A fixed-bed reactor is a tube packed with porous catalyst pellets. Gas flows through the voids between the pellets; the reaction happens on the internal surface of the pellets, which the gas must reach by diffusing into the pores.
Three length scales matter, and the model resolves all three:
| Scale | What happens there | How the model treats it |
|---|---|---|
| Bed (cm–m) | Gas flows along the tube; concentration changes with position and time | Axial coordinate , cells |
| Pellet (mm) | Gas diffuses radially inward while reacting | Radial coordinate , nodes per cell |
| Surface (nm) | Molecules adsorb, react, and desorb on active sites | Fractional coverages |
The separation of these scales is what makes the problem stiff: surface processes equilibrate in microseconds to milliseconds, while the gas composition along the bed evolves over seconds.
Why periodic operation is interesting¶
Under steady feed, the catalyst settles into a fixed surface composition. The selectivity to ethylene is then set by the balance between the desired partial hydrogenation and the undesired full hydrogenation to ethane — a balance that cannot be improved by changing a constant temperature or pressure alone.
Forcing the inlet composition to vary in time breaks that equilibrium. Because the two reaction pathways have different dependencies on the surface hydrogen coverage, a periodic inlet can drive the surface through states that a steady feed never reaches. The open question is which input profiles do this best — and that question requires a dynamic model to answer, which is what this work provides.