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7/29/2019 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.
T. Mongkhonsi, L. Kershenbaum / Applied Catalysis A: General 170 (1998) 3348 37
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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
38 T. Mongkhonsi, L. Kershenbaum / Applied Catalysis A: General 170 (1998) 3348
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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.
40 T. Mongkhonsi, L. Kershenbaum / Applied Catalysis A: General 170 (1998) 3348
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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.
T. Mongkhonsi, L. Kershenbaum / Applied Catalysis A: General 170 (1998) 3348 41
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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).
42 T. Mongkhonsi, L. Kershenbaum / Applied Catalysis A: General 170 (1998) 3348
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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
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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.
44 T. Mongkhonsi, L. Kershenbaum / Applied Catalysis A: General 170 (1998) 3348
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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.
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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).
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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
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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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