Effect of Mixed Thermodynamic and Kinetic Hydrate Promoters on

Mar 5, 2013 - In this communication, formation kinetics and stability of methane hydrate in mixtures of (propanone + sodium lauryl sulfate (SDS)), ...
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Effect of Mixed Thermodynamic and Kinetic Hydrate Promoters on Methane Hydrate Phase Boundary and Formation Kinetics Behzad Partoon† and Jafar Javanmardi* Department of Chemical Engineering, Shiraz University of Technology, 71555-313, Shiraz, Iran ABSTRACT: In this communication, formation kinetics and stability of methane hydrate in mixtures of (propanone + sodium lauryl sulfate (SDS)), as thermodynamic and kinetic hydrate promoter, have been studied. Results show that both the induction time and initial formation rate have been increased in the presence of propanone and the mixture of (propanone + SDS). Propanone and mixtures of (propanone + SDS) solutions decrease the self-preservation at low pressures and temperatures below the ice point. Moreover, the presence of propanone decreases the storage capacity of methane hydrate. The addition of SDS, however, increases the experimental storage capacity in comparison with the theoretical value.

1. INTRODUCTION Gas hydrate, a crystalline compound consisting of water molecules and small gas molecules, is a result of strong hydrogen bonding between water molecules. This compound causes the blockage and pressure drop in oil and gas transmission pipelines. Whereas oil and gas industries try to overcome this problem in a cheap and safe way, some researchers investigate its production in terms of economical scales. The ability to store a large amount of natural gas in the hydrate form makes it a good candidate for the storage and safe transport of of natural gas. However, there are so many problems in developing this technology: Hydrate formation is a slow process and normally gas uptake is far from its maximum capacity. Therefore, scientific studies dating to the last decade of 20th century, focus on the methods of increasing the formation rate as well as gas uptake. Promoters are the results of these investigations. Promoters are special chemicals which, being added to water, enhance the hydrate formation process. Depending on their effects, promoters are divided into thermodynamic and kinetic classes. Thermodynamic promoters shift the three phase boundaries of gas hydrate, liquid water and gas, LHV, to the higher temperature/lower pressure while kinetic promoters increase the hydrate formation rate and normally the gas uptake. The promotion effect of propanone, as a hydrate former, on the formation conditions of methane hydrate was studied by Ng and Robinson1 for the first time. At low concentrations, less than 6 mol percent, propanone promotes the formation conditions of methane hydrate. Later, Saito et al.2 showed that low concentrations of a group of water-soluble hydrocarbons can stabilize gas hydrates. Later, Mainusch et al.3 confirmed Ng and Robinson’s finding regarding the effect of propanone (acetone) on the formation conditions of methane hydrate. Their study also showed that the presence of propanone in concentrations lower than 0.06 mol fraction increases the methane hydrate formation temperature. At concentration above 0.3 mol fraction, however, this effect is altered to inhibition. The effect of 1,4-dioxane, a water-soluble © 2013 American Chemical Society

hydrocarbon, on methane hydrate boundary conditions was studied by Jager et al.4 Results confirmed the promotion effect of 1, 4-dioxane on methane hydrate formation conditions. Recently, Illbeigi et al.5 modeled a set of methane hydrate equilibrium data in the presence of propanone solutions as well as other water-soluble and insoluble hydrate formers. Their investigation verified that usage of the SRK equation of state in prediction of (propanone + water + methane hydrate) phase boundaries could lead to more accurate results. In contrast to the thermodynamic promoters, kinetic promoters, as a second type of promoters, do not have any influence on equilibrium conditions. The kinetic promotion effect of surfactants was studied by Kalogerakis et al.6 These additives showed significant effects on the gas solubility in water and therefore, on the rate of hydrate formation. Zhong and Rogers7 have studied the effect of surfactants on ethane hydrate formation rate in a quiescent reactor. They determined that at surfactant concentrations above the critical micellar concentration, CMC, ethane hydrate could be formed up to 700 times faster. They reported that the CMC of an aqueous solution of SDS, also known as sodium dodecyl sulfate, was equal to 2.42 × 10−4 wt fraction at hydrate forming conditions. This value was determined using the hydrate induction time. This relation between formation of micelles in solution and promotion of hydrate formation, however, was refuted by some researchers.8−12 Karaaslan and Parlaktuna13−16 studied the promotion effect of different surfactants and polymers and their concentrations on hydrate formation rate. The effect of surfactants on structures of gas hydrate was studied by Karaaslan et al.17 They concluded that the increase of hydrate formation rate for sII hydrate is relatively small in comparison with sI hydrate. Link et al.18 and Sun et al.19,20 reported that SDS has the greatest effect on the promotion of hydrate Received: April 27, 2012 Accepted: February 19, 2013 Published: March 5, 2013 501

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supersaturated system to remain in the metastable state for a certain time.30 Therefore, this parameter as well as the initial rate of gas hydrate formation depends on the system supersaturation, which is known as the hydrate formation driving force. Arjmandi et al.31 have shown that for a wide pressure range, subcooling is a good representative of pure components hydrate formation driving force, that is, chemical potential difference. In this work, the following procedure is applied to measure this parameter: In each run, 30 cm3 of the aqueous solution is fed into the reactor cell. Then the cell is pressurized by methane. The starting conditions are set at (5.0 ± 0.1) MPa and 288 K for all experiments. This condition is not in the hydrate formation region. Each run consists of three steps. The kinetics of methane hydrate formation is studied during the first step. In this step, temperature is decreased to 275 K in 13 h and remains at this temperature for 12 h. With this small cooling rate, the system temperature passes through the LHV phase boundary very slowly. One method of measuring the induction time is to fix the system temperature below the LHV equilibrium temperature and measure the time required for hydrate formation and crystals agglomeration. In this study, however, the presence of an additive, that is, propanone, affects the LHV equilibrium conditions. So, to achieve consistent and uniform subcooling conditions for all solutions with different concentrations of the additives, the system temperature decreases slowly, about 1 K·h−1, in this way, it can be assumed that the system remains at thermal equilibrium; and for all experimental runs, temperature follows through a uniform temperature history. Consequently, the effect of time on the start of hydrate formation can be studied. In other words, under these conditions this time is lower for systems with lower induction time. The induction time and initial rate of hydrate formation in the presence of different concentrations of propanone and SDS are measured in this step. In the second step, the stability of produced hydrate at atmospheric pressure and 268 K as well as self-preservation are studied. For this purpose, at first, the temperature is decreased to 268 K in 5 h, and then extra gases in the reactor are purged by opening valve 3, V-3 in Figure 1. After that, this valve is closed again and the system remains at this temperature for another 10 h, then the system is warmed to 271 K in about 3 h and remains at this temperature for another 2 h to study the upper margin of the self-preservation region. The details of measurements will be discussed in the Results and Discussion section. In the third step, the system is gradually warmed to 285 K and the LHV equilibrium conditions of methane hydrate in the presence of (propanone + SDS) are measured. 3.2. Storage Capacity. The first procedure is performed at isochoric conditions. Therefore, to study the ultimate storage capacity of methane hydrate in the presence of (propanone + SDS), the second procedure using the same amount of solution, 30 cm3, is introduced. This procedure is at isobaric conditions. The starting condition is set at a regulated pressure of (5.0 ± 0.05) MPa and 275 K. The system is held at this condition for at least 14 h. After that valve 2, V-2 in Figure 1, is closed and the system pressure is monitored for 2 h. In this interval, any pressure drops more than 0.05 MPa will result incomplete cycle of hydrate formation. In this case, valve 2 is opened again for 1 h. After that valve 2 is closed again and the system is monitored for any pressure drop in the next 2 h cycle. These cycles are repeated until no pressure drop is observed. In the next part, the system is cooled down to 263 K in 3 h and

formation rate and gas uptake in comparison with other surfactants. The effects of hydrotropes, the other type of kinetic promoters, were studied by Gnanendran and Amin.21,22 Their investigation showed that hydrotropes increased the hydrate formation rate and gas uptake. They have also claimed that hydrotropes could change the hydrate phase boundaries. This postulate, however, was rejected by Rovetto et al.23 Other researchers also believed that kinetic promoters had no effect on hydrate phase boundaries and only affect the kinetics of hydrate formation6,8,13,14,24 Ganji et al.25 concluded that some surfactants such as hexadecyl-trimethyl-ammonium bromide (CTAB) and 2-(2-nonylphenoxy)ethanol (ENP) had promotion effects on hydrate kinetics only at 0.001 wt fraction and the presence of them in lower concentrations decreases the hydrate formation rate. They also mentioned that the presence of surfactants increased the dissociation rate of gas hydrate. Wang et al.26 also studied the dissociation conditions of methane hydrate in the presence of SDS. Their results revealed that process of hydrate formation has significant effect on the dissociation rate of methane hydrate below the ice point. The effects of propanone as a thermodynamic promoter on the methane hydrate induction time and initial formation rate as well as its effect on self-preservation phenomenon and gas uptake were studied by Partoon et al.27 The induction time is defined as the time elapsed until a detectable volume of hydrate phase forms. The “self-preservation” is a phenomenon that makes hydrate crystals remain stable for an extended period of time beyond their phase boundaries (near atmospheric pressure condition and temperatures lower than the ice point). Storage capacity of gas hydrate is defined as the volume of gas uptake by the hydrate crystals at STP condition per unit volume of hydrate.28 In this work, simultaneous effects of propanone and SDS on these parameters are studied.

2. APPARATUS AND MATERIALS Purities and suppliers of materials used in this work are reported in Table 1. The main part of the experimental setup is Table 1. Purities and Suppliers of Materials Used in This Work chemical

supplier

purity

methane propanone SDSa water

Air Product Merck Merck

0.9995 mol fraction 0.99 wt fraction 0.99 wt fraction deionized water

a

SDS = sodium lauryl sulfate (IUPAC name), also known as sodium dodecyl sulfate.

a stainless-steel equilibrium cell with effective volume of 75 cm3 as shown in Figure 1. The cell temperature is controlled using a thermostatted bath. The stirring system of the equilibrium cell consists of a DC motor equipped with a magnetic impeller with 1000-rpm speed. Temperature and pressure of the cell are measured and recorded on a computer every 30 s. The accuracies of the measured temperatures and pressures are within ± 0.1 K and ± 0.1 MPa, respectively. This rig is described in more details by Javanmardi et al.29

3. EXPERIMENTAL SECTION 3.1. Induction Time, Initial Formation Rate, and Stability. The induction time demonstrates the ability of a 502

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Figure 1. Schematic diagram of the apparatus used in this work. V, valve; R, regulator; T, thermometer AT, ambient temperature thermometer; PT, pressure transducer; PG, pressure gage; DAS, data acquisition system; AC, AC motor; E, exhaust; TB, thermostatted bath; GI, gas in; GO, gas out; C, cell; VP, vacuum pump.

definition, is the time that the system resists phase change in the supersaturation condition. Therefore, the difference between the time associated with the sharp pressure drop and the moment at which the system passed through the equilibrium phase boundary is considered as the induction time. The equilibrium pressure was predicted using the method of Javanmardi et al.32 They assumed that the propanone molecules are entrapped in the hydrate lattice and act as hydrate former. On the basis of this assumption, suitable expressions are needed for the propanone fugacity and its Langmuir constant in the van der Waals and Platteeuw hydrate equation. The propanone fugacity can be correlated using the basic equation of:

remains at this temperature for 2 h. Then extra gases of the system are purged. After that, the system remains at this temperature for 5 h and then is warmed to 268 K in 1 h. Again, the system is held at this temperature for 5 h and then is warmed to 278 K in 2 h. In this way, the stability of the system as well as its response to temperature rising especially around the ice point temperature can be studied. After that, the system is warmed at a rate of 1 K·h−1 until all the hydrate crystals are melted down. The storage capacity of methane hydrate, as the last kinetic parameter, and the LHV equilibrium conditions of methane hydrate are measured in this procedure.

4. RESULTS AND DISCUSSION 4.1. Induction Time and Initial Formation Rate. Figure 2 shows pressure as a function of time for a solution of (0.05

fa = xaγaf ao

(1)

where xa is mole fraction of propanone in liquid phase; this is equal to 1.0 − mole fraction of water − mole fraction of dissolved methane in the liquid phase. γa is the activity coefficient of propanone in the water phase, and foa is the fugacity of propanone in the standard state, that is, pure propanone at temperature and pressure of the system. The fugacity of propanone in the standard state has been estimated by considering the effect of the poynting factor. The optimized Kihara potential parameters have been shown in Table 2. Table 2. Optimized Kihara Potential Parameters for Propanone32

Figure 2. the pressure of the system as a function of time for a mixture of (0.05 mol fraction of propanone +2.5 × 10−4 wt fraction of SDS). ●, equilibrium point; −, pressure profile; IT, induction time; EP, equilibrium point.

2aa/nm

σb/nm

(ε/k)c/K

0.19357

0.28349

306.66

a is the radius of the spherical molecular core. bσ is the collision diameter. cε is the characteristic energy, and k is the Boltzmann constant. a

propanone mole fraction + 2.50 × 10−4 SDS wt fraction), obtained in this work. The system temperature, as discussed in the procedure section, decreases from 288 to 275 K in 13 h. As shown in this figure, the sharp pressure drop indicates the initiation of hydrate formation. The equilibrium hydrate formation pressure, associated to the system temperature, is also shown in this figure. During the cooling part, when the system condition (pressure and temperature) passes through the LHV equilibrium phase boundary, the system is in a subcooling/supersaturating condition. Induction time, by its

Figure 3 shows gas consumption as a function of experiment time for the above solution. The slope of this curve in the beginning of the hydrate formation process is assigned as the initial formation rate of methane hydrate. The induction time can also be determined from this diagram. Figure 4 shows the induction time as a function of propanone concentration for different solutions of SDS. As it seen in this figure, the induction time increases at low 503

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Figure 6 demonstrates the initial rate of gas consumption as a function of propanone concentration. As shown in this figure,

Figure 3. Gas consumption (GC) for the mixture of (0.05 propanone mole fraction +2.5 × 10−4 SDS wt fraction). ●, equilibrium point; −, pressure profile; EP, equilibrium point. Figure 6. Initial rate of gas consumption (IG) as a function of propanone concentration. ■, without SDS; ●, 1.0 × 10−4 SDS wt fraction; ▲, 2.5 × 10−4 SDS wt fraction; ⧫, 5.0 × 10−4 SDS wt fraction SDS.

the presence of propanone at low concentrations, less than 0.03 mol fraction, has no significant effect on the formation rate, but the presence of propane at higher concentrations increases the formation rate.27 As mentioned before, hydrate formation is a stochastic phenomenon. Therefore, with an increase in the amount of propanone molecules in solution, the chance of formation of sII hydrate would increase. This can explain the effect of propanone concentration on the hydrate formation rate. In addition, the presence of SDS, at any concentration of propanone, increases the formation rate of hydrates. This behavior was also reported by other researchers.24,37 It should be noted that SDS even at a very low concentration of 1.0 × 10−4 wt fraction, has considerable effect on the formation rate. This behavior is shown in Figure 6. This is in contrast with the Zhong and Rogers’s statement,7 which claimed that only at concentrations higher than CMC, that is 2.42 × 10−4 wt fraction, would SDS show significant effect on the induction time and formation rate. Consequently, it could be concluded that the formation of micelles in the solution could not be the major reason of hydrate formation promotion by surfactants. It should be noted that SDS as a surfactant can decrease the surface tension of water molecules at the gas− liquid interface. Therefore, gas molecules can enter the liquid phase easily and consequently the gas solubility in the water would increase even at SDS concentrations smaller that CMC. This behavior was also showed by Peng et al.38 in a study of methane solubility in aqueous solutions of SDS at various concentrations. The enhancement in gas solubility could be the reason of increasing gas hydrate formation rate in the presence of SDS. 4.2. Stability. Stability of gas hydrate is an important parameter for potential hydrate application in natural gas transportation or storage. Thus, the feasible hydrate equilibrium conditions are desirable.26,39−45 Somehow, the “self-preservation effect” may be the key parameter for this demand. The “self-preservation” represents the hydrates stability for an extended time beyond the hydrate stable region. This anomalous preservation region of methane hydrate is observed over the temperature range of (242 to 271) K after rapid depressurization of the system to atmospheric pressure. On the basis of the thermodynamic principles, methane hydrate is in an unstable condition and thus all of the hydrate crystals must be

Figure 4. Induction time (IT) as a function of propanone concentration. ■, without SDS; ●, 1.0 × 10−4 SDS wt fraction; ▲, 2.5 × 10−4 SDS wt fraction; ⧫, 5.0 × 10−4 SDS wt fraction.

propanone concentrations until approximately 0.03 mol fraction and then decreases. It should be noticed that the induction time is a stochastic property, particularly at low driving forces.28 This stochastic behavior has been reported by several researchers.33−36 For example, no systematic trend between the induction time and the SDS concentration was observed by Zhang et al.33 as shown in Figure 5. In addition, different values of induction times for natural gas hydrates, as large as 2400 s, have been reported by Tang et al.35 at constant subcooling conditions. As shown in Figure 4, the addition of SDS has no significant and orderly effect on the induction time.

Figure 5. Induction time (IT) at different SDS concentration and 7 MPa pressure, obtained by Zhang et al.33 ■, 271 K first run; ●, 271 K second run; ▲, 274 K first run; ⧫, 274 K second run. 504

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Figure 7. History of methane hydrate dissociation pressure in the presence of propanone obtained from the first procedure. +, pure water; ○, 0.01 propanone mole fraction; ▲, 0.02 propanone mole fraction; ⧫, 0.04 propanone mole fraction; ×, 0.05 propanone mole fraction; solid line, temperature profile.

rapidly melted at this situation. Although, after a short rapid dissociation stage and release of (5 to 20) percent of the total methane in the hydrate sample, the remaining portion remains stable for a long time depending on the dissociation temperature. Some believe that after the rapid dissociation stage, an ice shell forms around the sample and thus methane hydrate is “metastably” preserved.42,46,47 However, this is not the only postulate believed by the scientists. Zhang and Rogers,45 in a study regarding the preservation of methane and natural gas hydrates in the presence of SDS, declared that the stability of the basic hydrate particles is not due to an ice rind. They claimed that the formation of small hydrate particles by the water−surfactant solution would decrease the empty spaces and fractions among the hydrate network. As the strength of the small hydrate particles is greater than that of ice, equilibrium pressures can be better maintained within the defect spaces, and thus the hydrate bulk containing a smaller void has a better capability to sustain high pressures. The ultrastability of methane hydrate in the presence of SDS solution has been studied before.25,45,48 To study the effect of additives used in this work on methane hydrate “selfpreservation”, after purging the gas phase of the reactor at 268 K, the histories of methane dissociation pressure obtained from the first procedure have been recorded. The results are shown in Figures 7 through 9 for (water + propanone), (water + propanone +1.0 × 10−4 wt fraction of SDS), and (water + propanone +2.5 × 10−4 wt fraction of SDS) solutions, respectively. It should be noted that after purging the gas phase, about (5 to 20) percent of hydrates dissociate rapidly. The temperature profile, according to the first procedure discussed in the Experimental Section, is also shown in Figure 7. As shown in Figure 7, the dissociation of methane hydrate without any additive is continued at a constant but very low rate for the first 10 h at 268 K. The same behavior is observed with (1.0 × 10−4 and 2.5 × 10−4) wt fraction of SDS solutions, Figures 8 and 9. The dissociation rate, however, is higher than that of pure water. This behavior has been previously reported by other researchers for different surfactants and other SDS concentrations.25,49

Figure 8. History of methane hydrates dissociation pressure and temperature in the presence of propanone and 1.0 × 10−4 SDS wt fraction obtained from the first procedure. +, without propanone; ○, 0.01 propanone mole fraction; ▲, 0.02 propanone mole fraction; ⧫, 0.04 propanone mole fraction; ×, 0.05 propanone mole fraction; solid line, temperature profile.

Figure 9. History of methane hydrates dissociation pressure and temperature in the presence of propanone and 2.5 × 10−4 SDS wt fraction obtained from the first procedure. +, without propanone; ○, 0.01 propanone mole fraction; ▲, 0.02 propanone mole fraction; ⧫, 0.04 propanone mole fraction; ×, 0.05 propanone mole fraction; solid line, temperature profile. 505

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In the presence of propanone, however, the dissociation behavior is different. At first, about 20% of hydrates are dissociated after the depressurizing step and regardless of the presence of SDS no more considerable dissociation is observed at 268 K. As shown in Figure 7, for pure water, the rate of dissociation is very low until near 273 K. After that, a sharp dissociation is observed and all of the hydrate crystals are melted. In addition, in the second constant temperature interval, 271 K for 2 h, no significant change of the dissociation rate is detected for pure water. The same behaviors are observed for (1.0 × 10−4 and 2.5 × 10−4) wt fraction of SDS solutions in the absence of propanone as shown in Figures 8 and 9. Their dissociation rates, however, are higher than that of pure water. Stern et al.,44 during the study of the methane hydrate self-preservation data reported by different research groups, mentioned that “a common and substantiating observation is that the warming of the preserved material through the ice point induces both ice melting and full decomposition of the residual hydrate.” Later, the same behavior for sII natural gas hydrate dissociation was reported by Zhang and Rogers.45 The presence of propanone, however, changes the above observation as shown in Figures 7 through 9: the dissociation of hydrates starts at temperatures above 268 K. At the second constant temperature interval, 271 K for 2 h, the hydrate phase remains almost stable and dissociation is approximately equal to zero, either in the presence or absence of SDS. It should be noted that no significant effect on dissociation rate is observed near the ice point and the dissociation rate is proportional to the system temperature after this point. Figure 10 shows the dissociation history of methane hydrate based on the second procedure after purging the reactor gas phase at 263 K. As shown in this figure, the dissociation of methane hydrate without any additive is continued at a constant but very low rate even during the temperature increasing step, that is, the fifth hour to sixth hour, and all of the hydrate phase dissociates above the ice point temperature. A solution with 1.0 × 10−4 wt fraction of SDS shows the same behavior, although the produced pressure is higher than that of pure water as a result of the presence of the kinetic promoter. Therefore, in the absence of propanone, no significant change in dissociation is observed until near the ice point; after that, a very rapid dissociation occurs and for the case of pure water, all of hydrate crystals are melted, and then pressure remains constant. For 1.0 × 10−4 wt fraction of SDS solution, however, pressure is built up around 4.0 MPa and rapid dissociation is terminated due to reaching the equilibrium hydrate formation pressure. This observation is consistent with the first procedure. In the presence of propanone, after the initial dissociation, no more decomposition occurs at 263 K. As shown in Figure 10, the stability of the remaining hydrates depends on temperature; that is, more hydrate is dissociated when the temperature increases to 268 K during the fifth to sixth hours after the purging step. The same trend is observed in the second warming step to 278 K, and no significant change in the dissociation rate is distinguished around 273 K. These observations are in contrast to that of pure water or SDS solution. Consequently, it can be concluded that the presence of propanone can change the stability profile of methane hydrate at temperatures below the ice point in the self-preservation region. However, the variation would lead somehow to more

Figure 10. History of methane hydrate dissociation pressure and temperature according to the second procedure. □, pure water; ■, 1.0 × 10−4 SDS wt fraction solution; Δ, 0.02 mol fraction propanone solution; ▲, 0.02 mol fraction propanone +1.0 × 10−4 SDS wt fraction solution; ○, 0.05 mol fraction propanone solution; ●, 0.05 mol fraction propanone +1.0 × 10−4 SDS wt fraction solution; solid line, temperature profile.

stable conditions such as the transportation of methane in hydrate form. 4.3. Storage Capacity. The storage capacity parameter is measured from the second procedure of experiments. Figure 11 shows the experimentally measured standard volume of methane per unit volume of gas hydrate as a function of propanone concentration for (0 and 1.0 × 10−4) wt fraction of SDS solutions. Owing to the first procedure results, in these

Figure 11. Storage capacity (SG) of methane hydrate as a function of propanone concentration. ■, pure water; ●, 1.0 × 10−4 SDS wt fraction solution; ▲, theoretically calculated. 506

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concentrations, kinetic parameters are not an appreciable function of SDS concentration. However, in the absence of propanone, as shown in Figures 4 and 6, SDS aqueous solution with 1.0 × 10−4 wt fraction shows the smallest induction time, while the initial formation rate is almost equal for different concentrations of SDS. Therefore, the second series of experiments were conducted only with 1.0 × 10−4 wt fraction of SDS in the solution. As shown in Figure 11, the presence of SDS significantly increases the storage capacity of hydrate in any concentration of propanone. This behavior can be expected from SDS as a kinetic hydrate promoter. Decreasing methane uptake with propanone concentration can be explained by the occupation of some of the large cavities of sII hydrate with propanone molecules. Some researchers believe that propanone changes the structure of methane hydrate to sII.5,50 In the absence of SDS, however, propanone concentration has no significant effect on the methane uptake. Theoretically calculated values of methane uptake for different propanone concentrations are also shown in Figure 11. To obtain these values it is assumed that all water molecules in the solution are converted to sII hydrate crystals. This assumption would lead to a maximum storage capacity around 174 sm3 of methane per unit volume of hydrate. It is also assumed that propanone molecules occupy the large cavities of sII hydrate as much as they are present in the solutions. As shown in this figure, SDS increases the storage capacity in comparison to the theoretical values. Thus, it seems that the presence of SDS can help methane molecules to be stored in large cavities of sII hydrate more than the expectation. 4.4. Equilibrium Data. The last parameter which can be measured from each run of experiments is the methane hydrate equilibrium condition for the system of (methane + water + propanone). These values have been measured based on the pressure search method and heating curve obtained in each run. Experimental data have also been predicted using the model of Javanmardi et al.32 These data and calculation results are given in Table 3. As it was mentioned earlier, the presence of SDS has no effect on the thermodynamic equilibrium condition of gas hydrate. The same phenomenon is observed in this study. Therefore, the reported data are just tabulated against propanone concentration, while some of the reported data are measured in the presence of (propanone + SDS). Additionally, in Figure 12 the results are compared with some available literature data. As shown in this figure, the measured data are in good agreement with literature.

Table 3. Experimental and Calculated Hydrate Dissociation Temperatures, (Texp) and (Tcal), at Different Pressures (P) and Propanone Mole Fraction (x2) for the System of Methane (1) + Propanone (2) + Water (3) x2a

5. CONCLUSIONS Experimental study on methane hydrate kinetics, stability and storage capacity in the presence of propanone as a thermodynamic promoter and SDS as a kinetic promoter has been performed. Results have shown that the presence of propanone in the liquid phase increases the induction time of methane hydrate formation. At low concentrations, approximately up to 0.03 mol fraction, the induction time has increased and at higher concentrations this parameter has decreased due to the presence of propanone. In addition, the induction time of the mixtures of (propanone + SDS) solutions have not shown any considerable difference with propanone solutions. The presence of propanone at low concentrations, less than 0.03 mol fractions, has not had any significant effect on the formation rate, but at higher concentrations it has increased it.

Pb/MPa

Texpc/K

Tcald/K

AAEe/K

AEf/K

0.010 0.010 0.010 0.010 0.010

1.09 1.38 2.95 2.98 9.72

273.6 274.4 280.0 279.7 289.3

272.9 274.5 280.0 280.1 289.1

0.28

0.08

0.020 0.020 0.020 0.020 0.020 0.020 0.020

1.03 2.03 2.79 2.98 3.17 3.21 9.26

274.5 278.7 281.5 281.8 282.0 282.4 290.5

274.6 279.1 281.3 281.8 282.2 282.3 290.1

0.20

0.00

0.030 0.030 0.030 0.030 0.030

1.52 1.84 3.25 3.36 8.63

279.0 279.6 283.3 283.4 290.8

278.3 279.6 283.1 283.4 290.2

0.30

0.30

0.041 0.041 0.041 0.041 0.041 0.041

1.52 1.84 3.40 3.45 3.58 8.23

279.0 279.6 283.8 284.1 284.4 290.6

278.3 279.6 283.8 283.9 284.2 290.1

0.27

0.27

0.050 0.050 0.050 0.050 0.050 0.050 0.050

2.27 3.73 3.85 4.01 4.55 4.57 7.90

281.6 284.9 285.2 285.5 286.3 286.5 289.6

281.1 284.5 284.8 285.0 285.9 286.0 289.9

0.43

0.34

0.070 0.070 0.070 0.070 0.070 0.070 0.070 Overall

1.99 2.70 2.97 3.04 3.31 3.36 3.50

280.6 282.7 282.8 283.2 284.0 284.0 284.4

280.1 282.1 282.8 283.0 283.5 283.7 283.9

0.37

0.37

0.30

0.22

The uncertainties U(x) in the reported mole fractions are about ± 0.001. bThe uncertainties U(P) in the reported pressures are about ± 0.01 MPa. cThe uncertainties U(T) in the reported temperature are about ± 0.1 K. dCalculated hydrate dissociation temperature based on the model of Javanmardi et al.32 eAverage absolute error (AAE) = 1/N ∑|Texp − Tcal|. fAverage error (AE) = 1/N ∑(Texp − Tcal). a

In addition, the presence of SDS with any concentration of propanone has increased the formation rate of hydrate. The presence of propanone in the solution has increased the stability of methane hydrate at constant temperatures of 263 K and 268 K. However, methane hydrate in the presence of propanone has not shown any self-preservation. The presence of propanone has decreased the gas uptake of methane by hydrate crystals, but the addition of SDS helped the 507

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Figure 12. Comparison of measured HLwV of methane + water + propanone hydrate in this work with literature data. (a) ■, 0.0098 wt fraction, Ng and Robinson1; ●, 0.0167 wt fraction, Mainush et al.3; ▲, 0.0102 wt fraction, this work; (b) ■, 0.0333 wt fraction, Ng and Robinson1; ●, 0.0333 wt fraction, Mainush et al.3; ▲, 0.0300 wt fraction, this work; (c) ■, 0.0519 wt fraction, Ng and Robinson1; ●, 0.0528 wt fraction, Mainush et al.3; ▲, 00500 wt fraction, this work; (d) ●, 0.0721 wt fraction, Mainush et al.3; ▲, 0.0700 wt fraction, this work. Hydrates of Methane Plus Water-Soluble or -Insoluble Hydrate Former. Ind. Eng. Chem. Res. 2011, 50, 9437−9450. (6) Kalogerakis, N.; Jamaluddin, A. K. M.; Dholabhai, P. D.; Bishnoi, P. R. In Effects of Surfactants on Hydrate Formation Kinetics. SPE International Symposium on Oilfield Chemistry, New Orleans, March 2−5; New Orleans, 1993. (7) Zhong, Y.; Rogers, R. E. Surfactant Effects on Gas Hydrate Formation. Chem. Eng. Sci. 2000, 55, 4175−4187. (8) 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. (9) Di Profio, P.; Arca, S.; Germani, R.; Savelli, G. Surfactant Promoting Effects on Clathrate Hydrate Formation: Are Micelles Really Involved? Chem. Eng. Sci. 2005, 60, 4141−4145. (10) Watanabe, K.; Imai, S.; Mori, Y. H. Surfactant Effects on Hydrate Formation in an Unstirred Gas/Liquid System: An Experimental Study Using HFC-32 and Sodium Dodecyl Sulfate. Chem. Eng. Sci. 2005, 60, 4846−4857. (11) Watanabe, K.; Niwa, S.; Mori, Y. H. Surface Tensions of Aqueous Solutions of Sodium Alkyl Sulfates in Contact with Methane under Hydrate-Forming Conditions. J. Chem. Eng. Data 2005, 50, 1672−1676. (12) Zhang, J. S.; Lee, S.; Lee, J. Does SDS Micellize under Methane Hydrate-Forming Conditions Below the Normal Krafft Point? J. Colloid Interface Sci. 2007, 315, 313−318. (13) Karaaslan, U.; Parlaktuna, M. Surfactants as Hydrate Promoters? Energy Fuels 2000, 14, 1103−1107. (14) Karaaslan, U.; Parlaktuna, M. Effect of Surfactants on Hydrate Formation Rate. Ann. New York Acad. Sci. 2000, 912, 735−743. (15) Karaaslan, U.; Parlaktuna, M. On the Dissociation of Natural Gas Hydrates from Surfactant Solutions. Energy Fuels 2001, 15, 241− 246. (16) Karaaslan, U.; Parlaktuna, M. Promotion Effect of Polymers and Surfactants on Hydrate Formation Rate. Energy Fuels 2002, 16, 1413− 1416.

methane molecules to occupy some cages that were expected to be occupied with propanone molecules. Some equilibrium data for methane−propanone hydrate have been presented as a final result of these experiments.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +98-711-7279416. Fax: +98-711-7354520. Present Address †

(B.P.) Phase Separation Laboratory, Research Center for CO2 Capture, Chemical Engineering Department, Universiti Teknologi PETRONAS, 31750 Tronoh, Perak, Malaysia. Notes

The authors declare no competing financial interest.

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