Volumetric Gas-liquid Mass Transfer Coefficient Research Articles

Effects of internals on macroscopic fluid dynamics in a bubble column

The effects of internals on liquid mixing and gas–liquid mass transfer have rarely been investigated in bubble columns, and the commonly used measurement method overestimates significantly overall gas holdup. Firstly, gas holdup measurement method is improved by conducting multi-point liquid level measurement and using net fluid volume instead of bed volume to calculate gas holdup. Then, a stable conductivity method for liquid macromixing has been established by shielding large bubbles using #16 nylon mesh. Subsequently, the influences of internal coverage (=12.6%, 18.9% and 25.1%) on macroscopic fluid dynamics in a bubble column with a free wall area are systematically investigated. It is found that the presence of internals has a notable effect on macroscopic fluid dynamics. The overall gas holdup and gas–liquid volumetric mass transfer coefficient decrease, and the macromixing time decreases with the increase of internal cross-sectional area coverage. These are mainly caused by the uneven distribution of airflow due to the low resistance in the free wall area. This design makes maintenance easier, but in reality, the reactor performance has decreased. Further improvements will be made to the reactor performance based on such a configuration through flow guidance using baffles.

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.

Unlocking the Potential of Nanobubbles: Achieving Exceptional Gas Efficiency in Electrogeneration of Hydrogen Peroxide.

The electrogeneration of hydrogen peroxide (H2 O2 ) via the oxygen reduction reaction is a crucial process for advanced water treatment technologies. While significant effort is being devoted to developing highly reactive materials, gas provision systems used in these processes are receiving less attention. Here, using oxygen nanobubbles to improve the gas efficiency of the electrogeneration of H2 O2 is proposed. Aeration with nanobubbles is compared to aeration with macrobubbles under an identical experimental set-up, with nanobubbles showing a much higher gas-liquid volumetric mass transfer coefficient (KL a) of 2.6 × 10-2 min-1 compared to 2.7 × 10-4 min-1 for macrobubbles. Consequently, nanobubbles exhibit a much higher gas efficiency using 60% of O2 delivered to the system compared to 0.19% for macrobubbles. Further, it is observed that the electrogeneration of H2 O2 using carbon felt electrodes is enhanced using nanobubbles. Under the same dissolved oxygen levels, nanobubbles boost the reaction yield to 84%, while macrobubbles yield only 53.8%. To the authors' knowledge, this is the first study to investigate the use of nanobubbles in electrochemical reactions and demonstrate their ability to enhance gas efficiency and electrocatalytic response. These findings have important implications for developing more efficient chemical and electrochemical processes operating under gas-starving systems.

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.

Magnetic-field-induced simultaneous charge separation and oxygen transfer in photocatalytic oxygen activation for algae inactivation

Photocatalytic oxygen activation is an excellent strategy for algae control in water. However, the fast recombination of photogenerated charge and slow rate of oxygen transfer limit the reactive oxygen species generation efficiency for algae inactivation. Herein, to solve above issues, magnetic field was introduced to the BiO2−x/Bi3NbO7 system to effectively covert oxygen into reactive radicals. The electrochemical experiment and DFT calculation results indicated the charge separation could be accelerated by the Lorentz force generated by the magnetic field, resulting in increase of electron concentration. Meanwhile, the value of volumetric gas-liquid mass transfer coefficient was increased by 59.79 % with magnetic field, thus more oxygen could be reduced to superoxide radical. Photocatalytic algae inactivation rate by BiO2−x/Bi3NbO7 with magnetic field could be increased by 2.07 times than that without magnet filed. This work further extends the strategy of using magnetic field to simultaneously facilitate the charge separation and oxygen transfer rate.

Effect of particles on hydrodynamics and mass transfer in a slurry bubble column: Correlation of experimental data

AbstractThe effects of particle concentration and size on hydrodynamics and mass transport in an air–water slurry bubble column were experimentally studied. When the particle concentration αs increased from 0% to 20%, the averaged gas holdup decreased by ~30%, gas holdup of small bubbles and gas–liquid volumetric mass transfer coefficient decreased by up to 50%, while the gas holdup of large bubbles increased slightly. The overall effect of particle size was insignificant. A liquid turbulence attenuation model which could quantitatively describe the effects of particle concentration and size was first proposed. Semi‐empirical correlations were obtained based on extensive experimental data in a wide range of operating conditions and corrected liquid properties. The gas holdup and mass transfer coefficient calculated by the correlations agreed with the experimental data from both two‐phase and three‐phase bubble columns, with a maximum error <25%.

Circulating jet for enhancing the mass transfer in a gas–liquid stirred tank reactor

AbstractThere are some problems in gas–liquid stirred tank reactor (STR), such as low mass transfer efficiency, high exhaust gas oxygen concentration, and low product conversion rate, due to limitations of stirring speed and input power. This article summarizes the effect of adding circulating jet internals on bubble size distribution, overall gas holdup, gas–liquid volumetric mass transfer coefficient, and power per unit volume of the STR. When circulating jet is added, the average bubble Sauter mean diameter in the STR is reduced to 1.26 mm, and the overall gas holdup is increased to 8.23%, which is an increase of 3.62 times of the original STR. The gas–liquid volumetric mass transfer coefficient is increased to 0.05556 s−1, which is 4.84 times of the original STR. The power per unit volume is increased by only 1.4 times. These data provide references for the design and scale‐up of new jet STRs.

Mass transfer in a single-use angled-shaft aerated stirred bioreactor applicable for animal cell culture

The single-use stirred bioreactors are increasingly recognized as a viable alternative in animal cell culture due to the higher production capacity, increased flexibility, prevention of cross contamination, reduction of the cleaning cost, and shortened downtime. In this paper, the effects of the impeller speed, the volumetric gas flow rate, and the impeller type on the volumetric gas-liquid mass transfer coefficient in a single-use unbaffled angled-shaft bioreactor applicable for the animal cell culture were analyzed and compared with those attained for the baffled vertical-shaft bioreactors. The volumetric gas-liquid mass transfer coefficient (KLa) was experimentally determined by the simplified dynamic pressure method. The lowest P/V values as well as the highest KLa values were achieved for an unbaffled angled-shaft bioreactor. It was found that the Rushton impeller was the most efficient impeller in term of mass transfer for this bioreactor. An empirical correlation for the mass transfer coefficient was also developed.

Hydrodynamics, mixing and mass transfer in a pilot-scale bubble column with dense internals

Bubble column reactors with exothermic reactions are often equipped with dense tube bundle heat exchangers. While there is some knowledge about the impact of such internals on hydrodynamics and mass transfer for narrow columns, its role in pilot-scale columns is less clear. In this paper we report on a study of hydrodynamics and mass transfer in a BCR of 4.2 m height and 0.392 m diameter. We investigated different tube arrangements with triangular and square pitch and tube diameters of 32 × 10−3 m and 45 × 10−3 m at the same cross-sectional coverage (∼25%). The column was operated at homogeneous and heterogeneous flow conditions. A customized three-layer wire-mesh sensor was utilized to visualize gas phase dynamics and liquid mixing characteristics in the column’s cross-section. We found that sub-channel size is the most crucial geometric design parameter. Tracer mixing experiments revealed that internals reduce the mixing time due to the induction of large-scale liquid circulation. Mass transfer was studied with the oxygen stripping method. Here we found, that the effect of the internals on the gas-liquid mass transfer is almost negligible. Eventually, correlations for gas holdup, axial liquid dispersion and the volumetric gas-liquid mass transfer coefficient are given, which take the internals’ geometry into account.

Gas to liquid mass transfer in mixing system with application of rotating magnetic field

This study reports a new multiphase device, the mixer equipped with the rotating magnetic field (RMF) generator, which shows high rates of gas-liquid mass transfer in comparison to a convectional mixer with impeller. The volumetric gas-liquid mass transfer coefficient kLa in the tested apparatus with application of the RMF increases with increasing magnetic field and superficial gas velocity. It is shown that the mass transfer process under the RMF action has significantly higher values of the kLa coefficient than the conventional Rushton turbine or the synergistic effect of the rotational Rushton turbine and the RMF generator. The empirical relationships between this coefficient and the operational parameters obtained in this work can provide helpful guidance for design and operation of set-up equipped with the RMF or the mechanical mixer supported by RMF.

Bubble coalescence suppression driven carbon monoxide (CO)-water mass transfer increase by electrolyte addition in a hollow fiber membrane bioreactor (HFMBR) for microbial CO conversion to ethanol

This study investigated the effects of electrolytes (CaCl2, K2HPO4, MgSO4, NaCl, and NH4Cl) on CO mass transfer and ethanol production in a HFMBR. The hollow fiber membranes (HFM) were found to generate tiny gas bubbles; the bubble coalescence was significantly suppressed in electrolyte solution. The volumetric gas-liquid mass transfer coefficients (kLa) increased up to 414% compared to the control. Saturated CO (C∗) decreased as electrolyte concentrations increased. Overall, the maximum mass transfer rate (Rmax) in electrolyte solution ranged from 106% to 339% of the value obtained in water. The electrolyte toxicity on cell growth was tested using Clostridium autoethanogenum. Most electrolytes, except for MgSO4, inhibited cell growth. The HFMBR operation using a medium containing 1% MgSO4 achieved 119% ethanol production compared to that without electrolytes. Finally, a kinetic simulation using the parameters got from the 1% MgSO4 medium predicted a higher ethanol production compared to the control.

Gas-liquid mass transfer studies: The influence of single- and double-impeller configurations in stirred tanks

The influence of impeller structure on the mass transfer characteristics was studied with the steady-state method for gas-liquid volumetric mass transfer coefficient (k L a). The single-impeller configurations included eight impeller types (three radial flow impellers, four axial flow impellers and one mixed flow impeller), and the doubleimpeller included three configurations (RT+RT, RT+WH D , WH D +WH D ). For single-impeller, the gas-liquid mass transfer rates of radial flow impellers were better than those of axial flow impellers under the same rotation speed and gas flow rate. The mass transfer performance (defined as the volumetric mass transfer coefficient per unit power input) of radial flow impellers were also better than that of axial flow impellers. With the same kLa value under a certain gas flow rate, the local bubble size distribution between radial flow impeller and axial flow impeller was similar. As for double impellers, RT+RT provided the highest mass transfer rate under certain rotation speed and gas flow rate, while WH D +WH D gave the highest values of gas-liquid mass transfer coefficient with the same power consumption.

Influence of rotating magnetic field on gas-liquid volumetric mass transfer coefficient

Abstract The main objective of these experiments was to study the oxygen mass transfer rate through the volumetric mass transfer coefficient (kLa) for an experimental set-up equipped with a rotating magnetic field (RMF) generator and various liquids. The experimental results indicated that kLa increased along the magnetic strength and the superficial gas velocity. Mathematical correlations defining the influence of the considered factors on kLa were proposed.

Predicting hydrodynamic parameters and volumetric gas–liquid mass transfer coefficient in an external-loop airlift reactor by support vector regression

For modeling, design and scale-up of the airlift reactors, it is crucial to estimate hydrodynamic parameters and volumetric gas–liquid mass transfer coefficient for different flow regimes. Prediction of these variables had begun by applying empirical power low correlations and later evolved in use of the artificial neural networks (ANN) as the best option available in the literature. The objective of this study was to present the support vector regression (SVR) model that predicts the gas holdup, downcomer liquid velocity and volumetric gas–liquid mass transfer coefficient values in the external-loop airlift reactor better than ANN. Furthermore, to demonstrate the applicability of the SVR model, it was used on the different literature data sets with wide-ranging databanks. The statistical error analysis revealed that the proposed generalized SVR model had more precisely prediction than ANN with an average absolute relative error (AARE) of 2.17%, 1.32% and 9.64% for gas holdup, downcomer liquid velocity and volumetric gas-liquid mass transfer coefficient values, respectively.

Determination of volumetric gas–liquid mass transfer coefficient of carbon monoxide in a batch cultivation system using kinetic simulations

A mathematical model of microbial kinetics was introduced to predict the overall volumetric gas–liquid mass transfer coefficient (kLa) of carbon monoxide (CO) in a batch cultivation system. The cell concentration (X), acetate concentration (Cace), headspace gas (Nco and Nco2), dissolved CO concentration in the fermentation medium (Cco), and mass transfer rate (R) were simulated using a variety of kLa values. The simulated results showed excellent agreement with the experimental data for a kLa of 13/hr. The Cco values decreased with increase in cultivation times, whereas the maximum mass transfer rate was achieved at the mid-log phase due to vigorous microbial CO consumption rate higher than R. The model suggested in this study may be applied to a variety of microbial systems involving gaseous substrates.

A three-phase Robinson-Mahoney reactor as a tool for intrinsic kinetic measurements: Determination of gas-liquid hold up and volumetric mass transfer coefficient

The liquid hold-up, εL, as well as the volumetric gas-liquid mass transfer coefficient, kLa, have been measured in a bench-scale, gradientless, three-phase Robinson-Mahoney reactor at high temperature and pressure conditions using realistic feeds to verify its suitability for the acquisition of intrinsic chemical kinetics. kLa has been determined via the so called ‘dynamic gas absorption technique’. Even without stirring, the reactor internals cause already interaction between the gas and the liquid, resulting in a mass transfer coefficient amounting to 1.17×10−2s−1. Initially, a small increase in the agitator speed results in a moderate increase of kLa. However, once an agitator speed of 10rps is reached, kLa was found to increase more rapidly. This observation is captured in the following correlation of the volumetric gas-liquid mass transfer with the power input per volume: kLa=1.06×10−2(P/V)1.13+1.17×10−2. The trend as well as the order of magnitude of our results are similar to those reported by Pitault et al. at ambient conditions, albeit that the kLa in the absence of stirring is significantly higher and the variation with the agitator speed is less pronounced in our case. The liquid hold-up was investigated at ambient conditions using a H2-water mixture as well as at high temperature and pressure (HTP) conditions using a H2-Halpasol™ mixture by injecting an impulse of tracer or by imposing a step change in the liquid phase fed to the reactor, respectively. At HTP conditions, the temperature and total pressure, which were varying between 523–583K and 2.5–5.5MPa respectively, were found to only exert a minimal impact on the liquid hold-up, i.e., the liquid hold-up remained relatively constant at about 70%. A decrease to a liquid hold-up of about 50% was observed by changing the volumetric inlet gas-liquid ratio from 5 to 250m3NTPm−3 at these high temperature and pressure conditions. At ambient conditions, a change in the volumetric inlet gas-liquid ratio from 0 to 580m3NTPm−3 was necessary to achieve a similar reduction in liquid hold-up. This difference was attributed to differences in surface tension of the fluids used rather than in operating conditions such as temperature and total pressure. The experimentally observed liquid hold-up was higher compared to that obtained from vapour-liquid equilibrium calculations at similar volumetric inlet gas liquid ratio. The evolution of the liquid hold-up with increasing volumetric inlet gas-liquid ratio suggests the phase within the reactor changes from completely filled liquid with gas bubbles dispersed at low inlet gas-liquid ratio and potentially to continuous gas phase with entrained liquid droplets at very high inlet gas-liquid ratio.

Pilot-scale semi-continuous cultivation of microalgae Chlorella vulgaris in bubble column photobioreactor (BC-PBR): Hydrodynamics and gas–liquid mass transfer study

A 56L compressed-air aerated bubble column photobioreactor (BC-PBR) was used to cultivate Chlorella vulgaris under indoor conditions via semi-continuous cultivation method. The effects of hydrodynamics and gas–liquid mass transfer limitations towards the biomass production of C. vulgaris at different compressed-air aeration rates were studied. The optimum biomass production of microalgae was achieved at compressed-air aeration rate of 0.16VVM, with superficial velocity, UG of 0.185ms−1. The experimental volumetric gas–liquid mass transfer coefficient, kLaL(CO2), was 4.5×10−3s−1 under the optimum growth condition. Besides that, the physical properties such as density, viscosity and surface tension remained constant throughout the semi-continuous cultivation mode. Low biomass production was observed at the highest aeration rate (0.19VVM), which indicated that shear sensitivity has significant effect on microalgae growth in BC-PBR. Apart from that, the effect of shear stress on the growth of C. vulgaris was also reported, especially on the reproduction of microalgae cells. The semi-continuous cultivation system had shown the possibility to partially harvest microalgae biomass during the exponential growth phase and thus, shorten the overall cultivation time in the subsequent cycles (elimination of lag phase) which resulted in higher biomass production.

Volumetric gas-liquid mass transfer coefficient in an external-loop airlift reactor with inserted membrane

The effects of the inserted membrane in the downcomer of an external-loop airlift reactor, the gas sparger type (single orifice and sinter plate) and added alcohol (ethanol, n-butanol, or n-hexanol) on the volumetric gas-liquid mass transfer coefficient (kLa) were studied. Due to the presence of the membrane in the downcomer, kLa did not change significantly; the differences were smaller than 10%. The highest values of the kLa were obtained using the sinter plate. It was found that the addition of small amounts of alcohol increased the mass transfer. Using our experimental results and the data of other authors, the feed-forward back propagation neural network for prediction of kLa in external-loop airlift reactors with alcohol solutions was proposed.

Effect of ultrasonic irradiations on gas–liquid mass transfer coefficient (kLa); Experiments and modelling

Sonochemical reactors have proven to be very useful for intensification of various reaction systems. However, there is a lack of understanding in mass transfer mechanism under ultrasonication due to dependency of mass transfer on various parameters. The present work aims at investigating the effect of ultrasonic intensity on volumetric gas–liquid mass transfer coefficient, kLa, as a function of gas flow rate and temperature. Response surface methodology (RSM) coupled with central composite design (CCD) was used for design, statistical analysis and evaluation of the interaction between operational parameters. The maximum value of kLa was found to be 0.0128s−1 in an optimum range of ultrasonic intensity between 320 and 360W, though gas flow rate was the most influential parameter for kLa. In the next part of the study, a model was developed based on ANFIS to map the input variables to the outputs. Satisfactory agreement was observed between the ANFIS predictions and experimental data.

Gas–liquid mass transfer studies in inverse fluidized bed biofilm reactor for the biodegradation of industrial effluent rich in phenolic compounds

Gas–liquid mass transfer studies were carried out in inverse fluidized bed biofilm reactor (IFBBR) for the biodegradation of phenol. Studies were done to analyze the effect of superficial air velocity (Ug), gas holdup (ɛg), and biofilm characteristics such as biofilm thickness, biofilm dry density, suspended and attached biomass concentration, and bioparticle density on oxygen transfer rate (OTR) and gas–liquid volumetric mass transfer coefficient (kLa) for different superficial air velocities and various particle sizes (2.9, 3.5, and 3.8 mm). Average OTR and kLa was found to be high [(OTR)avg. = 0.0159 min−1; (kLa)avg.= 1.8823 g/(L min)] for the particle size of 3.5 mm at the optimum superficial air velocity (Ugm) of 0.220 m/s which created high turbulence with smaller bubble size. Higher volumetric mass transfer coefficient and OTR resulted in higher percentage of Chemical Oxygen Demand (COD) removal (98%) and phenol degradation (100%) in IFBBR. The gas holdup dominated over the smaller size bubbles resulting in higher mass (oxygen) transfer rate. Thin, dense, and stable biofilm was produced at Ugm. Above Ugm, thickness of the biofilm was increased where the detachment force did not control the outgrowth of biofilm anymore and thus the kLa was found to be decreasing. © 2015 American Institute of Chemical Engineers Environ Prog, 35: 433–438, 2016