Effect of Gas Composition on sII Hydrate Growth Kinetics

Abstract

Kinetics of hydrate formation is of paramount importance for hydrate prevention in pipelines and gas storage in a hydrate state. The effect of gas composition on the kinetics of structure II (sII) hydrate growth has been examined by using two different synthetic natural gases, one with two components (SNG2) and the other with seven components (SNG7). The hydrate equilibrium properties of SNG2 and SNG7 were comparable and the initial degree of subcooling was thus approximately the same during all growth experiments. The same PT conditions and stirring rate were also set for both SNGs enabling us to examine the effect of gas composition on the growth behavior of the same crystal structure. The amount of gas consumed and the structure of the growth curves have been investigated to understand the effect of gas composition on the kinetics of sII hydrate formation. In addition, several chemicals (MeOH, PVP, PVCap) have been tested for their effects on the amount of hydrate formation and structure of the growth curves. The results showed that the amount of sII hydrate formed from SNG7 was doubled as compared to SNG2 with and without the additives. This could be explained by the higher amount of large cavity preferring gas components in SNG7 as compared to SNG2. On the other hand, the growth rate of sII hydrate made from SNG7 first increased toward a maximum value after hydrate onset, and then decreased toward zero at the end of experiment. This growth behavior was not observed for sII hydrate structure produced from SNG2. The SNG2 growth rate showed a maximum at onset and then decreased toward zero eventually. The addition of the chemicals did not change the characteristic growth behavior produced by the individual gas mixtures. The chemicals had an effect in shifting the initial growth rates either up or down based on concentration and type of the chemicals used while the structure of growth rate versus time was kept the same. This study confirmed that gas composition alone is an important parameter for sII hydrate growth kinetics, and the same hydrate structure produced from different gas compositions does not necessarily show the same growth behavior.