Experimental correlation for the formation rate of CO2 hydrate with THF (tetrahydrofuran) for cooling application

Energy 91 (2015) 712e719

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Experimental correlation for the formation rate of CO2 hydrate with THF (tetrahydrofuran) for cooling application Qibei Sun, Yong Tae Kang* School of Mechanical Engineering, Korea University, 145 Anam-ro, Seongbuk-gu, Seoul 136-701, South Korea

a r t i c l e i n f o

a b s t r a c t

Article history: Received 16 March 2015 Received in revised form 26 August 2015 Accepted 27 August 2015 Available online xxx

The CO2 hydrate formation experiments with THF (tetrahydrofuran) are performed in a stirred semi-bath reactor. The experimental data on CO2 hydrate formation are obtained at constant pressure and temperature with the low driving force conditions. The experimental temperature is above 279 K, which is high enough to prevent the formation of only THF hydrate. The Gibbs free energy difference by the pressure variation is chosen as the driving force. The experimental results confirm that the THF drastically reduces the required CO2 hydrate formation pressure. A two-parameter kinetic model based on the Chen-Guo model is developed to predict the CO2 hydrate formation rate and to correlate the experimental data. It is found that the experimental correlation based on the present estimation model fits well with the experimental results and can predict the CO2 hydrate formation rate satisfyingly for cooling application. © 2015 Elsevier Ltd. All rights reserved.

Keywords: CO2 Experimental correlation Hydrate formation Kinetic model THF

1. Introduction As is well known, CO2 is a greenhouse gas and it causes the global warming problem [1]. Researchers focus on different ways to capture CO2, including absorption, adsorption, membrane separation and so on. It is also found that clathrate hydrate is a novel approach to mitigate CO2 emissions [2,3]. Gas hydrate is crystalline solid which is formed by water molecules with small guest molecules. All common gas hydrates belong to three distinct structures, SI, SII and SH, which depending on the cavity size. The structure of CO2 hydrate is SI, and the structure of THF (tetrahydrofuran) hydrate is SII, however, after adding THF, the CO2 hydrate structure changes to structure SII [4]. In summer, the air conditioner takes 25%e30% of the national total electrical power. Some researches focus on the traditional storage media, but have paid little attention on the CO2 hydrate for using as a secondary refrigerant [5]. Jerbi et al. [6] investigated the rheological characteristics of CO2 hydrate slurry to use it as a secondary refrigerant in refrigeration systems. Shi and Zhang [7] proposed a cooling system that used TBAB (Tetra-n-butylammonium bromide) hydrates as a refrigerant. CO2 hydrate is suitable for the air conditioning field due to positive temperature conditions

* Corresponding author. Tel.: þ82 2 3290 5952. E-mail address: [email protected] (Y.T. Kang). http://dx.doi.org/10.1016/j.energy.2015.08.089 0360-5442/© 2015 Elsevier Ltd. All rights reserved.

(the temperature of the phase change is above 0
C). The ice-slurry systems have gained wide attention due to the high heat capacity [8,9]. The dissociation enthalpy of CO2 hydrate is high enough (500 kJ/kg), compared with the dissociation enthalpy of ice (333 kJ/ kg) [10,11], R11 (334 kJ/kg) and R141b (344 kJ/kg) [5], and it has a high thermal storage capacity than most of the traditional materials used in the cooling applications. Therefore, CO2 hydrate slurry is considered as a phase change secondary refrigerant for a higher energy conversion efficiency. However, for the cooling applications such as the air conditioner, the suitable equilibrium temperature should be 0e7
C and suitable working pressure be 0.1 MPae0.5 MPa. This pressure is much lower than the required CO2 hydrate formation pressure. To apply the CO2 hydrate slurry for cooling applications, it has been found that adding THF could reduce the equilibrium formation pressure of hydrate drastically [12,13]. The experiments at atmospheric pressure were conducted by adding THF in our previous study [14]. Linga and coworkers did a series of studies on the kinetics of the hydrate with THF [15e17]. In the literature [15], a new apparatus which allowed feeding CO2 gas (open system), was developed, and the gas uptake with THF obtained in this work was higher than that in the closed system. This open system is adopted in the present study. Sabil et al. [18] compared the kinetic phenomenon of the formation for CO2 hydrate with the kinetic phenomenon of the formation for CO2 þ THF hydrate. Both of Linga et al. [15e17] and Sabil et al. [18] carried out a series of experiments to get the kinetic data, such as equilibrium

Q. Sun, Y.T. Kang / Energy 91 (2015) 712e719

Nomenclature A Aij a B Bε Bij C Cij f f0 G m n N P Pε R

parameter for calculating f0, Pa parameter for calculating the Langmuir parameter, K/ Pa amount of hydrate formation, mol parameter for calculating f0, Pa bias error, % parameter for calculating the Langmuir parameter, K parameter for calculating f0, Pa Langmuir parameter fugacity, Pa fugacity of gas species in equilibrium with the unfilled basic hydrate, Pa Gibbs free energy, kJ/mol numbers of linked small cavities per water molecule numbers of linked big cavities per water molecule finite value pressure, Pa standard deviation, % gas constant

pressure and temperature, induction time, and the formation rate. However, in the present study, adding THF is to decrease the formation pressure which is suitable for the cooling application, and the experiments were done to get the data of the hydrate formation rate, which are an important factor for the design of a cooling system in our further study. The experimental data of the hydrate formation rate are also used to develop the correlation of the present kinetic model. The biggest difference is that the hydrate formation pressure is low (3e5 bar) in the present study while it is very high (15e110 bar) in Linga and coworkers [15e17] and Sabil et al. [18]. There have been several hydrate growth models. Based on the experimental observation, Glew and Hagget [19] attempted to make a kinetic model for ethylene oxide hydrates. The model is based on the heat transfer resistances with convection on the hydrateesolution interface (film theory) and conduction through the wall. One of the most famous and basic growth kinetics is the EnglezoseBishnoi model [20], which is based on the experiments of methane and ethane hydrates, combining the theories of crystallization and mass transfer at a gaseliquid interface. Skovborg and Rasmussen [21] studied the Englezos model and suggested that no secondary nucleation existed and all particles had the same size, so the PBE (population balance equation) [22] could be removed. Herri [23] revised the model by taking into the particle size distributions into account. Since all these model components form SI hydrate, the model should be used for SII and SH discreetly. Different kinds of driving force are chosen to develop the hydrate growth models. Svandal [24] established the model based on phase field theory, while Buanes’ model [25] based on Monte Carlo cellular automata. Both Uchida [26] and Mori [27] developed the theory of curved film front growth on water-hydrate former interface. The positive application of CO2 hydrate in the cooling system has gained increasing attention. However, most experiments of the existing literature were conducted at high pressure (above 10 bar), which are not suitable for the cooling application system, and the purpose of their studies was to achieve the high formation rate. Some researchers began to study on the thermodynamics of the CO2 hydrate, such as the dissociation enthalpy or adding some

r T Uε X X

713

hydrate formation rate, mol=molH2 O$S temperature, K Uncertainty, % measurement value measurement mean value

Greek letters a activity b structural parameter, K/Pa q proportion of cavity, % m chemical potential, J/mol m0H hydrate chemical potential without gas, J/mol m0THF chemical potential of pure THF, J/mol l number of linked cavities per water molecule Subscripts H combined hydrate i species of gas j different types of cavities w water

additives to measure the equlibirium conditions. The objectives of the present work are to obtain the kinetic data for the cooling application, and to develop the experimental correlation of the CO2 hydrate formation rate ate low pressure. There is no other literature giving the data which can be compared for the cooling application system. In the present study, the Gibbs free energy difference is chosen as the driving force, which is directly related with the operating pressure. Compared with the literature at the high working pressure of 30 bar [28], the present experiments are conducted at the low driving forces ranges (3, 4, 5 bars) in a stirred semi-bath reactor with THF. The reason of using the THF is that, the suitable working pressure should be near the atmospheric pressure to apply the CO2 hydrate for the cooling system. A modified model based on Chen-Guo model [29] is developed to compare the experimental results with the CO2 hydrate formation rate with the modified model.

2. Experiment 2.1. Experimental apparatus and procedure The schematic of the experimental equipment is shown in Fig. 2. The experimental apparatus consists of a test section (a diameter of 5.3 cm, and a height of 23.5 cm), which combines a transparent column and three stainless steel lids, an external refrigeration, gas tank, water bath, a flow meter, a pressure sensor and some valves. The test section is immersed into a water bath, which is controlled by an external refrigerator to keep the constant temperature. A RTD (Resistance Temperature Detector) with a measurement error of ±0.01
C is inserted from the top lid into the liquid to monitor the temperature variation during the reaction process. A flow meter with a measurement error of ±1.0% is installed to measure the CO2 gas consumption rate. A needle valve, a check valve and a relief valve are installed for controlling the flow rate and preventing the gas backflow. The supply vessel is employed to support the gas during the crystal formation process. A pressure sensor with a measurement error of ±8 Pa is used to check the pressure in the test section. Stirring is provided by a magnetic stirrer which is placed at

714

Q. Sun, Y.T. Kang / Energy 91 (2015) 712e719

Fig. 1. Schematic of the three hydrate crystal structures.

the bottom of the test section. The experimental data are recorded for every second by a data acquisition unit. The THF (the purity is 99%) and distilled water are mixed as base fluid in this study. The CO2 (the purity is 99.99%) is initially injected

into the test section to a fixed pressure. When the pressure is stabilized at the initial value for 1 h, the vacuum pump will be used to evacuate the gas. THF of 300 g (19.06 wt%, stoichiometric concentration for THF hydrate) and distilled water is charged into the test

Fig. 2. Schematic of the experiment equipment.

Q. Sun, Y.T. Kang / Energy 91 (2015) 712e719

section. The test section is cooled down by the refrigerator to reach the experimental temperature, which is around or above 279 K. The temperature is lower than the equilibrium one of CO2 þ THF hydrate, but high enough to avoid forming only THF hydrate. Then CO2 is injected into the test section to reach the desired pressure, and the combined liquid is mixed well by the magnetic stirrer bar. The pressure of the test section decreases during CO2 absorption and hydrate formation processes. When the pressure in the test section becomes lower than the desired pressure, the valve is open again to supply the CO2 gas to reach the desired pressure. After the completing of experiments, the water bath temperature is increased to decompose the hydrate completely. The formation process is monitored when the CO2 hydrate formation starts, and the CO2 gas consumption aCO2 during the formation period is recorded by the data acquisition. The average CO2 hydrate formation rate rCO2 is calculated from the aCO2 and the formation period t. The unit of the hydrate formation rate is mol=molH2 O$S . The kinetic data is shown in Table 1.

2.2. Experimental error analysis The uncertainty in an experimental measurement mainly caused by bias errors and random errors. The random uncertainty is computed based on the RSS (root-sum-squares) [30]. The uncertainty is expressed as

Uε ¼

715

Table 2 Summary of the uncertainty analysis. Uncertainty [%] Bε 0.8 1 1

Pressure Temperature Mass flow rate

Pε 0.3 0.04 0.02

Uε 1.21 1.42 1.42

3. Kinetic model The formation of CO2-THF hydrate is expressed as:

H2 O þ mCO2 þ nTHF!CO2m $THFn $H2 O

(3)

The ideal unit cell formula for SII is 8(51264)$16(512)$136 H2O (8 large cavities, 16 small cavities and 136 H2O molecules), which is shown in Fig. 1. If all the large cavities are occupied by THF and only the small cavities are occupied by CO2, then m ¼ 2/17, n ¼ 1/17. However, it is impossible for all cavities to be occupied. Hydrates always have more water molecules than the ideal composition. The fractional occupation can be expressed as the following equation [31].

qij ¼

1þ

Cij fi P

Cij fi

; Cij ¼

  Aij Bij exp T T

(4)

i¼species

rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi  2 B2ε þ P 2ε

(1)

vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u N  u 1 X 2 where Pε ¼ t XX N  1 n¼1

(2)

The results of the uncertainty analysis are summarized in Table 2. The uncertainties of pressure, temperature and mass flow rate are estimated 1.21, 1.42 and 1.41%, respectively.

where fCO2 is calculated by the PengeRobinson [32] equation, and Cij can be calculated from Aij and Bij, which are summarized in Table 3 [13,33]. With the assumption that temperature and pressure are constant during the experimental procedure, and the THF and H2O form the basic hydrate;

nTHF þ H2 O !THFn $H2 O

(5)

the Gibbs free energy difference is chosen as the driving force as follows [29]: Table 1 Experimental conditions along with measured induction time and hydrate formation rate. System

Exp. No. P (bar) T (K)

Induction time r  108 mol=molH2 O$S (min)

CO2/THF/H2O

1 2 3 4 5 6 7 8 9 10 1 2 3 4 5 6 7 8 1 2 3 4 5 6

45.3 108.5 53.5 172.1 112.6 284.1 194.7 161.6 197.3 301.0 71.9 140.2 100.8 90.0 164.9 122.0 295.2 323.3 34.6 46.2 50.5 60.7 78.5 124.3

3 3 3 3 3 3 3 3 3 3 4 4 4 4 4 4 4 4 5 5 5 5 5 5

278.7 278.8 279.0 279.3 279.5 279.6 279.7 279.9 280.2 280.3 279.4 279.4 279.7 279.8 279.9 280.5 281.3 281.4 279.4 279.7 279.8 280.1 280.4 280.6

5.62 3.80 4.79 2.94 3.21 1.82 2.41 2.81 2.54 1.46 8.29 8.14 6.73 7.04 6.45 6.19 4.01 3.35 13.0 11.1 10.1 7.41 7.78 6.73

DG ¼ mH  mW 

1 m 17 THF

(6)

After injecting the CO2, the mH is calculated by van der Waals and Platteeuw [34].

mH ¼ m0H þ RT

X

  lij ln 1  qij ;

(7)

i ¼ species j ¼ cavities

mTHF ¼ m0THF þ RT ln fTHF

(8)

where lij ¼ 1/17 and 2/17 for big and small cavities, respectively, and fTHF is calculated by the virial equation [35]. Combining Eqs. (6)e(8), the following equation is obtained.

DG ¼ m0H mW 

1 0 m þlij RT 17 THF

X i ¼ species j ¼ cavities

  1 ln 1qij  RT ln fTHF 17 (9)

Now, the first three terms on the right hand side in Eq. (9) is calculated by the Chen-Guo model [29] as follows:

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Table 3 Parameters for calculating the Langmuir parameters. CO2

THF

A (K/Pa) Structure II

m0H  mW 

Large cavities Small cavities

6

8.51  10 8.45  1010

B (K)

A (K/Pa)

B (K)

2025 3615

6.59772 e

1003.22 e

1 0 1 m RT ln f 0 ¼ 17 THF 17

    T bP 1n þ C exp aw and f 0 ¼ A exp B T

(10)

(11)

where f0 is considered as the function of pressure, temperature and the water activity. For SII, b ¼ 10.224 K/MPa, aw is calculated by the NRTL equation [36] and A, B, C are correlated by the equilibrium data of temperature and pressure of THF-CO2 double hydrate, using Eq. (12). Finally, the CO2 hydrate formation rate r, is calculated by the following equations;



  DG r ¼ C1  1  exp C2  RT 2 3  0  X   DG 4 1 f ¼ ln and þ lj ln 1  qj 5 RT 17 fTHF j¼cavities

(12)

(13)

where two parameters C1 and C2 are determined by the experimental data regression of CO2-THF system. 4. Results and discussion In the present study, the experiment of CO2 hydrate formation with THF was carried out at the pressures of 3, 4, 5 bar and the temperatures of 279e281 K. If there is no addition of THF, the equilibrium condition for CO2 hydrate at 279 K is 2.5 MPa. After adding THF, the equilibrium pressure at 281 K decreases to around 2 bar, and at 279 K the equilibrium pressure becomes even lower [13]. So they reported the THF could significantly decrease the required pressure of CO2 hydrate formation. THF is a thermodynamic additive, which can form the hydrate as a guest molecule. For THF hydrate, the THF molecule enters into the large cavities, forming the structure SII, and after injecting CO2 gas into the solution, the small guest molecules are entrapped into the small cavities of the structure SII, and this is the mechanism how the structure changes into SII by adding the THF. Fig. 3 shows a typical gas uptake curve along with temperature profile as a function of time. (at 280.5 K, and 4 bar). It can be found that a sharp increase temperature happens around 125 min since hydrate formation is an exothermic process. The period before hydrate formation is the nucleation process of CO2 hydrate, and the corresponding time is defined as the induction time. The time 0 is chosen after the nucleation period (at the induction time). After the temperature reaches the highest point, it decreases until the set temperature point by the water bath controller. Fig. 4 shows the amount of CO2 gas consumed per mol H2O during the hydrate formation, aCO2 in this experiment at different pressures and temperatures. It is found that the amount of CO2 hydrate formation has almost linear relationship with the reaction time. Therefore, in this study, the amount of CO2 hydrate formation during 90 min is chosen to calculate the average CO2 hydrate

formation rate rCO2 . The linear relationship of the hydrate formation and the time is different from other papers [18,37] in which there are non-linear relationships during 90 min. The reasons are that the pressure in the present study is much lower than the others, which results in the low driving force, and the volume of the THF and distilled water is much larger than the others, therefore, the consumption rate of the CO2 is small. These two reasons lead to the slow reaction process and result in the linear profiles. The error band is estimated as ±7.5%. Fig. 5 shows the CO2 hydrate formation rate as a function of temperature for each pressure. The solid lines denote the results from the present kinetic model [Eqs. (12) and (13)]. From Figs. 4 and 5, it is found that the CO2 hydrate formation rate is strongly dependent on the experimental temperature and pressure. When the pressure is constant, the CO2 hydrate formation rate decreases with increasing the temperature. The reason is that the hydrate formation rate is proportional to the subcooling. Moreover, when the pressure becomes higher, the effect of the temperature on the CO2 hydrate formation becomes more significant. It can also be found the formation rate increases with increasing the pressure. The reason is that the fugacity difference between the gas phase and the hydrate phase is the main driving force for the hydrate formation process. The hydrate formation rate increases with increasing the fugacity difference. If the pressure becomes higher, the fugacity of the gas phase becomes higher, while the hydrate phase doesn't change much. Therefore, the fugacity difference becomes higher, leading to a higher CO2 hydrate formation rate. Fig. 6 shows the relationship between the CO2 hydrate formation rate and the Gibbs free energy difference. The CO2 hydrate formation rate has an exponential relationship with DG/RT. The Boxlucas nonlinear model [38] is used to fit the experimental data. The estimation model for CO2 hydrate formation rate is expressed as Eq. (14), and the experimental coefficients C1 and C2 with the experimental errors are summarized in Table 4.



  DG r ¼ 2:03  1  exp 19:2  RT

(14)

In the present model Eq. (14), the reaction driving force (the Gibbs free energy difference) is a function of temperature, fugacity and qj. Fugacity is mainly determined by the experimental pressure and it increases with the increase of pressure, and qj depends on

Fig. 3. A typical gas uptake curve along with temperature profile as a function of time at 280.5 K, 4 bar.

Q. Sun, Y.T. Kang / Energy 91 (2015) 712e719

Fig. 4. The amount of CO2 gas consumed per mol H2O during the hydrate formation at different pressure and temperature.

717

Fig. 6. The relationship between the CO2 hydrate formation rate and the Gibbs free energy difference.

Table 4 Values of model parameters.

both experimental temperature and pressure. The temperature is in the denominator of Eq. (14), so the driving force is very sensitive to the experimental temperature. From the calculation, the DG/RT increases with the decrease of the temperature and the increase of the pressure. As shown in Fig. 6, the present model predicts the experimental results with the error bands of ±23%. Figs. 7, 8 and 9 show the comparisons between the literature [14,18,37,39] and the present model prediction of the CO2 hydrate formation rate. Sabil et al. [18]’s data is for CO2 hydrate with THF, Roosta et al. [37] is for only CO2 hydrate without THF while Sharma et al. [39] is for the methane hydrate with THF. In Fig. 7, it is found that the CO2 hydrate formation rate from the present model has the same trend but lower than that from the literature [18], the difference is about 30%. There are several reasons causing this difference. Firstly, in Sabil et al. [18], they conducted the experiments with THF concentration of 5 mol %, while the present study did the experiments with THF concentration of 5.56 mol %. Secondly, the hydrate formation pressure in the present study is much lower than

that in Sabil et al. [18]. Thirdly, the stirring rate affects the hydrate formation rate. The stirring rates for the present study and Sabil et al. [18] are 300 rpm and 500 rpm, respectively. However, it is difficult to come to a conclusion that if the CO2 hydrate formation rate increases with decreasing the concentration of THF. Roosta et al. [37] stated that THF at very low concentrations was more effective than that at higher concentrations, while several researchers found that the CO2 hydrate formation rate would decrease with the increasing concentration of THF [16,18]. Compared with our previous work [14], the CO2 hydrate formation

Fig. 5. The hydrate formation rate at different temperatures and pressure.

Fig. 7. Comparison between literature [14,18] and the modelling prediction of the hydrate formation rate.

Parameters

Values

Error

C1 C2

2.03 19.2

0.54 2.53

718

Q. Sun, Y.T. Kang / Energy 91 (2015) 712e719

Fig. 8. Comparison between literature [37] and the modelling prediction of the hydrate formation rate.

rate in the case of THF (6 mol %) is 7.04  109 mol=molH2 O$S at 1 bar 0
C, and the CO2 hydrate formation rate calculated by the present model is 5.48  109 mol=molH2 O$S , and the difference is about 28% which may be caused by the experimental conditions such as the stirring methods, main reason is that in our previous work [14], the researcher used the overhead stirring bar, which gives better effect on the hydrate formation rate than the magnetic stirring bar. In Fig. 8, the present model and Roosta et al. [37] predict that the CO2 hydrate formation rate increases with increasing the pressure and decreasing the temperature. However, it is found that the present model gives higher formation rate at 275.2 K than Roosta et al. [37] while the present model gives lower formation rate at 276.2 K. The experimental result from Ref. [37] doesn't match the present model. This is because Roosta et al. [37] used only CO2 gas without THF, and the CO2 hydrate structure is SI. However, in the present study, THF hydrate is chosen as the basic hydrate which occupies the big cavities and forming the structure SII. So the mechanism of the CO2 hydrate formation doesn't match each other. In Fig. 9, the model [39] is for the methane hydrate formation with THF, and it is found that the trend of the present model is different from the experimental data in Division [39]. The reason is that the gas is methane in Division [39] and CO2 in the present study, respectively. The parameters C1 and C2 are related with the solution components and gas species. It is not suitable to use the present C1 and C2 for different kinds of gas species. From the above discussion, it is concluded that the present model should be applied only for the case of CO2 with THF. Fig. 10 shows the schematic of the CO2 hydrate cooling system proposed in the present study. The CO2 gas is introduced to the hydrate reactor to form the CO2 hydrate, and then the CO2 hydrate with THF is transported to the CO2 dissociation unit, which functions as a heat exchanger like the evaporator in the cooling system. In the dissociation unit, the CO2 gas is dissociated by the heat exchange with the chilled water of which inlet temperature is 12
C. The chilled water is cooled down to around 7
C by the high dissociation enthalpy from the CO2 hydrate with THF, and then the chilled water is supplied to the district cooling zone for cooling application.

5. Conclusions

Fig. 9. Comparison between literature [39] and the modelling prediction of the hydrate formation rate.

In the present study, the CO2 hydrate with THF is formed in a stirred semi-bath reactor. The experimental pressure and temperatures varied from 278.7 to 280.6 K and from 3 to 5 bar, respectively. The kinetic data at the low driving force conditions, which are suitable for the cooling application, was given, and the concept of how to apply the CO2 hydrate in the cooling system was shown in Fig 10. Meanwhile, a model was developed to predict the CO2 hydrate formation rate, in which the Gibbs free energy difference was chosen as the driving force. The results are as follows:

Fig. 10. The schematic of the CO2 hydrate cooling system developed in the present study.

Q. Sun, Y.T. Kang / Energy 91 (2015) 712e719

(1) It is confirmed that the presence of THF drastically reduces the CO2 hydrate formation pressure. Therefore, it can be used for the cooling application near the atmospheric pressure by adding THF. (2) The CO2 hydrate formation rate increases with increasing pressure and decreasing temperature in the test section. (3) An experimental correlation was developed to predict the CO2 hydrate formation rate with THF. It fits well the experimental data with the error bands of ±23%. (4) Based on the comparisons of the present study and the literature, it is concluded that the present model should be applied only for the CO2 hydrate formation process with THF at low pressures (lower than 5 bar). Acknowledgements This work was supported by the Korea CCS R&D Center (KCRC) grant funded by the Korea government (Ministry of Science, ICT & Future planning) (No.NRF-2014M1A8A1049304). References [1] Baik Y-J, Kim M, Ra H-S. Simulation on the performance of carbon dioxide and hydrocarbon heat pumps for moderate to high temperature heating. Int J AirCond Refrig 2014;22:1450001. [2] Aaron D, Tsouris C. Separation of CO2 from flue gas: a review. Sep Sci Technol 2005;40:321e48. [3] D'Alessandro DM, Smit B, Long JR. Carbon dioxide capture: prospects for new materials. Angew Chem Int Ed Engl 2010;49:6058e82. [4] Kang S-P, Lee H, Lee C-S, Sung W-M. Hydrate phase equilibria of the guest mixtures containing CO2, N2 and tetrahydrofuran. Fluid Phase Equilib 2001;185:101e9. [5] Liu N, Gong G, Liu D, Xie Y. Effects of additives on carbon dioxide hydrate formation. Proc 6th Int Conf Gas Hydrates (ICGH 2008), Vancouver, Br Columbia, CANADA. 2008. [6] Jerbi S, Delahaye A, Fournaison L, Haberschill P. Characterization of CO2 hydrate formation and dissociation kinetics in a flow loop. Int J Refrig 2010;33: 1625e31. [7] Shi XJ, Zhang P. A comparative study of different methods for the generation of tetra-n-butyl ammonium bromide clathrate hydrate slurry in a cold storage air-conditioning system. Appl Energy 2013;112:1393e402. [8] Chung JD, Moon S-J, Lee Y-P, Lee J-H, Lee C-J, Yoo H. Feasibility of ice-slurry application to the district cooling system in Korea. Int J Air-Cond Refrig 2014;22:1450018. [9] Xu A, Liu Z, Zhao T, Wang X. Population balance model of ice crystals size distribution during ice slurry storage. Int J Air-Cond Refrig 2014;22:1440001. [10] Fournaison L, Delahaye A, Chatti I, Petitet J-P. CO2 hydrates in refrigeration processes. Ind Eng Chem Res 2004;43:6521e6. [11] Marinhas S, Delahaye A, Fournaison L, Dalmazzone D, Fürst W, Petitet J-P. Modelling of the available latent heat of a CO2 hydrate slurry in an experimental loop applied to secondary refrigeration. Chem Eng Process Process Intensif 2006;45:184e92. [12] Kang S-P, Lee H, Ryu B-J. Enthalpies of dissociation of clathrate hydrates of carbon dioxide, nitrogen, (carbon dioxide þ nitrogen), and (carbon dioxide þ nitrogen þ tetrahydrofuran). J Chem Thermodyn 2001;33:513e21. [13] Delahaye A, Fournaison L, Marinhas S, Chatti I. Effect of THF on equilibrium pressure and dissociation enthalpy of CO2 hydrates applied to secondary refrigeration. Ind Eng Res 2006;45:391e7.

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