With the rise of process intensification and innovative reactor technologies traditional reactor design methods, which predefine reactor types based on heuristics and optimize the parameters of the chosen reactor afterwards, possess limited capability especially when dealing with complex reaction systems. In this work, we propose a new method to identify the best reaction and process conditions for multiphase systems with severe mass transfer limitation. The method allows for the design of innovative and tailor-made multiphase reactor concepts including different intensification options, which is exemplified by the reactive absorption of carbon dioxide.
As basis for the optimization, a recently proposed design methodology  based on the concept of elementary process functions  is applied, and appropriately extended in order to include the rigorous treatment of multiple phases as described in detail below. The idea is to track fluid elements which travel through the process and change their states within, and to continuously provide them with the optimal fluxes according to the states, in order to achieve optimal reaction and process conditions. The optimal fluxes can be calculated by solving a dynamic optimization problem, which is formulated based on the apparatus-independent model equations.
In this contribution, we present a significant extension of the design methodology in order to rigorously consider multiple phases simultaneously, and to deal with the large concentration gradient in the phase. Considering the gas phase as a homogeneous fluid element and the liquid phase as an inhomogeneous fluid element, balance equations are applied to each phase and coupled on the length scale by the interfacial transfer, with the diffusion term accounting for the limiting mass transfer in the liquid phase and Henry’s law accounting for the limiting mass transfer between two phases. Reactor characteristics such as liquid hold-up and specific gas-liquid interfacial area are initially assigned values in a typical range, while the influence of these quantities is investigated in the further optimization by sensitivity analysis.
The optimal inlet conditions and the optimal temperature profile are obtained to maximize efficiency. Sensitivity analysis with simultaneous optimization reveals maximum efficiency improvement resulting from better reactor characteristics. These results serve as the basis of the intensified reaction concept and detailed reactor design.
This extension of the optimization methodology is not limited to the specific example presented here, but is generally applicable also to other multiphase reaction systems. In this regard, it presents an important contribution in the field of process intensification in multiphase reactor design.
 A. Peschel, H. Freund, K. Sundmacher, Methodology for the design of optimal chemical reactors based on the concept of elementary process functions, Ind. Eng. Chem. Res., 49 (2010) 10535-10548.
 H. Freund, K. Sundmacher, Towards a methodology for the systematic analysis and design of efficient chemical processes. Part 1. From unit operations to elementary process functions, Chem. Eng. Process., 47 (2008) 2051-2060.