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Figure 25 shows the use of a size factor for resins which is used to develop a pressure factor. This pressure factor can then be used to calculate the pressure drop under conditions of different solution viscosities, flow rates or particle size distributions once the pressure drop is known at one viscosity, flow rate and particle size distribution.

Figure 25. Size factor calculations for use in determining estimations for pressure drops in ion exchange columns.

5.7 Ion Exchange Resin Limitations

When ion exchange resins are used for an extended period of time, the exchange capacities are gradually decreased. Possible causes for these decreases include organic contamination due to the irreversible adsorption of organics dissolved in the feedstream, the oxidative decomposition due to the cleaving ofthe polymer cross-links by their contact with oxidants, the thermal decomposition of functional groups due to the use of the resin at high temperature and the inorganic contaminations due to the adsorption of inorganic ions.

When a resin bed has been contaminated with organic foulants, a procedure is available that can help to restore the resin.f811 Three bed volumes of 10% NaCl - 2% NaOH solution are passed through the resin bed at 50°C. The first bed volume is passed through at the same flow rate, followed by a through rinsing and two regenerations with the standard regenerate.

If the fouling is due to microbial contamination, the same authors recommend backwashing the bed and then filling the entire vessel with a dilute solution (< 0.05%) of organic chlorine. This solution should be circulated through the bed for 8 hours at a warm (50°C) temperature. After this treatment, the resin should be backwashed, regenerated and rinsed before returning it to service. This procedure may cause some oxidation of the polymeric resin, thereby reducing its effective cross-linking and strength. Therefore, treating the resin in this fashion should not be a part of the normal resin maintenance program.

Physical stability of a properly made cation or anion exchange resin is more than adequate for any of the typical conditions of operation. These resins can be made to have a physical crush strength in excess of300 grams per bead.

Perhaps more important are the limitations inherent in the structure of certain polymers or functional groups due to thermal and chemical degradation. Thermally, styrene-based cation exchange resins can maintain their chemical and physical characteristics at temperatures in excess of 125°C. At temperatures higher than this, the rate of degradation increases. Operating temperatures as high as 150°C might be used, depending on the required life for a particular operation to be economically attractive.

Strong base anion exchange resins, on the other hand, are thermally degraded at the amine functional group. Operating in the chloride form, this is not a severe limitation, with temperatures quite similar to those for cation exchange resins being tolerable. However, most strong base anion exchange resins used involve either the hydroxide form, the carbonate or bicarbonate form. In these ionic forms, the amine functionality degrades to form lower amines and alcohols. Operating temperatures in excess of 50°C should be avoided for Type I strong base anion exchange resins in the hydroxide form. Type II strong base resins in the hydroxide form are more susceptible to thermal degradation and temperatures in excess of 35°C should be avoided.

The amine functionality of weak base resins is more stable in the freebase form than that of strong base resins. Styrene-divinylbenzene weak base resins may be used at temperatures up to 100°C with no adverse effects.

Chemical attack most frequently involves degradation due to oxidation. This occurs primarily at the cross-linking sites with cation exchange resins and primarily on the amine sites of the anion exchange resins. From an operating standpoint and, more importantly, from a safety standpoint, severe oxidizing conditions are to be avoided in ion exchange columns.

Oxidizing agents, whether peroxide or chlorine, will degrade ion exchange resins.I82] On cation resins, it is the tertiary hydrogen attached to a carbon involved in a double bond that is most vulnerable to oxidative degradation. In the presence of oxygen, this tertiary hydrogen is transformed first to the hydroperoxide and then to the ketone, resulting in chain scission. The small chains become soluble and are leached from the resin. This chain scission may also be positioned such that the cross-linking of the resin is decreased, as evidenced by the gradual increase in water retention values.

The degradation of anion resins occurs not only by chain scission, but also at the more vulnerable nitrogen on the amine functionality. As an example, the quaternary nitrogen on Type I strong base anion resins is progressively transformed to tertiary, secondary, primary nitrogen and finally to a nonbasic product.

Oxidative studies on resins with different polymer backbones and functionalities have been performed as accelerated tests.[83] The data is shown in Table 17 for polystyrene and polydiallylamine resins. Although the polydiallylamine resins have a higher initial capacity, they are much more susceptible to oxidative degradation. When the polystyrene resin has a mixture of primary and secondary amino groups or when a hydroxy-containing group is attached to the amine of the functional group, the susceptibility to oxidation is enhanced. Thus one can understand the lower thermal limit for Type II anion resins compared to Type I resins.

The effect of thermal cycling on strong base anion resins has been studied by Kysela and Brabec.[84] The average drop per cycle in strong base capacity was 2.1 x 10"4mmol(OH")/mloverthe480cyclesbetween20°Cand 80°C. Figure 26 shows the decrease in total exchange capacity {open circles) and in strong base (salt splitting) capacity (solid circles) for each of the individual resins included in the study.

Table 17. Oxidation of Polystyrene and Polylamine Resins in a One-Week Accelerated Test at 90°C[83]

Initial Base Base Capacity Lost

Table 17. Oxidation of Polystyrene and Polylamine Resins in a One-Week Accelerated Test at 90°C[83]

Initial Base Base Capacity Lost

Resin Backbone

Functional GrouD

Capacitv imeq/al

During „Test (%)

Polystyrene

r-ch2n(ch3)2

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