Different configurations. Different applications.
in water. And beyond.
The ion exchange (IX) technology generally removes hardness of any feed water by reducing the total dissolved solids (TDS). But by varying the process designs and using different types of resins, the technology can be used to remove specific types of undesired ionic contaminants for different applications:-
- Softening (removal of hardness).
- Demineralisation (removal of all ions).
- Mixed bed polishing.
- De-alkalisation (removal of bicarbonate).
- Decationisation (removal of all cations).
- Nitrate removal.
- Selective removal of various contaminants.
As the chemical processes of these are rather complex, we mainly address the process designs or flows and do away with the details of chemical reactions that will place in our content.
IX process – service & regeneration.
Over a period of service cycles in any IX system, resins will get exhausted, a stage whereby the resins can no longer effectively facilitate any desired ion exchange reaction. This happens when contaminant ions have bound to nearly all available active sites on the resin matrix. All resins, when exhausted, must go through the regeneration process where anionic or cationic functional groups are restored to the spent resin matrix. The regeneration cycle may be through either a Co-flow Regeneration (CFR) or a Reverse-flow Regeneration (RFR) configuration.
In a CFR process, the regenerant solution is injected as the feed water is to be treated, which is usually from the top to the bottom of an IX column. CFR is not typically used for large IX capacities or when high output water qualities are desired, since excessive quantities of regenerant solution would be required to uniformly regenerate the resins. In RFR configuration (or counter-flow regeneration), on the other hand, the injection of the regenerant solution is in the opposite direction of the service flow. In this case, the regenerant solution contacts the less exhausted resin layers first, making the regeneration process more efficient. But this process is effective only if the resin layers stay in place throughout regeneration. Hence, RFR should be used with packed bed IX columns, or if some retention device is used to prevent the resins from moving within the column. In whichever case, a full operating cycle of any IX process typically involves service, backwash (for CFR only), regeneration and rinse.
Typical operating cycles (with CFR configuration)
During the service cycle,
the ionic contaminants are
absorbed by the resins,
producing the soft water’s
qualities desired. This cycle
continues until the resins in
the column are exhausted,
and regeneration process is
required to restore the
Backwash process is
performed in CFR only, and
involves rinsing the resin to
remove suspended solids
and redistribute compacted
resin beads. The agitation of
the beads helps remove any
fine particles and deposits
from the resin surface.
During the regeneration
process, the regenerant
solution is injected into the
IX column at a low flow rate
to allow adequate contact
time with the resins. In the
case of CFR configuration,
the regenerant is injected in
the same direction of the
The regenerant is flushed
out gradually by the slow
flow of feed water, typically
at the same flow rate as the
regenerant solution. The
flow rate of this “slow rinse”
stage must be carefully
managed to avoid damage
to the resin beads.
In the final step, the resins
are rinsed with water at the
same flow rate as the service
process. The rinse cycle
should continue until a
target water quality level is
reached before the service
cycle commences again.
Regeneration. Materials used.
Each resin type has a rather narrow range of potential chemical regenerants. Below are some common regenerant solutions by resin types and their alternatives:-
SAC resins can be regenerated with strong acids – sodium chloride (NaCl) is the most common regenerant for softening applications. It is relatively cheap and readily available. Potassium chloride (KCl) is a common alternative to NaCl when sodium is undesirable in treated water. Ammonium chloride (NH4Cl) is often used for hot condensate softening applications. Hydrochloric acid (HCl) is the most efficient and widely-used regenerant for decationization applications. Sulphuric acid (H2SO4), while a more affordable and less hazardous alternative to HCl, has a lower operating capacity, and can lead to calcium sulphate precipitation if applied in too high a concentration.
HCl is the safest and most effective regenerant for dealkalization applications. H2SO4 can be used as an alternative to HCl, though it must be kept in low concentration to avoid calcium sulphate precipitation. Other alternatives include weak acids, like acetic acid (CH3COOH) or citric acid, which are also sometimes used to regenerate WAC resins.
SBA resins can be regenerated with strong bases. Caustic soda (NaOH) is almost always used as an SBA regenerant for demineralization. Caustic potash can also be used, though it is a lot more expensive.
NaOH is almost always used for WBA regeneration, though weaker alkalis can also be used, such as Ammonia (NH3), Sodium carbonate (Na2CO3), or lime suspensions.
Water softening. Removing Ca2+ & Mg2+.
Water softening process is the most basic of the IX technology. It removes calcium and magnesium ions most frequently found in natural water. These cations are hardness ions, together with the less common and less soluble strontium and barium cations. When water evaporates, this cations precipitate and form scale in pipes, boilers and even kitchen appliances. SAC resins are used in the sodium form to remove these hardness cations from the water. When exhausted, they can be regenerated with sodium chloride (NaCl). However, softening the water does not reduce its salinity, it merely removes the hardness ions they don’t form scale or deposits. For more stringent industrial use, simple water softening process may not be adequate.
These days, zeolite water softening process is most widely used in domestic and commercial water softening applications. There are natural and synthetic/artificial. The natural zeolite that is used for water softening is gluconites or greensand. Permutit is the synthetic zeolite that is most used in commercial water softening applications. This is used as ion exchanger and odor removal in water softener. Permutit are more porous, glassy, and have higher softening capacity than greensand.
Demineralisation. Removing all ions.
Many industrial applications require all ions in the water to be removed, and this can be attained through a demineralisation (also called deionisation) process. As there are cations and anions in the water, both cation exchanger and an anion exchanger must be applied to produce the pure water desired. The cation resin is used in the hydrogen form (H+) and the anion resin in the hydroxyl form (OH–), and the cation resin can be regenerated with an acid while the anion resin with an alkali. The cation exchanger is usually configured before the anion exchanger, and subject to various factors and consideration, a demineralisation process can be configured in a few ways, from a simple 2-bed demineralising process to a 4-bed combination in sequence. A degasifier may be added in a train of demineralisation systems to remove the carbon dioxide created after cation exchange, if the water contains a significant concentration of bicarbonate.
Demineralised water is generally free of ions, except some residual traces of sodium and silica because the SAC and SBA resins have their lowest selectivity for these. In a simple demineralisation line’s regenerated in reverse flow, the treated water should have conductivity of only about 1.0 μS/cm, with some silica residual (of between 0.005 and 0.05 ppm).
Mixed bed polishing.
Removing salinity & silica.
Where last traces of salinity and silica are to be removed from water, a mixed bed polishing process, where highly regenerated strong acid cation and strong base anion resins are mixed, can be used to produce excellent results. However, these beds are complicated to be regenerated as the resins must first be separated by backwashing before the regeneration cycles. They also require large amounts of chemicals for regeneration. Mixed beds are usually used to to treat pre-demineralised water when the service cycles are long. Mixed bed polishing produces water with less than 0.1 μS/cm conductivity (or of pure water standard of 0.055 μS/cm with more sophisticated designs), and silica residual value lower than 0.01 ppm.
Dealklisation. Removing hardness & alkalinity.
Dealkilisation process uses WAC resins where relatively large concentration of hardness and alkalinity are present in the feed water. After this process, the treated water contains carbon dioxide and this can be removed with a degasifier. Dealkalisation process can produce water with zero temporary hardness. SAC exchanger may be added as the final polishing step to remove the remaining cations in the water for the desired qualities needed for a certain applications.
Removing all cations.
The removal of all cations is not often used except as the first stage of the demineralisation process, or it is sometimes used for condensate polishing where the decationiser is followed with a mixed bed process. A SAC exchanger is used in the H+ form for decationisation process.
Using SAC exchanger.
Nitrate can be removed selectively from water using SBA exchangers in the regeneration cycle with NaCl brine.
Of other contaminants.
Selective removal of metals and other contaminants (e.g. boron, Cd, Cr, Fe, Ni, Pb, Zn etc) is also possible with special resins.