Mixed Bed(DI) Resin Regeneration Process. Deionization (DI) is highly practicable in water treatment technology. Deionization removes dissolved solids in water. The ion exchange technology involves in the DI process. There are many soluble anions and cations in water that need to be removed. An ion exchange system exchanges their H+ and OH– negative and positive ions, and ultra-pure water results.
Why DI water?
Some many technologies and industries use Mixed Bed(DI) Resin Regeneration Process technology. Some are insulating/cooling, pharmaceuticals, electronic industries, power stations, dialysis, wastewater discharge, laboratories, etc. DI water system is a combination of several DI units or one DI vessel and other components to produce purified water. Depending on the requirement, a system can be simple with one unit and complex with several units using RO, NF, or UV filtration. According to the design, DI water purification plants can be duel bed systems or mixed bed systems.
Deionization using an ion exchanging system is the standard method used worldwide. It is an efficient, user-friendly, cost-effective system that produces ultra-pure water.
The common ion exchangers are resins. Experts use different types of resins as per their requirements for water polishing. We can identify ion exchangers into two main sectors; Natural and synthetic. There are inorganic and organic resins in each sector. Some examples of natural inorganic resins are Zeolite and Vermiculite. Usually, synthetic organic resins are the most abundant resin type within the field. They are plastic beads produced with polystyrene crosslink with divinylbenzene.
How are Mixed Bed(DI) Resins working in water polishing?
The plastic beads contain charged anions and cations. The beads with the negative charge are anion resins, and the resins with a positive form are cation resins. Cation resins are present in the form of H+ in deionization, and anion resins are present in the form of OH–. These cations released their cations into the water and captured the cations in the raw water. Anion resins do the same to the negatively charged particles in raw water.
Strong acid cation (SAC) resins and strong base anion (SBA) resins are the prominent resin types used in deionization plants. SAC is created by adding the SO3-H group, and the SBA is with a quaternary amine group. In some cases, the weak base anion is also used with SAC, and car washing spot-free rinsing is a good application. The mixed bed DI plant contains SAC and SBA resin mixer in one single bed.
Why regeneration?
When the resin beads continue their exchanging process, the resin bed gradually saturates with the dissolved contaminants in raw water. The effluent water shows increasing conductivity as a failing DI unit indicator. Therefore, it is essential to refresh the resin bed for efficient work. Releasing bonded contaminants from the resin bed and attaching the relevant resin forms by exchanging methods is regeneration. There are differences in the regeneration of duel bed Di plant and mixed bed DI plant.
How to regenerate the ion exchange resin bed in a duel bed DI plant?
The regeneration of anion and cations in the DI duel bed plant is complicated. The regeneration process includes strong acid and base; therefore, you should follow the appropriate safety conditions in handling chemicals and removing generated waste.
The active ions, Hydrogen in the SAC column, exchange the positive ions such as Mg2+, Ca2+, Na+, and the SBA unit exchanges sulfate, chloride, alkalinity, and silica with their active ions, Hydroxides. Reducing the concentration of available active ions due to ion exchanging and becoming to a lower level is considered as resins are exhausted. When the conductivity setpoint indicates endpoint leakage, regeneration should be carried out.
We can identify a few steps in this process;
- Backwashing
- Introducing regenerants
- Slow rinsing
- Fast rinsing
Backwashing
The key point of backwashing is to loosen the compacted resin bed. The resin bed collects suspended impurities and some broken fine media particles (beads) during its service cycle. In the opposite direction of the service cycle, a calculated flow rate appropriate to the given specifications of resins loosen up the resin bed and expands up to its freeboard. This water flow removes fine particles, reduces the likelihood of channeling, excessive pressure drops, and loses its compaction. Please refer to the consultant specification sheets given for anions and cations due to quite different densities of the two resins.
Introducing regeneration
Strong acid cations need to be regenerated with sulfuric or hydrochloric acid (HCl). Strong base anions are regenerated with Sodium hydroxide or caustic soda. The strength and flow rate of diluted regenerant is essential to prevent resin fouling.
Acid solutions supply H+ ions to the resin bed. The solution substituted H+ ions to the bed and stripped out the positive ions attached to the resin beads. Typically, HCl concentration varies from 4% to 6%. The application of sulfuric acid needs to be calculated depending on the calcium ion concentration of the water to avoid the precipitation of calcium sulfate. The sulfuric addition should be done with supervisory advice. The contact time of acid is around 30 minutes.
Hydroxyl ions in a caustic solution stripped negative ions in water and substituted OH– into the resin beads. 4% concentration is commonly used in anion regeneration, and a contact time of 45 to 60 minutes is preferred. Temperature and the contact time need to be concerned due to silica availability and its amount.
Slow rinsing
Slow rinse needs to be similar to the flow rate of diluted regenerant introduced. The displacement of regenerant from the systems starts with this step.
Fast rinsing
The flow rates should be introduced to the system at a higher rate than slow rinsing to remove excess regenerant from the system. The resin beds are in the activated form and will be ready to start the regular service run.
Regeneration of mixed bed resin
Mixed bed (MB) resin combines SAC and SBA in a 2:3 ratio. The resistivity or electrical conductivity is the only way to measure the mixed bed water because it is deficient in dissolved ions. The main point of success of the MB unit is continually working to neutralize the pH values. MB resins can deliver high-quality water with higher capacities to the given quantity of MB resins than the two-bed resin DI system.
Regeneration is sensitive and complicated compared to duel bed resin regeneration. The steps in mixed bed resin regeneration are;
- Separation of two resin types.
- Settling and draining.
- Regenerant Injection
- Slow rinsing
- Draining
- Mixing
- Water filling
- Final rinsing
Separation
Anion resins and cation resins couldn’t regenerate in the position of a mixed bed. It is essential to separate these two resin types. Backwashing is an excellent method to separate mixed beds into two separated layers. Different densities are the factor in separating them; anions have a lower density, and cations have a high density. Therefore, backwashing the resin bed supports cations to accumulate at the bottom of the vessel and the anions at the top of the cations. Flow rates need to be adjusted to have enough bed expansion. Also, anion resins can be entrapped at the bottom layer; therefore, changing flow rates helps prevent accumulation at the cation resin layer. Backwashing can be conducted within 20-30 minutes, but it can be much longer according to the suspended load.
Settling
After backwashing, the two layers will settle down without no cross mixing of anions and cations. There is a sharp line of separation line after settling occurs.
Regenerate Injection
Regenerate injection can be arranged in “simultaneous injection” and “sequential injection.” Simultaneous injection refers to acid injection and the base at once after separation. When sequential injection starts with the base (Sodium Hydroxide), there should be a buffer flow of water to prevent the contamination of cations with the base solution. Acid injection starts after dosing the base and displacement of the base. Also, a buffer water flow is required to prevent the contamination of the anion layer.
Slow rinsing
Slow rinsing supports removing the regenerate in separated resin layers, and continuous diluted water flow is used.
Draining
To obtain a homogenous resin mixture, the water in the shell needs to drain up to the resin level. But, there should be a 2-inch height of water above the resin layer.
Mixing
Mixing is recommended with oil-free air (be keen with your compressor), and it can be nitrogen or fresh air. Recommended air pressure needs to be maintained, and mixing time should vary from 5 – to 15 minutes.
Water filling
The ion exchange vessel is filled with water inserted from the top of the tank. The filling rate must be followed the service flow rate, and it should be able to reduce the disturbance of the resin bed after mixing.
Final rinse
Fast rinsing or the final rinsing completes the regeneration process, and it should be continued until the required effluent quality is obtained. Usually, successful regeneration of quality resin consumes a ten-bed volume of water for its final rinse.
Conclusion
- Deionized water is used in many industries.
- Producing DI water is a sensitive procedure but a widespread, cost-effective technology with ion exchanging worldwide.
- Resins do heavy-duty in ion exchange, and when they are gradually saturated with contaminants, they should be refreshed into their originated form.
- Resins in DI plants are in the form of H+ and OH–. Regeneration is a must to reactivate the resin beds.
- SAC and SBA resins are regenerated with strong acid and a strong base.
- Duel bed resin regeneration is more manageable than mixed bed resin regeneration.
- The regeneration procedure must be carried out with the guidance of the service provider.
