Base catalysed Mechanism
You want to mix the base (KOH,NaOH) with the alcohol to make a reactive anion
KOH + ROH -> RO- + H2O
KOH and NaOH are strong bases, so the reaction equilibrium is far to the right.
The ROH needs to be very dry. Any water in the alcohol will reduce the amount of RO- that gets formed.
The RO- is a reactive guy, so you must be very careful with this stuff. Often in chemistry alcohols are mixed with KOH to make a "base bath" for cleaning glass. It actually dissolves the surface of the glass, so be really careful with this stuff.
Once the RO- group is formed, it is added to the triglyceride. The Sn2 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 2 more RO's react via this mechanism at the other 2 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 variance, but 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) to speed up the reaction and the reaction takes place. Recommended reaction time varies from 1 to 8 hours, and some systems recommend the reaction take place at room 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 others 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.
