SWS Effluent Acidification

[EmmanuelTop]

Due to the nature of NH3 and H2S components, pH value of the effluent water from Sour Water Stripper is often higher than 10 (compared to pH=8 of sour water feed). Although SWS is working efficiently, high pH value (from residual NH3) causes unacceptable conditions in the wastewater treatment plant.

So, the question is: how to reduce the pH of effluent stream and have a complete stripping of H2S (which is the case now), at the same time?

I have an idea of SWS effluent acidification, but I do not know if this is applied somewhere in industrial practice. In such case, pH controller should take the information from SWS feed control valve (flow) and effluent pH (pH analyzer) and automatically adjust the amount of acidic neutralizer that is going to be injected downstream of the feed/effluent heat exchangers. Target pH should be maintained between 7 and 8.

- What type of acid ahould be used for such purpose?
- Does anyone of forum members have similar experiences, and how did you battle against these kind of problems?

Regards,


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[CJKruger]

EmmanuelTop, I don't know about your specific issue, but some companies inject caustic into the SWS to free NH3. Location is critical - low enough to ensure all H2S is already stripped, but a few trays remaining to strip the NH3 that is liberated.

CKruger

 

[EmmanuelTop]

Two-staged stripping (addition of acid to remove H2S and weak acids, and subsequent base injection to remove NH3) is a common SWS design practice in last several years.
In such design (the same one you're writing about), effluent pH is even higher than without 2-stage stripping - due to addition of caustic near the tower bottom. So, especially in those circumstances, effluent acidification/neutralization with acid agent would still remain necessary.

I was wandering if this can be done with some industrially approved acid agent (CH3COOH or something similar), which would provide for stabile and effective acidification without corrosion concerns and further wastewater contamination.



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[RogerH]

EmmanuelTop
Even with a good remeoval of NH3 the stripped sour water usually ends up in the 8.5-9.0 pH region.
What I have seen and been using in the WWTP is addition of acids, preferentailly in an equalisation basin (if such one exist).
The acid used usually is sulphuric acid since this seems to be the cheapest acid on the market. Pure Sulphuric acid is also cheaper to build since the material used usually is CS. One could inject the same amount of acid in the pipe but that requires high alloy material (or possibly PVC) but one has to take care of good mixing and possibly any surplus heat release, particulalry during upset conditions that will happen.