1 reply to this topic

[jorgek93]

Hi, I have a little problem trying to understand a process. I have a mix (methanol + biodiesel), I was asked to find the pressure at which methanol would evaporate keeping temperature constant. The main idea is to lower the vapor pressure by lowering the pressure inside the drum.  Doing some research I found that it's a property known, but I can't find a formula that will tell me the exact pressure I would need to achieve that...I was trying to find some heuristic results but nothing. 

 

Thank you 

[MrShorty]

I am not sure what formulas you found. This should be a fairly "typical" VLE type problem. The approach I would expect:

 

1) Choose a thermodynamic model. Modified Raoult's law seems like the easiest, if you have no other preference. P(MeOH)=x(MeOH)*gamma(MeOH)*P0(MeOH) where:

P(MeOH) is the partial pressure of methanol. If we assume the biodiesel is non-volatile, this will be the same as total pressure

x(MeOH) is the desired measure of liquid concentration of MeOH -- usually mole fraction

gamma(MeOH) is the activity coefficient -- usually computed from your favorite activity coefficient equation (Wilson, NRTL, Van Laar, etc.)

P0(MeOH) is the vapor pressure of pure MeOH at T.

 

2) "Solve" this equation for x at your specified T and P. Since gamma(MeOH) is a function of x, this will probably require a numerical algorithm (eg Excel's Solver utility).

 

3) Check the result to see if it seems reasonable.