5 research outputs found
Fickian and Non-Fickian Diffusion in Heavy Oil + Light Hydrocarbon Mixtures
Diffusive mass transfer is expected
to play a key role in existing
and proposed solvent-added processes for heavy oil production. Composition–distance
profiles arising during free diffusion scale as a function of the
joint variable (distance/time^<i>n</i><sub><i>w</i></sub>). Simple fluids are governed by Fickian diffusion, where <i>n</i><sub><i>w</i></sub> = 0.5. For nanostructured
fluids, the value of <i>n</i><sub><i>w</i></sub> can be as low as <i>n</i><sub><i>w</i></sub> = 0.25, known as the single-file limit, but more typically, the
value for the exponent falls between these two limits and is composition-dependent.
In this work, five published data sets, comprising free diffusion
composition profiles for Athabasca bitumen fractions and for Cold
Lake bitumen + light hydrocarbons obtained using diverse apparatus,
are probed from this perspective. Additional experimental results
are provided for Athabasca bitumen + toluene mixtures over the temperature
range of 273–313 K, and results from positive and negative
control experiments for two well-defined mixturesî—¸(0.25 mass
fraction carbon nanotubes + polybutene) + toluene, and polybutene
+ tolueneî—¸are also provided. The value of <i>n</i><sub><i>w</i></sub> for the negative control experiment
remains at 0.50 ± 0.05 over the entire composition range, and
for the positive control experiment, the value drops to <i>n</i><sub><i>w</i></sub> = 0.30 ± 0.02 at low toluene mass
fraction. Although the quality of the diffusion profile data in the
data sets analyzed is variable, the values of the exponent <i>n</i><sub><i>w</i></sub> are shown to be light-hydrocarbon-dependent
and increase from <i>n</i><sub><i>w</i></sub> ∼
0.25 at low light-hydrocarbon mass fraction up to <i>n</i><sub><i>w</i></sub> ∼ 0.50 at high light-hydrocarbon
mass fraction. Secondary convective effects are also noted in free
diffusion experiment outcomes at long times. The industrial applications
of these findings are currently being evaluated, but it is clear that
the time for light hydrocarbons to penetrate a fixed distance into
nano- and micro-structured hydrocarbon resources is greater than the
value anticipated for unstructured fluids