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Biodiesel is the name of a fuel alternative of the conventional, petroleum based diesel engine fuel, which is manufactured from vegetable oils or animal fats by catalytically reacting these with a short chain aliphatic alcohol, methanol, or ethanol using a process, which is called variously transesterification, or alcoholysis. The application of this process in an industrial scale is what is meant by biodiesel production.

Esterification

Biodiesel can be produced from straight vegetable oil, animal oil/fats, tallow and waste oils. There are three basic routes to biodiesel production from oils and fats: Base catalysed transesterification of the oil; Direct acid catalysed transesterification of the oil, and; Conversion of the oil to its fatty acids and then to biodiesel (Drewette, Dwyer, Farrell & Miller, 2003).

Almost all biodiesel is produced using base catalysed transesterification as this is the most economical process. It requires only low temperatures and pressures and produces a 98% conversion yield. The Transesterification process is the reaction of a triglyceride (fat/oil) with an alcohol to form esters and glycerol. A triglyceride has a glycerine molecule as its base with three long-chain fatty acids attached. The characteristics of the fat are determined by the nature of the fatty acids attached to the glycerine. During the esterification process, the triglyceride is reacted with alcohol in the presence of a catalyst, usually a strong alkaline like sodium hydroxide. The alcohol reacts with the fatty acids to form the mono-alkyl ester, or biodiesel, and crude glycerol. In most cases, methanol or ethanol is the alcohol used, where methanol produces methyl esters, and ethanol produces ethyl esters. Potassium hydroxide has been found to be more suitable for the ethyl ester biodiesel production. Either base catalyst can be used for the methyl ester. A common product of the transesterification process is Rape Methyl Ester (RME) produced from raw rapeseed oil reacted with methanol (Drewette, Dwyer, Farrell & Miller, 2003).

Figure 13 shows the chemical process for methyl ester biodiesel. The reaction between the fat or oil and the alcohol is a reversible reaction so that alcohol must be added in excess to drive the reaction towards the right and ensure complete conversion. The products of the reaction are the biodiesel itself and glycerol.

Figure 13 The chemical process for methyl ester biodiesel (courtesy of the “Biofuels for Transport” by the University of Strathclyde).

A successful transesterification reaction is signified by the separation of the ester and glycerol layers after the reaction time. The heavier co-product, glycerol, settles out and may be sold as it is or it may be purified for use in other industries, e.g. the pharmaceutical, cosmetics, etc. (Drewette, Dwyer, Farrell & Miller, 2003). Biodiesel is a less toxic and more biodegradable fuel than is petroleum diesel and is often blended with petroleum diesel to provide a renewable energy component in the fuel (Australian Business Council for Sustainable Energy, 2005).

Transesterification chemistry
A reaction scheme is as follows:



Animal and plant fats and oils are typically made of triglycerides which are esters of free fatty acids with the trihydric alcohol, glycerol. In the transesterification process, the alcohol is deprotonated with a base to make it a stronger nucleophile. Commonly, ethanol or methanol are used. As can be seen, the reaction has no other inputs than the triglyceride and the alcohol.

Normally, this reaction will proceed either exceedingly slowly or not at all. Heat, as well as an acid or base are used to help the reaction proceed more quickly. It is important to note that the acid or base are not consumed by the transesterification reaction, thus they are not reactants but catalysts.

Almost all biodiesel is produced from virgin vegetable oils using the base-catalyzed technique as it is the most economical process for treating virgin vegetable oils, requiring only low temperatures and pressures and producing over 98% conversion yield (provided the starting oil is low in moisture and free fatty acids). However, biodiesel produced from other sources or by other methods may require acid catalysis which is much slower. Since it is the predominant method for commercial-scale production, only the base-catalyzed transesterification process will be described below.

The following steps can be performed in a small, home-based biodiesel processor, or in large industrial facilities. The chemistry is similar in either case.

Steps in the process
The major steps required to synthesize biodiesel are as follows:

Purification
If waste vegetable oil (WVO) is used, it is filtered to remove dirt, charred food, and other non-oil material often found.

Water is removed because its presence causes the triglycerides to hydrolyze to give salts of the fatty acids instead of undergoing transesterification to give biodiesel.

At home, this is often accomplished by heating the filtered oil to approximately 120 °C. At this point, dissolved or suspended water will boil off. When the water boils, it spatters (chemists refer to it as "bumping"). To prevent injury, this operation should be done in a sufficiently large container (at most two thirds full) which is closed but not sealed.

In the laboratory, the crude oil may be stirred with a drying agent such as magnesium sulfate to remove the water in the form of water of crystallization. The drying agent can be separated by decanting or by filtration. However, the viscosity of the oil may not allow the drying agent to mix thoroughly.

Neutralization of free fatty acids
A sample of the cleaned oil is titrated against a standard solution of base in order to determine the concentration of free fatty acids (RCOOH) present in the waste vegetable oil sample. The quantity (in moles) of base required to neutralize the acid is then calculated.

Transesterification
While adding the base, a slight excess is factored in to provide the catalyst for the transesterification.

The calculated quantity of base (usually sodium hydroxide) is added slowly to the alcohol and it is stirred until it dissolves. Sufficient alcohol is added to make up three full equivalents of the triglyceride, and an excess of usually six parts alcohol to one part triglyceride is added to drive the reaction to completion.

The solution of sodium hydroxide in the alcohol is then added to a warm solution of the waste oil, and the mixture is heated (typically 50 °C) for several hours (4 to 8 typically) to allow the transesterification to proceed. A condenser may be used to prevent the evaporative losses of the alcohol. Care must be taken not to create a closed system which can explode.

Final process
The lower layer of the process is composed primarily of glycerine and other waste products. The top layer, a mixture of biodiesel and alcohol, is decanted. The excess alcohol can be distilled off, or it can be extracted with water. If the latter, the biodiesel should be dried by distillation or with a drying agent.

Reaction
An example of the transesterification reaction equation, shown in skeletal formulas:



Since natural oils are typically used in this process, the alkyl groups of the triglyceride are not necessarily the same. Therefore, distinguishing these different alkyl groups, we have a more accurate depiction of the reaction:



R1, R2, R3 : Alkyl group.

During the esterification process, the triglyceride is reacted with alcohol in the presence of a catalyst, usually a strong alkaline (NaOH, KOH, or Alkoxides). The main reason for doing a titration to produce biodiesel, is to find out how much alkaline is needed to completely neutralize any free fatty acids present, thus ensuring a complete transesterfication. Empirically 6.25 g / L NaOH produces a very usable fuel. One uses about 6 g NaOH when the WVO is light in colour and about 7 g NaOH when it is dark in colour.

The alcohol reacts with the fatty acids to form the mono-alkyl ester (or biodiesel) and crude glycerol. The reaction between the biolipid (fat or oil) and the alcohol is a reversible reaction so the alcohol must be added in excess to drive the reaction towards the right and ensure complete conversion.

Base catalysed mechanism
This reaction is base catalysed. Any strong base will do, e.g. NaOH, KOH, Sodium methoxide, etc. Commonly the base (KOH,NaOH) is dissolved in the alcohol to make a convenient method of dispersing the otherwise solid catalyst into the oil. The ROH needs to be very dry. Any water in the process promotes the saponification reaction and inhibits the transesterification reaction.

Once the alcohol mixture is made, it is added to the triglyceride. The SN2 reaction that follows replaces the alkyl group on the triglyceride in a series of reactions.

The carbon on the ester of the triglyceride has a slight positive charge, and the oxygens have a slight negative charge, most of which is located on the oxygen in the double bond. This charge is what attracts the RO- to the reaction site R1  Polarized attraction | RO- ————————————————>  C=O |                       O-CH2-CH-CH2-O-C=O |       |                              O-C=O    R3                                | R2

This yields a transition state that has a pair of electrons from the C=O bond now located on the oxygen that was in the C=O bond.

R1  | RO-C-O- (pair of electrons) |  O-CH2-CH-CH2-O-C=O |       |         O-C=O    R3           | R2

These electrons then fall back to the carbon and push off the glycol forming the ester.

R1  | RO-C=O +    -O-CH2-CH-CH2-O-C=O |       |          O-C=O    R3            | R2

Then two more RO groups react via this mechanism at the other two C=O groups. This type of reaction has several limiting factors. RO- has to fit in the space where there is a slight positive charge on the C=O. So MeO- works well because it is small. As the R on RO- gets bigger, reaction rates decrease. This effect is called steric hindrance. That is why methanol and ethanol are typically used.

There are several competing reactions, so care must be taken to ensure the desired reaction pathway occurs. Most methods do this by using an excess of RO-.

The acid-catalyzed method is a slight variant, that is also affected by steric hindrance.

Batch process

 * Preparation: care must be taken to monitor the amount of water and free fatty acids in the incoming biolipid (oil or fat). If the free fatty acid level or water level is too high it may cause problems with soap formation (saponification) and the separation of the glycerin by-product downstream.


 * Catalyst is dissolved in the alcohol using a standard agitator or mixer.


 * The alcohol/catalyst mix is then charged into a closed reaction vessel and the biolipid (vegetable or animal oil or fat) is added. The system from here on is totally closed to the atmosphere to prevent the loss of alcohol.


 * The reaction mix is kept just above the boiling point of the alcohol (around 70 °C, 158 °F) to speed up the reaction though some systems recommend the reaction take place anywhere from room temperature to 55 °C (131 °F) for safety reasons. Recommended reaction time varies from 1 to 8 hours; under normal conditions the reaction rate will double with every 10 °C increase in reaction temperature. Excess alcohol is normally used to ensure total conversion of the fat or oil to its esters.


 * The glycerin phase is much more dense than biodiesel phase and the two can be gravity separated with glycerin simply drawn off the bottom of the settling vessel. In some cases, a centrifuge is used to separate the two materials faster.


 * Once the glycerin and biodiesel phases have been separated, the excess alcohol in each phase is removed with a flash evaporation process or by distillation. In other systems, the alcohol is removed and the mixture neutralized before the glycerin and esters have been separated. In either case, the alcohol is recovered using distillation equipment and is re-used. Care must be taken to ensure no water accumulates in the recovered alcohol stream.


 * The glycerin by-product contains unused catalyst and soaps that are neutralized with an acid and sent to storage as crude glycerin (water and alcohol are removed later, chiefly using evaporation, to produce 80-88% pure glycerin).


 * Once separated from the glycerin, the biodiesel is sometimes purified by washing gently with warm water to remove residual catalyst or soaps, dried, and sent to storage.

Supercritical process
An alternative, catalyst-free method for transesterification uses supercritical methanol at high temperatures and pressures in a continuous process. In the supercritical state, the oil and methanol are in a single phase, and reaction occurs spontaneously and rapidly. The process can tolerate water in the feedstock, free fatty acids are converted to methyl esters instead of soap, so a wide variety of feedstocks can be used. Also the catalyst removal step is eliminated. High temperatures and pressures are required, but energy costs of production are similar or less than catalytic production routes.

Ultra- and high-shear in-line reactors
Ultra- and High Shear in-line reactors allow to produce biodiesel continuously, therefore, reduces drastically production time and increases production volume. Ultra – Shear, up to three sets of rotor and stator which converts mechanical energy to high tip speed, high shear stress, high shear-frequencies. Droplet size range expected in the low micrometer until sub-micrometer range after one pass.

The reaction takes place in the high-energetic shear zone of the Ultra- and High Shear mixer by reducing the droplet size of the immiscible liquids such as oil or fats and methanol. Therefore, the smaller the droplet size the larger the surface area the faster the catalyst can react.

Ultra- and High Shear mixers are used for the pre-treatment of crude vegetable oil or animal fats such as:
 * the dispersion of citric/phosphoric acid and crude oil within the de-gumming process to remove Phosphatides (Gums).
 * the dispersion of caustic and de-gummed oil within the neutralization process to remove FFA (Free Fatty Acid)
 * Furthermore, for the transesterification of pre-treated vegetable oil or animal fats into Methyl Ester.

Finally, for the water wash process of Methyl Ester. Water amount to be used in conjunction with high-shear is around 3%. Citric Acid amount is ~ 0.2% within the 1st water wash process.

Ultrasonic-reactor method
In the ultrasonic reactor method, the ultrasonic waves cause the reaction mixture to produce and collapse bubbles constantly. This cavitation provides simultaneously the mixing and heating required to carry out the tranesterification process. Thus using an ultrasonic reactor for biodiesel production drastically reduces the reaction time, reaction temperatures, and energy input. Hence the process of transesterification can run inline rather than using the time consuming batch processing. Industrial scale ultrasonic devices allow for the industrial scale processing of several thousand barrels per day.

Microwave method
Current research is being directed into using commercial microwave ovens to provide the heat needed in the transesterification process. The microwaves provide intense localized heating that may be higher than the recorded temperature of the reaction vessel. A continuous flow process producing 6 liters/minute at a 99% conversion rate has been developed and shown to consume only one-fourth of the energy required in the batch process. Although it is still in the lab-scale, development stage, the microwave method holds great potential to be an efficient and cost-competitive method for commercial-scale biodiesel production.