142
5 Nucleation of Gas Hydrates
5.4.6 Experimental Observations
Given that the nucleation of clathrate hydrates in the absence of salts is still full
of unresolved issues as detailed in Sect. 5.2, one may conclude that it would be
premature to investigate the impact of salts on clathrate hydrate nucleation. Still,
some limited progress has been made. The basic idea behind such endeavor was that
any insight obtained from the investigations of clathrate hydrate nucleation in the
presence of salts might shed new light to the general issues of clathrate nucleation.
Sowa et al. found that (1) concentrated (>1 M) strong monovalent salt solutions
acted as thermodynamic inhibitors as expected, (2) some strong monovalent salts
might kinetically promote natural clathrate hydrate formation at low concentrations
(<1 M), (3) the probability distribution of 90 mol% methane–10 mol% propane
(C1/C3) mixed hydrate formation in strong monovalent salt solutions was consistently narrower than that in pure water for the entire range of salt concentrations
investigated (between 10
−5 M and the solubility limit of each salt) [57]. A narrow
probability distribution means that the survival probability, F, and the natural logarithm of F, lnF, changes more steeply in a narrower range of T. That the slope
d(lnF)/d(T ) is steeper than that of pure water suggests that the nucleation rate
becomes higher with the addition of strong monovalent salts, as we detailed in
Chap. 2, which cannot be explained in the context of traditional colligative properties
of the strong monovalent salts.
The impact of multivalent salts and urea was unclear beyond the expected thermodynamic inhibition at high concentrations [116]. Still, a consistent trend stood
out that was also observed for the strong monovalent salts; the probability distribution of 90 mol% methane–10 mol% propane (C1/C3) mixed hydrate formation in a
multivalent salt solution or urea was narrower than that in pure water for the entire
range of salt concentrations investigated (between 10
−5 M and the solubility limit
of each salt) [116]. However, the extent of the narrowing of the probability distributions differed among the studied multivalent salts and it was difficult to determine
systematic correlations between the types of the multivalent salts and the extent of
the narrowing [116]. More work is required to decouple several factors that are likely
in play.
5.5 Concluding Remarks: The End of the Beginning
We have seen throughout the course of this book that the nucleation of clathrate
hydrates is in its infancy as a research field. Therefore, the focus has been on what
relevant issues need to be taken into account during an investigation of nucleation
of gas hydrates, not on a comprehensive coverage of a mature research field. Nucleation rates are central to any nucleation phenomena and yet they have rarely been
determined for clathrate hydrates. A primary difficulty in the investigation of clathrate
hydrate nucleation has been the inability of the researchers to determine and compare
5 Nucleation of Gas Hydrates
5.4.6 Experimental Observations
Given that the nucleation of clathrate hydrates in the absence of salts is still full
of unresolved issues as detailed in Sect. 5.2, one may conclude that it would be
premature to investigate the impact of salts on clathrate hydrate nucleation. Still,
some limited progress has been made. The basic idea behind such endeavor was that
any insight obtained from the investigations of clathrate hydrate nucleation in the
presence of salts might shed new light to the general issues of clathrate nucleation.
Sowa et al. found that (1) concentrated (>1 M) strong monovalent salt solutions
acted as thermodynamic inhibitors as expected, (2) some strong monovalent salts
might kinetically promote natural clathrate hydrate formation at low concentrations
(<1 M), (3) the probability distribution of 90 mol% methane–10 mol% propane
(C1/C3) mixed hydrate formation in strong monovalent salt solutions was consistently narrower than that in pure water for the entire range of salt concentrations
investigated (between 10
−5 M and the solubility limit of each salt) [57]. A narrow
probability distribution means that the survival probability, F, and the natural logarithm of F, lnF, changes more steeply in a narrower range of T. That the slope
d(lnF)/d(T ) is steeper than that of pure water suggests that the nucleation rate
becomes higher with the addition of strong monovalent salts, as we detailed in
Chap. 2, which cannot be explained in the context of traditional colligative properties
of the strong monovalent salts.
The impact of multivalent salts and urea was unclear beyond the expected thermodynamic inhibition at high concentrations [116]. Still, a consistent trend stood
out that was also observed for the strong monovalent salts; the probability distribution of 90 mol% methane–10 mol% propane (C1/C3) mixed hydrate formation in a
multivalent salt solution or urea was narrower than that in pure water for the entire
range of salt concentrations investigated (between 10
−5 M and the solubility limit
of each salt) [116]. However, the extent of the narrowing of the probability distributions differed among the studied multivalent salts and it was difficult to determine
systematic correlations between the types of the multivalent salts and the extent of
the narrowing [116]. More work is required to decouple several factors that are likely
in play.
5.5 Concluding Remarks: The End of the Beginning
We have seen throughout the course of this book that the nucleation of clathrate
hydrates is in its infancy as a research field. Therefore, the focus has been on what
relevant issues need to be taken into account during an investigation of nucleation
of gas hydrates, not on a comprehensive coverage of a mature research field. Nucleation rates are central to any nucleation phenomena and yet they have rarely been
determined for clathrate hydrates. A primary difficulty in the investigation of clathrate
hydrate nucleation has been the inability of the researchers to determine and compare