The Chemistry of Biodiesel Production

Making Biodiesel From Vegetable Oil
This article is a simplified version of what happens at the molecular level when Biodiesel is made. It’s meant to help you understand what’s going on when Biodiesel is produced and can help you trouble shoot any problems you may run into when producing Biodiesel.

Biodiesel starts out in the form of an organic oil molecule called a triglyceride. These are molecules with three fatty acid chains connected to a glycerol backbone.

G–FATTY-ACID-CHAIN
G
G–FATTY-ACID-CHAIN
G
G–FATTY-ACID-CHAIN

To make Biodiesel, a catalyst is needed; either Potassium Hydroxide (KOH) or Sodium Hydroxide (NaOH). The catalyst is then dissolved into methanol to create what is called methoxide. The oil is heated and the methoxide is mixed in and the chemical reaction begins.

As the catalyst is mixed with the oil, it breaks the chemical bond between the fatty acid chain and the glycerol back bone. A methanol molecule then attaches to the fatty acid chain and a Biodiesel molecule is created (technically called a Fatty Acid Methyl Ester).

G [METHANOL–FATTY-ACID-CHAIN]
G
G–FATTY-ACID-CHAIN
G
G–FATTY-ACID-CHAIN

This process, technically called transesterification, repeats itself until all the fatty acid chains are stripped away from the glycerol backbone and reacted into Biodiesel.

[METHANOL–FATTY-ACID-CHAIN] = BIODIESEL

G [METHANOL–FATTY-ACID-CHAIN]
G
G [METHANOL–FATTY-ACID-CHAIN]
G
G [METHANOL–FATTY-ACID-CHAIN]

So, this is the way it happens in theory. Break the bond between the fatty acid chain and glycerol and attach each fatty acid to a methanol resulting in Biodiesel molecules. This all happens in sequence meaning that each fatty acid chain is broken away one at a time.

To help the explanation out, it’s important to know that a glycerol molecule with 3 fatty acid chains is called a triglyceride. If it’s lost one fatty acid chain and only has two still connected it’s called a diglyceride. And if it only has one fatty acid chain left, it’s called a monoglyceride (Tri=three, Di=two, and Mono=one). If there are no fatty acid chains attached, it’s called Free Glycerin.

So, that’s how it happens in theory….in the real world, things are much different. What usually happens is that before the catalyst & methanol even get added, many of the oil molecules are already missing fatty acid chains.

This occurs naturally as the oil ages or as the oil is used in fryers under extreme heat. This means that waste vegetable oil tends to have a lot of diglycerides and monoglycerides in it (glycerol molecules missing some of their fatty acid chains). The older the oil is or the longer it was used under heat, the more of them there are.

When the fatty acid chains break off through aging or through heat, we refer to them as free fatty acids (or FFA’s). This is because they’re not hooked to any other molecules. There’s a lot of fancy chemistry that explains how they break away from the glycerol molecules, but for the purpose of this article, let’s just assume that the chemical bond that held the fatty acid to the glycerol has “rusted” away. This means that each “free-fatty acid” has one end that’s rusted and there’s a bit of rust still left on the glycerol backbone. We’ll designate the rust with an r.

(MONOGLYCERIDE EXAMPLE WITH “RUSTED” BONDS)
Gr rFATTY-ACID-CHAIN
G
Gr rFATTY-ACID-CHAIN
G
G–FATTY-ACID-CHAIN

When we try to make Biodiesel with free fatty acids in the oil, these free fatty acid chains start attacking the catalyst and form soap.

[CATALYST-rFATTY-ACID-CHAIN] = SOAP

If there’s enough free fatty acids in the oil, we end up with lots of soap and not enough catalyst left over to break all the bonds of the remaining tri, di, and monoglcyrides molecules. In other words, we end up with soap, unconverted oils (tri, di, & mono’s), and some Biodiesel. I like to call this mixture partially reacted Biodiesel; because there’s still so much unreacted oil left in it.

To counter this problem, we do something called an oil titration on the oil. What we’re doing when we titrate oil is measuring how many free fatty acids there are in the oil. Think of it like literally counting all the fatty acid chains that have broken away from glycerol molecules.

Once the free fatty acids are all counted (the titration number), we use that number to tell us how much extra catalyst to add to the reaction to ensure we react all the free fatty acids into soap. Then, when we’re done making soap, we still will have enough catalyst left over to break the bonds on fatty acid chains still attached to glycerol molecules which is then reacted with methanol into Biodiesel. We call making Biodiesel this way a “base reaction”. A strong base (KOH or NaOH) reacts with the oil and methanol to make soap and Biodiesel.

The more free fatty acids that a given oil has, the less Biodiesel we will get out of it. This is because we’re reacting free fatty acids into soap instead of Biodiesel–remember, it’s the fatty acids combined with methanol that make Biodiesel (not the free fatty acids). If there’s a lot of free fatty acids in the oil, that means there’s a lot less fatty acids left that we can make into Biodiesel. So, the higher the titration a given oil has, the less Biodiesel you’re going to be able to get out of it.

So, to get well converted Biodiesel, we always titrate the oil, add the extra amount of catalyst required to “neutralize” the free fatty acids into soap and then still have enough left over to fully strip the remaining tri, di, and mono glycerides of their fatty acid chains and react them into Biodiesel.

And that’s the basics of the chemistry behind how Biodiesel is made.

SPECIAL NOTE ABOUT WATER:
Just a note about water in the oil. When water is present, it makes the soap problem even worse.
This is because Oil + Water + Catalyst = Soap (not Biodiesel). So if your oil is wet, not only are you dealing with free fatty acids eating up your catalyst, but the water is also going to use some of it to make soap out of not only free fatty acids but it’ll pull from the good fatty acids as well. (That’s actually how they make soap. They take organic oils, mix in some catalyst–either Sodium Hydroxide or Potassium Hydroxide–, add water and make soap!)

One of the absolute best things you can do to help your reactions go well is to remove any water from the oil before you get started. We call this “dewatering the oil”. My recommendation is to keep the water content in oil under 1500 ppm (or 0.15% water content). The dryer the oil, the better chance you have of getting better reactions.

Check out the video below for a visual example of this article using my hands.

If the oil titrates too high, Sulfuric Acid can be utilized to effectively lower the titration and still give us decent yields. Click here to see how that process works

Additional Resources
Introduction To Oil Chemistry And Transesterification – John Bush
Video presentation – John Bush
Biodiesel Composition and It’s Effect on Fuel Properties Gerhard Knothe
Video Presentation Gerhard Knothe

About Utah Biodiesel Supply

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