Question

when regenerating ion exchanger, why do they do it by backwashing regenerant (flowing regenerant such as sulphuric acid from bottom to top, instead of from top to bottom). I understand if someone wants to regenerate sand filter they have to do it this way, since there's deposit sediment in the lower part of sand filter, but in ion exchanger, feed of water coming to exchanger is relatively clean, there shouldn't be sediment anywhere in ion exchanger. I don't think there's an advantage of flowing regenerant from bottom to top, if there is, could you please explain it to me? thanks

Answer 1

The backwash expands the resin bed from its settled and packed condition(which will not occur if washing is done from top to bottom), and cleans the resin by flushing out any suspended solids that may have been filtered during the service run. Resin particles can act as effective filter media because they have ionic charges, which can coagulate fine particles. During the backwash, the resin beads rub against each other, and this scrubbing action helps clean accumulated dirt or iron from the surface of the beads.

On the other hand, it also removes any broken resin particles or resin fines. In addition, the backwash cycle classifies the bed with the larger resin beads on the bottom, and the finer resins on top. This provides the best conditions for a good uniform flow of regenerant solution.

so, these are the two reasons to select backwashing over co-current washing.