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(a) Cation exchanger

Sodium salts (sodium cycle) or

Acids (hydrogen cycle)

FIGURE 16.1 Unit operations of ion exchange.

Sodium salts (sodium cycle) or

Acids (hydrogen cycle)

Regenerant Backwash waste (Rinsing waste)

Anion exchanger

Regenerant

(b) Anion exchanger

Regenerant

(b) Anion exchanger

FIGURE 16.1 Unit operations of ion exchange.

of ions takes place. This exchange of ions is the chemical reaction of the unit process of ion exchange; the mere physical passing through of the water with the attendant head loss and pumping consideration is the unit operation of ion exchange. The unit operations of head losses are similar to those of granular filtration discussed in Part II. The unit operation of pumping was also discussed in Part II. After ion exchange, product waters are withdrawn at the respective bottoms of the vessels.

16.3 SODIUM, HYDROGEN CYCLE, AND REGENERATION

As shown in Table 16.1, sodium, lithium, and hydrogen are the logical choices for the exchangeable ions. In practice, however, sodium and hydrogen are the ions of choice. The cation exchange resin using sodium may be represented by (R~n)r(Na+)rn. Its exchange reaction with Ca+2 and similar cations is shown below:

As shown, Ca+ has become embedded in the resin, thus removed from solution, and Na+ has become solubilized. Similar reactions may be formulated for the rest of the ions in Table 16.1.

As soon as the resin is exhausted, it may be regenerated. As shown in Equation (16.3), by the Law of Mass Action, the reaction may be driven to the left by increasing the concentration of the sodium ion on the right. In practice, this is what is actually done. The resin is regenerated by using a concentration of NaCl of about 5 to 10%, thus, driving the reaction to the left. Operations where regeneration is done using NaCl is said to run on the sodium cycle. Regeneration may also be made using acids,

NaOH

nh4oh

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