Ind. Eng. Chem. Res. 1991,30, 1819-1824
1819
Selective Oxidation of n -Butane to Maleic Anhydride. 1. Optimization Studies Shyamal K. Bej and Musti S. b o * Department of Chemical Engineering, Indian I n s t i t u t e of Technology, K a n p u r 208 016,
U.P., India
Optimization studies have been carried out for the selective oxidation of n-butane to maleic anhydride (MA) over vanadium-phosphorus oxide (VPO) catalysts promoted with Mo and Ce. It is known that Mo increases the selectivity to MA while Ce increases the conversion of n-butane. Response surface methodology has been used for finding out the optimum catalyst composition and optimum values for the process variables, viz., inlet feed composition, reaction temperature, and contact time for the reaction. It is observed that a catalyst having a P / V atomic ratio equal to 1.08, Mo/V weight ratio of 0.06, and Ce/V weight ratio of 0.022 gave the maximum yield of MA (49.5%) a t a temperature of 423 "C and an inlet n-butane concentration of 1.75% of air and n-butane mixture. 1. Introduction
Maleic Anhydride (MA), an industrially important chemical, is generally produced by the selective oxidation of either benzene or n-butane. For the past few years, due to higher prices of benzene and restrictions imposed on using benzene from a pollution point of view, C4fractions were found to be better feedstocks for the production of this important chemical. Generally, vanadium-phosphorus oxide (VPO)with or without promoter is used as a catalyst for this reaction. The selective oxidation of C4fractions has been reviewed by Hucknall (1974), Varma and Saraf (1979), Hodnett (1985), and most recently Centi et al. (1988). Factors like the method of preparation of VPO catalyst, phosphorus/vanadium (P/V) atomic ratio of the catalyst, temperature, and atmosphere of calcination of the catalyst affect the selectivity toward MA. Several investigators (Varma and Saraf, 1978; Cavani et al., 1984,1985a; Centi et al., 198413) have extensively studied the influence of these variables on the selective oxidation of 1-butene to MA. Similarly, the influence of different factors on the selective oxidation of n-butane to MA has been studied by a number of investigators (Cavani et al., 1985b; Centi et al., 1984a; Wenig and Schrader 1986; Hodnett and Delmon, 1985; Buchanan and Sundaresan 1986; Moser and Schrader 1985). Different promoters have been used in the VPO catalysts so as to improve the yield of MA. Most of the information on promoted catalysts is in patented form. Hodnett (1985) had summarized the information on the activity and selectivity (to MA) of promoted VPO catalysts. Though there are many publications regarding the chemistry of VPO catalysts and possible mechanistic pathways for this reaction, one finds hardly any information regarding the optimization of all the variables together to get the maximum yield of MA. A detailed one-dimensional, heterogeneous model for the selective oxidation of n-butane to MA in fixed-bed reactors accounting for interfacial and intraparticle gradients has been presented by Wellauer et al. (1986). In the present investigation, we have tried to find out the optimum values of the variables to obtain the maximum yield of MA over VPO catalysts promoted by molybdenum and cerium added simultaneously under negligible interparticle and intraparticle mass transfer resistances with a view to obtain a better insight into the effects of catalyst composition, reaction temperature, contact time, and feed n-butane
* To whom
correspondence should be addressed.
0888-5885/91/2630-1819$02.50/0
Table I. First-Order Response S u r f a c e Strategy (First Move): Levels of Factors lower higher factor code base level level (-) level (+) n-butane concn in the x1 0.7 0.4 1.0 feed, vol % of air-n-butane mixture 360 390 reaction temp., "C x2 375 0.04 0.14 Mo/V wt ratio %3 0.09 ~1 0.04 0.02 0.06 Ce/V wt ratio 10.0 15.0 W/F,g/(mg-mql/s) ~5 12.5 ~6 1.05 0.95 1.15 P/V atomic ratio
concentration, respectively, on the intrinsic yield of MA. Integral reactor data were obtained with a powdered catalyst. One of the aims was to obtain a suitable catalyst composition which will be used in parts 2 and 3 of this series under differential reactor conditions for modeling the intrinsic kinetics of this reaction. While extrapolating the results of this study to the operational design of practical reactors, caution should, however, be exercised. Methods of obtaining the nonisothermal effectiveness factors are available in literature (Smith, 1981). Also the extrapolation of these results to the region, where higher values of n-butane conversion are prevailing, should be carefully avoided without any evidence from experimental support. We have selected molybdenum and cerium as promoters since cerium was reported to increase the conversion of n-butane while molybdenum is expected to enhance the selectivity to MA.
2. Experimental Section 2.1. Catalyst Preparation. The catalysts used in the present investigation were prepared in an organic medium. Catalysts prepared in an organic medium have greater surface areas as compared to catalysts prepared in an aqueous medium. The method of catalyst preparation was the same as that reported by Katsumoto and Marquis (1979). Vanadium pentoxide was suspended in a 1:3 (v/v) mixture of allyl alcohol and isobutyl alcohol. Allyl alcohol was used as a reducing agent and isobutyl alcohol was used as a solvent. The mixture was refluxed at 115 "C for 2 h under constant stirring. Then it was cooled to 40 "C. The proper amount of orthophosphoric acid (needed to maintain the desired P/V atomic ratio) was mixed with isobutyl alcohol and then added to the slurry of reduced vanadium. The whole mixture was then again refluxed for 2 h at 115 "C under constant stirring. After the mixture was cooled to 40 "C, molybdic acid was added to it while it was stirred 0 1991 American Chemical Society
1820
Ind. Eng. Chem. Res., Vol. 30, No. 8, 1991
Table 11. The 2er First-Order (First Move) Design Mntrix nnd Vnluee of t h e Reeponses factors in coded form expt no. *I X2 x3 x4 x5 ( X I X 3 ) x6 (xIx4) 1 2 3 4
-
-
+
+ + -
-
-
-
-
+ +
5 6 7 8 9 10 11 12 13 14 15 16
+ + + + +
+ +
17 18 19
+ + +
-
+ + + +
-
+ +
-
+ + + +
-
-
-
+ + +
-
-
18.7 19.9 19.5
83.2 86.1 85.8
15.5 17.1 16.7
-
-
+
Rewat Trials
-
-
-
-
+ +
-
+ +
18.3 12.8 23.2 19.0 3.4 16.2 12.9 21.9 14.9 13.1 20.5 18.5 7.3 16.3 12.3 21.2
+ -
+ + + +
+ + + + + + + +
Y
90.4 91.8 87.1 78.4 98.6 84.4 96.3 84.8 70.8 81.2 65.3 84.6 78.1 79.7 76.5 72.1
-
-
-
S
20.2 13.9 26.7 24.2 3.5 19.2 13.4 25.8 21.1 16.2 31.4 21.9 9.3 20.5 16.1 29.4
+ + + +
-
-
-
+ +
+ +
-
-
+ + +
values of responses X
Table 111. Firet-Order Responee Surface Methodology (Firet Move): Test for Model Adequacy Conversion j = 19.551 + 1 . 8 2 4 ~+~4 . 0 7 1 ~- ~2 . 3 8 5 ~+~ 1 . 1 8 0 ~+~ 4.732 xg 0.582~8 (4)
ANOVA source due to regression residual pure error lack of fit total
sum of sauares 794.42 17.83 0.74
17.M 812.55
deg of freedom
mean sauare
6 12 3 9 18
0.246 1.899
Fdcd = 7.72; Fom(e,3, = 8.81; model is adequate
Selectivity
= 82.523 - 0.371~1- 1.862~2+ 1.301X3 - 6.474~4- 3.176~6+ 3.739x, (5)
valve
Figure 1. Schematic diagram of the experimental setup.
ANOVA
deg of source sum of squares freedom mean square due to regression 1202.58 6 residual 73.94 12 pure error 5.26 3 1.753 lack of fit 68.68 9 7.631 total 1276.52 18 F,, = 4.35; F0,,e,3, = 8.81; model is adequate Yield
9 = 15.775 + 1.611~1+ 2.970~1- 1.77413 - 0.098~4+ 3.299~6+ 0.121~0 (6)
ANOVA
deg of source sum of squares freedom mean square due to regression 415.30 6 residual 21.42 12 pure error 1.348 3 0.449 lack of fit 20.073 9 2.230 total 436.72 18 Fdd = 4.97; Fo,mce,3, = 8.81; model is adequate
constantly. Then the slurry was filtered and ceric ammonium nitrate was added to the solid. Both molybdic acid and ceric ammonium nitrate were added in predetermined quantities so as to maintain the desired Mo/V and Ce/V weight ratios. The solid mass was then dried at 150 "C for 12 h, and it was calcined at 450 "C for 1 h
in an atmosphere of flowing air. 2.2. Experimental Setup. The schematic diagram of the experimental setup is given in Figure 1. Compressed air was allowed to pass through a sodium hydroxide tower (for removing small quantitiea of carbon dioxide) first and then through a drying tube. The flow of air was controlled by a needle valve and measured by a rotameter. Similarly the flow of high-purity n-butane was controlled by a needle valve and measured by a soapbubble flowmeter. This was then passed through a drying tube and finally mixed with air in a mixing chamber. The mixture was then preheated and fed to the reactor. The reactor w a a stainless steel tube of 1 cm i d . and a total height of approximately 45 cm, which consisted of inlet and outlet calming sections and the reactor section. Two Chromel-Alumel thermocouples were used to measure the temperature of the catalyst bed, The product gases were then bubbled through water so as to dissolve MA and then passed through a condenser. The temperatures of the abeorber and the condenser were kept at a 10 and 6 O C , respectively, by passing refrigerated water through their jackets. The noncondensed gases were allowed to pass to a gas chromatograph for analysis. 2.3. Experimental Procedure. A known amount of catalyst (in the form of fine powder) diluted with silica (1:l weight ratio), was taken in the reactor and sandwiched between silica at the exit and inlet sections. Air was peeeed at the rate of 100 cm3/min, and the reactor was heated in
Ind. Eng. Chem. Res., Vol. 30,No. 8, 1991 1821 Table IV. Calculation of the Path of Steepest Ascent factor x2
x3
x4
XS
x6
375 15 2.97 44.55 6.0
0.09 0.05 -1.7745 -0.0887 -0.012
0.04 0.02 -0.0981 -0.00196 -0.00026
12.5 2.5 3.2988 8.247 1.11
1.05 0.1 0.1213 0.012 0.0016
X1
0.1
base level unit estimated slope (b) unit X b change in level per 6 OC change in x 2
0.3 1.611 0.4833 0.065
Path of Steepest Ascent As Represented by a Series of Trial Points trial Doint X1 x., XQ XA 0.078 0.0397 1 0.165 381 0.0394 0.066 2 0.830 387 0.895 393 0.054 0.0391 3 0.0389 0.960 399 0.042 4 0.030 0.0386 1.025 405 5 0.0383 0.018 6 1.090 411
exDt no. 20 21 22 23 24 25
XI.
XR
13.61 14.72 15.83 16.94 18.05 19.16
1.0516 1.0532 1.0548 1.0564 1.0580 1.0596
and then cooled in a condenser (kept at 6 "C). The products which were dissolved in water were analyzed by gas chromatography using a 2-m-long Porapak-QS column and a flame ionization detector. The product was found to be entirely MA. The noncondensed gases were analyzed by use of two different columns, viz., Porapak-Q and molecular sieve 5A. The hydrocarbons and carbon dioxide were separated in a 2-m-long Porapak-Q column and analyzed by use of a thermal conductivity detector. Oxygen, nitrogen, and carbon monoxide were separated in a 2-mlong molecular sieve 5A column and analyzed with the use of a thermal conductivity detector. 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.
Table V. Results of Experiments along the Path of Steepest Ascent expt no. trial point exptl yield predicted yield 20 1 20.3 19.1 21 2 23.9 22.6 22 3 29.9 26.1 23 4 37.1 29.5 38.5 32.9 24 5 25 6 38.7 36.4 Table VI. First-Order Response Surface Strategy (Second Move): Levels of Factors lower higher factor code base level level (-) level (+) n-butane concn in the rl 1.3 0.9 1.1 feed, vol W of air-n-butane mixture reaction temp, "C XZ 405 390 420 x3 0.04 0.01 0.07 Mo/V wt ratio x4 0.02 0.01 0.03 Ce/V wt ratio W/F,g/(mg-mol/s) ~6 18.5 15.0 22.0 P/V atomic ratio xg 1.125 1.o 1.25
3. Results and Discussion All experiments were carried out in the presence of negligible intra- and interparticle resistances. The absence of intraparticle resistance was verified by comparing the rates of MA formation and n-butane consumption using the catalyst in the form of pellets and fine powder. The interparticle transport effect was checked by changing the feed rate at a constant contact time of the reactants. The difference between the inlet and outlet temperatures of the catalyst bed was found to be not more than 1 "C. This study has been carried out with a view to determine the effects of different variables on the yield of MA and their optimum levels for obtaining the maximum yield of
a controlled manner. When the desired temperature was reached, air flow was controlled according to the requirement and n-butane was allowed to pass through. The reaction was allowed to proceed for about 2 h when steady state was reached. At that stage the products were withdrawn and analyzed. 2.4. Analytical Methods. The reactor effluent was allowed to pass through a water bubbler (kept at 10 "C)
Table VII. The p*First-Order (Second Move) Design Matrix and Values of the Responses factors in coded form values of responses expt no. xl X2 x3 x4 %6 (Xlx3) x6 (xlx4) X S Y 26 51.3 74.5 38.2 21 59.5 68.2 40.6 28 + + +66.4 63.2 42.0 29 75.1 51.5 43.5 30 39.5 92.3 36.5 + 31 + 62.8 66.5 41.8 32 51.2 82.1 42.0 33 74.6 54.8 40.9 34 + + 73.8 53.2 39.3 35 + + + 57.5 70.5 40.5 36 + + + 89.8 41.8 37.5 + 37 + + 71.3 57.5 41.0 38 + + 56.5 72.3 40.8 39 ++ ++60.4 69.6 42.0 40 + + 72.6 60.3 43.8 41 + + + + + 74.5 56.8 42.3
+ + + +
+
+
+ +
+ + +
+
+
+ +
+ +
42 43 44
+ + +
+ + +
+
+
-
-
-
Repeat Trials -
-
-
76.9 76.5 74.8
56.7 57.9 58.5
43.6 44.3 43.1
1822 Ind. Eng. Chem. Res., Vol. 30, No. 8, 1991
MA. We have adopted response surface methodology (RSM) for this purpose. RSM is discussed extensively by Kittrell and Erjavec (1968), Box et al., (1978), Davies (1978), and Khuri and Cornel1 (1987). RSM coltsists of several steps. The first step is to design a set of experiments and conduct them to get reliable values of the response. Factorial and fractional factorial designs are used for this purpose. The details of these designs are discussed by Box et al. (1978) and Rao and Iyengar (1984). The second step is to propose a suitable mathematical model to fit the experimental data and test for model adequacy through lack-of-fit F testa (Draper and Smith, 1981). The third and last step is to find out the optimum conditions of the independent variables which will produce the maximum (or minimum) value of the response. A t a point that is remote from the optimum, there is little curvature in the true response surface and first-order models will be satisfactory. In the vicinity of the optimum, higher order models will be required due to the presence of curvature in the response surface. RSM is sequential in nature. Some experiments are carried out, valuable information is gathered, and the next stage is designed for getting better values of the response. Generally, the method of steepest ascent (or descent) is applied for moving sequentially along the direction of maximum (or minimum) increase (or decrease) in response. 3.1. First-Order Design (First Move). 3.1.1. Identification of Variables and Their Levels. Important variables that influence the selective oxidation of n-butane to MA are n-butane concentration in the feed stream (q), reaction temperature (q), Mo/V weight ratio (xJ, Ce/V weight ratio (xJ, contact time (XJ, and P/V atomic ratio of the catalyst ( x 6 ) . The levels of these variables in the first stage of design are given in Table I. The responses are conversion of n-butane, selectivity to MA, and yield of MA. These are defined as
Table VIII. First-Order Response Surface Strategy (Second Move): Test for Model Adeauacv Conversion 4 = 64.837 + 2.210~1+ 7.175X2 - 3.325~3+ 4.712~4+ 4.362~5~~
5.825~6 (7)
ANOVA source due to regression residual pure error lack of fit total F&d
sum of squares
deg of freedom
2590.28 32.76 2.587 30.173 26 33.0 4
6 12 3 9 18
= 3.89;
F0.05(9,3)
mean square 0.862 3.352
= 8.81; model is adequate
Selectivity
3 = 65.025 - 2.287~1- 6 . 0 6 2 ~+~4.425X3
- 5.025~4- 4.612r5 + 5.386~8 (8)
ANOVA deg of source sum of sauares freedom mean souare 2367.67 6 due to regression 8.76 12 residual pure error 1.71 3 0.570 7.05 9 0.783 lack of fit 2376.43 18 total FCdd= 1.37; FO,o6(9,3) = 8.81; model is adequate Yield
4 = 40.794 + 0 . 7 8 1 ~ 1+ 0.831~2+ 0.46%~ + 0.106~4- 0.294~50.231X6
(9)
ANOVA deg of source sum of squares freedom mean square 19.94 6 due to regression 42.46 12 residual 0.387 3 0.129 pure error lack of fit 42.07 9 4.675 total 62.40 18 Fcalcd= 36.24; FO,O6(9,3) = 8.81; model is not adequate
Ji
X = YO conversion = no. of moles of n-butane consumed x 100 no. of moles of n-butane fed
Yield = 40.794 + 0.781~1+ 0.831~2 0.469~3+ 0.106~4- 0.294~50.231~6- 0 . 4 8 1 ~ 1 ~-20 . 5 8 1 ~ 2 ~-40 . 6 5 6 ~ 2 ~+5 0.431XzX6 +
+
S = 7'0 selectivity = no. of moles of MA formed x 100 no. of moles of n-butane consumed
0 . 8 5 6 ~ 3 ~ 4(10)
ANOVA deg of source sum of squares freedom mean square due to regression 56.65 6 5.75 12 residual 0.387 3 0.129 pure error 5.363 9 0.596 lack of fit 63.40 18 total F&d = 4.62; Fo.O5(9,3) = 8.81; model is adequate
Y = 5% yield of MA = no. of moles of MA formed no. of moles of n-butane fed x 100
xs
=-
100
Table IX. Calculation of the Path of Steepest Ascent factor XP
XI
x3 0.04 0.03 0.469 0.0141 0.00679
x4
base level 1.3 405 0.02 unit 0.4 15 0.01 estimated slope ( b ) 0.781 0.831 0.106 unit X b 0.3124 12.46 0.0011 change in level per 6 "C change in x 2 0.1504 6 0.00053 Path of Steepest Ascent as Represented by a Series of Trial Points expt no. trial point XI X2 X3 X4 45 46
47 48 49
1
2 3 4 5
1.4504 1.6008 1.7510 1.9016 2.0520
411 417 423 429 435
0.046 79 0.053 58 0.060 37 0.067 16 0.073 95
0.020 53 0.021 06 0.021 59 0.022 12 0.022 65
X5
18.5 3.5 -0.294 -1.029 4.495
X6
1.125 0.125 -0.231
-0.0289 -0.0139
X6
X6
18.005 17.510 17.051 16.556 16.061
1.1110 1.0971 1.0832 1.0693 1.0554
Ind. Eng. Chem. Res., Vol. 30, No. 8, 1991 1823 Table X. Results of Experiments along the Path of Steepest Ascent expt no. trial point exptl yield predicted yield 45 1 42.1 41.5 46 2 46.4 42.2 47 3 49.5 42.8 48 4 45.7 43.2 49 5 42.6 43.1 ~
3.1.2. First-Order 26-2Fractional Factorial Design (First Move). Since there are six variables, a 26-2 fractional factorial design around the base levels was employed. The base levels of the factors were chosen on the basis of an a priori knowledge. The design matrix is shown in Table 11. The basis for choosing x 5 as ~ 1 x and 3 x 6 as ~ 1 x 4 is that these combinations are expected to have minimum interactions. Experiments were carried out in a randomized sequence to avoid bias. Values of all the responses (conversion, selectivity, and yield) for the first move of experiments as planned in the design matrix are given in Table 11. Experiment 6 was repeated four times so as to obtain an estimate of the pure error. 3.1.3. Model Fitting. The following first-order model has been fitted to the experimental data. 9 = 60 + 61x1 + 62x2 + 63x3 + 64x4 65x5 + 66x6 (1)
+
where h0, hl, b2, b3, h4, b5, and 6 6 are the parameters, xi is the ith variable in its coded form, and j , is the predicted value of the response (conversion, selectivity, or yield). The coefficients can be estimated by the least-squares technique as
6 = (XTX)-lXTy (2) wher? X is the design matrix, Y is the vector of responses, and b is the vector of coefficients. The details of the method are given by Draper and Smith (1981). This is then tested for its adequacy by a lack-of-fit F test. With the experimental results as given in Table 11, the fitted first-order models for conversion, selectivity, and yield are given in Table 111. Models for conversion and selectivity are not required for optimizing yield. However, these are given to have information on the effect of different variables on conversion and selectivity. 3.1.4. Calculation of the Path of Steepest Ascent and Conduct of Experiments along This Path. Information obtained from the models was used to locate the path of maximum increase in yield. The calculation of the path of steepest ascent in which maximum value of the yield would be expected is given in Table IV. Additional experiments were then performed according to trial points 1-6. The experimental results and values predicted by the first-order model are given in Table V. It is observed that, after point 3, experimental yields were nearly constant. Here, though the yield of MA was becoming steady, the first-order model was fitting the data, indicating that the point might not be the true optimum. A possible reason for this type of ambiguous result may be due to the wrong choice of levels of variables in the first set of design. Therefore, a second set of Ze2 fractional factorial design (first order, second move) was done around the new levels of variables and conducted to study the effect of the variables at their new levels. 3.2. First-Order Strategy (Second Move). 3.2.1. Variables and Their Levels. The base levels of the factors for the second move were chosen on the basis of the optimum conditions obtained in the first move, the magnitude of the effects of factors as obtained from the coefficients of the models, and the constraint of not using high reaction temperature so as to avoid explosion. The
new levels of different variables for the second move using the first-order design are given in Table VI. 3.2.2. First-Order 2"2 Fractional Factorial Design (Second Move). The design matrix is exactly same as in the first move. The values of the responses are given in Table VII. 3.2.3. Model Fitting. The models that fitted the experimental values of the responses are given in Table VIII. The first-order model did not fit the yield data. Therefore, a higher order model including some important cross product terms was fitted. The cross products that are confounded with the main effect cannot be determined from here. The cross products that are confounded with the main effects can be identified as follows. For this type of design, generators are I = 135 = 146 = 3456. Neglecting third-order and higher order interactions, we get the confounding pattern as follows: 1=35=46;
3=15; 4=16;
5=13; 6 = 1 4
Besides the cross product terms that are confounded with the main effects, the other cross product terms that have a strong influence on yield are 612 = 4.481; 624 4.581; 6 2 5 = -0.656; b26 = +0.431; b34 = +0.856 ~
Therefore, a better model is proposed incorporating these terms with the first-order model. The model then takes the following form 9 = 60 + 61x1 62x2 + 63%3 + 64x4 6 5 x 5 612xlx2 + 624xZx4 + b25X2X5 + b 2 6 X 2 X S + b34X3X4 (3)
+
+
+
The inadequacy of the first-order model indicates the proximity to the optimum. 3.2.4. Calculation of the Path of Steepest Ascent and Conduct of Experiments Along This Path. The path of the steepest ascent to obtain maximum yield is calculated in Table 1X. Results of additional experiments performed at those points that would give better yields are given in Table X. The results show that at trial point 3 a maximum yield of MA (49.5%) was obtained. 4. Conclusions
The promoted VPO catalyst having a P/V atomic ratio of 1.08, Mo/V weight ratio of 0.06, and Ce/V weight ratio of 0.022 gave the maximum yield of MA at an inlet n-butane concentration of 1.75% and a W / F ratio of 17.051 g/ (mg-mol/s). The responses (conversion, selectivity, and yield) can be described in terms of fitted models. An increase in the value of reaction temperature, WIF, n-butane concentration in the inlet feed, or Ce/V weight ratio increases the conversion of n-butane at the cost of selectivity towards MA. On the other hand, an increase in the value of P/V atomic ratio of Mo/V weight-ratio increases the selectivity at the cost of conversion. This optimization technique selects those values of the variables which keep the conversion and selectivity at such levels which maximize the yield of MA.
Nomenclature bo, bl, ..., b6 = parameters in eq 1 F = total feed flow rate, mg-mol/s;also used for the ratio of two independent estimates of the same variance S = selectivity toward MA T = transpose of matrix W = weight of catalyst, g x i = ith factor in coded form X = conversion of n-butane
X = design matrix
1824
Ind. Eng. Chem. Res. 1991,30, 1824-1828
y = response Y = yield of MA Y = vector of responses = least-squares estimate Registry No. PO, 1314-56-3; MA, 108-31-6; Mo, 7439-98-7; Ce, 7440-45-1; H(CH2),H, 106-97-8; V, 7440-62-2.
Literature Cited Box, G. E. P.; Hunter, W. C.;Hunter, J. S. Statistics for Experimenters; Wiley: New York, 1978; Chapters 10 and 12. 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.; Trifiro, F. The Chemistry of Catalysts Based on Vanadium-Phosphorus Oxides Note IV Catalytic Behaviour of Catalysts Prepared in Organic Medium in the Oxidation of C4 Fraction. Appl. Catal. 1984,9, 191-202. Cavani, F.; Centi, G.; Manenti, I.; Trifiro, F. Catalytic Conversion of C, Hydrocarbons on Vanadium-Phosphorus Oxide: Factors Influencing the Selectivity of l-Butene Oxidation. Ind. Eng. Chem. Prod. Res. Deu. 1985a, 24, 221-226. Cavani, F.; Centi, G.; Trifiro, F. Study of n-Butane Oxidation to Maleic Anhydride in a Tubular Flow Stacked-Pellet Reactor. Influence of Phosphorus on the Selectivity. Appl. Catal. 1985b, 15, 151-160. Centi, G.; Fornasari, G.; Trifiro, F. On the Mechanism of n-Butane Oxidation to Maleic Anhydride: Oxidation in Oxygen-Stiochiometry-Controlled Condtions. J . Catal. 1984a, 89, 44-51. Centi, G.; Manenti, I.; Riva, A.; Trifiro, F. The Chemistry of Catalysts Based on Vanadium-Phosphorus Oxides Note I11 Catalytic Behaviour of Different Phases in 1-Butene Oxidation to Maleic Anhydride. Appl. Catal. 1984b,9, 177-190. Centi, G.; Trifiro, F.; Ebner, J. R.; Franchetti, V. M. Mechanistic Aspects of Maleic Anhydride Synthesis from C4 Hydrocarbons over Phosphorus Vanadium Oxide. Chem. Reu. 1988,88,55-80. Davies, 0. L. The Design and Analysis of Industrial Experiments; Longman Group: New York, 1978; Chapter 11.
Draper, N. R.; Smith, H. Applied Regression Analysis, 2nd ed.; Wiley: New York, 1981; pp 1-125. Hodnett, B. K. Vanadium-Phosphorus Oxide Catalysts for the Selective Oxidation of C4 Hydrocarbons to Maleic Anhydride. Catal. Rev. Sci. Eng. 1985, 27 (3), 373-424. Hodnett, B. K.; Delmon, B. Factors Influencing the Selectivity of Vanadium-Phosphorus Oxide Catalysts for n-Butane Oxidation to Maleic Anhydride. Appl. Catal. 1985, 15, 141-150. Hucknall, D. J. Selective Oxidation of Hydrocarbons; Academic Press: London, 1974; Chapter 4. Katsumoto, K.; Marquis, D. M. US. Patent 4 132 670, 1979. Khuri, A. I.; Cornell, J. A. Response Surfaces Designs and Analyses; Marcel Dekker: New York, 1987; Chapters 1, 2, 3, and 5. Kittrell, J. R.; Erjavec, J. Response Surface Methods in Heterogeneous Kinetic Modelling. Ind. Eng. Chem. Process Des. Deu. 1968, 7 (3),321-327. Moser, T. P.; Schrader, G. L. Selective Oxidation of n-Butane to Maleic Anhydride by Model V-P-0 Catalysts. J . Catal. 1985,92, 216-231. Rao, M. S.; Iyengar, S. S. In Computer Modelling of Complex Biological Systems; Iyengar, S. s.,Ed.; CRC Press: Boca Raton, FL, 1984; pp 29-53. Smith, J. M. Chemical Engineering Kinetics, 3rd ed.; McGraw-Hill: New York, 1981; Chapter 11. Varma, R. L.; Saraf, D. N. Oxidation of Butene to Maleic Anydride I. Kinetics and Mechanism. J . Catal. 1978,55, 361-372. Varma, R. L.; Saraf, D. N. Selective Oxidation of C, Hydrocarbons to Maleic Anhydride. Ind. Eng. Chem. Prod. Res. Deu. 1979, 18 (11, 7-13. 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 ) , 765-772. Wenig, R. W.; Scharder, G. L. Vanadium-Phosphorus-Oxygen Industrial Catalysts for n-Butane Oxidation: Characterization and Kinetic Measurements. Ind. Eng. Chem. Fundam. 1986, 25, 612-620. Receiued for review July 2, 1990 Reuised manuscript received February 6, 1991 Accepted February 12,1991
Selective Oxidation of 11 -Butane to Maleic Anhydride. 2. Identification of Rate Expression for the Reaction Shyamal K. Bej and Musti S. Rao* Department of Chemical Engineering, Indian Institute of Technology, K a n p u r 208 016,
U.P., India
The modeling of the selective oxidation of n-butane to MA has been done in a systematic manner. Both Langmuir-Hinshelwood and redox models have been proposed and tested with differential rate data. Four redox models have been fitted to the experimental data. Discrimination among these models has been achieved with the use of a sequential experimental design. The redox model having first-order dependency on n-butane partial pressure and zero-order dependency on oxygen partial pressure was found to be the most appropriate. 1. Introduction
The selective oxidation of n-butane to maleic anhydride (MA) is a complex reaction from the mechanistic and modeling point of view. Besides the main reaction of selective formation of MA from n-butane, two other side reactions, viz., the complete oxidation of n-butane and of MA to oxides of carbon, take place simultaneously. Different types of models have been proposed by different investigators (e.g., Wohlfahrt and Hofmann (1980), Centi et al. (19851, Buchanan and Sundaresan (1986), Lerou and Weiher (1986), and Schneider et al. (1987)). Wohlfahrt and Hofmann (1980), Centi et al. (1985),and
* T o whom correspondence should be addressed. 0888-5885/91/2630-1824$02.50/0
Buchanan and Sundaresan (1986) have proposed different forms of redox models for this reaction. An adsorption model has been proposed by Lerou and Weiher (1986). Though a number of studies are available on the modeling of this reaction, there is some doubt regarding the order of reaction with respect to oxygen and also the activation energy. In this present work we have tried to clarify some of these doubts by systematic modeling studies. The reaction has been studied over a Mo- and Ce-promoted VPO catalyst (P/V atomic ratio = 1.0s; weight ratio of Mo/V = 0.06; weight ratio of Ce/V = 0.0221, the optimum composition of which was established in part 1 of this series. The catalyst had a surface area of 20 m2/g. The details of catalyst preparation were also given in part 1 of this series. 0 1991 American Chemical Society
