Optimal Policies of Operation of a Fixed-Bed Reactor for Oxidation of o

Using the methods of mathematical simulation, the opportunities for application of a dual salt bath and a dual catalyst bed to the conventional and lo...
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Ind. Eng. Chem. Res. 1998, 37, 3424-3433

Optimal Policies of Operation of a Fixed-Bed Reactor for Oxidation of o-Xylene into Phthalic Anhydride Asen I. Anastasov* and Valentin A. Nikolov Institute of Chemical Engineering, Bulgarian Academy of Sciences, Acad. G. Bontchev Street, bl. 103, Sofia 1113, Bulgaria

Using the methods of mathematical simulation, the opportunities for application of a dual salt bath and a dual catalyst bed to the conventional and low air ratio processes of oxidation of o-xylene into phthalic anhydride are studied. It is established that both systems lead to a significant increase of the yield of the raw phthalic anhydride, as well as to a drastic reduction of the undesired side product phthalide. The optimal characteristics of a dual salt bath and a dual catalyst bed are determined. Experimental temperature profiles obtained in an industrial reactor for oxidation of o-xylene during all the time of catalyst use are studied. Both the nature of variation of the catalyst activity along the bed and its influence on the effectiveness of the process are defined. I. Introduction It is well-known that high exothermic catalytic processes of partial oxidation as oxidation of o-xylene and/ or naphthalene to phthalic anhydride, benzene or n-butane into maleic anhydride, anthracene to anthraquinone, durene into pyromellitic anhydride, ethylene into ethylene oxide, and so forth are realized mainly in the fixed bed of the catalyst (Froment, 1974). The analysis of the processes mentioned (Lo´pez-Isunza, 1983; Wellauer, 1985; Henning and Perez, 1986; Nikolov et al., 1989, 1991) shows that the catalyst does not operate in a satisfactory way along the fixed bed, and the reactor effectiveness declines. This is due mainly to the character of the temperature profile realized in the bed. In the conventional fixed-bed reactor the temperature in the beginning of the bed is too high, while it drops sharply in the rest part. Various methods to improve the temperature regime are suggested in the literature. Borio et al. (1989, 1995) propose three different schemes of cooling the reaction gasscountercurrent, cocurrent, and perfectly mixed coolant. According to these authors, cocurrent operation (the cooling agent flows parallel with the reaction mixture) leads to the best reactor performance. Using the same cooling method, Bucala et al. (1997) present the optimal shape of the temperature profile for the process of oxidation of ethylene into ethylene oxide. Varying the catalyst activity along the bed by means of changing the number of active sites, or simple dilution of the bed with inert particles (Caldwell and Calderbank, 1969; Sadhukan and Petersen, 1976; Pirkle and Wachs, 1987; Kotter et al., 1991), a suitable technological regime can also be achieved. Pirkle and Wachs (1987) have investigated the process of oxidation of o-xylene into phthalic anhydride using a mathematical model and have determined that a lower activity of the catalyst in the front of the bed results in a more stable operation of the reactor. A very interesting opportunity for improving the temperature regime is to maintain higher temperature of the * To whom all correspondence should be addressed. Phone: +359 (2) 70-41-18. Fax: +359 (2) 70-75-23. E-mail: [email protected].

coolant in the low-temperature second zone of the bed in comparison with the temperature in the first one. It should be mentioned that a cooling system with two or more temperatures of the coolant (the so-called dual salt bath (DSB)) can be attached only to new-built units, while catalysts with different activity (the so-called dual catalyst bed (DCB)) (Nikolov et al., 1991) can be used in operative reactors with minimum additional expenses. To put into practice these methods for optimization of the performance of a fixed-bed reactor, the following questions need to be answered. A. With respect to a dual salt bath (DSB) 1. What must be the length of every cooling section? 2. What must be the temperature of the coolant in every section? B. With respect to a dual catalyst bed (DCB) 1. What must be the activity of the catalyst used? 2. What must be the length of a section composed by a catalyst of given activity? 3. How the deactivation of the catalyst influences the characteristics of the process? The reason to carry out the present investigation is the fact that, except for the thesis of Wellauer (1985), there is practically no information in the scientific literature about what application of DSB systems to high exothermic catalytic processes operated in the industry. The published studies regarding DCB are very scarce too, and unfortunately, they cannot be used directly for improving the exploitation of the industrial reactors. Having in mind the above-mentioned, our aim was to study in detail using a mathematical model the influence of DSB and DCB systems on the temperature regime in the fixed bed, as well as on the effectiveness of the catalytic process; at the same time, we wished to confirm experimentally a part of the simulation results. We chose the important industrial process of oxidation of o-xylene into phthalic anhydride, which had been studied by us for several years. The final task is to obtain such results, which can be used for improving the performance of the conventional units operated by initial concentration of o-xylene 40 g/Nm3 and those applying the modern LAR (low air ratio) process (initial

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Ind. Eng. Chem. Res., Vol. 37, No. 8, 1998 3425

o-xylene concentration 60 g/Nm3). Both installations are presented in Bulgaria and produce phthalic anhydride. II. Mathematical Model The numerical experiments presented here were carried out by a two-dimensional heterogeneous model. It is considered to describe as best as possible a heterogeneous catalytic process taking place in a single reactor tube of cylindrical form (Froment, 1974). The model is presented by the following system of partial differential equations:

gas phase:

(

proposed by Calderbank et al. (1977). Our industrial experience has shown that these kinetic equations predict comparatively well the location of the hot spot, as well as the o-xylene conversion and phthalic anhydride yield. Besides, the kinetics discussed recognizes the formation of a phthalide-undesired side product, whose quantity influences violently the quality of the raw phthalic anhydride, and as a result the effectiveness of the whole process of obtaining phthalic anhydride. Calderbank et al. (1977) suggest the following network:

)

∂Ci ∂2Ci 1 ∂Ci ∂Ci a ) Dr,i + kgp,i (Cp,i - Ci) + - Vb 2 ∂t r ∂r ∂l  ∂r (1) Cg

(

)

t ) 0; Ci ) C0,i, T ) T0

(3)

where A, B, C, and D are o-xylene, o-tolualdehyde, phthalide, and phthalic anhydride. According to Calderbank et al. (1977) stage 6, as kinetically insignificant, can be neglected. Therefore, it is not included in the kinetic model. The kinetic equations in partial pressures, as given by the authors, are the following:

l ) 0; Ci ) C0,i, T ) T0

(4)

r1 ) K1RPA

(10)

∂Ci ∂T ) 0, )0 ∂r ∂r

r2 ) K2RPB

(11)

(5)

r3 ) K3RPA

(12)

∂Ci ∂T ) 0; -λr ) hgc(T - Tc) ∂r ∂r

(6)

r4 ) K4RPA

(13)

r5 ) K5RPC

(14)

∂T ∂2T 1 ∂T ∂T a ) λr 2 + + hgp (Tp - T) (2) - VbCg ∂t r ∂r ∂l  ∂r

with boundary conditions

r ) 0; r ) R; solid phase:

∂Cp,i a ) kgp (C - Cp,i) + Wp,iFA(l)Fp ∂t (1 - ) i Cc

∂Tp

where

(7) R)

s a rp,mFA(l)(-∆Hm)Fp ) hgp (T - Tp) + ∂t (1 - ) m)1 (8)

∑

with boundary conditions t ) 0; Cp,i ) Cp,0,i, Tp ) Tp,0

(9)

The fundamental assumptions of the model are given by Froment (1974) and do not need to be discussed here. Equations 1-9 are transformed in dimensionless coordinates, concentrations, and temperatures, then approximated by an implicit finite-difference scheme, and solved iteratively until a steady state is achieved (Kalitkin, 1978). To avoid the erroneous influence of a constant gas velocity assumed in the model (in the case of significant difference between the inlet gas temperature and the coolant temperature), the dimensionless concentrations are corrected with respect to the temperature at every segment along the dimensionless reactor length. According to the numerical method of solution, it is divided into a given number (M) of segments (intervals). More details regarding the method of solution are given by Nikolov and Anastasov (1992a). The model coefficients are determined using dependencies shown in the paper by Anastasov et al. (1988). III. Kinetics The simulation was carried out using the kinetic model of oxidation of o-xylene into phthalic anhydride

KCPox KCPox + (K1 + 6.5K3 + 3K4)PA + K2PB + K5PC (15)

The values of the preexponential factors and energies of activation are given in Table 4 of the paper by Calderbank et al. (1977). As suggested, KCPox with air at near atmospheric pressure is 0.722 10-5 kmol/(kg s). IV. Experimental Section The physical experiments were carried out in a pilot unit and in an industrial reactor, too. (A) Pilot Installation. The integral nonisothermal, nonadiabatic reactor in the pilot unit represents a single tube (325-cm length and 25-mm inner diameter) of the industrial reactor. The maintenance of the cooling temperature along the tube is realized by electric heaters located around it. In this way various temperatures of cooling can be achieved, which is a great facilitation with respect to carry out experiments with more than one cooling temperature along the bed (dual salt bath). The temperature in the bed is measured by means of a mobile NiCr-Ni thermocouple situated coaxially in the center of the bed. More information about the pilot installation, as well as its scheme, are given by Nikolov and Anastasov (1992a,b). (B) Industrial Reactor. The industrial reactor for phthalic anhydride production operates with inlet oxylene concentration 40 g/Nm3. It contains 8920 tubes and is cooled by a molten salt NaNO2/KNO3. The apparatus has a built-in steam generator. The temper-

3426 Ind. Eng. Chem. Res., Vol. 37, No. 8, 1998 Table 1. Basic Conditions Maintained in the Numerical and Physical Experiments parameters gas flow rate through a single tube inlet concentration of o-xylene temperature of the coolant inlet temperature of the reactant mixture activity of the catalyst

conventional process

LAR process

4.5 Nm3/h 40.0 g/Nm3 variable 230 °C

4.0 Nm3/h 60.0 g/Nm3 variable 230 °C

variable

variable

Figure 2. o-Xylene conversion and yields as a function of the bed length for a conventional processsmodel predictions; V, C0,1, Tc, and T0 as in Figure 1. Curves 1, 2, 3, and 4, represent o-xylene, o-tolualdehyde, phthalide, and phthalic anhydride, respectively.

Figure 1. Temperature profiles along the bed for a conventional processsmodel predictions; V ) 4.5 Nm3/h (Nm3 defined as m3 at 0 °C and 101.325 kPa), C0,1 ) 40 g/Nm3, Tc ) 370 °C, T0 ) 230 °C. Curves 1 and 2 represent the solid and gas phase, respectively.

ature in the bed is measured in nine contact tubes located in a definite way in the bundle. In each of the tubes mentioned a mobile NiCr-Ni measuring thermocouple is installed. More details about the industrial reactor accompanied by its scheme can be find out in the papers by Nikolov and Anastasov (1989, 1992b). (C) Catalyst. The catalyst used in the experiments carried out in both the pilot unit and the industrial reactor is a vanadia-titania (V2O5-TiO2) catalyst promoted by P2O5 and Al2O3. The active substance (0.10.2-mm thick) is laid over porcelain spheres with a diameter of 6 mm. The length of the fixed bed is 280 cm and the bulk density is 1500 kg/m3. (D) Analyses. The analyses of o-xylene, phthalic anhydride, and side products were done by means of gas chromatography (gas chromatograph Perkin-Elmer 8500) and by polarography. All measurements of the temperatures and concentrations were performed 24 h after a stable regime was realized. V. Results and Discussion In all numerical and physical experiments the following basic conditions corresponding to the industrial ones were maintained (Table 1). Before we discuss the main results, let us discuss the temperature and concentration profiles for a conventional process (CP; inlet o-xylene concentration 40 g/Nm3) presented in Figures 1 and 2. The character of the same profiles for the LAR process (LARP; inlet o-xylene concentration 60 g/Nm3) does not change; therefore, they are not presented in the figures. It is quite evident that the oxidation of o-xylene is almost completed in the front zone of the fixed bed. The temperature of the catalyst rises to about 475 °C (hot

spot) and then drops with about 100 °C in the first half of the bed, while in the second one it decreases only with 10 °C more. The concentration profiles along the bed (Figure 2, curves 1-4) correspond to the temperature regime obtained. In the first 140 cm of the tube the o-xylene conversion reaches its final value, whereas the phthalic anhydride yield is about 84% (62 vs 74%) of the final one (Figure 2, curve 4). Actually, an additional oxidation of the undesired side products (o-tolualdehyde and phthalide) is mainly realized in the second part of the bed (Figure 2, curves 2 and 3). Unfortunately, due to the low temperature there, these processes are not intensive enough. This fact is very important, as the phthalide contents define the quality of the raw phthalic anhydride. Obviously, the temperature profile discussed is to be improved in order to completely use the second half of the bed. This can be achieved if a higher cooling temperature is maintained in the rear of the bed in comparison with the hot spot zone, or in other words, if a DSB is applied to the bed. It is mentioned above that another opportunity for a better utilization of the bed is to combine it by catalysts with different activity, or to organize a DCB. The catalyst of lower activity is located in the first part of the tube, while the more active one forms the second fixed bed. Let us examine at first the dual salt bath as an instrument to improve the reactor performance. Dual Salt Bath (DSB). As mentioned above, to apply a DSB successfully, the temperature of the coolant in each of the cooling sections, as well as their length, must be determined. Simulation results which can be used to define the most suitable coolant temperature in the first 140 cm of the bed are presented in Figure 3. A highest coolant temperature of 450 °C is settled in the second part of the reactor tube. The molten salt decomposes over this temperature. Dotted lines (a-d) parallel to the ordinate indicate the cooling temperatures in the first half of the bed under which the maximum admissible temperature for the catalyst (500 °C) is reached. Coolant temperatures situated out of the interval limited by both dotted lines for the corresponding curve lead to an inadmissible temperature of the hot spot. It can be seen in Figure 3 (curves 1 and 2) that the phthalic anhydride yield as a function of the coolant temperature in the first half of the bed has a minimum value for both the conventional and LAR processes. Obviously, the curve branch to the left of the corresponding dotted line (“a” for CP and “b” for LARP) has no practical importance, as the hot spot exceeds 500

Ind. Eng. Chem. Res., Vol. 37, No. 8, 1998 3427

Figure 3. Dependence of the phthalic anhydride (YD) and phthalide (YC) yields on the cooling temperature in the first half of the bed Tc,I at Tc,II ) 450 °Csmodel predictions. Curves 1 and 1asconventional process (V ) 4.5 Nm3/h, C0,1 ) 40 g/Nm3, T0 ) 230 °C). Curves 2 and 2asLAR process (V ) 4.0 Nm3/h, C0,1 ) 60 g/Nm3, T0 ) 230 °C).

°C there. According to expectations, the hot spot location is in the second half of the bed, which due to the high coolant temperature, operates more intensively than the first one. In fact, the cooling temperature in the first half of the bed can be determined by using the rising brunch of the curves situated between both dotted lines, “a” and “c” for CP and “b” and “d” for LARP. For the conventional (Figure 3, curve 1) and LAR processes (Figure 3, curve 2) the maximum admissible temperatures of the coolant are 385 and 342 °C, respectively. In both cases the hot spot is located in the first half of the bed. It is worth noting that the industrial cooling temperature for the conventional process is 370 °C (i.e., the accepted reserve is 15 °C). In contrast to the phthalic anhydride yield, the yield of phthalide (Figure 3, curves 1a and 2a) decreases continuously with respect to the increase of the cooling temperature, which confirms the choice of the temperatures discussed above. Using Figure 3 one can conclude that the optimal cooling temperature in the first part of the bed is the temperature under which the hot spot reaches the maximum admissible value for the catalyst, and besides, its location is in the front of the reactor. In brief, regardless of the extremely high temperature of the second salt bath, the first half of the bed should operate more intensively than the second one. The results presented in Figures 4 and 5 answer the question of what must be the cooling temperature in the second part of the bed. In Figure 4 the yield of phthalic anhydride and phthalide as a function of the cooling temperature in this part for the CP are shown. The same yields for the LARP are given in Figure 5. The cooling temperature in the first 140 cm for the conventional process is accepted to be equal to the industrial cooling temperature of a single bath (370 °C), while for the LAR process a temperature of 340 °C, which is quite near to the maximum admissible temperature (342 °C) of a single bath, is used. The great advantages of the DSB for both the conventional and LAR processes can be seen at once. The rise of the temperature of the second salt bath from 370 to 450 °C (Figure 4) and from 340 to 450 °C (Figure 5)

Figure 4. Influence of the cooling temperature of the second salt bath Tc,II on the phthalic anhydride and phthalide yields for a conventional process at Tc,I ) 370 °Csmodel predictions.

Figure 5. Influence of Tc,II on YD and YC for a LAR process at Tc,I ) 340 °Csmodel predictions.

increases the phthalic anhydride yield for the CP from 74.3 to 79% (Figure 4), while for the LARP the increase is still larger, from 70.3 to 79.1% (Figure 5). The growth of the productivity in both cases is very essential, 6.3 and 12.4%, respectively. More important is the fact that the increased cooling temperature in the second half of the bed stimulates violently the conversion of the undesired side products o-tolualdehyde and phthalide into phthalic anhydride. For the CP (Figure 4) the contents of phthalide in the raw phthalic anhydride drops from about 4.2% (single salt bath) to about 0.34% at the temperature of the second salt bath 450 °C. For the LARP these values are 7 and 0.26%, respectively (Figure 5). The characteristics achieved improve sharply the quality of the crude product. Moreover, the decreased expenses for the purification of the phthalic anhydride compensate many times the energy needed for the rise of the temperature in the second salt bath. It is worthwhile to complement here that the hot spot does not change its temperature; it remains the same as that under a single salt bath (catalyst temperature 475 °C for the CP and about 496 °C for the LARP).

3428 Ind. Eng. Chem. Res., Vol. 37, No. 8, 1998

Figure 6. Dual salt bathsexperimental and predicted temperatures in the pilot reactor. Points: experimental, solid line: model predictions. V ) 4.5 Nm3/h, C0,1 ) 39.4 g/Nm3, T0 ) 230 °C, Tc,I ) 370 °C, Tc,II ) 450 °C.

Figure 7. Phthalic anhydride and phthalide yields as a function of the position of introduction of the second salt bath along the dimensionless bed lengthsmodel predictions. Curves 1 1as conventional process; curves 2, 2asLAR process.

It is interesting to analyze the experimental and calculated temperature profiles presented in Figure 6. The solid line shows the predicted temperature profile obtained under two salt baths with temperatures 370 and 450 °C, respectively. They are located from 0 to 140 cm and from 140 cm to the end of the bed. The predicted temperature Tx is an arithmetic means of the temperatures of the gas and solid phases. In comparison, with a fixed bed supplied with a single salt bath (Figure 1, curves 1 and 2) the temperature in the second half of the bed under DSB is significantly higher. A region of a second hot spot is observed, which is much more smooth than this one in the first half of the bed. In fact, the temperature from 180 cm to the end of the tube is practically constant and near the cooling temperature. As noted above, the quasi-isothermal regime of high temperature realized in the last 140 cm of the bed is very suitable for the oxidation of phthalide into phthalic anhydride, which enables a final product of high purity to be obtained. The experiments shown in the figure are carried out in the pilot reactor discussed above. The temperature is measured at intervals of 20 cm by the mobile NiCrNi thermocouple situated coaxially in the center of the reactor tube. Additional measurements in the regions of the hot spot (50 cm) and of the change of the cooling temperature (140 cm) are performed. The mean initial concentration of o-xylene was about 39.4 g/Nm3. As Figure 6 shows, the coincidence between experimental and predicted temperatures is comparatively well. More significant differences reaching about 40 °C are observed in the region from 120 to 150 cm of the tube. Probably, this is due to a certain reciprocal influence of the electric heaters providing a definite temperature of the tube wall, as the change of the cooling temperature is realized just in this zone. Doubtless, the differences discussed are produced also by the kinetics of Calderbank et al. (1977), which describes not quite precisely the real process. In our opinion this affects also the location of the hot spot; in comparison with the experimental hot spot the predicted one is removed 10-15 cm to the beginning of the bed. After definition of the cooling temperature in both cooling sections, their length is to be determined. In Figure 7 the dependence of phthalic anhydride and phthalide yields on the place of introduction of the second salt bath along the dimensionless length of the bed for the CP (curves 1 and 1a) and for the LARP

(curves 2 and 2a) is presented. According to the results presented above, the cooling temperature in the second half of the bed for both processes is 450 °C. In the first half of the bed the temperature of the salt bath is 370 °C (CP) and 340 °C (LARP), respectively. It can be seen in Figure 7 that in the boundary case at L ) 1, when in fact there is only one salt bath in the reactor (temperatures 370 and 340 °C), the yield of phthalic anhydride is lowest (curves 1 and 2) and the phthalide contents is highest (curves 1a and 2a). This fact was discussed above, but in order for things to be more clear, it was mentioned here again. The more of the second salt bath that is introduced to the beginning of the bed, the more the yield of phthalic anhydride and phthalide increases and decreases, respectively. At the left border (L ) 0) the bed has again only one salt bath, but its temperature is 450 °C, now. Of course, the characteristics of the process are the best, but there is a “runaway” of the hot spot there (hot spot temperature is much higher than 500 °C). In fact, the position where the second salt bath for the LARP (Figure 7, curves 2 and 2a) is to be introduced lies on the second dotted line (line b) and to the right of it. This line defines a dimensionless bed length L ) 0.28 (78.4 cm). In the point L ) 0.28 the phthalic anhydride yield is highest and that of phthalide is lowest. At the same time the hot spot temperature is the same as in the case of a single salt bath of low temperature (340 °C). With respect to the conventional process (Figure 7, curves 1 and 1a) it is most proper to introduce the second salt bath at the place of dotted line “a” and to the right of it. Line a is disposed at L ) 0.22 (61.6 cm). In the interval between both dotted lines (a and b) the hot spot is higher than that under a single bath of temperature of 370 °C, but is lower than 500 °C. To the right of the dotted line b (L ) 0.28) its temperature is now equal to that of a single bath of 370 °C. Thus, the optimal lengths of DSB for both processes are about 80 cm for the first and about 200 cm for the second salt bath. If the DSB is organized as discussed above, a yield of phthalic anhydride 79.5% (Figure 7, curve 1, CP) and 79.8% (Figure 7, curve 2, LARP) is achieved under phthalide contents lower than 0.1% (Figure 7, curves 1a and 2a). Besides, the temperature of the hot spot is quite admissible. For example, in the case of a single salt bath these characteristics are 74.3 and 70.3%, respectively. If the course of curves 1 and 2 is studied, it is seen that in the region between L ) 0.3 and L ) 0.5 a low grade section, where the phthalic

Ind. Eng. Chem. Res., Vol. 37, No. 8, 1998 3429 Table 2. Influence of Changes in Operating Variables on the Magnitude of the Hot Spots and Phthalic Anhydride Yield (Conventional Process, Predicted Results) case normal raised T0 raised V decreased C0 decreased Tc,I decreased Tc,I

V C0 T0 Tc,I Tc,II (Nm3/h) (g/Nm3) (°C) (°C) (°C) 4.5 4.5 7.0 4.5 4.5 4.5

40 40 40 30 40 40

230 370 230 230 230 230

370 370 370 370 330 303

450 450 450 450 450 450

Ths,I (°C)

Ths,II (°C)

YD (%)

474.7 446.4 444.3 441.2 400.4 346.5

454.7 456.7 468.2 454.2 471.7 499.8

79.0 78.1 76.2 78.5 77.3 76.7

anhydride yield changes slightly, is observed. This enables the length of both salt baths to be one and the sames140 cm. Unfortunately, the phthalide contents grow up to about 0.3% (Figure 7, curves 1a and 2a). Doubtless, the application of a DSB system is connected with the process stability in the rear of the bed. Having in mind that the coolant temperature Tc,II is extremely high (450 °C) there, this problem is especially important. A sudden change in the operating variables can lead to a sharp increase of the temperature in the second half of the bed and to a “runaway” of the hot spot, respectively. To evaluate such an opportunity, numerical experiments regarding the CP were carried out. The results are shown in Table 2. It is seen that drastic changes in the inlet temperature T0, flow rate V, and inlet o-xylene concentration C0 do not affect practically the hot spot temperature Ths,II (Ths,I is the temperature of the hot spot in the front of the bed) in the second part of the bed. In fact, this temperature is most sensitive to a decrease of the coolant temperature Tc,I in the first salt bath. But even in the case of a violent drop of Tc,I (from 370 to 303 °C; this is practically impossible in the industry), the hot spot temperature in the second bed section remains equal to the critical one (500 °C). In other words, it can be concluded that, in addition to the advantages discussed above, the application of a DSB system leads also to a reduction of the parametric sensitivity of the process and improves its stability. Another interesting phenomenon is observed here (Table 2, last column). Regardless of the significant changes in the operating variables, the phthalic anhydride yield YD does not decrease very much in comparison with that of the normal case. This will be studied elsewhere. Dual Catalyst Bed (DCB). The analysis of the modern patent literature (Nikolov et al., 1991) shows that almost all companies producing phthalic anhydride use a dual catalyst bed composed of catalysts of different activity and acid-base characteristics. Unfortunately, there are almost no investigations connected with the influence of a DCB on the effectiveness of the process. Two opportunities exist for creating a DCB of catalysts of different activity: (1) The reactor is loaded with a catalyst of given activity, which is deactivated during its exploitation in the region of the hot spot (i.e., in the front of the bed) due to the high temperatures there. So, two or more catalyst beds of different activity are formed. (2) The reactor is initially loaded with two catalyst beds having different activity. In our paper (Nikolov and Anastasov, 1992b) devoted to the pretreatment of a fresh vanadia-titania catalyst for oxidation of o-xylene into phthalic anhydride and its training for a long industrial exploitation, we settled that during the period of pretreatment (duration 50

Figure 8. Temperature profile in the industrial reactor at the third day of catalyst use. Points: experimental. Solid line: model predictions. V ) 3.0 Nm3/h, C0,1 ) 31.5 g/Nm3, Tc ) 385 °C, T0 ) 247 °C. FA ) 1.08 from 0 to 60 cm, FA ) 1 from 60 to 280 cm.

Figure 9. Temperature profile in the industrial reactor at the 50th day of catalyst use. Points: experimental. Solid line: model predictions. V ) 4.5 Nm3/h, C0,1 ) 40.0 g/Nm3, Tc ) 372 °C, T0 ) 230 °C, FA ) 0.8 from 0 to 60 cm, FA ) 1 from 60 to 280 cm.

days) the original catalyst bed of a given activity is divided into two beds of different activity in a natural way. In the first 3 days of use of the bed, its activity in the zone from 0 to about 60 cm (hot spot zone) increases from 1 to 1.08 (this fact is confirmed experimentally), while in the last 220 cm it remains equal to 1. The predicted temperature profile under industrial conditions at the third day of exploitation is presented in Figure 8. The higher activity in the first 60 cm of the bed is considered. As in Figure 6, the predicted temperature is calculated from the arithmetic means of the temperatures of both phases. The experimental points are defined as mean values of the corresponding measurements in the industrial reactor. A good coincidence between experimental and predicted results is observed. The predicted selectivity to phthalic anhydride is a little higher (with about 3%) in comparison with that determined experimentally. As a result of the high temperature in the front part of the bed (about 490 °Csnear the maximum admissible temperature) during the pretreatment, the specific surface of the catalyst in this zone (0-60 cm) reduces (sintering) and its activity drops from 1.08 to 0.8 relative units. In the remaining part of the bed (60-280 cm) it does not change (equal to 1). The temperature regime in the reactor at the end of the pretreatment (at 50th day) is shown in Figure 9. As stated above, the solid line represents the results of the numerical experiments, while the points show the

3430 Ind. Eng. Chem. Res., Vol. 37, No. 8, 1998

Figure 10. Temperature regime in the bed (industrial reactor) after 20 months exploitation of the catalyst. Points: experimental. Solid lines: model predictions. V ) 4.5 Nm3/h, C0,1 ) 39.6 g/Nm3, Tc ) 370 °C, T0 ) 230 °C. Curve 1sFA ) 0.8 from 0 to 60 cm, FA ) 1 from 60 to 280 cm. Curve 2sFA ) 0.5 from 0 to 50 cm, FA ) 0.8 from 50 to 140 cm, FA ) 1 from 140 to 280 cm.

Figure 11. Temperature regime in the bed (industrial reactor) after 36 months exploitation of the catalyst (the catalyst must be changed). Points: experimental. Solid lines: model predictions. V ) 4.5 Nm3/h, C0,1 ) 39.6 g/Nm3, Tc ) 378 °C, T0 ) 230 °C. Curve 1spredicted temperature profile under activities as on curve 2. Curve 2spredicted activity of the catalyst along the bed.

physical experiments carried out in the industrial reactor. The good coincidence between predicted and experimental temperature profiles confirms the values of the relative catalyst activity in both regions specified above. Moreover, they are also supported by the analyses of the specific surface of catalyst samples (by BET method) taken from the front and bottom parts of the bed (Nikolov and Anastasov, 1992b). It is of definite interest to study the behavior of the catalyst during all its life, or to investigate the redistribution of the activity along the reactor tube, respectively. Figures 10 and 11 show the temperature regimes in the bed for a catalyst which is in the middle and at the end (3 years old) of its life. Curve 1 (Figure 10) represents the temperature profile obtained by the model at activities of the two beds identical to those of a catalyst operated only 50 days. A significant difference between experimental and numerical results is observed (Figure 10, curve 1). In our opinion without reservation it is due to the changed activity of the catalyst in the different zones of the bed on account of its 20-months operation. Really, the prognosticated temperature regime obtained under activities 0.5 at 50 cm, 0.8 from 50 to 140 cm, and 1 from 140 cm to the

end of the bed (Figure 10, curve 2) interprets comparatively well the experimental temperatures, as well as the location of the hot spot. The variation of the activity of a catalyst operated about 36 months and liable to a change is much more significant (Figure 11). Curve 1 demonstrates the calculated temperature profile, while curve 2 shows the distribution of the activity along the bed, giving the best approximation to the experimental temperatures. Quite briefly, an optimization procedure based on the method of the “fortuity search” is used to find out the most suitable distribution of the catalyst activity along the bed. The sum of the squares of the differences between experimental and calculated by the model temperatures at a given activity profiling (changed at every iteration) is minimized. It is impressionable that the catalyst activity to 70-80 cm of the bed is very low (under 0.1 relative units). It is even under 0.05-50 cm. This is rather surprising, having in mind that a completely fresh catalyst operated only 50 days produces a hot spot after 50 cm (i.e., the catalyst is not exposed to the influence of high temperatures in the region before 50 cm). Making an attempt to explain this fact, we are going to remember that vanadia-titania catalysts for oxidation of o-xylene are deactivated reversibly and irreversibly in the course of their exploitation. The irreversible deactivation is caused by the high temperatures during the calcination of the catalyst (Nikolov and Anastasov, 1992b; Galantowicz et al., 1994). It is observed also in the zone of the hot spot and is due to the transformation of anatase to rutile, to the reduction of the specific surface of the catalyst (sintering), and to the decrease of the contents of phosphorus. The reversible deactivation is a result of (a) modification of the valence of V along the bed length, caused by the different ratio “oxygen-hydrocarbon” (Lopez-Isunza and Kershenbaum, 1992), and (b) deposition of volatile residual (TAR) products on the catalyst surface (Bond and Konig, 1982; Dias et al., 1994; 1996; Cheng et al., 1996). It seems that the very low catalyst activity observed in the first 80 cm of the bed (Figure 11, curve 2) is due mainly to the reversible deactivation of the catalyst. Thus, the catalyst is most reduced (having least activity) in the front part of the bed where the ratio oxygenhydrocarbon and temperature are lowest. According to Lo´pez-Isunza and Kershenbaum (1992) who have investigated the process of oxidation of o-xylene and studied this reason for deactivation in detail, the activity in the first 30 cm is about zero, increases slightly from 50 to 80 cm, and reaches high value (a little under 1) in the remaining part of the bed. These conclusions confirm our experimental results completely. According to Dias et al. (1996) in the temperature interval 260340 °C under high flow rates and o-xylene conversion below 20% (i.e., in the beginning of the bed), a large quantity of TAR products blocking the catalyst surface reversibly is generated. Cheng et al. (1996) show an activity below 1 in the first 40 cm of the bed, which agrees with the results obtained by us. But it increases sharply to 1 immediately after 40 cm. We do not observe such a phenomenon in our investigation. Cheng et al. (1993) present a higher activity in the front part (about 0.7), growing up to about 0.95 at 95 cm of the bed. It is worth noting that any information about the period of exploitation of the catalyst studied in the papers discussed above cannot be found. Doubtless, in

Ind. Eng. Chem. Res., Vol. 37, No. 8, 1998 3431 Table 3. Hot Spot Characteristics and Phthalic Anhydride Yield Depending on the Catalyst Life for a Conventional Process (Experimental Results) time of catalyst use hot spot

3 days

50 days

location (cm) 30-35 50-55 magnitude (°C) 475-480 455-460 phthalic (%) 79 74 anhydride yield

20 months 36 months 65-70 450-455 71.3

130-140 435-440 61.1

addition to the reversible deactivation, the catalyst undergoes an irreversible one in the region of the hot spot due to the sintering. It is interesting to see how the location and the magnitude of the hot spot are changed as a result of the catalyst deactivation (Table 3). It becomes clear from all of the above-mentioned that the formation of two or more beds of catalyst of different activity as a result of the natural deactivation of the catalyst during its use is an undesirable phenomenon. It leads to a decrease of the phthalic anhydride yield and to an increase of the contents of the undesired side products, especially phthalide. For example, in the beginning of the catalyst life (50 days old) the yield of phthalic anhydride is about 74%, while it drops to about 71.3% after 20-months exploitation. In the industry this is compensated by rising the temperature of the coolant. In the case discussed the temperature is to be increased from 370 to 380 °C in order to achieve the performance of a fresh catalyst. Indeed, the coolant temperature chosen in the industry for a catalyst being subject to a change is 378 °C (Figure 11). However, in the case shown in Figure 11, this temperature is no more sufficient and both the phthalic anhydride and phthalide yields differ significantly from the standard ones. Actually, the coolant temperature is to be risen continuously according to the reduction of the activity starting from 15 to 16 months of the catalyst life until its change. The results of the simulation show that it must exceed 400 °C when the catalyst is wasted. We made an attempt above to determine how the activity of a catalyst for the oxidation of o-xylene (conventional process) changed in a natural way during the period of exploitation, and how regions of different catalyst activity were formed in the bed. The problem of how the deactivation of the catalyst affected the effectiveness of the process was also our concern to some extent. However, it is important to study the reactor as it is charged initially by two beds of catalysts having different activity. The dependence of the phthalic anhydride and phthalide yields (obtained numerically) on the activity of the catalyst in the second catalyst bed, 140-cm long, is presented in Figure 12. The first catalyst bed has a relative activity of 1 and is located in the first half of the tube. According to expectation, the risen activity in the second zone of the reactor intensifies the process of additional oxidation of the side products to phthalic anhydride. In contrast to a DSB (Figure 6), in the case of DCB the temperature in the second catalyst bed remains low and is near to that of the coolant. It is evident that in the interval A ) 1-2.5 the slope of curves 1 and 1a (conventional process) and 2 and 2a (LAR process) is high, whereupon it goes down significantly. This fact defines the activity of the catalyst in the second bed as being 2-2.5 times higher than that of the first bed, as is most suitable. The results obtained come into line with those published by

Figure 12. Prediction of phthalic anhydride and phthalide yields as a function of the activity of the second catalyst bed (from 140 to 280 cm). Activity of the first catalyst bed (from 0 to 140 cm) equal to 1. Curves 1 and 1asconventional process; curves 2 and 2asLAR process.

Figure 13. Phthalic anhydride and phthalide yields versus dimensionless length of DCB-model predictions. Curves 1 and 1as conventional process; curves 2 and 2asLAR process. Activity of first and second catalyst bed 1 and 2, respectively.

Papageorgiou and Froment (1996). They have optimized the process of receiving phthalic anhydride by organization of a triple bed of catalyst whose activity increases from a base one (equal to 1) to three times larger (equal to 3) in the last third bed. An additional quantity of oxygen is fed in at the first and second meter along the length of the apparatus. From the last figure (Figure 13) it can be defined which position along the reactor length is most suitable, the second catalyst bed having activity 2 to be introduced. Curves 1 and 1a are related to the conventional process, while curves 2 and 2a are related to the LAR process. As in Figure 7, in the boundary case at L ) 1, the bed is single and composed of a catalyst of base activity equal to 1. To the left of L ) 1 there are two beds formed of catalysts of activity 1 and 2. At the left border (L ) 0) the bed is single again, but the activity of the catalyst is 2 times higher. If the second bed of activity 2 is introduced at the point L ) 0.14 (where the dotted line a is drawn) for the conventional process (curves 1 and 1a), the yields of phthalic anhydride and phthalide are at the maximum and minimum, respectively. The temperature of the hot spot there is equal

3432 Ind. Eng. Chem. Res., Vol. 37, No. 8, 1998 Table 4. Predicted Data for DSB and DCB Systems conventional process yields

ORB DCB DSB

P.A. (%) 74.3 PH. (%) 4.2

77.9 1.3

79.0 0.3

LAR process

DSB + DSB + DCB ORB DCB DSB DCB 79.4 0.02

70.3 7.0

75.5 3.3

79.1 0.3

79.4 0.01

to the maximum admissible temperature (500 °C). We would like to explain that in this case (second bed introduced at L ) 0.14) the length of the first bed of activity 1 is about 40 cm, while that of the second bed (activity 2) is 240 cm. To the left of line a the hot spot temperature is over 500 °C. In the interval between dotted lines a and b for the conventional process the temperature of the hot spot is between 500 and 475 °C. It is worth noting that a hot spot temperature of 475 °C for the solid phase is characteristic of a single bed of activity 1. To the right of line b, regardless of the fact that there are two beds of different activity in the reactor, the hot spot temperature is 475 °C. In the case of the LAR process (curves 2 and 2a) if the second bed is introduced at the point L ) 0.22 (dotted line “b”) and to the right of it, the temperature of the hot spot is constant and equal to 496 °C. To the left of line b this temperature exceeds the maximum admissible one. Hence, the optimal lengths of the first and second catalyst bed are 60 and 220 cm, respectively. After studying how DSB and DCB influence the effectiveness of the oxidation process one by one, it is interesting to investigate what is the effect if both means are combined. For this purpose we present in Table 4 the phthalic anhydride and phthalide yields obtained in the following cases: (1) Ordinary bed (ORB); single bed of activity 1 and single salt bath of temperatures 370 for the conventional process and 340 °C for the LAR process. (2) Dual catalyst bed (DCB); two beds of the same length having activities 1 and 2 and a single salt bath as for the ordinary bed. (3) Dual salt bath (DSB); two salt baths (equal in length) of temperatures 370 and 450 °C for the conventional process and 340 and 450 °C for the LAR process and a single catalyst bed of activity 1. (4) Combination of DSB and DCB (DSB + DCB); characteristics of the two catalyst beds and the two salt baths as in items 2 and 3, respectively. Doubtless, the combination of DSB and DCB (DSB + DCB) leads to the best characteristics of the oxidation process. In comparison with the ordinary bed (ORB) the effect is very large, especially with respect to the LAR process. The situation is not the same if the case DSB + DCB is compared with a DSB regarding the phthalic anhydride yield. The absolute increase is only 0.4 and 0.3% for the conventional and LAR processes, respectively. But in relation to the yield of the undesired side product phthalide, the differences are drastic. For the conventional process phthalide decreases 15 times, while for the LAR process this reduction is about 30 times. Hence, phthalic anhydride of the best quality can be achieved under a combination of DSB and DCB. Actually, no additional expenses for cleaning are needed. VI. Conclusions The present investigations show the following:

(1) For the process of oxidation of o-xylene to phthalic anhydride, the application of a dual salt bath as well as a dual catalyst bed, leads to significant advantages expressed in a raised yield of the main product and a sharp decrease of the undesired product phthalide under a suitable temperature regime. (2) In the case of a dual salt bath, irrespective of the temperature of the second salt bath, the first half of the bed is to operate more intensively in comparison with the second one, the hot spot being located in the front part of the bed. (3) A final product practically pure can be obtained by using DCB combined with DSB, which are characterized by a higher activity of the catalyst and a higher cooling temperature in the second catalyst bed. (4) The optimal parameters of DSB and DCB for a conventional process and a LAR process are determined. (5) A strong reduction of the activity of the industrial catalyst in the first 80 cm of the bed, which is due to the reversible deactivation during its exploitation, is established. (6) The character of changing the activity of the industrial catalyst along the bed during the entire period of use is determined. Notation A ) relative activity of catalyst a ) specific external surface area of pellets, m2/m3 C ) concentration of component in gas phase, kmol/m3 Cc ) heat capacity of catalyst, J/(m3 K) Cg ) heat capacity of gas, J/(m3 K) Cp ) concentration of component on the surface of pellets, kmol/m3 Cp,0 ) initial concentration of component on the surface of pellets, kmol/m3 C0 ) inlet concentration of component in gas phase, kmol/ m3, g/Nm3; (Nm3 defined as m3 at 0 °C and 101.325 kPa) Dr ) coefficient of effective mass diffusivity in radial direction for gas, m2/s FA ) factor accounting for in a formal way the catalyst activity along the bed ∆H ) heat effect of corresponding reaction, J/kmol hgc ) heat-transfer coefficient between gas and coolant, W/(m2 K) hgp ) heat-transfer coefficient between gas and catalyst pellets, W/(m2 K) kgp ) mass-transfer coefficient between gas and catalyst pellets, m/s K ) reaction rate constant, (kmol m2)/(kg s N) Kc ) rate constant of catalyst reoxidation, (kmol m2)/(kg s N) L ) length of bed, cm, as well as dimensionless length of bed l ) axial coordinate, m P ) partial pressure of component, N/m2 Pox ) partial pressure of oxygen, N/m2 R ) tube radius, m r ) radial coordinate, m, as well as reaction rate by steps, kmol/(kg s) rp ) reaction rate by steps on catalyst pellets, kmol/(kg s) T ) temperature of gas, K, °C Tc ) temperature of coolant, K, °C Tp ) temperature of pellets, K, °C T0 ) inlet temperature of gas, K, °C Tp,0 ) initial temperature of pellets, K Tx ) temperature in bed, °C; measured experimentally, as well as mean temperature (obtained from the arithmetic means of the temperatures of gas and solid phases) for model predictions, °C

Ind. Eng. Chem. Res., Vol. 37, No. 8, 1998 3433 t ) time, s V ) flow rate of gas, Nm3/h Vb ) gas velocity with respect to the full cross section of the tube, m/s Wp ) reaction rate by component on catalyst pellets, kmol/ (kg s) X ) o-xylene conversion, mol % Y ) yield, mol % Greek Letters R ) defined in kinetic equations  ) void fraction of fixed bed λr ) coefficient of effective conductivity in radial direction for gas, W/(m K) Fp ) density of pellets, kg/m3 Subscripts A ) o-xylene B ) o-tolualdehyde C ) phthalide D ) phthalic anhydride i ) component (1 ) A, 2 ) B, 3 ) C, 4 ) D) m ) reaction step I ) first half of bed (first 140 cm) II ) second half of bed (second 140 cm)

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Received for review December 4, 1997 Revised manuscript received April 15, 1998 Accepted April 20, 1998 IE9708844