Aspects of acid-cleaning operations in MSF plants Original Research Article

In most large MSF plant, control of alkaline-scale deposition involves continuous dosing with polymeric inhibitors together with periodic circulation of sponge balls. This strategy is successful in substantially retarding the fall-off in plant performance but it does become eventually necessary to conduct an acid-cleaning operation to remove alkaline-scale deposits. The first segment of this paper comprises a survey of the acid-cleaning equipment, environmental conditions and procedures in a number of large MSF plant in the Middle East. This reveals some distinctive features in terms of: • the incorporation of acid-cleaning facilities in the original design and construction of MSF units, • the parts of the plant through which the acid is circulated, • the actual acid employed, the “base liquid” (seawater or distillate), the temperature and times at which the cleaning is undertaken. A feature of recent experience has been the substantial extension of the period of the period between acid cleans in MSF plant. This poses the question of the optimum frequency of acid cleans in terms of the cost of the cleaning operation and the gradual decline in plant performance in the intervening periods. The paper provides a discussion of the relevant factors that are likely to influence the optimum frequency of acid cleaning. It is demonstrated that the relative importance of these factors varies from plant-to-plant and from location-to-location but an illustrative case study of one particular plant is provided. The major benefit of an acid clean of a fouled plant is to reduce the steam consumption. The increase in production is relatively small. For example in a typical 5 Migd MSF unit if the recovery section fouling factor is reduced, say, by 0.10 m2. K/kW during an acid clean, and no other changes are made to the process conditions, then the result would be a drop in the steam consumption of the order of 10% and an increase of only 1% in distillate production. Illustrative optimisation calculations for a sample plant will be presented and discussed giving an indication of the potential savings to be made in operational costs.