Foam breaking foams

Quick-breaking foams consist of a miscible solvent system such ethanol (qv) [64-17-5] and water, and a surfactant that is soluble in one of the solvents but not in both. These foams are advantageous for topical appHcation of pharmaceuticals because, once the foam hits the affected area, the foam coUapses, deUvering the product to the wound without further injury from mechanical dispersion. This method is especially usehil for treatment of bums. Some personal products such as nail poHsh remover and after-shave lotion have also been formulated as quick-breaking foams. 

Foam Inhibitors. Methyl sihcone polymers of 300-1000 mm /s(= cSt)) at 40°C are effective additives at only 3—150 ppm for defoaming oils in internal combustion engines, turbines, gears, and aircraft appHcations. Without these additives, severe churning and mixing of oil with air may sometimes cause foam to overflow from the lubrication system or interfere with normal oil circulation. Because sihcone oil is not completely soluble in oil, it forms a dispersion of minute droplets of low surface tension that aid in breaking foam bubbles. 

Foam Breaking It is usually desirable to collapse the overflowing foam. This can be accomphshed by chemical means (Bikerman, op. cit.) if external reflux is not employed or by thermal means [Kishi-moto, Kolloid Z., 192, 66 (1963)] if degradation of the overhead product is not a fac tor. 

Foam can also be broken with a rotating perforated basket [Lem-lich, Principles of Foam Fractionation, in Periy (ed.), Progre.ss in Separation and Purification, vol. 1, Interscience, New York, 1968, chap. 1]. If the foamate is aqueous (as it usually is), the operation can be improved by discharging onto Teflon instead of glass [Haas and Johnson, Am. In.st. Chem. Fng. J., II, 319 (1965)]. A turbine can be used to break foam [Ng, Mueller, and Walden, Can. J. Chem. Fng., 55, 439 (1977)]. Foam which is not overly stable has been broken by running foamate onto it [Brunner and Stephan, Ind. Fng. Chem., 57(5), 40 (1965)]. Foam can also be broken by sound or ultrasound, a rotating disk, and other means [Ohkawa, Sakagama, Sakai, Futai, and Takahara,y. Ferment. Technol, 56,428, 532 (1978)]. 

If desired, dephlegmation (partial collapse of the foam to give reflux) can be accomplished by simply widening the top of the column, provided the foam is not too stable. Otherwise, one of the more positive methods of foam breaking can be employed to achieve dephlegmation. 

This carry-off of film drops, which may also occur with breaking foam, in certain cases can partially convert water pollution into air poH lution. If such is the case, it may be desirable to recirculate the gas. Such recirculation is also indicated if hydrocarbon vapors or otlier volatiles are incorporated in the gas stream to improve adsorptive selectivity [Maas, Sep. Sci, 4, 457 (1969)]. 

Goldberg, M. and Rubin, E. Ind. Eng. Chem. Process Design and Development 6 (1967) 195-200. Mechanical foam breaking. 

Foam breaks down and vaporizes its water content when exposed to heat and flame. Therefore, it must be applied to a burning surface at a sufficient rate to compensate for this loss and to ensure a residual foam layer over the liquid. 

Cas-o l separators ran be plagued by solids buildups, especially il solids are a known problem This may be particularly so i( the demulsifier is injected prior to the separator resulting in release of the solids from the emulsion. A suitable demulsifier will break foam and release the gas but if excessive foam is a problem and the chemical does not readily break the emulsion to release the gas at this potni, it is possible for loam to build up and carry emulsion out the gas outlet. The lack of adequate gas removal will cause undesirable agitation at points downstream. It can cause some rolling of fluids ui vessels at atmospheric pressure or in pressure vessels where operating pressures are exceeded. 

Foam breaking Injection port provided for chemical breaking mechanical breaker optional, consists of a double disk rotated at high speed with its own drive... 

While the emphasis of this section has been on kinetics of liquid crystal formation, the rate of liquid crystal dissolution may also be of interest—e.g., in connection with breaking foams and emulsions by adding materials which destroy the liquid crystalline structure. 

Antifoam agents Prevent foam formation. Foaming of lubricants is detrimental for two reasons. First, in an oil-circulation system foaming may lead to a break-down of the lubrication transport. Second, the increased surface area in a foam accelerates oxidation. 

Bendure indicates 10 ways to increase foam stability (1) increase bulk liquid viscosity, (2) increase surface viscosity, (3) maintain thick walls (higher liquid-to-gas ratio), (4) reduce liquid surface tension, (5) increase surface elasticity, (6) increase surface concentration, (7) reduce surfactant-adsorption rate, (8) prevent liquid evaporation, (9) avoid mechanical stresses, and (10) eliminate foam inhibitors. Obviously, the reverse of each of these actions, when possible, is a way to control and break foam. 

In the case where foam instability is desirable, it is essential to choose surfactants that weaken the Gibbs-Marangoni effect. A more surface-active material such as a poly(alkyl) siloxane is added to destabilize the foam. The siloxane surfactant adsorbs preferentially at the air/liquid interface, thus displacing the original surfactant that stabilizes the foam. In many cases, the siloxane surfactant is produced as an emulsion which also contains hydrophobic silica particles. This combination produces a synergetic effect for foam breaking. 

F. Examining the Foam. Break or cut through one of your samples to reveal the inside. Look carefully at the sample and describe the structure that you see. 

There is a qualitative agreement between the results for the rate of decrease in the specific foam surface area estimated by the increase in surfactant concentration during gravitational foam breaking and those from the kinetic studies of foam dispersity changes. 

It is necessary to bear in mind that although Eqs. (4.9) and (4.10) are rigorously fulfilled at any hydrostatically equilibrium state of the foam, the capillary pressure exerts a strong influence on the drainage and foam stability. At a certain value of the capillary pressure, depending of foam dispersity and the foam film type, the foam lifetime becomes very short and the foam breaks down instantaneously. 

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