Technologies > Electrodeionization
It is a development that comes out from the conventional process of ionic exchange technology. The EDI allows the system to demineralize water continuously with 90% or higher recovery.
Like conventional ionic exchange processes, cations and anions in the feeding water are exchanged by hydrogen and hydroxyl ions in the ionic exchange resin, producing demineralized water. The operational difference is that EDI regenerates continuously, while conventional ionic exchange method does it intermittently.
The continuous regeneration in the EDI is electrochemically possible by ion conducting membranes and electrical current application. Hydrogen and hydroxyl ions need to the regeneration are made in-situ, without chemical reagents addition, by the well-known water dissociation reaction.
In order to understand the water ion removal mechanism by an EDI cell, it is necessary to know previously the EDI stack or the cell components.
An EDI cell is made of many beds of mix exchange resin; driving membranes and open channels in between two electrodes.
Two electrodes are at the opposite extremes of the cell, as it is shown in the picture. These electrodes provide the electrical current to the water that flows inside the cells. One electrode is the cathode negatively charged, which constitutes an electron source. The cathode attracts cations (positively charged ions); the other electrode is the anode, which positive charge attracts anions (negatively charge ions). Both, attraction and repulsion of the charge is the answer to the opposite charge.
Some other elements to consider are the membranes separating the chambers. The figure shows the EDI cells emphasizing the membranes. There are two types of membranes: anionic membrane, that allows only negative ion passage and cationic membrane, that allows only the positive ion to pass. Water is not going to pass through these membranes.
The membranes are displaced to be the walls that separate the chambers. There are three types of chambers: dilute chamber (D), concentrate chamber (C) and electrolyte chamber (E).
The water to be treated is fed in the dilute chamber (D). This chamber has the cationic and anionic resins which are placed between the anionic and cationic membranes. The resins allow the ion to adsorb into their respective beds and traveling till the membranes through each resin particle.
After the polluting ions pass through the membranes they will find the C chamber. Ions are removed by a recycle flow from which a part is drained and balanced with the replacement of fresh water.
Chamber D, cationic membrane, chamber C and anionic membrane combination is called the cell. Many cells working in parallel make an EDI cell. Terminal chambers are the electrolyte (E) ones. Both has the electrodes and are feed with water from the concentrate knot. E chamber receives polluting ions from the closest membrane. To the E chamber cathode pass small amounts of hydrogen gas generated by hydrogen ion reduction.
2H+ + 2e- H2
Chamber E anode receives small amounts of oxygen and chlorine gas generated by hydroxyl and chloride ions oxidation.
4OH- 4e- + 2H2O + O2
2Cl- 2e- + Cl2
Chamber E flow is send to discharge to avoid membrane damage because of the chlorine.
Consists of three linked processes:
a) Ionic exchange: water pass through a ionic exchange resin beds.
b) Ions continuous removal: by transport ship through the ionic exchange resin and membranes to the concentrated stream.
c) Continuous regeneration: by hydrogen and hydroxyl ions presence preserve by water dissociation reaction cause by current application.
The water to be treated gets into the EDI cell through D chamber. When it meets the resin spheres ionic exchange ocurrs. Ions are absorbed into the resin, emitting hydrogen and hydroxyl ions.
Ions in resin beds are moved by electric current application in the respective electrode. Sodium ion positively charge (for example) travels through the cationic resin till cationic membrane. Being this permeable to the cations allows sodium ion to pass through and get into the concentrated chamber (C). Sodium ion is going to be trapped in the concentrated chamber moving to the cathode through the water, but it could not pass the anionic membrane. At this point, the recycle flow carry away sodium ions.
At the same time, negative charge of chlorine ion (for example), is going to the opposite direction. Negative ion is going to the anode, traveling through anionic resin, passing anionic membrane to the concentrated chamber at the other side of D chamber. After that is carried out by the recycle flow.
The electric field applied through EDI cell is enough to produce significant amount of water molecules dissociation into hydrogen and hydroxyl ions. The hydrogen ions are adsorbed in the ionic resin, replacing the other ions. Scrolling ions are free to go to the next ionic exchange site. This displaying process by hydrogen and hydroxyl ions represents the regeneration.
COMPARISION BETWEEN EDI AND MIX BEDS CONVENTIONAL DEIONIZATION
The EDI and mix beds demineralizators commonly take the same place in the ultra-pure water attainment process: after a reverse osmosis unit.
Ionic exchange mix beds use resins, that act like a sponge to remove ionizable pollutes. When the resin satieties it regenerates in a batch operation using strong basis and acids. The regeneration produces great volumes of waste and requires constantly an operator to assure a successful result.
On the other hand, with the EDI there is no need of handling and consumption of aggressive chemical products for the regeneration. There is no investment cost on regeneration equipment for chemical products manipulation like special valves, water pipe line, pumps, instruments, storage tanks, interconnection water pipe lines, neutralization tanks and other acquaintance components to continuous chemical products replacement and component maintenance costs.
The EDI only requires a chemical cleaning in case RO is not working properly or it is working outside design restrictions. EDI internal components are designed to support periodical chemical cleaning.
If salt concentration is too low, the EDI requires a sodium chloride injection on the concentrate stream. This is necessary to provide conductive ions that allow a voltage to establish continuous demineralization mechanism.
Principal outlook to remark:
It is continuous. For cleaning, each cell can be removed individually while the other may work with higher flow for a while.
Comparing to batch regeneration mix bed systems the EDI requires minimum intervention.
The EDI has a few automatic valves and simple control sequence, that may require an operator attention.
Instrumentation and control are simple.
EDI product quality reaches the 18Mohm/cm resistivity in design conditions. Feeding changes do not mark any difference in product quality and it does not vary or decline in the course of time.
SiO2 reduction and ionizable TOC in an EDI system is comparable to a dep final polish mix bed results. However, depending on design particular conditions, product quality of mix bed demineralizators is better and on feeding water is less demanding.
Feeding water requirements
Generally mix beds have no restrictions, except those imposed by the service cycle.
EDI´s behavior is restricted by STD, hardness and weakly ionized compounds (CO2 and SiO2).
It does not generate waste because of C chamber discard stream may be used in other process unit, as OI system feedback obtaining 99% recovery. EDI recovery is between 90-95%. It’s unique restriction is the chamber C precipitation.
It requires small spaces because it does not need tanks and auxiliary pumps for the regeneration.
Feeding water specifications