A Deep Foam Story
This sort of behavior by a pure liquid is thermodynamically unfavorable. Some of the liquids do foam very bad. Foaming in such cases can be explained by the existence of stabilizing solute that is positively absorbed in the surface. Such solutes often present as impurities that possess hydrophobic and hydrophilic character and behave like surfactants in a particular system. These surfactants concentrate at the surface with their “hydrophilic” portion in the continuous surface with their “hydrophobic part” try to emerge into the gas phase. The lower forces of attraction between the hydrophobic ends of the molecules will produce a new surface of lower surface tension than previously. The surfactant system will reduce the surface tension of water from 72 to around 30 dyn/cm. The positive absorption of the surfactant at the interface means that work is required to move it to the bulk. The increase in free energy resulting from such a transfer will if large enough, make it energetically more favorable for the foam to persist.
To destroy the foam an antifoam must be incorporated into the system that will displace the foam stabilizer from the interface. The foam stabilizing mechanism will be destroyed and the foam will collapse. The antifoam must have low surface tension to operate efficiently and it must be insoluble in the foaming medium to spread on the film lamella. There have been several discussions of the exact model of the action of antifoams and the many different types available. Most organic antifoams for aqueous systems consist of an insoluble oil compounded with a surfactant to increase spreading.
Antifoams based on polydimethylsiloxane has most of the desirable features for control the foam like fast knockdown, long-lasting action, high efficiency, low cost, and ease of handling. These silicone fluids are clear, colorless liquids with surface tensions of 20-22 dyn/cm. They may compound with silica to improve dispersibility.
Therefore the criteria for a successful foam control are ;
- A lower surface tension than the foaming medium
- Insolubility in the foaming media
- Dispersibility in the foaming media
Let’s have a look at the chemical processing industries that Sour and acid gas scrubbing: The antifoam must be added to the absorbent stream and in many plants, there is a separate antifoam addition point feeding antifoams to the top of the absorber. Alternatively, the antifoam can be metered into the suction of the absorber feed pump.
Refinery processing: Antifoams can be used in crude processing and vacuum units. The foam may be controlled by adding low concentrations (1-2 parts/million) of a high viscosity polydimethylsiloxane antifoam.
Asphalt production and handling: Foam in the strippers of a deasphalting unit will cause heavy fractions to be carried into the recycle system. The addition of antifoam compound (Maxant Series that produced into Latro R&D lab. with high technology), either to the flash drum or to the strippers will eliminate this problem.
Petrochemical operations: The addition of an antifoam compound prediluted in toluene or xylene to the stripper feed will eliminate solvent loss and has been known to double unit throughputs.
Monomer recovery: The best solution is adding our special Maxant series for optimum foam control during polymerization during stripping for PVC production and synthetic latices. The lattices in latex are often high pH (9 or more) and are heated. Under these conditions, special high stability Maxant products should be used.
Chlorinated solvent systems: The only area where the polydimethylsiloxane-based antifoam is ineffective where it is dissolved by components of the foaming system. An important example of such a system occurs in the processing of chlorinated hydrocarbons. Under these conditions, our fluorosilicone based Maxant antifoams must be used. Such antifoams may be prediluted in a solvent such as a cellosolve acetate or methyl isobutyl ketone.