Ind. Eng. Chem. Res. 2010, 49, 8267–8270
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Methane Enclathration with Sodium Dodecyl Sulfate: Effect of Cyclopentane and Two Salts on Formation Kinetics Junshe S. Zhang, Jo A. Salera, and Jae W. Lee* Department of Chemical Engineering, The City College of New York, New York, New York 10031
This work presents the effect of NaCl and NaClO4 on the kinetics of methane enclathration with cyclopentane (CP) and sodium dodecyl sulfate (SDS) in a nonstirred batch reactor. Methane and 1 cm3 of CP were charged sequentially to 150 cm3 of solutions in a high-pressure vessel and the reaction system was cooled down to 274.6 K with an initial pressure of 7.1 MPa. Hydrates are visually observed within 1 h after the onset of cooling at a SDS concentration range of 0-200 ppm. At the end of a growth period of 2.5 h, the pressure reduces to 6.4 MPa for SDS concentrations below 20 ppm, whereas it decreases to 3.2 MPa for SDS concentrations above 50 ppm without any salts, which is very close to the hydrate equilibrium pressure. With 20 ppm SDS and 1 cm3 of CP, the average enclathration rate maximizes at 1.0 mM NaCl or 5.0 mM NaClO4 as the salt concentration increases from 0 to 100 mM. However, with 100 ppm SDS, it decreases monotonically with the increased salt concentration. These results not only provide an implication of reducing the SDS dosage (down to 50 ppm or less) in regard to fast enclathration but also further our understanding of the promoting role of surfactants. 1. Introduction Clathrate hydrates are nonstoichiometric crystalline compounds in which small molecules such as methane, carbon dioxide, and hydrogen stabilize cavities formed by hydrogenbonded water molecules.1 They have been considered as a promising storage medium for natural gas and hydrogen as well as a potential option for CO2 capture and sequestration. Putting these hydrate-based technologies into industrial practice depends on a solid understanding of the enclathration kinetic; including both nucleation and growth.2 The enclathration in the absence of mechanical agitation is usually carried out with a small amount of surfactants to shorten the enclathration period.3 The promoting role of surfactants has yet to be understood, although a lot of effort has been made to clarify the mechanism to develop novel promoters. One proposed picture at this moment is that fast enclathration rate is not due to the formation of micelles at temperatures below the normal Krafft point but most likely due to the interaction between surfactant monomers and hydrate crystals.4 Two recent studies found that sodium dodecyl sulfate anions (DS-) can adsorb onto clathrate hydrate particles.5,6 The adsorbed DSforms hemimicelles on the hydrate surface, into which the hydrate formers like methane solubilize and thus the enclathration rate is enhanced.6 The adsorption of DS- could also change the water structure at hydrate-water interface because the aqueous environment of methyl groups is found to be compatible with the structure of the hydrate surface;7 therefore, the enclathration rate is accelerated. Previous studies5,6 found that the surface of clathrate hydrates is negatively charged, which is attributed to the adsorption of anions such as hydroxide, bicarbonate, and carbonate. This negative charge produces an unfavorable electrostatic force for DS- adsorption. The repulsion force between negatively charged surface and an anion relaxes as the ionic strength increases, which enhances anion adsorption. On the other hand, other anions possibly compete with DS- adsorption for the adsorption sites. Therefore, the presence of electrolyte could affect the enclathration kinetics. * To whom correspondence should be addressed. Tel: 212-650-6688. Fax: 212-650-6660. E-mail:
[email protected].
In this work, we study the effect of cyclopentane (CP) on the methane enclathration with sodium dodecyl sulfate (SDS). The main reason to introduce a very small amount of CP is that it readily forms hydrates at the atmospheric pressure and temperatures up to 280 K;8 thus, it acts as a thermodynamic promoter. Second, we present the methane enclathration kinetics with two salts, NaCl and NaClO4, in the presence of both SDS and CP. We find that the addition of CP reduces the dosage of SDS down to 50 ppm or less with respect to achieving fast enclathration. With 20 ppm SDS, the average enclathration rate, correlated to the pressure decrease in a certain growth period, reaches a maximum at 1.0 mM NaCl or 5.0 mM NaClO4 as the salt concentration increases from 0 to 100 mM. However, it decreases as the salt concentration becomes higher with 100 ppm SDS. This work is the first attempt to reduce the dosage of SDS for methane enclathration by adding a small amount of salt. The effect of salt is discussed in terms of electrostatic force and DS- adsorption on the hydrate surface. 2. Experimental Section 2.1. Materials. Sodium dodecyl sulfate (SDS) and NaClO4 with a certified purity of 99+% and 98+%, respectively, were purchased from Sigma-Aldrich. Cyclopentane (CP) and NaCl with a purity of >98.5% and >99.5%, respectively, were obtained from Fluka. Methane (CH4) was supplied by T. W. Smith with a purity of 99.97%. All chemicals were used as received without further purification. Deionized water was produced in our lab with a resistivity of 18 MΩ cm-1. 2.2. Apparatus and Procedures. A detailed description of the reactor was given in our previous papers.9,10 We charged 150 cm3 of surfactant solutions with or without salts to the reactor followed by setting the reactor temperature to 288.2 K. The reactor was purged with methane twice and then pressurized with methane to 7.1 MPa. We injected 1 cm3 of CP at 1.0 cm3 min-1 after the methane charge was completed for 15 min. Five minutes after the CP injection, the reactor was cooled to 274.6 K and the reactor was maintained at that temperature for the rest of run. The uncertainty of pressure measurement at the end
10.1021/ie100759p 2010 American Chemical Society Published on Web 07/28/2010
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Ind. Eng. Chem. Res., Vol. 49, No. 17, 2010 Table 1. Induction Time As Function of SDS Concentrations CSDS (ppm) t ind. (hour)
Figure 1. Profiles of pressure and temperature during the enclathration with 20 (a) and 100 ppm SDS (b).
of a given growth period is estimated to be within 0.1 MPa based on triplicate measurements. 3. Results and Discussion 3.1. Methane Enclathration with SDS and CP. Figure 1 shows profiles of pressure and temperature during the enclathration with 20 and 100 ppm SDS. Here, the SDS concentration is the one at the start of cooling and the time is also set to be zero at this point. As shown in Figure 1a, a temperature spike is observed at 0.48 h with 20 ppm SDS, which is due to the crystallization of CH4 + CP binary hydrates because the pressure (6.97 MPa) is lower than the equilibrium pressure (7.52 MPa) of CH4 hydrate formation at the peak temperature (283.5 K). With 100 ppm SDS, hydrates are visually detected at 0.34 h, which is also attributed to simultaneously enclathrating CH4 and CP into small and large cavities of binary hydrates, respectively, because the temperature (286.2 K) at this point (see Figure 1b) is higher than the dissociation temperatures of CH4 hydrates (282.8 K at 6.95 MPa1) and CP hydrates (280.3 K8). A further examination of Figure 1b reveals that the pressure decreases from 6.95 to 3.15 MPa within 2.5 h after the onset of enclathration. This pressure (3.15 MPa) is very close to the equilibrium value (3 MPa1) of CH4 hydrates at 274.6 K, indicating that the enclathration reaches equilibrium in this period. The same trend is also found for 50 and 200 ppm SDS. With 20 ppm SDS, however, the pressure maintains at 6.4 MPa
0 0.58
10 0.82
20 0.49
50 0.47
100 0.34
200 0.56
for 3 h after hydrates are visually observed (Figure 1a), which is much higher than the equilibrium pressure of CH4 hydrates at 274.6 K. The mole number of CH4 enclathrated at the end of run is 0.07 and 0.62 with 20 and 100 ppm SDS, respectively. A detailed calculation of the amount of CH4 converted to hydrates is given in our previous paper.9 It is expected that all added CP is encaged into the binary hydrates in which CP and CH4 occupy the large and small cavities, respectively, and then the maximum amount of CH4 encaged in the binary hydrates is about 0.02 mol. Therefore, the formation of CH4 hydrates is dominant even with 20 ppm SDS. The water conversion at the end of run with 100 ppm is around 44% by assuming that the hydrate number of CH4 hydrates is 5.8. The effect of SDS on the induction time is given in Table 1. Here, the induction time is defined as the period between the start of cooling and the onset of the temperature spike. As Table 1 shows, no trend is observed between the SDS concentration and the induction time, suggesting the hydrate nucleation is not much affected by the presence of SDS as reported before.11 The short induction times less than 1 h may come from the relatively high degree of supercooling with respect to CH4 + CP binary hydrates under the experimental conditions as observed in the CO2 + CP binary hydrate.10 The pressure at 2.5 h after the onset of the first temperature spike (or the onset of the hydrate growth) is presented in Figure 2. It shows that the pressure almost keeps constant at SDS concentrations below 20 ppm and above 50 ppm; however, it sharply decreases from 6.4 to 3.2 MPa as the SDS concentration increases from 20 to 50 ppm. These observations indicate that SDS does not accelerate the enclathration at its concentration below 20 ppm and its promoting effect is constant at the concentration above 50 ppm. At this point, whether 50 ppm is the critical concentration above which the enclathration kinetics cannot be enhanced further is unclear. However, the critical SDS concentration falls between 20 to 50 ppm and it is 1 order of magnitude lower than that without CP under the same condition.9 Our previous work suggests that the promoting role of SDS in hydrate crystallization comes from the interaction between DS- and hydrate crystals5,6 and the accelerating effect becomes significant with increased adsorption density of DSon hydrates, which could be affected by both the ionic strength and other anions in the aqueous phase. Thus, the next sections
Figure 2. Pressure at 2.5 h after the onset of the first temperature spike as a function of SDS concentrations.
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Figure 3. Effect of NaClO4 (a) and NaCl (b) on the pressure at 16 h after the start of the enclathration with 20 ppm SDS.
present the effect of NaCl and NaClO4 on methane enclathration kinetics with 20 and 100 ppm SDS, below and above 50 ppm SDS which is assumed as the critical concentration for the promoted enclathration. 3.2. Methane Enclathration with 20 ppm SDS. Figure 3 presents the pressure at the end of a growth period of 16 h with respect to NaCl and NaClO4 concentrations. The reason we chose this period is that the enclathration is completed and no further decrease in pressure is observed at a specific salt concentration. Here, the salt concentration is the one at the start of cooling. As Figure 3a shows, the pressure decreases to a minimum of 3.08 MPa at 5 mM NaClO4 and then it increases again as its concentration increases further, indicating that the average enclathration rate, the amount of methane consumed over a certain period, reaches a maximum with 5 mM NaClO4. The same pattern is observed for NaCl except that the minimum pressure occurs at 1 mM and the pressure slightly decreases again at NaCl concentrations above 20 mM as shown in Figure 3b. The occurrence of an optimum salt concentration regarding to the average enclathration rate can be interpreted in terms of the shrinkage of electric double layer with salts and the competitive adsorption of electrolyte anions with DS- on hydrate crystals.12 As mentioned before, the surface charge of hydrate crystals is negative without addition of any acids or bases, which is due to adsorption of hydroxyl ion, bicarbonate, or carbonate.5,6 The surface charge of hydrate particle becomes less negative as the salt concentration increases due to the electrical double layer compression, causing an increase in the adsorption density of DS- on hydrates. More DS- adsorbed at the hydrate-water interface makes the enclathration kinetics
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become more favorable. Therefore, the promoting effect becomes stronger with increased electrolyte concentrations. This can explain the increase in the average enclathration rate as the NaCl (NaClO4) concentration changes from 0 to 1 mM (5 mM). Although the adsorption density of DS- on the hydrate surface at different salt concentration is not available at this point, the enhanced adsorption can be inferred from the water-CP interfacial tension measurements. The water-CP interfacial tension with 20 ppm SDS (Figure 1 in Supporting Information) decreases with the increased salt concentration, indicating that more DS- adsorption at the water-CP interface (which is also negative13,14) occurs at higher salt concentrations. Marinova et al.13 and Beattie and Djerdjev14 found that other anions such as Cl-, ClO4-, and bicarbonate are not responsible for the negative charge of the oil-water interface, which may not be true for the hydrate-water interface because these anions could adsorb on the ice surface whose structure is similar to the structure of hydrates.15,16 Interestingly, our results suggest that the adsorption of Cl- and ClO4- on hydrates is not significant at the low electrolyte concentrations (up to 1 mM NaCl and 5 mM NaClO4). These two anions, however, possibly compete with DS- for the adsorption sites at the higher salt concentrations and subsequently reduce the surface coverage of DS- on the hydrate surface, causing a decrease in the average enclathration rate as shown in Figure 3. In addition to the competitive adsorption between anions, two other factors that can contribute to the decreased average enclathration rate with the increased salt concentration are (1) the decrease in the particle charge because of the electric double layer shrinkage at high ionic strengths, which reduces repulsion force between hydrate particles; and (2) the change in the water structure at the hydrate-water interface. The reduced repulsion force may enhance agglomeration, causing a decrease in the total surface area of hydrate-water interface which is the crystallization front, accelerating the formation of a nonporous hydrate layer at the hydrocarbon-water interface, or both. The change in water structure is because these two anions are reported to be structure breaking ions.17 This effect, however, is plausibly negligible at low salt concentrations. The above argument could not justify the slight increase in the average enclathration rate again at NaCl concentrations greater than 20 mM. At this moment, we cannot provide an explanation for this observation. 3.3. Methane Enclathration with 100 ppm SDS. The variation of pressure after hydrates were visually observed with 100 ppm SDS is given in Figure 4. The pressure monotonically increases with the increased NaClO4 (or NaCl) concentration, suggesting the average enclathration rate decreases gradually as more salt is added to the system. The unfavorable effect of salt on the enclathration is due to the three factors as mentioned in the previous section, including the competitive adsorption between Cl- (or ClO4-) and DS-, shrinkage of the electrical double layer, or the change in the water structure at high salt concentrations. The first one reduces the adsorption density of DS-, while the second enhances the possibility of particle agglomeration, and the third one plausibly decreases the intrinsic enclathration constant (water cage formation). Although the enclathration could be controlled by the transfer of methane from gas phase to the crystallized zone, the third one cannot be excluded because some growing hydrates locate at the hydrocarbon-water interface due to low density of hydrates compared with water. However, at this stage, we cannot identify which factor is more important than the others; thus, more study is required to clarify this point.
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in promoting the enclathration. However, these two salts inhibit the hydrate growth with both CP and 100 ppm SDS. Moreover, this inhibition effect becomes stronger as the salt concentration increases from 0 to 100 mM. The favorable enclathration kinetics is attributed to the increased surface coverage of DSon hydrates in the presence electrolytes, which reduces the repulsion force between DS- and negative charged hydrates; whereas the competitive adsorption between DS- and other anions, or the presence of structure-breaking anions like Cland ClO4-, or both could contribute to the unfavorable enclathration kinetics at high salt concentrations. Supporting Information Available: Additional information on water-CP interfacial tension measurements under SDS and two salts and micro-DSC measurements. This material is available free of charge via the Internet at http://pubs.acs.org. Acknowledgment We are grateful for the support of the National Science Foundation for this work (under grant CBET-0854210). Literature Cited
Figure 4. Effect of NaClO4 (a) and NaCl (b) on the pressure at 3 h after the onset of the enclathration with 100 ppm SDS.
One last point to be emphasized is that the phase boundary of hydrates will shift when salt is added to the system. To elucidate this point, we determined the dissociated temperature of methane hydrates with 200 mM NaClO4 by using a high-pressure differential scanning calorimeter (DSC). We choose this concentration because the maximum salt concentration is less than 200 mM at the end of enclathration as the maximum water conversion to hydrates is lower than 50%. The measurements (Figure 2 in Supporting Information) indicate that salts have little effect on the equilibrium at this concentration. Therefore, the unfavorable effect cannot be explained by the decrease in the driving force at a specific temperature and pressure. 4. Conclusions In this work, the effect of the two salts and cyclopentane on methane enclathration was investigated in a nonstirred batch reactor. The results show that a small amount of CP reduces the SDS dosage, above which the enclathration kinetic cannot be accelerated further, from hundreds of ppm down to 50 ppm or less. We also observed that simultaneously enclathrating methane and CP into the binary hydrate occurs first as the system is cooled to an operating temperature. Additionally, the induction time, a period elapsed before hydrates were visually observed, varies from 0.34 to 0.82 h at SDS concentrations of 0-200 ppm. However, no correlation is observed between these two variables. At a concentration of 20 ppm or less, SDS cannot promote methane enclathration even with CP. Under the above condition, methane enclathration is accelerated by adding salts. Among the two salts investigated, NaCl is more effective than NaClO4
(1) Sloan, E. D.; Koh, C. A. Clathrate Hydrate of Natural Gases, 3rd ed.; CRC Press: Boca Raton, FL, 2008. (2) Sum, A. K.; Koh, C. A.; Sloan, E. D. Clathrate hydrates: From laboratory science to engineering practice. Ind. Eng. Chem. Res. 2009, 48, 7457–7465. (3) Zhong, Y.; Rogers, R. E. Surfactant effects on gas hydrate formation. Chem. Eng. Sci. 2000, 55, 4175–4187. (4) Zhang, J. S.; Lee, S. Y.; Lee, J. W. Does SDS micellize under methane hydrate-forming conditions below the normal ambient Krafft point? J. Colloid Interface Sci. 2007, 315, 313–318. (5) Lo, C.; Zhang, J. S.; Somasundarant, P.; Couzis, A.; Lee, J. W. Adsorption of surfactants on two different hydrates. Langmuir 2008, 24, 12723–12726. (6) Zhang, J. S.; Lo, C.; Somasundarant, P.; Couzis, A.; Lee, J. W. Adsorption of sodium dodecyl sulfate at THF hydrate/liquid interface. J. Phys. Chem. C 2008, 112, 12381–12385. (7) Storr, M. T.; Taylor, P. C.; Monfort, J. P.; Rodger, P. M. Kinetic inhibitor of hydrate crystallization. J. Am. Chem. Soc. 2004, 126, 1569– 1576. (8) Zhang, J. S.; Lee, J. S. Equilibrium of hydrogen + cyclopentane and carbon dioxide + cyclopentane binary hydrates. J. Chem. Eng. Data 2009, 54, 659–661. (9) Zhang, J. S.; Lee, S. Y.; Lee, J. W. Kinetics of methane hydrate formation from SDS solution. Ind. Eng. Chem. Res. 2007, 46, 6353–6359. (10) Zhang, J. S.; Lee, J. W. Enhanced kinetics of CO2 hydrate formation under static conditions. Ind. Eng. Chem. Res. 2009, 48, 5934–5942. (11) Zhang, J. S.; Lee, J. S. Effect of sodium dodecyl sulfate on the supercooling point of ice and clathrate hydrates. Energy Fuels 2009, 23, 3045–3047. (12) Zhang, J. S.; Lo, C.; Somasundaran, P.; Lee, J. W. Competitive adsorption between SDS and bicarbonates on THF hydrates using pyrene fluorescence. J. Colloid Interface Sci. 2010, 341, 286–288. (13) Marinova, K. G.; Alagova, R. G.; Denkov, N. D.; Velev, O. D.; Petsev, D. N.; Ivanov, I. B.; Borwankar, R. P. Charging of oil-water interface due to spontaneous adsorption of hydroxyl ions. Langmuir 1996, 12, 2045– 2051. (14) Beattie, J. K.; Djerdjev, A. M. The pristine oil/water interface: Surfactant-free hydroxide-charged emulsions. Angew. Chem., Int. Ed. 2004, 43, 3568–3571. (15) Drzmala, J.; Sadowski, Z.; Holysz, L.; Chibowski, E. Ice/water interface: Zeta potential, point of zero charge, and hydrophobicity. J. Colloid Interface Sci. 1999, 220, 229–234. (16) Suga, H.; Matsuo, T.; Yamamuro, O. Thermodynamic study of ice and clathrate hydrates. Pure Appl. Chem. 1992, 64, 17–26. (17) Marcu, Y. Effect of ions on the structure of water: Structure making and breaking. Chem. ReV. 2009, 109, 1346–1370.
ReceiVed for reView March 30, 2010 ReVised manuscript receiVed June 28, 2010 Accepted July 15, 2010 IE100759P
