Adsorption of isopentane on 5A adsorbents with different degree of calcium exchange was investigated at 100 and 250°C using a microbalance. In spite the ...
CHEMICAL ENGINEERING TRANSACTIONS, Volume 3, 2003 Edited by Sauro Pierucci First-edition 2003 Copyright © 2003, AIDIC Servizi S.r.l. ISBN 88-900775-2-2 Printed in Italy
Edited by Sauro Pierucci Copyright © 2003, AIDIC Servizi S.r.l., ISBN 88-900775-2-2 CHEMICAL ENGINEERING TRANSACTIONS, Volume 3, 2003
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Calcic exchange effect on molecular encumbrance during isopentane adsorption on A zeolite F. Benaliouche and Y. Boucheffa UER de Chimie Appliquée, EMP BP 17 Bordj El-Bahri, Algiers, Algeria Adsorption of isopentane on 5A adsorbents with different degree of calcium exchange was investigated at 100 and 250°C using a microbalance. In spite the porous volume of the adsorbents decreasing with the calcic exchange degree, the isopentane (∅ > 5Å) penetrates into pores of all adsorbents. The penetration induces a signal disruption in adsorption isotherms which is attributed to a strong difficulty of isopentane molecule to enter into α cavity of 5A adsorbent. The decrease of disruption signal amplitude was related to the increase of calcic exchange degree.
1. Introduction 5A zeolite is one of the most used adsorbents in hydrocarbon separation processes, notably for n/iso paraffin mixtures which is carried out in order to improve the octane number of gasoline (Jonhson and Oroskar, 1989; Jullian et al., 1993). During this separation, despite their important sizes, the isoalkanes molecules such as isopentane (iC5), 2-methylpentane (2MP), 3-methylpentane (3MP), 2,3-dimethylbutane (23DMB) and 2,2-dimethylbutane (22DMB) can be adsorbed and lead to deactivate the molecular sieves by blocking 5A zeolite pores (Magnoux et al., 2000; Benaliouche et al., 2002). The thermogravimetric measurements at 100°C and 27kPa show a disturbance isotherm signal. The amplitude of this disturbance increases with molecular encumbrance according to : iC5 < 2MP < 3MP < 23DMB < 22DMB when the adsorption is operated at 100°C. At 250°C, the adsorbed molecules can be transformed partially into various products (oligomers and aromatics) by acid reaction. In this case, the evolution of disturbance amplitude is not in agreement with the increase of the size of adsorbed molecules (Benaliouche et al., 2002). The objective of this study is to investigate the effect of the calcic exchange degrees (33, 44, 57 and 75%) in 5A adsorbent during the isopentane adsorption. The disturbance isotherm will be exanimate in order to understand the relationship between the isopentane molecular encumbrance and the calcic exchange degree.
2. Experimental section The adsorption isotherms were obtained at constant pressure of 27kPa using a Setaram microbalance (sensitivity of 10-8 g) linked to a computer by way of Cobra interface. The Cobra software used in the thermogravimetric method allows to obtain a curve with 1008 dots. This number appears very sufficient to visualize the signal disturbance of kinetic sorption. The 5A adsorbents (5A33, 5A44, 5A57) were supplied by IFP (80% wt.-
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% of zeolite and 20 wt.-% of clay binder). The 5A adsorbent exchanged at about 75% (5A75) was supplied by Rhône-Poulenc. Adsorption isotherms of nitrogen over the samples were established by microbalance at –196°C. Before the introduction of isopentane, the sample (0.085g) was pretreated in vacuum (10-3 Pa) at 420°C for 5 hours. After drying on 3A zeolites, isopentane (purity > 99%) from Fluka, was introduced in microbalance system. The adsorption isotherms had been monitored at 100 and 250°C for a period of 5 hours.
3. Results and discussion 3.1 Adsorption capacity of 5A adsorbents To calculate the volume of nitrogen adsorbed at –196°C, its density was taken as that of liquid phase at the adsorption temperature. Results are : 0.222, 0.216, 0.165 and 0.02 cm3/g, respectively, for adsorbents 5A75, 5A57, 5A44 and 5A33. In the case of 5A75,, the result (0.222 cm3/g) is lower than nitrogen capacity adsorption obtained with pure 5A (% Ca2+ =74) which is equal to 0.26 cm3/g (Boucheffa et al., 1997). The 20 wt.-% of clay binder in the composition of adsorbent explains this reduction in porous volume. When the degree of exchange is at 33%, the porosity of adsorbent is nearly inaccessible. But then, from 44%, α cavities begin to open and nitrogen molecules become completely free to enter at 75% of calcic exchange in agreement with Takaichi diagrams (Takaishi et al., 1975). 3.2 Adsorption isotherm of isopentane The effect of time on the increase in weight was determined at 100 and 250°C from isopentane on 5A adsorbents (figure 1). (a)
Wt %
(b)
Wt % 2
2 5A75 5A57
1
1
5A75 5A57 5A44 5A33
5A44 5A33
0 0
1
2 3 Time (h)
4
5
0 0
1
2 3 Time (h)
4
5
Figure 1: Adsorption isotherms of isopentane on 5A adsorbents (P = 27 kPa) at 100°C (a) and at 250°C (b). At 100 and 250°C, the adsorption of isopentane operates on all 5A adsorbents. It is extremely weak on 5A33 (lower than 0,2%) and a light increase appears on the 5A44 sample (≈ 0.5%). For the two temperatures, adsorbed quantities increase with the exchange rate and this increase is more important at 100°C. No adsorption was operated on 4A adsorbent (zero calcic exchange). The determination of initial adsorption rate
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from origins of curves shows that these values and quantities adsorbed after 5 hours increase also with the calcic exchange degree (figure 2).
Wt %
1.5 1 1 0.5
0.5 0
0 0
20
40
60
80
Calcium e xchange (%)
2
2.5 2
1.5 Wt %
2
1.5
Vi (10 -3 . mol . h -1 . g -1 )
2.5
1.5 1 1 0.5
0.5
Vi (10 -3 . mol . h -1 . g -1 )
(b)
(a) 2
0
0 0
20
40
60
80
Calcium e xchange (%)
Figure 2 : Initial rate of isopentane adsorption and adsorbed amounts after 5 hours versus the degree of calcium exchange on 5A adsorbents, (a) at 100°C, (b) at 250°C In previous studies (Magnoux et al., 1998), an analysis by GC and GC/MS coupling of adsorbed materials on pure 5A (calcic exchange equal to 74%) after dissolution of zeolite in hydrofluoric acid solution in order to liberate the compounds trapped in the pores shows that the isopentane was the only trapped molecule in the zeolite pores at 100°C. However, at 250°C, the isopentane is accompanied by a small amount of cracked products (olefin and paraffin) and by branched (saturated and unsaturated) C15 to C24 hydrocarbons. These products result from acids transformation of isopentane over acid sites of 5A zeolite. In fact, it is known that Ca2+ cations can be hydrolyzed during the heating of zeolite and a protonic sites can be formed according to the reaction (Mix et al., 1988).: Ca2+ + H2O ⎯→ [Ca(OH)]+ + H+ This mode of formation of protonic sites has been found in various zeolites exchanged by alkaline-earth or rare-earth cations (Ward, 1976, p. 118). Therefore, an increase of the exchange rate would not only increase the opening of cages but also increases the acidity (favorable to the heavy product formation) (Magnoux et al., 1999). 3.3 Molecular encumbrance of isopentane during adsorption When we examine the sorption kinetic of the isopentane (∅Kinetic = 5.2Å) and n-pentane (∅Kinetic = 4.9Å) over 5A75, we can see that the adsorption is accompanied by a visible disruption of isotherm (figure 3). This phenomenon is assigned to the molecular encumbrance of the isopentane which has some difficulties to penetrate into the α cavity of the zeolite. On the other hand, the values of apparent diffusion coefficient (D/r02) indicate that a diffusion in 5A75 adsorbent of n-pentane is approximately 20 times greater than that for isopentane. The coefficient estimated by using Fick’s law for small time (Goddard and Ruthven, 1986, p. 283) :
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1 2
where mt and m∞ are respectively the adsorbed amounts at time t and equilibrium, D is the diffusion coefficient, r the radius of the zeolite crystallite and t the sorption time. m t/m oo 1
n-pe ntane
0.8 isope ntane
0.6 0.4 0.2 0 0
0.2
0.4 Time
0.6 1/2
0.8
1
-1
(h )
Figure 3 : Sorption kinetic of n-pentane and isopentane on 5A75 adsorbent at 100°C Wt % 1
Wt % 10 8
(a)
0 .5
6
Wt % 1
Wt % 10 8
0 .5
6
0
0 0
4 2
2
0
0 0
1
2 3 Time (h)
4
(c)
1
8
0
2 3 Time (h)
4
5
(d)
0 .5 0
4
0 .3 0 .6 Ti m e (h )
0 .3 0 .6 Ti m e (h )
Wt % 1
Wt % 10
6
0
4
1
8
0 .5
6
0
5
Wt %
Wt % 10
0
4
0 .3 0 .6 Ti m e (h )
(b)
0
0 .3 0 .6 Ti m e (h )
2
2
0
0 0
1
2 3 Time (h)
4
5
0
1
2
3
4
5
Time (h)
Figure 4 : Weight increase versus time of isopentane adsorption on 5A adsorbents, (a) 5A75, (b) 5A57, (c) 5A44 and (d) 5A33.
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In the same way, the adsorption of isopentane on 5A33, 5A44, 5A57 and 5A75 adsorbents is accompanied by disruption which increase according to the calcic exchange degree. It should be remarked in figure 4 (presented with a zoom on the first hour), that in absence of heavy products formation (i.e. 100°C), this disruption increases with degree of calcium exchange, particularly for 5A33, 5A44 and 5A57 adsorbents. The 5A75 adsorbent supplied by Rhône-Poulenc, has disruption isotherm quasi-similar to 5A57 adsorbent despite its more important exchange degree and adsorption capacity. The nature and amount of binder used during the preparation of adsorbent might be different. If we consider the weight increase at 100°C after 5 hours of isopentane adsorption as equivalent to m∞, (amount adsorbed at equilibrium), the amplitude of this disruption of the curve mt/m∞ = f (t1/2) (expressed in arbitrary unit) increases from 1 unit with 5A75 to 11 units with 5A33. At 250°C, as observed in figure 1, amounts adsorbed are too weak and the evaluation of the amplitude shows a constancy in the disruption. A relatively more important disruption is observed nevertheless on the 5A75 adsorbent.
4. Conclusion Despite its encumbrance, the isopentane with a diameter greater than 5Å penetrates into the narrow pores of 5A adsorbent. We showed through this work that this adsorption can operate even for values of calcic exchange degree lower than 57%. The adsorption isotherms plotted using thermogravimetric method were characterized by a disruption phenomenon. This one was previously observed during adsorption of various isoalkanes (2MP, 3MP, 23DMB and 22DMB) and also olefins (propene and isobutene). We have established that, in absence of heavy compounds (T = 100°C), the evolution of amplitude disruption follows an increasing function with regards to calcic exchange degree. This evolution was attributed to the difficult penetration of the guest molecule (isopentane) into cavities controlled by the number and locations of calcium cations. At 250°C, the protonic sites resulting from calcium hydrolysis lead to the formation of heavy products and function disruption does not follow the same evolution.
5. References Benaliouche F., Y. Boucheffa and P. Magnoux, 2002, Algerian Chemistry Journal, submitted. Boucheffa Y., C. Thomazeau, P. Cartraud, P. Magnoux, M. Guisnet and S. Jullian, 1997, Industrial and Engineering Chemistry Research, 36, No 8, p. 3198, American Chemical Society. Johnson J.A. and A.R. Oroskar, 1989, Studies in Surfaces Science and Catalysis, 46 p. 451. Jullian, S.; L. Mank and A. Minkkiner, Fr. Patent 2.679.245, 1993; U.S. Patent 5,233, 120, 1993. Magnoux P., Y. Boucheffa, M. Guisnet, G. Joly and S. Jullian, 2000, Oil and Gas Science and Technology-Rev. IFP, 55 No 3, p. 307, Technip Eds.
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Magnoux P., Y. Boucheffa, M. Guisnet, G. Joly and S. Jullian, 1999, Studies in Surface Science and Catalysis, 126 p.427. Magnoux P.; Y. Boucheffa, M. Guisnet; G. Joly and S. Jullian, 1998, Fundamentals of Adsorption VI, F. Meunier Eds., Elsevier, p. 135 Mix H., H. Pfeifer, B. Standte, 1988, Chem. Phys. Lett. 146, p. 541. Takaishi T., Y. Yatsurugi, A. Yusa and T. Kuratomi, 1975, J. Chem. Soc. Far. Trans., 1, p. 97. Ward J.W., 1976, Zeolite Chemistry and Catalysis, J.A. Rabo, Eds., ACS Monograph 171, American Chemical Society, Washington DC, p. 118.
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