Mass Transfer Enhancement in Carbon Dioxide Gas Hydrate Formation for Effective Carbon Separation and Storage
Creators: name: Zhang, Fengyuan
Publisher: name: The Australian National University
Year: 2022

description: Carbon dioxide (CO2) is widely acknowledged as a significant contributor to global warming. Hydrate-based carbon capture (HBCC) technology holds high potential in delivering cost-effective and environmentally friendly carbon capture solutions. However, the relatively severe formation conditions and low formation rate of gas hydrates limit its practical applications. This thesis focuses on the mass transfer enhancement methods for effective CO2 hydrate formation through experimental and numerical studies. The thermodynamic and kinetic promotion experiments on CO2 hydrate formation using chemical promoters are implemented in tetra-n-butyl ammonium bromide (TBAB) solution with surfactants. TBAB, as a thermodynamic promoter, can moderate hydrate phase equilibrium by forming CO2-TBAB semiclathrate hydrates. However, it decreases CO2 gas uptake yields. Three kinds of surfactants, namely anionic surfactant sodium dodecyl sulfate (SDS), cationic surfactant dodecyl-trimethylammonium chloride (DTAC), and non-ionic surfactant Tween 80 (T-80), are added in the system to increase the formation rate and offset the low gas uptake yields. Induction time, normalized gas uptake, split fraction and separation factor are the performance metrics. The results in TBAB systems show that the hydrate formation is most accelerated with the addition of SDS, but DTAC shows better CO2 separation performance. Similar results of rapid formation rate with the addition of non-ionic surfactant T-80 are also found. Analysis of variance is used to analyze the difference among experimental results, and a decision box is proposed to evaluate the performance of the systems studied. Compared with SDS and DTAC, 2000-ppm T-80 shows the best CO2 separation performance in semiclathrate hydrates. The mass transfer can also be enhanced by adding microparticles due to their considerable surface areas. The kinetic promotion experiments of CO2 hydrate formation are thus further studied in "dry water" and silica gel (SG) microparticles of different sizes. The experimental results reveal that "dry water" particles with 8-wt% silica has the highest normalized gas uptakes. However, "dry water" are broken after a repeat cycle. SDS and DTAC are added to the SG system to further enhance gas-water mass transfer. With the addition of surfactants in 100-nm SGs, SDS systems save up to 23.7%-49.3% time to achieve the same amount of gas uptake, while DTAC systems save 16% of the time. SGs show better stability and promotion effect than "dry water". A modified shrinking core model (SCM) is established to study the CO2 hydrate formation kinetics in both "dry water" particles and SG pores. It is the first model that integrates the effects of CO2 solubility, capillary effect, volume expansion, and heat transfer model. The hydrate formation in both pure CO2 and CO2/N2 gas mixtures are simulated to reveal the different roles of CO2 and N2 molecule diffusion and reaction in hydrate formation. In "dry water" particles, the water consumed through capillaries accounts for less than 10% of the total water consumed. The decoupled heat transfer model reveals that the instantaneous temperature gradient in the hydrate shell is of a small magnitude of 10-2 K m-1. In SG pores, the initial proportion of water consumed by capillary effect is only 1%-26.6%, but it can be up to 74.9% in small pores with surfactants. This work provides comprehensive insights into gas hydrate formation in both water systems and microparticles. It contributes a theoretical basis for the improvement of gas hydrate kinetics through mass transfer enhancement. The modeling strategies in this work can be applied to hydrate formation mechanisms in other porous materials.