Acetone/water separation idea 3

[dufftobeat]

I'm working on an idea that involves the use of an acetone/water binary mixture as a working substance.

Here is some background:
The working substance (acetone/water - low H2O concentration <5%) picks up water in a subsequent step, and is redirected to a distillation step where the water is stripped back out. The overhead stream then becomes the working substance, etc. Thus the purpose of the separation is not to purify acetone, but to cycle as much water as possible for the least amount of energy input.

My question is this: is this arrangement energy intensive, and how much energy must I expend per pound of cycled water, for instance. I know I can calculate this and am in the process of doing so, I just want some ChemE's with direct experience with this mixture (or MEOH/water for instance) to give me a feel for the applicability of this choice (plus, I haven't pulled out my textbooks in a while). Ideally, I want to put in no more heat than the heat of vaporization/condensation of H2O.

 

[Montemayor]

Duff:

If you yourself are a graduate ChemE, you already know that when you mention the need to "put in no more heat than the heat of vaporization/condensation of H2O", you're already talking of a hell of a lot of heat. Water, as everyone knows, has one of the highest latent heats of vaporization. That's one of its key characteristics. Nevertheless, I don’t think this hinders your proposal.

I believe a logical question is: why would you want to furnish the latent heat of water? Aren't you proposing to boil out the "low-boiler" - which is the acetone (atm. boiling point = 56 oC)? And if you are using a methanol-water feed to a distillation/stripping operation, doesn’t the methanol (atm boiling point = 65 oC) also boil up first – especially since water boils at 100 oC?

In other words - neglecting the presence of an azeotrope - aren’t you proposing to boil out the organic phase out of the aqueous phase? And if so, aren’t you proposing to furnish the latent heat of the organic fluid (whether acetone or methanol)? Please correct me on this.

 

[dufftobeat]

Montemayor,

Thanks for your post.

Let me clarify further. The process that I am considering is to replace a process that cycles water. The process being replaced operates at a relatively high temperature (250+ Farenheit), which is necessary to release the water from the bound state (condensed on solid). The reason for this is to make use of low temperature / passive heat sources.

I'm considering distillation instead with a low-boiler like acetone (and sorry for the confusion, methanol is not included, it's just an alternative...), so that I can operate at 212 or below (the max. temperature at the bottom of the column). The reference to the latent heat of water is just because the amount of latent heat to boil out the organic phase is equivalent to the latent heat of the water to be stripped (correct me if I'm wrong...)


Yes, water is a high latent heat substance, but I only need to maintain a 1-to-1 ratio, or close, of heat furnished versus heat of condensation per pound of water stripped. My concern is that, as with similar compounds, lots of reflux is needed to achieve a reasonable separation. This drives up the heat requirements, which would make my idea unreasonable.

Does this help?

 

[kenvlach]

Your real problem is drying the hydrated solid; distillation to separate acetone & water is a self-created one.

Investigate drying the solid at reduced pressure.

 

[Latexman]

If I recall correctly (it's been > 20 years since I did some work for my company's acetone derivatives unit), at 50 psig acetone and water form a binary azeotrope. At atmospheric pressure, the xy diagram equilibrium curve asymptotes very, *VERY* close to the diagonal (y=x), and getting < 5% water overhead will be tough. Under a vacuum the acetone/water separation gets easier, but you need chilled water on the condenser. I believe most acetone/water separations use extractive distillation these days with EG or PG.

Good luck,
Latexman

 

[25362]

For heterogeneous low boiling azeotropes you may select other solvents as cyclohexane, toluene, octane, etc

 

[dufftobeat]

Ken,

Yes, my real problem is drying the hydrated solid. I want to try and do that by introducing a "dry" stream with a low partial pressure of H2O into which the water can desorb. I can't operate at reduced pressure.

I was considering this method so I can operate at lower temperatures. The fact that an organic is introduced doesn't matter if it can be made to be economical.

Latex,

If I can't get acetone to 5%, how about methanol?

 

[Latexman]

Never trust an old memory when it comes to data! An acetone/water azeotrope exists at 20 psig. It's 98.7 wt. % acetone. At atmospheric pressure, an azeotrope does not exist, but there's a quite a pinch near pure acetone.

Methanol does not azeotrope with water, so it'll probably be easier, plus it's heat of vaporization on a mass basis is a lot less than acetone's.

Good luck,
Latexman

 

[nbucska]

Both acetone and methanol are explosion hasards.

I would rather try to recycle the heat from the condenser
to the evaporator.

<nbucska@pcperipherals DOT com> subj: eng-tips
read FAQ240-1032

 

[unclesyd]

Early on the process for making our product had a stage where an anhydrous organic acid and anhydrous organic base were reacted in methanol to make a salt. This accomplished a couple of things the salt was extremely pure as the product was insoluble in methanol while all the impurities were and the salt was very easy to dry for the next step in the process, polymerization. Once the capacity increased, the mechanics of handling a dry material got to be problem. Essentially to transport we added water and put the salt into solution to be later evaporated off. Also the methanol became the bad actor in process, both from the flammability and toxicity standpoints. The whole process was changed to a water base system so we ended up with a 75% salt solution that had to be dehydrated prior to polymerization.

Over the years various investigations took place to reduce the energy requirements of the dehydration step by using low boiling azeotropes to dehydrate the salt and minimize the polymerization with resultant fouling of He’s that occurred while dehydrating. We utilized one bay in our pilot plant to test about every possible azeotropic combination from the butane/water to toluene. At different stages we had each system working to varying degrees. We were continually running into problems with the final stages of achieving complete dehydration and pure salt. In our process we were never able to get final product completely free of the organic phase of the azeotrope without an extra step.
I would stay away from acetone.

 

[dufftobeat]

unclesyd,

I'd be interested to know more about your approach to reducing energy requirements and dehydration through low-boiling azeotropes. Why do you use azeotropes as opposed to non-azeotropes?

Is your process liquid-phase dehydration followed by distillation?