2.1. Kinetic Results
The results of the catalytic activity measurements are presented in Figure 1
It can be seen that all prepared catalyst samples revealed complete NO conversion (Figure 1
a). The best activity was obtained by Cu/USY/s—catalysts prepared by sonochemical preparation route. The complete conversion was achieved at 225 °C. The comparable activity was obtained by its counterpart prepared by conventional ion-exchange. However, it is worth mentioning that the comparison of the copper content, determined by atomic absorption spectroscopy (AAS) (Table 1
), in sonochemically prepared sample (Cu/USY/s) is ca. 25% lower than in Cu/USY sample.
When comparing the activity of ZSM-5 catalysts, the best was obtained by sample prepared by conventional ion-exchange method, Cu/ZSM-5. The complete NO conversion was achieved at 350 °C, whereas for catalysts prepared by sonochemical method, the maximum NO conversion was shifted to 400 °C. However, to comprehensively compare the overall performance of copper exchange zeolite catalysts prepared by both conventional and sonochemical methods, the selectivity towards N2
should also be considered. The selectivity curves are presented in Figure 1
b. It must be emphasized that for all catalysts prepared by sonochemical route, the selectivity is almost constant, and for Cu/ZSM-5/s sample, varies between 99% and 100%, whereas for the most active catalyst sample, Cu/USY/s, the selectivity varies from 99% at 150 °C to 98% at 500 °C. For Cu/USY catalyst sample, the most significant decrease in selectivity can be noticed at 400 °C, reaching the selectivity to N2
equals 94%. The lowest and the most visible decrease in selectivity was observed for Cu/ZSM-5 sample, for which the selectivity drops from 100% at 250 °C to 86% at 500 °C. When considering the deNOx
reaction selectivity, the impact of N2
O that could be formed during the reaction cannot be neglected. Since the N2
O have a strong greenhouse gas, it may affect the whole SCR process. In this study, the series of the Cu/USY and Cu/ZSM-5 catalysts varying the method of preparation are described. As presented in the catalytic activity tests (Figure 1
a), the high catalytic activity windows are different for both catalysts. The Cu/USY and Cu/USY/s catalysts are active in low temperature region (starting from 200 °C), which is below the temperature region for commercially used V2
. At low temperature regions, the selectivity to N2
O is low and does not exceed 6%. On the other hand, when considering the high temperature deNOx
catalysts (Cu/ZSM-5/s, Figure 1
b), the selectivity to N2
O was below 1%.
To compare the catalysts performance, the Arrhenius equation parameters were determined according to the Arrhenius equation (Equation (9)). The Arrhenius plots are presented in Figure 1
c. It can be inferred that the lowest activation energies were obtained for faujasite catalysts, where apparent activation energy was equal to 42.76 and 36.21 kJ/mol for Cu/USY and Cu/USY/s catalysts, respectively. The higher activation energies were obtained for Cu/ZSM-5 and Cu/ZSM-5/s and equal to 78.39 and 96.01 kJ/mol, respectively. The calculated values are typical for copper exchange zeolite catalysts for deNOx
abatement and are close to those found in the literature for similar catalytic systems [19
Based on the catalytic activity tests of the prepared zeolite catalysts, for modelling purposes, the modelling of structured reactors in SCR deNOx was performed for catalysts prepared by sonochemical route, Cu/USY/s and Cu/ZSM-5/s, respectively.
2.2. Modelling Results
The modelling results for both Cu/USY/s and Cu/ZSM-5/s catalysts are presented in Figure 2
The reactor performance is presented in a form of conversion profiles along the reactor in Figure 2
a,b for both Cu/USY/s and Cu/ZSM-5/s, respectively. The reactor performance was also presented as a temperature distribution along the reactor (Figure 2
c,d). Due to the sufficient heat exchange between the surface and the bulk phase, the differences between their temperatures can be neglected. The maximum difference between those two areas does not exceed 3 K at the reactor entrance for the monolith structure. For the wire gauze and foam structured supports, the difference in bulk and surface temperature was as low as 0.3 K at the reactor entrance. Thus, for comparison reasons, the temperature along the reactor was presented only for the bulk phase.
As can be noticed, the performance of the Cu/USY/s catalysts during the NOx
SCR reaction is substantial. It is due to the high activity of the Cu/USY/s obtained during the activity tests. The most significant impact on the modelling results can be attributed to calculated activation energy (Figure 1
c). The calculated Ea
values for Cu/USY/s and Cu/ZSM-5/s substantially differ and they equal 36.21 and 96.01 kJ/mol, respectively (Table 1
). Moreover, the differences between the modelled reactor supports represented by wire gauzes and metal foam are not substantial. The best performance was achieved by the reactor composed of wire gauze structures with deposited Cu/USY/s catalyst (Figure 2
a). The nearly complete conversion was achieved after 0.1 m of the reactor. The reactor composed by the foam structure at similar reactor conditions achieves 97% conversion of NOx
’s. The small differences between performance of the modelled structured supports are due to the differences between the hydraulic diameters of both wire gauze and foam structures. Despite the slightly slower NO conversion for foam structure, the comparison of the shape of the light-off curve, the most significant differences can be noticed at the very beginning of the reactor. However, the maximum difference in NOx
conversion was noticed at 0.05 m of the reactor. The NO conversion at 0.05 m of the reactor with wire gauze support was equal to 76%, whereas for foam-supported reactor the conversion was 56%. The differences in NO conversion between both foam and wire gauze structures disappear at complete NOx
conversion. On the contrary, the most significant differences at the reactor performance can be observed for modelled 100 cpsi (channels (cells) per square inch) ceramic monolith. The complete conversion achieved by Cu/USY/s catalysts deposited on monolith structure was at 0.46 m reactor. Similar trends can be observed when comparing the surface temperature (Figure 2
c). The maximum temperature, ca. 704 K, is achieved and all NO are reduced during the reaction.
On the contrary, the simulation performed for the Cu/ZSM-5/s catalysts reveals that the reactor length required for complete NO conversion in case of reactor composed of wire gauzes is equal to 0.96 m, whereas for reactor composed of foam structure, the required length is equal to 1.44 m. The worst performance was presented by reactor composed of 100 cpsi ceramic monolith. The complete conversion of 2500 ppm of NO is achieved at 2.88 m.
For the assessment of the performance of the reactors composed of all structured supports, the evaluation criteria including reactor length (LR, 90%
), pressure drop (∆P90%
) and catalyst mass present on the geometric surface area of the carrier (Mcat, 90%
) were compared and presented in Figure 3
Over the three modelled structured supports, the wire gauze and foam metallic carriers allow for the substantial shortening of the reactor. The required length of the reactor to achieve 90% conversion of 2500 ppm of NO at 673 K at 1 m/s is equal to 0.06 and 0.08 m for wire gauze and foam carriers, respectively. At the same time, the required length of the reactor with ceramic monolith is almost three times higher and equals 0.18 m. Catalysts loading in both monolith and wire gauze structures, represented by Mcat, 90%
do not differ substantially. The required amounts of catalysts are equal to 3.86 and 3.84 kg/m2
, for monolith and wire gauze, respectively. Nevertheless, the Mcat, 90%
calculated for foam structure is slightly higher and equals 4.63 kg/m. The most significant impact on that parameter can be attributed to the specific surface area (a, m−1
). Despite the fact that the required lengths of the modelled reactors are three times shorter than in the case of the monolith structure, their specific surface area is more than two times higher (cf. Table 2
). When comparing another important engineering characteristic—flow resistance represented by ∆P—the unarguable lowest value was achieved by monolith structure (0.21 kPa). Over the metallic supports, the lowest pressure drop was achieved by foam structure and was equal to 1.34 kPa (Figure 3
The comparison of three reactor performance parameters for Cu/ZSM-5/s catalysts reveals similar trends. All calculated reactor characteristics are almost one order of magnitude higher than those obtained for Cu/USY/s catalyst.
Over presented simulations, it is also worth considering the reactor performance at different inlet superficial gas velocities. The results of simulations are summarized in Table 2
The differences between the calculated reactor lengths required to achieve 90% NO conversion at different gas velocities are substantial. It can be inferred that decreasing the gas velocity to 0.5 m/s results in decrease of all parameters. The reactor length and catalyst loading are almost two times lower than at 1 m/s gas velocity. Furthermore, the decrease of gas velocity result in four times the decrease of the pressure drop.
The reversed tendency can be observed for higher velocities. Increasing the superficial gas velocity to 2 m/s results in an increase of the required reactor length to 0.12 and 0.16 m for wire gauze and metallic foam structure, whereas for the monolith structure the reactor length increases to 0.36 m.