Sedimentation emulsions

Consider now a bidisperse emulsion, distributed uniformly in a layer between parallel horizontal planes z = 0 and z = H at the initial moment. The vertical z-axis is directed parallel to gravity. The initial volume concentrations of drops are small (Wio 1 and W20 1), so the hindered character of sedimentation can be neglected. After a certain time, all large drops of kind 1 will settle. We can select a column of a unit cross section in the emulsion volume and ask how many small drops will remain inside this column. A similar problem is of interest in laboratory studies on how the electric field affects the emulsion sedimentation rate. 

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. 

Additionally, mechanical (primarily shear), freeze—thaw, and thermal stabiHty the tendency to form sediment on long-term standing and compatibiHty with other dispersions, salts, surfactants, and pigments of acryHc dispersions are often evaluated. Details on the determination of the properties of emulsion polymers are available (60). 

Proper emulsification is essential to the satisfactory performance of a carrier. A weU-formulated carrier readily disperses when poured into water, and forms a milky emulsion upon agitation or steaming. It should not cause oil separation upon heating or crystallization and sedimentation upon cooling. 

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. 

Emulsions and Dispersions The mixture of hquids leaving a mixer is a cloudy dispersion which must be settled, coalesced, and separated into its liquid phases in order to be withdrawn as separate liquids from a stage. For a dispersion to Freak into separate phases, both sedimentation and coalescence of the drops of the dispersed... 

We have an emulsion of oil in water that we need to separate. The oil droplets have a mean diameter of lO " m, and the specific gravity Of the oil is 0.91. Applying a sedimentation centrifuge to effect the separation at a spedd of 5,000 rpm, and assuming that the distance of a droplet to the axis of rotation is 0.1 m, determine the droplet s radial settling velocity. 

It has been known for many years that microbial contaminants may effect the spoilage of pharmaceutical products through chemical, ply sical or aesthetic changes in the nature of the product, thereby rendering it unfit for use (see Chapter 18). Active drug constituents may be metabolized to less potent or chemically inactive forms. Physical changes commonly seen are the breakdown of emulsions, visible surface growth on solids and the formahon of slimes, pellicles or sediments in hquids, sometimes... 

Separation of phases is sometimes a problem, especially in the case of mixtures of alkaline aqueous solutions and organic liquids. Addition of materials that decrease the surface tension, filtration of the emulsion to remove interfacial contamination, and the use of sedimentation centrifuges should be considered if the separation time is too long. 

Content uniformity and long-term stability of a pharmaceutical product are required for a consistent and accurate dosing. Aggregation of dispersed particles and resulting instabilities such as flocculation, sedimentation (in suspensions), or creaming and coalescence (in emulsions) often represent major problems in formulating pharmaceutical disperse systems. 

For parenteral products specific consideration needs to be included for tonicity adjustment, emulsion globule size, ease of resuspension and sedimentation rate, particle size and particle size distribution, viscosity and syringeability, and crystal form changes. Full consideration should be included of the proposed instructions for dilution or reconstitution of products and of compatibility with the proposed solvents or diluents. This should include a demonstration that the proposed storage temperature and extremes of concentration are suitable. 

This section describes the treatment techniques currently used or available to remove or recover wastewater pollutants normally generated by aluminum forming facilities. In general, these pollutants are removed by oil removal (skimming, emulsion breaking, and flotation), chemical precipitation and sedimentation, or filtration.6- 3... 

Option 1 Emulsion breaking and oil separation by skimming, cyanide oxidation, chromium reduction, chemical precipitation and sedimentation, and sludge drying beds. 

The typical viscous behavior for many non-Newtonian fluids (e.g., polymeric fluids, flocculated suspensions, colloids, foams, gels) is illustrated by the curves labeled structural in Figs. 3-5 and 3-6. These fluids exhibit Newtonian behavior at very low and very high shear rates, with shear thinning or pseudoplastic behavior at intermediate shear rates. In some materials this can be attributed to a reversible structure or network that forms in the rest or equilibrium state. When the material is sheared, the structure breaks down, resulting in a shear-dependent (shear thinning) behavior. Some real examples of this type of behavior are shown in Fig. 3-7. These show that structural viscosity behavior is exhibited by fluids as diverse as polymer solutions, blood, latex emulsions, and mud (sediment). Equations (i.e., models) that represent this type of behavior are described below. 

An emulsion is a dispersed system of two immiscible phases. Emulsions are present in several food systems. In general, the disperse phase in an emulsion is normally in globules 0.1-10 microns in diameter. Emulsions are commonly classed as either oil in water (O/W) or water in oil (W/O). In sugar confectionery, O/W emulsions are most usually encountered, or perhaps more accurately, oil in sugar syrup. One of the most important properties of an emulsion is its stability, normally referred to as its emulsion stability. Emulsions normally break by one of three processes creaming (or sedimentation), flocculation or droplet coalescence. Creaming and sedimentation originate in density differences between the two phases. Emulsions often break by a mixture of the processes. The time it takes for an emulsion to break can vary from seconds to years. Emulsions are not normally inherently stable since they are not a thermodynamic state of matter. A stable emulsion normally needs some material to make the emulsion stable. Food law complicates this issue since various substances are listed as emulsifiers and stabilisers. Unfortunately, some natural substances that are extremely effective as emulsifiers in practice are not emulsifiers in law. An examination of those materials that do stabilise emulsions allows them to be classified as follows ... 

DOM also tends to be concentrated at boimdaries, such as the sediment-water and air-sea interfeces. The latter is also characterized by enrichments of POM, such as bacteria. Under calm conditions, the DOM and POM that collects at the air-sea interface forms a visible surfece slick or microlayer. On windy days, this organic matter can be whipped up into an emulsion that has the appearance of a very sturdy foam. DOM can also be transferred into the POM pool by adsorbing onto organic particles. 

Another technique consists of submitting the emulsion to centrifugation and determining the droplet volume fraction < / at the top (bottom) of the cream (sediment). The centrifugation typically takes several hours until the equilibrium volume fraction is achieved. After equilibration, if the droplets occupy a distance much less than that of the centrifuge lever arm, the spatial gradient in the acceleration can be neglected, and the osmotic pressure can be determined (see Fig. 4.1) ... 

---