Semi-clathrate hydrate process of methane in porous media-mesoporous materials of SBA-15

Fuel 220 (2018) 446–452

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Full Length Article

Semi-clathrate hydrate process of methane in porous media-mesoporous materials of SBA-15 ⁎

T

⁎

Jianzhong Zhaoa,b, , Yangsheng Zhaoa,b, , Weiguo Liangb,c, Su Songd, Qiang Gaoa a

Mining Technology Institute, Taiyuan University of Technology, Taiyuan 030024, PR China Key Laboratory of In-situ Property Improving Mining of Ministry of Education, Taiyuan University of Technology, Taiyuan 030024, PR China c College of Mining Engineering, Taiyuan University of Technology, Taiyuan 030024, PR China d Department of Mechanical Engineering, Keio University, Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan b

A R T I C L E I N F O

A B S T R A C T

Keywords: SBA-15 Semi-clathrate hydrate Tetrahydrofuran(THF) Tetra-n-butyl ammonium bromide (TBAB) Formation

The hydrate based technology process in porous media is an effective method for gas storage and separation. The formation of methane hydrate in SBA-15, an ordered mesoporous silicon material, was measured with tetrahydrofuran (THF) and tetra-n-butyl ammonium bromide (TBAB) aqueous solution at different pressure and temperature·THF and TBAB were selected to lower the equilibrium pressure at which the gas hydrates can efficiently formed. At the experimental condition of 1.8 MPa–2.0 MPa, SBA-15 was proved to be effective to methane uptake in the hydrates. The results showed that mesoporous silica can help to get the fast-kinetic formation of methane hydrates. The temperature was almost steady and only had two slight increase no more than 0.5 K. The pressure dropped rapidly and then became gentle, which translated into an increase in gas uptake rapidly in the beginning of 100 min and then maintained stable till the end. The reaction rate quickly reached the peak then rapidly dropped and slowly decayed. The desirable kinetics parameters of final gas uptake capacity, average of reaction rate and conversion of gas to hydrate (%) were obtained at higher pressure and lower temperature while THF was better than TBAB. A gas uptake capacity of 91.13 mmol methane gas per mole water was achieved when the hydrates were formed in the presence of THF with SBA-15.

1. Introduction Gas hydrates (or clathrate hydrates) are non-stoichiometric ice-like solid compounds consisting of gas molecules and water molecules [1,2]. Generally, gas hydrates have three basic crystal structures (structure I, structure II, and structure H) where gas molecules (guest molecules) are enclathrated in cavity structures that are formed by hydrogen-bonded water molecules (host molecules) [3,4]. Methane hydrates are formed at high pressure and low temperature with solid crystalline structure that is made of hydrogen bonded water cages in which a methane molecule is encaged [5,6]. Upon dissociation, one volume of solid methane hydrates can release 150–180 volumes methane gases at a standard temperature and pressure (STP) [7]. Gas hydrates have very unique properties of heat-mass transfer and are being applied in many areas such as the gas storage, separation [8] and capturing medium [9–11]. However, the large-scale application of methane hydrates has been hindered by a few practical problems, including the slow formation kinetics of gas hydrates and low gas uptake capacity [12]. Increasing hydrates formation rate might compromise the gas capacity, therefore ⁎

reducing its effectiveness [13]. The un-reacted interstitial water trapped between solid hydrates particles is also another problem, which occupies a large percentage of the total volume, therefore not only increases the maintenance cost but also decreases the gas uptake capacity [14]. There are two approaches to overcome the slow kinetics of hydrate formation: the first approach is innovative reactor designs including the use of different porous media and the second approach is the use of kinetic promoters (predominantly surfactants) in order to enhance the rate of hydrate formation [15]. Many studies have been carried out to improve the kinetics of hydrate formation by enhancing the mass and heat transfer during the hydrate formation process in different materials [16,17]. Some researchers have used porous media to make water molecules scatter on the pore surfaces of the materials [18,19], which increases the contact area of the gas and water, thereby accelerating gas hydrate formation [20–22]. Zhou et al. have reported that using water-wetted activated carbon can promote the rate of natural gas hydrate formation, reduce the equilibrium pressure, and save costs [23,24]. Especially, over the last 10 years, there has been growing interest in new materials engineering methods for enhancing the rate of gas hydrate formation, which involved in sand packs [25–27], hollow

Corresponding authors at: Mining Technology Institute, Taiyuan University of Technology, Taiyuan 030024, PR China. E-mail addresses: [email protected] (J. Zhao), [email protected] (Y. Zhao).

https://doi.org/10.1016/j.fuel.2018.01.010 Received 30 September 2017; Received in revised form 20 November 2017; Accepted 3 January 2018 0016-2361/ © 2018 Elsevier Ltd. All rights reserved.

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with time and issued commands of experiments according to a prescribed program.

silica [28], and hydrogels [29]. A common observation in all of these studies was that the rate of gas hydrate formation increased as the water saturation was decreased [30,31]. Though the kinetics of hydrate formation is improved by a choice of suitable reactor design or kinetic promoter, there is no reduction of the high energy requirement due to low temperature and high-pressure conditions of hydrate formation. However, some problems still need to be studied. For example, how to choose appropriate promoters to further reduce the hydrate formation pressure, and what changes will happen to the structure of hydrates after adding the promoter [32–34]. The most important is what size of pore structure is the most suitable for hydrate formation and application. Apart from different materials, different thermodynamic and kinetic promoters are being evaluated for improving the hydrate formation conditions and the kinetics respectively [35]. The presence of THF or TBAB can significantly shift the hydrate phase equilibrium to lower pressure for flue gas mixtures (CH4/CO2/N2). Especially, some major advances have been achieved on mixed hydrate formation in the presence of additive likes THF whereby there is no need for even the addition of any materials as THF can do both the job of providing thermodynamic and kinetic promotion [36]. THF can form structure II (S-II) hydrates with water and small molecules such as CH4, CO2 or N2 while TBAB forms the title semi-clathrate hydrate crystal, C16H36N+·Br-·38H2O [37]. There has been a series of works on the use of bio-friendly amino acids to promote the methane hydrate formation got better enhancement of methane hydrate formation [38,39]. But the thermodynamic promoter can enter the clathrate cage, resulting in a penalty on the storage capacity of methane is inevitable. Thus, a hybrid combination approach is required to address all the major challenges of the hydrate technology process chain wherein the improvement in kinetics of hydrate formation along with milder hydrate storage conditions is necessary. Ordered mesoporous silica (pores diameters between 2 and 50 nm) has been brought to academic attention due to its unique properties compared to dense materials [40,41], for example, SBA-15 with a 2-D hexagonal structure have high surface-area-to-volume ratios, which may make them more reactive due to their mesoporous structures. Such mesoporous silica powders have been used for catalysis in chemical engineering [42]. Hydrate formation in the mesoporous space is a new attempt which can reduce gas hydrate formation pressure by adding promoter. Theoretically, the mesoporous silica not only can accommodate adsorption sites for molecule of methane and water on its surface, but also helps the fast-kinetic formation of methane gas hydrates. Hence, the formation of methane hydrates was examined in ordered mesoporous silicon SBA-15 at the different promoters and pressure-temperature condition. Kinetics of methane hydrate formation for TBAB and THF aqueous systems had been investigated at an initial pressure conditions of 2.0 and 1.8 MPa and at 279.15 K and 282.15 K with SBA-15.

2.2. Materials Mesoporous silicon is porous media and also as promoting media for methane hydrate formation. SBA-15 was supplied by Jiaxing Tanli new materials R&D Co., Ltd. Table 1 listed the detailed related properties of SBA-15 which were provided by the manufacturer and several important pore properties have been characterized in Section 3.1. In the present analysis, deionized water and 99.9% methane gas (Beijing AP BAIF Gases Industry Co., LTD, Beijing, China) were used. THF was purchased from Tianjing Beicheng Chemical Co., Ltd., with a certified mass purity higher than 99%. THF was dissolved in deionized water to form THF solutions with concentrations of 5.56 mol %. (The theoretical proportion of the semi-clathrate hydrates, THF: H2O = 1:17) Analytical-grade TBAB with the purity of 99.0 mol% was from Tianjing Fucheng Chemicals Ltd. TBAB was dissolved in deionized water to form TBAB solutions with concentrations of 2.56 mol %. (The best proportion of the semi-clathrate hydrates theory, C16H36N+·Br-·38H2O) THF or TBAB solution with desired concentration was then slowly added to the SBA-15 by mechanical mixing to prepare the wet sample. 2.3. Experimental procedure Firstly, after the reactor was cleaned and dried, the mesoporous silicon SBA-15 presaturated with aqueous solution of THF or TBAB were introduced into the tube reactor. After that, the reactor was connected with the gas pipeline and put into the water bath. Subsequently, the reactor was purged with methane gas to a pressure of 0.5 MPa three times to remove residual air in the porous. Following that, the reactor compartment was pressurized to experimental pressure with CH4 gas and allowed to stabilize at 290 K under conditions outside the hydrate equilibrium region. After the stabilization of pressure and temperature, the reactor was cooled to predetermined temperature of water bath. Exothermic methane hydrate formation is distinguished by an increase in temperature accompanied with drastic pressure drop. Once the pressure and temperature within the reactor stabilized around the set point, the desired hydrate was attained, and hydrate formation stage was considered complete. In this case, the volume and the bath temperature are constant while the pressure decreased during the experiment. As the cell temperature is lowered the pressure decreases, principally due to gas contraction as well as increased gas solubility upon cooling at constant volume. Neither gas nor water were added to the system during the experiment. During the whole hydrate formation process, the temperature and pressure were recorded by the data collection system.

2. Experiment

2.4. Calculation of kinetics and gas uptake capacity

2.1. Experimental apparatus

In the beginning, the pressure of the reactor dropped continuously due to the enclathration of methane into the hydrate cages. Hydrate formation is considered complete when the pressure of the reactor system stabilized with no further drop [43]. At the beginning, amount of methane gas was introduced into the reactor ng,0 is calculated from Eq. (1):

Experiments were carried out on a setup schematically shown in Fig. 1. Water saturated mesoporous silicon SBA-15 was packed into a tube reactor of length 1000 mm and inner diameter 11.0 mm. The pressure transducer was bought from Micro Sensor Co., Ltd., the type is MPM480 and the precision is 0.01 MPa. The thermocouples were supplied by Beijing Kunlun Ocean Instrument Company Ltd., the type is KYW-T1 and the precision is ± 0.01 K. The zero point of transducers was adjusted automatically to compensate for the fluctuation of room temperature. There were eight thermocouples set along the tube reactor to record the distribution of temperature. All parts were connected by stainless steel capillary tubes of inner diameter 2.0 mm and wall thickness is 0.5 mm. The computer recorded the variation of signals

ng,0 =

P0 Vg z 0 RT0,avg

(1)

where P0 is the pressure in the reactor, Vg is the volume of reactor by which methane gas occupies, z0 is the compressibility factor at P0 and T0,avg computed using Pitzer correlations, R is the universal gas constant and T0,avg is the average temperature over all measured points within 447

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feed gas

1

1. gas valve;

T

P

2

2. back-pressure regulator;

8

4 3

3. three-way valve; 4. pressure regulator;

9

5. Connector; 6. water bath;

5

7

off gas

7. Reactor; 8. P-T transducer;

6

9 T-type thermocouples

Fig. 1. Schematic flowchart of experimental setup.

dNt d Δnt ⎞ Δnt + Δt −Δnt =⎛ = dt Δt ⎝ dt ⎠t

Table 1 The pore structure parameters of SBA-15. Feature

Value

Crystal symmetry Space group symmetry Composition wt% Impurities: Fe2O3 wt% Granularity mesh SBET m2/g SMicro m2/g Pore size nm Pore volume cm3/g

Hexagonal P6mm 99.9% silica < 0.01 100 816 128 9.7 1.19

(4)

where, Δt is the time difference between two observations, and is taken as 30 s and the average rate of methane hydrate formation was calculated every 60 min and plotted. Conversion of gas to hydrate at the end of experiment is calculated from Eq. (5):

Cgh (%) = Δnt∗

100 ng,0−neq

(5)

is the final consumption of moles gas. neq is the number of where, moles of methane in the reactor at equilibrium (P and T) that will remain in the gas phase of reactor. It is noted that neq should be accounted in the denominator since that quantity will never participate in hydrate formation due to thermodynamic limit which would otherwise under estimate the gas conversion to hydrates significantly.

Δnt∗

the reactor. When hydrates started forming, the pressure in the reactor reduces and the consumption of moles of gas uptake during hydrate/semiclathrate hydrate formation have been calculated using Eq. (2):

3. Results and discussion

P0 Pt ⎞ Δnt = Vg ⎜⎛ − ⎟ z RT z RT t t ,avg ⎠ ⎝ 0 0,avg

(2)

3.1. Characterization of SBA-15

where, Tt,avg is the average temperature of the reactor at time t, Pt is the pressure of the reactor at time t, zt is the compressibility factor of the gas in the reactor at t. z0 and zt are calculated using Pitzer correlation [44]. The moles of methane gas uptake (Δnt ) per moles of water (nw) are calculated using Eq. (3):

NGt = Δn↓ (mol/ mol) = Δnt / n w

Generally, SBA-15 has a hexagonal ordered ‘meso’ structure, possesses large cell parameters (12–37 nm), high specific areas (690–1040 m2/g), various pore sizes (4.6–30 nm) and thick pore walls (3.1–6.4 nm). The X-ray diffractometer (XRD) using Cu Kα (λ = 1.5406 Å) radiation with a scan rate of 2°/min in the 2θ range of 2° to 20° patterns of the mesoporous silica samples were shown in Fig. 2(a). As shown in Fig. 2(a), the XRD pattern of SBA-15 included one main diffraction peak and two additional peaks, corresponding to the 100, 110, and 200 reflections of 2D hexagonal phase. Combined the cell parameter a = 11.1 nm acquired from the XRD pattern the pore-wall thickness of SBA-15 framework was estimated to be around 6 nm. An ASAP 2020 apparatus was used to measure the nitrogen

(3)

Where, nw is the number of moles of water taken in the reactor for hydrate formation. The average rate of methane hydrate formation has been calculated using a discrete forward difference method as shown in Eq. (4):

a

b 8800

0.4

3

0.3

0.2

0.1

3

600

dV/dD /(cm /(g

d/nm a0 9.6 11.1 5.8 6.7 5.0 5.8

Vads /(cm STP/g)

Intesity (a.u)

hkl 1 100 110 200

nm))

100

110

1

200

400

0.0

0

5

10

15

20

D /nm

200 2

3

4

0.0

5

0.2

0.4

0.6

0.8

p /p 0

2 (Degree)

Fig. 2. Characterization pore structure of SBA-15. a. XRD patterns of SBA – 15. b. N2 sorption isotherms and pore distribution.

448

1.0

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281.5

1.7

281.0

1.6

280.5

280.0

1.5

2.0MPa,282.15K,Pressure 1.8MPa,279.15K,Pressure 2.0MPa,282.15K,Temperature 1.8MPa,279.15K,Temperature

1.8

Pressure (MPa)

1.8

283.0

b. THF Solution

1.9

Temperature (K)

Pressure (MPa)

2.0MPa,282.15K,Pressure 1.8MPa,279.15K,Pressure 2.0MPa,282.15K,Temperature 1.8MPa,279.15K,Temperature

282.0

282.5 282.0

1.7

281.5

1.6

281.0

1.5

280.5

Temperature (K)

2.0

a. TBAB Solution 1.9

1.4 280.0 1.3

279.5

1.4

279.5 1.2 279.0

279.0

1.3 0

200

400

600

800

1000

0

1200

200

400

Time (min)

600

800

1000

1200

1400

Time (min)

Fig. 3. Pressure and temperature profiles of crystallizer during hydrate formation in different promoter with SBA-15.

then became gentle. The temperature kept stable during the whole hydrate formation process because of the special characteristic of SBA15. SBA-15 is one kind of silicon aluminate with homogeneous aperture, polar adsorption function and extremely high specific surface area which has good thermal conductivity and hydrophilicity. The average aperture of SBA-15 is 9.7 nm that water molecule and methane molecule can freely enter into interior. Therefore, the methane hydrate formation process can occur separately on each fine SBA-15 surface and mesoporous. SBA-15 can provide the nucleation site for the methane hydrate formation, reducing the nucleation randomicity and promote hydrate formation. Under the presence of SBA-15, methane hydrate formation process carries on evenly and sufficiently. The reaction heat can be transferred in time, so the system temperature almost maintains stable.

adsorption−desorption isotherms on the SBA-15 at 77 K and the results were shown in Fig. 2(b). The specific surface area of SBA-15 calculated from the BET equation was 816 m2/g. The pore-size distribution determined from the Barrett–Joyner–Halenda equation was shown in Fig. 2(b). The SBA-15 sample showed a very narrow pore-size distribution, with diameters of the pores in the range of around 9.7 nm. 3.2. Profiles of pressure and temperature The hydrate formation process is an exothermic reaction and generally may experiences three stages: induction stage, catastrophic hydrate growth stage and ending stage. If a large amount of hydrates was formed in a short time, the reaction heat cannot be transferred in time which result to rapid increase of the temperature in the reactor. The temperature and pressure variation were shown in Fig. 3 during methane hydrate formation process of 6.0 g SBA-15 powder and 10 ml (according to saturated water absorption of SBA-15) distilled water solution system (THF and TBAB). As shown in Fig. 3, the temperature and pressure profiles of different experimental systems had a similar trend. Moreover, the temperature of reaction process was stable at each stage during the formation. For the entire formation process of hydrate, the system temperature increased slightly two times in a stepwise manner. It was obviously shown in Fig. 3 that the temperature firstly raised about 0.2–0.3 K after 50–80 min of the high-pressure gaseous methane entered into the reactor during the induction stage. The temperature in the THF aqueous solution raised earlier and the duration was shorter than that of the TBAB solution. The system temperature raised for 0.3–0.5 K after around 400–600 min as a second “stage” when the gas uptake process during catastrophic hydrate growth stage has completed, which indicated that lots of methane hydrates formed during this period. The temperatures with different promoter (THF and TBAB) of SBA-15 had similar change tendency. Afterwards, the temperature maintained constant until the reaction finished at the ending stage. It also can be seen from Fig. 3 that temperature did not vary obviously in the last reaction stage, which indicated that the reaction rate almost maintained constant. The gaseous methane can react sufficiently and evenly with water to form methane hydrate over 20 h under the experimental condition. Compared with Fig. 3a and b, the experiments conducted at THF aqueous solution shows higher pressure drops as compared with TBAB aqueous solutions (see Fig. 3(a, b)) at initial pressure of 2.0 and 1.8 MPa. However, the temperature of THF was observed to be more uniform than TBAB aqueous systems. It was also can be shown in Fig. 3, the pressure dropped rapidly and

3.3. Gas uptake capacity variation All the methane hydrate formation experiments were done at 37.5% water saturation (10 ml solution + 6 g SBA-15) at 282.15 K and 279.15 K with the initial pressure of 2.0 MPa and 1.8 MPa. The results obtained during hydrate formation were summarized in Table 2. As shown in Table 2, the adding of SBA-15 and the promoter can promote methane hydrate formation and the gas uptake capacity significantly. The gas uptake capacity was calculated with reaction pressure by the Eq. (2). The final gas uptake capacity of methane hydrate was 11.39 mmol CH4/mol H2O in TBAB solution at 1.8 MPa and 282.15 K. When the pressure increased to 2.0 MPa, the final gas uptake capacity enhanced to 34.29 mmol CH4/mol H2O correspondingly at 282.15 K while it was 76.74 mmol CH4/mol H2O in THF aqueous Table 2 Summary of experimental results during formation of methane hydrates. Solution

Reaction condition (MPa/K)*

Pressure drops (MPa)

Final gas uptake NGt (mmol CH4/ molH2O)

Average rate dNt/dt (mmol CH4/hr)

TBAB TBAB TBAB TBAB THF THF THF THF

2.0/282.15 2.0/279.15 1.8/282.15 1.8/279.15 2.0/282.15 2.0/279.15 1.8/282.15 1.8/279.15

0.14 0.42 0.07 0.40 0.49 0.59 0.21 0.58

34.29 78.50 11.39 64.47 76.74 91.13 29.69 82.32

0.276 0.853 0.135 0.874 0.837 1.025 0.447 1.004

* The first digit indicates pressure of reaction while the second digit indicate temperature in column reaction condition.

449

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J. Zhao et al. 2.0MPa,282.15K,Gas uptake 2.0MPa,279.15K,Gas uptake 1.8MPa,282.15K,Gas uptake 1.8MPa,279.15K,Gas uptake

1.7

60

1.6 40 1.5 20

1.4

1.3

100

b. THF Solution

1.9 80

1.8

Pressure (MPa)

80

1.8

Pressure (MPa)

2.0

100

a. TBAB Solution

Gas uptake (mmol of gas/mol of water)

1.9

1.7

60

1.6 1.5

40

1.4 20

1.3 1.2

0

0 0

200

400

600

800

1000

Gas uptake (mmol of gas/mol of water)

2.0MPa,282.15K,Pressure 2.0MPa,279.15K,Pressure 1.8MPa,282.15K,Pressure 1.8MPa,279.15K,Pressure

0

1200

200

400

600

800

1000

1200

1400

Time (min)

Time (min)

50

60

a. TBAB solution

2.0MPa,282.15K 2.0MPa,279.15K 1.8MPa,282.15K 1.8MPa,279.15K

50

40

40

30

30 20

20 10 0

10

0

50

100

150

200

0

Average rate of hydrate formation (mmol/hr)

Average rate of hydrate formation (mmol/hr)

Fig. 4. Pressure and methane uptake profiles during hydrate formation in different promoter with SBA-15.

b. THF solution

50

2.0MPa,282.15K 2.0MPa,279.15K 1.8MPa,282.15K 1.8MPa,279.15K

60 50

40

40 30

30

20 20

10 0

10

0

50

100

150

200

0

0

200

400

600

800

1000

1200

0

Time (min)

200

400

600

800

1000

1200

1400

Time (min)

Fig. 5. Reaction rate profiles of crystallizer during hydrate formation in different promoter with SBA-15.

was 91.13 mmol CH4/mol H2O. So, the THF can increase the gas uptake capacity obviously and the maximum increase rate was about 13.9% more than that of TBAB. The temperature of the reaction also affected the increase of gas consumption, where the higher temperature resulted in a lower gas consumption. Fig. 4(a, b) showed comparison on the gas consumption for the selected best sets of hydrate system at 2.0 and 1.8 MPa. On the other hand, the initial pressure had some effect on the final gas uptake (Table 2). The final gas uptake increased with the reaction pressure. These results indicated that the pressure and temperature also had significant influence on gas uptake capacity of methane hydrate in SBA15 system. In semi-clathrate hydrate of TBAB, the bromide ions (Br-) takes part in the formation of hydrate cages along with water molecules, while the tetra n-alkyl ammonium cation (TBA+) encapsulate the larger cages of hydrate along with gas molecules which fills the small cages [45]. THF is observed to form structure II hydrate system in which the large (hexadecahedron) cages are filled with THF molecules, while the small pentagonal dodecahedron cages are available for gas molecules to occupy. So, it causes the THF aqueous solution have the better efficiency than that of TBAB in SBA-15.

solution at same reaction condition. Fig. 4(a, b) showed the moles of gas consumed per mole of water and pressure during methane hydrate formation in the presence of THF and TBAB with SBA-15 for a period of 24 h. The growth of gas uptake capacity had the same morphology in different experimental condition. As shown in Fig. 4, the SBA-15 with solution was easy to react with gaseous methane in the preliminary methane hydrate formation stage. The pressure started to decrease rapidly in 100 min after the gas intake process was completed which indicated that a large amount of methane hydrates formed at this stage. From 400 min, the decrease range of pressure lessened and the pressure maintained stable till the end. It can be seen from Table 2 that the final pressure drop was associated with the temperature and the type of promoter. The pressure drops at 279.15 K for THF solution (0.49–0.59 MPa) was far more than that of 282.15 K (0.21–0.58) and TBAB solution. The experiments conducted at THF aqueous solution showed higher moles of gas consumption per mole of water and pressure drops as compared with TBAB aqueous solutions (see Fig. 4 (a, b)) at initial pressure of 2.0 and 1.8 MPa. Similar with TBAB solution, the methane hydrate had maximum gas uptake capacity when the reaction condition is 2.0 MPa and 279.15 K. Under the experimental condition, the gas uptake capacity of hydrate 450

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Conversion of gas to hydrate (%)

70 60

THF TBAB 59.09

concentration of promoters. In order to further increase the actual uptake capacity of methane and reaction rate in some of these hydrate systems, new types of reactor configurations which can provide improved gas-liquid contact can be used. It should be noted here that the obtained results correspond to the experimental set-up used in mesoporous materials. However, the inference that was drawn on the use of mesoporous materials, such as SBA-15 in this study, for the formation of methane hydrate system may form the basis for further investigations.

69.34 63.46

59.72

54.66

50 40 30

26.4

25.5

4. Conclusion

20 10

The results from this study showed that the existence of mesoporous Silicon SBA‑15 can promote methane hydrate formation and enable THF and TBAB solution to react with gaseous methane to form hydrate. The temperature was almost steady and only had two slight increase no more than 0.5 K. The pressure dropped rapidly and then became gentle compare with the gas uptake increased rapidly in the beginning of 100 min and then maintained stable till the end after. The reaction rate quickly reached the peak (within first stage) and then rapidly dropped and slowly decayed to near zero growth. The desirable kinetics parameters, such as a higher final gas uptake capacity, average of reaction rate and conversion of gas to hydrate (%) were achieved at higher pressure and lower temperature while THF was better than TBAB. From 2.0 MPa to 1.8 MPa the gas uptake of hydrate increase about 13.9–61.6%. A gas uptake capacity of 91.13 mmol methane gas per mole water was achieved when the hydrate formed in the presence of THF with SBA-15. Simultaneously the reaction rate on hydrate formation of THF solution was 12.9–69.8% more than that of TBAB solution when the pressure was 2.0 MPa and 1.8 MPa at the experimental conditions.

9.77

0 2.0MPa,282.15K 2.0MPa,279.15K 1.8MPa,282.15K 1.8MPa,279.15K

Fig. 6. Comparison of final conversion of gas to hydrate for various promoters.

3.4. Gas hydrate reaction rate and conversion of gas to hydrate The reaction rate of the hydrate formation process was shown in Fig. 5. At the reaction introduction stage, the wet SBA-15 can adsorb methane molecule and water molecule. It played a key role on methane hydrate formation rate because of its dual polar adsorption effect, leading to an increase in the methane hydrate formation rate and the suitable interior temperature. Therefore, the promotion effect of SBA15 on methane hydrate formation became strong, and the methane hydrate formation rate raised rapidly. The reaction rate quickly reached the peak after the reaction began. Then it quickly dropped and slowly decayed until the end of the reaction. The higher the reaction pressure, the faster the formation rate. While the better formation was achieved at lower temperature (279.15 K). At the same time, the average of reaction rate on hydrate formation in THF solution was much more (ranging between 12.9 and 69.8% higher) than that of TBAB solution when the pressure was 2.0 MPa and 1.8 MPa at the same experimental conditions. The normalized rate of hydrate formation for the experiments reported by Linga et al. was calculated and found to be 0.00485 mol/(h mol of water), which is obviously lower than the range in this study [46]. SBA-15 can cause the water to contact fully with methane gas to form hydrate, and the reasons can be analyzed from the aspects below. The inner pore size of SBA-15 is 9.7 nm, and the molecule dynamic diameters of methane and water are approximately 0.436 nm and 0.29 nm, respectively. The SBA-15 can adsorb water molecule and methane molecule under the experimental condition. Its mesoporous cage can supply third contact surface between water and methane molecules, and reduce the surface energy and chemistry potential barrier which the formation must overcome. Thus, it has positive effect on the hydrate formation. On the other hand, SBA-15 plays a role as an additional heterogeneous phase in promoting hydrate formation. This is why there is a stable temperature and continuous pressure drop in hydrate formation process of SBA-15. Fig. 6 showed the conversion of gas to hydrate (in percentage) for methane hydrate formed from TBAB/THF aqueous solutions with SBA15. It can be observed from the Fig. 6 that, at 2.0 MPa, the conversion of gas to hydrate (%) was observed to be highest for THF at 279.15 K, followed by THF (1.8 MPa, 279.15 K), TBAB (2.0 MPa, 279.15 K), THF (2.0 MPa, 282.15 K), TBAB (1.8 MPa, 279.15 K), TBAB (2.0 MPa, 282.15 K), THF (1.8 MPa, 282.15 K), and TBAB (1.8 MPa, 282.15 K) aqueous solutions, respectively. For the same initial hydrate formation pressure conditions, conversion of gas to hydrate (%) for aqueous system containing THF was found to be higher than TBAB. In addition, it has been observed that the conversion of gas to hydrate are found to be lower at 282.15 K as compared with 279.15 K for the same set of

Acknowledgements The financial support from the National (11772213) and Natural Science Foundation of Shanxi Province (201701D121135) are greatly appreciated. Jianzhong Zhao also greatly acknowledges the support from the State Scholarship Fund from the China Scholarship Council for his one-year visit to NUS Singapore. References [1] Koh CA. Towards a fundamental understanding of natural gas hydrates. Chem Soc Rev 2002;31:157–67. [2] Sloan ED. Clathrate Hydrates of Natural Gases. 2nd ed. New York: Marcel Dekker; 1998. [3] Englezos P, Kalogerakis N, Dholabhai PD, Bishnoi PR. Kinetics of gas hydrate formation from mixtures of methane and ethane. Chem Eng Sci 1987;42:2659–66. [4] Kumar R, Linga P, Moudrakovski I, Ripmeester JA, Englezos P. Structure and kinetics of gas hydrates from methane/ethane/propane mixtures relevant to the design of natural gas hydrate storage and transport facilities. AIChE J 2008;54:2132–44. [5] Makogon YF. Hydrates of Natural Gas. Tulsa (Oklaoma): PennWell Books; 1981. [6] Makogon YF. Hydrates of hydrocarbons. Tulsa (Oklaoma): PennWell Books; 1997. [7] Sloan ED. Fundamental principles and applications of natural gas hydrates. Nature 2003;426:353–9. [8] Zhong Do-Liang, Li Zheng, Yi-Yu Lu, Wang J-Le, Yan Jin. Evaluation of CO2 removal from a CO2 + CH4 gas mixture using gas hydrate formation in liquid water and THF solutions. Appl Energy 2015;158:133–41. [9] Linga P, Kumar R, Englezos P. The clathrate hydrate process for post and pre combustion capture of carbon dioxide. J Hazard Mater 2007;149:625–9. [10] Lee HJ, Lee JD, Linga P, Englezos P, Kim YS, Lee MS, et al. Gas hydrate formation process for pre-combustion capture of carbon dioxide. Energy 2010;35:2729–33. [11] Babu P, Kumar R, Linga P. Pre-combustion capture of carbon dioxide in a fixed bed reactor using the clathrate hydrate process. Energy 2013;50:364–73. [12] Lang XM, Fan SS, Wang YH. Intensification of methane and hydrogen storage in clathrate hydrate and future prospect. J Nat Gas Chem 2010;9:203–9. [13] Vysniauskas A, Bishnoi PR. A kinetic-study of methane hydrate formation. Chem Eng Sci 1983;38:1061–72. [14] Zhong Y, Rogers RE. Surfactant effects on gas hydrate formation. Chem Eng Sci 2000;55:4175–87. [15] Chong ZR, Yang SHB, Babu P, Linga P, Li X-S. Review of natural gas hydrates as an energy resource: prospects and challenges. Appl Energy 2016;162:1633–52.

451

Fuel 220 (2018) 446–452

J. Zhao et al.

[16] Zhong DL, Englezos P. Methane separation from coal mine methane gas by tetra-nbutyl ammonium bromide semiclathrate hydrate formation. Energy Fuels 2012;26:2098–106. [17] Zhao Jianzhong, Zhao Yangsheng, Liang Weiguo. Hydrate-based gas separation for methane recovery from coal mine gas using tetrahydrofuran. Energy Technol 2016;4:864–9. [18] Zhao Yangsheng, Zhao Jianzhong, Shi Dinxian, Feng Zengchao, Liang Weiguo, Yang Dong. Micro-CT analysis of structural characteristics of natural gas hydrate in porous media during decomposition. J Nat Gas Sci Eng 2016;31:139–48. [19] Yang M, Liu W, Song Y, Ruan X, Wang X, Zhao J, et al. Effects of additive mixture (THF/SDS) on the thermodynamic and kinetic properties of CO2/H2 hydrate in porous media. Ind Eng Chem Res 2013;52:4911–8. [20] Zhou Y, Wang Y, Chen H, Zhou L. Methane storage in wet activated carbon: Studies on the charging/discharging process. Carbon 2005;43:2007–12. [21] Sun Y, Wang Y, Zhang Y, Zhou Y, Zhou L. CO2 sorption in activated carbon in the presence of water. Chem Phys Lett 2007;437:14–6. [22] Liu J, Zhou Y, Sun Y, Su W, Zhou L. Methane storage in wet carbon of tailored pore sizes. Carbon 2011;49:3731–6. [23] Zhou L, Liu J, Su W, Sun Y, Zhou Y. Progress in studies of natural gas storage with wet adsorbents. Energy Fuels 2010;24:3789–95. [24] Zhou Y, Dai M, Zhou L, Sun Y, Su W. Storage of methane on wet activated carbon: Influence of pore size distribution. Carbon 2004;42:1855–8. [25] Linga P, Haligva C, Nam SC, Ripmeester JA, Englezos P. Gas hydrate formation in a variable volume bed of silica sand particles. Energy Fuels 2009;23:5496–507. [26] Chong ZR, Pujar GA, Yang M, Linga P. Methane hydrate formation in excess water simulating marine locations and the impact of thermal stimulation on energy recovery. Appl Energy 2016;177:409–21. [27] Chong ZR, Yang M, Khoo BC, Linga P. Size effect of porous media on methane hydrate formation and dissociation in an excess gas environment. Ind Eng Chem Res 2016;55:7981–91. [28] Veluswamy HP, Prasad PSR, Linga P. Mechanism of methane hydrate formation in the presence of hollow silica. Korean J Chem Eng 2016;33:2050–62. [29] Su F, Bray CL, Tan B, Cooper AI. Rapid and reversible hydrogen storage in clathrate hydrates using emulsion-templated polymers. Adv Mater 2008;20:2663–6. [30] Linga P, Clarke MA. A review of reactor designs and materials employed for increasing the rate of gas hydrate formation. Energy Fuels 2017;31:1–13. [31] Chong ZR, Yin Z, Tan JHC, Linga P. Experimental investigations on energy recovery from water-saturated hydrate bearing sediments via depressurization approach. Appl Energy 2017;204:1513–25. [32] Li XS, Zhan H, Xu CG, Zeng ZY, Lv QN, Yan KF. Effects of tetrabutyl-(ammonium/

[33]

[34]

[35]

[36]

[37]

[38]

[39] [40] [41] [42] [43]

[44] [45]

[46]

452

phosphonium) salts on clathrate hydrate capture of CO2 from simulated flue gas. Energy Fuels 2012;26:2518–27. Babu P, Ho CY, Kumar R, Linga P. Enhanced kinetics for the clathrate process in a fixed bed reactor in the presence of liquid promoters for pre-combustion carbon dioxide capture. Energy 2014;70:664–73. Yang Mingjun, Jing Wen, Zhao Jiafei, Ling Zheng, Song Yongchen. Promotion of hydrate-based CO2 capture from flue gas by additive mixtures (THF (tetrahydrofuran) + TBAB (tetra-n-butyl ammonium bromide)). Energy 2016;106:546–53. Veluswamy HP, Wong AJH, Babu P, Kumar R, Kulprathipanja S, Rangsunvigit P, et al. Rapid methane hydrate formation to develop a cost effective large scale energy storage system. Chem Eng J 2016;290:161–73. Veluswamy HP, Kumar S, Kumar R, Rangsunvigit P, Linga P. Enhanced clathrate hydrate formation kinetics at near ambient temperatures and moderate pressures: application to natural gas storage. Fuel 2016;182:907–91. Shimada Wataru, Shiro Motoo, Kondo Hidemasa, Takeya Satoshi, Oyama Hiroyuki, Ebinuma Takao, et al. Tetra-n-butylammonium bromide water (1/38). Acta Cryst 2005;C61:65–6. Veluswamy HP, Kumar A, Kumar R, Linga P. An innovative approach to enhance methane hydrate formation kinetics with leucine for energy storage application. Appl Energy 2017;188:190–9. Veluswamy HP, Lee PY, Premasinghe K, Linga P. Ind Eng Chem Res 2017;56:6145–54. Hartmann M. Ordered mesoporous materials for bioadsorption and biocatalysis. Chem Mater 2005;17:4577–93. Hoffmann F, Cornelius M, Morell J, Fröba M. Silica-based mesoporous organic-inorganic hybrid materials. Angew Chem Int Ed 2006;45:3216–51. Moller K, Bein T. Inclusion chemistry in periodic mesoporous hosts. Chem Mater 1998;10:2950–63. Mech Deepjyoti, Gupta Pawan, Sangwai Jitendra S. Kinetics of methane hydrate formation in an aqueous solution of thermodynamic promoters (THF and TBAB) with and without kinetic promoter (SDS). J Nat Gas Sci Eng 2016;35:1519–34. Smith JM, Van Ness HC, Abbot M. Introduction to Chemical Engineering Thermodynamics. sixth ed. New York: McGraw-Hill; 2001. Shimada W, Ebinuma T, Oyama H, Kamata Y, Narita H. Free-growth forms and growth kinetics of tetra-n-butyl ammonium bromide semi-clathrate hydrate crystals. J Cryst Growth 2005;274:246–50. Linga P, Kumar R, Lee JD, Ripmeester J, Englezos P. A new apparatus to enhance the rate of gas hydrate formation: application to capture of carbon dioxide. Int J Greenhouse Gas Control 2010;4:630–7.