Dispersion, in liquids

Practical separation techniques for gases dispersed in liquids are discussed. Processes and methods for dispersing gas in hquid have been discussed earlier in this section, together with information for predicting the bubble size produced. Gas-in-hquid dispersions are also produced in chemical reactions and elec trochemic cells in which a gas is liberated. Such dispersions are likely to be much finer than those produced by the dispersion of a gas. Dispersions may also be uninten-tionaUy created in the vaporization of a hquid. 

Figure 5-5X. Type R-500. Very high shear radial flow impeller for particle size reduction and uniform dispersion in liquids. By permission, Lightnin, (Formerly Mixing Equipment Co.) a unit of General Signal.

Molybdenum disulhde (M0S2), graphite, hexagonal boron nitride, and boric acid are examples of lamella materials commonly applied as solid lubricants. The self-lubricating nature of the materials results from the lamella crystalline structure that can shear easily to provide low friction. Some of these materials used to be added to oils and greases in powder forms to enhance their lubricity. Attention has been shifted in recent years to the production and use of nanosize particles of M0S2, WS2, and graphite to be dispersed in liquid lubricants, which yields substantial decreases in friction and wear. 

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). 

Powders dispersed in liquids consist of agglomerates—a collection of aggregates—which in turn are composed of primary particles. Agglomerates... 

The USP has numerous requirements, e.g., ophthalmic solutions [need be] essentially free from foreign particles, suitably compounded and packaged for instillation into the eye, or ophthalmic suspensions [need contain] solid particles dispersed in liquid vehicle intended for application to the eye [1]. Ophthalmic suspensions are required to be made with the insoluble drug in a micronized form to prevent irritation or scratching of the cornea. A finished ophthalmic ointment must be free from large particles and must meet the requirements for leakage and for metal particles under ophthalmic ointments . These and other requirements will be discussed further in subsequent sections. 

Particulate solids dispersed in liquid and devoid of order... 

H N Stein, The preparation of dispersions in liquids (New York Marcel Dekker, 1995). 

Figure 4.8. Longitudinal dispersion in liquids in packed beds. CP—Cairns and Prausnitz(37), CB — Carberry and Bretton(32) EW—Ebach and White(38) H — Hiby(39) LG—Liles and Geankoplis(40)...

Fig. 8.9 Different methods for spinning CNT fibers and scanning electron micrographs of representative samples, (a) Wet spinning of nanocarbons dispersed in liquid, (b) drawing from a forest of aligned CNTs and (c) direct spinning from the gas phase during CNT synthesis by CVD. Images from references [53,59, 60, 61,62], With kind permission from AMS (2000, 2013), Elsevier (2007, 2011), Wiley (2010).

Just as with the metal surface reactions described above, the efficiency of heterogeneous reactions involving solids dispersed in liquids will depend upon the available reactive surface area and mass transfer. 

The study of the interfacial liquid-liquid phase however is complicated by several factors, of which the chief is the mutual solubility of the liquids. No two liquids are completely immiscible even in such extreme cases as water and mercury or water and petroleum the interfacial energy between two pure liquids will thus be affected by such inter-solution of the two homogeneous phases. In cases of complete intersolubility there is evidently no boundary interface and consequently no interfacial energy. On addition of a solute to one of the liquids a partition of the solute between all three phases, the two liquids and the interfacial phase, takes place. Thus we obtain an apparent interfacial concentration of the added solute. The most varied possibilities, such as positive or negative adsorption from both liquids or positive adsorption from one and negative adsorption from the other, are evidently open to us. In spite of the complexity of such systems it is necessary that information on such points should be available, since one of the most important colloidal systems, the emulsions, consisting of liquids dispersed in liquids, owe their properties and peculiarities to an extended interfacial phase of this character. 

The Van Deemter equation (1) was the first rate equation to be developed and this took place as long ago as 1956. However, it is only relatively recently that the equation has been validated by careful experimental measurement (2). As a result, the Van Deemter equation has been shown to be the most appropriate equation for the accurate prediction of dispersion in liquid chromatography columns, The Van Deemter equation is particularly pertinent at mobile phase velocities around the optimum velocity (a concept that will shortly be explained). Furthermore, as all LC columns should be operated at, or close to, the optimum velocity for maximum efficiency, the Van Deemter equation is particularly important in column design. Other rate equations that have been developed for liquid chromatography will be discussed in the next chapter and compared with the Van Deemter equation... 

Figure 3.35 Axial dispersion in liquid-solid fixed beds (for e = 0.45 and Sc = 1000). Average value is estimated by data given by Levenspiel (1972).

Gases are dispersed in liquids usually to facilitate mass transfer between the phases or mass transfer to be followed by chemical reaction. In some situations gases are dispersed adequately with spargers or porous distributors, but the main concern here is with the more intense effects achievable with impeller driven agitators. 

Another method, the emulsion process, was developed in Europe to meet the demand for a resin with small particle size, high bulk density, and low plasticizer absorption properties. These characteristics are especially desirable in making plastisols and organosols, where the resins are dispersed in liquid plasticizers. In this polymerization process, emulsifiers are added, and the solution is agitated to keep the monomer droplets dispersed. The initiators must be water soluble it is usually an inorganic persulfate or an organic hyperperoxide. 

Lithium shot or dispersion in liquid paraffin can be exposed to air during handling without deterioration. It may be transferred by pouring through a wide-necked funnel. Small quantities of the dispersion may be destroyed by washing with water to allow the lithium metal to react with water. Larger quantities should be suspended in ether and treated in a fume cupboard with dry t-butyl alcohol. Hydrogen is liberated in this reaction. 

For bubble systems (gases dispersed in liquids) fractional holdup can approach 0.5 as shown by Fig. 14-104. However, before reaching this holdup, the bubble systems shift to an unstable mix of bubbles and vapor jets. Hence an exact... 

T. Reith, Physical aspects of bubble dispersions in liquids, Ph.D. Thesis, Delft Techn. Univ., 1968. 

A most important physical property of colloidal dispersions is the tendency of the particles to aggregate. Encounters between particles dispersed in liquid media occur frequently and the stability of a dispersion is determined by the interaction between the particles during these encounters. 

Tamir et al. [109] also studied an impinging stream absorber operated in bubbling mode, as shown in Fig. 7.3. The absorber takes liquid as the continuous phase while gas is dispersed in liquid, so it actually belongs to liquid-continuous impinging streams (LIS). The experimental results obtained showed that this flow configuration exhibits a higher absorption rate than that shown in Fig. 7.2(a). Combining them with the results... 

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