I recently got a couple of vacuum pumps and am "playing around" with them trying to decides which of the two is better for my needs. The small one will pull an 19" vacuum. The bigger one a 25" vacuum.

The thing I am trying to figure out is would water (for drying the oil before processing) and methanol (for distilling out the extra after processing) boil at 50C at these vacuums? I've found the vapor pressure charts for these compounds, but am having a failure of memory in trying to convert my inches of vacuum to absolute mm of Hg pressure.

Can anyone let me know the formula for converting between the two? Also anyone care to offer a recommendation (aside from the technical boiling points) as to which vaccum pump might serve best for these purposes?

TIA


PS - Using a vacuum to load the oil into my hot water heater based processor is definitely the easiest, quickest, and cleanest way to get used oil into the thing.

returning to the roots- Dale Scroggins' Touchless Processor was the original water heater based processor, and it loaded oil this way. I'm sorry I don't have the info handy on vacuum conversions, but I do think I have it in a file SOMEWHERE if you dont get an answer out of one of the engineers on this forum.

Mark

Ok, I think I figured this out, that inches of vacuum are talking about an inch of Hg (somehow I was thinking water column before). If this is the case then 18" of vacuum should be roughly 280mm/Hg of absolute pressure and 25" of vacuum should be 125mm/Hg.

If this logic is right, then the 18-19" of vacuum should be able to boil off the methanol just fine (boiling at 280mm is about 41C). But it won't do a whole lot for boiling off water which still has a bp of 75C at that pressure.

I'll be building my condensor and should be able to test this out in the next few weeks. But if this is correct, then folks with hot water tank type processors, should be able to use a fairly small vacuum pump for loading oil and distilling unused methanol. (I have a 1/10hp gast rotary vane pump that is pulling the 18-19" of vacuum)

If my unit conversions are wrong, someone please let me know.


Steve

Actually a drop of 25C is a pretty nice reduction.

Steve,
18 inches of Hg is 457.2 mm of Hg, 244.7 inches of water, 60.95 kPa, 8.84 psi, and 0.6 atm. (of course that’s mercury at a temperature of 32 deg F.)

Factors:
For one inch of Hg multiply by:
13.595 for inches of water
3.386 for kPa
0.491 for psi
and 0.033 for atm.
HTH. If you need any other factors, just ask.
Joe

Joe-

I was figuring that 19" of vacuum was 1A (760mm) less the 19" (483.6mm)of vacuum or 276mm.

I assumed this is the right way of looking at it, as this gauge starts at 0" of vaccum when open to the atmosphere, and goes to increasing inches as the vacuum increases...


mark-

Not that 25C is worthless, it certainly is a decent return on the energy to run a 0.1hp pump for 5 min. I was just trying to get all the vapor extractions to go at processing temps of 40-50C...

At 30in/hg vacuum, water will boil at its atmospheric freezing point (although you'd have a difficult time pulling that much of a vacuum). At 20in/hg, it will boil at approximately 160degF. 10in/hg at about 190degF. And of course 212degF at 0in/hg vacuum. Water which is thoroughly dispersed in oil appears to boil off at slightly higher temperatures. Pure water will boil at 50degC (122degF) at around 25in/hg.

I built a vacuum distillation setup a couple of months ago in an effort to dry my WVO prior to processing into biodiesel. A few things to keep in mind:
1) Make sure your tank is of sufficiently thick steel to withstand the atmospheric pressure exerted on a partially- or fully- evacuated vessel. At 29in/hg of vacuum there is over a ton of force per square foot on the outside surface of your tank! I've found that common 55-gallon (ribbed) steel drums are safe to about 10in/hg. Too much beyond that and you risk collapse (see photo). You'll want a minimum of 1/8" mild steel to work safely beyond that limit. Pressure tanks, propane tanks, and I assume intact water heater tanks will work well.

2) The tank and plumbing must be airtight to maintain vacuum.

3) The tank must be well insulated to prevent vapor from condensing on the inside of the tank before it reaches your condenser, and should also be sized to accomodate your batch for best efficiency (don't use a 120-gallon still for a 30 gallon batch).

4) If you manage to get the water to boil at ambient temperature without the addition of any outside source of heat (you'll need a 28in/hg vacuum to achieve this -- more or less depending on your climate), you must devise a way to cool your condenser and your condensate collection tank below ambient or the vapor will not condense.

Excelent post.
One way of ensuring that the water condenses, is to put the condenser AFTER the compressor. This will ensure that the pressure is such that the BP of the water is back at 100'C
Tony

I've never tried placing the condenser after the pump. Will this work? It would seem that once the desired vacuum has been reached, the still is effectively a closed system, and will transfer neither air nor vapor beyond the pump. I'd be curious to hear of any experience with this that anyone might have.

Given that a lyquid is transforming into a gas wouldn't the vacuum pump have to work at keeping the vacuum level constant as the water boils out. Just as steam creates pressure it will reduce vacuum until all the water in out same goes for methanol.

If there is a condenser that changes the vapor back to a liquid, the volume of the whole system will remain in equilibrium and the vacuum pump can be turned off. What can require that it be running to maintain a vacuum at a certain level are leaks. They can be difficult to find in a vacuum system.

Put the condenser befor the recovery tank, and the vacuum pump after the tank. I use an old propane tank for a recovery tank. They can be picked up for free at any propane filling station.

hi!!! This is an interesting treat....I have a vacum but I am not sure how it works and if I install it ok.... the person who install it for me told me that this will absorb the vapour and I could do a faster evaporation of methanol and water...it needs water to work (I think is for lubricating it) .... but I am not sure if it's working all right..and maybe is better if I use it to create a vacum how you said and can evaporate methanol and water at lower temperatures....could anybody help me???

quote:

Originally posted by quemala:
it needs water to work (I think is for lubricating it) .... but I am not sure if it's working all right..

It sounds like a liquid ring vacuum pump, in which case it needs a constant flow of water which enters typically just below the centre line on the front or side of the pump housing.
Do you have the details of the vacuum pump, Make Model Size etc. from this you may be able to get sufficient info from the suppliers website to be able to set it up correctly.

With a liquid ring vacuum pump you can actually condense the methanol in the seal water (subject to to the level of vacuum required) but that would then mean that the water is contaminated and would need disposing of (unless the pump is a seal water recovery unit which recirculates and just bleeds the excess volume that condenses)
Post some details of the pump and we may be able to give you further help.

Whatever happens, if it is an LRVP then you must have sufficient water flow otherwise the pump will cavitate like mad and destroy itself if left long enough.

Fuzznag...thanks a lot for the answer I will look for the details and post it later

Here are some data from the vacuum, its caudal is 48m3/H and the vacuum is 30mmHg, its a Liquid ring vacum and I am trying to get the manual but for the moment I can't find it