Unrefined petroleum enters a tall distillation column, heated from below. Hydrocarbons with progressively higher boiling points evaporate and collect on trays and condensers. The Australian Institute of Petroleum website details specific components, temperature ranges and diagrams pertaining to fractional distillation.
Two or more compounds may collect in a condenser and receiver as a mixture. Precise temperature control in distillation may not separate the substances past a certain point; liquid and vapor phases retain unchanging fractions of each compound. This is an azeotropic mixture.
Assume substances A and B exist as an azeotropic mixture AB. Furthermore, assume a third liquid C exists that mixes with A but not with B. Adding substance C gives two immiscible states, AC and B, that are in distinct layers, just like water and oil. Mechanical methods can separate substance B. AC remains as a mixture, but AC does not form an azeotrope. Distillation of AC would isolate A and C.