Crude oils, alkane content

The high viscosity of heavy crude oils is essentially due to the high levels of asphaltene content. Asphaltene is the highest MW component of crude oil, is a friable, amorphous dark solid, which is colloidally dispersed, in the oily portion of the crude. Asphaltenes are considered to be heavily condensed aromatic molecules with aliphatic side chains and with high heteroatom content (S, N, and O) as well as high-metal content. The asphaltene fraction is physically defined as that fraction insoluble in n-alkanes, but soluble in toluene and is the most polar fraction of oil. 

Eckart and co-workers have published a series of papers on laboratory studies of biodesulfurization of petroleum and petroleum fractions. The ability of various aerobic mixed cultures to desulfurize Romashkino crude oil (1.69 wt.% S) was addressed by Eckart et al. (21). After 5 days of incubation at 30°C in sulfur-free mineral medium with oil as sole source of carbon and sulfur, approximately 55% of the total sulfur was recovered in the aqueous phase from two of the most active cultures. In another study, gas oil (1.2 to 2 wt.% S), vacuum distillates (1.8 to 2 wt.% S) and fuel oil (up to 4 wt.% S) were used as sole carbon and sulfur sources for the oil-degrading microorganisms (36). The addition of an emulsifying agent was required to enhance desulfurization. Sulfur removals of up to 20% from the gas oil, 5% from the vacuum distillates, and 25% from the fuel oil were observed after 5 to 7 days of incubation. In a later study (37). approximately 30% of the sulfur was removed from fuel-D-oil by a mixed population of bacteria. The removal of benzothiophene, dibenzothiophene and naphthobenzothiophene was shown by high resolution MS analysis. Hydrocarbon degradation was observed in each of these studies. For example, in the latter study with fuel-D-oil, the decreases in the n-alkane and aromatic content were 59% and 14%, respectively. 

Paraffinic base oils are produced from crude oils of relatively high alkane content typical crudes are from the Middle East, North Sea and US mid-continent. This is not an exclusive list, nor does it follow that all North Sea crudes, for example, are suitable for production of paraffinic base oils. The manufacturing process requires aromatics removal (usually by solvent extraction) and de-waxing. 

The strongest films are formed under the most acidic conditions. Finally, the films least able to relax under reduced surface stress conditions were observed to form the most stable emulsions. They also found that components other than asphaltenes, such as alkanes (paraffin waxes), resins, and aged interfacially active components of relatively high H/C ratio (1 5), can also contribute to the stabilization of water-in-crude oil emulsions. They concluded that the state of solvation of asphaltenes in the oil phase plays an important role in their ability to stabilize emulsions, which may explain the indirect action of resins on emulsion stability. The temperature is also an important factor when it comes to the stability of crude oil emulsions, especially true when the wax content is relatively high. The most important factor influencing the water-in-crude oil emulsion stability at low temperature is the interaction at the water-oil interface between wax crystals and the heavy fractions of the crude. 

NMR and to some extent NMR spectroscopy are very important analytical tools to measure the content and composition of the alkanes and other hydrocarbons in crude oils , naphtha, bitumenous rocks, liquified coal products " , gasoline ", diesel fuels ° and the like. Rapid and reliable detection of the main components is of high practical importance, and gave rise to numerous reports on analytical procedures and results. Quantitative methods involving NMR have been developed, and the results checked by statistical methods such as principal component analysis ... 

The chemical structure and physicochemical properties of asphaltenes and resins are not well understood. The operational definitions of asphaltenes and resins are based on their solubility in different diluents. Asphaltenes are defined as the fraction of crude oil insoluble in excess normal alkanes such as n-pentane but soluble in excess benzene and toluene at room temperature. Resins are defined as the fraction of crude oil insoluble in excess liquid propane at room temperature. Resins are adsorbed on silica, alumina, or other surface-active material. Figure 5.8 shows the precipitation when a bitumen oil is mixed with various diluents. The normal alkanes used are n-pentane, n-hexane, n-heptane, n-octane, n-nonane, and n-decane. According to the operational definition stated above, the asphaltene content of the bitumen... 

Crude oils and refined middle distillate products such as diesel fuels, kerosene (jet fuel) or heating oil contain an important fraction of paraffins (alkanes) of high energy content with a broad linear (w-paraffins) and branched chain length distribution (Fig.l). 

Petroleum is a mixture of liquid hydrocarbons of variable composition depending on its origin alkanes, cyclic hydrocarbons (cyclopentane, cyclohexane), and aromatic hydrocarbons. Crude oil always contains light gases (methane, ethane), particles in suspension, more or less brackish fossil water, and sulphur compounds. The sulphur content ranges from 0.5 to 3 %, depending on its origin. 

Distillation concentrates the metallic constituents in the residua (Table 3-5) some can appear in the higher-boiling distillates but the latter may, in part, be due to entrainment. Nevertheless, there is evidence that a portion of the metallic constituents may occur in the distillates by volatilization of the organometallic compounds present in the petroleum. In fact, as the percentage overhead obtained by vacuum distillation of reduced crude is increased, the amount of metallic constituents in the overhead oil is also increased. The majority of the vanadium, nickel, iron, and copper in residual stocks may be precipitated along with the asphaltenes by low-boiling alkane hydrocarbon solvents. Thus, removal of the asphaltenes with n-pentane reduces the vanadium content of the oil by up to 95% with substantial reductions in the amounts of iron and nickel. 

Previous pyrolysis studies at NRL stressed fractions of crude shale oil residua, measured the yields of JP-5, and determined the content of potential n-alkanes in the JP-5... 

---