What Is a Petrochemical Separator: Types, Sizing and Applications Answering the core question: What is a petrochemical separator and how is it sized? A petrochemical separator is a pressure vessel that splits a mixed process stream into its vapour and liquid phases, and in three-phase duty into vapour, hydrocarbon liquid and free water, using gravity settling assisted by inlet momentum reduction and a mist elimination device. Its gas handling capacity is set by the Souders