Properties of alcohol mixtures.

[dwalter68]

Hello- I need some expertise about what properties result when different alcohols are mixed in liquid form. I particularly need to know what vapor pressure will result from a mixture of 1-octanol (vp of 0.1 mmHg @20C) and 1-propanol (vp of 15 mmHg @20C). If I mix them will get a vp that is between the two? More importantly, if this mixture was left sitting at room temp. in an open bottle would the 1-prop evaporate first and leave the 1-oct behind or would the mixture evaporate uniformly? My goal in all this is find some way to increase my rate of 1-octanol evaporation.

Thanks for the help.

 

[sshep]

For this mixture you are not far off to assume an ideal system. The vapor pressure of the mix is:

Pmix=sum(X*Psat); where X is molfrac and Psat the vapor pressure of the corresponding pure components. An inert gas will make up the difference with respect to atmospheric pressure.

also

Psat=f(T); per Antoines Equation log(Psat)=A-B/(T+C) which can be regressed from a few data points.

From these equations all manner of flash calcs can be done.

With respect to your objective, the evaporation rate of octanol can be increased by raising the temp, or flow of non-condensibles which will carry the alcohol vapors away. Unfortunately there is no practical way to raise the evaporation of octanol with respect to propanol in a simple evaporation. If you want to concentrate your propanol, you need to do a distillation and take propanol off as the distillate and octanol as the residue.

best wishes, sshep