Crude Oil Blending

[S.G]

Dear All,

I found below in a test paper. but still searching for the API/ASTM crude reference which puts why API’s cannot be averages into understandable language.

Question 1

Equal volumes of a 30°API, 20°API, and 15°API crude are blended to produce a feed to refinery. Estimate the °API of the resulting blend. Clearly state your assumptions and comment on your result. (Hint: Would °API blend linearly?) 15 pts
Answer:
Blend: Equal volumes of 30°API, 20°API, and 15°API
°API = 141.5/SG60oF - 131.5

°Theoretically APIs cannot be averaged, but SGs can be, assuming that the volume change upon mixing the three crudes is negligible.
Determine SG of each crude, calculate average SG, and use this value to calculate the blend °API.

30= 141.5/SG1–131.5, 20= 141.5/SG2–131.5, 15=141.5/SG3 – 131.5
SG1 = 0.876 SG2 = 0.933 SG3= 0.966

SGavg = AVG (0.876:0.933:0.966)
SGavg = 0.925

°APIblend = 141.5/0.863 – 131.5
°APIblend = 21.4

Note that the blend °API is not a simple a mean of the component °API’s (AVG 30:20:15) = 21.7. Although the difference appears to be small, it is important to use the precise °API in several calculations and correlations that are used to predict the quality and yield of refinery products.

 

[LittleInch]

I think it's because one part of the answer is inversely proportional (141.5/SG) and the second part(-131.5) is a fixed number regardless of SG. I think.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[S.G]

Thank you, LittleInch. I found this:

the relationship between °API and specific gravity is not linear. Therefore, the °API gravity of crude blends cannot be calculated by linear averaging of the component °APIs. Specific gravities of the components can be averaged, though, to determine the specific gravity of the resulting blend. In practice, averaging °APIs is usually accepted because the error involved in averaging is small.

 

[shvet]

In practice, averaging °APIs is usually accepted because the error involved in averaging is small.

Note that in practice of fiscal metering / mutual settlement this small error has an opportunity to lead to a huge loss in profit.

 

[S.G]

Agreed, Shvet.