Water wash systems have been used by refiners in distillation overheads for decades to assist with corrosion control. Some of these wash systems have worked well reducing overhead corrosion from HCl salts providing the operator with valuable flexibility in operation of the crude unit. However, some wash systems struggle to protect the system from salt corrosion despite implementing many industry-popular features in the wash design. This paper proposes a theory for how a successful water wash achieves salt corrosion control, the mass-transfer considerations driving this process and how popular practices may contribute to or hinder successful outcomes.
In the refining of crude oils into usable products the oil is first distilled and separated by boiling point ranges in a crude distillation unit. These units first send the crude oil to a desalter vessel with emulsified water to remove water soluble inorganic chloride salts found naturally in the crude oil. The crude oil is further heated to boiling temperatures and passed into a larger distillation column. Steam is also added to aid successful distillation at the desired operating temperatures. Lighter lower boiling oils rise as vapor towards the top of the column and heavier higher boiling oils descend as liquid. The lightest hydrocarbons (dissolved natural gasses and naphtha) and steam rise to the top of the column and exit the overhead vapor line. These vapors are cooled to condense the naphtha for separation from the gasses.
Conditions in the overhead system typically favor corrosion of the equipment by a couple of mechanisms. The first mechanism is aqueous acidic corrosion of equipment, resulting from condensed water with low pH due to presence of a variety of acids. This mechanism is easily mitigated by the application of neutralizing amines to raise pH and filming amine corrosion inhibitors. The second mechanism, which accounts for most overhead system corrosion failures, is deposition of hydrochloride salts that form when HCl reacts with ammonia (NH3) and/or amines. These salts are acidic and, when conditions favor formation at temperatures above the water dew point, the acidity leads to localized corrosion beneath the salt deposits. For systems where the temperature of salt formation is lower than the water dew point, the salts are highly soluble in condensing water and do not accumulate.
