온도범위 268.15에서 318.15K까지 HC-290과 HFC-32의 기-액 상평형측정

화학공학의 이론과 응용 제8권 제1호 2002년

양웅조, 이병권, 이윤우, 임종성

Theories and Applications of Chem. Eng., 2002, Vol. 8, No. 1

한국과학기술연구원 CFC대체연구센터

Measurements of the vapor-liquid equilibrium of HC-290 + HFC-32

in the temperature range from 268.15 to 318.15K

Theories and Applications of Chem. Eng., 2002, Vol. 8, No. 1

Woong Jo Yang,  Byung Gwon Lee, Youn Woo Lee, Jong Sung Lim

CFC Alternatives Research Center, KIST

Introduction

Alternative refrigerants have become increasingly important to industrial applications as the ozone-depleting refrigerants are being phased out under the requirements of the Montreal Protocol. Some countries have volunteered to phase out hydrochlorocarb-

ons (HCFCs) much earlier than the Montreal protocol schedules.

The purpose of this study is to improve the understanding of the vapor-liquid equilibrium (VLE) measurement on the systems of HFC and HC Refrigerant Mixture-

s. There are two main reasons for studying refrigerant mixtures containing HC-290.

One of the difficulties with the chlorine-free alternative refrigerants is their immiscibility with the oils used in refrigeration cycles. The addition of HC-290 to the mixture to enhance oil solubility is under investigation. Another reason for studying propane is the complex phase behavior exhibited by propane mixtures and allows the models to be rigorously tested [1].

The important works of the present study are as follows; 1) measuring the VLE data over wide ranges of temperature, 2) investigating the azeotropes, 3) calculating the binary parameters (constant k, □12 and □21.) Intensive VLE data of difluoromethane (HFC-32, CH2F2) + propane (HC-290, C3H8) system  were obtained in a circulation- type equilibrium apparatus over the temperature range from 268.15K to 318.15K. This system has azeotropes which is one of the most important types of information in evaluating the performance of refrigeration cycles and determining their optimal compositions. The experimental data were correlated with the Peng-Robinson equation of state using the Wong-Sandler mixing rule [2] within the uncertainty of ±1.0%.

Experimental

The vapor-liquid equilibrium data were measured in a circulation type apparatus, in which both vapor and liquid phases were continuously recirculated. This experimental apparatus was to be classified into five major parts. [3]

Part 1. The equilibrium cell made of a stainless steel was used for the VLE measurements and equipped with dual glass windows in order to make it possible to observe the inside during operation.

Part 2. The temperature in the cell was measured with a platinum resistance sensor located in the middle part of the equilibrium cell which was immersed in the thermostated water-bath. The accuracy of the temperature measurement was estimated to be within 0.01 K.

Part 3. A pressure transducer was used in order to measure a pressure and its uncertainty was estimated to be within  0.05 bar.

Part 4. A dual-head circulation pump were continuously recirculated in order to reach the equilibrium of the vapor and liquid phases rapidly in the equilibrium cell.

Part 5. The composition of the phases was determined by means of a gas chromatograph using a Porapak Q column which was connected on-line to the equilibrium cell. The response of the thermal conductivity detector was carefully calibrated using standard mixtures prepared gravimetrically. Data derived from gas chromatograph were treated with computer program.

The results of calibration were regressed with polynomial equations expressing the mole fraction of the higher saturated pressure compound as a function of the percentage of its peak area. At least five analyses were performed for each phase.

Results and Discussion

The experimental results for the VLE were correlated with the Peng-Robinson equation of state (PR-EOS). [4]

Table 1. Comparison of the Measured Pure Components Vapor Pressures with

the Calculated values from the Database REFPROP 6.01 (1998) [5]

component	T [K]	P [MPa]		△Pa [MPa]	△P/Pb [%]
		measd	calcd
R-32	268.15 278.15 288.15 298.15 308.15 318.15	0.6954 0.9520 1.2812 1.6856 2.1852 2.7890	0.6901 0.9509 1.2800 1.6890 2.1890 2.7940	0.0053 0.0011 0.0006 0.0012 0.0038 0.0050	0.7622 0.0116 0.0542 0.0937 0.1739 0.1793
R-290	268.15 278.15 288.15 298.15 308.15 318.15	0.4079 0.5494 0.7350 0.9548 1.2130 1.5308	0.4059 0.5510 0.7314 0.9519 1.2180 1.5340	0.0020 0.0016 0.0036 0.0029 0.0050 0.0032	0.4903 0.2912 0.4898 0.3037 0.4122 0.2090

a△P=[eqn: LEFT | { P}_{exp}- { P}_{cal } RIGHT |], and b△P/P=[eqn: LEFT |LEFT({P}_{exp}- {P}_{cal}/ {P}_{exp} RIGHT) RIGHT| times 100]

Table 1 is a comparison of measured vapor pressures of pure HFC-32 and HC-290 with those calculated from the database REFPROP 6.01 [5] which is considered to be reliable for the pure compounds considered and consistent with other literature data. The average deviation (△P/P) between measured and calculated values from the data of REFPROP 6.01 is 0.212% for HFC-32, 0.366% for HC-290.

We measured mole fraction of the liquid and vapor phases, pressures, and temperatures in equilibrium and calculated the deviations between measured and calculated pressures (△P/P) and vapor compositions(△y), point by point for the R-32 + R-290 system.

Fig 1. P-x-y Diagram for the HFC-32(1) + HC-290(2) system		Fig 2. Deviation of the HFC-32(1) + HC-290(2) binary system
(◇), experimental at 268.15K; (◆), experimental at 278.15K; (△), experimental at 288.15K; (▲), experimental at 298.15K; (○), experimental at 308.15 K; (●), experimental at 318.15 K; (―), calculated with PR EOS using Wong-Sandler mixing rules.

Measurements were made along six isotherms, i.e., 268.15, 278.15, 288.15, 298.15, 308.15 and 318.15K. The experimental results are shown in Figs. 1 and 2. In Fig. 1, the solid lines represent the calculated values using PR equation of state with Wong-Sandler mixing rule. Table 2 lists the interaction parameters of binary mixtures for each isotherm, the binary parameters of the NRTL model with Wong-Sandler mixing rules, and the average deviations of pressure (□P) and vapor phase composition (□y) between measured and calculated values. As can be seen in Table 2, the average deviations of pressures are 0.317% and those of vapor phase compositions are 0.0103 for the HFC-32 + HC-290 system.

Table 2. Values of Binary Parameters and Average Deviations of P and y

System	T [K]	NRTL parameters [kJ·mol-1]			□Pc [%]	□yd
R-32/R-290	268.15	kij=0.218	Aij=0.378	Aji=0.348	0.266	0.0154
	278.15	kij=0.183	Aij=0.374	Aji=0.382	0.256	0.0107
	288.15	kij=0.167	Aij=0.389	Aji=0.381	0.261	0.0059
	298.15	kij=0.254	Aij=0.321	Aji=0.322	0.328	0.0079
	308.15	kij=0.168	Aij=0.336	Aji=0.400	0.442	0.0128
	318.15	kij=0.369	Aij=0.307	Aji=1.086	0.349	0.0095

c□P=[eqn: { 1} over {N } SUM LEFT | LEFT ( { P}_{exp }- { P}_{cal } RIGHT )/{ P}_{exp } RIGHT | TIMES 100], and d□y=[eqn: { 1} over {N } SUM LEFT | { y}_{exp } - { y}_{ cal} RIGHT |]

From Figs. 1 and 2 and the low average deviations of pressure (□P <0.442%) and vapor phase compositions (□y <0.0154) in Table 2, we conclude that the values calculated with the PR equation of state using the Wong-Sandler mixing rules give comparatively good agreement with experi-

mental data.

In this system, there exist azeotropes. Figure 3 shows variation of the compositi

Fig 3. Variation of the azeotropic composition

for the HFC-32(1)+HC-290(2) binary        system

(a) versus pressure, (b) versus temperature

(c) versus y1-x1

(◇), experimental at 268.15K

(◆), experimental at 278.15K

(□), experimental at 288.15K

(▲), experimental at 298.15K

(○), experimental at 308.15K

(●), experimental at 318.15K

-

on at azeotropes. The azeotropic points change with temperature and they were x1 = 0.642 (mole fraction), P = 0.9086 MPa at 268.15 K, x1 = 0.661, P = 1.220 MPa at 278.15 K, x1 = 0.667, P = 1.605 MPa at 288.15 K, x1 = 0.678, P = 2.093 MPa at 298.15 K, x1 = 0.697, P = 2.641 MPa at 308.15 K and x1 = 0.700, P = 3.275 MPa at 318.15 K for HFC-32 + HC-290 system, respectively. The experimental data were fitted with a cubic polynomial equation. The azeotropic compositions were calculat- ed as one of the solutions of these cubic equations. [6]

It is especially interesting to note that the PR equation of state using the WS mixing rules did correctly predict the azeotropic behaviors comparing with experimental data at over range temperatures.

Literature Cited

1. D. Holcomb, J. W. Magee, J.L. Scott, S.L. Outcalt, W.M. Haynes, NIST Technical Note 1397 (1997).

2. H. Orbey, S. I. Sandler., Modeling Vapor-Liquid Equilibria-cubic equation of state and their mixing rules. Cambridge University Press (1998) 1-74.

3. J. S. Lim, J. Y. Park, J. Chem. Eng. Data. 45 (2000) 734-737.

4. D. Y. Peng, D. B. Robinson, A New Two-Constant Equation of State. Ind. Eng. Chem. Fundam. 15 (1976) 59-64.

5. M. O. McLinden, S. A. Klein, E. W. Lemmon, A. P. Peskin, REFPROP Version 6.01, NIST. Thermophysics Division (1998).

6. M. B. Shiflett and S. I. Sandler, Fluid Phase Equilibria. 147 (1998) 145-162