Cation Bed

Because during the 1st step (Cation exchanger) the positive ions (cations) are exchanged for hydrogen ions (H+), thusly creating an acidic solution. The acid solution is then passed through an anion exchanger where the acid is neutralized by the exchange of the acid ion (Cl-) for the hydroxyl (OH-) ion. In the end, the cations are replaced by H+ and the anions by OH- and we have H + OH = H2O (DI water).

.......which makes the anion bed smaller. Good point.

farrukh dar

The number one reason is that anion resins cost about five times the cost of cation resins. If you do the reverse process, the metals Ca, Mg, iron, etc will precipitate onto the anion media and plug or cover the exchange sites. The recharging and rebedding of anion medias are more complicated so it should be protected. The hydrogen (H+) ion should be the only cation that the anion resin will see and result in combining with the (OH-) ion. It is the easiest way to protect the anion resin and makes economical sense.

I have also heard that anion exchange media is mores suceptible to damage and fouling in presence of suspended impurities comapred to cation exchangers. The upstream cation exchanger also filter out these suspended impurities thus protecting the anion exchanger and increasing its life time.

You have the right idea. The protection of the anion resin is important but so is the cation resin. If turbidity is a problem you may have to consider disinfection and fine filtration to one micron. The cation resin becomes very susceptible to molds, yeast, and fungi due to unavoidable low pH on this media. Usually the anion media remains clean for a long time but the cation fails early. Cation media is the least cost to rebed or rehabilitate. However, that does not mean you don't provide a decent quality of water before the cation and treatment should reflect the quality required.

If you put the anion resin first the high pH (OH ion) will cause the metals to come out of solution, so you always do the cation first. Before any de-mineralization, the water should be of high quality in terms of colour, turbidity, bacteria, and nutrients.

I'm prepared to marked off-topic but in exchange (if I can use that term) would someone explain what function is the interposition of a mixed-bed column between anion and cation beds?

I've been in the deionization business since mid-60's and in all that time, have never seen a mixed bed column used between a cation and anion exchange column. I cannot conceive any useful function of such an arrangement. The mixed bed column follows the cation/anion exchange process as a polishing step in the production of highly deionized water. The ionic leakage from a "standard" deionizer system of cation/anion varies depending on the feedwater quality and ion distribution characteristics but is roughly in the range of maybe 1 to 5 mg/l. The mixed bed in its polishing role will easily reduce this to 0.1 mg/l or lower.

There are some systems using membrane treatment that use what is called a "working mixed bed exchanger" operated at relatively higher inlet dissolved solids concentration to remove the residual salt passage from the membrane process. The membrane process removes those components such as calcium to a sufficiently low level to avoid chemistry side issues.

Thanks or clearing that up. The units that I refer to produce DI water for sawing IC chips from silicon wafers. The quality is measured in terms of the resistance and the units produce water of 18Mohm. I guess I got the location of the mixed bed mixed up.

The production of so-called "holy water" used in the electronics industry in the highest tech areas is the ultimate in terms of sought-after quality. This is a mult-technology flow sheet with many stages focused on specific impurity removal. All aspects of contamination are attacked in this treatment scheme.

There is a considerable pretreatment scheme employed, membrane treatment, UV, ozonation, micro/ultrafiiltration, ion exchange and so on. Mixed beds play a role in this and in the electronic industry, sometime as in point-of use.

This area of ultra-treatment was not my bag in my career, my focus being in the power industry, but I do know that to make the "holy water" that the technology is highly intense.

please give answer of question anin bed installed before cation bed