PVT properties, vapor pressures and critical parameters of HFC-32

RIHDPHAS[ EOUlUDRIA ELSEVIER

Fluid Phase Equilibria 111 (1995) 273-286

PVT properties, vapor pressures and critical parameters of HFC-32 Y i - D o n g Fu, L i - Z h o n g Han, M i n g - S h a n Zhu Dept. of Thermal Engineering, Tsinghua University, Beijing 100084, China Received 20 December 1994; accepted 9 April 1995

Abstract

One hundred twenty three PVT data points for HFC-32 in the gaseous phase have been measured using Burnett method along fourteen isotherms for temperatures from 243 to 373 K, pressures from 0.07 to 5.7 MPa and densities from 1.8 to 240 kg m -3. The present experimental PVT data, compared with the EOS developed by Piao et al., has an RMS deviation of 0.17%. Sixty vapor pressure data points for HFC-32 have also been measured for the temperature range from 233 to 351 K. The RMS deviation of the pressures in the present data from the vapor pressure equation developed by Piao et al. is 0.063%. Based on the present data and selected data from other investigators, a new vapor pressure equation for HFC-32 has been developed. By means of visual observation of the disappearance of the meniscus in an optical cell, the critical temperature, density and pressure for HFC-32 have been determined to be 351.295 __+0.010 K, 425 + 3 kg m 3 and 5.785 + 0.002 MPa, respectively. The purity of the HFC-32 was 99.95 wt.%. Keywords: PVT property; Vapor pressure; Critical parameter; HFC-32

1. I n t r o d u c t i o n

On Nov. 25, 1992, the Copenhagen revisions to the Montreal Protocol states that HCFCs will be phased out by 2030. As a traditional and effective refrigerant, HCFC-22 is widely used in numerous applications, such as heat pumps, airconditioners and refrigerating machines. Alternatives to HCFC-22 must be developed that are harmless to the environment and allow high capacity, high efficiency applications. HFC-32 (Difluoromethane, CH2F 2) whose ozone depletion potential (ODP) is zero, is considered a promising alternative, especially as a component in mixtures, to replace HCFC-22. Therefore, the thermophysical properties of HFC-32 are of great interest. PVT properties, vapor pressures and the critical parameters are among the most fundamental thermophysical properties. Some information on the properties of HFC-32 has been published previously. Malbrunot et al. (1968) measured the vapor pressure between 191 and 351 K, the vapor and liquid phase PVT properties and the critical parameters. Kanungo et al. (1987) published a vapor pressure curve for temperatures between 149 K 0378-3812/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved

SSDI 0 3 7 8 - 3 8 1 2 ( 9 5 ) 0 2 7 7 6 - 9

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Y. -D. Fu et al. / Fluid Phase Equilibria 111 (1995) 2 73-286

and 245 K. Sato et al. (1992) measured the vapor and liquid PVT data as well as the vapor pressure. Widiatmo et al. (1992) published the vapor pressure and saturated liquid density. In 1993, several publications provided measurements of the thermodynamic properties of HFC-32. Zhu et al. (1993) measured 32 vapor pressure data points from 273.39 to 347.29 K. Kuwabara et al. (1993) determined the critical temperature and density by visual observation of the disappearance of the meniscus at the vapor-liquid interface. Qian et al. (1993) published the compressibility factors in the gaseous phase as well as the vapor pressure. Holcomb et al. (1993) published experiment results for vapor pressures and coexisting densities. Weber and Goodwin (1993) published 27 data points of vapor pressure from 208 to 237 K. Defibaugh et al. (1993) measured PVT properties using a vibrating tube densimeter apparatus and a Burnett/isochoric apparatus. Baroncini et al. (1993) published vapor pressure and PVT data in the superheated vapor region. Recently, Schmidt and Moldover (1994) published refractive index data and capillary rise data and estimated the critical density. This paper reports the measurement of PVT properties, vapor pressures and critical parameters. The data is compared with previously reported measurements. Based upon those measurements, an analytical correlation for vapor pressure has also been developed.

2. Apparatus A Burnett apparatus has been described by Zhu et al. (1992) in detail. A modified apparatus shown in Fig. 1 is used in this work. It includes a high-accuracy thermostat bath, a temperature-measurement system, a pressure-measurement and control system, a vacuum system, and a sample cell.

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275

Y.-D. Fu et a L / Fluid Phase Equilibria 111 (1995) 273-286

Table 1 Experimental PVT data for HFC-32 T (K)

P (MPa)

p (kg m -3)

T (K)

P (MPa)

p (kg m -3)

243.15 243.15 243.15 243.15 243.15 243.15 253.15 253.15 253.15 253.15 263.15 263.15 263.15 263.15 263.15 263.15 273.15 273.15 273.15 273.15 273.15 273.15 273.15 283.15 283.15 283.15 283.15 283.15 283.15 293.15 293.15 293.15 293.15 293.15 293.15 303.15 303.15 303.15 303.15 303.15 303.15 303.15 303.15 313.15 313.15 313.15 313.15 313.15 313.15

0.2674 0.2062 0.1586 0.1215 0.0928 0.0708 0.3952 0.3069 0.2365 0.1811 0.5713 0.4468 0.3465 0.2671 0.2046 0.1561 0.7901 0.6226 0.4856 0.3759 0.2893 0.2217 0.1696 0.8922 0.7025 0.5473 0.4233 0.3255 0.2492 1.3945 1.1205 0.8864 0.6933 0.5372 0.4140 1.6812 1.3575 1.0776 0.8450 0.6563 0.5067 0.3892 0.2978 2.2660 1.8652 1.5018 1.1901 0.9323 0.7239

7.45 5.64 4.27 3.24 2.45 1.86 10.76 8.15 6.17 4.68 15.40 11.67 8.84 6.70 5.08 3.85 21.21 16.07 12.17 9.22 6.99 5.29 4.01 23.02 17.44 13.21 10.01 7.59 5.75 37.77 28.56 21.59 16.32 12.34 9.33 45.07 34.07 25.76 19.47 14.72 11.13 8.41 6.36 63.47 47.98 36.27 27.42 20.73 15.67

313.15 313.15 313.15 323.15 323.15 323.15 323.15 323.15 323.15 323.15 323.15 323.15 323.15 323.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 343.15 343.15 343.15 343.15 343.15 343.15 343.15 343.15 343.15 343.15 343.15 343.15 343.15 343.15 353.15 353.15 353.15 353.15 353.15 353.15 353.15 353.15 353.15

0.4285 0.3278 0.2502 2.1045 1.6900 1.3371 1.0459 0.8116 0.6252 0.4798 0.3670 0.2799 0.2131 0.1620 3.7574 3.2470 2.7128 2.2105 1.7681 1.3947 1.0887 0.8435 0.6492 0.4978 0.3805 0.2901 4.7646 4.2904 3.7046 3.0969 2.5264 2.0231 1.5976 1.2488 0.9680 0.7459 0.5718 0.4373 0.3331 0.2530 5.7347 5.2883 4.6762 3.9900 3.2339 2.6159 2.0828 1.6373 1.2747

8.95 6.77 5.12 52.18 39.44 29.82 22.54 17.04 12.88 9.74 7.36 5.57 4.2l 3.18 121.14 91.58 69.23 52.34 39.56 29.91 22.61 17.09 12.92 9.77 7.38 5.58 178.76 135.14 102.16 77.23 58.38 44.14 33.37 25.22 19.07 14.41 10.90 8.24 6.23 4.71 240.23 181.61 137.29 103.78 75.89 57.37 43.37 32.79 24.78

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Y.-D. Fu et al. / Fluid Phase Equilibria 111 (1995) 273-286

Table 1 (continued) T (K)

P (MPa)

p (kg m 3)

T (K)

P (MPa)

p (kg m - 3)

313.15 353.15 353.15 353.15 353.15 353.15 353.15 363.15 363.15 363.15 363.15 363.15 363.15

0.5582 0.7570 0.5798 0.4426 0.3374 0.2571 0.1952 4.9985 4.2134 3.4646 2.7934 2.2172 1.7380

11.85 14.16 10.71 8.09 6.12 4.63 3.50 135.94 102.76 77.69 58.73 44.40 33.56

353.15 363.15 363.15 373.15 373.15 373.15 373.15 373.15 373.15 373.15 373.15 373.15

0.9846 1.3504 1.0428 4.7722 3.9245 3.1665 2.5163 1.9763 1.5378 1.1877 0.9125 0.6986

18.74 25.37 19.18 114.07 86.23 65.19 49.28 37.26 28.16 21.29 16.10 12.17

The high-accuracy thermostat bath has transparent viewing windows and a large capacity bath, 350 X 350 X 450 mm 3. The temperature can be varied from 223 to 453 K. The temperature instability in the bath is less than + 5 mK in 8 h. It can be used to measure PVT properties, vapor pressures and critical parameters. Silicone oil, distilled deionized water or alcohol is used as the fluid in the bath depending on the temperature range. The temperature-measurement system includes a high-grade platinum electric resistance thermometer (5187SA) with an uncertainty of ___2 mK (ITS-90), a precision Tinsley thermometer bridge (5840D), accuracy within _ 1 mK, a select switch (5840CS/6T) and a personal computer. The overall temperature uncertainty for the bath and temperature-measurement system is less than +__10 mK. The pressure-measurement system includes a piston-type pressure gauge, a pressure transducer and an atmosphere pressure gauge. The accuracy of the piston-type pressure gauge is less than 0.005% in the range of 0 . 1 - 6 MPa. A very sensitive diaphragm pressure transducer (405 T) separates the sample from an N2-filled system including the precision piston-type pressure gauge. The accuracy of the transducer is 0.2%, the pressure difference adjustable range is 6 - 3 8 kPa, the temperature range is 2 3 3 - 4 0 0 K, and the maximum allowable pressure is 17.8 MPa. The pressure-measurement system uses a large oil area without Hg and is designed for stability and for setting the oil level at the bottom of piston-type pressure gauge. The accuracy of the atmosphere pressure gauge is 0.05% and the pressure range is 1-160 kPa: The whole pressure-measurement system has an uncertainty of _ 500 Pa. The extremely high vacuum is about 1 × 10 - 6 torr. Usually, the Burnett cell is a heavy-walled metal vessel having two chambers separated by a valve. The resultant experimental data distribution is uneven, namely the test points are rare in the high pressure area, but dense in the low pressure area. Therefore, a three-chamber Burnett cell is adapted in this apparatus that provides a set of volume ratios, Ni, between the total volume and that of any chamber by combining any two of the three chambers. The experimental data are well-distributed using different combinations of chambers once the cell is filled. Because the volume of the chamber varies linearly with temperature and pressure changes in the apparatus, the volume ratios, N i, become constant, N, for special combination of chambers. The volume ratio, N, is determined by calibration

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Y.-D. Fu et a l . / Fluid Phase Equilibria 111 (1995) 273-286

Table 2 Studies of PVT property in gaseous phase for HFC-32

First author

Year

Data points

Malbrunot Sato Qian Defibaugh Baroncini This work

1968 79 1992 51 1993 95 1993 161 1993 93 1 9 9 4 123

T Range (K) 298-473 330-420 290-370 268-373 273-360 243-373

8 T (K)

Pressure Range (MPa)

0.080 0.007 0.010 0.010 0.010 0.010

0.9-16.4 3.5-9.8 0.1-6.5 0.2-7.7 0.7-2.7 0.07-5.7

8P (kPa)

Density Range (kg m-3)

~p (%)

Sample purity (wt.%)

1 2 0.6 0.5 0.5 0.5

19.7-56.9 111-425 2.6-303 4.1-310 19-57 1.8-240

0.1 0.2 kg m -3 -

99.95 99.998 99.98 99.99 99.6 99.95

with high purity helium. A c c o r d i n g to the calibration test, the v o l u m e ratio N for V1 = 600 ml and V2 = 200 ml is 1.32281 and 1.31990. The f o r m e r value was obtained before disassembling the cell and the latter one after reassembling the cell. The R M S deviation is 0.058%. The purity of the sample provided by the Zhejiang Fluoro-Chemical T e c h n o l o g y Research Institute w a s 99.95 wt.%.

3. PVT data and analysis One hundred twenty three P V T data points for H F C - 3 2 are s h o w n in Table 1. Table 2 s u m m a r i z e s previously published experimental studies o f P V T property in gaseous phase for HFC-32. The uncertainties o f the temperature, pressure and density, 6T, 8P and 6p, are also s h o w n in Table 2. The present experimental conditions are very close to those o f other researchers, except for Malbrunot et al. T h e present experimental P V T data as well as those reported by Malbrunot et al., 1968, Sato et al., 1992, Qian et al., 1993, Defibaugh et al., 1993 and Baroncini et al., 1993 were c o m p a r e d with the 18-term M B W R - t y p e equation o f state developed b y Piao et al. (1993) (Table 3). Fig. 2 shows the pressure deviations o f the experimental data f r o m the equation o f state developed by Piao et al., 1993. T a b l e 3 and Fig. 2 show that the present experimental data appears to be consistent with the data m e a s u r e d b y the other authors, except for Malbrunot et al. The experimental range is extended to lower temperatures (243 K) in this work. At lower temperatures ( < 320 K), the pressures measured

Table 3 Max. Dev. and RMS Dev. between experimental data and values calculated from Piao's equation (Piao et al. (1993)) First author

Max. positive Dev. (%)

Malbrunot Sato Qian Defibaugh Baroncini This work

3.41 0.32 0.24 0.18 0.11 0.27

Max. negative Dev. (%) -

3.48 0.40 0.50 0.51 - 14.36 - 0.51

RMS Dev. (%) 1.99 0.17 0.16 0.14 1.99 0.17

Y.-D. Fu et al. / Fluid Phase Equilibria 111 (1995) 273-286

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by Defibaugh et al., 1993, Qian et al., 1993, Baroncini et al., 1993 and in the present work are less than predicted by Piao et al., 1993. It is estimated that the calculated data from Piao's equation may be less accurate in the lower temperature range from 243 to 320 K.

4. Vapor pressure data and analysis Table 4 shows sixty vapor pressure data points for HFC-32 from 233 to 351 K. There have been twelve previously published sets of HFC-32 vapor pressure data (Table 5). From Table 5, the uncertainties of the temperature data for Malbrunot et al., 1968 and Holcomb et al., 1993 are much larger than those of the other authors. The uncertainty for Baroncini et al., 1993 is also large. The uncertainty of the pressure data for Widiatmo et al., 1992 is larger than those of the other authors.

279

Y.-D. Fu et al. // Fluid Phase Equilibria 111 (1995) 273-286

Table 4 Experimental vapor pressure data for HFC-32 Temperature (K)

Pressure (MPa)

Temperature (K)

Pressure (MPa)

233.15 235.15 237.15 239.15 241.15 243.15 245.15 247.15 249.15 251.15 253.15 255.15 257.15 259.15 261.15 263.15 265.15 267.15 269.15 271.15 273.15 275.15 277.15 279.15 281.15 283.15 285.15 287.15 289.15 291.15

0.1770 0.1937 0.2115 0.2307 0.2513 0.2735 0.2967 0.3212 0.3477 0.3758 0.4056 0.4371 0.4705 0.5057 0.5430 0.5823 0.6238 0.6675 0.7143 0.7624 0.8135 0.8668 0.9226 0.9816 0.0427 0.1069 0.1743 0.2448 0.3185 1.3943

293.15 295.15 297.15 299.15 301.15 303.15 305.15 307.15 309.15 311.15 313.15 315.15 317.15 319.15 321.15 323.15 325.15 327.15 329.15 331.15 333.15 335.15 337.15 339.15 341.15 343.15 345.15 347.15 349.15 351.15

1.4747 1.5583 1.6451 1.7356 1.8299 1.9275 2.0288 2.1345 2.2450 2.3593 2.4776 2.6003 2.7280 2.8606 2.9984 3.1402 3.2876 3.4405 3.5994 3.7637 3.9327 4.1085 4.2904 4.4787 4.6753 4.8768 5.0861 5.3070 5.5323 5.7679

The authors listed in Table 5 have proposed some vapor pressure equations for HFC-32. However, the range of temperature and pressure data used to establish the various equations are very different. A comprehensive vapor pressure equation, based on vapor pressure data from several authors, was developed by Piao et al., 1993. Therefore, the vapor pressure equation developed by Piao et al., 1993 was selected to compare the data of various authors. The present experimental vapor pressure data, as well as those reported in Table 5, are compared with the vapor pressure equation developed by Piao et al., 1993 (Table 6). Fig. 3 shows the pressure deviations of the experimental data from the values predicted by the equation developed by Piao et al., 1993. From Fig. 3, it can be seen that: 1. The present data agrees very well with Piao's equation except near the critical point. The RMS deviation from Piao's equation is 0.063% and the maximum absolute deviation is less than 3 kPa.

28O

Y.-D. Fu et al. / Fluid Phase Equilibria 111 (1995) 273-286

Table 5 Vapor pressure of HFC-32 First author

Malbrunot Sato Widiatrno Qian Weber Defibaugh Holcomb Baroncini Zhu Wang Xiang This work

Year

1968 1992 1992 1993 1993 1993 1993 1993 1993 1993 1993 1994

Data points

29 23 25 9 27 18 25 56 32 35 25 60

Temperature

Pressure

Range (K)

6 T (mK)

Range (MPa)

6 P (kPa)

Sample purity (wt.%)

191-Tc 320-351 220-325 280-350 208-237 268-348 295-349 238-Tc 273-347 233-Tc 290-348 233-351

80 7 15 10 10 10 100 20 10 30 8.3 10

0.02-Pc 2.9-Pc 0.09-3.3 1-5.6 0.05-0.2 0.7-5.4 1.6-5.5 0.2-Pc 0.8-5.3 0.2-Pc 1.3-5.4 0.17-5.76

1 2 10 0.6 0.5 0.5 3.5 0.5 0.5 1.5 0.05% 0.5

99.95 99.998 99.998 99.98 99.98 99.99 99.9 99.6 99.95 99.95 99.95 99.95

2. D a t a m e a s u r e d b y W e b e r a n d G o o d w i n ,

1 9 9 3 a n d D e f i b a u g h et al., 1 9 9 3 a l s o a g r e e v e r y w e l l w i t h

Piao's equation. The RMS deviations are 0.046%

and 0.023%, respectively, for relatively smaller

temperature ranges than those of the present work. 3. T h e m a x i m u m

a b s o l u t e d e v i a t i o n o f t h e d a t a m e a s u r e d b y Q i a n et al., 1 9 9 3 is a l s o l e s s t h a n 2 k P a ,

b u t t h e r e a r e o n l y n i n e d a t a p o i n t s at i n t e r m e d i a t e a n d h i g h e r t e m p e r a t u r e s . T h e d a t a o f S a t o et al., 1 9 9 2 a n d B a r o n c i n i et al., 1 9 9 3 a l s o a g r e e w e l l w i t h P i a o ' s e q u a t i o n e x c e p t f o r s e v e r a l s p e c i a l points. 4. T h e d a t a m e a s u r e d b y W i d i a t m o et al., 1 9 9 2 , X i a n g , 1 9 9 3 a n d Z h u et al., 1 9 9 3 h a s r e l a t i v e l y l a r g e deviations when compared

to P i a o ' s e q u a t i o n .

Table 6 Deviations between experimental data and calculated values from Piao's equation (Piao et al. (1993)) First author

Malbrunot Sato Widiatmo Qian Weber Defibaugh Holcomb Baroncini Zhu Wang Xiang This work

Absolute Dev. (kPa)

Relative Dev. (%)

Max. ( + ) Dev.

Max. ( - ) Dev.

RMS Dev.

Max. ( + ) Dev.

Max. ( - ) Dev.

RMS Dev.

18.11 5.85 2.99 1.72 0.079 0.88 9.64 7.83 13.46 21.45 6.67 1.52

- 13.38 - 0.81 - 2.84 - 1.84 - 0.060 - 1.21 - 6.89 -- 2.41 - 11.08 - 5.67 - 2.98

6.80 1.91 1.73 1.01 0.046 0.61 4.50 2.38 6.82 13.14 3.64 0.60

0.65 0.20 1.19 0.047 0.083 0.036 0.60 0.22 0.62 3.04 0.25 0.035

- 0.42 - 0.01 - 1.24 - 0.082 - 0.12 - 0.055 - 0.13 -- 0.12 - 0.43 - 0.32 - 0.20

0.28 0.061 0.56 0.040 0.046 0.023 0.20 0.083 0.30 1.10 0.15 0.063

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360

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283

Table 7 Absolute and relative deviations between experimental data and values calculated from Eq. (1) First author Malbrunot Sato Widiatmo Qian Weber Defibaugh Holcomb Baroncini Zhu Wang Xiang This work

Relative Dev. (%)

Absolute Dev. (kPa) Max. ( + ) Dev.

Max.( - ) Dev.

RMS Dev.

Max.( + ) Dev.

Max.( - ) Dev.

RMS Dev.

19.65 4.79 2.15 1.84 0.067 0.57 9.07 9.58 14.94 22.98 6.01 3.21

- 14.29 - 0.74 - 2.66 - 1.25 - 0.041 - 1.10 - 5.44 - 3.19 - 12.21 - 5.01 - 2.85

6.97 1.74 1.49 1.01 0.039 0.48 4.36 2.79 6.67 12.99 3.20 0.82

0.61 0.16 1.16 0.040 0.056 0.023 0.58 0.23 0.58 3.04 0.21 0.065

-

0.30 0.050 0.55 0.042 0.037 0.016 0.18 0.092 0.29 1.11 0.14 0.055

0.45 0.013 1.28 0.061 0.081 0.035 0.10 0.16 0.47 - 0.34 - 0.19

5. Data measured by H o l c o m b et al., 1993 and Malbrunot et al., 1968 are not consistent with P i a o ' s equation. The results of W a n g et al., 1993 show large positive deviations in the whole temperature range.

5. Vapor pressure equation The present vapor pressure data was combined with the data of Weber and Goodwin, 1993, Defibaugh et al., 1993, Qian et al., 1993, Sato et al., 1992 and Baroncini et al., 1993 to cover the temperature range o f 2 0 8 - 3 5 1 K. The combined data set was correlated with the following equation: I n ( P / P c ) = ( A i r + A 2 rl"2s + A3 r3 +

A4"t'7)Tc/T

(1)

Where r - - - 1 - T / T c, A 1 = - 7 . 8 5 7 9 3 , A 2 = 1.59907, A 3 = - 2 . 0 3 3 6 0 , A 4 - - - - 5 . 5 5 0 6 4 , To= 351.295 K, Pc = 5.789 MPa. Eq. (1) fits the data with an absolute deviation of 1.2 kPa. The temperature range is from 200 K to Tc. The critical temperature was fixed at the value (351.295 K) measured in this work, as described below. The critical pressure, which was an adjustable parameter in the correlation, was found to be 5.789 MPa. The absolute and relative deviations between the experimental data and the values calculated from Eq. (1) are shown in Fig. 4 and summarized in Table 7. Table 8 shows the absolute R M S deviations o f the present data as well as the data of W e b e r and Goodwin, 1993, Defibaugh et al., 1993, Qian et al., 1993, Sato et al., 1992 and Baroncini et al., 1993 from the vapor pressure equations developed by Piao et al., 1993, Defibaugh et al., 1993 and Eq. (1), respectively. All the vapor pressure equations fitted the data very well. However, Eq. (1) has the widest temperature range o f the three equations.

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Table 8 Comparison of Eq. (1) with equations of Piao et al., 1993 and Defibaugh et al., 1993 Data source

Absolute RMS Dev. (kPa)

Sato Qian Weber Defibaugh Baroncini This work

Eq. (1)

Piao's equation

Defibaugh's equation

1.74 1.01 0.039 0.48 2.79 0.82

1.91 1.01 0.046 0.61 2.38 0.60

2.52 0.94 0.030 0.36 2.87 0.69

Table 9 Experimental determined critical parameters for HFC-32 First author

Year

Tc (K)

Pc (MPa)

Pc (kg m - 3 )

Sample purity

(wt.%) Malbrunot Schmidt Holcomb Kuwabara Qian Sato Wang This work

1968 1994 1993 1993 1993 1992 1993 1994

351.52 +_0.20 351.36 _+0.02 351.255 _+0.01 351.26 _+0.05 351.295 ___0.01

5.830 5.780 5.784 5.801 5.785

_+0.006

_+0.002 + 0.0025 _+0.008 _+0.002

429.6 + 1.2 419 _+7 428.50 +_ 1.83 424 -4-1 430 ___4 425 +_3

99.95 99.9 99.9 99.98 99.98 99.998 99.95 99.95

6. Experimental determination of critical parameters The optical cell with two windows was used to observe the meniscus of the sample refrigerant, Fig. 5. This cell was placed on a frame and the frame was installed in the thermostated bath described previously. The volume of the optical cell was determined by calibration with distilled, deionized water. According to the calibration test, the volume of the cell was 9.94594 c m 3 at 16 °C. Considering the effect of temperature on the volume of the cell, the volume of the cell is 9.97668 c m 3 at 78 °C.

Fig. 5. Experimental cell for observing the sample meniscus.

Y.-D. Fu et al. / Fluid Phase Equilibria 111 (1995) 273-286

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The experimentally determined critical temperature was 351.295 K. As mentioned above, the uncertainty o f the temperature measurement was less than + 10 inK. Therefore, the critical temperature of HFC-32 was determined to be Tc = 351.295 + 0.010 K. The experimentally determined critical density is 425 kg m - 3 with an uncertainty of about + 3 kg m - 3 . Therefore, the critical density o f HFC-32 is Pc = 425 + 3 kg m - 3 . The critical pressure, determined from the vapor pressure Eq. (1) using the critical temperature, was 5.785 MPa with an uncertainty of about + 2 kPa. Therefore, the critical pressure o f HFC-32 is Pc = 5.785 _ 0.002 MPa. Table 9 compares the experimentally determined critical parameters with those determined by other authors.

7. Conclusion A total of 123 P V T data points were obtained for HFC-32 in the gaseous phase over a wide range of temperatures, 2 4 3 - 3 7 3 K, pressures, 0 . 0 7 - 5 . 7 MPa, and densities, 1 . 8 - 2 4 0 kg m -3. 60 vapor pressure data points for HFC-32 were measured in the temperature range from 233 to 351K. A vapor pressure correlation which accurately reproduces the measured data was also developed. The critical parameters for HFC-32 were also determined.

Acknowledgements This work was supported by the National Natural Science Fundation of China. W e are grateful to Zhejiang Fluoro-Chemical T e c h n o l o g y Research Institute and U.S. E P A for providing the HFC-32 sample.

References Baroncini, C., Camporese, R., Giuliani, G., Latini, G. and Polonara, F., 1993. Experimental study of thermodynamic properties of difluoromethane (R32). Private communication. Defibaugh, D.R., Morrison, G. and Weber, L.A., 1993. Thermodynamic properties of difluoromethane(R32), to be published. Holcomb, C.D., Niesen, V.G., Poolen, L.J.V. and Outcalt, S.L., 1993. Coexisting densities, vapor pressures and critical densities of refrigerants R-32 and R-152a, at 300-385K. Fluid Phase Equilibria, 91: 145-147. Kanungo, A., Oi, T., Popowicz, A. and Ishida T., 1987. Vapor pressure isotope effects in liquid methylene difluoride. J. Phys. Chem., 91(15): 4198-4203. Kuwabara, S., Sato, H. and Watanabe, K., 1993. Measurements of the vapor-liquid coexistence curve in the critical region and the critical parameters for several alternative refrigerants. 13th Euro. Conf. Thermophys. Prop., Lisboa, Portugal. Malbrunot, P.F., Meunier, P.A., Scatena, G.W., Mears, W.H., Murphy, K.P. and Sinka, J.V., 1968. Pressure-volume-temperature behavior of difluoromethane. J. Chem. Eng. Data, 13(1): 16-21. Piao, C.C., Noguchi, M., Sato, H. and Watanabe, K., 1993. Thermodynamic properties of new fluid HFC-32. Private communication (in Japanese). Qian, Z.Y., Nishimura, A., Sato, H. and Watanabe, K., 1993. Compressibility factors and virial coefficients of difluoromethane(HFC-32) determined by Burnett method. JSME Int. J., 36(4): 665-670.

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Sato, T., Sato, H. and Watanabe, K., 1992. A study of PVT properties of HFC-32. Proc. 13th Japan Symp. Thermophys. Prop., Akita, 1992, pp. 49-52. Schmidt, J.W. and Moldover, M.R., 1994. Alternative refrigerants CH2F 2 and C2HFs: critical temperature, refractive index, surface tension, and estimates of liquid, vapor and critical densities. J. Chem. Eng. Data, 39(1): 39-44. Wang, H.X., Li, Z., Ma, Y.T. and Lu, C.R., 1993. Experimental research on vapor pressure and critical parameters of HFC-32, Proc. Eng. Thermodynamics and Utilization of Energy Conf., Jiangxi, China, 1993, IV-104-IV-109 (in Chinese). Weber, L.A. and Goodwin, A.R.H., 1993. Ebulliometric measurement of the vapor pressure of difluoromethane. J. Chem. Eng. Data, 38(2): 254-256. Widiatmo, J.V., Sato, H. and Watanabe, K., 1992. Measurement of vapor pressure and liquid densities of HFC-32 and HFC-125. Proc. of the 3rd Asian Thermophys. Prop. Conf., Beijing, China, 1992, pp. 364-369. Xiang, H.W., 1993. Theoretical studies on the equations of thermophysical properties of fluids and experimental measurements of vapor pressure of R32 and PVTx of R32/R152a. M.S. Thesis, Xi'an Jiaotong University (in Chinese). Zhu, M.S., Fu, Y.D. and Han, L.Z., 1992. An experimental study of PVT properties of CFC alternative HFC-134a. Fluid Phase Equilibria, 80: 149-156. Zhu, M.S., Li, J. and Wang, B.X., 1993. Vapor pressure of difluoromethane (HFC-32). Int. J. Thermophys., 14: 1221-1227.