Suspension Sedimentation

Sediments and Suspensions Sediments may be due to insoluble or sparingly soluble tablet or capsule excipients such as talc, starch, or calcium phosphate, certain poisons such as arsenious oxide, or sometimes to an interaction of the added contaminant with die beverage or drink (e.g. batteiy acid producing a coagulation of protein material). Microscopic examination of the sediment or filtered suspension may give a clue to the nature of die material. 

The Bingham plastic model usually provides a good representation for the viscosity of concentrated slurries, suspensions, sediments, emulsions, foams, etc. Such materials often exhibit a yield stress,... 

Additional tests would be required for an ophthalmic suspension or other type of dosage form. Such tests might be one for particle size and one to show whether the suspension sediments can be readily dispersed on shaking to enable the correct dose to be delivered. 

Let us make a proper account of the dependence of particle sedimentation velocity on the volume concentration (or, which is the same thing, on the mass concentration p) of particles in the process of suspension sedimentation, and take a look at the results. 

Since the diameters of bubbles range from Di to D2, at t > 0 there are three regions in the considered volume (Fig. 24.1), divided by flat surfaces of contact discontinuities (these surfaces are already familiar to us from the similar problem of suspension sedimentation (Section 8.5)). Consider a plane (z, t) (Fig. 24.2) and straight lines... 

Also termed sedimentation diameter. This is usually referred to as the equivalent spherical diameter in the case of emulsion and suspension sedimentation. 

Stokesian dynamics is a numerical technique for simulating the dynamic hehaviour of colloidal suspensions (sedimentation, rheology), where the motions of the individual particles is driven hy Brownian and volume forces (including particle interactions) and coupled by hydrodynamic interaction. In a more general approach than in Eq. (4.69), the hydrodynamic forces are traced back to the generalised particle velocities Vp and the velocity gradients E ... 

Write one sentence to explain each term suspension sediment aqueous solution... 

Figure 4.5. Photosedimemometers monitor the sedimenting of a suspension by monitorir the light intensity as the suspension sediments, a) A simple photosedimentometer showing the solid angle subtended by the photodetector, b) Light energy scattered for various suspension concentrations.

The variation of additives showed no defined impact on the determined suspension densities or pH values. Opposite to that, the varied additive types showed an effect on the resulting suspension sedimentation speed, viscosity, and dynamic mobility of the primary particles (Table 11.17). 

The larger particles of suspensions sediment rather rapidly. The flocculation velocity of a suspension is slow, owing to the relatively smalinumber of particles. Therefore the sedimentation velocities of a flocculated and a stable suspension do not differ pronouncedly, although in special cases this difference is applied (blood sedimentation). 

Although it is hard to draw a sharp distinction, emulsions and foams are somewhat different from systems normally referred to as colloidal. Thus, whereas ordinary cream is an oil-in-water emulsion, the very fine aqueous suspension of oil droplets that results from the condensation of oily steam is essentially colloidal and is called an oil hydrosol. In this case the oil occupies only a small fraction of the volume of the system, and the particles of oil are small enough that their natural sedimentation rate is so slow that even small thermal convection currents suffice to keep them suspended for a cream, on the other hand, as also is the case for foams, the inner phase constitutes a sizable fraction of the total volume, and the system consists of a network of interfaces that are prevented from collapsing or coalescing by virtue of adsorbed films or electrical repulsions. 

In most colloidal suspensions tire particles have a tendency to sediment. At infinite dilution, spherical particles with a density difference Ap with tire solvent will move at tire Stokes velocity... 

In practice, sedimentation is an important property of colloidal suspensions. In fonnulated products, sedimentation tends to be a problem and some products are shipped in the fonn of weak gels, to prevent settling. On the other hand, in applications such as water clarification, a rapid sedimentation of impurities is desirable. 

SEP mixers are hori2ontaHy installed on the side of the tank and near the bottom. By rotating the propeller, a spiralling jet is produced (Pig. 27) near the tank floor which provides the desired thmst to dislodge and entrain the sediments. SEP mixers should be designed to provide the suspension velocity at the farthest distance on the tank floor. 

A larger sedimentation force can be developed by centrifuging the suspension. At an angular velocity of CO radians pet second the centrifugal... 

In sohd—sohd separation, the soHds are separated iato fractions according to size, density, shape, or other particle property (see Size reduction). Sedimentation is also used for size separation, ie, classification of soHds (see Separation, size separation). One of the simplest ways to remove the coarse or dense soHds from a feed suspension is by sedimentation. Successive decantation ia a batch system produces closely controUed size fractions of the product. Generally, however, particle classification by sedimentation does not give sharp separation (see Size MEASUREMENT OF PARTICLES). 

Most authors who have studied the consohdation process of soflds in compression use the basic model of a porous medium having point contacts which yield a general equation of the mass-and-momentum balances. This must be supplemented by a model describing filtration and deformation properties. Probably the best model to date (ca 1996) uses two parameters to define characteristic behavior of suspensions (9). This model can be potentially appHed to sedimentation, thickening, cake filtration, and expression. 

Hindered Settling When particle concentration increases, particle settling velocities decrease oecause of hydrodynamic interaction between particles and the upward motion of displaced liquid. The suspension viscosity increases. Hindered setthng is normally encountered in sedimentation and transport of concentrated slurries. Below 0.1 percent volumetric particle concentration, there is less than a 1 percent reduction in settling velocity. Several expressions have been given to estimate the effect of particle volume fraction on settling velocity. Maude and Whitmore Br. J. Appl. Fhys., 9, 477—482 [1958]) give, for uniformly sized spheres,... 

Tyj)e of dryer Applicable with dry-product recirculation True and colloidal solutions emulsions. Examples inorganic salt solutions, extracts, milk, blood, waste liquors, rubber latex, etc. Pumpable suspensions. Examples pigment slurries, soap and detergents, calcium carbonate, bentonite, clay sbp, lead concentrates, etc. does not dust. Recirculation of product may prevent sticking Examples filter-press cakes, sedimentation sludges, centrifuged sobds, starch, etc. 

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