densest
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concentrating sulfuric acid electrolytically
http://www.docstoc.com/docs/41773598/Sulfurless-Electrolytic...
is interesting in that it asserts a method of concentrating sulfuric acid beyond 100% at moderate ( < 100C ) temperatures.
The patent uses chlorine or chlorine compounds as oxidizing agents to prevent reduction of sulfur at the cathode. This could render the acid useless
though HCl is considerably volatile and might be removed with heat. Alternatively, H2O2 added locally might be sufficient to prevent sulfur
deposition.
The electrolysis might conceivably move metal contaminants to the region of the cathode. Fractional removal to gradually purify the acid might be
possible.
Has anyone heard anything about anyone using this in practice?
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hissingnoise
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This subject has been discussed before; the obvious stumbling block being the need for substantial Pt electrodes.
And there seems be be some doubt as to whether oleum would be the product if the reaction went beyond 100% H2SO4.
If one had deep enough pockets it would be a fascinating project with the promise that oleum of almost any concentration might be produced at will.
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watson.fawkes
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Google Patents link: U.S. Patent 4003816
What strikes me about this patent is that the purity of the sulfuric acid just doesn't matter for the purpose of the device, which is to remove water.
This isn't so much a sulfuric acid concentrator as a water removal device that uses sulfuric acid as a transfer medium for water. The way it
eliminates sulfur reduction is with what is essentially a sacrificial reductant, which they call an oxidizer (not inaccurately); this is the role of
the chlorine compounds in the process. The upshot is that there's going to be some steady-state concentration of the sacrificial reductant in the
solution; it's necessary for the concentration process to work. So the question comes to that whether a particular application for sulfuric acid can
use a mixture containing some concentration of oxidizer (and oxidizer/electrolysis byproducts). It doesn't seem like a feasible technique for
producing reagent-grade acid.
H2O2 seems promising, but the problem is that you're adding water as well, and that water needs removal. At best this decreases the current efficiency
of the process. The molar efficiency of Cl2 as a sacrificial reductant is pretty good. In Example 4 in the patent, they used 6.3 mmol of Cl2 to remove
306 mmol of H2O. (I should point out that it's really clear that the patent was filed based on lab-scale experiments and they hadn't yet built a
pilot-scale plant.) If the molar efficiency of H2O2 is as good as Cl2, then it might work. You'd still have residual H2O2, but that might be solved
with a bit of SO2.
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hissingnoise
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It's unlikely that sulphur deposition will be a problem at the labscale if you're not processing large quantities of acid at high current densities.
With suitable electrodes, removing water from say, 50% H2SO4 should be fairly straightforward.
The co-production of H2S2O8 might complicate things however. . .
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