[TortoiseWrath]

I am using acetone to remove water from silica gels used in the production of aerogels. With the system I am currently using, I am replacing the acetone four times for every gel. However, I am looking to increase my production dramatically on account of demand, and this would lead to the use of 200-liter drums of acetone instead of small bowls.

Consuming 800 liters of acetone for every batch seems inefficient to me, so I'm wondering if it would be possible to distill the used acetone and reuse it. Because the boiling point of acetone is 43 K lower than that of water, it seems to me that it would be feasible to use simple distillation for this purpose.

The problem I'm running into is how I could possibly distill 200 liters at a time of mostly acetone. Would the following procedure work?

• Have another drum on the ceiling above the drum containing the mixture
• Connect a pipe with a valve in it between the drums
• Heat the lower drum (or both drums?) to, say, 70°C
• Wait a while
• Close the valve to trap the acetone gas in the upper drum
• Turn off the heat
• Replace the lower drum, now hopefully containing just water, with an empty one
• Open the valve, allowing the acetone to flow back into the lower drum

This seems like it would be far too simple to work, so what would the problem be? It's been a while since I've done distillation and I'm terribly unfamiliar with equipment on this scale, so please explain like I'm five.

[Enthalpy]

Water and acetone make no azeotrope at normal pressure so some distillation method will work. The purity you need decides the number of steps - hum.
http://www.solvent--recycling.com/azeotrope_1.html

To the very least, the setup you propose is far too dangerous. Acetone is highly flammable and its vapour shouldn't be breathed, so heating a drum is the wrong way, worse within a building.

Companies supply distillation apparatus, you should consult them. Or ask your acetone supplier if it would recycle the product.

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Until you have the distillation apparatus, you can save much acetone by using successive acetone baths for the silica gel batches. That is:
- Each silica batch passes through successive baths, the first acetone bath contains some water, only the last is very dry.
- Acetone will absorb humidity. Once the wettest bath gets unusable, discard or recycle it - but only this one.
- Introduce a new very dry acetone bath. Degrade all others one step: the previous very dry bath is the new almost-dry one, the previous almost-wet bath is the new wettest one.

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My preferred process would go a bit differently: I'd let acetone vapour dehydrate the silica gel, as is done with vapour degreasing, vapour cleaning...

Have the acetone at the bottom of a container where you heat gently. Put the silica gel at the top, outside the liquid, and keep it colder than the liquid - or cool the acetone vapour independently, above the silica gel.

That way, you can accept more water in the acetone, since evaporation favours acetone. The vapour, possibly after condensation, will catch water from the silica gel and fall back into the liquid bottom.

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The multistep scheme previously described applies here as well, and you can combine the successive dehydration stations of silica gel with a fractional distillation of acetone and water, in a single apparatus.

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Please be careful with that! My intuition tells you lack the necessary qualifications for the process. Ask a specialized company. To the very least, don't tinker within a building! Maybe you can adapt a dry cleaning machine
http://en.wikipedia.org/wiki/Dry_cleaning
but chances are that acetone destroys the seals, and you get vapour everywhere even before.

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Could dry nitrogen be good enough, instead of acetone? End users just put the silica gel in an oven to dry it. You could cycle the nitrogen, with a good heat exchanger, between a chilled end where water condensates away, and a hot end where the gas carries humidity away from the silica gel. A regenerative heat exchanger and a pulsating gas movement, like in a Stirling engine, would make it more compact.
http://en.wikipedia.org/wiki/Stirling_engine

[TortoiseWrath]

the setup you propose is far too dangerous

Yeah, that's about what I figured. Stupid chemicals, always having properties... 

Or ask your acetone supplier if it would recycle the product.

This is not applicable, for reasons that will become evident below.

let acetone vapour dehydrate the silica gel

I'll try this; I hadn't thought of that. I don't know how well it will work in my application.

My intuition tells you lack the necessary qualifications for the process.

Indeed...

Ask a specialized company.

...unfortunately, not indeed. I live in an incredibly rural area (not something I can easily change). "Incredibly rural" as in "the distance to the nearest people qualified to do this is about 500 km." Random Internet forums are about as close as I can get.

To the very least, don't tinker within a building!

I don't intend to; I have dedicated outdoors tinkering sites.

Could dry nitrogen be good enough, instead of acetone?

Alas, the application won't provide for this. I have some other ideas about acetone replacements; some experimentation will be necessary. The distillation idea is what I'm primarily thinking of at this time, however.

I'd previously been thinking that distillation apparatus were the way to go; I'd just been having trouble finding one with adequate volume capacity. Any recommendations?

[curiouscat]

My preferred process would go a bit differently: I'd let acetone vapour dehydrate the silica gel, as is done with vapour degreasing, vapour cleaning...

Have the acetone at the bottom of a container where you heat gently. Put the silica gel at the top, outside the liquid, and keep it colder than the liquid - or cool the acetone vapour independently, above the silica gel.

That way, you can accept more water in the acetone, since evaporation favours acetone. The vapour, possibly after condensation, will catch water from the silica gel and fall back into the liquid bottom.

-----

Good idea. But it has the same hazards as distillation.

[curiouscat]

Wonder if fractional freezing might be viable. They have a huge difference in Freezing Points.

Where are you located? In winter you might have an excellent, free freezing source.

If you adopt a modified stage-wise fractional freezing thawing cycle you might be able to get a large enhancement. With none of the dangers of heating acetone.

[Enthalpy]

I like the freezing method, but at what temperature?

For instance, ethanol and methanol are commonly used to produce real cold (like -70°C), by mixing them with water ice that they force to melt and absorb heat. I expect acetone to act the same way: prevent diluted water from freezing at 0°C - much like any antifreeze.

What kind of acetone dryness is sought? If it must be dried from 5% to 0.5% water, the temperature that lets the residual water freeze out must be really chilly.

[curiouscat]

I like the freezing method, but at what temperature?

For instance, ethanol and methanol are commonly used to produce real cold (like -70°C), by mixing them with water ice that they force to melt and absorb heat. I expect acetone to act the same way: prevent diluted water from freezing at 0°C - much like any antifreeze.

What kind of acetone dryness is sought? If it must be dried from 5% to 0.5% water, the temperature that lets the residual water freeze out must be really chilly.

True. I was trying to find a freezing curve for the system. That'd let us answer these questions more accurately.

[Archer]

(posted this on the wrong thread by mistake 

You definitely need a safer set up.

If you need it just for your process you can get a falling film, short path distiller which is a closed system (and therefore much safer). The solvent runs over a heat exchanger set to just above acetone bp. Then it gets condensed on a cooling plate which is in close proximity. You control the flow rate so that you have a continuous process.

This will at least work as a fist step, you may find that with the right conditions that this is all you need.

[Enthalpy]

This shall illustrate the successive acetone baths suggested in reply #1, which saves acetone: it gets a dry product but uses acetone until very moist. The upper part of the illustration is done often, with product batches; the lower part is done from time to time, when the baths are used. I believe the process is very well-known.

[Enthalpy]

This shall illustrate the combined fractional disitllation of the dessicating fluid (here acetone) with the progressive dessication of the product (silica gel in the original post), as suggested in reply #1.

It's a fractional distillation column where the product to be dessicated serves as random packing material - somewhat similar to the dry cleaning of clothes. The product enters the column where the dessicant is less dry and leaves the column where the dessicant is driest. The dessicant condenses and vapourizes at the product, getting driest at the top (in the case of acetone, which boils more easily than water).

Since the column can further eliminate water from the dessicant, the input dessicant needs not be very dry; then its inlet is lower than the top of the column. It should be pre-heated, or even vaporized partly, to disturb minimally the column's temperature profile.

To save dessicant, its outlet can contain much water - up to essentially water - in which case some permanent packing material can replace the product at the bottom.

Some, err, details are not shown on the sketch: how to introduce and extract the product (a screw pump may contribute to the solution), how to move the product in the column (superimposed trays? A dessicant boiling less easily than water, to reverse the product's path?)...

I didn't check if this is commonly done. Comments welcome as usual!
Marc Schaefer, aka Enthalpy

[curiouscat]

how to move the product in the column (superimposed trays?

Minor details?

[Enthalpy]

how to move the product in the column (superimposed trays?

Minor details?

  Mechanical engineers too will have the right to contribute their creative work...

Trays carrying the product could have slides that hold them to an other, or they could hang to chains running at the sides of the column. Trays are then introduced at the bottom with the product to be dessicated and removed at the top with the dry product.

A dessicant more difficult to boil than water would ease this point. Glycerine or polyols fit this criterion, but are viscous (possibly a general difficulty) and hard to separate from water. I suppose acetone has definite advantages.

[Enthalpy]

Maybe reverse osmosis can remove moisture from the many fluid compounds chemists want dry.

The overpressure is higher than usual, to leave permil or ppm moisture rather than a brine when desalinating seawater. The osmosis elements may need an adapted mechanical design.

At a company producing ultra-dry compounds, the adaptation could be worth it.

Marc Schaefer, aka Enthalpy