Evaluation of the mixing quality of high-viscosity yield stress fluids in a tubular reactor

In this study, the mixing quality of high-viscosity yield stress fluid (Carbopol aqueous solution) under laminar and turbulent flow regimes was evaluated through a numerical experimental study. A three-dimensional computational fluid dynamics large-eddy simulation (CFD-LES) model was employed to capture large-scale vortex structures. The proposed CFD model was validated by the experimental data in terms of mean velocity profiles and velocity-time history. Thereafter, the CFD model was applied to simulate the residence time distribution using the tracking technique: tracer pulse method and step method. In addition, the non-ideal flow phenomena caused by molecular diffusion and eddy diffusion were evaluated. The effects of the rheological properties on the mixing performance were also investigated. The presented results can provide useful guidance to enhance mass transfer in reactors with high-viscosity fluids.

Investigation of the flow patterns and power requirements in agitated systems: effects of the design of baffles and vessel base

The flow patterns and power consumption of a six-blade Rushton turbine (RT) in a cylindrical vessel are characterized in this paper. We focus on the effects of the shape of the vessel base by studying two cases: a conical and a dished shape. In addition, the effects of the height of the vessel base (h2) are explored and four cases are considered, namely: h2/D = 1/10, 1/6, 1/5 and 1/3 (D: vessel diameter). In the second part of our investigation, a new design of baffles (a triangular-shaped baffle) is suggested and a comparison is made between the performance of the standard and the triangular baffles. The main findings revealed that the conical shape of the vessel base provides a slight enhancement in the axial circulation at almost the same power input for the dished bottomed vessel. For Re < 2 × 104, the power required by both types of baffles is the same; however, above this value of Re, a reduction by about 4% in power consumption is given by the standard baffles. Also, and for all shapes of baffles and vessel bases, a reduction in power consumption may be obtained by increasing the height of the vessel base.

Enhancement of the performance of a static mixer by combining the converging/diverging tube shapes and the baffling techniques

The performance of a newly designed static mixer is explored in this paper by numerical simulations. It concerns a converging/diverging circular tube equipped with opposing and staggered baffles. This new design of static mixer presents a cost-effective, easy-to-design, easy-to-maintain choice, with a minimum pressure drop and a good mixing index at low Reynolds numbers (from 60 to 700), to the currently used static mixers. The investigation was carried out numerically for different baffle clearances (cl/D = 0.3, 0.5, 0.7 and 1.0, respectively) and inclination angles (α = 0°, 30° and 45°, respectively), different ratios of inlet diameter-to-outlet diameter (Dd = d/D = 0.6, 0.8, 1.0, 1.2 and 1.4, respectively) and different Reynolds number values. The obtained results revealed that the diverging tube favors the mixing index while reducing the pressure drop. For a dimensionless baffle clearance (cl/D) of 0.5 and Re = 700, the mixing index has been increased from 0.78 for a simple tube (d/D = 1) having vertical baffles (α = 0°) until 0.95 for a diverging tube at the ratio (d/D) of 1.4 with the same vertical baffles (α = 0°). The increase in the ratio d/D has also yielded a decrease in pressure losses. Compared to the simple static mixer without baffles and having the same inlet as outlet sections, the maximum enhancement in mixing index was about 315.84% when d/D = 1.4 and Cl/D = 0.3 and α = 0°. However, a reduction in pressure drop by about 92% may be obtained when α = 30° with only a reduction in mixing index by 14% (compared to α = 0°).

Investigation of Heat Transfer Efficiency of Improved Intermig Impellers in a Stirred Tank Equipped with Vertical Tubes

The heat transfer of a reactor with improved Intermig impellers was numerically investigated by the finite element method (FEM) simulation software Fluent (V.19). A turbulence model utilized the standard k-ε model, and the turbulent flows in two large vortexes between vertical tubes were collided to form a strong convection. The influence of heat and mass transfer developing from the impeller diameters, the distance between the two impellers (C1), the rotational speed and the installation height of the bottom impeller (C2) were studied. The reactor was equipped with special structure vertical tubes to increase the heat exchange areas. The rate of heat transfer, including criteria such as the convective heat transfer coefficient, the Nusselt number of outside vertical tubes, and the temperature boundary layer thickness, assured the accurate control of the heat exchange mixing state. The experimental testing platform was designed to validate the simulated results, which revealed the influence order of related factors. The Nusselt number Nu was affected by various related factors, resulting in the rotation and diameter of impellers extending far beyond the distance between the two impellers (C1) and the installation height of the impeller (C2). The average temperature boundary layer thicknesses of the symmetrical and middle sections were 3.24 mm and 3.48 mm, respectively. Adjusting the appropriate parameters can accurately control the heat exchange process in such a reactor, and the conclusions provide a significant reference for engineering applications.

Data on the agitation of a viscous Newtonian fluid by radial impellers in a cylindrical tank

In this paper, the data assembled concerning the agitation of a Newtonian fluid in a cylindrical vessel is disclosed. The stirred vessel is not provided with baffles and has a flat-bottom. The data presents some information on the characteristics of two impellers: a six-blade Rushton turbine and a six-blade paddle impeller. The flow patterns generated by both impellers are depicted and compared. Also, the power required when changing the impeller rotational speed is given. The data summarized here via three-dimensional calculations of velocities and viscous dissipation in the whole volume of the tank provides additional knowledge for the best choice of impellers for each industrial process.