T
Ethanol is also produced from petrochemical plant derived chemicals where it is manufactured by reacting ethene with steam. The catalyst used is solid silicon dioxide coated with phosphoric(V) acid. The reaction is reversible. ... By removing the ethanol from the equilibrium mixture and recycling the ethene, it is possible to achieve an overall 95% conversion. I have not been able to find out what percentage of ethanol is produced this way.
Peter
Synthetic ethanol , produced from ethylene, is much more than 95% efficient , more like 99.5%.
Fermentation ethanol is produced from fermentable carbohydrates, this is now generally referred to as Bioethanol.
When I left the chemical industry the split on a global basis was about 50:50, but since then fermentation has increased, thanks to subsidies (esp in USA) and other inducements, and synthetic has fallen as the value added to ethylene is smaller than other options.
The big issue with all fermentation ethanol is that the fermentation process will only produce something like 15 to 20% ethanol in an aqueous process, you now need to get rid of the water which is done by distillation (which uses energy). The highest concentration you can achieve by distillation is 95 to 96% as it forms an azeotrope (constant boiling mixture) This 95:5 product is used in Brazil as a neat fuel (due to their plentiful sugar cane production of ethanol) . Elsewhere the presence of water is gasoline systems causes all sorts of issues so the minimum concentration of ethanol for blending is normally 99+% , the higher the better, therefore the water has to be removed by another distillation process using a co-solvent or entrainer (used to be benzene or mono ethylene glycol)
Ethanol burns without significant difference to hydrocarbon fuels with the possible exception of traces of acetaldehyde or acetic acid, the other combustion products are the same, however there are some real benefits:
Due to the latent heat of vaporisation the incoming charge is cooled so the engine will run cooler
The octane rating is higher so blenders do not have to use so much of the alternative higher value hydrocarbons.
Because there is oxygen in the fuel (compared to hydrocarbons) the Carbon Monoxide produced is much lower.
I have probably bored you all to death by now , so I will happily step down off my soap box
