Sorption isosteres of CO2 on CarboTech activated carbon, D47/2. Solid dashed line is the .... (Germany), Kansai Coke & Chemicals (Japan), Osaka Gas (Japan) and WestCavo (USA), were evaluated for ... area carbon with ac. 10 mol/kg, and ...
Evaluation of Commercial Activated Carbons for Self-chilling by Desorption of Carbon Dioxide by
Dongmin Shen and Martin Bülow BOC Gases Technology Murray Hill, NJ, December 1999
CONTENTS
LIST OF FIGURES .....................................................................................................................2 LIST OF TABLES ......................................................................................................................3 SUMMARY ................................................................................................................................4 1. BACKGROUND OF CHILLCAN PROJECT........................................................................4 2. METHODICAL OF ADSORBENT EVALUATION .............................................................5 2.1. The Isosteric Principle .......................................................................................................5 2.2. The Isosteric Apparatus .....................................................................................................6 3. ACTIVATED CARBONS ......................................................................................................8 4. RESULTS AND DISCUSSION .............................................................................................9 4.1. Sorption Isosteres of Carbon Dioxide on Activated Carbons ............................................9 4.2. Sorption Thermodynamics For CO2 On Activated Charcoals .........................................25 4.3. Differential CO2 Sorption Capacities For Self-Chilling ..................................................28 4.4. Integral Sorption Heats Of CO2 For Self-Chilling ...........................................................29 4.5. Comparison of Sorption Isotherms ..................................................................................36 5. CONCLUSIONS ...................................................................................................................39 6. REFERENCES.....................................................................................................................40
1
LIST OF FIGURES Figure 1. Scheme of the isosteric apparatus. (1. Gas supply, 2. Circulating pump, 3&4. Gas cylinders, 5&6. Pressure sensors, 7. Mass spectrometer, 8. Sample holder, 9&10. Cryostat, 11-15, Vacuum systems). ...................................... 6 Figure 2. Sublimation curve for carbon dioxide measured isosterically in absence of sorbent (empty and full symbols denote, respectively, experiments performed from low to high temperature and in reverse direction). ............................................. 7 Figure 3. Sorption isosteres of CO2 on CarboTech activated carbon, D47/2. Solid dashed line is the pure CO2 sublimation curve. ...................................................... 12 Figure 4. Sorption isosteres of CO2 on Osaka activated carbon, M30. Solid dashed line is the pure CO2 sublimation curve. .................................................................. 15 Figure 5. Sorption isosteres of CO2 on WestVaco activated carbon, 241-R-99. Solid dashed line is the pure CO2 sublimation curve. ...................................................... 18 Figure 6. Sorption isosteres of CO2 on WestVaco activated carbon, 1091-R-99. Solid dashed line is the pure CO2 sublimation curve. ...................................................... 21 Figure 7. Sorption isosteres of CO2 on Kansai Coke activated carbon, MWS30. Solid dashed line is the pure CO2 sublimation curve. ...................................................... 24 Figure 8. Isosteric sorption heats for CO2 on activated charcoals, obtained experimentally by the isosteric method. .................................................................. 26 Figure 9. Standard sorption entropies for CO2 on activated charcoals, obtained experimentally by the isosteric method. .................................................................. 27 Figure 10. Standard Gibbs free sorption energies for CO2 on activated charcoals at the sublimation temperature, obtained experimentally by the isosteric method. .......... 27 Figure 11. Standard Gibbs free sorption energies for CO2 on activated charcoals at 298 K, obtained experimentally by the isosteric method. .............................................. 28 Figure 12. Sorption isotherms of CO2 on activated charcoals at 298 K, obtained from sorption isosteres. .................................................................................................... 28 Figure 13. Sorption isotherms of CO2 on CarboTech’s activated charcoals at 25 and 10 o C, obtained from sorption isosteres. ...................................................................... 29 Figure 14. Integral heat of sorption for CO2 on CarboTech activated carbon D47/2, calculated before chilling (25 oC, 12 bar) and after chilling (10 oC, 1 bar)............. 30 Figure 15. Integral heat of sorption for CO2 on Osaka activated carbon M30, calculated before chilling (25 oC, 12 bar) and after chilling (10 oC, 1 bar). ............................. 30 Figure 16. Integral heat of sorption for CO2 on WestVaco activated carbon 241-R-99, calculated before chilling (25 oC, 12 bar) and after chilling (10 oC, 1 bar)............. 31 Figure 17. Integral heat of sorption for CO2 on WestVaco activated carbon 1091-R-99, calculated before chilling (25 oC, 12 bar) and after chilling (10 oC, 1 bar)............. 31 Figure 18. Integral heat of sorption for CO2 on Kansai Coke activated carbon MWS30, calculated before chilling (25 oC, 12 bar) and after chilling (10 oC, 1 bar)............. 32
Figure 19. Sorption isotherms for CO2 on activated carbons measured by VTI high pressure apparatus at 25 oC. .................................................................................... 36 Figure 20. Comparison of sorption isotherms for CO2 on D47/2 activated carbons measured by isosteric and VTI high pressure apparatuses at 25 oC. ....................... 37 Figure 21. Comparison of sorption isotherms for CO2 on Osaka M30 activated carbons measured by isosteric and VTI high pressure apparatuses at 25 oC. .......... 37 Figure 22. Comparison of sorption isotherms for CO2 on WestVaco 241-R-99 activated carbons measured by isosteric and VTI high pressure apparatuses at 25 oC. ................................................................................................................... 38 Figure 23. Comparison of sorption isotherms for CO2 on WestVaco 1091-R-99 activated carbons measured by isosteric and VTI high pressure apparatuses at 25 oC. ................................................................................................................... 38 Figure 24. Comparison of sorption isotherms for CO2 on Kansai MWS30 activated carbons measured by isosteric and VTI high pressure apparatuses at 25 oC. .......... 39
LIST OF TABLES Table 1. Characteristics of Activated Carbons used. .................................................................. 8 Table 2. Sorption isosteres and thermodynamic functions of CO2 on CarboTech activated carbon, D47/2. ............................................................................................ 10 Table 3. Sorption isosteres and thermodynamic functions of CO2 on Osaka activated carbon, M30. .............................................................................................................. 13 Table 4. Sorption isosteres and thermodynamic functions of CO2 on WestVaco activated carbon, 241-R-99. ....................................................................................... 16 Table 5. Sorption isosteres and thermodynamic functions of CO2 on WestVaco activated carbon, 1091-R-99. ..................................................................................... 19 Table 6. Sorption isosteres and thermodynamic functions of CO2 on Kansai Coke activated carbon, MWS30. ......................................................................................... 22 Table 7. Cooling Capacity obtained using the isotherm calculated sorption isosteres. ............ 33 Table 8. Cooling Capacity obtained using the isotherm measured by VTI apparatus. ............. 34 Table 9. Average cooling Capacity of the two sets of isotherms. ............................................. 36
SUMMARY Five commercial activated carbons from four commercial suppliers: CarboTech (Germany), Kansai Coke & Chemicals (Japan), Osaka Gas (Japan) and WestCavo (USA), were evaluated for BOC ChillCan Project, based on CO2 sorption thermodynamics measured by the Sorption Isosteric method and high pressure CO2 sorption isotherms measured by VTI high pressure apparatus. The following conclusions can be drawn from the evaluation: (1). In terms of the isosteric heat of adsorption for CO2, CarboTech’s D47/2 and WestVaco’s 1091-R-99 carbons shows the highest sorption heat, ca. 23 kJ/mol, in the concentration range related to CO2 sorption equilibrium pressures between 1 to 12 bars, followed by Osaka’s M-30, ca. 21 kJ/mol, and then by WestVaco’s 241-R-99 and Kansai Coke MWS-30, ca. 18 kJ/mol. (2). In terms of the differential CO2 sorption capacity before releasing CO2 (12 bar and 25 C) and after self-chilling process (1 bar and 10 oC), Kansai high surface area supercarbon provides the largest amount of CO2 loading, ca. 13 mol/kg, followed by Osaka’s high surface area carbon with ac. 10 mol/kg, and then by WestVaco wood chips with ca. 7~9 mol/kg, and then by CarboTech with ca. 5.4 mol/kg. o
(3). In terms of overall self-chilling power, i.e. the integral heat of sorption over the concentration range before and after releasing CO2, the high surface area supercarbons from Kansai and Osaka provide the largest chilling power, ca. (210-234) J/g, followed by WestVaco’s wood chips with c. 170 J/g, and then by CarboTech with ca. 122 J/g. (4). Another important factor influencing overall chill-can performance is the density of a charcoal material. A high density material may reduce the volume of a capsule for containing the same amount of charcoal, or for a given volume of a capsule, a high density material may store more CO2 molecules, therefore, providing more chilling power. Based on these results, it is recommended that: Isosteric sorption heat, differential CO2 loading, and packing density are “equally” important factors that have to be considered in selecting a chillcan adsorbent. For a constant volume of a capsule, the activated charcoal from WestVaco, USA, is recommended.
1. BACKGROUND OF CHILLCAN PROJECT The self-chilling can is one of the latest BOC inventions, which takes advantage of ad adsorption-desorption processes to chill liquids in a container utilizing a simple principle: when adsorbed carbon dioxide (CO2) molecules release from an charcoal material, the desorption of the gas molecules will take up energy and cool down the material as a result. In order to provide enough self-chilling power, a large integral heat of sorption, depending on a high isosteric heat of sorption and a large differential amount of sorption, for CO2 on a charcoal material over a pressure range from 12 to 1 bar is essential to maximize the selfchilling effect of a ChillCan and to lead to successful commercialization of the BOC ChillCan Project.
2. METHODICAL OF ADSORBENT EVALUATION 2.1. The Isosteric Principle A modern version of the isosteric technique used for the evaluation, is described in Figure1. Its principle is to measure the equilibrium pressure as a function of temperature, while a (nearly) constant sorption phase concentration is maintained. Sorption thermodynamic functions are obtained as dependences on concentration by repeating measurements of isosteres for different concentrations controlled by volumetric dosing procedures. The thermodynamic definition for the isosteric heat of a pure gas is the molar enthalpy in the gas phase minus the differential enthalpy in the sorption phase. For a real gas, the isothermal variation of its enthalpy with pressure introduces residual enthalpy nuisance terms into the thermodynamic equations. Since our experiments are performed at sub-atmospheric pressure, the following equation is written for the case when the bulk gas obeys the perfect gas law: ln P q st R . (1 / T ) nm
(1)
Plots, lnP vs. 1/T, at constant sorption phase concentration, n, are called sorption isosteres. Eq. (1) shows that the isosteric heat is determined by their slope. If isosteres, or data sets (T, p, n) for a given system, are available over the entire sorbate concentration region, the sorption equilibrium is comprehensively and fully described. To evaluate sorption thermodynamic functions for single gases, such as the isosteric sorption heat and, thus, the differential sorption enthalpy, H, the standard sorption entropy, S°, and the standard Gibbs free sorption energy, G°, the following equations are used: q isosteric ( n) RT
(2)
p H (n) R ln o T p
(3)
G o (n) H (n) TS o (n) .
(4)
ln p const .
S o (n)
Integral sorption heats were calculated from the concentration dependence of the isosteric sorption heat as obtained directly from isosteres,
q int
1 = n
n
0
q st
dn.
(5)
One should have in mind, that an integral sorption heat does not represent a single-phase property but that of an equilibrium between two phases in a sense that should be imagined as moving from one isotherm to another when moving from one concentration, n, to another, n + dn. This is a process connected with a pressure change, viz., from p to p + dp, which again is connected with a finite value of mechanical work executed. Thus, the mechanic work does play a role for an integral sorption heat.
1 7
5 X5
X7
X6
X1
X12
X10
X3
X11 X14
3 X2
4
6
11
X15 X13
X4
12
2
8
X8
X9
14
9
13 10
15
T=const.
Figure 1. Scheme of the isosteric apparatus. (1. Gas supply, 2. Circulating pump, 3&4. Gas cylinders, 5&6. Pressure sensors, 7. Mass spectrometer, 8. Sample holder, 9&10. Cryostat, 1115, Vacuum systems).
2.2. The Isosteric Apparatus The experimental isosteric system is outlined in Figure 1. Its principle is to measure the equilibrium pressure as a function of temperature, while a (nearly) constant value of sorption phase concentration is being maintained. Repeating the procedure for various sorbate concentrations, n, sorption thermodynamic functions are obtained as concentration dependences after subsequent dosing. The main features of the currently utilized version of the isosteric technique are as follows: (i) A large amount of adsorbent used to minimize errors, 3 ~ 15 g; (ii) minimum dead gas-phase-volume to sorption-phase-volume ratio, Vg /Vs < 5; (iii) low equilibrium pressure, (0.1 ~ 99) torr; (iv) small temperature increments, ca. 2 - 5 K, in regions of cryogenic temperature; (v) strongly controlled equilibration criteria; (vi) gas phase circulation in the sorption vessel; (vii) sophisticated data acquisition and evaluation software (viii) apparatus layout for measurements at cryogenic temperature; (ix) any violation of isosteric condition becomes visible directly. The accuracy of the isosteric apparatus was checked by measuring the sublimation curve of carbon dioxide (CO2) in the absence of sorbent, as shown in Figure 2. The changes of enthalpy, - 25.26 kJ/mol, and entropy, -129.57 J/mol K, typical of CO2 sublimation as determined by isosteric experiments without sorbent, agree well with literature data that
amount to - 25.23 kJ/mol and - 129.63 J/mol K, respectively. In terms of sublimation energy, the experimental accuracy is ca. ± 70 J/mol. This assessment is based on two full data sets gathered by experiments performed independently of each other for both increasing and decreasing temperature. Concerning differential sorption enthalpy, the experimental accuracy of underlying basic values, qisosteric, can be increased further by choosing sections of sorption isosteres with highest slope at given concentration, to calculate the isosteric sorption heat, qisosteric. This approach is based on the experimental experience that any external influence on the system leads to a decrease of the slope of an isostere. The determination of highest slopes represents a special feature of the data acquisition software utilized, in conjunction with a highperformance helium cryostat system designed and manufactured by Leybold AG, Germany, specifically for the purpose of the isosteric technique described in this reprot. In addition, accuracy is gained in regions of cryogenic temperature. This is because for a given constant temperature interval, T, the interval, (1 / T), on the abscissa scale is spread out at low absolute temperature compared with that at high absolute temperature. This leads to a more accurate determination of the slope of an isostere measured at cryogenic temperature over the same temperature interval, T. Altogether, this combination enables the current technique to minimize the experimental error to ca. ± 20 J/mol. The accuracy with which caloric data were gathered is comparable with that of most advanced sorption calorimetric techniques.
p, Pa
10000
1000
H = -25.26 kJ/mol 100
5.75
o
S = -129.57 J/mol
6.00
6.25
6.50
6.75
7.00
7.25
7.50
1000/(T, K) 1/22/98 10:02 Dongmin Shen Figure 2. Sublimation curve for carbon dioxide measured isosterically in absence of sorbent (empty and full symbols denote, respectively, experiments performed from low to high temperature and in reverse direction).
3. ACTIVATED CARBONS Five commercial activated carbon materials were tested. The characteristics of these materials are listed in Table 1. The dry weights of the activated carbons, utilized in isosteric measurements, were between 3 to 6 g, dependent on their packing densities. 6.20 g of CarboTech D47/2, 2.71 g of Osaka M30, 3.02 g of WestVaco 241-R-99, 2.59 g of WestVaco 1091-R-99, and 2.80 g for Kansai MWS30, were used, respectively. Prior to sorption experiments, the sample were carefully activated in the sample holder of the apparatus. The activation temperature was slowly increased from ambient one to 673 K in vacuo over 24 hours and maintained at 673 K and 10 5 torr over night. Isosteric measurements were performed over a region of temperature, c. (100 to 280) K and sorbate equilibrium pressures, c. (0.1 to 99) torr. Altogether, over 100 sorption isosteres were obtained for CO2 on CarboTech, Osaka, WestVaco and Kansai samples. By dosing procedure, it was aimed to cover entire solid phase concentration from nearly zero CO2 coverage to saturation capacity, i.e. from ca. (0.0610 to 12.4212) mol/kg for CarboTech, ca. (0.0434 to 19.6976) mol/kg for Osaka, ca. (0.0604 to 27.4707) mol/kg for WestVaco 241R-99, ca. (0.0696 to 18.9731) mol/kg for WestVaco 1091-R-99 and ca. (0.0672 to 28. 9608) mol/kg for Kansai, respectively.
Table 1. Characteristics of Activated Carbons used. Manufacturer:
CarboTech Germany
Osaka Gas Japan
WestVaco USA
WestVaco USA
Kansai Japan
D47/2
M30
241-R-99
1091-R-99
MWS30
extrudates
extrudates
woodchip
woodchip
beads
Size:
2 mm
2 mm
0.7 to 2 mm
6x8 mesh
1 mm
Packing Density g/cm3
0.47
0.30
0.23 ??
Surface Area m2/g
>1050
2449
2370
Pore Volume cm3/g
0.85
1.10
1.70
Type: Shape:
4. RESULTS AND DISCUSSION 4.1. Sorption Isosteres of Carbon Dioxide on Activated Carbons Sorption isosteres of CO2 as measured on the five activated carbons are presented in Figures 3-7, respectively for CarboTech D47/2, Osaka M30, WestVaco 241-R-99, WsetVaco 1091-R-99 and Kansai MWS30. The sorption isostere for a low sorption phase concentration appears on the left hand side of the lnp vs. 1/T plot. As the sorption phase concentration increases incrementally, a new isostere appears to the right of the previous isostere. This is because for a given temperature the equilibrium pressure in the gas phase is higher for the isostere with a higher sorption phase concentration. The slope of these isosteres may change with sorption phase concentration if the sorption enthalpy is concentration dependent. Except those near saturation capacity and those on 241-R-99 and MWS30 at high loadings, the sorption isosteres determined within the experimental conditions appear to be linear, indicating that there were no sorption phase transitions occurring. Therefore, the sorption isosteres were characteristic of sorption equilibrium processes of CO2 on the activated charcoals, providing, thus, fundamental physical parameters of the sorption process. Isosteric sorption enthalpy and entropy were calculated from the isosteres. They are attributed to the corresponding sorption phase concentrations that were set through dosing procedures. From the CO2 sorption isosteres on WestVaco 241-R-99 and Kansai MWS30, it can be found that the isosteres are not straight lines at high loadings, they bent at certain points into two sections, and the slopes became larger as temperature increased. The bead sizes of these activated carbons are small and they have very small packing densities. Therefore, this unnormal behavior might be due to CO2 condensation in maropores of or between fine corban particles. As a result, mixed sorption isosteric behaviors, e.g. CO2 sorption inside micropores and CO2 evaporization from condensed phase of CO2 in macropores or between carbon particles, migh have been measured. Tables 2-6 give lists of numerical values of sorption phase concentration together with the corresponding isosteric sorption enthalpy, H, standard sorption entropy (determined from the highest slope part of an isostere), S°, and standard Gibbs free sorption energy, G°, for each sorption isostere obtained. In the case of Gibbs free sorption energy, the sublimation temperature of 194.65 K for CO2 was chosen as the standard temperature. This choice provides also an opportunity to check thermodynamic consistency of the experimental data because the standard Gibbs free sorption energy should be zero when the sorption phase concentration exceeds the saturation capacity of a sorbent at the boiling point temperature. The standard Gibbs free sorption energy for any other temperatures can be calculated from values of sorption enthalpy and standard sorption entropy listed, using the Gibbs equation. Thereafter, the sorption equilibrium mass distribution, i.e., the equilibrium pressure corresponding to the parameters n, H, S° and G°, can be determined.
Table 2. Sorption isosteres and thermodynamic functions of CO2 on CarboTech activated carbon, D47/2. Concentration of CO2 Sorbed
Related Symbol of Isostere in Figure 3
Sorption Enthalpy
Standard Sorption Entropy
Standard Gibbs Free Sorption Energy at 194.65 K
n, mol/kg
-,-
-H, kJ/mol
S°, J/mol K
G°, kJ/mol
0.0610
30.678
-62.336
-18.522
0.1554
O
28.798
-64.907
-16.141
0.2725
27.966
-67.801
-14.745
0.4634
27.007
-70.591
-13.242
0.7420
26.122
-73.676
-11.755
1.0622
+
25.054
-74.288
-10.568
1.4280
x
24.453
-76.303
-9.574
1.8439
24.122
-79.089
-8.699
2.2448
23.634
-80.042
-8.026
2.7630
23.269
-81.845
-7.309
3.3117
23.054
-83.673
-6.738
4.2451
22.573
-85.472
-5.906
5.0819
22.655
-87.795
-5.535
5.9277
,+
22.438
-89.801
-4.927
6.9626
O,+
22.719
-93.704
-4.447
8.0704
,+
22.296
-94.934
-3.784
9.1657
,+
22.114
-97.491
-3.103
10.2454
,+
21.625
-98.610
-2.396
11.3336
,x
23.488
-114.72
-1.116
12.4212
O,x
24.499
-121.889
-0.731
100
n, m ol/kg: 0.0610 0.1554 0.2725 0.4634 0.7420 1.0622
10
1.428
p, torr
1.8439 2.2448 2.7630 3.3117 4.2451
1
5.0819 5.9277 6.9626 8.0704 9.1657 10.2454 11.3336
0.1
12.4212
3.5
4.0
4.5
5.0
5.5
6.0
6.5
7.0
7.5
8.0
1000/(T, K) Dongm in Shen
11
6/14/99 17:16
Figure 3. Sorption isosteres of CO2 on CarboTech activated carbon, D47/2. Solid dashed line is the pure CO2 sublimation curve.
Table 3. Sorption isosteres and thermodynamic functions of CO2 on Osaka activated carbon, M30. Concentration of CO2 Sorbed
Related Symbol of Isostere in Figure 4
Sorption Enthalpy
Standard Sorption Entropy
Standard Gibbs Free Sorption Energy at 194.65 K
n, mol/kg
-,-
-H, kJ/mol
S°, J/mol K
G°, kJ/mol
0.0434
+
29.28
64.80
16.65
0.0826
x
27.81
65.94
14.95
0.1249
27.01
67.31
13.89
0.1627
26.19
67.03
13.12
0.1997
25.48
66.15
12.58
0.2491
24.05
64.15
11.54
0.2655
23.91
64.45
11.33
0.5499
22.65
66.81
9.62
1.0880
21.63
69.68
8.04
1.0884
21.47
69.46
7.93
2.0491
20.92
74.87
6.32
4.0495
20.76
82.15
4.74
7.7428
20.43
88.71
3.13
20.76
92.79
2.67
15.1620
22.89
114.78
0.51
19.6976
24.06
123.03
0.07
11.1800
*
13
Adsorption isosteres of CO 2 on CMS-4 100 n, m ol/kg 0.2491 0.2655 0.5499
10
1.0880
p, torr
1.0884 2.0491 4.0495 7.7428
1
11.1800 15.1612
0.1
3.5
4.0
4.5
5.0
5.5
6.0
6.5
7.0
7.5
8.0
8.5
9.0
9.5
3
10 /(T, K) Dongm in Shen
14
6/25/99 16:12
Figure 4. Sorption isosteres of CO2 on Osaka activated carbon, M30. Solid dashed line is the pure CO2 sublimation curve.
Table 4. Sorption isosteres and thermodynamic functions of CO2 on WestVaco activated carbon, 241-R-99. Concentration of CO2 Sorbed
Related Symbol of Isostere in Figure 5
Sorption Enthalpy
Standard Sorption Entropy
Standard Gibbs Free Sorption Energy at 194.65 K
n, mol/kg
-,-
-H, kJ/mol
S°, J/mol K
G°, kJ/mol
0.0604 0.1275 0.3214
30.238 27.213 25.887
64.047 60.951 65.893
17.749 15.328 13.038
0.6548
23.320
63.975
10.845
1.0353
+ x *
21.564
62.715
9.335
19.074 18.353 18.331 17.916 17.629 17.324
58.819 60.080 63.952 65.033 65.873 65.671
7.604 6.637 5.860 5.235 4.784 4.518
17.381
67.608
4.197
17.161 16.971
67.568 68.223
3.985 3.668
68.330 71.255
3.430 3.282
1.8041 2.649 3.6691 4.8249 5.8467 6.7766
O
7.7046 8.5367 9.4522
,+
10.3628 11.5267
O,+ ,+
16.754 17.177
12.6462
,+
17.227
72.597
3.071
13.7964
,+ ,x O,x
17.095
73.016
2.857
,x
17.187 17.070 17.154
74.917 75.172 76.853
2.578 2.411 2.168
18.8712
,x
16.889
75.921
2.084
20.0452
,x
16.818
76.516
1.897
21.3831 22.7123 24.2908
--O--
16.874 16.983 17.257
78.414 79.708 81.749
1.583 1.440 1.316
25.9130
--
18.058
86.648
1.162
27.4707
--
19.058
92.958
0.931
15.0134 16.2240 17.6159
16
100 0.0604 0.1275 0.3214 0.6548 1.0353 1.8041 2.6490 3.6691 4.8249
10
5.8467
p, torr
6.7766 7.7046 8.5367 9.4522 10.3628 11.5267 12.6462 13.7964 15.0134
1
16.2236 17.6159 18.8712 20.0452 21.3831 22.7123 24.2908 25.9130 27.4707
0.1 3
4
5
6
7
8
9
10
11
1000/(T, K) Dongm in Shen
17
7/8/99 15:34
Figure 5. Sorption isosteres of CO2 on WestVaco activated carbon, 241-R-99. Solid dashed line is the pure CO2 sublimation curve.
Table 5. Sorption isosteres and thermodynamic functions of CO2 on WestVaco activated carbon, 1091-R-99. Concentration of CO2 Sorbed
Related Symbol of Isostere in Figure 6
Sorption Enthalpy
Standard Sorption Entropy
Standard Gibbs Free Sorption Energy at 194.65 K
n, mol/kg
-,-
-H, kJ/mol
S°, J/mol K
G°, kJ/mol
0.0696
30.445
63.849
17.994
0.1440
O
29.115
66.197
16.206
0.2892
28.303
70.780
14.501
0.7155
26.298
73.955
11.877
1.3246
24.924
76.480
10.010
1.8961
+
23.790
77.139
8.748
2.6998
x
23.053
78.967
7.654
3.6756
22.605
81.673
6.679
4.6928
22.184
83.174
5.965
6.2902
21.719
85.654
5.016
8.0733
21.635
89.586
4.166
9.8237
21.469
91.510
3.625
11.7444
21.209
93.326
3.010
13.4030
,+
21.014
94.850
2.518
15.1762
O,+
20.857
96.638
2.013
17.0021
,+
21.885
106.063
1.203
18.9731
,+
24.505
123.574
0.408
19
100 n, mol/kg: 0.0696 0.1440 0.2892 0.7155
10
1.3246 1.8961
p, torr
2.6998 3.6756 4.6928 6.2901 8.0733
1
9.8237 11.7444 13.4030 15.1762 17.0021 18.9731
0.1 3
4
5
6
1000/(T, K)
20
7
8
9
Figure 6. Sorption isosteres of CO2 on WestVaco activated carbon, 1091-R-99. Solid dashed line is the pure CO2 sublimation curve.
Table 6. Sorption isosteres and thermodynamic functions of CO2 on Kansai Coke activated carbon, MWS30. Concentration of CO2 Sorbed
Related Symbol of Isostere in Figure 7
Sorption Enthalpy
Standard Sorption Entropy
Standard Gibbs Free Sorption Energy at 194.65 K
n, mol/kg
-,-
-H, kJ/mol
S°, J/mol K
G°, kJ/mol
0.0672
28.612
60.920
16.733
0.1387
O
26.730
60.397
14.953
0.2705
20.779
42.858
12.422
0.8226
19.856
49.812
10.143
1.5126
19.084
55.475
8.266
2.1359
+
18.977
58.877
7.496
3.2272
x
18.735
62.268
6.593
4.3692
19.196
68.184
5.900
5.5918
18.309
66.629
5.316
7.2780
18.069
68.265
4.757
9.1601
18.397
72.802
4.200
11.0808
17.977
72.414
3.856
12.9309
17.966
73.436
3.646
14.9262
,+
17.806
74.241
3.329
17.0064
O,+
17.564
74.521
3.032
19.0176
,+
17.385
75.072
2.746
21.0761
17.152
75.114
2.505
22.9706
,+
17.551
79.025
2.141
24.9485
O,+
18.053
83.189
1.831
26.9836
,+
17.798
82.867
1.639
28.9608
,+
18.124
85.751
1.403
22
n, m ol/kg
100
0.0672 0.1387 0.2705 0.8226 1.5126 2.1359
p, torr
3.2272
10
4.3692 5.5918 7.2780 9.1601 11.0808 12.9309 14.9262
1
17.0064 19.0176 21.0761 22.9706 24.9485 26.9836 28.9608
0.1 3
4
5
6
7
1000/(T, K)
23
8
9
10
Figure 7. Sorption isosteres of CO2 on Kansai Coke activated carbon, MWS30. Solid dashed line is the pure CO2 sublimation curve.
4.2. Sorption Thermodynamics For CO2 On Activated Charcoals Sorption thermodynamic quantities for CO2 on the activated carbons were obtained from their sorption isosteres as functions of sorption phase concentration using eqs (1-3). Figure 8 shows the isosteric sorption enthalpy for CO2 on the five carbons for comparison. The initial sorption enthalpies are about 28 to 31 kJ/mol for the five samples. However, the different concentration dependence of sorption enthalpy are found for different activated carbons measured. In terms of the isosteric heat of adsorption for the middle range of CO2 loading, i.e. in the concentration range related to CO2 sorption equilibrium pressures between 1 to 12 bars, CarboTech’s D47/2 and WestVaco’s 1091-R-99 carbons shows the highest sorption heat, ca. 23 kJ/mol, followed by Osaka’s M-30, ca. 21 kJ/mol, and then by WestVaco’s 241-R-99 and Kansai Coke MWS-30, ca. 18 kJ/mol. The saturation capacity for CO2 adsorption on these carbons are significantly different. Extremely high sorption capacities for CO2 on WestVaco 1091-R-99 and Kansai MWS30 carbons, over 25 mol/kg, was observed, which was nearly twice as large as that on CarboTech D47/2, ca. 13 mol/kg. Sorption capacities for CO2 on WestVaco 241-R-99 and Osaka M30 carbons were also high, ca. 20 mol/kg. The values of standard sorption entropy, S°, which are referred to 760 torr of the gas phase pressure as the standard state, show different concentration dependences for CO2 on these carbons, cf., Figure 9. The concentration profiles for CarboTech, WestVaco 241-R-99 and Osaka materials are similar, and the loss of entropy increases with increasing the concentration, while the entropy loss for CO2 on WestVaco 1019-R-99 and Kansai carbons are relatively small and of weak concentration dependence. Figure 10 shows dependences of standard Gibbs free sorption energies, G°, on sorption phase concentration for CO2 on the five adsorbents, as referred to the sublimation temperature and 760 torr. The values, G°, for CO2 on the adsorbents change from negative values to zero as the concentration increases and exceeds micropore saturation capacities of the sorbent. This demonstrates that the experimental thermodynamic data determined by the isosteric method are thermodynamically consistent and correct. If the values of standard Gibbs free sorption energy, G°, as dependences on sorption phase concentration are referred to other temperatures, for example, 298 K, as shown in Figure 11, sorption equilibria for CO2 at the temperature of 298 K becomes available, e.g. Figure 12 showing sorption isotherms obtained from those standard Gibbs free sorption energies at 298 K. Gibbs free sorption energy is the true thermodynamic quantity that determines sorption equilibrium properties of a gas-adsorbent system. A large negative value of G° indicates a strong adsorption system. Although CarboTech and WestVaco 241-R-99 materials have large isosteric heats of sorption, the sorption capacities of CO2 are much smaller than that on WestVaco 1091-R-99 and Kansai MWS30 materials in the equilibrium pressure range up to 3500 torr. Therefore, to select an adsorbent for the ChillCan project, one must consider both sorption enthalpy and entropy, because the Gibbs free sorption energy is determined by both of these two thermodynamic quantities. In general, these five activated carbons fall into two groups: (1) WestVaco 1091-R-99 and Kansai MWS30, which have much high CO2 sorption capacities, but low isosteric heats of adsorption; (2) CarboTech, Osaka and WestVaco 241-R-99, which have relatively low CO2 sorption capacities, but relatively high isosteric heats of adsorption. Since the best materials for
25
self-chilling are not only dependent on the isosteric heat, but also on the differential loadings, these two factors have to be considered together. 35
30
- H, kJ/m ol
25
20
15 W estVaco, 241-R -99
10
C arboTech, D 47/2 O saka, M -30 W estVaco, 1091-R -99
5
Kansai, M W S-30
0 0
2
4
6
8
10
12
14
16
18
20
22
24
26
28
n, m ol/kg Figure 8. Isosteric sorption heats for CO2 on activated charcoals, obtained experimentally by the isosteric method. -20 W estVaco, 241-R -99 C arboTech, D 47/2
-40
O saka, M -30
O saka, M W S-30
-80
o
S , J/m ol K
W estVaco, 1091-R -99
-60
-100
-120
-140 0
2
4
6
8
10
12
14
16
n, m ol/kg
18
20
22
24
26
28
Figure 9. Standard sorption entropies for CO2 on activated charcoals, obtained experimentally by the isosteric method. 5
-5
o
G , kJ/m ol
0
-10
W estVaco, 241-R -99 C arboTech, D 47/2 O saka, M -30
-15
W estVaco, 1091-R -99 Kansai, M W S-30
-20 0
2
4
6
8
10
12
14
16
18
20
22
24
26
28
n, m ol/kg Figure 10. Standard Gibbs free sorption energies for CO2 on activated charcoals at the sublimation temperature, obtained experimentally by the isosteric method. 20 15
o
G , kJ/m ol
10 5 0 -5
W estVaco, 241-R -99 C arboTech, D 47/2
-10
O saka, M -30 W estVaco, 1091-R -99
-15
Kansai, M W S-30
-20 0
2
4
6
8
10
12
14
16
n, m ol/kg
18
20
22
24
26
28
Figure 11. Standard Gibbs free sorption energies for CO2 on activated charcoals at 298 K, obtained experimentally by the isosteric method. 30 T = 298 K W estVaco, 241-R-99
25
CarboTech, D47/2 Osaka, M-30 W estVaco, 1091-R-99
n, m ol/kg
20
Kansai, MW S-30
15
10
5
0 0
2
4
6
8
10
12
14
16
18
20
p, bar Figure 12. Sorption isotherms of CO2 on activated charcoals at 298 K, obtained from sorption isosteres.
4.3. Differential CO2 Sorption Capacities For Self-Chilling Self-chilling power of a Chillcan is determined by the integration of CO2 molecules desorbed from carbon adsorbents over a certain concentration range, e.g. from an adsorbed amount at room temperature and relatively high gas phase pressure, e.g. 25 oC and 12 bars, to a final equilibrium amount at a low temperature and atmospheric pressure, e.g. 10 oC and 1 bar, after self-chilling. In this Section, the five materials will be evaluated against their differential loadings between these two points. Figure 13 shows the sorption isotherms of CO2 on CarboTech’s charcoal at 25 and 10 C. The loadings are ca. 8.2 mol/kg at 25 oC and 12 bars and 2.8 mol/kg at 10 oC and 1 bar, respectively. The differential loading for other carbons are listed in Table 7 obtained from the isotherms calculated from the sorption isosteres, and in Table 8 obtained from the isotherms directly measured by the VTI high pressure apparatus. For CarboTech and WestVaco 1091-R99 materials the sorption isotherms obtained from the two different experimental methods agree very well, whereas for other systems, the isotherms obtained from the isosteric measurements are much higher than those measured by VTI apparatus. Therefore, the differential loadings obtained from these two experimental methods are different for some of these carbons (see Section 4.5 for detail discussions). o
If the sorption isotherms measured by VTI are used for loading evaluation, in terms of the differential CO2 sorption capacity before releasing CO2 (12 bar and 25 oC) and after self-
chilling process (1 bar and 10 oC), the following can be drawn. Kansai high surface area super carbon provides the largest amount of CO2 loading, ca. 13 mol/kg, followed by Osaka’s high surface area carbon with ca. 10 mol/kg, and then by WestVaco wood chips with ca. 7~9 mol/kg, and then by CarboTech with ca. 5.4 mol/kg.
12 11 10 9
n, m ol/kg
8 7 6 5 4 3 o
2
10 C
1
25 C
o
0 0
2
4
6
8
10
12
14
16
18
20
p, bar Figure 13. Sorption isotherms of CO2 on CarboTech’s activated charcoals at 25 and 10 oC, obtained from sorption isosteres.
4.4. Integral Sorption Heats Of CO2 For Self-Chilling Over the loading range obtained from the sorption isotherms and the middle range of the isosteric heat of adsorption, the total integral heat was calculated as shown in Figures 14 18, respectively for the five samples. The integral heats are also listed in Tables 7 and 8. The integral heats from the two different isotherms measured were averaged and listed in Table 9. In terms of overall self-chilling power, i.e. the integral heat of sorption over the concentration range before and after releasing CO2, the high surface area super carbons from Kansai and Osaka provide the largest chilling power, ca. (210-234) J/g, followed by WestVaco’s wood chips with ca. 170 J/g, and then by CarboTech with ca. 122 J/g.
35
30
0
H intg (25 - 10 C) = 121.5 kJ/kg
- H, kJ/m ol
25
20
15
10
5
0 0
2
4
6
8
10
12
14
n, m ol/kg 7/8/99 15:57 in Shen D47/2, Figure 14. Integral heat of sorption for CO2 on CarboTech activatedDongm carbon calculated before chilling (25 oC, 12 bar) and after chilling (10 oC, 1 bar).
35
30 0
H intg (25 - 10 C) = 134.4 kJ/kg
- H, kJ/m ol
25
20
15
10
5
0 0
2
4
6
8
10
12
14
16
n, m ol/kg Dongm in Shen 7/8/99 15:57 Figure 15. Integral heat of sorption for CO2 on Osaka activated carbon M30, calculated before chilling (25 oC, 12 bar) and after chilling (10 oC, 1 bar).
35
30
- H, kJ/m ol
25 o
H intg (25 - 10 C) = 276.5 kJ/kg
20
15
10
5
0 0
2
4
6
8
10
12
14
16
18
20
22
24
26
28
n, m ol/kg 7/8/99 16:04 in Shen241-R-99, Figure 16. Integral heat of sorption for CO2 on WestVaco activated Dongm carbon calculated before chilling (25 oC, 12 bar) and after chilling (10 oC, 1 bar).
35
30
o
H intg (25 - 10 C) = 170 J/g
- H, kJ/m ol
25
20
15
10
5
0 0
2
4
6
8
10
12
14
16
18
20
n, m ol/kg Figure 17. Integral heat of sorption for CO2 on WestVaco activated carbon 1091-R-99, calculated before chilling (25 oC, 12 bar) and after chilling (10 oC, 1 bar).
35
30 o
H intg (25 - 10 C) = 342 J/g
- H, kJ/m ol
25
20
15
10
5
0 0
2
4
6
8
10
12
14
16
18
20
22
24
26
28
30
n, m ol/kg Figure 18. Integral heat of sorption for CO2 on Kansai Coke activated carbon MWS30, calculated before chilling (25 oC, 12 bar) and after chilling (10 oC, 1 bar).
Table 7. Cooling Capacity obtained using the isotherm calculated sorption isosteres. ID#
n@12 bar
n@1 bar
n, mol/kg
Qist, kJ/mol
Hint, J/g
D47/2
8.2
2.8
5.4
22.5
-122
M-30
8.1
1.9
6.2
21
-130
1091-R-99
10.5
3.1
7.4
23
-170
241-R-99
19.9
4.9
15.9
18
-286
MWS-30
23.4
4.4
19.9
18
-342
Table 8. Cooling Capacity for different pressure windows using the isotherm calculated sorption isosteres. 8 to 1 bar
12 to 1 bar
16 to 1 bar
ID#
n, mol/kg
Hint, J/g
n, mol/kg
Hint, J/g
n, mol/kg
Hint, J/g
D47/2
4.1
-92
5.4
-122
6.3
-142
M-30
4.0
-84
6.2
-130
9.0
-189
1091-R-99
5.1
-117
7.4
-170
9.1
-209
241-R-99
11.8
-212
15.9
-286
19.0
-342
MWS-30
15.6
-281
19.0
-342
21.6
-389
0
H int, j/g
-100
-200
18 - 1 bars
-300
12 - 1 bars 8 - 1 bars
-400 D47/2
M -30
1091-R-99 241-R-99
M W S-30
Activated carbons Figure 19. Cooling Capacity for different pressure windows using the isotherm calculated sorption isosteres.
Table 9. Cooling Capacity obtained using the isotherm measured by VTI apparatus. ID#
n@12 bar
n@1 bar
n, mol/kg
Qist, kJ/mol
Hint, J/g
D47/2
8.2
2.8
5.4
22.5
-122
M-30
11.5
2.0
9.5
21
-200
1091-R-99
9.5
2.5
7.0
23
-161
241-R-99
11.0
2.5
8.5
18
-153
MWS-30
15.4
3.0
12.6
18
-227
Table 10. Cooling Capacity for different pressure windows using the isotherm measured by VTI apparatus. 8 to 1 bar
12 to 1 bar
16 to 1 bar
ID#
n, mol/kg
Hint, J/g
n, mol/kg
Hint, J/g
n, mol/kg
Hint, J/g
D47/2
4.2
-95
5.4
-122
6.1
-137
M-30
7.2
-151
10.0
-200
11.5
-242
1091-R-99
5.3
-122
7.0
-161
8.5
-196
241-R-99
6.1
-110
9.0
-153
10.3
-185
MWS-30
9.2
-166
12.6
-227
15.0
-270
0
H int, j/g
-50
-100
-150
-200 18 - 1 bars
-250
12 - 1 bars 8 - 1 bars
D47/2
M-30
1091-R-99 241-R-99
MW S-30
Activated carbons Figure 20. Cooling Capacity for different pressure windows using the isotherm calculated sorption isosteres.
Table 11. Average cooling Capacity of the two sets of isotherms (12 to 1 bar). Isosteric
VTI
Average
ID#
Hint, J/g
Hint, J/g
Hint, J/g
D47/2
-122
-122
-122
M-30
-130
-200
-165
1091-R-99
-170
-161
-166
241-R-99
-286
-153
-220
MWS-30
-342
-227
-285
4.5. Comparison of Sorption Isotherms 25 o
T = 25 C 241-R -99
20
1091-R -99 M SW -30
n, m ol/kg
M -30 D 47/2
15
10
5
0 0
2
4
6
8
10
12
14
16
18
20
22
p, bar Figure 21. Sorption isotherms for CO2 on activated carbons measured by VTI high pressure apparatus at 25 oC.
12
10
n, mol/kg
8
6
4 o
C O 2 / D 47/2 at 25 C Isosteric data
2
VTI data 0 0
2
4
6
8
10
12
14
16
18
20
22
24
26
p, bar Figure 22. Comparison of sorption isotherms for CO2 on D47/2 activated carbons measured by isosteric and VTI high pressure apparatuses at 25 oC. 24 22
o
C O 2 / M -30 at 25 C
20
Isosteric data
18
VTI, first run VTI, second run
n, m ol/kg
16 14 12 10 8 6 4 2 0 0
2
4
6
8
10
12
14
16
18
20
22
24
p, bar
Figure 23. Comparison of sorption isotherms for CO2 on Osaka M30 activated carbons measured by isosteric and VTI high pressure apparatuses at 25 oC.
30 o
C O 2 / 241-R -99 at 25 C Isosteric data
25
V TI data, rptd V TI data
n, m ol/kg
20
15
10
5
0 0
2
4
6
8
10
12
14
16
18
20
22
p, bar Figure 24. Comparison of sorption isotherms for CO2 on WestVaco 241-R-99 activated carbons measured by isosteric and VTI high pressure apparatuses at 25 oC. 16 14 12
n, m ol/kg
10 8 6 4
o
CO 2 / 1091-R -99 at 25 C Isosteric data
2
VTI data
0 0
2
4
6
8
10
12
14
16
18
20
22
24
26
p, bar Figure 25. Comparison of sorption isotherms for CO2 on WestVaco 1091-R-99 activated carbons measured by isosteric and VTI high pressure apparatuses at 25 oC.
28 26 24 22 20
n, m ol/kg
18 16 14 12 10 o
8
C O 2 / M W S-30 at 25 C
6
Isosteric data
4
VTI
2 0 0
2
4
6
8
10
12
14
16
18
20
22
p, bar
Figure 26. Comparison of sorption isotherms for CO2 on Kansai MWS30 activated carbons measured by isosteric and VTI high pressure apparatuses at 25 oC.
5. CONCLUSIONS Five commercial activated carbons from four commercial suppliers: CarboTech (Germany), Kansai Coke & Chemicals (Japan), Osaka Gas (Japan) and WestCavo (USA), were evaluated for BOC ChillCan Project, based on CO2 sorption thermodynamics measured by the Sorption Isosteric method and high pressure CO2 sorption isotherms measured by VTI high pressure apparatus. The following conclusions can be drawn from the evaluation: (1). In terms of the isosteric heat of adsorption for CO2, CarboTech’s D47/2 and WestVaco’s 1091-R-99 carbons shows the highest sorption heat, ca. 23 kJ/mol, in the concentration range related to CO2 sorption equilibrium pressures between 1 to 12 bars, followed by Osaka’s M-30, ca. 21 kJ/mol, and then by WestVaco’s 241-R-99 and Kansai Coke MWS-30, ca. 18 kJ/mol. (2). In terms of the differential CO2 sorption capacity before releasing CO2 (12 bar and 25 C) and after self-chilling process (1 bar and 10 oC), Kansai high surface area supercarbon provides the largest amount of CO2 loading, ca. 13 mol/kg, followed by Osaka’s high surface area carbon with ca. 10 mol/kg, and then by WestVaco wood chips with ca. 7~9 mol/kg, and then by CarboTech with ca. 5.4 mol/kg. o
(3). In terms of overall self-chilling power, i.e. the integral heat of sorption over the concentration range before and after releasing CO2, the high surface area supercarbons from Kansai and Osaka provide the largest chilling power, ca. (210-234) J/g, followed by WestVaco’s wood chips with ca. 170 J/g, and then by CarboTech with ca. 122 J/g.
(4). Another important factor influencing overall chill-can performance is the density of a charcoal material. A high density material may reduce the volume of a capsule for containing the same amount of charcoal, or for a given volume of a capsule, a high density material may store more CO2 molecules, therefore, providing more chilling power. Based on these results, it is recommended that: Isosteric sorption heat, differential CO2 loading, and packing density are “equally” important factors that has to be considered in selecting a chillcan adsorbent. For a constant volume of a capsule, the activated charcoal from WestVaco, USA, is recommended.
6. REFERENCES 1.
P.W. Atkins: Physical Chemistry, University Press, p. 112, Oxford (1978).
The author thanks officials of the former BOC Group for the permission to publish this report. Dierhagen, May 25, 2017
