Review pubs.acs.org/IECR
Role of Surfactants in Promoting Gas Hydrate Formation Asheesh Kumar, Gaurav Bhattacharjee, B. D. Kulkarni,* and Rajnish Kumar* Chemical Engineering and Process Development Division, CSIR − National Chemical Laboratory, Pune, India S Supporting Information *
ABSTRACT: Gas hydrates have been proposed as a potential technology for a number of applications, such as separation of gas mixtures, CO2 capture, transportation, and sequestration, methane storage and transport, and seawater desalination. Most of these applications will benefit from reduced induction time of hydrate nucleation, enhanced hydrate growth rate, and maximum water-to-hydrate conversion. The addition of surfactants to the gas−water system serves this purpose in a very effective manner. This review focuses on different surfactants that were utilized for gas hydrate formation studies; insights have been provided on the possible mechanisms of action through which these surfactants affect hydrate formation kinetics. A thorough analysis of the existing literature on surfactants suggests that enhanced rate of hydrate nucleation and growth kinetics may not be directly linked to micelle formation. Conversely, reduced surface tension in the presence of surfactants not only enhances the mass transfer but also changes the morphology of hydrate formation, which in turn enhances gas−water interactions for faster hydrate growth rate.
1. INTRODUCTION Gas hydrates are ice-like crystalline solids that can be synthesized in the laboratory by mixing water and gas(es), such as methane, carbon dioxide, and so forth, at low temperatures and/or high pressures. According to Jeffrey and McMullan, the guest molecules can be hydrophobic compounds, water-soluble acid gases, water-soluble polar compounds, and water-soluble ternary or quaternary alkyl ammonium salts.1 Once trapped inside the hydrogen bonded water cages, the guest molecules interact with water molecules through van der Waals forces and stabilize the structure.2−5 Gas hydrate formation is a crystallization process that is characterized by nucleation (dispersion of water and gas clusters that go on until a critically stable-sized nucleus has been formed)5 followed by growth and agglomeration.6,7 A natural gas hydrate sample of 1 m3 can produce ∼163 m3 of gas upon dissociation at standard temperature and pressure conditions and hence are considered to be a potential energy source. Natural gas hydrates exist under the sea bed and permafrost regions with a potential estimate of over 1.5 × 1016 m3 of gas reserves contained within them.8−14 It has been suggested that natural gas hydrates also exist on a number of planets, such as Mars, Saturn, Uranus, and Neptune and their several moons.15,16 Gas hydrates have also been considered a nuisance as they can plug the pipelines used in the oil and gas industries.17−19 The use of additives in gas hydrate-related studies was first explored with the objective of inhibiting gas hydrate formation in oil and gas pipelines so as to prevent oil and gas industries from suffering major losses. With the development of this research, two different classes of inhibitors have been identified: (i) those that shift the three phase boundary to extreme conditions (thermodynamic hydrate inhibitors (THIs)) in the presence of these additives in aqueous mixtures with the hydrates nucleating at a higher pressure (for a given temperature) or at a lower temperature (for a given pressure) and (ii) those that either inhibit the nucleation of stable gas hydrates (kinetic hydrate inhibitors (KHIs)) or prevent the agglomeration of formed solid gas hydrate crystals (anti-agglomerants (AAs)), which results in a transportable hydrate slurry formed in a liquid hydrocarbon © XXXX American Chemical Society
phase (Figure S1). KHIs and AAs are broadly classified together as low dosage hydrate inhibitors (LDHIs). To date, LDHIs seem to be the way to go as far as hydrate inhibition in oil and gas pipelines is concerned as these are required at very low concentrations (0.1−5 wt %) relative to those of THIs (20−40 wt %) and pose a much lower threat to the environment relative to typical THIs.20−22 While the search for better and more environmentally friendly hydrate inhibitors goes on, a different class of additives known as hydrate promoters that promote hydrate crystallization (nucleation and growth) have become essential in many gas hydrate-related applications, where faster hydrate growth is desired. In the recent past, several gas hydrate-based processes have been identified through lab scale demonstrations. Hydrate formation and decomposition cycles have been utilized in a number of technological applications, such as gas fractionation23−27 methane or hydrogen storage and transportation,28−30 carbon dioxide capture, transport, and sequestration,31−36 and desalination37,38 (Figure S2 represents a schematic of one of these applications). Figure 1 lists the major applications of gas hydrates in various industries. The hydrate-based gas separation (HBGS) process for CO2 separation from a gas mixture has been studied extensively. It has been identified that a commercially viable hydrate-based CO2 separation process would require a rapid hydrate formation rate.39 Hydrate formation is basically a crystallization process; upon successful hydrate nucleation, a thin hydrate film forms on the water−gas interface, which grows further in a mass transferlimited regime.40 It has been identified that other than utilizing better reactor designs, higher solubility of hydrate forming guests in water and a larger contact area between the hydrate formers and water can reduce the mass transfer resistance and ensure faster hydrate growth.29,41 Kalogerakis et al. suggested that the Received: September 17, 2015 Revised: November 17, 2015 Accepted: November 19, 2015
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Figure 1. Various applications of gas hydrates.
the induction time. Once nucleation occurs, hydrate formation enters into the growth phase. The growth phase continues for a certain period of time and can be concluded to have finished when no more gas uptake takes place, i.e., hydrate growth reaches saturation. The slope of the gas uptake curve can be utilized to measure the hydrate growth rate.45 In a typical stirred tank batch reactor, hydrate growth rate slows over time due to (a) limited heat and mass transfer and (b) a drop in driving force due to a drop in total pressure and/or due to preferential enclathration in the case of a gas mixture. A gas hydrate promoter is expected to reduce the induction time and sustain a fast hydrate growth rate for an extended period of time. Thus, a classical gas uptake experiment and the ensuing gas uptake curve in a stirred tank reactor can be utilized for identifying a hydrate promoter. The addition of surfactants to the aqueous phase results in fairly fast hydrate crystallization; thus, the use of such additives can be beneficial for several technological applications that operate through hydrate formation and decomposition cycles.
addition of surface active agents to water can enhance gas uptake rate during clathrate hydrate formation without affecting the three phase equilibrium.42 Literature also suggests that the presence of a small percentage of surfactant in the hydrateforming mixture changes the hydrate growth kinetics and its morphology. This changed morphology results in better gas− water contact and thus sustains faster hydrate growth kinetics for longer periods of time, resulting in better water-to-hydrate conversion.42−44 Figure 2 shows a typical gas uptake curve with its corresponding temperature profile and induction time. A general
2. SURFACE ACTIVE AGENTS (SURFACTANTS) AND THEIR ROLE IN GAS HYDRATE STUDIES Surface active agents (surfactants) are compounds whose molecules contain both lipophilic (hydrophobic) and hydrophilic moieties, i.e., they are amphiphilic (exhibit affinity for both polar and nonpolar substances (Figure 3(a)). The lipophilic and hydrophilic groups characteristic of each surfactant are the property-determining factors. Surfactants can diffuse from the bulk phase to an interface, altering the surface or interfacial tension, modifying the contact angle between the phases and wettability of solid surfaces, and thus changing surface charge and surface viscosity.46 At suitable concentrations, the surfactant molecules in water aggregate to form various kinds of structures (called micelles) with diverse shapes and orientations (spherical, rod-like micelles, multilayer structures, etc.; Figure 3(b)).47,48 Surfactants can mainly be classified into three categories depending on the moieties they contain, namely, anionic, cationic, and nonionic surfactants. Zwitterionic surfactants are another major class that is distinguished from others as these
Figure 2. A typical gas uptake curve along with the temperature profile showing the induction time for gas hydrate formation. Inset figures: (A) showing gas dissolution, (B) hydrate nucleation, and (C) hydrate growth.
gas uptake curve, as discussed by Natarajan et al., begins with a gas dissolution phase shown by a gradual drop in pressure of the reactor (in batch mode). This is followed by hydrate nucleation; the nucleation point is identified by a sudden rise in gas uptake rate and a corresponding sharp exothermic peak in the temperature profile. The time from the start of the experiment at which stable hydrate nucleation is observed is also known as B
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of the requirements, surfactants play a significant role.49 The use of surfactants in gas hydrate-related studies has been ongoing since the early 1990s.42,50,51 A thorough search of relevant literature clearly shows that the use of surfactants greatly enhances the kinetics of hydrate formation, and they have been widely used in different lab scale studies. However, there is still a lot of ambiguity as to which surfactant is going to work favorably for a particular system. It has been reported that the efficacy of these surfactants for enhanced kinetics are system specific.52,53 Literature suggests that the gas mixture being used for hydrate formation plays a role as does the presence/absence of any other additive, such as a thermodynamic promoter. To the best of our knowledge, a thorough review on this aspect is missing from the literature. In this paper, we present an extensive review of the work that has been conducted to date using surfactants for gas hydrate formation studies. Different mechanisms by which such surfactants may enhance the hydrate formation rate have been addressed, and how certain combinations of surfactants help in improving gas hydrate growth has been discussed. 2.1. Surfactants as Kinetic Hydrate Promoters. Table S2 presents a list of all the surfactants with their structures used so far in gas hydrate-based studies. In one such study, Karaaslan et al. used three different types of surfactants (anionic, cationic, and nonionic); they reported that hydrate formation kinetics was significantly better in the presence of the anionic surfactant as compared to a system without any surfactant. The effect of the nonionic surfactant was less pronounced, whereas the cationic surfactant showed behavior exactly opposite to that of the anionic surfactant at low and high concentrations.54,55 In a different study, Karaaslan et al. studied the effect of three
Figure 3. (a) A surfactant molecule with hydrophilic and lipophilic groups and (b) micelle formation in aqueous media through association of surfactant molecules.
compounds contain both cationic and anionic centers attached to the same molecule (Figure 4). Surfactants find applications in a number of industries, including petroleum and food. The addition of surfactants to a multiphase system enhances surface activity, which favorably affects the spreadability, wetting, foaming, detergency, and so forth, of the system. In industries where micelle formation is one
Figure 4. Different classes of surfactants and their corresponding structures. C
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was varied from 260 to 10000 ppm.64 Much earlier in a similar study performed on ethane hydrate, Zhong and Rogers found that by adding ∼284 ppm of SDS to an ethane−water system, the rate of hydrate formation increased by a factor of greater than ∼700 as compared to a system having only pure water. They also reported good reproducibility of the induction time in the surfactant solution compared with a no surfactant experiment. Furthermore, it was suggested that the formation of micelles in the presence of surfactants not only enhanced ethane solubility but the micelles themselves acted as nucleating sites for faster hydrate growth.43 Authors report that the CMC value of the SDS−water solution decreases with pressure from 2725 ppm (at atmospheric pressure and room temperature) to 242 ppm (at hydrate-forming conditions). At 242 ppm of SDS, there was a significant change in the hydrate induction time, which was used to define the CMC. Zhang et al. found that pure nonionic surfactant Tween-40 (T40) performed better in promoting a stable crystal nuclei and shortening the induction time compared to T40 (Tween-40)/T80 (Tween-80) (1:1) and T40/T80 (4:1) mixtures.65 It is important to note that the micellization tendency of surfactant mixtures are completely different from those of pure species, and mixed surfactants show superior performance as compared to individual surfactants.49 Mandal and Laik studied the effects of anionic surfactant (SDS, 300 and 500 ppm) on ethane hydrate formation, dissociation, and storage capacity in a static system. They concluded that in the presence of SDS, hydrates grow as very fine particles with enhanced gas consumption and storage capacities. They measured the hydrate dissociation rate and showed that the presence of SDS lowers the self-preservation effect and increases the dissociation rate. However, their finding that the presence of SDS shows a thermodynamic effect on hydrate formation resulting in a shift in the formation temperature (at a given pressure, compared to pure water) contradicts other work in the literature.66 A critical commentary on this contradictory claim was provided by Y.H. Mori. Mori pointed out the doubtful experimental data provided in the paper discussed above66 and also argued that there was no evidence obtained to support the surfactant micelle hypothesis as claimed by Mandal and Laik.67 In light of such contradicting reports regarding faster hydrate nucleation in the presence of micelles, it is concluded that the surfactant micelle hypothesis has not been tested extensively, and at this point in time, there is no concrete evidence in the literature to support this claim. A detailed discussion on the same has been provided in sections 3 and 3.1. Gayet et al. performed studies on methane hydrate equilibrium conditions in the presence of 0.02 wt % of SDS and found that SDS did not have any effect on the gas hydrate equilibrium but rather enhanced the hydrate formation rate. Additionally, they visually observed that, for a pure water system, nucleation and growth of hydrates usually occurred at the water− gas interface, whereas for a water−SDS system, hydrates grow as a porous structure on the reactor wall. It was observed that liquid migrates from the bulk phase to the gaseous phase through the porous hydrate structure (capillary-driven water supply).40 In a similar study, Torré et al., while studying the growth of CO2 hydrate in the presence of 0.3 wt % of SDS, suggests that SDS does not influence the thermodynamic phase boundary; however, it enhances the rate of hydrate formation.68 SDS has also been used in combination with thermodynamic promoters like THF69,70 and CP71,72 (refer to Table S1 for more details). It has been observed that the presence of thermodynamic promoters like THF and CP reduces the influence of SDS and its effect as a hydrate promoter compared to that of a pure water
nonionic surfactants: polyoxyethylene (5) nonylphenyl ether (IGEPAL-520), Brij-58, and Tween-40. The authors concluded that IGEPAL-520 is the most effective hydrate formation promoter among the three. An amount of 1 wt % of IGEPAL520 accelerates the methane hydrate formation rate by a factor of 2.4 compared that in pure water.56 Okutani et al. studied the effects of three homologue anionic surfactants (sodium dodecyl sulfate (SDS), sodium tetradecyl sulfate (STS), and sodium hexadecyl sulfate (SHS)) on methane hydrate formation. At concentrations of ∼1000 ppm or above, SDS was found to be very effective in increasing both the rate of pure methane hydrate formation and the final water-to-hydrate conversion. However, an equivalent promotional effect was seen with STS at a much lower concentration of ∼100 ppm. It was concluded that STS is more favorable than SDS as far as methane hydrate formation is concerned. SHS was found to be less effective compared to SDS and STS.57 According to Yoslim et al., the addition of SDS (concentration range between 242 and 2200 ppm) increases the gas uptake rate for a mixed hydrate of methane and propane (CH4/C3H8) by 14 times as compared to that for pure water. At SDS concentrations of 2200 and 645 ppm, the drop in the reactor pressure, which relates to the extent of water-to-hydrate conversion, was found to be maximum. They suggest that, in the presence of the surfactant (SDS), the liquid− gas interface does not get covered with an impermeable solid hydrate film but rather that the hydrate grows as a porous surface, allowing efficient water-to-gas contact for better conversion. Hydrate growth was also seen on the walls of the reactor, suggesting better water-to-gas contact by a capillary effect.58 The capillary mechanism of hydrate-layer growth mentioned above was first observed by Mel’nikov and Nesterow. They observed the growth of a porous hydrate layer with a thickness of approximately 5−10 mm at the surface of a sand sample saturated with water.59 Kumar et al. studied the three types of surfactants (anionic, cationic, and nonionic) for CO2 hydrate formation kinetics; anionic surfactant (SDS) was found to be most effective in enhancing the rate of hydrate formation as well as reducing the induction time. Nonionic surfactant (Tween-80) was found to be better than the cationic surfactant DTACl.33 Recently, Veluswamy et al. also used such types of cationic and nonionic surfactants (DTACl and Tween-20, respectively) for mixed hydrogen/tetrahydrofuran (THF) and methane/THF hydrate formation. They observed a marginal improvement in hydrogen THF hydrate formation rates, whereas a reduction of hydrate formation rate of methane/THF mixed hydrate were observed. Thus, the effect of surfactants depends upon the guest gas and the system.60 Kang et al., through their experimental work, have concluded that the use of an optimum concentration of SDS acts as a promoter, but an excess amount of the same can inhibit hydrate growth. In the presence of SDS, initial hydrate formation rates were found to increase the gas consumption, resulting in faster hydrate growth.61 Han et al., in their study on natural gas hydrates, concluded that maximum gas hydrate formation for natural gas was achieved at a 300 ppm concentration of SDS.62 Link et al. tested a large selection of surfactants to gauge their kinetic-promoting properties on methane hydrate formation. The authors have reported that, out of all the surfactants tested, SDS was the best surfactant for promoting methane hydrate formation.63 Zhang et al., working with methane hydrate, reported that the use of SDS reduces the induction time; however, a systematic trend was not observed between induction times and SDS concentrations when the concentration of SDS D
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studied as additives for hydrate formation. Kwona et al. synthesized a Gemini-type multichain surfactant and measured methane hydrate storage capacity in the presence of these additives. The experiments were conducted in a stirred tank reactor having a volume of 350 cm3. They found that the multichain disulfonate surfactant synthesized exhibited higher methane storage capacity than SDS (sodium dodecyl sulfate) even at lower surfactant concentrations. This observation was attributed to the lower CMC and surface tension value for the Gemini surfactant as compared to an SDS−water solution having a similar concentration.76 Verrett and Servio investigated the effects of Gemini (DOWFAX 8390 surfactants) and conventional (sodium dodecyl sulfate) surfactants on hydrate growth in a stirred tank reactor with a volume of 600 cm3. DOWFAX 8390 (DOWFAX) contains 35.6% active ingredients, 0.6% sodium sulfate, and 0.1% sodium chloride. Here, active ingredients are a mixture consisting of roughly 75% disodium hexadecyldiphenyloxide disulfonate (monoalkyl disulfonate surfactant) and 25% disodium dihexadecyldiphenyloxide disulfonate (gemini surfactant). The authors have reported enhanced kinetics for methane hydrate formation in the presence of these Gemini surfactants.77 Few studies in the literature report that, at certain concentrations, surfactants like SDS, cetyltrimethylammonium bromide (CTAB), and LS-54 (fatty alcohol EO/PO derivative) may act as hydrate inhibitors. 78,79 Certain zwitterionic surfactants have also been suggested to work as antiagglomerants or as kinetic hydrate inhibitors.80−82 However, a thorough impurity profile of the surfactants used in these studies has not been discussed, and thus, we are not discussing these manuscripts in this review. Even though the data presented in the literature overwhelmingly support the utility of surfactants as promoters for hydrate crystallization, the qualitative knowledge obtained so far is system specific. Dependencies on the surfactant concentration for hydrate-formation rate and the final waterto-hydrate conversion ratio have been established for many guest species. However, it is not clear whether such dependencies actually exist or if it is more to do with the different reactor configurations used in these studies. As discussed, SDS is one of the most common surfactants used as a hydrate promoter; Table
system. Zhong et al. studied the influence of cyclopentane (CP) and SDS on methane separation from low-concentration coal mine gas. They found that the gas uptake and rate of hydrate formation were dependent on SDS concentration, but the presence of SDS did not show any clear influence on methane recovery. The methane recovery obtained in the presence of SDS was 33.3%, whereas that obtained without SDS was 33.1%.71,73 They have reported that SDS was not very effective in promoting methane enclathration in the presence of CP. However, methane enclathration is accelerated by adding salts like NaCl and NaClO4. Li et al. found that the methane hydrate formation rate for a cyclopentane/water emulsion with Tween-80 was better than that obtained in the absence of Tween-80, and a higher gas/ liquid contact area in the presence of surfactant was identified as one of the reasons for the obtained results.74 Gemini surfactants are a new and unique class of surfactants; they are dimeric surfactants having two hydrophilic head groups and two hydrophobic tails. The hydrophilic head groups of the surfactants are linked by a spacer group of varying length (most commonly an ethylene spacer or an oxyethylene spacer) as shown in Figure 5.75 Gemini surfactants not only have lower
Figure 5. General structure of a Gemini surfactant.
CMC values but also show lower surface tension at their respective CMC values. Gemini surfactants have also been
Figure 6. Normalized rate of CO2 hydrate formation for different fixed bed media and STR configurations with the effect of surfactants (SDS). E
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Figure 7. Properties of surfactant molecules (data adopted from Rosen,86 Rosen et al.,120 and Swisher121).
On the basis of these discussions (section 2.1), one can say that the surfactant (SDS) performs best in unstirred systems (quiescent or in porous medium); in a stirred tank configuration, enhancement of the kinetics of hydrate formation is not very significant probably due to enhanced mass transfer in a stirred tank reactor (with or without surfactant) and the mixing effect that could affect the alignment of SDS at the interface.
S1 compares the effect of SDS at different conditions on different experimental systems with the variables being the gas used for hydrate formation, the nature of the reactor (STR or fixed bed), the packing media used in the case of fixed bed reactors, the presence of other additives in the system, such as CP and THF, and the experimental pressure and temperature conditions. It is clear from the data provided in Table S1 that the optimum concentration of surfactant required for a system is a characteristic property of the system studied. For a quick comparison of the effect of surfactants on the rate of hydrate formation, in some of our previous works we have calculated the rate of hydrate formation (R30) by giving a linear fit for the hydrate growth (gas consumed) data versus time and subsequently presented the normalized rate of hydrate formation (NR30) using the equation given below.53,83 normalized rate of hydrate formation (NR30) R = 30 (mole of gas min−1 m−3) Vw
3. MECHANISM OF ACTION OF DIFFERENT SURFACE ACTIVE AGENTS ON CLATHRATE HYDRATE FORMATION In general it has been observed that surfactants with large hydrophobic groups and large hydrophilic groups show lower interfacial tension values than similar surfactants with lower molecular weights and with the same balance of hydrophilic and lipophilic groups.84,85 Surfactants with the hydrophilic group in the center of the molecule and with 12−14 carbon atom hydrophobic chains are found to be excellent wetting agents.86 Closer packing of the surfactant molecules at the interface may decrease the rate of diffusion of the gas. Consistent with this, interfacial resistance to gas diffusion has been shown to increase with an increase in the number of carbon atoms in the hydrophobic group and with a decrease in the molecular mass of the hydrophilic group.87 When a hydrophilic group is shifted to a more central position in the chain, the CMC of the surfactant increases, which might have an impact on the kinetics of hydrate
(1)
where Vw is the volume of water taken for the experiment in m3. A comparison of the initial rate of pure CO2 hydrate formation (for the first 30 min after nucleation) from data available in the literature is shown in Figure 6; it can clearly be seen that the presence of SDS promotes hydrate growth rate irrespective of the reactor configuration (fixed bed media or STR). F
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Figure 8. Relationships between surfactant solubility, CMC, and the Krafft point.
surfaces.89 Watanabe et al. reported the importance of surfactant concentration in the system and how surfactant solubility in aqueous solution may have a role to play in affecting hydrate growth. In their study, however, Watanabe et al. reported their strong suspicion on the surfactant-micelle hypothesis for enhanced hydrate formation kinetics. Their suspicion was based on the fact that micelles cannot form below a certain temperature known as the Krafft temperature, which for SDS, the surfactant of choice in most hydrate formation studies, is above the typical hydrate formation temperature. The authors concede however that it is not entirely impossible for micelle formation to take place under hydrate formation conditions as the Krafft point data have all been obtained under atmospheric pressure and may very well vary under hydrate formation conditions. Conversely, there is no data available in the literature to suggest otherwise, thus greatly rationalizing the suspicions of the surfactant-micelle hypothesis.90 3.1. Krafft Temperature and CMC of Surfactants and Their Role in Gas Hydrate Formation. A sharp increase in the solubility of surfactants occurs above a certain temperature, which is a characteristic of each compound. This temperature is termed the Krafft point (Figure 8). The Krafft point is the minimum temperature at which micelles can form.47,90 Table 1 lists the Krafft temperature of selected surfactants. As shown in Figure 8, the solubility of a surfactant increases exponentially above the Krafft point, thereby indicating micellization and reduction in surface tension of the aqueous phase.91 Nonionic surfactants do not exhibit a Krafft point; their solubility decreases with increasing temperature, and these surfactants may begin to lose their surface active properties above a transition temperature referred to as the cloud point.92 A surfactant does not micellize at temperatures below its Krafft point or below a certain concentration known as the critical micelle concentration (CMC). It can be gauged from Figure 8 that, at the Krafft point, the solubility of the surfactant is equal to
formation and decomposition. Figure 7 represents the properties of surfactant molecules that in some way might affect the hydrate formation kinetics. Mass transfer limitations are identified as one of the major reasons for low (∼20−30%) conversion of water to hydrate in a stirred tank reactor. As discussed above, literature suggests that the use of surface-active agents allows for better water-to-hydrate conversion. In general, anionic surfactants have been identified as the best candidates to enhance the rate of hydrate growth. Kalogerakis et al. investigated the effect of surfactants (cationic, anionic, and nonionic) on methane hydrate formation kinetics and suggested that the surfactants used in the study form a micellar structure (spherical aggregation of surfactant molecules, as shown in Figure 3(b)) in the solution, accelerating hydrate growth by reducing the hydrate nucleation barrier (presence of surfactant results in lower induction time). Additionally, it was suggested that surfactants do not take part in hydrate formation and thus do not influence the thermodynamic phase boundary.42 Subsequently, quite a few studies have quoted the above reference in support of enhanced hydrate growth due to the formation of micelles in the presence of surfactants. Zhong and Rogers, while studying the effect of surfactants of nonpolar gases, suggested that the formation of micelles not only increases the solubility of hydrocarbon gas in the aqueous phase but that it also acts as a nucleating site, inducing the formation of hydrate crystals around the micelle in the bulk water phase.43 Sun et al. showed that micellar surfactant solutions increase the gas hydrate formation rate and storage capacity.73,88 Woods et al. suggested that surfactant molecules may adsorb and orient as micelles on appropriate solid interfacial surfaces in the gas hydrate system. Evidence of this is seen especially in porous media saturated with water/biosurfactant solutions where adsorption of the biosurfactant to promote hydrate growth depends on the specific mineral surfaces and the molecular orientations of the biosurfactants adsorbed on those G
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Krafft temperature.64,95 It is therefore quite safe to conclude that the promoting effect of SDS in gas hydrate nucleation and growth at temperatures below the Krafft point may not be explained by SDS micellization alone. Prajapati and Bhagwat,96 in one of their studies, suggested that the Krafft point of an anionic surfactant, such as SDS, can be reduced by the addition of certain chemicals, which they term as foam boosters. In a follow up to this work, Prajapati and Bhagwat97 also reported that the CMC of anionic surfactant solutions can be decreased by the addition of the same foam boosters. In both studies, it was observed that a zwitterionic betaine, cocoamidopropyl betaine (CAPB), obtained from coconut oil, has a synergistic effect with SDS. They reported that only a small amount of CAPB (0.038 mol %) drastically reduces the Krafft point of SDS. Thus, a low value of CMC might be a critical factor in the enhancement of hydrate formation as well as in other process industries. As can be seen in Figure 8, the solubility of a surfactant increases drastically once the CMC value is reached. It has also been reported that the addition of CAPB to SDS triggers a synergistic change in the shape of the micelles from spherical to cylindrical or rod-like. However, whether this phenomenon can actually affect hydrate formation and if so, how, needs to be understood and can only be achieved through rigorous experimentation.98 Some of the other techniques that can be employed to obtain a low CMC are (i) by increasing the molecular mass of the hydrophobic moiety (ii) reducing the temperature (iii) adding an electrolyte to the system (iv) the existence of polyoxypropylene group92 (v) having a fluorocarbon in the structure of surfactant (vi) the coexistence of polar organic compounds (such as alcohols and amides) (vii) the addition of xylose and fructose99,100 Table S3 lists the CMC values of some common surfactants at different temperatures and employing different methods of measurement of CMC. In addition, Table S3 also shows the effect of different additives on the CMC values of SDS. We believe that a detailed experimental study (gas uptake data) using two similar surfactants, one with the Krafft point below the gas hydrate formation temperature and one with the Krafft point above the hydrate formation temperature, would clearly suggest if micelle formation actually helps enhance hydrate growth. In one such study, Ando et al. used the surfactants lithium dodecyl sulfate (LDS), dodecyl benzenesulfonic acid (DBSA), and sodium oleate (SO), which have sufficiently low Krafft points to allow for micelle formation under typical hydrate forming conditions (experimental temperature of 275 K and pressure of 3.9−4.0 MPa using CH4 gas in an unstirred reactor). Significant increases in the rate of hydrate formation and the final water-to-hydrate conversion ratio were simultaneously observed. However, there is no appreciable change in either the qualitative hydrate-formation behavior or quantitative hydrate formation-related parameters as a result of an increase in the LDS or DBSA concentration in the aqueous phase crossing their relevant CMC values, respectively. That is, the micelle formation in the aqueous phase neither promotes nor retards hydrate formation. The effect of LDS and SDS was almost comparable, and the addition of SO to any concentration, either below or above the CMC, hardly promotes hydrate formation.101 3.2. Role of Surfactant Adsorption on the Hydrate Interface. Anionic and cationic surfactants tend to adsorb on
Table 1. Selected Surfactants and their Krafft points S. No.a 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
surfactant C12H25SO−3Na+ C12H25OSO−3Na+ (SDS) C14H29SO−3Na+ C14H29OSO−3Na+ C16H33SO−3Na+ C16H33OSO−3Na+ sodium 4-(3-nonyl)benzenesulfonate sodium 4-(5-decyl)benzenesulfonate C12H25C6H4SO3Cs (cesium 4dodecylbenzenesulfonate) C6F13COONa C7F15COONa C8F17COOH C8F17COONa C10F21COONa MV(C10H21SO3)2 (methylviologen bisdecanesulfonate) C8BP(C14)2 (1,1′-(1,ω-octanediyl)-bispyridinium tetradecanesulfonate) C10BP(C14)2 (1,1′-(1,ω-decanediyl)-bispyridinium tetradecanesulfonate)
Krafft point (°C) 38 16 48 30 57 45
