Experimental investigation for the solubility of R1234ze(E) in pentaerythritol tetrahexanoate and pentaerythritol tetraoctanoate

Fluid Phase Equilibria 400 (2015) 38–42

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Fluid Phase Equilibria j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / fl u i d

Experimental investigation for the solubility of R1234ze(E) in pentaerythritol tetrahexanoate and pentaerythritol tetraoctanoate Xiaopo Wang * , Yanjun Sun, Kai Kang Key Laboratory of Thermo-Fluid Science and Engineering, Ministry of Education, Xi’an Jiaotong University, Xi’an 710049, China

A R T I C L E I N F O

A B S T R A C T

Article history: Received 23 March 2015 Received in revised form 29 April 2015 Accepted 4 May 2015 Available online 6 May 2015

The phase behavior of refrigerant with lubricant oil is an important issue for designing the refrigeration system. R1234ze(E) was proposed as a possible replacement for R134a recently. In this work, the p–T–x data of R1234ze(E) with pentaerythritol tetrahexanoate (PEC6) and pentaerythritol tetraoctanoate (PEC8) were obtained based on the isochoric method at temperatures ranging from 283.15 to 353.15 K. The experimental data were correlated by Peng–Robinson equation of state with HVOS mixing rule in which the excess Gibbs energy at infinite pressure was represented by Wilson equation. The absolute average deviation and maximum deviation of the pressure from the calculated values was 0.79% and 2.67% for R1234ze(E) + PEC6 system, respectively. For R1234ze(E) + PEC8 system, the absolute average deviation and maximum deviation of pressure were 0.98% and 2.89%. ã 2015 Elsevier B.V. All rights reserved.

Keywords: R1234ze(E) Pentaerythritol tetrahexanoate Pentaerythritol tetraoctanoate Solubility

1. Introduction Due to the high global warming potential (GWP) values of many hydrofluorocarbons (HFCs), more research and development efforts have been focused on potential refrigerants possessing low GWPs during the last several years. Trans-1,3,3,3-tetrafluoropropene (R1234ze(E)), proposed by Honeywell and Dupont, has the atmospheric lifetime of only 18 days and GWP of 6 relative to CO2 on a 100-year time horizon [1,2]. It is being widely considered as a possible replacement for R134a in many different applications, including refrigeration systems and electronic thermal management. Experimental and theoretical investigations of thermodynamic and transport properties for R1234ze(E) have been reported elsewhere [3–13]. However, the investigation of phase equilibrium of R1234ze(E) with lubricant oil is still very limited in the literature. The knowledge of the phase behavior of refrigerant with lubricant oil is important in order to design various refrigeration plant components. For the different kind of lubricant oils, polyol ester oils (POEs) have been found to have favorable properties compared to polyalkylene glycols (PAGs). For example, POEs are less hygroscopic, have higher miscibility and are miscible with mineral oils [14,15]. POEs are usually synthesized from the alcohol pentaerythritol, which is combined with four equal acids to a pentaerythritol tetraalkyl ester. The main components of POEs are

* Corresponding author. Tel.: +86 29 82668210; fax: +86 29 82668789. E-mail address: [email protected] (X. Wang). http://dx.doi.org/10.1016/j.fluid.2015.05.008 0378-3812/ ã 2015 Elsevier B.V. All rights reserved.

pentaerythritol esters, which can be linear, branched, and cyclic chained esters [16]. In order to investigate the effect of the oil structure on the phase behavior of R1234ze(E) and POE oil, a series of measurements for the phase equilibrium (or solubility) of R1234ze(E) with different linear chained esters are ongoing in our laboratory. The solubilities of R1234ze(E) in pentaerythritol tetrapentanoate (PEC5), pentaerythritol ester heptanoic acid (PEC7), and pentaerythritol tetranonanoate (PEC9) have been reported previously [17,18]. In this work, the solubility of R1234ze(E) in pentaerythritol tetrahexanoate (PEC6) and in pentaerythritol tetraoctanoate (PEC8) was reported for the first time at temperatures ranging from 283.15 to 353.15 K. In addition, the comparison of different structure of linear chained esters on solubility was performed. 2. Experimental 2.1. Materials R1234ze(E) (CF3CH¼CHF; CAS No. 29118-24-9) was supplied by Honeywell with declared mass purity of 99.9%. The sample was purified several times by using freeze–pump–thaw cycles with liquid nitrogen and a high vacuum pump to eliminate the effect of non-condensable gases. Pentaerythritol tetrahexanoate (PEC6, C29H52O8, CAS No. 744547-8) and pentaerythritol tetraoctanoate (PEC8, C37H68O8, CAS No. 3008-50-2) were synthesized by Chemipan (Poland) on a laboratory scale with a declared mass purity higher than 98% and were used directly without further purification. Table 1 shows

X. Wang et al. / Fluid Phase Equilibria 400 (2015) 38–42

39

Table 1 Sample used in the measurements. Chemical name

Source

Initial mass fraction purity (%)

Purification method

Analysis method

R1234ze(E)a PEC6b PEC8c

Honeywell Chemipan (Poland) Chemipan (Poland)

99.9 98.0 98.0

Freeze–pump–thaw None None

Gas chromatography None None

a b c

R1234ze(E) = trans-1,3,3,3-tetrafluoropropene. PEC6 = [3-octanoyloxy-2,2-bis(octanoyloxymethyl)propyl] octanoate. PEC8 = 3-(hexanoyloxy)-2,2-bis[(hexanoyloxy)methyl]propyl hexanoate.

the sample descriptions used in the present work. Fig. 1 shows the chemical structure of PEC6 and PEC8. 2.2. Apparatus and procedure In this work, the solubility was determined with an apparatus based on the isochoric method which has been described in our previous work [17–19]. The principle of this method is that measurements are performed by observing the amount of gas absorbed in a known quantity of solvent from the pressure change in a gas system of known volume [20–22]. Two stainless steel cells, named as equilibrium cell and gas system cell, were immersed in a thermostat bath which was supplied by Fluke. The calibrated volumes of the gas system cell and the equilibrium cell were 31.33 cm3 and 73.26 cm3, respectively. The temperature stability and uniformity of the bath was better than 0.01 K. Temperature was determined by a 100 V platinum resistance thermometer which was inserted in the equilibrium cell. The thermometer was supplied by Fluke and calibrated by National Institute of Metrology of China. The combined expanded uncertainty of temperature was within 0.03 K with level of confidence 0.95 (k = 2). The pressure was measured by means of a pressure transducer (Rosemount 3051S) with a full scale of 5500 kPa, and the accuracy of the pressure transducer is 0.025%. The combined expanded uncertainty (k = 2) in pressure measurement was within 2.0 kPa. The detailed solubility measurement procedure has been given in the literature [18]. A well-known amount of PEC6 or PEC8 was charged directly into the equilibrium cell by a glass syringe. Then, the cell was evacuated with a vacuum pump to remove the impurities. And the charged mass of esters was determined by an analytical balance (Mettler Toledo ME204, 220 g full scale) with an uncertainty of 0.002 g. R1234ze(E) was introduced into the gas system cell from the gas bottle, and the initial pressure of gas system cell was recorded. After the R1234ze(E) entered the equilibrium cell from the gas system cell, the pressure values in the equilibrium was also measured once the system reached

equilibrium. The next step was to change the temperature of the system to obtain the new equilibrium pressure. For temperature higher than the ambient temperature, R1234ze(E) condensation will occur in the tubes outside the thermostat bath. In order to avoid the density deviations due to temperature differences between the bath and the ambient temperature, all the connection pipes outside the bath and the pressure transducer were heated with a wire resistance regulated by a PID controller. It should be noted that the vapor pressure of PEC6 and PEC8 is very low. According to Razzouk et al. [23], the vapor pressures of PEC5 and PEC7 at about 374 K were 2.61 102 Pa and 2.58  104 Pa, respectively. The vapor pressure of PEC9 was 5.57  105 Pa at 394.55 K. Due to the homologous series of PECs, the vapor pressure of PEC6 and PEC8 at temperatures from 283.15 to 353.15 K is even lower than the above values. Hence, it was considered negligible in the range of the present case. The mole fraction x1 of absorbed R1234ze(E) in PEC6 or PEC8 can be calculated by x1 = n1/(n1 + n2), where n2 is the number of moles of PEC6 or PEC8. n1 is the number of moles of R1234ze(E) absorbed in the solvent and can be obtained as follows: n1 ¼ n01  n11

(1)

where n01 is the initial number of moles of R1234ze(E) in the system and n11 is the number of moles of R1234ze(E) remaining in the system after it reached equilibrium. Those two mole numbers are given by: n01 ¼

n11 ¼

V sys

(2)

ygas ðT ini ; pini Þ V sys

ygas ðT equilib Þ

þ

V cell  V 2;cell

ygas ðT equilib ; pequilib Þ



R O

O O R

C

O

CH2 O

CH2

C

CH2

O

C

R

PEC6 R CH2 CH2

CH2

CH2

CH3

PEC8 R CH2 CH2

CH2

CH2

CH2

CH2 O C

O

(3)

The subscripts ‘ini’ and ‘equilib’ represent the conditions at the initial measurement of the gas system and the equilibrium state of the system, respectively. T and p are temperature and pressure of

[(Fig._1)TD$IG] C

V abs;gas

ygas ðT equilib ; pequilib Þ

R Fig. 1. Chemical structure of PEC6 and PEC8.

CH2

CH3

40

X. Wang et al. / Fluid Phase Equilibria 400 (2015) 38–42

[(Fig._2)TD$IG]

Table 2 Solubility of R1234ze(E) in pentaerythritol tetrahexanoate (PEC6).a p (MPa)

x1

p (MPa)

x1

p (MPa)

x1

p (MPa)

T = 283.15 K 0.2105 0.050 0.4553 0.127 0.5867 0.177 0.6979 0.222 0.7965 0.259 0.8680 0.280 0.9193 0.292

T = 293.15 K 0.1858 0.061 0.4178 0.159 0.5506 0.228 0.6692 0.293 0.7789 0.351 0.8595 0.386 0.9161 0.403

T = 303.15 K 0.1609 0.072 0.3781 0.191 0.5093 0.279 0.6329 0.37 0.7546 0.459 0.8475 0.516 0.9115 0.544

T = 313.15 K 0.1391 0.082 0.3386 0.221 0.4650 0.329 0.5902 0.447 0.7218 0.578 0.8300 0.672 0.9050 0.725

T = 323.15 K 0.1197 0.092 0.3015 0.249 0.4207 0.376 0.5435 0.521 0.6804 0.699 0.8047 0.849 0.8956 0.945

T = 333.15 K 0.1034 0.101 0.2675 0.275 0.3786 0.418 0.4964 0.589 0.6330 0.813 0.7690 1.036 0.8811 1.203

T = 343.15 K 0.0890 0.109 0.2374 0.299 0.3404 0.457 0.4514 0.650 0.5840 0.916 0.7237 1.215 0.8582 1.495

T = 353.15 K 0.0771 0.116 0.2110 0.327 0.3062 0.498 0.4100 0.705 0.5366 1.008 0.6740 1.375 0.8219 1.795

1.6

1.2

p /MPa

x1

2.0

0.8

0.4

0.0 0.0

0.2

0.4

The combined expanded uncertainties Uc are Uc(T) = 0.03 K, Uc(p) = 2.0 kPa, and Uc,r(x1) = 3.0%, with a 0.95 level of confidence (k = 2).

equilib

2

3. Results and discussion 3 3.1. Experimental data

5

equilib

The solubility of R1234ze(E) in PEC6 and PEC8 has been measured in the temperatures ranging from 283.15 to 353.15 K and pressures up to 1.86 MPa. The experimental values are given in Tables 2 and 3. The solubilities for the two R1234ze(E) + PECs systems in terms of mole fraction of R1234ze(E) in liquid phase are presented in Figs. 2 and 3. For the two systems, the gas solubility

3

=41 

y  abs;gas 5 ygas T equilib ; pequilib

(5)

The volume of R1234ze(E) in the vapor phase, ygas, is calculated by Refprop 9.1 database [25]. Since the present measurement temperature T is lower than the critical temperature of R1234ze(E), the molar volume of the absorbed gas in the PECs, yabs,gas, is calculated as the liquid specific volume at equilibrium conditions T

[(Fig._3)TD$IG] 2.0

Table 3 Solubility of R1234ze(E) in pentaerythritol tetraoctanoate (PEC8).a p (MPa)

x1

p (MPa)

x1

p (MPa)

x1

1.6

p (MPa)

T = 283.15 K 0.2393 0.061 0.4890 0.148 0.6259 0.206 0.7355 0.252 0.8271 0.283 0.8910 0.296 0.9355 0.300

T = 293.15 K 0.2089 0.073 0.4448 0.181 0.5839 0.260 0.7029 0.330 0.8084 0.384 0.8829 0.408 0.9326 0.416

T = 303.15 K 0.1805 0.084 0.3994 0.213 0.5363 0.313 0.6611 0.412 0.7813 0.502 0.8709 0.547 0.9286 0.561

T = 313.15 K 0.1549 0.094 0.3555 0.242 0.4866 0.362 0.6122 0.491 0.7431 0.630 0.8526 0.715 0.9228 0.745

T = 323.15 K 0.1326 0.103 0.3150 0.268 0.4381 0.407 0.5605 0.563 0.6948 0.754 0.8239 0.907 0.9141 0.969

T = 333.15 K 0.1135 0.112 0.2787 0.292 0.3930 0.447 0.5097 0.627 0.6411 0.864 0.7815 1.103 0.9002 1.241

T = 343.15 K 0.0970 0.120 0.2468 0.314 0.3523 0.483 0.4624 0.683 0.5876 0.959 0.7284 1.279 0.8751 1.551

T = 353.15 K 0.0824 0.126 0.2191 0.343 0.3164 0.519 0.4198 0.735 0.5377 1.043 0.6739 1.428 0.8306 1.860

1.2

p /MPa

x1

1.0

and p.Taking into account the uncertainties of temperature, pressure, volume of cell, and the density of fluid, the relative expanded uncertainty of the measured solubility with level of confidence of 0.95 (k = 2) was less than 3.0% in this work.

(4)

Considering Eqs. (1)–(4), n1 can be calculated: 2 V  V cell V sys V
sys þ  2;cell  n1 ¼ 4 ygas ðT ini ; pini Þ ygas T equilib ygas T ;p

0.8

Fig. 2. Solubility in mole fraction for R1234ze(E) in PEC6 at different temperatures. &, 283.15 K; *, 293.15 K; ~, 303.15 K; !, 313.15 K; &, 323.15 K; , 333.15 K; 4, 343.15 K; 5, 353,15 K.

the system. Vsys and Vcell are the volume of the gas system and the equilibrium cell, respectively. The volume of PEC6 or PEC8 in the equilibrium cell, V2,cell, is calculated from the charged mass and its density, the densities of PECs were obtained from Fedele et al. [24]. Vabs,gas is the volume of R1234ze(E) absorbed in the solvent and can be expressed as: V abs;gas ¼ n1 yabs;gas

0.6

x1

a

0.8

0.4

a

The combined expanded uncertainties Uc are Uc(T) = 0.03 K, Uc(p) = 2.0 kPa, and Uc,r(x1) = 3.0%, with a 0.95 level of confidence (k = 2).

0.0 0.0

0.2

0.4

0.6

0.8

1.0

x1 Fig. 3. Solubility in mole fraction for R1234ze(E) in PEC8 at different temperatures. &, 283.15 K; *, 293.15 K; ~, 303.15 K; !, 313.15 K; &, 323.15 K; , 333.15 K; 4, 343.15 K; 5, 353,15 K.

X. Wang et al. / Fluid Phase Equilibria 400 (2015) 38–42

41

[(Fig._4)TD$IG]

Table 4 Pure compound parameters used in the data correlation

2

Compound

Molar mass (g/mol)

Tc (K)

pc (kPa)

Acentric factor

R1234ze(E) PEC6 PEC8

114.04a 528.73b 640.94b

382.51a 931.2b 972.5b

3634.9a 848.7b 681.1b

0.313a 1.546b 1.527b

b

The values from the Refprop 9.1 database [25]. The values from Razzouk et al. [23].

decreases when the temperature increases, and the solubility increases with the pressure increasing at a constant temperature.

100(pexp-pcal)/pexp

a

1

3.2. Data correlation

b ¼ x1 b1 þx2 b2

(6b)

For the PR EoS, the parameter C, pffiffiffi  ln 2  1 pffiffiffi C¼ 2

(7)

gE1 is represented by the Wilson equation,

 gE1 ¼ x1 ln x1 þ L12 x2  x2 ln x2 þ L21 x1 RT

(8)

The subscripts 1 and 2 represent R1234ze(E) and PEC6 or PEC8, respectively. x is the mole fraction of component; R is the universal ideal gas constant (R = 8.31445 J K1 mol1); T is temperature (K); L12 and L21 are the parameters of Wilson equation and is expressed as:

L12 ¼ L12;0 þ L12;1 ðT  273:15Þ

(9)

L21 ¼ L21;0 þ L21;1 ðT  273:15Þ

(10)

-3 0.0

0.2

0.4

0.6

0.8

1.0

x1 Fig. 4. Deviations between experimental data and the calculated values for R1234ze(E) + PEC6. &, 283.15 K; *, 293.15 K; ~, 303.15 K; !, 313.15 K; &, 323.15 K; , 333.15 K; 4, 343.15 K; 5, 353.15 K.

(E) + PEC8 system, respectively. The maximum deviation was 2.67% for R1234ze(E) + PEC6 system, and was 2.89% for R1234ze (E) + PEC8 system. Our group has reported the solubility data of R1234ze(E) in PEC5, PEC7, and PEC9 at different temperatures in the literature [17,18]. In Fig. 6, the solubilities of R1234ze(E) in the linear chained esters (PEC5, PEC6, PEC7, PEC8, and PEC9) at 283.15 K are plotted together in terms of mole fraction. At mole factions of R1234ze(E) higher than about 0.3, the solubility decreases from PEC5 to PEC9, that is, the solubility decreases with the length of the ester chains or molecular weight. Similar behavior was also obtained by Pernechele for the CO2 + linear chained esters systems at the mole fractions higher than roughly 0.8 [29]. It can be deduced that, for the homologous series of PECs with known structure, the higher the molecular weight, the lower the solubility. In addition, Fig. 6 also shows that at the very low mole fractions of R1234ze(E), the solubilities of different systems are almost identical.

[(Fig._5)TD$IG] 3

where L12,0, L12,1, L21,0, and L21,1 are the coefficients of the correlations. The values of the coefficients are obtained by fitting the experimental data, the objective function is N X jpexp  pcal i j i

2

1

(11)

pexp i

where N is the number of the experimental data. The optimized values of L12,0, L12,1, L21,0 and L21,1 for R1234ze(E) + PEC6 and R1234ze(E) + PEC8 system are tabulated in Table 5. The deviations in pressure are presented in Figs. 4 and 5. The absolute average deviation between experimental data and calculated values was 0.79% and 0.98% for R1234ze(E) + PEC6 system and R1234ze

100(pexp-pcal)/pexp

i¼1

-1

-2

The experimental solubility data was correlated by Peng– Robinson equation of state [26] using HVOS mixing rule [27] and Wilson equation [74_TD$IF][28] for the excess Gibbs energy at infinite pressure (gE1 ). The critical parameters and the acentric factor of R1234ze(E) and PECs are shown in Table 4. The HVOS mixing rule is,       a1 a2 1 gE1 b b þx2 þ þ x1 ln a ¼ bRT x1 þx2 ln (6a) b1 b2 b1 RT b2 RT C RT

obj ¼

0

0

-1

-2

-3

Table 5 Coefficients of the correlations for the two systems.

0.0

System

L12,0

L12,1

L21,0

L21,1

R1234ze(E) + PEC6 R1234ze(E) + PEC8

6.6236 6.1070

0.0175 0.0295

0.8388 1.0493

0.0047 0.0067

0.2

0.4

0.6

0.8

1.0

x1 Fig. 5. Deviations between experimental data and the calculated values for R1234ze(E) + PEC8. &, 283.15 K; *, 293.15 K; ~, 303.15 K; !, 313.15 K; &, 323.15 K; , 333.15 K; 4, 343.15 K; 5, 353,15 K.

42

X. Wang et al. / Fluid Phase Equilibria 400 (2015) 38–42

[(Fig._6)TD$IG]

References 0.30

0.25

p /MPa

0.20

0.15

0.10

0.05 0.2

0.4

0.6

0.8

1.0

x1 Fig. 6. Solubility in mole fraction of R1234ze(E) in different PECs at 283.15 K. &, PEC5; &, PEC6; *, PEC7; , PEC8; ~, PEC9. , obtained from the model.

4. Conclusions Solubility of R1234ze(E) in PEC6 and PEC8 was measured from 283.15 to 353.15 K. The solubility data were correlated by a model based on the Peng–Robinson equation of state with HVOS mixing rule and Wilson equation for the gE at infinite pressures. Moreover, the experimental solubility data for five systems (R1234ze(E) with PEC5, PEC6, PEC7, PEC8, and PEC9) were compared at the same temperature, a similar solubility trend was observed, and at mole factions of R1234ze(E) higher than about 0.3, the solubility decreases from PEC5 to PEC9. [75_TD$IF]Acknowledgements This research was supported by the [76_TD$IF]2National Natural Science Foundation of China (Grant 51476129) and the Fundamental Research Funds for the Central Universities.

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