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Thermophysical properties of new working fluid systems for absorption processes
Thermophysical properties of new working fluid systems for absorption processes U. N o w a c z y k and F. Steimle Institut fiir Angewandte Thermodynamik und Klimatechnik, Universit~it Essen, Germany Received 5 M a y 1991
Research on new working fluid pairs for use in absorption heat pumps has been continued. The selection of suitable substances was made by following a systematic approach based on the molecular properties of the refrigerant solvents. The refrigerants investigated are trifluoroethanol (TFE) and hexafluoroisopropanol (HFIP). They are combined with solvents from the group of organic heterocycles. Experimental data are presented on solubility, heat of mixing and vapou~liquid equilibrium. Finally, the new systems are compared with those used currently. (Keywords:heat pumps;trifluoroethanol;hexafluoroisopropanol)
Propri&6s thermophysiques de nouveaux syst6mes de fluides actifs pour des proc6d6s a absorption On a poursuivi les recherches sur les nouveaux couples de fluides actifs, utilisables dans les pompes h chaleur 3 absorption. On a s~lectionnb les substances approprikes en se r~f~rant 3 une approche syst~matique fondke sur les propri~tbs molbculaires des solvants frigorifiques. Les frigorig~nes btudi~s sont le trifluorobthanol ( TFE) et l'hexafluoroisopropanol ( HFIP). Ils sont combinks avec des solvants du groupe des h~t~rocycles organiques. On prbsente des donnkes expbrimentales sur la solubilitb, la chaleur de mklange et I'bquilibre vapeur-liquide. Enfin, on compare les nouveaux syst~mes avec les syst~mes actuellement utilisbs.
(Mots cl6s: pompe ~ chaleur; trifluoro&hanol; hexafluoroisopropanol)
When new improved absorption heat pumps and absorption heat transformers are to be developed, the choice of the appropriate working fluid system is crucial. The demands to be fulfilled by an optimal system have already been presented and thoroughly discussed. There is no ideal working pair suitable for every application because several of the many requirements made of such a pair are contradictory. A summary of the requirements for working fluids is given: Refrigerant
• • •
high specific enthalpy of evaporation; favourable pressure range; low pressure difference;
Refrigerant and absorbent
• • • • • •
high thermal and chemical stability; good solubility of refrigerant in the absorbent; low specific mass flowrate; low heat of mixing; low specific heat capacity; high difference in boiling points.
A suitable compromise has to be found therefore, and the properties have to be thoroughly compared to find the best working pair. Table 1 lists the working fuid systems investigated between 1981 and 1989 at the Institut ffir Angewandte Thermodynamik und Klimatechnik, Universit~it Essen. The working fluid system T F E - N M P shows favourable properties for use in absorption heat pumps. However, measurements of vapour-liquid equili0140-7007/92/01001006 © 1992 Butterworth-HeinemannLtd and IIR 10 Int. J. Refrig. 1992 Vo115, No 1
brium revealed a relatively high fraction of absorbent in the vapour phase. Therefore, it seems desirable to make further investigations with fluoroalcohols and organic solvents with high boiling points. Selection of appropriate working fluid mixtures The most important criteria for a working pair are thermal and chemical stability and large solubility of the refrigerant in the absorbent. The molecular basis for good solubility and thermal stability can be found with fluoridized alcohols. The properties of the fluoroalcohols and their mixtures with other compounds are related to the hydrogen bonding properties of the alcohols. These substances have the activated hydrogen atom of their hydroxyl group available for interaction with a proton acceptor. The suitable absorbent therefore must be an electron donor that can achieve effective hydrogen bonds with a centre of high electron density. Fluoroalcohols as refrigerants Based on the above ideas the following refrigerants were chosen: trifluoroethanol (TFE) and hexafluoroisopropanol (HFIP). In Table 2 the physical and chemical properties of the refrigerants investigated are listed. They are all polar 'water-white' liquids having a high density and low viscosity and a high degree of thermal stability. All substances listed are miscible with water and also many organic solvents.
Thermophysical properties of working fluids for absorption processes: U. Nowaczyk and F. Steimle Table 1 Working fluid systems investigated for absorption processes (references) Tableau 1 SystOmes de fluides actifs btudibs pour les proc~d~s h absorption (rbJ~rences) Absorbents
Refrigerants: Methanol
Lithiumbromide Lithiumthiocyanate Zincbromide Lithiumbromide-zincbromide Water Ethyleneglycol Triethyleneglycol-dimethylether(DTrG) Tetraethyleneglycol-dimethylether(DTG) N-Methylpyrrolidone (NMP) Dimethylpropylenurea (DMPU) N-Formylmorpholine (NFM) Sulpholane (Sul)
Ammonia-lithium-nitrate Methylamine
R22
R123a
8
2
TFE
HFIP
19 19 19 19 4,8 8
3,5,15,16 2 3,5,15,16
3,5,15 5,16 5
Table 2 Physicalproperties of refrigerants and absorbents Tableau 2 Propribtbs physiques des J?igorig~nes et des absorbants Abbreviation Molecular formula
TFE C2H3F30 FI HI F--C--C~OH
Structural formula
I F
Molecular weight (g mol ~) CAS Nr. Boiling point (*C) Melting point (*C) Critical temperature (°C) Critical pressure (bar) Enthalpy of evaporation at 0°C (kJ kg ~) Specific heat at 50°C (kJ kg ~K ~) Density at 20*C (kg m 3) Dynamic viscosity at 20°C (mPa s ~) Heat conductivity at25*C(Wm ~K ~) Surface tension at 25°C (mN m ~) Dipole moment p_(Debye) at 25°C in cyclohexane Dielectric constant at 250C pK, values in water at 25°C Danger LD50, oral (mouse/rats) "Ref. 2 hRef. 20
JRef. 10 ,Ref. 13
HFIP C3H2F60 Fi H~ FI F~C--C--C~F
I H
I F
168.04 920~6-1 58 - 3 195~ 34.2b 270~ 1.51 1618.37 1.96
20.4k 2.0M
16.3h 2.05~
240 mg kg J , IRef. 14 ~Ref.21
hRef. 24 ,Ref. 26
,Ref. 9 ~Ref. 18
16.7t 9.3~ eye and skin irritation 600 mg kg J ~
~Ref. 1 '"Ref.22
In contrast to T F E , H F I P is n o t corrosive n o r inflamm a b l e 24. The insertion of two CF3 groups in a molecule, as with H F I P , leads to a decrease in boiling p o i n t a n d e n t h a l p y of e v a p o r a t i o n . The reason for this negative effect is the i n t r a m o l e c u l a r h y d r o g e n b o n d i n g interaction of the O H g r o u p to two fluorine atoms, one in each CF3 group. F a v o u r a b l e solubility a n d good thermal stability c o n t r a s t with a relatively low e n t h a l p y o f e v a p o r a t i o n . Because of the nearly complete fluoridization, the toxicity of the H F I P is essentially smaller t h a n that o f T F E , which is c o m p a r a b l e in its toxicity to a m m o n i a .
Polar solvents as appropriate absorbents A p p r o p r i a t e solvents with an active d o n o r centre, a stable m o l e c u l a r c o n s t r u c t i o n a n d high boiling p o i n t can be f o u n d in the organic heterocycle group, e.g. cyclic derivates o f N-alkyl urea, lactams, oxazolidinones, carb o n a t e s a n d sulphones. These are strong polar solvents having high dipole m o m e n t s a n d c o n s e q u e n t l y a high electron density s u r r o u n d i n g their acceptor atoms. The organic solvents chosen for this investigation are presented with their most i m p o r t a n t properties in Table
128.18 7226-23 5 230 < - 20 453~
222.28 143-24-8 275-276 - 30 446m
2.0° 1064.5 3.51
494.3 (at 25°C)" 2.03 1010.8 3.58 0.2322q
H~C-
.CH3
Sul C4H~O2S o%~.o
CH~CH2CH~4CH, J
I I OH F
100.04 75 89 8 73.6 - 45 226.7, 49.3, 449,, 1.9, 1391 2.03
26.67,' 12.4, eye irritation
N~N
DTG C~0H2205
DMPU C6H~2N20
"Ref. 11 "Ref.23
31.1, 4.23~ 36.12/
9.16q
1300 mg kg ~~ pRef. 12 ~Ref. 25
5140 mg kg ~,
120.17 126 33M) 287 28 577~
1261.4 10.25 35.5J 4.81• 43.3r
1941 mg kg ~
rRef. 6
2. They all are i m p o r t a n t technical solvents with high thermal stability. These d i p o l a r a p r o t o n i c solvents are clear, colourless liquids with high boiling points a n d different basicity.
Experimental procedure To verify the suitability of T F E a n d H F I P systems for a b s o r p t i o n processes an experimental p r o g r a m m e was carried out in which the following m e a s u r e m e n t s were made: thermal stability; solubility; v a p o u r pressure; v a p o u r - l i q u i d equilibrium; calorific properties; a n d density a n d viscosity. The results of these investigations are published in References 3, 5, 7, 16 a n d 17. The present paper reports the results o f m e a s u r i n g solubility, v a p o u r - l i q u i d e q u i l i b r i u m a n d heat of mixing.
Results The solubility o f T F E was d e t e r m i n e d with a variety of basic solvents. The results are presented in Figure ! by curves o f c o n s t a n t pressure for a n e v a p o r a t i o n temperature of 0°C. In this d i a g r a m the t e m p e r a t u r e is plotted
Rev. Int. F r o i d 1 9 9 2 V o 1 1 5 , N o 1
11
Thermophysical properties of working fluids for absorption processes: U. Nowaczyk and F. Steimle 0,7
0,85 0,6 ~TFE
0,75
0,5
~:HFIP
\
0,65
0,4 0,55
0,3
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30
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Temperature t [°C]
[°C]
Figure 1 Solubility isobars at 0*C for TFE system
Figure 2 Solubility isobars at 10°C for HFIP system
Figure 1 Isobares de solubilitd it O°C pour le systOme TFE
Figure 2 lsobares de solubilitb h 10"C pour le systbme HFIP
versus the concentration. The quality of a system can be assessed by this graph. Promising working systems have a high refrigerant content of solution and further indicate a high change in concentration with temperature. These characteristics result in a low specific mass flowrate. As expected, the weak basic absorbents NMO and D T G have a smaller absorption power for TFE. On the other hand, DMEU, D M P U and NMC have good absorbtion properties for TFE. The solubility of H F I P was determined with several different types of absorbents in order to determine the influence of interaction between acidity and basicity. The results are presented in Figure 2 by curves of constant pressure for an evaporation temperature of 10°C. The basic solvents show a very good solubility for HFIP, likewise the non-basic solvents, which are unsuitable for TFE. The slope of the curve in the working range of the absorber is flat and consequently there is a smaller value for the specific mass flowrate. To construct provisional p,t,x-charts, additional equilibrium measurements at a specified refrigerant temperature of 50"C for T F E systems and 40°C for H F I P were made. Provisional p,t,x-charts of two systems designed by means of these results are shown in Figures 3 and 4. These charts give a first impression of the qualities of the systems. Promising working fluid systems show a broad solution field with an even distribution of the lines of constant concentration. High refrigerant concentrations can be achieved within the working range of an absorption heat pump and result in very low values for the specific mass flowrate. Furthermore, experimental data of the heat of mixing and the vapour-liquid equilibrium are presented. The enthalpy of mixing of T F E and H F I P with a variety of solvents were measured along the 30°C isotherms with a Setaram microcalorimeter. The depen-
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Int. J. Refrig. 1992 Vo115, No 1
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Diagramme provisoire de l'bquilibre pour TFE D M P U
dence of the enthalpy of mixing on the concentration is shown for the 30°C isotherm in Figures 5 and 6. Both graphs clearly show the dependence of the acidity of the refrigerant on the mixing enthalpy. The values of the H F I P systems are higher than those for TFE. Graduation of the values for hm can be explained by the difference in power of the hydrogen bridges formed. The lower values for N F M may be due to self-association in the compound. The boiling behaviour of the mixtures can be appreciated from the t,~-charts, which are based on v a p o u ~ liquid equilibria measurements. The vapour-liquid equilibrium (VLE) of the T F E and H F I P systems was measured isobarically by using a dynamic circulation apparatus 6f the Gillespie type. Within the group of T F E systems there was none with sufficiently high difference
Thermophysical properties of working fluids for absorption processes: U. Nowaczyk and F. Steimle 1000 rnbor 50C
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VLE measurements of the H F I P systems indicated that working fluid combinations with differences in boiling points over 200 K ( H F I P - D T G , HFIP-Sulpholane) do not require rectification. Some of the detailed measurements of the T F E and H F I P systems are presented in Figures 7 and 8. A complete summary of these measurements can be found in References 5 and 16.
~NFIP
Figure 6 Enthalpyof mixing for HFIP systems Figure 6 Enthalpie de mblange pour les systdmes HFIP
in boiling points and therefore rectification might be needed. Nevertheless the new systems have a considerably lower fraction of absorbent in the vapour phase than the system T F E - N M P .
Discussion
Finally the proposed working systems should be compared with the well known systems a m m o n i a - w a t e r and T F E - N M P . To illustrate the comparison, some characteristic numbers are established. In Table 3 the T F E and H F I P systems investigated are presented by using characteristic properties defined as follows:
Rev. Int. Froid 1992 Vo115, No 1
13
Thermophysical properties of working fluids for absorption processes: U. Nowaczyk and F. Steimle 2~ °C 200
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Vapour-liquid equilibrium and McCabe Thiele charts of HFIP-DTG at 750 mbar Figure 8 Equilibre vapeur-liquide et diagrammes McCabe Thiele pour HFIP et DTG gl 750 millibars
Mass concentration of refrigerant in the poor solution Mass concentration of refrigerant in the rich solution Specific mass flowrate, (1 - ~.)/(~r - ~.) f Difference of boiling points between absorbent Ats and refrigerant Specific enthalpy of evaporation of the refrigerr0 ant at 0°C Ps0-c Vapour pressure of the refrigerant at 50°C Minimum pumping energy related to the capacity no of the evaporator, fAp/roPr, where Ap is the pressure difference between evaporator and condenser, and pr is the density of the rich solution Heat flux of the solution heat exchanger related E/L to the capacity of the evaporator, (f - 1)cLAt/ro, where CL is the specific heat capacity of the poor ~a
14
Int. J. Refrig. 1992 Vo115, No 1
solution, calculated by CL = ~aCK + (1 -- ~a)CA for a temperature of approximately 50°C and At is the temperature difference between generation and absorption Relation of enthalpy of mixing and enthalpy of evaporation, hd/ro
To assess the merits of H F I P in comparison to T F E systems the specific mass flowratefand the characteristic number//p for the pumping power were identically calculated for the reference temperatures 0, 50 and 150°C, even though the working range of H F I P can be considerably higher, Table 3 shows that the seven systems that were established in the frame of this research programme might be alternatives to the well known and discussed systems ammonia-water and T F E - N M P . The systems T F E DMEU, T F E - D M P U and T F E NMC yield several positive characteristics for their application in absorption heat pumps. High refrigerant content can be achieved within the working range of an absorption heat pump, which gives, in combination with favourable behaviour of the lines of constant pressure, a very low value for the specific mass flowrate. A low pressure difference in combination with the low specific mass flowrate and also the relatively high enthalpy of evaporation gives, as a result, a very low value for the characteristic number rtp (pumping power). Results for vapour-liquid equilibrium measurements show that the new T F E systems have a considerably lower fraction of absorbent in the vapour phase than the system T F E - N M P because the difference of boiling point is higher. Finally, it must be admitted that for all T F E systems presented, certain rectification efforts have to be taken into account, although considerably less than for the T F E - N M P system. The thermal stability of T F E systems proved satisfactory by respective measurements. It should also be mentioned that the toxicity of T F E cannot be evaluated to be lower than that of ammonia and that T F E is inflammable. Table 3 further indicates particularly favourable values for H F I P systems for the characteristic numbers np and nL. Outside the group of H F I P systems there are two systems for which rectification can most probably be avoided. These are H F I P - D T G and HFIP-sulpholane. The thermal stabilities of these H F I P systems proved to be satisfactory by respective measurements. Further advantages are the non-inflammability and low toxicity of H F I P systems. The use of these working fluid systems was restricted by the relatively high melting point ( - 4°C) of HFIP. References 1 2
Barker, B. J., Rosenfarb, J., Caruso, A. Harnstoffe als L6sungsmittel in der chem. Forschung Angew. Chem. (1979)91 560 Bokelmann, H. Auswahl, Messung thermophysikalischer Eigenschaften und Beurteilung der Eignung von Niederdruckstoffsystemen fi.ir Absorptionswfirmepumpen Dissertation Universitfit Essen (1984) Forschungsbericht des Deutschen Kiilteund Klirnatechnischen Vereins ( D K V ) Nr. 12
3
Bothe, A., Nowaezyk, U., Schmidt, E. L., Steimle, F. New working fluid systems for absorption heat pumps present and future work Proceedings of an International Workshop on Absorption Heat Pumps, London (Ed. P. Zegers and J. Miriam), CEC (1988) 13
4 5
Bothe, A. Das Stoffsystem NH3-LiNO3/H20 fiir den Einsatz in Absorptionskreislfiufen Dissertation Universit/it Essen (1989) Bothe, A., Nowaczyk, U., Schmidt, E. L. Untersuchungen zum
Thermophysical properties of working fluids for absorption processes. U. Nowaczyk and F. Steimle Table 3 Characteristicsof working fluids Tableau 3
Caract~ristiques des fluides actifs
Systems
Refrigerant r0 (kJ k g ' )
Pso'c(bar)
TFE-DMEU TFE DMPU TFE-NMC
449
0.358
HFIP-DMPU HFIP-NFM HFIP-DTG HFIP-Sul
270
TFE NMP NH3 H20
449 1261
evaporation temperature, 0°C
6 7 8
9
10 11 12 13 14 15
0.720
0.358 20.3
Ats(K)
~a
~r
f
I0~np
nu
n,
Rectification
146 156 162
0.143 0.232 0.168
0.367 0.401 0.370
3.83 4.54 4.12
0.25 0.30 0.28
1.25 1.55 1.38
~0.09
yes yes yes
174 186 217 229
0.57 0.482 0.396 0.306
0.69 0.683 0.643 0.55
3.58 2.58 2.45 2.84
0.67 0.46 0.48 0.51
1.62 0.98 0.98 1.26
~0.26 ~ 0.28
yes maybe no no
129 134
0.15 0.23
0.43 0.41
3.04 4.28
0.21 6.56
0.86 1.15
~0.10 ~0.30
yes yes
condensation temperature, 500C absorption end temperature, 50°C
W/irme- und Stoffaustausch von neuen Arbeitsstoffpaaren in Absorptionsw/irmepumpen Abschluflbericht des yon der Europfiischen Gemeinschaft finanzierten Forschungsvorhabens Nr. EN-3E-OO24-D Brfissel (1989) Casteel, J. F., Sears, P. G. Dielectric constants, viscosities, and related physical properties of 10 liquid sulfoxides and sulfones at several temperatures J. Chem. Eng. Data (1974) 19 196 Chiapetta, C., Nowaczyk, U., Steimle, F. Thermophysical properties of hexafluorisopropanol Ki Klima Kiilte Heizung, International Edition '89 Ehmke, H. J. Stoffsysteme f/Jr Absorptionsw/irmepumpen Experimentelle Bestimmung thermophysikalischer Eigenschaften von L6sungen der K/iltemittel Methylamin, Ammoniak und Monochlordifluormethan (R22) Dissertation Universit/it Essen (1984) Forschungsbericht des DKV Nr.13 Kirk-Othmer Encyclopedia of Chemical Technology: Vol 10, Fluorine Compounds, Organic; Vol 21, Sulfolanes and sulJbnes John Wiley & Sons (1986) Kivinen, A., Murto, J., Lehtonen, M. Fluoroalcohols: Part 8 Suomen Kemistilehti B (1968) 41 359 Kneisl, P., Zondlo, J. W. Vapor pressure, liquid density, and the latent heat of vaporization as a function of temperature for four dipolar aprotonic solvents J. Chem. Eng. Data (1987) 32 11 13 Lien, E. J., Kumler, W. D. Dipole moments and pharmacologial activity J. Med. Chem. (1968) 11 214 Murto, J., Heino, E. L. Fluoroalcohols: Part 1 Suomen Kemistilehti B (1966) 39 263 Murto, J., Kivinen, A., Kivimaa, S., Laakso, R. Fluoroalcohols: Part 4 Suomen Kemistilehti B (1967) 40 250 Nowaczyk, U., Schmidt, E. L., Steirale, F. New working fluid
16 17 18 19 20 21 22 23 24 25 26
generation end temperature, 150°C
systems for absorption heat pumps and absorption heat transformers Proc XVllth Int Congr Refrig: Vol B, Thermodynamics, Transport Processes, Refrigerating Machinery IlR, Paris, France (1987) 1169 Nowaezyk, U., Sehmidt, E. L., Steimle, F. Stoffsysteme mit Trifluorethanol und Hexafluorisopropanol DKV-Tagungsbericht Band 2; K/iltetagung Hannover 1989 Nowaezyk, U., Schmidt, E. L. Stelmle, F. Kalorische Messungen an neuen Arbeitsmitteln fiir Absorptions-und ORC-Prozesse Ki Klima Kiilte Heizung (1990) 11 487 Reeves, R. L., Maggla, M. S., Costa, L. F. Importance of solvent cohesion and structure in solvent effects on binding site probes J. Am. Chem. Soc. (1974) 96, 5917 Renz, M. Bestimmung thermodynamischer Eigenschaften w/issriger und methanolischer Salzl6sungen Dissertation Universit/it Essen (1980) Forschungsbericht des DKV Nr. 5 Rochester, C. H., Symonfls, J. R. Thermodynamic studies of fluoroalcohols Trans. Farad. Soc. 1 (1974)69 1267-1281 Sherry, A. D., Purcell, K. F. J. Am. Chcm. Soc. (1972) 94 1848 Somayajulu, G. R. Estimation procedures for critical constants J. Chem. Eng. Data (1989) 34 106 VDI- Wiirmeatlas 4 Auflage DI-Verlag, Dfisseldorf (1984) Tech. Rep. Hexafluorisopropanol Du Pont De Nemours & Co, Freon Products Division, USA (1986) Produktinformation." Tetraethylenglykol-dimethylether Datenblatt: 1,1,1.3,3,3-Hexafluorpropanol-(2) Hoechst, Frankfurt, Germany (1986) Produktinformation." Hexafluorisopropanol 1,3 Dimethyl-3,4,5,6tetrahydro-2(1H)pyrimidinon SulJblan Merck-Schuchardt, Hohenbrunn, Germany (1989)
Rev. Int. Froid 1992 Vo115, No 1
15
Research on new working fluid pairs for use in absorption heat pumps has been continued. The selection of suitable substances was made by following a systematic approach based on the molecular properties of the refrigerant solvents. The refrigerants investigated are trifluoroethanol (TFE) and hexafluoroisopropanol (HFIP). They are combined with solvents from the group of organic heterocycles. Experimental data are presented on solubility, heat of mixing and vapou~liquid equilibrium. Finally, the new systems are compared with those used currently. (Keywords:heat pumps;trifluoroethanol;hexafluoroisopropanol)
Propri&6s thermophysiques de nouveaux syst6mes de fluides actifs pour des proc6d6s a absorption On a poursuivi les recherches sur les nouveaux couples de fluides actifs, utilisables dans les pompes h chaleur 3 absorption. On a s~lectionnb les substances approprikes en se r~f~rant 3 une approche syst~matique fondke sur les propri~tbs molbculaires des solvants frigorifiques. Les frigorig~nes btudi~s sont le trifluorobthanol ( TFE) et l'hexafluoroisopropanol ( HFIP). Ils sont combinks avec des solvants du groupe des h~t~rocycles organiques. On prbsente des donnkes expbrimentales sur la solubilitb, la chaleur de mklange et I'bquilibre vapeur-liquide. Enfin, on compare les nouveaux syst~mes avec les syst~mes actuellement utilisbs.
(Mots cl6s: pompe ~ chaleur; trifluoro&hanol; hexafluoroisopropanol)
When new improved absorption heat pumps and absorption heat transformers are to be developed, the choice of the appropriate working fluid system is crucial. The demands to be fulfilled by an optimal system have already been presented and thoroughly discussed. There is no ideal working pair suitable for every application because several of the many requirements made of such a pair are contradictory. A summary of the requirements for working fluids is given: Refrigerant
• • •
high specific enthalpy of evaporation; favourable pressure range; low pressure difference;
Refrigerant and absorbent
• • • • • •
high thermal and chemical stability; good solubility of refrigerant in the absorbent; low specific mass flowrate; low heat of mixing; low specific heat capacity; high difference in boiling points.
A suitable compromise has to be found therefore, and the properties have to be thoroughly compared to find the best working pair. Table 1 lists the working fuid systems investigated between 1981 and 1989 at the Institut ffir Angewandte Thermodynamik und Klimatechnik, Universit~it Essen. The working fluid system T F E - N M P shows favourable properties for use in absorption heat pumps. However, measurements of vapour-liquid equili0140-7007/92/01001006 © 1992 Butterworth-HeinemannLtd and IIR 10 Int. J. Refrig. 1992 Vo115, No 1
brium revealed a relatively high fraction of absorbent in the vapour phase. Therefore, it seems desirable to make further investigations with fluoroalcohols and organic solvents with high boiling points. Selection of appropriate working fluid mixtures The most important criteria for a working pair are thermal and chemical stability and large solubility of the refrigerant in the absorbent. The molecular basis for good solubility and thermal stability can be found with fluoridized alcohols. The properties of the fluoroalcohols and their mixtures with other compounds are related to the hydrogen bonding properties of the alcohols. These substances have the activated hydrogen atom of their hydroxyl group available for interaction with a proton acceptor. The suitable absorbent therefore must be an electron donor that can achieve effective hydrogen bonds with a centre of high electron density. Fluoroalcohols as refrigerants Based on the above ideas the following refrigerants were chosen: trifluoroethanol (TFE) and hexafluoroisopropanol (HFIP). In Table 2 the physical and chemical properties of the refrigerants investigated are listed. They are all polar 'water-white' liquids having a high density and low viscosity and a high degree of thermal stability. All substances listed are miscible with water and also many organic solvents.
Thermophysical properties of working fluids for absorption processes: U. Nowaczyk and F. Steimle Table 1 Working fluid systems investigated for absorption processes (references) Tableau 1 SystOmes de fluides actifs btudibs pour les proc~d~s h absorption (rbJ~rences) Absorbents
Refrigerants: Methanol
Lithiumbromide Lithiumthiocyanate Zincbromide Lithiumbromide-zincbromide Water Ethyleneglycol Triethyleneglycol-dimethylether(DTrG) Tetraethyleneglycol-dimethylether(DTG) N-Methylpyrrolidone (NMP) Dimethylpropylenurea (DMPU) N-Formylmorpholine (NFM) Sulpholane (Sul)
Ammonia-lithium-nitrate Methylamine
R22
R123a
8
2
TFE
HFIP
19 19 19 19 4,8 8
3,5,15,16 2 3,5,15,16
3,5,15 5,16 5
Table 2 Physicalproperties of refrigerants and absorbents Tableau 2 Propribtbs physiques des J?igorig~nes et des absorbants Abbreviation Molecular formula
TFE C2H3F30 FI HI F--C--C~OH
Structural formula
I F
Molecular weight (g mol ~) CAS Nr. Boiling point (*C) Melting point (*C) Critical temperature (°C) Critical pressure (bar) Enthalpy of evaporation at 0°C (kJ kg ~) Specific heat at 50°C (kJ kg ~K ~) Density at 20*C (kg m 3) Dynamic viscosity at 20°C (mPa s ~) Heat conductivity at25*C(Wm ~K ~) Surface tension at 25°C (mN m ~) Dipole moment p_(Debye) at 25°C in cyclohexane Dielectric constant at 250C pK, values in water at 25°C Danger LD50, oral (mouse/rats) "Ref. 2 hRef. 20
JRef. 10 ,Ref. 13
HFIP C3H2F60 Fi H~ FI F~C--C--C~F
I H
I F
168.04 920~6-1 58 - 3 195~ 34.2b 270~ 1.51 1618.37 1.96
20.4k 2.0M
16.3h 2.05~
240 mg kg J , IRef. 14 ~Ref.21
hRef. 24 ,Ref. 26
,Ref. 9 ~Ref. 18
16.7t 9.3~ eye and skin irritation 600 mg kg J ~
~Ref. 1 '"Ref.22
In contrast to T F E , H F I P is n o t corrosive n o r inflamm a b l e 24. The insertion of two CF3 groups in a molecule, as with H F I P , leads to a decrease in boiling p o i n t a n d e n t h a l p y of e v a p o r a t i o n . The reason for this negative effect is the i n t r a m o l e c u l a r h y d r o g e n b o n d i n g interaction of the O H g r o u p to two fluorine atoms, one in each CF3 group. F a v o u r a b l e solubility a n d good thermal stability c o n t r a s t with a relatively low e n t h a l p y o f e v a p o r a t i o n . Because of the nearly complete fluoridization, the toxicity of the H F I P is essentially smaller t h a n that o f T F E , which is c o m p a r a b l e in its toxicity to a m m o n i a .
Polar solvents as appropriate absorbents A p p r o p r i a t e solvents with an active d o n o r centre, a stable m o l e c u l a r c o n s t r u c t i o n a n d high boiling p o i n t can be f o u n d in the organic heterocycle group, e.g. cyclic derivates o f N-alkyl urea, lactams, oxazolidinones, carb o n a t e s a n d sulphones. These are strong polar solvents having high dipole m o m e n t s a n d c o n s e q u e n t l y a high electron density s u r r o u n d i n g their acceptor atoms. The organic solvents chosen for this investigation are presented with their most i m p o r t a n t properties in Table
128.18 7226-23 5 230 < - 20 453~
222.28 143-24-8 275-276 - 30 446m
2.0° 1064.5 3.51
494.3 (at 25°C)" 2.03 1010.8 3.58 0.2322q
H~C-
.CH3
Sul C4H~O2S o%~.o
CH~CH2CH~4CH, J
I I OH F
100.04 75 89 8 73.6 - 45 226.7, 49.3, 449,, 1.9, 1391 2.03
26.67,' 12.4, eye irritation
N~N
DTG C~0H2205
DMPU C6H~2N20
"Ref. 11 "Ref.23
31.1, 4.23~ 36.12/
9.16q
1300 mg kg ~~ pRef. 12 ~Ref. 25
5140 mg kg ~,
120.17 126 33M) 287 28 577~
1261.4 10.25 35.5J 4.81• 43.3r
1941 mg kg ~
rRef. 6
2. They all are i m p o r t a n t technical solvents with high thermal stability. These d i p o l a r a p r o t o n i c solvents are clear, colourless liquids with high boiling points a n d different basicity.
Experimental procedure To verify the suitability of T F E a n d H F I P systems for a b s o r p t i o n processes an experimental p r o g r a m m e was carried out in which the following m e a s u r e m e n t s were made: thermal stability; solubility; v a p o u r pressure; v a p o u r - l i q u i d equilibrium; calorific properties; a n d density a n d viscosity. The results of these investigations are published in References 3, 5, 7, 16 a n d 17. The present paper reports the results o f m e a s u r i n g solubility, v a p o u r - l i q u i d e q u i l i b r i u m a n d heat of mixing.
Results The solubility o f T F E was d e t e r m i n e d with a variety of basic solvents. The results are presented in Figure ! by curves o f c o n s t a n t pressure for a n e v a p o r a t i o n temperature of 0°C. In this d i a g r a m the t e m p e r a t u r e is plotted
Rev. Int. F r o i d 1 9 9 2 V o 1 1 5 , N o 1
11
Thermophysical properties of working fluids for absorption processes: U. Nowaczyk and F. Steimle 0,7
0,85 0,6 ~TFE
0,75
0,5
~:HFIP
\
0,65
0,4 0,55
0,3
o,t/-o.Eu
NMP DMEU
0,45
-~
DMPU
-~-
NMC
DMPU
NMO
--)(-- NMC - ~ - NMO
O, 1
- ~ - DTO
NMP
0
10
20
\
NMF
0,35
SUI 30
40
50
Ternperature
60
t
70
80
90
0,25 - 20
100
i
I
I
q
J
I
i
30
40
50
60
70
80
90
Temperature t [°C]
[°C]
Figure 1 Solubility isobars at 0*C for TFE system
Figure 2 Solubility isobars at 10°C for HFIP system
Figure 1 Isobares de solubilitd it O°C pour le systOme TFE
Figure 2 lsobares de solubilitb h 10"C pour le systbme HFIP
versus the concentration. The quality of a system can be assessed by this graph. Promising working systems have a high refrigerant content of solution and further indicate a high change in concentration with temperature. These characteristics result in a low specific mass flowrate. As expected, the weak basic absorbents NMO and D T G have a smaller absorption power for TFE. On the other hand, DMEU, D M P U and NMC have good absorbtion properties for TFE. The solubility of H F I P was determined with several different types of absorbents in order to determine the influence of interaction between acidity and basicity. The results are presented in Figure 2 by curves of constant pressure for an evaporation temperature of 10°C. The basic solvents show a very good solubility for HFIP, likewise the non-basic solvents, which are unsuitable for TFE. The slope of the curve in the working range of the absorber is flat and consequently there is a smaller value for the specific mass flowrate. To construct provisional p,t,x-charts, additional equilibrium measurements at a specified refrigerant temperature of 50"C for T F E systems and 40°C for H F I P were made. Provisional p,t,x-charts of two systems designed by means of these results are shown in Figures 3 and 4. These charts give a first impression of the qualities of the systems. Promising working fluid systems show a broad solution field with an even distribution of the lines of constant concentration. High refrigerant concentrations can be achieved within the working range of an absorption heat pump and result in very low values for the specific mass flowrate. Furthermore, experimental data of the heat of mixing and the vapour-liquid equilibrium are presented. The enthalpy of mixing of T F E and H F I P with a variety of solvents were measured along the 30°C isotherms with a Setaram microcalorimeter. The depen-
1000 ml~r 5OO
12
Int. J. Refrig. 1992 Vo115, No 1
100
/ 4 P/ /' " " /
e"
/ /
/
" / " / l k"
100 •
I
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•
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/
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~
//// 0
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/
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t
fl
!
t •
l ¢ [l
i
///,// i
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60
80
100 1~ ~
160 =C200
Figure 3 Provisional equilibrium chart for TFE-DMPU Figure 3
Diagramme provisoire de l'bquilibre pour TFE D M P U
dence of the enthalpy of mixing on the concentration is shown for the 30°C isotherm in Figures 5 and 6. Both graphs clearly show the dependence of the acidity of the refrigerant on the mixing enthalpy. The values of the H F I P systems are higher than those for TFE. Graduation of the values for hm can be explained by the difference in power of the hydrogen bridges formed. The lower values for N F M may be due to self-association in the compound. The boiling behaviour of the mixtures can be appreciated from the t,~-charts, which are based on v a p o u ~ liquid equilibria measurements. The vapour-liquid equilibrium (VLE) of the T F E and H F I P systems was measured isobarically by using a dynamic circulation apparatus 6f the Gillespie type. Within the group of T F E systems there was none with sufficiently high difference
Thermophysical properties of working fluids for absorption processes: U. Nowaczyk and F. Steimle 1000 rnbor 50C
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Figure 4 Provisional equilibrium chart for HFIP-DTG Figure 4 Diagramme provisoire de l'bquilibre pour HFIP~DTG
%.
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0,7
0,8
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Figure 5 Enthalpyof mixing for TFE systems Figure 5 Enthalpie de mblange pour les syst~mes TFE
P
O,too / !
o.ooo O.OOO
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0.200
0.4-00
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Figure 7 Vapour-liquid equilibrium and McCabe-Thiele charts of TFE-DMPU at 350 mbar Figure 7 Equilibre vapeur-liquide et diagrammes McCab~ Thiele pour TFE et DMPU ~ 350 millibars
-20
-40
-60 30oC isotherms "80 0
] 0,1
I 0,2
I 0,3
D
T
~
I
[
J
I
J
0,4
0,5
0,6
0,7
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I 0,9
VLE measurements of the H F I P systems indicated that working fluid combinations with differences in boiling points over 200 K ( H F I P - D T G , HFIP-Sulpholane) do not require rectification. Some of the detailed measurements of the T F E and H F I P systems are presented in Figures 7 and 8. A complete summary of these measurements can be found in References 5 and 16.
~NFIP
Figure 6 Enthalpyof mixing for HFIP systems Figure 6 Enthalpie de mblange pour les systdmes HFIP
in boiling points and therefore rectification might be needed. Nevertheless the new systems have a considerably lower fraction of absorbent in the vapour phase than the system T F E - N M P .
Discussion
Finally the proposed working systems should be compared with the well known systems a m m o n i a - w a t e r and T F E - N M P . To illustrate the comparison, some characteristic numbers are established. In Table 3 the T F E and H F I P systems investigated are presented by using characteristic properties defined as follows:
Rev. Int. Froid 1992 Vo115, No 1
13
Thermophysical properties of working fluids for absorption processes: U. Nowaczyk and F. Steimle 2~ °C 200
t
I-
24O
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22O
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,,,,
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I00 ~0
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/
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!
, o.,-K)o
i
i
t
0.~oo
I
~
~ '~
i 1.9o(3
Figure 8
Vapour-liquid equilibrium and McCabe Thiele charts of HFIP-DTG at 750 mbar Figure 8 Equilibre vapeur-liquide et diagrammes McCabe Thiele pour HFIP et DTG gl 750 millibars
Mass concentration of refrigerant in the poor solution Mass concentration of refrigerant in the rich solution Specific mass flowrate, (1 - ~.)/(~r - ~.) f Difference of boiling points between absorbent Ats and refrigerant Specific enthalpy of evaporation of the refrigerr0 ant at 0°C Ps0-c Vapour pressure of the refrigerant at 50°C Minimum pumping energy related to the capacity no of the evaporator, fAp/roPr, where Ap is the pressure difference between evaporator and condenser, and pr is the density of the rich solution Heat flux of the solution heat exchanger related E/L to the capacity of the evaporator, (f - 1)cLAt/ro, where CL is the specific heat capacity of the poor ~a
14
Int. J. Refrig. 1992 Vo115, No 1
solution, calculated by CL = ~aCK + (1 -- ~a)CA for a temperature of approximately 50°C and At is the temperature difference between generation and absorption Relation of enthalpy of mixing and enthalpy of evaporation, hd/ro
To assess the merits of H F I P in comparison to T F E systems the specific mass flowratefand the characteristic number//p for the pumping power were identically calculated for the reference temperatures 0, 50 and 150°C, even though the working range of H F I P can be considerably higher, Table 3 shows that the seven systems that were established in the frame of this research programme might be alternatives to the well known and discussed systems ammonia-water and T F E - N M P . The systems T F E DMEU, T F E - D M P U and T F E NMC yield several positive characteristics for their application in absorption heat pumps. High refrigerant content can be achieved within the working range of an absorption heat pump, which gives, in combination with favourable behaviour of the lines of constant pressure, a very low value for the specific mass flowrate. A low pressure difference in combination with the low specific mass flowrate and also the relatively high enthalpy of evaporation gives, as a result, a very low value for the characteristic number rtp (pumping power). Results for vapour-liquid equilibrium measurements show that the new T F E systems have a considerably lower fraction of absorbent in the vapour phase than the system T F E - N M P because the difference of boiling point is higher. Finally, it must be admitted that for all T F E systems presented, certain rectification efforts have to be taken into account, although considerably less than for the T F E - N M P system. The thermal stability of T F E systems proved satisfactory by respective measurements. It should also be mentioned that the toxicity of T F E cannot be evaluated to be lower than that of ammonia and that T F E is inflammable. Table 3 further indicates particularly favourable values for H F I P systems for the characteristic numbers np and nL. Outside the group of H F I P systems there are two systems for which rectification can most probably be avoided. These are H F I P - D T G and HFIP-sulpholane. The thermal stabilities of these H F I P systems proved to be satisfactory by respective measurements. Further advantages are the non-inflammability and low toxicity of H F I P systems. The use of these working fluid systems was restricted by the relatively high melting point ( - 4°C) of HFIP. References 1 2
Barker, B. J., Rosenfarb, J., Caruso, A. Harnstoffe als L6sungsmittel in der chem. Forschung Angew. Chem. (1979)91 560 Bokelmann, H. Auswahl, Messung thermophysikalischer Eigenschaften und Beurteilung der Eignung von Niederdruckstoffsystemen fi.ir Absorptionswfirmepumpen Dissertation Universitfit Essen (1984) Forschungsbericht des Deutschen Kiilteund Klirnatechnischen Vereins ( D K V ) Nr. 12
3
Bothe, A., Nowaezyk, U., Schmidt, E. L., Steimle, F. New working fluid systems for absorption heat pumps present and future work Proceedings of an International Workshop on Absorption Heat Pumps, London (Ed. P. Zegers and J. Miriam), CEC (1988) 13
4 5
Bothe, A. Das Stoffsystem NH3-LiNO3/H20 fiir den Einsatz in Absorptionskreislfiufen Dissertation Universit/it Essen (1989) Bothe, A., Nowaczyk, U., Schmidt, E. L. Untersuchungen zum
Thermophysical properties of working fluids for absorption processes. U. Nowaczyk and F. Steimle Table 3 Characteristicsof working fluids Tableau 3
Caract~ristiques des fluides actifs
Systems
Refrigerant r0 (kJ k g ' )
Pso'c(bar)
TFE-DMEU TFE DMPU TFE-NMC
449
0.358
HFIP-DMPU HFIP-NFM HFIP-DTG HFIP-Sul
270
TFE NMP NH3 H20
449 1261
evaporation temperature, 0°C
6 7 8
9
10 11 12 13 14 15
0.720
0.358 20.3
Ats(K)
~a
~r
f
I0~np
nu
n,
Rectification
146 156 162
0.143 0.232 0.168
0.367 0.401 0.370
3.83 4.54 4.12
0.25 0.30 0.28
1.25 1.55 1.38
~0.09
yes yes yes
174 186 217 229
0.57 0.482 0.396 0.306
0.69 0.683 0.643 0.55
3.58 2.58 2.45 2.84
0.67 0.46 0.48 0.51
1.62 0.98 0.98 1.26
~0.26 ~ 0.28
yes maybe no no
129 134
0.15 0.23
0.43 0.41
3.04 4.28
0.21 6.56
0.86 1.15
~0.10 ~0.30
yes yes
condensation temperature, 500C absorption end temperature, 50°C
W/irme- und Stoffaustausch von neuen Arbeitsstoffpaaren in Absorptionsw/irmepumpen Abschluflbericht des yon der Europfiischen Gemeinschaft finanzierten Forschungsvorhabens Nr. EN-3E-OO24-D Brfissel (1989) Casteel, J. F., Sears, P. G. Dielectric constants, viscosities, and related physical properties of 10 liquid sulfoxides and sulfones at several temperatures J. Chem. Eng. Data (1974) 19 196 Chiapetta, C., Nowaczyk, U., Steimle, F. Thermophysical properties of hexafluorisopropanol Ki Klima Kiilte Heizung, International Edition '89 Ehmke, H. J. Stoffsysteme f/Jr Absorptionsw/irmepumpen Experimentelle Bestimmung thermophysikalischer Eigenschaften von L6sungen der K/iltemittel Methylamin, Ammoniak und Monochlordifluormethan (R22) Dissertation Universit/it Essen (1984) Forschungsbericht des DKV Nr.13 Kirk-Othmer Encyclopedia of Chemical Technology: Vol 10, Fluorine Compounds, Organic; Vol 21, Sulfolanes and sulJbnes John Wiley & Sons (1986) Kivinen, A., Murto, J., Lehtonen, M. Fluoroalcohols: Part 8 Suomen Kemistilehti B (1968) 41 359 Kneisl, P., Zondlo, J. W. Vapor pressure, liquid density, and the latent heat of vaporization as a function of temperature for four dipolar aprotonic solvents J. Chem. Eng. Data (1987) 32 11 13 Lien, E. J., Kumler, W. D. Dipole moments and pharmacologial activity J. Med. Chem. (1968) 11 214 Murto, J., Heino, E. L. Fluoroalcohols: Part 1 Suomen Kemistilehti B (1966) 39 263 Murto, J., Kivinen, A., Kivimaa, S., Laakso, R. Fluoroalcohols: Part 4 Suomen Kemistilehti B (1967) 40 250 Nowaczyk, U., Schmidt, E. L., Steirale, F. New working fluid
16 17 18 19 20 21 22 23 24 25 26
generation end temperature, 150°C
systems for absorption heat pumps and absorption heat transformers Proc XVllth Int Congr Refrig: Vol B, Thermodynamics, Transport Processes, Refrigerating Machinery IlR, Paris, France (1987) 1169 Nowaezyk, U., Sehmidt, E. L., Steimle, F. Stoffsysteme mit Trifluorethanol und Hexafluorisopropanol DKV-Tagungsbericht Band 2; K/iltetagung Hannover 1989 Nowaezyk, U., Schmidt, E. L. Stelmle, F. Kalorische Messungen an neuen Arbeitsmitteln fiir Absorptions-und ORC-Prozesse Ki Klima Kiilte Heizung (1990) 11 487 Reeves, R. L., Maggla, M. S., Costa, L. F. Importance of solvent cohesion and structure in solvent effects on binding site probes J. Am. Chem. Soc. (1974) 96, 5917 Renz, M. Bestimmung thermodynamischer Eigenschaften w/issriger und methanolischer Salzl6sungen Dissertation Universit/it Essen (1980) Forschungsbericht des DKV Nr. 5 Rochester, C. H., Symonfls, J. R. Thermodynamic studies of fluoroalcohols Trans. Farad. Soc. 1 (1974)69 1267-1281 Sherry, A. D., Purcell, K. F. J. Am. Chcm. Soc. (1972) 94 1848 Somayajulu, G. R. Estimation procedures for critical constants J. Chem. Eng. Data (1989) 34 106 VDI- Wiirmeatlas 4 Auflage DI-Verlag, Dfisseldorf (1984) Tech. Rep. Hexafluorisopropanol Du Pont De Nemours & Co, Freon Products Division, USA (1986) Produktinformation." Tetraethylenglykol-dimethylether Datenblatt: 1,1,1.3,3,3-Hexafluorpropanol-(2) Hoechst, Frankfurt, Germany (1986) Produktinformation." Hexafluorisopropanol 1,3 Dimethyl-3,4,5,6tetrahydro-2(1H)pyrimidinon SulJblan Merck-Schuchardt, Hohenbrunn, Germany (1989)
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