Gas-liquid Mass Transfer Process Research Articles

Mass transfer and bubble hydrodynamics in stirred tank with multiple properties fluid via a CFD‐PBM method

AbstractThe performance of a stirred bioreactor was evaluated in this study in terms of the bubble hydrodynamics and the mass transfer efficiency, using a non‐viscous Newtonian fluid of water, a viscous non‐Newtonian fluid of xanthan, and a viscous non‐Newtonian fluids of xanthan with dispersed soybean powder, respectively. The computational fluid dynamics–population balance model (CFD‐PBM) method coupled with the viscosity model and the mass transfer model is established to simulate the gas–liquid mass transfer process and bubble size distribution in the stirred bioreactor. The results demonstrate that the rheological properties of the fluid play an important role in determining the gas holdup, the mass transfer efficiency, and the bubble size distribution. Viscosity of the fluid exhibits a negative impact on gas–liquid mass transfer rate and gas holdup. Moreover, by properly adjusting the operating conditions such as the stirrer speed, it is possible to modulate the gas dispersion and mass transfer rate in the reactor.

Effects of viscosity on hydrodynamics and mass transfer under a wire mesh-coupled solid particles method

Many practical industrial processes require gas–liquid mass transfer in highly viscous liquids, and liquid viscosity affects bubble characteristics and gas–liquid mass transfer. The current study investigated the effects of liquid viscosity on bubble dynamics and gas–liquid mass transfer via shadow imaging and dynamic oxygen dissolution methods, and the influence of fluid viscosity on the hydrodynamic effect when using a wire mesh-coupled solid particles method. The coupling strategy was associated with a bubble size regulation effect, with greater viscosity increasing the gas–liquid interface area by 27%–55% compared with unreinforced gas–liquid flow, which was superior to embedded wire mesh and added solid particles methods. Increased viscosity weakened the mass transfer enhancement effect of the coupling method, but the coupling method still effectively enhanced the gas–liquid mass transfer process, increasing the volumetric mass transfer coefficient (KLa) by 80%–130% compared to non-enhanced gas–liquid flow. Novel empirical KLa correlation equations were developed to predict the effects of the coupling method on gas–liquid mass transfer processes, and those equations exhibited good reliability and predictive capacity.

Exploring the multiphase flow and mass transfer characteristics for desulfurization in an upflow biogas reactor

With the increasing requirements for biogas desulfurization, the LO-CAT process design needed to be optimized to meet the production requirements. The purpose of this study was to establish a joint optimization model based on two-film theory and response surface methodology (RSM) to investigate the gas-liquid flow, mass transfer, and chemical reaction processes of the biogas reactor and to obtain the optimal operating conditions. The model was validated based on the measured data of the biogas reactor in a 355 KW desulfurization unit at the landfill. It was revealed that the optimal parameters for the gas-liquid flow field were a particle diameter of 3100 µm, a liquid/gas ratio of 19 L/m3, a nozzle angle of 55°, and an inlet velocity of 16 m/s. The effect of inlet velocity was most significant in the LO-CAT process due to its internal extended entrance. The synergistic modulation between gas and liquid then improved the homogeneity of the flow field through vortices, increasing flow field uniformity by 20.8%. The absorption of H2S was closely related to the change in flow pattern; the mass transfer process was mainly limited by the "liquid membrane limitation." Optimized parameter desulphurization efficiency increased by 28.76%. This study provided practical significance for the rational design and optimized operation of industrial biogas purification plants based on multiphase flow reaction transfer.

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.

Investigation on interphase mass transfer coefficient of a deformable bubble with density contrast using phase-field lattice Boltzmann method

The gas–liquid mass transfer process with density contrast is modeled in this paper using a phase-field Lattice Boltzmann (LB) method. In this method, the phase-field model used to calculate the flow field and the continuous species transfer model used to calculate the concentration distribution are coupled by introducing the divergence condition of the velocity in the presence of mass transfer into the Allen-Cahn equation. For a deformable bubble rising in stagnant liquid, a two-dimensional parameter dependence study is conducted and the results indicate that an increase in initial diameter, as well as a decrease in liquid diffusion coefficient or liquid viscosity, can improve the steady-state value for the Sherwood number, whereas the distribution coefficient has a little impact on this steady-state value. Furthermore, a correlation originally proposed by Takemura and Yabe (1998) modified with a shape factor can be used to predict the steady-state Sherwood number in the numerical results.

Research on carbon dioxide capture and mass transfer in hydro-jet cyclone

Steel smelting has a large amount of flue gas emissions and high carbon capture costs, making it one of the “heavy disaster areas” for carbon dioxide (CO2) emissions. This article develops a hydro-jet cyclone (HJC) by coupling the gas phase swirling flow field with the liquid phase jet flow field, and proposes a carbon capture technology for smelting flue gas. During the dispersion process of liquid phase microelements, there was simultaneous rotation and revolution of droplets, which enhances the gas–liquid mass transfer process. The surface renewal effect increases the effective specific interface area, and the interface oscillation effect was beneficial for reducing the mass transfer resistance on the liquid side. A model for the effective phase interface area (ae) of HJC was first proposed for different jet hole, with a maximum ae of 5358.95 m2·m−3.In the CO2-NaOH system, the absorption efficiency of CO2 was 88.9 %. This article supplements HJC's research on carbon capture and gas–liquid mass transfer by optimizing the size of the jet hole, which was beneficial for the development of efficient carbon capture equipment in the steel industry and further promotes the sustainable development of the steel industry.

Experimental and molecular dynamics simulation study of toluene absorption by nanofluids

It is well known that the emission of volatile organic compounds (VOCs) is a significant threat to human health and the ecological environment, and toluene is a typical representative of aromatic VOCs, whose elimination makes an important contribution to the prevention and control of air pollution. Absorption is the classical process to eliminate air pollutants, but the application of absorption is often constrained by absorbent and gas-liquid mass transfer. In recent years, process intensification has become an important development in the chemical industry, and nanotechnology can effectively reduce mass transfer resistance and improve gas-liquid mass transfer effectiveness. This work aims to evaluate the effect of adding nanoparticles on the gas-liquid mass transfer process. The effects of TiO2, Al2O3, and SiO2 nanoparticles on the removal of toluene by green solvent low transition temperature mixtures (LTTM) were investigated experimentally and theoretically. The mass transfer enhancement factors of the toluene absorption process were measured at different concentrations and sizes of TiO2, Al2O3, and SiO2 nanoparticles, absorption temperatures and number of cycles. Molecular dynamics (MD) simulations of the microscopic properties of LTTM and toluene were performed. Based on the MD simulation results, the toluene density distribution, interaction energy, radial distribution function, spatial distribution function, and self-diffusion coefficient were calculated, which helped to explain the absorption mechanism of toluene gas. The findings demonstrated that nanoparticles can alter the gas-liquid mass transfer process, and provided a method and theoretical reference for enhancing the toluene absorption process.

Process Intensification in Gas–Liquid Mass Transfer by Modification of Reactor Design: A Review

Process intensification is one way of developing new efficient production pathways for the chemical industry to overcome the increasing global energy demand and solve global energy and environmental problems. Process intensification plays an important role in the gas–liquid mass transfer processes. Herein, an overview of the development in gas–liquid mass transfer enhancement is provided. Classical mass transfer models such as two‐film theory, penetration theory, and surface renewal theory are analyzed. Major enhancement methods, namely, mechanical modification of reactor, are summarized and discussed here. The most recent accomplishments in gas–liquid mass transfer engineering are also provided. This review is expected to inspire new research on gas–liquid mass transfer engineering to enhance the future development and potential applications of gas–liquid mass transfer in scientific and industrial fields. Suggestions for the enhancement of gas–liquid mass transfer are also provided.

Study on the process intensification of superaerophobic materials to improve CO2 mass transfer efficiency

Chemical absorption is the most widely used method in capturing CO2 from flue gas, in which the gas-liquid mass transfer process is the control step of the absorption rate; the spherical structure and easy flow characteristics of bubbles in the liquid phase affect the gas-liquid mass transfer efficiency. This study carefully examines the effect of superaerophobic surfaces on the mass transfer efficiency of CO2 bubbles on both dynamic and static behaviors. The results show that the 0.5 mol/L MDEA solution can absorb CO2 bubbles within 70 s when the CA (Contact Angle) of the superaerophobic surface bubbles reaches more than 150°. However, after contacting the superaerophilic substrate, the mass transfer area between the gas and liquid phases increases instantly and the CO2 bubbles burst rapidly and form a gas film within 18 ms. After that, the absorption saturation time of the absorption tower using superaerophilic packing is 25% less than that using untreated packing. Therefore, the superaerophilic packing can significantly increase the gas-liquid contact area and the mass transfer efficiency.

New insights into elemental mercury removal from flue gas by a MoS2 heterogeneous system

Herein, MoS2 was used in a MoS2/H2O heterogeneous system for the heterogeneously catalyzed removal of Hg0 from flue gas. Hg0 removal efficiency reached 77.6% at 25 °C when the dosage of MoS2 was 0.200 g/L in aqueous suspension. The observed effects of MoS2 dosage and inlet Hg0 concentration on Hg0 removal efficiency revealed that the gas–liquid mass transfer process is the limiting step of Hg0 removal. Low pH and low temperature were conducive to Hg0 removal. SO2 in the flue gas was absorbed by MoS2, and the simultaneous decrease in solution pH resulted in a slight increase in Hg0 removal efficiency. The presence of NO in the flue gas enhanced Hg0 removal efficiency. Common anions, such as NO3−, SO42−, and CO32−, have no obvious effect on Hg0 removal. However, Cl− improved Hg0 removal efficiency by 11% because of the production of HgCln2−n. The kinetics and mechanism of Hg0 removal were studied, and the results indicated that Mo acted as a bridge and catalyst for Hg0 oxidation, while S immobilized Hg2+ by formation of HgS on MoS2.

Effect of ultrasonic waveforms on gas–liquid mass transfer in microreactors

AbstractThis study aims to investigate the effect of ultrasonic waveforms on the gas–liquid mass transfer process. For a given load power (P), continuous rectangular wave yielded stronger bubble oscillation and higher mass transfer coefficient (kLa) than continuous triangular and sinusoidal wave. For pulsed ultrasound, the kLa decreased monotonically with decreasing duty ratio (D), resulting in weak enhancement at low D (≤33%). For a given average load power (PA), concentrating the P for a shorter period resulted in a higher kLa due to stronger cavitation behavior. For a given PA and D, decreasing the pulse period (T) led to an increase in kLa, which reached a constant high level when the T fell below a critical value. By optimizing the D and T, a kLa equivalent to 92% of that under continuous ultrasound was obtained under pulsed ultrasound at a D of 67%, saving 33% in power consumption.

Phase-field lattice Boltzmann method for the simulation of gas-liquid mass transfer

The present work establishes a lattice Boltzmann (LB) method to simulate the gas-liquid mass transfer process. In this work, the flow field is solved by the conservative phase-field method proposed by Fakhari et al. (2016), which contains two LB equations to track the interface and calculate the pressure distribution. The concentration field, together with the concentration jump across the interface described by Henry’s law, is implemented by the continuous species transfer method through a LB equation. To couple these two methods, a source term for the phase-field equation is derived in the cases of static and moving interface. Four test cases containing both the steady and transient gas-liquid mass transfer processes with straight or curved interfaces are conducted to validate the proposed method. The results show that the present method is competent to describe the evolution of mass transfer coefficient in the gas-liquid mass transfer processes.

High-gravity deoxygenation of jet fuels using rotating packed bed

It has been proven that dissolved oxygen has fatal effect on thermal oxidation stability of jet fuels, although it is present in extremely low levels (70 ppm). In this paper, high-gravity deoxygenation method of jet fuels via rotating packed bed (RPB) was proposed to intensify gas–liquid mass transfer process. The deoxygenation efficiency of jet fuels was up to 99% (less than 1 ppm), and the liquid volumetric mass transfer coefficient (kLae) is 0.01 ∼ 0.06 s−1. The effects of operation parameters on deoxygenation efficiency and liquid volumetric mass transfer coefficient (kLae) were studied systematically, including gas–liquid ratio, rotating speed, liquid flow rate and packing structure. This method showed remarkable advantages in many aspects, such as superior mass transfer performance, extremely high deoxygenation efficiency, small device volume, very high process speed, low gas consumption and high operation stability. In addition, the mass-transfer model of high-gravity deoxygenation of jet fuel was established basing on the surface renewal theory, which considered the influences of gas flow and other operation parameters. The model is in good agreement with the experimental results, and the deviation is within ± 15%. This research is of great significance for the development of innovative deoxygenation technology of jet fuels.

Improved Model for Predicting Total Dissolved Gas Generation With the Residence Time of the Water in the Stilling Phase

Quantitative predictions of total dissolved gas (TDG) super-saturation are essential for developing operation schemes for high dams. Most TDG generation prediction models have various shortcomings that affect the accuracy of TDG super-saturation estimation, such as oversimplification of influencing factors and uncertainty in parameter values. In this study, the TDG generation process was divided into three parts, gas-liquid mass transfer process in the stilling phase, dilution resulting from the water jet plunging into the stilling phase, and outflow of TDG–super-saturated water from the stilling phase, while considering the water body and bubbles in the stilling phase as a whole. The residence time of the water in the stilling phase (Tr) was introduced to estimate mass transfer time, along with dimensional analysis methods. The properties of TDG generation were evaluated experimentally under varying Tr values. Based on the theoretical analysis and experimental results, a basic water renewal model was proposed and was validated using experimental data. Furthermore, prediction results of this model were compared with those of a classical empirical model and mechanical model based on observed data from a field survey at Xiluodu Dam. The results show that the relative errors between the predicted and experimental measurements were all less than 5%, indicating that the developed prediction model has a good performance. Compared with the mechanism model, the developed model could reduce the standard error (SE), normalized mean error (NME), and error of maximum (REMAX) by 60, 96, and 15%, respectively. Meanwhile, the developed model could reduce the SE, NME, REMAX by 17.4, 36, and 23%, respectively, compared with the empirical model. Considering all the error indexes, it can be concluded that the prediction performance of the water renewal model is the best among the three models. The proposed model was also more generically versatile than the existing models. Prediction results of water regeneration model for TDG could aid the drafting of governing strategies to minimize the risk of super-saturated TDG.

Process enhancement of organic pollutant removal by micro channel excited dielectric barrier discharge

In this study, a micro-channel excited DBD plasma water treatment system is proposed to enhance the discharge and gas-liquid mass transfer processes simultaneously. As the core of the reactor, the micro-channel plays a crucial role in the process intensification. On the one hand, the micro-channel can reduce the initial discharge voltage and enhances the discharge intensity, because of the increased space charge density initiated by the transverse electric field formed by the charges accumulated on the micro-channel wall, as a result, the generation efficiency of active particles can be improved. On the other hand, the micro-channel can also reduce the air pressure resistance and enhance the bubbling process, due to the exciting effect of micro-channel discharge on air flow and plasma-water interaction, as a result, the utilization efficiency of active particles can be improved. Tetrabromobisphenol-S (TBBPS) is used as the target pollutant, with the initial condition of 500 mL, 0.2 mmol/L, the maximum removal is over 98 % after a 30 min treatment under an applied voltage of 18.4 kV. Applied voltage, air flow rate and water flow rate are the key parameters affecting the treatment efficiency. It can be concluded from the results of this study that the micro-channel excited DBD plasma is of significance favorable for the treatment of wastewater.

Dual effects of long-chain alcohols in phase change capture of SO2: Increasing mutual solubility and decreasing product viscosity

To realize the goal of energy-saving SO2 capture process, a series of novel SO2 phase change absorbents (SPCAs) composed of 2-[2-(Dimethylamino)ethoxy]ethanol (DMEE)/long-chain alcohols/hexadecane were investigated in this work. The long-chain alcohols had dual effects in SPCAs, i.e., increasing mutual solubility of DMEE and hexadecane, and decreasing viscosity of the absorption products. Three alcohols, including butyl, hexyl and decyl alcohol, could turn the immiscible mixture of DMEE and hexadecane into homogeneous solution to form SPCAs. The decyl alcohol in absorption product could efficiently decrease the viscosity from 2338.1 mPa·s to 362.02 mPa·s, which is beneficial to gas-liquid mass transfer and fluid transport process. Meanwhile, the NMR and FTIR results indicated that DMEE chemically absorbed SO2, forming a zwitterionic alkyl sulfite compound. The long-chain alcohol formed hydrogen bonds with the alkyl sulfite and it didn't change the reaction mechanism. Noteworthy, the SPCA can absorb 0.823 mol SO2/mol DMEE (0.396 g SO2/g DMEE) at 293.15 K and 0.002 atm, which was exceptionally attractive. Furthermore, the absorbed SO2 could be partly released under vacuum at 110 °C and the net cycle capacity can reach 73% ~ 74% of the maximum capacity, which indicated the SPCAs have a promising application prospect.

Kinetics and mechanistic aspects of removal of heavy metal through gas-liquid sulfide precipitation: A computational and experimental study

The production of fine particles from extremely high supersaturation has challenged the application of sulfide precipitation in treating heavy metal wastewater due to the difficulty of solid-liquid separation. To this end, a gas-liquid sulfide precipitation reactor for the removal of Cu2+ was designed by controlling the mass transfer and supersaturation levels during sulfidation processes. Particularly, a computational fluid dynamics (CFD) model of the reactor, integrating sulfidation reaction kinetics with two-phase flow hydrodynamics, was first built, followed by examining the effects of H2S(g) bubble diameter and flow rate. Based on the CFD simulation, the rate-limiting step of the gas-liquid sulfide precipitation reaction is the gas-liquid mass transfer process. Either reducing H2S(g) bubble diameter or increasing H2S(g) flow rate can result in the control of reaction rate and supersaturation level in the system. In order to validate the CFD simulations, we measured Cu2+ concentrations during the sulfidation process with the batch experiments. The agreement between computational and experimental results indicated that our mechanistic model can provide a protocol for the design and optimization of the reaction system, allowing one to visualize the time-dependent reaction process and evaluate the performance of a reactor.

Ultrasound-assisted gas–liquid mass transfer process in microreactors: The influence of surfactant, channel size and ultrasound frequency

Ultrasound effect on the hydrodynamics and mass transfer behavior of gas–liquid Taylor flow is studied in ultrasonic microreactors with different frequencies (20, 28, 40 kHz) and channel dimensions (0.5 × 0.5, 1.0 × 1.0, 1.5 × 1.5, 2.0 × 2.0 mm2). Upon ultrasound irradiation, intense bubble oscillation is excited on the slug bubble, accompanied by cavitation microstreaming vortices around it. The amplitude of bubble oscillation decreases with the decrease of channel dimension as a result of the confinement effect. Channel dimension of 1.0 × 1.0 mm2 is considered as the critical dimension above which the confinement effect would be eliminated. More intensive bubble oscillation and cavitation microstreaming are observed at lower ultrasound frequency, where more significant mass transfer enhancement is also observed. At frequency of 20 kHz, the overall volumetric mass transfer coefficient is improved by 22 times at the power density of 0.14 W/mL. Adding surfactant (SDS) in the liquid increases the amplitude of bubble oscillation due to the decrease of interfacial tension. Such an increase in the oscillation amplitude enlarges the specific surface area, leading to an increase in the overall volumetric mass transfer coefficient with the increase of surfactant concentration. The detailed effect that how gas–liquid mass transfer would be enhanced by ultrasound is quantified by a mass transfer model, which could predict both the liquid side mass transfer coefficient and the specific surface area satisfactorily.

Investigations on Ozone-Based and UV/US-Assisted Synergistic Digestion Methods for the Determination of Total Dissolved Nitrogen in Waters

Over the past two decades, the alkaline persulfate oxidation (PO) with thermal and/or ultraviolet (UV) assisted digestion method has been widely used for digestion of nitrogen containing compounds (N-compounds) in water quality routine analysis in laboratory or on-line analysis, due to its simple principle, high conversion rate, high percent recovery, low-cost. However, this digestion method still has some inevitable problems such as complex operations, high contamination potential, batch N blanks, higher reaction temperature (120–124 °C) and time-consuming (30–60 min). In this study, ozone (O3) was selected as the oxidant for digestion of N-compounds through analysis and comparison firstly. Secondly, we proposed and compared the UV and/or ultrasound (US) combined with ozone (UV/O3, US/O3 and UV/US/O3) synergistic digestion methods based on O3 with sole O3 oxidation method on digestion efficiency (digestion time and conversion rate) of standard N-compounds. Simultaneously, the influence of reaction temperature, pH of water sample, concentration of O3 and mass flow rate, UV intensity, US frequency and power on digestion efficiency were investigated, and then the optimum parameters for digestion system were obtained. Experimental results indicated that UV radiation can effectively induce and promote the decomposition and photolysis of O3 in water to generate hydroxyl radicals (•OH), while US can promote the diffusion and dissolution of O3 in water and intensify the gas-liquid mass transfer process for the reaction system. Meanwhile, results showed that the UV/US/O3 synergistic digestion method had the best digestion efficiency under the optimum conditions: water sample volume, 10 mL; pH of water sample, 11; O3 mass flow rate, 3200 mg/h; reaction temperature, 30 °C; digestion time, 25 min; UV lamp power, 18 W; distance between UV lamp and reactor, 2 cm; US frequency, 20 kHz; US power, 75 W. The conversion rate (CR) of synthetic wastewater samples varied from 99.6% to 101.4% for total dissolved nitrogen (TDN) in the range of 1.0~4.0 mg/L. The UV/US/O3 synergistic digestion method would be an effective and potential alternative for digestion of N-compounds in water quality routine analysis in laboratory or on-line analysis.

Dispersion behaviors of droplet impacting on wire mesh and process intensification by surface micro/nano-structure

Droplet dispersion exists in a variety of industrial applications. Droplet impacting on the wire mesh could be dispersed into many tiny droplets and exposed larger surface area, which is better for the gas-liquid mass transfer process. In this work, the dispersion mechanisms and characteristics of droplet was investigated, as well as considering the influence of surface wettability. By increasing the surface hydrophobicity of the wire mesh, the cone angle of dispersion was enlarged maximally by 80%, the average diameter of daughter droplets decreased maximally by 70% and the liquid surface area could be eventually highly improved. Finally, based on the dispersion characteristics, the mass transfer model of droplet was established and verified by a gas-liquid absorption system. This study provides a better understanding of droplet dispersion after impacting on the wire mesh and also has major implications in the field of gas-liquid interface interaction enhancement.