September 19, 2011
Ethanol can be a source of motor fuel hydrocarbons. Ethanol is being used in increasing quantities as a gasoline additive in the United States. Following the demise of methyl-tert-butyl ether (MTBE), ethanol is used as a means to increase octane rating and as an oxygenate to promote cleaner burning and fewer emissions. However, ethanol has many performance problems as a gasoline additive.
Ethanol has only ≈70% of the energy content of gasoline; it is water-miscible (which means it must be added to the gasoline at the end of the supply chain to prevent dissolved water from rusting pipelines and storage vessels); and it is too volatile to use in diesel fuels. Nevertheless, many argue that, because ethanol is made from domestic renewable materials, its use is justified for political and environmental reasons.
V. Coupard and co-inventors disclose a solution for mitigating many of ethanol’s performance negatives as a transportation fuel. Their concept is to dehydrate ethanol (EtOH) to ethylene and oligomerize the ethylene to hydrocarbon fractions that can be used as components in gasoline or diesel blends. The inventors found alumina materials that can catalyze the dehydration and oligomerization reactions sequentially in one reactor.
In one example, a commercial solid acid alumina catalyst is loaded into a traversed bed reactor and activated at 550 °C in an air stream for 2 h. EtOH feedstock is flashed at 20 °C and 3 MPa pressure and then heated to 400 °C and passed over the catalyst. The EtOH is dehydrated to ethylene with conversions up to 97%, and the oligomerization reaction progresses at an ethylene conversion rate of 40%.
For the optimum production of hydrocarbon fractions suitable for use in gasoline and diesel, light hydrocarbon output is recycled to the reactor with unreacted ethylene. The gasoline and gas oil yields (gas oil is suitable for diesel) can be controlled by changing the temperature used to fractionate the recycle stream feed. The inventors report gasoline–gas oil yield combinations from 28% and 23%, respectively, to 49% and 10%, respectively.
In all cases, total liquid hydrocarbon yields range from 50% to 59%. The yield loss is attributed to the presence of water formed from EtOH dehydration and to the formation of smaller amounts of gaseous hydrocarbons. (IFP Energies Nouvelles [Rueil Malmaison, France]. US Patent 7,994,377, Aug. 9, 2011; Jeffrey S. Plotkin)