What is industrial chemistry?
Industrial chemistry is making chemicals in very large amounts, safely and at low cost. A raw material is what we start with (for example air, natural gas, salt, crude oil). The finished product is what we sell. Auxiliary materials help but do not end up in the product: catalysts, cooling water, steam, solvents.
We draw a plant as a block diagram. Each block is one step: preparing feed, reacting, separating, purifying. Arrows are streams of material. A more detailed drawing with pumps and valves is a process flow diagram.
Raw materials and how they are prepared
Inorganic raw materials: ores (rock with a useful metal compound), air (gives nitrogen and oxygen), water, salt, limestone. Organic raw materials: coal, crude oil, natural gas, wood and biomass (plant waste). Fossil ones run out; biomass grows back.
- Sampling: a small, fair sample is taken from many places in a heap and tested in the lab, so we know what is inside.
- Flotation: crushed ore is stirred in water with bubbles; the useful mineral sticks to bubbles and floats, rock sinks.
- Water treatment: settling, filtering, softening and disinfecting so water does not damage boilers.
- Dry distillation: heating coal or wood without air gives coke, tar and gas.
Unit operations, measuring and material balance
A unit operation is one physical step that appears in many plants.
- Mechanical: crushing, grinding, sieving, mixing (crushers, mills, sieves, mixers).
- Hydrodynamic (moving fluids): pumping, settling, filtering, centrifuging (pumps, pipes, settling tanks, filter presses, centrifuges).
- Others: heating and cooling (heat exchangers), distillation, drying.
Plants measure temperature (thermometer/thermocouple, °C or K), pressure (manometer, Pa or bar), flow rate (flow meter, kg/s or m³/h) and level.
Material (mass) balance: matter is not created or lost, so for a steady plant: mass in = mass out. If 1000 kg/h goes in and 820 kg/h comes out as product, 180 kg/h must leave as another stream.
Reaction conditions: rate, yield and the compromise
A plant wants product fast (rate) and a lot (yield). Higher temperature, higher concentration or pressure, finer powder and a catalyst all raise the rate. But for a reversible exothermic reaction such as N₂ + 3H₂ ⇌ 2NH₃, higher temperature lowers the yield. So engineers choose a compromise: about 400–450 °C, about 150–250 atm and an iron catalyst. Very high pressure would give more yield but needs costly, dangerous equipment.
Separation, recycle streams and process calculations
After the reactor, the mixture goes to a separator (cooling to liquefy, distillation, filtering). Unused reactants are sent back: a recycle stream. A small purge removes unreactive gases that would build up.
Conversion per pass = reacted ÷ fed into the reactor. With recycling, overall conversion can be near 100% even if one pass only converts 15%.
Calculations use the equation and moles: mass → moles → mole ratio → moles → mass, then % yield = actual ÷ theoretical × 100.
Safety, sustainability and the chemical company
Safety: pressure relief valves, temperature and leak sensors, automatic shut-down, protective clothing, and storing dangerous chemicals apart. Sustainability: reuse heat with heat exchangers, choose routes with high atom economy, recycle water and solvents, sell by-products, use renewable feedstock.
A chemical company is chosen and judged on: raw materials, energy, transport, jobs, product price, and its effect on the people living nearby and on the planet (emissions of CO₂, water use). Products like fertilisers, medicines and plastics raise the quality of life worldwide, but only if their costs to health and nature are kept low.
Try it
At home, make a mini plant: mix sand and salt (raw material), add water and stir (dissolve), filter (separator), and evaporate the clear liquid in a sunny spot (product). Weigh before and after: is mass in = mass out? Then in the 3D, set the recycle slider to 0% and to 90% and write the overall conversion each time.
Key formulas and definitions
- Mass balance (steady): total mass in = total mass out
- % yield = actual yield ÷ theoretical yield × 100
- Conversion per pass = amount reacted ÷ amount fed to reactor
- Overall conversion with recycle r: c ÷ (c + (1 − c)(1 − r))
- Atom economy = mass of wanted product ÷ total mass of reactants × 100
Worked examples
1. A reactor is fed 500 kg/h. Products leave at 430 kg/h. What is the mass flow of the other outlet stream?
Mass in = mass out, so 500 = 430 + x, x = 70 kg/h.
2. From 28 kg of N₂ a plant could make 34 kg of NH₃ in theory. It actually gets 5.1 kg in one pass. Find the % conversion per pass.
5.1 ÷ 34 × 100 = 15%.
3. Per-pass conversion is 15% and 90% of the unused gas is recycled. Find the overall conversion.
0.15 ÷ (0.15 + 0.85 × 0.10) = 0.15 ÷ 0.235 = 0.638, so about 64%.
4. How many tonnes of NH₃ can 14 t of N₂ make? (N = 14, H = 1)
N₂ + 3H₂ → 2NH₃. 28 t N₂ gives 34 t NH₃, so 14 t gives 17 t.
5. Why is ammonia made at about 450 °C and not 250 °C, even though 250 °C gives a higher yield?
At 250 °C the reaction is far too slow. 450 °C with a catalyst gives a good rate with an acceptable yield: a compromise.
6. Name the unit operation: (a) separating sand from water with a cloth, (b) breaking limestone rocks, (c) moving water up a pipe.
(a) filtration, (b) crushing, (c) pumping.
Common mistakes
- Thinking a catalyst increases the yield. It only makes equilibrium come faster; the yield at equilibrium stays the same.
- Forgetting the recycle stream when doing a mass balance on the whole plant.
- Mixing up conversion per pass and overall conversion.
- Saying the highest temperature is always best. For exothermic reversible reactions, very high temperature lowers the yield.