Methanol Recovery Systems
The theoretical amount of methanol needed to produce Biodiesel is about 12-15% of the oil being processed. However, since the chemical reaction is reversible, the amount needed to make a complete and full reaction is 20+%. This leaves left-over methanol in the finished product and especially in the glycerol. Methanol should be removed from Biodiesel before use as a fuel. Glycerol requires further treatment (methanol removal) before it can be further processed into a more pure glycerin for other recycling purposes.
Methanol recovery may not be worth it for everyone. The average amount of methanol which can be recovered from the biodiesel after a 24 hour settling period is about 1.5%-2% the volume of the biodiesel. Even with the use of solar technology for free heating, I personally have found the time and effort needed to distill 3 to 6 liters of methanol from my 95 gallon batches of biodiesel a waste of time. I prefer to evaporate my methanol into the air through agitation from a fishtank compressor. The remaining excess methanol will be in the glycerol, which can also be recovered by distillation or evaporated into the air. Whether it is worth the time and energy put into it or not is up to the user, and based on how expensive the methanol cost is and time needed to do the distillation.
To recover methanol, distillation must be used. Distillation is when the methanol is boiled out of the other fluids it is mixed in with. Since methanol boils at a much lower temperature than biodiesel, water and the glycerin, this is not too hard to do. However, it is important to have a fully closed metal vessel with gradual heating and pressure gauges to be sure the methanol does not cause rupture of the tank and that the tank is sure to survive the pressure and heat. Most plastic tanks can only handle about 140°F and not much pressure. methanol needs to be about 150°F at normal atmospheric pressure before it will boil.
Also, when methanol is dissolved into biodiesel or glycerin, it will require good amount of agitation or higher temperatures to get all of it out. Instead of expending lots of energy trying to use only heat, , it is best to utilize a vacuum pump or diaphragm pump. When the methanol is put under vacuum, it will boil at lower temperatures. The vacuum suction from a diaphragm pump also helps force the fumes through the condenser, so there is less waiting. I personally use our 40 psi diaphragm pumps, which will pull up to a 15mm HG vacuum, plenty enough to boil out all of the methanol with low enough temperatures. On a final note, remember to never allow your fluid temperature to go above the boiling point of water so that water in your biodiesel/glycerin will not end up in the methanol. Keep in mind that the boiling point of water will also drop when under a vacuum.
If a diaphragm pump is used for creating the vacuum in the vessel, the outlet of the diaphragm pump can push the methanol into a closed container. A check valve can be installed on the top of that container to blow out air when pressure is gained from the diaphragm pump pushing air and methanol into it. The check valve will keep the container closed from ambient air but prevent dangerous pressure build-up.
Once the methanol has begun boiling off, it will need to be captured and directed through a tube into a condenser. a condenser is simply a heat exchanger where a cooler fluid is used to cool the methanol vapors, causing it to condense back into a liquid and drop into a new container, free of the other fluids it was once in. Condensers are easy to build. You can use a simple copper tubing going through a bucket of water as a simple and cost-effective method. However, if you want to use something a bit more effective, it would be ideal to use one of our plate heat exchangers, pumping cold water from a container through one side of the heat exchanger
and allowing the methanol to travel through the other side, condensing into a sealed container. You can also hook up the exchanger directly to a water line if you choose not to use a pump but this will waste a lot of water. The methanol does not take too much heat loss to reduce it back to a liquid, and so having the coldest water is not so important unless the flow is very slow. I usually use a drum of water for my batches, circulating through a plate heat exchanger. When finished, the warm water remains in the drum so heat can slowly bleed off back into the ambient air.
You can find metal fabricators who can build closed vessels for methanol recovery out of drums, welding off most areas for a complete seal. Usually, a drum with a tight lever seal and gasket is good enough to prevent methanol leakage as well. Ports will need to be drilled and welded together to allow for the methanol to escape, draining of the treated fluid after demethylization, and other ports for filling and possibly circulation with a pump. We have found that using old water heater tanks is ideal when there is a stainless steel lining, since it is already fully closed and the stainless steel is ideal for resisting biodiesel and the glycerol.
One methanol has been recovered, it can be reused in your next batch. The less water that gets into the methanol, the better. It is best to add recovered methanol directly to a fresh drum of methanol so any water in it will evenly distribute to the larger volume of pure methanol, this keeping a much more pure methanol and less chance of soaps in the next batch.
After recovering methanol, it is good to test the purity of it before using it. To test the purity is simple. Take a 100 ml sample of the methanol, weigh it in grams. Take the number of grams you weighed, divide by 100ml and you get the density in g/ml. Check the density you find against a methanol density chart to see what percentage you have. This is also a great method to test old methanol that has been stored for a long time. When the purity is in question, a density check can tell you if it’s still good and free of water.
Please take caution when recovering methanol vapors. Methanol is poisonous and can cause blindness or death when inhaled in large concentrations. be sure everything is sealed off, and use in a very well-ventilated area.