Embodiments of a process for producing paraffins from a petroleum-based composition comprising long chain aromatics comprise mixing a supercritical water stream with a pressurized, heated petroleum-based composition to create a combined feed stream, introducing the combined feed stream to a first reactor through an inlet port of the first reactor, where the first reactor operates at supercritical pressure and temperature, cracking at least a portion of the long chain aromatics in the first reactor to form a first reactor product, and then introducing the first reactor product to a second reactor through an upper inlet port of the second reactor operating at supercritical pressure and temperature, where the second reactor is a downflow reactor comprising an upper inlet port, a lower outlet port, and a middle outlet port are provided. The middle outlet product passing out of the middle outlet port comprises paraffins and short chain aromatics.