Turbulent suspension

Wilson, K. C. and Puoh, F. ]. Can. Ji. Chem. Eng. 66 (1988) 721. Dispersive-force modelling of turbulent suspensions in heterogeneous slurry flow. 

The second step in Ten Cate s two-step approach was to focus on crystal-crystal interaction by means of an explicit two-phase DNS of the turbulent suspension that completely resolves the translational and rotational motions and collisions of the spherical particles plus the turbulence of the liquid between the particles. The particle motions are driven by the turbulent flow and the particles affect the turbulent flow of the liquid in between. When particles approach one another down to a distance smaller than the grid spacing, lubrication theory is exploited to bridge the gap between them. 

Froth flotation is an application of foams that is used to separate mineral components from each other based on their having different surface properties, typically their wettability and surface electrical charge. For example, froth flotation is the classic process used to separate copper from lead ore. The process involves having hydrophobic particles attach to gas bubbles which rise through a turbulent suspension to create a surface foam called a froth. Figure 10.2 shows an illustration of a mechanical flotation cell. This is the classic flotation device [53,91,625], First, the flotation feed particles are well dispersed into a particle suspension. Together with chemical flotation aids, such as collectors and frothers, this constitutes what is called the flotation pulp. In a mechanical flotation cell, air is fed in the form of fine bubbles and introduced near the impeller (see Figure 10.2). In addition to mechanical flotation cells, there are also pneumatic cells and cyclone flotation cells. Pneumatic... 

In the case of slurry flow in a rotary drum, turbulent suspension of the solids can occur due to the axial liquid velocity coupled with the radial liquid velocity due to the drum rotation and mixing action created by the drum internals. Typically, the peripheral drum velocity is much higher than the slurry axial velocity, and, consequently, the drum rotational speed becomes very important for solids turbulent suspension and, hence, transport. 

EBR hydroprocessing is a three-phase system in which the recycle gas bubbles up through the hydrocarbon mixture and the catalyst particles, creating a turbulent suspension [69]. The oil feed and hydrogen are delivered to the EBR in upflow... 

Unfortunately CFD codes, despite the extensive effort and computational power deployed, have in general not matched experimental data any better than the mechanistic models in challenge problem exercises conducted at two recent (1995 and 2001) International Fluidization Conferences. While numerical (CFD) approaches will no doubt make major future contributions, there are currently no codes capable of giving consistently reliable predictions for concentrated and turbulent suspension flows of the kind found in CFB risers. 

Ippen (1971) conducted an analysis of turbulent suspensions in open channel flows. This work showed that the concentration close to the lower boundary was the most important factor suppressing the Von Karman constant. This may not be astonishing when we consider that beds of coarse particles form in this region at low speeds. 

A concentration gradient occurs in the flow region above a sliding bed. This gradient is due to either turbulent suspension or shearing action of flow at the top of the bed. The shape of the measured concentration profiles gives an indication whether particle dispersion is due to turbulent suspension or due to shearing. 

The analysis of concentration profiles measured in flows of sand particles of different sizes in a horizontal 150-mm pipe showed that turbulent suspension was a prevailing suspension mechanism in flows of particles smaller than approximately 0.5 mm. Shearing determined a concentration profile for coarser sand particles. 

Concentration gradients due to shearing are considerably steeper than gradients in turbulent suspension. In many cases the distribution of solids within a shear layer can be considered linear [3]. The measured solids distribution within a shear layer can be approximated by a line that matches the inflexion point of the measured-profile curve, and has the slope given by the derivative of the curve in the inflexion point (Fig. 2). 

Fig. 1 Solids dispersion parameter for turbulent suspensions at different solids concentrations...

Sand particles finer than approximately 0.5 mm were dispersed owing to the mixing action of turbulent eddies developed in the flow of the carrying water. The concentration profile in turbulent suspension may be successfully simulated by the Rouse-Schmidt turbulent-diffusion model with an implemented hindered settling effect. In highly concentrated flows the theoretical profile should be linked to the contact bed at the reference level with a local concentration of about 0.35-0.38. 

Next we determine whether the mass loading effect (saltation) will occur. According to the MM, the amount of solids that the gas phase can hold in turbulent suspension upon its entrance into a cyclone depends on the mass average (the median) particle size of the feed, the cut-point of the inner vortex, X50, and, to a lesser extent, on the inlet loading itself, Cq. This limiting or limit-loading is ... 

In his earlier papers, Muschelknautz arrived at the following expression for the critical load or limit load in terms of kg dust (that the gas could keep in turbulent suspension) per kg of gas ... 

In situations where a low concentration of suspended solids needs to be separated from a liquid, then cross-flow filtration can be used. The most common design uses a porous tube. The suspension is passed through the tube at high velocity and is concentrated as the liquid flows through the porous medium. The turbulent flow prevents the formation of a filter cake, and the solids are removed as a more concentrated slurry. 

The criterion of maintaining equal power per unit volume has been commonly used for dupHcating dispersion qualities on the two scales of mixing. However, this criterion would be conservative if only dispersion homogeneity is desired. The scale-up criterion based on laminar shear mechanism (9) consists of constant > typical for suspension polymerization. The turbulence model gives constant tip speed %ND for scale-up. 

The overall superficial fluid velocity, mentioned earlier, should be proportional to the settling velocity o the sohds if that were the main mechanism for solid suspension. If this were the case, the requirement for power if the setthng velocity were doubled should be eight times. Experimentally, it is found that the increase in power is more nearly four times, so that some effect of the shear rate in macro-scale turbulence is effec tive in providing uphft and motion in the system. 

A fluid-bed incinerator uses hot sand as a heat reservoir for dewatering the sludge and combusting the organics. The turbulence created By the incoming air and the sand suspension requires the effluent gases to be treated in a wet scrubber prior to final discharge. The ash is removed from the scrubber water by a cyclone separator. The scrubber water is normally returned to the treatment process and diluted with the total plant effluent. The ash is normally buried. 

Further details ean be found in texts eoneerning fluid mixing and partiele suspension (Sterbacek and Tausk, 1965 Holland and Chapman, 1966 Oldshue, 1983 Uhl and Gray, 1986 Allen, 1990 Coulson and Riehardson, 1991 Harnby etal., 1992 and Gibilaro, 2001), the theory partieulate of proeesses (Randolph and Larson, 1988 Ramkrishna, 2000) and turbulent flows (Pope, 2000). 

The experimental results are eonsistent with erystal attrition oeeurring via both erystal-impeller impaets and turbulent disruption with no signifieant effeet of erystal-erystal eollisions on the number of fine fragments produeed in the dilute agitated suspensions. 

Wojcik, J. and Jones, A.G., 1998b. Particle disruption of precipitated CaC03 crystal agglomerates in turbulently agitated suspensions. Chemical Engineering Science, 53, 1097-1101. 

Operations such as blending, solids-suspension, dissolving, heat transfer and liquid-liquid extraction are typical of systems requiring high flow relative to turbulence, while gas-liquid reactions and some liquid-liquid contacting require high turbulence relative to flow. The case of (1) 100% of suspension—requires head to keep particles suspended and (2) 100% uniformity of distribution of particles—requires head for suspension plus flow for dis-tiibution. 

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