Suspensions settling

Stokes law is rigorously applicable only for the ideal situation in which uniform and perfectly spherical particles in a very dilute suspension settle without turbulence, interparticle collisions, and without che-mical/physical attraction or affinity for the dispersion medium [79]. Obviously, the equation does not apply precisely to common pharmaceutical suspensions in which the above-mentioned assumptions are most often not completely fulfilled. However, the basic concept of the equation does provide a valid indication of the many important factors controlling the rate of particle sedimentation and, therefore, a guideline for possible adjustments that can be made to a suspension formulation. 

Figure 4.11 Flocculation and formation effects in a chemically pulped bleached softwood pulp (slightly refined). Fibre suspensions settled for 40 min. Sheets (60 gm 2) photographed in transmitted light, (a) no additives, (b) polyelectrolyte added to induce flocculation. Scale bar = 2 cm.

In the sedimenter or gravity settler, the particles in the feed suspension settle due to difference in densities between the particles and the fluid. The settling particle velocity reaches a constant value - the terminal velocity - shortly after the start of sedimentation. The terminal velocity is defined by the following balance of forces acting on the particle ... 

For many purposes, lumps of materials of intermediate sizes are the most desirable forms, neither too small nor too large. For instance, beds of overly small granules of catalysts exhibit too great resistance to flow of reacting fluids, and too small particles in suspensions settle out or filter too slowly. Other situations that benefit from size enlargement of particles are listed in Table 12.11. 

Lay unit on its side. Tilt unit back and forth at approximate 45° angles three times to mix the cell suspension. Place unit on its side again to let the suspension settle evenly among the 10 trays. Turn upright. 

Powders vary dramatically in particle size on the basis of their origin. It is common for catalyst manufacturers to classify powders in order to assure users of consistency from batch to batch since suspension, settling rates, filtration, and performance in slurry-phase reactions are all dependent on particle size. The effect on suspension, settling rates, and filtration is obvious. However, factors that favor these are unfavorable for kinetics. For reactions controlled by transport rates from the bulk fluid to the surface of the catalyst, the overall reaction rate is a strong function of geometric surface area and thus is favored by small particles. Pore diffusion resistance is also minimized by smaller particles since reaction paths to active sites are smaller. The only mode of reaction control not influenced by particle size is for those reactions in which rate is controlled by reaction at active sites. Therefore, a compromise for optimum filtration and maximum reaction rates must be made. 

Concentration changes in a suspension settling under gravity... 

The Werner and Travis methods [81,82] also operate on the layer principle but their methods have found little favor due to the basic instability of the system a dense liquid on top of a less dense liquid being responsible for a phenomenon known as streaming in which the suspension settles en masse in the form of pockets of particles which fall rapidly through the clear liquid leaving a tail of particles behind. 

The Prussian Blue Reaction. On addition to an alkaline solution of hydrocyanic acid of a few ml. of ferrous sulphate solution and of ferric chloride solution, then shaking and warming, a blue precipitate of ferric ferrocyanide appears on acidification with hydrochloric acid. In presence of only a trace of hydrocyanic acid, only a greenish-blue colouration appears, due to the formation of a colloidal suspension of the ferric ferrocyanide. On standing (sometimes for as long as 12 hours) the suspension settles to blue floes, leaving the supernatant solution colourless. 

Aluminum (400 g) is melted and heated to 1200°. Cubes of nickel (300 g) are added to the melt all at once (cubic nickel is more suitable than compact, mechanically worked metal for preparation of the alloys). The nickel dissolves in a lively reaction, the temperature rising to about 1500°. After cooling, the alloy is broken and powdered. The alloy (250 g) is added in small portions to an ice-cold, approximately 25% sodium hydroxide solution (100 ml), whereupon decomposition sets in exothermally with lively evolution of hydrogen (foaming and spitting). When all the alloy has been added, the temperature is raised to 90-100° and kept there until evolution of hydrogen ceases. The metal is allowed to settle and is then decanted, and the treatment with alkali is repeated twice (11 of fresh solution each time). After decantation of the last alkali the precipitated nickel is washed with water by suspension, settling, and decantation until the wash-water reacts neutral to phenolphthalein. The water is then replaced by alcohol. The catalyst is stored under alcohol in bottles. 

Figure 8. Graphs showing the change in fines concentration (<22 fim material) as tailings volume is reduced with time in the tailings suspension settling pond.

Most early observations of sedimentation in inclined channels indicated that a quasi-steady interface shape between the clarified layer and suspension was formed rapidly in times short compared with the characteristic suspension settling time. Moreover, the clarified liquid layer thickness below the upper channel wall was observed to be much thinner than the channel width b. Most of the clarified fluid accumulated above the horizontal interface at the top of the suspension. It was this kinematic shock interface that was observed to fall with a vertical velocity larger than the hindered settling velocity U measured in vertical settlers. 

Acid and calcium bentonites are easy to disperse without forming lumps. Suspensions settle rapidly, leaving the liquid turbid but with a relatively light deposit. Protein adsorption is limited. 

Experimental Method for Immobilisation of Cells. The procedure for cell immobilisation is very simple and is illustrated by the following example. A suspension (in 10 ml 0.9w/v saline) of Escherichia coli cells (A qq 0.216) was mixed with a sample of the metal hydroxide as prepared as above (pH 5-7) and agitated gently for 5 min at room temperature. The mixture was then allowed to stand at room temperature and the suspension settled out, leaving a clear supernatant... 

The basic concern involves the fact that suspensions settle, and it is necessary to redistribute ihein before using or dispensing them as products. A desirable suspension should be easily redispersed by shaking, should remain suspended tong enough to withdraw an accurate dose, and should have the desired flow... 

In the field of stability, however, suspensions do not show such a marked difference between stable and flocculated systems as do sols, for the simple reason that even the individual particles of a stable suspension settle down in a comparatively short time. 

A suspension in water of uniformly sized sphere (diameter 150 pm, density 1140 kg/m has a solids concentration of 25% by volume. The suspension settles to a bed of solids concentration of 55% by volume. Calculate ... 

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