MATERIALS AND INTERFACES Effect of the Cl/Br Molar Ratio of a CaCl2-CaBr2 Mixture Used as an Ammonia Storage Material Chun Yi Liu and Ken-ichi Aika* Department of Environmental Chemistry and Engineering, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama, Kanagawa 226-8502, Japan, and CREST, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama, 332-0012, Japan The behavior of ammonia absorption and desorption was studied for CaCl2-CaBr2 halide mixtures with various molar ratios prepared via the aqueous solution method. The CaCl2- CaBr2 halide mixture was revealed to form solid solutions in any molar contents. Ammonia absorption abruptly increased at some pressure, forming the ammine complex, and the (step) pressures were different for the samples with different molar contents. The step pressure decreased when the Cl/Br ratio was decreased for both absorption and desorption cycles. All samples absorbed ammonia irreversibly, up to two molecules of ammonia coordination. Three samplessCaCl1.33Br0.67, CaClBr, and CaCl0.67Br1.33sproved to separate a great deal of ammonia at pressures of 60-10 kPa at 298 K. These samples are promising candidates for ammonia storage material to be used for pressure swing separation in a new ammonia synthesis process. Especially, CaCl1.33Br0.67 treated at 523 K showed the best separation capacity (27.1 mmol/g). Introduction Alkaline-earth metal halides are expected as am- monia storage material. In the small-scale ammonia synthesis process with the new ruthenium catalysts, effective ammonia storage material that is workable under an ammonia pressure of 40-80 kPa is required.1 As reported in our previous work, some of the simple halides (MgClOH, CaCl2, CaBr2, and SrBr2) were suit- able for the temperature swing absorption (TSA) method.2 However, ammonia storage with the pressure swing absorption (TSA) method is economically superior to the TSA method. Ammonia absorption into alkaline-earth metal halide mixtures has been studied, together with their charac- terization.3 The alkaline-earth metal halide mixture with a common cation (CaCl2-CaBr2 and SrCl2-SrBr2) makes a solid solution,3,4 and the absorption isotherm is not the sum of individual isotherms of each salt component. This means a new phase interacts with ammonia differently from the way it interacts with each component. Among them, the CaCl2-CaBr2 halide mixture was determined to be a promising ammonia storage material that is workable using the PSA method at 60-10 kPa at 298 K. The storage capacity of the CaCl2-CaBr2 solid solution was high enough (16.4 mmol/g) to be used as a practical ammonia storage material. As has been reported,3 the CaCl2-CaBr2 halide mixture forms a solid solution with a slightly distorted rutile-type structure that is similar to that of pure CaCl2 and CaBr2. Here, one cation (Ca2+) is surrounded by six anions (Cl- or Br-), and one anion is surrounded by three cations. The coordination of different anions is thought to change the electric condition of cations, which controls the ammonia absorption behavior. The effect of the molar ratio on the CaCl2-CaBr2 halide mixture has only been studied in view of the desorption tem- perature control in a chemical heat pump.5 Now, studies such as absorption behavior are necessary to develop materials for the PSA method. Ammonia absorption into CaCl2 and CaBr2 is princi- pally an ammine complex formation; thus, the number of coordinated ammonia to the host cation (Ca2+) is an important index. Here, the process of the ammine complex formation of CaCl2 is divided into four reac- tions:2,6,7 Also, the ammine complex formation of CaBr2 is divided into four reactions: * Author to whom correspondence should be addressed. Tel.: +81-45-924-5416. Fax: +81-45-924-5441. E-mail:
[email protected]. CaCl2 + NH3 a CaCl2‚NH3 (1) CaCl2‚NH3 + NH3 a CaCl2‚2NH3 (2) CaCl2‚2NH3 + 2NH3 a CaCl2‚4NH3 (3) CaCl2‚4NH3 + 4NH3 a CaCl2‚8NH3 (4) CaBr2 + NH3 a CaBr2‚NH3 (5) CaBr2‚NH3 + NH3 a CaBr2‚2NH3 (6) CaBr2‚2NH3 + 4NH3 a CaBr2‚6NH3 (7) CaBr2‚6NH3 + 2NH3 a CaBr2‚8NH3 (8) 6994 Ind. Eng. Chem. Res. 2004, 43, 6994-7000 10.1021/ie049873i CCC: $27.50 © 2004 American Chemical Society Published on Web 09/30/2004 The equilibrium pressures of these ammine complex formation reactions are calculated from the Clausius- Clapeyron equation as follows: where ∆H and ∆S represent the enthalpy and entropy changes for the reaction, respectively, and these ther- modynamic data are summarized in Table 1.2,6,7 Also, the parameter P298 indicates the calculated pressure of the ammine complex formation at 298 K. Any such data about CaCl2-CaBr2 halide mixtures have not been reported. In this work, CaCl2-CaBr2 halide mixtures with various molar ratios (5:1, 4:2, 3:3, 2:4, and 1:5) were prepared to investigate the effect of the molar ratio on the solid-solution formation and the ammonia absorp- tion behavior. Finally, the possibilities of CaCl2-CaBr2 halide mixtures as an ammonia storage material for the PSA method are discussed. Experimental Section Sample Preparation and Characterization. CaCl2-CaBr2 halide mixtures were prepared by the aqueous solution-mixing method.3,5 CaCl2‚2H2O (99.9%; Wako Pure Chemicals) and CaBr2‚2H2O (99.5%; Wako Pure Chemicals) were used as the precursors for each mixed halide. The precursors were mixed in an aqueous solution with a prescribed molar ratio (between 5:1 and 1:5), stirred for a few minutes at room temperature, evaporated at 343 K, and dried in air at 383 K. Before the experiment, the sample was evacuated for 2 h at 723 or 523 K. Here, a pretreated sample is described as CaCl1.33Br0.67(523), when the CaCl2-CaBr2 mixture with a molar ratio of 4:2 is evacuated at 523 K. The sample structure was analyzed via X-ray diffrac- tometry (XRD) with an in situ XRD cell. The method of XRD measurement was the same as the method de- scribed earlier.3 Ammonia Absorption and Desorption Measure- ment. The ammonia absorption and desorption iso- therms were obtained using a volumetric method with an automatic gas adsorption apparatus (Beckman Coulter model OMNISORP 100CX).2,3 Sample grains (50 mg) were placed in the cell and pretreated at either 723 or 523 K with evacuation (at where V and Mw represent the amount of absorbed ammonia (in units of mmol/g) and the sample molecular weight (in units of g/mol), respectively. The molecular weight of the CaCl2-CaBr2 halide mixture is listed in Table 2. It is assumed that CaCl2-CaBr2 halide mix- tures are completely dehydrated, according to the ther- mogravimetry (TG) results of each component.2,3 The results of ammonia absorption measurement are summarized in Table 3.2 The absorbed amounts at 40, 60, and 80 kPa for the absorption cycle (Vabs) and those at 20, 10, and 5 kPa for the desorption cycle (Vdes) are shown. At the same time, the pressures at the step positions are also shown as Pabs for the absorption cycle and Pdes for the desorption cycle, and the step pressure was calculated from the inflection point of the isotherm if not clear. The results of ammonia absorption in the first measurement (total absorption) for CaCl2-CaBr2 halide mixtures pretreated at 523 K are shown in Figures 3-7, together with those for CaCl2 and CaBr2 treated at 523 K as references. The scale of the coordination number (N) in Figures 3-7 is the same scale as that for the CaCl2 sample (NCaCl2), for conven- ience. For the halide mixture, the number of ammonia molecules coordinated to the Ca2+ ion is calculated as given below: The conversion factor f for each sample is calculated from the molecular weight of the sample shown in each figure caption. For the first absorption cycle into CaCl1.67Br0.33(523) (shown in Figure 3), the amount of absorbed ammonia gently increased as the ammonia pressure increased from 10 kPa to 60 kPa and increased sharply at pressures of >60 kPa, reaching a value of 43.3 mmol/g at 80 kPa. For the first desorption cycle, the amount of absorbed ammonia was decreased in two steps, at 25.7 kPa and 10.6 kPa, and reached a value of 22.8 mmol/g at 10 kPa. For the second absorption cycle, the amount Figure 2. XRD peak position of the (111) surface for CaCl2-CaBr2 halide mixtures pretreated at (b) 523 K and (O) 723 K. Table 2. Molecular Weights of the CaCl2-CaBr2 Halide Mixtures sample composition molecular weight (g/mol) CaCl2 110.98 CaCl1.67Br0.33 125.80 CaCl1.33Br0.67 140.62 CaClBr 155.44 CaCl0.67Br1.33 170.25 CaCl0.33Br1.67 185.07 CaBr2 199.89 Table 3. Ammonia Absorption Capacity (Vabs, Vdes) and Phase-Changing Pressure (Pabs, Pdes) of CaCl2-CaBr2 Halide Mixtures at 298 K Absorption Cycle Desorption Cycle Vabs (mmol/g) Vdes (mmol/g) samplea run Pabs (kPa) at 40 kPa at 60 kPa at 80 kPa Pdes (kPa) at 20 kPa at 10 kPa at 5 kPa CaCl2(723)b first 67.3 0.2 0.4 27.5 2.4 26.8 26.6 26.0 second 76.2 0.5 0.7 10.8 38.2, 3.5 4.6 4.0 2.8 CaCl2(523)b first 29.3, 74.2 14.2 18.2 38.0 56.2, 5.7 30.6 29.7 21.4 second 76.6 0.5 0.6 13.2 47.0, 6.4 4.8 3.7 1.7 CaCl1.67Br0.33(723) first 76.6 7.8 11.5 28.5 17.8 26.5 19.4 19.2 second 62.8, 78.6 5.2 6.0 24.1 22.6 12.0 9.5 9.3 CaCl1.67Br0.33(523) first 64.2, 76.0 7.5 15.2 43.3 25.7, 10.6 28.0 22.8 20.1 second 5.3, 52.3, 73.6 4.3 11.6 36.2 25.7, 24.3 5.8 5.7 5.6 CaCl1.33Br0.67(723) first 75.0 5.5 20.7 47.1 21.2 30.4 26.3 26.1 second 61.1, 73.0 8.7 11.0 39.8 26.9 13.0 12.4 11.9 CaCl1.33Br0.67(523) first 12.6, 58.9, 72.8 16.2 29.5 54.1 18.2 42.4 29.0 28.1 second 5.4, 45.8, 55.9 8.8 40.9 43.1 13.7 39.3 13.8 11.0 CaClBr(723) first 26.5, 58.2 12.4 38.0 46.7 23.6 27.2 26.8 26.7second 6.9, 54.5 11.1 30.7 32.9 25.9 14.1 14.0 13.8 CaClBr(523) first 5.2, 56.9 17.9 41.2 47.0 25.1 27.7 27.0 26.8second 6.4, 56.7 10.5 30.3 33.1 27.4 14.1 13.9 13.8 CaCl0.67Br1.33(723) first 22.5, 59.4 20.1 35.2 45.6 26.6 32.7 32.3 32.2 second 6.4, 15.0, 57.8 14.7 29.0 31.2 29.1 19.4 19.2 19.0 CaCl0.67Br1.33(523) first 3.2, 41.4 25.0 44.3 45.9 13.3 43.6 31.8 31.0 second 3.2, 9.3, 43.7 15.7 31.4 33.6 15.1 30.5 18.6 18.4 CaCl0.33Br1.67(723) first 27.4, 68.3 20.5 26.6 31.8 28.1 29.7 29.6 29.6 second 13.2, 47.5, 62.8 15.6 19.2 20.8 33.6 19.3 19.2 19.2 CaCl0.33Br1.67(523) first 1.5, 53.7, 61.9 24.0 30.7 35.1 26.4 32.5 32.5 32.4 second 12.8, 59.2 17.3 21.2 22.6 32.6 20.8 20.8 20.8 CaBr2(723)a first 7.6 28.8 29.1 29.4 n.d. 29.0 28.9 28.9 second 4.8 19.4 19.8 20.1 n.d. 19.6 19.5 19.4 CaBr2(523)a first 0.7, 5.5 27.9 28.1 28.2 n.d. 28.2 28.2 28.2 second 4.7 18.6 18.7 18.9 n.d. 18.9 18.9 18.9 a Values given in parentheses represent the pretreatment temperature. b From ref 2. Nsample ) NCaCl2f (11) 6996 Ind. Eng. Chem. Res., Vol. 43, No. 22, 2004 of absorption increased at 51 kPa and reached a value of 36.2 mmol/g at 80 kPa. The difference in the absorbed amount of ammonia at 80 kPa between the first and second cycle indicates the irreversible ammonia absorp- tion into the sample. However, if the reversible absorp- tion-desorption cycle is operated in the pressure range of 60-10 kPa, only 6 mmol/g of ammonia can be separated. For the first absorption cycle, the amount of absorbed ammonia in CaCl1.33Br0.67(523) (shown in Figure 4) was gradually increased from 14 kPa to 60 kPa with a higher slope than that of CaCl1.67Br0.33(523) (see Figure 3) and increased at 60 kPa, reaching 54.1 mmol/g at 80 kPa. For the first desorption cycle, the amount of absorbed ammonia decreased at 40 kPa and decreased to a value of 29.0 mmol/g at 10 kPa. For the second absorption cycle, the amount of absorption was increased in two steps at 46 and 56 kPa, and reached a value of 43.1 mmol/g at 80 kPa. For the second desorption cycle, ammonia was reversibly desorbed at 20 kPa and the amount decreased to 13.8 mmol/g at 10 kPa. Ammonia was observed to be reversibly absorbed at 60 kPa and reversibly desorbed at 10 kPa, in the case of this sample, which could be a suitable material for ammonia separation. This shall be discussed in detail in the next section. Figure 5 shows the results of ammonia absorption into CaClBr(523) and desorption from CaClBr(523), which was reported earlier.3 If we examine the second absorp- tion and desorption, ∼20 mmol/g of ammonia was absorbed at 60 kPa and desorbed at 10 kPa reversibly. Thus, this sample is also suitable for the purpose of ammonia separation. Figure 6 shows the ammonia absorption and desorp- tion isotherms, in regard to CaCl0.67Br1.33(523). There are two steps of ammonia absorption, at 3 and 41 kPa, in the first cycle. The amount reached a value of 45.9 mmol/g. In the first desorption cycle, ammonia was desorbed at a pressure of 13 kPa and the amount decreased to 32 mmol/g at 10 kPa. In the second absorption and desorption cycles, ∼15 mmol/g of am- monia can be absorbed at 60 kPa and desorbed at 10 kPa reversibly. Thus, this sample also is a suitable material for our purpose. Figure 7 shows the ammonia absorption and desorp- tion isotherms, in regard to CaCl0.33Br1.67(523). The amount of absorbed ammonia sharply increased at low pressures and reached a value of 35.1 mmol/g at 80 kPa in the first absorption cycle. In the first desorption cycle, ammonia was desorbed in the pressure range of 30-20 kPa; however, the desorbed amount was small and the absorbed amount was still 32.5 mmol/g at 10 kPa. The second absorption and desorption cycles were similar to the first cycles. The ammonia separation capacity in Figure 3. Ammonia absorption isotherms (closed symbols) and desorption isotherms (open symbols) of CaCl2(523) (f ) 1.00), CaBr2(523) (f ) 1.80), and CaCl1.67Br0.33(523) (f ) 1.13) measured at 298 K: (2) CaCl2(523) first measurement, ([) CaBr2(523) first measurement, (b, O) CaCl1.67Br0.33(523) first measurement, and (9, 0) CaCl1.67Br0.33(523) second measurement. Figure 4. Ammonia absorption isotherms (closed symbols) and desorption isotherms (open symbols) of CaCl2(523) (f ) 1.00), CaBr2(523) (f ) 1.80), and CaCl1.33Br0.67(523) (f ) 1.27) measured at 298 K: (2) CaCl2(523) first measurement, ([) CaBr2(523) first measurement, (b, O) CaCl1.33Br0.67(523) first measurement, and (9, 0) CaCl1.33Br0.67(523) second measurement. Figure 5. Ammonia absorption isotherms (closed symbols) and desorption isotherms (open symbols) of CaCl2(523) (f ) 1.00), CaBr2(523) (f ) 1.80), and CaClBr(523) (f ) 1.40) measured at 298 K: (2) CaCl2(523) first measurement, ([) CaBr2(523) first measurement, (b, O) CaClBr(523) first measurement, and (9, 0) CaClBr(523) second measurement. Ind. Eng. Chem. Res., Vol. 43, No. 22, 2004 6997 the reversible absorption and desorption cycles a pres- sures of 60-10 kPa is estimated to be as low as 0.4 mmol/g. Discussion Ammonia Affinity to the CaCl2-CaBr2 Halide Mixture. From the results of ammonia absorption into the CaCl2-CaBr2 halide mixture (shown in Figures 3-7), the pressure of initial ammonia absorption at N ) 0.5 was defined as the index of ammonia affinity to these mixed halides. The pressures of initial ammonia absorption into CaCl2-CaBr2 halide mixtures were 27.8 kPa for CaCl1.67Br0.33(523), 14.1 kPa for CaCl1.33- Br0.67(523), 5.2 kPa for CaClBr(523), 3.2 kPa for CaCl0.67- Br1.33(523), and 1.2 kPa for CaCl0.33Br1.67(523). These pressures decreased as the molar ratio of Br- in the mixtures increased. This observation indicates the interaction between Ca2+, the host cation of the ammine complex formation, and ammonia, which increases as the amount of Br- cations increases in the Cl/Br mixture. This means that the affinity between ammonia and the CaCl2-CaBr2 halide mixture can be controlled by changing the molar ratio. Estimation of the Kinetic Factor. In the case of pure CaCl2 and CaBr2, the observed pressures of the ammine complex formation were higher than the cal- culated equilibrium pressure.2 This means that the measurement conditions of this work, which are the same as the previous conditions, are different from the “real” equilibrium conditions. This observation indicates that rate factors such as an excess of ammonia pressure are required to promote the ammonia absorption in the case of the CaCl2-CaBr2 halide mixture. The driving force for ammonia absorption into metal salt is afforded by the difference between the chemical potential of ammonia in the gas phase (µNH3,gas) and the chemical potential of ammonia equili- brated with the solid phase (µNH3,solid), where µNH3,gas ) RT ln PNH3,gas and µNH3,solid ) RT ln PNH3,solid. The fictitious pressure of ammonia equilibrated with the solid phase (PNH3,solid) is considered to be the equilib- rium pressure of each ammonia absorption reaction calculated from eq 9. However, the thermodynamic values (∆H and ∆S) for the CaCl2-CaBr2 halide mix- ture have not been reported; therefore, these values are assumed to be linearly related to the ratio of Cl/(Cl + Br). The calculated values are summarized in Table 4.2,6,7 The driving force (∆µ) is expressed as follows: where R and T are the gas constant and temperature, respectively. Now, the ∆µ value for the initial ammonia absorption into CaClBr(523) in the first measurement at 298 K is calculated as an example. The enthalpy and entropy values for CaClBr are calculated as ∆H ) 70185 J/mol and ∆S ) 230.18 J mol-1 K-1 (average value of CaCl2 and CaBr2), respectively, giving a pressure of PNH3,solid ) 0.534 Pa. From the shape of the ammonia absorption isotherm (see Figure 5), PNH3,gas ) 5.2 kPa. Thus, the driving force for initial ammonia absorption into CaClBr(523) is calculated as ∆µ ) 22.8 kJ/mol under these experimental conditions. The ∆µ values for each CaCl2-CaBr2 mixture for the first absorption run at 298 K are summarized in Table 4. Interestingly, note that all ∆µ values for mixed halides are in the range of 20-27 kJ/mol. If we try to find any tendency, ∆µ slightly decreases as the amount of Br- increases in the forma- tion of the mono-ammine complex but slightly increases as the amount of Br- increases in the formation of the di-ammine complex. Application of CaCl2-CaBr2 Halide Mixtures Using the Pressure Swing Absorption (PSA) Method for Ammonia Storage. The ammonia storage capacity with the pressure swing absorption (PSA) method was defined as the amount of absorption at 60 kPa (or 80 kPa) in the second absorption cycle sub- tracted by that at 10 kPa in the second desorption Figure 6. Ammonia absorption isotherms (closed symbols) and desorption isotherms (open symbols) of CaCl2(523) (f ) 1.00), CaBr2(523) (f ) 1.80), and CaCl0.67Br1.33(523) (f ) 1.53) measured at 298 K: (2) CaCl2(523) first measurement, ([) CaBr2(523) first measurement, (b, O) CaCl0.67Br1.33(523) first measurement, and (9, 0) CaCl0.67Br1.33(523) second measurement. Figure 7. Ammonia absorption isotherms (closed symbols) and desorption isotherms (open symbols) of CaCl2(523) (f ) 1.00), CaBr2(523) (f ) 1.80), and CaCl0.33Br1.67(523) (f ) 1.67) measured at 298 K: (2) CaCl2(523) first measurement, ([) CaBr2(523) first measurement, (b, O) CaCl0.33Br1.67(523) first measurement, and (9, 0) CaCl0.33Br1.67(523) second measurement. ∆µ ) µNH3,gas - µNH3,solid ) RT ln( PNH3,gasPNH3,solid) (12) 6998 Ind. Eng. Chem. Res., Vol. 43, No. 22, 2004 cycle.2,3 The calculation was performed using the data of Table 3 and is summarized in Table 5. Some of the data (the sample treated at 523 K) is illustrated in Figure 8. As shown in Table 5 and Figure 8, the ammonia storage capacity of CaCl2-CaBr2 halide mixtures with molar ratios of 4:2, 3:3, and 2:4 is greater than that of other halide mixtures (with molar ratios of 5:1 and 1:5). These samples (CaCl1.33Br0.67, CaClBr, and CaCl0.67- Br1.33) can absorb and desorb a sufficient amount of ammonia reversibly at 60-10 kPa. Among these samples, the ammonia storage capacity of CaCl1.33Br0.67(523) is 27.1 mmol/g, which is 21 times as high as that of the Na form of Y-zeolite as a reference sample, under the same conditions. The affinity between ammonia and CaCl2 is lower than that between ammonia and CaBr2. Here, the affinity between ammonia and CaCl2 is increased by the Br- mixing, so that the step pressure of ammonia absorption into and desorption from CaCl2 becomes lower and suitable. Because the step pressure falls into the range of 60-10 kPa on samples such as CaCl1.33- Br0.67, CaClBr, and CaCl0.67Br1.33, the intended PSA cycle can be achieved. Conclusion The behavior of ammonia absorption into and desorp- tion from CaCl2-CaBr2 halide mixtures with various molar ratios prepared by the aqueous solution method were studied. From the X-ray diffractometry (XRD) measurement, any CaCl2-CaBr2 halide mixture was revealed to form a solid solution with a single phase. The ammonia pressure at which stepwise absorption occurred was decreased gradually when the amount of Br- was increased in the CaCl2-CaBr2 mixture for both absorption and desorption cycles. Three samples (CaCl1.33- Br0.67, CaClBr, and CaCl0.67Br1.33) proved to have a step pressure (in the ammonia absorption and desorption cycles) between the ranges of the absorption and de- sorption pressure of pressure swing absorption (PSA). These samples can be used as ammonia storage material when working with the PSA method for a small-scale ammonia synthesis process. Especially, the CaCl1.33- Table 4. Thermodynamic Data (∆H, ∆S) and Driving Force (∆µ) for Ammine Complex Formation of the CaCl2-CaBr2 Halide Mixtures at 298 K Pobs (kPa)c ∆µ (kJ mol) sample composition ∆H (J/mol)a ∆S (J mol-1 K-1)a P298 (Pa)b 523 Kd 723 Kd 523 Kd 723 Kd Mono-ammoniate Formatione CaCl2 69,052 234.14 1.36 29.3 67.3 24.7 26.8 CaCl1.67Br0.33 69,430 232.82 0.994 27.8 47.8 25.4 26.7 CaCl1.33Br0.67 69,807 231.50 0.728 14.1 34.7 24.5 26.7 CaClBr 70,185 230.18 0.534 5.2 24.2 22.8 26.6 CaCl0.67Br1.33 70,563 228.86 0.391 3.2 16.8 22.3 26.4 CaCl0.33Br1.67 70,940 227.54 0.287 1.2 23.3 20.7 28.0 CaBr2 71,318 226.22 0.210 0.7 6.6 20.1 25.7 Di-ammoniate Formationf CaCl2 63,193 237.34 21.2 37.6 67.3 18.5 20.0 CaCl1.67Br0.33 65,834 240.09 10.2 52.0 60.6 21.2 21.5 CaCl1.33Br0.67 68,474 242.83 4.87 27.3 50.3 21.4 22.9 CaClBr 71,115 245.58 2.34 11.3 34.1 21.0 23.8 CaCl0.67Br1.33 73,756 248.33 1.12 5.5 23.6 21.1 24.7 CaCl0.33Br1.67 76,397 251.07 0.537 4.0 27.4 22.1 26.9 CaBr2 79,037 253.82 0.258 1.5 7.6 21.5 25.5 a The values for CaCl2 and CaBr2 are obtained from refs 6 and 7, and the values for mixture samples are calculated based on the Cl/Br ratio. b Calculated from the Clausius-Clapeyron equation (ref 2). c Pressure through N ) 0.5 (for mono-ammoniate formation) or N ) 1.5 (for di-ammoniate formation). d Pressure at the given pretreatment temperature. e Reaction of CaCl2-xBrx f CaCl2-xBrx‚NH3. f Reaction of CaCl2-xBrx‚NH3 f CaCl2-xBrxf2NH3. Table 5. Ammonia Storage Capacity of CaCl2-CaBr2 Halide Mixture for Pressure Swing Absorption Operated at Absorption Pressures of 60-80 kPa and a Desorption Pressure of 10 kPa at 298 K ∆V (mmol/g)a sample absorption at 80 kPa absorption at 60 KPa CaCl2(723)b 6.8 0.0 CaCl2(523)b 9.5 0.0 CaCl1.67Br0.33(723) 14.6 0.0 CaCl1.67Br0.33(523) 30.5 5.9 CaCl1.33Br0.67(723) 27.4 0.0 CaCl1.33Br0.67(523) 29.3 27.1 CaClBr(723) 18.9 16.7 CaClBr(523) 19.2 16.4 CaCl0.67Br1.33(723) 12.0 9.8 CaCl0.67Br1.33(523) 15.0 12.8 CaCl0.33Br1.67(723) 1.6 0.0 CaCl0.33Br1.67(523) 1.8 0.4 CaBr2(723)b 0.6 0.3 CaBr2(523)b 0.0 0.0 Na-Y(723)c 1.3 a Amount of absorption at 80 or 60 kPa subtracted from that at 10 kPa in the desorption cycle (from Table 3). b Data taken from ref 2. c Used as a reference sample. Figure 8. Ammonia storage capacity using the pressure swing absorption (PSA) method at 298 K in the pressure range of 60- 10 kPa for CaCl2-CaBr2 halide mixture pretreated at 523 K. Ind. Eng. Chem. Res., Vol. 43, No. 22, 2004 6999 Br0.67 sample treated at 523 K showed the best perfor- mance (27.1 mmol/g) of the ammonia storage capacities. Literature Cited (1) Aika, K.; Kakegawa, T. On-Site Ammonia Synthesis in De- NOx Process. Catal. Today 1991, 10, 73-80. (2) Liu, C. Y.; Aika, K. Ammonia Absorption on Alkaline Earth Metal Halides as Ammonia Separation and Storage Procedure. Bull. Chem. Soc. Jpn., 2004, 77, 123-131. (3) Liu, C. Y.; Aika, K. Ammonia Absorption into Alkaline Earth Metal Halide Mixture as an Ammonia Separation Material. Ind. Eng. Chem. Res., in press. (4) Hodorowicz, S. A.; Eick, H. A. Phase Relationships in the System SrBr2-SrCl2. J. Solid State Chem. 1982, 43, 271-277. (5) Saito, T. Jpn. Kokai Tokkyo Koho 1994, JP 06136357. (6) Neveu, P.; Castaing, J. Solid-Gas Chemical Heat Pumps: Field of Application and Performance of the Internal Heat of Reaction Recovery Process. Heat Recovery Syst. CHP 1993, 13, 233-251. (7) In International Critical Tables of Numerical Data, Physics, Chemistry and Technology; McGraw-Hill: New York, 1929; Vol. 7, pp 224-313. Received for review February 16, 2004 Revised manuscript received June 22, 2004 Accepted July 28, 2004 IE049873I 7000 Ind. Eng. Chem. Res., Vol. 43, No. 22, 2004