Ž .Fluid Phase Equilibria 162 1999 171–179

Bubble point pressure for binary mixtures of difluoromethane withpentafluoroethane and 1,1,1,2-tetrafluoroethane

T. Takagi a,) , T. Sakura a, T. Tsuji b, M. Hongo b

a Department of Chemistry, Faculty of Engineering and Design, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku,Kyoto 606-8585, Japan

b Department of Industrial Chemistry, College of Industrial Technology, Nihon UniÕersity, Narashino, Chiba 275-8575,Japan

Received 14 September 1998; accepted 22 March 1999

Abstract

Bubble point pressures for the two binary hydrofluorocarbon mixtures difluoromethane, CH F with2 2

pentafluoroethane CHF CF and 1,1,1,2-tetrafluoroethane CF CH F were measured using an acoustic absorp-2 3 3 2

tion technique. Results cover the temperature range from 243 K to 333 K with an uncertainty of "20 kPaŽ .except at 333.15 K. For the 1yx CH F qxCHF CF system, bubble point pressures show a maximum2 2 2 3

around xs0.2 at 248.15 K, corresponding to the azeotropic mixture. This point shifts to the CH F -rich region2 2

with increasing temperature, and finally vanishes out at temperatures higher than 313.15 K. Bubble pressures forŽ .the 1yx CH F qxCF CH F system decrease monotonously with increasing composition of CF CH F at2 2 3 2 3 2

each temperature. These experimental data were fairly well correlated with the Peng–Robinson equation of stateincluding the azeotropic point. q 1999 Elsevier Science B.V. All rights reserved.

Keywords: Vapor–liquid equilibria; Acoustic absorption technique; Mixture; Hydrofluorocarbons; HFC32; HFC125;HFC134a

1. Introduction

In previous works Takagi reported the ultrasonic speeds for the liquid phase of the purew x w x w x w xhydrofluorocarbons: CH F 1 , CHF CF 2 , CF CH F 3 , and CF CH 4 , and binary mixture of2 2 2 3 3 2 3 3

w xCF CH FqCHF CF 5 measured in wide temperature and pressure ranges. In these studies,3 2 2 3Ž .vapor–liquid equilibrium VLE data are essential to estimate the ultrasonic speed in the saturated

liquid.

) Corresponding author. Fax: q81-75-724-7525; E-mail: takagi@ipc.kit.ac.jp

0378-3812r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.Ž .PII: S0378-3812 99 00174-0

( )T. Takagi et al.rFluid Phase Equilibria 162 1999 171–179172

For hydrofluorocarbon mixtures, a lot of VLE data are available elsewhere. However, in manycases the temperature range in these studies is limited to room temperature or close and it is difficultto obtain VLE values at arbitrary conditions, especially within wide temperature ranges. In this work,

Ž . Ž .the bubble point pressures for the binary mixtures of 1yx CH F qxCHF CF and 1yx2 2 2 3

CH F qxCF CH F were measured using an apparatus suitable for ultrasonic speed measurements2 2 3 2

from 243.15 K to 333.15 K. Experimental results were compared literature data and were correlatedŽ .using the Peng–Robinson P–R equation of state.

2. Experimental

2.1. Materials

Pure samples of CH F and CF CH F were supplied from Daikin Industries and that of CHF CF2 2 3 2 2 3

was supplied from Asahi Glass Ind. The purities of these samples were better than 99.96 mole%. Thephysical properties for pure compounds are listed in Table 1. Mixtures were prepared by weighingwith an uncertainty of "0.0005 mole.

2.2. Bubble point pressures

Bubble point pressures for mixture fluids were measured using an ultrasonic speed high pressurew xapparatus, as shown in Fig. 1, similar to the one outlined in previous works 1,2 . The ultrasonic speed

was obtained by measuring the period between the first and second echoes of a short acoustic pulseŽ .traveling a known distance between the transducer and reflector sing-around technique . The

Ž .transducer PZT, 20 mm diameter, 2 MHz was fixed horizontally to the upper wall in the sampleŽ y4 y1 .chamber. Pressure was generated by a hand oil pump using a silicon oil 1=10 m s in viscosity

and transmitted through a piston moving in two cylinders on the interferometer and outside cylinder.The volume the sample enclosed in the cylinder where the free piston is moving was about 60 cm3,and the one in the upper space above the transducer was about 3 cm3 at most.

The acoustic wave excited in the sample for the speed of sound measurement was stronglyabsorbed in the gas phase when compared to absorption in the liquid phase. In the case of the

Table 1Thermophysical properties for pure compounds

Compounds CH F CHF CF CF CH F2 2 2 3 3 2

y1Ž .Molecular weight g mol 50.024 120.022 102.031Ž .Boiling temperature K 221.5 224.6 247.1

Critical constantsŽ .Temperature K 351.26 339.19 374.27

Ž .Pressure MPa 5.777 3.618 4.065y3Ž .Density kg m 424 424 568Ž .Acentric factor – 0.277 0.301 0.326

y30Ž .Dipole moment 10 C m 1.98 1.563 2.058

( )T. Takagi et al.rFluid Phase Equilibria 162 1999 171–179 173

Ž .Fig. 1. Pressure vessel acoustic interferometer used for bubble point pressure measurement.

instrument used here, the exciting electric power is limited to a minimum to observe adiabatically thespeed of sound in the liquid only and therefore the gas phase in the acoustic traveling-path causes thedisappearance of the echo signal from a monitor. When the level of the interface is kept at the lowerlevel of the transducer by monitoring the form of the pulse-echo, that is, only a limited gas phaseexists on the upper side of the sample chamber, the bubble point pressure being accurately measuredwith a precision strain gauge of maximum pressure 5 MPa through the strain of the Teflon capsuleŽ .0.2 mm thickness used as the sample-oil separator. This gauge was held in a constant temperaturebath by a bobbin heater, controlled within 308"2 K, and was calibrated by using a quartz crystal

Ž .pressure gauge Paroscientific, 31K-101, 730 within "5 kPa. The sample vessel was immersed inthe thermostat controlled to within "20 mK, the temperature being observed with a quartzthermometer calibrated against ITS-90 using a platinum resistance within "7 mK by the NationalInstitute of Metrology of Japan.

3. Results and discussion

Ž . Ž .The experimental results of bubble point pressures, p for 1yx CH F qxCHF CF and 1yx2 2 2 3

CH F qxCF CH F at several temperatures, T and mole fractions x are presented in Table 2. The2 2 3 2w xvapor pressures for the pure substance were taken from the data reported by Defibaugh et al. 7 for

w x w xCH F , Magee 9 for CHF CF and JAR Tables 10 for CF CH F, respectively. Among these2 2 2 3 3 2w xcompounds, Takagi measured in previous works the vapor pressure for CHF CF 3 and CF CH F2 3 3 2

w x2 using the same method presented here, and confirmed the reliability to within "10 kPa, as alsolisted in Table 2, when compared with the reference data.

( )T. Takagi et al.rFluid Phase Equilibria 162 1999 171–179174

Table 2Ž .Experimental bubble point pressure, p kPa at various composition, x and temperature, T

Ž .x T K

248.15 263.15 283.15 298.15 313.15 333.15

( )1y x CH F q xCHF CF2 2 2 3a Ž . Ž . Ž . Ž . Ž . Ž .0 – 334 581 582 1104 1107 1687 1689 2.478 2477 – 3932

0.0593 345 591 1118 1695 2479 39340.1186 342 589 1113 1694 2477 39210.1683 344 590 1111 1685 2465 39040.3238 342 588 1095 1660 2418 38350.4993 342 577 1073 1619 2341 36780.7307 310 531 994 1504 2167 3429

b Ž . Ž . Ž . Ž . Ž . Ž .1 284 277 484 482 908 907 1377 1374 2014 2005 3182 3167

( )1y x CH F q xCF CH F2 2 3 2a d dŽ . Ž . Ž . Ž . Ž . Ž .0 273 273 581 582 1104 1107 1687 1689 2.478 2477 – 3932

d0.1922 238 520 973 1452 2111 3341d0.4313 207 430 810 1225 1798 2750d0.6107 132 334 674 1071 1503 2385d0.8161 116 255 499 818 1219 1977

c d1 84 201 415 666 1017 1681

a w xDefibaugh et al. 7 .b w x w xTakagi 3 ; data in parentheses, Magee 9 .c w x w xTakagi 2 ; data in parentheses, JAR Tables 10 .dAt 243.15 K.

Ž .For 1yx CH F qxCHF CF , the vapor pressure differences between the pure substances are2 2 2 3

small, especially the ones at low temperature, while the bubble point pressures show an interestingbehavior with composition change. That is, those at low temperature increase at first with increasingx, and then decrease indicating a convex curve through a small peak around xs0.2, which is theazeotrope, as shown in Fig. 2. For this binary system, the VLE data have been reported by Fujiwara et

w x w x w xal. 11 at 273.15 K, Nagel and Bier 6 for 205.861–342.381 K, and Higashi 12,13 for 283.05–313.06w xK. Widiatmo et al. 8 reported the bubble point pressures from 279.987 to 309.985 K. Their values

also indicate a small maximum point on the composition curve. According to reports of Widiatmo etw x w xal. 8 and Higashi 12,13 , they have discussed that the azeotropic composition appeared near xs0.1

and was almost temperature independent in the ranges from 280 to 313 K. While the results of Nagelw xand Bier 6 clearly show the azeotrope in the vicinity of xs0.1 for temperatures lower than 243 K,

and at 303 K the peak point against the composition having not been observed.For this mixture, the specification of azeotropic composition is difficult because the peak against x

is small and the data points are scarce for the CH F -rich region. In this work, to investigate in detail2 2

the state of the azeotrope, the measurement was carried out in a narrow x interval around xs0.1.Fig. 2b is an enlargements graph at 283.15 K, which is available the many data in the references. Ascan be seen in Fig. 2a and b, the maximum point at xs0.22 observed at 248.15 K shifts clearly tothe low x region of CHF CF with increasing temperature, and it diffuses at 313.15 K and above.2 3

From these facts, it seems reasonable to assume that for this system the azeotropic point shifts to the

( )T. Takagi et al.rFluid Phase Equilibria 162 1999 171–179 175

Ž . Ž .Fig. 2. Bubble point pressure, p with mole fraction, x for 1y x CH F q xCHF CF at several temperatures a , at2 2 2 3Ž . Ž . <283.15 K b and deviation, d from P–R equation c . `, D, =, , , Bubble point; ', %, ——, Dew point; `,

Ž . w x w x w xThis work; =, Fujiwara et al. 273.15 K 11 ; D, ', Higashi 12,13 ; =, %, Nagel and Bier 6 ; ®, Widiatmo et al.Ž Ž . Ž .. w x309.986 K a , 279.987 K b 8 ; full and dotted lines, P–R equation.

CH F -rich region and disappears finally with increasing temperature as obtained experimentally2 2

here.Ž .On the other hand, for the system 1yx CH F qxCF CH F, which have the large difference in2 2 3 2

the vapor pressures between both pure substances, the bubble point pressure at low temperatures isnearly a straight line against composition, x, that is, Raoult’s law is followed, while at highertemperatures the curvature becomes more apparent. For this mixtures, VLE data have been reported

w x w x w xby Fujiwara et al. 11 measured at 273.15 K, Higashi 14 for 283.15–313.15 K, Nagel and Bier 6w x w xfor 203.016–356.970 K and Chung and Kim 15 for 263.15–323.15 K. Also, Widiatmo et al. 16

observed the bubble point pressures at temperatures from 280 to 310 K. Their data are also plotted inFig. 3a, and an enlargements plot in Fig. 3b with the present results.

ŽThe experimental results thus obtained were correlated by the P–R equation of state Peng andw x.Robinson 17 as,

RT aPs y 1Ž .2 2Õyb Õ q2bÕyb

where p is the pressure and R, T and Õ are the perfect gas constant, the temperature and the molarvolume, respectively. The coefficients a and b were derived from the critical temperature, T ,c

w xpressure, p and the acentric factor, v, with a rule given in Ref. 17 . Further, the values for binaryc

mixture, a and b were estimated by using the van der Waals one-fluid mixing rule;m m

1r2a s x x 1yk a a , b s x b . 2Ž . Ž .Ž .Ý Ý Ým i j i j i j m j j

i j j

( )T. Takagi et al.rFluid Phase Equilibria 162 1999 171–179176

Ž . Ž . Ž .Fig. 3. Bubble point pressure, p for 1y x CH F q xCF CH F at several temperatures a , at 283.15 K b and deviation,2 2 3 2Ž . <d from P–R equation c . `, D, =, I, , , Bubble point; ', %, B, ——, Dew point; `, This work; =,

Ž . w x w x w x w xFujiwara et al. 273.15 K 11 ; D, ', Higashi 12,13 ; =, %, Nagel and Bier 6 ; I, B, Chung and Kim 15 ; ®,Ž Ž . Ž ..Widiatmo et al. 319.985 K a , 279.988 K b ; full and dotted lines, P–R equation.

where x is the mole fraction, and subscripts i, j are the values for both pure substances i and j. Thephysical properties and acentric factor, v for pure fluids required in these calculations are listed inTable 1. The binary interaction parameter, k , was estimated to minimize the deviations of relativei j

bubble point pressures between experimental and calculated values. Fig. 4 shows that k decreasesi jw xwith increasing temperature similarly to the curvatures reported by Higashi 12,13 and Chung and

w xKim 15 .

Ž . Ž .Fig. 4. Binary interaction parameter, k with temperature, T. `, v, 1y x CH F q xCHF CF ; D, ', 1y xi j 2 2 2 3w x w xCH F q xCF CH F; `, D, This work; v, Higashi 12,13 ; ', Chung and Kim 15 .2 2 3 2

( )T. Takagi et al.rFluid Phase Equilibria 162 1999 171–179 177

Ž .For the system 1yx CH F qxCHF CF having the azeotropic point, when the P–R equation is2 2 2 3

calculated in the whole composition range with k , there is a systematic error in the deviationi j

between the experimental and calculated values. That is, it is difficult to reproduce faithfully theŽ .azeotrope behavior with x. Similar errors can be found in the deviation plots, Fig. 4 proposed by

w xNagel and Bier 6 . In present study, the azeotropic point was obtained graphically based on theexperimental values of p–x at each temperature, the equation of state being calculated separately in

Žboth concentration sides from that point using the same k . Bubble point pressures solid lines in Fig.i j.2a and b thus estimated are conveniently reproduced within experimental values, and the absolute

average deviation of relative pressure with x from the equation are to be within "2.2% at most atŽ .283.15 K, as shown in Fig. 2c. Also, the dew point pressures dotted line in Fig. 2b are in good

w xagreement with the available data. At 248.15 K, the deviations from Nagel and Bier 6 are somewhatlarger on the whole than the present data, as illustrated against temperature in Fig. 5a, but thatmaximum deviation of "3.6% corresponding to only 11 kPa may be due to the error in the pressuremeasurement.

Ž .As shown in Fig. 3a and b, the values of bubble and dew point pressures for 1yx CH F q2 2Ž . Ž .xCF CH F estimated from Eqs. 1 and 2 are reproduced well when compared with our experimen-3 2

tal data, and the relative deviations of our results from the equation of state are within "3.0% atŽ .283.15 K except the data at xs0.8161 Fig. 3c . In the case of this mixture, with increasing

temperature, the differences of absolute percent values becomes larger, as drawn in Fig. 5b, that is,our data seems to be lower than the available data at higher temperatures.

In the present study, the bubble point pressure was obtained in conditions where limited availablegas phase was left in the upper side of the sample chamber. This little available gas is almostindependent of the result at low temperatures, but with increasing temperature that gives rise to theirregularity of the measurements, especially in the case of systems having a large difference in vaporpressures between the pure substances, such as CH F qCF CH F. Another problem of this method2 2 3 2

is that the acoustic signal excited in the sample suffers absorption near the saturation line at higher

Ž . Ž .Fig. 5. Relative deviation, d of our results from P–R equation with temperature, T for 1y x CH F q x CHF CF a and2 2 2 3Ž . Ž . w x w x w x1y x CH F q x CF CH F b . `, This work; D, Higashi 12,13 ; =, Nagel and Bier 6 ; I, Chung and Kim 15 .2 2 3 2

( )T. Takagi et al.rFluid Phase Equilibria 162 1999 171–179178

temperatures, especially near the critical, where it approaches a maximum. This absorption alsoencourages the difficulty of the measurements although its degree differs according to molecularcharacteristics in the components of the system. Consequently, at higher temperatures, the differencein the present values from the available data, especially those for CH F qxCF CH F, may be2 2 3 2

inferred based on the remaining gas phase andror absorption of acoustic wave described above.

4. Conclusion

Ž . Ž .Bubble point pressures for binary mixtures of 1yx CH F qxCHF CF and 1yx CH F q2 2 2 3 2 2

xCF CH F has been measured at several mole fractions, x by an acoustic absorption technique3 2

within wide temperature ranges. For the former mixture, the measurements in the CH F -rich region,2 2

where the azeotrope appears, were carried out in detail for narrow composition intervals. In this caseit is apparent that the azeotrope composition, observed near xs0.22 at 243.15 K, shift to lowerCHF CF mole fractions with increasing temperature, and it disappears above 313.15 K. The results2 3

Ž .for 1yx CH F qxCF CH F system decrease monotonously indicating a concave curve with2 2 3 2

increasing x. The data thus obtained are correlated with the P–R equation of state and found to bewithin "2.5% including the azeotropic point for the former system and "4.3% for the other one,respectively. The probable uncertainties in the present results rise somewhat at high temperatureregion compared with the available data reported elsewhere. These errors may be caused on theremaining limited gas phase in the sample andror absorption of an acoustic wave. While it is foundthat the acoustic absorption technique used is useful to measurement the bubble point pressure ofmixture, especially to obtain those in the wide temperature range.

Acknowledgements

We are grateful to Daikin Industrials, and Asahi Glass, for furnishing the samples. Also, this workŽ .was supported jointly by the Grant-in-Aid for Scientific Research Fund 1997, 1998, No. 09555230

of the Ministry of Education, Science and Culture of Japan.

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