Measurement and Correlation of Vapor–Liquid Equilibrium for R124 (1

Article pubs.acs.org/jced

Measurement and Correlation of Vapor−Liquid Equilibrium for R124 (1-Chloro-1,2,2,2-Tetrafluoroethane)-NMP (N‑Methyl-2-Pyrrolidone) and R124-DMF (N,N‑Dimethylformamide) Mixtures Shiming Xu,* Wei Wang, Xi Wu, Jungyong Hu, and Mengnan Jiang Key Laboratory of Ocean Energy Utilization and Energy Conservation of Ministry of Education, School of Energy and Power, Dalian University of Technology, Dalian 116023, China ABSTRACT: Vapor−liquid equilibrium data of binary working pairs play a crucial part in the research of absorption process and absorption refrigeration technology. In this paper, the VLE data of binary mixtures, R124-NMP and R124-DMF, were measured in a temperature range from 303.15 to 363.15 K, using a VLE measurement apparatus. The activity coefficient model of five-parameter nonrandom two liquid (NRTL) was selected to correlate all the VLE data acquired by measurement. The maximum relative deviation of the pressure between the measurement data and calculated data for R124-NMP and R124-DMF were 3.80% and 3.27% respectively, and the average relative deviation of the pressure were 1.25% and 1.17% respectively. The correlated results coincide well with the measurement data and show that the VLE characteristics of R124-NMP and R124-DMF mixtures have a negative deviation from Raoul’s law. Comparing three absorbents, DMF, DMAC and NMP, the activity coefficient of R124 in DMF is largest, that in DMAC is second, and that in NMP is smallest.

1. INTRODUCTION As the problem of energy shortage has become increasingly prominent, making full use of low-grade heat energy is playing increasingly important action at present. The absorption refrigeration systems (ARSs) can recover low-grade heat energy and convert it into cold energy to meet the air conditioning requirement of industrial, civil or domestic buildings.1 It can not only reduce consumption of primary energy significantly but also decrease the environmental pollution obviously. However, working fluids used in ARSs play an important role in their application. Two kinds of working fluids namely ammonia−water (NH3−H2O) and lithium bromide−water (LiBr−H2O) are commonly used in ARSs. Nevertheless, there are some inherent drawbacks in them.2 For the working fluid of ammonia−water, application fields for ARSs are restricted due to ammonia toxicity and flammability. It is forbidden to be used in civil or domestic buildings in China. Additionally, because of a relative small boiling point temperature difference between ammonia and water, a rectifier should be used in the ARSs to purify ammonia from a generator, which makes ARSs complicated and energy consumption increased. For the working fluid of lithium bromide−water, application fields for ARSs are restricted because water is used as refrigerant and the evaporation temperature must be above 0 °C. In order to replace two traditional working fluids, many efforts have been paid to research and develop new organic working fluids in which the refrigerants are hydrochlorofluorocarbons (HCFCs) or hydrofluorocarbons (HFCs), such as R22, R124, and R134a, and the absorbents are chemical solvents, such as N,N-dimethylformamide (DMF), N,N-dimethylacetamide © 2017 American Chemical Society

(DMAC), N-methyl-2-pyrrolidone (NMP), dimethyl ether diethylene glycol (DMEDEG), dimethyl ether tetraethylene glycol (DMETEG), and dimethyl ether triethylene glycol (DMETrEG).3−9 Although the ozone depletion potential (ODP) of these refrigerants may not be zero and the global warming potential (GWP) of these refrigerants may be high, the influence on the environment is comparatively small when refrigerants leak from ARSs due to most refrigerants stay in ARSs in the form of solution. The primary basis for the application of new organic working fluids in ARSs is to measure their vapor−liquid equilibrium (VLE) data first. Then their thermal properties can be obtained by the VLE data. In order to explore the possibility of using R134a as a refrigerant in combination with different organic absorbents in ARSs, Zehioua et al.4,5 measured the VLE data of three binary mixtures, R134a-DMF, R134a-DMEDEG, and R134a-DMETrEG, by the static analytic method, respectively. The measured temperature range for R134a-DMF mixture was from 303.30 to 353.24 K. The range for R134a-DMEDEG and R134a-DMETrEG mixtures was from 303 to 353 K. Later, Han et al.6 measured the solubility of R134a-DMF mixture in a wider temperature range from 263.15 to 363.15 K. Borde et al.7 constructed p−T−x curves of R134a-DMETEG, R134a-Nmethyl-epsilon-caprolactam (MCL) and R134a-dimethyl-ethylene urea (DMEU) mixtures based on VLE measurement data. Han et al.8 also took R32-DMF mixture as a promising new working fluid for ARSs and measured its VLE data in a Received: May 17, 2017 Accepted: August 11, 2017 Published: August 28, 2017 3414

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temperature range from 283.15 to 363.15K. Li et al.9 measured the VLE data of R32-DMEDEG and R32-DMAC mixtures and correlated the VLE data with the five-parameter Non-Random Two-Liquid (NRTL) model. Jing et al.10 measured the VLE data of R161-DMAC, R161-NMP, and R134a-DMAC mixtures in a temperature range from 293.15 to 353.15 K. All of the data were also correlated by the five-parameter NRTL model. Yan et al.11 measured the VLE data of R236fa-DMF and R236fa-NMP mixtures in a temperature range from 293.15 to 353.15 K. The results make clear that two mixtures can be used as potential working fluids for absorption power cycle. The results are also compared with their previous works.12 Although the refrigerants of HFCs mentioned above are with zero ODP, they are not suit to be used in the ARSs cooled by air due to high condensation pressure. Under air cooling condition, the binary mixture with R124 as refrigerant has relatively low condensation pressure, which causes low generation temperature, low solution circulation ratio and high coefficient of performance.13,14 These mean low power consumption of the solution pump and small size and weight of the devices in ARSs for air-conditioning. Recently, Xu et al.15−19 proposed an absorption compression hybrid refrigeration cycle (ACHRC) to recover the waste heat from vehicle engines. The R124-DMAC mixture was used in the cycle and some theoretical analysis and experiment study had been done for the cycle. The vertical fin-tube bubble absorber cooled by air was applied in the system.20−23 The reason why choosing R124-DMAC mixture used as working fluid is that its VLE data had been measured and the correlations of its thermal properties had been given by Borde et al.13 Nevertheless, other binary mixture such as R124-NMP or R124-DMF can be considered as working fluid in the ACHRC. Unfortunately, the VLE data of R124-NMP or R124-DMF mixture are never published in open literatures, so it is difficult to confirm which working fluid in three binary mixtures, R124-NMP, R124-DMF and R124-DMAC, is more suitable to be used in the ACHRC. The objective of this work is to measure the VLE data of R124-NMP and R124-DMF mixtures and to obtain p−T−x correlations and activity coefficients of each pair first and then to compare the properties of two pairs with that of R124-DMAC mixture.

Figure 1. Schematic diagram of the VLE measurement system and apparatus.

volume was 114 cm3. The still is immersed in the water bath after a certain amount of the measured mixture is charged into it. The temperature of the mixture in the still will reach that of the water bath after sufficient time and kept constant. The temperature and pressure sensors are used to measure the temperature and pressure of the mixture in the still and the data are recorded by the computer automatically. A liquid six-way metering valve is used to control the volume of the measured solution that will be injected into the gas chromatography (GC). The refrigerant or absorbent mass fraction of the solution in the still at different temperature and pressure could be measured online by the GC. The vacuum pump is applied to draw out the air inside of the still and provides necessary negative pressure in the still, which could not only avoid the air component affecting on the pressure measurement but also easily charge the measured solution into the still. In the GC, a glass-packed column is used as chromatographic separating column. The thermal conductivity detector is used. For R124-NMP mixture, during the measurement process the operation parameters of the GC are set as 260 °C injection port temperature, 240 °C column temperature, and 260 °C detector temperature. For R124-DMF mixture, the operation parameters of the GC are set as 210 °C injection port temperature, 191 °C column temperature, and 210 °C detector temperature. The operation parameters of the GC between R124-NMP and R124-DMF are different due to the boiling-point difference between the refrigerant and absorbent. Helium with a purity of 99.999% is used as carrier gas to improve the accuracy of the GC and its flow rate was 45 mL/min. The calculation method of measurement uncertainties given in ref 25 is adopted. The uncertainties in the measurement system are caused by the measurements of temperature (T), pressure (p), and composition (x). The standard uncertainty of measurement data, u, for the system can be described as follows

2. VLE MEASUREMENT SECTION 2.1. Measurement System and Apparatus. In order to measure the VLE data of R124-NMP and R124-DMF binary mixtures, the experimental principle given by refs 5, 8, and 10 was adopted in this paper and a VLE measurement system showed in Figure 1 was set. The system composed of some apparatuses which are (1) thermostatic water bath, (2) equilibrium still, (3) temperature controller, (4) refrigerator, (5) vacuum pump, (6) temperature sensors, (7) pressure sensors, (8) refrigerant cylinder, (9) refrigerant storage tank, (10) valves, (11) computer, (12) liquid injector, (13) six-way metering valve, and (14) gas chromatography (GC). The types, operation parameters and measurement accuracy of those apparatuses are listed in Table 1. In the system, the water temperature in the thermostatic water bath is adjusted by a heater or refrigerator and controlled by a temperature controller. The temperature in the bath is precisely controlled within ±0.15 K of the set temperature and kept uniform temperature by stirring water in the bath. The equilibrium still is made of stainless steel and its inner

n

u=

∑ u(2i) i=1

(1)

where, u(i) is the error of each measurement instrument in the system. The temperature controller is controlled by a platinum resistance thermometer which the accuracy is ±0.15 K, the 3415

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Table 1. Specification of Main Components and Instruments item

type/material

temperature controller temperature sensor pressure sensor six-way metering valve thermostatic water bath equilibrium still vacuum pump gas chromatography24

Julabo F-34ME Pt-100 DRUCK-PDCR5021 Vaclo,4-port-2 pos Int vol valve Thermal H10S stainless steel

±0.15K ±0.15K 0.04%FS BSL

accuracy

HP4890

≤0.02 mV/15 min

measuring rage

remarks

−30−200 °C −50−250 °C 0−750 psig 1 μL −20−200 °C

calibration calibration calibration 1/16*.75 mm 175C/1000 psi 114 cm3 TCD detector

Table 2. Sample Materials Used in the Measurements chemical

CAS registry no.

source

mass fraction purity

purification method

1-chloro-1,2,2,2-tetrafluoroethane (R124) 1,1,1,2-tetrafluoroethane (R134a) N-methyl-2-pyrrolidone (NMP) N,N-dimethylformamide (DMF) N,N-dimethylacetamide (DMAC)

2837-89-0 811-97-2 872-50-4 68-12-2 127-19-5

Zhejiang Zhonglong refrigerant Co. Ltd. Zhejiang Zhonglong refrigerant Co. Ltd. Aladdin Aladdin Aladdin

99.9% 99.9% 99.9% 99.9% 99.9%

none none none none none

Table 3. Basic Data for the Pure Components4,10,13 R124, R134a, NMP, DMF, and DMAC

a

chemical

M

Tc/K

pc/MPa

normal boiling pointa/K

1-chloro-1,2,2,2-tetrafluoroethane (R124) 1,1,1,2-tetrafluoroethane (R134a) N-methyl-2-pyrrolidone (NMP) N,N-dimethylformamide (DMF) N,N-dimethylacetamide (DMAC)

136.50 102.03 99.13 73.09 87.12

395.425 374.21 721.800 650.000 655.55

3.6243 4.0593 4.7175 5.499 4.211

261.187 247.076 477.720 425.950 438.15

Boiling point at pressure 101.325 kPa.

materials’ information used is listed in Table 2. The basic data needed for the pure components, R124, R134a, NMP, DMF, and DMAC, are listed in Table 3.4,10,13 2.3. VLE Measurement Procedures. Before the measured mixture was charged into the measurement system, the equilibrium still and pipeline were thoroughly washed by anhydrous alcohol first. Then they were dried at room temperature for more than 1 h to make sure that there were not any impurities in them. After that, the measurement processes were as follows. The thermostatic water bath began to work and the water temperature in the bath was set at 333.15 K. The vacuum pump ran to draw out the air from the still until the absolute pressure in the still was lower than 3 kPa. After that, the water temperature in the bath was down to 303.15 K and kept for 24 h. The absolute pressure in the still was no more than 3 kPa to ensure the airtightness of the measurement system. Then the absorbent, NMP or DMF, was injected into the equilibrium still about 75 mL by an injector and the pressure in the still will rise slightly due to partial space of the still was occupied by the injected absorbent. The vacuum pump was started again to draw out the air from the still. The pressure in the still dropped. The vacuum pump was stopped when the pressure was lower than 3 kPa again. In this process, a small amount of absorbent vapor will be drawn out together with air from the still, which can make sure that there is not any air in the still. The water temperature in the bath was kept at 303.15 K. A certain amount of refrigerant, R124, was slowly injected into the still and the pressure in the still rose. After the pressure reached a certain value, the operation of refrigerant injection was ended. The refrigerant and absorbent were fully mixed by waggling the still. Then, the still was immersed into the bath and allowed stand for more than 2 h. The mixture in the still

temperature stability is 0.03 K, and the resolution of digital data acquisition device is 0.01 K. In this work, the maximum experimental temperature is 90 °C. Therefore, the temperature uncertainties can be expressed26,27 as u(1)(T) = 0.15/ 3 = 0.087, u(2)(T) = 0.03/ 3 = 0.017 and u(3)(T) = 0.5 × 0.01/ 3 = 0.0029. Thus, the combined standard uncertainty of temperature in the range of this work is 3

u(T ) =

∑ u(i)2(T )

= 0.089 ≈ 0.09K (2)

i=1

For the pressure measurement, the accuracy and the maximum scale of the pressure sensor are 0.04% and 5.17 MPa, respectively. The resolution of pressure monitor is 0.001 MPa. Therefore, the pressure uncertainties can be expressed26,27 as u(1)(p) = 0.04% × 5.17/2.58 = 0.0008 in which the 2.58 denominator is the coverage factor while the confidence probability is 99%,27 and u(2)(p) = 0.5 × 0.001/ 3 = 0.0003. Thus, the combined standard uncertainty of pressure in the range of this work is 2

u(p) =

∑ u(2i)(p) i=1

= 0.0008 MPa = 0.8 kPa (3)

Similarly, for the composition measurement, u(i)(x) contains the error of the gas chromatography u(1)(x) and error caused by calibrations u(2)(x), which are 0.015 and 0.01 respectively. Thus, the standard uncertainty of composition is u(x) = 0.018. 2.2. Sample Materials. The refrigerants, R124 and R134a, are provided by Zhejiang Zhonglong refrigerant Co. Ltd. The absorbents, NMP, DMF and DMAC, are purchased from Shanghai Aladdin Bio-Chem Technology Co. The sample 3416

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Table 4. VLE Data of R134a-DMACa Mixture from Experiment, Reference 10, and Reference 32 T/Kb

x1,ref.10

pref.10/kPa

x1,expb

pexp/kPab

pcal/kPab

x1,ref.32

pref.32/MPa

313.15

0.1737 0.2421 0.3850 0.5208 0.6558 0.7398 0.7842 0.8540 0.9548

158.7 223.6 359.4 492.6 623.7 708.7 756.6 835.7 952.4

0.2083 0.2928 0.3364 0.4139 0.4681 0.5301 0.5823

192.6 271.2 310.8 395.4 454.2 510.6 560.4

190.2 269.5 310.8 384.9 437.1 497.2 548.1

0.5482 0.5909 0.6151 0.6522 0.6699 0.6970 0.7152 0.7335 0.7461 0.7754 0.7958 0.8155 0.8242 0.8333 0.8434 0.8526 0.8647 0.8888 0.9127 0.9272

0.4783 0.5228 0.5508 0.5910 0.6110 0.6413 0.6623 0.6826 0.6996 0.7365 0.7630 0.7865 0.7965 0.8082 0.8213 0.8316 0.8450 0.8748 0.9033 0.9204

exp, experimental data; ref, data from reference; cal, calculated data by using NRTL model. bThe combined standard uncertainties uc are uc(T) ≈ 0.09 K, uc(p) = 0.0008 MPa and uc(x1) = 0.018. a

was considered to reach equilibrium when its temperature no longer had any change. The temperature and pressure were measured and recorded. After the GC was started and its operation parameters were set, rotating the six-way metering valve and the sample fluid would inject automatically into the GC. Online composition detection results from the GC would be recorded by a computer. Keeping the component in the still, the water temperature in the bath was first raised from 303.15 to 363.15 K step by step. The temperature was risen 10 K for every step and stood a period of time until the temperature and pressure in the still keeping constant. The temperature, pressure, and refrigerant composition or mole fraction were measured. Then, the water temperature in the bath was reduced from 363.15 to 303.15K step by step for every 10 K and repeated above measurements. Average values of temperature, pressure, and composition measured two times were taken as valid measurement datum. Then, the refrigerant, R124, was injected into the still again, which makes the mass fraction of refrigerant in the binary mixture in the still rise. The above measurement processes were repeated until the mole fraction of refrigerant in binary mixture in the still reaching more than 70%. 2.4. Reliability Validation of Measurement Method. In order to validate the reliability of measurement method mentioned above, the VLE data of binary mixture, R134aDMAC, at 313.15 K were measured. Comparison of the measurement data and literature data10,32 was listed in Table 4. Variations of pressure parameters, pref.10, pref.32, pcal, and pexp with x1 are shown in Figure 2 in which the curve of pcal is achieved by the five-parameter NRTL model given in ref 10. The maximum relative deviation between pexp and pcal is 3.76% and the average relative deviation is 1.87%. Also, the maximum relative deviation between pref.32 and pcal is 8.03% at the point of x1 = 0.54 and average relative deviation is 2.48%. It can be seen from Figure 2 that the deviation between pref.32 and pcal increases gradually with x1 decreasing. Comparing pcal with

Figure 2. Reference data and curve and experimental data of R134a (1)−DMAC (2) at 313.15K.

pexp from Table 4 or Figure 2, the measurement data of this work is good in accordance with the data from ref 10. The validation result indicates that the measurement system and method in this work is reliable and meets the requirement to measure the VLE data of R124-NMP and R124-DMF mixtures.

3. CORRELATION EQUATION The NRTL model is an excellent predictive tool for the VLE of nonideal systems.28The five-parameter NRTL model had been successfully employed in correlating the experimental VLE data for binary mixtures, like R161-DMAC or NMP, R236fa-DMF or NMP.10,11 So, the NRTL model with five parameters was also selected to correlate the VLE data of R124-NMP and R124-DMF from 303.15 to 363.15 K in this paper. The model is expressed as follows ⎤ ⎡ ⎛ ⎞2 G21 τ12G12 ⎥ ln γ1 = x 22⎢τ21⎜ ⎟ + ⎢⎣ ⎝ x1 + x 2G21 ⎠ (x 2 + x1G12)2 ⎥⎦ (4) 3417

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G12 and G21 are defined as G12 = exp( −α12τ12)

(5)

G21 = exp( −α12τ21)

(6)

Table 6. Measured and Calculated VLE Data for R124NMPa Binary Mixture

where α12 is the nonrandomness parameter of the NRTL model. The binary interaction parameters, τ12 and τ21, considered to be temperature-dependent, are defined as τ12 = τ21 =

(g12 − g22) RT (g21 − g11) RT

[a + b1 ln(T )] = 1 RT

(7)

[a 2 + b2 ln(T )] RT

(8)

=

T/K

x1

pexp/kPa

pcal/kPa

δp/%b

303.15

0.5492 0.6466 0.7154 0.8233 0.4692 0.5051 0.5480 0.6459 0.7146 0.8229 0.3976 0.4681 0.5047 0.5471 0.6451 0.7137 0.8221 0.3213 0.3957 0.4670 0.5038 0.5458 0.6445 0.7123 0.8217 0.2629 0.3207 0.3942 0.4654 0.5029 0.5445 0.6437 0.7117 0.8209 0.2048 0.2617 0.3195 0.3931 0.4639 0.5019 0.5437 0.6431 0.7108 0.8202 0.2036 0.2609 0.3194 0.3921 0.4628 0.5008 0.5428 0.6425 0.7099 0.8198

104.4 159.5 211.9 301.7 105.9 123.5 139.4 212.3 280.1 412.1 103.4 142.3 165.8 186.9 278.1 365.2 523.2 101.4 139.7 184.9 217.6 244.2 371.2 466.5 670.8 104.4 134.8 180.7 240.9 278.4 311.7 459.6 587.0 847.0 108.1 136.8 174.3 240.2 304.3 351.9 394.4 571.1 745.6 1055.1 135.7 176.4 224.9 296.5 383.4 435.9 485.9 701.7 892.1 1297.2

107.0 159.4 207.9 301.3 104.3 121.3 144.7 213.5 276.7 400.4 104.6 139.9 162.2 191.9 280.4 361.3 521.2 101.8 138.9 184.3 212.6 249.7 361.8 462.8 667.9 104.9 135.1 181.4 237.9 273.7 319.3 459.2 585.3 841.6 104.4 137.4 175.4 232.7 302.2 346.7 402.9 574.6 728.4 1046.1 135.3 177.0 224.6 294.4 378.4 432.2 500.6 708.9 895.2 1284.7

2.49 0.06 1.89 0.13 1.51 1.78 3.80 0.57 1.21 2.84 1.16 1.69 2.17 2.68 0.83 1.07 0.38 0.39 0.57 0.32 2.30 2.25 2.53 0.79 0.43 0.48 0.22 0.39 1.25 1.69 2.44 0.09 0.29 0.64 3.42 0.44 0.63 3.12 0.69 1.48 2.16 0.61 2.31 0.85 0.29 0.06 0.13 0.71 1.30 0.85 3.03 1.03 0.35 0.96

313.15

323.15

In eqs 4 to 8, (g12−g22) and (g21−g11) are energy parameters, a1, a2, b1, b2, and α12 are the five parameters in the NRTL model, γ1 is the activity coefficient of the refrigerant, x1 and x2 are the liquid mole fractions of the refrigerant and absorbent, and R stands for the general gas constant. The five parameters in the NRTL model can be obtained by minimizing the objective function (OBF) using the VLE measurement data

333.15

Np

OBF =

∑ (ln γ1,cal − ln γ1,exp)2

(9)

i=1

where γ1,cal is the calculated activity coefficients of the refrigerant, γ1,exp is the experimental activity coefficients of the refrigerant, Np is the number of the measurement data. The criteria of phase equilibrium can be described by following equation v

yi ϕi p =

pis xiγi

⎡ V sl(p − ps ) ⎤ i i ⎥ exp⎢ ⎢⎣ ⎥⎦ RT

343.15

i = (1, 2, 3.... N ) (10)

where φvi is the fugacity coefficient of component i in the vapor phase and N is the number of components in a mixture. When the vapor phase component, 1, is assumed perfect for binary mixture, eq 10 can be described by following equation ⎡ V sl(p − p s ) ⎤ 1 1 ⎥ y1p = p1s x1γ1 exp⎢ ⎢⎣ ⎥⎦ RT

353.15

(11)

where y1 is the mole fraction of absorbent in vapor phase, p and T are the vapor pressure and temperature of the mixture, respectively, ps1 is the vapor pressure of pure refrigerant, VL1 is the mole volume of the saturated liquid at T, γ1 is the activity coefficient of the refrigerant, x1 is the liquid mole fractions of the refrigerant, and R stands for the general gas constant. The values of ps1 and VL1 for R124 at T = 303.15−363.15 K can be calculated by REFPROP29 and are listed in Table 5.

363.15

Table 5. Pressure and Molar Volume for R124 Saturated Liquid Calculated by REFPROP29 T/K

ps1/kPa

Vsl1 /m3·kmol−1

303.15 313.15 323.15 333.15 343.15 353.15 363.15

445.30 593.45 775.77 996.95 1262.0 1576.4 1946.2

0.102025562 0.104870928 0.108067453 0.111720413 0.115982666 0.121118012 0.127558172

The combined standard uncertainties uc are uc(T) ≈ 0.09 K, uc(p) = 0.0008 MPa and uc(x1) = 0.018. bRelative deviation of the pressure (δp): δp = (|pexp − pcal|)/pexp × 100. a

Zehioua et al.4 and Han et al.6 measured the VLE data of R134a+DMF binary mixture and found that there was almost no absorbent, DMF, in the vapor phase. Jing et al.10 reported 3418

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Table 7. Measured and Calculated VLE Data for R124-DMFa Binary Mixture T/K

x1

pexp/kPa

pcal/kPa

δp/%b

T/K

x1

pexp/kPa

pcal/kPa

δp/%b

303.15

0.3467 0.3908 0.4658 0.5542 0.6627 0.7631 0.3096 0.3450 0.3897 0.4642 0.5537 0.6619 0.7624 0.2450 0.3077 0.3425 0.3878 0.4631 0.5531 0.6612 0.7618 0.1924 0.2430 0.3059 0.3417 0.3861 0.4621 0.5526 0.6603 0.7611

103.9 123.9 161.7 210.5 262.3 312.2 117.7 135.1 165.1 217.1 272.0 337.1 420.4 108.3 150.4 175.8 211.6 278.5 348.3 453.7 544.7 102.8 137.5 197.8 225.1 271.6 360.6 441.6 573.6 689.2

103.6 124.7 162.1 207.5 263.1 313.1 115.6 136.4 163.8 212.5 272.2 345.3 411.9 107.9 150.9 176.7 211.9 273.8 350.9 444.8 531.5 102.3 141.2 195.1 228.1 271.1 348.8 445.6 563.5 673.8

0.29 0.65 0.25 1.43 0.30 0.29 1.78 0.96 0.79 2.12 0.07 2.43 2.02 0.37 0.33 0.51 0.14 1.69 0.75 1.96 2.42 0.49 2.69 1.37 1.33 0.18 3.27 0.91 1.76 2.23

343.15

0.1524 0.1904 0.2414 0.3045 0.3405 0.3849 0.4608 0.5518 0.6596 0.7601 0.1504 0.1890 0.2405 0.3037 0.3387 0.3835 0.4591 0.5511 0.6588 0.7591 0.1485 0.1875 0.2383 0.3021 0.3365 0.3814 0.4578 0.5502 0.6581 0.7582

103.1 136.0 180.6 252.1 288.9 344.3 450.5 555.2 725.6 844.5 133.8 177.8 229.2 317.1 363.4 432.2 551.3 687.4 873.5 1061.0 174.3 228.1 294.1 393.1 451.2 532.6 673.5 841.7 1065.0 1269.0

101.2 134.5 183.2 250.3 291.1 343.7 438.6 557.9 703.6 840.7 132.8 175.3 236.7 318.5 366.7 430.9 544.8 690.4 867.1 1034.7 172.0 225.4 299.7 399.8 456.6 533.6 670.5 844.5 1055.9 1258.0

1.84 1.10 1.44 0.71 0.76 0.17 2.64 0.49 3.03 0.45 0.75 1.41 3.27 0.44 0.91 0.30 1.18 0.44 0.73 2.48 1.32 1.18 1.90 1.70 1.20 0.19 0.45 0.33 0.85 0.87

313.15

323.15

333.15

353.15

363.15

a The combined standard uncertainties uc are uc(T) ≈ 0.09K, uc(p) = 0.0008 MPa and uc(x1) = 0.018. bRelative deviation of the pressure (δp): δp = (|pexp − pcal|)/pexp × 100.

Table 8. Five Parameters of NRTL model for R124-NMP and R124-DMF Binary Mixtures parameters

R124-NMP

R124-DMF

a1 b1 a2 b2 α12

242408 −46295.3 −166747 30875.3 −0.0515

−52406.6 4671.8 −88324 14735.9 −0.5116

the VLE data of R134a-DMAC, R161-DMAC and R161-NMP. They also found that the content of absorbent DMAC or NMP in the vapor was too little to be negligible. The absorbent, NMP and DMF, can be considered to be nonvolatile and there is not any absorbent in vapor. So, the mole fraction of absorbent in vapor, y1, is considered to be equal 1.

4. VLE DATA OF TWO MIXTURES 4.1. Measurement and Calculation Data. The VLE data of two mixtures, R124-NMP and R124-DMF, from 303.15 to 363.15 K were measured by the above method and are listed in Table 6 and Table 7, respectively. The VLE data calculated by the five-parameter NRTL model are listed in them. The relative deviations between measured and calculated pressures are also calculated and listed in Table 6 and Table 7. The five parameters in the NRTL model for two mixtures are calculated by eq 9 through the VLE measurement data and are listed in Table 8.

Figure 3. Isothermal VLE data for R124 (1)−NMP (2) at different temperatures; the symbols are experimental data points and the curve is calculated by using the five-parameter NRTL model.

The VLE data of the R124-DMAC mixture from 303.15 to 363.15 K was given in ref 13. The relationship among the mass fraction (ξ), temperature (T), and pressure (p) were expressed as a polynomial and the p−T−ξ chart was given in the literature. In order to compare the properties of the three mixture, R124-DMAC, R124-NMP, and R124-DMF, the mass 3419

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the absorption pressure will be helpful for the refrigerant absorbed by absorbents. Figure 6 is a p−T−x chart of three binary mixtures, R124-NMP, R124-DMF, and R124-DMAC, at the temperature

fraction of R124-DMAC mixture given by the literature was converted into mole fraction and its p−T−x chart was drawn in this paper. The refrigerant activity coefficient of R124-DMAC mixture at the temperature of 303.15 and 363.15K was calculated by eq 11. Figures 3, 4, and 5 illustrate the p−T−x charts of R124-NMP, R124-DMF, and R124-DMAC mixtures at phase equilibrium.

Figure 6. Comparison of the VLE data for three binary systems at the temperature of 303.15 and 363.15 K.

Figure 4. Isothermal VLE data for R124 (1)−DMF (2) at different temperatures; the symbols are experimental data points and the curve is calculated by using the five-parameter NRTL model.

of 303.15 and 363.15 K. It shows that at the same temperature (T) and mole fraction of refrigerant (x1), the equilibrium pressure (p) of R124-DMF is highest, that of R124-DMAC is secondary, and that of R124-NMP is lowest for the three mixtures. It means that the solubility of R124 in NMP is highest, that in DMAC is secondary, and that in DMAC is lowest under the same temperature and absorption pressure. 4.2. Deviation Analysis. Figure 7 illustrates the relative deviation distribution of equilibrium pressures for the

Figure 5. p−T−x chart for R124 (1)−DMAC (2) at different temperatures.

In Figures 3 and 4, the solid lines are the correlation results by the NRTL model with the five parameters that are listed in Table 8. The tabs nearby the solid lines are the points of measurement data. In Figure 5, the solid lines are the regression fitting results for the data given by ref 13 in which the mass fraction is replaced by mole fraction. It can be seen from Figures 3 and 4 that the correlation curves coincide well with measurement data, which means that the five-parameter NRTL model can be used to describe VLE properties of R124-NMP and R124-DMF mixtures in measurement range. Figures 3, 4, and 5 also show that the mole fraction of R124, x1, will reduce with temperature increasing at the same pressure and the pressure will increase with x1 rising at the same temperature. So, all measures to drop the absorption temperature or to raise

Figure 7. Distribution of relative deviation for R124-NMP/DMF systems between experimental and calculated pressures employing NRTL model in temperatures range from 303.15 to 363.15 K.

R124-NMP and R124-DMF mixtures between measurement data and calculated data using the five-parameter NRTL model in the temperature range from 303.15 to 363.15 K. The relative deviation values are listed in Tables 6 and 7. It can be found from the figure that relative deviation distributions of the pressures are randomly and the relative deviation values change 3420

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Figure 8. Refrigerant activity coefficient of three binary systems with respect to mole fraction at temperatures of 303.15 and 363.15 K.

between −4.0% and 4.0%. The average relative deviations are 1.25% and 1.17% respectively. The maximum relative deviations are 3.80% and 3.27% respectively. So, the five-parameter NRTL model given by this work can be used to accurately calculate and predict p−T−x data for R124-NMP and R124-DMF mixtures in a given range. 4.3. Activity Coefficient. The ideal solution obeys Raoult’s law and its activity coefficient equals 1. When the activity coefficients of refrigerant composition in the binary solutions are less than 1, the solutions appear as a negative deviation from Raoult’s law and the absorbent in them has a strong affinity for refrigerant. On the contrary, when the activity coefficient of refrigerant composition in the solutions is more than 1, the solutions appear as a positive deviation from Raoult’s law and the absorbent in them have a weak affinity for refrigerant.2,30,31 Figure 8 illustrates the variations of activity coefficients of R124 (γ1) in three binary solutions, R124-NMP, R124-DMF, and R124-DMAC with respect to mole fraction (x1) at the temperature of 303.15 and 363.15 K, respectively. In the figure, two solid curves are results correlated by the five-parameter NRTL model, the tabs near the curves are measurement data points and the isolated tabs are results calculated by the regression equation given in ref 13. It can be seen from the figure that the activity coefficients of refrigerant are less than 1, which means that all of the three binary solutions have a negative deviation from Raoult’s law. All three kinds of absorbents, NMP, DMF, and DMAC have a strong affinity for R124 refrigerant. The three binary solutions are suitable to be used as working fluids in ARSs. It can be also seen from the figure that the γ1 rises with the x1 increasing, which means that the degree of negative deviation from Raoult’s law will decrease with x1 increasing. It indicates that the affinity or mass transfer force between the refrigerant and the absorbents will reduce when refrigerant composition or mole fraction in the binary mixture increases. The measurement and correlation results indicate that in the three binary mixtures, the activity coefficient of refrigerant in R124-NMP is lowest, that in R124-DMAC is secondary, and

that in R124-DMF is highest under the same temperature, pressure, and refrigerant composition. It means that the absorbent NMP has strongest affinity for the refrigeration, R124, DMAC has secondary affinity for R124, and DMF has weakest affinity for R124. It also express as that the solubility of refrigerant, R124, in NMP is largest, that in DMAC is secondary, and that in DMF is smallest among the three binary mixtures.

5. CONCLUSIONS The VLE data of R124-NMP and R124-DMF binary mixtures were measured from 303.15 to 363.15 K at temperature intervals of 10 K. The five parameters in the NRTL correlation model were obtained by measurement data. The maximum relative deviations between measurement and correlated pressures for two mixtures were 3.80% and 3.27%, respectively, and the average relative deviations were 1.25% and 1.17%, respectively. Correlated results coincide well with measurement data. The NRTL model with five parameters given by this work can be used to correlate and predict p−T−x equilibrium data for the two mixtures in range of measurement data. The activity coefficient of R124 in three binary mixtures, R124-NMP, R124-DMF, and R124-DMAC was calculated at the temperature of 303.15 and 363.15 K, respectively and the γ−x chart was plotted. The results indicated that all three mixtures appear a negative deviation from Raoult’s law and absorbents in three mixtures have a strong affinity for R124. Under the same temperature and pressure, the solubility of refrigerant, R124, in NMP is largest, that in DMAC is secondary, and that in DMF is smallest among the three binary mixtures. So, the R124-NMP and R124-DMF binary mixtures can also be used as working fluid in ARSs.

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AUTHOR INFORMATION

ORCID

Wei Wang: 0000-0001-7374-2310 Notes

The authors declare no competing financial interest. 3421

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Funding

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This work is financially supported by National Natural Science Foundation of China (51376032,51776029), China Postdoctoral Science Foundation (2016M591427), and Doctoral Startup Funds of Liaoning Province (201601053).

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DOI: 10.1021/acs.jced.7b00444 J. Chem. Eng. Data 2017, 62, 3414−3422