2 replies to this topic

Im a fourth year chemist trying to design the following process unit. Ive run an Aspen simulation to determine a suitable number of stages ect and also completed an energy balance accross the column using stream enthalpy data but was wondering how i would go about this design by hand(to determine number of stages, reflux ratio ect). Im a fairly competent user of computer simulation models but to be honest I dont have the first idea how to design it by hand using vapour-liquid Equilibrium Data or where to get this information. (being a chemist not an engineer)






The product leaving fermentor and entering the distillation column is 8.88% ethanol, the rest of the stream consisting of water and impurities(ratio unknown but conc. of impurities is very low). Assuming that the distillation column removes all impurities and that no (or a negligable amount 0.002% w/w) ethanol is lost in the bottoms.




Mass flow of stream in is 2.011*10^5 kg/hr

For my simulation I have chosen atmospheric pressure and 95C as feed conditions although these are not set in stone and can be manipulated as needed.

Im looking for a general method rather than a complete solution but any input at all would be welcomed.

[ARAZA]

Im a fourth year chemist trying to design the following process unit. Ive run an Aspen simulation to determine a suitable number of stages ect and also completed an energy balance accross the column using stream enthalpy data but was wondering how i would go about this design by hand(to determine number of stages, reflux ratio ect). Im a fairly competent user of computer simulation models but to be honest I dont have the first idea how to design it by hand using vapour-liquid Equilibrium Data or where to get this information. (being a chemist not an engineer)




Please refer to a book called Unit Operations of Chemical engineering by Mc Cabe Smith which deals with fairly extensive method of determining number of stages, reflus ratios ect..

Good Luck.

ARAZA

The product leaving fermentor and entering the distillation column is 8.88% ethanol, the rest of the stream consisting of water and impurities(ratio unknown but conc. of impurities is very low). Assuming that the distillation column removes all impurities and that no (or a negligable amount 0.002% w/w) ethanol is lost in the bottoms.




Mass flow of stream in is 2.011*10^5 kg/hr

For my simulation I have chosen atmospheric pressure and 95C as feed conditions although these are not set in stone and can be manipulated as needed.

Im looking for a general method rather than a complete solution but any input at all would be welcomed.

[Andrei]

Danny,

First, you should search the forum, there were some recent posts on this subject.

Something doesn't seam right to me. You say that the feed is at 95 ºC and atmospheric pressure. That means your feed is partially vaporised. Is that what you intended to do? Ethanol-Water azeotrope boiling point is around 78 ºC at atmospheric conditions, I am sure you are aware about this. It's not that this is a big deal, but if everything is all right in your simulation, this should push down your feed tray. Can you see that?

Regarding the manual number of trays determination, look for McCabe Thiele Method, graphical; or Fenske, analytical. It will be tricky to get a result because of the azeotrope, but if you use as light key component the azeotrope and as heavy key component water you should get something.
And if you prefer something more elaborate you can try Lewis-Mathesson Method, a trial and error method prety similar with Inside-Out algorithm from the simulators.

Good Luck