Making Biodiesel

PS67-10 PTB 05/07 © CLEAPSS®, The Gardiner Building, Brunel Science Park, Kingston Lane, Uxbridge UB8 3PQPage 1 of 3 Tel: 01895 251496; Fax: 01895 814372; E-mail: [email protected]; Web site: www.cleapss.org.uk

Making Bio-diesel1. IntroductionBio-diesel is quite easily manufactured in the laboratory and a number of interesting observations can bemade.

If the main chain of the oil is based upon linoleic acid, then the following reaction occurs, catalysed byhydroxide ions. It is described as a trans-esterification reaction.

During the reaction, the same type of covalent bonds in the oil and the methanol are broken andsubsequently reformed in a different molecular environment. Thus, theoretically, the enthalpy for thereaction is about zero. There is, though, an increase in disorder and a rise in entropy. The reactionproceeds to near completion because propane-1,2,3-triol is insoluble in the product, methyl linoleate, andis removed from the reaction.

2. Technician and teacher instructionsMaking Bio-diesel involves potassium hydroxide (CORROSIVE) and methanol (VERY TOXIC & EXTREMELYFLAMMABLE). In an attempt to reduce the hazards for students at KS4, the activity has been tried withethanol in place of methanol but the chemistry appears more complicated. Therefore, despite the natureof the reactants, we believe the method described here is the most effective for students. The potassiumhydroxide acts as a catalyst for the reaction, rather than a reactant. (We do not recommend using sodiumhydroxide instead of potassium hydroxide because it has a lower solubility in ethanol.)

Methanol The workplace exposure limit (short-term) for methanol is 333 mg m-3, This means that80 g of methanol would need to vaporise in an average laboratory (with a volume ofabout 240 m3) to exceed the limit. In the procedure below, only about 30 cm3 ofmethanol is used per lesson. In addition, methanol is consumed in the reaction so riskof exposure is quickly reduced.


Potassiumhydroxide

Some recipes for this reaction use 9 mol dm-3 potassium hydroxide solution, which ishazardous to make up and use by students. However, a safer procedure is for thetechnician to prepare a 5% (w/v) potassium hydroxide solution in methanol in a fumecupboard. Only 1.5 cm3 is required per group; this quantity should be dispensed instoppered test tubes for students to use, further minimising potential exposure.

3. Preparing the catalyst• Wear goggles and use a fume cupboard.

• Pour 100 cm3 of methanol (VERY TOXIC & EXTREMELY FLAMMABLE) in a beaker or conical flask on astirrer, add 5 g of potassium hydroxide (CORROSIVE) and start the stirrer. It will take some time forthe potassium hydroxide to dissolve.

• Pour the solution into a bottle and label it, adding the hazard symbols CORROSIVE, TOXIC andEXTREMELY FLAMMABLE.

• Dispense 1.5 cm3 of the solution into as many test tubes as required and stopper them.

4. Making bio-dieselStudents will be able to observe a reaction almost immediately on mixing the oil and catalyst. The oil willquickly become less viscous. However, it is better to prepare bio-diesel in one lesson, store it in labelledtest tubes to allow the layers to separate fully, and perform any follow-up tests in the next lesson.

• Wear goggles.

• Place 10 cm3 of vegetable oil in a test tube.

• Add the contents of the test tube containing the methanol / potassium hydroxide catalyst(EXTREMELY FLAMMABLE, TOXIC & CORROSIVE) and firmly insert the stopper from the test tubepreviously containing the catalyst.

• Now invert the test tube SLOWLY over 30 times to ensure adequate mixing. DO NOT SHAKE THETEST TUBE AS THE METHANOL MAY SQUIRT OUT! If any solution gets onto your fingers,immediately wash them under a stream of water from a cold tap.

• Observe the contents of the test tube. A lower layer of glycerol gradually forms. The top layer isbio-diesel.

• To carry out the tests described below, it is best to wait at least 24 hours for complete separation.Make sure the test tube is labelled with your name.

5. Testing the bio-diesel5.1. ViscometryOne method of determining that a new substance has been created is by comparing the flow rate of thebio-diesel with the original vegetable oil.

• Prepare a pipette viscometer. (See the box below).

• Wear eye protection.

• Suck up the bio-diesel and time how long it takes to flow out between the two marks.

• Make sure the pipette viscometer is empty and repeat the procedure with the vegetable oil.

Preparation of the pipette viscometer (see PS67-5 for more details)• You will need a plastic pipette and a short piece of wire.

• Light a Bunsen burner.

• Heat the end of the piece of wire and pierce the top of the pipette bulb to make a small hole.

• Using a permanent black marker pen, draw a line against the 0.5 and 2.5 cm3 positions on the pipette. The position ofthese marks is not critical, as long as there is a sensible distance between them for timing the fall of oil.


5.2 Burning the bio-diesel• Wear eye protection.

• Use a small crucible or bottle top with a wad of mineral wool on it.

• Add about 2 cm3 of bio-diesel to it.

• Set fire to the bio-diesel with a lighted splint.

• Compare the ease with which the bio-diesel can be lit and burns with the same procedure for theoriginal vegetable oil.

6. VariationsWe used corn oil but other vegetable oils could be used. Commercial applications appear to be centred onsoya and rape oil.

It is possible to fill a spirit burner with the newly-made bio-diesel and measure its calorific value.

The calorific value could be compared with that of real diesel. Always use fresh, low-sulfur diesel.Alternatively, you could use kerosene or paraffin instead.

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Making Biodiesel