Analysis of potential alternatives to manage the heat removal in multitubular fixed bed reactors for the Fischer-Tropsch synthesis

The catalytic conversion of synthesis gas into long chain hydrocarbons through the low temperature Fischer-Tropsch synthesis over cobalt-based catalysts is receiving a growing attentions as a powerful way of exploiting non-oil carbonaceous sources such as natural gas, coal and biomasses. The reaction is strongly exothermic and internal mass transfer restrictions can control the overall reaction rate. Multitubular packed-bed reactors are adopted at the industrial scale to run the process: the removal of the reaction heat is well known to be the most critical aspect in developing and operating such units. Accordingly, the adoption of structured catalysts based on highly conductive supports, such as open-cell foams and honeycomb monoliths, has been recently proposed to improve the heat removal. In this case, in fact, conduction within the solid matrix of the supports can be exploited as heat transfer mechanism complementary to convection. Aim of this thesis work is the analysis of the thermal behavior of a conventional packed-bed reactor for the Fischer-Tropsch synthesis loaded with egg-shell pellets and of one of the alternatives proposed to better manage the heat removal: the adoption of open-cell metal foams, washcoated with a thin layer of catalyst. This goal has been pursued by developing two heterogeneous, pseudocontinous, non-adiabatic, bidimensional reactor models, describing the material and thermal behaviors of a single tube, loaded with packed pellets or with washcoated foams. Simulation results show that the management of the heat removal in conventional packed-bed reactors can be afforded only by adopting thin and long tubes, decreasing the catalyst inventory, diminishing the CO conversion per-pass or cofeeding significant amounts of liquid products. On the contrary, the adoption of aluminum foams allows to reach a radial effective thermal conductivity three time higher than that of conventional packed-bed reactor. Such feature allow to effectively manage the heat removal inside the tubes, guarantying flatter temperature profiles and increased catalyst inventory with respect to packed-beds. As a consequence, not only strategies aimed at reducing the heat released inside the reactor are no more needed, but also catalysts with higher activity factors can be safely adopted, thus maximizing the reactor yield.