Energy & Fuels 2009, 23, 617–623
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Chromatographic Study on the Adsorption and Diffusion of Light Hydrocarbons in ZSM-5 and USY Zeolites Jiajia Zhang,†,‡ Zhen Zhao,*,† Aijun Duan,† Guiyuan Jiang,† Jian Liu,† and Dengqian Zhang† State Key Laboratory of HeaVy Oil Processing, China UniVersity of Petroleum, Beijing 102249, People’s Republic of China, and Dongying Vocational College, Shandong, Dongying 257091, People’s Republic of China ReceiVed August 21, 2008. ReVised Manuscript ReceiVed NoVember 6, 2008
Single-component sorption/diffusion of methane, ethane, propane, n-butane, isobutane, cyclohexane, and toluene in ZSM-5 and USY zeolites has been measured by a chromatographic method in the temperature range of 373-598 K. It is shown that the adsorption equilibrium constants decreased with the increase of the temperature, while it increased with the increase of the atomic number of carbon in the adsorbate molecule. The adsorption energies increased with the increase of molecular weights of the adsorbate molecules. The adsorption energy follows the order: methane < ethane < propane < n-butane < cyclohexane < toluene. The diffusivities in the micropore were larger at higher temperatures. The diffusivities in the micropore increased with the decrease of critical sizes of adsorbate molecules, for example, ethane > propane > n-butane > cyclohexane > toluene. The diffusion coefficients are found to be influenced by the critical molecular size, electronic effect, and configuration of sorbate molecules. In comparison to the diffusion data in ZSM-5 and USY zeolites, it is confirmed that the zeolite channel also plays a vital controlling/deciding role in the penetration and diffusion.
1. Introduction Zeolite catalysts have fine micropores within their crystals, the diameters of which are almost equal to the molecular size of light hydrocarbons; thus, the special configuration gives the zeolite a high molecular sieving effect.1 Zeolite catalysts are widely used in hydrocarbon conversion processes, i.e., catalytic cracking and isomerization processes, because of their high activities and selectivities. This catalytic performance is strongly dependent upon the intracrystalline diffusivity of reactant molecules.2 When zeolites are used in various processes as adsorbents and/or catalysts, reactant molecules must diffuse into (adsorption process) or leave from (desorption process) the zeolite crystals. Diffusion plays a vital role in determining the product selectivity of catalytic processes through diffusionreaction interactions. Furthermore, diffusion also plays a very important role in determining kinetic selectivity in the adsorption separation processes. Therefore, a full understanding of the diffusion process and its controlling factors is very essential to obtain insight into the catalytic processes in these zeolite catalysts, and it can provide theoretical bases for designing the highly effective catalysts. The study of the diffusion of sorbates in the channels of zeolites and other microporous materials has driven more attention. Many experimental techniques3 have been applied in * To whom correspondence should be addressed. Telephone: +86-01089731586. E-mail:
[email protected]. † China University of Petroleum. ‡ Dongying Vocational College. (1) Fujikata, Y.; Masuda, T.; Ikeda, H.; Hshimoto, K. Microporous Mesoporous Mater. 1998, 21, 679–686. (2) Masuda, T.; Fukada, K.; Fujikata, Y.; Ikeda, H.; Hashimoto, K. Chem. Eng. Sci. 1996, 51, 1879–1888. (3) Theory and Applications; Ruthven, D. M., Brandani, S., Fraissard, J., Eds.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1997; Vol. 8, p 261.
detecting the diffusion performance of hydrocarbon molecules, including micro- and macroscopic methods, transient and steadystate measurements, etc. As new techniques have been developed producing results of great accuracy and complexity, the theoretical interpretation of the transport of molecules in the well-defined, regular pore structures of these frameworks has become more and more exciting and fascinating and the detailed modeling of such transport processes has begun to gain much attention. Chromatographic separation is a physicochemical process based on convection, diffusion, adsorption and liquid dissolution, and the technique used today mainly for chemical analysis purposes. Indeed, analysis by gas chromatography (GC) suffers from the so-called broadening factors, the most important of which are related to nonfulfillment of the assumptions under which the central chromatographic equation is derived, namely: (1) non-negligible axial diffusion of the solute gas in the chromatographic column, (2) nonlinearity of the distribution (e.g., adsorption) isotherm, and (3) non-instantaneous equilibration of the solute component between the mobile and stationary phases. It is through these broadening factors, embraced by the Van Deemter equation, that gas chromatography offers many possibilities for physicochemical measurements, leading to very precise and accurate results with relatively cheap instrumentation and very simple experimental setups.4 These methods are widely used today. Thus, GC5 is believed to be a rapid and dependable means for measuring the effective diffusivities in porous catalysts. As a result of the recent development, the GC method can provide the effective diffusivity values in both pore systems of bidisperse structured catalysts. The measurement can be conducted under near reaction conditions, and thus, the hazard(4) Nicholas, A. K.; Richard, T.; Fani, R. K. J. Chromatogr., A 1998, 795, 133–184. (5) Henry, W.; Haynes, J. R. Chem. Eng. Sci. 1975, 30, 955–961.
10.1021/ef800689g CCC: $40.75 2009 American Chemical Society Published on Web 12/18/2008
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ous procedure of extrapolating effective diffusivities obtained at ambient conditions is avoided. When the concentration pulse of adsorbate was introduced to the carrier gas stream at the inlet of the adsorbent column, the concentration elution curve at the outlet of the column was measured. The adsorption equilibrium constant and micropore diffusivity were determined by means of moment analysis of elution curves. In this flow system,6 good heat transfer is ensured, and thus, there is no problem to keep the sample at constant temperature. A lot of work was carried out regarding the adsorption and diffusion of big molecules, such as aromatics, which possess side chains.7 Thus far, few systematical research works have been performed in the diffusion behavior of different types of small hydrocarbon molecules in the faujasite structure. In this paper, the GC method was used to measure the diffusion coefficients of C1-C4 alkanes, cyclohexane, and toluene in ZSM-5 and USY zeolites, and their activation energies were also determined. The important effects of critical size and configuration of sorbate molecules on their diffusions in USY zeolite are discussed. At last, small hydrocarbon molecule diffusions in ZSM-5 zeolite were compared to those in USY zeolite. The aim of the present study is also to investigate the effect of the zeolite channel on the diffusion. 2. Theory The chromatographic adsorption and diffusion measurement involves injecting a pulse of some amount of adsorbate at a packed column inlet and measuring the concentration response of the absorbate at the column outlet.8 The mean retention time (or retention volume) of the concentration response defines the holdup in the column, from which the adsorption equilibrium constant can be calculated. The retention time distribution of the concentration response defines the combined effects of axial dispersion and mass-transfer resistance. A bidisperse structured catalyst can be prepared by compressing, extruding, or in some other manner, compacting finely powdered microporous material into pellets. In theory, the micropores are rewarded by the porosity inherent in the individual microparticle of catalyst and the macropores are voids among the microparticles after pelletization or extrusion. Mass transport of the gas through a packed catalyst bed undergoes the following processes: axial dispersion in the interpellet voids, bulk gas-pellet surface transfer, intrapellet diffusion in the macropores, intraparticle transport through the micropores of catalyst particles, and adsorption in the cavities of catalyst particles. Adsorption on the walls of the macropores is relatively insignificant, because the surface of the macropore is negligible compared to that of the catalyst crystallites. The macro-micropore model9 and the transport processes in the catalysts are studied in this work. A simple method of analyzing chromatographic data is by matching the first and second moments of the concentration response peaks to the theoretical expressions derived from a dynamic model. Early models for mass-transfer resistance in a chromatographic column were based on the equilibrium stage concept. Kucera10 showed the relationship of the height (6) Chiang, A. S.; Dixon, A. G.; Ma, Y. H. Chem. Eng. Sci. 1984, 39, 1451–1459. (7) Zaman, S. F.; Loughin, K. F.; Al-Khattaf, S. S. Ind. Eng. Chem. ReV. 2005, 44, 2027–2035. (8) Chen, S. Y.; Peng, S. Y.; Zhong, B. Chem. Eng. Sci. 1979, 1, 102– 110. (9) Ruthven, D. M. Principles of Adsorption and Adsorption Processes; John Wiley and Sons: New York, 1984; pp 124-127. (10) Kucera, E. J. Chromatogr. 1965, 19, 237–248.
Zhang et al.
Figure 1. Schematic diagram of the GC apparatus: (1) pulse gas, (2) carrier gas, (3) cut-off valve, (4) mass flow controller, (5) unilateral valve, (6) six-port gas sampling valve, (7) chromatography, and (8) personal computer.
equivalent to a theoretical plate (HETP) and the diffusional time constant. This provides the theoretical basis for the chromatographic approach to the measurement of intraparticle diffusivities. Sarma and Haynes,11 Hashimoto and Smith,12,13 Shah and Ruthven14 developed the bidisperse pellet models. Haq et al.15,16 provided the generalization of the bidisperse particle model, which includes axial dispersion, external fluid film mass-transfer resistance, and macro- and micropore diffusion. The relevant expressions for the first and second moments of the concentration response peaks are as following: the first moment: µ1 ≡
∫
∞
0
ctdt
∫
∞
0
1-ε L 1+ K (1) ν ε
[ (
)
cdt
the second moment: µ2 ) σ2 ≡
∫
∞
0
)]
c(t - µ)2dt
∫
∞
0
(2)
cdt
HETP: HETP )
(
2ν
2DL σ2 + L) 2 ν µ1
[
]
r2(K - εp) ε R R2 ε + + 1+ 1 - ε 3kf 15εpDp (1 - ε)K 15K2DC
)
[
-2
]
(3)
(1) The analysis of the first moment: One beeline that crosses the origin point was obtained by plots of µ1 versus L/ν, and the slope is B
[ ( 1 -ε ε )K]
B) 1+
(4)
Thus, the value of K was obtained from eq 4. (2) The analysis of the second moment: σ2 L DL HETP ) ) + 2ν 2µ 2 ν ν2
[
1
(
]
r2(K - εp) ε R R2 ε + + 1+ 1 - ε 3kf 15εpDp (1 - ε)K 15K2DC
)
[
-2
]
(5)
(11) Haynes, H. W.; Phanindra, J.; Sarma, N. AlChE J. 1973, 19, 1043– 1046. (12) Hashimoto, N.; Smith, J. M. Ind. Eng. Chem. Fundam. 1973, 12, 353–359. (13) Hashimoto, N.; Smith, J. M. Ind. Eng. Chem. Fundam. 1974, 13, 115–120.
Light Hydrocarbons in ZSM-5 and USY Zeolites
Energy & Fuels, Vol. 23, 2009 619
Table 1. Details of Adsorbents and Columns absorbent
ZSM-5 zeolite
particle density (g/cm3) ture density (g/cm3) total pore volume (cm3/g zeolite) total porosity crystal mean radius (um) mesh size average particle diameter (cm) length of the packed bed (cm) column diameter (mm) mass of adsorbent (g) bed voidage (εp)
USY zeolite
0.93 1.84 0.53 0.49 2.5 20–30 0.038 35.5 8 11.4 0.31
0.77 1.46 0.62 0.47 4.8 20–30 0.038 14.5 5 1.86 0.4
Transport properties are determined from the second moment or variance of the chromatographic peak. A simple expression for estimating the second moment is given by Hufton17 µ2 ) σ2 ) [(1/2.3548)ω(50)]2
(6)
The value of ω(50) was obtained from chromatographic peaks, because it is just the peak width at a peak height that equals to half of the maximum. This approximation is strictly valid for Gaussian peaks only. The detector responses measured during this study, however, were nearly symmetric; therefore, the approximation made in eq 6 is justifiable. In the low Reynolds number region, as Sh ≈ 2.0 or kf = Dm/R and when molecular diffusion is the dominant mechanism of transport within the macropores, DP = Dm/τ (assuming a tortuosity factor of 3.0), so that eq 5 becomes σ2 L DL ε HETP ) ) 2+ 2ν 2ν 1 ε 2µ ν
(
1
[
) ( DR )( 31 + 15ετ ) + 2
m
r (Κ - εp) 2
2
15K DC
][
p
1+
ε (1 - ε)K
-2
]
(7)
According to eq 7, for plots of HETP/2ν versus 1/ν2, the slope is DL and the intercept is B1 )
(
ε 1-ε
[
)(
)(
)
][
r2(Κ - εp) τ R2 1 ε + + 1+ Dm 3 15εp (1 - ε)εp 15K2DC
]
-2
(8)
According to eq 8, the intracrystalline diffusivity can be obtained from the intercepts of plots of (HETP/2ν) versus 1/ν2 3. Experimental Section The measurements of the adsorption and diffusion of light hydrocarbons in ZSM-5 and USY zeolites were carried out in a standard GC (SP3420) equipped with a flame ionization detector and gas sampling injection valve. A computer system was used to log and analyze the concentration distribution peak data from the gas chromatograph. The schematic diagram of the experimental system in this study is shown in Figure 1. The properties of the adsorbents and columns used in the experiments are summarized in Table 1. The average particle diameter was the arithmetic average of the maximum and minimum of sieve sizes. The apparent density of the crushed catalyst pellets was measured by the mercury porosimetry of the micromeritics at 4.98 psia. The catalyst skeletal density was determined at 29 996.47 psia. The pore-size distribution of catalyst is shown in Figure 2. (14) (15) 163. (16) 169. (17)
Shah, D. B.; Ruthven, D. M. AlChE J. 1977, 23, 804–809. Haq, N.; Ruthven, D. M. J. Colloid Interface Sci. 1986, 112, 154– Haq, N.; Ruthven, D. M. J. Colloid Interface Sci. 1986, 112, 164– Hufton, J. R.; Danner, R. P. AlChE J. 1993, 39, 954–961.
Figure 2. Pore-size distribution of zeolite pellets (larger range): (A) ZSM-5 zeolite and (B) USY zeolite (measured by the mercury porosimetry of micromeritics).
From the physical properties of the known sieve, the bed voidage of the column was calculated from the known adsorbent particle density and the weight of the packed column. Dead volume fittings were employed, and the columns was sufficiently long to minimize the effect of dead volume. It is also necessary to obtain the moments of the bed from the observed moments. The columns were packed with crushed pellets with 40-80 mesh, which were obtained by crushing and sieving 20-30 mesh of ZSM-5 and/or USY zeolite pellets. For pretreatment, the helium (at 20-30 cm3/min) was flowing through the packed columns for 15 h at 360 °C. Linearity of the system was confirmed in the usual way by measuring the response for different kinds of molecular sizes of injection pulse. For each gas sample, runs were made at several temperatures. To separate the axial dispersion contribution from the second moments and to obtain more reliable adsorption equilibrium constants, the flow rate of carrier gas were varied from 80 to 150 mL/min. For the liquid samples, the bubbling system to inject the reactant samples was used. Cyclohexane and toluene samples were kept in a device, maintained at 40 °C and a constant carrier gas flow rate controlled by the mass flow controller. It can obtain a definite concentration of samples to keep the linearity of the system.
4. Results and Discussion 4.1. Adsorption Equilibrium Data. The plots of the first moment against reciprocal velocity were linear for all species, as shown in Figure 3, indicating that equilibrium is approached in the column and eq 1 is valid over the entire velocity range. The Henry constants derived from the slopes of such plots plotted against the reciprocal temperature are shown in Figure 4. Because K is a dimensionless equilibrium constant based on concentration rather than pressure, the temperature dependence is given by eq 9. K ) K0e-∆U0/RT
(9)
The change in internal energy with adsorption is given by the Vant Hoff equation,17 as eq 10 ln(K) - ln(K0) ) -∆U0/RT
(10)
The change in enthalpy with adsorption or the isosteric heat of adsorption can be determined from the following eq 11: ∆H0 ) -∆U0 + RT
(11)
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Figure 4. Henry constants of hydrocarbons in ZSM-5 and USY zeolites.
Figure 5. Effect of the carbon number on ∆U0 for the adsorption of linear alkanes in zeolite. Figure 3. Representative plots showing variation of the first moment with reciprocal fluid velocity.
The values of Henry constants and adsorption energies for hydrocarbons in ZSM-5 and USY zeolites were determined from the linear least-squares regressions of the data in Figure 4. The results are listed in Tables 2 and 3. Tables 2 and 3 show that adsorption equilibrium constants increased with the decrease of the temperature or the increase of molecular weights of the hydrocarbons. For example, the Henry constant follows the order: methane < ethane < propane < n-butane < cyclohexane < toluene. The internal energy and the enthalpy of adsorption had the same tread of the Henry constant. In comparison to the diffusion of n-butane and isobutane in ZSM-5 zeolite, the effect of chain branching on the Henry
constants can be examined on the basis of the butane data. The value of Ki for isobutene was found to be lower by 40% than that measured for n-butane. This behavior is also obtained by the Hufton et al.17 This presumably reflects the rod-like shape of n-butane. The values of ∆U0 for the n-alkanes as a function of the number of carbon atoms in the adsorbate molecule, NC, are shown in Figure 5. The following correlations were obtained by regression of the data in Figure 5: In the ZSM-5 zeolite: -∆U0 ) 2.06NC + 5.92
(12)
-∆H0 ) 2.01NC + 5.17
(13)
Light Hydrocarbons in ZSM-5 and USY Zeolites
Energy & Fuels, Vol. 23, 2009 621
Table 2. Henry Constants and Adsorption Energies for Hydrocarbons in ZSM-5 Zeolite adsorbate
methane
ethane
propane isobutane n-butane cyclohexane
temperature (K)
K (dimensionless)
373 398 423 448 473 398 423 448 473 448 473 498 448 473 498 448 473 498 548 573 598
0.92 0.52 0.29 0.12 0.013 6.14 3.02 1.67 0.94 8.08 4.36 2.42 24.60 11.85 6.15 41.40 18.32 9.09 9.63 4.99 3.16
-∆U0 (kcal/mol)
7.23
9.33
10.79 11.92 13.45 14.52
Table 3. Henry Constants and Adsorption Energies for Hydrocarbons in USY Zeolite adsorbate
methane
ethane
propane
isobutane
n-butane
cyclohexane
toluene
temperature (K)
K (dimensionless)
373 398 423 448 473 373 398 423 448 473 373 398 423 448 473 498 448 473 498 423 448 473 498 473 498 523 548 573 598 523 548 573
0.28 0.15 0.14 0.12 0.1 1.16 0.74 0.51 0.36 0.26 4.88 2.78 1.66 1.14 0.7 0.59 3.63 2.13 1.36 6.4 3.81 2.3 1.53 38.26 23.16 14.04 9.28 6.34 4.54 225.66 116.1 57.92
-∆U0 (kcal/mol)
Figure 6. Representative plots showing variation of (HETP/2ν) versus 1/ν2.
3.57
Table 4. Experimental Zeolitic Diffusion Coefficients in ZSM-5 Zeolite adsorbate
5.21
ethane
propane 6.19 isobutane n-butane
8.7
cyclohexane
8.02
9.64
16.18
When NC is 6, the value of ∆U0 calculated from eq 12 is 18.28 kcal/mol, which is 76.48 kJ/mol. This result is very similar to the data reported by Wu et al.18 In the USY zeolite: -∆U0 ) 1.43NC + 2.17
(14)
-∆H0 ) 1.46NC + 2.96
(15)
When NC is 6, the value of ∆U0 calculated from eq 14 is 10.75 kcal/mol, which is 44.98 kJ/mol. This result is very similar to the data reported by Wu et al.18 (18) Wu, Z. B.; Zhao, W. R. The Principle and Application of EnVironment Catalysis; Chemical Industry Publisher: Beijing, China, 2005; pp 132-135.
temperature (K) 398 423 448 473 448 473 498 448 473 498 448 473 498 548 573 598
zeolitic diffusivity (cm2/s) 4.09 × 10-9 8.78 × 10-9 13.44 × 10-9 15.91 × 10-9 3.68 × 10-9 5.86 × 10-9 9.53 × 10-9 1.01 × 10-9 1.87 × 10-9 3.29 × 10-9 0.35 × 10-9 1.78 × 10-9 2.64 × 10-9 0.32 × 10-9 0.59 × 10-9 0.82 × 10-9
The agreement between the prediction of eqs 12 and 14 and the literature data of n-pentane is remarkable. Thus, the validity of eqs 12 and 14 as the approximate predictive tool for ∆U0 of linear alkane absorption in ZSM-5 and USY zeolites is reasonable. 4.2. Kinetic Data. According to eq 7, the intracrystalline diffusivity can be obtained from the intercept, by plotting (HETP/2ν) versus 1/ν2, as shown in Figure 6. In addition, the derived parameters are summarized in Tables 4 and 5. The plots show evidently linear. The molecular diffusion coefficient, Dm, was determined by using the Wilke-Lee19 formulas. Tables 4 and 5 showed that the diffusion coefficients inside the micropore were increased with the increase of the temperature and the molecular weight of the hydrocarbon molecules. The molecular diameters of the propane, n-butane, and toluene are 0.43, 0.49, and 0.67 nm, respectively. The increase of the molecular size of light hydrocarbon results in an apparent decrease in the diffusion coefficients and, meanwhile, causes a large increase in the activation energy of diffusion. This is (19) Li, R. H. Basic of Transport; Aviation College Publisher: Beijing, China, 1987; pp 47-68.
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Table 5. Experimental Zeolitic Diffusion Coefficients in USY Zeolite adsorbate
propane
n-butane
isobutane
cyclohexane
toluene
temperature (K)
zeolitic diffusivity (cm2/s)
373 398 423 448 473 423 448 473 498 448 473 498 473 498 523 548 573 598 523 548 573
1.61 × 10-8 2.68 × 10-8 4.01 × 10-8 5.23 × 10-8 5.44 × 10-8 0.95 × 10-8 1.69 × 10-8 2.67 × 10-8 4.03 × 10-8 1.45 × 10-8 2.31 × 10-8 3.45 × 10-8 0.36 × 10-8 0.58 × 10-8 0.91 × 10-8 1.13 × 10-8 1.64 × 10-8 2.26 × 10-8 0.048 × 10-8 0.11 × 10-8 0.18 × 10-8
expected because of the drastic increase in the steric hindrance for the sorption/diffusion in the zeolite. Table 4 shows that the diffusion coefficients in ZSM-5 zeolite increased with the decrease of the sizes of adsorbate molecules; i.e., the order of the diffusion coefficient follows: ethane > propane > isobutane > n-butane > cyclohexane. Among them, the critical molecular diameters of n-butane and isobutane are 0.43 and 0.50 nm, respectively, but the diffusion coefficient of isobutane is greater than that of n-butane at the same temperature. The reason is that the shape of n-butane is rod-like and the methyl in the isobutane makes the shape of isobutane similar to a sphere. As a result, isobutane is easy to diffuse in the channel of the zeolite. In Table 5, the diffusion coefficients in USY zeolite increased with the decrease of critical sizes of adsorbate molecules. The trend is propane > isobutane > n-butane > cyclohexane > toluene. The critical molecular diameters of cyclohexane (0.69 nm) and toluene (0.67 nm) are very similar, but in comparison to the diffusion properties of cyclohexane, toluene shows a very slow diffusion and a very high activation energy, which is attributed to the intrinsic difference in the structures of chemical bonds of these two adsorbed molecules. In the toluene molecule, there is a phenyl ring in which a conjugated and delocalized large π bond exist. Moreover, the electron cloud density of the phenyl ring further increased by the conjugation between the phenyl ring and methyl group or by the electron-clouding property of the methyl group. The characteristics of the configuration structure and electron of toluene are favorable for the interaction between the toluene molecule and the cations in the zeolite channels.2 Therefore, the adsorption energy for the diffusion of toluene is much higher, while the diffusion coefficient is much smaller than that of cyclohexane. To calculate the diffusion coefficients of methane and ethane based on the eqs 4 and 13, it is assumed that K . 1.0, which means that the reversible adsorption process within the zeolite crystal was intrinsically rapid. Thus, their adsorption equilibrium constants were smaller than 1.0, which is beyond the extension of this method. Intracrystalline diffusion of the smaller molecules was too rapid to obtain a reliable evaluation of the intracrystalline diffusivities. 4.3. Comparison of the Data between USY and ZSM-5 Zeolites. It can be seen from Figure 7 that the channel diameter distribution of ZSM-5 zeolite is mainly at 1.42 nm,
Figure 7. Pore-size distribution of zeolite pellets (smaller range): (A) ZSM-5 zeolite and (B) USY zeolite (measured by the N2 adsorption method).
while a little is at 3.76 nm. The channel diameter distribution of USY zeolite is mainly at 3.82 nm, while some is at 5.55 nm. The channel diameter of USY zeolite is larger than that of ZSM-5 zeolite; thus, the diffusion coefficient in USY zeolite is greater than that in ZSM-5 zeolite. For the same adsorbate, the adsorbent channel diameter played an important role in controlling the diffusion of adsorbate molecules, with the increase in the adsorbent channel diameter resulting in an increase in the diffusion coefficient. 5. Conclusions Single-component sorption/diffusion of methane, ethane, propane, n-butane, isobutane, cyclohexane, and toluene in ZSM-5 and USY zeolites has been measured by a chromatographic method. (1) All of the adsorption equilibrium constants decreased with the increase of the temperature and follow the order: methane < ethane < propane < n-butane < cyclohexane < toluene. (2) The adsorption energies increased with the increase of molecular weights of the adsorbate molecules. The deducted equations for the adsorption energies of C1-C4 adsorbates in ZSM-5 and USY zeolites are reasonable, which is an approximate predictive tool for adsorption energy of chain alkanes in ZSM-5 and USY zeolites. (3) The diffusion coefficients in the micropore were larger at higher temperatures. The diffusion coefficients in the micropore increased with the decrease of critical sizes of adsorbate molecules. The influence of various factors (critical size, configuration, and electronic effect) of adsorbate molecules on the intracrystalline diffusion/mass transfer has been clearly discussed, and these factors play an important role in the sorption/diffusion processes. (4) From the diffusion data of different adsorbates in ZSM-5 and USY zeolites, it can be concluded that the zeolite channel also plays an important role in the sorption/diffusion processes. Acknowledgment. This work was supported by the National Basic Research Program of China (Grants 2004CB217806 and 2005CB221402), the Scientific Research Key Foundation for the Returned Overseas Chinese Scholars of State Education Ministry, the National Natural Science Foundation of China (20773163) and Specialized Research Fund for the Doctoral Program of Higher Education (no. 20070425010).
Nomenclature HETP ) height equivalent to a theoretical plate (cm) (numerically equal to the column length divided by the number of theoretical plates in the column) t ) time (s) L ) length of packed column (cm) V ) interstitial gas velocity in the column (cm s-1) K ) dimensionless Henry’s law constant (the ratio of moles of adsorbate per volume in gas to moles of adsorbate per volume in the absorbent)
Light Hydrocarbons in ZSM-5 and USY Zeolites Ki ) equilibrium constant of component i K0 ) pre-exponential factor in eq 9 DL ) axial dispersion coefficient (cm2 s-1) DP ) macropore diffusivity (cm2 s-1) DC ) intracrystalline diffusivity (cm2 s-1) Dm ) molecular diffusivity R ) radius of particle (cm) r ) radius of zeolite crystal (cm) kf ) external mass-transfer coefficient (cm s-1) ∆U0 ) change in internal energy upon adsorption (kJ mol-1) T ) temperature (°C) ∆H0 ) change in enthalpy upon adsorption (kJ mol-1) NC ) number of carbon atoms in a linear alkane molecule
Energy & Fuels, Vol. 23, 2009 623 ω(50) ) width of peak at a height equal to half of the maximum peak height Sh ) Sherwood number Subscripts µ ) mean retention time (s) µ1 ) first moment of the response peak (s) µ2 ) second moment of the response peak (s2) σ2 ) variance of the response peak ε ) voidage of the adsorbent bed εp ) porosity of the adsorbent bed τ ) tortuosity factor EF800689G
