Strong acid cation exchange resin (SAC) is the workhorse of water treatment: a sulphonated styrene-divinylbenzene copolymer that exchanges cations across the full pH range. Supplied in the sodium form for softening or in the hydrogen form for demineralisation, it is the resin most plants specify first and the one most often taken for granted. Values below are typical for the type; the exact figures for a grade are confirmed on its technical data sheet.
Typical specification – gel type
| Parameter | Typical value | Note |
|---|---|---|
| Polymer matrix | Styrene-divinylbenzene copolymer, gel structure | Gel resins have no permanent pores; the network opens on swelling |
| Functional group | Sulphonic acid (-SO3H) | Strongly acidic – fully ionised across the whole pH range |
| Ionic form as shipped | Sodium (Na+) or hydrogen (H+) | Determines the regeneration chemical and the treated-water chemistry |
| Total exchange capacity | Of the order of 4.5 mmol/g dry, or 1.8 eq/L wet, in the Na form | Always confirm which basis and which form a figure is quoted on |
| Moisture content | About 45-55% (Na form) | Higher in the H form; relates to crosslink and porosity |
| Crosslink (DVB) | About 7-8% | More crosslink gives more selectivity and less swelling, but slower kinetics |
| Particle size | About 0.315-1.25 mm, 95% min in range | Standard Gaussian distribution |
| Effective size / uniformity coefficient | About 0.40-0.60 mm / 1.6 max | Uniform grades reach a coefficient near 1.1 |
| Wet true density | About 1.24-1.28 g/mL (Na form) | Sets the backwash flow rate limit |
| Wet bulk density | About 0.77-0.87 g/mL | Used to convert resin volume to weight |
| Swelling, Na to H | Of the order of 8-10% | Allow for it in vessel design and in the first conversion |
| Operating pH | 0 – 14 | A strong acid resin is fully functional even at low pH |
| Maximum operating temperature | About 120 deg C (Na form), 100 deg C (H form) | Confirm by form – the limits are not the same |
Macroporous variant
| Parameter | Gel type | Macroporous type |
|---|---|---|
| Capacity | Higher, of the order of 1.8 eq/L wet | Lower, of the order of 1.6 eq/L wet |
| Kinetics | Slower – diffusion through the gel network | Faster – the permanent pore structure shortens the diffusion path |
| Organic fouling | Sensitive; once fouled, hard to clean | Much more tolerant, and cleanable with caustic brine |
| Osmotic shock | Sensitive to rapid ionic strength changes | More resistant, which matters in cycling service |
| Oxidation resistance | Moderate | Better, though free chlorine still attacks the polymer |
| Where it is chosen | Clean feed water, softening, demineralisation | Surface water with organics, high-chlorine or cycling duty |
What it is used for
| Application | Form used | Function |
|---|---|---|
| Water softening | Na form, regenerated with NaCl brine | Exchanges calcium and magnesium for sodium; alkalinity and anions pass through |
| Demineralisation (first stage) | H form, regenerated with HCl or H2SO4 | Converts salts to their acids, which the anion resin then removes |
| Dealkalisation in combination | H form ahead of a degasser | Converts bicarbonate to CO2 for stripping; often paired with a weak acid resin |
| Condensate polishing | H form, usually uniform particle size | Removes corrosion products and ionic impurities at high flow |
| Industrial process water | Either form | Hardness control for boilers, cooling and rinsing |
| Food and potable treatment | Approved grades, Na or H form | Softening where the grade must meet the relevant approval |
Regeneration
| Regenerant | Used when | Practical note |
|---|---|---|
| Sodium chloride brine | Softening service | Brine concentration and contact time matter more than the total salt; a two-stage or counter-current arrangement cuts consumption |
| Hydrochloric acid | Demineralisation (H form) | Efficient and easy to handle; needs acid-resistant materials throughout |
| Sulphuric acid | Demineralisation (H form) | Cheaper per equivalent, but calcium sulphate can precipitate – use staged dilution and adequate flow |
Packaging and handling
Supplied as moist beads in 25 litre bags, fibre drums or bulk bags, with the moisture content part of the specification. Resin must not be allowed to dry out in storage – dried beads crack on re-wetting. Keep out of direct sunlight and frost, and do not stack heavy items on bags. Each delivery should carry a technical data sheet and a certificate of analysis covering capacity, moisture, particle size and density.
For the selection framework that puts this resin alongside the weak acid, strong base and weak base types, see the ion exchange resin selection guide; for the softening application in detail, see ion exchange resin for water softening.
