Article pubs.acs.org/jced
Adsorption Equilibria of Water Vapor on Zeolite 3A, Zeolite 13X, and Dealuminated Y Zeolite Kyung-Min Kim, Hyun-Taek Oh, Seung-Jun Lim, Keon Ho, Yongha Park, and Chang-Ha Lee* Department of Chemical and Biomolecular Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-Gu, Seoul, 120-749, Republic of Korea
ABSTRACT: The adsorption equilibria of water vapor on zeolite 3A, zeolite 13X, and dealuminated Y zeolite (DAY) were measured using a volumetric method. Equilibrium experiments were conducted at 293.15, 303.15, and 313.15 K and at relative pressure (P/Ps) up to 0.95. Experimental data were correlated using Aranovich−Donohue and Frenkel−Halsey−Hill models, using Langmuir, Toth, UNILAN, and Sips isotherms.
1. INTRODUCTION In various industrial processes, many effluent gases contain some water vapor. For example, in petrochemical processes such as steam cracking, steam reforming, and water gas shift reaction, water is injected or is generated during the reaction. The effluent gases from iron/steel processing, coal gasification, olefin/paraffin processing, and so forth, also include water vapor. In particular, natural gas is typically saturated with water vapor under normal production conditions. In gas separation, water vapor is the most common undesirable impurity to be removed from the desirable component of effluent gas mixtures. Furthermore, water removal is crucial in air dryer and air separation process. Accordingly, the dehydration process is generally carried out in a pretreatment unit to avoid unexpected side effects during further processing. Water removal from gas streams reduces the potential for undesirable effects, such as corrosion in the presence of acid contaminants, hydrate formation, hydrolysis reaction with the fluid itself, and condensation or freezing of moisture.1,2 In addition, because water vapor can act as a catalyst poison or a source of undesirable side reactions, gas streams are occasionally dehydrated in catalytic processes.3−5 The demands for moisture removal have continuously increased along with the increasing stringency of requirements for product quality control and production energy efficiency.6 Therefore, various dehydration methods are used in industrial practice, such as adsorption, absorption, and compression. Among them, adsorption is known as the most efficient method.1,2,7 Moisture removal can be carried out by means of temperature © XXXX American Chemical Society
swing adsorption and pressure swing adsorption processes over a wide range of operating conditions.1,8−10 Because adsorption is purely a surface phenomenon and the degree of adsorption is a function of temperature and pressure, the adsorption equilibrium of water vapor is essential in selecting the proper adsorbent and designing an efficient adsorption process.8,10,11 Zeolites are widely applied for adsorptive dehydration processes when gases having extremely low dew points are required for further processing. On the other hand, for purification of adsorptive waste gas and recovery of volatile organic compounds, highly dealuminated Y zeolite (DAY) is often selected as the adsorbent because of its hydrophobicity and relatively high ignition resistance. This study presents adsorption equilibria of water vapor on zeolite 3A, zeolite 13X, and dealuminated Y zeolite (DAY). Because pellet- or bead-type adsorbents are generally applied to adsorptive packed beds for dehydration, zeolite pellets, or beads were studied. Equilibrium experiments were performed at 293.15, 303.15, and 313.15 K and at relative pressures up to P/Ps = 0.95 (2.2, 4.0, and 7.0 kPa, respectively). The experimental data obtained were correlated using the Aranovich−Donohue (AD) and Frenkel−Halsey−Hill (FHH) models. Received: November 2, 2015 Accepted: March 1, 2016
A
DOI: 10.1021/acs.jced.5b00927 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
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2. EXPERIMENTAL SECTION Materials. Three different types of zeolite pellets or beads were selected: zeolite 3A (EPG MOLSIV, UOP, USA), zeolite 13X (APG MOLSIV, UOP, USA), and DAY (DAY 20F, 20:1 Si:Al ratio, Degussa AG, Hanau, Germany). Table 1 lists the
3. RESULTS AND DISCUSSION Adsorption Isotherm. Adsorption isotherm data of water vapor on zeolite 3A, zeolite 13X, and DAY at 293.15, 303.15, and 313.15 K are plotted in Figures 1−3, and the experimental
Table 1. Physical Properties of Zeolite 3A, Zeolite 13X, and DAY property
Zeolite 13X
Zeolite 3A
DAY
type bulk density/kg·m−3 BET surface area/m2·g−1 micropore volume/cm3·g−1 micropore size/Åc particle size/mm average pore diameter/nm total porosityd
bead 640 743a 0.3964a 8−9 2.1 1.79a 0.2923
pellet 660
pellet 500 704b 0.268b 7.4 2 2.17b 0.4342
3 3.7 0.4219
a
Data obtained by CO2 adsorption and desorption isotherm. bData obtained by N2 adsorption and desorption isotherm. cReferences 12 and 13. dData obtained by measuring mercury porosimeter.
Figure 1. Experimental and correlated isotherms for water vapor on Zeolite 3A at various temperatures (triangle up: 293.15 K, circle: 303.15 K, square: 313.15 K, filled triangle up: 298.15 K at zeolite 5A, filled square: 323.15 K at zeolite 5A (Y. Wang et al. 2009), solid lines: AD with Toth, dash lines: AD with Langmuir, dash-dot lines: AD with Sips, dotted lines: Sips).
physical properties of zeolite 3A, zeolite 13X, and DAY. Water used in the experiments was double-distilled water treated by an evaporator and deionizer with ion-exchange resin. Apparatus. Experiments to determine adsorption equilibria were conducted using an automatic volumetric sorption analyzer (Quantachrome, ASIQM0 V000-4) with a vapor sorption unit. The vapor unit consists of a manifold heater and circulation fan inside the insulated manifold compartment, plus a vapor generator and control. The vapor generator consists of a glass vial, similar to a small Erlenmeyer flask, approximately 5 cm long. The liquid (vapor source) is held in the vial, and the stem portion of the vial attaches to the vacuum-tight fitting behind the access. The generated vapor is supplied to the volumetric sorption analyzer without any condensation. Prior to each experiment, about 0.1 g of the relevant adsorbent was placed into an adsorption cell. In the method used, the total amount of gas admitted into the system and the amount of gas in the vapor phase, remaining after reaching the adsorption equilibrium, were determined from the results of appropriate P−V−T measurements. Procedure. All adsorbents were kept in dried vials at room temperature. After thermal regeneration (573.15 K) under vacuum, the mass of each adsorbent sample was measured using a microbalance with an accuracy of ±100 μg. To eliminate moisture and pollutants adsorbed during the installation of the adsorption cell, the adsorbents were the regenerated again at 573.15 K under high vacuum for 12 h. Then, the adsorption cell was placed in a water bath and maintained at a constant temperature within ±0.05 K by a water circulator (type RW, Jeio Tech Co.). To keep the water vapor from condensation in the system, the temperature of the manifold in the system was maintained over 318.15 K by a heater. The maximum experimental pressure was limited to remain at or below the point where the relative pressure of water vapor at each temperature was 0.95. The saturated vapor pressure was calculated by using the Antoine equation with the parameters obtained from the Dortmund Data Bank (DDBST) Web site. The saturated vapor pressures were 2.3295, 4.2316, and 7.3585 kPa at 293.15, 303.15, and 313.15 K, respectively. Thus, the respective maximum experimental pressures were set to 2.2, 4.0, and 7.0 kPa, according to the maximum of P/Ps = 0.95.
Figure 2. Experimental and correlated isotherms for water vapor on Zeolite 13X at various temperatures (triangle up: 293.15 K, circle: 303.15 K, square: 313.15 K, solid lines: AD with Toth, dash lines: AD with Langmuir, dash−dot lines: AD with Sips, dotted lines: Sips; filled triangle up: 298.15 K (Wang et al., 2009), filled circle: 293.15 K and filled square: 313.15 K (Kim et al., 2003).
data are presented in Tables 2−4. Experimental reproducibility within 3% was confirmed by means of duplicate experiments. The trend in water adsorption on the three zeolites was zeolite 13X > zeolite 3A > DAY for all pressure and temperature conditions tested. At all temperatures, the adsorption isotherms of zeolite 3A and zeolite 13X were classified as Type II in the BDDT classification (Figures 1 and 2). Each adsorption isotherm indicated noticeable uptakes at very low pressure region. They showed multilayer adsorption and condensation near the relative pressures P/PS of 0.87 and 0.90 on zeolite 3A and zeolite 13X, respectively. Monolayer coverage was completed at a very low pressure, showing very high adsorption affinity, and the first isotherm inflection was observed near P/PS of 0.18 for both B
DOI: 10.1021/acs.jced.5b00927 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
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below 0.65. Then, the adsorbed moles were steeply increased by multilayer adsorption and condensation. The adsorbed amount of water and the affinity of water on zeolite 13X were higher than those of water on zeolite 3A at each temperature condition tested (Figures 1 and 2, insets). In the pressure range of 0.1−0.7 kPa, the adsorbed amount of water on zeolite 13X at 313.15 K was actually also larger than that on zeolite 3A at 293.15 K. The adsorbed amount on DAY was much smaller than that on the other two zeolites because of its hydrophobicity (Figure 3). The experimental results on zeolite 13X at 293.15 and 313.15 K were compared with published results;14,15 the shape of the isotherm of zeolite 13X at 293.15 K was similar to that of the reference, but the adsorbed amount observed in the present study was higher than that of the reference (Figure 2). As listed in Table 1, the surface area (726 m2·g−1) and particle density (0.69 g·cm−2) differed somewhat from the reference values. The experimental data of zeolite 3A was compared with the previously reported data of zeolite 5A15 because it was hard to find any reported data of zeolite 3A. The adsorption capacity of zeolite 5A was higher than that of zeolite 3A. The experimental results on DAY at 293.15 and 313.15 K were also compared with the results of a previous study;16 the adsorbed amount in the present study was less than the reference value, and the
Figure 3. Experimental and correlated isotherms for water vapor on DAY at various temperatures (triangle up: 293.15 K, circle: 303.15 K, square: 313.15 K, solid lines: AD with Toth, dash lines: AD with Langmuir, dash−dot lines: AD with Sips; plus sign: 293.15 K and X sign: 313.15 K (Kim et al., 2005).
zeolite 3A and zeolite 13X. The adsorption isotherms of DAY showed type III of a convex shape in Figure 3. After an indistinct inflection near P/PS = 0.01, the adsorbed amount of water was almost linearly increased with a relative pressure
Table 2. Experimental Adsorption Isotherm Data for Water Vapor on Zeolite 3Aa P/kPa
q/mol·kg−1
P/kPa
q/mol·kg−1
0.003 0.004 0.006 0.008 0.011 0.013 0.015 0.017 0.019 0.021
0.043 0.899 1.813 3.065 5.959 6.741 7.030 7.212 7.340 7.449
0.043 0.063 0.084 0.113 0.134 0.157 0.183 0.205 0.229 0.346
7.923 8.394 8.652 8.796 8.905 9.018 9.133 9.223 9.314 9.602
0.005 0.008 0.014 0.017 0.021 0.026 0.028 0.031 0.034 0.038
0.001 0.363 0.992 1.735 2.563 3.442 4.733 6.678 7.145 7.364
0.080 0.115 0.162 0.204 0.247 0.285 0.332 0.377 0.419 0.646
7.810 7.985 8.213 8.353 8.464 8.552 8.640 8.726 8.789 9.071
0.008 0.014 0.020 0.028 0.037 0.041 0.047 0.056 0.066 0.069
0.196 0.637 1.311 2.245 3.275 4.264 5.509 6.663 6.939 7.197
0.139 0.199 0.277 0.356 0.433 0.500 0.577 0.658 0.723 1.150
7.608 7.867 8.075 8.234 8.356 8.447 8.538 8.625 8.689 8.983
P/kPa
q/mol·kg−1
P/kPa
q/mol·kg−1
0.473 0.583 0.704 0.815 0.949 1.055 1.171 1.286 1.399 1.520
9.876 10.09 10.29 10.45 10.61 10.74 10.85 10.95 11.04 11.16
1.637 1.740 1.863 1.986 2.102 2.118 2.141 2.167 2.185 2.211
11.27 11.39 11.57 11.85 12.39 12.54 12.73 12.97 13.16 13.45
0.853 1.085 1.263 1.480 1.684 1.915 2.120 2.320 2.558 2.760
9.270 9.455 9.568 9.694 9.814 9.935 10.03 10.13 10.23 10.35
2.967 3.180 3.387 3.605 3.839 3.857 3.893 3.944 3.994 4.018
10.48 10.66 10.85 11.15 11.75 11.87 12.03 12.24 12.49 12.70
1.489 1.870 2.224 2.560 2.984 3.328 3.702 4.038 4.421 4.817
9.161 9.332 9.467 9.577 9.711 9.810 9.914 10.02 10.13 10.27
5.165 5.703 6.286 6.644 6.693 6.768 6.851 6.918 6.990
10.40 10.61 10.85 11.22 11.33 11.52 11.71 11.90 12.09
293.15 K
303.15 K
313.15 K
a
Uncertainties: ΔT = 0.05 K, P/Ps < 0.001. C
DOI: 10.1021/acs.jced.5b00927 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
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Table 3. Experimental Adsorption Isotherm Data for Water Vapor on Zeolite 13Xa P/kPa
q/mol·kg−1
P/kPa
q/mol·kg−1
0.003 0.004 0.006 0.009 0.011 0.013 0.015 0.018 0.019 0.021
0.023 1.763 5.383 6.316 7.257 8.200 9.125 10.47 11.04 11.26
0.044 0.065 0.087 0.114 0.138 0.157 0.182 0.203 0.228 0.355
12.04 12.53 12.87 13.05 13.39 13.57 13.75 13.90 14.04 14.55
0.005 0.012 0.018 0.026 0.029 0.032 0.036 0.038 0.077 0.114
0.014 1.493 3.276 5.734 7.707 9.213 10.31 10.89 11.82 12.27
0.165 0.204 0.243 0.290 0.332 0.371 0.415 0.632 0.876 1.065
12.73 13.00 13.23 13.45 13.62 13.76 13.90 14.38 14.77 15.01
0.009 0.014 0.023 0.027 0.037 0.041 0.048 0.057 0.061 0.068
0.631 1.887 3.615 5.362 7.291 8.686 9.686 10.30 10.58 10.74
0.125 0.224 0.273 0.348 0.441 0.501 0.573 0.654 0.717 1.122
11.37 12.15 12.45 12.75 13.05 13.22 13.39 13.56 13.68 14.20
P/kPa
q/mol·kg−1
P/kPa
q/mol·kg−1
0.470 0.583 0.705 0.823 0.929 1.052 1.159 1.287 1.401 1.522
14.90 15.17 15.43 15.64 15.82 16.00 16.15 16.33 16.48 16.65
1.625 1.744 1.863 1.978 2.100 2.118 2.138 2.164 2.186 2.208
16.80 16.99 17.22 17.50 17.97 18.09 18.25 18.44 18.63 18.86
1.282 1.503 1.698 1.920 2.138 2.329 2.558 2.771 2.972 3.170
15.26 15.46 15.63 15.80 15.96 16.09 16.25 16.40 16.54 16.70
3.394 3.601 3.822 3.852 3.894 3.936 3.977 4.017
16.89 17.13 17.47 17.57 17.67 17.76 17.90 18.04
1.454 1.835 2.252 2.606 2.991 3.307 3.669 4.038 4.420 4.792
14.50 14.77 15.02 15.21 15.40 15.53 15.67 15.81 15.95 16.09
5.159 5.521 5.881 6.283 6.643 6.696 6.775 6.884 6.934 7.022
16.23 16.37 16.51 16.70 16.88 16.94 16.99 17.10 17.12 17.40
293.15 K
303.15 K
313.15 K
a
Uncertainties: ΔT = 0.05 K, P/Ps < 0.001.
where q is the number of adsorbed molecules in moles, and the term 1/(1−P/PS)d describes the divergence of q under pressure approaching saturation. Given the definition of f(P), the monolayer capacity qm can be determined from f(PS) because f(PS) corresponds to the maximum monolayer adsorption. The following Langmuir, Toth, UNILAN, and Sips equations were used to model f(P) in eq 1. Langmuir isotherm model:
isotherm shape was also quite different from that of the reference isotherm, which was nearly linear. Because of this difference, the isotherms were measured repeatedly and confirmed to be reproducible within 3%. It is expected that the observed difference in isotherms arose from differences in the properties of adsorbents manufactured by different companies, because the crystal size and the binder type can be different among the zeolites used in the study and references. In addition, differences in the experimental method and system may have led to some differences in the resulting isotherms. Isotherm Models. The adsorption isotherms of zeolite 3A and zeolite 13X were classified as type H3 in the classification system of Giles et al.17 In the adsorption isotherms each had an inflection point at which the isotherm changed from convex to concave (high adsorption affinity at low pressure to condensation with pressure increase; see Figures 1 and 2). On the other hand, the adsorption isotherms of DAY were of the L3 type. Although the L3 type is similar to the H3 type, the slope of the L3 isotherm remained almost constant throughout the low pressure region (Figure 3). In the study, the AD and FHH models were used to correlate the experimental data.18−20 The AD equation is as follows: q = f (P)/(1 − P /PS)d
f (P ) =
qmbP (1 + bP)
(2)
Toth isotherm model: f (P ) =
qmbP (1 + (bP)t )1/ t
(3)
UNILAN isotherm model: f (P ) =
qm ln⎡⎣(c + Pe+s)/(c + P e−s)⎤⎦ 2s
(5)
Sips isotherm model: f (P ) =
(1) D
qm(bP)1/ n 1 + (bP)1/ n
(6) DOI: 10.1021/acs.jced.5b00927 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
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Table 4. Experimental Adsorption Isotherm Data for Water Vapor on DAYa P/kPa
q/mol·kg−1
P/kPa
q/mol·kg−1
0.004 0.005 0.006 0.009 0.011 0.013 0.015 0.017 0.019 0.022
0.010 0.026 0.037 0.052 0.061 0.070 0.076 0.082 0.086 0.089
0.042 0.068 0.090 0.114 0.137 0.162 0.186 0.206 0.227 0.353
0.116 0.143 0.162 0.179 0.194 0.210 0.225 0.237 0.249 0.310
0.005 0.008 0.017 0.019 0.023 0.027 0.031 0.037 0.042 0.082
0.001 0.041 0.067 0.075 0.081 0.088 0.093 0.100 0.105 0.133
0.121 0.168 0.202 0.242 0.285 0.330 0.370 0.415 0.645 0.869
0.152 0.173 0.187 0.201 0.216 0.229 0.242 0.256 0.314 0.376
0.008 0.022 0.023 0.038 0.043 0.048 0.056 0.061 0.069 0.140
0.001 0.036 0.036 0.046 0.048 0.051 0.057 0.060 0.066 0.097
0.211 0.292 0.358 0.428 0.504 0.583 0.656 0.731 1.112 1.505
0.12 0.141 0.155 0.170 0.185 0.198 0.212 0.224 0.283 0.341
P/kPa
q/mol·kg−1
P/kPa
q/mol·kg−1
0.466 0.576 0.705 0.822 0.938 1.063 1.163 1.286 1.404 1.518
0.370 0.434 0.514 0.595 0.683 0.792 0.896 1.045 1.221 1.435
1.635 1.741 1.862 1.975 2.092 2.115 2.139 2.167 2.188 2.208
1.749 2.213 3.145 4.860 7.677 8.623 9.609 10.61 11.59 12.36
1.084 1.272 1.496 1.707 1.908 2.132 2.351 2.551 2.754 2.959
0.438 0.494 0.568 0.643 0.733 0.845 0.989 1.147 1.352 1.610
3.171 3.381 3.600 3.818 3.847 3.896 3.952 3.979 4.018
2.069 2.875 4.438 7.382 8.115 8.697 9.462 10.50 11.25
1.854 2.192 2.585 2.984 3.334 3.732 4.072 4.468 4.776 5.169
0.398 0.463 0.535 0.624 0.712 0.844 0.975 1.158 1.350 1.675
5.523 5.887 6.261 6.619 6.743 6.775 6.839 6.921 6.994
2.087 2.914 4.363 6.246 7.390 7.607 8.058 9.400 10.56
293.15 K
303.15 K
313.15 K
a
Uncertainties: ΔT = 0.05 K, P/Ps < 0.001.
Here, qm, b, t, c, s, n, A, B, and d are isotherm parameters that are numerically determined. In addition, adsorption isotherm data in the low pressure range (up to P/PS = 0.3) were correlated using the Sips isotherm model given in eq 6. The FHH equation was originally developed by assuming that the adsorption potential varies with distance from the surface.19 The FHH equation can describe the range of multilayer adsorption on a homogeneous planar surface and is attractive for its simple mathematical form: ⎛P⎞ ln⎜ ⎟ = − A(q)−B ⎝ PS ⎠
where k is the number of data points at a given temperature, and where qexp and qcal refer to experimentally observed and calculated numbers of adsorbed moles, respectively. The adsorbed moles at very low relative pressures were commonly very small; hence these experimental data definitely had much greater deviations than others measured at higher relative pressures. As a result, deviation calculations including all experimental points could lead to some skewed results due to the inclusion of these data. For an example, using the AD equation with the Sips isotherm yielded a calculation of 327% average deviation from experimental data on zeolite 3A at
(7)
Table 5. AD Equation Parameters for Water Vapor on Zeolite 3A, Zeolite 13X, and DAY for f(P) = Langmuir Equation
The parameter B is regarded as a standard of the rate of decline in the adsorption potential with distance from the surface. B is usually less than 3, and it is assumed that observed deviations from the theoretical value arise from the roughness of a real adsorbent surface. The fitting of the models to experimental data was performed using Matlab 7.10 software (Mathworks, Inc.), using a nonlinear curve fitting procedure. In this study, the average percent deviation (Δq) was calculated using the following equations: 100 Δq = k
k
∑ j=1
T/K
qm/mol·kg−1
b/kPa
d
Δq/%
Zeolite 3A
293.15 303.15 313.15 293.15 303.15 313.15 293.15 303.15 313.15
10.25 9.5 9.326 15.45 15.5 15.13 0.564 0.4872 0.4412
61.56 33.01 20.93 63.63 26.93 18.47 2.742 2.3 1.284
0.08859 0.09621 0.08936 0.07048 0.05668 0.0514 1.097 1.101 1.108
5.36 4.45 3.67 4.31 3.96 5.3 21.58 17.94 15.56
Zeolite 13X
qjexp − qjcal qjexp
adsorbent
DAY
(8) E
DOI: 10.1021/acs.jced.5b00927 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
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Table 6. AD Equation Parameters for Water Vapor on Zeolite 3A, Zeolite 13X, and DAY for f(P) = Toth Equation adsorbent
T/K
qm/mol·kg−1
b/kPa
t
d
Δq/%
Zeolite 3A
293.15 303.15 313.15 293.15 303.15 313.15 293.15 303.15 313.15
11.24 10.04 10.4 19.43 18.64 19.06 6.591 10.1 31.62
3641 2837 2371 9.35 × 104 8415 1.11 × 104 11.18 27.88 9.198
0.3836 0.4031 0.36 0.2472 0.2938 0.2652 0.1801 0.1404 0.117
0.0826 0.09479 0.07506 0.05246 0.04113 0.02983 1.05 1.042 1.012
1.22 4.74 2.57 2.58 1.52 1.64 10.33 9.46 5.03
Zeolite 13X
DAY
Table 7. AD Equation Parameters for Water Vapor on Zeolite 3A, Zeolite 13X, and DAY for f(P) = UNILAN Equation adsorbent
T/K
qm/mol·kg−1
c/kPa
s
D
Δq/%
Zeolite 3A
293.15 303.15 313.15 293.15 303.15 313.15 293.15 303.15 313.15
20.27 18.16 17.48 29.04 30.16 26.76 1.687 1.979 2.233
0.9465 0.9881 0.9816 0.4078 1.277 0.6102 29.2 736.5 3124
15.25 18.12 14.61 13.25 12.63 11.33 6.505 9.899 10.16
0.06952 0.0823 0.06074 0.04414 0.02864 0.01895 1.078 1.079 1.067
1.4 5.83 3.43 3.26 2.13 2.1 14.7 11.36 10.16
Zeolite 13X
DAY
Table 8. AD Equation Parameters for Water Vapor on Zeolite 3A, Zeolite 13X, and DAY for f(P) = Sips Equation adsorbent
T/K
qm/mol·kg−1
b/kPa
n
d
Δq/%
Zeolite 3A
293.15 303.15 313.15 293.15 303.15 313.15 293.15 303.15 313.15
10.92 9.802 10.26 16.01 16.62 19.74 3.514 2.446 1.61
172.1 147.9 82.75 102 54.62 23.59 0.01194 0.01113 0.03329
2.083 1.948 2.434 1.557 1.894 3.526 2.182 2.353 1.9
0.0829 0.09637 0.07548 0.06618 0.04752 0.0258 1.049 1.047 1.023
1.41 4.55 2.6 2.22 1.61 1.6 6.57 6.4 4.55
Zeolite 13X
DAY
Table 9. FHH Equation Parameters for Water Vapor on Zeolite 3A, Zeolite 13X, and DAY adsorbent
T/K
Zeolite 3A
293.15 303.15 313.15 293.15 303.15 313.15 293.15 303.15 313.15
Zeolite 13X
DAY
A/mol·kg−1 4.47 × 1.49 × 2.98 × 1.48 × 8.92 × 1.50 × 0.6144 0.6136 0.5208
1010 1010 109 1013 1010 1012
B/kPa
Δq/%
10.56 10.39 9.8 11.15 9.287 10.53 0.9704 0.9806 0.9682
5.38 8.75 7.28 9.49 8.45 9.1 14.39 9.03 19
Table 10. Sips Equation Parameters for Water Vapor on Zeolite 3A, Zeolite 13X, and DAY at Relative Pressure (P/PS) up to 0.3 adsorbent
T/K
qm/mol·kg−1
b/kPa
n
Δq/%
Zeolite 3A
293.15 303.15 313.15 293.15 303.15 313.15 293.15 303.15 313.15
15.64 9.153 8.786 18.06 15.31 15.05 42.88 41.76 28.78
28.6 50.1 28.71 124.7 40.11 30.05 4.47 × 10−4 1.55 × 10−4 3.75 × 10−4
4.757 0.802 0.706 2.633 1.215 1.256 1.803 1.919 1.705
1.46 8.32 4.99 4.37 3.89 4.26 10.08 9.89 5.05
Zeolite 13X
DAY
293.15 K, due to the initial few data in the very low pressure range, even though the model showed good agreement with the experimental data overall (Figure 2). Therefore, in the present study, the first three experimental points corresponding to the lowest pressures tested were excluded from the deviation calculation. Tables 5−10 summarize the model parameters. Correlated results from the AD equation, used with the Sips isotherm, the Langmuir isotherm, and the Toth isotherm, respectively, are presented in Figures 1−3, and the results of the Sips monolayer adsorption model at relative pressures up to 0.3 are shown in
the insets of Figures 1−3. Overall, the AD equation with the Sips isotherm provided better fits than the other models, but the difference in deviation among the models was not significant and the steep slope of the isotherm in the low pressure regions of the zeolite 3A and 13X (H3 type) isotherms was also predicted well. Isosteric Heat of Adsorption. Temperature variation arising from adsorption or desorption affects adsorption dynamics in an adsorption bed regardless of the feed temperature. Therefore, the isosteric heat of adsorption is a key F
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zeolite 13X > zeolite 3A > DAY at all experimental temperatures, and the trend in isosteric heats of adsorption was zeolite 3A > zeolite 13X > DAY. The adsorption isotherms of zeolite 3A and zeolite 13X were Type II isotherms in the BDDT classification, and Type H3 isotherms in the classification of Giles and co-workers. High adsorption affinity of water on zeolite 3A and zeolite 13X was observed in the low pressure range, and completion of a monolayer and formation of a second layer started from P/PS = 0.18. On the other hand, the DAY isotherm was classified as Type III in the BDDT classification and Type L2 in the classification of Giles and co-workers. Experimental data were correlated using the AD equation with the Langmuir, Toth, UNILAN, or Sips isotherms, and also using the FHH equation. Among the models applied, the AD equation with Sips isotherm yielded the best predictions for all adsorbents. These results may be useful for designing adsorptive dehydrogenation and/or solvent recovery processes.
thermodynamic variable for designing practical gas separation processes such as pressure swing adsorption, vacuum swing adsorption, and temperature swing adsorption processes. In this study, the isosteric heats of adsorption were calculated from the temperature dependence of the equilibrium capacity by using the Clausius−Clapeyron equation.20 The heats of adsorption were estimated based on the isotherms at three different temperatures. The ln p was plotted to 1/T, which was obtained from Figures 1−3 at a fixed adsorbed amount. Then, the ΔQst could be calculated from the slope of −ΔQst/R in a straight fitting line:
ΔQ st RT
2
⎡ ∂ ln P ⎤ =⎢ ⎣ ∂T ⎥⎦q
(9)
where ΔQst is the isosteric heat of adsorption, P is pressure, T is temperature, and R is the gas constant. The isosteric heats of adsorption are presented in Figure 4. The values of zeolites 3A and 13X were obtained from the
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AUTHOR INFORMATION
Corresponding Author
*Tel.: +82 2 2123 2762; Fax: +82 2 312 6401. E-mail address:
[email protected] (C.H. Lee). Funding
We would like to acknowledge the financial support from the R&D Convergence Program of MSIP (Ministry of Science, ICT and Future Planning) and NST (National Research Council of Science & Technology) of Republic of Korea (CRC14-1-KRICT). Notes
The authors declare no competing financial interest.
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A B b c d n P Qst q Δq qm R T t
Figure 4. Isosteric heats of adsorption on various adsorbents with respect to experimental data: Zeolite 3A (solid line), Zeolite 13X (dash−dot line), and DAY (dash−dot−dot line).
linear isotherm region, showing very steep linear increase with pressure in Figures 1 and 2, but the values of DAY were calculated from the FHH isotherms. The isosteric heats of adsorption varied within the range of 75 to 45 kJ·mol−1, and the results corresponded to the range for zeolites reported in the literature.21 The heats of adsorption followed the trend of zeolite 3A > zeolite 13X > DAY. For all zeolites, the decrease in the isosteric heat was observed with increasing adsorbed amount. Therefore, the vertical interaction between water vapor and adsorbent was dominant in the low pressure range. Comparing the variations in the heat of adsorption among the three zeolite materials, zeolites 3A and 13X showed steep decrease in the heat of adsorption during initial loading. And their heats of adsorption continuously decreased with increasing loading amount. On the other hand, the heat of adsorption of DAY became almost constant with increasing loading amount after an initial decrease. This implied that condensation was dominant after initial loading up to 4 mol·kg−1.
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LIST OF SYMBOLS FHH equation parameter FHH equation parameter Langmuir, Sips, and Toth isotherm parameter (kPa−1) UNILAN isotherm parameter (kPa) AD equation parameter (−) Sips isotherm parameter (−) Pressure (kPa) Isosteric heat of adsorption (kJ·mol−1) Adsorbed amount moles (mol·kg−1) Average percent deviation (%) Langmuir, Sips, and Toth isotherm parameter (mol·kg−1) Ideal gas constant (J·mol−1·K−1) Temperature (K) Toth isotherm parameter (−) REFERENCES
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3. CONCLUSION Adsorption isotherms of water vapor on zeolite 3A, zeolite 13X, and DAY were measured by a volumetric method conducted at 293.15, 303.15, and 313.15 K and at relative pressures up to 0.95. The trend in water adsorption on the three zeolites was G
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