What is cracking severity in steam cracking and how does it affect yields?

Cracking severity measures the extent of thermal decomposition, typically expressed as COT (coil outlet temperature) or conversion to methane/propylene. Higher severity: more ethylene production, more methane and hydrogen, less propylene and C4s, more aromatics in pyrolysis gasoline. Lower severity: more propylene and C4 olefins, less ethylene, better yields of liquid products. Severity selection based on feedstock (ethane needs higher severity than naphtha) and desired product slate (maximize ethylene vs. maximize C3/C4). Typical COT: 800-850C for naphtha, 850-900C for ethane. Residence time is very short (100-500 ms). Severity also affects coking rate - higher severity increases tube coking, requiring more frequent decoking. Optimize for product value minus operating costs.