3 replies to this topic

[bluefish20008]

Hi there! I have encountered a problem which I couldn't find any solution from books or journals. And I need to settle it within the next few hours!

I'm now designing a VAM distillation column, which receives feed from the previous acetic acid-VAM-H2O azeotropic DC. The feed contains 92% of VAM, 4% of H2O, ethylene and acetic acid as the balance. My production target is VAM with 99% purity.

From simulation, VAM exits as bottom while H2O goes up as overhead. Usually, heavy key is among bottom products and light ket is among top products.
However, VAM has higher vapor pressure compared to H2O, which is impossible to make VAM a heavy key. Unable to determine keys, I'm now stuck in tray calculation for weeks.

Some suggested using VAM-acetic acid pair instead of VAM-H2O pair to calculate tray number. I'm don't know the rationale behind it, but it's obvious that the vapor pressure for VAM is also higher than that of acetic acid.

On what basis should I make a decision on key determination for this weird case? TQ!
(Well, literature doesn't say much about solely VAM-H2O. The only thing I found was only the ideal number of stages for this process ... 35 trays.)

[MrShorty]

From simulation, VAM exits as bottom while H2O goes up as overhead. Usually, heavy key is among bottom products and light ket is among top products.

On what basis should I make a decision on key determination for this weird case?

How do you define "usual" and "weird"? This behavior is actually quite common. This is how a system behaves that forms a minimum boiling azeotrope. Using acetic acid - VAM will give you a different answer than H2O - VAM because acetic acid - VAM (assuming VAM stands for vinyl acetate monomer) is much more ideal than H2O - VAM.

I'm not familiar with all the ins and outs of using this to determine number of trays. Recognize that the top of the column is going to approach the azeotrope composition, and the bottom of the column is going to approach the pure component composition.

Edited by MrShorty, 20 April 2010 - 01:37 PM.

[latexman]

The VAM/water azeotrope (light key) is pulling all the water out of the top of the column, leaving the VAM to go out the bottom.

[bluefish20008]

Hi there!

Read through the replies. However, I am still puzzled over the calculation.

Now, we have an heterogeneous azeotropic distillation.

• My feed contains water, vinyl acetate (92 mole%), acetic acid, ethylene, oxygen, and CO2.
• I target purification of VAM to 99 mole% in bottom product.
• My top product contains all three of them, with 68 mole% VAM.

I wish to get a value of theoretical tray number and reflux ratio as the starting values for simulation input. So, I selected Fenske-Underwood-Gilliland method for getting the starting values. And that's where the problem comes from.
Since the major component at bottom is VAM, I set it as the heavy key. On the other hand, water is set as the light key. I used this standard to calculate relative volatiity (α) at feed dew point, and I got these.

Component αd αb xd xb xf
Water 0.474670499 0.433382676 0.3186 0.01 0.0772
Vinyl Acetate 1 1 0.6814 0.99 0.9228

αLK = SQRT(αLD * αLB) = 0.454

By using an α of less than 1, I obtained negative values for minimum tray number and reflux ratio. That's the place that I was stuck for days (and now weeks), and I couldn't proceed further.

So, I wish to know whether I have made a mistake on selecting light key and heavy key.
If yes, what should be the selection criteria?
If no, where goes wrong?

Thank you!

Edited by bluefish20008, 25 April 2010 - 04:32 AM.