Equilibrium Isotherm and Mass Transfer Coefficient for Adsorption of

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Equilibrium Isotherm and Mass Transfer Coefficient for Adsorption of Pure Argon on Small Particles of Pelletized LiLSX Zeolite CHIN-WEN WU, Mayuresh V Kothare, and Shivaji Sircar Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/ie504633s • Publication Date (Web): 06 Feb 2015 Downloaded from http://pubs.acs.org on February 18, 2015

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Industrial & Engineering Chemistry Research

Equilibrium Isotherm and Mass Transfer Coefficient for Adsorption of Pure Argon on Small Particles of Pelletized LiLSX Zeolite

Chin-Wen Wu, Mayuresh V. Kothare, and Shivaji Sircar* Department of Chemical Engineering, Lehigh University, Bethlehem, Pennsylvania 18015, U.S.A. Author information Corresponding author: *[email protected] Key words Equilibrium adsorption isotherm, mass transfer coefficient, LiLSX zeolite, argon, Column dynamics, skin resistance

Abstract

New experimental data are reported for equilibrium adsorption isotherm and mass transfer of pure argon on a sample of pelletized LiLSX zeolite. Model analysis of the data indicate that the 1

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zeolite behaved like a nearly homogeneous adsorbent for Ar adsorption while it exhibited substantial heterogeneity for adsorption of N2 and milder heterogeneity for adsorption of O2. The over-all mass transfer coefficient for Ar adsorption was comparable in magnitude with those of N2 and O2. The coefficient increased with increasing pressure and decreased with increasing temperature like those for N2 and O2. A large skin resistance at the adsorbent particle surface was observed for Ar mass transfer like that for adsorption of O2 and N2.

Introduction

The effects of the presence of ~ 1.0 % argon in the atmospheric air on the over-all performance of a rapid pressure swing adsorption (RPSA) process for production of ~ 90 % oxygen enriched air was recently measured using a pelletized, lithium-exchanged low silica X (LiLSX) zeolite as the nitrogen selective adsorbent in a continuous RPSA process system.1 The adsorbent (diameter ~ 500 – 550 µm, bulk density = 0.62 g/cm3) was a commercial sample made by Zeochem Corporation of U. S. A. The compressed air feed (with and without argon) was at 4 atm and the system temperature was ambient. The final desorption pressure was atmospheric. The total cycle time of the process was varied between 3 – 9 seconds. Figure 1 shows the steady state process performance of the RPSA system. It plots the bed size factor (BSF; Kg of adsorbent in the unit/ton per day of O2 product/ day), and percent oxygen recovery (R; moles of O2 in product gas per mole of O2 in the feed air per cycle) as functions of total cycle time of the process. BSF and R are the key performance variables for the process. Lower BSF (translates to smaller adsorbent inventory) and higher R (translates to lower air compressor power) are the desired process design goals. It may be seen from Figure 1 that the 2


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BSF was increased and the R was decreased by the presence of 1% argon in the feed air of the RPSA process at all total cycle times of the process in the range of the data compared to those for the argon - free feed air. More details of the RPSA process design, operating conditions, and data analysis can be found elsewhere.1 Argon is a minor (~1%) component of air and it dilutes the gas phase oxygen purity inside a PSA adsorber as well as the purity of the O2 - enriched product gas since the O2 - Ar selectivity on most commercial zeolites is near unity (non-selective vis a vis O2). However, the detrimental effects of the presence of argon in the feed air on the over-all RPSA air separation process performances can be complex and not insignificant. According to the data of Figure 1, the presence of 1 % Ar in feed air increases the adsorber size and the air compressor power by ~ 11% and 5%, respectively, when the RPSA process is operated at the conditions of minimum BSF (total cycle time of 5.7 s).

Figure 1. Effects of 1% argon in feed air on performance of a RPSA process. 3



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We recently measured (a) the pure and binary gas equilibrium adsorption isotherms of N2 and O2, and (b) the effective mass transfer coefficients of the pure gases into the adsorbent particles using the above mentioned LiLSX sample under different conditions of pressure and temperature.2-4 Isosteric heats of adsorption of the pure gases were estimated by thermodynamic analysis of the isotherm data.2 An analysis was also carried out to find an analytical isotherm model which describes the pure gas isotherms adequately and can be used to estimate binary adsorption isotherm from the pure gas isotherms.2, 3 Some of the key findings were that (a) the adsorbent exhibited a higher degree of energetic heterogeneity for adsorption of pure N2 than that for pure O2, (b) an analytical, thermodynamically consistent equilibrium isotherm model for pure and mixed gas adsorption, which accounted for the differences in the heterogeneity of adsorption of the components, provided a better prediction of the binary isotherm data from pure gas adsorption data for this system compared to many traditional isotherm prediction models,3,

5

(c) the mass transfer

coefficients for adsorption of pure N2 and O2 on the material were similar in magnitude , and they increased with increasing gas pressure and decreased with increasing system temperature, and (d) a skin resistance at the particle surface dominated (~ 65%) the over-all mass transfer coefficient.4

4


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Table 1. Summary of published articles on adsorption characteristics of Ar on various types of LiX, LiLSX and Ag-LiX zeolites. Data Range Authors and Adsorbent

P

T

(atm)

( K)

0 – 1.0

298

Adsorptive Properties Test Method

Isotherm

Isosteric

Data

Heat

yes

no

**

Zeolite

Mass

Source &

Transfer

Form

Data

Hutson et al. (1999)6

volumetric

Homemade

no

Powder

Ag-LiX Maurin et al. (2005)7

Commercial 0 – 0.5

300

calorimetric

yes

3.23

no

Air Liquid

LiX

Powder

Park et al. (2006)8

293 0 – 0.8

LiX Ferreira et al. (2014)

9

0 – 7.0

303

no

298

308

volumetric

yes

3.59

yes

LiLSX

273 0 – 6.0

303 338

Air Products 1.60 mm Bead Commercial

volumetric

yes

3.11

a

no

323

This work

UOP

Commercial

298

LiX

4.19a

288

Park et al. 0 – 9.8

yes

1.70 mm Bead

308

(2014)

gravimetric

313

Ag-LiLSX 10

Commercial

Zeochem 1.5-1.7 mm Bead Commercial

column

yes

dynamic

2.75

yes

Zeochem ~0.52 mm Bead

a

** Henry’s Law region (Kcal/mole), extracted from figure in reference

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The purpose of this work was to measure pure Ar adsorption isotherms and mass transfer coefficients on the same sample of LiLSX zeolite in order to measure the degree of heterogeneity of the adsorbent for Ar adsorption, if any, and to carry out a similar model analysis of the pure gas isotherm data so that a thermodynamically consistent equilibrium isotherm model can be established for all three components of air (H2O and CO2 free) for use in numerical simulation of adiabatic performance of a RPSA process using real air feed containing argon, particularly for ascertaining O2 product purity. A literature search on the subject indicates that there are several publications reporting the equilibrium adsorption isotherms of argon on various home-made and commercial samples of LiX, LiLSX and Ag-LiX zeolite at different conditions of temperature and pressure.6-10 Table 1 summarizes the search results. One article also reports experimental mass transfer data for argon adsorption.9

Figure 2. Argon adsorption isotherms on various commercial samples of pelletized LiLSX zeolite at ~ 300K. 6


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Figure 2 is a comparative plot of the published Ar adsorption isotherms at ~ 300 K in the pressure range of 0 to 0.7 atm on various commercial samples of LiLSX listed in Table 1. It shows that there are appreciable differences in the Ar isotherms presumably due to different degrees of Li ion exchange, and variations in nature and amount of binder used as well as method and conditions of reactivation. The Henry’s Law isosteric heats of adsorption of Ar on these materials also differ significantly as shown by Table 1. This suggests that independent measurement of basic adsorption characteristics of Ar on the adsorbent used in a RPSA air separation process is needed.

Experimental Method

An isothermal-isobaric column dynamic method was used to measure the pure argon adsorption isotherms and mass transfer coefficients on a sample of Zeochem LiLSX (particle diameter: 500 – 550 µm, bulk density: 0.62 g/cm3). The adsorption column was 1.5 cm in diameter and 30 cm long, and it was kept isothermal by flowing water from a constant temperature bath through a surrounding jacket. Details of the apparatus are given elsewhere.2 The experiment consisted of equilibrating the adsorption column packed with pure helium (inert) at pressure P0 and temperature T0 followed by flowing an argon + helium mixture at P0 and T0 [feed gas mass flow rate: Q0 moles/cm2 of empty cross sectional area of the column/s, feed gas argon mole fraction y0] and monitoring the effluent gas mass flow rate [Q(t)] and argon mole fraction [y(t)] as functions of time (t) [breakthrough curves].The argon composition in the

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column effluent gas was continuously measured by a Gow Mac Dual Pass analyzer (series 20). Details of the experimental protocol is given elsewhere.2 Figure 3 is a typical example of measured breakthrough plots where dimensionless groups λ [= y/y0] and φ [= (Q – Qs)/(Q0 - Qs)] are plotted against dimensionless time τ [= t/t*]. The variable Qs is the effluent gas mass flow rate until incipient breakthrough of argon from the column, t* [= ∙ / ] is the stoichiometric breakthrough time, L (cm) is the column length, and is the total specific argon capacity (moles/g) in the column (adsorbed + void) in equilibrium with the feed gas as at P0, T0 and y0. The parameters λ and φ vary between zero and unity across the length of the argon mass transfer zone (MTZ) in the column. Some data extrapolation and smoothing was necessary in the higher φ region of the φ – τ plot (dashed line) due to fluctuations in the raw Q – t data. The Figure corresponds to a test where pure helium was displaced by pure argon at 273 .1 K and at 6.0 atm pressure.

Figure 3. Dimensionless breakthrough plots for pure argon displacing pure helium.

8


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An argon mass balance between the total column inlet and exit quantities during the breakthrough test was used to estimate the specific equilibrium Gibbsian surface excess (GSE) of pure argon ( , moles/g] at pressure (P = P0y0) and temperature T0. A detailed protocol for this estimation method can be found elsewhere.2 A pure argon adsorption isotherm [ (P, T0)] can be constructed by repeating the experiment using different y0. An effective over-all mass transfer coefficient (ke, s-1) for adsorption of pure argon (y0 =1.0) on the zeolite at P and T0 was estimated from the breakthrough data of Figure 3 by using a specific model based on the assumption of constant pattern MTZ formation in the column and linear driving force (LDF) mechanism for transport of argon from the gas phase to the adosprtion site, in conjunction with the equilibrium adsorption isotherm data for Ar at T0. Details of the model can be found elsewhere.4

Experimental Results

Adsorption Equilibrium and Isosteric Heats Figure 4a shows the adsorption isotherms ( vs P at constant T) of pure argon on the LiLSX zeolite sample at three temperatures (273.1, 303.1 and 338.1 K) in the pressure range of 0 – 6 atm. The experimental pure gas isotherm data points can be found in a tabular form as supporting information for the article. The isotherms are Type I by IUPAC classification. Figure 4b shows the argon isotherms in the Henry’s Law region at different temperatures, where → K(T)∙P as P→ 0. The variable K [= K0 exp (q0/RT)] is the Henry’s Law constant (moles/kg/atm) for pure argon at T, q0(Kcal/mole) is the isosteric heat of adsorption of argon in the Henry’s Law region, K0 is a constant, and R is the gas constant. 9



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Figure 4. (a) Pure argon adsorption isotherm, (b) Isotherms in Henry’s law region. Table 2. Henry’s Law constants for Argon, Oxygen and Nitogen on LiLSX. q0 (Kcal/mole)

K (moles/kg/atm)

Temperature (K)

Ar

O2

273.1

0.262

0.300

328.1

0.110

303.1

0.156

0.157

338.1

0.0984

0.096

N2

Ar

O2

N2

1.15 0.611

5.55 2.75

0.458

3.22

5.87

1.01 0.98

4.77

Table 2 lists the values of K for Ar on the present sample of LiLSX zeolite at different temperatures measured in this work, along with those for O2 and N2 on the same LiLSX sample measured earlier.2 The isosteric heats of adsorption in the Henry’s law region for these gases are also given in the Table. The last two columns of Table 2 show that the Henry’s law selectivity of adsorption of O2 over Ar ( = / ) is nearly unity in the data range indicating that 10


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these two gases cannot be practically separated by LiLSX zeolite, while the selectivity of adsorption of N2 over O2 ( = / ) is large enough for separation of these gases. The argon isotherm at 303.1 K in the low pressure region (P< 1 atm) reported in Figure 4a is also shown in Figure 2 (circles) for comparison. The differences in the argon adsorption characteristics on different commercially available samples of LiX zeolite family are clearly demonstrated. The isotherm data of Figure 4a was used to estimate the isosteric heats of adsorption (q) of pure argon on the LiLSX zeolite sample as a function of GSE by using a thermodynamic relationship, q [= - RT2

].5 Figure 5a shows the q vs plot for Ar along with similar

plots for pure N2 and O2 isosteric heats on the same material reported earlier.2, 3 Figure 5 (b) plots the deviations of isosteric heat of those gases on the LiLSX sample, relative to their isosteric heats in the Henry’s Law region [qo – q], with increase in GSE. It may be seen from Figure 5a that the isosteric heats of adsorption of all three gases decrease with increasing GSE, which indicates that the adsorbent exhibits energetic heterogeneity for these gases. However, Figure 5b shows that the degree of heterogeneity for the gases are different. The adsorption of Ar is nearly homogeneous [q ( ) ~ q0], while O2 adsorption is moderately heterogeneous and N2 adsorption is substantially heterogeneous. The relative magnitudes of qo for these gases increase in the order of Ar < O2< N2 which indicates the the strength of adsorption of N2 > O2 > Ar on the zeolite. Figure 5c is a plot which shows that the isosteric heat of adsorption of a gas on the LiLSX zeolite increases as the permanent quadrupole moment of the gas increases. The isosteric heat of adsorption of CO2 on LiLSX zeolite shown in Figure 5c was obtained from the literature.10 An exponential relationship empirically correlates the isosteric heat of adsorption and the permanent 11



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quadrupole moment of the adsorbate gas very well. Consequently the electrostatic interactions between the quadrupole moment of the adsorbate gas and the – zeolite cation is dominating for adsorption of these gases.

Figure 5. (a) Isosteric heats of adsorption of pure N2, O2 and Ar on LiLSX zeolite sample, (b) Deviations from Henry’s Law region isosteric heats for the components, (c) Henry’s Law region isosteric heat on LiLSX zeolite vs quadrupole moment of adsorbate gas.

Analytical Isotherm Model

The heterogeneous nature of adsorption of pure N2 and O2 on the present sample of LiLSX zeolite could be described well by a thermodynamically consistent, analytical model which accounted for different degrees of heterogeneity for different gases.2, 3, 5 The model can also be used to estimate binary N2+O2 mixture adsorption data reasonably well.3 The key parameters of the model were (a) the saturation adsorption capacity (m, moles/kg), which is same for all gases in order to satisfy thermodynamic consistency, (b) the parameters (K0 and q0) describing the 12


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Henry’s Law region for each gas, and (c) a parameter (ψ) to account for the degree of heterogeneity of adsorption of a gas. A detailed description of the analytical mathematical expressions for pure and mixed gas adsorption isotherms and the corresponding isosteric heats of adsorption can be found elsewhere.3 The same model was used to describe the argon adsorption isotherms on LiLSX zeolite at different temperatures reported in this work. Table 3 shows the values of these model parameters for adsorption of O2, N2, and Ar on the LiX zeolite sample of this work. The model provides analytical expressions for pure and mixed gas adsorption isotherms, and pure and mixed gas isosteric heats of the components as functions of GSE. The model was derived by assuming Langmuir isotherm model for a homogeneous site and a uniform distribution of Langmuir gassolid interaction parameter to account for adsorbent heterogeneity.5 Table 3. Heterogeneous adsorption isotherm model parameters. T

m

K0

q0

(K)

(moles/kg)

(moles/kg/atm)

(Kcal/mole)

Gas

ψ 273.1 303.1

0.995 2.87

7.37×10-5

5.87

0.884

N2 338.1

0.834

273.1

0.240

303.1

2.87

7.94×10-4

3.22

O2

0.060

338.1

3.0x10-4

273.1

1.4x10-3

303.1

2.87

1.64×10-3

Ar

2.75

1.6x10-6 7.0x10-7

338.1

13



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The dashed lines in Figures 4a and 5a, respectively, show the best fit of the pure gas adsorption isotherms and isosteric heats of adsorption for N2, O2, and Ar on the LiLSX zeolite sample of this study. It may be seen that the model is flexible enough to adequately account for the different degrees of heterogeneity of adsorption for these gases. Thus it may be very useful in numerical modeling of a PSA process for air separation using the LiLSX zeolite.

Mass Transfer Coefficients for Argon

The estimated effective, over-all coefficients (ke, s-1) for transport of pure argon from the gas phase to the adsorption sites inside the LiLSX particle at different P and T are given in Table 4 and plotted in Figure 6 in conjunction with those for pure N2 and O2. The model protocol used for estimation of the transport coefficients from the experimental column breakthrough data can be found elsewhere.4 Table 4. Overall pure argon mass transfer coefficients at different P and T.

P (atm)

T (K)

ke (s-1)

skin resistance as % of overall resistance

2.0

303.1

1.78

75.3

4.0

303.1

0.90

75.0

6.0

273.1

0.36

75.2

6.0

303.1

0.59

75.4

6.0

338.1

0.94

74.6

Figure 6a shows that 1/ke linearly increases with increasing P at any given T for all three gases. In other words, ke decreases with increasing P at constant T. Figure 6b shows that lnke is a linear 14


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function of T. In other words, ke increases exponentially with increasing T at constant P. Another important observation from Figure 6 is that mass transfer coefficients for all three gases have comparable magnitudes.

Figure 6. Plots of (a) 1/ke vs P at constant T, and (b) 1/ke vs T at constant P for N2, O2, Ar on LiLSX zeolite.

Model Analysis of Mass Transfer Resistance

For isothermal and isobaric transport of an adsorbate having a linear adsorption isotherm into a pelletized zeolite particle, the overall mass transfer resistance (1/ke) can be assumed to be a summation of various component resistances in series, which are encountered by an adsorbate 15



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molecule to reach the adsorption site from the external gas film. These resistances include external film (1/kf), axial dispersion (1/kax), particle surface skin (1/ksk), macropore diffusion through binder (1/KM), and zeolite micropore diffusion (1/km).4,

11

Magnitudes of these

component resistances can be estimated using simplified models except for the skin resistance, which must be empirically determined from the measured value of ke. 4, 11 The adsorption isotherms of pure O2 on the present LiLSX zeolite sample were approximately linear over a large range of P and T.2 Consequently, the above-described approach was used to establish that the ratio of skin/overall resistance for O2 transport into the present sample of LiLSX zeolite was ~ 65%.4 The adsorption isotherms of Ar on the present LiLSX sample are also approximately linear over the range of data in Figure 4a. Thus, we could estimate an approximate ratio of skin to overall resistance by a similar model analysis. The results are shown in Table3. Approximately 75 % of overall resistance to Argon transport was caused by a skin at the particle surface for all values of P and T. The ratio of skin to overall resistance was not sensitive to system pressure. An observation from Figure 6 is that the transport coefficient from argon is smaller than those for N2 and O2 under similar conditions of P and T, which is consistent with the effectively larger size of argon among these gases as evidenced by relative transport rates of these gases into a molecular sieve carbon.12

Comparison with Published data

We used a commonly used correlation [ke = 60De/ ] between an effective overall diffusivity (De, m2/s) and an overall mass transfer coefficient (ke, s-1) for transport of a pure gas into an 16


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adsorbent particle of diameter (cm) in order to calculate De for Ar at a pressure of 1 atm and ~300 K on the LiLSX zeolite sample ( = 0.052 cm) of the present work. The value of ke (= 3.61 s-1) under these conditions was obtained by extrapolation of 1/ke vs P plot of Figure 6a. The estimated value of De was 1.63 x 10-8 m2/s. A value of ke = 0.23 s-1 was reported in the literature for transport of pure Ar under similar conditions (P = 1, atm, T = 298

o

K) using an Ag exchanged LiX zeolite sample ( = 0.16

cm).99 Thus, the estimated value of De for Ar on that material was 0.98 x 10

-8

m2/s.

Consequently, the LiLSX zeolite sample of the present work exhibited ~ 66 % larger overall diffusivity than the Ag-LiX zeolite sample. This re-emphasizes the need for independent measurement of adsorption characteristics of each different sample before use in practice.

Conclusions

The present study led to the following important conclusions: 1. The adsorption characteristics (equilibrium isotherm, isosteric heats, and mass transfer coefficients) of pure argon on different commercial samples of LiLSX can be substantially different. 2. The zeolite sample used in this study exhibits different degrees of adsorbent heterogeneity for N2, O2, and Ar. 3. The adsorbent is weakly heterogeneous (nearly homogeneous), mildly heterogeneous, and moderately heterogeneous for adsorption of, respectively, pure Ar, O2 and N2.

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4. An analytical isotherm model which can account for different degrees of heterogeneities for gases can be used to describe equilibrium isotherms and isosteric heats of N2, O2, and Ar on the LiLSX zeolite. 5. The isoseric heat of adsorption of a pure gas on the zeolite is found to be an exponential function of the quadrupole moment of the gas. 6. The overall mass transfer resistances of Ar on the LiLSX zeolite is comparable to those of N2 and O2, and it increases with increasing P, and decreases with increasing T analogous to the behavior exhibited by pure N2 and O2. 7. A skin resistance at the adsorbent particle surface constitutes ~ 75 % of the experimental overall transport resistance for Ar transport into the zeolite. 8. The sample of the present study exhibited larger overall Ar diffusivity than that reported for a sample from another source.

Associated Content

The experimental gas adsorption isotherm data on LiLSX of pure gas Ar at different temperatures is provided. This information is available free of charge via the Internet at http://pubs.acs.org/.

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Park, Y-J.; Lee, S-J.; Moon, J-H.; Choi, D-K.; Lee, C-H. Adsorption Equilibria of O2, N2 and Ar on Carbon Molecular Sieves and Zeolites, 10X, 13X, and LiX. J. Chem. Eng. Data 2006, 51, 1001.

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(10) Park, Y.; Moon, D- K.; Kim, Y-H.; Ahn, H.; Lee, C. H. Adsorption isotherms of CO2, CO, N2, CH4, Ar, and H2 on Activated carbon and Zeolite LiX up to 1.0 MPa. Adsorption 2014 20, 631. (11) Ruthven, D. M. Principles of Adsorption and Adsorption Processes; John Wiley and Sons, Inc.: New York, 1984. (12) Rege, S. U.; Yang. Y. T. Kinetic Separation of Oxygen and Argon Using Molecular Sieve Carbon. Adsorption 2000 6, 15.

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Equilibrium Isotherm and Mass Transfer Coefficient for Adsorption of

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