Fire broke out Thursday morning at Lyondell Basell Industries' 268,000-barrel-per-day refinery in Houston, Reuters reports.

At about 5:30 a.m., fire broke out in a hydrotreater, a catalytic chemical process used to remove sulfur from refined petroleum products. The refinery specializes in transforming heavy high-sulfur crude oil into reformulated gasoline and low-sulfur diesel.

Although the refinery’s in-house fire brigade dealt with the emergency, the hydrotreater remains shut down Friday. The emergency marked the second fire at the Houston refinery this year.