Additive versus non-additive properties in lube oil blending: which listed property is considered “additive” (i.e., can be estimated by linear blending rules)?

Introduction:
When blending petroleum fractions or lubricants, some properties can be estimated by simple linear mixing rules (additive properties), while others require nonlinear correlations or cannot be predicted accurately by simple averaging. Recognizing which is which is vital for quick blend calculations.

Given Data / Assumptions:

• Additive properties combine approximately linearly on a mass or volume basis.
• Non-additive properties often require blending indices (e.g., VBN for viscosity).
• The options include density-based and thermal properties.

Concept / Approach:
Specific gravity (or density) is commonly treated as additive for blending on a mass basis (overall mass and volume balances yield a predictable average density for many petroleum mixtures, within practical engineering accuracy). °API gravity is a nonlinear function of specific gravity and hence not additive. Viscosity is notoriously non-additive, requiring logarithmic or VBN-type methods. Flash point also does not follow simple linear blending because it depends strongly on the most volatile components.

Step-by-Step Solution:
Identify property with near-linear blending behavior: specific gravity.Eliminate non-additive: viscosity (needs blending charts), flash point (dominated by light ends), °API (inverse nonlinear transform of SG).Conclude: specific gravity is the additive property among the options.

Verification / Alternative check:
Refinery planning tools routinely blend densities using mass-volume balances; API values are calculated afterward from the blended specific gravity.

Why Other Options Are Wrong:

• °API gravity: nonlinear function, not blend-additive.
• Viscosity: strongly non-additive; requires empirical blending rules.
• Flash point: sensitive to light components; not reliably additive.

Common Pitfalls:
Blending on volume vs. mass without consistent basis; always maintain a consistent blending basis and convert at the end as needed.

Final Answer:
Specific gravity