A Millisecond Furnace is a device used for cracking naphtha into ethylene, by extremely short (50 to 100 millisecond) exposure to temperatures of about 900 degrees Celsius, followed by a rapid quenching below 750 degrees Celsius.
It was developed by M W Kellogg and Idemitsu in the 1970s.
References
Millisecond furnace Wikipedia(Text) CC BY-SA