Excessive foaming in multistage flash desalination plants can give rise to salt-contaminated distillate and reduced operating efficiency. Plant operating variations in pressure, temperature, flow, and seawater feed composition and alkalinity can perturb vapor/liquid equilibria, resulting in increased levels of foam. Since such foaming is not always controlled by mechanical means, additives are often needed to control foaming level. In addition to controlling foam, a useful additive must not interfere with heat transfer or with the performance of the scale control additive; it must not contain steam-volatile components that can contaminate the distillate; it must be stable at MSF operating temperatures and residence times; and it should be compatible with the antiscalant feed solution. Antifoam performance data for over 60 candidates in 7 chemical classes were determined according to these criteria. The most effective structural class was found to be the polyglycol monoalkyl ethers. Foam control effectiveness within this class was shown to be a function of ethylene oxide content and alkyl chain length. The most effective additive found in this study is offered commercially as FLOCON Antifoam AF-4. FLOCON AF-4 has been used successfully in high-temperature MSF plants.

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