8 replies to this topic

[Ben]

I am currently designing a distillation column for seperation of acetone and isopropanol as part of my project. The objective is to achieve a purity of 0.999 for acetone. The problem now lies with determining the optimum reflux ratio. I need to get the optimum relux ratio to plot the operating line for the maccabe-thiele diagram before able to find the numbers of stages. Can anyone enlighten me or list down a couple of steps to aid my designing work? Thanks you very much.

[Ankur Shah]

Hey!

The biggest problem here is that you are looking for high purity separations. here is where McCabe Thiele looks a lil bit more difficult. As the equilibrium curve approaches very close to the y=x curve. So u will have to plot a log-log plot of the y-x plot. (check coulson and richardson, Vol 6 for this.)

For such a system. optimum reflux is difficult to obtain. hence, after finding Rmin, the normal assumption of R=1.5Rmin is inaccurate. Here the log-log plot will help. coluson and richardson have a solved example for this case. So u MUST refer to it. and it is given in detail too. so it shouldnt be soo difficult then.

If more doubts, please post back!

Ankur

P>S: Mr Ben, you are the 1,999th member of ChE!!! awesome!! who is member 2k???

[joerd]

...
Ankur

P>S: Mr Ben, you are the 1,999th member of ChE!!! awesome!! who is member 2k???

That's gotta be AggieChemE ! http://www.cheresour...p?showuser=2000
Congratulations!

[Ben]

Ok, lets put the MacCabe Thiele diagram away for a while. Now, i found my Nmin to be 9 trays. As i proceed on to determine my Rmin with underwood equation, i got a Rmin of 0.6555. When i key in the data into hysys, it does not converge. Based on trial and error is hysys, the Rmin should be 3. This is something very different from the calculated value.

Basically, i decided to preheat my feed to the bubble point before entering the column where the q is now 1. Since this is a binary mixture, my 'alpha a/b' and 'alpha b/b' is found to be 2.8823 and 1 respectively. My 'dite' lies at 1.15. When i sub all the known value in, i got a Rmin of 0.6555. Is there anything that might be wrong or does the values looks kinda unusual?

You help is really appreciated! Thank you

[joerd]

did you try to use a shortcut column in Hysys to check your Rmin values? You can use the results for a first estimate for the Hysys column.

[Ben]

did you try to use a shortcut column in Hysys to check your Rmin values? You can use the results for a first estimate for the Hysys column.

Sorry, can you elaborate more on shortcut column in hysys. Cause i am not too sure about that. Can you give me a rough idea? Thank you

[joerd]

Hysys has a unit operation called Shortcut column. You can attach your feed stream as usual. Then, define your light key and heavy key components, and the desired purity of the distillate and bottoms streams.

The Shortcut Column performs Fenske-Underwood short cut
calculations for simple refluxed towers. The Fenske minimum
number of trays and the Underwood minimum reflux are
calculated. A specified reflux ratio can then be used to calculate
the vapour and liquid traffic rates in the enriching and stripping
sections, the condenser duty and reboiler duty, the number of
ideal trays, and the optimal feed location.

[Ben]

Thanks for your help.

I finally managed solve the distillation problem. It been found my distillation requires 24 trays and a reflux ratio of 0.784. But, i got another question related to the distllation column. My feed temperature is currently at 323K. I am thinking of adding preheater(s) to bring up my feed to the bubble point which is at around 333K. Do you think this is feasible? I thought of using the bottom product (352.9K) as the hot fluid to heat up my feed, but i doubt that only one heat exchange alone will do the work due to the huge different in tempertature. Actually my purpose behind doing this is to get a q of 1, which makes finding the optimum feed location on the McCabe Thiele diagram much easier since the feed line is vertical when q=1.

Anyone can comment on this? Thank you very much.

[Ankur Shah]

intereting question.. the reason u want to add a heat exchanger is because u want to find feed tray location easier??? interesting!! i guess HYSYS would have done all this for you...

also... yes its very much feasible to used the bottoms stream as yout delT is enarlt 20K.. which is fine

Ankur