Thermophysical properties of the trifluoroethanol-pyrrolidone system for absorption heat transformers

Thermophysical properties of the trifluoroethanol-pyrrolidone system for absorption heat transformers C. Z. Zhuo and C. H. M. Machielsen Laboratory for Refrigerating Engineering, Delft University of Technology, Mekelweg 2, 2628 CD Delft, The Netherlands Received 7 August 1992; revised 14 December 1992

Thermophysical properties of the 2,2,2-trifluoroethanol-2-pyrrolidone system have been investigated. The scope of the study includes density, viscosity, thermal conductivity, the equilibrium vapour pressure concentratio~temperature relationship, specific heat capacity, heat of mixing and enthalpy. Experimental measurements were carried out. Mathematical correlations for various properties were mainly derived from measured data. The good combination of physical and thermal properties of trifluoroethanol and pyrrolidone shows that they can be used as a working pair for absorption heat transformers. (Keywords: heat transformer; absorption; trifluoroethanol; TFE)

Propri6t6s thermophysiques du syst6me trifluoro6thanolpyrrolidone pour les transformateurs de chaleur fi absorption On a btudik les propribtbs thermophysiques du syst~me 2,2,2-trifluorobthanoL2-pyrrolidone, et plus prbcisbment la densitb, la viscositk, la conductivit~ thermique, la relation d'bquilibre pression de vapeur-concentrationtemperature, la chaleur spbcifique, la chaleur de mblange et l'enthalpie. On a effectub des rnesures expbrimentales. Les donnbes obtenues ont permis essentiellement d'obtenir des corrblations mathkmatiques sur plusieurs propribtbs. Gr6ce gt ces bonnes propribtks physiques et thermiques, le couple trifluorobthanol-pyrrolidone peut ~tre utilisb comme fluide actif dans les transformateurs de chaleur h absorption.

(Mots cl6s: transformateur de chaleur; absorption; trifluoro~thanol; TFE)

Nomenclature Cp

c~ h Ahv Ahm M P T V x

Specific heat at constant pressure (kJ kg -j K -I) Specific heat for the change in enthalpy of a saturated liquid with temperature (kJ kg i K - i ) Enthalpy (kJ kg l) Enthalpy of evaporation (kJ kg ~) Heat of mixing (kJ kg-~) Molecular weight (g mol-1) Vapour pressure (mbar) Temperature (K) Mole volume (cm -3 mol-1) Mole fraction of T F E in liquid phase

Nowadays, people are more and more concerned about energy conservation. One of the efficient ways to recover waste heat in the process industries is the use of absorption heat transformers, which can upgrade waste heat at an intermediate temperature to useful heat at a high temperature. Until now, all commercial absorption heat transformers have utilized the H20-LiBr system as the working pair. The use of the H20-LiBr system has many advantages, such as high enthalpy of evaporation, high heat and mass transfer, non-toxicity, no need of rectifi0140-7007/93/050357~07 ~3 1993 Butterworth Heinemannand IIR

Greek letters

2 /t P

Thermal conductivity (W m-~ K-~) Dynamic viscosity (Pa s) Density (g cm -3) Mass fraction of T F E Volume fraction

Subscripts

1 v Pyr TFE

liquid vapour pyrrolidone trifluoroethanol

cation apparatus, etc. However, because of corrosion and the solubility characteristics of H20-LiBr, the maximum temperature of the output heat and the temperature lift are restricted. The demand by industry for advanced absorption heat transformers with high output heat temperatures, high temperature lifts and high efficiency has stimulated much research on developing new working fluids I 5. Since the thermophysical properties of a working pair have a great influence on the performance of an absorpRev. Int. Froid 1993 Vol 16 No 5

357

Thermophysical properties of the trifluoroethanol-pyrrolidone system. C. Z. Zhuo and C. H. M. Machielsen "Fable 1 General physical properties Tableau 1 Propri~tbsphysiques gbnbrales Name

Contents

Chemical name Molecular formula Molecular weight (g mol ~) Boiling point (°C) Melting point (°C) Density (g cm ~) Refractive index Flash point (°C) Viscosity (mPa s) Critical temperature (K) Critical pressure (bar) Heat of combustion Heat of evaporation (kJ kg ~) Specific heat (kJ kg ~ K ~)

2,2,2-trifluoroethanol C2H3F30 100.04 74 - 43.5 1.4680 at 20°C 1.2907 at 22*C 35 1.24 at 25°C 498.57 48.25 -886,6 (kJ tool ') 449 at 0°C 1.9 at 50°C

tion heat transformer, it is essential to know well the properties of a new working pair before it can be technically applied in engineering. The 2,2,2-trifluoroethanol (TFE) and 2-pyrrolidone (Pyr) system has been chosen as a new working pair for an advanced absorption heat transformer at our laboratory. T F E acts as the working fluid and Pyr acts as the absorbent. The T F E - P y r working pair is probably a promising new working pair for absorption heat transformers, as proposed by Bokelmann and Steimle ~ and Westra 4. However, the properties of the working pair can rarely be found in the literature. Therefore, the thermophysical properties of T F E - P y r are described in this paper, based on a series of our own measurements and computer calculations, as well as references where available. The T F E and Pyr were obtained from Merck Corporation in the Netherlands. They have a purity of more than 99% and contain less than 0.2% water, respectively. With the help of the properties presented in this paper, the performance of an absorption heat transformer using T F E - P y r as the working pair can be well evaluated.

General physical properties The general physical properties of T F E and Pyr are presented in Table 1. T F E is a colourless liquid with an ethanol-like odour. It is completely miscible with many solvents. It has high thermal stability, relatively high heat of evaporation, low working pressure and no corrosion problems with most common metals. Although T F E has a flash point of 350C and is considered flammable, it will not sustain its own flame, because of its low heat of combustion. The main disadvantage of T F E is its toxicity; it has the same toxicity class as NH3 in Switzerland. In experiments with animals, it has been found that T F E is toxic by inhalation, ingestion and also when brought into contact with unprotected skin. Contact with the eyes should be avoided. Below a concentration of 10 ppm, no significant harmful effect has been found; the effect will be transient after exposure to 50 ppm. However, for safety reasons, it is recommended that a concentration in the environment greater than 5 ppm be avoided. At atmospheric pressure and temperatures above 25°C, Pyr is a liquid, miscible with water, ethanol, ether, chloroform, benzene, ethyl acetate and carbon disulfide. Pyr irritates the eyes to a small degree and possesses a 358

Int. J. Refrig. 1993 Vo116 No 5

Reference

Contents 2-pyrrolidone C~HTNO 85.11 245 24.6°C 1.120 at 20°C 1.4806 at 30°C 138 10.5 at 31°C 519.4 85.9 27266 (kJ kg ~) 676 at 7 mbar

6 6 6 6 15 12 11 I1 12 10 10

/.96 at 50°C

Reference

5 6 15 6 15 15 5 5 15 15 15

very weak irritating effect on skin and mucous membranes. It has good chemical and thermal stability, and will not cause corrosion with carbon steel and stainless steel. The temperature of thermal stability for T F E is recommended ~2 as high as 315°C in the presence of normal metals. Experiments ~4 for thousands of hours at 350°F (177°C) and 600°F (315°C) have shown that T F E has a high thermal stability with carbon steel, cast iron. low alloys, copper and aluminium. Product information 15for Pyr shows that it will begin decomposition at over 240°C. Tests at our laboratory show that both Pyr and an equimolar T F E - P y r mixture are compatible with normal metals, except copper which shows relatively significant decomposition. Therefore, the working pair T F E - P y r is suitable for use in absorption heat transformers at high temperatures, at least to 200°C. T F E is miscible with Pyr at the operating temperatures of absorption heat transformers over a wide concentration range. Crystallization should not occur at the operating conditions of heat transformers. The boiling point difference between T F E and Pyr is 171 K, and rectification apparatus may be unnecessary. By using the T F E - P y r system as the working pair for absorption heat transformers, a higher useful temperature and a higher temperature lift between the waste heat and the useful heat than for the H , O - L i B r system can be reached.

Density The liquid density of T F E , P y r was measured with an Anton Paar D M A 45 density meterl During the measure' ment, the sample was injected into an oscillator celf and kept at the desired temperature under control of an ultrathermostat. The natural frequency o f the oscillator was measured, from which the density was automatically determined by a built-in arithmetic processor in the density meter. Different samples gave different values of natural frequency, so that densities o f sampleS were determined, The density meter was calibrated with water and air. The accuracy o f the instrument is 0.0001 g c m -3. From the measured data using the m e t h o d of regression, a polynomial expression for the density p at concentration ~ and temperature T was derived: 6

3

i=1

j=l

p=

TJ-,

(1)

Thermophysical properties of the trifluoroethanol-pyrrolidone system: C. Z. Zhuo and C. H. M. Machielsen Table 3

C o n s t a n t s f o r the c o r r e l a t i o n o f viscosity T a b l e a u 3 Constantes dans la correlation pour la viscositk

Table 2

C o n s t a n t s o f the c o r r e l a t i o n o f d e n s i t y T a b l e a u 2 Constantes dans la corrblation pour la densitb A,j i= 1 i= 2 i=3 i=4 i=5 i=6

j=l

j=2

1.4158 7.6418 x 10 4 0.84890 -1.9671x10 ~ 2.1913 -0.92356

j=3

- 1.1924 4.8911 x 10 - 2 . 8 8 8 6 x 10 7.8993x10 1.6749 x 10 0.0

5.3463 x 10 - 1.0055 x 10 - 1.1364 x 10 -1.2091x10 0.0 0.0

4 4 4 3

7 6 6 6

B,j

j=l

j=2

i= 1 i= 2 i=3 i=4 i=5 i=6

4.5248 x 10' - 2 . 0 7 0 5 x 102 2.4207 x 102 -1.2372x102 7.6347 x 10 ~ -3.6711x10 ~

j=3

- 3 . 7 2 5 6 x 1@ 1.4047 x 105 - 1.4909 x 105 2.9993x104 1.1711 x 104 0.0

6.9704 x l 0 ~ - 2 . 5 9 3 2 x 107 3.0488 x 107 -1.0505x107 0.0 0.0

P a s)

~:os --. ~

0 0 7

•

~:o8

l

12

I

~:o~-

-

~,:~o+ /

. . . . .

i =O9

~'~

0 20

Figure I Figure 1

30

40

50

60

Density ofTFE

70

80

90

100

110

120 ~C

Pyr

20

I 3O

4O

5O

60

70

80

90

100

110

120"C

Figure 2

Viscosity p. as a f u n c t i o n o f t e m p e r a t u r e T a n d c o n c e n t r a t i o n

Figure 2

Viscositk ,u en fonction de la tempbrature T et de la concen-

Densitb du TFE-I°vr tration

where p is in g cm 3, ~ is the mass fraction of T F E in the solutions, T is in K and A~ are constants as shown in Table 2. Equation (1) has a mean deviation of0.113% in comparison with measured data. Figure 1 shows the density of T F E - P y r for temperatures from 20°C to 120°C and mass fractions of T F E from 0 to 1.

Thermal conductivity The Riedel correlation 7 is used to predict the thermal conductivity of T F E as:

Viscosity Viscosities were measured with a Ubbelohde viscometer. About a 15 ml sample was poured into the viscometer, and put into a constant temperature bath, where the temperature was controlled at a required constant + 0.01 °C. Measurement took place after 15 min for equilibration. A personal computer and automatic instruments were used to control the measuring process. The method of measurement gave the kinematic viscosity, which was then converted to dynamic viscosity using the density information. The accuracy of the measurements was within 1%. A polynomial correlation for the dynamic viscosity/1 of T F E - P y r at concentration ~ and temperature T was fitted to our measurements: 6 3 In/1 = Z ~ Bij ~i i=l /=1

the viscosity of T F E - P y r based on the calculation of Equation (2).

I

1 t j-'

2 = ,q,c[1 + 6.667(1 - T/Tc) o.667]

(3)

Since 2 at 293.15K is 0.1272 W m -~ K ~, the critical temperature T~ is 498.57 K; the constant ~,~ is calculated as 0.02712. Therefore, Equation (3) becomes: ~rFE = 2.712 X 10 2 [1 + 6.667 (1 -- 7"/498.57) °667]

(4)

According to product information ~3, the thermal conductivity of T F E at 104°F is 0.071 Btu h -~ ft ~ °F ~, i.e. 0.123 W m ~K-~ at 40°C, while the predicted value from Equation (4) at the same temperature is 0.121 W m K-~; they are very close to each other. The thermal conductivity data for Pyr can be found in a data bank s. A simple polynomial relationship with temperature is fitted to the data:

(2)

where/t is in Pa s, Tis in K and Bg/are constants given in Table 3. Equation (2) is valid over the temperature range from 20°C to 100°C and mass fractions of T F E from 0.3 to 1.0. Comparison of measured and calculated results shows that the mean deviation is 0.029%. Figure 2 shows

/I-Pyr =

el + e2T + e3 T2 + e4 T3

(5)

where e~ = 0.2880, e2 = - 4 . 5 4 9 2 × 1 0 - 4 , e3 = 5.2783 x l0 7, e4 = - 4 . 0 1 7 2 x 10 ~0. The mean deviation between calculated data and the data bank data is 0.014%. Since no measured data for the thermal conductivity

Rev. Int. Froid 1993 Vo116 No 5

359

Thermophysical properties of the trifluoroethanol-pyrrolidone system: C. Z Zhuo and C. H. M. Machielsen X (WmlK ~) 0.2 ~. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . )~X" o18~-N~

r _ i t=20C

k-'X\\

]

~n~N~X" ~

0.14 ~

~

-~-

I t= 100'C

-

-

~ _. . . . . . . . . . . . . 1=40C t = 60'C

A--

~

=120"C t= 140'0

] t=80Ci -~!

i

t= 160'C

~

02

0.3

04

05

06

07

08

09

t ~

Figure 3 Thermal conductivity of TFE Pyr Figure 3 Conductivitbthermiquedu TFE-Pyr

of T F E - P y r mixtures are available, a correlation 9 which is based on the thermal conductivities of pure components is used. '~ =

1~] '~TFE ~- I ~ / ~ P y r -1"- 2 4h

qb2/(l/J, rFE +

1/Zpy,)

(6)

n~0

where Z is in W m -~ K -~, x~ is the mole fraction of component i, ¢~ is the volume fraction of component i and Vi is the molar volume of pure liquid i. Figure 3 shows the thermal conductivity at temperatures from 20°C to ! 60°C over the whole concentration range. It has been pointed out 9 that Equation (6) is a good estimation method, with errors mostly within 3 - 4 % in comparison with those determined experimentally for a large number of binary liquid mixtures.

Equilibrium vapour pressure-temperature-concentration relationship Based on the measurement data of Bokelmann "), we derived a correlation for the equilibrium pressure-temperature-concentration relationship of T F E - P y r as: 2 /=l

n=0

Z'~;~"

T 2 : ,'; 1)

(9)

The constants D,, E, and F~ in Equation (9) are given in Table 5, and were determined from experimental data by the least squares method. The mean deviation between calculated values and the experimental data is 0.46%: A graphical presentation of the specific heat capacity is shown in Figure 5. The specific heat capacities of pure components in the vapour phase were predicted by an estimation method 9 based on chemical structures. Equations (10) and (11) are for the pure T F E vapour and the pure Pyr vapour, respectively.

Cp.TW = 0.2256 + 0.3135 X 10 : T + 0.5637 x 10-9T ~

0.2222 x 10 5T~ (10)

C p , p y r ~--- 0.04585

+ 0.5253 x 10 2 T - 0.3360 x 10-~T: + 1.3287 x 10-gT 3 (11)

8

~, Cij (1 - ~)'(T{,/T)J-'

(7)

Heat of mixing

i=1

where p is the pressure of T F E - P y r in mbar, T0 is the reference temperature in 100 K and C~ are constants given in Table 4. PTVEis the pressure of pure T F E in m b a r expressed by the correlation as: In PlvE = 16.375 - 1.5946 x 103/T - 6.0094 x 105/T2 + 6.8462 x IO~I/T 5 (8) A chart of the equilibrium vapour pressure of T F E Pyr versus temperature and concentration is shown in Figure 4, which is based on the calculated data of Equation (7).

A differential type heat-flow calorimeter was used to measure the heat of mixing (excess enthalpy). It consists of two containers, the sample and the reference parts, which are constructed identically. The reference container is filled with water, and the sample container is filled with an amount of the sample. The recorded signal is the difference between the heat flows to/from the sample container and to/from the reference container. Using different electrical heat inputs, the Calorimeter was calibrated to an accuracy within 1%. This calorimeter was used to measure heat effects which took place in mixing T F E with Pyr at temperature of 298.15 ± 0.10K for various concentrations. The measured data were correlated by means of the RedlichKister type equation as:

Specific heat capacity A differential scanning calorimeter (S~taram PSC-I i 1) was used for the measurements. The calorimeter was

360

~.~,, + r E3 E.~' +

cp =

•bi - - xl V~Xi+V ix: V2

In (P/PTvE) = ~,

calibrated by measuring the heat capacities of a standard sample synthetic sapphire ( e - AI203) at different temperatures; the m a x i m u m deviation from the standard data was within 1%. A sample of 0.2 ml was placed in a stainless steel sample container, which was then placed in the calorimeter. Meanwhile another empty container of the same size was put in the calorimeter as a reference. The temperature of the calorimeter was varied over the range of 293.15 K to 453.15 K at 5 K intervals. The measurement results from the sample container were corrected by the reference container. A total of seven samples (TFE mass fractions 1, 0.790, 0.680, 0.516, (1.341, 0.275 and 0) were measured. It should be pointed out that the measurement results are very close to Co (specific heat capacity for the change in enthatpy of a saturated liquid with temperature) and less close to Cp (specific heat capacity at constant pressure). However, the distinction between Co and Cp at low vapour pressures is negligibleL and therefore no correction is made in this paper~ A polynomial correlation is fitted to the measured data for different temperatures T and mass fractions ~ as:

Int. J. Refrig. 1 9 9 3 V o 1 1 6 N o 5

Ah m =

4 ~(! -- ~) E Gn(1-20" n=0

(12)

Thermophysical properties of the trifluoroethanol-pyrrolidone system: C. Z Zhuo and C. H. M. Machielsen Table 4 Constants for the correlation of vapour pressure Tableau 4 Constantes clans la correlation pour la pression de vapeur C,,

i=1

j= 1 j=2

i=2

1.4294 x 10J 6.1165 x 101

i=3

- 1.0834 x 103 2.1864 x 104

i=5

i=4

9.9196x 103 -2.3246 x 104

-3.3527 x 104 9.0161x 104

i=6

6.4651 x 104 - 1.7776 x 105

i=7

- 6.9030 x 104 1.9063 x 10s

i=8

3.8609x 104 - 8.8309 x 103 - 1.05894 x 105 2.3830x 104

Table 6 Constants for the correlation of heat mixing Tableau 6 Constantes dans la corrdlation pour la chaleur de m~lange

5(x) 0(x)!

G0

G~

-101.871

G2

G)

18.0257 -0.734142

G4

31.2014

25.4218 -80.3393

o

/

Ah (kJ 1 kg ~}

i

5 I 20

N1

211

,I

~)

80

I00

120

140 160

180

200 lo

\

Figure 4

\ \

Figure 4 Pressure temperature-concentration chart -is I

Graphique pression tempdrature-concentration

Gs

\

-20

-25 t

Table 5 Constants for the correlation of specific heat capacity Tableau 5 Constantes clans la corrdlation pour la chaleur massique

n

D,,

E,

0 1 2 3

1.42119 -4.21382 9.96416 -7.63183

8.14861 x 1.94421 x -4.97430 x 3.96786x

F, 10 10 10 10

" 2 2 2

-30 0

01

0.2

03

04

05

06

07

0.8

0.9

Figure 6 Heats of mixing at 250C

3.20324x -2.68915 x 6.29593x -4.86568x

10 10 5 10 5 10 5

Chaleurs de mdlange (J 250C

Figure 6

h (kJ k g u)

7(1(I

J

6C)0 ()q

0q

(I,

0~ 0, 0 0

O. I

0.2

{1.3

0.4

(1.5

(1.6

07

08

(1.9

Figure 7 Enthalpy of TFE Pyr in the liquid phase Figure 7 Enthalpie du TFE I~vr pour la phase liquide "~0

01

02

03

04

05

06

07

08

09

1

Figure 5 Specific heat capacities of TFE-Pyr Figure 5

Chaleurs massiques du TFE Pyr

T h e c o e f f i c i e n t s G, a r e s h o w n in T a b l e 6. C a l c u l a t e d v a l u e s f r o m E q u a t i o n (12) h a v e a m e a n d e v i a t i o n o f 0.76% compared with the experimental data. Figure 6 shows the calculated curves and experimental points.

Enthalpy W i t h t h e h e l p o f t h e p r o p e r t i e s o f specific h e a t c a p a c i t y , heat of mixing, liquid-vapour equilibrium, the enthalpy c o n c e n t r a t i o n d i a g r a m is c o n s t r u c t e d as s h o w n in F i g u r e 7, w h e r e i s o b a r lines a r e g i v e n in t h e r a n g e o f I m b a r t o 1 b a r , a n d i s o t h e r m lines in t h e r a n g e o f 0°C t o 240°C. E n t h a l p i e s at liquid p h a s e a n d v a p o u r p h a s e are calculated f r o m the equations:

Rev. Int. Froid 1993 Vol 16 No 5

361

Thermophysical properties of the trifluoroethanol-pyrrolidone system. C. Z Zhuo and C. H. M. Machieisen 1300 12

h (kJ kgl~

m

l

jF

O0

'

i

T i

I (101 1000[ ~ ....

2 < <

....... -

~ % ....... l ..... 5"'--

?~......:

>.< 70O - 6()0 500

I

!

!

Figure 8 Enthalpy of TFE Pyr in the vapour phase Figure 8 Enthalpie du TFE-Pyr pour la phase vapeur

hi = ~

Cp,TF E d T

+

Tm

+ (1 - ~) To Cp,p;,~ d T + Ahm

Cp d T + 100

(13)

hv = ~vh,.~ + ( 1 - ~v)h,,.2 h~i = •

To

CpdT+

AhvT b + '

(14)

r~

Cp.vdT+

100.

(15)

where To (To = 273.15 K) is the reference temperature, 100 is the reference enthalpy in kJ kg -~ K ~and T~ is the temperature at which the heat of mixing A h m is measured. Given Tm = 298.15 K, Ahm can be obtained from Equation (12). The specific heat capacities at liquid phase ( C p , Cp,TFE, Cp,Pyr)are calculated from Equation (9), while the specific heat capacities at vapour phase (Cp,~) are calculated from Equation (10) or (11). ~ is the mass fraction of T F E in the vapour phase, hv,~is the enthalpy of the pure component i, Tb is the boiling temperature and Ahv,Tbis the heat of evaporation at Tb. The results are presented in Figures 7 and 8. Analysis and conclusions

To develop a new working pair for absorption heat transformers, the thermophysical properties of the working pair need to be investigated. This paper presents the thermophysical properties of a new working pair, 2,2,2trifluoroethanol-2-pyrrolidone, based on experimental measurements and prediction methods. Mathematical correlations are derived, and comparison of calculated data with measured data shows quite good agreement. The T F E - P y r mixture has a high thermal stability to at least 200°C, This characteristic is favourable for absorption heat transformers to provide a high temperature output heat. High temperature heat is needed in great quantity in industry. The boiling point difference between T F E and Pyr is 171 K. The T F E working fluid can be easily separated in the generator of an absorption heat transformer, and rectification apparatus, which increases investment cost and reduces the heat ratio of the heat transformer, may be unnecessary. The evaporation enthalpy of T F E (449 kJ kg- t at 0°C) is smaller than that of water (2500 kJ kg-L at 0"C) and ammonia (1260 kJ kg -~ at 0°C). However, it is higher 362

Int. J. Refrig. 1 9 9 3 V o 1 1 6 No 5

than some other proposed working fluids lor absorption systems, such as H F I P (270 kJ kg ~Rat 0°C), PFPA (255 kJ kg-t at 0°C), R133a (211 kJ k g ~at 0°C) and R22 (205 kJ kg ~ at 0°C). A working fluid with a higher evaporation enthalpy has the advantage of higher output heat. less heat consumption, small size of' apparatus and less pumping work. The main disadvantage of using T F E Pyr as the working pair for an absorption heat transformer is that T F E is toxic, with the same toxicity class as NH3. It is recommended that continuous exposure to atmospheric concentrations greater than 5 ppm be restricted. Densities of T F E - P y r were given for temperatures from 20°C to 90°C and mass fractions of T F E from 0 to 1. A polynomial correlation was derived with a mean deviation of 0.113% compared with measured data. Viscosities were measured over the range of T F E mass fractions of 0.3 to 1 and 20°C to 100°C. Measured data were correlated as a function of temperature and mass fraction, with an average deviation of 0.029%. Although Pyr is relatively viscous, the viscosities of TFE--Pyr at the operating temperature of absorption heat transformers will always be below 2.5 cP, which is similar to that of the commercially used working pair H:O-LiBr~ and fulfils the requirement as a working pair for heat translbrmers. Thermal conductivities of T F E Pyr mixtures were predicted for temperatures from 20°C to t60°C over the whole mass fraction range, Errors are estimated to be within 3 4 % . It seems that lhermal conductivities of T F E - P y r mixtures are about 3 times lower than those of H20 LiBr mixtures at the same temperature: this is a disadvantage for heat transformers. The T F E . Pyr mixture possesses a flat vapour pressure curve. At the operating range of absorption heat transtk~rmers, high temperature lift and high temperature output heat are easier to be achieved; the difference between high and low pressure is small, and consequently the electrical energy for liquid pumps is low. Also, given the temperature values of waste heat and cooling water for a heat transformer, the low and high working pressures inside a heat transformer can be estimated, One finds that the working pressure range is quite suitable, which results in less investment cost, because a light component structure will be sufficient for heat transformers. The T F E - P y r mixture presents a negative deviation from Raoult's law, which satisfies the requirement of working pairs for heat transformers. Specific heat capacities were measured for temperatures from 293.15 K to 453.15 K and concentrations from 0 to 1 T F E mass fraction. The accuracy of measurements was within 1%. A correlation was developed which was able to fit the measured data with an average absolute deviation of 0.64%. Since specific heat capacities of TFE--Pyr are lower than those of H20-LiBr, larger heat exchangers are needed for the TFE--Pyr system. Heats of mixing were measured at 25°C, with an accuracy within 1%. A Redlich-Kister type correlation was obtained. The mean deviation between calculated data and measured data is 0.76%. Heats of mixing of T F E Pyr mixtures are exothermal, which provides an advantage to increase the heat of absorption in an absorption heat transformer• It is concluded that the T F E Pyr system is well suitable as the working pair for absorption heat trans-

Thermophysical properties of the trifluoroethanol-pyrrolidone system: C. 7-. Zhuo and C. H. M. Machielsen formers. T h e w o r k i n g p a i r has the a d v a n t a g e s o f high t h e r m a l stability, low w o r k i n g pressure, fiat v a p o u r pressure curve, high boiling difference between the w o r k i n g fluid a n d a b s o r b e n t , s t r o n g negative d e v i a t i o n f r o m R a o u l t ' s law, relatively high h e a t o f e v a p o r a t i o n a n d is n o t corrosive to m o s t metals. Its d i s a d v a n t a g e s are its toxicity a n d relatively low t h e r m a l conductivity.

6 7 8

Acknowledgement This project is financially s u p p o r t e d Utrecht, the N e t h e r l a n d s .

5

by

NOVEM

9 10

References 1 2

3 4

Bokelmann, H., Steimle, F. Development of advanced heat transformers utilizing new working fluids lnt J Refrig (1986) 9 51-59 Nowaczyk, U., Sehmldt, E.L., Steimle, F. New working fluids systems for absorption heat pumps and absorption heat transformers Proc XVllth Int Congr Refrig: Vol B, Vienna, Austria (1987) 1169-1176 Matthys, H., Trepp, C. Working fluids for high temperature sorption cycles lnt J Refrig (1989) 12 322331 Westra, J.J.W. Development of an advanced absorption heat

11 12 13 14 15

transformer, Dissertation, Delft University of Technology, The Netherlands (1990) 56~0 Borde, L Development of absorption heat pumps with organic working fluids for refrigeration and heating applications, Proc Jap Assoc of Refrig lnt Syrup on Recent Developments in Heat Pump Technology, Tokyo, Japan (1988) 123-132 David, R.L. CRC Handbook of Chemistry and Physics Chemical

Rubber, Florida, USA (1990) 3.237 3.439 Jamieson, D.T., Irving, J.B., Tudhope, J.S. Liquid Thermal Conductivity a Data Survey to 1973, HMSO, Edinburgh, UK (1975) 1-3 Uittenhout, G.C.J., van Nuland, J.B.P. Berekening van fysische en thermodynamishe eigenschappen van organische stoffen Procestechnologie (1986) 2 29-34 Reid, R.C., Prausnitz, J.M., Sherwood, T.K. The Properties of Gases and Liquids McGraw-Hill, New York, USA (1977) 149 534 Bokelmann,H., Ehmke, H.J. Arbeitsstoffsysteme f/ir eine sorptionw~irmepumpe, Report of the EEC Sponsored Research Programme EE-A4-O31-D(B), Essen, Germany (1984) 217 228 Bier, K., Tiirk, M., Zhai, J. Vapour pressure of trifluoroethanol, IIF-IIR Commission B1, Herzlia, Israel (1990) 129-136 Product Information: 2,2,2-Trifluoroethanol Kali-Chemie, Hannover, Germany (1989) Product Information: Trifluoroethanol Halocarbon Products Corp, NJ, USA (1980) Product Information: Fluorinols Halcocarbon Products Corp, NJ, USA (1979) Product Information: 2,2,2-Trifluoroethanol and 2-pyrrolidone

Merck-Schuchardt, Hohenbrunn, Germany (1984)

Rev. Int. Froid 1993 Vo116 No 5

363