Ammonia sorption on composites CaCl 2 in inorganic host matrix : isosteric chart and its performance

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1 Ammonia sorption on composites CaCl 2 in inorganic host matrix : isosteric chart and its performance Vasily E. Sharonov 1, Janna V. Veselovskaya 2 and Yury I. Aristov 1 (corresponding author) 1 Boreskov Institute of Catalysis, Pr. Lavrentieva, 5, Novosibirsk, Russia 2 Novosibirsk State University, Ul. Pirogova, 2, Novosibirsk, Russia aristov@catalysis.ru Abstract The isosteres of ammonia sorption on new composite sorbents CaCl 2 /g-al 2 O 3 and CaCl 2 /vermiculite were measured at T = C and P = bar. It was found that the modification of host matrices by the salt dramatically increases the ammonia uptake due to the formation of CaCl 2 nnh 3 complexes. The isobaric enthalpy and entropy of ammonia sorption by the salt confined to alumina pores were significantly lower than those for the bulk one. For a basic cycle, the values of COP = were calculated at T e = 18 C, T c = C and T g = C. High COP could make these sorbents promising for application in cooling units driven by relatively low temperature heat. Keywords ammonia sorption; calcium chloride; alumina; vermiculite; cooling COP Nomenclature T temperature, C or K P pressure, bar COP coefficient of performance w uptake, g/g m mass, g R universal gas constant, 8.31 J/(mol K) V volume, m 3 H enthalpy, kj/mol S entropy, J/(mol K) Q heat, J/kg C p specific heat, J/(g K) w salt content, g/g Subscripts 0 initial eq equilibrium e evaporator c condenser g energy source Introduction Three main processes in sorption heat pumps can be categorised as a gas adsorption on a porous solid, a chemical reaction between a gas and a solid, and a gas absorp-

2 192 V. E. Sharonov, J. V. Veselovskaya and Y. I. Aristov tion by a liquid. Intelligent combination of these three processes in composites salt confined to porous host matrix can give a valuable opportunity to reach an augmentation and even synergism of advantages of each process [1]. It was proved by a comprehensive study of advanced composites for sorption of water vapour (so called Selective Water Sorbents) [1 3]. Contribution of each process mentioned was shown to strongly depend on the chemical nature of the salt and host matrix, pore structure of the matrix, salt content, water uptake, etc. [3]. Similar composites for ammonia sorption have been synthesized on a basis of carbon matrices: the expanded graphite mixed with a variety of salts [4 6], the synthetic carbon Busofit [7, 8] and the activated carbons [9] impregnated with CaCl 2. These materials have higher sorption capacity in comparison with non-impregnated carbons and better kinetics than the bulk salt. However, the salt swelling due to its reaction with ammonia could destruct the carbon matrix [9], so the choice of a proper host matrix is important from a practical point of view. First attempts to combine CaCl 2 with inorganic matrices were done in [10, 11]. It was found that the modification of alumina by the salt can significantly enhance its ability to absorb ammonia. An additional amount of NH 3 was assumed to be absorbed due to the reversible chemical reaction between the salt and ammonia; CaCl 2 nnh 3 + mnh 3 CaCl 2 (n + m)nh 3 No detailed study on the equilibrium of ammonia sorption by the salt confined to inorganic host matrices has been done so far. Here we present experimental isosteres of NH 3 sorption on composites CaCl 2 /g-alumina and CaCl 2 /expanded vermiculite as well as make a comparison with the bulk salt. Experiment The sorbents 21.5 wt. % CaCl 2 /g-al 2 O 3 (II) and 63.5 wt. % CaCl 2 /vermiculite (III) were prepared by the immersion of the matrix with an aqueous 40% wt. solution of CaCl 2 followed by its drying at 200 C. For synthesis of the sorbent 14.5 wt. % CaCl 2 /g-al 2 O 3 (I) the matrix was impregnated with an aqueous 25% wt. solution of CaCl 2 followed by drying at 200 C. The main characteristics of used matrices are listed in Table 1. The typical grain size was mm. Isosteres of the ammonia sorption were measured using a set-up illustrated in Fig. 1. The core of this plant was a stainless steel cylindrical adsorber (1) 4 cm in diameter (volume cm 3 ) filled with the sorbent. Two thermocouples were placed in the middle of the adsorber and near the wall to provide a direct reading of the sorbent Table 1. The characteristics of matrices used for sorbent preparation Matrix Pore volume, cm 3 /g Surface area, m 2 /g Average pore diameter, nm g-al 2 O Vermiculite

3 Ammonia sorption on composites CaCl 2 in inorganic host matrix 193 Figure 1. The scheme of experimental set-up for measuring the isosteres of NH 3 sorption. temperature and its gradient along the bed. The adsorber was placed into a water thermostat (2) to maintain the sorbent at a constant temperature in the range of 20 to 90 C. The temperature of the bath could be held constant within a variation of ±0.1 K. Before isosteric experiments the sample was heated in an oven at 200 C for 12 h, cooled down to room temperature in dry atmosphere and its dry weight m o was measured. Typical dry weight was 97, 102 and 46 g for samples I, II and III, respectively. After that, the sample was placed in the adsorber and evacuated at 50 C for 1h down to the residual pressure 0.1 mbar (valves V1 V3 were open, V4 was closed). A certain amount of ammonia was absorbed on the sample at room temperature T according to the following procedure: a) valves V1 and V3 were closed; b) valve V4 was open to connect a buffer tank (3) with a high-pressure container of NH 3 (4) and increase the pressure in the tank to P 0 (P bar). The pressure was measured by a pressure gauge DM 5001E Y2 (5) with the accuracy of 0.01 bar; c) valve V4 was closed and valve V1 was open to start ammonia adsorption on the sample; d) valve V2 was closed when the pressure in the tank decreased to P 1, which was preliminary estimated to reach a given (minimal) ammonia uptake in the sample; e) equilibrium pressure over the sample P eq was measured after the sorption was finished (2 20 h). The initial uptake was calculated as: w 1 È( P0 - P1) V PV eq 1 m = Í - Î RT RT, m0 (1)

4 194 V. E. Sharonov, J. V. Veselovskaya and Y. I. Aristov where T is the room temperature (K), V is the tank volume (V = m 3 ), V 1 is the volume of the gas phase restricted by valve V2, R is a universal gas constant (R = 8.31 J/(mol K)), m is the molecular weight of ammonia. The dead volume V 1 included the volume of the pressure gauge, the valve V1, the connecting lines and the volume of the gas phase in the adsorber. The latter volume is the volume of adsorber minus the volume occupied by a solid phase of adsorbent skeleton. The dead volume was estimated to be V 1 = (0.93 ± 0.05) 10 4, (0.95 ± 0.05) 10 4 and 1.16 ± 0.05) 10 3 m 3 for samples I, II and III. It is worthy to mention that the second term in equation (1) is much smaller than the first one as V 1 < V and P eq << (P 0 P 1 ). The accuracy of the uptake determination was about g/g. The NH 3 pressure over the sorbent with the initial uptake w 1 was measured as a function of temperature at constant volume V 1 (valve V1 was open, valve V2 was closed). After the first reading at minimal temperature (about C) the bath temperature and, hence, the sorbent temperature was increased and new equilibrium pressure P(T) of ammonia was recorded. Plotting ln(p) vs. 1/T, we obtained the isostere of ammonia sorption, which corresponds to the uptake w 1. Then, the uptake was increased to w 2 > w 1, and new isostere was measured, and so on. To increase the NH 3 uptake an additional amount of ammonia was absorbed on the sample from the buffer tank. The procedure was as follows: a) the sample was cooled down to the room temperature, b) valve V2 was open for a certain time until the pressure in the tank decreased to preliminary estimated value P 2 < P 1, c) equilibrium pressure over the sample P eq was measured after the sorption was finished (2 20 h). The equilibrium uptake w 2 can be calculated according to similar methodology as described above. The drawback of this procedure is that during measuring a single isostere the uptake is not strictly constant because some amount of ammonia is desorbed from the sample as the temperature increases. During our tests the maximal increase of NH 3 pressure in the dead volume V 1 was less than 6 8 bar even at high uptakes ( g/g), which corresponds to the desorption of 0.5 g NH 3 or g/g. At low uptakes ( g/g) the amount of ammonia desorbed is g NH 3 or g/g. In both the cases the change in uptake is much less than the average uptake. Further verification of the experimental procedure applied in this work was done by measuring the isostere of ammonia sorption by a bulk CaCl 2 which corresponds to the transformation CaCl 2 2NH 3 + 2NH 3 CaCl 2 4NH 3 (see Appendix). Results and discussion Isosteres of ammonia sorption Composite sorbent CaCl 2 /γ-al 2 O 3 Isosteres of the ammonia sorption on the composite I are shown in Fig. 2. They are straight lines Ln P = A + B/T, so that the slop B = H/R allows the determination

5 Ammonia sorption on composites CaCl 2 in inorganic host matrix 195 Figure 2. The isosteres of NH 3 sorption on composite I, corresponding to different uptakes (symbols) and the pressure of NH 3 over bulk CaCl 2 4NH 3 (- -) and CaCl 2 8NH 3 ( -) [12]. Table 2. The enthalpy and entropy of NH 3 sorption on composite I Ammonia uptake H, S, g/g mol/mol CaCl 2 kj/mol J/(mol K) of enthalpy of the ammonia sorption, while the intercept A = S/R gives the sorption entropy. The average values of H and S over temperature range C are displayed in Tables 2 and 3. The increase of CaCl 2 content in the composite leads to the rise in the equilibrium ammonia uptake (see isotherms plotted from the experimental isosteres, Fig. 3). Besides, the transitions between various ammonia-salt complexes became sharper (Fig. 3). As, in principle, it is profitable to increase the amount of salt inside

6 196 V. E. Sharonov, J. V. Veselovskaya and Y. I. Aristov Figure 3. The isotherms of ammonia sorption on composites I (squares), II (circles) and III (triangles) at T = 50 C calculated from experimental isosteres. Table 3. The enthalpy and entropy of ammonia sorption on composite II Ammonia uptake H, S, g/g mol/mol CaCl 2 kj/mol J/(mol K) pores, materials with large pore volume could be attractive as hosts for new composite sorbents of NH 3, for instance, vermiculite that will be considered below. One can see from Figs 2 and 3 that the sorption properties of the confined salt differ from those of a bulk salt: a) transitions between complexes CaCl 2 nnh 3 are shifted to higher ammonia pressure in comparison with those in bulk [12]; b) both the enthalpy and entropy of reaction (1) for the confined salt are significantly reduced from those for the bulk one (Tables 2 and 3); c) the equilibrium of reaction (1) in the confined state is divariant that means the NH 3 uptake increases continuously as the ammonia pressure rises;

7 Ammonia sorption on composites CaCl 2 in inorganic host matrix 197 d) the solid compound CaCl 2 8NH 3 does not form inside alumina pores. Indeed, when the molar ratio NH 3 :CaCl 2 increased to 6 :1 7 :1, the pressure above the sample increases drastically and the enthalpy and entropy tend to reach the corresponding values for liquid ammonia. This could be possibly caused by the formation of a CaCl 2 /ammonia solution inside the pores. Simultaneous reduction of the enthalpy and entropy of ammonia sorption indicates that in the confined state ammonia is bound with the salt less strongly than in the bulk state. For understanding this experimental finding a detailed study of the structure and molecular vibrations of the confined salt and its complex with ammonia is necessary. In general, the difference in the thermodynamic properties of the confined and bulk CaCl 2 could be caused either by the contribution of the surface energy of salt nanocrystallites confined to the pores of alumina or by the interaction between the salt and the host matrix. Composite sorbent CaCl 2 /vermiculite As it was mentioned above, one of the most promising host matrices could be an expanded vermiculite, of which pore volume reaches 3.2 cm 3 /g [13]. It allows inserting inside the pores a large amount of salt, for instance, 63.5 wt % of CaCl 2 in our case. The sorption capacity of composite III is about 5 times higher than that for II (Fig. 3). The enthalpy and entropy of sorption are close to the literature values for the bulk salt (Table 4). This is probably due to the big enough crystallites of the salts confined to the relatively large pores of vermiculite (Table 1). One more very important advantage of vermiculite, which results from its large pore volume, is that it can accommodate the salt swelling due to its reaction with ammonia discussed in [9]. Estimation of the freezing COP The coefficient of performance (COP) is accepted as the most important thermodynamic parameter of sorption heating/cooling device. The experimentally obtained isosteres were used for plotting a single-effect non-regenerative freezing cycle based on the working pair NH 3 CaCl 2 /matrix (g-al 2 O 3, vermiculite) as well as for a Table 4. The enthalpy and entropy of ammonia sorption on composite III Ammonia uptake H, S, g/g mol/mol CaCl 2 kj/mol J/(mol K)

8 198 V. E. Sharonov, J. V. Veselovskaya and Y. I. Aristov Figure 4. Freezing cycle: the liquid ammonia curve (1) and experimental isosters of ammonia sorption on composite II at the NH 3 uptakes 0.20 g/g (2) and g/g (3). bulk CaCl 2 as it is shown in Fig. 4. The evaporator temperature was fixed at T e = 18 C, while T c and T g were obtained geometrically and appeared slightly different for various adsorbents (Table 5). The input parameters of the cycle (T ev, T c, T g ) are presented in Table 5 and Fig. 4. We estimated the COP as COP Qev ( w2 -w1) = Q ( w -w )+ C w + C ( 1-w )+ C w T T ( )( - ) des 2 1 pcacl2 CaCl2 pmatrix CaCl2 pnh3 2 g c where Q ev is the heat of NH 3 evaporation, Q des is the isosteric heat of NH 3 desorption, w 1 and w 2 are the NH 3 uptakes corresponding to weak and rich isosteres (g/g), C pcacl2, C pmatrix and C pnh3 is the specific heat of CaCl 2, matrix and ammonia absorbed, w CaCl2 is the salt content. For our estimations we took Q ev = 1370 J/g, C pcacl2 = 0.7 J/(g K), C pmatrix = 0.9 J/(g K) and C pnh3 = 3.2 J/(g K). We consider the isosteric heat averaged between the weak and rich isosteres as Q des, namely, 2.1 kj/g for CaCl 2 /alumina, 2.3 kj/g for CaCl 2 /vermiculite and 2.48 kj/g for bulk CaCl 2. Tc 1- Tg The Carnot COP was calculated according to [14] as COPCarnot = T. The c -1 Tev results are listed in Table 5. The estimated values of COP for the tested sorbents are rather large ( ) for such low T e, high T c and relatively low T g, still remaining times smaller than the Carnot COP. The main reason for such difference is probably the entropy generation due to the irreversibility of the energy transfer process from an external heat source at the temperature T g to the adsorber at temperature T, for which the following relation is valid T c T T g.

9 Ammonia sorption on composites CaCl 2 in inorganic host matrix 199 Table 5. The input parameters, calculated and theoretical (Carnot) COP of freezing cycle involved Sorbent w 1, g/g w 2, g/g T e, C T c, C T g, C COP, calc. COP, Carnot I II III bulk CaCl Conclusions Composite ammonia sorbents 14.5 wt. % CaCl 2 /g-al 2 O 3, 21.5 wt. % CaCl 2 /g- Al 2 O 3 and 63.5 wt. % CaCl 2 /vermiculite were synthesized and studied. It was found that the modification of host matrices by the salt dramatically increases the ammonia uptake (up to 0.7 g/g) due to the formation of CaCl 2 nnh 3 complexes. Thermodynamic properties of CaCl 2 confined to alumina appeared to differ from the properties of the bulk salt, namely, both the enthalpy and entropy of ammonia sorption were significantly lower in the confined state. The transitions between complexes CaCl 2 nnh 3 confined to alumina pores were shifted to higher ammonia pressure in comparison with those in bulk. For composite based on vermiculite the enthalpy and entropy of sorption were close to the literature values for the bulk salt. The COP of the single-effect non-regenerative freezing cycle was estimated to be The high COP makes these sorbents attractive for application in freezing units driven by relatively low temperature heat. This is especially true for CaCl 2 /vermiculite which can accommodate the salt swelling during reaction between ammonia and the salt. Acknowledgements The authors thank the Russian Foundation for Basic Researches (grants , and ) for partial financial support. References [1] Yu. I. Aristov, G. Restuccia, G. Cacciola, V. N. Parmon, A family of new working materials for solid sorption air conditioning systems, Appl. Therm. Engn. 22 (2002), [2] Yu. I. Aristov, M. M. Tokarev, G. Cacciola, G. Restuccia, Selective water sorbents for multiple applications: 1. CaCl 2 confined in mesopores of the silica gel: sorption properties, React. Kinet. Cat. Lett., 59 (1996), [3] Yu. I. Aristov, An optimal sorbent for adsorption heat pumps: thermodynamic requirements and molecular design in Proc. V Minsk International Seminar Heat Pipes, Heat Pumps, Refrigerators, Minsk, September, 2005, [4] M. Balat, B. Spinner, Optimization of a chemical heat pump: energetic density and power, Heat Recovery Systems & CHP, 13 (1993),

10 200 V. E. Sharonov, J. V. Veselovskaya and Y. I. Aristov [5] B. Spinner, Ph. Touzain, Application of exfoliated graphite intercalation compounds in order to improve the performance of thermochemical energy conversion process in Workshop Carbone, Paris, July, [6] K. Wang, J. Y. Wu, R. Z. Wang, L. W. Wang, Composite adsorbent of CaCl 2 and expended graphite for adsorption ice makers on fishing boats, Int. J. Refrig., 29 (2006), [7] L. L. Vasiliev, L. E. Kanonchik, A. A. Antujh, A. G. Kulakov, NaX, carbon fibre and CaCl 2 ammonia reactors for heat pumps and refrigerators, Adsorption, 2 (1996), [8] L. Vasiliev, D. Nikanpour, A. Antukh, K. Snelson, L. Vasiliev Jr., A. Lebru, Multisalt-carbon chemical cooler for space applications, J. Engineering Physics & Thermophysics, 72 (1998), [9] L. W. Wang, R. Z. Wang, J. Y. Wo, K. Wang, Compound adsorbent for adsorption ice maker on fishing boats, Int. J. of Refrigeration, 27 (2004), [10] R. Critoph, Properties of a Hybrid Adsorbents with Ammonia Refrigerant in 3rd Int. Heat Powered Cycles Conference, Larnaca, October, [11] V. E. Sharonov, Yu. I. Aristov, Ammonia adsorption by MgCl 2, CaCl 2 and BaCl 2 confined to porous alumina: fixed bed adsorber, Reaction Kinetics and Catalysis Lett., 85 (2005), [12] Ph. Tuzan, Thermodynamic values of ammonia-salts reactions for chemical sorption heat pumps in ISHPC 99, Proc. Of the Int. Sorption Heat Pump Conf., Munich, March, 1999, [13] L. G. Gordeeva, E. M. Moroz, N. A. Rudina, Yu. I. Aristov, Formation of vermiculite pore structure during its expantion, Rus. J Appl. Chem., 75 (2002), [14] W. M. Raldow, W. E. Wentworth, Chemical heat pumps a basic thermodynamic analysis, Solar Energy, 23 (1979), [15] R. Carling, Dissociation pressures and enthalpies of reaction in MgCl 2 nh 2 O and CaCl 2 nnh 3, J. Chem. Thermodynamics, 13 (1981), Appendix Verification of the method of isostere measurement To verify the experimental procedure applied in this work we measured the isostere of ammonia sorption by a bulk CaCl 2 which corresponds to the transformation CaCl 2 2NH 3 + 2NH 3 = CaCl 2 4NH 3. A mixture of 15.4 g CaCl 2 (crystal size less than 0.25 mm) and 250 g of cast-iron fillings was placed in the adsorber to decrease its dead volume and to improve the heat transfer. The initial average uptake was 2.7 mol of ammonia per 1 mol of CaCl 2. The average isosteric enthalpy and entropy of ammonia sorption at T = C were measured to be 42.6 kj/mol and J/(mol K), respectively. Good agreement with literature data [15] ( H = 42.1 kj/mol, S = J/(mol K) gives the evidence of applicability of this method for measuring isosteres of the ammonia sorption.

AN OPTIMAL SORBENT FOR ADSORPTION HEAT PUMPS: THERMODYNAMIC REQUIREMENTS AND MOLECULAR DESIGN

VI Minsk International Seminar Heat Pipes, Heat Pumps, Refrigerators AN OPTIMAL SORBENT FOR ADSORPTION HEAT PUMPS: THERMODYNAMIC REQUIREMENTS AND MOLECULAR DESIGN Yury I. Aristov Boreskov Institute of