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Influence of kinetic and thermodynamic promoters on post-combustion carbon dioxide capture through gas hydrate crystallization
Accepted Manuscript Title: Influence of kinetic and thermodynamic promoters on post-combustion carbon dioxide capture through gas hydrate crystallization Author: Asheesh Kumar Gaurav Bhattacharjee Vivek Barmecha Saee Diwan Omkar S. Kushwaha PII: DOI: Reference:
S2213-3437(16)30100-2 http://dx.doi.org/doi:10.1016/j.jece.2016.03.021 JECE 1023
To appear in: Received date: Revised date: Accepted date:
15-10-2015 6-2-2016 12-3-2016
Please cite this article as: Asheesh Kumar, Gaurav Bhattacharjee, Vivek Barmecha, Saee Diwan, Omkar S.Kushwaha, Influence of kinetic and thermodynamic promoters on post-combustion carbon dioxide capture through gas hydrate crystallization, Journal of Environmental Chemical Engineering http://dx.doi.org/10.1016/j.jece.2016.03.021 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Influence of kinetic and thermodynamic promoters on post-combustion carbon dioxide capture through gas hydrate crystallization
Asheesh Kumar1*, Gaurav Bhattacharjee1, Vivek Barmecha2, Saee Diwan3, Omkar S. Kushwaha1
1
Chemical Engineering
and
Process
Development Division,
CSIR-
National
Chemical
Laboratory, Pune, India 2
Petroleum Engineering Department, Maharashtra Institute of Technology, Pune, India
3
Chemical Engineering Department, Vishwakarma Institute of Technology, Pune, India
*
corresponding authors: Tel: 91-20-2590 3090; Email: [email protected],
1
Graphical abstract
Highlights
SDS enhanced the kinetics of hydrate formation for CO2/N2/SO2 gas mixture Enhanced kinetics of hydrate formation in a fixed bed media (of silica sand) is observed Use of THF in high concentration (5.56 mol %) enhanced the final gas uptake Corrosive effect of SO2 impurity was observed on metallic components of the reactor
Abstract In the present work, we report enhanced kinetics of hydrate formation in the presence of kinetic and thermodynamic promoters, SDS (sodium dodecyl sulphate) and THF (tetrahydrofuran) respectively. Hydrate formation was carried out in a fixed bed reactor for post-combustion capture of CO2. Silica sand was used as a fixed bed medium to capture CO2 from a CO2/N2/SO2 (17.7 mol % CO2, 1.05 mol % SO2 and balance N2) gas mixture by hydrate crystallisation. 2
Experiments were performed at a constant temperature (273.65 K) and at different pressures (9.5 and 2.45 MPa) in batch mode. It was found that the addition of SDS enhances the rate and gas uptake of gas hydrate formation. A higher gas consumption was achieved by using 5.56 mol % THF compared to 1.0 and 3.0 mol % THF.
Key words: Carbon dioxide capture, flue gas, gas hydrates, sulfur dioxide, sodium dodecyl sulphate
1. Introduction Global warming which occurs mainly due to increase in atmospheric greenhouse gases has long become a key environmental issue. It is well known that CO2 is one of the major greenhouse gases and contributes severely to the earth’s greenhouse outcome. The main source of anthropogenic CO2 emissions are usually point sources such as fossil fueled power stations. Efficient carbon capture and storage (CCS) of emissions from such fossil fueled power stations is a way out but the development of such technology will result in an increase in the cost of fossil fuel-based production of electricity. CO2 capture and separation from post-combustion gas mixture (flue gas) through hydrate formation is one of the novel approaches for reducing carbon emissions that is being actively researched on at the present. (Adeyemo et al., 2010; Kang et al., 2000; Linga et al., 2007a,b; Kumar et.al, 2014). Douglas Aaron & Costas Tsouris have reported that usually used amine scrubbers, pressure swing adsorption (PSA), and other conventional methods impose energy penalties up to 35 % for coal-fired plants. However, the hydrate based
3
gas separation (HBGS) process is believed to impose an energy penalty of only 4.4 % in an integrated gasification combined cycle (IGCC) system (on a commercial scale HBGS process is still not operational) (Pruschek et al., 1997; Aaron and Tsouris, 2005; Mukherjee et al., 2015). Gas hydrates (clathrates) are nonstoichiometric, crystalline, ice like solids formed by water and gas molecules. When a hydrate-forming gas and water come in contact at appropriate pressure and temperature conditions, the water molecules arrange themselves in three-dimensional hydrogen-bonded cage like structures while the small gas molecules get entrapped within these cages (Sloan and Koh, 2008). Gas hydrates find applications in large and varied number of fields such as gas capture, gas storage and transport and desalination (Kumar et.al. 2014; Bhattacharjee et.al, 2015; Velluswamy et al., 2014; Babu et.al, 2014; Park et al., 2011). The HBGS process is currently being looked as most promising for carbon dioxide capture from various gas mixtures containing CO2 (CO2+N2 , CO2+H2 gas mixtures etc) (D’ Alessandro et.al, 2010; Kumar et.al, 2014; Kumar et.al, 2015a). The selective partition of CO2 between the hydrate and gas phases is the basis for the HBGS process (Li et al., 2011). Three factors which enhance the efficiency of the HBGS process are 1) faster hydrate formation rate, 2) maximum water to hydrate conversion and 3) higher separation factor and CO2 recovery. Hydrate formation is essentially a gas-liquid-solid multiphase reaction and thus higher interfacial area is desirable for better gas-water contact to form solid hydrates. Stirring at high speeds is one of the approaches to increase water to gas contact, thus allowing faster hydrate formation. However, due to agglomeration of hydrate crystals at the interface, mass transfer limitations arise and the final water to hydrate conversion remains less than 20 %. Moreover, it has been shown that stirred tank reactor arrangement cannot be used for a large scale applications (such as CO2 capture from flue gas stream) due to stirring costs turn out to be significant portion of total
4
process costs, making it a highly energy intensive process (Linga et al., 2010). Higher gas-water contact can be ensured by using a fixed bed setup with a high surface area to volume ratio. The limitations of mass transfer as discussed above can be avoided by using a fixed bed setup as in this case the water is evenly distributed throughout the system (Bhattacharjee et.al, 2015; Kumar et.al, 2016). Fossil fuels such as coal, used in coal-fired power stations contain a significant amount of carbon and sulfur which primarily get converted into CO2 and SO2 during combustion respectively. Gas mixtures emerging from such thermal power plants and other point combustion sources are broadly classified as flue gas and contain mostly N2 and CO2 along with trace amounts of many other contaminants such as SO2, NO2 and fly ash. These unwanted harmful gases are largely removed by using various pre processing techniques, before a flue gas is released into the atmosphere. However due to a realistic benefit-cost ratio, the cleaning process is barely complete and the resulting flue gas still contains traces of such impurities (Beeskow et al., 2011, Daraboina et al., 2013). For an efficient CO2 capture and seperation process through gas hydrate formation, it is imepartive to study the impact of these impurities on the thermodynamics and kinetics of hydrate formation. Recently Kumar et al., have investigated the impact of fly ash on the efficiency of the HBGS process. It was observed that the hydrate equilibrium conditions did not change in the presence of fly ash. However, the presence of fly ash enhanced the separation efficiency of the HBGS process by reducing the induction time and increasing the kinetics of hydrate formation. It was concluded that the presence of fly ash in a flue gas mixture is not detrimental to the HBGS process (Kumar et al., 2014). The impact of 1 mol % SO2 on postcombustion capture of CO2 has also been investigated and it was found that the presence of SO2 shifts the hydrate equilibrium to milder conditions (Daraboina et al., 2013). Beeskow-Strauch et
5
al, reported that the addition of 1 mol % SO2 to CO2 enhances the hydrate stability and kinetics of formation of pure CO2 hydrate (Beeskow et al., 2011). However, as reported in the literature (Daraboina et al., 2013), the rate of hydrate formation for flue gas mixture is very slow even in presence of SO2. A commercially viable hydrate based CO2 separation process demands a rapid hydrate formation rate. It has been reported in the literature that the use of surfactants is one of the ways to enhance the rate of hydrate formation. Surfactants may enhance hydrate formation kinetics through a number of mechanisms (Kumar et.al, 2015). Sodium dodecylsulphate (SDS) is clearly the surfactant of choice for enhancement of gas hydrate formation kinetics (Gayet et al., 2005; Okutani et al., 2008; Yoslim et al., 2010; Pang et al., 2007; Kumar et al., 2015a,b, 2013). For an efficient HBGS process, another important condition is to minimize the operating pressure. As previously mentioned, SO2 helps in lowering the equilibrium hydrate conditions; the equilibrium hydrate formation pressure for the CO2/N2 mixture was reduced from 7.7 MPa to 7.2 MPa in the presence of 1 mol % SO2 at 273.7 K (Daraboina et al, 2013). A CO2 separation process through hydrate formation though demands much lower operating pressures. Kang and Lee, proposed a thermodynamic promoter, THF which reduces the operating pressure significantly. The addition of 1 mol% of THF into the system was found to lower the hydrate formation pressure from 8.4 to 0.5 MPa for a 17 mol % CO2/83 mol % N2 mixture at 275.15K (Kang and lee 2001a, b). The
objective
of
the
present
thermodynamic promotors on
study
is
to
post-combustion
investigate carbon
the
dioxide
effect of
kinetic and
capture during hydrate
formation in porous media where the gas mixture contains a small amount of impurity in the form of SO2 (1 mol%). 1wt% of SDS has been used as a kinetic hydrate promoter to enhance the rate of hydrate formation and THF in two different concentrations (3 and 5.56 mol %) has been
6
used as a thermodynamic promoter to shift the hydrate formation conditions to much milder conditions which indirectly contributes to reducing high compression costs.
2. Experimental Section: 2.1. Materials The gas mixture used was a CO2/N2/SO2 mixture containing 17.7 mol % CO2, 1.05 mol % SO2 and balance N2 corresponding to a typical composition of a flue gas mixture and was supplied by Vadilal Gases Ltd. Pune, India. Silica sand was purchased from Sakalchand & Company Pune, India. The particle size distribution of silica sand was in the range of 50-400 μm (Kumar et al., 2015a). The volume of water required to completely fill the void space between the sand particles was ~ 0.20 cm3/g. SDS (SQ Grade) with minimum 98% purity was purchased from Fisher Scientific Ltd. India. Tetrahydrofuran (THF, 99.5% purity) was purchased from S.D. Fine Chemicals Ltd. (Mumbai, India). Deionized and distilled water was used for all the experiments.
2.2. Apparatus Figure 1 shows the schematic of the used experimental setup. It consists of a crystallizer (CR) which is a cylindrical vessel made up of SS-316 with a volume of 400 cm3. The crystallizer was immersed in a temperature controlled water bath. The temperature of the water bath was controlled by an external refrigerator (Julabo make). A Pressure transducer (Wika make) was employed to record pressure data with a maximum uncertainty of 0.1% of the span (0-25 MPa). All hydrate formation experiments were conducted at a constant temperature. The temperature of the hydrate phase (silica sand bed) of the crystallizer was measured using a thermocouple (RTD) with an uncertainty of 0.1 K. The data acquisition system was coupled with a computer to record
7
and analyze the data. The pressure of the CR drops upon hydrate formation, which was used to calculate the number of moles of gas consumed for the hydrate formation experiments. It is noted that all the experiments were conducted in batch mode and thus effective driving force changes as hydrate formation proceeds, as more and more gas leaves the gaseous phase to occupy the solid hydrate phase. The CR was also equipped with a safety pressure valve in case of emergencies.
2.3. Experimental Procedure 2.3.1. Preparation of Silica sand Bed: The amount of silica sand placed in the crystallizer was 250g. The volume of water required to fill the void space with water was ~0.20 cm3/g, which is the interstitial, or pore volume of the bed of sand particles. Accordingly, 50 cm3 of water is added into the sand so as to achieve 100 water saturation of the fixed bed. 2.3.2. Hydrate Formation Procedure: Once the crystallizer bed was setup, the thermocouple was positioned and the crystallizer was closed. The crystallizer was pressurized with CO2/N2/SO2 gas mixture and depressurized to atmospheric pressure three times in order to eliminate the presence of any air bubble in the system. Circulating ethylene glycol / water mixture from the water bath was employed to cool the CR at the experimental temperature of 273.65 K. Once the desired temperature was reached, the CR was pressurized with the experimental gas mixture to a pre-determined experimental pressure. At this point, gas uptake measurement was initiated. All the gas uptake measurements were carried out in batch mode with THF (3 and 5.56 mol %) and fixed amount of SDS solution or water (50 cm3) at the constant experimental temperature mentioned above.
8
2.3.3. Calculation of the amount of gas consumed during hydrate formation: As the gas in the crystallizer was consumed for hydrate formation the pressure in the crystallizer dropped. The total number of moles of the gas that were consumed for hydrate formation was calculated by the following equation:
P P ( n H , ) t VCR VCR zRT 0 zRT t
(1)
Where z is the compressibility factor calculated using the Pitzer’s correlation given below (Smith et al., 2001); z depends on the gas uptake data and thus varies with time. VCR is the volume of the gas phase of the crystallizer; P & T are pressure and temperature of the crystallizer respectively.
Z 1 0
Where 0.083 0
Tr
Texp Tcritical
,
Pr
Pr P 1 r Tr Tr
(2)
0.172 0.422 1 0 . 139 , Tr4.2 Tr1.6
Pexp Pcritical
, ω = Acentric factor
Gas uptake (mol of gas / mol of water) =
( n H , ) t
nW ( nH , )t = Number of moles of gas consumed at time t
(3)
nW = Number of moles of water
9
3. Results and discussion Table 1 summarizes all the experiments performed during the course of this study and includes the experimental temperature and pressure, induction time and the total gas uptake (moles of gas consumed/ mol of water) at the end of each experiment. Operating temperature and pressure were estimated from available literature. The driving force (difference of experimental and equilibrium pressure) for hydrate formation at 273.7 K is 2.3 MPa for CO2/N2/SO2 system. (Daraboina, et al., 2013). Table 1 helps us to compare the induction time for the two different systems studied (CO2/N2/SO2/SDS and CO2/N2/SO2/THF). Induction time in gas hydrate crystallization is an important characteristic of the kinetic studies. Induction time is defined as the time elapsed until the appearance of a detectable volume of hydrate phase. The induction time is often also termed as the hydrate nucleation or lag time. (Sloan and Koh, 2008). As can be seen in Table 1, the presence of THF in the system drastically lowers the induction time as compared to the other systems studied. This is an important feature that will play a huge role when finally employing the HBGS process for CO2 capture on a commercial scale (Babu et al., 2013, 2014).
3.1. Kinetics of Hydrate Formation in the Presence of kinetic promoter (SDS) Figure 2 shows a typical gas uptake curve along with the corresponding temperature profile for a CO2/N2/SO2/SDS system for an experiment conducted at 273.65 K and 9.5 MPa pressure. The general shape of the curve agrees with the gas uptake profile described in detail by Natarajan et al. which involve gas dissolution, nucleation and hydrate growth (Natarajan et al. 1994). Initially, the gas diffuses into the water present between the interstitial spaces in the silica sand bed. The next stage is the super saturation and nucleation of hydrates which involves the formation of
10
critically stable hydrate nuclei and is characterized by a sudden increase in temperature due to the exothermic nature of hydrate crystallization. However, the crystallizer is cooled to the experimental temperature by an external refrigerator which results in the overall temperature of the crystallizer remaining essentially constant. The final stage of a gas hydrate formation process is the hydrate growth, where hydrate formation continues to occur in the bed until hydrate growth reaches saturation. In the presence of surfactants in general, an increase in both the rate of hydrate formation and the final water to hydrate conversion can be observed. In a standard gas-water system, hydrates and occluded water create a slushy hydrate mass which prevents efficient mass transfer of gas through the gas/water interface (Linga et al., 2012) However, in the presence of a surfactant, these mass transfer limitations can be avoided (Suradkar and Bhagwat, 2005). A water-soluble surfactant such as SDS produces a lower resistance to interfacial mass transfer which favorably affects hydrate formation rate by enhancing gas-water contact through efficient diffusion of hydrate forming gases into the water while higher solubility of hydrate forming gases in water in the presence of surfactants may also play a significant role in enhancing hydrate formation kinetics. Capillary driven supply of water into the porous hydrate layers in the presence of surfactants further enhances hydrate formation by ensuring greater gaswater contact. This significantly alters the hydrate morphology resulting in catastrophic hydrate growth on the walls of the reactor. The use of surfactants thus in more ways than one, contributes to enhancing the kinetics of hydrate formation. (Kumar et al., 2014; Caskey et al.,1992; Hanwright et al., 2005; Kumar et.al, 2015c ).
11
3.2. Kinetics of Hydrate Formation in the Presence of THF The use of THF as a thermodynamic promoter is well known and docuemnted. Since the gas mixture being used forms hydrates at a relatively high operating pressure, the use of THF can help to perform the experiment at much milder operating pressures. Daraboina et. al, 2013 reported the phase equilibria data for CO2/N2/SO2 mixture in presence of 1 mol% THF. The addition of 1 mol% THF offers a huge reduction in operating pressure while a significant reduction in the induction time was also observed. On the other hand, the addition of THF also reduces the gas uptake. (Linga et al., 2007; Daraboina et al., 2013). To enhance the rate of hydrtae formation, in the present work, larger amounts of THF were used (3 and 5.56 mol%). 5.56 mol% is the amount of THF required to completely fill the large cages in structure II hydrates. Figure 3 compares the gas consumption (mol of gas/ mol of water; hydrate growth) measured for different amounts of THF in the system (3 and 5.56 mol %) at 2.45 MPa and 273.65K pressure and temperature respectively. Time zero in the figure corresponds to nucleation point (induction time) for the experiments. It has been reported in the literature that the kinetics of hydrate formation may depend on the concentration of THF used and hydrate formation rate decreases with the addition of THF at higher concentrations due to the likelihood of THF occupying the large cages readily and thus stabilising the structure II (sII) hydrates. This results in lower gas uptake of the guest gas due to mass transfer resistance; more specifically, guest gas molecules can no longeer occupy the large cages due to the already present THF molecules in the same (Lee et al., 2010). However, in the present study, as can be seen in Figure 3, the kinetics of hydrate formation is favorably affected when a larger amount of
THF (5.56 mol %) is present
in the system. Furthermore, it can be seen in Figure 3 that the final gas uptake is higher in case of
12
the 5.56 mol % THF system as compared to the 3 mol % THF systems. Similar results were reported by Babu et al., for pre- combustion CO2 capture where the total hydrate growth for the experiment using 5.53 mol % THF greatly surpassed that for the experiment using 1 mol % THF with a 1.8 times higher yield (Babu et al., 2014). Veluswamy et al., observed a similar trend with THF for H2 storage in the form of gas hydrates (Veluswamy et al., 2013, 2014). In the present study, in addition to the higher final gas uptake, as mentioned before, it was also observed that the addition of THF to the system (3 mol% and 5.56 mol%) greatly reduces the induction time (Table 1) which is in agreement to the observations made by Daraboina et.al, 2013 (Daraboina et.al, 2013).
3.3. Comparison of gas uptake for different systems investigated Figure 4 compares the final gas uptake after 10 hours from nucleation for the different systems used in this study with those obtained in the study performed by Daraboina et.al 2013. As can be seen in the figure, gas uptake is maximum for the system containing the surfactant SDS. This increase in gas uptake in the presence of SDS has been discussed in detail in section 3.1. It can also be noted from Figure 4 that the addition of THF to the system lowers the final gas uptake. This phenomenon is followed even on the addition of larger amounts of THF to the system (3 mol% and 5.56 mol %). Details on hydrate formation in the presence of THF in the system is given earlier in section 3.2.
13
3.4. Safety concerns in the use of gas mixture containing SO2 It is well known that the presence of SO2 in the atmosphere (at high humidities) enhances the
corrosion
of
metals. The corrosive action of sulfur dioxide on metallic surfaces is
determined by the particular state in which the metal exists and its purity. (Mattson et al., 1963; Vannerberg et al., 1970). SO2 molecules present in the gas phase adsorb onto the surface of the metals and reduce them. For example, Fe3+ is reduced by SO2 to Fe2+ ions. Teresa et al., have observed that a SO2 complex forms when a water surface is exposed to an atmosphere of SO2 gas (Teresa et al., 2005). SO2 + H2O ⇌ HSO3− + H+ Figure 5 shows the image of a rupture disc from our crystallizer which was corroded by the action of SO2 present in the gas mixture used in this study. Rupture disc was made up of Monel, a nickel alloy comprising nickel (67%) and copper with some amount of iron and other trace elements. As can be seen in the figure, the action of SO2 has had a major impact on the state of the rupture disc corroding the alloy to a large extent. Hydrate formation and dissociation conditions (temperature transitions from low to high temperature) are believed to be favorable for the corrosive action of SO2 further aiding in the corrosion of the rupture disc which took place during our experiments. However, a detailed study needs to be carried out in order to understand the corrosive effect of SO2. Disclaimer: Due to the corrosive and toxic nature of SO2, it is highly recommended to follow standard safety precautions in the laboratory. Please refer MSDS of SO2.
14
4. Conclusion Hydrate formation kinetics and high operating pressure, which are the foremost issues that arise when dealing with CO2 capture through the HBGS process, have been addressed in this work. Kinetics of CO2 hydrate formation from a CO2/N2/SO2 gas mixture was enhanced by using 1 wt % of SDS solution at 9.5 MPa pressure and 273.65 K temperature. Gas uptake was increased by almost three folds as compared to that for a system in the absence of SDS. Operating pressure for formation of CO2 hydrate was reduced by using THF as a thermodynamic promoter (two different concentrations: 3 and 5.56 mol %) and its corresponding effect on hydrate formation kinetics was studied. It was observed that at 5.56 mol % THF, the final gas uptake achieved was the highest. It was also observed that SO2 reacted corrosively with some of the material of construction of the reactor. Due to the high corrosive activity of SO2, it is recommended to follow standard safety precautions in the laboratory; more specifically material selection for reactor design should be done carefully and after due diligence.
Acknowledgements The authors gratefully acknowledge the financial support (Senior Research Fellowship (SRF)) received for this work from the Council of Scientific and Industrial Research (CSIR). The authors are grateful to the Director, CSIR-NCL, Pune for allowing us to carry out this research work in India’s most prestigious chemical laboratory. Authors would like to thank Dr. Rajnish Kumar for intellectually stimulating technical discussions and for providing the necessary laboratory facilities.
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hydrogen storage: The impact of tetrahydrofuran, tetra-n-butylammonium bromide and cyclopentane as promoters on the macroscopic kinetics, international journal of hydrogen energy xxx (2014) 1-10 (36)
Veluswamy, H.P., Linga, P., 2013. Macroscopic kinetics of hydrate formation of
mixed hydrates of hydrogen/tetrahydrofuran for hydrogen storage. International Journal of Hydrogen Energy 38, 4587-4596. (37)
E. Mattson, I.V.A. Medd. 137, 100, Stockholm (1963).
(38)
Nils-Gosta Vannerberg And Tomas Sydberger, Reaction Between So2 And
Wet Metal Surfaces, Corroslon science, 1970, Vol. 10, pp. 43 to 49. Pergamon Press. Printed in Great Britain. (39)
Teresa L. Tarbuck and Geraldine L. Richmond, Adsorption and Reaction of CO2
and SO2 at a Water Surface, J. Am. Chem. Soc. 2006, 128, 3256-3267 (40)
Li, X.S., Xu, C.G., Chen, Z.Y., Wu, H.J., 2011. Hydrate-based pre-combustion
carbon dioxide capture process in the system with tetra-n-butyl ammonium bromide solution in the presence of cyclopentane. Energy 36, 1394-1403. (41)
Linga, P., Kumar, R., Lee, J.D., Ripmeester, J., Englezos, P., 2010. A new
apparatus to enhance the rate of gas hydrate formation: application to capture of carbon dioxide. Int. J. Greenh. Gas Con. 4, 630-637. (42)
Levenspiel, O., 2004. Chemical Reaction Engineering, third ed. John Wiley &
Sons Inc, New York. (43) Kumar A, Bhattacharjee G, Kulkarni B D, Kumar R. Role of surfactants in promoting gas hydrate formation, Ind Eng Chem Res 2015, DOI: 10.1021/acs.iecr.5b03476 (44) Asheesh Kumar, Tushar Sakpal, Gaurav Bhattacharjee, Anupam Kumar, and Rajnish Kumar, Impact of H2S Impurity on Carbon Dioxide Hydrate Formation Kinetics in Fixed Bed Arrangements, Ind. Eng. Chem. Res., DOI: 10.1021/acs.iecr.5b04079
19
Figure captions Figure 1. Schematic of the experimental apparatus Figure 2. Typical gas uptake measurement curve along with temperature profile at 9.5 MPa and 273.65K (Experiment no. SS2) Figure 3. Comparison of gas consumption (mol of gas/mol of water) for hydrate growth measured for different amount of THF (3 and 5.56 mol%) at 2.45 MPa and 273.65K.Time zero in the graph corresponds to nucleation point (induction time) for the experiments. Figure 4. Comparison of gas consumption for hydrate growth (mol of gas/mol of water after ~10h from induction) measured in presence of 1wt% SDS and with varying amounts of THF (1 mol% (Daraboina et al., 2013), 3 and 5.56 mol%THF) Figure 5. Image of rupture disc, damaged by the action of SO2
20
Figure 1
21
Figure 2
22
Figure 3.
23
5.56mol% THF
Hydrate growth in ~10 h from induction time
3mol% THF
1mol% THF
Daraboina et al. 2013
CO2/N2/SO2/SDS
CO2/N2/SO2 0.000
Daraboina et al.2013
0.008
0.016
0.024
0.032
0.040
0.048
Gas Uptake (mol of gas/mol of water)
Figure 4
24
Figure 5
25
Table 1. Summary of experiments, gas consumption; experimental pressure and temperature; Amount of water used for all the experiment was 50 cm3. System
Pressure (MPa)
Temperature (K)
THF (mol%)
SDS (wt%)
Induction Time (min)
SS1
9.5
273.65
0
1
24
Total Gas uptake (mol of gas/mol of water) 0.058
SS2
9.5
273.65
0
1
342
0.047
SS3
9.5
273.65
0
1
104
0.045
ST1
2.45
273.65
3
0
5
0.0067
ST2
2.45
273.65
3
0
7
0.0062
ST3
2.45
273.65
3
0
4
0.0075
ST4
2.45
273.65
5.56
0
4
0.0112
ST5
2.45
273.65
5.56
0
3
0.0121
ST6
2.45
273.65
5.56
0
5
0.0122
Experiment No. CO2/N2/SO2/ SDS
CO2/N2/SO2/ THF
Average total Gas uptake [±SD] 0.050 (±0.007)
0.0068 (±0.00066)
0.012 (±0.00055)
Utmost care was taken in the gas uptake calculations please note that the change in gas composition at the end of experiments was ignored for gas uptake calculations; a small change in gas composition may however have an impact on total gas uptake
26
S2213-3437(16)30100-2 http://dx.doi.org/doi:10.1016/j.jece.2016.03.021 JECE 1023
To appear in: Received date: Revised date: Accepted date:
15-10-2015 6-2-2016 12-3-2016
Please cite this article as: Asheesh Kumar, Gaurav Bhattacharjee, Vivek Barmecha, Saee Diwan, Omkar S.Kushwaha, Influence of kinetic and thermodynamic promoters on post-combustion carbon dioxide capture through gas hydrate crystallization, Journal of Environmental Chemical Engineering http://dx.doi.org/10.1016/j.jece.2016.03.021 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Influence of kinetic and thermodynamic promoters on post-combustion carbon dioxide capture through gas hydrate crystallization
Asheesh Kumar1*, Gaurav Bhattacharjee1, Vivek Barmecha2, Saee Diwan3, Omkar S. Kushwaha1
1
Chemical Engineering
and
Process
Development Division,
CSIR-
National
Chemical
Laboratory, Pune, India 2
Petroleum Engineering Department, Maharashtra Institute of Technology, Pune, India
3
Chemical Engineering Department, Vishwakarma Institute of Technology, Pune, India
*
corresponding authors: Tel: 91-20-2590 3090; Email: [email protected],
1
Graphical abstract
Highlights
SDS enhanced the kinetics of hydrate formation for CO2/N2/SO2 gas mixture Enhanced kinetics of hydrate formation in a fixed bed media (of silica sand) is observed Use of THF in high concentration (5.56 mol %) enhanced the final gas uptake Corrosive effect of SO2 impurity was observed on metallic components of the reactor
Abstract In the present work, we report enhanced kinetics of hydrate formation in the presence of kinetic and thermodynamic promoters, SDS (sodium dodecyl sulphate) and THF (tetrahydrofuran) respectively. Hydrate formation was carried out in a fixed bed reactor for post-combustion capture of CO2. Silica sand was used as a fixed bed medium to capture CO2 from a CO2/N2/SO2 (17.7 mol % CO2, 1.05 mol % SO2 and balance N2) gas mixture by hydrate crystallisation. 2
Experiments were performed at a constant temperature (273.65 K) and at different pressures (9.5 and 2.45 MPa) in batch mode. It was found that the addition of SDS enhances the rate and gas uptake of gas hydrate formation. A higher gas consumption was achieved by using 5.56 mol % THF compared to 1.0 and 3.0 mol % THF.
Key words: Carbon dioxide capture, flue gas, gas hydrates, sulfur dioxide, sodium dodecyl sulphate
1. Introduction Global warming which occurs mainly due to increase in atmospheric greenhouse gases has long become a key environmental issue. It is well known that CO2 is one of the major greenhouse gases and contributes severely to the earth’s greenhouse outcome. The main source of anthropogenic CO2 emissions are usually point sources such as fossil fueled power stations. Efficient carbon capture and storage (CCS) of emissions from such fossil fueled power stations is a way out but the development of such technology will result in an increase in the cost of fossil fuel-based production of electricity. CO2 capture and separation from post-combustion gas mixture (flue gas) through hydrate formation is one of the novel approaches for reducing carbon emissions that is being actively researched on at the present. (Adeyemo et al., 2010; Kang et al., 2000; Linga et al., 2007a,b; Kumar et.al, 2014). Douglas Aaron & Costas Tsouris have reported that usually used amine scrubbers, pressure swing adsorption (PSA), and other conventional methods impose energy penalties up to 35 % for coal-fired plants. However, the hydrate based
3
gas separation (HBGS) process is believed to impose an energy penalty of only 4.4 % in an integrated gasification combined cycle (IGCC) system (on a commercial scale HBGS process is still not operational) (Pruschek et al., 1997; Aaron and Tsouris, 2005; Mukherjee et al., 2015). Gas hydrates (clathrates) are nonstoichiometric, crystalline, ice like solids formed by water and gas molecules. When a hydrate-forming gas and water come in contact at appropriate pressure and temperature conditions, the water molecules arrange themselves in three-dimensional hydrogen-bonded cage like structures while the small gas molecules get entrapped within these cages (Sloan and Koh, 2008). Gas hydrates find applications in large and varied number of fields such as gas capture, gas storage and transport and desalination (Kumar et.al. 2014; Bhattacharjee et.al, 2015; Velluswamy et al., 2014; Babu et.al, 2014; Park et al., 2011). The HBGS process is currently being looked as most promising for carbon dioxide capture from various gas mixtures containing CO2 (CO2+N2 , CO2+H2 gas mixtures etc) (D’ Alessandro et.al, 2010; Kumar et.al, 2014; Kumar et.al, 2015a). The selective partition of CO2 between the hydrate and gas phases is the basis for the HBGS process (Li et al., 2011). Three factors which enhance the efficiency of the HBGS process are 1) faster hydrate formation rate, 2) maximum water to hydrate conversion and 3) higher separation factor and CO2 recovery. Hydrate formation is essentially a gas-liquid-solid multiphase reaction and thus higher interfacial area is desirable for better gas-water contact to form solid hydrates. Stirring at high speeds is one of the approaches to increase water to gas contact, thus allowing faster hydrate formation. However, due to agglomeration of hydrate crystals at the interface, mass transfer limitations arise and the final water to hydrate conversion remains less than 20 %. Moreover, it has been shown that stirred tank reactor arrangement cannot be used for a large scale applications (such as CO2 capture from flue gas stream) due to stirring costs turn out to be significant portion of total
4
process costs, making it a highly energy intensive process (Linga et al., 2010). Higher gas-water contact can be ensured by using a fixed bed setup with a high surface area to volume ratio. The limitations of mass transfer as discussed above can be avoided by using a fixed bed setup as in this case the water is evenly distributed throughout the system (Bhattacharjee et.al, 2015; Kumar et.al, 2016). Fossil fuels such as coal, used in coal-fired power stations contain a significant amount of carbon and sulfur which primarily get converted into CO2 and SO2 during combustion respectively. Gas mixtures emerging from such thermal power plants and other point combustion sources are broadly classified as flue gas and contain mostly N2 and CO2 along with trace amounts of many other contaminants such as SO2, NO2 and fly ash. These unwanted harmful gases are largely removed by using various pre processing techniques, before a flue gas is released into the atmosphere. However due to a realistic benefit-cost ratio, the cleaning process is barely complete and the resulting flue gas still contains traces of such impurities (Beeskow et al., 2011, Daraboina et al., 2013). For an efficient CO2 capture and seperation process through gas hydrate formation, it is imepartive to study the impact of these impurities on the thermodynamics and kinetics of hydrate formation. Recently Kumar et al., have investigated the impact of fly ash on the efficiency of the HBGS process. It was observed that the hydrate equilibrium conditions did not change in the presence of fly ash. However, the presence of fly ash enhanced the separation efficiency of the HBGS process by reducing the induction time and increasing the kinetics of hydrate formation. It was concluded that the presence of fly ash in a flue gas mixture is not detrimental to the HBGS process (Kumar et al., 2014). The impact of 1 mol % SO2 on postcombustion capture of CO2 has also been investigated and it was found that the presence of SO2 shifts the hydrate equilibrium to milder conditions (Daraboina et al., 2013). Beeskow-Strauch et
5
al, reported that the addition of 1 mol % SO2 to CO2 enhances the hydrate stability and kinetics of formation of pure CO2 hydrate (Beeskow et al., 2011). However, as reported in the literature (Daraboina et al., 2013), the rate of hydrate formation for flue gas mixture is very slow even in presence of SO2. A commercially viable hydrate based CO2 separation process demands a rapid hydrate formation rate. It has been reported in the literature that the use of surfactants is one of the ways to enhance the rate of hydrate formation. Surfactants may enhance hydrate formation kinetics through a number of mechanisms (Kumar et.al, 2015). Sodium dodecylsulphate (SDS) is clearly the surfactant of choice for enhancement of gas hydrate formation kinetics (Gayet et al., 2005; Okutani et al., 2008; Yoslim et al., 2010; Pang et al., 2007; Kumar et al., 2015a,b, 2013). For an efficient HBGS process, another important condition is to minimize the operating pressure. As previously mentioned, SO2 helps in lowering the equilibrium hydrate conditions; the equilibrium hydrate formation pressure for the CO2/N2 mixture was reduced from 7.7 MPa to 7.2 MPa in the presence of 1 mol % SO2 at 273.7 K (Daraboina et al, 2013). A CO2 separation process through hydrate formation though demands much lower operating pressures. Kang and Lee, proposed a thermodynamic promoter, THF which reduces the operating pressure significantly. The addition of 1 mol% of THF into the system was found to lower the hydrate formation pressure from 8.4 to 0.5 MPa for a 17 mol % CO2/83 mol % N2 mixture at 275.15K (Kang and lee 2001a, b). The
objective
of
the
present
thermodynamic promotors on
study
is
to
post-combustion
investigate carbon
the
dioxide
effect of
kinetic and
capture during hydrate
formation in porous media where the gas mixture contains a small amount of impurity in the form of SO2 (1 mol%). 1wt% of SDS has been used as a kinetic hydrate promoter to enhance the rate of hydrate formation and THF in two different concentrations (3 and 5.56 mol %) has been
6
used as a thermodynamic promoter to shift the hydrate formation conditions to much milder conditions which indirectly contributes to reducing high compression costs.
2. Experimental Section: 2.1. Materials The gas mixture used was a CO2/N2/SO2 mixture containing 17.7 mol % CO2, 1.05 mol % SO2 and balance N2 corresponding to a typical composition of a flue gas mixture and was supplied by Vadilal Gases Ltd. Pune, India. Silica sand was purchased from Sakalchand & Company Pune, India. The particle size distribution of silica sand was in the range of 50-400 μm (Kumar et al., 2015a). The volume of water required to completely fill the void space between the sand particles was ~ 0.20 cm3/g. SDS (SQ Grade) with minimum 98% purity was purchased from Fisher Scientific Ltd. India. Tetrahydrofuran (THF, 99.5% purity) was purchased from S.D. Fine Chemicals Ltd. (Mumbai, India). Deionized and distilled water was used for all the experiments.
2.2. Apparatus Figure 1 shows the schematic of the used experimental setup. It consists of a crystallizer (CR) which is a cylindrical vessel made up of SS-316 with a volume of 400 cm3. The crystallizer was immersed in a temperature controlled water bath. The temperature of the water bath was controlled by an external refrigerator (Julabo make). A Pressure transducer (Wika make) was employed to record pressure data with a maximum uncertainty of 0.1% of the span (0-25 MPa). All hydrate formation experiments were conducted at a constant temperature. The temperature of the hydrate phase (silica sand bed) of the crystallizer was measured using a thermocouple (RTD) with an uncertainty of 0.1 K. The data acquisition system was coupled with a computer to record
7
and analyze the data. The pressure of the CR drops upon hydrate formation, which was used to calculate the number of moles of gas consumed for the hydrate formation experiments. It is noted that all the experiments were conducted in batch mode and thus effective driving force changes as hydrate formation proceeds, as more and more gas leaves the gaseous phase to occupy the solid hydrate phase. The CR was also equipped with a safety pressure valve in case of emergencies.
2.3. Experimental Procedure 2.3.1. Preparation of Silica sand Bed: The amount of silica sand placed in the crystallizer was 250g. The volume of water required to fill the void space with water was ~0.20 cm3/g, which is the interstitial, or pore volume of the bed of sand particles. Accordingly, 50 cm3 of water is added into the sand so as to achieve 100 water saturation of the fixed bed. 2.3.2. Hydrate Formation Procedure: Once the crystallizer bed was setup, the thermocouple was positioned and the crystallizer was closed. The crystallizer was pressurized with CO2/N2/SO2 gas mixture and depressurized to atmospheric pressure three times in order to eliminate the presence of any air bubble in the system. Circulating ethylene glycol / water mixture from the water bath was employed to cool the CR at the experimental temperature of 273.65 K. Once the desired temperature was reached, the CR was pressurized with the experimental gas mixture to a pre-determined experimental pressure. At this point, gas uptake measurement was initiated. All the gas uptake measurements were carried out in batch mode with THF (3 and 5.56 mol %) and fixed amount of SDS solution or water (50 cm3) at the constant experimental temperature mentioned above.
8
2.3.3. Calculation of the amount of gas consumed during hydrate formation: As the gas in the crystallizer was consumed for hydrate formation the pressure in the crystallizer dropped. The total number of moles of the gas that were consumed for hydrate formation was calculated by the following equation:
P P ( n H , ) t VCR VCR zRT 0 zRT t
(1)
Where z is the compressibility factor calculated using the Pitzer’s correlation given below (Smith et al., 2001); z depends on the gas uptake data and thus varies with time. VCR is the volume of the gas phase of the crystallizer; P & T are pressure and temperature of the crystallizer respectively.
Z 1 0
Where 0.083 0
Tr
Texp Tcritical
,
Pr
Pr P 1 r Tr Tr
(2)
0.172 0.422 1 0 . 139 , Tr4.2 Tr1.6
Pexp Pcritical
, ω = Acentric factor
Gas uptake (mol of gas / mol of water) =
( n H , ) t
nW ( nH , )t = Number of moles of gas consumed at time t
(3)
nW = Number of moles of water
9
3. Results and discussion Table 1 summarizes all the experiments performed during the course of this study and includes the experimental temperature and pressure, induction time and the total gas uptake (moles of gas consumed/ mol of water) at the end of each experiment. Operating temperature and pressure were estimated from available literature. The driving force (difference of experimental and equilibrium pressure) for hydrate formation at 273.7 K is 2.3 MPa for CO2/N2/SO2 system. (Daraboina, et al., 2013). Table 1 helps us to compare the induction time for the two different systems studied (CO2/N2/SO2/SDS and CO2/N2/SO2/THF). Induction time in gas hydrate crystallization is an important characteristic of the kinetic studies. Induction time is defined as the time elapsed until the appearance of a detectable volume of hydrate phase. The induction time is often also termed as the hydrate nucleation or lag time. (Sloan and Koh, 2008). As can be seen in Table 1, the presence of THF in the system drastically lowers the induction time as compared to the other systems studied. This is an important feature that will play a huge role when finally employing the HBGS process for CO2 capture on a commercial scale (Babu et al., 2013, 2014).
3.1. Kinetics of Hydrate Formation in the Presence of kinetic promoter (SDS) Figure 2 shows a typical gas uptake curve along with the corresponding temperature profile for a CO2/N2/SO2/SDS system for an experiment conducted at 273.65 K and 9.5 MPa pressure. The general shape of the curve agrees with the gas uptake profile described in detail by Natarajan et al. which involve gas dissolution, nucleation and hydrate growth (Natarajan et al. 1994). Initially, the gas diffuses into the water present between the interstitial spaces in the silica sand bed. The next stage is the super saturation and nucleation of hydrates which involves the formation of
10
critically stable hydrate nuclei and is characterized by a sudden increase in temperature due to the exothermic nature of hydrate crystallization. However, the crystallizer is cooled to the experimental temperature by an external refrigerator which results in the overall temperature of the crystallizer remaining essentially constant. The final stage of a gas hydrate formation process is the hydrate growth, where hydrate formation continues to occur in the bed until hydrate growth reaches saturation. In the presence of surfactants in general, an increase in both the rate of hydrate formation and the final water to hydrate conversion can be observed. In a standard gas-water system, hydrates and occluded water create a slushy hydrate mass which prevents efficient mass transfer of gas through the gas/water interface (Linga et al., 2012) However, in the presence of a surfactant, these mass transfer limitations can be avoided (Suradkar and Bhagwat, 2005). A water-soluble surfactant such as SDS produces a lower resistance to interfacial mass transfer which favorably affects hydrate formation rate by enhancing gas-water contact through efficient diffusion of hydrate forming gases into the water while higher solubility of hydrate forming gases in water in the presence of surfactants may also play a significant role in enhancing hydrate formation kinetics. Capillary driven supply of water into the porous hydrate layers in the presence of surfactants further enhances hydrate formation by ensuring greater gaswater contact. This significantly alters the hydrate morphology resulting in catastrophic hydrate growth on the walls of the reactor. The use of surfactants thus in more ways than one, contributes to enhancing the kinetics of hydrate formation. (Kumar et al., 2014; Caskey et al.,1992; Hanwright et al., 2005; Kumar et.al, 2015c ).
11
3.2. Kinetics of Hydrate Formation in the Presence of THF The use of THF as a thermodynamic promoter is well known and docuemnted. Since the gas mixture being used forms hydrates at a relatively high operating pressure, the use of THF can help to perform the experiment at much milder operating pressures. Daraboina et. al, 2013 reported the phase equilibria data for CO2/N2/SO2 mixture in presence of 1 mol% THF. The addition of 1 mol% THF offers a huge reduction in operating pressure while a significant reduction in the induction time was also observed. On the other hand, the addition of THF also reduces the gas uptake. (Linga et al., 2007; Daraboina et al., 2013). To enhance the rate of hydrtae formation, in the present work, larger amounts of THF were used (3 and 5.56 mol%). 5.56 mol% is the amount of THF required to completely fill the large cages in structure II hydrates. Figure 3 compares the gas consumption (mol of gas/ mol of water; hydrate growth) measured for different amounts of THF in the system (3 and 5.56 mol %) at 2.45 MPa and 273.65K pressure and temperature respectively. Time zero in the figure corresponds to nucleation point (induction time) for the experiments. It has been reported in the literature that the kinetics of hydrate formation may depend on the concentration of THF used and hydrate formation rate decreases with the addition of THF at higher concentrations due to the likelihood of THF occupying the large cages readily and thus stabilising the structure II (sII) hydrates. This results in lower gas uptake of the guest gas due to mass transfer resistance; more specifically, guest gas molecules can no longeer occupy the large cages due to the already present THF molecules in the same (Lee et al., 2010). However, in the present study, as can be seen in Figure 3, the kinetics of hydrate formation is favorably affected when a larger amount of
THF (5.56 mol %) is present
in the system. Furthermore, it can be seen in Figure 3 that the final gas uptake is higher in case of
12
the 5.56 mol % THF system as compared to the 3 mol % THF systems. Similar results were reported by Babu et al., for pre- combustion CO2 capture where the total hydrate growth for the experiment using 5.53 mol % THF greatly surpassed that for the experiment using 1 mol % THF with a 1.8 times higher yield (Babu et al., 2014). Veluswamy et al., observed a similar trend with THF for H2 storage in the form of gas hydrates (Veluswamy et al., 2013, 2014). In the present study, in addition to the higher final gas uptake, as mentioned before, it was also observed that the addition of THF to the system (3 mol% and 5.56 mol%) greatly reduces the induction time (Table 1) which is in agreement to the observations made by Daraboina et.al, 2013 (Daraboina et.al, 2013).
3.3. Comparison of gas uptake for different systems investigated Figure 4 compares the final gas uptake after 10 hours from nucleation for the different systems used in this study with those obtained in the study performed by Daraboina et.al 2013. As can be seen in the figure, gas uptake is maximum for the system containing the surfactant SDS. This increase in gas uptake in the presence of SDS has been discussed in detail in section 3.1. It can also be noted from Figure 4 that the addition of THF to the system lowers the final gas uptake. This phenomenon is followed even on the addition of larger amounts of THF to the system (3 mol% and 5.56 mol %). Details on hydrate formation in the presence of THF in the system is given earlier in section 3.2.
13
3.4. Safety concerns in the use of gas mixture containing SO2 It is well known that the presence of SO2 in the atmosphere (at high humidities) enhances the
corrosion
of
metals. The corrosive action of sulfur dioxide on metallic surfaces is
determined by the particular state in which the metal exists and its purity. (Mattson et al., 1963; Vannerberg et al., 1970). SO2 molecules present in the gas phase adsorb onto the surface of the metals and reduce them. For example, Fe3+ is reduced by SO2 to Fe2+ ions. Teresa et al., have observed that a SO2 complex forms when a water surface is exposed to an atmosphere of SO2 gas (Teresa et al., 2005). SO2 + H2O ⇌ HSO3− + H+ Figure 5 shows the image of a rupture disc from our crystallizer which was corroded by the action of SO2 present in the gas mixture used in this study. Rupture disc was made up of Monel, a nickel alloy comprising nickel (67%) and copper with some amount of iron and other trace elements. As can be seen in the figure, the action of SO2 has had a major impact on the state of the rupture disc corroding the alloy to a large extent. Hydrate formation and dissociation conditions (temperature transitions from low to high temperature) are believed to be favorable for the corrosive action of SO2 further aiding in the corrosion of the rupture disc which took place during our experiments. However, a detailed study needs to be carried out in order to understand the corrosive effect of SO2. Disclaimer: Due to the corrosive and toxic nature of SO2, it is highly recommended to follow standard safety precautions in the laboratory. Please refer MSDS of SO2.
14
4. Conclusion Hydrate formation kinetics and high operating pressure, which are the foremost issues that arise when dealing with CO2 capture through the HBGS process, have been addressed in this work. Kinetics of CO2 hydrate formation from a CO2/N2/SO2 gas mixture was enhanced by using 1 wt % of SDS solution at 9.5 MPa pressure and 273.65 K temperature. Gas uptake was increased by almost three folds as compared to that for a system in the absence of SDS. Operating pressure for formation of CO2 hydrate was reduced by using THF as a thermodynamic promoter (two different concentrations: 3 and 5.56 mol %) and its corresponding effect on hydrate formation kinetics was studied. It was observed that at 5.56 mol % THF, the final gas uptake achieved was the highest. It was also observed that SO2 reacted corrosively with some of the material of construction of the reactor. Due to the high corrosive activity of SO2, it is recommended to follow standard safety precautions in the laboratory; more specifically material selection for reactor design should be done carefully and after due diligence.
Acknowledgements The authors gratefully acknowledge the financial support (Senior Research Fellowship (SRF)) received for this work from the Council of Scientific and Industrial Research (CSIR). The authors are grateful to the Director, CSIR-NCL, Pune for allowing us to carry out this research work in India’s most prestigious chemical laboratory. Authors would like to thank Dr. Rajnish Kumar for intellectually stimulating technical discussions and for providing the necessary laboratory facilities.
15
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19
Figure captions Figure 1. Schematic of the experimental apparatus Figure 2. Typical gas uptake measurement curve along with temperature profile at 9.5 MPa and 273.65K (Experiment no. SS2) Figure 3. Comparison of gas consumption (mol of gas/mol of water) for hydrate growth measured for different amount of THF (3 and 5.56 mol%) at 2.45 MPa and 273.65K.Time zero in the graph corresponds to nucleation point (induction time) for the experiments. Figure 4. Comparison of gas consumption for hydrate growth (mol of gas/mol of water after ~10h from induction) measured in presence of 1wt% SDS and with varying amounts of THF (1 mol% (Daraboina et al., 2013), 3 and 5.56 mol%THF) Figure 5. Image of rupture disc, damaged by the action of SO2
20
Figure 1
21
Figure 2
22
Figure 3.
23
5.56mol% THF
Hydrate growth in ~10 h from induction time
3mol% THF
1mol% THF
Daraboina et al. 2013
CO2/N2/SO2/SDS
CO2/N2/SO2 0.000
Daraboina et al.2013
0.008
0.016
0.024
0.032
0.040
0.048
Gas Uptake (mol of gas/mol of water)
Figure 4
24
Figure 5
25
Table 1. Summary of experiments, gas consumption; experimental pressure and temperature; Amount of water used for all the experiment was 50 cm3. System
Pressure (MPa)
Temperature (K)
THF (mol%)
SDS (wt%)
Induction Time (min)
SS1
9.5
273.65
0
1
24
Total Gas uptake (mol of gas/mol of water) 0.058
SS2
9.5
273.65
0
1
342
0.047
SS3
9.5
273.65
0
1
104
0.045
ST1
2.45
273.65
3
0
5
0.0067
ST2
2.45
273.65
3
0
7
0.0062
ST3
2.45
273.65
3
0
4
0.0075
ST4
2.45
273.65
5.56
0
4
0.0112
ST5
2.45
273.65
5.56
0
3
0.0121
ST6
2.45
273.65
5.56
0
5
0.0122
Experiment No. CO2/N2/SO2/ SDS
CO2/N2/SO2/ THF
Average total Gas uptake [±SD] 0.050 (±0.007)
0.0068 (±0.00066)
0.012 (±0.00055)
Utmost care was taken in the gas uptake calculations please note that the change in gas composition at the end of experiments was ignored for gas uptake calculations; a small change in gas composition may however have an impact on total gas uptake
26