Chemistry – Organic chemistry - Fuels | e-Consult

As petroleum fractions are separated in a fractionating column, the fractions collected at the bottom are progressively more complex and have different properties compared to those collected at the top. Here's a description of the changes:

• (a) Decreasing Chain Length: The fractions at the bottom of the column contain larger hydrocarbon molecules with longer carbon chains. As you move up the column, the hydrocarbon molecules become progressively smaller, with shorter carbon chains.
• (b) Higher Volatility: Fractions at the top of the column are composed of smaller, lighter molecules. These molecules have higher volatility, meaning they vaporize more easily at lower temperatures. Fractions at the bottom are less volatile.
• (c) Lower Boiling Points: Fractions with shorter carbon chains have weaker intermolecular forces (van der Waals forces). Therefore, they have lower boiling points compared to fractions with longer carbon chains. The boiling points generally increase as you move down the column.
• (d) Lower Viscosity: Fractions with shorter carbon chains are less viscous. This is because the smaller molecules can move past each other more easily. Fractions with longer carbon chains are more viscous due to increased intermolecular attraction and resistance to flow. Viscosity generally increases as you move down the column.