efecto de la degradacion del catalizador

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The effect of deactivation of a V2O5/TiO2 (anatase) industrial

catalyst on reactor behaviour during the partial oxidation ofo-xylene to phthalic anhydride

Tharathon Mongkhonsi1

, Lester Kershenbaum*

Department of Chemical Engineering and Chemical Technology, Imperial College, London SW7 2BY, UK

Received 28 September 1997; received in revised form 10 November 1997; accepted 12 January 1998

Abstract

It is well known that during the partial oxidation of o-xylene to phthalic anhydride, on a V2O5/TiO2 (anatase) catalyst under

industrial conditions, the catalyst can experience both reversible and irreversible deactivation. Experimental evidence

presented here suggests that the reversible deactivation can be attributed to the deposition of some carbonaceous compounds.

Experiments were carried out in both a microreactor and an industrial-scale pilot-plant reactor. Catalyst samples from themicroreactor were analysed by elemental CHN analysis, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy

(XPS). These analyses suggest that the decrease in the disappearance rate of o-xylene at high o-xylene concentrations, which

the standard redox model cannot predict, is most likely to be caused by the deposition of carbonaceous compounds rather than

by the over-reduction of the catalyst. Two types of reversible deactivation are postulated from the experimental results: (1) by

easily removable and (2) by strongly adsorbed carbonaceous compounds.

Experiments on the pilot-plant reactor exhibited some unusual dynamic behaviour such as multiple steady-state operation,

travelling hot spots and decreasedcatalyst activity, following an attempted reactivation process at a low temperature. These

were all found to be consistent with a model based upon the postulated deactivation mechanism and kinetics; corresponding

models based upon constant activity proles could not reproduce the observed results. # 1998 Elsevier Science B.V.

Keywords: Partial oxidation; Catalyst deactivation; Coke formation; Reaction kinetics; Fixed-bed reactors; Reactor modelling

1. Introduction

V2O5/TiO2 catalysts are widely employed in selec-

tive oxidation reactions, including the partial oxida-

tion of o-xylene to phthalic anhydride in xed-bed

reactors. Despite the fact that the kinetics of this

reaction have been studied for several decades, gen-

erally accepted reaction networks with corresponding

kinetic parameters are still subject to uncertainty.

The reaction mechanism most widely accepted is

the redox mechanism, proposed by Mars and van

Krevelen [1] in which the hydrocarbon reduces the

catalyst and the catalyst is re-oxidised by the oxygen

in the feed. However, the redox model, which predicts

a zero-order reaction rate with respect to hydrocarbon

Applied Catalysis A: General 170 (1998) 3348

*Corresponding author. Tel.: (+44) 171-594-5566; fax: (+44)

171-594-5638; e-mail: [email protected] address: Department of Chemical Engineering, Faculty

of Engineering, Chulalongkorn University, Bangkok 10330 Thai-

land.

0926-860X/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved.

P I I S 0 9 2 6 - 8 6 0 X ( 9 8 ) 0 0 0 3 4 - 9


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concentration at high concentrations (i.e. >1.0 mol%),

cannot explain the observed drop in reaction rate in

that region [2,3]. This phenomenon has been attributed

to the possibility that, at high hydrocarbon concentra-tions, the vanadium is over-reduced to a lower oxida-

tion state, possibly V3, which is less active than the

V5 state [2,4]. This explanation, however, conicts

with the results of several workers who reported that

several low-oxidation state compounds of vanadium

have higher activity than the V2O5; moreover, the V3

species could be rapidly oxidised to a higher oxidation

state despite the presence of hydrocarbon [5,6].

Skrzypek et al. [7] tried to use the Langmuir

Hinshelwood (LH) model to explain the decrease

in the reaction rate by assuming that the surfacereaction between oxygen and xylene chemisorbed

on separate active sites was a rate determining step.

However, this mechanism is not consistent with the

experimental results of Blanchard and Louguet [8]

who demonstrated by using O18 isotope that the

catalyst does, indeed, supply its oxygen to the hydro-

carbon. These conicting hypotheses make it difcult

to choose between a redox model which can explain

the observed changing oxidation state of the catalyst

and the LH model which can explain the observed

decrease in rate at the high hydrocarbon concentra-tions.

In addition, it was also observed from carbon

balances during the rst minutes of catalyst life on-

stream that, at low reaction temperatures (less than

those in real industrial reactors), not all the carbon fed

to the reactor came out as measurable oxidation

products [5,9]. It was later revealed that hydrocarbons,

especially those with unsaturated bonds, could form

some adsorbed species on the catalyst surface [9,10].

The imbalance of carbon, however, was reported to

disappear when the reaction temperature wasincreased [9].

The reversible deactivation discussed above takes

place in addition to the well-known irreversible de-

activation of these catalysts upon exposure to high

temperatures for extended periods of time. The latter

phenomenon and its causes have been studied by

numerous workers [11,12]. A comprehensive review

of the various mechanisms postulated for o-xylene

oxidation is given by Nikolov et al. [13]. A more

recent review of many aspects of o-xylene oxidation

on V2O5/TiO2 catalysts including catalyst activity,

transient behaviour, kinetics, and possible mechan-

isms for catalyst deactivation has been presented in a

series of papers by Dias et al. [1418].

In the present paper, we report the observed de-activation of a V2O5/TiO2 (anatase) industrial catalyst

during operations under industrial conditions. The

aims of our work are to reveal the most likely causes

of reversible deactivation and to examine how this

affects the behaviour of an industrial-scale pilot-plant

reactor. Subsequently, an appropriate model of the

kinetics of the catalyst deactivation and reactivation

together with a suitable reactor model, should enhance

the predictions of reactor performance in both, steady

and transient states.

2. Experimental

The industrial catalyst utilised in this study was

supplied by von Heyden and consisted of an inert

spherical carrier of ca. 8 mm diameter, pellet density

2800 kg/m3, and covered with a thin active coating

surface containing V2O5 and TiO2 (anatase).

The composition of the active surface coating and

the oxidation states of vanadium were determined by

means of XRD and XPS techniques. X-rays at awavelength of 1.54178 A were used in the XRD

analysis. In the XPS analysis, aluminium was used

as the X-ray source. Any carbonaceous compounds

that might have deposited on the active surface were

analysed by means of elemental CHN analysis. In

all cases, the active surface coatings were removed

from their inert carrier before analysis.

Catalytic properties of the catalyst were determined

in a `string-of-beads' microreactor containing one-to-

three pellets as well as in an industrial-scale pilot-plant

reactor. The pilot-plant reactor was a single tube,25 mm diameter3 m length, packed with catalystpellets of bulk density 1480 kg/m3 and cooled by a

jacket containing molten salt. Continuous measure-

ment of temperature and periodic sampling for com-

position measurement was possible at 25 axial

positions along the reactor. Experiments were carried

out using an air ow rate of 4 Nm3/h, feed composi-

tions ranging from 0.31.0 mol% o-xylene, and cool-

ant temperatures ranging from 3804038C. This led to

hot-spot temperatures ranging between 4505308C,

which depended as well upon the activity of the

34 T. Mongkhonsi, L. Kershenbaum / Applied Catalysis A: General 170 (1998) 3348


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catalyst. Details of the pilot-plant reactor, its start-up,

operation, and control have been presented elsewhere

[3,1922]; steady-state was generally achieved one-

to-two hours after attaining constant feed conditions.The microreactor was constructed from a 1/2"

SS304 tube. The reactor was placed in a tube furnace

and the catalyst was located in the constant tempera-

ture zone of the furnace. Silicon carbide was used as

an inert packing before and after the catalyst to

improve heat transfer and ow distribution. Blank

runs did not show any reaction between o-xylene

and SiC/stainless steel in the experimental region.

Thermocouples were placed immediately upstream

and downstream of the catalyst pellets to estimate

catalyst temperature. The concentration ofo-xylene inthe feed gas was controlled by manipulating the

temperature of a saturator, which was used to vaporise

liquid o-xylene into a stream of owing air; this could

be diluted further with more air. After the system

reached a steady-state (generally within 3060 min),

on-line analysis of the feed and product streams was

performed by a PerkinElmer 8500 gas chromato-

graph equipped with ame-ionisation and hot-wire

detectors. The separation of organic compounds

was performed on an 1/8" O.D.2 m long SS columnpacked with 0.25% PPE-20, 0.1% H3PO4 supported

on 80/100 mesh Carbopack-CHT. The separation of

inorganic compounds was carried out on a 1/8"O.D.1.8 m long SS column packed with 80/100 mesh Carbonsphere. Further details of the micro-

reactor, control, and data acquisition systems are

described elsewhere [3].

To preserve the state of the catalyst following an

experimental run, when the reaction was terminated,

nitrogen at ambient temperature was ushed through

the microreactor. The pellets were then stored under an

atmosphere of nitrogen before analysis of the surface.

3. Experimental results and discussion

3.1. Microreactor studies

The rst set of experiments sought to reproduce and

quantify Calderbank's observations [2] of the inhibi-

tion of the rate of reaction by high concentrations ofo-xylene above ca. 1 mol%. Typical results for a space

velocity of 10 ml (g cat)1 s1 are shown in Fig. 1 for

Fig. 1. The disappearance rate of o-xylene as a function of composition at 384 and 3988C (space velocity10 ml (g cat)1 s1).

T. Mongkhonsi, L. Kershenbaum / Applied Catalysis A: General 170 (1998) 3348 35


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a fresh catalyst (F3) operating at 3848C and a partially

deactivated catalyst (F1) operating at 3988C.

In order to distinguish between the various possible

mechanisms for this inhibition (e.g. deactivation of thecatalyst by the over-reduction of active V5 sites, or

by carbonaceous deposits, or by strong adsorption via

an LH kinetic model), the surface of the catalyst was

examined in some detail. Following reaction in the

microreactor, it was observed that the colour of cat-

alyst pellets had changed uniformly from light green

to dark grey. The colour change is clearly observable

for exposure times as little as 1 h [3]. However, the

colour change was seen to be reversible; it could be

partially reversed by reactivating the catalyst pellet in

an air stream at 4108C for 14 h and completelyreturned to its initial colour by exposing to air at

higher temperatures for a longer regeneration period.

Fig. 2 illustrates the XRD pattern of a fresh catalyst

pellet when compared with that of a pellet which has

been exposed to a stream of 2.5 mol% o-xylene in air

(space velocity of 10 ml (g cat)1 s1) at 3848C for

1 h; similar results are available for pellets aged under

other conditions [3]. The XRD results do not reveal the

presence of vanadium oxides of V valency


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lysed by XPS. The interpretation of XPS data in this

case is not simple, partly because the poor conductiv-

ity of the catalyst leads to some charging of the

samples with a resulting shift in peaks. Nevertheless,by examining the spectra relative to the C 1s peak as a

reference (not shown), it was found that the shifts due

to the charging effect were 6.6 and 8.1 eV for the fresh

and used catalysts, respectively. Taking this into

account, the results of Fig. 3 indicate a V 2p3/2 signal

at ca. 517.2 eV for both samples. This value corre-

sponds to V5 [23]. Nevertheless, it should be noted

that, because there is only a slight shift in the bindingenergy of vanadium when its oxidation states change

between 3 and 5, it is possible that a small fractionof low-oxidation state vanadium is present but not

detectable. In a recent study using XRD and XPS on

Fig. 3. XPS analyses of (a) fresh and (b) used catalyst pellets.



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similar catalyst samples, Nobbenhuis et al. [23] could

not detect any low-oxidation states of vanadium.

However, in an earlier work using ESR spectroscopy,

Gasior et al. [24,25] have detected reduced oxidationstates of vanadium which depended upon the form of

titania used in the catalyst. Similarly, Centi et al. [26

28] detected a signicant fraction of V4 sites even in

the absence of a reducing agent and studied the role of

these sites in the partial oxidation of o-xylene. An

explanation of many of these discrepancies has been

proposed by Nobbenhuis et al. [23], but the subject is

still a matter of some considerable controversy.

Finally, elemental CHN analysis was carried out

on the fresh and used catalysts. No carbon or hydrogen

can be detected in suitably activated fresh catalyst; theresults for catalyst pellets aged under a variety of

operating conditions are shown in Table 1. It was

found that some carbonaceous materials could deposit

on the catalyst surface at most high reaction tempera-

tures, especially at high hydrocarbon concentrations.

The absence of hydrogen also suggested that during

the oxidation reaction, there were some dehydrogena-

tion or oxidative dehydrogenation reactions which

occurred on the catalyst surface (the mass ratio of

H/C for xylene%0.1 which is well within the detection

range of the instrument used).The presence of carbon on the used catalyst pellets

and the absence of any low oxidation state of vana-

dium from the XRD and XPS analyses indicate that

the deposition of some carbonaceous materials is

likely to be a major cause of catalyst deactivation

when it was used in regions of high hydrocarbon

concentration. This deposited carbon compounds

may slow down the reaction rate by a fouling process

which reduces the effective active surface area of the

catalyst for further hydrocarbon adsorption. Despite

the oxidising atmosphere, at high hydrocarbon con-

centrations, a steady state can be established whereby

the rate of oxidation of the carbonaceous material is

balanced by the rate of further depositions.

Given the above qualitative hypotheses, experi-ments were carried out to determine the kinetics of

the rates of deactivation and reactivation of the cata-

lyst, and their temperature dependence. Bond and

Konig [9] had observed a decrease in catalyst activity

at low temperatures (well below that of industrial

operation) during the rst hours of catalyst life on-

stream. However, the observed activity decrease dis-

appeared when the reaction temperature was increased

up to 3418C. In this study, deactivationreactivation

experiments were carried out to measure the tempera-

ture and composition dependence of these rate pro-cesses at more realistic temperatures.

Fig. 4 shows typical recorded temperature and con-

centration proles during reaction/deactivation

(between A and B, between C and D, and following

E), and reactivation in pure air at various temperatures

(between B and C, and between D and E). In this case,

the reaction process was carried out at a low inlet o-

xylene concentration of ca. 0.4 mol%, a space velocity

of 10 ml (g cat)1 s1 and a constant reaction tem-

perature of ca. 3858C. Reactivation was carried out at

405 and 4258C. Fig. 5 shows the experimental resultsobtained from a similar experiment in which the feed

composition was increased to ca. 0.6 mol% o-xylene

and reactivation took place at somewhat higher tem-

peratures. In both cases, there is a clear decrease in the

amount of phthalic anhydride (and intermediate reac-

tion products) formed with time-on-stream during

each reaction period, due to the deactivation of the

catalyst. Following reactivation in air at a higher

temperature, a partial recovery of catalyst activity is

achieved; full recovery of activity requires a longer

exposure to air which indicates that the reactivation

Table 1

Surface analyses from microreactor experiments

Sample No. Temperature Xylene feed Space velocity Reaction time C content H2 content

(8C) (mol%) (ml/(g cat) s) (min) (mass %) (mass %)

1 367 1.22 10.0 370 0.34 nil

2 389 1.22 3.3 410 0.27 nil

3 466 1.75 3.6 268 0.46 0.03

4 499 1.75 3.3 285 0.62 nil

5 384 2.12 3.4 435 0.70 nil

6 385 2.12 2.9 385 0.83 nil



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rate was slower than the deactivation rate. Moreover, if

`reactivation' occurs at too low a temperature, activity

can actually decrease; this will be discussed in more

detail below.

In contrast, similar experiments carried out at lower

reaction temperatures show lower activity as expected,

but no signicant change of activity with time. This

result implies that there was either no deactivation or,

more likely, any deactivation took place quickly and

the activity then stabilised.

Numerous other experiments, similar to the ones

described above, were carried out to measure the

disappearance rate of o-xylene as a function of both

temperature and composition for catalyst pellets

which had achieved a constant activity following

cycles of reaction and reactivation [3]. The data were

then used, together with data generated in the pilot-

plant reactor to be described below, to t a suitable

expression for the kinetics of deactivation and reacti-

vation.

Fig. 4. Results of cyclic microreactor experiments using 0.4% o-xylene feed (space velocity10 ml (g cat)1 s1): (a) temperature; (b) exit

composition. (&) o-xylene; (!) o-tolualdehyde; (*) phthalide; (~) phthalic anhydride; and () total C8.

T. Mongkhonsi, L. Kershenbaum/ Applied Catalysis A: General 170 (1998) 3348 39


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3.2. Pilot-plant studies

The variable activity of catalyst within a xed-bed

reactor can have a profound effect on the behaviour of

the reactor; completely different temperature and

composition proles can exist for an apparently iden-

tical set of operating conditions. Fig. 6 shows the

comparison between three experimentally obtained

temperature proles with identical operating condi-

tions of an o-xylene feed concentration of 1.0 mol%,

an air-feed ow rate of 4 Nm3/h and a salt bath jacket

temperature of 3838C. For reference, the temperature

prole of the reactor in the absence of any reaction

(pure air feed) is also shown (prole 4). It was

observed that under these conditions, the location of

the hot spot could vary by 0.5 m and its magnitude

varied between 4605508C. The difference between

the three proles lies solely in the way the catalyst was

`reactivated' prior to the run.

Prole 1 in Fig. 6 is typical of those obtained for the

existing operating conditions for catalyst reactivated

periodically by exposure to air at temperatures at or

Fig. 5. Results of cyclic microreactor experiments using 0.6% o-xylene feed (space velocity10 ml (g cat)1 s1): (a) temperature; (b) exitcomposition. (&) o-xylene; (!) o-tolualdehyde; (*) phthalide; (~) phthalic anhydride; and () total C8.



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>4008C for 20 h or longer. Prole 2 in Fig. 6 was

obtained when `reactivation' was carried out in a

similar ow rate of air but at signicantly lower

temperatures (3703808C). It can be noted that the

hot spot and the reactor temperature, especially

between 0.51.0 m were lower than those recorded

before the catalyst bed was `reactivated'. Clearly, the

activity of the catalyst in that section was decreased

after the catalyst bed was exposed to owing air at a

lower temperature. This low hot spot temperature (orlow catalyst activity) after a reactivation process was

not observed when the catalyst was reactivated at a

higher temperature or a longer period. Prole 3 in

Fig. 6 represents operation with fresh catalyst; such an

operation is not sustainable because of potential per-

manent deactivation of the catalyst at such high tem-

peratures for extended periods of time.

This curious behaviour is consistent with the micro-

reactor experiments and surface examinations which

indicated that dehydrogenation, oxidative dehydro-

genation and/or polymerisation of adsorbed carbon-aceous species could form more strongly adsorbed, ormore difcult to oxidise, species. When the catalyst

was exposed to air at a low reactivation temperature, it

is likely that only the more reactive parts of the

deposited compounds are gradually removed, leaving

the rest to combine into larger molecules. The forma-

tion of macromolecules or tar products has been

reported by several other workers [9,10,29]. These

macromolecules, which are not easily desorbed or

oxidised at the low reactivation temperature, would

require exposure to air at higher temperatures for

longer periods of time for their removal. Thus, under

some operating conditions, they could be regarded as

contributing to a `quasi-reversible' deactivation.

The effects of reactivation can be seen clearly indynamic experiments performed on the pilot-plant

reactor. Fig. 7(a) and (b) show the response of the

reactor over a 90 min period to a ramp increase in the

jacket temperature from 383 to 4038C(atarateof18C/

min) and the subsequent decrease back to 3838C (at a

rate of 0.228C/min). Comparison of the initial prole

in Fig. 7(a) with the nal prole in Fig. 7(b) (espe-

cially the steepness of the slopes in the region between

0.5 and 1.0 m length) reveals that the catalyst has been

reactivated during the transition to a higher operating

temperature, despite the fact that the hydrocarbon feedcontinued throughout the experiment. Oxidation of

carbonaceous material is the most likely cause of this

reactivation.

Many other dynamic experiments were carried out

and reported in detail elsewhere [3]; some of these will

be referred to below in the development of an appro-

priate expression for the deactivation-reactivation

kinetics and in the comparison of experiments with

reactor simulations.

4. Kinetics of deactivationreactivation

The steady-state and dynamic data resulting from

the microreactor and pilot-plant reactor experiments

were used to nd suitable parameters in an appropriate

expression for the rates of catalyst deactivation and

reactivation. The activity is arbitrarily dened as the

ratio between the measured disappearance rate of o-

xylene and that predicted by the kinetics of Calder-

bank et al. [30,31]. Vanhove and Blanchard [32] and

others have reported that high selectivity to phthalicanhydride is obtained via the oxidised intermediates

o-tolualdehyde and phthalide; hence, it can be

assumed that the adsorbed species responsible for

the reversible deactivation are formed from the non-

selective oxidation ofo-xylene. This reaction was also

suggested by Bond and Konig [9] who also reported

that these adsorbed species could be removed by

increasing oxygen partial pressure. It can be further

assumed that the kinetic parameters of o-xylene oxi-

dation to phthalic anhydride are not affected by the

deactivation process other than by a single multi-

Fig. 6. Multiple steady-state temperature profiles in the pilot-plant

reactor (feed composition1 mol% o-xylene, air-flow rate4 Nm3/h): 1, () reactivated catalyst; 2, (&) catalyst reactivated at lowtemperature; 3, (S) fresh catalyst; and 4, (*) no reaction.



10/16

plicative factor, namely the activity; that is, the

kinetics for the oxidation of o-xylene and for the

catalyst deactivation are assumed to be separable.

With these results and supported by microreactor

experiments and surface measurements described

above, the simplest deactivationreactivation model,

based on the balance of active sites, is:

da

dt k1poxa k2pO2am a (1)

where

ki kiY0exp EaiaRT (2)

Fig. 7. Dynamic response of the pilot-plant reactor to a change in jacket temperature (feed composition1 mol% o-xylene, air-flowrate4 Nm3/h). (a) 208C Ramp increase (18C/min); and (b) 208C ramp decrease (0.228C/min).



11/16

Here, pox and pO2 are the partial pressures ofo-xylene

and oxygen, respectively, k1 and k2 the temperature-

dependent rate constants for deactivation and reacti-

vation, respectively, and the parameter am the maxi-mum activity under present conditions, a term which

is introduced to compensate, in an approximate way,

for the `quasi-reversible' deactivation caused by

strongly adsorbed species as described above. If no

such species are present and all the deposited com-

pounds can be removed by exposure to owing air,

then am will be equal to one. If the regenerating

condition has not been strong enough, e.g. too low

a temperature or too short a time, part of the catalyst

will still be covered by the deposited compound, and

am would be less than one. In cases in which incom-plete reactivation was suspected, am was used as a

single adjustable parameter in the model equations

describing reactor performance.

The rst term on the right hand side of Eq. (1)

represents the rate of deactivation. It is assumed to be

proportional to the o-xylene partial pressure and the

fraction of the surface which is still active. The second

term represents the reactivation rate, assumed to be

proportional to the oxygen partial pressure and the

fraction of surface which is inactive but recoverable.

The data from the microreactor experiments can beused directly to estimate parameters in Eq. (2). The

data from the pilot plant experiments provide addi-

tional information on the deactivationreactivation

kinetics. However, in this case, pointwise measure-

ments of reaction rate are not available; only tem-

peratures as a function of time at 25 points in the

reactor and steady-state measurements of composition

at a few isolated axial positions can be measured.

Hence, a reactor model is also required in order to

estimate the reaction rate (and hence the activity) at

each point. This was accomplished using a standardtwo-dimensional pseudo-homogeneous model of thereactor [33]. In dimensionless form, this becomes

dxidt

dxidz

1

Pmr

d2xidr2

1

r

dxidr

a

nj1

)ijDajfjXYy

(3)

Mdy

dt

dy

dz

1

Phr

d2y

dr2

1

r

dy

dr

a

nj1

jDajfjXYy

(4)

for the mass balance on the ith component and the

energy balance, respectively. Here, the various Dam-

kohler numbers, Daj, contain the nominal dimension-

less rate constants for the jth reaction step at thereference temperature and the functions fj represent

the temperature and composition dependence of the

rate of reaction [3]. The kinetic network and param-

eters were taken from the results published by Cal-

derbank et al. [30,31]. The other quantities in Eqs. (3)

and (4) are dened in Section 7.

Based upon the steady-state and dynamic experi-

ments in the microreactor and pilot-plant reactor

described above, a non-linear regression routine [3]

was used to nd a suitable set of kinetic parameters for

the deactivationreactivation steps described byEqs. (1) and (2). This led to the values: k1,0%0.22kPa1 s1, Ea1/R%3600 K, k2,0%2.410

15 kPa1 s1,

Ea2/R%30 000 K. Especially noteworthy is the high-temperature sensitivity of the reactivation process as

indicated by the very high activation energy. These

results do not necessarily conrm that the proposed

simple mechanism is the correct one; nevertheless, as

will be shown below, they are able to reproduce the

fairly complex reactor behaviour which was observed

experimentally.

5. Comparison of experiments and simulations

The deactivation- and reactivation-rate parameters

estimated above were incorporated into the model

for the dynamic behaviour of the pilot-plant reactor,

including catalyst deactivation and reactivation as

given by Eqs. (1)(4). Experimental and simula-

tion results for several steady-state and dynamic

operating conditions are shown below; results

obtained from simulations which ignore catalystdeactivationreactivation are also presented for com-

parison. A more complete set of results is available

elsewhere [3].

A typical steady-state operating condition for a low-

activity catalyst is shown in Fig. 8. In this case, the

reactor was started-up after a reactivation at a low

jacket temperature and a relatively short period of time

(3788C for 19 h). Therefore, it was estimated that the

catalyst activity had not been fully restored. The

operating conditions were at the o-xylene feed con-

centration of 0.7 mol% and a jacket temperature of



12/16

3828C. It is found that with am0.5, the temperatureproles predicted from the reactor model with the

catalyst activity equation can t the temperature pro-

le obtained experimentally very well. However, there

are substantial differences between the prole

obtained experimentally and that predicted whendeactivation and reactivation are ignored; this discre-

pancy is especially large taking into account the

magnitude and the location of the hot spot.

Fig. 9 illustrates the case of a more highly reacti-

vated catalyst in which the feed concentration of o-

xylene is 1.0 mol% and the jacket temperature 4008C.

Once again, the reactor model with the activity equa-tion can determine the shape and position of the

Fig. 8. Comparison between (&) measured steady-state temperature profiles and simulations (~) with and (!) without considering catalyst

deactivationreactivation. Jacket temperature3828C; feed composition0.7% o-xylene; and air-flow rate4 Nm3/h.

Fig. 9. Comparison between (&) measured steady-state temperature profiles and simulations (*) with and (!) without considering catalyst

deactivationreactivation. Jacket temperature4038C; feed composition1.0% o-xylene; and air-flow rate4 Nm3/h.



13/16

temperature prole very well, with the value of

am0.8. When a reactor model is used without thedeactivationreactivation kinetics, there is ca. 308C

temperature difference between the predicted and theobserved values of the hot spot and its position is

incorrectly located.

In these, and other results not shown here, it is also

observed that when the activity equation is excluded

from the reactor model, the predicted temperature

proles exhibit a smooth increase from the inlettemperature to the maximum hot-spot temperature

without the inection point, or `shoulder', which is

Fig. 10. Simulated dynamic response of the pilot-plant reactor to a 208C ramp decrease in the jacket temperature, conditions as in Fig. 7(b):

(a) ignoring catalyst deactivation-reactivation; and (b) including deactivationreactivation.



14/16

typically present in the experimental temperature

proles and the simulation results which include

deactivationreactivation. As will be shown in the

dynamic results below, the `shoulder' region is pre-cisely that zone in which there is a steep change in

catalyst activity as a function of both position and

time.

Fig. 7(b) illustrated the results of a dynamic

experiment on the pilot-plant reactor in which the

reactor was operated at steady state and, at time

t0, the jacket temperature was decreased by 208Cat a constant rate of 0.228C/min. The feed composi-

tion and ow rate were kept constant at 1.0 mol%

o-xylene and of 4 Nm3/h of air, respectively. Simula-

tion results, both ignoring and including the catalystdeactivation and reactivation for the same operating

conditions, are shown in Fig. 10(a) and 10(b), respec-

tively.

Without the deactivationreactivation kinetics, the

model predicted a smooth temperature increase from

the inlet gas temperature to the maximum hot-spot

temperature without an inection point, both at steadystate and in response to the change in jacket tempera-

ture. There is little or no correspondence to the

experimental results of Fig. 7(b): higher hot-spot

temperatures than those observed experimentally

are predicted; the location of the hot spot and its

movement downstream is not predicted at all.

A different and much better result is obtained whenthe activity equation is included into the reactor

model. The predicted responses using the reactor

model with the activity equation and with am0.8are illustrated in Fig. 10(b). There is a very close

agreement with that observed experimentally, given

the fact that only one adjustable parameter, am is used

in the simulations. The reason for the improved simu-

lation result can be seen from Fig. 11 which shows the

calculated activity proles as a function of time for

this dynamic experiment. It can be seen that, within

the region of interest (between 0.3 and 0.8 m of reactorlength), the decrease in jacket temperature has led to a

signicant decrease in catalyst activity. This, in turn,

has led to less reaction and less exothermicity until a

new steady state was reached in which most of the

reaction was occurring in the 0.71.2 m region of

reactor length, rather in the upstream region (0.3

0.8 m) corresponding to the initial steady state. Notealso that the calculated `activity' is indeterminate and

somewhat meaningless in regions where no reaction

takes place: the reactor entrance (where the tempera-

Fig. 11. Simulated values of the catalyst-activity profiles as a function of time during a 208C ramp decrease in the jacket temperature:

conditions same as in Fig. 7(b).



15/16

ture is too low) and the reactor exit (where all the

reactant has been consumed).

Other dynamic experiments and comparisons with

simulation are discussed by Mongkhonsi [3], but thegeneral behaviour is similar to the results shown

above.

Finally, it should be pointed out that the ability of

the reactor model (with deactivationreactivation

kinetics) to describe and predict reactor behaviour

has potentially important industrial applications. It

has been demonstrated by Cheng et al. [34,35], that,

together with a suitable estimation algorithm, such an

approach can be used to control reactors and adjust

their operating conditions to maintain high yield and/

or selectivity in the face of catalyst deactivation.

6. Conclusions

The aim of this study was to determine the factors

that affected the dynamic behaviour of an industrial

scale pilot-plant reactor using a V2O5/TiO2 catalyst

for o-xylene oxidation to phthalic anhydride. In order

to obtain useful information, several techniques were

applied. These techniques included testing of catalytic

activity under various conditions in a microreactorwith XPS, XRD and elemental CHN analyses as

well as experiments on an industrial scale pilot-plant

reactor. The conclusions postulated from this study

can be summarised as follows:

1. Deposition of some carbonaceous compounds can

occur on the catalyst surface under industrial

conditions, i.e. relatively high hydrocarbon con-

centration (%1.0 mol%) and relatively high reac-tion temperatures (!4008C) despite the presenceof a high oxygen concentration (%20.0 mol%).

2. XRD, XPS, and CHN analyses indicate that thedecrease in the disappearance rate of o-xylene at

high o-xylene concentrations is most likely due to

such deposition rather than to the over-reduction of

vanadium.

3. Nevertheless, operation at a very high o-xylene

concentrations can cause the layer of the deposited

compounds to become thick enough to prevent

both, the hydrocarbon and the oxygen from react-

ing with the catalyst. This may lead to the presence

of lower oxidation states of vanadium observed by

some workers.

4. Two types of reversible deactivation are postulated:

(a) deactivation by the easily removable adsorbed

compounds and (b) deactivation by some strongly

adsorbed species. To effectively remove all theadsorbed compounds, the catalyst requires reacti-

vation in an air stream at a temperature not less than

4008C over several hours, depending upon the past

history of the catalyst.

5. The reactivation of the catalyst in air is relatively

slower than the deactivation kinetics and, more

significantly, its kinetics are much more tempera-

ture sensitive. Fitting of data to a simple model of

the reactivation process led to an activation energy

above 200 000 J mol1 K1.

6. The simulation results demonstrate that when theactivity equation is incorporated in the reactor

model, the model predictions are substantially

enhanced, especially in the prediction of tempera-

ture profiles and the location of the hot spots. The

inflection point of the temperature profiles, nor-

mally observed during the experiments, is also

predicted; without the activity equation, the reactormodel fails to predict this effect and the magnitude

and location of the hot spot are also poorly pre-

dicted.

7. Notation

a catalyst activity

am maximum catalyst activity

Daj Damkohler number (dimensionless rate

constant) for reaction step j

Ea1 , Ea2 activation energy for deactivation, reacti-

vation

fj dimensionless rate expression for reactionj

k1, k2 rate constant for deactivation, reactivation

k1,0, k2,0 pre-exponential factors in rate constants

for deactivation, reactivation

M ratio of thermal capacities of the solid and

gas phases

Phr, Pmr Peclet numbers for heat transfer, mass

transfer

pox, pO2 partial pressures of o-xylene, oxygen

r dimensionless radial position in the re-

actor



16/16

R gas constant

t dimensionless time

T temperature

xi mole fraction of component iX vector of all compositions, xiy dimensionless temperaturez dimensionless axial position in the reactor

j dimensionless heat of reaction for reactionstep j

)ij stoichiometric coefficient for component iin reaction step j

Acknowledgements

The nancial support of the British Council to

T. Mongkhonsi is gratefully acknowledged.

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