Degasifier

Introduction

In water demineralisation, a degasifier, or degasser, is often used to remove dissolved carbon dioxide after cation exchange. The most common degassers are of the so-called forced draft or atmospheric type. One is represented below:

A forced draft degasifier

The water "rains" unto a bed of packing material, and the carbon dioxide is forced to escape to the top of the column by an ascending flow of clean filtered air.

Principle and details

After cation exchange, the bicarbonate and carbonate (if any) ions are converted to carbonic acid, or carbon dioxide. CO₂ is soluble in water (see graph below), but it tends to escape into the air, much as it does in a glass of Coca-Cola when you stir it. Using a degasser to remove CO₂ reduces the ionic load on the strong base anion resin, and the consumption of caustic soda is thus lower. To be effective, the degasifier must be placed after the cation exchange column. Before cation exchange, the water is containing bicarbonate. After it, the cations in water (Ca⁺⁺, Mg⁺⁺ and Na⁺ principally) are converted to H⁺ ions, which combine with the HCO₃^— bicarbonate anions to produce carbonic acid (H₂CO₃ = CO₂ + H₂O).
In practice, the water coming out of the cation exchange unit is introduced into a column where it is sprinkled over a bed of filling material, often polypropylene shaped as saddles to leave a maximum volume of voids in the bed. Air is introduced at the bottom of the column by a blower, and escapes at the top, loaded with carbon dioxide from the water.
The solubility of CO₂