For bitumens and crude oils, the volume of n-paraffin at the flocculation point, which is the point of incipient asphaltene precipitation, increases as the n-paraffin carbon number increases, reaching a maximum at a carbon number of 9 or 10, and then decreases. Thus, asphaltenes often can begin precipitating with a smaller volume of n-hexadecane than with n-pentane, even though large volumes of n-hexadecane precipitate much less (and more aromatic) asphaltenes than large volumes of n-pentane. How can n-hexadecane be both a better and a poorer solvent than n-pentane for asphaltenes? This paradox of solvent quality can be resolved by combining the entropy of mixing of molecules of different sizes with the heat of mixing from solubility parameters, as expressed by the regular Flory−Huggins model. With sufficient characterization data, the approximations and methods of Yarranton et al. can quantitatively describe asphaltene precipitation from the flocculation point to large excesses of n-paraffins from pentane to hexadecane. To describe only the flocculation point data of bitumens and crude oils, the oil compatibility model of Wiehe can be used. Although the oil compatibility model was derived on the basis that the solubility parameter is constant for a given oil at the flocculation point, using “effective” solubility parameters, flocculation points can be predicted with little characterization data.
Fouling mechanisms of a light conventional crude were investigated by characterizing the crude oil, performing fouling tests using a bench-scale Alcor hot liquid process simulator (HLPS) unit and characterizing fouling deposits by means of elemental analysis, scanned electron microscopy (SEM), thermogravimetric analysis (TGA), and photoacoustic infrared spectroscopy (PAS-IR). In addition, a mathematical fouling model was developed under a laminar flow regime following Epstein's methodology. Fouling tests were conducted at different temperatures and bulk velocities. Although the asphaltene content in the crude oil is low, the asphaltenes are still unstable because of a high saturate content and this crude oil has a high fouling propensity. On the basis of the fouling test results, fouling model analysis, and characterization of fouling deposits, the fouling mechanism of this crude oil can be explained as follows: In a laminar flow regime, unstable asphaltenes transport to the hot surface, become attached to the surface, and then, through chemical reactions, form fouling deposits. Mass transfer of entrained suspended particulates in the crude oil also contributes to fouling, although it is not the main cause. However, under turbulent flow conditions, such as those that prevail in industrial operations, it is expected that suspended particles would play a greater role in fouling.
The fouling characteristics of a coker gas oil (CGO) containing measurable amounts of olefins and conjugated olefins were investigated using a bench-scale hot liquid process simulator and a batch autoclave reactor at different temperatures. At low surface temperatures (∼200 °C), the fouling propensity of CGO is very low, and at surface temperatures between ∼250 and 325 °C, the fouling propensity is slightly higher because of polymerization of unsaturated olefins. When the surface temperature is further increased to above 350 °C, fouling is observed to increase significantly because of familiar coking reactions. Slow polymerization of olefins and conjugated olefins was observed at temperatures of 270 and 300 °C. However, at 350 °C, thermal cracking reactions that produce olefins and conjugated olefins take place and significant amounts of fouling deposits can be formed at this temperature. The fouling deposits of CGO exhibit typical polyaromatic coke structures. A better understanding of how the various mechanisms interact with temperature will improve management of fouling in process streams containing olefins and conjugated olefins.
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