Engineering ToolBox - Resources, Tools and Basic Information for Engineering and Design of Technical Applications!

# Hydrocarbon Mixtures - Average Boiling Points vs. Gravity and Molecular Weights

## Formulas and examples of calculation of boiling point of hydrocarbon mixtures from gravity and molecular weight.

Boiling point: The temperature at which a liquid turns into a gas

For many purposes in chemical engineering, there is important to know the boiling point of the fluids in process streams. When the fluid consists of a mixture of many different compounds, the average boiling point (ABP) can be useful. But boiling point information from real distillation or gas chromatographic analysis is not always available. However, there are several models calculating average boiling point, based on other properties of the fluid.  For hydrocarbons, such as crude oil and distillation fractions of crude oil, the best input pair of parameters to predict boiling point is molecular weight and specific gravity. Two correlations based on these from Riazi-Daubert are given below:

Riazi-Daubert, for  molecular weights of 70-300:    Tb = 3.76587*e(3.7741E-03*M+2.98404*S-4.25288E-03*M*S)*M0.40167*S-1.58262         (1)

Riazi-Daubert, for molecular weights of 300-700:   Tb = 9.3369*e(1.6514E-04*M+1.4103*S-7.5152E-04*M*S)*M0.5369*S-0.7276                  (2)
Eq. (2) is also applicable to hydrocarbons having molecular weight range of 70-300, but with less accuracy

Where
Tb = Average boiling point, in Kelvin
M = Average molecular weight
S = Specific gravity at 60 °F  (= 15.6°C)

Eq. (1) and (2) are used to calculate Tb for a number of specific gravities and molecular weights and the results, in °C and °F, are given in the figures below.

Example 1:  Average boiling point of naphtha
Calculate the average boiling point of a naphtha with specific gravtity,  S = 0.763  and a molecular weight of 125.

Naphtha is in the low molecular weight range, and equation (1) should be applied.

ABP = 3.76587*e(3.7741E-03*125+2.98404*0.763-4.25288E-03*125*0.763)*1250.40167*0.763-1.58262 = 418 K

Converted to °C and °F: 418 K = 145°C = 293 °F

Applying equation (2) gives

ABP =  9.3369*e(1.6514E-04*125+1.4103*0.763-7.5152E-04*125*0.763)*1250.5369*0.763-0.7276 = 423 K = 150°C = 302°F

NB! For low molecular weights (< 300) this ABP is consider to be less accurate than calculated from Eq. (1).

Example 2: Average boiling point of a vacuum gas oil.
Calculate the average boiling point of a vacuum gas oil of gravity of 16.7°API and an average molecular weight of 391.

The vacuum gas oil has a molecular weight > 300, and Eq. (2) must be applied.

But first, we have to convert the API gravity to specific gravity:

S, specific gravity = 141.5/(16.7°API +131.5) = 0.955

ABP =  9.3369*e(1.6514E-04*391+1.4103*0.955-7.5152E-04*391*0.955)*3910.5369*0.955-0.7276 = 737 K = 464°C = 867°F

## Related Topics

• ### Boiling Points

Boiling points of elements, products and chemical species at varying conditions.
• ### Fluid Mechanics

The study of fluids - liquids and gases. Involving velocity, pressure, density and temperature as functions of space and time.
• ### Material Properties

Material properties of gases, fluids and solids - densities, specific heats, viscosities and more.

## Related Documents

• ### Alcohols and Carboxylic Acids - Physical Data

Molweight, melting and boiling point, density, pKa-values, as well as number of carbon and hydrogen atoms in molecules are given for 150 different alcohols and acids.
• ### API Gravity

API expresses the gravity or density of liquid petroleum products. Online API to Specific Gravity calculator.
• ### Crude Oil - Density vs. Temperature

Variations in crude oil density are shown as function of temperatur, together with volume correction factors.
• ### Density vs. Specific Weight and Specific Gravity

An introduction to density, specific weight and specific gravity.
• ### Fuel Oils - Densities vs. Temperature

Variations in fuel oils density as function of temperatur, together with volume correction factors.
• ### Hydrocarbon Mixtures - Molecular Weight vs. Gravity and Average Boiling Point

Formulas and examples of calculation of average molecular weight of hydrocarbon mixtures from gravity and average boiling point, achieved from distillation data.
• ### Hydrocarbons - Physical Data

Molweight, melting and boiling point, density, flash point and autoignition temperature, as well as number of carbon and hydrogen atoms in each molecule for 200 different hydrocarbons.
• ### Hydrocarbons - Melting Point vs. Molecular Weight

Calculate melting point of hydrocarbons from molecular weight (molar mass).
• ### Jet Fuel - Density vs. Temperature

Variations in jet fuel density as function of temperatur, together with volume correction factors.
• ### Lubricating Oil - Densities vs. Temperature

Variations in lubricating oil density as function of temperatur, together with volume correction factors.
• ### Organic Nitrogen Compounds - Physical Data

Boiling and melting points of amines, diamines, pyrroles, pyridines, piperidines and quinolines shown together with their molecular structures, as well as molweights and density.
• ### Organic Sulfur Compounds - Physical Data

Boiling and melting points of thoils, sulfides, disulfides and thiophenes shown together with molecular structures, as well as molweights and density.
• ### Petroleum Products - Average Boiling Points

Definition, explanation and examples of calculation of various types of average boiling point of petroleum products and other mixtures of hydrocarbons: VABP, MABP, WABP, CABP and MeABP.
• ### Temperature

Introduction to temperature - including Celsius, Fahrenheit, Kelvin and Rankine definitions - and an online temperature converter.

## Engineering ToolBox - SketchUp Extension - Online 3D modeling!

Add standard and customized parametric components - like flange beams, lumbers, piping, stairs and more - to your Sketchup model with the Engineering ToolBox - SketchUp Extension - enabled for use with older versions of the amazing SketchUp Make and the newer "up to date" SketchUp Pro . Add the Engineering ToolBox extension to your SketchUp Make/Pro from the Extension Warehouse !

We don't collect information from our users. More about

## Citation

• The Engineering ToolBox (2017). Hydrocarbon Mixtures - Average Boiling Points vs. Gravity and Molecular Weights. [online] Available at: https://www.engineeringtoolbox.com/average-boiling-point-mixtures-calculation-prediction-estimation-gravity-density-molecular-weight-d_1996.html [Accessed Day Month Year].

Modify the access date according your visit.

10.2.10