Znd. Eng. Chem. Res. 1992,31,2075-2079
2075
Selective Oxidation of n -Butane to Maleic Anhydride. 4. Recycle Reactor Studies Shyamal K. Bej and Musti S. Rao* Department of Chemical Engineering, Indian Institute of Technology, Kanpur 208016, U.P.,India
The selective oxidation of n-butane to maleic anhydride has been modeled using recycle reactor data. Two different types of models have been tested based on the concept that V5+is the selective site and V4+is the nonselective site and vice versa. Recycle reactor data support the model which assumes V5+as the selective site and V4+as the nonselective site. The model has been used to predict the performance of an integral reactor and tested with experimental integral reactor data. 1. Introduction n-Butane, a cheap and clean feedstock, has almost replaced benzene for the past 2 decades in the production of maleic anhydride (MA), an industrially important chemical. The selective oxidation of n-butane to MA takes place over vanadium-phosphorus oxide (VPO) catalyst with or without some promoters. The kinetics of the reaction is rather complex. The kinetics and the optimization of the reaction have been studied by several investigators (Wohlfahrt and Hofmann, 1980; Sharma and Creeswel, 1984; Centi et al., 1985; Buchanan and Sundaresan, 1986, Lerou and Weiher, 1986, Wellauer et al. 1986, Schneider et al., 1987; Suter et al., 1990; Sharma et al., 1991; Bej and Rao, 1991a-c). Different investigators (Centi et al., 1984,1988, Cavani et al., 1985; Hodnett, 1985) have reported the presence of three oxidation states of vanadium (viz., Vs+, V4+, and V3+)in the VPO catalyst after its reaction with the hydrocarbon. But the role of different oxidation states in this complex reaction was not clear. From doping and poisoning experiments, Centi et al. (1989) have postulated that the V5+ site is responsible for the oxidation of MA to oxides of carbon. The presence of two different types of sites, viz., selective sites and combustion sites, has been identified by Bej and Rao (1991~).Selective sites are responsible for the formation of MA from n-butane. Reaction of either n-butane or MA with combustion sites leads to the production of oxides of carbon. Through modeling studies Bej and Rao (1991~)have suggested that the V5+site acts as the selective site and V4+ as the combustion site. Differential reactor results were used for testing the proposed models in the above investigation. The extrapolation of these (differentialreactor) results of low conversions to the region, where higher conversions are prevailing, should carefully be avoided without any evidence from experimental support. In the present investigation, the conclusions reported in our earlier work (Bej and Rao, 1991c), using differential reactor results, are tested and confirmed using recycle reactor data. A comparison is then made among differential, recycle, and integral reactor results up to 80% conversion. In industrial practice, conversions are kept below 80% with a view to limit further oxidation of MA to oxides of carbon. This study was carried out over a VPO catalyst promoted with molybdenum and cerium (added simultaneously). Molybdenum and cerium were selected as promoters, since molybdenum is expected to enhance the selectivity toward MA while cerium is reported to increase the conversion of n-butane (Hucknall, 1974). The catalyst had a P / V atomic ratio of 1.08, a Mo/V weight ratio of 0.06, and a Ce/V weight ratio of 0.02. This was the optimum com-
* To whom correspondence should be addressed.
position obtained from our earlier study (Bej and Rao, 1991a). 2. Experimental Section 2.1. Catalyst Preparation. The method of preparation of the catalyst was the same as described by Katsumoto and Marquis (1979). The details of the method are given elsewhere (Bej and Rao, 1991a). In brief, Vz05was reduced by a mixture of allyl alcohol and isobutyl alcohol. Phosphoric acid was then added to the reduced mass of vanadium. After the reaction of phosphoric acid with the reduced vanadium, appropriate amounts of the promoters were added. The catalyst was dried at 150 "C for 12 h and calcined at 450 OC for 1h in an atmosphere of flowing air. X-ray diffraction measurements revealed that the uncalcined catalyst contained mainly (VO)zP20,*2H20(Bej and Rao, 1992). The compounds formed in the calcined catalyst were a mixture of a-VOPO, and fl-VOPOI. The catalyst had a surface area of 20 m2/g and a pore volume of 0.012 cm3/g. 2.2. Experimental Setup. The experimental setup used for the present study was essentially the same as the one used earlier (Bej and Rao, 1991b) except for the inclusion of a recycle pump. The schematic diagram of the setup used for the present study is shown in Figure 1. A stream of high purity n-butane, of known flow rate, was mixed with nitrogen and oxygen of known flow rates in a mixing chamber. This mixture of gases was fed to the reactor after preheating in a preheater. The temperature of the reactor was controlled by using two temperature controllers, The reactor was a stainless steel tube of l-cm i.d. and 45-cm length (including the inlet and outlet calming sections and the reactor section). The effluent gases from the reactor were bubbled through a water bubbler to dissolve the MA and other liquid products. The noncondensed gases were recycled back by an external recycle pump (Model B 86 SE, supplied by Charles Austen P u m p Ltd.).A part of it was taken out from the diechatge side of the recycle pump and sent to the exhaust line via the gas sampling valve of a gas chromatograph. The remainder of the effluent stream was mixed with the fresh feed before the preheater. The flow rates of these two streams were regulated by control valves. 2.3. Experimental Procedure. The experimental procedure for conducting runs usingthe recycle reactor was as follows. The catalyst, in the form of a fine powder, was taken in the reactor. The desired volume of a mixture of oxygen and nitrogen (in the desired proportion) was allowed to pass through the reactor which was heated in a controlled manner. During this period the recycle pump was shut off. When the desired temperature was reached in the reactor, the recycle pump was started. With adjustment of valves 1 and 2, the flow rates of the recycle
oaaa-5aa5/92/2631-2075$03.00/0 0 1992 American Chemical Society
2076 Ind. Eng. Chem. Res., Vol. 31, No. 9,1992
TEMPERATURE CONTROLLER
1
'
PRE HEATER
1 7 -
I-
RECYCLE PUMP
3
4NT 3UT
Ll-
EXHAUST
U GAS SAMPLING VALVE
8
6 d
1 DRYING TUBE 2 ROTA M ET E R 3 C02 REMOVAL TOWER 4 SOAP BUBBLE METER 5 n -BUTANE CYLINDER 6 OXYGEN CYLINDER 7 NITROGEN CYLINDER 8 NEEDLE VALVE
WATER
l-
LIQUID SAMPLING VA LV E
Figure 1. Schematic diagram of the setup used for the recycle reactor. Table I. Concentrations of Different Components at the Inlet and Outlet of the Recycle Reactor and Rate of I)-Butane Consumption (Weight of Catalyst = 0.54 g; Weight of Inert Material = 0.54 g) outlet concentration (mol %) temp feed flow rate of reacn inlet n-butane rate of n-butane ("c) mixture (mg-mol/s) concn (mol % ) n-butane MA oxides of carbon consumpn X 103 (g-mol/gh) 400 0.0618 1.2 0.2990 0.7949 0.4568 3.7076 0.0610 1.6 0.5010 0.9912 0.4468 4.4725 0.0568 2.0 0.6990 1.1709 0.5232 4.9278 0.0525 2.4 0.9010 1.3542 0.6232 5.2470 0.0483 2.8 1.0990 1.5479 0.6248 5.4784 0.0439 3.2 1.2680 1.7388 0.7724 5.6571 420 0.0746 1.2 0.2256 0.7308 0.9744 4.8440 0.0780 1.6 0.4135 0.9492 0.9496 6.1671 0.0787 2.0 0.6519 1.1424 0.8804 7.0684 0.0704 2.4 0.8089 1.4010 0.7636 7.4641 0.0600 2.8 0.8970 1.6229 1.1460 7.6328 0.0603 3.2 1.1830 1.6943 1.2780 8.1057 440 0.0953 1.6 0.3514 0.8116 1.7656 7.9303 0.0841 2.0 0.4539 0.9860 2.4116 8.6687 0.0801 2.4 0.5869 1.0879 2.8720 9.3863 0.0796 2.8 0.8477 1.1714 3.1240 10.3604 0.0726 3.2 0.9826 1.2861 3.7652 10.7310 0.0767 3.6 1.3530 1.3482 3.7924 11.4870
stream and exhaust stream were controlled. Then n-butane was allowed to pass in the feed stream. Finally the flow rates of oxygen and nitrogen were checked and adjusted to the initial set values. The flow rate of the exhaust stream was kept equal to the flow rate of the fresh feed. A recycle ratio of 30 was used in the present investigation. After 3 h of operation, when steady state was reached, as evidenced by the constancy in the outlet gas composition, samples were withdrawn for analysis. 2.4. Analytical Methods. The products, which were dissolved in water, were analyzed by gas chromatography using a 2-m-length Porapak-QS column and a flame ionization detector. The product was found to be entirely MA. The noncondensed gases were analyzed using two different columns, viz., Porapak-Q (for hydrocarbons and COz) and molecular sieve 5A (for oxygen, nitrogen, and carbon monoxide) using thermal conductivity detectors. Each column was 2 m long. After all the analyses were completed, checks on carbon balance were done. In all cases, MA, carbon dioxide, and carbon monoxide were found to be the only products.
3. Results and Discussion It was reported earlier that V5+,V4+, and V3+ oxidation states were present in the VPO catalyst. The chemical analysis of reacted catalyst also indicated the presence of V5+,V4+,and V3+oxidation states. Investigation with the help of a poisoning study indicated that two different typea of sites (viz., selective sites and combustion sites) were present in the VPO catalyst (Bej and b o , 1991~).On the basis of this information, two models have been derived for the reaction. Model I is based on the concept that V6+ acts as the selective site and V4+as the nonselective site. Model I1 is based on the reverse concept; i.e., V4+ is the selective site, and V5+ is the nonselective site. 3.1. Derivation of Model I. The following redox cycles are involved in model I (according to our assumption) for the different reactions:
for MA formation
V5+ +e V4+
for CO, formation
~ 4 +
-e
-e
v3+
Ind. Eng. Chem. Res., Vol. 31, No. 9,1992 2077
The sequential steps can be written as follows: kl
OZ+R-X
B + X - ~ A + R
B + R ~ ~ - c + F
MA+R&+F k6
O,+F-R (1) where X, R, and F stand for V5+ (oxidized site), V4+(reduced site), and V3+ (further reduced site), respectively. B, MA, and C represent n-butane, maleic anhydride, and oxides of carbon, respectively. The total rate of depletion of n-butane is derived on the basis of the above sequential steps. The details of the derivation are given elsewhere (Bejand Rao, 1991~).The derived rate expression is given by the following equation: -rB
Table 11. Values of the Parameters and Orders of the Reaction for Model I (Obtained from Recycle Reactor Results) temp ("C) parameters 400 420 440 kl 0.0163 f 0.0001 0.0225f O.oo00 0.0290 t O.oo00 k2 3.1916 f 0.0030 4.8170t 0.0004 6.8824t 0.0005 k3 0.3178 f 0.0008 0.4926t 0.0001 0.7707 t 0.0001 k5 0.0446f 0.0002 0.0637 t O.oo00 0.0897 t O.oo00 n 0.0 0.001 0.0 r 0.9040 0.8817 0.8985 RSS 0.1376 x lo4 0.1638 X lo-" 0.2870 x "Units of k's are g-mol/(gatm.h). The confidence intervals for the parameters are calculated at the 95% confidence level.
the experimental setup, the outlet stream of the reactor had been passed through a condenser before being recycled. The MA formed in the reactor was condensed out in the condenser. Therefore, the inlet stream to the reactor did not contain any MA. Furthermore, the per pass conversion in the recycle reactor, corresponding to an overall conversion of 70% and a recycle ratio of 30, works out to be only 7%. Hence the concentration of MA for a single pass in the reactor is low enough for us to neglect the reaction MA COX. Thus both models are slightly reduced. These are given as follows: model I
-
where k,' = k,/a. Here, ki is the rate constant for the ith step of reaction 1. a is the number of moles of oxygen required to oxidize 1 mol of n-butane to MA, is the number of moles of oxygen required to oxidize 1 mol of n-butane to oxides of carbon, and y is the number of moles of oxygen required to oxidize 1 mol of MA to oxides of carbon. m is the order of the reaction with respect to n-butane for its oxidation to MA, n is the order of the reaction with respect to oxygen for the oxidation of reduced catalyst, r is the order of the reaction with respect to nbutane for its oxidation to oxides of carbon, and s is the order of the reaction with respect to MA for ita oxidation to oxides of carbon. 3.2. Derivation of Model 11. The redox cycles which are involved for different reactions for model I1 are given as follows: for MA formation for CO, formation
22 + v3+ v&+ 22 v4+
~
4
-e
-e
where kl' = k , / a and model I1
where kgl = k5//3. 3.3. Determination of the Rate of Reaction and Estimation of Parameters. In order to obtain the values of the parameters of eqs 5 and 6, experiments were conducted at temperatures of 400,420, and 440 OC, respectively. The results of the experiments along with the outlet compositions of n-butane and oxygen are given in Table I. The rates of n-butane consumption are calculated using the following relation.
On the basis of these redox cycles, the following steps can be written for model 11: 02
+ F-R
kl
B + R -!L MA + F
B + X ~ + R MA+Xk'-C+R
+ R-Xk6
(3) The derived rate expression for the total rate of consumption of n-butane is given by 0 2
(4)
where D = B k g b + yk&MA. The notations used for the derivation of model 11 are the same as those for model I. The above two models (eqs 2 and 4) got slightly modified for their use with recycle reactor data. As evident from
(7) Here, yBois the initial mole fraction of n-butane, YBf is the final mole of fraction of n-butane, W is the weight of catalyst, and F is the molar flow rate of the feed stream. Using the calculated rates of n-butane depletion, the values of the parameters of eqs 5 and 6 were calculated assuming m = 1. This assumption was taken since most of the earlier investigators (Centi et al., 1985, 1988; Buchanan and Sundaresan, 1986) have already reported the reaction to be first order with respect to n-butane. Estimation of the parameters was done by nonlinear regression using Marquardt's Bsolve algorithm (Kuester and Mize, 1973). The values of initial guesses for the parameters were obtained from the following linearized form of the eqs 5 and 6 (assuming m = 1): h'P8, - BkgS = kP8 + k3-Pb, - - r ~ ~ G B k&, -rB kflB k5& -+-----= - r ~ B(-~B)
(YkflB
klpa,
k;PP82 kapb
(8) 1
(9)
2078 Ind. Eng. Chem. Res., Vol. 31, No. 9, 1992 Table V. Conditions and Resultr of Experiments Conducted in Integral Reactor (Reaction Temperature = 430 OC; Feed n -Butane Concentration = 1.6 vol % of Air-n -Butane Mixture) % conversion % yield W / F , (g/(g-mol/h)) of n-butane of MA 36.45 31.8 27.8 36.3 49.83 44.2 41.6 61.25 53.2 46.5 84.39 64.9 47.8 108.75 75.7 47.2 125.64 83.8 46.4 160.32 92.3
Table 111. Values of the Parameters and Orders of the Reactions for Model I1 (Obtained from Recycle Reactor Results) temp ("C) parameterso 400 420 440 0.0521 kl 6.1232 0.0347 3.6191 4.5401 k2 0.0366 0.0697 -0.0149 k3 2.9293 0.0004 -0.0697 k5 0.0352 0.0 0.0 n 0.0 1.0 0.4801 r 0.9944 0.9992 X RSS 0.4283 X 0,1131 X
~~
"Units of k's are g-mol/(gatm.h).
Table IV. Values of Activation Energies (Obtained from Recycle Reactor Results) E3 = 21.04,kcal/mT E, = 14.17 kcal/mol E5= 16.65 kcal/mol E, = 18.26 kcal/mol
I
nnL
n-butane, W/F,, were calculated using the following relation (Froment and Bischoff, 1990).
where x is the fractional conversion of n-butane at the outlet of the reactor. The values of W/Fmthus calculated from eq 10 were then compared with the experimental values obtained from the integral reactor. 3.4.1. Prediction of the Performance of the Integral Reactor from Differential Reactor Results. Substituting the rate expression (with m = 1 and n = 0) as obtained from the differential study in eq 10, one gets r
-6.0
c
L
I
-a 0 1 1 38 E-003
1PZ
I
1 L6E-003
E-003
I 150E-003
1 / T (K-')
Figure 2. Arrhenius plot of various rate constants (obtained from recycle reactor results).
and using the least squares technique. The valuea of the parameters as estimated by nonlinear regression are given in Tables I1 and I11 for models I and 11, respectively. The values of the parameters for model I showed Arrhenius dependency (increasing trend) with respect to an increase in temperature. On the other hand, the values of parameters for model I1 are rather suspicious since some of them are negative and others are not following the expected (increasing) trend of the Arrhenius relationship with an increase in temperature. Thus,model I1 has been concluded to be untenable and for this reason no confidence limits are reported for this model. Hence it is concluded that model I is the most appropriate one for the reaction. Finally, the values of the activation energies are calculated based on an Arrhenius plot (Figure 2) and are given in Table IV. The values are comparable with thoee obtained from a differential study. Thus both the differential reactor study (Bej and Rao, 1991~)and recycle reactor study (present investigation) confirm the model which is based on the concept that V5+is the selective site and V4+is the nonselective site. 3.4. Comparison of the Differential Reactor, Recycle Reactor, and Integral Reactor Results. Some further experiments were conducted in an integral reactor, and the results were compared with those predicted by differential and recycle reactors, respectively. The experimental conditions and results of the experiments conducted in the integral reactor are given in Table V. The prediction of the behavior of the integral reactor from the results of the differential reactor and recycle reactor waa done as follows. The values of the contact times of
Also PB
= PBO(1 - x )
(12)
YPBO (13) where pBois the partial pressure of n-butane in the feed stream and Y is the yield of MA in the outlet stream. From the data of the integral reactor (as given in Table V), the following correlation can be proposed between Y (the yield of MA) and x (fractional conversion): Y = 0.90129~+ 0 . 0 6 7 9 1 ~- ~0 . 5 4 3 0 8 ~ ~ (14) After substitution of eq 14 in eq 13, the final expression for p m can be written as follows: PMA = p~o[O.90129X+ 0.06'791~~ - 0.54308~~1 (15) These two relations (eqs 12 and 15) are then substituted in eq 11, which makes the integration of the latter much easier. The values of W / F , were obtained after integration of the expression for a desired value of x . The integration was done numerically. The values of kl to k5, a,8, y, r, and s were taken for 430 O C from our earlier studies (Bej and Rao, 1991~).The results of these integrations are plotted in Figure 3 as W/Fm vs n. 3.4.2. Prediction of the Performance of the Integral Reactor from Recycle Reactor Results. Using the recycle reactor results (with m = 1and n = o), the expression for W/FBocan be written as follows: -W- PMA
FBO
Equation 12 is then substituted for pe, and the resulting expression for w / F o was integrated for a desired value
Ind. Eng. Chem. Res.,Vol. 31, No. 9, 1992 2079
X = oxidized state of the catalyst (i.e., V5+) c
I
= final mole fraction of n-butane ym = initial mole fraction of n-butane a = number of moles of oxygen required to oxidize 1mol of YBf
n-butane to MA 0 = number of moles of oxygen required to oxidize 1mol of n-butane to oxides of carbon y = number of moles of oxygen required to oxidize 1mol of MA to oxides of carbon Registry No. V, 7440-62-2;P,7723-14-0;Mo, 7439-98-7;Ce,
v
0 0.3
0.4
0.5 0.6 0. Fractional conversionp
0.8
Figure 3. Comparison of differential, recycle, and integral reactor
results. [-. -1 Predicted from differential reactor results; [---I predicted from recycle reactor results, [-(O)-] obtained from experimental integral reactor results.
of x . The values of klto k3 and k5 for 430 “C were taken from the Arrhenius plots of the rate constants obtained from recycle reactor results. The value of r was approximated as 0.90 (see Table 11). The values of a and @ were the same as those used for the differential study. The results of integration are plotted in Figure 3 as W / F , vs X.
For ease of comparison, the experimental results of the integral reactor are also plotted in Figure 3. It is evident from Figure 3 that for conversions up to 60% all three results match nicely. At higher conversions (greater than 60%),the reaction MA CO, cannot be neglected. This also necessitates a different mode of operation of the recycle reactor, viz., recycling of the MA (which means heating the recycle pump as well as the recycle lines) and determination of kq.
-
4. Conclusions The recycle reactor study confirms the model which assumes V5+as the selective site and V4+as the nonselective site. A similar conclusion was also obtained from differential reactor studies carried out earlier by the present authors. The values of the activation energies are also in the same range as those obtained from differential reactor studies. The use of the differential reactor and recycle reactor results for predicting the performance of an integral reactor has been made. The results match well for conversions up to 60%.
Nomenclature B = n-butane C = oxides of carbon e = electron F = further oxidized state (i.e., V3+) F = total flow rate of reaction mixture, g-mol/h Fm = flow rate of n-butane in the feed stream, g-mol/h kj = rate constant for the ith step of the reaction, g-mol/ (ghoatm); i = 1...5 m = order of the reaction with respect to n-butane for ita oxidation to MA n = order of the reaction with respect to oxygen for the oxidation of the reduced catalyst pB = partial pressure of n-butane, atm pBO = partial pressure of n-butane in the feed stream, atm pw = partial pressure of maleic anhydride r = order of the reaction with respect to n-butane for ita oxidation to oxides of carbon -rg = rate of n-butane consumption, g-mol/(gh) R = reduced state of the catalyst (Le., V4+) s = order of the reaction with respect to MA for ita oxidation to oxides of carbon W = weight of catalyst, g x = fractional conversion
7440-45-1;butane, 106-97-8;maleic anhydride, 108-31-6.
Literature Cited Bej, S. K.;.Rao, M. S. Selective Oxidation of n-Butane to Maleic Anhydride. 1. Optimization Studies. Znd. Eng. Chem. Res. 1991a,30, 1819-1824. Bej, S. K.; Rao, M. S. Selective Oxidation of n-Butane to Maleic Anhydride. 2. Identification of Rate Expression for the Reaction. Znd. Eng. Chem. Res. 1991b,30,1824-1828. Bej, S. K.;Rao, M. S. Selective Oxidation of n-Butane to Maleic Anhydride. 3. Modeling Studies. Ind. Eng. Chem. Res. 199lc, 30, 1829-1832. Bej, S. K.;Rao, M. S. Selective Oxidation of n-Butane to Maleic Anhydride-A Comparative Study between Promoted and Unpromoted VPO Catalysts. Appl. Catal. 1992,83,149-163. Buchanan, J. S.; Sundaresan, S. Kinetics and Redox Properties of Vanadium Phosphate Catalysts for Butane Oxidation. Appl. Catal. 1986,26,211-226. Cavani, F.; Centi, G.; Manenti, I.; Trifiro, F. Catalytic Conversion of C, Hydrocarbons on Vanadium-Phosphorus Oxide: Factors Influencing the Selectivity of 1-Butene Oxidation. Ind. Eng. Chem. Prod. Res. Dev. 1985,24,221-226. Centi, G.; Fornasari, G.; Trifiro, F. On the Mechanism of n-Butane Oxidation to Maleic Anhydride: Oxidation in Oxygen-Stoichiometry Controlled Conditions. J. Catal. 1984,89,44-51. Centi, G.; Fornasari, F.; Trifiro, F. n-Butane Oxidation to Maleic Anhydride on Vanadium-Phosphorus Oxides: Kinetic Analysis with a Tubular Flow Stacked-Pellet Reactor. Znd. Eng. Chem. Prod. Res. Deu. 1985,24,32-37. Centi, G.; Trifiro, F.; Ebner, J. R.; Franchetti, V. M.; Mechanistic Aspects of Maleic Anhydride Synthesis from C, Hydrocarbons over Phosphorus Vanadium Oxide. Chem. Rev. 1988,88,55-80. Centi, G.; Golinelli, G.; Trifiro, F. Nature of the Active Sites of (VO)2P207in the Selective Oxidation of n-Butane. Evidence from Doping Experiments. Appl. Catal. 1989,48,13-24. Froment, G. F.; Bischoff, K. B. Chemical Reactor Analysis and Design, 2nd ed.; John Wiley & Sons: Singapore, 1990. Hodnett, B. K. Vanadium-Phosphorus Oxide catalysts for the Selective Oxidation of C,, Hydrocarbons to Maleic Anhydride. Catal. Rev.-Sci. Eng. 1985,27 (3),373-424. Hucknall, D. J. Selective Oxidation of Hydrocarbons; Academic Press: London, 1974;Chapter 4. Katsumoto, K.; Marquis, D. M. U.S. Patent 4,132,670,1979. Kuester, J. L.; Mize, J. H. Optimization Techniques with Fortran; McGraw-Hill: New York, 1973;Chapter 6. Lerou, J. J.; Weiher, J. F. Paper presented a t the Pittsburgh/ Cleveland Catalysis Society Meeting, 1986. Schneider, P.; Emig, G.; Hofmann, H. Kinetic Investigation and Reactor Simulation for the Catalytic Gas-Phase Oxidation of nButane to Maleic Anhydride. Znd. Eng. Chem. Res. 1987, 22, 2236-2241. Sharma, R. K.; Cresswell, D. L. Paper presented at AIChE Annual Meeting, San Francisco, 1984. Sharma, R. K.; Cresswell, D. L.; Newson, E. J. Kinetics and FixedBed Reactor Modelling of Butane Oxidation to Maleic Anhydride. AIChE J. 1991,37 (l), 39-47. Suter, D.; Bartroli, F.; Schneider, F.; Rippin, D. W. T.; Newson, E. J. Radial Flow Reactor Optimization for Highly Exothermic Selective Oxidation Reactions. Chem. Eng. Sci. 1990, 45 (a), 2169-2176. Wellauer, T. P.; Cresswell, D. L.; Newson, E. J. Optimal Policies in Maleic Anhydride Production Through Detailed Reactor Modelling. Chem. Eng. Sci. 1986,41 (4),756-772. Wohlfahrt, K.; Hofmann, H. Kinetics of the Synthesis of Maleic Anhydride from n-Butane. Chem.-Zng.-Tech. 1980, 52, (lo), 811-814. Received for reuiew March 18,1992 Revised manuscript received June 14,1992
