# Effect of Process Parameters on Catalytic Incineration of Solvent Emissions

Authors: Satu Ojala; Ulla Lassi; Paavo Perämäki; Riitta L. Keiski
Journal: Journal of Automated Methods and Management in Chemistry
 (2008)

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## Abstract

Catalytic oxidation is a feasible and affordable technology for solvent emission abatement. However, finding optimal operation conditions is important, since they are strongly dependent on the application area of VOC incineration. This paper presents the results of the laboratory experiments concerning four most central parameters, that is, effects of concentration, gas hourly space velocity (GHSV), temperature, and moisture on the oxidation of n-butyl acetate. Both fresh and industrially aged commercial Pt/Al2O3 catalysts were tested to determine optimal process conditions and the significance order and level of selected parameters. The effects of these parameters were evaluated by computer-aided statistical experimental design. According to the results, GHSV was the most dominant parameter in the oxidation of n-butyl acetate. Decreasing GHSV and increasing temperature increased the conversion of n-butyl acetate. The interaction effect of GHSV and temperature was more significant than the effect of concentration. Both of these affected the reaction by increasing the conversion of n-butyl acetate. Moisture had only a minor decreasing effect on the conversion, but it also decreased slightly the formation of by products. Ageing did not change the significance order of the above-mentioned parameters, however, the effects of individual parameters increased slightly as a function of ageing.


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## Body

## 1. Introduction

Volatile organic compounds (VOCs)
together with nitrogen oxides are the major contributors to the formation of
photochemical ozone.  In 1999, the EU adopted the VOC solvent emissions directive
[1], the goals of which
should be implemented by the year 2007. The wood-coating processes are among
those that have to follow the VOC solvent emission directive. According to our
experience, the solvent emissions from wood-coating processes include usually
ethanol and n-butyl acetate [2] of which n-butyl acetate has the typical
solvent odor.The most feasible abatement technologies
for emissions that are mixtures of different VOCs are destruction-based, that
is, thermal and catalytic oxidation or biodegradation. If the total VOC
concentrations are not very high, catalytic incineration with heat recovery is
the most cost-effective alternative [3, 4]. The normal operation temperature of
an industrial scale catalytic VOC incinerator in solvent emissions abatement is
about 350°C, but it varies depending on the VOC
compound to be oxidized [5]. For example, minimum reactor inlet
temperatures reported by Hayes and Kolaczkowski [6] vary from 190°C to 350°C. The economy of catalytic incineration may be further
improved by employing a heat recovery system and, furthermore, the operation of
the incinerator may be autothermal, which means that no additional heating of
emission gases are needed to maintain oxidation reactions. The autothermal
operation may be achieved in practice, for example, with the aid of flow-reversal
and regenerative heat exchangers [3, 4, 7].In general, there are several process or
operation parameters that may have an effect on the total oxidation of VOCs.
These involve, for example, concentration of emission gases, gas hourly space
velocity (GHSV), temperature, and moisture content of the emission gas. In this
study, the effects of different process parameters on the oxidation of n-butyl
acetate are investigated at a laboratory-scale. Similar studies are carried out
over both fresh and industrially aged Pt/Al2O3 catalysts. The objective of this study
is to determine the optimal process conditions for the catalytic incineration
of n-butyl acetate as well as to find out how significant the effects of these
parameters are in practice. Computer-aided statistical experimental design is
used as a tool in experimental design, and in evaluation of the results. The
results are discussed “in the light of flow-reversal process,” since the
industrial process associated to this research is operated by flow-reversal. In
addition, industrial ageing of Pt/Al2O3 catalyst was carried out in a
flow-reversal incinerator.


## 2. Experimental

### 2.1. Laboratory Experiments

Laboratory
experiments have been carried out in a continuous-flow tubular quartz reactor
with a diameter of 9 mm. Liquid-phase n-butyl acetate, which was selected as
the model compound based on industrial measurements, was first vaporized and
fed with air to the reactor by the aid of a calibrator (Temet Instruments Inc.
Type Ø0009). Water in the moisture containing experiments was fed with a
separate syringe pump, vaporized, and mixed with the reaction mixture prior to
the reactor inlet. The reactor was heated up to the reaction temperature in a
tubular furnace. The continuous gas flow was analyzed at the outlet of the
reactor by GC/FID (flame ionization detector) and by GC/TCD (thermal
conductivity detector) (Agilent Technologies model 6892N). The catalyst bed
height (30 mm, giving a volume of ~1.9cm3) was kept constant during the
experiments. Temperature was measured at the catalyst inlet with a K-type
thermo element.The experiments
were carried out over fresh and industrially aged Pt/Al2O3 metallic monoliths. The Pt/Al2O3 catalyst was aged in an
industrial solvent-emission abatement process for 25 months. Sample catalysts
were installed in an industrial-scale incinerator between a catalyst bed and
regenerative heat exchangers in such a way that they were exposed to either
once treated or untreated emission flow depending on the operation of the reverse-flow
process. During the ageing period, the industrial incinerator was working in
solvent emission abatement application, where the most dominant emission
compound was n-butyl acetate. Total concentration of emission gases was
fluctuating according to solvent-using process operation. The temperature of
the catalytic incinerator during the ageing period was varying roughly between
350–400°C. More detailed description of industrial ageing is presented in [8].Prior to the laboratory
experiments, a fresh catalyst was heated up to600°C and cooled to room
temperature in air. This procedure was not carried out for the aged catalyst,
since it may change the state of the catalyst surface and even regenerate it. The
activity of each catalyst was tested by light-off tests before and after the factorial
experiments. In the light-off experiments, the n-butyl acetate concentration
was 2000 ppm, GHSV was 31 500 h−1, and heating rate 5°C min−1 from room
temperature to about 700°C. BET surface areas were also measured (Coulter Omnisorp
360CX) before and after the laboratory experiments.


### 2.2. Experimental Design

MODDE 6.0 program
(Umetric AB)
was used as a tool in the statistical experimental design and in evaluation of
the effects of the selected parameters (temperature, GHSV, concentration, and
moisture) affecting the catalytic oxidation of n-butyl acetate. Further, the
effect of ageing of the catalyst was considered, that is, does ageing affect
process parameters or does it even change the significance order of them. A set
of experiments was done with a full two-level factorial design. The effect of a
single factor was evaluated at all levels of other factors, which enabled study
of the effects of the interaction of selected parameters. The used response was
conversion of n-butyl acetate over the fresh and aged catalysts, which was
calculated from the measured concentrations. The validity of the empirical
models fitted with the multiple linear regression (MLR) was tested with the
analysis of variance (ANOVA). The used confidence level was 95%.The
parameters and the used levels were based on the earlier results achieved from
industrial measurements, from solvent emission sources, and from catalytic
incinerator used in the solvent emission abatement. The metal-supported Pt/Al2O3 catalyst was selected to these experiments based on catalyst screening tests,
where it showed the highest activity in n-butyl acetate oxidation [3, 9]. As
mentioned, the selected operation parameters were temperature, GHSV,
concentration, and moisture content. Temperatures of the experiments were
selected to be higher than the catalysts light-off, that is, above the
temperature of 50% conversion, of which the lower temperature, 300°C, is close to a normal industrial
operation temperature of the incinerator, and the higher temperature level, 500°C, is above it. The basis of the selection
of GHSV levels, that is, 31 500 h−1 and 63 000 h−1, was
similar to the selection of temperature levels. In the experiments, the GHSV
levels were set by adjusting the total flow of the reacting gas mixture. The
lower level of concentration (2000 ppm) was close to the concentration of
n-butyl acetate in the solvent emission measured [2], the higher level
was 4000 ppm. The higher level for moisture was 2.5 vol-% and at the lower
level water was not introduced into the system (i.e., zero level moisture).


## 2.1. Laboratory Experiments

Laboratory
experiments have been carried out in a continuous-flow tubular quartz reactor
with a diameter of 9 mm. Liquid-phase n-butyl acetate, which was selected as
the model compound based on industrial measurements, was first vaporized and
fed with air to the reactor by the aid of a calibrator (Temet Instruments Inc.
Type Ø0009). Water in the moisture containing experiments was fed with a
separate syringe pump, vaporized, and mixed with the reaction mixture prior to
the reactor inlet. The reactor was heated up to the reaction temperature in a
tubular furnace. The continuous gas flow was analyzed at the outlet of the
reactor by GC/FID (flame ionization detector) and by GC/TCD (thermal
conductivity detector) (Agilent Technologies model 6892N). The catalyst bed
height (30 mm, giving a volume of ~1.9cm3) was kept constant during the
experiments. Temperature was measured at the catalyst inlet with a K-type
thermo element.The experiments
were carried out over fresh and industrially aged Pt/Al2O3 metallic monoliths. The Pt/Al2O3 catalyst was aged in an
industrial solvent-emission abatement process for 25 months. Sample catalysts
were installed in an industrial-scale incinerator between a catalyst bed and
regenerative heat exchangers in such a way that they were exposed to either
once treated or untreated emission flow depending on the operation of the reverse-flow
process. During the ageing period, the industrial incinerator was working in
solvent emission abatement application, where the most dominant emission
compound was n-butyl acetate. Total concentration of emission gases was
fluctuating according to solvent-using process operation. The temperature of
the catalytic incinerator during the ageing period was varying roughly between
350–400°C. More detailed description of industrial ageing is presented in [8].Prior to the laboratory
experiments, a fresh catalyst was heated up to600°C and cooled to room
temperature in air. This procedure was not carried out for the aged catalyst,
since it may change the state of the catalyst surface and even regenerate it. The
activity of each catalyst was tested by light-off tests before and after the factorial
experiments. In the light-off experiments, the n-butyl acetate concentration
was 2000 ppm, GHSV was 31 500 h−1, and heating rate 5°C min−1 from room
temperature to about 700°C. BET surface areas were also measured (Coulter Omnisorp
360CX) before and after the laboratory experiments.


## 2.2. Experimental Design

MODDE 6.0 program
(Umetric AB)
was used as a tool in the statistical experimental design and in evaluation of
the effects of the selected parameters (temperature, GHSV, concentration, and
moisture) affecting the catalytic oxidation of n-butyl acetate. Further, the
effect of ageing of the catalyst was considered, that is, does ageing affect
process parameters or does it even change the significance order of them. A set
of experiments was done with a full two-level factorial design. The effect of a
single factor was evaluated at all levels of other factors, which enabled study
of the effects of the interaction of selected parameters. The used response was
conversion of n-butyl acetate over the fresh and aged catalysts, which was
calculated from the measured concentrations. The validity of the empirical
models fitted with the multiple linear regression (MLR) was tested with the
analysis of variance (ANOVA). The used confidence level was 95%.The
parameters and the used levels were based on the earlier results achieved from
industrial measurements, from solvent emission sources, and from catalytic
incinerator used in the solvent emission abatement. The metal-supported Pt/Al2O3 catalyst was selected to these experiments based on catalyst screening tests,
where it showed the highest activity in n-butyl acetate oxidation [3, 9]. As
mentioned, the selected operation parameters were temperature, GHSV,
concentration, and moisture content. Temperatures of the experiments were
selected to be higher than the catalysts light-off, that is, above the
temperature of 50% conversion, of which the lower temperature, 300°C, is close to a normal industrial
operation temperature of the incinerator, and the higher temperature level, 500°C, is above it. The basis of the selection
of GHSV levels, that is, 31 500 h−1 and 63 000 h−1, was
similar to the selection of temperature levels. In the experiments, the GHSV
levels were set by adjusting the total flow of the reacting gas mixture. The
lower level of concentration (2000 ppm) was close to the concentration of
n-butyl acetate in the solvent emission measured [2], the higher level
was 4000 ppm. The higher level for moisture was 2.5 vol-% and at the lower
level water was not introduced into the system (i.e., zero level moisture).


## 3. Results and Discussion

Objectives
of this study were to determine the optimal process conditions for the catalytic
incineration of n-butyl acetate and to find out significance levels of the selected
parameters. The experimental data was analyzed with a statistical design
software in order to have also a new insight into the simultaneous effects of
several process parameters. Table1 shows the array of experiments where
higher and lower parameter values are indicated. Figure 1 shows responses
(n-butyl acetate conversions) over the fresh and aged monoliths. In general,
the conversions achieved are rather high (over 94%) in all experiments. The
aged catalyst seems to give slightly smaller conversion values than the fresh
one,  which implies that the catalyst has actually lost its activity slightly
during the 25 months of ageing. This is especially observed at higher GHSV
values (experiments 5–8 and 13–16). It can also be clearly seen from Figure 1,
that lower GHSV gives higher conversion values, as expected, but GHSV seems to
have the largest main effect on the conversion of n-butyl acetate compared to
the effects of other parameters as well.Table 1
Array of experiments.
Number of experimentMoisture [%]Concentration [ppm]GHSV [h−1]Temperature [C°]1—20003150030022.52000315003003—40003150030042.54000315003005—20006300030062.52000630003007—40006300030082.54000630003009—200031500500102.520003150050011—400031500500122.540003150050013—200063000500142.520006300050015—400063000500162.5400063000500Figure 1
Conversions of n-butyl acetate over the fresh (⋄) and aged (∘) catalysts.To get more detailed information,
the responses were fitted with the aid of MODDE program. In general, the
fitting of responses showed that the parameters studied here had similar
effects on the conversion of n-butyl acetate independent whether the catalyst
was fresh or aged—the significance order of the parameters was
not changed. The calculated effects of parameters for fresh and aged catalysts
are presented in Figures2(a) and 2(b), respectively. In more detail, all
single effects as well as interaction terms are included in Figure 2, even if
the effects of all these terms are not significant. The error is indicated in
each effect-indicating bar separately. If the error is greater than the
calculated effect, the effect can be removed from the final model.Calculated effects of GHSV, moisture (Moist), concentration (c), and temperature (T) on n-butyl acetate conversion over (a) fresh and (b) aged catalysts.
(a)(b)When the validity of the fitted
model (MLR) was evaluated with ANOVA, the results showed that the model was
statistically significant with a 95% confidence level. Square of the multiple
correlation coefficient of the model, that is, the response variation
percentage explained by the model,R2, for fresh and aged catalysts
were 0.976 and 0.986, respectively. Response variation percentages predicted by
the model, Q2, were 0.945 for the fresh and 0.968 for the aged
catalyst. However, one has to remember that the model is valid only in the used
range of parameters.Figures1, 2(a), and 2(b) show
that GHSV has the largest and negative main effect on conversion, that is, when GHSV is increased, the conversion is
decreased. Furthermore, the effect of GHSV is more significant at low temperatures
and it is slightly more important over the aged catalyst than over the fresh
catalyst. Due to the selected temperature levels, the model reaction occurs in
the mass transfer limited area (See Figure 3). Increasing of GHSV moves
reaction slightly closer to the transition phase where reaction switches from
kinetic to mass transfer controlled region. At a mass transfer restricted area,
the reaction rate is first affected by pore diffusion (at lower temperature
level) and later by bulk diffusion. This affects the apparent reaction rate,
and in more detail, bulk mass transfer phase of catalytic reaction. This step
of reaction has smaller relative temperature dependence than other steps of
reaction (i.e., pore diffusion, sorptions, and surface reaction), but it is
more affected by flow conditions. The conversion is also affected by the fact
that the residence time of reactants inside a catalyst at a higher GHSV level
is also smaller than at a lower GHSV level.Figure 3
Light-off curves and BET surface areas for fresh and aged Pt/Al2O3 catalysts before and after the experiments. (cn-butyl acetate=2000 ppm, GHSV=31500 h−1, heating rate 5°C min−1.)Temperature has the second largest
but positive effect on conversion, thus when temperature is increased the
conversion is increased as well. As the experiments have been carried out at
mass transfer limited area, increasing temperature increases the apparent
reaction rate less than decreasing the GHSV. For example, bulk molecular
diffusion rates vary approximately byT3/2 while surface reactions depend on
temperature according to exponential Arrhenius law. Several authors [7, 10, 11]
have reported that if the catalytic incinerator is operated autothermally with
a flow reversal, small changes, for example, in the flow rate, inlet
temperature, and concentration may not affect the end conversion at all. This
is due to a quasisteady state operation where the temperature profile in a
tubular reactor has a maximum value near the center of the reactor and it
slowly oscillates toward the outlets of the reactor when the flow direction is
changed. This effect is called a heat trap and it can be used to achieve and
maintain higher reaction temperatures compared to a once-through reactor. Higher
catalyst surface temperature compared to bulk gas temperature may be formed
also in a once-trough system due to exothermic reactions. In a once-through
system, this temperature maximum is close to the outlet of the catalyst [12].Third significant term is the
interaction effect between temperature and GHSV. It can be observed from Figure2, that the interaction effect is positive and increases the end conversion of
n-butyl acetate. In general, the effects of other parameters are enhanced with
a higher GHSV value. However, higher temperature is balancing this effect. If
we consider the light-off curve presented in Figure 3, an increase in GHSV
moves the light-off curve toward a higher temperature region. If the reaction
temperature is kept the same, the conversion of n-butyl acetate decreases. Then
when temperature is increased, the conversion of n-butyl acetate is enhanced
again. At a higher flow rates, the compounds have not as much time to react on
the catalytic surface as at lower flow rates, and therefore, higher temperature
is needed to enhance surface reactions so that a similar oxidation efficiency
of n-butyl acetate can be maintained in both levels of GHSV. In the temperature
range used in this study, thermal (i.e., gas phase) conversion of n-butyl
acetate is less than 30% and it may only have a minor effect on the end
conversion [3]. If considering GHSV values in practice, it is also reported for
flow-reversal systems that too high GHSV will probably extinct the reactions
due to too short contact time at a certain reaction temperature [13].The effect of concentration is the
fourth important effect in this evaluation. The concentration has a more
significant effect on conversion when GHSV is high than at lower GHSV level. The
positive effect of higher concentration can be explained, for example, with the
increased temperature. This effect is most pronounced at the lower temperature
level and high GHSV level, when the reaction phase is moved away from bulk mass
transfer controlled phase of reaction. When there is more n-butyl acetate
available, more reaction heat is generated and surface reactions are enhanced
as long as there is enough oxygen available. These experiments are carried out
in lean conditions, and surplus oxygen is available. In addition to surface
reaction, chemisorption and desorption phases may be enhanced, as they are more
temperature dependent than pore and bulk mass transfer phases. It is good to
notice that in industrial flow-reversal applications, oxidation of 2000 ppm of n-butyl
acetate is enough to allow autothermal operation. The “thermal effect,” however, is probably not the only explanation to enhanced conversions at the higher
concentration level, but also probability of reactions at the catalytic surface
increases when more reacting molecules exist in the reacting mixture. This is
especially a more pronounced effect at laboratory-scale experiments, where
autothermal operation or even adiabatic conditions were not present.Despite the lack of previous
information on oxidation of solvent compounds in the presence of water, methane
oxidation (in the presence of water) has been studied quite considerably. It
has been observed that water has an inhibiting effect on the Pd/Al2O3 catalysts [14]
as well as Pt-Pd/Al2O3 catalysts [15]. On
contrary, Li et al. [16] reported that water vapor might have a promoting
effect on the catalytic oxidation of methane over Co/Mn mixed oxides. Keeping
in mind that the oxidation mechanism for methane and n-butyl acetate is
different, it was found out that in the case of n-butyl acetate oxidation 2.5 vol-% of water did not show any significant effect on the conversion of n-butyl
acetate, in general (See Figures 2(a) and 2(b)). However, the slight decreasing
effect of water on the end conversions seemed to depend on the values of GHSV,
concentration, and temperature.In our previous study [3], it has
been observed that close to the catalyst light-off temperature some by products
are formed. These by products are normally partially oxidized compounds that
escape from the catalyst surface before the total oxidation. Most of these
organic compounds have smaller molecular weight than n-butyl acetate and the quality
as well as quantity of these compounds depends strongly on the used catalysts.
The formation of by products is decreased when temperature is increased further
and the conversion of n-butyl acetate proceeds 100%. When the conversion of
n-butyl acetate is not complete, as in these experiments, it is noteworthy to
consider also the possible by product formation, that is, the selectivity of
the catalyst. The by product formation was followed also during these
experiments, and the results showed that some organic by products are formed in
these experimental conditions. Total concentration of detected by products was
always less than 20 ppm, even in the worst case. The formation of by products
is slightly more significant when GHSV is at the higher level. This can be
explained by the shorter residence time of reactants inside the reactor. Interestingly,
at higher levels of GHSV and temperature, the moisture somewhat decreases the formation
of organic by products. However, at the same time end conversion is very
slightly decreased. Perhaps water molecules occupy sites where n-butyl acetate
is adsorbed and as conversion of n-butyl acetate is suppressed, also the formation
and escape of by products is suppressed as well. Confirmation of this would,
however, require much deeper studies on the mechanism of n-butyl acetate oxidation
in the presence of moisture than studies carried out in this case. Ageing, in
this case, did not change the formation of by products significantly. Only a
slight increase in the by product formation is observed when the GHSV is at the
higher level and temperature is at the lower level.During the experimental procedure,
the BET surface area of the fresh catalyst decreased slightly from its initial
value (65.2m2g−1). However, the activity of the catalyst
was improved probably due to further calcination of the catalyst [17]. The BET value
and light-off temperature of the aged catalyst did not change significantly
from the initial values during these experiments, only the end conversions were
improved significantly after the experiments (see Figure 3). It seems that the
aged catalyst has been regenerated during the experimental procedure, which
shows that the slight deactivation is reversible. The shape of the light-off
curve (in Figure 3) for the aged catalyst before the factorial experiments
indicates the pore blockage as being one of the possible deactivation
mechanisms. Pore blocking might be due to coke formation in the oxidation of
carbon-containing compounds and the regeneration of the catalyst could be then
carried out, for example, by increasing the temperature momentarily (see also
Figure 3 aged before and after).In summation, the results show
that increasing the operation temperature and inlet concentration, and
decreasing the GHSV improves the conversion of n-butyl acetate. Moisture has
only a minor effect, which is even decreased when temperature is increased.
These results are illustrated more visually in Figure4.Contour-plots for parameters.
Left column shows other parameters versus temperature for the fresh catalyst
and right column for the aged catalyst. The other parameters are in the
mid-level, that is,GHSV=47250 h−1, c=3000 ppm, and moisture=1.2 if they are not presented in the figure.
(a)(b)(c)(d)(e)(f)The experiments were carried out
close to the optimum (see Figure4), and thus clear limited optimum area for
the parameters in n-butyl acetate oxidation was not found according to these
experiments. However, in practice, for example, temperature has a limiting
value due to heating costs and durability of construction materials of the
incinerator. Further, this sets a limit to the maximum GHSV when the total
oxidation of n-butyl acetate is desired. The concentration is limited by
flammability limits and regulations. For example, in Finland
the maximum limit for the
VOC concentration is 25% of LEL, which means in practice approximately 8 g of solvents
in 1 m3 of air [18]. Furthermore, as literature and discussions
above show, these results are not directly applicable for the catalytic
incinerator, which operates with flow reversal.


## 4. Conclusions

According to the study, space velocity, concentration,
temperature, and water have an effect on the activity of the catalyst, as
expected. GHSV has the largest and negative effect on the conversion of n-butyl
acetate over the Pt/Al2O3 catalyst. The interaction of
GHSV and temperature had more important effect than concentration on n-butyl
acetate oxidation. Increasing temperature and concentration and decreasing GHSV
enhance the oxidation. Moisture (2.5%) had only a minor decreasing impact on
the n-butyl acetate conversion, but it also decreased somewhat the formation of
organic by products. Similar results were achieved with fresh and aged
catalysts. However, the aged catalyst was slightly more affected by changes in
GHSV than the fresh one. These results and the model are valid only in the used
experimental region, which in this case, lays in mass transfer limited region
of the catalytic oxidation of n-butyl acetate.



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*Source: 759141-2008-06-19.xml*