Gas-liquid Mass Transfer Performance Research Articles

Experimental study on gas-liquid flow regimes of coaxial mixers equipped with a Rushton/pitched blade turbine and anchor

In view of the close relationship between gas-liquid mass transfer performance and flow regime, the coaxial mixer was selected as the research object, and the effects of rotation mode, inner impeller diameter, gas flow rate, and viscosity on flow regimes were experimentally investigated. The results showed that the main growth interval of the mass transfer coefficient was the loading regime, and the critical complete dispersion regime was an efficient and economic operation condition. In this state, the co-rotation mode had a low-power advantage compared to the counter-rotation mode. The anchor speed and gas flow rate could improve mass transfer performance, but at the expense of higher power consumption. Although it was easier to achieve the complete dispersion regime with a larger diameter impeller, the mass transfer performance was reduced at this time. The viscosity had a significant negative influence on gas-liquid dispersion and mass transfer performance. In terms of impeller combinations, PBTD+Anchor and RT+Anchor performed well in low and high viscous systems, respectively. In addition, applying the artificial neural network model to estimate the Ncd for the coaxial mixer correlated with the studied parameters to recognize the complete dispersion regime.

NETmix technology as ozone gas injection system: Assessment of the gas-liquid mass transfer

This work proposes an innovative high-intensity mixer based on the NETmix technology to promote the contact between ozone (O3) gas and water. The NETmix consists of a network of unit cells comprising mixing cylindrical chambers interconnected by prismatic channels. Its exclusive geometry generates successive contacting and splitting of flows, boosting mass transfer rates and rapidly developing an O3-enriched liquid stream. The gas-liquid mass transfer performance of NETmix was evaluated by measuring the volumetric mass transfer coefficient (kLa), standard oxygen transfer rate, standard oxygen transfer efficiency, and energy expenditure parameters. A co-current continuous flow condition was used to introduce a gas stream (consisting of either O3/oxygen(O2) or synthetic air) and a liquid stream (water), whereby it was varied the direction of co-current fluid flow (downwards or upwards) and the injected gas (QG) and liquid (QL) flowrates. The downward flow of gas and liquid streams enhanced the mass transfer phenomenon by at least 43 %. Moreover, the kLa values increased with QL and QG, reaching 2.46 s−1 at a low specific power consumption (25 kW m−3). The NETmix showed kLa values at least one order of magnitude larger than conventional contactors, maintaining a high-energy utilization efficiency (up to 19.4 kgO3 kWh−1).

Numerical and experimental investigation of 3D flow field bipolar plates for PEMFCs by metal 3D printing

The bipolar plate is one of the core components of a proton exchange membrane fuel cell (PEMFC). Its flow field structure directly affects the drainage and heat dissipation performance of the cell, as well as the transmission and distribution of the reaction gas. Compared with conventional 2D flow fields, novel 3D flow fields can provide superior gas–liquid mass transfer performance, significantly improving the performance of PEMFCs. However, traditional manufacturing processes fail to meet the associated molding needs, due to the complex structure of such plates. This research aims to investigate the metal 3D printing process, mass transfer law, and efficiency of 3D flow field bipolar plates, focusing on the influence of structural parameters on the gas–liquid mass transfer performance, by combining CFD multi-phase flow numerical simulations and experiments. The results indicate that the use of a 3D stepped flow field enhances the convection effect in the vertical direction due to the increase in Z-directional inflow, thus promoting oxygen transfer and reducing the accumulation of water. In particular, better gas–liquid mass transfer performance is achieved when the number of steps is 4. At 0.55 V, the measured power density of the Stepped-IV cell reached as high as 6727 W/m2, which is about 26 % higher than the conventional parallel flow field. This study provides an important technical and theoretical reference for the design and preparation of novel 3D complex flow fields.

A convenient method for measuring gas-liquid volumetric mass transfer coefficient in micro reactors

The research on gas-liquid multiphase reactions using micro reactors is becoming increasingly widespread, given their excellent mass transfer performance. Establishing an accurate and reliable method to measure the gas-liquid mass transfer performance of micro reactors is crucial for evaluating and optimizing the design of micro reactor structure. In this paper, the physical absorption method of aqueous solution-CO2 and the chemical absorption method of sodium carbonate solution-CO2 were proposed. By analyzing the chemical reaction equilibrium during the absorption process, the relationship between the mass transfer of CO2 and the solubility of hydroxide ions in the solution was established, and the total gas-liquid mass transfer coefficient was immediately obtained by measuring the pH value. The corresponding testing platform and process have been established based on the characteristics of the proposed method to ensure fast and accurate measurement. In addition, the chemical absorption method takes into account temperature factors that were not previously considered. The volumetric mass transfer coefficient measured by these two methods is in the same range as those measured by other methods using the same microchannel structure in previous literature. The methods have the advantages of low equipment cost, faster measurement speed, and simpler procedures, which can facilitate its wide application to the evaluation of the mass transfer performance and hence can guide the structure optimization of microchannel reactors.

CO2 chemical absorption into AMP aqueous solution and mass transfer intensification in cascade sudden expansion microchannels

Carbon dioxide chemical absorption into 2-amino-2-methyl-1-propanol (AMP) aqueous solution and mass transfer enhancement in cascade sudden expansion microchannels were investigated by a high-speed digital camera. The influences of the flow rates of gas and liquid phases, the sudden expansion units and the concentration of AMP on the gas-liquid two-phase flow and mass transfer performance are studied systematically. Results showed that the increase of sudden expansion unit is conductive to increase liquid side volumetric mass transfer coefficient and enhancement factor. The maximal enhancement factor could reach to 1.33, and the maximally effective mass transfer efficiency could reach 1.6 under taking the energy consumption into account. The gas phase flow rate has slight effect on mass transfer enhancement factor E, while E decreases with increasing liquid phase flow rate. With the increase of AMP concentration, the volumetric mass transfer coefficient increases, but the enhancement factor decreases. A new correlation with good prediction performance was proposed for the volumetric mass transfer coefficient. Furthermore, the energy consumption and effective mass transfer efficiency were assessed.

Effects of blade structures on the dissolution and gas-liquid mass transfer performance of cup-shaped blade mixers

BackgroundA mixer with the advantages of excellent mixing performance and easy manufacturing is widely needed in industrial applications. In this work, the solid-liquid and gas-liquid mass transfer characteristics of a cup-shaped blade mixer, which is obtained by cutting a 90° stainless-steel elbow, are investigated. MethodsThe effects of blade structure, blade angle, blade height, baffle presence, surface tension, and gas holding on mass transfer characteristics were investigated by the conductivity probe and the dissolved oxygen dynamic method. Significant findingsResults show that the solid-liquid and gas-liquid mass transfer performance is enhanced as the inlet area of the cup-shaped blade is increased and the optimum blade angles are 22.5° and 15° respectively. The solid-liquid mass transfer performance can be improved when the baffles are removed. The gas-liquid mass transfer performance first deteriorates but then improves as the surfactant concentration is increased. Moreover, the mass transfer performance of the cup-shaped blade mixer is superior to that of the 45° -pitched blade and Rushton turbine. Meanwhile, the correlations of the solid-liquid and gas-liquid mass transfer coefficients are obtained, which would provide insight for the industrial design, optimization, and scale-up of cup-shaped blade mixers for multiphase systems.

Gas–liquid mass transfer in the gas–liquid–solid mini fluidized beds

The gas–liquid–solid mini fluidized bed (GLSMFB) combines the advantages of fluidized bed and micro-reactor, and meets the requirements for safety and efficiency of green development of process industry. However, there are few studies on its flow performance and no studies on its mass and heat transfer performance. In this paper, the characteristics of gas–liquid mass transfer in a GLSMFB were studied in order to provide basic guidance for the study of GLSMFB reaction performance and application. Using CO2 absorption by NaOH as the model process, the gas–liquid mass transfer performance of GLSMFB was investigated. The results show that the liquid volumetric mass transfer coefficient and the gas–liquid interfacial area both increase with the increase of the superficial gas velocity within the experimental parameter range under the same given superficial liquid velocity. At the same ratio of superficial gas to liquid velocity, the liquid volumetric mass transfer coefficient increases with the increase of the superficial liquid velocity. Fluidized solid particles strengthen the liquid mass transfer process, and the liquid volumetric mass transfer coefficient is about 13% higher than that of gas–liquid mini bubble column.

Microbubbles intensification and mechanism of wet air oxidation process of MDEA-containing wastewater

ABSTRACT In order to the intensification of gas–liquid mass transfer of MDEA-containing wastewater during wet air oxidation (WAO) process, the microbubbles and millimetre bubbles were applied by fine-pore sparger (5 and 20–30 μm) and single pore sparger (6.35 mm), respectively. Effect of the superficial gas velocity on the average microbubble size, gas holdup and oxygen mass transfer coefficient (KLa) of MDEA-containing wastewater at the ambient conditions was studied. The results showed that the microbubbles (less than 1 mm) were beneficial to enhance mass transfer process and had a higher dissolved oxygen concentration during WAO process of MDEA-containing wastewater owing to higher gas holdup and larger oxygen mass transfer coefficient. The COD removal ratio was 66% at low superficial gas velocity (ug = 0.3 cm/s) in WAO process by microbubbles, while it achieved at high superficial gas velocity (ug = 3.0 cm/s) by millimetre bubbles. The critical oxygen mass transfer coefficient KLa was 0.183 min−1 of MDEA-containing wastewater by 20–30 and 5 μm fine pore sparger, which was 2∼5 times more than that of single pore sparger (<0.1 min−1). The microbubbles could improve dissolved oxygen concentration and enhance the formation of hydroxyl radical at short time with atmospheric pressure. During the WAO process, the MDEA would be converted into intermediates including formic acid, acetic acid, ammonium, nitrite and nitrate. The WAO process with microbubbles could significantly improve the gas–liquid mass transfer performance at low superficial gas velocity and greatly reduce air consumption for MDEA-containing wastewater.

Mass transfer intensification mechanism of Al2O3 sphere packing in a rotating packed bed

In this work, the gas–liquid mass transfer performance of rotating packed bed (RPB) with Al2O3 sphere packing, in term of volumetric liquid-side mass transfer coefficient (kLae), was investigated by CO2-NaOH chemical absorption system. The adsorption performance of Al2O3 spheres was also evaluated by an ink adsorption method. Results showed that the kLae values of RPB with Al2O3 sphere packing were higher than those of RPB with nonporous sphere packing, and over one magnitude order higher than those of trickle bed reactor (TBR) with Al2O3 sphere packing. The ink adsorption, film diffusion and pore diffusion rates of Al2O3 spheres have been greatly improved under the high gravity field, and it has been found that the surface tension was the main driving force of ink adsorption. Moreover, the adsorption kinetics was investigated and the experimental data fitted well with the pseudo-first-order adsorption kinetics model.

A machine learning model for predicting the mass transfer performance of rotating packed beds based on a least squares support vector machine approach

Rotating packed beds (RPBs) have been widely noted due to its superior gas-liquid mass transfer performance compared to conventional packed bed for CO2 absorption. The overall volumetric gas-side mass transfer coefficient (KGa) is selected as one of the key parameters for the screening and evaluation of RPBs. Existing theoretical and semi-empirical models for the KGa are easy to be used but have a poor accuracy and generalization ability. In this paper, a machine learning model based on least squares support vector machine (LSSVM) is developed to predict the KGa more accurately for CO2-NaOH chemical absorption system in different types of RPBs. Unlike the conventional prediction models, the input parameters are selected by multiple correlation analysis in the model establishment. Then, the proposed model is comprehensively evaluated by using four evaluation indicators, including determination coefficient, mean relative error, Root mean square error and standard deviations. The results show that the proposed model has the prediction performance with R2 = 0.9808 and RMSE = 0.0055 for testing set. In addition, the model performance is compared with the multiple nonlinear regression and artificial neutral networks. The results show that the proposed model has a superior performance for predicting the KGa of CO2 absorption in RPBs.

Efficient Synthesis of α-FeOOH from Pickling Wastewater in Falling Film Tower and Its Kinetics.

An efficient way to synthesize α-FeOOH from pickling wastewater in a falling film tower was proposed for the first time. This method overcomes the shortcomings of the traditional air oxidation method, and its production efficiency is increased by 16 times. The purity of α-FeOOH synthesized from pickling wastewater can reach 96.3%, and the iron recovery rate is greater than 90%. At the same time, we have systematically studied its kinetics in the falling film tower. The reaction rate constant k at different temperatures was also determined with the activation energy Ea = 32.2497 kJ/mol and the pre-exponential A = 47.4132 s–1. In addition, based on the double-film theory, a corresponding macrokinetic model was established. Also, the Hatta number in the reaction system was obtained, which proved the excellent gas–liquid mass transfer performance in the falling film tower. This work provides a promising method for the efficient production of α-FeOOH and the recycling of pickling wastewater.

Investigation of gas-liquid mass transfer and power consumption characteristics in jet-flow high shear mixers

Experimental measurements were performed to systematically investigate the effects of operating conditions and structural configurations of the rotor-stator head on gas-liquid characteristics (volumetric mass transfer coefficient, gas hold-up and bubble dispersion) and power consumption of the jet-flow high shear mixer (HSM). Gas hold-up distribution, bubble dispersion and flow fields under different structural configurations were also analyzed and verified by the computational fluid dynamics (CFD). The results indicated that the gas-liquid mass transfer performance and power consumption in jet-flow HSM were significantly impacted by the structural parameters involving blade angle, blade arc and stator bottom opening. Increasing gas flow rate or reducing surface tension could enhance the gas-liquid mass transfer performance to some extent. In addition, gas hold-up distribution and bubble dispersion in the gas-liquid system were more uniform with the modified configuration of the rotor-stator head. The dimensionless correlations were established based on the experimental data to predict Pog and kLa in gas-liquid operation of the jet-flow HSM: Pog = 0.958⋅(sinθ)1.567(D/(Ds – Db))-0.164 with the standard deviation (σ) of 16%; ShL = 7.92 × 104⋅FlQ0.38Fr0.69Pog0.29 with σ of 17%. This work can provide useful guidance on scaleup design and optimization of the jet-flow HSM to intensify the transfer properties for gas-liquid operation systems.

Wetting Behavior of the Stainless Steel Wire Mesh with Al2O3 Coatings and Mass Transfer Intensification in a Rotating Packed Bed

An Al2O3-coated stainless steel wire mesh packing was prepared, aiming to serve as the monolithic catalyst support applied in a rotating packed bed (RPB) reactor. Before the application, the wetting behavior and gas–liquid mass transfer performance of Al2O3-coated wire mesh packing were investigated in this work. The results showed that the Al2O3-coated stainless steel fiber had on average 18.8% longer liquid spreading length compared with the uncoated stainless steel fiber. The gas–liquid effective interfacial area (ae) and the volumetric liquid-side mass transfer coefficient (kLae) of the RPB with Al2O3-coated wire mesh packing were on average 25.9 and 45.7% higher than those with uncoated wire mesh packing, respectively. The Al2O3-coated wire mesh packing has great prospects for the catalytic reactions in the RPB reactor by providing double functions of mass transfer intensification and the monolithic catalyst support.

In silico screening of venturi designs and operational conditions for gas-liquid mass transfer applications

Venturi ejectors are commonly used to facilitate gas-liquid mass transfer, but their mass transfer characteristics depend on various operational and ejector design parameters, which make experimental identification of optimal conditions cumbersome. A CFD model using a two-phase Eulerian approach with a free surface model to describe local venturi hydrodynamics was developed and used to characterize the influence of gas-liquid flow ratio and venturi design on venturi-mediated gas-liquid mass transfer in bulk liquid. When standardized to gas injection rate, computed venturi-local gas-liquid interfacial area showed same dependency on gas injection rate as experimentally determined kLa during aeration of a 1 m3 tank. The model consequently enables in silico characterization of the venturi flow regime, which is critical to the venturi’s mass transfer performance. An experimental factorial design study investigated the influence of venturi geometry on gas-liquid mass transfer and found that the venturi throat diameter is a critical design parameter. The computational model revealed that this was due to its significant impact on generated gas-liquid interfacial area. The ability to characterize venturi flow regimes and qualitatively predict the relative effect of process parameters on kLa by modelling local venturi dynamics rather than the complete reactor system greatly reduces computational complexity, suggesting the model’s usage as an efficient screening methodology to increase venturi gas-liquid mass transfer performance.

Enhancement effect and mechanism of gas-liquid mass transfer by baffles embedded in the microchannel

The enhancement of gas-liquid mass transfer by baffles embedded in a microchannel was investigated experimentally for CO2 absorption into monoethanolamine/1-butyl-3-methylimidazolium tetrafluoroborate (MEA/[Bmim][BF4]) aqueous solutions. The influences of baffle size on mass transfer rate and pressure drop were studied through varying the MEA/[Bmim][BF4] concentration ratio, gas-liquid flow rate and blockage ratio. Experimental results showed that the proper baffle configuration could improve effectively gas-liquid mass transfer performance. Although the pressure drop of the baffled microchannel has slight increase (the maximum growth 0.3 kPa, or 20% in relative term) in comparison with the unobstructed microchannel, it is acceptable for industrial application. Under lower MEA/[Bmim][BF4] concentration ratios, higher two-phase flow rates and blockage ratios, the mass transfer enhancement is more pronounced and the enhancement factor could reach to 1.5. A new empirical correlation of enhancement factor is proposed with good predictive performance.

Effect of liquid phase rheology and gas–liquid interface property on mass transfer characteristics in bubble columns

Gas–liquid mass transfer in non-Newtonian fluid systems is important in many chemical and biochemical processes. Having a good understanding of gas–liquid mass transfer contributes to achieving the optimization of design and the energy-efficient operation. The mass transfer performance in tap water and carboxyl methyl cellulose (CMC) aqueous solutions with different weight percentages (0.3wt%, 0.4wt%, 0.5wt% and 0.6wt%) were investigated experimentally. The CMC aqueous solution was shear-thinning non-Newtonian fluid. In this study the apparent viscosity of the solutions ranged from 0.01 to 0.1Pas. As expected, the liquid phase rheology had a great effect on the gas–liquid mass transfer performance. The volumetric mass transfer coefficients in CMC aqueous solutions were much smaller than those in tap water, and the value decreased from 3.04×10−3s−1 to 2.8×10−4s−1 with the increase of the weight percentage of CMC aqueous solutions. The mobility of the gas–liquid interface in tap water and CMC aqueous solutions was identified by the dimensionless diameter number. It was found that the mobile gas–liquid interface prevailed in tap water, while bubbles behaved as rigid particles in relatively high weight percentage of CMC aqueous solutions (0.5wt% and 0.6wt%). The liquid film mass transfer coefficient could be approximated by theories for mobile and immobile gas–liquid interfaces in tap water and CMC aqueous solutions, respectively. Based on the liquid phase rheology and the mobility of the gas–liquid interface, the dimensionless correlations were proposed to describe the mass transfer process in Newtonian and non-Newtonian fluids.

The effect of flow distribution on mass transfer of gas-liquid two-phase flow in two parallelized microchannels in a microfluidic loop

The gas-liquid two-phase flow distribution and mass transfer performance of CO2 absorption into 2-amino-2-methyl-1-propanol (AMP) with ethylene glycol (EG) in two parallelized microchannels in a microfluidic loop were investigated. Four flow patterns were observed: filtering flow, repartition flow, breakup flow and irregular flow. The results showed that the uniformity of flow distribution is jointly affected by bubble length at T-junction and the hydrodynamic feedback of the downstream channels. Meanwhile, the uniform flow distribution facilitates the mass transfer performance. Furthermore, a prediction model of liquid side volumetric mass transfer coefficient under breakup flow in parallel microchannels was established by considering the relative error Er characterizing the flow maldistribution.

Effect of the height‐to‐diameter ratio on the mass transfer and mixing performance of a biotrickling filter

AbstractBACKGROUNDBiotrickling filters (BTFs) are among the most widely used biological technologies for air pollution control. The pollutant removal rate in BTFs relies to a large extent on its gas–liquid mass transfer performance. Therefore, knowledge of the design parameters affecting the mass transfer and mixing performance of full‐scale BTFs is of paramount importance.RESULTSThis work showed that the height‐to‐diameter (H/D) ratio is an important parameter in the design of BTFs devoted to air pollution control. The H/D ratio significantly affected the mass transfer and mixing performance of the BTF with and without additional stirring in the holding tank. It was observed that under the same liquid velocity, the mass transfer coefficient (kLa) and the mixing time (tmix) can be optimized only by selecting an adequate H/D ratio. Additional stirring in the holding tank also affected kLa and tmix, this effect being different for each H/D ratio tested.CONCLUSIONSThe H/D ratio should be considered in the design of full‐scale BTFs since this parameter will significantly affect the mass transfer and mixing performance, regardless of the agitation conditions prevailing in the holding tank. As far as the authors know this is first report addressing the combined effect of the H/D ratio, liquid velocity and holding tank stirring on kLa and tmix in BTFs devoted to air pollution control. © 2017 Society of Chemical Industry

Gas-liquid reaction and mass transfer in microstructured coiled flow inverter

Microstructured coiled flow inverter (MCFI) as a helically coiled tubular device with 90° alternating bends provides enhanced radial mixing due to secondary flow (Dean vortices) in different planes. Liquid-liquid mass transfer characterization in MCFI revealed higher mass transfer rates compared to other capillary setups. However, the influence of Dean vortices and 90° bends on gas-liquid mass transfer has not been investigated and described yet. Different reactor setups, i.e. MCFI, straight capillary, helical coil, and bend reactor were fabricated from FEP tubes (ID=1mm). Gas-liquid mass transfer performance was investigated for a gas-liquid reaction system, i.e. cobalt (II) catalyzed air oxidation of sodium sulfite. Two-phase slug flow hydrodynamics and pressure drop were experimentally characterized, while the influence of operating and geometrical reactor parameters on conversion was investigated for reactor setups. MCFI offers up to 14% higher conversion in comparison to other capillary setups. Mixing within the liquid phase is enhanced by the formation of Dean vortices and additional direction change via 90° bends, which contribute to the internal diffusion in the liquid phase.

Improving mass transfer in an inclined tubular photobioreactor

The mass transfer and hydrodynamics of two outdoor tubular photobioreactor designs were compared, a Tredici-design near-horizontal tubular photobioreactor (NHTR) and an enhanced version of this reactor (ENHTR), for the purpose of improving algal growth via improved hydrodynamics. The enhancements included addition of vertical bubble columns at the sparger end and a larger degasser with a diffuser. Gas-liquid mass transfer and other performance measures were assessed for a range of gas sparging rates. The ENHTR modifications proved to be very successful, increasing oxygen stripping and carbon dioxide dissolution by 120–220 % and 0–50 %, respectively. There was an increase in axial mixing and a fourfold decrease in total mixing time. Experiments were conducted to determine that approximately 50 % of the mass transfer occurred in the vertical bubble columns, while 85–90 % of the mass transfer in the near-horizontal tubes occurred in the lower half of the tubes. These improvements can lead to increased algae productivity depending upon culture-specific parameters. The theoretical maximum productivity of a hypothetical algal culture would be 1.6 g m−2 h−1 in the NHTR, and we have previously achieved a maximum of 1.5 g m−2 h−1 growing Arthrospira at densities up to 7.5 g L−1 in this reactor. Due to enhanced mass transfer in the ENHTR, the predicted maximum productivity should increase to 4.75 g m−2 h−1. The potential for further improvements in productivity due to various additional enhancements is described.