Example 1
A process theoretically makes 250kg product but isolates 200kg. Find percentage yield.
Yield =250200×100=80%. The calculation compares isolated product with the stoichiometric maximum.
Year 12 · SACE Stage 2
Industrial conditions, flow charts and yield
Practise rate–yield compromises and material-flow calculations.
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Method and worked examples
An industrial process balances useful production rate, equilibrium yield, separation, safety and operating cost. For exothermic ammonia synthesis, lower temperature favours equilibrium yield but slows reaction. A moderate temperature with a catalyst gives a useful compromise. Higher pressure favours ammonia because fewer gas moles form, but compression and stronger equipment cost energy and money.
A process flow chart distinguishes fresh feed, reaction, product separation, recycling and purge streams. Recycling unreacted feed can increase overall conversion without changing single-pass equilibrium. Percentage yield is actual product divided by theoretical product times 100. Catalysts and heat recovery can reduce energy demand. These benefits do not remove the need to account for by-products, waste and the source of raw materials.
Example 1
Example 2
A reactor converts 30 mol of a 100 mol feed. All 70 mol unreacted feed is recycled; 30% of it reacts on its next pass. Find total reacted after two passes.
Second pass reacts 0.30×70=21 mol. Total reacted =30+21=51 mol. Recycling improves overall conversion without requiring a changed equilibrium constant.
Watch out: An extremely slow high-yield reaction may produce less per hour; practical conditions balance yield, rate, separation and cost.