5.2 Nucleation Rate of Gas Hydrates
127
small number of active nucleation sites were indeed concentrated in a much smaller
“real” system size, such active nucleation sites could not be ubiquitous. What kind
of active nucleation sites could have more nucleation potency than silver iodide?
As we saw in Sect. 5.1, silver iodide’s lattice constants match those of ice to within
a few percent [9]. However, after decades of commercial cloud seeding with silver
iodide, the detailed mechanism by which silver iodide and many other inorganic
substrates facilitate ice freezing still remains unclear [9]. Thus, silver iodide could
in fact be a poor nucleation promotor of ice and of clathrate hydrates remains a real
possibility that could explain the negative results of Sowa et al.
5.2.6 Nucleation Pathways
Another possibility that could account for the very large discrepancy in the kinetic
parameter is to assume a much “longer” nucleation pathway that involves many more
kinetic steps than what is accounted for in classical nucleation theory. As detailed in
Chap. 1, regardless of the nature of a particular nucleation pathway or the complexity
of a system, a nucleation event can only be realized when a continuous path forms
from the beginning to the end or all the pieces of the puzzle are in place. And the
nucleation rate represents the rate at which such occurrence is realized, over a whole
nucleation pathway, in a unit system size. It means that presence of a bottleneck
anywhere along such nucleation pathway would lower the overall nucleation rate.
Such a bottleneck that is not accounted for in classical nucleation theory is all it
takes to cause a massive discrepancy between the experimental and the theoretical
nucleation rates.
A primary candidate for such a bottleneck in clathrate hydrate nucleation is the
typically very low solubility of a hydrocarbon guest gas in liquid water. The much
lower solubility of the guest gas compared to the guest gas content in the clathrate
form would pose a mass transfer barrier and hence require a larger number of kinetic
steps. In other words, nucleation of clathrate hydrates has a “longer” nucleation
pathway than nucleation of, say, a salt from a supersaturated solution. Any factors
that “lengthen” the nucleation pathways would lower the system-wide nucleation
rates, because the probability for the system to “clear” an extra hurdle within a given
timeframe cannot be greater than 1 and the overall nucleation rate is expected to be a
product of nucleation rates of each sub-step that constitutes the nucleation pathway.
That the nucleation rate of the clathrate hydrate of more soluble CO 2 was significantly
greater than that of methane hydrate or 90–10 mol% (C1/C3) mixed gas hydrate for a
given subcooling suggests that the mass transfer barrier to the nucleation of clathrate
hydrates is indeed a significant factor [56].
Another candidate that could “lengthen” the nucleation pathway is viscous slowdowns we detailed above while we covered ice nucleation. Subcoolings of more than
30 K were routinely encountered in the nucleation of clathrate hydrate in a quiescent
system of a sparingly soluble hydrocarbon guest gas [24]. Such large subcoolings
of more than 30 K in pure water would experience significant viscous slowdowns
127
small number of active nucleation sites were indeed concentrated in a much smaller
“real” system size, such active nucleation sites could not be ubiquitous. What kind
of active nucleation sites could have more nucleation potency than silver iodide?
As we saw in Sect. 5.1, silver iodide’s lattice constants match those of ice to within
a few percent [9]. However, after decades of commercial cloud seeding with silver
iodide, the detailed mechanism by which silver iodide and many other inorganic
substrates facilitate ice freezing still remains unclear [9]. Thus, silver iodide could
in fact be a poor nucleation promotor of ice and of clathrate hydrates remains a real
possibility that could explain the negative results of Sowa et al.
5.2.6 Nucleation Pathways
Another possibility that could account for the very large discrepancy in the kinetic
parameter is to assume a much “longer” nucleation pathway that involves many more
kinetic steps than what is accounted for in classical nucleation theory. As detailed in
Chap. 1, regardless of the nature of a particular nucleation pathway or the complexity
of a system, a nucleation event can only be realized when a continuous path forms
from the beginning to the end or all the pieces of the puzzle are in place. And the
nucleation rate represents the rate at which such occurrence is realized, over a whole
nucleation pathway, in a unit system size. It means that presence of a bottleneck
anywhere along such nucleation pathway would lower the overall nucleation rate.
Such a bottleneck that is not accounted for in classical nucleation theory is all it
takes to cause a massive discrepancy between the experimental and the theoretical
nucleation rates.
A primary candidate for such a bottleneck in clathrate hydrate nucleation is the
typically very low solubility of a hydrocarbon guest gas in liquid water. The much
lower solubility of the guest gas compared to the guest gas content in the clathrate
form would pose a mass transfer barrier and hence require a larger number of kinetic
steps. In other words, nucleation of clathrate hydrates has a “longer” nucleation
pathway than nucleation of, say, a salt from a supersaturated solution. Any factors
that “lengthen” the nucleation pathways would lower the system-wide nucleation
rates, because the probability for the system to “clear” an extra hurdle within a given
timeframe cannot be greater than 1 and the overall nucleation rate is expected to be a
product of nucleation rates of each sub-step that constitutes the nucleation pathway.
That the nucleation rate of the clathrate hydrate of more soluble CO 2 was significantly
greater than that of methane hydrate or 90–10 mol% (C1/C3) mixed gas hydrate for a
given subcooling suggests that the mass transfer barrier to the nucleation of clathrate
hydrates is indeed a significant factor [56].
Another candidate that could “lengthen” the nucleation pathway is viscous slowdowns we detailed above while we covered ice nucleation. Subcoolings of more than
30 K were routinely encountered in the nucleation of clathrate hydrate in a quiescent
system of a sparingly soluble hydrocarbon guest gas [24]. Such large subcoolings
of more than 30 K in pure water would experience significant viscous slowdowns