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Instability, precipitation and fouling in heavy oil systems E, Hong

Abstract

Asphaltenes are the most polar, highest molecular mass species found in crude oils and bitumens. Depending on the nature of the surrounding species, the temperature and pressure, asphaltene may be dissolved, or may flocculate resulting in precipitation. Asphaltene precipitation during oil production and processing is a very serious problem in many areas throughout the world. To avoid precipitation, much research has been directed to the solubility of asphaltenes in petroleum liquids as a function of temperature, pressure and composition. Bitumen from oil sands deposits contains about 13.5% asphaltenes, and during its processing is mixed with low molecular mass diluents, which can lead to precipitation and to fouling of processing equipment. In this work, the effects of diluent composition on asphaltene precipitation from Cold Lake vacuum residue (VR) and Athabasca atmospheric tower bottoms (ATB) were determined using the hot filtration method at 60-85°C. The selected diluents include pure n-alkanes (heptane, decane and dodecane), a lube oil basestock—paraflex (PFX), a heavy vacuum gas oil (HVGO) and a resin enriched fraction (REF) recovered from Cold Lake vacuum residue by supercritical fluid extraction and fractionation. The latter three complex diluents were tested alone and in blends, in order to cover a range of saturates from 56 to 99.4wt%, aromatics from 0.6 to 25wt%, and resins from 0 to 19wt%. For pure n-alkanes, the amount of asphaltene precipitation at a given diluent/residue ratio decreases as the molecular mass of n-alkanes increases. With the increase of diluent-to-residue ratios R, the amount of precipitated asphaltene, W, increases sharply at first and then levels off. Similar behaviour can be found for the mixtures of both feedstocks with the aliphatic diluent PFX. With more aromatic diluents such as pure HVGO or blends of HVGO/PFX=l, the increases in W with R are modest or slight within the range of experiment. The addition of the resin and aromatic-rich REF has a strong inhibition effect on asphaltene precipitation. For selected mixtures, the temperature effect on precipitation was investigated in the range of 60°C to 300°C. The results indicate that for the mixtures used in this work the solubility of asphaltenes increases monotonically with temperature. Asphaltene solubility and precipitation using simple diluents has been analyzed traditionally using thermodynamic models; however, these models are based on the assumption that asphaltene precipitation is a reversible process. This has been a controversial issue, and because of lack of precise experimental data, it has remained unresolved. Another approach is a scaling model based on aggregation of clusters. The original form of scaling equation proposed by Rassamdana et al. gave good agreement of the precipitation data as a function of R for the pure n-alkane diluents (heptane, decane and dodecane). For the complex multi-component diluents of this work, an extended scaling equation with two additional variables (the density and saturate content of the diluents) was developed and provided good agreement with the data over a wide range of diluents to feed ratios. The scaling equation was also extended to correlate asphaltene precipitation from different feed oils by incorporating the colloidal instability index (CII) of the feed oil. This extended scaling equation correlates the experimental data from the two different feed oils well for both single and multi-component diluents. The scaling equation can also be put into a form more useful for thermal fouling studies by converting the ratio W to the precipitated asphaltene concentration (g precipitated asphaltene/L mixture) in the mixture. The effects of different diluents on the stability of colloidally dispersed heavy oil systems were tested by determining asphaltene precipitation onset points by titration. Cold Lake vacuum residue (VR), Athabasca atmospheric tower bottoms (ATB) and Cold Lake heavy oil (HO) were selected as sources of asphaltene. The diluents included toluene, Paraflex (PFX), heavy vacuum gas oil (HVGO), resin enriched fraction (REF), de-asphalted oil (DAO) and fuel

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