An Analysis Of Fractional Distillation

Describe, with the help of a diagram, how the different hydrocarbon fractions in crude oil are separated by fractional distillation. Identify the main fractions by name and according to the approximate number of carbon atoms in the molecules.

In the world today the primary source of fuel for most of our needs come from fossil fuels. These are in the form of coal, natural gas and crude oil. In the petrochemical industry the crude oil is refined through a process of fractional distillation. This breaks apart the larger hydrocarbons into their smaller fractions at varying temperatures in the distillation tower. The heavier fractions containing the most carbon atoms are drawn off at the highest temperatures’ and the lighter the hydrocarbons the lower the temperature needs to be to draw off the fraction.

The process of fractional distillation provides the following groups of products:

1. Liquid petroleum gas

2. Naphtha

3. Gasoline

4. Kerosene

5. Diesel oil

6. Lubricating oils

7. Fuel oils

8. Residues

8. Residues

Furnace

Liquid petroleum gas id the lightest fraction and is drawn off as a gas from the top of the distillation tower. It contains between 1 and 5 carbon atoms. It is the lightest fraction and is not very dense, because of this it can be drawn off at a temperature of 20°C. It is used as an alternative to petrol in cars following a conversion.

FurnaceNaphtha is a fraction that contains between 5 and 9 carbon atoms, with this increase in carbon atoms the density and boiling points increase. This means that the fraction will come off at a higher temperature of 70°C. This fraction can be further broken down in the process of thermal cracking to produce ethylene and benzene which are important in the plastics and pharmaceutical industry.

Gasoline contains between 5 and 10 carbon atoms and is drawn of at a temperature of 120°C. This is the lightest fraction of the liquid grade fuels that are commonly used. It is what we know as petrol and widely used in cars and small 2 stroke engines.

Kerosene contains between 10 and 16 carbon atoms and is drawn off at a temperature of 170°C. This is the principal fuel in the aviation industry where it is graded further depending on the final use. It is widely used for powering gas turbines and other internal combustion engines in propeller driven aircraft. The use of kerosene to fuel aircraft is because it must have a high specific enthalpy of combustion per gram as this will release a lot of energy when it burns this means it will also have a high enthalpy density. This is important as the fuel must be stored. A less dense fuel will use more space and provide less energy for the space it takes up on the aircraft.

Diesel oils are drawn off at a temperature of 270°C and contain between 14 and 20 carbon atoms. This is very similar to kerosene and has a similar consistency with the increase in viscosity and increase in boiling point it become necessary to keep the fuel warm in cold environments as the fuel becomes thick and refuses to flow through fuel systems, this problem if needed can be solved by lighting a fire under the fuel tank to bring the fuel up to temperature and enable it to flow. Also as this fuel is not dissimilar to kerosene some gas turbines like the Rolls Royce Pegasus 11-61 turbofan will operate with no appreciable loss of power or function.

Lubricating oils come off at a temperature of between 300 and 375°C and contain 20 to 50 carbon atoms. This high content of carbon atoms gives the lubricating oils their high viscosity and high melting point making them ideal for this use.

Fuel oil is the heaviest of the fractions to come of other than the residue and is drawn off at a temperature of 600°C and has between 20 and 70 carbon atoms, this is a very heavy thick fuel which often requires pre heating before use in the fuel system. The main uses for this fuel are for large diesel engines in ships trains and factories. It is also used as a reserve supply for power stations at peak demand when gas supplies are limited. This fraction is also used to quench the residue in the process of thermal cracking to produce heavy fuel oil. This is also known as bunker oil, the lowest grade of fuel oil for use in the shipping industry as a cheaper alternative to fuel oil.

The residue from the distillation process is drawn off from the bottom of the distillation tower and contains a high number of carbon atoms >70 this carbon rich mixture is washed in fuel oil in the steam cracking process to make heavy fuel oil. This process leaves the residue coke. The raw residues are used in the manufacture of asphalts for road surfacing and bitumen’s for sealing roofing materials that also may be made from this residue sold as roofing felts.

b. Why might fractional distillation be carried out under reduced pressure?

Fractional distillation is carried out under reduced pressure because when the pressure of the substance is reduced the intermolecular forces between the molecules become weaker. These molecules are then able to escape as vapour more rapidly and means that the process of fractional distillation can take place at a lower temperature which both reduces the energy needed and ultimately cost.

c. Some heavier fractions are processed using cracking. Explain what is meant by cracking and why it is carried out.

The term cracking is used to refer to the breaking of larger hydrocarbons into smaller constituent parts to produce the smaller and more useful alkanes and an alkene and to further process the residues from fractional distillation. This can be done in several ways, and can be split into thermal cracking and catalytic cracking. Both of these processes are used in the petrochemical industry to process some of the heavier fractions of crude oil further into gasoline and other useful products the lighter fractions of naphtha and butane are also processed into compounds of ethylene and benzene for use in the pharmaceutical and plastics industry The demands of consumers for large amounts of gasoline for cars means that over 50% of the crude oil has to be turned into gasoline to meet demand and as this fraction only forms 30-40% of crude oil the demand has to be met by way of other process’s .

d. Distinguish between thermal cracking and catalytic cracking. Give examples of products formed by each method.

Thermal cracking of hydrocarbons is done in 3 ways. steam, vis breaking and coking. In these processes the hydrocarbons are heated to a high temperature until they break into there component parts.

Steam cracking is where the hydrocarbons are briefly heated with high temperature steam to 816 °C Ethane and Naphtha produce light alkenes such as ethylene and propylene the heavier naphtha’s are cracked into gasoline. Benzene comes off at the higher temperature of around 1000°C and is an important molecule in the pharmaceuticals industry. Ethylene is used in the manufacture of plastics

Vis breaking is carried out to process the residues from the distillation process. This is done by heating the residue to 482°C and then quenching it with fuel oil. This then poured down a distillation tower and then flashed without oxygen. This flashing of the residue produces the product heavy fuel oil and tar. Coke is the final residue from the process of steam cracking and is deposited on the sides of the furnace which is then periodically cleaned off in the process of decoking the furnace. This coke is then sold on for use in industry as a fuel.

Fluid catalytic cracking is the most important process of converting the fractions of crude oil into the more valuable gasoline, olefinic and other products. This process has largely replaced the thermal cracking process as it yields a higher return of gasoline with a higher octane rating. This process of fluid catalytic cracking runs as a continuous process in the refinery without interruption 24 hours a day for a number of years without before routine maintenance. The modern fluid catalytic cracking is a complex process in which the recycled fractions are injected into a riser with a high boiling point stock which is at a temperature between 315°C and 430°C and a pressure of 1.72barg. This vaporises the larger hydrocarbons and on contact with the catalyst it cracks them into smaller hydrocarbons. The catalyst that is used in the process is then cleaned of the hydrocarbon deposits and then recycled back through a regenerator. This is done by blowing air into the regenerator and burning off the coke deposits. This regenerator operates at a temperature of 715°C. The burning off of the coke from the catalyst is exothermic a heats it up. This recycled catalyst is then returned to the process and provides the heat to vaporise the stock and residue mixture and provide the energy for the endothermic cracking reaction. This process is dependant on both the physical and chemical properties of the catalyst. There are four main components to the modern catalyst in the FCC. These are crystalline zeolite, matrix, binder and filler. The zeolite is the main provider of catalytic activity. The matrix contains amorpous alumina which also provides some catalyst activity within the sites of large pores. This enables the cracking of larger hydrocarbons than the zeolite alone. The binders and fillers of the catalyst provide the physical strength and maintain its integrity. Contaminants in the stock from metals all have a detrimental effect on the catalyst which can be mitigated in some way by avoiding stock with contaminant, fresh catalyst, demetalisation though this is expensive, and by adding other metals to form compounds that are less troublesome to the catalyst.

e. Write a possible equation for the cracking of dodecane, C12H26.

Heat + CH3(CH2)10CH3 CH3(CH2)4CH3 +H2C=CH(CH2)3CH3

Dodecane _ hexane 1-hexane.

The thermal cracking of the dodecane is exothermic and the heat that is used in the process is largely taken up by the catalyst in an endothermic reaction. This heat energy is then partly recovered in the re use of the catalyst up until the point it requires cleaning or replacing.

2

a.(i) Define what is meant by a ‘catalyst’.

A catalyst is a substance that alters the rate of a chemical reaction without being changed chemically or consumed in the reaction. They can however change physically. Catalysis is the process of increasing or decreasing the rate of a reaction. These can be both positive and negative. The catalysts that speed up reaction are called positive catalysts and ones that slow down reactions are called inhibitors. There are also substances that increase the activity of the catalysts which are called promotors; the substances that deactivate the catalysts are called catalytic poisons. The use of catalysts is a means of making the process viable if it would not happen without it and also as a means of making the process cost efficient in terms of energy use and cost. The ability of the catalyst to be reused in most cases reduces the cost further.

a.(ii) How do catalysts work?

During chemical reactions the addition of a catalyst will increase the rate of the reaction at a lower energy level than the reaction without an addition of a catalyst. The combination of product and catalyst reactants raises it above the activation barrier and raises the potential energy of the reactant molecules as they come together. The energy of the activated complex reaches its maximum and form an activated complex, this activation energy then falls and the product forms and the molecules of the catalyst and product separate. This means the catalyst can be recovered from the product and used again following some reprocessing in some instances. The use of catalysts can be for both cost saving and also as some reactions may take a long time or not even be possible without the addition of a catalyst.

b. Why is it important that catalytic converters start working at as low a temperature as possible?

It is important for the catalyst converter to work at as lower temperature as possible as reaction rates are almost always increased when the temperature rises. This is because as the temperature increases so do collision rates of the molecules between the catalyst and exhaust gases. To ensure that the rich mix of gases emitted by a cool engine is converted fully at low temperature’s it is necessary for the catalyst to be active at a low temperature. This also means as the temperature increases the reaction rate will increase too.

c. Why do you think the catalytic converter is sited close to the engine?

The catalytic converter is sited close to the engine to ensure that the exhaust gases are as hot as possible before entering the converter. By keeping the gases hot it will increase the catalysis rate due to the molecules being more active as the temperature is increased.

d. Why are harmful emissions on short car journeys or in cold weather particularly high?

Short journeys will not raise the temperature of the engine block or manifold assembly, this leads to the hot combustion gases being cooled in the short journey to the converter. Operating in low temperatures will also cool the manifold assembly on a continuous basis as cold air passes the manifold leading to the cooling of the exhaust gases If the temperature is consistently cold the injectors or carburettors can be retuned to burn a leaner mix which raises the burn temperature in the cylinder and reduce the emission of un burnt fuel and rich exhaust gases into the catalytic converter. The burning of a lean mix at normal temperature will raise the cylinder temperature and cause pitting on the piston head and possible damage to the cylinder which in time will cause oil to pass into the cylinder producing soot and partially burnt oil into the exhaust system.

e. Explain what is meant by ‘poisoning’ a catalyst?

The catalyst converter can be poisoned by containments in fuel and by burning the wrong type. This leads to the coating of the catalyst with contaminants like lead from leaded fuels and manganese which is used as additive to gasoline. Gasket failure in the cylinder head will lead to oil and coolant entering the cylinder and being ejected out in the exhaust gases. Depositing partially burnt oil and silicon from the coolant in the converter. This reduces the contact between the catalyst and exhaust gases. Some of this reversible over time but the catalytic converter will be less effective in the mean time and may never fully recover its full working capacity.

f. What is done to the catalyst metals to increase their efficiency?

The catalytic converter is made of several components witch all have important roles to play in the functioning of the catalytic converter. The core or substrate in modern catalytic converters is made up of a ceramic honeycomb or a stainless steel foil. This is so that it doesn’t react with the catalyst and also providing an extremely large surface area to support the washcoat. This washcoat is to make the converter more efficient and is often a mixture of silica and alumina. This provides a rough surface area on the substrate which greatly increases the surface area compared to the honeycomb structure alone. The catalyst and washcoat are mixed and then added to the substrate. This catalyst is made up of precious metals such as platinum which is the most widely used metal as it is the most active catalyst but not necessarily suitable in all situations. There are several different metals used as catalysts as some are more suitable than others in certain circumstances. To reduce the cost and reduce unwanted reactions palladium and rhodium are also used. Platinum and rhodium are used as a reduction catalyst; platinum and palladium are used as an oxidising catalyst. There are several other metals used in catalytic converters though their uses have limitations and legislation also prevents use in certain countries due to the toxicity of the substances they produce.

g. What is meant by ‘homogeneous catalysis? Is the catalysis taking place in a catalytic converter a good example of homogeneous catalysis? Discuss.

Homogeneous catalysis is when the catalyst is in the same phase as the reactants and product. The homogeneous catalysts are more selective for a single product, more active and easily modified for optimising selectivity. Though these catalysts are more prone to permanent deactivation and are difficult to separate from the product so the catalytic converter is not a good example of a homogeneous catalyst. The catalytic converter in the exhaust system of a car is a heterogeneous catalyst as the catalyst product and reactant are not in the same phase. As the metals are coated onto a ceramic honeycomb surface the gases from the engine must diffuse to the catalyst surface and absorb onto it. This is why the catalyst needs to be coated thinly onto the honeycomb structure to increase the chance of the molecules coming into contact with the catalyst. The 3 main reactions that take place in the converter are as follows.

2CO(g) + O2(g) → 2CO2(g)

2NO(g) + 2CO(g) → N2(g) + 2CO2(g)

2C6H6(g) + 15O2 → 12CO2(g) +6H2O(l)

h. Suggest a reason why the catalytic converter has to be replaced eventually.

The catalytic converter will eventually have to be replaced as the physical structure and catalyst will become worn out and polluted by contaminants. This will reduce the capability of gases to be converted as there will be less contact both from soiling and reduced surface area due to physical and chemical degradation of both the substrate and the catalyst. As this catalyst is coated onto the substrate in the manufacturing process it is not possible to reuse the catalyst and a new one will be required.

i. Catalytic converters convert the pollutant gases carbon monoxide, C7H16 and nitrogen monoxide into harmless gases. However, this is still only a partial solution to the emission problem. Explain why.

Even though the catalytic converter converts the more harmful gases into carbon dioxide, nitrogen gas and waters it is only a partial solution to the problem of huge amounts of waste gases being emitted into the atmosphere? This is because the carbon dioxide and nitrogen gases are both pollutants that both contribute to the problem of global warming and in the case of CO2 a contributor to the acidification of the oceans. This latter problem could have far reaching consequences in relation to marine life for which 2 billion people rely upon for protein and the associated industries that harvest the sea for commercial gain.

j. An. oxygen sensor monitors the oxygen flowing through the exhaust system and feeds back to control the fuel-air mixture entering the engine. Why do you think too little oxygen flowing over the catalyst would be a bad thing? Why do you think too much oxygen flowing over the catalyst might be a bad thing?

The sensor in modern cars monitors the oxygen that flows through the catalytic converter. This provides the information for the engine management system to feed oxygen into the exhaust gases when insufficient oxygen is present from either cold starting or fuel rich gases being emitted this ensures the catalyst can completely react and convert the gases. If there is insufficient oxygen the gases will not be completely converted leading to the emissions of the more harmful gases. The presence of too much oxygen will cause an increase in temperature and also oxidisation of the catalyst which will cause degradation from heat or corrosion.

k. It has been suggested that battery-powered cars, which do not emit pollutants, are a solution to the problems of environmental pollution. What effect would the increased use of battery-powered cars have on the demand for electricity? What would be the consequences for the environment of this demand?

The global car count to date is approximately 600 million and is expected to double in the next 30 years to a huge 1.2 billion cars. This is clearly going to cause problems both for the demand of fuel to build and power them and also in the pollutants they emit. The use of gasoline and diesel oils to fuel this demand is eventually going to rapidly deplete world reserves at an ever increasing rate. This will happen even if car numbers stayed at today’s level as extraction is almost at its peak level. The current theories on how much is left is a secret closely guarded by opec and the oil rich nations like Saudi Arabia who for the last 30 years have never changed the forecast for the reserves they hold. This is clearly untrue, and in the future will cause a sudden collapse of the availability of oil. It has been suggested that electric powered cars could be a solution to the problems of both pollution and consumption of one of the most valuable resources mankind has. As a solution is this possible? There are a number of factors to take into account.

Cost of replacing 600-1 billion cars in both raw materials and energy to make them and scrap the old one.

Increase in the demand for electricity and can this demand be met without a net increase in global emissions from power stations without burning huge amounts of carbon fuels.

Will the electric car be able to meet the demands of the travel hungry consumers?

Is there the global political will to address the issue?

Resistance of the world population to change there ways, this is important as all the thinking in the world is of no use if nobody will do anything.

On the cost of replacing 600-1.2 billion cars, it is going to cost a huge amount of money, approximately 12 trillion pounds for the new ones and 3 trillion for scrapping the old ones. This is a total of £15 trillion. An estimate on the total amount of dollars in the world is 908 billion in use as of 2009 (source: Federal Reserve). In addition to this there is $1,655.6 billion in current accounts, $8,326.8 billion in savings and approximately $10 trillion in bank deposit certificate and stocks. As this latter one is money that is not freely available it could be said there is approximately $8.3 trillion of accessible cash. This is a huge amount of money, far short of the money needed to replace the global car collection. Although this is not all the money in the world it is a considerable amount of it and can be taken as a fair marker of how difficult it is going to be to fund the change.

Although the electric powered car is efficient at approximately 40% compared to gasoline at15%. Could the demand be met from the electric industry without increasing emissions? The answer to this is yes at a cost. global electricity production annually is 13.7 trillion watts. The requirements on demand if we were all to go electric would be approximately 60.44 billion watts though the annual cost of charging the cars up would be cheap compared to the use of gasoline.

There needs to be a global political will to change and this may not come until problems from global warming have started to have an economic effect

Last of all will the people of the world do the right thing for the good of the planet and make the change, it would be nice to think they would but this will be the most difficult challenge of them all as most people are not keen on a change.