Ion Exchange Resin Selection
Ion exchange resins function much differently in EDI devices than in a conventional demineralizer, or even than in a collection/discharge type EDI device. In EDI, the ability of the resin filler to rapidly transport ions to the surface of the ion exchange membranes is much more important than the ion exchange capacity of the resin. The resins are therefore not optimized for capacity, but for other properties that influence transport, such as water retention and selectivity.
Membrane/resin combinations must also be carefully chosen to selectively catalyze the electrochemical splitting of water at various locations within the EDI device, as mentioned previously. Considerable research has gone into optimization of resin fillers for EDI devices, mostly by the manufacturers of the EDI devices rather than the manufacturers of the ion exchange resins.
Ion Exchange Membrane Selection
Ion exchange membranes are different from the many types of filtration membranes in that they are essentially impermeable to water. They combine the ability to act as a separation wall between two solutions (the diluting and concentrating streams) with the chemical and electrochemical properties of ion exchange resin beads. Ion exchange membranes are selectively permeable, as they will allow the passage of counter ions while excluding co-ions. When placed in a water solution and an electric field, a cation membrane will permit the passage of cations only, while an anion membrane will allow the passage of anions only. An in-depth discussion of the theory and properties of permselective membranes is available elsewhere.
There are two main types of commercially available ion exchange membranes, heterogeneous and homogeneous. Homogeneous membranes consist of thin films of continuous ion exchange material, typically on a fabric support. These are essentially equivalent to an ion exchange resin bead, only in the form of a thin sheet. Heterogeneous membranes consist of small ion exchanger particles embedded in an inert binder, with or without any support.
Some of the more important properties of ion exchange membranes used in EDI devices include the following:
- Low water permeability
- Low electrical resistance
- High permselectivity
- High strength
- Resistance to contraction or expansion
- Resistance to high and low pH
Ion exchange membranes that were developed for electrodialysis may not have sufficient mechanical strength and handling properties for use in assembly of EDI devices, so most manufacturers have developed special ion exchange membranes that are optimized for their EDI devices. Extruded heterogeneous membranes based on a polyolefin binder have become very popular for this application. They are relatively low in cost, offer flexibility in formulation, and have been shown to be fouling resistant.
Mixed bed Resin Filler (EDI-MB) - Intermembrane Spacing
The first commercial EDI devices used mixed-bed ion exchange resin as a conductive media in the diluting compartments. For devices using a mixed-bed resin filler, one of the most important design constraints is the distance between the ion exchange membranes. In order for the resin to transport an ion to the membrane, there must be a continuous path of the appropriate type of ion exchange resin, i.e. cation resin for transfer of cations and anion resin for transfer of anions. For simple cubic packing and equal quantities of equal diameter anion and cation beads, the probability of a direct conductive path can be related to the number of resin beads between the membranes by Equation 1.
This shows that the probability of a direct conductive path decreases as the intermembrane spacing increases. The effect of intermembrane spacing on salt removal in a EDI-MB device has also been demonstrated experimentally, as shown in Table 1.
|Cell Thickness, mm
||Salt Removal, %
Relationship between cell thickness and performance for a EDI-MB device
Mixed bed Resin Filler (EDI-MB) - Resin Packing
It has also been shown that the performance of a EDI-MB device can be improved significantly by the use of uniform particle size ion exchange resins instead of conventional resins, which have a Gaussian distribution of bead sizes. The uniform beads allow a higher packing density, approaching a hexagonal close-packed structure. The effect of packing density on salt removal is illustrated by the data in Table 2.
||Product, MegOhm-cm non-uniform beads
||Product, MegOhm-cm uniform beads
Resin particle size distribution and performance for a EDI-MB deviceLayered Bed Resin Filler (EDI-LB)
In the late 1980s and early 1990s there was considerable activity in the development of layered bed (EDI-LB) devices. In this configuration the media comprise separate, sometimes alternating layers (or in one variation, clusters) of ion-exchange resin, each layer containing mainly one type of resin: e.g., either anion or cation resin. Liquid to be deionized flows sequentially through the layers of resins.
For EDI-LB devices there is essentially no "enhanced transfer" regime and less limitation on the intermembrane spacing. This is because transfer of only one type (polarity) ion is enhanced at any given time. In order to maintain electroneutrality, the ion that is transferred out is replaced by a co-ion resulting from splitting of water. This is illustrated in Figure 5. One of the main design constraints is the choice of ion exchange resin, which must catalyze the water splitting reaction at the resin/membrane interface. Resin selection must also ensure that the electrical resistance of the layers is similar, so that the DC current is fairly evenly distributed through the cell instead of preferentially passing through a single type of layer. It is likely that the use of uniform particle size resins will offer some benefit to the performance of thick-cell layered-bed devices, but that the difference will not be as dramatic as it is for a thin-cell mixed-bed.
One of the main advantages to the use of thicker cells is that it greatly reduces the amount of ion exchange membrane used to construct the device, which significantly reduces the assembly cost (both materials and labor). The tradeoff is that the performance for salt removal is lower than for thin cell devices, due to the higher flow per unit membrane area and greater distance that ions need to travel across the cell to reach the ion exchange membrane. The EDI-LB module performance is more sensitive to increases in feed water concentration and to decreases in feed water temperature. However, this is less important now than when EDI was first commercialized, due to improvements in reverse osmosis and gas transfer membranes that have reduced the typical ionic load on the EDI device. The performance of thick-cell EDI devices is sufficient for their use in most ultrapure water applications, given proper system design.
The other significant advantage of thick-cell devices is that the thicker resin chambers are considerably stronger than thin spacers. They also offer more flexibility in the design of the intercompartment sealing, such as the use of grooves and O-ring seals. This allows construction of modules without external leaks and with higher pressure rating. The only commercial EDI devices that are capable of operating continuously at 7 bar (100 psig) are thick-cell type. Even the spiral-wound devices in a pressure vessel are limited to 4 bar (60 psig) or less.
Separate Bed Resin Filler (EDI-SB)
Another electrodeionization device uses completely separate compartments for the cation and anion resins, and is somewhat analogous to a two-bed demineralizer. The cation exchange resin is placed in a compartment between a cation membrane and the anode, with the resin in direct contact with the electrode. The anion exchange resin is between an anion membrane and the cathode. The two ion exchange membranes create a concentrate compartment at the center of the cell. This configuration is shown in Figure 5.
Removal mechanism in thick-cell, separate-bed EDI cell
Instead of splitting water at a resin/membrane or resin/resin interface, this process obtains the hydrogen (H+) or hydroxyl (OH-) ions needed to regenerate the resin from the electrode reactions; hydrogen ions being generated at the anode and hydroxyl ions at the cathode.
Since the resins are in the electrode compartments, the O2, H2, and Cl2 gas that is created remains in the product water, which may require an additional gas removal process step. It is possible that the electrode reaction could produce enough chlorine to reduce the life of the ion exchange resin, depending upon the amount of chloride in the feed water.
It has been shown that the salt removal by EDI-SB device with 10 mm intermembrane spacing, is not nearly as good as for a EDI-MB device with 2.5 mm spacing. But the main disadvantage of the EDI-SB device is that it requires a set of electrodes for each cell. Since the electrodes are by far the most costly component of a EDI device, this approach is only cost effective for low flow rate applications where a single cell is sufficient. There have been some attempts to produce a multi-cell device using bipolar ion exchange membranes, but these have not been commercialized due to the short life of the bipolar membranes.