UNIT 2 CHEMISTRY · PEARSON AAQ · D1.6
Application of Green Chemistry in Industry
FREE CHEMISTRY REVISION · TOPIC 49
Application of Green Chemistry in Industry
Pearson BTEC Level 3 AAQ Applied Science
Unit 2: Principles and Applications of Chemistry · D1.6
Evaluate how a process can prevent waste, conserve resources and energy, reduce hazards, and plan for a product’s end-of-life. Calculate atom economy from a balanced equation, then combine that result with yield, separation, catalyst performance and life-cycle evidence. Every numerical process comparison below is illustrative teaching data.
Green chemistry aims to reduce or prevent hazardous substances and waste through the design of chemical products and processes. A single attractive number does not establish that a process is environmentally preferable. Consider raw-material extraction, transport, reaction, purification, use and end-of-life.
| Measure | What it tells you | What it leaves out |
|---|---|---|
| Atom economy | The theoretical fraction of reactant mass assigned to the desired product in the balanced reaction | Actual yield, solvents, excess feed, energy and many work-up materials |
| Percentage yield | Actual isolated product compared with the theoretical maximum from the limiting reactant | The fraction of atoms assigned to other products |
| Process efficiency | A defined practical measure, such as kg saleable product per kWh or per hour | There is no single universal efficiency percentage; state the metric. |
Atom economy / % = [stoichiometric mass of desired product ÷ total stoichiometric mass of reactants] × 100. Use balanced coefficients × relative formula masses. Equivalently, use the mass of all products in the denominator: conservation of mass makes the totals equal for a balanced equation.
Decide which product is desired before calculating. Include all stoichiometric reactants, even a reactant used to make an unwanted co-product. Catalysts are not consumed overall and are not included as stoichiometric reactants. Solvents and actual excess quantities are normally outside this equation-based measure, but still matter environmentally. Use the minimum balanced reaction amounts, not arbitrary experimental masses.
For these calculations use Ar: H=1.0, C=12.0, O=16.0, Na=23.0, Cl=35.5, Ca=40.0. Atom economy is dimensionless and is usually reported as a percentage.
C₂H₄ + H₂ → C₂H₆. Desired product: C₂H₆. Relative masses:28.0 + 2.0 → 30.0. Atom economy = 30.0/(28.0 + 2.0) × 100 = 100%.
Every atom in the balanced reactants can enter the one product. This does not guarantee 100% yield, a safe process or low energy use. Feedstock origin and purification still matter.
CaCO₃ → CaO + CO₂. Desired product: CaO. Mr(CaCO₃)=100.0; Mr(CaO)=56.0; Mr(CO₂)=44.0.
Atom economy = 56.0/100.0 × 100 = 56.0%. The remaining 44.0% of the stoichiometric reactant mass is assigned to CO₂, even if all CaCO₃ reacts perfectly. Collecting CO₂ for a verified useful application can reduce waste or displace another supply, but it does not change the atom economy for CaO alone.
2NaHCO₃ → Na₂CO₃ + CO₂ + H₂O. Desired product: Na₂CO₃. Mr(NaHCO₃)=84.0; Mr(Na₂CO₃)=106.0.
Total reactant mass for the balanced equation = 2 × 84.0 =168.0. Atom economy =106.0/168.0 ×100 = 63.1% (3 s.f.). Product mass check:106.0 +44.0 +18.0 =168.0.
Using 84.0 alone gives 126%, signalling an error: a product cannot contain more than 100% of the stoichiometric reactant mass.
HCl + NaOH → NaCl + H₂O. Desired product: NaCl. Mr values are 36.5,40.0,58.5 and 18.0 respectively.
Atom economy =58.5/(36.5 +40.0) ×100 = 76.5% (3 s.f.). Water is a co-product, so its atoms do not count as desired NaCl. Even a relatively low-hazard co-product reduces atom economy for a single selected product.
For the CaCO₃ reaction in example 2,100.0 g pure CaCO₃ can theoretically produce 56.0 g CaO. A process isolates 50.4 g CaO.
Percentage yield =50.4/56.0 ×100 = 90.0%. Atom economy remains 56.0%. One measures actual product recovery relative to the theoretical CaO amount; the other describes the balanced reaction’s allocation of atoms.
Here actual isolated CaO/reactant mass =50.4/100.0 ×100 =50.4%. Under these specified stoichiometric-feed assumptions, it equals 0.900 ×56.0%. It is a separate mass-based metric, not a new atom economy. Excess reactant, solvents or additives would change a broader process-mass calculation.
A process with high atom economy can have poor yield because of incomplete conversion, competing reactions or product lost during isolation. Conversely, excellent yield cannot remove the co-products required by the chosen balanced equation. Improve both measures while considering hazards and energy.
A co-product may become a useful raw material for another process if its composition, purity, quantity and demand are suitable. This can reduce disposal and extraction, but separation, transport and treatment have impacts too. Selling a waste stream does not automatically make the original reaction greener. Preventing waste is usually preferable to treating it after formation.
Recovering unreacted feed can reduce fresh-resource demand. A separation stage must distinguish reactants from product and contaminants. A recycle stream can accumulate inert substances or impurities; a controlled purge may be needed. Recycle improves overall conversion without changing the equilibrium constant at fixed temperature.
A reactor receives 100 kg of one reactant per pass and converts 60 kg. Of the 40 kg remaining, a separator recovers 90.0% for return. Assume the reported masses refer only to this reactant and no side consumption occurs.
Recovered feed =0.900 ×40 = 36.0 kg. Unrecovered reactant =40 −36 = 4.0 kg. To maintain a 100 kg reactor feed next pass, fresh top-up =100 −36 = 64.0 kg.
This is 36.0 kg less fresh feed than an otherwise identical once-through process. Check the steady-state fresh-feed balance:64.0 kg =60.0 kg consumed +4.0 kg lost. This is not a complete product-mass balance if another reactant also contributes atoms. Include separator energy and disposal before evaluating the overall benefit.
| Resource choice | Possible benefit | Evidence needed |
|---|---|---|
| Renewable biomass | Can be replenished on a relevant timescale | Regrowth rate, land use, water, fertiliser, biodiversity and competing food uses |
| Recycled feedstock | Can reduce virgin extraction and waste | Collection rate, contamination, sorting, transport, purification and usable recovery yield |
| Virgin finite resource | May offer consistent purity or a lower processing burden in a particular comparison | Extraction impacts, depletion, energy and available alternatives |
“Renewable” describes replenishment, not guaranteed sustainability. “Recycled” describes material recovery, not guaranteed low energy use. Compare equivalent amounts of usable product over a stated boundary. A mixed recycled stream may need more treatment; a renewable crop may still require substantial energy or water.
Reduce unnecessary heating and cooling, insulate equipment, recover useful heat, improve separation and use suitable catalysts. Low reaction temperature can save heating energy, but a dilute product requiring energy-intensive separation can offset that saving. Report energy per unit of saleable product, including the stages specified in the comparison.
Catalyst efficiency can improve through greater accessible active surface area, suitable supports, better contact and mixing, an appropriate operating temperature, improved selectivity and longer useful lifetime. Smaller catalyst particles can increase accessible area, but excessively fine particles may be hard to recover or create a large pressure drop. Select conditions using data rather than assuming “smaller is always better”.
An inhibitor decreases catalytic activity. It may compete for sites or interfere with a catalytic pathway. A poison can bind strongly and deactivate sites; some catalysts also lose activity through fouling or sintering. Do not claim that every inhibitor can be removed by heating. Catalyst manufacture, recovery, metal loss and regeneration energy belong in the evaluation.
An illustrative process initially uses 6.0 kWh per kg saleable product. Improved catalysis and heat recovery reduce this to 4.2 kWh kg⁻¹ at the same product quality.
Saving =6.0 −4.2 = 1.8 kWh kg⁻¹; percentage reduction =1.8/6.0 ×100 = 30%. At 100 kg h⁻¹ output the energy-use rate falls from 600 to 420 kWh h⁻¹, a reduction of 180 kW.
If 0.500 kg catalyst produces 75.0 kg product in one hour, its mass-based productivity is 75.0/(0.500 ×1.00) = 150 kg product kg⁻¹ catalyst h⁻¹. This metric alone does not give selectivity, lifetime or total environmental impact. A catalyst does not change equilibrium yield at unchanged temperature and pressure.
A hazard is a substance’s or situation’s potential to cause harm. Risk concerns how likely harm is, and how severe it could be, under the actual conditions of exposure. Consider reactants, products, intermediates, solvents and waste—not only the desired product. Amount, concentration, volatility, temperature, pressure and exposure route can all affect risk.
Evaluate controls in order: eliminate the hazard if possible; substitute a less hazardous material or method; use suitable enclosure or extraction and engineering controls; apply appropriate procedures, training and monitoring; then add suitable personal protective equipment. Choose controls using a task-specific assessment and substance information. These are evaluation examples, not practical instructions for carrying out the reactions.
| Illustrative hazard | Risk to evaluate | Appropriate control to justify |
|---|---|---|
| Volatile hazardous vapour | Inhalation during transfer or reaction | Substitution where feasible; contained transfer and suitable local extraction |
| Corrosive solution | Skin/eye exposure from a splash | Reduce inventory/concentration if suitable; containment and splash protection; compatible gloves and eye protection |
| Flammable material near heating | Ignition and fire | Safer alternative where possible; controlled ignition sources and suitable equipment |
| Hot pressurised system | Burns, leaks or containment failure | Suitable rated plant, temperature/pressure controls, maintenance and trained operation |
| Hazardous waste stream | Exposure or environmental release | Segregated compatible containment and appropriate recovery or treatment |
Precautions must address the identified exposure route and conditions. “Wear gloves” alone is not a complete response to an inhalation hazard. Check glove compatibility rather than assuming every glove protects against every chemical. A lower-hazard product may still require hazardous reactants, so assess the full route.
Control approach reference: HSE control measures and HSE hierarchy of controls.
Design for useful life, repair, reuse and realistic recycling. Recycling may preserve material value, but collection, sorting and contamination can limit it. Mechanical recycling and chemical recovery have different energy and quality implications; assess the supplied data instead of assuming a universal ranking.
Degradation breaks a material into smaller substances under particular conditions. Biodegradation depends on organisms, temperature, oxygen, moisture and time. A material that degrades in an industrial treatment facility may persist elsewhere. Fragmentation into small pieces is not necessarily complete biodegradation, and degradation products must also be evaluated for hazard. Durable products can avoid repeated replacement; rapid degradation is not always desirable during use.
For the same service and 100 kg of finished material, illustrative route A uses 250 kWh in manufacture plus 100 kWh in collection/recycling. Route B uses 200 kWh in manufacture plus 180 kWh in end-of-life treatment.
Within this stated boundary, A totals 350 kWh and B totals 380 kWh. A uses 30 kWh less, a reduction of 30/380 ×100 ≈ 7.9% relative to B. Manufacture alone would favour B, so omitting end-of-life reverses the conclusion.
This is an energy comparison, not a complete environmental verdict. Compare emissions from the energy supply, usable recovery, toxicity, transport, durability and actual disposal routes. Do not add an assumed recycling credit unless the data specify what it replaces.
- Atom economy equals percentage yield: one comes from the equation; the other from actual product.
- Ignore balanced coefficients:2NaHCO₃ has twice the formula mass contribution.
- Useful waste changes atom economy for the original selected product: it does not change the equation-based calculation.
- A renewable feedstock is automatically sustainable: replenishment and environmental impacts are different questions.
- Catalysts always remain efficient: poisoning, inhibition and physical deactivation can reduce activity.
- PPE removes the hazard: it is a protective control, usually additional to stronger exposure controls.
- Degradable means harmless in every environment: conditions and breakdown products matter.
- State the desired product before calculating atom economy.
- Show coefficient × Mr for each stoichiometric term.
- Keep atom economy, yield, conversion and productivity labels distinct.
- Use matched boundaries and equal service/product quality in comparisons.
- For each improvement, explain a benefit, a possible trade-off and the evidence needed.
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