November 5, 2012
Here’s how to make light olefins with catalytic cracking. A long-time goal of the petrochemical industry has been to catalytically crack liquid petroleum distillates to make ethylene and propylene in good yield. Refinery operators routinely practice fuel-oil catalytic cracking; the intent, however, is not to make olefins but to make molecules of the right size for gasoline production. In a typical fluidized catalytic cracker (FCC) refinery, propylene byproduct is only 4–5%.
This source of propylene is important in the United States because the feed to many U.S. steam crackers is inexpensive ethane from shale gas. This decreases the coproduct propylene supply, and propylene prices are increasing.
S. Choi and co-inventors disclose a so-called “fast-fluidization” catalytic cracking process that substantially boosts propylene and ethylene production. To test their system, they catalytically cracked a naphtha feedstock with the composition of 36.2% normal paraffins, 49.3% isoparaffins, 11.3% naphthenes, and 3.2% and aromatics in a fluidized bed reactor. The cracking catalyst was HZSM-5 zeolite with a lanthanum promoter. The fluidized bed reaction system consisted of a riser, a regenerator, a stripper, and a stabilizer with the following dimensions:
|Height, m||Diam, cm|
The feedstock, steam, and catalyst were mixed into the inlet of the riser. The naphtha and steam were fed at 390 g/h and 195 g/h, respectively, at a temperature of 400 °C. The catalyst circulated at a rate of 22 kg/h and was maintained at 725 °C. The catalyst circulation rate was equivalent to 88.1 kg/(m2·s) in the riser. The gas velocity at the riser inlet was 2.2 m/s.
The product mixture consisted of methane (11.0%), ethylene (14.6%), propylene (16.9%), C4s (9.2%), C5s (8.3%), and C6+ (27.4%). When the reaction was repeated with a riser inlet gas velocity of only 0.83 m/s, ethylene and propylene yields significantly increased to 20.9% and 20.1%, respectively. (SK Innovation [Seoul].US Patent 8,293,961, Oct. 23, 2012; Jeffrey S. Plotkin)