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Author: Subject: Separate Isopropyl Alcohol and Xylene
nimgoldman
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[*] posted on 11-6-2018 at 10:45
Separate Isopropyl Alcohol and Xylene


This is my first topic and I am just looking around, so please excuse me if this question has already been aswered somewhere else here.

I tried to purify isopropanol contaminated with aqueous HCl, xylene (mix of isomers) and water.

These are the actions I took:

1) neutralization of HCl with excess sodium carbonate
2) filtering
3) simple distillation
4) drying with magnesium sulfate (this should remove water and break the possible IPA-xylene azeotrope, right?)
5) fractional distillation (this should separate IPA and xylene)

Unfortunately, in the last step, the thermometer shows constant 79.6 °C (slowly raising to 80.0 as half of the alcohol is distilled) instead of 82.6 °C, which would be the correct b.p. for IPA.

The product still somewhat smells of xylene even though I am using very long Vigreux column and removed the initial low boiling fraction.

Is there a way to remove the xylene? I've read that aromatic hydrocarbons (BTX) can be destroyed by sulfonation, i.e. adding conc. sulfuric acid, but I haven't found a proper way to do it and I don't want to cause explosion...

Another tips I found is to treat the alcohol with sodium bicarbonate and activated charcoal to remove smells. This works for ehtanol, I am not sure about IPA though...

I have consulted the "Purification of laboratory chemicals" textbook, the section about general purification of monohydric alcohol, though I have not found a step that would remove traces of aromatic hydrocarbons.

As a last resort, I will put the xylene-smelling isopropanol in a separate bottle and use it only with xylene as it seems removing it completely is too much pain.
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DavidJR
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[*] posted on 11-6-2018 at 11:26


The conditions for sulphonation of xylenes (hot sulphuric acid) would lead to the formation of diisopropyl ether.

One method of separating xylenes and isopropanol would be to add water. Xylenes are practically insoluble in water, but isopropanol is miscible with water in every proportion. Of course, washing with water will mean your isopropanol will become diluted significantly.

How much of this do you have? If it's only a small amount i'd just discard it or use it for non-critical applications. Isopropanol is cheap.

[Edited on 11-6-2018 by DavidJR]




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happyfooddance
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[*] posted on 11-6-2018 at 11:39


Their might be a ternary azeotrope, similar to the ethanol/xylene/water one.

The only thing I see glaring is your step #4. MgSO4 is not strong enough a dessicant to dry IPA thoroughly. Try NaOH.
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nimgoldman
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[*] posted on 11-6-2018 at 11:42


I have less than a liter of it, so it's not a big issue. But I will often use it with xylene so regularly throwing out liters of it (contaminated with an environmental pollutant) is not an option for me. The xylene content is okay as long as there is no water in it, so I will keep it in a separate bottle.

I have mixed the initial fraction with excess water in the erlenmeyer flask. The water immediately turned cloudy with a droplets travelling and combining at the top. This looks like xylene. It seems even that amount of water (10:1) is still not enough to drive all the xylene from the alcohol.

I wonder why it stays in it since xylene-IPA azeotrope (neither xylene-IPA-water) does not exist (AFAIK, at least I have not found it in azeotropic tables) and the boiling points are far away from each other. Could it be that xylene is so volatile that isopropanol vapours would drive it all the way through the column?

[Edited on 11-6-2018 by nimgoldman]

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[Edited on 11-6-2018 by nimgoldman]

[Edited on 11-6-2018 by nimgoldman]

[Edited on 11-6-2018 by nimgoldman]
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DavidJR
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[*] posted on 11-6-2018 at 12:07


Quote: Originally posted by nimgoldman  
I have less than a liter of it, so it's not a big issue. But I will often use it with xylene so regularly throwing out liters of it (contaminated with an environmental pollutant) is not an option for me. The xylene content is okay as long as there is no water in it, so I will keep it in a separate bottle.

I have mixed the initial fraction with excess water in the erlenmeyer flask. The water immediately turned cloudy with a droplets travelling and combining at the top. This looks like xylene. It seems even that amount of water (10:1) is still not enough to drive all the xylene from the alcohol.

I wonder why it stays in it since xylene-IPA azeotrope (neither xylene-IPA-water) does not exist (AFAIK, at least I have not found it in azeotropic tables) and the boiling points are far away from each other. Could it be that xylene is so volatile that isopropanol vapours would drive it all the way through the column?



If it's cloudy then you have a suspension of droplets - you'd need to leave it longer for it to settle out (or sometimes adding a salt to increase the density of the aqueous layer helps - then again, if you add too much you'll salt out the isopropanol). I wouldn't think it's necessary to use anywhere near 10:1 water:isopropanol.

Consider using it as a fuel - should burn cleanly if it's mostly isopropanol.

Given that there apparently is no azeotrope you should be able to separate the mixture by fractional distillation. Perhaps you need to run the distillation slower (and insulate the column) or obtain a more efficient column.


[Edited on 11-6-2018 by DavidJR]




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nimgoldman
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[*] posted on 11-6-2018 at 15:49


> I wouldn't think it's necessary to use anywhere near 10:1 water:isopropanol.

This was just a test on a sample so I used excess water to push all the xylene out.

> Consider using it as a fuel - should burn cleanly if it's mostly isopropanol.

I don't have any use of that unfortunately. Would it work in an alcohol lamp?

> Given that there apparently is no azeotrope you should be able to separate the mixture by fractional distillation. Perhaps you need to run the distillation slower (and insulate the column) or obtain a more efficient column.

I distilled very slowly while mixing to prevent bumping. The temperature never exceeded 80.6 °C which is nowhere near the b.p. of xylene NOR isopropanol - this puzzles me completely.

Maybe my thermocouple thermometer is not accurate enough and shows temperature 2.0 degrees below the actual one.

I am skeptical about the column insulation. I read on the forum that insulation really works only if you have a splitter head. Having the column insulated otherwise removes any possibility for condensation on the plates, effectively reducing the fractional distillation into simple distillation. It makes sense only for high b.p. liquids where the vapours would not even reach the head without insulation.

For example, I usually get very good separation of ethanol:water (94% from 80%) using the 60 cm uninsulated Vigreux column, but for some reason it does not work for isopropanol:xylene even though the boiling points of the two liquids much further away than in ethanol-water mixture. Actually simple distillation should work just fine for that purpose.

I have dried the IPA with NaOH to remove traces of water, then stirred it several minutes with activated charcoal. I will then attemp another fractional distillation over NaOH and be super-careful with the heating ... and maybe try different thermometer too.

I really want to know why the two compounds don't separate even though they should.
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happyfooddance
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[*] posted on 11-6-2018 at 15:57


"Isopropyl alcohol forms an azeotrope with water, which gives a boiling point of 80.37 °C"

I don't see what is so confusing. As I already said, magnesium sulfate is not a good drying agent for isopropanol.

Also, I have seen no conclusive anything that says there is no ternary ipa/xylene/water azeotrope, and there is one with ethanol/xylene/water, but at the very least try legitimately drying your mix first, that's kinda standard for getting a clean separation.
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SWIM
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[*] posted on 11-6-2018 at 16:04


I'd be tempted to try adding it drop-wise to hot water in a still:

The isopropanol may separate cleanly because of the high ratio of water to solvent, and then boil off as a 91% mixture leaving most of the xylene behind. Just a bit of water will make for a high ratio, because the isopropanol will be continuously removed as it's added.

Co-distillation could be a problem, but might not be too bad with such a large BP difference.

I am surprised the fractionation didn't do the job. What kind of column did you use?





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walruslover69
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[*] posted on 12-6-2018 at 05:08


Is the solution a mixture of xylene and 91% IPA or pure IPA? xylene does have an azeotrope with water that boils at 92C. The difference in Bp between the IPA azeotrope and xylene azeotrope would therefore only be 12 degrees.
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weilawei
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[*] posted on 12-6-2018 at 05:20


12°C ought to be enough with slow, careful fractionation on a good column. I've been working with an ethyl acetate/n-hexane/MeOH system lately, and that gives about a 13°C separation. It works fine if you go slowly (less than a drip a second).
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nimgoldman
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[*] posted on 12-6-2018 at 06:51


Quote: Originally posted by walruslover69  
Is the solution a mixture of xylene and 91% IPA or pure IPA? xylene does have an azeotrope with water that boils at 92C. The difference in Bp between the IPA azeotrope and xylene azeotrope would therefore only be 12 degrees.


It's pure IPA. It was 99.5% from the vendor (0.5% being mostly water) but dried with excess anhydrous MgSO4 prior to mixing with xylene. Addition of NaOH to further dry the IPA-xylene mixture has not caused any clumping showing the mixture was really dry.

So now it should be practically just water and xylene.

I will try more careful distillation, thanks.
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[*] posted on 12-6-2018 at 07:32


NaOH doesn't clump in IPA, like MgSO4 does. It liquefies, or the flakes/prills/powder swells (even in dry IPA NaOH expands if given time).

It will look like a snowglobe, but it doesn't clump.
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[*] posted on 12-6-2018 at 08:15


If you aren't sure about how accurate your means of measuring the temperature is just boil some pure isopropanol and/or water and measure its temperature.

You didn't mention your location, are you trying to distill alcohol on top of a mountain?
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nimgoldman
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[*] posted on 13-6-2018 at 13:33


Quote: Originally posted by kulep  
If you aren't sure about how accurate your means of measuring the temperature is just boil some pure isopropanol and/or water and measure its temperature.

You didn't mention your location, are you trying to distill alcohol on top of a mountain?


Central Europe.

Today I distilled practically pure toluene and the thermometer at the still head said 109.7 °C (correct should be 111 °C) ... It's the TM902-C with wire probe placed in a glass thermometer adapter. I don't know. Maybe it's because of the stormy weather (lower atmospheric pressure?)

I tried several thermometers but they are not accurate enough. Maybe if I could somehow stick the probe right in the still head (but I have no idea how to insulate the probe so vapours won't escape), it would measure more accurately. I also tried adding little bit of liquid in the thermometer adapter so the probe is submerged instead of just touching the glass, but the result was the same (showing somewhat lower temp.)

I live about 300 meters above the sea level and one b.p. correction calculator showed me over 1 degree centigrade offset for b.p. of water. This is interesting. Maybe it would be a good idea to use a barometer and correct the boiling point to achieve proper separation.

In practice, it seems that once the isopropanol is contaminated with xylene, it will be stinky forever. Is there any way to purify it back to "reagent/synthesis grade" ?

I really like to reuse/recycle my solvents, even if it's cheap IPA. First, for enviornmental reasons and second, it would be a pain to re-order liter of solvent every month or so.

[Edited on 13-6-2018 by nimgoldman]

[Edited on 13-6-2018 by nimgoldman]
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walruslover69
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[*] posted on 14-6-2018 at 05:49


are you 100% sure your contaminant is xylene? What source of xylene are you using? unless its analytical grade there might be an impurity of toluene or benzene which have azeotropes at 80.6C and 71.9C respectively.

I think what is most likely happening is the main contaminant is probably toluene or benzene. This is causing the water to be cloudy.
I think there is probably very little xylene in your solution. from what i have read the detection threshold for smelling xylene is something like 0.1PPM so you might have have a continent of a few ppm.
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[*] posted on 14-6-2018 at 08:16


Xylene that's technical or industrial grade can have 10-15% ethylbenzene in it as well.

That has some limited solubility in water, but I don't know what it smells like.




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