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

The study addresses the longstanding mass and heat transfer limitations that hinder the scale-up of gas hydrate production when using kinetic promoters (e.g., surfactants, amino acids). Conventional promoter-driven hydrate growth often exhibits a wall-climbing effect, where solution is drawn up the reactor wall to create larger interfacial area, which accelerates formation but introduces critical scale-up challenges: (1) hydrate formation rate decreases as solution load increases for a given reactor configuration and thermodynamic conditions, impeding throughput; (2) exothermic heat release during rapid formation raises liquid temperature by approximately 4–8 K within 0.5–2.7 h, which feeds back negatively on formation; and (3) inefficient reactor space utilization demands large gas holdup (65–90% of reactor volume) to avoid inlet/outlet blockage. Additional issues include hollow hydrate morphology that lowers apparent gas storage density and bubble/foam generation upon dissociation. To overcome these constraints, the authors propose an active ice approach—an ice medium containing an unfrozen surfactant solution layer that enables rapid, uniform, and low-heat gas hydrate formation suitable for scale-up.

Kinetic promoters such as surfactants and amino acids are known to accelerate hydrate formation, achieving reported times as low as ~11 minutes with >80% water conversion. However, promoter-induced wall-adhering, upward growth patterns create scale-up drawbacks. Prior studies quantified large formation heats (e.g., ~54.44 kJ mol−1 for methane hydrate from water) and noted temperature rises during formation in bulk or porous media. Typical promoter-assisted operations require high gas void fractions (65–90%). The literature also highlights various reactor strategies (e.g., stirred tanks, bubble columns, porous matrices, aluminum foams) and additives (SDS, amino acids) to improve kinetics. These inform the current work’s focus on a new medium—active ice—that retains promoter benefits while mitigating scale-up penalties.

Materials: High-purity methane, carbon dioxide, hydrogen, and ethane (99.99%); surfactants including SDS (>99%), sodium oleate (>97%), N-Carbobenzoxy DL-leucine (>98%), dodecylbenzenesulfonic acid (>90%), sodium dodecyl benzene sulfonate (>95%), sodium laurylsulfonate (>98%); deionized water. Experimental setup: A high-pressure sapphire reactor (ID 2.54 cm, 61.90 cm³) with bottom piston for compression and an internal reciprocating iron ring stirrer (magnetically driven) was used, coupled to a gas reservoir (130.23 cm³) for precooled gas, all housed in an air bath (±0.1 K). Pressures were measured by calibrated transducers (0–20 MPa, 0–10 MPa; ±0.1% FS), logged every 1 min. A buffer tank (80.10 cm³) precooled the pressurizing fluid. Scale-up tests used a 220.00 cm³ stainless-steel reactor (ID 5.07 cm) and a larger gas reservoir (750.09 cm³). An in situ Raman-coupled reactor with opposing sapphire windows, ~1.4 cm³ chamber, annular coolant passage, Pt100 (±0.05 K), and a 0.02 MPa accuracy pressure transducer enabled spectroscopic characterization. Preparation of active ice: Route (1) Hydrate formation–dissociation: Prepare SDS solution, load 10 g into sapphire reactor, purge (methane charge–vacuum cycles ×3), charge gas reservoir to >10 MPa, set 277.15 K, equilibrate (~3 h). Pressurize reactor to 6.0 MPa, stir to nucleate, then stop stirring for quiescent growth. After ~3 h when pressure drop <20 kPa h−1, primary hydrate formation is deemed complete. Reset bath to 272.65 K, depressurize to atmospheric pressure within ~20 s, and dissociate hydrate at 272.65 K and 1 atm for ≥3 h to release gas. The remaining ice-like medium is the active ice. Route (2) CO2 hydrate formation–dissociation: identical to (1) but with CO2 as the hydrate-forming gas. Route (3) Ice powder/natural snow mixing: grind ice to powder, mix with surfactant under dry ice cooling to yield active ice. Compression of active ice: Cool active ice to 268.15 K. Insert precooled PTFE disk in reactor on top of active ice. Raise piston with hand pump to compress between piston and disk. Precool pressurizing fluid in buffer tank to prevent dissociation. Remove PTFE disk, introduce 0.2 MPa methane to return piston, then depressurize to atmospheric pressure. All steps at 268.15 K. Hydrate formation tests: After preparing active ice in situ, set formation temperature, charge gas reservoir with methane to 10 MPa and equilibrate (~3 h), then charge reactor to 6.0 MPa. Calculate gas uptake from reactor pressure decline. Control experiments replaced active ice with SDS solution under identical procedures. Gas separation: Prepare active ice, set temperature, load gas mixture into reservoir to target pressure, pressurize reactor. When pressure stabilizes, analyze residual reactor gas by GC (Agilent 7890A). Data treatment and characterization: Gas storage/separation results were obtained by molar balance between gas and hydrate phases. Raman, PXRD, SEM, and DSC methods are detailed in Supplementary Information.

- Active ice enables ultrafast methane hydrate formation: typically completes within 5 minutes at 272.65 K with gas uptake >170 Vg/Vw−1. No wall-climbing effect is observed; formation is uniform across particles. - Mechanism: Active ice is porous with an unfrozen surfactant (e.g., SDS) solution layer at temperatures slightly below the ice point. Hydrate formation in this layer triggers a virtuous cycle of ice melt–hydrate formation–ice melt that sustains rapid growth. The porous/powdery morphology allows simultaneous gas contact to many ice particles, producing rapid, multipoint nucleation and growth. - Promoter generality: Six kinetic promoters tested; all enhanced formation when used to produce active ice. Sodium dodecanoate and sodium dodecyl benzene sulfonate performed best among those tested. A promoter’s performance in active ice correlates with its efficacy in aqueous solution. - Recyclability and storage: Active ice can be recycled and stored long-term without loss of gas uptake performance. Optimal SDS dose in active ice is as low as 600 mg L−1. - Lower driving force operation: At 272.65 K and constant pressure 3.0 ± 0.02 MPa (driving force ≈0.69 MPa), cumulative uptake reached 174.8 V/Vw within 5 minutes, significantly lower driving force than typical SDS-solution cases. - Reduced formation heat: Methane hydrate formation heat in active ice is ~18.13 kJ mol−1 CH4 (vs >50 kJ mol−1 gas from water), mitigating temperature rise during rapid formation and favoring scale-up. - Scale-up behavior: Increasing active ice mass from 5.0 to 79.0 g raised peak temperature only from 278.76 to 281.53 K during rapid uptake (<5 min), while uptake reached ~91% of final values in both cases (181.10 and 173.21 V/Vw, or 91.19% and 90.97% of final). Formation completed within ~5 minutes across scales, indicating multipoint, bed-wide formation with minimal dependence on bed mass. - Compression–capacity tradeoff: Although active ice’s porous morphology lowers apparent storage capacity, moderate compression alleviates this with negligible performance loss. Under optimized loosening coefficient α=1.492 (α defined as bed specific volume relative to perfect ice crystal) and residual pressure 5.76 MPa, apparent methane storage reached 126.36 Vg Vbed−1, 2.44× that of compressed natural gas at the same T,P, and higher than ZIF-8 under similar conditions (105.37 Vg Vbed−1 at 269.15 K and 5.25 MPa). Negative impacts on rate and final uptake become significant only at higher compression (α<~1.398). - Apparent bed capacity without compression is low (e.g., 83.92 ± 2.6 Vg Vbed−1 for a hollow active ice column), motivating compression for practical deployment.

The active ice approach directly addresses key scale-up barriers in promoter-assisted hydrate formation. By providing a porous, uniformly accessible medium with an unfrozen surfactant solution layer near 0 °C, it enables near-simultaneous, multipoint hydrate formation without the wall-climbing growth mode. The endothermic ice melting accompanying exothermic hydrate formation buffers heat release, limiting temperature rise and sustaining kinetics. Experiments show rapid formation (<5 min) with high uptakes (>170 V/Vw) at modest driving forces and minimal sensitivity to bed mass, indicating favorable heat/mass transport. The method generalizes across multiple kinetic promoters with performance mirroring their aqueous efficacy, and it offers practical advantages of recyclability, long-term storage, low surfactant dose, and modest operating pressures. Compression strategies can overcome low apparent bed density while maintaining uptake and kinetics at moderate compaction. Overall, active ice presents a scalable pathway to hydrate-based gas storage and separation by decoupling fast formation from the detrimental reactor-scale effects observed in conventional promoter systems.

This work introduces active ice as an effective medium for ultrafast, scalable gas hydrate formation. The porous ice with an unfrozen surfactant solution layer supports a self-sustaining cycle of ice melt and hydrate growth, producing uniform, rapid formation with high gas uptake at low driving force and reduced heat release. The approach is versatile across promoters, recyclable, storable, and compatible with moderate compression to enhance apparent storage capacity, outperforming compressed natural gas and certain adsorbents (e.g., ZIF-8) under comparable conditions. These attributes unlock practical potential for large-scale gas storage and gas mixture separation using hydrates. Future work should optimize heat exchange in larger packing beds, refine compression protocols to balance density and kinetics, and further expand promoter systems and gas mixtures to broaden industrial applicability.

- The porous/powdery morphology leads to a large specific bed volume and low apparent storage capacity without compression; compression is therefore required for practical capacities. - Excessive compression (low loosening coefficient α) degrades uptake rate and final capacity; performance remains robust only under moderate compaction (e.g., α>~1.398). - As bed size increases, peak temperature still rises due to limited heat removal; dedicated heat exchange optimization in large beds is needed. - Preparation via hydrate formation–dissociation can be operationally involved; although improved routes (e.g., CO2 route, ice powder mixing) are proposed, additional process simplification and scale-up engineering may be necessary for industrial deployment.