Foam breaker

For small instaUafions, column foam separators are more suitable. Waste flows downward in the column whereas gas spargers, located at the bottom, give countercurrent flow. The foam generated is carried upward to a foam breaker and coUector. 

Both high bulk and surface shear viscosity delay film thinning and stretching deformations that precede bubble bursting. The development of ordered stmctures in the surface region can also have a stabilizing effect. Liquid crystalline phases in foam films enhance stabiUty (18). In water-surfactant-fatty alcohol systems the alcohol components may serve as a foam stabilizer or a foam breaker depending on concentration (18). 

The LT evaporator shown in Fig. 11-122/ is typical of those commonly used, especially for black hquor. Feed enters at the bottom of the tube and starts to boil partway up the tube, and the mixture of liquid and vapor leaving at the top at nigh velocity impinges against a deflector placed above the tube sheet. This deflector is effective both as a primary separator and as a foam breaker. 

The formation of micelles in the foam breaker does not affect the limiting equations because of the theoretically unhmited opportunity in a sufficiently tall column for their transfer from the reflux to the ascending stream [Lemhch, Principles of Foam Fractionation, in Perry (ed.), Progress in Separation and Purification, vol. 1, Interscience, New York, 1968, chap. 1]. 

Fig ure 11-12. Reaotors with meohanioal stirring M, moter G, gas (air) SB, baffles LR, oonduit tube SZ, foam breaker W, roller F, liquid. (Source Schuger [11].)... 

Schaum-bestflndigkeit, /. foam-holding capacity. -bier, n. foaming beer, -bildner, m. frothing agent, foamer. -bildtmg. /. formation of foam or froth, -blaae, /. bubble, -brecher, m. foam breaker, froth killer, schaumen, v.i. froth, foam (of wine, etc.) sparkle, fizz (of soap) lather. — v.t. skim, scum. 

This may double as a foam breaker foam will slow down the rotation or need an increase in power to maintain speed. 

The two other foam sensors mentioned above are ultrasound and rotating disks. The ultrasound sensor is a transmitter and receiver mounted opposite to each other and operating at 25—40 kHz. In the bioreactor, the waves are absorbed by the foam and the signal is generated. The rotational disk foam sensor is a mechanical foam breaker which is used by increasing the rotational resistance. 

Eig. 7.9. Aeration vessel with foam breaker for SCP production. 

Foam in the bioreactor is troublesome it can reduce the oxygen transfer rate (OTR). Antifoam is use to prevent foam formation. However, excess antifoam may cause growth inhibition in the course of fermentation. The simplest device is known as a foam breaker, which is mounted on the stirrer shaft located on the surface of liquid. It is a flat blade. 

Defoamers (foam breakers) destroy foams that have already occurred. 

Properties of Egg Albumen. Egg albumen is normally used in foods for two reasons it whips into a foam and the foam can be set irreversibly by heat. One advantage of egg albumen is that it is relatively unaffected by the presence of fat. Fat in these systems acts as a foam breaker. Some other whipping agents are badly affected by the presence of fat. In confectionery systems egg albumen is usually set by beating the reconstituted egg albumen into a hot sugar syrup. The coagulation... 

Here, the foam breaker will be scaled-up according to its tip speed u = nnd in model experiments, which will also moderately depend on the foam yield (q). 

Zlokarnik M. Design and scale-up of mechanical foam breakers. Ger Chem Eng 1986 9 314-320. 

In many applications we want to prevent the formation of foam or get rid of already existing foam (see, for example, Table 11.1). Usually chemicals are added to achieve this (see Ref. [564] for an introduction). We distinguish between antifoamers and defoamers. An-tifoamers are added to the liquid prior to foam formation and act to prevent or inhibit foam formation. Defoamers or foam breakers are added to eliminate existing foam. They can only reach the outer surface of a foam. 

Chemical Defoamers The addition of chemical foam breakers is the most elegant way to break a foam. Effective defoamers cause very rapid disintegration of the foam and frequently need be present only in parts per million. The great diversity of compounds used for defoamers and the many different systems in which they are applied make a brief and orderly discussion of their selection difficult. Compounds needed to break aqueous foams may be different from those needed for aqueous-free systems. The majority of defoamers are insoluble or nonmiscible in the foam continuous phase, but some work best because of their ready solubility. Lichtman (Defoamers, 3d ed., Wiley, New York, 1979) has presented a concise summary of the application and use of defoamers. Rubel (Antifoaming and Defoaming... 

Although many factors, such as film thickness and adsorption behaviour, have to be taken into account, the ability of a surfactant to reduce surface tension and contribute to surface elasticity are among the most important features of foam stabilization (see Section 5.4.2). The relation between Marangoni surface elasticity and foam stability [201,204,305,443] partially explains why some surfactants will act to promote foaming while others reduce foam stability (foam breakers or defoamers), and still others prevent foam formation in the first place (foam preventatives, foam inhibitors). Continued research into the dynamic physical properties of thin-liquid films and bubble surfaces is necessary to more fully understand foaming behaviour. Schramm et al. [306] discuss some of the factors that must be considered in the selection of practical foam-forming surfactants for industrial processes. 

Not all foams are wanted though. Foams, other than flotation froths, are generally not wanted in the process industries where they tend to interfere with process unit operations and may cause upsets. Some agents will act to reduce the foam stability of a system (termed foam-breakers or defoamers) while others can prevent foam formation in the first place (foam preventatives, foam inhibitors). There are many such agents and Kerner [327] describes several hundred different formulations for foam... 

Any substance that acts to reduce the stability of a foam it can also act to prevent foam formation. Terms such as antifoamer or foam inhibitor specify the prevention of foaming, and terms such as defoamer or foam breaker specify the reduction or elimination of foam stability. Example Poly(dimethylsiloxane)s, (CH3)3SiO[(CH3)2SiO]xR, where R represents any of a number of organic functional groups. Antifoamers can act by any of a number of mechanisms. 

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