INTERNATIONAL COUNCIL
OF ASSOCIATIONS FOR
SCIENCE EDUCATION
INVESTIGATING THE USE OF
VEGETABLE OILS AS A FUEL
Dr Jack B. Holbrook
Dr Jack B. Holbrook
This unit provides an example of a teaching resource to support Project 2000+ goals for scientific and technological literacy (STL) for all.
Introduction
The fuel for many vehicles, especially those used commercially, is diesel. Normal diesel is obtained from the distillation of crude petroleum, which is usually regarded as a non-renewable resource. Any limitation on the supply of petroleum can thus have dramatic effects on the commercial life of a country.
Conventional diesel causes much pollution in the form of hydrocarbon and sulfur emissions. This is a problem associated with most fossil fuels. There is strong environmental pressure to eliminate or at least greatly reduce these emissions, specially in areas of environmental sensitivity such as lakes and inland waterways and in inner city areas.
If alternative ways could be found to produce diesel, this could help countries plan into the future, ensure that supplies of fuel can be obtained and hopefully provide a cheaper alternative than replacing the diesel engine with other mechanisms. (If an alternative way was found to give a diesel product that was more environmentally friendly both in production and in use, this could be an added incentive).
This unit involves students in a project to explore alternatives to diesel.
Fuels based on vegetable oils produce much less hydrocarbon emissions and practically no sulfur emissions. However, direct use of the oil itself is possible only with modification to existing diesel engines.
Objectives
The objectives of this exercise are:
STUDENT'S GUIDE
You are asked to embark on a project to determine an acceptable alternative fuel to diesel made from crude petroleum. In particular you are asked to consider vegetable oils.
The project
A. Determine whether vegetable oils can be alternative fuels for diesel burning vehicles.
If so:
B. Which vegetable oil is 'best'?
C. Is the use of a vegetable oil a 'viable' alternative?
D. Is the use of a vegetable oil as a fuel acceptable ethically?
Notes:
Below you will find additional notes and procedures that should aid you in undertaking your project and in making your decisions.
1. Properties of vegetable oils
(a) One property of vegetable oils is that they burn. Does this mean the reaction of vegetable oils with oxygen is exothermic? (If the answer to this question is not obvious, discuss this with your teacher before continuing).
(c) Vegetable oils, without modification, are not considered suitable as fuels for diesel engines. Would you suggest this is a valid conclusion from your experiments from (a) and (b)?
A suggestion is to breakdown the oil in some way to create smaller molecules that will be less viscous, but still flammable. If these molecules are similar in size to the hydrocarbons used in diesel, then it will be possible to utilise them in standard diesel engines.
2. Breaking down vegetable oils
We are familiar with breaking down vegetable oils using an acid or an alkali. (If you are not familiar with this, consult your teacher before continuing).
But by reacting the oil with an aqueous substance, we have a problem of extracting the flammable part. How great a problem is this? Do you have a simple solution to extracting a flammable product (if the solution is not simple, the cost of extraction will stop the process from being viable)?
One cheap and simple method of breaking down vegetable oils is known as transesterification. This means making one ester from another. Vegetable oils are triglycerides (they are based on the alcohol, glycerol, which has three OH groups). It is possible by transesterification to base the ester on methanol or ethanol and thus create simpler molecules. Three simpler ester molecules are formed from the original triglyceride.
(a) How well does transesterification work? Is the product sufficiently flammable and suitable for use in a diesel engine? Does the process work for any vegetable oil? If so, which oil seems the most suitable?
Follow the transesterification process described below and carefully collect the washed and dried product. This product can be called biodiesel. Then devise a procedure for comparing flammability, viscosity, suitability of flame and calorific value with that of diesel.
Also suggest which vegetable oil gives the best biodiesel (don't forget to indicate the parameters chosen to determine the meaning you have attached to 'best').
TEACHER'S GUIDE
This project relates to biodiesel and the process of transesterification. Neither are usual topics within a science (chemistry) curriculum. However an understanding of the process and the development of the skills in making the actual product are secondary to the educational skills of devising procedures to measure ease of burning, viscosity, suitability of flame and calorific value.
To prepare a number of samples of biodiesel by one student or one group of students is obviously a time consuming process. It is recommended that for this project different groups of students work with different vegetable oils and results are compared between groups.
Outline of strategy
The teacher needs to determine a suitable manner by which to introduce this topic. As fuels based on vegetable oils produce much less hydrocarbon emissions and practically no sulfur emissions, a possible introduction involves a consideration of the environmental problems of diesel fuel and the search for alternatives. Vegetable oils cannot be used direct in a diesel engine and hence there is the need to either modify the engine or the vegetable oil, the latter being more practicable.
From the introduction the conceptual understanding of diesel fuel and how it is used, the reason for vegetable oils to produce less hydrocarbon emission on burning can be reinforced.
Once the desirability of transforming vegetable oils is appreciated, it is important to establish the need for a very economical process and that the viability of using vegetable oils depends on this. The conversion process will be more suitable for some vegetable oils and the initial cost of the vegetable oil will need to be considered. Add to that the calorific value of the fuel formed on transformation, the ease of burning and rate of combustion and there are many factors to be considered before the 'best fuel' can be suggested. The project thus involves the experimental conversion, determination of the parameters by which this fuel can be compared to others and consideration of the factors to include in deciding on the 'best' fuel, plus the weighting to be place on the various factors in arriving at the final suggestion.
Handouts
As this is a project in which students devise their own procedures, there is little need for full instructions. The main instruction is given in the introduction, that is, you are asked to embark on a project to determine an acceptable alternative fuel to diesel made from crude petroleum. In particular you are asked to consider vegetable oils.
A. Determine whether vegetable oils can be alternative fuels for diesel burning vehicles.
If so:
B. Which vegetable oil is 'best'?
C. Is the use of a vegetable oil a 'viable' alternative?
D. Is the use of a vegetable oil as a fuel acceptable ethically?
Additional guidelines as suggested in the earlier notes accompanying the project can be given to the students if appropriate.
Students will need to be given the experimental procedure for preparing biodiesel at a suitable time.
Preparation of biodiesel
100 cm3 vegetable oil
15 cm3 95% ethanol
1 cm3 9 mol dm-3 aq. potassium hydroxide solution
(1) Pour the vegetable oil and ethanol into a 250 cm3 beaker.
(2) Slowly add the potassium hydroxide solution from a 1 cm3 plastic syringe or a small dropping pipette, over about 1 minute.
(3) Stir continuously for a further 2-3 minutes and then stir occasionally (5-10 seconds every 2-3 minutes) for 2-3 hours or until 2 layers are formed on settling. Do not stir too vigorously as this may lead to emulsification.
(4) Pour into a separating funnel and allow to settle for 1 hour.
(5) Run off the lower layer. This layer contains most of the glycerol which is released during the reaction. The lower layer is discarded.
(6) Add 10 cm3 of distilled water to the crude product and mix well (shaking is not advisable since an emulsion can form which will take a long time to separate). Leave to stand for 1 hour.
(7) Run off and discard the lower layer. (This washing procedure can be repeated if the product is not clear).
(8) Add 0.5g anhydrous sodium sulfate. Stir for about 15 minutes.
(9) Allow the sodium sulfate to settle.
(10) Decant the biodiesel into a sample bottle.
Devise your own procedure for the following and compare with diesel.
Test the product for:
(a) flammability
(b) viscosity
(c) calorific value
(d) suitability of flame
Achieving objectives
Cooperate as part of a team
This is developed by team work within the group as individual students undertake different tasks.
Identify the steps needed in developing solutions to a problem
Again the teamwork leads to a common solution to the problem. By allowing everyone in the group to contribute, the burden of the project is shared over the team members.
Devise procedures to solve the problem of making biodiesel
This is a planning exercise and allows the students to utilise their chemistry skills.
Be able to carry out an experiment to make biodiesel
This objective is related to the ability to actually manipulate the apparatus and carry out the procedures on the worksheet.
Devise procedures for testing the biodiesel
This is further planning and calls upon conceptual knowledge related to the various factors.
Suggest parameters for deciding on the 'best' biodiesel
This objective is intended to illustrate that there is not one single solution to the problem and that by weighting various factors the final, preferred biodiesel may change. This objective thus tests the student's ability to make decisions.
Additional notes
Determining:
(a) Flammability
Intended here is a simple test of how easy it is to burn the product. Putting a match to a little of the sample on a watch glass is perhaps the simplest manner in which this test can be performed. If this does not lead to a noticeable difference between the various biodiesels or between a biodiesel and ordinary diesel, then more sophisticated tests can be devised. [Industrially the temperature at which a biodiesel burns after ignition by an electric spark is obtained. Also measured is the flash point - the temperature at which the fuel self ignites. THESE TESTS ARE NOT SUGGESTED.]
(b) Suitability of flame
Is it possible to burn biodiesel, or ordinary diesel in the standard spirit burner? If so the 'sootiness' of the flame can be compared. A sooty flame indicates incomplete combustion and gives a measure of whether the fuel will be efficient and whether it leads to greater pollution of the atmosphere.
(c) Viscosity
Again the emphasis is on a simple test such as the time it takes a weight (ball bearing) to fall through the biodiesel for a given length.
(d) Calorific value
The emphasis is on simple apparatus and if necessary students can devise ways to minimise heat losses by draughts, etc. The suggestion is to burn a known quantity of fuel in a spirit burner and to use this to heat a tin can containing a known quantity of water. The quantity of fuel needed to raise the temperature of the water by a standard temperature rise (5 degrees C) is determined and used as a measure of the calorific value. (Whether students undertake the actual calculation depends on the level of the students).
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