122
5 Nucleation of Gas Hydrates
proportional to the system volume. Likewise, it has been considered that a suitable
measure of the system size for heterogeneous nucleation is the area of a surface
or an interface that is responsible for the heterogeneous nucleation. However, these
conventional ideas are unlikely to be valid for clathrate hydrates, as we detailed
above. Perhaps as expected, it has been experimentally found that the nucleation rate
of 90–10 mol% (C1/C3) mixed clathrate hydrate in the presence of a foreign solid
wall was substantially greater than that in the absence of a solid wall when both
nucleation rates were normalized to the area of the guest–aqueous interface in the
system [24]. This discrepancy between the nucleation rate of a system that involves
a foreign solid wall and one that does not may be expected since the presence of a
foreign solid wall was not at all accounted for when the experimentally determined
nucleation rate was normalized to the guest–aqueous interfacial area [48].
Maeda and Shen recently applied the general scaling law we detailed in Chap. 1
to clathrate hydrate nucleation in an attempt to identify the suitable measure of the
“system size” [22]. They used the nucleation rate of 90–10 mol% (C1/C3) mixed
gas hydrate determined by an HP-ALTA to calculate the nucleation rate of methane
hydrate in a flow loop (model pipeline) that had a much larger system scale. The
nucleation rate of 90–10 mol% (C1/C3) mixed gas hydrate and the nucleation rate
of methane hydrate had been found to be broadly similar at comparable system
subcoolings [24]. They examined five possible measures of the “system size” that
could be used for scaling; (1) total nominal area of water–guest gas interface in the
presence of the pipe wall, (2) total area of water–guest gas interface in the absence
of the pipeline wall, (3) total lengths of the three-phase lines where the three phases
of the guest gas, water, and the pipeline wall met, (4) total wetted area of the pipeline
walls, (5) total volume of water in the pipeline [22]. Among these five measures,
only the total lengths of the three-phase lines scaled the nucleation rates between a
system of millimeter scale and a system of meter scale to within the same order of
magnitude [22]. Given that the effects of flows were ignored, the agreement within
an order of magnitude appears fortuitously good, which supports Maeda’s original
point that the appropriate measure of the system size in a quiescent clathrate hydrate
system is the total lengths of the three-phase lines [48].
5.2.5 Theoretical Front and the Applicability of Classical
Nucleation Theory to Gas Hydrate Systems
Nucleation of any system has been difficult to investigate computationally for two
reasons. First, it is impossible to predict when and where a nucleation event will
eventually occur in a given system. Second, the typically long induction times are
taxing in terms of computational resources. Consequently, the progress in computationally determining the nucleation rates of clathrate hydrates has been limited.
The few reported computational estimates of nucleation rates of clathrate hydrates
5 Nucleation of Gas Hydrates
proportional to the system volume. Likewise, it has been considered that a suitable
measure of the system size for heterogeneous nucleation is the area of a surface
or an interface that is responsible for the heterogeneous nucleation. However, these
conventional ideas are unlikely to be valid for clathrate hydrates, as we detailed
above. Perhaps as expected, it has been experimentally found that the nucleation rate
of 90–10 mol% (C1/C3) mixed clathrate hydrate in the presence of a foreign solid
wall was substantially greater than that in the absence of a solid wall when both
nucleation rates were normalized to the area of the guest–aqueous interface in the
system [24]. This discrepancy between the nucleation rate of a system that involves
a foreign solid wall and one that does not may be expected since the presence of a
foreign solid wall was not at all accounted for when the experimentally determined
nucleation rate was normalized to the guest–aqueous interfacial area [48].
Maeda and Shen recently applied the general scaling law we detailed in Chap. 1
to clathrate hydrate nucleation in an attempt to identify the suitable measure of the
“system size” [22]. They used the nucleation rate of 90–10 mol% (C1/C3) mixed
gas hydrate determined by an HP-ALTA to calculate the nucleation rate of methane
hydrate in a flow loop (model pipeline) that had a much larger system scale. The
nucleation rate of 90–10 mol% (C1/C3) mixed gas hydrate and the nucleation rate
of methane hydrate had been found to be broadly similar at comparable system
subcoolings [24]. They examined five possible measures of the “system size” that
could be used for scaling; (1) total nominal area of water–guest gas interface in the
presence of the pipe wall, (2) total area of water–guest gas interface in the absence
of the pipeline wall, (3) total lengths of the three-phase lines where the three phases
of the guest gas, water, and the pipeline wall met, (4) total wetted area of the pipeline
walls, (5) total volume of water in the pipeline [22]. Among these five measures,
only the total lengths of the three-phase lines scaled the nucleation rates between a
system of millimeter scale and a system of meter scale to within the same order of
magnitude [22]. Given that the effects of flows were ignored, the agreement within
an order of magnitude appears fortuitously good, which supports Maeda’s original
point that the appropriate measure of the system size in a quiescent clathrate hydrate
system is the total lengths of the three-phase lines [48].
5.2.5 Theoretical Front and the Applicability of Classical
Nucleation Theory to Gas Hydrate Systems
Nucleation of any system has been difficult to investigate computationally for two
reasons. First, it is impossible to predict when and where a nucleation event will
eventually occur in a given system. Second, the typically long induction times are
taxing in terms of computational resources. Consequently, the progress in computationally determining the nucleation rates of clathrate hydrates has been limited.
The few reported computational estimates of nucleation rates of clathrate hydrates