Help, Amount of CO2 Absorbed by NaOH while scrubbing H2S

[GhazaliSyed]

This thread is a continuation of thread127-153907.

I am designing a biogas scrubber to scrub H2S, but we do not want to scrub CO2. CO2 is 40% of the gas and H2S is 3000ppm.
In theory CO2 it will use a lot of NaOH while scrubbing H2S. Cana ny one let me know the ratio of CO2 absorbed with amount of H2S absorbed?
Or how can we control the system to reduce the CO2 absorption?
The gas flow is 624m3/h.

thanks

 

[bimr]

I don't think that it is possible to preferentially scrub the H2S without taking out the CO2.

Numerous other postings have addressed H2S removal. 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.

 

[dcasto]

NaOH with H2S and CO2 is an acid base reaction, selectivity is not going to happen.

You might be able to remove H2S with FeO like with sulfa-treat.

Selective amines like MDEA could get all the H2S an leave a large portion of the CO2

 

[siretb]

Selectity does happen because the mass tranfer coefficient of CO2 --> NaOH is not good.
By using NaOCl that oxidizes and "drags" H2S ,enhances the mass transfer coefficient, you'll keep CO2 capture to a minimum. pH and ORP control is critical. Beware of scaling with calcium, also.

 

[GhazaliSyed]

thank you all for your help.