NaOCl+NaOH scrubber 2

[gncm]

am trying to design a two stage scrubbing treatment for the waste water treatment plant. The first stage with H2SO4 to threat NH3, and the second one with (NaClO+NaOH)

My problem is that I'm not sure which are the reactions of the second stage, and in which order they occur.
In the gas flow, there are H2S + CH3SH +DMS +DMDS.

 

[NeedAHoliday]

gncm,

You state your chemical so this should be bread and butter for you. I will also assume this is for a waste water treatment facility as this would explain the breakdown. In any event the reactions you want are below:

H2S + 4NaOCl + 2NaOH --> Na2SO4 + 4NaCl + 2H2O
CH3-SH + NaOH + 3NaOCl --> R-SO3Na + 3NaCl + H2O
CH3CH2-SH + NaOH + 3NaOCl --> R-SO3Na + 3NaCl + H2O
CH3-S-CH3 + NaOCl --> CH3-SO-CH3 + NaCl

With respect to your query over which are harder to scrub? I can only suggest that you contact a vendor experienced in this type of design work. However I will try to explain briefly.

H2S can be easily scrubbed at high pH due to NaOH and you will see high conversions. The DMS, DMDS and Mercaptan are harder to remove and are dependent on the NaOCl conc. Also to put a spanner in the works the DMS and I would assume DMDS are better removed at almost nuetral pH.

The design needs to therefore be done so as to have the correct operating pH and ORP. It is not uncommon to see very high packing heights for this type of application and I have done designs wherein I have had 7.5m of high efficiency lower pressure drop packing to maintain a 99.9% efficiency on outlet stream. Alot of factors will impact on the design so as I said if you are not familiar with this type of design work I strongly suggest you contact a packing supplier and ask them for the base process design along with process guarantees for aforementioned design.

Remember to take care of the pH and ORP set points as these will make or break the process efficiency for this type of scrubber.

Good Luck!!

 

[NeedAHoliday]

Re-read your post, I don't have to assume waste water as it is written.

Should have read a little slower.

Cheers.

 

[bimr]

Not sure why you are trying to design something like this. You can buy this equipment already designed. What you need to do is to characterize the concentration of pollutants so that the equipment can be sized correctly. Contact this firm for the scrubber:

http://www.usfilter.com/NR/rdonlyres/96FEC24D-09D3-444C-BBB6-B798E4947836/0/LOPRObrochure0804.pdf

 

[gncm]

NeedAHoliday,

Your answer has been very helpful. You were right in thinking that the design was for a wastewater treatment plant.

I have a doubt of the reactions:

CH3-SH + NaOH + 3NaOCl --> R-SO3Na + 3NaCl + H2O
CH3CH2-SH + NaOH + 3NaOCl --> R-SO3Na + 3NaCl + H2O
CH3-S-CH3 + NaOCl --> CH3-SO-CH3 + NaCl

If there is NaOCl in excess the oxidation of CH3SH and CH3CH2-SH won’t finish in R-SO42-.
And why in the last reaction doesn’t participate the basic conditions.

 

[watapro]

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.