Sd  mainly because r1  increases steeper than r3  (up to εM r0:25) or 65

60

decreases slower than r3  (for   M 40:25). This trend is the same 55

everywhere in the bed and hence we obtain an increasing  integral 50

MAN selectivity as it was shown in Fig.   6. 45

This means the diffusion–reaction balance has a different effect 40

on each inpidual reaction. With that it is possible to not only 35

30

promote conversion, but also the selectivity by changing the   pore 25

reactor scale. At this point, one can deduce that the fractions of macro- and micro-pores inside the catalyst pellet have effects on

0 1 2 3 4 5 6 7 8 9 10

dm [nm]

both the concentration profiles and reaction kinetics on the pellet scale and on the reactor scale. In addition to catalyst synthesis, optimizing   the   pellet   pore   structure   offers   the   possibility to

Fig. 11.  Effect of the micro-pore diameter  dm

performance for n-butane oxidation.

on the simulated fixed-bed  reactor

improve  industrial  maleic  anhydride synthesis.

The other two parameters studied in this work were the macro- pore diameter dM and micro-pore diameter dm. Simulations with independent variation of dM and dm were carried out while the fraction of the macro- and micro-porosity (εM ¼ εm ¼ 0:25) was fixed. From the presented results above, εM ¼ 0:25 is the optimal value for dM ¼ 100 nm and dm ¼ 1 nm. In the following section, the main goal is to investigate if further improvement is possible by

tuning the pore size. Fig. 10 shows the overall conversion, selec- tivity, yield and hot spot temperature with respect to the macro- pore diameter. Both conversion and selectivity increase with increasing macro-pore diameter slightly. The overall yield of the product is improved, e.g. by 3% by increasing dM from 100 nm to 200 nm. The increase of the overall reactor performance can be explained by the acceleration of the diffusion rate since the Knudsen diffusivity increases linearly with the pore diameters. On the other hand, the specific area for reaction decreases with increasing dM but to a less significant extent than the promotion of the diffusion. However, the hot spot temperature increases further with increasing conversion and eventually will lead to destruction of the catalyst. Therefore, a maximum of 200 nm of the macro- pore diameter can be suggested for this  case.

The influence of micro-pore diameter dm  on the overall  reactor

performance is shown in Fig. 11. The conversion of n-butane decreases rapidly with increasing dm. Even though the selectivity increases slightly, the overall yield of the product decreases with increasing dm. The decrease of conversion is a consequence of the decreasing specific area with decreasing dm. Since the micro-pore diameter is in the denominator for calculating the specific area, a reduced value of dm will lead to much higher surface area and thus an enhanced reaction rate. While, the diffusivity decreases with