Process streams management:\nRejecting a portion of the outlet stream to control impurity build-up in a recycle system is called…

Introduction / Context:
Many continuous chemical processes include recycle loops to improve conversion or recover unreacted materials. However, inert or slowly reacting impurities can accumulate in the loop. A standard remedy is to intentionally remove a small slipstream to maintain impurity levels within specification. Knowing the correct terminology helps in design reviews and PFD/P&ID communication.

Given Data / Assumptions:

• Process contains a recycle loop with a nonreactive impurity.
• Objective is to prevent impurity build-up to unacceptable levels.
• Unit operations and controls are otherwise steady.

Concept / Approach:
Definitions:
• Purge: A deliberate withdrawal of a portion of the recycle or outlet stream to remove accumulated inerts or impurities.
• Recycle: Returning part of an output stream to an upstream unit to improve utilization or stability.
• Bypass: Diverting a portion of a feed around a unit without contacting it, usually for control (e.g., temperature or composition).
• Recirculation: Often used synonymously with recycle in fluid systems, referring to repeated circulation without necessarily implying impurity control.

Step-by-Step Reasoning:

Verification / Alternative check:
Material balance on the impurity at steady state shows that the purge fraction sets the outlet impurity concentration by balancing impurity inflow against removal.

Why Other Options Are Wrong:

• Recycling/Recirculation: These describe returning streams, not removing impurities.
• Bypassing: Skips a unit operation; it does not reduce impurity inventory in the loop.

Common Pitfalls:
Confusing “bypass” and “purge”; assuming that increasing recycle alone can control impurities (it cannot without a purge path).

Final Answer:
Purging