HFC-227ea를 포함한 2성분계 혼합냉매의 상평형

김용호, 박지영, 김재덕, 이윤우, 이병권, 이창하*, 임종성

한국과학기술연구원 환경공정연구부

연세대학교 화학공학과*

Phase Equilibria of Binary Refrigerant Mixtures Containing HFC-227ea

Yong-Ho Kim, Ji-Young Park, Jae-Duck Kim, Youn-Woo Lee,

Byung-Gwon Lee, Chang-Ha Lee*, Jong Sung Lim

Environment and Process Technology Division, KIST

Dept. of Chem. Eng., Yonsei University*

Introduction

Increasing concern about ozone depletion in the stratosphere will accelerate the regulation of uses of chlorofluorocarbons (CFCs)  and  hydrochlorofluoro- carbons (HCFCs). The hydrofluorocarbon (HFC) binary mixtures may provide some solutions to the problem of the very limited number of fluids which have suitable properties to provide alternatives to CFCs, since their ozone depletion potentials (ODP) and global warming potentials (GWP) are low. Vapor-liquid equilibria data are required as one of the most important types of basic information in evaluating the performance of refrigeration cycles and determining their optimal compositions.  In this work, we measured VLE data for two binary systems of 1,1,1,2,3,3,3-Heptafluoropropane (HFC-227ea) + difluoromethane (HFC-32) , + Hexafluoropropylene (HFP).  The experimental data were correlated with the NRTL (Non-Random Two Liquid) model[1].  In addition, the non-ideality of HFC-227ea + HFC-32, + HFP, and + iso-butane was discussed by calculating the excess gibbs energy.

Theory

The VLE experimental data were correlated with the NRTL (Non-Random Two Liquid) model[1].  Local composition was defined as

[eqn: {x_ji} over {x_ii } = { x_j exp(-alpha_ji g_ji /RT) } over { x_j exp(-alpha_ji g_ii /RT) }]                           [eqn: (1)]

Here [eqn: alpha_ji] should be in the range of 0.1 to 0.3. Consider a molecule of component i, surrounded by molecules of component i and j. The total residual Gibbs energy of cell i in a binary mixture, [eqn: g^(i)], is defined as

[eqn: g^(i) = x_1i g_1i + x_2i g_2i]    [eqn: (2)]

The molar excess Gibbs energy, [eqn: g^-E], for a binary solution is the sum of all changes in residual Gibbs energy in taking xi molecules from a cell of pure i, to a cell i of the solution.

[eqn: g^-E = x_1 ( g^(1) - g_pure^(1) ) + x_2 ( g^(2) - g_pure^(2) )]                  [eqn: (3)]

Substituting the relationships found in Equation (2), the following form of the excess Gibbs energy is derived.

[eqn: g^-E = x_1 x_21 ( g_21 - g_11 ) + x_2 x_12 ( g_12 - g_22 )]                  [eqn: (4)]

Substituting Equation (1) and rearranging, we arrive at the final form of the NRTL equation :

[eqn: g^-E over RT = x_1 x_2 [ {tau_21 G_21 } over { x_1 + x_2 G_21 } + { tau_12 G_12 } over { x_2 + x_1 G_12 } ]]                 [eqn: (5)]

Where :

[eqn: tau_ji = { g_ji - g_ji } over { RT }]       [eqn: G_ji = exp ( -alpha_ji tau_ji )]

For a binary system, there are three adjustable parameters : [eqn: ( g_12 - g_22 )], [eqn: ( g_21 - g_11 )], and [eqn: alpha_12] .

Table 1. Characteristic properties of pure chemicals

	formular	Tc(K)	Pc(MPa)	Vc(□/mol)	Zc	w
HFC-32[2]	CH2F2	351.26	5.782	122.7	0.243	0.2768
HFC-227ea[2]	C3HF7	375.95	2.980	293.9	0.279	0.3632
HFP[3]	C3F6	367.1	2.898	293.2	0.279	0.2045

Experimental Procedure

Experiments for a binary system were performed by the following procedure. The whole system was evacuated by a vacuum pump to remove all the inert gases. A certain amount of HFC-227ea was introduced into the cell, and then the temperature of the entire system was maintained constant by controlling the temperature of the water bath. After the desired temperature was attained, the pressure of the pure component was measured. A proper amount of HFC-32 (for HFC-32 + HFC-227ea) or HFP (for HFP + HFC-227ea) was supplied to the cell from a charging cylinder. Both vapor and

Table 2. The NRTL parameters and AADP(%)

	T/K	AADP(%)	NRTL parameters
HFC-227ea + HFC-32	283.15 303.15	0.8819 0.2195	□12=-0.1820 □12= 0.0820	□21=-0.0193 □21=-0.0044
HFC-227ea + HFP	293.15 303.15	0.1047 0.7543	□12=-0.0710 □12=-0.1045	□21=-0.0014 □21=-0.0003

[eqn: AAD P (%) = I OVER N SUM { LEFT | P_exp - P_cal RIGHT | } over {P_exp } TIMES 100] ,   [eqn: tau_12 , tau_21] : Dimensionless

liquid phases  were  allowed  to  circulate  for  about an  hour  to  ensure equilibrium. When the equilibrium was attained, pressure was  measured  and

then vapor  and liquid samples were taken  from  the  recycling lines by  the vapor and liquid sampling valves, respectively. The composition of the samples were measured by immediately injecting them into the gas chromatograph which is connected on-line to both vapor and liquid sampling valves. We measured the equilibrium concentration at least three times in a row to obtain reliable values. Average deviations of the equilibrium concentration were within ±0.001 (mole fraction basis) for both phases.

Results and Discussion

The VLE of HFC-227ea + HFC-32 and HFC-227ea + HFP systems were predicted with the NRTL model.  Figs. 1 and 2 show comparison between experimental data and predicted values. Characteristic properties of pure chemicals and the parameters in NRTL model were listed in Tables 1 and 2. The average absolute deviations(AAD, %) between experimental and calculated pressures for these system were listed in Table 2.

Fig. 3 shows the point to point pressure deviations (MPa) between experimental and calculated values as a function of composition. As can be seen in this figure, the pressure deviations were randomly distributed within ±0.015 (MPa) for each systems.  Fig 4. shows the excess gibbs energy ([eqn: g^-E]) as a function of composition.  The maximum values of [eqn: g^-E] for the binary mixtures containing HFC-227ea were 781.693 J/mole (HFC-227ea + iso-butane[4]), 48.25 J/mole (HFC-227ea + HFC-32), and -66.9 J/mole (HFC-227ea + HFP) at 303.15K, respectively.  As can be seen in this figure, the HFC-227ea + HFC-32, and + HFP systems show small positive and small negative deviations, respectively, while the HFC-227ea + iso-butane (hydrocarbon) system shows a strong positive deviation from Raoult's law. From the results of this work and our previous work[4], the non-ideality of HFC-227ea + HFCs mixtures were much lower than that of HFC-227ea + hydrocarbon (iso-butane) mixture. Both isothermal systems exhibit no azeotropes.

Literature Cited

1. Mark A. Gess, Ronald P. Danner, Manoj Nagvekar, Thermodynamic Analysis of Vapor-Liquid Equilibria, 26-27, (1991)

2. M. Huber, J. Gallagher, M. O. McLinden, G. Morrison, REFPROP, V.6.01, NIST, (1996)

3. W. Braker, A L. Mossman, Matheson Gas Data Book, Matheson Gas Products. Inc, 326-364, (1980)

4. J. Y. Park, J. S. Lim, B. G. Lee, Y. Y. Lee, J. D. Lee, Phase Behavior of Refrigerant Mixtures containing HFC-227ea, Theories and Applications of Chem. Eng., Vol.6, No.1, 977-980, (2000)