Hey everyone,

I recover my methanol with a reflux column and plumbers delight. I get excellent purity at about 98% using glycerin containing a 5% prewash but it is very slow. If I utilize a venturi will my purity go down? Will I need to increase my reflux column area to account for the vacuum pulling the vapors through it?

You may want to take a look at THIS thread.

quote:

Originally posted by stevep:
. . . Will I need to increase my reflux column area to account for the vacuum pulling the vapors through it?

Yes, if your venturi pulls vapors though the condenser in a single pass with the vapors that pass through the venturi being vented to the atmosphere.

No, if your venturi is set up to recycle vapors out of the condenser and back to the still pot.

Producer,

Thanks for the reply. Even if I recirculate the vapors I am still concerned that the vacuum will pull the water vapor through the column condensing both the water vapor and methanol vapor. This in fact my not be true, I am just trying to convince myself. You said recirculate the vapors though the condenser. Are you using “condenser” to mean the column and condenser?

I misread your first post, so let me respond to the question you asked, not the one I thought you asked.

As you know, adding heat or pulling a vacuum on a mixture of methanol and water in biodiesel will cause both water vapors and methanol vapors to form above the liquid. You can't get one vapor without the other.

The ratio of the concentration of methanol vapors versus water vapors is related to the temperature, vacuum, concentration of remaining methanol in the mixture at any point in time, and concentration of remaining water in the mixture at any point in time.

Increasing the distillation temperature while holding the pressure at atmospheric, primarily, increases the speed at which the vapors will form and, secondarily, will slightly change the ratio of vapors relatively to each other.

Decreasing the pressure of distillation while holding the temperature constant, primarily, increases the speed at which the vapors will form, and, to a lesser extent, will slightly change the ratio of vapors relatively to each other.

You have a "pot still" followed by a "reflux column" followed by a "condenser" followed by a "vent".

If you add a vacuum between the condenser and the vent and vent the vacuum direct to the atmosphere, then you can operate your still at a lower temperature than would be required without the vacuum and achieve an equal rate of methanol removal. Operating at a lower pressure and a corresponding lower temperature to achieve a relatively constant rate compared to no vacuum will allow your reflux column to operate just slightly more efficiently. The condenser, operating under vacuum and lower temperature, must be capable of removing the same amount of heat energy from the vapors. The lower pressure will make your condenser slightly less efficient, so must be made a little larger then the "atmospheric pressure" condenser.

If you add a vacuum between the condenser and the vent and direct the vacuum discharge back into the still pot, then the vacuum created at the top of the condenser will be offset by the pressure created where the vapors are reintroduced into the still pot. There would, probably, be no increase in the efficiency of the reflux column. The condenser would be operating at a lower pressure, would loose some efficiency, so need to be slightly larger then the "atmospheric pressure" condenser.

The GL1 type setup is designed to maximize methanol removal percentage from the biodiesel. It is not, necessarily, designed to maximize the purity of the recovered methanol.

Your setup is designed to maximize the purity of the recovered methanol. It is not, necessarily, designed to maximize the total volume of methanol recovered.

Producer,

Thank you for taking the time to explain it.