Q11Engineering Chemistry

Question

Explain Zeolite process for water softening with diagram.

Answer 1

The Zeolite process, also known as the Permutit process, is a highly efficient, chemical ion-exchange method utilized industrially and domestically to completely remove both temporary and permanent hardness from water.

Chemical Principle and Mechanism

Zeolites are naturally occurring or synthetically manufactured hydrated sodium aluminosilicates. Their complex chemical formula is often written as $N a_{2} O \cdot A l_{2} O_{3} \cdot x S i O_{2} \cdot y H_{2} O$ . However, for practical chemical equations, the entire complex aluminosilicate anionic framework is simply represented as ' $Z e$ '. Therefore, the active, functional zeolite is represented as $N a_{2} Z e$ .

The defining characteristic of zeolites is their highly porous, rigid, cage-like three-dimensional crystal lattice. The sodium ions ( $N a^{+}$ ) are loosely held within the microscopic pores of this lattice and are highly mobile. When hard water—which is rich in dissolved Calcium ( $C a^{2 +}$ ) and Magnesium ( $M g^{2 +}$ ) ions—is forced to percolate down through a thick cylindrical bed of granular zeolite, a spontaneous and rapid ion-exchange reaction occurs.

Because the zeolite matrix possesses a much higher chemical affinity for divalent cations ( $C a^{2 +}, M g^{2 +}$ ) than for monovalent cations ( $N a^{+}$ ), it actively grabs the hardness-causing ions out of the water and permanently locks them into its crystalline cages. In exchange, it releases two sodium ions into the water for every divalent cation captured.

Removal of Temporary Hardness: $N a_{2} Z e (solid) + C a (H C O_{3})_{2} (aq) \to C a Z e (solid) + 2 N a H C O_{3} (aq)$ $N a_{2} Z e (solid) + M g (H C O_{3})_{2} (aq) \to M g Z e (solid) + 2 N a H C O_{3} (aq)$
Removal of Permanent Hardness: $N a_{2} Z e (solid) + C a S O_{4} (aq) \to C a Z e (solid) + N a_{2} S O_{4} (aq)$ $N a_{2} Z e (solid) + M g C l_{2} (aq) \to M g Z e (solid) + 2 N a C l (aq)$

The effluent water flowing out of the bottom of the tank contains only highly soluble sodium salts ( $N a H C O_{3}, N a_{2} S O_{4}, N a C l$ ). Since sodium salts absolutely do not cause hardness (they do not form scum with soap or scale in boilers), the output water is considered 'soft' (hardness usually drops to an incredible $0 - 15 ppm$ ).

Regeneration of Exhausted Zeolite

After treating a large volume of hard water, all the available sodium ions in the zeolite bed become completely replaced by calcium and magnesium ions ( $C a Z e, M g Z e$ ). At this point, the zeolite bed is 'exhausted' and can no longer soften water. The bed must be regenerated. This is achieved by shutting off the raw water and flushing a highly concentrated $10%$ brine solution (Sodium Chloride, $N a C l$ ) backwards through the bed. The overwhelming concentration of sodium ions forces the equilibrium to reverse. The sodium ions aggressively displace the trapped calcium and magnesium ions, washing them away as waste chlorides and completely restoring the active $N a_{2} Z e$ bed.

Regeneration Reaction: $C a Z e (exhausted) + 2 N a C l (conc. brine) \to N a_{2} Z e (restored) + C a C l_{2} (waste washings to drain)$

Q11Engineering Chemistry

Question

Answer

Chemical Principle and Mechanism

Regeneration of Exhausted Zeolite