Fluid properties in petroleum products: As the density of a petroleum fraction increases (keeping other conditions comparable), how does its viscosity typically change?

Introduction:
Understanding the relationship between density and viscosity helps engineers select, pump, heat, and atomise petroleum products safely and efficiently. The question asks how viscosity generally trends as density increases for comparable petroleum fractions.

Given Data / Assumptions:

• We consider typical petroleum liquids (e.g., gas oil, lube base stocks, residual fuels).
• Comparison is at similar temperature and pressure conditions.
• “Increase in density” implies a heavier, more complex hydrocarbon mixture.

Concept / Approach:
Heavier petroleum streams contain larger, more complex molecules with stronger intermolecular interactions. These interactions resist flow, leading to higher viscosity. While temperature strongly affects viscosity, for a fixed temperature, heavier (denser) fractions are typically more viscous than lighter (less dense) ones.

Step-by-Step Solution:

Verification / Alternative check:
Empirical property tables for refinery streams show increasing density alongside increasing kinematic viscosity from naphtha → kerosene → gas oil → residual fractions.

Why Other Options Are Wrong:

• Decreases: Opposite to the common trend for comparable fractions.
• Remains the same: Ignores well-established composition effects.
• Either (a) or (b) depending only on temperature: Temperature changes viscosity, but the underlying composition/density trend remains.
• Becomes negligible: Viscosity cannot vanish in liquids under normal conditions.

Common Pitfalls:
Confusing temperature effects (warming cuts viscosity) with composition effects (heavier cuts are denser and more viscous at the same temperature).

Final Answer:
Increases