MIXED BED ION EXCHANGE (IX)

Mixed bed ion exchange is an ion exchange process for polishing of demineralised water, meaning for removal of trace dissolved solids from water.
A mixed bed ion exchanger (also: mixed bed polisher, mixed bed filter) is a vessel filled with a mixture of cation exchange resin and anion exchange resin. During service, water flows through this resin mixture. Cations dissolved in the water are then exchanged for hydrogen ions (H+), while anions dissolved in the water are exchanged for hydroxide ions (OH-). Hydrogen ions and hydroxide ions react to water.
With increasing service life, the ion exchange resins deplete. Depleted mixed bed resins are regenerated, usually with both hydrochloric acid (HCl) and sodium hydroxide (NaOH). With each regeneration, potentially acidic or alkaline waste water is produced, which may need to be neutralised. In case of smaller mixed bed exchangers, the ion exchange resin is sometimes also discarded and exchanged once depleted, rather than regenerated.
Usually, a mixed bed ion exchanger is used either as a last polishing step downstream of another demineralisation process, or as a working mixed bed for demineralisation of already partly demineralised or low-TDS water (e.g. condensate).


FEED WATER QUALITY AND TREATMENT TECHNOLOGY


MIXED BED EXCHANGER

For removing the residual cations and anions and producing demineralized water with maximum conductivity of 0.2µs/cm, after anion exchanger the water enters the mixed bed which contains two types of resins, strong acid cation resins and strong base anion resins.
The mixed bed exchanger is pressurized, cylindrical and vertical vessel with dished plate bottoms .Carbon steel is the main material for mixed bed exchanger.

The mixed bed vessel is designed for 100% resin expansion in backwash mode. The vessel consists of one chamber with perforated flat bottom and injectors for the passage and distribution of the outlet water. Each vessel is equipped with a PDS to check and control the pressure drop of each vessel.
One conductivity meter after each mixed bed controls the quality of outlet water .The regeneration mode is subjected to mixed bed when the outlet conductivity is more than 0.1µS/cm.

Total flow which passes through mixed bed exchanger during service time is measured with flow transmitter and indicator after the mixed bed exchanger. In addition to rising conductivity, by rinsing the flow quantity from specified value, the service line is changed and the regeneration of the mixed bed which has gone out of service will be started. The running time of each mixed bed filter is 96 hours.


When the outlet conductivity is more than 0.2µS/cm, the conductivity meter in the outlet line of mixed bed changes over the operating mixed bed to another stand by mixed bed by closing the inlet and outlet valves, and also opening the inlet and outlet valves of stand by mixed bed.
In the outlet line of each mixed bed a resin catcher is foreseen for catching carried over resins.
Guidelines for Acceptable Resin Feed Water

Max. level Unit Component


Mixed Bed Column Regeneration
Each vessel is consisted of central system of collectors with nozzles for collecting regeneration effluents of anionic and cationic resins. In regeneration sequence the first step is hydraulic separation (backwash). By backwashing, the strong cation resins settle in the lower portion because of their higher density and strong anion resin stay in the upper portion of vessel. After separation of the resins, the regeneration mode can be started.
The cationic resins are regenerated by diluted acid 5% in upward direction and anionic resins are regenerated by diluted caustic soda 4% in downward direction.

mixed bed exchanger system operation and regeneration

After regenerating of both kinds of resins, the acid and caustic soda will be drained from the middle of column into neutralization pit.
After regeneration, for removing residual acid and caustic soda, slow rinse will be started. For this sequence injection of acid and caustic soda will be stopped and demin water will pass through the column upward and downward in the same time and will be drained from the middle of the column. The last step is fast rinse, After the regeneration mode is finished, for mixing the resins, air is injected to the mixed bed and the vessel will get into service.

Design principles of Mixed bed resin system

CLASSIFICATIONS OF ION EXCHANGE RESINS
The development of a sulfonated coal cation exchange medium, referred to as carbonaceous zeolite, extended the application of ion exchange to hydrogen cycle operation, allowing for the reduction of alkalinity as well as hardness. Soon, an anion exchange resin (a condensation product of polyamines and formaldehyde) was developed. The new anion resin was used with the hydrogen cycle cation resin in an attempt to demineralize (remove all dissolved salts from) water. However, early anion exchangers were unstable and could not remove such weakly ionized acids as silicic and carbonic acid.

Ionizable groups attached to the resin bead determine the functional capability of the resin. Industrial water treatment resins are classified into four basic categories:
• Strong Acid Cation (SAC)
• Weak Acid Cation (WAC)
• Strong Base Anion (SBA)
• Weak Base Anion (WBA)
SAC resins can neutralize strong bases and convert neutral salts into their corresponding acids. SBA resins can neutralize strong acids and convert neutral salts into their corresponding bases. These resins are utilized in most softening and full demineralization applications. WAC and WBA resins are able to neutralize strong bases and acids, respectively. These resins are used for dealkalization, partial demineralization, or (in combination with strong resins) full demineralization.
SAC resins derive their functionality from sulfonic acid groups (HSO3¯). When used in demineralization, SAC resins remove nearly all raw water cations, replacing them with hydrogen ions, as shown below:

The exchange reaction is reversible. When its capacity is exhausted, the resin can be regenerated with an excess of mineral acid.
Strong acid cation exchangers function well at all pH ranges. These resins have found a wide range of applications. For example, they are used in the sodium cycle (sodium as the mobile ion) for softening and in the hydrogen cycle for decationization.
Weak acid cation exchange resins derive their exchange activity from a carboxylic group (-COOH). When operated in the hydrogen form, WAC resins remove cations that are associated with alkalinity, producing carbonic acid as shown:

These reactions are also reversible and permit the return of the exhausted WAC resin to the regenerated form. WAC resins are not able to remove all of the cations in most water supplies. Their primary asset is their high regeneration efficiency in comparison with SAC resins. This high efficiency reduces the amount of acid required to regenerate the resin, thereby reducing the waste acid and minimizing disposal problems.
Weak acid cation resins are used primarily for softening and dealkalization of high-hardness, high-alkalinity waters, frequently in conjunction with SAC sodium cycle polishing systems. In full demineralization systems, the use of WAC and SAC resins in combination provides the economy of the more efficient WAC resin along with the full exchange capabilities of the SAC resin.
SBA resins derive their functionality from quaternary ammonium functional groups. Two types of quaternary ammonium groups, referred to as Type I and Type II, are used. Type I sites have three methyl groups:

In a Type II resin one of the methyl groups is replaced with an ethanol group. The Type I resin has a greater stability than the Type II resin and is able to remove more of the weakly ionized acids. Type II resins provide a greater regeneration efficiency and a greater capacity for the same amount of regenerant chemical used.
When in the hydroxide form, SBA resins remove all commonly encountered anions as shown below:

As with the cation resins, these reactions are reversible, allowing for the regeneration of the resin with a strong alkali, such as caustic soda, to return the resin to the hydroxide form.
Weak base resin functionality originates in primary (R-NH2), secondary (R-NHR’), or tertiary (R-NR’2) amine groups. WBA resins readily re-move sulfuric, nitric, and hydrochloric acids, as represented by the following reaction:

Microscopic view of cellular resin beads (20-50 mesh) of a sulfonated styrene-divinylbenzene strong acid cation exhcanger. (Courtesy of Rohm and Haas Company.)


Applications
• Boiler feed water treament: Demineralisation of the →make-up water, e.g. downstream of an →EDI plant or of an →ion exchange demineralisation plant.
• Boiler feed water treament: →Condensate polishing, with an up- and downstream heat transfer system.
• Ultra-pure water treatment: Polishing of demineralised process water or ingredient water