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New working pairs for medium and high temperature industrial absorption heat pumps
Heat Recovery Systems & Clip Vol. 8, No. 5, pp. 459--468, 1988
08904332/88 $3.00+ .00 Perllamon Pressplc
Printed in Great Britain.
NEW WORKING PAIRS FOR MEDIUM AND HIGH TEMPERATURE INDUSTRIAL ABSORPTION HEAT PUMPS M. NARODOSLAWSKY, G. O H - ~ and F. MOSER Institute of Chemical Engineering, Graz University of Technology, lnffeldgasse25, A-8010 Graz, Austria (Received 6 April 1988)
Almngt--The application of absorption cycles for high temperature heat recovery systems c.alls for the investigation of new working pairs. To qualify as a potential working pair, a mixture of two substances has to fulfill stringent requirements with respect to thermodynamic properties, corrosion and safety hazards like toxicity and inflammabifity. Based on a thermodynamic analysis of an absorption heat pump cycle a systematic search for new working pairs has been conducted. The investigation dealt exclusivelywith organic compounch. To get a first estimate of the properties of mixing, a molecular group contribution model (UNIFAC) was employed. This method has so far been widely used for chemical engineering purpmes and renders agreeable mtimatm for the thermodynamic behavior of organic mixtures. With the help of the UNIFAC method, various pairings of functional molecular groups have been investigated for their possible potential to form effectiveworking pairs for medium and high temperature absorption heat pump cycles.As a consequence of this analysis, ten alternative working pairs are proposed and their respective theoretical performance data and their toxicity characteristics are given.
NOMENCLATURE d G h
coefficientof temperature dependence Gibbs free energy enthalpy P pressure x mole fraction of refrigerant COP coefficientof performance L T V limit threshold value T temperature
Subscripts b
normal boiling point
c max
0
critical m a x i m u m value referencestate
r
reducedproperty
Superscripts
E ev
excessproperty evaporation 1. I N T R O D U C T I O N
Absorption cycles are, until now, c o m m o n l y used for chilling and air-conditioning applications. Industrial employment o f absorption cycles other than for chilling has so far been limited to exceptions. There is, however, a great potential for this energy recovery technique in various industries. A recent review [1] showed that for some applications in oil refinery processes, pay out times between one and two years m a y be realised by the use o f absorption heat p u m p s for energy recovery duties. A major break through for industrial applications, however, m a y be contingent on finding new working pairs for industrial requirements. The application o f heat p u m p s in industrial processes usually calls for higher temperature levels than for either chilling or residential heating purposes. This is true for both the heat accepted as waste heat input and the useful heat rejected by the heat pump. CI__a~__'cai absorption cycle working pairs like LiBr-water or w a t e r - a m m o n i a are no longer optimal choices for absorption heat p u m p s which work entirely above ambient temperature levels. This has led to a n u m b e r o f investigations to look for new working pairs more compatible with these requirements. The endeavors to find 459
460
M. NARODOSLAWSKYet al.
a solution to this problem fall roughly into two categories: the search for additives to the working pairs now in use in order to extend their field of application; the search for completely new working pairs. In this work, the latter approach has been taken. If one adopts this direction of investigations, it is crucial to use an effective screening method to discriminate promising new working pairs from the almost infinite number of possible mixtures. As there are a number of requirements the screening has to be done according to more than one property. The most important criteria for a good working pair are: to guarantee high efficiency of the heat pumps; to show good heat and mass transfer; to be non-corrosive; to be nontoxic; to guarantee secure operation. The first two criteria address the economic viability of the process, whereas the latter three are responsible for the operational applicability of a heat pump. These criteria do not render themselves equally well for a screening process. Namely, heat and mass transfer and corrosivity require extensive experimental investigations. To embark on these costly and time consuming experiments is only advisable if a working pair shows the potential to turn out as a possible commercial success. It was therefore decided to screen a great number of mixtures on the basis of their possible effectiveness as working pairs in medium and high temperature heat pump applications. On top of this, a survey on their respective toxicity has been conducted. In this way, a substantial reduction in the number of candidates for further investigations could be achieved. 2. T H E R M O D Y N A M I C C R I T E R I A FOR GOOD AHP W O R K I N G PAIRS In order to rate possible working pairs according to their performance in absorption heat pumps, criteria are needed linking the performance of a heat pump to the physical properties of the working pair. The criteria should be reliable; based on properties easily available for a great number of mixtures. One approach to this is to link molecular mechanisms to the performance of a working pair. An attempt in this direction using the tendency of a mixture to form hydrogen bonds has been reported in the literature [2-4]. The results from investigations in this direction have so far been encouraging. However, hydrogen bond formation is only one aspect positively contributing to the overall performance of a working pair. Moreover, the behavior of electrolyte solutions, which may be well suited as heat pump fluids, may not easily be explained in terms of hydrogen bond formation. In the search for more versatile criteria to find promising absorption heat pump working pairs, the authors made a study [5] on the influence of both the absorbent and the refrigerant. To take into account the influence of the properties of mixing, mixtures were characterized by their respective extreme values of the excess Gibbs free energy G r ~ and the location of this extreme value x,~x on a binary molcfraction scale between the absorbent and the refrigerant. The excess heat of mixing, h E, was related to the excess Gibbs free energy by a simple equation, proposed by Gaube [6, 7] (see Fig. l).
Ge(x, T,) = \'~l.] Ge(x' TO).
(I)
Using GE.~ and x,~, the concentration dependence of tbe excess Gibbs freeenergy can be described by simple models as the Van Laar ~luation [8] GE =
A,.2x (! -- x) X I (Ai.2/A2.1) +
(2)
(1 -- X)'
or Lagendre-polynornials [9]: Gr R-T = xl (1 - xl ) ~. a r L r ( x )
(3a)
K
L r ( x ) = [(2K - l).(2x - I).Lr_, ( x ) - ( K - I ) ' L K _ 2 ( x ) ] / K
(3b)
Lo(x) = I
(3c)
LI (x) = 2x - I.
(ad)
M e d i u m and high temperature absorption heat pumps
461
gE hE
h~mnx /
/
\
/ / / /
\
\ \
/ /
gCmox
0.0
OS
lo
x mog
XR~rigerant i-a gElx,T)" gE(x.T.).(_~.) d gE(x.T) = d. h~x,T) Fig. I. Simplified connection between the heat of mixing and the Gibbs free energy.
With the assumption that the excess heat of mixing is not a function of temperature in the considered temperature interval and with the basic thermodynamic relationship between G E and he:
01 T
equation (1) leads to: GE=dh
E.
(5)
Thus, with the definition of the values for G ~E, X,.x and d at a reference temperature To the vapor-liquid equilibrium as well as the enthalpy of the mixture at any composition x and temperature, is accessible. This method was then included in a computer model to describe the performance of a mixture acting as a working fluid in an absorption heat pump. Comparison between results obtained with this model using only experimental values for G,ffi,, 8 x , ~ and d for well-known working pairs, and performance data derived from comprehensive experimental data for the same fluids, showed that this simple model is capable of representing the performance of absorption heat pumps sufficiently well. A great number of simulation runs were subsequently conducted to deduce criteria for good working fluids. In these runs, pure component properties as well as the parameters G ~• , x ~ and d, characterizing the properties of mixing, were changed in a systematic manner. From the results of these simulation runs, the criteria listed in Table 1 were found. The main influence on the Table 1. Criteria for optimal working pairs Refrigerant Absorbens
Vapor-liquid equilibrium
high low
low high
high low
[J mole- ']
[mole mole- i]
[_ ]
- 1000 to -2000
>0.6
> 1.0
462
M. NARODOSL^WSK¥et al.
performance of an absorption heat pump according to this study are wielded by the properties of mixing, in particular by Ge~x and x~x and by the latent heat of the refrigerant at the normal boiling point. For more information on the choice of criteria, the reader is kindly referred to the original paper [5].
3. S C R E E N I N G FOR P R O M I S I N G W O R K I N G FLUIDS As the properties of mixing turned out to be prominently influencing the suitability of any mixture to be employed as an absorption heat pump working fluid, a reliable method to calculate G E and h E is requisite in the screening process. This method should be reliable and generally applicable. Though there is no general answer to these requirements, group contribution methods like UNIFAC [9] and ASOG [10] seem to be the closest thing to it, at least for organic compounds. Group contribution methods, specially UNIFAC, have proved to be powerful tools to estimate vapor-liquid equilibria for various chemical engineering purposes. The idea behind these methods is to deduce the behavior of a mixture from interactions beween functional molecular groups which build up the molecules of the constituent mixtures. The required interaction parameters for the functional groups are fitted to a great number of experimental data. Therefore, these methods are sufficiently reliable for a screening process as envisaged in this work. For reasons of greater versatility and better data access, UNIFAC was chosen as the base for the screening. Unfortunately, there does not exist an equally versatile and reliable method for electrolyte solutions or inorganic mixtures. Therefore the screening has to be confined to organic mixtures. Besides the possibility of estimating the properties of mixing, group contribution methods allow for the investigation of the influence of a given molecular functional group, either the absorbent or the refrigerant, exerts on the performance of a heat pump. Therefore, the screening process has been divided into two parts: first, the influences on the performance exerted by different pairings of molecular functional groups in the working fluid were investigated; second, the most promising pairings were used to construct possible working fluids. Two problems arise if one tries to apply group contribution methods to find new absorption heat pump working pairs. The first concerns the extent of the investigations. Despite the great number of functional groups encompassed by UNIFAC, the number of pairs of such groups, for which there actually exist interaction parameters, is considerably smaller than the maximum number of possible combinations. Therefore, only a relatively small number of functional groups can be investigated. The second problem concerns the representation of the heat of mixing by UNIFAC. Although the vapor-liquid equilibria calculated with the help of UNIFAC are usually very close to experimental data, this is not true for the heats of mixing. As a matter of fact, UNIFAC even predicts incorrect trends for some combinations of molecular groups [l l, 12]. Although this is a serious drawback, the main influence on the COP is exerted by GE~,~and xm,~, whereas the factor d in equation (l) is of lesser importance. These two parameters, however, are sufficiently correctly represented by UNIFAC. The screening according to G,,~ 8 and x,,~ is therefore possible with UNIFAC and gives the most important informations on the optimal pairing of molecular groups in the refrigerant and the absorbent.
4. RESULTS OF THE S C R E E N I N G A C C O R D I N G TO P R O P E R T I E S OF MIXING In the first step of the analysis, optimal combinations of functional molecular groups were looked for. In addition, the influence of the number of such groups per molecule was investigated. To achieve this goal a matrix of 12 "refrigerants" and 36 "absorbents" was established. The refrigerants represent alcohols, hydrocarbons and amines. The absorbents contain in addition, ketones, cyclic molecules, amides and ethers. Tables 2a and 2b show which substances were used in this analysis, Table 3 offers an insight into which combinations could be calculated with the help of UNIFAC.
Medium and high temperature absorption heat pumps Table Refrigerant Tetrachlorocarbon Chloroform n-Hexane n-Propanol Ethanol Methanol n-Propylamine Ethylamine Methylamine Dimethylamine Trimethylamine Diaminoethane
463
2a. Substances used as refrigerants Formula Tb[°C] Ah7 [J mole- '] CCI, 76.6 30010 CHCI3 61. I 29720 C6H ,4 68.7 28870 C3HTOH 97.2 41780 C..2HsOH 78.4 38760 CHsOH 64.6 35270 C3H~NH2 48.7 29 720 C2HsNH: 16.5 28050 CH3NH2 - 6.2 24 570 (CH 3)2NH 6.8 26 500 (CH3)3N 2.9 24110 (CH2]~['[2) 2 117.3 41860
Table 2b. Substances used as absorbents
Absorbens Undekane
Formula C~H2~
Tb[°C]
CTHIsOH CsHITOH C2H602 C3H802 C4HIoO2 CsHI202 C6H,402 C~H,602 C3HIO3 C5Ht203 C7HI603 CTH,eO4 C2H~NO C6H60 C6H602 C6HoO3 C7H80 C7H80 C4H,oO3 CsHnOs CeH,40~ C8HI804 C,oH220~
176 195 197 214 235 239 250 259 289
MEA PHE DHB THB KRE BAL DEG TEG DMEDEG DMETrEG DMETEG
Heptanol Oktanol Ethanediol Propanediol Butanediol Pentanediol Hexanediol Heptanediol Glycerol Pentanetriol Hexanetriol Polyhydroxiheptane Monoethanolamine Phenol Dihydroxibenzene Trihydroxibenzene rn-Kt~lol Benzylalcohol Diethyleneglycol Tetraethyleneglycol Dimethylether diethykneglycol Dimethylether triethyleneglycol Dimethylether tetraethyleneglycol
170 182 285 309 202 205 246 327 162 216 275
Amides
DMF DMA DEF
N,N -Dimethylformamide N,N -Dimethylacetamide N,N-Diethylformamide
C3HTNO C4H,NO CsH. NO
153 165 180
Ketones
APN NMP 2,4-PTDION
Acetophenone N-Methylpyrrolidone Pentanedione
C$H80 CsHgNO C~HeO2
204 201 139
Esters
DEM DEO
Diethylmalonate Diethyloxalate
C,Ht204 CeHleO4
199 186
Amines
DMAN ANIL OKAM
N,N-Dimethylaniline Aniline Octylamine
CsHjIN CeHTN CsHIgN
194 184 182
Group Alcan Alcohols
UNDEK HEPOH OKTOH EG 1,3-PD
1,4-BD 1.5-PTD 1,6-HXD 1,7-HPD GLY ! ,3,5-PTT 1,4,7-HPT 1,3,5,7-PHHP
Ethers
196
In the following paragraphs, only those combinations which hold the potential to make up good absorption working fluids will be discussed. All other pairings are, from a thermodynamic point of view, disadvantageous. 4.1. Halogenated hydrocarbons as refrigerants
Halogenated hydrocarbons are represented in this analysis by chloroform and tetrachlorocarbon, thus giving an insight into the influence of the degree of substitution on heat pump performance. Unfortunately, no other halogen atoms other than chlorine are covered by UNIFAC. Therefore, an investigation on the influence of different forms of hydrogen substitution in hydrocarbons by halogen atoms is not possible at this time. The overall trend is that fully halogen substituted hydrocarbons show lesser suitability for application in absorption heat pump fluids (see Table 4). Incompletely substituted chlorohydrocarbons show potentially promising behavior with ethers, esters and ketones. Chloroform H.R.S.
8/5-..-F
M, NARODOSLAWSKY et al.
464
Table 3, Overview of selected pairs
U~.~EK HEPOH OKTOH E6 !, 3-PD 1)4=[~0 I~5-PID
1,6-HID DT"HPD GLY 1)3)5"PTT 1,4)7"WT I)3~5,7-f~4HP ~A PHE ~B THB kRE BAt TEG
IqET~'EG DltETE6
OEF ~N
2y4-PTOION PEH PEO ~IL OKAM
Crosses indicate for which pairings calculations were made,
may also be paired with dimethylaniline (DMAN). In the case of anilines, a group sensitivity could not be investigated due to the constriction of the UNIFAC model. The best results were obtained with the pairing of chloroform and various ethers. According to these results, all chloroform-ether combinations investigated show excellent values for x , ~ of about 0.6. With rising number of ether groups in the absorbent, G,~x E keeps falling, thus indicating improving suitability of these substances as absorbents. 4.2. Alcohols as refrigerants Alcohols seem to offer great potential for new working pairs. The results from the analysis of properties of mixing indicate that alcoholic refrigerants show promising behavior with other alcohols, ethers, amides and ketones as absorbents. In the first analysis, alcohols with different chainlengths as refrigerants were investigated to get a first impression on the influence of this parameter on the COP of an absorption heat pump. Let us first have a look at the combination of an alcohol as refrigerant and another alcohol as absorbent. The latter may either be an aromatic or a long chain alcohol. For straight chain alcohols as absorbents, long chainlength and a high degree of substitution seem to be favorable. The addition of an amino group to a straight chain alcohol also results in better values for G..~ E and xm~. This is especially true for combination of amino-alcohols with low molecular weight alcohols as refrigerants (see Table 5). From a purely thermodynamic viewpoint, aromatic alcohols are very interesting absorbents for alcoholic refrigerants. With a rising number of hydroxylic groups in the absorbent molecule, improving COPs for heat pumps operated with these working pairs are to be anticipated.
Medium and high temperature absorption heat pumps
465
Table 4. Results for chiorohydrocarbomas refrigerant(GE.=~in J m o l e - ' ) Refrigerant CCl, CHCI 3 Absorbens E E G~= x=~ G,,= x,~, Alcan Alcohols
UNDEK
- 113
HEPOH OKTOH EG 1,3-PD
1,4-BD 1,5-PTD 1,6-HXD 1,7-HPD GLY 1,3,5-PTT 1,4,7-HPT 1,3,5,7-PHHP MEA PHE DHB THB KRE BAL DEG TEG
0.58
+430 +2330 + 1997 + 1730
0.67 0.59 0.62 0.63
+2792
0.63
+ 1466
0.60
+ 1472 + 828
0.62 0.63
+ 1160
0.67
+ 137
0.70
+214 + 153 + 1180 +956 + 785 +642 +523 +420 +1315 +917 + 639 + 846
0.70 0.75 0.65 0.65 0.70 0.70 0.75 0.74 0.70 0.75 0.80 0.80
+466 -602
0.75 0.55
- ! 223 - 1481 - 1711
0.60 0.60 0.60
Ethers
DMEDEG DMETrEG DMETEG
+ 136 + 65
0.60 0.72
Amides
DMF DMA DEF
+715 +334 +200
0.650.58 0.77
- 1745
0.60
+439 +171 + 1150
0.63 0.60 0.58
-630 -1720 - 558
0.58 0.57 0.60
+ 254 +399
0.63 0.63
- 1080 -851
0.60 0.60
- 159 + 1000 + 132
0.43 0.53 0.67
- 1587
0.55
Ketones
APN NMP 2,4-PTDION
Esters
DEM DEO
Amines
DMAN ANIL OKAM
Abbreviationsaccordingto Table 2b.
Alcoholic refrigerants may also be used, to a certain extent, with ethers as absorbents. These combinations show falling G=~ E with an increasing number of ether groups and chainlength of the absorbent. The location of the extremum of G E shifts to higher refrigerant concentrations with falling mole weight of the refrigerant. For these reasons the most interesting working pair out of these combinations may be methanol as refrigerant and dimethylether-tetraethyleneglycol (DMETEG). Another possible combination is an alcoholic refrigerant and an amide as absorbent. This combination gets more favorable the greater the chainlength of the absorbing amide is. One possible pairing could be methanol with dimethylamide. Ketones are also interesting absorbents for alcoholic refrigerants. This is especially true for the heterocyclic N-methyl-pyrrolidone (NMP). From the viewpoint of the properties of mixing, the lower the moleweight of the refrigerant, the better the working pair will be. The best combination, therefore, is methanol with NMP. 4.3. Amines as refrigerants Amines have been widely proposed as refrigerants for absorption heat pumps. In this analysis, the combination of amines with long chain alcohols was investigated. The trend of the results (see Table 6) indicate that this combination is increasingly interesting the longer chains the absorbing alcohol has and the more hydroxyl groups there are in the alcohol molecule. 5. SCREENING
ACCORDING
TO
PURE
COMPONENT
PROPERTIES
T h e most important pure c o m p o n e n t properties o f the working fluid o f an absorption heat p u m p are the boiling point difference o f the refrigerant and the absorbent, and the latent heat o f the
M. NARODOSLAWSKY et al.
466
Table 5. Results for alcohols as refrigerant (G,,~,E in J mole-~ )
Refrigerant C~H.7OH G~, x~,
Absorbens
AIcan
UNDEK
Alcohols
HEPOH OKTOH EG 1,3-PD 1,4-BD 1,5-PTD 1,6-HXD 1,7-HPD GLY 1,3,5-PTT 1,4,7-HPT 1,3,5,7-PHHP MEA PHE DHB THB KRE BAL DEG TEG
C,HsOH G~ x~x
CH3OH G~, x,,,~~
+ 1510
0.48
+ 1680
0.52
+ 1813
0.53
+ 53 + 71 +315 + 148 +32 -52 - 114 - 161 +397 +59 - 79 -88
0.57 0.53 0.53 0.53 0.60 0.53 0.55 0.57 0.58 0.67 0.47 0.54
-1119 -1800 -2119 - 1248 + 201
0.45 0.55 0.60 0.48 0.55
+ 108 + 133 +ll6 - 15 - 101 - 161 - 203 -232 +92 - 181 - 362 -164 -128 -1150 -1720 -1948 - 1205 + 201
0.58 0.60 0.55 0.48 0.55 0.57 0.57 0.58 0.63 0.57 0.58 0.55 0.65 0,50 0.60 0.65 0.42 0.58
+ 270 + 296 -128 - 196 -236 -259 - 270 -276 -370 -515 - 591 -797 -912 -580 -1264 -1661
0.58 0.62 0.48 0.53 0.55 0.57 0.57 0.58 0.55 0.58 0.61 0.63 0.65 0.40 0.55 0.60
- 72
0.48
-84
0.50
-250
0.58
-835
0.62
Ethers
DMEDEG DMETrEG DMETEG
+ 537 + 520 +480
0.53 0.58 0.63
+ 568 + 526 +466
0.56 0.58 0.63
+ 158
0.38
- 170
0.75
Amides
DMF DMA DEF
-357 -115 -360
0.55 0.54 0.54
-438 -111 -344
0.55 0.48 0.54
-340 -790 - 141
0.54 0.58 0.53
Ketones
APN NMP 2,4-PTD1ON
+883 -1090 + 840
0.53 0.48 0.58
+931 -1190 + 750
0.57 0.48 0.61
+658 -724 + 258
0.55 0.60 0.53
Esters
DEM DEO
+ 774 +510
0.60 0.58
+ 480 +500
0.62 0.62
+ 516 +494
0.56 0.55
Amines
DMAN ANIL OKAM
+320 +390 - 36
0.60 0.50 0.48
+422 +405 + 95
0.60 0.54 0.66
-219 +421 - 593
0.53 0.55 0.62
Abbreviations according to Table 2b.
refrigerant. The boiling point difference should be at least 150°C to minimize the distillation effort in the absorption cycle. From the viewpoint of the latent heat, alcohols are the most recommendable refrigerants. However, the heat of vaporization of these substances is rising with their molecular weight and so is their normal boiling point. This leads to a trade-off between latent heat and the normal boiling point of the refrigerant, usually leaving only ethanol and methanol in consideration. At the other extreme, methylamine has a low latent heat but also a low normal boiling point. This leads to high pressures for industrial applications, but on the other hand reduces the Table 6. Results for amines as refrigerant ( G ~
in J m o k - ' )
Refrigerant C3HTNH 2
G ~,E
Absorbens Alcan Alcohols
x,,,~
C~H~NH 2
G ,,~,
x,,~,
UNDEK HEPOH
OKTOH EG 1,3-PD
1,4-BD 1,5-PTD 1,6-HXD 1,7-HPD GLY 1,3,5-PTT 1,4,7-HPT 1,3,5,7-PHHP MEA
-649 -694 +25 -222 -413 -565 -691 -797 +32 -432 -755 -646 +313
Abbreviations according to Table 2b.
0.48 0.50 0.48 0.53 0.53 0.54 0.55 0.58 0.55 0.57 0.58 0.58 0.50
-746 -791 -306 -526 -695 -830 -941 -969 -434 -840 -1122 -1125 +77
0.52 0.52 0.53 0.53 0.54 0.57 0.57 0.58 0.57 0.58 0.58 0.62 0.55
CHjNH 2
G ,,~F"
x,,m~
+918
0.59
-755 -780 -872 -1020 -1130 -1213 -1281 -1122 -1206 -1483 -1664 -1869 -284
0.48 0.48 0.53 0.53 0.55 0.57 0.57 0.58 0.58 0.58 0.59 0.62 0.55
(CH3)2NH
(CH3)3N
(CH2NH2)2
G ~,.
x,,,~
G ~.,
x,,.~"
G ~.~
x~
-777 -828 -300 -520 -689 -826 -941 -1039 -430 -832 -1115 -1116 +323
0.50 0.55 0.55 0.55 0.55 0.55 0.55 0.55 0.58 0.58 0.60 0.63 0.55
-793 -861 +179 -166 -359 -530 -676 -802 +241 -314 -665 -482 +441
0.60 0.60 0.40 0.75 0.65 0.65 0.65 0.65 0.45 0.70 0.65 0.70 0.50
-477 -461 -814 -935 -1012 -1064 -1097 -1120 -1077 -1330 -1476 -1687 -160
0.35 0.30 0.45 0.45 0.50 0.50 0:50 0.50 0.50 0.55 0.55 0.55 0.50
Medium and high temperature absorption heat pumps
467
distillation effort for low temperature application. Chloroform is in between these two extremes according to the normal boiling point as well as the latent heat. 6. SCREENING ACCORDING TO NON-PHYSICAL PROPERTIES Good performance characteristics of an alternative absorption heat pump working fluid is a prerogative of its possible application. However, non-physical qualities such as toxicity, stability and corrosivity are at least as decisive for the application as a good performance for a new working pair. For the most promising working fluids resulting from the thermodynamic screening, toxicity and stability have been investigated. Corrosivity data are mostly not known. Therefore, besides the compilation of thermophysical data and heat and mass transfer characteristics of newly proposed working fluids, their potential corrosivity should be investigated experimentally at a very early stage. Although thermodynamic screening shows that chloroform with ethers exhibit favorable performance characteristics, these mixtures may not be applicable to absorption heat pumps because of the carcinogenic nature of chloroform. Other promising pairs are aromatic alcohols as absorbents and alcoholic refrigerants. Although these pairs show excellent coefficients of performance, application may be hampered by the fact that the aromatic alcohols tend to decompose at high temperatures. This first screening leaves, as alternative working fluids "for absorption heat pumps, systems of alcohol-alcohol mixtures, alcoholic absorbent with amines as refrigerants and alcoholic refrigerants with either amides or ketones as absorbents. There may be, however, a drawback to this system. Though NMP is widely reported to be stable even at high temperatures, methanol may exhibit a tendency for decomposition at elevated temperatures. The resulting dimethylether has a low vapor pressure and therefore may contribute to problems in heat and mass transfer. Ultimate assessment of this working pair, therefore, should be contingent on further experimental tests. A first choice of alternative working pairs is given in Table 7, along with performance and toxicity data. 7. CONCLUSIONS High temperature applications of absorption heat pumps have gained increasing interest in the last few years. However, most absorption heat pump working pairs are not applicable at elevated temperatures. This is especially troublesome for industrial heat recovery problems, which require the production of steam from industrial waste heat. In the last couple of years new salt and water mixtures [13] as refrigerants have been proposed for this purpose. Although this seem to be a valuable direction of investigation, organic refrigerant-absorption systems may prove interesting in this area, too. A first screening of possible pairings applicable for medium and high temperature applications reveals that there exists a potential for some alternative absorption heat pump fluids. On the base of a systematic screening some working pairs have been proposed. These proposed working fluids
Table 7. Toxicity and theoretical C O P for a first choice of alternative working pairs
Temperature of heat source/sink [ ° q LTV
Refrigerant
[ppm]
CHCI3 CHCI 3 CHCIj CHsOH C2HsOH C2HsOH C3H~OH C3HtOH (CH2NH2)2 (CH2NH2)2
10 10 10 200 1000 1000 400 400 n.a. n.a.
LTV
Absorbens
[ppm]
20/70
60/120
90/150
APN DEM NMP NMP DHB THB DHB THB 1,4-BD GLY
n.a. n.a. 100 100 2 n.a. 2 n.a. n.a. n.a.
1.61 1.65 1.65 1.72 1.76 1.76 1.75 1.74 1.65 1.65
1.53 1.57 1.57 1.63 1.71 1.71 1.68 1.68 1.55 1.57
1.54 1.57 1.57 1.64 1.70 1.70 1.68 1.68 1.56 1.57
n.a. not available. Abbreviations according to Table 2b.
468
M. NARODOSLAWSKYet al.
should d r a w the interest o f experimental scientists to establish their real potential as absorption heat p u m p working fluids. A p a r t f r o m the compilation o f the t h e r m o d y n a m i c data base, a t h o r o u g h investigation in heat and mass transfer properties, toxicology and stability is needed. The report has shown that there exist possible alternatives for well-known working pairs for high temperature applications. These alternatives include mixtures o f substances widely used in chemical engineering, thereby assuring high availability and a g o o d database. However, further experimental investigations are needed on these working pairs. Acknowledgemems--The authors are grateful to the Austrian Research Council for providing the financial base for this research within the framework of the "W.~*rmepumpenschwerpunkt'', project $31. Also the authors want to thank Dr Hans Huemer for his very constructive criticism.
REFERENCES 1. W. F. Davidson and W. v. L. Campagne, Hydrocarbon Process, p. 30 (1987). 2. U. Nowaczyk, E. L. Sehmidt and F. Steimle, Selection and investigation of new working fluid systems for absorption heat pumps, in Proc. Int. Workshop on Research Activities on Advanced Heat Pumps, Graz (1986). 3. U. Nowaczyk, E. L. Schmidt and F. Stein'de, New working fluids systems for absorption heat pumps and absorption heat transformers, in Proc. ){;tilth Int. Congress of Refriferation, Wien (1987). 4. U. Nowaczyk, E. L. Schmidt and F. Steimle, New working fluid systems for absorption heat pumps--present and future work, in Proc. Int. Absorption Experts Meeting, Dallas (1988). 5. M. Narodoslawsky, G. Otter and F. Moser, Heat Recovery Systems & ClIP 8, 221-233 (1988). 6. J. Gaube and E. Koenen, Chem. Ing. Techn. 51, 496 (1979). 7. J. Gaube and E. Koenen, Bet Bun.venges. Phys. Chem. 86, 31 (1982). 8. J. J. Van Laar, Z. phys. Chem. 72, 723 (1910). 9. A. Fredenslund, J. Gmehling and P. Rasmussen, Vapor-Liquid Equilibrium Using UNIFAC. Elsevier, Amsterdam, Oxford, New York (1977). 10. G. M. Wilson and C. H. Deal, Ind. Engng Chem. Fund. 1, 20 (1962). 11. U. Weidlich and F. Crmeh~ng, Ind. F,nfng Chem. Res. 26, 1372 (1987). 12. B. L. Larse~, P. ~ u s s e n and A. Fredenslond, Ind. Enfns Chem. Res. 26, 2274 (1987). 13. D, C. Erickson, New Absorption Working Pairs Create New Absorption Heat Pump Possibilities, Absorption Experts Meeting, Feb. 4--6, Dallas, USA (1988).
08904332/88 $3.00+ .00 Perllamon Pressplc
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NEW WORKING PAIRS FOR MEDIUM AND HIGH TEMPERATURE INDUSTRIAL ABSORPTION HEAT PUMPS M. NARODOSLAWSKY, G. O H - ~ and F. MOSER Institute of Chemical Engineering, Graz University of Technology, lnffeldgasse25, A-8010 Graz, Austria (Received 6 April 1988)
Almngt--The application of absorption cycles for high temperature heat recovery systems c.alls for the investigation of new working pairs. To qualify as a potential working pair, a mixture of two substances has to fulfill stringent requirements with respect to thermodynamic properties, corrosion and safety hazards like toxicity and inflammabifity. Based on a thermodynamic analysis of an absorption heat pump cycle a systematic search for new working pairs has been conducted. The investigation dealt exclusivelywith organic compounch. To get a first estimate of the properties of mixing, a molecular group contribution model (UNIFAC) was employed. This method has so far been widely used for chemical engineering purpmes and renders agreeable mtimatm for the thermodynamic behavior of organic mixtures. With the help of the UNIFAC method, various pairings of functional molecular groups have been investigated for their possible potential to form effectiveworking pairs for medium and high temperature absorption heat pump cycles.As a consequence of this analysis, ten alternative working pairs are proposed and their respective theoretical performance data and their toxicity characteristics are given.
NOMENCLATURE d G h
coefficientof temperature dependence Gibbs free energy enthalpy P pressure x mole fraction of refrigerant COP coefficientof performance L T V limit threshold value T temperature
Subscripts b
normal boiling point
c max
0
critical m a x i m u m value referencestate
r
reducedproperty
Superscripts
E ev
excessproperty evaporation 1. I N T R O D U C T I O N
Absorption cycles are, until now, c o m m o n l y used for chilling and air-conditioning applications. Industrial employment o f absorption cycles other than for chilling has so far been limited to exceptions. There is, however, a great potential for this energy recovery technique in various industries. A recent review [1] showed that for some applications in oil refinery processes, pay out times between one and two years m a y be realised by the use o f absorption heat p u m p s for energy recovery duties. A major break through for industrial applications, however, m a y be contingent on finding new working pairs for industrial requirements. The application o f heat p u m p s in industrial processes usually calls for higher temperature levels than for either chilling or residential heating purposes. This is true for both the heat accepted as waste heat input and the useful heat rejected by the heat pump. CI__a~__'cai absorption cycle working pairs like LiBr-water or w a t e r - a m m o n i a are no longer optimal choices for absorption heat p u m p s which work entirely above ambient temperature levels. This has led to a n u m b e r o f investigations to look for new working pairs more compatible with these requirements. The endeavors to find 459
460
M. NARODOSLAWSKYet al.
a solution to this problem fall roughly into two categories: the search for additives to the working pairs now in use in order to extend their field of application; the search for completely new working pairs. In this work, the latter approach has been taken. If one adopts this direction of investigations, it is crucial to use an effective screening method to discriminate promising new working pairs from the almost infinite number of possible mixtures. As there are a number of requirements the screening has to be done according to more than one property. The most important criteria for a good working pair are: to guarantee high efficiency of the heat pumps; to show good heat and mass transfer; to be non-corrosive; to be nontoxic; to guarantee secure operation. The first two criteria address the economic viability of the process, whereas the latter three are responsible for the operational applicability of a heat pump. These criteria do not render themselves equally well for a screening process. Namely, heat and mass transfer and corrosivity require extensive experimental investigations. To embark on these costly and time consuming experiments is only advisable if a working pair shows the potential to turn out as a possible commercial success. It was therefore decided to screen a great number of mixtures on the basis of their possible effectiveness as working pairs in medium and high temperature heat pump applications. On top of this, a survey on their respective toxicity has been conducted. In this way, a substantial reduction in the number of candidates for further investigations could be achieved. 2. T H E R M O D Y N A M I C C R I T E R I A FOR GOOD AHP W O R K I N G PAIRS In order to rate possible working pairs according to their performance in absorption heat pumps, criteria are needed linking the performance of a heat pump to the physical properties of the working pair. The criteria should be reliable; based on properties easily available for a great number of mixtures. One approach to this is to link molecular mechanisms to the performance of a working pair. An attempt in this direction using the tendency of a mixture to form hydrogen bonds has been reported in the literature [2-4]. The results from investigations in this direction have so far been encouraging. However, hydrogen bond formation is only one aspect positively contributing to the overall performance of a working pair. Moreover, the behavior of electrolyte solutions, which may be well suited as heat pump fluids, may not easily be explained in terms of hydrogen bond formation. In the search for more versatile criteria to find promising absorption heat pump working pairs, the authors made a study [5] on the influence of both the absorbent and the refrigerant. To take into account the influence of the properties of mixing, mixtures were characterized by their respective extreme values of the excess Gibbs free energy G r ~ and the location of this extreme value x,~x on a binary molcfraction scale between the absorbent and the refrigerant. The excess heat of mixing, h E, was related to the excess Gibbs free energy by a simple equation, proposed by Gaube [6, 7] (see Fig. l).
Ge(x, T,) = \'~l.] Ge(x' TO).
(I)
Using GE.~ and x,~, the concentration dependence of tbe excess Gibbs freeenergy can be described by simple models as the Van Laar ~luation [8] GE =
A,.2x (! -- x) X I (Ai.2/A2.1) +
(2)
(1 -- X)'
or Lagendre-polynornials [9]: Gr R-T = xl (1 - xl ) ~. a r L r ( x )
(3a)
K
L r ( x ) = [(2K - l).(2x - I).Lr_, ( x ) - ( K - I ) ' L K _ 2 ( x ) ] / K
(3b)
Lo(x) = I
(3c)
LI (x) = 2x - I.
(ad)
M e d i u m and high temperature absorption heat pumps
461
gE hE
h~mnx /
/
\
/ / / /
\
\ \
/ /
gCmox
0.0
OS
lo
x mog
XR~rigerant i-a gElx,T)" gE(x.T.).(_~.) d gE(x.T) = d. h~x,T) Fig. I. Simplified connection between the heat of mixing and the Gibbs free energy.
With the assumption that the excess heat of mixing is not a function of temperature in the considered temperature interval and with the basic thermodynamic relationship between G E and he:
01 T
equation (1) leads to: GE=dh
E.
(5)
Thus, with the definition of the values for G ~E, X,.x and d at a reference temperature To the vapor-liquid equilibrium as well as the enthalpy of the mixture at any composition x and temperature, is accessible. This method was then included in a computer model to describe the performance of a mixture acting as a working fluid in an absorption heat pump. Comparison between results obtained with this model using only experimental values for G,ffi,, 8 x , ~ and d for well-known working pairs, and performance data derived from comprehensive experimental data for the same fluids, showed that this simple model is capable of representing the performance of absorption heat pumps sufficiently well. A great number of simulation runs were subsequently conducted to deduce criteria for good working fluids. In these runs, pure component properties as well as the parameters G ~• , x ~ and d, characterizing the properties of mixing, were changed in a systematic manner. From the results of these simulation runs, the criteria listed in Table 1 were found. The main influence on the Table 1. Criteria for optimal working pairs Refrigerant Absorbens
Vapor-liquid equilibrium
high low
low high
high low
[J mole- ']
[mole mole- i]
[_ ]
- 1000 to -2000
>0.6
> 1.0
462
M. NARODOSL^WSK¥et al.
performance of an absorption heat pump according to this study are wielded by the properties of mixing, in particular by Ge~x and x~x and by the latent heat of the refrigerant at the normal boiling point. For more information on the choice of criteria, the reader is kindly referred to the original paper [5].
3. S C R E E N I N G FOR P R O M I S I N G W O R K I N G FLUIDS As the properties of mixing turned out to be prominently influencing the suitability of any mixture to be employed as an absorption heat pump working fluid, a reliable method to calculate G E and h E is requisite in the screening process. This method should be reliable and generally applicable. Though there is no general answer to these requirements, group contribution methods like UNIFAC [9] and ASOG [10] seem to be the closest thing to it, at least for organic compounds. Group contribution methods, specially UNIFAC, have proved to be powerful tools to estimate vapor-liquid equilibria for various chemical engineering purposes. The idea behind these methods is to deduce the behavior of a mixture from interactions beween functional molecular groups which build up the molecules of the constituent mixtures. The required interaction parameters for the functional groups are fitted to a great number of experimental data. Therefore, these methods are sufficiently reliable for a screening process as envisaged in this work. For reasons of greater versatility and better data access, UNIFAC was chosen as the base for the screening. Unfortunately, there does not exist an equally versatile and reliable method for electrolyte solutions or inorganic mixtures. Therefore the screening has to be confined to organic mixtures. Besides the possibility of estimating the properties of mixing, group contribution methods allow for the investigation of the influence of a given molecular functional group, either the absorbent or the refrigerant, exerts on the performance of a heat pump. Therefore, the screening process has been divided into two parts: first, the influences on the performance exerted by different pairings of molecular functional groups in the working fluid were investigated; second, the most promising pairings were used to construct possible working fluids. Two problems arise if one tries to apply group contribution methods to find new absorption heat pump working pairs. The first concerns the extent of the investigations. Despite the great number of functional groups encompassed by UNIFAC, the number of pairs of such groups, for which there actually exist interaction parameters, is considerably smaller than the maximum number of possible combinations. Therefore, only a relatively small number of functional groups can be investigated. The second problem concerns the representation of the heat of mixing by UNIFAC. Although the vapor-liquid equilibria calculated with the help of UNIFAC are usually very close to experimental data, this is not true for the heats of mixing. As a matter of fact, UNIFAC even predicts incorrect trends for some combinations of molecular groups [l l, 12]. Although this is a serious drawback, the main influence on the COP is exerted by GE~,~and xm,~, whereas the factor d in equation (l) is of lesser importance. These two parameters, however, are sufficiently correctly represented by UNIFAC. The screening according to G,,~ 8 and x,,~ is therefore possible with UNIFAC and gives the most important informations on the optimal pairing of molecular groups in the refrigerant and the absorbent.
4. RESULTS OF THE S C R E E N I N G A C C O R D I N G TO P R O P E R T I E S OF MIXING In the first step of the analysis, optimal combinations of functional molecular groups were looked for. In addition, the influence of the number of such groups per molecule was investigated. To achieve this goal a matrix of 12 "refrigerants" and 36 "absorbents" was established. The refrigerants represent alcohols, hydrocarbons and amines. The absorbents contain in addition, ketones, cyclic molecules, amides and ethers. Tables 2a and 2b show which substances were used in this analysis, Table 3 offers an insight into which combinations could be calculated with the help of UNIFAC.
Medium and high temperature absorption heat pumps Table Refrigerant Tetrachlorocarbon Chloroform n-Hexane n-Propanol Ethanol Methanol n-Propylamine Ethylamine Methylamine Dimethylamine Trimethylamine Diaminoethane
463
2a. Substances used as refrigerants Formula Tb[°C] Ah7 [J mole- '] CCI, 76.6 30010 CHCI3 61. I 29720 C6H ,4 68.7 28870 C3HTOH 97.2 41780 C..2HsOH 78.4 38760 CHsOH 64.6 35270 C3H~NH2 48.7 29 720 C2HsNH: 16.5 28050 CH3NH2 - 6.2 24 570 (CH 3)2NH 6.8 26 500 (CH3)3N 2.9 24110 (CH2]~['[2) 2 117.3 41860
Table 2b. Substances used as absorbents
Absorbens Undekane
Formula C~H2~
Tb[°C]
CTHIsOH CsHITOH C2H602 C3H802 C4HIoO2 CsHI202 C6H,402 C~H,602 C3HIO3 C5Ht203 C7HI603 CTH,eO4 C2H~NO C6H60 C6H602 C6HoO3 C7H80 C7H80 C4H,oO3 CsHnOs CeH,40~ C8HI804 C,oH220~
176 195 197 214 235 239 250 259 289
MEA PHE DHB THB KRE BAL DEG TEG DMEDEG DMETrEG DMETEG
Heptanol Oktanol Ethanediol Propanediol Butanediol Pentanediol Hexanediol Heptanediol Glycerol Pentanetriol Hexanetriol Polyhydroxiheptane Monoethanolamine Phenol Dihydroxibenzene Trihydroxibenzene rn-Kt~lol Benzylalcohol Diethyleneglycol Tetraethyleneglycol Dimethylether diethykneglycol Dimethylether triethyleneglycol Dimethylether tetraethyleneglycol
170 182 285 309 202 205 246 327 162 216 275
Amides
DMF DMA DEF
N,N -Dimethylformamide N,N -Dimethylacetamide N,N-Diethylformamide
C3HTNO C4H,NO CsH. NO
153 165 180
Ketones
APN NMP 2,4-PTDION
Acetophenone N-Methylpyrrolidone Pentanedione
C$H80 CsHgNO C~HeO2
204 201 139
Esters
DEM DEO
Diethylmalonate Diethyloxalate
C,Ht204 CeHleO4
199 186
Amines
DMAN ANIL OKAM
N,N-Dimethylaniline Aniline Octylamine
CsHjIN CeHTN CsHIgN
194 184 182
Group Alcan Alcohols
UNDEK HEPOH OKTOH EG 1,3-PD
1,4-BD 1.5-PTD 1,6-HXD 1,7-HPD GLY ! ,3,5-PTT 1,4,7-HPT 1,3,5,7-PHHP
Ethers
196
In the following paragraphs, only those combinations which hold the potential to make up good absorption working fluids will be discussed. All other pairings are, from a thermodynamic point of view, disadvantageous. 4.1. Halogenated hydrocarbons as refrigerants
Halogenated hydrocarbons are represented in this analysis by chloroform and tetrachlorocarbon, thus giving an insight into the influence of the degree of substitution on heat pump performance. Unfortunately, no other halogen atoms other than chlorine are covered by UNIFAC. Therefore, an investigation on the influence of different forms of hydrogen substitution in hydrocarbons by halogen atoms is not possible at this time. The overall trend is that fully halogen substituted hydrocarbons show lesser suitability for application in absorption heat pump fluids (see Table 4). Incompletely substituted chlorohydrocarbons show potentially promising behavior with ethers, esters and ketones. Chloroform H.R.S.
8/5-..-F
M, NARODOSLAWSKY et al.
464
Table 3, Overview of selected pairs
U~.~EK HEPOH OKTOH E6 !, 3-PD 1)4=[~0 I~5-PID
1,6-HID DT"HPD GLY 1)3)5"PTT 1,4)7"WT I)3~5,7-f~4HP ~A PHE ~B THB kRE BAt TEG
IqET~'EG DltETE6
OEF ~N
2y4-PTOION PEH PEO ~IL OKAM
Crosses indicate for which pairings calculations were made,
may also be paired with dimethylaniline (DMAN). In the case of anilines, a group sensitivity could not be investigated due to the constriction of the UNIFAC model. The best results were obtained with the pairing of chloroform and various ethers. According to these results, all chloroform-ether combinations investigated show excellent values for x , ~ of about 0.6. With rising number of ether groups in the absorbent, G,~x E keeps falling, thus indicating improving suitability of these substances as absorbents. 4.2. Alcohols as refrigerants Alcohols seem to offer great potential for new working pairs. The results from the analysis of properties of mixing indicate that alcoholic refrigerants show promising behavior with other alcohols, ethers, amides and ketones as absorbents. In the first analysis, alcohols with different chainlengths as refrigerants were investigated to get a first impression on the influence of this parameter on the COP of an absorption heat pump. Let us first have a look at the combination of an alcohol as refrigerant and another alcohol as absorbent. The latter may either be an aromatic or a long chain alcohol. For straight chain alcohols as absorbents, long chainlength and a high degree of substitution seem to be favorable. The addition of an amino group to a straight chain alcohol also results in better values for G..~ E and xm~. This is especially true for combination of amino-alcohols with low molecular weight alcohols as refrigerants (see Table 5). From a purely thermodynamic viewpoint, aromatic alcohols are very interesting absorbents for alcoholic refrigerants. With a rising number of hydroxylic groups in the absorbent molecule, improving COPs for heat pumps operated with these working pairs are to be anticipated.
Medium and high temperature absorption heat pumps
465
Table 4. Results for chiorohydrocarbomas refrigerant(GE.=~in J m o l e - ' ) Refrigerant CCl, CHCI 3 Absorbens E E G~= x=~ G,,= x,~, Alcan Alcohols
UNDEK
- 113
HEPOH OKTOH EG 1,3-PD
1,4-BD 1,5-PTD 1,6-HXD 1,7-HPD GLY 1,3,5-PTT 1,4,7-HPT 1,3,5,7-PHHP MEA PHE DHB THB KRE BAL DEG TEG
0.58
+430 +2330 + 1997 + 1730
0.67 0.59 0.62 0.63
+2792
0.63
+ 1466
0.60
+ 1472 + 828
0.62 0.63
+ 1160
0.67
+ 137
0.70
+214 + 153 + 1180 +956 + 785 +642 +523 +420 +1315 +917 + 639 + 846
0.70 0.75 0.65 0.65 0.70 0.70 0.75 0.74 0.70 0.75 0.80 0.80
+466 -602
0.75 0.55
- ! 223 - 1481 - 1711
0.60 0.60 0.60
Ethers
DMEDEG DMETrEG DMETEG
+ 136 + 65
0.60 0.72
Amides
DMF DMA DEF
+715 +334 +200
0.650.58 0.77
- 1745
0.60
+439 +171 + 1150
0.63 0.60 0.58
-630 -1720 - 558
0.58 0.57 0.60
+ 254 +399
0.63 0.63
- 1080 -851
0.60 0.60
- 159 + 1000 + 132
0.43 0.53 0.67
- 1587
0.55
Ketones
APN NMP 2,4-PTDION
Esters
DEM DEO
Amines
DMAN ANIL OKAM
Abbreviationsaccordingto Table 2b.
Alcoholic refrigerants may also be used, to a certain extent, with ethers as absorbents. These combinations show falling G=~ E with an increasing number of ether groups and chainlength of the absorbent. The location of the extremum of G E shifts to higher refrigerant concentrations with falling mole weight of the refrigerant. For these reasons the most interesting working pair out of these combinations may be methanol as refrigerant and dimethylether-tetraethyleneglycol (DMETEG). Another possible combination is an alcoholic refrigerant and an amide as absorbent. This combination gets more favorable the greater the chainlength of the absorbing amide is. One possible pairing could be methanol with dimethylamide. Ketones are also interesting absorbents for alcoholic refrigerants. This is especially true for the heterocyclic N-methyl-pyrrolidone (NMP). From the viewpoint of the properties of mixing, the lower the moleweight of the refrigerant, the better the working pair will be. The best combination, therefore, is methanol with NMP. 4.3. Amines as refrigerants Amines have been widely proposed as refrigerants for absorption heat pumps. In this analysis, the combination of amines with long chain alcohols was investigated. The trend of the results (see Table 6) indicate that this combination is increasingly interesting the longer chains the absorbing alcohol has and the more hydroxyl groups there are in the alcohol molecule. 5. SCREENING
ACCORDING
TO
PURE
COMPONENT
PROPERTIES
T h e most important pure c o m p o n e n t properties o f the working fluid o f an absorption heat p u m p are the boiling point difference o f the refrigerant and the absorbent, and the latent heat o f the
M. NARODOSLAWSKY et al.
466
Table 5. Results for alcohols as refrigerant (G,,~,E in J mole-~ )
Refrigerant C~H.7OH G~, x~,
Absorbens
AIcan
UNDEK
Alcohols
HEPOH OKTOH EG 1,3-PD 1,4-BD 1,5-PTD 1,6-HXD 1,7-HPD GLY 1,3,5-PTT 1,4,7-HPT 1,3,5,7-PHHP MEA PHE DHB THB KRE BAL DEG TEG
C,HsOH G~ x~x
CH3OH G~, x,,,~~
+ 1510
0.48
+ 1680
0.52
+ 1813
0.53
+ 53 + 71 +315 + 148 +32 -52 - 114 - 161 +397 +59 - 79 -88
0.57 0.53 0.53 0.53 0.60 0.53 0.55 0.57 0.58 0.67 0.47 0.54
-1119 -1800 -2119 - 1248 + 201
0.45 0.55 0.60 0.48 0.55
+ 108 + 133 +ll6 - 15 - 101 - 161 - 203 -232 +92 - 181 - 362 -164 -128 -1150 -1720 -1948 - 1205 + 201
0.58 0.60 0.55 0.48 0.55 0.57 0.57 0.58 0.63 0.57 0.58 0.55 0.65 0,50 0.60 0.65 0.42 0.58
+ 270 + 296 -128 - 196 -236 -259 - 270 -276 -370 -515 - 591 -797 -912 -580 -1264 -1661
0.58 0.62 0.48 0.53 0.55 0.57 0.57 0.58 0.55 0.58 0.61 0.63 0.65 0.40 0.55 0.60
- 72
0.48
-84
0.50
-250
0.58
-835
0.62
Ethers
DMEDEG DMETrEG DMETEG
+ 537 + 520 +480
0.53 0.58 0.63
+ 568 + 526 +466
0.56 0.58 0.63
+ 158
0.38
- 170
0.75
Amides
DMF DMA DEF
-357 -115 -360
0.55 0.54 0.54
-438 -111 -344
0.55 0.48 0.54
-340 -790 - 141
0.54 0.58 0.53
Ketones
APN NMP 2,4-PTD1ON
+883 -1090 + 840
0.53 0.48 0.58
+931 -1190 + 750
0.57 0.48 0.61
+658 -724 + 258
0.55 0.60 0.53
Esters
DEM DEO
+ 774 +510
0.60 0.58
+ 480 +500
0.62 0.62
+ 516 +494
0.56 0.55
Amines
DMAN ANIL OKAM
+320 +390 - 36
0.60 0.50 0.48
+422 +405 + 95
0.60 0.54 0.66
-219 +421 - 593
0.53 0.55 0.62
Abbreviations according to Table 2b.
refrigerant. The boiling point difference should be at least 150°C to minimize the distillation effort in the absorption cycle. From the viewpoint of the latent heat, alcohols are the most recommendable refrigerants. However, the heat of vaporization of these substances is rising with their molecular weight and so is their normal boiling point. This leads to a trade-off between latent heat and the normal boiling point of the refrigerant, usually leaving only ethanol and methanol in consideration. At the other extreme, methylamine has a low latent heat but also a low normal boiling point. This leads to high pressures for industrial applications, but on the other hand reduces the Table 6. Results for amines as refrigerant ( G ~
in J m o k - ' )
Refrigerant C3HTNH 2
G ~,E
Absorbens Alcan Alcohols
x,,,~
C~H~NH 2
G ,,~,
x,,~,
UNDEK HEPOH
OKTOH EG 1,3-PD
1,4-BD 1,5-PTD 1,6-HXD 1,7-HPD GLY 1,3,5-PTT 1,4,7-HPT 1,3,5,7-PHHP MEA
-649 -694 +25 -222 -413 -565 -691 -797 +32 -432 -755 -646 +313
Abbreviations according to Table 2b.
0.48 0.50 0.48 0.53 0.53 0.54 0.55 0.58 0.55 0.57 0.58 0.58 0.50
-746 -791 -306 -526 -695 -830 -941 -969 -434 -840 -1122 -1125 +77
0.52 0.52 0.53 0.53 0.54 0.57 0.57 0.58 0.57 0.58 0.58 0.62 0.55
CHjNH 2
G ,,~F"
x,,m~
+918
0.59
-755 -780 -872 -1020 -1130 -1213 -1281 -1122 -1206 -1483 -1664 -1869 -284
0.48 0.48 0.53 0.53 0.55 0.57 0.57 0.58 0.58 0.58 0.59 0.62 0.55
(CH3)2NH
(CH3)3N
(CH2NH2)2
G ~,.
x,,,~
G ~.,
x,,.~"
G ~.~
x~
-777 -828 -300 -520 -689 -826 -941 -1039 -430 -832 -1115 -1116 +323
0.50 0.55 0.55 0.55 0.55 0.55 0.55 0.55 0.58 0.58 0.60 0.63 0.55
-793 -861 +179 -166 -359 -530 -676 -802 +241 -314 -665 -482 +441
0.60 0.60 0.40 0.75 0.65 0.65 0.65 0.65 0.45 0.70 0.65 0.70 0.50
-477 -461 -814 -935 -1012 -1064 -1097 -1120 -1077 -1330 -1476 -1687 -160
0.35 0.30 0.45 0.45 0.50 0.50 0:50 0.50 0.50 0.55 0.55 0.55 0.50
Medium and high temperature absorption heat pumps
467
distillation effort for low temperature application. Chloroform is in between these two extremes according to the normal boiling point as well as the latent heat. 6. SCREENING ACCORDING TO NON-PHYSICAL PROPERTIES Good performance characteristics of an alternative absorption heat pump working fluid is a prerogative of its possible application. However, non-physical qualities such as toxicity, stability and corrosivity are at least as decisive for the application as a good performance for a new working pair. For the most promising working fluids resulting from the thermodynamic screening, toxicity and stability have been investigated. Corrosivity data are mostly not known. Therefore, besides the compilation of thermophysical data and heat and mass transfer characteristics of newly proposed working fluids, their potential corrosivity should be investigated experimentally at a very early stage. Although thermodynamic screening shows that chloroform with ethers exhibit favorable performance characteristics, these mixtures may not be applicable to absorption heat pumps because of the carcinogenic nature of chloroform. Other promising pairs are aromatic alcohols as absorbents and alcoholic refrigerants. Although these pairs show excellent coefficients of performance, application may be hampered by the fact that the aromatic alcohols tend to decompose at high temperatures. This first screening leaves, as alternative working fluids "for absorption heat pumps, systems of alcohol-alcohol mixtures, alcoholic absorbent with amines as refrigerants and alcoholic refrigerants with either amides or ketones as absorbents. There may be, however, a drawback to this system. Though NMP is widely reported to be stable even at high temperatures, methanol may exhibit a tendency for decomposition at elevated temperatures. The resulting dimethylether has a low vapor pressure and therefore may contribute to problems in heat and mass transfer. Ultimate assessment of this working pair, therefore, should be contingent on further experimental tests. A first choice of alternative working pairs is given in Table 7, along with performance and toxicity data. 7. CONCLUSIONS High temperature applications of absorption heat pumps have gained increasing interest in the last few years. However, most absorption heat pump working pairs are not applicable at elevated temperatures. This is especially troublesome for industrial heat recovery problems, which require the production of steam from industrial waste heat. In the last couple of years new salt and water mixtures [13] as refrigerants have been proposed for this purpose. Although this seem to be a valuable direction of investigation, organic refrigerant-absorption systems may prove interesting in this area, too. A first screening of possible pairings applicable for medium and high temperature applications reveals that there exists a potential for some alternative absorption heat pump fluids. On the base of a systematic screening some working pairs have been proposed. These proposed working fluids
Table 7. Toxicity and theoretical C O P for a first choice of alternative working pairs
Temperature of heat source/sink [ ° q LTV
Refrigerant
[ppm]
CHCI3 CHCI 3 CHCIj CHsOH C2HsOH C2HsOH C3H~OH C3HtOH (CH2NH2)2 (CH2NH2)2
10 10 10 200 1000 1000 400 400 n.a. n.a.
LTV
Absorbens
[ppm]
20/70
60/120
90/150
APN DEM NMP NMP DHB THB DHB THB 1,4-BD GLY
n.a. n.a. 100 100 2 n.a. 2 n.a. n.a. n.a.
1.61 1.65 1.65 1.72 1.76 1.76 1.75 1.74 1.65 1.65
1.53 1.57 1.57 1.63 1.71 1.71 1.68 1.68 1.55 1.57
1.54 1.57 1.57 1.64 1.70 1.70 1.68 1.68 1.56 1.57
n.a. not available. Abbreviations according to Table 2b.
468
M. NARODOSLAWSKYet al.
should d r a w the interest o f experimental scientists to establish their real potential as absorption heat p u m p working fluids. A p a r t f r o m the compilation o f the t h e r m o d y n a m i c data base, a t h o r o u g h investigation in heat and mass transfer properties, toxicology and stability is needed. The report has shown that there exist possible alternatives for well-known working pairs for high temperature applications. These alternatives include mixtures o f substances widely used in chemical engineering, thereby assuring high availability and a g o o d database. However, further experimental investigations are needed on these working pairs. Acknowledgemems--The authors are grateful to the Austrian Research Council for providing the financial base for this research within the framework of the "W.~*rmepumpenschwerpunkt'', project $31. Also the authors want to thank Dr Hans Huemer for his very constructive criticism.
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