3964
Ind. Eng. Chem. Res. 1996, 35, 3964-3972
Isomerization of C8 Aromatics over a Pt/Mordenite Catalyst. A Statistical Model Horacio Gonza´ lez, Abante Rodrı´guez, Luis Ceden ˜ o, and Jorge Ramı´rez* UNICAT, Departamento de Ingenierı´a Quı´mica, Facultad de Quı´mica, UNAM, Me´ xico, D.F. 04510
Jose´ Aracil Departamento de Ingenierı´a Quı´mica, Universidad Complutense, 28040 Madrid, Espan˜ a
A statistical approach was used to analyze the behavior of the isomerization of the C8 aromatics fraction on a commercial Pt/mordenite catalyst. In particular, the effects that the operating variables, temperature, pressure, space velocity, and hydrogen to hydrocarbon ratio had on the yield of p-xylene and loss of xylenes were studied. To this end, a sequential experimental design with a linear model in the first stage and a quadratic model in the second stage was used. The results of the linear model indicated that the temperature, pressure, and space velocity were the most influential factors for the yield of p-xylene, while for the loss of xylenes, pressure and space velocity were 2 times more important than temperature. The results from the quadratic model indicate the existence of an important curvature effect, especially with respect to the space velocity, and therefore the linear model by itself cannot describe adequately the behavior of the reaction system in the whole range of operating conditions. Introduction The C8 aromatics fraction in a refinery consists mainly of four isomers, i.e., o-, m-, and p-xylene and ethylbenzene. Of all these isomers, p-xylene is the one with major industrial importance since it is widely used in the manufacture of synthetic fibers. However, the equilibrium amount of the para isomer obtained in the catalytic reformers is only 24% and therefore is not sufficient to cover the industrial demand. In view of this, a separation of the para isomer is performed after the reformer and the ortho- and meta-rich fractions are then selectively isomerized. The standard catalyst for the isomerization step is a precious metal supported on an acidic carrier. The metal site is used for hydrogenation and dehydrogenation, and the acid support provides the skeletal rearrangement function. In the past, silica-alumina catalysts have been the object of many studies (Hanson and Engel, 1967; Corma and Cortes, 1980); however, lately zeolitic catalysts have gained importance due to their high activity and predominant shape selectivity. Among the zeolitic catalysts ZSM-5 (Collins et al., 1983; Cappellazzo et al., 1991; Li et al., 1992) and mordenite (Norman et al., 1976; Aboul-Gheit et al., 1993; Benazzi et al., 1994) have been of the most widely studied. The reaction system is highly complex, and as has been pointed out previously (Young et al., 1982), besides isomerization, hydrogenolysis, disproportionation, transalkylation, and hydrocracking might also occur during the reaction. Although several attempts have been made to obtain kinetic models for the isomerization of the C8 aromatics fraction using zeolitic catalysts, the influence of diffusional effects which may affect the product distribution (Li et al., 1992) and the complexity of the reaction system, which has led to the proposal of different reaction schemes (Cappellazzo et al., 1991; Li et al., 1992; Hsu et al., 1988), have posed great obstacles to the construction of detailed kinetic models. Furthermore, the catalytic activity and selectivity of the zeolitic * Author to whom correspondence is addressed. Fax: (525) 622-5366. E-mail:
[email protected].
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catalyst are also affected by changes in the degree of acidity and by the extent of poisoning of the stronger acid sites in the catalyst (Nayak and Choudhary, 1982). Clearly, the above considerations point out the difficulty of using literature kinetic data to define the best operating conditions of the isomerization process. In these situations, the difficulty of using a phenomenological model, due to the lack of knowledge of the intrinsic kinetic parameters for a particular catalyst, may be surmounted by the use of a statistical model which would require a minimum of experimental runs to predict the best operating conditions for the particular catalytic system in use within the typical range of the process variables. Statistical models have been used with success in the optimization of the operating conditions in the study of different reaction systems. Coteron et al. (1993) demonstrated the usefulness of this approach in the study of the synthesis of an analogue of jojoba oil using a fully central composite design which allowed them to assess the influence of single variables like temperature, operating pressure, and catalyst concentration as well as their interactions on the yield of the desired product. In a different field, Ramirez et al. (1994) also showed the benefits of the statistical approach in the synthesis of mixed oxides catalytic supports of controlled surface area and porosity. Recently, Prieto et al. (1992) used a linear statistical model to study the catalyst performance under different operating variables in the isomerization of C8 aromatics in the vapor phase in industrial conditions. With this approach they built an empirical model to relate the reaction temperature, pressure, H2 to hydrocarbon ratio (H2/HC), and space velocity to the activity and selectivity of the catalyst in the isomerization of a mixture of o-, m-, and p-xylene and ethylbenzene. Using a statistical linear model, they found that the temperature and pressure were the most significative variables. However, the results of this study do not show the magnitude of the curvature effects which would be expected in a complex reaction system such as the isomerization of the C8 aromatics fraction. To obtain information on the © 1996 American Chemical Society
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curvature effects, it would be necessary to consider an experimental design which would allow one to fit the experimental data to a quadratic model. It is the purpose of this work to make use of a statistical approach to analyze further the behavior of the xylene isomerization reactions of an industrial feed over a Pt/mordenite catalyst, by means of a sequential experimental design in order to study the effect of the principal operating variables on the yield of p-xylene and loss of xylenes, using in the first stage a linear model which will allow the identification of the most important operating variables and in a second stage, with additional experimental work, a quadratic model which will permit one to estimate the magnitude of the curvature effects, from the shape of the surface response plots, in this complex reaction system. Experimental Section In all the experimental runs the feed to the reactor consisted of an industrial feedstock which contained 3.19% non-aromatics, 0.14% benzene, 6.55% toluene, 4.0% ethyl benzene, 9.33% p-xylene, 24.04% o-xylene, 48.6% m-xylene, and 4.16% C9+. The hydrogen gas in the feed was 99.9% purity provided by Linde. The experiments were performed in a 0.0095 m internal diameter and 0.30 m length stainless steel tubular reactor using 2-6 g of a Pt/mordenite/Al2O3 commercial catalyst (0.002 m diameter per 0.007 m length extrudates) with surface area ) 260 m2/g and a density of acid sites of 2.03 mequiv/g of catalyst, measured by the butylamine titration method (Cid and Pecchi, 1985). The liquid feed was introduced to the system by means of a Milton Roy high-pressure pump, and it was preheated to the reaction temperature before reaching the catalyst bed. The mixture coming out of the reactor was passed through a condenser maintained at 288 K and later to a gas separator. The liquids were analyzed by a gas chromatograph fitted with a FID cell and a 1/8 in. diameter, 6 m length column packed with 5% Bentona 34 and 5% diisodecyl phthalate on Chromosorb W, which operated at 363 K. Before the activity measurements, the catalyst was purged with nitrogen during 1/ h, then heated at 723 K, and reduced under hydrogen 2 flow during 3 h. All activity measurements were made when the system had reached steady state, and no significant deactivation was observed. Statistical Analysis Statistically designed experiments have the advantage that the systematic errors can be eliminated and also an estimate of the experimental error can be obtained. Furthermore, with this approach the effects of the main process variables and their interactions can be assessed with specific confidence levels (Box et al., 1978). The sequential experimental design used in the present work to analyze the effect of the main operating variables (reaction temperature (T), operating pressure (P), reactor space velocity (WHSV), and hydrogen to hydrocarbon molar ratio (R)) on the two responses (yield of p-xylene (YPX) and percent loss of xylenes (LX)) is presented in Figure 1. The selection of the maximum and minimum levels of the experimental factors or variables was made according to the typical operating conditions used in industrial practice. Accordingly, the selected ranges of study for the reaction temperature (T), reactor operating
Figure 1. Flowchart of the sequential experimental design.
pressure (P), hydrogen to hydrocarbon molar ratio (R), and weight hourly space velocity (WHSV) were as follows: T, 663-763 K; P, 5-10 kg/cm2; R, 4-7; WHSV, 3-8 h-1. Linear Model. The experimental design used in this first part of the study was a 2k full factorial design with two levels and four factors. In this model, the behavior of the dependent variable (Y) can be expressed as follows:
Y ) a0 + a1XT + a2XP + a3XWHSV + a4XR + a12XTXP + a13XTXWHSV + a14XTXR + a23XPXWHSV + a24XPXR + a34XRXWHSV + a123XTXPXWHSV + a124XTXPXR + a134XTXWHSVXR + a234XPXWHSVXR + a1234XTXPXRXWHSV (1) where Xi (i ) T, P, R, WHSV) ) codified values of the factor i. The codified values of the factors were obtained as follows:
Xi ) (xi - (xi)m)/d
(2)
where xi is the experimental value of the variable i, (xi)m is the mean value of xi, and d is the absolute difference between xi and (xi)m. Quadratic Model. This type of model is useful when after the application of a linear model it is considered necessary to explore the relationship between the different factors and the dependent variable within the experimental region and not only at the borders as in the 2k factorial designs (Box and Draper, 1987). Thus, in order to quantify the curvature effects, the following model was used:
3966 Ind. Eng. Chem. Res., Vol. 35, No. 11, 1996 k
Y ) a0 +
∑ i)1
k
aiXi +
∑ i)1
k-1
aiiXi2 +
k
∑ ∑ i)1 i
