Study on the phenomena and mechanism of reformation during hydrate dissociation of N-vinyl caprolactam system in pipeline

Abstract

Natural gas hydrate blockage is a major threat to the flow safety of long-distance transmission pipelines in deep water. Although the addition of hydrate inhibitors can effectively mitigate hydrate blockage in pipelines, during the dissociation of these inhibitor-containing hydrates, the inhibitor slows the hydrate dissociation rate, which is detrimental to hydrate removal from pipelines in the field. The dissociation characteristics of hydrates in inhibitor-containing systems require further investigation. This work investigated the dissociation of hydrates in a system containing the kinetic inhibitor monomer N-vinyl caprolactam (NVCap). The dissociation process of hydrates under high subcooling conditions could be divided into three stages: 1) initial hydrate dissociation, 2) hydrate reformation, and 3) residual hydrate dissociation, whereas hydrate reformation does not occur under low subcooling conditions. The differences in the dissociation phenomena between the NVCap system and the pure water system in these three stages were compared, and the reason why the hydrate reformation stage of the NVCap system lasts longer was discovered and explained. The phenomenon of a sudden increase in the number of bubbles in the reformation stage was explained, and the coordination relationship between the different effect of inhibitor concentration on dissociation in different stages was discussed. This study combined the analysis of temperature and pressure changes with the analysis of macroscopic gas phase transport laws. It also combined the microscopic observations via particle video microscopy to demonstrate the dissociation kinetics of inhibitor-containing hydrates, providing a theoretical basis for controlling the dissociation of hydrate inhibitors in deep-water pipelines and ensuring safe flow.