| 摘要 |
Water, e.g. sea water, which is to be distilled is first treated with a water-soluble polyphosphate and then heated, after which an acid (or a salt which forms acid on hydrolysis) is added to the water before it is introduced into the still, e.g. a vapour-compression still provided with a de-aerator. Suitable polyphosphates are:-tetrasodium pyrophosphate, sodium tripolyphosphate, and sodium hexametaphosphate. In the case of sea water, 0.5 to 10 parts of polyphosphate are added per million parts of feed water. After addition of the polyphosphate, the feed water is divided into two streams, one of which is heated by passing it in heat-exchange with the distillate, while the other stream is heated in heat-exchange with the brine blowdown. The two streams, which leave the heat-exchangers at a temperature of about 190 DEG F., are then reunited, and the acid substance is added. Suitable acid substances are:-sulphuric, hydrochloric, phosphoric, nitric, sulphamic and citric acids, sodium bisulphate, ferric chloride, and aluminium sulphate. The acidified acid is then passed, via a de-aerator, into the still. The pH of the brine withdrawn from the still is continuously checked by an instrument which automatically controls the acid addition to the feed to maintain the brine at a pH of 7.0 to 7.4. The pH of the distillate may be adjusted, as desired, with acid or alkali. The distillate may also be treated with chlorine-water.
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