Working Pair Selection of Compression and Absorption Hybrid Cycles

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Working Pair Selection of Compression and Absorption Hybrid Cycles through Predicting the Activity Coefficients of Hydrofluorocarbon + Ionic Liquid Systems by the UNIFAC Model Li Dong, Danxing Zheng,* and Xianghong Wu College of Chemical Engineering, Beijing University of Chemical Technology, Beijing 100029, China ABSTRACT: Hydrofluorocarbon (HFC) + ionic liquid (IL) systems can be adopted as working pairs for compression/absorption hybrid cycles. A method of selecting novel working pairs has been proposed in this paper, that the infinite dilution activity coefficients (γ1∞) of working pair system need to be obtained, and the UNIFAC model is implemented to predict the activity coefficients of the working pair system. With the help of published experimental vapor−liquid equilibrium (VLE) data of 18 HFC + IL systems, 16 new group interaction parameters for four anion ILs and nine HFCs have been fitted to extend the evaluating and predicting range of HFC + IL systems for developing alternative working pairs. To validate the reliability of the method, the model parameters have been used to calculate the VLE data with the average relative deviations (ARDs) of pressures less than 8.5%. The prediction of the γ1∞ shows that the solubility and affinity between HFCs and ILs increase with the decrease of the γ1∞, which is in agreement with the experimental results. It is found by the prediction that the combinations of R32/R134 and [Tf2N]-based ILs may be two kinds of potential working pairs to improve the performances of compression/absorption hybrid cycles.

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INTRODUCTION To use low-temperature heat effectively, e.g., waste heat or solar heat, research into the absorption heat pump cycle and the refrigeration cycle has attracted more and more attention. In recent years, ionic liquids (ILs) have received increasing attention due to their potential uses as novel absorbent species of absorption cycle working pairs. Many research groups1−5 have investigated various refrigerant−IL systems, involving H2O, NH3, hydrofluorocarbons (HFCs), CO2, 2,2,2-trifluoroethanol, etc. The compression/absorption hybrid cycle, which combines a vapor compression cycle with an absorption cycle, has been investigated to improve energy efficiency by utilizing waste heat or extending the operating temperature range.6−8 The performance of the hybrid cycle is influenced significantly by the absorption effect of the absorbent for the refrigerant. Several working pairs have been discussed for the hybrid cycles.9 Restriction in the use of chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) as working fluids has led to the use of HFCs as alternatives.10,11 HFCs must be developed that are harmless to the environment and allow high capacity, high efficiency applications.12 HFCs not only have much lower toxicities than ammonia, but they can also achieve lower chilling temperatures below 0 °C than water. As refrigerants, HFCs, combined with appropriate absorbents, can be exploited as working pairs for the compression/absorption hybrid cycle.13,14 Popular refrigerants such as R134a have some limitations. Recently, many efforts have been made to search for new refrigerants, which should have zero or a low ozone depletion potential, have a low global warming potential, be nonflammable and nontoxic, and have comparable cycle efficiency. Table 1 lists the physical, safety, and environmental data for common HFCs.15,16 The selection of suitable HFCs and corresponding absorbents is important. There are some critical evaluation criteria for selecting novel working pairs such as to ensure high efficiency of absorption cycles, to show good heat and mass transfer, to be noncorrosive, © 2012 American Chemical Society

to be nontoxic, to ensure secure operation, and so on. To assess possible working pairs, criteria needed linking the performance of the absorption cycle to the physical properties of the working pair were proposed, to make the criteria reliable and easily available for numerous mixtures. Some researchers17−19 presented that the extreme values of the excess Gibbs function (GEmax) should be the evaluation tool for the working pairs of the absorption cycle. The systems of strong absorption performance usually exhibit negative deviation from Raoult’s law. The excess Gibbs function of the negative deviation system is less than 0, and the activity coefficient is less than 1. Wu et al.20 studied the affinity between the two components of the working pair and found that γ1∞ is an extrapolated state point providing important physical/chemical understandings about the interactions between absorbent and absorbate. Therefore, γ1∞ can be chosen as a thermodynamic evaluation tool. The evaluation result on the criteria is that γ1∞ is consistent with GEmax. The γ1∞ of a binary system can be calculated from vapor−liquid equilibrium (VLE) data. However, the experimental data are only available for limited systems from the literature. Therefore, the group contribution methods, which are on the essential information of working pair compounds, have been developed to get more information about the systems that have not been determined experimentally before by using the known VLE data of the related systems. The UNIFAC model is especially popular. The UNIFAC method can be used to calculate activity coefficients based on the group contribution concept. In the UNIFAC method, the mixtures are regarded as consisting of functional groups instead of molecules. Thus the required activity coefficient can be predicted on the basis of the interactions of Received: Revised: Accepted: Published: 4741

September 6, 2011 January 6, 2012 February 23, 2012 February 23, 2012 dx.doi.org/10.1021/ie202029d | Ind. Eng. Chem. Res. 2012, 51, 4741−4747

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Table 1. Physical, Safety, and Environmental Data for Common HFCs no.

chemical formula

mol mass (g·mol−1)

NBP (°C)

Tc (°C)

pc (MPa)

ODP

GWP

ASHRAE 34 safety group

CH2F2 C2H4F2 C2HF5 C2H2F4 C2H2F4 C2H3F3 C2H5F CHF3 CH3F

52.02 66.05 120.02 102.03 102.03 84.04 48.06 70.01 34.03

−51.7 −24.0 −48.1 −26.1 −23.0 −47.2 −37.6 −82.0 −78.3

78.1 113.3 66.0 101.1 118.6 72.7 102.2 26.1 44.1

5.78 4.52 3.62 4.06 4.64 3.76 5.09 4.83 5.90

0 0 0 0 0 0 0 0 0

675 124 3500 1430 1000 4470 12 14760 92

A2 A2 A1 A1

a

R32 R152aa R125a R134aa R134b R143aa R161a R23a R41a

A2 A1

a

Reference 15. bReference 16. NBP = normal boiling point; Tc = critical temperature; pc = critical pressure; ODP = ozone depletion potential (semiempirical); GWP = global warming potential (for 100-year integration).

where p is the system pressure, pis expresses the saturated vapor pressure for the ith species, yi, xi, and γi are the vapor phase mole fraction, the liquid phase mole fraction, and the activity coefficient for the ith species, respectively, and Φi is the correction factor for the ith species (1 at sufficiently low pressures). For the HFC (1) + IL (2) binary system, because the concentration of the IL in the vapor phase can be neglected, i.e., p2s ≈ 0, eq 1 can be written as eq 2.

the limited number of functional groups. Virtually thousands of compounds and numerous kinds of solutions were replaced by limited numbers of groups. Furthermore, after the group parameters and interaction coefficients calculated from the VLE data have been obtained and tabulated, even if the experimental data are absent, the liquid equilibrium information of the unknown solutions can still be estimated with reasonable accuracy.21 Narodoslawsky et al.22 estimated the properties of organic refrigerant−absorption systems with the help of UNIFAC. The refrigerants involved are alcohols, hydrocarbons, and amines. The absorbents contained ketones, cyclic molecules, amides, and ethers. Kleiber23 and Hou24,25 extended new main groups to describe the various forms of halogenated refrigerants. Also, the UNIFAC group interaction parameters were obtained to predict VLE of mixtures containing fluorinated hydrocarbons. The range of applicability of UNIFAC has been continuously extended. Many researchers have endeavored to extend the UNIFAC model to IL-containing systems.26−32 Alevizou et al.33 predicted phase equilibria in mixtures of ILs, which consisted of the imidazolium cation and the hexafluorophosphate anion, and alkanes, cycloalkanes, alcohols, or water. Lei et al.34 added the group parameters for 12 main groups and 24 subgroups including ILs, CH2, CH3OH, and H2O into the current UNIFAC parameter matrix by means of correlating the activity coefficients of solutes at infinite dilution in ILs at different temperatures. Based on the available VLE data of HFC + IL systems, the UNIFAC method can be used to predict the solubilities of HFCs in ILs quantitatively, analyze the influences of the absorbent species structures and the mixture composition on the affinity between HFC and IL, and establish an appropriate thermodynamic model to depict the VLE behaviors of HFC + IL systems. In this work, the UNIFAC method and the γ1∞ were adopted as the evaluation criteria for the selection of novel working pairs for the absorption/compression hybrid cycle. The investigation is based on the solubility data of 18 systems, which were obtained from the combination of nine different HFCs and eight different ILs.35−40 The new interaction parameters extended the group parameters of ILs in the current UNIFAC parameter matrix. Also, the VLE data and γ1∞ for working pairs of HFC + IL systems are predicted at various compositions and temperatures.

γ1 =

⎡ v L (p − p s ) ⎤ 1 1 ⎥ Φ1 ≡ s exp⎢ − ⎢ ⎥⎦ ϕ1 RT ⎣ ϕ̂1

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

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

(3)

where ϕ̂ 1 is the vapor phase fugacity coefficient for HFC, ϕ1s is the fugacity coefficient for pure HFC as saturated vapor, v1L is the saturated molar liquid volume for HFC, B1 is the second virial coefficient for HFC at the system temperature T, which can be calculated as well as v1L by the REFPROP program, and R is the universal gas constant. UNIFAC Model. The UNIFAC model was developed by Fredenslund42 in 1975 on the basis of the group contribution method. The activity coefficient is calculated as a sum of two terms.43 ln γi = ln γiC + ln γi R

(4)

where γi is the activity coefficient of component i, γiC is the combinatorial term, and γiR is the residual term. The combinatorial term takes into account the different sizes and shapes of the molecules. The residual term considers the attractive forces between the molecules and is calculated from the group activity coefficients in the mixture of the pure substances. The combinatorial term of the UNIFAC equation is expressed as ⎛ V V⎞ ln γiC = 1 − Vi + ln Vi − 5qi⎜1 − i + ln i ⎟ Fi Fi ⎠ ⎝

METHODOLOGY VLE of HFC + IL Systems. For an N-component system, the gamma/phi formulation of vapor/liquid equilibrium can be described by eq 1.41 yi Φip = xi γipi

(2)

The correction factor Φ1 can be calculated by definition

■

s

pΦ1 x1p1s

(5)

where the parameters Vi and Fi are given as Vi = ri/∑j rjxj and Fi = qi/∑j qjxj. The parameters rj and qj represent the volume and surface area of component j, respectively. They can be

(1) 4742

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calculated using the van der Waals volumes Rk and surface areas Qk of the individual functional groups k according to ri = ∑k νk(i)Rk and qi = ∑k νk(i)Qk, where νk(i) denotes the number of group k in component i. The residual term of the UNIFAC is determined using the following expression: ln γi R =

Table 2. Fragmentation for ILs and HFCs for UNIFAC Model group composition (main group) molecule [BMIM]BF4 [HMIM]BF4 [BMIM]PF6 [HMIM]PF6 [EMIM]OTf [BMIM]OTf [EMIM]Tf2N [HMIM]Tf2N

∑ νk(i)[ln Γk − ln Γk(i)] (6)

k

where Γk is the residual activity coefficient of functional group k and Γk(i) is the residual activity coefficient of group k in the reference solution of component i. Both Γk and Γk(i) are expressed as ln Γk = Q k[1 − ln(∑ θmψmk)

R125 R134 R134a R143a R152a R161 R23 R32 R41

m

−

∑ (θmψkm/∑ θnψnm)] m

n

(7)

where the summations cover all groups m and n, θm = θmXm/ ∑n QnXn is the surface area fraction of group m in the mixture, and Xm = ∑i νm(i)xi/∑i∑k νk(i)xi is the fraction of group m in the mixture. The interaction term ψnm is expressed as ψnm = exp[− (anm/T )]

(8)

Fragmentation of ILs 1 [MIM]BF4 1 [MIM]BF4 1 [MIM]PF6 1 [MIM]PF6 1 [MIM]OTf 1 [MIM]OTf 1 [MIM]Tf2N 1 [MIM]Tf2N Fragmentation of HFCs 1 CHF2 2 CHF2 1 CHF2 1 CHF3 1 CHF2 1 CHF 1 CHF3 1 CHF2 1 CHF

4 6 4 6 2 4 2 6

CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2

1 CHF3 1 1 1 1

CHF3 CH2 CH2 CH2

Table 3. Group Volume and Surface Area Parameters Used for UNIFAC Model

where anm (K) is a temperature-independent group interaction parameter. By definition, ann = 0, and there are two group− group interaction parameters for each pair of functional groups m and n: anm and amn, where anm ≠ amn. Group Assignment and Group Parameters. Using the original UNIFAC model, ILs and HFCs are split into groups. ILs were decomposed in the same way as proposed by Kim44 and Lei.34 The groups presently needed for ILs included [MIM]Tf2N (methylimidazolium bis(trifluoromethylsulfonyl)imide), [MIM]BF4 (methylimidazolium tetrafluoroborate), [MIM]OTf (methylimidazolium trifluoromethanesulfonate), and [MIM]PF6 (methylimidazolium hexafluorophosphate). For HFCs, the groups defined by Kleiber23 are used. The combining rule was as follows: if a CH3 segment in an ethane derivative is changed by H or a CF3 segment is changed by F, it turns into the corresponding methane derivative. As shown in Table 1, the concerned HFCs included R134a (1,1,1,2-tetrafluoroethane, CH2FCF3), R143a (1,1,1-trifluoroethane, CH3CF3), R134 (1,1,2,2-tetrafluoroethane, CHF2CHF2), R152a (1,1-difluoroethane, CH3CHF2), R32 (difluoromethane, CH2F2), R23 (trifluoromethane, CHF3), R41 (fluoromethane, CH3F), R161 (fluoroethane, CH3CH2F), and R125 (pentafluoroethane, CHF2CF3). The ILs and HFCs studied are segmented into corresponding groups as listed in Table 2. The group parameters Rk and Qk of subgroups can be determined according to the method given by Bondi.45 The Rk and Qk for ILs and HFCs adopted in this paper are from the literature.23,34 Some of the group interaction parameters, anm and amn, have been presented before.23,34 The available parameters are listed in Tables 3 and 4. The new group interaction parameters are regressed to fill gaps in the existing parameter table. To maximize the scope of UNIFAC for HFC + IL systems, all available VLE data of binary systems containing one IL and one fluorinated refrigerant have been taken into account. As a result, 18 different binary systems were collected from the literature, including those had been measured in our previous work.40 The group interaction parameters, anm and amn, were determined by fitting these literature data. The objective function F was calculated as eq 9. For the

no. 1

a

subgroup.

Rk

Qk

CH3 CH2 CH C [MIM]Tf2N [IM]Tf2N [MIM]BF4 [IM]BF4 [MIM]OTf [IM]OTf [MIM]PF6 [IM]PF6 CH3F CH2F CHF CH2F2 CHF2 (CF3−)CH2F CHF3 (CH2−)CF3

0.9011 0.6744 0.4469 0.2195 8.3145 7.4134 6.5669 5.6658 9.5357 8.6346 7.6909 6.7898 1.2966 1.0699 0.8420 1.4654 1.2380 1.0699 1.6335 1.4060

0.8480 0.5400 0.2280 0.0000 7.3920 6.5440 4.0050 3.1570 5.0500 4.2020 4.6930 3.8450 1.3080 1.0000 0.6880 1.4600 1.2320 1.0000 1.6080 1.3800

main group CH2a

14

[MIM]Tf2Nb

20

[MIM]BF4b

22

[MIM]OTfb

23

[MIM]PF6b

51

CHFa

52

CHF2a

53

CHF3a

Reference 23. bReference 34.

minimization of F, the fitting procedure was done with the sequential search method.21 N

F=

∑ (γi ,exp − γi ,UNIFAC)2 i=1

(9)

γi,exp is the experimental activity coefficient of the solute, γi,UNIFAC is the calculated activity coefficient of the solute, and N is the number of experimental points. The optimization was performed sequentially for the various n, m pairs in order to decrease the number of fitted parameters in the F. The parameters for group CHF were first regressed, 4743

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data for the 18 HFC + IL systems studied using the UNIFAC group interaction parameters. The results show that the UNIFAC model is applicable for all systems studied with the average relative deviations (ARDs) of pressures less than 8.5%. The p−x curves of R32 with [BMIM]PF6 at temperatures between 283.2 and 348.2 K are shown in Figure 1. The predicted

Table 4. Group Interaction Parameters for UNIFAC Model m

n

CH2a CH2a CH2a CH2a CH2b CH2b CH2b CHF2b CHF CHF2 CHF2 CHF2 CHF2 CHF3 CHF3 CHF3 a

amn

Available Parameters [MIM]Tf2N 400.89 [MIM]BF4 1108.51 [MIM]OTf 405.39 [MIM]PF6 692.26 CHF 527.08 CHF2 134.38 CHF3 156.08 CHF3 224.25 New Parameters [MIM]PF6 71.49 [MIM]Tf2N 76.76 [MIM]BF4 347.38 [MIM]OTf −6.95 [MIM]PF6 235.77 [MIM]PF6 −47.24 [MIM]Tf2N 424.39 [MIM]BF4 267.65

anm 145.80 588.74 284.37 401.54 105.48 35.69 96.28 −131.47 196.90 47.15 57.54 159.50 −121.43 581.10 −100.69 5327.76

Figure 1. Isothermal p−x diagram for R32 with IL [BMIM]PF6. Symbols: experimental data from ref 37. ■, 283.2 K; ○, 298.2 K; ▲, 323.2 K; ▽, 348.2 K. Lines: UNIFAC model calculations.

Reference 34. bReference 23.

and experimental vapor pressure data for the three binary systems R134a−[EMIM]Tf2N, R32−[BMIM]BF4, and R41− [BMIM]PF6 at 298.2 K are compared in Figure 2.

then the parameters for group CHF2 were regressed, and finally the parameters for group CHF3 were obtained. Parameters for eight group combinations have been added to the UNIFAC parameter table. The resulting new group interaction parameters are also listed in Table 4, which can be used to predict VLE of systems containing fluorinated refrigerants and four kinds of anion-based ILs at various compositions and temperatures.

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RESULTS AND DISCUSSION

Correlation of VLE of HFC + IL Systems. Table 5 presents a comparison of the experimental and predicted VLE Table 5. Calculated Results of UNIFAC Model for Vapor Pressure of the Studied HFC + IL Systems system

T/K

no. of data points

ARDg for p (%)

R134a + [HMIM]Tf2Na R134a + [EMIM]Tf2Nb R134a + [HMIM]PF6a R134a + [HMIM]BF4a R134a + [BMIM]PF6c R143a + [BMIM]PF6c R134 + [BMIM]PF6d R134 + [EMIM]Tf2Nb R152a + [BMIM]PF6c R32 + [BMIM]BF4c R32 + [BMIM]PF6c R23 + [BMIM]PF6c R41 + [BMIM]PF6d R161 + [BMIM]PF6d R125 + [BMIM]PF6c R32 + [EMIM]OTfe R32 + [BMIM]OTfe R32 + [EMIM]Tf2Nf

298.1−348.1 283.1−348.1 298.1−348.1 298.1−348.1 283.1−348.1 285.1−348.1 283.1−348.1 283.1−348.1 283.1−348.1 283.1−348.1 283.2−348.2 283.1−348.1 283.1−348.1 283.1−348.1 283.1−348.1 273.1−348.1 273.1−348.1 283.1−348.1

26 29 19 15 27 33 29 30 25 30 28 31 31 27 31 37 37 30

8.49 7.06 6.41 6.64 3.89 2.48 5.31 3.60 5.41 3.61 2.90 5.01 3.90 1.72 1.86 4.27 6.87 3.60

Figure 2. Isothermal p−x diagram for three HFC−IL systems at 298.2 K. Symbols: experimental data. ■, R134a−[EMIM]Tf2N;36 ○, R32−[BMIM]BF4;37 ▲, R41−[BMIM]PF6.38 Lines: UNIFAC model calculations.

Figure 3. Effect of cations on solubility of R32 in [PF6]-based ILs at 298.2 K. The symbols are the experimental data from ref 37: ■, [BMIM]PF6. Lines: UNIFAC model calculations; − · −, [EMIM]PF6; ―, [BMIM]PF6; ---, [HMIM]PF6; ···, [OMIM]PF6.

As shown in Figures 1 and 2, the experimental data fall close to the lines obtained by the UNIFAC model. The results indicate that the UNIFAC interaction parameters derived from VLE data can be used to predict the vapor pressure of HFC + IL binary systems. Influence of Chemical Structures on the Solubility of HFCs in ILs. The systems of strong absorption performance

a

Reference 35. bReference 36. cReference 37. dReference 38. N Reference 39. fReference 40. gARD = 100∑i=1 |(expi − cali)/expi|/N, where N is the number of measurement points, expi is the experimental value, and cali is the calculated value. e

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in Figure 4, the solubility of R32 in [BMIM]Tf2N is larger than that in [BMIM]PF6. The solubility of R32 in [BMIM]BF4 is smaller than that in [BMIM]OTf at lower pressure, while it is larger than that in [BMIM]OTf at higher pressure. The solubility generally increases with more fluorine atoms in the anion. However, for the solubilities of different HFCs in the same IL, due to the differences in saturation vapor pressure of HFCs, the solubilities were compared using normalized pressure (or normalized fugacity).36 The normalized fugacity was given by the vapor phase fugacity of the solute, f, divided by the saturation fugacity, f 0. Figure 5 is a plot of the normalized fugacity of various HFCs versus mole fraction in [BMIM]PF6 at 298.2 K. The solubilities can easily be seen when comparing one HFC versus another at the same f/f 0 ratio. The results show that the solubilities of HFCs decrease in the order R32 > R134 > R41 > R23 > R152a > R161 > R134a > R125 > R143a, which is consistent with the experimental results. R32 and R134 have greater solubilities and better compatibilities in ILs. Meanwhile, both R32 and R134 have relatively short atmospheric lifetimes and lower values of ozone depletion potential and global warming potential. Therefore, environmentally friendly R32 and R134 can be adopted with [Tf2N]-based ILs as promising working pairs for compression/absorption hybrid cycles. Prediction of Activity Coefficients at Infinite Dilution. The γ1∞ values can be used as evaluation criteria for working pairs. The smaller the γ1∞ value of the absorption system is, the better the absorption effect of the absorbent will be. The γ1∞ of HFCs in ILs can be derived from the UNIFAC model by setting x1 = 0 and x2 = 1. As listed in Table 6, the γ1∞ values decrease with increasing alkyl chain length in the cation of the IL. As listed in Table 7, except [BMIM]OTf, the γ1∞ values decrease with increasing fluorine atoms in the anion of the IL. The γ1∞ values for nine HFCs in the IL [BMIM]PF6 are shown in Table 8. It can be seen that the γ1∞ values for HFCs increase in the following order: R32 < R134 < R41 < R23 < R152a < R161 < R134a < R125 < R143a. The result is in accordance with solubility experimental investigations. The γ1∞ values decrease with increasing solubilities of HFCs in ILs. In summary, the correlation and prediction results of the γ1∞ values using the UNIFAC model for HFC−IL systems are satisfactory. The UNIFAC method is useful in selecting abovementioned working pairs, which deserve more thorough investigations, and provides guidance in the search for novel working pairs for the absorption/compression hybrid cycles.

Figure 4. Effect of the anions on the solubility of R32 in [BMIM]based ILs at 298.2 K. The symbols were the experimental data: ■, [BMIM]PF6;37 ○, [BMIM]BF4;37 ▲, [BMIM]OTf.39 Lines: UNIFAC model calculations; − · −, [BMIM]Tf2N; ―, [BMIM]PF6; ---, [BMIM]OTf; ···, [BMIM]BF4.

Figure 5. Normalized fugacity versus mole fraction of various HFCs in [Bmim]PF6 at 298.2 K. Symbols: experimental data. ■, R23;37 □, R32;37 ●, R41;38 ○, R125;37 ▲, R134;38 △, R134a;37 ▼, R143a;37 ▽, R152a;37 ⧫, R161.38 Solid lines: UNIFAC model calculations. Dotted line: Raoult’s law.

Table 6. Activity Coefficients at Infinite Dilution of R32 in [PF6]-Based ILs at 298.2 K γ1∞

[EMIM]PF6

[BMIM]PF6

[HMIM]PF6

[OMIM]PF6

1.1028

0.7328

0.5559

0.4525

Table 7. Activity Coefficients at Infinite Dilution of R32 in [BMIM]-Based ILs at 298.2 K γ1∞

[BMIM]Tf2N

[BMIM]PF6

[BMIM]BF4

[BMIM]OTf

0.5026

0.7328

0.8794

0.7112

usually exhibit negative deviation from Raoult’s law.46 The interaction between HFC and IL is associated with solubility. There is a large difference in the solubilities of HFCs in ILs. By the calculation and prediction of the UNIFAC model, the results show that the affinity between HFC and IL was influenced by structural variations such as changing the length of the alkyl chain in the cation and substituting the anion of the IL. For the solubilities of the same HFC in different ILs, the predicted results indicate that when the IL anion remains the same, increasing the alkyl chain length on the cation increases the solubility. For example, the solubilities are in the order [EMIM]X < [BMIM]X < [HMIM]X < [OMIM]X. Figure 3 shows the effect of the cations on the solubility of R32 in [PF6]based ILs at 298.2 K. When the IL cation is the same, as shown

■

CONCLUSIONS The method of using the UNIFAC model to calculate activity coefficients combined with γ1∞ is proposed in this paper for evaluating novel working pairs for absorption/compression hybrid cycles. On the reported VLE data of 18 HFC + IL systems, 16 group interaction parameters have been regressed. The new group interaction parameters can be used to predict VLE of systems containing fluorinated refrigerants and four kinds of anion-based ILs at various compositions and temperatures, to extend the evaluation and prediction range of HFC + IL systems for developing alternative working pairs. The vapor pressures for all systems studied have been calculated with the ARDs of pressures less than 8.5%. By the

Table 8. Activity Coefficients at Infinite Dilution of Various HFCs in IL [BMIM]PF6 at 298.2 K γ1∞

R32

R152a

R23

R134a

R125

R143a

R134

R41

R161

0.7328

1.3957

1.2847

1.8615

3.1726

3.6674

0.8289

0.8677

1.4969

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calculation and prediction of the UNIFAC model, the results are in agreement with the experimental results, showing the validity of the proposed working pair selection method. Also, it is found by the prediction that the combinations of R32/R134 and [Tf2N]-based ILs could be considered as two kinds of potential working pairs for improving the performances of compression/absorption hybrid cycles.

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AUTHOR INFORMATION

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*Tel./fax: +86-10-6441-6406. E-mail: [email protected].

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

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REFERENCES

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