Sodium thiosulfate 3

[nuig]

Helllo,

I'm interested in regenerating 20% caustic solution for a H2S scrubbing process, < 1 atm, 25'C gas conditions.
The gas contains 60% CH4, 38% CO2 and ~2% H2S, liquid dispersion is via spraying nozzles to minimize CO2 absorption by minimizing contact time.

During the regenerative process, sodium thiosulfate comes out in the equations.

My question is, should I be particularily concerned about this compound with regards pumping it back via recirc system?

Thanking you in advance...

 

[kenvlach]

Are you using just caustic solution or caustic + oxidizing agent (like hypochlorite)?
Thiosulfate is a reducing agent and will react with your oxidizing agent to form sodium sulfate Na2SO4, which has a lower solubility than thiosulfate.
So, I see only 2 problems: Consumption of oxidizing agent & possible deposition of solids.
[at ~neutral pH, thiosulfate can be broken down by bacteria, but I doubt any effect in a caustic solution]

 

[pleckner]

I worked for almost 10 years developing aned designing Wellman-Lord SO2 Recover Plants (flue gas desulfurization). We scrubbed SO2 out of flue gases using a sodium sulfite solution. Upon scrubbing, we formed sodium bisulfite. We would then regenerate the sodium bisulfite solution in evaporators, recovering the SO2 and reforming sodium sulfite. In the evaporators, we also formed sodium thiosulfate. This compound had to be purged. It is inert to the absorption process and was auto-catalytic in nature; the more you had of the stuff, the more that was made.

 

[TomSD]

You have tweaked my curiosity. Is this a real scrubbing process because I am not familiar with the use of caustic (sodium hydroxide solution) for scrubbing H2S? If so, I am also curious about what the caustic regeneration step is.

CO2 and H2S are commonly removed from natural gas using DEA (diethylamine) as an absorbent.

My experience with caustic scrubbing systems tells me that CO2 will be quickly and easily &quot;chemi-sorbed&quot; forming sodium bicarbonate (NaOH + CO2 -> NaHCO3) and primarily sodium carbonate (NaOH + NaHCO3 -> Na2CO3 + H2O). Starting with 20% w/w NaOH and depleting most of the NaOH may result in some crystallization of the carbonate species. At the conditions mentioned, I don't think there would be much absorption of H2S except a limited physical solubility unless a strong oxidizing compound is added. With hydrogen peroxide added to the solution, there will be a production of sodium sulfate; any intermediate thiosulfate would be reduced by peroxide to sulfate.

Regards

 

[nuig]

kenvlach:
Thanks for reminding me that I should be looking at hypoclorite as well as caustic.
When you say that sodium sulfate has a lower solubility than thiosulfate, does that imply that it could be more easily extracted?

TomSD:
Is (caustic + hypo) a more common process from your experience than just caustic alone?

As for the regeneration, don't have the process finalised, but it involves aerating the spent scrubbing solution in the presence of an oxidising catalyst. This is where the thiosulfate is formed.

Regarding the CO2 being &quot;chemi-sorbed&quot;, I'm trying to get info on selective H2S amine based solutions. Sent info requests to Dow but nothing back yet, so I've been pressing on with the caustic (+hypochlorite) option in the meantime.

pleckner:
From your post, I gather that I should aviod thiosulfate due to its auot-catalytic nature.

Does anyone have a contact in Dow that I could e-mail directly?
Also, for this application, batch or continious replinishment? (Gas flowrate is only 70m3/hr)

 

[kenvlach]

I think the major problem with your system is that you will create lots of solids: sodium carbonates & sodium sulfate.

Please explain how you plan to regenerate; I don't see how a reducing type of compound like sodium thiosulfate will form in an oxidizing environment, like hypochlorite or peroxide or even air (w/catalysis).

Your best bet is to get a selective amine to remove the H2S and not the CO2 (unless that is also an objective).

 

[pleckner]

Yes, you want to avoid the formation of thio. At the very least, you want to keep it's concentration in the solution at 0.5% or less by purging if you have to.

 

[TomSD]

nuig
As far as my experience goes, caustic solution is mostly used to scrub acid gases such as chlorine (Cl2), hydrogen chloride (HCl), sulfur dioxide (SO2), sulfur trioxide (SO3), carbon dioxide (CO2), etc. These gases react with caustic forming salts dissolved in the solution and this maintains the highest concentration difference of the absorbed component between the gas and liquid phase such that the mass transfer rate is also high.

Gases such as carbon monoxide (CO), hydrogen sulfide (H2S), nitrogen monoxide (NO), do not react with the caustic or only do so very slowly unless a strong oxidizing compound is present as well. The oxidizing compound is essentially turning these gases into stronger acids that then react with the caustic (NaOH).

I am not aware of caustic + hypochlorite solution being a common scrubbing solution.

Hypochlorite is a fairly strong oxidant and it is most commonly produced by absorbing chlorine into caustic solution. The overall reaction is
2NaOH + Cl2 --> NaOCl + NaCl + H2O
where NaOCl is sodium hypochlorite. This is the common method of producing bleach solution that can be used for water treatment and laundry. A small excess of NaOH (~0.25% to ~1%) is maintained in the produced hypo bleach solution to increase the stability; the hypochlorite can slowly react to form chlorate (NaClO3) and can also decompose (NaOCl --> 1/2 O2 + NaCl), facilitated by metallic impurities (Ni, Fe, Cu, Co) and by higher temperature. Some tricky methods are used to remove/reduce the co-produced sodium chloride, but this is unusual. You could buy bleach solution and mix it with caustic to obtain whatever hypochlorite concentration is appropriate. You could also make your own caustic + hypochlorite solution by absorbing some chlorine into a caustic solution, feeding flow controlled amounts of each to obtain the desired concentration of hypochlorite. This entails a separate caustic scrubbing system, that will probably have a recirculation loop and a cooler to remove the heat of reaction.

A caustic + hypochlorite solution should provide for the oxidation of H2S such that sodium sulfate is produced. However, I believe you will have to allow for the absorption of the CO2 producing sodium carbonate. If the NaOH is fully depleted due to CO2 absorption before hypochlorite is fully reacted then you will get some release of chlorine as the solution becomes slightly acidic. There may also be a small amount of hypochlorite (or chlorine) reacting with the methane (CH4) producing chlorinated hydrocarbons, possibly chloroform (CHCl3) or carbon tetrachloride (CCl4), hopefully very little.

If I had to scrub H2S from this gas using a caustic solution, I would add hydrogen peroxide as my oxidizing agent to avoid corrosive chlorides and the especially corrosive active-chlorine species (namely hypochlorite). Hydrogen peroxide is also a bit corrosive but much more evironmentally friendly since it decomposes to oxygen and water. It is more expensive than chlorine or hypochlorite but I think you avoid other problems that will cost money. In a small scale operation, the higher cost of hydrogen peroxide is worth it.

However, I can't see how the scrubbing solution can regenerated to caustic in the manner you mentioned. I'd like to see the process flowsheet and more about the catalyst. There are electrochemical methods for recovering sodium hydroxide from sodium carbonate and sodium sulfate solutions but these are somewhat expensive to install and operate plus the solution of recovered caustic may have to be concentrated by evaporation. The by-product streams would be CO2 gas and a weak sulfuric acid. You can't recover all of the caustic value so some caustic make-up would be necessary.

Other schemes might be considered. For example, if chlorine gas is injected into the gas stream, the H2S might be converted into SO2, SO3, H2SO4 vapors all of which could be absorbed in water. This may only work at higher temperatures and the issue of chlorinated hydrocarbons is probably greater. Only a little of the CO2 would be absorbed. Now the acidic water stream has to be dealt with.

The use of absorbents such as DEA is likely the better way to go but this is outside my experience. Perhaps you will get more help from someone in the petroleum or natural gas industry. Otherwise, I would suggest going to a forum for petroleum and/or natural gas for help, I haven't checked if there is one here. A lot of natural gas is &quot;sour&quot; (containing sulfur compounds such as H2S and mercaptons); they remove the sulfur compounds and produce elemental sulfur as a by-product.

Best regards

 

[nuig]

To all,

Sounds to me that you've all put in a strong case for selective scrubbing.
As for the regeneration process, I was refering to using process that I come across on the web, and the details I have are sketchy at best.

Thanks again for all the help and an amine-based solution is the way i'll probably go if the cost compares well with other options.

Best regards.

 

[ihg]

Suggest that you look in a copy of the GPSA Engineering Handbook (one of the best value handbooks available - see

http://www.gasprocessors.com/gpsa_book.html).

This is a fairly standard natural gas purification process, using a variety of amines but the scale is not apparent. Disposal of the sulfur may be a problem; oxidizing H2S to elemental sulfur via SO2+2H2S -> 3S + 2H2O may be friendlier

 

[bkmuduli]

We are facing corrosion problem in the overhead circuit of Naphtha Desulphurization plant. The pH of boot water in the settler vessel comes to be acidic. Corrosion takes place in the piping near the weld joints.

Does anybody have such experience?

What is the probable reason of low pH?

What is the possible solution?

Thanks

 

[moltenmetal]

I know that the ethanolamines are commonly used for H2S/CO2 removal from steams of the sort you're talking about. I also know that sulphide oxidation gives you a mixture of sulphur and sulphate, tending to less and less sulphur as the pH increases. I presume that CO2 removal, though not essential, is desirable- but your case may be different.

I'm interested to know what is done with the stripped H2S afterward! What's the next step? Is is the generation of sulphur, and if so, how exactly is it done?