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Morphology of CO2/H2 Hydrates in the Presence of Cyclopentane with/without Sodium Dodecyl Sulfate Yu An Lim, Ponnivalavan Babu, Rajnish Kumar, and Praveen Linga Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/cg400118p • Publication Date (Web): 27 Feb 2013 Downloaded from http://pubs.acs.org on February 28, 2013

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Crystal Growth & Design

Morphology of CO2/H2 Hydrates in the Presence of Cyclopentane with/without Sodium Dodecyl Sulfate Yu-An Lim 1δ, Ponnivalavan Babu1δ, Rajnish Kumar2, Praveen Linga1,* 1

Department of Chemical and Biomolecular Engineering, National University of Singapore, Singapore, Singapore 117 576

2

Chemical Engineering and Process Development Division, CSIR- National Chemical Laboratory, Pune, India

Abstract In this study, effects of cyclopentane (CP) and sodium dodecyl sulfate (SDS) on the hydrate formation morphology were investigated. A gas mixture of 40.0 mol% carbon dioxide & 60.0 mol% hydrogen was used in an unstirred system with sub-cooling as the driving force. Experimental pressure is at 6.0 MPa and experimental temperatures used are at 275.65 K and 277.65 K (∆ = 15.15K and 13.15 K). Formation of hydrates started at the cyclopentane-liquid water interface. Cloud-like, equiaxed skewed dendritic, equiaxed orthogonal dendritic, long dendritic and cactus-like crystals could be observed for the experiments in the absence of surfactants. Rapid hydrate formation was observed for the experiments with 0.9 ml CP with or without the presence of surfactants compared to the experiments with 0.45 ml CP system at the same experimental conditions. The addition of SDS had led to a change in the hydrate crystal morphology, forming fiber-like crystals from the hydrate layer. Hydrates had also shown affinity to comparatively colder metal surfaces and tend to grow rapidly due to better heat transfer capacity. Gas uptake measurements were found to correlate well with the morphological observations. Based on the morphological observations, the mechanism of the CO2/H2/CP system in an unstirred system is presented. Keywords: Gas hydrates, cyclopentane, sodium dodecyl sulfate, carbon dioxide capture, fuel gas, crystal morphology δ

Both authors equally contributed to the work

*corresponding author, Tel: (65) 6601-1487; e-mail: [email protected]; Fax: (65) 6779-1936.

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Introduction Gas hydrates are non-stoichiometric solids existing in lattice structures that are made up of gas molecules and water. They are formed when the gaseous phase and the water liquid phase come into contact under extreme conditions of high pressures and low temperatures.1,2 The water represents the host molecule, while the gas compounds such as hydrogen, carbon dioxide, methane are encapsulated as the guest molecules. Capture of carbon dioxide from precombustion (fuel) and post combustion (flue) gas streams employing the hydrate based gas separation (HBGS) process is a promising application for gas hydrates.3-9 Other novel applications like hydrogen storage10-12, cool storage13, sea water desalination14, 15, concentration of dilute aqueous solutions in food engineering16,

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and gas separations through clathrates

hydrate formation18, 19 are undergoing research. With CO2 being a huge contributor to the greenhouse effect leading to global warming, the removal of CO2 before the combustion (pre-combustion capture) is highly desired and sought after. 3, 4 The integrated gasification combined cycle (IGCC) is a technology that converts coal and/or biomass to fuel gas via a water shift reaction and subsequently removing CO2 from H2 before combusting the carbon-free fuel. The novel technology proposed is the removal of CO2 from the fuel gas mixture via the formation of gas hydrates.5, 20-22 The equilibrium pressure of CO2 at 280 K is determined at 2.91 MPa, while the dissociation pressure of H2 at the same temperature is 300 MPa, indicating that CO2 is more stable in the hydrate cavities and thus preferably entrapped in the gas hydrates cages over H2.23 Moreover, it has been known that pure hydrogen gas is quite small for the smallest hydrate cage and requires significant pressure to form a stable gas hydrate.24 Thus, this brings about the possibility of capturing the carbon dioxide in the hydrates from the CO2/H2 gas mixture. However, extreme conditions of low temperature and high pressures are required for the formation of CO2/H2 hydrates (274.6 K, 6.04 MPa for 39.2 mol% CO2/ 60.8 mol% H2).25 Therefore to reduce the equilibrium hydrate formation conditions, thermodynamic promoters such as propane, tetrahydrofuran (THF), tetra butyl ammonium bromide (TBAB) and cyclopentane (CP) are employed for CO2 capture.20, 23, 2628

The addition of cyclopentane (CP) is known to decrease the equilibrium pressure of CO2 +H2

hydrates.23 Zhang et al.23 investigated the phase equilibrium of CP-CO2-H2 hydrate at the various vapor phase CO2 mole fractions and at 0.3998 CO2 composition the equilibrium temperature at


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6.0 MPa is 290.8 K, as compared to 274.6K25 in the absence of CP. Another problem faced in developing this separation technology via hydrate formation is due to the high costs involved in large scale industrial purposes, where high mixing of the guest molecules and water using mechanical means is required.29-32 Zhang and Lee33 reported that the presence of CP enhances the kinetics of CO2 hydrates and suggested that it can be employed for post and pre-combustion capture of carbon dioxide using an unstirred reactor configuration. Recently, Ho et al.34,

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reported a significant enhancement in the kinetics of hydrate formation for CO2 capture from a fuel gas (40% CO2-60% H2) mixture by employing CP in an unstirred reactor configuration compared to a stirred tank reactor. While there is a need to develop innovative reactors for gas/liquid contact to enhance the kinetics of hydrate formation at the same time the use of kinetic promoters such as surfactants is an area of study that cannot be neglected. Anionic surfactants have known to increase the rate of gas uptake during hydrate formation, without shifting the equilibrium conditions of the formation process.36, 37 The promoting effect of sodium dodecyl sulfate (SDS) is believed to be due to the adsorption of DS- ions on hydrate crystals38, reducing the energy barrier of hydrate nucleation.39 The addition of surfactants also keeps the hydrates in a scattered form due to electrostatic repulsion between hydrate particles, thus becoming a potential method in reducing the amount of unreacted water trapped between hydrates and allows the hydrates to be continuously permeable to diffusion of CO2 to reach the liquid phase for hydrate formation in a non-stirred system.37-40 Moreover, SDS was found to be the most effective in increasing the kinetics of hydrate formation and decreasing induction time among the 3 surfactants (Tween-80, Dodecyl trimethyl ammonium chloride and Sodium Dodecyl Sulfate) investigated on carbon dioxide hydrates.41 Morphology is the study of the size, shape and structure of hydrates whereby the length scales are larger than molecular structure and much smaller than system dimensions.36,

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Understanding the mechanism of the hydrate formation will be useful in industrial process optimizations, where we will be able to predict the macroscopic flow characteristics and transport characteristics.42 To the best of our knowledge, there are no reports on the morphology of CO2/H2 gas hydrates in the presence of cyclopentane. Understanding the size and shape of crystals is important if unstirred reactor configurations are to be employed for capturing carbon dioxide from pre- and post-combustion streams.33-35


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This paper thus aims to report the effects of CP on the morphology on CO2/H2 (fuel gas mixture) clathrates in a non-stirred system using sub-cooling as a driving force, where hydrates are produced at a temperature below the equilibrium. The effect of SDS surfactant on the crystal growth and morphology is also investigated. Lastly, the mechanism of the CO2-H2-CP hydrate system based on the morphological observations is presented. Experimental Section Materials A gas mixture of 40 mol% CO2 and 60 mol% H2 from Soxal Pte Ltd were used in the experiment. Cyclopentane of purity 99.98% and Sodium Docedyl Sulfate (SDS) of purity 99% were supplied by Alfa Aesar and Amresco respectively. Distilled and deionized water were used for all the experiments. Apparatus The schematic of the experimental apparatus is shown in Figure 1. It consists of a crystallizer immersed in a temperature controlled water bath. The crystallizer has three parts namely middle transparent hollow cylindrical poly-methyl methacrylate (PMMA) column of inner diameter of 2.5 cm and length of 7.5 cm, top and bottom lids of outer diameter 12.5 cm made up of 316 stainless steel. The top lid has two ports for gas inlet and outlet and one port for thermocouple. The lids and column are held together firmly with hexagonal nuts and bolts. Orings are also used in both the top and bottom lids to prevent leakage of crystallizer contents from the crystallizer. The volume of the crystallizer is 36.8 cm3. Figure 2 shows the top view and front view of the crystallizer. The temperature of the system is maintained by an external refrigerator (PolyScience 9012). An Omega copper-constantan thermocouple with an uncertainty of 0.1 K was used to measure the temperature of the aqueous phase inside the crystallizer. A pressure transmitter (Rosemount 3051S) with a maximum uncertainty of 0.1% of the span (0-20 MPa) and a Wika pressure gauge was employed to measure the pressure of the crystallizer. The pressure and temperature data was recorded using a data acquisition system (National Instruments) coupled with computer. LabView 2011 software was used to record the pressure


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and temperature data. A microscope (Nikon SMZ-1000) coupled with a digital camera (Nikon DS-Fi1) was used to record the images during the experiment. Experimental Procedure 12.0 ml of distilled water or SDS solution was first injected into the crystallizer followed by 0.9 ml or 0.45 ml of CP. The crystallizer was then assembled and the thermocouple was connected. The crystallizer was placed inside the water bath and then the system was cooled to the experimental temperature using an external refrigerator. The crystallizer was pressurized to 0.5 MPa and depressurized to atmospheric pressure thrice to remove any air bubble in the system. The crystallizer was then pressurized to 6.0 MPa with the predetermined gas mixture. The pressure and temperature data was recorded for every 20 s. Pressure in the system dropped due to gas consumption for hydrate formation. The formation and growth of hydrate crystals were monitored and recorded using the microscope coupled with the digital camera. The experiment was stopped when there was no further drop in the pressure of the crystallizer. It is noted that all the experiments were conducted on fresh solutions. Results and Discussion Table 1 summarizes the volume of CP, concentration of SDS and experimental conditions. The equilibrium temperature for hydrate formation for the CP-CO2-H2-H2O system at 6.0 MPa is 291.5 K.23 Experiment sets with different conditions are represented by alphabets AJ, and the numerical denote the number of experiments conducted with similar experimental conditions on fresh solutions. CP forms a clear layer above water since it is immiscible and lighter than water. Ho et al.34, 35 reported better kinetic performance in unstirred reactor configuration than a stirred tank reactor for CO2 capture from fuel gas in the presence of CP as a promoter. In order to understand the hydrate growth characteristics and mechanism of hydrate growth in unstirred crystallizer, the experimental conditions of 6.0 MPa and 275.65 K was chosen. To replicate the experimental conditions of our previous study34, 35 i.e 0.9 mm and 1.8 mm thick CP layer, 0.45 and 0.9 ml of CP was selected for our experiments.


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CO2-H2-CP hydrates Hydrate formation and growth at 6.0 MPa, 0.9 ml CP and 275.65 K (Exp A-1) is shown in Figure 3. It can be observed that hydrate formation (first nucleation event) started at the CPwater interface, where there was an upward dent on the right side of the crystallizer due to hydrate formation at 150 min observed in the CP-water interface (fig. 3b). As the hydrate formation continued, the upward dent grew larger. The growth of hydrates was observed in the upwards direction where the hydrates climbed the walls of the crystallizer in the gas phase (fig. 3b-3h). A second hydrate nucleation event was observed on the left side of the crystallizer at 153 min. The two hydrate fronts started growing simultaneously as seen in fig. 3e-3h and finally merged at 160 min (fig. 3i). Downward growth of the hydrates subsequently started and the water level started to decrease, indicating that a large amount of water was used up for the hydrate formation (fig. 3i-3k). As the hydrate formation proceeded downwards, mushy hydrates were seen sinking downwards and a mushy layer of hydrate was formed at the CP-water interface (fig. 3j & 3k). The hydrates in both the gas and water phase grew denser, and in 60 minutes after first nucleation event, it was observed that the whole surface of the crystallizer wall which was within the microscopic view was filled with hydrates. In fig. 4 (Exp A-2), first nucleation event started at 126 min after the start of experiment and an upward dent into the CP layer was observed (fig. 4b). As the hydrates grew, it formed a cluster of hydrates surrounding the thermocouple in the gas, CP and water layer (fig. 4c & 4d) and also the hydrate grew on the walls of the crystallizer. At 168 min (fig. 4e), a thin layer of hydrates started covering the surface of the thermocouple and it grew drastically as can be seen through fig. 4f-4h. The diameter of the hydrate surrounding the thermocouple increased from 2.4 mm (fig. 3f) to 3.7 mm (fig. 4g) in about 9 min and eventually reached 7.4 mm (fig. 4h) in 16 min. This phenomenon occurring in both the gas and liquid phases show that gas hydrates have affinity to grow on metal surfaces. The hydrate grew downwards and finally at the end of the experiment, the dense hydrate was observed throughout the crystallizer wall. Fig. 5 shows the morphological observations in the presence of 0.9 ml CP at 6.0 MPa and 275.65 K. The experiment was done with memory water, whereby this batch of water had already undergone 1 cycle of hydrate formation and decomposition. Hence, this hydrate formation in this memory water already started at 5 minutes after the pressurization of the


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crystallizer, where nucleation was observed and an upward dent was formed on the CP–water interface. In fig.5a-5b, the microscope was focused on the left hand side of the crystallizer, where the hydrates grew in the upward and lateral direction. Cloud-like hydrates could also be seen sinking into the water phase and two fronts of hydrates created on both sides of the crystallizer were observed (fig. 5c). The microscope was then shifted back to the center of the crystallizer, where the 2 fronts of hydrates were approaching each other, and a mass of cloud-like hydrates was extending downwards in the water phase (fig. 5d). The microscope was finally focused on the water phase, where these hydrates were seen to be filling up the entire water phase (fig. 5e-5g). A hydrate layer was observed to form around the thermocouple as shown in fig. 5h. The water level subsequently started to decrease and the whole water phase and walls of the crystallizer were covered with hydrates in 55 min (fig. 5g-5i). When the amount of cyclopentane used in the system was reduced by half from 0.9 ml to 0.45 ml (Exp B-1), no upward dent was seen at the CP-water interface. Instead at 78 min, floating equiaxed skewed dendrites were seen throughout the crystallizer (fig. 6a-6b). An equiaxed orthogonal dendrite crystal of a much larger size was subsequently observed to be hovering around the thermocouple (fig. 6c & 6d). It is noted that such equiaxed skewed and equiaxed orthogonal dendrites were observed in the methane (90.5%) -propane (9.5%) hydrate formation experiment performed by Lee et al.42 Fig 7 shows the sequential images for the repeat experiment with the same parameters of 0.45 ml of cyclopentane in the absence of SDS (Exp. B-2). As seen in fig. 7a, hydrate formation for this experiment started at the bottom of the crystallizer. A long dendritic hydrate grew from a height of 1.5 mm to 3.1 mm, 4.7 mm and lastly 5.8 mm in intervals of 1 h.(fig. 7a-7e). This is possible only if the conditions at the bottom were met for hydrate formation. As CP was not present at the bottom of the crystallizer in this unstirred system, hydrate formation could only involve guest gas by diffusion in quiescent conditions. The bulk water should be saturated with gas molecules as hydrate formation occurred after a long period of time (~15 hr) from the bottom of the crystallizer. It is noted that, for water – gas quiescent system without mixing, it would take sufficiently long time for the gas molecules to diffuse through the water and hence it is unlikely that the saturation of gas in the water for nucleation to occur is possible in the time scale reported in this work. We believe that the presence of CP aids in the diffusion of guest gas to the liquid


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water in a quiescent/unstirred contact mode. We conducted a simple morphology experiments with water and the CO2/H2 gas mixture without CP at 6.0 MPa. As can be seen in fig. S1 (given in supporting information), even after ~80hr of reaction time, there was no hydrate formation suggesting that CP aids the diffusion of guest gas to the bulk water phase. Moreover, it is also noted that the equilibrium hydrate formation pressure for a CO2 (40%)/H2 (60%) fuel gas mixture at 275.65 K is 6.84 MPa.25 After another 4 hours at t = 23 h, the long dendritic structure of the hydrate had developed into a cactus-like structure, where thin flanges developed from the tip and sides of the hydrate (as seen in fig. 7f). Subsequently, this hydrate cactus started to branch out at the bottom of its stem, and the branches grew and followed a cactus-like structure similar to the main stem (as seen in fig. 7g & 7h). Other long dendritic crystals also started growing, whereby one could be seen in fig. 7g, and it also developed into a cactus-like hydrates stem eventually (fig. 7h). The growth of such hydrates spread throughout the bottom surface, with many long dendritic hydrates believed to be stemming out from the bottom surface and finally developed into a family of cactus-like hydrates of different lengths and thickness (fig. 7h). In summary, for the experiments with 0.45 ml CP, we did not observe drastic hydrate formation as was the case for the experiments in the presence of 0.9 ml CP. This observation is also supported by fig. 11, where the gas uptake in Exp. A-2 is more than twice that of Exp. B-1. CO2-H2-CP hydrates in the presence of SDS With the addition of SDS as the surfactant, the morphology of the gas hydrates was observed to be different from those without SDS. It is noted that the clarity of the water layer has decreased due to the addition of SDS. Moreover, the liquid spots shown in fig. 8a are CP spots created during the addition of CP into the crystallizer, whereby some of the CP sunk into the water layer due to the injection force from the pipette and they were attached to the surface of the crystallizer wall. The attachment of CP to the walls of the crystallizer had led to a slight decrease in the CP layer although 0.9 ml of CP had been used. The first nucleation event started at the CPwater interface (fig. 8b) and a second nucleation event was observed 5 min later close to the first nucleation (fig. 8c). Both nucleation front created an upward dent at the CP-water interface and was similar to what was observed for experiment A-1 (fig. 3b) in the absence of SDS. The two nucleation fronts merged quickly (fig. 8d) and the merged fronts grew rapidly. Many equiaxed orthogonal dendrites were seen floating in the liquid water in the presence of SDS, and they


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continued to stay in the water layer when the fiber-like crystals were growing into water phase (fig. 8g-8i). The bubbles seen in fig. 8g and 8h are CP. As the weight of the hydrates became heavier, a large chunk of hydrates detached from the main bulk of hydrates and fell to the bottom of the crystallizer (fig. 8j & 8k). Finally, the whole crystallizer was filled up with hydrates in 168 min (fig. 8j). With a reduction of CP to 0.45 ml in the presence of SDS, a tree-like crystal of 1.05 mm appeared at 150 min (expanded and shown in fig. 9b) and at 161 min upward dent at the CPwater interface appeared (Fig 9c) and subsequently upward growth of hydrates was observed (fig 9c-9d). Later fiber-like crystals started to grow downwards from the CP-water interface, forming a 1.2 mm layer of fiber-like structures in the water phase (fig. 9d-9e). It is also noted that the tree-like crystal shape (shown in 9b) can be seen to be clinging to the fiber-like crystals in fig 9e. It was observed that external branching of the stem took place as the fiber like hydrates developed further, and the branches also followed the fiber-like shape of the main stem (fig. 9f & 9g). Hydrate formation and morphological observations were also carried out at a lower subcooling of 13.15 K (277.65 K). The morphology of the hydrates was similar to the systems carried out at a sub-cooling of 15.15 K. Sequential images of two experiments conducted at 277.65 K (sub cooling of 13.15 K) with 0.9 ml CP with and without the presence of 300 ppm SDS are compiled and presented as Figure S2 and Figure S3 (given in the supporting information). Gas uptake measurements Fig. 10 shows the typical gas uptake profile of Exp A-2. Hydrate formation is an exothermic process and hence there is an increase in the temperature when hydrate formation occurs. As can be seen in the figure, multiple temperature spikes were observed for the experiment and the temperature was restored back to the experimental temperature due to external cooling. In addition every time a temperature spike was observed, there was an increase in the gas uptake indicating additional gas consumption for hydrate formation. The gas uptake curves also directly compliments our morphological observations of rapid hydrate growth in the crystallizer for the 0.9 ml CP experiments with/without the presence of SDS. In fig. 4b, the


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nucleation was observed at 126 min and in fig. 10 there was a corresponding small steep gas uptake increase at 128 min. In fig. 4c at t = 146 min, the rapid growth of hydrates shown correlated well to the second temperature spike at t = 145 min in fig. 10 too. As the CP-water interface dropped downwards and approached the tip of the thermocouple, the temperature spikes recorded also corresponded accurately to the images observed. In fig. 10, there was a third large temperature spike between 200 min and 220 min. This spike corresponded to the drastic hydrate formation as seen in fig. 4f to 4h taking place between 204 min and 220 min in the crystallizer and surrounding the thermocouple. It is notable to recall that as we observed through morphological observations, there was no significant hydrate formation for the experiments conducted with CP of 0.45 ml; this can also be seen in the gas uptake curve for an experiment conducted with 0.45 ml CP that is shown in fig. 11. The gas uptake for the experiment with 0.9 ml CP was 2.3 times higher compared to the gas uptake for 0.45 ml CP experiment. It is also noted from fig. 11 that rate of gas diffusion depended on the CP layer thickness. Gas consumption due to diffusion into CP/water at 2 hr was 0.0076 mol of gas/mol of water for 0.9 ml CP experiments where as for the 0.45 ml CP experiment it was 0.0055 mol of gas/mol of water. Based on the gas uptake profiles we observed that the presence of surfactant did not have a significant effect on the extent of hydrate formation as can be seen in fig. 12. However, it is noted that even though the gas uptake characteristics were similar (see fig. 12), the crystal morphology of the hydrates for the experiments with and without SDS were different (see fig. 7 and 9). If unstirred reactor configurations have to be employed for capturing carbon dioxide in the presence of cyclopentane, then knowing the size and shape of crystal formation along with understanding the mechanism of hydrate formation is necessary for process design and scale up. Morphological observations coupled with gas uptake measurements can provide useful insights to the kinetics of hydrate formation including the size and shape of the crystals formed as shown in our study. Caution should be employed when the gas uptake is directly quantified and compared to the hydrate crystal morphology viewed through the microscope for our system involving gascyclopentane-water. This is because, gas uptake only quantifies the amount of guest gas dissolved and/or consumed for hydrate formation. However, in the microscopic images showing hydrate formation, there is another guest (cyclopentane) that involves in hydrate cage occupation and is not quantified in the gas uptake. In addition, the fractionation effect occurring in the gas


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phase during hydrate formation is well known in the literature49, 50 which could further hinder hydrate formation in the crystallizer. It is noted that all the morphological experiments were conducted in a batch manner. The preferential enclathration of carbon dioxide in the hydrate phase will enrich the hydrogen composition in the gas phase.5, 21 Mechanism of CO2-H2-CP hydrates The equilibrium hydrate formation pressure for a 40% CO2/60% H2 fuel gas mixture at 275.65 K is 6.84 MPa.25 This gas mixture is reported to form structure sI.51 It is noted that the equilibrium temperature for the CP-40% CO2-60% H2 system at 6.0 MPa is 290.8 K and it forms structure sII.23 The experimental conditions are not favorable for CO2-H2 hydrate (sI) formation since the temperature of the system is higher than that of equilibrium temperature. Hence the hydrate observed in our study is CP-CO2-H2 hydrate (sII). Fig. 13 shows a pictorial illustration of the mechanism of CO2-H2-CP hydrates (sII) in an unstirred system based on our morphological observations at a pressure of 6.0 MPa and temperature of 275.65 K. Fig. 13a shows a distinct gas phase, CP layer and water layer at the start of the experiment, along with a thermocouple with its tip in the bulk water. The system is at a dynamic state where a driving force of 15.15 K is used for hydrate formation. Diffusion of guest molecules happens due to the high pressure, and they diffuse into both the CP and water layers. The next step is the start of nucleation that occurs at the CP-water interface (fig. 13b). Upward dents are observed at the CP-water interface (fig. 13b) and hydrate fronts grow upward along the crystallizer walls and radially inward towards the center of the crystallizer (fig. 13c). After the merger of the 2 fronts, downward growth starts to occur and water level rapidly decreases as the hydrates continue to grow in all 3 directions (upwards, radially inward and downwards). Thin hydrate clusters are formed on the thermocouple due to their affinity towards metal surfaces and they continue to grow as hydrate formation progresses (fig. 13d–13f). As seen in fig. 13f, the bottom layer will be fully filled with hydrates, while in the top gas phase layer the amount of hydrates filled up are lower. Finally, reaction stops or reaches a steady state when there is insufficient contact between the water, CP and guest molecules. Conclusion


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The morphology of the CO2/H2 hydrates was studied, in the presence of cyclopentane and sodium dodecyl sulfate. Experiments were conducted at 6.0 MPa, two sub-cooling temperatures 15.15 K and 13.15 K), 0.90 ml and 0.45 ml of CP, and at 100 and 300 ppm of SDS. Results are summarized as follow: 1.

The nucleation and formation of hydrates took place at the CP-water interface in both presence and absence of surfactant, and the hydrates grew upwards in the gas mixture along the crystallizer walls first before penetrating into the water layer. During the downward growth of hydrates, the water layer could be seen to decrease rapidly.

2.

Equiaxed skewed dendrites, equiaxed orthogonal dendrites and long dendritic and cactus-like crystals could be observed in the CO2-H2-CP-Water System.

3.

In the absence of surfactant, mushy and cloud-like hydrates could be seen sinking down in the bulk water and even form a mushy layer just below the CP-water interface.

4.

The addition of surfactant (SDS) had led to a change in the hydrate crystal morphology, forming fiber-like crystals from the hydrate layer. In addition, it was seen that the equiaxed orthogonal dendritic crystals (that floated in the water layer) could co-exist with the fiber-like crystals that were growing from the hydrate film.

5.

At low CP concentrations, no drastic hydrate formation was observed and the location of nucleation was not restricted to the CP-water interface, as the bulk water layer gets saturated and hence allowing hydrates to grow from the bottom of the surface where equilibrium conditions were met at that location as well.

6.

The presence of a higher amount of CP (0.9 mL) had shown a more rapid formation of gas hydrates, with multiple temperature spikes observed as compared to the experiments where lesser amount of CP was used.

7.

The gas uptake measurements correlated well with the morphological observations.


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8.

Based on our morphological observations, the mechanism of hydrate formation for the CO2-H2-CP in an unstirred system is presented.

Acknowledgement The financial support from the Ministry of Education’s AcRF Tier 1 (R-279-000-317133) and the National University of Singapore is greatly appreciated. Rajnish Kumar thanks the Council of Scientific and Industrial Research (CSIR) for the financial support. Supporting Information Sequential illustrations of hydrate crystals at the cyclopentane-liquid water interface for the experiment conducted with 0.9 ml CP and at 277.65 K and 6.0 MPa (Experiment E-2) and sequential illustrations of hydrate crystals at the cyclopentane-liquid water interface for the experiment conducted with 0.9 ml CP and at 277.65 K and 6.0 MPa in the presence of 300 ppm SDS (Experiment G-1) are given in the supporting information. This material is available free of charge via the Internet at http://pubs.acs.org. References

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Table Captions: Table 1 - Experimental conditions indicating the experimental temperature, CP volume and SDS concentrations. All experiments were conducted at 6.0 MPa Figure Captions: Figure 1. Schematic diagram of the experimental apparatus Figure 2. Assembled crystallizer with the top, bottom and middle portions Figure 3. Sequential illustrations of hydrate crystals at the cyclopentane-liquid water interface (Experiment A-1) Figure 4. Hydrate growth in the absence of surfactant in 0.90 ml CP (Experiment A-2) Figure 5. Hydrate growth in the absence of surfactant in 0.90 ml CP (Memory water experiment conducted after A-2) Figure 6. Hydrate growth in the absence of surfactant with 0.45ml CP (Experiment B-1) Figure 7. Cactus-like hydrates in the absence of surfactant with 0.45 ml CP (Experiment B-2) Figure 8. Hydrate growth in the presence of 300 ppm SDS with 0.90ml CP (Experiment C-2) Figure 9. Hydrate growth in the presence of 300 ppm SDS with 0.45 ml CP (Experiment D-2) Figure 10. Gas uptake measurement curve with temperature profile of 0.9 ml CP experiment conducted at 6.0 MPa and 275.65 K (Exp A-2) Figure 11. Gas uptake profiles for the experiments with 0.9 and 0.45 mL CP solutions. Figure 12. Gas uptake measurement curve for 0.9 ml CP with and without 300 ppm SDS Figure 13. Pictorial illustration of the mechanism of CO2/H2/CP hydrates in an unstirred system


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Table 1 - Experimental conditions indicating the experimental temperature, CP volume and SDS concentrations. All experiments were conducted at 6.0 MPa

Exp No.

CP (ml)

Thickness of

SDS

Texpt

CP Layer (cm)

(ppm)

(K)

Sub-cooling driving force ∆ (K)

A-1

0.90

0.18

-

275.65

15.15

A-2

0.90

0.18

-

275.65

15.15

B-1

0.45

0.09

-

275.65

15.15

B-2

0.45

0.09

-

275.65

15.15

C-1

0.90

0.18

300

275.65

15.15

C-2

0.90

0.18

300

275.65

15.15

D-1

0.45

0.09

300

275.65

15.15

D-2

0.45

0.09

300

275.65

15.15

E-1

0.90

0.18

-

277.65

13.15

E-2

0.90

0.18

-

277.65

13.15

F-1

0.45

0.09

-

277.65

13.15

F-2

0.45

0.09

-

277.65

13.15

G-1

0.90

0.18

300

277.65

13.15

G-2

0.90

0.18

300

277.65

13.15

H-1

0.45

0.09

300

277.65

13.15

H-2

0.45

0.09

300

277.65

13.15

I-1

0.90

0.18

100

275.65

15.15

I-2

0.90

0.18

100

275.65

15.15

J-1

0.90

0.18

100

277.65

13.15

J-2

0.90

0.18

100

277.65

13.15


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For Table of Contents Use Only

Morphology of CO2/H2 Hydrates in the Presence of Cyclopentane with/without Sodium Dodecyl Sulfate Yu-An Lim 1δ, Ponnivalavan Babu1δ, Rajnish Kumar2, Praveen Linga1,*

Synopsis: The graphic abstract shows an intermediate stage of hydrate growth in a CO2-H2-CP system at 6.0 MPa and 275.65 K in an unstirred system. The unstirred system gives 3 distinct phase: gas, cyclopentane (CP) and water. Nucleation starts at the CP-water interface and there are upward and radial growth along the crystallizer walls.


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Figure 1. Schematic Diagram of the experimental apparatus. 127x127mm (300 x 300 DPI)



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Figure 2. Assembled crystallizer with the top, bottom and middle portions. 83x38mm (300 x 300 DPI)


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Figure 3. Sequential illustrations of hydrate crystals at the cyclopentane-liquid water interface. 123x169mm (300 x 300 DPI)



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Figure 4. Hydrate growth in the absence of surfactant in 0.9 ml CP (Experiment A-2). 96x198mm (300 x 300 DPI)


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Figure 5. Hydrate growth in the absence of surfactant in 0.90 ml CP (Memory water experiment after A-2). 169x137mm (300 x 300 DPI)



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Figure 6. Hydrate growth in the absence of surfactant with 0.45 ml CP (Experiment B-1). 91x77mm (300 x 300 DPI)


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Figure 7. Cactus-like hydrates in the absence of surfactant with 0.45 ml CP (Experiment B-2). 121x169mm (300 x 300 DPI)



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Figure 8. Hydrate growth in the presence of 300 ppm SDS with 0.90 ml CP (Experiment C-2). 128x139mm (300 x 300 DPI)


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Figure 9. Hydrate growth in the presence of 300 ppm SDS with 0.45 ml CP (Experiment D-2). 97x169mm (300 x 300 DPI)



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Figure 10. Gas Uptake Profile of 0.9 ml CP experiment conducted at 6.0 MPa and 275.65K (Exp A-2). 53x33mm (300 x 300 DPI)


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Figure 11. Gas uptake profiles for the experiments with 0.9 and 0.45 mL CP solutions. 163x120mm (300 x 300 DPI)



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Figure 12. Gas uptake measurement curve for 0.9 ml CP with and without 300 ppm SDS. 163x120mm (300 x 300 DPI)


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Figure 13. Pictorial illustration of the mechanism of CO2-H2-CP hydrates in an unstirred system. 166x169mm (300 x 300 DPI)


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