혼합냉매의 상평형 및 냉동성능 연구

박지영, 이병권, 임종성

한국과학기술연구원 CFC 대체연구센터

A study of vapor-liquid equilibria and refrigeration performance

of refrigerant mixtures

Ji-Young Park, Byung Gwon Lee, Jong Sung Lim

CFC Alternatives research center, KIST

Introduction

Production and use of CFCs have been regulated under international agreement, Montreal protocol, because of their high ozone-depleting potentials. HCFCs and HFCs have been suggested as strong candidates for the replace- ment of CFCs. However, HCFCs were also revealed to have significantly high ozone depleting potentials. Thus, it has been finally decided in the Vienna Meeting of 1995 that HCFCs also have to be phase out by 2030 in the developed countries and by 2040 in the developing countries, respectively.

As a result, it became a very urgent issues to search and develop CFC and HCFC alternatives. This question has been recognized as well at home and the research about CFC alternatives has been performed by CFC alternatives research center in KIST since 1990.  HCFC-22 has been widely used as refrigerant in the room air conditioner. While HFC-134a was found to be a very ideal alternative refrigerant of CFC-12, HCFC-22 alternative has not been unfortunately found yet. It is generally accepted that the only choice to replace HCFC-22 is to mix two or more refrigerants together so that the mixture can bear the similar performance to HCFC-22. In general, the performance of refrigerant is determined  by COP (Coefficient of Performance), VCR (Volumetric Capacity of Refrigeration), the pressure of evaporator and condenser. 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 of twenty six binary mixtures composed of eight well known HFCs and HC refrigerant (HFC-23,  32, 125, 134a, 143a, 152a, 227ea, and R-600a). These VLE data were used in the procedure for  developing HCFC-22 alternatives, such as prior thermodynamic simulation for selecting candidate mixtures and performance evaluation of candidate refrigerant mixtures to determine final HCFC-22 alternatives. And, the thermodynamic studies such as data correlation using four types of cubic equation of state with three mixing rules and the calculation of azeotropic points were performed. From these research, we could suggest several systems of refrigerant mixtures for HCFC-22 alternatives. These results were expected to be used as important technical information in the refrigerant industry.

Selection of various refrigerants candidates HFC-23, HFC-32, HFC-125, HFC-134a, HFC-143a, HFC-152a, HFC-227ea, R-600a
↓ ↓
Development of VLE Measurement apparatus		Development of refrigeration cycle simulator for estimation of refrigerant
↓		↓
Measurement of VLE for refrigerant mixtures	→	Estimation of COP, VCR, ... for binary, ternary, quaternary systems
↓		↓
Thermodynamic studies		Selection of refrigerant mixtures
Correlation from EOS ; □1, □12, □12, □12, . . Types of P-x-y diagram ; Azeotropes, zeotropes, near azeotropes		↓
		Performance evaluation of selected refrigerant mixtures
		↓
		Selection of CFC alternative refrigerant mixtures
Figure. 1. Procedure to develop CFC alternative refrigerant mixtures

Research content

Selection of single components composing refrigerant mixtures and data collection : Eight well known HFCs and HC refrigerant whose ODP is zero were chosen for developing the HCFC-22 alternatives.

Manufacture of VLE apparatus and VLE measurement : VLE of twenty six binary systems were measured and studied thermodynamically.

1) Correlation : The cubic EOS and mixing rules used for correlation are:

< Equation of state >	< Mixing rules >
Carnahan Staring De Santis (CSD) [1] Peng Robinson (PR) [2] Peng Robinson Stryjek Vera (PRSV) [3] Soave Redlich Kwong (SRK) [4]	Van der waals mixing rule [3] Wong Sandler mixing rule [5] Huron Vial mixing rule [3]

2) Modeling of equation for azeotropic point calculation : Azeotropes areattrac -tive as refrigerants because they behave just like pure materials. To predict azeotropic composition and pressure, we derived a simple equation from the fundamental equation of equilibria and NRTL model.

[eqn: P=x_1 P_1^sat exp LEFT ( (1-x_1 )^2 LEFT [ □_21 LEFT ( G_21 OVER { x_1 + (1-x_1 ) G_21}RIGHT )^2 + { □_21 G_12 } OVER { ((1-x_1 ) + x_1 G_12 )^2 } RIGHT ] RIGHT )]

[eqn: (1)][eqn: + (1-x_1 ) P_1 ^sat exp x_1 ^2 LEFT [ □_12 LEFT ( G_12 OVER { (1- x_1 ) + x_1 G_12}RIGHT ) ^2 + { □_21 G_21 } OVER { (x_1 + (1- x_1 ) G_21 )^2 } RIGHT ]]

for azeotropic composition we have

[eqn: dP OVER dx = 0]	[eqn: (2)]

Thermodynamic simulation of refrigerant : The candidate refrigerant mixtures for alternating HCFC-22 were selected through prior thermodynamic simulation.

Performance evaluation of various refrigerant mixtures : Performance evaluation of binary, ternary, quaternary systems was performed by calorimeter.

	iso | b u t a n e	H F C | 23	H F C | 32	H F C | 125	H F C | 134 a	H F C | 143 a	H F C | 152 a	H F C | 227 ea
		○	○	●	○	○	○	●	isobutane
			●	●	○	○	○	○	HFC-23
				◎	◎	◎	◎	○	HFC-32
						●	●	○	HFC-125
	◎ CSD					◆		○	HFC-134a
	◆ SRK						◆	○	HFC-143a
	○ PR							○	HFC-152a
	● PRSV								HFC-227ea
Figure 3. The application of four EOS to twenty six binary systems

Suggestion for HCFC-22 substitutes : HCFC-22 alternatives were selected finally by comparing the refrigeration performance of candidate refrigerant mixtures with that of HCFC-22.

iso-butane						Zeotropes
HFC-23						Azeotropes
HFC-32						Near azeotropes
HFC-125
HFC-134a
HFC-143a
HFC-152a
HFC-227ea
	iso | b u t a n e	H F C | 23	H F C | 32	H F C | 125	H F C | 134 a	H F C | 143 a	H F C | 152 a	H F C | 227 ea
Figure. 2. Types of phase behavior of twenty six binary systems

Results and Discussion

Figure 1. shows the procedure for developing CFC alternative refrigerant mixture. In this procedure, the simulator predict theoretically the performance of all given refrigerant mixtures to select candidate mixtures. The measurement of vapor liquid equilibria informs more exact thermodynamic data in predicting the performance of refrigerant mixtures. The VLE of twenty six binary systems composed of eight refrigerants were measured. The types of phase behavior of twenty six binary systems were expressed in Figure 2. Among these mixtures, four mixtures formed azeotropes, six mixtures formed near azeotropes, and the others formed zeotropes. All VLE data were correlated with four equation of state, CSD, SRK, PR, PRSV EOS with three mixing rules. Almost all the calculated values using these models give good agreement with the experimental data. The most suitable equation of state to correlate them were shown in Figure 3. Table 1. compares the measured azeotropic point with calculated values by using the simple equations derived in this work. As can be seen in Table 1, they show relatively good agreement with each other within 0.06 mole fraction.

Evaluations of refrigeration performance for the candidate refrigerant mixtures were performed. From the result of this study, we found HCFC-22 alternatives having similar or better performance comparing with HCFC-22. They are the systems of  R-32/134a/600a (41.8/47.3/10.9 wt%), R-32/125/143a/ 600 (45/10/40/5wt%), R-32/125/134a/152a (66.2/19/9.8/5wt%), and R-32/143a/ 152a/227ea (30/21/39/10wt%), etc. COP and VCR values of these new systems were compared with HCFC-22 and shown in Figure 4. These research are expected to be used as important technical information in the refrigerant industry and the detailed study on the development of HCFC-22 alternatives is now in progress.

Figure 4. Results of refrigerant performance evaluation, (a)COP, (b)VCR

1 : HCFC-22, 2 : R-32/134a/600a=41.8/47.3/10.9 wt%, 3 : R-32/125/143a/600=45/10/40/5 wt%

4 : R-32/125/134a/152a=66.2/19/9.8/5 wt%, 5 : R-32/143a/152a/227ea=30/21/39/10 wt%

Table 1. Azeotropic composition

	T/K	Azeotropic composition
		xexp	xcal	Pexp/MPa	Pcal/MPa	△xa	|△P/P|(%)b
HFC-32/isobutane [6]	301.75 321.75	0.912 0.934	0.932 0.957	1.920 3.080	1.885 3.057	0.020 0.023	1.822 0.746
HFC-134a/isobutane [6]	303.15 323.15	0.745 0.762	0.755 0.779	0.882 1.448	0.877 1.451	0.010 0.017	0.566 0.207
HFC-152a/isobutane [6]	293.15 303.15 313.15 323.15 333.15	0.791 0.792 0.794 0.796 0.798	0.751 0.763 0.755 0.797 0.807	0.561 0.758 0.988 1.266 1.601	0.578 0.762 0.997 1.270 1.599	0.040 0.029 0.039 0.001 0.009	3.030 0.527 0.910 0.316 0.124
HFC-227ea/isobutane [7]	303.15 313.15 323.15	0.599 0.608 0.614	0.642 0.665 0.649	0.665 0.845 1.081	0.646 0.841 1.081	0.043 0.057 0.035	2.857 0.473 0.000

a△x=|xexp-xcal|, b|△P/P|(%)=|(Pexp-Pcal)|/Pexp|×100

Literature Cited

[1] G. Morrison, M. McLinden, Application of a Hard Sphere Equation of State to Refrigerants and Refrigerant Mixtures, National Bureau of Standards Technical Note 1226, (1986) 1-11

[2] D. Y. Peng and D. B. Robinson, Int. Eng. Chem. Fundam., 15 (1976) 59.

[3] H. Orbey, and S. I. Sandler, Modeling Vapor-Liquid Equilibria-cubic equation of state and their mixing rules, Cambridge University Press, 1998.

[4] Smith, J. M., Van Ness, H. C., Introduction to chemical engineering thermodynamics,

3rd., McGraw-Hill, New York, (1975).

[5] D. S. H. Wong and S. I. Sandler, AIChE J. 38: 671 (1992).

[6] J. S. Lim, J. Y. Park, B. G. Lee, Y. W. Lee, J. Chem. Eng. Data, 44 (1999) 1226.

[7] J. S. Lim, J. Y. Park, B. G. Lee, J. D. Kim, J. Chem. Eng. Data, 45 (2000) 734.