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Author: Subject: How do you filter the air going to your vacuum pump?
Siconic
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[*] posted on 7-3-2010 at 18:32
How do you filter the air going to your vacuum pump?


So I have done a lot of chemistry, and I try to be safe as I can be. I haven`t delt with a lot of distillations under vacuum and one of my first concerns is filtering the air going into the pump. The reason this is a concern is two fold:

1. I am using my distillation setup to make pure nitric acid, with low NO2 concentrations. I will be using the same vacuum pump to distill H2O2 to higher concentrations for catalyst reactions.

2. My petcock on the pump is copper(or brass).

Knowing what I know about chemistry, I know that HNO3 and H202 mixing, could be very bad especially in a pump. During my first test run, in which I took Red Fuming Nitric and put it under vacuum to draw off the NO2, the gasses began to attack my petcock on the pump. I suspect this is partially because there was some moisture in the vacuum line, but even so, I would like to filter the gasses before they get to the pump.

Any suggestions?

The first thing that comes to my mind is water filtration, but if I am dealing with a substance that evaporates at a higher atmospheric pressure than water, I may get some water in my final product.
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DJF90
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[*] posted on 7-3-2010 at 18:43


Generally you can protect a vacuum pump by:
> Employing a cold trap to condense out organic vapors. Obviously its effectiveness depends on the cooling media, e.g. dry ice/acetone or LN2; with the latter, be cautious of oxygen condensing in the trap.
> Employing a "drying tube" filled with NaOH prills or the like; this will remove acid gases on their way to the pump. This would be a great idea to use for HNO3 distillations.
>For some molecules, a drying tube filled with activated carbon will prevent them reaching the pump; ammonia is one such molecule that is generally absorbed.

Using a combination of these traps will help keep your pump in a healthy condition. Often if you have been neglectful of your pump and it has started to corrode, changing the oil may help. Ideally its best not to let your pump corrode in the first place. These are only the suggestions I could think of off the top of my head; I'm sure there are many others out there.
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hissingnoise
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[*] posted on 8-3-2010 at 03:53


Quote: Originally posted by Siconic  
During my first test run, in which I took Red Fuming Nitric and put it under vacuum to draw off the NO2, the gasses began to attack my petcock on the pump.

You could have simply blown dry air through the acid to decolourise it.
Or added a small quantity of urea. . .
And why use vacuum for HNO3; distillation at ordinary pressure will give you acid strong enough for practically any nitration if your H2SO4 is 98% and your KNO3 id dry?

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ninefingers
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sad.gif posted on 8-3-2010 at 07:06


I don't know Any way of completely removing vapors/corrosives from the intake of a vacc pump. I've tried a liquid trap, then a filter of dried salt; still lost a Welch 1500.:(

The best thing to do is use a plastic/glass, or diaphragm pump you can just wash out.

Or just something you don't care that much about. I use my Harbor Freight $89 compressor--hook input to what I'm exhausting, then open the drain so air can flow unimpeded.

Even after spraying a lot of WD-40 after wards, I still's have to replace reed valves every so often. A complete head is still available for my comp for only $9. I will try stainless valves some day.

I do a lot of vacuum filtering of Chlorates I am refining by freezing--they must be filtered Quickly before the ice melts.

You can as also try a cheap plastic ($9) vacuum aspirator from a pet store. I have no close source of water to my shop for one, though. and water is Expensive in this desert.
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[*] posted on 8-3-2010 at 09:43



Quote:

You could have simply blown dry air through the acid to decolourise it. Or added a small quantity of urea. . . And why use vacuum for HNO3; distillation at ordinary pressure will give you acid strong enough for practically any nitration if your H2SO4 is 98% and your KNO3 id dry?


Not worried about it being decolorized... I need it to have almost no nitrogen oxides present. I have heard that method works, but Nitric acid decomposes in open air. Plus its an extra step. I could just distill the H2SO4 and KNO3 under vacuum and get a more pure product, and higher percentage.

Wouldn`t urea reduce the concentration? And potentialy form urea nitrate? Not to mention making my nitric Impure.... But I do agree, 70% is plenty good far almost any reaction.

Speaking of, is there any way to distill 70% with the H2SO4 and KNO3, or do I just have to dilute it after distillation?
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[*] posted on 8-3-2010 at 10:19


Use weaker H2SO4, and you should get azeotropic HNO3 (68%) coming over when you distil. You should be able to find its boiling point online so you know what to look for..
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