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Separating gasses

  1. Oct 10, 2006 #1
    When you have a mixture of HCl, NH3, SO2 and N2 gases. How would you separate these gases?

    HCL and NH3, when perfectly dry, apparently do not react. Even the tiniest amounts of water vapor make them react and then you obtain solid NH4Cl. This solid I would filter electrically.

    Separation of these 4 gases:

    NH3 + HCl react, giving a solid. But HCl or NH3 may remain, if their amounts are not precisely balanced. Next, bubble the remaining mix through H2SO4: Any excess NH3 will be absorbed, the rest is bubbling through. Of course, you separated the NH3, but not as free gas anymore.
    Next, bubble through a solution of NaOH. Both SO2 and HCl are captured. N2 is bubbling through.
    Finally, add excess dilute H2SO4 to the solution with NaOH/SO2/HCl and heat to near boiling. All SO2 is driven off, while hardly any HCl is driven off. The problem here is, I haven't separated the gases, I just put them all in water and threw them in the sea...So the end product is water with ions from: Cl-, SO42-, Na+. Of course you could boil this solution to get remove H2O then boil it to recuperate H2SO4 and then you get salt. Also, the SO2 that is repelled can be used to make H2SO4 with O2 combustion.

    Now, what about using CaO or CaCO3 as wet scrubber? I've read somewhere CaO would react with 2 HCl to form CaCl2 and water. Also CaO and SO2 reacts to CaSO3. I'm not sure at which temperature this occurs... Problem is, the slurry is a real mess and you can't recuperate any of it or am I wrong?

    There's also this one: using Zeolite as molecular sieve you can get rid of SO2, HCl and water vapor. Again a solution of molecules that needs to be taken care of.

    And a bit weird: When you only have HCl and SO2 in N2 you could decrease the temperature to -20 to make liquid SO2. How about that? Then you could recuperate HCl by bubbling it through water.

    Apparently you could also just burn it all up and make fertilizer with HNO3, H2SO4. But I think this isn't an option when the concentrations aren't that high...

    So all these alternatives, which one is really the best...

    PS: Now you use H2SO4 to get rid of NH3. Maybe it is better to turn SO2 into H2SO4 with combustion with O2. So you don't need to buy H2SO4 all the time. The problem is: would HCl react at this high temperature? An alternative is using a selective solvent to capture HCl and then desorbing it to have pure HCl. The SO2 would then be pure and you can make H2SO4 from it to reflux it back to NH3. But I don't know if there is a solvent that selects HCl more than SO2...
    Last edited: Oct 10, 2006
  2. jcsd
  3. Oct 10, 2006 #2


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    Are you looking for "clean" gases? Or, just separate "sinks" for each component?
  4. Oct 11, 2006 #3
    Those concentrations of HCl, SO2 and NH3 are low. So I think just separating them from N2 is the goal. But if there is an efficient way to recuperate any of it, it's better.

    I would really think NH3 + H2SO4 then, CaCO3 + HCl and then CaOH2 + SO2 would be the best solution.

    The CaSO4 and CaCl2 could be dumped as waste.

    Note: the absolute main goal is that the air does not exceed the emission standards of VLAREMII.
  5. Oct 11, 2006 #4


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    Anything wrong with water scrubbing?
  6. Oct 12, 2006 #5
    Yeah, SO2 is not really reacting good without alkaline solution scrubbing.

    I'm talking about:

    54 kg/h HCl
    47 kg/h SO2
    15 kg/h NH3
    maximum: 1000 m^3/h

    1-2 tonnes SO2 or HCl a day, is that worth separating?
    Last edited: Oct 12, 2006
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