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The effect of experimental conditions on natural gas hydrate formation

Longinos, Sotirios
Natural gas hydrates (NGH) are proposed as gas storage and transportation media owing to their high gas content and long-term stability of hydrate crystal structure at common refrigeration temperatures and atmospheric pressure. Technically feasible, cost efficient hydrate production is one of the crucial items of the whole chain of storage and transportation of gas by means of NGH technology. This study investigated the effects of types of impellers and baffles, and the use of promoters on natural gas hydrate formation. Up-pumping pitched blade turbine (PBTU) and Rushton turbine (RT) were the two types of impellers tested. The reactor was equipped with different designs of baffles: full, half and surface baffles, or no baffles. In total 48 experiments were completed with different impeller – baffle combinations. Single (PBTU or RT), dual (PBTU/PBTU or RT/RT) and dual-mixed (PBTU/RT or RT/PBTU) use of impellers with full (FB), half (HB), surface (SB) and no baffle (NB) combinations formed two sets of 24 experiments. The first set was completed with the use of pure methane as hydrate forming gas and in the second set methane – propane mixture (95% - 5%) was used. Experimental data was analyzed in terms of induction time, hydrate formation rate, overall power consumption, hydrate productivity and conversion of water to hydrate. In addition, split fraction of methane and propane in free gas and solid hydrate during hydrate formation from mixture gas is determined. The reactor was operated in batch mode in all the experiments. Single impeller experiments with methane showed that Rushton turbine has better performance than up-pumping pitched blade turbine, for all kind of baffles. Use of dual impellers, either the same type or mixed, produced similar results of single impeller. The initial hydrate formation rate is generally higher with the use of Rushton turbine, but the decline rate of hydrate formation was also high compared to up-pumping pitched blade turbine (PBTU). The higher amount of hydrate formed at the gas-water interface initially restricts the mass transfer between gas and water phases and results in a higher decline in the formation rate of hydrate. The other fact causing the decline in hydrate formation rate is the exothermic nature of hydrate formation. Hydrate formation process by gas mixtures become more complex because of different partition of gas components in free gas and solid hydrate phases. Propane is consumed more if hydrate is formed from a gas mixture of methane – propane. This fact brings another complexity of varying hydrate equilibrium curve during hydrate formation. As propane is consumed, hydrate equilibrium pressure gets higher for the given process temperature. Aqueous solutions of amino acids tested in this study showed shorter induction time and generally higher hydrate formation rates compared to distilled water. This indicates a potential of finding hydrate promoters among these amino acids.