Scale reduction in saline water conversion

Abstract

Distillation of sea water and other naturally occurring saline waters gives rise to the formation of scale deposits, causing a reduction in capacity or efficiency, or both. Several methods suggested to reduce the amount of potential scale have met with only partial success. Recently, it was found that the addition of small amounts of polymeric materials of selected molecular weight range to the sea water feed, had a remarkable effect in reducing the adverse effects of scale formation on the heat transfer surfaces. The work reported here was carried out in an attempt to establish the optimum dosage of these organic polymers (sodium polyacrylate and sodium polymethacrylate)in saline water feed, by experiments with a single effect evaporator. For batch type operations, using a synthetic sea water of 7,000 ppm total hardness as CaCO 3 and a pH of 7.5, sodium polyacrylate at a 4 ppm concentration, gave 98% scale reduction. Molecular weight of the polymer being tested was 125,000. Similar experiments with sodium polymethacrylate, molecular weight 8,000 to 10,000, with rather concentrated feed (10,400 ppm total hardness) indicated it to be effective at 100 ppm concentration. Scale reduction in this case was 92%. Three different molecular weights of each polymer were investigated for their effectiveness. Sodium polymethacrylate, having a molecular weight of 4,500, was found to be most effective as a scale preventive additive. In the case of sodium polyacrylate, both a lower molecular weight of 84,500 and a higher one of 190,000 were found to be almost equally effective. However, a molecular weight of 125,000 gave very poor results. Scale reduction was found to be only 17.1%, as compared to 69.2%, obtained with the other homologue.