Transient Kinetics of N-Butane Oxidation to Maleic Anhydride Over a VPO Catalyst

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Transient Kinetics of n-Butane Oxidation to Maleic Anhydride over a VPO Catalyst

Xiao-Feng Huang, Cheng-Yue Li, and Biao-Hua Chen The Key Laboratory of Science and Technology of Controllable Chemical Reactions, Ministry of Education, Beijing University of Chemical Technology, Beijng 100029, China P. L. Silveston Dept. of Chemical Engineering, University of Waterloo, Waterloo, Ontario, Canada

Transient reaction kinetics is usually needed to model the reactor system operated under unsteady-state mode to impro®e the performance of the catalyst packed in such a reactor system and to match operating conditions of different reaction steps in this system. The transient reaction kinetics of n-butane oxidation to maleic anhydride o®er a commercial VPO catalyst was systematically studied, using online MSrGC measurements, transient response, and forced periodic operation methods. A simplified scheme with respect to the reaction network is proposed based on the pre®ious studies and the obser®ations in this work. Combining the results on the roles of lattice and adsorbed oxygen in the reaction process with the data obtained in transient response experiments, a transient kinetic model taking account of the oxygen storage and diffusion in the bulk-phase of the catalyst was introduced and its parameters were estimated. The model was examined by the transient response experimental data and time-a®erage concentrations of butane and maleic anhydride under forced unsteady-state operation.

Introduction

The selective catalytic oxidation of hydrocarbon is an im- were proposed based on a triangular reaction network portant reaction for producing oxygenates, mostly using com- ŽEscardino et al., 1973; Wohlfahrt et al., 1980; Buchanan and plex oxides of metals as catalyst and running according to the Sundaresan, 1986; Schneider et al., 1987; Sharma et al., 1991; redox mechanism. The direct oxidation of light alkanes to Bej and Rao, 1991. . Some information on the reaction rate oxygenates, among all of the selective oxidation technologies, models is summarized in the Appendix. Recently, Contractor is particularly attractive because alkanes cost less than other et al. proposed a new unsteady-state reaction mode to syn- feedstock, such as olefins and aromatics. Up to now, the par- thesize maleic anhydride from n-butane by spatial separation tial oxidation of n-butane to maleic anhydride over vanadium of n-butane oxidation and VPO catalyst reoxidation in a CFB phosphorus oxideŽ. VPO catalyst is the only alkane selective reactor system and demonstrated its advantages over the tra- oxidation reaction used in the industry. This reaction system ditional technologiesŽ Contractor et al., 1987, 1994; Contrac- has been studied extensively in recent years because of its tor, 1999. . Temporal separation of selective oxidation of n- significance and because it is representative of the reaction butane and catalyst reoxidation in the fixed-bed reactors of mechanism. Besides the structure of the reaction network different scales was systematically investigated as a possible Ž.Centi et al., 1988; Xue and Schrader, 1999 , the identifica- alternative method, and obvious improvements in selectivity tion of the active phasesŽ. Hutchings et al., 1994 , and reac- and yield were observedŽ Huang, 1999; Huang et al., 2002a,b; tion mechanismŽ Centi et al., 1984; Schuurman and Gleaves, 2001. . In fact, this is another variety of unsteady-state opera- 1994. , the kinetic features of the VPO catalyst, which is tion of a chemical reactor system where the catalytic cycle needed to model the reactor, has been investigated in a se- involved is precisely controlled. The selective oxidation of n- ries of works. In most of these studies, the steady-state kinet- butane to maleic anhydride in the riser of a CFB reactor sys- ics was experimentally studied, and intrinsic rate equations tem was modeled by Pugsley et al.Ž. 1992 , based on steady- state reaction rate equation proposed by Centi et al.Ž. 1985 Correspondence concerning this article should be addressed to C.-Y. Li. and a correction factor corresponding to catalyst deactivation

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with a decrease in oxygen loading. The initial oxygen loading assumed that the surface lattice oxygen was the only source of the catalyst entering the riser and its effect on the oxida- of active oxygen and proposed a reaction network consisting tion kinetics were not considered in this work. Golbig and of three parallel reactions between the surface lattice oxygen WertherŽ. 1997 performed a systematic study of the selective and gas-phase n-butane to directly form maleic anhydride,

synthesis of maleic anhydride by spatial separation of n- CO2 , and CO, taking the lattice oxygen diffusion in the butane oxidation and catalyst reoxidation. In their model, with bulk-phase of the catalysts into account. respect to the riser, the butane conversion rate was assumed As a powerful tool for studying the mechanisms and the to be proportional to the n-butane concentration and the av- transient kinetics of heterogeneous catalytic reactions, a tran- erage oxygen loading, which was experimentally correlated sient-response method has been proposed by BennettŽ. 1976 , with the regeneration duration in the fluidized-bed regenera- Kobayashi et al.Ž. 1974, 1982a,b , and more recent works. The tor for the catalyst specified. purpose of this work is to develop a transient kinetic model It should be emphasized that it is very important to take using the transient-response and periodic-operation methods. the dynamic behavior of the reaction steps in gaseous and The model is generally applicable to the unsteady-state oxi- solid phases into account for unsteady-state operation of the dation of n-butane to maleic anhydride by spatial or tempo- chemical reactor system so as to match different reaction steps ral separation processes of n-butane oxidation and catalyst with respective operation conditions, and to improve the reoxidation. property of the catalyst used in the reactor. Especially for the selective oxidation reaction of butane to maleic anhydride, it is generally accepted that the ability of an oxide catalyst to Experimental Studies store lattice oxygen in its bulk phase and release it for sur- Catalyst face reactions plays a very important role in the dynamic characteristics of the catalystŽ. Arnold and Sundaresan, 1987 The catalyst used in this study was a domestic commercial and of the reactor loaded with this catalystŽ Arnold and Sun- VPO catalyst developed by the Beijing Research Institute of daresan, 1989.Ž . Schuurman and Gleaves 1994. investigated Chemical Technology. It had undergone a 3,000-h steady-state the transient kinetics of n-butane oxidation to maleic anhy- run in a single-tube wall-cooled reactor and gave a selectivity to maleic anhydride of approximately 63ᎏ60% at 85ᎏ90% dride over a ‘‘reactor-equilibrated’’Ž. VO22 P O 7 catalyst, using the temporal analysis of the productsŽ. TAP reactor sys- conversion. XRD measurement showed that the catalyst was r tem and roughly lumped the conversion of n-butane into a mainlyŽ. VO22 P O 7 . Chemical analysis indicated a P V ratio single reaction. It was shown that the selectivity is strongly of 1.0 and vanadium oxidation state of 4.10. The catalyst influenced by the oxidation state of the VPO catalyst and packed in the microreactor was crushed and sieved into 60 to that selectivity increases with the oxygen availability. Further 80 meshes. study indicated that the activation energy for the lumped reaction of n-butane conversion varied between 53.8 and 98.8 kJrmol, depending on the oxidation state of the catalyst Experimental setup Ž. Schuurman and Gleaves, 1997 . The studies for identifying the structure of the reaction None of the just-mentioned studies were based on the more network, transient response experiments, temporal separa- rigorous transient reaction kinetics involved in the whole re- tion experiments of selective oxidation and reduction of the duction and reoxidation processes of the catalyst, and took catalyst, and forced unsteady-state operation of a fixed-bed no account of the diffusion of the subsurface lattice oxygen. reactor were performed with the experimental setup shown Ž. Recently, Barteau and Wang 2000 reported the transient in Figure 1. This laboratory system comprised three parts: kinetics of the reduction of an equilibrated catalyst in bu- the gas-blending station for preparing the gas mixture with r tane He, and concluded that the reduction of the subsurface different compositions, the assembly of the fixed-bed mi- layer of the catalyst is not limited by lattice oxygen diffusion. croreactor᎐electric oven with a multichannel temperature Ž. Coulston et al. 1997 explored the transient rate of maleic controller and an on-line mass-spectrometer᎐gas chromato- ␣ r anhydride formation when either 142-VOPO SiO or graph analysis system. Ž. r VO22 P O 7SiO 2 catalyst was exposed to n-butane, using The gas-blending station consisted of five mass flowmeter time-resolved in situ X-ray absorption spectroscopy. Their re- controllers, two three-way switching valves, and one four-way sults showed that the rate of maleic anhydride formation is 5q switching valve. The two-position four-way valve was used to proportional to the rate of decay of the V species in the switch between the reactant mixtures for the selective oxida- catalyst, even though the rate data were quantitatively not tion of n-butane and reoxidation of the catalyst. The opening Ž. correlated. Huang et al. 2002a investigated reoxidation ki- of Valve 6 must be preregulated to ensure flow stability be- netics of a VPO catalyst and proposed a transient rate model fore and after switching. Valve VS1, a two-position three-way Ž. of this process. Mills et al. 1999 reported the research re- valve, was used to switch between the standard gas with its sults on the redox kinetics of the VOPO4 catalysts with n- composition preblended and He, as the sweeping gas. Valve butane and oxygen using the TAP reactor system. For reoxi- VS2 was used to switch between the reactant mixture and the dation processes of reduced VOPO4 catalysts, the transient sweepingrstandard gas. responses to O2 step-up could satisfactorily be modeled by a The main parts of the microreactor assembly are one-dimensional axial dispersion model of the fixed-bed mi- ⅷ croreactor used in that work, taking the single-surface oxida- Fixed-bed microreactor: 4.7 mm ID; 302 mm length tion reaction with subsurface oxygen diffusion into account. ⅷ Electric oven: 800 W To satisfactorily interpret the transient responses for the re- The temperature of the reaction section was measured with

duction processes of the VOPO4 catalysts with n-butane, they a microthermol couple inserted in its center through a small AIChE Journal April 2002 Vol. 48, No. 4 847 中国科技论文在线 http://www.paper.edu.cn

difference between the two methods was less than 3%. This fact suggests that the total acids measured by using the titration method is almost completely maleic acid, even though trace amounts of other species, such as acetic acid and acrylic acid, were also detected by an online mass spectrometer. The transient concentrations of the maleic anhydride in the gas phase involved in the transient-response experiments had to be calculated according to the first method just mentioned. Further details of the analysis are presented elsewhere Ž.Huang, 1999 .

Preliminary experiments Using MS and GC, experiments were performed to determine suitable analytical conditions, to confirm the absence of internal and external diffusion effects on the reaction kinetics and to determine the flow pattern and the hydrodynamic lag time from the reactor outlet to the MS. It was shown that the assumptions of step-change in feed composition and plug Figure 1. Experimental setup. flow are acceptable, and that the hydrodynamic lag time from the switching values of feed composition to the detector of y1 jacketed tube and controlled with a temperature-programma- the MS was about 20 s at a space velocity of 4,000 h . ble controller. Temperature fluctuation in the reaction section was no more than 1ЊC, so it was assumed to be an Experimental procedures isothermal operation. The reaction section of the microreac- For temporal separation of butane selective oxidation in r tor was packed with about 0.6 g of 60 80 mesh catalyst, and the fixed-bed reactors, the ratio of butane to oxygen is gener- quartz chips were loaded in both sides of the reaction section ally switched between higher and lower valves. To compare to preheat the feed gas and to stop possible flames when the the time-average performances of the unsteady-state opera- concentration of butane is higher than its low flammability tion with those of the steady-state operation of industrial limitŽ. LFL . The body of the reactor was grounded. wall-cooled fixed-bed reactors, where the feed concentration The analysis system consisted of an on-line mass spectrom- of butane is about 1.5% and approaches its low flammability eter, VG70-SE, that was connected by a capillary tube, which limitŽ. LFL in air, of 1.6%Ž Crowl et al., 1990. , the periodic was as short as possible, to the microreactor outlet, and a operation experiment had to be performed by switching the series of two online GCs, which were used to quantitatively butane concentration between those that were higher and analyze the stationary or transient composition of the gas lower than LFL. Therefore, transient kinetic information was mixture. The first on-line GC was equipped with a 12-chan- needed in the area that is higher than LFL and some tran- nel gas sampler, which could collect 11 samples during the sient-response experiments in the area had to be done. It period and analyze them later. should be emphasized that these experiments can only be undertaken in a microreactor, and some measures must be taken Products analysis to prevent fires and explosions, for example, adding quartz A series of two GCs with a multichannel sampler was used chips to both sides of the catalyst bed and grounding the re-

to monitor the transient concentrations of n-butaneŽ. C4 , actor body as described in the section on the experimental CO22 , CO, and O . The first GC, with a 3.5-m-long GDX-501 setup. For the unsteady-state operation of a large-size fixed- column, was used to analyze C410 H and CO 2 , while the sec- bed reactor and with a composition modulated reaction sys- ond GC, with a 3.0-m-long 13X molecular sieve column was tem, it is strongly suggested that the concentration of oxygen

used to analyze O22 , N , and CO. Two gas chromatographs in the butane-rich duration should be lower than the mini- were equipped with thermal conductivity detectorsŽ. TCD , mum oxygen concentrationŽ. MOC required to propagate a ᎐ ᎐ and helium was supplied as carrier gas. The signals of the flame, such as 10.4% O2 for a butane oxygen inert mixture GCs were calibrated with standard gases. The relative correc- Ž.Crowl et al., 1990 .

tion factor for O2241 , CO, CO , and n-C H0 were 1.0, 1.02, Three different types of experiments were undertaken in 0.85, and 0.53, respectively. The tail gas from the GCs was this investigation. To identify possible intermediates and get introduced into a water-absorber to collect maleic anhydride, a greater understanding of the reaction network scheme, a then flowed through a soap-film meter to measure the flow group of steady-state experiments with a mixture of 3% n- rate. The solution containing maleic anhydride was sampled butane, 15% oxygen, and balanced helium as feed gas were from the absorber and titrated with the standard solution of conducted at 372ЊC, 402ЊC, and 440ЊC, respectively. The sam- NaOH. The stationary and time-average concentrations of ples of product gas were analyzed by GC-MS to detect prod- maleic anhydride in the product gas could be calculated ac- ucts in the n-butane oxidation reactions. To probe the possi- cording to either the titration values of the maleic anhydride ble decomposition products of maleic anhydride, a few ana-

solution or the gas-phase concentrations of n-butane, CO2 , lytically pure maleic anhydride crystals were packed in a small CO, and O2 , and the flow rates of the feed and tail gases, container situated upstream of the microreactor. The con- based on the carbon balance. It was verified that the relative tainer was placed in an additional oven and heated to subli-

848 April 2002 Vol. 48, No. 4 AIChE Journal 中国科技论文在线 http://www.paper.edu.cn

mate the maleic anhydride crystals. Helium gas was passed Further sequential transient experiments provided more through the container and into the microreactor where the information relevant to the roles of adsorbed and lattice oxy- reaction temperature was kept at 402ЊC. The effluent of the gen, and it was concluded that the surface lattice oxygen and reactor was also analyzed by GC-MS. adsorbed oxygen are directly responsible for the formation of

To determine the parameters in the transient kinetic model, maleic anhydride and CO2 , respectively. The adsorbed and two groups of transient-response experiments were carried lattice oxygen on the surface of the catalyst can be trans- out at different temperatures or different feed conditions. formed to one another, so the diffusion of the lattice oxygen ᎏ q Three kinds of gas mixture 1.55% C4221.62% O , 2.78% in the bulk phase of the catalyst will indirectly influence the q q ᎏ C4215.10% O , and 3.34% C 4222.03% O were used in response behavior of COx for a long time under transient these experiments, and the temperature varied from 400ЊCto reaction conditions. More details can be found in another 430ЊC. The flow rate was always kept at 20 mLŽ. STP rmin, articleŽ. Huang et al., 2002b . and pressure in the reactor was kept at 200 kPa. To stabilize and keep the same oxidation state of the catalyst, before each Transient response and period operation experiment the catalyst was pretreated at 415ЊC for one hour The results of transient responses to step-change in feed with 20% oxygen-containing gas, after which a reaction mix- composition at different temperatures and different feeds arc ture of 1.5% n-butane and 20% oxygen was introduced to the shown in Figures 2 and 3. The concentration response of MA reactor for one hour. After the reactor was swept with he- was rapid right after switching from oxygen-containing gas to lium gas, the reaction temperature was regulated to a speci- butane-containing feed in each experiment, but it soon de- fied value and the catalyst was reoxidized at this temperature clined to a steady level. This suggests the possibility of im- for a given duration with 20% oxygen-containing gas. Finally, proving the reactor performance by periodically changing feed the feed was switched to a mixture of butane and oxygen. composition. The periodic-operation experiment was performed to ex- During the periodic-operation experiment, the time-aver- amine the transient model, typically using the strategy of cy- age concentration of each component approached a nearly cling between 2.78% C q15.45% O qbalanced He and 20% 42 invariant value after about 20 cycles, which suggested that O qbalanced He for a period of 6 min at 415ЊC. An online 2 the cycling steady state was achieved. The transient concen- mass spectrometer was used to monitor the concentration re- tration responses of MA monitored by the on-line mass spec- sponses and confirm that the cycling steady state was trometer are shown in Figure 4, while the quantitative mea- achieved. Then, the reactor effluent was sampled and quanti- surements at cycling steady state by a series of two GCs are tatively analyzed with GC. shown in Figure 5. The overshoots of MA can be seen in both figures, but the one in Figure 4 is much stronger than Results and Discussion that in Figure 5. This probably can be explained by the rela- Structure of the reaction network tively long time interval between the two samplings in GC measurements, even when the multichannel sampler is used. For the steady-state reaction at 372ЊC, 402ЊC, and 440ЊC, Even so, the results of the measurement with GC are really and a feed with 3% n-butane, 15% oxygen, and balance he- the time-average values for a short time. Thus, the measure- lium, the major products were found to be carbon oxides, ment by an online mass spectrometer is closer to the real water, and maleic anhydride. Trace amounts of butene, acetic transient response. However, because of the difficulty in acid, acrylic acid, furan, and benzene were also found. While q Њ for an oxygen-poor feed of 12% C425% O at 402 C, besides the products just mentioned, trace amounts of 2,5-dihy- drofuran and methylbenzene were also observed. However, butadiene, crotonaldehyde, and methyl vinyl keton, which Centi et al.Ž. 1984 observed under similar conditions, were not found. The catalytic decomposition products of maleic anhydride at 402ЊC were found to be trace amounts of acetic acid, acrylic acid, benzene and methylbenzene, carbon dioxide, and water. Only the small amounts of carbon dioxide and water can be explained by using a maleic anhydride-containing gaseous mixture without oxygen as the feed to the catalytic decomposition. This suggests that a series of deeply oxidized reactions of maleic anhydride occurred. Observed facts relevant to the kinds and the amounts of the intermediate products from n-butane to maleic anhy-

dride, COx and H2 O are basically consistent with most of the previous works. Considering that all products, except maleic anhydride, CO , and H O, appear in trace amounts, it can x 2 Figure 2. Concentration response of MA to step-change be seen that this reaction network pattern is factually very in feed composition at different temperatures. similar to those presented by Cavani and TrifiroŽ. 1994 and q q Ž. Feed composition was switched to 1.5%C4220%O Gleaves et al. 1988 , from the viewpoint of a simplifying ki- balance helium after a 5-min reoxidation at different reac- netic representation. tion temperatures.

AIChE Journal April 2002 Vol. 48, No. 4 849 中国科技论文在线 http://www.paper.edu.cn

Figure 3. Transient response of MA to step-change in Figure 5. Product concentration at cycling steady state feed composition at different concentrations for feed-composition modulation. of butane. Transient concentration of different species in a period duration of 6 min. Feed composition was switched from 20% Feed was switched to the compositions given after a 5-min O qbalance helium to 2.78% C q15.50% O qbalance Њ 242 reoxidation at 415 C. helium at the third minute reaction temperature: 415ЊC.

quantifying MA by online mass spectrometer, the GC mea- CH lqOl™CH lqHOq l3b surement results are still used for estimating kinetic parame- 48Ž. Ž.46 Ž. 2 Ž. Ž . q ™ q ters. CH46Ž. l Ol Ž.CHOl46 Ž. Ž.l3Ž.c q ™ q q Transient kinetics model CHOl46Ž.Ol Ž.CHOl44 Ž.HO 2 Ž.l3d Ž . q ™ q q As a result of the investigation of transient reoxidation ki- CHOl44Ž.3O Ž. l CHO423 Ž. l HO 2 3l Ž. Ž 3e . netics for the VPO catalyst used in this work, a reoxidation k |4 q dynamic model, including the following surface reaction steps CHO423Ž. l X CHO 423 Ž.l4Ž. proposed by Arnold et al.Ž. 1989 , has been adopted here k 4

k1 It is well known that the initial oxidation rate of 1-butene O q2s6 2Os 1 2 Ž. Ž. Ž. is several tens times higher than that of n-butaneŽ Cavani k 2 and Trifiro, 1994. . Moreover, all products except maleic an- OsŽ.q Ž.l | Ol Ž.q Ž.s2Ž. X hydride, CO , and H O are almost trace amounts, just as we k 2 x 2 observed in our experiments. This suggests that the surface

Integrating the reoxidation kinetic model with the reaction concentrations of C48 HŽ. l , C 46 H Ž. l , C 46 H O Ž. l , and C44 H OŽ. l network pattern and the roles of oxygen species discussed in Reactions 3a᎐3e can be considered to be zero, and Reac- earlier, a set of surface reactions involved in the formation of tion 3a dominates the sequence of processes from Reactions maleic anhydride can be devised as follows 3a to 3e. Obviously, this is also the reason why the processes of releasing intermediates from the surface of the catalyst to q | q CH410 OlŽ.CH48 Ž. l HO 2 Ž.3a the gas phase have been left out from the sequence just mentioned. Therefore, by summing all the reactions from Reac- tions 3a to 3e, a lumping reaction should be obtained as follows

k 3 q 6 q q CH410 7OŽ. l C423 H OŽ. l 4H 2 O 6l Ž. Ž 3 .

The rate expression of this lumped reaction can be deduced according to the mass action law based on ReactionŽ. 3a . Because MA could be adsorbed on the surface of the cata-

lyst, whereas C410 H could not be, and the adsorbed oxygen is responsible for the formation of COx Ž. Huang et al., 2002b , a set of formation reactions of COx can be approximated as follows

k5 Figure 4. Transient concentration response of MA un- q 6 q q CH410 9OŽ. s 4CO 5H2 O 9sŽ. Ž. 5 der periodic operation monitored by online k 6 q 6 q q mass spectrometer. CH410 13OŽ. s 4CO25H 2 O 13Ž. s Ž. 6

850 April 2002 Vol. 48, No. 4 AIChE Journal 中国科技论文在线 http://www.paper.edu.cn

s Table 1. Overall Reaction Scheme and Rate Equations RMA r4 Ž.18 Reactions Rate Expression sy RO1r Ž.19 ␪ 2 5 ky5C S 4␪ s q 6 ky6C S RCO4r 54r 7 Ž.20 ␪␪4 7 k7 MA S ␪␪ R s4r q4r 21 8 k8 MA S CO2 6 8 Ž. R s4r q5r q5r q r q r 22 HC2 3 5 6 7 8 Ž.

k s y y y y y 7 R␪ 2r12r 9r 513r 62r 76r 8 Ž.23 q 6 q q q S CHO423Ž. l 2O Ž. s 4CO HO2 Ž.l 2s Ž. Ž 7 . R␪ s r y7r y r 24 k 8 L 23D Ž. q 6 q q q CHO423Ž. l 6O Ž. s 4CO2HO 2 Ž.l 6s Ž. Ž 8 . R␪ s r y r y r y r 25 L , MA 3478 Ž. Further simplifications are needed to model the transient s R␪ rD Ž.26 kinetic behavior of the reaction-diffusion system, owing to the B undetermined structure of the elementary step sequence for In these equations the ␪ and ␪ are the coverage of ᎐ SS,V Reactions 5 8 and the complexity of oxygen diffusion in the adsorbed oxygen sites and vacancies on the surface of the bulk phase of the catalyst. The following basic assumptions ␪ ␪ ␪ catalyst where oxygen may be adsorbed; LL, ,VL, and , MA are adopted to simplify the descriptions of the reaction and are, respectively, the coverage of surface lattice oxygen, sur- transport processes in the fixed-bed microreactor: face lattice oxygen vacancies, and surface lattice oxygen Ž.1 The reaction rates of Reactions 5᎐8 can be expressed ␪ bonded with the precursor of maleic anhydride; and B is a as the rates of their controlling steps, as listed in Table 1; ratio of lattice oxygen concentration in bulk phase to its satu- Ž. 2 The diffusion rate of the lattice oxygen in the bulk ration value. They are assumed to be subject to phase of the catalyst is considered to be directly proportional ␪ ␪ s to the difference between LBand ; in other words, rD ␪ q␪ s12Ž.7 k Ž.␪ y␪ ; SS,V DL B ␪ q␪ q␪ s Ž.3 The axial dispersion in the catalyst bed is negligible, LL, MA L,V 12Ž.8 and a one-dimensional plug-flow model for the fixed-bed mi- ␪ ␪ croreactor can be adopted. The S,VL,Vand in all of equations with r and R terms can Obviously, the second of these assumptions masks partly be easily deducted using these two equalities. the truth. It followed an example where a simplified Based on the principle of differential mass balances of the lumped-parameter model, which included the bulk phase dif- species considered, the transient model of the fixed-bed mi- fusion of the nitrogen atom in an iron-based catalyst, success- croreactor used in this investigation can elicit a set of partial fully predicted the transient responses to step changes in feed differential equations with corresponding initial and bound- composition and the time-average performance of unsteady- ary conditions state ammonia synthesisŽ. Li et al., 1985 . A more rigorous Ѩ Ѩ consideration should lead to a set of PDEs, as was done re- NiiN q F␳ s F␳ RyŽ.,␪ 0FtFT ,0FwFw cently in Mills et al.Ž. 1999 . Ѩ t B Ѩ w Bi 0 Based on these assumptions, all the rates of eight reactions Ž.29 can be respectively described by the following elementary Ѩ␪ processes j Ž.N s RyŽ.,␪ Ž.30 tjj Ѩ t s ␪ 2 r11Oky S, V Ž.9 s = 2 NtifiŽ.,0 N y , f Ž.31 s ␪␪ r22k SL,V Ž.10 ␪ s␪ swxT Ž.0,w 0 0.0, 0.1, 0.1, 0.0 Ž.32 s ␪ r33Cky L Ž.11 4 where s ␪ y ␪ r44k L, MA ky4 MA L,V Ž.12 T Ns N , N , N , N , N r s ky ␪ 13 C42O MA CO CO2 55C4 S Ž. ␪ swx␪ , ␪ , ␪ , ␪ T r s ky ␪ 14 SLBL, MA 66C4 S Ž. A study of the reoxidation kinetics of the VPO catalyst used r s k ␪␪ Ž.15 77L, MA S in this work showed that the catalyst weight increase after s ␪␪ Њ q r88k L, MA S Ž.16 reaction equilibration at 415 C in the mixture, 1.5% C4 20% q O2 balance He, and a successive reoxidation for several ; Њ The transformation rates of different species in the gas minutes at 400 430 C in the mixture, 20% O2 and balance phase, bulk phase, and on the surface are, respectively He, as done in the transient-response experiments in this investigation, was about a tenth part of the weight increase of the completely reoxidated catalyst relative to the completely R sy r y r y r Ž.17 C3564 reduced catalystŽ. Huang et al., 2002a . Therefore, the initial

AIChE Journal April 2002 Vol. 48, No. 4 851 中国科技论文在线 http://www.paper.edu.cn

␪ ␪ value for BLand may be approximately considered 0.1 as expressed is Eq. 32. The inlet concentration of each gaseous component was varied by a step change in feed, while the concentrations of adsorbed components were assumed to be independent of the reactor coordinate and were almost equal to the measurement after the steady-state reaction. ␳ The density of the catalyst bed, B, and the mass of the = 3 r 3 loaded catalyst, w0, are known to be 0.91 10 kg m and 0.5997 g, respectively. The concentrations of active sites were estimated in the reoxidation kinetics studiesŽ Huang et al., 1999.

y2 y2 N ␪ s4.73=10 molrkg, N ␪ s9.51=10 molrkg, t, SLt,

y1 N ␪ s3.10=10 molrkg t, B

The variable-step finite difference method was used to solve Figure 6. Simulated results based on the bulk inventory the partial differential equations. It was numerically stable, model vs. experimental data. even when steep concentration gradients occurred in the reactor. In this study, the parameters in the Arrhenius expressions for k were estimated from transient experimental data, i A residual distribution analysis is shown in Figure 8. Almost which spanned the range of feed concentrations and temper- all of the relative deviations between simulated and experi- atures. A distributed hybrid genetic algorithm developed by mental transient responses occur in an area limited by two HuangŽ. 1999 is employed to minimize the objective function straight lines of ysŽ.1"0.15 x, where y and x denote simu- defined as lated and experimental transient responses, respectively. The 2 average relative deviation is 4.01%. s EMMy r RSS ÝÝÝÄ4yti, giŽ.yt, gi Ž.y ,max Ž35 . The model was also examined using the data of the forced g ti periodic operation. As shown in Figure 9, the simulated results approaching a cycling steady state are very close to the where y stands for the concentration of each component, and experimental data. g, t, and i stand for the ordinal numbers with regard to the As shown in Figure 10, a kinetic model that is the same as transient-response experiments, the sampled time, and the the previously described model except that the oxygen diffu- species in tail gas. sion in the bulk phase of the catalyst was not taken into ac- The estimated transient kinetic parameters are summa- count did not fit the transient response data. This suggests rized in Table 2. that the lattice oxygen stored in the bulk phase of the VPO Comparisons between the model predictions of the tran- catalyst and its diffusion play a very important role in transient responses to step-change in feed composition and the sient reaction kinetics, which is a similar conclusion to that experimental data are shown in Figures 6 and 7. It can be drawn by Mills et al.Ž. 1999 . seen that the model predictions are in good agreement with the experimental data, except for the higher overshoot in the simulation. However, the profiles of the model predictions of the transient-response curves are qualitatively in very good agreement with the MS measurements. Statistical measures for the proposed model and its relevant parameters were obtained to identify their significance.

Table 2. Estimated Reaction Kinetics Parameters Under Unsteady State Rate Exponential Factors Activation Energy ConstantŽ.Ž. molؒgy1 ؒsky1 Jؒmoly1 = 3 k1 1.402 10 56.8 = 2 k2 8.774 10 58.9 = 4 kD 4.596 10 98.7 = 4 k3 7.038 10 66.4 = 3 k4 1.047 10 45.4 X = 4 k4 1.260 10 67.0 = 8 k5 4.396 10 131.5 = 8 k6 7.254 10 133.3 = 5 k7 3.866 10 87.0 Figure 7. Simulated results vs. experimental data with k 7.353=105 90.4 8 different feed composition.

852 April 2002 Vol. 48, No. 4 AIChE Journal 中国科技论文在线 http://www.paper.edu.cn

Figure 8. Residual distribution for the model with bulk inventory. Figure 10. Experimental data vs. simulated results without bulk inventory. Table 3 shows some results of activation energies obtained in previous studies, using a TAP reactor system. As a comparison, our results gained in this work are also listed. Taking This transient kinetic model was satisfactorily used to pre- the strong dependence of the activation energy on the oxida- dict the transient and time-average performances of the tion state of the catalyst and the significant differences in the fixed-bed reactors of different scales for maleic anhydride basic assumptions adopted in these works into account, it is synthesis from n-butane, and the relative results will be re- believed that there is good agreement between the values of ported in other articles. the activation energy with respect to the reaction from n- butane to maleic anhydride obtained in these works. As far Conclusions as concerns the activation energies with respect to other reactions, as indicated in Table 3, there are obvious differences Based on the previous studies and some information with between the results from this work and the previous study. respect to the composition of the reaction products obtained These differences can probably be attributed to the differ- in this work, a reaction network scheme of the n-butane oxi- ences in the kinds of the catalyst employed, the structure of dation to maleic anhydride is proposed. reaction network, the kinds of active oxygen, and the forms Transient-response experiments in which the composition of reaction rate expressions accepted. It is generally believed of tail gas using by an online GC equipped with a multichan- that the activation energy of the complete oxidation reaction nel sampler were carried out. Starting from the data col- is greater than that of partial oxidation in a selective oxida- lected in the experiments, and integrating the results in this tion reaction of a hydrocarbon, as seen in some kinetic stud- work with respect to the reaction network and the roles of ies for n-butane oxidation to maleic anhydride over VPO cat- different kinds of oxygen in the reaction processes, a tran- alystŽ. Bej and Rao, 1991; Buchanan and Sundaresan, 1986 . sient kinetic model of n-butane oxidation to maleic anhy- Our results seem consistent with this viewpoint. dride over a commercial VPO catalyst has been developed. This model considers that the molecular oxygen in the gas phase is dissociatively adsorbed on the surface of the catalyst and can be further transformed to lattice oxygen, and that the n-butane in the gas phase reacts directly with the lattice and adsorbed oxygen on the surface to form partial and complete oxidation products, respectively. Moreover, the storage and diffusion of the lattice oxygen in the bulk phase of the catalyst must be taken into account. A set of transient kinetic parameters has been estimated. It has been shown that the model can fit the data coming

Table 3. Activation Energy of n-Butane Oxidation to Maleic Anhydride E Ž.kJؒmoly1 ™ ™ ™ ™ Source C4424MA C CO C CO MA COx Schuurman and 53.8;98.8 ᎏᎏ ᎏ GleavesŽ. 1997 Figure 9. Simulation vs. experimental data under com- Mills et al.Ž. 1999 71.0 13.0 67.0 ᎏ ; position modulation. This work 66.4 133.3 131.5 87.0 90.4

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from the transient-response experiments and predict the data over Phosphorus Vanadium Oxide,’’ Chem. Re®., 88, 55Ž. 1988 . of forced periodic operation very well. Contractor, R. M., H. E. Bergna, H. S. Horowitz, C. M. Blackstone, U. Chowdhry, and A. W. Sleight, ‘‘Butane Oxidation to Maleic Anhydride in a Recirculating Solids Reactor,’’Stud. Surf. Sci. Catal., Acknowledgments 38, 645Ž. 1987 . Contractor, R. M., D. I. Garnett, H. S. Horowitz, H. E. Bergna, G. S. The authors gratefully acknowledge the financial support of the Patience, J. T. Schwartz, and G. M. Sisler, ‘‘A New Commercial National Natural Science Foundation of China under Grant No. Scale Process for n-Butane Oxidation to Maleic Anhydride Using 29792073-3. a Circulating Fluidized Bed Reactor,’’ New De®elopment in Selec- ti®e Oxidation II, V. C. Corberan and S. V. Bellon, eds., Elsevier, Amsterdam, p. 233Ž. 1994 . Notation Contractor, R. M., ‘‘Dupont’s CFB Technology for Maleic Anhy- dride,’’ Chem. Eng. 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Schuurman, Y., and J. T. Gleaves, ‘‘A Comparison of Steady-State Wohlfahrt, K., and H. Hofmann, ‘‘Kinetics of the Synthesis of Maleic and Unsteady-State Reaction Kinetics of n-Butane Oxidation over Anhydride from n-Butane,’’ Chem. Ing. Tech., 52, 811Ž. 1980 . VPO Catalysts Using a TAP-2 Reactor System,’’ Catal. Today, 33, Xue, Z. Y., and G. L. Schrader, ‘‘Transient FTIR Studies of the 25Ž. 1997 . Reaction Pathway for n-Butane Selective Oxidation over Vanadyl Sharma, R. K., D. L. Cresswell, and E. J. Newson, ‘‘Kinetics and Pyrophosphate,’’ J. Catal., 184, 87Ž. 1999 . Fixed-Bed Reactor Modeling of Butane Oxidation to Maleic An- hyride,’’ AIChE J., 37, 39Ž. 1991 . Manuscript recei®ed July 7, 2000, and re®ision recei®ed Aug. 3, 2001.

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