Hydrate-based Gas Separation Process Research Articles

Hydrate-based capture of blowing agents: Thermodynamic investigation of model gas mixtures consisting of HCFC-22, HCFC-142b, and N2

Hydrochlorofluorocarbons (HCFCs) are representative powerful greenhouse gases that contribute significantly to global warming. As an alternative strategy for their existing separation and recovery methods, hydrate-based gas separation (HBGS) has been of great. A preliminary investigation was performed to measure the hydrate phase equilibria of pure HCFC-22, pure HCFC-142b, and two different gas mixtures of these with N2 (at molar composition ratios of 8.01:12.01:79.98 and 20.01:30.00:49.99), which is indispensable to the design of a HBGS process and determination of its operating conditions. Hydrate dissociation enthalpies of the investigated systems were calculated from the measured equilibrium data using the Clausius-Clapeyron equation, and their values were 82.12 (HCFC-22), 153.65 (HCFC-142b), 127.90 (HCFC-22 8.01%), and 133.51 (HCFC-22 20.01%) kJ/mol, respectively. Their crystal structures were determined using powder X-ray diffraction, and results revealed that HCFC-22 merely formed structure I hydrate. Kinetic hydrate formation tests were carried out with the two different gas mixtures, and the compositions of the vapor and hydrate phases were examined. The hydrate nucleation and separation performance were profoundly affected by the initial system pressure, proven by showing the shortest induction time and the highest separation factor for HCFCs.

Thermodynamic behavior and spectroscopic properties of CO and C3H8 mixed gas hydrates: Implications for hydrate-based gas separation

Gas mixtures containing carbon monoxide (CO) and propane (C3H8) are encountered in a variety of chemical processes such as the incomplete combustion of fossil fuels, syngas production, and steel plants. Here we investigate the thermodynamic stability and spectroscopic properties of CO-C3H8 mixed gas hydrates for potential applications related to hydrate-based gas separation (HBGS). The CO-C3H8 mixed gas hydrates were formed from CO/C3H8 gas mixtures at pressures of 0.5–1.4 MPa and 274 K. Three-phase (H–Lw–V) equilibrium conditions of the CO-C3H8 mixed gas hydrates were shifted to higher pressure and lower temperature regions with increasing CO concentration in the vapor phase. X-ray diffraction patterns of the CO-C3H8 mixed gas hydrates suggest the formation of cubic Fd3m structures (sII hydrate). Sold-state 13C cross polarization/magic angle spinning NMR and Raman spectroscopy demonstrated the preferential occupation behavior of CO and C3H8 molecules in the hydrate cages. Thermodynamic predictions using the predictive Soave–Redlich–Kwong model with the UNIFAC group contribution method were in excellent agreement with the experimental phase equilibria and the cage occupancy analysis by 13C NMR and Raman spectroscopy. The compositions of the CO-C3H8 mixed gas hydrates were examined by gas measurements after hydrate dissociations, which were very close to the 13C NMR analysis results and theoretical predictions. The recovery ratio of C3H8 was always higher than 1 and gradually increased with decreasing C3H8 concentration in the vapor phases. These results indicate that the HBGS process can effectively be used for selectively separating CO and C3H8 from CO/C3H8 gas mixtures.

Enhanced methane recovery from low-concentration coalbed methane by gas hydrate formation in graphite nanofluids

Coalbed methane (CBM) is a primary type of unconventional natural gas adsorbed in coal seams. Its amount around the world is estimated to be 260 × 1012 standard cubic meters (SCM). The low-concentration coalbed methane (LCCBM) with CH4 content less than 30 mol% is difficult to utilize because of its low combustion efficiency. This work reports an investigation of employing a graphite nanofluid to enhance CH4 recovery from LCCBM via gas hydrate formation. The CH4 separation efficiency was evaluated by comparing CH4 recovery ratio, the separation factor, and gas consumption with those reported in the literature, and the behavior of gas hydrate growth in graphite nanofluid was observed. The results indicated that hydrate nucleation was promoted when adding a mass fraction of 0.5% graphite nanoparticles (GNP) into the tetrahydrofuran and sodium dodecyl sulfate solution (THF/SDS solution) and gas consumption increased significantly in comparison with that obtained without GNP. The graphite nanofluid containing 0.5% GNP is preferred for CH4 separation among the three GNP mass concentrations (0.1%, 0.5%, and 3.0%) tested in this work. CH4 recovery ratio and the separation factor obtained in graphite nanofluid were higher than those obtained in other systems such as water-in-oil emulsions. CH4 concentration in LCCBM was increased from 30 mol% to 57.1 mol% via a one-stage hydrate based gas separation process. As a consequence, it has the potential to be an efficient approach to recover CH4 from LCCBM by forming gas hydrates in graphite nanofluid.

Experimental and modeling phase equilibria of gas hydrate systems for post-combustion CO2 capture

Abstract Accurate knowledge of hydrate phase equilibria is of fundament in terms of the hydrate-based gas separation process (HBGS) for post-combustion CO2 capture. In this work, we experimentally investigated the phase equilibria of flue gas during hydrate-based CO2 capture. Additionally, new experimental data for dissociation pressure of flue gases with different CO2 concentration were reported. Subsequently, a more accurate thermodynamic model combining Cubic-Plus-Association Equation of State (CPA-EoS) and Chen-Guo model was employed to predict the dissociation pressure of flue gases. To better perform the hydrate equilibria, a temperature dependent binary interaction parameter kij and new parameters for Chen-Guo model were developed. The results showed that the general tender of the new experimental data was in line with previous results. In addition, an improved accuracy was noticed for the mixtures with an Average Absolution Deviation (AAD) approximately to 4.07%, through comparing the predicted results with the experimental data. Especially, in terms of the gases with a CO2 concentration less than 85.32%, the improvement was significant. Lastly, this work also utilized the thermodynamic model and the Clausius-Clapeyron equation to predict the dissociation enthalpies of CO2 hydrate. The results showed that the prediction results were located in the margin of experimental data, which demonstrated the thermodynamic model proposed in our work was capable of describing the gas hydrate behavior with high accuracy.

Effects of additives on continuous hydrate-based flue gas separation

CO2 capture from fossil fuel power plants is the main method of CO2 storage. Hydrate-based gas separation is regarded as a potential method for CO2 capture from flue gas. In this study, hydrate-based gas separation (HBGS) was used to capture CO2 from flue gas (19.96 mol% CO2 and 80.04 mol% N2), and the continuous experimental process was monitored using magnetic resonance imaging (MRI). The effects of two additives (5 wt% TBAB + 5 wt% THF and 19 wt% THF), two gas injection methods (constant pressure and constant flow rate processes), and of different pressures and flow rates on the hydrate saturation and solution movement were investigated. The results show that both additives effectively promote hydrate formation. The constant pressure process was superior to the constant flow rate process for hydrate formation. Furthermore, the flow rate had little influence on the hydrate saturation. The process was most efficient when a hydrate formation stage time of approximately 80 min was used. The solution movement resulting from the continuous multiple cycles tended to decrease during subsequent cycles. Moreover, the addition of 19 wt% THF had a more obvious effect on the solution movement than 5 wt% TBAB + 5 wt% THF. Solution concentration phenomena were observed in the presence of 19 wt% THF at 285.15 K; these phenomena may have been affected by the formation and dissociation of hydrates. Due to solution movement during the continuous industrial hydrate-based gas separation process, the solution might need to be replenished. Finally, in terms of the resulting the hydrate saturation, the use of 5 wt% TBAB + 5 wt% THF was found to be more suitable in this study, while the 19 wt% THF was more suitable for a higher experimental temperature.

Semiclathrate based CO2 capture from fuel gas mixture at ambient temperature: Effect of concentrations of tetra-n-butylammonium fluoride (TBAF) and kinetic additives

Hydrate-based gas separation (HBGS) for pre-combustion CO2 capture from fuel gas is one of the attractive methods to reduce carbon emission. To further advance the HBGS technology, there is a need to identify promoters that can moderate the operating conditions and enhance the kinetics and separation efficiency. Tetra-n-butylammonium fluoride (TBAF) is a semiclathrate former that can enable the HBGS process to be operated at ambient temperatures. In this study, the kinetic performance of CO2/H2/TBAF semiclathrate formation was evaluated under various TBAF concentrations in a stirred tank reactor at 6.0 MPa with a temperature driving force of 4.1 K. Compared with other concentrations, 0.8 and 1.5 mol% solutions exhibited higher gas uptake (normalized by the solution volume), hydrate formation rate, and CO2 composition in the captured gas. In addition, the effects of three kinetic additives on hydrate formation were tested, including sodium dodecyl sulfate (SDS), leucine, and tryptophan. All additives could reduce the induction time and increase the hydrate formation rate. The visual images of hydrate formation indicated two morphology styles, with the occurrence of style II identified to be highly related to the long induction time and the presence of leucine or tryptophan.

CO<sub align="right">2 capture by adsorption and hydrate-based separation: a technological review

The rising concern over global climate change has geared up a desire for reduction in greenhouse gas emissions. In this work, a systematic review of the literature work done so far on the application of adsorption and hydrate-based gas separation for the capture of CO2 from both fuel gas and flue gas mixtures has been carried out. Different adsorbents and operating conditions have been studied. Improvements in the adsorption capacities by thermal, mechanical and chemical activation have also been analysed. A range of reactor designs and the role of thermodynamic and kinetic promoters in the hydrate-based gas separation process for CO2 capture have been reviewed. Each experimental work has been scrutinised independently and compared finally. Furthermore, the advantages and limitations of adsorption process and hydrate-based gas separation process have been pointed out.

Phase Equilibrium Data for the Hydrates of Synthesized Ternary CH4/CO2/N2 Biogas Mixtures

To enhance the energy efficiency of biogas and reduce environmental pollution, impurities such as CO2 and N2 must be removed. In this context, hydrate-based gas separation (HBGS) is a novel technology that has been utilized in gas purification. As such, the hydrate phase equilibrium data of the CH4/CO2/N2 gas mixture are crucial for application of the HBGS process in biogas separation. This study presents our investigation into the phase equilibrium conditions for the hydrates of synthesized ternary CH4/CO2/N2 biogas mixtures. All experiments were conducted at 276.2–286.3 K and 2.59–8.84 MPa using an isochoric method with different ternary CH4/CO2/N2 gas concentrations. The results obtained herein indicated that the hydrate phase equilibrium conditions shifted to lower pressures and higher temperatures with increasing CO2 contents. In addition, with a N2 content of about 10%, the hydrate phase equilibrium data of the ternary gas mixtures approached that of pure CH4.

Semiclathrate-based CO2 capture from fuel gas in the presence of tetra-n-butyl ammonium bromide and silica gel pore structure

Pre-combustion CO2 capture technology was investigated via a hydrate-based gas separation process. Semiclathrate phase equilibria were measured for the simulated fuel gas from the integrated gasification combined cycle (IGCC) consisting of 40mol% CO2 and balanced H2 in the presence of tetra-n-butyl ammonium bromide (TBAB) and porous silica gel particles containing water inside. The basic idea is an extension of our previous work [34], that is, using a porous silica gel structure made it possible to get rid of stirring the content inside the hydrate reactor and ensure the rapid hydrate formation with relatively short induction. TBAB was introduced to confined aqueous solution in silica gel pore structure, having nominal pore diameter of 100nm, as a thermodynamic promoter and formed semiclathrate with the gas mixture under milder temperature and pressure conditions. Compositional analysis of semiclathrate and coexisting gas phases was also performed at various temperatures and TBAB concentrations. The highest CO2 concentration obtained from the dissociated semiclathrate was 94mol% formed at 277.15K and 3.0MPa, with 20wt% TBAB solution in 100nm silica gel. To demonstrate the highly-concentrated CO2 outlet stream from the single-stage semiclathrate reactor, the kinetic study of CO2 distribution through fixed-bed type reactor charged with silica gel particles containing aqueous TBAB solution was also investigated. Utilization of porous silica gel makes it possible to capture higher concentration of CO2 in the semiclathrate phase, but it also reduced the total amount of gas consumed for semiclathrate conversion. However, the use of silica gel particles with TBAB enabled formation of semiclathrate hydrate under milder conditions and unstirred continuous method to a high extent and a high rate.

Semiclathrate hydrate process for pre-combustion capture of CO2 at near ambient temperatures

CO2 capture has become an important part in building a sustainable energy system featuring the clean use of fossil fuels with low carbon footprint. Hydrate-based gas separation (HBGS) is one of the potential technologies to capture carbon dioxide from pre-combustion (fuel gas) stream. Promoters are often employed to moderate the formation conditions of hydrates. Tetra-n-butylammonium fluoride (TBAF) exhibits great thermodynamic promotion on hydrate formation. In this study the kinetic performance of the formation process of CO2-H2-TBAF semiclathrate hydrate was evaluated under different experimental pressures (6MPa, 4MPa and 2MPa) and temperatures (298K, 292K and 286K), with the stoichiometric TBAF concentration (3.38mol%). Gas uptake measurement and visual observations showed that at a given pressure, the total gas uptake decreased with the decrease of experimental temperature due to high mass transfer resistance caused by rapid growth of TBAF hydrate. The highest gas uptake was achieved by experiments conducted at 6.0MPa and 298.0K. It was the highest among semiclathrate promoters with stoichiometric concentration, but lower compared with THF. The major advantage of the use of TBAF is that the HBGS process can be operated at near ambient temperatures compared to other promoters. The CO2 composition in the hydrate phase was between 65.2 and 93.1mol%, highly dependent on the experimental conditions. Variance analysis was employed to evaluate the impact of pressure and temperature on gas uptake. Gas solubility measurements were conducted to provide further insights into the kinetic performance of CO2-H2-TBAF semiclathrate hydrate formation.

Impact of fixed bed reactor orientation, liquid saturation, bed volume and temperature on the clathrate hydrate process for pre-combustion carbon capture

Hydrate based gas separation (HBGS) process is a promising technology for carbon capture from pre-combustion streams of power generation. Recently, fixed bed reactor (FBR) configuration has been reported to significantly enhance the kinetics of hydrate formation for the HBGS process. In this work, silica sand bed reactor was employed along with 5.56 mol% THF solution to capture CO2 from fuel gas mixture (CO2/H2) at 6.0 MPa, to investigate the effects of reactor orientation (vertical, horizontal), liquid saturations in the bed (50%, 75%, 100%), and fixed bed volume. Horizontal configuration showed a major improvement in terms of gas uptake and normalized rate of hydrate formation than vertical configuration, due to the larger cross sectional area in the horizontal configuration. 50% liquid saturation performed better than the other saturations from water utilization perspective, whereas 100% saturation was better from space utilization perspective. While bed volume did not influence the kinetics of hydrate formation much, smaller bed volume showed better dissociation kinetics. In addition, the effect of operating temperatures (279.2 K, 282.2 K and 285.2 K) were evaluated for a chosen configuration. Operating temperature of 282.2 K presented slightly lower performance compared with 279.2 K, but had the advantage of energy saving. The short induction time and high CO2 composition in hydrate phase (more than 91%) further enhanced the potential and feasibility of employing this horizontal FBR configuration for pre-combustion CO2 capture with the use of THF.

Precombustion CO2 capture using a hybrid process of adsorption and gas hydrate formation

In this study, a fixed bed of pulverized coal particles was employed to enhance the precombustion CO2 capture from fuel gas (40 mol% CO2/H2). The performance of the HAHF (hybrid adsorption-hydrate formation) process occurred in the fixed bed was evaluated at different liquid saturations and operation pressures. The experiments were carried out at 274.2 K with the pressure varying from 3.0 MPa to 6.0 MPa. 1.0 mol% THF (tetrahydrofuran) solution instead of liquid water was used to saturate the fixed bed and reduce the hydrate phase equilibrium conditions. The results indicated that gas hydrate nucleation in the fixed bed was quickly induced by gas adsorption in coal particles. The HAHF process was dominated by hydrate formation as the fixed bed saturation increased, and the CO2 recovery and separation factor were increased. Gas consumption during the HAHF process was promoted with the increase of operation pressure, but the CO2 recovery and separation factor decreased. In the fixed bed of coal particles saturated by 1.0 mol% THF solution the highest gas uptake obtained was 51.9 mmol of gas/mol of water, which was comparable with that obtained in the fixed bed of silica sand in the presence of 5.53 mol% THF. Therefore, compared with the hydrate based gas separation process, the hybrid adsorption-hydrate formation process proposed in this study could be a viable option for CO2 capture from fuel gas.

CH4Separation from Coal Bed Methane by Hydrate in the SDS and THF Solution

Hydrate-based separation experiments on simulate coal bed methane gas have been conducted in THF solution and SDS solution. In this work, a novel hydrate-based gas separation process was used to enhance CH4separation from a 65.7% CH4/20.2% N2/O2gas mixture in the presence of 300 ppm SDS and 19% THF solution. The characteristics of the CH4separation efficiency, fluctuation of temperature, and pressure were studied at different promoter solution. It was found that hydrate formation was induced by promoter in the solution and occurred immediately as the experiments were started. THF performed better than SDS for CH4separation from the CH4/N2/O2gas mixture. In particular, the separation coefficients of CH4and N2were compared in two solutions. The gas mixture S.Fr. or CH4recovery is increased from 1.056 to 1.259 while SF of N2is decreased from 1.183 to 0.634 in THF solution.

Evaluation of CO2 removal from a CO2 + CH4 gas mixture using gas hydrate formation in liquid water and THF solutions

In this work, the performance of hydrate formation for CO2 removal from a simulated recovered shale gas (40mol% CO2 and 60mol% CH4) was evaluated in liquid water, and THF and THF/SDS solutions. Experiments were carried out at a fixed temperature of 277.15K and in the pressure range of (2.8–6.7) MPa. The impact of driving force (overpressure) on hydrate growth and CO2 selectivity was studied both in liquid water and in THF solutions. It was found that higher driving force resulted in a reduction of the final gas uptake as well as a significant decrease of the CO2 recovery and separation factor. The results indicated that mass transfer for hydrate growth was greatly hindered at high driving force, and the competition between CH4 and CO2 molecules for the occupancy of hydrate cavities became stronger with the increase of driving force. Although the kinetics of hydrate formation with the CO2/CH4 gas mixture was improved by adding THF and SDS into liquid water, the selectivity for CO2 removal from the CO2/CH4 gas mixture was compromised as compared to that in liquid water. The highest CO2 recovery (52%) and separation factor (8.8) were obtained at a low driving force (2.5MPa) in liquid water, which were much higher than those obtained in THF and THF/SDS solutions. As a result, enhancing the kinetics of hydrate formation without reducing CO2 selectivity is the key factor to improve the hydrate based gas separation process for CO2 removal from the CO2/CH4 gas mixture.

CO2 capture using the clathrate hydrate process employing cellulose foam as a porous media

A new biodegradable porous medium has been employed in this work for the hydrate-based gas separation (HBGS) process to capture carbon dioxide in a fixed bed column from a precombustion stream. Propane (2.5 mol%) was added as a promoter to reduce the operating pressure of the HBGS process. Experiments were conducted at 6 MPa and 274.2 K at different water saturation levels (50% and 100%) in a cellulose foam bed. It was found that a normalized rate of hydrate formation was more than double for 50% as compared to 100% water-saturated level. In addition, kinetic modelling of hydrate formation in porous media has been carried out using Avrami model by utilizing the experimental gas uptake data from current and published works. The Avrami model was found to fit the hydrate growth kinetics very well, up to 40 min of hydrate growth for different porous media like silica sand, polyurethane foam, and cellulose foam, and for different guest gas and gas mixtures.

Role of Metallic Packing and Kinetic Promoter in Designing a Hydrate-Based Gas Separation Process

In the present work, kinetics of the hydrate-based gas separation (HBGS) process has been studied employing an unstirred reactor configuration. Gas uptake measurements were carried out for studying hydrate formation kinetics and final water to hydrate conversion using fuel gas mixtures (mixture 1, 40.4% CO2 + 59.6% H2; mixture 2, 40.9% CO2 + 58.05% H2 + 1.05% H2S). Silica sand and stainless-steel-structured packing (SSP) were used as contact media in the presence of an anionic surfactant sodium dodecyl sulfate (SDS) as a kinetic promoter. Experiments were conducted with three different concentrations of SDS in water to enhance hydrate formation kinetics. On the basis of induction time and rate of hydrate growth, 1 wt % SDS was determined to be the best concentration for carbon dioxide capture at 7.0 MPa and 273.65 K from the CO2 + H2 gas mixture. In comparison to silica sand packing, use of SSP was found to improve the final water to hydrate conversion (71.0 ± 4.1%). The addition of H2S impurity in the fu...

Impact of Fly Ash Impurity on the Hydrate-Based Gas Separation Process for Carbon Dioxide Capture from a Flue Gas Mixture

The hydrate-based gas separation (HBGS) process for gas mixtures of CO2 + N2 (flue gas) and CO2 + H2 (fuel gas) has proven to be very efficient and highly selective for CO2 capture. In addition to CO2 and N2, flue gas from coal-based thermal power stations can contain impurities such as nitrogen oxides (NOx), sulfur oxides (SOx), and fly ash. In this work, the impact of fly ash on the HBGS process efficiency was investigated. Tetrahydrofuran (THF, 1 mol %) was used as a thermodynamic promoter, and sodium dodecyl sulfate (SDS) and sodium dodecyl benzenesulfonate (SDBS) (anionic surfactants) were used as kinetic promoters. Whereas the use of THF in the HBGS process reduces the operating pressure significantly, both SDS and SDBS were found to enhance the rate of hydrate formation. It was observed that the hydrate equilibrium conditions did not change in the presence of fly ash. However, the presence of fly ash enhanced the separation efficiency of the HBGS process by reducing the induction time and increasin...

A New Porous Material to Enhance the Kinetics of Clathrate Process: Application to Precombustion Carbon Dioxide Capture

In this work, the performance of a new porous medium, polyurethane (PU) foam in a fixed bed reactor for carbon dioxide separation from fuel gas mixture using the hydrate based gas separation process is evaluated. The kinetics of hydrate formation in the presence of 2.5 mol % propane as thermodynamic promoter was investigated at 4.5, 5.5, and 6.0 MPa and 274.2 K. Significantly higher gas consumption and water conversion to hydrate was achieved when PU foam was employed. PU foam as a porous medium can help convert 54% of water to hydrate in two hours of hydrate formation. In addition the induction times were very low (<3.67 min at 6.0 MPa). A normalized rate of hydrate formation of 64.48 (±3.82) mol x min(-1) x m(-3) was obtained at 6.0 MPa and 274.2 K. Based on a morphological study, the mechanism of hydrate formation from water dispersed in interstitial pore space of the porous medium is presented. Finally, we propose a four step operation of the hydrate based gas separation process to scale up.

Medium pressure hydrate based gas separation (HBGS) process for pre-combustion capture of carbon dioxide employing a novel fixed bed reactor

This work presents an effective medium pressure hydrate based gas separation (HBGS) process for pre-combustion carbon dioxide capture in a novel fixed bed column. 2.5mol% propane was added to the fuel gas mixture as an additive to decrease the operating pressure of the HBGS process. Hydrate formation kinetics was investigated at three different pressures (4.5, 5.5 and 6.0MPa respectively) and at 274.15K. The performance of silica sand and silica gel as a medium was evaluated. In silica sand bed, multiple nucleation events were observed. In silica gel bed, the gas uptake and water conversion to hydrates was significantly low at any given driving force than that obtained in silica sand bed. Experiments at different water saturation levels (50, 75 and 100%) in silica sand bed were investigated at 6.0MPa and 274.15K. It was found that at 50% water saturation, gas consumed for hydrate formation and water conversion to hydrates was almost three times that at 100% saturation. Water to hydrate conversions of up to 64.3% was achieved after 4h of hydrate formation for the 50% water saturated silica sand bed. Our study presents an opportunity to scale up the HBGS process for CO2 capture with enhanced kinetics by employing a fixed bed reactor configuration. Decomposition experiments at a driving force of ΔT of 10K and 23K were carried out to recover the gas consumed for hydrate formation and it was found that ΔT of 23K was sufficient to recover the hydrated gas.

Recovery of CO2 from Flue Gas Using Gas Hydrate: Thermodynamic Verification through Phase Equilibrium Measurements

The main purpose of this study was to develop a new hydrate-based gas separation (HBGS) process especially for recovering CO2 from flue gas. Temperature and pressure conditions for hydrate formation have been closely examined at the various CO2 concentrations of flue gases. Tetrahydrofuran (THF) chosen as a hydrate promoter can also participate in forming hydrates and produces a mixed hydrate together with CO2. The hydrate stability region was greatly expanded by using THF for lowering the equilibrium formation pressure. To confirm thermodynamic validity of the HBGS process, the three-phase equilibria of hydrate, liquid, and vapor were measured for the systems comprising CO2, N2 and water with or without THF in the temperature range of 272−295 K. In addition, two phase equilibria of hydrate and vapor were experimentally investigated for the same systems at several temperatures. Through close examination of the overall experimental results, it was firmly verified that the HBGS process makes it possible to recover more than 99 mol % of CO2 from the flue gas. The key unit operations of the HBGS lie in hydrate formation and subsequent dissociation similarly to gas absorption and desorption using the sterically hindered amines. The HBGS provides several advantages over the conventional ones. First, the operational temperature is moderate in the range of 273−283 K, and continuous operation allows this process to treat a large amount of gaseous stream. Second, only a small amount of THF is needed together with water and therefore severe corrosion problem can be avoided. Third, the aqueous solution containing THF after dissociation can be easily recycled to the hydrator.