Melting Freezing Point Molar Mass Molecular Weight Hydrocarbon Aromatics Alkanes Cycloalkanes Paraffins Naphthenes Estimate Predict Calculate

Overview

Engineering reference data for Melting Freezing Point Molar Mass Molecular Weight Hydrocarbon Aromatics Alkanes Cycloalkanes Paraffins Naphthenes Estimate Predict Calculate in thermodynamics.

Key Formulas

First Law

$\Delta U = Q - W$

Energy is conserved — heat added minus work done.

Ideal Gas Law

$PV = nRT$

Relates pressure, volume, and temperature of an ideal gas.

Heat Transfer

$Q = mc\Delta T$

Sensible heat transfer.

Carnot Efficiency

$\eta = 1 - T_C/T_H$

Maximum efficiency between two temperatures.

Variables

Melting & Freezing Behavior in Mixtures

For petroleum or hydrocarbon mixtures, melting and freezing occur over a temperature range rather than at a single point.

• The initial melting point (during heating) is close to the melting point of the lightest compound in the mixture.
• The initial freezing point (during cooling) is close to the freezing point (melting point) of the heaviest compound in the mixture.
• For a wax mixture, the liquid will start to freeze above the calculated average freezing point, and the solid will start to melt below the calculated average melting point. The width of this range depends on the molecular weight distribution.

Hydrocarbon-Specific Correlations

Equations developed by Riazi and Sahhaf can estimate the melting point (in Kelvin) for pure hydrocarbon classes based on molecular weight $M$ (g/mol).

n-Alkanes (Paraffins, C5-C40): $T_{MP} = 397 - \exp\left(6.5096 - 0.14187M^{0.47}\right)$

n-Alkylcyclopentanes (Naphthenes, C7-C41): $T_{MCP} = 370 - \exp\left(6.52504 - 0.04945M^{2/3}\right)$

n-Alkylcyclohexanes (Naphthenes, C7-C20): $T_{MCH} = 360 - \exp\left(6.55942 - 0.04681M^{0.7}\right)$

n-Alkylbenzenes (Aromatics, C9-C42): $T_{MA} = \exp\left(6.55942 - 0.04681M^{0.7}\right)$

For mixtures, insert the average molecular weight of each hydrocarbon group into its respective equation to estimate the average melting/freezing point for that fraction.

Practical Notes

• In wax precipitation modeling, very light hydrocarbons (C1-C15) and aromatics are typically absent, so their melting points are often not required.
• These correlations yield good practical accuracy (error ~1-1.5%) for pure components within the specified carbon number ranges. Extrapolation outside these ranges may introduce larger errors.
• The calculated temperatures are for 1 atm pressure. Both melting and freezing points change with pressure, but standard data is usually reported at 1 atm.

References

• Original Source