Question

When petroleum is heated gradually, first batch of vapours evolved will be rich in: (a) Kerosene (b) Petroleum ether (c) Diesel (d) Lubricating oil

Short answer

The first vapours evolved will be rich in petroleum ether.

Step-by-step solution

Understanding the Petroleum Distillation Process

Petroleum is a mixture of hydrocarbons that need to be separated through a process known as fractional distillation. This process involves heating petroleum so that it evaporates and then condenses at various levels in a distillation tower, with different products being collected at different heights based on their boiling points.

Identifying the Volatility of Different Fractions

Each component of petroleum has different boiling points. Petroleum ether, kerosene, diesel, and lubricating oil are all products with rising boiling points in that order. Petroleum ether is the most volatile and will evaporate at lower temperatures compared to kerosene, diesel, and lubricating oil.

Determining the First Vapors Evolved

Since petroleum is heated gradually in the distillation process, the most volatile components will evaporate first. Therefore, the vapor that evolves initially is rich in the component with the lowest boiling point, which is petroleum ether.

Key concepts

Fractional Distillation

Fractional distillation is a critical process used extensively in petroleum refining to separate a mixture of hydrocarbons into individual components. The method hinges on differences in boiling points. Let’s break this down simply:
Petroleum is a complex mixture, with each component having its own unique boiling point. This characteristic is key because it means that when petroleum is heated, these components evaporate at different temperatures.
In a fractional distillation tower, petroleum is heated gradually. As it heats up, the various components, based on their boiling points, become vapor and rise through trays.
The tower is cooler at the top, allowing for condensation of different vapors at specific heights. For instance:

• Lower boiling point products condense near the top.
• Higher boiling point products condense lower down or exit as heavy residues.

This method enables refineries to effectively extract different petroleum products, such as gasoline, diesel, and kerosene, based on where they condense in the tower.

Hydrocarbons in Petroleum

Hydrocarbons are organic compounds made solely of hydrogen and carbon atoms. They are the primary components of petroleum. Understanding their structure and behavior is essential for grasping how petroleum distillation works.
There are several types of hydrocarbons within petroleum:

• Alkanes – These are saturated hydrocarbons with single bonds. They are often referred to as paraffins.
• Cycloalkanes – Also known as naphthenes, these have ring structures and are quite prevalent in crude oil.
• Aromatics – These hydrocarbons, such as benzene, contain at least one aromatic ring and are known for their stability.

Each type of hydrocarbon has distinct properties impacting their boiling points and behavior during the distillation process. Knowing these differences allows for more precise separation of petroleum into various useful products.

Boiling Points of Petroleum Products

The boiling point of a substance is the temperature at which its vapor pressure equals the atmospheric pressure, causing it to transition from liquid to gas.
In the context of petroleum products, understanding boiling points is crucial for the effectiveness of fractional distillation. Different products have distinct boiling points:

• Petroleum Ether – Evaporates at lower temperatures, making it one of the first to rise during distillation.
• Kerosene – Has a higher boiling point compared to petroleum ether, condensing in the middle of the tower.
• Diesel – Boils at an even higher temperature, condensing lower in the tower.
• Lubricating Oil – Has a very high boiling point, making it one of the last to distill or remaining as a residue.

This hierarchy of boiling points ensures that each product is efficiently separated based on temperature gradients within the distillation column. Understanding these points is vital for refining processes to produce the desired petroleum products effectively.