Baffles solids suspension

The latter two types of stirrers are produced by Ekato Company, Schop-fheim, Germany. These are low-speed stirrers with a dx/d, ratio of 1.5 in the presence of baffles and 1.1 in the absence of baffles. The stirrers are versatile and are used for liquid homogenization, gas-liquid or liquid-liquid dispersion, and solids suspension in slurry reactors. 

Baffles are essential for solids suspension operations involving solids that are heavier than the liquid. They convert the swirling motion into top-down or axial fluid motion, which helps to lift and suspend the solids. 

Liquid circulation is reduced by baffles. Figure 11.21 shows radial baffles with central openings. Figures 11.21a and b show flat and conical baffles, respectively (conical shape is preferable for a solid suspension). These baffles minimize liquid circulation. The resulting flow pattern is shown... 

Geometry For a stirred tank, the geometry is cylindrical, with a small aspect ratio (the height of fluid in the tank [H], is one to three times the tank diameter [T]). Although many industrial vessels have a dished bottom (especially if solids suspension is involved), simulations to date have used the simpler flat bottom geometry. The impeller, of diameter D = T/4 to T/2, is placed at the desired off-bottom clearance (C). Around the tank walls, two to four rectangular baffles are evenly... 

In steel or alloy vessels, the recommended baffle design for solid suspension of settling solids is four flat blade baffles, each with width, B, equal to T/12 at a wall clearance of at least T/72. The baffles should extend to the lower edge of the lower impeller or to the lower tangent line. 

There are analogies between the minimum impeller speed Njs for solids suspension and Nmm for drop suspension. Both depend on density difference, continuous phase viscosity, and impeller diameter. However, Njs depends directly on particle size, while Nmin depends instead on interfacial tension and the other physical properties that determine drop size. Skelland and Seksaria (1978) determined the minimum speed to form a liquid-liquid dispersion from two settled (separated) phases of different density and included the sensitivity to impeller location. The vessels used were fully baffled. They determined Nmin for systems of equal volumes of light and heavy phase. Studies included use of single impellers placed midway in the dense phase (C = H/4), at the o/w interface (C = H/2) and midway in the lighter phase (C = 3H/4). They also examined the use of dual impellers located midway in both phases. Several impeller types were tested, including a propeller (Prop), a 45° pitched blade tmbine (PBT), a flat-blade turbine (FBT), and a curved-blade turbine (CBT). Their results are correlated by the following equation, which is dimensionless ... 

Baffled Tanks For vigorous agitation of thin suspensions, the tank is provided with baffles which are flat vertical strips set radially along the tank wall, as illustrated in Figs. 18-15 and 18-16. Four baffles are almost always adequate. A common baffle width is one-tenth to one-twelfth of the tank diameter (radial dimension). For agitating slurries, the baffles often are located one-half of their width from the vessel wall to minimize accumulation of solids on or behind them. 

Solid partieles in liquids generally tend to settle to the bottom of a vessel under gravity due to their exeess density. To maintain a suspension, some form of agitation is normally provided together with wall baffles to prevent vortex formation in the swirling flow (Figure 2.14). 

Settling tanks are used to separate low solids concentration suspensions (solid-liquid separation devices usually comprising a vessel containing a single baffle which directs the suspension to the base from where it rises to the outlet, as illustrated in Figure 4.1. So long as the... 

A rotating impeller in a fluid imparts flow and shear to it, the shear resulting from the flow of one portion of the fluid past another. Limiting cases of flow are in the axial or radial directions so that impellers are classified conveniently according to which of these flows is dominant. By reason of reflections from vessel surfaces and obstruction by baffles and other internals, however, flow patterns in most cases are mixed. When a close approach to axial flow is particularly desirable, as for suspension of the solids of a slurry, the impeller may be housed in a draft tube and when radial flow is needed, a shrouded turbine consisting of a rotor and a stator may be employed. 

Pitch-blade turbine (paddle stirrer with pitched blades) and propeller stirrers provide high mixing with an axial flow pattern. Both of these stirrers are normally used for low-viscosity liquids and in vessels with baffles. They are well suited for providing liquid homogenization and suspension of solids in slurry reactors. The stirrers can also be used in viscous fluids and for vessels with H/dT > 1, which are generally encountered in fermentation processes. For these situations, axial flow is increased with the use of multistage stirrers with pitched stirring surfaces. 

Additional work on the suspension of solid particles in agitated systems has been reported by Oyama and Endoh (013), on suspension of sands and resin particles, in baffled vessels 5.5, 6.7, and 10.8 in. in diameter. These authors used a 3.6-in. vaned disk, and 2.6- and 3.6-in. flat-blade turbines. A light beam passed through the vessel onto a photoelectric tube was used to monitor the particle concentration in a horizontal plane at a height about of the total liquid height. At low speeds the particles tended to congregate near the vessel bottom. As the... 

A.W. Nienow, Suspension of solid paricles in turbine agitated baffled vessels, CES, 23, 1453-1459 (1968). 

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