Desalting Fundamentals: A Process Engineer’s Perspective

Desalting in oil refineries removes residual water and contaminants from the crude oil before further processing. The vast majority of conventional refineries have one or more desalters as part of the refinery’s crude distillation units. In a recent survey of 65 crude units in 48 locations, 57 units included desalting in the crude distillation unit.

Desalting has been a major contributor to increasing time between crude unit turnarounds. Average crude unit run length is now 4-5 years. Some refiners are attempting 7-year runs between crude unit shutdowns. Effective desalting is critical to extended unit runs.

Crude desalting provides multiple benefits:

• Reduced corrosion of the crude tower and overhead systems
• Reduced exchanger fouling downstream of the desalter
• Reduced pressure drop in the downstream heat exchangers and fired heaters
• Reduced energy consumption in the fired heaters
• Increased capacity in the distillation column
• Improved residue quality
• Decreased fouling and plugging in the atmospheric column, vacuum column, and condensing heat exchangers

The intent of this discussion is to cover the fundamentals of desalting from the perspective of crude unit process engineers.

The discussion here focuses on the following points:

• Contaminants and their consequences
• Salt stability and required desalter performance for salt removal
• Water addition locations
• Water mixing
• Desalter operation
  • Settling
  • Coalescence and electrostatic attraction
  • Temperature