Ionic Exchange

Why an ion exchanger and common uses in everyday life and in industry?

Very rarely, man meets water in ideal conditions to be used. It usually contains “impurities” of a particular, ionic or biological nature that must be removed for their use due to health, technical and / or economic issues.
Intercambio ionico

In many daily uses we are presented water qualities that do not meet the requirements of the end user. The most common case can be seen with the       so-called softeners, in which, by means of a cation exchanger, the hardness of a water can be reduced, exchanging sodium cations for calcium and magnesium, or cases where a source of drinking water of underground origin is close to a septic tank and can have water nitrates. In the latter case, the use of ion exchange is one of the ways to reduce the concentrations of this compound in water, exchanging it for chlorides.

There are also numerous applications of industrial nature with “ultra pure” water qualities where ion exchangers are used, one of the most common is boiler water, where to avoid corrosion, imbalance in turbines, inlays or deposits that then prevent the heat transfer, it is necessary to reduce to very low values ​​the contents of alkalinity, hardness, silica, etc. They are also less frequent, ion exchangers in units “condensate polishers” also for boiler water, thus reducing the operating costs of electric power plants.

Ion exchange is a reversible process in which ions are exchanged between a solid matrix (resin) and a liquid medium (water or process fluid) without physically altering the first. Although this description refers to process waters, in many other industries (pharmaceutical, mining, food processing, etc.) this process is used as a way to separate other liquids or fluids.  This resin has been, for years, an inert plastic material (with average diameters in the order of 500 microns) in which active groups have been fixed that allow the exchange of cations or anions.

*An ion is an atom or combination of atoms (molecule) that has a positive (cation) or negative (anion) net electric charge.*

In general, this matter can be indicated as follows:

RA+ (s) + B+ (l) RB+ (s) + A+ (l)

Where the exchanger or resin (R-A) carries cations (A +) as interchangeable ions with those of the medium in solution (B +). This equilibrium represented in the above equation is an example of cation exchange. In an analogous way, an anion exchange can be expressed:

R+A (s) + B (l) R+B (s) + A (l)

It is well known that there are ions that are more easily interchangeable than others, that is to say that the exchange resins exhibit a certain selectivity or affinity for various ions.

AblandadoresThe exchange capacity of a resin is a measure of the total number of interchangeable ions it possesses, commonly expressed in milli equivalents / gram in the dry state and for a given monovalent ionic form.

These ions that we mentioned can be classified according to their net electric charge in cations (positive – sodium, potassium, calcium, magnesium, iron, manganese, etc.) and anions (negative – carbonates, bicarbonates, chlorides, sulfates, nitrates, silica, etc.).

The types of commercial exchangers that we can find frequently are the following:

  • Weak cationics.
  • Strong cationics.
  • Weak anionics
  • Strong anionics (Type I).
  • Strong anionics (Type II).

Materialization of an Ion Exchanger

The traditional way of carrying out an ion exchanger, regardless of the type, is by means of a vertical cylindrical column under pressure, where the exchange resin is contained within it and the water to be treated passes through it. Supports or false bottoms together with a system of nozzles keep the resin inside the column.

The operation of the exchangers has a sequence comprising in general the following stages.

 

  1. Production
  2. Regeneration
    • Backwash
    • Regeneration
    • Displacement
  3. Final Rinse

 

  1. Production:

In this phase the actual use of the ion exchanger takes place, obtaining the  water in the quantity and quality established by the installed capacity.

  1. Regeneration
    • Backwash:

Once the productive cycle is finished, the resin is regenerated. For this it is necessary to decomparate the bed first and simultaneously remove the impurities physically retained throughout the cycle, using an ascending stream of water to achieve the expansion of the resin.

  • Regeneration:

During this operation, the reversibility of the ion exchange phenomenon following the principle of mass conservation is achieved by using a regenerating solution, compensating for the greater ionic selectivity of the fixed ions by a considerable increase in the concentration of the regenerating solution.

  • Displacement:

When the regeneration stage is finished, the entire exchanger is full of regenerating solution, which must be evacuated to leave the column in a position to restart production.

  • Final Rinse:

The resin in the presence of high concentrations of regenerants physically absorbs the reagents, so it is necessary to carry out a final rinse at a flow rate greater than that of the regeneration and which is commonly taken as the same as the operation.

At the end of this period, the desired water quality must be obtained practically before affecting the equipment to the productive cycle.

In turn, the operation of these exchangers can be carried out in co-current or counter-current, that is to say, in the first case the direction of the flow in production is the same as during the regeneration (generally downwards).  In this mode of operation, the concentrated regenerant first comes into contact with the upper layer of the resin mantle, which is the one that most exhausts with high affinity ions for the resin. Finally, the ions exchanged in the regeneration of the upper strata of the column have to cross the whole mantle before being evacuated from it. The final result of this operation is that the upper strata are regenerated, but the lower strata can be partially regenerated, shortening the production cycle before the leakage of the ion to be removed.

In the counter-current operation exchangers, the direction of flow in production is opposite to that of regeneration. Both cases can be given for the same production in the descending or ascending direction. Here, in this mode of operation, the incoming concentrated regenerant comes into contact first with the region of the mantle that is less exhausted and only by low affinity ions with the resin. Therefore, the final result is a regeneration of greater efficiency due to the high degree of reconversion of the resin and therefore a better product water quality.

Ion exchange processes in the field of industrial water or human consumption

The following table shows the most common processes and with the greatest amount of facilities for industrial or drinking water and the type of resin used:

ProcessType of resin
SofteningStrong cationic*
 Weak cationic
 
DealcalizationWeak cationic *
 Strong anionic
 
DemineralizationStrong cationic *
 Weak cationic
 Strong anionic *
 Weak anionic
 
Removal of organic matterStrong anionic *
 Weak anionic
 
Nitrate removalStrong anionic

* Main type of resin used.

Each of these processes has its particularities. Some will be treated in more detail because they are more frequent as softeners and demineralizers, however, the general characteristics of each of those mentioned in the table above are the following:

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