HFCs와 Hydrocarbon 냉매 혼합물의 공비 거동

박지영, 임종성, 이병권, 이윤우, 김재덕

한국과학기술연구원 환경공정연구부

Azeotropic Behavior of HFCs and Hydrocarbon Refrigerant Mixtures

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

Environment and Process Technology Division, KIST

Introduction

Azeotropic mixtures are attractive as refrigerants because they behave very nearly as pure materials.  The method for predicting azeotropic composition and pressure was very useful and valuable.  In this study, we derived a simple equation from the fundamental  equation of equilibria and activity coefficient model (NRTL model) for predicting azeotropic composition and pressure. The VLE data for the binary mixtures of hydrocarbon (iso-butane) and HFCs (HFC-32, HFC-134a, HFC-152a, and HFC-227ea) were used to test this equation. These mixtures were selected as a promising alternative refrigerants in place of chlorofluorocarbons (CFCs) because of their low ozone depletion potentials (ODP) and global warming potentials (GWP).  HFCs are stable but expensive, on the other hand, hydrocarbons are flammable but very cheap. The mixtures of these two compounds may have good potential for alternative refrigerants.

In this work, we presented activity coefficient model parameters and the deviation between calculated and experimental values of azeotropic compositions and pressures.

Theory

If a gaseous phase and liquid phase are in equilibrium, the fundamental equation of phase equilibria can be expressed as follow;

[eqn: Py_i PHI_i = x_i gamma _i P_i ^sat]                            [eqn: (1)]

where [eqn: P_i^sat] is the saturated vapor pressure, [eqn: gamma _i] is the a activity coefficient and [eqn: PHI _i] is the fugacity coefficient of i.

We assumed that the vapor phase behaves ideally

[eqn: P = y_1 P + y_2 P= x_1 P_1 ^sat gamma _1 + x_2 P_2 ^sat gamma _2]                    [eqn: (2)]

and, NRTL model was used to obtain activity coefficients.

[eqn: ln gamma_1 = x_2 ^2 [□_21 ( G_21 OVER { x_1 + x_2 G_21})^2 + { □_21 G_12 } OVER { (x_2 + x_1 G_12 )^2 } ]]                        [eqn: (3)]

[eqn: ln gamma _2 = x_1 ^2 [□_12 ( G_12 OVER { x_2 + x_1 G_12})^2 + { □_21 G_21 } OVER { (x_1 + x_2 G_21 )^2 } ]]                        [eqn: (4)]

[eqn: gamma_1 = 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: (5)]    [eqn: gamma _2 = 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 ]]                                  [eqn: (6)]

[eqn: □_12 = { g_ 12 -g _21 } OVER RT]                              [eqn: (7)]

[eqn: G_12 = exp(-□_12 □_12 )]                         [eqn: (8)]

Where [eqn: g_12, g_21] are the parameters of interaction between component 1 and 2, and[eqn: □_12] is nonrandomness parameter and is set 0.3.

The following equation (9) is obtained by combining equation (2), (5) and (6).

[eqn: P=x_1 P_1^sat gamma_1 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-x_1 ) P_1 ^sat gamma _2 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 ]] [eqn: (9)]

for azeotropic composition we have

[eqn: dP OVER dx = 0]                                    [eqn: (10)]

The azeotropic compositions and pressures can be obtained from equations (9) and (10).

Experimental section

The experimental equipment used to measure the isothermal VLE in this work is a circulation-type apparatus in which both phases were continuously recirculated. The explanation of the experimental apparatus and the procedure has been reported in our previous publications[3]. Experiments for a binary

Table 1. The NRTL parameters for the [eqn: G_ gamma^ex] calculation

	T/K	□12	□21	G12	G21
HFC-32+iso-butane [2]	301.75 321.75	1.9340 1.8430	0.0715 -0.0627	0.5597 0.5752	0.9787 1.0189
HFC-134a+iso-butane [2]	303.15 323.15	1.6902 1.5905	0.1056 0.0017	0.6022 0.6205	0.9688 0.9994
HFC-152a+iso-butane [2]	293.15 303.15 313.15 323.15 333.15	1.6508 1.5532 1.2364 1.7638 1.6007	-0.0744 -0.1033 0.1017 -0.3587 -0.3138	0.6094 0.6275 0.6900 0.5891 0.6186	1.0225 1.0314 0.9699 1.1136 1.0987
HFC-227ea+iso-butane [3]	303.15 313.15 333.15	1.2587 1.3957 0.9409	0.2136 0.0290 0.3400	0.6854 0.6578 0.7540	0.9379 0.9913 0.9030

※ the unit of □12,□12, G12, and G12 is dimensionless

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 iso-butane was introduced into the cell, and then the temperature of the

entire system was maintained constant by controlling the temperature of the

air bath. After the desired temperature was attained, the pressure of the pure component was measured. A proper amount of HFC compound was supplied to the cell from a charging cylinder. Both vapor and liquid phases were allowed to circulate for about an hour to ensure equilibrium. When the equilibrium was attained, pressure was measured and then the composition of the vapor and liquid samples were obtained by immediately injecting them into the gas chromatography connected on-line to both vapor and liquid sampling valves.

Results and discussion

We derived the simple equation to calculate the azeotropic compositions and pressures from the basic equation of equilibria and activity coefficient model.  Using this equation, we calculated the azeotropic compositions and pressures in four systems ; HFC-32 + iso-butane, HFC-134a + iso-butane, HFC-152a + iso-butane, and HFC-227ea + iso-butane.  The experimental and calculated azeotropic compositions and pressures were given in Table 2. The values calculated with this equation showed some deviations from experimental ones.  We considered that these deviations come from the assumption of ideal solution. In this method, the derived equation have some limitations because the parameters of activity coefficient model were obtained by applying  experimental data.  However, this method could be extrapolated to other temperature and used to calculate azeotropic points for similar binary azeotropic mixtures.

Table 2. Azeotropic compositions

	T/K	Azeotropic compositions
		xexp	xcal	Pexp /MPa	Pcal /MPa	□x	□P×100
HFC-32 + iso-Butane [2]	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 + iso-Butane [2]	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 + iso-Butane [2]	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 + iso-Butane [3]	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

[eqn: DELTA P = { LEFT | P_exp - P_cal RIGHT | } over {P_exp }],  [eqn: DELTA x= { LEFT | x_exp - x_cal RIGHT | }]

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