17.1 Modern Chemical Manufacturers
🎯 Learning outcomes
By the end of this chapter, Students should be able to:
- 17.1 State and show the manufacture of ammonia by Haber’s process (principle and flow sheet diagram).
- 17.2 State and show the manufacture of nitric acid by Ostwald’s process (principle and flow-sheet diagram).
- 17.3 State and show the manufacture of sulphuric acid by the contact process (principle and flow-sheet diagram).
- 17.4 State and show the manufacture of sodium hydroxide by the diaphragm cell (principle and flow-sheet diagram).
- 17.5 State and show the manufacture of sodium carbonate by ammonia soda or the Solvay process (principle and flow-sheet diagram).
Ever wonder how the ingredients in your smartphone, medicine, farm fertiliser, or sneakers actually get made? Behind almost everything we touch is the chemical industry—a high-tech world using automation, clever science, and eco-friendly practices to mass-produce the building blocks of modern life.
In this chapter, you’ll step inside the factory to master the recipe books for five chemical superstars:
- Ammonia (powering agriculture)
- Nitric Acid (vital for fertilisers and explosives)
- Sulfuric Acid (the backbone of modern manufacturing)
- Sodium Hydroxide (essential for soap and paper)
- Sodium Carbonate (key to making glass and detergents)
For each chemical, you will learn the full production process: the starting ingredients (raw materials), the key chemical reactions, the exact temperature and pressure conditions needed, and the visual flow diagrams that bring these industrial-scale operations to life.
Principle
- Nitrogen does not combine with hydrogen under ordinary conditions.
- But when a mixture of nitrogen and hydrogen in the ratio 1:3 by volume is heated to about 450 °C under a pressure of 200-900 atmospheres in the presence of finely divided Iron as a catalyst and Molybdenum as a promoter, ammonia is obtained.
Conditions for maximum yield of ammonia:
The reaction is reversible and exothermic. It also produces fewer gas molecules. Therefore, Le Chatelier’s principle helps us choose suitable conditions for ammonia production.
- High pressure: It has been shown that at a particular temperature, the yield of ammonia increases with increasing pressure.
Why ~200 atm?
Higher pressure → higher ammonia yield
But very high pressure → expensive + greater safety requirements
Therefore: ~200 atm = practical compromise
- Low temperature: As the reaction is exothermic, it is favoured by low temperature. But if the temperature is too low, the reaction will be too slow. Therefore, an optimum temperature of 450 °C is used.
Lower temperature → higher equilibrium yield → but reaction too slow
Higher temperature → faster reaction → but lower equilibrium yield
450 °C = compromise between yield and rate
- High concentration: If the reactants are used in excess, a larger amount of products is formed. Hence, pure reactants are used, and one of the reactants is used in excess.
- Presence of a catalyst: Finely divided iron is used as a catalyst to speed up the reaction under given conditions. To increase the efficiency of the catalyst, Molybdenum is used as a promoter. Moreover, the purity of the reactants is essential to avoid poisoning the catalyst.
Flow sheet diagram:
Ammonia is essential for making nitrogen fertilisers, which support global food production— it is arguably the most important chemical reaction in human history.
(Technical Production of HNO₃ by Catalytic Oxidation of NH₃) (VVVI)
Principle
The first and most important step in this process is the catalytic oxidation of ammonia by the oxygen of the air, carried out in the presence of Platinum-rhodium gauze heated to about 850 °C. Under these conditions, ammonia is oxidised to NO, which is then cooled to about 50 °C -100 °C. NO is oxidised by oxygen in air to NO₂. Nitrogen dioxide is then allowed to react with water and more air to form HNO₃.
Flow sheet diagram
Sulphuric acid is a widely used chemical across various industries; therefore, it is also called the “king of chemicals”. The amount of sulphuric acid consumed in a country can be taken as the index of its industrial advancement.
It is manufactured by the contact process.
Principle:
In this industrial process, sulphuric acid is produced in the following steps
- Production of sulphur dioxide: SO₂ gas can be prepared either by burning sulphur or roasting iron pyrites
- Oxidation of SO₂ to SO₃: Sulphur dioxide is oxidised to sulphur trioxide in the presence of the catalyst vanadium pentoxide at about 450°C and 2 atm pressure. Since this reaction is carried out in contact with the catalyst, it is also called the contact process.
Conditions for the optimum yield of H₂SO₄
The formation of SO₃ from SO₂ is a reversible process. The optimum condition for the formation of SO₃ can be explained by Le Chatelier’s principle.
- Temperature: The formation of SO₃ is an exothermic process. So, according to Le Chatelier’s principle, the reaction should be carried out at a low temperature. However, too low a temperature makes the reaction rate too slow. Therefore, an optimum temperature of about 450 °C is maintained.
- Pressure: According to Le Chatelier’s principle, the pressure should be high. High pressure increases the equilibrium yield, but the increase is not large enough to justify the high cost and energy needed for high-pressure equipment. Therefore, the process operates at only a slightly elevated pressure, around 1–2 atm.
- Concentration: High concentrations of reactants are used for a higher yield of the product.
- Catalyst: The rate of the reaction can be increased by using catalysts such as vanadium pentoxide.
- Purity of gas: Pure gases should be used to protect the catalyst from poisoning.
SO₃ is not directly added to water because the reaction is highly exothermic and produces a fine acid mist. Instead, SO₃ is absorbed in concentrated H₂SO₄ to form oleum.
Flow-Sheet Diagram:
Principle:
In the diaphragm cell, electrolysis of brine solution is performed in order to obtain sodium hydroxide in the presence of a coating of titanium on a Ru–Ti oxide anode and a steel cathode. In this process, chlorine is discharged through the anodic outlet, and hydrogen is discharged through the cathodic outlet.
Reactions at the Anode:
Between Cl⁻ and OH⁻, OH⁻ remains unaffected, but Cl₂ is discharged from the anode in the diaphragm cell.
Reactions at the Cathode:
Water is reduced at the cathode; sodium ions remain in solution. Molecular hydrogen is discharged from the outlet of the cathode.
Formation of Sodium Hydroxide:
During electrolysis, the hydrogen and chlorine gases are removed separately. Sodium ions move with the brine through the porous diaphragm into the cathode compartment, where they remain in solution with the OH⁻ ions to form NaOH.
The hydroxide ions are prevented from reacting with chlorine gas by the diaphragm. During electrolysis, the solution from the cathode is periodically removed and concentrated by the evaporation process. A dilute solution of NaOH is obtained, which is about 10-12 % (w/w) and about 16% NaCl. Sodium chloride is crystallised out and can be recycled for the same process.
Flow sheet diagram for the manufacture of sodium hydroxide by the diaphragm cell:
The diaphragm cell is an older method of producing NaOH. Modern chlor-alkali plants commonly use ion-exchange membrane cells, which are more energy-efficient and produce purer NaOH.
Principle
Brine solution is saturated with ammonia by bubbling ammonia gas, and the ammoniacal brine is then treated with carbon dioxide. First, ammonium bicarbonate is formed, which further reacts with sodium chloride to give sodium bicarbonate
Sodium bicarbonate is sparingly soluble in water in the presence of sodium chloride. Hence, the sodium bicarbonate formed precipitates out which is separated by filtration. The sodium bicarbonate is then calcined to get sodium carbonate.
Flow-Sheet Diagram:
Working of the Plant:
- Saturation of brine
In the ammonia absorber, brine solution (about 30% NaCl) is saturated with ammonia gas. Impurities like CaCl₂, MgCl₂, etc, present in brine are precipitated as hydroxides, and carbonates are removed.
NH₃ + H₂O → NH₄OHMgCl₂ + 2NH₄OH → Mg(OH)₂ + 2NH₄Cl2NH₃ + H₂O + CO₂ → (NH₄)₂CO₃CaCl₂ + (NH₄)₂CO₃ → CaCO₃ + 2NH₄ClAmmoniated brine is then pumped to the carbonation tower.
- Carbonation
The carbonation tower works on the counter-current principle.
Ammoniated brine is allowed to trickle down a tower, and carbon dioxide is passed from the bottom at about 2 atm pressure; the temperature is maintained at about 30 ºC. Sodium bicarbonate and ammonium chloride are formed.
NH₃ + H₂O + CO₂ → NH₄HCO₃NaCl + NH₄HCO₃ → NaHCO₃ + NH₄ClSodium bicarbonate is filtered, and the filtrate (NH₄Cl with little NH₄HCO₃) is pumped to the ammonia generator.
- Recovery of ammonia
In an ammonia generator, ammonium chloride reacts with lime to generate ammonia gas.
NH₄Cl + Ca(OH)₂ → CaCl₂ + NH₃ + H₂ONH₄HCO₃ → NH₃ + H₂O + CO₂ - Generation of CO₂
In a lime kiln, limestone is heated to produce CO₂ required for carbonation.
CaCO₃ → CaO + CO₂CaO is treated with water to get slaked lime and pumped into the ammonia generator.
CaO + H₂O → Ca(OH)₂ - Calcination
Sodium bicarbonate obtained by filtration is heated to get anhydrous sodium carbonate.
- Crystallisation
Aqueous sodium carbonate solution may be subjected to crystallisation to get washing soda.
Na₂CO₃ (aq.) → Na₂CO₃.10H₂O
↓ + NH₃
Ammoniated brine
↓ + CO₂
NaHCO₃ precipitates
↓ filtration
NaHCO₃
↓ heating
Na₂CO₃
Meanwhile: NH₄Cl + Ca(OH)₂ → NH₃ recycled
And CaCO₃ → CO₂ + CaO
17.1.6 Summary
| Process | Product | Main raw materials | Key condition |
|---|---|---|---|
| Haber | NH₃ | N₂ + H₂ | ~450 °C, ~200 atm, Fe |
| Ostwald | HNO₃ | NH₃ + air | Pt–Rh, ~850–900 °C |
| Contact | H₂SO₄ | SO₂ + O₂ | V₂O₅, ~450 °C |
| Diaphragm cell | NaOH | Brine + electricity | Electrolysis |
| Solvay | Na₂CO₃ | Brine + NH₃ + CO₂ | Carbonation + heating |