THE PRODUCTION OF ETHENE Many reactions proceed too slowly under normal conditions of temperature and pressure. Some reactions proceed at very fast rates but produce very small quantities of product. In order to maximise profits and to reduce costs to consumers, industries aim to minimise the costs of industrial processes. This involves a consideration of yields and rates. The reactions that cause greatest concerns to industries include:
Reactions with low equilibrium constants. Low equilibrium constants result in low yields
of product. For example: The production of ammonia, nitric acid and sulfuric acid. Reactions with slow reaction rates.
Exothermic processes.
Lower temperatures are required to increase yields, however, this results in slower reaction rates. Industries will generally employ lower temperatures and use catalysts to compromise on the decreased reaction rates. For example: The production of sulfuric acid, nitric acid and ammonia.
THE GREATEST COSTS ASSOCIATED WITH INDUSTRIAL PROCESSES INCLUDE:
The costs of raw materials. To maximise profits, yields are maximised. Generating high pressures. Industries avoid using extremes of pressure to maximise the yield of product as high
pressures require very powerful and expensive pumping equipment together with vessels that can withstand the high pressures. These added costs may not justify the use of higher pressures, and in many cases, it is more profitable to lower the pressure and obtain a lower yield of product.
Generating high temperatures. Industries decrease these costs by using heat evolved in exothermic processes to fuel
other reactions in the plant. The time required to produce the product. Rates are increased by using appropriate catalysts.
Note: Industries use processes that use less energy to decrease costs and preserve finite sources. For example: The heat produced in one stage of a chemical process is frequently recycled and used
to heat other stages of the process.
The heat exchangers which remove and recycle heat operate 24 hours per day so that the enormous costs associated with warming up equipment are avoided.
Industries often exist as integrated complexes i.e. A collection of related industries are located within a close proximity of one another. The by products of one industry (eg. heat) can then be used as a raw material for another industry, reducing wastage, environmental pollution and costs.
If sufficient thermal energy is produced, it may be possible to convert it to electrical energy for use in the plant. In some cases, excess supplies are sold to an electricity supply grid.
MAXIMISING YIELDS Industries will attempt to maximise yields by manipulating Le Chatelier’s Principle. Yields may be cost effectively increased by changing the following reaction conditions: Adding an excess amount of the cheaper reactant.
Periodically removing products.
Changing the temperature and pressure of the reaction system.
MAXIMISING RATES As time has a significant impact on the cost of products and staff, industries will also attempt to maximise the speed or time taken to produce a product. Conditions that favour fast reaction rates include: High reactant concentrations.
FACTORS INFLUENCING THE METHOD IN WHICH A CHEMICAL IS PRODUCED
The operating conditions and any compromises that are required are determined by running small scale experiments, and choosing the set of conditions that will maximise profits i.e. those conditions that result in the highest possible yield of product in the shortest possible time. Other considerations include: Raw materials – cost, availability, purity, safety.
Environmental impact – pollution, storage/hazards of waste products, use of water
bodies to cool equipment.
Transporting of raw materials and product.
Location of plant.
Availability of necessary technology.
Availability of appropriately qualified staff.
TYPES OF CHEMICAL PROCESSES
Batch Processing In this process, fixed amounts of reactants are mixed to produce fixed amounts products. This method is usually reserved for the production of small amounts of product and/or reactions that display high equilibrium constants. Continuous Flow Processing
In this process, reactants are continuously supplied at one end, to produce a continual supply of products, which are then removed at the other end of the processing line. This process is only cost effective if sufficient demand exists for the large amounts of products derived via the process. Continuous flow processing also allows for greater control over reaction conditions, making it the preferred technique for many large scale operations. Reactants may be added or products removed at any stage of a process to increase product yields.
PLANT LOCATION AND STRUCTURE Factors to Consider: Accessibility to raw materials.
Transportation costs.
Availability of energy/power sources.
The price of land.
Availability of water supplies.
Storage of raw materials and waste products.
Disposal of waste products.
Pollution and its effects on the environment.
Recycling energy, water and waste products.
GREEN CHEMISTRY Green chemistry involves the design of chemical processes and products that reduce or eliminate the use and generation of hazardous substances in the manufacture and application of the products. By eliminating and reducing waste from chemical processes, green chemistry aims to develop a sustainable approach to a cleaner environment that is beneficial to both our society and the economy. The hazards that green chemistry aims to avoid completely include: Toxicity. Physical hazards like explosions. Impact on global climate change. Depletion of resources. The major difference between green and environmental chemistry is that environmental chemistry focuses on pollution control once the pollutants have been produced whereas green chemistry aims to avoid pollution in the first place.
It is better to design chemical processes to prevent waste than to treat waste or clean it up after it is formed.
2. Design safer chemicals and products
Design chemical products to be fully effective, yet have little or no toxicity. 3. Design less hazardous chemical syntheses
Methods should be designed that use and generate substances with little or no toxicity to humans and the environment.
4. Use renewable raw materials
Use starting materials that are derived from renewable resources such as plant material rather than those such as from fossil fuels that will eventually run out.
5. Use catalysts, not stoichiometric reagents
Minimise waste by using catalysts in small amounts that can carry out a single reaction many times. They are preferable to stoichiometric reagents, which are used in excess and work only once.
6. Avoid chemical derivatives
Avoid using blocking or protecting groups or any temporary modifications if possible. Derivatives use additional reagents and generate waste.
7. Maximise atom economy
Design syntheses so that the final product contains the maximum proportion of the starting materials. There should be few, if any, wasted atoms.
8. Use safer solvents and reaction conditions Avoid using toxic solvents to dissolve reactants or extract products. 9. Increase energy efficiency
Energy requirements should be minimised. Run chemical reactions at room temperature and pressure whenever possible.
10. Design for degradation
Chemical products should be designed to break down to harmless substances after use so that they do not accumulate in the environment.
Include continuous monitoring and control during process to minimise or eliminate the formation of by-products.
12. Minimise the potential for accidents
Design chemicals and their forms (solid, liquid or gas) to minimise the potential for chemical accidents including explosions, fires and releases to the environment.
BENEFITS OF GREEN CHEMISTRY Some of the many benefits of a green chemistry approach include: Higher atom economy.
Advocating energy efficient processes.
Lowers cost of production and regulation.
Less wastes.
Safer products.
Healthier workplaces and communities.
Protects human health (end-users) and the environment.
Offers businesses a competitive advantage in the market place.
YIELD VERSUS ATOM ECONOMY The yield of a reaction tells us how efficient a reaction is in terms of the amount of product we obtain, relative to the maximum we could get from the amount of reactants we used. It is calculated using the formula:
% Yield = mass of product obtained (g) x 100 theoretical yield (g)
However, it does not take into account the waste products. Efficient chemical processes have high atom economy, and are important for sustainable development. Atom economy is determined by measuring the amount of starting materials that are incorporated into the desired products, and distinguishing them from those that are wasted (incorporated into undesirable products). Atom economy can be calculated by: % Atom economy = Relative Molar Mass of Desired Product X 100 Sum of Relative Molar Masses of all Products
A given chemical reaction might have high yield but low atom economy, hence not be seen as a adhering to green chemistry guidelines.
WORKED EXAMPLE 1 (a) Calculate the percentage atom economy of 2 2CH Cl , which is formed according to the
following chemical equation: 4( ) 2( ) 2 2( ) ( )2 2g g aq aqCH Cl CH Cl HCl
% Atom economy 85
100 53.8%85 36.6
(b) Would this method of 2 2CH Cl production be considered as a “Green” process?
Give a reason for your answer.
An atom economy of 53.8% is particularly poor, and this is a very wasteful process. This would not be considered a green process, as one the key principles of green chemistry is that it is better to develop reactions with fewer waste products than to have to clean up the waste (eg. achieve high atom economy).
(c) How could a chemical company maximise their profits from this chemical process?
Use waste products in other chemical reactions. The by-product is hydrogen chloride, which can be sold as a gas or made into hydrochloric acid. These useful substances can then be sold, reducing the potential wastage from the initial process. Alternatively, waste products that are non-toxic and biodegradable are favourable.
WASTE MANAGEMENT AND POLLUTION IN THE CHEMICAL INDUSTRY
A waste product is an unusable or unwanted substance produced during or as a result of a chemical process. Chemical waste is generated in many chemical processes and if not managed correctly, can impose adverse effects on human health and the environment. Responsible industries therefore practise sound waste management by implementing the following actions: 1. Prevention
2. Elimination
3. Reduction
4. Recycling
5. Treatment
6. Disposal
WASTE TREATMENT There are many different forms of waste treatment including: Landfill
Dumping at sea
Dispersion in controlled amounts in water or air
Vitrification (sealing in molten slag)
High-temperature incineration (1100oC )
Removal of pollutants from waste gases and liquids
Storage in sealed drums in secure locations
High-temperature steam and water treatments Which treatment process is used by industries depends upon: The physical form of the waste
PRODUCTION OF ETHENE Ethene or ethylene )( 42HC is the first member of the alkene series. These molecules are characterised by the presence of one or more double bonds between carbon atoms, and therefore, ethene is referred to as an unsaturated hydrocarbon.
PROPERTIES OF ETHENE
Ethene is a non polar molecule.
Ethene is a flammable gas.
Due to the weak dispersion forces acting between molecules,
ethene displays a low boiling temperature ( 0-104 C ).
USES OF ETHENE Ethene is the leading synthetic organic chemical in the world in terms of scale of production. Although ethene is widely used as a starting material in the production of many chemicals and products, most of the ethene produced in Australia (about 60%) is used to make polyethene, which is used to produce: Plastic equipment eg. bowls, buckets, bins, glad wrap.
Covers for metal containers eg. piping that is susceptible to corrosion.
Insulating materials. Other uses of ethene include: The production of ethanol, which is used as:
Solvents such as methylated spirits.
Solvents in cosmetics, drugs and perfumes.
A raw material in the manufacture of other chemicals.
Herbicides. The production of ethylene glycol which is used as an antifreeze solution.
The production of teflon which is used to make “non stick” cooking utensils.
The production of PVC (polyvinyl chloride) which is used for:
Insulation around electrical cables.
Packaging.
Products traditionally produced from glass, rubber.
THE PRODUCTION OF ETHENE Ethene is made from a variety of feedstocks, all of which mainly consist of saturated hydrocarbons. Example of such feedstocks include: Natural gas, which primarily consists of methane (about 90%) as well as small
quantities of ethane, propane and butane. Petroleum (crude oil) which is a mixture of solution of hundreds of saturated
hydrocarbons and results from the decay of marine plants and animals. Feedstocks are first refined and impurities are removed and then sent to a refinery for “cracking” – a process which converts large saturated hydrocarbons into smaller saturated and unsaturated products. Impurities such as water and hydrogen sulfide are removed from natural gas. Petroleum is separated into fractions using “fractional distillation”.
PETROLEUM: The crude oil component of petroleum is separated into various fractions containing hydrocarbons of similar molecular weights, via a process known as fractionation. This procedure is carried out in a fractionating tower. The fractionating tower is divided into a series of horizontal trays that contain hundreds of bubble caps; raised holes in the trays that contain loose fitting covers or caps. The bubble caps ensure that the rising vapour percolates through the liquid that has already condensed on each tray.
The sample is heated to approximately Co350 to form a mixture of gaseous vapour and liquid. The vapour moves up the fractionating tower, whereas the liquid settles at the bottom of the tower and is removed.
The temperature in the fractionating tower changes from approximately Co350 at the
bottom of the tower to Co20 at the top of the tower. This temperature gradient is created by the rising vapour that is cooling as the gas moves up the tower. This means that the trays are arranged in such a manner, that each tray is at a lower temperature than the tray below it.
As the vapour moves up the tower: Hydrocarbons whose boiling points are equal to or higher than that of the condensed fraction will condense (become a liquid). Hydrocarbons whose boiling points are lower than that of the condensed fraction will again vaporise, and move up to the next tray. Eventually, each tray will contain a mixture of hydrocarbons with similar boiling points. As the boiling temperatures of compounds depends upon the strength of the intermolecular forces, the smaller hydrocarbon molecules condense near the top of the tower, whereas the heavier molecules condense near the bottom. The desired fractions are then collected (usually naphtha 106 CC and gas oil 2014 CC ),
and sent to the refinery for cracking. Note: The exact operating conditions and the fractions that are obtained depend upon the
nature of the raw materials and market demands. The liquid remaining at the bottom of the tower is usually redistilled at low pressures so
CRACKING The distillation process is insufficient to meet consumer demands for the lighter hydrocarbon fractions. Greater quantities of the lighter fractions are therefore produced via a process referred to as cracking. Cracking is the process that breaks large molecules into smaller molecules, and may be achieved by two different methods; thermal cracking and catalytic cracking. The technique is used not only to produce greater quantities of lighter fractions from crude oil, it is used to produce unsaturated hydrocarbons for various chemical industries. Eg. ethene.
THERMAL CRACKING Thermal Cracking involves the production of small molecules by subjecting large molecules
to high temperatures ( Co900750 ). The products of thermal cracking are mainly smaller molecular weight alkanes, along with some unsaturated hydrocarbons such as ethene and propene. Ethene is primarily produced by a type of thermal cracking called steam cracking. The raw materials or feed stocks for steam cracking include: Ethane and/or propane from natural gas.
Naphtha or gas oil fractions obtained from the fractional distillation of crude oil.
Steam and feed stocks enter a furnace at temperatures between Co750 and Co900 , and travel through coiled metal tubes where cracking occurs. Typical reactions that occur in the metal tubes include:
2 6 2 4 2 138( g ) ( g ) ( g )C H C H H H kJ / mol
Ethane ethene
3 8 2 4 4 81( g ) ( g ) ( g )C H C H CH H kJ / mol
Propane ethene
10 22 8 18 2 2C H C H CH CH /H kJ mol
Decane octane ethene
13 28 11 24 2 2( g ) ( g ) ( g )C H C H CH CH 93.5 /H kJ mol
Gas oil octane ethene General Equation: Alkane Alkene + Smaller Alkane
Reaction Conditions: As the cracking process is endothermic, Le Chatelier’s Principle suggests that high equilibrium yields of ethene will be favoured by higher temperatures and lower pressures (less than 1 atm). In practice, however, the gas is only allowed to remain in the furnace for less than one second so as to prevent ethene from cracking into smaller molecules (over-cracking). Not only does over-cracking result in decreased ethene yields, it results in the production of carbon (coke), which reacts with water to produce carbon monoxide and hydrogen gas:
2 2( s ) ( g ) ( g ) ( g )C H O CO H
Note: Typical yields of ethene from ethane feedstock are in the order of 50%. As the size of the hydrocarbons in the feedstock increases, the products from the
cracking process become more diverse and the yield of ethene decreases.
The gas leaving the furnace is quickly cooled to below Co100 in a quench tower to avoid secondary reactions (further cracking).
STEP 3: DISTILLATION AND WASTE TREATMENT
The components of the cracking process are further cooled to below Co100 by gradually compressing and expanding it and the components are then separated by distillation.
2H and 2CO are removed, and any un-reacted ethane is recycled back into the feedstock.
STEP 4:
Residue fractions containing hydrocarbons are returned to the refinery.
DISADVANTAGES OF THERMAL CRACKING It is difficult to control the quality of products produced. Over-cracking may occur,
resulting in the formation of lighter fractions than that required. The process is expensive.
CATALYTIC CRACKING Catalytic Cracking involves the production of smaller molecules by subjecting large
molecules to lower temperatures ( CO500 ) and an appropriate catalyst such as zeolite. As lower temperatures are employed, it becomes easier to control the products produced in the cracking process. For example: 221882210 CHCHHCHC decane octane ethylene (ethene) Note: This equation only represents ONE of the possible outcomes of the cracking of decane. Decane is able to crack at a number of different points, resulting in the formation of many different molecules. The products of cracking are then separated by fractional distillation.
DISADVANTAGES OF CATALYTIC CRACKING The technique is expensive.
Desulfurisation involves the removal of sulfur containing compounds and is carried out before fractions are sent to a refinery for cracking. This process is carried out for the following reasons: To reduce 2SO emissions when fuels are burned. Sulfur acts as a catalytic poison, impeding or destroying the catalytic activity of
many catalysts that are used in the refinery. The sulfur that is removed during the desulfurisation process is used as a raw material for the production of chemicals such as sulfuric acid.
HEALTH AND SAFETY
Working with gases at pressures below 1 atmospheric is hazardous as a leak would cause air to be drawn in, which may form an explosive mixture.
Ethene is an asphyxiant and at moderate to high concentrations can cause nausea and
headaches. Butadiene gas emissions are carefully monitored because high exposures may cause damage to the nervous system, and other illnesses.
Ethene is a highly volatile and flammable gas posing a great threat to the environment
through explosions. Having industries that use the products of the refining process in close proximity means that the explosive materials do not have to be transported across large distances, reducing the threat of explosions and consequential damage to the surrounding environment. In addition, extensive fire prevention and fire fighting strategies are employed by the industry.
The biggest risk to workers in the petrochemical industry is usually considered to be
explosions and fire. Like other hydrocarbons, ethene can readily form explosive mixtures with air. As a consequence,
Since both high temperature and low temperature stages are involved in ethene
production, special attention is also given to the prevention of burning and freezing injuries.
QUESTION 1 If you had to transport ethene in a cylinder, which safety sticker would you place on the cylinder? A Non-flammable liquid B Non-flammable gas C Flammable liquid D Flammable gas QUESTION 2 The major source of unsaturated hydrocarbons for industrial use is: A Distillation of natural petroleum B Catalytic cracking of natural petroleum C Hydrolysis of starch D Combustion of natural gas QUESTION 3 In the petroleum refining industry, crude oil is subjected to fractional distillation. In this process: A Various mixtures of chemicals, each boiling within a selected temperature range are separated B High molar mass molecules are vaporised at low temperatures and high pressures C Each of the individual chemicals in crude oil are separated D The fractionating column is kept at constant temperature throughout so that equilibrium can be attained QUESTION 4 Fractional distillation is a process by which crude oil is separated into different fractions. Each fraction contains molecules which: A Are from one homologous series B Are vastly different in molar mass C Differ greatly in boiling temperature D Are relatively insoluble in each other
The following information relates to Questions 5, 6 and 7. Below is a diagram of a gas processing tower used for the fractional distillation of natural gas. The natural gas can be sourced from various locations and consists mainly of methane, ethane, propane and butane.
QUESTION 5 The identities of W, X, Y and Z respectively are: A Methane, ethane, propane, butane B Ethane, methane, propane, butane C Ethane, propane, butane, methane D Butane, propane, ethane, methane QUESTION 6 Which of W, X, Y and Z is removed at the lowest temperature? A W B X C Y D Z QUESTION 7 Process A immediately follows the fractional distillation. This process is most likely to be: A Reduction B Hydration C Chlorination D Cracking
QUESTION 8 What property determines how hydrocarbons are separated in the fractionating tower? Solution QUESTION 9 Sulfur compounds are often present in crude oil. If a sample of crude oil containing 1% sulphur was burnt in air, which of the following compounds would be produced in the smallest amount in the flame? A 3SO
B 2SO
C OH2
D 2CO QUESTION 10 The cracking of petroleum fractions involves: A Breaking of long chain hydrocarbons into shorter ones. B Joining of short chain hydrocarbons to form longer ones. C Polymerisation of alkenes. D Conversion of linear hydrocarbons to cyclic structures.
QUESTION 11 Unsaturated hydrocarbons such as ethylene (ethene) are produced in industry by cracking of crude oil fractions. The process of cracking involves: A The separation of the components of crude oil according to their boiling temperatures. B Reducing the pressure in order to separate hydrocarbons of high molecular mass. C Heating crude oil in the presence of pure oxygen to ensure complete reaction of the oil’s components. D Breaking long chained hydrocarbons into shorter ones by heating crude oil in the absence of air. QUESTION 12 Which one of the following is most likely to have been produced by a ‘cracking’ process? A Paraffin wax B Lubricating oil C Diesel fuel D Petrol QUESTION 13 Which one of the following reactions is an example of a ‘cracking’ reaction of a hydrocarbon? A 422323 CHCHCHCHCHCH
B 33222 CHCHHCHCH
C OHCOOCH 2224 22
D OHCHCHOHCHCH 23222
QUESTION 14 When the gas propane is cracked, several different products can be obtained. Write the formulas for four likely products of this process. Solution
QUESTION 15 The products of thermal cracking include an alkane and an alkene. Explain why alkenes are produced during the process. Solution QUESTION 16 Why is the cracking of ethene produced using catalysts rather than high temperatures? Solution QUESTION 17 Discuss the conditions that are required to maximise the yield of ethene, during thermal cracking.
C2H6(g) C2H4(g) + H2(g) H = +138 kJ/mol ethane ethene
QUESTION 18 Use Le Chatelier’s principle to describe the theoretical conditions that should be used to maximise the yield of ethene from cracking reactions. Are these the conditions actually used? If not, why not? Solution QUESTION 19 Catalytic cracking of alkanes is carried out by passing the hydrocarbon vapour over a heated catalyst in the absence of air. Which of the following is not a possible product of the catalytic cracking of hexane? A Propene B Methane C Hydrogen D Carbon dioxide QUESTION 20 What is the purpose of using a catalyst in catalytic cracking processes? Solution
Why is the gas that leaves the furnace cooled to below Co100 ? Solution QUESTION 22 (a) What is the physical property used in fractional distillation? (b) What is the difference between thermal and catalytic cracking? (c) State 3 properties of ethene.
QUESTION 23 Ethene is obtained in large quantities by the catalytic cracking of naphtha. (a) (i) Write an equation for the production of ethene from the cracking of butane. Use this equation to explain why the products of cracking reactions will also consist of saturated and unsaturated hydrocarbon. (ii) How does the use of a catalyst reduce the costs of producing ethene from naphtha?
QUESTION 24 Explain the differences between fractional distillation and thermal cracking, and the products that are produced in each process. Solution QUESTION 25 What are the potential waste products of this industrial process? What risks are posed to human beings or the environment by these waste products? Solution
QUESTION 26 When crude oil is fractionally distilled, six fractions are obtained, as shown in the following table.
Fraction Size Range of Molecules Boiling Point Range ( Co )
Gas C1 - C5 -160 to 30
Gasoline C5 - C12 30 to 200
Kerosene, Fuel oil C12 - C18 180 to 400
Lubricants C16 and up 350 and up
Paraffins C20 and up >350
Asphalt C36 and up >350
(a) What is the general trend in boiling points as the number of carbon atoms increases? Explain the observed trend in terms of bonding. (b) What process can be used to convert some of the heavier fractions to lighter fractions? (c) As the world reserve of liquid hydrocarbons are depleted, how could gaseous hydrocarbons be used as a replacement for gasoline?
QUESTION 28 The crude oil component of petroleum is separated into various fractions via a process known as fractionation. This procedure is carried out in a fractionating tower, as illustrated below.
(a) Crude oil consists of a large number of different compounds. Explain how fractional distillation is used to produce useful compounds from crude oil.
3 marks The distillation process is insufficient to meet consumer demands for the lighter hydrocarbon fractions. Greater quantities of the lighter fractions are therefore produced via a process referred to as cracking. (b) Give an equation for the production of ethene from hexane from a cracking process.
The production of ethene from larger hydrocarbons are endothermic processes. (c) (i) State the conditions of temperature and pressure which would result in the greatest yield of ethene in cracking processes.
QUESTION 30 (a) Calculate the atom economy of ethylene oxide, created in the following reaction:
(b) Would this method of production of ethylene oxide be considered as a “Green” process? Give a reason for your answer.
(c) Recently, a method of synthesising ethylene oxide from ethene and oxygen using a silver catalyst was developed. What’s the atom economy of this alternative reaction?
