Process Synthesis — Cleaning Air of SO₂ and NO₂ with a Regenerable Mg(OH)₂ Scrubber: 23-Chem-A5 Chemical Plant Design and Economics, December 2017, Q1

Question 1: Process Synthesis — Cleaning Air of SO₂ and NO₂ with a Regenerable Mg(OH)₂ Scrubber (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. A polluted-air feed containing N₂, O₂, SO₂ and NO₂; a regenerable magnesium reagent; and the physical-property table below. The table is the design lever — it tells us which species are volatile at the calcination temperatures (so they leave as gas) and which magnesium salts are soluble (so a filter can separate them).

Species	Melting pt (°C)	Boiling pt (°C)	Solubility in water
N₂ / O₂	−210 / −218	−196 / −183	—
SO₂	−73	−10	— (gas)
NO₂	−11	21	— (gas)
H₂O	0	100	—
MgSO₃	200 (decomp.)	—	Insoluble
Mg(NO₂)₂	250 (decomp.)	—	Highly soluble
Mg(OH)₂	350 (decomp.)	—	Insoluble
MgO	2852	3600	Insoluble

Find. A closed-loop flowsheet (unit operations + streams) that delivers clean air overhead, aqueous SO₂ and aqueous NO₂ as separate products, and regenerates the magnesium reagent so the only net raw materials are make-up MgO, water and air.

Figure 1. Magnesium-oxide (MgO) regenerable wet-scrubbing flowsheet. The magnesium carrier circulates (green loop): it is consumed in the scrubber and rebuilt in the slaker, so the only net inputs are make-up MgO, water and air and the only net outputs are clean air, aqueous SO₂ and aqueous NO₂.

Approach. Exploit two property differences from the table — solubility (to split the insoluble sulfite/hydroxide solids from the soluble nitrite by filtration) and volatility (to release SO₂ and NO₂ as gases at their respective decomposition temperatures) — then close the magnesium loop by re-hydrating the MgO back to Mg(OH)₂.

The process is built from five operations, traced in Figure 1:

Scrubbing (absorption + reaction). Polluted air is bubbled through an aqueous suspension of excess Mg(OH)₂ in a bubble-column (or spray/packed) scrubber. The two acid gases are captured by $$\text{SO}_2 + \text{Mg(OH)}_2 \rightarrow \text{MgSO}_3 + \text{H}_2\text{O}, \qquad 2\text{NO}_2 + \text{Mg(OH)}_2 \rightarrow \text{Mg(NO}_2)_2 + \text{H}_2\text{O} + \tfrac12\text{O}_2.$$ The excess reagent guarantees complete SO₂/NO₂ removal, so only inert N₂ and O₂ leave overhead as clean air. The spent liquor leaving the bottom carries insoluble MgSO₃(s), unreacted insoluble Mg(OH)₂(s), and dissolved (highly soluble) Mg(NO₂)₂.
Filtration (solubility split). Because MgSO₃ and Mg(OH)₂ are insoluble while Mg(NO₂)₂ is highly soluble, a single filter cleanly separates the streams: the cake is MgSO₃ + excess Mg(OH)₂, and the filtrate is the Mg(NO₂)₂ solution. This one property difference does the hardest separation for free.
Solids calcination in two temperature stages (volatility split for SO₂). The cake is heated in stages so the two solids decompose separately. At 200 °C only the sulfite reacts, MgSO₃ → MgO + SO₂; since SO₂ boils at −10 °C it leaves as a gas and is absorbed in water to give the aqueous-SO₂ product (feed to a downstream H₂SO₄ plant). Raising the same solids to 350 °C decomposes the excess hydroxide, Mg(OH)₂ → MgO + H₂O, driving off water vapour and leaving pure MgO. Staging the temperatures keeps the SO₂ off-gas undiluted by the hydroxide's steam.
Nitrite decomposition (volatility split for NO₂). The filtrate is de-watered in an evaporator, then the dry Mg(NO₂)₂ is calcined at 250 °C under an excess-air (O₂) sweep — the reaction ½O₂ + Mg(NO₂)₂ → MgO + 2NO₂ consumes oxygen. NO₂ (boiling at 21 °C) leaves as a gas and is absorbed in water to give the aqueous-NO₂ product (feed to a downstream HNO₃ plant); solid MgO remains.
Regeneration (closing the magnesium loop). The MgO from both calciners is combined and slaked with water at 20 °C: MgO + H₂O → Mg(OH)₂. The regenerated Mg(OH)₂ suspension is recycled to the scrubber. Magnesium therefore acts as a circulating carrier — consumed in step 1, rebuilt in step 5 — so the only net raw materials are make-up MgO/water (to cover losses) and the sweep air, and the only net products are clean air plus the two acid-gas solutions.

Operation	Purpose / driving property	Key outlet(s)
Scrubber (bubble column)	Absorb + react SO₂, NO₂ into excess Mg(OH)₂	Clean air; spent liquor
Filter	Solubility: insoluble solids vs. soluble nitrite	Cake (MgSO₃, Mg(OH)₂); filtrate Mg(NO₂)₂
Solids calciner 200 → 350 °C	Volatility: release SO₂, then decompose Mg(OH)₂	SO₂ gas → SO₂(aq); MgO
Evaporator + nitrite calciner 250 °C	Volatility: release NO₂ under excess O₂	NO₂ gas → NO₂(aq); MgO
Slaker 20 °C	Regenerate reagent (close Mg loop)	Mg(OH)₂ recycle to scrubber

Check / assumptions Excess Mg(OH)₂ is assumed sufficient for complete SO₂/NO₂ capture (per the problem statement). SO₂ and NO₂ are collected as aqueous solutions (permitted). Heat integration is available but not required for the concept: the hot calciner off-gases and the warm clean air can preheat the evaporator and slaker feeds. Make-up MgO and water cover mechanical/solubility losses around the loop.

Concept & Theory

Topic. Process synthesis with a regenerable reagent — this is essentially the industrial magnesium-oxide (MgO) wet-scrubbing route for flue-gas desulfurization, extended here to co-capture NO₂.

Key relations. Reaction stoichiometry (capture, thermal decomposition, regeneration) combined with phase behaviour read from the property table: solubility governs the filtration split; boiling point governs which species leaves as gas at each calcination temperature.

Why this works. A good flowsheet chooses each separation to exploit the largest available property difference. Here the highly-soluble nitrite versus insoluble sulfite/hydroxide makes filtration trivial, and the −10 °C / 21 °C boiling points of SO₂/NO₂ mean that heating the dry salts past their decomposition temperatures liberates the acid gases cleanly. Recycling magnesium as MgO↔Mg(OH)₂ turns an otherwise reagent-hungry scrubber into a nearly closed loop.

Common pitfall. Forgetting that the excess Mg(OH)₂ also reports to the solids and must be decomposed/regenerated (skip it and magnesium is lost), or collecting SO₂ and NO₂ in a single step — the different decomposition temperatures are exactly what let you recover them separately.

Source. Turton et al., Analysis, Synthesis, and Design of Chemical Processes, Ch. 2 (process synthesis / input–output structure); Towler & Sinnott, Chemical Engineering Design, Ch. 4.

Question 1 of 6: Process Synthesis — Cleaning Air of SO₂ and NO₂ with a Regenerable Mg(OH)₂ Scrubber

Question 1: Process Synthesis — Cleaning Air of SO₂ and NO₂ with a Regenerable Mg(OH)₂ Scrubber (20 marks)