A process and a device implementing the process, for separating 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, to obtain water with necessary characteristics for its injection without pollution of underground aquifers, and/or when the mixture is dominant in crude petroleum, acceleration of 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 of reduced pressure on part of the free surface of a liquid to be processed, within a closed processing tank, without the overall pressure inside the closed processing tank being affected.