Desalting is a separation process and it is also the first process that a crude oil will go through.  Desalting removes NaCl and some metals from crude oils due to the costly damage they cause if not removed (the reason for placing this process first).  NaCl will cause corrosion on heat transfer surfaces and metals foul expensive catalysts.  Previously, if NaCl exceeded 10 lb./bbl it was desalted, but now heavier crudes* are being processed and many companies are now desalting all crudes.  Typically, heavier (high specific gravity) crudes are younger than light crudes and contain more impurities such as salt, sulfur, and metals.

    The salt is removed using a water wash because NaCl is more soluble in water than petroleum.

*Old, light crudes are most desirable because nature slowly removes sulfur, metals, and reduces the viscosity of crude oil as time passes.  But, due to the high consumption of crude oil, these old, light crude reserves are being depleted and heavier crudes are now being processed..