Ion-exchange resin

Ion-exchange resin (or ion-exchange polymer).
(>link to French Hydro-Land site for more specifies)

Generalities.
An ion-exchange resin or ion-exchange polymer is an insoluble matrix (or support structure) normally in the form of small (0.5-1 mm diameter) beads, usually white or yellowish, fabricated from an organic polymer substrate. The beads are typically porous, providing a high surface area. The trapping of ions occurs with concomitant releasing of other ions; thus the process is called ion-exchange. There are multiple types of ion-exchange resin. The most commercial resins are made of polystyrene sulfonate.
Ion-exchange resins are widely used in different separation, purification, and decontamination processes. The most common examples are water softening and water purification. In many cases ion-exchange resins were introduced in such processes as a more flexible alternative to the use of natural or artificial zeolites. Also, ion exchange resins are highly effective in the biodiesel filtration process.

Types of resins.
Most typical ion-exchange resins are based on crosslinked polystyrene. The actual ion exchanging sites are introduced after polymerization. Additionally, in the case of polystyrene, crosslinking is introduced via copolymerization of styrene and a few percent of divinylbenzene (non-crosslinked polymers are soluble in water). Crosslinking decreases ion-exchange capacity of the resin and prolongs the time needed to accomplish the ion exchange processes but improves the robustness of the resin. Particle size also influences the resin parameters; smaller particles have larger outer surface, but cause larger head loss in the column processes.
Besides being made as bead-shaped materials, ion exchange resins are produced as membranes. The membranes, which are made of highly cross-linked ion exchange resins that allow passage of ions, but not of water, are used for electrodialysis.
Four main types of ion exchange resins differ in their functional groups:

• strongly acidic, typically featuring sulfonic acid groups, e.g. sodium polystyrene sulfonate or polyAMPS.
• strongly basic, typically featuring quaternary amino groups, for example, trimethylammonium groups, e.g. polyAPTAC)
• weakly acidic, typically featuring carboxylic acid groups.
• weakly basic, typically featuring primary, secondary, and/or ternary amino groups, e.g. polyethylene amine.

Specialised ion exchange resins are also known such as chelating resins (iminodiacetic acid, thiourea-based resins, and many others).

Properties.
The general properties of resins are:

• the degree of crosslinking,
  > for example : Divinylbenzene (DVB) in the polystyrene sulfonate resin, 4 to 16 %.
• the porosity of the skeleton (organic portion of the resin),
  > existence of channels, pores of defined size variable according to their type.
• stability and longevity,
  > Quality of the skeleton to remain stable long term, keeping its exchange capacity (degradation due to heat, light, etc.).
• the density,
• particle size,
  > generally 0.3 to 1 mm (average diameter: 0.6/07).
• moisture retention,
  > affects the resin swelling when immersed.
  selectivity,
  > distinguishes resins according to their propensity to fix certain ions
• Exchange Capacity (EC)
  >the amount of ions that can fix a mass or a given volume (often expressed as equivalents per liter of resin).
• the pressure drop (the loss of energy in m water / m of resin, by passing through the bed)
• expansion (by lifting the bed of resin, its% expansion is a function of the flow rate velocity)

Water softening.
Reaction mechanism of ions fixation on cationic resin (softening resins):
The resins are highly ionized in salt (R-SO3Na) form.
When in contact with a solution containing magnesium and calcium ions (but a low concentration of sodium ions), the magnesium and calcium ions preferentially migrate out of solution to the active sites on the resin, being replaced in solution by sodium ions. This process reaches equilibrium with a much lower concentration of magnesium and calcium ions in solution than was started with.
Idealized image of water softening process, involving replacement of calcium ions in water with sodium ions donated by a cation exchange resin.
The resin can be recharged by washing it with a solution containing a high concentration of sodium ions (e.g. it has large amounts of common salt (NaCl) dissolved in it). The calcium and magnesium ions migrate off the resin, being replaced by sodium ions from the solution until a new equilibrium is reached. The salt is used to recharge an ion-exchange resin which itself is used to soften the water.

Cationic resin saturated sodium: [R - Na +]

• Main cations present in the water: Ca²⁺, Mg²⁺, Na⁺, etc.. (total B⁺)
• Major anions present in water: HCO₃^-, Cl^-, SO₄^2-, NO₃^-(total B^-)

exchange reaction:

[R^-- Na⁺] ...... + ...... [B⁺- B^-] >>>>>> [R^-- B⁺] ...... + ...... Na⁺- B^-

resin...........................water................ .fixed ions..............released ions

Total capacity.
This is the maximum number of ions that the resin may determine. It depends on the type of resin. In general the total capacity of cationic to anionic exceed.
It is expressed as the case in:

• equivalents per liter (resin), eq /L,
• milliequivalents per liter (meq / L),
• equivalents per cubic meter (eq/m3)
• French degree per liter or m3 (°F /L or ° F/m3)
• gram (an ion) per liter or m3.

Available basis capacity.
This is the number of ions that can attach resin, depending on the type of resin and the rate of regeneration.
Specific capacity.
This is the amount of a resin can fix that, depending on the type of resin, and level regeneration, and flow rate direction during regeneration step.
Example: apacity of hardness (Ca + Mg) according to cocurrent regeneration (NaCl regenerant).

Standardization.
Standard products used for the production of drinking water : French bulletin officiel - Liste B,
-- Circulars 23 and 24 July 1985 and Decree of 29 May 1997
- Circular of 27 May 1987 and Decree of 29 May 1997.

Uses.
This technology is used in paints, adhesives, cleaning products, computers, construction materials.
Resins are used for:

• the treatment of industrial and domestic water
• effluent treatment
• production of ultra pure water in the electronics industry
• food and pharmaceutical applications
• catalyst in various industrial processes

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