In this section we will go through the process of making biodiesel using a simple one-stage base-catalysed transesterification. This is the method used by many biodiesel ‘homebrewers’ throughout the world, with many variations possible in the detail. With care, biodiesel made by this method will work in any Diesel engine. It will fit the criteria for the lower duty rate in the UK though it is unlikely to meet all the stipulations of the European Biodiesel Standard EN14214. Later on in the manual we will look at some elaborations on this method that can improve the quality of the final product or allow you to use different feedstocks.
Raw materials
Feedstock
The most important raw material is of course the feedstock. This process will work with fresh oil, but we are presuming that you will be using waste oil (WVO).
Most small-scale biodieselers will want to collect oil themselves. This is generally quite easy as catering outlets are often charged for disposal by collectors and are more than happy for you to take their oil away for free. Many of them will admit in private that they dispose of their waste oil by improper and illegal methods. Either way you are doing them a favour by collecting it. Bear in mind that larger outlets may be tied in to contracts with waste oil collectors who might not appreciate you on their turf. Most restaurants and fast food outlets store WVO in the 20-litre drum it came in originally. If you have a choice, try to get light oil that is very liquid: Chinese restaurants are a good source. Some caterers may use palm oil that makes biodiesel that gels when it gets cold. Others may use animal fats. While it is perfectly possible to make good biodiesel with these feedstocks its better to start with easier, more liquid vegetable oils. You may also encounter very dark oil that might be the result of over-heating or overuse. Avoid this oil as it’s very high in FFAs. You will also probably want to avoid these restaurants from a health point of view! You can expect to get 2 x 20-litre cans from the average fast food outlet per week.
You need to prepare yourself with gloves and dirty clothes, as it can be a messy job. Try to make sure that no oil is spilt during transport and storage: it’s surprising what an environmental hazard vegetable oil can be, especially if it enters watercourses.
Alcohol
Biodiesel can be made with either methanol (methyl esters) or ethanol (ethyl esters). Methanol may be preferable for a number of reasons:
- the reaction is much easier to do with methanol
- methanol is cheaper
- there is more available research on methyl esters
In some ways it would be better to use ethanol from an environmental perspective. Even though methanol was originally produced from wood biomass it is now almost exclusively produced from fossil fuels, whereas it is possible to get ethanol from non-fossil sources. We’re hoping that it will become possible to get biomethanol in the UK at some point so that the climate change impact of methyl esters is reduced still further. Making ethyl esters remains an advanced technique.
Methanol is available from a number of chemical suppliers. You may have trouble with some who are unsure about delivering to a non-industrial address. Generally you need to buy a 200l barrel to get a decent price. 98% purity methanol or ‘technical grade’ is what you are after.
Lye
The lye we use to catalyse the reaction and neutralise the FFAs can be sodium hydroxide (NaOH) or potassium hydroxide (KOH). NaOH is somewhat cheaper, compounded by the fact that less is needed for the reaction, but we use KOH for a number of reasons:
- KOH makes for an easier reaction
- KOH is slightly less dangerous
- not much is used, so the price difference is not so significant
NaOH and KOH are available from chemical supply houses. Ideally you want technical grade (98%). Supply companies are likely to be less worried about supplying lye than they are about methanol.
Only use fresh chemicals. Buy as much as you’re going to use in the coming few months, and use them, don’t store them – they’re dangerous, and they go off. Sodium / potassium hydroxide is carbonated by CO2 in the air, for example.
Lye can be very dangerous (see ‘health and safety’), especially if it gets into the eyes. One grain in the eye is enough to cause terrible damage, and a minimum three-month healing period (if it heals at all). KOH tends to consist of bigger, more visible and less mobile flakes, and is therefore safer. You can also get both types of lye in pellet form.
NaOH is sometimes sold as drain cleaner but is unlikely to be very pure.
Equipment
The equipment you need varies with the scale on which you make biodiesel. In order to describe the process we will suppose that you are making ‘mini batches’ based on 1 litre of WVO. For more information on scaling up the process see the ‘small-scale reactor’ section.
You can get the equipment from a number of sources including DIY shops, online tool catalogues, laboratory suppliers, jumble sales, your loft etc. In the following list we have suggested possible sources for some of the items in square brackets
safety equipment:
- eye-wash
- goggles
- chemical-resistant (nitrile) gloves
- vinegar in squeezy bottle [washing-up liquid bottle]
- hose connected to tap, with spray nozzle on other end
- sawdust to absorb spills [from sawmill or timber yard]
- paper towels or rags
titration kit:
- 20ml beaker
- isopropyl (rubbing) alcohol [lab supplies or pharmacy]
- de-ionized water [car parts or DIY shop]
- pH meter or phenolphthalein indicator (not universal indicator)
- calibrated pipette or syringe
- stirrer
- precise scales (to 0.1g)
mixing and reaction:
- 250ml stoppered flask (glass) [lab supplies]
- hot plate (electric) [jumble sale]
- petri dish or similar
- 1500ml calibrated beaker
- strong jar around 2 litre. [e.g. bulk mayo jar]
- old blender or drill with paint stirrer
- old saucepan
- thermometer
other useful stuff:
- pen and paper
- assorted containers and vessels
- sticky labels
- ladle
The process
Caution: first, make sure you have read the health and safety section, and that you are wearing goggles, mask and chemical-resistant gloves.
Step 1: WVO pre-treatment
You want to start with about 2 litres of WVO for a 1-litre mini-batch, so we can pull off the best oil. The oil for your first batches should be liquid at room temperature. If you have some oil stored use oil from the top as the heavier fats move to the bottom over time.
Water in the oil will cause problems with the reaction. Too much water combined with the soap formed during the reaction could form an emulsion and your biodiesel could end up looking like chicken soup. For mini-batches we heat up the oil to over 100°C to boil off the water. Don’t heat it too much though or you may char the oil, and create FFAs. Be careful: do this outside as the oil will spit and writhe.
When making full batches we use a different dewatering method: raise the temperature of the oil to 50-55°C and allow to cool slowly (you can insulate the vessel with an old blanket or loft insulation) and settle. Water won’t settle during heating because of convection currents. Water will fall to the bottom of the vessel because it’s heavier than the oil, and can be tapped off the bottom. This method uses much less energy than trying to drive it off by heating to over 100°C, and is also quite effective.
Pour the oil into a jar through a sieve to remove any large bits like chips, cigarette ends, or large visible chunks. You can also use a J-cloth as a filter. This does a good job of getting smaller particles out of old oil, though it can take some time to drip through.
Step 2: titrate
In the chemistry section we saw that we use lye (in this case, KOH) for 2 purposes: as a catalyst for the transesterification and to neutralise the FFAs.
The amount of KOH we need to perform the transesterification is fixed: for every litre of oil, you need 9 grams of KOH (or 3.5 grams of NaOH). This amount has been worked out using the transesterification formula based on the average composition of vegetable oils.
If we were using fresh vegetable oil, we would just use 9g of KOH. But with WVO we need to add an amount to neutralize the FFAs. This amount will depend on the oil in question. To work out how much we need, we perform a titration.
For every batch of oil, you need only one titration, although you do need to mix the oil well first, to make sure that it is homogenized, and so has the same properties throughout. You do, however, need to do a titration for every new batch of oil, as the free fatty acid content of each batch will be different.
It’s best to perform the titration in a warm area. Whenever you do a titration you will need a standard solution of KOH in distilled water. We will start by preparing this. It should last for a long time if kept stoppered.
Making standard solution
- weigh 1 gram of KOH on a petri dish on a scale
- take a 1 litre stoppered container and fill with de-ionised water. You will probably have bought the de-ionised water in such a container
- pour the KOH into the distilled water and swirl well to dissolve. It should dissolve fairly easily
- label this bottle ‘KOH standard solution’
Titration
- Add 10ml of isopropyl alcohol to a beaker.
- Add a few drops of phenolphthalein and stir.
- Use a calibrated pipette / eye dropper to add 0.5ml of the KOH standard solution into the oil / alcohol and stir.
- Observe the colour of the mixture. You will notice a flare of red as you add the KOH solution, which will disappear as you stir.
- Continue to add the KOH standard solution, 0.5ml at a time, until the red colour persists after stirring. This occurs when the isopropyl alcohol itself has been neutralised. This is called a ‘blank titration’; we have now accounted for the acidity of the isopropyl alcohol.
- With a calibrated pipette / eye dropper, add 1ml of the WVO to the isopropyl alcohol and stir. It’s best to take 1ml of oil from your sample after you’ve filtered and de-watered it. You need to dissolve the oil in the alcohol completely. You will notice that the red colour disappears again: the FFAs in the oil have made the mixture more acid. At this point you start keeping a record of the amount of KOH solution you are adding.
- Continue to add the KOH standard solution, 0.5ml at a time, until the red colour persists after stirring. (this is when the free fatty acids have been neutralised). The exact colour will vary with the colour of the WVO: what you are looking for is a definite and persistent colour change, normally to a brick red or purple.
- Note down the amount of KOH standard solution you have added since completing the blank titration; this figure will normally be no more than 3ml.
We used to use a pH meter to perform the titration. The technique is similar: you keep adding KOH standard solution and plotting pH readings on a graph to find the point at which the pH rapidly rises . We have moved to using phenolphthalein on our courses and in the lab because cheap pH meters are often inaccurate and all pH meters need careful cleaning and recalibration. However you titrate, remember that you are looking for a sudden change in pH as the FFAs are neutralised. This neutralising point is not at pH 7 because we are neutralising a weak acid with a strong base which forces the pH upwards (‘the weak acid is incompletely ionised in solution’ for all you chemists!). Don’t worry too much about precise pH: it’s the sudden change we are looking for. Some biodiesel recipes leave out the blank titration stage, but when you have got the hang of it, it only adds minimal complication for a more precise result.
Calculations
The number of ml of KOH standard solution it takes to neutralise the FFAs in 1ml of WVO equates to the number of grams of KOH it takes to neutralise the FFAs in 1 litre of the same WVO.
x ml of KOH standard solution for 1 ml of WVO
x litres of standard solution for 1 litre of WVO
x litres of standard solution contains x grams of KOH
This is rather handy! To find the total amount of KOH we need we just add the amount needed as catalyst for fresh oil (9g) to the result of the titration. So if the titration endpoint was reached after 2.5ml of KOH standard solution was dripped into the oil/alcohol mixture, the total amount of KOH needed would be 9+2.5 = 11.5 gram per litre of WVO
That baseline amount of 9g is the amount of KOH needed to catalyse the reaction . This will actually be different for different feedstocks but a figure of 9g gives good results in our experience across all the types of WVO we have worked with. If you are using NaOH (Sodium Hydroxide) the baseline amount is normally given as 3.5g. Of course if you use NaOH you should also titrate with NaOH standard solution rather than KOH.
Step 3: mix methoxide
For every 1 litre of oil, you will need 200ml of methanol. This needs to be mixed with the amount of KOH we calculated from the titration.
Put 200ml of methanol in the stoppered flask. Weigh the correct amount of KOH and add to the methanol. Swirl or stir the flask until all the KOH is dissolved in the methanol. You will notice that as the KOH dissolves it will release heat and the flask will become warm.
Take care with this step as it involves the two most dangerous chemicals we use in simple biodiesel making. Mixed together they’re no nicer!
Biodieselers often call this mixture methoxide. Remember that it is not a chemical compound but a mixture: there are no bonds between the KOH and the methanol.
You might be wondering about the 200ml figure. Like the baseline amounts given for catalyst, this 20% methanol figure is subject to some debate. In large-scale biodiesel manufacture, 25% is often used. On the other hand, in making crude biodiesel, quantities as low as 12% have been used. In general the more you use up to 25%, the more complete the transesterification reaction is, but the more excess methanol you have to recover once the reaction is over.
Step 4: the reaction
Add the methoxide you have prepared to the oil. Although it will work at a range of temperatures, the reaction works best if the oil is at about 45°C. Too much higher and the methanol (which boils at 65°C) starts to evaporate off. As soon as the reactants are mixed, you need to start stirring the mixture with the blender or other stirring device. A violent stir in the first few minutes of the reaction ensures best conversion to biodiesel.
You need to be careful to avoid breathing in methanol vapours. Also if you are using an electric motor to stir the reaction, be aware that a spark from the motor could ignite methanol vapours. Never use a drill or similar motor on anything bigger than a mini-batch.
On a mini-batch, 10-20 minutes of stirring will easily suffice. On larger batches it may be hard to get efficient stirring and some biodieselers stir for 2 hours or more. When oil and methoxide are first mixed, the mixture is a milky colour; when it’s ready, it’s dark – look for the colour change as an indication that the reaction is complete.
Step 5: settle
Once the reaction is complete you need to leave the mixture to stand overnight. The next day, a thick brown liquid will have settled at the bottom. This is glycerine, and the lighter-coloured liquid above it is biodiesel. Biodieselers tend to use the term ‘glycerine’ quite loosely: in this layer there will be glycerol, most of the surplus methanol, most of the catalyst, and most of the soaps formed when the FFAs were neutralised. You can expect to have around 120-180ml of this ‘glycerine’ layer, which is more accurately called the ‘by-product phase’. Of this, perhaps 80ml will be glycerol, up to 80ml could be methanol, the rest is mostly soap.
Step 6: separate
The biodiesel and the glycerine are easy to separate. In a jar you can pour most of the biodiesel off the top as the glycerine is much more viscous. In a reactor you would draw the glycerine off the bottom.
Congratulations: you have made biodiesel! Or have you? There are a few simple tests you can perform to see whether your reaction has been successful
Step 7: test
The simplest test is the visual test. You are expecting to see a very clear distinction between biodiesel and glycerine. If that is present then you have most likely succeeded to some degree. If you have a very small amount of glycerine (i.e. less than 100ml) then it is likely that your reaction did not proceed to completion. The likely cause is using too little catalyst. You might want to check the titration again, or reprocess your product to see if more glycerine will come out. If there are large amounts of visible soaps between the two layers you may have over-catalysed or failed to de-water the oil correctly. Over-catalysing will occasionally lead to the formation of a gel or emulsion.
You should have about the same volume of biodiesel as you had vegetable oil in the beginning, or a bit less if it was high in FFAs, as these will have turned to soap.
You can do simple tests for density and viscosity. Using a hydrometer with a scale from 800-1000 you can check the density of your fuel. It should be between 880 and 900 (that is, your biodiesel should weigh between 880 and 900g per litre). You can also check this with a set of scales.
It is not easy to get an absolute measure of viscosity but you can get a comparative measure. Pour a measured amount of biodiesel into a tin with a hole in it and time how long it takes the tin to empty. Do the same for a sample of the original WVO and for a sample of petrodiesel if you have some. The biodiesel should go through the hole much faster than the original WVO, showing that you have succeeded in reducing the viscosity. The measured times for the biodiesel and the petrodiesel should be similar, though don’t worry if your biodiesel is a little more viscous.
Step 8: wash the fuel
At this stage you have succeeded in making a small amount of basic biodiesel. You could decide to scale up production using the same method, and in fact this is what many homebrewers across the world have done, some of them with reported success. However the fuel we have made is still rather crude. Not all engines will run happily on it as it stands, or if they do, they may not continue running for all that long! Although most of the impurities are in the glycerine, there are still small amounts of lye, soap, methanol and water in the biodiesel. These will cause trouble in your fuel system and engine. The easiest way to remove them is by washing the fuel with water. You can try this with a mini-batch. Add water to a jar containing your biodiesel. Add the water carefully, dribbling it down the side of the glass. You will notice that the biodiesel floats on top of the water (remember that it is less dense than water: the density is 0.88 to 0.9 percent that of water). Keep adding until you have about 300ml of water to your litre of biodiesel. Now stir the mixture quite gently, but enough so that the two layers are mixed together. You will notice that the water becomes cloudy. As you mix, the lye, soaps and methanol are becoming dissolved in the water. This is because the water is better at dissolving the impurities than the biodiesel is: it is a more polar solvent.
You need to mix carefully because of the presence of soap. Soap is an emulsifier: if you mix too violently the biodiesel and the water will become bound together in an emulsion: the dreaded ‘chicken soup’. As you wash, more of the soap becomes dissolved in the water.
Carefully remove a sample of the biodiesel from the top of the jar using a ladle or similar. Add this to another jar and repeat the washing process. This time you will find that you can stir more violently without forming an emulsion, as much of the soap will have been removed. One of the less obvious benefits of water washing is that as the soap is removed from the biodiesel, the biodiesel becomes less able to bind water molecules, so in effect we are using water to dry our fuel! Water washing also removes the small amount of free methanol in the fuel. This stops the reaction which would otherwise continue very slowly, dropping out small amounts of glycerine which can clog the fuel filter of your vehicle.
The initial results of water washing are often rather disappointing. The lovely transparent golden liquid you found after your morning’s settling has been transformed into a straw-yellow and somewhat cloudy liquid. Don’t panic. The haziness is due to a small amount of water in the fuel which will quickly settle out. Just take another look at that wash water and you can no longer doubt that washing was a good idea. Even better – test the pH. It will normally be basic (pH >7). We want our biodiesel to be as near neutral (pH 7) as possible.
Step 9: filter & de-water the fuel
It’s always necessary to give the fuel a final filtration before using it in a vehicle. Although the fuel system incorporates a fuel filter which removes particles and traps water, it’s best not to overburden it. The simplest way to filter a mini-batch is to pass it through a J-cloth or similar tightly-woven cloth. This filters to about 10 microns. Even If you already did this with the WVO at the beginning of the process, it is still worth doing again as there are plenty of opportunities for unwanted bits to get into the fuel during the process.
Biodiesel, like petrodiesel, is hygroscopic, meaning that it absorbs water from the atmosphere. Normal biodiesel has 1200-1500 ppm (parts per million) of water. Any water above this level will damage your engine if it gets past the fuel filter. If your fuel is well washed, it will not hold on to much water. However if it remains cloudy after a day or so of settling, it is advisable to de-water. You can do this using the same technique as for WVO: heat to 50-55°C and allow the water to settle out.
Step 10: deal with the by-products
Wash water contains soaps, lye, and a small amount of methanol. In mini-batch size quantities it should be ok to put this down the drain. You can think of it as bath water into which you accidentally spilled some of your bathtime whisky.
The glycerine will contain soaps, glycerol, lye and quite a lot of methanol. The 150-odd ml you get from your mini-batch can be left somewhere well-ventilated to evaporate the methanol, after which you can use it to make soap or compost it.
Of course in larger quantities the by-products need to be dealt with more comprehensively.