A process for cracking a light hydrocarbon feedstock containing non-volatile components and/or coke precursors, wherein a heavy hydrocarbon feedstock is added to the contaminated light hydrocarbon feedstock to form a contaminated hydrocarbon feedstock blend which is thereafter separated into a vapor phase and a liquid phase by flashing in a flash/separation vessel, separating and cracking the vapor phase, and recovering cracked product. The heavy hydrocarbon feedstock allows operation of the flash/separation vessel at a higher temperature, within the operating temperature range of the separation vessel.