The Effect of Hydrophilic and Hydrophobic Multi-Wall Carbon

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The Effect of Hydrophilic and Hydrophobic Multi-Wall Carbon Nanotubes on Methane Dissolution Rates in Water at Three Phase Equilibrium (V-Lw-H) Conditions James Pasieka, Sylvain Coulombe, and Phillip Servio Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/ie502457c • Publication Date (Web): 22 Aug 2014 Downloaded from http://pubs.acs.org on August 29, 2014

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Industrial & Engineering Chemistry Research

The Effect of Hydrophilic and Hydrophobic MultiWall Carbon Nanotubes on Methane Dissolution Rates in Water at Three Phase Equilibrium (V-Lw-H) Conditions James Pasieka a, Sylvain Coulombe b, Phillip Servio a,* a

Department of Chemical Engineering, McGill University, Montréal, Québec, Canada, H3A 0C5 b

Plasma Processing Laboratory, Department of Chemical Engineering, McGill University, Montréal, Québec, Canada, H3A 0C5 * Corresponding author’s email: [email protected], ph: 514-398-1026.

ACS Paragon Plus Environment

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Abstract Presently, gas hydrates are being studied for their potential applications in technologies involving natural gas transportation, carbon dioxide sequestration, and component separation. In order to optimize their use, research has focused on finding hydrate promoting agents and understanding how they work. One such promoter, multi-wall carbon nanotubes (MWNTs), was found to enhance hydrate growth. The current study investigates the effects of adding plasmafunctionalized hydrophilic MWNTs and as-produced hydrophobic MWNTs on the dissolution stage of methane hydrate formation. It was found that the addition of the hydrophilic MWNTs increased methane dissolution rates with an increase in MWNT loading. Furthermore, the hydrophobic MWNTs initially enhanced dissolution up until a concentration of 5 ppm, at which point the rates began to return to their nominal values. It was also found that the addition of either type of MWNT did not significantly affect the total number of moles of methane dissolved in the water.


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1.0 Introduction Clathrate hydrates are non-stoichiometric crystalline compounds that form when inclusion molecules are caged within a thermodynamically stable water lattice. Their discovery in 1810 by Sir Humphry Davy sparked an interest that initially drove research for purely academic purposes.1 It was only in the 1930’s that an industrial relevance surfaced when it was found that these inclusion compounds were blocking oil and gas pipelines.2 Presently, hydrates are being studied for novel industrial applications due to their high gas storage potential and selective formation processes. Examples of these applications include the transportation and storage of natural gases, green house gas reduction via carbon dioxide sequestration, and component separation techniques.3-7 Hydrate formation is a process that can be divided into three steps: saturation, induction and growth.3 A schematic representing the process in terms of the hydrate former consumption versus time can be found in Figure 1. The first step involves the dissolution of the hydrate former into an aqueous phase until it is completely saturated with respect to the three phase equilibrium condition. This occurs in the figure at time tEquilibrium at which point nSat moles have been dissolved. The amount of moles dissolved in the water at the saturation point is governed by thermodynamics, however the time required to reach this value is influenced by mass transfer considerations. For hydrates to form, the system has to surpass this saturation value by continually dissolving gas during a metastable state known as supersaturation.3 This occurs during the induction step of the formation process where small hydrate nuclei stochastically form and dissociate until a critical cluster radius is formed.3 At this point in time, known as


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tTurbidity, hydrate growth is energetically favorable. This marks the onset of the growth step of the formation, where the hydrate former consumption increases linearly with time.

Figure 1: Representation of the three phases of hydrate formation in terms of gas consumption.

In order to optimize the use of hydrates for novel technologies, it is of great interest to find agents that can aid in their formation process. One such example are multi-wall carbon nanotubes (MWNTs). These carbon allotropes were found to increase the yield of methane hydrates when introduced into a water/methane system.8-10 The issue associated with utilizing as-produced MWNTs is that they are naturally hydrophobic, causing them to agglomerate and settle out of an aqueous solution rapidly.11 In the past, researchers have overcome this by either mixing the MWNTs with ionic surfactants or through chemically treating their surfaces.10,12 Recent advancements have led to the ability to stabilize the MWNTs in aqueous dispersions


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through the modification of their surfaces using plasma treatments.12 This functionalization process produces oxygen functionalities at the MWNT surface causing them to remain stable for extended periods of time (>2 years).13 These functionalized MWNTs were used in a study along side the as-produced samples that investigated their effects on methane hydrate growth rates.14 The study found that the growth was enhanced through their addition and the trend associated with MWNT loading depended strongly on whether the MWNTs were functionalized or not.14 While previous investigations studied the effects of MWNTs on the overall hydrate yield and on growth rates, to the best of our knowledge, there have been no studies done on their effect during the saturation stage of hydrate formation.8-10,14 The current study aims to investigate the effects of both as-produced (hydrophobic) MWNTs and plasma-functionalized (hydrophilic) MWNTs on the dissolution of methane in water. By maintaining the system at the incipient three phase equilibrium conditions (vapor, liquid water and hydrate or V-LW-H), the gas consumption will increase until it plateaus when the water becomes fully saturated. Due to the impossibility of supersaturation at these conditions, there is no risk of entering the induction period. Using this methodology, it is possible to analyze the effects of MWNT loading on the total number of moles of methane dissolved, as well as the time required to achieve this level of saturation. This information can provide valuable insight towards understanding MWNT-based enhancement in gas hydrate systems.

2.0 Materials and Experimental Apparatus A simplified schematic of the experimental apparatus used in the study can be found in Figure 2. A complete description of the equipment can be found in Pasieka et al.14 The set up consists of a stainless steel 316 crystallizer placed in a cooling bath filled with a 50/50 volume


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mixture of ethylene glycol and water. The temperature of the bath is controlled by a Neslab RTE chiller that can operate in the -40 to 200 °C temperature range. Three gas cylinders are also submerged in the bath. One of these cylinders, called the reservoir, is used to supply the crystallizer with methane in order to maintain isobaric conditions. This was accomplished through employing a Baumann 51000 Low Flow control valve. The function of the remaining two cylinders is to provide bias values for both the reservoir and the crystallizer. The temperatures of the liquid and gas phases in the crystallizer as well as the gas phase inside the reservoir are monitored with Omega Class A platinum RTDs. Pressure readings are obtained via Rosemount 3051 Smart Pressure Transmitters. In order to promote proper mixing, a magnetic stir bar was placed in the crystallizer and was spun with the use of a Leeson Electric Motor. Furthermore, a Leeson Direct Current Permanent Magnet Motor is used to mix the fluid in the cooling bath to produce a homogeneous temperature distribution. All data is recorded with the use of a National Instruments LabVIEW™ virtual instrument (VI).


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Figure 2: Simplified schematic of the experimental apparatus. 1- Methane cylinder, 2- Liquid sample injection port, 3- Crystallizer, 4- Magnetic Stirrer, 5- Electric Stirrer, 6- Chiller, 7Reservoir, 8-Reservoir bias, 9- Reactor bias. T, P, DP, and CV represent temperature, absolute pressure, differential pressure and control valve respectively. The water used in the experiments underwent reverse osmosis treatment with a 0.22 µm filter. This produced water with a conductivity of 10 µS and a total organic content of less than 10 ppb. The methane used was purchased from MEGS and is of Ultra High Purity grade (99.99%). Both hydrophobic and hydrophilic MWNTs were obtained from McGill University’s Plasma Processing Laboratory. The MWNTs were produced via a thermal chemical vapor deposition technique that uses acetylene as the carbon source and stainless steel meshes as the growth surface. The lengths and diameters of both sets of MWNTs are approximately 31.4 nm and 1.2 µm, respectively.15 In order to produce the hydrophilic MWNTs samples, as-produced MWNTs were plasma functionalized using a capacitively-coupled RF glow discharge sustained


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in a Ar/C2H6/O2 mixture environment.11 Analysis of the surface of these MWNTs using x-ray photoelectron spectroscopy (XPS) showed that approximately 21% of the atomic composition of their surface was composed of oxygen.11 Further analysis revealed that the oxygen functionalities at the surface were composed of hydroxyl (COH), carbonyl (C=O), and carboxyl (COOH) groups.11 In order to quantitatively describe the degree of hydrophobicity and/or hydrophilicity of the samples, contact angle (CA) measurements were preformed. Measurements of the hydrophobic MWNTs grown on the stainless steel substrate gave a CA of 103 °, whereas the hydrophilic MWNTs produced a CA of < 5°.12 For further information including the characterization, imaging, and functionalization of the MWNT samples used in the dissolution experiments consult Hordy et al.11 and Vandsburger et al.12 Once produced, the MWNTs are broken off their stainless steel substrates and dispersed in reverse osmosis treated water via ultrasonication.

3.0 Procedure The first step of the experimental protocol involves setting the Neslab Chiller to reach the experimental temperature of 2 °C. Before introducing any of the liquid samples, the crystallizer is rinsed out four times with 360 mL of reverse osmosis water. Once this is done, 300 mL of the experimental sample is inserted through the liquid injection port. At this point, methane is used to flush the crystallizer three times at a pressure of 500 kPa in order to rid the system of air or any other undesirable gases. After the system cools down to the experimental temperature, the crystallizer is pressurized to the three-phase equilibrium condition. For a temperature of 2 °C, this corresponds to a pressure of 3146 kPa.16 Once the temperature and pressure readings are stable, the control valve is set to maintain the pressure and the magnetic stirrer is initiated. At


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this point, the LabVIEW™ VI begins to take temperature and pressure readings once per second. The pressure differential between the reservoir and the reservoir bias is later used to calculate the total number of moles dissolved into the liquid phase through the use of the Trebble-Bishnoi equation of state.17 The experiments terminate when the gas consumption curve stabilizes, which corresponds to when the liquid phase becomes saturated. This occurs within an hour of initiating the experiment. Each experimental condition is tested under this protocol with 4 replicates. Three types of liquid samples were injected into the crystallizer for the results reported in this article. Initially, reverse osmosis water was tested in order to obtain baseline values. Throughout the analysis of the data, two main aspects were investigated, namely, the number of moles of methane at saturation, as well as the time frame required to reach this value. More on how this was done can be found in the Results and Discussion section. The other two liquid samples were aqueous dispersions of hydrophilic and hydrophobic MWNTs. These MWNT containing samples were injected individually at concentrations of 0.1, 0.5, 1, 5, and 10 ppm respectively. Preparing samples of concentrations less than 0.1 ppm was not possible as such minute masses were problematic to measure with an acceptable reproducibility. Additionally, it was not possible to conduct experiments above the 10 ppm threshold due to production limitations in the Plasma Processing Laboratory.

4.0 Results and Discussion 4.1 Water Baselines In order to be able to compare the MWNT suspensions to baseline value results, dissolution experiments were conducted with reverse osmosis water. An example of a typical methane dissolution curve obtained can be seen in Figure 3. In order to evaluate the accuracy of the data


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acquired, a comparison to literature values was conducted. Servio and Englezos18 reported the solubility values of methane in water at the three phase equilibrium conditions over a range of temperatures. Based on extrapolated data from this study, the mole fraction of methane at saturation in water for 2 °C is 1.10 ×10-3. Using the techniques described in the current study, mole fractions obtained for water were consistently 92 % of the value of this extrapolation. The 8% difference between the studies is due to the different measuring techniques. In the work by Servio and Englezos18, the solubility was obtained by taking a liquid sample and measuring the amount of methane that can be evolved from the sample using a gasometer. This was done for conditions with and without the presence of a hydrate phase. In the current study, the dissolved methane is measured through the amount of gas that is sent from the reservoir to the reactor in order to maintain isobaric conditions. The benefit of using this technique is that dynamic dissolution curves can be obtained which give insight on the rates of dissolution. The use of the gasometer technique, which must be employed if hydrates are formed, unfortunately cannot give such information, but instead only final saturation values. However, the drawback of using the methodology in this study is that the data is only recorded once the stir bar is initiated. Therefore, the small amount of methane that can dissolve during the cool-down period after pressurization is not accounted for. It is believed that this small amount accounts for the difference in the data.


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Figure 3: Typical methane consumption curve during a baseline dissolution experiment. The next objective of the study is to quantitatively characterize how quickly the saturation values can be obtained by a system. The dissolution process itself can be modeled as a first order response. Analysis of the dissolution rates using this technique was previously conducted in a study by Zhu et al.19. Using this functionality to depict the consumption plot allows for the use of tau (τ) to describe the rate of dissolution. Tau, or the time constant, is used to designate the point in time at which 63.2 % of the asymptotic value is achieved. In order to obtain this value, experimental data was fit to a first order response model where resulting τ values were obtained. By comparing the τ values of different conditions, the effect of the MWNTs on dissolution rates be calculated and quantified. A reduction in τ would signify an increase in the dissolution rate while an increase in τ would suggest the process is slowing down. Water baseline τ values can be found in the following results sections.


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4.2 Hydrophilic MWNTs Aqueous dispersions containing the plasma-functionalized MWNTs were prepared and injected into the crystallizer at concentrations of 0.1, 0.5, 1, 5, and 10 ppm. Before investigating their effect on the kinetics of dissolution, the total amount of moles of methane dissolved at the end of the experiments were compared with water baseline values. The number of moles dissolved as well as the resulting deviation associated with adding the functionalized MWNTs can be seen in Table 1. For both values, 95 % confidence intervals on the data were included.

Condition

Moles

95%

Deviation from

95% Confidence

Dissolved at

Confidence

Baseline Water

Interval

1 Hour

Interval

Value

(%)

(mol)

(mol)

(%)

Water Baseline

1.687×10-2

1.841×10-4

0.000

1.099

0.1 ppm

1.697×10-2

2.740×10-4

0.6026

1.624

0.5 ppm

1.689×10-2

2.596×10-4

0.0987

1.539

1 ppm

1.697×10-2

2.278×10-4

0.5730

1.350

5 ppm

1.705×10-2

9.476×10-5

1.066

0.5617

10 ppm

1.718×10-2

4.366×10-4

1.482

1.057

Table 1: The effect of functionalized MWNT loading on methane solubility.


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The data presented in Table 1 suggest that the effect of adding the functionalized MWNTs on the methane solubility in water is negligible. Deviations range from 0.0987 to 1.482 % which just barely exceeds the baseline 95 % confidence interval. This demonstrates that the effect of the MWNTs is not thermodynamic in nature. Thermodynamic inhibitors or promoters for example, are known to alter the system parameters at the three phase equilibrium condition.3 Standard observations include a shift in equilibrium temperature, pressure, or in this case mole fraction. Based on the data from this current study, there are no substantial effects on the amount of methane found in the liquid phase. In order to investigate the effect on the rate at which the methane was dissolving in the liquid phase, the value of τ was calculated for every condition. If the value of τ is smaller than the baseline values, then this means that the time required obtaining 63.2 % of the saturation is shortened and the mass transfer is enhanced. Similar logic follows for the opposite situation. Table 2 lists the average values of τ and their respective 95 % confidence intervals for the hydrophilic MWNTs.


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Condition

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Average τ

95% Confidence Interval

(s)

(s)

Water Baseline

407

6.83

0.1 ppm

410

7.14

0.5 ppm

398

6.66

1 ppm

391

8.55

5 ppm

376

9.43

10 ppm

346

2.6

Table 2: Measured τ values for the functionalized (hydrophilic) MWNTs. The data from Table 2 demonstrate a general trend where an increase in concentration of the hydrophilic MWNTs causes τ to decrease. In order to better visualize this effect, the percent reduction in τ was plotted versus the MWNT loading in Figure 4. This finding agrees well with previous studies that investigated the effects of nanoparticles on gas dissolution.19-22 The reduction in τ with increased MWNT loading is similar to the observations by Olle et al.21, who measured the oxygen mass transfer enhancement as a function of functionalized magnetic nanoparticle concentrations. Potential explanations of this observation are generally attributed to one of three phenomena: adsorption of the gas on the surface of the nanoparticles, mass transfer coefficient enhancement or an increase in the gas-liquid interfacial area.19-22 Due to the nature of the compounds used in this study, the first explanation is unlikely, as methane generally doesn’t


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have an affinity to the surfaces of the MWNTs. The research that focused on this explanation strategically functionalized nanoparticles with chemical groups that would attract gas molecules.19 More specifically, Zhu et al.19 functionalized silica nanoparticles (MCM41) with hydroxyl groups that were used to attract carbon monoxide. The remaining two mechanisms, mass transfer coefficient enhancement and an increase in interfacial area, offer a more plausible explanation of the findings of the current study. When investigating oxygen mass transfer aided by the addition of functionalized magnetic nanoparticles, Olle et al.21 were able to conclude with their set-up that both the mass transfer coefficient and the interfacial area were enhanced. The majority of the improved oxygen mass transfer was found to be caused by the latter explanation.21 This was determined with the use of a system undergoing a chemical reaction, where the concentrations of certain catalysts were modified in order to isolate the mechanism.21 In the case of the current study, it is believed that the interfacial area between the gas and liquid phases could be increased due to the surface disruptions caused by the MWNTs. With respect to the theory that added nanoparticles improve mass transfer coefficients, in quiescent systems, this is linked to an increase in molecular diffusivity.22-24 These studies have found that nanoparticles produce Brownian agitation, which at the macroscopic level improves the mass diffusivity.23,24 However in this case, since the system in agitated with a magnetic stir bar, Brownian motion effects are likely not the dominant cause for the changes in τ. Instead, it is believed that the MWNTs improve the mixing in an already turbulent system. Here, the MWNTs can be analogous to microscopic stirrers which produce localized fluid displacements. Similar results were seen in a study by Pasieka et al.14, which found an increase in methane hydrate growth rates with the addition of both functionalized and as-produced MWNTs. While with the given system


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it is difficult to discern which mechanism associated with the MWNTs is more prevalent in the reduction of τ, their effects on the rates of dissolution are apparent.

Figure 4: The effect of hydrophilic MWNT loading on τ. 4.3 Hydrophobic MWNTs As-produced (hydrophobic) MWNTs were also dispersed in water and injected into the crystallizer. Under a range of concentrations identical to the plasma-functionalized samples, the effect of loading on dissolution was investigated. Again, before determining if the MWNTs influence τ, methane dissolution values at the end of each experiment were measured. The number of moles dissolved as well as the respective deviation from the baselines can be found in Table 3. Furthermore, 95% confidence intervals on this data are included in the table.


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Condition

Moles

95%

Deviation from

95% Confidence

Dissolved at

Confidence

Baseline Water

Interval

1 Hour

Interval

Value

(%)

(mol)

(mol)

(%)

Water Baseline

1.687×10-2

1.841×10-4

0.000

1.099

0.1 ppm

1.695×10-2

3.664×10-4

0.5038

2.171

0.5 ppm

1.693×10-2

3.459×10-4

0.4090

2.050

1 ppm

1.674×10-2

1.490×10-4

-0.7644

0.8835

5 ppm

1.665×10-2

2.800×10-4

-1.318

1.659

10 ppm

1.661×10-2

1.697×10-4

-1.297

1.007

Table 3: The effect of as-produced MWNT loading on methane solubility. Similar to the experiments with the plasma-functionalized MWNTs, the naturally hydrophobic MWNTs had no significant affect on methane solubility. For all concentrations, the departure from the water baseline value was minimal. Deviations span from 0.4090 % higher to 1.318 % lower than the pure water values. Once again, this demonstrates that the effect of these MWNTs is not thermodynamic in nature. In order to obtain insight on their effect on the rate of dissolution, τ was determined for each of the conditions. This data can be seen in Table 4.


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Condition

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Average τ

95% Confidence Interval

(s)

(s)

Water Baseline

407

6.83

0.1 ppm

393

10.2

0.5 ppm

389

7.46

1 ppm

379

10.1

5 ppm

396

18.6

10 ppm

409

8.03

Table 4: Measured τ values for the as-produced (hydrophobic) MWNTs. The τ values obtained show a similar trend as the hydrophilic MWNTs in that τ decreases with an increasing MWNT loading. However, this trend reverses when samples with MWNT concentrations of 5 ppm or higher are used. In order to better visualize this, the effect on τ relative to water baselines was plotted as a function of MWNT loading in Figure 5. This phenomenon can be explained by the nature of the surface of the MWNTs. As they are naturally hydrophobic, the MWNTs tend to agglomerate out of aqueous solutions.11 At higher concentrations, large-scale agglomeration becomes more prevalent. Furthermore, in a study investigating the effects of MWNTs on methane hydrate growth rates, Pasieka et al.14 found that at concentrations higher than 5 ppm, the hydrophobic MWNT’s associated growth enhancement plateaus. This is the same concentration where in the current study the value of τ begins to


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increase back to nominal values. It is hypothesized that at these concentrations, the MWNTs form larger clusters that decrease their effective surface area as well as their micro-scale mixing properties.14 The effect of nanoparticle size on gas-liquid mass transfer was previously studied by Zhu et al.19 They found that mass transfer enhancement decreased with increasing particle size. This supports and explains the agglomeration hypothesis. Furthermore, it can be noted that the error bars in the experiments with the hydrophobic MWNTs are generally larger than their hydrophilic counterparts. Again this can be attributed to agglomeration trends and the variation in cluster sizes. With respect to the mechanisms governing the MWNT’s influence on dissolution, it is likely that they behave in a manner similar to the hydrophilic samples. The surface adsorption mechanism is not likely present, however, mass transfer coefficient and interfacial area related effects are probable causes for the changes in τ.


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Figure 5: The effect of hydrophobic MWNT loading on τ.

5.0 Conclusions The addition of either hydrophilic or hydrophobic MWNTs affects the rate at which methane gas dissolves into liquid water at the three phase hydrate condition. The methane saturation values however are not significantly changed with the addition of the MWNTs. By comparing the kinetics (i.e. the τ values) of the mole consumption process, it was found that the hydrophilic MWNTs increase the rate of dissolution with increasing MWNT concentration. This can be attributed to an increase in the mass transfer coefficient and interfacial area, which is often associated with the addition nanoparticles. With the addition of hydrophobic MWNTs, the dissolution kinetics are enhanced up until concentrations of 5 ppm, where the τ values begin to increase back to baseline water values with increasing concentrations. Contrary to the


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hydrophilic MWNTs which remain stable over the range of concentrations investigated, agglomeration of the hydrophobic MWNTs is likely taking place at this increased loading.

Acknowledgements The authors would like to acknowledge the financial support received from the Natural Sciences and Engineering Research Council of Canada (NSERC), the Fonds de recherche du Québec – Nature et technologies (FRQNT), the Chemical Engineering Department at McGill University (EUL funds), and Hydro-Québec.

References (1) Davy, H. On a Combination of Oxymuriatic Gas and Oxygene Gas. Trans. R. Soc. London. Ser. A. 1811, 101, 155. (2) Hammerschmidt, E. G. Formation of Gas Hydrates in Natural Gas Transmission Lines. J. Ind. Eng. Chem. 1934, 26, 851. (3) Sloan, E. D.; Koh, C. A. Clathrate Hydrates of Natural Gases; CRC Press: New York, 2008. (4) Gudmundsson, J.; Borrehaug, A. Frozen Hydrate for Transportation of Natural Gas. Proceedings of the 2nd International Conference on Gas Hydrates. France, 1996. (5) Harrison, W. J.; Wendlandt, R. F.; Sloan, E. D. Geochemical Interactions Resulting from Carbon Dioxide Disposal on the Sea Floor. Appl. Geochem. 1995, 10, 461. (6) Linga, P.; Kumar, R.; Englezos, P. The Clathrate Hydrate Process for Post and PreCombustion Capture of Carbon Dioxide. J. Hazard. Mater. 2007, 149, 625.


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The Effect of Hydrophilic and Hydrophobic Multi-Wall Carbon

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