September 3, 2012
These catalysts improve steam cracking conversions. Steam cracking hydrocarbon feedstocks to give ethylene is perhaps the most important process technology in the petrochemical industry. Curiously, it is the only large-volume process in the industry that does not use a catalyst. A catalytic process for converting natural gas liquids or naphtha to ethylene is one of the industry’s “holy grails”. Despite much research and many patents in this area, a catalytic process for making ethylene remains elusive.
J.-s. Choi and co-inventors disclose catalyst compositions with excellent thermal stability that give improved olefin yields. The catalysts are based on chromium, zirconium, and, in some cases, phosphorus oxides. The inventors tested the efficacy of these catalysts on hexane as a model hydrocarbon feedstock.
In the patent’s examples, a quartz tube with 0.25-in. o.d. was filled with catalyst to 5 cm height. Hexane (2.75 mL/h) and water (0.92 mL/h) were injected by a syringe pump and vaporized at 400–500 °C. The two gases were mixed well and contacted with the catalyst layer.
In one example, run at 800 °C, the catalyst consisted of chromium and zirconium oxides. Hexane conversion was 73.9%, an increase of ≈14% over the conversion from inactive γ-Al2O3. The ethylene/propylene ratio was 1.2:1, whereas it was 1.6:1 when γ-Al2O3 was used. When the catalyst consisted of chromium, zirconium, and phosphate oxides, hexane conversion increased to 83.5%, and the ethylene/propylene ratio remained at 1.2:1.
For this method to become commercial, extensive life testing must be performed. This type of data was not included in the patent, but the inventors did state that the catalysts could be reused after the coke was burned off. (LG Chem [Seoul]. US Patent 8,242,047, Aug 14, 2012; Jeffrey S. Plotkin)