Basic Biodiesel Chemistry

alt Diesel Fuel and Vegetable oil are similar in that they both have a lot of carbon and hydrogen atoms bound together in a big molecule, giving both a high energy density and making them great fuels for motor vehicles. The big difference is that vegetable oil is thicker and less runny than diesel fuel. That's not that big a problem with the older extremely robust Indirect Diesel Injection type engines using mechanical fuel Injection Pumps. The problem arises with the push to make diesel engines more fuel efficient. As they become more efficient, they have become less tolerant of fuels that don't meet the official definition of diesel fuel.

The difference in thickness (or viscosity) is due to way the atoms are arranged in their respective molecules. Diesel Fuel is one long chain of carbon atoms studded with hydrogen atoms. With vegetable oil, three of those long chains are stuck onto a glycerin backbone. Biodiesel chemistry is all about breaking those long carbon chains off the glycerin backbone, so that the result is something that is closer to diesel fuel than vegetable oil.

One Liter Batch of Biodiesel The chemical process that works best is called transesterification. That's a long word that simply means removing the long chain of carbon atoms from the glycerin and sticking them onto another alcohol, like methanol.

To get the carbon atoms to jump from the glycerin to the methanol we need something to drive things along. We call that a catalyst. The catalyst is needed for chemical step in the middle that bridges the gap from vegetable oil to biodiesel. It is only used to bridge the chemical gap. It is returned to its original state at the end of the reaction so, is not used up.

To make biodiesel we dissolve the catalyst, usually either NaOH or KOH, into the methanol and mix with our vegetable oil, usually heated to about 50°C. Most of the reaction takes place in the first few minutes, but we continue mixing for up to two hours in order to maximize the conversion from vegetable oil to biodiesel. Once the mixing is complete we let it settle. Over time the glycerin will settle to the bottom with the biodiesel on top.

Forcing Biodiesel

They call biodiesel chemistry “Reversible” meaning that once the chemistry starts it will go in both directions from vegetable oil to biodiesel and back to vegetable oil. It will eventually reach a balance with some biodiesel and some vegetable oil. Scientists have found that they can force that balance to be better than 97% biodiesel with the right conditions.

One of those conditions is that we keep water contamination of our vegetable oil to an absolute minimum. The reason is that water will react with the vegetable oil and catalyst to make soap in a process called saponification. With enough water, this unwanted chemical reaction eats away at our catalyst, and throws off the conditions needed to make good biodiesel. There is no absolute cutoff, the drier the vegetable oil, the better the biodiesel. You can have too much water. Once you get more than about 5% water in your oil, the unwanted side reaction cascades uncontrollably, using up all the catalyst making soap, without making any biodiesel. If you are using NaOH when this happens you get a tank full of glop. Glop is when soap goes solid trapping oil inside the soap matrix.

Glop or Badly Brewed Biodiesel Another condition is limiting how much free fatty acids are present in our vegetable oil. Free Fatty Acids, or FFA, are the result of vegetable oil breaking down during cooking. The long carbon chains are broken away from the glycerin in a way that makes them acidic and unstable. When these FFA come in contact with our catalyst, they react with it to make soap and water. The soap part is not to bad. We know how to wash the soap out. As we have already seen, the water is our problem. The higher the FFA, the more water we will produce when the FFA turns into soap. This gives us a fuzzy limit on how much FFA we can have in our vegetable oil.

We measure the FFA in our vegetable oil by titrating it. When we titrate we are actually measuring how much catalyst will be used up by the FFA. There are two articles on the site that explain how to titrate WVO.

The articles use different descriptions and tools to arrive at the same end point. That is, ‘how many grams of catalyst are needed to deal with the FFA.’ With that knowledge we can build a recipe that has enough catalyst left over to make good biodiesel.

Another major condition is that we make our biodiesel using excess methanol. In fact we usually use about twice as much alcohol as is needed theoretically. That works out to about 20% of the volume of the vegetable oil we will be converting.

The Results

All that extra methanol and catalyst has to go somewhere and it ends up being divided between the glycerin and the biodiesel. About 1/3rd of the methanol and catalyst stay in the biodiesel. More specifically the biodiesel is a mixture of partially converted vegetable oil, biodiesel, soap, methanol, water, and catalyst. A purification process is needed to remove the contamination from the biodiesel. Biodiesel can be purified either by water washing or by drywashing, both discussed extensively elsewhere. Whatever method of purification you use on your biodiesel, the worst contamination is methanol. It is the primary solvent holding all the bad stuff in the biodiesel, and removing the methanol will certainly make your biodiesel better.

The glycerin byproduct is a mixture of glycerin, water, methanol, soap, biodiesel, and catalyst. It needs treatment before it can be safely used or trashed. It contains up to 2/3rds of the excess methanol, meaning the raw glycerin is over half methanol. It is well worth the effort to distill the methanol out of the raw glycerin. Once the methanol has been removed, there are dozens of great uses for your glycerin from feeding livestock to making soap.

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How to make biodiesel at home

WARNING

Making Biodiesel requires the use of flammable, toxic liquids and strong caustics to make a fuel. No matter what safety precautions are put in place or what equipment you use, making biodiesel will never be a safe hobby and can place you, your property, and your family at risk of injury or even death. Make Biodiesel at your own risk.