How to avoid and destroy foams

Foaming is not always desired. There are many processes where foaming can be a major problem. Excessive foaming can lead to production stop or malfunction and loss of valuable materials. Some of 

Antifoamers are usually added to the aqueous phase, prior to foam formation, and prevent or inhibit foam formation from within the aqueous phase. Defoamers or foam breakers are added to eliminate an existing foam and usually act on the outer surface of the foam (a foam is a closed system and the defoa-mer can only reach the outer surfaces). Frequently, the separation is confusing but often the mechanisms are different for example, alcohols such as octanol are effective defoamers, but ineffective as antifoamers (Pugh, 1996). There are many commercial formulations available and it is quite a challenge to find the optimal ones to use in a given situation. Some are recommended for special purposes, but usually some practical experience is needed. In addition, we must consider whether the defoamer is compatible with the product or process (environmental issues, cost, availability etc.). Defoamers will often end up in the product being produced and this can sometimes be a problem if the product is an intermediate product to be used for further processing. 

The mechanisms of antifoaming and defoaming are not so well understood. However, these compounds interfere in one or more ways with the stabilization methods discussed earlier. Potential mechanisms are to increase surface tension or to decrease elasticity, bulk- or surface viscosity, or electrostatic repulsion in the lamellae. Defoamers are formulations (multicomponent products) and typically contain various oils (e.g. silicone oUs) and hydrophobic particles making the possible interactions quite versatile. Some of the basic requirements of a defoamer are limited solubility in aqueous phase so that it goes to the interface, surface tension should be below the value of foam liquid to enhance entry and spreading at air-hquid interphase, and low interfacial tension with foaming liquid to enhance spreading at air-hquid interface. Formation and use of oil lenses is an often used route to foam destruction. 

As stated by Pugh (1996), the synergistic antifoam-ing effect of mixtures of insoluble hydrophobic 

Example 13.3. Compounds performing defoaming action in beer foam. 

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