Residence Time

In steam cracking processes, olefins are formed as primary products. Aromatics and higher hydrocarbon compounds result from secondary reactions of the formed olefins. Accordingly, short residence times are required for high olefin yield. When ethane and light hydrocarbon gases are used as feeds, shorter residence times are used to maximize olefin production and minimize BTX and liquid yields; residence times of 0.5–1.2 sec are typical. Cracking liquid feedstocks for the dual purpose of producing olefins plus BTX aromatics requires relatively longer residence times than for ethane. However, residence time is a compromise between the reaction temperature and other variables.

A fairly new development in cracking liquid feeds that improves ethylene yield is the Millisecond furnace, which operates between 0.03–0.1 sec with an outlet temperature range of 870–925°C. “The Millisecond furnace probably represents the last step that can be taken with respect to this critical variable because contact times below the .01 sec range lead to production of acetylenes in large quantities.”