We have discussed about two desalination methods : Distillation and Reverse Osmosis (RO) in the previous article. Now we will discuss the third desalination method, namely Ion Exchange Demineralization or Demin Plant.
What is a Demin Plant? As we know TDS is the amount of dissolved salt in water. The dissolved salts release positive ions (cations) and negative ions (anions) into the water, including the following:
- Cations, for example: Ca2+, Mg2+, Na+, dan K+.
- Anions, for example: SO42-, CO32-, Cl–, HCO3–, NO3–
Demin Plant is a process of reducing the TDS content by exchanging cations in water with H+ ions and anions with OH- ions present in ion exchange resins.
Ion exchange resins are insoluble resins/polymers, in the form of small grains (0.25-1.43mm radius), porous both on the surface and inside, so that the surface area is very large for the ion exchange process to occur. Some resin matrices are gel (transparent) and some are macroporous (opaque). Each ion exchange resin has a fixed ion structure and a counter ion to keep it neutral. Counter ions are mobile so they can go in and out of the resin granules. Now, it is these counter ions that will exchange with ions in TDS water, of course they must be with the same charge ions, cations with cations and anions with anions, so that the charge in the resin remains neutral. So ion exchange resins are definitely cation exchange resins or anion exchange resins, they cannot exchange anions and cations at the same time in one resin.
The following are resin types based on the type of ion exchanged:
- Strong Acid Cation (SAC) Resin àall cations: Ca2+, Mg2+, Na+, K+,etc.
- Weak Acid Cation (WAC) Resin à cation hardness associated with alkalinity
- Strong Base Anion (SBA) Resin àall anions: SO42-, CO32-, Cl–, SiO2, etc.
- Weak Base Anion (WBA) Resin àfree mineral acids: HCl, H2SO4, HNO3
The selection of equipment in the Demin plant depends on the desired TDS input and TDS output. The simplest form consists of SAC Exchanger – SBA Exchanger. Several options that can be done to achieve the desired TDS output are as follows:
- Add a degassifier between SAC and SBA, if the HCO3 content is > 1 meq/l and the flowrate capacity is large, so that the SBA load is lighter
- Add WAC before SAC if the weak cation content is significant enough
- Add WBA before SBA if the weak anion content is significant enough
- Add Mixed Bed Exchanger (SAC and SBA resins are combined in one vessel) if the desired TDS output quality is less than 5 mg/l.
So to design a good demin plant, everything starts with a complete feed water analysis data, flowrate, and the desired running / cycle time. Complete water analysis data is required to calculate the number of cations and anions to be exchanged, the cation and anion load, resin volume, and so on.
- Cation load = Flowrate (l/hour) x cycle time (hours) x number of cations (meq/l)
- Anion load = Flowrate (l/hour) x cycle time (hours) x number of anions (meq/l)
- Operating exchange capacity resin is usually 60-90% of the total exchange capacity resin
- Cation resin volume = Cation load / Operating exchange capacity of cation resin
- Anion resin volume = Anion load / Operating exchange capacity of anion resin
- Hydraulic loading (BV/hour) = Flowrate (m3/hour) / Resin volume (m3) typical 8-40 BV/hour
Once the resins are saturated, they must be regenerated to restore its ion-exchangeability. The cation resin is regenerated using HCl or H2SO4, while the anion resin is regenerated using NaOH. Based on the direction of regeneration flow there are 2 kinds of systems used: co-flow and counter flow. Co-flow means the regeneration flow is in the same direction as the operating flow, while counter flow means the regeneration flow is in the opposite direction to the operating flow.
A good demin plant design does not only take into account low manufacturing costs, but also considers optimal operational costs such as consumption of chemicals, power, labor and replacement of consumables and spare parts. For more details about the Demin Plant, please contact and discuss further with our engineering team.
Well, that’s all our article this time, we hope you find it useful. See you in the next article.