Clathrate hydrate dissociation conditions of refrigerants R404A, R406A, R408A and R427A: Experimental measurements and thermodynamic modeling

Accepted Manuscript Clathrate Hydrate Dissociation Conditions of Refrigerants R404A, R406A, R408A and R427A: Experimental Measurements and Thermodynamic Modeling Hamed Hashemi, Saeedeh Babaee, Amir H. Mohammadi, Paramespri Naidoo, Deresh Ramjugernath PII: DOI: Reference:

S0021-9614(15)00211-6 http://dx.doi.org/10.1016/j.jct.2015.06.035 YJCHT 4294

To appear in:

J. Chem. Thermodynamics

Received Date: Revised Date: Accepted Date:

20 March 2015 2 June 2015 4 June 2015

Please cite this article as: H. Hashemi, S. Babaee, A.H. Mohammadi, P. Naidoo, D. Ramjugernath, Clathrate Hydrate Dissociation Conditions of Refrigerants R404A, R406A, R408A and R427A: Experimental Measurements and Thermodynamic Modeling, J. Chem. Thermodynamics (2015), doi: http://dx.doi.org/10.1016/j.jct.2015.06.035

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Clathrate Hydrate Dissociation Conditions of Refrigerants R404A, R406A, R408A and R427A: Experimental Measurements and Thermodynamic Modeling

Hamed Hashemi,1 Saeedeh Babaee,1 Amir H. Mohammadi,1,2,3* Paramespri Naidoo,1 and Deresh Ramjugernath1, ** 1

Thermodynamics Research Unit, School of Engineering, University of KwaZulu-Natal, Howard College

Campus, King George V, Avenue, Durban, 4041, South Africa 2

Institut de Recherche en Génie Chimique et Pétrolier (IRGCP), Paris Cedex, France Département de Génie des Mines, de la Métallurgie et des Matériaux, Faculté des Sciences et de Génie,

3

Université Laval, Québec (QC), G1V 0A6, Canada

Abstract-

Clathrate hydrate dissociation conditions were measured for four “alternative”

refrigerants, viz. R404A, R406A, R408A and R427A. The experimental measurements were performed within the pressure range of (0.079-9.08) MPa and temperatures ranging from (272.7-288.8) K. A thermodynamic model based on the van der Waals–Platteeuw (vdW-P) was applied for the prediction of the dissociation conditions which were compared to the experimental measurements. The fluid phase was modeled using the MHV2 GE-EoS mixing rule along with the UNIFAC (original) activity model. The van der Waals–Platteeuw (vdWP) model was used for the modeling of the hydrate phase. There was reasonable agreement between the experimental and predicted values.

Keywords: Gas hydrate; Clathrate hydrate; Refrigerant; Cool storage; Experimental; Thermodynamic modeling. *

Corresponding author: e-mail: [email protected] AND [email protected] (Amir H. Mohammadi),

**

Corresponding author: [email protected] (Deresh Ramjugernath)

1

1. Introduction Gas/clathrate hydrates, also known as “warm ice”, are inclusion compounds comprised of water molecules and guest molecule(s) in which water molecules produce cavities via hydrogen bonding which can be occupied by the guest molecules (the hydrate former) with appropriate size and shape. Depending on the size of the guest molecule(s) the clathrate hydrate structure can be classified into three common structures, viz. sI, sII and sH [1]. In spite of their formation being avoided in transportation pipelines of gas industries [2], gas hydrates can be utilized in numerous, so called “positive” applications such as gas storage and transportation, purification of saline water, air-conditioning systems, producing fluid concentrates in food industry, etc. [2-7]. The potential for gas hydrates to be considered as an alternative to conventional cold storage materials (e.g. ice, chilled fluids, water eutectic salts) is related to their unique properties, such as appropriate phase change temperature and high enthalpy of dissociation [5, 8-11]. The study of the application of gas hydrates as phase change materials (PCM) in cold storage technology commenced in 1982 and there has been a surge in publications and extended reviews in literature [9, 12, 13]. Carbajo [7] studied the hydrate dissociation conditions for R11 (CCl3F), R12 (CCl2F2), and their mixtures. Mori and Mori [14] studied the hydrate formation characteristics of R12 in a direct-contact charge cold storage system. The study of R12 hydrate formation in heat pump cool storage applications was carried out by Ternes [15]. Although CFC hydrates showed the most favorable results in cold storage systems, they have been completely phased out due to the Montreal Protocol. Therefore the study of CFC alternatives was considered towards the end of 1980s [16]. Mori and Mori [16] and Oowa et al. [17] studied hydrate formation of the CFC alternative R134a as a replacement for R12. Formation of R134a hydrates in a direct-contact charge cold storage system in the presence of surfactants was performed by Isob and Mori [18] in order to decrease the degree 2

of sub-cooling. Guo et al. [8] studied the plant design and economics for the application of gas hydrates in cold storage technology. They found that a mixture of high and low pressure refrigerants would provide a mixed hydrate within the applicable pressure (near the atmospheric pressure) and temperature range (278.15-288.15) K in cold storage technology. Tani et al. [19] studied the cold storage system using R141b hydrates in the presence of surfactants and ethylene glycol in order to lower the dissociation temperature to within the applicable temperature range. The hydrate dissociation conditions for refrigerant blends including R410A and R507C were measured by Akya et al. [20]. Kinetic studies of refrigerant hydrate formation in cold storage systems have been covered by various investigations [11, 21-23]. The effect of the volumetric flow rate of the crystallizer on the cold storage efficiency and the crystallization process of R141b hydrates in the presence of kinetic additives was studied by Bi et al. [24]. In designing a gas hydrate based cold storage system it is crucial that one is aware of the gas hydrate dissociation conditions. Hence, in the present study hydrate dissociation conditions for some refrigerant blends which are commercially available and environmentally friendly were measured using an experimental apparatus based on isochoric pressure search method. A step heating procedure was applied to measure the clathrate hydrate dissociation conditions. The

refrigerants used in this study consisted of refrigerant blends with the

ASHRAE numbers of R404A [certified composition of R143a/R125A/R134a (52/44/4 mass%)], R406A [certified composition of R22/R142b/R600a (55/41/4 mass%)], R408A [certified composition of R143A/R125/R22 (46/7/47 mass%)], and R427A [certified composition of 134a/R125a/R32/R143a (50/25/15/10 mass%)]. Furthermore, the enthalpy of hydrate dissociation was determined. The measured experimental dissociation data were compared with a correlative thermodynamic model developed in a previous study [6]. The hydrate phase was modeled using van der Waals–Platteeuw (vdW-P)[25]. For the modeling

3

of the vapor and liquid phases the modified Peng-Robinson equation of state [6, 26] along with the MHV2 GE-EoS mixing rule [6, 27] coupled with UNIFAC activity model was used [6, 28].

2. Experimental Section

2.1. Materials The details of the suppliers of the materials used in this study, as well as the chemical purities are listed in Table 1. The reported purities are as stated by the suppliers in their product certificates.

2.2. Apparatus A schematic diagram of the experimental apparatus used in this study is shown in Figure 1. The experimental setup is comprised of a high pressure stainless steel equilibrium cell with the internal volume of approximately 40 cm3 which can withstand pressures up to 20 MPa. A mechanical stirrer which is connected to a Heidolph overhead stirrer was used to mix the cell contents. The temperature of the cell is controlled using a thermostatted bath which has a temperature stability of 0.1 K. The cell temperature was measured using a Pt100 (platinum temperature probe) within an uncertainty of ± 0.1 K. The Pt100 was calibrated using a WIKA primary temperature probe which was connected to WIKA CTH 6500 multimeter. The pressure of the cell is measured using a WIKA pressure transducer with an uncertainty of ± 0.005 MPa, as stated by the manufacturer.

4

2.3. Procedure

The equilibrium cell was washed with distilled water, drained and thereafter placed under vacuum before starting the measurements. After injection of an appropriate amount of distilled water (approximately 16 cm3) into the equilibrium cell, it was evacuated again to ensure that all volatile gases were removed. The isochoric pressure search method was applied in this study for the measurement of hydrate dissociation conditions of the considered refrigerants [29]. After pressurizing the equilibrium cell to a desired pressure, the cell temperature was set to a value below the expected hydrate formation temperature. For the pressures above the upper quadruple point, Q2, (Hydrate-Liquid water- Liquid refrigerant) pure distilled water was injected into the cell using a hydraulic hand pump. Hydrate formation was confirmed by the rapid decline of the cell pressure. After the formation of a suitable amount of gas hydrate and once the pressure of the cell approached a constant value, the temperature was increased in steps. As it is demonstrated by Tohidi et al. step heating combined with adequate time at each temperature step results in the generation of reliable experimental data [30]. Hence in this study a step heating procedure is applied. For this purpose at the beginning of the heating process, the temperature of the cell was increased in steps of 0.5 K. As the heating curve approached the final dissociation point, steps of 0.1 K were chosen. The time for different systems to achieve the equilibrium conditions at each step is different, depending on the system under investigation, mixing efficiency and amount of water [30]. In this study, for the refrigerants investigated, approximately two hours were required for the systems to achieve equilibrium conditions. Hence at each step the pressure was allowed to settle for two hours to reach a constant value; this process is depicted in Figure 2 where the final dissociation point was taken as the equilibrium dissociation condition at which an abrupt change in the slope of the heating curve was observed.

5

3. Thermodynamic modeling In this study the equality of fugacity of water, fw, in adjacent phases is used as the equilibrium criteria:

(1)

f wAqueous = f wHydrate

3.1. Hydrate phase The solid solution theory of van der Waals and Platteeuw was used for the calculation of the water fugacity in the hydrate phase as follows [31, 32]:

 − ∆ µ wβ − H f wH = f wβ exp   RT

  

(2)

in which

 ∆µ β − L f wβ = f wL exp w  RT

  

(3)

In the abovementioned equations, f wβ denotes the fugacity of water in the empty hydrate lattice and f wL represents the fugacity of pure water in the liquid phase. Furthermore R stands for the Universal Gas constant, T denotes temperature, and ∆µwβ −H is the difference between the chemical potential of water in the empty hydrate lattice and the hydrate phase which is obtained using the following equation [1]:

  ∆µwβ − H = µwβ − µwH = RT ∑ vm ln1 + ∑ C jm f j  m j  

(4)

6

In equation (4) vm is defined as the number of cavities of type m per water molecule in the unit hydrate cell, fj is the fugacity of the gas component j. Cjm denotes the Langmuir constant which represents the interaction between gas and water molecules in the cavity and can be an be evaluated as follows [1, 6, 29]: C mj (T ) =

4π kT

R /2

∫ 0

 w(r )  2 exp  − r dr  k T 

(5)

Where k represents the Boltzmann's constant, R is the cell radius, w(r) is the spherically symmetric cell potential function, and r indicates distance from center of the cavity. In this study the Kihara potential function was used for the evaluation of the w(r) as shown below:

 (σ )12  a 11  σ 6  4 a 5  w(r ) = 2 zε  11 δ 10 + δ  −  δ + δ  R R  Rr   R r 

(6)

where,

δ

N

1 = N

−N −N  r a  r a   1 − − −  − 1 +  R  R   R  R

  

(7)

In the abovementioned equations z is the coordinate number, ε is the characteristic energy, a represents the radius of the spherical molecular core, σ denotes the collision diameter, and N is an integer equal to 4, 5, 10 or 11. ∆µ wβ − L in equation (3) which is defined as the

difference between the chemical potential of water in the empty hydrate lattice and that in the liquid phase is calculated using the following equation [31]:

β −L β −L ∆µ wβ − L µ wβ (T , P ) µ wL (T , P ) ∆µ w0 T ∆h w P ∆v w = − = − ∫T dT + ∫ P dP 0 0 RT RT RT RT RT RT

(8)

in which, µwβ and µ wL represent the chemical potential of the empty hydrate lattice and that of pure water in the liquid (L) phase respectively. P is the equilibrium pressure and T0 is the

7

reference temperature (ice point). ∆µ w0 is defined as the reference chemical potential difference between water in the empty hydrate lattice and pure water in the ice phase at 273.15 K. ∆vwβ − L and ∆hwβ − L are the volume differences and molar enthalpy differences between an empty hydrate lattice and liquid water respectively [1, 6, 29]. The phase transition parameters from water to sI and sII clathrate hydrates are reported in Table 2 [1, 6, 29].

3.2. Fluid phases

The modified version of the Peng-Robinson equation of state by Stryjek and Vera (PRSV) along with the MHV2 EoS-GE mixing rule was used for the calculation of the water fugacity in the fluid phases, f i , as follows [26, 27]:

ln

fi b α  1 ∂n 2 a bi   Z + 2.414B)   = i (Z − 1) − ln( Z − B) − −  ln  yi P bm 2 2  n ∂ni bm   Z − 0.414B 

(9)

The pure component parameters TC, PC, ω and k1 of the PRSV EoS [26] are given in Table 3. In equation (9), P is the pressure of the system, yi stands for the compositions of component i in the fluid phases, Z is the compressibility factor and other parameters are defined as follows [27]:

  ∂(nα ) 1 b b  q1α i + ln γ i + q2 (α i 2 + α 2 ) + ln m + i − 1 = ∂ni (q1 + αq2 )  bi bm 

(10)

1 ∂ (n 2 a) ∂ ( nα ) = αbi RT + bm RT n ∂ni ∂ (ni )

(11)

α=

a b RT

(12)

8

bm = ∑ xi bi

(13)

bm P RT

(14)

B=

∂ (nb m ) = bi ∂n i

(15)

in which n stands for the number of molecules and xi is the mole fraction of component i. For the calculation of the activity coefficient of refrigerants and water in equation (10), γ i , the UNIFAC activity model was applied [28]. More details regarding the modelling of the refrigerant hydrate dissociation conditions can be found elsewhere [6].

4. Results and discussion

Hydrate dissociation conditions for four refrigerants, namely R404A, R406A, R408A, and R427A were measured at pressures up 10 MPa and are reported in Figures 3-6 and Table 4. To the best of our knowlwdge there is no experimental data reported in the open literature for the refrigerants considered in this study. The Kihara parameters for the constituent components of the aforementioned refrigerants used in the modeling are taken from our previous study and are reported in Table 5 [6]. For R600a the model parameters were calculated in this study using the following objective function:

 1  NDP  Texp − Tcal Fobj =  ∑ Texp  NDP  k =1 

   k

(16)

9

in which Texp and Tcal denote the experimental and calculated hydrate dissociation temperatures, respectively. NDP is the number of data points. A comparison between experimental data obtained in this study and data values calculated is shown in Figures 3-6. Table 4 lists the Absolute errors (|Texp-Tcalc.|) between the experimental data obtained in this study and the model predictions. The maximum absolute error between experimental data and model predictions is 0.9. For the estimation of the enthalpy of dissociation of the refrigerant hydrates, which is a key factor in the design of cold storage systems, the Clausius- Clapeyron equation was used:

∆ H = − RZ

d ln( P ) d (1 / T )

(17)

in which Z is the compressibility factor obtained using the PRSV [26] equation of state and R is the Universal Gas constant. In equation (14), dln(P)/dT was calculated using the three phase (H-Lw-V) experimental hydrate dissociation data reported in Table 4 and Figures 3-6. The enthalpy of hydrate dissociation for the four refrigerants, viz. R427A, R404a, R406a and R408A at different temperatures is reported in Table 4. The temperature dependency of the hydrate dissociation enthalpy is shown in Figure 7. The enthalpy of hydrate dissociation decreases slowly with an increase in temperature. Among the refrigerants chosen in this study, R404A and R408A showed a maximum enthalpy of dissociation with a value of 142 kJ/mol at temperatures close to 275 K. The enthalpies of hydrate dissociation at the upper quadruple points are also reported in Table 4. R408A and R406A showed larger enthalpy of hydrate dissociation values compared to other refrigerants considered in this study.

10

Conclusions

Equilibrium hydrate dissociation data for four refrigerants, viz. R404, R406A, R408A and R427A within the equilibrium regions of hydrate-aqueous solution–liquid refrigerant and hydrate-aqueous solution–vapor were measured, modelled, and are reported. The experimental measurements were carried out in the temperature range of (272.7-286.5) K and pressures up to 10 MPa. R427A showed the lowest hydrate dissociation pressures amongst the aforementioned refrigerants. A previously developed model based on the van der Waals– Platteeuw (vdW-P) model was used for the correlation of the measured experimental data [6]. The liquid phase was modeled using the PRSV equation of state combined with the MHV2 GE-EoS mixing rule. The activity coefficients of the molecules in the fluid phases were obtained using the UNIFAC (original) activity model. The Kihara potential function parameters for different refrigerants were determined and are reported. The model predictions were in acceptable agreement with the experimental data. The enthalpies of hydrate dissociation for the refrigerants considered were obtained. R408A and R406A have maximum enthalpies of hydrate dissociation. A maximum enthalpy of dissociation of 142 kJ/mol was determined for R404A and R408A at a temperature close to 275 K, a value which is larger than that for R134a, R125a, R410A and R32 [20]. Regarding the equilibrium pressure of the clathrate hydrate, R427A showed the most suitable behaviour amongst the refrigerants studied, and with respect to the enthalpy of hydrate dissociation, R404A and R408A are better candidates for use in cold storage applications. From a thermodynamic perspective the results of this study indicate that the refrigerants studied are appropriate substitutes for CFC’s in cold storage applications. However, their kinetics of hydrate formation is still unclear and more kinetic studies need to be performed.

11

Acknowledgement

This work is based upon research supported by the South African Research Chairs Initiative of the Department of Science and Technology and National Research Foundation.

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[19] T. Tanii, M. Minemoto, K. Nakazawa, Y. Ando, Study on a cool storage system using HCFC (Hydro‐Chloro‐Fluoro‐Carbon)‐141B (CCI2 FCH3)(1, 1‐dichloro‐1‐fluoro‐ethane) clathrate, The Canadian Journal of Chemical Engineering, 75 (1997) 353-361. [20] T. Akiya, T. Shimazaki, M. Oowa, M. Matsuo, Y. Yoshida, Formation Conditions of Clathrates Between HFC Alternative Refrigerants and Water, International Journal of Thermophysics, 20 (1999) 1753-1763. [21] Y. Bi, T. Guo, L. Zhang, H. Zhang, L. Chen, Experimental study on cool release process of gas-hydrate with additives, Energy and Buildings, 41 (2009) 120-124. [22] J. Li, D. Liang, K. Guo, R. Wang, S. Fan, Formation and dissociation of HFC134a gas hydrate in nano-copper suspension, Energy Conversion and Management, 47 (2006) 201-210. [23] J. Li, K. Guo, D. Liang, R. Wang, Experiments on fast nucleation and growth of HCFC141b gas hydrate in static water columns, International Journal of Refrigeration, 27 (2004) 932-939. [24] Y. Bi, T. Guo, T. Zhu, L. Zhang, L. Chen, Influences of additives on the gas hydrate cool storage process in a new gas hydrate cool storage system, Energy Conversion and Management, 47 (2006) 2974-2982. [25] J.H.v.d. Waals, J.C. Platteeuw, Clathrate Solutions, in: Advances in Chemical Physics, John Wiley & Sons, Inc., 2007, pp. 1-57. [26] R. Stryjek, J. Vera, PRSV: An improved Peng—Robinson equation of state for pure compounds and mixtures, The Canadian journal of chemical engineering, 64 (1986) 323-333. [27] S. Dahl, M.L. Michelsen, High‐pressure vapor‐liquid equilibrium with a UNIFAC‐based equation of state, AIChE journal, 36 (1990) 1829-1836. [28] T. Magnussen, P. Rasmussen, A. Fredenslund, UNIFAC parameter table for prediction of liquid-liquid equilibriums, Industrial & Engineering Chemistry Process Design and Development, 20 (1981) 331-339. [29] A.H. Mohammadi, R. Anderson, B. Tohidi, Carbon monoxide clathrate hydrates: equilibrium data and thermodynamic modeling, AIChE journal, 51 (2005) 2825-2833. [30] B. Tohidi, R.W. Burgass, A. Danesh, K.K. ØStergaard, A.C. Todd, Improving the Accuracy of Gas Hydrate Dissociation Point Measurements, Annals of the New York Academy of Sciences, 912 (2000) 924-931. [31] G. Holder, G. Corbin, K. Papadopoulos, Thermodynamic and molecular properties of gas hydrates from mixtures containing methane, argon, and krypton, Industrial & Engineering Chemistry Fundamentals, 19 (1980) 282-286. [32] F. Anderson, J. Prausnitz, Inhibition of gas hydrates by methanol, AIChE journal, 32 (1986) 1321-1333. [33] P. Proust, J. Vera, PRSV: The stryjek‐vera modification of the peng‐robinson equation of state. Parameters for other pure compounds of industrial interest, The Canadian Journal of Chemical Engineering, 67 (1989) 170-173. [34] H. Orbey, S.I. Sandler, Equation of state modeling of refrigerant mixtures, Industrial & engineering chemistry research, 34 (1995) 2520-2525. [35] M. McLinden, S. Klein, E. Lemmon, A. Peskin, Thermodynamic properties of refrigerants and refrigerant mixtures database (REFPROP), in: O1 r R]. Gaithersburg, MD: NIST, 1998. [36] H.A. Duarte-Garza, C.E. Stouffer, K.R. Hall, J.C. Holste, K.N. Marsh, B.E. Gammon, Experimental Critical Constants, Vapor Pressures, and Vapor and Liquid Densities for Pentafluoroethane (R-125), Journal of Chemical & Engineering Data, 42 (1997) 745753. 14

Table 1. Purities and suppliers of the chemicals studied. Chemical d

distilled water

Formula

a

H2O

b

c

GWP

Supplier

-

-

-

UKZN

0.52 C2H3F3/0.44 C2HF5/ 0.04 C2H2F4

0.998

0

3260

Afrox

0.55 CHClF2/ 0.41 C2H3ClF2/0.04 C4H10

0.998

0.036

n.a.

Afrox

0.46 C2H3F3/ 0.07 C2HF5/ 0.47 CHClF2

0.997

0.026

3020

Afrox

0.5C2H2F4/ 0.25 C2HF5/

0.995

0

1800

Afrox

R404A

e

R406A

e

R408A

e

R427A

e

Purity

0.15 CH2F2/ 0.1C2H3F3 a

As indicated by supplier

Ozone depletion potential

b

Global warming potential

c

d

Ultrapure Millipore Q water with an electrical conductivity of 18 MΩ.cm

e

mass fraction

16

ODP

Table 2. Phase transition parameters [29] from water to hydrate used in this study.

Structure

∆µ w0 /Jmol-1

∆hw0 /Jmol-1

∆ v w0 /cm3.mol-1

I

1297

-4620.5

4.6

II

937

-5201

5.0

∆µ w0 : Chemical potential difference between empty hydrate lattice and pure water ∆hw0 : Molar enthalpy difference between empty hydrate lattice and ice at the ice point and zero pressure ∆v w0 : Volume difference between empty hydrate lattice and pure water

17

Table 3. Pure component PRSV EoS parameters used in this study. Refrigerant

a

R22(CHClF2)

TC/K

b

PC/MPa

k1

Ref

c

d

369.3 4.989

0.21974

0.04513

R32(CH2F2)

351.6 5.830

0.27704

-0.02874

R600a(C4H10)

408.1 3.655

0.18466

-0.00238

R134a(C2H2F4) 374.3 4.068

0.32610

0.0

[34]

R143a(C2H3F3) 346.0 3.776

0.26110

e

n.a

[35]

R142b(C2H2F4) 409.6 4.330

0.25100

n.a

R125a(C2HF5)

0.30380

n.a

339.4 3.639

a

Tc= critical temperature

b

Pc=critical pressure

ω

ω = acentric factor

c

d

k1= PRSV EoS constant

n.a.= not available

e

18

[33]

[36]

Table 4. Equilibrium hydrate dissociation data and the enthalpy of hydrate dissociation reaction in this studya .

Texp/ K 272.7 274.6 277.2 281.2 283.9 286.3 287.6 287.9 288.1 288.7

Pexp/ MPa 0.079 0.095 0.150 0.286 0.442 0.639 0.830 1.422 2.702 7.623

275.8 276.9 278.8 279.5 281.1 282.7 283.7 284.8 1.301 2.098 3.512 9.081

0.112 0.140 0.199 0.203 0.310 0.439 0.536 0.626 285.0 285.2 285.4 286.2

a

b

R427A Tcalc./ b AE ∆H/kJ/mol K 273.4 0.7 102.5 274.5 0.1 102.2 277.1 0.1 101.1 281.1 0.1 98.5 283.8 0.1 95.4 286.2 0.1 91.4 287.8 0.2 87.3 289.1 0.6 288.6 0.5 288.5 0.4 R406A 276.0 0.2 124.6 277.1 0.2 123.8 278.9 0.1 122.3 279.0 0.5 122.2 281.1 0.0 119.4 282.8 0.1 115.9 283.8 0.1 113.3 284.4 0.4 110.7 286.4 0.2 285.6 0.2 285.4 0.2 285.3 0.3

Texp/ K 274.7 275.8 276.6 277.3 279.0 280.9 282.4 283.3 283.7 284.3 285.3 285.3 285.6 286.5

R404A Pexp/ Tcalc./ MPa K 0.128 274.1 0.165 275.4 0.188 276.1 0.214 276.7 0.314 278.6 0.492 280.8 0.692 282.3 0.849 283.2 0.946 283.7 1.136 284.4 1.531 285.5 2.003 285.4 2.830 285.4 9.995 285.6

AE

∆H/kJ/mol

0.6 0.4 0.5 0.6 0.4 0.1 0.1 0.1 0.0 0.1 0.2 0.1 0.1 0.9

142.7 141.6 141.0 140.2 137.3 132.0 125.7 120.4 116.9 109.6 89.2 -

b

R408A 274.9 275.6 278.1 279.8 281.3 282.1 282.6 283.7 284.2 284.6 285.0

u(T) = ±0.1 K, u(P) = ±0.005 MPa

AE=|Texp -Tcalc.|

19

0.148 0.182 0.301 0.393 0.555 0.711 0.787 0.926 1.577 2.666 6.100

274.9 275.9 278.4 279.7 281.3 282.5 282.9 283.6 283.8 284.1 285.1

0.0 0.3 0.3 0.1 0.0 0.4 0.3 0.1 0.4 0.5 0.1

142.1 141.1 137.7 134.9 130.0 125.0 122.5 117.7 -

Table 5. Kihara potential parameters used in this study. Refrigerant

a/Å

σ/Å

ε/k

R32(CH2F2)

0.790 2.890 198.60

R134a (C2H2F4)

1.311 2.722 241.32

R143a (C2H3F3)

1.220 3.100 202.30

Ref.

[6]

R142b (C2H3ClF2) 1.430 3.172 232.70 R125a (C2HF5)

1.321 2.574 270.12

R600a (C4H10)

0.910 3.590 215.98 This work

a:radius of the spherical molecular core σ:collision diameter ε:characteristic energy

20

Figure 1. Schematic diagram of the apparatus used in this study; C, cell; CF, cold finger; DAS, data acquisition system; GC, gas cylinder; MS, mechanical stirrer; MJ; mechanical jack; PT, pressure transmitter; TB, thermostatted bath; TP, temperature probe; TPC, temperature programmable circulator; V-i, valve; VP, vacuum pump.

21

P/MPaMpa Pressure,

0.9

0.75

0.6

0.45 282

283

284

285

286

287

288

289

Temperature, T/K K

Figure 2. Primary heating and cooling curve for R427A hydrate obtained in this study.

22

10

8 6 5 4 3

P/ MPa

2 1

0.8 0.6 0.5 0.4 0.3 0.2 0.1

0.08 0.06 270

272

274

276

278

280

282

284

286

288

290

T/ K

Figure 3. Hydrate dissociation conditions of R427A, experimental data: ●, (This work); Solid lines, Model predictions.

23

15

P/ MPa

10 8 6 5 4 3 2 1.5 1 0.8 0.6 0.5 0.4 0.3 0.2 0.15 0.1 0.08 274

276

278

280

282

284

286

288

T/ K

Figure 4. Hydrate dissociation conditions of R406A, experimental data: ▲, (This work); Solid lines, Model predictions.

24

10 8 6 5 4 3

P/MPa

2 1 0.8 0.6 0.5 0.4 0.3 0.2 0.1 0.08 0.06 272

274

276

278

280

282

284

286

288

T /K

Figure 5. Hydrate dissociation conditions of R404A, experimental data: ■, (This work);; Solid lines, Model predictions.

25

10 7 6 5 4 3

P/MPa

2

1 0.7 0.6 0.5 0.4 0.3 0.2

0.1 0.07 272

274

276

278

280

282

284

286

T /K

Figure 6. Hydrate dissociation conditions of R408A, experimental data: ●, (This work); Solid lines, Model predictions.

26

Figure 7. Temperature dependency of the hydrate dissociation enthalpies of ♦, R427A; ●, R406A; ▲, R408A, ■ R404A.

27

Highlights

•

The application of refrigerant hydrates in cold storage systems is investigated.

•

Hydrate dissociation conditions of various refrigerants have been measured.

•

A correlative thermodynamic model was applied to the data.

•

Enthalpy of dissociation for the refrigerants studied calculated.

•

Experimental measurements performed over a wide range of pressures.

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