How does 'severity' influence product yield in steam cracking?

In steam cracking, 'severity' refers to the intensity of the cracking conditions, primarily determined by the furnace outlet temperature and residence time. Increasing the severity generally leads to a higher yield of ethylene but also results in a higher yield of lighter products, such as methane and hydrogen, and increased coke formation. At lower severities, the cracking reactions are less complete, resulting in a lower ethylene yield and a higher yield of heavier products, such as propylene, butadiene, and pyrolysis gasoline (pygas). As the severity increases, more of the feed hydrocarbons are cracked into smaller molecules, leading to a higher ethylene yield. However, at very high severities, secondary cracking reactions become more significant, breaking down the ethylene into methane, hydrogen, and coke. Therefore, the optimal severity for steam cracking is a balance between maximizing ethylene yield and minimizing the formation of undesirable byproducts. The specific severity that is used in a steam cracking unit depends on the feed composition, the desired product slate, and the design of the furnace.