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Analysis of Growth Rates and Nucleation Dynamics in Gas Hydrates

This study explores crucial growth rate parameters and nucleation behaviors within gas hydrates, highlighting how various factors such as defect sizes and kinetic inhibitors influence these processes. A notable transition observed at Dm/kT = 0.2 marks a shift from step growth to island nucleation. We analyze crystal structures of clathrate hydrates, emphasizing their dependence on temperature and pressure conditions. Our findings also reveal that while decreasing defect width has minimal impact on growth rates, increasing fy significantly reduces them, offering insights into the thermodynamics of hydrate formation.

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Analysis of Growth Rates and Nucleation Dynamics in Gas Hydrates

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  1. MC Results - Growth Rate fy/kT = 0.7 kJ mol-1 fy/kT = 1.0 kJ mol-1

  2. MC Results for Dm/kT = 0.2 0 impurities Length=10 units Length = 20 units Length = 15 units

  3. MC results - summary • Sudden change in slope at Dm/kT = 0.2 signals transition from step growth to island nucleation • glass ceiling for low dosage (kinetic) inhibitors? • For fx/kT = 0.7 • (Dm/kT)* = 0.1 for defects of length = 10 units • (Dm/kT)* = 0.2 for defects of length = 15 and 20 units • decreasing defect width has little effect on growth rate • increasing fx has little effect on growth rate • increasing fy reduces growth rate significantly

  4. Gas Hydrates (Clathrate Hydrates) • Two components • open tetrahedral water lattice • hydrophobic “guest” molecules • Three main crystal structures; • probably two important for environment & industry • Each built from two water polyhedra • diameter 8–9 Å type II type I

  5. Clathrate Hydrates: Guests • hydrophobic (natural gas) • Acid Gases (H2S, CO2) • cyclic ethers (DMO, TMO, THF) • NOT ionic or strongly polar • Size matches/determines structure

  6. Typical of sub-sea pipelines, continental shoulders, tundra Hydrates: Phase Diagram • Favoured by low temperatures and high pressures

  7. Favourable Unfavourable Crystal Nucleation • Activated process: • Favourable “bulk” energy • Unfavourable interfacial energy • Critical “cluster” size • Classical Nucleation Theory • Fundamentally Stochastic • Long and random induction times (~ days)

  8. Nucleation in Other Aqueous Systems • Matsumoto, Saito & Ohmine, Nature 2002 • MD • Mild subcooling (ca 10 K) • Multiple ms trajectories • Critical nucleus built around on long-lived (1-2 ns) hydrogen bonds

  9. Nature of the nucleus • Non-spherical & not compact for ice (from Matsumo et al, Nature 2002)

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