What is IX resin regeneration?
Over the course of one or more service cycles, an IX resin will become exhausted, meaning that it can no longer facilitate ion exchange reactions. This happens when contaminant ions have bound to nearly all available active sites on the resin matrix. Put simply, regeneration is a process where anionic or cationic functional groups are restored to the spent resin matrix. This is accomplished through the application of a chemical regenerant solution, though the exact process and regenerants used will depend upon several process factors.
Types of IX resin regeneration processes
IX systems typically take the form of columns that contain one or more varieties of resin. During a service cycle, a stream is directed into the IX column where it reacts with the resin. The regeneration cycle may be one of two types, depending on the path that the regenerant solution takes. These include:
1）Co-flow regeneration (CFR). In CFR, the regenerant solution follows the same path as the solution to be treated, which is usually top to bottom in an IX column. CFR is not typically used when large flows require treatment or higher quality is needed, for strong acid cation (SAC) and strong base anion (SBA) resin beds since excessive quantities of regenerant solution would be required to uniformly regenerate the resin. Without full regeneration, the resin may leak contaminant ions into the treated stream on the next service run.
2）Reverse flow regeneration (RFR). Also known as counterflow regeneration, RFR involves injection of the regenerant solution in the opposite direction of the service flow. This can mean an upflow loading/downflow regeneration or downflow loading/upflow regeneration cycle. In either case, the regenerant solution contacts the less exhausted resin layers first, making the regeneration process more efficient. As a result, RFR requires less regenerant solution and results in less contaminant leakage, though it is important to note that RFR only functions effectively if the resin layers stay in place throughout regeneration. Therefore, RFR should be used only with packed bed IX columns, or if some type of retention device is used to prevent the resin from moving within the column.
Steps involved in IX resin regeneration
The basic steps in a regeneration cycle consist of the following:
Backwash. Backwashing is performed in CFR only, and involves rinsing the resin to remove suspended solids and redistribute compacted resin beads. The agitation of the beads helps remove any fine particles and deposits from the resin surface.
Regenerant injection. The regenerant solution is injected into the IX column at a low flow rate to allow adequate contact time with the resin. The regeneration process is more complex for mixed bed units that house both anion and cation resins. In mixed bed IX polishing, for example, the resins are first separated, then a caustic regenerant is applied, followed by an acid regenerant.
Regenerant displacement. The regenerant is flushed out gradually by the slow introduction of dilution water, typically at the same flow rate as the regenerant solution. For mixed bed units, displacement takes place after the application of each of the regenerant solutions, and the resins are then mixed with compressed air or nitrogen. The flow rate of this “slow rinse” stage must be carefully managed to avoid damage to the resin beads.
Rinse. Lastly, the resin is rinsed with water at the same flow rate as the service cycle. The rinse cycle should continue until a target water quality level is reached.
What materials are used for IX resin regeneration?
Each resin type calls for a narrow set of potential chemical regenerants. Here, we have outlined common regenerant solutions by resin type, and summarized alternatives where applicable.
Strong acid cation (SAC) regenerants
SAC resins can only be regenerated with strong acids. Sodium chloride (NaCl) is the most common regenerant for softening applications, as it is relatively cheap and readily available. Potassium chloride (KCl) a common alternative to NaCl when sodium is undesirable in treated solution, while ammonium chloride (NH4Cl) is often substituted for hot condensate softening applications.
Demineralization is a two-step process, the first of which involves removal of cations using an SAC resin. Hydrochloric acid (HCl) is the most efficient and widely-used regenerant for decationization applications. Sulphuric acid (H2SO4), while a more affordable and less hazardous alternative to HCl, has a lower operating capacity, and can lead to calcium sulphate precipitation if applied in too high a concentration.
Weak acid cation (WAC) regenerants
HCl is the safest, most effective regenerant for dealkalization applications. H2SO4 can be used as an alternative to HCl, though it must be kept in low concentration to avoid calcium sulphate precipitation. Other alternatives include weak acids, like acetic acid (CH3COOH) or citric acid, which are also sometimes used to regenerate WAC resins.
Strong Base Anion (SBA) regenerants
SBA resins can only be regenerated with strong bases. Caustic soda (NaOH) is almost always used as an SBA regenerant for demineralization. Caustic potash can also be used, though it is expensive.
Weak Base Anion (WBA) resins
NaOH almost always used for WBA regeneration, though weaker alkalis can also be used, such as Ammonia (NH3), Sodium carbonate (Na2CO3), or lime suspensions.
Post time: Jun-16-2021