Surfactant Effects on the Crystal Growth of Clathrate Hydrate at the

Publication Date (Web): January 5, 2015. Copyright © 2015 ... The effect of surfactants on clathrate hydrate crystal growth at the interface of water...
0 downloads 0 Views 2MB Size
Download PDF
Article pubs.acs.org/crystal

Surfactant Effects on the Crystal Growth of Clathrate Hydrate at the Interface of Water and Hydrophobic-Guest Liquid Makoto Mitarai, Masatoshi Kishimoto, Donguk Suh, and Ryo Ohmura* Department of Mechanical Engineering, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan ABSTRACT: Visual observations of the effect of surfactants on clathrate hydrate crystal growth at the interface of water and cyclopentane has been studied. Surfactants used in the present study are sorbitan monooleate (Span 80), naphthenic acid, and polypropylene glycol. The surfactants were each used at a mass fraction of 0.001% and 0.01%. All the surfactants were soluble in cyclopentane. The crystalline morphology and crystal growth behavior of the hydrate at the interface were found to be substantially affected by the addition of all surfactants. The size of the individual hydrate crystals in the surfactant system was larger than those in the pure cyclopentane system. The observations showed a distinct variation in the behavior of cyclopentane hydrate crystal growth depending on the chemical species of the surfactants, their concentration, and ΔTsub, which is defined as the difference between the equilibrium temperature and the experimental temperature. For the system with surfactants, the growing hydrate crystals did not engulf the interface, because the crystal grains that have grown detached from the interface and fallen into the water. From the observations, it is clear that hydrate crystal production increased in comparison to systems without surfactants due to the interface area being preserved. These observations explain the physical mechanism of two surfactant effects, where one is the prevention of hydrate agglomeration (interface area preservation) and the other is the promotion of hydrate production.

■

INTRODUCTION Clathrate hydrates are crystalline solid compounds each consisting of host water molecules forming a cage-like structure encapsulating guest molecules. Hydrocarbons and noble gases are typical guest substances that form clathrate hydrates. Depending on the size and shape of the guest substance, water molecules form several different cage structures interconnected to yield different crystallographic structures, such as structures I, II, and H.1 Hydrates have a large gas-storage capacity, large heat of formation and decomposition, and can separate guest gas mixtures. On the basis of these properties, there are various industrial applications, especially in the field of energy and environmental technology. For example, hydrates are used as a media for storage and transportation of hydrogen2 and natural gases,3 cool energy,4,5 carbon sequestration,6 and developing highly efficient heat pumps.7 Therefore, the promotion of hydrate formation is essential to advance various industrial technologies. On the other hand, it is well-known that hydrates cause a serious problem in flow assurance of oil and gas pipelines.8 The formed hydrates plug pipelines since they are nonflowing crystalline solids. The simplest method for prevention of pipeline plugging is excluding water from the pipeline completely; however, this method is usually expensive and difficult to implement. Therefore, the use of antiagglomeration agents to prevent gas line plugging has been suggested. Recently, antiagglomeration agents, typically surfactants, are being used to avoid the formation of hydrate plugs. In this © 2015 American Chemical Society

study, we focus on the effect of surfactants on crystal growth of clathrate hydrates. According to the previous studies, it has been reported that surfactants have two different effects on hydrates. The first effect is the prevention of hydrate agglomeration.9 Aman et al.10,11 measured adhesion forces between two hydrate particles with surfactants and reported that Span 80, naphthenic acid, and polypropylene glycol (PPG) reduce the adhesion force of hydrate particles. Kelland et al.12 demonstrated a mechanism to prevent the agglomeration of gas hydrates in multiphase systems, which rely on using additives that result in good mixing of water and liquid hydrocarbon phases. Polypropoxylates were also shown to work well with this antiagglomeration mechanism. On the other hand, it has also been reported that surfactants such as SDS reduce the induction time of hydrate crystallization and thereby increase the rate of formation.13−16These two effects, antiagglomeration and promotion of hydrate formation, are evidently contradictory. To clarify this paradox, it is necessary to understand the physical mechanism of hydrate formation in surfactant systems. Studies on the dynamic/kinetic characteristics of hydrate crystal growth provide valuable information on the physical aspects of nucleation, growth, and decomposition of hydrate crystals. Microscopic observations of the crystal morphology may provide insight into the physical effect of surfactants on Received: November 4, 2014 Revised: December 23, 2014 Published: January 5, 2015 812

DOI: 10.1021/cg501613a Cryst. Growth Des. 2015, 15, 812−821

Article

Crystal Growth & Design hydrate crystal growth. The developments of novel hydratebased technologies require prediction and understanding of the morphologies of hydrate crystals. Furthermore, knowledge of the crystal morphology is also important because it can critically impact process efficiency when handling hydrate crystals, specifically, dewatering and storage of hydrates, and pumping of hydrate slurries.17,18 The core of the understanding comes from clarifying the morphological behavior of hydrate crystals formed at the guest−water interface, and therefore, this topic has been extensively studied in previous studies.19−24 In recent years, there has been substantial progress in studies on the morphology of hydrate crystals formed at the guest− water interface in surfactant systems. Several previous studies25−30 investigated the formation of cyclopentane hydrates in water−oil emulsion. Experimental results have led to the conclusion that hydrate formation takes place primarily at the interface between water drops and the continuous oil phase. In the absence of surfactants, a robust hydrate “shell” develops around the water drop, limiting transport of the hydrate former to the free water, which remains trapped inside the hydrate layer. Direct visualization of hydrate formation in larger water drops under the influence of oil-soluble surfactants shows that hydrate crystals have much smaller features and appear “hairy or mushy”, and the surfactant is the key factor in the mechanism leading to the hollow-conical hydrate growth configuration.25−27 These studies have shown excellent examples of various crystal morphologies in surfactant systems. There has, however, not yet been a study that clarifies the dynamic crystal growth process, where the addition of surfactants prevents agglomeration while enhances hydrate formation simultaneously. This paper reports a visual study of the formation and growth of hydrate crystals created at the interface between cyclopentane and liquid water. Span 80, naphthenic acid, and polypropylene glycol were the surfactants that were used. Two mass fractions (0.001% and 0.01%) were examined for each surfactant in the cyclopentane solution. The objective of this study is to understand the effect of surfactants on hydrate growth by analyzing the visually observed dynamics of the clathrate hydrate growth process and individual crystal morphology.

■

Figure 1. Schematic diagram of the major portion of the apparatus. experiments. For this, the system temperature was first set to a prescribed temperature in the range of 250.0−273.1 K. A sufficient amount of hydrate crystals was formed in the test tube; thereafter the system temperature was increased stepwise in increments of 0.1 K. At each step, the temperature was kept steady for at least 1 h. If no noticeable hydrate dissociation was observed within 1 h, the system temperature was increased. By repeating this procedure of visual observation and temperature increase, the equilibrium temperature of cyclopentane hydrate was determined. The equilibrium temperature measurements were performed by using the same apparatus as that used for the crystal growth observations. Cyclopentane hydrate crystals (approximately 5 mg) were formed in advance, and a small bit was used as a seed by placing it on the liquid−liquid interface (the cyclopentane hydrate density was in between that of pure water and the surfactant solution in the cyclopentane). In the initial growth process, a seed was inserted at the interface without any use of the memory effect. It should be noted that the densities of the hydrates formed with the gas mixture used in the present experiments are estimated to be about 970 kg/cm3. This procedure artificially induces nucleation and growth of hydrates in the test tube. The instant the seed crystal was placed in the test tube was set as the starting time for crystal growth (t = 0 min). A minimum of three replications were conducted for each case. We defined the system subcooling ΔTsub as the difference between the experiment temperature and the hydrate equilibrium temperature (ΔTsub = Teq − Tex), which acts as the magnitude of the driving force for crystal growth. The lateral growth rates of the hydrate crystals formed at the interface between liquid water and the surfactants in the cyclopentane solution were determined from the observations. Table 1 lists the specific values of Teq, Tex, and ΔTsub set in the experiments.

EXPERIMENTAL APPARATUS AND PROCEDURES

The liquid samples utilized in the experiments contained deionized water and liquid cyclopentane with a certified purity of 99% from Aldrich Chemical Co. The surfactants used in the study were sorbitan monooleate (Span 80), naphthenic acid, and PPG. All surfactants were used at mass fractions of 0.001% and 0.01%, and soluble in the cyclopentane liquid phase. Cyclopentane was used as the guest hydrate former because it creates structure II hydrates at atmospheric pressure conditions similar to natural gas hydrate. Figure 1 illustrates a set up schematic of the apparatus used to observe hydrate crystal growth at the interface of pure water and liquid cyclopentane mixed with the surfactants. Pure water was first injected into a glass tube (external diameter 10 mm, inner diameter 8 mm, and height 90 mm), followed by the liquid cyclopentane and surfactant mixture. Hydrate crystals formed at the liquid−liquid interface were observed using a microscope (EdmundOptics, model VZM450). A digital camera (Fortissimo, model CMOS300-USB2) was also attached to the microscope to acquire the images of the hydrate crystals. The system temperature was controlled by a chiller system (Tokyo Rikakikai Co., CTP-300). The temperature was measured using a thermistor temperature sensor with an uncertainty of ±0.1 K. Separate experiments were performed to determine the equilibrium temperature of cyclopentane hydrate in all the systems as preliminary

Table 1. Experimental Temperature for Hydrate Crystal Growth in Surfactants + Cyclopentane Solutiona Span 80, naphthenic acid, PPG (mass fraction 0.001, 0.01)

a

Teq/K

Tex/K

ΔTsub/K

280.2 280.2 280.2

278.9 277.8 275.9

1.3 2.4 4.3

Uncertainty of the temperature measurements: ± 0.1 K.

■

RESULTS AND DISCUSSION Figure 2 shows the sequential photographs and diagrams of the cyclopentane-hydrate crystal growth at the interface of water and cyclopentane with/without surfactants at Tex =278.5 K (ΔTsub = 3.1 K) /Tex = 277.8 K (ΔTsub = 2.4 K) under atmospheric pressure. As seen in the images of the system without surfactants, hydrate crystals grew at the interface to 813

DOI: 10.1021/cg501613a Cryst. Growth Des. 2015, 15, 812−821

Article

Crystal Growth & Design

Figure 2. Sequential photographs and diagrams of the cyclopentane hydrate crystal growth at the water and cyclopentane interface with and without Span 80 at atmospheric pressure.

Figure 3. Sequential images of cyclopentane hydrate crystal growth at the mass fraction 0.001% Span 80 + cyclopentane solution and water interface at atmospheric pressure and Tex = 278.9 K, 277.8 K, 275.9 K (ΔTsub = 1.3 K, 2.4 K, 4.3 K).

also observed in a previous study by Keranjkar et al.24 The hydrate crystals on the interface grew until the interface became unobservable. The observation shows that hydrate crystal production increased in comparison with those in the system without surfactants. The interface disappeared at 28 h for this experiment (see Figure 2 (2c)). The following are the results of the cyclopentane-hydrate crystal formation for different surfactants (Span 80, naphthenic acid, and PPG), concentrations (mass fraction of 0.001% and 0.01%), and subcooling (ΔTsub = 1.3, 2.4, and 4.3 K). Observations. Span 80. Figure 3 shows sequential images of cyclopentane-hydrate crystal growth at the water/cyclopentane interface with 0.001% Span 80 for different ΔTsub. After the nucleation process, hydrate crystals grew until the

create a polycrystalline layer as shown in Figure 2 (1b). The crystals preferentially form a layer that covers the entire interface, which took 15 h for this experiment (see panel (1c)). After the interface was completely covered, no noticeable hydrate crystal growth could be further observed. On the other hand, the observations for systems with surfactants showed a distinct difference in the behavior of cyclopentane hydrate crystal growth. The representative system was a combination of water and cyclopentane with a mass fraction of 0.001% Span 80. In the cyclopentane + Span 80 systems during crystal growth at the interface, the hydrate crystal grains did not aggregate and fully coat the interface but rather detached from the interface and floated into the lower water phase (see Figure 2 (2b)). The detachment of the growing hydrate crystals was 814

DOI: 10.1021/cg501613a Cryst. Growth Des. 2015, 15, 812−821

Article

Crystal Growth & Design

Figure 4. Sequential images of the hydrate crystal growth in the mass fraction 0.001% naphthenic acid + cyclopentane solutions and water interface at atmospheric pressure and Tex = 278.9 K, 277.8 K, 275.9 K (ΔTsub = 1.3 K, 2.4 K, 4.3 K).

Figure 5. Sequential images of the hydrate crystal growth for the mass fraction 0.001% naphthenic acid + cyclopentane solutions and water interface at atmospheric pressure and Tex = 278.9 K, 277.8 K, 275.9 K (ΔTsub = 1.3 K, 2.4 K, 4.3 K).

only took 24 and 3.3 h at ΔTsub = 2.4 and 4.3 K, respectively (Figure 4 (1d), (2d), and (3d)). Naphthenic Acid. The results for 0.001% naphthenic acid in the cyclopentane phase are shown in Figure 5. The findings are similar to those for Span 80 systems, where the hydrate crystals detached from the interface and floated in the water phase (Figure 5 (1b) and (3b)), while the interface submerged. At ΔTsub = 1.3 K, the time for the disappearance of the interface was 234 h, whereas it was only 7 and 2.5 h at ΔTsub = 2.4 and 4.3 K, respectively (Figure 5 (1d), (2d), and (3d)). For 0.01% concentration of naphthenic acid, the pictures are shown in Figure 6. For naphthenic acid at ΔTsub of 1.3 and 2.4 K, no hydrate crystal formation could be observed for a week after seeding on the interface. With a ΔTsub of 4.2 K, the phenomenon of the hydrate crystal growth was generally the same as that of the Span 80 systems, where the hydrate crystals sank into the water phase. The time for the disappearance of the interface was 5 h at ΔTsub = 4.2 K (Figure 6 (3d)).

interface became invisible, since most of the water converted to clathrate hydrate. As stated previously, at all ΔTsub, the hydrate crystals detached from the interface and floated without growing (see Figure 3 (1b), (2b), and (3b)). The time for disappearance of the interface decreased with increasing ΔTsub. At ΔTsub = 1.3 K, the time for the disappearance of the interface took over 112 h, whereas it only took 28 and 5 h at ΔTsub = 2.4 and 4.3 K, respectively (Figure 3 (1d), (2d), and (3d)). Figure 4 shows the behavior of hydrate crystal growth for the system with 0.01% Span 80 at the same driving forces as in Figure 3. At all ΔTsub, the hydrate crystals separated from the interface and floated in the water phase (Figure 4 (1b), (2a), (3c)), which was the same as 0.001% Span 80 for lower ΔTsub. Hydrate crystal growth continued until the interfaces disappeared. As seen in the images, hydrate crystals grew and the interface eventually became invisible, which in this experiment occurred at 229 h at ΔTsub = 1.3 K, whereas it 815

DOI: 10.1021/cg501613a Cryst. Growth Des. 2015, 15, 812−821

Article

Crystal Growth & Design

Figure 6. Sequential images of the hydrate crystal growth in the mass fraction 0.01% naphthenic acid + cyclopentane solutions and water interface at atmospheric pressure and Tex = 278.9 K, 277.8 K, 275.9 K (ΔTsub = 1.3 K, 2.4 K, 4.3 K).

Figure 7. Sequential images of the hydrate crystal growth in the mass fraction 0.001% PPG + cyclopentane solutions and water interface at atmospheric pressure and Tex = 278.9 K, 277.8 K, 275.9 K (ΔTsub = 1.3 K, 2.4 K, 4.3 K).

Polypropylene Glycol. Figure 7 shows the behavior of hydrate crystal growth for the system with 0.001% PPG at the same driving force and concentration as the other surfactants. At all ΔTsub, the hydrate crystal growth behavior was similar to those in systems without surfactants. The hydrate crystals grew at the interface of water and cyclopentane to form a polycrystalline layer and the crystals stopped growing after covering the interface entirely, which was completed in 20.5, 17.5, and 1.5 h at ΔTsub = 1.3 K, 2.4 and 4.2 K, respectively (Figure 7 (1d), (2d), and (3d)). Figure 8 shows the behavior of hydrate crystal growth for the system with 0.01% PPG. With all ΔTsub, hydrate crystals did not detach entirely from the interface (Figure 8 (1c), (2c), and (3c)). Like systems with Span 80, the interface was not wholly covered by crystal growth, so hydrate formation persisted until the interface completely disappeared. For systems with surfactant effects, we believe the hydrate crystal layer at the interface to actually be a porous film, where the pores allow perpetual contact between water and

cyclopentane. The observations show that hydrate crystal production increased in comparison with that of systems without surfactants. The time required for the disappearance of the interface was 315, 14, and 3.5 h at ΔTsub = 1.3, 2.4, and 4.2 K, respectively (Figure 8 (1d), (2d), and (3d)). Classification. The observations above can be divided into three distinct types of hydrate crystal growth: (a) There is no crystal growth with 0.01% naphthenic acid at Tex = 278.9 K and 277.8 K (ΔTsub = 1.3 and 2.4 K). The observations are in Figure 6. (b) When the cyclopentane hydrate crystals grew at the water−cyclopentane interface, they detached from the interface and floated in the water phase. The hydrate crystals grew until the interface disappeared, thus increasing crystal production compared to other types of hydrate crystal growth. Type (b) was observed for all naphthenic acid systems except for those in (a), all Span 80 systems, and 0.01% PPG Tex = 278.9 K, 277.8 K, and 275.9 K (ΔTsub = 1.3, 2.4, and 4.3 K). The observations are shown in Figures 3, 4, 5, 6, and 8. 816

DOI: 10.1021/cg501613a Cryst. Growth Des. 2015, 15, 812−821

Article

Crystal Growth & Design

Figure 8. Sequential images of the hydrate crystal growth in the mass fraction 0.01% PPG + cyclopentane solutions and water interface at atmospheric pressure and Tex = 278.9K, 277.8 K, 275.9 K (ΔTsub = 1.3 K, 2.4 K, 4.3 K).

Figure 9. Sequential images of the hydrate crystal growth in the mass fraction 0.001% and 0.01% surfactants solution systems at atmospheric pressure and Tex = 278.9 K, 277.8 K, 275.9 K (ΔTsub = 1.3 K, 2.4 K, 4.3 K).

water to hydrates in the systems of 0.001% PPG Tex = 278.9 K, 277.8 K, and 275.9 K (ΔTsub = 1.3, 2.4, and 4.3 K). The observations are in Figure 7.

(c) Cyclopentane hydrate crystals grew at the liquid−liquid interface to form a polycrystalline layer and covered the interface completely, thus preventing further conversion of 817

DOI: 10.1021/cg501613a Cryst. Growth Des. 2015, 15, 812−821

Article

Crystal Growth & Design

Figure 10. Summary of cyclopentane hydrate crystal growth and morphology based on subcooling ΔTsub. The red outline in the images identifies typical hydrate crystal shapes. Cyclopentane without surfactants was reprinted with permission from ref 24. Copyright 2010 American Chemical Society.

Type (c) was the same as that of the system without surfactants; the hydrate crystals grew at the liquid−liquid interface and covered the interface completely. This behavior suggests that PPG has a weaker effect in changing the wettability of hydrate on water compared to other surfactants. Type (c) was observed at 0.001% PPG Tex = 278.9 K, 277.8 K, and 275.9 K (ΔTsub = 1.3, 2.4, and 4.3 K). Crystal Morphology. Figure 10 is an ensemble of crystal morphology observations by pictures taken from the current study based on systems with different surfactants, mass fractions (0.001% and 0.01%), and ΔTsub, and pictures from Sakemoto et al.24 that show cyclopentane−water systems without surfactants. In this figure, the grain boundaries of characteristic hydrate crystals are outlined in red. As seen in Figure 10, there is no notable difference in the hydrate crystal configuration with and without surfactants. It is readily observed that the size of individual cyclopentane hydrate crystals decrease with increasing ΔTsub. The individual hydrate crystal sizes in the presence of surfactants were larger than those in the system without surfactants. In systems without surfactants, hydrate crystals continued to grow until they aggregated. The growth and the size of the individual crystals are limited by the hydrate film created from the contact of the growing crystals. On the other hand, since detachment of crystals from the interface typically occurred in the system with surfactants, there was less aggregation between crystals. As a result, most of the individual crystals were able to grow without ever meeting any other crystals, so the individual crystal size became larger. The representative crystal morphology observed in this study was polygonal. The hollow-conical crystals

The hydrate crystal growth behaviors (type (a), type (b), and type (c)) of each surfactant at different concentrations and subcooling temperatures are summarized in Figure 9. Type (a) exemplifies that naphthenic acid has a hydrate crystal growth inhibition effect for ΔTsub = 1.3 and 2.4 K at the concentration of 0.01%. The crystal growth rate proportionally increases with subcooling, which is the driving force for hydrate nucleation. Clathrate hydrate formation only occurred at the highest subcooling (ΔTsub = 4.2) with 0.01% naphthenic acid meaning that the surfactant increased the minimum ΔTsub required to form hydrate crystals. Among the surfactants experimented, this inhibition characteristic is unique to naphthenic acid, where the others enhanced hydrate production. Type (b), which is representative of surfactant effects on hydrate growth, shows that hydrate crystals sometimes detach from the interface and float in the water phase when cyclopentane crystals grew at the interface. This can be explained from an increase in wettability between watercyclopentane-hydrate with the addition of surfactants. The hydrate crystals at the interface could not conjoin to form a typical film that separated the water and guest phases. As a result, the contact of water and cyclopentane persisted, which led to an increase in hydrate crystal production in comparison with systems without surfactants. Type (b) was observed for all naphthenic acid systems except for 0.01% Tex = 278.9 K and 277.8 K (ΔTsub = 1.3 K, 2.4 K), all Span 80 systems, and all 0.01% PPG systems. Type (b), as stated before, is the characteristic phenomenon for the effect of surfactants on clathrate hydrate growth. 818

DOI: 10.1021/cg501613a Cryst. Growth Des. 2015, 15, 812−821

Article

Crystal Growth & Design

Figure 11. Growth rate of hydrate crystals at the interface of liquid cyclopentane with surfactants and water. If the variation is smaller than the symbol size, there is no error bar.

Figure 12. Illustration of the definition of the lateral growth rate of the hydrate v ≡ l/Δt. v is defined by the length of the lateral growth of a crystal over Δt, which is the time required for the growth.

previously reported by Keranjkar et al.26 was also observed in this study. Figure 11 shows the relation between ΔTsub and the lateral hydrate growth rates that were determined from the observations in the present study. The lateral growth rate v is defined as v = l/Δt, where l is the length of the lateral growth of a crystal and Δt is the time required for the growth. This definition is illustrated in Figure 12. The lateral growth rate represents the rate of one-dimensional hydrate crystal growth, sometimes referred to as the “growth rate of hydrate-film/layer propagation”. We have performed three independent measurements at a given thermodynamic condition to determine the crystal growth rate. Consistent with the previous studies,19,31−33 the lateral hydrate growth rate decreased with decreasing ΔTsub for all systems. In this study, except for the system of mass fraction 0.01% Span 80 and mass fraction 0.001% naphthenic acid at ΔTsub = 4.2 K, there was no clear surfactant effect for the lateral crystal growth rate. The hydrate growth rate is particularly high in these systems. The plot shows the average value of hydrate crystal growth of at least three independent measurements. From the plots, it is evident that 0.001% naphthenic acid promotes the growth of hydrate crystals at a faster rate compared to any other surfactant at a driving force of 4.2 K. In the surfactant systems, hydrate crystals generally detached from the interface and did not form polycrystalline layers. One can infer that the detachment of the hydrate crystals promoted mass and heat transfer, which are controlling factors in hydrate crystal growth. When hydrate crystals detached from the interface, the distance between each single hydrate crystal became greater. Therefore, the removal of heat increased resulting in the enhancement of heat transfer and promoted hydrate crystal growth.

At 0.01% Span 80 and 0.001% naphthenic acid systems, the hydrate crystals clearly detached from the interface. Moreover, from the observation, it can be inferred that the rate of hydrate detachment increased with increasing ΔTsub, so the rate of hydrate crystal growth was the highest at ΔTsub = 4.2 K. Furthermore, with hydrate formation, the surfactant absorption density on the interface and the surface tension changed. The gradient of surface tension will naturally cause Marangoni convection, which would increase mass transfer on the interface and again promote hydrate crystal growth.

■

CONCLUSION We performed a set of observations of hydrate formation and growth at the interface between liquid water and surfactant in a cyclopentane solution to clarify the effect of surfactants on hydrate crystal growth. The resulting crystal growth behavior was divided into three distinct types: (a) there is no crystal growth with 0.01% naphthenic acid at Tex = 278.9K and 277.8 K (ΔTsub = 1.3 and 2.4 K). (b) When the cyclopentane hydrate crystals grew at the interface, they detached from the interface and sank into the water phase. The hydrate crystal production thus increased for all naphthenic acid systems except for the aforementioned systems in (a), all span 80 systems, and 0.01% PPG Tex = 278.9 K, 277.8 K, and 275.9 K (ΔTsub = 1.3, 2.4, and 4.3 K). (c) Cyclopentane hydrate crystals grew at the liquid− liquid interface to form a polycrystalline layer and covered the interface completely in the systems of 0.001% PPG Tex = 278.9 K, 277.8 K, and 275.9 K (ΔTsub = 1.3, 2.4, and 4.3 K). Type (b) is considered as the representative phenomenon for the effect of surfactants on clathrate hydrate growth. From the observations, one can deduce that the surfactants made the hydrate crystal more wettable, so an impenetrable hydrate film 819

DOI: 10.1021/cg501613a Cryst. Growth Des. 2015, 15, 812−821

Article

Crystal Growth & Design

(12) Kelland, A. M.; Svartas, M. T.; Andersen, D. L. Gas hydrate antiagglomerant properties of polypropoxylates and some other demulsifiers. J. Pet. Sci. Eng. 2009, 64, 1−10. (13) 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, 21, 5441−5445. (14) Okutani, K.; Kuwabara, Y.; Mori, Y. H. Surfactant effects on hydrate formation in an unstirred gas/liquid system: An experimental study using methane and sodium alkyl sulfates. Chem. Eng. Sci. 2008, 63, 183−194. (15) 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. (16) Pietoro, D. P.; Simone, A.; R, G.; Gianfranco, S. Novel Nanostructured Media for Gas Storage and Transport: Clathrate Hydrates of Methane and Hydrogen, J. Fuel Cell Sci. Technol. 2007, Vol.4/49. (17) Takahashi, M.; Moriya, H.; Katoh, Y.; Iwasaki, T. Development of natural gas hydrate (NGH) pellet production system by bench scale unit for transportation and storage of NGH pellet. In Proceedings of the Sixth International Conference on Gas Hydrates, 2008; paper P-166 (5566). (18) Watanabe, S.; Takahashi, S.; Mizubayashi, H.; Murata, S.; Murakami, H. A demonstration project of NGH land transportation system. In Proceedings of the Sixth International Conference on Gas Hydrates, 2008; paper 4-B3 (5442). (19) Servio, P.; Englezos, P. Morphology of methane and carbon dioxide hydrates formed from water droplets. AIChE J. 2003, 49, 269− 276. (20) Tanaka, R.; Sakemoto, R.; Ohmura, R. Crystal growth of clathrate hydrates formed at the interface of liquid water and gaseous methane, ethane, propane; variations in crystal morphology. Cryst. Growth Des. 2009, 9, 2529−2536. (21) Saito, K.; Kishimoto, M.; Tanaka, R.; Ohmura, R. Crystal growth of clathrate hydrate at the interface between hydrocarbon gas mixture and liquid water. Cryst. Growth Des. 2011, 11, 295−301. (22) Kishimoto, M.; Ohmura, R. Correlation of the growth of the hydrate layer at a guest/liquid-water interface to mass transfer resistance. Energies 2012, 5, 92−100. (23) Watanabe, S.; Saito, K.; Ohmura, R. Crystal growth of clathrate hydrate in liquid water saturated with a simulated natural gas. Cryst. Growth Des. 2011, 11, 3235−3242. (24) Sakemoto, R.; Sakamoto, H.; Shiraiwa, K.; Ohmura, R.; Uchida, T. Clathrate hydrate crystal growth at the seawater/hydrophobic − guest − liquid interface. Cryst. Growth Des. 2010, 10, 1296−1300. (25) Nakajima, M.; Ohmura, R.; Mori, H.; Y. Clathrate Hydrate Formation from Cyclopentane-in-Water Emulsion. Ind. Eng. Chem. Res. 2008, 47, 8933−8939. (26) Karanjkar, P. U.; Lee, J. W.; Morris, J. F. Calorimetric investigation of cyclopentane hydrate formation in an emulsion. Chem. Eng. Sci. 2012, 68, 481−491. (27) Karanjkar, P. U.; Jae, W. Lee.; Morris, J. F. Surfactant Effect on hydrate crystallizations at the Water-Oil interface: Hollow-Conical Crystals. Cryst. Growth Des. 2012, 12, 3817−3824. (28) Aman, M. Z.; Olcott, K.; Pfeiffer, K.; Sloan, E. D., Jr.; Sum, K. A.; Koh, A. C. Surfactant adsorption and interfacial tension investigations on cyclopentane hydrate. Langmuir 2013, 29, 2676− 2682. (29) Lim, Y.; Babu, P.; Kumar, R.; Linga, P. Morphology of carbon dioxide-hydrogen-cyclopentane hydrates with or without sodium dodecyl sulfate. Cryst. Growth Des. 2013, 13, 2047−2059. (30) Lo, C.; Zhang, S. J.; Couzis, A.; Somasundaran, P.; Lee, W. J. Adsorption of cationic and anionic surfactants on cyclopentane hydrates. J. Phys. Chem. C 2010, 114, 13385−13389. (31) Freer, E. M.; Selim, M. S.; Sloan, E. D., Jr. Methane hydrate film growth kinetics. Fluid Phase Equilib. 2001, 185, 65−75.

could not form. Because of the detachment of hydrate crystals, the contact of water and cyclopentane was maintained, thereby increasing water−hydrate conversion and hydrate production. In the surfactant system, individual hydrate crystal sizes were larger than those in cyclopentane without surfactants. An increase in the hydrate crystal growth rate was observed for systems with a mass fraction of 0.01% Span 80 and 0.001% naphthenic acid at ΔTsub = 4.2 K. The increase in the crystal size and growth rate may both be ascribed to the enhancement of convective heat and mass transfer around hydrate crystals induced from the detachment of the crystals. In conclusion, the paradox of simultaneous antiagglomeration and hydrate promotion when using surfactants in producing cyclopentane hydrates has been clarified. The findings are, however, not limited to cyclopentane hydrate crystal, and can be further applied to other guest molecules as well as other applications such as increasing storage capacity in clathrate hydrates.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This study was supported by Keirin-racing-based research promotion fund from the JKA foundation Grant Number 26102 and by JSPS KAKENHI Grant Number 25289045.

■

REFERENCES

(1) Sloan, E. D.; Jr., Koh, C. A. Clathrate Hydrates of Natural Gases, 3rd ed.; CRC Press: Boca Raton, FL, 2008. (2) Mori, Y. H. Recent advances in hydrate-based technologies for natural gas storage-A review. Ind. Eng. Chem. Res. 2003, 54, 1−17. (3) Mao, W. L.; Mao, H. Hydrogen storage in molecular compounds. Proc. Natl. Acad. Sci. U. S. A. 2004, 101, 708−710. (4) Ogoshi, H.; Takao, S. Air-conditioning system using clathrate hydrate slurry. JFE Techn. Rep. 2004, 3, 1−5. (5) Sakamoto, H.; Sato, K.; Shiraiwa, S.; Takeya, M.; Nakajima, R. Ohmura. Synthesis, characterization and thermal-property measurements of ionic semi-clathrate hydrates formed with tetrabutylphosphonium chloride and tetrabutylammonium acrylate. RSC Adv. 2011, 1, 315−322. (6) Seo, Y.-T.; Moudravski, I. L.; Ripmeester, J. A.; Lee, J. W.; Lee, H. Efficient recovery of CO2 from flue gas by clathrate hydrate formation in porous silica gels. Environ. Sci. Technol. 2005, 39, 2315−2319. (7) Ogawa, T.; Ito, T.; Watanabe, K.; Tahara, K. I.; Hiraoka, R.; Ochiai, J. I.; Ohmura, R.; Mori, Y. H. Development of a novel hydratebased refrigeration system: A preliminary overview. Appl. Therm. Eng. 2006, 26, 2157−2167. (8) Sloan, E. D. Fundamental principles and applications of natural gas hydrates. Nature 2003, 426, 353−359. (9) Li, X.; Negadi, L.; Firoozabadi, A. Anti-agglomeration in cyclopentane hydrates from bio-and co-surfactants. Energy Fuels 2010, 24, 4397−4943. (10) Aman, Z.; Dieker, L. E.; Aspenes, G.; Sum, A. K.; Sloan, E. D. Influence of Model Oil with Surfactants and Amphiphilic Polymers on Cyclopentane Hydrate Adhesion Fo0rces. Energy Fuels 2010, 24, 5441−5445 cvx. (11) Dieker, L. E.; Aman, Z. M.; George, N. C.; Sum, A. K.; Sloan, E. D.; Koh, C. A, Micromechanical Adhesion Force Measurements between Hydrate Particles in Hydrocarbon Oils and Their Modifications. Energy Fuels 2009, 23, 5966−5971. 820

DOI: 10.1021/cg501613a Cryst. Growth Des. 2015, 15, 812−821

Article

Crystal Growth & Design (32) Saito, K.; Sum, A. K.; Ohmura, R. Correlation of hydrate-film growth rate at the guest/liquid-water interface to mass transfer resistance. Ind. Eng. Chem. Res. 2010, 49, 7102−7103. (33) Taylor, C. J.; Miller, K. T.; Koh, C. A.; Sloan, E. D. J. Macroscopic investigation of hydrate film growth at the hydrocarbon/ water interface. Chem. Eng. Sci. 2007, 62, 6524−6533.

821

DOI: 10.1021/cg501613a Cryst. Growth Des. 2015, 15, 812−821