Influence of the carbon chain length of a sulfate-based surfactant on the formation of CO 2 , CH 4 and CO 2 –CH 4 gas hydrates

This study investigates how the length of the carbon chain of homologous surfactants affects the amount and growth rate of gas hydrates formed in quiescent CO2/CH4/water systems. The hydrate formation experiments were conducted using sodium alkyl sulfates with different carbon-chain lengths (C8, C10, C11, C12, C13, C14, C16 and C18), at a concentration of 10.4 mol m−3. The CO2:CH4 ratios investigated were 0:100, 25:75, 75:25, and 100:0. Hydrate formation was studied in a batch reactor at an initial subcooling of about 5 K. It was observed to be efficient only for those surfactants that promote the formation of a water-wettable porous hydrate structure, which spreads over the inner sidewalls of the hydrate-formation cell. For the CO2:CH4 ratios of 0:100, 25:75, 75:25 and 100:0, hydrate formation was efficient for the surfactants with respectively 8 to 14, 11 to 13, 11 to 12, and 12 carbon atoms in their alkyl chain. Only the surfactant with 12 carbon atoms was found to promote and accelerate hydrate growth for all the gas-phase compositions tested. The much lesser surfactant effect on hydrate growth rate observed with the increase in the initial CO2 fraction in the gas phase is ascribed to a modification of the adsorption behavior of the surfactant molecules on the hydrate surface, which, as already suggested by Zhang et al. (2010), is probably due to competitive adsorption between the surfactant anions and bicarbonat

Using Microscopic Observations of Cyclopentane Hydrate Crystal Morphology and Growth Patterns To Estimate the Antiagglomeration Capacity of Surfactants

The crystal growth and morphology of cyclopentane (CP) hydrates at a quiescent water/oil interface in the presence of 10 different surfactants were observed under a microscope. In most cases, the oil was CP, but for some of the observations a 50/50 vol % mixture of CP and n-octane (n-C8) (or n-dodecane (n-C12)) was used instead. For some of the surfactants, gas hydrates formed from a methane (CH4)/propane (C3H8) gas mixture at a quiescent water/n-C8 interface were also observed. The capacity of the surfactants to prevent the hydrate particles from agglomerating was assessed by measuring torque on oil-dominated systems (70 vol %) in a stirred autoclave at subcoolings of 6 and 10 °C for the CP hydrates and CH4/C3H8 hydrates, respectively. The oil phases were the same as those used in the morphology study. In the case of CP hydrates, the agglomeration state of the system was directly observed by opening the autoclave at the end of the hydrate formation. The size of the CP hydrate particles was measured, and their wettability was determined. The effect of the presence of salt (NaCl) on the crystal morphology and AA performance was also studied for some systems. All the surfactants that induced the formation of hydrate crystals that rapidly agglomerated at the water/CP interface showed poor AA performance. Whenever the surfactants induced the formation of individual oil-wettable crystals, their AA performance was good. If the individual crystals formed were water-wettable, two main behaviors were observed: (1) when the surfactant induced a very low water/CP interfacial tension (1 mN/m), it exhibited poor AA performance. These trends in the AA performance of the surfactants were observed on both hydrate systems (CP hydrates and CH4/C3H8 hydrates). From the experimental results obtained in this work, we can infer that the microscopic observation of the morphology and growth pattern of CP hydrate crystals formed at a quiescent water/CP interface might be a simple way to rapidly assess if a surface-active molecule has an antiagglomeration effect on sII gas hydrates

Hydroquinone clathrate based gas separation (HCBGS): Application to the CO2/CH4 gas mixture

Hydroquinone (HQ) clathrates have recently been identified as promising candidates for selective gas capture and storage processes. This study evaluates the effectiveness of HQ clathrates in the separation of CO2 from CO2/CH4 gas mixtures, through direct gas/solid reactions in a fixed-bed reactor. The influence of the process operating parameters (i.e. reaction time, pressure, temperature and feed gas composition) on the CO2 capture kinetics, selectivity towards CO2, and transient storage capacity were investigated. The experiments were performed using either pure HQ or HQ-based composite materials, with temperatures ranging from about 283 to 343 K, pressures from 3.0 to 9.0 MPa, and CO2 mole fraction in the gas mixture ranging from 0.2 to 1. The experimental results show that over the range of gas composition investigated, the enclathration reaction is selective to CO2. This preferential CO2 capture is enhanced at high CO2 mole fractions, low temperatures and high pressures. Regarding gas capture kinetics, it was confirmed that the composite material is much more efficient than pure HQ crystals. The CO2 enclathration rate increases with temperature, pressure and CO2 fraction in the feed gas. For the first time, the feasibility of such gas separation techniques using HQ clathrates was demonstrated at bench scale

CO2 capture by hydrate formation in quiescent conditions: In search of efficient kinetic additives

International audienceAs a preliminary step to the development of a CO2 capture process under high pressure conditions, an experimental kinetic study of CO2 hydrate formation has been carried out in a high-pressure batch reactor, using as water-soluble additives a mixture of tetrahydrofuran (THF) and surfactant (sodium dodecyl sulfate, SDS). Used together and in suitable concentrations, these two additives were found to be very efficient for promoting CO2 capture

Effect of a Hydrophilic Cationic Surfactant on Cyclopentane Hydrate Crystal Growth at the Water/Cyclopentane Interface

The effects of a water-soluble quaternary ammonium surfactant (called DA 50) on cyclopentane (CP) hydrate growth were studied at the water/CP interface. Microscopic observations were made without and with DA 50 at concentrations of 0.01, 0.1, and 1 wt % (in the aqueous phase). In addition, the effect of NaCl at different concentrations (0 to 4 wt %) was also investigated, in the presence and absence of the surfactant. Systems with 0, 0.1, or 1 wt % DA 50 and 0 or 2 wt % NaCl, as well as those with 0.01 wt % DA 50 and 2 or 3 wt % NaCl, all led to the formation of a hydrate layer, composed of an assembly of smooth and/or striated plates, at the water/CP interface. With 0.01 wt % DA 50 and without NaCl, hydrate needles formed at the interface before aggregating into a thick unconsolidated layer in the aqueous phase. For systems without DA 50 and with 3 or 4 wt % NaCl, and for the one with 0.01 wt % DA50 and 4 wt % NaCl, a few hexagonal, triangular, and needle-like crystals grew very slowly at the interface, and most of the interface remained free of hydrate crystals for several hours after the onset of crystallization. Spectacular changes in the hydrate growth pattern and morphology were observed for the systems with 3 or 4 wt % NaCl and 0.1 or 1 wt % DA 50, where small individual crystals, in the shape of step pyramids with their vertex pointing to the CP phase, formed at the interface. Results of interfacial tension measurements showed that the adsorption kinetics of the surfactant molecules and the amount of surfactant adsorbed on the water/CP interface increased significantly with NaCl concentration. A formation mechanism of the pyramidal hydrate crystals is proposed

Effects of a Quaternary Ammonium Salt on the Growth, Wettability, and Agglomeration of Structure II Hydrate Crystals

This work studied the effects of a water-soluble quaternary ammonium salt (called DA 50) on the growth, wettability, and agglomeration of cyclopentane (CP) hydrate crystals and methane (CH4)/propane (C3H8) hydrate crystals. The impact on these properties of adding 4 wt % NaCl to the DA 50 solution was also investigated. The hydrates were formed from water/CP, water/(CP + n-octane (n-C8)), and water/(CP + n-dodecane (n-C12)) mixtures at atmospheric pressure and from a water/n-C8/(CH4 + C3H8) mixture under pressure (about 67 bar). Experiments were performed at a subcooling of 6 °C in the case of the CP hydrates and 9–10 °C in the case of the CH4/C3H8 hydrates. In both hydrate systems, adding NaCl to the surfactant solution of 0.1 or 1 wt % DA 50 led to the formation of individual oil-wettable pyramidal crystals. Without salt, the hydrate formed a water-wettable shell that covered the water/oil interface just as the system without surfactant did. The antiagglomeration performance of the 1 wt % DA 50 solution was evaluated by performing torque measurements in an agitated batch reactor at a water cut of 30 vol %. Without NaCl, torque increased with the amount of CP hydrates. The system formed a nonflowable jelly-like phase, with water as the continuous phase, until a phase inversion occurred. From there on torque significantly decreased and the system became a flowable dispersion of large hydrate particles (∼700 μm) in the CP phase. With 4 wt % NaCl, the system consisted of small (∼70 μm) hydrate particles dispersed in the CP phase and the torque signal remained constant throughout the hydrate crystallization process. The torque profiles obtained at concentrations of 0 or 4 wt % NaCl for the CP hydrates and the CH4/C3H8 hydrates were similar, suggesting analogous states for both systems. For both hydrate systems, adding NaCl to the DA 50 solution led to the formation of oil-wettable hydrates and drastically improved the antiagglomeration performance of the surfactant molecules, revealing a correlation between the formation of individual crystals and the antiagglomeration performance of the surfactant. The similarity between the growth patterns and shapes of the CP–hydrate crystals and the CH4/C3H8–hydrate crystals confirmed that CP hydrates are an interesting model for evaluating the antiagglomeration performance of surfactants

Carbon dioxide gas hydrate crystallization in porous silica gel particles partially saturated with a surfactant solution

This paper reports on investigations into the way carbon dioxide (CO2) hydrate forms in porous silica gel partially saturated with pure water or with a surfactant solution. The experiments, conducted at two different temperatures (278.2 and 279.2 K) and under a loading pressure of 3.8 MPa, used silica particles of different nominal pore diameters (30 and 100 nm), saturated at 80% pore volume with pure water or with a 100 ppm solution of either sodium dodecyl sulfate (SDS) or polyoxyethylenesorbitan monoleate (Tween-80). They were run following the “hydrate precursor method” developed in previous works (Duchateau et al., 2009, 2010) to form bulk hydrate under controlled subcooling conditions, and adapted for studying hydrate formation behavior in porous media. The work demonstrated that the successive hydrate formation and decomposition cycles involved in this method do not alter the pore size distribution in the porous media. At the two temperatures investigated, silica gel particles with a nominal pore diameter of 100 nm proved better suited to comparing the CO2-hydrate formation behaviors: higher water-to-hydrate conversions (>90 mol%) were effectively obtained for all the conditions tested making comparison of the results much easier. Of the two surfactants used, only SDS was found to produce a positive effect on both the hydrate formation kinetics and the amount of hydrate formed. Our visual observations of quiescent bulk systems (without porous silica gel) suggest that when SDS is present, CO2 hydrate forms not only at the w/g interface (where it occurs without SDS too), but also in the bulk water phase. This may explain the beneficial effect observed on the porous medium

Anti-agglomerant performance of surfactants evaluated in cyclopentane hydrate and CH4/C3H8 gas hydrate systems

The main objective of this work was to evaluate and compare the AA performance of surfactants of different affinity in cyclopentane (CP) hydrate and gas (methane – propane mixture) hydrate systems. The study was performed with two surfactants: Noramium® DA 50 and Inipol® AH 81, which are respectively water-soluble and dispersible in water. Their AA performance was evaluated and compared without or with 4 wt% NaCl. A comparative study of the effect of the surfactants on the water/CP interfacial activity was carried out by surface pressure measurements. The impact of the surfactants on hydrate formation and morphology was observed by microscopic observations at the water/CP interface. Lastly, the AA performance of DA 50 and AH 81, was evaluated at the macroscopic scale in a batch reactor under agitation. The experiments in reactor were performed in oil-dominated systems (70 vol%) with CP as the oil phase in the CP hydrate system, and n-octane in the gas hydrate one

Effect of the concentration and carbon chain length of a sulfate-based surfactant on hydrate formation kinetics

This paper reports an experimental study on the effect of the concentration and the carbon chain length of surfactants on the formation kinetics of gas hydrates in a quiescent CO2/CH4/water system. Sodium alkyl sulfates with different carbon chain length (C8, C10, C12, C14, C16 and C18) were tested at different concentrations. The experiments were conducted in a batch configuration, at a temperature of 275 K and with an initial gas pressure of about 3.3 MPa. For each system studied, hydrate crystallization was triggered by suddenly injecting a small amount of THF (4,000 ppm) directly into the surfactant solution in contact with the gas-hydrate-former phase at 275 K and 3.3 MPa. The long induction time for hydrate formation usually observed for these systems at the pressure and temperature conditions used in this study was thus eliminated. The experimental results show that, of the six surfactants tested, only the sodium dodecyl (C12) sulfate efficiently promotes the formation of CO2-CH4 binary hydrate under quiescent conditions. A minimum concentration of 500 ppm of the C12 surfactant was however necessary to obtain a beneficial effect on hydrate formation, and the rate of hydrate crystallization was observed to level off for the surfactant concentrations higher than 3,000 ppm. For the systems containing the C8 and C10 surfactants, which have a Krafft temperature lower than 275 K, the presence or absence of micelles in the surfactant solution does not have any effect on the hydrate formation kinetics

New Insights on Gas Hydroquinone Clathrates Using in Situ Raman Spectroscopy: Formation/Dissociation Mechanisms, Kinetics, and Capture Selectivity

Hydroquinone (HQ) is known to form organic clathrates with different gaseous species over a wide range of pressures and temperatures. However, the enclathration reaction involving HQ is not fully understood. This work offers new elements of understanding HQ clathrate formation and dissociation mechanisms. The kinetics and selectivity of the enclathration reaction were also investigated. The focus was placed on HQ clathrates formed with CO2 and CH4 as guest molecules for potential use in practical applications for the separation of a CO2/CH4 gas mixture. The structural transition from the native form (α-HQ) to the clathrate form (β-HQ), as well as the reverse process, were tracked using in situ Raman spectroscopy. The clathrate formation was conducted at 323 K and 3.0 MPa, and the dissociation was conducted at 343 K and 1.0 kPa. The experiments with CH4 confirmed that a small amount of gas can fill the α-HQ before the phase transition from α- to β-HQ begins. The dissociation of the CO2–HQ clathrates highlighted the presence of a clathrate structure with no guest molecules. We can therefore conclude that HQ clathrate formation and dissociation are two-step reactions that pass through two distinct reaction intermediates: guest-loaded α-HQ and guest-free β-HQ. When an equimolar CO2/CH4 gas mixture is put in contact with either the α-HQ or the guest-free β-HQ, the CO2 is preferentially captured. Moreover, the guest-free β-HQ can retain the CO2 quicker and more selectively