Mixed Beds

Ion exchange resins are used on the industry for the replacement of dissolved ions for those available on the resins surface. They are typically used for softening and water demineralization, and on a variety of different processes such as chemical purification, syrup demineralization, etc.

When an anionic resin is regenerated with a basic solution and a cationic resin is regenerated with an acid one, they can exchange all minerals dissolved on water by putting them together in contact with that fluid. The resin's matrix is formed by an insoluble polymer and all the reactive groups are attached to either a proton (on the cationic resin) or a hydroxide (on the anionic resin). When resins come in contact with water, ion exchange reactions take place reversibly.

Advantages of water treatment with Ion Exchange

• Resins have now higher operating capacity, allowing compact processes with moderate investment.
• High chemical stability, long service life and easy regeneration.
• Automated or manual stations can be installed, depending on the requirements.

It is important to keep in mind that effluents from regenerations are usually corrosive and after mixing, a neutralization might be needed before dumping.

Nowadays most resins are synthetic, based on a co-polymer: Styrene-DVB, especially treated to have functional groups attached to it. When sulphonated, cationic resins are obtained and when aminated, anionic ones.
There are four kinds of resins: Strong Acid Cation (SAC), Weak Acid Cation (WAC), Strong Basic Anion (SBA) and Weak Basic Anion (WBA).

The most notorious difference is that strong resins operate under a wider range of pH and are more versatile on ion exchanging.

STRONG ACID CATION (SAC)

These products are capable of retaining all cations from water. Their maximum selectivity is towards trivalent cations, intermediate for divalent cations and minimum for monovalent cations.
The exchange reaction is fast and yields low leakage. These are highly stable resin and can last 20 years or more. When switching from the sodium form to the acid form, the swelling is only about 8%. They can withstand temperatures higher than 100°C.

SRONG BASIC ANION (SBA)

These resins are capable of removing all anions, weak and strong ones, operating on practically any pH.
Its selectivity towards bivalent anions is higher than towards monovalent ones. They are less stable than the Strong Acid Cation resins, last a lot less than them, and withstand lower temperatures (35 to 60°C). The swelling when changing to the hydroxyl form is lower than 12%.

DESMINERALIZATION

There are number of ways that the four kinds of resins can be combined. Some of them require a degasifyer (for CO2 removal) in order to improve its performance.
Ion exchange treatment units should be preceded by the correct pre-treatment to eliminate the suspended organic matter and colloidal matter that can foul the resin surface, reducing the speed of reaction by slowing the diffusion and allowing for preferred paths, reducing efficiency. A similar effect can occur when oils and gases are present on feed water.

According to water composition and the quality requirements, all for kinds of resin can be used, either as single beds or twin beds. When superior water quality is to be achieved, a mixed bed is placed as a final polishing unit, after the cationic and anionic bed. The mixed bed is a single column filled with both cationic and anionic resin. This configuration can produce water with less than 1 microS/cm of conductivity, and silica concentrations of less than 5 ppb.