In catalytic alkylation (e.g., isobutane with C3–C4 olefins), how does increasing the iso-butane-to-olefin (I/O) ratio affect product properties and yields?
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AIt produces a product with a lower final boiling point (cleaner, more desirable cut).
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BIt increases the overall alkylate yield.
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CBoth (a) and (b).
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DNeither (a) nor (b).
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EIt mainly increases polymer gasoline rather than alkylate.
Answer
Correct Answer: Both (a) and (b).
Explanation
Introduction:Alkylation combines isobutane with light olefins in a strong acid medium to make high-octane, low-endpoint alkylate. The iso-butane-to-olefin (I/O) ratio is a critical severity knob that shapes selectivity, endpoint, and yield.
Given Data / Assumptions:
- Reaction: isobutane + olefins → branched C7–C9 isoparaffins (alkylate).
- Variables: I/O ratio adjusted at constant temperature and acid strength.
- Targets: maximize yield and lower final boiling point (better product quality).
Concept / Approach:A higher I/O ratio supplies excess isobutane, suppressing oligomerization/polymer gasoline formation, reducing heavy-end side reactions, and driving cleaner C7–C9 isoparaffin production. This typically lowers final boiling point and raises alkylate yield and quality.
Step-by-Step Solution:Increase I/O → carbocation reactions favor desired alkylation over polymerization.Desired outcome → lower endpoint (final boiling point) and higher liquid yield.Therefore, both statements (a) and (b) are correct.
Verification / Alternative check:Operating guidelines for H2SO4/HF alkylation units recommend high I/O ratios to improve selectivity and product quality, evidenced by lower endpoint and higher yields.
Why Other Options Are Wrong:a or b alone: Incomplete; both are observed.d: Contradicts established practice.e: Higher I/O actually suppresses polymer gasoline formation.
Common Pitfalls:Confusing alkylation with polymerization units; in alkylation, excess isobutane is beneficial.
Final Answer:Both (a) and (b).