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Effect of synthesized silver nanoparticles in promoting methane hydrate formation at 4.7 MPa and 5.7 MPa
ARTICLE IN PRESS
CHERD-1135; No. of Pages 5
chemical engineering research and design x x x ( 2 0 1 3 ) xxx–xxx
Contents lists available at SciVerse ScienceDirect
Chemical Engineering Research and Design journal homepage: www.elsevier.com/locate/cherd
Effect of synthesized silver nanoparticles in promoting methane hydrate formation at 4.7 MPa and 5.7 MPa Samad Arjang, Mehrdad Manteghian ∗ , Abolfazl Mohammadi Faculty of Chemical Engineering, Tarbiat Modares University, Jalal Al Ahmad Highway, P.O. Box 14115-143, Tehran, Iran
a b s t r a c t Using gas hydrates as materials for storage and transportation of natural gas have attracted much attention in recent years. However, there are two barriers in industrializing this new method. Firstly, methane hydrate induction time is relatively high. On the other hand the amount of gas trapped in methane hydrate crystals is too low. In this survey, silver nanoparticles were synthesized using a chemical reduction method and introduced to the hydrate reactor. Experiments were conducted at initial reactor pressures of 4.7 MPa and 5.7 MPa. At each pressure three independent experiments were performed. According to the results, in the presence of silver nanoparticles, methane hydrate induction time decreased by 85% and 73.9%, and the amount of methane trapped in hydrate crystals increased by 33.7% and 7.4% at the pressures of 4.7 MPa and 5.7 MPa respectively. © 2012 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. Keywords: Silver nanoparticles; Methane; Hydrate; Induction time
1.
Introduction
Many researchers have studied methane hydrate formation conditions aimed normally at promotion (Ganji et al., 2007b) or inhibition (Xiao et al., 2010). Since each volume of methane hydrate can contain as much as 184 volumes of methane at standard temperature and pressure (Sloan and Koh, 2008), hydrates are currently considered as a new technology for natural gas storage and transport. To industrialize this new technology, it is necessary to increase methane hydrate formation rate and storage capacity. In fact, the reaction between methane and water is very slow during formation process. Various methods were used to promote gas hydrates formation. Addition of surfactants (Ganji et al., 2007a,b; Kwon et al., 2011; Mohammadi et al., 2011, accepted for publicationa, accepted for publication-b) has been the most popular one. Ganji et al. (2007a,b) have reported remarkable promotional effects of SDS (an anionic surfactant) on methane hydrate formation process. Some other methods like applying ultrasonic (Liu et al., 2003a) and magnetic fields (Liu et al., 2003b) have been studied too. Although favorable effects were reported of these methods, but it is hard to use them in practice.
Li et al. (2006) employed copper nanoparticles as an additive to promote HFC134a gas hydrate formation for the first time. Lee et al. (2007) showed poor promotional effects of triple nanoparticles on methane hydrate formation process. In addition, increasing methane hydrate storage capacity and decreasing of the induction time were reported by Park and Kim (2010) using carbon nanotubes. Increasing heat transfer coefficient in the presence of these conductive nanoparticles was mentioned as the main reason for this promotional effect in these researches. In this survey silver nanoparticles were chosen. Silver has the highest heat transfer coefficient among all metals.
2.
Experimental
2.1.
Synthesis of silver nanoparticles
Various methods have been proposed to synthesize silver nanoparticles (Aswathy et al., 2011; Bonsak, 2010; Ghader et al., 2007; Lupu, 2010; Scaiano et al., 2006; Zhang et al., 2006). In this study, a suitable chemical method has been chosen to synthesize these nanoparticles. This method has been described below.
∗
Corresponding author. Tel.: +98 2182883333; fax: +98 2182884931. E-mail address: [email protected] (M. Manteghian). Received 23 May 2012; Received in revised form 17 November 2012; Accepted 7 December 2012 0263-8762/$ – see front matter © 2012 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cherd.2012.12.001 Please cite this article in press as: Arjang, S., et al., Effect of synthesized silver nanoparticles in promoting methane hydrate formation at 4.7 MPa and 5.7 MPa. Chem. Eng. Res. Des. (2013), http://dx.doi.org/10.1016/j.cherd.2012.12.001
CHERD-1135; No. of Pages 5
2
ARTICLE IN PRESS chemical engineering research and design x x x ( 2 0 1 3 ) xxx–xxx
Nomenclature P T t n V Z R
2.1.1.
pressure, MPa absolute temperature, K time, min number of moles volume, m3 compressibility factor (dimensionless) universal gas constant, 8.314 J mol−1 K−1
Materials
Sodium borohydride (NaBH4 ) and tri-sodium citrate (C6 H5 Na3 O7 ·2H2 O) were purchased from Merck, Germany. Silver nitrate (AgNO3 ) was obtained from PanReac, Spain. All of these chemicals were used as received.
2.1.2.
Synthesis method
A chemical reduction method (Bonsak, 2010; Jana et al., 2001) was employed to synthesize silver nanoparticles. First 20 ml aqueous solution with concentration of 0.25 mM AgNO3 and 0.25 mM trisodium citrate was prepared. During stirring this solution, 0.6 ml of 10 mM NaBH4 was added. Stirring was stopped after 3 min. In this route sodium borohydride acted as the reducing agent and trisodium citrate as the stabilizer. The chemical reaction of this synthesis process can be written as (Solomon et al., 2007): AgNO3 + NaBH4 → Ag + 0.5H2 + 0.5B2 H6 + NaNO3 Some samples were prepared on glass slides and was imaged using SEM. The image is shown in Fig. 1. Using Image J 1.43 software, the size distribution of silver nanoparticles was obtained. This is shown near SEM image. According to this image, silver nanoparticles diameter is between 50 and 75 nm. EDX analysis showed the purity of synthesized silver nanoparticles. The spectrum is shown in Fig. 2. Presence of calcium and silicon is believed to be from the glass slide. UV–vis absorption spectra peak is on 400 nm, the conventional peak of silver metal nanoparticles (Aswathy et al., 2011; Bonsak, 2010; Ghader et al., 2007; Jana et al., 2001; Lupu, 2010; Scaiano et al., 2006; Solomon et al., 2007). The spectrum is shown in Fig. 3.
Fig. 1 – (a) SEM image and (b) size distribution of synthesized silver nanoparticles.
2.2.2.
Experimental procedure
To do each experiment first of all, water or aqueous solution containing silver nanoparticles or trisodium citrate was poured into the reactor. Then methane gas with purity of 99.9% was injected until the final pressure was reached at 283.15 K. After that cooling system was turned on while its temperature was adjusted on 275.15 K. When the electromotor was turned on, data recording was started. The reactor temperature and pressure were recorded each 10 s. These data are plotted using Excel 2007 software.
2.2. Effects of silver nanoparticles on methane hydrate formation process 2.2.1.
Apparatus
The schematic diagram of methane hydrate formation apparatus is shown in Fig. 4. The reactor is a batch one and its volume is 460 cm3 . A mixture of water and alcohol is used to cool the system. In this setup a mechanical system accompanied with an electromotor was employed to agitate the mixture of water and methane in the reactor. The whole reactor was insulated to prevent any heat loss. One platinum resistance thermometer (Pt 100) is inserted into the reactor to measure temperature of the system within a precision of ±0.01 K. Pressure is measured by a BD pressure transducer within a precision of ±0.01 MPa for the operating range. The temperature and pressure are monitored and recorded in a computer by means of a data acquisition interface.
Fig. 2 – EDX analysis of synthesized nanoparticles.
Please cite this article in press as: Arjang, S., et al., Effect of synthesized silver nanoparticles in promoting methane hydrate formation at 4.7 MPa and 5.7 MPa. Chem. Eng. Res. Des. (2013), http://dx.doi.org/10.1016/j.cherd.2012.12.001
ARTICLE IN PRESS
CHERD-1135; No. of Pages 5
chemical engineering research and design x x x ( 2 0 1 3 ) xxx–xxx
3
2
Absorbance (AU)
1.6
1.2
0.8
0.4
0 300
400
500
600
700
800
900
wavelength (nm)
Fig. 3 – UV–vis spectra of obtained nanoparticles suspension.
2.2.3.
Fig. 5 – P–t and T–t profiles of methane hydrate formation in the presence of pure water at target temperature of 275.15 K and initial pressure of 4.7 MPa.
Results and discussion
To find out effect of silver nanoparticles on methane hydrate formation process, three different experiments were done at each pressure. At initial pressure of 4.7 MPa, 100 ml of distilled water was poured into the reactor. Pressure–time and temperature–time diagrams of this experiment are plotted in Fig. 5. Based on these data, methane hydrate induction time was 485 min. In the second experiment at this pressure, the suspension containing silver nanoparticles diluted to 100 ml was injected to the reactor. Obtained pressure–time and temperature–time profiles are plotted and shown in Fig. 6. In order to investigate the effect of sodium citrate an experiment with the
same conditions was designed. For this purpose, 20 ml of 0.25 mM trisodium citrate was diluted to 100 ml with distilled water. In this solution the concentration of this chemical was the same as the second experiment. After gathering and plotting data (Fig. 7) of this experiment, very interesting results have been observed. According to the obtained graphs for these three experiments, the induction time of methane hydrate formation decreased in the presence of trisodium citrate and silver nanoparticles which is 379.8 and 71.4 min respectively. But the decrease is remarkable in the presence of silver nanoparticles. There are three reasons for this effect. First, the presence of silver nanoparticles creates a nanofluid
Fig. 4 – A schematic diagram of the methane hydrate apparatus. Please cite this article in press as: Arjang, S., et al., Effect of synthesized silver nanoparticles in promoting methane hydrate formation at 4.7 MPa and 5.7 MPa. Chem. Eng. Res. Des. (2013), http://dx.doi.org/10.1016/j.cherd.2012.12.001
CHERD-1135; No. of Pages 5
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ARTICLE IN PRESS chemical engineering research and design x x x ( 2 0 1 3 ) xxx–xxx
Table 1 – Induction time of methane hydrate formation at 5.7 MPa in different environments. Environment Induction time (min)
In pure water
In nanofluid containing silver nanoparticles
In trisodium citrate solution
380.3
99.2
272.9
Table 2 – The amount of methane gas consumed after 900 min for experiments with initial pressure of 4.7 MPa. Environment Moles of gas consumed
In pure water
In nanofluid
In trisodium citrate solution
0.014
0.019
0.016
Table 3 – The amount of methane gas consumed after 900 min for experiments with initial pressure of 5.7 MPa. Environment Moles of gas consumed
In pure water
In nanofluid
In trisodium citrate solution
0.027
0.029
0.0275
with higher heat transfer coefficient. Methane hydrate nucleation is an exothermic process. If this heat dissipates more easily, methane hydrate nucleation takes place faster. The required work needed for heterogeneous nucleation is less
Fig. 6 – P–t and T–t profiles of methane hydrate formation in the presence of synthesized silver nanoparticles at target temperature of 275.15 K and initial pressure of 4.7 MPa.
than homogenous nucleation (Kashchiev and Firoozabadi, 2002). So, when silver nanoparticles exist in water, methane hydrate nucleates easier and faster. On the other hand, the surface to volume ratio increases significantly in nanometric dimensions. Now considering favorable Park and Kim (2010) could decrease methane hydrate induction time by 52% while this parameter was decreased by 85% (from 485 min for pure water case to 71.8 min) in this investigation at the same conditions. Comparison of our results with Ganji et al. (2007b) show that utilizing of 500 ppm SDS is more effective than silver nanoparticle and trisodium citrate to reduce the induction time of methane hydrate formation. Their experiments show that in presence of SDS with concentration of 500 ppm, the induction time is very short and hydrate attains steady state in about one hour. These results are observed in the experiments with initial pressure of 5.7 MPa too. As it is mentioned in Table 1, methane hydrate formation time decreased noticeably in the presence of silver nanoparticles. Initial and final pressures have been employed to investigate the amount of methane gas trapped in methane hydrate crystals:
n =
Fig. 7 – P–t and T–t profiles of methane hydrate formation in the presence of trisodium citrate at target temperature of 275.15 K and initial pressure of 4.7 MPa.
PV ZRT
i
−
PV ZRT
f
The compressibility factor has been calculated using Lee–Kesler equations (Smith et al., 2005). The amount of methane gas trapped in methane hydrate crystals was calculated after 900 min in each experiment. The results were reported in Tables 2 and 3. In the presence of silver nanofluid, the amount of methane gas trapped in methane hydrate crystals increased by 33.7% and 7.4% for experiments with initial pressures of 4.7 and 5.7 MPa respectively. But, when trisodium citrate was injected to the reactor with water, this parameter has been increased by 14.2% and 1.8% at initial pressures of 4.7 and 5.7 MPa respectively. These results show that silver nanoparticles have a considerable effect on increasing gas to water ratio of methane hydrate.
Please cite this article in press as: Arjang, S., et al., Effect of synthesized silver nanoparticles in promoting methane hydrate formation at 4.7 MPa and 5.7 MPa. Chem. Eng. Res. Des. (2013), http://dx.doi.org/10.1016/j.cherd.2012.12.001
CHERD-1135; No. of Pages 5
ARTICLE IN PRESS chemical engineering research and design x x x ( 2 0 1 3 ) xxx–xxx
3.
Conclusion
Silver nanoparticles were synthesized by a chemical method in which trisodium citrate acted as stabilizer. Using these nanoparticles as promoter, methane hydrate induction time has been decreased by 85% and 73.9% for the experiments with initial pressure of 4.7 MPa and 5.7 MPa respectively. However the amount of methane consumed to form methane hydrate increased by 33.7% and 7.4% on molar basis in the presence of silver nanoparitcles, respectively at pressures of 4.7 MPa and 5.7 MPa. Trisodium citrate was injected at the same concentration in independent experiments to verify the net effect of this chemical on methane hydrate formation process. The results show that this stabilizing agent decrease induction time and increase moles of trapped gas in methane hydrate crystals, but it did not supersede the effect of silver nanoparticles.
References Aswathy, B., Avadhani, G.S., Sumithra, I.S., Suji, S., Sony, G., 2011. Microwave assisted synthesis and UV–Vis spectroscopic studies of silver nanoparticles synthesized using vanillin as a reducing agent. J. Mol. Liq. 159, 165–169. Bonsak, J., 2010. Chemical Synthesis of Silver Nanoparticles for Light Trapping Applications in Silicon Solar Cells. Faculty of Mathematics and Natural sciences, Oslo, Norway. Ganji, H., Manteghian, M., Rahimi Mofrad, H., 2007a. Effect of mixed compounds on methane hydrate formation and dissociation rates and storage capacity. Fuel Process. Technol. 88, 891–895. Ganji, H., Manteghian, M., Sadaghiani zadeh, K., Omidkhah, M.R., Rahimi Mofrad, H., 2007b. Effect of different surfactants on methane hydrate formation rate, stability and storage capacity. Fuel 86, 434–441. Ghader, S., Manteghian, M., Kokabi, M., Mamoory, R.S., 2007. Preparation of truncated triangular silver nanoparticles by a simple and rapid method in aqueous solution. Anglais 81, 1555–1565. Jana, N.R., Gearheart, L., Murphy, C.J., 2001. Wet chemical synthesis of silver nanorods and nanowires of controllable aspect ratio. Chem. Commun., 617–618. Kashchiev, D., Firoozabadi, A., 2002. Nucleation of gas hydrates. J. Cryst. Growth 243, 476–489. Kwon, Y.-A., Park, J.-M., Jeong, K.-E., Kim, C.-U., Kim, T.-W., Chae, H.-J., Jeong, S.-Y., Yim, J.-H., Park, Y.-K., Lee, J.-d., 2011.
5
Synthesis of anionic multichain type surfactant and its effect on methane gas hydrate formation. J. Ind. Eng. Chem. 17, 120–124. Lee, J.D., Kim, H.C., Kim, Y.S., Kim, Y.D., Lee, M.S., 2007. Synthesis of nanosized TiO2 –Ag–SiO2 sols by modified sol–gel method and their application for methane hydrate formation. Solid State Phenom. 124–126, 1059–1062. Li, J., Liang, D., Guo, K., Wang, R., Fan, S., 2006. Formation and dissociation of HFC134a gas hydrate in nano-copper suspension. Energy Convers. Manage. 47, 201–210. Liu, Y., Guo, K., Liang, D., Fan, S., 2003a. Experimental study on crystallizing process of HCFC-141b hydrate by ultrasonic. J. Wuhan Univ. Technol. (China) 24, 21–23. Liu, Y., Guo, K., Liang, D., Fan, S., 2003b. Refrigerant gas hydrate growth under influence of magnetic field. Sci. China (Ser. B) 33, 89–96. Lupu, N., 2010. Nanowires Science and Technology. InTech, Croatia. Mohammadi, A., Manteghian, M., Mirzaei, M., 2011. Effect of -cyclodextrin on dissolution of methane in water. Chem. Eng. Res. Des. 89 (4), 421–427. Mohammadi, A., Manteghian, M., Hoseini Nasab, S.M., Mousavi, S.M. Effect of multi walled carbon nanotubes on solubility of methane in water. Petrol. Sci. Technol., accepted for publication-a. Mohammadi, A., Manteghian, Mousavi, S.M. Solubility of methane in water at presence of -cyclodextrin, SDS and multi walled carbon nanotubes. Petrol. Sci. Technol., accepted for publication-b. Park, S.-S., Kim, N.-J., 2010. Multi-walled carbon nano tubes effects for methane hydrate formation. In: The 2nd International Conference on Computer and Automation Engineering (ICCAE), pp. 294–297. Scaiano, J.C., Aliaga, C., Maguire, S., Wang, D., 2006. Magnetic field control of photoinduced silver nanoparticle formation. J. Phys. Chem. B 110, 12856–12859. Sloan, E.D., Koh, C.A., 2008. Clathrate Hydrates of Natural Gas. Talyor & Francis Group. Smith, J.M., Ness, H.C.V., Abbott, M.M., 2005. Introduction to Chemical Engineering Thermodynamics, seventh ed. Mc Graw-Hill. Solomon, S.D., Bahadory, M., Jeyarajasingam, A.V., Rutkowsky, S.A., Boritz, C., Mulfinger, L., 2007. Synthesis and study of silver nanoparticles. J. Chem. Educ. 84, 322–325. Xiao, C., Wibisono, N., Adidharma, H., 2010. Dialkylimidazolium halide ionic liquids as dual function inhibitors for methane hydrate. Chem. Eng. Sci. 65, 3080–3087. Zhang, W., Qiao, X., Chen, J., 2006. Synthesis and characterization of silver nanoparticles in AOT microemulsion system. Chem. Phys. 330, 495–500.
Please cite this article in press as: Arjang, S., et al., Effect of synthesized silver nanoparticles in promoting methane hydrate formation at 4.7 MPa and 5.7 MPa. Chem. Eng. Res. Des. (2013), http://dx.doi.org/10.1016/j.cherd.2012.12.001
CHERD-1135; No. of Pages 5
chemical engineering research and design x x x ( 2 0 1 3 ) xxx–xxx
Contents lists available at SciVerse ScienceDirect
Chemical Engineering Research and Design journal homepage: www.elsevier.com/locate/cherd
Effect of synthesized silver nanoparticles in promoting methane hydrate formation at 4.7 MPa and 5.7 MPa Samad Arjang, Mehrdad Manteghian ∗ , Abolfazl Mohammadi Faculty of Chemical Engineering, Tarbiat Modares University, Jalal Al Ahmad Highway, P.O. Box 14115-143, Tehran, Iran
a b s t r a c t Using gas hydrates as materials for storage and transportation of natural gas have attracted much attention in recent years. However, there are two barriers in industrializing this new method. Firstly, methane hydrate induction time is relatively high. On the other hand the amount of gas trapped in methane hydrate crystals is too low. In this survey, silver nanoparticles were synthesized using a chemical reduction method and introduced to the hydrate reactor. Experiments were conducted at initial reactor pressures of 4.7 MPa and 5.7 MPa. At each pressure three independent experiments were performed. According to the results, in the presence of silver nanoparticles, methane hydrate induction time decreased by 85% and 73.9%, and the amount of methane trapped in hydrate crystals increased by 33.7% and 7.4% at the pressures of 4.7 MPa and 5.7 MPa respectively. © 2012 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. Keywords: Silver nanoparticles; Methane; Hydrate; Induction time
1.
Introduction
Many researchers have studied methane hydrate formation conditions aimed normally at promotion (Ganji et al., 2007b) or inhibition (Xiao et al., 2010). Since each volume of methane hydrate can contain as much as 184 volumes of methane at standard temperature and pressure (Sloan and Koh, 2008), hydrates are currently considered as a new technology for natural gas storage and transport. To industrialize this new technology, it is necessary to increase methane hydrate formation rate and storage capacity. In fact, the reaction between methane and water is very slow during formation process. Various methods were used to promote gas hydrates formation. Addition of surfactants (Ganji et al., 2007a,b; Kwon et al., 2011; Mohammadi et al., 2011, accepted for publicationa, accepted for publication-b) has been the most popular one. Ganji et al. (2007a,b) have reported remarkable promotional effects of SDS (an anionic surfactant) on methane hydrate formation process. Some other methods like applying ultrasonic (Liu et al., 2003a) and magnetic fields (Liu et al., 2003b) have been studied too. Although favorable effects were reported of these methods, but it is hard to use them in practice.
Li et al. (2006) employed copper nanoparticles as an additive to promote HFC134a gas hydrate formation for the first time. Lee et al. (2007) showed poor promotional effects of triple nanoparticles on methane hydrate formation process. In addition, increasing methane hydrate storage capacity and decreasing of the induction time were reported by Park and Kim (2010) using carbon nanotubes. Increasing heat transfer coefficient in the presence of these conductive nanoparticles was mentioned as the main reason for this promotional effect in these researches. In this survey silver nanoparticles were chosen. Silver has the highest heat transfer coefficient among all metals.
2.
Experimental
2.1.
Synthesis of silver nanoparticles
Various methods have been proposed to synthesize silver nanoparticles (Aswathy et al., 2011; Bonsak, 2010; Ghader et al., 2007; Lupu, 2010; Scaiano et al., 2006; Zhang et al., 2006). In this study, a suitable chemical method has been chosen to synthesize these nanoparticles. This method has been described below.
∗
Corresponding author. Tel.: +98 2182883333; fax: +98 2182884931. E-mail address: [email protected] (M. Manteghian). Received 23 May 2012; Received in revised form 17 November 2012; Accepted 7 December 2012 0263-8762/$ – see front matter © 2012 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cherd.2012.12.001 Please cite this article in press as: Arjang, S., et al., Effect of synthesized silver nanoparticles in promoting methane hydrate formation at 4.7 MPa and 5.7 MPa. Chem. Eng. Res. Des. (2013), http://dx.doi.org/10.1016/j.cherd.2012.12.001
CHERD-1135; No. of Pages 5
2
ARTICLE IN PRESS chemical engineering research and design x x x ( 2 0 1 3 ) xxx–xxx
Nomenclature P T t n V Z R
2.1.1.
pressure, MPa absolute temperature, K time, min number of moles volume, m3 compressibility factor (dimensionless) universal gas constant, 8.314 J mol−1 K−1
Materials
Sodium borohydride (NaBH4 ) and tri-sodium citrate (C6 H5 Na3 O7 ·2H2 O) were purchased from Merck, Germany. Silver nitrate (AgNO3 ) was obtained from PanReac, Spain. All of these chemicals were used as received.
2.1.2.
Synthesis method
A chemical reduction method (Bonsak, 2010; Jana et al., 2001) was employed to synthesize silver nanoparticles. First 20 ml aqueous solution with concentration of 0.25 mM AgNO3 and 0.25 mM trisodium citrate was prepared. During stirring this solution, 0.6 ml of 10 mM NaBH4 was added. Stirring was stopped after 3 min. In this route sodium borohydride acted as the reducing agent and trisodium citrate as the stabilizer. The chemical reaction of this synthesis process can be written as (Solomon et al., 2007): AgNO3 + NaBH4 → Ag + 0.5H2 + 0.5B2 H6 + NaNO3 Some samples were prepared on glass slides and was imaged using SEM. The image is shown in Fig. 1. Using Image J 1.43 software, the size distribution of silver nanoparticles was obtained. This is shown near SEM image. According to this image, silver nanoparticles diameter is between 50 and 75 nm. EDX analysis showed the purity of synthesized silver nanoparticles. The spectrum is shown in Fig. 2. Presence of calcium and silicon is believed to be from the glass slide. UV–vis absorption spectra peak is on 400 nm, the conventional peak of silver metal nanoparticles (Aswathy et al., 2011; Bonsak, 2010; Ghader et al., 2007; Jana et al., 2001; Lupu, 2010; Scaiano et al., 2006; Solomon et al., 2007). The spectrum is shown in Fig. 3.
Fig. 1 – (a) SEM image and (b) size distribution of synthesized silver nanoparticles.
2.2.2.
Experimental procedure
To do each experiment first of all, water or aqueous solution containing silver nanoparticles or trisodium citrate was poured into the reactor. Then methane gas with purity of 99.9% was injected until the final pressure was reached at 283.15 K. After that cooling system was turned on while its temperature was adjusted on 275.15 K. When the electromotor was turned on, data recording was started. The reactor temperature and pressure were recorded each 10 s. These data are plotted using Excel 2007 software.
2.2. Effects of silver nanoparticles on methane hydrate formation process 2.2.1.
Apparatus
The schematic diagram of methane hydrate formation apparatus is shown in Fig. 4. The reactor is a batch one and its volume is 460 cm3 . A mixture of water and alcohol is used to cool the system. In this setup a mechanical system accompanied with an electromotor was employed to agitate the mixture of water and methane in the reactor. The whole reactor was insulated to prevent any heat loss. One platinum resistance thermometer (Pt 100) is inserted into the reactor to measure temperature of the system within a precision of ±0.01 K. Pressure is measured by a BD pressure transducer within a precision of ±0.01 MPa for the operating range. The temperature and pressure are monitored and recorded in a computer by means of a data acquisition interface.
Fig. 2 – EDX analysis of synthesized nanoparticles.
Please cite this article in press as: Arjang, S., et al., Effect of synthesized silver nanoparticles in promoting methane hydrate formation at 4.7 MPa and 5.7 MPa. Chem. Eng. Res. Des. (2013), http://dx.doi.org/10.1016/j.cherd.2012.12.001
ARTICLE IN PRESS
CHERD-1135; No. of Pages 5
chemical engineering research and design x x x ( 2 0 1 3 ) xxx–xxx
3
2
Absorbance (AU)
1.6
1.2
0.8
0.4
0 300
400
500
600
700
800
900
wavelength (nm)
Fig. 3 – UV–vis spectra of obtained nanoparticles suspension.
2.2.3.
Fig. 5 – P–t and T–t profiles of methane hydrate formation in the presence of pure water at target temperature of 275.15 K and initial pressure of 4.7 MPa.
Results and discussion
To find out effect of silver nanoparticles on methane hydrate formation process, three different experiments were done at each pressure. At initial pressure of 4.7 MPa, 100 ml of distilled water was poured into the reactor. Pressure–time and temperature–time diagrams of this experiment are plotted in Fig. 5. Based on these data, methane hydrate induction time was 485 min. In the second experiment at this pressure, the suspension containing silver nanoparticles diluted to 100 ml was injected to the reactor. Obtained pressure–time and temperature–time profiles are plotted and shown in Fig. 6. In order to investigate the effect of sodium citrate an experiment with the
same conditions was designed. For this purpose, 20 ml of 0.25 mM trisodium citrate was diluted to 100 ml with distilled water. In this solution the concentration of this chemical was the same as the second experiment. After gathering and plotting data (Fig. 7) of this experiment, very interesting results have been observed. According to the obtained graphs for these three experiments, the induction time of methane hydrate formation decreased in the presence of trisodium citrate and silver nanoparticles which is 379.8 and 71.4 min respectively. But the decrease is remarkable in the presence of silver nanoparticles. There are three reasons for this effect. First, the presence of silver nanoparticles creates a nanofluid
Fig. 4 – A schematic diagram of the methane hydrate apparatus. Please cite this article in press as: Arjang, S., et al., Effect of synthesized silver nanoparticles in promoting methane hydrate formation at 4.7 MPa and 5.7 MPa. Chem. Eng. Res. Des. (2013), http://dx.doi.org/10.1016/j.cherd.2012.12.001
CHERD-1135; No. of Pages 5
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ARTICLE IN PRESS chemical engineering research and design x x x ( 2 0 1 3 ) xxx–xxx
Table 1 – Induction time of methane hydrate formation at 5.7 MPa in different environments. Environment Induction time (min)
In pure water
In nanofluid containing silver nanoparticles
In trisodium citrate solution
380.3
99.2
272.9
Table 2 – The amount of methane gas consumed after 900 min for experiments with initial pressure of 4.7 MPa. Environment Moles of gas consumed
In pure water
In nanofluid
In trisodium citrate solution
0.014
0.019
0.016
Table 3 – The amount of methane gas consumed after 900 min for experiments with initial pressure of 5.7 MPa. Environment Moles of gas consumed
In pure water
In nanofluid
In trisodium citrate solution
0.027
0.029
0.0275
with higher heat transfer coefficient. Methane hydrate nucleation is an exothermic process. If this heat dissipates more easily, methane hydrate nucleation takes place faster. The required work needed for heterogeneous nucleation is less
Fig. 6 – P–t and T–t profiles of methane hydrate formation in the presence of synthesized silver nanoparticles at target temperature of 275.15 K and initial pressure of 4.7 MPa.
than homogenous nucleation (Kashchiev and Firoozabadi, 2002). So, when silver nanoparticles exist in water, methane hydrate nucleates easier and faster. On the other hand, the surface to volume ratio increases significantly in nanometric dimensions. Now considering favorable Park and Kim (2010) could decrease methane hydrate induction time by 52% while this parameter was decreased by 85% (from 485 min for pure water case to 71.8 min) in this investigation at the same conditions. Comparison of our results with Ganji et al. (2007b) show that utilizing of 500 ppm SDS is more effective than silver nanoparticle and trisodium citrate to reduce the induction time of methane hydrate formation. Their experiments show that in presence of SDS with concentration of 500 ppm, the induction time is very short and hydrate attains steady state in about one hour. These results are observed in the experiments with initial pressure of 5.7 MPa too. As it is mentioned in Table 1, methane hydrate formation time decreased noticeably in the presence of silver nanoparticles. Initial and final pressures have been employed to investigate the amount of methane gas trapped in methane hydrate crystals:
n =
Fig. 7 – P–t and T–t profiles of methane hydrate formation in the presence of trisodium citrate at target temperature of 275.15 K and initial pressure of 4.7 MPa.
PV ZRT
i
−
PV ZRT
f
The compressibility factor has been calculated using Lee–Kesler equations (Smith et al., 2005). The amount of methane gas trapped in methane hydrate crystals was calculated after 900 min in each experiment. The results were reported in Tables 2 and 3. In the presence of silver nanofluid, the amount of methane gas trapped in methane hydrate crystals increased by 33.7% and 7.4% for experiments with initial pressures of 4.7 and 5.7 MPa respectively. But, when trisodium citrate was injected to the reactor with water, this parameter has been increased by 14.2% and 1.8% at initial pressures of 4.7 and 5.7 MPa respectively. These results show that silver nanoparticles have a considerable effect on increasing gas to water ratio of methane hydrate.
Please cite this article in press as: Arjang, S., et al., Effect of synthesized silver nanoparticles in promoting methane hydrate formation at 4.7 MPa and 5.7 MPa. Chem. Eng. Res. Des. (2013), http://dx.doi.org/10.1016/j.cherd.2012.12.001
CHERD-1135; No. of Pages 5
ARTICLE IN PRESS chemical engineering research and design x x x ( 2 0 1 3 ) xxx–xxx
3.
Conclusion
Silver nanoparticles were synthesized by a chemical method in which trisodium citrate acted as stabilizer. Using these nanoparticles as promoter, methane hydrate induction time has been decreased by 85% and 73.9% for the experiments with initial pressure of 4.7 MPa and 5.7 MPa respectively. However the amount of methane consumed to form methane hydrate increased by 33.7% and 7.4% on molar basis in the presence of silver nanoparitcles, respectively at pressures of 4.7 MPa and 5.7 MPa. Trisodium citrate was injected at the same concentration in independent experiments to verify the net effect of this chemical on methane hydrate formation process. The results show that this stabilizing agent decrease induction time and increase moles of trapped gas in methane hydrate crystals, but it did not supersede the effect of silver nanoparticles.
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Please cite this article in press as: Arjang, S., et al., Effect of synthesized silver nanoparticles in promoting methane hydrate formation at 4.7 MPa and 5.7 MPa. Chem. Eng. Res. Des. (2013), http://dx.doi.org/10.1016/j.cherd.2012.12.001