When you make larger amounts of biodiesel you will inevitably end up with quite a large store of glycerine. Each time you make 100 litres of fuel you can expect to get somewhere between 8 and 15 litres of crude glycerine. This glycerine contains glycerol (about 8 litres) and excess lye (the catalyst is always left over after the reaction). If you did a single-stage reaction with WVO, the crude glycerine will contain soaps formed by the neutralisation of the FFAs. If you have not done methanol recovery yet most of the methanol will be in the glycerine.
You would normally start by recovering methanol from the glycerine. This takes less energy than recovering it from the whole reaction mixture as there is less to heat. You will have to be careful as immersion heaters can easily burn out in glycerine: the heat does not spread easily. Most homebrewers who do this build a small still using water bath heating or a heat exchanger.
The equipment you need to recover methanol from glycerine is very similar to that used to distil alcohol. In fact it has long been the custom of moonshine-makers to throw the first cupful from the still onto the fire as it contains poisonous methanol. You can find a lot of relevant advice on websites dedicated to the art of backwoods distillation… bearing in mind that it is illegal in the UK to produce your own spirits for drinking purposes.
The basic principle is to bring about uniform heating of the glycerine inside a well-sealed vessel with a pipe coming out of the top. Methanol evaporates at 65C, but in practice you will need to get the glycerine to around 80C before significant quantities of vapour form. A small amount of pressure will build up, forcing the methanol vapour out of the pipe in the top of the vessel.
The pipe leads to a condenser, which cools the vapour rapidly, turning it back into liquid methanol which drips into a collection vessel. The condenser can be as simple as a coil of flexible copper pipe in the air. If you are doing this on anything larger than a mini-batch basis you will probably need to cool the condenser coil with water or possibly with a large cooling fan. Some people use a pipe-within-pipe arrangement as a condenser.
Methanol recovery can be a time-consuming process. Some homebrewers accelerate it by using a vacuum pump to pull the vapours through the condenser. At all times it is worth bearing in mind that you are working with methanol vapour which is hazardous to health and extremely flammable. Any equipment you construct needs to be properly sealed and completely free of the possibility of sparks that could ignite the vapour.
It’s easy to see why the commercially-available small biodiesel kits invariably ignore the challenge of methanol recovery and just leave it in the glycerine. We are still experimenting with different setups in order to find a cheap and safe design. The best advice we can give is to do your own research, and to seek advice from others before
implementing any design. Internet forums are probably the best medium for this.
Glycerol is very valuable in its pure form and large biodiesel plants always include equipment to purify it. Unfortunately this involves distillation techniques which are very complex and require expensive equipment. For the small-scale producer this is impractical. To sell crude glycerine you need to produce many tonnes. Therefore we have to find other uses for glycerine.
Purification
Although we can’t distil it, it is possible to purify the glycerine to the point where a soap-maker could use it. We can do this by adding phosphoric acid to the crude glycerine. This converts the soaps back to FFAs which separate out. The lye also precipitates to the bottom as a powder. The FFAs can be collected and esterified to make biodiesel. The remaining glycerine is about 95% glycerol.
Burning
Glycerine burns well but must be combusted at a high temperature or it releases acrolein, which is a very hazardous gas. It is also possible to etherify the glycerine to make a fuel.
Composting
Crude glycerine will compost well and the lye is a fertiliser. It will need mixing with cardboard or similar to avoid it turning into a slimy mess.
Soap-making
This is perhaps the most common use for crude glycerine on a small scale. You can make a simple liquid soap from crude glycerine: heat it to reduce viscosity, filter it, add about 5% essential oils to make it smell nice, allow to cool and stand in an open-top vessel for at least a week.
To make hard soap you can use materials you already have: KOH and water. Gloves and goggles on as always when handling lye.
Calculate the amount of catalyst you need to add to the glycerine: you need to use around 100g KOH for every litre of glycerine. Measure how much glycerine you have (in litres) at the end of your reaction, and multiply by 100 to see how many grams of catalyst you need. Take away the original amount of catalyst that you used in the reaction (which is already in the glycerine) and this is how much KOH you need to add (in grams).
- Calculate the volume of water you need to dissolve the catalyst: for every litre of glycerine you need 340ml of water. So multiply the number of litres by 0.34 to find the total amount of water you need in litres
- Heat the water to 32°C
- Slowly add the amount of catalyst you calculated in 1
- Stir until all the catalyst is dissolved
- In a large pot, heat the glycerine to 45°C
- Add the catalyst solution slowly
- Continue to stir slowly and keep the mixture at 45°C for 10 minutes
- Add around 5% essential oils to make the soap smell nice
- Pour the mixture into a container lined with a damp cloth or tea-towel
- Put a lid or a piece of board on top and wrap in a blanket
- Leave it for 24 hours
- Pull the block out of the container using the cloth
- Cut into smaller blocks the right size for bars of soap
- Let the soap sit for at least a month before use