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Molecular Simulation Studies on the Vapor−Liquid Equilibria of the cis- and trans-HCFO-1233zd and the cis- and trans-HFO-1336mzz Gabriele Raabe* Institut für Thermodynamik, Technische Universität Braunschweig, Hans-Sommer-Str. 5, 38106 Braunschweig, Germany ABSTRACT: Hydrofluoroolefins (HFO) are considered as the fourth generation of working fluids as they exhibit a low Global Warming Potential, though at the moment, only a few members of the HFO family are commercialized. For most HFO compounds, experimental data for their thermophysical properties are rare, which hampers the exploration of their performance in technical applications. In our earlier work [Raabe, G.; Maginn, E. J. J. Phys. Chem. B 2010, 114, 10133−10142; Raabe, G. J. Phys. Chem. B 2012, 116, 5744−5751], we have introduced a transferable force field for fluoropropenes, which enables reliable predictions of their thermophysical properties by molecular simulations. In this work, we apply the force field model for simulation studies on the VLE of the hexafluorobutenes cis- and trans-HFO-1336mzz. We additionally present an extension of the force field to the chlorinated compounds cis- and trans-HCFO1233zd, and provide molecular simulation results for the VLE properties of both isomers. As both trans-1233zd and cis-1336mzz are discussed as working fluids for low grade Organic Rankine Cycles (ORC), we also compare their predicted thermophysical properties with those of the widely used ORC-working fluid R-245fa. negligible due to its short atmospheric lifetime.7 Hulse et al.7 proposed the HCFO compound as a cleaning solvent or expansion agent, and Moles et al.5 studied the performance of HCFO-1233zd(E) as a working fluid in low-grade ORC processes in comparison with HFO-1336mzz(Z) and the widely used HFC-245fa. The exploration of the performance of new working fluids in potential technical applications requires a detailed knowledge of their thermophysical properties, though experimental data for these compounds in literature are rare. Experimental data for the HCFO compound R-1233zd(E) in literature only comprise vapor pressure, saturated liquid densities, and liquid viscosities.7 Studies on the performance of HFO-1336mzz(Z) as a working fluid are based on data from DuPont,5 which are not available in open literature. In our earlier work,8−10 we have introduced a transferable force field for fluoropropenes to enable predictions of the properties of these refrigerants by molecular simulations. Simulation studies on the vapor−liquid phase equilibria and liquid phase properties have proven that the new force field yields reliable reproductions and predictions for a wide range of thermophysical properties of fluoropropenes, as the simulated data in general agree well with available experimental data and correlations.9,10 In this paper, we employ the force field model to molecular simulation studies on the fluorinated butene HFO-1336mzz. Additionally, we present an extension of the molecular model

1. INTRODUCTION Due to their low Global Warming Potential (GWP), hydrofluoroolefins (HFOs) are regarded as a new generation of refrigerants or working fluids.1 Commercialized members of the HFO family are for instance 2,3,3,3-tetrafluoropropene R1234yf and trans-1,3,3,3-tetrafluoropropene R-1234ze(E), which are both discussed as alternative refrigerants in mobile air conditioning (MAC) systems, either as a pure compound (R-1234yf2), or as components in a refrigerant blend (R1234ze(E) in R-445A3). Furthermore, several blends of R1234yf or R-1234ze(E) with hydrofluorocarbon (HFC) compounds such as R-32, R-134a, or R-152a were proposed as alternative refrigerant blends to replace the high GWP refrigerants R-407C, R-410A, or R404A in air conditioning and refrigeration application. An overview of alternative refrigerant blends based on HFO compounds is, for instance, given by Brown et al.4 Studies on the thermophysical properties and technical applications of HFO compounds have mainly focuses on fluorinated propenes such as R-1234yf or R-1234ze(E). Though recently, also HFO compounds based on fluorinated butenes and also chlorinated compounds (hydrochlorofluoroolefine, HCFO) were introduced as working fluids. A commercialized HFO-butene compound is the cis-1,1,1,4,4,4hexafluoro-2-butene, 1336mzz(Z) also known as DR-2,5 which is discussed as working fluid for chillers, air conditioning systems, high temperature heat pumps, or Organic Rankine Cycles.1 The HCFO compound trans-1-chloro-3,3,3-trifluoropropene 1233zd(E) was approved by the U.S. EPA for chiller applications. 6 Though chlorine containing, the ozonedepletion potential (ODP) of the HCFO-1233zd(E) is © XXXX American Chemical Society

Received: March 26, 2015 Accepted: July 8, 2015

A

DOI: 10.1021/acs.jced.5b00286 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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(1−4 interactions) are scaled by a factor of 1/2 and 1/1.2, respectively. The force field model is transferable with regard to both the intramolecular terms and the LJ parameters, and only the partial charges are calculated for all compounds individually from ab initio simulations. More details on the strategy used in the parametrization of the molecular model are given in our earlier work.9,10 The force field in its original form only comprises different fluoropropene compounds. Supporting Information in ref 10 provides a complete list of the force field parameters for fluoropropenes. In order to allow for the modeling of HCFO compounds, LJ parameters for the Cl atom type and parameters for the stretching of the Cl−CM bond and bending of the Cl− CM−CM and Cl−CM−H1 angles are required. Following the same strategy as in our earlier work on the fluoropropenes modeling, we started the parametrization with ab initio simulations on the B3LYP12,13/DGDZVP14 level of theory to determine the energy minimized structure of the isolated molecules. From the optimized geometries we obtained the averaged nominal bond lengths r0 and bond angles θ0 of the intramolecular terms involving the Cl atom. The force constants were also derived from ab initio simulations by perturbing the bond length and angles around their equilibrium value and fitting of the harmonic potential to the resulting energy change. The torsion around the ClCMCMHC and ClCMCMCT dihedral is described with the same XCMCMX term we used for the modeling of the fluoropropenes. The phase angles δ and multiplicities n are assigned accordingly to distinguish between the cis- and transisomer of HCFO-1233zd. The partial charges of both compounds were calculated from ab initio simulations by the ESP approach with the CHELPG fitting scheme15 at the HF/631G* level of theory. The electrostatic potentials have been determined for the isolated molecules whose geometries were optimized as described above. All ab initio simulations were performed using the Gaussian 03 package.16 The LJ parameters for the Cl atom type were then established to reproduce experimental data for trans-1233zd(E). For the fluorobutenes 1336mzz, the partial charges were derived from ab initio simulations following the procedure described above. In general, all other force field parameters for LJ interactions and the intramolecular terms can be transferred from the force field for fluoropropenes. However, in our earlier work,10 we found that the fluorine LJ parameters used to describe fluoropropenes with up to four fluorine atoms result in an underestimation of vapor pressures of heavier HFO compounds. In ref 10, we therefore derived a modified FCMh parameter for compounds with more than four fluorines, by slightly reducing the ε parameter from 0.23617 to 0.21784 kJ· mol−1. The HFO-1336mzz compounds also comprise six fluorine atoms, but not of the FCM but of the FCT atom type. We therefore reduced accordingly the ε value of the FCT atom type to 0.21784 kJ·mol−1 (to yield FCTh). It should be noted that the LJ parameters for the FCTh atom type in 1336mzz were adjusted only based on our findings from earlier work, but not to improve agreement with experimental data for these compounds. The additional force fields parameters for the HCFO and fluorobutene compounds are summarized in Table 1; Table 2 provides the partial charges in the different compounds from ab initio simulations (see Figure 1 for the atom numbers). Also given there are the resulting estimates for the dipole moment μ of the isolated molecules.

to allow simulation studies on the HCFO compound 1233zd. For both, HFO-1336mzz and HCFO-1233zd, our studies comprise the cis- and trans-isomer to gain insight into the influence of conformational isometry on the thermophysical properties. Thus, we here present Gibbs Ensemble Monte Carlo simulations on the vapor−liquid equilibria of cis1336mzz(Z), trans-1336mzz(E), cis-1233zd(Z), and trans1233z(E) in comparison to experimental data where available. As both trans-1233zd and cis-1336mzz are considered as working fluids for low grade ORC processes, we additionally relate their predicted VLE properties to that of the commonly used working fluid R-245fa.

2. COMPUTATIONAL METHODS Force Field Model. The structure of the compounds studied in this work, and the nomenclature used for the different Lennard−Jones (LJ) atom types are shown in Figure

Figure 1. Structures of the working fluids studied in this work: trans-1chloro-3,3,3-trifluoropropene (HCFO-1233zd(E)), cis-1-chloro-3,3,3trifluoropropene (HCFO-1233zd(Z), cis-1,1,1,4,4,4-hexafluorobutene (HFO-1336mzz(Z)) and trans-1,1,1,4,4,4-hexafluorobutene (HFO1336mzz(E)), and nomenclature for the different Lennard−Jones atom types.

1. In our force field model, the potential energy is expressed by the following standard functional form11 Uconf =

∑

kr(r − r0)2 +

bonds

+

∑

∑

kθ(θ − θ0)2

angles

kχ [1 + cos(nχ − δ)]

dihedral

⎧ ⎡⎛ ⎞12 ⎛ ⎞6 ⎤ qiqj ⎫ σij ⎥ ⎪ ⎪ ⎢ σij 1 ⎬ + ∑ ∑ ⎨4εij⎢⎜⎜ ⎟⎟ − ⎜⎜ ⎟⎟ ⎥ + r 4πε0 rij ⎪ ⎝ rij ⎠ ⎦ i j>i ⎪ ⎭ ⎩ ⎣⎝ ij ⎠ (1)

Therein, the calculation of nonbonded interactions is based on Lennard−Jones (LJ) site−site terms and electrostatic interactions between fixed partial charges on the atomic sites. The intramolecular potential energy is modeled by harmonic terms for bond stretching and angle bending, and a cosine term as torsion potential. Nonbonded LJ and electrostatic interactions between atoms separated by exactly three bonds B



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output of TOWHEE then yielded simulation results for the equilibrium pressures, saturated densities and heats of vaporization derived from the block averaging technique. The “final” simulation results were then determined as simple averages of the 3−5 independent TOWHEE simulation results, and the stated uncertainties (see Tables 3 to 6) are the standard deviations of the final results from the 3−5 independent measurements. The critical properties of the HCFO and fluorobutene compounds were estimated by fitting the simulation results at subcritical conditions to the scaling law20

Table 1. Additional Force Field Parameters for the Molecular Modelling of HFCO-1233zd and HFO-1336mzz atom

ε/kJ·mol−1

σ/A

Cl FCTh bond

1.09752 0.21784 kr/kJ·mol−1·Å−2

3.55 2.94 r0/Å

CMCl angle

■

953.8 kθ/kJ·mol−1 ·rad−2

1.734 θ0/deg

280.8 152.6

122.95 113.8

ClCMCM ClCMH1

ρ L − ρ V = Aτ β

SIMULATION DETAILS Vapor−liquid coexistence curves for the pure compounds were calculated via Monte Carlo Gibbs ensemble17 in the NVTensemble using the simulation code TOWHEE.18 The systems consisted of N = 300 molecules in studies on the 1233zd compounds and N = 256 for fluorobutene molecules. The Ewald sum technique19,20 was employed to deal with the electrostatic interactions with a cutoff radius adjusted to half the box length. The cutoff radius for the LJ interactions was set to 12 Å, and standard long-range corrections to the energy and pressure were applied.20 The simulations were equilibrated for 200000 cycles to ensure that the simulation results show no drift and that the simulated chemical potentials in both phases agreed within their error bars. The productions consist of 300 000−500 000 cycles, each cycle involving N attempted moves, such as a volume move, translation of the center-of-mass, rotation about the center-of-mass, and a configurational-bias exchange move21 between the boxes. The different moves were selected at random with a fixed probability. The pressure was calculated in TOWHEE via the pressure virial equation. The heats of vaporization ΔHvap were determined from the energies U and densities ρ of the coexisting phases and the vapor pressure p: ⎛ 1 1⎞ ΔH vap = UV − U L + p⎜ V − L ⎟ ρ ⎠ ⎝ρ

with

τ=1−

T TC

(3)

and the law of rectilinear diameters ρL + ρ V = ρc + Bτ 2

(4)

by employing the TOWHEE utility routine fitcoex.18 As before for the fluoropropenes, we assumed that the models obey the Ising exponent β = 0.32. The critical pressures in the fitcoex routine are estimated by extrapolating the vapor pressure curves using the Clausius−Clapeyron equation. From this, fitcoex also provides estimates of the normal boiling point of the compounds. The stated uncertainties of the derived properties are those given by the fitcoex routine, accounting for both the uncertainties of the simulated properties as input data and the errors of the fitting procedure.

3. RESULTS AND DISCUSSION The application of the force field to the fluorobutene and its extension to the HCFO compounds has been tested by performing Gibbs ensemble simulations on the vapor−liquid equilibrium (VLE) of 1,1,1,4,4,4-hexafluoro-2-butene (cis- and trans- HFO-1336mzz), and 1-chloro-3,3,3-trifluoropropene (cisand trans-HCFO-1233zd). In the following section we discuss the performance of the force field regarding the prediction of the vapor−liquid phase equilibria and critical properties of these compounds. Numerical results for the simulated vapor pressures, saturated densities and heats of vaporization of these compounds are given as in the Tables 3−6. In Table 7, we have summarized the predictions for their critical properties and

(2)

For determining the ensemble averages and standard deviations, each production run was subdivided in 3−5 successive TOWHEE simulations each of 100 000 cycles. These were again divided into ten blocks, so that the standard

Table 2. Partial Charges q and Resulting Dipole Moments μ for the cis- and trans-Isomers of HFCO-1233zd and HFO-1336mzz trans-1233zd

cis-1233zd

cis-1336mzz

trans-1336mzz

no.

type

q/e

type

q/e

type

q/e

type

q/e

1 2 3 4 5 6 7 8 9 10 11 12 μ/D

CM CM CT Cl H1 HC FCT FCT FCT

−0.00108 −0.30674 0.78148 −0.09402 0.17109 0.19567 −0.24880 −0.24880 −0.24880

CM CM CT Cl H1 HC FCT FCT FCT

−0.00427 −0.35183 0.97116 −0.07583 0.15783 0.16832 −0.28846 −0.28846 −0.28846

CT CM CM CT FCTh FCTh FCTh HC HC FCTh FCTh FCTh

0.83591 −0.25029 −0.25029 0.83591 −0.25389 −0.25389 −0.25389 0.17605 0.17605 −0.25389 −0.25389 −0.25389

CT CM CM CT FCTh FCTh FCTh HC HC FCTh FCTh FCTh

0.81189 −0.24485 −0.24485 0.81189 −0.25015 −0.25015 −0.25015 0.18341 0.18341 −0.25015 −0.25015 −0.25015

1.1428

3.054

3.1946 C

0.0026 DOI: 10.1021/acs.jced.5b00286 J. Chem. Eng. Data XXXX, XXX, XXX−XXX


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Table 3. GEMC Simulation Results for the Vapor Pressure p, Saturated Liquid Density ρL, Saturated Vapor Density ρV, and Heats of Vaporization ΔHvap of the VLE for (trans-) HCFO-1233zd(E)a

a

T/K

p/MPa

u(p)/MPa

ρL/kg·m−3

u(ρL)/kg·m−3

ρV/kg·m−3

u(ρV)/kg·m−3

ΔHvap/kJ·mol−1

u(ΔHvap)/kJ·mol−1

273.15 298.15 303.15 313.15 323.15 353.15 373.15 383.15

0.043 0.124 0.158 0.206 0.300 0.683 1.084 1.326

0.005 0.019 0.005 0.011 0.007 0.004 0.006 0.018

1324.3 1262.3 1251.1 1225.0 1201.6 1113.7 1045.5 1007.2

0.9 1.0 1.0 0.8 1.9 1.6 1.0 1.6

2.55 6.84 8.63 11.02 15.86 35.49 57.18 70.15

0.27 0.10 0.24 0.62 0.42 0.30 0.55 1.25

28.14 26.32 25.95 25.24 24.43 21.74 19.52 18.35

0.02 0.03 0.04 0.03 0.05 0.05 0.17 0.12

u(p), u(ρL), u(ρV), and u(ΔHvap) are the standard deviations of the simulation results.

Table 4. GEMC Simulation Results for the Vapor Pressure p, Saturated Liquid Density ρL, Saturated Vapor Density ρV, and Heats of Vaporization ΔHvap of the VLE for (cis-) HCFO-1233zd(Z)a

a

T/K

p/MPa

u(p)/MPa

ρL/kg·m−3

u(ρL)/kg·m−3

ρV/kg·m−3

u(ρV)/kg·m−3

ΔHvap/kJ·mol−1


293.15 313.15 333.15 353.15 373.15 393.15 413.15

0.048 0.108 0.211 0.381 0.630 0.991 1.450

0.002 0.007 0.012 0.013 0.003 0.027 0.029

1309.9 1264.1 1216.2 1166.1 1110.1 1047.6 971.9

1.0 1.0 1.3 2.6 3.0 4.9 4.0

2.64 5.65 10.70 18.96 31.23 49.39 74.11

0.13 0.42 0.63 0.76 0.24 1.78 2.18

29.53 28.20 26.72 25.09 23.24 21.17 18.73

0.07 0.05 0.03 0.04 0.07 0.06 0.09


Table 5. GEMC Simulation Results for the Vapor Pressure p, Saturated Liquid Density ρL, Saturated Vapor Density ρV, and Heats of Vaporization ΔHvap of the VLE for (cis-) HFO-1336mzz(Z)a

a

T/K

p/MPa

u(p)/MPa

ρL/kg·m−3

u(ρL)/kg·m−3

ρV/kg·m−3

u(ρV)/kg·m−3

ΔHvap/kJ·mol−1


303.15 313.15 323.15 333.15 343.15 353.15 363.15 383.15 393.15 403.15

0.090 0.125 0.193 0.259 0.335 0.419 0.585 0.891 1.185 1.422

0.018 0.021 0.023 0.019 0.025 0.026 0.064 0.030 0.085 0.051

1351.4 1319.3 1289.8 1263.4 1225.6 1191.2 1157.6 1069.8 1031.1 970.6

3.7 4.5 4.2 2.3 4.8 8.2 5.6 8.5 12.4 5.7

6.19 8.34 12.88 17.02 21.92 27.13 38.44 58.63 82.67 102.33

1.38 1.53 1.66 1.17 1.78 2.15 2.67 3.22 7.97 5.65

28.63 27.79 26.78 25.81 24.81 23.92 22.66 20.21 18.48 16.85

0.17 0.20 0.12 0.18 0.16 0.21 0.09 0.14 0.30 0.15


Table 6. GEMC Simulation Results for the Vapor Pressure p, Saturated Liquid Density ρL, Saturated Vapor Density ρV, and Heats of Vaporization ΔHvap of the VLE for (trans-) HFO-1336mzz(E)a

a

T/K

p/MPa

u(p)/MPa

ρL/kg·m−3

u(ρL)/kg·m−3

ρV/kg·m−3

u(ρV)/kg·m−3

ΔHvap/kJ·mol−1


293.15 303.15 313.15 323.15 333.15 343.15 353.15 363.15

0.158 0.265 0.348 0.456 0.605 0.798 1.017 1.318

0.013 0.019 0.022 0.028 0.038 0.018 0.019 0.017

1322.2 1297.6 1258.1 1217.6 1177.6 1138.9 1089.9 1036.6

2.5 3.6 4.2 2.6 7.0 2.7 6.0 4.3

11.29 19.01 24.60 32.17 42.26 57.15 72.48 97.47

1.00 1.27 1.82 2.36 3.50 2.16 1.94 0.34

25.47 24.46 23.42 22.21 21.11 19.83 18.36 16.71

0.03 0.01 0.10 0.13 0.17 0.04 0.15 0.15


respectively. Figures 2 and 3 show molecular simulation results for the vapor pressure and saturated densities of the trans compound HCFO-1233zd(E) in comparison with experimental data by Hulse et al.7 and Tanaka et al.,22 and with calculations by the latest equation of state by Mondejar et al.23 As the LJ parameters for the chlorine atoms were fine-tuned to reproduce

normal boiling points in comparison with available experimental data.1,5,7,23 HCFO-1233zd. The GEMC simulation results for vapor pressure, saturated densities, and heats of vaporization of the vapor−liquid equilibria of the trans- and cis-isomers of 1-chloro3,3,3-trifluoropropene are summarized in Tables 3 and 4, D



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Table 7. Estimated Critical Properties and Normal Boiling Points of the Compounds Studied in This Work in Comparison with Experimental Data Where Available

a

Tc,sim

u(Tc,sim)

Tc,exp

ρc.sim

u(ρc,sim)

ρc,exp

pc,sim

u(pc,sim)

pc,exp

Tb,sim

u(Tb,sim)

K

K

K

kg·m−3

kg·m−3

kg·m−3

MPa

MPa

MPa

K

K

K

trans-1233zd(E)

440.8

1.9

438.75a −439.6c

475.6

4.0

480.2c

3.90

0.43

3.6237c

290.9

3.5

291.12b 291.41c

cis-1233zd(Z) cis-1336mzz trans-1336mzz

468.6 438.0 401.6

3.6 6.7 5.6

466.6 491.3 513.0

7.6 20.0 17.0

3.89 2.70 2.92

0.56 0.56 0.64

2.9

311.5 305.5 278.8

4.5 5.7 6.3

444.45b

Tb,exp

306.5

Hulse et al.7 bKontomaris,1 Moras et al.5 cMondejar et al.23

reproduction of both the vapor pressure curve and the vapor− liquid coexistence curve (VLCC) of HCFO-1233zd(E) results in an excellent estimate of the critical temperature, critical density, and the normal boiling point, as they deviate by less than 1% from the reported experimental values.1,7,23 Though the simulation results yield a higher critical temperature then the experimental Tc value, which than results in an overestimation of the critical pressure. As HCFO-1233zd(E) is discussed as a working fluid in lowgrade ORC processes to replace HFC-245fa, the vapor pressure curve and VLCC of R-245fa are also shown in Figures 2 and 3 (as gray chain line). HCFO-1233zd(E) shows lower vapor pressure than HFC-245fa, which allow for higher operation temperatures,1 or result in lower pump power consumption5 for HCFO-1233zd(E) when operating the ORC cycle at the same temperature as the HFC-245fa process. Though the HCFO-1233zd(E) exhibits lower densities that in turn affects the turbine work output. To our best knowledge, there are no experimental data available for the heats of vaporization of HCFO-1233zd(E). Figure 4 illustrates that our molecular simulations predict higher ΔHvap values than those determined by the new EOS by Mondejar et al. (ΔΔHvap = 1.3 kJ·mol−1, i.e., ≈5% at 303 K), though both approaches yield heats of vaporization of HCFO1233zd(E) comparable to that of HFC-245fa.

Figure 2. Gibbs ensemble (GEMC) simulation results for the vapor pressure of HCFO-1233zd(E) (blue ⊕) and HCFO-1233zd(Z) (red ⊗) obtained in this work, experimental data (gray ●,7 black ●22) and calculated vapor pressure curve (solid line, ) using REFPROP23,24 for HCFO-1233zd(E). Also shown is the vapor pressure curve of R245fa for comparison (gray chain line, −·−, calculated by REFPROP24).

Figure 3. Gibbs ensemble (GEMC) simulation results for the saturated densities of HCFO-1233zd(E) (blue ⊕) and HCFO1233zd(Z) (red ⊗) obtained in this work, experimental data (gray ●,7 black ●22) and calculated VLCC curve (solid line, ) using REFPROP23,24 for HCFO-1233zd(E). Also shown is the VLCC curve of R-245fa for comparison (gray chain line, −·−, calculated by REFPROP24).

Figure 4. Gibbs ensemble (GEMC) simulation results for the heats of vaporization of HCFO-1233zd(E) (blue ⊕) and HCFO-1233zd(Z) (red ⊗) obtained in this work, and the calculated ΔHvap(T) curve (solid line, ) for HCFO-1233zd(E) using REFPROP.23,24 For the sake of comparison, the heats of vaporization of R-245fa are also shown (gray chain line, −·−, calculated by REFPROP24).

experimental data for the vapor pressure and liquid densities of trans-1233zd(E), the good agreement between simulation and experiment for these properties is to be expected. The good E



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The same LJ parameters with individually derived partial charges were then used in simulation studies on the cis- HCFO1233zd(Z), for which, to our best knowledge, no experimental data are available. Figures 2−4 show that the molecular simulations yield significantly lower vapor pressure, higher saturated liquid densities, and heats of vaporization for the cis isomer compared to the trans compound HFO-1233zd(E). Thus, similarly to the finding for the cis- and trans isomers of HFO-1234ze,10 the cis-isomer of HCFO-1233zd represents the higher boiling compound. This can again be attributed to its higher dipole moment (μ1233zd(E) = 1.1428 D, μ1233zd(E) = 3.054 D, see Table 2). The molecular simulations predict a difference in the normal boiling points of the cis- and trans-isomers of ΔTb ≈ 21 K. With this, the ΔTb due to the different positions of the chlorine atom in HCFO-1233zd seems to be less pronounced than the effect of the fluorine position in the cis- and trans isomer of HFO-1234ze compound with ΔTb = 28 K. HFO-1336mzz. Figures 5 and 6 show simulation results for the vapor pressures and saturated densities of the cis- and trans-

Figure 6. Gibbs ensemble (GEMC) simulation results for the saturated densities of HFO-1336mzz(Z) (orange ⊕) and HFO1336mzz(E) (green □ with ×) obtained in this work in comparison with experimental data (gray ●,5 black ●22). Also shown is the VLCC curve of R-245fa for comparison (gray chain line, −·−, calculated by REFPROP24).

As cis-1336mzz(E) is also discussed as a HFC-245fa alternative for ORC processes,1,5 Figures 5 and 6 again show the vapor pressure curve and VLCC of HFC-245fa as gray chain lines for the sake of comparison. Figure 5 illustrates that cis-1336mzz has considerably lower vapor pressures than HFC245fa, which would again allow for higher operation temperatures as discussed before for HCFO-1233zd(E). In fact the vapor pressure curve of R-245fa is closer to that of the transisomer of 1336mzz. The liquid densities of cis-1336mzz(E) are very similar to those of HFC-245fa, but slightly higher. Our molecular simulations predict heats of vaporization of cis1336mzz(E) that are significantly higher than those of HFC245fa, as shown by Figure 7. The predicted ΔHvap values of the trans-isomer though are lower than the values of the R-245fa. The higher heats of vaporization and liquid densities of the cis1336mzz should results in higher turbine25 output compared to

Figure 5. Gibbs ensemble (GEMC) simulation results for the vapor pressure of HFO-1336mzz(Z) (orange ⊕) and HFO-1336mzz(E) (green □ with ×) obtained in this work, and experimental data for HFO-1336mzz(Z) (gray ●5) and for HFO-1336mzz(E) (black ●22). Dash lines connecting the experimental data points are added to guide the eyes. Also shown is the vapor pressure curve of R-245fa for comparison (gray chain line, −·−, calculated by REFPROP24).

1336mzz compounds in comparison with experimental data. Experimental information though only comprises a few single data points for the cis-compound5 and measurements for p and ρL from Tanaka22 for the trans-isomer. As no LJ parameters have been adjusted for this compound, the simulations for both the cis- and trans-HFO-1336mzz are purely predictive. In spite of this, the simulation results for the vapor pressures of the cis1336mzz(Z) agree very well with the experimental data. For the trans-isomer though, the simulations tend to overestimate the vapor pressures for temperatures above 340 K. The comparison of our simulation results with the experimental data for the saturated liquid densities of trans-1336mzz(E) show good agreement over the whole temperature range studied. Once more, the generally good reproduction of the VLE properties of the cis- and trans-isomer of HFO-1336mzz(E) using the same LJ parameters as for fluoropropene compounds with six fluorine atoms attest the good transferability of our force field parameters.

Figure 7. Gibbs ensemble (GEMC) simulation results for the heats of vaporization of HFO-1336mzz(Z) (orange ⊕) and HFO-1336mzz(E) (green □ with ×) obtained in this work. For the sake of comparison, the heats of vaporization of R-245fa are also shown (gray chain line, −·−, calculated by REFPROP24). F



Article

the R-245fa, which again identifies the cis-isomer as a suitable working fluid for ORC processes. The properties of the transisomer though are unfavorable. The predicted normal boiling point and critical temperature of cis-1336mzz(E) deviate by 0.3% and 1.4% from the data given by Molés et al.5 The agreement with experimental values for Tb and Tc is quite satisfying considering that our simulation results for this compound are purely predictive. The simulations underestimate the critical temperature by Molés et al.,5 which then yields an estimate for the critical pressure that is 6.9% below the value given there.5 To our best knowledge, no experimental critical data are available for trans-HFO-1336mzz(E) to allow for an evaluation of our estimated critical properties. Our molecular simulation results predict a difference in the normal boiling point between the cis- and trans-isomers of HFO-1336mzz of ΔTb ≈ 27 K. Again the cis-isomer of HFO-1336mzz is the higher boiling compound due to its higher dipole moment (see Table 2). The difference between the dipole moment of the cis-and transisomer of HFO-1336mzz (Δμiso = μ1336mzz(Z) − μ1336mzz(E) = 3.19 D) is more pronounced as for the HCFO-1233zd (Δμiso = 1.91 D) and also the HFO-1234ze compounds (Δμiso = 2.23 D).

the HCFO-1233zd(E) are similar to that of R-245fa, and its liquid densities are lower.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +49 531 391 2628. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. Katsuyuki Tanaka, Nihon University, Funabashi, Japan for sharing his experimental information on HCFO1233zd(E) and trans-1336mzz(E) with us. We also thank Dr. Eric Lemmon from the NIST, Boulder for proving the new HCFO-1233zd(E) EOS prior to publication.

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REFERENCES

(1) Kontomaris K. HFO-1336mzz-Z: High Temperature Chemical Stability and Use as a working Fluid in Organic Rankine Cycles. 15th Int. Refrigeration and Air Cond. Conference, Purdue University, West Lafayette, IN, 2014. (2) Spatz, M.; Minor, B. HFO-1234yf, a low GWP refrigerant for MAC. VDA Alternative Refrigerant Winter Meeting, Saalfelden, Austria, 2008. (3) Peral-Antunez, E. MAC refrigerant blend cooperative research program − Update. TMSS Symposium, SAE International, Scottsdale, AZ, 2012. (4) Brown, J. S.; Zilio, C.; Brignoli, R.; Cavallini, A. Thermodynamic properties and heat transfer and pressure drop Performance potentials of hydrofluoroolefins, hydrochloro-olefins, and their blend. HVACR Res. 2014, 20, 203−220. (5) Molés, F.; Navarro-Esbrí, J.; Peris, B.; Mota-Babiloni, A.; Barragán-Cervera, Á .; Kontomaris, K. Low GWP alternatives to HFC-245fa in Organic Rankine cycles for low temperature heat recovery: HCFO-1233zd-E and HFO-1336mzz-Z. Appl. Therm. Eng. 2014, 71, 204−212. (6) McLinden, M. O.; Kazakov, A. F.; Brown, J. S.; Domanski, P. A. A thermodynamic analysis of refrigerants: possibilities and tradeoffs for low-GWP refrigerants. Int. J. Refrig. 2014, 38, 80−92. (7) Hulse, R. J.; Basu, R. S.; Singh, R. R.; Thomas, R. H. P. Physical properties of HCFO-1233zd(E). J. Chem. Eng. Data 2012, 57, 3581− 3586. (8) Raabe, G.; Maginn, E. J. Molecular modeling of the vapor-liquid equilibrium properties of the alternative refrigerant 2,3,3,3-tetrafluoro1-propene HFO-1234yf. J. Phys. Chem. Lett. 2010, 1, 93−96. (9) Raabe, G.; Maginn, E. J. A Force Field for 3,3,3-fluoro-1propenes, including HFO-1234yf. J. Phys. Chem. B 2010, 114, 10133− 10142. (10) Raabe, G. Molecular Modeling of Fluoropropene Refrigerants. J. Phys. Chem. B 2012, 116, 5744−5751. (11) Cornell, W. D.; Cieplak, P.; Bayly, C. I.; Gould, I. R.; Merz, K. M., Jr.; Ferguson, D. M.; Spellmeyer, D. C.; Fox, T.; Caldwell, J. W.; Kollman, P. A. A Second Generation Force Field for the Simulation of Proteins, Nucleic Acids, and Organic Molecules. J. Am. Chem. Soc. 1995, 117, 5179−5197. (12) Becke, A. D. Density-functional thermochemistry III. The role of exact exchange. J. Chem. Phys. 1993, 98, 5648−5652. (13) Lee, D.; Yang, W.; Parr, R. G. Development of the Colle-Salvetti correlation-energy formula into a functional of the electron density. Phys. Rev. B: Condens. Matter Mater. Phys. 1988, 37, 785−789. (14) Godbout, N.; Salahub, D. R.; Andzelm, J.; Wimmer, E. Optimization of Gaussian-type basis sets for local spin density functional calculations. Part I, Boron through neon, optimization technique and validation. Can. J. Chem. 1992, 70, 560−571. (15) Breneman, C. M.; Wiberg, K. B. Determining Atom-Centered Monopoles from Molecular Electrostatic Potentials. The Need for

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CONCLUSION We have presented GEMC molecular simulation studies for the vapor−liquid phase equilibria of the HCFO compounds cisand trans-1-chloro-3,3,3-trifluoropropene 1233zd and the fluorinated butenes cis- and trans-1,1,1,4,4,4-hexafluoro-2butene 1336mzz. In order to allow for simulation studies on the chlorinated compounds, we have derived new force field parameters for the intramolecular terms involving the bonded Cl atom. The LJ parameter for the chlorine atom was then adjusted to fine-tune agreement with experimental vapor pressures and saturated densities of the trans-1233zd(E) compound. For simulation studies on the butene compounds, force field parameters for LJ interactions and the intramolecular terms were transferred from our force field for fluoropropenes including the slight modification of the LJ parameters for the fluorine atoms in compounds with five or more fluorine atoms. With this, the simulation studies on the cis- and trans-1336mzz are purely predictive. The generally good reproduction of the VLE properties of both isomers using the same parameters as for fluoropropenes attests the good transferability of our force field parameters. The comparison of the simulation results for the cis- and trans-isomers of both compounds studied here illustrates that the cis-isomer is always the higher boiling compound. This agrees with our earlier studies on the cis- and trans-isomer of 1234ze. In all cases this can be attributed to the significantly higher dipole moment of the cis compound in comparison to the trans-isomer. The effect on the difference in the molecular dipole moment and the normal boiling point seems to be more pronounced for cis- and trans-configurations involving fluorine atoms than for those with chlorine. Both trans-1233zd and cis-1336mzz are discussed as alternative working fluids to R-245fa in ORC processes, and they both show lower vapor pressures than R-245fa. Our molecular simulation studies yield higher liquid densities and significantly higher heats of vaporization for cis-1336mzz compared to R-245fa, whereas our predicted ΔHvap values of G



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