Petroleum Size Exclusion Chromatography: Mechanisms Explaining the NMP Front Peak

Size exclusion chromatography (SEC) is abundantly used in analysis and fractionation of petroleum fractions despite numerous challenges with the techniques. Changing the eluent type among classical solvents in SEC normally leads to smaller changes in the elution profile of asphaltenes. Changing the eluent to N-methyl pyrrolidone (NMP) however leads to very different chromatographic profiles. Especially, the occurrence of a very early eluting peak has been severely debated in the literature without reaching a clear conclusion. NMP has particular solvent properties that target the solubility of aromatic molecules, and therefore asphaltenes are only partially soluble. In order to understand the occurrence of the front peak in NMP a systematic study is undertaken by changing eluent solvents and temperature. We discuss the different mechanisms involved in SEC such as phase behavior, gel swelling, pore size, molecular size, and unwanted adsorption that are all involved in the chromatographic process. To investigate the phase behavior and effects of reduced solubility, the elution in mixtures of heptane and toluene is presented. This leads to an increase adsorption but no indication of larger-sized aggregates as could have been expected. Analysis of the difference between NMP, THF, and toluene profiles indicate that gel swelling and void volume variation are relatively insignificant and cannot explain the early peak. This therefore points at a different mechanism based on fluid dynamic effects known from hydrodynamic chromatography (HDC) of particles where larger particles travel faster than the average fluid velocity in a laminar flow and arrive at the detector before the average void volume. Based on the observations on petroleum systems in NMP, we therefore hypothesize that HDC is the driver behind the appearance of the peak in NMP. The insights revealed by the present study show that SEC can be used also to understand changes in solubility in the eluent solvent, for example, caused by alteration in chemistry.