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Contents lists available at ScienceDirect

International Journal of Greenhouse Gas Control

journa l homepage: www.e lsev ier .com/ locate / i jggc

inetic of formation for single carbon dioxide and mixed carbon dioxide andetrahydrofuran hydrates in water and sodium chloride aqueous solution

halik M. Sabil a,b, Ana Rita C. Duartea, John Zevenbergenc, Murni M. Ahmadb, Suzana Yusupb,bdul A. Omarb, Cor J. Petersa,d,∗

Laboratory of Process Equipment, Delft University of Technology, Mechanical, Maritime & Materials Engineering, Leeghwaterstraat 44, 2628 CA Delft, The NetherlandsChemical Engineering Department, Universiti Teknologi PETRONAS, Bandar Seri Iskandar, 31750 Tronoh, Perak, MalaysiaTNO Defence, Security and Safety, Lange Kleiweg 137, 2288 GJ Rijswijk, The NetherlandsChemical Engineering Program, The Petroleum Institute, Bu Hasa Building, Room 2207A, P.O. Box 2533, Abu Dhabi, United Arab Emirates

r t i c l e i n f o

rticle history:eceived 22 January 2010eceived in revised form 26 May 2010ccepted 26 May 2010vailable online xxx

eywords:

a b s t r a c t

A laboratory-scale reactor system is built and operated to measure the kinetic of formation for singleand mixed carbon dioxide–tetrahydrofuran hydrates. The T-cycle method, which is used to collect thekinetic data, is briefly discussed. For single carbon dioxide hydrate, the induction time decreases with theincrease of the initial carbon dioxide pressure up to 2.96 MPa. Beyond this pressure, the induction time isbecoming relatively constant with the increase of initial carbon dioxide pressure indicating that the liquidphase is completely supersaturated with carbon dioxide. Experimental results show that the inclusion of

arbon dioxide hydrateinetic of hydrate formation

nduction periodydrate massive growthetrahydrofuran

tetrahydrofuran reduces the induction time required for hydrate formation. These observations indicatehydrate nucleation process and onset growth are more readily to occur in the presence of tetrahydrofuran.In contrast, the presence of sodium chloride prolongs the induction time due to clustering of watermolecules with the ions and the salting-out effects. It is also shown that the degree of subcooling requiredfor hydrate formation is affected by the presence of tetrahydrofuran and sodium chloride in the hydrateforming system. The presence of tetrahydrofuran in the hydrate system significantly reduces the amount

. The

of carbon dioxide uptake

. Introduction

Carbon dioxide has been known to be among a number ofolecules that can form clathrate hydrate. As a simple hydrate,

arbon dioxide forms structure I hydrate under appropriate pres-ure and temperature conditions. If all the hydrate cavities areccupied, the chemical formula is 8CO2·46H2O or CO2·5.75H2O.xtended compilations of published hydrate equilibrium condi-ions of carbon dioxide in pure water can be found in Sloan andoh (2008). Like other fields of research with a focus on carbonioxide, the interest of carbon dioxide hydrates is mainly based onhe possibilities of using the formation of carbon dioxide hydratesor carbon dioxide separation, capture and storage (CCS). Any tech-

Please cite this article in press as: Sabil, K.M., et al., Kinetic of formationdrofuran hydrates in water and sodium chloride aqueous solution. Int. J

ique that prevents or reverses the release of carbon dioxide tohe atmosphere and diverts the carbon to a viable carbon sinkan be considered as carbon capture. Currently, there are a fewechniques available for carbon dioxide capture and separation

∗ Corresponding author at: Laboratory of Process Equipment, Delft University ofechnology, Mechanical, Maritime & Materials Engineering, Leeghwaterstraat 44,628 CA Delft, The Netherlands. Tel.: +31 15 2782660; fax: +31 15 2788047.

E-mail addresses: cpeters@pi.ac.ae, C.J.Peters@tudelft.nl (C.J. Peters).

750-5836/$ – see front matter © 2010 Published by Elsevier B.V.oi:10.1016/j.ijggc.2010.05.010

apparent rate constant, k, for those systems are reported.© 2010 Published by Elsevier B.V.

with varied degree of success such as chemical solvents, physi-cal absorption, physical adsorption, chemisorptions and chemicalbonding through mineralization. However, the main concerns ofthese techniques are the amount of chemicals to be used in theseprocesses, the energy penalties and the costs associated with theseprocesses in their present form, making these processes becomeless attractive for large-scale carbon capture (GCEP Carbon CaptureTechnology Assessment, 2005).

In this retrospect, clathrate hydrate technique offers a couple ofadvantages. Firstly, the main chemical required for carbon dioxidehydrate formation is water, which provides the process with abun-dant (cheap) and green raw chemical. Secondly, reduction of energyrequirements for hydrate formation can be achieved by inclusion ofcertain organic chemical in low concentrations, known as hydratepromoter in the hydrate forming system (Mooijer-van den Heuvel,2004). The inclusion of a hydrate promoter will reduce the pressurerequirement or increase the temperature at which the clathratehydrates are stable. This leads to a reduction in the energy required

for single carbon dioxide and mixed carbon dioxide and tetrahy-. Greenhouse Gas Control (2010), doi:10.1016/j.ijggc.2010.05.010

for pressurization or cooling of the targeted systems. Moreover,the interest in carbon dioxide hydrates is not limited to carboncapture and sequestration. Desalination of water (Javanmardi andMoshfeghian, 2003), carbon dioxide separation from fuel and fluegases (Seo et al., 2000; Kang and Lee, 2000; Kumar et al., 2006;

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inga et al., 2007), cold storage application (Darbouret et al., 2005)re among the various areas of interest of application of carbonioxide hydrate technology.

One important aspect to be considered in order to success-ully develop CO2 hydrate based technology for carbon dioxideapture and storage applications is the insights of the kineticsf formation and dissociation of either single or mixed carbonioxide hydrate in water and aqueous electrolyte solutions. Unfor-unately, time-dependent hydrate kinetics are more difficult to

easure compared to time-independent phenomena, i.e. structuralnd thermodynamic information (Sloan and Koh, 2008). Thereforet is not surprising that the reported kinetic data of either formationr dissociation of carbon dioxide hydrates in literature are rathercarce. Some of the available literature sources on carbon diox-de hydrate are summarized in the following paragraph. Ohgakit al. (1993) measured the formation of carbon dioxide hydraten pure water and in seawater. They obtained the apparent rateonstant of hydration under the assumption of the process is aseudo-first-order reaction with respect to the CO2 concentration.hindo et al. (1993) proposed a mechanism of the formation ofO2 hydrate at the interface between liquid CO2 and water. Chunnd Lee (1996) carefully measured the kinetics of carbon dioxideydrate formation in a semi-batch stirred tank reactor with stirringate of 500 rpm at three different temperatures between 275.2 and79.2 K, and at pressures ranging from 2.0 to 3.5 MPa. They con-luded that the hydrate kinetic model developed by Englezos et al.1987) adopted in their study, was able to predict the kinetic curvesf formation of carbon dioxide hydrate. Malegaonkar et al. (1997)easured the kinetics of carbon dioxide hydrate formation and its

olubility in distilled water in a semi-batch stirred tank reactor.ith these data, they tested a modified kinetic model of Englezos

t al. (1987) and determined the intrinsic kinetic rate constant forhe carbon dioxide hydrate formation. They established that theate was higher than that of methane hydrate. A similar study wasonducted by North et al. (1998), and it was found that there muste a minimum amount of carbon dioxide available in all phases inrder to form gas hydrate, and this amount varied with pressure.o simulate the deep ocean sediment, Lee et al. (2002) studied theinetics of carbon dioxide hydrate in sodium chloride solution in alay system. They discovered that when clay was included, the finalonsumption of carbon dioxide for hydrate formation decreasedut the initial formation rate increased compared to the systemithout clay. A Raman spectroscopy study conducted by Kawamura

t al. (2002) showed that the mechanism of carbon dioxide hydrateormation depends on temperature. Clarke and Bishnoi (2004) pre-ented a method for calculating the moments of the particle sizeistribution. In their study, they used a semi-batch stirred tankeactor with an in situ particle analyzer to determine the intrin-ic rate of carbon dioxide hydrate and its activation energy. Theffect of pressure and vessel size on the formation of gas hydrateas studied by McCallum et al. (2007) using a 72 L vessel. Based

n their experimental work, the reduction in overpressure and/ornduction time required for the accumulation of hydrates may bettributed to the increase in bubble surface area, gas concentration,ifetime of bubbles, total volume of the vessel, or any combinationf all these factors. Giavarini et al. (2007) studied the formationnd dissociation of carbon dioxide hydrate and found that carbonioxide hydrate did not present any anomalous self-preservationffect (memory effect). They also found that the dissociation of car-on dioxide is not affected by subcooling before storage. However,hey pointed out that the preservation of carbon dioxide hydrate

Please cite this article in press as: Sabil, K.M., et al., Kinetic of formationdrofuran hydrates in water and sodium chloride aqueous solution. Int. J

s more affected by temperature rather than by pressure, which ishe opposite of that of methane hydrate.

Unfortunately, only limited information of formation kineticf mixed carbon dioxide hydrates are available in open litera-ure. Therefore, it is our goal to fill this gap by measuring the

Fig. 1. Schematic representation of the high-pressure kinetics measurement appa-ratus: A: data acquisition system, B: high-pressure vessel, C: liquid sample, D: stirrer,E: thermocouple, F: pressure gauge, G: thermostatic bath, H: RPM controller, J:cryostat, K: valve, L: thermostatic liquid, M: one-way valve.

kinetics of formation of simple carbon dioxide and mixed carbondioxide and tetrahydrofuran hydrates in water and aqueous elec-trolyte solutions. The main focus of this study is to obtain thekinetic data under rapid formation of simple and mixed carbondioxide hydrates. In this work, an apparatus for the measurementof the pressure–temperature data during hydrate formation wasdesigned and built. The kinetics of formation of simple carbon diox-ide and mixed carbon dioxide and tetrahydrofuran hydrates werestudied in water and aqueous solutions of sodium chloride. Theclosed loop method is used to obtain kinetic data on hydrate forma-tion is briefly discussed. The induction time, induction temperature,the carbon dioxide consumption and the apparent rate constant forsimple carbon dioxide and mixed carbon dioxide and tetrahydro-furan hydrates in water and sodium chloride solution are reportedand discussed.

2. Methodology

2.1. Materials

Carbon dioxide was purchased from Messer Griesheim witha certified purity higher than 99.95 vol%. Tetrahydrofuran andsodium chloride were purchased from Aldrich–Sigma with certi-fied purities of >0.99 and >0.95, respectively. All chemicals wereused without any further purification. Distilled and deionized waterwere used after careful degassing.

2.2. Kinetic measurement apparatus

A schematic representation of the high-pressure system usedfor the measurement of kinetic of formation of clathrate hydrate inthe present work is depicted in Fig. 1. The main part of the systemis a high-pressure stainless steel vessel with an internal volume ofapproximately 150 cm3. The maximum working pressure for thevessel is 100 MPa. The vessel is immersed in a thermostatic bathto keep the temperature constant at a desired value. The tempera-

for single carbon dioxide and mixed carbon dioxide and tetrahy-. Greenhouse Gas Control (2010), doi:10.1016/j.ijggc.2010.05.010

ture of the thermostatic bath is controlled by a PID controller thatmaintained the temperature within ±0.1 ◦C. The thermostat liq-uid used is a mixture of ethylene glycol + water and the heatingand cooling of the thermostat liquid is carried out by a cryostat.The temperature inside the vessel was monitored both in the gas

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hase and in the liquid phase by two thermocouples (New Portmega type T) with an accuracy of ±0.01 ◦C. To achieve properixing in the liquid sample, a magnetic stirrer is placed in the

essel. The pressure inside the cell is measured with a pressureransducer (a piezo-resistive AE SML series pressure sensor) withressure reading range between 0 and 250 bars. The pressure andemperatures readings are recorded and stored in a data acquisitionystem.

.3. Kinetic measurement procedure

In the present work, a closed loop cycle method is employedor the measurement of formation and decomposition of clathrateydrate kinetics. In this method, the amount of carbon dioxidend liquid sample are kept constant and no addition of theseomponents is made to the system during the experiment. Forach cycle, the high-pressure vessel is loaded with a known vol-me and mass of a liquid sample. Throughout the present work,hree different liquid samples are used depending on the systems

easured. The liquid samples are demineralized water, a mix-ure of water and tetrahydrofuran (5 mol%) or a mixture of water,etrahydrofuran (5 mol%) and sodium chloride (1 mol%). Then, theessel is flushed with carbon dioxide by loading the vessel witharbon dioxide and subsequently releasing the pressure. Duringhe flushing, the stirrer is switched off to prevent solubility ofarbon dioxide in the aqueous sample. After the flushing proce-ure is completed, the vessel is ready to be used for the kineticeasurements. Since the presence of other gases may affects the

ormation conditions, the procedure is repeated five times forach sample to ensure that the repeatability of the experimentalesults.

Due to high solubility of carbon dioxide in the aqueous mixture,he loading of carbon dioxide is conducted at an initial tempera-ure 2–3 K higher than the hydrate equilibrium temperature at apecified pressure. Firstly, when the temperature is stabilized, theessel is pressurized with carbon dioxide to the desired pressurealue. When the pressure and the temperature in the liquid andapour phases in the vessel are relatively constant, the stirrer iswitched on. The rate of stirring is kept at 500 rpm. This value ishosen because the stirring rate is sufficient to remove the clathrateydrates from the gas–liquid interface and to keep the hydrate par-icles as small crystals in slurry but not high enough to inducexcessive rippling at the surface or bubble entrapment by vor-ex (Vysniauskas and Bishnoi, 1983). Throughout the experimentalork, a decrease in pressure is observed during this step, suggesting

hat the dissolution of carbon dioxide in the aqueous mixture is tak-ng place. When the pressure regains stability, the temperature ofhe system is slowly reduced to 5 K below the hydrate equilibriumemperature of the system. Meanwhile, the system is continuouslytirred. The temperature is kept above 273 K to prevent the for-ation of ice instead of the gas hydrate. When the desired final

emperature condition is achieved, the system is left for 2–3 h tonsure that the clathrate hydrate formation is completed. Through-ut the process, pressure and temperature changes are monitorednd recorded by the data acquisition system at a time interval of 1 s.hen the pressure and temperature stabilize, the measurement of

lathrate hydrate formation kinetic is completed.When the clathrate hydrate formation is completed, i.e. the

ressure of the system is stabilized at a preset temperature belowhe hydrate equilibrium temperature, the stirring is switched off.

Please cite this article in press as: Sabil, K.M., et al., Kinetic of formationdrofuran hydrates in water and sodium chloride aqueous solution. Int. J

hen, the system is slowly heated up to the initial temperature,hich is 2–3 K higher than the hydrate equilibrium temperature.

he system is allowed to reach equilibrium, normally in about 2–3 hhile the pressure and temperature data are recorded by the data

cquisition system.

Fig. 2. Pressure and temperature trajectory during formation and decomposition ofsimple carbon dioxide hydrate starting at an initial pressure of 2.25 MPa.

3. Results and discussion

3.1. The closed loop (T-Cycle) method

In this work, the closed loop (T-cycle) method used by Ohgakiet al. (1993) is adapted for the measurement of the kinetic of for-mation of single and mixed carbon dioxide hydrates. An exampleof a working diagram of the method for the determination of theformation, turbidity point and stationary point of carbon dioxidehydration is represented in Fig. 2. Initially, a sharp decrease ofpressure is observed at the beginning of the experiment althoughthe temperature is kept constant above the hydrate equilibriumtemperature. This is mainly attributed by the solubility of carbondioxide in water. Once the liquid phase is saturated with carbondioxide, the pressure stabilizes as shown by Point A in Fig. 2, indi-cating that a steady-state is achieved.

From A to B, it is observed that the pressure was decreasing asthe temperature decreased. This trend is mainly due to two factors:the solubility of the gas in the liquid phase and gas contraction uponcooling. At point B, which is also known as the turbidity point, a sud-den decrease of pressure is observed. This pressure drop is mainlydue to the hydrate formation during which a rapid entrapment ofthe gas molecules in the solid phase occurred. In general, the for-mation of gas hydrates starts rapidly and the rate then decreaseswith time until maximum sorption is attained at point C. Point C isdenotes as the stationary point of the reaction. After the formationof the hydrate is completed, the process is reversed to let the formedhydrate to dissociate again. The dissociation of the gas hydrates isachieved by heating the system from Point C to Point A. During theheating process, an increase of pressure is observed until all car-bon dioxide molecules entrapped in the solid phase are completelyreleased at Point A. The number of moles of carbon dioxide at eachpoint is calculated from the pressure–temperature data by applyingthe Peng–Robinson equation of state (Peng and Robinson, 1976).

In the present work, three different hydrate forma-tion systems are studied, i.e. carbon dioxide + water,carbon dioxide + tetrahydrofuran + water and carbon diox-ide + tetrahydrofuran + sodium chloride + water systems. Theconcentration of tetrahydrofuran and sodium chloride are fixed at5 and 1 mol%, respectively, in the aqueous solution when they arepresent in the systems studied. The amount of aqueous solutions

for single carbon dioxide and mixed carbon dioxide and tetrahy-. Greenhouse Gas Control (2010), doi:10.1016/j.ijggc.2010.05.010

i.e. water or aqueous tetrahydrofuran or aqueous tetrahydrofu-ran + sodium chloride solutions, used in each experiment is 50 ml.The formation of clathrate hydrates is generally divided into twoprocesses, the nucleation process and the stable growth process

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ig. 3. The induction time of simple carbon dioxide and mixed carbon dioxide andetrahydrofuran hydrates in water and an aqueous solution of NaCl at different initialressure.

Vysniauskas and Bishnoi, 1983; Englezos et al., 1987; Skovborg etl., 1993; Natarajan et al., 1994; Kashchiev and Firoozabadi, 2003).loan and Koh (2008) stated that hydrate nucleation is a processuring which small cluster of hydrate nuclei grow and disperse

n an attempt to achieve critical size for continuous growth. Thisucleation step is a microscopic phenomenon involving tens ofhousands of molecules and is difficult to observe experimen-ally especially in the experimental set-up such as used in thisork. Moreover, they stated that the induction time is defined inractice as the time elapsed until the appearance of a detectableolume of hydrate phase or equivalently, until the consumptionf a detectable number of moles of hydrate former gas. Basedn these definitions, the induction time is defined as the timeaken for hydrates to be detected macroscopically, after nucleationnd onset of growth have occurred. In this work, the inductionoint corresponds with the turbidity point, at which significantressure drop is first detected in the system. Beyond the inductionime, massive hydrate growth process is studied through the

easurements of gas consumption and the calculation of apparentate constant.

.2. Induction time

One of the advantages of the T-cycle method or measurement ofhe kinetic data through a constant cooling rate is that the hydrateormation is less stochastic (Sloan and Koh, 2008). This enableshe measurement of the induction time, i.e. the time required forydrates to be detected macroscopically after nucleation and onsetf growth have taken place. The induction time for simple carbonioxide and mixed carbon dioxide and tetrahydrofuran hydrates inater and an aqueous sodium chloride solution at different initialressures are plotted in Fig. 3. The induction time is calculated fromhe point where the dissolution of carbon dioxide in the aqueoushase is completed and the turbidity point has been reached. Dueo the stochastic nature of hydrate nucleation, the experiment isepeated at least five times and the values of the induction timealls within ±2 min of each others for each initial carbon dioxideressure. The average values of the induction times are plotted inig. 3.

Please cite this article in press as: Sabil, K.M., et al., Kinetic of formationdrofuran hydrates in water and sodium chloride aqueous solution. Int. J

As depicted in Fig. 3, at initial pressure <1.95 MPa for the sys-em of carbon dioxide and water, the induction time is observedo go to infinity suggesting that no massive hydrate growth cane detected macroscopically. North et al. (1998) and Jensen et al.2008) mentioned that the induction time for hydrate forming sys-

PRESSnhouse Gas Control xxx (2010) xxx–xxx

tems is dependent on the supersaturation of the guest componentin the aqueous phase and if the supersaturation critical point isnot achieved, no hydrate will be formed in the system. Therefore,below an initial pressure of 1.95 MPa, it is deduced that a criti-cal point of supersaturation of the carbon dioxide in water is notachieved resulting in no massive hydrate formation As the initialcarbon dioxide pressure increases from 1.95 MPa to 2.96 MPa, theinduction time decreases from 76.4 min to almost half of its value,i.e. 39.5 min. At higher pressure values, the induction time is deter-mined at ±38 min. This trend shows that the supersaturation ofcarbon dioxide is increasing with the increase of carbon dioxideinitial pressure until 2.96 MPa. Since the induction time is observedto be almost constant above this value, it is assumed that the liquidphase is supersaturated with carbon dioxide beyond this value.

For the carbon dioxide–tetrahydrofuran–water system, it isobserved that clathrate hydrates are formed for all initial carbondioxide pressures used in this study. Since tetrahydrofuran is alsoa hydrate former and it is completely miscible in water, this trendsuggested that the amount of carbon dioxide required to achievesupersaturation condition for hydrate nucleation is reduced sig-nificantly. Moreover, the induction time for hydrate formation isfound to be decreasing from 25.62 to 12.25 min as the initial pres-sure increases from 1.53 to 3.76 MPa. For comparison, the inductiontime for the ternary system at initial carbon dioxide of 1.93 MPa isapproximately four times lower than that of the binary system. Asthe initial pressure increases, the difference between the inductiontimes of the ternary and binary systems decreases. This compari-son indicates that the hydrate nucleation process and onset hydrategrowth is more readily to occur in presence of tetrahydrofuran.

As shown in Fig. 3, the presence of sodium chloride prolongs theinduction time in the mixed carbon dioxide and tetrahydrofuranhydrate forming system. The difference in the induction times forthe mixed hydrate forming system in water and sodium chloridesolution decreases with an increase in the initial carbon dioxidepressure. It is well known that the presence of ions in the aque-ous phase hinders hydrate formation due to the clustering of watermolecules with the ions and the salting-out effect, which reducethe solubility of both tetrahydrofuran and carbon dioxide in water.These two factors are suspected to create a mass transfer resistancethat prolongs the process of hydrate nucleation and onset growth inthe presence of electrolytes. Although the induction time increases,it is still significantly lower than the induction time of simple carbondioxide hydrates. Based on these comparisons, it can be concludedthat the inclusion of tetrahydrofuran in hydrate forming systems isnot only thermodynamically favours hydrate formation but it alsoreduces the induction time for clathrate hydrate formation.

The temperature at the turbidity point (induction point) for eachinitial carbon dioxide pressure is taken as an average value of thefive experimental repetitions. The average error of these tempera-tures is ±1 ◦C. These average temperature values are plotted againstthe initial carbon dioxide pressures and depicted in Figs. 4–6 forcarbon dioxide + water, carbon dioxide + tetrahydrofuran + waterand carbon dioxide + tetrahydrofuran + sodium chloride solutionsystems, respectively. For comparison, the hydrate equilibria forthe measured systems (Sabil et al., 2009, 2010) are included alsoincluded in these figures. It is clearly shown that for the formation ofclathrate hydrate, subcooling is required to overcome the metasta-bility condition (Sloan and Koh, 2008) in the hydrate formingsystem. Furthermore, it is shown that for all systems investigated inthis study, the temperature at the turbidity point increases whenthe initial carbon dioxide pressure in the system increases. This

for single carbon dioxide and mixed carbon dioxide and tetrahy-. Greenhouse Gas Control (2010), doi:10.1016/j.ijggc.2010.05.010

behaviour is expected as an increase in carbon dioxide causes aricher environment of supersaturation at the liquid–vapour inter-face, which is required in order for the hydrate nucleation and onsetgrowth to take place. Due to this factor, the degree of subcoolingrequired to enrich the interface with carbon dioxide molecules for

Please cite this article in press as: Sabil, K.M., et al., Kinetic of formationdrofuran hydrates in water and sodium chloride aqueous solution. Int. J

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Fig. 4. The temperature at the turbidity point for different initial carbon dioxidepressures for the CO2 + water system. The corresponding hydrate equilibrium curvefor each system is also included (solid line, Sabil et al., 2009).

Fig. 5. The temperature at the turbidity point for different initial carbon dioxidepressures for the CO2 + THF + water system. The corresponding hydrate equilibriumcurve for each system is also included (solid line, Sabil et al., 2009).

Fig. 6. The temperature at turbidity point for different initial carbon dioxide pres-sures CO2 + THF in NaCl solution system. The corresponding hydrate equilibriumcurve for each system is also included (solid line, Sabil et al., 2010).

Table 1The required subcooling (�Tsub) for different hydrate forming systems.

CO2 + water CO2 + THF + water CO2 + THF in NaCl solution

Pressure �Tsub Pressure �Tsub Pressure �Tsub

– – 1.53 3.91 – –– – 1.76 3.17 1.73 3.292.06 1.49 1.98 3.31 2.04 3.052.21 1.75 2.25 2.70 2.27 2.472.48 2.22 2.50 2.82 2.53 1.652.77 2.56 2.71 2.70 2.76 1.27

2.95 2.70 3.02 1.75 3.03 0.923.25 3.05 3.25 1.12 – –– – 3.47 0.91 – –

higher initial concentrations of carbon dioxide, i.e. higher initialcarbon dioxide pressure, is expected to be less than at lower carbondioxide concentrations.

To gain some more insights in the degree of subcooling required,the difference between the hydrate equilibrium temperature andthe temperature at the turbidity point, �Tsub, at a specified pressureis calculated and tabulated in Table 1. In the case of simple carbondioxide, the degree of subcooling is increasing with the increase ininitial pressure of carbon dioxide. Arjmandi et al. (2005) showedthat for a simple hydrate such as carbon dioxide hydrate, subcool-ing is proportional to the driving force for hydrate formation. Inthis case, the higher degree of subcooling is due to the increase ofthe initial carbon dioxide pressure which results in a higher drivingforce for hydrate formation. With the increase of driving force forhydrate formation, it is expected that the system is more suscepti-ble for hydrate formation resulting in the reduction of the inductiontime as previously shown in Fig. 3.

In contrast, the degree of subcooling required for hydrate forma-tion is found to decrease with the increasing initial carbon dioxidepressure for mixed carbon dioxide and tetrahydrofuran hydratesin water and an aqueous sodium chloride solution. For both cases,a significant reduction of more than 2 ◦C of the degree of subcool-ing is observed when pressure of the system increases from ∼1.7 to∼3.0 MPa. The induction time also decreases in this pressure regionas shown in Fig. 3. This behaviour might be explained by consider-ing the degree of supersaturation in the hydrate forming systems.It is forecasted that the presence of tetrahydrofuran in both sys-tems reduces the driving force requirement for hydrate formation.As the initial carbon dioxide pressure increases in the systems, theamount of carbon dioxide molecules at the liquid–vapour interfacewill also increase. The combination of these two factors results in areduced driving force required for hydrate formation. As the driv-ing force of the hydrate formation can be related to the degree ofsubcooling, at least to some extent (Arjmandi et al., 2005), the sub-cooling requirement decreases with the decrease of the requireddriving force. In addition, no significant effect of the presence ofelectrolytes can be observed in the hydrate forming systems on thedegree of subcooling requirement for hydrate formation.

3.3. Massive hydrate growth

In this section, the massive hydrate growth process is consideredto start from the turbidity point, at which the hydrate formation isnoticeable, to the stationary point, at which the maximum amountof carbon dioxide consumed for the hydrate formation is observed.The amount of carbon dioxide dissolved in the liquid phase is notconsidered during hydrate growth because it is assumed that the

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supersaturation condition is maintained throughout the hydrateformation process (North et al., 1998) and the hydrate particlesformed are in three-phase equilibrium with the liquid and vapourphases (Englezos et al., 1987). The amount of carbon dioxide con-sumed during this process for simple carbon dioxide hydrates and

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Fig. 7. Carbon dioxide consumption during massive hydrate growth for simple car-bon dioxide hydrates at different initial pressures of carbon dioxide.

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Fig. 9. Carbon dioxide consumption during massive hydrate growth for mixed car-bon dioxide and tetrahydrofuran hydrates in an aqueous sodium chloride solutionat different initial pressures of carbon dioxide.

ig. 8. Carbon dioxide consumption during massive hydrate growth for mixed car-on dioxide and tetrahydrofuran hydrates at different initial pressures of carbonioxide.

oth mixed carbon dioxide and tetrahydrofuran hydrates in waternd sodium chloride at different initial pressure of carbon dioxidere plotted in Figs. 7–9, respectively. For all the hydrate formingystems, the carbon dioxide consumption during the process isound to be dependent on the initial pressure of carbon dioxiden the system, which relates to the initial amount of carbon dioxiden the system. By comparing all the figures, it is noticed that theime taken from the turbidity point to the stationary point for sim-le carbon dioxide hydrates (∼60 min) is about three times longerhat that of mixed carbon dioxide and tetrahydrofuran hydrates∼20 min) regardless of the initial carbon dioxide pressure in theystems. The time is slightly prolonged with the presence of sodiumhloride in the system for the mixed carbon dioxide and tetrahy-rofuran hydrates (∼30 min). This observation implies that the ratef massive hydrate growth is much faster with the presence ofetrahydrofuran in the system while the presence of sodium chlo-ide slightly hinders the hydrate growth process.

For comparison, the maximum carbon dioxide consumption

Please cite this article in press as: Sabil, K.M., et al., Kinetic of formationdrofuran hydrates in water and sodium chloride aqueous solution. Int. J

uring the process at different initial carbon dioxide pressures forach system studied is plotted in Fig. 10. As shown in Fig. 10,t initial pressures >2.5 MPa, the amount of carbon dioxide con-umed during the massive growth in the simple carbon dioxide

Fig. 10. Maximum carbon dioxide consumption during massive growth at differentinitial carbon dioxide pressures.

hydrate system is higher than that of mixed carbon dioxide andtetrahydrofuran hydrate system at similar initial pressures. Thisobservation suggested that the inclusion of tetrahydrofuran in thehydrate forming systems reduces the amount of carbon dioxideentrapped in the hydrate structure and can be explained by con-sidering the type of hydrate structure in each system. In the caseof simple carbon dioxide hydrate, structure I is formed and carbondioxide molecules fill the large cavities of the structure and possi-bly some of the small cavities because the ratio of carbon dioxidemolecule diameter versus the effective diameter of the 512 is equalto 1 (Sloan and Koh, 2008). If all cavities are occupied, 8 moleculesof carbon dioxide molecules are entrapped per unit cell of clathratehydrate with 46 molecules of water giving a ratio of carbon dioxideto water per unit cell of 0.1304. In contrast, structure II hydrate isformed in the case of the mixed carbon dioxide and tetrahydrofu-ran hydrates where 136 water molecules are present per unit cellof the structure. Due to their size, tetrahydrofuran molecules onlyoccupy the large cavities of this structure, leaving the smaller cav-ities for the carbon dioxide molecules. Although the size ratio ofcarbon dioxide molecule is slightly larger than that of the small

for single carbon dioxide and mixed carbon dioxide and tetrahy-. Greenhouse Gas Control (2010), doi:10.1016/j.ijggc.2010.05.010

cavity of structure II hydrate, there is evidence showing that car-bon dioxide molecule can fit in the small cavities of structure IIhydrate, especially when there is a larger hydrate former present inthe system (Teng, 1996; Ripmeester and Ratcliffe, 1998; Mooijer-

ARTICLE IN PRESSG ModelIJGGC-315; No. of Pages 8

K.M. Sabil et al. / International Journal of Greenhouse Gas Control xxx (2010) xxx–xxx 7

Fig. 11. Plot of ln[C0 − Cs/C − Cs] versus time in pure water at different initial pres-sures of carbon dioxide for simple carbon dioxide hydrates.

Fs

vccrdpdsisd

TT

ig. 12. Plot of ln[C0 − Cs/C − Cs] versus time in pure water at different initial pres-ures of carbon dioxide for mixed carbon dioxide and tetrahydrofuran hydrates.

an den Heuvel, 2004). If the small cavities are fully occupied byarbon dioxide, the ratio of carbon dioxide versus water per unitell is equal to 0.118. This simple calculation shows that there is aeduction of carbon dioxide consumption among the formation ofifferent hydrate structures, even if all possible cavities are occu-ied with carbon dioxide. Obviously, if the size ratio of carbonioxide is slightly larger than the diameter of the small cavity of

Please cite this article in press as: Sabil, K.M., et al., Kinetic of formationdrofuran hydrates in water and sodium chloride aqueous solution. Int. J

tructure II hydrate, the occupancy of carbon dioxide in this cav-ty is expected to be less than in the ideal case which leads to aignificantly less carbon dioxide consumption in the mixed carbonioxide and tetrahydrofuran hydrates.

able 2he apparent rate constant for simple carbon dioxide and mixed carbon dioxide and tetra

Simple CO2 hydrate Mixed CO2 and THF hydrat

T Pint. k T Pint.

275.6 2.06 0.0237 284.4 1.53275.9 2.21 0.0287 285.7 1.76277.0 2.48 0.0325 286.0 1.98277.5 2.77 0.0452 287.1 2.25277.8 2.95 0.0516 287.3 2.50278.2 3.25 0.0627 287.8 2.71

289.1 3.02290.1 3.25290.5 3.47

Fig. 13. Plot of ln[C0 − Cs/C − Cs] versus time in sodium chloride solution at differentinitial pressures of carbon dioxide for mixed carbon dioxide and tetrahydrofuranhydrates.

At initial pressure of <2.20 MPa, the consumption of carbon diox-ide in simple carbon dioxide hydrate is found to be less than thatof the mixed carbon dioxide and tetrahydrofuran hydrate. Refer-ring to Fig. 3, at these lower initial pressure conditions the hydrateformation of simple carbon dioxide hydrate occurs at a temper-ature near to that of the equilibrium temperature of the hydrateequilibrium, resulting in a smaller driving force for hydrate forma-tion. Furthermore, the equilibrium condition limits the formationof more hydrate particles in the system. The combination of thesetwo factors reduces the amount of carbon dioxide consumption forhydrate formation in the case of simple carbon dioxide at these lowinitial pressure values.

As shown in Figs. 8 and 9, the presence of sodium chloride in theaqueous solution is found to prolong the hydrate growth periodby ∼30%, regardless the initial pressure of carbon dioxide in thesystem. Moreover, the amount of carbon dioxide consumed is alsofound to be slightly less in the presence of sodium chloride in thesystem, especially at lower initial pressure conditions as shown inFig. 10. This observation can be explained by taking into accountthe effect of ionization of salt in aqueous solution. Due to ioniza-tion, the ions of sodium chloride form Coulombic bond with thedipoles of water and this bond is much stronger than either hydro-gen bond or van der Waals forces. The formation of these bondscauses clustering around the water molecules and inhibits the for-mation of hydrate since water is more attracted to ions than to

for single carbon dioxide and mixed carbon dioxide and tetrahy-. Greenhouse Gas Control (2010), doi:10.1016/j.ijggc.2010.05.010

the hydrate structure (Sloan and Koh, 2008). Therefore, the reduc-tion of carbon dioxide consumption in presence of sodium chloridemight be attributed to the clustering and salting-out effects of elec-trolytes.

hydrofuran hydrates.

e Mixed CO2 and THF hydrates in NaCl solution

k T Pint. k

0.1500 283.9 1.73 0.07750.1670 284.7 2.04 0.08230.1825 285.7 2.27 0.09240.1980 286.9 2.53 0.12000.1989 287.6 2.76 0.11240.2007 288.3 3.03 0.13670.20080.21420.2559

INI

8 f Gree

tF

wic1

firiccea

4

tcitficTatcmitdcwtoc

A

ItgSe

R

A

A

Sloan Jr., E.D., Koh, C.A., 2008. Clathrate Hydrates of Natural Gas, 2nd ed. CRC Press,Florida.

Teng, H., 1996. Comment: characteristics of the hydrate layer formed at the liquid

ARTICLEG ModelJGGC-315; No. of Pages 8

K.M. Sabil et al. / International Journal o

In this work, the apparent rate constant of the first-order reac-ion is calculated from the slope of the straight line shown inigs. 11–13. The equation is shown as

dCh

dt= k

(C2 − Cs

2

)(1)

here k is the apparent rate constant, Ch is concentration of CO2n hydrate, C2 is concentration of CO2 and C2

S is the saturated con-entration of carbon dioxide at the stationary point (Ohgaki et al.,993).

From these plots, the apparent rate constant k is calculatedrom the slope of each line and the calculated data are tabulatedn Table 2. From the calculated data, it is shown that the apparentate constant is dependent on the initial pressure of carbon dioxiden the system. Moreover, the apparent rate constant for the mixedarbon dioxide hydrate is found to be higher than that of the simplearbon dioxide hydrate at similar initial pressure values. The pres-nce of sodium chloride in the hydrate forming system reduces thepparent rate constant of the system.

. Conclusion

A series of experiments was conducted to obtainhe kinetic data of hydrate formation in carbon dioxide,arbon dioxide + tetrahydrofuran + water and carbon diox-de + tetrahydrofuran + sodium chloride solution systems. Forhat purpose, the T-cycle method was applied. The induction timeor the measured hydrate systems was found to be much lowern the ternary system of carbon dioxide + tetrahydrofuran + waterompared to that of the binary system of carbon dioxide + water.his comparison indicates that the hydrate nucleation processnd onset growth is more readily to occur in the presence ofetrahydrofuran in the aqueous solution. The presence of sodiumhloride is found to slightly prolong the induction time in theixed hydrate system. In general, the carbon dioxide consumption

s found to be higher for simple carbon dioxide hydrates comparedo mixed carbon dioxide and tetrahydrofuran hydrates. It wasetermined that the apparent rate constant for these system isoncentration dependent. Moreover, the apparent rate constantas found to be higher in the mixed hydrate system suggesting

hat it is more susceptible for hydrate formation. The presencef sodium chloride is found to slightly reduce the carbon dioxideonsumption during the hydrate formation process.

cknowledgements

The authors gratefully acknowledge financial support fromnstitute Technology PETRONAS (Ltd.), Malaysia. Partial support byhe European Commission FP6 (NESSHY contract no. 518271) isratefully acknowledged. In addition, the assistance of Dr. Alireezahariati in designing and fabrication the reactor system is acknowl-dged.

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