Characterization of Precipitated Asphaltenes and Deasphalted Oils of the Medium Crude Oil-CO_2 and Medium Crude Oil- n-Pentane Systems

摘要

In this paper, asphaltenes are precipitated from a medium crude oil using a non-hydrocarbon precipitant (CO_2) or a hydrocarbon precipitant (n-pentane). First; the onset pressure of asphaltene precipitation from the medium crude oil-CO_2 system is determined using a visual high-pressure saturation cell. Second, the morphology of CO_2-precipitated asphaltenes (CO_2-asphaltenes) is examined and compared to that of n-pentane-precipitated asphaltenes (C5-asphaltenes). Third, the precipitated asphaltenes and deasphalted oils (i.e., maltenes) of the medium crude oil-CO_2 and medium crude oil-n-pentane systems are characterized by applying a variety of analytical techniques, such as the density and molecular-weight measurements, nuclear magnetic resonance (NMR) spectrometry, Fourier transform infrared (FTIR) spectrometry, and simulated distillation. It is found that the onset pressure of asphaltene precipitation from die medium crude oil-CO_2 system is equal to 3.8 MPa. The yields and physicochemical properties of the precipitated asphaltenes and deasphalted oils strongly depend upon the specific precipitant used. CO_2-asphaltenes are small particles dispersed in the co-precipitated resins, whereas C_5-asphaltenes are solid-like dark brown and brittle powders. In addition, CO_2-asphaltenes have a lower molecular weight, low aromaticity, and shorter aliphatic side chains than C_5-asphaltenes. On the other hand, CO_2-deasphalted oil (CO_2-maltenes) has a higher molecular weight, high aromaticity, and longer aliphatic side chains than n-pentane-deasphalted oil (C_5-maltenes). The compositional analysis results of CO_2-maltenes and C_5-maltenes indicate that the carbon numbers of most precipitated asphaltenes are larger than C_(50) and that CO_2-maltenes are heavier than C_5-maltenes.