Flow field and residence time distribution simulation of a cross-flow gas-liquid wastewater treatment reactor using CFD

A three-dimensional Eulerian-Eulerian two-phase approach has been used for the simulation of a cross-flow gas-liquid wastewater treatment reactor. Two different turbulence models have been tested: the k-ε and Reynolds Stress Model (RSM) models. Bubble induced turbulence source terms have been added to these models. Numerical results have been validated using Laser Doppler Velocimetry (LDV) measurements. Simulations with both turbulence models successfully predicted the hydrodynamics of the reactor. Then particle tracking with a stochastic approach has been used to calculate residence time distributions (RTD) with the flow previously simulated. It has been shown that dispersion in the reactor is primarily due to turbulence. Results have been compared with experimental RTD for various liquid and gas flowrates both on a bench scale and full scale plant. The RSM model accurately predicted the dispersion whereas the standard k-ε model slightly underestimated the dispersion.     

Liquid-solid mass transfer in a rotating packed bed reactor with structured foam packing

A rotating packed bed (RPB) reactor has substantially potential for the process intensification of heterogeneous catalytic reactions. However, the scarce knowledge of the liquid–solid mass transfer in the RPB reactor is a barrier for its design and scale-up. In this work, the liquid–solid mass transfer in a RPB reactor installed with structured foam packing was experimentally studied using copper dissolution by potassium dichromate. Effects of rotational speed, liquid and gas volumetric flow rate on the liquid–solid mass transfer coefficient (k_LS) have been investigated. The correlation for predicting k_LS was proposed, and the deviation between the experimental and predicted values was within ± 12%. The liquid–solid volumetric mass transfer coefficient (k_LSa_LS) ranged from 0.04–0.14 1⁻¹, which was approximately 5 times larger than that in the packed bed reactor. This work lays the foundation for modeling of the RPB reactor packed with structured foam packing for heterogeneous catalytic reaction.     

Mass transfer at gas-evolving vertical electrodes

Various models have been proposed to describe the mass transfer of indicator ions to gas-evolving electrodes. For verification of the proposed models, the dependence of the mass transfer coefficient of indicator ions,k _j, on the length,L _e, of a gas-evolving electrode may be very useful. Experimental relations betweenk _j andL _e have been determined for oxygen-evolving as well as hydrogen-evolving vertical electrodes in a supporting electrolyte of 1 M KOH. Moreover, a modified hydrodynamic model, where a laminar solution flow is induced by rising bubbles, has been proposed in order to calculatek _j. It has been found that this model is not useful for both types of gas-evolving electrodes. The experimental results support the earlier proposed convection-penetration model for the oxygen-evolving electrode. The solution flow near a vertical electrode, induced by rising bubbles, behaves in a turbulent manner.     

Hydrodynamic and mass transfer study in a rectangular three-phase air-lift loop reactor

This study concerns a three-phase rectangular air-lift reactor. The first part deals with the reactor’s gas–liquid two-phase flow hydrodynamics. Parameters, such as the gas hold-up, the liquid velocity, the mixing parameters, and the solid hold-up, were quantified in the presence of two different plastic solids—a regular shape solid called P1, and an heterogeneous shape solid, P4. No fundamental influence of the solids on the studied parameters was noticed, except for the gas hold-up. Indeed, a bubble coalescence phenomenon was highlighted for high solid concentrations (ε_s > 16%). Moreover, a comparison of these two materials led to the conclusion that P4 exhibited better hydrodynamic performances. In a second set of experiments, the oxygen mass transfer was characterised in three-phase flow, with both kinds of plastic materials. The k_La coefficient was deduced from a mass balance on the gas phase. It was found to be weakly influenced by the presence, the nature or the quantity of solid.     

Numerical simulation of mass transport in a filter press type electrochemical reactor FM01-LC: Comparison of predicted and experimental mass transfer coefficient

This paper studies flow characteristics and their effect on local mass transfer rate to a flat plate electrode in a FM01-LC electrochemical reactor. 3D reactor simulations under limiting current and turbulent flow conditions were performed using potassium ferro-ferricyanide electrochemical system with sodium sulfate as supporting electrolyte. The model consists of mass-transport equations coupled to hydrodynamic solution obtained from Reynolds-averaged Navier–Stokes equations using standard k–? turbulence model, where the average velocity field, the turbulence level given by the eddy kinetic energy and the turbulent viscosity of the hydrodynamic calculation were used to evaluate the convection, turbulent diffusion and the concentration wall function. The turbulent mass diffusivity was evaluated by Kays–Crawford equation using heat and mass transfer analogies, while wall functions, for mass transport, were adapted from Launder–Spalding equations. Simulation results describe main flow properties, concentration profiles throughout the entire volume of the reactor and local diffusion flux over the electrode. Overall mass transfer coefficients estimated by simulation, without fitting parameters, agree closely with experimental coefficients determined from limiting current measurements (1.85% average error) for Re between 187 and 1407.     

Numerical simulation of turbulent flow in a baffled stirred tank with an explicit algebraic stress model

An explicit algebraic stress model (EASM) was used to simulate anisotropic turbulent flows in baffled stirred tanks equipped with a standard Rushton turbine. The quantitative predictions of velocity components, turbulence kinetic energy, Reynolds stresses and turbulence energy dissipation rate in the context of anisotropic turbulence were conducted to assess the comprehensive performance of the EASM. A lot of efforts have been made to ensure numerical stability during the calculations such as using a good initial flow field, manipulating source terms and adjusting under-relaxation factors. The predicted results were also compared with experimental data and other simulation results obtained using the standard k–ε model, algebraic stress model (ASM), Reynolds stress model (RSM) and large eddy simulation (LES). All the simulations were run with in-house codes. The simulation results show that agreement between the EASM predictions and experimental values is satisfactory. The EASM is consistently superior to the standard k–ε model when predicting both peak values and trend of variation in velocities and turbulence quantities. In comparison to the RSM, the EASM has almost the same predictive accuracy. The EASM is inferior to the LES on the prediction of turbulence kinetic energy. Nevertheless, the computational cost of the EASM is significantly lower than that of the LES, which is an obvious advantage in practical applications.     

Computational fluid dynamics modeling of immobilized photocatalytic reactors for water treatment

A computational fluid dynamics (CFD) model for the simulation of immobilized photocatalytic reactors used for water treatment was developed and evaluated experimentally. The model integrated hydrodynamics, species mass transport, chemical reaction kinetics, and irradiance distribution within the reactor. The experimental evaluation was performed using various configurations of annular reactors and ultraviolet lamp sizes over a wide range of hydrodynamic conditions (350 < Re < 11,000). The evaluation showed that the developed CFD model was able to successfully predict the photocatalytic degradation rate of a model pollutant in the analyzed reactors. In terms of hydrodynamic models, the results demonstrated that the laminar model performs well for systems under laminar flow conditions, whereas the Abe‐Kondoh‐Nagano low Reynolds number and the Reynolds stress turbulence models give accurate predictions for photoreactors under transitional or turbulent flow regimes. The performed analysis confirmed that degradation rates of organic contaminants in immobilized photocatalytic reactors are strongly limited by external mass transfer; as a consequence, the degradation prediction capability of the CFD model is largely determined by the external mass transfer prediction performance of the hydrodynamic models used. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

Simulation of swirling gas–particle flows using USM and k–ε–kp two-phase turbulence models

The turbulent swirling gas–particle flows with swirl numbers 0.47 and 1.5 are simulated using a unified second-order moment (USM) (two-phase Reynolds stress equations) and a k–ε–kp two-phase turbulence models. The results are compared with experiments. Both two models can well predict the axial time-averaged two-phase velocities in case of s=0.47, but the USM model is better than the k–ε–kp model in predicting the tangential time-averaged two-phase velocities of strongly swirling flows (S=1.5). The anisotropic two-phase turbulence can well be described only using the USM model. The results give the difference in flow behavior between weakly swirling and strongly swirling gas–particle flows.     

Influence of recycling on mass transfer and reaction in a G-L jet loop reactor with variable interfacial area

The effect of recycling on mass transfer, characterized by the volumetric mass transfer coefficient k_La, was investigated by absorbing CO₂ into water in a laboratory jet loop reactor. Based on a mathematical model, which includes the correlation function k_La = k_La(r_L), a first order chemical reaction and the variation of interfacial area, a = a(ε_G), caused by strong absorption effects, the interaction ‘recycling – mass transfer – reaction’ was simulated and its influence on the reactor performance is discussed.     

Oxygen mass transfer coefficient in bubble column slurry reactor with ultrafine suspended particles and neural network prediction

The gas–liquid volumetric mass transfer coefficient was determined by the dynamic oxygen absorption technique using a polarographic dissolved oxygen probe and the gas–liquid interfacial area was measured using dual‐tip conductivity probes in a bubble column slurry reactor at ambient temperature and normal pressure. The solid particles used were ultrafine hollow glass microspheres with a mean diameter of 8.624 µm. The effects of various axial locations (height–diameter ratio = 1–12), superficial gas velocity (u_G = 0.011–0.085 m/s) and solid concentration (ε_S = 0–30 wt.%) on the gas–liquid volumetric mass transfer coefficient k_La_L and liquid‐side mass transfer coefficient k_L were discussed in detail in the range of operating variables investigated. Empirical correlations by dimensional analysis were obtained and feed‐forward back propagation neural network models were employed to predict the gas–liquid volumetric mass transfer coefficient and liquid‐side mass transfer coefficient for an air–water–hollow glass microspheres system in a commercial‐scale bubble column slurry reactor. © 2012 Canadian Society for Chemical Engineering     

CFD models of jet mixing and their validation by tracer experiments

The classical theory of RTD was applied to characterize a flow in a laboratory jet mixer using both numerical and experimental approaches. Detailed information about flow field in the reactor was obtained through computational fluid dynamics (CFD) simulations. Three different turbulence models have been tested: the standard k-?, RNG k-? and Reynolds Stress Model (RSM). The CFD models predicted slight yet relevant differences in flow patterns. The experimental RTD can be used to identify erroneous numerical results. This paper points out differences in the predicted flow velocities. Such discrepancy may have significant impact on the assessment of the reactor's performance. Thus, the role of experimental verification is emphasized. A dedicated experiment is proposed to resolve the potential validation problem.     

The cocurrent downflow contactor–a novel reactor for gas-liquid-solid catalysed reactions

The Cocurrent Downflow Contactor (CDC) has been developed as a mass transfer and reactor device, with and without addition of tangential (swirl) flow, giving gas hold-up (E_g) values of 0.5–0.75, interfacial areas in the range 1000–6000 m²m^?3 liquid and k_La values in the range of 0.15–1.55 s^?1 for absorption using the O₂/H₂O system. It has been studied as a catalytic slurry reactor for the hydrogenation of (i) itaconic acid and (ii) triglycerides catalysed by Pd and Ni catalysts. The reactions were observed to be largely surface-reaction rate controlled, due to the very efficient mass transfer (k_La up to 11.75 s^?1 under reaction conditions) and application of swirl flow-enhanced reaction rates. The CDC has recently been found to be capable of operating as a fixed bed reactor, thus eliminating a downstream catalyst separation problem (therefore more cost effective), and is superior in its mass transfer characteristics to other known devices. Scale-up can be undertaken without loss of performance efficiency.     

Residence time distribution and liquid holdup in Kenics KMX static mixer with hydrogenated nitrile butadiene rubber solution and hydrogen gas system

A Kenics^® KMX static mixer that has curved-open blade internal structure was investigated to study its hydrodynamic performance related to residence time distribution and liquid holdup in a gas/liquid system. The static mixer reactor had 24 mixing elements arranged in line along the length of the reactor such that the angle between two neighboring elements is 90°. The length of the reactor was 0.98 m with an internal diameter of 3.8 cm and was operated cocurrently with vertical upflow. The fluids used were hydrogen (gas phase), monochlorobenzene (liquid phase) and hydrogenated nitrile butadiene rubber solution (liquid phase). In all the experiments, the polymer solution was maintained as a continuous phase while hydrogen gas was in the dispersed phase. All experiments were conducted in the laminar flow regime with the liquid side hydraulic Reynolds number in the range of 0.04-0.36 and the gas side hydraulic Reynolds number in the range of 3-18. Different polymer concentrations and different operating conditions with respect to gas/liquid flow rates were used to study the corresponding effects on the hydrodynamic parameters such as Peclet number (Pe) and the liquid holdup (ε_L). Empirical correlations were obtained for the axial dispersion coefficient (D_a) and liquid holdup in liquid system alone and for the gas/liquid system separately. It was observed that the Peclet number decreased with the introduction of gas in to the reactor while in the liquid system alone, an increase in viscosity decreased the Peclet number. The liquid holdup was empirically correlated as a function of the physical properties of the fluids used in addition to the operating flow rates.     

Gas–liquid mass transfer in periodically operated trickle-bed reactors: Influence of the liquid-phase physical properties and flow modulation

The influence of periodic operation on trickle-bed reactor (TBR) hydrodynamics and gas–liquid mass transfer was investigated. Two-phase pressure drop, dynamic liquid hold-up and gas–liquid mass transfer coefficient (k_La) were determined at various liquid flow rates and for different modes of liquid flow variation (increasing and decreasing liquid flow rate). The results reveal the considerable influence of type of liquid flow rate modulation on k_La values (deviations of up to 80% in k_La). Simulation studies on gas-limited reaction in a periodically operated TBR indicate that an enhancement in conversion of about 14% can be expected from an appropriate selection of the operating mode, thus clearly demonstrating the quantitative process intensification feasible through increased gas–liquid mass transfer.     

Turbulence modelling of multiphase flow in high-pressure trickle-bed reactors

Computational fluid dynamics (CFD) has been used as a successful tool for single-phase reactors. However, fixed-bed reactors design depends overly in empirical correlations for the prediction of heat and mass transfer phenomena. Therefore, the aim of this work is to present the application of CFD to the simulation of three-dimensional interstitial flow in a multiphase reactor. A case study comprising a high-pressure trickle-bed reactor (30 bar) was modelled by means of an Euler-Euler CFD model. The numerical simulations were evaluated quantitatively by experimental data from the literature. During grid optimization and validation, the effects of mesh size, time step and convergence criteria were evaluated plotting the hydrodynamic predictions as a function of liquid flow rate. Among the discretization methods for the momentum equation, a monotonic upwind scheme for conservation laws was found to give better computed results for either liquid holdup or two-phase pressure drop since it reduces effectively the numerical dispersion in convective terms of transport equation.After the parametric optimization of numerical solution parameters, four RANS multiphase turbulence models were investigated in the whole range of simulated gas and liquid flow rates. During RANS turbulence modelling, standard k-ε dispersed turbulence model gave the better compromise between computer expense and numerical accuracy in comparison with both realizable, renormalization group and Reynolds stress based models. Finally, several computational runs were performed at different temperatures for the evaluation of either axial averaged velocity and turbulent kinetic energy profiles for gas and liquid phases. Flow disequilibrium and strong heterogeneities detected along the packed bed demonstrated liquid distribution issues with slighter impact at high temperatures.     

Mass transfer modeling and simulation at a rotating cylinder electrode (RCE) reactor under turbulent flow for copper recovery

This work presents the numerical simulation of a laboratory reactor with rotating cylinder electrode (RCE) and a six-plate counter electrode that is used in studies on heavy metal recovery. The rate of electrode rotation and the potential applied are of such magnitude that the electrochemical reactor works in conditions of mass transport control under turbulent flow to obtain high recovery rates and formation of dendritic metal deposits. For hydrodynamics, the Reynolds averaged Navier–Stokes (RANS) equations were solved using the standard k–ε turbulence model, as well as wall functions based on the universal velocity distribution in the near-wall region. Results of 3-D simulations of the velocity field show clearly the formation of the turbulence Taylor vortex flow. For mass transfer, convection–diffusion equation was solved using the Kays–Crawford model for turbulent Schmidt number and Launder–Spalding wall functions adapted for mass transfer. Kinetics of copper recovery from aqueous solutions containing 0.019 M CuSO₄ and 1 M H₂SO₄, in the range of rotation speed of 400–1100 rpm, was adequately fit (error <8%) during the electrolysis time to achieve a final recovery of 85% for potentiostatic and 60% for galvanostatic experiments. The fitting parameter of the concentration wall function used in all experiments was A=2.9.     

Toward Understanding of Two‐Phase Eccentric Helical Reactor Performance

Mass transfer investigations in a two‐phase gas‐liquid Couette‐Taylor flow (CTF) reactor and a numerical flow simulation are reported. The CTF reactor is characterized by high values of the mass transfer parameters. Previous mass transfer investigations have yielded high values of the volumetric mass transfer coefficients (of the order of 10^–1 s^–1) and the specific interfacial area, compared to those obtained in a stirred tank (10³ m² m^–3). In order to intensify mass transfer in the CTF reactor, an eccentric rotor (rotating inner cylinder) was used. In the eccentric annulus with rotating inner cylinder, due to frequent variation of the hydrodynamic flow field parameters, nonlinear hydrodynamic conditions occurred. These conditions can influence the rate of mass transfer. The experimental results of benzaldehyde oxidation in an eccentric CTF reactor confirmed an increase in mass transfer, as against a concentric CTF reactor. Numerical simulation of the Couette‐Taylor (helical) flow was performed in a concentric and in an eccentric annulus. Calculation of parameters such as velocity, static pressure, kinetic energy and energy dissipation rate revealed a significant effect of gap eccentricity on the flow behavior.     

CFD simulation of stirred tanks: Comparison of turbulence models. Part I: Radial flow impellers

A critical review of the published literature regarding the computational fluid dynamics (CFD) modelling of single‐phase turbulent flow in stirred tank reactors is presented. In this part of review, CFD simulations of radial flow impellers (mainly disc turbine (DT)) in a fully baffled vessel operating in a turbulent regime have been presented. Simulated results obtained with different impeller modelling approaches (impeller boundary condition, multiple reference frame, computational snap shot and the sliding mesh approaches) and different turbulence models (standard k ? ε model, RNG k ? ε model, the Reynolds stress model (RSM) and large eddy simulation) have been compared with the in‐house laser Doppler anemometry (LDA) experimental data. In addition, recently proposed modifications to the standard k ? ε models were also evaluated. The model predictions (of all the mean velocities, turbulent kinetic energy and its dissipation rate) have been compared with the experimental measurements at various locations in the tank. A discussion is presented to highlight strengths and weaknesses of currently used CFD models. A preliminary analysis of sensitivity of modelling assumptions in the k ? ε models and RSM has been carried out using LES database. The quantitative comparison of exact and modelled turbulence production, transport and dissipation terms has highlighted the reasons behind the partial success of various modifications of standard k ? ε model as well as RSM. The volume integral of predicted energy dissipation rate is compared with the energy input rate. Based on these results, suggestions have been made for the future work in this area.     

CFD modelling of fast chemical reactions in turbulent liquid flows

In this work an in-house CFD code is used to simulate a single-phase acid–base neutralisation in a tubular reactor. The Reynolds averaged Navier–Stokes (RANS) equations including the k–ε-turbulence model is used to simulate the turbulent flow. Different models are tested to describe the chemical reaction, including the Eddy dissipation concept (EDC) and the presumed probability density function (PDF) models. The EDC-model was developed for gas phase reactions and the objective of this work was to modify the model to make it more suitable for liquid phase reactions. Two different PDF-models are tested, namely the battlement- and the beta-PDF. The simulation results are compared to experimental data and the results has shown that the standard EDC-model is not suitable for liquid phase reactions, a modified version of the model has shown good results. The most promising PDF-model is shown to be the beta-PDF-model.     

Effect of the radical scavenger t-butanol on gas-liquid mass transfer

The alcohol t-butanol has been used as a radical scavenger in the studies of ozone reactions in water and has been found to affect the gas-liquid mass transfer rates. An understanding of the effects of t-butanol on mass transfer parameters, including bubble size, gas holdup, mass transfer coefficient and the mass transfer specific surface area, is of key importance to not only improve the knowledge of this particular system but also to gain fundamental understanding about the effects of gas/liquid surface modifiers on the contact between phases and the mass transfer rates. An experimental study has been carried out to investigate the effects of t-butanol concentrations on the physical properties of aqueous solutions, including surface tension and viscosity. It was found that t-butanol reduced both properties-by 4% for surface tension and by a surprising 30% for viscosity. These reductions in the solution physical properties were correlated to enhancement in the mass transfer coefficient, k_L. The hydrodynamic behaviour of the system used in this work was characterised by a homogeneous bubbling regime. It was also found that the gas holdup was significantly enhanced by the addition of t-butanol. An equation to predict the gas holdup from the gas flow rate and t-butanol concentration was proposed to describe the experimental data. Moreover, the addition of t-butanol was found to significantly reduce the size of gas bubbles, leading to enhancement in the volumetric mass transfer coefficient, k_La. Bubble mean diameter was predicted using an equation developed by the Radial Basis Function Neural Network architecture obtained from the literature, and the mass transfer coefficient, k_L, was predicted using an equation based on the surface coverage ratio model. The ratio was found not to depend either on t-butanol concentration or on gas flow rate. A significant increase in the volumetric mass transfer coefficient, k_La, due to an increase in both k_L and a, was obtained following the addition of t-butanol, even at low concentrations.