Volumetric Mass Transfer Coefficient kLa Research Articles

Mixing behavior and mass transfer of liquid–liquid two-phase flow in an annular microchannel with helical wires

Combining the advantages of high efficiency, low-pressure drop, and large throughput, the pore array-enhanced tube-in-tube microchannel (PA-TMC) is a promising microreactor for industrial applications. However, most of the mass transfer takes place in the upstream pore region, while the contribution of the downstream annulus is limited. In this work, helical wires were introduced into the annulus by adhering to the outer surface of the inner tube. Mixing behavior and mass transfer of liquid–liquid two-phase flow in PA-TMC with different helical wires have been systematically studied by a combination of experiments and volume of fluid (VOF) method. The introduction of helical wires improves the overall volumetric mass transfer coefficient KLa by up to 133% and the mass transfer efficiency E by up to 117%. The simulation results show that the helical wire brings extra phase mixing regions and increases the specific interface area, while accelerating the fluid flow and expanding the area of enhanced turbulent dissipation rate. Influences of helical wires in various configurations are compared by the comprehensive index I concerning the pressure drop and mass transfer performance simultaneously and a new correlation between KLa and specific energy consumption φ is proposed. This research deepens the understanding of the mixing behavior and mass transfer in the PA-TMCs and provides practical experience for the process intensification of microchannel reactors.

Microfluidic mass transfer of supercritical CO2 in brine

In the context of carbon sequestration in saline aquifers, we investigate CO2 transport dynamics in brine under reservoir-like conditions (P = 8 MPa, T = 50∘C) using microfluidics. We quantify the mass transfer of supercritical CO2 in brine across a range of concentrations (0–1 M) and liquid flow rates (15–60 μL/min) for the first time. We find the volumetric mass transfer coefficient kLa ranges from 50.3 to 144.0 s−1, increasing with flow rate and, to a lesser extent, with salinity. More importantly, the kLa value for supercritical CO2 in brine at a higher temperature shows a significant enhancement of >50% compared to gas and liquid CO2 in water. Our analysis, incorporating a theoretical mass transfer model, points to a dominant contribution from the liquid film to the overall kLa, nearly doubling that of the emulsion caps, attributed to the thin film's larger surface area and dynamic renewal.

Characterization Data for the Establishment of Scale-Up and Process Transfer Strategies between Stainless Steel and Single-Use Bioreactors

The reliable transfer of bioprocesses from single-use bioreactors (SUBs) of different scales to conventional stainless steel stirred-tank bioreactors is of steadily growing interest. In this publication, a scale-up study for SUBs with volumes of 200 L and 2000 L and the transfer to an industrial-scale conventional stainless steel stirred-tank bioreactor with a volume of 15,000 L is presented. The scale-up and transfer are based on a comparison of mixing times and the modeling of volumetric mass transfer coefficients kLa, measured in all three reactors in aqueous PBS/Kolliphor solution. The mass transfer coefficients are compared with the widely used correlation of van’t Riet at constant stirrer tip speeds. It can be shown that a van’t Riet correlation enables a robust and reliable prediction of mass transfer coefficients on each scale for a wide range of stirrer tip speeds and aeration rates. The process transfer from single-use bioreactors to conventional stainless steel stirred-tank bioreactors is proven to be uncritical concerning mass transfer performance. This provides higher flexibility with respect to bioreactor equipment considered for specific processes.

Intensification of a biocatalytic oxidation under fine bubble aeration in a rotating bed reactor

The utilization of oxidases suffers from insufficient oxygen availability due to its low solubility in gas-liquid multiphase systems. To overcome this limitation, oxygen is continuously supplied to the reaction medium during the course of the reaction, however, it causes significant gas consumption. In this study, a novel experimental set-up was established by installing a sintered frit porous sparger in the SpinChem® rotating bed reactor. Covalently immobilized glucose oxidase on epoxy-functionalized carriers was used in the rotating bed reactor with an activity yield of the immobilization of over 98%. For fine bubble aeration, the volumetric mass transfer coefficient kLa (67.3 h−1) tripled compared to macrobubble aeration (22.5 h−1), while the volume-specific aeration rate remained constant. A maximum reaction yield of 96.5% was reached. The reaction rate was improved by a factor of 12.9 for the fine bubble aeration compared to macrobubble aeration under identical conditions. With fine bubble aeration, the oxygen consumption was reduced by 87.5% over macrobubble aeration, establishing the same enzyme-specific reaction rates at a comparable kLa. Validation was carried out in repetitive batches, achieving the same glucose oxidase specific activity. This study demonstrates the advantages of fine bubble aeration and presents promising results for addressing the sustainable biotransformation processes.

A novel synergistic intensification method for CO2 absorption by metal salt and task-specific ionic liquids hybrid solvent in microchannel reactor

A novel metal salt-promoting mass transfer intensification method for CO2 absorption into task-specific ionic liquids aqueous solution was proposed. The mass transfer intensification and intrinsic mechanisms of different concentrations of metal salts (NaBF4, KBF4, KCl, and CuCl2) with task-specific ionic liquids (1-aminopropyl-3-methylimidazolium tetrafluoroborate [Apmim] [BF4]) aqueous solutions for CO2 capture process were visually investigated using a microchannel reactor. The results showed that adding metal salts could remarkably enhance the interaction between metal cations and ionic liquid anions, thereby decreasing the interaction between anions and cations within the ionic liquid molecules and promoting the chemical reaction between ionic liquid cations and CO2. Under the condition of the same salt concentration, the addition of KBF4 has more obvious enhancement for mass transfer in CO2 absorption process. The mass transfer coefficient increases as the gas-liquid phase flow rate rise under slug flow condition. In addition, based on the effects of metal salt concentration, absorbent density, and gas-liquid phase flow rate on the CO2 volumetric mass transfer coefficient kLa, a new correlation for predicting the volumetric mass transfer coefficient of CO2 absorption into the hybrid solvent in microchannel reactor was proposed and validated.

Mass transfer enhancement and hydrodynamic performance with wire mesh coupling solid particles in bubble column reactor

It is of vital significance to investigate mass transfer enhancements for chemical engineering processes. This work focuses on investigating the coupling influence of embedding wire mesh and adding solid particles on bubble motion and gas–liquid mass transfer process in a bubble column. Particle image velocimetry (PIV) technology was employed to analyze the flow field and bubble motion behavior, and dynamic oxygen absorption technology was used to measure the gas–liquid volumetric mass transfer coefficient kLa. The effect of embedding wire mesh, adding solid particles, and wire mesh coupling solid particles on the flow characteristic and kLa were analyzed and compared. The results show that the gas–liquid interface area increases by 33% to 72% when using the wire mesh coupling solid particles strategy compared to the gas–liquid two-phase flow, which is superior to the other two strengthening methods. Compared with the system without reinforcement, kLa in the bubble column increased by 0.5–1.8 times with wire mesh coupling solid particles method, which is higher than the sum of kLa increases with inserting wire mesh and adding particles, and the coupling reinforcement mechanism for affecting gas–liquid mass transfer process was discussed to provide a new idea for enhancing gas–liquid mass transfer.

Enhanced liquid–liquid mass transfer in a monolithic reactor with multi-jet-channel in the circumferential array

A monolithic reactor with multi-jet-channel in the circumferential array (MR-MJCCA) is designed to intensify mass transfer. Effects of operating conditions and geometric parameters on the mass transfer efficiency E and overall volumetric mass transfer coefficient KLa are illustrated through experimental and numerical methods. Results show that the increased jet impingement intensity due to a higher flow rate and smaller jet diameter Dp effectively enhances mass transfer. E and KLa in the MR-MJCCA possessing eight channels increase with reducing the channel depth De and helical pitch p, yet first rise and then decline with enlarging the jet unaligned-impinging distance w. A higher two-phase interfacial area and energy dissipation rate are conducive to improving mass transfer performance in configurations with Dp = 0.3 mm, De = 1 mm, p = 5 mm, and w = 1.5 mm. Increasing the axial length of reactor elevates E whereas KLa diminishes. A reliable artificial neural network model was used to correlate KLa with key parameters. MR-MJCCA exhibits a higher KLa compared to most microreactors and the throughput increases by over tenfold, demonstrating its scalability and broader industrial applications.

Micro-interface enhanced mass transfer sodium carbonate absorption carbon dioxide reaction

Micro-interface intensified reactor (MIR) can be applied in series/parallel in the absorption of CO2 in industrial gases by Na2CO3 due to the ability to produce large numbers of stable microbubbles. This work focuses on the variation pattern of mass transfer characteristics parameters of the reaction gas in Na2CO3 solution under the influence of different solution properties and operating parameters in the reaction of CO2 absorption by Na2CO3. The mass transfer characteristics parameters include bubble Sauter mean diameter, gas holdup, interfacial area, liquid side mass transfer coefficient, and liquid side volume mass transfer coefficient kLa. The solution properties and operating parameters include Na2CO3 concentration (0.05–2.0 mol·L−1), superficial gas velocity (0.00221–0.01989 m·s−1), superficial liquid velocity (0.00332–0.02984 m·s−1), and ionic strength (1.42456–1.59588 mol·kg−1). And volumetric mass transfer coefficients kLa and superficial reaction rates r of the MIR and the bubble column reactor are compared in the reaction of sodium carbonate absorption of carbon dioxide, and the former shows a greater improvement under different solution properties and operating parameters. The enhanced role of MIR in mass transfer in non-homogeneous reactions is verified and the feasibility of industrial practical applications of MIR is demonstrated.

Influence of Liquid Properties and Design Parameters on Mass Transfer in Gas–Liquid Flow through a Miniaturized Reactor

This paper presents the results of oxygen absorption from air in various liquid media within a rectangular milli-channel equipped with exchangeable staggered herringbone-like mixing inserts. We have determined the volumetric liquid-side mass transfer coefficient kLa using three different liquid substances (water, ethanol, and polypropylene glycol) in a range of superficial liquid velocities of 0.047–0.210 m s–1 and superficial gas velocities of 0.012–0.210 m s–1. From this data, we derived an empirical kLa-correlation. Overall, kLa values ranging from 0.036–0.515 s–1 were obtained, and the correlation predicts kLa with a coefficient of determination of R2 = 0.93. We have also examined the influences of changing the static mixer design parameters on the mass transfer performance and evaluated the mixer efficiency by taking the power consumption into account. Depending on liquid properties, the right choice of a mixer design leads to a significant improvement in mass transfer and/or efficiency.

Mass transfer intensification of slug flow by interfacial deformation at low flow rate in the microchannels with periodic expansion units

Liquid-liquid slug flow has great application potential in many fields. The effective intensification of mass transfer is an important impetus for its wider application. However, it still lacks a method which can intensify its mass transfer at low flow-rate without slug breakup. Hereon, the periodic expansion microchannel is used to induce the interface deformation of the droplets in slug flow to strengthen mass transfer. Concentration field obtained by experimental analysis, and velocity field obtained through numerical simulation are used to investigate the influence of expansion structure on the mass transfer mechanism of liquid–liquid slug flow. The regulating mechanism of mass transfer is disclosed by investigating the effect of operating conditions on the volumetric mass transfer coefficient kLa and intensification factor E. Optimized E can reach up to 1.5. Introducing expansion structures could intensify the mass transfer of slug flow in a microchannel with low energy consumption.

Gas holdup, bubble size distribution, and mass transfer in an airlift reactor with ceramic membrane and perforated plate distributor

AbstractThe airlift reactor is one of the most commonly used gas–liquid two‐phase reactors in chemical and biological processes. The objective of this study is to generate different‐sized bubbles in an internal loop airlift reactor and characterize the behaviours of the bubbly flows. The bubble size, gas holdup, liquid circulation velocity, and the volumetric mass transfer coefficient of gas–liquid two‐phase co‐current flow in an internal loop airlift reactor equipped with a ceramic membrane module (CMM) and a perforated‐plate distributor (PPD) are measured. Experimental results show that CMM can generate small bubbles with Sauter mean diameter d32 less than 2.5 mm. As the liquid inlet velocity increases, the bubble size decreases and the gas holdup increases. In contrast, PPD can generate large bubbles with 4 mm < d32 < 10 mm. The bubble size and liquid circulation velocity increase as the superficial gas velocity increases. Multiscale bubbles with 0.5 mm < d32 < 10 mm can be generated by the CMM and PPD together. The volumetric mass transfer coefficient kLa of the multiscale bubbles is 0.033–0.062 s−1, while that of small bubbles is 0.011–0.057 s−1. Under the same flow rate of oxygen, the kLa of the multiscale bubbles increases by up to 160% in comparison to that of the small bubbles. Finally, empirical correlations for kLa are obtained.

Liquid-liquid extraction using combined hydrophilic-hydrophobic emulsion templated macroporous polymer micromixer-settlers

Continuous liquid-liquid extraction and separation of 4-aminoacetophenone, a product of the hydrogenation of 4-nitroacetophenone, was performed within emulsion templated macroporous polymer (polyHIPE) extraction units combined with a miniaturized gravity-based settler. PolyHIPEs with interconnected and tailorable macroporous structures are effective micromixers allowing to mix fluids in both axial and radial directions. We fabricated extraction units by combining hydrophilic and hydrophobic polyHIPEs, which improved the extraction efficiency by inverting the liquid/liquid dispersion type from oil/water to water/oil (or vice versa) during the extraction. The dispersion type is governed by the wettability of the porous medium. The extraction efficiency of our combined polyHIPE micromixer reached 98%, while that of control experiments performed using a blank tube or commercial Kenics® static mixer was 78%. The overall volumetric mass transfer coefficient kLa in polyHIPE micromixer-settlers was significantly higher 0.011 s−1 reaching interfacial areas a of 17900 m2/m3, much larger compared to a blank tube (kLa = 0.0035 s−1 and a = 5700 m2/m3) and static mixer (kLa = 0.0041 s−1 and a = 6800 m2/m3). PolyHIPE micromixer-settlers could be potentially useful to intensify continuous L-L extractions.

Unified Approach for Prediction of the Volumetric Mass Transfer Coefficients in a Homogeneous and Heterogeneous Bubble Column Based on the Non-Corrected Penetration Theory: Case Studies

A critical review on the improvement of penetration theory is presented in this work. The volumetric liquid-phase mass transfer coefficients kLa in seven different liquids (1-butanol, 2-propanol, anilin, decalin, nitrobenzene, tetralin, and ethylene glycol) aerated with air in a small bubble column (BC) (inner diameter: 0.095 m) were measured at ambient conditions and further analyzed. It was found that the kLa values can be predicted satisfactorily on the basis of the classical Higbie’s penetration model. The gas–liquid contact time was defined as the ratio of the Sauter-mean bubble diameter to bubble rise velocity. Moreover, the experimental kLa values were well predicted, not only in the homogeneous regime, but also in the transition and heterogeneous regimes. This is a new finding, since to date, it was considered that the penetration theory needs a correction factor for a successful application to any liquid, even in the homogeneous regime. The predictions of the mass transfer coefficients kLa in the above-mentioned seven liquids imply that the mean bubble diameters are always ellipsoidal or spherical, which is the key condition for the applicability (without a correction) of penetration theory. In the presented (in this work) model-based kLa predictions, the Sauter-mean bubble diameters were estimated by means of the reliable correlation of Wilkinson et al., which always predicts a gradually decreasing bubble size at higher gas velocities.

Gas mass transfer in syngas fermentation broths is enhanced by ethanol

In gas fermentations (using O2, CO, H2, CH4 or CO2), gas-to-liquid mass transfer is often regarded as one of the limiting processes. However, it is widely known that components in fermentation broths (e.g., salts, biomass, proteins, antifoam, and organic products such as alcohols and acids) have tremendous impact on the volumetric mass transfer coefficient kLa. We studied the influence of ethanol on mass transfer in three fermentation broths derived from syngas fermentation. In demineralized water, we observed that the addition of ethanol, the expected product, increased kLa two-fold in the 0–5 g L−1 range, after which near-constant kLa values were obtained. In the fermentation broths, kLa was increased significantly (2–4 fold compared to water) by ethanol supplementation, and to be highly influenced by broth salinity. Our results indicate that kLa is a dynamic parameter in gas fermentation experiments and can be significantly increased due to broth components.

A comprehensive comparison of mixing and mass transfer in shake flasks and their relationship with MAb productivity of CHO cells

The selection of highly recombinant protein (RP)-productive Chinese hamster ovary (CHO) cell lines is widely carried out in shake flasks. It is assumed that increases in the operating parameters in shake flasks lead to impairments in cell growth and RP production. These effects in cells metabolism are widely associated with high mass transfers and hydrodynamic stress. This study examined the impact of commonly used operational parameters on growth and specific productivity (qP) of two CHO cell lines differentially secreting a humanized anti-hIL8 monoclonal antibody (mAb) and cultured in 250ml flasks. The evaluated parameters are filling volume (10, 15, and 20%), shaking frequency (60 and 120 revolutions per minute -rpm-), and orbital diameter (25.4 and 19mm). The analysis of the oxygen transfer was done in terms of the measured volumetric mass transfer coefficient (kLa) and of the hydrodynamics in terms of power input per unit volume of liquid (P/V), the turbulent eddy length scale measured by the Kolmogorov's microscale of turbulence, the energy dissipation rate, the average shear stress, and the shear rate. Though almost all measured kinetic and stoichiometric parameters remained unchanged, mAb titer included, significant differences were found in maximum cell concentration, 10-45% higher in conditions with lower values of kLa and P/V. Changes in glucose metabolism contributing to qP were only shown in the higher producer cell line. Non-lethal responses to elevated oxygen transfer and shear stress might be present and must be considered when evaluating CHO cell cultures in shake flasks.

Influence of Sparger Type on Mass Transfer in a Pilot-Scale Internal Loop Airlift Reactor

In a pilot-scale internal loop airlift reactor with a height of 5.5 m and a main column diameter of 0.484 m, the influence of three gas sparger structures (ladder distributor, tri-nozzle sparger and perforated plate) on the volumetric mass transfer coefficient kLa was investigated. It was found that the perforated plate produces the highest gas holdup difference and circulating liquid velocity between the riser and the downcomer. The perforated plate provides the most efficient mass transfer due to the more uniform gas distribution and higher circulating liquid velocity, followed by the ladder distributor and tri-nozzle spargers. Compared with the tri-nozzle sparger, the perforated plate increases the value of kLa by up to 16% at a superficial velocity of 0.15 m/s. Interestingly, the analysis of the liquid-phase mass transfer coefficient kL and specific area a with respect to gas velocity shows that the mass transfer rate is primarily controlled by a. By comparing the predictions of different mass transfer models, the slip velocity model based on penetration theory yields a satisfactory agreement with the experimental results within ±15% error. Meanwhile, empirical correlations regarding gas holdup and kLa were developed and were found to have good consistency with experimental values.

Gas Hold-Up and Mass Transfer in a Vessel with an Unsteady Rotating Concave Blade Impeller

The steady mixing of gas-liquid systems is used where a large development of the interfacial area is required. However, the presence of gas in the liquid reduces the efficiency of mass transfer by reducing the mixing power, due to the creation of gas formations behind the impeller blades and the reduction in density. The efficiency of mass transfer can be increased by using a concave blade impeller or unsteady mixing. Mass transfer efficiency studies for these impellers and unsteady mixing are limited. This paper presents an analysis of the influence of the impeller construction on the gas hold-up and volumetric mass transfer coefficient kLa. Impellers with a different number of concave blades, and with alternatively arranged concave blades, were analyzed. The obtained results were compared with the standard flat blade turbine. The obtained results indicate that the arrangement of the concave blades has the greatest effect on reducing the gas hold-up and kLa. Higher values were obtained for the four-bladed and six-bladed impellers. A comparison of the gas hold-up rate for the unsteady and steady mixing has shown that for steady mixing greater gas hold-up is achieved. The volumetric mass transfer coefficient for unsteady mixing is also greater compared to steady mixing, indicating greater efficiency in mass transfer.

Influence of counterdiffusion effects on mass transfer coefficients in stirred tank reactors

Accurate knowledge of volumetric oxygen mass transfer coefficients in multiphase systems is important for successfully designing and efficient operation of process plants. As a result, a large number of papers on the determination of volumetric mass transfer coefficients kLa have been published in the literature. The dynamic degassing method is probably the most common method for determining the volumetric mass transfer coefficient. However, little work is known on the influence of countercurrent diffusion effects, arising from further dissolved components, on the volumetric mass transfer coefficient.For this reason, the effect of countercurrent diffusion is investigated in the present work by using different stripping gases to determine the volumetric oxygen mass transfer coefficient. Furthermore, the effect of counterdiffusion is investigated for both conventional aeration and aeration with microbubbles.The present work shows that the choice of stripping gas has a decisive influence on the determination of the volumetric oxygen mass transfer coefficient. Moreover, it can be shown that this effect is directly proportional to the solubility of the stripping gas in the aqueous phase. Simultaneous measurements of bubble size distributions allow determining mass transfer coefficients kL. A model is developed describing the decrease in the mass transfer coefficient as a function of the solubility of the stripping gas.Furthermore, it can be shown that for the experimental determination of volumetric oxygen mass transfer coefficients, the choice of stripping gas should be adapted to secondary gas types occurring within real processes achieving a better comparability.

Hydrodynamics and Mass Transfer in a Concentric Internal Jet-Loop Airlift Bioreactor Equipped with a Deflector

The gas–liquid hydrodynamics and mass transfer were studied in a concentric tube internal jet-loop airlift reactor with a conical bottom. Comparing with a standard design, the gas separator was equipped with an adjustable deflector placed above the riser. The effect of riser superficial gas velocity uSGR on the total gas holdup εGT, homogenization time tH, and overall volumetric liquid-phase mass transfer coefficient kLa was investigated in a laboratory bioreactor, of 300 mm in inner diameter, in a two-phase air–water system and three-phase air–water–PVC–particle system with the volumetric solid fraction of 1% for various deflector clearances. The airlift was operated in the range of riser superficial gas velocity from 0.011 to 0.045 m/s. For the gas–liquid system, when reducing the deflector clearance, the total gas holdup decreased, the homogenization time increased twice compared to the highest deflector clearance tested, and the overall volumetric mass transfer coefficient slightly increased by 10–17%. The presence of a solid phase shortened the homogenization time, especially for lower uSGR and deflector clearance, and reduced the mass transfer coefficient by 15–35%. Compared to the gas–liquid system, the noticeable effect of deflector clearance was found for the kLa coefficient, which was found approx. 20–29% higher for the lowest tested deflector clearance.

Influence of Interfacial Force Models and Population Balance Models on the kLa Value in Stirred Bioreactors

Optimal oxygen supply is vitally important for the cultivation of aerobically growing cells, as it has a direct influence on cell growth and product formation. A process engineering parameter directly related to oxygen supply is the volumetric oxygen mass transfer coefficient kLa. It is the influences on kLa and computing time of different interfacial force and population balance models in stirred bioreactors that have been evaluated in this study. For this investigation, the OpenFOAM 7 open-source toolbox was utilized. Firstly, the Euler–Euler model with a constant bubble diameter was applied to a 2L scale bioreactor to statistically examine the influence of different interfacial models on the kLa value. It was shown that the kL model and the constant bubble diameter have the greatest influence on the calculated kLa value. To eliminate the problem of a constant bubble diameter and to take effects such as bubble breakup and coalescence into account, the Euler–Euler model was coupled with population balance models (PBM). For this purpose, four coalescence and five bubble breakup models were examined. Ultimately, it was established that, for all of the models tested, coupling computational fluid dynamics (CFD) with PBM resulted in better agreement with the experimental data than using the Euler–Euler model. However, it should be noted that the higher accuracy of the PBM coupled models requires twice the computation time.