Effluent pH control with sulfuric acid 1

[ed666]

Here is the situation:
1. Process changes require to modify pH control from a caustic injection to an acid base system cause the nature of the effluent will be affected
2. i need roughly 60 kg/d of H2SO4 (97%) to treat about 75m3/d in order to neutralize pH 10 effluent. very low flow...
3. H2SO4 is already available at the location
4. Gravity can be used to bring the sulfuric acid to injection point
5. Would like to use a 'day' tank
6. Injection point has already a small mixing chamber with pH probe

Here are the questions:
a) is it better to dilute sulfuric acid (to about 20%) or use concentrated.
b) if diluted what type of tank/equipment would you use to make the mixing
c) in any case is it better to use a control valve or a dosing pump for the injection (any recommendations on material selection, type of valves and pumps)

Thanks Eric

 

[bimr]

If it was necessary to pump over a long distance or the volume of acid was small would be reasons to dilute the acid. In your application, it does not appear warranted to dilute the acid. Note that 20% acid is more corrosive as well.

The metering pump will be more accurate than the valve.

The corrosivity of sulfuric acid depends on many factors including temperature, concentration, the presence of oxidizing or reducing impurities, velocity effects, and solids in suspension. For concentrated acid (93% or >) at 40 to 60 °C and pipe diameters > 3 inches, the recommended materials are 14% silicon-iron, Alloy B-2, Alloy C-276, glass, PVDF or PTFE lined pipe, and high silicon stainless steel.

 

[ed666]

thank you BIMR,

I think i will go pure for what you said and the heat generation, the extra step to do the dilution

i will need a very small amount 0-100 ml/min. what type of pump will work best?

how would you inject it? submersed or non submersed pipe.

my injection point is a 2 compartments chamber with each a pH probe and agitator. I think of adding the acid in the first chamber and have the pH probe control the pump flow. The pH probe in the second chamber will be for regulatory use and X-checking.

finally, you mentionned pipe size >3 in, is the acid very viscous at temp -30C to +30C so it would cause problem in small pipes?

Eric

 

[bimr]

A standard metering pump (milton roy) would provide good results.

One would prefer a non-submersed pipe so that you can see the acid is flowing. The pipe should be close to the fluid level to avoid excessive splashing.

There should not be a problem with viscosity at the temperatures that you reference.