6.2 [Hydrocarbons] Flashcards
What are the 3 main processes used to convert crude oil into fuels?
Fractional distillation
Cracking
Reforming
These processes are used in oil refineries that are located all over the world.
What is fractional distillation?
Crude oil is a complex mixture of compounds including hydrocarbons. Composition of mixtures vary depending on were the crude oil came from. It involves converting crude oil into a small number of fraction, the number of fractions vary between different refineries takes place in a distillation column.
Fractional distillation is the process used to separate a liquid mixture into fractions by boiling and condensing
What is the process for fractional distillation?
crude oil is heated in a furnace which turns most of the liquid into vapour, it is then passed into the column near the bottom. There is a temperature gradient in the column hotter at the bottom cooler at the top.
As vapour passes up the column different fractions condense at different highest in the column depending on the boiling temperature range of the molecules in the fraction.
Near the bottom the fractions contain larger molecules with longer chains and higher boiling temperature.
Near the top of the column the fractions contain smaller molecules with shorter chains and lower boiling temperatures.
Some hydrocarbons in crude oil are dissolved gases and they rise to top of column without condensing.
Why can’t some fractions undergo fractional distillation?
Some fractions contain many different compounds so may go further fractional distillation separately.
What is cracking?
Demand for shorter chain hydrocarbons is much higher as they are better fuels can be used to make other substances like polymers.
Cracking is the breakdown of molecules into shorter ones by heating with a catalyst.
What is the process involved for cracking?
Cracking is done by passing the hydrocarbons in the heavier fractions through a heated catalyst, usually of zeolite , this causes larger molecules to break up into smaller ones. One large molecule may form 2 small ones.
What is reforming?
During very rapid combustion that occurs in vehicle engines, not all hydrocarbons of the right size burn in the same way. Those with straight chains burn less efficiently than those with branched chain molecules and those with rings. Reforming is the process of the conversion of straight chain hydrocarbons into branched chain cyclic hydrocarbons.
What are Alkanes burned for?
Alkanes can burn, Alkanes can be burned in vast quantities to provide worlds energy.
What are the products of combustion?
One of the products of combustion is water, which adds to the total quantity of H20.
Other products is CO2. This is a greenhouse gas and its increasing production is considered to be responsible for global warming climate change and other problems.
Other problems are caused by using alkanes as fuels, water and carbon dioxide formed not considered to be pollutants but other compounds formed during combustion of alkanes are pollutants.
What is incomplete combustion?
The combustion of alkanes can be incomplete if there is insufficient oxygen or because the combustion is very rapid. Some of the atoms in the fuel are not fully oxidised. All of the hydrogen atoms in the alkane molecules are converted into water but some of the carbon atoms can form carbon monoxide or solid carbon (soot) smoke. Some of the products of incomplete combustion can be fatal.
What is carbon monoxide?
Carbon monoxide is a toxic gas that causes death to many people. It prevents the transport of oxygen around the body, it is colourless and odourless so breathe it in without knowing.
What occurs with unburned hydrocarbons?
An example of incomplete combustion is when the hydrocarbon does not burn at all, a small proportion of the hydrocarbons in a fuel are released into the atmosphere unchanged, they are known as unburned hydrocarbons.
How does combustion link to oxides of sulfur?
Some of the molecules in crude oil contain atoms of sulfur, and these many not be removed by the fractional distillation, cracking or reforming processes. During the combustion of alkanes these atoms of sulfur form sulfur dioxide and then react in atmosphere to form sulfur trioxide
Both of these acids contribute to formation of acid rain, responsible for environmental damages to aquatic life in lakes and rivers and damage to crops and forest
How does combustion link to oxides of nitrogen
Very few molecules used as alkane fuels contain atoms of nitrogen, combustion of these atoms occurs at a very high temperature, Under these conditions nitrogen molecules in the air react with oxygen molecules. these reactions leads to the formation of oxides of nitrogen represented by the formula NOx Nitrogen monoxide and Nitrogen dioxide contribute to environmental damage in the same way as sulfuric acid.
How do catalytic converts fight pollution?
Cars and other road vehicles are responsible for a lot of air pollution, widespread use of catalytic converters fitted to exhaust system made pollution less of a problem.
Different types but all use small quantities f precious metals like platinum these metals speady thinly over a honeycomb mesh to increase surface area for reaction. One common type is a three way catalyst it can remove 3 different types of pollutants CO, unburned hydrocarbons and oxides of nitrogen
As the exhaust gases from the engine passes through the catalytic converters different reactions occur. Examples are the oxidation of the carbon monoxide and the oxidation of unburned hydrocarbons.
Catalyst not very good at removing sulfur compounds best way to remove sulfur based pollution is to remove the sulfur compounds from the fuel before the fuel is burned.
Why are there needs for alternative fuels?
Other than pollution caused by the combustion of alkanes the other factors that need to be considered are the depletion of natural resources and global warming and climate change.
The alternatives to fossil fuels are biofuels (obtained from living matter) , furthermore renewable resources
What is carbon neutrality?
Fuels are considered in terms of their carbon neutrality. A fuel should be completely carbon neutral.
What does carbon neutral mean?
Carbon neutral is a term used to represent the idea of carbon dioxide neutrality.
Fossil fuels such as those from tress described as carbon neutral because they too absorb carbon dioxide during their growth to form the same amount of carbon dioxide when they are burned.
What are Biofuels?
Biofuels are fuels obtained from living matter that has died recently. Biofuels are not carbon neutral because the plants have been harvested and processed in factories. Both of these stages involve the use of energy from formation of carbon dioxide so overall use of biofuel involves forming more carbon dioxide than is absorbed. Much closer to being carbon neutral than other fossil fuels.
What is Halogenation?
Reactions between alkenes and bromine examples of halogenation products of these reactions are dihalogenalkanes. Reactions with chlorine are chlorination.
What are hydrations?
Hydrations is adding water adding H and OH to 2 C=C double bonds this reaction is usually done by heating the alkene with steam passing mixture over a catalyst of phosphoric acid.
What is addition of hydrogen halides?
Another addition reaction is addition of a hydrogen halide often hydrogen bromide or chlo9ride to form halogenoalkane.
What happens in oxidation to diols?
Both addition and oxidation A diol is a compound containing to OH groups. Oxidising agent is potassium manganate (VII) in acidic conditions. The potassium manganate provides an oxygen at, and the water in the solution provides another oxygen atom and 2 hydrogen
atoms addition of 2 OH groups across double bond.
During this reaction Potassium manganate (VII) solution changes from purple to colourless this col9our used to distinguish between alkanes and alkenes
