IX resin regeneration

Need for effective ion exchange resin

In a normal H-OH form operation, the cation resin release H+ ions and anion resin release OH- ions. Eventually, the IX resins become exhausted as their ion exchange sites are saturated. Separation is the most important step as improper separation leads to cross-contamination. The anion resin is subsequently regenerated with NaOH (caustic soda), When cation resin contaminate anion resin layer, the entrapped cation resin in the anion resin will completely saturated with sodium ions. The cation resin is subsequently regenerated with mineral acid such as HCl. When some anion resin contaminate cation resin, the entrapped anion resin will completely saturated with chloride ions if the acid used to regenerate the cation is HCl. This cross-contamination will leads to equilibrium leakage of sodium and chloride ions into the treated water. If the water produced from the mixed beds need to be maintained at ultra-low concentration, it is essential to separate the IX resins completely.

Density Seperation

Common mixed bed separation process are rely on different in density and falling speed of the IX resins. Denser cation resin beads settle more quickly than anion resin due to greater ion exchange capacity. Effective separation will result in anion resins upper layer and cation resin lower layer. Then, top anion layer will transported to anion regeneration vessel while bottom cation layer are transported to cation regeneration vessel. Resins are regenerated separately, returned and air mixed before rinsing.

Step 1:Density Separation and transportation

Step 2: Regeneration

  1. anion resins upper layer is transfer into anion resin regeneration vessel.
  2. cation resin lower layer is transported to cation regeneration vessel.
  3. Anion: Cation = 2:1, as cation minerals in the treated water is more than anions. (Resin amount 60g)
  4. Each bed was fed with 60g resin. Based on DuPont, HCl= 75 - 150 g/L, 5% conc for DM polishing. NaOH = 80 - 160 g/L, 4% conc fo DM polishing.
  5. The flowrate derives from regenerant volume and contact time, contact time > 30 minutes for each regenerant (35mins)
  6. reverse-flow regeneration (counter-flow) with air or water hold-down is applied.
  7. BV: bed volume (1 m3 water per m3 resin or 7.5 gallons per ft3 resin), SAC: strong acidic cation exchange resin, SBA: strong base anion exchange resin
  8. 2 type regeneration on RFR, downflow loading, upflow
    regeneration, vise versa. (Variable)
  9. For RFR, bed depth = 1400 – 3000 mm, a higher bed depth produces a higher operating capacity. The limiting parameter for bed depth and linear flowrate is pressure drop. For design purposes a target pressure drop, ∆P of 100 kPa for anion should be used. During service, max ∆P for cation resins is 300 kPa and 200 kPa for anion resins. Specific flow rate= 6 – 60 BV/h. Linear flowrate for SAC ≥ 25 m/h. Ambient water temperature, Note: operational temperature may impact resin lifetime and/or removal of specific ions.

Demineralizing

Resin used

Strong Acid Cation (Principal resin type Used)

Weak Acid Cation

Strong Base Anion (Principle resin type Used)

Weak Base Anion

References:

Harland, C.E., 1994. Ion exchange: theory and practice (Vol. 6). Royal society of Chemistry.

References

Gopalakrishnan, B., 2011. Separation of resin types in mixed bed ion exchange columns. Oklahoma State University.

Reference

Gopalakrishnan, B., 2011. Separation of resin types in mixed bed ion exchange columns. Oklahoma State University.

Reference

  1. DuPont, 2020. DuPont Ion Exchange Resins: Recommended Operating Conditions for Mixed Bed Ion Exchange Unit. DuPont de Nemours Inc.

Equipment

Three of 12 inches columns, a large feed tank and all required plumbing. Water can be fed from top or bottom of each vessel. Water and resin can be transferred among 3 columns. The service column has 6 sampling valves spaced 6 inches apart that allow both sampling and resin transfer above and below an interface. A 440V pump moves water through the system. Sensors for conductivity and pH are connected to sampling lines that can be removed to multiple locations. The DI water feed tank includes a heater to operate the system at any steady state temperature and a level controller to prevent equipment problems. The cation resin used was DOWEX 650C and the anion resin was DOWEX 550A in a 2:1 volume ratio.

Chonan, K., Koyama, K. and Hagiwara, M., Ebara Corp, 1990. Method of separating and transferring ion-exchange resin. U.S. Patent 4,891,138.

Solutions of 5 vol% of HCl and 0.1 vol% of NaOH each in 100 mL distilled water were prepared. The cation tube was filled with the 5% HCl solution and the anion tube with the 0.1% NaOH solution; each containing 20 g polymeric resin. The process started by pumping the hard water through the column, and water was allowed to pass through the outlet tubes. Once a steady flow is passing through the outlet, the conductivity of the water at the outlet was recorded at time intervals of 10 seconds. The pump turned off when the conductivity values start to increase until reaching a steady state. The experiment was repeated with other concentrations of HCl and NaOH with 0.5 and 0.1 increments, respectively.


For the purpose of studying the effect of amount of water treated, the process was repeated at certain concentrations of HCl and NaOH with the tubes filled with different amounts of hard water. Also, the effect of amount of resin used was studied by pacing different amounts of resin at the different runs for given concentrations of HCl and NaOH.


In the regeneration process, acid or
caustic is fed into the ion exchangers from below using the same pump. The acid and
caustic used is collected in the collecting tank. The flow rate of the pump is adjustable, and can be read from a flow meter before it enters the first ion exchanger. For
continuous evaluation of the process, a conductivity sensor is installed upstream of
the inlet into the collecting tank. The measured values can be read from a conductivity meter. Samples can be taken at all relevant points. Tap water can be used as
raw water.


or continuous evaluation of the process, a conductivity sensor is installed upstream of the inlet into the collecting tank. The measured values can be read from a conductivity meter. Samples can be taken at all relevant points. Tap water can be used as raw water.


as the amount of resin decreases, the conductivity of water increases indicating that fewer ions were removed. So, resin volume should be high.


lowest conductivity is achieved when using 1-vol% NaOH and 5-vol% HCl in the cathodic and anodic resin tubes, respectively.

Procedure

  1. Backwashing a mixed bed results in the separation of the anion resin from the cation resin.
  2. Done for at least 20 – 25 minutes, preferably 40 – 45 minutes to ensure perfect separation of the two resin layers. (Flow rate = variable 1)
  3. Take few minutes for the layers to settle.
  4. Sluicing water is introduced into the vessel through a perforated transferring intermediate sluicing pipe positioned below the interface between the separated layer. Pressurized water was introduced into the vessel from above, while anion layer which is the bigger portion of the IE resins with smaller specific gravity is transfer through a resin transfer pipe positioned above the intermediate sluicing pipe, and then positioned at anion regeneration vessel.
  5. Backwashing water is introduced through a lower water collecting unit and through intermediate sluicing pipe to form backwashing.
  6. After resin settling, sluicing water is introduced in high flow rate through intermediate sluicing pipe so that the cation layer is lifted, while pressurized water is introduced from above to direct the cation IE resins to the opening of resin transfer pipe to be transfer to cation regeneration vessel.

This method make use of repeated backwash and settling steps. Resin separation is directly proportional to the degree of fluidization of the bed. Degree of fluidization can be affected by the velocity of the backwash water, duration of flow and bed expansion during isothermal conditions. During backwashing, the DI water enters from from bottom of the service vessel at a set flow rate thereby fluidizing the resin bed and causing it to expand. Although there is a sharp difference in particle density between anion and cation resins, there still have small quantities of resin intermixing both below and above the interface.

(EAR) Putting capacitor or battery in the fixed bed

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References

  1. DuPont, 2020. DuPont Ion Exchange Resins: Recommended operating conditions for separate beds in water


    treatment. DuPont de Nemours Inc


  2. Al-Asheh, S. and Aidan, A., 2020. A Comprehensive Method of Ion Exchange Resins Regeneration and Its Optimization for Water Treatment. IntechOpen.

Testing Method

Effectiveness or degree of regeneration

Characterization of Resin

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Additional step (Heating temperature)

Heating to 60C