Abstract
NMR spectroscopy in combination with statistical methods was used to study vacuum residues and vacuum gas oils from 32 crude oils of different origin. Two chemometric metodes were applied. First- ly, principal component analysis on complete spectra was used to perform classification of samples and clear distinction between vacuum residues and vacuum light and heavy gas oils were obtained. To quanti- tatively predict the composition of asphaltenes, principal component regression models using areas of res- onance signals spaned by 11 frequency bins of the 1 H NMR spectra were build. The first 5 principal com- ponents accounted for more than 94 % of variations in the input data set and coefficient of determination for correlation between measured and predicted values was R 2 = 0.7421. Although this value is not signifi- cant, it shows the underlying linear dependence in the data. Pseudo two-dimensional DOSY NMR exper- iments were used to assess the composition and structural properties of asphaltenes in a selected crude oil and its vacuum residue on the basis of their different hydrodynamic behavior and translational diffusion coefficients. DOSY spectra showed the presence of several asphaltene aggregates differing in size and interactions they formed. The obtained results have shown that NMR techniques in combina- tion with chemometrics are very useful to analyze vacuum residues and vacuum gas oils. Furthermore, we expect that our ongoing investigation of asphaltenes from crude oils of different origin will elucidate in more details composition, structure and properties of these complex molecular systems.
Highlights
A common characteristic of crude oil is a very complex chemical composition
Asphaltenes can be present in both crude oil and its vacuum residue which is one of the main byproducts of oil refining.[15,16]
The typical 1H and 13C NMR spectra of the crude oil sample are displayed in the Figure 1
Summary
A common characteristic of crude oil is a very complex chemical composition. Crude oils consist of thousands of individual components (organic molecules), ranging in size, mainly composed of carbon and hydrogen atoms. Asphaltenesare complex organic macromolecular mixtures containing condensed aromatic and saturated rings, aliphatic moieties and different heteroatoms.[13] Asphaltenes may aggregate and precipitate during petroleum processing and form aggregates causing serious problems including reduction in oil flow, blocking of production pipes and some drawbacks during the processing of heavy ends such as coke formation, catalyst deactivation and poisioning.[14] Asphaltenes can be present in both crude oil and its vacuum residue which is one of the main byproducts of oil refining.[15,16] Despite many efforts and variety of analytical methods used, accurate data on molecular weight, size, shape and structure of asphaltenes are still scarce in the literature and their properties not completely elucidated.[1,2,14] We applied here a pseudo two dimensional DOSY NMR technique to preliminary characterize composition and structure of these complex molecular systems. Estimated mean square error of prediction and the determination coefficient (R2) of the predictive model were used for evaluating the PCR model
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