3 replies to this topic

[saeed.s]

Dear friends,
As you know, Formic acid and water form maximum boiling azeotrope and With increasing pressure , formic acid concentration moves higher.
Do you think it is possible to purify a mixture of formic acid and water ( formic mass frac=60%) and take formic acid about 85% in the bottom of a batch distillation column at 3.5 atm pressure? I attached K-value & x-y diagram at 3.5 atm that are calculated by aspen plus and UNIQUAC property method.
Thanks in advance.

Attached Files

•  k-value.xls   201KB   57 downloads

[riven]

Not sure exactly what you mean but

At a 60% feed the best a single column can hope for is a product stream containing mostly water and another product stream containing the 85% azeotrope. Shunting this azeotrope stream to another column will not achieve anything unless for example you have a different pressure in the column or you use extractive distillation.

I do not know which is the preferred process but pressure swing does seem possible. Your azeotrope at 3 bar is 85% formic acid (but remember your produict will be somewhat different t but according to this
http://web.inc.bme.h...stillation.html
the azeotrope at atmospheric pressure is approx 78%-80% formic acid (get aspen to give you data on this).

What I would suggest if you want to avoid extractive distillation is shunt your azeotrope product stream to a vacuum distillation column (what level of vacuum will depend on your economics) in which case the azeotrope stream at 3 bar will now be beyond the azeotrope at the lower pressure and you will produce pure formic acid.

[saeed.s]

HI Riven,
Thank s for your reply.I think my question needs more explanation. As you said, the usual way to separate formic acid from water is extractive distillation but in my project amount of mixture is low and I like to investigate other ways.
First of all, I compared azeotropic data from page 1233 perry s hand book with different property methods data to select best of them that is UNIQUAC.
As you know, in maximum boiling azeotrop mixture , we can take azeotrope point from the bottom of batch distillation , my feed is about 2000 lit per batch with 60
% formic acid and I need formic acid 85% , as x-y diagrams show with increase pressure formic acid concentration in azeotrope point moves higher for example at 1 atm , formic acid mass frac = 0.63 and at 3 atm mass frac = 0.838 ,and I want to take formic acid 83% ( it depends on pressure) from bottom of batch tower at 3 atm or higher and I m not sure about that can I decrease amount of formic acid in distillate or not? . Because it s a hard distillation and k value is low and because of similarity between some properties of water and formic, there is not any third component to break the azeotrope and also any pressure.
Another problem I will have if I use this method, is material selection and corrosion of formic because of temperature increment that I think it has comprehensive affect on project cost.
As you suggest, another way to avoid temperature increment , in first step I can take formic acid in azeotrope point at 2 atm from bottom and in second step , separate mixture at vaccum pressure and take formic from top, but in simulation (at vacuum) when I use mixture with 70% formic ,separation is not good but with 80% formic it s better and it s make me to use 2 atm pressure in first step.

Edited by behnam, 19 May 2010 - 01:52 PM.

[pawan]

Probably you are looking for a confirmation on whether Pressure Swing Distillation will work or not in this case. It will work definitely (as you can see on my blog post) but I am not sure about your objective & data for this particular combination so I can not suggest pressure is better or vacuum is better.