Solid-liquid, suspension dispersion

A dispersion is a system consisting of two or more phases in which a material is finely distributed throughout another. Dispersions can be liquid/liquid (emulsions) or solid/liquid (suspensions). Polymer dispersions consists of an internal phase, the polymer or copolymer and an external phase, water. 

The choice of impeller for three-phase systems is a compromise between dispersing gas in the liquid and suspending the particles in the liquid. We recall that the axial-flow impellers are usually used for solid-liquid systems, while radial impellers are used for gas-liquid systems. Introducing gas from the bottom of a tank containing a solid-liquid suspension will destroy the flow pattern created by an axial impeller. Therefore, radial impellers are usually more effective in three phase systems even if they require more power for the same level of suspension [65]. Another solution is to apply multiple impellers, one to fulfill the criterion for gas dispersion and another one to fulfill the criterion for solids suspension [87]. The existence of solid particles might also modify the interfacial area between gas and liquid compared to gas-liquid systems. 

Different types of dispersions are encountered in industrial applications, the most common ones include solid/liquid (suspension), liquid/liquid (emulsions), gas/liquid (foams), liquid/solid (gels), and liquid/gas (aerosols). These dispersions are encountered in almost every industry in some form or the other during the preparation or as end product. Examples of industrial applications of dispersions include paints, dyestuffs, printing ink, paper coatings, cosmetics, ceramics, microelectronics, agrochemical and pharmaceutical formulations, and various household products. In the following sections, the characterization and properties of solid/liquid suspensions will be described. However, the same concepts would be valid for other kinds of dispersions also. 

Jafari R, Tanguy PA, Chaouki J. (2012c) Experimental investigation on solid dispersion, power consumption and scale-up in moderate to dense solid-liquid suspensions. Chem. Eng. Res. Des., 90 201-212. 

Annenante and Huang (1992) and Armenante et al. (1992) found practically no advantage in using multiple impellers for determining Nmin- This is similar to the result for solid-liquid suspension. However, multiple impellers were useful in improving dispersed phase uniformity. Results agreed with the work of Skelland and Seksaria (1978). 

Foams are agglomerations of gas bubbles separated from each other by thin films (5). Mainly, the problem is concerned with one class of colloidal systems —gas dispersed in liquid—but liquid dispersed in gas, solids dispersed in liquid (suspensions), and liquids dispersed in liquids (emulsions) cannot be ignored. The dispersion of a gas into a liquid must be studied and observed by the food technologist to improve the contact between the liquid and gas phases, the agitation of the liquid phase, and most important, the production of foam 10). 

In terms of the two-phase system which comprises dispersions of solids in liquids, the minimum energy requirement is met if the total interfacial energy of the system has been minimized. If this requirement has been met, chemically, the fine state of subdivision is the most stable state, and the dispersion will thus avoid changing physically with time, except for the tendency to settle manifest by all dispersions whose phases have different densities. A suspension can be stable and yet undergo sedimentation, if a true equilibrium exists at the solid-liquid interface. If sedimentation were to be cited as evidence of instability, no dispersion would fit the requirements except by accident—e.g., if densities of the phases were identical, or if the dispersed particles were sufficiently small to be buoyed up by Brownian movement. 

Disperse systems can be classified in various ways. Classification based on the physical state of the two constituent phases is presented in Table 1. The dispersed phase and the dispersion medium can be either solids, liquids, or gases. Pharmaceutically most important are suspensions, emulsions, and aerosols. (Suspensions and emulsions are described in detail in Secs. IV and V pharmaceutical aerosols are treated in Chapter 14.) A suspension is a solid/liquid dispersion, e.g., a solid drug that is dispersed within a liquid that is a poor solvent for the drug. An emulsion is a li-quid/liquid dispersion in which the two phases are either completely immiscible or saturated with each other. In the case of aerosols, either a liquid (e.g., drug solution) or a solid (e.g., fine drug particles) is dispersed within a gaseous phase. There is no disperse system in which both phases are gases. 

Of special interest in liquid dispersions are the surface-active agents that tend to accumulate at air/ liquid, liquid/liquid, and/or solid/liquid interfaces. Surfactants can arrange themselves to form a coherent film surrounding the dispersed droplets (in emulsions) or suspended particles (in suspensions). This process is an oriented physical adsorption. Adsorption at the interface tends to increase with increasing thermodynamic activity of the surfactant in solution until a complete monolayer is formed at the interface or until the active sites are saturated with surfactant molecules. Also, a multilayer of adsorbed surfactant molecules may occur, resulting in more complex adsorption isotherms. 

Adsorption on Kaolinite. For kaolinite, the polymer adsorption density is strongly dependent on the solid/liquid ratio, S/L, of the clay suspension. As S/L increases, adsorption decreases. This S/L dependence cannot be due totally to autocoagulation of the clay particles since this dependence is observed even in the absence of Ca2+ at pH 7 and at low ionic strength where auto-coagulation as measured by the Bingham yield stress is relatively weak (21). Furthermore, complete dispersion of the particles in solvent by ultra-sonication before addition of... 

SUSPENSION. A liquid medium having small, solid particles uniformly dispersed through it. 

Colloid chemistry investigates substance mixtures. These substance mixtures can be heterogenous, such as emulsions (in which tiny droplets of one liquid are dispersed in another), suspensions (consisting of a fine dispersion of solid particles in a liquid volume phase), and aerosols (in which liquid droplets are dispersed in the gas phase). However, there are also homogenous mixtures in which the solute is present in larger, supermolecular aggregates. These homogenous mixtures include micellar solutions and liquid crystalline... 

Dispersed Systems. Many fluids of commercial and biological importance are dispersed systems, such as solids suspended in liquids (dispersions) and liquid—liquid suspensions (emulsions). Examples of the former include inks, paints, pigment slurries, and concrete examples of the latter include mayonnaise, butter, margarine, oil-and-vinegar salad dressing, and milk. Blood seems to fall in between as it is a suspension of deformable but not liquid particles, and it does not behave like either a dispersion or an emulsion (69) it thus has an interesting rheology (70). 

Disperse systems often necessitate particle size reduction, whether it is an integral part of product processing, as in the process of liquid-liquid emulsification, or an additional requirement insofar as solid particle suspensions are concerned. (It should be noted that solid particles suspended in liquids often tend to agglomerate. Although milling of such suspensions tends to disrupt such agglomerates and produce a more homogeneous suspension. 

Phenomena at Liquid Interfaces. The area of contact between two phases is called the interface three phases can have only a line of contact, and only a point of mutual contact is possible between four or more phases. Combinations of phases encountered in surfactant systems are L—G, L—L—G, L—S—G, L—S—S—G, L—L, L—L—L, L—S—S, L—L—S—S—G, L—S, L—L—S, and L—L—S—G, where G = gas, L = liquid, and S = solid. An example of an L—L—S—G system is an aqueous surfactant solution containing an emulsified oil, suspended solid, and entrained air (see Emulsions Foams). This embodies several conditions common to practical surfactant systems. First, because the surface area of a phase increases as particle size decreases, the emulsion, suspension, and entrained gas each have large areas of contact with the surfactant solution. Next, because interfaces can only exist between two phases, analysis of phenomena in the L—L—S—G system breaks down into a series of analyses, ie, surfactant solution to the emulsion, solid, and gas. It is also apparent that the surfactant must be stabilizing the system by preventing contact between the emulsified oil and dispersed solid. Finally, the dispersed phases are in equilibrium with each other through their common equilibrium with the surfactant solution. 

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