Effects of Stirring and Cooling on Methane Hydrate Formation in a High-Pressure Isochoric Cell

Abstract

Abstract:Higher mixing rates applied to hydrate-forming liquids in laboratory cell systems are commonly assumed to cause a more thorough dispersion of newly formed hydrate nuclei from gas-liquid interface into bulk liquid. There is, however, no simple and direct correlation between stirring rate and nucleation rate. Most experimental studies on hydrate kinetics in literature have been carried out at isobaric conditions. Studies in cooled systems at isochoric conditions (temperature-dependent pressures) are in scarcity. This paper presents an experimental study on effects of varying stirring rates (220-660 rpm) and varying cooling rates (0.5-10 °C/h) on the point of spontaneous nucleation (PSN) of structure-I methane hydrate. The experiments were conducted in an isochoric system with temperature and pressure both decreasing along with the continuous cooling process. Initial growth rates after nucleation were also monitored. The results suggest that higher stirring rates lead to reduced hydrate induction time and increased hydrate growth rate. The nucleation rate appeared to be slightly reduced at the lowest stirring rate (220 rpm). The degree of sub-cooling at PSN seemed to be of a more stochastic character at cooling rates lower than at 2 °C/h. Increased cooling rates trend to reduce induction time while the sub-cooling at PSN appeared to first oscillate at lower cooling rates then approach some constant level at higher cooling rates. For higher cooling rates, the nucleation rate decreased faster. Hydrate growth behavior in cooling rate experiments (0.5-10 °C/h) suggests that the total gas intake and the gas consumption rate increased with increasing cooling rate towards a constant level at 6-10 °C/h. A careful selection of stirring rate and cooling rate is essential to hydrate study.

Enhancing Methane Storage in Clathrate Hydrates Using Leaf Extract Catalysts: Advancing Sustainable Energy Storage

“…Typically, elevating the stirring speed is known to improve gas uptake and hasten process kinetics. 42,43 Achieving optimal hydrate formation requires a careful selection of stirring rates that strike a balance between their impacts on the nucleation and growth. The optimum RPM, where overall hydrate conversion is maximized, depends on the reactor configuration and impeller design.…”

“…Additionally, stirring prevents the agglomeration of gas bubbles, which could impede hydrate growth, and aids in the formation of nucleation sites, providing starting points for the growth of gas hydrate crystals. 42,43 The kinetic profile of the process at 500 rpm is depicted in Figure 2b. It represents pure water, and the t 90 value is 141.67 ± 72.83 min.…”

“…Typically, increasing stirring speed enhances gas uptake and accelerates process kinetics. 42,43 It is crucial to carefully choose a stirring rate that strikes a balance between its effects on both nucleation and growth for optimal hydrate formation. Bioadditives, with their diverse constituents, require a higher subcooling for effective hydrate nucleation.…”

“…This remarkable rise is largely attributed to the effect of agitation, which intensifies the interaction between methane gas and water molecules, leading to more efficient gas dissolution. 42 In contrast, the additives exhibit varying responses to agitation, and SDS maintains a consistent performance, with an absolute consumption of 2.83 L at 0 rpm and 3.25 L at 500 rpm, indicating a percentage increase of around ∼14.84%. SDS's stability in enhancing methane gas dissolution irrespective of agitation levels suggests its robust ability to facilitate gas−liquid interactions.…”

Methane and hydrogen storage in clathrate hydrates

“…Higher pressure was associated with an increase in CH4 gas consumption, while an inverse was observed with temperature. Subsequent investigations into using a stirred tank reactor for pure CH4 hydrate formation were conducted by various research groups [87,[121][122][123][124][125][126]. While a stirred tank reactor demonstrated a maximum CH4 storage capacity of 10.0 wt.% (0.12 mol CH4/mol H2O) with a shorter induction time, the energy consumption induced by stirring becomes a significant consideration if the total time required to achieve this conversion exceeds 3 days [127].…”