Scale prevention in sea water evaporators. Part III. Decarbonation, filtration, and acidification

The problem of supplying fresh water to meet the requirements for normal operation of ocean-going vessels has been of major importance for centuries in maritime circles. An early solution of this problem was the storing of fresh water on the vessel while in port for use white at sea. This solution was not satisfactory because it meant added tonnage and lost storage space. The present approach in the production of fresh water from salt water by distillation.

The distillation of sea water to produce fresh enter has been widely used in maritime circles throughout the world, but in spite of its wide adoption there are still matters existing that have not been settled. The most troublesome of these is the formation of scale on the heating surfaces of the distillation equipment. These scale deposits are caused by the precipitation of the solid matter in the sea water during the distillation process. As a result of the scale, the efficiency of the equipment is reduced and the production of fresh water is thereby diminished.

Much attention has been given to the elimination of scale deposits in sea water distilling apparatus. Chemical pretreatments, which had been successful in preventing scale in evaporators using ground waters, were tried on sea water equipment but no completely satisfactory processing has been found. Mechanical and chemical methods of scale removal are now being used, but these methods offer no solution to the scale formation problem.

A possible solution has been offered by the Armour Research Foundation of Chicago, Illinois. Basing their theory on the fact that the scale deposits formed are composed primarily of carbonates, the Armour Research Foundation has worked out, on a small laboratory scale, a heart-aeration process that eliminates the carbonates from the sea water before it enters the distilling plant. The process is divided into three separate steps. The first step, the removal of carbonates, is accomplished by heating the seat water to a temperature above 200 °F, holding the water at this condition for a period of time, and then aerating the water with compressed air. This step removes part of the carbonates present, but in the process magnesium hydroxide is precipitated. The second step, the removal of the precipitated magnesium hydroxide, is accomplished by filtering the decarbonated sea water. The third step, the neutralization of the high alkalinity resulting from decarbonation, is accomplished by injecting sulfuric acid into the sea water before it is fed to the evaporator. The sea water fed to the evaporator after this treatment is neutral and partially free of carbonates. The use of this treated sea water should greatly reduce the formation of scale in the evaporator.

A treatment and distillation plant has been erected at the Distillation Test Station, Fort Story, Virginia. Evaluation of the Armour Research Foundation process for the prevention of scale was made with the use of this equipment.

The specific purpose of this investigation is to evaluate the complete decarbonation-filtration-acidification process for scale prevention on sea water distillation equipment.