Scale prevention in sea water evaporators Part II: decarbonation-filtration treatment

Abstract

The use of distillation for converting sea water to non-saline water has a history dating back over 400 years. This problem has been especially troublesome in regard to the installation of evaporator units on shipboard, in which operation costs, steam consumption, and space requirements are critical.

The major difficulty in the operation of the evaporators has been the deposition of an insulating scale composed of calcium sulfate, calcium carbonate, and magnesium hydroxide on the heat transfer surfaces of the evaporators. Various chemical and mechanical treatments have been successfully used by industry to prevent scale formation in fresh water distilling plants but these processes have not proved successfully used by industry to prevent scale formation in fresh water distilling plants but these processes have not proved successful when applied to salt water evaporators.

A possible process for preventing scale formation in salt water evaporators has been proposed bt Armour Research Foundation whereby the carbon dioxide gas present in the sea water either as dissolved gas or combined in the form of carbonate and bicarbonate ions can be removed by heating the feed water to a temperature above 200°F, holding it at this temperature for at least 30 minutes and aerating the feed using compressed air. After about 50 percent of the carbon dioxide has been removed, the feed becomes saturated with magnesium hydroxide. The precipitate can be removed from the feed water by filtration, and the high alkalinity of the saturated solution can be reduced by injecting sufficient sulfuric acid into the feed water to the evaporator. Laboratory tests can indicate that feed water treated in this manner will not deposit calcium carbonate or magnesium hydroxide scale on the heat transfer surfaces of the evaporator.

Full scale operational tests of this process are to be conducted at the Army Engineering Research and Development Laboratory, Distillation Test Station, Fort Story, Virginia.

The purpose of this investigation is to determine the effect of decarbonation-filtration feed treatment of sea water in reducing scale formation in a 4,000-gallon per day, double effect, low pressure, Solo-shell evaporator.