Effect of the Promoter on Gas Hydrate Formation and Dissociation

Jun 19, 2008 - The hydrate formation rate increases with the surfactant concentration above the critical micelle concentration. The gas storage capaci...
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Energy & Fuels 2008, 22, 2527–2532

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Effect of the Promoter on Gas Hydrate Formation and Dissociation Ajay Mandal* and Sukumar Laik Department of Petroleum Engineering, Indian School of Mines UniVersity, Dhanbad 826 004, India ReceiVed April 7, 2008. ReVised Manuscript ReceiVed May 6, 2008

In this study, ethane hydrates were produced in the presence of a surfactant to investigate the effect of a surfactant on hydrate formation and dissociation. The hydrate formation rate increases with the surfactant concentration above the critical micelle concentration. The gas storage capacities in the presence of a surfactant were calculated at the system pressure and temperature. The dissociation behaviors of hydrates were also studied. The dissociation of hydrates increases with the surfactant concentration under similar operating pressure and temperature.

1. Introduction The gas hydrates are a group of solid crystalline nonstoichiometric compounds called clathrates, in which gas molecules remain engaged as guest inside the host lattice of water molecules. Such compounds resembling ice or snow are formed when some light gases, such as the components of natural gas come into contact with water under particular conditions of temperature and pressure. The main reason of gas hydrate existence is the ability of water molecules to form a lattice structure through hydrogen bonding under suitable conditions. This crystal lattice structure alone is thermodynamically unstable. The inclusion of nonpolar gas molecules into the hydrate structure makes the structure stabilized; therefore, hydrates can form at temperatures above the freezing point of liquid water. Beside pressure, temperature, and composition of the gas mixture, the formation and dissociation of hydrates are affected by different chemicals; some of these act as inhibitors, and some act as promoters. Surfactants are responsible for the rapid formation of hydrates under similar conditions. The importances of studying hydrate formation and dissociation in the presence of a surfactant promoter are as follows: (1) There are some surfactants, which are naturally formed from the crude oil itself under suitable conditions.1,20,28 These surfactants act as a hydrate promoter, causing the problem in transportation of oil and gas through pipelines at low temperature. The hydrate deposit in a permafrost region and deep-sea sediments may also be enhanced in the presence of such natural surfactants. Thus, to solve the transportation problem and also to develop proper exploitation technology of gas hydrates, substantial research work on hydrate formation and dissociation in the presence of a surfactant is necessary. (2) Gas hydrates have drawn much attention these days as not only a new natural energy resource but also a new means for natural gas storage and transport. * To whom correspondence should be addressed. E-mail: [email protected]. (1) Gafonova, O. V.; Yarranton, H. W. The stabilization of waterin-hydrocarbon emulsions by asphaltenes and resins. J. Colloid Interface Sci. 2001, 241, 469–478. (2) Ganji, H.; Manteghian, M.; Sadaghiani zadeh, K.; Omidkhah, M. R.; Rahimi Mofrad, H. Effect of different surfactants on methane hydrate formation rate, stability and storage capacity. Fuel 2007, 86, 434–441. (3) Gayet, P.; Dicharry, C.; Marion, G.; Graciaa, A.; Lachaise, J.; Nesterov, A. Experimental determination of methane hydrate dissociation curve up to 55 MPa by using a small amount of surfactant as hydrate promoter. Chem. Eng. Sci. 2005, 60, 5751–5758.

Natural gas storage in hydrates has been investigated because hydrates store large quantities of natural gas (∼180 Sm3/m3 of hydrate).15,19 Gudmundsson and Parlaktuna6 showed that the hydrate can be stored at -15 °C under atmospheric pressure for 15 days, retaining almost all the gas. They also showed a substantial cost saving (24%) for the transport of natural gas in the form of hydrates compared to liquefied natural gas. Hydrate formation with promoters for the purpose of natural gas storage and transport has been reported in some recent literature.7,22,25 However, industrial applications of hydrate storage processes have been hindered by some problems, such as slow formation rates, unreacted interstitial water as a large percentage of the hydrate mass, reliability of hydrate storage capacity, and economy of process scale-up. The surfactant molecules help minimize mass-transfer and kinetic difficulties during hydrate formation. During the last 2 decades, several studies have been reported showing a significantly increased hydrate formation rate with the addition of surfactant molecules.12–14,26 Zhong and Rogers30 (4) Giavarini, C.; Maccioni, F. Self-preservation at low pressures of methane hydrate with various gas contents. Ind. Eng. Chem. Res. 2004, 43, 6616–6621. (5) Gnanendran, N.; Amin, R. The effect of hydrotropes on gas hydrates formation. J. Pet. Sci. Eng. 2003, 40, 37–46. (6) Gudmundsson, J.; Parlaktuna, M.; Khokhar, A. A. Storing natural gas as frozen hydrates. SPE Prod. Facil. 1994, 69–73. (7) Guo, Y.; Fan, S.; Guo, K.; Chen, Y. Storage capacity of methane in hydrate using calcium hypochlorite as additive. In Proceedings of the 4th International Conference on Natural Gas Hydrates, Yokohama, Japan, 2002; 1040-3. (8) Guo, Y. K.; Fan, S. S.; Guo, K. H.; Chem, Y. Storage capacity of methane in hydrate using calcium hypochlorate as additive. In Proceedings of the 4th International Conference on Gas Hydrates, Yokohama, Japan, 2000; P-1040. (9) Han, X.; Wang, S.; Chen, X.; Liu, F. Surfactant accelerates gas hydrate formation. In Proceedings of the 4th International Conference on Natural Gas Hydrates, Yokohama, Japan, 2002; 1036-9. (10) Handa, Y. P. Compositions, enthalpies of dissociation, and heat capacities in the range 85 to 270 K for clathrate hydrates of methane, ethane, and propane, and enthalpy of dissociation of isobutane hydrate as determined by heat-flow calorimeter. J. Chem. Thermodyn. 1986, 18, 915–921. (11) Hussain, S. M. T.; Kumar, A.; Laik, S.; Mandal, A.; Ahmad, I. Study of the kinetics and morphology of gas hydrate formation. Chem. Eng. Technol. 2006, 29, 937–943. (12) Kalogerakis, N.; Jamaluddin, A. K. M.; Dholabhai, P. D.; Bishnoi, P. R. Effect of surfactants on hydrate formation kinetics. SPE25188, 1993; pp 375-383. (13) Karaaslan, U.; Parlaktuna, M. Surfactants as hydrate promoters. Energy Fuels 2000, 14, 1103–1107.

10.1021/ef800240n CCC: $40.75  2008 American Chemical Society Published on Web 06/19/2008

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Figure 1. Schematic diagram of the experimental setup.

reported that the formation rates of gas hydrates can be increased multiple orders of magnitude if the host water is a micellar solution containing sodium dodecyl sulfate (SDS) or related surfactant. They found that the critical micelle concentration (cmc) for ethane and natural gas-hydrate-forming conditions of a SDS solution is approximately 250 ppm. Han et al.9 obtained the maximum gas hydrate content for natural gas containing 90 wt % methane at 300 ppm SDS concentration. Gou et al.7,8 investigated the effect of calcium hypochlorite on the methane hydrate gas content. Karaaslan et al.14 studied the effect of linear alkyl benzene sulfonic acid on the formation rate of the hydrate rate of structure I and II. Their results revealed that this compound increases the rate of hydrate but its effect on structure I is more pronounced. Sun et al.27 showed that the effect of a non-ionic surfactant on hydrate storage capacity is less pronounced compared to that of an anionic surfactant. Gnanendran and Amin5 discussed the hydrate promotion capabilities of hydrotrope para-toluene sulfonic acid (pTSA) when used as an additive in water for natural gas-hydrate formation. They tested the hydrate promotion capability of p-TSA by forming hydrates with different concentration solutions in the presence of natural gas by subjecting it to hydrate formation conditions and, subsequently, after adequate formation, dissociating and measuring the gas/water ratio in hydrates. Link et al.17 found that the addition of sodium dodecyl benzene sulfonate and potassium oxalate monohydrate increases the formation rate of natural gas hydrate and its storage capacity. Some recent works21,23,29 also described the effect of surfactants on hydrate formation and dissociation characteristics. (14) Karaaslan, U.; Uluneye, E.; Parlaktuna, M. Effect of an anionic surfactant on different type of hydrate structures. J. Pet. Sci. Eng. 2002, 35, 49–57. (15) Khokhar, A. A.; Gudmundsson, J. S.; Sloan, E. D. Gas storage in structure H hydrates. Fluid Phase Equilib. 1998, 150-151, 383–392. (16) Lin, W.; Chen, G.-J.; Sun, C.-Y.; Guo, X.-Q.; Wu, Z.-K.; Liang, M.-Y.; Chen, L.-T.; Yang, L.-Y. Effect of surfactant on the formation and dissociation kinetic behavior of methane hydrate. Chem. Eng. Sci. 2004, 59, 4449–4455. (17) Link, D. D.; Ladner, E. P.; Elsen, H. A.; Taylor, C. E. Formation and dissociation studies for optimizing the uptake of methane hydrates. Fluid Phase Equilib. 2003, 211, 1–10. (18) MacKerell, A. D., Jr. Molecular dynamics simulation analysis of a sodium dodecyl sulfate micelle in aqueous solution: Decreased fluidity of the micelle hydrocarbon interior. J. Phys. Chem. 1995, 99, 1846–1855. (19) Makogon, Y. F. Hydrates of Hdrocarbon; Pennwell Publishing Co.: Tulsa, OK, 1997. (20) Moran, K.; Czarnecki, J. Competitive adsorption of sodium naphthenates and naturally occurring species at water-in-crude oil emulsion droplet surfaces. Colloids Surf., A 2007, 292, 87–98.

Though significant work is being carried out, more research work is still required to know the exact behavior of the surfactant in hydrate formation and dissociation. The main purpose of this work is to study the effect of the surfactant on hydrate formation, particularly to investigate its ability of increasing the formation rate of gas hydrate and improving hydrate storage capacity in a quiescent system. An extensive observation of pressuretemperature equilibrium during formation and decomposition of ethane hydrate under various conditions is also discussed. 2. Experimental Section 2.1. Apparatus. The schematic diagram of the experimental apparatus is shown in Figure 1. It has both the provision to operate in semibatch or batch manner. The reactor bomb is made of stainless steel, having a capacity of 500 cc, and is provided with two plexiglass windows for visual observation. There are three temperature sensors within the reactor to measure the instantaneous temperature of the liquid, gas-liquid interface, and gas phase, respectively. The reactor pressure is directly measured from the attached pressure gauge (bourdon type, accuracy +1%). The reactor is also provided with an elaborate refrigeration system for cooling to 255 K, by using ethanol as a cooling media. Temperature variations during the experiments are displayed digitally in a data logger (Electronet SC-240 digital temperature scanner) and can be taken out in printed form. 2.2. Chemical Used. Deionized water produced from a Millipore system (Millipore SA, 67120 Molshein, France) and research-grade ethane (99.99 mol %), obtained from Chemtron Science Laboratory, India, were used for hydrate formation. SDS (analytical reagent) collected from Central Drug House, New Delhi, India, was used as a surfactant for the present study. 2.2.1. Characteristics of Surfactants. Surfactants are substances that, when present even at very low concentrations, have the ability to significantly alter the surface or interfacial free energies.24 Therefore, surfactants have a direct effect on the hydrate formation process. The formed nuclei in the presence of surfactants possesses a different critical radius and hence a higher surface area per unit (21) Okutani, K.; Kuwabara, Y.; Mori, Y. H. Surfactant effects on hydrate formation in an unstirred gas/liquid system: An experimental study using methane and sodium alkyl sulfates. Chem. Eng. Sci. 2008, 63, 183– 194. (22) Pang, W. X.; Chen, G. J.; Dandekar, A.; Sun, C. Y.; Zhang, C. L. Experimental study on the scale-up effect of gas storage in the form of hydrate in a quiescent reactor. Chem. Eng. Sci. 2007, 62, 2198–2208. (23) Rogers, R.; Zhang, G.; Dearman, J.; Woods, C. Investigations into surfactant/gas hydrate relationship. J. Pet. Sci. Eng. 2007, 56, 82–88.

Promoter in Gas Hydrate Reactions

Figure 2. cmc for SDS at ambient conditions.

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Figure 4. Hydrate formation and dissociation behavior for the pure water-ethane system.

system, and the temperature of the system was decreased slowly by using ethanol as a refrigerant. Initially, there was a slow decrease of pressure because of dissolution of ethane gas in the solution phase. After a certain period, the mixture becomes a little translucent and sudden fall of pressure at a higher rate indicates growth and nucleation of the hydrate. The temperature and pressure were recorded during hydrate formation. To obtain the amount of moles (gas) consumed during hydrate formation, the following equation has been used: PV (1) ZRT in which P, V, and T are the gas pressure, volume, and temperature, respectively, R is the gas constant, and Z is the compressibility factor obtained from the generalized plot (standing Katz method) of compressibility factors at low reduced pressures and reduced temperatures. After the formation of hydrates, to study dissociation, the process was reversed; i.e., temperature was slowly increased after a certain period. The pressure started to rise with the rise in temperature, indicating the start of decomposition. Because the temperature gradients were maintained very low during decomposition, the temperature was taken as the equilibrium temperature for the hydrate at the corresponding reactor pressure. Each experiment was repeated to check the reproducibility. n)

Figure 3. Model of SDS micelle in the hydrate-forming system.30

volume of the system, which may have a significant effect on the hydrate formation rate. Surfactants form colloidal-size clusters in solutions called micelles. The cmc is the minimum concentration of the dissolved surfactant above which the clusters (micelles) begin to appear in the solution. At cmc, an abrupt change occurs in almost every physical property that depends upon the size or number of particles in solution.24 From Figure 2, one may see that, above the cmc, there is no further change of surface tension. This behavior is true for all types of surfactants, non-ionic, anionic, cationic, and zwitterionic, in aqueous solutions. The rate of hydrate formation is increased in the presence of surfactants near their cmc levels because it provides maximum reduction in surface tension.30 MacKerell18 discussed the chemistry behind the higher rate of hydrate formation in the presence of SDS. A model of the SDS micelle is depicted in Figure 3. 2.3. Experimental Procedure. A required amount of sodium dodecyl surfactant was weighed on an electronic balance and is dissolved in 300 cc of reverse osmosis water from a Millipore water system. The solution was then poured into the reactor bomb. Air was removed from the reactor by a vacuum pump and ethane purging. The reactor was then pressurized by ethane to a preset experimental pressure. The reaction was carried in a quiescent (24) Rosen, M. J. Surfactants and Interfacial Phenomena, 2nd ed.; Wiley: New York, 1989.

3. Results and Discussion 3.1. Pressure-Temperature Trace of Formation and Dissociation. Anionic surfactant SDS was selected, and its promotion effect on hydrate formation rate was investigated. The experiments were carried out at different ranges of pressure and temperature. To observe the effect of the surfactant on the formation and dissociation of gas hydrates, experiments were performed with two different concentrations of surfactant (300 and 500 ppm of SDS). The variation of pressure with temperature was recorded, and the amount of moles (gas) consumed during formation was calculated by eq 1. For comparison, a set of experiments were also performed in the absence of the surfactant. The pressure and temperature traces of hydrate formation and dissociation for the three different systems are presented in Figures 4–6, respectively. For the SDS solutions, larger pressure drops caused by the formation of hydrates were observed upon cooling. For every system, a certain degree of supercooling is required for spontaneous crystallization. The higher rate of hydrate formation is also evident from the shifting of the hydrate formation temperature from 265 to 272 K when

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Figure 5. Hydrate formation and dissociation behavior for the water-ethane-SDS system.

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Figure 7. Temperature and pressure trace for the formation of the water-ethane-SDS system.

Figure 8. Picture of the hydrate cluster in an opened reactor cell. Figure 6. Hydrate formation and dissociation behavior for the water-ethane-SDS system.

concentrations of SDS were changed from 0 to 500 ppm for the same range of operating pressures. After crystallization began, the pressure dropped abruptly at a fairly constant temperature. Decomposition occurred as the system is reheated to the equilibrium point; gas evolved from the hydrates; and the system returned to the original pressure and ambient temperature. Therefore, all together, they give a hysteresis-type phenomenon. In loop, the first drop in pressure is due to the dissolution of the gas in water or aqueous surfactant solutions. The rapid drop of pressure within a small temperature range indicates the formation of gas hydrates, and then a rise in pressure with an increase in temperature is due to dissociation of hydrates. The upper part of the loop expands at higher pressures, whereas the width of the lower part contracts. The results keep good parity with the experimental observation made by other researchers.3,14 The simultaneous variations of pressure and temperature with time during nucleation and growth of hydrate for an ethanewater-surfactant system has been presented in Figure 7. During the process of formation, it was assumed that the water remained (25) Saito, Y.; Kawasaki, T.; Okui, T.; Kondo, T.; Hisaoka, R. Methane storage in hydrates phase with soluble guests. In Proceedings of the 2nd International Conference on Natural Gas Hydrate, Toulouse, France, June 2-6, 1996; P-459.

saturated with gas throughout and the pressure drop in constant volume reactor was solely due to the formation of the hydrate. To inspect the status of hydrate growth, the reactor was opened and several photographs were taken. A typical photograph showing the cluster of hydrates formed around the interior wall of the reactor is depicted in Figure 8. 3.2. Gas Consumption Rate. The gas consumption rates are shown in Figure 9. Higher formation rates were observed in a quiescent water-ethane system in the presence of selected surfactants. The key function expected from an ethane hydratepromoting agent is to improve the solubility of the hydrate forming gas in water. Zhong and Rogers30 reported that above the cmc of surfactants the solubility of ethane increases, while at the surfactant concentration below or around the cmc, the gas solubility remains similar to that of pure water. From Figure 9, it may be seen that rate of gas consumption gradually decreases with time. This is due to the fact that all of the experiments were performed in a constant volume batch reactor, where the pressure of the system gradually decreases because of consumption of the gas. Further, with the progress of the hydrate formation, the overall mass-transfer coefficient gradually decreases because of the increased resistance for the transfer of ethane gas from the gas to liquid phase.11 The rate curves (26) Sun, Z.; Ma, R.; Fan, S.; Guo, K.; Wang, R. J. Nat. Gas Chem. 2004, 13, 107–112.

Promoter in Gas Hydrate Reactions

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Figure 9. Rate of gas consumption during nucleation and growth.

after passing through maxima slowly decrease, which indicates the starting of hydrate dissociation. 3.3. Storage Capacity of Ethane Gas in Hydrates. Expediency of gas storage in a hydrate state comes from a gas-water molar ratio and an extremely high density of gas in a hydrate state. The advantages of gas storage in a hydrate state result from a comparison of volumes of gas contained in a free state and in a hydrate state at various pressures. The volume of gas stored in a unit volume of hydrate at the hydrate formation conditions of pressure and temperature may be calculated by the expression VH )

103VGF MH

(2)

where VG is the molecular volume of gas and MH is the molecular weight of hydrate and is defined as MH ) M + 18.02nh

(3)

where M is the molecular weight of hydrate forming gas and nh is the hydrate number. The volume of gas stored in a unit volume of hydrate at standard conditions is defined by the equation VSC )

VHPT0 ZP0T

(4)

where P and T are the operating pressure and temperature, respectively, and the suffix zero refers to standard conditions. The density of hydrate formed by the gas mixture, F, is given by the expression19 F)

∑ N (M + 18.02n ∑ 18.02N V n i

hi)

i

(5)

i i hi

In the present study, different experiments were performed under similar conditions only by changing the concentration of surfactants. Thus, the storage capacity of gas in the gas hydrate has been qualitatively explained by the total gas consumed as a function of the SDS concentration as shown in Figure 10. Storage capacity increases with an increase in the SDS concentration because of higher consumption of gas. Use of the surfactant causes the higher solubility of gas into water that enhances the formation of a finer hydrate particle. These finer hydrate particles possess a higher surface area compared to that of the simple ethane-water system. Thus, because of a higher

Figure 10. Influence of the SDS concentration on the hydrate storage capacity.

surface area, the mass-transfer rate also increases, causing a higher rate of formation. Zhong and Rogers30 also found that storage capacity increases with an increase in the SDS concentration above 242 ppm. Lin et al.16 found the same trend in the lower concentration range of surfactant, but they reported that storage capacity decreases with the SDS concentration in a higher concentration region (>800 ppm) because of a lower final percent conversion of water into hydrate. 3.4. Dissociation of Ethane Hydrates. For a closed system, the dissociation proceeded in a constant volume system and the dissociation kinetic behavior was manifested by the increasing rate of the system pressure. Three sets of experiments on dissociation with different SDS concentrations were conducted at their corresponding decomposition temperature. Because the amount of hydrate formed in a definite period is different for different pressures, the pressure rise during decomposition will be proportional to the initial hydrate crystal population. Therefore, the rates of decomposition cannot be directly compared for different sets. The mole percentage decomposition curve with the temperature gives a better comparison of the rates of decomposition (Figure 11). The mole percentage decomposition for definite intervals was calculated by expressing the pressure rise in terms of moles of gas released. The percentages were made on the basis of moles of gas released in a definite period, divided by the total moles of gas released at the end and then multiplying by 100. The dissociation behavior for the three different systems is given in Figure 11. From the figure, it may be seen that the hydrate dissociation is higher in the presence of the surfactant compared to the pure water-ethane system. The higher dissociation rate of hydrate in the presence of the surfactant is due to a finer size of the hydrate particle and a higher gas content as explained by Ganji et al.2 Similar findings were also made by Lin et al.16 and Giavarini and Maccioni.4 Certain degrees of supercooling is required to induce nucleation of hydrate formation. Thus, the temperature at which decomposition started was assumed to be the equilibrium hydrate (27) Sun, Z.; Wang, R.; Ma, R.; Guo, K.; Fan, S. Natural gas storage in hydrate with the presence of promoters. Energy ConVers. Manage. 2003, 44, 2733–2742. (28) Yarranton, H., W.; Hussein, H.; Masliyah, J. Water-in-hydrocarbon emulsions stabilized by asphaltenes at low concentrations. J. Colloid Interface Sci. 2000, 228, 52–63. (29) Zhang, G.; Rogers, R. E. Ultra-stability of gas hydrates at 1 atm and 268.2 K. Chem. Eng. Sci. 2008, 63, 2066–2074. (30) Zhong, Y.; Rogers, R. E. Surfactant effects on gas hydrate formation. Chem. Eng. Sci. 2000, 55, 4175–4187.

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component system changes the energy of the intermolecular interaction and changes the equilibrium between molecules of water and gas. Thus, the presence of SDS shows a thermodynamic effect on hydrate formation, which causes a shift in the formation temperature. The hydrate dissociation rate measured at the operating pressure and temperature showed that the presence of SDS lowers the self-preservation effect and increases the dissociation rate. Acknowledgment. We gratefully acknowledge the financial assistance provided by University Grants Commission, Delhi, India, under the Special Assistance Program (SAP) to the Department of Petroleum Engineering, Indian School of Mines University, Dhanbad, India. Thanks are also extended to all individuals associated with the project.

Nomenclature Figure 11. Dissociation percentage profile of ethane hydrate in the presence of SDS.

temperature or hydrate formation temperature for the corresponding pressure. 4. Conclusion Experiments were performed to observe the effects of the anionic surfactant, SDS on ethane hydrate formation, dissociation, and storage capacity at a quiescent state. It has been found that the pressure-temperature trace for complete formation and dissociation gives a hysteresis loop. For comparison, the experiments were performed at two different SDS concentrations along with a pure ethane-water system. In the presence of SDS, hydrates grow as very fine particles, which increase the gas consumption and hence the storage capacities. The addition of a surfactant in the ethane-water two-

M ) molecular weight of hydrate former gas MH ) molecular weight of hydrate Mi ) molecular weight of ith hydrate former gas n ) number of moles nhi ) hydrate number or ith component of gas in the gas mixture nh ) hydrate number Ni ) molecular fraction of the ith hydrate former gas component P ) pressure T ) temperature P0 ) standard pressure T0 ) standard temperature V ) volume VG ) molecular volume of gas VH ) volume of gas stored in a unit volume of hydrate Vi ) specific volume of water in the hydrate formed by component i VSC ) volume of gas stored in a unit volume of the hydrate at standard conditions Z ) compression factor F ) density EF800240N