Waldemar Bujalski

The dependency on scale of power numbers of Rushton disc turbines

Extensive and very accurate power numbers have been obtained for a wide range of Rushton disc turbines using water as the working fluid in six fully baffled vessels from 0.22 to 1.83 m diameter. The results show that for Reynolds numbers ⩾ 2 × 104, the average peak power number Po is dependent on the disc thickness (χ1) to impeller diameter (D) ratio and to the vessel diameter (T). Provided χ1/D is constant, Po is independent of impeller to vessel ratio in the range 0.25 ⩾ D/T ⩽ 0.70. For 0.01 ⩽ χ1/D ⩽ 0.05 and for the range of vessels studied, the equation fitted the data to within ± 3% where To is a 1 m diameter vessel. An F-test shows that the inclusion of the small effect of scale, (T/To), is statistically significant at the 99% level. The implications of these scale and geometrical effects for mixing research, process design and scale-up are discussed.

CFD Study of Homogenization with Dual Rushton Turbines—Comparison with Experimental Results: Part I: Initial Studies☆

The transient responses of six conductivity probes distributed throughout a stirred tank of diameter T (= 0.72m) and an aspect ratio of 2, equipped with dual-Rushton turbines ( D = T/2 ), were measured. Three impeller speeds (75, 100 and 150 rpm) were used and the terminal mixing times, u 95, were also determined. These experiments were also modelled using the structured Fluent CFD code based on the finite volume method with a sliding mesh option. The k − ɛ and RNG k − ɛ models both predicted similar local axial and radial mean velocities. However, both also predicted large areas of tangential circulation in a direction opposite to the impeller rotation and all the local turbulence quantities and the tangential component of mean velocity were considerably different. Stability problems with the RNG k − ɛ model in the version of the Fluent™ CFD code available for this work meant that transient conductivity responses could not be computed using this method. On the other hand, the k − ɛ model gave stable solution and many features of the experimental transient responses and those from the model were similar. However, the computed θ 95 were about two to three times longer than the measured values. It is suggested this difference arises because the mass exchange between the four distinct axial-radial circulation loops was highly underpredicted by CFD.

CFD Study of Homogenization with Dual Rushton Turbines—Comparison with Experimental Results: Part II: The Multiple Reference Frame☆

Experimental transient responses of six conductivity probes distributed throughout a stirred tank have been used to determine the terminal mixing times, θ95, in the turbulent flow regime. These transients were simulated using a fully predictive mode of the structured Fluent CFD code based on a sliding mesh method1. Here, a new procedure based on a multiple reference frame (MRF) method with grid refinement is employed. Both k-E and the standard RNG k-ɛ models with the MRF substantially eliminated the wrongly predicted large areas of tangential circulation in a direction opposite to the impeller rotation reported elsewhere. The simulated distribution of the tracer in time using the new approach is also more realistic and the transient responses are shorter, though the computed mixing times were still about two times longer than the measured values. These computations by k-ɛ E and RNG k-ɛ E models with MRF and mesh refinements for momentum transfer and calculating concentration fields in a single, stationary frame of reference represent an improvement in comparison with those predictions obtained without it.

Suspension and Liquid Homogenization in High Solids Concentration Stirred Chemical Reactors

In many crystallization processes and other solid-liquid reactors, solids are present in high concentrations. Often these reactors are operated in the semi-batch mode and the purity, productivity and selectivity of the reaction (and in the case of precipitation and crystallization, the size distribution and morphology too) depends on the relative rates of mixing (or homogenization) compared to chemical reaction. It has usually been assumed that, provided the solids are all fully suspended, the rate of mixing is similar to that found in single-phase systems. Recent extensive work carried out by the above team at The University of Birmingham with support from DuPont has analysed the processes of homogenization and particle suspension. It has shown that, for typical industrial conditions, there is a range of operating conditions in which the solid particles at high concentration are fully suspended but have a clear liquid layer above them in which the local specific energy dissipation rate appears to be very low. Under these conditions, the mixing time may be two or more orders of magnitude longer than in the single phase case. This paper describes the phenomenon, analyses its importance, develops a physical model and proposes ways of overcoming the problem.

Study of micromixing in a stirred tank using a Rushton turbine: Comparison of feed positions and other mixing devices

The consecutive-competing iodide-iodate reaction scheme has been used to study micromixing phenomena in a baffled vessel of 0.29 m diameter agitated by a Rushton turbine. It has been confirmed that, by using successive injections, this reaction scheme is very efficient for such a study. Four agitator speeds giving mean specific energy dissipation rates, e T from ∼0.2 to ∼1.2 W/kg have been used, with sub-surface feeding at one of four points. For a given speed, addition at each of these four points gave different local values of e T , ranging from less than e T very close to the top of the liquid to much greater close to the impeller. The point closest to the impeller was chosen to be such that feeding was estimated to be at the point of maximum e T . For the maximum speed, the segregation index, as a measure of the amount of waste product, was ∼20% with feed at the top of the liquid (as preferred industrially because of its convenience). This waste was reduced to ∼5% by feeding at the point of maximum e T close to the impeller. A comparison was also made with results reported in the literature using the same reaction for two new devices developed for improved micromixing. By feeding at the carefully chosen position close to the impeller, the results with the Rushton turbine were as good as or better than with the special devices even at the comparatively low e T of ∼1.2 W/kg. It was estimated that the maximum local specific energy dissipation rate was ∼70 times the mean, in reasonable agreement with a very recent study where the same pair of reactions and LDV were both used.

The Influence of the Addition Position of a Tracer on CFD Simulated Mixing Times in a Vessel Agitated by a Rushton Turbine

Previous papers on simulated mixing times in stirred vessels using CFD have sometimes given predictions in good agreement with empirical equations based on experiments and some have not. In this study, mixing times have been measured for a vessel agitated by a Rushton turbine and compared with those predicted by CFD. The flow field was developed using the sliding mesh approach and computational parameters and the point of addition of the tracer have been varied. The simulations were very insensitive to the former whilst the radial distance from the wall of the latter had a very profound effect on both the mixing time and the development of the concentration field. When the addition point was close to the sliding mesh surface, the simulation was in good agreement with experimental values and empirical predictions whilst that for a point close to the wall was much too long. This finding may explain the contradictions in the literature.

Recent Studies on Agitated Three-Phase (Gas–Solid–Liquid) Systems in the Turbulent Regime

Earlier work using Rushton turbines and down-(MFD) and up-(MFU) pumping, 45°-pitched blade turbines at relatively low concentrations of solids of different size and density in vessels up to 1.8 m diameter enabled correlations to be developed for predicting the minimum speed for solid suspension, NJSg. Further work has been conducted with these impellers and with two modern impellers, a Scaba 6SRGT, a typical hollow-blade radial flow impeller, and a typical up-pumping, axial flow, wide-blade hydrofoil, the Lightnin’ A315, in a vessel of 0.45 m diameter with solids up to 40% by weight. The earlier correlations were validated for the new higher concentration conditions. In all cases, increases in solids concentration increased NJSg and the specific energy dissipation rate required to suspend solids, (ɛT)JSg On the other hand, the large increase (up to two orders of magnitude) in mixing time found at solids concentration >∼15% by weight in two phase systems at NJSg as compared with the case without solids, is essentially eliminated in the three-phase case. At low gassing rates, QGv (vvm), down-pumping impellers achieve suspension and relatively good vertical solids distribution at the lowest (ɛT)jsg-However, with increased QGv UP to 3 vvm, NJSg and (ɛT)JSg increase rapidly, solids maldistribution develops, the flow pattern is very unstable and gross torque fluctuations occur. For the 6SRGT, the operation is very stable and although NjSg and (ɛT)JSg are both high they are insensitive to QGv This high value of (ɛT)JSg may not be a disadvantage if high rates of gas–liquid mass transfer are required. For both the 6MFU and especially the A315(U), the flow pattern, NJSg and (ɛT)JSg are again all very insensitive to QGv and a good vertical solids distribution is maintained. For the A315(U) at high QGv values, (ɛT)JSg is the least amongst the impellers tested, so that wide-blade, up-pumping axial flow hydrofoils are considered to be the optimum impeller when just physical suspension is required or solid–liquid reactions are rate limiting.

The use of upward pumping 45° pitched blade turbine impellers in three-phase reactors

Abstract Earlier work on a range of geometries showed that an upward pumping six-blade 45°-pitch agitator (6MFU) of impeller to tank diameter ratio equal to one half with a sparger of about 0.8 the impeller diameter was the most versatile for solid suspension, gas dispersion and a combination of the two even at gas rates up to 3.5 vvm in low-viscosity liquids. That work also showed that at the same specific power input and superficial gas velocity approximately equal hold-ups were achieved even at different scales. Here it is shown that, by using such 6MFU—sparger combinations, k L a values comparable with those of Rushton impellers are also obtained. It is concluded that, because of its superior hydrodynamic versatility and comparable mass transfer performance, overall this geometry offers the best operating characteristics amongst the geometries tested.

The Transition from Homogeneous to Heterogeneous Flow in a Gassed, Stirred Vessel

Much ’old’ work has been undertaken studying gas-liquid systems in stirred reactors (Nienow1). However, this ‘old’ work has generally been limited to specific ungassed energy dissipation rates, ( ɛ T ) 0 , up to ∼2 Wkg −1 and gas flow rates of up to ∼2 vvm (or superficial gas velocity, ν s , of −1 ). Recent developments have led to the need to operate modern reactors at much higher values of ɛ T and ν s . This preliminary study reports work under such conditions and also builds on studies at very high gas velocities in bubble columns. Under such conditions, a transition from homogeneous to heterogeneous flow occurs. The results show that under well dispersed conditions, the transition occurs at lower gas velocities for the coalescence-inhibited solutions in comparison to water. Possible reasons for these findings and for the formation of large bubbles at this transition are discussed. A simple model on the basis of a gas volume balance is proposed which matches the trend. The impact of the intense operating conditions on a range of other mixing parameters is also reported.

Mass transfer and hold-up characteristics in a gassed, stirred vessel at intensified operating conditions

The steady state mass transfer coefficient, k L a, and the hold-up, e, have been measured in a vessel 0.29 m in diameter, sparged with air using 6RT (single and dual) or 6SRGT (single) impellers at unaerated energy dissipation rates of up to ∼100 kW/m 3 and superficial gas velocities of up to 0.13 m/s in deionized water (coalescing) and an aqueous solution of 0.2 M -1 sodium sulphate (non-coalescing). Under these intense conditions, the flow structure changed from homogeneous to heterogeneous, the hold-up could be as high as ∼65% and the gassed to ungassed power ratio for the RT and 6SRGT as low as 0.2 and 0.6, respectively, k L a values up to ∼2.0 s were found and the results could be correlated well by k L a=k(P/V L ) a v S β with a single parameter set for each fluid, based on a well-mixed model and a χ 2 -method of data fitting.