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Crystallization Temperature, Vapor Pressure, Density, Viscosity, and Specific Heat Capacity of the LiNO3/[BMIM]Cl/H2O Ternary System Chunhuan Luo,†,‡ Kang Chen,† Yiqun Li,† and Qingquan Su*,†,§ †

School of Energy and Environmental Engineering, University of Science and Technology Beijing, 30 Xueyuan Road, Haidian District, Beijing 100083, China ‡ Beijing Key Laboratory of Energy Conservation and Emission Reduction for Metallurgical Industry, Beijing, 100083, China § Beijing Higher Institution Engineering Research Center of Energy Conservation and Environmental Protection, Beijing, 100083, China ABSTRACT: The crystallization temperature, vapor pressure, density, viscosity, and specific heat capacity for the ternary system of LiNO3/[BMIM]Cl/H2O with a mass ratio (LiNO3:[BMIM]Cl) of 2:1 were measured. The crystallization temperature was measured from w = 0.65 to 0.75. The vapor pressure was measured from w = 0.55 to 0.75 and 298.15 to 416.45 K. The density, viscosity, and specific heat capacity were measured from w = 0.55 to 0.75 and 293.15 to 373.15 K. Regression equations for the measured values were obtained by a least-squares method. Results showed that LiNO3/[BMIM]Cl/H2O had vapor pressure nearly identical to that for LiNO3/H2O at a 10% lower mass fraction. The crystallization temperature for LiNO3/[BMIM]Cl/H2O was much lower than that for LiNO3/H2O with the same absorption ability. LiNO3/ [BMIM]Cl/H2O has a great potential as an alternative working pair.

1. INTRODUCTION Absorption heat pumps have gained interest recently as they can be driven by low grade industrial waste heat and renewable energy such as solar energy and geothermal energy to supply heat, refrigeration, and steam.1−3 The thermophysical properties of a working pair have great impacts on the performance of an absorption heat pump. LiBr/H2O is usually used as a traditional working pair but has the shortcomings of easy crystallization and severe corrosivity, with which applications for use in an absorption heat pump have been greatly limited until now. To replace LiBr/H2O, our research group previously proposed LiNO3/H2O.4−6 Results showed that LiNO3/H2O was helpful to reduce the temperature of the driving heat source under the same operation conditions. Moreover, its corrosivity to the structure material and heat exchange material of the absorption heat pump is much less severe than that of LiBr/H2O. The coefficient of performance (COP) for LiNO3/ H2O is also slightly higher than that for LiBr/H2O. However, LiNO3/H2O still faces the problem of easy crystallization. Recently, ionic liquids (ILs), which show unique properties including low melting points and very low saturated vapor pressures, were widely studied as alternative absorbents for the absorption heat pump.7−13 Kim et al. investigated the feasibility of several refrigerant/IL working pairs for the absorption refrigeration system by a numerical method. Among the refrigerants of H2O, R32 and R124, H2O combined with the absorbent of [EMIM][BF4] (1-ethyl-3-methylimidazolium tetrafluoroborate) gave the highest COP.14,15 Dong and Zheng et al. carried out a series of studies on imidazolium ILs/H2O with different kinds of anions (Cl−, Br−, DMP, BF4, etc.) and established criteria for selecting working pairs. On the © 2017 American Chemical Society

basis of measured thermophysical properties, they predicted the performance of an absorption heat pump using [MMIM][DMP] (1,3-dimethylimidazolium dimethylphosphate)/H2O as a working pair, showing that the problems of crystallization and corrosion were relieved, although the performance was slightly lower than that with LiBr/H2O as the working pair.16−19 Zheng et al. also reviewed the research on the working pairs based on imidazolium IL with particular focus on the methods of evaluation and selection, thermophysical property measurement and modeling, future prospect assessment, and developing potential in the applications.20 The thermophysical properties of ILs/H2O as well as performance predictions of the absorption heat pump based on ILs/H2O have also been reported in other studies.21−25 Generally, compared with LiBr/ H2O and LiNO3/H2O, ILs/H2O have much lower crystallization temperatures and corrosivity while with much higher working absorbent mass fractions and viscosities, leading to a reduction in the performance of the absorption heat pump.20 Ionic liquids with imidazolium cations and halide anions have the characteristics of good chemical and thermal stability as well as strong hydrophilicity. Adding IL into LiNO3/H2O is helpful to overcome the crystallization problem for LiNO3/H2O. Meanwhile, adding LiNO3 into IL/H2O is able to reduce the viscosity for IL/H2O. Consequently, a ternary working pair of LiNO3/[BMIM]Cl (1-butyl-3-methylimidazolium chloride)/ H2O was proposed, and its thermophysical properties, including the crystallization temperature, vapor pressure, Received: January 20, 2017 Accepted: July 21, 2017 Published: August 4, 2017 3043

DOI: 10.1021/acs.jced.7b00059 J. Chem. Eng. Data 2017, 62, 3043−3052

Journal of Chemical & Engineering Data

Article

Table 1. Detailed Information for Chemical Samples chemical name phenol salicylate [BMIM]Cl lithium nitrate lithium bromide sodium chloride silver nitrate potassium chromate

source Shandong Xiya Chemical Co., Ltd. Shanghai Chengjie Chemical Co., Ltd. Tianjin Jinke Institute of Fine Chemicals Tianjin Jinke Institute of Fine Chemicals Tianjin Jinke Institute of Fine Chemicals Tianjin Fengchuan Chemical Co., Ltd. Tianjin Guangfu Chemical Co., Ltd.

initial mass fraction purity

purification method

melting temperature (K)

thermal decomposition temperature (K)

w > 0.99 w > 0.99

none none

315.0526 338.15,27 343.15a

537.1528

w > 0.995

none

533.1529

873.1529

w > 0.995

none

822.1529

w > 0.995

none

1074.1529

w > 0.998

none

485.1529

w > 0.995

none

a

The experimental pressure p was p = 0.1 MPa, and its standard uncertainty u was u(p) = 3.0 kPa. The standard uncertainty u in the melting temperature Tm measurement was u(Tm) = 2 K. N

density, viscosity, and specific heat capacity, were measured systematically.

AAD =

2. EXPERIMENTAL SECTION 2.1. Materials. LiNO3, LiBr, and NaCl reagents were supplied by Tianjin Jinke Institute of Fine Chemicals (China). [BMIM]Cl reagent was supplied by Shanghai Chengjie Chemical Co., Ltd. (China). C13H10O3 (phenol salicylate) was supplied by Shandong Xiya Chemical Co., Ltd. (China). AgNO3 was supplied by Tianjin Fengchuan Chemical Co., Ltd. (China). K2CrO4 was supplied by Tianjin Guangfu Chemical Co., Ltd. (China). All reagents were used without further purification. Pure water with an electric resistance of 18.2 MΩ· cm was used to prepare solutions. The melting point temperature of [BMIM]Cl was measured by a microreaction calorimeter (μRC, THT Co., UK). The melting point of phenol salicylate was scanned to verify this measurement. The deviation between the measured value and the literature value was 1.60 K.26 The detailed sample information is given in Table 1. 2.2. Apparatus and Procedure. The measurement apparatus for crystallization temperature consisted of a precision thermostat with a measuring range from 243.15 to 372.15 K (HX-3010, Bilang, Shanghai), a sets of Erlenmeyer flasks (500 and 150 mL), and a precision balance (PL2002, Mettler Toledo, Switzerland). A solution of LiNO3/[BMIM]Cl/H2O with a certain mass fraction was prepared in a ground 500 mL Erlenmeyer flask and then equally poured into three smaller Erlenmeyer flasks (150 mL). After being sealed with rubber stoppers, the samples were immersed in the same thermostat at a certain setting temperature for 24 h. If the solution in any Erlenmeyer flask was not crystallized, the setting temperature of the thermostat was reduced, and the samples were observed for another 24 h. If crystallization occurred in any Erlenmeyer flask, the setting temperature was increased, and the samples were observed in the thermostat for another 24 h after the crystal was dissolved by heating. After repeating the above operations, as the sample was crystallized at temperature T K and was not crystallized at temperature T+1 K, the temperature T K was determined to be the crystallization temperature. For each absorbent mass fraction, the measurements of crystallization temperature were replicated three times, and the averages were adopted. The overall average absolute deviation (AAD) for all measurements was determined to be 1.6 K through the following equation:

∑i = 1 |Tc − Tm| N

(1)

where Tc (K) is the crystallization temperature for each measurement, Tm is the mean value of the three repetitions, and N is the overall measurement number. The largest deviation in three repetitions for all measurements was 5.0 K at w = 0.71 due to the larger undercooling degree. The validity of the measurement was verified through measuring the crystallization temperatures of LiBr solutions at w = 0.60 and 0.65. The differences between the measurement values and the literature data were 1.2 and 0.5 K, respectively.30 The equilibrium solid phases were also investigated. After reaching equilibrium, the wet solid phase at the bottom of the Erlenmeyer flask was filtered with glass cloth and dewatered with a centrifuge (TGL-20B, Anke, Shanghai) lasting 3 min at 3000 r·min−1. After the adhesive solution was removed, the equilibrium solid phase was analyzed through TGA (thermogravimetric analysis) using a differential scanning calorimeter (Labsys Evo, Setaram, France) and baking with a Muffle furnace at 523.15 K. The weight loss should correspond to the crystal water. The amount of [BMIM]Cl in the solid phase was determined by precipitating the Cl− ion with 0.1 mol·L−1 AgNO3 solution using K2CrO4 solution (w = 0.05) as an indicator. However, as one drop of AgNO3 solution was added into the prepared solution, precipitate with red-brick color suddenly appeared. It was indicated that there was no Cl− ion in the solid phase. To further determine the amount of nitrate in the solid phase, an atomic absorption spectrophotometer (TA990, Purkinje General Co. Beijing) was used to measure the content of Li+ ion. The detailed procedures were reported in our previous research.5 In this study, a calibration curve (R2 = 0.99996) was obtained by one set of standard lithium solutions (0, 0.1, 0.2, 0.3, 0.4, 0.5, and 0.6 μg·mL−1). A certain amount of solid phases were weighed on a Sartorius balance (BSA224S) and diluted 50 000 times with volumetric flasks. On the basis of the calibration curve, the absorbance (abs.) and corresponding concentrations of Li+ ion for the diluted solutions were obtained. The amounts of LiNO3 in the solid phases were also calculated. In this study, two crystalline morphologies were observed in the equilibrium solid phases. Thus, the amounts of Li+ ions in the solid phases at w = 0.69 and 0.73 with different crystalline morphologies were measured. The measurements of the concentrations of Li+ ion for the two typical mass fractions were also carried out with three parallel 3044



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Table 2. Determination of the amount of LiNO3 in the Solid Phase sample

dry solid phase weight (g)

abs.

concentration of Li+ (μg·mL−1)

calculated weight for LiNO3 (g)

recovery rate (%)

w = 0.69 w = 0.73

0.1193 0.1364

0.110 0.125

0.247 0.279

0.1226 0.1385

102.77 101.54

After detection of the sealing performance of the system, about 500 mL of solution with 2 g of water added (to reduce the error caused by mass loss in purging and vacuuming) was poured into the autoclave and sealed with Teflon ring. To degas the dissolved and adsorbed air, the system was purged about 5 min with helium (He), which has a very low solubility in water. Then, the autoclave was vacated with a vacuum pump and placed in the oil bath. After thermal equilibrium was reached, the temperature of the solution and the corresponding pressure were measured. The temperature and pressure at each state point was measured three times and the average value was used. The overall average absolute relative deviation (AARD) of vapor pressure for all measurements was determined to be 0.55% through the following equation:

samples, and the mean values were used. As shown in Table 2, according to the content of Li+, the amounts of LiNO3 in the solid phases were calculated. The calculated weights of LiNO3 were approximately identical with the weight of the solid phase. The recovery rates were 102.77 and 101.54% at w = 0.69 and 0.73, respectively. Taking into account the measurement errors, we conclude that the nitrate in the equilibrium solid phases with different crystalline morphologies was LiNO3. Then, the composition of the equilibrium solid phase was identified. The static method was used to measure the vapor pressure for LiNO 3 /[BMIM]Cl/H2 O system. The measurement apparatus primarily consisted of a constant temperature oil bath (DKU-30, Jinghong, Shanghai), a thermocouple (Pt-100), a solution autoclave (700 mL) made of duplex stainless steel, two vacuum valves and stainless steel pipes for purging and vacuating, a vacuum pump, and three precision digital absolute pressure meters with ranges from 0 to 6 kPa (MIK-P300, Asmik, Hangzhou), 0 to 20 kPa (MIK-P3000, Asmik, Hangzhou), and 0 to 110 kPa (AX-110, Aoxin, Xi’an), respectively. The connecting pipe was wrapped with the cotton insulation. The detailed schematic figure is shown in Figure 1.

N

AARD =

∑i = 1 |(p − pm )/pm | N

(2)

where p (kPa) is the vapor pressure for each measurement, pm is the mean value of the three repetitions, and N is the overall measurement number. The largest relative deviation was 1.64%. To verify the validity of the vapor pressure measurement, the vapor pressures of LiBr solution at w = 0.55 were measured from 303.95 to 416.35 K and compared with the literature data.31 The overall AARD between the measured values and the literature data was 1.69%. Density measurement was measured with a capillary pycnometer. The measurement apparatus consisted of a precision viscometer oil bath (SYP1003-H, Zhongxi, Beijing), a capillary pycnometer (50 mL) with a capillary diameter of about 1 mm, and a Sartorius balance (BSA224S, Sartorius, Germany). The detailed procedure was reported in previous research.5 Density measurements for each mass fraction and temperature were performed for three repetitions, and the averages were obtained. The AARD for all measurements was determined to be 0.04%, and the largest relative deviation was 0.07%. The calibration of capillary pycnometer was made through measuring the density of a NaCl solution at w = 0.10 from 303.45 to 353.15 K with a step of 10 K. The AARD between the measured values and the literature data was 0.21%.29 Viscosities were measured by using Ubbelohde capillary viscometers with different fine capillaries (0.46, 0.58, 0.73, and 0.91 mm) and viscosity constants. The detailed measurement

Figure 1. Schematic of experimental apparatus for vapor pressure.

Figure 2. Schematic of measurement apparatus for specific heat capacity. 3045



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balance (BSA224S) with a precision of 0.1 mg. The combined standard uncertainties of the crystallization temperature, density, and specific heat capacity were estimated to be 2 K except for w = 0.71, 0.003 g·cm−3, and 0.05 J·g−1·K−1, respectively. The relative standard uncertainties of the vapor pressure and viscosity were ur(p) = 0.02 and ur(ν) = 0.03, respectively.

schematic and procedure were reported in our previous research.5 The measurements for each absorbent mass fraction and temperature were repeated three times, and the averages were obtained. The viscosities were calculated through eq 3: ν = Ctm

(3)

where v (mm2·s) is the kinematic viscosity for LiNO3/ [BMIM]Cl/H2O, C (mm2·s−2) is the viscosity constant, and tm (s) is the mean time of the three repetitions. The AARD value of viscosity for all measurements was determined to be 0.16%, and the largest relative deviation was 0.25%. The capillary viscometers were also validated using water from 303.15 to 353.15 K with a step of 10 K, and the AARD between the measured values and the literature data was 1.46%.29 The specific heat capacity for LiNO3/[BMIM]Cl/H2O was also measured by the microreaction calorimeter. The measurement apparatus consisted of an air cylinder, a drying operation cabinet, a microreaction calorimeter, and a computer for data analysis. The detailed apparatus is shown in Figure 2. The measurement of specific heat capacity was conducted by making a “step-change” in the measurement temperature. A step-change of 1 K was applied in this work. For example, at a measurement temperature of 293.15 K, the step-change corresponds to a shift from 292.65 to 293.65 K, then back to 292.65 K, where 293.15 K is the median temperature. In the measurement of the specific heat capacity, a blank measurement, in which two empty vials of approximately 1.5 mL were placed in the sample and reference positions, was conducted first. After running the blank measurement, an empty vial was left in the reference position. The sample solution was prepared in the drying operation cabinet filled with dry air due to the strong hygroscopicity of LiNO3 and [BMIM]Cl. Then, suitable amount of solution (about half of a vial volume) was added in another vial and weighed on a Sartorius balance, and the vial was carefully set in the sample position. The steps for the sample measurement were the same as the steps for the blank measurement. The specific heat capacity was obtained through subtracting the measured blank value from the measured sample value in the μRC analysis software. Measurements of specific heat capacity were also carried out for three repetitions for each mass fraction, and the mean value was obtained. The AARD for all measurements was determined to be 0.95%, and the largest relative deviation was 1.22%. Pure water was used to validate the method from 293.15 to 353.15 K with a step of 10 K. The AARD between the measured values and the literature data29 was 1.2%. Compared with a differential scanning calorimeter, the microreaction calorimeter has better reproducibility, especially at high temperatures, due to the hundredfold greater sample mass. The temperatures for the density and viscosity measurements were obtained by the thermometer placed inside the precision viscometer oil bath, and the standard uncertainty of the temperature measurement was 0.05 K. The crystallization temperature was measured by the thermometer in the precision thermostat, and the standard uncertainty was 0.2 K. The temperature for the vapor pressure was measured by a Pt-100 thermometer, and the standard uncertainty was 0.1 K. For the specific heat capacity, the temperature was measured by the microreaction calorimeter with an accuracy of 0.001 K. The solutions prepared for crystallization temperature and vapor pressure measurements were weighed on a Mettler balance (PL2002) with a precision of 0.01 g. For density, viscosity, and specific heat capacity, the reagents were weighed on a Sartorius

3. RESULTS AND DISCUSSION 3.1. Influence of IL on the Crystallization Temperature and Vapor Pressure. The crystallization temperatures for LiNO3/[BMIM]Cl/H2O at w = 0.65, 0.69, and 0.73 with different mass ratios m 1 /m 2 (LiNO 3 :[BMIM]Cl) were measured, and the results are given in Table 3. At a constant Table 3. Crystallization Temperature Tc at Absorbent Mass Fraction w for the System of LiNO3 (1) + 1-Butyl-3methylimidazolium Chloride (2) + H2O (3) at Different Mass Ratios m1/m2 and p = 0.1 MPaa w1+2 = 0.65 m1/m2

Tc (K)

2:1 2.5:1 3:1

270 275 274

w1+2 = 0.69

solid phase

Tc (K)

LiNO3·3H2O LiNO3·3H2O LiNO3·3H2O

271 274 283

w1+2 = 0.73

solid phase

Tc (K)

solid phase


282 311 323

LiNO3 LiNO3 LiNO3

a

Standard uncertainties u were u(m1/m2) = 0.01, u(w1+2) = 0.002, u(p) = 3.0 kPa, and u(Tc) = 2 K (0 °C = 273 K).

absorbent mass fraction, the crystallization temperature substantially decreased with decreasing mass ratio, demonstrating that the addition of [BMIM]Cl reduced the crystallization temperature for LiNO3/[BMIM]Cl/H2O. The equilibrium solid phases were the same for different mass ratios. The influence of [BMIM]Cl on the vapor pressure for LiNO3/[BMIM]Cl/H2O was also investigated through measuring the vapor pressures at w = 0.70 with different mass ratios. As shown in Figure 3, the vapor pressure increased as the m1/ m2 mass ratio decreased, indicating that the addition of [BMIM]Cl reduced the absorption ability of LiNO3/[BMIM]Cl/H2O. At w = 0.70 with mass ratios of m1/m2 (2:1, 2.5:1, and 3:1), LiNO3/[BMIM]Cl/H2O had almost the same absorption ability as LiNO3/H2O at w = 0.60. On the basis of the overall consideration of the crystallization temperature and absorption ability, LiNO3/[BMIM]Cl/H2O with the mass ratio of 2:1 was further studied as a new working pair. 3.2. Properties for LiNO3/[BMIM]Cl/H2O with the Mass Ratio of 2:1. 3.2.1. Vapor Pressure. Vapor pressures for LiNO3/[BMIM]Cl/H2O with the mass ratio of 2:1 were measured from w = 0.55 to 0.75 and 298.15 to 416.45 K. The measured values are shown in Table 4. The vapor pressures over the absorbent mass fractions and temperatures were fitted to an Antoine eq 4.32,33 4

log(p) =

∑ [Ai + {Bi/(T − Ci)}]wi i=0

(4)

where p (kPa) is the vapor pressure, T (K) is the absolute temperature, and w is the absorbent mass fraction. Ai, Bi, and Ci are the regression parameters, which were determined by a least-squares method and are listed in Table 5. The overall AARD value between the measured values and the calculated results was determined to be 0.49%. The measured values 3046



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Figure 3. Vapor pressures of LiNO3/[BMIM]Cl/H2O at w = 0.70 with different mass ratios ●, w = 0.70 for LiNO3/[BMIM]Cl/H2O (2:1); ▲, w = 0.70 for LiNO3/[BMIM]Cl/H2O (2.5:1); ■, w = 0.70 for LiNO3/[BMIM]Cl/H2O (3:1); ○, w = 0.55; △, w = 0.60; □, w = 0.65 for LiNO3/H2O.

Table 4. Vapor Pressure p at Temperature T and Absorbent Mass Fraction w for the System of LiNO3 (1) + 1-Butyl-3methylimidazolium Chloride (2)+ H2O (3)a w1+2 = 0.55

w1+2 = 0.60

w1+2 = 0.65

w1+2 = 0.70

w1+2 = 0.75

T (K)

P (kPa)

T (K)

P (kPa)

T (K)

P (kPa)

T (K)

P (kPa)

T (K)

P (kPa)

298.45 302.65 308.75 313.55 318.45 323.55 328.25 333.85 338.55 343.45 353.85 363.85 373.45 384.85 392.05

1.598 2.059 2.879 3.741 4.875 6.421 8.140 10.693 13.462 16.683 26.072 38.759 54.597 78.841 97.680

298.45 303.15 308.05 312.85 317.85 322.55 328.15 333.65 338.35 343.05 352.35 362.65 373.45 383.15 388.95 396.75

1.271 1.703 2.233 2.896 3.795 4.881 6.544 8.645 10.862 13.518 20.418 31.293 46.637 64.716 77.696 97.613

298.15 303.75 308.25 313.35 318.45 323.45 328.45 333.35 339.15 342.75 353.55 363.65 373.65 383.15 393.65 402.45

0.971 1.389 1.791 2.356 3.102 4.053 5.260 6.787 8.869 10.746 17.583 26.759 39.160 55.062 76.406 98.298

298.55 302.65 307.75 313.05 318.55 323.35 328.85 334.15 338.65 344.05 353.15 362.25 373.35 382.45 393.55 404.35 409.15

0.752 1.004 1.348 1.792 2.407 3.105 4.167 5.395 6.873 8.936 13.655 20.187 31.315 43.544 62.981 86.639 98.559

323.15 328.25 332.55 344.15 353.55 363.75 373.25 383.75 393.65 403.15 408.15 416.45

2.331 3.060 3.818 6.861 10.761 16.774 24.613 36.345 50.937 68.791 79.442 97.541

a

The mass ratio of LiNO3 to1-butyl-3-methylimidazolium chloride was 2:1, and the uncertainty was u(m1/m2) = 0.01. Standard uncertainties u were u(T) = 0.1 K, u(w1+2) = 0.002, and the relative standard uncertainty ur was ur(p) = 0.02. 3

(solid points) and the calculated results (solid lines) are plotted against temperature and compared with those (hollow points) for LiNO3/H2O in Figure 4. As shown in Figure 4, the measured values were in good agreement with the calculated results. The vapor pressure for LiNO3/[BMIM]Cl/H2O was almost the same as that for LiNO3/H2O at a 10% lower mass fraction. 3.2.2. Crystallization Temperature. Crystallization temperatures for LiNO3/[BMIM]Cl/H2O were measured from w = 0.65 to 0.75 with a step of 2%. The measured values are shown in Table 6. Polynomial eq 5 for the crystallization temperature was obtained as a function of absolute temperature from the measured values.

(Tc) =

∑ Ai(w)i i=0

(5)

where Tc (K) is the crystallization temperature, w (wt %) is the absorbent mass fraction, and Ai are the regression parameters. The parameters were determined by a least-squares method and are listed in Table 5. The identified equilibrium solid phase was LiNO3·3H2O from w = 0.65 to 0.71 and anhydrous LiNO3 at w = 0.73 and 0.75. These results were confirmed through crystalline morphology. Bulk crystals were observed at the lower mass fractions, which generally corresponded to hydrated crystals, whereas granular crystals were observed at the higher mass fractions, typically corresponding to anhydrous crystals. 3047



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Table 5. Values of Ai, Bi, Ci, and AARD for Eqs 4−8 eq 4

eq 5

eq 6

eq 7

eq 8

i

Ai

Bi

Ci

AARD (%)

0 1 2 3 4 0 1 2 3 0 1 2 0 1 2 3 0 1 2

16.823266 −21.326553 79.049897 −62.057604 22.251215 −5.689991 × 104 2.512619 × 105 −3.678522 × 105 1.793982 × 105 −5.495414 21.745478 −16.831896 65.272400 −1.877324 × 102 1.216458 × 102 14.736149 10.222776 −21.165475 15.361451

−4.485143 × 103 8.100416 × 103 2.026742 × 103 −6.367834 × 103 1.696996 × 103

−1.061026 × 102 1.006537 × 103 2.985851 × 102 63.335921 1.852958 × 102

0.49

0.22

3.946918 × 10−2 −0.127859 0.101844 −3.779491 × 104 9.629412 × 104 −3.796766 × 104 −3.093659 × 104 −4.167569 × 10−2 0.116856 −8.984805 × 10−2

−6.034304 × 10−5 1.925331 × 10−4 −1.536450 × 10−4 5.103506 × 106 −1.029599 × 107 −2.096252 × 106 9.474969 × 106 6.960093 × 10−5 −1.871806 × 10−4 1.423247 × 10−4

0.06

0.72

0.18

Figure 4. Plot of vapor pressure for LiNO3/[BMIM]Cl/H2O at temperatures from 298.15 to 416.45 K ▲, w = 0.55; ■, w = 0.60; ⧫, w = 0.65; ●, w = 0.70; ★, w = 0.75; , calculated results; △, w = 0.45; □, w = 0.50; ◇, w = 0.55; ○, w = 0.60; ☆, w = 0.65 for LiNO3/H2O system.

Table 6. Crystallization Temperature Tc at Absorbent Mass Fraction w for the System of LiNO3 (1) + 1-Butyl-3methylimidazolium Chloride (2) + H2O (3) at p = 0.1 MPaa w1+2 0.65 0.67 0.69

Tc (K)

solid phase

270 272 271


w1+2 0.71 0.73 0.75

b

Tc (K)

solid phase

269 282 313

LiNO3·3H2O LiNO3 LiNO3

points) for LiNO3/H2O with the same absorption ability in Figure 5. Results indicated that the measured values agreed well with the calculated results. The crystallization temperature for LiNO3/[BMIM]Cl/H2O was lower than that for LiNO3/H2O with the same absorption ability. Particularly, as w ≤ 0.71%, the crystallization temperature for the former was about 30 K lower than that for the latter, indicating that the addition of [BMIM] Cl to LiNO3/H2O was able to solve the crystallization problem. 3.2.3. Density. Densities for LiNO3/[BMIM]Cl/H2O were measured from 293.15 to 373.15 K and w = 0.55 to 0.75. The measured values are listed in Table 7. Polynomial eq 6 for density was obtained as a function of absolute temperature and absorbent mass fraction by a least-squares method from the measured values.34

a

The mass ratio of LiNO3 to 1-butyl-3-methylimidazolium chloride was 2:1, and the uncertainty was u(m1/m2) = 0.01. Standard uncertainties u were u(w1+2) = 0.002, u(p) = 3.0 kPa and u(Tc)= 2 K except for w1+2 = 0.71. bLiNO3/[BMIM]Cl/H2O ternary system may have a larger undercooling degree at this point (0 °C = 273 K).

The measured crystallization temperatures (solid points) and the calculated results (solid lines) were plotted against the absorbent mass fraction and compared with those (hollow 3048



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Figure 5. Plot of crystallization temperature for LiNO3/[BMIM]Cl/H2O from w = 0.65 to 0.75 ●, measured values; , calculated values for LiNO3/[BMIM]Cl/H2O (2:1); ○, measured values for LiNO3/H2O with the same absorption ability.

Table 7. Density ρ at Temperature T and Absorbent Mass Fraction w for the System of LiNO3 (1) + 1-Butyl-3methylimidazolium Chloride (2) + H2O (3) at p = 0.1 MPaa w1+2 = 0.55

w1+2 = 0.60 −3

w1+2 = 0.65 −3

w1+2 = 0.70 −3

w1+2 = 0.75 −3

T (K)

ρ (g·cm )

T (K)

ρ (g·cm )

T (K)

ρ (g·cm )

T (K)

ρ (g·cm )

T (K)

ρ (g·cm−3)

293.15 303.15 313.15 323.15 333.15 343.15 353.15 363.15 373.15

1.280 1.275 1.267 1.262 1.255 1.249 1.242 1.235 1.227

293.15 303.15 313.15 323.15 333.15 343.15 353.15 363.15 373.15

1.309 1.302 1.296 1.288 1.282 1.275 1.268 1.261 1.255

293.15 303.15 313.15 323.15 333.15 343.15 353.15 363.15 373.15

1.340 1.333 1.327 1.320 1.313 1.306 1.298 1.291 1.285

293.15 303.15 313.15 323.15 333.15 343.15 353.15 363.15 373.15

1.367 1.360 1.355 1.348 1.341 1.334 1.327 1.320 1.313

323.15 333.15 343.15 353.15 363.15 373.15

1.377 1.369 1.362 1.355 1.345 1.337

a

The mass ratio of LiNO3 to 1-butyl-3-methylimidazolium chloride was 2:1, and the uncertainty was u(m1/m2) = 0.01. Standard uncertainties u were u(T) = 0.05 K, u(w1+2) = 0.002, and u(p) = 3.0 kPa, and the combined standard uncertainty uc was uc(ρ) = 0.003 g·cm−3.

Figure 6. Plot of density for LiNO3/[BMIM]Cl/H2O at temperatures from 293.15 to 373.15 K ▲, w = 0.55; ■, w = 0.60; ⧫, w = 0.65; ●, w = 0.70; ★, w = 0.75; , calculated results; ○, w = 0.60 for LiNO3/H2O system.

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Table 8. Viscosity ν at Temperature T and Absorbent Mass Fraction w for the System of LiNO3 (1) + 1-Butyl-3methylimidazolium Chloride (2) + H2O (3) at p = 0.1 MPaa w1+2 = 0.55

w1+2 = 0.60

w1+2 = 0.65

w1+2 = 0.70

w1+2 = 0.75

T (K)

ν (mm2·s−1)

T (K)

ν (mm2·s−1)

T (K)

ν (mm2·s−1)

T (K)

ν (mm2·s−1)

T (K)

ν (mm2·s−1)

293.15 303.15 313.15 323.15 333.15 343.15 353.15 363.15 373.15

5.283 4.026 3.162 2.531 2.100 1.767 1.538 1.376 1.244

293.15 303.15 313.15 323.15 333.15 343.15 353.15 363.15 373.15

7.617 5.784 4.518 3.570 2.885 2.392 2.023 1.760 1.537

293.15 303.15 313.15 323.15 333.15 343.15 353.15 363.15 373.15

11.351 8.561 6.569 5.222 4.138 3.311 2.756 2.327 2.025

293.15 303.15 313.15 323.15 333.15 343.15 353.15 363.15 373.15

17.706 13.274 10.071 7.719 6.053 4.789 3.905 3.235 2.741

323.15 333.15 343.15 353.15 363.15 373.15

12.177 9.259 7.251 5.783 4.720 4.004

a

The mass ratio of LiNO3 to 1-butyl-3-methylimidazolium chloride was 2:1, and the uncertainty was u(m1/m2) = 0.01. Standard uncertainties u were u(T) = 0.05 K, u(w1+2) = 0.002, and u(p) = 3.0 kPa, and the relative uncertainty ur was ur(ν) = 0.03. 2

ρ=

∑ [(Ai + Bi T + CiT 2)wi] i=0

(6)

−3

where ρ (g·cm ) is the density, T (K) is the absolute temperature, w is the absorbent mass fraction, and Ai, Bi, and Ci are the regression parameters. The values of Ai, Bi, Ci, and AARD are given in Table 5. The measured values (solid points) and the calculated results (solid lines) were plotted against temperature in Figure 6 and compared with those (○) for LiNO3/H2O. Results showed that the measured values were in good agreement with the calculated results. The density for LiNO3/[BMIM]Cl/H2O at w = 0.70 was smaller than that for LiNO3/H2O at w = 0.60, indicating that the former had a lower density compared with the latter with the same absorption ability. 3.2.4. Viscosity. Viscosities for LiNO3/[BMIM]Cl/H2O were measured from 293.15 to 373.15 K and w = 0.55 to 0.75. The measured values shown in Table 8 were fitted to polynomial eq 7 as a function of absolute temperature and absorbent mass fraction by a least-squares method.35 3

log(ν) =

⎡⎛

∑ ⎢⎣⎜⎝Ai + i=0

Bi C ⎞ ⎤ + 2i ⎟wi ⎥ T T ⎠ ⎦

Figure 7. Plot of viscosity for LiNO3/[BMIM]Cl/H2O at temperatures from 293.15 to 373.15 K ▲, w = 0.55; ■, w = 0.60; ⧫, w = 0.65; ●, w = 0.70; ★, w = 0.75; , calculated results ○, w = 0.60 for LiNO3/H2O; □, w = 0.80; ☆, w = 0.90 for [BMIM]Cl/H2O. 2

Cp =

(7)

∑ [(Ai + Bi T + CiT 2)wi] i=0

where v (mm2·s) is the kinematic viscosity, T (K) is the absolute temperature, w is the absorbent mass fraction, and Ai, Bi, and Ci are the coefficients in eq 7. The values of Ai, Bi, Ci, and AARD are given in Table 5. The measured values (solid points) and the calculated results (solid lines) are plotted against temperature in Figure 7 and compared with those for LiNO3/H2O (○) and [BMIM]Cl/H2O (□, ☆). Results showed that the measured values agreed well with the calculated results. The viscosity for LiNO3/[BMIM]Cl/H2O at w = 0.70 was larger than that for LiNO3/H2O at w = 0.60 with the same absorption ability. High viscosity not only increases the transport pressure drop for the working pair but also blocks the heat and mass transfer in the heat pump system, leading to a reduction in the performance of the absorption heat pump. However, compared with [BMIM]Cl/H2O, which is usually highly concentrated as a working pair, LiNO3/ [BMIM]Cl/H2O had a much lower viscosity. Therefore, the weakness of high viscosity for [BMIM]Cl/H2O was improved. 3.2.5. Specific Heat Capacity. Specific heat capacities for LiNO3/[BMIM]Cl/H2O were measured from 293.15 to 373.15 K and w = 0.55 to 0.75. The measured values listed in Table 9 were fitted to the following eq 8.36

(8)

where Cp (J·g−1·K−1) is the specific heat capacity, T (K) is the absolute temperature, w is the absorbent mass fraction, and Ai, Bi, and Ci are the regression parameters. The values of Ai, Bi, Ci, and AARD are given in Table 5. The measured values (solid points) and the calculated results (solid lines) are plotted against temperature in Figure 8 and compared with those (○) for LiNO3/H2O. Results indicated that the measured values were in good agreement with the calculated results. The specific heat capacity for LiNO3/[BMIM]Cl/H2O increased with increasing temperature and decreased with increasing absorbent mass fraction. Compared with that of LiNO3/H2O with the same absorption ability, LiNO3/[BMIM]Cl/H2O had a lower specific heat capacity.

4. CONCLUSIONS The crystallization temperature, vapor pressure, density, viscosity, and specific heat capacity for the ternary system of LiNO3/[BMIM]Cl/H2O with a mass ratio of 2:1 were measured at various temperatures and absorbent mass fractions. Regression equations for the measured values with high accuracy were obtained by the least-squares method. Results 3050



Article

Table 9. Specific Heat Capacities Cp at Temperature T and Absorbent Mass Fraction w for the System of LiNO3 (1) + 1-Butyl-3methylimidazolium Chloride (2) + H2O (3) at p = 0.1 MPaa w1+2 = 0.55

w1+2 = 0.60

w1+2 = 0.65

w1+2 = 0.70

w1+2 = 0.75

T (K)

Cp (J·g−1·K−1)

T (K)

Cp (J·g−1·K−1)

T (K)

Cp (J·g−1·K−1)

T (K)

Cp (J·g−1·K−1)

T (K)

Cp (J·g−1·K−1)

293.15 303.15 313.15 323.15 333.15 343.15 353.15 363.15 373.15

2.71 2.73 2.75 2.77 2.78 2.79 2.81 2.84 2.88

293.15 303.15 313.15 323.15 333.15 343.15 353.15 363.15 373.15

2.64 2.66 2.67 2.69 2.70 2.72 2.73 2.75 2.79

293.15 303.15 313.15 323.15 333.15 343.15 353.15 363.15 373.15

2.57 2.58 2.59 2.61 2.63 2.64 2.66 2.68 2.71

293.15 303.15 313.15 323.15 333.15 343.15 353.15 363.15 373.15

2.50 2.51 2.53 2.54 2.56 2.57 2.59 2.61 2.64

323.15 333.15 343.15 353.15 363.15 373.15

2.49 2.50 2.51 2.52 2.55 2.58

a

The mass ratio of LiNO3 to 1-butyl-3-methylimidazolium chloride was 2:1, and the uncertainty was u(m1/m2) = 0.01. Standard uncertainties u were u(T) = 0.01 K, u(w1+2) = 0.002, and u(p) = 3.0 kPa, and the combined standard uncertainty uc was uc(Cp) = 0.05 J·g−1·K−1.

Notes

The authors declare no competing financial interest.

■

Figure 8. Plot of specific heat capacity for LiNO3/[BMIM]Cl/H2O at temperatures from 293.15 to 373.15 K ▲, w = 0.55; ■, w = 0.60; ⧫, w = 0.65; ●, w = 0.70; ★, w = 0.75; , calculated results; ○, w = 0.60 for LiNO3/H2O.

showed that the vapor pressure for LiNO3/[BMIM]Cl/H2O was nearly identical with that for LiNO3/H2O at a 10% lower mass fraction. Compared with LiNO3/H2O with the same absorption ability, LiNO3/[BMIM]Cl/H2O had a great advantage in terms of crystallization temperature, and compared to that of [BMIM]Cl/H2O with the same absorption ability, LiNO3/[BMIM]Cl/H2O also had an advantage in terms of viscosity. This work is of significance for the design and evaluation of an absorption heat pump using LiNO3/[BMIM]Cl/H2O as a working pair.

■

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 10 62333542; Fax: +86 10 62333542; E-mail: [email protected]. ORCID

Chunhuan Luo: 0000-0002-6985-6777 Funding

The authors gratefully acknowledge financial support from the Fundamental Research Funds for the Center Universities (Grant FRF-BD-16-009A), the National Nature Science Foundation of China (Grant 51506005), and China Postdoctoral Science Foundation (Grant 2014M560049). 3051



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