CFD simulations of gas–liquid–solid stirred reactor: Prediction of critical impeller speed for solid suspension

CFD evaluation of hydrophobic feedstock bench-scale fermenters for efficient high agitation volumetric mass transfer

A new biomanufacturing platform combining intracellular metabolic engineering of the oleaginous yeast Yarrowia lipolytica and extracellular bioreaction engineering provides efficient bioconversion of plant oils/animal fats into high-value products. However, predicting the hydrodynamics and mass transfer parameters is difficult due to the high agitation and sparging required to create dispersed oil droplets in an aqueous medium for efficient yeast fermentation. In the current study, commercial computational fluid dynamic (CFD) solver Ansys CFX coupled with the MUSIG model first predicts two-phase system (oil/water and air/water) mixing dynamics and their particle size distributions. Then, a three-phase model (oil, air, and water) utilizing dispersed air bubbles and a polydispersed oil phase was implemented to explore fermenter mixing, gas dispersion efficiency, and volumetric mass transfer coefficient estimations (kL a). The study analyzed the effect of the impeller type, agitation speed, and power input on the tank's flow field and revealed that upward-pumping pitched blade impellers (PBI) in the top two positions (compared to Rushton-type) provided advantageous oil phase homogeneity and similar estimated kL a values with reduced power. These results show good agreement with the experimental mixing and kL a data.

Turbulent CFD Simulation of Two Rotor-Stator Agitators for High Homogeneity and Liquid Level Stability in Stirred Tank

Good solid-liquid mixing homogeneity and liquid level stability are necessary conditions for the preparation of high-quality composite materials. In this study, two rotor-stator agitators were utilized, including the cross-structure rotor-stator (CSRS) agitator and the half-cross structure rotor-stator (HCSRS) agitator. The performances of the two types of rotor-stator agitators and the conventional A200 (an axial-flow agitator) and Rushton (a radial-flow agitator) in the solid-liquid mixing operations were compared through CFD modeling, including the homogeneity, power consumption and liquid level stability. The Eulerian-Eulerian multi-fluid model coupling with the RNG k-ε turbulence model were used to simulate the granular flow and the turbulence effects. When the optimum solid-liquid mixing homogeneity was achieved in both conventional agitators, further increasing stirring speed would worsen the homogeneity significantly, while the two rotor-stator agitators still achieving good mixing homogeneity at the stirring speed of 600 rpm. The CSRS agitator attained the minimum standard deviation of particle concentration σ of 0.15, which was 42% smaller than that achieved by the A200 agitators. Moreover, the average liquid level velocity corresponding to the minimum σ obtained by the CSRS agitator was 0.31 m/s, which was less than half of those of the other three mixers.

Experimental and simulation study on mixing time and suspension quality of liquid-solid flow field in stirred reactor with draft tube

The solid-liquid flow field is established in the stirred reactor with baffles and draft tube. Mixing time and suspension quality of the flow field are studied. Based on the Eulerian multiphase flow model and RNG k−ε turbulence model, the CFD model is established for the simulation research. By comparing the experimental data of solid concentration distribution and mixing time with the simulated values, it is proved that the CFD model established can be used to study the flow field in the stirred reactor. The effects of stirring speed, liquid viscosity and solid particle size on mixing time and suspension quality are considered. With the increase of stirring speed, mixing time decreases, mixing uniformity is improved. Mixing time decreases first, then increases slightly with the increase of liquid viscosity, and the suspension quality is improved. When the solid particle size increases, mixing time increases rapidly, but the mixing uniformity becomes worse.

Bioengineered bioreactors: a review on enhancing biomethane and biohydrogen production by CFD modeling

Computational fluid dynamics (CFD) is numerical strategy developed for simulating the behavior of liquid and gas flow. CFD may be applied starting from aerospace, engine design, vehicle aerodynamics, power plants and chemical industries for analyzing and solving relevant system design and process issues. Biogas produced during anaerobic digestion (AD) is sustainable and renewable alternative to fossil fuels. AD may improve the controlled production of biogas and offers significant environmental benefits. This review focuses on research outcomes relevant for enhanced biogas production by exploring the possible applications of CFD in AD technology. CFD-related research performed in AD conditions in order to improve mixing performance, reduce power consumption, and understand the effects of total solid (TS) concentrations on flow behavior have been discussed. In addition, the use of AD for bio-hydrogen production, wastewater treatment, and sludge treatment are looked in. This review also identifies novel areas for AD technology advancement where there is potential for economic improvement in renewable energy production. Finally, future research needs have been identified, focusing on the opportunities to integrate conceptual and mathematical models for advancing CFD simulations for bioenergy.

CFD Simulation of Solid Suspension for a Liquid–Solid Industrial Stirred Reactor

The present study examines the possibility of using an industrial stirred chemical reactor, originally employed for liquid–liquid mixtures, for operating with two-phase liquid–solid suspensions. It is critical when obtaining a high-quality chemical product that the solid phase remains suspended in the liquid phase long enough that the chemical reaction takes place. The impeller was designed for the preparation of a chemical product with a prescribed composition. The present study aims at finding, using a numerical simulation analysis, if the performance of the original impeller is suitable for obtaining a new chemical product with a different composition. The Eulerian multiphase model was employed along with the renormalization (RNG) k-ε turbulence model to simulate liquid–solid flow with a free surface in a stirred tank. A sliding-mesh approach was used to model the impeller rotation with the commercial CFD code, FLUENT. The results obtained underline that 25% to 40% of the solid phase is sedimented on the lower part of the reactor, depending on the initial conditions. It results that the impeller does not perform as needed; hence, the suspension time of the solid phase is not long enough for the chemical reaction to be properly completed.

Improving the Homogenization of the Liquid-Solid Mixture Using a Tandem of Impellers in a Baffled Industrial Reactor

The paper explores a tandem configuration of three-blade impellers in a stirred reactor. The working fluid is a liquid-solid mixture and the stirring mechanism fitted with the two impellers must prevent the sedimentation of solid particles while homogenously dispersing them in the bulk liquid. The present numerical investigation, performed with the expert software Ansys® Fluent, Release 16, employs the Eulerian multiphase model along with the RNG k–ε turbulence model to simulate the free-surface liquid–solid flows in the baffled stirred reactor. A sliding mesh approach is used to model the impellers rotation. The tandem configuration is clearly superior to a single impeller, while the existing electrical motor that drives the stirring mechanism still provides the necessary power.

CFD Assessment of the Hydrodynamic Performance of Two Impellers for a Baffled Stirred Reactor

When converting a baffled stirred reactor to work with a different fluid, usually the original impeller must be replaced with a customized one. If the original impeller was designed for mixing liquids, its performance for liquid–solid suspensions may not be satisfactory. A case study is presented, where a two-blade original impeller is replaced with a new three-blade design. The new impeller shows clear improvements in mixing a liquid–solid suspension, while keeping the shaft power practically at the same level. As a result, a practically homogenous liquid–solid mixture is obtained, thus ensuring the required quality of the final product. The present numerical investigations employ the Eulerian multiphase model with renormalization (RNG) k–ε turbulence model to simulate the three-dimensional unsteady free-surface liquid–solid flow in a stirred tank. A sliding mesh approach was used to account for the impeller rotation within the expert code, FLUENT 16. The comparative quantitative analysis of the solid phase distribution and the relevant velocity profiles show that the new design of three-blade-impeller is significantly increasing the sedimentation time of the solid phase beyond the chemical reaction specific time. The necessary power to drive the new impeller has a slightly higher value than for the original impeller but it can be sustained by the existing driving system.

CFD simulations of stirred-tank reactors for gas-liquid and gas-liquid-solid systems using OpenFOAM®

An open-source CFD software OpenFOAM® is used to simulate two multiphase stirred-tank reactors relevant to industrial processes such as slurry polymerization and fuel production. Gas-liquid simulations are first performed in a single-impeller stirred-tank reactor, studied experimentally by Ford, J. J., T. J. Heindel, T. C. Jensen, and J. B. Drake. 2008. “X-Ray Computed Tomography of a Gas-Sparged Stirred-Tank Reactor.” Chemical Engineering Science 63: 2075–85. Three impeller rotation speeds (200, 350 and 700 rpm) with three different bubble diameters (0.5, 1.5 and 2.5 mm) are investigated. Flow patterns compared qualitatively to those from experiments. Compared to the experimental data, the simulations are in relatively good agreement for gas holdup in the reactor. The second multiphase system is a multi-impeller stirred-tank reactor, studied experimentally by Shewale, S. D., and A. B. Pandit. 2006. “Studies in Multiple Impeller Agitated Gas-Liquid Contractors.” Chemical Engineering Science 61: 486–504. Gas-liquid simulations are performed at two impeller rotation speeds (3.75 and 5.08 RPS). The simulated flow patterns agree with published pictures from the experiments. Gas-liquid-solid simulations of the multi-impeller stirred-tank reactor are also carried out at impeller rotation speed 5.08 RPS. The addition of solid particles with a volume fraction characteristic of slurry reactors changes the flow pattern significantly. The bottom Rushton turbine becomes flooded, while the upper pitched-blade downflow turbines present a radial-pumping flow pattern instead of down-pumping. Nonetheless, the solid phase has a similar flow pattern to the liquid phase, indicating that the particles modify the effective density of the fluid.

Another Critical Look at Three-Phase Catalysis

Three-phase catalysis, for example, hydrogenation, is a special branch of chemical reactions involving a hydrogen reactant (gas) and a solvent (liquid) in the presence of a metal porous catalyst (solid) to produce a liquid product. Currently, many reactors are being used for three-phase catalysis from packed bed to slurry vessel; the uniqueness for this type of reaction in countless processes is the requirement of transferring gas into liquid, as yet there is not a unified system of quantifying and comparing reactor performances. This article reviews current methodologies in carrying out such heterogeneous catalysis in different reactors and focuses on how to enhance reactor performance from gas transfer perspectives. This article also suggests that the mass transfer rate over energy dissipation may represent a fairer method for comparison of reactor performance accounting for different types/designs of reactors and catalyst structures as well as operating conditions.

A General Review of the Current Development of Mechanically Agitated Vessels

The mixing process in a mechanically agitated vessel is a widespread phenomenon which plays an important role among industrial processes. In that process, one of the crucial parameters, the mixing efficiency, depends on a large number of geometrical factors, as well as process parameters and complex interactions between the phases which are still not well understood. In the last decade, large progress has been made in optimisation, construction and numerical and experimental analysis of mechanically agitated vessels. In this review, the current state in this field has been presented. It shows that advanced computational fluid dynamic techniques for multiphase flow analysis with reactions and modern experimental techniques can be used with success to analyse in detail mixing features in liquid-liquid, gas-liquid, solid-liquid and in more than two-phase flows. The objective is to show the most important research recently carried out.

Critical review of different aspects of liquid-solid mixing operations

Mechanically stirred slurry tanks are utilized in several industries to perform various unit operations such as crystallization, adsorption, ion-exchange, suspensions polymerization, dispersion of solid particles, leaching and dissolution, and activated sludge processes. The major goal of this review paper is to critically and thoroughly analyse the different aspects of previous research works reported in the literature in the field of liquid-solid mixing. This paper sheds light on the advantages and limitations of various particle concentration measurement methods employed to assess the suspension quality and the extent of solid suspensions in slurry reactors. Attempts are being made to identify and compare various mathematical models and methods to quantify particle dispersion and distribution in slurry reactors. It has been shown that various factors such as geometric configurations, agitation conditions, and physical characteristics of liquid and solid have pronounced influence on local suspension quality and power consumption. Computational fluid dynamics (CFD) modeling can be extremely useful in assessing the suspension of solid particles in slurry tanks. A critical review of different scale-up procedures employed for solid suspension and distribution in liquid-solid systems is presented as well. The findings of this review paper can be useful for future research works in liquid-solid mixing.

Computational Fluid Dynamics Study of the Effects of Drill Cuttings on the Open Channel Flow

A three-dimensional computational fluid dynamics (CFD) study was carried out for drilling fluid flow with drill cuttings in open channels. The flow is similar to the return flow when drilling, stream containing drilling fluid, and drill cuttings. The computational model is under the framework of the Eulerian multifluid volume of the fluid model. The Herschel–Bulkley rheological model was used to describe the non-Newtonian rheology of the drilling fluid, and the computational model was validated with experimental results for two-phase flow in the literature. The effect of flow depth and flow velocity in an open channel was studied for drill cutting size of up to 5 mm and for a solid volume fraction of up to 10%. For constant cross section and short open channels, the effect of drill cuttings on flow depth and mean velocity was found to be small for particle sizes less than 5 mm and solid volume fractions less than 10%. High momentum force in the downward direction can carry the solid-liquid mixture at higher velocities than a lower density mixture. Higher inclination angles mean that the gravity effect upon the flow direction is more significant than the particle friction for short channels.

Dissolution of nuclear materials in aqueous acid solutions

The quantitative understanding of the dissolution of nuclear fuel materials is essential for the process design and development of an industrial-scale nuclear fuel reprocessing plant. The main objective of this review article is to analyze the published data related to the dissolution of important nuclear materials, namely, urania, plutonia, thoria, and their oxides in the existing literature. The published results on rate-controlling step and reaction mechanism of dissolution processes are reconciled and reviewed in this work. Clear suggestions are made for future research work for the identification of rate-controlling step. Suggestions are also provided to overcome the shortfalls in the published data for the identification of intrinsic kinetics and mass-transfer rates.

Numerical Simulation of Gas-Liquid Dispersion in A Stirred Tank Agitated by Punched Rigid-Flexible Impeller

The hydrodynamics of gas-liquid dispersion process in a stirred tank with rigid impellers, rigid-flexible impellers, and punched rigid-flexible impellers were investigated using a combined computational fluid dynamics (CFD) and population balance model (PBM) approach. A classical Eulerian-Eulerian approach coupled with standard k-e turbulence model was employed to simulate gas-liquid turbulent flow in the stirred tank. The multiple reference frame (MRF) approach was used to simulate impeller rotation. The effects of impeller type, flexible connection piece length, aperture size/ratio, impeller speed and superficial gas velocity on the local gas holdup and bubble size distribution for the gas-liquid dispersion process were investigated. Results showed that a long flexible connection piece length was conductive to the gas-liquid dispersion process. The optimum aperture ratio and aperture diameter were 12 % and 8 mm, respectively, for the gas-liquid dispersion process. Punched rigid-flexible impeller could reduce the differential pressure ahead and behind the impeller blade, decrease the size of low-pressure zone, and improve the water shear strain rate compared with rigid impeller and rigid-flexible impeller at the same Pg,v. It was found that punched rigid-flexible impeller was more efficient in terms of gas dispersibility than rigid impeller and rigid-flexible impeller.

SIMULATION OF SOLID–LIQUID SUSPENSION AND SCALE-UP OF AGAROSE GEL ACTIVATION REACTOR

Agarose gel activation reaction, which is of great importance in preparing the carrier of the column packing material for blood purification, would be significantly influenced by the configuration and parameter of reactor. In order to optimize the structure design of the reactor and operating parameters of the process, the characteristics of suspension system composed of agarose gel, NaOH, water, 3-allyl bromide, and activated alumina were simulated numerically utilizing an Eulerian multiphase flow model and multi-reference frame (MRF) approach. The effect of impeller configuration was studied with three typical impellers, including Rushton disk turbine (DT), pitched blade downflow turbine (PBTD45), and pitched blade upflow turbine (PBTU45). The results showed that the optimum solid suspension was obtained using PBTD45 impeller with a diameter of 100[Formula: see text]mm and critical suspension speed of 570[Formula: see text]rpm in a 20[Formula: see text]L stirred reactor. In addition, the critical suspension speeds using the three impellers were calculated and the errors were all within the engineering allowance. Finally, the feasibility of scale-up design for agarose gel activation reactor was preliminarily discussed. The results would be useful to the optimization and scale-up of relevant reactor design.

Solid-liquid mixing analysis in stirred vessels

This review evaluates computational fluid dynamic applications to analyze solid suspension quality in stirred vessels. Most researchers typically employ either Eulerian-Eulerian or Eulerian-Lagrangian approach to investigate multiphase flow in stirred vessels. With sufficient computational resources, the E-L approach simulates flow structures with higher spatial resolution for dispersed multiphase flows. Common turbulence models such as the two-equation eddy-viscosity models (