Small and medium sized biodiesel producers are growingly interested in exploring simpler, more environment friendly ways to make biodiesel. Several factors make these alternative production methods appealing: (1) producers can avoid the labor-intensive glycerol removal process, (2) utilize ethanol instead of methanol and (3) sidestep the hazardous use of sodium hydroxide or other strong alkali. Currently, methanol is preferred over ethanol only because it is less sensitive to water in the alkali procedure. Imagine a process where one doesn't have to worry about this (and where less toxic ethanol could be used).

Processes that allow biodiesel producers to simply mix alcohol and oil, pour the liquid over a catalyst and collect the products as they drip into a receiver, are trouble-free, flexible and growingly viable. Glycerol is still produced, but there is no need to wash the biodiesel esters and neutralize the corrosive alkali.

Drawbacks of standard methods
Standard methods for producing biodiesel, a mixture of alkyl (methyl or ethyl) esters of fatty acids, is to heat vegetable oils or mixed animal-vegetable oils with (a) a strong alkali and (b) methanol or ethanol. Biodiesel forms during a process known as transesterifcation, which is simply the exchange of the original alkyl group of the fat (oil) with the methyl group of the methanol (or ethyl group of the ethanol). The diagram below displays transesterification.

RCOOR' (fat or oil) + NaOH (alkali) + MeOH (methanol) ? RCOOMe + glycerol (biodiesel ester)

The RCOOMe (mixture of several methyl esters) is the biodiesel (methyl ester of fatty acids from the animal or vegetable oil).

The final biodiesel product still has to be washed with water, neutralized and "dried" or "salted out" (to remove water) to be suitable for use as a biodiesel fuel or fuel additive.

The initial alkali reaction transesterification reaction requires the removal of glycerol, partway through the reaction, which is a real time consumer and is labor intensive.

Methanol is convenient, because of its insensitivity to water, but it is a questionable environmental choice. Methanol is highly toxic to humans. Even a few milliliters can cause blindness and violent nausea, as historians can tell us. During the Prohibition, when ethanol was not readily available, people drank "wood alcohol" (methanol) and went blind, or even died. Vapors of methanol are irritating to the lungs and can damage the eyes, even in small doses. Methanol is metabolized by the body to formaldehyde (which is also very toxic) and then to formic acid (which is also toxic). Formic acid is found in the bodies of stinging ants.

Sodium hydroxide, also known as "lye," is very corrosive. It also dissolves slowly in methanol. On a large scale, solid sodium hydroxide is exothermic (evolves heat) and can cause the alcohol to heat or boil. Handling sodium hydroxide and methanol, can be dangerous to workers-especially those without chemistry education and laboratory safety experience. A far safer procedure is to avoid the sodium hydroxide altogether.

Another problem is the interference of glycerol (glycerine or propane-1, 2, 3-triol). The glycerol has to be removed in order to drive the equilibrium of the reaction to the right, that is, to get a good yield of the biodiesel methyl esters (instead of returning to the original oil and alcohol).

Using a Simple, Greener Chemistry
For many producers-and future producers-the idea of using a process in which you could simply mix alcohol and oil, pour it over a catalyst and collect the products as they drip into a receiver (flask, jar or other collection vessel) is very attractive.

There's good news. These non-alkali methods have been developed by research chemists. These processes are not only simple, but flexible as well.

First, a biodiesel producer could easily use ethanol in place of methanol, even if the ethanol contains a little water. Ethanol is much less toxic than methanol, and you've probably tasted it if you've ever consumed gin, vodka, rum or wine. Ethanol is metabolized in the body to acetaldehyde, which quickly converts to acetic acid (which in dilute form is ordinary vinegar). The author doesn't recommend that you start drinking, but at least you'll know that you and your staff are less likely to go blind if you're exposed to a little alcohol vapor from the ethanol (caution: nobody suggests drinking the ethanol at home).

Ethanol is readily available from fermentation of corn, of course. Now since we know that there's plenty of vegetable oil (corn oil from the Midwest, palm kernel oil, cottonseed oil, soybean oil, canola oil from Canada and avocado oil from California) and plenty of Midwestern corn-produced ethanol, we can turn on the "green chemistry" machine and start making biodiesel fuel with a minimum of environmental waste disposal problems and fewer worker safety hazards. Let's call this "ethyl biodiesel," to distinguish from the "methyl biodiesel " derived from methanol.

Catalysts: Enzymes or OrganometallicsWhat catalysts should we use if we're not using an alkali, like sodium hydroxide? Well, for one thing, NaOH is not a catalyst but actually a co-reactant, because you have to weigh out a certain amount. Catalysts can be used in small amounts. They do not enter into the reaction. They can be used over and over again without replacement.

These new catalysts for converting vegetable oil into ethyl biodiesel are not the same as the platinum catalysts in your auto engine. Two types of catalysts can be used: (a) biochemical enzymes or (b) "organometallics" (that is, inert catalysts based on organic and inorganic chemistry).

You might know something about biochemically-derived enzymes, even if you are not a scientist. Enzymes are used in converting (a) milk into cheese, and (b) hops into beer. They are also used in modern enzyme based detergents.

Enzymes for making biodiesel esters include lipases and other "esterases." The enzymes can be derived from several different sources, ranging from bacterial and yeast cells to pork tissue (depending on your preferences).

The enzymatic process could simply be called the "trickle down theory." All you need to do is to mix the reactants (oil and alcohol/EtOH) and pour them over the column containing the enzyme. You can already buy the enzymes supported on an inexpensive material to prepare your enzyme column.

The advantages? Well, there's (a) no heating (the reaction takes place at room temperature), (b) no two-step reactions (the reaction goes to completion in one step), (c) no neutralization for the caustic alkali (this is a neutral pH reaction), (d) no water washing (no acid to remove), and (e) no drying to remove excess water. If you want the "pure" biodiesel esters-free from the glycerol and anything else-you can remove the glycerol by gravity (just use a large glass, gravity separator).

Organometallic catalysts are even better. They're commercially available and can also be supported on inexpensive clay, diatomaceous earth, zeolites and other common materials. You can also use the "trickle-down theory" and just pour the oil and alcohol over the column and watch it "trickle down" the column into your waiting container. For many small producers, especially in "environmentally aware" states, this method is much more suitable than using the harsh, corrosive chemicals and worrying about the chemical waste.

Dr. Novinson works with chemists, chemical engineers, biologists and other scientists with skills in environmental engineering, toxicology, government regulations, and analytical chemistry.