Chemical descaling of acid dosed desalination plants

Abstract

The heat transfer efficiency of sea water desalters is very often impaired by the deposition of scale of various compositions on the heat transfer surfaces. Four major classifications of such scales can be recognised—corrosion products such as iron and copper oxides, alkaline scale Ca(OH) 2 and MgO, additive scales consisting of calcium and magnesium combined with additive materials that are usually soft and easily removed, and calcium sulphate. Acid cleaning techniques have been developed and successfully applied to the problem of removing in the first 3 categories of scale and these are reviewed. However for calcium sulphate scale there is as yet no widely accepted chemical cleaning process and mechanical methods are usually adopted. These also are briefly considered and shown to be inadequate to deal effectively with every instance of calcium sulphate scaling. The need therefore for chemical descaling techniques able to adequately remove calcium sulphate is shown to be clear. A number of chelating agents which can dissolve calcium sulphate and therefore are potential candidates for the formulation of a suitable reagent have been reviewed and two such reagents taken to pilot scale trials. These trials included corrosion tests on a typical range of desalter construction materials and in later work used actual scale removed from operational plants. From the data obtained an economic assessment of the use of these reagents for the descaling of a typical desalter heat input vessel was made, from which it was concluded that the once through application of either reagent could only be considered feasible for limited treatment. To overcome this economic problem, processes for the regeneration of the spent reagents have been developed to the pilot plant stage. From this data one reagent has been chosen and following continuing development, a design for a mobile chemical descaling unit has been evolved. This unit, which also has the capability of carrying out a wide range of descaling operations is described.