Use of Shallow-Shell Resins to Reduce Demineralizer Cost
By By Francis Boodoo, Fabio Sousa, Carmen Mihaela Iesan, PhD and Sean Kennedy (Purolite Corp.)
Over the last 20 years, savings in acid and salt usage with the use of shallow-shell type cation ion exchange resins have been well documented. Four years ago, complimentary Type I and Type II shallow-shell strongly basic anion resins were developed. Laboratory studies showed that caustic dosages could be reduced by 15%, while simultaneously reducing silica leakages by up to 37%. Results for both co-flow and counter-flow regeneration are presented in this article. The report also includes summaries from two enduser sites.
Despite the introduction of competing membrane technologies more than 50 years ago, a large fraction of high quality demineralized water is still being produced today using ion exchange (IX) demineralizers. Water scarcity and comparatively large volumes of wastewater generated by membrane technology are key reasons for this. Although the adoption of counter-flow regeneration techniques has been the major driver in reducing demineralizer costs, the invention of shallow-shell cation resins more than 20 years ago has served to further entrench the use of IX demineralizers by generating significant savings in operating cost for acid and water during cation resin regeneration.
Such savings in regenerant use are possible when it is realized that conventional demineralizer resins use a very large excess of chemicals, generally ranging from 200% to 400% of theoretical quantities. When using conventional resins in co-flow regenerated plants, it is necessary to use such large excess of chemicals in order to drive the chemical reactions to completion so as to produce deionized (DI) water with acceptably low leakage levels for sodium and silica. Shallow-shell cation resins were invented to improve the efficiency of the regeneration process compared to conventional resins by using a manufacturing process that shortens the length of the diffusion path for IX. This shorter diffusion path allows use of lower dosages of regenerant while simultaneously producing lower leakage of sodium.
The complimentary introduction of shallow-shell anion resins about four years ago has further expanded savings in operating cost for caustic, acid and water. This article provides details on laboratory testing that has been done on Type I and Type II strongly basic anion resins manufactured to shell-and-core specifications. Field results are discussed for a number of commercial installations
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