Foam breakdown

Research into the effectiveness of separator internals for foam breakdown is warranted. Parallel plates do reduce foam height, but there is need for further improvement... 

The mechanical ways of foam breakdown are realised by various rotating devices (mixers, disks, centrifuges, propellers, etc.) or by employing stream of air, vapour or liquid or by running the foam through cyclones and narrow gaps [1,2]. In all these cases the liquid outflow from the foam is accelerated which creates critical deformations and stresses in films and other foam elements. A convenient classification of the mechanical means of foam breakdown and the methods designing such mechanical devices are considered in [3]. The... 

The physical means of defoaming involve foam breakdown by thermal treatment (heating, processing with overheated vapour, freezing), by acoustic (mainly ultrasound) waves, vibrations, a-particles irradiation, creation of high capillary pressure in the foam, etc. 

Since antifoams can be either foam breakers or foam inhibitors [8,9], this should be taken into account in the estimation of their foam breakdown ability. For example, addition of one drop of buthanol or siloxane antifoams breaks down a foam from a saponin solution. However, further shaking of the solution containing the buthanol drop leads to intensive foaming and a large volume of a stable foam is formed. On the contrary, shaking the solution containing siloxane antifoam does not lead to formation of a foam [8]. 

The antifoams were studied on a foam from 1% commercial sulphonol solution. The rate of foam breakdown, caused by the antifoam sprayed, is determined by the foam volume destroyed for 1 min and by the lifetime of one half of the foam column. As tests showed the change in the content of the foaming composition affects the activity of the antifoaming agent but the relative foam stability almost does not change. 

A number of authors investigated the process of foam breakdown by antifoams introduced in the foaming system or the foam such as O/W emulsion and solutions in water-soluble organic solvents. Under such defoaming conditions the mechanism of antifoaming action becomes more complex and a destabilisation of the films is possible to occur by an entirely different (heterogeneous) mechanism (see below). 

It has been established [55] that hydrocarbon (decane, benzene) promoted foam breakdown, occurring with formation of unstable films (barrierless rupture), is only possible at very low surfactant concentrations (less than 0.003-0.004% saponin and OP-10). At higher surfactant concentrations the defoaming ability of the antifoam results from the lowering of the energy barrier of film rupture. The latter is determined by the properties of the adsorption layers and other film parameters. 

Even more important proved to be the fact that under reduced pressure the foam breakdown was initiated at its contact with the saturated hydrocarbons (tetradecane and hexadecane). However, when no reduced pressure was applied the contact layer did not undergo any changes. A sharp increase in the number of microcavities was also observed in the initial period when the pressure was reduced. It should be noted that the reduced pressure affected significantly only the rate of destruction of the lowest foam layer that contacted the... 

In these studies, however, it remained unclear whether the accelerated breakdown of a foam in contact with a hydrocarbon was due to the formation of unstable asymmetric aqueous films (including inside the Plateau borders) or because of the worsened conditions of wetting of foam borders contacting the hydrocarbon are worsen compared to that on glass walls. The latter mechanism of foam breakdown is realised when hydrophobic bridges of solid particles or oils were formed in the foam film [19,20,57]. This mechanism will be considered in Section 9.4. 

The kinetic regularities of heterogeneous defoaming, i.e. the acceleration of foam breakdown with the decrease in hydrocarbon molecular weight (respectively, the decrease in hydrocarbon surface tension and the increase in the contact angle between the meniscus and the substrate) and with the increase in both foam expansion ratio and reduced pressure, indicate that foam breakdown can occur due to the lowered stability of film borders at their contact with the substrate (worsened wetting). The fact that the foam situated between the cuvette wall and the capillary decayed at a considerably slower rate is in benefit to this statement. 

In contrast to the pure heterogeneous defoaming by hydrocarbons, the foam breakdown by typical diphilic antifoams occurs as a result of homogenous and heterogeneous... 

Maximum effectiveness in foam inhibition is exhibited by the alcohols in the middle of the homologous series (C7 - C9). In the whole range of their concentrations the foam inhibition occurs under heterogeneous conditions and hence, is a consequence of the joint action of the homogenous and heterogeneous adsorption mechanisms of foam breakdown. 

Similar results have been obtained in the study of foam breakdown by alcohols using a special device for determination of the rate of contact foam breakdown [69]. The foam was supplied at constant rate over the surface of organic solvent. The rate of foam breakdown was determined in two regimes impulse and continuous. In the impulse regime, after contacting the organic solvent, the foam expanded as a consequence of its breakdown in the contact zone and detached itself from the solvent surface. Since the foam was constantly supplied, after a... 

Fig. 9.12. Change in the rate of contact foam breakdown by saturated alcohols within a homologous...

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