Oil refining, fuels and thermochemistry Flashcards
Q: What are fossil fuels?
- Fossil fuels are fuels formed from the remains of plants and animals that lived millions of years ago
Examples: Hydrocarbons such as coal, natural gas crude oil (petroleum)
Natural gas
Natural gas is a mixture of hydrocarbons – primarily methane
Methane = (CH4)
Q: How is methane produced?
1) Animal waste and dead plants are allowed to be decomposed by anaerobic bacteria producing methane
2) Naturally produced in slurry pits, coal mines, and refuse dumps , landfills
Q: Give an advantage of methane production
- Methane is a very good fuel
Outline a hazard of methane production
- Accidents have occurred in mines as methane forms an explosive mixture with air and explosions have occurred
Q: Give a disadvantage of methane production
- Methane is one of the leading gases that causes the “greenhouse effect” leading to global warming
Q: What is crude oil/petroleum?
- Crude oil is a type of oil that is a mixture of many different hydrocarbons
Note: Crude oil is found and extracted from beneath the Earth’s surface
Q: What is fractional distillation/fractionation of crude oil?
- Fractional distillation is a process that is used to separate crude oil into its component hydrocarbons based on their specific molecular mass /boiling points
Note: Fractional distillation is carried out in fractionating columns/towers in an oil refinery
Q: Why is fractional distillation of crude oil carried out?
- Crude oil itself is of little use – its component hydrocarbons are extremely useful
Q: How is fractional distillation carried out?
- The crude oil is heated to high temperatures and fed into large fractionating towers up to 80m high.
- Towers kept at high temperatures at the bottom
- There are trays present going up the toes to collect he fractions.
- Temperature decreases as you ascend tower
- As crude oil rises up, the fractions will reach a temperature that’s just below boiling point and turn into liquid which is collected on the tray.
- Heavier hydrocarbons condense first, lighter hydrocarbons rise higher and higher up & condense at top of tower.
Q: Based on what two properties of the hydrocarbons are they separated into their different fractions?
1) Their molecular mass 2) Their boiling points
Refinery Gas
Chain Length: C1- C4
Uses: Domestic Gas (LPG)
Light Gasoline
Chain length : C5 - C10
Uses: Petrol for cars
Naphtha
Chain length: C7-C10
Uses: petrol for cars, Feedstock for the petrochemical industry to make plastics, solvents , detergents.
Kerosene (paraffin oil)
Chain Length - C10- C14
Uses : Fuel for jet aircraft (aviation)
Oil fir hole central heating
Gas Oil (Diesel oil)
Chain length : C14- C40
Uses: Diesel for vehicles, Lubricant for engines
Residue
Chain Length: C35 and higher
Uses: Bitumen for road surfacing, roofing and waterproofing
Q: Why is the bottom fraction known as the residue fraction?
- This is the fraction that is left over at the bottom of the fractionating column as the other, more volatile (lower B.P) fractions remain as gases and travel upwards
Q: What does the abbreviation LPG stand for? What gases are the main components of LPG? What is LPG used for?
- Liquid Petroleum Gas
- Consists of propane and butane mixture
- Sold as domestic gas for heating and cooking
Q: What are mercaptans and why are they used?
- Mercaptans are sulfur containing compounds
- They are added to natural gas (methane) and liquid petroleum gas to give them an unpleasant smell to warn of gas leaks
Q: What is petrol composed of and how is it used as motor fuel?
- Petrol is a mixture of different hydrocarbons, mostly chain length C5 – C10 alkanes
- In the (internal combustion) engine of a car, a mixture of petrol and oxygen is ignited by spark plugs
- The “explosion” provides the power to turn the wheels
Petrol rich in branched chained hydrocarbons burns more smoothly and efficiently. An example of a branched-chained hydrocarbon is 2,2,4-trimethylpentane (iso-octane).
Auto-Ignition (knocking)
The early explosion of a petrol-air mixture caused by increasing pressure in the engine
Results in loss of power in a car and damages the engine.
Q: What property of petrol leads to auto-ignition?
- Petrol containing long, straight chained alkanes ignites very easily with air Examples: heptane, octane, nonane
Q: What is meant by octane number?
The octane number of a fuel is the measure of its tendency to resist Auto-ignition/’knocking’.
Note: Higher octane number = more resistant to auto-ignition = better fuel
Factors affecting a fuel’s octane number
a) Length of chain: The shorter the chain length, the higher the octane number
b) Degree of branching: The more branched the chain, the higher the octane number
c) Cyclic structure: Cyclic compounds have a higher-octane number than straight chain compounds
Factors affecting a fuel’s octane number
a) Length of chain: The shorter the chain length, the higher the octane number
b) Degree of branching: The more branched the chain, the higher the octane number
c) Cyclic structure: Cyclic compounds have a higher-octane number than straight chain compounds
Q: How is a fuel given an octane number?
- Two reference compounds are used:
➢ Heptane has a large tendency to auto ignite – given octane number of 0
➢ 2, 2, 4 -trimethylpentane (iso-octane) has a low tendency to auto-ignite – given octane number of 100 - The performance of the fuel is compared with known mixtures of heptane and 2,2,4 - trimethylpentane and is given a suitable octane number
Q: Until the early 2000’s in Ireland lead compounds were added to petrol to increase its octane number and reduce its tendency to auto-ignite before being phased out. Why were lead compounds phased out?
- Lead is harmful to the environment and to people’s health
- Lead poisons the catalytic converters in cars
Q: What alternatives to adding lead compounds are used to improve the octane number of a fuel/reduce its tendency to auto-ignite
1.) ADD LEAD TO PETROL (ADDITIVE) - Lead was added in the form of tetraethyl lead. This allowed the petrol to burn smoothly. However, leaded petrol is toxic, therefore harmful to the environment and people’s health. Leaded petrol is being phased out due to these reasons.
2.) ADDITION OF OXYGENATES (ADDITIVES) - - this is the addition of oxygen compounds to the petrol. These tend to be of two types
(1) Addition of alcohols i.e. methanol (octane no. of 114)
(2) addition of ethers i.e. methyl tert-butyl ether (MTBE) (octane no. of 118).
These compounds,
(a) increase the octane number of the petrol and (b) they also cause less pollution as they reduce the level of carbon monoxide in the exhaust fumes.
3.ISOMERISTATION (REFORMING) - this involves changing straight-chained hydrocarbons into branched-chained isomers. The branched-chained isomers burn more smoothly and therefore have a higher octane number.
4.) DEHYDROCYCLISATION (REFORMING) - this process involves the conversion of straight-chained hydrocarbons to form ring compounds. The ring compounds are then converted into aromatic compounds. This is done in the presence of a catalyst. As hydrogen is a by-product of this reaction it is called ‘dehydro’. Aromatic compounds have a high octane number but are also carcinogenic.
5.) CATAYLTIC CRACKING (REFORMING) - this process involves breaking down long chained hydrocarbons for which there is low demand into short chained molecules for which there is high demand. These short-chained hydrocarbons have a higher octane number and also tend to be highly branched.
Reforming
Reforming involves changing straight chained hydrocarbons into branched chained hydrocarbons or cyclic hydrocarbons.
Cracking
Cracking involves changing long chained hydrocarbons for which there is low demand into short chained hydrocarbons for which there is high demand.
Auto-ignition
The early explosion of a petrol-air mixture caused by increasing pressure in the engine.
Q: How is hydrogen manufactured on a large scale?
- Steam reforming of natural gas
- Electrolysis of water
- Dehydrocyclisation of a hydrocarbon – produces hydrogen gas as a bi-product
Q: Give three uses of hydrogen gas
1) Manufacture of Ammonia (NH3) – Known as Haber process: used to make ammonia which is
used in cleaners and bleaches
2) Hydrogenation of vegetable oil to make margarine
3) Potential to use as a fuel
Notice: An iron catalyst is used for the Haber process - transition metals are excellent catalysts
Q: Give two advantages of using hydrogen as a fuel
1) Hydrogen is a “clean fuel” – water is the only product of its combustion
2) Hydrogen burns efficiently as a fuel – it has a high kilogram calorific value: a lot of energy is given out per kg burned in oxygen
Q: Give a disadvantage of using hydrogen as a fuel
- Hydrogen is explosive with oxygen – problems regarding storage and transportation
Q: What is an exothermic reaction?
- A reaction where heat is lost from the reaction to the surroundings
associated with a rise in temperature - Given a negative ΔH value (ΔH = - )
Δ = delta = charge
H = Heat
Example: Combustion (burning in oxygen) of hydrocarbons
Q: What is an endothermic reaction?
- A reaction where heat is taken in by the reaction from the surroundings. (associated with a fall in temperature)
- Given a positive ΔH value ( ΔH = +)
eg, = Reaction of Herbert with water
HEAT OF FORMATION
The heat change that occurs when 1 mole of a substance is formed from its elements in their standard states.
Note: When writing a heat of formation chemical equation for a substance, the substance must be present as 1 mole on the right hand side
Examples: Write equations for the heats of formation of:
CH4 : C + 2H2 + CH4
C2H5OH : 2C + 3H2 + 1/2O2 = C2H5OH
HEAT OF FORMATION
The heat change that occurs when 1 mole of a substance is formed from its elements in their standard states.
Note: When writing a heat of formation chemical equation for a substance, the substance must be present as 1 mole on the right hand side
Examples: Write equations for the heats of formation of:
CH4 : C + 2H2 + CH4
C2H5OH : 2C + 3H2 + 1/2O2 = C2H5OH
HEAT OF COMBUSTION
The heat change that occurs when 1 mole of a substance is burned in an excess of oxygen.
Note: All organics burned in o2 produce CO2 + H2O
Example: The following equation describes butane (C4H10) in a cylinder being burned in a gas cooker
2C4H10(g) + 13O2 (g) 8CO2(g) +10H2O(g) ΔH=-5754kJ
Q: What piece of apparatus in the laboratory can be used to measure the heats of combustions of substances such as fuel or foodstuffs? Describe how it is used.
➢ A bomb calorimeter
* A known mass of the substance is placed in the crucible in the bomb
* Bomb is placed in a known volume of water
* The substance is ignited electrically and allowed to burn in excess
oxygen
* The change in temperature of the water is recorded
* The heat produced can be calculation using the formula;
m X c X ΔT. (m=mass, c=specific heat capacity, ΔT= change of temp)
* The heat produced for one mole of the substance (Heat of combustion) can then be calculated
Q: A bomb calorimeter is used to determine the kilogram calorific value of a fuel e.g. a food. What is meant by kilogram calorific value?
- Kilogram calorific value of a fuel is the heat energy produced when 1 kg of a fuel is burned completely in excess oxygen
Q: Give two reasons why methane is an excellent fuel
1) Methane has a very high kilogram calorific value – a lot of energy is given out per kg burned in oxygen
2) Methane is a pure substance and burns with a clean flame
Q: Define bond energy
- Bond energy is the average energy required to break one mole of a particular covalent bond and to separate the neutral atoms completely from each other
Q: Define bond energy
- Bond energy is the average energy required to break one mole of a particular covalent bond and to separate the neutral atoms completely from each other
Q: Explain why the average bond energy for the C≡C triple bond is less than three times the average bond energy for the C—C single bond
- The C– C single bond is made up of a sigma bond
- The C≡ C triple bond is made up of a sigma bond and two pi bonds
- Pi bonds are weaker than sigma bonds and require less energy than sigma bonds to be broken
- Explains why the average bond density for the C≡ C triple bond is less than three times
the average bond energy for the C–C single bond
HEAT OF REACTION
The heat change that occurs when a reaction takes place according to a balanced chemical equation.
Examples:
H2 (g) + 1⁄2 O2 (g) —–> H2O (g) ΔH = - 242 kJ mol-1
C8H18 ———> C8H10 + 4H2 ΔH = +237.6 kJ mol-1
Q: What is Hess’s Law?
States when a chemical reaction takes place in stages the sum of the heat changes of the individual stages equals the heat change if the reaction were to take place in one stage.
2022 Question: Explain why the same amount of energy is released when one mole of hydrogen is used up in a fuel-cell as when one mole of hydrogen is burned in excess oxygen
Hess’s Law - when a chemical reaction takes place in stages the sum of the heat changes of the individual stages equals the heat change if the reaction were to take place in one stage.
CATALYTIC CONVERTERS IN CARS
- A catalytic converter is fitted to car exhausts to reduce pollution from the exhaust fumes.
- Exhaust fumes contain carbon monoxide, nitrogen monoxide, nitrogen dioxide and lead compounds if leaded petrol is used. It also contains unburned hydrocarbons.
- The catalytic converter changes these harmful gases into harmless gases.
CO + NO —> + 1/2 N2 - The catalytic converter contains catalysts such as platinum, palladium and rhodium inside a stainless steel case. The inside of the converter looks like a honeycomb. This honeycomb arrangement allows for a large surface area so increased removal of harmful gases into harmless gases such as carbon dioxide and nitrogen..
- The lifetime of a converter is dependent on the type of petrol used. If leaded petrol is used then the lifetime of the converter is shorter than if unleaded petrol is used. The lead poisons the catalysts.
- It is policy in the EU since 1993 that all new cars are fitted with catalytic converters.
