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0.5 while the macro and micro-pore diameters were kept to the values of the reference case. Fig. 6 shows the simulated reactor performance, i.e the overall conversion, selectivity and yield at the
Yield ¼ Conversion · Selectivity; ð39Þ
where the NC4 H10 stands for the molar flux of C4H10 in unit of mol m— 2 s— 1. The hot spot temperature was evaluated as the maximum temperature in the calculation domain.
The macro-pore porosity controls the distribution of macro- and micro-pores which influences both diffusion and specific surface area for reaction. If the pellet porosity is made up by a higher fraction of macro-pores diffusion is faster but less active
surface is available for reaction. With increasing εM, molecules can
diffuse faster in and out of the catalyst pellet and hence the con- version will increase at first. At some value of εM (here between εM ¼ 0:2— 0:25) the loss of active surface becomes too high and conversion decreases. Therefore, an optimal value exists for con-
version with respect to the macro-pore porosity as shown in Fig. 6. The selectivity at the reactor exit increases gradually with
Y. Dong et al. / Chemical Engineering Science 142 (2016) 299–309 305
increasing εM except for the case when εM ¼ 0:5. As we will discuss in more detail later on Fig. 9, this is due to the fact that the local selectivity to MAN in each catalyst pellet increases with increasing εM. For industry, yield is important as n-butane is not recycled. As one can see in Fig. 6, the yield has similar shape as the conversion
with respect to εM but the maximum is slight shifted to εM ¼ 0:25 which is also the reference case used in this work. The hot spot
temperature follows the same trend as the conversion due to the reaction progress. The highest hot spot temperature for εM ¼ 0:2 is 36 K higher than the inlet temperature. This temperature rise is still within the tolerance temperature rise (Max. 60 K in the lit- erature) (Guettel and Turek, 2010). The resulting reactor perfor- mance indicates that catalysts with bimodal pore structure can out-perform those with only micro-pores or only macro-pores
being the two limiting cases. The optimal distribution of the macro- and micro- pores in the presented study is achieved when εM ¼ 0:25 if the maximum yield is the concerned criteria. This optimum is a compromise between the high reactive surface mostly created by the micro-pores, and high diffusion rate facili- tated by the macro-pores (transport pores).
increases gradually due to the higher concentration of C4H10 and thus higher reaction rates. The maximum found around ζ¼ 0.80 is likely because of the accumulation of the C4H2O3 molecules when the diffusion rate is still low compared to the reaction. With increasing fraction of the macro-pores inside the pellet, the con- centration profiles for both species flatten out. It is not only because of the faster diffusion of the reactants inwards and pro- ducts outwards of the pore, but also because of the loss of reaction sites due to the reduced specific surface. Above εM ¼ 0:3, the cat-
alyst performance is limited by the reduction of the active surfaces.
Reaction rates averaged over the pellet at position 1 were evaluated and depicted in Fig. 9. Reaction rates r1 and r2 show the same trend with respect to εM. This is expected from the kinetic expressions in Eqs. (5) and (6). The decomposition reaction rate r3 shows a different behavior as a function of εM. It seems that the influence of accelerated diffusion rate on r3 is less pronounced than the reduction of the surface area as one can see from the slope of the increasing part (r1, r2) compared to the decreasing part (r3). This has an impact on the differential (local) selectivity