Professional Analysis Tool for Petroleum Engineering
API Gravity (Standard)
Specific Gravity (SG)
Select the input parameter available from your lab report.
Typical range: 10 (Heavy) to 60 (Light).
Please enter a valid API between 0 and 100.
Relative density to water (approx 0.6 to 1.1).
Please enter a valid SG between 0.6 and 1.2.
Estimated Molecular Weight
212.8g/mol
Based on Cragoe's Relation
Oil Density
849.8
kg/m³
Specific Gravity
0.850
Unitless
Classification
Light Oil
Grade
Property Breakdown
Parameter
Value
Unit
Molecular Weight (MW)
212.8
g/mol
API Gravity
35.0
°API
Specific Gravity 60/60°F
0.850
–
Density @ 15.6°C
849.8
kg/m³
Table 1: Calculated properties derived from input gravity.
Molecular Weight vs. API Gravity Curve
― Cragoe's Correlation Curve● Your Oil Sample
Figure 1: Relationship showing how Molecular Weight decreases as API Gravity increases.
How to Calculate Molecular Weight of Oil: A Comprehensive Guide
Understanding how to calculate molecular weight of oil is a fundamental task in petroleum engineering, chemical processing, and refinery operations. Unlike pure substances such as water or methane, crude oil is a complex mixture of thousands of different hydrocarbons. Therefore, when we calculate molecular weight for oil, we are determining the average molecular weight of the mixture.
This guide provides the mathematical frameworks, specifically focusing on API gravity to molecular weight conversions, to help engineers and analysts estimate this critical property for process simulation and quality control.
What is Molecular Weight of Oil?
The molecular weight (MW) of crude oil represents the average mass of a mole of the oil mixture, measured in grams per mole (g/mol). Because oil consists of paraffins, naphthenes, and aromatics ranging from simple volatile compounds to heavy asphaltenes, there is no single fixed value.
Who needs this calculation?
Refinery Engineers: To model distillation columns and heat exchangers.
Reservoir Engineers: To calculate equation of state (EOS) models for reservoir simulation.
Chemists: For characterizing unknown hydrocarbon samples.
A common misconception is that density alone defines the oil quality. While density (Specific Gravity) is related, the molecular weight provides deeper insight into the boiling point distribution and the carbon chain length of the mixture.
Molecular Weight Formula and Mathematical Explanation
The most widely accepted method to estimate the molecular weight of petroleum fractions when only density is known is Cragoe's Correlation (1929). This empirical formula relates the physical property of API Gravity to the average molecular weight.
The Cragoe Formula
The formula is expressed as:
MW = 6084 / (API + 5.9)
Where:
MW = Average Molecular Weight (g/mol)
API = API Gravity of the oil (degrees)
Alternative Using Specific Gravity (SG)
If you have Specific Gravity (SG) instead of API, you must first convert it:
API = (141.5 / SG) – 131.5
Variable
Meaning
Unit
Typical Range
MW
Molecular Weight
g/mol
70 – 600+
API
API Gravity
Degrees
10 (Heavy) – 60 (Light)
SG
Specific Gravity
Dimensionless
0.65 – 1.05
Table 2: Key variables in hydrocarbon property estimation.
Practical Examples (Real-World Use Cases)
Example 1: Light Sweet Crude (West Texas Intermediate)
Scenario: An analyst is working with a sample of WTI crude oil which has an API gravity of 39.6°. They need to input the molecular weight into a process simulator.
Calculation:
Input API: 39.6
Formula: MW = 6084 / (39.6 + 5.9)
Denominator: 45.5
Result: 133.7 g/mol
Interpretation: This low molecular weight indicates a fluid rich in lighter hydrocarbons (gasoline and diesel range), making it valuable for refining.
Example 2: Heavy Fuel Oil
Scenario: A refinery is processing a heavy residue stream with a Specific Gravity (SG) of 0.98.
Calculation:
Step 1: Convert SG to API. API = (141.5 / 0.98) – 131.5 = 144.39 – 131.5 = 12.89° API
Step 2: Calculate MW. MW = 6084 / (12.89 + 5.9)
Denominator: 18.79
Result: 323.8 g/mol
Interpretation: The high molecular weight confirms the presence of long-chain hydrocarbons, requiring cracking processes to be useful for transportation fuels.
How to Use This Molecular Weight Calculator
Our tool is designed for quick, accurate estimations on the field or in the lab. Follow these steps:
Select Input Mode: Choose "API Gravity" if you are using standard oilfield units, or "Specific Gravity" for lab density measurements.
Enter Value: Input your known value. Ensure it falls within realistic limits (API 0-100).
Review Classification: The calculator identifies if the oil is Light, Medium, Heavy, or Extra Heavy.
Analyze the Chart: Check where your sample lies on the curve. Lower API values (heavier oil) result in exponentially higher molecular weights.
Copy Data: Use the "Copy Results" button to paste the data directly into your Excel sheets or reports.
Key Factors That Affect Molecular Weight Results
When studying how to calculate molecular weight of oil, consider these six influencing factors:
Paraffin Content: Oils with high paraffin wax content tend to have higher molecular weights compared to aromatic oils of similar density.
Temperature: While MW is a chemical property, the input density depends on temperature. Always ensure SG is corrected to standard conditions (60°F/15.6°C).
Light End Loss (Weathering): If a sample sits open, volatile components evaporate. This increases the remaining density and molecular weight.
Water and Sediment (BS&W): Impurities can skew density readings, leading to incorrect API values and flawed MW calculations.
Sulfur Content: "Sour" crudes often have different density-MW relationships than "Sweet" crudes, though Cragoe's correlation is a general approximation.
Asphaltenes: High concentrations of asphaltenes (found in heavy oils) drastically increase average molecular weight and viscosity.
Frequently Asked Questions (FAQ)
1. Can I calculate the exact molecular weight of crude oil?
No, because crude oil is a mixture, not a pure compound. You can only calculate the average molecular weight.
2. Is Cragoe's correlation accurate for all oils?
It is most accurate for standard crude oils. It may be less accurate for highly refined products or synthetic oils.
3. What is the difference between Specific Gravity and API?
They measure the same property (density relative to water) but on different scales. API expands the scale for better differentiation of oil grades.
4. How does molecular weight affect refinery profit?
Lower MW oils (Lighter) generally contain more gasoline and diesel, which are higher value products, increasing refinery margins.
5. What is the unit of Molecular Weight?
The standard scientific unit is grams per mole (g/mol) or Daltons (Da).
6. Why does the calculator show "Extra Heavy"?
If the API is below 10, the oil is denser than water (bitumen), classified as extra heavy.
7. Does pressure affect molecular weight?
Pressure affects the volume (density) but not the mass of the molecules. However, in reservoir conditions, dissolved gas changes the mixture's apparent MW.
8. Can I use this for vegetable oils?
No. This calculator uses correlations specifically derived for petroleum hydrocarbons.
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