cadimodo - 30-1-2007 at 09:19
I want to clean H2S from Biogas. So I use NaOH to absorb H2S, then electrolyse this solution to recycle NaOH.
NaOH + H2S -> Na2S + H2O
Na2S + 2H2O --eletrolyse--> 2NaOH + H2 + 2S
Do you think that it will be successful ? Help me.
[Edited on 30-1-2007 by cadimodo]
[Edited on 30-1-2007 by cadimodo]
[Edited on 31-1-2007 by cadimodo]
YT2095 - 30-1-2007 at 09:23
it would form a polysulphide and I`m not entirely sure it would conduct either.
sodium sulphide + water would give you H2S again also.
you`re probably favored to bubble the H2S through H202 to make your sulpher that way.
[Edited on 30-1-2007 by YT2095]
cadimodo - 30-1-2007 at 09:47
But this is biogas (CH4) + H2O2 -> flammable
YT2095 - 30-1-2007 at 09:53
if you`re using Conc H2O2 yes, but a ~3% soln would act just fine and without problem.
cadimodo - 30-1-2007 at 10:19
So did you success in using 3% solution of H2O2 to oxidize H2S ? Help me !
YT2095 - 30-1-2007 at 10:55
I don`t make Biogas, however the Chem is sound.
Rosco Bodine - 30-1-2007 at 11:28
Maybe use a ferrous sulfate/ ferrous hydroxide solution as a scrubber and tie up the sulfur as insoluble ferrous sulfide which could be filtered out .
cadimodo - 30-1-2007 at 19:26
thanks, I think it works, but this is an expensive solution. So do you have any idea ?
Rosco Bodine - 30-1-2007 at 20:26
Try limewater .
cadimodo - 30-1-2007 at 21:31
Sorry, but I had searched, CaS is hydrolyzed by water. So limewater is inutile.
Elimination H2S seem to be a simple problem. But I need an economic solution. Anyone help me ?
[Edited on 31-1-2007 by cadimodo]
Rosco Bodine - 30-1-2007 at 23:10
Limewater is exactly what you need and it is the most ecomomic solution .
I don't know where you searched , but the use of
lime as a scrubber for H2S is a long established practice
and it works very well .
Lambda - 31-1-2007 at 02:40
Personally I would go for Iron(II)Hydroxide for the following reason:
1 - You want to remove H2S according to the following equation:
H2S + Fe(OH)2 ---> FeS + 2 H2O
2 - Seeing that you are using Bio Gas, there are other concerns to be acknowledged, namely:
a - Thiols (Mercaptans).
b - Biodegradable Volatile and semi-volatile hydrocarbons odors together with Amine odors.
This stench is also removed by Iron(II)hydroxide.
Remember, all sulfur containing products of Bio Gas give exactly the same undesirable end product upon combustion, namely SO2 gas. H2S has an added
disadvantage of being highly toxic, and corrosive to equipment and piping systems used in bio gas handling.
Iron(II)Hyroxide can be impregnated into a Calcined (Like Ca(OH)2) packing Matrix to increase absorption, hardness, porosity, and surface area.
Iron(II)Oxide may also give good results with or without Iron(II)Hydroxide in the same.
A heat reversible and recyclable system that produces elemental Sulfur, without electricity !!, would however be wonderful. Stable at room
temperature, absorbing all Sulfur compounds, and then recycled by heat to give you the same starting product + evaporated Sulfur. I bet my girlfriends
dick
that this would be a real market banger 
Regards,
Lambda.
cadimodo - 31-1-2007 at 04:01
Thanks so much. You have given to me some intial ideas. But :
*To Rosco Bodine : I found out at this site :
http://www.reference.com/browse/wiki/Calcium_sulfide
So CaS is hydrolyzed by water.
*To Lambda : Do you have any document about this method ? I find out this in Internet :
http://www.connellygpm.com/ironsponge.html
Rosco Bodine - 31-1-2007 at 09:21
Those are not the reactions which apply when H2S
is being bubbled through limewater . CaS is never present and is irrelevant .
2 H2S + Ca(OH)2 ----> Ca(SH)2
The spent limewater can be purged of the H2S it contains
by CO2 which will result in CaCO3 precipitating .
Ca(SH)2 + CO2 + HOH -----> CaCO3 + 2 H2S
randolph_carter - 31-1-2007 at 11:53
well well.....
finally a "new" discussion around here....
ha!!!
well i have done my share of engineering of biogas plants in amerika and overseas.....
here is some basic info on biogas and "scrubbing" as it is practiced in meat space........
General Composition of Bio Gas
Methane CH4 54~70%
Carbon Dioxide CO2 27~45%
Nitrogen N2 0.5~3%
Hydrogen H2 1~10%
Carbon Monoxide CO 0.1%
Oxygen O2 0.1%
Hydrogen Sulfide H2S trace
Biogas H2S Scrubbing
Scrubbing is the operation that removes unwanted compounds from the biogas before it is used. Usually these compounds are those that will cause us
some grief in some way. The main culprit to be scrubbed will be Hydrogen Sulphide, or 'Rotten Egg' gas, because this will combine with the moisture in
the biogas to form sulphurous acids and these can corrode almost anything. The way to get rid of it is to give it something to corrode that you don't
want; like some steel wool, for instance, in a wide-necked bottle or flagon. It must be of clear glass with the gas inlet pipe running down to the
bottom of the container and an outlet pipe coming away near the top. Of course, the whole thing needs to be gas-tight. As you use the gas, the steel
wool will corrode from the bottom upwards taking up the hydrogen sulphide by conversion to black iron sulphide which can later be reused after being
oxidised to rust (ferric oxide) by exposure to air, although the process is slower than the initial scrubbing one was. When the black corrosion
reaches 75% of the height of the container, or so, it's time to change the steel wool or ferric oxide for fresh, sacrificial stuff. It's probably
better to run two or more similar bottles or containers connected one after the other to give some flexibility by providing some 'back-up' scrubbing
capability if you are away for a period.
hope this info is of use....
i have an IMMENSE library of info on this subject, but this is probably what you are looking for....
enjoy.....
randolph_carter - 31-1-2007 at 12:10
oh i forgot another simple one....
also an aqueous medium could be used ......
where h2s is ionized to H+ and S2-...
2Fe3+ + S2 ----> 2Fe2+ + S
then oxidation of ferrous ions by air.....
4Fe2+ + O2- -----> 4Fe3+ + H2O
just raving in da pm into da sat link......
cadimodo - 31-1-2007 at 17:41
Thank to Rosco Bodine and randolph_carter.
I have thinked about using Fe3+. But in the reaction : Fe2+ +O2 ----> Fe3+ O2-, do we need the bacteria thiobacillus ferroxidans to increase the
velocity ? I don't have this bacteria .
Rosco Bodine - 31-1-2007 at 18:10
The thing I am more concerned about is the carbon dioxide content of the biogas . That could easily present a problem .
Ozone - 31-1-2007 at 18:32
Wash that with a solution of diethanolamine.
I would think that you are primarily interested in H2 and Methane. A long tube* should work since:
pH2 = 0.09g/L STP
pCH4 = 0.72g/L STP
and,
pH2S = 1.36 g/L STP
pCO2 = 1.98 g/L STP
Pull you gas off ot the top (you other permanent gasses are coming along for the ride) and scrub the gas (probably first passing through a U tube
level with the "1.00g/L" mark) from the bottom.
Just a thought,
O3
*With some sort of ballast tank to keep pressure transients from messing up your density gradient.
[Edited on 1-2-2007 by Ozone]
Rosco Bodine - 1-2-2007 at 11:39
Probably a mixture of limewater with NaOH , or with
Na2CO3 .....which would produce the NaOH in situ ,
would be the cheapest effective scrubber system .
The CO2 would be stripped and precipitate as CaCO3 ,
and the H2S would be stripped and remain in solution
first as Na2S and later as NaHS if the H2S capacity
was run to its maximum absorption of the extra mole of
H2S .
By controlling and monitoring , the operation of these scrubber(s) as a *pair* of units could be optimized , so
that when one scrubber is being emptied and its
useful byproducts collected , the other freshly recharged scrubber unit is in operation .
The solid CaCO3 is filtered out and from the effluent ,
after cooling and concentration ....
Crystalline Na2S as the hexahydrate or nonahydrate ,
could be obtained .
If the scrubber had been maxed out in its absorption of
H2S , using a sensor of H2S blowthrough to indicate the
time for recharge , then the solution would be NaHS and
would require a molar equivalent of NaOH to be added
to convert the highly soluble NaHS to Na2S which could be crystallized out .
The supernatant alkaline liquid could be recycled in the
recharging of the scrubber with additional NaOH or Na2CO3 and lime .
See
US2346550
US2409392
[Edited on 1-2-2007 by Rosco Bodine]
unionised - 1-2-2007 at 12:23
This is a "solved problem"; how did they remove H2S from coal gas? I'm sure it was some Fe oxide that was recycled by oxidation with air.
A spot of googling confirms...
http://www.sotacarbo.it/uploads/documenti/1158736575_2005%20...
Page 5
Rosco Bodine - 1-2-2007 at 14:24
The thing is that Na2S is required in huge quantities by the paper pulp industry , so there is reason to want to make use of a sulfur recovery scheme
where the scrubbers produce a byproduct that can be sold ....
and doesn't return sulfur to the atmosphere , but ties it up in a useful compound which has value to industry .
I am not familiar with that amine based CO2 stripping
method . How is the MEA recycled and reused ?
cadimodo - 1-2-2007 at 20:56
MEA is recycled by using a thermal desorption process. Desorpted H2S will be sent to Claus process and transfered to S. But this method is used for
large-scale industrial purpose.
To unionised : This problem is common in chemical industry. But I want to design a small-scale device that could treat H2S and CO2 in a economical
way.
[Edited on 2-2-2007 by cadimodo]
Ozone - 1-2-2007 at 21:27
OK,
How small of a scale? I thought that if you were troubling yourself to produce biogas... then you were interested in volume, hence the ChE approach.
If you are talking a couple of L, a strategy would be oxidation to SO2. For small scale, a length of garden-type hose filled with a mixture of coarse
KMNO2 and C *should* suffice. OTT, the Fe fixation route sounds OK.
Best of luck,
O3
[Edited on 2-2-2007 by Ozone]
not_important - 1-2-2007 at 22:48
The iron-based route mentioned by unionised may be the best. Hydrated ferric oxide reacts with the H2S to give iron sulfides and water. The ferric
oxide is regenerated by exposure to air, which also forms elemental sulfur.
After a number of regenerations the sulfur content became high enough to reduce the efficiency of the process, the mixed iron oxideds and sulfur would
be worked up to recover both sulfur and iron.
The main consumables are air and some water to keep the moisture content in the proper range, and the occasional replace of the iron oxides. It may be
practical to solvent extract most of the sulfur from the iron oxides and then reuse the oxides, don't know.
cadimodo - 2-2-2007 at 07:31
Thanks so much. Next week, I'll do some experiments and I will tell you my results.
Keep posting, because your experiences are very important to me.