Consideraciones de la presentación de resultados

The thermal conductivity of the wall is an important design parameter, since it influences heat communication between channels as well as along the reactor. Calculations presented so far are based on typical metallic wall thermal conductivity. In the following, reactor behaviour is studied for the case where thermal conductivity is 50 times lower than the base case, keeping everything else constant. This ratio of thermal conductivities is typical of metal/ceramic materials. Figure 3.10 presents channel centreline dimensionless temperature profiles along the reactor for both sides. It can be seen that significant radial temperature differences appear. In addition, hot spots in the axial profiles are observed, while these are not present for the base case calculations

Modelling o f a CPR fo r Dehydrogenation-Combustion Coupling

(see Figure 3.3). The above behaviour is due to the fact that heat conduction is not efficient enough to transport the large amount of heat generated by the catalytic combustion axially downstream the reactor, as well as radially to the endothermie reaction location. Combustion rate becomes higher and gas temperature increases.

800 Combustion channel 760 U o 720 Dehydrogenation 680 chaniel 640 0.0 0.2 0.4 0.6 0.8 1.0

Axial C o-o rd in ate, m

Figure 3.10. Variation of temperature at channel centrelines along the reactor. Solid thermal conductivity in 50 times smaller than that of the base-case.

Temperature differences between specific location across the radial direction, as defined in Figure 3.4, are shown in Figure 3.11. Several aspects can be identified by comparing with the base case results of Figure 3.5. Large temperature difference between the two sides of the wall is observed, and reaches a maximum of 22 °C (see Figure 3.11 b). For the base case though, the corresponding maximum does not exceed 0.4 °C (see Figure 3.5b). This is consistent with increased heat transport resistance across the wall. The most significant temperature difference is observed between channel centrelines, as opposed to the base case, where the highest temperature difference is obtained between the gas phase and the channel wall of the dehydrogenation side (compare Figures 3.5

Modelling o f a CPR for Dehydrogenation-Combustion Coupling

and 3.11). This is due to the shift of the prineipal heat transfer resistance from the gas phase to the solid phase. Slower axial heat conduction results not only to hot spots as mentioned above, but also it reduces the difference between gas and wall temperatures at the entrance of the reactor, particularly for the dehydrogenation channel (compare Figures 3.5c and 3.11c). An interesting feature is that the gas temperatures can exceed wall temperatures in the second half of the reactor (see Figures 3.11c and 3.1 Id), something that was not observed for the base case, which indicates the increased contribution of convection as compared to solid phase conduction for heat transport.

25 -- 20 y E 0.0 0.2 0.4 0.6 0.8 1.0 25 20 15 10 5 0 0.0 0.2 0.4 0.6 0.8 1.0 Axial Co-ordinate, m 10 -- 0.0 0.2 0.4 0.6 0.8 1.0 -2 -6 -14 4 0.6 0 0.2 0.4 0.8 1 Axial Co-ordinate, m

Figure 3.11 Variation of temperature differences (for definitions see Figure 3.4) along the reactor. Solid thermal conductivity is 50 times lower than that of base ease.

Modelling o f a CPR fo r Dehydrogenation-Combustion Coupling

3.5 Conclusions

Catalytic Plate Reactors (CPRs) represent a novel reactor design (Reay 1993, Charlesworth 1996) that despite its attractiveness has not been implemented in practice yet. Its feasibility is demonstrated from the theoretical point o f view by means of the present work. It is proved that such reactors combine reaction with heat exchange in an intensified manner. Although, the autothermal coupling o f the exothermic and endothermie reactions by means o f indirect heat transfer has been previously presented in the literature (Frauhammer et al 1999, Kolios et al 2001) such approach refers to monolith channels rather than alternate plates. The model is based on a simplified ID approach and is limited to study mainly the axial temperature profile for a counter- current flow arrangement. Therefore, the features of the present model represent an element o f novelty not only due to its formulation based on a 2D- approach, but also due to parametric studies performed. The 2D-approach makes the model more realistic by increasing its capability to capture significant parameters design (i.e. wall thickness, channel gap) and by eliminating the uncertainties introduced by heat and mass transfer coefficients used in a ID model. The parametric studies performed are valuable to provide guidance for CPR design for a different reaction system than before in a co­ current flow arrangement.

In this work, the endothermie catalytic ethane dehydrogenation in a CPR was modelled and the reactor operation was simulated. Heat required for the reaction was supplied by catalytic combustion of methane. The feasibility o f using a CPR for the above reaction system was demonstrated utilising a 2-dimensional theoretical model. By adjusting the ratio of heat generation to heat consumption on the two sides o f the reactor smooth axial temperature profiles can be obtained and hot spots avoided. The metallic wall, due to its high thermal conductivity made possible efficient heat transfer between endothermie and exothermic catalyst locations even for small temperature differences. If a wall with low thermal conductivity is employed, not only significantly radial temperature gradients appear but also poor heat transport along axial direction gives rise to hot spots. The ratio of catalyst loadings for the two reactions is a key variable and must be

Modelling o f a CPR fo r Steam Reforming -Combustion Coupling

CHAPTER 4