Fig. 8-Flow curves in the system with and without Anti-agglomerants agent under shutdown and restart conditions
First, CO2 hydrates need induction time before random crystallization and nucleation. After the induction period, CO2 hydrate began to form and grow rapidly on the wall. Since the temperature at the pipe wall is lower than that at the flow center, only a small amount of hydrate is formed in the liquid body. Due to the hydrophilic surface of hydrate particles, CO2hydrates continue to form, aggregate and adhere to the formed thin hydrate layer. The viscous and moist surfaces of the two CO2 hydrate layers become thicker and thicker, and the flow area decreases. Then, the system was shut down and restarted. Due to the sudden increase of pump frequency, the increase of flow promoted and destroyed some of the non-compact deposits. Secondly, as the liquid phase begins to flow again, it provides better mass and heat transfer conditions for hydrate formation, thus promoting the formation, growth and adhesion of a large number of hydrates to the wall and the previously formed hydrate layer. This leads to a decrease in the flow cross section at the location of the previously formed hydrate layer. The new hydrate layer will also adhere and deposit on the wall. Finally, for the blockage after pump shutdown, even if the pump frequency is restored to the initial value, the driving force of the flow will decrease. With the continuous formation of a large number of CO2 hydrates, secondary deposition occurs at the position of the first hydrate layer. v Due to the compaction of accumulated CO2 hydrates, the plugging cannot be irreversible by adjusting the pump frequency to the flow rate of the original system. At the same time, other new hydrate layers and plugging points are formed in the flow circuit, which also aggravate the risk of hydrate plugging. Fig. 9 shows the schematic diagram of hydrate blockage process under shutdown and restart conditions.