SDS) on the Thermodynamic and

Article pubs.acs.org/IECR

Effects of Additive Mixture (THF/SDS) on the Thermodynamic and Kinetic Properties of CO2/H2 Hydrate in Porous Media Mingjun Yang,† Weiguo Liu,† Yongchen Song,*,† Xuke Ruan,‡ Xiaojing Wang,† Jiafei Zhao,† Lanlan Jiang,† and Qingping Li§ †

Key Laboratory of Ocean Energy Utilization and Energy Conservation of the Ministry of Education, Dalian University of Technology, Dalian 116024, China ‡ Key Laboratory of Renewable Energy and Gas Hydrate, Guangzhou Institute of Energy Conversion, Guangzhou 510640, China § China National Offshore Oil Corporation Research Center, Beijing 100027, China ABSTRACT: The separation of CO2 from fuel gas (CO2/H2) as hydrates was studied. In this investigation, the effects and mechanism thereof of the additive mixture (1, 2, 3, and 4 mol % tetrahydrofuran (THF), with 1000 mg/L sodium dodecyl sulfate) on the thermodynamic and kinetic properties of the hydrate in porous media were measured using an isochoric method, keeping the volume constant. The experimental results show that an increasing THF concentration increases the driving force for hydrate formation and decreases the hydrate induction time. The Langmuir constants of H2 and CO2 showed that H2 may occupy the small cavities of s-II hydrate in the H2−CO2−THF−H2O system. The presence of THF results in a drastic decrease of the hydrate phase equilibrium pressure. Higher THF concentrations correspond to lower hydrate phase equilibrium pressures, but the decrease in pressure with concentration slows when the THF concentration exceeds 3 mol %. An improved thermodynamic model was used to predict the hydrate phase equilibrium, and the calculations agreed well with the experimental data.

1. INTRODUCTION The increase in CO2 concentration in the atmosphere has enhanced the “greenhouse effect”. The International Energy Agency (IEA) predicted that the total cost of climate change control will decrease significantly with the application of CO2 capture and storage (CCS).1 Considering that fossil fuel electric power plants are producing approximately one-third of all CO2 emissions worldwide, the disposal of their CO2 emissions has become an issue of international concern. The first step of CCS is to separate CO2 from the flue gas (CO2/N2) or fuel gas (CO2/ H2), corresponding to post- and precombustion capture, respectively. Hydrate-based CO2 separation is a promising option for fossil fuel power plants. It is a novel concept entailing the formation of hydrates to trap CO2 molecules in an icelike crystalline lattice.2,3 The U.S. Department of Energy (DOE) considers it the most promising long-term CO2 capture technology because of its small energy penalty.4 Laboratory work at Los Alamos National Laboratory (LANL) has shown that hydrates can be produced in a flow-through system, which is essential for hydrate technology industrial implementations.5 Hydrate-based CO2 separation technology can be used in the precombustion capture of CO2, which refers to the separation of CO2 from the CO2/H2 mixture gas emitted from an integrated gasification combined cycle (IGCC) plant. Many investigations have been conducted to mitigate hydrate formation conditions and increase the hydrate formation rate and gas capacity. Some additives can reduce the hydrate equilibrium conditions, such as tetrahydrofuran (THF),6,7 tetrabutylammonium bromide (TBAB),8,9 cyclopentane (CP),10−12 and cyclobutanone.13 Linga et al. formed gas hydrates from CO2/H2 gas mixtures in a semibatch stirred vessel, finding that the presence of THF not only reduces the hydrate equilibrium pressure but also reduces © 2013 American Chemical Society

the induction time and increases the hydrate growth rate. Later, they reported that THF is superior to TBAB and tetrabutylammonium fluoride (TBAF) for gas uptake and CO2 recovery from CO2/H2 gas mixtures.14,15 Zhang et al. tested the hydrate phase equilibrium of CO2−H2−CP−H2O systems and proposed a staged-separation scheme based on the experimental data.10 Kumar reported the thermodynamic and kinetic properties of CO2/H2 and CO2/H2/C3H8 hydrates. He found that the addition of 2.5 mol % C3H8 in the fuel gas mixture can reduce the hydrate formation pressure, and the power penalty for a 500 MW power plant is estimated to be approximately 2.5% of the power output when using his three separation stages.16 Sabil et al. proved that THF significantly reduces the hydrate equilibrium pressure and that the equilibrium conditions are dependent on the ratio of CO2 and H2 in the gas mixture.17 Kim et al. investigated the effect of TBAB on the separation of CO2 from CO2/H2 gas mixtures via hydrate crystallization and found that the phase equilibrium conditions shifted to milder conditions as the amount of TBAB increased up to 3.0 mol %.18 Li et al. also reported that the hydrate equilibrium pressures of CO2−H2− TBAB−H2O are much lower than those of CO2−H2−H2O at the same temperature and that the pressure decreases with increasing TBAB concentration.19 Mohammadi et al. concluded that the optimal TBAB concentrations for the promotional effect on H2 hydrate are 0.100 and 0.150 mass fraction and that the effect decreases gradually after the stoichiometric ratio.20 Belandria et al. measured the compositions of CO2/H2 in the presence of Received: Revised: Accepted: Published: 4911

November 28, 2012 March 7, 2013 March 10, 2013 March 10, 2013 dx.doi.org/10.1021/ie303280e | Ind. Eng. Chem. Res. 2013, 52, 4911−4918

Industrial & Engineering Chemistry Research

Article

hydrate using an isochoric technique and proposed that H2 is not trapped in the hydrate cavities.21 After determining the hydrate phase equilibrium conditions, researchers paid more attention to accelerating hydrate formation and to enlarging the captured gas capacity. They found that the surfactants and porous medium are potential means to achieve the two purposes. Relative to other surfactants, sodium dodecyl sulfate (SDS) is very effective for increasing the hydrate formation rate and gas capacity.22−25 Okutani et al. reported that the optimal SDS concentration for increasing both the hydrate formation rate and the final water-to-hydrate conversion ratio is 1000 mg/L.22 Although most of the investigations have focused on hydrate-based natural gas storage, SDS is also a clear potential kinetic accelerant for hydrate-based CO2 capture from fuel gas. Furthermore, the prerequisite for the industrial use of hydrate technology is that hydrate crystallization must be performed without mechanical agitation.14 Seo et al. found that dispersed water in silica gel pores reacts readily with the gas; thus, there is no need for a stirred reactor and excess water.26 Moreover, a microimaging study showed the rapid conversion of water to hydrate in silica gel pores, which indicated that the formation of hydrate in silica gel pores might allow the recovery of CO2 from flue gases. Adeyemo found a nearly 4-fold increase of the gas/water ratio in silica gel and an improvement in CO2 recovery from 42 to 51% compared with stirred-tank reactors.27 Kang et al. found that the gas obtained from hydrate dissociation in porous media contains over 95 mol % CO2 (70 mol % in bulk water hydrate) for fuel gas, which further validates the feasibility of using hydrate-based CO2 separation in porous silica gel.28 Later, they found that the presence of SDS can increase the initial hydrate formation rates in silica gel.29 As is clear in the introduction above, many studies have investigated hydrate-based CO2 capture technology; however, only very limited literature data exist for the effects and mechanism of additive mixture (THF/SDS) on CO2 separation in porous media, especially for that of CO2/H2 gas mixtures. To elucidate the mechanism data of hydrate-based CO2 capture from fuel gas, the effects of porous media and additive mixture on hydrate thermodynamics and dynamics were investigated experimentally. A thermodynamic model was used to calculate the hydrate phase equilibrium conditions. This investigation can be used to provide fundamentals for the conceptual design of hydrate-based CO2 capture for IGCC power plants. The knowledge obtained from this work is sufficiently general and is expected to be useful in other applications, such as H2 purification.

Figure 1. Scheme of the gas hydrate experimental apparatus.

GmbH) filled with glycol−water was used to precisely control the temperature. The amounts of additives were weighed using a high-precision balance with a minimum reading of up to 0.0001 g (type JA10003N; Shanghai Minqiao Precision Scientific Instruments Co., Ltd., China). A gas chromatograph (type GS-101T; Dalian Replete Science and Technology Co., Ltd., China) was used to analyze the gas composition. Glass beads (As-One Co., Ltd., Japan) were used to form the porous medium, whose porosity was approximately 36.4%. The mixture gas was provided by Dalian Guangming Special Gas Co., Ltd., China. None of the chemicals were purified, and deionized water was used in all experiments. The experimental materials are described in detail in Table 1. Table 1. Material Properties and Suppliers material

purity or composition

particle size

CO2/H2

39.6 mol % CO2, 60.4 mol % H2

−

BZ-01

soda glass

0.105−0.125 mm

THF

≥99.0%

−

SDS

≥91.0%

−

supplier Dalian Date Special Gas Co., Ltd., China As-One Co., Ltd., Japan Sinopharm Chemical Reagent Co., Ltd., China Tianjing Bodi Chemical Holding Co., Ltd., China

2.2. Experimental Procedures. The solutions were prepared before each experiment. The SDS and THF were weighed and then poured into a volumetric flask (500 mL) to obtain the desired solutions. Next, a graphic method was used to measure the hydrate phase equilibrium conditions by keeping the volume constant. After the glass beads were added to fill the vessel, solutions and mixture gas were used to saturate the glass beads, and then the bath temperature was changed to form or decompose CO2 hydrate. Nothing was added to the closed system during the experimental process. The driving force for hydrate formation increased with decreasing bath temperature. Hydrate then began to form, and a rapid temperature increase was observed due to the exothermic reaction of hydrate formation. The hydrate formation was considered complete when there was no pressure change in the system. After a pause, the vessel was heated to dissociate the hydrate. The hydrate phase equilibrium condition was obtained by measuring the intersection point of the cooling and heating isochors.

2. EXPERIMENTAL INVESTIGATION 2.1. Experimental Apparatus. The experimental apparatus used in this study is shown in Figure 1, and further details of the experimental apparatus can be found in our previous publication.30 The high-pressure-resistant vessel was connected to five thermocouples (Sakaguchi E. H. Voc Corp., Japan) and two pressure transducers (Nagano Keiki, Japan). The thermocouple and pressure transducer signals were collected by an analog-to-digital module (Advantech Co., Ltd., Taiwan) and sent to a PC. The estimated errors of temperature and pressure measurements were ±0.1 K and ±0.1 MPa, respectively. A high-pressure pump (D-250L, Haian Oil Scientific Research Apparatus Co., Ltd., China) was used to increase the pore pressure by injecting water or gas. A large-scale and highprecision thermostat bath (type F-25 me; JULABO Labortechnik 4912

dx.doi.org/10.1021/ie303280e | Ind. Eng. Chem. Res. 2013, 52, 4911−4918


Article

3. RESULTS AND DISCUSSION Hydrates were formed and dissociated in CO2−H2−THF− SDS−H2O systems. Four THF concentrations (1, 2, 3, and 4 mol %) and 1000 mg/L SDS were used as experimental solutions. The hydrate formation and dissociation processes, gas components in the hydrate, and hydrate phase equilibrium conditions were analyzed and are discussed below. 3.1. Hydrate Formation and Dissociation Processes. During hydrate formation and dissociation, the pressure (p) and temperature (T) are important control and indication parameters. Typical p and T curves during hydrate formation are shown in Figure 2. The abrupt p decrease and T increase at

Figure 3. Typical p−T curve for CO2/H2 gas hydrate in aqueous solution with 3 mol % THF and 1000 mg/L SDS.

dissociation process finishes at point E, after which the p−T curve returns to point A along the BA curve with increasing T, as shown in Figure 3. For comparison, hydrates were formed from a CO2−H2−H2O system in glass beads. Most of the p−T curves during the experiments are similar to that shown in Figure 3. Additionally, a novel p−T curve is obtained, as shown in Figure 4. After T

Figure 2. Typical p and T changes with time for CO2/H2 gas hydrate formation in an aqueous solution with 3 mol % THF and 1000 mg/L SDS (five different temperatures).

approximately 55 min indicate the beginning of hydrate formation. Because the THF hydrate crystallization process exhibits exothermic and volume-increasing behavior, p and T increase simultaneously as the THF hydrate forms in the closed vessel.31 This finding conflicts with the experimental results shown in Figure 2. Therefore, the gas phase must be enclathrated into hydrate cavities as the THF hydrate forms, causing p to decrease. T1, T2, T3, T4, and T5 represent the temperatures at five different positions in the vessel. Because the hydrate formation is inhomogeneous, the inflections of these T curves occur at different times in Figure 2. Since p is a global variable and T is a local variable, the different T curves produce the different profiles of the B−C−D section for the p−T curve in Figure 3. Figure 3 shows the p−T curve corresponding to that in Figure 2. The vessel is cooled from point A to point B by the highprecision thermostat bath. When the hydrates begin to form at point B (approximately 280.00 K and 2.35 MPa), p decreases and T dramatically increases. Because p represents a macroscopic parameter of the vessel and the T value is a local parameter, the profile of the B−C−D curve is controlled by T in Figure 3; that is, five different profiles may be obtained with selected different thermocouples. Since the hydrate phase equilibrium condition is obtained during the hydrate dissociation process, the B−C−D curve profile does not affect the measured hydrate phase equilibrium conditions, and it is not discussed further. Because the T value for hydrate formation is 280 K, T decreases again after the exothermic energy of hydrate formation can no longer counterbalance the refrigerating output from the bath, finally reaching point D in Figure 3. After the formation is complete, the vessel is heated and the hydrates begin to dissociate. The

Figure 4. Novel p−T curve for the ternary CO2−H2−H2O system.

reaches approximately 266.00 K, p and T increase dramatically. Considering that CO2 is a vapor under this p−T condition and there is little literature evidence that empty hydrate cavities can exist, we propose that the p and T increases are caused by ice formation. When ice is formed in the vessel, both T and p increase (due to the exothermicity and density decrease of the ice crystallization process). It is unclear whether hydrate formed in this experimental cycle; thus, the intersection of the heating and cooling curves in Figure 4 is not observed in the hydrate phase equilibrium data. For ease of comparison, the induction time is defined as the time from the beginning of the cooling process to a detected significant T increase in this investigation (the initial temperatures are identical). Although this time is not technically the true induction time for hydrate nucleation, this definition is convenient for the discussion of the results and has no effect on the conclusions. Many researchers have claimed that SDS can decrease the induction time and significantly improve the 4913



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hydrate formation rate.32 Sabil et al. found that the presence of 5 mol % THF significantly decreased the induction time of the CO2 hydrate.33 The question is whether the concentration of THF influences the induction time. In the literature, little evidence can be found for the effects of THF concentration on the hydrate induction time. As can be seen in Figure 5, the

THF concentration is below 2 mol %. As the initial experimental p increases from 3.5 to 8.0 MPa with 1 mol % THF, the induction time decreases from 135 min to almost 70 min. This behavior can also be attributed to the supersaturation of the hydrate-forming component in the aqueous phase. Because the hydrate mixture enclathrated THF and gas molecules, the combined saturation of THF and gas is important for hydrate formation. When the THF concentration is lower, the driving force for hydrate formation is also lower and a higher p is needed to improve the gas saturation and driving force for hydrate formation. After the THF concentration reaches 3 mol %, the induction time changes slightly with the changes in p. Thus, we assume that the liquid phase is supersaturated with THF and that the driving force is high enough for hydrate formation under these conditions.33 The changes in induction time are small as the THF concentration increases from 3 to 4 mol %. That is, the hydrate induction times get close to each other for 3 and 4 mol % THF concentration at the same pressure. The induction times decrease from 55 min to almost 40 min as p increases from 3.0 to 7.5 MPa at 3 and 4 mol % THF. In other words, 3 mol % THF is optimal from a hydrate induction time perspective. 3.2. Gas Component in Hydrates. Sugahara et al. claimed that the hydrates generated in H2−CO2−H2O (without THF) ternary systems can be regarded as pure CO2 hydrate crystal (s-I) based on Raman spectroscopy.37 Kumar et al. reported that hydrogen occupies the small cavities of structure I (s-I, for the H2−CO2−H2O system) and structure II hydrate (s-II, for the C3H8−H2−CO2−H2O system).38 Are hydrogen molecules present in the clathrate hydrate cavities in this study? With no THF in the systems, the fugacity of CO2 in the mixture gas is calculated based on the experimental data for hydrate equilibrium conditions and compared with that of pure CO2 hydrate.39 The results are shown in Figure 6. As mentioned by

Figure 5. Induction time of CO2/H2 gas hydrate for different THF concentrations with 1000 mg/L SDS, with glass beads forming the porous medium.

increase in THF concentration decreases the induction time. This phenomenon may be explained by the supersaturation of the guest component. Investigators have mentioned that the hydrate induction time depends on the supersaturation of the hydrate-forming component in the aqueous phase,34−36 and that if the supersaturation critical point is not achieved, no hydrate will be formed in the system. Kashchieva et al. claimed that the conditions of gas dissolution in the aqueous phase are of prime importance for creating a driving force for hydrate crystallization,35 which can be applied to the THF−hydrate−water−gas system. As a hydrate-forming component, the increase in THF concentration enhances the driving force for hydrate formation and decreases the hydrate induction time. As shown in Figure 5 and Table 2, the hydrate formation induction times decrease significantly with increasing p when the Table 2. Induction Times of CO2/H2 Hydrate for Different THF Concentrations with 1000 mg/L SDS, with Glass Beads Forming the Porous Medium THF concn/ mol %

p/ MPa

induction time/min

THF concn/ mol %

p/ MPa

induction time/min

1

7.9 7.8 6.8 6.1 5.6 4.8 3.6 8.2 8.2 7.5 5.7 4.9 4.2 2.6

91 72 81 91 109 121 133 48 52 54 71 78 80 84

3

7.5 6.4 5.1 4.3 3.6 2.4 8.8 7.5 6.2 5.3 4.2 3

38 44 48 51 49 57 27 36 39 44 49 54

2

4

Figure 6. Fugacity of CO2 in CO2/H2 gas hydrate and pure CO2 hydrate.

Sugahara et al., CO2 hydrates are generated when the CO2 fugacity in the gas mixture exceeds the equilibrium fugacity of the pure CO2 hydrate system.37 Once pure CO2 hydrate is generated from the CO2/H2 mixture, the CO2 fugacity in the gas mixture is equal to the equilibrium fugacity of pure CO2 hydrate. The comparisons in Figure 6 indicate that the CO2 fugacity in the gas mixture is high, suggesting the presence of H2 in the hydrate. However, this finding is not proof of the presence of H2 in hydrate because the change in gas components caused by hydrate 4914



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formation has been neglected in the calculation of the CO2 fugacity in the gas mixture. In the presence of THF, many studies have reported that H2 is found in hydrate cavities for the CO2−H2−THF−H2O system. Anderson et al. reported that small cavities are occupied by only one H2 molecule,40 with large cavities unambiguously occupied by THF molecules, regardless of the initial aqueous THF concentration for the H2−THF−H2O system. Hashimoto et al. verified that it is possible for H2 molecules to occupy the small cavities to generate the H2−CO2−THF mixed hydrate using Raman spectroscopy.41 Later, they reported that the Raman spectra of H2 and THF for the mixed gas hydrate do not change with the variation of the THF mole fraction from 0.010 to 0.130.42 According the equation proposed by Munck et al., the Langmuir constant for CO2 in small cavities of s-II hydrates is 8.40 MPa−1 at 282.00 K.43 Because of the relatively recent discovery of hydrogen in clathrate hydrates, few Kihara potential parameters are available in the literature for H2 in hydrate cavities. Strobel et al. calculated the H2 Kihara parameters and obtained the Langmuir constant for H2 in small cavities of s-II hydrates (0.03 MPa−1 at 282 K),44 which is 2 orders of magnitude higher than the value of Munck et al.43 Considering that the occupation probability of cavities is related to the Langmuir constant and gas fugacity,10 H2 may have occupied the small cavities of s-II hydrate as CO2 fugacity decreased sharply in this investigation. Considering that CO2 molecules can also form and stabilize s-I hydrate, the presence of s-I hydrate is also indeterminate for the CO2−H2−THF−H2O system. Although Kumar et al. have found that the ternary CO2−H2−C3H8 mixtures form pure s-II hydrate at 3.8 MPa by introducing 2.6% C3H8,38 this phenomenon must also be investigated for the CO2−H2− THF−H2O system, which may be an important and interesting topic for further investigations. 3.3. Effects of THF on Hydrate Phase Equilibrium Conditions. In the presence of THF, the significant decrease in the equilibrium p coupled with the increase in the equilibrium T expands the hydrate stability region of the CO2−H2−THF− H2O−hydrate system. The experimental data for hydrate phase equilibrium conditions are shown in Figure 7 and Table 3. Compared with that of pure water, the presence of THF in the system results in a dramatic decrease of the hydrate phase

Table 3. Hydrate Phase Equilibrium Conditions for CO2/H2 Gas Hydrate with Different THF Concentrations and 1000 mg/L SDS, with Glass Beads Forming the Porous Medium THF concn/ mol % 1

2

T/K

p/ MPa

THF concn/ mol %

283.75 284.55 283.85 282.85 282.65 282.45 281.75 279.75 278.55 288.15 287.65 287.65 286.75 286.15 285.55 283.25

8.23 7.55 6.49 5.78 5.36 4.57 3.97 3.41 3.00 7.97 7.30 6.58 5.51 4.74 4.09 3.03

3

4

T/K

p/MPa

290.15 289.65 288.85 288.15 286.95 286.25 286.05 284.65 290.95 290.15 289.55 288.75 287.65 286.35

8.52 7.33 6.23 4.98 4.15 3.485 2.925 2.295 8.60 7.32 6.01 5.19 4.07 2.90

equilibrium p. The hydrate equilibrium T increases by over 7 K due to the presence of 1 mol % THF and increases further with increasing THF concentrations. For example, the equilibrium conditions of CO2/H2 gas hydrate in deionized water are approximately 275.00 K and 6.00 MPa. T increases to approximately 283.00 K in the presence of 1 mol % THF, and the hydrate equilibrium T is as high as 287.00 K in aqueous solution in the presence of 2 mol % THF. When the THF concentration increases to 3 mol %, the hydrate equilibrium T reaches 289.00 K; however, the increase slows with further increases in THF concentration. In a previous study, we found that “pseudoretrograde” behavior exists at approximately 3.00 MPa with 3 mol % THF and 0−2000 mg/L SDS for pure CO2 gas hydrate.31 However, in this investigation, the “pseudoretrograde” phenomenon only appears at 1 mol % THF. When p reaches approximately 7.50 MPa, the highest equilibrium T (284.55 K) is obtained, at which point the CO2 fugacity is approximately 2.80 MPa. The hydrate equilibrium T decreases with further increase of p. As reported in the literature,45,46 the “pseudoretrograde” behavior usually occurs in a pseudobinary system containing two types (s-I and s-II) of hydrate formers with fairly low vapor pressure. s-II hydrates form first and rapidly in the CO2−H2−THF−H2O system, with THF occupying the large cavities and CO2/H2 occupying the small cavities. As the THF concentration decreases, the competition of CO2 with THF to occupy large cavities of s-II hydrate turns impetuous, and CO2 obtains the advantage. Because CO2 hydrate is more stable in s-I hydrate, a structural transition (s-II to s-I) will occur in the system, causing “pseudoretrograde” behavior. The fugacity of CO2 is below 3.00 MPa for the 3 mol % THF experiments, so there is no “pseudoretrograde” behavior in this investigation. 3.4. Modeling Hydrate Phase Equilibrium Conditions. The improved model of Song et al. was modified further and used to predict the hydrate equilibrium conditions for the CO2−H2− THF−H2O system in glass beads in the presence of SDS,47 which is based on the traditional model of van der Waals and Plateeuw.48 Because the presence of SDS has little effect on the hydrate phase equilibrium, it is neglected in these calculations. In the improved model, the mechanical equilibrium of force

Figure 7. Hydrate phase equilibrium conditions for CO2/H2 gas hydrate with different THF concentrations and 1000 mg/L SDS, with glass beads forming the porous medium. 4915



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Error analysis of the predicted hydrate equilibrium conditions in a porous medium is shown in Table 5. The overall ΔAADT

between the interfaces in a hydrate−liquid−vapor system is introduced and the interfacial energy between the liquid and gas is solved using Li’s method.49 The capillary effect in porous sediments is solved using a modified equation from Henry et al.50 The Peng−Robinson equation of state (P−R Eos) with the modification by Stryjek and Vera (PRSV) is chosen to calculate the fugacity of the hydrate former,51 which is associated with the modified Huron−Vidal second-order model (MHV2) mixing rule.52 The nonrandom two liquid (NTRL) model is used to obtain the excess free energy and the activity coefficient.53 The ́ reference values for the “empty” hydrate are taken from Martinez et al.54 The A and B parameters for Langmuir constants of CO2 and THF can also be found in ref 54. The Langmuir constants of H2 were calculated using the method of Strobel,44 which is based on the Kihara potential function. The parameters used to calculate Langmuir constants for H2, CO2, and THF are shown in Table 4.

Table 5. Absolute Average Deviations of Predicted Hydrate Equilibrium Conditions

A (kPa−1) structure II

a (Å)

large cavities small cavities

8.40 × 10−6 8.34 × 10−10 H244 σ (Å)

0.198

3.068

THF54 B (K−1)

A (kPa−1)

B (K−1)

2.025 3.615

6.5972 −

1003.22 −

ε/k (K)

75.01

Figure 8. Comparison of experimental and calculated results for CO2/ H2 gas hydrate phase equilibrium conditions.

AADT (%)

AADP (%)

0 1 2 3 4

272.15−277.85 278.55−283.75 283.25−288.15 284.65−290.15 286.35−289.95

3.81−8.30 3.02−8.21 3.02−7.98 2.29−8.51 2.93−8.59

6 5 5 6 6

0.09 0.33 0.17 0.20 0.25

6 28 17 22 24

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

Np

Notes

ΔAADT = (1/Np)∑ [|Tcal − Texp| /Texp]j ·100

The authors declare no competing financial interest.

■

(1)

ACKNOWLEDGMENTS This project is financially supported by the Key Program of the National Natural Science Foundation of China (50736001), the National Natural Science Foundation of China (51106018), the High-tech Research and Development Program of China

Np j=1

Np

■

predictions agree well with the experimental data. The absolute average deviations of the predicted T (ΔAADT) and p (ΔAADP) are defined as follows:

ΔAADP = (1/Np)∑ [|pcal − pexp | /pexp ]j ·100

P/MPa

4. CONCLUSIONS In this investigation, the effects of an additive mixture (THF/ SDS) on the thermodynamics and kinetics of CO2/H2 gas hydrates were investigated. The experiments were performed using an isochoric method, and the hydrate phase equilibrium conditions were calculated using an improved model. The experimental results showed that the increase in THF concentration enhanced the driving force for hydrate formation and decreased the hydrate induction time, which is favored for hydrate based CO2 capture. The Langmuir constants for H2 and CO2 showed that H2 may occupy the small cavities of s-II hydrate in the H2−CO2−THF−H2O system. This decreases the purity of captured CO2, and it is a disadvantage. The other advantage is that the presence of THF in the system resulted in a decrease in the hydrate phase equilibrium pressure, and higher THF concentrations led to more decreases in the hydrate equilibrium pressure. The hydrate equilibrium temperature increases by over 7.00 K due to the presence of 1 mol % THF, and increases further with increasing THF concentrations. The rate of decrease slows with a further increase of the THF concentration above 3 mol %. An improved hydrate thermodynamic model, based on the traditional model of van der Waals and Plateeuw, the PRSV equation of state, a modified Huron−Vidal 2 (MHV2) mixing rule, and the NRTL model, was used to predict the equilibrium conditions, and the deviations between the measurements and the modeling results are not surprising due to the complexity of the system.

Figure 8 compares the experimental data and calculated predictions for the hydrate phase equilibrium conditions. The

j=1

T/K

values for the improved model are 0.33%, and all the ΔAADP values are on the order of 20%, except for that for water without THF. The differences are mainly caused by the presence of THF, which makes the hydrate formation mechanism more complicated. The deviations between the measurements and the modeling results are not surprising due to the complexity of the system. The results show that the improved model provides acceptable predictions of the equilibrium conditions of the hydrate mixture.

Table 4. Calculation Parameters for Langmuir Constants of H2, CO2, and THF CO254

THF mole fraction

(2)

where Np denotes the number of data points. 4916



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(2006AA09A209-5), the Major State Basic Research Development Program of China (2011CB707304), the Scientific Research Foundation for Doctors of Liaoning Province (20111026), the Key Laboratory of Renewable Energy and Gas Hydrate (Chinese Academy of Sciences, y207k71001), and the Fundamental Research Funds for the Central Universities of China.

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