Multi Stage Scrubbing for H2S

[jerdog]

Can anyone give me a brief explanation why using two stage chemical scrubbing {NaOH (1st stage) & NaOH + NaOCL(second stage)} for H2s removal in wastewater off gasses makes sense over single stage scrubbing (NaOH + NaOCL) at certain concentrations of H2S?

 

[PeterAB]

If you have only hydrogen sulfide, then you should not need a hypo stage to scrub this.
Two stage scrubbing (first caustic, then hypo) generally is used when there are more reduced sulfur compounds present than just hydrogen sulfide. The mix of reduced sulfur compounds commonly detected includes hydrogen sulfide, methyl mercaptan, dimethyl sulfide (DMS) and dimethyl disulfide (DMDS). Hydrogen sulfide and methyl mercaptan are acids that are efficiently scrubbed with caustic, but DMS and DMDS require at least partial oxidation to render them soluble in aqueous solution. Unfortunately, hypo can also oxidize methyl mercaptan to DMDS, which is then harder to remove. Therefore caustic is used first to take out most of the methyl mercaptan, prior to using an oxidizing agent.

 

[moltenmetal]

A polishing step using an oxidant (NaOCl, H2O2 etc.) may reduce the size of scrubbing equipment required to achieve large % removal of H2S. You may also be able to get away with recirculating scrubbing solution in the polishing step, which can be dumped and replaced periodically rather than continuously.

 

[yorkjim]

1st stage - NaOH solubolizes the H2S as HS-.

2nd stage - NaOH / NaOCl - oxidizes the sulfide to sulfate, assuming sufficient chemical is present.

Chemicals are expensive, by pulling the H2S into solution and then oxidizing in a 2nd stage the most complete use of NaOCl is assured. Also, the blowdown from the 2nd stage can be sent to the 1st stage sump to assure that all of the oxidizing agent is consumed.

Theoretical chemical consumption can be estimated. Generally one will find at that at 50-100 ppm H2S inlet concentration in the air being scrubbed that conservation of chemicals will justify the expense of the 2nd scrubber. Above 100 ppm it is very hard not to justify a 2 stage system.

This approach has caused some problems over the years - remember - with caustic only the H2S is being solubolized but not destroyed. WWTPs with this 2 stage scrubbing have lost their NaOCl for various reasons but continued to scrub using caustic only to prevent odor problems in their neighborhoods. But the effluent sent downstream from the plant now contains a high HS- concentration. If pH rises downstream, the HS- will convert back to H2S and come out of solution. This type of scenario has killed workers in downstream sewer lines.

 

[EngineeringManger]

What is your objection to using multi stage chemical scrubbers?
i.e. - Are you not achieving the correct scrubbing effeciency? or Do you wish to increase capacity through the existing equipment? or do you wish to find an alternative (non haz) scrubbing medium.

As PeterAB pointed out - It must be remembered that odors are usually a cocktail of gasses and whilst one substance may be more easily detectable than others, it is usually the substances with the lowest threshold detection levels (mercaptans 5ppb and thiols 3 ppb)which create fugitive odor problems.

We have found that some process gas streams having "long chain" gas compounds (poreous structures) are difficult to neutralize and require multi stages of treatment.

If I can be of assistance please feel free to contact me at engineering@envirocontrol.com - Allen Botha

 

[yorkjim]

Eu Vey! HS- ion is held in solution at pH ~ 9.

If pH FALLS - then H2S will reform and will come out of solution. This will start at pH 8 and accelerate as pH falls further.

My apologies for the error in my previous post!

JimE

 

[EngineeringManger]

We have a low cost product that will neutralize the H2S in your scrubber. Be happy to send you a sample to lab test.

 

[jerdog]

Engineering Manager,

No real objection to using multi stage. Was looking at a saleman's cost analysis and just wanted to understand the chemistry. His numbers justified multi-stage at way lower concentrations than 50 ppm, although our target is 40 ppm and seems to make a good case for multi-stage.

 

[EngineeringManger]

If your process gas strean contains only 50 ppm H2S together with other odorous gasses traces (Mercaptans & Sulphides and the like), a single unit, misting scrubber system should surfice. We would be happy offer an onsite trial.

 

[FrancisM]

Folks,

Its important to remember that an oxidizer is used to destroy the reduced sulfur compounds, so that they will not re-release when the scrubber solution pH is dropped back to neutral, either at waste treatment or in sewer system.