4 replies to this topic

[gegio1960]

Dear Experts,

Unfortunately I've deleted the previous post with the same title, trying to reply. Sorry!

Anyway, I've the answer from PingPong (thank you!) and I'll restart from there....

 

In my original request, I asked some expert advice on an acetone water distillation.

The target is to recover more than 95% of acetone as distillate from a feed basically made of 50/50 acetone and water.

In more detail, water is salty and is contaminated by a little concentration of oil (about 200 MW)... but this can be considered only as a marginal complication since both salt and oil will go in the bottom with water.

One of my biggest doubts is about the reboiling system: the feed to the tower would be higher than 200 t/h and then I don't think steam heating is a viable option (due to the big surfaces).

As an initial gues, I would consider the following items:

- (moderate) vacuum conditions: about 0.5 bara at top;

- 6-7 theoretical stages should be enough;

 

- feed tray: 1 stage above the bottom.

...but I don't like to reinvent the wheel, so any expert advice (for example, from either designers or field process engineers) will be highly appreciated.

........

 

Ping Pong answered as follows:

I have no experience with distillation of acetone/water mixtures, so I have only some remarks:

If the oil were miscible with the water it would mostly go to the bottom, but in this case it suspect most of the water immiscible oil will end up in the acetone distillate; it will simply be stripped by the actone/water vapors like in a steam distillation. Oil with MW = 200 is kerosene like.

Required reboiler surface depends on LMTD, so on steam condensing pressure. Availability of steam may be a bigger problem than reboiler area.

Why would you want to operate at only 0,5 bara ? Is that what others do? That would mean that the acetone condensor outlet is say 0,4 bara and 31 ^oC, requiring a cooling medium much colder than 31 ^oC.

And if you are worried about required reboiler area: what about required condensor area? Condensor LMTD will be much smaller than reboiler LMTD.

Is that based on rigorous calculations? (referred to stage number and feed location)

Number of stages required depends on amount of acetone you allow in water product, and on water in acetone product, both of which you have not specified.

..............

 

PingPong remarks are reasonable but don't allow me to make big steps beyond, so I'll try to rediscuss the subject as follows:

1) Is anyone aware of running plant of this kind (ie acetone-water, neglecting the contaminants for the time being)?

2) are they operated under vacuum?

3) how are they reboiled?

4) also papers, articles, links, etc could be useful

 

Thank you in advance for any help/info.

 

Kind regards,

gegio

 

 

 

[gegio1960]

Hi Ping Pong,

thank you for your remarks that I'd like to further discuss/comment.

 

1) the only specification target of the process is to recover at least 95% of acetone (and to stay below 95°C as max temperature).

 

2) 0.5 bara is intended at condenser. by the way, at this pressure I found a condensing temp of 42°C, compatible with my cold utility (thank you for the lesson ;-) Latins used to say "repetita juvant!")

 

3) about steam, the wide quantities needed are another point against its utilization

 

4) vacuum system condensers are always big. Too big reboilers can be replaced by heaters

Edited by gegio1960, 09 January 2014 - 10:53 AM.

[Bobby Strain]

Eastman Chemical in Tennessee operated multiple columns for acetone recovery. The feed originated from washing exhaust air from fiber dryers. The columns operated about atmospheric pressure at the condenser and were reboiled by stem, inclucing live injection. There was large feed/bottoms exchange using plate exchangers to reduce steam consumption. I recall these columns to have 20-30 trays and they were making a clean separation, i.e. little acetone in the bottoms. There was some stuff in the water phase that was a big foulant, mainly depositing as the bottoms cooled. To minimize energy consumption in a new installation, you might want two systems where one operates at a higher pressure and reboils the lower pressure system.

 

Bobby

[PingPong]

1) the only specification target of the process is to recover at least 95% of acetone.

Nobody can design a distillation column with only that specification target.

 

You have to define both the target for max water in acetone product, and max acetone in water product.

[gegio1960]

pingpong,

yes (nobody can...) and no (you have...)

thanks for your efforts