Question

The purpose of a purge stream in a recycle process is to:

• A. Increase conversion of reactants
• B. Prevent accumulation of inerts
• C. Cool down the recycle stream
• D. Increase the feed flow rate

Hint:

At steady state, the material balance for any inert component is simply:
\[ \text{Inlets of Inert} = \text{Outlets of Inert (via Purge)} \]
Since inerts do not react, they can only leave the process through the purge stream.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
The question asks about the process engineering purpose of a "purge stream" in chemical plants that employ recycle loops.
This is a standard topic in mass balance and chemical process calculations.

Step 2: Key Formula or Approach:
In many chemical reactions, some species are present in the feed stream that do not participate in the reaction. These are called "inerts" (e.g., argon in ammonia synthesis, nitrogen in hydrocarbons).
If a recycle stream is used to return unreacted reactants back to the reactor and there is no outlet for these inerts, they will continuously build up in the system.

Step 3: Detailed Explanation:

• Recycle Loop Dynamics: A recycle stream is implemented to return unreacted raw materials to the reactor, which significantly increases the overall process conversion.

• Inert Accumulation: If the fresh feed contains even a tiny fraction of an inert species, and we recycle all unreacted material, the inerts cannot leave the system because they do not react.
Consequently, the inert concentration in the recycle loop will increase over time.
This accumulation increases the total system pressure or reduces the partial pressure of the reactants, reducing reactor performance and efficiency.

• Role of Purge: To prevent this accumulation, a small fraction of the recycle stream is continuously bled off or "purged."
The purge stream provides an exit route for the inerts, allowing the system to reach a steady state where the rate of inerts entering with the fresh feed equals the rate of inerts leaving through the purge.


Step 4: Final Answer:
The primary function of a purge stream is to prevent the continuous accumulation of inerts in a recycle loop.