INDUSTRIAL CHEMISTRY THE PRODUCTION OF NITRIC ACID

Transcription

1 INDUSTRIAL CHEMISTRY THE PRODUCTION OF NITRIC ACID 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. The School For Excellence 2012 Unit 4 Chemistry The Production of Nitric Acid Page 1

2 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. High pressures. High temperatures. High surface areas. Use of catalysts. The School For Excellence 2012 Unit 4 Chemistry The Production of Nitric Acid Page 2

3 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. The School For Excellence 2012 Unit 4 Chemistry The Production of Nitric Acid Page 3

4 Factors to Consider: PLANT LOCATION AND STRUCTURE 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. The School For Excellence 2012 Unit 4 Chemistry The Production of Nitric Acid Page 4

5 1. Prevent waste THE 12 PRINCIPLES OF GREEN CHEMISTRY Taken From Heinemann Chemistry 2 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. The School For Excellence 2012 Unit 4 Chemistry The Production of Nitric Acid Page 5

6 11. Analyse in real time to prevent pollution 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. Economical stimulus. The School For Excellence 2012 Unit 4 Chemistry The Production of Nitric Acid Page 6

7 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 CH2Cl 2, which is formed according to the following chemical equation: CH4( g ) 2Cl2( g) CH 2Cl2( aq) 2HCl( aq) % Atom economy % (b) Would this method of CH2Cl 2 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. The School For Excellence 2012 Unit 4 Chemistry The Production of Nitric Acid Page 7

8 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 (1100 o C ) 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 The hazardness of the waste Threats to animals, people and the environment The cost of the process The School For Excellence 2012 Unit 4 Chemistry The Production of Nitric Acid Page 8

9 NITRIC ACID (HNO3) USES OF NITRIC ACID In terms of production, nitric acid is the third most widely produced acid across the world. It has a wide range of uses in agriculture, industry and medicine where it is used as a fertiliser and in the manufacture of fireworks, explosives, medicines, dyes, food preservatives, pesticides and detergents. Nitric acid: PROPERTIES OF NITRIC ACID Is colourless in its pure form but may become orange or reddish in colour if contaminated by nitrogen oxides. Is highly corrosive. Is a poisonous liquid (freezing point -42 C, boiling point 83 C). Reacts with water or steam to produce heat and toxic, corrosive and flammable vapours. Can cause severe burns. Miscible in water at all concentrations. Has an acid dissociation constant (pk a ) of 1.4. In aqueous solution, it almost completely (93% at 0.1 mol/l). Will decompose at higher temperatures to form nitrogen oxides. Nitric acid is both a strong monoprotic acid and a strong oxidant, particularly when hot and concentrated. The School For Excellence 2012 Unit 4 Chemistry The Production of Nitric Acid Page 9

10 OXIDISING PROPERTIES The products of the reaction between nitric acid and metals depends upon the reactivity of the metal and the concentration of the acid. As a general rule, oxidising reactions occur primarily with the concentrated acid, favouring the formation of nitrogen dioxide (NO 2 ). Reaction between a reactive metal and dilute acid (<1M): Nitrogen has been reduced from +5 all the way to -3. Reaction between a less reactive metal and more concentrated acid: Acid concentration 3 to 6M: Nitrogen has been reduced from +5 to +2. Acid concentration 12M: Nitrogen has been reduced from +5 to +4. Since nitric acid is an oxidising agent, hydrogen (H 2 ) is rarely formed. Only magnesium (Mg), manganese (Mn) and calcium (Ca) react with cold, dilute nitric acid to give hydrogen: Mg(s) + 2 HNO 3 (aq) Mg(NO 3 ) 2 (aq) + H 2 (g) Reaction with non-metallic elements (with the exceptions of nitrogen, oxygen, noble gases, silicon and halogens) usually oxidises them to their highest oxidation states. The formation of nitrogen dioxide occurs for concentrated acid and nitric oxide for dilute acid. C (s) + 4 HNO 3(aq) CO 2(g) + 4 NO 2(g) + 2 H 2 O (l) 3 C (s) + 4 HNO 3(aq) 3 CO 2(aq) + 4 NO (q) + 2 H 2 O (l) The School For Excellence 2012 Unit 4 Chemistry The Production of Nitric Acid Page 10

11 ACIDIC PROPERTIES Nitric acid is a strong acid. In moderately dilute solution (~ 0.1 M) it is dissociated to an extent of about 93%, in accordance with the reaction, Being an acid, it reacts with alkalies to from nitrates It decomposes carbonates and bicarbonates as: The School For Excellence 2012 Unit 4 Chemistry The Production of Nitric Acid Page 11

12 THE PRODUCTION OF NITRIC ACID Nitric acid is made from ammonia in a three-step process known as the Ostwald process. Step 1: Oxidation of NH 3 to NO. Step 2: Oxidation of NO to NO 2. Step 3: Absorption and reaction of NO 2 with water. NO 2 STEP 1: CATALYTIC OXIDATION OF AMMONIA Air is preheated and mixed with ammonia (which is not preheated as it would decompose) and then passed through a converter where the following reaction occurs: 4NH 5O 4NO 6H O H 907 kjmol 3( g) 2( g) ( g) 2 ( g) 1 In this reaction, ammonia undergoes catalytic oxidation to form nitrogen monoxide (nitric oxide (NO) and water. This is the start of the oxidation process. The nitrogen in ammonia starts with an oxidation number of -3 (its lowest possible oxidation state) and is converted to +2 in nitrogen monoxide. The ratio of air/ammonia must be carefully monitored and is maintain at between 9 and 12%. If the concentration of ammonia rises much beyond this, the mixture becomes explosive. The catalyst used in this process is 90% platinum alloyed with 10% rhodium for increased strength. The catalyst consists of several woven or knitted gauzes formed from the alloy. The gauze mats are preheated so that the gases are directly heated as they pass over the catalyst. The catalyst may become poisoned by air pollution and contamination from the ammonia which reduces its efficiency. The cost of these catalysts are extremely high and need to be frequently replaced due to the wear and tear they experience under such severe conditions. A cheaper alternative is yet to be developed. The School For Excellence 2012 Unit 4 Chemistry The Production of Nitric Acid Page 12

13 Nitrous oxide, nitrogen and water are also simultaneously formed in this step, as shown below. 4NH3( g) 3O2( g) 2N2( g) 6H 2 O( g) H 1267 kjmol 2NH 2O N O 3H O 3( g) 2( g) 2 ( g) 2 ( g) 1 Conditions are carefully controlled in the converter in order to ensure that nitrogen monoxide (NO) is the main product, rather than nitrogen gas (N 2 ) or nitrogen (I) oxide ( NO). 2 The yield of nitric oxide depends on the pressure and temperature as shown below. Pressure (atm) Temperature ( C) NO yield (%) Below Above Typical conditions for the production of NO are therefore: High temperatures ( o C ) High pressures (11 atm) Temperature Considerations Even though higher yields would be obtained at lower temperatures (the forward reaction is exothermic), the process is carried out at high temperatures ( o C ). This is because the rate at which the reaction proceeds at low temperatures is too slow to be commercially viable. To compensate for the resultant loss in product yield, the gas mixture is passed over a catalyst a number of times to produce a moderate yield of NO. Pressure Considerations At the high temperatures employed, the NO formed decomposes to form nitrogen and oxygen. 2NO( g) N2( g) O2( g) To avoid this, the gas mixture is passed across the catalyst very rapidly (contact time is approximately sec). To achieve this high flow rate, the reaction is performed at high pressures even though lower pressures would result in a higher product yield. The consequential loss in yield of NO is compensated for by the increased reaction rates and the quality of product obtained. Note: Even with all the compromises that are required in this step, the yield of NO is in the order of 95%. The School For Excellence 2012 Unit 4 Chemistry The Production of Nitric Acid Page 13

14 STEP 2: OXIDATION OF NITROGEN MONOXIDE The waste heat from the gases leaving the converter is recycled and used in other sections of the plant. The temperature of the nitrogen monoxide mixture is reduced to around C in this process. The gases are then passed through a cooling chamber and their temperature reduced to approximately 50 C. Any condensed water is transferred to the absorption tower. As the gases are cooled, the nitrogen monoxide oxidises to nitrogen dioxide (the nitrogen in NO is oxidised from +2 to +4 in the nitrogen dioxide). The oxygen consumed in this step may be added from an external source or is provided by excess oxygen in the gaseous mixture exiting the converter. The reaction is: 2NO O 2NO H 114kJmol ( g) 2( g) 2( g) 1 Temperature Considerations As this reaction is exothermic, high yields of product can be achieved by using lower temperatures. This reaction is unusual in that its rate increases with decreasing temperature meaning that NO CONFLICT arises between the conditions required to optimise rates and yields. Pressure Considerations Yields can be further maximised by using high pressures. The system will respond to high pressures by favouring the reaction that will produce the fewer mole of gas which in this case is the formation of products. The School For Excellence 2012 Unit 4 Chemistry The Production of Nitric Acid Page 14

15 STEP 3: ABSORPTION OF NITROGEN DIOXIDE Water is mixed with the nitrogen dioxide gas in absorption towers to form dilute solutions of nitric acid according to the following overall reaction: 3NO H O 2HNO NO 2( g) 2 ( l) 3( aq) ( g) This is a redox reaction in which produces nitrogen in its highest oxidation state (+5 in nitric acid). The towers contain large number of inert plates packed with inert granular materials designed to increase the contact between the gases and water. This reaction is exothermic and continuous cooling is needed. The conversion is favoured by low temperatures and significant reaction occurs until the gases leave the towers. Nitrogen dioxide gas is pumped at 5 to 10 atm across the inert packing material, through which water is trickled from above. Reaction between the water and the gas produces nitric acid, which then dissolves in the remaining water. Small quantities of NO are also produced, which reacts with oxygen from the air in the tower to produce NO 2 which then reacts as before. A solution of nitric acid may be produced that is about 45 60% 3 HNO. This can easily be increased to 68% (equivalent to 16 M ) by distilling off some of the water. The School For Excellence 2012 Unit 4 Chemistry The Production of Nitric Acid Page 15

16 WASTES AND MANAGEMENT The Ostwald process is very energy efficient and produces little waste. The oxidation of ammonia is highly exothermic, generating sufficient heat energy to meet the energy needs of the rest of the plant. The main gaseous emissions from the Ostwald process include NO and NO 2. Both gases contribute to photochemical smog, and therefore, careful attention must be paid to minimising how much of these gases are emitted into the atmosphere. One approach involves the heating of these gases using a fuel such as natural gas, naphtha or hydrogen, over a catalyst, so that the NO x is reduced to N 2. CH 4(g) + 4NO 2(g) CO 2(g) + 2H 2 O (l) + 4NO (g) then, CH 4(g) + 4NO g) CO 2(g) + 2H 2 O (l) + 4N 2(g) Also: CH 4(g) + 4N 2 O g) CO 2(g) + 2H 2 O (l) + 2N 2(g) H 2(g) + NO 2(g) 2H 2 O (l) + NO (g) then, 2H 2(g) + 2NO (g) 2H 2 O (l) + N 2(g) Also: H 2(g) + N 2 O g) CO 2(g) + H 2 O (l) + 2N 2(g) In addition, the absorption tower may be modified by increasing its size or operating pressure so as to maximise conversion of NO to nitric acid. x The gas mixture entering the converter is filtered to remove catalytic poisons, which increases the efficiency of the catalysts and hence decreasing the pressure (and energy) required to force the gas through the catalyst bed. Heat exchangers are employed to remove heat released by the reaction in the converter and then using it to heat incoming gases or generate electricity. At the high temperatures and pressures used in the converter, the catalyst slowly vaporises and is lost. Gases leaving the converter are passed through a filter to recover the metals and minimise the impact of these vapours on the environment. Specific catalysts are added to the converter to decompose any 2 NO formed. Note: Nitrogen(I) oxide is a significant greenhouse gas. The School For Excellence 2012 Unit 4 Chemistry The Production of Nitric Acid Page 16

17 HEALTH AND SAFETY Concentrated nitric acid is corrosive and causes severe burns to the skin and eyes. Its fumes evolve nitrogen dioxide gas which at low concentrations may cause lung oedema (fluid in the lungs) and fatal with excessive exposure. As a strong oxidant, nitric acid reacts readily with a range of organic materials and metals to produce flammable and/or explosive products. NO x gases are significant greenhouse gases and some can react with water to form acid rain. Safety Measures Employed: There is careful monitoring in nitric acid plants for leaks and spills and all employees are trained to handle such if they do occur. Equipment must be carefully maintained to avoid corrosion. Acid spills are contained using materials such as earth, clay or sand, and then neutralised with a base such as slaked lime ( Ca( OH ) 2 ) or sodium carbonate. Full protective equipment and breathing apparatus is readily accessible across the plant. The ratio of ammonia to air in the gas entering the converter is continuously measured and controlled to ensure it does not reach explosive conditions. Un-reacted gases are recycled where possible. Various methods are employed to limit x NO emissions, maximise conversion efficiency, and minimise loss of energy. The School For Excellence 2012 Unit 4 Chemistry The Production of Nitric Acid Page 17

18 MIXED QUESTIONS QUESTION 1 A number of different oxidation states of nitrogen are involved in the industrial production of nitric acid from ammonia. State the various nitrogen containing compounds involved in the Ostwald Process and the corresponding oxidation states of nitrogen. Solution QUESTION 2 Describe the theoretical conditions that should be used to maximise the rate of the reaction of ammonia and oxygen to produce nitrogen monoxide. Are these the conditions actually used? If not, why not? Solution The School For Excellence 2012 Unit 4 Chemistry The Production of Nitric Acid Page 18

19 QUESTION 3 Which of the following is not a property of nitric acid? A B C D It is monoprotic. It is a strong acid. It is a good oxidant. It forms nitride salts. QUESTION 4 Write equations for the following reactions of nitric acid: (a) With water. (b) With ammonia to make ammonium nitrate. (c) With potassium hydroxide to make potassium nitrate. (d) With zinc metal to form zinc ions and ammonia. The School For Excellence 2012 Unit 4 Chemistry The Production of Nitric Acid Page 19

20 QUESTION 5 During the Ostwald process, nitrogen monoxide is made from ammonia at about 900 C and then cooled to 30 C before being reacted with air to make nitrogen(iv) oxide. Both these reactions are exothermic. Why are the temperatures used for these reactions so different? Solution QUESTION 6 As the gas passes through the catalyst bed in the converter during nitric acid manufacture, its temperature increases. The gas must be cooled before it is mixed with air. (a) (b) (c) Why does the temperature of the gas rise? Why is it necessary to cool the gas? What side benefit is obtained from the need to cool gases? Solution The School For Excellence 2012 Unit 4 Chemistry The Production of Nitric Acid Page 20

21 QUESTION 7 The flow chart below shows the processes leasing to the production of nitric acid on an industrial scale. (i) What is Gas A? What is Gas B? (ii) Write an equation for the process occurring in Reactor 1. (iii) What would be the effect of increasing the temperature in Reactor 1 on the rate of production of gas B? The School For Excellence 2012 Unit 4 Chemistry The Production of Nitric Acid Page 21

22 (iv) What would be the effect of increasing the temperature in Reactor 1 on the equilibrium yield of gas B? (v) What is the function of the catalyst in Reactor 2? (vi) At low temperatures, NO 2 is in equilibrium with another oxide of nitrogen. Write an equation for this equilibrium. (vii) What is reagent D, which is added into Reactor 4 with NO 2? Write an equation for the reaction occurring in Reactor 4. (viii) Reagent E is recycled back into the cooling tower as shown in the diagram. What is reagent F? The School For Excellence 2012 Unit 4 Chemistry The Production of Nitric Acid Page 22

23 QUESTION 8 Nitrogen oxide, NO, is a small but important component of the atmosphere and is produced commercially on a large scale during the manufacture of nitric acid. (i) Name two processes, one natural and one involving the activities of man, that contribute significant significantly to the NO present in the atmosphere. (ii) Write a balanced equation for the reaction by which NO is formed during the production of nitric acid by the Ostwald process. (iii) NO produced during the Ostwald process is oxidised to NO 2, according to the 1 equation 2NO( g) O2( g) 2NO2( g) H 181 kjmol. In a particular factory, energy is released at a rate of 570 kj per minute during this stage. What volume of NO 2 at STP is being released each minute? The School For Excellence 2012 Unit 4 Chemistry The Production of Nitric Acid Page 23

24 QUESTION 9 (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? The School For Excellence 2012 Unit 4 Chemistry The Production of Nitric Acid Page 24