2013: Advent Oil & Gas: Processing 2 – refining

by on December 9, 2013

Once stabilised, crude oil is processed at a refinery to turn it into useful products like petrol, diesel, and heavy oil.  This is a fiendishly complicated chain of operations – but there are not that many basic processes, and individually they aren’t too hard to get your head round.  I’ll run through them in broad terms and consider how they produce the array of end products.

The real key to refining is fractional distillation, which allows a mixture to be split into different components or ‘fractions’.  This depends on the fact that each component of crude oil has a different boiling point.  A fractional distillation tower is a huge construction that’s heated at the bottom and cooled at the top, with the temperature carefully controlled at all heights.  Lighter components like pentane and hexane boil very easily, and rise up the tower until they reach a point cold enough to condense; meanwhile the heavier ones sink to the bottom, condensing at a much higher temperature.  At each level the condensed liquid is drawn off, and at the very top the gas boils off and is recovered for use as fuel.

This leaves a sticky residuum at the bottom, which can’t be boiled without the heat breaking down the component.  How can we deal with this horrible stuff?  The answer is vacuum distillation, which unsurprisingly uses a vacuum to reduce the boiling point.  Residuum is heated up and fed in, and separates out just like in normal (atmospheric) distillation.  Sometimes the very heavy oil is used as a fuel for the heaters.  Anything that won’t boil in vacuum is considered vacuum residuum.

Most of the products of distillation still contain sulphur and as this is a contaminant it needs to be removed.  Pretty much every fraction goes through either hydrotreating or merox processes, where hydrogen or oxygen (respectively) are added to remove sulphur.  Even this isn’t wasted: elemental sulphur is produced as a by-product, which can be sold on as an industrial feedstock.  The hydrotreating process also serves to remove nitrogen, and to turn aromatics and alkenes into more useful alkanes.

We then have a large number of fractions, all of which are useful… or are they?  A constant problem is that the proportions of fractions don’t match up with demand.  Most commonly the distillation produces a lot of heavy fractions whereas we want mostly light-to-medium fractions for petrol, kerosene, and diesel.  Fortunately it’s possible to break down heavy hydrocarbons into lighter chunks through a family of processes called, simply enough, cracking.

The best-known cracking process is fluid catalytic cracking (FCC), which uses heat and a catalyst like palladium to break heavy oil down to lighter components.  It works well but does leave some residue – which is often treated by hydro-cracking, using hydrogen to completely break down a wider variety of hydrocarbons.  The very viscous vacuum residuum needs more heavy-duty processes: residue fluid catalytic cracking (a two-stage FCC process under increased heat and pressure), visbreaking (a thermal cracking process), and coking (a really severe thermal cracking process that can break down just about anything).

Some additional treatment is often necessary – such as alkylation, where branched hydrocarbons are converted into straight chains using an acid catalyst – but distillation, sulphur treatment, and cracking are the most important processes, and give us the basic building blocks of fuels and other products.

  • The lightest fractions are gasses: chemical feedstocks, refinery gas, and liquefied petroleum gas (LPG).
  • Middleweight fractions in increasing weight order are petrol, kerosene, and diesel.
  • Then there are heavy oils: lubricants as well as bunker oil for ships.
  • Heaviest of all is bitumen, used mostly for roads.

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