Nucleation of sodium acetate trihydrate in thermal heat storage cycles

Solar Energy Vol. 46, No. 2, pp. 97-100, 1991

0038-092X/91 $3.00 + .00 Copyright (c, 1991 Pergamon Press pie

Printed in the U.S.A.

N U C L E A T I O N OF SODIUM ACETATE T R I H Y D R A T E IN T H E R M A L HEAT STORAGE CYCLES J, GUION and M. TEISSEIRE Laboratoire de Chimie Physique, Universit6 de Nice, U.F.R. Sciences, Parc Valrose, 06034 Nice-cedex, France Abstract--Experimental results about nucleating abilities of some sodium-hydrogenophosphates hydrates are presented. An analysis of structural compatibilities of different nucleating agents is reviewed. A critical examination leads to some new orientations such as studying multicomponents systems in the corner rich in sodium acetate trihydrate including hydrogenophosphate ions as components of reciprocal mixtures.

1. INTRODUCTION

agreement for the compounds examined and does not agree with Wada's previous conclusions[6-8]. However, at least for the first cycles the heterogeneous nucleation ability of Na2HPO4 and Na4P207.10 H20 towards sodium acetate tri-hydrate may be verified. Our study concerns other hydrogenophosphate salts such as NaNH4HPO4.4 H20, Na2HPO4.12 H20 and Na2HPO4.7 H20 tested as nucleators of the same compound.

Among latent heat storage materials, sodium acetate trihydrate deserves a special attention for its large latent heat of fusion-crystallisation (264-289 J . g-~) and a melting temperature of 58°-58.4°C [ 1-3 ], However this material supercools dramatically, preventing any practical application on a large scale. Many attempts have been made to find ways of suppressing or reducing this phenomenon, but very few effective results may be claimed. Christensen and co-workers[4], then Barrett and Best[5] mentioned sodium carbonate decahydrate (Na:CO3. l 0 H20) as an effective nucleator. However, some tests we made for long cycling periods showed soon failure in initiating nucleation of sodium acetate trihydrate. This seems to be due to self-decomposition of sodium carbonate decahydrate and transformation into anhydrous sodium carbonate. Recently, T. Wada and his colleagues[6-8 ] studied the behaviour of anhydrous disodium hydrogenophosphate (Na2HPO4) and sodium pyrophosphate decahydrate (Na4P207 • 10 H20) as effective nucleation catalysts of sodium acetate trihydrate. These compounds, together with sodium bromide dihydrate, were already proposed by Kimura[ 9]. The interpretation of their effectiveness is based on the "crystalline adsorption model" for crystal nucleation, first elaborated by Richards [ 10 ], and some conclusions for epitaxial compatibility were drawn [ 6 ]. Nishizaki [ 11 ] patented barium and potassium bromates associated with hydrated calcium or/and potassium sulfate and barium hydroxyde as nucleators of sodium acetate trihydrate. Recently, T. Wada and his group [12 ] patented the use of alkali and ammonium fluorides and transition metal fluorides for the same purpose. We reexamine here the role of hydrogenophosphate salts in possible nucleation of sodium acetate trihydrate. Our quest is based on the poor agreement of structural compatibility of anhydrous disodium hydrogenophosphate or sodium pyrophosphate decahydrate and sodium acetate trihydrate. The proposed criterion related to lattice cell parameters does not lead to any serious

2. EXPERIMENTAl, NaCH3CO2 • 3 H20, NaNH4PO4 • 4 H20, Na2HPO4 • 12 H20 were pure reagents manufactured by Carlo Erba. Na2HPO4.7 H20 was prepared by partial dehydration of the dodecahydrate. Mixtures of sodium acetate trihydrate with any of the hydrogenophosphate salts were prepared in the same way. Components of the mixture are weighted, heated and mixed by careful stirring, the temperature being in the range 70°-75°C. In all cases, the phosphate salt is slightly soluble in the sodium acetate trihydrate, and the solutions are supersaturated. Typical compositions studied correspond to 5% by weight of the hydrogenophosphate salt. Solutions were kept liquid at 70°C, where part of them was sampled out for differential scanning calorimetry tests (DSC), the bulk solution being cycled between 40°C and 70°C in a temperature controlled water-bath, where temperature versus time curves were recorded for each of the hundreds of heating and cooling cycles performed. The DSC calorimeter used is a SETARAM D.S.C. 111. For each run, in the laboratory tube of the calorimeter, a blanck is made with the empty crucible plus the gasket and cap along the programmed range of temperatures to be investigated. The results are stored and will be substracted from the calorimetric signal corresponding to the experiment with the filled and sealed crucible, in the same range of temperatures. Each sample has a weight of about 150 mg, and is runned at a low heating rate (5°C • h ~), ensuring on one hand

97

98

J. GUION and M. TEISSEIRE

near equilibrium conditions, and working with about the same speed as in solar heat storage and discharge cycles on the other hand. Some cycling experiments are performed directly in the calorimeter. The results are presented either as dQ/dT = f ( T ) curves or as enthalpy variations with temperature AH = g ( T ) . 3. RESULTS AND DISCUSSION

All the hydrogenophosphate salts studied were found to act more or less as catalysts for nucleation of sodium acetate trihydrate. The nucleation ability of NaNH4HPO4.4 H:O is limited to few cycles, and even for the first cooling cycle, a still important supercooling is observed. Figure 1(a) shows the variation of dQ/dT versus temperature for the heating and cooling part of the first DSC run. The corresponding AH versus temperature curves are shown on Fig. 2(a). The temperature spread of the supercooling observed is A0 = 19°C for the first run, but it rapidly increases for following cycles. The crystallisation enthalpy AH recovered during the cooling is a poor fraction of what is necessary in the heating part of the cycle in order to melt the acetate solution. The results observed with Na2HPO4.12 H20 are more encouraging. Figures l(b) and 2(b) show the corresponding behaviour of this compound used as nucleating agent. We notice a supercooling reduced to only 6°-7°C (13°C reported for the best nucleating agent known, Na4P207.10 H20)[6], with a fair recovery of enthalpy at 52°C during cooling. Moreover, the shape of dQ/d T versus temperature curve, indicates for the heating part, the presence of a premelting zone somewhat similar in what we already

endo

observed in calcium chloride hexahydrate and ammonium chloride solutions [13 ] and with ammonium and potassium nitrates [ 14 ], which may be related to the presence of some free water molecules liberated in the system. This observation is important and may be indicative of new research hypothesis and working directions. The nucleating effectiveness of Na2HPO4.12 H20 has been proved for a hundred cycles for some samples. Moreover, samples left at ambient temperature for more than six months proved to be working perfectly well when recovering heating-cooling procedures. Many more repeated cycles are nevertheless necessary, including test procedures in a bigger scale, to conclude firmly. We looked for the possibility of finding correlations between cell parameters of crystal structures of all the phosphate compounds mentioned, anhydrous compounds and even subhydrates which could form like ( Na2HPO4 • 7 H~O), (Na2HPO4 • 2 H20). The simple examination of phase diagram of the Na2HPO4--H20 system[15] indicates the phase transformations under atmospheric pressure:

Na2HPO4.12H20(¢/)

35°C 48°C ~ Na2HPO4.7H20

Na2HPO4.2 H20

95°C --~ Na2HPO4

The (a) --~ (~) allotropic transformation has been omitted as it has never been observed in any of our experiments about disodiumhydrogenophosphate dodecahydrate, from 5 ° to 35°C back and forth. A recent reexamination of the phase diagram confirms this fact[16]

da/dT

t•lb ///'

I 40

I

~0

I

,.

60

'

T "c

--'1~ ,I

i: :! iiI: Ii

lb

q

I: Fig. 1. Calorimetric signals for heating (endotherms) and cooling (exotherms) parts of cycles. (a) NaNH4HPO4.4 H20 as nucleating agent of sodium acetate trihydrate. (b) Na2HPO4.12 HPO412 H20. (c) Na2HPO4-7 H20 (only the endotherm shown).

Nucleation of sodium acetate trihydrate

99

~.li (J.~ 4 )

2 . . . . . . . . . . . . . . . . . . _.. ; .

ii.

................................

................................

2a

....

.-~"'"~

,

30~

20~

100

~ 40

i

i

p

i

5O

T °c

50

Fig. 2. Enthalpy variations for the three nucleating agents tested vs. temperature.

The transformation of anhydrous disodium hydrogenophosphate into anhydrous pyrophosphate is usually occuring over 200°C. As we limited the higher temperature zone explored to 75°C, we might be able to meet each of the compounds existing between the dodecahydrate and the dihydrate. As discussed by Lane [ 17 ] and Telkes [ 18 ], there is about no possible chance of observing nucleation catalysis in such systems if cell parameters differ more than 10%-15% for both nucleant and nucleating agent. This seems to be a necessary condition but not sufficient as already observed for many systems. A first simple criterion to look at is that both components must belong to the same symmetry space group. Along such a direction it is possible to eliminate any nucleating potentiality of the anhydrous salt Na2P207 (orthorombic system) [ 19 ] towards sodium acetate trihydrate (monoclinic)[20]. For the same reason Na2HPO4-2 H20 and the hemihydrate are to be eliminated, just as NaNH4HPO4 • 4 H20 [ 21 ]. It is to be noted that Wada et aL claimed that anhydrous pyrophosphate was responsible for nucleation catalysis of sodium acetate trihydrate [ 7], a conclusion opposed to their previous studies where they proposed that the pyrophosphate decahydrate was the nucleation agent [ 6 ]. The sodium acetate trihydrate belongs to the monoclinic space-group, as the three other phosphate

hydrates whose cell parameters are compared with in Table 1. Included are also the cell parameters of sodium carbonate decahydrate, belonging to the same system as the barium bromate monohydrate and anhydrous sodium hydrogenophosphate. Obviously the 15% deviation rule is not fullfilled for any of the phosphate hydrates if all the cell parameters have to agree. Apart from the a cell parameter, the b and c values for sodium hydrogenophosphate heptahydrate fit quite well those of sodium acetate trihydrate, while those of the dodecahydrate and baryum bromate monohydrate may seem not too far, and those of pyrophosphate and sodium carbonate hydrates and anhydrous disodiumhydrogenophosphate far apart. Following these conclusions we tried to nucleate directly sodium acetate trihydrate with the heptahydrate. The dQ/d 7" and AH versus temperature curves are presented in Fig. l ( c ) and 2(c) only for the heating part. We notice a large flat signal for melting, and if incidently we get an effective nucleation of the acetate, quite soon--less than five cycles--the dQ/dTcurves became larger and flater bands, with nucleation effect disappearing completely. Turning back to the binary phase diagram of NaCH3CO2--H20 system[28-30], we note that the sodium-acetate trihydrate is non-congruent, and undergoes a peritectic decomposition reaction. 'The temperature corresponding to the peritectic reaction is not very different from the melting temperature expected

Table 1. Cell parameters for phosphate hydrates and related compounds.

NaCH3CO2 • 3 H20 (2°) Na4P207.10 H20 (22) Na2HPO4 • 12 H20 (23)(13) Na2HPO4 • 7 H20(24) Na2CO3 • 10 H20 (25) Ba(BO3)2 • H20 (z6) Na2HPO4 (27)

a (A)

b (A)

c (A)

fl (o)

12.353 16.960 15.718 9.258 12.756 9.660 9.726

10.466 6.960 9.018 11.007 8.980 7.92 6.846

10.401 14.850 12.769 10.437 13.462 9.060 5.762

111.69 111.76 121.39 95.60 122.66 93.05 90.28

J. GUION and M. TEISSEIRE

100

for the trihydrate c o m p o u n d , if congruently melting. This situation is quite similar to that observed for C a C I 2 - - H 2 0 system with peritectic decomposition of calcium chloride hexahydrate CaCI2.6 H20, (quasicongruent situations [ 17 ]). This kind of situation is suitable for looking to m u l t i c o m p o n e n t solutions and a close analysis of simple ternary diagrams might favour the possibility of stabilizing m u l t i c o m p o n e n t s mixtures rich in sodium acetate trihydrate. In the present study we formed reciprocal mixtures, and the possibility of getting lower hydrated phosphate c o m p o u n d s such as N a 2 H P O 4 . 7 H : O o r / a n d N a 2 H P O 4 . 2 H 2 0 is not excluded. For this kind o f solutions the possibility of observing ternary invariants must be explored. Moreover, formation of mixed c o m p o u n d s are to be looked for, just as for calcium c h l o r i d e - - ( N H ~ - - K +) (CI NOj)--H20[31]. Consequently the active nucleating catalyst species may not be so easily identified, if any. Further studies a b o u t stability of the solutions on cycling, identification of solid phases in equilibria, phase diagrams for multic o m p o n e n t equilibria are necessary. Noticeably, in the range of temperatures investigated, the presence of a n h y d r o u s Na2HPO4 is not expected, and c a n n o t account for the nucleation catalysis of sodium acetate trihydrate as mentioned in litterature. On the other hand, in the interval 5 0 ° - 5 8 ° C , the only k n o w n solid phase containing hydrogenophosphate ion, is N a 2 H P O 4 . 2 H20, which c a n n o t be an effective nucleation catalyst for sodium acetate trihydrate. This may support the idea of looking for hydrated mixed c o m p o u n d ( s ) in m u l t i c o m p o n e n t solutions in the corner rich in sodium acetate, which would crystallise a r o u n d 50°C.

REFERENCES

1. A. Pebler, Dissociation vapour pressure of sodium acetate trihydrate, Thermochim. Acta 13, 109 (1975). 2. M. Telkes, In: L. E. Murr (ed.), Solar materials science, Academic Press, New York (1980). 3. J. Guion, J. D. Sauzade, and M. Laugt, Critical examination and experimental determination of melting enthalpies and entropies of salt hydrates, Thermochim. Acta 67, 167 (1983). 4. L. Christensen, G. Keyser, E. Wedum, N. Cho, D. Lamb, and J. Hallet, Studies of nucleation and growth of hydrate crystals with application to thermal storage systems, Report NSF-RANN-AER 75,19601, Desert Research Institute, Reno, NV (1975), and ibid, Final Report, Nevada University, Reno, NV (1979). 5. P. F. Barrett and B. R. Best, Supercooled mixtures with Na2S203" 5 H20, Materials Chemistry and Physics 12, 529 (1985). 6. T. Wada and R. Yamamoto, Studies on salt hydrates for latent heat storage. I. Crystal nucleation of sodium acetate trihydrate catalyzed by tetrasodium pyrophosphate decahydrate, Bull. Chem. Soc. Jpn. 55, 3603 (1982).

7. T. Wada, K. Matsunaga, and Y. Matsuo, Studies on salt hydrates for latent heat storage. V. Preheating effect on crystallisation of sodium acetate trihydrate from aqueous solution with a small amount of sodium pyrophosphate decahydrate, Bull. Chem. Soc. Jpn. 57, 557 (1984). 8. T. Wada and Y. Matsuo, Studies on salt hydrates for latent heat storage. VI. Preheating effect on crystallisation of sodium acetate trihydrate from aqueous solution with a small amount of disodium hydrogenophosphate, Bull. C/win. Soc. Jpn. 57, 561 (1984). 9. H. Kimura, Nucleating agents for sodium acetate trihydrate J. Jpn. Ass. Crys. Growth 9(3), 73 (1982). I 0. W.T. Richards, Crystalline adsorption model for crystal nucleation, J. Am. Chem. Soc. 54, 478 (1932). 11. M. Nishizaki, Japan pat. N ° 54-50719 (6/11/80). 12. T. Wada, F. Yokotani, Y. Matsuo, and H. Yoneno, European pat., 0146304, A 1 (4/12/84). 13. J. Guion, M. Laugt, and M. Teisseire, Calcium chloride hexahydrate-ammonium chloride binary solutions: A D.S.C. study, Thermochim. Acta 138, 39 (1989). t4. J. Guion, M. Laugt, A. Jaffrin, and M. Teisseire, Brevet ('NRS, F 87/13485 (21/9/87). 15. D. L. Hammick, H. K. Goadby, H. Booth, Disodium hydrogen phosphate dodecahydrate, J. Chem. Soc. 117, 1589 (1920). 16. A. Hammami, Th~%ede 3Ome, Cycle Universit6 C. Bernard, Lyon 1, France (1989). 17. G. A. Lane, Solar heat storage latent heat material, Vol. 1, CRC Press Inc., Boca Raton, FL (1983). 18. M. Telkes, Nucleation of supersaturated inorganic salt solutions, Ind. and Eng. Chem. 44, 1308 (1952). 19. K. Y. Leung and C. Cairo, The structure of Na4P207 at 22°C. Can. J C71em. 50, 16, 2522 (1972). 20. K. T. Weu and D. L. Ward, Sodium acetate trihydrate: A redetermination, Acta Co,st. B.33, 522 (1977). 21. M. Catti, G. Ferraris, and M. Franchini-Angela, The crystal structure of Na2HPO4-2 HzO. Competition between coordination and hydrogen bonds, Acta Cryst. B.30, 504 (1974). 22. W. S. McDonald, D. W. Cruikshank, A reinvestigation of the structure of Na4P207.10 H~O, Acta Co'st. 22, 43 (1967). 23. M. Catti, G. Ferraris, and G. lvaldi, Disorder of HPO]and hydrogen bonds in the structure of ~-NazHPO4.12 H20, Acta Cr)'st. B.34, 369 (1978). 24. W. H. Baur and A. A. Khan, The crystal structures of disodium hydrogen orthoarsenate heptahydrate and of disodium hydrogen orthophosphate heptahydrate, Acta. ()3'st. B26, 1584 (1970). 25. T. Taga, Crystal structure ofNa2CO3- 10 H20, Acta Cryst. B25, 2656 (1969). 26. S. Khartha, The crystal structure determination of Ba(BO3)2. H20, Proc. Indian Aead. Sci. A38, 1 (1953). 27. D. R. Corbridge, F. R. Tromans, Identification of sodium phosphates with an x-ray focusing camera, Anal. Chem. 30, 1101 (1958). 28. W. F. Green, Sodium acetate--Water binary system, J. Ph.vs. Chem. 12, 655 (1908). 29. N. V. Sidgwick and J. A. Gentle, Phase diagram of binary system sodium acetate--Water. J. Chem. Soc. 121, 1837 (1922). 30. T. Wada, F. Yokotani, and Y. Matsuo, Equilibria in the aqueous ternary system containing Na +, CH3CO~ and P20~- between 38 and 85°C, Bull. Chem. Soc. Jpn. 57, 1671 (1984). 31. M. Teisseire, ThOse de Doctorat. Sciences de l'Ingrnieur, Universit6 de Nice, France (1989).