Hydrodynamics And Backmixing Properties Of In-line High Shear Mixers

As a novel type of process intensification equipment, high shear mixers (HSMs)have great potential to intensify chemical reactions with fast inherent reaction ratesbut relatively slow mass transfer, due to their locally intense turbulence and shear.Hydrodynamics and backmixing properties of pilot-scale in-line HSMs, instructive forthe design, selection and optimization of these novel chemical reactors, were bothexperimentally and numerically investigated in this thesis.Firstly, the radial and tangential velocities in the jet flow region of a pilot-scalein-line HSM with single stage, double rows of ultra-fine teeth (backward inclinedstator teeth) were measured using LDA. Power consumptions of the mixer wereinvestigated by the torque based method. The single phase flow field in the HSM wasestimated through CFD simulations for the assessments of the predictive capabilitiesof different turbulence models and discretization schemes. It is found that, for thehighly three-dimensional, strongly rotating, wall-bounded and anisotropic turbulentflow in the HSM, LES captures more accurately the mean and secondary flowcharacteristics than the standard k-ε turbulence model. Obvious deviation fromisotropic turbulence is observed in the rotor-stator region, as well as the jet regionsnear the outer stator and the clamp nut.Secondly, the intrinsic RTDs of the ultra-fine teethed in-line HSM were obtained bythe stimulus-response experiments with imperfect tracer pulse and a deconvolutiontechnique. The LES and combined species transport method was adopted for the RTDpredictions. The experimental and simulated RTD curves share overall features in theresponse peaks, and the predicted mean residence times approximate to the measuredvalues, indicating reliable CFD predictions of the RTDs. Results show that the teethedin-line HSM behaves like a mixed flow reactor with stagnant regions. The increase ofeither the rotor speed or the flowrate leads to improvement of the mixedness andelimination of the trivial channeling defect, but also increase of internal recirculation.Geometric parameters of the teethed in-line HSM have significant influences on theRTDs. The defects of channeling, short circuiting and fluid re-entrainment are resultedfrom inefficient geometric designs of HSMs, such as those with large shear gapwidths, large tip-to-base clearances, single rows of rotor and stator teeth, or axiallystraight stator teeth. Lastly, the pumping heads, pumping efficiencies, input shaft powers and powercurves were measured under both the pump-fed and free-pumping modes for therotor-stator teethed and blade-screen in-line HSMs under different rotor speeds,flowrates and fluid viscosities. The power number of both units with the bearing losssubtracted can be correlated by the Froude number modified Kowalski’s powerconsumption model. Both HSMs do not pump well at lower rotor speeds. While athigher rotor speeds, better pumping capacity is observed for the blade-screen unitover the teethed unit. Both units demonstrate pressure drops when operated withoutthe rotors attached. Local resistances from the teethed and screen stators areapproximate at different fluid viscosities. In the fully turbulent flow region with thesame flow number, the dual-row ultra-fine teethed HSM draws over50%net power tofluid than the Silverson with dual fine screens. The single-row blade-screen HSM hasapproximate turbulent power draw to the dual standard Silverson under the pump-fedmode, and to the dual fine Silverson when operated under the free-pumping mode.