Liquid dispersions

W, R Penney/ Ph D / P.E./ Profes.sor of Chemical Engineenng, University of Arkansas Member, Amencan In.stitute of Chemical Engineers. (Ga s-in-Liquid Dispersions)... 

The baffle plate operates with liquid dispersed and gas as the continuous phase and is used primarily in heat-transfer apphcations. 

Excessive foaming. This is a case of a gas-in-liquid dispersion (covered in the next subsection). 

Dispersion Characteristics The chief characteristics of gas-in-liquid dispersions, like those of hquid-in-gas suspensions, are heterogeneity and instabihty. The composition and structure of an unstable dispersion must be obsei ved in the dynamic situation by looking at the mixture, with or without the aid of optical devices, or by photographing it, preferably in nominal steady state photographs usually are required for quantitative treatment. Stable foams may be examined after the fact of their creation if they are sufficiently robust or if an immobilizing technique such as freezing is employed [Chang et al., Ind. Eng Chem., 48, 2035 (1956)]. 

Gases and liquids may be intentionally contacted as in absorption and distillation, or a mixture of phases may occur unintentionally as in vapor condensation from inadvertent cooling or liquid entrainment from a film. Regardless of the origin, it is usually desirable or necessary ultimately to separate gas-liquid dispersions. While separation will usually occur naturally, the rate is often economically intolerable and separation processes are employed to accelerate the step. 

Types of Gas-in-Liquid Dispersions Two types of dispersions exist. In one, gas bubbles produce an unstable dispersion which separates readily under the influence of gravity once the mixture has been removed from the influence of the dispersing force. Gas-hquid contacting means such as bubble towers and gas-dispersing agitators are typical examples of equipment producing such dispersions. More difficulties may result in separation when the gas is dispersed in the form of bubbles only a few micrometers in size. An example is the evolution of gas from a hquid in which it has been dissolved or released through chemical reaction such as electrolysis. Coalescence of the dispersed phase can be helpful in such circumstances. 

Gravity Settlers Decanters These are tanks in which a liqmd-liquid dispersion is continuously settled and coalesced and from wriich the settled liquids are continuously withdrawn. They can be either horizontal or vertical. Figure 15-24 shows some typical horizontal decanters. For an uninstrumented decanter the height of the heavy-phase-liquid leg above the interface is balanced against the height of the hght-hquid phase above the interface, Eq. 15-50. 

FIG. 15-37 Portion of a perforated-tray tower, arranged for light liquid dispersed. 

Gas-Liquid Dispersion This involves physical dispersion of gas bubbles by the impeller, and the effect of gas flow on the impeller. 

Liquid Dispersion Spray columns are used with slurries or when the reaction product is a solid. The absorption of SO9 by a hme slurry is an example. In the treatment of phosphate rock with sulfuric acid, offgases contain HF and SiF4. In a spray column with water, solid particles of fluorosilic acid are formed but do not harm the spray operation. The coefficient /cl in spray columns is about the same as in packed columns, but the spray interfacial area is much lower. Considerable backmixing of the gas also takes place, which helps to make the spray volumetri-caUy inefficient. Deentrainment at the outlet usually is needed. 

The application of liquid dispersion reac tors to the absorption of fluorine gases is described by Kohl and Riesenfeld (G .s Purification, Gulf, 1985, pp. 268-288). 

When a liquid is dispersed into droplets the surface area is increased, which enhances the rates of heat and mass transfer. For a particular liquid dispersed at constant concentration in air the MIE varies with approximately the cube of surface average droplet diameter, hence the MIE decreases by a factor of about 8 when the surface average diameter D is halved (A-5-1.4.4). Ease of ignition is greatly enhanced for finely divided mists with D less than about 20 /rm, whose MIE approaches that of the vapor. Below 10 /rm a high flash point liquid mist (tetrahydronaphthalene) was found to behave like vapor while above about 40/rm the droplets tended to burn individually [ 142]. Since liquid mists must partially evaporate and mix with air before they ignite, the ease with which a liquid evaporates also affects MIE (Eigure 5-1.4.4). 

Sample requirements Solids, liquids (dispersed or evaporated on a substrate), or powders must be vacuum compatible... 

Liquid-liquid dispersion, Solid-liquid dispersion. Local shear... 

Solid-Liquid Dispersions, Bohuslav Dobias, Xueping Qiu, and Wolfgang von Rybinski... 

Generate liguid-liquid dispersions (droplets) Highly Turbulent NRe > 100,000 Low Turbulence Nlie < 100,00 2 six-element modules 3 six-element modules... 

Foam is gas-liquid dispersion in which the liquid is the continuous phase and the gas is the discontinuous phase. The first use of foam in drilling was reported in 1964. 

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