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Nucleation Probability Distributions of Methane–Propane Mixed Gas Hydrates in Salt Solutions and Urea
journal contribution
posted on 2015-12-17, 10:15 authored by Barbara Sowa, Xue Hua Zhang, Karen A. Kozielski, Patrick G. Hartley, Dave E. Dunstan, Nobuo MaedaWe studied nucleation probability
distributions of a model natural
gas (a mixture of 90% methane and 10% propane; C1/C3) hydrate in aqueous
solutions of salts using the combinations of seven anions (F–, Cl–, Br–, I–, SCN–, NO3–, SO42–) and seven cations (NH4+, K+, Na+, Mg2+, Ca2+, Mn2+, Al3+) at a broad range of concentrations.
A high pressure automated lag time apparatus (HP-ALTA) was used for
the study. HP-ALTA can apply a large number (>100) of linear cooling
ramps under isobaric conditions to the sample and construct nucleation
probability distributions of C1/C3 mixed gas hydrate for each sample.
We found that (1) ions can promote or inhibit gas hydrate formation
at concentrations below 1 M, (2) this promotion or inhibition effect
of salts did not depend on the valency of the ions involved for the
range of salts studied, (3) the width of the nucleation probability
distributions (stochasticity) of gas hydrate decreased in the presence
of ions, (4) this decrease in the distribution width was largely independent
of the type of ions involved or the concentration of the salt below
100 mM. We also extended the study to include the effects of urea
and a quaternary ammonium salt (an antiagglomerate). The results showed
that the formation of C1/C3 mixed gas hydrate was inhibited more strongly
by urea than any of the inorganic salts at 1 M or above, suggesting
that urea may serve as an effective and environmentally friendly hydrate
inhibitor at high concentrations. The quaternary ammonium salt showed
a modest inhibition effect at all ranges of concentrations. The study
was extended to the formation of ice and model tetrahydrofuran hydrate,
for comparison.