What Is a Desalter? Crude Oil Processing

A desalter is a process vessel that removes dissolved salts, water, and suspended solids from crude oil before it enters the atmospheric distillation column in a refinery. Salt in crude oil—primarily sodium chloride, magnesium chloride, and calcium chloride—causes severe corrosion in downstream equipment, fouls heat exchanger surfaces, and poisons catalysts. Desalting is therefore the first process step in virtually every crude oil refinery.

When Desalting Is Required

Desalting is required whenever crude oil contains salt content above the refinery’s tolerance limit, typically 1 PTB (pound of salt per thousand barrels) at the distillation column inlet. Most crude oils contain 10-200 PTB at the refinery gate, making desalting necessary. The process reduces salt content by 90-99%.

Desalter Design and Specifications

Parameter	Typical Specification
Vessel type	Horizontal pressure vessel per ASME Section VIII
Desalting method	Electrostatic coalescence with wash water injection
Operating temperature	120-150 degrees C (to reduce oil viscosity and improve water separation)
Operating pressure	10-15 barg (to prevent vaporization at operating temperature)
Wash water rate	3-10% of crude oil volume
Electric field strength	1-5 kV/cm (AC, DC, or dual-polarity)
Salt removal efficiency	90-99% per stage
Inlet salt content	10-200 PTB (crude dependent)
Outlet salt content	Less than 1 PTB (single stage), less than 0.5 PTB (two stages)
Residence time	20-30 minutes
Materials	Carbon steel shell; internals may include CRA components

How the Desalting Process Works

The process involves three key steps:

1. Wash water injection: Fresh water (3-10% by volume) is injected into the crude oil upstream of the desalter. A mixing valve creates an emulsion that dissolves the salts from the crude into the water phase.

2. Electrostatic coalescence: Inside the desalter vessel, a high-voltage electric field causes the small water droplets (now containing dissolved salts) to coalesce into larger droplets.

3. Gravity settling: The larger water droplets settle by gravity to the bottom of the vessel, forming a distinct water layer (brine) that is drained off. Clean crude exits from the top.

Stage Configuration	Salt Inlet	Salt Outlet	When Used
Single-stage desalter	Less than 50 PTB	Less than 5 PTB	Light, low-salt crudes
Two-stage desalter	50-200 PTB	Less than 1 PTB	Heavy crudes, high-salt crudes
Three-stage desalter	Greater than 200 PTB	Less than 0.5 PTB	Very heavy or high-salt crudes (rare)

Common Mistake

Operating the desalter at too low a temperature increases crude oil viscosity, which slows water droplet settling and reduces separation efficiency. Conversely, operating at too high a temperature risks vaporization, which disrupts the electric field and causes arcing. Always maintain the desalter within the designed temperature window and monitor for process upsets.

Desalter effluent water (brine) is highly corrosive and must be treated before disposal. The brine is routed through the sour water system and eventually to a wastewater treatment facility. Proper desalter operation is critical to protecting downstream heat exchangers, distillation column overhead systems, and catalyst beds.

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