Research progress of hydrate-based CO₂ separation and capture from gas mixtures

Abstract

Hydrate-based CO₂ separation and capture from gas mixtures containing CO₂ has gained growing attention as a new technology for gas separation, and it is of significance for reducing anthropogenic CO₂ emissions. Previous studies of the technology include the thermodynamics and kinetics of hydrate formation/dissociation, hydrate formation additives, analytical methods, separation and capture progress, equipment and applications. Presently, the technology is still in the experimental research stages, and there are few reports of industrial application. This review examines research progress in the hydrate formation process and analytical methods with a special focus on laboratory studies, including the knowledge developed in analog computation, laboratory experiments, and industrial simulation. By comparing the various studies, we propose original comments and suggestions on further developing hydrate-based CO₂ separation and capture technology.

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Chun-Gang Xu

Chun-Gang Xu, Ph.D., Research fellow, graduated from Guangzhou Institute of Energy Conversion, Chinese Academy Science. Since 2008, he has worked at the Center of Gas Hydrate Research in Guangzhou Institute of Energy Conversion, the Chinese Academy of Sciences, as a research fellow. He has undertaken more than 10 projects in China, including the “National High Technology Research and Development Program of China (863 Program)”, the “National Natural Science Foundation of China”, CAS Knowledge Innovation Program, the “Natural and Science Foundation of Guangdong”, “Science and Technology Planning Project of Guangdong Province”, etc. He mainly works on the study of hydrate-based CO2 capture from flue and fuel gases.

Xiao-Sen Li

Xiao-Sen Li, Ph.D., Professor, graduated from the Department of Chemical Engineering, Tsinghua University for Doctoral Degree in 2000. From April 2000 to July 2005 he worked at the University of Alberta and the University of British Columbia. Since August 2005, he has worked at the Center of Gas Hydrate Research in Guangzhou Institute of Energy Conversion, the Chinese Academy of Sciences, and he is a chief scientist of the Guangzhou Center of Gas Hydrate Research, CAS, and an academic leader for the Innovation Program of CAS. His research areas include the equilibrium and kinetics of gas hydrates, the production technology and the utilization of natural gas hydrate and the application technology of gas hydrate.

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1. Introduction

Carbon dioxide (CO₂) is one of the most important anthropogenic greenhouse gases (GHG). Between 1970 and 2004, its annual emissions have grown by about 80%, from 21 to 38 gigatonnes (Gt), and CO₂ represented 77% of total anthropogenic GHG emissions in 2004.¹ According to predictions by the International Panel on Climate Change (IPCC), CO₂ concentrations may reach 570 ppm in the atmosphere by the year 2100, causing the mean global temperature to rise by approximately 1.9 °C.² To prevent global climate deterioration, fossil energy alternatives such as nuclear, biomass, solar energy, etc., are being developed. However, these energy sources cannot replace the fossil fuels sufficiently to meet our needs, and any rapid change to non-fossil energy resources may result in large disruptions to the existing energy supply infrastructure. Therefore, fossil fuels will continue to be the major energy supply in the near future because of their availability, competitiveness and ease of transport. Thus, highly effective technologies for CO₂ separation and capture from fossil fuels need to be further researched and developed in order to meet CO₂ reduction targets. The major CO₂ separation technologies that are commercially used involve chemical absorption, physical adsorption, cryogenic separation and membrane separation. However, conventional technologies have intensive energy consumption, chemical degradation, low capacity, etc.³ Therefore, some research has dedicated efforts to developing new technologies: gas hydrate separation technology, chemical looping technology and electro-chemical cells separation technology.⁴

Gas hydrates are non-stoichiometric compounds composed of water molecules and small gas molecules. Examples of gas hydrates include methane (CH₄), carbon dioxide (CO₂), nitrogen (N₂), hydrogen (H₂), sulfur dioxide (SO₂), hydrogen sulfide (H₂S), ethane (C₂H₆), propane (C₃H₈), iso-butane (i-C₄H₁₀), ethylene (C₂H₄), propylene (C₃H₆).^5,6 Water molecules (hosts) connect with each other by hydrogen-bonds to form cavities, and small gas molecules (guests) are stabilized in the cavities by van der Waals interaction forces.⁷ Currently, the most common gas hydrate structures include structure I (sI), structure II (sII), structure H (sH) and semi-clathrate (sc).^8,9 In general, the gas hydrate structure is mainly determined by size of the gas molecule if a single gas acts as a single guest. However, the structure is also affected by the composition and/or pressure when gas mixtures with multiple components act as guests.¹⁰ The structures of sI and sII hydrates were first determined by single crystal X-ray crystallography in the early 1950s. After the structures were discovered, van der Waals and Patteeuw were the first two people to describe the hydrates in terms of stability and composition.¹¹

In 1810, Humphrey Davy first reported gas hydrates in the Bakerian lecture to the Royal Society.¹² Hammerschmidt suggested that it was methane hydrate blocking gas pipelines rather than ice in 1934.¹³ However, researchers didn’t investigate the hydrates existing in nature until 50 years later.^7,14 The effects of gas hydrates on energy and environmental applications have been intensively researched since the early 1990s. The applications include production and transportation of gas in subsea flow assurance, the potential energy recovery from naturally occurring gas hydrates deposits, the storage of new fuels (natural gas or hydrogen) in hydrate materials, the role of gas hydrates in environmental safety (stability of seafloor and climate change), gas separation and purification from gas mixtures, and desalination of sea water.^8,15–17

Hydrate-based gas separation is based on the selective partitioning of the components in the hydrate phase and in the gas phase.^3,18–20 At the same temperature, different gases have their individual equilibrium hydrate formation pressures. A gas with a relatively low equilibrium hydrate formation pressure at a certain temperature is expected to be preferentially entrapped in water cavities to form a gas hydrate with higher thermodynamic stability, and it results in a gas-rich hydrate phase while the residual gas phase is gas-poor.^21–23 Relative to the other gas separation technologies, gas hydrate separation technology has the following advantages: (1) simple process, (2) low investment, (3) low material and energy loss, and (4) environmental friendliness.²⁴ Previous studies focused on the separation of CO₂, CH₄,^25–32 H₂,^33–35 N₂,³⁶ oil gas,^37,38 and other greenhouse gases.^39–44 The main research content includes kinetics and thermodynamics of gas hydrate formation/dissociation, separation and capture processes, analytical methods, and equipment and applications. For example, the studies of CO₂ separation focus on the equilibrium conditions of hydrate formation, hydrate formation additive/promoter approaches, and separation equipment and processes. This review paper examines research progress in the hydrate formation process and analytical methods with a special focus on laboratory studies of hydrate-based CO₂ separation, and its content includes the following parts: equilibrium hydrate formation conditions, additives for forming CO₂ hydrates, molecular-level measurements of the hydrates containing CO₂, process and apparatus, cost and comparison, and conclusions.

2. Equilibrium hydrate formation conditions

2.1 Computation of equilibrium formation conditions

Phase equilibrium conditions for hydrate formation for gas mixture–water (or solution in presence of additive/or promoter) systems need to be measured before conducting studies of CO₂ hydrate separation. In 1964, Saito et al. first used the van der Waals and Platteeuw model (vdWP model) to systematically predict hydrate formation temperatures and pressures.^11,45,46 Then, the approach was extended by Parrish and Prausnitz.⁴⁷ Later, the model was substantially simplified by John and Holder.⁴⁸ During the last 30 years, the vdWP model coupled with the simplified Parrish and Prausnitz algorithm was widely used to predict the equilibrium hydrate formation temperatures and pressures.^7,49–55

In fact, all prediction models are established based on parameters (e.g. gas fugacity, Langmuir constant) and correlations between the parameters. There are five major methods to determine the correlations. The first is the K-value method, which utilizes vapor–solid equilibrium constants to predict hydrate formation conditions.^56,57 The second is a gas-gravity plot developed by Katz.⁵⁸ The gas-gravity is defined as the apparent molecular weight of a gas mixture divided by the apparent molecular weight of air. The method is a simple graphical technique and it is useful for an initial estimate of hydrate formation conditions. The hydrate formation chart is made according to the limited experimental data and calculations based on the K-value method. But, the statistical accuracy analysis (Sloan⁵⁹) showed that the method is inaccurate, and different gas mixtures can lead to about 50% deviation in predicted pressure for the same gas-gravity. However, copious amounts of experimental data have been collected in the past 60 years, and a more accurate gas-gravity plot may be developed based on the data. Therefore, Holder et al.⁶⁰ and Makogon⁶¹ developed empirical correlations for selected pure gases, and this is the third method. Kobayashi et al.⁶² developed a new empirical equation for hydrate formation conditions of natural gases. However, the empirical equations have their limitation of temperature and pressure, and the method of the gas-gravity plot is seldom used for predicting the equilibrium conditions of gas hydrates containing CO₂. The fourth method for determining correlations between parameters involves a chart of permissible expansion, and it is based on the range of the permissible expansion that a natural gas can undergo without the possibility of hydrate formation. The chart of permissible expansion is drawn based on the gas-gravity chart using the Joule–Thomson cooling curve.⁵⁸ The method is suitable for rough design of throttling in valves and chokes where gas expansion normally occurs. However, it is not suitable for natural gases containing more CH₄. An average error of 10% can be obtained in general. The fifth is a statistical thermodynamic approach, developed by van der Waals and Platteeuw,¹¹ predicting the equilibrium hydrate formation temperatures and pressures on the criterion Δμ_H = Δμ_W at equilibrium.⁶⁰ Thus, from a statistical mechanics point of view, those constants that can impact the chemical potentials between the hypothetical empty and fully filled hydrate lattice become key for the approach.^11,63–65

The Langmuir constant is quite important, and different models have been developed to obtain more accurate values of the Langmuir constant, for example, the Kihara potential model.^66–68 Parrish and Prausnitz developed a correlation using the Kihara potential and experimental data of the hydrate formation, and the accuracy of the correlation is approximately 0.2%.⁴⁷ The parameters of the Kihara potential model are empirically regressed from the experimental data of the cavity occupancy and the phase equilibrium. Although the Kihara potential models can reproduce the experimental data, its capability for extrapolation is generally poor. For instance, the Parrish–Prausnitz model⁴⁷ and CSMHYD program⁷ can only predict phase equilibria below 40–50 MPa. Thus, the models cannot predict the structure changes in mixture hydrates (e.g., the transition from sI to sII in CH₄–C₂H₆ hydrate¹⁰). Furthermore, Tee et al.⁶⁹ found that the Kihara potential surface calculated from the experimental data was inconsistent with that calculated from the second virial coefficient and viscosity data, and the inconsistency was due to the pre-treatment of the model overrating the occupancy fraction of non-spherical guests in a small cage,^51,70 i.e., the Kihara potential model needs further optimization, especially in accurately describing the interaction between guest and water molecules. Accurate intermolecular potential between a guest molecule and a water molecule can be directly obtained by an ab initio quantum chemical method, and the intermolecular potential is considered to be strongly angle dependent.^71–75 The ab initio potential model can be used for predicting the hydrate number and the cage occupancies. Duan et al.⁷⁶ successfully used an atomic site–site Lennard-Jones formula plus an electrostatic term to fit the ab initio intermolecular potential energy surface of the CO₂–H₂O complex to account for the angle dependent molecular potential angle. They improved the model and predicted the equilibrium pressure of CO₂-hydrate in a wide T–P range with absolute average deviation less than 3%.

The best method for determining the hydrate formation conditions is to experimentally measure the hydrate formation at the temperature, pressure and composition of interest. However, it is impossible to satisfy the infinite number of conditions for which measurement are needed. Thus, the hydrate formation prediction methods need to interpolate between the measurements. However, such experimental measurements are both time consuming and expensive. Therefore, a comprehensive artificial neural network model (ANN model) was developed to enable the user to accurately predict hydrate formation conditions for a given gas mixture without having to make experimental measurements.⁷⁷

Previous studies on predicting CO₂ hydrate phase equilibrium are summarized in Table 1. The most recent predictions are rooted in the vdWP theory for hydrates.¹¹ Although the methods mentioned above have their individual characteristics for predicting the equilibrium conditions for forming or dissociating the different gas hydrates, they all have their limitations. To solve these problems, an integrated method combined with the above different methods must be developed.

Table 1 List of equilibrium conditions predictions for hydrates containing CO₂ via computation models

Authors	Temperature (K)	Pressure (MPa)	Study	N^a
a Number of measurements.
Deaton & Frost^184	273–283	1.3–4.3	K-charts, giving the vapor–solid equilibria for natural gases (including pure gases or gas mixtures) at lower than 273.15 K and higher than 273.15 K.	19
Carson & Katz^56	277–283	2.0–4.5	Katz method, using vapor–solid equilibrium constants to predict the hydrate formation conditions. Katz correlation is not recommended above 100–150 MPa, depending on the composition of the gas mixtures.	15
Katz^58	273–322	0.2–42.0	Method of gas-gravity plots, which relate the hydrate formation pressure and temperature to gas-gravity. The method was useful for an initial estimate of hydrate formation conditions and the prediction is rough.	128
van der Waals & Platteeuw^11			van der Waals–Platteeuw model, which was based on a statistical thermodynamic approach, accounting for the interactions between gas molecules and water molecules forming gas hydrates.
Larson^186	257–283	0.5–4.5	Predicted the equilibrium hydrate formation conditions of CO2 hydrates.	45
Miller & Smythe^192	151–193	0–0.000022	Dissociation pressure of CO2 hydrate with equations for CO2 hydrate dissociation pressures and vapor pressures.	8
Robinson & Mehta^188	274–283	1.3–4.5	The conditions for initial hydrate formation in the system of CO2–C3H8–H2O over a wide concentration range for the hydrate-water-rich liquid–gas phase region were measured and predicted in terms of solid–vapor K-factor.	7
Falabella^193	148.8–240.4	0.02–0.1	At a low pressure range, hydrates of CH4, C2H6, C2H4, C2H2 and CO2 are involved. The van der Waals–Platteeuw model was employed to predict the equilibria associated with experimental measurements.	5
Ng & Robinson^78,190	279–284	2.7–14.5	A modification of the Parrish and Prausnitz program, predicting hydrate forming conditions for pure gases in the presence of up to 20 wt% methanol solutions.	9
Holder et al.^60			Empirical correlations developed in different forms and with various numbers of parameters.
Adisasmito et al.^85	273–288	1.2–11.0	Verifying the work done by Unruh and Katz and by Berecz and Balla-Achs by experimental measurements.	9
Englezos^63	269–281	1.1–4.3	Because the solubility of CO2 in salt solutions cannot be computed accurately using rigorous thermodynamic models, thus CSMHYD in conjunction with Trebble–Bishnoi equation is adopted to predict the incipient CO2 hydrate formation pressure in NaCl solutions and the average deviation is around 7.2%.	57
Dholabhai et al.^83	273–279	1.3–2.5	Coupling model of statistical thermodynamic model of van der Waals and Platteeuw with coefficient models. Equilibrium conditions of CO2 hydrate in pure water and single and mixed electrolytes.	4
Englezos & Hall^194	275–283	1.5–4.2	CSMHYD model predicting and measuring CO2 hydrate formation pressure in electrolyte, water-soluble polymers and montmorillonite.	6
Breland & Englezos^195	275–280	1.6–3.0	An isothermal pressure search method is employed to measure the incipient equilibrium data for CO2 hydrate in glycerol solutions (10, 20, 30 mass%).	2
Tohidi et al.^81	268–284	1.0–5.0	The model based on a thermodynamic approach, in which an equation of state is combined with a modified Debye–Huckel electrostatic term, with only one adjustable parameter for the water-rich phase. Predicting phase equilibrium conditions for CO2 hydrates in presence of saline water.
Nakano et al.^82	289–294	100–500	The high-pressure phase equilibrium for CO2 in pure water and saturated liquid CO2 and Raman spectrum of CO2 hydrate.	13
Fan & Guo^196	264–284	0.5–5.0	Hydrate phase equilibria for CO2–CH4, CO2–C2H6, CO2–N2, CO2–CH4–C2H6–N2 in pure water and 10 mass% NaCl solution.	13
Wendland et al.^79	270–305	0.5–7.0	Equilibrium conditions for CO2–H2O system focusing on three- and four-phase equilibria including fluid, hydrate and ice phases. The experimental data are correlated with the equations of Clausius–Clapeyron type.	9
Seo & Lee^80,197	272–284	1.5–5.0	The three phase equilbria for aqueous phase containing CO2 and CH4 were predicted. The vapor and liquid phases were treated with SRK-EOS incorporated with the second-order modified Huron–Vidal (MHV2) mixed rule and hydrate phase with the van der Waals–Platteeuw model.	26
Duan & Sun^76,198	253–293	0.5–200	Ab initio potential model predicting initial hydrate formation conditions for CH4 and CO2. Compared to the models employing the Kihara potential or Lennard-Jones potential, atomic site–site potentials was more accurate either under low pressure or under high pressure.	20
Li & Englezos^199	298–313	5.0–11.0	SAFT equation of state was employed for the correlation and prediction of vapor–liquid equilibrium of eighteen binary mixtures. The predicted values agreed with the experimental data except for the H2O–CH3OH–CH4 at low CH3OH concentration in liquid phase of 60 wt%.	4
Bahadori & Vuthaluru^200	265–298	1.2–40.0	A novel correlation based on the extracted data from Katz gas-gravity charts was proposed to predict the hydrate formation conditions for gases with weights of 16–29, the absolute deviation in average around 0.18%.	44
Zeng & Li^201	270–282	0.8–4.0	PC-SAFT in conjunction with the van der Waals–Platteuw model and capillary Kelvin model was employed to predict CH4 and CO2 hydrates formation equilibrium conditions in porous media. The deviations for CH4 hydrate and CO2 hydrates were 1.66% and 2.76%, respectively.	18
Sloan^7	277–283	Up to 400 MPa	Presenting an alternative set of K-values for the Katz method, which are dependent upon gas composition and hydrate structures, the valid pressure up to 400 MPa.	20
Karamoddin & Varaminian^97	260–330	0–5.0	A method using PR equation of state and different mixing rules for predicting hydrate formation conditions for binary mixtures of CH4, C2H6, C3H8, i-C4H10, CO2 and H2S. The interaction parameters were optimized by using two phase equilibrium data (VLw), and then the optimized parameters were used for three phase equilibrium (VLwH) calculations.	63
Elgibaly & Elkamel^77	250–320	0.001–1000	Firstly proposing ANN compositional models to predict hydrate formation conditions based on the K-value method and gas-gravity chart method. The ANN models consist of four models. The predicted results were more accurate than those obtained by the conventional models. The new models have to be updated by being retrained by using extra collected data.	2387
Eslamimanesh et al. ^202	279–295	0–120	A thermodynamic model is proposed for representation/prediction of phase equilibria of semi-clathrate hydrates of the CO2, CH4, or N2 + tetra-n-butylammonium bromide (TBAB) aqueous solution. The van der Waals–Platteeuw (vdW–P) solid solution theory is used, revised with two modifications for evaluations of Langmuir constants and vapor pressure of water in the empty hydrate lattice, in which these values are supposed to be a function of TBAB concentration in aqueous solution. The Peng–Robinson (PR-EoS) equation of state along with re-tuned parameters of Mathias–Copeman alpha function is applied for calculation of the fugacity of gaseous hydrate former. For determination of the activity coefficient of the non-electrolyte species in the aqueous phase, the non-random two-liquid (NRTL) activity model is used.	40
Eslamimanesh et al.^203	276–294	2–500	The model based on conventional Clapeyron model. Considering that the effect of pressure on molar volume of gas hydrate could not ignored, the “Clausius–Clapeyron” equation was improved from to .	40
Shuker et al.^95	270–295	0–2.5	NN model was employed to predict hydrate formation conditions for pure gases and gas mixtures. The ANN model was relatively accurate for a given gas mixture and without having to do experimental measurements compared to the previous models of K-factor, HYDOFF, CSMHYD and HYSYS.	20
Heydari et al.^96	273–296.5	0.3–29.0	ANN models were used for prediction of hydrate formation temperature. The results included the calculation relative error and R^2 values between the experimental data and ANN predictions. The results showed that the ANN models could be improved based on the more collected data in a wider range of temperature and pressure.	167

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2.2 Experimental equilibrium conditions

The experimental equilibrium conditions are generally determined by two events during the hydrate formation: (1) initial occurrence or final disappearance of hydrate particles, and (2) a sharp decrease in pressure or a sharp increase in temperature. For the first, the pressure must be elevated above the hydrate equilibrium value, and then the hydrate formation leads to a certain meta-stable pressure. Then, the system is heated slowly or depressed slightly to dissociate the hydrate and to ensure no meta-stability. Thus, the endpoint of hydrate dissociation is ensured to be reproducible and is taken as the upper limit of the formation of meta-stability.⁷ Carson and Katz further verified the principle to judge the hydrate equilibrium point.⁵⁶ For the second, when the gas is enclosed in the hydrate, the system pressure decreases, and the disappearance of the last of the hydrate often accompanies a decrease of the slope of the pressure vs. the temperature trace. By this means, the hydrate formation equilibrium conditions can be obtained by measuring the intersection point of the cooling or heating isochore. Therefore, three primary methods (isothermal, isobaric and isochoric, respectively) were developed.

For the isothermal method, the system is first set at a pressure higher than the expected equilibrium pressure to form hydrates, and the system pressure must be maintained by an external reservoir for addition or withdrawal of gas. The pressure is reduced gradually after the hydrate formation. The equilibrium pressure can be obtained by the visual observation of the hydrate disappearance. Thus, the method requires an apparatus with windows mounted in both back and front.^7,15,78–85

For the isobaric method, the system pressure is first kept constant by an external reservoir for the addition of gas, then the temperature is decreased until a significant addition of gas is noted from the external reservoir, which indicates the hydrate formation. Then, the temperature is slowly increased to dissociate the hydrates. During the process of hydrate dissociation, the system pressure must be kept constant. As the last of the hydrate disappears, the point is taken as the equilibrium temperature of the hydrate formation at the constant pressure. Similar to the isothermal method, the isobaric method must use an apparatus with windows. Fig. 1 shows a typical P–T diagram of the three-phase and four-phase equilibria in the system of CO₂–H₂O by the method.⁷⁹

Fig. 1 P–T diagram of the three-and four-phase equilibria in the carbon dioxide–water system: comparison of the results reported by Wendland et al.⁷⁹ (·) with literature data by Kuenen and Robson¹⁸³ (⊗); Deaton and Frost¹⁸⁴ (×); Unruh and Katz¹⁸⁵ (+); Larson¹⁸⁶ (□); Takenouchi and Kennedy¹⁸⁷ (○); Robinson and Mehta¹⁸⁸ (Δ); Vlahakis et al.¹⁸⁹ (▲); Ng and Robinson¹⁹⁰ (◊); Nakayama et al.¹⁹¹ (□); Adisasmito et al.⁸⁵ (▼).

The isochoric method describes how the pressure in a closed vessel changes with the temperature and the phase transition. The temperature is first lowered from the vapor–liquid (V–L) region, and the isochoric cooling leads the pressure to slightly decrease. At a certain temperature, the hydrate forms, causing a remarkable pressure drop. Then the temperature is slowly increased to dissociate the hydrates, resulting in the pressure rising quickly. Then, we can find the point that is the intersection of the hydrate dissociation trace with the initial cooling trace, and this point is taken as the equilibrium point for the hydrate dissociation. The isochoric method is commonly used for hydrate formation at high pressure, as it does not need to be visually observed.

Based on the isochoric method, a method of isochoric step-heating (T-cycle method) was developed.^19,86–89 The experimental results obtained by this method are quite reliable and repeatable. However, it takes more than 24 hours for a measurement.⁸⁶ Herri and Kwaterski⁹⁰ improved the isochoric step heating method by sampling small quantities of gas and liquid during the heating procedure, and analyzing them by gas chromatography, ionic chromatography and refractive index measurement. By the mass balance, the improved method (a modified step heating method) allows one to determine the gas and hydrate compositions.

Differential scanning calorimetry (DSC) has also been used to determine binary phase (solid/solid or solid/liquid) equilibrium data in various fields.⁹¹ Recently, DSC was applied to determine the equilibrium conditions of hydrate.^92–94 The key is calibrating the temperature by measuring the melting temperatures of pure materials and then computing a correction function by taking into account the deviation between the measurements and the known melting temperatures at different heating rates within a special temperature range. Once the temperature calibration is complete, the DSC software can automatically correct the measured temperature according to the correction function. The method of DSC requires careful calibration, and the sample mass and heating rate must be chosen within reasonable limits.

Table 2 shows previous hydrate equilibrium measurements involving pure CO₂ or gas mixtures containing CO₂ in pure water, or electrolyte solution, or porous media systems. Presently, the hydrate equilibrium measurements mainly relate to the sequestration of pure CO₂ in the seafloor, while CO₂ separation and capture pertains to gas mixtures containing CO₂. However, due to the equilibrium equipment limitations, the measurements are operated at a relatively narrow range of pressures. The equilibrium data at higher pressure (for example, >100 MPa) still needs computational models. As mentioned in Section 2.1, more accurate prediction needs more experimental data. The ANN method should be further developed because it integrates four different models and saves computational time across a wide range of temperatures and pressures.^95–98

Table 2 List of experimental measurements of equilibrium conditions for hydrates containing CO₂

Author(s)	System	Study
Hashimoto et al.^102	H2–CO2–THF–water	Phase equilibria and Raman spectroscopic analysis for gas hydrate.
Shin et al.^103	CO2–3M1B–water	Thermodynamic stability, spectroscopic identification and cage occupation of binary CO2 hydrates.
	CO2–THF–water
	CO2–DXN–water
Kang & Lee^21	CO2–N2–water	Phase equilibrium measurements for CO2 recovery from flue gas using gas hydrate.
	CO2–N2–THF–water
Dholabhai et al.^84	CO2–water	CO2 hydrate equilibrium conditions in aqueous solutions.
	CO2–electrolytes–water
	CO2–methanol–water
Dholabhai & Bishnoi^204	CO2–CH4–electrolytes–water	Hydrate equilibrium conditions in aqueous electrolyte solutions.
Matsui et al.^205	CO2–C2H6–water	Phase equilibrium for binary hydrate systems.
	CO2–CF4–water
Makino et al.^206	CO2–CP–water	Phase equilibrium and structural transition in the CO2–CP mixed hydrates.
Yang et al.^207	CO2–Doda glass–water	Characteristics of CO2 hydrate formation and dissociation in glass beads and silica gel.
	CO2–silica gel–water
Li et al.^116	CO2–H2–TBAB–water	Phase equilibrium conditions for CO2–H2–TBAB–water mixed gas hydrate.
Kumar et al.^105	CO2–H2–C3H8–water	Hydrate phase equilibrium conditions for CO2–H2–C3H8–water mixed gas hydrate.
Belandria et al.^208	CO2–N2–water	Experimental and predicted hydrate phase equilibrium conditions for CO2–N2–water gas hydrate.
Lee et al.^209	CO2–electrolyte–water	Phase equilibria and kinetic behavior of CO2 hydrate in electrolyte and porous media solutions.
	CO2–porous media–water
Sabil et al.^210	CO2–THF–water	Phase equilibria in ternary system of CO2–THF–water.
Maekawa^211	CO2–alcohols–water	Hydrate equilibrium conditions for CO2 hydrates in presence of alcohols, glycols, and glycerol.
	CO2–glycols–water
	CO2–glycerol–water
Kim et al.^155	CO2–H2–TBAB–water	Hydrate-based CO2 capture for pre-combustion process in IGCC plant.
Lin et al.^212	CO2–TBAB–water	Hydrate phase equilibrium and dissociation enthalpy for CO2–TBAB hydrate.
Li et al.^120	CO2–TBAB (TBACl, TBAF)	Hydrate phase equilibrium for CO2 hydrate in presence of TBAB, TBACl, TBAF.
Kang et al.^100	CO2–N2–THF–water	Hydrate phase equilibrium for CO2–N2–THF hydrate.
Zhang et al.^108	CO2–H2–CP–water	Thermodynamic analysis of hydrate-based pre-combustion capture of CO2.
Sugahara et al.^213	CO2–H2–water	Hydrate phase equilibria for CO2–H2 hydrate.
Park et al.^214	CO2–N2–silica gel–water	Hydrate phase equilibrium and NMR analysis for CO2–N2–silica gel hydrate.
Mayoufi et al.^215	CO2–TBPB–water	Hydrate phase equilibria for TBPB hydrate and CO2–TBPB hydrate.
Mayoufi et al.^216	CO2–TBMAC–water	Hydrate phase equilibrium for CO2–TBMAC hydrate.
Lee et al.^22	CO2–H2–THF–water	Hydrate phase equilibrium, gas consumed and CO2 separation efficiency in the process of pre-combustion capture of CO2.
Lee et al.^118	CO2–CH4–N2–water	Thermodynamic stability, spectroscopic identification, and gas storage capacity of CO2–CH4–N2 hydrate.
Nakano et al.^82	CO2–water–liquid CO2	High-pressure phase equilibrium and Raman spectroscopic analysis for CO2 hydrate.
Ohgaki et al.^217	CO2–CH4–water	Hydrate phase equilibrium for CO2–CH4 gas hydrate and replacement of CH4 by CO2.
Bruusgaard et al.^218	CO2–CH4–water	Hydrate phase equilibrium for CO2–CH4 hydrate.
Adisasmito et al.^85	CO2–CH4–water	PVT studies on dissociation conditions of CO2–CH4 hydrate.
Belandria et al.^219	CO2–CH4–water	Phase equilibria in the CO2–CH4–H2O system has measured by the method of isochoric pressure-search method in the conditions of 233–373 K and up to 60 MPa.
Beltran & Servio^220	CO2–CH4–water	PVT studies on dissociation conditions and composition measurement of gas phase.
	CO2–CH4–neohexane-emulsion
Fan et al.^221	CO2–CH4–SDS–water	Experimental and modeling studies on the hydrate formation of CO2 and CO2-rich gas mixtures.
Deschamps & Dalmazzone^222	CO2–N2–TBAB–water	Dissociation enthalpies and phase equilibrium for TBAB semi-clathrate hydrates of N2, CO2, N2–CO2 and CH4–CO2.
	CO2–CH4–TBAB–water
Fan et al.^223	CO2–H2–TBAB–water	Efficient capture of CO2 from simulated flue gas by formation of TBAB or TBAF semi-clathrate hydrates.
	CO2–H2–THF–water
Meysel et al.^224	CO2–N2–TBAB–water	Incipient equilibrium conditions for the formation of semi-clathrate hydrates from quaternary mixtures of (CO2–N2–TBAB–H2O).
Mohammadi et al.^225	CO2–N2–TBAB–water	Experimental data for the hydrate dissociation conditions for the system comprising mixtures of CO2 (0.151/0.399 mole fraction) + N2 (0.849/0.601 mole fraction) + TBAB (0.05/0.15/0.30 mass fraction) in the conditions of 277.1–293.2 K and up to 16.21 MPa.
Kamata et al.^32	CO2–H2S–TBAB–water	A high-pressure vessel of separation gas from gas mixture.
Belandria et al.^226	CO2–H2–water	Molar compositions of carbon dioxide (and hydrogen) in the gas phase in equilibrium with gas hydrate and aqueous phases were measured for various (H2–CO2) gas mixtures t water systems in the temperature range of 273.6–281.2 K at pressures up to ∼9 MPa. The compositions of the gas phase were measured using an isochoric technique, in combination with the ROLSI capillary gas-phase sampling and a gas chromatography technique.

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3. Additives for forming CO₂ hydrates

In pure water, CO₂ hydrate needs high pressure and low temperature to form, and the extreme operation conditions lead to high costs in industrial applications. The hydrate formation rate is quite slow. Therefore, additives that can moderate the hydrate formation conditions and promote hydrate formation are developed for the process of forming CO₂ hydrates.

The additives are classified into thermodynamic and kinetics types. The thermodynamic additives have the tendency to moderate the equilibrium conditions to higher temperature or lower pressure, and they often consist of organic compounds, including tetrahydrofuran (THF), propane (C₃H₈), cyclopentane (CP), and tetra-n-butyl-ammonium (bromide, fluoride or chloride; TBAB, TBAF and TBACl, respectively). Kinetic additives accelerate the hydrate formation and they typically consist of surfactants, including sodium dodecyl sulfate (SDS), and dodecyl trimethyl ammonium chloride (DTAC).^7,99

Tetrahydrofuran (THF) solution has proved to be capable of significantly reducing the hydrate formation pressure at a given temperature.^100–102 Kang et al.,¹⁰⁰ Linga et al.,¹⁰¹ and Hashimoto et al.¹⁰² found that the equilibrium pressure of hydrates for both CO₂–N₂ and CO₂–H₂ mixtures in the presence of THF solution was considerably lower than that without the additive, and THF of 1.0 mol% proved to be the optimal concentration for CO₂ separation from the mixture.^22,100 However, THF can form sII THF-hydrates, competing with CO₂ for occupying large cavities (5¹²6⁴).^22,103 Therefore, THF cannot remarkably improve the gas consumption and CO₂ separation efficiency although it can moderate the conditions of forming gas hydrates for either CO₂–N₂ or CO₂–H₂.¹⁰⁴ Kumar et al.¹⁰⁵ compared the hydrate equilibrium conditions for CO₂–H₂ and CO₂–H₂–C₃H₈, and found that 3.2 mol% C₃H₈ added to a CO₂–H₂ mixture reduced the pressure by approximately 50%. They verified that the addition of C₃H₈ into the CO₂–H₂ mixture reduced the hydrate phase equilibrium pressure without comprising the CO₂ recovery and found that H₂ existed in both sI hydrates formed by CO₂–H₂ mixture and sII hydrates formed by CO₂–H₂–C₃H₈ mixture (C₃H₈ of 2.6 mol%) by Raman spectroscopy.¹⁰⁶ Cyclopentane (CP) can also reduce the hydrate phase equilibrium pressure. Zhang et al.^107,108 found that the equilibrium of ternary CO₂–H₂–CP hydrates was significantly lower than that of ternary CO₂–H₂–THF at a vapor-phase CO₂ mole fraction of 0.3204. However, because the CO₂–H₂–CP forms sII hydrate and CP occupies the large cavities of 5¹²6⁴ preferentially, CO₂ can only compete with H₂ to occupy the small cavities of 5¹², reducing the selectivity of CO₂ over H₂ in the hydrate phase.¹⁰⁸ TBAB in water forms a semi-clathrate hydrate under moderate conditions. In the semi-clathrate hydrate system, the anions (Br⁻) are strongly incorporated with the host water lattice and a single TBA cation (TBA⁺) occupies four cavities, leaving dodecahedral cavities for small gaseous molecules.^{9,88,109–112} Arjmandi et al.,⁸⁸ Oyama et al.¹¹³ and Duc et al.²⁴ found that the equilibrium conditions for binary CO₂–TBAB hydrates were considerably lower than those for pure CO₂ hydrate. Furthermore, the hydrate phase equilibrium pressure shifts to lower with the increase of the TBAB concentration.^32,114,115 TBAB of 0.29 mol% is considered as the optimum to recover CO₂ from either flue gas or IGCC synthesis gas because TBAB of more than 0.29 mol% makes no more contribution to the CO₂ recovery.^{19,23,88,116,117} However, adding TBAB reduces the gas consumption because the TBA⁺ occupies the big cavities in the sc hydrates. Therefore, there is some controversy over the use of TBAB as a gas hydrate promoter. Besides, both TBAF and TBACl can also reduce the hydrate equilibrium conditions.^118–121 TBAF is rarely used because it is much more expensive than TBAB, although the effect of reducing pressure of TBAF is superior to that of TBAB.

SDS is widely used as a kinetic additive. Zhong and Rogers¹²² studied the SDS effects on gas hydrate formation and found the hydrate formation rate can be increased by multiple orders of magnitude in the presence of SDS solution or a related surfactant solution. Tajima et al.,¹²³ Li and Chen,¹²⁴ Rossi et al.,¹²⁵ and Torre et al.¹²⁶ further verified that the SDS of critical micelle concentration (CMC) has the best effect on enhancing the hydrate formation rate.

However, either using thermodynamic additives or kinetic additives cannot resolve all the problems (extreme conditions, low gas consumption, low hydrate formation rate and low CO₂ recovery) of hydrate-based CO₂ separation and capture from the gas mixtures. Therefore, researchers have focused on the synergistic effect of thermodynamic additives and kinetics additives. Li et al.¹¹⁷ added DTAC into the 0.29 mol% TBAB solution to investigate hydrate-based CO₂ capture from the flue gas and considerably improved the formation conditions, the formation rate and CO₂ recovery. Ricaurte et al.¹²⁷ used THF in combination with SDS to investigate CO₂ removal from a CH₄–CO₂ gas mixture by hydrate formation and found that the combinational additive decreased the hydrate formation pressure and improved the selectivity of CO₂ capture. The combination of two thermodynamic additives or two kinetic additives has also been studied. Ding¹²⁸ used SDS associated with an anionic fluorosurfactant (FS-62) (FS-62/SDS: 100/1000 ppm) as a joint additive to hydrate-based CO₂ capture from a CO₂–N₂ mixture, but its effect on raising the gas consumption was limited. However, Li et al.¹⁹ found that the addition of CP into a 0.29 mol% TBAB solution could considerably improve hydrate-based CO₂ separation from IGCC synthesis gas, and the result was further verified by powder X-ray diffraction (PXRD) spectroscopy.¹⁸

In fact, additives are still in the course of being screened, i.e., which kind of additive is the best for hydrate-based CO₂ separation and capture from the gas mixtures is still undetermined. Currently, THF and TBAB are two of the most popular thermodynamic additives. Much work is needed to further screen additives, especially to screen one that can remarkably enhance the gas uptake.

4. Molecular-level measurements of the hydrates containing CO₂

The information (identification of the hydrate structure type, lattice parameters, guest occupancy and position in the cavity) of the hydrates structure (containing CO₂) can be accurately obtained using molecular-level measurement methods, including diffraction and spectroscopic methods. The hydrates containing CO₂ are measured in the form of protecting samples under liquid nitrogen (LN) (only for spectroscopic methods) or in situ. For in situ measurements, the original formed hydrates are detected, through a silica window with high-purity silica or a sapphire window, by a set of fiber optics or microprobe incorporated into the spectrograph. For samples, the hydrate samples are firstly handled and quickly transferred in dry LN vapor before being mounted for detection on a pre-cooled stage.

4.1 Diffraction methods

The diffraction methods consist of X-ray diffraction (XRD) and neutron diffraction. The earliest and most comprehensive diffraction method is XRD.^49,129,130 The hydrate structure, lattice constants and composition of hydrates containing CO₂ can be determined from crystal XRD data at a certain temperature.^131–134 Through XRD analysis, it was confirmed that CO₂ molecules were trapped in the small 5¹² cavities of the CO₂–THF binary sII hydrates.¹⁰³ In a ternary system, the XRD patterns showed that CO₂ molecules occupy both sI and sII CO₂–H₂–C₃H₈ hydrates at 5 MPa and 253.15 K.¹⁰⁶ For binary systems, e.g. CO₂–H₂ or CO₂–N₂, the XRD patterns showed that the gas hydrates were exemplary sI crystal structures.¹³⁵ However, the hydrate structures might shift with the change of CO₂ in the binary systems. The CO₂–N₂ gas mixture containing 3–20 mol% CO₂ formed sI hydrates, while that containing 1 mol% CO₂ formed sII hydrate, determined by analyzing the XRD patterns.¹³⁶ In 0.29 mol% TBAB solution in the presence of CP, CO₂ occupied sII and sc hydrates cavities. It is worthwhile to note that for sample detection, due to the high sensitivity of the XRD measurements, the samples must be carefully handled to remain flat on the sample disk.

Neutron diffraction studies are able to determine the positions of the guest and the host in a hydrate crystal, to trace the structural changes during the hydrate formation, and to measure the extent of guest–host interactions in a hydrate lattice.^137–142 However, few studies on CO₂ hydrate via neutron diffraction are reported. Henning et al.^139,143 observed complete conversion from the hexagonal ice to the sI type CO₂ hydrate as the temperature of the sample was slowly increased through the melting point of D₂O.

4.2 Spectroscopic methods

Two main types of spectroscopy have been used to investigate the hydrates containing CO₂: Raman spectroscopy and nuclear magnetic resonance (NMR) spectroscopy.

Raman spectroscopy is a good way to identify the hydrate structure and composition because the Raman peak is determined by the inter-atomic vibration.^144–146 Hydration number and relative cavity occupation can also be measured via Raman spectroscopy.^{103,147–152} Fig. 2 shows a typical Raman spectrum of CO₂.¹⁵³ The split peaks (located at 1276 cm⁻¹ and 1384 cm⁻¹, respectively) are caused by Fermi resonance effect corresponding to C–O symmetric stretching (ν₁) and O–C–O bending (2ν₂) modes of CO₂ molecules. Due to the various conditions of measurements and hydrate structures, the Raman shifts of CO₂ molecules may be changes in a small range.^103,106,154 The Raman peaks of molecules of CH₄, N₂, H₂ are around 2910 cm⁻¹, 2325 cm⁻¹ and 4120 cm⁻¹, corresponding to their vibration modes of ν₁ sym C–H stretching, ν₁ sym N–N stretching and H–H oscillation (pure molecular vibration). Thus, it is quite simple to identify CO₂ from CO₂–CH₄, CO₂–N₂, and CO₂–H₂ gas mixtures via Raman spectra.^{102,106,154–158}

Fig. 2 Raman spectra of solvated CO₂ and CO₂ hydrates. The black dotted lines located at the solvated CO₂ peak positions show the peak shift associated with hydrate formation.¹⁵³

NMR can be used to identify the hydrate structure and quantify the relative cavity occupancy. Presently, the study of NMR on hydrate-based CO₂ separation focuses on the spectra of ¹H, ¹²⁹Xe and ¹³C.^159–162 Among all the studies, ¹H NMR has been used for ethane, propane, and isobutene hydrates;^{159 129}Xe NMR has been used for identifying ratios of xenon atoms in small and large cavities;^160–162 and ¹³C NMR has been applied to study hydrates of CO₂, CH₄ and C₃H₈.^{149,163–165} Seo et al.¹⁶⁶ and Seo and Lee¹³⁶ found, based on the ¹³C NMR spectra for CO₂, that CO₂ molecules occupied the large cavities of sI, and as the CO₂ increased in the CO₂–N₂ vapor phase, the role of stabilizing both small and large cavities was transformed from N₂ to CO₂ molecules.

5. Process and apparatus

5.1 Process

The processes of hydrate-based CO₂ separation and capture from gas mixtures are investigated. During the process, the gas continuously dissolves into the solution and forms gas hydrates under reasonable conditions, resulting in the decrease of the pressure in the system. Thus, the gas from the supply vessel must be introduced into the system to maintain the pressure. Hydrate formation is an exothermic process, causing the system temperature to rise slightly when forming large amount of hydrates. Once the hydrate formation rate decreases, the system temperature returns to the setting temperature because of the heat transfer. The CO₂ separation efficiency changes with the continuous consumption of the gas in the process according to the formula of and (n is the number of moles, and the superscripts of H and G express hydrate phase and gas phase, respectively. The subscripts of CO₂ and other express CO₂ and other component, respectively).¹⁶⁷

Duc et al.²⁴ conducted experiments to separate CO₂ from a CO₂–N₂ mixture in the presence of TBAB at under suitable operation conditions, and they proposed a continuous multi-stage separation process. However, hydrate-based CO₂ separation becomes more difficult and the CO₂ recovery becomes lower with the decrease of CO₂ concentration in the gas mixtures, and the separation process with the single hydrate method cannot separate CO₂ from gas mixtures such as flue gas or fuel gas completely and efficiently.¹⁶⁸ Thus, various hybrid processes were proposed. In the hybrid processes, CO₂ was first separated by the hydrate method, leaving lean-CO₂ gas mixtures to be separated by other methods such as chemical adsorption, cryogenic separation, and membrane separation. Linga et al.¹⁶⁷ proposed a hybrid process that combined the hydrate method and membrane separation method for separating CO₂ from flue gas and fuel gas, respectively. Xu et al.¹⁶⁹ conducted experiments to separate CO₂ from IGCC synthesis gas by a hybrid process of two-stage hydrate separation in combination with a chemical absorption separation. Surovtseva et al.¹⁷⁰ designed a process combined cryogenic and hydrate methods to capture CO₂ from IGCC flue gases. Pure CO₂ (>95%) could be obtained by the above hybrid processes, which were considered to be more efficient and economic compared to conventional CO₂ separation methods.^{104,167,169–171} However, two problems retard even pilot-scale application of the hybrid processes: the gas hydrate capacity per volume of water (L) and the hydrate formation rate per unit of time (h).

The hydrate-based CO₂ separation process does not use large absorber towers or steam-reboiled regenerators, so both the capital and operating costs of the process are primarily in refrigeration and compression systems. Before designing parameters for the process, researchers must focus on some parameters and key processes:

(1) Hydrate number of the hydrate: the number of water molecules required for removing a mole of CO₂ has a great effect on the heat removal requirements in the reactor, and therefore the size of the refrigeration system.

(2) Slurry concentration: the amount of free water circulating in the system must be heated and cooled, and this also affects the size of the refrigeration system.

(3) Temperature of the reactor: the performance of the system is greatly affected by the temperature in either CO₂ removal or heat transfer.

SIMTECHE designed a test apparatus for a plant of coal feed 5 kt per day. The mixed gases and conditioned water were metered to a reactor, mixing with the reactants, and the reactants flowed through a tail tube (ID: 4.8 mm). Under the conditions of around 10 MPa and 269 K and at the feed rates of CO₂ (0.94 mol min⁻¹) and H₂O (28 mol min⁻¹), the test results showed that the hydrate production rate was 0.271 mol min⁻¹, which matched the design requirement.¹⁷² However, being subject to hydrate formation route, CO₂ separation efficiency and gas–liquid-hydrate separation in a special pressurised vessel, there are as yet no mature hydrate-based CO₂ separation process being applied in industry, even in a small-scale pilot application. Thus, we have a long way to go to achieve the final goal of hydrate-based CO₂ separation and capture from gas mixtures in industry, especially for the aspects of the gas–liquid-hydrate separation processes, improvement of the CO₂ separation efficiency and optimization of the process parameters.

5.2 Apparatus

Various apparatuses were also developed along with proposing processes for CO₂ separation and capture, and the development of the apparatus mainly focuses on the innovation of a mode of hydrate formation and design of a continuous flow reactor. The modes of hydrate formation generally include stirring, bubbling, and spraying, which can well mix the gas and water or solution. Szymcek et al.¹⁷³ designed a pilot-scale continuous-jet hydrate reactor (CJHR). A multiple capillary was mounted in the CJHR to maximize the surface area of interaction between reactants during the hydrate formation. Furthermore, the new design overcame the product-limit aspects of hydrate production, decreasing the amounts of unconverted CO₂ and H₂O. Li et al.¹⁷⁴ also invented a set of batch-flow apparatuses to capture CO₂ from the flue gases. The water (or solution) was jetted into the reactor filled with gases. The hydrate slurry formed in the reactor flowed into a decomposing tank via a special device. Xu et al.²³ designed a visual bubble reactor and conducted experiments of CO₂ capture from CO₂–H₂ mixture. The visual bubble reactor had a volume of 40 L (4 m in height and 0.01 m² in area), which was around 100 times as big as the general reactor in the laboratory. Via the reactor, while the gas bubbles move from the bottom to the top, the whole gas bubbles could convert to gas hydrates. Castellani et al.¹⁷⁵ developed a new apparatus to capture CO₂. The water (or solution) and the gas mixtures are sprayed from the top and the bottom into the reactor via arranged nozzles. Linga et al.¹⁷⁶ designed a new apparatus in which stirring and bubbling were combined together to enhance the contact of gases with water. Via the new apparatus, the hydrate formation rate, the gas uptake and the CO₂ recovery from the flue gases or the fuel gases are considerably improved compared to the results that were obtained in a smaller scale stirred tank reactor.^101,171 Yang et al.^177,178 developed a set of continuous flow reactors (Fig. 4) for CO₂ hydrate formation based on a block flow diagram of the SIMTECHE CO₂ capture process (Fig. 3). Using the continuous flow reactor, the effects of the gas carrier, the fluid velocity, the slurry concentration, and the temperature on the hydrate formation rate are investigated. The results indicated the reactor brought vigorous inter-phase mixing, reducing the heat and mass transfer resistances, and ultimately ensuring the global reaction rate to approach the intrinsic CO₂ hydrate formation rate under industrially relevant processing conditions.

Fig. 3 Block flow diagram of the SIMTECHE CO₂ capture process (for IGCC applications).¹⁷⁷

Fig. 4 Basic layout of flow rate reactors in the ETM system.¹⁷⁷

Although it has been subject to gas–solid–liquid separation in special pressured vessel and low CO₂ recovery, the continuous CO₂ separation process is still immature. Furthermore, no complete set of equipment has been developed and utilized for CO₂ hydrate-based separation and capture until now. Therefore, it is imperative to further develop the process associated with developing apparatus for hydrate-based CO₂ separation and capture.

6. Cost and comparison

Reduction of anthropogenic CO₂ emissions into the atmosphere can be obtained by different means, which have been summarized by Professor Yoichi Kaya of the University of Tokyo and can be expressed as follows:¹⁷⁹where is the total CO₂ released to the atmosphere, POP is population, GDP/POP is per capita gross domestic product and is a measure of living, BTU/GDP is energy consumption per unit of GDP and is a measure of energy intensity, is the amount of CO₂ released per unit of energy consumed and is a measure of carbon intensity, and is the amount of CO₂ stored/sequestrated in biosphere and geosphere sinks. Reducing the population or the standard of living is not likely to be considered. Therefore, only three methods, including reducing energy intensity, reducing carbon intensity and carbon storage are employed. Geosphere sinks have the capability to store large quantities of CO₂ on a geologic time scale of thousands of years, and the most important issue that limits the use of the geological sinks as mitigation options is cost.¹⁸⁰

The cost of disposing of CO₂ consists of four factors, including separation (i.e. capture/separation of CO₂ from combustion gases), compression, pipelining and injection (e.g. pumping and disposal wells). Capture/separation costs represent the largest financial impediment among the four factors, accounting for approximately three fourths of the total costs. Hence, it is necessary to develop efficient, cost-effective transportation and capture/separation technologies to allow large-scale use of geologic sinks. New CO₂ separation technologies such as gas hydrate and membrane are developed under the driving force.

Spencer et al.¹⁸¹ first proposed a relatively comprehensive economic analysis on the basis of a hybrid process of on-stage hydrate in conjunction with chemical absorption proposed by SIMTCHE. The engineering basis of the analysis was divided into four parts: on-stream factor, major equipment costs estimated from ASPEN “Icarus”, installation factor, and contingencies. In the process, two-stage hydrate separation was carried out at 22 °F and 108 to 625 psia. Under the conditions for 90% removal of CO₂, the cost for capturing one tonne CO₂ from the flue gas was about 18 dollars. The cost was much lower than those by chemical absorption, membrane, cryogenic separation and solid physical adsorption. According to Wong et al.'s analysis, the range for the cost of capturing CO₂ from flue gas using amine absorption is $30–$50 per tonne (t) of CO₂, and the costs for the other technologies such as solid physical adsorption (including pressure swing adsorption (PSA) and temperature swing adsorption (TSA)), cryogenic separation, membranes, hybrid membrane/amine processes, electrical swing adsorption (ESA) and sorbent energy transfer system are higher.^179,182 The gas hydrate technology has a certain advantage for the economy, however, there are some barriers to the technology, including the ability to release CO₂ from the hydrate in an energy efficient manner, efficient capture of CO₂, stale pre-hydrate, and trace contaminant interference with hydrate formation. The barriers limit the further development of the gas hydrate separation technology. Until now, there are quite few publications reporting on the process of CO₂ separation by gas hydrate and the relevant cost analysis. Thus, in order to evaluate the CO₂ separation cost by gas hydrate, the further development of gas hydrates, including kinetics and thermodynamics, is necessary, as well as CO₂ hydrate separation processes and equipment.

7. Conclusions

Hydrate-based CO₂ separation and capture from the gas mixtures containing CO₂ is considered as a new technology to reduce anthropogenic CO₂ emissions, and it is being extensively studied. In this work, we comprehensively discuss the hydrates containing CO₂ formation equilibrium conditions, hydrate formation promoters, molecular-level measurement methods, the hydrate-based CO₂ separation process and the relative equipment based on previous studies.

The present computation models are mainly based on the van der Waals–Platteeuw model and are developed according to the various revisions of relevant parameters. Although the models are considered to be increasingly accurate, they still have individual limitations such as the limitation of pressure range and the dependency on the experimental data. ANN, a new computation model consisting of the four conventional models, has been developed to save computation time and to obtain more accurate predictions. The experimental data for equilibrium conditions for hydrates containing CO₂ or gas mixtures have been obtained in different systems, including pure water, solutions with additives, and solutions with electrolytes in different measurement methods. The numerous data are the basis for hydrate-based CO₂ separation and capture.

Neither thermodynamic additives nor kinetic additives can resolve all the problems of hydrate-based CO₂ separation, thus, combination additives have developed. It was found that the combination additives of THF and SDS, TBAB and CP, THF and C₃H₈, etc., can considerably improve the CO₂ separation compared to the single additives. However, until now, there is no conclusion to determine which kind of additive is the best for hydrate-based CO₂ separation and capture from the gas mixtures.

The hydrate structures, compositions and the cage occupancies can be identified by diffraction methods and microscopic methods. However, due to the limitation of sampling, the molecular-level measurements are difficult to conduct. Thus, it is necessary to develop new apparatus or new ways to make the measurements simpler.

Because the equilibrium conditions become extreme as CO₂ decreases in gas mixtures such as CO₂–H₂ or CO₂–N₂, it is quite difficult to thoroughly separate and capture CO₂ from the gas mixtures by the hydrate method. Thus, integrated separation technologies such as hydrate/chemical absorption, hydrate/membrane and hydrate/cryogenic have been developed, and the experimental results indicate almost all the CO₂ can be completely separated and captured. But, being subject to the hydrate formation method, CO₂ separation efficiency and gas–liquid-hydrate separation in a special pressured vessel, there are no mature hydrate-based CO₂ separation processes that are applied in industry, even in small scale–scale pilot applications.

New apparatuses that can promote the gas hydrate formation have been developed, including a new type of reactor, the continuous flow reactor. The objective of developing a new apparatus is expected to bring the hydrate-based CO₂ separation from experiment to industrial practice. However, the present apparatus does not yet match the requirements.

Acknowledgements

This work was supported by the National Science Foundation for Distinguished Young Scholars of China (51225603), the Science & Technology Program of Guangzhou City (2012J5100012), and the National Natural Science Foundation of China (51106160, 51376184, 21306194). We gratefully acknowledge each of these supporting agencies.

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