Definition of Steady State in Continuous Reactors — Which statement correctly characterizes a continuous reactor operating at steady state?

Introduction:
Steady state is a central concept in continuous bioprocessing. It does not mean reactions stop; rather, the macroscopic concentrations inside the reactor become time-invariant because inflow, outflow, and reaction balance each other.

Given Data / Assumptions:

• Perfect mixing and constant volume.
• Constant dilution rate and feed composition.
• System has passed transients and reached equilibrium conditions.

Concept / Approach:

Component balances take the form dCi/dt = (in − out)/VR ± ri. At steady state, dCi/dt = 0 for each i, but the reaction rates ri are generally nonzero; they are simply balanced by flow terms. Therefore, concentrations remain constant even though metabolism proceeds.

Step-by-Step Solution:

Verification / Alternative check:

Time traces of reactor measurements flatten at steady state; however, reaction heat and gas exchange continue, demonstrating ongoing metabolism despite steady concentrations.

Why Other Options Are Wrong:

A: Reaction rates need not be zero; only accumulation terms are zero. B: Concentrations are not necessarily zero. D and E: These describe unsteady operation.

Common Pitfalls:

Equating steady state with chemical equilibrium; biological steady states are dynamic and far from thermodynamic equilibrium.

Final Answer:

Biomass, substrate, and product concentrations do not change with time (their time derivatives are zero).