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# Calculation Of Required Solvent Rate For Chemical Absorption

chemical absorption sovent rate naoh hydrogen sulfide biogas

10 replies to this topic
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### #1 SIVAMOORTHY

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Posted 09 December 2017 - 01:22 AM

Can anyone please explain how to calculate minimum solvent required for chemical absorption(Absorption of Hydrogen sulfide in NaOH). Will the required solvent be different at different concentrations of NaOH ( 1 M, 2 M, 3 M NaOH).

### #2 MrShorty

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Posted 12 December 2017 - 10:53 AM

From what little I know of this process, it is based on the acid+base neutralization reactions:

NaOH+H2S -> NaHS+H2O

NaHS+NaOH -> Na2S+H2O

So, yes, I would expect your overall solvent flow rate to be dependent on caustic concentration, as well as reaction stoichiometry and equilibrium constants, and maybe reaction rates. If nothing else, the reaction stoichiometry should allow you to calculate the absolute minimum required assuming the reaction(s) go all the way to completion even at pH 7 (which is probably a poor assumption). A quick search for the pKa's of H2S (and a refresher of those acid-base equilibrium problems from your earliest chemistry courses) would probably help understand how good/bad this assumption is.

Beyond that, and knowing that I have used aqueous NaOH to capture H2S in the lab before, I cannot speak to the calculation details. This is not a new technology, so I expect there is much in the literature about this reaction process.

### #3 Pilesar

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Posted 12 December 2017 - 02:22 PM

You might start with the 'SO2 and Acid Gas Controls' EPA paper https://www3.epa.gov...r1/cs5-2ch1.pdf for a nice overview.

### #4 SIVAMOORTHY

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Posted 14 December 2017 - 07:20 AM

MrShorty & Pilesar:

Thank you very much...

### #5 SIVAMOORTHY

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Posted 14 December 2017 - 07:32 AM

I am going to use adsorption technology for h2s removal. I can go for either using of sponge iron or activated carbon. I have read and heard that by using activated carbon, we can not able to regenerate once it reaches equilibrium. So, I prefer sponge iron which is regenerable by applying oxygen in the sponge iron. I approached manufacturing companies for inquiring about sponge iron usage for h2s removal. They are telling like they are not aware about h2s removal by using sponge iron. Also, they said that it will be used for making steel. Could anyone please suggest me for removing h2s removal by using adsorbent, which is regenerable after usage and also suggest me for where can i get the adsorbent...

### #6 sgkim

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Posted 14 December 2017 - 07:53 AM

Sivamoorthy,

Your idea seems to be related to the following chemistry.

Rather than iron, the Iron oxide can probably be used for the removal of H2S by the following chemistry:

Fe2O3 + H2S ------------------>Fe2S3 + H2O   .................(1) < ---- removal of  H2S

Fe2S3  + O2 (usualy  air) ---->Fe2O3 + 3S    .................(2)  <-----regeneration of Fe2O3

The regenerated sulfur may coat the porous iron oxide on the surface of the sponge iron.  After a while the sulfur should be removed properly for continuous service of the sponge iron bed.   But I have no idea how to remove the elemental sulfur from the bed.

Stefano Kim

### #7 MrShorty

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Posted 14 December 2017 - 11:29 AM

I put "sponge iron sulfide removal" into my favorite search engine, and this was the second page suggested: https://sourgas.ca/2...ss-h2s-removal/ They talk about the regeneration of the bed, suggesting that it is limited (~10 batch regenerations) due to solid sulfur build up as noted by sgkim. At that point, the author suggests that the bed must be replaced. Again, this seems like an old technology so I am surprised that you are having trouble getting information about it. Is it possible this is because this is a historical technology that is no longer commonly used?

I am also surprised that you have not mentioned one of the most common technologies for acid gas removal that I see -- amine based solvents (https://en.wikipedia...ne_gas_treating ). I don't know if this is because you are unaware of these technologies or if you have already discarded them as unsuitable for your specific scenario.

### #8 SIVAMOORTHY

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Posted 15 December 2017 - 12:55 AM

Sivamoorthy,

Your idea seems to be related to the following chemistry.

Rather than iron, the Iron oxide can probably be used for the removal of H2S by the following chemistry:

Fe2O3 + H2S ------------------>Fe2S3 + H2O   .................(1) < ---- removal of  H2S

Fe2S3  + O2 (usualy  air) ---->Fe2O3 + 3S    .................(2)  <-----regeneration of Fe2O3

The regenerated sulfur may coat the porous iron oxide on the surface of the sponge iron.  After a while the sulfur should be removed properly for continuous service of the sponge iron bed.   But I have no idea how to remove the elemental sulfur from the bed.

Stefano Kim

Thank you..

I have read that we can apply steam or hot air to remove sulfur from sponge iron bed. But it consumes more energy to regenerate since it needs temperature around 700 K.

### #9 SIVAMOORTHY

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Posted 15 December 2017 - 01:08 AM

I put "sponge iron sulfide removal" into my favorite search engine, and this was the second page suggested: https://sourgas.ca/2...ss-h2s-removal/ They talk about the regeneration of the bed, suggesting that it is limited (~10 batch regenerations) due to solid sulfur build up as noted by sgkim. At that point, the author suggests that the bed must be replaced. Again, this seems like an old technology so I am surprised that you are having trouble getting information about it. Is it possible this is because this is a historical technology that is no longer commonly used?

I am also surprised that you have not mentioned one of the most common technologies for acid gas removal that I see -- amine based solvents (https://en.wikipedia...ne_gas_treating ). I don't know if this is because you are unaware of these technologies or if you have already discarded them as unsuitable for your specific scenario.

Thank you..

Actually my problem is to remove H2S from biogas, which contains 60 % CH4, around 2000 ppm H2S and remaining CO2. I tried first by chemical absorption using NaOH as solvent. Since, Gas stream having around 40 % of CO2 and it also reacts with NaOH easily, this method leads to formation of carbonate precipitation and more consumption of NaOH. So, this process is economically not feasible. Then I came for using amine solvent (Methyldiethanolamine, which is selective for H2S and less reaction with CO2). But, for this also I have to regenerate by applying saturated steam in stripping section. This also, will consume more power for steam generation. Later, I came for adsorption technology using activated carbon and sponge iron. Could you please suggest me a best one for treating biogas economically...

Thank you much

Edited by SIVAMOORTHY, 15 December 2017 - 01:09 AM.

### #10 MrShorty

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Posted 15 December 2017 - 10:35 AM

I cannot make a recommendation, because this is beyond my area of expertise. However, you have given the experts some very useful information about your process that you have withheld until now:

1) Gas is 60:40:0.2% CH4:CO2:H2S

2) You want to only remove the H2S but not the CO2, which means you need something that is selective (at 40:0.2 CO2:H2S it probably needs to be very selective) for H2S.

It seems to me that the main difficulty you have is the selectivity that you require. I put "h2s selective gas process" into my favorite search engine and found several articles across the internet. Many of them seemed to be testing on gas streams with much lower CO2 than you have, but that should give you a starting place to do some of your own research while waiting for someone here to comment or inquire among those you are consulting with about H2S selective gas removal processes.

### #11 SIVAMOORTHY

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Posted 15 December 2017 - 11:20 AM