Biodiesel: using renewable resources: teacher’s notesActivity 1In this activity students make biodiesel from a vegetable-based cooking oil. The cooking oil is mixed with methanol and a catalyst (potassium hydroxide). Cooking oil is a lipid called a triglyceride. The structure of this type of lipid is characteristic of all animal and plant fats. These consist of propane-1,2,3-triol (glycerol) attached to three fatty acids. Differences among the fats are due to different fatty acids being connected to the glycerol.
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In making biodiesel the reaction breaks the bond between the propane-1,2,3-triol (glycerol) and the fatty acids. A methyl group is added to the end of the fatty acid. This is what we call biodiesel, with the other product being propane-1,2,3-triol (glycerol). The process is catalysed by potassium hydroxide. Each group of students will need
Safety
Answers to questions:Q 1. The cooking oil is very greasy and slippery. The methanol is a colourless liquid that has an odour and evaporates quickly. The potassium hydroxide solution is colourless and odourless and may be slippery when touched (not recommended as it is very caustic). Propane-1,2,3-triol (glycerol) is a slightly viscous liquid that is colourless and odourless. The appearance of the biodiesel will vary, but may have some slight colour and odour. Q 2. The mixture will separate into two layers, the biodiesel and the glycerol. Q 3. The washing removes the remaining potassium hydroxide catalyst. Q 4. Student answers will vary. In the example given of the commercial process, the yield is approximately 90%. Activity 2This activity is designed to test semi-quantitatively the characteristics of the biodiesel prepared by students. The idea is that the waste products given off by the combustion of the biodiesel include carbon dioxide and soot. A rough measure of the amount of carbon dioxide is gained by bubbling the waste gas through Universal Indicator. Because carbon dioxide forms an acid when it dissolves in water, the rate of change in colour of the universal indicator is proportional to the amount of carbon dioxide in the combustion products. Students are also asked to compare the amount of soot that collects. Soot is unburned carbon and represents incomplete combustion of the fuel. Less-efficient fuels generally produce more soot. Each group of students will need
Safety
Answers to questionsQ 1. Gas mixtures with a high concentration of carbon dioxide will cause a quicker colour change in the Universal Indicator. The amount of carbon dioxide released from combustion is an indication of the efficiency of the combustion reaction. Q 2. Soot occurs when a combustion reaction is not complete. The more soot, the less efficient the combustion process. Activity 3This activity gives students data to make an evaluation about how reasonable it is to consider replacing fossil diesel with biodiesel. The answer turns out to be ‘not very easy’. A rough approximation suggests that less than 20% of current use could be replaced by biodiesel, even if all current farm land was converted to biodiesel production. As in any such estimate, many simplifying assumptions have to be made. For example, there may be additional arable land in the US or abroad that could be used for growing oil seeds. Multiple crops might be harvested from a single field in one year. Technology might improve the process. Nevertheless, the basic limits are daunting. Much the same situation is true in Europe. In the United Kingdom estimates are that if all arable land were converted to biodiesel production only 7-10% of current use could be met. Even so, replacing even part of current fossil fuel supply could be a positive contribution Answers to questionsQ 1. The question uses unfamiliar units – those used by farmers in the US. It illustrates the difficulty of using data in different disciplines. Answers may vary in their approach, here is one sample calculation. = 5.38 x 108 acres per year required to produce enough biodiesel to replace the existing use. For comparison, using the values given that there are about a billion (109) acres of farmland in the US, the country would be able to meet less than one fifth of the requirements, even if every acre of farmland were used for biodiesel production. Q 2. Much the same situation is true in Europe as in the U.S. In the United Kingdom, estimates are that if all arable land were converted to biodiesel production only 7‑10% of current use could be met. Q 3. Europe tends to have a higher percentage of vehicles that use diesel and relatively less land available for crop production. Austria, Germany and Switzerland have removed all sales and road tax from biodiesel. Q 4. One green feature of biodiesel is that it does use renewable resources. It is also lower in sulfur content and produces fewer particulates. It is much less toxic than fossil diesel and is more biodegradable. Some ‘not so green’ features are that it still uses a combustion reaction and has approximately the same atom economy in releasing unwanted waste products into the atmosphere. The slightly lower energy content of biodiesel may result in having to use more to get equivalent results. References
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