Methane
solubility parameters are required when using a regular
solution approach to predict asphaltene precipitation from crude oils
that contain methane. To determine these parameters, asphaltene onsets
and yields from mixtures of bitumen, methane, and n-pentane were measured at temperatures of 21 and 130 °C and
pressures of 10 and 60 MPa. The onsets (solvent content at which asphaltene
precipitation was first observed) were measured by titrating bitumen
with a mixture of methane and n-pentane in a high-pressure
microscope. Asphaltene yields (mass of asphaltenes divided by mass
of bitumen in feed) were measured in a blind cell pressure-volume-temperature
apparatus for mixtures with the same methane content as the onset
measurements. Saturation pressures were also measured in the blind
cell apparatus for the mixtures with methane and were used to ensure
that the onset and yield measurements were performed in a vapor-free
condition. The onset and yield data were fitted with a Modified Regular
Solution (MRS) model. The inputs to the MRS model are temperature,
pressure, and the mole fraction, molar volume, and solubility parameters
of each component in the mixturein this case, methane, n-pentane, and the bitumen characterized into saturates,
aromatics, resins, and asphaltenes. The properties of the nonasphaltene
bitumen components and n-pentane were determined
from previously established correlations, and the only unknowns were
the asphaltene and methane solubility parameters. The respective unknown
solubility parameters were determined by fitting the MRS model to
asphaltene precipitation data from mixtures of bitumen and n-pentane and from mixtures of bitumen, n-pentane, and methane. The fitted methane solubility parameters ranged
from 6.1 to 9.5 MPa0.5 depending on the temperature. A
correlation for the parameter was proposed, and the MRS model with
this correlation matched the measured onsets and yields with average
deviations of 1 wt % solvent and 8 wt % compared with the experimental
uncertainties of ±1.5 wt % solvent and ±6 to 11 wt %.