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Article

Morphology of CO2/H2 Hydrates in the Presence ofCyclopentane with/without Sodium Dodecyl Sulfate

Yu An Lim, Ponnivalavan Babu, Rajnish Kumar, and Praveen LingaCryst. 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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1

Morphology of CO2/H2 Hydrates in the Presence of Cyclopentane

with/without Sodium Dodecyl Sulfate

Yu-An Lim 1δ

, Ponnivalavan Babu1δ

, Rajnish Kumar2, Praveen Linga

1,*

1Department of Chemical and Biomolecular Engineering, National University of Singapore,

Singapore, Singapore 117 576

2Chemical 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

δBoth authors equally contributed to the work

*corresponding author, Tel: (65) 6601-1487; e-mail: chepl@nus.edu.sg; 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 pre-

combustion (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, 17

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, 26-

28 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, 35

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 crystals

38, 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, 42-48

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. O-

rings 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 A-

J, 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 CP-

water 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 CP-

water 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 CP-

water 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 sub-

cooling 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 gas-

cyclopentane-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-279-000-317-

133) 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.

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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

CP Layer

(cm)

SDS

(ppm)

Texpt

(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 Linga

1,*

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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