Ion exchange system is a process of liquid separation. Generally this process uses ion exchange resin, adsorbent resin or catalytic resin.
Ion exchange resins are polymeric beads, granulas or powders having a functional acidic group called cation resin and basic group called anion. They work by exchanging ions in a solution with ions fixed inside the porous polymer matrix of the resin. Adsorbent resins are more porous and work by attracting materials onto the large surfaces of the resin. Catalytic resins are polymer beads or powders with acidic or basic groups and provide an effective, environmentally friendly alternative to liquid catalist.
In most cases involving ion exchange system the resin is confined in a column. The system may contain one or more columns filled with one or more types of resin of the same or different function. The particular advantage of such ion exchange liquid separation systems as described above lies in their effectiveness, high efficiency, simplicity and low operating cost compared to other separation processes such as evaporation, reverse osmosis and the like. Successful liquid separation processes using ion exchange system depend in major part, upon the characteristic of the system utilized. Among the desired characteristics are:                High purity of the processed product.                    The system must be able to ensure production of high quality of demineralized or adsorbed product. It should keep the resin bed in a compacted form and reduce the ion leakage.                        Low chemical consumption                    Regeneration of the exhausted resin should be performed efficiently for optimum chemical consumption and should maximize the resin bed operating capacity such as to produce the most highly regenerated resin at the service flow outlet.                        Less waste during regeneration                    Regeneration is to be performed efficiently, less time, less water consumption for backwashing and rinsing, and less chemical waste.                        Reduced pressure drop                    High flow rate increases pressure drop. High pressure drop increases possibility of breakage of the resin beads and causes higher pumping energy.                        Wide range of flow rates                    Since most processes are not constant the system should be operable for processes at wide range of flow rates and is still capable to produce highly purified product.                        Low operating and investment cost                    The system should be easily operable with low operating, investment and maintenance costs.The following basic ion exchange systems are generally available.                        
The first system is a cocurrent system where the direction of flow during service is from top to bottom while the regeneration process is also from top to bottom (DEGREMONT. WATER TREATMENT HANDBOOK, 5th EDITION, JOHN WILEY & SONS 1979 PAGE 319-320). In this system the resin bed is compacted during service and regeneration. During service the liquid is passed through a column which contains resin beads supported by underdrain which normally contains nozzles at the bottom of the tank. There is a free board above the surface of the resin almost as high as the resin bed to allow the resin bed to be effectively backwashed before the regeneration process to clean and remove the small broken particles.
This cocurrent system is widely used since it is simple but requires plenty of regenerant. Because the regenerant flow is from top to bottom the highly regenerated resin bed is at the service flow inlet while the lower regenerated resin bed is at the service flow outlet. Therefore the purity of the processed products is not very high. This system requires a lot of regenerant and should be backwashed before each regeneration process.
The second system is a counter current system where the direction of flow during service is from top to bottom while the flow of the regeneration process is from bottom to top (THE UPCORE SYSTEM, DOW CHEMICAL COMPANY, FORM No.17701527895 CH 17280 E 0895 R). The upper part of the column is equipped with upper bed containing nozzles identical to that of the underdrain bed. There is a free board above the resin bed to accommodate 5-10% expansion of the resin bed after being exhausted.
The disadvantage of This system is that during the regeneration process, where the regenerant flow is from bottom to top, the resin will not be fully compacted due to the tendency of fluidization which Causes the regeneration process to be less effective. Because of the low upflow speed of the limited quantity of regenerant as well as the tenency of the resin bed to fall down due to the higher specific gravity of the resin as compared to that of the liquid, more regenerant are required during regeneration although it is still less than that of the cocurrent system.
Tank diameter is correlated to the upflow speed of the regenerant. Increasing the upflow speed would result in smaller tank diameter and smaller resin volume and therefore shorter regeneration cycle.
There is no backwashing facility and the resin should be discharged from the column if the pressure drop of the system exceeds the limit due to the accumulation of the broken resin particles on the nozzles opening. This system requires specially sized resin with more uniform particles to reduce pressure drop.
The third system is a counter current system where the direction of flow during service is from bottom to top and the regeneration process is conducted from top to bottom (AMBERPACK BACKWASHABLE PACKED BED SYSTEM [EXTERNAL BACKWASHING] ROHM AND HASS; PRINT INF 9003 A DEC 93 IMPACTES RCS B 350424636). The upper part of the column is equipped with upper bed which contains nozzles identical to that of the underdrain bed. There is a free board above the resin bed to accommodate 5% to 10% expansion of the resin bed after being exhausted.
The disadvantage of this system is that during service, where the flow is from bottom to top, the flow speed must be relatively high in order to obtain a compacted resin bed. In many operations the service flow rate is fluctuating or interupted which causes fluidization or fall of the resin bed. This makes the resin bed uncompacted and contaminates the highly regenerated resin at the flow outlet which will reduce the water quality and exchange capacity.
Small broken resin particles tends to rise to the top and resin fines with the size smaller than the nozzle openings may be transported and contaminate the other ion exchange vessel. To protect the system a resin trap should be installed, but it will increase the pressure drop. There is no internal backwashing facility and the resin should be discharged to be backwashed outside of the column if the pressure drop of the system exceeds the limit due to the accumulation of the broken resin particles having sizes larger than the nozzle openings. This system requires specially sized resin with more uniform particles to reduce pressure drop.
U.S. Pat. No. 1,688,915 (year: 1928) utilizes two connected compartments as apparatus for treating liquids. This invention emphasized that no screen are required for carrying the weight of the softening material because the weight of the softening material is carried directly by the tanks or containers themselves. No compaction of the softening material and counter current effect was intended.
One can understand this phenomenon since at the time this invention was filed, softening material contain mostly of minerals which are much heavier than nowadays resin. This system does not have a separate backwashing port.
The tank diameter is correlated with the upflow speed during service where increasing the speed requires smaller tank diameter and therefore yields smaller resin volume and consequently shorter regeneration cycle.
The fourth system is a counter current system with water or air hold down (COUNTER CURRENT ION EXCHANGE SYSTEMS IN INDUSTRIAL AND UTILITY WATER TREATMENT. V. R. DAVIES. ROHM AND HAAS COMPANY, MARCH 1989). This system is a down flow service and upflow regeneration. In the water hold down system, there is a free board above the resin bed with distributor at the top. Regenerant exit distributor is buried just below the upper surface of the resin bed.
During regeneration cycle, the regenerant will flow upward from the bottom and exit through this collector while at the same time water will flow downward from the distributor through this exit distributor to maintain packed resin bed. In the air hold down system, air is used instead of water for the regeneration process.
The disadvantage of this system is that it requires a relatively large quantity of water or air during regeneration to compact the resin bed. Besides, the operation and maintenance are more complicated.
Counter current system shows advantages over cocurrent system. During regeneration process the resin bed at the regenerant inlet will be highly regenerated with the highest exchange capacity while the resin bed at the regenerant outlet will remain partially in the exhausted stage and has lower exchange capacity. In the cocurrent system where the service inlet is the same as the regenerant inlet, this partially exhausted stage will affect the leakage influencing the quality of the treated liquid. It is possible to increase the quality of the treated liquid by imposing higher regeneration level to convert the partially exhausted stage resin to become more highly regenerated but it will be less economical.
In the counter current system the outlet liquid will be in contact with highly regenerated portion of the resin during the service which yields much higher quality of processed liquid with less leakage and less regenerant cycle.