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 "sour" (containing sulfur compounds such as H2S and mercaptons); they remove the sulfur compounds and produce elemental sulfur as a by-product.
Best regards
