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Available online at www.sciencedirect.com

J. of Supercritical Fluids 46 (2008) 238–244

High-pressure phase equilibria for the binary system carbondioxide + dibenzofuran

Eduardo Perez, Albertina Cabanas, Yolanda Sanchez-Vicente,Juan A.R. Renuncio, Concepcion Pando ∗

Departamento de Quımica Fısica I, Facultad CC. Quımicas, Universidad Complutense,E-28040 Madrid, Spain

Received 4 October 2007; received in revised form 18 January 2008; accepted 18 January 2008

bstract

The experimental solubility of dibenzofuran in near-critical and supercritical carbon dioxide and the solid–liquid–vapor (SLV) equilibriumine for the CO2 + dibenzofuran system are reported. The built in-house static view cell apparatus used in these measurements is described.he solubility of naphthalene in supercritical CO2 and the CO2 + naphthalene SLV line are also determined in order to assess the reliabilitynd accuracy of the measurement technique. The solubility of dibenzofuran in carbon dioxide is determined at 301.3, 309.0, 319.2, 328.7 and

38.2 K in the 6–30 MPa pressure range. Solubility data are correlated using the Chrastil model and the Peng–Robinson equation of state. Thisquation is also used to predict the CO2 + dibenzofuran SLV line. Results show the feasibility of using supercritical CO2 to extract dibenzofu-an.

2008 Elsevier B.V. All rights reserved.

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uki

camoC[hgsctp

eywords: Supercritical carbon dioxide; Dibenzofuran; Solubility; Solid–liqui

. Introduction

Supercritical carbon dioxide (Tc = 304.2 K, Pc = 7.37 MPa1]) and supercritical or near-critical water (Tc = 647.1 K,c = 22.06 MPa [1]) are increasingly used as solvents or reactionedia in a variety of supercritical processes. Supercritical water

xidation is the method preferred for destroying hazardous wastehile either CO2 or H2O are used in supercritical fluid extrac-

ion (SFE) to separate organic pollutants from solid matrixes [2].ue to its accessible critical parameters, non-toxicity, low cost

nd non-flammability, carbon dioxide is very often used bothor analytical and clean up purposes. Many examples are foundf extractive treatment of solids using supercritical CO2. Fornstance, polycyclic aromatic hydrocarbons and polychlorinatediphenyls and dioxins were removed from soils [3–9]. Recently,abarra et al. [10] studied the feasibility of using CO2 SFE to

emove polychlorodibenzodioxins and dibenzofurans from flysh in a solid-waste incineration facility and Kawashima et al.11] studied the removal of similar compounds from fish oils

∗ Corresponding author. Tel.: +34 91394 4304; fax: +34 91394 4135.E-mail address: pando@quim.ucm.es (C. Pando).

(saisfl

896-8446/$ – see front matter © 2008 Elsevier B.V. All rights reserved.oi:10.1016/j.supflu.2008.01.009

or equilibrium; High-pressure phase equilibria

sing CO2 SFE. In order to design and optimize these processes,nowledge of the high-pressure phase equilibria of the mixturesnvolved is required.

Dibenzofuran (DB) is a flat organic molecule; it is a pre-ursor of chlorinated pollutants (polychlorodibenzofurans) andsolid at ambient conditions. The chemical structure, molarass, melting and boiling points and dipole moment of DB and

ther solutes considered in this paper are shown in Table 1.ritical parameters for DB were reported by Chirico et al.

14]: Tc = 824 K, Pc = 3.64 MPa, the critical temperature is muchigher than that of carbon dioxide. In this study, we investi-ate the behavior of the CO2 + dibenzofuran system that mayerve as a model for systems formed by carbon dioxide and thehlorinated dibenzofurans. The thermodynamic study includeswo types of measurements: (a) solubilities of DB in com-ressed CO2 at 301.3, 309.0, 319.2, 328.7 and 338.2 K andb) the decreasing of the melting point under CO2 pressure,olid–liquid–vapor (SLV) equilibrium line. A static view cell

pparatus was used in these measurements. Solubilities of DBn carbon dioxide are also compared to those previously mea-ured by Hansen at 308.2, 323.2 and 343.2 K using a dynamicow apparatus [15].

E. Perez et al. / J. of Supercritical Fluids 46 (2008) 238–244 239

Table 1Physical properties of solutes

Solute Structure MW Tm (K)a Tb (K)a μ (Debye)b

Naphthalene 128.2 353 490 0

Dibenzofuran 168.2 355 558 0.88

Xanthene 182.2 374 584 1.14

2

p(w

aadisTowamnctotpticT

Table 2Solid–liquid–vapor equilibrium for the CO2 + dibenzofuran system

T (K) P (MPa) T (K) P (MPa) T (K) P (MPa)

330.9 17.00 332.6 13.50 335.1 11.00331.2 18.00 332.7 18.90 335.8 9.50331.3 16.00 332.9 13.50 337.0 9.5033

rpthntSm±u±

AadcsatTits0wtain some cases a slightly different melting temperature was

a Tm and Tb taken from Ref. [12].b μ at 25 ◦C taken from Ref. [13].

. Experimental

The materials employed were CO2 (Air Liquide 99.98 mol%ure), dibenzofuran (Fluka ≥ 99 mol% pure) and naphthaleneFluka ≥ 99.0 mol% pure). Commercial materials were usedithout further purification.Fig. 1 is the schematic diagram of the static view cell

pparatus used to perform phase equilibria measurements. Thepparatus was designed and built in-house and is similar to thatescribed by McHugh and Krukonis [16]. The main components a high-pressure, variable-volume cell constructed of stainlessteel, working volume ≈15 cm3, fitted with a sapphire window.he image of the mixture in the cell was obtained by meansf a Fiegert Endotech boroscope placed against the sapphireindow, fitted with a Moticam 2000 camera and connected tocomputer. The cell was electrically heated up to 373 K byeans of a silicone heating tape (Omegalux SRT051-040) con-

ected to a Proportional Integral Derivative (PID) temperatureontroller (Micromega, model CN77322). A type J calibratedhermocouple was used to measure the temperature with a res-lution of ±0.05 K. The stability of the thermostat is estimatedo be ±0.2 K. A known amount of solute was introduced andurged several times at room temperature with carbon dioxideo remove the entrapped air. Carbon dioxide was transferred

nto the view cell gravimetrically by means of an auxiliaryell. The mole fraction precision was estimated to be ±0.1%.he contents of the cell were mixed using a magnetic stir-

o0a

Fig. 1. Schematic diagram of the static view cell apparat

31.6 15.00 333.8 12.50 339.0 8.0031.7 15.00 334.8 11.50 340.5 6.30

er. The solution can be compressed to the desired operatingressure (up to 30 MPa) by displacing a movable piston fit-ed within the cell using water pressurized by means of aigh-pressure generator. This high-pressure generator is con-ected to a water reservoir through valve V1 and to the cellhrough valve V2. The water pressure was determined using awagelok pressure gauge. The pressure inside the cell was deter-ined using a relative transducer (Druck, model PTX7511-1,0.15% uncertainty) provided with a digital display with an

ncertainty of 0.01 MPa. The pressure error was estimated to be(0.01 + 0.0015 P) MPa.A solubility data point was obtained in the following manner:

t a fixed temperature the mixture in the cell was compressed tosingle phase at high pressures. The pressure was then slowlyecreased until a condensed phase appears. The contents of theell were periodically agitated using the magnetic stirrer. Theolute can be alternatively solubilized and precipitated to obtainprecise pressure value. The solubility was determined from

he amounts of carbon dioxide and solute loaded into the cell.he SLV line was determined applying the so-called “first melt-

ng point” as follows. An excess of solid DB was placed intohe cell that was then filled with CO2 up to a certain pres-ure. The temperature was then isobarically increased about.05 K/min with periodic stirring until the solid in equilibriumith the CO2-rich gas phase started to melt. At this temperature

he three phases SLV coexist. The measurement was repeatedfter cooling down the cell. As may be seen in Figs. 3 and 4,

bserved for CO2 + naphthalene or CO2 + DB. Differences of.1 and 0.3 K are observed in Table 2 for CO2 + DB at 15.00nd 13.50 MPa, respectively. The greater difference observed at

us used to perform phase equilibria measurements.

240 E. Perez et al. / J. of Supercritical Fluids 46 (2008) 238–244

F 2 at:M ) Rev

9s

3

3

nsatottuufpKpo

Ft[

catJ

3

slvlmTiishould begin at the DB triple point (T = 355.31 K [14]) andend at the upper critical end point (UCEP) where it would

ig. 2. Solubility isotherms of naphthalene in near-critical and supercritical COcHugh and Paulaitis [18]; (�) Mitra et al. [19]; (�) Chung and Shing [20]; (×

.50 MPa (1.2 K) is considered the limit error of present mea-urements.

. Results and discussion

.1. Verification of the method

To test the reliability and accuracy of our measurement tech-ique, naphthalene was used as a calibration standard. Fig. 2hows a comparison of the solubility data of naphthalene in CO2t 308.2 and 328.2 K reported in this paper to those reported inhe literature [17–22]. The agreement is very good for mostf the pressure range. At high pressures, results obtained inhis paper are slightly lower than those reported in the litera-ure. These differences may be due to the different techniquessed; data presented in this paper are the only ones measuredsing a synthetic method. Fig. 3 shows a similar comparisonor the CO2 + naphthalene SLV line [23–26]. CO2 + naphthalene

resents a type III diagram in the classification of Scott and vanonynenburg [27]; the liquid–gas line intersects with a solidhase. Considerable differences occur between the literature setsf results for this system. Results presented in this paper are very

ig. 3. Solid–liquid–vapor equilibrium for the CO2 + naphthalene system: (�)his work; (�) McHugh [23]; (♦) Cheong et al. [24]; (©) Lemert and Johnston25]; (�) White and Lira [26]; ( ) upper critical end point [25].

ic

Fcsl

(a) 308.2 K and (b) 328.2 K; (�) this work; (�) Tsekhanskaya et al. [17]; (♦)erchon et al. [21]; (©) Sauceau et al. [22].

lose to those of Cheong et al. [24] (first freezing point method)nd White and Lira [26] (first melting point method), and lie inhe middle of those reported by McHugh [23], and Lemert andohnston [25].

.2. CO2 + dibenzofuran solid–liquid–vapor equilibrium

Temperatures and pressures for CO2 + dibenzofuranolid–liquid–vapor equilibrium are listed in Table 2. The SLVine is shown in Fig. 4 together with the critical point andapor pressure curve for carbon dioxide [1,12] and dashedines indicating the conditions of the solubility isotherms. A

inimum of the melting temperature is observed at 17 MPa.he shape of the SLV line suggests a liquid–gas type III diagram

n the classification of Scott and van Konynenburg [27], whichntersects with a solid phase. In this case, the three-phase line

ntersect the critical line. At the UCEP, a vapor–liquid mixtureritical point occurs in the presence of a solid phase. This

ig. 4. Solid–liquid–vapor equilibrium for the CO2 + dibenzofuran system, CO2

ritical point (©) [1] and vapor pressure curve (- - -) [12] and conditions of theolubility isotherms reported in this paper (- - -): (�) P, T coordinates of the SLVine, this work; (—) predicted using the Peng–Robinson EOS.

E. Perez et al. / J. of Supercritical Fluids 46 (2008) 238–244 241

Table 3Mole fraction solubility of dibenzofuran in near-critical and supercritical CO2

P (MPa) 103 y2 P (MPa) 103 y2 P (MPa) 103 y2

301.3 K6.74 3.00 13.30 4.65 17.39 5.379.27 3.56 15.00 5.14 20.10 6.00

10.48 3.97

309.0 K8.96 3.00 15.55 5.78 20.95 7.569.40 3.56 14.42 5.80 22.50 7.95

10.65 4.06 16.92 6.35 21.46 8.0412.11 4.65 18.41 6.89 27.48 8.6513.35 5.14

319.2 K10.66 3.00 13.40 6.00 16.55 8.0411.19 3.56 13.97 6.34 17.72 8.6511.37 3.97 13.65 6.43 19.36 9.4412.35 4.65 14.27 6.57 20.70 10.3912.50 4.83 14.84 6.85 22.40 11.1713.16 5.80 15.69 7.42

328.7 K12.80 3.00 15.22 6.33 16.24 8.6513.16 3.56 14.99 6.35 16.75 9.0713.48 4.06 14.96 6.43 17.27 9.9913.59 4.59 15.06 6.57 17.53 10.0413.75 4.65 14.99 6.74 17.70 10.0814.63 5.32 15.20 6.85 17.83 10.3914.60 5.80 15.63 7.42 18.47 11.0714.83 6.00 15.80 8.04 18.51 11.17

338.2 K13.50 3.44 16.48 7.39 18.90 11.1014.10 4.04 16.77 7.83 18.92 10.7014.94 5.03 17.24 7.66 19.43 11.5211

SCltpe

3

b3robtit

iogt

FCc

mppsoiaptuauxotsaf

taaiAttfrtbt

3p

5.49 5.76 17.56 8.60 19.97 12.205.97 6.58 18.37 9.79

LV line is similar to that reported in the literature for theO2 + naphthalene system. For the CO2 + DB system, the SLV

ine appears at temperatures higher than those measured forhe former system. At temperatures close to a critical endoint, there is a solid solubility enhancement that is currentlyxploited in SFE experiments.

.3. Dibenzofuran solubility

The mole fraction solubility (y2) of dibenzofuran in car-on dioxide was determined at 301.3, 309.0, 319.2, 328.7 and38.2 K in the 6.5–30 MPa pressure range. Results are summa-ized in Table 3. Fig. 5 shows plots of the five solubility isothermsbtained. An inspection of Fig. 4 and Table 3 indicates that car-on dioxide is a liquid for data taken at 301.3 K while for dataaken at 309.0, 319.2 and 328.7 K carbon dioxide is a supercrit-cal fluid. On the other hand, data taken at 338.2 K correspondo liquid–vapor equilibrium.

At the studied conditions, dibenzofuran mole fraction solubil-

ty ranges from 10−3 to 10−2. These values show the feasibilityf using supercritical CO2 fluid to extract dibenzofuran. For aiven temperature, the solubility increases with pressure due tohe higher density of the solvent. The effect of temperature is

ca

ig. 5. Solubility isotherms of dibenzofuran in near-critical and supercriticalO2 at: (�) 301.3 K; (�) 309.0 K; (�) 319.2 K; (�) 328.7 K; (�) 338.2 K; (—),orrelated using a second degree polynomial.

ore complex and a crossover is observed in the solubility versusressure curves at approximately 15 MPa for the different tem-eratures studied. At pressures above the crossover pressure, theolubility increases with temperature, while the opposite trend isbserved at pressures below the crossover pressure. This behav-or results from a compromise between the decreasing of densitys the temperature rises (that leads to lower solubilities at lowerressures) and the higher vapor pressure of the solute (that leadso higher solubilities at higher pressures). On the other hand, sol-bilities of dibenzofuran in carbon dioxide at 309.0 and 328.7 Kre lower than those of naphthalene at 308.2 and 328.2 K. Sol-bilities of dibenzofuran at 309.0 K are very close to those ofanthene at 308.2 K [28]. This could be expected: an inspectionf Table 1 reveals that the three compounds have aromatic ringshat lead to low solubilities but dibenzofuran and xanthene haveimilar molar masses much higher than those of naphthalenend similar chemical structures with polar groups that cause aurther decrease of the solubility in carbon dioxide.

The solubility isotherms reported in this paper are comparedo those obtained by Hansen at 308.2, 323.2 and 353.2 K usingdynamic flow apparatus [15] in Fig. 6. Except for data taken

t 308.2–309.2 K, the agreement is satisfactory. The 323.2 Ksotherm is intermediate between those at 319.2 and 328.7 K.

15◦ temperature increase explains the differences betweenhe liquid–vapor equilibrium data taken at 338.2 and 353.2 K;he shape of the two isotherms is similar and clearly differentrom those of solid–vapor equilibrium data. The solubility dataeported by Hansen at 308.2 K are lower than data reported inhis paper at 309.2 K both at high and low pressure and will note considered in the simultaneous data correlation described inhe next section.

.4. Correlation of experimental solubility data andrediction of the SLV line

Solubility data of solid dibenzofuran in liquid and supercriti-al CO2 obtained in this paper (301.3, 309.0, 319.2 and 328.7 K)nd those reported by Hansen at 323.2 K were correlated using

242 E. Perez et al. / J. of Supercritical Fluids 46 (2008) 238–244

F orted( �) 33

tt

l

wwuace

A

wtiFtet

Fo(

sd

he

y

wcVl

P

A

ig. 6. Comparison of solubility isotherms of dibenzofuran in carbon dioxide repa): (�) 308.2 K; (�) 309.2 K. (b): (�) 319.2 K; (©) 323.2 K; (�) 328.7 K. (c) (

he Chrastil model [29] that relates the solute solubility, y2, andhe solvent density, ρ:

ny2 = a0 + a1 lnρ + a2

T(1)

here a0, a1 and a2 are adjustable parameters. Solvent densitiesere obtained from NIST [12]. A least square-procedure wassed to determine values for a0, a1 and a2 by minimizing theverage absolute relative deviation between experimental andalculated solubilities (AARD) and giving the same weight toach isotherm. AARD is defined as:

ARD = 1

4

∑T

1

NT

NT∑i=1

|ycal − yexp|yexp

(2)

here NT is the number of experimental points for a givenemperature, ycal is the calculated solubility, and yexp is the exper-mental solubility. Results from this correlation are shown in

ig. 7. Chrastil model is shown to describe well the tempera-

ure effect on solubilities; three parameters are used to fit all thexperimental data. At high pressures, the solubility is observedo increase monotonically and linearly with pure solvent den-

ig. 7. Solubility of dibenzofuran in carbon dioxide as a function of the densityf pure CO2: (�) 301.3 K; (�) 309.0 K; (�) 319.2 K; (�) 328.7 K, this paper;©) 323.2 K Hansen [15]; simultaneous correlation using Chrastil model [29].

m

φ

wbPe

wb

acfb(c

in this paper (full symbols) and those reported by Hansen [15] (empty symbols).8.2 K; (�) 353.2 K.

ity. At the low pressures, solubilities deviate from the linearependence.

The solubility of a solid solute in equilibrium with a fluid atigh pressure at a given temperature can be calculated using thequation:

2 = P sat2

P

1

φG2

exp

[vs

2(P − P sat2 )

RT

](3)

here P sat2 is the saturated vapor pressure, φG

2 is the fugacityoefficient and vs

2 is the solid-state molar volume of the solute.alues used for the vapor pressure were obtained from the fol-

owing equation [30]:

sat2 = 302.724

R− 92761.912

RT− 35.96

Rln

(T

298.15

)(4)

value of 1.30 × 10−4 m3 mol−1 [31] was used for the solidolar volume. The fugacity coefficient is given by

G2 = 1

RT+

∫ V

∞

[(∂P

∂ni

)T,V,ni=j

− RT

V

]dV − ln z (5)

here z is the compressibility factor. This coefficient maye calculated using an equation of state (EOS) such as theeng–Robinson EOS [32]. The mixture constants a and b werevaluated using the classical mixing rule:

a =∑

i

∑j

xixjaij; aij = (aiiajj)1/2(1 − kij)

b =∑

i

∑j

xixjbij; bij = (bii + bjj)

2(1 − δij)

(6)

here kij = kji and δij = δji denote binary interaction parametersetween unlike molecules i and j.

The pure component constants aii and bii for carbon dioxidend dibenzofuran were evaluated using the values of the criticalonstants given in the introduction and values of 0.225 and 0.397

or the CO2 and DB acentric factors, respectively [1,14]. Solu-ility data of solid dibenzofuran in liquid and supercritical CO2301.3, 309.0, 319.2, 323.2 and 328.7 K) were simultaneouslyorrelated using one binary parameter (k12) or two binary param-

E. Perez et al. / J. of Supercritical

Table 4Parameters and standard deviations, σ, for CO2 + dibenzofuran solubility corre-lations using the Chrastil model and the Peng–Robinson EOS with one or twobinary interaction parameters

Chrastil model PR EOS

a0 −14.637 k12 −0.08441 k12 0.01937aaσ

esedriPwa

σ

w

mnmoSnilAe

4

itsscuiCtsw

3bCoCP

ttcOpo

A

UPSttfi

R

[

[

[[

[

[

1 3.6805 − − δ12 −0.1486

2/K −4675 − − − −5.8 × 10−4 σ 15 × 10−4 σ 10 × 10−4

ters (k12 and δ12). The Peng–Robinson EOS correlation is notubstantially improved if binary interaction parameters are fit toach set of isothermal data. A least square-procedure was used toetermine parameter values by minimizing the average absoluteelative deviation between experimental and calculated solubil-ties (AARD) defined by Eq. (2). Results for the Chrastil andeng–Robinson EOS correlations are shown in Table 4 alongith values for the standard deviations between experimental

nd calculated solubilities, σ.

=[

1

(N − 1)

N∑i=1

(ycal − yexp)2

]1/2

(7)

here N is the total number of experimental points.A prediction of the CO2 + dibenzofuran SLV line was also

ade following the procedure described by McHugh and Kukro-is [16]. To this end, the Peng–Robinson EOS and the classicalixing rule with values for the binary parameters k12 and δ12

btained from the solubility data correlation were used. TheLV line calculated using two binary interaction parameters isot accurate: predicted melting temperatures rise as pressurencreases. The predicted SLV line shown in Fig. 4 was calcu-ated using one binary interaction parameter (k12 = −0.08441).

value of 1.9 K is obtained for the standard deviation betweenxperimental and predicted melting temperatures.

. Conclusions

The solubility behavior of dibenzofuran in pure CO2 wasnvestigated at 301.3, 309.0, 319.2, 328.7 and 338.2 K inhe 6.5–30 MPa pressure range. Data taken at 338.2 K corre-pond to vapor–liquid equilibrium. At the other temperaturestudied, the solubility of solid dibenzofuran in liquid or super-ritical CO2 is reported. These data show the feasibility ofsing supercritical CO2 to extract dibenzofuran from contam-nated soils. The solid–liquid–vapor (SLV) equilibrium for theO2 + dibenzofuran system was also measured. The shape of

he SLV line suggests a liquid–gas type III diagram in the clas-ification of Scott and van Konynenburg [27] which intersectsith a solid phase.The solubility data obtained in this paper at 301.3, 309.0,

19.2 and 328.7 K and those previously reported at 323.12 Ky Hansen [15] were simultaneously correlated using the

hrastil density-based model and the Peng–Robinson equationf state. In spite of its empirical character and simplicity, thehrastil model provided a correlation of better accuracy than theeng–Robinson correlation. Nevertheless, it must be noted that

[

[

Fluids 46 (2008) 238–244 243

he Chrastil model requires three adjustable parameters whilehe Peng–Robinson equation used in conjunction with classi-al mixing rule requires only one or two adjustable parameters.n the other hand, the Peng–Robinson equation may be used toredict the SLV line using a value for the binary parameter k12btained from the solubility data correlation.

cknowledgements

We gratefully acknowledge the financial support of theniversidad Complutense de Madrid (UCM), research projectR1/06-14425-A and the Spanish Ministry of Education andcience (MEC), research project CTQ2006-07172/PPQ. A.C.

hanks MEC for its support through a “Ramon y Cajal” con-ract. Y.S.V. and E.P. thank MEC and UCM, respectively, fornancial support through FPI predoctoral grants.

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2 ritical

[

[

[

[

[

[

[

[

[

[

[

[

[Phys. Chem. Chem. Phys. 5 (2003) 710–712.

44 E. Perez et al. / J. of Superc

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