Suggestions for vacuum pumps for dewatering and whole batch demething for the appleseed user please.
Rotary/piston/diaphragm or ?
Safest to handle methanol fumes is a liquid ring vacuum pump....but expensive...
... a venturi is also a very safe vacuum pump with sufficient flow and good heat control...
Jon Heron uses a large water cooled compressor tank for a vacuum chamber before the vaccuum pump to minimize those vapors....
I've done demething using vacuum distillation mostly in a one litre glass boiling flask in thermal shock resistant glass. A problem can be trying to do it too fast where it foams over the still head. Or if the liquid is too warm/hot when the vacuum is turned on. There's something called explosive boiling where it all comes out at once if the pressure is dropped quickly. I found a graph on the internet that tells what the boiling point of methanol is at a particular atmospheric pressure. If you drop the pressure enough the condenser has to be very cold or the vapor will not condense and will go out the exhaust of the vacuum pump. The exhaust of the vacuum pump might be vented outside since it may contain methanol vapor. I used a diaphram vacuum pump that I salvaged from a trash pile when a medical supply was going out of business. It pulled vacuum down to about 1/3 atmospheric pressure. My still head thermometer temperature was 32 degrees centigrade. I used ice water and an underwater pump system to cool my condenser. A problem you might consider with vacuum distillation in a large reactor is if you pull too much vacuum your plastic hoses and reactor vessel will implode. That might be a problem. I have a better vane vacuum pump that pulss something like 100 millimeters of vacuum but I have not used it yet. There is a chemistry mathmatical formula that I would have to look up. It was first year college chemistry. It was the mole percentages of two liquids Biodiesel + methanol or water then a physical constant then the atmospheric pressure the boiling point of the lower boiling liquid was in there somewhere to predict the actual boiling point of methanol to demeth or dewater biodiesel or vegetable oil. I would have to look it up to describe it better, or you could just do it to determine what temperature to heat your still pot to at the particular atmospheric pressure you decide to do your distillation at.
I am leaning toward a diaphragm pump. Having a large pretank full of methanol vapors, even if cooled, sounds like a big firecracker. I use a GL system with a venturi and presently blow air with a small pump through my venturi to increase the air flow and condense it through a car radiator with a box fan blowing on it.
Various posts mention their vacuum technique but none reference a specific pump type. I wondered about recycling an Automotive AC Pump except the rotary vane pumps need oil and I would think the methanol would like to mix in with it.
A venturi is the safest since your pumping methanol vapors. ....
Do you have any pics?
Wesley - I love air chair engineering, any formula link appreciated.
Danodiesl - I have the ventui on my GL processor but it just moves vapor around, doesn't really create a vacuum.
Do you mean the venturi for demething on the GL push-pill processor?
I found a Thomas diaphragm vacuum pump at the junk store. Now just have to plumb it.
Photos will have to wait until I clean it up a bit Ha ha, it's biodiesel!
Sooty; I'm not sure what air chair engineering is. But there is empirical solutions to engineering questions, that is that rather that calculate it on paper to determine a boiling point you just do it and read the still head thermometer. When I salvaged (trash picker) the diaphram vacuum pump from the medical supply's trash pile when it was going out of business, I actually got two that I carried for about three miles to my home. New they cost about 5-6 hundred dollars. I got them home and removed the head over the rubber diaphram. There was a biological hazard white crud that I scraped off with a flat edged screw driver. After I removed the crud inside the used diaphram vacuum pump it worked as good as new. So if you can you might consider cleaning the inside of your used diaphram vacuum pump to make sure it operates with utmost efficency.
Sorry, my secretary is a poor typist.
I meant "armchair engineering".
I agree the empirical approach is faster and less of a headache.
More to come, I will try to dewater WVO first.
My diaphragm pump would only draw to 13 in.Hg and had low CFM.
So, I now have a piston pump that can take the processor down to 26 in. Hg.
But it cannot hold that vacuum through my entire condenser/collector system because of leaks at so many connections and almost crushed my collection vessel.
Do any of you using vacuum to de-water and/or de-meth place your pump BETWEEN the processor and the condenser? The output of the pump would be at atmosphere and subsequent condensing should need less cooling.
OK, so I found out the usual way, by trying it out.
Vacuum at 20" Hg, WVO at 160F.
The pump gets really hot being between the processor and atmosphere. Once the water vapor hits the atmosphere side of the pump it expands, releasing heat which melted my hose AND the pump got too hot to touch.
So, I won't try to dewater WVO this way.
On to de-mething next at a much lower temperature
Put a water cooled condenser and receiving container between your boiling pot and your vacuum pump. It's a standard vacuum still design. A problem with larger scale applications is not collapsing your equipment with high vacuum (implosion). So thicker wall containers would be used.
I think what happens is that to expel the vapor, air and gasses the pump has to work real hard pumping them into a higher pressure - atmospheric - so the material being ejected doesn't expand, it compresses. This in itself raises the temperature of the material plus you have the energy contributed by the motor. It's complex.
Paulus, yes of course you're right. I had it backwards in my mind, it's just a heat pump.
I wanted to avoid getting water into my methanol condenser circuit so I bypassed all that but now I see I would have to cool the pump in that case.
Better to set up as mentioned above and not try to dewater my WVO this way.
In manufacturing the synthetic liquid fuel, biodiesel for example, the earth receives energy from the sun which photosynthesis captures such there is an energy gain on earth that can be tapped and used by people. Dewatering vegetable oil at atmospheric pressure requires making the oil hot, which is energy expensive. I expect it's probable that dewatering and demething crude biodiesel under vacuum would produce an energy obtained compared to the cost of making the fuel as a profit. I demethed methanol based biodiesel under vacuum with a still head temperature at 35 degrees centigrade at twelve inches of atmospheric pressure above zero, that's something like 300 millimeters of pressure above zero (a perfect vacuum) . Methanol boils at 65 degrees centigrade at atmospheric pressure normaly, but how much energy does the vacuum pump consume in electricity cost.
Jon Heron might be able to give practical advice here as he uses vacuum for his large setup.
How many amps does the vacuum pump draw, at what voltage, for how many hours, at what cost per kilowatt hour?
For example: 5 amps at 220 volts running continuously for 24 hours, at 10 cents per kWh
0.1*5*220*24/1000=$2.64 per day
I never had much success with a venturi so I now use a large Busch rotary vane vacuum pump.
The condenser is ahead of the pump and as mentioned I chill the collection tank to minimize the methanol that goes through the pump.
You can read about what I did here. http://biodiesel.infopop.cc/ev...=473104352#473104352
It works great, other than the condenser pipe should be as big as you can afford, 3/8" is too small for a processor that size.
Simple schematic for a pump and heater control with a high limit
Sensor for the biodiesel/glycerin layer
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