Promotion Effect of Carbon Nanotubes-Doped SDS on Methane

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Promotion Effect of Carbon Nanotubesdoped SDS on Methane Hydrate Formation Yuanmei Song, Fei Wang, Guo-Qiang Liu, Sheng-Jun Luo, and Rong-Bo Guo Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b02418 • Publication Date (Web): 05 Jan 2017 Downloaded from http://pubs.acs.org on January 6, 2017

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Promotion Effect of Carbon Nanotubes-doped SDS on Methane Hydrate Formation Yuanmei Song,†,‡ Fei Wang,†,‡ Guoqiang Liu,§ Shengjun Luo,†,* and Rongbo Guo†,* †

Shandong Industrial Engineering Laboratory of Biogas Production & Utilization,

Key Laboratory of Biofuels, Qingdao Institute of Bioenergy and Bioprocess, Qingdao 266101, China ‡

University of Chinese Academy of Science, Beijing 100049, China.

§

Qingdao University of Science&Technology, Qingdao 266042, China

ABSTRACT: To achieve greater performance of low concentration of sodium dodecyl sulfate (SDS) in the methane hydrate formation, the SDS solutions were doped with pristine carbon nanotubes (pCNTs) and the oxidized forms (OCNTs). With nanotubes as the predominant accelerant in the mixed promoters, the nucleation stage could be shorten efficiently due to the continuous Brownian motion of more nanoparticles. The mixed promoters exerted more pronounced influence on the methane hydrate growth rate compared to the pure SDS, with OCNTs-SDS system performed slightly better owing to the high dispersion and stability of the OCNTs in the aqueous SDS solutions. Nevertheless, the promotion effects of high concentrated pCNTs or OCNTs could be weakened possibly due to the aggregation of nanotubes. KEYWORDS: Carbon nanotubes; mixed promoters; methane hydrate growth rate; induction time 1.

INTRODUCTION The gas hydrate is ice-like crystalline clathrate, which is externally bonded by

water molecules through hydrogen bonds and internally by the gas molecules such as methane through Vander Waals bonds1. In recent years the hydrate technology have been applied in Gas engineering due to its unique gas storage capacity of 180 volumes methane per volume of hydrate, assured safety and economic feasibility2. However, the further development of hydrate-based technology for Gas storage and 1

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transportation was limited by long induction time and slow growth rate during the hydrate formation process3-5. In spite of numerous proposed methods to deal with these problems, addition of chemical reagent which can alter the kinetic or thermodynamic properties of the hydrate formation was regarded as more attractive way6. Surfactants have been proved to be an effective and low-cost promoters3, 7-9, among which the sodium dodecyl sulfate (SDS) performed best in promoting the methane hydrate formation process especially in expediting the hydrate growth rate. However, the methane hydrate doped with surfactant can generate considerable foams during the dissociation process which exerts negative influence on the practical application of gas hydrate 10. Application of nanoparticles in the reagent has been also proved to be a promising idea in gas hydrate formation according to many researchers10-13. For example, some nanoparticles such as ternary SiO2-Ag-TiO2 nanoparticles, silver nanoparticles and polystyrene nanospheres not only improved the hydrate formation process but also altered the methane hydrate growth pattern10. A fancy one-dimensional material, the carbon nanotubes (CNTs), has been ranked as one efficient accelerant to improve methane hydrate formation. The hydrophobic CNTs exhibited enhanced growth rate at higher concentrations, whereas the hydrophilic MWCNTs exhibit initial local maximum effect at lower concentrations14. Moreover, the nanotubes of shorter length resulted in shorter hydrate formation time and consumed higher amount of gas as well15. Basically, there are three theories that account for the promotion mechanism: carbon nanotubes are equipped with great adsorptive property for gases due to highly uniform pore size, high surface areas, and attractive surface potentials; the nanoparticles in the liquid phase behave as numerous microstirrers which keep agitating in forms of Brownian motion to update the gas-liquid interface; and the existence of nanoparticles are supposed to enhance the mass transfer coefficient16. A limitation for the as-produced CNTs was the easy agglomeration and sediment of the more nanotubes in aqueous system due to the their higher respective surface energy17. Many methods including mechanical ways (e.g. 2


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milling, stirring and sonication) and chemical (covalent18, 19 and nonvalent20, 21 means) ways have been reported to effectively solubilize nanotubes in aqueous system, among which surfactants at appropriate concentration played important role in improving CNTs dispersion and stability. It was extrapolated that the nanotube individuals could be exfoliated from bundles by SDS adsorbed on their surface in forms of cylindrical micelle or hemimicelles22, 23. Inspired by the literature on the good dispersion ability of nanotubes in SDS as well as the excellent promotion effects of both SDS and the nanotubes on the hydrate formation process, this work is highlighted that the addition of nanotubes particles into the surfactants could achieve higher growth rate and shorter induction time during the methane hydrate formation. Thus, the nucleation stage and growth rate of the methane hydrate formation were investigated with the mixed promoters systems. 2.

EXPERIMENTAL SECTION 2.1. Materials. Methane (purity > 99.99%) was provided by Heli Gas Company; sodium dodecyl

sulfonate (SDS,A.R.) (purity >99.8%) was provided by Xiya Reagent Company; the pristine MWCNTs (purity >99.9%, diameter~8-15nm and length ~50µm) was purchased from Xfnano, Inc synthesized by chemical vapor deposition. Nitric acid (95%-98%, A.R.) and sulfuric acid (65%-68%, A.R.) were purchased from Sinopharm Chemical Reagent Co., Ltd. The deionized water used in this experiment was laboratory-made with conductivity of 1.1±0.1µs/cm at 298.15K. 2.2. Chemical treatment of as-received CNTs. The pristine CNTs (pCNTs) were added to 40 mL oxidant composed of sulfuric acid and nitric acid (volume ratio of 1:3) based on the traditional oxidized method24. The oxidation process was performed at 110 oC in oil bath for 24 h. Then the slurry was diluted with deionized water, followed by washing and filtration until the pH close to neutral. The oxidized CNTs (OCNTs) were obtained after dried in vacuum at 55 oC overnight. 2.3. Characterization of carbon nanotubes. 3


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2.3.1. CNTs dispersed in aqueous phase. The water was chosen as the polar solvent to dissolve CNTs to prepare 1 g/L suspensions. After sonication at 40 W(Power) for 1 hour in the sonicator, the suspension was left undisturbed for a certain time. 2.3.2. Transmission electron microscopy. TEM was performed on the microscopy of JEM-1200EX at 100 kV. Sample preparation involved sonicating materials in ethanol for approximately 5 minutes and a drop of suspension was put onto a carbon film supported by copper grinds. 2.3.3. Raman spectroscopy. The Raman spectra of CNTs were obtained through in-Via Raman Spectrometer manufactured from Renishaw. The light source was Ar+ laser at a wavelength of 532 nm and a power of 100 mW. The spectrum ranges is 500-3500 nm-1. 2.4 Procedures for methane hydrate formation. The diagrammatic representation of the experimental set-up is shown in Figure 1. It is an interconnected four-reactor set-up made of 316L stainless steel (roughness≤0.2 µm). The other main compartment is the temperature-controlled liquid bath filled with mixture of glycol and water with volume ratio of 1:2 as the cooling fluid. The temperature was controlled at 275.15 K with the accuracy of 0.01 K. Besides, a magnetic stirring set-up was fixed beneath the reactors in order to enhance the proper suspension mixing.

Figure 1.Schematic diagram for experimental set-up 4


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Initially, the reactor was washed and rinsed with deionized water for 6 times to remove residual hydrate. Then prepared 10 mL-solution was injected into the reactor and the reactor was flushed 3 times with methane to evacuate air from the cell. When the temperature reached equilibrium temperature of 275.15 K, the reactor was pressurized with methane till the required 6 MPa was obtained and the stirrer was adjusted to 300 rpm. To avoid the melting of hydrate, the reactor was depressurized immediately after hydrate formation completed, which caused the temperature decrease rapidly lower than 273.15 K. The photograph of the hydrate morphology was taken after the reactor was opened. 3.

RESULTS AND DISCUSSIONS 3.1. Characterization of MWCNTs. 3.1.1. Stability of nanotube suspension

Figure 2. nanotubes dispersion in water (a) before sonication; (b) after sonication; (c) 30 minutes later; (d) 20 hours later

In Figure 2(a) and Figure 2(b), the oxidized nanotubes were tiny particles which were difficult to aggregate while the pristine CNTs with oversize grains performed poorly in the aqueous phase as they appeared to settle down at the bottom of the test tube as soon as taken out from sonicator. Moreover, the OCNTs suspension tend to keep homogeneous even 20 hours later (shown in Figure 2(c)) whereas the CNTs suspension become transparent with sedimentation of all particles at the tube bottom (shown in Figure 2 (d)). It was obvious that the acid treatment of pristine carbon 5


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nanotubes would generate smaller CNTs particles and enhance their solubility and stability in the aqueous phase. 3.1.2. Transmission electron microscopy

Figure 3. TEM images for pristine CNTs (a) and oxidized CNTs (b)

The possible morphological changes of the CNTs before and after acid treatment could be observed directly in TEM images in Figure 4. The pristine CNTs were intertwined with each other and forming into mass while the acid-functionalized nanotubes were shorter fragments with uneven length. It indicated that strong oxidant could act as scissors to cut long CNTs into short tubes. Thus, it was difficult for OCNTs to intertwine and agglomerate in aqueous phase, which could explain the phenomenon in Figure 2. Similar phenomenon was reported by Ziegler25 who concluded that the oxidant attacked CNTs to generate vacancies in the damage sites of the graphene sidewall and longer time reaction would further broke the graphene structure to yield shorter nanotubes. 3.1.3. Raman spectroscopy. Raman spectroscopy is one valuable technique to characterize the quality and microscopic structure of the carbon nanotubes. The spectrums of as-received CNTs and OCNTs can be found in Figure 5. Both of two spectrums consisted of three characteristic bands. Generally, the D-band at ~1350 cm-1 results from the lattice defects and structural disorders. G-band at around 1580 cm-1 is usually attributed to the vibration peak of the tubular graphite structure and the 6


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G’ band at ~2700 cm-1 originates from the second order mode of the D band26.

Figure 5. Raman spectrum before and after acid treatment

The Raman spectrum of oxidized CNTs is similar to that of the pristine CNTs with almost the same peak positions. It revealed that chemical modification had little effect on carbon structure which was related to many investigations18,

27, 28

. The

intensity of the D-band was enhanced obviously after acid treatment. Integrating the areas of the D and G peaks for both samples was used to compare the defective degree and structural disorder of the carbon nanotubes29. Thus an important parameter R is referred as the intensity ratio of the D band to G band:

, where ID and IG

are the integral areas of the D-band and G-band.. The detailed calculation results are shown as Table S1 where intensity ratio of OCNTs (R1=0.69) is approximate four times higher than that of pCNTs (R2=0.16). Therefore, the acid treatment of the pCNTs produced appreciable defects on the CNTs surface, further resulting in the subsequent destruction of the graphitic integrity and formation of small graphitic fragments. This result was also supported TEM result where a number of short tubes existed after acid oxidation. Therefore, the produced defective sites with more oxygen-containing groups contributed to the higher dispersion and stability of nanotubes in aqueous phase and it is crucial to apply those nanotubes in the methane hydrate formation. 3.2. Methane hydrate formation experiments. 7


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3.2.1. Pure SDS and water. The recorded data including the pressure and temperature during the hydrate formation process were converted to molar numbers of methane referring to our previous method as described by Wang et al.10. For further investigation of the promotion effects of mixed promoters, the SDS solutions and deionized water were used to react with methane as the blank groups under the identical condition. Figure 6(a) describes the evolution of methane consumption in the hydrate formation with different SDS solutions. A small amount of methane was consumed within 300 min when the deionized water was used, which indicated that the hydrate formed quite slowly without additives in reagent. Using SDS as promoters, the hydrate formation could be completed within 250 min. Moreover, two distinct stages were observed: the hydrate nucleation stage and hydrate growth process as marked on the green curve. The former was also called induction period from the time when charging gas into the reactor to the time obvious increase of methane consumption was observed. To obtain the concrete value of the induction time, the methane consumption of 0.002 mol was regarded as demarcation point for the completion of nuclei period and the beginning of hydrate growth10. The hydrate growth period began once the critical size was reached after nucleation stage and end with the balance state that there was no more methane consumption. In view of stochastic nucleation, the hydrate formations with different SDS solutions were repeated for five times. The detailed data for this process were listed in Table S2 and further discussed in session 3.2.4 when compared with the mixed promoters.

8


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Figure 6. (a)Methane consumption evolution during hydrate formation with different SDS solutions; (b) The corresponding hydrate morphology in the reactor

The hydrate morphologies in the reactor was described in Figure 5(b). With the inner reactor of 4.5 cm height and the 0.6 cm liquid level, the hydrates tend to grow upward in forms of hollow structure with increasing SDS concentration. This phenomenon was possibly caused by the capillary effect of the formed porous hydrate crystals with the aid of SDS solutions9, 30. In spite of a little tendency to upward growth at SDS concentration lower than 0.4 mmol/L, the hydrate layers appeared as mushy mass. Consequently, the gas hydrates produced under above conditions were all detrimental to the gas loading and transportation. Therefore the addition of carbon nanoparticles is expected to improve the hydrate formation as discussed in the following sessions. 3.2.2. Various nanotubes loadings in the mixed promoters. In order to minimize the SDS impact on the hydrate formation, 0.1 mmol/L SDS was chosen to mix with nanotubes to form two kinds of mixed promoters system marked as pCNTs-0.1 mmol/L SDS and OCNTs-0.1 mmol/L SDS. Figure 7 describes the moles of methane consumed during the hydrate formation process with different nanotubes loadings in the mixed promoters. Compared with pure water, hydrate formation process completed faster in the mixed promoters systems. When 0.1 mmol/L SDS solution was used, the nucleation lasted for about 60 min, whereas in the mixed promoters systems, the methane consumption kept increasing steadily from the beginning time to equilibrium state. No clear demarcation points between the nucleation stage and hydrate growth process were observed. Therefore, it was extrapolated that there was no induction period when the nanotubes played dominant role in hydrate formation with a minimum concentration of 0.1 mmol/L SDS contained in the mixed promoter 9


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systems.

Figure 7. (a) The evolution of methane consumption during hydrate formation in mixed promoters system with 0.1 mmol/L SDS (b) The corresponding hydrate morphology in the reactor

In Figure 7(a), the hydrate formation tend to slow down when pCNTs loadings exceeded 0.4 mg/mL or OCNTs loadings exceeded 0.05 mg/mL, which showed the promotion effect of the nanotubes could be limited by the high-concentrated nanoparticles. The nanotubes particles were analogical to micro stirrers in the liquid system, with continuous agitation in forms of Brownian motion to promote local displacements and update the gas-liquid interface. In this way, the continuous hydrate growth was available due to the increasing overall surface area of the hydrate42, 43. Nanotubes with higher concentrations could be used to accelerate the methane hydrate formation due to more effective micro stirring. The oxidized nanotubes performed better than pristine CNTs at lower concentrations due to stronger agitation of numerous tiny nanoparticles in the hydrate formation. Nevertheless, the promotion effect was reduced when the pristine CNTs concentration was higher than 0.4 mg/mL or OCNTs concentration higher than 0.05 mg/mL, which was possibly resulted from 10


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the aggregation of nanotubes at higher concentrations. Park et al. reported that the pCNTs and OCNTs content below 0.004% and 0.003% respectively could promote the gas consumption efficiently, which was consistent with the present work17. Figure 6(b) depicts the final morphology and growth pattern of methane hydrate in the reactor. The methane hydrate mainly formed at the bottom of the reactor. Compared with Figure 5(b), the formed hydrates were a little more compact and concentrated at the bottom of the reactor. This phenomenon suggested that nanotubes’ continuous stirring in liquid phase were conductive to compact the hydrate through curbing the hydrate upward growth. Therefore, the addition of small amount of nanotubes especially the OCNTs into reagent could improve the methane hydrate formation process and increase the compactness of the hydrate, which contributed a lot to the gas transportation and storage. 3.2.3. Mixed promoters with various SDS concentrations. The influence of mixed reagents composed of 0.1 mg/mL nanotubes and various SDS concentrations on the methane hydrate formation was also studied. Every experiment was also performed for 5 repetitions given the stochastic nucleation. The induction time and methane hydrate growth rate in the hydrate formation with different nanotubes-SDS systems are listed in Table 1. When the SDS concentration was increased from 0 to 0.8 mmol/L in pCNTs-SDS or OCNTs-SDS system, the average value of the nucleation duration all showed slight tendency to increase firstly and then decrease at 0.8 mmol/L SDS. In spite of that, the SDS concentration within 1 mmol/L had little impact on the induction time taking the standard deviation into account, which was consistent with previous studies3, 31. Zhang31 concluded that the induction time was not changed when the SDS concentration varied from 260 to 10000 ppm under 274 K and 7 MPa. In comparison, the average induction times in table 2(b) were slightly shorter than the results in table 2(a) resulting from oxidized nanotubes in the reaction system. The mechanism of nucleation period in the hydrate formation was still poor understood. Posteraro and Pasieka32 pointed out that SDS molecules arranged at the 11


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gas-liquid interface would reduce liquid surface tension to facilitate more gas diffusion into liquid phase32. Furthermore, SDS molecules could also absorb on the surface of hydrate nuclei, which could also enhance the solubility of the methane and reduce the nuclei-liquid interfacial energy8,

33

. Besides, the main promotion

mechanism of nanotubes was the improved gas-liquid interface through agitation instead of gas adsorption on the nanotubes because the nanotubes did not affect the solubility of gas in the suspensions16, 34. As the oxidized nanotubes had more defective sites than the pristine nanotubes, it was assumed that the SDS molecules were easy to adhere to the surface of oxidized nanotubes in forms of micelle-like structure22. Thus, the local concentrated SDS on the stirring oxidized nanotubes was more efficient in enhancing nuclei growth10. However, the detailed nucleation mechanisms needed further investigations. Table 1. The induction time and growth rate for the hydrate formation with different SDS content in the mixed promoters (a) pCNTs-SDS system SDS content (mmol/L)

Induction time (min) 1st

2nd

3th

4th

5th

Growth rate (mmol/min) av

sd

1st

2nd

3th

4th

5th

av

sd

0

18

14

21

18

13

16.80

3.27

0.22

0.22

0.18

0.16

0.15

0.19

0.03

0.1

23

37

25

20

24

25.80

6.53

0.75

0.43

0.26

0.40

0.31

0.43

0.19

0.2

33

22

20

32

37

28.80

7.40

0.51

0.43

0.64

0.59

0.9

0.61

0.18

0.4

47

33

39

13

24

31.20

13.20

1.44

1.13

2.43

2.81

2.08

1.98

0.69

0.8

8

27

11

6

10

12.40

8.38

1.67

3.23

4.65

4.44

4.25

3.65

1.23

(b) OCNTs-SDS system SDS content

Induction time (min)

Growth rate (mmol/min)

(mmol/L)

1st

2nd

3rd

4th

5th

av

sd

1st

2nd

3rd

4th

5th

av

sd

0

23

19

28

20

20

22.00

3.67

0.24

0.32

0.21

0.19

0.22

0.24

0.05

0.1

17

28

18

19

22

20.80

4.44

0.57

0.42

0.36

0.26

0.90

0.50

0.25

0.2

5

10

24

15

20

14.80

7.60

0.92

1.23

0.27

0.56

0.63

0.72

0.37

0.4

12

16

14

10

21

14.60

4.22

2.00

2.66

2.03

1.75

1.83

2.05

0.36

0.8

19

6

13

20

11

13.80

5.81

2.84

4.19

4.85

5.18

3.79

4.17

0.92

Note: av-average; sd-standard deviation

12


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Figure 8. Hydrate morphology in the reactor with different SDS content in the mixed promoters

As listed in Table 1, when the SDS concentration increased from 0.1 to 0.8 mmol/L, the hydrate growth rate was also elevated from 0.43 to 3.65 mmol/min in the pCNTs-SDS system and 0.5 to 4.17 mmol/min in OCNTs-SDS system. Thus, with fixed nanotubes loadings, the hydrate growth rate was improved greatly when the SDS concentration was elevated in the reagents compared with single nanotubes suspensions, which was in accordance with a number of previous studies35-37. The SDS molecules had influence not only on mass transfer at gas-liquid interface, but also on the liquid-hydrate interface8, 33. Once onset of hydrate formation, hydrate growth rate could be enhanced dramatically through the formed microdomain by surfactant adsorption on the hydrate surface33. The difference of the growth rate between pCNTs-SDS and OCNTs-SDS systems would be discussed in the session 3.2.4. The corresponding hydrate morphologies were shown in Figure 7. Similar to the photographs in Figure 5(b), the higher-concentrated SDS in the mixed promoters also caused the hydrate upward growth along the reactor sidewall. Subtle difference between Figure 5(b) and Figure 7 were that more hydrates aggregated at the bottom of the reactor in the mixed promoters systems than in the SDS solutions, which could also be explained by the agitation effect of nanotube particles in the hydrate formation. In conclusion, the SDS doped with a certain amount of nanotubes were still equipped with excellent ability in expediting the hydrate growth rate, whereas, some induction time was still required before the fast hydrate growth. 13


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3.2.4. Effect enhancement. The differences in the promotion effect between the various mixed promoters were further studied by observing the variations compared with baseline SDS experiments. The different SDS solutions doped with 0.1 mg/mL were also chosen in the comparative analysis. To better understand the differences, the comparative data on the induction time and growth rate in Table S3 were charted in Figure 9. In Figure 8(a), the negative value of induction time enhancement represented for faster nucleation of hydrate formation in the mixed promoters than in the SDS solutions. Both the OCNTs-SDS system and pCNTs-SDS system exerted prominent influence in shortening the induction time at SDS concentration lower than 0.2 mmol/L, which was consistent with the results in session 3.2.2 where the nucleation stage was too short to be detected in 0.1mmol/L SDS-nanotubes system. Moreover, the induction time enhancement in the OCNTs-SDS system was better than in the pCNTs-SDS system within 0.4 mmol/L SDS due to the excellent properties of oxidized nanotubes in the aqueous phase as discussed in Session 3.2.3. However, the gap of induction time enhancement between the mixed promoters system and pure SDS solutions become narrowed with the increasing SDS content. At the concentration of SDS lower than 0.4 mmol/L in the mixed promoters, 0.1 mg/mL nanotubes may play the primary role in the nucleation stage while the advantages of high-concentrated SDS were highlighted in the hydrate nucleation process when SDS content was further elevated. Therefore, the induction times in the three kinds of accelerator systems were almost close at SDS concentration higher than 0.4 mmol/L.

14


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Figure 9. Enhancement of promotion effect of the mixed promoter system compared to SDS solutions: (a) induction time enhancement; (b) hydrate growth rate enhancement

In Figure 9(b), all positive values of the growth rate enhancements suggested more excellent promotion effect of the mixture than that of pure SDS solutions. More individual nanotubes particles would take part in the continuous agitation due to the exfoliation of bundled CNTs with the aid of SDS molecules23, which could improve the hydrate growth rate greatly. Moreover, the growth rate enhancement in the OCNTs-SDS systems was higher than in CNTs-SDS systems, indicating that OCNTs performed best in the hydrate formation possibly due to greater solubility of OCNTs in SDS solutions. To evaluate the degree of dispersion of nanotubes particles in SDS solutions, the absorbance at the maximum absorption wavelength (~254 nm obtained by full spectrum scan in Lambda 25 UV/VIS spectrometer) was measured in a UV-Vis spectrophotometer (UV2000, UNICO). The detailed procedures were described in supporting material. The individual CNTs rather than the bundled ones can exhibit their characteristic bands due to the activity in the UV-vis region20. Therefore, a relationship can be established between the amount of dispersed nanotubes individually in the SDS and the intensity of the maximum absorption wavelength. Figure 10 depicts the absorbance evolutions when nanotubes were dispersed in different SDS solutions. Evidently, the increasing SDS concentration resulted in higher absorbance values, suggesting more individual nanotubes existed in the SDS solutions which were in accordance with previous work20. Furthermore, the absorbance values of OCNTs-SDS systems were all above that of pCNTs-SDS systems, which highlighted the greater dispersion of OCNTs in the SDS solutions. As a result, a large amount of individual OCNTs with Brownian motion in the SDS solutions exhibited stronger agitation. This accounted for why the oxidized nanotubes were more effective in promoting hydrate growth rate.

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Figure 10. Evolution of the value of the absorbance at different SDS concentrations in the mixed promoter system

Regardless of the decreasing enhancement with the growing SDS concentration, the addition of nanotubes into the aqueous surfactant solutions still improved the promotion effect of low concentration of SDS solution both on reducing the induction time and increasing hydrate growth rate. 4.

CONCULSION Impacts of sodium dodecyl sulfate doped with carbon nanotubes on the methane

hydrate formation were investigated in this study. Defective sites that produced during the acid treatment of as-received CNTs improved the stability and dispersion of nanotubes in aqueous phase. Due to the continuous Brownian motion of nanoparticles, the induction time of the methane hydrate formation was shortened obviously by the addition of nanotube particles into SDS solutions at concentration lower than 0.4 mmol/L. Compared with the pure SDS, mixed promoter was more effective in the enhancement of the hydrate growth rate. Especially, the higher dispersion of OCNTs in aqueous phase led to the best performance of OCNTs-SDS system. However, high-concentrated nanotubes could weaken the promotion effect probably due to nanotubes particles’ aggregation. In conclusion, the addition of a small amount of nanoparticles into surfactants could be a promising method for accelerating the hydrate formation process.

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ASSOCIATED CONTENT

Supporting information The supporting information is available free of charge on the ACS Publication website. Calculation results for Intensity ratios (Table S1) Induction time and hydrate growth rate of hydrate formation in pure SDS solutions (Table S2) The effect enhancement between the mixed promoters system and the SDS baseline (Table S3) The procedures for measuring the dispersion of nanotubes in SDS solutions

AUTHOR INFORMATION

Corresponding Author *Telephone: 0532-66060239. E-mail: [email protected] (R.B.Guo); Telephone: 0532-58782861. E-mail: [email protected] (S.J.Luo) Notes The authors declare no competing financial interest.

ACKNOWLEGMENTS The financial was supported by the National Science and Technology Pillar

Program (20140015), Qingdao Science and Technology and People’s Livelihood Project (14-2-3-69-nsh) and The Integrated and Industrialized Research of the Gasified Grain-based Residuals (2014BAC31B01)

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Promotion Effect of Carbon Nanotubes-Doped SDS on Methane

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