Solid-liquid mixing turbulence

In solid-liquid mixing design problems, the main features to be determined are the flow patterns in the vessel, the impeller power draw, and the solid concentration profile versus the solid concentration. In principle, they could be readily obtained by resorting to the CFD (computational fluid dynamics) resolution of the appropriate multiphase fluid mechanics equations. Historically, simplified methods have first been proposed in the literature, which do not use numerical intensive computation. The most common approach is the dispersion-sedimentation phenomenological model. It postulates equilibrium between the particle flux due to sedimentation and the particle flux resuspended by the turbulent diffusion created by the rotating impeller. 

The application of CFD in the modeling of solid-liquid mixing is fairly recent. In 1994, Bakker et al. developed a two-dimensional computational approach to predict the particle concentration distribution in stirred vessels. In their model, the velocity field of the liquid phase is first simulated taking into account the flow turbulence. Then, using a finite volume approach, the diffusion-sedimentation equation along with the convective terms is solved, which includes Ds, a... 

Hartmann, H., Detailed simulations of liquid and liquid-solid mixing—turbulent agitated flow and mass transfer , Ph.D. Thesis, Delft University of Technology, Delft, Netherlands (2005). 

Acoustic/ultrasonic techniques that have been developed into flow-monitoring instruments are Doppler, cross-correlation, and transit-time methods. An ultrasonic Doppler flowmeter has been applied to single-phase turbulent flows and mixed-phase (solid/liquid or gas/liquid) flows. The crosscorrelation technique is mainly for mixed-phase flows, whereas the transit-time method has been applied to single-phase flows, either liquid or gas, in large pipes. 

The Doppler technique measures the frequency shift of scattered waves with respect to incident sound waves. The technique, therefore, requires the presence of scatterers in the flow that is being monitored. The scatterers could be turbulent eddies or vortex shedding for liquid single-phase flows, and solid particles for solid/fluid mixed-phase flows. The basic geometry of a Doppler... 

Thus, increase of intensity of turbulent mixing allows production of fine systems "solid-liquid". Furthermore, reactions underlying the condensate method of their reception proceed fast. This determines expediency of studying of tubular turbulent apparatus application possibility in particular divergent-convergent design for preparation of homogeneous fine suspensions by condensate method under fast chemical reaction. 

The literature is small on rotor-stator devices, as discussed in Chapter 8. [They are also discussed in Chapter 12, since this is the process result (liquid-liquid and solid-liquid) that these devices are most used for.] It should be mentioned that because of their very high speed they can produce turbulent motion in some rather high viscosity fluids. Also see Cohen (1998), Dietsche (1998), and Myers et al. (1999) for general information about the industrial application of rotor-stator mixers versus other mixing options. 

Applicability/Limitations The system is capable of treating solids, sludges, slurries and liquids. The high degree of turbulence and mixing ensures treatment of a wide variety of wastes. The waste... 

Fluidised beds may be divided into two classes. In the first, there is a uniform dispersion of the particles within the fluid and the bed expands in a regular manner as the fluid velocity is increased. This behaviour, termed particulate fluidisation, is exhibited by most liquid-solids systems, the only important exceptions being those composed of fine particles of high density. This behaviour is also exhibited by certain gas-solids systems over a very small range of velocities just in excess of the minimum fluidising velocity—particularly where the particles are approximately spherical and have very low free-falling velocities. In particulate fluidisation the rate of movement of the particles is comparatively low, and the fluid is predominantly in piston-type flow with some back-mixing, particularly at low flowrates. Overall turbulence normally exists in the system. 

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