methyl mercaptan odor control 1

[mruby]

There was considerable discussion of sulfides control in an exhaust gas stream in thread127-72509 but the specific subject of methyl mercaptan remains illusive. We are trying to control methyl mercaptan using a two-stage scrubber - first stage is acid and second stage is caustic with NaOH/ClO2 (see thread127-157289 for details). Methyl mercaptan cuts through both like a hot knife through butter. The alternatives we are looking into are replacement of the oxidizer in either the caustic or the acid scrubber (if we used the acid scrubber we would raise the pH to about 4.5) with a proprietary hydrogen peroxide plus catalyst solution or with potassium permanganate. We have had real difficulty finding anyone who has successfully controlled methyl mercaptan using wet scrubbing. Papers in the literature seem to be all over the map as to the correct pH for max solubility. Would appreciate any comments on the alternative scrubbing solutions that might be successful.

 

[jppmj]

Dear MRUBY
What are the flow rate, temperature, Humidity, methylmercaptan concentration you want to treat by scrubber.
Regards
JPPMJ

 

[mruby]

The air stream is about 2000 acfm at about 85 F. There are currently two scrubbers in line following a venturi scrubber so the gas stream is fully saturated. The incoming air stream seems to be about 5 ppm H2S and 5 ppm methyl mercaptan. The current exhaust gas seems to be about 0 ppm H2S, 3 ppm methyl mercaptan and 2 ppm dimethyl disulfide. What we really need to know most is what does the solubility curve for methyl mercaptan look like? We are currently at pH 3.5 in the acid scrubber and pH 8.5 in the caustic scrubber. The next thing we need to know, is there anything that will react fast enough that we can get the methyl mercaptan at the top of the tower down to zero. The DMDS says to me that we are partially oxiding the MM. We need to take it all the way to sulfonic acid.

If changing the chemistry of the scrubber doesn't work, our other option seems to be to follow the scrubber with a deep bed of something like Purafil. But out of the scrubber we are supersaturated. I do have a boiler nearby that I could steal some heat from to boost the exit temp up about 10 degrees and dry it out.

 

[25362]

Thread798-90462 advises adsorption by activated carbon.
Googleing around you may find plenty of sites on the subject. Worth to explore.

 

[jppmj]

Dear MRUBY

I don't understand why you use an acid scrubber. Usually for H2S and RSH removal, we use one or two caustic scrubber with an oxidant like sodium hypochlorite.

I designed the folowing scrubber :
Material PEHD
Diametre 700 mm
Packing Pall 50 (2") or equivalent (NOT METAL).
Packing height 5m
Liquid NaOH + NaClO aqueous solution
Liquid flow rate 8 m3/h
pH 10.5 NaClO 1g/liter
(Sorry for units, but I live in Europe)
You can change your acid scrubber in a caustic + oxidant scubber.
Regards
JPPMJ

 

[mruby]

The acid scrubber is being used to remove horrendous amounts of ammonia and amines. I have not been mentioning that because it is working fine. The caustic scrubber is very similar (slighly more effective packing) to the specs you provide and has been operated at several pH levels, including 10.5 with the same lack of success.

 

[jppmj]

Dear MRUBY
Normally this kind of scrubber works very well. I think there is something working wrong in your scrubber.
Please check the following points:
Reactives injection (NaOH+ Oxidant) has to be located at the top of the scrubber.
pH and oxidant concentration (ORP for example) has to be measured at the bottom of the scrubber, so you are sure that the reactives concentrations are good in all the packing.
You have to use softened water, in a caustic scrubber, in order to avoid CaCO3 precipitation.
If there is not enough oxidant , you can get sulphur precipitation (as a yellow powder) in the packing.
An orifice distributor is better than sprayers
For packing height more than 3 m it is better to have packing in 2 stages with liquid re-center.
Could you write how is designed your scrubber ( packing, diameter, liquid flow rate, pressure drop, pH and ORP control loops, etc...)
Regards
JPPMJ