Dramatic footage from the scene shows the moment flames were sent billowing into the sky.
Three employees have been injured and all staff are accounted for, plant owner TPC Group confirmed.
The blast, at 01:00 local time (07:00 GMT) on Wednesday, was felt 30 miles (48km) away. A mandatory evacuation zone is in place for everyone within half a mile of the site.
TPC Group said in a statement that it has activated its emergency response plan and requested assistance from Port Neches Fire Department.
Port Neches residents told local news outlet KDFM that windows had blown out miles away from the plant.