"We woke up to glass all over us and parts of the ceiling caved in, doors blown in," said Ryan Mathewson, who lives roughly two minutes from the plant.
The 25-year-old told AFP he and his family were "shook up and scared" following the explosion.
TPC said two plant employees and a contractor were injured.
County Judge Jeff Branick told KFDM News that one person suffered burns, one had a broken wrist and another a broken leg. He called it "a miracle" that there were no fatalities.
The local Nederland Volunteer Fire Department said a mandatory evacuation had been ordered for everyone living within a 1/2 mile radius of the plant.
The TPC Group said the explosion happened in what it said was a "processing unit" at around 1:00 am local time (0700 GMT).
"TPC Group has activated its Emergency Response Plan and requested assistance from Port Neches Fire Department and Huntsman," it said in a statement.
"The event is ongoing, but will be brought under control as quickly and safely as possible.