Process dynamics terminology: When is a chemical process said to be operating under unsteady (transient) state conditions?
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AWhen inventory changes do not take place
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BWhen ratios of inlet and outlet streams are independent of time
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CWhen both flow rates and compositions vary with time
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DNone of these
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EWhen only temperature varies with time but flows are constant
Answer
Correct Answer: When both flow rates and compositions vary with time
Explanation
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:
Identify time-dependence: if concentrations or flow rates change with time, the process is unsteady.Option (c) explicitly states that both flow and composition are time-dependent; this is a textbook indicator of unsteady state.Options (a) and (b) imply constant conditions; they correspond to steady-state behavior.Option (e) is incomplete; temperature alone may vary, but the definition emphasizes process variables broadly, including compositions and flows.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