Maximum Freeboard for Ion Exchange Backwash 3

[htwoOtreat]

Does anyone know what the maximum recommended freeboard height to accomplish backwash of a single bed ion exchanger. I have observed industry standards for minimum freeboard at approx. 75% based on 50% bed expansion, but I have not seen guidelines for maximum freeboard and the impact.

 

[GarySCWSVI]

You can find that info on most ion exchange resin manufacturers Technical Data Bulletins. "In general" 75% freeboard based on 50% bed expansion is okay to use. However, 4 other factors come into play.

1. Is is cation resin or anion resin or mixed bed resin.
2. What is the water temperature.
3. What is the backwash flow rate.
4. What is the resin beads particule size.

Cation resins are heavier than anion resins. Bed lifting is different for different water temps and flow rates.

Most of the Tech Data Bulletins have graphs and curves that show how much the bed will expand.

Gary Schreiber, CWS VI
The Purolite Co.

 

[htwoOtreat]

Thanks Gary

As you mentioned, most resin suppliers specify minimum freeboard (i.e. 50-75%) based on 30-50% bed expansion as you describe, but is there a limit to how much freeboard. One document rule of thumb states that you need 3 volumes (bed + freeboard) needed to achieve a 95% flush out of the solids from a completely stirred tank reactor (CSTR). Does this apply to IX

 

[GarySCWSVI]

The only limit is not having enough freeboard. That rule of thumb is a good one BUT that all depends upon the make up of the solids that the resin bed may filter out or the resin fines (broken beads) that backwashing causes. I am not personally aware of having too much freeboard being a problem. Too little, yes. Too much, no.

Gary Schreiber, CWS VI
The Purolite Co.

 

[bimr]

I agree with GarySCWSVI's comments. The comments are appropriate for most ion exchange water applications. Ion exchange equipment manufacturers generally do not supply more freeboard than the minimum because it costs them money with little benefit.