HW 7 - p 9.6 - Distillation of an Acetone-Water Mixture: Fenske vs. McCabe Thiele - 10 pts
Please post all of your questions about this problem as comments on this post. I will respond with my own comments.
posted by Dr. B at 5:50 PM
4 Comments:
I got this question by email b/c I had not posted the blog. Just thought I would share.
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I found Nmin=2.66 which is ~ 3 instead of 4..
I've calculated all the partial pressure for both components in Reboiler and Condenser and I've also calculated the relative volatilities and used the Fenske equation to find the Nmin.
This is what i got:
alpha lk/hk for condenser = 10.7966
alpha lk/hk for reboiler = 15.443
Could you please verify the answer or suggest where I may have gone wrong?
Thank you so much. =)
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Yep, your Nmin is too low. Let's see if I can tell why. The alphas are not correct.
The first thing you want to do is use modified Raoult's Law to solve a bubble point problem based on the bottoms composition to determine Treboiler and another bubble point problem based on the distillate composition to determine Tcondenser. It is a bubble point for Tcondenser b/c we assume it is a total condenser. They would have told us if it were a partial condenser.
By adding the mod Raoult eqn for each species, we get:
P = x(1) gamma(1) P*(1) + x(2) gamma(2) P*(2)
Use Antine for P* = vapor pressure (not partial pressure) and then use Solver to determine T.
Once you know Tcond and Treb use Mod Raoult to calculate K values and alpha values.
I hope this helps. I am not sure what you did to get the values of alpha you provided.
what happens to problem 9.7? hints? blog?
amom:
I deleted 9.7 from this assignment. I just noticed I did not delete it from the problem list on the main or central homework page. I just fixed it :)
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