March 26, 2012
Make olefins from natural gas– or coal-derived alcohols. Ethylene and propylene traditionally are made by steam-cracking natural gas liquids (e.g., ethane, propane, or butane) or by cracking liquid feedstocks such as naphtha or gas oils. Methanol-to-olefins (MTO) technology now allows natural gas or coal to be used as a feedstock for ethylene and propylene. In 2011, China began to use MTO technology with coal-derived methanol as the feedstock.
Methanol is synthesized by gasifying coal to synthesis gas, which is converted to methanol. With the advent of shale gas in the United States, natural gas prices are so low that MTO technology using gas-derived methanol may be an attractive route to olefins.
An alternative approach for converting methane or coal to olefins is to convert synthesis gas to so-called mixed alcohols rather than methanol. A mixture of ethanol, propanol, and even butanol can be converted easily to the corresponding olefins by catalytic dehydration.
M. M. Tirtowidjojo and nine co-inventors at Dow Chemical disclose a catalyst system and the operating conditions for converting synthesis gas to mixed alcohols. In addition, they found that any methanol formed in the mixed alcohol synthesis can be separated and recycled to give significantly higher yields of ethanol.
The catalyst consists of potassium-modified cobalt molybdenum disulfide catalyst compounded with a clay binder. The components are 66 wt% CoMo2Sx powder (x = 4–6), 20 wt% bentonite clay, 10 wt% K2CO3, and 4 wt% of a lubricant.
In the inventors’ first example, the crushed catalyst pellets are loaded into a stainless steel tubular reactor. The feedstock to the reactor is a gas mixture composed of 47.5 vol% H2, 47.5 vol% CO, and 5 vol% N2. A small amount of H2S is added as a sulfiding agent. The reactor temperature is 320 °C; the pressure is ≈20 MPa; and the gaseous hourly space velocity is ≈3000 h–1. The conversion of CO is 41.1%; and the methanol, ethanol, and propanol productivity values are 8.2, 11.7, and 2.2 lb/(ft3·h), respectively.
In a second case in this example, methanol is added at the same rate as it is produced in the first experiment with no changes to the other conditions. The product mixture contains no additional methanol, but ethanol productivity increases from 11.7 to 15.7 lb/(ft3·h). This finding is important because excess methanol produced by using this method is not useful as an olefin precursor. (Dow Global Technologies [Midland, MI]. US Patent 8,129,436, March 6, 2012; Jeffrey S. Plotkin)