Process and apparatus for desalting crude petroleum

Claim 1

A process for desalting crude petroleum, said process comprising the following sequential steps: (A) continuously introducing into a desalter an unstable aqueous emulsion of crude petroleum, (B) resolving said unstable emulsion in said desalter into a water phase and a crude phase, said resolving step being capable of resulting in the formation of an intermediate phase composed of a stable emulsion located between said water phase and said crude phase, and (C) continuously drawing off the water phase and crude phase from said desalter, wherein the improvement comprises the following combination of additional steps: (a) continuously adjusting the level of the top surface of said water phase alternately between predetermined upper and lower limiting levels whereby at a predetermined level situated between said upper and lower limiting levels there are arranged successively said water phase, said intermediate phase of stable emulsion when formed, and said crude phase; (b) taking a sample at said predetermined level; (c) testing whether said sample contains a predetermined minimum quantity of said stable emulsion, and (d) if said sample contains said minimum quantity of said stable emulsion, injecting a second demulsifying agent into said desalter at said predetermined level.