Question 1 of 6: Flowsheet Synthesis for the Solvay (Ammonia-Soda) Production of Na 2 CO 3

Question 1: Flowsheet Synthesis for the Solvay (Ammonia-Soda) Production of Na₂CO₃ (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Given. Five stoichiometric reactions and a set of physical-property facts (solubilities and states) that must dictate where each separation goes. Raw materials are limestone (CaCO₃) and salt (NaCl); the target is Na₂CO₃, and CaCl₂ is the only unavoidable by-product of the overall reaction.

Find. A labelled process flowsheet with the components identified in every stream, arranged so that ammonia and CO₂ are recovered and recycled (minimizing reactant purchase) and only the terminal by-product leaves as waste.

Approach

Let the properties place each unit: a solid/liquid property difference (slightly-soluble NaHCO₃ vs. everything else dissolved) calls for a filter; a thermal-stability difference (ionic solids decompose on heating) calls for calciners; and every species that is neither raw material nor product (NH₃, CO₂) must be closed into a recycle loop so it is regenerated rather than repurchased.

The synthesis logic, unit by unit:

1. Lime kiln (decomposition, 1000 °C). Calcining limestone serves two purposes at once: it makes the CaO that will later regenerate ammonia, and it makes the CO₂ that carbonation consumes. Because ionic solids decompose on heating (fact g), a high-temperature kiln is the natural unit. Output streams: CaO (solid, to the slaker) and CO₂ (gas, to carbonation).
2. Slaker. Quenching the hot lime with water, CaO + H₂O → Ca(OH)₂, produces milk-of-lime. This is the reagent that will strip ammonia back out of the spent NH₄Cl solution. Stream: Ca(OH)₂ slurry to ammonia recovery.
3. Carbonation tower. Saturated NaCl brine is contacted with ammonia and the kiln CO₂: NH₄OH + CO₂ + NaCl → NH₄Cl + NaHCO₃. Because NaHCO₃ is only slightly soluble (fact d) it precipitates, while NH₄Cl stays dissolved (fact c). The solubility gap is what makes the whole process work: it separates the sodium from the ammonia without any added reagent.
4. Filter. A simple solid/liquid split isolates the precipitated NaHCO₃ crystals from the NH₄Cl mother liquor. Solid NaHCO₃ goes forward to calcination; the NH₄Cl(aq) is sent to ammonia recovery.
5. Soda calciner (calcination, 300 °C). Heating the bicarbonate decomposes it: 2NaHCO₃ → Na₂CO₃ + CO₂ + H₂O. Solid Na₂CO₃ is the product; the released CO₂ is recycled to carbonation, cutting the CO₂ the kiln must supply.
6. Ammonia recovery. Milk-of-lime displaces ammonia from the spent liquor: Ca(OH)₂ + 2NH₄Cl → 2NH₄OH + CaCl₂. The regenerated NH₄OH returns to carbonation (closing the ammonia loop), and CaCl₂ — which cannot be recycled because it will not react with CO₂ (fact h) — leaves as the terminal waste.

Collecting the components carried by each stream:

Summing the recycles collapses the plant to exactly the stated overall reaction, CaCO₃ + 2NaCl → Na₂CO₃ + CaCl₂: the ammonia is neither consumed nor purchased after start-up, and the CO₂ balance closes because carbonation needs 2 mol CO₂ per 2 NaHCO₃ while the kiln (1 mol) plus the calciner (1 mol) together supply exactly 2. Only limestone and salt are bought, and only CaCl₂ is discharged — the “minimum waste and reactants” the question demands.