User:Milton Beychok/Sandbox

Condensate polishing is a process used to purify the steam condensate produced in high-pressure steam generation facilities such as those in large thermal power plants. Steam condensate is the water formed by condensing the exhaust steam from the steam-driven turbines in thermal power plants and which is recycled for reuse as the major part of the steam generation feedwater.

Condensate polishing is a unique application of ion-exchange resins that removes suspended and dissolved impurities from the condensate. It is essential for the very stringent quality required of high-pressure steam generation feedwater.

Types of condensate polishing

(CC) Image: Milton Beychok
Schematic diagram of a mixed-bed condensate polisher.

How condensate polishing works

(CC) Diagram: Milton Beychok
Diagram depicting how ion exchange resins work.

Ion exchange is a reversible chemical reaction wherein ions (atoms or molecules that have lost or gained an electron and thus acquired an electrical charge) present in a solution are exchanged for a similarly charged ion attached to an immobile solid particle of ion exchange material.

By charging (i.e., treating) ion exchange material with an acid, usually sulfuric acid (H₂SO₄) or hydrochloric acid (HCl), positively charged hydrogen ions (H⁺) are attached to sites on the ion exchange material. Those sites will attract other positively charged ions (called cations) present in a solution brought into contact with the positively charged H⁺ ions.

Similarly, by charging ion exchange material with a caustic such as sodium hydroxide (NaOH), negatively charged hydroxide ions (OH^-) are attached to sites on the ion exchange material. Those sites will attract other negatively charged ions (called anions) present in a solution brought into contact with the negatively charged OH^- ions.

The adjacent diagram schematically depicts an acid treated bead of ion exchange resin with its attached H⁺ ions (referred to as cation resin) and a caustic treated bead of ion exchange resin with its attached OH^- ions (referred to as an "anion resin").

The water first passes through the cation bed. The resins beads have been charged with acid, usually sulfuric or hydrochloric. The resin has a greater attraction for the incoming cations, usually calcium, magnesium, sodium and iron and so retains them, releasing the H+ ions. So the effluent of the cation bed is acidic.

The cation effluent then goes to the anion bed, that has been charged with OH -ions from sodium hydroxide. The anions, Cl-, SO4--, CO3--, etc. are exchanged for the OH- by the resins.

Amazingly enough, the H+ ions from the cation resins and the OH- ions released by the anion resins neutralize each other and form water, so if everything is working correctly, the effluent of the anion bed is at a neutral pH.