Zeolite 13X

Article pubs.acs.org/jced

Adsorption Equilibria of Water Vapor on an Alumina/Zeolite 13X Composite and Silica Gel Hyun-Taek Oh,†,§ Seung-Jun Lim,†,§ Jeong Hoon Kim,‡ and Chang-Ha Lee*,† †

Department of Chemical and Biomolecular Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-Gu, Seoul 120-749, Republic of Korea ‡ Carbon Resources Institute, Korea Research Institute of Chemical Technology, 141 Gajeongro, Yuseong, Daejeon, Korea ABSTRACT: The adsorption equilibria of water vapor on an alumina/zeolite 13X composite and silica gel were measured at 293.15, 303.15, and 313.15 K and at relative pressures (P/Ps) of up to 0.95. The experimental isotherms were compared with those of zeolite 13X and alumina. The data were correlated by the Dubinin−Astakhov (DA) model and the Aranovich− Donohue (AD) model using Sips and Toth isotherms. The heats of adsorption were in the range of 50−59 kJ·mol−1 for silica gel but decreased from 75 to 45 kJ·mol−1 for the alumina/zeolite 13X composite with increased loading. The composite showed much higher adsorption affinities and adsorption amounts than did silica gel in the low-pressure range, owing to the characteristics of zeolite 13X. However, at increased pressures, the isotherm shape for the composite was similar to that for alumina.

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INTRODUCTION

following adsorbent layer plays a role of a purifier to produce the product gas.4−7 Activated alumina, silica gel, and zeolites are representative adsorbents that are commonly used in industry. Activated alumina is widely used in the drying of compressed air, natural gas, and saturated hydrocarbons, as well as in TSA and PSA dryers.2,8,9 This adsorbent has a large surface area, high strength against crushing, and stable physical and chemical characteristics, even in high temperature and corrosive environments. Silica gel is also widely applied for dehydration processes. Recently, it has been used as a refrigerant in water adsorption refrigeration systems driven by waste heat and solar energy.10 In addition, silica gel is suitable for the bulk separation of water in the engineering plastic and pharmaceutical industries.9 Zeolites are used for adsorptive dehydration processes to produce gases with extremely low dew points because they have very strong adsorption affinities for water molecules. However, as zeolites have lower saturation capacities than activated alumina and silica gel, a layered bed TSA process using both zeolite 13X and silica gel was suggested to improve the separation and thermal efficiencies of the drying process.4 New adsorbents are continuously developed to achieve improved adsorption capacities while maintaining the desired adsorption affinity characteristics. It is generally reported that zeolite shows a high affinity for polar molecules in a broad range and activated alumina high adsorption capacity at high humidity conditions.2,11 Recently, a new hybrid adsorbent,

Water vapor is one of the most undesirable impurities in many processes. Hence, the demand for moisture removal from products has continuously increased with the increasing stringency of requirements for product quality control and process energy efficiency. In various gas separation processes, such as H2 separation, natural gas purification, air separation, and N2O recovery, water vapor can deteriorate the separation efficiency. For example, during CO2 capture, water vapor is one of the most detrimental components because it decreases the CO2 capture capacity and increases energy consumption.1 Furthermore, in many petrochemical processes, a dehydration process is often required to reduce undesirable side effects, such as corrosion, hydrate formation, hydrolysis reactions, and condensation. Therefore, for many industrial effluent gases with a certain level of moisture, a pretreatment unit is generally essential to remove water vapor before further processing. Among the various available dehydration methods, adsorption technology, such as a packed bed, temperature swing adsorption (TSA), and pressure swing adsorption (PSA) processes, is known as to be the most efficient method for dehydration of gases with low dew points.2,3 A packed bed can be used to remove trace amount of water vapor from various gas mixtures. After saturation, the bed is replaced by a new adsorbent bed or regenerated by inert gas purge or hot gas. TSA and PSA processes are designed to purify the feed gas by a cyclic operation using multibeds consisting of a single adsorbent or multiadsorbents. Generally, in the case of a layered bed, the first adsorbent layer works as a bulk separator to remove water or large molecular impurities, and the © 2017 American Chemical Society

Received: September 30, 2016 Accepted: December 22, 2016 Published: January 5, 2017 804

DOI: 10.1021/acs.jced.6b00850 J. Chem. Eng. Data 2017, 62, 804−811

Journal of Chemical & Engineering Data

Article

g−1) and alumina (340 m2·g−1).12 Silica gel had a high surface area of 952 m2·g−1 and a micropore volume of 0.317 cm3·g−1. Using the Dubinin−Radushkevich (DR) method, the micropore size and volume of the alumina/zeolite 13X composite were determined as 9.21 Å and 0.161 cm3·g−1, respectively, which are somewhat smaller than those of silica gel. Owing to the presence of alumina in the composite, the average pore diameter was 3.91 nm. Apparatus and Procedure. Adsorption equilibria experiments were conducted using an automatic volumetric sorption analyzer (ASIQM0 V000-4, Quantachrome, USA) with a vapor sorption unit. Double-distilled water treated with an evaporator and a deionizer with an ion-exchange resin was used in all experiments. The vapor unit consisted of a manifold heater and circulation fan inside an insulated manifold compartment, as well as a vapor generator and controller. The vapor generator consisted of a glass vial, similar to a small Erlenmeyer flask, approximately 5 cm in length. Water (vapor source) was held in the vial, and the stem of the vial was attached to a vacuum-tight fitting on the vapor sorption unit. The generated water vapor was supplied to the volumetric sorption analyzer without any condensation. All adsorbents were kept in dried vials at room temperature. To eliminate any moisture or pollutants adsorbed during installation of the adsorption cell, the adsorbents were regenerated at 423.15 K for silica gel and 573.15 K for the alumina/zeolite 13X composite under vacuum conditions for 12 h. Then, the mass of each adsorbent sample was measured using a microbalance with an accuracy of ±100 μg. The adsorption cell was placed in a circulating water bath (RW model, Jeio Tech Co., Republic of Korea) and maintained at a constant temperature (±0.05 K). To prevent water vapor from condensing in the system, a heater was used to maintain the manifold at a temperature greater than 318.15 K. The maximum experimental pressure was limited to that at which the relative pressure of water vapor at each temperature was 0.95. The saturated vapor pressure was calculated using the Antoine equation with 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 at each corresponding experimental temperature. The adsorption data were obtained by automatically adding or subtracting a known quantity of vapor from the adsorption cell at a constant temperature. A sample is considered to reach equilibrium when the pressure changes less than 0.0008 bar for the period of 10 min. In this method, the quantity of vapor adsorbed at the equilibrium pressure was determined from the difference between the supplied amount of vapor and final amount remaining in the adsorption cell.

which is an alumina/zeolite 13X composite, has been commercialized to utilize the advantageous characteristics of both alumina and zeolite. It is worthwhile to examine the thermodynamic data of the alumina/zeolite 13X composite because the results help to understand the adsorption characteristics of the composites. By comparing the adsorption characteristics of silica gel, which is one of the representative adsorbents for dehydration, the feasibility and applicability of the alumina/zeolite 13X composite can be evaluated for dehydration. The purpose of this study is to provide adsorption equilibrium data of water on two different adsorbents, which can be applied to adsorptive dehydration processes. The adsorption characteristics of the newly developed adsorbent, the alumina/zeolite 13X composite, were compared with those of silica gel which is widely used for dehydration. The adsorption equilibria of water vapor on these two adsorbents were determined using a volumetric method. Bead (or granule)-type adsorbents were used to measure the adsorption isotherms 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 collected data in the experimental temperature range can be used to select a guideline for adsorbents and design a packed bed or adsorptive cyclic processes. The experimental data were correlated by the Dubinin−Astakhov (DA) model and the Aranovich−Donohue (AD) model using Sips and Toth isotherms. However, as it was difficult to fit the full data sets to the isotherm models, Henry’s law was applied to fit the experimental data in the very low-pressure region instead of the DA and AD models. In addition, the changes in entropy and heat of adsorption with adsorption amount are discussed using the results of the DA equation.

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EXPERIMENTAL SECTION Materials. Alumina/zeolite 13X composite beads (AZ-300, UOP, USA) and silica gel granules (grade 40, Grace Davison, USA) were used for the experiments. The mole ratio of Al to Si in the alumina/zeolite 13X composite was determined as 3:1 using inductively coupled plasma (5100 ICP-OES, Agilent Technologies, USA) analysis, which implies that the composite is hydrophilic. In the composite, the high Al content was expected, as it is a component of both alumina and zeolite 13X. The physical properties of the alumina/zeolite 13X composite and silica gel were measured using an automatic volumetric sorption analyzer (ASIQM0 V000-4, Quantachrome, USA), and the results are listed in Table 1. The Brunauer−Emmett−Teller (BET) surface area of the alumina/zeolite 13X composite was found to be 455 m2·g−1, which is intermediate between those of zeolite 13X (726 m2· Table 1. Physical Properties of the Alumina/Zeolite 13X Composite and Silica Gel property type

alumina/zeolite bead

silica gel granule

bulk density (kg·m−3) BET surface areaa (m2·g−1) micropore volumea (cm3·g−1) micropore sizea (Å) average pore diametera (nm) total porosityb

721 455 0.161 9.21 3.91 0.283

736 952 0.317 14.64 2.25 0.251

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RESULTS AND DISCUSSION Adsorption Isotherms. The adsorption isotherms of water vapor on silica gel and the alumina/zeolite 13X composite are plotted in Figures 1 and 2, and the experimental data are presented in Tables 2 and 3. Experimental reproducibility (within 3%) was confirmed by means of duplicate experiments. At all temperatures, the adsorption isotherms for silica gel were Type IV in the IUPAC classification (Figure 1). Overall, the isotherm shape appeared linear from low pressures to near the highest pressure. However, a close examination reveals that

a

Data obtained from N2 adsorption and desorption isotherms. bData obtained using a mercury porosimeter. 805



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Interestingly, when compared with the isotherms of activated alumina2 and zeolite 13X,11 the shape of the alumina/zeolite 13X composite isotherm was similar to the combined isotherms of alumina and zeolite 13X. Owing to the characteristics of zeolite 13X, the adsorption affinity and adsorption amount of the composite in the very low-pressure region were much higher than those of activated alumina. After the isotherm inflection, the adsorption capacity remained higher than that of activated alumina but was smaller than that of zeolite 13X. Moreover, the adsorption amount at P/PS = 0.95 was greater than that of zeolite 13X due to the role of alumina. The alumina/zeolite 13X composite also had much a higher adsorption amount than silica gel in the very lowpressure region. Prediction by Isotherm Models. The Dubinin and Radushkevich (DR) equation was derived from adsorption potential theory of Polanyi, assuming that adsorption progresses by volume filling not building up adsorption layer one by one and that pore makes Gaussian distribution. Then, the Dubinin and Astakov (DA) equation was extended from the concept of the DR equation and made it more common form, which allows for surface heterogeneity.14−16 If the degree of heterogeneity increases owing to a wide pore size distribution, the DR equation does not describe the equilibrium data well, and thus, the DA equation was developed. The DA equation is as follows:17,18

Figure 1. Experimental and correlated adsorption isotherms for water vapor on silica gel at various temperatures. Reference data: filled triangle: silica gel at 293.15 K, filled square: silica gel at 313.15 K.13

⎛ ⎛ A ⎞n ⎞ q = V exp⎜ −⎜ ⎟ ⎟ ⎝ ⎝E⎠ ⎠

(1)

⎛P ⎞ A = RT ln⎜ s ⎟ ⎝P⎠

(2)

where n is an empirical parameter that describes the surface heterogeneity. When n = 2, the DA equation reduces to the DR equation. The parameter A describes the adsorption potential, which is defined in eq 2, V is the limiting micropore volume, and q represents the adsorption amount at temperature T and relative pressure P/Ps. The adsorption isotherms of the alumina/zeolite 13X composite were Type II according to the IUPAC classification. The characteristic feature of Type II multilayer adsorption isotherms is an apparent divergence of the adsorption amount as the pressure approaches saturation, and such isotherms are represented by the following AD equation:12,19

Figure 2. Experimental and correlated adsorption isotherms for water vapor on the alumina/zeolite 13X composite at various temperatures. Reference data: filled blue triangle: zeolite 13X at 293.15 K, filled blue circle: zeolite 13X at 303.15 K,11 filled red triangle: alumina at 288.15 K, filled red circle: alumina at 298.15 K.2

the changes in the isotherms with pressure were quite complex. The isotherm could be divided into three parts. Up to P/Ps = 0.18, the isotherm is slightly convex. Then, the isotherm has a somewhat concave shape until P/Ps = 0.45. The final part of the isotherm has a clearly convex shape. The isotherms of silica gel in this study were slightly higher at 293.15 K and lower at 313.15 K than those reported previously,13 which is a result of the somewhat different physical properties of the silica gels used. The adsorption isotherms of the alumina/zeolite 13X composite were classified as Type II in the IUPAC classification (Figure 2). The isotherms obtained for the composite were compared with those of zeolite 13X and activated alumina reported previously.2,11 The isotherms of activated alumina were not found at the same temperature range, but the relative comparison was feasible for understanding the adsorption isotherm of the alumina/zeolite 13X composite. Then, a steady increase driven by multilayer adsorption was observed.

q = f (P)/(1 − P /Ps)d

(3)

where q is the adsorbate loading and the term 1/(1 − P/Ps)d describes the divergence of q when the pressure approaches saturation. The Toth and Sips equations were used in the study to determine f(P) in eq 3. Sips isotherm model: f (P ) =

qm(bP)1/ n 1 + (bP)1/ n

(4)

Toth isotherm model: f (P ) =

qmbP (1 + (bP)t )1/ t

(5)

where qm, b, t, and n are numerically determined isotherm parameters. 806



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Table 2. Experimental Adsorption Isotherm Data for Water Vapor on the Alumina/Zeolite 13X Compositea P (kPa)

q (mol·kg−1)

P (kPa)

q (mol·kg−1)

0.002 0.004 0.006 0.009 0.011 0.013 0.015 0.017 0.019 0.021

0.032 1.501 2.807 3.540 4.174 4.702 5.126 5.394 5.557 5.681

0.043 0.064 0.087 0.112 0.134 0.157 0.181 0.207 0.229 0.358

6.172 6.512 6.787 7.004 7.220 7.393 7.566 7.733 7.865 8.449

0.004 0.008 0.013 0.016 0.021 0.023 0.027 0.031 0.035 0.039

0.285 0.859 1.659 2.466 3.362 4.128 5.105 5.320 5.475 5.574

0.080 0.119 0.162 0.204 0.244 0.287 0.334 0.415 0.632 0.855

6.093 6.382 6.675 6.903 7.089 7.278 7.460 7.731 8.268 8.779

0.008 0.021 0.032 0.036 0.040 0.049 0.054 0.062 0.141 0.208

0.538 2.040 3.273 4.265 4.943 5.231 5.348 5.471 6.007 6.339

0.278 0.361 0.430 0.508 0.569 0.660 0.727 1.123 1.493 1.831

6.601 6.857 7.049 7.237 7.376 7.559 7.688 8.219 8.692 9.083

P (kPa)

q (mol·kg−1)

P (kPa)

q (mol·kg−1)

0.469 0.593 0.705 0.810 0.939 1.055 1.163 1.277 1.407 1.511

8.905 9.362 9.804 10.26 10.87 11.51 12.16 12.81 13.67 14.41

1.628 1.746 1.860 1.981 2.095 2.117 2.137 2.163 2.187 2.209

15.27 16.13 17.00 18.09 19.30 19.59 19.89 20.27 20.62 20.90

1.054 1.281 1.488 1.706 1.907 2.121 2.324 2.550 2.743 2.959

9.197 9.675 10.15 10.71 11.27 11.94 12.62 13.38 14.05 14.84

3.173 3.398 3.613 3.816 3.854 3.891 3.941 3.978 4.022

15.61 16.46 17.40 18.49 18.79 19.05 19.36 19.66 19.96

2.230 2.586 2.967 3.314 3.696 4.052 4.405 4.808 5.160 5.524

9.551 10.00 10.53 11.06 11.72 12.39 13.07 13.90 14.71 15.54

5.878 6.266 6.649 6.841 6.911 7.008

16.37 17.48 18.63 19.64 19.88 20.46

293.15 K

303.15 K

313.15 K

a

Uncertainties: ΔT = 0.05 K, P/Ps < 0.001.

model showed good overall agreement with the experimental data (Figure 2), the calculated deviation was unreliable because of the initial data points in the very low-pressure region. Therefore, in the present study, the first four experimental points corresponding to the lowest pressures tested were excluded from the deviation calculation. The obtained best-fitting parameters and the average relative error are summarized in Tables 4−7. The correlated results from the DA and AD equations, experimental data, and reference data are presented in Figures 1 and 2. The parameter V of the DA equation indicates the limiting micropore volume, but it is noted that the DA equation for water adsorption does not necessarily follow a micropore filling mechanism.14 Even though the micropore volume of silica gel is much higher than that of the alumina/zeolite 13X composite in Table 1, the adsorption isotherms of two adsorbents showed opposite results. It implies that the parameter V is not exactly followed by a limiting micropore volume. Furthermore, the DR and DA equations are suggested for the estimation of the micropore volume from the low- and medium-relative pressure parts of the adsorption isotherm.20 Since the DR or DA equations were applied to a full-pressure range in the study, it was hard to have a physical meaning for the parameter V but have a fitting parameter to estimate the isotherms.

The AD and DA models were applied to the experimental isotherms for both adsorbents. In addition, the Sips and Toth models were also used, but they were applied to the isotherms for the alumina/zeolite 13X in a low relative pressure range (up to P/Ps = 0.3) because the isotherm showed Type II. The parameters obtained from fitting the experimental data are summarized in Tables 4−7. In this study, the best-fitting parameters were found by minimizing the average relative error (ARE), which was calculated by the following equation: 100 ARE = k

k

∑ j=1

qjexp − qjcal qjexp

(6)

where k is the number of experimental data points at a given temperature, and qexp and qcal are the experimental and calculated numbers of moles adsorbed, respectively. The adsorbate loading at very low relative pressures was commonly very small; hence, the deviation between these experimental data and the calculated amounts was much greater than that at higher relative pressures when the full data set was fitted to one isotherm model. As a result, calculating the deviation by including the low relative pressure data could lead to skewed results. For example, the DA equation yielded an average deviation of 344% from the experimental data for the alumina/zeolite 13X composite at 293.15 K. Even though the 807



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Table 3. Experimental Adsorption Isotherm Data for Water Vapor on Silica Gela P (kPa)

q (mol·kg−1)

P (kPa)

q (mol·kg−1)

0.004 0.005 0.006 0.009 0.011 0.016 0.017 0.019 0.021 0.042

0.033 0.073 0.148 0.244 0.314 0.466 0.516 0.570 0.630 1.085

0.065 0.085 0.113 0.136 0.160 0.180 0.229 0.350 0.469 0.580

1.524 1.851 2.181 2.508 2.837 3.100 3.742 5.081 6.544 7.991

0.007 0.008 0.012 0.016 0.020 0.024 0.028 0.031 0.036 0.039

0.001 0.043 0.132 0.221 0.308 0.370 0.430 0.491 0.551 0.591

0.081 0.120 0.160 0.202 0.247 0.285 0.335 0.369 0.422 0.628

1.079 1.419 1.763 2.108 2.450 2.724 3.065 3.280 3.621 4.865

0.008 0.014 0.022 0.029 0.035 0.042 0.050 0.055 0.060 0.067

0.001 0.056 0.142 0.219 0.275 0.332 0.391 0.429 0.469 0.510

0.133 0.217 0.284 0.357 0.439 0.503 0.571 0.656 0.729 1.099

0.809 1.194 1.495 1.795 2.087 2.319 2.549 2.840 3.069 4.128

P (kPa)

q (mol·kg−1)

P (kPa)

q (mol·kg−1)

0.696 0.811 0.925 1.052 1.159 1.281 1.392 1.530 1.643 1.753

9.604 11.20 12.79 14.36 15.46 16.57 17.27 17.77 18.03 18.23

1.857 1.989 2.093 2.114 2.139 2.162 2.185 2.212

18.41 18.62 18.79 18.84 18.89 18.94 18.98 19.04

0.844 1.057 1.260 1.486 1.684 1.917 2.113 2.342 2.546 2.753

6.220 7.575 8.905 10.40 11.70 13.12 14.06 15.10 15.87 16.42

2.959 3.190 3.410 3.616 3.812 3.849 3.891 3.947 3.984 4.030

16.80 17.07 17.26 17.45 17.60 17.62 17.66 17.70 17.73 17.76

1.493 1.821 2.207 2.557 2.942 3.316 3.706 4.070 4.410 4.799

5.329 6.320 7.495 8.647 9.891 11.04 12.16 13.07 13.89 14.47

5.179 5.563 5.967 6.274 6.623 6.732 6.771 6.877 6.941 7.000

14.92 15.24 15.43 15.59 15.74 15.83 15.85 15.91 16.04 16.18

293.15 K

303.15 K

313.15 K

a

Uncertainties: ΔT = 0.05 K, P/Ps < 0.001.

Table 4. AD Equation Parameters for Water Vapor Adsorption on the Alumina/Zeolite 13X Composite and Silica Gel for f(P) = Sips Equation adsorbent alumina/zeolite

silica gel

T (K) 293.15 303.15 313.15 293.15 303.15 313.15

qm (mol·kg−1) 37.84 37.7 37.12 21.63 21.37 20.52

b (kPa) −2

0.900 × 10 0.756 × 10−2 0.233 × 10−2 1.349 0.6847 0.3373

n

d

ARE (%)

4.725 4.311 4.754 0.5258 0.5906 0.6262

0.2191 0.2005 0.2228 0.357 × 10−2 0.234 × 10−2 0.089 × 10−2

4.103 5.124 3.778 32.59 32.25 31.41

Table 5. AD Equation Parameters for Water Vapor Adsorption on the Alumina/Zeolite 13X Composite and Silica Gel for f(P) = Toth Equation adsorbent

T (K)

qm (mol·kg−1)

b (kPa)

t

d

ARE (%)

alumina/zeolite

293.15 303.15 313.15 293.15 303.15 313.15

23.15 22.56 21.28 19.14 18.24 16.65

6149 2691 10490 0.7537 0.4092 0.2131

0.1993 0.2038 0.179 5.064 3.785 4.143

0.2207 0.2085 0.2544 0.120 × 10−2 0.384 × 10−2 0.242 × 10−2

6.086 5.992 5.146 14.46 15.84 16.59

silica gel

reducing the accuracy difference between the DA and AD equations. The difference in prediction accuracy between the two models was more significant in the results of silica gel. It implied that the consideration of surface heterogeneity in water

For the experimental isotherms of silica gel and the alumina/ zeolite 13X composite, the DA equation provided a better fit than the AD model by showing a smaller average relative error. The AD equation combined with the Toth model resulted in 808



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Table 6. DA Equation Parameters for Water Vapor Adsorption on the Alumina/Zeolite 13X Composite and Silica Gel

alumina/ zeolite silica gel

T (K)

E (J·mol−1)

V (mol·kg−1)

293.15 303.15 313.15 293.15 303.15 313.15

1355

35

0.2811

4030

16.83

1.281

n

Table 8. Henry’s Constant for Various Adsorbents adsorbent

T (K)

KH (mol·kg−1·kPa−1)

alumina/zeolite

293.15 303.15 313.15 293.15 303.15 313.15

368.5 169.8 108.0 29.13 14.96 7.62

ARE (%) 3.148 3.877 1.673 18.36 18.55 14.34

silica gel

thermodynamic variable for designing practical cyclic adsorption processes.24−26 In this study, the isosteric heats of adsorption were calculated using the Gibbs−Helmholtz equation and Clausius−Clapeyron equation.14,23,27 Qst is the isosteric heat of adsorption, where Qst is the isosteric heat of adsorption, hv is the heat of condensation of the adsorbate, T is the temperature, ΔS is the differential entropy, and A is the Helmholtz energy, which is given by16

adsorption (DA model) is more important than an apparent divergence of the adsorption amount with pressure (AD model). In addition, it was noted that the prediction accuracy of Sips or Toth models for silica gel was higher than that of the DA and AD equations. However, it was noted that they were worse than the DA equation for the alumina/zeolite 13X even though these two models were applied to the data at relative pressure (P/Ps) up to 0.3. Henry’s Law Region. As mentioned before, it was difficult to correlate the experimental data in the very low-pressure region using the DA model or the AD model using Sips and Toth equations. Thus, the experimental data in the lowpressure region were correlated using the Henry’s law equation, as follows:21

q = KHP

Q ad = hv + A − T ΔS

(8)

⎛P ⎞ A = RT ln⎜ s ⎟ = −ΔG ⎝P⎠

(9)

The differential entropy is obtained from

28

⎛ ∂A ⎞ ΔS = αq⎜ ⎟ ⎝ ∂q ⎠T

(10)

where α is the thermal expansion coefficient of the adsorbate and q is the adsorption amount.

(7)

Q st

where q is the adsorption amount, P is the pressure, and KH is the Henry constant, which is an indicator of the interaction between the adsorbate and the adsorbent.22 The fitted experimental data are shown in the insets of Figures 1 and 2 for silica gel and the alumina/zeolite 13X composite, respectively, and the resultant Henry constants are listed in Table 8. The Henry constants for silica gel were similar to those previously reported23 and showed a decrease with increasing temperature. However, the Henry constants of the alumina/zeolite 13X composite were much higher than those of silica gel at all temperatures. Thus, the interaction between the composite and water molecules was much stronger than that between silica gel and water molecules. Isosteric Heat of Adsorption. The isosteric heat of adsorption is derived from the interaction between adsorbent lattice atoms and adsorbate molecules. Furthermore, heats arising from adsorption or desorption lead to temperature variations in an adsorption bed and affect the separation performance. Therefore, the isosteric heat of adsorption is a key

RT 2

⎡ ∂ ln P ⎤ =⎢ ⎣ ∂T ⎥⎦q

(11)

The variation of the differential entropy with loading is presented in Figure 3. The value of differential entropy of both adsorbents decreased with increased loading in the negative region. The differential entropies were not exact zero at q = 0 but approached to almost zero. As mentioned in the isotherms, the results might stem from the experimental deviation at a very low pressure range. The isosteric heats of adsorption are presented in Figure 4. Since the parameters of the DA equation were obtained from fitting the experimental data in a full range, the heats of adsorption from the Gibbs−Helmholtz equation were compared with the results from the Clausius−Clapeyron equation to confirm the validation. Both results for silica gel were almost identical, but a certain difference in a low loading region was observed in the alumina/zeolite 13X composite because of a large fitting deviation as mentioned above.

Table 7. Parameters of Sips and Toth Models for Water Vapor Adsorption on the Alumina/Zeolite 13X Composite and Silica Gel Sips

Toth −1

adsorbent

T (K)

qm (mol·kg )

b (kPa)

n

ARE (%)

T (K)

qm (mol·kg )

b (kPa)

t

ARE (%)

alumina/zeolitea

293.15 303.15 313.15 293.15 303.15 313.15

26.18 26.23 26.21 11.44 11 10.57

1.048 0.5019 0.2258 26.94 14.08 10.62

0.7835 0.8512 0.8838 1.811 1.623 1.758

20.40 16.98 16.44 7.434 8.409 7.278

293.15 303.15 313.15 293.15 303.15 313.15

19.19 18.54 16.82 12.26 11.74 11.21

0.7489 0.4073 0.2138 290.6 84.7 89.32

5.091 3.698 3.919 0.417 0.4728 0.4416

15.16 15.61 16.31 7.168 8.051 7.043

silica gel

a

−1

Fitted data up to P/Ps = 0.3. 809



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between solid surface molecules and gas molecules decrease as coverage increases, while the lateral interactions between adsorbed molecules increase with coverage.32,33 According to the data shown in Figures 3 and 4, different adsorption phenomena are active in each adsorbent. In silica gel, the vertical interaction is dominant at initial loading, but the contribution of the lateral interaction to adsorption increases with loading. On the other hand, the vertical interaction is the main mechanism in the alumina/zeolite 13X composite. Moreover, the heats of adsorption decreased sharply in the initial loading range and approached the latent heat of condensation of water. The changes in the heats of adsorption shown in Figure 4 are coincident with the differential entropy results shown in Figure 3.

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CONCLUSIONS The adsorption isotherms of water vapor on an alumina/zeolite 13X composite and silica gel were measured using a volumetric method at 293.15, 303.15, and 313.15 K and at relative pressures up to 0.95. The adsorption isotherms of the alumina/ zeolite 13X composite were Type II isotherms in the IUPAC classification. High adsorption affinities were observed for water on the alumina/zeolite 13X composite in the low-pressure region, and the completion of a monolayer and formation of a second layer started at P/Ps = 0.17. On the other hand, the silica gel isotherms were classified as Type IV in the IUPAC classification. The adsorption amounts of silica gel were higher than those of the composite in the middle range of relative saturated pressures at all experimental temperatures. The DA equation could correlate the experimental data of water adsorption on both adsorbents better than the other models used in the study. The isosteric heats of adsorption of the alumina/zeolite 13X composite were higher than those of silica gel until 10 mol· kg−1, but the opposite result was observed with further adsorption of water. In the low-pressure region, the adsorption affinities and amounts of water on the alumina/zeolite 13X composite were much higher than those of silica gel. With increasing pressure, the isotherm shape became similar to that of alumina because zeolite 13X was saturated. Therefore, the adsorption amount of the composite was higher than that of zeolite 13X in the high-pressure region. The results can be used to design an adsorption bed for removing a trace amount of water vapor or PSA processes for dehydration. However, since the experimental temperature range was narrow, further study at the thermal desorption temperature is needed to design TSA processes for dehydration.

Figure 3. Differential entropy changes for the alumina/zeolite 13X composite (solid line) and silica gel (dash−dot line).

Figure 4. Isosteric heats of adsorption for the alumina/zeolite 13X composite (solid line) and silica gel (dash−dot line) from the Gibbs− Helmholtz equation (symbols from the Clausius−Clapeyron equation) and the heat of condensation of water at 303 K (dashed line).

The isosteric heats of adsorption of both adsorbents decreased with increased loading and approached to the heat of condensation of water. The results of the alumina/zeolite 13X composite decreased from 75 to 45 kJ·mol−1 with increased loading, which corresponds to the range of reported values for zeolites.29 On the other hand, the isosteric heats of adsorption of silica gel were in the range of 45−55 kJ·mol−1. The heats of adsorption of the alumina/zeolite 13X composite were higher than those of silica gel but became approximately same value of isosteric heats of adsorption at loadings of 10 mol·kg−1 and higher. When the surfaces are energetically homogeneous and there is no interaction between the adsorbed molecules, the isosteric enthalpy of adsorption is independent of the amount adsorbed. However, it was reported that the interactions between the adsorbed molecules cannot be neglected and the isosteric heat of adsorption varies with the surface coverage in various adsorbent−adsorbate systems when different levels of surface energy exist.11,30,31 For heterogeneous surfaces in the micro/ nanopores of some adsorbents, the vertical interactions

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +82 2 2123 2762; fax: +82 2 312 6401; e-mail address: [email protected]. ORCID

Chang-Ha Lee: 0000-0001-8769-7936 Author Contributions §

H.T.-O. and S.-J.L. contributed equally.

Funding

We would like to acknowledge financial support from the Ministry of Environment and the Korea Research Institute of Chemical Technology (KRICT: E616-00169-0605-1). 810



Article

Notes

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The authors declare no competing financial interest.

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LIST OF SYMBOLS thermal expansion coefficient (K−1) adsorption potential (J·mol−1), Helmholtz energy (J· mol−1) ARE average relative error (%) b Sips and Toth isotherm parameter (kPa−1) d AD equation parameter (−) E characteristic energy (J·mol−1) ΔG Gibbs free energy (J·mol−1) hv heat of condensation KH Henry’s constant n surface heterogeneity P pressure (kPa) Ps saturated pressure (kPa) Qst isosteric heat of adsorption (kJ·mol−1) q adsorption amount moles (mol·kg−1) cal q calculated adsorbate loading qexp experimental adsorbate loading qm Sips and Toth isotherm parameter (mol·kg−1) R ideal gas constant (J·mol−1·K−1) ΔS differential entropy (J·kg−1·K−1) T temperature (K) t Toth isotherm parameter (−) V limiting micropore volume α A

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