Possibilities of caustic scrubber H2S without oxidizer

[daniellekeijzer]

I have a question about so-called caustic scrubbers. One of our clients have requested for a scrubber for H2S gas (without impurities like CO2 and SO2 etc), scrubbing with NaOH.

Is it possible to remove H2S by using a 15% NaOH solution?
I have been searching the internet, but so far I have only found scrubbers where a combination of NaOH and an oxidizing agent (like NaOCl or H2O2) is used.

What kind of reaction takes place?
Is this: H2S + 2 NaOH --> Na2S + 2 H2O the reaction, or are there more possibilities?

I hope someone can help me with this, because I do not have lots of time to find it out by myself...

 

[Auflu]

Caustic soda scrubbers are very commonly used to remove H2S (or any acid gas) from gas streams. The reaction you've shown is correct, but in practical terms the species will remain as ions in solution (Na+, S-2 etc.) Precipitation as Na2S won't occur until it is saturated. Typical applications are for scrubbing H2S from a gas stream where the H2S concentration is less than 5%(v); higher concentrations are certainly doable but at that point the sulfur may be worth something and you might as well consider a Claus plant.

The addition of an oxidizer is twofold. 1. To oxidize the sulfur ions so that they can be removed from the system and not become gaseous again. 2. To provide a driving force for the reaction by removing one of the products.

15%(wt) caustic is much more concentrated than generally needed to scrub H2S in most applications. It may be used as a feedstock to water so that the resultant pH of the scrubbing solution is basic (although 50% is a standard commodity). H2S, being an acid gas, will readily dissolve in a basic solution. Depending on pH, the H2S will dissociate first as HS- up to pH ~9.1, above that it will be S-2. As long as the scrubber solution pH remains high the sulfide can be held in solution, but you'll need to blow it down somewhere or convert it to a non-volitile form (e.g. S elem., SO4=,). What do you want to do with the spent scrubbing solution if you don't want to oxidize it? How will you dispose of it?