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Biodiesel Production

Chemistry of the Biodiesel Reaction

The following is a quick description of the biodiesel reaction and the minimum laboratory methods and apparatus required for production control and product quality. There are many variations on this theme but the simplest uses potassium hydroxide as the catalyst and methanol as the alcohol to form methoxide. Sometimes, methylate is used instead. Methylate is similar to methoxide but has less water for side reactions and avoids the use of solid hydroxide (thus reducing some OH&S issues).

The feedstock generally consists of clean and dry triglyceride oil contaminated with free fatty acid. Triglycerides are fatty acids attached (therefore not "free") to a glycerol backbone. The triglyceride oil reacts with an alcohol such as methanol using an hydroxide catalyst. The hydroxide actually reacts with the alcohol to make the real catalyst; the alkoxide or methoxide ion. Water interferes in this process. Water reduces the amount of catalyst available as the alkoxide anion by converting it back to hydroxide and the alcohol and also through saponification side reactions (i.e. making soap). Also, any free fatty acid reacts with the hydroxide anion to make soap.

Then, any left-over hydroxide will catalyse the trans-esterification reaction where the glycerol backbone of the triglyceride is replaced by the alcohol to form fatty acid alkyl esters. If methanol is used as the alcohol then then the product is fatty acid methyl ester (FAME) commonly known as biodiesel. Catalyst concentration is critical. Too little and the hydroxide will be consumed by side reactions and the reaction might not proceed. Too much and the reaction forms excessive amounts of soap.

On completion of the reaction, the mix forms two phases that differ in density and volume and therefore can be separated by gravity (slow) or by a centrifuge (fast). The phases consist of mixtures of the reactants and the products of reaction. For example, the heavier bottom phase might consist of, by weight, glycerol (37%), methanol (24%), FAME (22%), soap (13%) and water (3%). The lighter top phase might consist of, by weight, FAME (96%), methanol (3%), glycerol (0.5%), soap (0.3%) and water (0.02%).

Soap causes the biodiesel to entrain in the glycerol phase. The higher the free fatty acid content of the oil, the more soap is made and the more biodiesel is lost into the glycerol phase. By volume, the phases would be about 18 to 20L of glycerol phase for every 100L of biodiesel phase for an oil with an free fatty acid (FFA) content of 2%. Higher free fatty acid levels give larger volumes of the glycerol phase. Above 5% FFA, the densities of the top and bottom phases become similar enough to make the two phases harder to separate.

An acid wash can then titrate any soap back into FFA to meet both Acid Value (ASTM D664) and alkaline and alkaline-earth metals (EN 14538). The acid can be a non-oxidising mineral acid or an organic acid such as acetic or citric. Citric has the benefit that it can sequester transition metal cations to improve oxidation stability (EN 14112, prEN 15751) but it can be corrosive to moving metal parts. There is the choice between water washes, magnesium silicate or ion exchange cleanup for the biodiesel depending on reaction and environmental circumstances. After phase separation, some biodiesel can be recovered for rework from the glycerol phase if it is left to stand.

Flash distillation of the biodiesel phase and the glycerol phase can recover any excess alcohol for rectification and re-use. This might or might not be economic or problematic. For example, if sodium is used as the cation, then methanol removal from the glycerol phase will cause it to harden (sodium soaps are less soluble than potassium soaps) and the glycerol phase then becomes difficult to handle. The glycerol phase might also be separated into its other constituents but this is not without its problems and is generally not an economic process except for some recovery of the biodiesel.

Laboratory Tests

It would be unusual for a biodiesel production laboratory to have all of the equipment and staff required for a full analysis required by the various biodiesel fuel quality standards (see the Australian summary of the current Biodiesel Fuel Quality Standard. However, production quality control can be provided by the following list of procedures:

Acid-base titrations for soaps and free fatty acids (implies Acid Value (ASTM D664) and alkaline and alkaline-earth metals (EN 14538)). The equipment consists of burettes, titration flasks, indicator solutions (phenolphthalein, bromophenol blue), and solvents (acetone, isopropanol).

Moisture analysis is required for quality control of feedstock as well as finished product. Generally, the biodiesel fuel specification requirement for liquid water and sediment (ASTM 2709) is a fairly gross test requiring expensive equipment. A Karl Fischer titrator for water in methanol, oil and biodiesel can provide better quality control. Requires a Karl Fischer titrator, solvents and titrants.

Total and free glycerol (TFG). There are a number of methods ranging from estimates that can be carried out by the operators to analysis by ASTM D6584 requiring laboratory staff.

• Quick production test for unreacted oil using methanol, biodiesel product and a fat soluble dye. This test uses a measuring cylinder.
• Saftest apparatus using enzymes and a colorimeter (this site provides a good illustration of the equipment).
• Near Infra Red test (uses a NIR machine and glass cuvettes to test for unreacted oil.
• Gas chromatography (ASTM D6584). Requires line filters, columns, instrument air, hydrogen and nitrogen for chromatography.

Flash Point (ASTM D93). Requires a semi-automatic flash point apparatus. This is a check to ensure that materials are or are not dangerous goods. Some biodiesels such as coconut biodiesel will have a flash point around 110C which is below the standard of 120C but above that for dangerous goods. This test could also indicate the presence of methanol (implying (EN 14110)) in the products.

Filter Blocking Tendency (IP 387): Apparatus to IP 387. Requires FBT filter papers and beakers. This is part of the Australian Diesel Fuel Quality Standard but is a very useful test for biodiesel.

There are further associated tests and apparatus such as chemicals, electronic balances, glassware, drying oven, tripods, burette stands and rings that are typical of a general laboratory. Generally, a production laboratory needs laboratory tests that can be used to provide production feedback to the operators, with some of those tests sufficiently uncomplicated so that they can be carried out by the operators as part of their immediate feedback for quality control.

The other simple tests required for the fuel standards such as a temperature-compensating density (ASTM D1298) meter (used for billing), cloud point tester and FAME (EN 14103) can be used for the on-site blending of final product. Some biodiesels such as coconut biodiesel will always fail the FAME standard designed for canola biodiesel and, even though perfectly good biodiesel, will never meet the FAME standard even with blending. Others, such as used cooking oil (UCO) biodiesel can be blended with tallow, soy or canola biodiesel to meet the FAME standard.

Legislation

The Australian Institute of Petroleum has a useful discussion paper on some of the difficulties that biodiesel blends face in meeting the diesel fuel quality standards. Under the Australian Energy Grants (Cleaner Fuels) Scheme Act 2004 and the Cleaner Fuels Grant Scheme, where the grant offsets excise and customs duty, biodiesel must meet the biodiesel fuel quality standard under the Fuel Tax Act 2006 and the Fuel Quality Standards Act 2000 (see the Australian Tax Office fact sheet NAT 15418) and blends of biodiesel with diesel must meet the diesel fuel quality standard. Further information can be obtained from The cleaner fuels grant scheme (NAT 9886). Similarly, to qualify for the off-road diesel fuel rebate, the biodiesel must meet the biodiesel fuel quality standard and then the biodiesel/diesel blend must meet the diesel fuel quality standard.