Foam-breaking

Finally Obemosterer [402] discussed the origin of the coalescence-inhibiting effect of tensides and came to the following conclusions  

Thus there is still a need for research into the field of coalescence inhibition through tensides. 

Coalescence processes are not only important in industrial chemistry and in biotechnology, but are ever more important in aerobic waste water purification, since so-called fine bubble aeration became established over surface aeration. It is therefore hardly surprising, that there has recently been strong research activity in this field [32, 570], (Bubble coalescence is not involved in surface aeration [625].) 

Since tensides become concentrated in the G/L interfacial area, particular nonionic tensides are often used as anti-foaming agents. Several of them have been tested regarding their effect on kia values [630], It was shown that in the case of particularly effective anti-foaming agents 3 ppm was sufficient to halve the kia value for pure water, see Fig. 4.20. This is connected with the fact, that these substances reduce both Icl and a. This shows, that the use of chemical anti-foaming agents should be dispensed with. 

Foams can occur in any chemical, biological or industrial process to such an extent, that process control is made considerably more difficult or even becomes impossible, Thus, for example, chemical reaction systems tend to foaming, if nascent ( in statu nascendi ) gas is produced in them, because such fine gas bubbles do not coalesce into larger ones and therefore remain in the system. Major foaming problems are often connected with the expulsion of the residual monomer after the end of emulsion polymerization (e.g. Buna rubber manufacture), because here the finest gas bubbles are formed in a material system, which contains emulsifiers, e.g. foam-forming surface active substances. 

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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. 

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. 

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. 

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

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. 

The soapy foam in which bubbles are stabilized by surface-adsorbed components. These foams cover a wide range of system factors. Sometimes chemical foam-breaking additives are used. 

Thermal foam breaking (by direct injection of steam into the foam space or by indirect heating of the walls) aims at bursting the inflated gas bubbles, while simultaneously lowering the viscosity of the liquid lamellae, ft has proved to be ineffective, frequently resulting in (thermal) damage to the product and incrustation of the heating surface. 

Mechanical foam breaking, the most frequently used method, is essentially based on subjection the foam lamelae to shear stress. In addition, the pressure changes... 

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