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Effect of desorption temperature on CO2 adsorption equilibria of the honeycomb zeolite beds
Applied Energy 72 (2002) 555–564 www.elsevier.com/locate/apenergy
Effect of desorption temperature on CO2 adsorption equilibria of the honeycomb zeolite beds K. Kamiutoa,*, S. Abeb, Ermalinab a
Department of Production Systems Engineering, Oita University, Oita 870-1192, Japan b Graduate School of Oita University, Oita 870-1192, Japan Received 7 May 2002; received in revised form 30 May 2002; accepted 2 June 2002
Abstract The behaviours of MS-13X and MS-4A were examined. The fitness of the Dubinin–Astakhov equation to the adsorption equilibria of the fully-desorbed honeycomb beds was examined. The validity of the Langmuir approximation to the obtained adsorption equilibria for relatively low CO2 partial pressures was also discussed. The CO2 adsorption equilibria of the honeycomb zeolite beds are not affected by it’s desorption temperature, as long as the examined honeycomb beds are desorbed at a temperature greater than 250 C, and are well correlated with the Dubinin–Astakhov equation, irrespective of adsorption temperature. The Langmuir approximation works well for CO2 partial pressures less than about 0.2 bar, but the accuracy of the approximation deteriorates as the adsorption temperature increases. # 2002 Elsevier Science Ltd. All rights reserved.
1. Introduction Honeycomb zeolite beds have been utilized for dehumidification and recovery of evaporated organic-solvents in many industries because of their enormous specific surface areas and small flow resistances. In addition to these traditional applications, the honeycomb zeolites have also been considered for use for the CO2 recovery from power plants [1] and, in relation to this application, the CO2 adsorption characteristics
* Corresponding author. Tel.: +81-097-554-7797; fax: +81-097-554-7790. E-mail address: [email protected] (K. Kamiuto). 0306-2619/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S0306-2619(02)00048-X
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Nomenclature A a,b C E n P Ps(T) q R T w
adsorption potential (kJ/mol) Langmuir parameters CO2 concentration (kg-CO2/m3) characteristic energy (kJ/mol) exponent partial pressure of CO2 (Pa) saturated-vapour pressure of CO2 (Pa) amount adsorbed per unit mass of a bed (kg-CO2/ kg-bed) general gas constant (kJ/mol K) temperature (K) maximum volume of adsorption space (m3/kg-bed)
Greek letters * liquid density of adsorbate (kg/m3) Physical properties of CO2 required for the Dubinin-Astakhov plots (1) Density of adsorbed liquid CO2 at T (K) *=1.0276103[1.912573.0 (T/1000)],(kg-CO2/m3) (2) Saturated-vapour pressure of CO2 for T < 304.19(K): Ps=107[15.54953+175.8642(T/1000)682.9149(T/1000)2+923.1018 (T/1000)3], (Pa) for T > 304.19 (K), Ps=7.382106exp(9.98144 3.036254103/T), (Pa).
of the honeycomb zeolites are needed. Under these circumstances, the authors [2] have determined the CO2 adsorption equilibria of the honeycomb zeolite beds consisting of MS-13X or MS-4A1 from breakthrough curves for various CO2–N2 mixtures at 293 K and examined the fitness of the Dubinin–Astakhov equation [3] with the exponent n=3 to the obtained data. However, the desorption temperature of the tested beds were relatively low, i.e., 150 C and thus the desorption might be incomplete in our previous study. The purpose of the present study is threefold: first, to examine the effect of the desorption temperature on the CO2 adsorption equilibria of the honeycomb zeolite beds, which are composed of thin fibrous walls with the porosity about 0.8 and hold MS-13X MS-4A as an adsorbent, by using the constant volume method; second, to address the fitness of the Dubinin–Astakhov equation to the CO2 adsorption equilibria not influenced by the desorption temperature; and finally, to discuss the 1 MS stands for ‘‘molecular sieve’’. The rational formula of 13X-type zeolite is Na86[(Al2)86 (SiO2)86].276H2O, whereas that of 4A-type zeolite is Na12[(AlO2)]12(SiO2)12].27H2O. Moreover, the micro pore sizes of MS-4A and MS-13X are 3.5 A˚ and 9 A˚ respectively.
K. Kamiuto et al. / Applied Energy 72 (2002) 555–564
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validity of the Langmuir approximation to the obtained adsorption equilibria for relatively low CO2 partial pressures.
2. The Experiment A schematic diagram of the experimental apparatus—BELSORP-HP(BELL JAPAN INC)—for measuring the CO2 adsorption characteristics of the honeycomb zeolite adsorbents is shown in Fig. 1. The major part of the measuring system is made of a Pyrex glass tube with a stopcock, in which an about 0.4 g honeycomb adsorbent was placed. The zeolite contents of the utilized honeycomb beds were about 43 wt.% of the beds. The wall thickness is 2104 m. More details of the examined honeycomb zeolite beds were described in Ref. [2] and, hence, are not repeated here. Before the experiment, the desorption of the adsorbent was performed: the test tube including a sample was heated in an electrically-heated furnace, kept at various temperatures ranging from 150 to 375 C, and was evacuated continuously below 2 Pa using a vacuum pump. This process took about 4 h. At the end of the desorption, the attached stopcock was closed and then the test tube was naturally cooled-down in the air. In the adsorption experiment, the test tube was installed in a thermostat, which was controlled at various temperatures, i.e., 20, 30, 50 and 70 C and was evacuated to below 2 Pa. After CO2 gas at a low pressure was introduced into the test tube, a pressure change within the test tube due to adsorption was measured and the amount of CO2 gas adsorbed was calculated. Similar procedures were consecutively repeated by introducing more CO2 gas into the test tube. These processes were performed automatically according to a prescribed schedule.
Fig. 1. Schematic diagram of the experimental apparatus.
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Fig. 2. Effect of desorption temperature on relations between the amount adsorbed and the CO2 concentration for the honeycomb MS-13X bed at 303.15 K.
3. Results and Discussion The effect of the desorption temperature on the CO2 adsorption equilibria of MS13X and MS-4A at 35 C is shown in Figs. 2 and 3. It is found from these figures that, when the desorption temperature is higher than 250 C, the CO2 adsorption equilibria are hardly affected by the desorption temperature. On account of this fact, in what follows, we examined only the CO2 adsorption equilibria of MS-13X and MS-4A that were desorbed at 375 C. Relations between the volume of adsorption space q/*(T) and adsorption potential A [=RT ln(Ps(T)/P)] are shown in Fig. 4. The volume of adsorption space of MS-13X is greater than that of MS-4A for on adsorption potential less than 18.79 (kJ/mol), and thus MS-13X is more advantageous for CO2 recovery than MS-4A, as long as CO2 partial pressure is greater than some specified pressure depending on temperature; say, 0.035 bar at 300 K. The fitness of the Dubinin–Astakhov equation represented by Eq. (1) to the experimental data of the relation between the volume of adsorption space and the adsorption potential was examined in Figs. 5 and 6. q= ðT Þ ¼ w exp½ðA=EÞn :
ð1Þ
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Fig. 3. Effect of desorption temperature on relations between the amount adsorbed and the CO2 concentration for the honeycomb MS-4A bed at 303.15 K.
Fig. 4. Relations between the volume of adsorption space and the adsorption potential.
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Fig. 5. The fitness of the Dubinin–Astakhov equation to the CO2 adsorption equilibria of the honeycomb MS-13X bed.
Fig. 6. The fitness of the Dubinin–Astakhov equation to the CO2 adsorption equilibria of the honeycomb MS-4A bed.
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Fig. 7. Relations between the Langmuir parameters and the temperature.
Here, w (m3/kg-bed), E (kJ/mol) and n are adjustable parameters and were determined by a least-squares method: we obtained n=2.28, w =1.63104 and E=18.7 for MS-13X and n=3.31, w=9.14105 and E=24.2 for MS-4A. In the previous report [2], we assigned 3 to the exponent n to reduce a number of adjustable parameters because the experimental data of the adsorption equilibria determined by the breakthrough method were quite restricted and it is well recognized that when a ratio of the micro pore size to the molecular diameter is less than 3, the exponent n becomes 3 for non-polar gases and is slightly greater than 3 for polar gases [4]. The present results substantiate the validity of our previous assumption, but, needless to say, the values of n determined here are more accurate and should be used for future applications.
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Fig. 8. Relations between the amount adsorbed and the CO2 concentration for the honeycomb MS-13X bed.
The Dubinin–Astakhov equation well reproduces the experimental results irrespective of adsporption temperature, but, strictly speaking, the fitness of the Dubinin-Astakhov equation to the experimental data of MS-13X is superior to that for the experimental data of MS-4A: the standard deviation for MS-13X is 3.14103, while that for MS-4A is 6.28103. Although the Dubinin–Astakhov equation is the most general expression for the equilibrium of physical adsorption and can be used at arbitrary temperatures and pressures, it is represented by an exponential function and, hence, tends to cause instability in numerical computation of dynamic adsorption processes. To circumvent this difficulty, use of the Langmuir approximation defined by Eq. (2) to the experimental data of the relation between the amount adsorbed q (kg-CO2/kg-bed) and CO2 concentration C (kg-CO2/m3) seems to be promising for a relatively low CO2 pressure region. q ¼ bC=ð1 þ aCÞ:
ð2Þ
Since the relation between q and C depends on temperature, the Langmuir parameters, a and b, must also depend on temperature and were determined using a least-squares fit of Eq. (2) to the experimental data of the adsorption equilibria. Results for a and b are shown in Fig. 7. As seen from this figure, ln a and ln b vary
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Fig. 9. Relations between the amount adsorbed and the CO2 concentration for the honeycomb MS-4A bed.
linearly with T, and thus these can be approximated by the following linear equations in the temperature range of T < 345 K: For MS-13X, ln a ¼ 9:6250:0244T; ln b ¼ 9:8400:0319T;
ð3Þ
For MS-4A, ln a ¼ 11:9510:0288T; ln b ¼ 10:0750:0307T:
ð4Þ
The estimated Langmuir curves are depicted in Figs. 8 and 9 by the solid lines. The Langmuir approximation is quite effective in a relatively low temperature region, but its accuracy deteriorates as the adsorption temperature rises.
4. Conclusions
1. The CO2 adsorption equilibrium of the honeycomb zeolite beds are not influenced by the desorption temperature, as long as the examined honeycomb beds are desorbed at a temperature greater than about 250 C.
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2. The CO2 adsorption equilibria of the honeycomb zeolite beds can be mathematically represented by the Dubinin–Astakhov equation, irrespective of adsorption temperature. 3. The adsorption capacity of MS-13X is greater than that of MS-4A for the adsorption potential less than 18.79 kJ/mol. 4. The CO2 adsorption equilibria of the honeycomb zeolite beds can be approximated by the Langmuir equation for CO2 partial-pressures less than about 0.2 bar, but the Langmuir approximation becomes less accurate as the adsorption temperature increases.
Acknowledgements The authors express their sincere gratitude to Mr. T. Miyawaki (Nichias Co.Ltd.) for providing them with samples of the honeycomb zeolite beds.
References [1] Takeuchi Y, editor. Handbook of adsorbtion technology. Tokyo: NTS Inc; 1999. [2] Kamiuto K, Ermalina, Ihara K. CO2 adsorbtion equilibria of the honeycomb zeolite beds. Applied Energy 2001;69:285–92. [3] Dubinin MM, Astakhov VA. Description of adsorption equilibria of vapors on zeolite over wide ranges of temperature and pressure. In: Flanigen M, Sand LB, editors. Molecular sieves zeolite II. Washington: American Chemical Society; 1971. [4] Kawazoe K, Kawai T, Eguchi Y, Itoga K. Correlation of adsorption equilibrium data of various gases and vapors on molecular-sieving carbon. Journal of Chemical Engineering of Japan 1974;7: 158–61.
Effect of desorption temperature on CO2 adsorption equilibria of the honeycomb zeolite beds K. Kamiutoa,*, S. Abeb, Ermalinab a
Department of Production Systems Engineering, Oita University, Oita 870-1192, Japan b Graduate School of Oita University, Oita 870-1192, Japan Received 7 May 2002; received in revised form 30 May 2002; accepted 2 June 2002
Abstract The behaviours of MS-13X and MS-4A were examined. The fitness of the Dubinin–Astakhov equation to the adsorption equilibria of the fully-desorbed honeycomb beds was examined. The validity of the Langmuir approximation to the obtained adsorption equilibria for relatively low CO2 partial pressures was also discussed. The CO2 adsorption equilibria of the honeycomb zeolite beds are not affected by it’s desorption temperature, as long as the examined honeycomb beds are desorbed at a temperature greater than 250 C, and are well correlated with the Dubinin–Astakhov equation, irrespective of adsorption temperature. The Langmuir approximation works well for CO2 partial pressures less than about 0.2 bar, but the accuracy of the approximation deteriorates as the adsorption temperature increases. # 2002 Elsevier Science Ltd. All rights reserved.
1. Introduction Honeycomb zeolite beds have been utilized for dehumidification and recovery of evaporated organic-solvents in many industries because of their enormous specific surface areas and small flow resistances. In addition to these traditional applications, the honeycomb zeolites have also been considered for use for the CO2 recovery from power plants [1] and, in relation to this application, the CO2 adsorption characteristics
* Corresponding author. Tel.: +81-097-554-7797; fax: +81-097-554-7790. E-mail address: [email protected] (K. Kamiuto). 0306-2619/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S0306-2619(02)00048-X
556
K. Kamiuto et al. / Applied Energy 72 (2002) 555–564
Nomenclature A a,b C E n P Ps(T) q R T w
adsorption potential (kJ/mol) Langmuir parameters CO2 concentration (kg-CO2/m3) characteristic energy (kJ/mol) exponent partial pressure of CO2 (Pa) saturated-vapour pressure of CO2 (Pa) amount adsorbed per unit mass of a bed (kg-CO2/ kg-bed) general gas constant (kJ/mol K) temperature (K) maximum volume of adsorption space (m3/kg-bed)
Greek letters * liquid density of adsorbate (kg/m3) Physical properties of CO2 required for the Dubinin-Astakhov plots (1) Density of adsorbed liquid CO2 at T (K) *=1.0276103[1.912573.0 (T/1000)],(kg-CO2/m3) (2) Saturated-vapour pressure of CO2 for T < 304.19(K): Ps=107[15.54953+175.8642(T/1000)682.9149(T/1000)2+923.1018 (T/1000)3], (Pa) for T > 304.19 (K), Ps=7.382106exp(9.98144 3.036254103/T), (Pa).
of the honeycomb zeolites are needed. Under these circumstances, the authors [2] have determined the CO2 adsorption equilibria of the honeycomb zeolite beds consisting of MS-13X or MS-4A1 from breakthrough curves for various CO2–N2 mixtures at 293 K and examined the fitness of the Dubinin–Astakhov equation [3] with the exponent n=3 to the obtained data. However, the desorption temperature of the tested beds were relatively low, i.e., 150 C and thus the desorption might be incomplete in our previous study. The purpose of the present study is threefold: first, to examine the effect of the desorption temperature on the CO2 adsorption equilibria of the honeycomb zeolite beds, which are composed of thin fibrous walls with the porosity about 0.8 and hold MS-13X MS-4A as an adsorbent, by using the constant volume method; second, to address the fitness of the Dubinin–Astakhov equation to the CO2 adsorption equilibria not influenced by the desorption temperature; and finally, to discuss the 1 MS stands for ‘‘molecular sieve’’. The rational formula of 13X-type zeolite is Na86[(Al2)86 (SiO2)86].276H2O, whereas that of 4A-type zeolite is Na12[(AlO2)]12(SiO2)12].27H2O. Moreover, the micro pore sizes of MS-4A and MS-13X are 3.5 A˚ and 9 A˚ respectively.
K. Kamiuto et al. / Applied Energy 72 (2002) 555–564
557
validity of the Langmuir approximation to the obtained adsorption equilibria for relatively low CO2 partial pressures.
2. The Experiment A schematic diagram of the experimental apparatus—BELSORP-HP(BELL JAPAN INC)—for measuring the CO2 adsorption characteristics of the honeycomb zeolite adsorbents is shown in Fig. 1. The major part of the measuring system is made of a Pyrex glass tube with a stopcock, in which an about 0.4 g honeycomb adsorbent was placed. The zeolite contents of the utilized honeycomb beds were about 43 wt.% of the beds. The wall thickness is 2104 m. More details of the examined honeycomb zeolite beds were described in Ref. [2] and, hence, are not repeated here. Before the experiment, the desorption of the adsorbent was performed: the test tube including a sample was heated in an electrically-heated furnace, kept at various temperatures ranging from 150 to 375 C, and was evacuated continuously below 2 Pa using a vacuum pump. This process took about 4 h. At the end of the desorption, the attached stopcock was closed and then the test tube was naturally cooled-down in the air. In the adsorption experiment, the test tube was installed in a thermostat, which was controlled at various temperatures, i.e., 20, 30, 50 and 70 C and was evacuated to below 2 Pa. After CO2 gas at a low pressure was introduced into the test tube, a pressure change within the test tube due to adsorption was measured and the amount of CO2 gas adsorbed was calculated. Similar procedures were consecutively repeated by introducing more CO2 gas into the test tube. These processes were performed automatically according to a prescribed schedule.
Fig. 1. Schematic diagram of the experimental apparatus.
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Fig. 2. Effect of desorption temperature on relations between the amount adsorbed and the CO2 concentration for the honeycomb MS-13X bed at 303.15 K.
3. Results and Discussion The effect of the desorption temperature on the CO2 adsorption equilibria of MS13X and MS-4A at 35 C is shown in Figs. 2 and 3. It is found from these figures that, when the desorption temperature is higher than 250 C, the CO2 adsorption equilibria are hardly affected by the desorption temperature. On account of this fact, in what follows, we examined only the CO2 adsorption equilibria of MS-13X and MS-4A that were desorbed at 375 C. Relations between the volume of adsorption space q/*(T) and adsorption potential A [=RT ln(Ps(T)/P)] are shown in Fig. 4. The volume of adsorption space of MS-13X is greater than that of MS-4A for on adsorption potential less than 18.79 (kJ/mol), and thus MS-13X is more advantageous for CO2 recovery than MS-4A, as long as CO2 partial pressure is greater than some specified pressure depending on temperature; say, 0.035 bar at 300 K. The fitness of the Dubinin–Astakhov equation represented by Eq. (1) to the experimental data of the relation between the volume of adsorption space and the adsorption potential was examined in Figs. 5 and 6. q= ðT Þ ¼ w exp½ðA=EÞn :
ð1Þ
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559
Fig. 3. Effect of desorption temperature on relations between the amount adsorbed and the CO2 concentration for the honeycomb MS-4A bed at 303.15 K.
Fig. 4. Relations between the volume of adsorption space and the adsorption potential.
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Fig. 5. The fitness of the Dubinin–Astakhov equation to the CO2 adsorption equilibria of the honeycomb MS-13X bed.
Fig. 6. The fitness of the Dubinin–Astakhov equation to the CO2 adsorption equilibria of the honeycomb MS-4A bed.
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Fig. 7. Relations between the Langmuir parameters and the temperature.
Here, w (m3/kg-bed), E (kJ/mol) and n are adjustable parameters and were determined by a least-squares method: we obtained n=2.28, w =1.63104 and E=18.7 for MS-13X and n=3.31, w=9.14105 and E=24.2 for MS-4A. In the previous report [2], we assigned 3 to the exponent n to reduce a number of adjustable parameters because the experimental data of the adsorption equilibria determined by the breakthrough method were quite restricted and it is well recognized that when a ratio of the micro pore size to the molecular diameter is less than 3, the exponent n becomes 3 for non-polar gases and is slightly greater than 3 for polar gases [4]. The present results substantiate the validity of our previous assumption, but, needless to say, the values of n determined here are more accurate and should be used for future applications.
562
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Fig. 8. Relations between the amount adsorbed and the CO2 concentration for the honeycomb MS-13X bed.
The Dubinin–Astakhov equation well reproduces the experimental results irrespective of adsporption temperature, but, strictly speaking, the fitness of the Dubinin-Astakhov equation to the experimental data of MS-13X is superior to that for the experimental data of MS-4A: the standard deviation for MS-13X is 3.14103, while that for MS-4A is 6.28103. Although the Dubinin–Astakhov equation is the most general expression for the equilibrium of physical adsorption and can be used at arbitrary temperatures and pressures, it is represented by an exponential function and, hence, tends to cause instability in numerical computation of dynamic adsorption processes. To circumvent this difficulty, use of the Langmuir approximation defined by Eq. (2) to the experimental data of the relation between the amount adsorbed q (kg-CO2/kg-bed) and CO2 concentration C (kg-CO2/m3) seems to be promising for a relatively low CO2 pressure region. q ¼ bC=ð1 þ aCÞ:
ð2Þ
Since the relation between q and C depends on temperature, the Langmuir parameters, a and b, must also depend on temperature and were determined using a least-squares fit of Eq. (2) to the experimental data of the adsorption equilibria. Results for a and b are shown in Fig. 7. As seen from this figure, ln a and ln b vary
K. Kamiuto et al. / Applied Energy 72 (2002) 555–564
563
Fig. 9. Relations between the amount adsorbed and the CO2 concentration for the honeycomb MS-4A bed.
linearly with T, and thus these can be approximated by the following linear equations in the temperature range of T < 345 K: For MS-13X, ln a ¼ 9:6250:0244T; ln b ¼ 9:8400:0319T;
ð3Þ
For MS-4A, ln a ¼ 11:9510:0288T; ln b ¼ 10:0750:0307T:
ð4Þ
The estimated Langmuir curves are depicted in Figs. 8 and 9 by the solid lines. The Langmuir approximation is quite effective in a relatively low temperature region, but its accuracy deteriorates as the adsorption temperature rises.
4. Conclusions
1. The CO2 adsorption equilibrium of the honeycomb zeolite beds are not influenced by the desorption temperature, as long as the examined honeycomb beds are desorbed at a temperature greater than about 250 C.
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2. The CO2 adsorption equilibria of the honeycomb zeolite beds can be mathematically represented by the Dubinin–Astakhov equation, irrespective of adsorption temperature. 3. The adsorption capacity of MS-13X is greater than that of MS-4A for the adsorption potential less than 18.79 kJ/mol. 4. The CO2 adsorption equilibria of the honeycomb zeolite beds can be approximated by the Langmuir equation for CO2 partial-pressures less than about 0.2 bar, but the Langmuir approximation becomes less accurate as the adsorption temperature increases.
Acknowledgements The authors express their sincere gratitude to Mr. T. Miyawaki (Nichias Co.Ltd.) for providing them with samples of the honeycomb zeolite beds.
References [1] Takeuchi Y, editor. Handbook of adsorbtion technology. Tokyo: NTS Inc; 1999. [2] Kamiuto K, Ermalina, Ihara K. CO2 adsorbtion equilibria of the honeycomb zeolite beds. Applied Energy 2001;69:285–92. [3] Dubinin MM, Astakhov VA. Description of adsorption equilibria of vapors on zeolite over wide ranges of temperature and pressure. In: Flanigen M, Sand LB, editors. Molecular sieves zeolite II. Washington: American Chemical Society; 1971. [4] Kawazoe K, Kawai T, Eguchi Y, Itoga K. Correlation of adsorption equilibrium data of various gases and vapors on molecular-sieving carbon. Journal of Chemical Engineering of Japan 1974;7: 158–61.