Equilibrium Adsorption Isotherms of Pure N2 and O2 and Their Binary

Article pubs.acs.org/IECR

Equilibrium Adsorption Isotherms of Pure N2 and O2 and Their Binary Mixtures on LiLSX Zeolite: Experimental Data and Thermodynamic Analysis Chin-Wen Wu, Mayuresh V. Kothare, and Shivaji Sircar* Department of Chemical Engineering, Lehigh University, Bethlehem, Pennsylvania 18015, United States S Supporting Information *

ABSTRACT: An isothermal−isobaric column dynamic test apparatus was built to measure equilibrium adsorption isotherms of pure N2 and O2 and their binary mixtures at 0, 30, and 65 °C in the pressure range of 0−6 atm on LiLSX zeolite. The data successfully passed an integral and a differential thermodynamic consistency test between pure and binary gas isotherms. The binary selectivities of N2 over O2 were functions of the gas-phase mole fraction and pressure, and the pure gas isosteric heats of adsorption decreased with increased loadings. Thus, the adsorbent was energetically heterogeneous for both gases. The degree of heterogeneity for N2 was larger than that for O2.

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INTRODUCTION Currently, the commercially preferred adsorbent for air separation by pressure swing, vacuum swing, or pressure− vacuum swing adsorption processes are N2-selective Liexchanged X zeolites (standard LiX or low silica LiLSX with or without doping with other cations) because they offer relatively a high N2 working capacity, moderate-to-high N2 selectivity over O2 and Ar, moderate isosteric heats of adsorption for N2 and O2, and favorable isotherm shapes for ease of desorption by these processes.1−4 They are frequently used for both tonnage-scale plants5,6 and compact medical oxygen concentrators7−9 for production of ∼90+% O2 from air. An adsorber engaged in these processes may experience large cyclic swings in gas-phase pressures (e.g., 0.2−4 atm), adsorbent temperatures (e.g., 10−50 °C), and gas-phase N2 mole fractions (e.g., 0.05−1) during a cycle. Consequently, the design and optimization of these processes require reliable experimental data and correlations for data inter extrapolation for all conditions prevailing inside the adsorber. Interestingly, the published data for equilibrium adsorption of N2, O2, Ar, and their mixtures on LiX or LiLSX zeolites are sporadic in the literature. Table 1 is a list of key publications on the subject. It also reports the source, the types, and the forms of the adsorbent used in these measurements. Most of the reported data were measured on homemade samples of LiX zeolite. It may be seen that nearly all publications report only pure gas N2 and O2 adsorption isotherms except two, which additionally report a few binary gas adsorption isotherm data for this system. Furthermore, the published data range is very limited, particularly for the binary data, which covers only one pressure and one temperature. Thus, the scarcity of data for this system is evident. The goals of this work are to (a) construct an isothermal− isobaric column dynamic test apparatus for measuring equilibrium adsorption isotherms of pure and binary gases, (b) systematically measure pure and binary gas isotherms for N2 and O2 on a commercial sample of LiLSX zeolite over an extensive range of conditions of pressure, temperature, and © 2014 American Chemical Society

mixture compositions, (c) check the thermodynamic consistency of the measured data using integral and differential tests, and (d) estimate pure gas isosteric heats of adsorption as functions of the adsorbate loadings to evaluate whether the adsorbent is energetically homogeneous or heterogeneous in nature. Thus, this work provides a detailed characterization of the equilibrium adsorptive properties for this system, which cannot be found elsewhere.

■

EXPERIMENTAL APPARATUS, MEASUREMENT PROTOCOL, AND DATA PROCESSING Figure 1a is a schematic drawing of the isothermal column dynamic test apparatus constructed in our laboratory and used in adsorption isotherm measurements, and Figure 1b is a photograph of the assembled unit. It consists of a waterjacketed adsorption column (i.d. = 1.5 cm; length = 30 cm) packed with a screened sample of commercial, pelletized LiLSX zeolite (particle diameter = 500−550 μm, bulk density = 0.62 g/cm3, and weight = 32.8 g) obtained from Zeochem Corp. The helium void fraction in the column was 0.7 cm3/cm3. The column was thermostated at a chosen experimental temperature (T) by circulating water through the jacket at that temperature from a constant-temperature bath. The column isothermality was ascertained by monitoring the column temperatures at the inlet and outlet. Flow control meters monitored the flow rates of the individual feed gas components before mixing and the effluent gas flow rate. A back-pressure control valve was used to maintain a constant column pressure (P) during a test. LiLSX zeolite was regenerated by heating at 633 K under a flow of dry CO2 free of N2 for 8 h in a separate apparatus. It was cooled under a N2 flow and very quickly poured into the Received: Revised: Accepted: Published: 7195

January 19, 2014 April 2, 2014 April 4, 2014 April 4, 2014 dx.doi.org/10.1021/ie500268s | Ind. Eng. Chem. Res. 2014, 53, 7195−7201

O2

√ √ √ × √ √

×

√ √ ×

√ √ √

√ √ √ × √

√

N2

√ √ √ √ √ √

√

√ √ √

√ √ √

√ √ √ √ √

√

authors

Baksh et al. (1992)10 Chao et al. (1995)11 Yang et al. (1996)12 Bajusz et al. (1997)13 Rege et al. (1997)14 Hutson et al. (1999)15

Labasque et al. (1999)16 Jale et al. (2000)17 Shen et al. (2001)18 Yoshida et al. (2001)19 Sircar et al. (2003)3 Kim (2003)20 Bulow et al. (2004)21

Jobic et al. (2005)22 Todd et al. (2005)23 Park et al. (2006)24 Pillai et al. (2010)25 Zanota et al. (2010)26

Zanota et al. (2011)27

gas

7196

√

× × × × √

× × ×

× × ×

×

× × × × × ×

binary

P (atm)

0−30 (pure gas) 4 (binary)

0−1 0−5 0−0.8 0.001−1.2 0−30 (pure gas) 4 (binary)

0−1.5 0−1.3 0−2

0−4 0−5 0−0.2; 0−1.2

0−1.4

0−1; 5−25 0−5 0−1 0−1.1 0−1 0−1

T (K)

303 (binary at 5 compositions)

303 (binary at 5 and 8 compositions for homemade and commercial LiLSX, respectively) 288, 303, 318 (pure gas)

260 273.65, 297.45, 321.85 293, 303 313 303 288, 303, 318 (pure gas)

298 293 298

298 298 186, 194, 200, 216, 245, 273, 398, 313

293

298 not given 298 303, 318, 328, 338 298 298

data range source

homemade

N/A N/A commercial (UOP) homemade homemade and commercial (Tricat Zeolites GmbH)

commercial (Zeochem) homemade homemade

homemade homemade homemade

homemade

homemade homemade homemade homemade homemade homemade

Table 1. Publications Reporting Pure N2 and O2 and Their Binary Adsorption Isotherms on LiX or LiLSX Zeolite

pellet

N/A crystalline powder Li, RELSX (bead) LiLSX (crystal) N/A pellet sphere N/A N/A

binderless powder binderless particle binderless pellet

beads N/A binderless powder N/A N/A binderless hydrated powder N/A

form

zeolite type size

dp: 3 mm; L: 5−15 mm dp: 5.6 mm; L:4 mm

dp: 1.9 mm ; L: 3−10 mm

N/A dp: 1.7 ± 0.2 mm 1.7 mm N/A N/A

N/A N/A N/A

dp: 1.5−2.0 mm ≈ 1.5 mm dp: 0.5−2.0 mm

N/A

8 × 12 mesh N/A N/A N/A N/A N/A

Industrial & Engineering Chemistry Research Article

dx.doi.org/10.1021/ie500268s | Ind. Eng. Chem. Res. 2014, 53, 7195−7201

Industrial & Engineering Chemistry Research

Article

Figure 1. (a) Schematic diagram of the column dynamic test apparatus (1, mass flow controller; 2, pressure gauge; 3, circulating water bath; 4, isothermal adsorption column; 5, back-pressure regulator; 6, effluent gas mass flowmeter; 7, gas analyzer; 8, data acquisition system; 9, recorder). (b) Photograph of the fully assembled apparatus.

Figure 2. Pure gas adsorption isotherms on LiLSX at three temperatures: (a) N2 (all pressures); (b) N2 (Henry’s law region); (c) O2 (all pressures); (d) O2 (Henry’s law region).

equilibrated with the feed gas. The column effluent gas flow rate and composition were measured as a function of time during the process. Two mass flow controllers at the feed end of the apparatus measured the flow rates of the components of the feed gas before mixing and entering the column, and a mass flow meter measured the effluent gas flow rate. A back-pressure control valve maintained a constant pressure in the column during a test. The effluent gas composition was continuously monitored using a thermal conductivity analyzer (manufactured by Gowmac Corp.) for N2 + He mixtures, and a membranebased ZrO2000 O2 analyzer (manufactured by Alpha Omega Instruments) for O2 + He and O2 + N2 mixtures. A data acquisition system (OMB-DAQ-3000 series, manufactured by Omega) was used for continuously gathering and recording all data.

adsorption column of Figure 1. Dry He was then passed through the column for 1 h before the helium void fraction of the column was measured. The experiments for measurement of the adsorption isotherms of pure N2 or O2 consisted of equilibrating the column with pure He (nonadsorbing) at P and T, followed by flowing a binary gas mixture of N2 (or O2) + He of different compositions at P and T until the column was equilibrated with the feed gas. The column effluent gas flow rate and composition were measured as a function of time during the process (breakthrough curves). The isobaric and isothermal experiments for measurement of the component adsorption isotherms of a binary N2 + O2 mixture consisted of equilibrating the column with pure O2 at P and T, followed by flowing a binary mixture of N2 + O2 of different compositions at P and T until the column was 7197



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Table 2. Thermodynamic Properties in the Henry’s Law Region on LiLSX N2

O2

T (K)

Ki (mol/kg/atm)

qi* (kcal/mol)

T (K)

Ki (mol/kg/atm)

qi* (kcal/mol)

323.1 328.1 333.1 338.1

0.683 0.611 0.523 0.458

5.87

273.1 303.1 318.1 328.1

0.297 0.175 0.127 0.110

3.22

Figure 3. Binary gas adsorption isotherms for N2 + O2 mixtures at three different temperatures and two different total gas pressures: (a) N2; (b) O2.

It may be seen from eq 2 that the equilibrium GSE for component i at the feed gas conditions (nmF i ) can be estimated by the dynamic test if the GSE for that component at the conditions of initial equilibration (nmS i ) is known. For the case mS mS of pure N2 or O2 GSE measurement, nmS N2 = nO2 = nHe = 0, when the initial equilibrating gas is pure nonadsorbing He, and for the case of binary N2 + O2 GSE measurement, nmS O2 = GSE for mS pure O2 at P and T and nN2 = 0.

The pure and binary gas equilibrium adsorption isotherms were estimated in terms of Gibbsian surface excess (GSE), which can be directly estimated unambiguously from the abovedescribed experimental data as follows:28 The total specific amount of component i (Ni, mol/g) in a packed adsorbent column with a total specific helium void volume of v (cm3/g) when it is equilibrated with an ideal gas mixture (mole fraction of component i = yi) at pressure (P, atm) and temperature (T, K) is given by28 Ni = nim(P , T , yi ) + vρg yi

i = 1, 2

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EXPERIMENTAL PURE AND BINARY GAS EQUILIBRIUM ADSORPTION ISOTHERMS FOR N2 AND O2 ON LILSX ZEOLITE We employed the mass balance protocol in terms of GSE as variables (eq 2) to estimate pure gas GSE isotherms for component i [nmi ° as a function of P at constant T] and binary gas GSE isotherms for component i [nmi as a function of yi at constant P and T] for N2 and O2 on LiLSX zeolite at three temperatures (0, 30, and 65 °C) and in the pressure range of 0−6 atm. A large volume of data were gathered. Pure Gas Isotherms. Figure 2 shows the pure gas adsorption isotherms at three temperatures. Parts a and c of Figure 2 show the N2 and O2 isotherms in full, respectively, and parts b and d of Figure 2 show the corresponding isotherms at low pressures. Both gases exhibited a Henry’s law region at low pressures where the GSE was a linear function of pressure P [nmi ° = Ki(T) P; P → 0, where Ki(T) is Henry’s law constant for pure gas i at temperature T].28 All isotherms are type I according to the Bruauner classification.29 The pure gas isotherm data points can be found in tabulated form for easy access as Supporting Information for this article.

(1)

where nmi (P,T,yi) is the specific equilibrium GSE (mol/g) of component i at P, T, and yi and ρg = (P/RT) is the molar density (mol/cm3) of the ideal gas at P and T. The component i mass balance for an isothermal−isobaric dynamic test, where the column is initially equilibrated with a gas mixture characterized by P, T, and ySi , followed by passage of a feed gas mixture characterized by P, T, and yFi , where ySi and yFi (>ySi ) are respectively the mole fractions of component i in the initial equilibrating and feed gases, can be written as Q FyiF t −

∫0

t

Q E(t ) yiE (t ) dt

= [nimF(P , T , yiF ) − nimS(P , T , yiS )] − [vρg (yiF − yiS )] (2) F

E

where Q and Q (t) are respectively the specific feed gas and column effluent gas flow rates at time t (mol/g/s) and yEi (t) is the mole fraction of component i in the effluent gas at time t (s). Equation 1 is used to derive eq 2. 7198



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Table 3. Binary N2 (1) + O2 (2) Gas Adsorption Selectivity on LiLSX at Constant P and T 273.1 K

303.1 K

P = 1 atm y1 0.00 0.30 0.40 0.50 0.60 0.70 0.80 1.00

P = 6 atm S12

y1

8.31 8.77 8.80 8.12 7.89 7.71

0.00 0.30 0.40 0.50 0.60 0.70 0.80 1.00

S12

y1

5.83 5.74 5.52 5.13 4.85 4.66

0.00 0.30 0.40 0.50 0.60 0.70 1.00

P = 6 atm S12

y1

6.30 6.51 6.32 6.33 6.46

0.00 0.50 0.60 0.70 0.80 0.90 1.00

d

( T1 )

=

qi* R

;

K i = K i* exp(qi*/RT ) (3)

⎡q * − q *⎤ K1* 2 exp⎢ 1 ⎥ K 2* ⎣ RT ⎦

y1

4.75 4.57 4.37 4.48 4.26

0.00 0.30 0.40 0.50 0.60 0.70 1.00

S12

y1

S12

4.26 4.32 4.14 4.89 4.58

0.00 0.20 0.40 0.60 0.80 1.00

4.21 4.22 4.17 3.91

ENERGETIC HETEROGENEITY OF LILSX ZEOLITE FOR ADSORPTION OF N2 AND O2 According to the GSE framework, the isosteric heat of adsorption of a pure gas i, qi°(nmi °), at an adsorbate loading of nmi ° is given by28 qi° ⎡ ∂ ln(P) ⎤ ⎥ =− ⎢ ⎣ ∂T ⎦n m° RT 2 i

(6)

Equation 6 can be used to calculate qi° as a function of nmi ° from a set of pure gas GSE isotherms at different temperatures, as shown by the data in Figure 2a,c. A plot of ln P versus 1/T at a constant nmi ° should yield a straight line (if qi° is independent of T) with a slope equal to qi°/R]. The isosteric heat of adsorption of a pure gas should be the same as the Henry’s law isosteric heat at all adsorbate loadings for an energetically homogeneous adsorbent, while the isosteric heat will decrease with increased loading for an energetically heterogeneous adsorbent.30 Pure Gas Isosteric Heats for N2 and O2 on LiLSX. We calculated the isosteric heats of adsorption of pure N2 and O2 as a function of the equilibrium GSE using eq 6 and isotherms of Figure 2. Figure 4 shows the results. It plots the difference between the isosteric heat of adsorption of pure gas i at a specific value of GSE [qi°(nmi °)] and the Henry’s law isosteric heat [qi*] of that component as a function of GSE. The difference vanishes for an energetically homogeneous adsorbent. Figure 4 shows that the difference (qi° − qi*) increases with increasing GSE for both N2 and O2, indicating that the zeolite is energetically heterogeneous for both gases. This supports the selectivity characteristics of the adsorbent discussed earlier. The figure also shows that the adsorbent is weakly heterogeneous

(4)

The binary gas isotherms in the low pressure region (P → 0) also follow Henry’s law for the mixture29 [nmi (P,T,yi) = Ki(T) P yi; P → 0]. Hence, the binary selectivity in the Henry’s law region (S12*), according to eqs 3 and 4, is given by S12*(T ) =

S12

P = 6 atm

■

where Ki* is a constant and R is the gas constant. Table 2 gives the values of Ki and qi* obtained from the data of Figure 2b,d. The Henry’s law isosteric heat of adsorption of N2 is nearly double that for O2, demonstrating that N2 is much more strongly adsorbed than O2 on LiLSX. This is caused by a stronger quadrupole moment−ion interaction between N2 and LiLSX zeolite than that for O2. Binary Gas Isotherms. Parts a and b of Figure 3 plot the adsorption isotherms for N2 and O2 from binary mixtures of these gases at different temperatures at total gas pressures of 1.0 and 6.0 atm, respectively. The component GSEs are plotted as functions of the equilibrium compositions at constant P and T. The experimental binary gas isotherm data points are given in tabulated form for easy access as Supporting Information for this article. The binary component isotherms shown by Figure 3 are typical for systems with type I pure gas adsorption isotherms where component 1 (N2) is more selectively adsorbed than component 2 (O2). Binary Gas Selectivity of Adsorption. The selectivity of adsorption of component 1 over component 2 of a binary gas mixture (S12) is defined in the GSE framework by28 ⎡ n my ⎤ ⎢S12 = 1m 2 ⎥ ⎢⎣ n2 y1 ⎥⎦

P = 1 atm

Table 3 shows estimated values of S12 for each measured binary gas isotherm data point. The following observations can be made for the system of interest: (a) S12 for a given P and y1 decreases with increasing T. (b) S12 at constant P and T remains approximately constant or decreases weakly as y1 is increased. The effect is more pronounced at lower T. (c) S12 at constant T and y1 decreases as P (or n1) is increased. Observation a is a general principle of binary gas adsorption, which can be very easily demonstrated for binary gas adsorption in the Henry’s law region. According to eq 5, (dS12*/dT) < 0 when q1* > q2*. Observations b and c are typically exhibited when the adsorbent is energetically heterogeneous for at least one of the components of a binary gas mixture.30

It should be mentioned here that the isotherms reported in this work cannot be justifiably compared with the others reported in Table 1 because the amounts of N2 and O2 adsorbed on the zeolite sample depend on various factors such as the degree of Li exchange in the zeolite, amount of binder in the final product, types and conditions of regeneration, etc.,3 which may be different for different samples of LiX described in Table 1. The isosteric heat of adsorption (qi*) of pure gas i in the Henry’s law region is thermodynamically given by28 d ln K i

338.1 K

P = 1 atm

(5) 7199



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Differential Test: n1m°(P , T ) = nm(P , T , y1) ⎡ ∂ ⎢ +P ∂P ⎢ ⎣

∫y

m 1⎧ ⎪ n1 y 2

⎨ ⎩

⎪

1

⎤ − n2my1 ⎫ ⎬ dy1⎥ ; ⎥ y1y2 ⎭P ,T ⎦ T ,y ⎪

⎪

1

m

n =

n1m

+

n2m

(9)

n2m°(P , T ) = nm(P , T , y1) ⎡ ∂ ⎢ −P ∂P ⎢ ⎣

∫0

⎧ n1my y1 ⎪ 2 ⎨ ⎩

⎪

⎤ − n2my1 ⎫ ⎬ dy1⎥ ; ⎥ y1y2 ⎭P ,T ⎦ T ,y ⎪

⎪

1

m

n =

n1m

+

n2m

(10)

The quantities on the left-hand sides of eqs 9 and 10 can be obtained from the pure gas isotherms only. The quantities on the right-hand sides of these equations can be estimated from binary gas isotherms at constant P and T and at constant T and y1 only. Hence, these identities check the thermodynamic consistency between pure gas and binary gas isotherms. The data presented in Figures 2 and 3 were used to apply the above-described consistency tests. Tables 4 and 5 respectively

Figure 4. Isosteric heats of adsorption of pure N2 and O2 on LiLSX zeolite.

Table 4. Integral Consistency Tests of Pure and Binary Gas Isotherm Data for N2 and O2 on LiLSX

(smaller deviation from Henry’s law heat with increasing loadings) for O2 adsorption and it is moderately heterogeneous (larger deviation from Henry’s law heat with increasing loading) for N2 adsorption. Thus, the same adsorbent exhibits substantially different degrees of heterogeneity of adsorption for the two gases.

P = 1 atm

■

THERMODYNAMIC CONSISTENCY OF EQUILIBRIUM ADSORPTION DATA The pure gas and binary isotherm data presented in this work were subjected to integral and differential thermodynamic consistency tests.31 These tests provide relationships (identities) between the thermodynamic quantities, which can be independently estimated only from pure gas isotherms of the components of a binary mixture and only from a binary gas isotherm. The relevant identities using the GSE framework are given as follows:

P = 6 atm

T (K)

left-hand side of eq 7 (mol/kg)

right-hand side of eq 7 (mol/kg)



273.1 303.1 338.1

1.72 0.77 0.31

1.68 0.85 0.36

3.96 2.37 1.23

3.94 1.98 1.21

report the results of the integral and differential consistency tests. It can be concluded that the data presented in this work pass both thermodynamic consistency tests. It should be Table 5. Differential Consistency Tests of Pure and Binary Gas Isotherm Data for N2 and O2 on LiLSX P = 1 atm

Integral Test: ϕ2°(P) − ϕ1°(P) RT

=

∫0

1

n1my2 − n2my1 y1y2

dy1 ;

constant P , T

ϕ° (P ) i

=−

∫0

P

y1 + y2 = 1

T (K)

y1


273.1

0 0.2 0.4 0.6 0.8 0 0.2 0.4 0.6 0.8 0 0.2 0.4 0.6 0.8

1.36 1.28 1.32 1.40 1.32 0.77 0.79 0.83 0.82 0.81 0.42 0.43 0.43 0.43 0.41

(7)

nim°(P) dP P

constant T

(8)

303.1

where ϕi°(P) is the surface potential of pure gas i at P and T. It can be calculated from the pure gas isotherm of component i by the integral of eq 8. Thus, the quantity on the left-hand side of eq 7 can be estimated from the pure gas isotherms of components 1 and 2 only. The quantity on the right-hand side of eq 7 can be estimated using a binary isotherm of components 1 and 2 at constant T and P only. Hence, eq 7 checks a thermodynamic consistency between pure gas and binary gas isotherms.

338.1

RT

7200

P = 6 atm




1.38

2.21 2.01 2.31 2.45 2.52 1.34 1.40 1.56 1.70 1.81 1.11 1.09 1.18 1.24 1.29

2.47

0.79

0.38

1.86

1.29



Article

(9) Jagger, T. W.; et al. Personal Oxygen Concentrator. U.S. Patent 7,121,276, 2006. (10) Baksh, M. S. A.; Kikkinides, E. S.; Yang, R. T. Lithium Type-X Zeolite as a Superior Sorbent for Air Separation. Sep. Sci. Technol. 1992, 27, 277. (11) Chao, C. C.; et al. Mixed ion-exchanged zeolites and processes for the use thereof in gas separations. U.S. Patent 5,413,625, 1995 (12) Yang, R. T.; et al. Zeolites containing mixed cations for air separation by weak chemisorption-assisted adsorption. Ind. Eng. Chem. Res. 1996, 35, 3093. (13) Bajusz, I. G.; Goodwin, J. G. N2 adsorption in LiX zeolite: Isotopic transient analysis. Langmuir 1997, 13, 6550. (14) Rege, S. U.; Yang, R. T. Limits for air separation by adsorption with LiX zeolite. Ind. Eng. Chem. Res. 1997, 36, 5358. (15) Hutson, N. D.; Rege, S. U.; Yang, R. T. Mixed cation zeolites: LixAgy-X as a superior adsorbent for air separation. AIChE J. 1999, 45, 724. (16) Labasque, J.; Moreau, S.; Rouge, D. Process for the separation of gas mixtures containing oxygen and nitrogen. U.S. Patent 5,922,107, 1999. (17) Jale, S. R.; et al. Monte Carlo simulation of sorption equilibria for nitrogen and oxygen on LiLSX zeolite. J. Phys. Chem. B 2000, 104, 5272. (18) Shen, D. M.; et al. Thermodynamics of nitrogen and oxygen sorption on zeolites LiLSX and CaA. Microporous Mesoporous Mater. 2001, 48, 211. (19) Yoshida, S.; et al. Nitrogen adsorption properties of cubic and orthorhombic Li-exchanged low silica X. Microporous Mesoporous Mater. 2001, 46, 203. (20) Kim, J. B. Li+- and H+-exchanged low-silica X zeolite as selective nitrogen adsorbent for air separation. Bull. Korean Chem. Soc. 2003, 24, 1814. (21) Bulow, M.; et al. Sorption equilibria of nitrogen and oxygen on Li,Re-LSX zeolite for oxygen PVSA processes. Rec. Adv. Sci. Technol. Zeolites Relat. Mater., Parts A−C 2004, 154, 1961. (22) Jobic, H.; Schober, H.; Pullumbi, P. Adsorption and diffusion of N2 and O2 in LiLSX studied by neutron scattering techniques. Adsorption 2005, 11, 449. (23) Todd, R. S.; Webley, P. A. Macropore diffusion dusty-gas coefficient for pelletised zeolites from breakthrough experiments in the O2/N2 system. Chem. Eng. Sci. 2005, 60, 4593. (24) Park, Y. J.; et al. Adsorption equilibria of O2, N2, and Ar on carbon molecular sieve and zeolites 10X, 13X, and LiX. J. Chem. Eng. Data 2006, 51, 1001. (25) Pillai, R. S.; Sethia, G.; Jasra, R. V. Sorption of CO, CH4, and N2 in Alkali Metal Ion Exchanged Zeolite-X: Grand Canonical Monte Carlo Simulation and Volumetric Measurements. Ind. Eng. Chem. Res. 2010, 49, 5816. (26) Zanota, M. L.; et al. Adsorption Isotherms of Pure Gas and Binary Mixtures of Air Compounds on Faujasite Zeolite Adsorbents: Effect of Compensation Cation. J. Chem. Eng. Data 2010, 55, 448. (27) Zanota, M. L.; et al. Thermodynamic study of LiNaKLSX zeolites with different Li exchange rate for N2/O2 separation process. Microporous Mesoporous Mater. 2011, 143, 302. (28) Sircar, S. Gibbsian surface excess for gas adsorptionRevisited. Ind. Eng. Chem. Res. 1999, 38, 3670. (29) Young, D. M.; Crowell, A.D. Physical adsorption of gases; Butterworths: London, 1962; p xi, 426 p. (30) Sircar, S. Role of Adsorbent Heterogeneity on Mixed GasAdsorption. Ind. Eng. Chem. Res. 1991, 30, 1032. (31) Rao, M. B.; Sircar, S. Thermodynamic consistency for binary gas adsorption equilibria. Langmuir 1999, 15, 7258.

mentioned that the binary isotherm data at 4 atm and 303 K, reported in Table 1, was also tested by the integral consistency test.

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SUMMARY An isothermal−isobaric column dynamic test apparatus was constructed and used for measurement of the adsorption isotherms of pure N2 and O2 and their binary mixtures on a commercial sample of LiLSX zeolite at 0, 30, and 65 °C and in the pressure range of 0−6 atm. The data passed both integral and differential thermodynamic consistency tests between pure gas and binary gas isotherms. The binary selectivity of adsorption of N2 over O2 at constant pressure and temperature was practically independent of the gas-phase N2 mole fraction at higher temperatures, but it decreased with increasing N2 mole fraction at lower temperatures. The binary selectivity at constant temperature and N2 mole fraction decreased. These characteristics implied adsorbent heterogeneity (energetic) for adsorption of these gases. Estimation of pure gas isosteric heats of adsorption as a function of the adsorbate loadings by using thermodynamic relationships to the isotherm data showed that the heats for both gases decreased with increasing loadings, which proved that the sample of LiLSX zeolite was energetically heterogeneous. The heat data also indicated that the degree of heterogeneity exhibited by the adsorbent for O2 adsorption was mild, while that for N2 adsorption was moderately strong.

■

ASSOCIATED CONTENT

S Supporting Information *

Experimental gas adsorption isotherm data on LiLSX of pure gas N2 and O2 at different temperatures and binary N2 (1) + O2 (2) at constant pressure and temperature. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

(1) Chao, C. C. Process for Separating Nitrogen from Mixtures Thereof with Less Polar Substances. U.S. Patent 4,859,217, 1989. (2) Leavitt, F. W. Air separation pressure swing adsorption process. U.S. Patent 5,074,892, 1991. (3) Sircar, S.; Myers, A. L. In Gas Separation by Zeolite, in Handbook of zeolite science and technology; Aurbach, S. M., Carrado, K. A., Dutta, P. K., Eds.; Marcel Dekker: New York, 2003; pp 1063−1104. (4) Yang, R. T. Sorbents for Applications. Adsorbents: Fundamentals and Applications; Wiley-Interscience: Hoboken, NJ, 2003; pp 280− 303. (5) Leavitt, F. W. Vacuum Pressure Swing Adsorption Process. U.S. Patent 5,415,683, 1995. (6) Sircar, S.; Naheiri, T.; Fisher, J. R. Oxygen Generation Process and System with Single Adsorber and Single Blower. U.S. Patent 6,146,447, 2000. (7) Kulish, S.; Swank, R. P.Rapid Cycle Pressure Swing Adsorption Oxygen Concentration Method and Apparatus. U.S. Patent 5,827,358, 1998. (8) Ackley, M. W.; Zhong, G. M. Medical Oxygen Concentrator. U.S. Patent 6,551,384, 2003. 7201


Equilibrium Adsorption Isotherms of Pure N2 and O2 and Their Binary

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