Hydrate formation and particle distributions in gas-water systems

Methane hydrate formation rate and resistance to flow were measured for gas-water systems in a high-pressure visual autoclave over a range of mixture velocities (300-5000 Reynolds number). A transition from a homogeneous to heterogeneous particle distribution, proposed following flowloop studies by others, was observed directly in the autoclave through three independent measurements: motor current increase (resistance to flow), pressure consumption rate (hydrate growth rate), and visual observation. The hydrate volume fraction at the transition, ϕ_transition, generally increased with increasing turbulence, although the relationship between Reynolds number and ϕ_transition was not the same as that observed in flowloop experiments. The addition of a thermodynamic inhibitor below the full inhibition threshold (i.e. under-inhibited) increased the transition point by about 10 vol% hydrate, without affecting the initial hydrate growth rate. A simple mass transport-limited formation model with no adjustable parameters was implemented to enable quantitative predictions of hydrate formation rate. In sufficiently turbulent systems the model's predictions were in excellent agreement with the observed growth rates. At lower Reynolds numbers, two mechanisms are proposed to explain the deviations between the observed and predicted growth rates. Prior to ϕ_transition the low shear means that hydrate formation is limited by the rate at which the aqueous phase can be re-saturated with methane. This rate is increased greatly by the formation of a hydrate bed after ϕ_transition, which increases the gas-liquid interfacial area.