Article pubs.acs.org/IECR
Model Analysis of Equilibrium Adsorption Isotherms of Pure N2, O2, and Their Binary Mixtures on LiLSX Zeolite Chin-Wen Wu, Mayuresh V. Kothare, and Shivaji Sircar* Department of Chemical Engineering, Lehigh University, Bethlehem, Pennsylvania 18015, United States S Supporting Information *
ABSTRACT: Three analytical adsorption isotherm models for energetically homogeneous (Langmuir) and heterogeneous (Toth, Sircar) adsorbents were tested for describing recently measured pure gas isotherm data for adsorption of N2 and O2 on pelletized LiLSX zeolite at three different temperatures and in the pressure range of 0−6 atm. Estimated binary gas isotherms for this system by these models and the Ideal Adsorbed Solution Theory (IAST) were compared with the corresponding experimental data. All three models describe the pure gas isotherms reasonably well, but the model that accounts for the differences in the degrees of adsorbent heterogeneity of the components of a mixture (Sircar) provided better correlation between experimental and estimated binary selectivities than the other models and IAST under various conditions of pressure, temperature, and gas composition. Predictions of multicomponent isotherm data by models must be extensively tested experimentally, since their ability to describe pure gas isotherms does not guarantee the quality of multicomponent isotherm predictions.
■
INTRODUCTION Lithium-exchanged X zeolites in various forms (standard (LiX) or low-silica (LiLSX) with or without doping with other cations) are currently favored N2 selective adsorbent for air separation by cyclic pressure or vacuum swing adsorption (PSA/VSA) processes, particularly for production of ∼90% O2 from ambient air in both tonnage-scale plants and compact medical oxygen concentrators.1−9 The design and optimization of these processes require reliable experimental data and explicit correlations (models describing pure and mixed gas adsorption isotherms and isosteric heats of adsorption) for data interpolation/extrapolation for all conditions prevailing inside an adsorber. Analytical models are necessary for easier and time-efficient simulation of adiabatic process performances using numerical process models. We recently reported extensive adsorption isotherm data for pure N2, O2, and their binary mixtures on a commercial sample of pelletized LiLSX zeolite (obtained from Zeochem Corp.) at temperatures of 0, 30 and 65 °C, and in the pressure range of 0−6 atm, which covers much of the operating conditions encountered in a typical air separation PSA/VSA process.10 The estimated measurement uncertainities were less than ±5%. The isotherms were Type I in shape according to the Brunauer (or IUPAC) classification, which is typical for a microporous solid.11 Low-pressure data for all pure gas isotherms exhibited a Henry’s Law region. The data passed an integral and a differential thermodynamic consistency test, proving their reliability.12 We also found that the zeolite exhibited different degrees of energetic heterogeneity for adsorption of pure N2 and O2, which is a behavior that was previously not reported in the literature. This is presumably caused by the subtle differences in the quadrupole-ion interactions between the gas molecules and the cationic adsorption sites in the adsorbent. N2 exhibits a higher degree of heterogeneity, because it has a larger quadrupole moment than O2. © 2014 American Chemical Society
The purpose of this work is to analyze (i) how different analytical pure gas adsorption isotherm models for homogeneous and heterogeneous adsorbents describe the measured pure gas isotherms for the present system, (ii) how well the analytical mixed gas isotherm models describe the experimental binary adsorption data using parameters obtained from the pure gas isotherm fits, and (iii) how well the classic Ideal Adsorbed Solution Theory (IAST) work for this system. It should be emphasized here that the experimentally measured adsorption isotherms for pure and binary gases actually report Gibbsian surface excesses (GSE).13 We estimated the upper bounds of differences between the GSE and actual amount adsorbed for the present system by assuming that the entire micropore volume within the zeolite crystal constitutes the adsorbed phase under all conditions of measurements.13 This indicated that the maximum possible differences under the worst-case scenario for the reported data (highest pressure and lowest temperature) were less than ∼4% Table 1. List of Models Used To Correlate Pure N2 and O2 and Binary Gas Adsorption Data on LiLSX Zeolite 19
Isotherm Model authors
LiX
Santos et al. (2007)15 Zhong et al. (2010)18 Rama Rao et al. (2010)19
Li-Ag-LSX LiX Li-Ag-X
Received: Revised: Accepted: Published: 12428
adsorbent
Rege and Yang (1997)14
pure gas
binary gas
empirical modified Langmuir11 16 Sips IAST17 Langmuir11 Langmuir11
April 28, 2014 July 16, 2014 July 17, 2014 July 17, 2014 dx.doi.org/10.1021/ie501750h | Ind. Eng. Chem. Res. 2014, 53, 12428−12434
Industrial & Engineering Chemistry Research
Article
Figure 1. Model fit of pure N2 and O2 adsorption isotherms of LiLSX zeolite at three temperatures, Circles and triangles represent experimental data; dotted line (- - -), Langmuir model; solid line (), Toth model; and dashed line (− − −), Sircar model.
Table 2. Analytical Model Isotherms for Pure and Multicomponent Gas Adsorption pure gas
multicomponent gas
thermodynamic and physical constrains
Langmuir Model (1914)10 (Homogeneous)
θi° =
biP 1 + biP
mi = m ≠ m(T )
biPyi
θi =
1 + ∑i biPyi
⎛ q* ⎞ bi(T ) = bi0 exp⎜⎜ i ⎟⎟ ⎝ RT ⎠
θi =
ni mi
ni° mi
θi° =
Toth Model (1962)19 (Heterogeneous)
θi° =
biP
θi =
[1 + (biP)k i ]1/ k i
biPyi [1 + (∑i biPyi ) ]
⎛ q* ⎞ bi(T ) = bi0 exp⎜⎜ i ⎟⎟ ⎝ RT ⎠ θi° =
mi = m ≠ m(T ) k i 1/ k i
ki(T ) = k(T )
n θi = i mi
00 dT
ni° mi Sircar Model (1991)20 (Heterogeneous)
⎡ ⎤ °) (1 − θiH ° ⎢1 − θi° = θiH f (zi°)⎥ ° θiH ⎣ ⎦ μ iP ° = θiH [1 + μi P] °; zi° = ψθ i iH
ψi =
⎡σ ⎤ 3⎢ i⎥ ⎢⎣ μi ⎥⎦
⎛ q* ⎞ μi = μ*i exp⎜⎜ i ⎟⎟ ⎝ RT ⎠ ⎛ λ*⎞ σi = σi* exp⎜ i ⎟ ⎝ RT ⎠ θi° = f (zi°) =
⎡ ⎤ (ψ − z ) θi = θiH ⎢1 − i f (z)⎥ z ⎣ ⎦ θiH =
mi = m ≠ m(T ) 0 ≤ ψi(T ) < 1
μiPyi
dψi
1 + ∑i μiPyi
dT
z=
