With the aim at finding a method for the selection of a suitable demulsifier for crude oil dehydration, five crude/associated water/effective demulsifier real systems were evaluated in this study. Likewise, based on the existing knowledge about the fundamentals of emulsion stabilization process, the typical characteristics of each element of the systems, which represent the variables or factors governing the process, were chosen. These variables were: in the crude oil, the acidity number, as a measure of its polarity; in the associated water, the salinity; and in the demulsifier, the relative solubility number (RSN).

A hypothesis based on the surfactant affinity difference (SAD) model is proposed. This model was adapted to the specific case of crude oil chemical demulsification. A simplified mathematical expression was obtained, which establishes a relation between the crude oil nature, the associated water salinity, and the demulsifier hydrophilicity.

The data obtained from the evaluation of the selected characteristics were introduced into the model. As a result, the following empirical relationship was obtained:
lnS=-0.77A-0.28RSN+8.17

where S is the associated water salinity and A is the crude oil acidity number, variables which are easily measured.

RSN values were calculated and compared with those obtained from experiments. Except in one case, the calculated RSN value approximates very closely the experimental one. It is therefore concluded that this correlation serves to select the appropriate demulsifier to dehydrate a given crude oil, based on the simple procedure outlined above.

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