March 10, 2014
Crack propane to propylene with a specially pretreated catalyst. Shale gas has been a game-changer for the US ethylene business, but it has led to dwindling supplies of propylene. Steam-cracker operators with the flexibility to crack ethane instead of heavier feedstocks do so because an overabundance from shale gas extraction has made ethane prices unprecedentedly low. The downside of this choice that propylene is not produced from ethane cracking.
Among US propylene suppliers, the technology of choice to close the supply gap is propane dehydrogenation (PDH). This is not a new a technology, but it is finding new life because of this unique market situation. The world’s largest PDH plant was built in 2010 by Petrologistics in Houston. Seven new PDH plants will be in operation over the next 5 years in North America. They will provide an estimated 5 million metric tons of propylene capacity.
Almost all of the world’s PDH plants operate via nonoxidative dehydrogenation of propane to give propylene and byproduct hydrogen. This process is highly energy intensive, and the selectivity to propylene is just ≈90–92%.
S. Ahmed and co-inventors disclose catalysts for oxidatively dehydrogenating propane to propylene with 100% selectivity. The catalysts are based on crystalline MoVOx powders with various Mo/V oxide ratios. The inventors found that washing the metal oxide catalyst with i-PrOH and drying it in a supercritical CO2 dryer dramatically improves propylene selectivity.
In one example, a stream of propane was passed over a MoVO5.5 catalyst that had been washed and dried according to the inventors’ method. In the temperature range 350–450 ºC, propane conversion ranged from 2.74 to 3.43%. Propylene selectivity was 100% at all temperatures. In a control experiment at 450 ºC, the catalyst was not washed or dried, and propylene selectivity plummeted to 3.1%. At 500 ºC with the pretreated catalyst, propylene selectivity declined significantly to 31.6%. (King Faud University of Petroleum and Minerals [Dhahran, Saudi Arabia]. US Patent 8,623,781, Jan. 7, 2014; Jeffrey S. Plotkin)