Theory of Technology
Basic Principles of Ion Exchange
Ion exchange refers to a process where different ions in solution are exchanged, or replaced, by other ions. An ion exchange media, or resin, is used to accomplish this.
Ion exchange resin is an insoluble, porous, polymer bead. The beads have a very high molecular weight and carry a functional group with either positive (+) or negative (-) charge, known as exchange sites. Negatively charged resin is called cation resin and attracts positive ions, or cations. Positively charged resin is called anion resin and attracts negative ions, or anions. They can be further classified as weak and strong acid cation resins and weak and strong base anion resins. The porosity of the bead allows water to flow through the bead, increasing the amount of contact with the exchange sites.
The strength and characteristics of the exchange sites, along with the characteristics of the ions, determine a resin’s affinity for certain ions. For example, ions with multiple charges, (e.g. Ca++) have a stronger attraction to the resin than ions with single charges. Ions of equal charge are selected by the resin based on molecular weight. Heavier ions are selected first. A resin’s selectivity is also based on an equilibrium principle. Basic water softening theory originated from this principle. In general, calcium has a +2 charge, while sodium has a +1 charge. A cation exchange resin has a higher affinity for calcium over sodium in a weak solution, such as tap water. However, in a concentrated sodium chloride brine solution, the selectivity reverses. Thus, cation exchange resin in a water softener is rinsed with a brine solution to remove the calcium from the resin bed. This is known as regeneration.
The ion exchange process is used to soften water, deionize water, scavenge metals, and recycle waste water (another form of deionization). All of these processes are accomplished utilizing the above methods.
When designing an ion exchange system, the characteristics of the influent stream need to be examined. Based on this data, a determination is made as to the most beneficial type of ion exchange treatment. The following explains these parameters.
The total dissolved solids (TDS) of the inlet water provides the total quantity of contaminants in the water and is reported as parts per million (ppm) or mg/l. The higher level of TDS, the more often the system will regenerate. Levels above 750 mg/l TDS should not be treated with ion exchange. Often times the TDS of the water is equated with the conductivity. Due to different conductive characteristics of different ions, that is not always accurate. For example, a stream with no contaminants other than silica will be very low in conductivity, yet may have very high TDS. While there are charts and equations that correlate TDS with conductance, it is best to initially analyze specifically for TDS and individual ions.
The total suspended solids (TSS) is the quantity of solids in the water that can be removed by filtration. These solids are not dissolved in the water and will not be removed by ion exchange resin. High concentrations of TSS require additional filtration. Levels should not be above 5 mg/l entering the ion exchanger, or the chance for suspended solids fouling greatly increases.
The level of suspended oils and greases coming into an ion exchange bed should not be above 0.1 mg/l. Higher levels will cause a fouling of the ion exchange resin, which would prevent it from operating properly. The inlet temperature of the water should not exceed 100oF. If the temperature does exceed this level, special design considerations are required. A temperature of 140oF can be deionized with special resins; however, expected resin life at this temperature is not more than two years. The level of organics can greatly affect the structure and ability of ion exchange resins. This is also true for free chlorine. When levels of chlorine or organics exceed 1 mg/l, additional treatment through carbon or other absorbent material is recommended. Both the maximum and average values for each of these parameters should be taken into account. While the maximum averages may be well within specifications, peak levels may drastically affect the deionization process.
Prefiltration of the water is almost always a must. Solids, which are in the water, if not removed, will plug any ion exchange column. This is especially true in the packed-bed designs of high purity systems. Adequate filtration for deionization applications is less than 5 micron filtration, usually through replaceable bags or cartridges. Particles smaller than this size are usually passed through the resin bed, causing it no harm.
This first step of the actual deionization process uses cation resins to remove positively charged ions from the water. Using the “opposites attract” rule, the negatively charged cation resin binds and removes positively charged molecules from the water. These constituents include sodium, calcium, magnesium, iron, and other metals. As these positively charged molecules are removed, hydrogen ions are released from the resin. This is the element which is exchanged off the cation resin in the deionization process.
Following the cation exchange process, the negatively charged molecules in the water are removed by the anion exchange process. Typical anion molecules present in water include chlorides, sulfates, nitrates, carbon dioxide, and silica. As the resin removes these molecules, an hydroxide ion (OH) is released. It is important that the cation exchanger be functioning properly and located before the anion. Should multivalent cations come into contact with the anion resin, they will usually precipitate within the resin bed and foul it. This is the most common type of fouling which can occur, and especially common with calcium. The H+ ions from the cation exchanger and the OH– ions from the anion exchanger immediately recombine to form water.
A polishing unit, either a cation or a mixed bed (a combination of both cation and anion resin) can be used following the cation/anion process. Without any polishing, the minimum quality of water that can be produced is between 50,000 ohm/cm (50 K) and 1,000,000 ohm/cm (1 meg). With the polishing unit, the quality can be raised all the way to 18,300,000 ohm/cm. This quality is achieved by the use of a mixed bed polisher. A cation polisher will raise the quality to above 5,000,000 ohm/cm (5 meg). When any of the resins used in the deionization process become exhausted, or have exchanged off all of their respective ions, the resins must be regenerated.
Cation resins use acids to regenerate (25-50% strength). Hydrochloric is the preferred and easiest acid to use. While other acids may be used, special regeneration techniques are required. Anion resins use sodium hydroxide (caustic) for their regeneration. The regeneration process can either be co-current flow or countercurrent. This means in the same direction as the process flow, or in the opposite direction. A countercurrent system will maximize the chemical’s ability to regenerate the resin and minimize the volume of waste.
The solution produced from this regeneration sequence will contain the same constituents as the incoming water, however, they will be concentrated and in a strong acid or caustic background. By combining the regeneration from the cations and anions, a partial neutralization can be made. However, the pH range can vary greatly in this type of mixture. It is usually necessary to send this solution to a neutralization system prior to its being discharged.
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