Multiscale Modeling of Interfacial Mass Transport in Liquid-Liquid Dispersions

Abstract

Crude oil is a complex system which consists of many different compounds and substantial amounts of naphthenic acids which can create naphthenate deposits. This can cause significant fouling in the petroleum transport and separation process. Considerable efforts were applied to reveal the mechanism of the calcium naphthenate precipitation problem but the details of it are still not clear. It was found that indigenous tetrameric acids are the main constituent of calcium naphthenate deposits. They are characterized as highly interfacial active components which react with calcium ions at the water-oil interface during the degassing process. Several model compounds were synthesised to match interfacial and bulk properties of indigenous tetrameric acids. The BP10 model compound was able to mimic the properties of a family of tetra-acids called "ARN acids". Therefore it became possible to experimentally study these tetra-acids at the water-oil interface and in the bulk as well as with the use of computational tools.

The purpose of this thesis is to employ molecular dynamic simulations to study the structure and orientation of tetrameric acids at the water-oil interface and in the bulk water and oil phases and, principally, to focus on parameterization of a molecular mixed monolayer isotherm with the aim to employ it in a multi-component mass transport model which can predict the interfacial concentration of tetra-acid at the water-oil interface.

The structure, arrangements and orientation of the tetra-acids at the interface and bulk water and oil phases obtained from molecular dynamic studies correlate well with experimental observations. It is demonstrated that phase-space trajectories determined from molecular dynamic simulations of charged and uncharged carboxylic acids can be employed to develop methods to calculate the area per molecule in the adsorbed phase to parameterize the molecular mixed monolayer isotherm and predict the interfacial tension of the surfactant mixtures at acidic and basic conditions of water phase. The agreement between experiment and theory was found to be good. The molecular mixed monolayer isotherm for surfactants in charged and uncharged states is successfully applied as a constitutive equation in a continuum mass transport model in order to predict interfacial concentration of tetra-acids in liquid-liquid dispersions (emulsions) with a high surface to volume ratio. Effects of pH, phase partitioning and micelle formation in the aqueous phase were considered. Precipitation behaviour observed in real systems correlates with the trends predicted by model.