For ideal tracer studies, the residence time distribution (RTD) of a vessel reflects macromixing and flow patterns. Which factor does RTD NOT depend upon?

Introduction / Context:
Residence time distribution (RTD) characterizes how long fluid elements spend inside a reactor and is measured using non-reactive tracers. RTD is central to diagnosing deviations from ideal plug flow or well-mixed behavior at the macroscopic level.

Given Data / Assumptions:

• Non-reactive tracer experiments under steady flow.
• Focus on macromixing and hydrodynamic residence times, not on molecular-scale events.
• Geometric and hydraulic features can shape flow patterns appreciably.

Concept / Approach:
RTD captures macroscopic dispersion and by-pass/recirculation zones. Micromixing refers to molecular-level mixing inside eddies or fluid packets and affects reaction outcomes where kinetics compete with diffusion, but it does not alter the time a packet spends inside the vessel as measured by a passive, non-reactive tracer.

Step-by-Step Solution:
1) Identify macroscopic determinants: inlet/outlet placement, baffles, volume, aspect ratio, and flow rate sculpt the E(t) curve.2) Recognize that RTD is measured by tracking tracer concentration at the outlet versus time; it integrates path-length distributions, not internal molecular-scale mixing states.3) Therefore, RTD is independent of micromixing but depends on vessel geometry and hydrodynamics (volume, height, width).

Verification / Alternative check:
Two reactors with identical geometry and flow rates but different impeller micro-scales can show similar RTDs if macropatterns are unchanged; however, selectivity in fast reactions may differ due to micromixing.

Why Other Options Are Wrong:
Volume, height, and width all affect flow regime, dead zones, and circulation loops and thus shape RTD.

Common Pitfalls:
Conflating micromixing with macromixing; assuming better micromixing shortens RTD peak width—RTD does not quantify molecular-scale mixing quality.

Final Answer:
Micromixing (molecular-scale mixing within fluid elements)