Viscosity comparison among refinery streams (same temperature) Which of the following petroleum fractions has the lowest viscosity at a given temperature?

Introduction / Context:
Viscosity directly affects pumping, atomization, and processing of petroleum fractions. Lighter fractions flow more easily; heavier fractions resist flow. Identifying relative viscosity among common streams is fundamental in refinery operations and equipment sizing.

Given Data / Assumptions:

• All streams are compared at the same temperature.
• Typical cuts: naphtha (light), kerosene (middle), diesel (heavier middle), lube oil (heavy).
• No additives or unusual compositions considered.

Concept / Approach:
For hydrocarbon mixtures, viscosity generally increases with average molecular weight and boiling range. Therefore, among the listed options, the lightest fraction (naphtha) exhibits the lowest viscosity at a given temperature.

Step-by-Step Solution:

Verification / Alternative check:
Viscosity–temperature charts and ASTM D445 data for typical refinery streams show naphtha viscosities near water-like values at ambient, while lube oils are orders of magnitude higher.

Why Other Options Are Wrong:

• Kerosene, diesel: Heavier than naphtha; higher viscosities.
• Lube oil: Intentionally high viscosity for lubrication film strength; the highest among choices.

Common Pitfalls:
Assuming aromatic content alone governs viscosity; cut boiling range and molecular weight distribution are primary drivers for these bulk comparisons.

Final Answer:
Naphtha