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katchum
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If someone replies, but thanks a lot for all your help!
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katchum
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Just to make sure:
Can someone please verify to me that when I add NaOH to a level of pH 12 that all SO2 will convert to SO3--?
See this figure:
Attachment: so2.pdf (61kB)
This file has been downloaded 729 times
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woelen
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Keep in mind that adding SO2 reduces the pH, so you have to add fresh NaOH all the time to keep pH at 12.
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unionised
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If you add enough NaOH to get the pH up to 12 all the SO2 will be converted to sulphite. In fact, most of it would be converted at pH9
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katchum
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Okay, I'll make sure the pH is kept at 12.
I could calculate that this way:
When you have 0,2 mol/s SO2 and you want to have 0,01 mol/l OH- (pH 12) in 2 l/s liquid in the end (or 0,02 mol/s), we can calculate the begin pH
necessary to keep pH at 12.
I know 1 mole of SO2 reacts with 2 moles of OH- so 0,2 mol/s SO2 reacts with 0,4 mol/s OH-. 0,4 mol/s + 0,02 mol/s = 0,42 mol/s at 2 l/s.
So the pH at the beginning is: 0,21 mol/l or 13,3 pH.
Summary: To keep a pH of 12 I need a pH of 13,3 in the beginning to absorb 0,2 mol/s SO2 with liquid flow of 2 l/s.
I could use a pH of 10, maybe that's better for cost. But I have to be sure that the reaction occurs super fast. So it's for some safety protocol or
something.
[Edited on 27-11-2006 by katchum]
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woelen
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You should also keep in mind that the curves you posted are equilibrium curves. They do not tell how fast an SO2/NaOH mix reaches the equilibrium and
how well the SO2 is absorbed by the solution. With a pH, equal to 10 (and sufficient buffering to keep it equal to 10), most SO2 will be absorbed,
when left in contact for sufficiently long time, but I can imagine that simply bubbling SO2/air mix through such a liquid does not remove all of it.
Maybe you could use some opposite flow mechanism. Liquid is gpoing in one direction and SO2 is going in the other direction. Where highest
concentration of SO2 is encountered, the liquid has lowest pH, but at that point, this is not of real concern, because a lot of SO2 can be absorbed
yet. Higher up in the scrubber, more alkaline liquid is used and at the top, it is most alkaline. Over there, the last traces of SO2 can be absorbed.
In this way, you could save on costs of NaOH, using a pH equal to 12 or even higher near the end, where concentration of SO2 already has dropped
considerably, while using a lower pH of 10 or so, at the start of the SO2 stream, where concentration still is high.
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Maya
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You can do away completely with almost all the NaOH by freezing the SO2 first below the B.P. , -15 or so?
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katchum
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I'm doing it counterflow, yes.
How do I estimate the kinetics of the reaction? I really don't know, so keeping the pH high enough is the message. It's a matter of mass transfer,
people who sell scrubbers will give HTU data and we can rely on that. Also, I'm not just bubbling it through NaOH, I'm using a packed scrubber which
gives huge turbulence and good mass transfer. So I hope it will reach equilibrium at every HTU.
Freezing the gas. Hmm, maybe, but you need an installation for cooling which requires space and besides you still need to build that scrubber. Also
that liquid SO2 has to be neutralised so it won't get into the air again. How are we going to do that? (with NaOH? So work for nothing)
You could think, maybe make H2SO4 out of that SO2 but I know there will be a huge amount of HCl in the liquid making things difficult.
One simple and clean scrubber is the solution.
[Edited on 27-11-2006 by katchum]
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