Fundamentals of catalytic cracking: Identify the correct qualitative outcome associated with the catalytic cracking process for producing gasoline from heavier feedstocks.

Difficulty: Medium

Correct Answer: gasoline obtained has very high aromatic content

Explanation:


Introduction / Context:
Catalytic cracking converts heavy gas oils into lighter products including gasoline with superior antiknock quality. Compared with thermal cracking, catalytic routes favour reactions that increase branching and aromatics, improving octane ratings of the gasoline fraction.


Given Data / Assumptions:

  • Feed: vacuum gas oils or heavy distillates.
  • Process: fluid catalytic cracking (FCC) as the archetype.
  • Qualitative comparisons to thermal cracking outcomes.


Concept / Approach:
Catalysts promote skeletal isomerization, cyclization, and dehydrogenation/aromatization. The result is a gasoline richer in isoparaffins and aromatics, giving higher octane. Operating pressure is typically low to moderate (close to atmospheric in FCC risers), so a blanket statement of “very high pressure and temperature” is misleading. Gum tendency is lower than for purely thermal cracking when measured under controlled stabilization and treatment, though cracked gasoline is more olefinic than straight-run and still needs care in handling.


Step-by-Step Solution:

1) Determine which listed effect is characteristic of catalytic cracking.2) Elevated aromatics (and isoparaffins) in cracked gasoline are expected and directly raise octane → supports option (c).3) Exclude options inconsistent with practice: very low octane (false), very high pressure (false for FCC), systematically “very high gum” (overstated/false relative to thermal).4) Select the correct qualitative descriptor.


Verification / Alternative check:
Standard refinery texts describe FCC gasoline as higher in aromatics and olefins than straight-run with improved octane number due to increased ring structures and branching.


Why Other Options Are Wrong:

(a) Octane is higher, not very low.(b) Pressure is not “very high”; temperatures are high but typical for cracking, and pressure is relatively low.(d) Gum formation risk is managed; thermal gasoline typically has worse gum tendencies.(e) Although qualitatively close, it is broader than the stem allows; the key attribute commonly emphasized is increased aromatics content.


Common Pitfalls:
Assuming any “cracked” gasoline must always have extreme gum levels or that severe operating conditions automatically mean high pressure in FCC.


Final Answer:
Gasoline obtained has very high aromatic content

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