NaOCl + NaOH for biogas treatment

[lfbg]

Hello,

Could anyone help me establish the real chemical reaction in a biogas treatment facility to remove H2S?
Biogas composition is 50% CH4 and 40% CO2 and can have up to 1300 ppm of H2S. It is now treated as I told before, with NaOH and NaClO.
I have so far that:
H2S + 4NaOCl + 2NaOH --> Na2SO4 + 4NaCl + 2H2O
but, because of the amount of CO2 I am trying to find what other reactions are taking place in order to optimize the plant performance.

thanks,

 

[siretb]

Because pH is well above 9, you also capture some CO2
CO2 + 2NaOH --> Na2CO3
Depending of the water hardness, you may scale or precipitate.

 

[lfbg]

Thanks,
So, I have so far three reactions to purify biogas:

- H2S + 4NaClO + 2NaOH --> Na2SO4 + 4NaCl + 2H2O
- H2S + NaClO --> NaCl + H2O + S
- 2NaOH + CO2 --> Na2CO3 + H2O

is this correct? how can I find the proportion of each one? (I found that the second one is only 1% of the whole reaction)

 

[bimr]

Numerous other postings have addressed this issue. See thread127-121447 for example.

Where removal of ammonia and other nitrogen compounds is required, a two-stage scrubber system using a dilute sulfuric acid solution in the first stage is typically used. The ammonia reacts with the sulfuric acid to form ammonium sulfate, a soluble, non-volatile salt, which is removed from the scrubber effluent through the overflow.

1. NH3 absorption section

Gas passes through the main scrubber. In the first stage, absorption reactions are as follows:

2NH3+H2SO4 -> (NH4)2SO4-------------(1)
(CH3)3N+H2SO4 -> ((CH3)3NH)2SO4-----(2)

2. H2S absorption section
In the second stage H2S and other acid gases are absorbed. For example,

H2S+4NaClO -> Na2SO4 + 4NaCl-----(3)
H2S+2NaOH -> Na2S+2H2O-----------(4)

By using NaClO, acid odor gases can be efficiently absorbed. It is very important to control pH and ORP for the purpose of successful absorption. If not controlled, trouble such as sulfur particles and Cl2 gas will arise

The most common method of control of H2S gas is to pass the smelly gas through a vertical, packed bed wet scrubber. The air passes up the tower as the scrubbing liquid containing caustic (NaOH) and oxidizing agent (most often bleach or NaOCl, sodium hypochlorite) flows down the tower in counter-current fashion. The high pH provided by the caustic drives the mass transfer from gas to liquid phase by solubolizing H2S as HS- bisulfide and S-2 sulfide ions. Once in solution, the reaction between hydrogen sulfide and oxidizing agent is almost instantaneous (assuming sufficient oxidizing agent is present). This reaction converts the sulfide to sulfate (SO4-2) ion. The overall chemical reaction is described by the following equation:

H2S + 4NaOCl + 2NaOH Na2SO4 + 4NaCl + 2H2O

Therefore, theoretically, for each molecule of H2S destroyed, four molecules of bleach and two molecules of caustic are consumed. However, the chemistry is not quite so simple, as partial oxidation of H2S also takes place that forms elemental sulfur:

H2S + NaOCl NaCl + H2O + S?

This reaction represents about 1% of the chemistry present in a wet scrubber. The presence of excess bleach helps to minimize the formation of elemental sulfur. But bleach is an expensive chemical. The use of two stage scrubbing is often employed both to minimize chemical consumption as well as to control sulfur deposits when scrubbing H2S. The first stage operates at~ 80% efficiency and uses a caustic only scrub at high pH (~ 12.5). The air then passes to the second stage, where the remaining H2S is scrubbed with caustic / bleach solution at pH ~9.5. The H2S present is destroyed at 99%+ efficiency. The blowdown from the 2nd stage, which will contain some amount of unused NaOCl, is sent to the sump of the 1st stage. In this way, additional H2S is destroyed and maximum consumption of expensive oxidizing agent is assured.

Never the less, there are losses of chemicals which cannot be prevented, which of course raise the cost of odor control scrubbing. These losses are due to the facts that bleach, NaOCl, slowly decomposes in storage as well as the fact that some amount of caustic is constantly lost to CO2 absorption in both scrubbing stages.

 

[lfbg]

Thanks

 

[jarabina]

An alternative to chemical removal of H2S is a system I saw in a WWTP here in Slovakia. Regulated ammount of air is being supplied into anaerobic tank and sulphide-oxidizing bacteria do their job. Efficiency of H2S removal is around 80%. Air in biogas is safely under 1%. The supplier for this is

http://www.KH-KINETIC.cz

jarabina