The electrostatic desalter is the first processing unit in a petroleum refinery crude distillation unit (CDU). Its primary objective is to remove water-soluble inorganic salts (primarily NaCl, MgCl2, and CaCl2) and suspended sediment/solids from the crude feed. In the atmospheric furnace, thermal hydrolysis of MgCl2 and CaCl2 produces highly corrosive hydrochloric acid (HCl) gas that corrodes distillation overhead systems. Desalting reduces crude salt from 50-200 PTB down to under 1.0-2.5 PTB, protecting crude heaters and exchangers from fouling and corrosion.

Water is a polar molecule with a high dielectric constant (~80) compared to non-polar crude oil (~2.2). When a fine water-in-oil emulsion passes through an AC or pulsed DC high-voltage electric field (typically 1.5 to 3.0 kV/cm), the electric field polarizes the microscopic water droplets, inducing electric dipoles. Opposite charges on adjacent droplets attract each other, accelerating droplet collision and coalescence rate by hundreds of times. The merged droplets grow to 100-200 microns and rapidly settle out by gravity into the bottom brine layer.

Fresh wash water (4% to 8% vol) is injected into the raw crude upstream of the mixing valve. The mixing valve creates controlled turbulent shear to disperse the fresh water into uniform droplets, contacting and extracting dissolved salts from the indigenous crude water droplets. If ΔP is too low (<0.7 bar), mixing is insufficient and salt extraction fails. If ΔP is too high (>2.5 bar), the water is sheared into sub-micron droplets, forming an unbreakable tight emulsion or "rag layer" that short-circuits the electric grid.

A single-stage desalter achieves 90% to 95% salt removal efficiency, suitable for light, low-salt crudes (<20 PTB). A two-stage desalter arranges two vessels in series with counter-current wash water flow (fresh wash water is injected into the second stage, and effluent brine from stage 2 is recycled as wash water for stage 1). Two-stage systems achieve 98% to 99.5% salt removal, essential for heavy, high-conductivity, or high-salt crudes (>50 PTB) to achieve <1.5 PTB effluent.

A rag layer is an intermediate stable emulsion zone that accumulates at the oil-water interface, stabilized by asphaltenes, waxes, clays, iron sulfide solids, and excessive shear. As the rag layer expands upward toward the energized electric grid, its high electrical conductivity draws excessive current, causing the high-voltage power unit (transformer-rectifier) to trip on over-current. Desalters use chemical demulsifiers (5-20 ppm) and interface level control to prevent rag layer expansion.