Miscibility Measurement and Evaluation for the Binary Refrigerant

May 6, 2015 - ABSTRACT: The miscibility of refrigerants with mineral oils is very important in the process of refrigerant substitution. The experiment...
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Miscibility Measurement and Evaluation for the Binary Refrigerant Mixture Isobutane (R600a) + 1,1,1,2,3,3,3-Heptafluoropropane (R227ea) with a Mineral Oil Zhao Yang,*,†,‡ Tian Tian,†,‡ Xi Wu,†,‡ Rui Zhai,† and Biao Feng† †

Key Laboratory of Efficient Utilization of Low and Medium Grade Energy, MOE, School of Mechanical Engineering, and ‡State Key Laboratory of Engines, Tianjin University, 92 Weijin Road, Tianjin, 300072, P. R. China ABSTRACT: The miscibility of refrigerants with mineral oils is very important in the process of refrigerant substitution. The experimental critical solubility temperature (TCST) for the binary refrigerant mixture isobutane (R600a) + 1,1,1,2,3,3,3heptafluoropropane (R227ea) in varying proportions with a 3GS (viscosity, ISO 32) naphthenic mineral oil was determined over a wide temperature range from 223.15 K to 303.15 K, and over an oil mass fraction range from 5 % to 20 %. A method based on analysis of the element type and number was proposed not only for evaluating the miscibility degree of pure refrigerants, but also for fitting miscibility data of the binary refrigerant mixture R600a/R227ea with the mineral oil. The miscibility evaluation result of R600a/R227ea with the mineral oil was calculated and shown on a triangular diagram which included five regions, standing for the area of TCST over 303.15 K, between 273.15 K and 303.15 K, between 253.15 K and 273.15 K, between 233.15 K and 253.15 K, and below 233.15 K, respectively. The diagram showed that when the mass fraction of R600a was below 20 % in the R600a/R227ea/mineral oil solution, a second liquid phase (oil-rich phase) would appear under common air-conditioning refrigeration temperatures; when the mass fraction of R600a was over 30 %, the liquid phase would not appear.

1. INTRODUCTION Recently, hydrochlorofluorocarbon (HCFC) refrigerants were scheduled to be phased out by the Montreal Protocol (including its subsequent amendments and adjustments) because of the large ozone depletion potential (ODP) and high global warming potential (GWP) of these HCFCs.1 In China, over the past several decades, HCFCs, especially HCFC-22, have been widely used in various refrigeration and air-conditioning equipment, such as cold storage warehouses, household air conditioners, and chillers.2 The miscibility of refrigerants with lubricants has a large influence on the oil return, evaporator heat transfer, and compressor working efficiency.3 In a HCFC refrigeration system, mineral oils are commonly used for their good mutual solubility, relatively cheap price, low solubility for moisture, and low chemical reactivity.4 It is well-known that in the refrigeration systems working fluids are a mixture solution of the refrigerant and lubricant, not the refrigerant itself.5 Lubricant spurts from the compressor outlet valve and flows in a circular path by the push of the refrigerant. If an oil-rich phase appears in the solution, oil may accumulate in the evaporator or condenser, which decreases the heat transfer effect of heat exchangers and also results in oil shortage in the compressor. Therefore, in the “drop-in” substitution for HCFCs by new nonozone depleting, low GWP refrigerants, whether these new fluids are miscible with mineral oils is important for refrigeration workers to © 2015 American Chemical Society

simplify replacement procedures and avoid immiscibility problems.6 Mixtures of hydrocarbons (HCs) and hydrofluorocarbons (HFCs) are considered as potential drop-in alternatives, and they may have the unsurpassable environmental advantage of low GWP, zero ODP, nonflammability, and high cycle performance.7 HCs are miscible with mineral oils because they have a similar weak polarity, but the majority of HFCs have a certain polarity, resulting in immiscibility with mineral oils and only miscibility with synthetic oils, such as polyesters (POEs) and polyglycols, (PAGs), which are highly hygroscopic and expensive. Whether HCs/HFCs mixture is miscible with mineral oils depends on the proportion of HCs, for example R416A (R134a/124/600 (59.0/39.5/1.5, mass fraction)) does not mix with mineral oils up to 299.85 K because the amount of R600 is too small,8 while R430A (R152a/600a (76.0/24.0)) is miscible with mineral oils because of the relatively high content of R600a.9 A miscibility test of pure refrigerants R22, R32, R123, R124, R125, R134, R134a, R142b, R143a, R152a with mineral oils was performed by Pate et al.10 The test performed over a temperature range of 223.15 K to 363.15 K. An interesting Received: December 27, 2014 Accepted: April 21, 2015 Published: May 6, 2015 1781

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discovery from the literature8,10 was that the upper critical solubility temperature of HCFCs refrigerants (R22, R124, R142b, and R416A) with mineral oils appeared at the point near an oil mass fraction of 20 %. Inoue et al.11 studied the mutual solubility of refrigerants with lubricants by 1H-NMR spectra and Hansen solubility parameters. The literature11 determined the Hansen solubility parameters of R134a, R125, R143a, R32, and esters. A major conclusion from that paper was that the polar parameter δd played a key role in understanding TCST. Remigy et al.12 tested Hansen solubility parameters and solubility volumes of R12, R134a, a POE, a poly-α-olefin base oil (PAO), and an alkylbenzene base oil (AB). That paper drew a conclusion that classical criterion of solubility parameters was not enough to predict the miscibility of refrigerants with oils, while the distance between the centers of solubility volumes was valuable. Yokozeki13 modeled solubility data of refrigerants/lubricants with Soave−Redlich−Kwong (SRK) and Peng−Robinson (PR) cubic equations of state (EOS) and demonstrated that the miscibility gaps could be correctly predicted by using a common cubic EOS with a special mixing rule. Similar work had been done by Teodorescu et al.14 That paper showed that the SRK equation with the classical quadratic mixing rule was appropriate for describing the phase behavior of HFCs with lubricants. However, miscibility of HCs/HFCs with mineral oils was not systematically researched by experiment. A view is approved by refrigerant scholars that adding HCs into HFCs benefits the oilreturn. Lee et al.15 studied vapor−liquid equilibria for R600a/ R227ea and azeotropy phenomenon was discovered at all temperatures studied in their work which suggested that R600a/R227ea might be a promising mixture. In this work, the miscibility of refrigerant mixture R600a/R227ea with a mineral oil was researched based on China standard SH/T 0699-2000 and the appendix D of Japan standard JIS K2211-2009. TCST was tested over an oil mass fraction range from 5 % to 20 %. The miscibility data were fitted by a method based on analysis of the number of fluorine (F), chlorine (Cl), and hydrogen (H) atoms in the refrigerant mixture. Using this evaluation method, a triangular diagram was proposed, which could show the relationship of TCST with each component mass fraction in the solution of R600a/R227ea/mineral oil.

Table 1. Properties of the Mineral Oil16 property

method

SUNISO 3GS

density 15 °C g/cm3 color ASTM viscosity 40 °C mm2/s viscosity 100 °C mm2/s flash point COC °C pour point °C aniline point °C floc point (R12) °C

ASTM D 1250 ASTM D 1500 ASTM D 445 ASTM D 445 ASTM D 92 ASTM D 97 ASTM D 611 ANSI/ASHRAE 86

0.909 L0.5 29.5 4.31 178 −40 75.4 −53

Figure 1. Schematic diagram of miscibility measurement apparatus.

thermostat (model AI518, from Udian company, China). A transparent quartz test tube withstanding a pressure of 3 MPa was used as the test cell, which had a volume of 137 cm3. The temperature of the refrigerants/oil solution in the test cell was measured by an inserted 100 Ω platinum resistance thermometer (model KYW-010, China) with an accuracy of ± 0.1 K (calibrated by secondary standard platinum resistance thermometers). The mass of refrigerants and the oil was measured by an electronic scale (model CP4202C, from Ohaus agent in Shanghai, China) with accuracy of 0.01 g. The temperature range in this test was from 223.15 K to 303.15 K. Experimental procedures were developed for testing the miscibility of refrigerants with oils. The desired amount of mineral oil was first weighed and injected into the cell. Then the cell was evacuated to remove noncondensable gas and moisture in the oil and piping system. Then the desired amount of R227ea and R600a was charged into the cell which had been submerged in the cooled oil bath in advance. The test cell was filled to more than 50% of its volume in order to decrease the experimental uncertainty of liquid concentration due to the existence of refrigerants vapor. The temperature of the bath was decreased from the beginning of 303.15 K and suspended cooling at every 5 K interval for 15 min so that no large temperature difference appeared between the cell and bath. At the same time, a steady state (near phase equilibria) in the cell could also be reached. If a second liquid layer, cloudiness, flocs, or precipitates were observed in the test cell as seen through the observation window, the cell was shaken until these signs disappeared and then the temperature of the bath was appropriately adjusted until they appeared again. At last, if the temperature difference between the solution and the bath was within 1 °C, the temperature of the bath was recorded as the critical solubility temperature (TCST) for that concentration. Through increasing the amount of R600a or R227ea and repeating the previous procedures, different TCST values and the corresponding concentrations were obtained.

2. EXPERIMENTAL SECTION 2.1. Materials. R600a (isobutane, CAS Registry No. 75-285) (≥ 99.5 %, mass fraction) and R227ea (1,1,1,2,3,3,3heptafluoropropane, CAS Registry No. 431-89-0) (≥ 99.96 %, mass fraction) were obtained from Zhejiang Lantian Environmental Protection Co., Ltd. (FLTCO). The mineral oil used in the experiment was obtained from Japan Sun Oil Co., Ltd. (SUNOCO), which was refined from specially selected naphthenic crude oils. The typical physical properties of the mineral oil are shown in Table 1. All materials were used without any further purification. 2.2. Apparatus and Procedures. The apparatus used for the miscibility measurement is shown in Figure 1. A cascade refrigeration system was designed to cool down the thermostatic bath to 223.15 K, in which a special kind of antifreezing oil was used as the refrigerating medium. Heat source of the thermostatic bath was offered by an electrical heater. The temperature of the thermostatic bath (stability within 0.05 K) was measured by a 100 Ω platinum resistance thermometer (model TR/02022, from Docorom company, Germany) withan accuracy of ± 0.15 K and controlled by a high accuracy 1782

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the temperature was a little higher than TCST, flocs or precipitates would form in the cell’s bottom (Figure 2c). When the temperature was slightly lower than TCST, cloudiness and flocs increased quickly (Figure 2d). If an obvious oil-rich layer formed in the cell, it indicated that the temperature was much lower than TCST (Figure 2e). Miscibility measurement data consisted of TCST and mass fraction of mineral oil, R600a, and R227ea, shown in Table 2.

The mass fraction of oil, R600a, and R227ea in the liquid phase could be calculated by eq 1. xi = (mi − ρ v V vyi )/(∑ mi − ρ v V v)

(1)

where x was the mass fraction in the liquid phase; m stood for mass; ρv was the density of the vapor phase calculated by the REFPROP software17 under the conditions that the temperature equaled TCST, that the vapor phase was saturated, and that the charged concentration of refrigerants was the hypothetical liquid phase concentration; y was the mass fraction in the vapor phase obtained with ρv at the same time; Vv was the vapor phase volume in the test cell. For the mineral oil, i equaled 1, y1 equaled zero. For R600a and R227ea, i equaled 2 and 3, respectively. 2.3. Analysis of the Experimental Uncertainty. In this paper, the experimental uncertainty might be introduced by temperature measurement and concentration in the liquid phase. The temperature uncertainty of the thermostatic bath and the test cell was within ± 0.5 K. However, because of difficulty in the visual observation of the appearance of flocs, according to standard, the reappearance of large flocs in different laboratories was within 5 K when using the same refrigerants and oils. The mass of oil, R600a and R227ea was weighed by an electronic scale with an accuracy of 0.01 g. The inner diameter of the cell was 2.7 cm and the height measurement error of the cell was 0.05 cm. So the uncertainty of vapor phase volume measurement in the test cell was ± 0.286 cm3. The vapor density and vapor phase concentration were determined by REFPROP software17 as described in the last paragraph. Assuming that the error of the vapor density calculated by REFPROP 9.0 was 10 %, the error quantification was concluded as eq 2. In view of the error from the volume measurement, REFPROP software and electronic scale, the total uncertainty of liquid phase concentration was within ± 0.01. Δx =

∑

ρ V ΔV V + V V Δρ V + Δm weigh

n

m

3. MISCIBILITY OF PURE REFRIGERANTS WITH MINERAL OILS In essence, the theory for (vapor) liquid−liquid equilibrium was appropriate for the miscibility evaluation of the refrigerants/oil mixture. However, determining the concentration of the oil-rich phase and refrigerant-rich phase was difficult in low temperature conditions.18 Therefore, the activity coefficient models such as nonrandom two-liquid (NRTL) or Flory−Huggins theory was less convenient. The Hansen solubility parameters were good methods to predict the miscibility, but a large number of solvents were needed to determine these parameters for refrigerants and oils. Even so, some solubility parameters such as the polar parameter (related to hydrogen bond) always had considerable error.19 On the basis of the analysis about the element number and element contribution in the refrigerants, miscibility degree of refrigerants with mineral oils can be predicted easily. F blocked the miscibility, while Cl and H promoted the miscibility. Element contribution20 could be reflected in eq 3. n1 Z= n1 + 1.9n2 + n3/4 (3) where n1 was the number of F; n2 was the number of Cl, n3 was the number of H. The miscibility degree for common refrigerants with mineral oils was obtained from the literature8,10 as shown in Table 3. Seen from the table, the larger the parameter Z was, the worse the miscibility became.

/n

4. MISCIBILITY OF REFRIGERANT MIXTURES WITH MINERAL OILS As compared with other methods, the element contribution evaluation method (ECEM) was convenient to predict the miscibility of HFCs, HCFCs, HCs, and their mixtures with mineral oils. Shown in Table 2, the parameter Z for R600a/ R227ea in various proportions was calculated by eq 4, which was derived through considering the mole fraction of R600a/ R227ea/oil.

= 0.005 % + 0.744 % + 0.100 % = 0.849 %

(2)

2.4. Experimental Result. In the miscibility measurement, when the temperature of the refrigerants/oil solution exceeded TCST, the solution appeared to be clear as in Figure 2b. When

Z=

n1R227eax3 MR227ea n1R227eax3 MR227ea

+

(

n3R227eax3 MR227ea

+

n3R600ax 2 MR600a

)/4

(4)

where M was the molar mass and its subscript stood for the corresponding refrigerant. The superscript of n also stood for the corresponding refrigerant. The value of M was obtained from REFPROP software,17 shown in Table 3. The relationship between parameter Z and TCST is shown in Figure 3. Without regard to the oil mass fraction, the larger the parameter Z was, the higher TCST became (in other words, the worse the miscibility became). Through fitting many math models, such as polynomial and logarithm models using MATLAB software, eq 5 was proposed

Figure 2. Miscibility measurement: (a) pure mineral oil, (b) clear mixture solution, (c) a little flocs or precipitates in the solution, (d) large flocs in the solution and an (e) oil-rich liquid layer. 1783

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Table 2. Miscibility Data from 223.15 K to 303.15 K of Various Oil Ratiosa and Calculation Results for R600a/R227ea in Mineral Oil x1

x2

Z

TCST/K

TCAL/K

0.2752 0.1994 0.2449 0.1927 0.1869 0.1045 0.0775 0.1821 0.1263 0.0767 0.1801 0.0805 0.1249 0.1288 0.1024 0.1317 0.0834 0.1126 0.1774 0.1353 0.0753 0.1229 0.0865 0.1008 0.1387 0.1744 0.0878 0.0748 0.1196 0.1402 0.1214 0.0900 0.0991 0.1714 0.1451 0.0738 0.2208 0.0744

0.1712 0.2007 0.2625 0.2274 0.2508 0.2526 0.2405 0.2699 0.2610 0.2483 0.2782 0.2496 0.2695 0.2660 0.2677 0.2721 0.2588 0.2720 0.2891 0.2795 0.2623 0.2812 0.2682 0.2791 0.2865 0.3011 0.2724 0.2677 0.2891 0.2922 0.2900 0.2791 0.2911 0.3131 0.3024 0.2771 0.3351 0.2779

0.73600 0.72185 0.62799 0.69179 0.66592 0.69144 0.71212 0.64504 0.67532 0.70414 0.63602 0.70177 0.66640 0.66891 0.67582 0.66110 0.69114 0.66774 0.62429 0.65160 0.68994 0.65420 0.68025 0.66401 0.64243 0.61148 0.67543 0.68444 0.64656 0.63549 0.64493 0.66766 0.65163 0.59876 0.62209 0.67489 0.54825 0.67387

> 303.15 > 303.15 > 303.15 > 303.15 > 303.15 310.15 305.15 301.15 298.12 294.15 292.73 290.65 290.15 289.95 283.65 283.15 280.65 280.15 279.15 275.65 273.15 272.65 272.15 272.15 269.15 268.35 268.15 268.15 266.15 263.15 262.65 260.15 260.15 259.65 259.15 258.15 257.15 257.15

650.40 473.15 306.69 370.02 322.13 301.93 309.69 296.14 297.06 294.46 287.11 294.73 286.05 291.23 280.53 284.92 282.66 279.34 276.87 278.18 273.55 273.36 272.75 267.94 272.49 267.23 268.96 267.00 265.43 268.11 265.17 263.47 257.09 258.94 261.55 257.15 251.70 256.79

TCAL − TCST

−8.22 4.54 −5.01 −1.06 0.31 −5.62 4.08 −4.10 1.28 −3.12 1.77 2.01 −0.81 −2.28 2.53 0.40 0.71 0.60 −4.21 3.34 −1.12 0.81 −1.15 −0.72 4.96 2.52 3.32 −3.06 −0.71 2.40 −1.00 −5.45 −0.36

x1

x2

Z

TCST/K

TCAL/K

TCAL − TCST

0.0731 0.0932 0.1272 0.1683 0.1184 0.0962 0.0784 0.0720 0.1587 0.1173 0.1657 0.1652 0.1162 0.0712 0.1001 0.0703 0.1374 0.2082 0.1724 0.0589 0.0693 0.0948 0.1148 0.1611 0.1799 0.1138 0.1480 0.2023 0.1088 0.1117 0.0680 0.0919 0.1880 0.1556 0.1962

0.2845 0.2893 0.3073 0.3256 0.3074 0.2984 0.2926 0.2944 0.3307 0.3137 0.3454 0.3378 0.3204 0.3028 0.3105 0.3113 0.3320 0.3730 0.3592 0.2897 0.3217 0.3218 0.3287 0.3544 0.3748 0.3340 0.3576 0.3908 0.3374 0.3465 0.3345 0.3430 0.3917 0.3761 0.4089

0.66740 0.65568 0.62362 0.58557 0.62698 0.64490 0.65713 0.65739 0.58400 0.62045 0.56376 0.57276 0.61356 0.64888 0.63046 0.64039 0.59197 0.50858 0.54443 0.66655 0.62994 0.62028 0.60508 0.55563 0.52217 0.59961 0.55826 0.49045 0.59802 0.58687 0.61710 0.59882 0.49764 0.53333 0.47217

254.15 253.15 252.15 250.15 249.15 248.15 247.65 247.15 246.65 245.50 241.65 240.65 239.65 239.65 238.65 238.65 238.65 238.15 238.15 238.15 236.65 236.65 236.15 235.15 233.65 233.65 232.65 232.15 229.15 228.15 228.15 228.15 227.15 227.15 226.15

250.51 256.26 254.29 251.43 252.00 250.77 247.20 242.84 247.43 247.91 241.77 245.01 243.92 237.18 244.56 232.19 243.04 235.85 237.19 238.40 226.79 236.77 239.42 237.51 232.68 236.73 234.30 229.97 233.93 231.03 221.05 226.74 228.45 229.23 224.69

−3.64 3.11 2.14 1.28 2.85 2.62 −0.45 −4.31 0.78 2.41 0.12 4.36 4.27 −2.47 5.91 −6.46 4.39 −2.30 −0.96 0.25 −9.86 0.12 3.27 2.36 −0.97 3.08 1.65 −2.18 4.78 2.88 −7.10 −1.41 1.30 2.08 −1.46

Standard type B uncertainties u are u(T) = ± 0.29 K, and u(x) = ± 0.0058. a

Table 3. Parameter Z of Common Pure Refrigerants in an Order of Decreasing Miscibility from Left to Right formula molar mass Z

R600a

R12

R142b

R161

R22

R124

R152a

R227ea

C4H10 58.12 0

CF2Cl2 120.91 0.3448

C2H3F2Cl 100.5 0.4301

C2H5F 48.06 0.4444

CHF2Cl 86.468 0.4819

C2HF4Cl 136.48 0.6504

C2H4F2 66.051 0.6667

C3HF7 170.03 0.9655

to fit miscibility data, and the calculation result is shown in Table 2. TCAL =

β1x1 β2 1 − β3Z

+ β4

refrigerants and oil. This paper proposed a triangular diagram to represent the miscibility degree of the binary refrigerant mixture with oil. According to the scope of calculated TCST, the triangular diagram was divided into five regions, shown in Figure 4. From the top left corner to the bottom right corner, five regions stood for the area of TCST over 303.15 K, between 273.15 K and 303.15 K, between 253.15 K and 273.15 K, between 233.15 K and 253.15 K and below 233.15 K, respectively. The scale in each axis stood for the mass fraction of corresponding refrigerants and the mineral oil. Seen from the figure, if the mass fraction of R600a was smaller than 20 % in the mixed solution, an oil-rich liquid phase might appear even though the

(5)

β1 to β4 were fitted using the miscibility data in Table 2; β1 = 72.3512, β2 = 0.6365, β3 = 1.2704, β4 = 160.5577. Shown in Table 2, the maximum deviation between TCAL and TCST was 9.86 K, and the average deviation was 3.47 K. On the basis of eq 5, TCST in different component concentrations could be calculated. Certainly, derived eqs 4 and 5 were related only to one branch (lean-oil) of solubility and the same sample of 1784

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +86 22 27890627. Funding

This work has been supported by the National Natural Science Foundation of China (Grant No. 51476111 and 51276124), Research Fund for the Doctoral Program of Higher Education of China (No.20130032130006), and the Science and Technology Project of Tianjin City (Grant No. 12ZCDGGX49400). Notes

The authors declare no competing financial interest.

■

REFERENCES

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Figure 3. Relationship between TCST and parameter Z.

Figure 4. Evaluation result of miscibility degree for R600a/R227ea with mineral oil.

oil rate was within 2.5 % to 5 % (typical oil circulation concentration) under common air-conditioning refrigeration temperature. When the mass fraction of R600a was larger than 30 %, the miscibility was good enough for a common refrigeration system.

5. CONCLUSION The critical solubility temperature for R600a/R227ea with a mineral oil was tested over a temperature range from 223.15 K to 303.15 K and an oil mass fraction range from 5 % to 20 %. A method based on the element type and number was used to evaluate miscibility of pure refrigerants with mineral oils and fit miscibility data of R600a/R227ea with a mineral oil. The fitting result was shown in a triangular diagram, and the diagram was divided into five regions according to the scope of TCST. The result showed that when the mass fraction of R600a changed between 20 % and 30 % in the mixed solution, the miscibility degree of R600a/R227ea with the mineral oil was sensitive. In a common refrigeration air-conditioning system, when the R600a mass fraction was below 20 % in the R600a/R227ea/oil solution, the other liquid phase would appear; when the R600a mass fraction is over 30 %, the refrigerants/oil solution was believed to be clear enough. 1785

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DOI: 10.1021/je5011714 J. Chem. Eng. Data 2015, 60, 1781−1786