Fractional distillation of crude oilIndustrial Cracking

By Andrew Barker

Why I am doing this presentation

• This presentation ties in to the section on alkenes

• I am doing my presentation on fractional distillation and Thermal, Industrial and Catalytic Cracking.

What is fractional Distillation?• Fractional distillation is the

separation of mixtures into it component parts, or its fractions.

• We do this by heat it up and collecting the fractions that boil over different ranges of temperature.

• Each fraction is a mixture of hydrocarbons of similar chain length.

Steps of Fractional Distillation • The crude oil is first heated in the furnace.• A mixture of liquid and vapour passes into a

tower that is cooler at the top than at the bottom.

• The vapour pass up the tower via a series of trays containing bubble caps until they arrive at a tray that is sufficiently cool (at a lower temperature then their boiling point). Then they condense to liquid.

• The mixture of liquids that condenses on each tray is piped off.

• The shorter chain hydrocarbons condense in the tray nearer to the top of the tower, where it is cooler, because they have lower boiling points.

• The thick residue that collects at the base of the tower is called tar or bitumen. it can be used for road surfacing but, as supply often exceeds demand, this fraction is often further processed to give more valuable products.

The composition of a typical North Sea oil

Product Gases Petrol Naptha Kerosene Gas oil Fuel oil and wax

Approximate boiling temperature / K

310 310 - 450 400 - 490 430 - 523 590 - 620 Above 620

Chain length 1 - 5 5 - 10 8 - 12 11 - 16 16 - 24 25+

Percentage present 2 8 10 14 21 45

Industrial Cracking• The naphtha fraction is in huge demand for petrol and chemical industry

• The longer chain fractions are not useful and therefore of lower value economically

• To meet demand for the shorter chain hydro carbons, many of the longer chain fractions are broken into smaller lengths (cracked)

• This has two useful results:

– Shorter chains are produced

– Some of the products is alkenes, which are more reactive then alkanes

Thermal Cracking• This is when Alkanes are heated up to 700 – 1200k and under high pressure, 7000 kPa.

• Carbon-carbon bonds break in such a way that one electron from the pair in the covalent bond goes to each carbon atom

• Initially two shorter chains are produced each ending in a carbon atom with an unpaired electron

• These fragment are called free radicals

• Free radicals are highly reactive

CH3CH3 → 2 CH3•

Catalytic cracking • Catalytic cracking takes place at low

temperatures (720k) and low pressure (but more then atmospheric)

• Using a zeolites catalyst, consisting of silicon dioxide and aluminum oxide.

• Zeolites have a honeycomb structure with an enormous surface are

• This form of cracking is used mainly to produce motor fuel

• The products obtained are separated by fractional distillation

• The products are branched alkanet, cycloalkanes and aromatic compounds