What is Residence time?
How long the gas stays inside a reactor — the time available for reactions.
Reactions take time. If the residence time is too short, the gas leaves before reactions finish.
In the same tube, a faster flow means a shorter residence time.
Shown in PFR results. In a PFR it follows from length, area and flow rather than being an input.
For constant density, with volume V and volumetric flow Q:
\tau = \frac{V}{Q}
When temperature and density change along the reactor, integrate:
\tau = \int_0^L \frac{\rho A}{\dot{m}}\, dx