Question:
For an irreversible reaction containing inerts in the feed, the single-pass conversion in a reactor is 70%. Which one of the following statements is CORRECT?
For an irreversible reaction containing inerts in the feed, the single-pass conversion in a reactor is 70%. Which one of the following statements is CORRECT?
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Overall (fresh-feed-basis) conversion in a recycle loop is always higher than the single-pass conversion for an irreversible reaction; also, inerts in the feed always require a purge stream, recycle never removes that need.
Updated On: Jul 17, 2026
- Recycle eliminates the need for a purge stream.
- Recycle reduces the concentration of inerts in the system.
- Recycle increases the overall conversion.
- Recycle decreases the overall conversion.
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The Correct Option is C
Solution and Explanation
Step 1: Set up the recycle reactor system.
Consider a reactor that processes a fresh feed containing the reactant along with inerts. The stream leaving the reactor is separated into a product stream and an unconverted-reactant stream, and the unconverted-reactant stream is recycled back and mixed with the fresh feed before it re-enters the reactor. The reactor gives a single-pass conversion of X_s = 0.70, meaning 70% of the reactant entering the reactor (fresh + recycle) is converted to product on every pass.
Step 2: Recall single-pass versus overall conversion.
Single-pass conversion X_s is defined on the reactor inlet basis. Overall conversion X_ov is defined on the fresh feed basis. Because unconverted reactant that would otherwise leave the system is sent back for another pass, very little unconverted reactant escapes, so X_ov is always greater than or equal to X_s for an irreversible reaction, approaching 1 as the recycle ratio increases (with a small purge to bleed inerts).
Step 3: Why a purge is still required.
The feed contains inerts, which are never consumed. If the unconverted stream is recycled with no bleed-off, inert material keeps entering with fresh feed but has no route to leave, so it would build up without bound. Whenever inerts enter with the feed of a recycle system, a purge stream is mandatory. Recycle does not eliminate the need for a purge; the presence of recycle together with inerts is exactly the situation that makes a purge necessary.
Step 4: What recycle does to inert concentration.
Because reactant is consumed each pass while inert is not, the inert fraction in the recycle loop rises with each pass, not falls. Inert concentration in the recycle stream is higher than in the fresh feed, not lower.
Step 5: Confirm the effect on overall conversion.
Since the reaction is irreversible, every mole of recycled reactant gets another opportunity to convert at the same single-pass conversion on its next pass. Only the small fraction leaving through purge escapes unconverted. So overall conversion is always higher than single-pass conversion, increasing further as the recycle ratio increases.
Step 6: Conclude.
Recycle increases the overall conversion achieved from the fresh feed for an irreversible reaction.
\[ \boxed{\text{Recycle increases the overall conversion} \Rightarrow \text{Option (C)}} \]Was this answer helpful?
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