Liquid-side Mass Transfer Coefficient Research Articles

Dynamic and interfacial properties for dichloromethane absorption in ionic liquids: Experimental and molecular dynamic insights

In this study, a strategy for dichloromethane (DCM) capture using ionic liquids (ILs) was systematically investigated from the perspective of dynamic analysis. The diffusion coefficient, Henry’s law constant (H), saturated absorption capacity, and overall volumetric liquid-side mass-transfer coefficient (KLao) of four types of ILs were obtained. Among all the ILs studied, tetrabutylphosphonium caproate ([P4444][C5COO]) demonstrated superior DCM absorption capabilities, and its corresponding H value (3.13 Pa m3/mol at 25 °C) was markedly lower than those of solvents documented in the literature. Furthermore, its KLao value was determined to be 2.77 × 10−3/s at 25 °C, which was measured in a customized cylindrical absorption bottle with a height of 150 mm and an inner diameter of 16 mm. The absorption bottle, fitted with a gas inlet tube having an inner diameter of 5 mm and positioned 5 mm above its base, utilized a simple bubbling technique to introduce gas into the liquid phase, eliminating the need for gas-dispersion equipment. Nevertheless, ILs with high affinities showed relatively high initial mass-transfer rates for DCM. Based on molecular dynamics (MD) simulations, the vapor–liquid interface properties, including mass and number density profiles, molecular orientation, and surface tension, were investigated. This revealed that in the pure [P4444][C5COO] system, the alkyl chain of the anion was oriented toward the gas phase, while the carboxyl group was oriented toward the bulk. The tilt angle of [C5COO]− with respect to the interface was within the range of 37° < θ′ < 70°. For the [P4444][C5COO]/DCM mixture, at the interface, the anion exhibits an enhanced ordered orientation, while the cation shows a more disordered orientation.

Experimental study of interfacial mass transfer from single CO2 bubbles ascending in stagnant water

The study focuses on the interfacial mass transfer from single CO2 bubbles with initial diameter in the range of 0.7−3.0 mm in a vertical column with stagnant deionized water. The objective is to enhance the understanding of the intricate phenomena involved in interfacial mass transfer by examining the individual effects of the liquid-side mass transfer coefficient and the bubble size. The liquid-side mass transfer coefficient is found in this study to be influenced by both the initial bubble diameter and on the bubble age, i.e., the time the bubble is exposed to the liquid influences the obtained liquid-side mass transfer coefficient. Existing literature suggests that bubbles of diameter <2−3 mm behave as rigid spheres with immobile interface and reduced internal gas circulation, which negatively impact the interfacial mass transfer. The results obtained from this study show that the maximum value of the liquid-side mass transfer coefficient is obtained for bubbles of diameter 2.1−2.3 mm. For bubbles with a mean diameter ≤1.4 mm, the mean liquid-side mass transfer coefficient decreases with decreasing mean bubble diameter. A Lagrangian model description is used with various correlations for the liquid-side mass transfer coefficients to perform numerical simulations of the change in bubble volume during the bubble rise. The existing correlations for the liquid-side mass transfer coefficient give very different simulation results and fail to accurately capture the transient evolution of the bubble as it exposes to the mass transfer phenomenon and hydrostatic pressure gradient. The correlation for the liquid-side mass transfer coefficient proposed by Clift et al. (1978) is modified in this work, and the experimentally obtained data are well predicted by the new modified correlation.

Effect of surfactant frequently used in soil flushing on oxygen mass transfer in micro-nano-bubble aeration system

In-site soil flushing and aeration are the typical synergetic remediation technology for contaminated sites. The surfactant presented in flushing solutions is bound to affect the aeration efficiency. The purpose of this study is to evaluate the effect of surfactant frequently used in soil flushing on the oxygen mass transfer in micro-nano-bubble (MNB) aeration system. Firstly, bio-surfactants and chemical surfactants were used to investigate their effects on Sauter mean diameter of bubble (dBS), gas holdup (ε), volumetric mass-transfer coefficient (kLa) and liquid-side mass-transfer coefficient (kL) in the MNB aeration system. Then, based upon the experimental results, the Sardeing’s and Frössling’s models were modified to describe the effect of surfactant on kL in the MNB aeration. The results showed that, for the twenty aqueous surfactant solutions, with the increase in surfactant concentration, the value of dBS, kLa and kL decreased, while the value of ε and gas–liquid interfacial area (a) increased. These phenomena were mainly attributed to the synergistic effects of immobile bubble surface and the suppression of coalescence in the surfactant solutions. In addition, with the presence of electric charge, MNBs in anionic surfactant solutions were smaller and higher in number than in non-ionic surfactant solutions. Furthermore, the accumulation of surfactant on the gas–liquid interface was more conspicuous for small MNB, so the reduction of kL in anionic surfactant solutions was larger than that in non-ionic surfactant solutions. Besides, the modified Frössling’s model predicted the effect of surfactant on kL in MNB aeration system with reasonable accuracy.

Heat and mass transfer of a novel CO2 desorption process integrated with highly turbulent catalytic heat exchanger

This was the first work that pioneered the development a novel catalytic CO2 desorption process using an agitated jacket vessel with a coil heat exchanger (JVC) into a bench-scale pilot plant for post-combustion carbon capture technology. The key parameters of heat and mass transport e.g., overall heat transfer coefficient, overall volumetric liquid-side mass transfer coefficient and the enhancement factor for catalyst effect, were experimentally investigated by varying the operating conditions, including temperature, stirring speed, and heating methods. The proposed investigations revealed that the utilization of the JVC was a promising technology for improving heat transfer and mass transfer performance in the desorption process due to the high turbulence flow and more favorable thermal distributions achieved by the JVC. In comparison to the conventional process with a fixed catalyst bed, the JVC demonstrated a remarkable approximately 30 % catalytic enhancement and a substantial 50 % reduction in catalyst demand. Additionally, other potential benefits and challenges of further practical operations were discussed in this paper. This research carried significant potential to contribute to the advancement of knowledge and offer valuable insights into the practicability of the catalytic desorption process for further commercial application in carbon capture. The integration of catalyst and JVC could potentially motivate the utilization of catalyst towards commercial-scale applications.

Prediction and optimization of liquid dispersion of monoethanolamine in a rotating packed bed for CO2 absorption

Rotating packed bed (RPB) has recently gained traction as a preferred technology for the absorptive removal of CO2. In this device, solvent dispersion is the most crucial hydrodynamic parameter that directly affects the liquid-side mass transfer coefficient. The current study focuses on the optimization of the dispersion of monoethanolamine (MEA), a commonly used CO2 absorption solvent, in the RPB. Computational fluid dynamics (CFD) model was developed and validated against the published experimental data. The validated model was used to investigate the effect of operational parameters such as solvent concentration, rotational speed of the packed bed, inlet velocity, and the contact angle of packing material on MEA dispersion. The liquid dispersion of MEA was modeled using response surface methodology (RSM), artificial neural network (ANN) and adaptive neuro-fuzzy inference systems (ANFIS). The comparison of the modeled data revealed that ANN has the least mean square error (0.16) and is, therefore, the most capable model to predict the liquid dispersion of MEA in the RPB. Based on the ANOVA analysis, the inlet velocity of MEA is found to be an insignificant parameter and, therefore, does not contribute significantly to the liquid dispersion of MEA in the RPB. At optimized operating conditions, the MEA dispersion increased from 5346 to 10485 m−1, thus enhancing the physical absorption of CO2 up to eight times. CFD coupled with ANN for the prediction and RSM for the optimization of liquid dispersion can be employed to optimize industrial-scale RPB for the effective absorptive removal of CO2.

Developing aqueous porous carbons for biogas upgrading

Developing novel sorbents is essential for biogas upgrading. In this study, mixed sorbents of aqueous porous carbons were developed to separate CO2 from the biogas, where the porous carbon with the developed micropore structure was identified as the most desirable constituent. Both thermodynamics and kinetics were studied experimentally, and Henry’s constant (KH) and the liquid-side mass-transfer coefficient (kL) of CO2 in the mixed sorbent as well as the selectivity of CO2/CH4 were obtained accordingly. Furthermore, the CO2 separation performance was evaluated with a proposed index, and the cost of biogas upgrading using the mixed sorbent was estimated and compared. The results showed that the porous carbon with the developed micropore structure led to better performance on KH and kL of CO2 in the mixed sorbent, and the mixed sorbent with 3.03 wt% porous carbon exhibited the best CO2 separation performance, reducing 36.2 % in cost compared to the current technologies.

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.

Gas/Liquid Operations in the Taylor-Couette Disc Contactor: Continuous Chemisorption of CO2

Gas/liquid contactors are widely used in chemical and biotechnological applications. The selection and design of bubble-column-type gas/liquid contactors requires knowledge about the gas distributor design to provide appropriate gas flow patterns. This study presents the continuous chemisorption of CO2 in 0.1 molar sodium hydroxide solution in a counter currently operated gas/liquid Taylor-Couette disc contactor (TCDC). This vertical-column-type contactor is a multi-stage agitated gas/liquid contactor. The performance of a lab-size TCDC contactor in gas/liquid mass transfer operations was investigated. The apparatus design was adjusted for gas/liquid operations by installing perforated rotor discs to provide a rotational-speed-dependent dispersed gas phase holdup in the column. The parameters of dispersed gas phase holdup, volumetric mass transfer coefficient and residence time distribution were measured. In the first step, hydraulic characterization was performed. Then, the efficiency in gas/liquid operations was investigated by continuous neutralization of 0.1 molar sodium hydroxide with a gas mixture of 30 vol% CO2 and 70 vol% N2. Temperature, rotational speed and gas flow rate were varied. The desired pH value of pH 9 at the column outlet was kept constant by adjusting the sodium hydroxide feed. From the experimental results, the volume-based liquid-side mass transfer coefficient kLa was deduced in order to model the reaction according to the two-film theory over the column height. The CSTR cascade model fitted the experimental data best. The experimental results confirm stable and efficient reactive gas/liquid contact in the Taylor-Couette disc contactor.

Analysis of Reaction Kinetics of the Ozonation of Phenolic Compounds and Assessment of the Role of Mass Transfer in the Overall Rate

The ozonation technique for biorefractory phenolic wastewater is preferred as a clean technology. However, its effectiveness and successful implementation on the commercial scale is crucial. The understanding of the rate-determining step and reliable reaction kinetics is the essence to the design and scale-up of ozonation reactors. In the case of phenolic wastewater, it is found that both mass transfer and chemical reaction contribute to the overall reaction rates. However, the mass transfer resistance is often significant in studies that determine the ozonation rate constant. This has resulted in discrepancies in the reported values of the intrinsic reaction kinetic parameters, and such apparent rate constants cannot be useful for suitable reactor design. The aim of this work is to analyze past works on the reaction kinetics of the ozonation of phenol, chlorophenols, and nitrophenols and determine the role of mass transfer steps in the overall reaction rate. The value of the volumetric liquid-side mass transfer coefficient (kLa) is estimated from the overall observed rates using the theory of mass transfer with chemical reaction, and these are found to be appropriate for the reactor geometry used in the respective works. In this way, our appraisal of the literature has provided key insights, which are yet unfamiliar, and renders useful the large body of data published in the literature, which otherwise remains unutilized.

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.

Interface mass transfer and properties of bubbly flows in a column with Newtonian and non-Newtonian liquids

The mass transfer of gas–liquid systems is commonly reported through the volumetric mass transfer coefficient, kLa, which is a function of several complex phenomena. To optimize the rate of mass transfer, increased knowledge about the individual effects of the interfacial area, a, and the liquid-side mass transfer coefficient, kL, on kLa is necessary. In this study, kLa was measured by monitoring the dissolved oxygen concentration in a bubble column. The bubble flows were recorded by a photographic method and the images were analyzed by means of artificial neural network to determine the bubble size. The effects of rheology, superficial gas velocity, and gas sparger design were analyzed. kL decreased with an increase in the superficial gas velocity and with an increase in the viscosity. The relative change in a was much larger compared to the relative change in kL, and hence, for the investigated operational conditions and liquid solutions, the change in kLa was mainly attributed to the change in a. Bubble clusters were formed in the non-Newtonian solutions but for the given operating conditions and liquid solutions, the bubble cluster formation did not have a prominent effect on the mass transfer.

Hydrodynamics and gas-liquid mass transfer in an oscillatory flow reactor: Influence of liquid properties

Liquid properties, such as, surface tension and viscosity are important parameters as they control gas-liquid mass transfer in bioprocesses. An oscillatory flow reactor provided with smooth periodic constrictions (OFR-SPC) was considered to evaluate its potential for mass transfer performance in non-pure gas-liquid systems. The effect of surface tension and viscosity on the volumetric (kLa) and liquid-side mass transfer coefficients (kL), interfacial area, (a), gas holdup (εG) and bubbles’ dynamics were investigated under different operational conditions (oscillation amplitude (x0) and frequency (f) and superficial gas velocity (ug)). Two liquid phases, ethanol and sucrose aqueous solutions covering a range of surface tension and viscosity values were used. For the bubble size distribution (BSD) measurements an image analysis technique was used. A Design of Experiment (DoE) methodology was implemented in this work to establish the relation of x0, f, ug, surface tension and viscosity with kLa. According to the results, changes in the liquid properties and operational conditions showed marked effects on bubble’s size and mass transfer. However, surface tension and viscosity had no significant influence on εG, contrary to the reported for common contactors, where εG increased in the presence of ethanol and decreased at moderate/high viscosities. Moreover, it was found that increasing the oscillatory movement notably improved kL, and kLa (2–6-fold), either in ethanol or sucrose solutions, compared to common reactors, even with moderate power consumption (∼105 W m−3). This improvement resulted from the bubbles’ breakage, which originates bubbles with small and approximately the same size (homogeneous regime) enhancing a, instead, lower oscillations resulted in large bubbles (heterogeneous regime). The results demonstrate that the OFR-SPC can ensure outstanding mass transfer rates, with potential and feasibility for use in gas-liquid bioprocesses, where mass transfer and liquid properties are important.

Glucosamine for CO2 Capture: Absorption Kinetics, Promoted Absorption Rate, and Comparison with Other Amino Sugars

Amino sugars─sugars wherein the amine group substitutes the hydroxyl group─are renewable and ecofriendly CO2 separation solvents. In this work, the kinetics of CO2 absorption in the amino sugar glucosamine (GA) was studied. Absorption rates in aqueous GA solutions (0.1–1 M) were measured in a stirred cell reactor in the 303–313 K range. At T = 308 K, the value of the reaction rate constant for GA (0.1 M) was 27 M–1 s–1. The two-step zwitterion mechanism was employed for describing the reaction pathway. Some useful characteristics of GA solutions, such as density and viscosity, were measured. Using the modified Stokes law and the N2O analogy method, the diffusivity and solubility of CO2 in solutions were determined. The liquid-side mass-transfer coefficient was found to be 0.005 cm/s at 308 K. Three amine promoters were chosen to improve the reactivity of GA, viz., monoethanolamine (MEA), 2-amino-2-methyl-1-propanol (AMP), and piperazine (PZ). Mixtures of GA/PZ (2.5/0.5 M) were the most reactive. The reactivity of two further amino sugars, N-acetyl-d-glucosamine (NAG) and N-methyl-d-glucamine (NMG), was also studied, and a comparison with GA was provided. NMG was the most reactive with CO2, whereas GA was the least reactive. This work will draw attention to this new category of CO2-capturing solvents.

Kinetics study and performance evaluation of a hybrid choline-glycine/polyethylene glycol/water absorbent for CO2 separation

Thermodynamic and kinetic properties of absorbents are beneficial in evaluating their CO2 separation performance. In this study, the kinetic properties of CO2 in a hybrid choline-glycine/polyethylene glycol/water absorbent, including the liquid-side mass-transfer coefficient, enhancement factor, and reaction rate constant, were systematically determined through experimental measurements and data processing. Furthermore, an index referred to as “absorbility” was proposed to combine the kinetic properties determined in this study with the thermodynamic properties obtained in our previous study to evaluate the CO2 separation performance. Additionally, the regeneration performance of the hybrid absorbent was also conducted. The results show that the performance of the hybrid absorbent (30 wt% [Cho][Gly] + 10 wt% PEG200 + 60 wt% H2O) is comparable to that of aqueous monoethanolamine, and is thus promising for CO2 separation, considering its low regeneration temperature and low environmental impact.

Modelling of absorption process by seawater droplets for flue gas desulfurization application

• A new model for calculating the mass transfer coefficient between the droplet and the gaseous phase has been implemented. • The new model takes additional physical parameters such as surface renewal intensity. • With the newly-implemented model in 3D CFD simulations better results are achieved. Flue gas desulfurization technology for sulfur dioxide removal from exhausts of combustion processes is becoming more widespread as allowed emission levels keep getting lowered. Spray scrubbers have gained traction in coastal and maritime applications, where seawater can be used as a scrubbing liquid. Detailed numerical models are required for accurate replication of processes in these applications, with complex physical and chemical phenomena considered. Among them, mass transfer modeling between phases has proven to be particularly important for obtaining accurate simulation results. Present work investigates a new model for calculating liquid side mass transfer coefficient in falling droplets, which considers additional parameters such as surface renewal rate, Reynolds number, and droplet diameter, compared to simplified and more common approaches. Initial modelling was performed on a single droplet level, followed by the implementation in the computational fluid dynamics framework and expansion to the entire spray. Desulfurization efficiencies for real cases modelled with the new approach were compared with the experimental data and the previously used penetration theory model results. The newly implemented model investigated the influence of operational parameters such as water and gas flow, sulfur dioxide concentration, droplet size, and distribution on desulfurization efficiency. The results obtained by the new model showed the expected trend of increased efficiency with the water flow increase, as well as greater sensitivity to operational conditions in different cases. Furthermore, compared to the previously used model, the present work more accurately replicated removal efficiencies in simulations of seawater spray scrubber applications, which is beneficial in designing new and more efficient equipment.

Development of an innovative process for post-combustion CO2 capture to produce high-value NaHCO3 nanomaterials

• An innovative process using SG for CO 2 capture from flue gas was developed in aspen plus v.10. • The flue gas flow rate used in the process was 44.75 tph and contained 0.0023 mol% SO 2 and 13.33 mol% CO 2 . • The process was designed to remove all SO 2 , and capture 90 mol% of CO 2 from the flue gas to produce valuable NaHCO 3 nanoparticles. • The hydraulics and mass transfer of the gas-liquid systems were obtained and the CAPEX, OPEX, and LCOC of the process were calculated. • The process was able to capture 8.144 tph of CO 2 and produce 15.545 tph of NaHCO 3 nanoparticles. An innovative post-combustion process using aqueous sodium glycinates solutions (SGS) for CO 2 capture from a split flue gas stream emitted from the 600 MWe coal power plant, used in the Wolverine Clean Energy Venture (WCEV) project, was developed in Aspen Plus v.10. The flue gas flow rate used in the process was 12.43 kg/s (at 353.15 K and 101.33 kPa) and contained 0.0023 and 13.33 mol% of SO 2 and CO 2 , respectively. The overall process includes 5 main units designed to remove all SO 2 and capture more than 90 mol% of CO 2 in the flue gas stream, while producing high-value, salable sodium bicarbonate (NaHCO 3 ) nanomaterials to offset the total process costs. The hydraulics, mass transfer characteristics, and process performance obtained using Aspen Plus were discussed. Also, the capital expenditure (CAPEX), operating expenditure (OPEX), and Levelized cost of CO 2 capture (LCOC) were calculated to assess the feasibility of this process. The hydraulics in the SO 2 washing and CO 2 capture units showed a pressure drop of 12 and 1 kPa, respectively, and the behaviors of the liquid holdup and normalized packing specific wetted surface area were similar in both units. The gas-side mass transfer coefficients were orders of magnitude greater than the liquid-side mass transfer coefficients. The process was able to capture 2.352 kg/s of CO 2 and produce 4.486 kg/s of valuable NaHCO 3 nanomaterials. Also, the calculated CAPEX, OPEX, and LCOC of the process for a 30-year plant lifetime were ($4,450,552), (233.00 $/h) and (35.49 $/ton of CO 2 captured), respectively.

Developing hybrid 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide/titanium dioxide/water absorbent for CO2 separation

The development of novel absorbents is essential for improving CO2 separation technology. In this study, 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide/titanium dioxide/water ([Hmim][NTf2]/TiO2-H2O) was developed to separate CO2, where the thermodynamic and kinetic experiments were conducted, and Henry’s constant and the liquid-side mass-transfer coefficient were determined accordingly. Furthermore, CO2 separation performance in a bubble tower was validated. A previously proposed index named “absorption ability” (AA) was used to predict and compare the experimental results. Additionally, the cost of biogas upgrading (i.e., CO2 removal for biogas purification) using [Hmim][NTf2]/TiO2-H2O was estimated. The results showed that for the developed [Hmim][NTf2]/TiO2-based technology, the average CO2 mass-transfer rate was increased by 20.0% compared with the current commercialized technology, and the contributions from the thermodynamic and kinetic aspects were 2.5% and 17.5%, respectively. The cost of biogas upgrading was 16.6% lower. In addition, AA successfully predicted the performance of CO2 separation technologies, achieving an average relative deviation of 8.1%.

The influence of electrolytes in aqueous solutions on gas-liquid mass transfer in an oscillatory flow reactor

In bioprocesses, the microorganisms’ growth is ruled by physical, chemical, and biological conditions being the electrolyte concentration one of the most important parameters to control. In this study, the effect of three electrolytes (HCl, NaOH, NaCl) in aqueous solutions on the volumetric (kLa) and liquid-side (kL) mass transfer coefficients and bubbles’ dynamics in an oscillatory flow reactor provided with smooth periodic constrictions (OFR-SPC) under different operational conditions (oscillation amplitude, x0 and frequency, f and superficial gas velocity, ug) were studied. Electrolyte concentrations below and above the transition concentration (ct) of bubbles coalescence were evaluated. The results showed insignificant inhibition of bubble coalescence, regardless of electrolyte kind and concentration. Also, no significant variations were found in kLa, kL, εG, and d32 values compared with water. The three electrolytes behave similarly. Moreover, contrary to the widespread consensus in which the presence of electrolytes below the ct in aqueous systems reduces kL, in the present study in the OFR-SPC the kL results were identical to pure systems and higher than common gas-liquid contactors. According to the results, kLa was improved by increasing the oscillatory conditions and ug. The results indicate that oscillations and ug govern the hydrodynamics and mass transfer changes in aqueous electrolyte solutions. These results suggest that efficient mass transfer rates (up to 5-fold higher face to bubble columns and airlift) can be achieved with moderate power consumption, 100 W m−3, and, hence, enabling the OFR-SPC with potential for bioprocesses, replacing bubble columns and airlift bioreactors.

Liquid-phase mass-transfer processes in packings with a segregated flow model

Liquid back mixing behavior has significant influences on the liquid-side mass-transfer coefficient in packed columns and rotating packed beds(RPBs). And the liquid-side mass-transfer coefficient without the liquid back mixing behavior is called a liquid-side intrinsic mass-transfer coefficient. A ratio of the liquid-side mass-transfer coefficient to the liquid-side intrinsic mass-transfer coefficient is defined as a liquid-side mass-transfer coefficient factor. The lower coefficient factor means the bigger reduction in the liquid-side mass-transfer coefficient. A segregated flow model in packings is developed. From the model, the liquid back mixing effect is first separated from the liquid-side mass-transfer coefficient and is then involved in the coefficient factor, and equations of the liquid-side intrinsic mass-transfer coefficient and coefficient factor are derived. Values of the liquid-side intrinsic mass-transfer coefficient and coefficient factor of oxygen desorption from water in Flexipac 2 structure packing in a packed column were calculated. The values of the intrinsic mass-transfer coefficient were in good agreement of a deviation of 0.58 % with the predictions by theoretical mass-transfer model. Values of the coefficient factor of oxygen desorption from water in a RPB with wire mesh packing were also obtained. Values of the coefficient factor in the RPB rotor were much lower than those in the packed column due to a highly intensified mass-transfer and a higher liquid back mixing in the RPB rotor as compared with those in the packed column. The best method to increase the coefficient factor is to reduce the liquid back mixing and to increase the liquid flow rates in packed columns and RPBs.

Taylor bubble formation and flowing in a straight millimetric channel with a cross-junction inlet geometry Part II: Gas-liquid mass transfer

• Mass transfer was studied during bubble formation at a cross-junction. • Accuracy of resazurin-based technique was improved by advanced image systems. • Different mechanisms were observed for the developing O 2 concentration fields. • Oxygen concentration fields were measured during and after bubble pinch-off. • A scaling law relating k L a and f c was proposed to gather literature data. Using the resazurin-based colorimetric technique and advanced image acquisition, the equivalent oxygen concentration fields inside the liquid slugs were measured during and after the bubble formation stage at a cross-junction in a straight millimetric channel. Firstly, two different mechanisms were identified for the development of oxygen concentration fields depending on the two-phase Reynolds numbers (Re TP ). Under low Re TP , a jet-like central oxygen concentration ‘finger’ occurred between the newly formed bubble and the gas finger at the bubble pinch-off point. Right after the bubble pinch-off, the dissolved oxygen was transported first by the entering liquid from two side inlets, and later by the developing recirculation loop inside the liquid slug. Under higher Re TP , two highly concentrated oxygen concentration spots were formed near the bubble rear and the channel wall region, and a much more complex flow structure in the liquid slug appeared. Then, whatever the operating conditions, it was observed that the averaged oxygen concentrations inside the liquid slug followed a nearly linear relation as a function of the axial position in the channel, and that the related mass flux density decreased as far as the bubbles flowed along the channel’s length. The bubble formation process could be decomposed into three stages when considering the temporal evolution of the cumulated oxygen mass and depending on the development of the liquid slug. At last, the overall volumetric liquid side mass transfer coefficients ( k L a ) were deduced from the concentration fields, and found to linearly increase with the recirculation frequencies, leading to a scaling law.