Thermodynamic and Physico-chemical Analyses of CH4 - CO2 Replacement in Various Clathrate Structures for Application to CH4 Recovery and CO2 Sequestration

Dissociation enthalpies of the initial CH4 + C3H8 hydrate, the substituted hydrates and the pure CO2 hydrate. 13C NMR spectra of the CH4 molecules encapsulated in the pure CH4 (sI), and CH4 + MCP (sH) hydrates.

Introduction

Historical Perspective

Molecular Structures of Gas Hydrates

Five representative polyhedral cavities—(a) pentagonal dodecahedron, (b) tetrakaidecahedron, (c) hexahedron, (d) irregular dodecahedron, and (e) icosahedron—construct three common structures of gas hydrates in nature: (f) structure I, ( g) structure II, and (h) structure H.

Figure 1.2.1. Five representative polyhedral cavities — (a) pentagonal dodecahedron, (b) tetrakaidecahedron, (c) hexakaidecahedron, (d) irregular dodecahedron, and (e) icosahedron — construct three common structures of gas hydrat

Exploitation of Naturally Occurring Gas Hydrates Using CO 2 /Flue Gas Injection

Moreover, the sequestration of CO2 in these natural gas hydrate reservoirs is attracting widespread interest in the field of CO2 capture and storage (CCS). Since energy security and global warming have been important issues, replacing CH4 with CO2 in the natural gas hydrate reservoirs will function as both effective CH4 recycling for ensuring future clean energy and innovative CO2.

Experimental Investigation

Materials

Experimental Apparatus and Procedure

Hydrate Phase Behavior and Sample Preparation
Hydrate Structure Identification
Gas Composition Measurement
Dissociation Enthalpy Measurement

The temperature was then increased at a heating step of 0.1 K at time intervals of 90 min until all hydrate crystals disappeared. For each experimental series, the exchange took place in a high-pressure cell for 72 h at 274.2 K. Hydrate samples were taken and crushed in a liquid nitrogen container for structure analysis and gas composition measurement.

Figure 2.2.2. Determination of phase equilibrium from formation and dissociation of CO 2 (20%) + N 2

Abstract
Hydrate Stability Condition

Stability Condition of CO 2 + N 2 Hydrates
Influence of Replacement on the Stability Condition

Hydrate Crystalline Structure

Structural Identification of CO 2 + N 2 Hydrates
Influence of Replacement on the Hydrate Structure and the Replacement Efficiency

Influence of Replacement on the Thermal Properties

Dissociation Enthalpy of CO 2 + N 2 Hydrates
Influence of Replacement on the Dissociation Enthalpy and Heat Flow

Summary

At each pressure condition, the equilibrium dissociation temperatures of CH4 + CO2 + N2 hydrates after exchange were obtained from endothermic dissociation thermograms. The DHd values of CO2 + N2 gas hydrates increased with increasing CO2 composition in the hydrate phase, as shown in Figure 3.4.2. Dissociation enthalpies of CO2 + N2 hydrates according to the composition of CO2 in the hydrate phase (a) ΔHd (J/g of water) and (b) ΔHd (kJ/mol of gas).

Accurate information about the dissociation enthalpies of the mixed gas hydrates is required to understand the dissociation behavior of CO2 + N2 hydrates. Comparison of the dissociation enthalpies of the gas hydrates before and after the replacement; (a) ΔHd (J/g water) and (b) ΔHd (kJ/mol gas).

Figure 3.2.1. Comparison of hydrate phase equilibrium conditions (pVT method versus DSC method)

Abstract

Furthermore, 13C NMR spectroscopy revealed that exchange by flue gas occurs without a structural transition and that N2 molecules preferentially attack CH4 molecules occupied in the small 512 cages of sH hydrates. During CH4 - flue gas exchange in sH hydrates, there was no significant change in the heat flow associated with the dissociation and subsequent reforming of gas hydrates. Dissociation enthalpies of gas hydrates before and after exchange, measured using a high-pressure micro-differential scanning calorimeter (HP μ-DSC), also supported isostructural exchange with the high degree of reaction.

The combined experimental results provide a better understanding of the thermodynamic and physicochemical background for CO2 enclathration in sH hydrates and its significance in CO2 based gas hydrates. In addition, this study reports the first experimental evidence of isostructural CH4 − flue gas exchange occurring in sH hydrates, and thus may contribute to expanding the potential areas of CH4 exploitation using flue gas exchange in sH natural gas hydrate reservoirs.

Stability Conditions of the CH 4 + CO 2 + NH Hydrates
Structure Identification of the CH 4 + CO 2 + NH Hydrates
Influence of the Structure-transitional Replacement

Pure CH4 hydrate exhibited two resonance peaks at -4.3 ppm and -6.6 ppm, which corresponded to CH4 molecules trapped in the small 512 cages and large 51262 cages of the sI hydrate, respectively. After the structural transition from sH to sI occurred at 60% CO2 concentration, the CH4 intensity ratio. It should be noted that the area ratio of the two Raman peaks corresponding to the large 51262 and small 512 cages of CH4.

A 13C NMR spectrum of the CH4 molecules encapsulated in the CH4 + NH hydrate has already been presented in Figure 4.2.2. In figure 4.2.5. intensity ratio of the CH4 molecules in the large 51262 cages and small 512 cages (IL/IS) of sI was 0.96 after the exchange, which also confirmed the preferential occupation of CO2 molecules in the large 51262 cages of sI hydrates.

Figure 4.2.1. Hydrate phase equilibria of the (a) CH 4 + CO 2 + NH + water systems and (b) the CH 4 + NH + water, CH 4 (40%) + CO 2 (60%) + NH + water, and CO 2 + NH + water systems

Influence of Replacement on Thermodynamic Stability
Structural Identification and Cage-dependent Distribution of CH 4 Molecules
Influence of Replacement on the Dissociation Enthalpy
Heat Flow Changes during the Replacement Process

The phase equilibrium conditions for the initial CH4 + NH hydrate and the CH4 + NH hydrates replaced with CO2 + N2 at each pressure condition are depicted in Figure 4.3.3. In this study, PXRD was adopted to accurately investigate the crystalline structure of CO2 + N2. In the isostructural systems of each sI and sH hydrate, the DHd values of the CO2 + N2 and CO2 + N2 + NH hydrates increased with an increase in CO2 concentration.

Dissociation enthalpies of CO2 + N2 systems and CO2 + N2 + NH systems. kJ/mole gas) Hydration number Description Structure. In the substitution system accompanying a structural transition, the DHd value of the initial CH4 + NH hydrate (sH) was found to be kJ/mole guest and that of the CO2 substituted CH4 + NH hydrate (sI) was 56.2.

Table 4.3.1. (a) Hydrate phase equilibrium data for the N 2 + NH and CO 2 + N 2 + NH systems

Summary

Heat flux changes monitored by HP m-DSC showed that significant parts of the sH hydrate frameworks were preserved during CH4 + NH − flue gas replacement despite the possible partial breakdown and restoration of some cage structures. The measured DHd values of substituted hydrates allowed a better understanding of the dissociation behavior and thermodynamic stability of isostructural and structural-transition substituted systems. This study reports the first experimental evidence for isostructural CH4 - flue gas exchange occurring in sH hydrates and thus may contribute to expanding the areas of NGH exploitation using CO2 exchange in sH hydrate reservoirs.

The overall experimental results covering the complex phase behavior, cage-specific distributions of external gas molecules, structural properties and dissociation enthalpies of the mixed gas hydrates indicate that the dual purpose of CH4 recovery and CO2 sequestration using CO2 or flue gas mixts. sH hydrates be feasible, even if the kinetics and extent of replacement in the actual natural gas hydrate reservoirs are dependent on several factors such as the sediment pore sizes, mass transfer, hydrate particle size and marine environment.

Abstract

Final Hydrate Composition after Replacement with Various Pressure of CO 2
Composition Changes during Replacement

As can be seen in Figure 5.2.1., the final CO2 compositions of the substituted hydrates were remarkably increased in the CH4 + C3H8 - CO2 substitution as PCO2 was increased. The increase in the replacement efficiency with an increase in PCO2 for the CH4 + C3H8 - CO2. The difference induced by PCO2 can be more clearly observed in the compositional changes of the hydrate phase during substitution, as shown in Figures 5.2.4.

Composition changes in the vapor phase during substitution at (a) 2.6 MPa of PCO2 and (b) 4.0 MPa of PCO2. Composition changes in the hydrate phase during substitution at (a) 2.6 MPa of PCO2 and (b) 4.0 MPa of PCO2.

Figure 5.2.2. Conceptual schema of replacement scenarios. (a) The initial sII CH 4 + C 3 H 8 hydrate goes through an isostructural conversion into the sII CH 4 + C 3 H 8 + CO 2 hydrate

Hydrate Crystalline Structure and Cage-dependent Guest Distribution

Structural Identification and Cage-filling Characteristics as Revealed by PXRD

Moreover, they also support the fact that the degree of structural transformation in the hydrate phase was dependent on PCO2. The trapped CO2 in the large 51262 cages containing sI hydrates showed almost complete cage occupancy for the replaced hydrates at both 2.4 and 3.9 MPa PCO2, as shown in Figure 5.3.4. Therefore, it could be reasonably expected that the higher CO2 occupancy in the small cages of the newly transformed sI hydrates at 2.4 MPa was attributed to the lower driving force at low PCO2, leading to low replacement efficiency.

In the large 51264 cages with sII hydrates, the CO2 and C3H8 enclathration behavior showed almost the same cage occupancy between the replaced hydrates after being replaced by 2.4 MPa and 3.9 MPa. The pure CH4 hydrate showed two resonance signals at -4.3 and -6.6 ppm corresponding to CH4. molecules in the small 512 cages and the large 51262 cages of sI hydrate, respectively.

Figure 5.3.2. (a) Comparison of PXRD profiles of the replaced hydrate in the the 2θ range of 28.5 to 31.5⁰, and (b) phase ratio of initial CH 4 + C 3 H 8 hydrate, and replaced hydrates at 2.4 and 3.9 MPa of CO 2

Replacement Behavior Observation by Time-dependent Raman Spectroscopy

Enclathration of CO 2 Molecules
Release Behavior of CH 4 Molecules

However, the enclathration behavior of CO2 molecules during the replacement at 3.6 MPa CO2, as shown in Figure 5.4.1. b) demonstrated the predominance of the CO2 molecules encapsulated in sI hydrate over the entire experimental time scale. This predominance of CO2 molecules embedded in sI hydrate from the beginning was attributed to the increased proportion of CO2-rich sI hydrates and the rate of replacement as increase in PCO2. On the other hand, the peak intensities of CO2 molecules embedded in both structures were almost constant after 10 h when replaced with 3.6 MPa CO2.

The inclusion rate of CO2 molecules in sI and sII hydrates during the replacement with (a) 2.3 MPa CO2 and (b) 3.6 MPa CO2. For the replacement with 3.6 MPa CO2, a dramatic decrease of the peak intensity corresponding to trapped CH4 molecules was observed, while relatively gradual release behavior was observed in the replacement with 2.3 MPa CO2.

Figure 5.4.1. Time-dependent Raman spectra of the CO 2 in the sI and sII hydrates during being replaced with (a) 2.3 MPa of CO 2 and (b) 3.6 MPa of CO 2 , and their top view: (c) and (d), respectively

Influence of Replacement on the Dissociation Behavior and Dissociation Enthalpy

The dissociation thermograms of gas hydrates substituted with CO2 at two PCO2 conditions were found to have broader and split endothermic peaks compared to the original CH4 + C3H8 hydrate. The broader endothermic peaks after replacement could be attributed to the compositional heterogeneity of the replaced hydrates. The structural coexistence of the sI and sII hydrate for CH4 + C3H8 - CO2 substitution was also confirmed using a HP μ-DSC.

Our previous study showed that for the substitution accompanying a complete structural transition (CH4 + neohexane - CO2 substitution, sH → sI), there was a significant difference in the ΔHd value before and after the substitution and in the ΔHd value of the substituted hydrate (CH4 .+ CO2) was even lower than that of gas hydrate formed with the injection gas itself (CO2) [95]. However, the ΔHd value of substituted hydrates in the isostructural substitution (CH4-CO2 substitution, sI → sI) was placed between that of the initial hydrate and that of the hydrate formed with the injection gas (CO2) itself [55].

Figure 5.5.2. Dissociation enthalpies of the initial CH 4 + C 3 H 8 hydrate, the replaced hydrates, and the pure CO 2 hydrate

Summary

Conclusion and Future Work

Conclusion

The PXRD patterns of the substituted hydrates showed that the higher substitution efficiency at higher PCO2 was attributed to a higher proportion of CO2-rich sI hydrate being converted from the initial sII hydrates. To identify the generation or absorption of heat during the replacement process and the subsequent effect on the thermal properties of the replaced hydrates, changes in heat flow and dissociation enthalpies were investigated by HP μ-DSC. The DHd value of the substituted hydrate in the structural transition system sH-sI was significantly lower than that of the initial CH4 + NH hydrate and even slightly lower than that of the pure CO2 hydrate.

On the other hand, the DHd values of the replaced hydrates in both sI and sH isostructural systems did not change as remarkably as in the sH-sI structure transitional replacement. However, in contrast to the full structure transition and isostructural replacement systems, the ΔHd values of the hydrates after CH4 + C3H8 - CO2 replacement were intermediate between those of the initial CH4 + C3H8 hydrates and the pure CO2.

Figure 6.1.1. The schematic illustration of (a) sI-isostructural CH 4 - flue gas replacement, (b) sH- sH-isostructural CH 4 + NH - flue gas replacement, (c) sH-sI structure-transitional CH 4 + NH - CO 2

Future Work

Experimental and Computational Investigation of the CH 4 - CO 2 Replacement in sH
Experimental Verification of CH 4 Recovery Induced by Flue Gas Injection into sII CH 4 +

Much research on the CH4 - CO2 exchange in gas hydrates has been done, mainly focusing on kinetic and thermodynamic approaches to the CH4 - CO2 exchange occurring in sI hydrates. CH4 recovery and CO2 sequestration using flue gas in natural gas hydrates as revealed by a micro-differential scanning calorimeter. Experimental verification of methane-carbon dioxide substitution in natural gas hydrates using a differential scanning calorimeter.

Methane extraction by carbon dioxide from gas hydrates - phase equilibria for CO2-CH4 mixed hydrate system. The driving forces of gas substitution in gas hydrates - a laser Raman study on CH4-CO2 exchange in the presence of impurities.