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Fig. 7 illustrates the specific surface Sg and the effective diffu-
S r r = r
r which is also displayed in Fig. 9 (right axis).
sivity of selected species Deff ;i as a function of εM. The specific surface area decreases linear with εM and the diffusivity increases quadratically with εM. Both trends can be mathematically
explained by Eqs. (8) and (11). The interplay between diffusion and reaction was already found a few decades ago (Hegedus, 1980). The question is how much the yield can be improved by tuning the pore-structure of the catalyst pellet. In the presented study, even with slight modification, from εM ¼ 0:3 to εM ¼ 0:25, 1% of yield improvement can be achieved which is of industrial interest (Trifirò and Grasselli, 2014) due to high production capacity of MAN. These results clearly demonstrate that the role of the pore
structure of the catalyst can be as important as the improvement of the catalyst. Even though the presented values are based on estimated values of pore structure, the trend observed from the calculations will be conserved in reality. Another message from Fig. 6 is that a not optimized fraction of macro-pores from the pelleting process can make a poor-performance catalyst even though the active ingredients are not altered.
In order to better understand the influence of the pore struc- ture parameter εM on the pellet scale, the concentration profiles of
d ¼ð 1 — 3Þ ð 1 þ 2Þ
The differential selectivity inside a catalyst pellet is influenced by numerous variables (temperature, maleic anhydride concentration in the pellet, diffusion rate of n-butane and maleic anhydride in
Dimensionless pellet coordinate
C4H10 and C4H2O3 inside the pellet located at position 1 are plotted
0 0.2 0.4 0.6 0.8 1
Dimensionless pellet coordinate
in Fig. 8. When εM is lower than 0.1, very steep concentration
gradient of C4H10 can be observed. This is a clear indication of the
diffusion limitation that barely any reactant molecules can enter
the interior active surface. After introducing the macro-pores, for example εM ¼ 0:1, the inner parts of the catalyst pellet become accessible. When εM o0:1, the concentration profile of C4H2O3 first
Fig. 8. Effect of the macro-pore porosity εM on concentration profiles of C4H10 and C4H2O3 inside the pellet located at position 1.
Fig. 7. Effect of the macro-pore porosity εM on the specific surface area Sg and effective diffusivity Deff ;i .
Fig. 9. Effect of the macro-pore porosity εM on the integrated average reaction rates
ri and the differential (local) selectivity of the pellet located at position 1. The differential selectivity is defined as Sd ¼ ðr1 — r3Þ=ðr1 þ r2 Þ.
306 Y. Dong et al. / Chemical Engineering Science 142 (2016) 299–309
the pore). Increasing εM leads to an increase in the local selectivity