The oil Industry

C.2.1 Compare the use of oil as an energy source and as a chemical feedstock C.2.2 Compare catalytic cracking, thermal cracking and steam cracking. ( Students should include the environmental impact of the processes and their products.) These are both “woffly”. Its hard to know exactly what information is needed. I will give you some information, but it would be useful to do some background reading as well!

Crude oil is a mixture of compounds. In order to be useful the crude oil must be refined. This is done through fractional distillation.

Make sure you can explain how fractional distillation works. You may want to add the number of carbon atoms present in each fraction. FractionNumber of carbon atoms Refinery gas1 - 4 Gasoline/naphtha Kerosene Diesel oil Residue>25

Lets have a debate. Lets have a debate. Motion: Dmitri Mendeleev was correct when he said that burning oil as a fuel was like “firing up a kitchen stove with banknotes”

Some points: 90% of all oil products are used as fuel. 10% of oil products are used to produce other products such as polymers, drugs, cosmetics, paints, fertilisers, pesticides, detergents and dyes. Petrol is a very concentrated and convenient energy source. ( a petrol pump supplies energy at about 34MW ; a medium sized power station produces about 700MW)

Burning hydrocarbons produces smog and global warming (maybe!) Plastics are non-biodegradable and it can be problematic to dispose of them. Crude oil is a non-renewable resource It will last longer if we conserve energy and recycle materials. Alternative energy sources are becoming available. Polymers and other organic chemicals can be made from coal or biological materials such as wood, starch or cotton.

There is no right or wrong answer. Just be prepared to discuss the different considerations.

Cracking Breaking larger molecules into shorter ones. This is done because shorter molecules (with 1 to 12 carbon atoms) are in greater demand. Cracking produces a mixture of alkanes and alkenes.

The total number of carbons is unchanged. We get a mixture of saturated and unsaturated products.

The short chain alkanes are useful as fuels. This is because they are easier to burn than longer alkanes. Why? Despite what you might think from all the equations (!), cracking often produces branched alkanes. These are better as car fuel because they prevent “knocking” They have a higher “octane” number.

The alkenes produced are useful as chemical feedstock. They are mainly used to produce polymers. Notice also that polymers don’t form straight lines. They zigzag. Why? The carbon atoms each have a tetrahedral structure.

Different ways of cracking There are (at least) 4 different ways of cracking long alkane molecules. You need to know the conditions for each one.

Thermal cracking (pyrolysis) Feedstock: long chain alkanes Reaction conditions: Temp: 800 – 850 °C Pressure: 70 atm No catalyst Long contact time Mechanism: Free radical (heterolytic fission) Products: short chain alkanes (for fuel) and short chain alkenes (esp. ethene for polymerisation)

Catalytic cracking Feedstock: long chain alkanes Reaction conditions: Temp: 500°C Pressure: low or moderate Powdered zeolite catalyst (usually a mix of silica and alumina) Short contact time Mechanism: Complicated – forms an ionic intermediate. Products: Branched alkanes and alkenes with high octane number (suitable for fuels). Some cyclic compounds are also produced.

Steam cracking Feedstock: Shorter alkanes – up to 10 carbons Reaction conditions: Temp: °C Pressure: low or moderate No catalyst Very short contact time Feedstock diluted with steam Mechanism: Free radical Products: Low molecular mass alkanes and alkenes (esp. ethene)

Hydrocracking Feedstock: Heavy hydrocarbon fractions (i.e. high Mr) Reaction conditions: Temp: °C Pressure: 80 atm Zeolite or platinum catalyst Feedstock mixed with hydrogen Mechanism: Products: High yield of branched alkanes and cycloalkanes. Some aromatic compounds. No alkenes!

Assessment statements - C3 Addition polymers C.3.1 Describe and explain how the properties of polymers depend on their structural features. ( Students should consider: different amounts of branching in low- and high- density polyethene; different positions of the methyl groups in isotactic and atactic polypropene. C.3.2 Describe the ways of modifying the properties of addition polymers. Examples include plasticizers in polyvinyl chloride and volatile hydrocarbons in the formation of expanded polystyrene. C.3.3 Discuss the advantages and disadvantages of polymer use. (Include strength, density, insulation, lack of reactivity, use of natural resources, disposal and biodegradability. Use polyethene (both LDPE and HDPE), polystyrene and polyvinyl chloride plastics as examples.

Addition polymerisation

We’ve already studied this!! Prove to me that you can remember it by drawing the repeat unit of polypropene (aka polypropylene)

A few other common polymers: Polystyrene

Polychloroethene (Polyvinylchloride – PVC)

Obviously the identity of the monomer changes the properties of the polymer formed. But there are also a number of other factors that determine the properties of a polymer.

Branching

Polyethene can have different properties depending on how it is produced. One of the factors that makes a difference is the presence of side chains. This is known as branching. HDPE has less branching LDPE has more branching

High density polyethene HDPE is defined by a density of greater or equal to g/cm 3. HDPE has a low degree of branching and thus stronger intermolecular forces. Strong, rigid, high MP (135°C) HDPE is used in products and packaging such as milk jugs, detergent bottles, margarine tubs, garbage containers, buckets and water pipes.

HDPE Straight chains Close packing Strong intermolecular forces

Low density polyethene LDPE is defined by a density range of 0.910– g/cm 3. LDPE has a high degree of short and long chain branching, which means that the chains do not pack as well. It has weaker intermolecular forces. More flexible, lower MP (100°C) LDPE is used for both rigid containers and plastic film applications such as plastic bags and film wrap

LDPE Branched chains Limits how close molecules can get Weak intermolecular forces

LDPE is produced at high pressure The mechanism is free radical. HDPE is produced at a higher temperature A catalyst is used.

Orientation

Extra credit question: Who is Kilgore Trout?

In any polymer with a “dangling” side group (such as the methyl group in polypropene) The orientation of the side group can affect the polymer properties. The side groups may be all on the same side Isotactic They may alternate sides Syndiotactic Or they may have a random arrangement Atactic

In isotactic PP the polymer chains can pack more closely So intermolecular forces are higher So the polymer is tougher It is used for car bumpers, or is drawn into strong fibres for carpets etc. Atactic PP is more flexible It is used in sealants.

Adding other substances to polymers

The properties of polymers can also be modified by adding other substances. For example expanded polystyrene (used for packing material and coffee cups – or to make houses!!) Pentane is added to the mixture during the processing stage. This doesn’t form a polymer, but as it is volatile it vaporises and expands the polystyrene. Hence the resulting polymer has low density and is a good thermal insulator

Plasticisers PVC contains a polar C-Cl bond and hence has high intermolecular forces. This leads to a very rigid plastic (UPVC or PVCU) which is used in windows, doors, drainpipes etc.

“UPVC is not sensitive to water, wind or sun exposure”

To make the plastic more flexible, we add small molecules known as plasticisers These reduce intermolecular forces and produce a more flexible polymer E.g. for use in blood bags or intravenous drips (not to mention pvc clothes!

A typical plasticiser would be D.O.P. DiOctylPhthalate – sometimes called DEHP DiEthylHexylPhthalate

This has the disadvantage of mimicking female hormones It leaches into materials such as oils and fats The US EPA sets a safe limit of 6ppb in water It lowers the sperm count in men and carries a high risk for male foetuses and males at or around puberty. It is also suspected to cause obesity and insulin resistance. 25% of American women have phthalate levels which are “a concern”

Copolymers Another common way of modifying polymers is to mix two or more monomers. The resulting copolymer will have a mix of properties. By choosing both the identity and relative proportions of the 2 monomers, we can modify the properties of the copolymer to exactly what we require. E.g. mechanical properties, solubility, crystallinity, glass transition temperature.

Common examples of copolymers include: EVA – ethyl vinyl acetate Used for hot melt adhesives

And ABS – acrylonitrile butadiene styrene Used to make Lego, canoes, car parts and tattoo ink!!

Copolymers may be of different types:

Thermoplastic and Thermoset Polymers In many plastics the only forces between polymer chains are Van Der Waal’s forces. These can be overcome if the ploymer is heated. Then the tangled polymer chains can slide past each other. The hot plastic can be moulded into any shape.

Certain polymers have strong covalent bonds between polymer chains These are known as cross linked polymers Rubber is probably the commonest example. These polymers do not soften when they are heated. They are thermoset plastics and cannot be moulded into different shapes.

Advantages of Polymers They can be specifically made to have the required properties: Strength: HDPE; rigid PVC; Polystyrene Flexibility: LDPE; Plasticised PVC Insulator: Expanded polystyrene Chemical resistant: PTFE (Teflon)

They are usually Light Waterproof Electrical insulators They are (mostly) thermoplastic, so they can be moulded into different shapes They can be easily and permanently coloured.

Disadvantages of polymers They are mostly sourced from petroleum – a non renewable resource. They are difficult to dispose of – Non-biodegradable – Produce toxic fumes when burned – Have a high volume in landfill

We can minimise problems by: Conserving plastics – Do we really need so much packaging? Recycling – Some plastics are very easy to melt and reuse. – E.g. PET from plastic bottles – The hardest bit is usually sorting the plastics out – This is very labour intensive – The amount recycled depends on government policy

Just for fun... In a 2007 national survey of American beliefs about recycling, for example, it was discovered that as much as 72 percent of Americans don't know that plastic is an oil- based product (around 10 percent of U.S. oil consumption goes into making plastic); while 40 percent of them think that plastic biodegrades underground, in composts, in landfills, or incredibly, out at sea.

Making biodegradable polymers from cellulose or starch. – These can be broken down by bacteria or fungi – Except... – The linings used to protect most landfill sites prevent decomposition of these plastics! Burning plastics – Most plastics can be burned and the energy given out used for heating or electricity generation. – If the Temp is not high enough toxic compounds such as CO or dioxins are produced – CO 2 is a greenhouse gas – Chlorinated polymers give HCl which causes acid rain – The printing on plastics often contains toxic heavy metals like lead or cadmium.

Review the Syllabus Statements