Interpreting specific gravity changes For petroleum fractions, an increase in specific gravity typically indicates what change in composition and properties?

Introduction / Context:
Specific gravity (or API gravity) is a quick indicator of hydrocarbon family distribution in petroleum fractions. Heavier, denser streams generally contain more aromatics and polars, while lighter, less dense streams are richer in paraffins. Understanding this relationship helps predict combustion behaviour and refining pathways.

Given Data / Assumptions:

• Comparisons are qualitative and typical.
• We relate density to composition (paraffins vs aromatics).
• H/C ratio trends mirror family distribution.

Concept / Approach:
Aromatics possess ring structures with lower hydrogen content per carbon than paraffins. This makes them denser on average. Therefore, as specific gravity increases (API decreases), the aromatic fraction usually rises, and the paraffinic fraction falls. H/C ratio declines with higher aromaticity. Net heating value per weight does not increase with aromatics; paraffins tend to have slightly higher heating value per unit mass.

Step-by-Step Solution:

Verification / Alternative check:
Assay correlations show API gravity inversely correlates with aromatic content; smoke point and aniline point trends corroborate this.

Why Other Options Are Wrong:

• Decrease in paraffin content: Also generally true, but the question asks for what the increase indicates most directly; the standard interpretation focuses on aromatics rising.
• Higher H/C ratio: Opposite; aromatics reduce H/C.
• Higher thermal energy per weight: Not supported; gravimetric heating value tends to be lower for more aromatic streams.

Common Pitfalls:
Confusing weight-based and volume-based heating values; also conflating API gravity with viscosity.

Final Answer:
Increase in aromatic content