Evaporators employed in the distillation of sea water have scale form on their heat transfer surfaces as calcium carbonate, magnesium hydroxide, and calcium sulfate in 300 to 500 hours of operation. Although the measures directed to overcome the formation of scale in sea water evaporators have been varied, none of these measures have been as successful as desired, and the problem is, therefore, still a major one.

In 1947, however, C. A. Hempel, Armour Research Foundation, Chicago, Ill., approached the scale problem of sea water evaporators rationally by saying that if the carbon dioxide content and the pH of sea water could be controlled by either physical or chemical means, that the scale deposition on the heat transfer surfaces would be reduced. By experimentation, Hampel developed a process whereby sea water is heated under pressure for a definite period of time, and then it is released to atmospheric pressure with aeration. This physical pretreatment process removes the carbon dioxide that is evolved from the decomposition and hydrolysis of the carbonate and bicarbonate content of sea water, and thus prevents the formation of insoluble calcium carbonate. The change in alkalinity does, however, cause insoluble magnesium hydroxide to form, but this insoluble material can be readily removed by filtration followed by acidification. Therefore, two of the scale forming salts, calcium carbonate and magnesium hydroxide, have been eliminated by this process.

In 1948, both the U. S. Coast Guard and the Bureau of Ships, Navy Department, became interested in this development of Hampel's, and they agreed to design a pretreatment plant for a 4000-gallon per day evaporator in order to evaluate further this process on a large scale basis. Such a plant was designed and constructed at the Norfolk Naval Shipyard, Portsmouth, Va., with the Coast Guard providing the necessary materials and equipment for the pretreatment plant, and the Navy furnishing the materials and equipment for the distilling unit. The Army Engineer's building and sea water facilities that were available at Fort Story, Va., made it a highly desirable location for the testing of the decarbonation and distilling units. So, after all the equipment had been fabricated and hydrostatically tested, it was shipped to Distillation Test Station at Fort Story where it was erected and made operational by Naval Shipyard personnel.

In September, 1949, a series of tests were started on this equipment at Fort Story to evaluate the design and construction of the pretreatment plant to decarbonate sea water es a means of reducing the scale in a 4000-gallon per day Grissom-Russell low pressure, double effect Soloshell evaporator. The first test of decarbonation calibration was made to determine the optimum operating conditions of the pretreatment plant which would give the maximum degree of carbon dioxide removal from sea water. The second test was a blank determination of evaporator scale, using untreated feed, by which a comparison could be made with all subsequent tests.

The pretreatment plant involved the operation of two pieces of equipment, the feedwater holding tank and the aerator tank. The procedure that was followed in this calibration was essentially this: the sea water was heated to a definite temperature at a specific feed rate, pumped to the feedwater holding tank and retained in this tank for definite periods of time; then the sea water was released to the aerator tank, again held for definite periods of time while being aerated with air, and a sample of sea water analyzed to determine the degree of carbon dioxide that was removed by these conditions. The variables that had their effect on the degree of decarbonation were, therefore, feed temperature, feed rate, feedwater tank holding time, aerator tank holding time, and air rate.

The effect of temperature on the degree of decarbonation was that the greater the temperature, the greater the carbon dioxide removal; 250 °F removed 47.5 to 50 per cent, 240 °F removed 38 to 42 per cent, and 228 °F removed 25.2 to 25.8 per cent.

The effect of feed rate at 250 °F on the degree of decarbonation was that the smaller feed rate, the greater the carbon dioxide removal; a feed rate of 5.25 gallons per minute removed 42.1 to 52.4 per cent whereas a feed rate of 10.5 gallons per minute only removed 36.6 to 47 per cent.

The effect of feedwater tank holding time on the degree of decarbonation et 250 °F and 5.25 gallons per minute feed was negligible since holding times of 25, 45, 66, 86, and 106 minutes removed 50.2 to 54 per cent of the carbon dioxide.

In the determination of the effect of aerator holding time on the degree of decarbonation a 250 °F and 5.25 gallons per minute feed, the greater the holding time, at 14 and 28 minutes, the greater the carbon dioxide removal, i.e., 46 end 50 per cent, respectively.

The effect of air on the degree of decarbonation at 250 °F, 5.25 gallons per minute feed, and 28 minutes aerator tank holding time, was that it gave the greater degree of carbon dioxide removal, but without air, the effect was that the greater the feedwater tank holding time, the less the carbon dioxide removal. The difference between the two air rates tried was negligible; 5 cubic feet per minute removed 51 to 54 per cent, whereas 20 cubic feet per minute only removed 44 to 52 per cent. With no air, however, a feedwater tank holding time of 25 minutes removed 50.8 per cent, 45 minutes removed 43.5 per cent, 66 minutes removed 39.5 per cent, and 106 minutes removed 38.5 per cent.

Therefore, from the results of the calibration, it is concluded that the optimum operating conditions for the pretreatment plant are a feed temperature of 250 °F, a feed rate of 5.25 gallons per minute, a feedwater tank holding time of 25 minutes, an aerator tank holding time of 28 minutes, and an air rate of 5 cubic feet per minute which will remove 50.8 per cent of the carbon dioxide in the sea water.

In the blank determination, the feed by-passed the pretreatment plant and went directly to the evaporator, which was operated at a feed rate of 5 gallons per minute, 150 °F, 2.5 pounds per square inch, gage of steam to the first effect, 97 pounds per square inch, gage of steam to the air ejector, 26.5 inches of vacuum, 70 gallons per minute of circulating water, and 1.5-thirty seconds overboard brine density. In 135 hours of operation, 19,400 gallons of fresh water were produced which had a salinity of less than 0.5 grain per gallon. Scale was formed at a rate of 0.23 pound per 1000 gallons of distillate produced, and whose composition wan 91.1 per cent calcium carbonate, 2.6 per cent magnesium hydroxide, 2.7 per cent calcium sulfate hemihydrate, 2.7 per cent silica dioxide, and 1.1 per cent ferric oxide.

From the operation of these tests, it is concluded that both the pretreatment plant and the distilling unit operated satisfactorily within the limits of their design.