(175e) Vacuum Distillation: Fractionating Wash Zones

Feed quality imposes limits on possible product qualities in refinery crude vacuum distillation. Impurities in the products come from a combination of volatile contaminants and non-volatile contaminants.

Non-volatile contaminants get into the product by mechanical entrainment in the flash zone and wash zone of the tower. Volatile contaminants come from a combination of vaporization in the feed plus possible entrainment. Consequences from both vaporization and entrainment vary with the contaminant, the quantity in the feed, and the distribution in the feed.

A properly functioning wash zone will remove entrainment. However, volatile contaminant concentration depends on the fractionation effectiveness of the wash zone. Higher fractionation effectiveness removes more volatile contaminants, but increases the hydraulic load of the wash bed. Higher hydraulic loads must be accommodated by either increasing the bed diameter or reducing the feed rate to the unit.

This review focuses on the types volatile contaminants. Examples based on typical volatile contaminants for a range of commonly processed crude oils are presented. The examples compare the product quality versus the unit capacity changes for varying fractionation stages in the wash zone.