Characterisation of Deposited Foulants and Asphaltenes using Advanced Vibrational Spectroscopy

Abstract

The magnitude and significance of crude oil fouling have led to a number of studies; however, the fundamentals of the complex fouling process are not fully understood. This thesis describes the use of the high chemical specificity, imaging capabilities and fast acquisition times offered by advanced vibrational spectroscopic techniques to characterise and understand the physicochemical behaviour of these complex materials. Rapid and reliable methodologies are developed to provide an important chemical characterisation tool which will advance research into crude oil fouling. An emerging and powerful imaging technique based on Fourier transform infrared (FTIR) spectroscopy is applied for the first time to the characterisation of deposited foulants and asphaltenes. Attenuated total reflection (ATR)-FTIR spectroscopic imaging has the advantage of being a non-destructive analytical technique and most importantly, is able to provide both chemical and spatial information about a sample. The novel application, of combining macro and micro ATR modes in FTIR imaging, yields important information about the spatial distribution of different components in deposited foulants and laboratory-extracted asphaltenes. Clusters of chemically different compounds in crude oil deposits from the refinery, such as asphaltenes, carbonates, sulphates, sulfoxides, oxalates and even “coke-like” materials, were identified and analysed. A lab-made aperture is utilised in the macro ATR diamond accessory to correct spectral distortions that occur for high refractive index materials. This approach has been extended to monitor the heating of crude oil in situ and the onset of asphaltene deposition was determined. Micro ATRFTIR imaging of the particulates formed in the crude oil after heating has identified seven chemically different species, namely, silicates, amides, sulphates, carbonates, sulfoxides, vitrinite compounds and “coke-like” materials which are products of different reactions in fouling. The complementary use of Raman and FTIR spectroscopy has been demonstrated to characterise the carbon structures in asphaltenes. The ID/IG and IV/IG parameters derived from the Raman spectra on real deposits showed that it has more ordered structures compared to petroleum asphaltenes which may be linked to the ageing effects of the deposit in heat exchangers. The ATR-FTIR spectra of petroleum asphaltenes suggest that the shape of an average asphaltene is more similar to a wide continental model than the archipelago model