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Fig. 3. Predicted velocity field (left), temperature profile (right) of the reference case.
Fig. 4. Predicted concentration profile for C4H10 (left) and C4H2O3 (right) of the reference case.
304 Y. Dong et al. / Chemical Engineering Science 142 (2016) 299–309
Position 1
Position 2
Position 3
Dimensionless pellet coordinate
Position 4
Position 5
Fig. 5. Concentration profiles of C4H10 (left top), C4H2O3 (right top), CO2 (left bottom) and temperature profile (right bottom) inside the pellet at five sampling positions in the reactor. Position 1: r ¼ 0 m, z ¼ 0.1 m; Position 2: r ¼ 0 m, z ¼ 0.25 m; Position 3: r ¼ 0 m, z ¼ 0.4 m; Position 4: r ¼ 5:25 · 10 — 3 m, z ¼ 0.1 m; Position 5: r ¼ 10:5 · 10— 3 m, z ¼ 0.1 m.
the progress of reaction, the concentration gradient is flattened as expected. The flattened profiles along the reactor are a sign for the decreasing reaction rates along the bed due to lower concentration of the reactants and decreasing temperature along the bed.
If the entire reactor for n-butane oxidation is packed with the same catalyst material, one can image that the catalysts in the lower part of the bed are not used as efficiently as at the bed inlet (Trifirò and Grasselli, 2014). From an economic point of view, a more structured catalyst packing where catalysts with different kinetics are packed at different sections of the reactor may be a good approach (Guettel and Turek, 2010). Positions 1, 4, and 5 are taken from the same axial coordinate but different radial coordi- nate. One can observe rather different concentration profiles
inside the pellet. Position 1 is almost in the hot-spot region as one
can see in the temperature plot and that may explain the higher concentration gradient with higher reaction rates. Figs. 4 and 5 show that the catalyst pellet located in different sections of the reactor may experience different conditions (concentration of the reactants and temperature). It is possible that these catalysts are chemically different from each other especially on the surface in
Fig. 6. Effect of the macro-pore porosity εM on the simulated fixed-bed reactor performance for n-butane oxidation.
outlet of the fixed-bed. These were calculated as integral average at the outlet:
N — N
accordance to the exposed conditions. In future work, incorpora-
tion of the surface dynamics in the model can be an insightful refinement of the model.
3.2. Effect of pore structure parameters
In total, there are four pore structure parameters in the model of Wakao and Smith (1962) which can be varied or optimized. In the presented study, only three parameters were studied since the total porosity of the catalyst pellet was kept fixed to 0.5. By fixing the total porosity, the pellet density and bed density were kept unchanged with varying distribution of the macro and micro pores porosity in the pellets. The following question is addressed in this work: which pore structure of the catalyst pellet is more efficient for a given mass of active catalyst and fixed pellet shape. All simulations were carried out at the same operational conditions as for the reference case.
Firstly, the macro-pore porosity εM was varied from 0 to