DEMINERALIZATION

DEMINERALIZATION:

Demineralization: removing ions
ION: Electrically charged atom;

can be positive (Cation) like:
Calcium (Ca++)
Magnesium (Mg++)
Iron (Fe+++)
Manganese (Mn++)
Sodium (Na+)
Hydrogen (H+)

Can be negative (Anion) like:
Chlorides (Cl-)
Sulfates (SO4=)
Nitrates (NO3=)
Carbonates (CO3=)
Silica (SiO2-)
Hydroxyl (OH-)

Demineralized Water is the only water source MUST be used to generate steam or be addd as an ingredient to the processes, because it has the least possible amount of impurities and, therefore, its electrical conductivity (The best indicator for impurities) is less than about 0.2 micro siemens/cm.

Demineralization is done by several methods. Ion Exchange process is one of them that uses resins to remove non-desirable ions which may cause corrosion, deposition on boiler tubes, boiler malfunction, etc.

Resins are organic polymeric beads and able to exchange ions with the
cations and anions:

Cations are removed by cation resins . Hence, cation resines have negatively charged functional group.

Anions are removed by anion resins. Hence, anion resins have positively functional group.

There are two types of cation resins:
1-WAC WEAK ACID CATION 
2-SAC STRONG ACID CATION

WAC is mainly used to decrease pH (Dealkalization), but SAC is the right choice for water demineralization.

There are two types of anion resins:
1-WBA WEAK BASE ANION
2-SBA STRONG BASE ANION

Both types are used for demineralization;

WBA can not remove Silica and Carbonates, only neutralize weak acids and produces water with pH less than 7, if used in a dual separate bed system.

SBA removes all anions and produces water with pH higher than 7, if used in a dual separate bed system.

SAC and SBA are mixed to make a mixed-bed for Ion Exchange process.

Different ions are attracted by different strengths. For example, Calcium with 2+ charges is attracted more strongly than Hydrogen ion with only one positive charge. Both Hydrogen and Hydroxyl ions are not attracted strongly and, therefore, can be exchanged easily with other stronger ions. This is the exact definition of Ion Exchange. As a result of these exchanges, Hydrogen and Hydroxyl ions are detached from the resins and combine to produce water.

But, resins have limited capacity . All their actives sites get occupied by stronger ions than Hydrogen and Hydroxyl ions. So they get saturated sooner or later.

Cation resines that already attracted cations MUST be regenerated by an Acid such as HCL or H2SO4. For example, Hydrogen ion in HCL is exchanged with Ca++ that was already attracted by the resines.

On the other side, Anion resins MUST be regenerated by a base like caustic( NaOH) to release negatively charged ions and reattract Hydroxyl ion.

After regeneration, they are ready to exchange Hydrogen and Hydroxyl ions with stronger ions again.

A question came to me:

Why resines are mixed during regeneration by air blower?

Ok. Let’s assume resines are not mixed in one vessel and there are two separate vessels, one of them is filled with cation resines and the other filled with anion resins. 

Suppose dirty water flows through cation bed first and exchange its own cations with Hydrogen ion. For example, CaCl2 can exchange its Ca++ with H+. Therefore, Ca++ is attached on the resin, but the released H+ combine with Cl- to produce HCL. So, effluent is a water stream with less than 7 pH. 

Then effluent passes through anion bed to exchange negatively charged ions with anion resins. Hydroxyl ions are released. 

The gnerated HCL is exchanged Cl- with OH-. H+ is added to OH- to produce water. 

You may say: 

Ok, there is no issue.

But what if the dirty water had some Sodium ion (Na+)?

If you take a look at positive ions in previous post, you see both H+ and Na+ have only one positive chrge. It means, cation resins can’t exchange Sodium ion with Hydrogen well. So, the effluent of cation bed contains some Sodium that leakes to anion bed. It is called ‘Sodium Leakage’. 

This Sodium combines with released OH- in next vessl to produce NaOH which increases electrical conductivity. Therefore, outlet stream of anion bed has high conductivity. 

Sodium leakage has direct effect on Silica leakage. In anion bed, silica as an negative ion is attracted on anion resins. The produced caustic due to sodium leakage, regenerate attracted silica. Silica is released and final effluent silica analyzer shows Silica leakage and operating time is decreased significantly.

Anion resins are much expensive and more susceptible to fouling by cations. Cation bed should be installed first to exchange cations and act as a filter to remove suspended impurities for anion bed. Therefore, there is no advantage to install an anion bed before a cation one.

All in all, resins should be mixed, because having them in very close proximity makes a stronger driving force for released Hydrogen ion to attract released Hydroxyl ion and produce water.

Order of selectivity