Methane Hydrate Formation
A novel spiral-agitated reactor was used, the schematic diagram of which is presented in Figure S1, and more details can be found in our previous publications24,35. The high-pressure reactor is composed of a stainless-steel cylinder and a stirring rod, and the effective volume of the reactor is 581.71 mL. The stirring rod is equipped with screw blade, the pitch of which is 10 mm (see in Figure S2). Two windows are set up at the reactor to observe inner hydrate morphologies. The experimental temperature is acquired by a PT100 temperature transducer (± 0.1 K), while pressure is recorded by a BP801 pressure transducer (± 0.01 MPa). The reactor is supported by a bracket, so the inclined angle of the spiral-agitated reactor can be adjusted. All of the above devices are put in a thermostatic chamber. The consumed methane is measured by a flow totalizer with an uncertainty of 0.01 mL/min, and the flow, pressure and temperature data are logged by a data acquisition system every second.
Before hydrate formation experiments, the reactor was firstly adjusted to a set inclined angle, and hydrate formation under three inclined angles (25°, 35° or 45°) was evaluated in this work. Subsequently, the reactor was washed with deionized water three times, the stirring was turned on at a revolving speed of 60 rpm during cleaning, and then the reactor was flushed with methane three times. 120 mL solution was injected into the reactor, and the cooling system was turned on to precool the solution and methane. When the temperature inner the reactor reached the experimental temperature (275.15 K), methane was fed into the reactor to a desired pressure, and then the stirring was turned on. When the stirring rod was stuck by formed hydrates, the stirring was turned off, and hydrate formation continues in a static system. Three trails were performed at each experimental condition to ensure the repeatability. Methane storage capacity was defined as the STP volume of methane stored in per unit volume of hydrates, which can be calculated using Eq. (S1), and the water-to-hydrate conversion was calculated using a hydration number of 6.136.