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International Journal of Greenhouse Gas Control 26 (2014) 185–192

Contents lists available at ScienceDirect

International Journal of Greenhouse Gas Control

j ourna l h o mepage: www.elsev ier .com/ locate / i jggc

ydrate phase equilibria of CO2 + N2 + aqueous solution of THF,BAB or TBAF system

men Ben Attouche Sfaxia,b, Isabelle Duranda, Rafael Lugoa,∗, Amir H. Mohammadib,c,∗∗,ominique Richonc,d

IFP Energies Nouvelles, 1-4 avenue de Bois-Préau, 92852 Rueil-Malmaison, FranceMINES ParisTech, CEP/TEP – Centre Énergétique et Procédés, 35 Rue Saint Honoré, 77305 Fontainebleau, FranceThermodynamics Research Unit, School of Engineering, University of KwaZulu-Natal, Howard College Campus, King George V Avenue, Durban 4041,outh AfricaTechnical University of Denmark, Center for Energy Resources Engineering (CERE), Department of Chemical and Biochemical Engineering, DK-2800 Kgs.yngby, Denmark

r t i c l e i n f o

rticle history:eceived 9 April 2013eceived in revised form 15 March 2014ccepted 9 April 2014

a b s t r a c t

We report hydrate dissociation conditions of CO2 (15 and 30 mol%) + N2 (85 and 70 mol%) in the presenceof aqueous solutions of THF, TBAB or TBAF. The concentrations of TBAB and TBAF in the aqueous solutionsare 5 wt% and 9 wt% while THF concentration in aqueous solution is 3 mol%. Two different experimen-tal techniques including isochoric pressure search method and a DSC method are used to measure thehydrate dissociation conditions. A comparison is finally made with the literature data. It is expected that

eywords:as hydrateslathrate hydratesO2 capturehase equilibrium dataromoter

this study provides better understanding of hydrate phase equilibria associated with CO2 capture.© 2014 Elsevier Ltd. All rights reserved.

alorimetry

. Introduction

The design of an innovative and environmental friendly post-ombustion CO2 capture process has been a subject of intensiveesearch in several groups around the world (Kang et al., 2001; Seot al., 2001; Linga et al., 2007a; Duc et al., 2007; Fan et al., 2009;hang et al., 2009; Eslamimanesh et al., 2012a). One of the cur-ent technological solutions that have been the subject of severalatents and scientific publications involves capturing CO2 in formf CO2 clathrates hydrates.

Clathrates hydrates are crystalline structures formed by waterolecules surrounding guest molecules which are in general small

olecules such as methane, ethane, etc. (Sloan, 1998). Other guestolecules include CO2, N2, H2S, etc. These structures are normally

table at high pressures and low temperatures. There are three

∗ Corresponding author.∗∗ Corresponding author at: MINES ParisTech, CEP/TEP – Centre Énergétique etrocédés, 35 Rue Saint Honoré, 77305 Fontainebleau, France. Tel.: +33 164694970;ax: +33 164694968.

E-mail addresses: Rafael.Lugo@IFP.fr (R. Lugo), amir h mohammadi@yahoo.com,.h.m@irgcp.fr (A.H. Mohammadi).

ttp://dx.doi.org/10.1016/j.ijggc.2014.04.013750-5836/© 2014 Elsevier Ltd. All rights reserved.

major types of hydrate crystalline structures: sI hydrate, sII hydrateand sH hydrate (Sloan, 1998). The most common structures in therelevant industrial applications (mainly the oil and gas industry) aresI and sII. Depending on their molecular diameter, some moleculeswill form sI hydrates (CH4, CO2, H2S etc.) whereas other moleculeswill form sII hydrates (propane, N2, etc.)

By analogy with the amine-based process (Raynal et al., 2011),the hydrate-based CO2 capture process follows the following steps(see Fig. 1):

• A formation stage, at low temperatures and high pressures, wherethe post-combustion flue gases are brought into contact with anaqueous solution in an “absorber” column. Flue gases containmainly CO2 diluted in N2 (15–17 mol%). Water will form hydrateswhich will trap the gas molecules. Hydrates of pure CO2 format less severe conditions (higher temperatures, lower pressures)than hydrates of N2. Thus, it is expected that the hydrate phasewill be enriched in CO2 whereas the lean gas will be highly con-

centrated in N2.

• The second stage, called dissociation, involves regenerating thehydrate-trapped gas from the absorber by heating it and/or low-ering its pressure.

186 I.B.A. Sfaxi et al. / International Journal of Greenhouse Gas Control 26 (2014) 185–192

CaNl

tptshspecttpiC

adrdhma

•

•

Fig. 1. Hydrate-based CO2 capture process.

The efficiency of such a process is measured by the amount ofO2 in the gaseous mixture obtained after regeneration i.e. themount of CO2 selectively trapped in the hydrate phase. Indeed,2 will also be trapped within the hydrate phase, but in a much

ess proportion.The hydrate-based process is thus based on the preferential cap-

ure of CO2 with respect to N2 and the other molecules present inost-combustion flue gases. In general, in a typical power plant,he post-combustion flue gases are emitted at atmospheric pres-ure and must be compressed to some hundred of bars for theydrate formation step to be performed. The cost of the compres-ion step may increase the energy penalty of the hydrate-basedrocess (Linga et al., 2007a,b,c). One of the solutions that have beenxplored by several authors includes the usage of specific chemi-al additives called hydrate promoters. These chemicals affect thehermodynamic stability conditions of the system by shifting themoward less severe conditions (i.e. higher temperatures or lowerressures). The regeneration can also be performed under pressure

n order to take advantage of the resulting energy saves in the globalO2 capture process.

It is clear that the simulation of such a process requires a suit-ble thermodynamic model. Therefore, reliable phase equilibriumata are necessary for tuning these models parameters. A literatureeview (Ben Attouche Sfaxi, 2011) shows that few thermodynamicata are available for phase equilibrium predictions of clathrateydrates of mixed gases in the presence of most of the current ther-odynamic promoters. Some of the most widely used promoters

re as follows:

Tetrahydrofuran (THF): The solid–liquid, liquid–liquid andvapour–liquid equilibria of binary solutions of THF and water arewell known (Shnitko and Kogan, 1968; Signer et al., 1969; Matouset al., 1972; Treiner et al., 1973; Hayduk et al., 1973; Wallbruchand Schneider, 1995; Riesco and Trusler, 2005; Delahaye et al.,2006). Few authors have investigated the phase equilibria of sys-tems containing CO2, N2 and THF (Kang et al., 2001; Seo et al.,2001; Sabil et al., 2010a,b) but only the work of Kang et al.(2001) provides data for different CO2/N2 load compositions (17and 70 mol%) and for different amounts of THF (1 and 3 mol% inwater).Tetra alkyl ammonium salts: In the scope of this work, we onlyconsider tetra n-butyl ammonium bromide (TBAB) and tetra n-butyl ammonium fluoride (TBAF). Several works can be foundinvestigating the hydrates of these compounds (Dyadin andUdachin, 1984; Aladko et al., 2002; Sun et al., 2008; Shimada et al.,2005; Oyama et al., 2005). It appears that the phase behavior ofbinary water + alkyl ammonium salts systems is quite complexand several structures have been identified depending on thecomposition of the system (Eslamimanesh et al., 2012a; Dyadinand Udachin, 1984; Aladko et al., 2002; Sun et al., 2008; Shimada

et al., 2005; Oyama et al., 2005; Deschamps and Dalmazzone,2009). These molecules form semi-hydrates in which they takepart in the hydrate lattice formed by the water molecules. Adetailed crystallographic description of the behavior of these

Fig. 2. Schematic of the experimental setup used for hydrate phase equilibriummeasurements.

systems is beyond the scope of the present work, which is mainlydevoted to further investigating the thermodynamic effect ofalkyl ammonium salts as additives in the hydrate-based captureprocess. Different authors provide thermodynamic data for semi-clathrate hydrates of CO2 and N2 in the presence of tetra alkylammonium salts (Deschamps and Dalmazzone, 2009; Duc et al.,2007; Belandria et al., 2012; Mohammadi et al., 2012; Arjmandiet al., 2007; Fan et al., 2009; Li et al., 2009, 2010; Ye and Zhang,2012; Lee et al., 2010, 2011; Zhong et al., 2011). For the semi-clathrate hydrates of TBAB, some discrepancies are observedbetween the data provided by different authors. However, forTBAF semi-clathrate hydrates, very few works are available in theliterature.

For an optimal design of the hydrate-based CO2 capture pro-cess, sufficient and reliable data are necessary to predict thethermodynamic stability conditions of hydrates in presence of pro-moters. Most of the published works involve phase equilibriumdata for a single gas (CO2, N2, CH4, etc.) and a single promotersuch as THF, TBAB or TBAF. Very few works (Duc et al., 2007;Deschamps and Dalmazzone, 2009) investigate the role of pro-moters in gas mixtures relevant to post-combustion applications.New measurements are thus required for the CO2 + N2 mixturesto validate and extent the existing thermodynamic data. In thepresent work, we provide new phase equilibrium data for the semi-clathrate hydrates of CO2 + N2 in the presence of THF, TBAB, orTBAF.

2. Experimental setup and procedure

2.1. Description of the experimental setups

The experimental work presented in this paper has been mainlycarried out in a high-pressure cell, originally designed for hydratekinetic measurements but adapted for phase equilibrium measure-ments. A schematic representation of the setup is provided in Fig. 2.The cell consists of two sapphire windows and a total volume of300 cc. It is a doubled-wall cell and a refrigerant flows throughthe interspace to control the internal temperature. A Keller/PA-21-100 pressure transducer allows measuring the system pressure inthe range 0–10 MPa. Two PT100 Prosensor temperature probes are

used to measure the temperatures of the liquid and the vapourphases. A Parr magnetic stirrer A2042HC with 300 rpm is used toimprove the heat and mass transfer between the phases to reachthermodynamic equilibrium.

I.B.A. Sfaxi et al. / International Journal of Greenhouse Gas Control 26 (2014) 185–192 187

ork for calorimetric measurements (Lin et al., 2008).

ummptifrw

pcdcspctieasadpdm

mrrveftifa

2

aeaaan

Table 1Experimental equipment.

Experimentalequipment

Experimentalmethod

CO2 load in the CO2/N2 gasmixture, promoterconcentration in aqueoussolution

Hydrate kinetic rig Isochoric pressuresearch method

- CO2 15% mol, TBAB 5% wt

- CO2 15% mol, TBAB 9% wt- CO2 30% mol, TBAB 9% wt- CO2 30% mol, TBAF 5% wt- CO2 30% mol, TBAF 9% wt

DSC (Fig. 3) Calorimetricmeasurement of

- CO2 15% mol, THF 3% mol -CO 15% mol

Mohammadi et al., 2008). A typical pressure-temperature (p–T) plotfor an isochoric experiment is provided in Fig. 4.

1.3

1.4

1.5

1.6

1.7

1.8

1.9

2

2.1

2.2

273 278 283 288

p/ M

Pa

Fig. 3. Schematic of the HP DSC VII used in this w

The pressure transducer was calibrated between 0 and 9 MPasing the DPI 605 precision pressure calibrator. We have esti-ated the maximum experimental uncertainty on the pressureeasurement to be 0.05 MPa in the whole range. Temperature

robes were calibrated between 268 and 298 K in an Ametek havingwo PT100 ATC 155B temperature probes. This calibration resultedn a maximum uncertainty of 0.1 K. This uncertainty is acceptableor carrying out kinetic experiments but might be considered asather high for thermodynamic measurements. No further workas performed to improve this accuracy.

For some complementary measurements, we also used a high-ressure (HP) micro DSC VII Setaram device. This differentialalorimeter allows investigating the conditions of formation andissociation of hydrates by monitoring the thermal effect asso-iated to the corresponding phase changes. It has been alreadyhown (Lin et al., 2008; Dalmazzone et al., 2003) that this techniquerovides interesting information on the thermodynamic stabilityonditions of hydrates as well as on the enthalpies corresponding toheir formation/dissociation. A schematic of this device is providedn Fig. 3 (Lin et al., 2008). It consists of two Hastelloy C276 cells, a ref-rence and a sample cell, that can work up to 400 bar. Each cell has

volume of 0.33 cc. The cooling effect is provided by a Peltier effectystem. The minimum temperature that can be reached is −45 ◦C,nd the maximum temperature is 120 ◦C. The measurement of theifference of heat flows between the reference cell and the sam-le cell is based on the Calvet principle. The fluxmeter used in thisevice ensures a high sensitivity and precision of the calorimetriceasurements.The DSC VII is calibrated every year using reference substances:

ercury, water, gallium and diphenylether. The heating/coolingate used for calibration and all other measurements is 1 K/h. Thisate is convenient for these measurements according to the pre-ious experience of the authors (see for example (Dalmazzonet al., 2003) for the use of this technique for the study of hydrateormation in drilling muds). For these conditions of calibration,he maximum experimental uncertainties are 0.5 K for the heat-ng/cooling rates applied in the present work. Table 1 brings out,or each studied system, the experimental equipment that is usednd the experimental method applied.

.2. Experimental procedure

In order to measure the stability conditions of hydrates, we havedopted the isochoric pressure-search technique (Dalmazzonet al., 2003; Tohidi et al., 2000; Ohmura et al., 2004). In this method,

gaseous load and about 80 cc of an aqueous solution are mutu-lly saturated at ambient temperature and at a given pressure in

constant volume cell. Once the system has reached thermody-amic equilibrium, hydrates are formed by decreasing the system

the dissociationtemperature

2

temperature (the final temperature is about 10 K below the esti-mated dissociation temperature). The formation of hydrates isobserved when a sudden pressure decrease, caused by gas trappingin the hydrates, occurs. Once hydrates have formed, the systemis slowly heated (about 1 K/h) till dissociation is observed. Thepoint where the formation and the dissociation thermodynamicpaths intersect corresponds to a point of the stability curve for thegiven load composition (Tohidi et al., 2000; Ohmura et al., 2004;

T / K

Fig. 4. Typical p–T plot obtained from the isochoric pressure search technique(Tohidi et al., 2000; Ohmura et al., 2004; Mohammadi et al., 2008).

1 f Greenhouse Gas Control 26 (2014) 185–192

amtppuattt1

3

3T

s

gc(oaCdp

Table 2Hydrate dissociation conditions for systems with gaseous loads of CO2/N2 (15 mol%and 30 mol% of CO2) and for aqueous solutions of TBAB (5 wt% and 9 wt%)a.

Dalmazzoneet al. (2003)

This work

mol% CO2/N2 15 15 15 30wt% TBAB aqueous solution 40 5 9 9mol% TBAB aqueous solution 3.6 0.3 0.55 0.55

T/K p/MPa T/K p/MPa T/K p/MPa T/K p/MPa

289.9 2.91 281.0 1.90 284.6 3.26 286.1 3.82291.3 4.65 283.0 3.86 286.0 4.88 287.1 4.85292.6 7.13 284.2 5.77 287.0 6.26 287.8 5.91293.3 9.18 285.5 7.79 287.8 7.34 288.1 6.41

FD

88 I.B.A. Sfaxi et al. / International Journal o

The calorimetric measurements carried out in the DSC VII deviceim mainly at measuring the stability conditions of hydrates byeasuring the thermal effect of their dissociation as a function of

emperature. These measurements are done by putting in the sam-le cell an amount 0.5 ml of the aqueous solution to be studied. Theressure of both cells is then increased up to a given pressure levelsing the gas mixture of interest. A temperature program is thenpplied where: in a first step, hydrates are formed by decreasinghe temperature of the system well below the estimated dissocia-ion temperature. Then, a second isothermal step allows stabilizinghe system. The final step involves heating the system at a rate of

K/h until hydrates dissociation is observed.

. Results and discussion

.1.1. Determination of the thermodynamic promoting effect ofBAB and TBAF

In this work, we performed measurements for the hydrate dis-ociation conditions of the following systems:

- CO2/N2, 15 mol% CO2 with an aqueous solution of TBAB (5 wt% or0.3 mol%).- CO2/N2, 15 mol% CO2 with an aqueous solution of TBAB (9 wt% or0.55 mol%).- CO2/N2, 30 mol% CO2 with an aqueous solution of TBAB (9 wt% or0.55 mol%).- CO2/N2, 30 mol% CO2 with an aqueous solution of TBAF (5 wt% or0.36 mol%).- CO2/N2, 30 mol% CO2 with an aqueous solution of TBAF (9 wt% or0.68 mol%).

The promoter concentrations investigated in this work are thoseenerally studied in single gas measurements, so that comparisonsan be made with the currently available data. See for exampleKang et al., 2001; Seo et al., 2001; Sabil et al., 2010a,b) for THFr (Duc et al., 2007; Arjmandi et al., 2007; Li et al., 2010) for alkyl

mmonium salts. The hydrate dissociation data measured for theO2/N2 loads with the TBAB promoter are given in Table 2. Theseata have also been plotted in a p–T diagram in Fig. 5 and com-ared to the data measured by Deschamps and Dalmazzone (2009).

ig. 5. Hydrate dissociation curves for different gaseous loads and for different amounalmazzone (2009).

a Temperatures and pressures are measured within ±0.5 K and ±0.05 MPa, respec-tively.

These authors carried out their measurements using a DSC for agaseous load containing 15 mol% CO2 and for an aqueous solutionhaving 40 wt% of TBAB (0.043 mol%). We have also investigatedthe thermodynamic behavior of loads with 30 mol% CO2 becausethis composition is relevant to cement industry post-combustionsgases (>20 mol%). Furthermore, as shown by Kang et al. (2001), at15–17 mol% CO2, there seems to be a structure transition that canlead to high experimental uncertainties.

It can first be noticed that our measurements are consistent withthe expected behavior since the stability of hydrates is increasedwhen the amount of TBAB in the solution increases. Analogously,the amount of CO2 in the gas load leads to lower pressure and/orhigher temperature of dissociation. These data are also consistentwith the measurements of Deschamps and Dalmazzone (2009). Italso seems that the slopes of the dissociation curves are similar. As afirst approximation, this means that the enthalpies of dissociationhave similar values, which in turn means that the crystal struc-tures are the same since the hydrate structure and its enthalpy ofdissociation are strongly related as shown by Sloan (1998). Theseobservations need to be further investigated since the applicationof the Clausius–Clapeyron equation to these types of data requires a

rigorous thermodynamic derivation of the phase equilibrium equa-tions and their relationship with the enthalpy of dissociation.

Duc et al. (2007) have measured dissociation data of hydratesof CO2/N2 in the presence of aqueous solutions containing 5 wt%

ts of TBAB in the aqueous solution. Comparison with data from Deschamps and

I.B.A. Sfaxi et al. / International Journal of Greenhouse Gas Control 26 (2014) 185–192 189

0

1

2

3

4

5

6

7

8

9

301296291286281276T /

p /

MP

a

15.50 % mol CO2 + TBAB 5% wt19.20 % mol CO2 + TBAB 5% wt23.40 % mol CO2 + TBAB 5% wt21.50 % mol CO2 + TBAB 5% wtthis work: 15%mol CO2 + TBAB 5% wtthis work: 15% mol CO2 +TBAB 9% wt

F of TBA

orc(fotchwcT

vhmwIh

Fw

ig. 6. Hydrate dissociation curves for different gaseous loads and various amounts

f TBAB. The gaseous loads investigated by these authors haveespectively a content of 15.5, 19.2, 21.5 and 23.4 mol% of CO2. Aomparison between our data and the measurements of Duc et al.2007) is presented in Fig. 6. As can be seen, our data and the datarom Duc et al. (2007) are not consistent. For instance, the slopesf the dissociation curves at 23.4 mol% CO2 and that determined inhis work at 15 mol% CO2 are very different. These discrepanciesould be attributed to a difference in terms of hydrate structure;owever, Lin et al. (2008) also observed important discrepanciesith some of the results of Duc et al. (2007) for pure CO2 semi-

lathrate hydrates in the presence of different concentrations ofBAB in aqueous solution.

Unlike TBAB, for which some data can be found in the literature,ery few data are available for hydrates of other alkyl ammoniumalide promoters such as TBAF. It is important to study other pro-

oters of this family of chemicals in order to find new additiveshich have more considerable effects on the stability of hydrates.

t should also be noticed that the anion has a strong effect on theydrate phase equilibrium when this type of chemicals are used as

0

1

2

3

4

5

6

7

8

28284282280278276

T /

p /

MP

a

30% mol CO2 + TBAB 9% wt

30% mol CO2 + TBAF 5% wt

30% mol CO2 + TBAF 9% wt

ig. 7. Hydrate dissociation curves for a gaseous load of CO2/N2 (30 mol% CO2) and for dith the measurements performed for the same gaseous load with an aqueous solution o

K

B in the aqueous solution. Comparison with experimental data of Duc et al. (2007).

promoters in the aqueous solution. A comparative study has beendone by Li et al. (2010) in this regard. These authors compared thepromoting power of TBAB, TBAC, and TBAF and concluded that TBAFhas the strongest promotion effect.

In this work, we performed hydrate dissociation measurementsusing gaseous loads of CO2 + N2 (30 mol% CO2) and aqueous solu-tions containing 5 wt% and 9 wt% of TBAF. The results are presentedin Table 3 and Fig. 7.

For comparison purposes, we have included in Fig. 7 our data forthe system containing 30 mol% CO2 and 9 wt% TBAB. It clearly seemsthat TBAF has a much stronger promotion effect for the same massfraction in the solution. Indeed, the dissociation curve is shiftedtoward much higher temperatures (about 15 K). These results arein good agreement with the observations of Li et al. (2010) andshow that the anion plays a major role in the stability of the hydrate

structure.

As mentioned earlier, tetra alkyl ammonium salts promotersare getting major interest in the hydrate-based CO2 capture pro-cess. Indeed, these additives have a promoting effect at a low

2962942922902886

K

ifferent amounts of TBAF in the aqueous solution (5 wt% and 9 wt%). Comparisonf 9 wt% TBAB.

190 I.B.A. Sfaxi et al. / International Journal of Greenhouse Gas Control 26 (2014) 185–192

Fig. 8. Calorimetric study of hydrates of CO2/N2 (15 mol% of CO2) at 9 MPa.

Table 3Hydrate dissociation conditions for systems with gaseous loads of CO2/N2 (30 mol%CO2) and for aqueous solutions of TBAF (5 wt% and 9 wt%)a.

mol% CO2/N2 30 30

wt% TBAF aqueous solution 5 9mol% TBAF aqueous solution 0.36 0.68

T/K p/MPa T/K p/MPa

287.7 2.21 291.9 2.43288.7 4.93 292.5 3.85289.4 6.74 293.7 5.96

t

dwbhenhoP(estws

3T

hptcw

Fig. 9. Comparison between the measured hydrate dissociation data for the CO2/N2

(15 mol% CO2) + water system and the experimental data of Kang et al. (2001) mea-sured using the DSC technique.

Table 4Hydrate dissociation conditions of clathrate hydrates of CO2 (15 mol%) + N2 in thepresence of THF aqueous solution (3 mol%)a.

T/K p/MPa (±0.05 MPa)

293.5 7.80291.5 5.90285.8 2.00290.6 5.00285.2 1.90288.9 3.90

a Temperatures and pressures are measured within ±0.5 K and ±0.05 MPa, respec-ively.

osage in the aqueous solutions. Moreover, they are not volatile,hich means that their losses in the process after regeneration can

e neglected. However, unlike the classic hydrate formers (smallydrocarbons, CO2, N2, etc.) there is still an important lack of phasequilibrium data for these new molecules. Furthermore, the ionicature of these additives and the fact that they form semi-clathrateydrates requires the use of complex models for both the aque-us and the hydrate phases. The reader may refer to the works ofaricaud (2011), Eslamimanesh et al. (2012b) and Eslamimanesh2012) on this subject. It should also be noticed that the phasequilibrium of these systems is also highly complex and still theubject of debates. For instance, although several authors have con-ributed to the determination of the solid–liquid phase diagram ofater–TBAB solutions, the number and structure of the different

emi-clathrate hydrates need to be further investigated.

.1.2. Calorimetric study of CO2/N2 hydrates in the presence ofHF

The thermodynamic equilibrium of hydrates of CO2/N2 mixturesave been already studied by Kang et al. (2001). These authors

rovide data for several load compositions. It is worth mentioninghat these authors observe a change in the slope of the dissociationurves for CO2/N2 load compositions around 17 mol% of CO2. It isidely accepted that CO2 forms sI hydrates, whereas the structure

a Temperatures and pressures are measured within ±0.5 K and ±0.05 MPa, respec-tively.

of pure N2 hydrates is still a subject of controversy, but most of theauthors assume that N2 forms structure sII.

In order to further investigate this possible structure transition,we have performed a calorimetric study using the DSC describedin Section 2. For these measurements, the sample cell was filledup with pure water (about 5 mg). A CO2/N2 mixture (15 mol% CO2)

I.B.A. Sfaxi et al. / International Journal of Greenhouse Gas Control 26 (2014) 185–192 191

F %) + Ne

wccbaspttfistgsws

maeitdopoC

(fanm

fsthrCK

ig. 10. Comparison of dissociation conditions of clathrate hydrates of CO2 (15 molxperimental data of Kang et al. (2001).

as later used to pressurize both the sample and the referenceells up to 9 MPa (since the pressure was not regulated, it actuallyhanged during the experiment, but these changes were negligi-le). The thermal program involved several cycles of cooling downnd heating up in order to reduce the amount of ice formed. Fig. 8hows the subsequent thermogram. In this Fig., the heat flow islotted against temperature. As can be seen, during the dissocia-ion step, two peaks are observed, corresponding to two differenthermal phenomena. Notice that the first peak is associated to arst-order transition phase-change, whereas the second one is aecond-order phase change. For this ternary system, a first orderransition implies the coexistence of four phases, i.e. liquid water,as and two coexisting hydrate structures. The first peak corre-ponds to the dissociation of sI hydrates, whereas the second peakould correspond to the dissociation of sII hydrates, the hydrate

tructure that is usually attributed to pure N2.The coexistence of both structures could probably correspond to

etastable phases formed during the cooling step and might prob-bly not correspond to the phases that are stable at thermodynamicquilibrium. The existence of sII hydrates, in which the formations most probably caused by the presence of N2, has an impact onhe isochoric measurements. Indeed, the final dissociation pointetected by this technique will correspond to the disappearancef sII hydrates. This assumption is consistent with our dissociationoint measurements, which are in good agreement with the dataf Kang et al. (2001) (see Fig. 9) for a mixture of CO2/N2 at 17 mol%O2.

The promoting effect of THF has been investigated by Kang et al.2001). They have provided thermodynamic measurements for dif-erent CO2/N2 mixtures and for different amounts of THF in thequeous solution. These data are currently used to fit thermody-amic models. No recent work has been done to corroborate theeasurements of Kang et al. (2001).We performed measurements of the hydrate dissociation curve

or a mixture of CO2/N2 (15 mol% CO2) in the presence of an aqueousolution of THF (3 mol%) using the device described in Fig. 3 withhe isochoric technique. The results are presented in Table 4. We

ave plotted our results in Fig. 10. As can be seen, our results are inather good agreement with the data of Kang et al. (2001) at 17 mol%O2, although the amount of CO2 in the mixture investigated byang et al. (2001) should lead to a dissociation curve shifted to the

2 in the presence of THF aqueous solution (3 mol%) measured in this work with the

right with respect to ours. This anomaly might be explained by theexperimental uncertainties of both sets of data. In any case, theconsistency is acceptable.

4. Conclusion

In this work, we performed thermodynamic equilibrium mea-surements to determine the hydrate stability conditions of CO2/N2[CO2 (15 and 30 mol%) + N2 (85 and 70 mol%)] in presence of THF(3 mol%), TBAB (5 wt% and 9 wt%) and TBAF (5 wt% and 9 wt%) ther-modynamic promoters. These data are of major importance for thedesign of hydrate-based CO2 capture processes. In particular, weprovide new experimental data for systems containing alkyl ammo-nium ionic additives. These additives are gaining interest sincethey have rather strong thermodynamic promoting effect com-pared to other classic promoters. Furthermore, these compoundsare non-volatile, which is a major advantage for their use in theCO2-capture process. It was shown that our data are generally con-sistent with the measurements performed by other authors. It wasalso indicated that the TBAF additive has a much stronger effectthan its bromide homologous molecule. Further measurements arerequired to have a better understanding of the phase equilibriumof these systems.

We have also performed measurements for systems containingCO2/N2 loads and aqueous solutions of THF, another well knownthermodynamic promoter, using the DSC calorimetric technique.The obtained results show that there might be coexistence of sIand sII structures in phase equilibria of these systems. This phe-nomenon could explain the apparent structure transition observedby Kang et al. (2001). We also provided new experimental dissoci-ation data for the aforementioned systems. Our data are in rathergood agreement with the data of Kang et al. (2001). Future worksshould also involve modeling the phase equilibria of these systemsand finding out what phases are stable for the conditions relevantto the hydrate-based CO2-capture process. Compositional analysisof the existing phases (particularly the vapor/gas phase) is alsorecommended.

Acknowledgements

The authors wish to acknowledge Christian Prioux, IsabelleBrunella, Sophie Dutarte and Anne Sinquin for their support.

1 f Gree

Dm

R

A

AB

|

D

D

DD

DE

E

EFHKLL

Treiner, C., Bocquet, J.F., Chemla, M.J., 1973. Chim. Phys. Phys. Chim. Biol. 70, 72–79.

92 I.B.A. Sfaxi et al. / International Journal o

r Ali Eslamimanesh is acknowledged for his comments on theanuscript.

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