Crude oil is the raw material from which different hydrocarbon products, such as petrol, are derived. It consists of various components with varying weights, boiling points, and molecular sizes. Because of these differences in boiling points, you can separate them easily through a process known as fractional distillation.
This process involves separating crude oil into its components or fractions. This is done through a series of evaporation and condensation processes at several boiling points in the fractional distillation column. Hence, this allows the refining of crude oil into other profitable products. Here’s a breakdown of how the distillation process occurs.
Fractional distillation steps
Step 1: Vapourising the crude oil
The first step is heating the crude oil until it vapourises in a furnace. The crude oil vapour is fed into a fractional distillation column where the constituent distillates or fractions separate as per their boiling points. The least dense ones, like butane and propane, rise to the top of the column, and the others separate lower in the column as per their molecular weight. This weight difference means the heaviest fractions, such as asphalt and lubricating oil, settle at the bottom of the column.
Step 2: Condensation
The distillation column has plates fitted with bubble caps or holes to allow the vapour to pass through. There are different plates at varying temperature levels to collect the fractions that pass through. The bubble caps increase the contact time between the vapour and the liquids at a particular level, thus allowing for them to separate.
As the vapour rises through the plates, it cools. When a fraction reaches a height in the column where its temperature equals its boiling point, it condenses. The temperature in the distillation column is highest at the bottom, where the heavier fractions liquefy and cool at the top.
Fractions are categorised based on their characteristics. The lightest ones are the top fractions, such as propane and other gases. The middle fractions, such as gasoline and petrol, form lower in the column. Below this point are the heavier products, such as fuel oil, which is used in marine time applications. Lastly, the lowest distillates are the residuals, such as bitumen.
Step 3: Collection of the distillates
After the fractions condense into a liquid, it’s collected and passed through another condenser to cool it further. Afterward, it is fed into a storage tank, as it awaits further processing. An example of such a process is stripping, which is done to every product after leaving the fractional distillation column.
Since perfect separation is not possible in the central separation column, unwanted components mix into the distillate collected, especially if their boiling points are close; stripping removes these volatile elements and thus reduces the flash point of the end product. Engineering manufacturing companies use this stripping process to purify petroleum products before utilising them.
Fractional distillation is an invaluable process in the oil refining industry. Its ability to separate substances with slight differences in their boiling points allows for the production of the many oil and petroleum products available today.
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