Methane Hydrate Formation and Dissociation in ... - ACS Publications

Article Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Methane Hydrate Formation and Dissociation in Synperonic PE/F127, CTAB, and SDS Surfactant Solutions Hugo I. Pérez-López,† Octavio Elizalde-Solis,*,† Juan Ramon Avendaño-Gómez,† Abel Zúñiga-Moreno,‡ and Felipe Sanchez-Minero† †

Departamento de Ingeniería Química Petrolera and Sección de Estudios de Posgrado e Investigación, Escuela Superior de Ingeniería Química e Industrias Extractivas, Instituto Politécnico Nacional, UPALM, Ed. 8, Lindavista, 07738, Ciudad de México, México ‡ Departamento de Ingeniería Química Industrial, Laboratorio de Investigación en Fisicoquímica y Materiales, Escuela Superior de Ingeniería Química e Industrias Extractivas, Instituto Politécnico Nacional, Edif. Z-5, 2° piso, UPALM, Lindavista, 07738, Ciudad de México, México S Supporting Information *

ABSTRACT: Formation and dissociation conditions for methane hydrates were investigated in the presence of aqueous solutions of a no-ionic triblock copolymer synperonic PE/F127 (PE/F127) at 377, 710, 1530 ppm with the aim of evaluating the promotion activity. A cationic cetyltrimethylammonium bromide (CTAB) and an anionic sodium dodecyl sulfate (SDS) surfactant were also tested and conducted at 356, 705, 1500 ppm and 376, 701, 1510 ppm respectively. Experiments with the three surfactants were carried out in an isochoric autoclave under the same conditions: near the ice normal melting point (273.15 K), as the cooling temperature of the autoclave, and five initial pressures stated from 3.5 to 12.0 MPa. Performance of these surfactants was assessed by reporting hydrate−liquid water−vapor (H−L−V) phase equilibria, enthalpies of dissociation estimated using the Clausius− Clapeyron equation, induction times, crystallization temperatures, growth rates and theoretical methane consumptions. In brief, the obtained induction time for PE/F127 was higher than CTAB but lower than SDS, the growth rate between PE/F127 and SDS were similar at lower concentrations, and the maximum gas consumption for PE/F127 was reached at 1530 ppm but this methane uptake was far from those results using SDS.

1. INTRODUCTION

formation in pipelines or to promote its formation for applications as storage and transport of methane. Surfactants have been used in order to promote or to inhibit the hydrate formation. These are usually water soluble and are dosed at low concentrations ranging from 0.1 to 1.0 wt %.3 Several authors have analyzed multiple effects of surfactants on gas hydrates in depth.5−29 SDS is one of the most studied additives on methane hydrates in a wide range of pressure and concentration because this anionic additive has an effective promotion activity. It has been determined that the induction time decreases with SDS in methane hydrates.5−10 This effect was ascribed to the increase on contact area using 300 ppm of SDS at 277.15 K, 6.0 and 7.0 MPa,6 and the reduction of methane mole fraction in the aqueous phase at 274.15 K and 4.2 MPa.5,8 Also, a nonsignificant but systematic decreasing was observed on

Gas hydrates are solid compounds crystallized from water molecules; gas molecules are surrounded by these water molecules.1 Cavities are formed by hydrogen bonds shaping different structures depending on the size of the guest molecule. In nature or under induced conditions, gas hydrate formation requires special environments (i.e., low temperatures and high pressures). Besides, the hydrate−liquid water−vapor phase equilibrium is achieved with an energy requirement known as enthalpy of dissociation. This depends on hydrogen bonds as well as on the cavities formed.2 At present, gas and oil industries are facing an important problem regarding gas hydrates formation because pipelines blocking causes significant losses in production.3 Natural gas extracted from deep underground is mainly formed of methane (>85%) and other compounds constituted by maximum six carbon atoms on their paraffinic chain. Methane is a greenhouse gas and has a valuable end-use. Hydrates could be an alternative for natural gas extraction, gas storage, and transportation.4 For these reasons, the study of hydrates formation or dissociation is relevant either to avoid its © XXXX American Chemical Society

Special Issue: Emerging Investigators Received: November 17, 2017 Accepted: April 19, 2018

A

DOI: 10.1021/acs.jced.7b01009 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

induction time as the surfactant concentration increased at 6 MPa.11 Opposite variations on this stage were observed with different SDS concentrations. Inhibition activity seemed to take place at about 86.5 ppm of SDS, whereas a promotion activity was observed at higher concentrations.12 Nevertheless, in the vast majority induction time did not have a systematic trend with concentration changes since its stochastic nature. The presence of SDS foam was reported to accelerate the nucleation stage.13 Critical micelle concentration (CMC) under gas hydrates conditions was proposed to be measured as the point where the induction time trend changed drastically as the surfactant concentration decreased. It was reported to be 242 ppm for SDS under gas hydrate conditions in a quiescent system.14 Methane hydrate formation rate was promoted using SDS solutions under stirring or even in quiescent conditions.7−10,14−19 Maximum growth rate using SDS was reported to be at about 500 ppm.20−23 In general, some surfactants enhance this rate with no agitation requirements in comparison with pure water under stirring.14,15,19 SDS is considered a promoter because of the improvement on the amount of water converted to hydrate.5,10,16,24,25 The maximum storage capacity was attained at 500 ppm of SDS21 in an unstirred system10 or 650 ppm below the ice point.18 Despite some studies that ensure the gas storage capacity was better using 1000 ppm of CTAB in comparison with the studied SDS concentrations,21 SDS is an excellent promoter for methane hydrates over other surfactants. In summary, several parameters have influence on gas hydrates. Then, the effect of surfactants requires sufficient exploration since the subcooling temperature, pressure formation, supersaturation conditions, and memory effect are involved on the driving forces that affect the nucleation, growth, and formation kinetics. Finally, the dissociation conditions on gas hydrates are expected to be kept using these additives at low dosage. This work contributes to study the performance of three surfactants, a no-ionic triblock copolymer PE/F127, a cationic CTAB, and an anionic SDS on the formation and dissociation processes for methane hydrates under the same experimental conditions. Hydrate formation using SDS and CTAB has been reported elsewhere; then, those results serve for indirect comparison. The evaluated parameters were enthalpy of dissociation, induction time, gas consumption, growth rate, and crystallization temperature. These were analyzed from experiments performed twice. Additives were used at low concentrations in a wide range of initial pressure and a cooling temperature at the ice melting point.

Table 1. Characteristics of Surfactants name

ion

PE/ F127 CTAB

nonionic polymeric cationic

SDS

anionic

a

characteristic

MW (g·mol−1)

CMC (ppm)

flakes

12600a

white powder white powder

364.46

40.3 (this work) 35.3b

288.38

2364.7b

Reference 30. bReference 31.

Apparatus. The experimental set up is shown in Figure 1. The main device was a cylindrical autoclave (1) made of

Figure 1. Schematic diagram of the experimental setup. 1, cylindrical autoclave; 2, thermoregulated reservoir; 3, temperature liquid bath controller; 4, external motor; 5, temperature indicator; 6, pressure transducer; 7, multimeter; 8, manual syringe pump; 9, personal computer.

stainless steel with 25 cm3. The setup was constituted by a magnetic bar located into the autoclave, a thermo-regulated reservoir (2), a temperature liquid bath controller (3) which used water−methanol 60%−40% solution as thermal fluid, and an external motor (4). Temperature was measured using a calibrated temperature probe inserted on the top of the autoclave and coupled to an indicator (5). Pressure was measured by means of a transducer (6) coupled to a multimeter (7), a manual syringe pump (8) and a computer for the data acquisition system (9) developed in python free-software. Temperature and pressure measurements were made with expanded uncertainties of ±0.01 K and ±0.1 MPa, respectively. Procedure. Experiments were based on the well-known isochoric method,2 which the gas hydrate formation and dissociation conditions can be determined by monitoring pressure and temperature variables. First, the aqueous surfactant solutions were synthesized at known concentration. Three different concentrations were prepared at about 350, 700, and 1500 ppm for each one (PE/ F127, CTAB, and SDS). Then, 11 cm3 of the surfactant solution with known concentration was poured into the autoclave, which was previously washed and air-dried. The sealed autoclave was connected to the additional devices and was submerged into the liquid reservoir. Air was evacuated with a vacuum pump from the autoclave and it was flushed three times with methane. The system was kept at the initial operating temperature of 293.15 K by means of the temperature controller and the gas was fed to the autoclave until the lowest required initial pressure reached 3.5 MPa. The

2. EXPERIMENTAL SECTION Materials. Gas Innovations supplied methane with a purity of 99.99%. Cetyltrimethylammonium bromide (CTAB) and sodium dodecyl sulfate (SDS) were purchased from SigmaAldrich with purities of 98 and 95%, respectively. The nonionic copolymer (Synperonic PE/F127) was manufactured by Uniqema (Germany). Characteristics of additives are listed in Table 1.30,31 Water was deionized in a Barnstead Easypure II RF obtaining a resistivity of 18.2 MΩ·cm. The CMC = 40.3 ppm for PE/F127 was experimentally determined by means of a force tensiometer (KRÜ SS, Model K20) at atmospheric pressure and 293.15 K. The CMC method suggested finding a slope change in the surfactant concentration against the interfacial tension plot. B



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increasing to reach initial conditions in A. Both trends were fitted to lineal equations and the dissociation point was determined by their intersection because the slope change occurred at this point. Temperature was proportional to pressure on the (H−L−V) phase equilibria for each surfactant concentration and these data are shown in Table 2. PE/F127 and the other surfactants did not act as thermodynamic promoters or inhibitors at the studied concentrations since dissociation conditions did not change respect to the methane hydrate system at zero concentration. The surfactant added in low concentration acted in the gas−liquid interface and did not affect the breakdown of hydrogen bonds from water molecules that constitute the crystal structure throughout the dissociation process. Equilibrium data are plotted in Figure 3. They seemed to converge in a single trend. Instead of experimental calorimetry, the enthalpy of dissociation (ΔHdis) was obtained indirectly using the Clausius−Clapeyron equation applied to nonideal gases

autoclave was made airtight and the external motor was turned on to activate the internal magnetic bar for a better mass transfer. Stirring was adjusted to 500 rpm from the initial conditions up to reach growing process. During this last process, stirring was blocked due to agglomeration of hydrate crystals in some cases. Initial pressure and temperature were kept stable. The autoclave was cooled at 273.15 K. This path is indicated from point A to B in Figure 2. Growth of the crystalline phase started

d ln p d

( T1 )

=−

ΔH dis ZR

(1)

where p, T, R, and Z denote pressure, temperature, the universal gas constant and the compressibility factor, respectively. The enthalpy of dissociation listed in Table 2 was quite similar among the experiments performed at the same initial pressure for any surfactant concentration. Enthalpy of dissociation is an endothermic process, then it was positive. Hydrate Formation. Induction time (tind) for this work is defined as the lifetime where a supersaturated system remains under metastable state.32,33 This period where the nucleation process takes place is indicated in Figure 2 between point D (dissociation) and B (initial crystallization). In this path, the difference between dissociation and initial crystallization is also known as the subcooling temperature. It affects the driving forces of hydrate formation since a greater value on subcooling temperature causes that the induction period might be less. The induction times are listed in Table 3. These were analyzed with respect to the surfactant concentration and initial pressure. These results corresponded to the average of two runs using the same sample; nevertheless, five initial conditions were randomly replicated three times. As reported elsewhere,9,11,28 it was expected that this period was not reproducible after several runs due to its stochastic nature. Some features such as autoclave volume, stirring conditions, and cooling rate might influence this phenomenon. For almost all the experiments, nucleation period for the second run was shorter than the first one, which confirms the memory effect reported elsewhere.27,33 Two phenomena could contribute before starting the second run: (1) microscopic residues of crystals are remaining on the system, and (2) a possible increase of methane quantity in the liquid phase that is inducing supersaturated aqueous phase. In this instance, the required subcooling temperature as well as the corresponding depressurizing in the growth could be less. Induction time was about the same between the absence and lowest surfactant concentration because of this concentration might not be sufficient to modify interfacial tension; on the other hand, this period reached maximum values at the highest concentration in all cases. This nucleation stage seemed to be greater as the concentration was raised. However, the standard deviation for each induction time indicates that the error bars

Figure 2. A typical P−T projection for methane hydrate formation and dissociation using PE/F127 at 376 ppm. Segments refer to stages: AD, solubility; DB, nucleation; BC, growth; CD, dissociation; DA, initial conditions recovery.

after point B and finished by point C. Maximum storage capacity was observed at point C where the measured variables achieved stable conditions. Afterward, the heating process was carried out at 0.5 K·h−1. Temperature increased and pressure slightly raised. Then, it exhibited a steep increment until the dissociation heat was sufficient to break down the first hydrate crystals. Later, pressure and temperature were raising to reach point D, which was considered as the phase equilibrium point. The heating was still carried out to return to the initial conditions. Experiments were repeated once for each pressure. Methane was again fed to the autoclave to reach the next initial pressures as follows: 5.5, 8.0, 10.0, and 12.0 MPa.

3. RESULTS AND DISCUSSION Formation and dissociation process for methane hydrates was investigated systematically in the presence of PE/F127, CTAB and SDS. This study was conducted near the ice normal melting point (273.15 K) covering the initial pressures interval of 3.5−12 MPa. Results stored in the acquisition system were assessed to determine phase equilibrium condition, dissociation enthalpy, induction time, crystallization temperature, growth rate, and theoretical gas consumption. H−L−V phase equilibrium data for methane + water, nitrogen + water, and carbon dioxide + water were previously measured using this setup in order to validate the methodology used. Hydrate Dissociation. After pressure and temperature stabilization in C, hydrate crystals broke down when the dissociation heat was enough. It allowed pressure increments because the gas was released from the cages. Pressure and temperature trend had a drastic slope change when the last hydrate crystal was dissociated but both variables were still C



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Table 2. Hydrate Equilibrium Conditions (H−L−V) for Methane + Water + Surfactant Systemsa C ppm

p MPa

377

3.02 5.42 7.76 9.63 12.12 3.37 5.16 7.66 9.63 11.72 3.27 5.18 7.65 9.64 11.67

710

1530

376

701

1510

a

TK

Z

Synperonic PE/F127 (nonionic) 275.16 0.916 280.95 0.866 284.37 0.828 286.22 0.806 288.19 0.787 276.35 0.908 280.47 0.871 284.27 0.830 286.12 0.805 287.99 0.789 276.05 0.911 280.59 0.871 284.17 0.830 286.24 0.806 287.77 0.789 SDS (anionic) 2.98 274.52 0.917 5.35 280.47 0.867 7.50 283.73 0.831 9.66 285.98 0.805 10.82 286.94 0.794 3.31 276.07 0.910 5.17 280.57 0.871 7.61 284.20 0.831 9.58 286.15 0.806 11.65 287.74 0.789 3.24 275.61 0.911 5.24 280.44 0.869 7.68 284.16 0.829 9.61 286.18 0.806 11.65 288.19 0.790

ΔHdis kJ·mol−1

C ppm

p MPa

7.146 8.080 8.525 8.758 9.042 7.342 8.013 8.510 8.752 9.000 7.290 8.022 8.504 8.760 8.983

356

2.99 5.17 7.54 9.69 11.74 3.27 5.16 7.65 9.69 11.62 3.26 5.23 7.68 9.63 11.65

705

1500

7.103 8.047 8.469 8.745 8.876 7.307 8.020 8.503 8.750 8.980 7.263 8.026 8.506 8.754 9.008

0

3.27 5.19 7.63 9.72 11.69

TK

Z

CTAB (cationic) 275.11 0.917 280.47 0.871 283.93 0.831 286.23 0.805 288.15 0.789 275.97 0.910 280.46 0.871 284.13 0.830 286.19 0.805 287.74 0.789 275.80 0.911 280.59 0.870 284.14 0.829 286.18 0.806 287.78 0.789 water 276.09 0.911 280.69 0.871 284.12 0.830 286.18 0.804 287.86 0.789

ΔHdis kJ·mol−1 7.128 8.014 8.482 8.762 9.011 7.288 8.012 8.502 8.760 8.978 7.274 8.031 8.505 8.756 8.983 7.292 8.029 8.500 8.761 8.990

The combined expanded uncertainty u(T) = 0.01 K, u(p) = 0.1 MPa. with 0.95 level of confidence (k ≈ 2).

between values for PE/F127 and CTAB at the same concentration except for PE/F127 at 1500 ppm and 3.5 MPa, it was 5.7 h as the maximum value. The crystallization temperature (Tc) as a function of the concentration for each surfactant are charted in Figures 4 to 6. The required crystallization temperature decreased as the concentration was raised at constant pressure; however, the subcooling temperature did not increase and did not have a systematic trend since the stochastic nature of nucleation. A suitable trend was observed at constant concentration and the crystallization temperature raised as initial pressure was high; meanwhile the subcooling temperature also exhibited an apparent increment. It meant an increment on the driving force caused by the subcooling temperature. Temperature and pressure sets from point B to C in Figure 2 allowed one to estimate the gas consumption. Methane was captured into the hydrate structures built by water molecules cavities. The end of pressure drops stood for the maximum quantity of methane captured into the hydrate cages. The main balance to calculate the theoretical mole number of the consumed methane was based by the nonideal gases law

Figure 3. H−L−V phase equilibria for methane + water + surfactant systems.

overlap between the values for each concentration; two runs are not so sufficient to support trends in our induction time. High initial pressures exerted the effect by shortening the nucleation period because of an increment of driving forces (the difference between the initial and the equilibrium pressure). Consequently, the elevated values for induction time were observed at the lowest initial pressure. Induction time was almost similar

ΔN = Nt = 0 − Nt

ΔN = D

⎛ pv ⎞ ⎛ pv ⎞ ⎜ ⎟ ⎟ −⎜ ⎝ ZRT ⎠t = 0 ⎝ ZRT ⎠t

(2)

(3) DOI: 10.1021/acs.jced.7b01009 J. Chem. Eng. Data XXXX, XXX, XXX−XXX


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Table 3. Induction Time for Methane Hydrate C ppm

tind h

pi MPa

Synperonic PE/F127 (nonionic) 377 1.1 ± 0.23 3.3 1.2 ± 0.34 5.7 0.2 ± 0.05a 8.1 0.3 ± 0.08 10.0 0.4 ± 0.25 12.5 710 2.9 ± 0.50 3.6 2.2 ± 0.43a 5.5 1.5 ± 0.34 8.0 1.4 ± 0.25 10.0 1.2 ± 0.12 12.1 1530 5.7 ± 0.89 3.5 2.4 ± 0.11 5.5 2.0 ± 0.02 8.0 2.4 ± 0.33 10.0 1.6 ± 0.30 12.0 SDS (anionic) 376 1.6 ± 0.31 3.3 0.6 ± 0.18 5.7 0.3 ± 0.11 7.9 0.6 ± 0.12 10.1 1.1 ± 0.21 11.2 701 11.5 ± 0.56 3.6 1.7 ± 0.29 5.5 2.1 ± 0.22 8.0 0.8 ± 0.34a 10.0 3.5 ± 1.16 12.1 1510 20.7 ± 8.23 3.5 2.9 ± 0.27 5.6 2.1 ± 0.05 8.0 2.1 ± 0.52 10.0 1.9 ± 0.20 12.0 a

C ppm 356

705

1500

0

tind h CTAB (cationic) 3.3 ± 0.19 1.1 ± 0.14 0.6 ± 0.05a 0.6 ± 0.15 0.6 ± 0.09 3.8 ± 0.28 1.2 ± 0.21 1.7 ± 0.33 0.9 ± 0.24 1.0 ± 0.11 4.4 ± 0.74 3.4 ± 0.35 3.6 ± 0.68 2.4 ± 0.30 1.6 ± 0.04 water 2.6 ± 0.53 0.8 ± 0.17 0.9 ± 0.38a 0.4 ± 0.08 1.0 ± 0.27

pi MPa 3.2 5.5 8.0 10.1 12.1 3.5 5.5 8.0 10.1 12.0 3.5 5.5 8.0 10.0 12.0

Figure 5. Crystallization temperature (Tc) for methane hydrate using CTAB at different initial pressures.

3.5 5.5 8.1 10.1 12.1

Measurements replicated thrice. Figure 6. Crystallization temperature (Tc) for methane hydrate using SDS at different initial pressures.

hydrate, it was calculated based on the procedure suggested by Pang et al.34 The compressibility factor was calculated by the Peng−Robinson equation of state35 Z3 − (1 − B)Z2 + (A − 3B2 − 2B)Z − (AB − B2 − B3) = 0 (4)

where the A and B parameters are dependent on reduced properties denoted with the subscript r, and the α parameter is a function of the acentric factor ω. αp A = 0.45724 2r Tr (5) B = 0.0778 Figure 4. Crystallization temperature (Tc) for methane hydrate using PE/F127 at different initial pressures.

pr Tr

(6)

α = [1 + (0.37464 + 1.54226ω − 0.26992ω2)(1 − Tr0.5)]2 (7)

where ΔN is the variation of mole number assumed as the theoretical gas consumption after certain period, Nt=0 is the initial methane mole number that represent the initial growth depicted as the point B in Figure 2, and N is the methane mole number at any time t. The gas volume, v, into the autoclave changed as a consequence of phase change from liquid water to

A typical methane hydrate consumption in the presence of PE/F127 at 300 ppm as a function of time is shown in Figure 7. Maximum value was achieved faster for the lowest initial pressure against the highest one. Methane consumption for hydrate formation is implicated directly with nucleation where the surfactant in aqueous solution acts as core during the E



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Figure 9. Methane consumption in CTAB solutions at different initial pressures.

Figure 7. Methane consumption evolution in the presence of CPE/F127 = 300 ppm.

crystallization process. Gas uptake and elapsed time showed good results if PE/F127 would be used for storage purposes. The gas consumption might be enhanced at high initial pressure. This behavior is observed for the initial pressures of 3.5, 5.5, 8.0, and 12 MPa with the exception of 10 MPa where the consumption is similar to the obtained for 8.0 MPa. We verified stabilization at the end of formation conditions and additional pressure drop was not observed. Therefore, we elucidate that the driving force provided by the magnetic stirring bar was not sufficient to obtain the maximum gas uptake. Results on methane consumption are shown in Figures 8 − 10 for the three concentrations. This variable tends to increase Figure 10. Methane consumption in SDS solutions at different initial pressures.

However, the system is above the critical micelle concentration (CMC) and seemed to decrease for SDS at high concentration (701 and 1510) ppm. Then, we elucidate that no more methane could be solubilized in water and the SDS molecules were saturated in the gas−liquid phase to promote gas uptake. Consumption was decreasing as the concentration increased for CTAB. Additional experiments to confirm it should be performed at those conditions. Macroscopic crystal formation appeared at low temperature (below 278 K) for CTAB when the aqueous surfactant solutions were prepared in all the concentrations; it could cause inconsistent gas consumption as a function of concentration. The mole gas consumption for methane in presence of CTAB was almost the same at any concentration against the water + methane reference system, except for the lowest concentration at the two highest initial pressures (10 and 12 MPa). PE/F127 had a better gas consumption compared with CTAB. The highest value of 0.046 mol was obtained at C = 1530 ppm for PE/F127. Gas consumption increased as initial pressure increased (5.5, 8, 10, and 12 MPa). Gas consumption was also plotted in terms of moles of methane per mole of water in Figures 8−10. The same ratio is also listed for the data reported in the literature6,20,27−29,36−39 in Table S1 as Supporting Information. A strict comparison cannot be achieved since each experiment was performed at

Figure 8. Methane consumption in PE/F127 solutions at different initial pressures.

as the initial pressure raised at constant concentration for PE/ F127 and SDS because there were higher quantities of methane. Gas consumption seemed to reach a limit at elevated pressures in particular for PE/F127; it could be a consequence of the gas saturation in the liquid phase. As reported by several authors, SDS (anionic) had the maximum gas consumption, yielding 0.079 mol at 376 ppm, followed by PE/F127, and finally by CTAB. Gas consumption using the anionic surfactant was enhanced by 58% against the corresponding maximum value for PE/F127; then, SDS had a better prevention in the agglomeration of hydrate particles in the gas−liquid phase. F



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different driving forces but both sets of data are similar or in the same magnitude order. Growth rate was determined using the following equations N = Nt = 0e−kt

Table 4. Growth Rate for Methane Hydrate Formation with Surfactants C ppm

(8) 377

The derivative with respect to time dN = Nt = 0k e−kt dt

(9)

N was gradually calculated up to the maximum gas consumption. Nevertheless, the velocity constant k was calculated taking into account the first slope for the lineal pathway between N0 and N as a function of time. The growth rates reflected the gas hydrates evolution and are listed in Table 4. In addition, surfactant concentration was related to gas consumption rate and hydrate formation efficiency. PE/F127 showed an accelerated growing rate at the lowest concentration compared with the absence of surfactant. However, the growth rate slowed down for higher concentrations. Then, we can assume that low concentration promotes better the hydrate formation. Maximum gas consumption for PE/F127 took less time than for CTAB. Mole gas depletion using PE/F127 and CTAB was lower than the corresponding for SDS. Methane hydrate growth is slow using CTAB in a similar way to those obtained in the absence of surfactant. Initial pressure did not have any systematic effect on the growth rate. The highest rate corresponded to SDS. This additive exerted a promotion activity on hydrates because the rate tended to grow upward as concentration was increasing. The chemical affinity model was used to represent the hydrate formation kinetics.40 The expression is the following ⎡ ⎤−A r / RT ⎛ Ni ti ti ⎞⎥ ⎢ = exp⎜⎜1 − ⎟⎟ ⎢⎣ t f Nf t f ⎠⎥⎦ ⎝

710

1530

356

705

1500

(10)

where the subscripts i and f denote any datum during hydrate growth and the maximum gas consumption, respectively. The growth rate is considered as the absolute value of Ar/RT. The obtained results using eq 10 are listed in Table 4 which showed agreement with the experimental data.

376

701

4. CONCLUSION In this research, we studied the parameters involved on the formation and dissociation processes for methane hydrates in the presence of three kinds of surfactants nonionic (PE/F127), cationic (CTAB), and anionic (SDS) over a range of 3.5 to 12 MPa as initial pressure. These additives were prepared in aqueous solutions at concentrations of about 350, 700, and 1500 ppm. Induction time for PE/F127 was higher than that reported for CTAB but lower than that for SDS at high concentration. This period tended to increase as initial pressure was reduced, being the maximum value for SDS; however two runs are not so sufficient to confirm the behavior of induction time. Trends of gas depletion were very different for each additive. Methane uptake for PE/F127 seemed to increase as the concentration raised at high initial pressures; meanwhile, the optimal SDS concentration for gas depletion was 350 ppm, which was consistent with that reported in the literature. The growth rate was enhanced for PE/F127 at low concentrations. On the opposite, the lower CTAB concentration tended to delay the growth rate. This rate was greatly increased for SDS as concentration and initial pressure raised. Hydrate−liquid

1510

0

k min−1

dN/dt mol·min−1

pi MPa

synperonic PE/F127 (nonionic) 3.3 0.0150 3.70 × 10−04 0.0148 5.48 × 10−04 5.7 0.1410 9.61 × 10−04 8.1 0.0227 2.03 × 10−03 10.0 0.0182 2.10 × 10−03 12.5 0.0021 6.06 × 10−05 3.6 0.0015 7.01 × 10−05 5.5 0.0025 1.62 × 10−04 8.0 0.0020 1.82 × 10−04 10.0 0.0014 1.58 × 10−05 12.1 0.0014 3.98 × 10−05 3.5 0.0018 8.47 × 10−05 5.5 0.0014 1.01 × 10−04 8.0 0.0015 1.40 × 10−04 10.0 0.0019 2.12 × 10−04 12.0 CTAB (cationic) 0.0013 3.38 × 10−05 3.2 0.0007 3.28 × 10−05 5.5 0.0024 1.66 × 10−05 8.0 0.0026 2.35 × 10−05 10.1 0.0052 5.94 × 10−05 12.1 0.0016 4.58 × 10−05 3.5 0.0012 5.62 × 10−05 5.5 0.0013 8.86 × 10−05 8.0 0.0011 1.00 × 10−05 10.1 0.0006 6.83 × 10−05 12.0 0.0016 4.33 × 10−05 3.5 0.0012 5.64 × 10−05 5.5 0.0011 7.97 × 10−05 8.0 0.0009 8.44 × 10−05 10.0 0.0013 1.47 × 10−05 12.0 SDS (anionic) 0.0206 5.03 × 10−04 3.3 0.0272 1.18 × 10−03 5.7 0.0841 5.30 × 10−03 7.9 0.0596 5.01 × 10−03 10.1 0.0186 1.93 × 10−03 11.2 0.0614 1.58 × 10−03 3.6 0.0896 3.30 × 10−03 5.5 0.0716 4.72 × 10−03 8.0 0.0535 4.42 × 10−03 10.0 0.0343 3.78 × 10−03 12.1 01016 2.20 × 10−03 3.5 0.0959 3.43 × 10−03 5.6 0.0588 4.14 × 10−03 8.0 0.0815 6.67 × 10−03 10.0 0.1151 3.75 × 10−03 12.0 water 0.0016 4.57 × 10−05 3.5 0.0020 9.23 × 10−05 5.5 0.0008 5.77 × 10−05 8.1 0.0008 7.50 × 10−05 10.1 0.0011 1.25 × 10−05 12.1

t min

Ar/RT

60 100 333 328 616 576 641 831 802 920 490 312 897 1454 875

−0.941 −1.427 −0.467 −0.382 −0.406 −0.904 −0.982 −1.377 −0.992 −0.978 −0.980 −1.201 −1.021 −0.737 −1.031

348 1840 638 5860 4560 642 1142 447 713 815 512 846 817 265 485

−0.840 −0.882 −0.614 −0.424 −0.403 −0.906 −0.916 −1.697 −0.956 −0.799 −1.596 −1.014 −0.818 −1.136 −1.121

84 85 91 224 233 37 108 840 263 80 145 137 254 329 787

−0.731 −0.559 −1.077 −1.021 −1.012 −0.698 −0.289 −0.313 −0.451 −0.309 −0.171 −0.191 −0.732 −0.605 −1.520

471 2084 648 1464 502

−0.984 −0.561 −0.957 −0.674 −0.888

water−vapor phase equilibrium conditions were the same for any surfactant concentration since the crystalline structure was not modified and thus the required enthalpy of dissociation was the same. Additional experiments for methane hydrate G



Article

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formation should be carried out at a cooling temperature above the ice normal melting point in order to deeply analyze the effects of concentration and pressure.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.7b01009. Table S1 includes the mole of methane consumption per mole of water reported in the literature using SDS and CTAB (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: (52) 55 5729-6000, Ext. 55120, 55124. ORCID

Octavio Elizalde-Solis: 0000-0002-7282-3554 Funding

This research was supported by Instituto Politécnico Nacional from Mexico. H.I.P.-L. was a recipient of a doctoral fellowship (CONACyT 392894; BEIFI-IPN; COMECyT). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Anonymous reviewers are acknowledged for their valuable comments. REFERENCES

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