Biodiesel production

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Biodiesel production is the process of making biodiesel. Biodiesel is a liquid fuel source largely compatible with petroleum based diesel fuel. It is synthesized by replacing glycerol with a short chain alcohol such as methanol or ethanol in a step known as transesterification.

Contents

1 Transesterification chemistry
2 Steps in the process

2.1 Purification
2.2 Neutralization of free fatty acids
2.3 Transesterification
2.4 Workup

3 Reaction

3.1 Base catalysed mechanism
3.2 Process

4 Alternate production methods
5 See also
6 Academic reviews

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, the alcohol is deprotonated with a base to make it a stronger nucleophile. Commonly, ethanol or methanol is 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 using the base-catalyzed technique as it is the most economical process requiring only low temperatures and pressures and producing over 98% conversion yield (provided the starting oil is low in moisture and free fatty acids). For this reason only this 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 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 to 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 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 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.

Workup

Once the reaction is complete, the glycerol should sink. When ethanol is used, it is reported that an emulsion often forms ^{[[Citing sources citation needed]]}. This emulsion can be broken by standing, centrifugation, or the addition of a low boiling (easily removed) nonpolar solvent, decanting, and distilling ^{[[Citing sources citation needed]]}.

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

The reaction may be shown

CH₂COOR1
|
CHCOOR1 + 3 CH₃OH → (CH₂OH)₂CH-OH + 3 CH₃COO-R1
|
CH₂COOR1

Since we are dealing with nature, the alkyl group on the triglycerides is probably different, so it would actually be more like

CH₂OC=OR1
|
CHOC=OR2 + 3 CH₃OH → (CH₂OH)₂CH-OH + CH₃COO-R1 + CH₃COO-R2 + CH₃OC=O-R1
|
CH₂COOR3

Triglyceride + methanol → Glycerol + Esters

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 sodium silicate). The main reason for doing a titration to produce biodiesel, is to find out how much alkaline is needed to ensure a complete transesterfication. ly 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.

A word on methoxide production: Claims that methoxide is produced by the reaction

KOH + ROH → RO^- + H₂O

are incorrect as the reaction constant is on the order of K_log -15. I.e. the reaction equilibrium is far to the left. While KOH and NaOH are strong bases, methoxide can only be produced by reacting e.g. sodium metal in alcohol. However, the following reaction mechanism using methoxide as an example are common in the literature as methoxide is an excellent base catalyst for this reaction.

Once the alcohol mixture is made, it is added to the triglyceride. The S_n2 reaction that follows replaces the alkyl group on the tricglyceride 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
backside attack    |
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 hinderance. 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 catalysed method is a slight variant, that is also affected by steric hinderance.

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.

Alternate production methods

Supercritical fluid methanol

[link](pdf)

[link](pdf)biodiesel production without a catalyst

[link] 2 step method with hydrolysis followed by esterification with supercritical methanol(pdf)

Ultrasonic reactor

Using an ultrasonic reactor for biodiesel production drastically reduces your reaction times.

Reaction time

5–10 h(using conventional agitation)
15 min(using ultrasound irradiation)

Static separation time

5–10 h(using conventional agitation)
10 min(using ultrasound irradiation)

[www.chemistry.org] ultrasonic irradiation

[www.nrel.gov] Biodiesel Production

Technology (pdf)

Academic reviews

Gerpen, J.V., Biodiesel processing and production, Fuel Processing Technology, 2005, 86, 1097-1107. [doi:10.1016/j.fuproc.2004.11.005]
Ma, F. & Hanna, M.A., Biodiesel production: a review, Bioresource Technology, 1999, 70, 1-15. [doi:10.1016/S0960-8524(99)00025-5]

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