Mechanically agitated gas-liquid reactors

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 − e model, RNG k − e 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 − e 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 − e 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 − e 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 simulation of stirred tanks: Comparison of turbulence models. Part I: Radial flow impellers

This work concerns itself with laminar mixing of viscous liquids. The primary focus is on mechanical mixing of immiscible fluids but mixing of diffusing and reacting fluids is also briefly considered. Operations such as thermal homogenization and reactive mixing usually not treated in the context of laminar mixing are also included here. The emphasis is on mixing of polymeric liquids and on basic experimentation and theory. Dispersion of solids, mechanisms of demixing (such as gradient coagulation), properties of mixtures, and the description of mixing equipment are not considered. There are no substantial reviews covering developments of theory and applications in laminar mixing in terms of a fundamental unifying theory and analyses relating operating variables to mixing quality. This review, although an attempt in this direction, is not self sufficient and cannot stand alone without recourse to the indicated references. The reader is assumed to have some familiarity with the literature on mixing. Thus, basic concepts are not explained but merely reviewed. Emphasis is given to recent theoretical developments; application examples are provided. The objective here is to condense results from several diverse sources, to present and extend recent developments, to point out the relation of the subject to seemingly unrelated work, and to suggest areas for further research.

Laminar mixing of polymeric liquids: a brief review and recent theoretical developments

The purpose of this paper is to extend basic information on mixing and circulation times for mixing systems equipped with helical ribbon impellers. In a first part, from a survey of existing literature, we have reviewed the different effects of the geometrical parameters of the helical mixing systems (clearance-wall, pitch size ratio, blade width, number of blades) on the mixing process. From this qualitative analysis, useful guidelines are provided on the influence of the helical impeller geometry on the mixing effectiveness. In a second part, the homogenization effectiveness of an atypical helical ribbon impeller was investigated and compared with the performance of classical helical ribbons impellers.

Circulation and mixing times for helical ribbon impellers. Review and experiments

A discussion of the effect of deformation on mixing leads to reorientation of fluid interfaces as a proposed mechanism for special “mixing sections” in single screw extruders. Based on the kinematics of mixing it is shown how a mixing section can greatly decrease the amount of work necessary to accomplish extensive mixing through orienting the fluid more favorably for mixing by subsequent shear. Mathematical development quantitatively describes favored configurations of mixing sections. An upper bound for the mixing performance of a simple model of an extruder with special sections is shown to be similar to turbulent mixing. Based on this theory the key actions of the special sections are identified.

Theory of mixing sections in single screw extruders

The characteristics of a mixing apparatus with two agitator axes having multidisks as a high viscosity polymer finisher was studied from a practical point of view, 1) Holdup : it could be correlated by using a term of (μnD/δ). 2) Power consumption and mixing time : the results were compared with those of other high-viscosity equipment. 3) Residence time distribution and dead space : the former almost agreed with the tanks-inseries model. The latter seemed to be small. 4) Surface renewal action : the rate of surface renewal was determined from the gas absorption and also estimated by the observation of liquid flow. As a model reaction polycondensation polymerization of polyamide in the presence of water and viscosity stabilizer is discussed.

On a high viscosity polymer finisher apparatus with two agitator axes having multidisks

There exists no universally accepted technique for evaluating the functional status of equipment for high-viscosity mixing based upon generalized basic information such as flow patterns and shearing deformation characteristics. This paper deals with the measurement of flow patterns. After obtaining the overall flow patterns in a mixing vessel, circulation capacity and fluid deformations which cover information about energy dissipation were correlated with the homogenizing time. The physical meanings of C1=nTM and C2=TM √Pυgc/μ are interpreted by using a concept of striation thickness. A measure of an averaged dimensionless shear rate number √Pυgc/μ/n is plotted against the dimensionless nearest distance between moving part and fixed wall in the mixing vessel. This diagram could give an useful comparison as to averaged shear rate characteristics for any device for high-viscosity mixing regardless of geometrical configuration. A novel technique is also proposed for quick evaluation of the shear distribution in a given apparatus.

Evaluation of performance of mixing apparatus for high viscosity fluids

This paper deals with an examination of the hypothesis for striation thickness by Mohr. The effect of viscosity ratio on striation thickness is examined in a cone rotor-type mixer as a uniform shear system, and it is concluded that the effect of the viscosity ratio might be negligible in a system where a minor component is inset in a miscible major component of different viscosity. The authors have shown the phenomenon of formation of particles of minor component in the range of viscosity ratio μd/μc>3 and μd/μc<0.6.

Katsumasa Fujimoto

Papers

On a high viscosity polymer finisher apparatus with two agitator axes having multidisks

Evaluation of performance of mixing apparatus for high viscosity fluids

The homogenizing process in high viscosity materials having different viscosities