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 asurfactant on hydrate formation and dissociation. The hydrate formation rate increases with the surfactantconcentration above the critical micelle concentration. The gas storage capacities in the presence of a surfactantwere calculated at the system pressure and temperature. The dissociation behaviors of hydrates were alsostudied. The dissociation of hydrates increases with the surfactant concentration under similar operating pressureand temperature.

1. Introduction

The gas hydrates are a group of solid crystalline non-stoichiometric compounds called clathrates, in which gasmolecules remain engaged as guest inside the host lattice ofwater molecules. Such compounds resembling ice or snow areformed when some light gases, such as the components ofnatural gas come into contact with water under particularconditions of temperature and pressure. The main reason of gashydrate existence is the ability of water molecules to form alattice structure through hydrogen bonding under suitableconditions. This crystal lattice structure alone is thermodynami-cally unstable. The inclusion of nonpolar gas molecules intothe hydrate structure makes the structure stabilized; therefore,hydrates can form at temperatures above the freezing point ofliquid water. Beside pressure, temperature, and composition ofthe gas mixture, the formation and dissociation of hydrates areaffected by different chemicals; some of these act as inhibitors,and some act as promoters. Surfactants are responsible for therapid formation of hydrates under similar conditions. Theimportances of studying hydrate formation and dissociation inthe presence of a surfactant promoter are as follows: (1) Thereare some surfactants, which are naturally formed from the crudeoil itself under suitable conditions.1,20,28 These surfactants actas a hydrate promoter, causing the problem in transportation ofoil and gas through pipelines at low temperature. The hydratedeposit in a permafrost region and deep-sea sediments may alsobe enhanced in the presence of such natural surfactants. Thus,to solve the transportation problem and also to develop properexploitation technology of gas hydrates, substantial researchwork on hydrate formation and dissociation in the presence ofa surfactant is necessary. (2) Gas hydrates have drawn muchattention these days as not only a new natural energy resourcebut also a new means for natural gas storage and transport.

Natural gas storage in hydrates has been investigated becausehydrates store large quantities of natural gas (∼180 Sm3/m3

of hydrate).15,19 Gudmundsson and Parlaktuna6 showed that thehydrate can be stored at -15 °C under atmospheric pressurefor 15 days, retaining almost all the gas. They also showed asubstantial cost saving (24%) for the transport of natural gas inthe form of hydrates compared to liquefied natural gas. Hydrateformation with promoters for the purpose of natural gas storageand transport has been reported in some recent literature.7,22,25

However, industrial applications of hydrate storage processeshave been hindered by some problems, such as slow formationrates, unreacted interstitial water as a large percentage of thehydrate mass, reliability of hydrate storage capacity, andeconomy of process scale-up. The surfactant molecules helpminimize mass-transfer and kinetic difficulties during hydrateformation.

During the last 2 decades, several studies have been reportedshowing a significantly increased hydrate formation rate withthe addition of surfactant molecules.12–14,26 Zhong and Rogers30

* To whom correspondence should be addressed. E-mail:mandal_ajay@hotmail.com.

(1) Gafonova, O. V.; Yarranton, H. W. The stabilization of water-in-hydrocarbon emulsions by asphaltenes and resins. J. Colloid InterfaceSci. 2001, 241, 469–478.

(2) Ganji, H.; Manteghian, M.; Sadaghiani zadeh, K.; Omidkhah, M. R.;Rahimi Mofrad, H. Effect of different surfactants on methane hydrateformation 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 dissociationcurve up to 55 MPa by using a small amount of surfactant as hydratepromoter. Chem. Eng. Sci. 2005, 60, 5751–5758.

(4) Giavarini, C.; Maccioni, F. Self-preservation at low pressures ofmethane hydrate with various gas contents. Ind. Eng. Chem. Res. 2004, 43,6616–6621.

(5) Gnanendran, N.; Amin, R. The effect of hydrotropes on gas hydratesformation. J. Pet. Sci. Eng. 2003, 40, 37–46.

(6) Gudmundsson, J.; Parlaktuna, M.; Khokhar, A. A. Storing naturalgas as frozen hydrates. SPE Prod. Facil. 1994, 69–73.

(7) Guo, Y.; Fan, S.; Guo, K.; Chen, Y. Storage capacity of methane inhydrate using calcium hypochlorite as additive. In Proceedings of the 4thInternational Conference on Natural Gas Hydrates, Yokohama, Japan, 2002;1040-3.

(8) Guo, Y. K.; Fan, S. S.; Guo, K. H.; Chem, Y. Storage capacity ofmethane in hydrate using calcium hypochlorate as additive. In Proceedingsof the 4th International Conference on Gas Hydrates, Yokohama, Japan,2000; P-1040.

(9) Han, X.; Wang, S.; Chen, X.; Liu, F. Surfactant accelerates gashydrate formation. In Proceedings of the 4th International Conference onNatural Gas Hydrates, Yokohama, Japan, 2002; 1036-9.

(10) Handa, Y. P. Compositions, enthalpies of dissociation, and heatcapacities in the range 85 to 270 K for clathrate hydrates of methane, ethane,and propane, and enthalpy of dissociation of isobutane hydrate as determinedby 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.

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10.1021/ef800240n CCC: $40.75 2008 American Chemical SocietyPublished on Web 06/19/2008

reported that the formation rates of gas hydrates can be increasedmultiple orders of magnitude if the host water is a micellarsolution containing sodium dodecyl sulfate (SDS) or relatedsurfactant. They found that the critical micelle concentration(cmc) for ethane and natural gas-hydrate-forming conditionsof a SDS solution is approximately 250 ppm. Han et al.9

obtained the maximum gas hydrate content for natural gascontaining 90 wt % methane at 300 ppm SDS concentration.Gou et al.7,8 investigated the effect of calcium hypochlorite onthe methane hydrate gas content. Karaaslan et al.14 studied theeffect of linear alkyl benzene sulfonic acid on the formationrate of the hydrate rate of structure I and II. Their resultsrevealed that this compound increases the rate of hydrate butits effect on structure I is more pronounced. Sun et al.27 showedthat the effect of a non-ionic surfactant on hydrate storagecapacity is less pronounced compared to that of an anionicsurfactant. Gnanendran and Amin5 discussed the hydrate promo-tion capabilities of hydrotrope para-toluene sulfonic acid (p-TSA) when used as an additive in water for natural gas-hydrateformation. They tested the hydrate promotion capability ofp-TSA by forming hydrates with different concentration solu-tions in the presence of natural gas by subjecting it to hydrateformation conditions and, subsequently, after adequate forma-tion, dissociating and measuring the gas/water ratio in hydrates.Link et al.17 found that the addition of sodium dodecyl benzenesulfonate and potassium oxalate monohydrate increases theformation rate of natural gas hydrate and its storage capacity.Some recent works21,23,29 also described the effect of surfactantson hydrate formation and dissociation characteristics.

Though significant work is being carried out, more researchwork is still required to know the exact behavior of the surfactantin hydrate formation and dissociation. The main purpose of thiswork is to study the effect of the surfactant on hydrate formation,particularly to investigate its ability of increasing the formationrate of gas hydrate and improving hydrate storage capacity ina quiescent system. An extensive observation of pressure-temperature equilibrium during formation and decompositionof ethane hydrate under various conditions is also discussed.

2. Experimental Section

2.1. Apparatus. The schematic diagram of the experimentalapparatus is shown in Figure 1. It has both the provision to operatein semibatch or batch manner. The reactor bomb is made of stainlesssteel, having a capacity of 500 cc, and is provided with twoplexiglass windows for visual observation. There are three tem-perature sensors within the reactor to measure the instantaneoustemperature of the liquid, gas-liquid interface, and gas phase,respectively.

The reactor pressure is directly measured from the attachedpressure gauge (bourdon type, accuracy +1%). The reactor is alsoprovided with an elaborate refrigeration system for cooling to 255K, by using ethanol as a cooling media. Temperature variationsduring the experiments are displayed digitally in a data logger(Electronet SC-240 digital temperature scanner) and can be takenout in printed form.

2.2. Chemical Used. Deionized water produced from a Milliporesystem (Millipore SA, 67120 Molshein, France) and research-gradeethane (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 usedas a surfactant for the present study.

2.2.1. Characteristics of Surfactants. Surfactants are substancesthat, when present even at very low concentrations, have the abilityto significantly alter the surface or interfacial free energies.24

Therefore, surfactants have a direct effect on the hydrate formationprocess. The formed nuclei in the presence of surfactants possessesa different critical radius and hence a higher surface area per unit

(14) Karaaslan, U.; Uluneye, E.; Parlaktuna, M. Effect of an anionicsurfactant 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 instructure 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 anddissociation 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. Formationand dissociation studies for optimizing the uptake of methane hydrates. FluidPhase Equilib. 2003, 211, 1–10.

(18) MacKerell, A. D., Jr. Molecular dynamics simulation analysis of asodium dodecyl sulfate micelle in aqueous solution: Decreased fluidity ofthe 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 sodiumnaphthenates and naturally occurring species at water-in-crude oil emulsiondroplet surfaces. Colloids Surf., A 2007, 292, 87–98.

(21) Okutani, K.; Kuwabara, Y.; Mori, Y. H. Surfactant effects onhydrate formation in an unstirred gas/liquid system: An experimental studyusing 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 ofhydrate in a quiescent reactor. Chem. Eng. Sci. 2007, 62, 2198–2208.

(23) Rogers, R.; Zhang, G.; Dearman, J.; Woods, C. Investigations intosurfactant/gas hydrate relationship. J. Pet. Sci. Eng. 2007, 56, 82–88.

Figure 1. Schematic diagram of the experimental setup.

2528 Energy & Fuels, Vol. 22, No. 4, 2008 Mandal and Laik

volume of the system, which may have a significant effect on thehydrate formation rate.

Surfactants form colloidal-size clusters in solutions calledmicelles. The cmc is the minimum concentration of the dissolvedsurfactant above which the clusters (micelles) begin to appear inthe solution. At cmc, an abrupt change occurs in almost everyphysical property that depends upon the size or number of particlesin solution.24 From Figure 2, one may see that, above the cmc,there is no further change of surface tension. This behavior is truefor all types of surfactants, non-ionic, anionic, cationic, andzwitterionic, in aqueous solutions. The rate of hydrate formationis increased in the presence of surfactants near their cmc levelsbecause it provides maximum reduction in surface tension.30

MacKerell18 discussed the chemistry behind the higher rate ofhydrate formation in the presence of SDS. A model of the SDSmicelle is depicted in Figure 3.

2.3. Experimental Procedure. A required amount of sodiumdodecyl surfactant was weighed on an electronic balance and isdissolved in 300 cc of reverse osmosis water from a Millipore watersystem. The solution was then poured into the reactor bomb. Airwas removed from the reactor by a vacuum pump and ethanepurging. The reactor was then pressurized by ethane to a presetexperimental pressure. The reaction was carried in a quiescent

system, and the temperature of the system was decreased slowlyby using ethanol as a refrigerant.

Initially, there was a slow decrease of pressure because ofdissolution of ethane gas in the solution phase. After a certainperiod, the mixture becomes a little translucent and sudden fall ofpressure at a higher rate indicates growth and nucleation of thehydrate. The temperature and pressure were recorded during hydrateformation. To obtain the amount of moles (gas) consumed duringhydrate formation, the following equation has been used:

n) PVZRT

(1)

in which P, V, and T are the gas pressure, volume, and temperature,respectively, R is the gas constant, and Z is the compressibilityfactor obtained from the generalized plot (standing Katz method)of compressibility factors at low reduced pressures and reducedtemperatures.

After the formation of hydrates, to study dissociation, the processwas reversed; i.e., temperature was slowly increased after a certainperiod. The pressure started to rise with the rise in temperature,indicating the start of decomposition. Because the temperaturegradients were maintained very low during decomposition, thetemperature was taken as the equilibrium temperature for the hydrateat the corresponding reactor pressure. Each experiment was repeatedto check the reproducibility.

3. Results and Discussion

3.1. Pressure-Temperature Trace of Formation andDissociation. Anionic surfactant SDS was selected, and itspromotion effect on hydrate formation rate was investigated.The experiments were carried out at different ranges of pressureand temperature. To observe the effect of the surfactant on theformation and dissociation of gas hydrates, experiments wereperformed with two different concentrations of surfactant (300and 500 ppm of SDS). The variation of pressure with temper-ature was recorded, and the amount of moles (gas) consumedduring formation was calculated by eq 1. For comparison, a setof experiments were also performed in the absence of thesurfactant. The pressure and temperature traces of hydrateformation and dissociation for the three different systems arepresented in Figures 4–6, respectively. For the SDS solutions,larger pressure drops caused by the formation of hydrates wereobserved upon cooling. For every system, a certain degree ofsupercooling is required for spontaneous crystallization. Thehigher rate of hydrate formation is also evident from the shiftingof the hydrate formation temperature from 265 to 272 K when

(24) Rosen, M. J. Surfactants and Interfacial Phenomena, 2nd ed.;Wiley: New York, 1989.

Figure 2. cmc for SDS at ambient conditions.

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

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

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concentrations of SDS were changed from 0 to 500 ppm forthe same range of operating pressures. After crystallizationbegan, the pressure dropped abruptly at a fairly constanttemperature. Decomposition occurred as the system is re-heated to the equilibrium point; gas evolved from the hydrates;and the system returned to the original pressure and ambienttemperature. Therefore, all together, they give a hysteresis-typephenomenon. In loop, the first drop in pressure is due to thedissolution of the gas in water or aqueous surfactant solutions.The rapid drop of pressure within a small temperature rangeindicates the formation of gas hydrates, and then a rise inpressure with an increase in temperature is due to dissociationof hydrates. The upper part of the loop expands at higherpressures, whereas the width of the lower part contracts. Theresults keep good parity with the experimental observation madeby other researchers.3,14

The simultaneous variations of pressure and temperature withtime during nucleation and growth of hydrate for an ethane-water-surfactant system has been presented in Figure 7. Duringthe process of formation, it was assumed that the water remained

saturated with gas throughout and the pressure drop in constantvolume reactor was solely due to the formation of the hydrate.

To inspect the status of hydrate growth, the reactor wasopened and several photographs were taken. A typical photo-graph showing the cluster of hydrates formed around the interiorwall of the reactor is depicted in Figure 8.

3.2. Gas Consumption Rate. The gas consumption rates areshown in Figure 9. Higher formation rates were observed in aquiescent water-ethane system in the presence of selectedsurfactants. The key function expected from an ethane hydrate-promoting agent is to improve the solubility of the hydrateforming gas in water. Zhong and Rogers30 reported thatabove 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. FromFigure 9, it may be seen that rate of gas consumption graduallydecreases with time. This is due to the fact that all of theexperiments were performed in a constant volume batch reactor,where the pressure of the system gradually decreases becauseof consumption of the gas. Further, with the progress of thehydrate formation, the overall mass-transfer coefficient graduallydecreases because of the increased resistance for the transferof ethane gas from the gas to liquid phase.11 The rate curves

(25) Saito, Y.; Kawasaki, T.; Okui, T.; Kondo, T.; Hisaoka, R. Methanestorage in hydrates phase with soluble guests. In Proceedings of the 2ndInternational Conference on Natural Gas Hydrate, Toulouse, France, June2-6, 1996; P-459.

(26) Sun, Z.; Ma, R.; Fan, S.; Guo, K.; Wang, R. J. Nat. Gas Chem.2004, 13, 107–112.

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

Figure 6. Hydrate formation and dissociation behavior for thewater-ethane-SDS system.

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

Figure 8. Picture of the hydrate cluster in an opened reactor cell.

2530 Energy & Fuels, Vol. 22, No. 4, 2008 Mandal and Laik

after passing through maxima slowly decrease, which indicatesthe starting of hydrate dissociation.

3.3. Storage Capacity of Ethane Gas in Hydrates. Expedi-ency of gas storage in a hydrate state comes from a gas-watermolar ratio and an extremely high density of gas in a hydratestate. The advantages of gas storage in a hydrate state resultfrom a comparison of volumes of gas contained in a free stateand in a hydrate state at various pressures. The volume of gasstored in a unit volume of hydrate at the hydrate formationconditions of pressure and temperature may be calculated bythe expression

VH )103VGF

MH(2)

where VG is the molecular volume of gas and MH is themolecular weight of hydrate and is defined as

MH )M+ 18.02nh (3)

where M is the molecular weight of hydrate forming gas andnh is the hydrate number. The volume of gas stored in a unitvolume of hydrate at standard conditions is defined by theequation

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 givenby the expression19

F)∑ Ni(Mi + 18.02nhi)

∑ 18.02NiVinhi

(5)

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

surface area, the mass-transfer rate also increases, causing ahigher rate of formation. Zhong and Rogers30 also found thatstorage capacity increases with an increase in the SDS concen-tration above 242 ppm. Lin et al.16 found the same trend in thelower concentration range of surfactant, but they reported thatstorage capacity decreases with the SDS concentration in ahigher concentration region (>800 ppm) because of a lowerfinal 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 thedissociation kinetic behavior was manifested by the increasingrate of the system pressure. Three sets of experiments ondissociation with different SDS concentrations were conductedat their corresponding decomposition temperature. Because theamount of hydrate formed in a definite period is different fordifferent pressures, the pressure rise during decomposition willbe proportional to the initial hydrate crystal population. There-fore, the rates of decomposition cannot be directly comparedfor different sets. The mole percentage decomposition curvewith the temperature gives a better comparison of the rates ofdecomposition (Figure 11). The mole percentage decompositionfor definite intervals was calculated by expressing the pressurerise in terms of moles of gas released. The percentages weremade on the basis of moles of gas released in a definite period,divided by the total moles of gas released at the end and thenmultiplying by 100. The dissociation behavior for the threedifferent systems is given in Figure 11. From the figure, it maybe seen that the hydrate dissociation is higher in the presenceof the surfactant compared to the pure water-ethane system.The higher dissociation rate of hydrate in the presence of thesurfactant is due to a finer size of the hydrate particle and ahigher gas content as explained by Ganji et al.2 Similar findingswere also made by Lin et al.16 and Giavarini and Maccioni.4

Certain degrees of supercooling is required to induce nucleationof hydrate formation. Thus, the temperature at which decom-position started was assumed to be the equilibrium hydrate

(27) Sun, Z.; Wang, R.; Ma, R.; Guo, K.; Fan, S. Natural gas storage inhydrate with the presence of promoters. Energy ConVers. Manage. 2003,44, 2733–2742.

(28) Yarranton, H., W.; Hussein, H.; Masliyah, J. Water-in-hydrocarbonemulsions stabilized by asphaltenes at low concentrations. J. ColloidInterface Sci. 2000, 228, 52–63.

(29) Zhang, G.; Rogers, R. E. Ultra-stability of gas hydrates at 1 atmand 268.2 K. Chem. Eng. Sci. 2008, 63, 2066–2074.

(30) Zhong, Y.; Rogers, R. E. Surfactant effects on gas hydrateformation. Chem. Eng. Sci. 2000, 55, 4175–4187.

Figure 9. Rate of gas consumption during nucleation and growth. Figure 10. Influence of the SDS concentration on the hydrate storagecapacity.

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temperature or hydrate formation temperature for the corre-sponding pressure.

4. Conclusion

Experiments were performed to observe the effects of theanionic surfactant, SDS on ethane hydrate formation, dissocia-tion, and storage capacity at a quiescent state. It has been foundthat the pressure-temperature trace for complete formation anddissociation gives a hysteresis loop. For comparison, theexperiments were performed at two different SDS concentrationsalong 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 storagecapacities. The addition of a surfactant in the ethane-water two-

component system changes the energy of the intermolecularinteraction and changes the equilibrium between molecules ofwater and gas. Thus, the presence of SDS shows a thermody-namic effect on hydrate formation, which causes a shift in theformation temperature. The hydrate dissociation rate measuredat the operating pressure and temperature showed that thepresence of SDS lowers the self-preservation effect and increasesthe dissociation rate.

Acknowledgment. We gratefully acknowledge the financialassistance provided by University Grants Commission, Delhi, India,under the Special Assistance Program (SAP) to the Department ofPetroleum Engineering, Indian School of Mines University, Dhan-bad, India. Thanks are also extended to all individuals associatedwith the project.

NomenclatureM ) molecular weight of hydrate former gasMH ) molecular weight of hydrateMi ) molecular weight of ith hydrate former gasn ) number of molesnhi ) hydrate number or ith component of gas in the gas mixturenh ) hydrate numberNi ) molecular fraction of the ith hydrate former gas componentP ) pressureT ) temperatureP0 ) standard pressureT0 ) standard temperatureV ) volumeVG ) molecular volume of gasVH ) volume of gas stored in a unit volume of hydrateVi ) specific volume of water in the hydrate formed by com-

ponent iVSC ) volume of gas stored in a unit volume of the hydrate at

standard conditionsZ ) compression factorF ) density

EF800240N

Figure 11. Dissociation percentage profile of ethane hydrate in thepresence of SDS.

2532 Energy & Fuels, Vol. 22, No. 4, 2008 Mandal and Laik