Nucleation and control of clathrate hydrates : insights from simulation

Clathrate hydrates are important in both industrial and geological settings. They give rise to many technological and environmental applications, including energy production, gas transport, global warming and CO2 capture and sequestration. In all of these applications there is a need to exert a high degree of control on the crystallisation process, either to promote or inhibit it according to the application. This crystallisation process involves the formation of a tetrahedral hydrogen bonding network (as occurs with ice), but is complicated by mass transport limitations due to the poor mixing of the common guest molecules, such as methane, and the water that forms the host lattice. The net effect is that the mechanisms for hydrate formation and growth are still poorly understood, with the consequence that development of additives to control nucleation and growth is still largely governed by trial-and-error approaches. In this paper we show how classical molecular dynamics simulations can be used to provide a direct simulation of the nucleation process for methane hydrate and consequently to allow direct simulation of the effect of additives on the nucleation and growth process. Data are presented for oligomers of PVP and compared with existing data for PDMAEMA. The results show that the two additives work by very different mechanisms, with PVP increasing the surface energy of the interfacial region and PDMAEMA adsorbing to the surface of hydrate nanocrystals. The surface energy effect is a mechanism that has not previously been considered for hydrate inhibitors