Model-based design of load flexible fixed bed reactors

Model-based methodologies for the design of chemical reactors gain increasing importance in the field of chemical reaction engineering. On the basis of a suitable mathematical model of the chemical reactor, a numerical optimization problem is solved and decision variables of the reactor such as reactor dimensions and operating conditions are determined that maximize a desired objective function while constraints are fulfilled. Initiated by a changing availability of raw materials and the transition to more renewable energy, an important aim of high current relevance are more tolerant and flexible chemical reactors. If a reactor is tolerant with respect to changes of the flow rate, it is referred to as a load flexible reactor. Such reactors are not operated at a constant steady state operating point but highly dynamically within a given load range. The design of load flexible reactors is a challenging task especially for highly exothermic reactions in wall-cooled fixed bed reactors. Fluctuations of the flow rate provoke changes of the axial temperature profile in the reactor which can damage the reactor and catalyst. Moreover, the changing residence time and temperature profile cause variations of the product composition. These challenges can be managed by a more conservative reactor design, but it creates a performance gap between conventional and load flexible reactors. For this reason, there is a strong need for the intensification of load flexible fixed bed reactors in order to reduce this performance gap. In the present thesis, novel numerical optimization strategies are presented that enable the model-based design of load flexible fixed bed reactors. The proposed optimization framework is demonstrated using the methanation of carbon dioxide as an example process. In the context of Power-to-Gas processes for energy storage, the methanation is a suitable case study of high current relevance for both research and society. One approach is based on an optimization of the reactor design while considering multiple steady state operating points within the desired load range simultaneously. The reactor dynamics are not explicitly considered in the problem formulation. Therefore, the feasibility of the reactor design to dynamic changes of the flow is verified in a second step by means of a simulation of an appropriate dynamic scenario. While this approach requires a comparatively low computational effort, there exist cases where the approach fails to yield a reactor design that is feasible for the dynamic transition periods. For these cases, a second optimization approach is proposed. The special concept is to consider the reactor dynamics already within the optimization problem formulation. The reactor is optimized not for some steady state operating points but for a worst case dynamic scenario. In this regard, a steady state and a dynamic reactor model are coupled and solved simultaneously. This approach is computationally more expensive but guarantees feasibility for the dynamic transition periods. The two proposed optimization strategies overcome major limitations of existing numerical works in that field. So far, no method was proposed in open literature that considers load flexibility and feasibility of the dynamic transition periods within one design procedure. Based on the presented optimization strategies, several methods to intensify the load flexible reactor design are determined. This includes an optimized control strategy, where the coolant temperature is dynamically changed in accordance to the current load level. A straightforward and effective intensification method is a staged reactor concept with a catalyst dilution profile. The heat management is considerably improved for the load flexible operation. An interesting potential is revealed for the intensification of the reactor by a distributed dosing strategy of the feed to different reactor stages. In this case, the axial velocity field can be adapted in accordance to the current load level. The outlined contributions to the model-based design of load flexible reactors are complemented by a comprehensive study on the dynamic behavior of fixed bed reactors and the suitable selection of reactor models. In this regard, a novel approach for the comparison of reactor models with different levels of complexity is presented. On the basis of this approach, the importance of the dimensionality and the phase treatment of a reactor model for the description of the steady state and dynamic behavior is systematically clarified. A key finding is that the predicted times of the dynamic transition periods considerably differ between heterogeneous and pseudohomogeneous reactor models. However, if the transport resistances are low, pseudohomogeneous models are also able to describe the dynamic transition periods well. The presented optimization strategies and findings of this thesis are well transferable to further applications and are able to unlock the potential of load flexible reactors.