The field assessment of a high temperature scale control additive and its effect on plant corrosion

Abstract

Distillation of seawater, currently the only aćcepted, method, for the large scale production of fresh water from seawater, produces two serious problems that cannot be easily “engineered” out of the process. These are scale formation on heat transfer surfaces and corrosion of the plant constructional materials and are a direct result of the nature and chemical composition of seawater. Scale formation can he controlled by the use of additives or acid. However, where fuel costs are high, it is important for efficiency to operate distillation plants at as high a temperature as possible. Until recently high temperature operation has only been possible by the use of acid treatment, as additives based on polyphosphates do not work at temperatures greater than 90°C because of hydrolysis of the polyphosphate to inactive phosphate. Use of acid, unless very carefully controlled, can lead to serious corrosion of the plant. Ideally acid addition should be controlled to leave a residual alkalinity in the seawater feed of 10–20 ppm, which in practice is very difficult, because of the problems in accurately controlling the pH of a large flowing volume of seawater. If acid is underdosed, to avoid corrosion problems then scale formation will result. The result of a joint programme between CIBA-GEIGY (UK) LTD and the UKAEA has been the introduction of a high temperature scale control additive. Results of plant trials with this chemical will be described and calculation of fouling factors from plant operational data show its effectiveness in controlling alkaline scale formation. Simple laboratory tests have shown the likely benefits, in terms of lower corrosion rates, to be gained from the use of additives which result in high recirculating brine pH. These observations have been substantiated by plant experience.