Adsorption & Adhesion of Asphaltenes at Complex Interfaces: An AFM Approach
Asphaltenes are the heaviest fraction of crude oil, pinpointed as the culprit for the fouling of components throughout the hydrocarbon value chain. The ill-defined nature of asphaltenes has challenged scientists and industry alike, to determine their molecular composition and mitigate their effects respectively. In this thesis, a link between the two approaches is drawn.
A model interfacial system was designed and optimised to study the onset of fouling due to the adsorption and adhesion of asphaltenes on relevant surfaces. The effect of surface chemistry on adsorption was originally investigated by examining deposition in the designed system, consisting of synthesised asphaltene molecules and well-defined, atomically flat surfaces. The model molecules examined displayed different deposition behaviour depending on their functionality. AFM morphological characterisation revealed that adsorption occurs at a range of lengthscales, from monomers to flocs, and challenged the concept of a discrete solubility limit.
Further optimisation of the system and a multitechnique characterisation approach led to the conclusion that the archipelago asphaltene molecule exhibits the least affinity for oxidised metal surfaces, a model for pipelines. The deposition found on those surfaces is a result of the aggregation of the asphaltene molecules that decreases their individual diffusivity leading to weak physisorption as confirmed by adhesion measurements. An effective mitigation strategy to counteract such adhesion would be the injection of an anti-agglomerant. However, such a strategy needs to be combined with an effort to mitigate the effect of surface roughness on adsorption. Morphological and nanomechanical characterisation of real-life fouled samples revealed the efficacy of an adhesion inhibitor in decreasing roughness and tuning surface chemistry by competitively adsorbing onto the relevant surfaces.
A combination of the two mitigating strategies suggested has shown to have an increased potential as an anti-fouling measure. The efficacy of the designed system in identifying such strategies as well as in providing a screening approach for the testing of inhibitors underlines the importance of this work.