2 replies to this topic

[Avancini]

Dear friends,

 

I'm doing some process simulation at Aspen Hysys and i got some results on the purification that i couldn't understand yet. I hope you can help me.

 

- My goal is to have a stream with the purest acetic acid that i can.

 

I got a Distillation column that is feeded with a stream with:

 

Acetic Acid:     0,4222

Water:             0,5474

Ethyl Acetate:  0,0214

Acethaldeyde: 0,0076

 

And, I did the simulation and when it finally converged, i got as result that in the bottom liquid outlet and in the liquid side draw I have pure water.

 

Why did i have water in the bottom liquid outlet?

 

I know that i'll have an azeotropic, but i'm not sure if it will cause such impact.

I'm doing some research, but i didn't find an answer yet.

 

Does anyone knows if this really can happen? And why did this happen?

 

If someone are able to help i 'll be very grateful.

 

Thank you!

 

Avancini

 

PS:

- I'm sorry for any writting mistake in my post. 

- I think it's good to say this: I'm such a newbie user of aspen Hysys, and i selected the basic Distillation Column. Maybe I'm comitting some mistake on that.

[Lucian Gomoescu]

Hello Avancini,

Components have the following n.b.p. [°C]: 118, 100, 77 and 21 respectively.
You may use NRTL package with UNIFAC estimation of VLE coeff.
There is only one azeotrope (homogeneous, water-acetate = 40.3 / 59.7 %mol) with n.b.p. of 70.7 °C.
Assume that the objective of the separation is a impurity of 0.001%mol water in bottoms and 0.001%mol acetic acid in distillate.

All these show that:
- an atmospheric column should be provided with a reboiler and a partial condenser (in case of total condenser, the pressure should be increased to around 2.8 barg in order to use cooling water in condenser);
- there is no need for a side draw;

- as specifications you can use the impurities value in distillate and bottoms + either overhead vapors rate, either condenser temperature;
- minimum number of theoretical plates (NTP) is around 25 (you can't use ShortCut Distillation to easily find it because of azeotrope presence, but you can find it by decreasing number of trays until reflux ratio gets very high, hundreds);
- as a start configuration, use 2*25 stages (48 trays + partial condenser + reboiler).

[Lucian Gomoescu]

I want to correct myself: impurities above should be 0.1%mol, not 0.001 %mol (this is not suitable for a distillation column ).
Also, i forgot to say that position of feed tray should be the 35th tray, numbering from the top.