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Author: Subject: separation of nitroethane from 2-nitropropane?
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thumbdown.gif posted on 8-2-2011 at 00:15
separation of nitroethane from 2-nitropropane?

the boiling points of nitroethane (112-116C) and 2-nitropropane (120C) are too close to fractionate, the only thing coming to mind is azeotropic distillation. US Patent # 3480517 states that heptane forms an azeotrope with nitroethane and 2-nitropropane does not. the heptane/nitroethane azeotrope is 72% heptane and 28% nitroethane and it boils at 89C. that's a nice enough gap to fractionate with no problem (89C vs 120C). there will be mechanic losses due to some solubility of nitroethane in heptane, the patent states that when the azeotrope distills over and separates into 2 different layers, that the top heptane layer will contain 10-12% heptane. that's not bad, BUT the problem with that patent is that the math doesn't add up and make sense in that patent and it turns out the mechanical losses will be very large due to the sheer volume of heptane in the azeotrope.

the patent states to only use 25% w/w heptane compared to the amount of nitroethane. that sounds completely backwards to me knowing the ratio of the azeotrope is primarily heptane. the examples in the patent use varying ratios of heptane, anywhere from approx 30-50%. the only way that makes sense is if they use, say, only 30% w/w heptane to nitroethane. then distill all the heptane over azeotropically then return the heptane back to the distilling flask and let it distill over azeotropically to bring more nitroethane, little bit by bit, continually returning the heptane to the distilling flask until the heptane comes over at it's normal bp which would signal that there is no more nitroethane to form an azeotrope with. is what what they're doing in the patent? the problem with that is that it'd take forever to distill because the heptane azeotrope only brings over 28% nitroethane when it distills over and nitroethane is 10-12% soluble in it. that means after the first pass, when you recycle the distilled heptane by putting it back to the distilling flask to continue azeotropically distilling the nitroethane it will have 10-12% nitroethane already, meaning you'll only be distilling over 16-18% nitroethane at a time from that point forward. sounds like it would take 9 distillations to get all the nitroethane at that pace. otherwise you use a ton of heptane and end up losing half of the nitroethane due to its 10-12% solubility.

am i missing something or does anyone have any suggestions?

i attached the patent

Attachment: Nitroethane, Purification by Azeotropic Distillation US3480517.pdf (118kB)
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