Autothermal radial flow reactors typically consist of a reactor setup of multiple catalyst-beds with internal heat exchange. These reactors are widely used because of their high efficiency due to the internal heat exchange, and radial flow arrangements are preferred due to their low pressure drops. Although an efficient multi-functional reactor arrangement, this setup has shown to provide for an additional destabilizing mechanism via the heat feedback. Thus, additional stability considerations are necessary when operating autothermal or non-adiabatic reactors at high conversions. This work proposes the formulation of a simplified model to investigate the effect of the heat transfer feedback on the stability of autothermal radial flow reactors. The present work focuses on a lumping approach to reduce the order of a complex distributed parameter system. The model is complex enough so as to preserve the intricacies of this reactor arrangement, but still yield a tractable dynamic formulation. The industrial ammonia synthesis process has been chosen as a case study to illustrate the proposed methodology. The lumped model predictions are qualitatively compared against numerical simulations of a detailed mathematical model.

Metakaolin-based geopolymer coatings (GCs) represent an environmentally friendly alternative to conventional protective coatings, but their effectiveness is highly influenced by formulation parameters. In this study, the effect of activator/binder (a/b) ratio on the microstructural development, interfacial adhesion, and mechanical performance of metakaolin-based geopolymers was systematically investigated. Three ratios (0.3, 0.4, 0.5) were evaluated for pull-off adhesion, pencil hardness, 3D profilometry, FTIR, XRD, and digital microscopy. The 0.3 ratio provided weak geopolymerization, high roughness (Sa = 42.8 µm), and low hardness, which were beyond the valid range. Both 0.4 and 0.5 yielded coherent coatings, but with distinct performance: 0.4 maximized adhesion due to moderate roughness (Sa = 27.1 µm) and optimal thickness; 0.5 maximized scratch hardness due to a smoother surface (Sa = 9.1 µm). Increasing the a/b ratio increased porosity via excess water evaporation and reduced cross‑linking density. Digital microscopy confirmed no early‑age drying damage. The processing window for metakaolin‑based GCs is a/b = 0.4–0.5, within which adhesion (0.4) or hardness (0.5) can be prioritized, providing a rational formulation framework for tunable geopolymer coatings.

Metakaolin-based geopolymer coatings (GCs) represent an environmentally friendly alternative to conventional protective coatings, but their effectiveness is highly influenced by formulation parameters. In this study, the effect of activator/binder (a/b) ratio on the microstructural development, interfacial adhesion, and mechanical performance of metakaolin-based geopolymers was systematically investigated. Three ratios (0.3, 0.4, and 0.5) were evaluated for pull-off adhesion, pencil hardness, 3D profilometry, FTIR, XRD, and digital microscopy. The 0.3 ratio provided weak geopolymerization, high roughness (Sa = 42.8 µm), and low hardness, which were beyond the valid range. Both 0.4 and 0.5 yielded coherent coatings, but with distinct performance: 0.4 maximized adhesion due to moderate roughness (Sa = 27.1 µm) and optimal thickness; 0.5 maximized scratch hardness due to a smoother surface (Sa = 9.1 µm). Increasing the a/b ratio increased porosity via excess water evaporation and reduced cross-linking density. Digital microscopy confirmed no early-age drying damage. Based on the tested range (a/b = 0.3, 0.4, 0.5), the preferred formulation range for metakaolin-based GCs is identified as a/b = 0.4–0.5, within which adhesion (0.4) or hardness (0.5) can be prioritized to provide a rational formulation framework for tunable geopolymer coatings.