Firefighters responded around 4:20 a.m. to a blaze on the second floor of the building on the 1600 block of West Anaheim Street.
More than 160 firefighters were on scene. It took them almost four hours to knock down the flames. No injuries were reported.
Officials said there was no threat to surrounding buildings. The blaze wasn't near chemicals or the refinery process.
The cause of the fire remains under investigation.