(24a) Impact of Mass Spectrometer Capillary Probe on the Measured Concentration in a Monolith Reactor

Impact
of Mass Spectrometer Capillary Probe on the

Measured
Concentration in a Monolith Reactor

Hoang Nguyen,
Dan Luss*, and Michael P. Harold*

Department of Chemical & Biomolecular
Engineering, University of Houston, Houston, TX 77204-4004, United States

(^*Corresponding
authors:  mharold@uh.edu, dluss@uh.edu)

Detailed spatiotemporal information
is crucial to elucidating the behavioral features of heterogeneous catalytic reactions.
The technique called capillary inlet mass spectrometry (Spaci-MS), first developed
by Partridge and coworkers at Oak Ridge National Laboratory (ORNL), enables spatially-resolved
concentration measurements using a fused-silica capillary positioned in a
monolith channel during a catalytic reaction [1]. Suction gas samples are
analyzed with a mass spectrometer to provide time-resolved species
concentrations while translation capability of the probe enables their spatial
dependencies. Currently, there is a debate on the veracity of the Spaci-MS technique
because the probe may influence the conversion in the measured channel due to blockage
of the flow by the probe. Research carried out by Deutschmann and coworkers [2] used computational fluid
dynamic modeling with spatially-resolved mass spectrometer measurements to show
that during catalytic methane partial oxidation the blockage effect was
significant even when the probe occupied only 3.5% of the channel area. It is
of interest to the catalysis and reaction engineering community to further
elucidate the utility of Spaci-MS method.

The
objective of this work is to systematically study the impact of the capillary probe
during in situ measurements in a monolith reactor. To this end, we carried out
propylene oxidation on Pt/Al₂O₃ washcoated monoliths with
a range of channel densities (100, 200, 400, and 600 cpi) with concentration
sampling by probes of two different sizes (OD = 170 & 363 µm). We
positioned another stationary optical fiber probe (OD = 125 µm) in an adjacent
channel to monitor the intra-channel temperature using the recently-developed
coherent Optical Frequency Domain Reflectrometry (OFDR) technique, applied for
the first time to catalytic reactions by Nguyen et al. [3].
Our approach is to compare the Spaci-MS measured propylene concentration
profile and the OFDR measured temperature profile. A spatial shift in the
location of the concentration (temperature) decrease (increase) would indicate
a disturbance by the probe on the extent of reaction. 

Steady
state profiles were obtained for lean propylene oxidation (0.2 vol.% C₃H₆,
10 vol.% O₂) for different combinations of the channel size and
probe diameter, and the feed gas space velocity and temperature. Our findings
reveal a complex interaction of several factors, including, in addition to the
probe diameter and channel size, the axial position of the probe, as well as
the feed flow rate and temperature. 

Typical
results in Figure 1 show the spatial dependence of propylene concentration and
temperature when the gas mixture having a space velocity of 17400 hr^-1 and
a temperature of 200°C was fed to a 600
cpi monolith (Pt loading = 23.8 g/.ft³).  In this experiment, the
363 µm OD probe occupied about 10% of the channel area.  A comparison of the
two profiles indicates a negligible shift between
the location of the maximum temperature and the propylene depletion
point (Dz ≈ 4
mm). In this case the probe had a small
impact. In contrast, Fig. 2 shows the results of an experiment in which the gas
was fed with a space velocity of 12630 hr^-1and a
temperature of 180˚C
to a 400 cpi monolith
(Pt loading = 96 g/.ft³). A comparison of the temperature and
concentration profiles revealed a major probe
impact (Dz ≈ 12
mm). Furthermore, the same shift was
obtained for two different capillary probe sizes (170
µm and 363 µm OD).
One might have expected that the larger probe would block more flow and lead to
a shift in the concentration profile upstream because of the lower flow rate
and higher residence time, following the findings
of Deutschmann et al. [2].
However,
we suspect that the larger probe increases both the
blockage and the flow suction, thereby compensating
for the blockage effect. To confirm this conjecture, we carried out
the same experiment but with a higher total flow rate (space velocity = 17400hr^-1)
while keeping other operating conditions constant. Figure
3 shows that the concentration profile measured
by the bigger probe was
shifted
towards the upstream of the reactor compared
to that of the smaller probe.
This result
indicates the stronger impact of the blockage effect
compare to the suction effect.

These
and
other experiments to be described
indicate that suction effect, channel density, and reaction
rate (catalytic activity) are important and should
be taken into account in the analysis of Spaci-MS data. Additional experiments
will be described for different combinations of the channel and probe size. We
will discuss the important factors that contribute to the observed trends, and
conditions are determined for which the impact of the
probe is negligible.   

Figure 1: Simultaneously measurements of temperature
and concentration inside the 600 cpi channel monolith reactors.

Figure 2: Intra-channel concentration measurements by
two capillary probes (170 µm and 363 µm) and temperature measurement by the
optical fiber (OD = 125 µm) in a 400 cpi monolith at 12630 hr^-1
space velocity respectively.

Figure 3: Intra-channel concentration measurements by
two capillary probes (170 µm and 363 µm) and temperature measurement by the
optical fiber (OD = 125 µm) at 17400 hr^-1 space velocity in a 400
cpi monolith respectively.

References:

[1]
J.S. Choi, W.P. Partridge, C.S. Daw, Spatially resolved in situ measurements of
transient species breakthrough during cyclic, low-temperature regeneration of a
monolithic Pt/K/Al2O3NOx storage-reduction catalyst, Appl Catal a-Gen 293
(2005) 24-40.

[2]
M. Hettel, C. Diehm, B. Torkashvand, O. Deutschmann, Critical evaluation of in
situ probe techniques for catalytic honeycomb monoliths, Catal Today 216 (2013)
2-10.

[3] H. Nguyen, M.P. Harold, D. Luss,
Optical frequency domain reflectometry measurements of spatio-temporal
temperature inside catalytic reactors: Applied to study wrong-way behavior,
Chem Eng J 234 (2013) 312-317.