Assessment of Conversion Technologies for Bioalcohol Fuel ...

Alcohol Synthesis
The direct synthesis of methanol is an established commercial technology, and the
catalysts for this process can be purchased from many suppliers. Copper/Zinc based
catalysts are typically used for this synthesis and achieve high productivity. The perpass
CO conversion is low (7-20%) because of equilibrium limitations and the need to
maintain mild conditions to prevent copper sintering, but selectivity is high (99.5%).
Because of the low cost ($20-30 l -1 ) and long useful life (3 to 5 years) of these
catalysts, the production of methanol from synthesis gas is a very cost-effective
process and is the starting point for many other useful chemicals like formaldehyde,
acetic acid, MTBE, plastic compounds, etc. Spath and Daton (2003) present a
thorough overview of methanol catalysts and systems related to methanol production.
The methanol catalysts have been used as a starting point for the manufacture of
ethanol and higher alcohols. Several processes for higher alcohol synthesis have
focused on modifying the hydrogenation catalysts to produce larger amounts of higher
alcohols including ethanol.
Most of the recent efforts on the conversion of syngas to ethanol have focused on
modifications of catalysts originally developed by Dow Chemical Company (U.S. Pat.
No. 4,675,344; 4,749,724; 4,752,622; 4,752,623; and 4,762,858). They developed a
supported catalyst based on molybdenum disulfide (MoS2) to produce mixed alcohols,
primarily C1-C4 (methanol—butanol), in a packed column or fluidized bed. The best
per-pass CO conversion is approximately 20%, with up to 85% selectivity to mixed
alcohols. The alcohol mix is typically comprised of 40% ethanol, 55% methanol and
about 5% C3-C5 alcohols.
Alcohol Purification
The resulting raw alcohol produced via catalytic synthesis requires purification to meet
market standards for alcohol products. Both methanol and ethanol have quality
standards for fuel grade and chemical grade products. Raw methanol can contain
water, higher alcohols, hydrocarbons and other byproducts. Raw mixed alcohols
contain a mixture of multiple linear alcohols and water (and possibly other trace
products). In order to separate these constituents into fuel grade components, a
combination of absorption and multi-step distillation can be used. The technology to
purify alcohols is technically feasibly with various components that have been
employed by methanol and ethanol production facilities around the world.
Some have proposed that alcohol mixtures be accepted as fuel additives without
further purification, but there are currently no accepted standards for these mixtures.
Italy successfully used a mixed alcohol product (MAS-Metanolo piu Alcoli Superiori) in
gasoline during the 1980’s produced by the Snamprogeti plant (Spath and Daton,
2003). Ultimately, this type of approach would save some of the cost of purification
steps in alcohol synthesis plants, but would require acceptance by vehicle
manufacturers and air quality regulatory agencies.
27