3 replies to this topic

[chemeng2014]

I am designing a distillation column to seperate ethanol and water up to azeotropic composition.

 

I have determined my feed and outlet compositions so then wanted to use the Fenske equation to determine the number of theoretical stages required. However, I have the stumbling block that I need the relative volatility of both components and I need the mole fraction of each substance in the liquid and vapour stages.

 

My problem is where do we get these mole fraction figures from? Is the vapour mole fraction from the distillate and the liquid mole fraction from the bottoms?

 

I have Perry's Chemical Engineering book and it says typically 60 trays are required so I'm hoping an answer somewhere beneath this will be realistic once I have the correct data for the Fenske equation (on the basis that tray efficiency is <100%)

 

This is probably going to be a relatively simple question for most posters on here and I'll probably get slightly ridiculed for it but my brain is mush at the minute so appreciate any help/advice.

Edited by chemeng2014, 10 February 2019 - 04:51 PM.

[breizh]

Hi,

Consider the resources attached to support your work .

 

https://checalc.com/...stillation.html

 

Hope this is helping you .

 

Breizh

Edited by breizh, 10 February 2019 - 11:47 PM.

[MrShorty]

My problem is where do we get these mole fraction figures from? Is the vapour mole fraction from the distillate and the liquid mole fraction from the bottoms?

According to Wikipedia (https://en.wikipedia...Fenske_equation ), you are correct. Xd is the mole fraction composition in the distillate and Xb is the mole fraction composition in the bottoms. The relative volatility is computed as an average, where the Wikipedia article suggests that you can take a simple geometric mean of the relative volatility at the top and bottom to get the average relative volatility.

 

Does that help?

[chemeng2014]

Thanks all, very useful.