Process Intensification of the Fischer-Tropsch synthesis using conductive packed POCS with skin: the role of the POCS design parameters

Heat management poses severe challenges when dealing with highly exothermic and endothermic reactions. In this paper we study the design of structured packed beds with highly conductive (Al) cellular internals during the Fischer-Tropsch (FT) synthesis in a tubular reactor with external cooling. A structured packed bed consists of an interconnected matrix with a well-defined geometry packed with catalyst particles. Herein we focus on 3D printed Periodic Open Cellular Structures (POCS), which allow a fully tailored design of their geometry. Moreover, the 3D printing technique enables introduction of elements in addition to just the cellular structure and adoption of different materials for manufacturing. An external skin on the outer diameter of the POCS lattice has been printed, starting from the same aluminum alloy powder, to form the so called POCS with skin. This choice was made to improve the contact with the reactor walls, which is considered one of the major heat transfer resistances. The POCS have been tested in the Fischer-Tropsch synthesis, a highly exothermic and temperature sensitive reaction, which makes heat management pivotal. The tests on POCS with skin have been performed in a semi pilot-scale rig equipped to fully qualify the heat transfer performances of the structured reactor. 3D printed elements have been tested to explore the effects of the following POCS features: porosities or relative density (RDlattice), specific surface area (Sv), and Gap between the external diameter of the POCS and the reactor walls. The results show the dominant effect of the relative density of the element when compared to the other parameters. Furthermore, it appears that a threshold value exists below which the structured catalytic bed is unaffected by the initial Gap, which from our experimental conditions is about 40–50 μm. This study paves the way for the optimization of those structures in terms of internal design and contact with the wall, aiming at improving the overall heat transfer coefficient, maximizing productivity and reducing the manufacturing efforts. During our tests, we were able to pack about 3.75 g/cm of catalyst in a 20 cm long, 1-inch I.D., single tube reactor, estimating an overall heat transfer coefficient larger than 1200 W/m2/K and enabling the removal of 1.2 MW/m3.