Some Theoretical Aspects of Tertiary Treatment of Water/Oil Emulsions by Adsorption and Coalescence Mechanisms: A Review

Abstract

The massive increase in the volumes of oily contaminated produced waters associated with various industrial sectors has initiated considerable technological and scientific efforts related to the development of new cleaning strategies. The petrochemical industry (oil and gas production and processing) contributes to those volumes by approximately 340 billion barrels per year. The removal of emulsified oily components is a matter of particular interest because the high emulsion stability necessitates sophisticated technological approaches as well as a deep theoretical understanding of key mechanisms of oil/water separation. This review deals with the theoretical aspects of the treatment of emulsified oil/water mixtures and is particularly focused on tertiary treatment, which means the reduction of the oil content from 70–100 ppm to below 10 ppm, depending on national regulations for water discharge. The review concerns the mechanisms of oil/water separation and it covers the (i) adsorption isotherms, (ii) kinetics of adsorption, (iii) interfacial interactions between oil/water mixtures and solid surfaces, and (iv) oil/water separation techniques based on the wettability of solid/oil/water interfaces. The advantages and drawbacks of commonly used as well as newly proposed kinetic and adsorption models are reviewed, and their applicability for the characterization of oil/water separation is discussed. The lack of suitable adsorption isotherms that can be correctly applied for a description of oil adsorption at external and internal solid surfaces of both nonporous and porous structures is pointed out. The direct using of common isotherms, which were originally developed for gas adsorption, often leads to the incorrect data description because the adsorption of oily components at solid surfaces does not fit the assumptions from which these models were originally derived. Particularly, it results in problematic calculations of the thermodynamic parameters of sorption. The importance of nonlinear analysis of data is discussed, since recent studies have indicated that the error structure of experimental data is usually changed if the original nonlinear adsorption isotherms are transformed into their linearized forms. The comparison between the pseudo-first-order and pseudo-second-order kinetic models was performed. It was shown that the correlation between data and models strongly depends on the selection of data, particularly on the frequency of collected data in time scale. The wettability of solid surfaces by oil in air and under water is discussed, regarding the surface morphology of surfaces. We demonstrate that the combination of surface chemistry and topology strongly influences the separation of oil/water emulsions.