Phase separation foam prevention

By definition osmotic pressure equals the pressure that has to be applied onto a mobile semi-permeable membrane (filter) separating foam and liquid in order to prevent liquid phase from entering the foam. This pressure can be calculated as the difference between pressure of liquid column and pressure of foam column, both having the same height [84]... 

Chemically, the preparation of a "stable" foam or emulsion requires the use of a surfactant to aid in dispersion of the internal phase and prevent the collapse of the foam (or emulsion) into separate bulk phases. The selection of a surfactant is made on the basis of severity of conditions to be encountered, the gas to be entrained (N2, C02, LPG, CH, or air), the continuous phase liquid (water, alcohol, or oil), and half-life of foam stability desired. 

The resulting solution is cooled to 0° and decomposed by careful addition of 500 ml. of 102V hydrochloric acid. The mixture is transferred to a separatory funnel, the aqueous phase is separated, and the toluene layer is extracted with two 250-ml. portions of 102V hydrochloric acid. The aqueous extracts are combined and heated under reflux for 15 hours to effect decarboxylation. The hot, dark-colored solution is treated with 10 g. of activated charcoal, filtered, and evaporated to dryness under reduced pressure. The residue is washed into a separatory funnel with 300 ml. of water. The solution is treated with saturated aqueous potassium carbonate solution until it is alkaline to litmus the carbonate solution must be added very carefully to prevent excessive foaming. Solid potassium carbonate is added until a thin slurry is obtained, and the slurry is extracted with four 400-ml. portions of ether. The combined ether extracts are dried for at least 60 minutes over calcined potassium carbonate and then filtered. 

The bicarbonate extractions must be performed quickly, since the product slowly hydrolyzes in the presence of water. The best yields were obtained when the phases were shaken briskly for 10 seconds and separated as soon as foaming ceased. Foaming also occurred when the pentane extracts were washed with water, but did not prevent the separation of phases. 

Nanoparticles are frequently used as a suspension in some kind of solvent. This is a two phase mixture of suspended solid and liquid solvent and is thus an example of a colloid. The solid doesn t separate out as a precipitate partially because the nanoparticles are so small and partially because they are stabilised by coating groups that prevent their aggregation into a precipitate and enhance their solubility. Colloidal gold, which has a typical red colour for particles of less than 100 nm, has been known since ancient times as a means of staining glass. Colloid science is a mature discipline that is much wider than the relatively recent field of nanoparticle research. Strictly a colloid can be defined as a stable system of small particles dispersed in a different medium. It represents a multi-phase system in which one dimension of a dispersed phase is of colloidal size. Thus, for example, a foam is a gas dispersed in a liquid or solid. A liquid aerosol is a liquid dispersed in gas, whereas a solid aerosol (or smoke) is a solid dispersed in a gas. An emulsion is a liquid dispersed in a liquid, a gel is liquid dispersed in a solid and a soils a solid dispersed in a liquid or solid. We saw in Section 14.7 the distinction between sol and gel in the sol gel process. 

If a low-viscosity liquid is beaten to form a foam, it will inevitably become a polyhedral foam. Liquid will always drain from it. For a volume fraction of 0.9, the overrun is 900%, for cp = 0.95, it is even 1900% such a foam would make a very fluffy food. Most aerated foods are different. They are dilute foams in the sense that the bubbles are separate from each other and they remain spherical. To prevent the bubbles from creaming, the continuous phase should have a yield stress. This can be achieved in several manners ... 

Again, the wetting and emulsification properties of the cleaning solution are very important, and in this case, the solution needs to be low foaming [28]. For example, end-capped , non-ionic surfactants used at or near their cloud point can provide these features (the low foaming nature resulting from having the separated phase acts as a defoamer). For heavily soiled surfaces, another non-ionic surfactant may have to be added in order to provide the necessary emulsification and prevent redeposition [21]. 

The scaled surface area and its variation with d> are of crucial importance in the definition and evaluation of the osmotic pressure , H, of a foam or emulsion. We introduced the concept in Ref 37, where it was referred to as the compressive pressure , P. It has turned out to be an extremely finitful concept (22,27,38). The term osmotic was chosen, with some hesitation, because of the operational similarity with the more familiar usage in solutions. In foams and emulsions, the role of the solute molecules is played by the drops or bubbles that of the solvent by the continuous phase, although it must be remembered that the nature of the interaetions is entirely different. Thus, the osmotic pressure is denned as the pressure that needs to be applied to a semipermeable, freely movable membrane, separating a fluid/fluid dispersion from its continuous phase, to prevent the latter from entering the former and to reduce thereby the augmented surface free energy (Fig. 4). The membrane is permeable to all the components of the continuous phase but not to the drops or bubbles. As we wish to postpone diseussion of compressibility effects in foams until latter, we assume that the total volume (and therefore the volume of the dispersed phase) is held constant. 

The above conditions assume two relatively pure liquids. The presence active agent or fine dispersed solids can interfere with the coalescing process and result in a stable emulsion. Many liquid-liquid separators form a stable emulsion at the interface called a rag layer because of these agents and may require draw-off nozzles near the interfoce to prevent accumulation. The rag layer is like foam in liquid-gas systems and is typically stabilized by very fine solids. If the rag is drawn off it may be de-emulsified or broken by filtration, heating, chemical addition, or reveising the phase that is dispersed. 

If a trayed column in high pressure service does not have sufficient downcomer residence time to clarify the liquid phase, the vapor/liquid mixture will be carried down to the next lower tray. This recycling of vapor lowers the capacity, as well as the separation efficiency, of the trays. Conditions preventing rapid disengagement of vapor and liquid in a tray downcomer are high liquid viscosity, low surface tension, and a small density difference between liquid and vapor. Also, any tendency for the liquid to foam will increase the downcomer residence time required. 

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. 

---