pH of demineralized water

[padovano]

The pH of water entering the demineralising chain is 7.02. The pH of this after passing through the cationic drum is 4. The pH of water leaving the anionic drum to the storage tank is 10.4. I would like to know if the pH of this demineralised water is normal to be 10.4. I understand there is escape of OH from the anionic drum but is this escape linked in anyway with the performance of the Resin?
Can the pH of the demineralised water be below 8?

 

[bimr]

You are correct in your thinking. The pH of demineralized water should be approximately 10. The elevatd pH is because the leakage of ions leaving the demineralizer will be in the form of NaOH. Na from the cation unit and OH from the anion unit. The OH will cause the elevated pH.

The pH of demineralized water is somewhat pointless because there is no buffering capacity inherent with low mineral water. A slight amount of chemical is capable of changing the pH.

 

[padovano]

we inject Morpholine to the demineralised water on its way to the storage tank. Given that, the pH of demineralised water is 10. Can we stop the injection of Morpholine? Will there be any consequences? Can CO2 be analysed in demineralised water?

 

[bimr]

The big problem and cause of corrosion is oxygen content. Oxygen is probably a bigger cause of corrosion than a lower pH.

Some power plants use nitrogen blanketing to prevent the demineralized water from absorbing oxygen. Vacuum degasification is also used.

CO2 is also absorbed from the air causing the demineralized water pH to drop.

What is the objection to morpholine?

 

[padovano]

what I mean is that, if the demineralised water has a pH of 10 is it "necessary to inject morpholine?. What I think is that, a pH of 10 guanrantees protection from corrosion. Since morpholine is there to prevent corrosion caused by the presence of CO2 can its injection be stopped or reduced?
Also I would like to know if the rate at which OH escapes from the anionic drum can be related in monitoring the life cycle of the resin.

 

[bimr]

Corrosion is more likely related to the oxygen concentration. If you keep the oxygen out, then the corrosion will not occur. Corrosion is not just a function of pH.

http://www.lenntech.com/boiler/corrosion.htm

http://www.gewater.com/handbook/boiler_water_systems/ch_11_preboiler.jsp

You probably can not stop use of the mopholine.

Cations and anions in water must balance. The Na leaks from the cation. In order to maintain the ionic water balance, the OH is then pulled from the anion. To reduce the concentration of OH, you have to decrease the Na leakage from the cation.

 

[GarySCWSVI]

I agree with bimr. Morpholine is also fed to be carried over in the steam to prevent corrosion outisde of the boiler drum.

Gary Schreiber, CWS VI
The Purolite Co.

 

[dougarthur42]

Sorry but i somewhat disagree. Our water going into our demineralizers is raw river water with a PH of around 6.2 to 6.8. Coming out of our Demineralizers the PH is between 6 and 8. If it is higher or lower we look for anion or cation leakage. We use no furthur chemical treatment of demin water.
Thanks Doug

 

[bimr]

Not sure what you are disagreeing with.

The raw source will not have much effect on the effluent pH of the demineralizer.

Cation leakage is important because it effects the purity of the effluent. A strong base anion exchanger can remove only the acidity, not the sodium. A strong base anion exchanger converts the sodium salts to sodium hydroxide by salt splitting, which then creates high conductivity and pH value in the effluent.

If you have a pH less than 8, then there is something going wrong in your demineralizer.

The morphonline that is being added has nothing to do with the demineralizer. One would assume that the morphonline has to do with a power plant downstream of the demineralizer.