Active ice: a novel formation medium for rapid and scalable gas hydrate production

Gas hydrates, crystalline water cages encapsulating small gas molecules, offer potential for gas storage and transportation. However, the slow formation rates of conventional methods hinder industrial applications. Kinetic promoters, such as surfactants and amino acids, have been investigated to accelerate hydrate formation. However, scaling up hydrate production using these promoters faces challenges: the "wall-climbing effect" (upward and wall-adhering hydrate growth) reduces the overall formation rate with increasing solution load; the exothermic nature of hydrate formation leads to temperature increases, inhibiting further formation; and significant gas space is needed, reducing reactor space utilization. This study aims to address these challenges by introducing a novel approach using "active ice" as a formation medium for gas hydrates.

Existing literature extensively documents the use of kinetic promoters like surfactants (e.g., SDS) and amino acids to enhance gas hydrate formation kinetics. Studies have shown significant improvements in formation rates, sometimes achieving water conversion exceeding 80% within 11 minutes. However, scaling up these processes has proven difficult due to limitations in mass and heat transfer. The wall-climbing effect, a characteristic growth pattern induced by surfactants, creates challenges due to its dependence on solution load and temperature sensitivity. The exothermic nature of hydrate formation causes temperature increases, while the need for significant gas space for the wall-climbing effect leads to inefficient reactor space utilization. Previous studies have explored various approaches to mitigate these limitations, including using porous materials and optimizing reactor designs, but a comprehensive solution remains elusive.

The study employed a two-step process to produce active ice: (1) Methane hydrate formation from a surfactant solution (e.g., 600-ppm SDS solution) at 277.15 K and 6.0 MPa, followed by (2) dissociation of the primary hydrate below the ice point (272.65 K and atmospheric pressure). The resultant porous ice, containing an unfrozen surfactant solution layer, is termed "active ice." The performance of active ice in methane hydrate formation was evaluated using a high-pressure sapphire cell reactor. The gas uptake was determined by monitoring pressure changes. Experiments were conducted with various kinetic promoters to examine the applicability beyond SDS. Recyclability and long-term storage capacity were also assessed. Furthermore, the formation heat of methane hydrate in active ice was determined and compared to conventional methods. A stainless-steel reactor with a larger volume was used for scale-up experiments. Finally, the active ice's performance under compression was studied, evaluating the trade-off between storage capacity and formation rate. Characterization techniques such as Raman spectroscopy, powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), and differential scanning calorimetry (DSC) were used to analyze the structure and properties of active ice and the formed hydrates. Gas chromatography was used for analyzing gas mixtures in separation experiments.

The active ice approach demonstrated remarkably fast methane hydrate formation, typically completing within 5 minutes with gas uptake exceeding 170 Vg/Vw⁻¹. This is significantly faster than conventional methods using SDS solutions. The active ice exhibited several advantages: good repeatability, recyclability, compressibility, ease of storage, and low heat generation (18.13 kJ mol⁻¹ CH₄ compared to >50 kJ mol⁻¹ CH₄ in conventional methods). The rapid formation is attributed to the porous nature of active ice, enabling simultaneous hydrate formation at multiple points, and the presence of an unfrozen surfactant solution layer that facilitates a continuous ice melt-hydrate formation cycle. Multiple kinetic promoters were successfully used to produce active ice, with sodium dodecanoate and sodium dodecyl benzene sulfonate showing the best performance. The active ice maintained its high gas uptake capacity even after multiple cycles of hydrate formation and dissociation. High hydrate formation rates and gas uptake were achieved under smaller driving forces (0.69 MPa), potentially reducing operating costs. Scale-up experiments confirmed the scalability of the active ice approach, with minimal impact on formation rates even with a significant increase in active ice mass. The apparent storage capacity was initially low due to the porous nature of the active ice. Compression improved the apparent gas storage capacity, reaching 126.36 Vg Vbed⁻¹ under optimized conditions—2.44 times higher than compressed natural gas under similar conditions and also higher than dry ZIF-8 packing beds.

The active ice approach successfully addresses the key limitations of conventional methods for gas hydrate production. The rapid formation rate, facilitated by the porous structure and the unfrozen surfactant layer, overcomes the mass and heat transfer limitations associated with the wall-climbing effect. The low heat generation eliminates the temperature increase that typically inhibits hydrate formation. The recyclability and ease of storage of active ice make it a viable option for industrial applications. While the initial storage capacity is lower due to the porous nature of active ice, compression offers a solution to increase the apparent storage density to levels surpassing compressed natural gas and dry ZIF-8. This highlights the potential for active ice as a highly efficient and scalable alternative for gas storage and separation applications.

This study demonstrates the active ice approach as a promising solution for rapid and scalable gas hydrate production. Its advantages, including superior kinetics, low heat generation, recyclability, and achievable high storage capacity through compression, provide a significant improvement over conventional methods. Future research can focus on optimizing the compression techniques to further enhance storage capacity without significantly affecting the formation rate and investigating the applicability of active ice for various gas mixtures and storage conditions. The findings pave the way for the development of more efficient and cost-effective gas hydrate-based technologies.

The study primarily focuses on methane hydrate formation. Further research is needed to investigate the performance of active ice with other gases. While compression improves storage capacity, it also affects the formation rate. Optimal compression levels need further refinement for various applications. The long-term stability of active ice under various operational conditions also warrants further investigation.