1. Removal of ionic substances

2. Reduction of conductivity

3. Control of pH

4. Filtration of corrosion products

Describe the principles of demineralizer operation.

The demineralizer system consists of one or more ion exchange resin columns, including strong cation unit and strong anion unit.  The cation resins exchange hydrogen for raw water cations . The anion resins exchange hydroxyl for raw water anions. Because anion used in demineralizer resin is strongly basic, it will also remove weakly ionized carbonic and silicic acids. 

Demineralization systems which produce water of extremely high purity use mixed bed demineralizers (a mixture of strongly acidic cation and strongly basic anion resins in same column).  A typical mixed demineralizer rinses to low conductivity and silica values after regeneration.  These values will remain low and nearly constant during service cycle, but when resin is exhausted, both values will increase.

State the reason for sampling both the inlet and outlet conductivity of a demineralizer.

The effectiveness of ion exchange process is monitored by checking important parameters of solution before and after it passes through ion exchanger.  If the demineralizer is used for ion removal (purification) conductivity readings will indicate exhaustion. When active resin is nearing exhaustion, the effluent conductivity will begin to rise. The influent conductivity then becomes a measure of comparison with the effluent reading.

Describe the effect of excessive differential pressure on demineralizer performance.

High dP due to crud build-up may cause a higher flow rate which can cause channeling and fines.

The pressure drop across demineralizer is function of flow rate.  Because demineralizer acts like filter, pressure drop (and flow rate drop) is desired.  Monitoring of differential pressure (D/P) provides useful information on demineralizer performance.  As corrosion products and suspended solids are accumulated over service run, flow resistance and D/P increase.

Describe how demineralizer differential pressure is used to determine the condition of a demineralizer resin bed.

D/P (and flow) influences the operation of the demineralizer.  Lower than normal D/P indicates that demineralizer is operating at reduced efficiency.  Higher than normal D/P indicates that flow through demineralizer is set too high or bed is clogged due to buildup of corrosion products and suspended solids.

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Excessive flow results in several adverse effects. 

1.)  Abnormally high flow reduces rate of ion exchange because there is insufficient time for exchange to take place.  Resin beads are forced through lower retention element into demineralizer effluent. 

2.)  High flow physically breaks down resin beads into "fines", which also pass through retention element. 

3.)  Finally, high flow rates result in channeling of bed, which decreases mechanical filtration and results in very low rate of exchange.

Channeling occurs as result of water finding or forcing low resistance flow path from inlet to outlet.  An indication of channeling is decrease in differential pressure, because resin bed no longer provides sufficient flow resistance to influent.  These conditions result in high ion concentrations and conductivity in demineralizer effluent, as well as high radiation readings.

Ion exchange resins are sensitive to elevated temperatures.  Although inert resin bead structure is stable up to fairly high temperatures (300°F), active exchange sites are not.  The anion resin begins to decompose at about 140°F, and decomposition becomes rapid above 180°F.  The cation resin is stable up to 250°F.  When temperature limits for OH - form anion resins are exceeded, ion exchange capabilities are reduced due to formation of inferior resins. 

In addition, weak base called trimethylamine (TMA) is formed, which has odor similar to that of dead fish.  Sulfuric acid is produced when H+ form cation resin temperature limit is exceeded.  All ion exchange capabilities are destroyed.  Hot fluids, such as water from vessel bottom head drain, must be cooled prior to entering reactor water cleanup demineralizer.

Demineralization Factor

Decontamination Factor