Chapter 2 Experimental Methodology
2.5 Catalytic Activity Testing
2.5.1 CO Oxidation Catalytic Testing
CO oxidation activity tests for the Co3O4 nanoparticles were carried out using 50
mg of catalyst per channel under a 2% CO/8% O2/N2 gas stream at a space velocity
of 60,000 ml-h−1-gcat−1 The reactor effluent composition was measured via FTIR
temperature, with ramp up and stabilization between temperatures requiring less than 10 minutes. Catalyst bed temperatures were measured for each channel and used in subsequent calculations. A single channel was left empty of catalytic material in each experiment to ensure zero conversion of the empty reactor tubes. All catalytic activities were tested under atmospheric pressure. CO conversion was calculated using the effluent CO and CO2 concentrations as shown in Equation 2.18.
COconversion= (1− COout
COout+CO2,out
)∗100% (2.18) Kinetic evaluations of catalysts were also performed by utilizing the Arrhenius equation in Equation 2.19 where A is a pre-exponential factor, Ea is the apparent
activation energy and r is the reaction rate [152]. The nomenclature of apparent ac- tivation energy is used to the actual activation energy from the measured activation energy, where the effects of reaction concentration at the catalyst surface on reaction temperature cannot be compensated for [152]. In general, measurements of apparent activation energy were made by measuring conversion over a temperature ramp at dif- ferential conversions. The Arrhenius expression in Equation 2.19 was then linearized in a plot of the natural log of rate versus inverse temperature such that the apparent activation energy could be estimated from the slope of the plot.
r=aexp(−Ea
RT ) (2.19)
Reaction orders were calculated using a standard power law kinetic model as shown in Equation 2.20. In a similar manner to the calculation of apparent activation energy, this expression is linearized such that a plot of the natural log of the reaction rate vs. the natural log of the partial pressure of each component can be used to estimate the reaction order of each component from the slope.
Figure 2.14: CO2and CO stretching IR absorption at concentrations between 0.25-2%
FTIR Calibrations for CO Oxidation Experiments
FTIR calibrations for CO and CO2 were carried out via GRAMS software and the
PLS-1 regression model. Calibration points were collected at a mesh of 0.25% between 0 and 2% CO2 with balance CO (CO+CO2 = 2%). Specifically, the model utilized
the infrared active CO stretch around 2150 cm−1 and CO2 assymetric stretch at 2350
cm−1 shown in Figure 2.14. R2 values for all 16 channels were <0.99 demonstrating
excellent goodness-of-fit. To further ensure calibration accuracy, the models were applied to validation points collected between 0.5 and 2% CO and CO2, the latter of
which are shown in Figure 2.15. Analysis of error showed that carbon mass balances were closed with an average error of <0.5%, further proving the adequacy of the calibrations.
Figure 2.15: Validation of CO2 FTIR calibration for HTR channels
2.5.2 Ethylene Epoxidation Catalytic Testing
Ethylene epoxidation activity tests for co-promoted Cu-Ag/α-Al2O3 catalysts were
carried out using 200 mg of catalyst per channel under a 10% ethylene and 10% oxygen gas stream at a space velocity of 4,000 h−1. Data was collected between 215-300◦C, allowing a 30 minute stabilization period at each temperature in order to be able to compare ethylene oxide selectivity at a fixed conversion (but variable temperature) level. Conversion and selectivity data collected over the temperature ramp was also used in the determination of apparent activation energies. Additionally, the partial pressure of both O2 and C2H4 was varied while the other was held constant to explore
the catalyst activity under oxygen rich and ethylene rich conditions and make a calculation of reaction order with respect to each component. Ethylene conversion and ethylene oxide selectivity were calculalated using the effluent ethylene, ethylene oxide, and carbon dioixde concentrations using Equations 2.21 and 2.22, respectively.
It was possible to simply omit water from the calculation of ethylene conversion since its concentration is related to that of CO2 by stoichiometry. This is desirable due
to the inherent difficulty of making an accurate quantitative measurement of water using FTIR.
C2H4conversion = (1−
C2H4,out
C2H4,out+ET Oout+ 0.5CO2,out
)∗100% (2.21)
ET Oselectivity = ET Oout
ET Oout+ 0.5CO2,out
x100% (2.22)
FTIR Calibrations for Ethylene Epoxidation Experiments
The limit of detection for ethylene in the previously described parallel FTIR spec- troscopic imaging apparatus was found to be between 0.05 and 0.125%. This means that, for a feed gas containing 10% ethylene by volume, conversions up to 99% can theoretically be measured in the experimental setup. In order to accomplish this, mul- tivariate calibration techniques were used to quantify effluent ethylene (0-10%), EO (0-1%), and CO2 (0-10%), where multivariate calibrations were necessary to quan-
tify ethylene and ethylene oxide due to their overlapping IR bands. Accuracy of all calibration files were established by collecting validation data sets including varying levels of ethylene, ethylene oxide, and carbon dioxide and comparing expected to predicted concentrations for each species.
Generating high quality quantitative FTIR data for ethylene epoxidation requires careful calibrations accounting for the overlapping IR bands of water, ethylene oxide, C2H4, and CO2 as shown via the representative IR spectrum of the gas phase effluent
during the ethylene epoxidation reaction in Figure 2.16. This figure gives light to the overlapping IR regions of ethylene and ethylene oxide in the C-H stretching region (2700-3200 cm−1) as well as water and ethylene in the 1200-1600 cm−1 range and
Figure 2.16: Sample FTIR spectrum during ethylene epoxidation reaction for different catalysts
stretching peak at 2350 cm−1 becoming saturated at concentrations greater than 2%
both the stretch and the CO2 overtone peak required calibration.
FTIR calibrations for ethylene epoxidation were carried out via GRAMS software and the PLS-1 regression model. To ensure calibration accuracy, the models were applied to validation points collected between 0-0.5% ethylene oxide and 0-6% C2H4.
The expected and predicted points are shown in Figure 2.17.
The average relative error of each channel is shown in Figure 2.18 showing that all errors are under 10% and are actually slightly higher for C2H4 predictions than
ethylene oxide.
Validation points for the CO2 stretch and overtone calibrations were collected
Figure 2.17: Validation of C2H4 and EO FTIR multivariate calibration for HTR
channels
these validations are shown in Figure 2.19.
2.5.3 Ethane Partial Oxidation Catalytic Testing
Ethane partial oxidation tests were performed using an ethane to oxygen ratio of 4:5 (40% ethane, 50% oxygen, 10% nitrogen). On stream results were analyzed with a Shimadzu Gas Chromatograph GC2014 equipped with a TCD detector to analyze the product distribution. The experiments were carried out in a quartz tube plug flow reactor operating at ambient pressure, where the space velocity was kept at a constant 1200 hr−1. The catalysts were used as synthesized and a temperature ramp ranging
from 120-460◦C was used with 30◦C step sizes to probe the temperature profile of each catalyst. Temperature measurements were taken inside the catalyst bed using an insulated K-type thermocouple. Ethane conversion and selectivity toward acetic acid,
Figure 2.18: Relative error for C2H4and EO calibration predictions
Figure 2.19: Validation of CO2 HTR FTIR calibrations for asymmetric stretching
ethylene, carbon monoxide, and carbon dioxide were calculated using Equations 2.23, 2.24, 2.25, 2.26, and 2.27, respectively. C2H6conversion = (1− C2H6,out C2H6,in )∗100% (2.23) AAselectivity = AAout
C2H4,out+ 0.5COout+ 0.5CO2,out+AAout
(2.24)
C2H4selectivity=
C2H4,out
C2H4,out+ 0.5COout+ 0.5CO2,out+AAout
(2.25)
COselectivity = 0.5COout
C2H4,out+ 0.5COout+ 0.5CO2,out+AAout
(2.26)
CO2selectivity=
0.5CO2,out
C2H4,out+ 0.5COout+ 0.5CO2,out+AAout
(2.27)