Mixed resin demineralization

Figure 2. Shows mixed resin demineralization scheme.

Non-reactive ion-exchange resin. Commonly employed at a specific intermediate density to separate cation and anion resins in a mixed-bed demineralization plant, in order to limit contaminant leakage, especially from regenerants. 

Styrene-DVB resins (cation and anion) First ion-specific resin synthesized Early industrial 2-stage demineralizing First commercial mixed bed demineralizing... 

MIX BED DEMINERALIZER - Having a mixture of cation and anion exchange resin in the same housing. 

In mixed-bed units, both the cation and the anion resins are mixed together thoroughly in the same vessel by compressed air. The cation and the anion resins being next to each other constitute an infinite number of cation and anion exchangers. The effluent quality obtainable from a well-designed and operated mixed-bed exchanger will readily produce demineralized water of conductivity less than 0.5 mmho and silica less than 10 ppb. 

Demineralized water is water from which all the minerals have been removed by ion-eNchange. It is used where pure water is needed for a process and as boiler feed water. Mixed and multiple-bed ion-exehange units are used for this purpose, one resin converting the cations to hydrogen and the other removing the acid radicals. Water can be produced that has less than I ppm of dissolved solids. 

Dowex ion exchange resins include a range of anion and cation resins for multibed demineralization, mixed-bed condensate polishing, as well as nuclear and other specialty applications. Most Dowex resins are based on styrene copolymerized with divinylbenzene (DVB). According to Dow, styrene/DVB structures are the preferred matrices for ion exchange resins because... 

However, for many potential uses such as boiler feed water it is too contamined, so that a polishing unit is also required. In the manufacture of ammonium nitrate or sulfate, a mixed-bed resin system can be used to polish the partially demineralized condensate. However, poUshing of... 

In this application a mixed bed is widely used to remove or polish the ion residuals remaining in demineralized water after prior stage-wise coflow or counterflow demineralization. The anion cation resin volume ratio often differs from 1 1 for polishing mixed beds depending upon the anticipated loading of cations (sodium) and anions ( silica ). Since no hardness cations are present, only sodium, precipitation reactions are absent and a well designed unit with clean resins... 

The two basic deionizer configurations are the two-bed and mixed-bed. Two-bed deionizers have separate vessels for the cation and anion resins. In mixed-bed deionizers, the anion and cation resins are blended in a single vessel. Generally, mixed-bed systems produce higher-quahty water but have a lower total capacity compared with two-bed systems. Figures 3 and 4 are schematics of typical ion exchange demineralization processes. 

Fig. 9.9-1 Mixed bed ion exchanger (a) Backwash (b) regeneration of anion exchange resin (c) regeneration of cation exchange resin (d) mixing of resins (next step demineralization of raw water) (Perry 1984,19 3)...

For purer water quality, the water may be totally demineralized by mixed beds of polymeric resins which exchange in turn hydrogen ions for all cations and hydroxyl ions for all anions, effectively producing pure HjO from a raw water stream. Such highly purified waters are required for boilers operating from about 6 MPa and for all nuclear boilers (to avoid radioactive half-life of water-borne salts). 

Strong-acid cation resins. Strongly acidic cation resins derive their functionality from the sulfonic acid groups. These strong-acid exchangers operate at any pH, split all salts, and require substantial amounts of regenerant. This is the resin of choice for almost all softening applications and as the first unit in a two-bed demineralizer or the cation component of a mixed bed. 

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