A process, and a device for implementing the process, for separation of fluids in emulsion and / or in solution, and / or for low pressure distillation, in particular of water and / or gaseous hydrocarbons dissolved in crude petroleum, and / or for separation of crude petroleum droplets emulsified in water, so as to obtain water with the necessary characteristics for its injection without pollution of underground aquifers, and / or when the mixture is dominant in crude petroleum, acceleration of the settling of the water in the lower part of the mixture, and / or for low pressure distillation of crude petroleum. The method creates a localized zone (5) of reduced pressure on part of the free surface of a liquid (2) to be processed, within a closed processing tank (1), without the overall pressure inside the closed processing tank being affected.