nucleation mechanism most common model: ¯methane solvation shell is dodecahedral hydrate cage...
Post on 22-Dec-2015
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Nucleation Mechanism
• Most common model: Methane solvation shell is
dodecahedral hydrate cage Nucleation occurs by
aggregation of dodecahedral water cages
• Other models based on long-range ordering of methane molecules
0.6 ps
Mechanism: Long Lived Fragments
Cluster GrowthSize of Biggest Cluster
0
20
40
60
80
100
120
140
160
0 0.2 0.4 0.6 0.8 1t / ns
nu
mb
er o
f h
ydra
te w
ater
s 2.75 A3.3 A3.5 A
• Large, rapid variations in cluster size
• Not dependent on clustering distance
Hydrate Inhibitors
• Thermodynamic e.g. methanol, glycols (salt)very large volumes (50% w/w water)
• KineticSince very late 1980’sAffect nucleation / growth – not thermodynamicssmall volumes (parts per thousand)lactam polymers (PVP, PVCap & copolymers)small is better (1–2 kDa)others include quaternary ammonium salts
Fish Anti-freeze Proteins
From Arctic Winter Flounder
Main ice growth face
(Lal et al., Faraday Discussions, 1993)
Relax
ca. 1 ns
Adding PVP
Water in Hydrate Environment
Fraction of Hydrate-Water
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0 10 20 30 40
time / ns
Control1
Control2
Inhib1
Inhib2
Control3
Methane-Methane RDFs
For methane within the water film
With PVP ; without PVP
Inhibitors & Nucleation?
Without PVP
With PVP
0.6 ns 10.5 ns 40.2 ns
10.8 ns5.4 ns0.9 ns
Hydrate Inhibitors in Motion
Inhibitors: Selected Conformations
• Typically no contact between inhibitor and hydrate cluster
• ca. 0.5 ns after adding inhibitor
• Bridging between inhibitor and hydrate cluster
Inhibitors: Selected Conformations
Conclusions
• Molecular Simulations can provide important information to inform coarse-graining
Parameterise models
Identify mechanisms to inform model-formulation
Identify regimes for different mechanisms – e.g. effect of different sub-coolings on crystal growth/additives
Oswald’s step rule – which unstable polymorphs never restructure?
Nature of the match between additive and crystal
Acknowledgements
• ModellingChangman MoonMark StorrRob HawtinDorothy Duffy
• FundingEPSRCBritish CouncilICIRF Rogaland
• Computer TimeMaterials ConsortiumWarwick Centre for
Scientific Computing