Year 12 Chemistry Assessment Task 1
Ethanol as an alternative fuel
The commercial production of ethanol fuel in Australia is the use of sugar cane and wheat using yeast in the fermentation process. Yeast is a fungus which can multiply in the absence of oxygen by using enzymes (e.g. zymase) to catalyse the decomposition of sugars. Those sugars are sucrose or maltose.
The conditions that promote the fermentation of sugar are:
* A suitable micro-organism such as yeast
* A suitable temperature for the fermenting yeast
* Low oxygen concentrations favouring the fermenting yeast * A small amount of yeast nutrients such as phosphate salt. * Once the ethanol concentration reaches 14-15% by volume, the yeast cannot survive, and the fermentation process stops.
Biomass fuels such as rice straw and sugar cane bagasse are being investigated as raw materials for ethanol production but the transportation costs are very high and do not justify their use. Genetically modified crops are being analysed and this could provide a cheap source of biomass fuels for the production of ethanol.
Production from Sugar Cane
Sucrose extracted from sugarcane accounts for little more than 30% of the chemical energy stored in the mature plant; 35% is in the leaves and stem tips, which are left in the fields during harvest, and 35% are in the fibrous material (bagasse) left over from pressing. Most of the industrial processing of sugarcane in Brazil is done through a very integrated production chain, allowing sugar production, industrial ethanol processing, and electricity generation from by-products. The typical steps for large scale production of sugar and ethanol include milling, electricity generation, fermentation, distillation of ethanol, and dehydration.
Replacement for distillation
With increasing attention being paid to saving energy, many methods have been proposed that avoid distillation altogether for dehydration. Of these methods, a third method has emerged and has been adopted by the majority of modern ethanol plants. This new process uses molecular sieves to remove water from fuel ethanol. In this process, ethanol vapour under pressure passes through a bed of molecular sieve beads. The bead's pores are sized to allow absorption of water while excluding ethanol. After a period of time, the bed is regenerated under vacuum or in the flow of inert atmosphere to remove the absorbed water. Two beds are used so that one is available to absorb water while the other is being regenerated. This dehydration technology can account for energy saving of 3,000 btus/gallon (840 kJ/l) compared to earlier azeotropic distillation.
| FORMULAC6H12O6 > 2CO2 + 2C2H5OH (ethanol) 180.00g > 88.00g + 92.00g
Ethanol vs. Fuel
Arguments for ethanol as a fuel
| Arguments against ethanol as a fuel
| It is a clean and efficient use of energy. It is much safer then petrol (Ethanol is biodegradable without harmful effects on the environment) and will greatly reduce the spread of pollution. Seeing that it is not a fossil-fuel, manufacturing it and burning it does not increase the greenhouse effect. * Ethanol can reduce net carbon dioxide emissions by up to 100% on a full life-cycle basis. High-level ethanol blends can reduce emissions of Volatile Organic Compounds (VOCs) by 30% or more (VOCs are major sources of ground-level ozone formation) * High-level ethanol blends reduce nitrogen oxide emissions
| Ethanol is clean but it only produces two-thirds the energy of octane, hence more is needed
| It is a much healthier alternative for people: * Sulphur dioxide and Particulate Matter (PM) emissions are significantly decreased with ethanol. * As an octane enhancer, ethanol can cut emissions of cancer-causing benzene and butadiene...
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