Nucleation Probability Distributions of Methane-Propane Mixed Gas Hydrates in Salt Solutions and Urea

We 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.