Crude Oil (1): Oil fractionation

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In this post we discuss a key chemical refinery process: Oil Fractionation or Fractional Distillation.

Oil fractionation is one of the likely starting points in the study of chemistry.

It is also discussed at several different levels and the reasons aren't hard to find.

Oil fractionation is a key starter process in chemical production that leads to a host of products often called petrochemicals and/or fuels e.g LPG, Petrol or Gasoline, Kerosene, Diesel are fuels whereas plastics, solvents and lubricants might be called petrochemicals.

Much of what we do in life today relies on these types of compounds and what chemists can derive from them like plastics.

These compounds begin life in a mixture of many different hydrocarbons: called petroleum or crude oil.

Several questions arise at this point:

#1. How can this mixture of hydrocarbons be separated?

#2. How did petroleum form in the first place?

#3. What is hydrocarbon?

In this post we are going to discuss how the petroleum mixture of hydrocarbons is separated.

Further posts will address the other two questions.

How to separate a mixture of hydrocarbons or how oil fractionation happens.

Oil fractionation or fractional distillation depends on the parts of the mixture being all liquids with different and distinct boiling points.

The fact is that the crude oil that is extracted from under the North Sea or from under the deserts of Saudi Arabia is mixture of molecules that have different boiling points.

So if this mixture is heated to a high temperature (say about 350°C) and pumped into long vertical tube (called a fractionating tower) then the smaller molecules with low boiling points rise to the cooler parts of the tube and the larger molecules with higher boiling points tend to remain in the lower parts of the tube.

You can find several diagrams of this on the net I've included a few for you to see how they are drawn.

If you are studying chemistry you will find that you will be questioned on how this process works so below I have reproduced a typical question from a very recent exam paper at GCSE that focuses on this area and also you can see something of the chemistry points that were expected in a typical high level answer.

You can find the full question and answer here.


So here are the bullet points looked for in the answer:

Examples of chemistry points made in the response could include:

•   Some / most of the hydrocarbons (or petrol) evaporate / form vapours or gases
  
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•   When some of / a fraction of the hydrocarbons (or petrol) cool to their boiling point they condense
  
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•   Hydrocarbons (or petrol) that have (relatively) low boiling points and are collected near the top of the fractionating column or hydrocarbons with (relatively) high boiling points are collected near the bottom of the fractionating column
  
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•   The process is fractional distillation
  
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•   Heat the crude oil / mixture of hydrocarbons or crude oil / mixture is heated to about
  350°C
  
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•   Some of the hydrocarbons remain as liquids
  
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•   Liquids flow to the bottom of the fractionating column
  
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•   Vapours / gases rise up the fractionating column
  
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•   Vapours / gases cool as they rise up the fractionating column
  
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•   The condensed fraction (or petrol) separates from the vapours / gases and flows out through a pipe
  
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•   Some of the hydrocarbons remain as vapours / gases
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•   Some vapours / gases rise out of the top of the fractionating column
  
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•   There is a temperature gradient in the fractionating column or the fractionating column is cool at the top and hot at the bottom.

These points help to clarify what is happening inside the fractionating column.  

Let's just note some features of the expected answer: 
the column is called a fractionating column, 
there is a temperature gradient: hot at the bottom to cool at the top because it is cooler the further from the heat source, 
fractions are just parts of the crude oil mixture and these fractions tend to boil over a very narrow range of temperatures for example 250-300°C, 
larger molecules have the higher boiling points so these molecules form the fractions emerging from the lower parts of the column. 

More about fractional distillation in my next post. 

If you appreciated this post please add a comment or question and I will post an answer on the blog. 

Here is a source of an experiment about fractional distillation.

Here is a task where you can cut and put together a sketch of a fractionation column:

Pages on the "Mole" and "Using the Mole" in chemical calculations are here

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