Process dynamics terminology: When is a chemical process said to be operating under unsteady (transient) state conditions?

Introduction / Context:In process control and mass/energy balance calculations, distinguishing steady state from unsteady (transient) state is fundamental. Most design correlations assume steady state, while start-up, shut-down, and disturbances are transient.

Given Data / Assumptions:

• Generic process (reactor, separator, or heat exchanger) with inlets and outlets.
• System boundaries remain fixed.

Concept / Approach:Steady state means that all macroscopic variables inside the control volume (e.g., holdup, composition, temperature) are time-invariant. Under unsteady state, at least one of these variables changes with time, often due to changing inflows/outflows or reactions not balanced by removal.

Step-by-Step Solution:

Verification / Alternative check:Write a dynamic mass balance: d(Inventory)/dt = In − Out + Generation − Consumption. If d(Inventory)/dt ≠ 0 for any species or energy, conditions are unsteady.

Why Other Options Are Wrong:

• (a) and (b): Describe time-invariant behavior.
• (d): Invalid because (c) correctly states unsteady state.
• (e): Too narrow; unsteady state typically involves broader time dependence.

Common Pitfalls:Confusing periodic steady state with true unsteady state, or assuming thermal transients alone define dynamics. Always check material and energy inventories.

Final Answer:When both flow rates and compositions vary with time