and nonionic SAAs on the specific interfacial area and overall volumetric mass‐transfer coefficient obtained in the TISR were investigated. It was found that the presence of a little amount of the above‐mentioned contaminants increases the specific interfacial area and decreases the overall volumetric mass‐transfer coefficient. On the basis of the experimental results obtained for various types of SAAs, correlations were derived for the interfacial area as well as the Sherwood number for the liquid phase in terms of Froude, Reynolds, Schmidt, and Morton numbers.     

Free‐Surface Turbulent Flow in an Eccentric Stirred Tank

This work aims at a reliable prediction of the flow dynamics in stirred tanks. The focus is on the free‐surface turbulent flow in an eccentric stirred tank by using the combination of detached‐eddy simulation and volume of fluid model. The flow field, profiles of the free surface, mean velocities, and the macroinstability phenomenon were explored and compared with the available experimental data. A reasonable representation of the free‐surface hydrodynamics was achieved. The findings indicate that the model and simulation strategies presented here can be used with sufficient confidence to predict the free‐surface hydrodynamics in stirred tanks.     

Aldol condensation of n‐butyraldehyde in a biphasic stirred tank reactor: Experiments and models

To model a biphasic stirred tank reactor, intrinsic reaction kinetics and interfacial area are required. In this study, reactor modeling for n‐butyraldehyde aldol condensation was investigated under industrially relevent conditions. The interfacial area in the reactor was directly measured using a borescope system under appropriate temperature, NaOH concentration and rpm conditions. To estimate the interfacial area, a semiempirical correlation was developed, which provides good estimates within ±15% error. The reactor model based on two‐film theory was developed, combining the interfacial area and intrinsic reaction kinetics reported in our prior work. The model was verified by reaction experiments in the range 0.05–1.9 M NaOH, 80–130°C, and 600–1000 rpm. The prediction errors using the interfacial area from direct measurements and the correlation were ±8% and ±15%, respectively, suggesting that the model accuracy may be improved with better interfacial area estimation. © 2015 American Institute of Chemical Engineers AIChE J, 61: 2228–2239, 2015     

Micellar Extraction of Zinc with Di(2‐ethylhexyl)phosphoric Acid and the Addition of the Cationic Surfactant Cetyltrimethylammonium Bromide

The addition of surfactants can influence extraction processes. In cases where the surfactant concentration is above the critical micelle concentration, the mass transfer rate increases due to the formation of inverse micelles and, thus, through the mechanism of micellar extraction of zinc. The influence of the added surfactant cetyltrimethylammonium bromide on zinc extraction with the cation exchanger di(2‐ethylhexyl)phosphoric acid was analyzed based on extraction equilibrium and mass transfer experiments. The observed experimental results are explained phenomenologically.     

Impact of Cellulose and Surfactants on Mass Transfer of Bubble Columns

The effects of cellulose, surfactants, and their combination on the hydrodynamic behavior and the liquid‐side mass transfer coefficient of a bubble column were evaluated. For that purpose, different aqueous solutions containing surfactants (sodium dodecyl sulfate) and cellulose (microcrystalline cellulose, MCC) were investigated. The interfacial areas were calculated from the bubble diameters, the bubble frequencies, and the terminal bubble rising velocities. The liquid‐side mass transfer coefficients were determined from the volumetric mass transfer coefficients measured by the dynamic method. In the concentration range under test, the experimental results proved that the addition of MCC to the studied liquid phases did not affect the mass transfer coefficient.     

Direct Extraction of Propionic Acid from Propionibacterium acidipropionici Broths with Tri‐n‐octylamine

The direct reactive extraction of propionic acid from Propionibacterium acidipropionici broths with solutions of tri‐n‐octylamine in dichloromethane, n‐butyl acetate or n‐heptane underlined the strong negative influence of the cells, due to the blockage of the interface by their adsorption. The magnitude of this effect <#>depends on the affinity of the cells for the organic phase, which is more important for n‐heptane, but only at biomass concentrations below 18 g L^–1 d.w. (dry weight). Moreover, the interfacial mass transfer of the acid is also controlled by the solvent polarity, and is accelerated from n‐heptane to dichloromethane and by the addition to the organic phase of 1‐octanol as a phase modifier. The influences of the biomass concentration, the rotation speed and the solvent dielectric constant were included in a mathematical model describing the solute mass flow from the aqueous to the organic phase.     

Effect of Carbon Dioxide Chemical Absorption on Bubble Diameter and Interfacial Area

The influence of a gas‐liquid chemical reaction on the interfacial area produced in a contactor is analyzed. Two different amines were used to capture carbon dioxide by chemical absorption. The effects of the operation time, the amine used, the concentration interval, and the gas flow rate on typical hydrodynamic parameters used in bubble columns such as the gas holdup and the Sauter mean diameter were investigated. These parameters were used to determine the interfacial area value. Significant influences on the gas‐liquid interfacial area were detected, mainly caused by the reaction rate intensity, the physicochemical properties of the liquid phase, and the gas flow rate fed to the contactor.     

Absorption of Carbon Dioxide into Aqueous Colloidal Silica Solution

Abstract

On the basis of experimental data for carbon dioxide absorption into aqueous nanometer sized colloidal silica solution as a non‐Newtonian fluid, a dimensionless correlation for volumetric liquid‐side mass transfer coefficient (k_La) of CO₂ in the flat‐stirred vessel was proposed. In addition to ordinary liquid properties and operating parameters such as impeller size and speed in the vessel, Deborah number, which is defined as the product of the characteristic material times of the liquid and agitation speed in the flat‐stirred vessel and represents the viscoelastic behavior of non‐Newtonian fluid, was used to present unified expressions for k_La in Newtonian as well as non‐Newtonian liquid. The values of k_La in the aqueous colloidal silica solution were reduced due to elasticity of the solution.