Nucleation of Methane Hydrate in Water-In-Oil Emulsions: Role of the

Apr 20, 2016 - Institute of Petroleum Chemistry, SB RAS, 4 Akademichesky Avenue, Tomsk 634021, Russian Federation. Energy Fuels , 2016, 30 (5), ...
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Nucleation of Methane Hydrate in Water-In-Oil Emulsions: Role of the Phase Boundary Andrey S. Stoporev,† Andrey Yu. Manakov,*,†,‡ Viktor I. Kosyakov,† Vladimir A. Shestakov,† Lubov’ K. Altunina,§ and Larisa A. Strelets§ †

Nikolaev Institute of Inorganic Chemistry, SB RAS, 3 Ac. Lavrentiev Avenue, Novosibirsk 630090, Russian Federation Novosibirsk State University, Pirogova Street 2, Novosibirsk 630090, Russian Federation § Institute of Petroleum Chemistry, SB RAS, 4 Akademichesky Avenue, Tomsk 634021, Russian Federation ‡

ABSTRACT: The processes of nucleation of the methane hydrate have been studied for water emulsions (50% (w/w)) in decane, toluene, oil, and their mixtures. The experimental data have been obtained by the polythermal technique under methane pressures of 12−13 MPa; each of the experiments has been carried out with a freshly prepared sample of the emulsion. The obtained data have been used to derive the survival curves of the samples describing the dependence of the probability Nu/N0 on temperature T, where Nu is the number of samples lacking the hydrate on overcooling to a particular temperature T and N0 is the total number of the samples. The observed curves constitute two families of similar nature: (1) emulsions in decane and toluene and (2) emulsions in oil and mixtures of the oil with decane or toluene. We suggest that the observed results are explained by adsorption of some components present in the systems (surfactant or heavy components of oil) at the interface between the water and organic phases and on the mineral particles present nearby. The result is the formation of a number of surfaces behaving as the centers of heterogeneous nucleation with similar activity in proximity to the water−organic interface (the region of the nucleation of the methane hydrate), thus explaining the similarity of the observed survival curves.

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INTRODUCTION Preventing gas hydrate formation and ensuring fault-free operation of multiphase (oil−brine−gas) flow in field pipelines is a significant part of current gas hydrate studies.1−3 Hydrate formation in multiphase flows most commonly occurs on the interaction of the associated petroleum gas dissolved in oil with the water emulsified in oil.1−3 The conditions appropriate for hydrate formation are present there: (1) large surface of the water−oil interface and (2) high concentration of the dissolved gas in the oil medium around the water droplets. The models of gas hydrate formation process in water-in-oil emulsions are suggested in several studies (e.g., refs 4−6). According to this model, the formation of a nucleating seed at the water−oil interface is rapidly followed by the growth of the hydrate shell at the surface of the water droplets. Furthermore, hydrate formation occurs within the shell limited by gas diffusion through it. It has been demonstrated7 that in water-rich emulsions primary hydrate/ice nucleation can be accompanied by secondary nucleation of these phases as well as repeating events of primary nucleation. To avoid ambiguity, only nucleation of the hydrate will be considered. The most probable mechanism of this process is the following. After development of the hydrate crystals on the surface of a droplet, they grow into the organic phase. They may be formed as whiskers, which rapidly reach the adjacent droplets, initiating hydrate formation on their surface. Propagation of this process triggers the development of the hydrate in some part of the sample right after its formation in one droplet. The data obtained by magnetic resonance microimaging8,9 show that the real picture of hydrate formation in water droplets does not always coincide with the models discussed above. It was demonstrated that the hydrate particle can be confined to © 2016 American Chemical Society

individual droplets for a long time without propagation. Hydrate nucleation may occur inside the water droplet, not necessarily at the surface of the droplet. Most probably, different models should be used for description of hydrate formation in different types of dispersive systems. A fast developing technique of prevention of hydrate blocks in multiphase flows is the usage of so-called low-dosage hydrate formation inhibitors: antiagglomeration agents and kinetic inhibitors.10,11 Antiagglomeration agents (AA) do not prevent hydrate formation in multiphase flows. They adsorb on the hydrate particles in the flow providing wetting with oil. This hinders adhesion of the particles and pipe blocking. Thus, the HYDRA FLOW12 technique employing AA has been suggested for combined transportation of oil and associated gas in the gas hydrate form.13 Kinetic inhibitors (KI) slow down the nucleation and/or growth of hydrate particles in the multiphase flow to the extent that the flow escapes the hydrate-forming region before the hydrate particles are formed. Typical KI are exemplified with water-soluble polymers;10,11 however, the possible application of antifreeze proteins14 and inorganic salts15,16 as KI has been demonstrated. As shown,17−20 some natural components present in oils can behave as KI and AA; their efficiency is comparable to that of the synthetic analogs.17 Similar studies of oil components adsorbing on the surface of the water droplets and hydrate particles as well as on the effect of oil components on the wettability of the hydrate particles have been carried out.18−20 In particular, in these studies it has been demonstrated that naphthenic acids are preferentially Received: September 30, 2015 Revised: February 16, 2016 Published: April 20, 2016 3735

DOI: 10.1021/acs.energyfuels.5b02279 Energy Fuels 2016, 30, 3735−3741

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Table 1. Compositions of the Dispersion Media Used in This Work and Size of Water Droplets in the Emulsion Obtained in These Disperse Mediaa media component or parameter

D

decane toluene oil average size of water droplets in the emulsion (standard deviation of the average size) (μm) area of interfacial surface (m2) viscosity (mPa·s) density (g/cm3) a

T

1

OD

1 38 (16) 0.051 0.85 0.730

OT

O

0.5 0.5 40 (25) 0.030 1.53 0.866

1 37 (23) 0.035 17.27 0.865

0.5

32 (14) 0.063 0.56 0.867

0.5 35 (17) 0.048 2.55 0.775

All compositions are presented in weight fractions.

nucleation of hydrates and (probably) ice (e.g., ref 41). In particular, development, adsorption, and deactivation of hydrate formation inductor particles may occur here. There are several studies34,37,38,40 using the statistical approach to the nucleation of hydrates. Thus, the study34 has been devoted to the nucleation of methane and xenon in water-in-oil emulsions. It was noticed that nucleation as a rule occurred at the cell walls and impurity particles. The distribution functions for the nucleation of ice and the hydrate were significantly different. The goal of this study was the examination of the water− organic phase interface on the methane hydrate nucleation in respective emulsions. In our experiments we used simplified systems that do not fully reproduce real multiphase flows. For example, pure methane gas was used as hydrate former instead of natural gas. The emulsion was not stirred in the cource of the experiments. These simplifications facilitate interpretation of our experimental data. In this work we present the data on methane hydrate nucleation in water-in-oil emulsions based on crude oil, decane, and toluene as well as the mixtures of the crude oil with decane or toluene.

adsorbed on the hydrate surface. Therefore, there are no doubts about the existence of the oil components’ influence on hydrate formation in water-in-oil emulsions as well as on the properties of the hydrate-in-oil suspensions formed. Nucleation of solid crystal phases is still an area of intensive studies where researchers have encountered many surprising phenomena.21−23 Nucleation theory in application to gas hydrates (GH) was considered in detail.24,25 Homogeneous and heterogeneous nucleation processes should be distinguished. To clarify, we will focus here on crystallization of the liquid phase. Homogeneous nucleation occurs because of a density fluctuation in a homogeneous metastable liquid, which results in the formation of a solid-phase nucleus that is able to grow on its own. Heterogeneous nucleation of a new phase occurs at a surface that is present in the liquid. This may be a vessel wall, suspended particles, and so on. A contact of the nucleation center surface with the nucleus surface results in a decrease of the nucleus surface energy and, consequently, makes it more stable. As a result, heterogeneous nucleation is characterized by activation energy lower than that of homogeneous nucleation. As a rule, the natural concentration of nucleation centers in a liquid cannot be controlled; however, there are numerous examples of successful control of nucleation in liquids by introducing promotors with pronounced catalytic activity.26−30 Similar examples also have been reported for gas hydrates.31−33 Nucleation of gas hydrates and ice has been studied (e.g., refs 7, 30, and 34−40). The common practice is to examine the nucleation induction time (isothermal method) or supercooling inducing the nucleation of the new phase (polythermal method) by repeatable formation−decomposition cycles (melting and dissolution of the solid phase).34,35,37,40 Sometimes, this approach is inapplicable because of decay of the sample after the first cool−warm cycle. Exactly this situation takes place on emulsion freezing and related gas hydrate formation. Ice melting or hydrate decomposition can be followed (e.g., emulsion destruction or growth of water droplets). In such cases (and also to avoid the “memory effect”), statistical averaging is gained with installations having multiple cells.35 As known, nucleation is a stochastic process characterized with a certain distribution function. Crystallization processes are most often characterized by so-called “survival function”, indicating the share of the noncrystallized samples at a certain moment or extent of supercooling.30,34,37,38,40 The above illustrates that the studies on nucleation of gas hydrates and ice in water-in-oil emulsions are essential for fundamental and applied science. The oil−water interface existing in such systems can positively effect on the processes of

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EXPERIMENTAL SECTION

The materials used in the study included distilled water, decane, and toluene (chemical grade, no additional purification), surfactant SPAN80 (Sigma-Aldrich), and CH4 with purity better than 99.98%. The oil studied was taken from Sovetskoe field (hereafter oil) with the following characteristics: density 866.7 kg/m3, 1.5% asphaltenes, 10.7% tar, and 1.9% paraffins (w/w). The compositions of the dispersion media used are given in Table 1 (w/w). The following notations are used: O, oil, D, decane, T, toluene, OD, mixture of oil and decane (50/50 wt %), OT, mixture of oil and toluene (50/50 wt %). Water/ oil emulsions were prepared in weight ratio of 1/1. SPAN-80 surfactant was used as the emulsifier for decane and toluene; other emulsions were prepared without additives. Water emulsions in OD, OT, and O were prepared on a magnetic stirrer (1250 rpm) during 20 min. Water emulsions in D and T were prepared using an ultrasonic disperser (22 kHz). On preparation, all emulsions were cooled with melting ice. The emulsions of water in oil were stable for a long time (months); the other emulsions were stable at least for 1 day. The sizes of emulsion droplets were determined by taking photographs with a microscope followed by measurement of the images. Table 1 gives average droplet sizes in the emulsions and areas of the water−oil interface. The nucleation of the hydrate and ice was carried out on the installation reported in ref 42. An aluminum sample holder was loaded with four Teflon cells charged with water/oil emulsions. The weights of the samples were 0.9017 ± 0.0018 g. The thickness of the emulsion layer in each of the cells was 10 mm. The assembly of the samples was placed in a high-pressure chamber. Type K chromel−alumel thermocouple was placed in each of the samples, with the junction being approximately in the middle of the sample. Then, the chamber was flushed with methane and pressurized to 15 MPa. After that, the 3736

DOI: 10.1021/acs.energyfuels.5b02279 Energy Fuels 2016, 30, 3735−3741

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Energy & Fuels dnv dns 1 1 , Js = , v(N − n) dt s(N − n) dt dnk 1 Jk = lk(N − n) dt

emulsions in the media D, T, OD, and OT were saturated with the gas for 4 h; special experiments indicated that this time is sufficient for saturation. Two series of experiments (4 and 15 h of saturation) were carried out for water emulsions in the medium O. It was found that the exposure of 4 h was insufficient, so the experiments with exposure of 15 h were used for data analysis. Then the cell was cooled/heated (+20 °C → −15 °C → +20 °C) at the constant rate of 0.14 °C/min. The temperature in each of the samples as well as the pressure in the chamber were digitally recorded. The pressure in the chamber was not adjusted and changed with the temperature. At the hydrate formation temperature, in all cases the pressure in the chamber was 12−13 MPa. The pressure was monitored with an electronic gauge calibrated against a high-precision Bourdon tube. The absolute temperature uncertainty was ±0.2 °C; pressure accuracy was ±0.25% of the measured value. The drift of the sensors on temperature and pressure was less than 0.1 °C and 0.01 MPa, respectively. The formation of the hydrate and ice was revealed by heat effect registered with the thermocouple.

Jv =

(1)

The overall nucleation rate in the sample is J = dn/dt or

J = vJv + sJs +

∑ lkJk

(2)

k

It appears from eqs 1 and 2 that ln φ(t ) = ln[1 − ρ(t )] = −

∫t

t

J (t ) d t

(3)

0

where t0 is the moment of the transition of the system in the metastable state. Assuming the constant cooling rate w = −dT/ dt, eq 3 can be reformulated as

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RESULTS AND DISCUSSION Let us briefly outline some aspects of the mathematical description of the nucleation processes applicable to the nonisothermal experiment carried out in this work. First, we note that our experimental data can be explained using the model described above.7,42 This model postulates the stochastic nature of the first seed nucleation within one of the water droplets emulsified in oil. A series of secondary nucleation events following formation of the first seed results in covering of some part of the water droplets by hydrate film. Further inward growth of these films is a slow process. This sequence of events manifests itself as a strongly asymmetric exothermic effect. Undoubtedly, new events of primary nucleation of hydrate or ice can occur in the water droplets not involved in the previous events, but our experiments were limited to observation of the first nucleation event. This allowed us to attribute all nucleation events to the samples characterized with the same water content and known area of the oil−water interface. Thus, the onset of the first thermal effect on the thermal curve can be attributed to formation of the first seed in the sample25 with some systematic shift. This makes it possible to examine nucleation by observing the fraction of the samples exhibiting the thermal effects as a function of time or temperature. We note that many workers noticed mechanistically different nucleation processes in the same sample. Turnbull26 observed two-stage nucleation in emulsion of mercury droplets covered with Hg2J2 in EtOH. It was demonstrated43 that in supercooled water both homogeneous and heterogeneous nucleation of ice occurs. Ref 44 describes a two-stage mechanism of nucleation of protein lysozyme in aqueous solution. Thus, in our experiments the first seed can originate from different mechanisms. Let us assume that inside the overcooled sample the seeds can be developed within water droplets, at the interface with the oil phase, or on solid particles (i.e., the centers of heterogeneous nucleation). These particles can be suspended in water or dispersed at the interface between the phases. Let us denote the volume of the water solution in a sample as v, the total surface of the droplets as s, and lk as the number of solid particles of the type k. The object is the system of N identical samples exhibiting heat effects in n samples at the moment t (N − n samples remain liquid). The probability of development of n first seeds in N samples is ρ(t) = n/N, and the survival probability of N − n samples from N is φ = 1 − ρ. For different mechanisms, the nucleation rates are determined as

ln φ(T ) = ln[1 − ρ(T )] =

1 w

∫T

T

J (T ) d T

(4)

0

where T0 is the equilibrium temperature of the hydrate at the experimental pressure. We note that eqs 3 and 4 are valid for continuous survival curves, suggesting an experiment with the infinite number of samples. For the real experimental data, it is convenient to present experimental dependences φ(t) and φ(T) as step functions and replace the integral with the relevant sum. These dependences can be approximated with a continuous function (e.g., with spline), and we define J(T) as J (T ) = w

d(ln φ(T )) dT

(5)

This expression gives the total nucleation rate. It is noteworthy that the results of the data treatment with this approach can be quite sensitive to the approximation used. Theoretical functions J(T) for gas hydrates are scrutinized.24 For a narrow temperature interval, the following approximation can be used 2

J = A e−B / T(ΔT )

(6)

Here A and B are constants, ΔT = T0 − T, and T0 is the equilibrium temperature of the hydrate at the experimental pressure. When several nucleation mechanisms are operating 2

2

J = A v′ e−Bv / T(ΔT ) + As′ e−Bs / T(ΔT ) +

2

∑ Ak′ e−B /T(ΔT) k

k

(7)

The characteristic feature of the sum of these exponents is that in a narrow temperature interval one of the addends can dominate. Hence, the function J(T) can be approximated with a piecewise smooth curve having the fragments separated by salient points. Different parts of the survival curve should correspond to different nucleation mechanisms. According to eq 4, such dependence J(T) can result in complex survival curves, not infrequently observed in experiments (e.g., ref 35). In our work, the studied dispersed systems are emulsions of water in organic liquids. In all cases, the metastable phase is the oversaturated (toward the hydrate) solution of methane in water. It is known that the hydrate nucleation in such systems occurs in the aqueous phase near the interface between the solutions of the hydrate-forming gas in water and in hydrocarbon.2,3,45 If the equilibrium distribution of the dissolved methane is not achieved in the system, then the 3737

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The temperature of the endothermic effect corresponded to ice melting, and the temperature interval of ice nucleation was in reasonable agreement with low-temperature effects observed in the experiments with methane. The nucleation temperature of the hydrate or ice was always determined as the onset of the corresponding heat effect. The set of the overcooling values, related to the nucleation of the hydrate or ice in a given emulsion, was used to derive the survival function for this system.30,34,37,38,40 The survival function was calculated as the dependence of Nu/N0 on temperature T (function φ(T), eq 4), where Nu is the number of samples lacking hydrate or ice formation when cooled to the temperature T and N0 is the total number of the samples. Because the experiments were carried out at the constant volume, heating or cooling of the installation affected the pressure inside it. Hence, each of the observed nucleation temperatures corresponded to a particular pressure of the gas. Actually, all the values fell within the interval 12−13 MPa. As a result of the invariant course of the experiments, all the systems exhibited almost identical linear dependences of hydrate nucleation pressure versus nucleation temperature. With this organization of the experiment, the nucleation rate in the integrand of eq 4 depends both on temperature and pressure, severely hindering the calculation of the methane hydrate nucleation rate from our data. One can assume that the effects of decreasing temperature and the pressure fall on the nucleation rate are somewhat compensated. The influence of the concentration of the dissolved gas on the observed survival function is clearly illustrated by the data for the dispersion medium O (Figure 2). As noted in the

driving force of hydrate nucleation will also depend on the methane concentration in a given part of the emulsion. In the expression for the nucleation rate, the value of the driving force is in the power of the exponent (eq 7). In turn, the nucleation rate is present in the power of the exponent of the survival function (eq 4). Therefore, minor variations in the thermodynamic oversaturation of the system should strongly affect the survival function. Typical experimental curves observed by us are illustrated in Figure 1. The cooling curve has two asymmetric exothermal

Figure 1. Typical experimental curve obtained in our experiments. Bottom line corresponds to temperature of the sample. Small exoeffect corresponds to methane hydrate formation; large exoeffect corresponds to ice formation. Large and small endoeffects correspond to ice melting and methane hydrate decomposition, respectively. Top line corresponds to pressure in the experimental apparatus. For more explanation, see the text.

effects. The outline of both effects indicates that the process begins with a fast growth of the temperature of the sample, and then the temperature reaches the maximum and relatively slowly decreases. As a rule, the baseline after the peak is higher than before. This can be associated with two factors. First, this can be related to heat evolution caused by hydrate formation in emulsion droplets covered with the hydrate shell. This process is restricted by gas diffusion through the hydrate shell and occurs slowly. The second possible factor is the difference in thermophysical properties of the sample before and after the heat effect, probably affecting the heat exchange between the sample and the sample holder. The magnitude of the second (lower-temperature) exothermal effect is always much larger than that of the first one (Figure 1). Relying on the data,7,34,46 we attribute the first (smaller) peak to the formation of the hydrate and the second (larger) to ice freezing. The observations (vide infra) confirm this interpretation. In all experiments, the heating curve exhibited two endothermic effects. The first begins at −0.9 to −0.7 °C, corresponding to ice melting at the given pressure (within experimental uncertainty). The onset of the second group of the effects occurs at 15.1−16.5 °C, in reasonable accordance with the temperature of methane hydrate decomposition at relevant pressures. The attribution of the effects was additionally checked in experiments with methane substituted with nitrogen (nitrogen does not form a hydrate under these conditions). In this case, the thermal curves always demonstrated a single exo- and endothermic effect on heating and cooling, respectively.

Figure 2. Survival curves obtained with methane saturation for the dispersion medium O (oil) during 4 and 15 h.

Experimental Section, in our experiments the exposure to pressurized methane during 4 h was enough for saturation of water emulsions in the dispersion media T, D, OT, and OD. As illustrated by Figure 2, this span of time is insufficient for more viscous dispersion medium O. Hence, gas saturation was carried out for 15 h giving unbiased survival curves. Prior to discussion of nucleation of methane hydrate and ice in the emulsions studied, two essential points should be noted. (1) Water, toluene, decane, and oil used in our experiments were taken from the same vessels; hence, the type and the content of the heterogeneous seeds in these liquids were invariable. (2) The volume of water in all studied samples was certainly identical. Experimental survival curves for ice and methane hydrate in different disperse media are illustrated in Figures 3 and 4. Hereafter, the survival curves will be denoted 3738

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Energy & Fuels with the relevant dispersion medium suffixed with h for hydrates and with i for ice.

centers). Development of a special mathematical procedure is necessary to calculate nucleation rates from these survival curves and is beyond the scope of this work. We plan to develop this procedure in future. Let us consider the obtained results from the viewpoint of the above speculations on the processes of heterogeneous nucleation. It is currently accepted that the nucleation of ice occurs in the bulk of water whereas that of the hydrate occurs at the water−gas or water−oil interface.2,3,45 Therefore, the area of the interface between water and organic phases can influence the position and shape of the survival curve (eq 4). At the same time, Table 1 shows that despite of the little variation of sizes of the water droplets in the emulsions studied (32−40 μm) the contact areas differ more than by a factor of 2. In particular, for almost similar curves Oh and Th the areas of the interface between the organic and the water phases differ nearly by 20%. The ratio of the interface areas for weakly different curves ODh and OTh is 1.6. This indicates that in the studied systems heterogeneous nucleation of the hydrate occurs not only at the water−oil interface. Most likely, the second and the most important mechanism is the nucleation of the hydrate at the heterogeneous particles present in the water phase (eq 7), with the most probable location of these nucleation seeds being the neighborhood of the water−oil interface. We believe that the influence of coalescence of the emulsion droplets on our results can be neglected. First, visual observations showed that all emulsion were stable during the experiment. Second, quite a good coincidence of the curves Dh and Th as well as that of Oh, ODh and Oth would be fairly unlikely because the coalescence process is sensitive to the composition of the dispersion medium. In general, keeping in mind the absence of positive correlations between the survival curves and the area of the water−organic interface, it is possible to suppose that the nucleation of the hydrate predominantly takes place not at this interface but on an individual seed located in proximity of the water−oil phase interface. The concentration of such centers is moderate. Methane hydrate nucleation in the studied emulsions has several specific features. Almost perfect coincidence of the survival curves Th and Dh is possible only when the amount and activity of the centers of heterogeneous nucleation of the hydrate are quite similar in these systems. Accidental coincidence of such is of low probability. As noticed above, here the nucleation of the hydrate occurs near the water−oil interface; hence, the centers of heterogeneous nucleation of the hydrate must also be located near this interface. Apparently, the molecules of the surfactant used for preparation of the emulsions are also concentrated at this interface. We believe that the observed coincidence of the curves Th and Dh can result from two simultaneous factors. First, the interfaces of the organic and water phases have similar nature in this case. At the side of the water phase, the surface is covered by hydrophilic groups of the surfactant SPAN-80 used in emulsion preparation (hydroxyl, ether, and ester groups). Hence, these surfaces should behave similarly on hydrate nucleation. Second, solid particles located near the interface of the organic and water phases are expected to adsorb the molecules of the surfactant; therefore, their surface properties will be changed. Nucleation seeds formed in this way should have comparable activity in hydrate nucleation. The survival curves for oil and oil-containing dispersion media (Oh, OTh, and ODh) are close together. They are shifted to higher temperatures as compared to the curves Th

Figure 3. Survival curves for methane hydrate formation from emulsions of water in different disperse media. The following notations for the disperse media are used: O, oil, D, decane, T, toluene, OD, mixture of oil and decane (50/50 wt %), OT, mixture of oil and toluene (50/50 wt %). Nu, number of experiments in which methane hydrate was not formed at given temperature; N0, total number of experiments. Symbols and lines are used to indicate different curves for readability.

Figure 4. Survival curves for ice formation from emulsions of water in different disperse media. The following notations for the dispersion media are used: O, oil, D, decane, T, toluene, OD, mixture of oil and decane (50/50 wt %), OT, mixture of oil and toluene (50/50 wt %). Nu, number of experiments in which methane hydrate was not formed at given temperature; N0, total number of experiments. Symbols and lines are used to indicate different curves for readability.

It is clear that the curves Dh and Th virtually coincide. The curves Oh, ODh, and OTh are also rather close. At the same time, the middle parts of these curves are shifted by 3−4 °C toward higher temperatures as compared to Th and Dh. It is noteworthy that all survivals curves of ice are relatively smooth, whereas survival curves of the majority of the oil-containing dispersion media have expressed peculiarities. The peculiarities on the curves Dh and Th are less pronounced in comparison with those of the oil-containing dispersion media. Thus, these peculiarities can be considered an important characteristic of the system related to the specific features of the structure and the composition of relevant samples. Analysis of eqs 4 and 7 indicates that these peculiarities may originate from occurrence of different nucleation pathways at different subcooling levels (e.g., nucleation on different types of heterogeneous nucleation 3739

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intermediate between the curves obtained for water emulsions in pure oil, decane, and toluene. The observed survival curves trend to the curve obtained for water emulsions in oil. Survival curves of water emulsions in chemically different toluene and decane practically coincided. We conclude that the observed results are explained by adsorption of some components present in the systems (surfactant and heavy components of oil) on the mineral particles and (in minor extent) at the interface between the water and organic phases. The result is the formation of a number of “unitized” heterogeneous nucleation centers with similar activity in proximity to the water−organic interface. Therefore, the nucleation of the methane hydrate at these surfaces occurs with comparable efficacy, thus explaining the similarity of the observed survival curves.

and Dh. In our opinion, the most probable reason for this is that this oil contains some components adsorbing at the water− organic interface and on solid particles present in the system. We note that similar ideas on the mechanism of action of kinetic inhibitors have been formulated in ref 47. The adsorption films formed by these components are preferential for the hydrate nucleation as compared to those formed by SPAN-80. Analogous temperature intervals and the slopes of the survival curves of all three samples indicate the similarity of the nucleation centers present in these systems. The presence of specific features at different parts of the survival curves may indicate that on emulsion preparation several types of nucleation centers with comparable activity can be formed. This can be caused by adsorption of the heavy oil components on the particles in the system. Mere mixing of nucleation centers originating from different sources can hardly result in such a situation. Another factor hindering the interpretation of the obtained results should be noted. Examination of the literature data on methane solubility in water, toluene, and decane indicates that the solubility drop caused by pressure decrease by 1 MPa is somewhat larger (by modulo) than the solubility gain achieved by cooling by 35 °C. Thus, the variation of temperature and pressure in the course of the experiment changes methane solubilities in water and the organic phases and development of methane concentration gradients in the system. It is very difficult to assess the influence of this factor on the results obtained. The equilibration of the methane concentration in the system is primarily determined by the diffusion coefficient of methane in the organic phase and hence, on its viscosity. We believe that the similarity of the survival curves Oh, ODh, and OTh, obtained for dispersion media with strongly different viscosity, indicates the minor importance of this factor. Additional studies are necessary to clarify this point. Survival curves for ice nucleation in the studied water-in-oil emulsions are shown in Figure 4. These curves are smoother than the hydrate nucleation curves and have higher slopes relative to the temperature axis (as compared to the hydrate). The temperature range of the curves does not exceed 2.5 °C, being considerably smaller than that for survival curves of the methane hydrate. In this series of experiments, temperature surges related to formation/decomposition of the hydrate were always much smaller than those related to freezing and melting of ice (Figure 1). Thus, the amount of the hydrate formed at the early stage of the experiment was small, and the amount of water available for ice formation changed insignificantly. The analysis of the obtained data reveals that nucleation of ice and the hydrate occurs at different types of particles. Thus, the survival curves for ice Di and Ti are essentially different, whereas the hydrate nucleation curves Dh and Th coincide. As in the case of hydrate nucleation, the mixing of the organic phases can result in some modification of the activity of the nucleation centers, so the curves Oi and Ti are at lower temperatures than the curve OTi. It is unlikely that this result can originate from mere redistribution of the nucleation centers already present in the reagents used.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +7 (383)-316-53-46. Fax: +7 (383)-330-94-89. Notes

The authors declare no competing financial interest.

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REFERENCES

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CONCLUSIONS Survival curves of methane hydrate nucleation have been studied for water emulsified in different organic liquids (decane, toluene, and oil) and their mixtures. It has been demonstrated for the first time that survival curves observed for water emulsions in oil diluted with decane or toluene are not 3740

DOI: 10.1021/acs.energyfuels.5b02279 Energy Fuels 2016, 30, 3735−3741

Article

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DOI: 10.1021/acs.energyfuels.5b02279 Energy Fuels 2016, 30, 3735−3741