Kinematic Study of Methane Hydrate Formation and Self-Preservation

Article Cite This: Energy Fuels XXXX, XXX, XXX−XXX

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Kinematic Study of Methane Hydrate Formation and SelfPreservation in the Presence of Functionalized Carbon Nanotubes Omar Nashed,†,‡ Bhajan Lal,*,†,‡ Behzad Partoon,‡ Khalik M. Sabil,§ and Yaman Hamed∥ †

Chemical Engineering Department, Universiti Teknologi PETRONAS, Bandar, Seri Iskandar, 32610 Perak, Malaysia CO2 Research Centre, Universiti Teknologi PETRONAS, Bandar, Seri Iskandar, 31750 Tronoh, Perak, Malaysia § Institute of Petroleum Engineering, School of Energy, Geoscience, Infrastructure, and Society, Heriot-Watt University Malaysia, No. 1 Jalan Venna P5/2, Precinct 5, 62200 Putrajaya, Malaysia ∥ Centre for Corrosion Research, Department of Mechanical Engineering, Universiti Teknologi PETRONAS, Bandar, Seri Iskandar, 32610 Perak, Darul Ridzuan, Malaysia

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‡

ABSTRACT: Transportation of natural gas in the form of gas hydrates is shown to have superiority, from economic, environment, and safety viewpoints, over liquefied natural gas (LNG), especially for transferring natural gas from stranded gas reserve. However, hydrate-based technology is still under development as there are still many technical challenges, including slow production rate and stability. In this study, the effect of various multiwall carbon nanotubes (MWCNTs) on the equilibrium phase boundaries, kinetics, and self-preservation of CH4 hydrates have been studied. The carboxylated carbon nanotubes (COOH-MWCNTs) and hydroxylated carbon nanotubes (OH-MWCNTs) along with pristine MWCNTs were chosen. The carbon nanotubes were suspended in a 0.03 wt % SDS aqueous solution, and the results were compared with the SDS aqueous solution at the same concentration of 0.03 wt % and with deionized water. The CH4 hydrate phase equilibrium and kinetic parameters of the CH4 hydrate formation, including induction time, the initial rate of the hydrate formation, gas uptake, storage capacity, water-to-hydrate conversion, half-completion time, t50, and semicompletion time, t95, have been studied. The results show that the nanofluids studied did not affect the equilibrium conditions of the CH4 hydrates. In addition, the 0.01 wt % COOH-MWCNTs mixed with 0.03 wt % SDS showed the best promotional effect. Furthermore, a comparison between the SDS and the COOH-MWCNTs (without the SDS stabilizer) at 0.03 wt % revealed that the SDS was a more effective CH4 promoter. However, the self-preservation phenomenon at atmospheric pressure was more pronounced in the presence of the COOH-MWCNTs compared to the SDS. additives, mainly surfactants, to overcome these issues.11 Sodium dodecyl sulfate (SDS) has been described as the most effective surfactant for a kinetic hydrate promoter.11 It increases the hydrate formation rate and gas uptake by reducing the surface tension at the water/gas interface.12 However, the use of surfactants could lead to some industrial challenges, such as foaming, precipitation, and hydrate formation on the reactor wall.13 The foam would be generated throughout the hydrate dissociation process because of the existence of surfactants. This does not just have an effect on the applications of gas hydrates, but it also results in the outflow of the surfactants.13 Additionally, the promotion effect of the surfactants on the formation of the hydrates is significantly influenced by the concentration of the surfactants; additionally, a specific concentration is needed in order to obtain an efficient promotion effect.11 Nevertheless, the solubility of many surfactants is significantly reduced under the hydrate formation conditions; this could, in turn, result in the precipitation of the surfactants.13 Furthermore, when surfactants are present, the hydrates will form on the wall of the reactor rather than at the water/gas interface. Consequently, only a limited amount of reactor space is available

1. INTRODUCTION Methane hydrates are crystalline inclusion compounds, formed by a physical combination between hydrogen-bonded water cages and methane gas under high pressure and low temperature.1 Although natural gas hydrates (which predominantly contain methane) have been reported as a major problem for the oil and gas industry, methane hydrates are available abundantly in nature and could be a main energy source in the future.2−6 Additionally, the unique merits of gas hydrates offer a potentially valuable alternative for many applications.7 In fact, every 1 m3 of gas hydrates contains up to 170 m3 of gas under standard conditions.8 Moreover, the hydrates could remain stable at atmospheric pressure and temperatures below the ice melting point.9 It has been reported that the transportation cost of gas in the form of dry hydrates, hydrate slurries, or pellets is expected to be 18−24% lower than with liquefied gas,10 given the nonexplosive nature of hydrates and that the considerably low concentration of chemicals (when necessary) renders the hydrate formation technology to be more environmentally friendly. Therefore, hydrates are considered as a promising technology for gas transportation and storage.8 The slow hydrate formation rate and the inadequate gas occupancy that could be attained are considered the critical issues challenging the industrial applications of gas hydrates. However, researchers have examined several chemical © XXXX American Chemical Society

Received: May 14, 2019 Revised: July 8, 2019 Published: July 8, 2019 A

DOI: 10.1021/acs.energyfuels.9b01531 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels Table 1. Name, Abbreviation, Properties, and Supplier of the Chemicals Used chemical multiwall carbon nanotube hydroxylated multiwall carbon nanotube carboxylated multiwall carbon nanotube sodium dodecyl sulfate methane

diameter (nm)

length (μm)

purity

MWCNT OH-MWCNT

20−30 20−30

10−30 10−30

>95% >95%

198.78 196.57

COOHMWCNT SDS CH4

20−30

10−30

>95%

192.53

NA NA

NA NA

99% 99.95%

NA NA

abbreviation

BET surface area (m2 g−1)

supplier Research Nanomaterials Inc., U.S.

Merk Air Product Sdn. Bhd.

promotion to be weakened; this is likely because of the aggregation of the nanotube particles.25 The positive effect of the functionalized MWCNTs has been ascribed to the considerable defects that formed on the surfaces of the MWCNTs, which resulted in small graphitic fragments being formed.19 Consequently, the defective sites that were produced with groups containing more oxygen resulted in the nanotubes in the aqueous phase being dispersed better and having more stability.27 Pesieka et al. functionalized carbon nanotubes via a plasma treatment to convert the MWCNTs to a hydrophilic nature from the hydrophobic nature.28 The greater hydrophilicity increased the mass transfer of the methane gas to the empty water cages. Moreover, the carbon nanotubes promoted not only the CH4 but also the CO2 hydrates and natural gas hydrates. They found that the nanotubes enhanced the dissolution rate of the carbon dioxide, which resulted in an increased gas consumption rate.29 In order to apply gas hydrate technology for gas transportation and storage, the stability of the produced hydrate crystals has a great importance. The literature data revealed that transporting the natural gas in hydrate form (NGH) is more cost-effective in comparison with liquefied natural gas (LNG) for medium distances.30 The main drawback of the NGH could be the high-pressure requirement, while LNG transport is conducted at near atmospheric pressure. Nonetheless, researchers found that gas hydrates could be stable at atmospheric pressure and at a few degrees below 273 K.31 The observed phenomenon was called self-preservation and referred to the hydrate’s steadiness outside the stable thermodynamic condition.9 The concept of the self-preservation of hydrates is based on suppressing the hydrate dissociation rate by forming an ice layer around the hydrates. However, some research has shown that the presence of kinetic promoters, such as SDS, decreases the stability of hydrates in the self-preservation zone.32−34 In this work, the effect of three types of carbon nanotubes on methane hydrate formation has been investigated. Besides the conventional multicarbon nanotubes, two other types of functionalized carbon nanotubes, namely, hydroxylated multiwall carbon nanotubes (OH-MWCNTs) and carboxylated multiwall carbon nanotubes (COOH-MWCNTs), were used in this work. To the best of our knowledge, the selected functionalized multiwall carbon nanotubes had yet to be tested for methane gas hydrates. The impact of the selected carbon nanotubes on the thermodynamic and kinetic CH4 hydrate formation and dissociation have been studied experimentally. The methane hydrate equilibrium temperature, induction time, initial rate, and amount of gas consumption, storage capacity, and water-to-hydrate conversion were measured during the hydrate formation. Moreover, for the effective application of the proposed promoters, the effect of the nanoparticles on the

for utilization, and no more separation or compaction can be made with the hydrates, for either transportation or storage.13 In order to find new promoters, in 2006, Li et al. introduced copper nanoparticles as a hydrate promoter for 1,1,1,2tetrafluoroethane (HFC134a).14 Copper had been selected based on its high thermal conductivity which aids the rapid removal of the heat released by the hydrate formation. Afterward, several studies investigated different categories of nanomaterials, such as silver, copper oxide, zinc oxide, and single and multiwall carbon nanotubes.15−21 The term nanofluid refers to the suspension of nanoparticles with sizes below 100 nm dispersed in a base liquid, such as water and ethylene glycol.22 Different dispersion methods, such as adding surfactants, surface modification, and ultrasonication, are used for the preparation of nanofluids.23 The impact of carbon nanotubes on methane hydrate formation has been discussed in the literature.20,24−27 For the thermodynamic effect, researchers found that multiwall carbon nanotubes (MWCNTs) and oxidized multiwall carbon nanotubes (OMWCNTs) had a slight promotional effect as they shifted the CH4 equilibrium curve around 0.5 K toward a higher temperature region.25,26 However, carbon nanotubes were selected to enhance the kinetics of gas hydrate formation. MWCNTs and OMWCNTs were reported as kinetic promoters for methane and tetrahydrofuran (THF) hydrates. The functionalized carbon nanotubes, OMWCNTs, showed significant enhancement on the induction time, rate of hydrate formation, and amount of methane consumed.24 In the experiments of Park et al., the results showed that the highest amount of methane was consumed in the presence of 0.003 wt % OMWCNT solution. The adhesion of methane to the carbon nanotubes has been reported as the cause of this. The concentrations higher than 0.003 wt % were found to be less effective despite the expectations. Park et al. believed that this observation was due to the extensive initial formation of the hydrates with the existence of the OMWCNTs that rapidly formed a thick barrier on the gas/water interface and, consequently, prevented any further formation of hydrates.26 The synergic effect of OMWCNTs and THF on the time it takes methane hydrate to go through the induction process was studied by Song et al.19 They were of the opinion that the effect on the time needed for the induction process was because of the nanoparticles undergoing a continuous Brownian motion. Additionally, the mixed promoter (SDS +OMWCNTs) enhanced the growth rate of the hydrate more efficiently as compared to the SDS solution; to be more specific, the best performance in the OMWCNTs-SDS system was found to be a result of the higher dispersion of OMWCNTs in the aqueous phase. Kim et al. observed that shorter MWCNTs led to more gas consumption in a 12-h time frame. On the other hand, it was stated that highly concentrated nanotubes could cause the effect of the B


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Figure 1. Experimental setup.

which represented the equilibrium point was measured. Tcycle methods were used by reducing the temperature down to 273.15 K. The system was left at a constant temperature until the hydrate formation was complete. Then, the stepwise heating method was applied with an increase in the temperature of 0.5 K every 2 h. The slow heating rate was crucial for detecting an accurate hydrate dissociation temperature. The experiments were conducted at four different pressures in the range of 3.1−8.2 MPa for the pure water and the nanofluid systems. 2.3.2. Kinetics of the Methane Hydrate Formation. For the kinetic experiments, fresh liquid sample was poured into the reactor. Then, the reactor was cooled to the temperature of 279.65 K, which was about 2 K higher than the hydrate equilibrium temperature. Then, the CH4 was compressed into the reactor up to 5.1 MPa. Afterward, the stirrer was turned on, and the system was allowed to reach the equilibrium state. Then, the system was cooled to 274.15 K without stirring during the cooling period. A decrease in the pressure was observed as the temperature decreased. When the pressure regained stability after 65 min at a constant temperature of 274.15 K, the stirrer was turned on. The hydrate formation was indicated by a sudden pressure drop and an increase in the temperature of the system. The changes in pressure and temperature were recorded every 10 s using a data acquisition system. When the pressure of the system remained unchanged for 2 to 3 h, this indicated that the hydrate formation was completed. All kinetic experiments were repeated at least three times, and the reported results are average values. 2.3.3. Methane Hydrate Dissociation Rate. In order to investigate the stability of methane hydrates, the experiments were conducted in the presence of pure water, 0.03 wt % SDS, and 0.03 wt % COOH-MWCNT aqueous solutions. After the hydrates were formed as described in the kinetic experiments, the system was cooled to 271.15 K and left for 5 h. Five hours was considered enough time to reach thermal equilibrium in the cell. Then, the vent valve was opened to depressurize the system. Once the system reached atmospheric pressure, the vent valve was closed again. The hydrates were left in the

hydrates’ self-preservation was evaluated by measuring the dissociation rate.

2. METHODOLOGY 2.1. Materials. The chemical name, supplier, purity, and other nanotube specifications are listed in Table 1. The chemicals were used as purchased without further purification. All the carbon nanotubes had the same diameter and length. Deionized water used to prepare all the nanofluids was obtained from an Ultra-Pure Water System. The concentrations of the carbon nanotubes in the nanofluids were in the range of 0.005−0.1 wt % suspended in an aqueous solution of 0.03 wt % SDS. SDS was used to stabilize the nanofluids and to compare them with the carbon nanotubes. For the comparison study, one sample of the 0.03 wt % COOH-MWCNTs was prepared without adding the surfactant as it was stable in the water via its own functional group interaction with water. All of the samples were prepared using an AND analytical balance with the accuracy of ±0.0001 g. 2.2. Apparatus. The experimental setup used in this work is shown in Figure 1. It consisted of a high-pressure stainlesssteel reactor with an internal volume of 423 mL. The reactor was equipped with two thermocouples with an accuracy of ±0.5 K to measure the temperatures in the gas and liquid phases. The reactor sustained a maximum pressure of 20 MPa and was connected to a pressure transducer to measure the pressure inside the vessel. The mixing of the different phases was achieved using a 2-bladed pitch impeller connected with a magnetic system that could provide stirring up to 600 rpm. The vessel was inserted into a water bath to keep the temperature constant at the desired value. Then, 100 mL of the hydrate forming liquid was inserted into the vessel (highpressure reactor). The system was then vacuumed, and the reactor was purged with gas thrice to remove any excess air inside of the reactor. The procedures of the three types of experiments that had been conducted are demonstrated in the next section. 2.3. Procedure. 2.3.1. Methane Hydrate Phase Equilibrium Measurement. The dissociation hydrate temperature C


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Figure 2. Pressure/temperature vs time profile of a kinetic hydrate formation experiment.

Figure 3. Amount of methane gas consumption curve vs time during the kinetic experiment.

water volume changes during hydrate formation. Therefore, the following equation is used for the isothermal experiment: ÄÅ ÉÑ V ÅÅÅij P yz P yz ÑÑÑ i j zz − jj zz ÑÑ Δng = ÅÅÅjj R ÅÅÇk ZT {0 k ZT {t ÑÑÑÖ (2)

reactor for 4 days at a constant temperature, and the pressure of the system was monitored and recorded. The dissociation rate was calculated from the slope of the P-t curve. 2.4. Calculation of the Kinetic Parameters. 2.4.1. Induction Time. The induction time of the CH4 hydrate formation was taken as the time needed to start forming massive amounts of hydrates. It was detected as the point when a sudden pressure drop and temperature increase was observed as illustrated in Figure 2. 2.4.2. Initial Rate of the CH4 Consumption. The initial rate of gas consumption is the main parameter for gas hydrate applications. It represents the rate of the CH4 hydrate formation and is calculated as follows in eq 1: r (t ) = −

nii − 1 − nii + 1 −1 nw ti − 1 − ti + 1 0

where, P and T are the system pressure and temperature, respectively. V is the gas phase volume; R is the universal gas constant. Z is the compressibility factor, which is calculated using the Peng−Robinson equation of state. Subscript 0 stands for the start time of the experiment, and t stands for the conditions at time t. Figure 3 demonstrates the gas consumption changes during the hydrate formation process. To normalize the amount of gas consumption and eliminate the size of the sample, the gas uptake is calculated as shown in eq 3. It represents the amount of gas trapped in one mole of a water sample:

(1)

where nii − 1 and nii + 1 are the mole numbers of gas in the gas phase at time intervals ti−1 and ti+1, respectively, and nw0 is the initial mole number of the water. 2.4.3. Amounts of the CH4 Consumption and Gas Uptake. The amount of CH4 consumed to perform the maximum hydrate formation is an important parameter to bring gas hydrate technology to the industrial scale. The gas consumption is calculated using eq 2. It is assumed that no

U=

Δng nw

(3)

2.4.4. Storage Capacity. In order to store the gas in hydrate form, it is crucial to study the storage capacity. The storage capacity (SC) expresses the ratio of the volume of gas captured under standard conditions (STP) to the volume of hydrate and is calculated using eq 4: D


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Figure 4. HLVE curve of methane for pure water, 0.03 wt % SDS, and 0.1 wt % nanofluid samples.

SC =

V gSTP VH

Δng RT STP/P STP

=

VH

Ci = (4)

(5)

where vβw is the molar volume of the blank hydrate lattice that is calculated following eq 6. νwβ = (11.835 + 2.217 × 10−5T + 2.242 × 10−6T 2)3 ×

conversion =

10−30NA − 8.006 × 10−9P + 5.448 × 10−12P 2 46 (6)

(7)

where M is the hydration number (water molecules per guest molecule). The hydration number is directly related to the fractional occupancy of the small and large cavities as follows (eq 8): M=

46 6θl + 2θs

(8)

where θl and θs are the fractional occupancy of large and small cavities, respectively. The fractional occupancy is calculated using the Langmuir Adsorption Theory, as follows (eq 9):35 θi =

CifCH

4

1 + CifCH

4

M × Δng n w0

(11)

3. RESULTS AND DISCUSSION 3.1. Hydrate Phase Equilibria of the Carbon Nanotube Fluids. To design any gas transportation and storage process, the phase boundary data of the system are required. Thermodynamic experiments have been conducted to study the impact of carbon nanotubes on CH4 hydrate phase equilibrium. The nanofluids were tested at 0.1 wt % carbon nanotubes prepared in an aqueous solution of 0.03 wt % SDS. First, the experiments were conducted for a blank sample and compared with the CSMGem software data to check the reliability of the equipment and method. The results were in accord with the predicted data as plotted in Figure 4. Then, the 0.03 wt % SDS and nanofluid samples were tested. It was reported in the literature that surfactants do not change the phases’ equilibria;3 meanwhile, the carbon nanotubes showed slight promotional effects.26,27 Also, the function group did not alter the thermodynamic effect of the carbon nanotubes.10 However, this work showed a negligible effect on the hydrate phase equilibria for all the tested kinetic promoters as illustrated in Figure 4. 3.2. Kinetics of the Methane Hydrate Formation. 3.2.1. Induction Time Measurement. The induction times of the methane hydrate formations are listed in Table 2. It can be observed from Table 2 that apart from the OH-MWCNTs and MWCNTs at 0.1 wt %, all the nanofluids diminished the

where NA is Avogadro’s number, and T and P are the temperature and pressure, respectively. The physical reaction for a CH4 gas hydrate can be expressed as eq 7: CH4 + M ⥂H 2O ↔ CH4 · (H 2O)M

(10)

where Ai and Bi are constants and T is the temperature in Kelvin. The fugacity of CH4 in the gas phase is calculated using the Peng−Robinson equation of state. 2.4.5. Water-to-Hydrate Conversion. The fraction of the mole number of water molecules in the hydrate phase per mole of the initial solution is called the water-to-hydrate conversion and is calculated as follows as defined in eq 11:

where VH is the gas hydrate volume and is calculated using eqs 5 and 17. VH = M × Δng × vwβ

Ai iB y expjjj i zzz T kT {

(9)

where Ci is the gas Langmuir constant of CH4 in a type i cavity and f CH4 is the fugacity of the CH4 in the gas phase. The Langmuir constant of gas (Ci) is formulated as shown below (eq 10): E


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industrial applications. Although the stirring system can achieve a higher formation rate, it causes problems, making it impractical. During the hydrate formation process, a thick slurry can be formed, which requires more energy for stirring. Not to forget the fact that mechanical stirring could increase equipment capital and maintenance costs. Besides the presence of nanoparticles providing more nucleation sites, they also act as a stirrer due to the Brownian motion.19 3.2.2. Initial Rate of the Gas Hydrate Consumption. The initial rate of the CH4 hydrate formation is shown in Figure 5. It can be observed that the 0.03 wt % SDS and all the concentrations of nanofluids enhanced the hydrate formation rate more as compared to water. In comparison to SDS, all the carbon nanotubes showed better performances at the concentration of 0.01 wt %. For 0.05 wt %, the COOHMWCNTs and OH-MWCNTs enhanced the CH4 hydrate formation in contrast to the MWCNTs. Moreover, very low concentrations of nanoparticles, i.e., 0.005 wt %, and higher concentrations of nanoparticles, i.e., 0.1 wt %, had lower hydrate formation rates compared to the 0.03 wt % SDS. Among all the samples studied, the 0.01 wt % COOHMWCNTs had the highest initial rate as it showed 2959.0% and 18.0% enhancement rates in comparison with water and the 0.03 wt % SDS, respectively. The carboxylated carbon nanotubes showed better performances compared to the pristine carbon nanotubes. The carboxyl group was more negatively charged and had a higher charge density. The charge density increased in the order of MWCNTs < OH-MWCNTs < COOH-MWCNTs (0.53, 1.54, and 4.00 C/g, respectively).37 That caused higher attractive forces between the carbon nanotubes and the water along with an increasing stability.38 The previous discussion could be confirmed by observing that the COOH-MWCNT nanofluid was stable in water without using a surfactant, converse to that of the OHMWCNTs and MWCNTs as shown in Figure 6. The stability

Table 2. Induction Time of CH4 Hydrates for a Blank, 0.03 wt % SDS, and Nanofluids sample group no.

aqueous phase

solid phase

induction time (min)

1 2 3 4 5 6 7 8 9 10 11 12 13 14

pure water 0.03 wt % SDS 0.03 wt % SDS 0.03 wt % SDS 0.03 wt % SDS 0.03 wt % SDS 0.03 wt % SDS 0.03 wt % SDS 0.03 wt % SDS 0.03 wt % SDS 0.03 wt % SDS 0.03 wt % SDS 0.03 wt % SDS 0.03 wt % SDS

− − 0.005 wt % MWCNT 0.01 wt % MWCNT 0.05 wt % MWCNT 0.1 wt % MWCNT 0.005 wt % OH-MWCNT 0.01 wt % OH-MWCNT 0.05 wt % OH-MWCNT 0.1 wt % OH-MWCNT 0.005 wt % COOH-MWCNT 0.01 wt % COOH-MWCNT 0.05 wt % COOH-MWCNT 0.1 wt % COOH-MWCNT

74.2 73.9 65.0 69.9 68.3 85.0 65.0 66.3 70.2 77.4 65.0 46.1 64.3 56.1

induction times in comparison to the SDS and water samples. Table 2 demonstrates that COOH-MWCNTs have a higher impact on the induction time. The 0.01 wt % COOHMWCNTs had the shortest induction time among the tested samples. As is well-known, hydrate formation is a probabilistic phenomenon resulting in scattered induction times. The general trend found in this work was that lower concentrations showed shorter induction times. However, the trend was inconsistent for all the nanofluids. Furthermore, it is worth noting that the induction times of the COOH-MWCNTs at the concentration range of 0.01−0.1 wt % were less than 65 min. This refers to the methane hydrate formation initiated prior to agitation via a stirrer. That was not observed for 0.03 wt % SDS. In fact, forming hydrates in a simple quiescent system is considered as an advantage for

Figure 5. Initial CH4 hydrate formation rate for the blank sample, 0.03 wt % SDS, and nanofluids (0.005−0.1 wt % carbon nanotube + 0.03 wt % SDS). F


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hydrophobicity of the nanoparticles is reached. After reaching this maximum, any further increase in the concentration of the surfactants leads to a second layer of surfactant molecules being adsorbed; this happens through a chain−chain interaction on the surfaces of the nanoparticles which, in turn, decreases the hydrophobicity of the particles as well as the adsorption.40 3.2.3. Amount of Gas Consumption and Gas Uptake. Due to the nature of isochoric experiments, the final gas consumption could be limited by the thermodynamic constraints of the system. In most of the experiments, as indicated by the results, the system reached to almost the same equilibrium pressure of 2.8−2.9 MPa at the temperature of 274.15 K. Subsequently, the amount of gas consumption obtained was in the range of 0.332−0.342 mol. However, at the end of the experiment with the pure water sample, only 0.209 mol of CH4 had been consumed, which indicated the uncompleted hydrate formation due to the limitation of the mass transfer in the system and not a thermodynamic limitation. Therefore, to have a better understanding of the amount of gas consumed during hydrate formation, the amounts of the gas uptake were reported both after 180 min and at the end of the experiments. It can be observed from Table 3 that all the samples showed massive improvement in the amount of methane consumption, either after 180 min or at the end of the experiments, compared to the water. The addition of carbon nanotubes slightly improved the gas consumption in comparison to 0.03 wt % SDS. Also, Table 3 revealed that there was no appreciable final gas uptake difference between nanofluid samples. However, the difference in gas uptake after 180 min was more pronounced. Figure 7 shows that the 0.01 and 0.05 wt % concentrations gave higher enhancement rates of the gas uptake after 180 min, whereas the 0.005 and 0.1 wt % had lower gas uptake compared to the 0.03 wt % SDS. It is worth observing that the gas uptake after 180 min for the concentrations of 0.01 and 0.05 wt % had the opposite trend compared to the initial formation rate. The concentration gave a high initial rate, showing a lower gas uptake. The observation can be speculated as being due to the rapid hydrate formation on the surface that slowed down the later hydrates’ growth.

study showed that the COOH group stabilized the carbon nanotubes in water for several months, whereas the MWCNTs and OH-MWCNTs had settled out in less than 1 h.

Figure 6. Stability test of pristine and functionalized carbon nanotubes in water at the concentration of 0.03 wt %.

The enhancement caused by the nanoparticles on the initial rate can be attributed to the high thermal conductivity of the carbon nanotubes.39 They absorbed the heat released due to the exothermic nature of crystallization. As a rule of thumb, surfactants improve the stability of nanoparticles in suspensions, which in turn, enhances the heat transfer. Low concentrations of carbon nanotubes lead to an increase in nanotube dispersion. Thus, no aggregation would occur in the nanofluid. At higher concentrations, the nanotube aggregates again due to the decrease in the intermolecular spaces. In the case of the 0.005 wt % carbon nanotubes suspended in 0.03 wt % SDS, the amount of carbon nanotubes may not be enough to perform effective enhancement. In addition, the relative high ratio of surfactants to nanotubes (6:1) causes collisions to take place between the surfactant molecules and the nanotubes; thus, a greater number of surfactant molecules are adsorbed onto the surfaces of the nanoparticles until the maximum

Table 3. Gas Uptake after 180 min, Final Gas Uptake, Storage Capacity, Conversion, t50, and t95 for Water, 0.03 wt % SDS, and Nanofluid at an Initial Pressure = 5.1 MPa and T = 274.15 K sample aqueous phase water 0.03 wt 0.03 wt 0.03 wt 0.03 wt 0.03 wt 0.03 wt 0.03 wt 0.03 wt 0.03 wt 0.03 wt 0.03 wt 0.03 wt 0.03 wt

% % % % % % % % % % % % %

SDS SDS SDS SDS SDS SDS SDS SDS SDS SDS SDS SDS SDS

solid phase

final gas uptake (molg/molw)

gas uptake after 180 min (molg/molw)

storage capacity (V/V)

conversion

t50 (min)

t95 (min)

− − 0.005 wt % MWCNT 0.01 wt % MWCNT 0.05 wt % MWCNT 0.1 wt % MWCNT 0.005 wt % OH-MWCNT 0.01 wt % OH-MWCNT 0.05 wt % OH-MWCNT 0.1 wt % OH-MWCNT 0.005 wt % COOH-MWCNT 0.01 wt % COOH-MWCNT 0.05 wt % COOH-MWCNT 0.1 wt % COOH-MWCNT

0.0380 0.0606 0.0613 0.0615 0.0622 0.0612 0.0610 0.0612 0.0625 0.0623 0.0593 0.0608 0.0621 0.0607

0.0038 0.0598 0.0580 0.0607 0.0617 0.0493 0.0597 0.0611 0.0605 0.0526 0.0580 0.0607 0.0615 0.0594

113.94 159.76 160.91 161.62 163.34 160.41 160.27 160.82 163.71 162.61 159.03 159.72 163.18 159.32

0.259 0.367 0.369 0.371 0.375 0.369 0.368 0.370 0.377 0.374 0.366 0.367 0.376 0.367

656.1 107.0 127.5 111.0 106.9 139.9 98.6 94.0 107.5 123.5 103.0 74.3 99.0 104.8

1937.6 149.4 182.3 152.8 151.4 188.5 147.1 135.3 151.7 175.3 146.2 114.3 149.3 159.1

G


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stage during the hydrate formation, the crystal grew fast, thus the pressure decreased dramatically. As a result, the subcooling degree reduced and a longer time was needed to complete the hydrate formation. However, for industrial applications where a continuous reactor is more favorable, reaching the maximum conversion requires a longer retention time and reactor length, which results in extra CAPEX. Therefore, achieving 95% of the hydrate formation in the crystallizer may be more reasonable from an economic viewpoint compare to a 100% conversion. Consequently, shorter t50 and t95 are more favorable for reactor design purposes. The shortest t50 and t95 were observed for the system containing the 0.01 wt % COOH-MWCNTs + 0.03 wt % SDS. 3.3. Comparison of COOH-MWCNTs with SDS. Although surfactants like SDS have been proven to be extraordinary kinetic promoters,their industrial applications, however, are still questionable. First, surfactants are wellknown foaming agents, and as far as their application as hydrate promoters are in mind, this foaming property may not be advantageous. In a hydrate production process, the foaming may hinder the proper contact between the gas and liquid phases. Moreover, foaming may lead to a carry-over in the reactor or even cause flooding. Second, the presence of surfactants in the aqueous solution reduces the stability of the produced hydrate and weakens the self-preservation phenomenon.36 Yet, the kinetic promotion effect of surfactants is essential for the development of the hydrate technology. Therefore, there is an imperative need for surfactant-free promoters. Showing a high dispersion stability without the aid of a surfactant, a COOH-MWCNT could be a good candidate for a surfactant-free hydrate promoter. Therefore, the experimental work was extended to investigate the performance of the 0.03 wt % COOH-MWCNT aqueous solution as a kinetic promoter in the absence of SDS as a stabilizer agent. The results were compared with the 0.03 wt % SDS to evaluate the efficiency of this carbon nanotube as a kinetic promoter. As can be seen in Table 4, the induction time of the CH4 hydrates in the presence of 0.03 wt % COOH-MWCNTs was 65 min, whereas the times were 73.9 and 74.2 min for the 0.03 wt % SDS and water samples. The hydrates formed fast in the presence of the nanoparticles because they acted as nucleation seeds. The seeds reduced the work needed to initiate the crystallization. Although the induction time in the presence of the nanofluid was shorter than with SDS, the initial hydrate formation rate was considerably lower than with SDS. In comparison with water, the 0.03 wt % SDS had an enhanced methane hydrate formation rate of 2489.0%, while the 0.03 wt % of the COOH-MWCNTs showed a 665.0% enhancement rate. The remarkable superiority of the surfactant over the nanofluid was attributed to the high solubility of the methane gas in the SDS solution. The presence of a hydrophobic tail in the surfactant was the main factor that could have enhanced the dissolution of the hydrocarbon gases in the aqueous phase. However, combining the SDS and the nanoparticles could have used the advantages of both promoters. Owing to the

Figure 7. Gas uptake enhancement rate after 180 min compared to 0.03 wt % SDS.

Consequently, the gas uptake after 180 min was lower. The promoters, either SDS or carbon nanotubes, reduced the surface tension, thus the mass transfer was enhanced. 3.2.4. Storage Capacity and Water-to-Hydrate Conversion. The hydration number for the current system was calculated theoretically using eq 11 and was found to be 6.0, which was close to the optimum value of 5.75. The methane formed structure I and could access the small and large cavities. The cage occupancy values obtained using eq 9 in the presence of the promoters were 0.880 and 0.975. Table 3 shows that all the promoters increased the storage capacity and water-tohydrate conversion more than the water sample. However, no significant difference was observed between the different types of promoters as they reached to almost the same equilibrium point. The storage capacity values ranged between 159.0 and 163.7 V/V. The storage capacity was relatively higher at the concentration of 0.05 wt %. Similarly, the water-to-hydrate conversions for the promoters were higher than for water. In addition, the conversion values were in the range of 0.366− 0.377 in the presence of the promoters. However, each system had reached the maximum gas uptake, storage capacity, and water-to-hydrate conversion at different times. Therefore, in this work, we defined the half completion time, t50, as the time elapsed to reach the 50% storage capacity and water-to-hydrate conversion. The half completion time is important for industrial applications. If the hydrates can achieve the nucleation and initial growth rate fast, it indicates that the most critical hydrate formation issue has been overcome. Table 3 demonstrates that, overall, the COOH-MWCNTs showed a shorter t50 compared to the other promoters. Among all the samples, the 0.01 wt % COOH-MWCNTs + 0.03 wt % SDS had the shortest t50. This agreed with the previous results which showed the shortest induction time and highest initial rate for the 0.01 wt % COOH-MWCNTs + 0.03 wt % SDS. Another parameter was introduced, namely, the semicompletion time t95. The t95 refers to the time needed to reach 95% of the maximum gas uptake, storage capacity, and water-to-hydrate conversion. At the early

Table 4. Comparison between the SDS and the COOH-MWCNTs at the Concentration of 0.03 wt % sample SDS COOHMWCNT

induction time (min)

initial rate (molg/molw min) × 104

final gas uptake (molg/molw)

gas uptake after 180 min (molg/molw)

t50

t95

SC

conversion

73.9 65

12.03 3.55

0.06067 0.06063

0.05981 0.01747

107.0 283.8

149.4 1538.6

159.76 159.32

0.3674 0.3657

H


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Energy & Fuels

Figure 8. Effect of different promoters on the morphology of the methane hydrates.

Figure 9. Pressure profile of the methane hydrate dissociation in the presence of water, 0.03 wt % SDS, and 0.03 wt % COOH-MWCNTs at 271.15 K.

higher gas consumption after 180 min due to the difference in the growth rate as discussed above. The final gas uptake, storage capacity, and water-to-hydrate conversion did not show any significant difference between the SDS and the COOHMWCNTs. By comparing the results presented in Tables 3 and 4, it can be concluded that using carbon nanotubes mixed with SDS results in more efficient kinetic promotion than individual promoter. The advantages of both promoters could be utilized and led to better performance. Another relevant factor that should be considered is the hydrate morphology. As illustrated in Figure 8, in the presence of pure water or pure COOH-MWCNT and in the absence of SDS, the hydrate formed at the gas/liquid interface and then grew downward as dendrites into the bulk water to form like a solid block around

significant difference in the enhancement rates between the SDS and the COOH-MWCNTs, the hydrate formation completed much faster in the presence of the SDS compared to the COOH-MWCNTs as listed in Table 4. Additionally, it can be observed that the t50 of the COOH-MWCNTs was higher than for the SDS by 165.0%, while the t95 of the COOH-MWCNTs was much longer than for the SDS (929.6%). It can be concluded that carbon nanotubes play a more important role at the early stage of hydrate formation by providing seeds. However, besides the low solubility of the methane in the surfactant-free nanofluid, forming hydrates rapidly at the surface could hinder further mass transfer. The gas uptake obtained after 180 min with the 0.03 wt % COOHMWCNTs was 0.0175 molg/molwr, which was 355.1% higher than that for water. However, the SDS solution showed much I


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Energy & Fuels Table 5. MCDA Ranking System for the Studied Promoters sample aqueous phase

solid phase

induction time

rate

gas uptake end

gas uptake after 180 min

t50

t95

score

decision

pure water 0.03 wt % SDS 0.03 wt % SDS

− − 0.005 wt % MWCNT 0.01 wt % MWCNT 0.05 wt % MWCNT 0.1 wt % MWCNT 0.005 wt % OH-MWCNT 0.01 wt % OH-MWCNT 0.05 wt % OH-MWCNT 0.1 wt % OH-MWCNT 0.005 wt % COOH-MWCNT 0.01 wt % COOH-MWCNT 0.05 wt % COOH-MWCNT 0.1 wt % COOH-MWCNT 0.03 wt % COOH- MWCNT

13 12 5.5 10 9 15 5.5 8 11 14 5.5 1 3 2 5.5

15 6 12 4.5 7 9 13 4.5 3 10 8 1 2 11 14

15 13 6 5 3 8 9 7 1 2 14 10 4 11 12

15 7 11 5 1 13 8 3 6 12 10 4 2 9 14

15 8 12 10 7 13 3 2 9 11 5 1 4 6 14

15 6 12 9 7 13 4 2 8 11 3 1 5 10 14

88 52 58.5 43.5 34 71 42.5 26.5 38 60 45.5 18 20 49 73.5

15 10 11 7 4 13 6 3 5 12 8 1 2 9 14

water

the impeller.13 In other words, COOH-MWCNTs did not change the morphology of the final hydrate block. As a result, the morphology does not contribute in the promoting of methane hydrates. On the other hand, the presence of SDS either in a pure SDS sample or SDS mixed with COOHMWCNT forms the hydrate along the reactor wall. A thin and highly porous hydrate layer grows upward under a capillary effect as proven in the literature.13 Hence, the interface area between the phases increased, which in turn enhanced the kinetics of the hydrate formation due to the enhanced mass transfer. According to previous scientific reports, such a formation of hydrates along the reactor wall could limit the reactor capacity for further hydrate production.13 In addition, it is noteworthy that combining SDS with carbon nanotubes forms a small amount of dendrite hydrate around the impeller at the expense of hydrate formed along the wall. Nevertheless, the SDS effect on the morphology still dominates the produced hydrates in the presence of mixed promoters. However, this issue was not encountered for the pure carbon nanotubes samples. In this work, pure and mixed promoters have reached to almost equal values of final gas uptake and water-to-hydrate conversion. This indicates that the reactor space utilization issue mentioned above is not applicable for all cases. The selection of promoter highly depends on the process design, reactor configuration, and production conditions. 3.4. Methane Hydrates’ Self-Preservation. The dissociation pressure profiles shown in Figure 9 allow us to compare the dissociation rates. It can be observed that the water sample had the most stable crystals followed by the COOH-MWCNTs and then the SDS. In addition, it can be seen that the hydrate dissociation occurred in two steps. The first step was the rapid hydrate crystal decomposition, which was assumed to take place at the surface of the hydrate block. Next, the dissociation rate was reduced due to the forming of an ice film surrounding the hydrate surface caused by the initial rapid dissociation. It should be noted that the hydrate dissociation is an endothermic phenomenon that could convert any produced water at the outer surface of the hydrate block to ice immediately. The phenomenon of self-preservation takes place due to an ice layer resulting in a slow dissociation rate. Second, as the dissociation occurs in a closed system, the release of methane

results in an increase in the vessel pressure. This pressure increases and then slows the rate of hydrate dissociation. As Figure 9 shows, the SDS increased the hydrate dissociation more as compare to water. Lin et al.23 reported similar results for the methane hydrate dissociation when SDS was present at 650 ppm. When surfactants are present, there are two factors to be considered in order to increase the dissociation rate of the hydrates. The first factor is that the hydrate particle sizes formed are finer when surfactants are present than in pure water.23 Additionally, referring to the hydrate’s morphology in the presence of SDS shown in Figure 8, the surface-to-volume ratio is much higher in comparison with pure water and COOH-MWCNT samples. As a result, the hydrate is prone to dissociate faster due to the difficulty of covering a large area by ice layer, and the gas captured in the hydrate phase has many ways to escape. The second factor is related to the hydrate structure being destabilized by the presence of surfactants. Although the higher amount of gas obtained through the presence of the surfactants requires a stronger hydrate structure in order to prevent the methane from returning to the gas phase, a massive amount of surfactant molecules will damage the uniformity of the molecular pattern and cause the crystalline structure to have defects. To put it another way, while surfactants lead to more methane being transferred to the structure of the hydrate because they lower the interface barriers, in the crystalline network, they are impurities that could possibly damage the uniform structure of the hydrate causing its destabilization. Giavarini and Maccioni found a similar result when they studied the dissociation rates of methane hydrate samples and came to the conclusion that the samples having higher gas/water ratios became dissociated faster.12,36 For the COOH-MWCNT case, the solid nanotubes entrapped between the water cavities reduce the integrity of hydrate crystals. The carbon nanotubes could weaken the hydrogen bonding by detaching the surrounding water molecules. Therefore, it accelerates the dissociation process in comparison with water. It is worth noting that the dissociation rate was fast in the presence of carbon nanotubes at the first stage. Looking at the morphology of hydrates in the presence of the pure COOH-MWCNT presented in Figure 8, the solid particles are available abundantly on the expense of water at the surface. This is due to the axial impeller pattern J


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Energy & Fuels

the final gas uptake, storage capacity, and water-to-hydrate conversion compared to water. However, a slight difference was observed in comparison with SDS. An investigation of the half completion time, t50, and the semicompletion time, t95, of the hydrate formation process demonstrated that the 0.01 wt % COOH-MWCNTs + 0.03 wt % SDS had the shortest t50. However, the sequence was changed for t95. The shortest t50 could achieve a high formation rate near the surface, which could reduce the mass transfer result in high t95. The comparison kinetic study between the 0.03 wt % SDS and the 0.03 wt % COOH-MWCNTs showed that the surfactants played a major role in the methane hydrate formation. The hydrophobic tail of the surfactant was able to dissolve methane more effectively compared to the surfactant-free nanofluid. The visual observation on the final produced hydrate morphology revealed that the presence of SDS provides a higher gas−liquid contact area due to hydrate growth along the reactor wall. Statistical analysis was done on the kinetic data to evaluate the samples based on the kinetic parameters. It was concluded that using mixed promoters of SDS and nanotubes is more effective than using an individual promoter. Finally, the kinetic promoters reduce the stability of the methane hydrates compared to water. However, a self-preservation phenomenon was more pronounced in the presence of COOH-MWCNTs compared to SDS. That could be useful for future hydratebased applications.

flow that pushes the extra solid particles, that do not act as hydrate crystal seed, toward the reactor wall. Therefore, the system needs to decompose more hydrates to provide enough water for an ice shield. However, the dissociation rate in the presence of carbon nanotubes becomes slower than SDS once an ice shield is formed.

4. STATISTICAL ANALYSIS As discussed earlier, the performance of each sample was evaluated based on eight parameters. These parameters were categorized into two main classes. Class I contained the parameters of induction times, t50, and t95, which indicated a better performance within a sample when their values were less than those of the other samples. On the contrary, the Class II parameters of rate of hydrate formation, final gas uptake, gas uptake after 180 min, storage capacity, and conversion indicated a better performance within a sample when their values were higher than those of the others. Considering that the parameters within both classes were of different natures, it was quite challenging to directly determine the promoter that had the best kinetic promotion performance. Therefore, a multicriteria decision analysis (MCDA) was performed to distinguish between the performance of the studied samples. The MCDA is a ranking-based statistical test that assigns a certain score to each promoter by giving a suitable rank within each parameter based on its efficiency. The ranking system standardizes the values within the different parameters which leads to a legitimate comparison. The MCDA results in a decision matrix. The promoter that has the best score in the decision matrix is considered to have the best performance. As presented in Tables 3 and 4 and Figure 5, the performance of each sample was evaluated based on eight parameters. However, the storage capacity and the conversion parameters were basically a component of the gas uptake parameter; therefore, both of them were eliminated from the MCDA procedure. The decision matrix of the studied samples is given in Table 5. The parameters of Class I were assigned with ascending ranks, whereas the parameters of Class II were assigned with descending ranks. The score of each promoter was then calculated as the summation of its ranks. The results presented in Table 5 that are based on kinetic parameters showed that the 0.01 wt % COOH-MWCNTs + 0.03 wt % SDS was the best promoter among the investigated samples. In addition, SDS was the major contributor in the methane promotion as indicated by the lowest promotion made by the surfactant free sample (water and 0.03 wt % COOH-MWCNTs). Moreover, statistical analysis presented in Table 5 confirmed that 0.01 and 0.05 wt % carbon nanotubes are the optimum concentrations of nanofluid which are more effective than the 0.03 wt % SDS.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel./Fax: +6053687684; +60103858473/+6053656176. ORCID

Bhajan Lal: 0000-0002-1731-4466 Behzad Partoon: 0000-0002-9567-9559 Khalik M. Sabil: 0000-0002-4365-7761 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Universiti Teknologi PETRONAS and the CO2 Research Centre (CO2RES) for providing the facilities used in this work.

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REFERENCES

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5. CONCLUSION In this study, the results showed no significant effect of carbon nanotubes on the hydrate phase boundaries. The kinetic experiments revealed that the nanofluid reduced the induction time by acting as seeds. Carboxylated carbon nanotubes showed the shortest induction time among the samples studied. In addition, the initial hydrate formation rate was enhanced the most by using the 0.01 wt % COOH-MWCNTs + 0.03 wt % SDS. It showed 2959.0% and 18.0% enhancement rates in comparison with water and 0.03 wt % SDS, respectively. Additionally, all of the promoters used enhanced K


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Kinematic Study of Methane Hydrate Formation and Self-Preservation

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