Major categories and structures.
There are four major classes of resins used in industrial
water treatment: strong acid cation, weak acid cation,
strong base anion and weak base anion. Each of these
major resin classes has several physical or chemical
variations within the class. The variations impart
different operating properties to the resin.
A good ion exchange system designer not only will design
the system to meet all design specifications but also will
utilise resins that will allow the system to operate at peak
efficiency and maximum cost effectiveness.
Strong Acid Cation (SAC) Resins
SAC resins are probably the most common resins used today. These resins are highly ionised in both the acid and salt of the sulfonic acid groups (-SO3H). These resins can convert a metal salt to the corresponding acid through the following process. The hydrogen and sodium forms of strong acid resins are highly dissociated and are readily available for exchange over the entire pH range – the exchange capability of SAC resins are independent of a solution’s pH. These resins are used in most softening and full demineralisation applications. This exchange reaction is reversible – the resin can be regenerated with an excess of mineral acid when its capacity is exhausted.
Weak Acid Cation (WAC) Resins
WAC resins can be used in demineralisation and dealkalisation systems. These resins derive their exchange capability from a carboxylic group (-COOH), and are very efficient when matched up with the proper influent water chemistry. While SAC resins remove all cations that are held tighter to the resins than the regenerant ion being used, WAC resins remove only cations associated with alkalinity. In most water treatment applications they are used to remove divalent ions such as calcium associated with carbonate alkalinity. Their main advantage is their high regeneration efficiency compared with SAC – this reduces the amount of acid needed for regeneration and hence minimises the waste acid and disposal hassle.
Strong Base Anion (SBA) Resins
SBA resins are used in demineralisation processes. They also are used in dealkalisation, desilicisation and organic trap applications. They can be used in the entire pH range. These resins are used in the hydroxide (OH) form for water deionisation. There are two types of SBA resins. Type I SBA resins are used where low levels of silica leakage is an important operating criterion or in warmer climates. They operate at improved efficiency when warm caustic (@120º F) is used to regenerate the resin bed. Type II SBA resins have an exchange site that is chemically weaker than Type I resins. Therefore, they must be regenerated at lower temperatures (@95º F.) and normally are not used in prolonged warm climates. Type II SBA resins have the advantage of a higher initial exchange capacity. They can be the resins of choice in applications that do not have heated caustic regenerant or where a low silica level is not a critical operating specification.
Weak Base Anion (WBA) Resins
WBA resins are acid absorbers as much as they are ion exchange resins. They remove only the anions of the strong mineral acids (sulfate, chloride and nitrate), and allow the carbonate/ bicarbonate and silica ions to pass through. Hence, they cannot be used to make demineralised water without a SBA resin bed in the downstream to remove the carbonate/bicarbonate and silica. The advantage of using the WBA resin is its efficiency. Like WAC resins, WBA resins can be regenerated using the spent caustic from the SBA resin bed. Their use is particularly very efficient when used on water having a high percentage of anion loading from sulfate, chloride or nitrate. Many WBA resins can also be used for removing organic substances from the water before they have a chance to reach and foul the SBA resins.
Applications – to each its own.
SAC resins are used widely for water softening applications. They are very effective in the complete removal of hardness ions including magnesium (Mg+), calcium (Ca2+) etc. These resins may also be used for split-stream dealkalisation where two SAC beds operate in parallel, with the first serving as a softener producing an alkaline solution, while the second works as a demineraliser, with the alkalinity eventually removed from the blended stream. Certain SAC resins have also been developed to remove barium and radium etc from water.
WAC resins are used for demineralisation and dealkalisation applications. Their high affinity for cation ions (Mg2+ and CA2+) is ideal for removing hardness ions associated with alkalinity. WAC resins have relatively high oxidation resistance and mechanical durability. They are good for dealing with water containing oxidants such as hydrogen peroxide and chlorine etc.
SBA resins are often used for demineralisation, dealkalisation and desilication, apart from being used t remove total organic carbon (TOC) and other organic matters depending on the type of resins used.
Type I SBA resins are used for selective removal of nitrates (NO3-), sulphates (SO3-), perchlorate (CIO4-), general demineralisation, desalination (where low level of silica is desired).
Type II SBA resins are used for total removal of anions, if needed,
especially if low use of caustic is desired, and if low silica level is
not critical for a particular water application.
WBA resins are used for partial demineralisation. For some larger capacity’s operations, WBA resin beds can be used with SBA systems for complete demineralisation applications. WBA beds, in this instance, can work as an effective total organic carbon (TOC) barrier for the downstream SBA process. WBA resins are also effective for acid adsorption to remove chloride, sulphate, nitrate and other anions associated with strong acid.
Resin structures – common types.
The majority of resins used today have a styrene-divinylbenzene copolymer bead structure. The other important resin bead structure for water treatment resins is the acrylic resin structure. These structures give the different resins different properties for use. The operating properties for acrylic resins are different from those of an equivalent styrene-divinylbenzene resin. Another difference in resin structure is the difference between gel and macroporous types. Gel resins generally can be characterized as having smaller pores (approx. 1 to 2 nm, hydrated) in the resin structure, higher initial exchange capacity and a lower price than macroporous resins of the same type. But macroporous resins are usually considered for their ability to elute foulants more effectively due to the larger pore structure (approx. 20 to 100 nm, hydrated) and can often stand up better in harsher operating environments.
has its challenges.
Resins’ fouling & degradation.
Similar to filtration membrane, resins can become fouled with contaminants or degraded over time. Severely fouled and/or degraded resins will have to be replaced. Without proper designs and operations, resins may be fouled by minerals (iron, manganese, aluminium etc), hardness precipitates (calcium sulphate, barium sulphates etc), silica, oil, microbes, organics etc. Irreversible degradation on resins may occur due to attacks by oxidising agents such as chlorine, oxygen-saturated water, or due to excessive temperature during service or regeneration cycles (especially for acrylic resins and Type II SBA, which have relatively low temperature tolerance). Exhausted resins, however, can be regenerated (“revived”). Learn more →
inspections & cleaning
of resins are the keys.
Resins’ fouling and degradation can be prevented. Samples representative of an entire resin bed should be collected periodically for tests and analyses to determine the best time for the resins to be cleaned. They shall be checked for physical stability (resins’ physical damages may include cracks and breakages of the beads), foulant levels and the ability to perform the required exchange process for a particular application.