Meeting the industrial requirements for ion exchanger methodology: Ca2+, Mg2+, Na+,Fe2+ , Mn2+

It must discharge the least possible polymerisation residue, measured through the TOC, especially in the case of food applications (resins approved by the health authorities), nuclear power stations and in ultra pure water applications;bead dimension and granulometry range must be calculated in such a way as to limit load losses from the exchanger columns.

The ion exchangers featured in the techniques described below use 0.3 to 1.2 mm diameter beads. Back flow applications, especially those involving blocked beds, often use single calibration resins (e.g. 0.65 mm).

For some special uses, resins will be crushed down to between 5 and 30 mm and delivered regenerated (condensate treatment). These micro-resins cannot be regenerated:

the beads are mechanically and osmotically stable (minimum of broken or cracked beads).

During the exchanges, the exchanger may be required to fix ions or ionised complexes of varying sizes and weights.

In some cases, this will induce a contraction or an expansion that cannot be ignored (up to 100% for some carboxylic resins (HCO2-R) between phase H and phase NH4). This expansion and contraction, quite clearly, must not cause any of the beads to burst. Furthermore, in the most difficult cases, equipment dimensioning must allow for the need to permit this expansion to take place without generating excessively high compression strain within the layer.

The use of ion exchangers will be subject to a certain number of conditions:

ion exchangers have been designed to fix ions and not to filter out suspended solids, colloids or oily emulsions. When these latter substances are present, they can only shorten the life of an ion exchanger;removal of soluble organic matter is a complex task and requires a special study;the presence of large quantities of dissolved gas in water can cause major disturbance to exchanger activity;powerful oxidants Cℓ2, O3, CℓO4 (perchlorate) damage resins;finally, the industrial exploitation of laboratory results or documents provided by ion exchanger manufacturers calls for proven experience.

Equipment design and operating rules are just as important as a knowledge of the theoretical performance of resins:

cation exchangers

Cation exchangers can be divided into two groups :

highly acid cation exchangers;slightly acid cation exchangers.

highly acid exchangers:

One of the characteristics of these exchangers is the presence of sulphonic radicals HSO3– that have an acidity similar to that of sulphuric acid. At present, these are sulphonated polystyrenes obtained from :

styrene and divinylbenzene copolymerisation carried out in the form of an emulsion in order to obtain perfect spheres on solidification;sulphonation of the beads thus obtained.

The products created by this preparation are virtually single function products. Their physical and chemical properties vary according to the percentage of divinylbenzene to styrene, called the reticulation or crossing rate and which will usually vary from 6 to 16%.

For conventional fixed bed applications, the cationic exchange reticulation rate is approximately 8%.

In the case of high velocity (continuous or intermittent), short cycle treatments or the treatment of water containing traces of oxidants, resins with higher reticulation levels are used for both gel and macroporous structures.

Low acid excahngers:

These exchangers are polyacrylic resins characterised by the presence of carboxylicradicals HCO2, that can be likened to certain organic acids such as formic acid or acetic acid. They differ from high acid exchangers on two counts :

they only fix cations Ca2+, Mg2+, Na+,Fe2+ , Mn2+ … bonded to bicarbonates but cannot exchange cations that are in equilibrium with strong anions (SO42-, Cℓ–, NO3–);they regenerate easily (reaction of a strong acid on weak acid salts); the reaction is total, with regeneration levels that are close to the stoichiometry.