Kinetic Properties of CO2 Hydrate Formation in the Wax-Containing System at Different Concentrations

Precipitation of wax crystals and hydrate formation during deep-sea multiphase pipeline transportation would limit pipeline transportation flow and even trigger accidents. It is necessary to study the hydrate formation behavior in the wax-containing system. This paper utilized a mixture of number 60 Kunlun paraffin wax and number 2 industrial white oil to simulate the wax-containing system, and the CO2 hydrate was formed in the wax-containing system. The morphology of CO2 hydrate formation was captured by the microscope and the charge-coupled device imaging system. In this study, the influences of the wax crystal concentrations on the induction time and cumulative gas consumption of hydrate formation were investigated, and the effects of wax crystal precipitation on the CO2 hydrate nucleation mechanism were discussed. The experimental results indicated that, for the wax crystal concentrations of 1.50 and 2.50 wt %, the induction time of CO2 hydrate formation in the wax-containing system was reduced by 29.23 and 69.23% compared to that in the pure water system, respectively. However, for the wax crystal concentration of 0.50 wt %, the induction time of CO2 hydrate formation in the wax-containing system was increased by 130.77% compared to that in the pure water system. The reason for this was that the precipitation of wax crystals at high concentrations increased the chance of heterogeneous nucleation and the probability of heterogeneous nucleation increased with the increase of wax crystal concentrations. However, for the wax crystals at low concentrations, it not only lowered the chance of cross nucleation but also increased the mass transfer resistance for gas diffusion, thereby resulting in the increase of the induction time of hydrate formation. It was also found that, with the increase of the wax crystal concentrations, the cumulative gas consumption during hydrate formation decreased gradually. When the wax crystal concentration increased to 2.50 wt %, the cumulative gas consumption was reduced by 32.96% compared to that of the pure water system. The experimental results obtained in this study were helpful for understanding the kinetic characteristics of hydrate formation in the wax-containing system, which is meaningful for the pipeline flow assurance, safety, and economic security of pipeline transportation as well as the development of carbon capture and storage.