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Demineralized water treatment systems play an important role in the petrochemical industry. The ability to consistently supply demineralized water that is subsequently used for steam generation is essential for all types of steam cracking processes. During the conceptual design of demineralized water treatment plants, special attention must be paid to raw water source identification and the associated quality and reliability of the demineralized water desired. Other considerations include the nature of demineralized water demand, condensate return rates and amount of storage. As mentioned in Part 1 of this article (published in the March issue of Hydrocarbon Processing), a typical design decision matrix should consider raw water source selection, the type of pretreatment for suspended solids removal and the quality desired by other users (utility and cooling water makeup). Part 2 discusses common technologies considered for water demineralization, along with some considerations for demineralized water treatment plant design optimization.
Ion exchange (IX) process. In the IX process, ions of a given species are displaced from an insoluble exchange material-usually an organic resin. IX resins consist of an organic or network structure with attached functional groups. The exchange capacity is determined by the number of functional groups per unit mass of resin. Once the exchange capacity of the resin is depleted, the resin will begin to cease exchanging dissolved ions and must be regenerated prior to reuse.
Four major classes of resins in water treatment exist: strong acid cation (SAC), weak acid cation (WAC), strong base anion (SBA) and weak base anion (WBA). The type of resin selected is based on source water chemistry and constituents that must be removed. SAC resins are the most common resins used for softening and demineralization applications. SAC resin removes the non-carbonate hardness, and WAC resin removes hardness associated with alkalinity. Hence, weak acid resins are more suitable for high alkalinity water and for when the hardness-to-alkalinity ratio in the feedwater is greater than 1. SBA resins are used for dealkalization, de-silicization and organic trap applications, while WBA resin removes only the anions of the strong mineral acids such as sulfate, chloride and nitrate. Carbonate/bicarbonate and silica ions will pass through a WBA resin.
Typically, the selectivity of SAC is best for calcium and magnesium, and SBA resin is ideal for...