CO2 composition in the hydrate phase increased to 77% in the TBAC semi-clathrate and decreased to 44% in THF hydrate from 50% CO2. The faster growth of CO2 than CH4 in the pure water system was found and this implies the kinetic selectivity of CO2 in the early stage of hydrate formation. Spectroscopic analysis implied that PZ and HZ are not sandwiched in the cages of CH4 hydrate and do not affect the structural transition of the hydrate.
The time-dependent growth patterns and induction time of CH4 hydrate in the presence of diamines were observed by in situ Raman spectroscopy. Concentration of CO2 in the vapor and clathrate phases at the end of hydrate formation at 3.0 MPa and ΔT = 5.0 K. Changes in the ratio of large cage area to small cage area of pure CH4 hydrate and CH4. Molecular structure configuration of 512 cage of sI CH4 hydrate and (a) PZ molecule, (b) HZ molecule and (c) methanol molecule.
Interaction energy between a 512 cage of CH4 hydrate and an inhibitor molecule calculated from density functional theory (B3LYP/6-31G*).
Introduction
- Natural gas production and transport
- Gas hydrate
- Clathrate based CO 2 separation
- Flow assurance in pipeline using hydrate inhibitors
Moreover, gas hydrate itself can be used in the different fields with the help of their physical and chemical properties. Gas hydrates can be applied for natural gas storage, solid gas transportation, carbon capture and storage (CCS) and desalination process [14-18]. The structure of gas hydrate can be classified as cubic structure I (sI), cubic structure II (sII) and hexagonal structure H (sH).
In the CH4 + CO2 + water system, CO2 can be trapped more in the hydrate cages than CH4 during hydrate formation due to the difference in thermodynamic stability. Separated and collected CO2 gas in the hydrate phase can be sequestered or used for the further process. Thermodynamic promoters can be used to improve the temperature and pressure condition, which is one of the important factors for the separation process.
As the hydrate was formed with thermodynamic promoters such as tetrahydrofuran (THF), neohexane (NH), and tetra-n-butylammonium chloride (TBAC), a structural transition from sI to sII, sH, and semiclathrate, respectively, occurred, and the thermodynamic stability of the clathrate could be change in this process.
Experimental Methods
- Materials and apparatus
- Phase equilibrium measurement
- Gas consumption and composition measurements
- Spectroscopic measurement
- Computational Analysis
The cell was slowly cooled from 293K until the pressure in the cell suddenly dropped due to the formation of gas hydrate (AB). As hydrate dissociated, the pressure in the cell increased due to the release of gas from the melting hydrate. The equilibrium cell is filled with the solutions and gas mixture to measure how many gas molecules have been consumed by the hydrate formation.
The pressure in the cell is constantly maintained at 3.0 MPa for the isobaric condition, and the driving force (ΔT) defined as the experimental temperature difference from the equilibrium temperature is set to 5.0 K. Microflow syringe pump (500D, Teledyne Isco, USA) was used to maintain constant maintaining pressure in the cell and measuring the volume consumed while the hydrate is being formed. Gas composition in the cell was observed every 10 min during hydrate formation using a gas chromatograph (GC, 7890A, Agilent, USA).
Gas molecules in the vapor phase were circulated by a high-pressure pump (Eldex, USA) to take a uniform sample, and taken for 10 seconds through a sampling valve (Rheodyne, USA) connected to a loop whose volume is 20μL. As for the gas composition in the hydrate phase, it can be measured at the end of the hydrate formation process. Once the hydrate had sufficiently formed, the cell was immersed in the liquid nitrogen to cool.
Consequently, the hydrate was left in the cell and diffused to get the gas molecules inside the hydrate. The composition of the released gas was measured by GC in the same way as the measurement of the vapor composition. In hydrate research fields, it is commonly used to identify the structure of gas hydrates and to observe the growth pattern during hydrate formation.
Ecombined is the optimized energy of the overall configuration of the hydrate cage and an inhibitor. The optimized energy of the gas hydrate in was used to estimate the interaction energy between the CH4 hydrate and the inhibitor in the aqueous system.
Results and discussions
CO 2 separation from sour natural gas using clathrate hydrate
- Thermodynamic promotion
- Structure confirmation
- Gas uptake and gas composition measurement
- Kinetic characteristic of CO 2 /CH 4 mixed hydrate
The NMR peak area is proportional to the number of CH4 molecules occupied in the hydrate cages, and the ratio of large to small cage areas (AL/AS) shows the ratio of the number of cavities where CH4 is present. Pure CH4 hydrate forms an sI hydrate consisting of two small cages (512) and six large cages (51262) in the unit cell, and the theoretical ratio of the number of large to small cages is 3.0. Therefore, the peak of the CH4 molecule was detected in the only small cage of the sII hydrate.
The CO2 composition in the vapor phase during hydrate formation with pure water gradually decreased until 80 min because CO2 is preferentially trapped in the hydrate cages. The CO2 composition in the TBAC semiclathrate showed the highest increase up to 76% from the 50% CO2 in the feed gas. On the other hand, the CO2 composition in the THF hydrate decreased to 45%, while it increased in the vapor phase.
Therefore, it should be noted that the CO2 composition in the vapor and hydrate phase is influenced by the thermodynamic stability between CH4 and CO2 hydrate and the hydrate structure. In both systems the peaks were detected at 2905 cm-1 and 2915 cm-1 and the peak intensity increased during the hydrate growth. The changes in the area ratio of the large cage to small cage (AL/As) during hydrate growth are plotted in Figure 3.1.7.
Changes in the ratio of large cage area to small cage area of pure CH4 hydrate and CH4. In the pure water system, the two peaks at 1276 cm-1 and 1380 cm-1 are observed which are assigned to the CO2 molecules in the sy-hydrate. The Raman spectra of pure water system had two peaks at 2905 cm-1 and 2915 cm-1, which are assigned to CH4 molecules in the large and small cage.
In the THF system, a peak at 2914 cm-1 corresponding to CH4 in sII small cage was observed. The amount of gas molecule trapped in the hydrate cages is proportional to the intensity and area of the peak. Although the CO2 composition in the hydrate phase was found to be 56%, and it could be higher in the early stage of hydrate formation.
The guest-dependent growth rate of CO2 is higher than that of CH4 and it could make the CO2 concentration in the hydrate phase richer than that of CH4.
Diamine hydrate inhibitors for flow assurance in pipeline
- Thermodynamic inhibition
- Hydrate structure conformation
- Kinetic characteristic of hydrate growth with diamines
- Effect of diamines on kinetic inhibition
- Interaction energy between a hydrate cage and an inhibitor
The resonance NMR peaks were detected at -4.3 ppm and -6.6 ppm in the pure CH4 hydrate sample and the peaks are assigned to CH4 molecules trapped in small 512 and large 51262 cages of structure I (sI) hydrate, respectively . The result of NMR measurement for CH4 hydrates with diamines showed two resonances at the same chemical shift with pure CH4 hydrate. The effect of diamines on the CH4 hydrate formation process was investigated using in-situ Raman spectroscopy.
The time-dependent Raman spectra of CH4 hydrate formation with and without diamines are shown in Figure 3.2.4. Both pure CH4 hydrate cages grew continuously until approximately 80 min, and then the intensity of each Raman peak remained nearly constant, indicating that hydrate growth was almost complete. However, the growth patterns of CH4 + diamine hydrate differed significantly from those of pure CH4 hydrate.
In the presence of PZ and HZ, the growth rate of CH4 hydrate was significantly slowed down. In particular, the addition of HZ inhibited the growth of CH4 hydrate more substantially compared to PZ. The different growth patterns between pure CH4 hydrate and CH4 + diamine hydrate were also clearly observed for the large 51262 and small 512 cages.
In Figure 6(a), At/Af for 51262 large cages of CH4 hydrate + PZ (1.0 mol%) increased relatively slower than that of pure CH4 hydrate up to 60 min, and then slowly approached that of pure CH4 hydrate. This indicates that HZ can delay the growth of large and small CH4 hydrate cages more than PZ. The induction time, which refers to the time required for gas hydrate nucleation, was measured to examine the effect of diamine kinetic inhibition on CH4 hydrate formation.
Optimized structural configurations of a 512-cage CH4 hydrate with each inhibitor molecule are shown in Figure 9. Moreover, they were more negative than the interaction energy between a water monomer and a 512-cage (-12.27 kJ/mol), indicating that all inhibitor molecules have an attraction with CH4 hydrate cages. From Table 3 and Figure 2, it is suggested that the magnitude of interaction energy between each inhibitor molecule and a cage containing 512 CH4 hydrate may be strongly related to the degree of thermodynamic inhibition of CH4 hydrate.
In this study, HZ, which has more negative interaction energy between an inhibitor molecule and a 512 cage than PZ, showed a longer induction time and a slower formation kinetics for CH4 hydrate at the same concentration, compared to PZ.
Applications
Conclusion
An overview of the hydrate-based gas separation (HBGS) process for carbon dioxide precombustion capture. CO2 removal from a CO2-CH4 gas mixture by clathrate hydrate formation using THF and SDS as water-soluble hydrate promoters. Dual inhibition effects of diamines on methane hydrate formation and their significance for natural gas production and transport.
Medium-pressure hydrate-based gas separation process (HBGS) for pre-combustion carbon dioxide capture using a new fixed-bed reactor. Macroscopic and spectroscopic identifications of the synergistic inhibition of an ionic liquid on hydrate formations. Hydrate formation in systems containing methane, ethane, propane, carbon dioxide or hydrogen sulfide in the presence of methanol.
Prediction of equilibrium conditions for the formation of gas hydrates in mixtures of electrolytes and alcohol. A study on the thermodynamic effect of [EMIM]-Cl and [OH-C 2 MIM]-Cl on the methane hydrate equilibrium line. Incorporation of ammonium fluoride into clathrate hydrate lattices and its importance in inhibiting hydrate formation.
Gas trapping in tetra-n-butylammonium chloride (TBAC) semiclathrates: potential application to natural gas storage and CO 2 capture. Thermodynamic and spectroscopic identification of guest gas inclusion in the double tetra-n-butylammonium fluoride semiclathrates.
![Figure 1.1.1. Sour natural gas reserves around world; North America Excluded [4]](https://thumb-ap.123doks.com/thumbv2/123dokinfo/10524203.0/11.892.195.696.501.766/figure-sour-natural-reserves-world-north-america-excluded.webp)
![Table 1.2.1. Crystallographic information on the structures of gas hydrates [11]](https://thumb-ap.123doks.com/thumbv2/123dokinfo/10524203.0/14.892.108.789.156.444/table-1-crystallographic-information-structures-gas-hydrates-11.webp)
