Distillation, Cracking and Catalyzing; Refining Petroleum

The process of refinement of petroleum.

by Arthur Caldicott

You know those coin sorters – a set of stacked trays punched with holes. Different sized holes for differ­ent coins. You toss your jar of coins in the top tray, shake them, and eve­rything falls through except twoonies, the biggest coin. All the remaining coins fall through the holes in the next tray, except loonies, then quarters, nickels, dimes, pennies? Oops, pen­nies, dimes.

That’s kind of how petroleum is refined. Of course, it is more compli­cated than sorting coins. Distillation, cracking and catalyzing are key parts of the process. But at the end it’s the same thing: bags of money. 

We’re talking about hydrocar­bons, that is, substances with mol­ecules made up of hydrogen and car­bon atoms.

These atoms combine and can be combined in precisely a gazillion different ways. In a petroleum refin­ery the valuable hydrocarbons are re­moved. What remains is then broken up and recombined into other valuable hydrocarbons. At the end, virtually all the hydrogen and carbon atoms that went into the refinery in the begin­ning, end up as, well, bags of money.

Number of Carbon Atoms

Fundamental properties of a hy­drocarbon begin with the number of carbon atoms in the molecule.

That number determines the weight, state (gas, liquid, or solid) and boiling point of the fuel.

To illustrate, the lightest hydro­carbon, methane, has one carbon atom. Its molecular formula is CH4. It is normally a gas, half the time re­ferred to as natural gas, and it boils – changes from a liquid (or liquefied natural gas or LNG) to a gas at -160°C. Ethane has two carbon atoms: C2H6. Propane has three: C3H8.

Fractional Distillation

Specific hydrocarbons are sepa­rated out of crude oil by the process of distillation, separating the gases from the liquids at different temperatures.

The fractionating column is a tall cylinder, cooler at the top, hottest at the bottom. As the hot hydrocarbon va­pours rise through the cooling tower, they condense into liquids at the level where the temperature is the boiling point for that hydrocarbon. The en­gineers have designed the tower with collecting trays at all the right levels. As these fractions condense out, they are drawn off, and more heated crude is introduced at the bottom.

Gasoline is the mix or fractions of hydrocarbons between 5 and 12 carbon atoms. Kerosene has 12 to 15 carbon atoms.

The heavier hydrocarbons could also be fractionally distilled out, for low value tar or asphalt. But refiner­ies can crack, or break, the remaining heavy hydrocarbons into lighter mol­ecules with fewer carbon atoms, and thereby convert a low value substance into something of much higher value.

The processes in a refinery don’t end here. We can increase the octane level and improve the gasoline in other ways by messing with the molecular bonds, and the molecular structure, of the fuel in a catalytic reformer unit.

Highly engineered fuels come out of modern refineries, as do other products and byproducts which are used in innumerable petrochemical processes to create fibres like nylon and polyester, plastics, detergents and solvents.

Bitumen from the Tar Sands

Bitumen from Alberta’s tar sands is on the heaviest end of the hydro­carbon scale. Once the bitumen is separated from the sand and water, it is upgraded to a synthetic crude oil. Upgrading involves the same processes but starts at higher tem­peratures – 500° C – and includes coking, a thermal cracking process which removes carbon while it breaks the heavy hydrocarbons into lighter forms. Coke can be gasified and used in the upgrading and refining process. It is a combustible substance similar to coal, and much of Alberta’s coke output ends up in Japan, though even more is stockpiled.

Other parts of bitumen upgrading include distillation, catalytic conver­sions and hydrotreating which re­moves sulphur and nitrogen and adds hydrogen to molecules.

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Information for this article was obtained from Wikipedia and other online sources. GHG emissions are from www.pollutionwatch.org.

[From WS September/October 2008]

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