Isothermal Vapor–Liquid Equilibrium Measurements of 1,1

Article pubs.acs.org/jced

Isothermal Vapor−Liquid Equilibrium Measurements of 1,1-Difluoroethane + N,N-Dimethylformamide and N,N-Dimethylacetamide Xuelin Meng,†,‡ Danxing Zheng,*,† Xinru Li,† and Yanshu Shen† †

College of Chemical Engineering, Beijing University of Chemical Technology, Beijing, 100029, China Guangxi Vocational & Technical Institute of Industry, Nanning, 530001, China

‡

ABSTRACT: Vapor−liquid equilibrium (VLE) data of working fluids play a vital role in the research and development of absorption refrigeration technology. In this paper, the VLE data of two new working fluids, 1,1-difluoroethane (R152a) + N,N-dimethylformamide (DMF) and R152a + N,N-dimethylacetamide (DMAC), were measured by the static analytical method at temperatures from (293.15 to 353.15) K. The experimental results show that R152a exhibits better solubility characteristics with DMF and DMAC, and the solubility of R152a in DMAC is slightly better than that in DMF. All experimental data were correlated by the activity coefficient model of five-parameter nonrandom two liquid (NRTL). The calculated results are in essential agreement with the experimental data, and the maximum relative deviation of pressure are 3.69 % and 3.51 %, the average relative deviation of pressure are 1.60 % and 1.11 %, for binary systems R152a + DMF and R152a + DMAC, respectively.

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potential (GWP) is slightly higher (GWP = 2800).17 R134a is also an ozone-safe refrigerant and is the leading replacement of R12 and R22 used in refrigeration systems. For the absorption refrigeration cycle, the performance of the working fluids R134a + absorbents had been tested by many researchers.9−12 As a refrigerant, R152a has zero ODP and it is especially advantage is that the global warming potential is very low (GWP = 140). Compared to R134a, R152a has similar characteristics of pressure levels and volumetric cooling capacity, while the mass flow, the energy efficiency, and the density of vapor are even more favorable. Therefore, it can be used in refrigeration systema as the replacement of R134a.17 Jelinek et al.15 have studied the cycle performance of different working fluids, using DMEU as absorbent and five refrigerants (R22, R32, R124, R125, and R152a) in a absorption refrigeration system, and the results showed that the cycle performance of R152a + DMEU is superior to R134a + DMEU when the generation temperature is higher than 100 °C. The absorbents DMF and DMAC are commonly used organic solvents. Moreover, in the solution with halogenated hydrocarbons, DMF and DMAC have a very low partial pressure. Compared to the solvents of DMEDEG, DMETEG, and DMEU, DMF and DMAC have the advantages of significantly lower viscosity, strong absorption capacity for halogenated hydrocarbons, and considerable lower price.18 Therefore, the combination of R152a with DMF and DMAC as working fluids used in an absorption refrigeration system seems to be worthy as a further research topic.

INTRODUCTION The absorption refrigeration cycle powered by low-grade heat has provided an effective way for fuel saving, rational use of existing energy sources, recycling of industrial waste heat, and reducing the environmental pollution. Therefore, the absorption refrigeration technology has attracted the attention of more and more researchers in recent years.1 For the application of the absorption refrigeration technology, the working fluids selected are very important. At present, the most common working fluids are NH3 + H2O and H2O + LiBr. However, the demerit of these two working fluids limits their application in the refrigeration system. Because the separating energy consumption is relatively higher, a slight toxicity exists when NH3 + H2O is used as working fluid. Additionally H2O + LiBr system crystallizes at moderate concentrations, and refrigerating temperatures cannot be below 0 °C. Therefore, in recent decades more and more researchers have paid a great deal of attention to developing new working fluids. Practical needs have made novel absorption working fluid development a focus of attention. For the refrigerants, they mostly are chlorodifuoromethane (R22),2−5 chlorotetrafluoroethane (R124),6 pentafluoroethane (R125),7,8 and 1,1,1,2-tetrafluoroethane (R134a),9−12 etc. For the absorbents, they mostly are N,Ndimethylformamide (DMF),2−4,13,14 N,N-dimethylacetamide (DMAC),10−12 dimethylether tetraethyleneglycol (DMETEG),3,4 dimethylether diethyleneglycol (DMEDEG),4,5 and N,N′-dimethylethylenurea (DMEU),15 etc. Because they contain the element chlorine, the refrigerants R22 and R124 have been banned for use in refrigeration systems all over the world in 2010.16 R125 is a hydrofluorocarbons (HFCs) refrigerant and has zero ozone depression potential (ODP), but the global warming © 2013 American Chemical Society

Received: August 5, 2012 Accepted: April 2, 2013 Published: April 16, 2013 1078

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Table 1. Specifications of the Chemicals Used chemical name R152a DMF DMAC carbon dioxide (CO2) ethanol (C2H5OH)

source Shandong Dongyue Fluorine & Silicon Material Co., Ltd. Sigma-Aldrich Co., Ltd. Sigma-Aldrich Co., Ltd. Zhaoge Gas Co., Ltd. Sinopharm Chemical Reagent Co., Ltd.

mass fraction purity

CAS No.

≥ 99.98 %

75-37-6

≥ 99.9 % ≥ 99.9 % ≥ 99.9 %

68-12-2 127-19-5 124-38-9

≥ 99.8 %

64-17-5

Article

EXPERIMENTAL SECTION

Materials. The refrigerant R152a (≥ 99.98 %, mass fraction) was purchased from Shandong Dongyue Fluorine & Silicon Material Co., Ltd. The absorbents DMF (≥ 99.9 %, mass fraction) and DMAC (≥ 99.9 %, mass fraction) were supplied by Sigma-Aldrich Co., Ltd. The specifications of all chemicals used are summarized in Table 1. Apparatus. In this study, the VLE data for R152a + DMF and R152a + DMAC binary systems were measured using a static analytical method. The experimental apparatus is illustrated in Figure 1, and the detailed information has been described in the earlier papers.30−33 The apparatus consists of five systems: equilibrium system, temperature controlling system, injection and detection systems, data acquisition system, and vacuum system. And the main units are an equilibrium cell, an isothermal water bath, a temperature controller, a pressure transducer, six-way valves, a gas chromatography, a vacuum pump, a magnetism mixer, and a refrigerator. The detail description of each unit present follows. The equilibrium cell was designed and processed by ourselves, the material was stainless steel, and the inner volume was about 160 cm3. The temperature controlling system includes two parts, the isothermal water bath and the equilibrium cell, and was controlled by a temperature controller (model LC-6, Julabo Co.) through the Pt-100 resistance temperature probe. The stability is ± 0.03 K. The pressure transducer (model PTX7533, GE Co.) was used to measure the pressure of the equilibrium cell. The maximum scale of the pressure transducer is 6 MPa. The liquid six-way valves (Valco model C1) was used to control the liquid injection volume. The gas chromatography (GC, model GC-9A) was used to analyze the equilibrium composition of binary mixture online. The function of vacuum pump is to exclude the air inside the system and then improve the accuracy of the experiment. To accelerate the equilibrium, the magnetism stirrer was used in the experiment system. The refrigerator (Eyela Co.) was used to control the equilibrium temperature when the experiment was carried out at low temperature. In this study, the thermal conductivity detector (TCD) was used and the operation conditions of GC were as follows: column temperature, 160 °C; injection temperature,160 °C; detector temperature, 180 °C; and detector current, 140 mA.

However, vapor−liquid equilibrium (VLE) data are the foundation of developing new working fluids. In the literature, the published VLE data mostly concern the working fluids of R134a + absorbent systems, rather than R152a + absorbent systems. For example, the VLE data concerning the systems of R134a + DMF, R134a + DMEDEG, and R134a + DMETrEG were studied by Zehioua et al.19,20 using the static method at temperatures from (303.30 to 353.24) K. Coronas et al.21 reported the solubility of R134a in DMETrEG at temperatures from (283.15 to 353.15) K. Lopez et al.22 presented the experimental solubility of R134a in DMETrEG and DMETEG at 101.33 kPa between (258.15 and 298.15) K. Han et al.23 measured the solubility of R134a in DMF from (263.15 to 363.15) K. Moreover, Agarwal and Bapat24 studied and analyzed comprehensively the isothermal VLE data of R22 + DMF from (248.15 to 393.15) K. He et al.25 and Han et al.26 measured the isothermal VLE data of R22 + DMF and R32 + DMF from (283.15 to 363.15) K, respectively. For the refrigerant R152a, Wahlstrom and Vamling27,28 have measured the solubility of R152a in n-eicosane, n-hexadecane, n-tridecane, and 2,6,10,14-tetramethylpentadecane at different temperatures. Yelisetty and Visco29 have reported the solubility of R152a in Pluracol 975, Pluracol 355, and Terol 352. However, the VLE data of R152a + DMF and R152a + DMAC are not available, limiting the author’s literature works. This paper is aimed at measuring the VLE data for two binary systems, R152a + DMF and R152a + DMAC, from (293.15 to 353.15) K using a static analytical method, and correlating the experimental data by the NRTL model to investigate the possibility that these systems be used as alternative working fluids.

Figure 1. Schematic diagram of the VLE experimental apparatus: 1, gas cylinder; 2, gas storage tank; 3, pressure monitor; 4, temperature controller; 5, isothermal water bath; 6, equilibrium cell; 7, stirrer; 8, magnetism mixer; 9, liquid injector; 10, refrigerator; 11, cushion tank; 12, vacuum pump; 13, gas chromatography; 14, six-way valve; 15, decompression valve; 16, computer. 1079

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Table 2. Comparison with Literature34 of VLE Data of CO2 + C2H5OH at 293.15 K and 303.15 Ka T = 293.15 K pexp/MPa xexp plit /MPa xlit a

0.7163 0.0530 0.68 0.0497

T = 303.15 K

1.6429 0.1273 1.67 0.1315

2.169 0.1848 2.14 0.1692

0.9306 0.0622 0.91 0.0596

1.9703 0.1282 2.03 0.1362

2.9035 0.2061 2.89 0.2014

Standard uncertainties u are u (T) = T ± 0.072 K and u(p) = p ± 0.0006 MPa.

Hydrogen was used as a carrier gas, and the flow rate was 50 mL/min. The chromatographic column used was a GDX-102. Experimental Procedures. To complete the experiment and obtain the experimental data, a series of experimental procedure must be carried out. Checking the airtightness of the apparatus is very important for the experiment because the next experimental step can not be done until the airtightness is confirmed. Then the system is evacuated to ensure that the air inside the system is thoroughly exhausted. After the system has been evacuated, a specified quantity (60 mL to 80 mL) of absorbent (DMF or DMAC) is loaded into the equilibrium cell from the liquid injector, and then the refrigerant R152a is also loaded into the equilibrium cell to a desired pressure. Next, controlling the temperature of the isothermal water bath makes sure that the temperature of the equilibrium cell is equal to the experimental temperature. If the pressure and temperature of the equilibrium cell fluctuate near a certain value for more than 30 min, it was believed that equilibrium was reached. Then the temperature and pressure of the equilibrium cell were recorded and the compositions of the liquid sample were analyzed online by GC. With the changing of the experimental pressure and temperature, a set of isothermal experiment data were obtained. Apparatus Reliability Validation. To validate the reliability of the experimental apparatus used in this work, the VLE data of the CO2 + C2H5OH system at 293.15 K and 303.15 K were measured. The results are shown compared with the available data34 in Table 2, where pexp and plit denote the pressures of the experiment and the literature, respectively, and xexp and xlit indicate the liquid mole fractions of CO2 of the experiment and the literature, respectively. The comparison between the results calculated by the literature data using the interpolation method and the experimental data shows that the experimental data and literature data are in good agreement, and the average relative deviation of the pressure was 1.66 %. This means that the experimental device has acceptable reliability and can be used to measure the data. Uncertainty Analysis. In this experimental system, the uncertainties are mainly caused by the measurement of temperature (T), pressure (p), and composition (x). The temperature controlled system in the experiment is a temperature controller (model LC-6, Julabo Co.) with a Pt-100 resistance temperature probe. The stability is ± 0.03 °C, the resolution of temperature monitor is 0.01 °C, and the measurement error of Pt-100 resistance temperature probe is ± (0.10 + 0.0017|t|) °C. So the temperature uncertainties can be calculated as follows:35,36 u1(t ) = 0.03/ 3 = 0.0017 °C

u 2(t ) = 0.5 × 0.01/ 3 = 0.0029 °C

and then the combined standard uncertainty of temperature is, 3

uc(t ) =

= 0.059 °C (4)

i=1

Therefore, on range of the experimental temperature, the average combined standard uncertainty of temperature is 0.072 °C, i.e., u (T) = T ± 0.072 K. The uncertainties of pressure measurement are same as that of temperature. In the experiment, the pressure transducer (model PTX7533, GE Co.) was used to measure the pressure, and the maximum scale and measurement error are 6 MPa and 0.025 %, respectively. The resolution of pressure monitor is 0.0001 MPa. So the pressure uncertainties can be calculated as follows:35,36 u1(p) = 0.025% × 6 MPa/2.58 = 0.0006 MPa

(5)

u 2(p) = 0.0001 MPa × 0.5/ 3 = 0.00003 MPa

(6)

2

uc(p) =

∑ ui2(p)

= 0.0006 MPa (7)

i=1

Therefore, the combined standard uncertainty of pressure is 0.0006 MPa; that is, u(p) = p ± 0.0006 MPa. The gas chromatography is used to analyze the liquid composition, and the quantitative analysis method is the corrected area normalization method. The uncertainties of composition measurement are mainly caused by the stability of the carrier gas flow rate, the baseline noise, the baseline wander, and the repeatability of the TCD measurement. The technical parameters of the gas chromatograph (model GC-9A) used in the experiment are the carrier gas flow rate is 1 %, the baseline noise is ≤ 0.1 mV, the baseline wander is ≤ 0.2 mV and the repeatability of the TCD measurement is ≤ 3 %. For the corrected area normalization method, the effect of the stability of the carrier gas flow rate is very small, so the uncertainty contribution by the stability of the carrier gas flow rate is negligible. Usually, the chromatography workstation baseline is stable at 30 mV, and the uncertainty contribution by the baseline noise and the baseline wander are 0.33 % and 0.67 %, respectively. The uncertainty calculation of the repeatability of the TCD measurement can use the same method of the temperature monitor resolution, and then the Table 3. Parameters of the NRTL Model in this Work values

(1) (2)

u3(t ) = (0.10 + 0.0017 × 30 °C)/2.58 = 0.059 °C (take the case of t = 30 °C)

∑ ui2(t )

(3) 1080

parameter

R152a(1) + DMF(2)

R152a(1) + DMAC(2)

α a12 b12 a21 b21

1.54 59294.75 −9961.76 56288.30 −5330.65

1.72 2918.02 −350.26 21444.17 −3786.88

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Table 4. Experimental and Calculated VLE Data for R152a (1) + DMF (2)a T/K

x1

pexp/MPa

pcal/MPa

δp/%b

u(x1)a

T/K

x1

pexp/MPa

pcal/MPa

δp/%b

u(x1)a

293.15 293.15 293.15 293.15 293.15 293.15 293.15 293.15 293.15 303.15 303.15 303.15 303.15 303.15 303.15 303.15 303.15 303.15 313.15 313.15 313.15 313.15 313.15 313.15 313.15 313.15 313.15 323.15 323.15 323.15 323.15 323.15 323.15 323.15

0.1689 0.2595 0.3769 0.4664 0.5726 0.6900 0.7795 0.8801 0.9305 0.1605 0.2687 0.3538 0.4366 0.4830 0.5914 0.6895 0.7639 0.8879 0.1382 0.2043 0.2797 0.3846 0.4893 0.6045 0.7197 0.7853 0.8973 0.1283 0.1981 0.2773 0.4158 0.5165 0.5859 0.6905

0.1084 0.1652 0.2356 0.2822 0.3340 0.3868 0.4238 0.4722 0.4971 0.1402 0.2303 0.2949 0.3535 0.3878 0.4541 0.5110 0.5542 0.6307 0.1548 0.2253 0.3105 0.4082 0.5105 0.6004 0.6904 0.7433 0.8370 0.1826 0.2767 0.3828 0.5485 0.6657 0.7522 0.8483

0.1045 0.1591 0.2277 0.2780 0.3346 0.3920 0.4308 0.4684 0.4855 0.1356 0.2237 0.2904 0.3529 0.3867 0.4615 0.5233 0.5659 0.6292 0.1557 0.2275 0.3071 0.4132 0.5128 0.6140 0.7056 0.7534 0.8308 0.1777 0.2705 0.3721 0.5396 0.6523 0.7255 0.8286

3.60 3.69 3.35 1.49 0.18 1.34 1.65 0.80 2.33 3.28 2.87 1.53 0.17 0.28 1.63 2.41 2.11 0.24 0.58 0.98 1.10 1.22 0.45 2.27 2.20 1.36 0.74 2.68 2.24 2.80 1.62 2.01 3.55 2.32

0.0034 0.0052 0.0075 0.0093 0.0115 0.0138 0.0156 0.0176 0.0186 0.0032 0.0054 0.0071 0.0087 0.0097 0.0118 0.0138 0.0153 0.0178 0.0028 0.0041 0.0056 0.0077 0.0098 0.0121 0.0144 0.0157 0.0179 0.0026 0.0040 0.0055 0.0083 0.0103 0.0117 0.0138

323.15 323.15 323.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 343.15 343.15 343.15 343.15 343.15 343.15 343.15 343.15 343.15 343.15 353.15 353.15 353.15 353.15 353.15 353.15 353.15 353.15 353.15 353.15

0.7726 0.8804 0.9314 0.1284 0.1920 0.3091 0.3777 0.4463 0.5211 0.5947 0.6516 0.7670 0.8691 0.9310 0.1251 0.1970 0.2656 0.3392 0.4345 0.5178 0.6182 0.6817 0.7419 0.8156 0.1064 0.1602 0.2406 0.2898 0.3344 0.4003 0.4617 0.5779 0.6632 0.8601

0.9347 1.0385 1.0816 0.2322 0.3408 0.5336 0.6423 0.7460 0.8549 0.9586 1.0326 1.1908 1.3143 1.3934 0.2866 0.4348 0.5781 0.7263 0.8993 1.0426 1.2303 1.3340 1.4329 1.5564 0.2976 0.4434 0.6474 0.7756 0.8898 1.0365 1.1834 1.4266 1.6196 2.0300

0.9044 1.0005 1.0472 0.2367 0.3487 0.5453 0.6546 0.7595 0.8687 0.9713 1.0475 1.1955 1.3237 1.4039 0.2842 0.4396 0.5826 0.7301 0.9126 1.0646 1.2396 1.3465 1.4461 1.5673 0.2925 0.4348 0.6406 0.7627 0.8709 1.0268 1.1682 1.4266 1.6106 2.0297

3.24 3.66 3.18 1.94 2.32 2.19 1.91 1.81 1.61 1.32 1.44 0.39 0.72 0.75 0.84 1.10 0.78 0.52 1.48 2.11 0.76 0.94 0.92 0.70 1.71 1.94 1.05 1.66 2.12 0.94 1.28 0.00 0.56 0.01

0.0155 0.0176 0.0186 0.0026 0.0038 0.0062 0.0076 0.0089 0.0104 0.0119 0.0130 0.0153 0.0174 0.0186 0.0025 0.0039 0.0053 0.0068 0.0087 0.0104 0.0124 0.0136 0.0148 0.0163 0.0021 0.0032 0.0048 0.0058 0.0067 0.0080 0.0092 0.0116 0.0133 0.0172

a Standard uncertainties u are u(T) = T ± 0.072 K and u(p) = p ± 0.0006 MPa. bThe relative deviation of pressure (δp): δp/% = (|pexpt − pcalc|/ pexpt)·100.

1 and 2 (absorbents DMF or DMAC). The parameters α, a12, b12, a21, and b21 in eqs 9 and 10 can be regressed by the experimental data. The objective function is

uncertainty contribution by the repeatability of the TCD measurement is 1.73 %, and finally, the combined standard uncertainty of composition measurement is less than 2.00 %.

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CORRELATION To correlate VLE data, several equations of state and activity coefficient models have been adopted by previous researchers.19−21,23−26 The VLE data of refrigerant + absorbent can be well correlated by the NRTL model.21,23,26 In this work, the five-parameter NRTL model was used, ⎡ τ g 2 ⎤ τ12g12 21 21 ⎥ ln γ1 = x 2 2⎢ + 2 2 ⎢⎣ (x1 + x 2g21) (x 2 + x1g12) ⎥⎦

N

F=

g21 = exp( − ατ21)

y1Φ1p = x1γ1p1s

(8)

τ21 =

a 21 + b21ln(T ) RT

(12)

where the parameter Φ1 is defined as (9)

Φ1 ≡ a12 + b12 ln(T ) RT

(11)

where N is the number of experimental points; pexpt and pcacl are the experimental pressure and the calculated pressure, respectively. For the binary system, the relation of the VLE can be described as follows:37

and τ12 =

− pcalc )i2

i=1

where g12 = exp( − ατ12)

∑ (pexpt

⎡ v L (p − p s ) ⎤ 1 ⎥ ⎢− 1 exp RT ϕ1s ⎢⎣ ⎥⎦ ϕ1̂

⎡ (B − v L)(p − p s ) ⎤ 1 1 1 ⎥ = exp⎢ RT ⎢⎣ ⎥⎦

(10)

where γ1 is the activity coefficient of species 1, refrigerant R152a and x1 and x2 are the liquid mole fractions of the species 1081

(13)

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Table 5. Experimental and Calculated VLE Data for R152a (1) + DMAC (2)a T/K

x1

pexp/MPa

pcal/MPa

δp/%b

u(x1)a

T/K

x1

pexp/MPa

pcal/MPa

δp/%b

u(x1)a

293.15 293.15 293.15 293.15 293.15 293.15 293.15 293.15 293.15 303.15 303.15 303.15 303.15 303.15 303.15 303.15 303.15 303.15 313.15 313.15 313.15 313.15 313.15 313.15 313.15 313.15 313.15 323.15 323.15 323.15 323.15 323.15 323.15

0.1832 0.2110 0.3086 0.4097 0.5223 0.6390 0.7626 0.8773 0.9649 0.1419 0.2242 0.3154 0.4103 0.5105 0.5916 0.7344 0.8605 0.9445 0.1194 0.2057 0.2905 0.3789 0.4797 0.5920 0.7158 0.8291 0.9373 0.1039 0.1653 0.2421 0.3229 0.4225 0.4926

0.1103 0.1248 0.1782 0.2308 0.2880 0.3436 0.4022 0.4542 0.4892 0.1114 0.1733 0.2388 0.3035 0.3709 0.4281 0.5220 0.5975 0.6460 0.1154 0.1939 0.2753 0.3584 0.4512 0.5518 0.6593 0.7548 0.8432 0.1187 0.1903 0.2829 0.3798 0.4985 0.5816

0.1080 0.1238 0.1779 0.2315 0.2884 0.3445 0.4015 0.4538 0.4953 0.1087 0.1703 0.2371 0.3046 0.3735 0.4276 0.5194 0.5987 0.6526 0.1160 0.1992 0.2801 0.3631 0.4559 0.5566 0.6643 0.7609 0.8536 0.1193 0.1900 0.2784 0.3712 0.4847 0.5637

2.09 0.80 0.17 0.30 0.14 0.26 0.17 0.09 1.25 2.42 1.73 0.71 0.36 0.70 0.12 0.50 0.20 1.02 0.52 2.73 1.74 1.31 1.04 0.87 0.76 0.81 1.23 0.51 0.16 1.59 2.26 2.77 3.08

0.0037 0.0042 0.0062 0.0082 0.0104 0.0128 0.0153 0.0175 0.0193 0.0028 0.0045 0.0063 0.0082 0.0102 0.0118 0.0147 0.0172 0.0189 0.0024 0.0041 0.0058 0.0076 0.0096 0.0118 0.0143 0.0166 0.0187 0.0021 0.0033 0.0048 0.0065 0.0085 0.0099

323.15 323.15 323.15 323.15 323.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 343.15 343.15 343.15 343.15 343.15 343.15 343.15 343.15 343.15 353.15 353.15 353.15 353.15 353.15 353.15 353.15 353.15 353.15

0.6054 0.6972 0.8049 0.8503 0.9656 0.1489 0.2158 0.2681 0.3619 0.4395 0.5199 0.5905 0.6509 0.7658 0.9421 0.1200 0.1934 0.2636 0.3492 0.4416 0.5398 0.6455 0.7669 0.8316 0.1251 0.1970 0.2831 0.3686 0.4723 0.5333 0.6179 0.7034 0.7896

0.7144 0.8100 0.9250 0.9688 1.1010 0.2174 0.3129 0.3943 0.5389 0.6576 0.7785 0.8834 0.9722 1.1381 1.3978 0.2188 0.3492 0.4839 0.6468 0.8208 1.0061 1.2038 1.4299 1.5499 0.2814 0.4294 0.6270 0.8298 1.0773 1.2248 1.4265 1.6299 1.8289

0.6893 0.7896 0.9057 0.9544 1.0789 0.2222 0.3231 0.4024 0.5449 0.6629 0.7850 0.8918 0.9826 1.1541 1.4147 0.2173 0.3525 0.4832 0.6444 0.8200 1.0080 1.2110 1.4437 1.5672 0.2729 0.4334 0.6290 0.8268 1.0710 1.2166 1.4204 1.6279 1.8376

3.51 2.52 2.09 1.49 2.01 2.21 3.26 2.05 1.11 0.81 0.83 0.95 1.07 1.41 1.21 0.69 0.95 0.14 0.37 0.10 0.19 0.60 0.97 1.12 3.02 0.93 0.32 0.36 0.58 0.67 0.43 0.12 0.48

0.0121 0.0139 0.0161 0.0170 0.0193 0.0030 0.0043 0.0054 0.0072 0.0088 0.0104 0.0118 0.0130 0.0153 0.0188 0.0024 0.0039 0.0053 0.0070 0.0088 0.0108 0.0129 0.0153 0.0166 0.0025 0.0039 0.0057 0.0074 0.0094 0.0107 0.0124 0.0141 0.0158

a Standard uncertainties u are u(T) = T ± 0.072 K and u(p) = p ± 0.0006 MPa. bThe relative deviation of pressure (δp): δp/% = (|pexpt − pcalc|/ pexpt)·100

Figure 2. Isothermal VLE data for R152a (1) + DMF (2): △, 353.15 K; ○, 343.15 K; □, 333.15 K; ▼, 323.15 K; ▲, 313.15K; ●, 303.15K; ■, 293.15 K; solid lines, calculated results using the NRTL model.

Figure 3. Isothermal VLE data for R152a (1) + DMAC(2): △, 353.15 K; ○, 343.15 K; □, 333.15 K; ▼, 323.15 K; ▲, 313.15K; ●, 303.15K; ■, 293.15 K; solid lines, calculated results using the NRTL model.

where p is the equilibrium pressure of the system; p1s is the saturated pressure of R152a; γ1, x1, and y1 are the activity coefficient, the liquid mole fraction and the vapor mole fraction of R152a, respectively; Φ1 is the modified coefficient of R152a, B1 is the second virial coefficient of R152a, V1L is the molar volume of the saturated liquid of R152a. In this study, the

values of p1s, B1, and V1L were cited from REFPROP.38 According to the calculated results, the values of Φ1 are almost equal to 1. It should be mentioned that the value of y1 is assumed equal to 1 in this paper, because the absorbents DMF and DMAC have a very low partial pressure in solution with R152a. Furthermore, Zehioua et al. and Han et al.23 have 1082

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Figure 4. Relative deviations of the pressure at temperatures from (293.15 to 353.15) K: ●, R152a (1) + DMF (2); ○, R152a (1) + DMAC.

Figure 7. Deviations of experimental results from Raoult’s law for R152a (1) + DMAC (2): △, 353.15 K; ○, 343.15 K; □, 333.15 K; ▼, 323.15 K; ▲, 313.15 K; ●, 303.15 K; ■, 293.15 K; solid lines, calculated results using the NRTL model.

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RESULTS AND DISCUSSION On the basis of the experimental VLE data measured at temperatures (293.15, 303.15, 313.15, 323.15, 333.15, 343.15, and 353.15) K for binary systems R152a + DMF and R152a + DMAC, the values of the parameters (α, a12, b12, a21, and b21) in the NRTL model were correlated, which are listed in Table 3. And the experimental VLE data and values estimated by the NRTL model are respectively illustrated in Tables 4 and 5. Figures 2 and 3 show the relation of the system pressure and the mole fraction of R152a (x1) at different experimental temperatures. Moreover, the relative deviations of pressure between the experimental data and the calculated values are described in Figure 4. In Figures 2 and 3, it can be seen that the pressure increases with the an increase of x1 at a given temperature. The x1 decreases with the increase of temperature. From Tables 4 and 5 and Figures 2 to 4, it was found that R152a exhibits better solubility characteristics with DMF and DMAC, and the results correlated by the NRTL model are in essential agreement with the experimental data. The maximum relative deviations of pressure are 3.69 % and 3.51 %, the average relative deviations of pressure are 1.60 % and 1.11 %, for binary systems R152a + DMF and R152a + DMAC, respectively. It indicates that the NRTL model can be used to express the VLE behaviors for binary systems R152a + DMF and R152a + DMAC. Figure 5 represents the comparison of the VLE data for these two binary systems at 303.15 K and 343.15 K. From Figure 5, it can be seen that the solubility of R152a in DMAC is slightly larger than that in DMF at the same temperature and pressure. It means that the solubility characteristic of R152a in DMAC is slightly better than that in DMF. This may be caused by the influence of the solvation effect.39 In an R152a molecule, there are two fluorine atoms with very strong electronegativity, which make the density of the electron cloud around the hydrogen atom decrease, causing R152a to exhibit electron affinity. DMF and DMAC both are nucleophilic reagents; the difference is that DMAC has one more electron-donating group, alkyl group, than DMF. So the nucleophilic effect of DMAC is stronger than that of DMF. Therefore, the solubility characteristic of R152a in DMAC is slightly better than that in DMF. Figures 6 and 7 show the deviations of experimental results from Raoult’s law at different temperatures for R152a + DMF and R152a + DMAC, respectively. It can be seen that the deviations

Figure 5. Comparison of the VLE data for two binary systems: ■, R152a (1) + DMF (2); □, R152a (1) + DMAC (2); solid lines, 303.15 K; dashed lines, 343.15 K.

Figure 6. Deviations of experimental results from Raoult’s law for R152a (1) + DMF (2): △, 353.15 K; ○, 343.15 K; □, 333.15 K; ▼, 323.15 K; ▲, 313.15 K; ●, 303.15 K; ■, 293.15 K; solid lines, calculated results using the NRTL model.

reported the VLE data of the R134a+DMF system, and the results showed that the vapor compositions of R152a were larger than 0.99 and there was almost no DMF in the vapor phase of the mixture. During our experiment, before and after injecting the absorbents into the equilibrium cell, the change of the vacuity degree is very small. 1083

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(6) Borde, I.; Jelinek, M.; Daitrophe, N. C. Working fluids for an absorption system based on R124 (2-chloro-l,l,l,2,-tetrafluoroethane) and organic absorbents. Int. J. Refrig. 1997, 20, 256−266. (7) Jelinek, M.; Levy, A.; Borde, I. Performance of a triple-pressurelevel absorption cycle with R125-N,N′-dimethylethylurea. Appl. Energy 2002, 71, 171−189. (8) Levy, A.; Jelinek, M.; Borde, I.; Ziegler, F. Performance of an advanced absorption cycle with R125 and different absorbents. Energy 2004, 29, 2501−2515. (9) Borde, I.; Jelinek, M.; Daitrophe, N. C. Absorption system based on the refrigerant R134a. Int. J. Refrig. 1995, 18, 387−394. (10) Arivazhagan, S.; Murugesan, S. N.; Saravanan, R.; Renganarayanan, S. Simulation studies on R134a-DMAC based half effect absorption cold storage systems. Energy Convers. Manage. 2005, 46, 1703−1713. (11) Arivazhagan, S.; Saravanan, R.; Renganarayanan, S. Experimental studies on HFC based two-stage half effect vapour absorption cooling system. Appl. Therm. Eng. 2006, 26, 1455−1462. (12) Muthu, V.; Saravanan, R.; Renganarayanan, S. Experimental studies on R134a-DMAC hot water based vapour absorption refrigeration systems. Int. J. Therm. Sci. 2008, 47, 175−181. (13) He, Y. J.; Chen, G. M. Experimental study on an absorption refrigeration system at low temperatures. Int. J. Therm. Sci. 2007, 46, 294−299. (14) He, L. J.; Tang, L. M.; Chen, G. M. Performance prediction of refrigerant−DMF solutions in a single-stage solar-powered absorption refrigeration system at low generating temperatures. Sol. Energy 2009, 83, 2029−2038. (15) Jelinek, M.; Levy, A.; Borde, I. The performance of a triple pressure level absorption cycle (TPLAC) with working fluids based on the absorbent DMEU and the refrigerants R22, R32, R124, R125, R134a, and R152a. Appl. Therm. Eng. 2008, 28, 1551−1555. (16) Bolaji, B. O. Experimental study of R152a and R32 to replace R134a in a domestic refrigerator. Energy 2010, 35, 3793−3798. (17) Sand, J. R.; Fischer, S. K.; Baxter, V. D. Energy and Global Warming Impacts of HFC Ref rigerants and Emerging Technologies; Report from project sponsored by DOE and AFEAS; Oak Ridge National Laboratory: TN, 1997. (18) Bhaduri, S. C.; Verma, H. K. Heat of mixing of R22-absorbent mixture. Int. J. Refrig. 1988, 11, 181−185. (19) Zehioua, R.; Coquelet, C.; Meniai, A. H.; Richon, D. Isothermal vapor−liquid equilibrium data of 1,1,1,2-tetrafluoroethane (R134a) + dimethylformamide (DMF) working fluids for an absorption heat transformer. J. Chem. Eng. Data 2010, 55, 985−988. (20) Zehioua, R.; Coquelet, C.; Meniai, A. H.; Richon, D. p−T−x Measurements for some working fluids for an absorption heat transformer: 1,1,1,2-tetrafluoroethane (R134a) + dimethylether diethylene glycol (DMEDEG) and dimethylether triethylene glycol (DMETrEG). J. Chem. Eng. Data 2010, 55, 2769−2775. (21) Coronas, A.; Mainar, A. M.; Patil, K. R.; Conesa, A.; Shen, S. B.; Zhu, S. M. Solubility of 1,1,1,2-tetrafluoroethane in triethylene glycol dimethyl ether. J. Chem. Eng. Data 2002, 47, 56−58. (22) Lopez, E. R.; Mainar, A. M.; Garcia, J.; Jose, S.; Urieta, J. S.; Fernandez, J. Experimental and predicted solubilities of HFC134a(1,1,1,2-tetrafluoroethane) in polyethers. Ind. Eng. Chem. Res. 2004, 43, 1523−1529. (23) Han, X. H.; Gao, Z. J.; Xu, Y. J.; Qiu, Y.; Min, X. W.; Cui, X. L.; Chen, G. M. Solubility of refrigerant 1,1,1,2-tetrafluoroethane in the N,N-dimethyl formamide in the temperature range from (263.15 to 363.15) K. J. Chem. Eng. Data 2011, 56, 1821−1826. (24) Agarwal, R. S.; Bapat, S. L. Solubility characteristics of R22DMF refrigerant-absorption combination. Int. J. Refrig. 1985, 8, 70− 74. (25) He, L. J.; Chen, G. M.; Cui, X. L. Vapor−liquid equilibria for R22 + N,N-dimethylformamide system at temperatures from 283.15 to 363.15 K. Fluid Phase Equilib. 2008, 266, 84−89. (26) Han, X. H.; Xu, Y. J.; Gao, Z. J.; Wang, Q.; Chen, G. M. Vapor− liquid equilibrium study of an absorption heat transformer working fluid of (HFC-32-DMF). J. Chem. Eng. Data 2011, 56, 1268−1272.

are dependent on temperature. The deviations of R152a + DMF show a positive deviation from Raoult’s law, and the deviations of R152a + DMAC show a little positive deviation from Raoult’s law at lower temperature, while exhibit a negative deviation from Raoult’s law at higher temperature. In spite of this, it is not affect the two binary systems as potential working fluids for absorption refrigeration system due to the lower GWP and zero ODP, excellent energy efficiency of R152a and the lower viscosity and considerable lower price of DMF and DMAC. It can be also seen that the deviations of R152a + DMAC are lower than that of R152a + DMF, or exhibit a negative deviation from Raoult’s law. It means that the potential of the system R152a + DMAC as working fluid for absorption refrigeration system is higher than that of R152a + DMF.

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CONCLUSIONS The VLE data of two binary systems R152a + DMF and R152a + DMAC were measured by means of a static analytical apparatus at temperatures from (293.15 to 353.15) K. The experimental data indicate that R152a exhibits better solubility characteristics with DMF and DMAC, and the solubility of R152a in DMAC is slightly better than that in DMF. Moreover, the NRTL model, containing five parameters, was selected to correlate the experimental data. The calculated results are in essential agreement with the experimental data, and the maximum relative deviations of pressure are 3.69 % and 3.51 %, and the average relative deviations of pressure are 1.60 % and 1.11 % for binary systems R152a + DMF and R152a + DMAC, respectively. This indicates that the NRTL model can appropriately express the VLE behaviors for binary systems R152a + DMF and R152a + DMAC. This work provides the VLE data of two new working fluids, and the study results show that they are potential options for research and development of novel absorption refrigeration systems.

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AUTHOR INFORMATION

Corresponding Author

* Tel./Fax: +86-010-6441-6406. E-mail: [email protected]. Funding

The supports provided by the National Natural Science Foundation of China (No. 50890184) and the National Basic Research Program of China (No. 2010CB227304) for the completion of the present work are gratefully acknowledged. Notes

The authors declare no competing financial interest.

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