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Kinetics of methane hydrate formation in the presence of activated carbon and nano-silica suspensions in pure water
Journal of Natural Gas Science and Engineering 26 (2015) 810e818
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Kinetics of methane hydrate formation in the presence of activated carbon and nano-silica suspensions in pure water Varun Govindaraj a, b, Deepjyoti Mech a, Gaurav Pandey a, R. Nagarajan b, Jitendra S. Sangwai a, * a Gas Hydrate and Flow Assurance Laboratory, Petroleum Engineering Program, Department of Ocean Engineering, Indian Institute of Technology Madras, Chennai 600036, India b Department of Chemical Engineering, Indian Institute of Technology Madras, Chennai 600036, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 May 2015 Received in revised form 7 July 2015 Accepted 8 July 2015 Available online 15 July 2015
New kinetic promoters must be researched for hydrate formation suitable for natural gas storage and transportation to push this technology towards economic feasibility. In this study, the kinetics of the methane hydrate formation have been investigated in the presence of activated carbon and nano-silica (0.5, 1.0 and 2.0 wt%) suspensions in an aqueous solution. Experiments were conducted at 8 MPa and 275.15 K using methane gas as the hydrate former. Particles (activated carbon and nano-silica) were analyzed using scanning electron microscope (SEM) and X-ray diffraction before use. Information on the number of moles of gas consumed during hydrate formation, induction time, rate of hydrate formation, water-to-hydrate and gas-to-hydrate conversion were investigated. It was seen that the kinetics of hydrate formation were more favorable at higher concentrations of particles of activated carbon and nanosilica in the suspension. The effect of the deactivation of activated carbon was also studied and has shown a reversed trend when compared to other particles, behaving as an inhibitor for methane hydrate formation. The rate of hydrate formation was enhanced in the presence of activated carbon and was on the higher side when compared to suspensions of nano-silica. The water-to-hydrate and gas-hydrate conversions observed were in-line with the trends seen in the moles of gas consumed, with activated carbon being more effective than the rest of the particles. The induction time was observed to be reduced in the presence of suspensions of activated carbon when compared to the other hydrate forming systems studied in this work. In general, the results show that both activated carbon and nano-silica have promoting effects on methane hydrate formation kinetics, however, the effect of activated carbon is significantly more pronounced. This study provides a precursor for an improved understanding on the role of particle suspensions for methane hydrate formation suitable for gas storage applications. © 2015 Elsevier B.V. All rights reserved.
Keywords: Activated carbon Clathrate hydrates Kinetics Methane Nano-silica Suspension
1. Introduction Gas hydrates are inclusion compounds formed when certain gases (the guest molecule) come in contact with water (the host molecule) at high pressures and low temperatures. Under these conditions, the molecules can get trapped within a lattice of water molecules and then stabilized by van der Waals forces. A number of possible hydrate structures may form, namely type I, type II and type H hydrates (Sloan, 2003) depending on both the guest molecule as well as the operating conditions. Vast reserves of these
* Corresponding author. E-mail address: [email protected] (J.S. Sangwai). http://dx.doi.org/10.1016/j.jngse.2015.07.011 1875-5100/© 2015 Elsevier B.V. All rights reserved.
natural gas hydrates are currently available in deep offshore environments and can serve as an energy source if properly produced (Lu et al., 2007; Sloan, 2003). They remain, however, inaccessible for the production of natural gas commercially due to the technological constraints involved in isolating and excavating hydrates. Safety in the production process is another major issue as there is a significant risk of huge amounts of methane gas being released into the atmosphere as a result of underwater blowouts if the hydrates are not produced with adequate care (Babu et al., 2013a, b). The other key relevant application of gas hydrates is in the field of storage and transportation of natural gas. At present, natural gas is liquefied and transported by sea in floating tankers. During this process the gas must be cooled to temperatures well below 110 K (Kanoglu, 2002), and as a result production and storage incur
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significant expenditures. This adds up as an extra cost for importers of natural gas. In contrast, gas hydrates are stable even above the freezing point of water; and therefore, a significant amount of energy will be conserved if they are used to transport natural gas (Javanmardi et al., 2005). Though the storage capacity of natural gas in hydrates is lower than liquefied natural gas, they can be used effectively for localized transportation under the right conditions. However, there are major impediments that prevent this means of natural gas transportation from being economically viable. In addition to the mass of the gas, a large amount of water must also be transported when moving gas hydrates which reduces efficiency of the process. Furthermore, gas hydrate crystallization is a slow process and hence, forms a bottleneck. The extra transportation costs coupled with a kinetically slow process have proven to be deterrents in the adoption of this technique by the industry. As a result, liquefaction is still the most commonly used method to store and transport natural gas. Indeed, hydrates as a means to store and transport natural gas may find potential applications for stranded gas fields where liquefaction of natural gas may not be a promising alternative. In order to overcome these issues, the kinetics of hydrate formation may need to be enhanced. New kinetic promoters must be developed for this process to push it towards economic feasibility. A number of possibilities are currently being investigated, the best known of which are surfactants such as the anionic compound sodium dodecyl sulphate (SDS). Studies have shown that the presence of even minute concentrations of SDS (above the critical micellar concentration) can improve hydrate formation rates by a factor of 700 (Zhong and Rogers, 2000). However, SDS also has its drawbacks. While SDS has a profound effect on hydrate crystallization rates, the effect it has on the initial induction time before the onset of crystallization is minimal (Zhang et al., 2007). It has been seen to even extend the induction time at higher concentrations (Ganji et al., 2007). Furthermore, the mechanism by which SDS micelles promote hydrate formation is still not very well understood and therefore, cannot be exploited to the maximum extent (Zhang et al., 2007). Cationic and non-ionic surfactants such as cetyl tri-methyl ammonium bromide (CTAB) and ethoxylated nonylphenol (ENP) have also been studied; however, they did not show consistently positive effects on the rate of crystallization in hydrates; both promoting and inhibiting characteristics are seen at different concentrations of the surfactant solution (Ganji et al., 2007). Linga et al. (2009) have investigated the role of silica sand of 329 mm size on the methane hydrate formation kinetics. They observed of about 74.0% of water to hydrate conversion in all experiments conducted at 4.0 and 1.0 C. Babu et al. (2013) have studied the efficacy of polyurethane (PU) foam as porous material for improving the kinetics of carbon dioxide hydrate formation, which indeed showed significant improvement. Various researchers (Ilani-Kashkouli et al., 2013; Kang et al., 2008; Lee et al., 2010; Seo et al., 2002; Smith et al., 2002) provided phase equilibrium data for hydrate of carbon dioxide, methane, ethane and propane hydrates in pure water with various porous media (mesoporous silica gel, porous glass, and silica gel) with marginal inhibition on hydrate formation. These studies in general provide a promising insight into the hydrate formation in porous media and their possible use to improve hydrate-based technology for various applications. In addition to surfactants and other kinetic promoters, nanoparticles are being tested for their potential as kinetic promoters as well. Research into the effect of nano-copper suspensions on HFC134a hydrates has shown a marked improvement in the formation time (Li et al., 2006). Studies of CO2 hydrate formation using nano-graphite as a kinetic promoter have clearly demonstrated improvements in both kinetics and in the gas carrying capacity (of
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up to 20% at equilibrium) of the hydrate (Zhou et al., 2014). For the case of methane hydrates in particular, experiments with nanosilica particles have shown yields of hydrate of up to 89.42% (Chari et al., 2013). Research on mesoporous carbon has shown a significant improvement in methane storage capacity. A strong linear correlation is seen between the amount of methane consumed and the weight of carbon added (Liu et al., 2006). Studies conducted on pre-wetted beds of activated carbon (Siangsai et al., 2015; Mahboub et al., 2012) have demonstrated water-hydrate conversions of even up to 96.5%. It is, therefore, evident that these materials have the potential to address the significant issues being faced by the hydrate formation process. In this study, the focus is on achieving a deeper understanding on the effects that nano-silica and activated carbon have on methane hydrate formation. To the best of our knowledge, there have been no studies till date on the effect of a suspension of activated carbon on the kinetics of hydrate formation and hence being explored in this study. Research in this field normally looks at fixed beds of particles with high mass ratios of activated carbon to water (Najibi et al., 2007; Liu et al., 2011; Siangsai et al., 2015). Certainly, these studies demonstrate the effectiveness of activated carbon as a kinetic promoter. However, there are numerous advantages of using suspensions of particles over a fixed bed. First, stirred suspensions would have a greater gas to liquid surface area contact, thereby increasing the rate of the hydrate crystallization process. Second, stirred suspensions would ensure hydrate formation more uniformly in the reactor than a fixed bed, which could result in localized hydrate crystallization. Finally, stirred suspensions would allow for a continuous production process of hydrates, as compared to a fixed bed which can operate primarily in a batch mode. Furthermore, the separation of hydrates from a suspension of particles would be easier than from a fixed bed. In addition to activated carbon, suspension of nano-silica is being studied here as the research so far on these material primarily focuses on particles of much larger sizes and in packed bed (Mekala et al., 2014; Linga et al., 2009; Chari et al., 2013). In this study, the kinetics of the methane hydrate formation has been investigated in the presence of activated carbon and nanosilica (0.5, 1.0 and 2.0 wt%) suspension in the aqueous solution. Experiments were conducted at 8 MPa and 275.15 K using methane gas as the hydrate former. Two types of experiments were performed, one for shorter reaction time of 9.5 h and 24 h post hydrate formation, i.e., after the hydrate induction time. Information on the number of moles of gas consumed during hydrate formation, induction time, rate of hydrate formation and water-to-hydrate and gas-to-hydrate conversion were investigated. 2. Experimental section 2.1. Materials The materials used in this study have been listed in Table 1 along with their properties. All quantities were weighed using an accurate mass balance (RADWAG AS-220/X) and are accurate to a ±0.00004 mass fraction. Suspensions of particles were made using deionized distilled water of ‘Type 3’ grade with the help of an equipment named Labostar (SIEMENS, Germany). 2.2. Characterization of particles Microscopic images were taken of the two samples (activated carbon and nano-silica) to understand the morphological differences between them using a high resolution Scanning Electron Microscope (SEM) (S4800 Type I, Hitachi, Japan). Fig. 1 (a) and 1 (b) illustrate these differences. It can be seen that the activated carbon
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Table 1 Properties and suppliers of materials used in this study. Chemical
Supplier
Purity mass fraction
Particle size
Specific surface area
Methane Nano-Silica Activated Carbon
Bhuruka Gas Agency, Banglore Sisco Research Laboratories, Mumbai HiMedia Laboratories, Mumbai
0.995 0.995 0.996
e 15 nm 250e425 mm
e 650 m2/g 372 m2/g
Fig. 1. SEM image of (a) activated carbon (240e425 mm); (b) nano-silica (15 nm).
particle is of a much larger size scale, with only a portion of a single particle visible at a range of 4 microns. In Fig. 1 (a), the pore is clearly visible as a dark cavity on the activated carbon surface. As seen in Fig. 1 (b), the smaller length scale of the nano-silica particle can be easily identified when compared to the activated carbon. The particles in the SEM image (Fig. 1 (b)) have agglomerated to form a larger section of terrain, however, the individual particle heads can still be seen at the given length scale. To better understand the particles being used in the analysis, an X-Ray Diffraction (XRD) analysis was run on both materials using a powder XRD apparatus (D8 Advance, Bruker, Germany). In the case of activated carbon (see Fig. 2 (a)), it was seen that the curve is not flat, with multiple peaks spread over the 2q range. Two of the peaks are broad, indicating an amorphous nature of the compound being analyzed which is in line with activated carbon. However, there are two sharp peaks seen at 22 and 27 indicating that there is also a crystalline component to the sample. In Fig. 2 (b), which shows the results of the XRD analysis on nano-silica, there is a broad peak going from 15 to 35 indicative of an amorphous sample. Furthermore, the broad peak seen here is also representative of a
nano-level material. The intensity profile here is in line with particles of quartz (which would show a peak at around 26 ). A porosimeter analysis of activated carbon was also carried out to calculate the surface area and pore width of the sample using a Micromeritics-ASAP 2020 porosimeter (Micromeritics, USA). A weight of 0.1755 g of the activated carbon sample was outgassed for 12 h at 373 K with nitrogen gas used as the adsorptive and an equilibrium interval of 5 s. The results showed a BET surface area of 372.59 ± 1.38 m2/g and a pore volume of 0.167 cm3/g. Furthermore, the HorvatheKawazoe median pore width was found to be 0.708 nm. The adsorption capacity of the activated carbon for the hydrate study was found to be in the range of 7e12 mol CH4/kg of activated carbon at 274.15 K and 6.5e7 MPa (Zanota et al., 2005). 2.3. Experimental set-up Table 2 gives the details on various experiments conducted in this study for the formation kinetics of methane gas hydrate in suspensions of activated carbon and nano-silica in pure water. The experimental set-up used for the study is shown in Fig. 3 (a). The
Fig. 2. XRD analysis of (a) activated carbon; (b) nano-silica used in this study.
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813
Table 2 Details on hydrate formation experiments conducted in this study. PW: pure water, NS: nano-silica, AC: activated carbon, DC: de-activated carbon. Experiment Solution T (K) number
Water to hydrate P Total experimental run time Induction Moles of gas consumed Moles of gas time (h) till induction time (milli- consumed post conversion, % (MPa) (h) (including induction induction time till mol) time) end of experiment (milli-mol/mol H2O)
1 2 3 4 Average: 5 6 Average: 7 8 Average: 9 10 Average: 11 12 13 Average: 14 15 16 17 Average: 18 19 20 21 Average: 22 23 24
8 8 8 8
10.32 10.94 39.13 e
0.83 1.51 15.12 NHF
NS 275.15 8 NS 275.15 8
11.97 41.85
2.48 17.84
NS 275.15 8 NS 275.15 8
9.72 25.36
0.22 1.36
NS 275.15 8 NS 275.15 8
12.57 24.65
3.07 0.65
AC 275.15 8 AC 275.15 8 AC 275.15 8
10 9.90 24.48
0.5 0.4 0.48
AC AC AC AC
275.15 275.15 275.15 275.15
8 8 8 8
9.91 10.75 26.55 24.44
0.41 1.25 2.55 0.44
AC AC AC AC
275.15 275.15 275.15 275.15
8 8 8 8
10.02 10.44 25.41 24.87
0.55 0.96 1.41 0.87
DC 275.15 8 DC 275.15 8 DC 275.15 8
24.27 27.18 24.74
0.27 3.18 0.74
PW PW PW PW
275.15 275.15 275.15 275.15
PW 0.5% 0.5% 0.5% NS 1.0% 1.0% 1.0% NS 2.0% 2.0% 2.0% NS 0.5% 0.5% 0.5% 0.5% AC 1.0% 1.0% 1.0% 1.0% 1.0% AC 2.0% 2.0% 2.0% 2.0% 2.0% AC 0.5% 1.0% 2.0%
22.0 23.3 20.1 e 21.8 20.7 20.9 20.8 19.5 26.2 22.8 39.2 22.5 30.8 25.8 23.3 22.4 23.8 20.5 24.5 25.4 25.9 24.1 27.4 34.2 31.1 26.0 29.7 30.4 19.7 27.6
set-up consists of a high pressure, stainless steel reactor (SS-316) with a capacity of 250 ml at a working pressure upto 10 MPa. A pressure transducer (A-10, Wilka, Germany) and a Pt-100 for temperature (with a working range of upto 673 K) measure the conditions within the reactor. The pressure and temperature sensors used for the study is having an uncertainty of ±0.005 MPa and ±0.05 K, respectively. The reactor is equipped with a magnetic stirrer which can rotate upto1500 revolutions per minute. The reactor is surrounded by a jacket through which a mixture of glycol and water is circulated as the coolant. This is done with the help of a temperature controlled water bath (HAAKE A25, Thermo-Fischer Scientific, USA). 2.4. Experimental procedure The various aqueous suspensions are prepared by mixing 0.8, 1.6 and 3.2 g of activated carbon or nano-silica in 160 ml of distilled water. This gives a required concentration of particles in the desired concentrations of 0.5, 1.0 and 2.0wt% in the aqueous system. The slurry is first stirred vigorously before filling it into the reactor using a magnetic stirrer at 400 rpm until the particles are completely dispersed in the fluid without any noticeable agglomeration. Before each experiment is conducted, the contents of the reactor are siphoned out and it is cleaned of all traces of the previous run using distilled water. The prepared slurry solution of activated carbon/nano-silica is then filled into the reactor and sealed to be airtight. The water bath and magnetic stirrer are then turned on, and the water-glycol mixture is allowed to flow through
± 0.7
± 0.1
± 2.3
± 5.9
± 0.8
± 1.1
± 1.6
Gas to hydrate conversion, %
9.5 h
24 h
9.5 h
24 h
9.5 h
24 h
10.4 9.6 11.1 e 10.4 11.4 12.1 11.8 10.3 11.5 10.9 11.9 11.0 11.5 11.2 10.8 10.7 11.5 13.3 12.6 13.5 12.8 13.1 16.4 17.8 15.5 14.8 16.1 11.1 12.7 9.5
e e 17.5 e 17.5 e 17.2 17.2 e 19.6 19.6 e 16.1 16.1 e e 18.9 18.9 e e 20.1 18.1 19.1 e e 20.3 18.3 19.3 14.0 15.0 13.1
6.3 5.8 6.7 e 6.3 ± 6.9 7.4 7.2 ± 6.3 7.0 6.6 ± 7.3 6.7 7.0 ± 6.9 6.6 6.5 6.7 ± 8.1 7.7 8.3 7.8 8.0 ± 10.0 10.9 9.4 9.0 9.8 ± 6.8 7.8 5.8
e e 10.6 e 10.6 ± 0.3 e 10.5 10.5 ± 0.2 e 11.9 11.9 ± 0.6 9.8 9.8 ± 0.3 e e 11.5 11.5 ± 0.2 e e 12.2 11.0 11.6 ± 0.6 e e 12.3 11.0 11.7 ± 0.6 8.5 9.1 8.0
25.1 23.2 25.4 e 24.6 28.5 28.9 28.7 25.0 27.8 26.4 29.6 27.0 28.3 27.2 26.1 26.3 26.5 31.6 30.3 33.2 31.3 31.6 39.7 44.2 37.8 36.1 39.4 26.0 30.8 23.6
e e 40.2 e 40.2 ± 1.0 e 41.0 41 ± 0.2 e 47.2 47.2 ± 1.8 e 39.4 39.4 ± 1.2 e e 46.1 46.1 ± 0.6 e e 49.1 44.2 46.7 ± 1.7 e e 49.5 44.7 47.1 ± 1.7 32.6 36.2 32.3
± 0.3
± 0.2
± 0.4
± 0.3
± 0.1
± 0.2
± 0.5
± 0.5
± 0.4
± 0.8
± 0.5
± 0.2
± 0.5
± 0.5
0.2
0.1
0.3
0.2
0.1
0.1
0.3
± 0.6
± 0.1
± 1.0
± 0.9
± 0.3
± 0.5
± 1.5
the reactor jacket until the temperature of its contents is allowed to stabilize at 275.15 K. Once this temperature has been reached, the stirrer is turned off and the reactor is then filled with methane gas upto a pressure of 8 MPa from a cylinder through the inlet valve. A syringe pump (500 D, Teledyne Isco, USA) was also used on a few occasions so as to reach the desired pressure in the reactor to conduct the experiment. Once the gas is injected, the magnetic stirrer is turned on once again to a speed of 1000 rpm. This mixing speed is sufficient to ensure that mass and heat transfer resistances are low enough for hydrate formation to be the rate limiting process. Once the experiment begins a data acquisition system is used to record the data on pressure and temperature at every 30 s intervals. After an initial induction period, hydrate crystallization takes place. The onset of this induction is visible either as an exothermic spike in the reactor temperature, or by a sudden increase in the rate of methane consumption, or by both phenomena. This time is termed as the induction time. Hydrate crystallization was then allowed to run for a specified amount of time (upto 9e24 h post induction time for various experiments). After the completion of formation experiments, the hydrate was completely dissociated by increasing the reactor temperature upto room temperature (299 K), following which the gas is vented. A typical pressure and temperature profile of an experiment is shown in Fig. 3(b). The reactor is opened to prepare it for the next experiment. The moles of gas consumed and rate of hydrate formation were then calculated using the pressure and temperature data recorded during the course of experiment.
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Fig. 3. (a)Experimental set-up used for hydrate formation study (b) Representative pressure and temperature profiles of hydrate formation experiment (Table 2: experiment #15).
2.5. Calculation of moles of gas consumed The moles of gas consumed at any time was calculated as the difference between the number of moles of gas in the reactor at the start of hydrate crystallization, i.e, induction time (the gas that was dissolved in the suspension before the onset of crystallization has been reported separately in Table 2) and at any time, t. It was calculated using the following formula:
DnY;H ¼ Vg
Pi Pt Zi RT Zt RT
where, nmethane is the moles of methane in the reactor at the beginning of the experiment
2.8. Calculation of rate of hydrate formation The rate of hydrate formation was calculated using a discrete forward difference method. It is given as:
where, DnY; H is the moles of methane consumed at time, t; Vg is the volume of gas in the reactor; Pi is the pressure at the start of hydrate crystallization within the reactor; Zi is the compressibility factor at the start of hydrate crystallization; R is the ideal gas constant; T is the average temperature of the gas over the crystallization experiment; Pt is the pressure of the reactor at time, t; Zt is the compressibility factor of the gas in the reactor at time, t. The compressibility factor is calculated using the Pitzer correlation (Smith et al., 2001). This DnY; H can be represented as moles of gas consumed (DnY; H )/moles of water (nwater) used in this study. It is calculated as:
DnY ðmol=molÞ ¼ DnY;H nwater
2.6. Calculation of water to hydrate conversion Water to hydrate conversion was calculated at the end of experiment (taken to be at t ¼ 9.5 or 24 h) as follows:
Water to hydrate conversion ¼ DnY;H *Hydration number*
100 Gas to hydrate conversion ¼ DnY;H * nmethane
100 nwater
where, the hydration number for methane hydrate is taken as 6.1 (Tulk et al., 2000). 2.7. Calculation of gas to hydrate conversion Gas to hydrate conversion was calculated at the end of experiment (taken to be at t ¼ 9.5 or 24 h) as follows:
DnY;H;tþDt DnY;H;t dDnY;H ¼ Dt dt t where, Dt is the time difference between two observations and is taken to be 30 s The average of this rate over a 30 min interval is calculated and reported.
3. Results and discussion All experiments were conducted at an initial pressure of 8 MPa and a temperature of 275.15 K (see Table 2). Three different kind of suspension particles have been tested, namely activated carbon, deactivated carbon and nano-silica. De-activated carbon has been studied to understand the effect of contamination of activated carbon on the hydrate formation kinetics. The term de-activated carbon refers to activated carbon that has been kept open in the atmosphere for a sufficient period of time, until no further increase in its weight was seen. Experiments were conducted over 2 distinct timeframes after induction time to study the effect of time on hydrate crystallization kinetics. The first set of experiments were run with a crystallization time of 9.5 h. The second set of experiments were run with a crystallization time of 24 h. The total duration of each experiment, including the initial induction time, has been reported in Table 2. In the information reported below, the results for the shorter (9.5 h) experiments have been reported as the mean of the experiments including initial 9.5 h of the longer (24 h) experiments and the repeat experiments. The experiments conducted for hydrate formation with pure water have been compared with those using suspensions of particles.
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3.1. Activated carbon Figs. 4 and 5 shows the moles of gas consumed during hydrate formation in suspension of activated carbon for two different time periods (9.5 and 24 h, respectively). It is to be noted that these figures are plotted post-hydrate induction, denoting the point of induction as t ¼ 0 on x-axis. In the case of experiments conducted over a 9.5 h crystallization period, a clear and positive correlation is seen between the amount of activated carbon in the suspension and the moles of methane consumed during hydrate formation. Interestingly, during the experiments conducted with 2.0 wt% activated carbon it was noticed that some of the methane gas that was consumed was not recovered at the end of the experiment possibly due to adsorption. Some authors (Zhou et al., 2005; Liu et al., 2006) have reported the ability of activated carbon to effectively adsorb methane gas onto its surface. This aspect was factored in while analyzing the data on the moles of gas consumed in hydrate and an adjusted number of moles of methane for the experiments of higher concentration (2 wt% activated carbon) was calculated accordingly and reported in the corresponding figures and Table 2. However, the experiments conducted with deactivated carbon at 2 wt% did not show any adsorption of methane at the end of experiments. Table 2 shows the information on induction time, number of moles of gas consumed and gas and water to hydrate conversion for various experiments. At the end of 9.5 h, 0.0161 mol CH4/mol water was consumed by the 2.0 wt% activated carbon solution followed by 0.0131 mol CH4/mol water in the case of 1.0 wt% activated carbon and 0.0115 mol CH4/mol water for 0.5 wt%. The benchmark, i.e., pure distilled water experiment showed a consumption of 0.0104 mol CH4/mol water at the end of this time period. This positive correlation is in-line with literature observations, albeit for fixed beds of activated carbon. One such study (Mahboub et al., 2012) looked at the gas carrying capacity of hydrates at three levels of R (the water to carbon weight ratio), namely 0.8, 1.0 and 1.4. At these three levels the methane storage on a V/V basis (defined as moles of methane stored per unit volume of the vessel) is 210, 237 and 248 V/V, respectively. Converting the results of this study to a V/V basis, a similar increasing trend was seen between pure water, 0.5, 1.0 and 2.0 wt% suspensions at 12.80, 14.28, 17.04 and 19.95 V/V, respectively, at the end of 9.5 h. The differences between the results in this study and the study by Mahboub et al. can be attributed to differences in the reactor setup, the amount of water used for hydrate formation, hydrate formation conditions and the free volume available for the methane gas. It is also
Fig. 4. Moles of gas consumed (mol/mol) vs. time (activated carbon t ¼ 9.5 h). Pure water; 0.5 wt% activated carbon; 1.0 wt% activated carbon; 2.0 wt% activated carbon.
Fig. 5. Moles of gas consumed (mol/mol) vs. time (activated carbon t ¼ 24 h). Pure water; 0.5 wt% activated carbon; 1.0 wt% activated carbon; 2.0 wt% activated carbon.
observed here that the rates of methane hydrate crystallization are significantly enhanced throughout the experiment. In the case of experiments carried for 24 h, a similar positive correlation is seen between concentration and moles of gas consumed as shown in Fig. 5. An average of values of two repeats experiments are reported for all the cases but the errors bars are not shown here for simplicity. The average values of gas consumed with standard deviation are reported in Table 2. The positive effect of activated carbon on the kinetics is evident in all these cases. While the gas consumption on hydrate for the case of 1.0 wt% and 2.0 wt% activated carbon approaches at a very similar levels towards the end of the experiment, it must be noted that the experiment conducted with 2.0 wt% reached the saturation point well before the 1.0 wt% experiment. At the end of 24 h, 0.5, 1.0 and 2.0 wt% activated carbon consumed 0.0189, 0.0191 and 0.0193 mol of methane (on a mol CH4/mol H2O basis) as compared to 0.0175 mol in the case of pure water. It is also to be noted here that of the different suspensions tested in this study, those with activated carbon had the most significant positive effect on hydrate formation kinetics, especially in the earlier stages of the experiment. 3.2. De-activated carbon During our investigation on hydrate formation studies on activated carbon, a different phenomenon of the effect of activated carbon was observed when the activated carbon sample was left out in the open atmosphere for a sufficient period of time (about 12 h). In such situations, it was expected that the activated carbon may have adsorbed heaver gas molecules from atmosphere including moisture, which might have resulted in its deactivation thereby reducing the kinetic promotion ability of the carbon sample (Najibi et al., 2007). One experiment was conducted for each of the three concentrations of deactivated carbon (0.5, 1.0 and 2.0 wt %). Fig. 6 shows the results of de-activated carbon over a 24 h long crystallization period. It can be seen that in the short run (9.5 h), 0.5 and 1.0 wt% de-activated carbon show mild kinetic promotion when compared to water. However, in the long run (more than 10 h), all three concentrations are inferior to even pure water. From Fig. 6, it is clear that de-activated carbon is not only acts as a kinetic inhibitor, but also reduces the gas carrying capacity of the methane hydrates formed. It is seen that, at the end of 24 h, the consumption of methane on a mol/mol H2O basis is 0.0140, 0.0150 and 0.0131 for 0.5, 1.0 and 2.0 wt% de-activated carbon, respectively, when compared to pure water which has a consumption of 0.0174 mol/ mol H2O after 24 h. It is clearly shown that a saturation is reached
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after about 12 h and no more hydrate formation takes place, as shown in Fig. 6 (also as compared to activated carbon experiments shown in Figs. 4 and 5). One reason for this would be that the gas to liquid contact area would have been reduced as contaminants from the air would already have adsorbed to the surface of the activated carbon, thereby diminishing the capability of the carbon sample to adsorb methane and help hydrate crystallization. In another study of natural gas storage on activated carbon (Najibi et al., 2007), it was noticed that the efficiency of activated carbon in natural gas storage drastically reduced after a few cycles of adsorption and desorption, possibly because of accumulation of the heavier components of natural gas. In this case, it was speculated that other gases from atmosphere in addition to moisture may have had adsorbed on the carbon sample affecting its ability. The results, in general, indicated that the deactivated carbon significantly deter hydrate formation. It was also noticed that consumption of methane was much lower than what was expected as compared to the experiments conducted with activated carbon (see Table 2). However, when the same carbon samples were reheated at 775 K for 6 h in a nitrogen atmosphere inside a tube furnace they were found to have regained their promoting abilities. 3.3. Nano-silica As in-line with experiments with suspension of activated carbon, experiments with nano-silica were also conducted over two timeframes post-hydrate crystallization (9.5 h and 24 h). Fig. 7 shows the number of moles of methane consumed over a 9.5 h timeframe. Interestingly, all three concentrations of nano-silica (0.5, 1.0 and 2.0 wt%) showed similar effects on the kinetics of methane hydrate formation for a significant part of the experiment. They are all equally effective in promoting the kinetics of the hydrate formation, although this is only by a small margin. This would indicate that in order to study the relationship that concentrations of nano-silica have on hydrate crystallization rates, smaller concentrations would need to be looked at to understand how the gradient varies with concentration. Above a specific threshold concentration a constant kinetic promotion is visible for all concentrations. Interestingly, the results here are noticeably different from another study on nano-silica particles (Chari et al., 2013) where there is a far more significant impact of nano-silica on the hydrate yield. It may be due to differences in the particle size and morphology (15 nm in this study vs. 250 nm from Chari et al., 2013) or the nature of the particle itself (hydrophilic in this study vs. hydrophobic from Chari et al., 2013). The experiments for 24 h hydrate formation time showed a significantly lower methane consumption when compared to
Fig. 6. Moles of gas consumed (mol/mol) vs. time (de-activated carbon t ¼ 24 h). Pure water; 0.5 wt% de-activated carbon; 1.0 wt% de-activated carbon; 2.0 wt% de-activated carbon.
activated carbon and is listed in Table 2. On comparing the 2.0 wt% suspensions, nano-silica shows a consumption of 0.0161 mol CH4/ mol water vs. activated carbon, which shows a consumption of 0.0203 mol CH4/mol water. In comparison with activated carbon, nano-silica does not seem to be as promising option as kinetic enhancer as the promotion seen in crystallization kinetics is marginal and also somewhat unpredictable. A comparison between the rates of hydrate crystallization of activated carbon and nano-silica are shown in Fig. 8 (a) and (b). It is obsrvable that 2.0 wt% activated carbon has a significantly higher rate throughout the run of the experiment when compared to other concentration of both activated carbon and nano-silica. 1.0 wt% activated carbon stays marginally above that of pure water, and while 0.5 wt% activated carbon starts strongly it later converges with pure water in terms of the rate of hydrate crystalization. Nanosilica does not show significantly different rates from pure water at any point of the experiment other than in the first half an hour. The rate observations are in concurrence with the moles of gas consumed, with activated carbon showing properties of kinetic promotion, while nano-silica is far less effective. Other than the variation of consumption of methane gas consumed during hydrate formation as discussed earlier, it is interesting to note that the induction time of the hydrate formation in the case of activated carbon is far lower than the other experiments. In the case of activated carbon, the median time is 31.5 min, while for all the remaining experiments (for pure water, deactivated carbon and nano-silica) it is 84.25 min. In particular, some experiments with nano-silica and pure water have even seen induction times go above even 15 h before initial crystallization was seen. As the time at which crystallization occurs is considered to be a random process, it is difficult to predict which experiments will have a longer or shorter induction time. However, from this study it can be inferred that the mean and probability distribution the induction time follows is influenced by the particle being used. Fig. 9 depicts the schematic of methane hydrate formation for (a) pure water and (b) activated carbon. In the presence of suspension of particles (such as activated carbon or nano-silica) in water, it is expected that the particles help in generation of multiple nucleation sites for hydrate formation resulting in a faster crystallization. This also helps in early induction time for hydrate formation as compared to pure water. Also, as the suspension of particles of activated carbon provides a large surface area and have higher tendency to adsorb methane gas thereby improving the solubility, this helps in improving hydrate formation kinetics. Fig. 10 shows the water-to-hydrate and gas-to-hydrate conversion as a percentage of the water available at the beginning of the
Fig. 7. Moles of gas consumed (mol/mol) vs. time (nano-silica t ¼ 9.5 h); 0.5 wt% nano-silica; 1.0 wt% nano-silica; 2.0 wt% nano-silica.
Pure water;
V. Govindaraj et al. / Journal of Natural Gas Science and Engineering 26 (2015) 810e818
Fig. 10.
Fig. 8. (a): Rate of gas consumption (mol/mol) vs. time (activated carbon t ¼ 9.5 h); Pure water; 0.5 wt% activated carbon; 1.0 wt% activated carbon; 2.0 wt% activated carbon (b): Rate of gas consumption (mol/mol) vs. time (nano-silica t ¼ 9.5 h). Pure water; 0.5 wt% nano-silica; 1.0 wt% nano-silica; 2.0 wt% nano-silica.
experiment and the moles of gas in the reactor at the beginning of the experiment, respectively. The values reported are the mean of those obtained for each of the individual experiments at the end of 9.5 h. Both sets of values are in-line with the observations made earlier with activated carbon showing a prominent positive correlation between the concentration of the suspension and the average water to hydrate conversion/gas to hydrate conversion for
Gas-to-hydrate and
817
water-to-hydrate conversion at t ¼ 9.5 h.
each suspension. Looking at the water to hydrate conversion ratios at the end of 9.5 h, 2.0 wt% activated carbon shows the highest value of 9.8%, followed by 1.0 and 0.5 wt% at conversion ratios of 8.0% and 6.7%, respectively, all of which are significant improvements over the case of pure water which has a water to hydrate conversion ratio of 6.3%. The lowest conversion ratio is shown by 2.0 wt% de-activated which is 5.8%. Similarly, the gas to hydrate conversion percentage observed to be higher for activated carbon, followed by nano-silica and lastly deactivated carbon. It is to be noted here that these values are primarily correspond to the experimental set-up used in this study and may differ on case-to-case basis. However, the conclusions drawn on the role of activated carbon and nano-silica may form basis for better understanding on the hydrate formation kinetics in the presence of their suspensions. 4. Conclusion Hydrate formation experiments were conducted to study the effect of activated carbon, deactivated carbon and nano-silica suspensions on methane hydrate formation kinetics. Activated carbon was shown to be an efficient kinetic promoter with a positive correlation seen between its promoting capabilities and the concentration of the particle in the suspension. An improvement of nearly 60% was seen in the gas carrying capacity of 2.0 wt% activate carbon, when compared to pure water. Deactivated carbon was
Fig. 9. Schematic of Methane hydrate formation conditions (a) Pure water (b) Activated carbon.
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shown to be a kinetic inhibitor and also reduced the gas carrying capacity of the methane hydrate. Nano-silica, however, did not have any significant effect on hydrate formation kinetics as compared to activated carbon, though the gas consumption was higher as compared to pure water system. Interestingly, activated carbon was also seen to improve the hydrate formation induction time with a significant improvement seen over cases of pure water, deactivated carbon and nano-silica. The study is, in general, useful for the development of suitable hydrate formation mechanism in the presence of suspension of particles for natural gas storage application. 5. Acknowledgments The authors would like to thank the Department of Chemistry, IIT Madras for the use of their powder XRD equipment (sponsored by DST) and porosimeter. They would also like to thank the Department of Chemical engineering for the use of their HR-SEM (sponsored by DST). Author also would like to thank the partial financial support provided by Earth System Science Organization, Ministry of Earth Sciencess, Government of India, through NIOT, Chennai, India (Grant: NIOT/F&A/PROJ/GHT/01/2K14, dated 02 May 2014). References Babu, P., Yee, D., Linga, P., Palmer, A., Khoo, B.C., Tan, T.S., Rangsunvigit, P., 2013a. Morphology of methane hydrate formation in porous media. Energy Fuels 27, 3364e3372. Babu, P., Kumar, R., Linga, P., 2013b. A new porous material to enhance the kinetics of clathrate process: application to precombustion carbon dioxide capture. Environ. Sci. Technol. 47, 13191e13198. Chari, D., Sharma, D., Prasad, P., Murthy, S., 2013. Methane hydrate formation and dissociation in nano silica suspension. J. Nat. Gas. Sci. Eng. 11, 7e11. Ganji, H., Manteghian, M., Sadaghiani zadeh, K., Omidkhah, M.R., Mofrad, H.R., 2007. Effect of different surfactants on methane hydrate formation rate, stability and storage capacity. Fuel 86, 434e441. Ilani-Kashkouli, P., Hashemi, H., Gharagheizi, F., Babaee, S., Mohammadi, A.H., Ramjugernath, D., 2013. Gas hydrate phase equilibrium in porous: an assessment test for experimental data. Fluid Phase Equilibria 360, 161e168. Javanmardi, J., Nasrifar, K., Najibi, S., Moshfeghian, M., 2005. Economic evaluation of natural gas hydrate as an alternative for natural gas transportation. Appl. Therm. Eng. 25, 1708e1723. Kang, S.P., Lee, J.W., Ryu, H.J., 2008. Phase behavior of methane and carbon dioxide hydrates in meso and macro-sized porous media. Fluid Phase Equilibria 274,
68e72. Kanoglu, M., 2002. Exergy analysis of multistage cascade refrigeration cycle used for natural gas liquefaction. Int. J. Energy Res. 26, 767e774. Lee, S., Cha, I., Seo, Y., 2010. Phase behavior and 13C NMR spectroscopic analysis of the mixed methane þ ethane þ propane hydrates in mesoporous silica gels. J. Phys. Chem. B 114, 15079e15084. Li, J., Liang, D., Guo, K., Wang, R., Fan, S., 2006. Formation and dissociation of HFC134a hydrate in nano-copper suspension. Energy Convers. Manage 47, 201e210. Linga, P., Haligva, C., Nam, S., Ripmeester, J.A., Englezos, P., 2009. Gas hydrate formation in a variable volume bed of silica sand particles. Energy Fuels 23, 5496e5507. Liu, J., Zhou, Y., Sun, Y., Su, W., Zhou, L., 2011. Methane storage in wet carbon of tailored pore sizes. Carbon 49, 3731e3736. Liu, X., Zhou, L., Li, J., Sun, Y., Su, W., Zhou, Y., 2006. Methane sorption on ordered mesoporous carbon in the presence of water. Carbon 44, 1386e1392. Lu, H., Seo, Y., Lee, J., Moudrakovski, I.W., Ripmeester, J.A., Chapman, N.R., Coffin, R.B., Gardner, G., Pohlman, J., 2007. Complex gas hydrates from cascadia margin. Nature 445, 303e306. Mahboub, M., Ahmadpour, A., Rashidi, H., 2012. Improving methane storage on wet activated carbons at various amounts of water. J. Fuel Chem. Technol. 40, 385e389. Mekala, P., Babu, P., Sangwai, S., Linga, P., 2014. Formation and dissociation kinetics of methane hydrates in seawater and silica sand. Energy Fuels 28, 2708e2716. Najibi, H., Chapoy, A., Tohidi, B., 2007. Methane/natural gas storage and delivered capacity for activated carbons in dry and wet conditions. Fuel 87, 7e13. Seo, Y., Lee, H., Uchida, T., 2002. Methane and carbon dioxide hydrate phase behavior in small porous silica gels: three-phase equilibrium determination and thermodynamic modeling. Langmuir 18, 9164e9170. Siangsai, A., Rangsunvigit, P., Kitiyanan, B., Kulprathipanja, S., Linga, P., 2015. Investigation on the roles of activated carbon particle sizes on methane hydrate formation and dissociation. Chem. Eng. Sci. 126, 383e389. Sloan, E., 2003. Fundamental principles and applications of natural gas hydrates. Nature 426, 353e359. Smith, J.M., Van Ness, H.C., Abbot, M., 2001. Introduction to Chemical Engineering Thermodynamics, sixth ed. McGraw-Hill, New York. Smith, D.H., Wider, J.W., Sheshadri, K., 2002. Methane hydrate equilibria in silica gels with broad pore-size distributions. AIChE J. 48, 393e400. Tulk, C., Ripmeester, J., Klug, D., 2000. The application of Raman spectroscopy to the study of gas hydrates. Ann. N.Y. Acad. Sci. 912, 859e872. Zanota, M.L., Perier-Camby, L., Chauvy, F., Brulle, Y., Herri, J.M., 2005. Improvement of methane storage in activated carbon using methane hydrate. In: Fifth International Conference on Gas Hydrates (ICGH 5). Zhang, J., Lee, S., Lee, J.W., 2007. Kinetics of methane hydrate formation from SDS solution. Ind. Eng. Chem. 46, 6353e6359. Zhong, Y., Rogers, R., 2000. Surfactant effects on gas hydrate formation. Chem. Eng. Sci. 55, 4175e4187. Zhou, S., Yu, Y., Zhao, M., Wang, S., Zhang, G., 2014. Effect of graphite nanoparticles on promoting CO2 hydrate formation. Energy Fuels 28, 4694e4698. Zhou, Y., Wang, Y., Chen, H., Zhou, L., 2005. Methane storage in wet activated carbon: studies on the charging/discharging process. Carbon 43, 2007e2012.
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Kinetics of methane hydrate formation in the presence of activated carbon and nano-silica suspensions in pure water Varun Govindaraj a, b, Deepjyoti Mech a, Gaurav Pandey a, R. Nagarajan b, Jitendra S. Sangwai a, * a Gas Hydrate and Flow Assurance Laboratory, Petroleum Engineering Program, Department of Ocean Engineering, Indian Institute of Technology Madras, Chennai 600036, India b Department of Chemical Engineering, Indian Institute of Technology Madras, Chennai 600036, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 May 2015 Received in revised form 7 July 2015 Accepted 8 July 2015 Available online 15 July 2015
New kinetic promoters must be researched for hydrate formation suitable for natural gas storage and transportation to push this technology towards economic feasibility. In this study, the kinetics of the methane hydrate formation have been investigated in the presence of activated carbon and nano-silica (0.5, 1.0 and 2.0 wt%) suspensions in an aqueous solution. Experiments were conducted at 8 MPa and 275.15 K using methane gas as the hydrate former. Particles (activated carbon and nano-silica) were analyzed using scanning electron microscope (SEM) and X-ray diffraction before use. Information on the number of moles of gas consumed during hydrate formation, induction time, rate of hydrate formation, water-to-hydrate and gas-to-hydrate conversion were investigated. It was seen that the kinetics of hydrate formation were more favorable at higher concentrations of particles of activated carbon and nanosilica in the suspension. The effect of the deactivation of activated carbon was also studied and has shown a reversed trend when compared to other particles, behaving as an inhibitor for methane hydrate formation. The rate of hydrate formation was enhanced in the presence of activated carbon and was on the higher side when compared to suspensions of nano-silica. The water-to-hydrate and gas-hydrate conversions observed were in-line with the trends seen in the moles of gas consumed, with activated carbon being more effective than the rest of the particles. The induction time was observed to be reduced in the presence of suspensions of activated carbon when compared to the other hydrate forming systems studied in this work. In general, the results show that both activated carbon and nano-silica have promoting effects on methane hydrate formation kinetics, however, the effect of activated carbon is significantly more pronounced. This study provides a precursor for an improved understanding on the role of particle suspensions for methane hydrate formation suitable for gas storage applications. © 2015 Elsevier B.V. All rights reserved.
Keywords: Activated carbon Clathrate hydrates Kinetics Methane Nano-silica Suspension
1. Introduction Gas hydrates are inclusion compounds formed when certain gases (the guest molecule) come in contact with water (the host molecule) at high pressures and low temperatures. Under these conditions, the molecules can get trapped within a lattice of water molecules and then stabilized by van der Waals forces. A number of possible hydrate structures may form, namely type I, type II and type H hydrates (Sloan, 2003) depending on both the guest molecule as well as the operating conditions. Vast reserves of these
* Corresponding author. E-mail address: [email protected] (J.S. Sangwai). http://dx.doi.org/10.1016/j.jngse.2015.07.011 1875-5100/© 2015 Elsevier B.V. All rights reserved.
natural gas hydrates are currently available in deep offshore environments and can serve as an energy source if properly produced (Lu et al., 2007; Sloan, 2003). They remain, however, inaccessible for the production of natural gas commercially due to the technological constraints involved in isolating and excavating hydrates. Safety in the production process is another major issue as there is a significant risk of huge amounts of methane gas being released into the atmosphere as a result of underwater blowouts if the hydrates are not produced with adequate care (Babu et al., 2013a, b). The other key relevant application of gas hydrates is in the field of storage and transportation of natural gas. At present, natural gas is liquefied and transported by sea in floating tankers. During this process the gas must be cooled to temperatures well below 110 K (Kanoglu, 2002), and as a result production and storage incur
V. Govindaraj et al. / Journal of Natural Gas Science and Engineering 26 (2015) 810e818
significant expenditures. This adds up as an extra cost for importers of natural gas. In contrast, gas hydrates are stable even above the freezing point of water; and therefore, a significant amount of energy will be conserved if they are used to transport natural gas (Javanmardi et al., 2005). Though the storage capacity of natural gas in hydrates is lower than liquefied natural gas, they can be used effectively for localized transportation under the right conditions. However, there are major impediments that prevent this means of natural gas transportation from being economically viable. In addition to the mass of the gas, a large amount of water must also be transported when moving gas hydrates which reduces efficiency of the process. Furthermore, gas hydrate crystallization is a slow process and hence, forms a bottleneck. The extra transportation costs coupled with a kinetically slow process have proven to be deterrents in the adoption of this technique by the industry. As a result, liquefaction is still the most commonly used method to store and transport natural gas. Indeed, hydrates as a means to store and transport natural gas may find potential applications for stranded gas fields where liquefaction of natural gas may not be a promising alternative. In order to overcome these issues, the kinetics of hydrate formation may need to be enhanced. New kinetic promoters must be developed for this process to push it towards economic feasibility. A number of possibilities are currently being investigated, the best known of which are surfactants such as the anionic compound sodium dodecyl sulphate (SDS). Studies have shown that the presence of even minute concentrations of SDS (above the critical micellar concentration) can improve hydrate formation rates by a factor of 700 (Zhong and Rogers, 2000). However, SDS also has its drawbacks. While SDS has a profound effect on hydrate crystallization rates, the effect it has on the initial induction time before the onset of crystallization is minimal (Zhang et al., 2007). It has been seen to even extend the induction time at higher concentrations (Ganji et al., 2007). Furthermore, the mechanism by which SDS micelles promote hydrate formation is still not very well understood and therefore, cannot be exploited to the maximum extent (Zhang et al., 2007). Cationic and non-ionic surfactants such as cetyl tri-methyl ammonium bromide (CTAB) and ethoxylated nonylphenol (ENP) have also been studied; however, they did not show consistently positive effects on the rate of crystallization in hydrates; both promoting and inhibiting characteristics are seen at different concentrations of the surfactant solution (Ganji et al., 2007). Linga et al. (2009) have investigated the role of silica sand of 329 mm size on the methane hydrate formation kinetics. They observed of about 74.0% of water to hydrate conversion in all experiments conducted at 4.0 and 1.0 C. Babu et al. (2013) have studied the efficacy of polyurethane (PU) foam as porous material for improving the kinetics of carbon dioxide hydrate formation, which indeed showed significant improvement. Various researchers (Ilani-Kashkouli et al., 2013; Kang et al., 2008; Lee et al., 2010; Seo et al., 2002; Smith et al., 2002) provided phase equilibrium data for hydrate of carbon dioxide, methane, ethane and propane hydrates in pure water with various porous media (mesoporous silica gel, porous glass, and silica gel) with marginal inhibition on hydrate formation. These studies in general provide a promising insight into the hydrate formation in porous media and their possible use to improve hydrate-based technology for various applications. In addition to surfactants and other kinetic promoters, nanoparticles are being tested for their potential as kinetic promoters as well. Research into the effect of nano-copper suspensions on HFC134a hydrates has shown a marked improvement in the formation time (Li et al., 2006). Studies of CO2 hydrate formation using nano-graphite as a kinetic promoter have clearly demonstrated improvements in both kinetics and in the gas carrying capacity (of
811
up to 20% at equilibrium) of the hydrate (Zhou et al., 2014). For the case of methane hydrates in particular, experiments with nanosilica particles have shown yields of hydrate of up to 89.42% (Chari et al., 2013). Research on mesoporous carbon has shown a significant improvement in methane storage capacity. A strong linear correlation is seen between the amount of methane consumed and the weight of carbon added (Liu et al., 2006). Studies conducted on pre-wetted beds of activated carbon (Siangsai et al., 2015; Mahboub et al., 2012) have demonstrated water-hydrate conversions of even up to 96.5%. It is, therefore, evident that these materials have the potential to address the significant issues being faced by the hydrate formation process. In this study, the focus is on achieving a deeper understanding on the effects that nano-silica and activated carbon have on methane hydrate formation. To the best of our knowledge, there have been no studies till date on the effect of a suspension of activated carbon on the kinetics of hydrate formation and hence being explored in this study. Research in this field normally looks at fixed beds of particles with high mass ratios of activated carbon to water (Najibi et al., 2007; Liu et al., 2011; Siangsai et al., 2015). Certainly, these studies demonstrate the effectiveness of activated carbon as a kinetic promoter. However, there are numerous advantages of using suspensions of particles over a fixed bed. First, stirred suspensions would have a greater gas to liquid surface area contact, thereby increasing the rate of the hydrate crystallization process. Second, stirred suspensions would ensure hydrate formation more uniformly in the reactor than a fixed bed, which could result in localized hydrate crystallization. Finally, stirred suspensions would allow for a continuous production process of hydrates, as compared to a fixed bed which can operate primarily in a batch mode. Furthermore, the separation of hydrates from a suspension of particles would be easier than from a fixed bed. In addition to activated carbon, suspension of nano-silica is being studied here as the research so far on these material primarily focuses on particles of much larger sizes and in packed bed (Mekala et al., 2014; Linga et al., 2009; Chari et al., 2013). In this study, the kinetics of the methane hydrate formation has been investigated in the presence of activated carbon and nanosilica (0.5, 1.0 and 2.0 wt%) suspension in the aqueous solution. Experiments were conducted at 8 MPa and 275.15 K using methane gas as the hydrate former. Two types of experiments were performed, one for shorter reaction time of 9.5 h and 24 h post hydrate formation, i.e., after the hydrate induction time. Information on the number of moles of gas consumed during hydrate formation, induction time, rate of hydrate formation and water-to-hydrate and gas-to-hydrate conversion were investigated. 2. Experimental section 2.1. Materials The materials used in this study have been listed in Table 1 along with their properties. All quantities were weighed using an accurate mass balance (RADWAG AS-220/X) and are accurate to a ±0.00004 mass fraction. Suspensions of particles were made using deionized distilled water of ‘Type 3’ grade with the help of an equipment named Labostar (SIEMENS, Germany). 2.2. Characterization of particles Microscopic images were taken of the two samples (activated carbon and nano-silica) to understand the morphological differences between them using a high resolution Scanning Electron Microscope (SEM) (S4800 Type I, Hitachi, Japan). Fig. 1 (a) and 1 (b) illustrate these differences. It can be seen that the activated carbon
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Table 1 Properties and suppliers of materials used in this study. Chemical
Supplier
Purity mass fraction
Particle size
Specific surface area
Methane Nano-Silica Activated Carbon
Bhuruka Gas Agency, Banglore Sisco Research Laboratories, Mumbai HiMedia Laboratories, Mumbai
0.995 0.995 0.996
e 15 nm 250e425 mm
e 650 m2/g 372 m2/g
Fig. 1. SEM image of (a) activated carbon (240e425 mm); (b) nano-silica (15 nm).
particle is of a much larger size scale, with only a portion of a single particle visible at a range of 4 microns. In Fig. 1 (a), the pore is clearly visible as a dark cavity on the activated carbon surface. As seen in Fig. 1 (b), the smaller length scale of the nano-silica particle can be easily identified when compared to the activated carbon. The particles in the SEM image (Fig. 1 (b)) have agglomerated to form a larger section of terrain, however, the individual particle heads can still be seen at the given length scale. To better understand the particles being used in the analysis, an X-Ray Diffraction (XRD) analysis was run on both materials using a powder XRD apparatus (D8 Advance, Bruker, Germany). In the case of activated carbon (see Fig. 2 (a)), it was seen that the curve is not flat, with multiple peaks spread over the 2q range. Two of the peaks are broad, indicating an amorphous nature of the compound being analyzed which is in line with activated carbon. However, there are two sharp peaks seen at 22 and 27 indicating that there is also a crystalline component to the sample. In Fig. 2 (b), which shows the results of the XRD analysis on nano-silica, there is a broad peak going from 15 to 35 indicative of an amorphous sample. Furthermore, the broad peak seen here is also representative of a
nano-level material. The intensity profile here is in line with particles of quartz (which would show a peak at around 26 ). A porosimeter analysis of activated carbon was also carried out to calculate the surface area and pore width of the sample using a Micromeritics-ASAP 2020 porosimeter (Micromeritics, USA). A weight of 0.1755 g of the activated carbon sample was outgassed for 12 h at 373 K with nitrogen gas used as the adsorptive and an equilibrium interval of 5 s. The results showed a BET surface area of 372.59 ± 1.38 m2/g and a pore volume of 0.167 cm3/g. Furthermore, the HorvatheKawazoe median pore width was found to be 0.708 nm. The adsorption capacity of the activated carbon for the hydrate study was found to be in the range of 7e12 mol CH4/kg of activated carbon at 274.15 K and 6.5e7 MPa (Zanota et al., 2005). 2.3. Experimental set-up Table 2 gives the details on various experiments conducted in this study for the formation kinetics of methane gas hydrate in suspensions of activated carbon and nano-silica in pure water. The experimental set-up used for the study is shown in Fig. 3 (a). The
Fig. 2. XRD analysis of (a) activated carbon; (b) nano-silica used in this study.
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813
Table 2 Details on hydrate formation experiments conducted in this study. PW: pure water, NS: nano-silica, AC: activated carbon, DC: de-activated carbon. Experiment Solution T (K) number
Water to hydrate P Total experimental run time Induction Moles of gas consumed Moles of gas time (h) till induction time (milli- consumed post conversion, % (MPa) (h) (including induction induction time till mol) time) end of experiment (milli-mol/mol H2O)
1 2 3 4 Average: 5 6 Average: 7 8 Average: 9 10 Average: 11 12 13 Average: 14 15 16 17 Average: 18 19 20 21 Average: 22 23 24
8 8 8 8
10.32 10.94 39.13 e
0.83 1.51 15.12 NHF
NS 275.15 8 NS 275.15 8
11.97 41.85
2.48 17.84
NS 275.15 8 NS 275.15 8
9.72 25.36
0.22 1.36
NS 275.15 8 NS 275.15 8
12.57 24.65
3.07 0.65
AC 275.15 8 AC 275.15 8 AC 275.15 8
10 9.90 24.48
0.5 0.4 0.48
AC AC AC AC
275.15 275.15 275.15 275.15
8 8 8 8
9.91 10.75 26.55 24.44
0.41 1.25 2.55 0.44
AC AC AC AC
275.15 275.15 275.15 275.15
8 8 8 8
10.02 10.44 25.41 24.87
0.55 0.96 1.41 0.87
DC 275.15 8 DC 275.15 8 DC 275.15 8
24.27 27.18 24.74
0.27 3.18 0.74
PW PW PW PW
275.15 275.15 275.15 275.15
PW 0.5% 0.5% 0.5% NS 1.0% 1.0% 1.0% NS 2.0% 2.0% 2.0% NS 0.5% 0.5% 0.5% 0.5% AC 1.0% 1.0% 1.0% 1.0% 1.0% AC 2.0% 2.0% 2.0% 2.0% 2.0% AC 0.5% 1.0% 2.0%
22.0 23.3 20.1 e 21.8 20.7 20.9 20.8 19.5 26.2 22.8 39.2 22.5 30.8 25.8 23.3 22.4 23.8 20.5 24.5 25.4 25.9 24.1 27.4 34.2 31.1 26.0 29.7 30.4 19.7 27.6
set-up consists of a high pressure, stainless steel reactor (SS-316) with a capacity of 250 ml at a working pressure upto 10 MPa. A pressure transducer (A-10, Wilka, Germany) and a Pt-100 for temperature (with a working range of upto 673 K) measure the conditions within the reactor. The pressure and temperature sensors used for the study is having an uncertainty of ±0.005 MPa and ±0.05 K, respectively. The reactor is equipped with a magnetic stirrer which can rotate upto1500 revolutions per minute. The reactor is surrounded by a jacket through which a mixture of glycol and water is circulated as the coolant. This is done with the help of a temperature controlled water bath (HAAKE A25, Thermo-Fischer Scientific, USA). 2.4. Experimental procedure The various aqueous suspensions are prepared by mixing 0.8, 1.6 and 3.2 g of activated carbon or nano-silica in 160 ml of distilled water. This gives a required concentration of particles in the desired concentrations of 0.5, 1.0 and 2.0wt% in the aqueous system. The slurry is first stirred vigorously before filling it into the reactor using a magnetic stirrer at 400 rpm until the particles are completely dispersed in the fluid without any noticeable agglomeration. Before each experiment is conducted, the contents of the reactor are siphoned out and it is cleaned of all traces of the previous run using distilled water. The prepared slurry solution of activated carbon/nano-silica is then filled into the reactor and sealed to be airtight. The water bath and magnetic stirrer are then turned on, and the water-glycol mixture is allowed to flow through
± 0.7
± 0.1
± 2.3
± 5.9
± 0.8
± 1.1
± 1.6
Gas to hydrate conversion, %
9.5 h
24 h
9.5 h
24 h
9.5 h
24 h
10.4 9.6 11.1 e 10.4 11.4 12.1 11.8 10.3 11.5 10.9 11.9 11.0 11.5 11.2 10.8 10.7 11.5 13.3 12.6 13.5 12.8 13.1 16.4 17.8 15.5 14.8 16.1 11.1 12.7 9.5
e e 17.5 e 17.5 e 17.2 17.2 e 19.6 19.6 e 16.1 16.1 e e 18.9 18.9 e e 20.1 18.1 19.1 e e 20.3 18.3 19.3 14.0 15.0 13.1
6.3 5.8 6.7 e 6.3 ± 6.9 7.4 7.2 ± 6.3 7.0 6.6 ± 7.3 6.7 7.0 ± 6.9 6.6 6.5 6.7 ± 8.1 7.7 8.3 7.8 8.0 ± 10.0 10.9 9.4 9.0 9.8 ± 6.8 7.8 5.8
e e 10.6 e 10.6 ± 0.3 e 10.5 10.5 ± 0.2 e 11.9 11.9 ± 0.6 9.8 9.8 ± 0.3 e e 11.5 11.5 ± 0.2 e e 12.2 11.0 11.6 ± 0.6 e e 12.3 11.0 11.7 ± 0.6 8.5 9.1 8.0
25.1 23.2 25.4 e 24.6 28.5 28.9 28.7 25.0 27.8 26.4 29.6 27.0 28.3 27.2 26.1 26.3 26.5 31.6 30.3 33.2 31.3 31.6 39.7 44.2 37.8 36.1 39.4 26.0 30.8 23.6
e e 40.2 e 40.2 ± 1.0 e 41.0 41 ± 0.2 e 47.2 47.2 ± 1.8 e 39.4 39.4 ± 1.2 e e 46.1 46.1 ± 0.6 e e 49.1 44.2 46.7 ± 1.7 e e 49.5 44.7 47.1 ± 1.7 32.6 36.2 32.3
± 0.3
± 0.2
± 0.4
± 0.3
± 0.1
± 0.2
± 0.5
± 0.5
± 0.4
± 0.8
± 0.5
± 0.2
± 0.5
± 0.5
0.2
0.1
0.3
0.2
0.1
0.1
0.3
± 0.6
± 0.1
± 1.0
± 0.9
± 0.3
± 0.5
± 1.5
the reactor jacket until the temperature of its contents is allowed to stabilize at 275.15 K. Once this temperature has been reached, the stirrer is turned off and the reactor is then filled with methane gas upto a pressure of 8 MPa from a cylinder through the inlet valve. A syringe pump (500 D, Teledyne Isco, USA) was also used on a few occasions so as to reach the desired pressure in the reactor to conduct the experiment. Once the gas is injected, the magnetic stirrer is turned on once again to a speed of 1000 rpm. This mixing speed is sufficient to ensure that mass and heat transfer resistances are low enough for hydrate formation to be the rate limiting process. Once the experiment begins a data acquisition system is used to record the data on pressure and temperature at every 30 s intervals. After an initial induction period, hydrate crystallization takes place. The onset of this induction is visible either as an exothermic spike in the reactor temperature, or by a sudden increase in the rate of methane consumption, or by both phenomena. This time is termed as the induction time. Hydrate crystallization was then allowed to run for a specified amount of time (upto 9e24 h post induction time for various experiments). After the completion of formation experiments, the hydrate was completely dissociated by increasing the reactor temperature upto room temperature (299 K), following which the gas is vented. A typical pressure and temperature profile of an experiment is shown in Fig. 3(b). The reactor is opened to prepare it for the next experiment. The moles of gas consumed and rate of hydrate formation were then calculated using the pressure and temperature data recorded during the course of experiment.
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Fig. 3. (a)Experimental set-up used for hydrate formation study (b) Representative pressure and temperature profiles of hydrate formation experiment (Table 2: experiment #15).
2.5. Calculation of moles of gas consumed The moles of gas consumed at any time was calculated as the difference between the number of moles of gas in the reactor at the start of hydrate crystallization, i.e, induction time (the gas that was dissolved in the suspension before the onset of crystallization has been reported separately in Table 2) and at any time, t. It was calculated using the following formula:
DnY;H ¼ Vg
Pi Pt Zi RT Zt RT
where, nmethane is the moles of methane in the reactor at the beginning of the experiment
2.8. Calculation of rate of hydrate formation The rate of hydrate formation was calculated using a discrete forward difference method. It is given as:
where, DnY; H is the moles of methane consumed at time, t; Vg is the volume of gas in the reactor; Pi is the pressure at the start of hydrate crystallization within the reactor; Zi is the compressibility factor at the start of hydrate crystallization; R is the ideal gas constant; T is the average temperature of the gas over the crystallization experiment; Pt is the pressure of the reactor at time, t; Zt is the compressibility factor of the gas in the reactor at time, t. The compressibility factor is calculated using the Pitzer correlation (Smith et al., 2001). This DnY; H can be represented as moles of gas consumed (DnY; H )/moles of water (nwater) used in this study. It is calculated as:
DnY ðmol=molÞ ¼ DnY;H nwater
2.6. Calculation of water to hydrate conversion Water to hydrate conversion was calculated at the end of experiment (taken to be at t ¼ 9.5 or 24 h) as follows:
Water to hydrate conversion ¼ DnY;H *Hydration number*
100 Gas to hydrate conversion ¼ DnY;H * nmethane
100 nwater
where, the hydration number for methane hydrate is taken as 6.1 (Tulk et al., 2000). 2.7. Calculation of gas to hydrate conversion Gas to hydrate conversion was calculated at the end of experiment (taken to be at t ¼ 9.5 or 24 h) as follows:
DnY;H;tþDt DnY;H;t dDnY;H ¼ Dt dt t where, Dt is the time difference between two observations and is taken to be 30 s The average of this rate over a 30 min interval is calculated and reported.
3. Results and discussion All experiments were conducted at an initial pressure of 8 MPa and a temperature of 275.15 K (see Table 2). Three different kind of suspension particles have been tested, namely activated carbon, deactivated carbon and nano-silica. De-activated carbon has been studied to understand the effect of contamination of activated carbon on the hydrate formation kinetics. The term de-activated carbon refers to activated carbon that has been kept open in the atmosphere for a sufficient period of time, until no further increase in its weight was seen. Experiments were conducted over 2 distinct timeframes after induction time to study the effect of time on hydrate crystallization kinetics. The first set of experiments were run with a crystallization time of 9.5 h. The second set of experiments were run with a crystallization time of 24 h. The total duration of each experiment, including the initial induction time, has been reported in Table 2. In the information reported below, the results for the shorter (9.5 h) experiments have been reported as the mean of the experiments including initial 9.5 h of the longer (24 h) experiments and the repeat experiments. The experiments conducted for hydrate formation with pure water have been compared with those using suspensions of particles.
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815
3.1. Activated carbon Figs. 4 and 5 shows the moles of gas consumed during hydrate formation in suspension of activated carbon for two different time periods (9.5 and 24 h, respectively). It is to be noted that these figures are plotted post-hydrate induction, denoting the point of induction as t ¼ 0 on x-axis. In the case of experiments conducted over a 9.5 h crystallization period, a clear and positive correlation is seen between the amount of activated carbon in the suspension and the moles of methane consumed during hydrate formation. Interestingly, during the experiments conducted with 2.0 wt% activated carbon it was noticed that some of the methane gas that was consumed was not recovered at the end of the experiment possibly due to adsorption. Some authors (Zhou et al., 2005; Liu et al., 2006) have reported the ability of activated carbon to effectively adsorb methane gas onto its surface. This aspect was factored in while analyzing the data on the moles of gas consumed in hydrate and an adjusted number of moles of methane for the experiments of higher concentration (2 wt% activated carbon) was calculated accordingly and reported in the corresponding figures and Table 2. However, the experiments conducted with deactivated carbon at 2 wt% did not show any adsorption of methane at the end of experiments. Table 2 shows the information on induction time, number of moles of gas consumed and gas and water to hydrate conversion for various experiments. At the end of 9.5 h, 0.0161 mol CH4/mol water was consumed by the 2.0 wt% activated carbon solution followed by 0.0131 mol CH4/mol water in the case of 1.0 wt% activated carbon and 0.0115 mol CH4/mol water for 0.5 wt%. The benchmark, i.e., pure distilled water experiment showed a consumption of 0.0104 mol CH4/mol water at the end of this time period. This positive correlation is in-line with literature observations, albeit for fixed beds of activated carbon. One such study (Mahboub et al., 2012) looked at the gas carrying capacity of hydrates at three levels of R (the water to carbon weight ratio), namely 0.8, 1.0 and 1.4. At these three levels the methane storage on a V/V basis (defined as moles of methane stored per unit volume of the vessel) is 210, 237 and 248 V/V, respectively. Converting the results of this study to a V/V basis, a similar increasing trend was seen between pure water, 0.5, 1.0 and 2.0 wt% suspensions at 12.80, 14.28, 17.04 and 19.95 V/V, respectively, at the end of 9.5 h. The differences between the results in this study and the study by Mahboub et al. can be attributed to differences in the reactor setup, the amount of water used for hydrate formation, hydrate formation conditions and the free volume available for the methane gas. It is also
Fig. 4. Moles of gas consumed (mol/mol) vs. time (activated carbon t ¼ 9.5 h). Pure water; 0.5 wt% activated carbon; 1.0 wt% activated carbon; 2.0 wt% activated carbon.
Fig. 5. Moles of gas consumed (mol/mol) vs. time (activated carbon t ¼ 24 h). Pure water; 0.5 wt% activated carbon; 1.0 wt% activated carbon; 2.0 wt% activated carbon.
observed here that the rates of methane hydrate crystallization are significantly enhanced throughout the experiment. In the case of experiments carried for 24 h, a similar positive correlation is seen between concentration and moles of gas consumed as shown in Fig. 5. An average of values of two repeats experiments are reported for all the cases but the errors bars are not shown here for simplicity. The average values of gas consumed with standard deviation are reported in Table 2. The positive effect of activated carbon on the kinetics is evident in all these cases. While the gas consumption on hydrate for the case of 1.0 wt% and 2.0 wt% activated carbon approaches at a very similar levels towards the end of the experiment, it must be noted that the experiment conducted with 2.0 wt% reached the saturation point well before the 1.0 wt% experiment. At the end of 24 h, 0.5, 1.0 and 2.0 wt% activated carbon consumed 0.0189, 0.0191 and 0.0193 mol of methane (on a mol CH4/mol H2O basis) as compared to 0.0175 mol in the case of pure water. It is also to be noted here that of the different suspensions tested in this study, those with activated carbon had the most significant positive effect on hydrate formation kinetics, especially in the earlier stages of the experiment. 3.2. De-activated carbon During our investigation on hydrate formation studies on activated carbon, a different phenomenon of the effect of activated carbon was observed when the activated carbon sample was left out in the open atmosphere for a sufficient period of time (about 12 h). In such situations, it was expected that the activated carbon may have adsorbed heaver gas molecules from atmosphere including moisture, which might have resulted in its deactivation thereby reducing the kinetic promotion ability of the carbon sample (Najibi et al., 2007). One experiment was conducted for each of the three concentrations of deactivated carbon (0.5, 1.0 and 2.0 wt %). Fig. 6 shows the results of de-activated carbon over a 24 h long crystallization period. It can be seen that in the short run (9.5 h), 0.5 and 1.0 wt% de-activated carbon show mild kinetic promotion when compared to water. However, in the long run (more than 10 h), all three concentrations are inferior to even pure water. From Fig. 6, it is clear that de-activated carbon is not only acts as a kinetic inhibitor, but also reduces the gas carrying capacity of the methane hydrates formed. It is seen that, at the end of 24 h, the consumption of methane on a mol/mol H2O basis is 0.0140, 0.0150 and 0.0131 for 0.5, 1.0 and 2.0 wt% de-activated carbon, respectively, when compared to pure water which has a consumption of 0.0174 mol/ mol H2O after 24 h. It is clearly shown that a saturation is reached
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after about 12 h and no more hydrate formation takes place, as shown in Fig. 6 (also as compared to activated carbon experiments shown in Figs. 4 and 5). One reason for this would be that the gas to liquid contact area would have been reduced as contaminants from the air would already have adsorbed to the surface of the activated carbon, thereby diminishing the capability of the carbon sample to adsorb methane and help hydrate crystallization. In another study of natural gas storage on activated carbon (Najibi et al., 2007), it was noticed that the efficiency of activated carbon in natural gas storage drastically reduced after a few cycles of adsorption and desorption, possibly because of accumulation of the heavier components of natural gas. In this case, it was speculated that other gases from atmosphere in addition to moisture may have had adsorbed on the carbon sample affecting its ability. The results, in general, indicated that the deactivated carbon significantly deter hydrate formation. It was also noticed that consumption of methane was much lower than what was expected as compared to the experiments conducted with activated carbon (see Table 2). However, when the same carbon samples were reheated at 775 K for 6 h in a nitrogen atmosphere inside a tube furnace they were found to have regained their promoting abilities. 3.3. Nano-silica As in-line with experiments with suspension of activated carbon, experiments with nano-silica were also conducted over two timeframes post-hydrate crystallization (9.5 h and 24 h). Fig. 7 shows the number of moles of methane consumed over a 9.5 h timeframe. Interestingly, all three concentrations of nano-silica (0.5, 1.0 and 2.0 wt%) showed similar effects on the kinetics of methane hydrate formation for a significant part of the experiment. They are all equally effective in promoting the kinetics of the hydrate formation, although this is only by a small margin. This would indicate that in order to study the relationship that concentrations of nano-silica have on hydrate crystallization rates, smaller concentrations would need to be looked at to understand how the gradient varies with concentration. Above a specific threshold concentration a constant kinetic promotion is visible for all concentrations. Interestingly, the results here are noticeably different from another study on nano-silica particles (Chari et al., 2013) where there is a far more significant impact of nano-silica on the hydrate yield. It may be due to differences in the particle size and morphology (15 nm in this study vs. 250 nm from Chari et al., 2013) or the nature of the particle itself (hydrophilic in this study vs. hydrophobic from Chari et al., 2013). The experiments for 24 h hydrate formation time showed a significantly lower methane consumption when compared to
Fig. 6. Moles of gas consumed (mol/mol) vs. time (de-activated carbon t ¼ 24 h). Pure water; 0.5 wt% de-activated carbon; 1.0 wt% de-activated carbon; 2.0 wt% de-activated carbon.
activated carbon and is listed in Table 2. On comparing the 2.0 wt% suspensions, nano-silica shows a consumption of 0.0161 mol CH4/ mol water vs. activated carbon, which shows a consumption of 0.0203 mol CH4/mol water. In comparison with activated carbon, nano-silica does not seem to be as promising option as kinetic enhancer as the promotion seen in crystallization kinetics is marginal and also somewhat unpredictable. A comparison between the rates of hydrate crystallization of activated carbon and nano-silica are shown in Fig. 8 (a) and (b). It is obsrvable that 2.0 wt% activated carbon has a significantly higher rate throughout the run of the experiment when compared to other concentration of both activated carbon and nano-silica. 1.0 wt% activated carbon stays marginally above that of pure water, and while 0.5 wt% activated carbon starts strongly it later converges with pure water in terms of the rate of hydrate crystalization. Nanosilica does not show significantly different rates from pure water at any point of the experiment other than in the first half an hour. The rate observations are in concurrence with the moles of gas consumed, with activated carbon showing properties of kinetic promotion, while nano-silica is far less effective. Other than the variation of consumption of methane gas consumed during hydrate formation as discussed earlier, it is interesting to note that the induction time of the hydrate formation in the case of activated carbon is far lower than the other experiments. In the case of activated carbon, the median time is 31.5 min, while for all the remaining experiments (for pure water, deactivated carbon and nano-silica) it is 84.25 min. In particular, some experiments with nano-silica and pure water have even seen induction times go above even 15 h before initial crystallization was seen. As the time at which crystallization occurs is considered to be a random process, it is difficult to predict which experiments will have a longer or shorter induction time. However, from this study it can be inferred that the mean and probability distribution the induction time follows is influenced by the particle being used. Fig. 9 depicts the schematic of methane hydrate formation for (a) pure water and (b) activated carbon. In the presence of suspension of particles (such as activated carbon or nano-silica) in water, it is expected that the particles help in generation of multiple nucleation sites for hydrate formation resulting in a faster crystallization. This also helps in early induction time for hydrate formation as compared to pure water. Also, as the suspension of particles of activated carbon provides a large surface area and have higher tendency to adsorb methane gas thereby improving the solubility, this helps in improving hydrate formation kinetics. Fig. 10 shows the water-to-hydrate and gas-to-hydrate conversion as a percentage of the water available at the beginning of the
Fig. 7. Moles of gas consumed (mol/mol) vs. time (nano-silica t ¼ 9.5 h); 0.5 wt% nano-silica; 1.0 wt% nano-silica; 2.0 wt% nano-silica.
Pure water;
V. Govindaraj et al. / Journal of Natural Gas Science and Engineering 26 (2015) 810e818
Fig. 10.
Fig. 8. (a): Rate of gas consumption (mol/mol) vs. time (activated carbon t ¼ 9.5 h); Pure water; 0.5 wt% activated carbon; 1.0 wt% activated carbon; 2.0 wt% activated carbon (b): Rate of gas consumption (mol/mol) vs. time (nano-silica t ¼ 9.5 h). Pure water; 0.5 wt% nano-silica; 1.0 wt% nano-silica; 2.0 wt% nano-silica.
experiment and the moles of gas in the reactor at the beginning of the experiment, respectively. The values reported are the mean of those obtained for each of the individual experiments at the end of 9.5 h. Both sets of values are in-line with the observations made earlier with activated carbon showing a prominent positive correlation between the concentration of the suspension and the average water to hydrate conversion/gas to hydrate conversion for
Gas-to-hydrate and
817
water-to-hydrate conversion at t ¼ 9.5 h.
each suspension. Looking at the water to hydrate conversion ratios at the end of 9.5 h, 2.0 wt% activated carbon shows the highest value of 9.8%, followed by 1.0 and 0.5 wt% at conversion ratios of 8.0% and 6.7%, respectively, all of which are significant improvements over the case of pure water which has a water to hydrate conversion ratio of 6.3%. The lowest conversion ratio is shown by 2.0 wt% de-activated which is 5.8%. Similarly, the gas to hydrate conversion percentage observed to be higher for activated carbon, followed by nano-silica and lastly deactivated carbon. It is to be noted here that these values are primarily correspond to the experimental set-up used in this study and may differ on case-to-case basis. However, the conclusions drawn on the role of activated carbon and nano-silica may form basis for better understanding on the hydrate formation kinetics in the presence of their suspensions. 4. Conclusion Hydrate formation experiments were conducted to study the effect of activated carbon, deactivated carbon and nano-silica suspensions on methane hydrate formation kinetics. Activated carbon was shown to be an efficient kinetic promoter with a positive correlation seen between its promoting capabilities and the concentration of the particle in the suspension. An improvement of nearly 60% was seen in the gas carrying capacity of 2.0 wt% activate carbon, when compared to pure water. Deactivated carbon was
Fig. 9. Schematic of Methane hydrate formation conditions (a) Pure water (b) Activated carbon.
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shown to be a kinetic inhibitor and also reduced the gas carrying capacity of the methane hydrate. Nano-silica, however, did not have any significant effect on hydrate formation kinetics as compared to activated carbon, though the gas consumption was higher as compared to pure water system. Interestingly, activated carbon was also seen to improve the hydrate formation induction time with a significant improvement seen over cases of pure water, deactivated carbon and nano-silica. The study is, in general, useful for the development of suitable hydrate formation mechanism in the presence of suspension of particles for natural gas storage application. 5. Acknowledgments The authors would like to thank the Department of Chemistry, IIT Madras for the use of their powder XRD equipment (sponsored by DST) and porosimeter. They would also like to thank the Department of Chemical engineering for the use of their HR-SEM (sponsored by DST). Author also would like to thank the partial financial support provided by Earth System Science Organization, Ministry of Earth Sciencess, Government of India, through NIOT, Chennai, India (Grant: NIOT/F&A/PROJ/GHT/01/2K14, dated 02 May 2014). References Babu, P., Yee, D., Linga, P., Palmer, A., Khoo, B.C., Tan, T.S., Rangsunvigit, P., 2013a. Morphology of methane hydrate formation in porous media. Energy Fuels 27, 3364e3372. Babu, P., Kumar, R., Linga, P., 2013b. A new porous material to enhance the kinetics of clathrate process: application to precombustion carbon dioxide capture. Environ. Sci. Technol. 47, 13191e13198. Chari, D., Sharma, D., Prasad, P., Murthy, S., 2013. Methane hydrate formation and dissociation in nano silica suspension. J. Nat. Gas. Sci. Eng. 11, 7e11. Ganji, H., Manteghian, M., Sadaghiani zadeh, K., Omidkhah, M.R., Mofrad, H.R., 2007. Effect of different surfactants on methane hydrate formation rate, stability and storage capacity. Fuel 86, 434e441. Ilani-Kashkouli, P., Hashemi, H., Gharagheizi, F., Babaee, S., Mohammadi, A.H., Ramjugernath, D., 2013. Gas hydrate phase equilibrium in porous: an assessment test for experimental data. Fluid Phase Equilibria 360, 161e168. Javanmardi, J., Nasrifar, K., Najibi, S., Moshfeghian, M., 2005. Economic evaluation of natural gas hydrate as an alternative for natural gas transportation. Appl. Therm. Eng. 25, 1708e1723. Kang, S.P., Lee, J.W., Ryu, H.J., 2008. Phase behavior of methane and carbon dioxide hydrates in meso and macro-sized porous media. Fluid Phase Equilibria 274,
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