Baffled Stirred Tank Reactor Research Articles

Characterization of vortical structures in a stirred tank

Data obtained from large eddy simulations of single-phase, turbulent flow of Newtonian and shear-thinning fluids in a baffled stirred tank reactor are considered to identify and characterize vortical structures. The identification proceeds through an objectivized Eulerian method, accounting for the inhomogeneities in the flow, which palliates some shortcomings of previous implementations. The characterization focuses on turbulent vortices larger than the dissipative scales and, to a lesser extent, on trailing and macro-instability vortices. The characterization performed through different statistical analyses includes aspects such as size, number density, shape, distribution and organization in space, and correlation with the kinetic energy due to turbulence and the periodic passage of the blades. To the authors' knowledge, some of these representative aspects have been rarely investigated or have not been addressed at all for the turbulent flow in a stirred vessel. The influence of changing the rotational speed of the tank and the rheology of the working fluid are explored as well. Finally, considering one-way coupling, some potential and practical implications for liquid–liquid and gas–liquid dispersed systems are briefly discussed.

Esterification reaction of scrap tyre oil blended with alcohol in baffled stirred tank reactors: CFD modeling and simulation

A distinct state admixture can not be formed between triglycerdes and alcohol. Establishment of strring enhances the face connection, response rate and also biodiesel product. In artificial approach, transesterification is generally sustained in stirred tank reactors on examing work of this stirred tank reactors is essential for successful plan, process and optimization. Experiments on inflow-fields and chemical responses are costly and time consuming, and therefore cannot meet this test perfectly. Computational fluid dynamics is another approach to model and simulate the stirred tank (CFD). As a result, computational fluid dynamics was used to simulate a transesterification process. Response rates can be determined in this way by looking at a set of discriminational equations that describe response kinetics. The kinetic model agreed with the attention biographics for the anticipated components, and the mass bit patterns demonstrated successful mixing.

Comparison of Different Impellers for Gas Dispersion in Power-Law Fluids

The 3D multiphase computational fluid dynamic (CFD) models have been developed for gas–liquid systems in a standard baffled stirred tank reactor (diameter 0.3 m) for non-Newtonian power-law fluids. The standard k-ε model has been used to model the turbulence, and drag has been modeled with the Grace drag model with Brucato modifications. CFD simulations have been carried out for three impellers, namely, the Rushton turbine, pitched blade turbine, and hydrofoil impeller. The effects of impeller design, power input per unit volume of the fluid (P/V) (1200–5200 W/m3), fluid rheology (K = 0–0.1561 Pa·sn, n = 0.687–1.000), and superficial gas velocity (7.1–28.3 mm/s) have been analyzed. The CFD model has been validated with the local distribution of axial, radial, and tangential velocities and overall gas holdup data reported in the literature. The CFD model has been used to predict the regime transition from flooding to loading and complete dispersion. A good agreement was observed for the prediction of regime transition for the Rushton turbine when compared with the experimental data. The average value of absolute error for the prediction of the gas holdup is 20%, which indicates that the developed CFD model can be successfully used to predict the gas holdup and regime transition in stirred tank reactors. Among the impellers studied, the hydrofoil impeller shows the highest gas holdup and uniform gas distribution in the vessel at low power consumption.

Production of metallic iron nanoparticles in a baffled stirred tank reactor: Optimization via computational fluid dynamics simulation

The aim of this work is to optimize iron nanoparticle production in stirred tank reactors equipped with two classical impellers: Rushton and four-pitched blade turbines, which are largely used in batch industrial synthesis and efficient scale-up. The main operative parameters of nanoparticle synthesis are the precursor initial concentration, reducing agent/precursor molar ratio, impeller–tank clearance, and impeller rotational velocity. These parameters were varied during the synthesis to find the optimal operating values based on the Fe(0) (%) production, zeta potential, particle size distribution, and powder X-ray diffraction pattern obtained. We found that the optimal operating conditions for nanoparticle production were an impeller velocity of 1500rpm, initial iron precursor concentration of 20mM, molar ratio of reducing agent to iron precursor of 3mol/mol, and impeller clearance of 0.25 and 0.4 times the vessel diameter for Rushton and four-pitched blade impellers, respectively. Setting these conditions achieved a total conversion of 0.94–0.98 and yielded a product with a unimodal size distribution and average diameters in the range 30–50nm. The computational fluid dynamics results agreed with the expectations, and the obtained mixing Damkohler numbers show that the process is mixed controlled.

Using PLIF/PIV techniques to investigate the reactive mixing in stirred tank reactors with Rushton and pitched blade turbines

Planar laser induced fluorescence (PLIF) technique was used to investigate concentration profiles and reaction progress of a turbulent reactive mixing process in a baffled stirred tank reactor. Rushton turbine, up-pumping pitched blade turbine (PBTU), and down-pumping pitched blade turbine (PBTD) were used to study the effect of mixing on the fast Fenton reaction progress. For determining the reaction progress, a criterion was developed, which was based on obtaining the number of mixed pixels with a defined uniformity percentage for the entire reactor area. It was found that impeller type, impeller clearance, and reactant injection location significantly altered concentration profiles and reaction progress. The reaction time scale along with the micro- and macromixing time scales were determined and it was concluded that the system performance was controlled by macromixing in the tank. Mean velocity field of the impellers was obtained by particle image velocimetry (PIV) technique and reasonably validated the concentration distribution results of PLIF technique.

Analysis of the turbulent flow and trailing vortices induced by new design grooved blade impellers in a baffled tank

U or V shaped grooves have shown reduction in energy and power consumption in blunt bodies immersed in turbulent streams. In this work, the performance of two new design turbines with four blades rotated 45°, one with U and the other with V grooved shape blades, were evaluated in a baffled stirred tank reactor and they were compared with the one of the regular 4PBT. The power consumption, mean velocities, turbulent kinetic energy production (TKE), energy dissipation rate (ε) and trailing vortices induced by new design impellers with grooved walls and 4PBT were evaluated experimentally and modeled by detached eddy simulation (DES) approach, which has not been tested in blade walls with small perturbations. Validation of the results was carried out by PIV (Particle image velocimetry) experimental tests. The evolution of new and distinctive trailing vortices located in the upper and lower faces of the grooved new design impellers is presented. They were measured by angled resolved PIV technique, and visualized in three dimensions by the DES method. The V-grooves shaped impeller generated a larger region of influence of the turbulent kinetic energy along the discharge region than the other impellers. Moreover, presented higher mean dissipation rate levels for the most of the measured angles and induced higher number of trailing vortices in the blades vicinity than the other turbines. It was found that in a wide range of the Reynolds number (Re) (40 × 103 < Re < 125 × 103) the dissipated power measured experimentally from shaft, presented reductions of the order of 6% and 4% for the U and V grooved new models, respectively, compared with that of the regular 4PBT impeller.

Investigations of the Gas-Liquid Multiphase System Involving Macro-Instability in a Baffled Stirred Tank Reactor

Bubble Sauter Mean Diameter (SMD) in gas-liquid multiphase system is of particular interest and the quantification of gas characteristics is still a challenge today. In this contribution, multiphase Computational Fluid Dynamic (CFD) simulations are combined with Population Balance Model (PBM) to investigate the bubble SMD in baffled stirred tank reactor (STR). Hereby, special attention is given to the phenomenon known as the fluid macro-instability (MI), which is a large-scale low-frequency fluid velocity variation in baffled STRs, since the fluid MIs have a dominating influence on the bubble breakage and coalescence processes. The simulations, regarding the fluid velocity, are validated with Laser Doppler Anemometry (LDA) experiments, in which the instant radial velocity is analyzed through Fast Fourier Transform (FFT) spectrum. The frequency peaks of the fluid MIs are found both in the simulation and in the experiment with a high degree of accuracy. After the validation, quantitative predictions of overall bubble SMD with and without MIs are carried out. Due to the accurate prediction of the fluid field, the influence of the fluid MI to bubble SMD is presented. This result provides more adequate information for engineers working in the field of estimating bubble SMDs in baffled STRs.

CFD Modeling and Simulation of Transesterification Reactions of Vegetable Oils with an Alcohol in Baffled Stirred Tank Reactors

Alcohol and triglycerides do not form a single phase mixture and thus there is a poor surface contact between them causing transesterification to proceed relatively slow. Introduction of stirring improves the surface contact and consequently the reaction rates and biodiesel yields. Thus, in industrial processes, transesterification is usually carried out in stirred tank reactors. Investigating how this type of reactor works is necessary for successful design, operation and optimization. Experimental methods for investigating flow-fields and chemical reactions are expensive and time demanding and cannot meet this challenge accurately. An alternate way is to model and simulate stirred tanks by computational fluid dynamics (CFD). Thus, in this work, a CFD simulation of transesterification was performed, with reaction rates being evaluated by solving a set of differential equations describing the reaction kinetics. The concentrations profiles for the expected components were in accordance with the kinetic model, and the mass fraction patterns showed efficient mixture.

Precipitation of amorphous colloidal silica from aqueous solutions—Aggregation problem

Batch process of silica precipitation was investigated experimentally and interpreted based on available theories and models. Silica was manufactured by neutralization of dilute sodium silicate solution with sulphuric acid in a baffled stirred tank reactor of diameter DT=145mm. Effects of pH, temperature, silica and electrolyte (NaCl) concentration were investigated. Particle size measurements were performed using a Malvern Zetasizer ZS ZEN 3500. Effects of process conditions on agglomeration of silica particles and stabilization of the silica nano-particle suspensions were interpreted using the DLVO based theories and models of surface composition. The stability ratio and surface composition were expressed in terms of the properties of the solution. Results of theoretical investigations together with experimental data show that effects resulting from colloidal forces including repulsion and attraction are not enough to explain aggregation phenomena and thus effects resulting from surface reactions should be considered in modeling as well.

Flow structure and the effect of macro-instabilities in a pitched-blade stirred tank

Turbulent flow inside a dished bottom baffled stirred tank reactor (STR) with a 45° pitched blade impeller is studied numerically and experimentally. Three different impeller rotational speeds are studied corresponding to impeller Reynolds numbers of 44,000, 88,000 and 132,000, respectively. The numerical study is based on a large-eddy simulation (LES) technique with a fixed body-fitted curvilinear mesh. The moving impeller geometries are modeled using an immersed boundary method (IBM). The experiments consist of particle image velocimeter (PIV) measurements of the flow field. The instantaneous as well as the time-averaged flow field suggests the formation of trailing vortex structures which are associated with higher levels of turbulent kinetic energy relative to the remaining flow field. Instabilities occurring at a frequency lower than the frequency of impeller rotation are identified from the time signal of the velocity components. The role of these lower frequency macro-instabilities (MI) is explored by observing changes in the three-dimensional circulation pattern within the stirred tank. The growth and dissipation of trailing edge vortices are shown to be appreciably influenced by the macro-instability. A significant amount of kinetic energy (velocity fluctuations) is observed to be associated with the low frequency dynamics of the trailing edge vortices during an MI cycle.

Simulation of laminar and turbulent impeller stirred tanks using immersed boundary method and large eddy simulation technique in multi-block curvilinear geometries

Impeller stirred tanks are commonly used in the chemical processing industries (CPI) for a variety of mixing and blending technologies. Such processes require accurate modeling of the turbulent flow in the tank over a range of operating conditions (e.g. impeller speed), and in addition, require a computationally efficient solution strategy that can represent moving rigid geometric parts (impellers) in the tank. In the present study, a methodology is proposed that combines the advantages of the immersed boundary method (IBM) to represent moving rigid geometries with the efficiency of multi-block structured curvilinear meshes (to minimize wasted grid points) for the representation of overall complex domains. The IBM implementation on a multi-block curvilinear mesh is advocated for the simulations of impeller stirred tank reactors (STR) and has distinct advantages over other competing methods. In the present work, the curvilinear-IBM methodology is further combined with the curvilinear coordinate implementation of large eddy simulation (LES) technique to address the issue of modeling unsteady turbulent flows in the STR. To verify the implementation of IBM in a multi-block curvilinear geometry, a laminar STR with a stack of four pitched blade impellers on a single shaft is simulated and compared against experimental data. Verification of the combined IBM–LES implementation strategy in curvilinear coordinates is done through comparisons with the measurements of turbulent flow in a baffled STR with a single pitched blade impeller. For both laminar and turbulent STR, the predictions are in very good agreement with measurements. It is suggested here that this methodology can be reliably used as a predictive tool for the flow fields in STRs with complex geometries.

Assessment of Large Eddy Simulations for Agitated Flows

Large eddy simulations (LES) on the single-phase flow driven by a pitched blade impeller in a baffled stirred tank reactor were performed. The geometry, and operation conditions (defined in terms of Re = 7,300) were chosen to comply with the experimental work by Schäfer et al. As the turbulence in stirred tanks is strongly off-equilibrium, no straightforward criteria with respect to spatial and temporal resolution of the simulations can be formulated, and experimental validation becomes of prime importance. In this study, the influence of the resolution, and the specific choice of the subgrid-scale model (standard Smagorinsky model, and structure function model) on the simulated flow field were investigated.

A critical assessment on the use of k– ε turbulence models for simulation of the turbulent liquid flow induced by a Rushton-turbine in baffled stirred-tank reactors

The stirred-tank reactor (standard configuration) equipped with a Rushton turbine is theoretically investigated by solving continuity, momentum and turbulence quantity balance equations. Modelling approaches for the Rushton turbine and further important modelling aspects concerning the stirred tank are discussed. In this work the Rushton turbine is modelled with the aid of experimental data. In combination with this special modelling approach for the stirrer, the accuracy of three ad hoc applicable k– ε turbulence models in predicting mean flow and turbulence quantities is tested with the aid of three-dimensional simulations. The differing simulation results are compared with experimental data. None of these k– ε models performs satisfactorily without modification. However, in considering experimental observations and the general construction of k– ε models it is possible to optimize one special k– ε model. Good agreement can then be achieved between measured flow details and the simulation results for vessels and impellers of different geometry.

Steady and Unsteady Computations of Turbulent Flows Induced by a 4/45° Pitched-Blade Impeller

The present paper summarizes steady and unsteady computations of turbulent flow induced by a pitched-blade turbine (four blades, 45° inclined) in a baffled stirred tank. Mean flow and turbulence characteristics were determined by solving the Reynolds averaged Navier-Stokes equations together with a standard k-ε turbulence model. The round vessel had a diameter of T = 152 mm. The turbine of diameter T/3 was located at a clearance of T/3. The Reynolds number (Re) of the experimental investigation was 7280, and computations were performed at Re = 7280 and Re = 29,000. Techniques of high-performance computing were applied to permit grid sensitivity studies in order to isolate errors resulting from deficiencies of the turbulence model and those resulting from insufficient grid resolution. Both steady and unsteady computations were performed and compared with respect to quality and computational effort. Unsteady computations considered the time-dependent geometry which is caused by the rotation of the impeller within the baffled stirred tank reactor. Steady-state computations also considered neglect the relative motion of impeller and baffles. By solving the governing equations of motion in a rotating frame of reference for the region attached to the impeller, the steady-state approach is able to capture trailing vortices. It is shown that this steady-state computational approach yields numerical results which are in excellent agreement with fully unsteady computations at a fraction of the time and expense for the stirred vessel configuration under consideration.

96/06425 Direct and large-eddy simulation of turbulent fluid flow using the lattice-Boltzmann scheme

Abstract This paper reports on direct and large-eddy simulations (LES) of turbulent flows using the lattice-Boltzmann scheme for the discretization of the Navier-Stokes equations. It is divided into two parts. The first part deals with direct simulation of fully developed turbulent channel flow with heat transfer in order to check the performance of our solver. It is shown that the present numerical results agree well with the numerical results reported by Kim et al. (1987). The second part deals with large-eddy simulation (LES) of the turbulent flow in a baffled stirred tank reactor. This kind of equipment is frequently used in the (petro)chemical industry for mixing applications and is, therefore, of direct practical relevance. Large-eddy simulation is a powerful tool to study such flows, as it accounts in a natural way for the unsteady and quasi-periodic behavior of these flows. The results of our first attempt to simulate this flow by means of modeling the impact of the mechanical impeller on the flow field via a varying force field in space and time, reveal a fair agreement with available experimental data. In accordance with measurements, it is shown that the thickness of the impeller blades plays an important role for the motion of the fluid in the vicinity of the impeller. In conclusion, the present application of LES to this engineering flow problem clearly shows the potential of LES as a tool to investigate turbulent flows in industrial applications of practical importance.

Direct and large-eddy simulation of turbulent fluid flow using the lattice-Boltzmann scheme

This paper reports on direct and large-eddy simulations (LES) of turbulent flows using the lattice-Boltzmann scheme for the discretization of the Navier-Stokes equations. It is divided into two parts. The first part deals with direct simulation of fully developed turbulent channel flow with heat transfer in order to check the performance of our solver. It is shown that the present numerical results agree well with the numerical results reported by Kim et al. (1987). The second part deals with large-eddy simulation (LES) of the turbulent flow in a baffled stirred tank reactor. This kind of equipment is frequently used in the (petro)chemical industry for mixing applications and is, therefore, of direct practical relevance. Large-eddy simulation is a powerful tool to study such flows, as it accounts in a natural way for the unsteady and quasi-periodic behavior of these flows. The results of our first attempt to simulate this flow by means of modeling the impact of the mechanical impeller on the flow field via a varying force field in space and time, reveal a fair agreement with available experimental data. In accordance with measurements, it is shown that the thickness of the impeller blades plays an important role for the motion of the fluid in the vicinity of the impeller. In conclusion, the present application of LES to this engineering flow problem clearly shows the potential of LES as a tool to investigate turbulent flows in industrial applications of practical importance.

Computational fluid dynamics for chemical reactor engineering

Computational Fluid Dynamics (CFD) involves the numerical solution of conservation equations for mass, momentum and energy in a flow geometry of interest, together with additional sets of equations reflecting the problem at hand. In this paper the current capabilities of CFD for chemical reactor engineering are illustrated by considering a series of examples from industrial practice. These examples form the basis for the identification of future trends and potential stumbling blocks. Recent progress in the predictions of flow in baffled stirred tank reactors is reviewed. Improved turbulence modelling and high-performance computing have a particularly important part to play in the future applications of CFD to this type of reactor. In the next example, which concerns non-Newtonian, non-isothermal flow in an extruder, CFD turns out to be helpful in understanding the rôle of temperature profiles and residence time distribution in determining the product quality. In this application area limitations in available computing power and experimental validation of rheological models are both important. CFD simulations of gaseous turbulent flow with competing parallel and consecutive reactions constitute the third example. Apart from the need for accurate kinetic rates, the main problem in this case is the optimal choice of closure of chemical source terms in the mean transport equations for species mass fractions. Several models have been implemented in a commercial CFD code and validated with plant data, thus allowing CFD model calculations to contribute to an improved reactor design. Finally, a discussion of future needs and expectations of the chemical processing industry with respect to CFD modelling, mainly focussing on multiphase flow, is given.

Development of a stirred tank reactor system for the production of lignin peroxidases (ligninases) byPhanerochaete chrysosporium BKM-F-1767

Lignin peroxidases produced byPhanerochaete chrysosporium have several important potential industrial applications based on their ability to degrade lignin and lignin-like compounds. A stirred tank reactor system for the production of lignin peroxidases is described here. Included in this study is an examination of the mechanics of pellet biocatalyst formation and the optimization of an acetate buffered medium. Higher levels of lignin peroxidase were obtained with acetate buffer compared to the other buffer systems tested. Concentrations of 0.05% (w/v) Tween 80 and 0.4 mM veratryl alcohol gave optimal lignin peroxidase activity in acetate buffered medium. In shake flask cultures, mycelial fragments in the inoculum aggregated into pellets during the first eight hours of incubation and thereafter increased in size through the eighth day. The agitation rate in shake flask cultures affected pellet size, the number of pellets formed, and lignin peroxidase activity. Transfer of fungal pellets from shake flask culture to a continuously oxygenated baffled stirred tank reactor (STR) resulted in production of high lignin peroxidase titres comparable to those of shake flask cultures when the agitation rate, oxygen dispersion and foaming were closely controlled.

The influence of agitation rate on xanthan production by Xanthomonas campestris

The influence of agitation rate on Xanthan production in a baffled stirred tank reactor is reported. To separate the hydrodynamic effect of agitator speed on any slime layer from the influence on oxygen mass transfer, oxygene-enriched air was used. Furthermore, the influence of agitation speed on the molecular weight distribution was studied

Influence of bioreactor design on the rate and amount of curdlan-type exopolysaccharide production byAlcaligenes faecalis

The amount of polymer recovered during lab-scale batch production of curdlan-type polysaccharide byAlcaligenes faecalis (ATCC 31749) was increased by 46% through the manipulation of the vessel configuration. When standard turbine impellers were used to provide mixing and agitation the specific rate of production, Qp, decreased significantly after 40–50 hours elapsed fermentation time, EFT. The Qp remained at a high level throughout the entire time course of production (90 hours) when (i) a propeller was substituted for a flat-blade turbine impeller in a conventional baffled stirred tank reactor, or (ii) agitation and mixing were accomplished in a non-rotary vibro-fermenter.