title: Efficient hydrate-based carbon capture and storage through biomimetic encapsulation
Creators: name: Zhang, Richard
Publisher: name: The Australian National University
Year: 2025

description: CO2 hydrate is a safe, clean, and cost-effective material that leverages the ability of gas hydrates to selectively store CO2 molecules, enabling the removal of CO2 gas from emitted gas mixtures through hydrate-based carbon capture (HBCC). However, the application of HBCC is hindered by the low hydrate formation rate due to insufficient gas-liquid contact area without external agitation. This research aims to develop a novel passive strategy, specifically encapsulation, to enhance CO2 hydrate formation kinetics without the need for agitation. The first part of this work reviews four main types of passive methods to stimulate hydrate formation without relying on mechanical techniques. These methods include nano-confinement, surface treatment, suspended nanoparticles, and distributed hydrate systems. The benefits and limitations of each method are summarized, along with overarching design guidance. The second section investigates the effects of various encapsulation geometries on the dynamic melting behaviour of a phase change material (PCM) under natural convection. Since CO2 hydrate can also be considered as a PCM that involves the adsorption and release of gas during its phase transition, this study helps identify the impact of encapsulation on heat transfer. Six capsule geometries are designed, and a numerical model considering the conjugate heat transfer through the surrounding air, capsule shell and PCM is developed to predict the dynamic melting process of PCM. It reveals that the pyramid and tetrahedron normally exhibit the shortest melting times, while the red-blood-cell(RBC)-shaped capsule exhibits the highest average convection heat transfer coefficient. Building on this foundation and inspired by the biological functions of red blood cells, the RBC-shaped encapsulation design is used to improve gas-liquid mass transfer and CO2-TBAB semi-clathrate hydrate formation kinetics. Results show that the gas uptake efficiency of the RBC-shaped capsule is 143% higher than that of the spherical one with the same internal volume under identical experimental conditions. Optimizing the RBC structure reveals that the peak average hydrate formation per surface area is achieved when the ratio of the height at the centre to the width of the ring is between 0.128-0.160, similar to that of real RBCs in human bodies. The subsequent section studies the dissociation behaviour of gas hydrate in individual capsules. Two distinct stages are observed under various operating conditions. A two-stage numerical model is developed to simulate the dissociation process. The effects of temperature, pressure, capsule volume, and geometry on dissociation kinetics are investigated. Results reveal that the model captures the two distinct dissociation stages, with surface-to-volume ratio and dissociation driving force being the main influencing factors. The ring-shaped capsule demonstrates the most efficient dissociation due to its high surface-to-volume ratio and long transition time to the slow dissociation stage. The final section designs and tests an efficient HBCC unit based on RBC-inspired encapsulation. A validated numerical model is developed to investigate the impact of capsule layouts and sizes on hydrate formation kinetics. Furthermore, a close-packing configuration of RBC capsules, inspired by the natural structure of beehives, achieves a space utilization rate of 64.3%. The CO2 capture performance of this configuration is examined in both closed and open systems under different pressures. In a closed system, a trade-off between water-to-hydrate conversion rate and average CO2 uptake per space volume is observed, with the highest conversion rate of 88.5% achieved at an initial pressure of 4.5 MPa, corresponding to an average CO2 uptake per unit space volume of 6.12 mol min-1 m-3. In an open system, high water-to-hydrate conversion rates and CO2 uptake efficiencies are attained even at low operating pressures.