This is the fifth in a series of 8 chemistry labs on the fundamentals of biodiesel.
In this lab we will learn about “Transesterification of Vegetable Oil and Alcohol to Produce Ethyl
Biodiesel”.
Written procedures and background information for all 8 labs is provided on the biodiesel
education website.
The process for making biodiesel called transesterification is a chemical reaction in which 1 mole of
triglycerides (vegetable oil or animal fats) reacts with 3 moles of alcohol to produce
3 moles of alkyl esters (biodiesel) and 1 mole of glycerol, a co-product.
Although there is a 3:1 molar ratio of alcohol to oil to satisfy a complete reaction, a 6:1
molar ratio is usually used to drive the reaction to the product side.
The excess alcohol that was not used in the reaction separates partly with the fuel and
partly with the glycerol at end of the process and can be recovered and reused through a
process of distillation.
Typically, a strong base such as sodium hydroxide or potassium hydroxide is used as the catalyst,
which normally would not be consumed during this reaction and would end up in the glycerol
layer.
A triglyceride is a molecule made up of three fatty acids attached to a glycerol backbone.
The three fatty acids on each molecule could be the same or they could be different.
The fatty acid makeup of the oil or fat is responsible for some of the properties found
in the resulting biodiesel.
The transesterification reaction will proceed quickly at first and eventually slow down
as it reaches equilibrium.
At this point, there will be some unreacted glycerides left in the biodiesel.
It is possible to improve the quality by removing the glycerin that has settled out and to run
another reaction, thus pushing the equilibrium point further to the products side.
For this lab, one reaction should be enough to get a good separation of the glycerin from
the biodiesel.
In this laboratory exercise we will be making a small batch of biodiesel.
To conduct the exercise, you will need a 125 mL and 250 mL flask all with stoppers, a 250
mL separatory funnel, a stirring hot plate and magnetic stir bar, thermometer, aluminum
foil, and a weighing scale.
For reagents you will need 100 grams of vegetable oil, 20 grams of 100% ethanol that is anhydrous,
and 1 gram of potassium hydroxide for the catalyst.
You will also need to prepare the data tables as shown here.
The first step is to calculate the molecular
weight of ethanol.
It can be very difficult to calculate the molecular weight of a fat or oil, since they
are made up of many different large molecules (triglycerides), each with different fatty
acids attached.
For this exercise we will use a value of 877 g/mol for canola.
Since we are using 100 g of oil, we can use the following relationship to calculate the
ethanol needed for a 6:1 ratio of methanol moles to triglyceride moles.
Following safe lab procedures carefully weigh out 1 gram of potassium hydroxide into a 125
mL Erlenmeyer flask.
Into this same flask add the amount of ethanol you calculated.
Add a stir bar, cover with aluminum foil, and place on a stirring hot plate.
Mix until the potassium hydroxide is completely dissolved.
Set this aside.
Now weigh an empty 250 mL Erlenmeyer flask and record the weight in your data table.
Into this same flask weigh out about 100 grams of vegetable oil to be converted to biodiesel
and record the exact weight in the data table.
Add a stir bar, put on a hot plate, and start to heat the oil to around 60 °C.
Once 60 °C is reached, add the alcohol/catalyst mixture
to the oil in the flask and cover with a watch glass or aluminum foil to keep the alcohol
vapors from escaping.
Stir this mixture at a moderate level 60 minutes while holding the temperature at 60 °C.
After 60 minutes stop heating and stirring and carefully transfer the mixture to a separatory
funnel and allow it to cool down and separate for 20 minutes.
Weigh a 125-mL flask and record the weight.
Carefully remove the lower layer of glycerol from the bottom of the funnel into this pre-weighed
flask.
The upper layer is the biodiesel phase and the separated lower layer is the crude glycerol
with some catalyst in it.
Weigh the flask with separated glycerol and record the net weight into the data table.
Based on the crude glycerol weight, calculate the estimated biodiesel yield.
Also, calculate the average molecular weight of the ethyl esters using the known molecular
weights of oil, ethanol and glycerol with the relationship shown here.
Thank you for watching the “Transesterification of Vegetable Oil and Alcohol to Produce Ethyl
Esters” lab on the fundamentals of biodiesel.
Please go to the biodiesel education website for a complete written transcript of this
lab exercise.