12 procESS EquipMEnT upDATE
Electro-deionisation modules help remove contaminants
Dr Michael Strahand looks at the requirement for chlorine control in continuous electro- deionisation modules and provides a review of methods of chemical control in ultrapure water loops.
of impurity from products. Continuous electro- deionisation (CEDI or EDI) units are routinely installed in manufacturing plants to produce this ultrapure deionised water. These units remove common contaminants, such as silica, carbon dioxide, bacteria, pyrogens (bacterial fragments) and metal ions, which are often found in the feed water supplied to manufacturing plants. Many parts of the world add chlorine to drinking water as part of their disinfection process and this can prove extremely damaging to CEDI units in ultrapure water loops. Resins utilised in CEDI units have very low tolerance to chlorine in water and if chlorine enters a CEDI module it can be very quickly oxidised. As the cost to rebuild these units is extremely high, due to the rapid effects of chlorine oxidation, it has become a requirement for CEDI-based ultrapure water systems to incorporate a chlorine removal stage before the water reaches the CEDI modules as well as prior to the reverse osmosis system.
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There are currently a number of ways to ensure the dechlorination of water in ultrapure water loops and the optimal method remains a topic of much debate.
Dechlorination
Chlorine removal from ultrapure water often takes place by adding a solution of sodium bisulphite or sodium sulphite into the water flow. This reacts with
any manufacturing processes require ultrapure water to ensure a sterile environment and remove very small traces
chlorine residuals and chlorine removal is guaranteed as the resulting sulphite ion is a strong reducing agent.
A typical method for monitoring chlorine removal is the use of a residual chlorine monitor, which acts as a final line of defence before the water flow reaches the CEDI unit. However this method has a number of limitations. Perhaps most notably, chlorine monitors do not perform well when left to run at 0.000 parts per million for extended periods and are often slow to respond when chlorine does enter the module. Chlorine monitors can also be misleading as a failed sensor will often read zero and therefore it is impossible to differentiate between instrument failure and zero chlorine.
In addition, most plants in need of
“New technologies, such as sulphite monitoring systems, are currently being developed with the aim of eliminating many of the
challenges associated with traditional approaches.”
Dr Michael Strahand,
General Manager Europe, Analytical Technology
www.engineerlive.com
dechlorination currently run relatively high sulphite residuals to ensure complete chlorine removal at all times. Many dosing systems have a fixed sulphite dose or a simple flow proportional dose. However this can result in excessive chemical consumption which can prove extremely costly. Another challenge of traditional chlorine monitors is that they do not indicate how much bisulphite is present in excess and therefore can lead to more chemicals than necessary being consumed in the water purification process. Other methods of protecting continuous deionisation units in ultrapure water loops include redox potential (ORP). ORP measurement is used to indicate the presence of oxidising or reducing
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