Some modifications in the Scaba 6SRGT impeller to enhance the mixing characteristics of Hershel–Bulkley fluids

Effect of H/D ratio and impeller type on power consumption of agitator in continuous stirred tank reactor for nitrocellulose production from cotton linter and nitric acid

In this study, the effect of height/diameter (H/D) ratio and type of impeller on the power consumption of the agitator in the Continuous Stirred Tank Reactor (CSTR) was analyzed. CSTR in the process of producing nitrocellulose from cotton linters with a production capacity of 10,000 tons/year was used as a case study. In designing a CSTR, power consumption is also considered because it is related to techno-economics. The results show that it is necessary to adjust the amount of impeller related to the H/D ratio value because it can affect the level of liquid in the reactor during the stirring process so that it also affects the reaction conversion. This work shows that with the higher H/D ratio, the greater number of impellers needed and increase the agitator power consumption. For specific applications, the number of impellers (NT) must be increased to meet the minimum power consumption. As the result, this work recommends the optimal H/D ratio for CSTR design is the maximum H/D ratio to get the NT-calculation as close as to the NT-taken while satisfying the minimum power consumption required. The optimal H/D ratio can be different depending on the impeller type and application.

Numerical investigation of hydrodynamics and heat transfer for Bingham-pseudoplastic fluids in an industrial coiled stirred tank

Computational fluid dynamics (CFD) was used to investigate the influence of complex rheological properties for Bingham-pseudoplastic fluids on hydrodynamics and heat transfer performance in an industrial polymerization coiled agitated reactor. The fluid rheology was described by the Herschel–Bulkley rheological model. The power consumption obtained by CFD simulation was in good agreement with the plant data. The relationship between the Metzner–Otto constant and power-law index for dual axial flow impellers was investigated. The hydrodynamics strongly depends on the rheological parameters and rotational speed. The flow domain is composed of two parts: the cavern around the impellers, and the stagnant zone adjacent to helical coils and tank wall which resulting in poor mixing and heat transfer performance. With the increase of rotational speed, the stagnant zone could be effectively eliminated, while the heat transfer performance could be significantly improved.

Simulation of Mixing Intensity Profile for Bioethanol Production via Two-Step Fermentation in an Unbaffled Agitator Reactor

Bioethanol synthesis techniques have been studied intensively due to the energy crisis and various environmental concerns. A two-step bioethanol production process was carried out multiple times in an unbaffled agitator tank. The parameters varied, including the fermentation temperature, the pH level, the amount of yeast, and the impeller type. Then, a simulation was used to obtain an image of the agitation behavior inside the agitator tank to compare the velocity profile of each type of impeller design. The impeller with eight blades was found to produce the highest flow velocity: 0.28 m/s. The highest concentration of bioethanol generated from the fermentation was 34 g/L, which was produced by using an eight-blade impeller at 30 °C, a pH level of 5, an agitation speed of 70 rpm, and 2 wt % yeast. The two-blade impeller produced the lowest bioethanol concentration, 18 g/L, under the same conditions. Ethanol concentration was found to peak at 40 °C and a pH level of 5. The geometry of the impeller, the fermentation temperature, and the pH level were each found to have a significant effect on the resulting bioethanol concentration according to the results of an ANOVA test. The amount of yeast had no effect on the fermentation reaction. Finally, the results demonstrated the possibility of using computational fluid dynamic modeling to determine the impeller’s behavior for the development of the bioethanol fermentation process. The simulation and experimental results from this research support the scaling up of a bioethanol production facility.

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.