No deaths were reported after the 1:25 p.m. CST explosion at the plant's K-resin unit, spokesman Norm Berkley said.
"We're doing everything possible, search and rescue, and we have some very sophisticated processes to try to make sure we account for everyone," he said. "It's a terrible day for us at the plant."
More than 10 years ago, the same complex was the scene of another explosion that killed 23 workers.
"I don't know what's happened in this case," Berkley said. "I think the only similarity is certainly that we've had a tragic, tragic day. We've had an explosion, we've had a fire, injured employees. We've got some employees unaccounted for. I couldn't feel worse about it."
The plant has about 850 workers but Berkley said about 600 would have been there at the time of the blast.
Hospitals treated workers for a range of symptoms, including anxiety, smoke inhalation, falls, burns and shrapnel wounds. All were in stable condition and some were released quickly, hospital officials said.
Speaking at the site Monday afternoon, Phillips chief executive officer Jim Mulva said the Bartlesville, Okla.-based company was concentrating on taking care of its injured employees and contractors.
"Our greatest concern and objective is to provide for care and assistance for all personnel in the vicinity, and their immediate families," Mulva said.
Workers at neighboring plants, area residents and schoolchildren initially were urged to remain indoors while officials tried to determine what was burning. That precaution was lifted after about 2-and-a-half hours.
Heavy black smoke turned white about two hours after the explosion, leaving behind tangles of scorched metal. Plant officials said there was no indication anyone outside the plant could be injured by the smoke but said breathing high concentrations could cause irritation of the eyes, nose and throat.
The fire burned cyclohexane, styrene and 1,3-butadiene, according to plant environmental director Cindy Simpson. The facility was shut down and stable by dusk, said plant manager Kevin Collins, who added that he did not know when systems would be restarted.
As a precaution, the U.S. Coast Guard ordered the chemical ship NCC Jizan to leave the complex until further notice. There were no anticipated traffic interruptions for the Port of Houston, officials said.
Other plants at the chemical complex were not damaged, the company said.
Pasadena Mayor Johnny Isbell said an emergency siren system intended to alert Pasadena residents did not sound immediately because it was being repaired Monday. However, a boom heard for several miles was warning enough fo many in the area.
"I was in the main shop area when I heard a loud explosion," said Tim Williams, a plant worker who estimated he was more than 200 yards from the blast site. "My ears hurt, and I took off running. I looked back and saw flames, and kept going."
The Phillips complex was the site of an Oct. 23, 1989, series of explosions and fires in a polyethylene reactor that killed 23 and injured 130 in one of the region's worst industrial calamities.
Last June, two people were killed and four were injured in an explosion that also damaged the K-resin unit. K-resin is a clear plastic used in drinking cups, food containers and other items.
Phillips was fined $204,000 by the Occupational Safety and Health Administration for 13 alleged safety and health violations in the wake of the June 23 explosion and fire.
"We have worked tremendously hard to make sure a situation like that has never, ever occurred again," Berkley said. "Obviously, it has."