HFC-227ea를 포함한 냉매 혼합물의 상거동

박지영, 임종성, 이병권, 이윤우, 김재덕

한국과학기술연구원 환경공정연구부

Phase Behavior of Refrigerant Mixtures with HFC-227ea

Ji-Young Park, Jong Sung Lim, Byung-Gwon Lee, Youn-Woo Lee, Jae-Duck Kim

Environment and Preocess Technology Division, KIST

Introduction

Hydrofluorocarbons (HFCs) have been considered as promising alternative refrigerants in place of chlorofluorocarbons (CFCs) since their ozone depletion potentials (ODP) and global warming potentials (GWP) are low.  The HFCs binary mixtures may have good potential for altermative refrigerants because of HFCs are stable and inflammable.  Vapor-liquid equilibrium 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)+1,1,2,2-tetrafluoroethane(HFC-134a), + 1,1-difluoroethane(HFC-152a).  The experimental data were correlated with Modified Huron-Vidal(MHV) mixing rules[1] combined with the Peng-Robinson

-Stryjek-Vera equation of state[1] and the NRTL excess free energy model[3].  In addition, the nonideality of HFC-227ea + HFC-134a, HFC-152a, and iso-butane was discussed by the excess gibbs energy.  The NRTL parameters obtained by these mixing rules and second virial coefficients and fugacity coefficients were presented.

Theory

The VLE experimental data were correlated with the Peng-Robinson-Stryjek-

Vera (PRSV) equation of state[1].  The □term in Peng-Robinson EOS was replaced by the temperature dependence □ term in PRSV EOS.  In this study, the modified Huron-Vidal(MHV) mixing rule of Michelsen[1] is used with PRSV EOS and expressed as follows ;

[eqn: { G_ gamma^ex } over {RT} =- SUM from { { i}}x_i ln LEFT ( b over b_i RIGHT ) +q_1 LEFT ( a over bRT - SUM from { i } x_i a_i over b_i RT RIGHT )],   [eqn: b= SUM from { i } x_i b_i]    [eqn: (1)]

[eqn: q_1 = C (V_mix^0 ) = C(V_i^0 )]                                     [eqn: (2)]

[eqn: C(V) = 1 OVER {2 SQRT {2 } } ln LEFT [ {V + (1- SQRT { 2})b } over {V + (1- SQRT { 2})b} RIGHT ]]                           [eqn: (3)]

where [eqn: a, b] are the EOS parameters in the mixture composition with activity coefficient or Gibbs free-energy models. These mixing rules were combined with [eqn: G_ gamma^ex](NRTL in this study).  In two systems, the excess gibbs energy were obtained by activity coefficient model to investigate the nonideality  The excess gibbs energy equation is used expressed as follows

[eqn: G_ gamma ^ex = RT(ln phi - SUM x_i ln phi_i )]                       [eqn: (4)]

where R is the gas constant, T is the absolute temperature, [eqn: phi] is the fugacity coefficient of the mixture and [eqn: phi_i] is the fugacity coefficient of the i component of mixture.

[eqn: ln phi_i =(2 SUM from { j }y_j B_ij - B) P OVER RT]                       [eqn: (5)]

[eqn: B=y_i^2 B_i + 2y_i y_j B_ij + y_j^2 B_j]                        [eqn: (6)]

where[eqn: B_i] and [eqn: B_j] are the pure second virial coefficients and [eqn: B_ij] is interaction virial coefficient.

Table 1. Characteristic properties of pure chemicals the k1, and interaction parameters in PRSV EOS with [eqn: G_ gamma^ex] model

Pure component properties
	iso-butane			HFC-134a		HFC-152a		HFC-227ea
Tc[K]	407.85			374.21		386.41		375.95
Pc[MPa]	3.640			4.059		4.517		2.980
Vc[□/mol]	259.001			199.316		179.483		293.155
Zc	0.278			0.260		0.252		0.279
□	0.185			0.326		0.275		0.363
k1 of PRSV (Range of T, K)	-0.1426 (303.15-323.15)			0.0678 (303.15-323.15)		-0.0150 (283.15-303.15)		0.1718 (283.15-303.15) 0.1394 (303.15-323.15)
Parameters in PRSV EOS with [eqn: G_ gamma^ex] model
		T/K	k12		□P(%)		NRTL parameter
HFC-227ea +HFC-134a		303.15 323.15	0.2743 -0.8980		0.312 0.148		□12= 3.320 □12= 1.354		□21=-1.842 □21= 2.611
HFC-227ea +HFC-134a		283.15 303.15	-0.0602 -0.6296		0.531 0.329		□12=-0.038 □12= 0.912		□21= 0.792 □21= 1.939

※ [eqn: DELTA P = I OVER N SUM { LEFT | P(exp)-P(cal)RIGHT | } over {P(exp) } TIMES 100]

Table 2. The fugacity coefficient and second virial coefficient

T/K	[eqn: phi _1]		[eqn: phi _2]		B11 (□□mol-1)		B22 (□□mol-1)		B12 (□□mol-1)
HFC-227ea(1) + HFC-134a(2)
303.15		0.977		0.931		-242.128		-180.618		-442.678
323.15		0.972		0.918		-200.201		-150.287		-375.776
HFC-227ea(1) + HFC-152a(2)
283.15		0.981		0.951		-161.138		-105.385		-574.768
303.15		0.977		0.940		-200.201		-129.472		-483.537

Results and discussion

The VLE of HFC-227ea/HFC-134a and HFC-227ea/HFC-152a systems were predicted using modified Huron-Vidal (MHV) mixing rules combined with NRTL excess free energy models and the PRSV equation of state (Figure 1~2.) The parameter, k1, of the pure components (iso-butane, HFC-227ea, HFC-134a, and HFC-152a) and all the binary parameters of the NRTL model with modified Huron-Vidal (MHV) mixing rules were listed in Table 1.  The modified Huron-Vidal (MHV) mixing rules combined with PRSV equation of state and an activity coefficient model(NRTL model) for the [eqn: G_ gamma^ex]x term show good correlations for the vapor-liquid equilibria of refrigerant mixtures within the uncertainty of ±0.5 %.  The deviation(%) of this system were listed in Table 1.  The excess gibbs energy[eqn: ( G_ gamma^ex )] of these mixtures were negative from Raoult's law in all system.  The values of [eqn: G_ gamma^ex](max) in HFCs mixtures and HFC-Hydrocarbon(iso-butane) mixture were 97.063J/mol(HFC-227ea+HFC-134a), 128.403J/mol, and 781.693J/mol(HFC-227ea+iso-butane).  The [eqn: G_ gamma^ex] and activity coefficient as a function of composition were shown in Figures 3~4, and second virial coefficients and fugacity coefficients were presented in Table 2.

In this result, the nonideality of HFC-227ea+iso-butane was higher than that of HFCs mixtures.  Both isothermal systems exhibit no azeotropes.

Literature Cited

1. H. Orbey., S. I. Sandler., Modeling Vapor-Liquid Equilibria-cubic equation

of state and their mixing rules, 19-74, (1998)

2. J. Y. Park, J. S. Lim, B. G. Lee, Y. Y. Lee, J. D. Lee, Prediction of vapor-liquid equilibria for refrigerant mixture, Theories and Applications of Chem. Eng., Vol. 5, No. 2 2977~2980, (1999)

3. Robert C. Reid, John M. Prausnitz, Bruce E. Poling, The properties of gases and liquids, fourth edition, McGRAW-HILL International edition

4. NIST, PEFPROP, Version 6.01, (1998)