I am wanting to set up a large vacume pump that i have sitting around to pull a vacume on the pressure vessel to boil the methanol and water off of my nearly finished fuel. The pump can pull a perfect vacume on the 80 gal tank in about 2-3 minutes. My question is how many IN of Mercury would it take to boil just the methanol and how much would it take to boil the water? If i was able to get this set up properly, how would i go about posibly recondensing the methanol for use later? Could i set up a condensing coil like in an old moonshine still? Any input would be greatly appreciated. Thanks

quote:

Originally posted by rgilland:
. . . My question is how many IN of Mercury would it take to boil just the methanol and how much would it take to boil the water? . . .

Short answer is: you can't.

Water has a boiling point of 100 deg C. Methanol has a boiling point of 65 deg C. Both at atmospheric pressure.

The boiling point of the mixture will be greater than 65 deg C and less than 100 deg C at atmospheric pressure. The relative percentages of each compound in the mixture dictates how close the mixture boiling point is to the pure material. i.e. a mixture of 5% methanol in 95% water will have a boiling point close to that of water or 100 deg C. Likewise a mixture of 95% methanol in 5% water will have a boiling point close to 65 deg C.

I don't know the exact values, but lets say pure methanol boils at 55 deg C at 28 inches of mercury (in Hg), and pure water boils at 80 deg C at 28 in Hg. The mixture would have boiling point above 55 deg C and below 80 deg C at 28 in Hg vacuum.

You may be thinking that if you apply a vacuum of 28 in Hg and heat the mixture to 75 deg C, then the methanol will boil off and the water will not. Well distillation doesn't quite work that way.

Any heat you apply to the mixture heats the entire mixture, not just the methanol or the water. As the temperature increases, both compounds will start to evaporate out of the mixture. There is no temperature or vacuum setpoint where you can drive off methanol without some water being driven off at the same time.

Many brewers add a reflux column to their recovery condenser. The reflux column helps to concentrate and purify the methanol. But, it is virtually impossible to capture methanol without some water in it.


In regard to your second question: Yes, you can use a condensing coil similar to what moonshiners used.

well if that is the case, than could i not pull a vacume to evaporate all that methanol and water off of the mixture, recondense the water meth mixture and then just distill it. Is there anyone out there that has been doing anthing like this that would have any input, pros cons do's and dont's. Thanks.

Yes, pull the maximum vacuum with the maximum heat to remove both water and methanol from your biodiesel.

Check the quality of your recovered methanol/water mixture. You can remove water from the methanol in a separate distillation step. At this point, a good reflux condenser, a mild vacuum, and good temperature control will aid in the recovery of high purity methanol.