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Water sorption in faujasite- and chabazite type zeolites of varying lattice composition for heat storage applications
Zeolites and Related Materials: Trends, Targets and Challenges Proceedings of 4th International FEZA Conference A. Gédéon, P. Massiani and F. Babonneau (Editors) © 2008 Elsevier B.V. All rights reserved.
599
Water sorption in faujasite- and chabazite type zeolites of varying lattice composition for heat storage applications Jochen Jänchen,a Helmut Stach,b Udo Hellwiga a
University of Applied Sciences Wildau, Bahnhofstr. 1, 15754 Wildau, Germany, email: [email protected]. b ZeoSolar e.V., Volmerstr. 13, 12489 Berlin-Adlershof, Germany.
Abstract The water sorption properties of faujasite type zeolites with different Si/Al ratios as well as of natural chabazite and two different SAPO-34 molecular sieves have been investigated systematically by thermogravimetry (TG), differential scanning calorimetry (DSC), microcalorimetry, and isotherm measurements. Via changing the lattice chemistry of the zeolites by dealumination and isomorphous substitution of T atoms the energetics of the water sorption have been tailored. According to this approach the charging and discharging properties of microporous molecular sieves in a thermochemical storage process can be controlled as has been demonstrated in a labscaled storage of 1.5 L volume. Keywords: water adsorption, faujasite, chabazite, thermochemical storage.
1. Introduction In recent years the optimization of zeolites for thermochemical heat storage [1, 2] was focused on a maximum storage density and discharging temperature which can be achieved by changing the nature of cations of the zeolites to get high heats of adsorption [3]. However, if the temperature level of the heat (solar or waste heat) available for the desorption (charging) of the zeolitic storage material is too low (<420 K) not all the water adsorbed can be removed. Hence, the potential high storage density of these materials cannot be utilized completely. The modification of the anionic skeleton of the zeolites concerning a lower electric charge density in the cavities reduces the heats of adsorption of water in the molecular sieves and thus, the temperature necessary for the water removal [4]. Therefore we have studied systematically the influence of the lattice composition of faujasite- and chabazite type zeolites on the water adsorption and on the storage properties in a thermochemical storage process.
2. Experimental The water adsorption behaviour of pelleted faujasite type zeolites with a Si/Al ratio between 1 and 30, a natural chabazite (Wassons Bluff, Nova Scotia) and two different SAPO-34 samples (E=Erlangen, T=Toufar) have been investigated in regard to their thermochemical storage application by different adsorption methods. The TG and DSC measurements were performed with a SETARAM TG-DSC 111 equipment with a heating rate of 3 K/min to a temperature of 723 K in a nitrogen stream of 1 L/h with water p/ps=0.3 saturated samples. Adsorption isotherms were measured gravimetrically using a McBain balance. The differential molar heats were estimated with a C 80
600
J. Jänchen et al.
microcalorimeter from SETARAM connected to a standard volumetric isotherm apparatus. A thermochemical storage of 1.5 L volume for an amount of about 1 kg material was used for tests of the zeolites as thermochemical storage materials (for more information compare [3]).
3. Results and Discussion Table 1 summarizes the results of the TG and DSC measurements to get an overview of the water adsorption properties. Column 2 of Table 1 gives information about the lattice composition of the samples. The integral heats of desorption, the maxima of the DTG profiles and the energy density shift systematically with the chemical composition of the lattice for both the FAU and CHA samples (including AlPO-18). Important is the decrease of the DTG T-maxima because lower desorption (charging) temperatures can Table 1 results of the TG/DSC measurements: integral heats of desorption Qint, temperature of the desorption maximum, Tmax, (from DTG) and energy density for FAU and CHA type zeolites Tmax (DTG) Energy density in Sample Lattice Si/Al Qint from DSC in kJ/mol in K Wh/kg ratio NaLSX 1 77.9 440 291 NaX
1.2
62.7
425
278
NaY
2.3
61.3
395
257
NaY(7)
7.4
49.0
375
165
NaY(11)
11.4
53.0
355
207
NaY(30)
30
-
345
123
CHA(nat)
2
81.3
410/500
326
SAPO-34(E)
5.8 Si/UC
67.9
385
254
SAPO-34(T)
3.0 Si/UC
61.8
370
259
AlPO-18(E)
(no Si)
65.1
365
203
Figure 1. Water adsorption isotherms at 293 K for faujasite type zeolites (left) and chabazite types (right); FAU (from left to right): NaX, NaY, NaY(7), NaY(11), NaY(30); CHA (from left to right): CHA(nat.), SAPO-34(E), SAPO-34(T).
601
Water sorption in faujasite- and chabazite type zeolites
Figure 2. Heat curves (differential molar heat of adsorption, Q) of water in FAU type zeolites (left) and in CHA types (right); dashed line denotes heat of condensation at 303 K; FAU: NaLSX (top), 313 K (triangles), 333 K (circles), 372 K (triangles inverted), NaY(7), 303 K (triangles), 313 K (squares), 333 K (filled squares); CHA: CHA(nat.) 303 K (top), SAPO-34(T) 303 K (bottom)
be expected. CHA has a complex DTG profile showing at least two different maxima due to different adsorption sites in combination with the small windows of this structure influencing the kinetics. Figure 1 shows the water isotherms at 293 K for a series of faujasite type zeolites with decreasing Si/Al ratios and chabazite type zeolites with varying lattice composition, respectively. Both dealumination of Y as well as isomorphous substitution in SAPO’s regarding a reduced electric field in the cavities shift the water isotherms towards higher pressure because of decreasing heats of adsorption as can be seen in Figure 2. As an example Figure 2 shows the differential molar heats of adsorption of water for two FAU and two CHA samples. A systematic influence of the lattice composition can be stated here also. Table 2 results of the tests in the lab-scaled storage for selected FAU and CHA type zeolites Sample Desorption Adsorbed Tmax (storage) Energy density in temperature in K amount in kg/kg in K Wh/kg NaLSX 470 0.234 370 159 NaX
470
0.200
370
156
NaY
470
0.225
363
156
NaY(7)
470/420
0.183
340
155
NaY(11)
420
0.149
340
125
NaY(30)
420
0.125
330
95
SAPO-34(E)
470/420
0.195
350
150
SAPO-34(T)
470/420
0.186
345
130
Not only the position of the isotherms in Figure 1 but also the shape of the isotherms is important for the performance of the storage or heat pump. Especially the steep course
602
J. Jänchen et al.
of the SAPO isotherms is an advantage (compared with silica gel having a shallow isotherm) for the performance of adsorption heat pumps and storages. A model based on the Dubinin equation could be employed successfully by Stach et al. [5] to model the storage performance expectations based on the measured thermodynamic data. However, decreasing heats of adsorption (Figure 2, Table 1) reduce the storage density as well because of less charges in the cavities of the modified materials. Consequently, an optimization has to be carried out between the charging and discharging temperature, the storage density and the materials composition. Equivalent investigations have been made in a 1.5-L-storage with selected materials of different composition. Table 2 summarizes the results with respect to their charging (desorption) temperature, adsorbed amount of water, the discharging temperature (Tmax), and the storage density. As can be seen from Table 2 all parameters change systematically with the lattice composition. The charging temperature of the dealuminated FAU samples and of the SAPO’s can be lowered by at least 50 K without losing storage density. This is not the case for X an Y zeolites. On the other hand this advantage goes at the costs of the discharging temperature. Dependent on the case of application (industrial waste heat >470 K or solar heat <420 K) the Si/Al ratio or the lattice composition of the storage material can be tailored.
4. Conclusion In conclusion it can be stated that the water desorption temperature of zeolitic materials and other parameters can be controlled by modification of the lattice chemistry such as dealumination and isomorphous substitution of T atoms. Following this new route, zeolitic materials can replace silica gel with benefit for utilisation of solar heat or low temperature waste heat in thermochemical storage application and heat transformation processes. Acknowledgement We thank W. Schwieger, University of Erlangen-Nürnberg, Germany and H. Toufar, Süd-Chemie Zeolites GmbH, Bitterfeld, Germany for the supply of the SAPO-34 samples. The financial support by the German Federal Ministry of Economics and Technology, Grand No. 0329525F, is acknowledged. References [1] F. Meunier, J. Solar Energy Eng., 108 (1986) 257 [2] D.I. Tshernev, in: D.L. Bish and D.W. Ming, Editors, Natural zeolites: occurrence, properties, applications. Reviews in Mineralogy&Geochemistry, 45 (2001) 589-617 [3] J. Jänchen, D. Ackermann, H. Stach and W. Brösicke, Solar Energy, 76 (2004) 339 [4] J. Jänchen, D. Ackermann, E. Weiler, H. Stach and W. Brösicke, Thermochmica Acta, 434 (2005) 37 [5] H. Stach, J. Mugele, J. Jänchen and E. Weiler, Adsorption, 11 (2005) 393
599
Water sorption in faujasite- and chabazite type zeolites of varying lattice composition for heat storage applications Jochen Jänchen,a Helmut Stach,b Udo Hellwiga a
University of Applied Sciences Wildau, Bahnhofstr. 1, 15754 Wildau, Germany, email: [email protected]. b ZeoSolar e.V., Volmerstr. 13, 12489 Berlin-Adlershof, Germany.
Abstract The water sorption properties of faujasite type zeolites with different Si/Al ratios as well as of natural chabazite and two different SAPO-34 molecular sieves have been investigated systematically by thermogravimetry (TG), differential scanning calorimetry (DSC), microcalorimetry, and isotherm measurements. Via changing the lattice chemistry of the zeolites by dealumination and isomorphous substitution of T atoms the energetics of the water sorption have been tailored. According to this approach the charging and discharging properties of microporous molecular sieves in a thermochemical storage process can be controlled as has been demonstrated in a labscaled storage of 1.5 L volume. Keywords: water adsorption, faujasite, chabazite, thermochemical storage.
1. Introduction In recent years the optimization of zeolites for thermochemical heat storage [1, 2] was focused on a maximum storage density and discharging temperature which can be achieved by changing the nature of cations of the zeolites to get high heats of adsorption [3]. However, if the temperature level of the heat (solar or waste heat) available for the desorption (charging) of the zeolitic storage material is too low (<420 K) not all the water adsorbed can be removed. Hence, the potential high storage density of these materials cannot be utilized completely. The modification of the anionic skeleton of the zeolites concerning a lower electric charge density in the cavities reduces the heats of adsorption of water in the molecular sieves and thus, the temperature necessary for the water removal [4]. Therefore we have studied systematically the influence of the lattice composition of faujasite- and chabazite type zeolites on the water adsorption and on the storage properties in a thermochemical storage process.
2. Experimental The water adsorption behaviour of pelleted faujasite type zeolites with a Si/Al ratio between 1 and 30, a natural chabazite (Wassons Bluff, Nova Scotia) and two different SAPO-34 samples (E=Erlangen, T=Toufar) have been investigated in regard to their thermochemical storage application by different adsorption methods. The TG and DSC measurements were performed with a SETARAM TG-DSC 111 equipment with a heating rate of 3 K/min to a temperature of 723 K in a nitrogen stream of 1 L/h with water p/ps=0.3 saturated samples. Adsorption isotherms were measured gravimetrically using a McBain balance. The differential molar heats were estimated with a C 80
600
J. Jänchen et al.
microcalorimeter from SETARAM connected to a standard volumetric isotherm apparatus. A thermochemical storage of 1.5 L volume for an amount of about 1 kg material was used for tests of the zeolites as thermochemical storage materials (for more information compare [3]).
3. Results and Discussion Table 1 summarizes the results of the TG and DSC measurements to get an overview of the water adsorption properties. Column 2 of Table 1 gives information about the lattice composition of the samples. The integral heats of desorption, the maxima of the DTG profiles and the energy density shift systematically with the chemical composition of the lattice for both the FAU and CHA samples (including AlPO-18). Important is the decrease of the DTG T-maxima because lower desorption (charging) temperatures can Table 1 results of the TG/DSC measurements: integral heats of desorption Qint, temperature of the desorption maximum, Tmax, (from DTG) and energy density for FAU and CHA type zeolites Tmax (DTG) Energy density in Sample Lattice Si/Al Qint from DSC in kJ/mol in K Wh/kg ratio NaLSX 1 77.9 440 291 NaX
1.2
62.7
425
278
NaY
2.3
61.3
395
257
NaY(7)
7.4
49.0
375
165
NaY(11)
11.4
53.0
355
207
NaY(30)
30
-
345
123
CHA(nat)
2
81.3
410/500
326
SAPO-34(E)
5.8 Si/UC
67.9
385
254
SAPO-34(T)
3.0 Si/UC
61.8
370
259
AlPO-18(E)
(no Si)
65.1
365
203
Figure 1. Water adsorption isotherms at 293 K for faujasite type zeolites (left) and chabazite types (right); FAU (from left to right): NaX, NaY, NaY(7), NaY(11), NaY(30); CHA (from left to right): CHA(nat.), SAPO-34(E), SAPO-34(T).
601
Water sorption in faujasite- and chabazite type zeolites
Figure 2. Heat curves (differential molar heat of adsorption, Q) of water in FAU type zeolites (left) and in CHA types (right); dashed line denotes heat of condensation at 303 K; FAU: NaLSX (top), 313 K (triangles), 333 K (circles), 372 K (triangles inverted), NaY(7), 303 K (triangles), 313 K (squares), 333 K (filled squares); CHA: CHA(nat.) 303 K (top), SAPO-34(T) 303 K (bottom)
be expected. CHA has a complex DTG profile showing at least two different maxima due to different adsorption sites in combination with the small windows of this structure influencing the kinetics. Figure 1 shows the water isotherms at 293 K for a series of faujasite type zeolites with decreasing Si/Al ratios and chabazite type zeolites with varying lattice composition, respectively. Both dealumination of Y as well as isomorphous substitution in SAPO’s regarding a reduced electric field in the cavities shift the water isotherms towards higher pressure because of decreasing heats of adsorption as can be seen in Figure 2. As an example Figure 2 shows the differential molar heats of adsorption of water for two FAU and two CHA samples. A systematic influence of the lattice composition can be stated here also. Table 2 results of the tests in the lab-scaled storage for selected FAU and CHA type zeolites Sample Desorption Adsorbed Tmax (storage) Energy density in temperature in K amount in kg/kg in K Wh/kg NaLSX 470 0.234 370 159 NaX
470
0.200
370
156
NaY
470
0.225
363
156
NaY(7)
470/420
0.183
340
155
NaY(11)
420
0.149
340
125
NaY(30)
420
0.125
330
95
SAPO-34(E)
470/420
0.195
350
150
SAPO-34(T)
470/420
0.186
345
130
Not only the position of the isotherms in Figure 1 but also the shape of the isotherms is important for the performance of the storage or heat pump. Especially the steep course
602
J. Jänchen et al.
of the SAPO isotherms is an advantage (compared with silica gel having a shallow isotherm) for the performance of adsorption heat pumps and storages. A model based on the Dubinin equation could be employed successfully by Stach et al. [5] to model the storage performance expectations based on the measured thermodynamic data. However, decreasing heats of adsorption (Figure 2, Table 1) reduce the storage density as well because of less charges in the cavities of the modified materials. Consequently, an optimization has to be carried out between the charging and discharging temperature, the storage density and the materials composition. Equivalent investigations have been made in a 1.5-L-storage with selected materials of different composition. Table 2 summarizes the results with respect to their charging (desorption) temperature, adsorbed amount of water, the discharging temperature (Tmax), and the storage density. As can be seen from Table 2 all parameters change systematically with the lattice composition. The charging temperature of the dealuminated FAU samples and of the SAPO’s can be lowered by at least 50 K without losing storage density. This is not the case for X an Y zeolites. On the other hand this advantage goes at the costs of the discharging temperature. Dependent on the case of application (industrial waste heat >470 K or solar heat <420 K) the Si/Al ratio or the lattice composition of the storage material can be tailored.
4. Conclusion In conclusion it can be stated that the water desorption temperature of zeolitic materials and other parameters can be controlled by modification of the lattice chemistry such as dealumination and isomorphous substitution of T atoms. Following this new route, zeolitic materials can replace silica gel with benefit for utilisation of solar heat or low temperature waste heat in thermochemical storage application and heat transformation processes. Acknowledgement We thank W. Schwieger, University of Erlangen-Nürnberg, Germany and H. Toufar, Süd-Chemie Zeolites GmbH, Bitterfeld, Germany for the supply of the SAPO-34 samples. The financial support by the German Federal Ministry of Economics and Technology, Grand No. 0329525F, is acknowledged. References [1] F. Meunier, J. Solar Energy Eng., 108 (1986) 257 [2] D.I. Tshernev, in: D.L. Bish and D.W. Ming, Editors, Natural zeolites: occurrence, properties, applications. Reviews in Mineralogy&Geochemistry, 45 (2001) 589-617 [3] J. Jänchen, D. Ackermann, H. Stach and W. Brösicke, Solar Energy, 76 (2004) 339 [4] J. Jänchen, D. Ackermann, E. Weiler, H. Stach and W. Brösicke, Thermochmica Acta, 434 (2005) 37 [5] H. Stach, J. Mugele, J. Jänchen and E. Weiler, Adsorption, 11 (2005) 393