May 5, 2014
Use a catalyst to improve propylene yield from an ethylene reactor. Ethylene is the most-widely used petrochemical building block. In 2013, the global demand for ethylene was 134 million tonnes. About 99% of the world’s ethylene is produced by the steam cracking process in which hydrocarbon feedstocks such as ethane, propane, butane, and naphtha are thermally cracked to yield ethylene and other olefins, mainly propylene and butadiene.
The ratio of ethylene to propylene and butadiene depends mostly on the carbon number of the cracking feedstock. Heavier feedstocks produce greater amounts of coproducts.
A curious feature of the steam cracking process is that it is not catalytic. Almost all process technologies in the petrochemical industry are catalytic; but the world’s most abundant product, ethylene, is made without a catalyst.
Inventors J.-H. Kang, J. Song, and J. Choi disclose catalysts that crack naphtha with improved yields and higher ethylene/propylene ratios than are obtained with standard noncatalytic conditions. They prepared a composite catalyst by mixing and sintering 5 wt% CrZr0.833Ti0.167Ox and 95 wt% SiC. They then added the catalyst to a reaction tube 4.3 cm in diam and 300 cm long. Naphtha and water (2:1 w/w) were pumped through the reactor at a liquid-hourly space velocity of 10 h–1. Primary and secondary heaters were maintained at 550 and 650 ºC, respectively.
The inventors varied the cracking temperature over the catalyst bed between 940 and 1060 ºC. At 980 ºC, ethylene yield was 27.7%; and propylene yield was 17.1%. This is equivalent to an overall light olefin yield of 44.8% and an ethylene/propylene ratio of 1.63:1. The same experiment run without a catalyst gave a light olefin yield of 39.2% and an ethylene/propylene ratio of 1.49:1. (LG Chem [Daejeon, Korea]. US Patent 8,674,158, March 18, 2014; Jeffrey S. Plotkin)