pH system - Design criteria

[misterd]

What is the design criteria for sizing a tank that would be used for pH adjustment? Assuming continuous feed of 25 gpm, proper mixing, what would be the proper dimensions for the mixing tank?

 

[bimr]

It depends on what you are trying to do.

If you are trying to adjust a solution with a widely variable pH, you may need 2 tanks. The first tank will bring you into a range and the second tank will fine tune the pH.

If you are trying to get a very precise pH, you may need two tanks.

If you are trying to adjust the pH with a slow reacting material, you may need larger tanks.

If you want a definite answer, provide a description of what you are trying to do.

 

[misterd]

This tank would be used to adjust pH before discharge. Once the pH is below discharge limit, we can dump it. No need for a highly precise tuning. The pH should not vary too much, but it is not possible to estimate precisely the range (too many variables for now).

I am just looking for a rule of thumb, since we often see that type of installation. Should we plan 30 seconds or 3 hours?

Thanks

 

[bimr]

A rule of thumb is to use 5 min. of residence time for liquid reagents and 20 min. of residence time for slurried reagents.

Another rule of thumb, turnover (mixing) time should be less than 20% of residence time.

If the solution contains concentrations of 1% and greater of either acid or alkali, then you should go with a two-stage system to minimize the oscillations caused by overfeeding of a reagent.

 

[rak74]

Why not perform a bench test to estimate?

 

[bimr]

It is very unlikely that you could make a bench test that could scale up to real world conditions. You have reaction times, instrumentation sensing and control functions, reagant feeding times, and mixing intensity issues that do not accurately scale up. A bench scale is probably a waste of time.

 

[Rhodrons]

For what it's worth, I have just received a quotation for a similar system requirement. We have to neutralise waste acid rinse water during the normal course of events but quarterly we need to treat the strong washing acid. The supplier has specified a two-step neutralisation system. The first step feeds caustic at a "high" rate and will switch automatically to a "low" dosing rate pump when the neutralisation tank begins to approach the allowable discharge rate to avoid overshoot. The allowable pH range is crucial for us as the factory is in Singapore and the Government there demands an automated penstock connected to a sealed pH probe. In the event that we try to dump outside the permitted pH range, the penstock shuts automatically and we will have to ask the nice man in the ministry to allow us to break the seal to retract the penstock. Obviously this puts us on the sinners list and brings a visit in due course. So it is not a case of how big the tanks should be but how well you can hit the target pH.