United States
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park, NC 27711
Research and Development
EPA/600/S7-88/017 Dec. 1988
&EPA Project Summary
Mixture Effects in the Catalytic
Oxidation of VOCs in Air
S. Gangwal, K. Ramanathan, P. Caffrey, M. Mullins, and J. Spivey
Most volatile organic compound
(VOC) releases into the environment
are mixtures. However, most
fundamental studies of the catalytic
deep oxidation of such compounds
are usually confined to single
components. This study examines
the deep oxidation of organic
mixtures over a heterogeneous
catalyst in an attempt to explain
earlier observations concerning the
apparent inhibition or enhancement
of destruction of some components
to establish a scientific basis for the
design and operation of catalytic
incineration systems for VOC control.
To elucidate these effects, the
oxidation kinetics of n-hexane,
benzene, ethyl acetate, and methyl
ethyl ketone in air were examined
over a commercial catalyst (0.1%
Pt/3% Ni on Y-a'umina.) Reaction
rates of these components
individually were determined at
temperatures of 150 to 360 °C from
differential reactor studies. When
these were compared to overall
destruction efficiencies from integral
reactor studies for both individual
compounds and mixtures, the
Mars/van Krevelen (MVK) reaction
rate model satisfactorily represented
the results for some single organic
compounds at lower temperatures.
By incorporating pore diffusion ef-
fects, the MVK model adequately
explains the single component data
over the entire temperature range for
some of the compounds. A multi-
component MVK model incorporating
competitive adsorption effects is
moderately successful in predicting
the observed behavior for a binary
mixture of benzene and n-hexane;
however, it cannot predict the
apparently enhanced reaction rate
observed for ethyl acetate at higher
temperatures (>220°C). Other
reaction pathways available for
compounds with carbon-oxygen
linkages and/or the advent of
catalytically supported homogeneous
combustion with free radical
precursors may explain this
phenomenon. The enhancement of
ethyl acetate conversions in
humidified air streams suggests that
autocatalysis by-product water may
be a possible mechanism.
This Project Summary was
developed by EPA's Air and Energy
Engineering Research Laboratory,
Research Triangle Park, NC, to
announce key findings of the research
project that is fully documented in a
separate report of the same title (see
Project Report ordering information at
back).
Introduction
The goal of this study was to establish
a scientific basis for the selection and
evaluation of heterogeneous catalysts
and operating conditions for the control of
gas streams containing mixtures of
volatile organic compounds (VOCs). The
research was devoted to both
experimental evaluation of the catalytic
oxidation of VOC-containing mixtures
and the kinetic interpretation/modeling of
the results.
Experimental
A schematic diagram of the experi-
mental setup is shown in Figure 1. Table
-------
Control Thermocouple
Measurement Thermocouples
GC Carrier Gas
Valve-Oven with 10-Port valve
FC: Differential flow controller
PR: Two-stage pressure regulator
BFM V Soap bubble flow meter and vent
Process lines
Electrical
Integrator
Actuator
Controller
and Timer
Figure 1. Microreactor system.
1 details the mixtures studied, and Table
2 describes the experimental conditions.
Results
Figure 2 shows the inhibition of n-
hexane conversion in mixtures. Figure 3
shows the enhancement of ethyl acetate
conversions in mixtures. Experiments
were conducted in humidified air streams
to ascertain if hydrolysis by-product
water may be responsible for the en-
hancement. Figure 4 shows the en-
hancement of ethyl acetate conversions
in humidified air streams.
Detailed experiments were carried out
to obtain fundamental rate constants for
the catalytic oxidation of the compounds
in Table 1 as single components. These
constants were incorporated into a
reactor model based on the Mars/van
Krevelen kinetic mechanism to try and
predict the observed mixture effects.
Figure 5 represents the fit of the model
to the single component benzene data.
The fits for n-hexane and ethyl acetate
were not as good. Mixture effects in a
binary n-hexane/benzene mixture were
also marginally predicted by the model.
Conclusions
• Conversions of components in a
mixture of organics may be
significantly higher or lower than
when present by themselves.
• Mars/van Krevelen (MVK) kinetic
mechanism is an adequate
representation for the deep
oxidation of single organ
compounds over the catalyst use
in this study.
• A reactor model incorporating poi
diffusion effects and MVK kinetic
adequately explained singl
component benzene data over tt
entire temperature range of intere
(150to360°C).
• A multicomponent reactor mod
incorporating pore diffusion effec
and a simple proposed extension
MVK kinetics was marginal
successful in predicting benzeni
n-hexane mixture behavior.
• Conversions of oxygenated speci<
such as ethyl acetate are higher
mixtures than in single con
-------
Table 1. Test Gas Mixtures
Concentration in Air (ppmv)
Single Hydrocarbon
Binary Mixtures
benzene: 9, 69, 163, 375, 525
ethyl acetate: 53, 109, 238, 450
n-hexane: 201, 410, 566
methyl ethyl ketone (MEK): 25, 50, 70, 135, 190, 298
1 93 ppm benzene + 1 72 ppm ethyl acetate
1 89 ppm benzene + 1 90 ppm n-hexane
184 ppm ethyl acetate + 190 ppm n-hexane
135 ppm MEK
135 ppm MEK
149 ppm n-hexane
160 ppm benzene +
Ternary Mixtures 143 ppm benzene
143 ppm benzene + 174 ppm ethyl acetate
90 ppm n-hexane; 103 ppm n-hexane + 10'
benzene + 93 ppm MEK
104 ppm
ponents. The MVK model appears
to explain the data in the kinetic
regime. MEK oxidation shows lower
apparent enhancement than ethyl
acetate. Other reaction pathways,
including thermally enhanced free
radical mechanisms and the
interactions of oxygen containing
species with partially reduced
metal surfaces may explain the
observed phenomena. Further
experimentation is necessary
before a specific model can be
postulated for the oxidation of
oxygenated compounds.
The presence of water vapor in the
gas stream increases the
conversion of ethyl acetate
significantly. This suggests that
autocatalysis by-product water
may be responsible for the
observed enhancement described
above. The fact that MEK
conversions are not significantly
affected by the presence of water
vapor lends credence to this
theory.
Table 2. Experimental Conditions
Pressure:
Temperature Range:
Space Velocity at ReactionTemperature:
Catalyst Bed:
Hydrocarbon Concentration in Air:
Catalyst Particle Size:
Ratio of Reactor Diameter to Particle Diameter
1 atm
140 to 360 °C (isothermal
operation)
50,000 to 1,000,000 rr1
17 to 20 mg
10 to 600 ppmv
120 to 170 mesh
20:1
-------
110
100
90
80
70 -
I 60
<3
O SO
I
u
">- 40 •
30 -
20 •
10-
410 ppm n-hexane in air
+ 190 ppm n-hexane with
184 ppm ethyl acetate in air
• 730 ppm n-hexane with
189 ppm benzene in air
• 90 ppm n-hexane with
174 ppm ethyl acetate in air
143 ppm benzene in air
17 mg, 120 x 170 mesh catalyst
WHS V = 209
l
260
I I 1 I
300 340 3SO
140 180
Temperature (°C>
Figure 2. Effect of multicomponent mixtures on n-hexane conversion.
-------
I
I
-------
• Dry
• 0.01 g Water/g Air
WHSV- 209
150 ppm ethyl acetate
10 I—
160
180 200 220 240 260 280 300
Temperature (°C)
320 340
Figure 4. Comparison of ethyl acetate conversions in dry and humidified air streams.
100
------- |