Gas Mass Transfer Coefficient Research Articles

The Study of Fluid Dynamics Simulation of the Internal Flow Field in a Novel Airlift Oscillation Loop Bioreactor

The three-dimensional flow and mass transfer conditions in 5 L and 40 L airlift oscillation loop reactors were studied and compared with existing two-dimensional simulation and experimental data to verify the accuracy of the method. Then, the fluid dynamics behavior of the 2500 L reactor was simulated via supercomputing and provided guidance for production data. The results indicate that the application of oscillation operation in the 40 L multi-guide tube reactor can effectively improve the gas holdup and mass transfer coefficient in the reactor, with a maximum increase of 38% and 29%. For the 2500 L multi guide tube reactor, oscillation operation oscillation operation can significantly improve gas holdup and mass transfer coefficient increase gas holdup by 46% under 0.5 vvm operating conditions; the mass transfer coefficient increased by 54%. Therefore, oscillation operation can greatly improve the mass transfer coefficient for actual production reactors. After digging a hole in the middle sleeve, the circulating liquid speed has no effect. Although the gas holdup and mass transfer coefficient decreased by 1.3%, the gas holdup inside the entire reactor was more uniform, effectively reducing the average bubble aggregation.

The Influence of Draft Tubes on the Mass Transfer and Mixing Performance of a Pilot-Scale Internal-Loop Airlift Reactor

The hydrodynamic characteristics, mass transfer, and mixing performance of three different reactors, a bubble column reactor (BCR), a single-stage internal-loop airlift reactor (SSALR), and a four-stage internal-loop airlift reactor (FSALR), were investigated systematically through cold model experiments to explore the influence of draft tube configurations on the pilot-scale internal-loop airlift reactor (ILAR). The findings indicated that the BCR yielded a higher gas holdup and mass transfer coefficient due to its longer bubble residence time. Segmenting the draft tube improved the gas holdup in both the riser and downcomer, and the overall gas holdup in the downcomer increased by 9%. Compared with the SSALR, the mass transfer coefficient of the FSALR in the riser and downcomer increased by 10.2% and 9.3% on average, respectively. In addition, a higher liquid circulating velocity was obtained with the ILARs due to a higher gas holdup difference between the riser and the downcomer. Specifically, the liquid circulating velocity of the FSALR was 134.1% higher than that of the BCR and 15.8% higher than that of the SSALR. The mixing time of the ILARs was reduced due to more intense overall circulation. The mixing effect of the FSALR was the best. The mixing time was reduced by 70.2% and 51.3% compared with the BCR and SSALR for UG ranging from 4.0 cm/s to 9.1 cm/s, respectively. Empirical correlations were proposed for the gas holdup, liquid circulating velocity, mass transfer coefficient, and mixing time on the superficial gas velocity, and agreement with experimental data was satisfactory.

Modeling and Optimal Operating Conditions of Hollow Fiber Membrane for CO2/CH4 Separation.

In this work, the capture of carbon dioxide using a dense hollow fiber membrane was studied experimentally and theoretically. The factors affecting the flux and recovery of carbon dioxide were studied using a lab-scale system. Experiments were conducted using a mixture of methane and carbon dioxide to simulate natural gas. The effect of changing the CO2 concentration from 2 to 10 mol%, the feed pressure from 2.5 to 7.5 bar, and the feed temperature from 20 to 40 °C, was investigated. Depending on the solution diffusion mechanism, coupled with the Dual sorption model, a comprehensive model was implemented to predict the CO2 flux through the membrane, based on resistance in the series model. Subsequently, a 2D axisymmetric model of a multilayer HFM was proposed to simulate the axial and radial diffusion of carbon dioxide in a membrane. In the three domains of fiber, the CFD technique was used to solve the equations for the transfer of momentum and mass transfer by using the COMSOL 5.6. Modeling results were validated with 27 experiments, and there was a good agreement between the simulation results and the experimental data. The experimental results show the effect of operational factors, such as the fact that temperature was directly on both gas diffusivity and mass transfer coefficient. Meanwhile, the effect of pressure was exactly the opposite, and the concentration of CO2 had almost no effect on both the diffusivity and the mass transfer coefficient. In addition, the CO2 recovery changed from 9% at a pressure equal to 2.5 bar, temperature equal to 20 °C, and a concentration of CO2 equal to 2 mol%, to 30.3% at a pressure equal to 7.5 bar, temperature equal to 30 °C, and concentration of CO2 equal 10 mol%; these conditions are the optimal operating point. The results also manifested that the operational factors that directly affect the flux are pressure and CO2 concentration, while there was no clear effect of temperature. This modeling offers valuable data about the feasibility studies and economic evaluation of a gas separation unit operation as a helpful unit in the industry.

Optimization Study of CO2 Gas Absorption with NaOH Absorbent Continuous System in Raschig Ring Packing Column Using Box–Behnken Design

Increasing CO2 gas emissions results in climate change by increasing air temperature and worsening environmental problems. It is necessary to control CO2 gas in the air to overcome this. This research aims to optimize the absorption of CO2 gas in the air with 0.1 M NaOH absorbent in the column of the Raschig ring stuffing material using the response surface methodology (RSM). This research was conducted using a continuous system of three independent variables by varying the contact time (10–80 min), the flow rate of NaOH absorbent (2–5 L/min), and the flow rate of CO2 gas (1–5 L/min). The response variables in this study were the absorption rate (L/min) and mass transfer coefficient, while the air flow rate was constant at 20 L/min. Air and CO2 gas mix before absorption occurs and flow into the Raschig ring packing column so that contact occurs with the NaOH absorbent. Mass transfer of CO2 gas occurs into the NaOH absorbent, resulting in absorption. The results showed that the effect of contact time (min), the flow rate of NaOH absorbent (L/min), and CO2 gas flow rate individually and the interaction on CO2 absorption rate and mass transfer coefficient were very significant at a p-value of 0.05. Chemical absorption of CO2 also occurred due to the reaction between CO2 and OH- to form CO32− and HCO3−, so the pH decreased, and the reaction was a function of pH. Optimization using Design Expert 13 RSM Box–Behnken Design (BBD) yielded optimal conditions at an absorption time of 80 min, NaOH absorbent flow rate of 5 L/min, CO2 gas flow rate of 5 L/min, absorption rate of CO2 gas of 3.97 L/min, and CO2 gas mass transfer coefficient of 1.443 mol/min m2 atm, with the desirability of 0.999 (≈100%).

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%.

Hydrodynamics and liquid–solid mass transfer in micropacked bed reactors with copper foam packing

Hydrodynamics and liquid–solid mass transfer in micropacked bed reactors (μPBRs) with copper foam packing are investigated. Effects of fluid superficial velocities, liquid viscosity, pore diameter of foam packing on the residence time distribution, liquid holdup, axial dispersion coefficient, and liquid–solid mass transfer coefficient are discussed. The liquid holdup of μPBRs can reach 0.23 ∼ 0.39, which is larger than that of packed bed reactors (0.05). The axial dispersion coefficient of μPBRs is 3.17 × 10-6 ∼ 2.30 × 10-5 m2/s, which is similar with that of packed bed reactors. The value of liquid–solid volumetric mass transfer coefficient with effect of axial dispersion coefficient in μPBRs can reach 0.033 ∼ 0.13 s−1, which is higher than that of packed bed reactors (0.006 ∼ 0.09 s−1). The empirical models of liquid holdup, axial dispersion coefficient, and liquid–solid mass transfer coefficient in single liquid phase flow and gas–liquid two phase flow of μPBRs are established and their predictions are in good agreement with experimental values.

Experimental and modeling investigation on char combustion and char-nitrogen evolution characteristics under different conditions

The proper description of char combustion and char-nitrogen (char-N) evolution characteristics are imperative to analyze the operating performance of decoupling combustion systems. The char-O2 combustion and char-CO2 gasification reactivity of three typical coal chars, including lignite, bituminous, and anthracite, was tested in a lab-scale fixed-bed reactor in the temperature range of 1023 K–1223 K. The relevant chemical kinetic parameters were determined by applying a one-dimensional packed-bed reactor model embedded with a lumped particle model. Meanwhile, a 1-dimensional spherical particle model is necessary for describing the char-N conversion behaviors. Experimental results show that the conversion rate of char-N in the entire bed to NO (XNO, bed) gradually decreases with increasing temperature, and the decrease of coal rank leads to a decline in the NOx emission. Further simulation results indicate that for a single char particle in the initial combustion stage, if the char particle size increases (0–1000 μm), the external gas mass transfer coefficient decreases (0–5 m·s−1), or the oxygen content decreases (0–20%), the char-N conversion to NO (XNO, p) will gradually decrease. However, due to the complicated effects of temperature on multiple factors, involving reactivity, CO/CO2 ratio in products, and dual nature of CO influence on char-NO reaction, the relationship between char-N conversion and temperature for a single particle is not fixed. When the oxygen concentration is low, the XNO, p increases as temperature rising; while, an opposite trend may be formed under high oxygen conditions.

Prediction and minimization of NOx emission in a circulating fluidized bed combustor: A comprehensive mathematical model for CFB combustion

A comprehensive 1-dimensional/1.5-dimensional hybrid mathematical model is developed for predicting NOx emission of a circulating fluidized bed (CFB) combustor under broader operating parameters. In this model, the local gas–solid fluidization state and gas/heat transfer conditions in different regions of a CFB combustor are specifically considered. Some two- or three-dimensional problems, such as bubble breakage over dense bed surface, secondary air injection, core-annular flow structure, and particle clusters in freeboard, are also taken into account in 1-D/1.5-D modeling. The detailed chemical kinetic mechanism is creatively used to describe the homogeneous reaction system towards CFB combustion simulation. In addition to operating parameters and fuel-specific inputs, no other model parameters can be trimmed from case to case. This integral CFB model is validated against the field test data obtained from three commercial CFB boilers with different capacities, some of which are first disclosed. Favorable comparisons are obtained between the predicted and measured results, involving particle size distributions, temperature and pressure profiles, and NOx/SO2 emissions. The final NO emission, as well as gas profiles, are somewhat different among the cases, which may be attributed to the discrepancy in boiler structure, fuel properties, and operating conditions. Further sensitivity analysis indicates that the proportion of volatile-N in total fuel-N, char combustion reactivity, and char-NO reactivity significantly impact the NOx emission for CFB combustion. Meanwhile, the gas–solid fluidization state also plays an essential role in the NOx emission and the in-furnace combustion efficiency, such as the gas flow distribution between phases, bubble size, secondary air penetration depth, etc. However, the NOx emission seems insensitive to the particle external gas mass transfer coefficients.

Implementation of two-phase gas transport into VERA for molten salt reactor analysis

Molten salt reactors (MSRs) are a class of next-generation nuclear reactors that have received recent industrial and research interest. A generalized species transport solver was implemented in the Virtual Environment for Reactor Applications (VERA) computing suite to extend this tool to analyze liquid-fueled MSRs. This core simulator has been extended to model the transport of fission product gases into a collection of circulating gas bubbles with the purpose of removing the gases. This paper presents the governing species transport equation, along with various nuclear source terms. Development of the source term for phase migration is discussed, along with a simplified interfacial area tracking method. Finally, a case study on a simplified MSR loop is presented in which modeling parameters were varied to assess their impact on gas removal. The steady state results show that parameters such as bubble diameter, gas injection rate and mass transfer coefficient have a low to moderate effect on the fraction of xenon in the core region. Removal efficiency has the greatest effect on the fraction in the core region. After the pump bowl, bubble diameter has a minor effect on the fraction of xenon in the gas void. These results point out that increasing parameters such as mass transfer coefficient, gas injection rate, and removal efficiency drives the xenon into the circulating gas void, while decreasing bubble diameter also drives xenon into the gas void by increasing interfacial area.

A tool for mass transfer evaluation in packed-bed columns for chemical engineering students

This paper presents a Microsoft Excel tool to calculate liquid-gas mass transfer coefficients in packed towers to support numerical design activities in the courses of Unit Operations for Industrial Process and Sustainable Process Design for the Master’s degree in Chemical Engineering of the University of Naples Federico II (Italy).The Mass Transfer Solver Tool (MT Solver Tool) uses several available models to estimate, separately, the values of liquid and gas mass-transfer coefficients and the wet surface area for 144 random and structured packings of interest for absorption/stripping and distillation processes. In addition, a separate spreadsheet can be used in a user-defined mode, to evaluate the mass transfer coefficients with new packing types or to interpret experimental data when the geometrical and physical characteristics of the packing are known. Eventually, the tool is supplied with a data library, where packing geometry and model fitting parameters can be retrieved.The software is aimed to support students and educators in the Unit Operations for Industrial Process and Sustainable Process Design courses. In particular, this is meant to be an example on how the accuracy of design algorithms adopted in unit operation processes is affected by the use of the underpinning correlations for mass transfer rate or pressure drops. Besides, this is aimed to encourage comparison of different correlations when exact field data are not available. Besides, chemical engineers and researchers interested in packed columns design and modelling data may also benefit from the utilization of the software. The MT Solver Tool was introduced to students in a dedicated tutorial lesson after lecturers on packed column design algorithms for distillation, absorption and stripping. Most of the students of the course participated to a group training aimed to simulate the design of an absorption column supported by the MT Solver Tool providing feedback on its application.After the training, an anonymous survey was proposed to the students to monitor the approval rating of the proposed activity and the use of the MT Solver Tool software to support numerical calculations.

Local Distribution of Oxygen Mass Transfer Coefficient in CMC Solutions in Bioreactors Furnished with Different Types of Coaxial Mixers

Aerated stirred tank bioreactors are one of the most commonly used systems in various processes such as biofermentation where ensuring adequate mass transfer rate is the sole purpose. In this study, the crucial role of the central impeller type (including Rushton turbine, pitched blade turbine, elephant ear impeller and their combinations) on the performance of an aerated bioreactor equipped with two central impellers and an anchor on the local and overall volumetric mass transfer coefficient was investigated at a variety of operating conditions such as non-Newtonian solution concentrations, and central and anchor impeller speeds. Carboxymethyl cellulose (CMC) solution was used as a power-law fluid. Through the use of electrical resistance tomography, simplified dynamic pressure method, and computational fluid dynamics, several aspects of the aerated coaxial mixer namely local and overall gas holdup and mass transfer were studied. The local mass transfer coefficients were measured using three dissolved oxygen sensors installed at three various heights of the bioreactor. The local gas holdup was also measured using tomography. The results showed the positive effect of the anchor impeller on the mass transfer coefficient up to 10 rpm for different coaxial mixing configurations with various impeller type combinations and CMC concentrations. Among different impeller types, the system with two Rushton turbines resulted in the highest volumetric mass transfer coefficient, which was attributed to the higher gas holdup provided by this configuration as well as the flow pattern generated by the coaxial mixer. Moreover, the local distribution of gas holdup and mass transfer coefficient demonstrated a reduction in both parameters from the bottom to the top of the bioreactor.

Parametric numerical study and optimization of mass transfer and bubble size distribution in a gas-liquid stirred tank bioreactor equipped with Rushton turbine using computational fluid dynamics

Abstract Designing and optimizing a bioreactor can be an especially challenging process. Computational modelling is an effective tool to investigate the effects of various operating parameters on bioreactor performance and identify the optimum ones. In this work, a computational fluid dynamics-population balance model (CFD-PBM) was developed to elucidate the effect of different geometrical and operating parameters on the hydrodynamics and mass transfer coefficient of a batch stirred tank bioreactor. The validated model was projected to predict the effect of different parameters including the gas flow rate, the impeller off-bottom clearance, the number of agitator blades, and rotational speed of the impeller on the velocity profiles, air volume fraction, bubble size distribution, and the local gas mass transfer coefficient (K l a) in the bioreactor. Air bubble breakup and coalescence phenomena were considered in all simulations. Factorial experimental design approach was employed to statistically investigate the impacts of the aforementioned operating and geometrical parameters on K l a and bubble size distribution in the bioreactor in order to determine the most significant parameters. This can give an essential insight into the most impactful factors when it comes to designing and scaling up a bioreactor.

Hydrodynamic and mass transfer parameters for CO2 absorption into amine solutions and its blend with nano heavy metal oxides using a bubble column

ABSTRACT In this work, hydrodynamic and mass transfer of CO2 absorption into metal oxide nanofluid includes TiO2, ZnO, and ZrO2 nanoparticles at Piperazine (Pz) solution in the bubble column was studied. The novel rigorous correlations were extended based on the variable parameters for the calculation of gas holdup, enhancement factor, and mass transfer coefficients in the reactive absorption processes using the π-Buckingham method. The correlations were constructed based on dimensionless numbers, including nanoparticle loading, film parameter, CO2 partial pressure ratio to the total pressure, the film thickness of phases, CO2 loading, and ratio of diffusion coefficients of CO2 in the gas and liquid phases. Experimental data were used to calculate the correlations, consisting of Pz concentration, CO2 partial pressure, nanoparticle loading, and stirrer speed in the range of 0.1–0.5 mol/l, 16.0–35.2 kPa, 0.0–0.1 wt%, and 0–300 rpm, respectively. The film parameter was applied to exert the influence of chemical reactions on the performance of absorption. The mean squared error (RMSEP), the average absolute error (%AARD), and the coefficient of correlation of the results (R2 ) for the correlations were in the range 0.021–2.604, 3.78–18.14, and 0.861–0.980, respectively, which represents the excellent accuracy of the proposed correlation.

Chaotic mixing and mass transfer characteristics of fractal impellers in gas-liquid stirred tank

Gas-liquid mixing characteristics in fractal impeller stirred tank were experimentally investigated by measuring multi-scale entropy (MSE), relative power demand (RPD), local gas holdup, bubble size, and mass transfer coefficient (KLa). Results showed that fractal impeller could enhance 21.69% of MSE and 11.94% of RPD on the basis of pitched-blade impeller. Fractal impeller can enhance the local gas holdup, decrease the bubble size, reduce the dispersion coefficient (σ) and improve the gas-liquid dispersion performance. Meanwhile, the regression based on d32− for pitched-blade impeller and fractal impeller were obtained. In addition, fractal impeller can also increase 11.07% of volume mass transfer coefficient (KLa) compared with pitched-blade impeller in the gas-liquid mixing process. The results can provide theoretical guidance for the optimal design of gas-liquid stirred tank.

Thermodynamic analysis of a gamma-type stirling engine driven by Scotch Yoke mechanism

ABSTRACT In this study, thermodynamic analysis of a gamma-type Stirling engine driven by Scotch Yoke mechanism was conducted. The advantages of Scotch Yoke mechanism over classical crank mechanism were discussed in detail. In the analysis hydrogen was used as working gas. Working space was divided into 16 nodal volumes. Solution of conservation of mass and energy equation were obtained for each nodal volume. Wall temperatures of nodal volumes were assumed to be constant and temperature variations in nodal volumes were calculated by means of the first law of the thermodynamics. Maximum engine power was obtained as 1105 W at 800 rpm engine speed and gas mass of 0.3 g for 1200 W/m2K heat transfer coefficient. The optimum phase angle, which was not affected significantly by hot end temperature, gas mass, and heat transfer coefficient variations, was determined as 90° degrees at engine speed of 600 rpm. However, it was determined that engine speed has a great impact on optimum phase angle and a variable phase angle mechanism may be useful to obtain maximum specific power from the engine at different engine speeds. At engine speed of 600 rpm, the maximum specific power of the engine was 695 W/l for 1.65 compression ratio and 90° degrees phase angle.

Kinetics of CaCO3 precipitation in seeded aeration softening of brackish water desalination concentrate

Desalination of brackish water is energetically attractive, but the concentrate produced poses a major environmental issue. Intensive research has been carried out to develop demineralization techniques of such concentrates to facilitate their utilization and disposal. Aeration was reported to allow chemical-free and effective demineralization through CaCO3 precipitation. When applied in concentrate treatment, the process was assumed to follow first order kinetics, but second order kinetics were proposed when the process was applied in potable groundwater treatment. In the current study the effect of aeration rates and seed concentration on reaction kinetics was studied in real brackish water desalination concentrates (BWDC). In the absence of or at low aeration, CaCO3(s) precipitation followed second order kinetics characteristic of simple CaCO3(s) precipitation. With increasing aeration rate, the reaction shifted toward first order kinetics, characteristic of gaseous CO2 release. Both aeration rate and seed concentration linearly increased the gas mass transfer coefficient. Under the experimental conditions, maximal removal rate was 1.05 g calcium-CaCO3 L−1 h−1. Calcium removal reached 73% and was found to depend on raw concentrate composition and reaction time. The composition of the softened concentrate corresponded well with the geochemical PHREEQC code simulation of the solution at equilibrium. The precipitates comprised at least 92% calcite, which may be of economic significance. The ability to use the intrinsic precipitates as seed particles may be advantageous in terms of CaCO3 recovery and simplicity of operation.

Microbubble ozonation of the antioxidant butylated hydroxytoluene: Degradation kinetics and toxicity reduction

Butylated hydroxytoluene (BHT) is recognized as a crucial pollutant in aquatic environments, but efforts to achieve its complete removal are without success. The aim of this study was to investigate the degradation efficiency of BHT in water using ozone microbubbles (OMB), coupled with toxicity change assessment at sub-lethal BHT concentrations (0.34, 0.45 and 0.90 μM) based on oxidative stress biomarkers in Daphnia magna. The efficiency of OMB on ozone gas mass transfer was assessed and the contribution of hydroxyl radicals (·OH) in the degradation of BHT was determined using p-chlorobenzoic acid (pCBA) probe compound and a ·OH radical scavenger (sodium carbonate, Na2CO3). The ozone gas mass transfer coefficient (kLa = 1.02 × 10−2 s−1) was much larger than the ozone self-decomposition rate (kd = 8 × 10−4 s−1) implying little influence of self-decomposing ozone in the volumetric ozone transfer during OMB generation. Generally, OMB improved ozone gas mass transfer (1.3–19-fold) relative to conventional ozone techniques, while indirect reaction of BHT with ·OH was dominant (82%) over the direct reaction with molecular ozone. Addition of 15, 25 and 35 mM Na2CO3 reduced BHT degradation by 30, 50 and 65%, respectively, indicating the significance of ·OH in the degradation of BHT. Increase in initial BHT concentration correspondingly reduced its removal rate by OMB possibly due to increase in metabolites produced during ozonation. Post BHT treatment exposure tests recorded significant (p < 0.05) reductions in oxidative stress (according to enzyme activities changes) in D. magna compared to pretreatment tests, demonstrating the effectiveness of OMB in detoxification of BHT. Overall, the results of the study indicate that OMB is extremely efficient in complete degradation of BHT in water and, consequently, significantly (p < 0.05) reducing its toxicity.

Experimental and Computational Studies of Aerated Stirred Tank with Dual Impeller

AbstractStirred tanks are commonly used in chemical and allied industries for reaction and separation. In order to improve the mixing performance, large scale reactors are often equipped with multiple impellers. In the case of gas-liquid systems, the gas hold-up, mass transfer coefficient, and interfacial area strongly depend on the size and type of impellers, clearance between impellers and superficial gas velocity. In the present work, the effect of the impeller speed, superficial gas velocity, and top impeller position has been investigated on gas hold-up, interfacial area, and mass transfer coefficient. Computational fluid dynamics have been carried out for the multiphase multi-impeller system and the model predictions have been compared with the experimental data.

3D CFD Simulation of Gas Hold-up and Mass Transfer in a Modified Airlift Reactor with Net Draft Tube

Abstract This paper documents CFD simulations of the gas hold-up (ɛg) and volumetric mass transfer coefficient (KLa) for three kinds of airlift reactors (ALRs) namely, conventional ALR, ALR with net draft tube (ALR-ndt), and packed-bed ALR with net draft tube (PBALR-ndt). The 3D two-fluid Eulerian-Eulerian model was adopted to predict the influence of superficial gas velocity on ɛg and KLa. The simulation results were consistent with the trends described previously in the experimental work regarding ɛg and KLa values and a good agreement was obtained (absolute error less than 20 %). Based on the simulation results, axial flow is the dominant flow in the conventional ALR while in ALR-ndt and PBALR-ndt radial flow streamlines are appeared in the reactors due to the presence of concentric net draft tube which improves their performance. The effective role of the net draft tube is proven since consequence of generation of small bubbles by passing through net draft tube is the entrainment of a larger percentage of gas bubbles from the riser into the downcomer which results in improvement of gas holdup and the KLa values. An exponential correlation is used for relating gas hold-up and mass transfer coefficient. Higher power obtained for ALR-ndt and PBALR-ndt (n ≈ 1.22) compared to ALR (n = 0.95) was indicative of high sensitivity of KLa value to gas hold-up in these reactors due to presence of the concentric net draft tube. The CFD modeling is considered to be an invaluable tool allowing us to analyze and visualize the impact of fluidic forces on hydrodynamic properties and consequently, reactor performance.

Mass transfer enhancement in a two-phase flow electrochemical reactor

Experimental investigations were carried out to determine the possible levels of augmentation in mass transfer coefficient in gas–liquid flow system using a string of inverted cones. It is noticed that the mass transfer coefficient nearly doubled due to insertion of this promoter. Studies revealed that gas velocity, liquid velocity, pitch and rod diameter have no effect on mass transfer coefficient. The mass transfer coefficient increased with base diameter of cone. An empirical correlation is also obtained to predict mass transfer coefficient.