(430g) Novel Multiplicity and Stability Criteria for Polytropic Fixed-Bed Reactors

Fuel syntheses from green hydrogen and carbon dioxide (CH₄, CH₃OH, jet fuel) are key process steps in future Power-to-X applications. In order to perform fuel synthesis on a large production scale at low cost, the catalytic fixed-bed reactor is the favored technical solution. Changing market environments and volatile process inputs (e.g., if renewable energies are involved) currently demand reactors that perform well not only at a fixed nominal load but also under varying partial loads (load flexibility). In this context, the resilience and flexibility to changing operating conditions become major objectives for the design and operation of real industrial-scale reactors. Therein, steady-state multiplicity and stability are essential measures, but so far, their explicit quantification is primarily accessible for ideal reactor concepts with zero or infinite back-mixing.

Based on a continuous stirred tank reactor (CSTR) cascade modeling approach, this work derives novel criteria for stability, multiplicity, and uniqueness applicable to real polytropic reactors with finite back-mixing. The novel criteria indicate that thermo-kinetic multiplicities induced by back-mixing remain relevant even for high Bodenstein numbers (see Fig. 1). In consequence, generally accepted back-mixing criteria (e.g., Mears’ criterion) appear insufficient for real polytropic reactors. The criteria derived in this work are applicable to any exothermic reaction and reactors at any scale. Ignoring uniqueness and multiplicity would disregard a broad operating range and thus a substantial potential for reactor resilience and flexibility. Furthermore, other features such as reactor runaway are demonstrated experimentally and underpin the relevance of the derived criteria. The novel criteria firstly present an a priori estimate of these characteristics for polytropic fixed-bed reactors with finite back-mixing, which makes them easy to use for reactor design, operation, and safety analysis.

Figure 1. Mass- and energy-based sensitivity for criteria evaluation on a broad axial dispersion range.