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.