Industrial processes and catalysis

What you'll learn

Industrial chemistry applies the principles of rates, equilibrium, energetics and catalysis to manufacture chemicals on a huge scale, safely and economically. The Caribbean is home to major chemical industries: Trinidad and Tobago's Point Lisas estate produces ammonia, urea and methanol from natural gas for export around the world, and Jamaica and Guyana refine bauxite into alumina. This guide covers the factors involved in siting and running a chemical plant, the Haber process for ammonia, the manufacture of methanol and urea, the Contact process for sulfuric acid, the Bayer process and the electrolytic extraction of aluminium, the role of catalysts, and the environmental and economic impacts of these industries.

CAPE questions ask you to explain the choice of conditions using rate, yield and cost, write equations, describe the stages of a process and evaluate the economic, social and environmental impacts of industries in the region.

Key terms and definitions

Feedstock — The raw material supplied to an industrial process.

Yield — The amount of product obtained, often expressed as a percentage of the theoretical maximum.

Compromise conditions — Conditions chosen to balance a reasonable rate, an acceptable yield and acceptable costs and safety.

Continuous process — A process that runs non-stop, with reactants fed in and products removed continuously.

Batch process — A process carried out in separate batches.

Recycling (in industry) — Returning unreacted materials to the reactor.

Heterogeneous catalyst — A catalyst in a different phase from the reactants, usually a solid with gaseous reactants.

Steam reforming — Reacting methane with steam to produce hydrogen and carbon monoxide (synthesis gas).

Synthesis gas (syngas) — A mixture of carbon monoxide and hydrogen.

Core concepts

Factors in running a chemical plant

When planning and operating an industrial process, chemists and engineers consider:

• availability and cost of raw materials — Trinidad's natural gas supplies both feedstock and fuel;
• energy costs — high temperatures and pressures are expensive;
• rate of reaction — faster production is more profitable;
• position of equilibrium and yield;
• catalysts — lower energy costs and increase rate;
• plant safety and environmental regulations;
• transport — access to ports for importing materials and exporting products;
• labour and infrastructure — skilled workers, water and electricity supplies;
• waste disposal and the potential for by-products to be sold or used.

Point Lisas is sited on the coast with a deep-water port, close to natural gas pipelines, and with industrial infrastructure shared between plants — an example of these factors in action.

Steam reforming: making hydrogen

Most industrial hydrogen comes from natural gas:

CH₄ + H₂O ⇌ CO + 3H₂ (nickel catalyst, high temperature)

The carbon monoxide is then converted to carbon dioxide in the shift reaction: CO + H₂O ⇌ CO₂ + H₂. The carbon dioxide can be used to make urea.

The Haber process (ammonia)

N₂(g) + 3H₂(g) ⇌ 2NH₃(g) ΔH = −92 kJ mol⁻¹

• Raw materials: nitrogen from the air; hydrogen from steam reforming of natural gas.
• Temperature: about 400–450 °C. A low temperature would give a higher equilibrium yield (exothermic forward reaction), but the rate would be too slow; this compromise gives a reasonable rate and yield.
• Pressure: about 200 atm. High pressure favours the side with fewer moles of gas (4 → 2), increasing the yield and rate, but very high pressures are costly and dangerous.
• Catalyst: finely divided iron (with promoters), increasing the rate.
• Yield per pass: only around 15%, but ammonia is removed by cooling and liquefying it, and unreacted nitrogen and hydrogen are recycled, so overall conversion is high.

Uses of ammonia: fertilisers (ammonium nitrate, ammonium sulfate, urea), nitric acid, explosives, cleaning products and refrigeration.

Urea

Ammonia reacts with carbon dioxide (from the hydrogen plant) to form urea:

2NH₃ + CO₂ → CO(NH₂)₂ + H₂O

Urea has a high nitrogen content and is a major fertiliser exported from Trinidad.

Methanol

Methanol is made from synthesis gas:

CO(g) + 2H₂(g) ⇌ CH₃OH(g) ΔH = −91 kJ mol⁻¹

• Conditions: about 250 °C, 50–100 atm, with a copper–zinc oxide–aluminium oxide catalyst.
• The same reasoning applies: lower temperatures favour the exothermic forward reaction but slow the rate; high pressure favours fewer moles of gas.

Uses: a solvent, a fuel and a feedstock for making methanal (formaldehyde), plastics, adhesives and other chemicals. Trinidad and Tobago has been one of the world's largest exporters of methanol.

The Contact process (sulfuric acid)

1. Sulfur is burned: S + O₂ → SO₂.
2. Sulfur dioxide is oxidised: 2SO₂ + O₂ ⇌ 2SO₃, ΔH = −197 kJ mol⁻¹, using a vanadium(V) oxide catalyst at about 450 °C and 1–2 atm. The equilibrium already lies far to the right, so high pressure is not needed.
3. Sulfur trioxide is absorbed in concentrated sulfuric acid to form oleum, H₂S₂O₇, which is then diluted with water: SO₃ + H₂SO₄ → H₂S₂O₇; H₂S₂O₇ + H₂O → 2H₂SO₄. SO₃ is not added directly to water because the reaction is very exothermic and forms a corrosive mist.

Uses: fertilisers (superphosphates and ammonium sulfate), detergents, paints, batteries and processing of ores.

The Bayer process and aluminium extraction

Bayer process (alumina from bauxite) — used in Jamaica and formerly in Guyana:

1. Crushed bauxite is digested with hot concentrated sodium hydroxide under pressure. The amphoteric aluminium oxide dissolves: Al₂O₃ + 2NaOH + 3H₂O → 2NaAl(OH)₄.
2. Insoluble impurities, mainly iron(III) oxide, are filtered off as red mud.
3. The solution is cooled and seeded with aluminium hydroxide crystals, causing Al(OH)₃ to precipitate.
4. The aluminium hydroxide is heated (calcined) to form pure alumina: 2Al(OH)₃ → Al₂O₃ + 3H₂O.

Hall–Héroult electrolysis (aluminium from alumina):

• Alumina is dissolved in molten cryolite, which lowers the operating temperature to about 950 °C, saving energy.
• Cathode: Al³⁺ + 3e⁻ → Al
• Anode: 2O²⁻ → O₂ + 4e⁻; the oxygen reacts with the carbon anodes, forming CO₂, so the anodes must be replaced regularly.
• The process needs enormous amounts of electricity, which is why most Caribbean alumina is exported for smelting where cheap electricity (often hydroelectric) is available.

Environmental and economic impacts

Benefits: foreign exchange earnings, jobs, skills development, industrial infrastructure and government revenue.

Costs and risks:

• Air pollution — emissions of CO₂, NOₓ and SO₂ from combustion and processing; greenhouse gas emissions from steam reforming.
• Water pollution — from effluents and run-off of fertilisers, causing eutrophication in rivers and coastal waters.
• Red mud from bauxite refining is highly alkaline and must be stored in lagoons; leaks can contaminate land and water.
• Land degradation from bauxite mining, requiring rehabilitation of mined-out areas.
• Safety hazards — ammonia leaks, explosions and fires.
• Economic vulnerability — dependence on world prices for natural gas, ammonia, methanol and aluminium.

Mitigation includes scrubbing flue gases, treating effluents, recycling heat and materials, land rehabilitation, emergency planning and diversifying the economy.

Worked examples

Example 1: Explaining Haber conditions

Explain why a temperature of about 450 °C is used in the Haber process rather than a lower temperature. (3 marks)

The forward reaction is exothermic, so a lower temperature would give a higher equilibrium yield (1). However, at lower temperatures the rate would be too slow, even with a catalyst (1). 450 °C is a compromise giving an acceptable yield at a reasonable rate (1).

Example 2: Contact process pressure

Why is the Contact process run at only 1–2 atm?

The equilibrium already lies well to the right at these conditions, giving a high conversion. Higher pressures would increase costs and safety risks for only a small gain in yield.

Example 3: Calculating mass of urea

What mass of urea can be made from 68 tonnes of ammonia? (Mᵣ NH₃ = 17, urea = 60)

Moles of NH₃ = 68 × 10⁶ g ÷ 17 = 4.0 × 10⁶ mol. 2 NH₃ → 1 urea, so moles urea = 2.0 × 10⁶ mol. Mass = 2.0 × 10⁶ × 60 = 1.2 × 10⁸ g = 120 tonnes.

Example 4: The Bayer process

Explain the role of sodium hydroxide in the Bayer process.

Aluminium oxide is amphoteric, so it dissolves in hot concentrated sodium hydroxide to form sodium aluminate, while basic iron(III) oxide does not dissolve. This separates the alumina from the iron oxide impurities, which are filtered off as red mud.

Example 5: Explaining methanol conditions

Methanol is made by CO(g) + 2H₂(g) ⇌ CH₃OH(g), ΔH = −91 kJ mol⁻¹, at about 250 °C and 50–100 atm with a copper-based catalyst. Explain these conditions. (4 marks)

The forward reaction is exothermic, so a lower temperature gives a higher equilibrium yield, but the rate would be too slow; 250 °C is a compromise (1). The forward reaction reduces the number of moles of gas from 3 to 1, so high pressure shifts the equilibrium to the right and increases the yield (1). Very high pressures would be expensive and dangerous, so a moderate pressure is chosen (1). The catalyst increases the rate at this relatively low temperature, saving energy (1).

Example 6: Comparing nitrogen fertilisers

Calculate the percentage by mass of nitrogen in urea, ammonium nitrate and ammonium sulfate. (Mᵣ: CO(NH₂)₂ = 60, NH₄NO₃ = 80, (NH₄)₂SO₄ = 132)

• Urea: 28 ÷ 60 × 100 = 46.7%
• Ammonium nitrate: 28 ÷ 80 × 100 = 35.0%
• Ammonium sulfate: 28 ÷ 132 × 100 = 21.2%

Urea's high nitrogen content makes it cheaper to transport per kilogram of nitrogen, one reason it is a major export from Trinidad and Tobago.

Example 7: Economic and environmental evaluation

Evaluate the impact of the ammonia and methanol industries on Trinidad and Tobago.

The industries convert natural gas into high-value exports, earning foreign exchange and government revenue, providing skilled jobs and supporting related businesses at Point Lisas. However, they consume large amounts of natural gas, produce carbon dioxide emissions, and carry risks of ammonia leaks and industrial accidents; fertiliser run-off from users can cause eutrophication. Their dependence on world gas and fertiliser prices also makes the economy vulnerable. Overall, the industries bring major economic benefits, but they must be managed with strict environmental controls and planned diversification.

Example 8: Recycling in the Haber process

Explain why the Haber process is economical even though only about 15% of the gases are converted in each pass over the catalyst.

Ammonia is removed by cooling and liquefying it, and the unreacted nitrogen and hydrogen are recycled back into the reactor. Over many passes, almost all the starting gases are eventually converted, so the overall yield is high even though the single-pass conversion is low.

Major processes at a glance

Common mistakes and how to avoid them

• Explaining conditions using yield only — always include rate and cost.
• Saying the catalyst increases yield — it increases rate only.
• Forgetting recycling of unreacted gases in the Haber process.
• Adding SO₃ directly to water in the Contact process.
• Forgetting cryolite's role in aluminium extraction.
• One-sided evaluations of industry — include both benefits and environmental costs.

How this links to your Internal Assessment

Industrial chemistry can inspire SBAs that model processes on a small scale: investigating how temperature affects the yield or rate of a reversible reaction, preparing ammonium sulfate fertiliser and determining its purity by titration, or analysing the pH and composition of soils before and after adding fertilisers. An environmental investigation could examine the alkalinity of soils near a bauxite area (with appropriate permission and safety). Strong work links practical findings to the principles of rate, equilibrium and yield used in real industries.

Exam technique for Industrial processes and catalysis

• For each process, learn the equation, ΔH, temperature, pressure and catalyst.
• Explain conditions using Le Chatelier's principle (yield), collision theory (rate) and economics (cost, safety).
• In evaluation questions, consider economic, social and environmental impacts.
• Use Caribbean examples — Point Lisas (ammonia, urea, methanol) and Jamaican alumina — where relevant.

Quick revision summary

• Plant siting: raw materials, energy, transport, labour, infrastructure, safety and environment.
• Steam reforming: CH₄ + H₂O → CO + 3H₂ (Ni catalyst).
• Haber: N₂ + 3H₂ ⇌ 2NH₃; ~450 °C, ~200 atm, iron catalyst; recycle unreacted gases.
• Urea: 2NH₃ + CO₂ → CO(NH₂)₂ + H₂O. Methanol: CO + 2H₂ ⇌ CH₃OH; ~250 °C, 50–100 atm, Cu/ZnO/Al₂O₃.
• Contact: 2SO₂ + O₂ ⇌ 2SO₃; ~450 °C, 1–2 atm, V₂O₅; absorb in H₂SO₄ to form oleum.
• Bayer: Al₂O₃ dissolves in NaOH; red mud removed; Al(OH)₃ precipitated and calcined. Electrolysis in molten cryolite.
• Impacts: economic benefits versus air and water pollution, red mud, land degradation and safety risks.