Horn, RaimundRaimundHorn2026-01-192026-01-192026-01-01Springer 978-3-662-69921-8: 559–623 (2026)https://hdl.handle.net/11420/60945Fixed bed reactors are the most commonly used reactor type in the chemical and petrochemical industry. They are also used for mobile and stationary exhaust gas cleaning. All fixed bed reactors have in common that a solid, usually a catalyst, is spatially fixed in the reactor and is flowed through or over by the reaction mixture. This chapter provides an overview of this important reactor type, with the focus on catalytic fixed bed reactors for gas reactions. After a brief introduction, catalytic fixed bed reactors are classified according to the type of involved phases, the structure of the catalyst phase, and the temperature control. The operating principle of the different reactor types is explained using examples. In addition to fixed bed reactors in industry, fixed bed reactors in laboratory and research are also discussed, which are used for high-throughput screening, for measuring kinetic data, or for operando spectroscopy on catalysts. Subsequently, the mathematical modeling of fixed bed reactors is discussed. The classification of the different modeling approaches includes the dimension of the modeling domain, the mathematical structure of the equations, the description of transport processes, the number and type of involved phases, and the description of the reaction kinetics. In addition to the classic pseudo-homogeneous, pseudo-heterogeneous and heterogeneous reactor models modern modeling approaches such as the CFD simulation of catalytic fixed bed reactors are presented using examples. Finally, the unsteady operation and the dynamic behavior of fixed bed reactors are addressed. In this context, the parametric sensitivity, the thermal runaway, and the periodic flow reversal are examined in more detail.enFixed bed reactorCatalystModelingDynamicsUnsteady operationNatural Sciences and Mathematics::540: ChemistryTechnology::660: Chemistry; Chemical Engineering::660.2: Chemical EngineeringBook Part10.1007/978-3-662-69921-8_18Book Chapter