The water-gas shift reaction, CO + H2O -> CO2 + H2, is used in syngas processing chiefly to

adjust the H2:CO ratio to what the downstream process needs

remove all carbon from the gas stream

adjust the H2:CO ratio to what the downstream process needs

cool the syngas to ambient temperature

In the contact process for sulfuric acid, SO3 is absorbed into

concentrated sulfuric acid, forming oleum, which is then diluted

Ammonia synthesis (Haber-Bosch) operates at high temperature (400-500 C) despite the exothermic reaction being favoured by low temperature at equilibrium, because

the reaction rate over the catalyst would be impractically slow at low temperature

high temperature increases the equilibrium conversion further

the reaction rate over the catalyst would be impractically slow at low temperature

high pressure is only achievable at high temperature

urea cannot be made from low-temperature ammonia

Which of the following are true of urea manufacture?

It proceeds via an ammonium carbamate intermediate.

The carbamate-to-urea dehydration step is the slower, equilibrium-limiting step.

Its CO2 feedstock is very often supplied by an adjacent ammonia plant's own byproduct.

It proceeds via an ammonium carbamate intermediate.

The carbamate-to-urea dehydration step is the slower, equilibrium-limiting step.

Its CO2 feedstock is very often supplied by an adjacent ammonia plant's own byproduct.

It requires no ammonia at all as feedstock.

The kraft process for chemical pulping is economically viable at scale chiefly because

its cooking chemicals are recovered and regenerated through an evaporation-and-combustion cycle

mechanical pulping gives a stronger fibre at the same cost

Ordinary distillation of a dilute ethanol-water fermentation broth can concentrate ethanol up to approximately

the ethanol-water azeotrope, about 95-96% by volume, and no further

0%, since distillation cannot separate ethanol from water at all

Catalytic reforming of naphtha is primarily used to

rearrange its molecular structure to raise octane number without much change in carbon count

break heavy molecules into much lighter ones

rearrange its molecular structure to raise octane number without much change in carbon count

remove all sulfur from the feedstock

produce sulfuric acid as a byproduct

LDPE and HDPE, both made from ethylene, differ mainly in density and mechanical properties because of

the polymerisation route and catalyst used (high-pressure free-radical versus Ziegler-Natta/metallocene)

using entirely different monomers

the polymerisation route and catalyst used (high-pressure free-radical versus Ziegler-Natta/metallocene)

one being made from styrene and the other from ethylene

Inorganic, Fertilizer, Natural-Product and Petrochemical Industries

Section 10 of the GATE Chemical Engineering (CH) paper, and its last — where every earlier tool (mass and energy balance, thermodynamics, reaction engineering, separations) is applied to name real industries rather than generic unit operations. Start with the inorganic heavy-chemical industries built on syngas, sulfuric acid, phosphoric acid and chlor-alkali electrolysis; then fertilizers, ammonia and urea specifically; then two natural-product industries, pulp-and-paper and sugar-and-ethanol; and finally petroleum refining, petrochemicals (ethylbenzene, styrene, ethylene oxide) and the polymerization industries (polyethylene and polyester) built on their output.

1. Inorganic chemical industries: syngas, sulfuric acid, phosphoric acid, chlor-alkali

Synthesis gas (syngas, a CO/H2 mixture) is made by steam reforming natural gas (CH4 + H2O -> CO + 3H2, over a nickel catalyst) or by partial oxidation/gasification of heavier feedstocks including coal, and the water-gas shift reaction (CO + H2O -> CO2 + H2) is then used to adjust the H2:CO ratio to whatever the downstream process — ammonia synthesis, methanol synthesis, or Fischer-Tropsch — actually needs. Sulfuric acid is made almost universally by the contact process: sulfur (or sulfide ores) is burned to SO2, SO2 is catalytically oxidised to SO3 over vanadium pentoxide (V2O5), and SO3 is absorbed into concentrated (not dilute or water) sulfuric acid to form oleum, which is then diluted — SO3 is never absorbed directly into water, because the reaction is so exothermic and the resulting mist so fine that it cannot be efficiently captured that way.

Phosphoric acid: two different routes to the same product
Route	Raw material and chemistry	Product purity
Wet process	Phosphate rock dissolved with sulfuric acid, forming phosphoric acid and gypsum byproduct	Lower purity, but far cheaper — the dominant route for fertilizer-grade acid
Thermal (furnace) process	Elemental phosphorus burned to P2O5, then hydrated to H3PO4	Much higher purity, needed for food and electronic applications

The chlor-alkali industry electrolyses brine (concentrated NaCl solution) to produce chlorine, hydrogen and sodium hydroxide simultaneously — the membrane cell has now displaced the older mercury cell almost everywhere, because it avoids mercury entirely and needs less electrical energy per tonne of product, even though it demands a more carefully controlled, high-purity brine feed to protect its ion-exchange membrane.

2. Fertilizers: ammonia and urea

Ammonia is synthesised by the Haber-Bosch process, N2 + 3H2 <-> 2NH3, over an iron catalyst; because the reaction is exothermic and reduces the number of gas moles, equilibrium conversion is favoured by LOW temperature and HIGH pressure (Le Chatelier), but the reaction is far too slow at low temperature for the catalyst to give a usable rate — so the actual operating condition (typically 400-500 degrees C, 150-300 bar) is a deliberate compromise between equilibrium favourability and reaction rate, with unconverted gas recycled after the small per-pass conversion this compromise allows.

ℹ️ Urea consumes both ammonia and its own CO2 byproduct

Urea, CO(NH2)2, is made from ammonia and carbon dioxide in two steps: ammonia and CO2 first form ammonium carbamate (fast, exothermic), which then dehydrates to urea and water (slower, the equilibrium-limiting step, needing excess ammonia to push it forward). The CO2 used is very often the same CO2 that is a byproduct of the ammonia plant's own steam-reforming and shift reactions next door, which is exactly why ammonia and urea plants are almost always built together on one site rather than separately.

3. Natural-product industries: pulp and paper, sugar and ethanol

Pulping separates cellulose fibre from wood's lignin binder, either mechanically (grinding wood, high yield but weak, short fibres — newsprint grade) or chemically (the kraft process dominates: wood chips cooked in a hot sodium hydroxide/sodium sulfide solution dissolves out most of the lignin, giving stronger fibre at lower yield); the kraft mill then recovers and regenerates its own cooking chemicals through an evaporation-and-combustion recovery cycle, which is what makes it economically viable at scale rather than a one-pass chemical consumption.

Sugar manufacture from sugarcane extracts juice by crushing (milling) or diffusion, clarifies it (removing non-sugar impurities), then concentrates and crystallises sucrose through multiple-effect evaporation followed by vacuum-pan boiling; ethanol production from the same feedstock (or from cane molasses, the sugar mill's own byproduct) ferments sugars to ethanol using yeast, then separates the dilute fermentation broth (typically 8-12% ethanol) by distillation up to the ethanol-water azeotrope (about 95-96% by volume), beyond which ordinary distillation cannot go and a separate dehydration step (molecular sieves, or azeotropic/extractive distillation) is needed for fuel-grade anhydrous ethanol.

4. Petroleum refining, petrochemicals and polymerization industries

Crude oil refining begins with atmospheric distillation (separating crude into fractions by boiling range: gases, naphtha, kerosene, diesel, atmospheric residue) and vacuum distillation of that residue (avoiding thermal cracking that atmospheric-pressure temperatures would otherwise cause); conversion units then reshape less valuable fractions into more valuable ones — catalytic cracking breaks heavy molecules into lighter, more valuable ones (mainly gasoline range), catalytic reforming rearranges naphtha's molecular structure to raise octane number without changing its carbon count much, and hydrocracking combines cracking with hydrogen addition for a cleaner, more saturated product slate.

Ethylbenzene is made by alkylating benzene with ethylene (Friedel-Crafts type, over a zeolite catalyst in modern plants); it is then dehydrogenated to styrene, the monomer for polystyrene and styrene-butadiene rubber — nearly all styrene manufactured worldwide follows exactly this two-step ethylbenzene route.
Ethylene oxide is made by direct catalytic (silver-catalysed) oxidation of ethylene with oxygen or air, and is itself the feedstock for ethylene glycol (via hydrolysis, the monomer route into polyester) and a wide range of surfactants and solvents — one epoxide feeding several entirely separate downstream chains.
Polyethylene is made by polymerising ethylene: high-pressure free-radical polymerisation gives low-density polyethylene (LDPE, branched, flexible film), while Ziegler-Natta or metallocene catalysis at lower pressure gives high-density polyethylene (HDPE, linear, rigid, used for bottles and pipe) — the SAME monomer, and the polymerisation route and catalyst chosen, not the monomer, is what sets the resulting polymer's density, branching and mechanical properties. Polyester (PET) is made by condensation polymerisation of ethylene glycol with either terephthalic acid or dimethyl terephthalate, releasing water (or methanol) as each ester linkage forms — a fundamentally different (step-growth, not chain-growth) polymerisation mechanism from polyethylene's.

Key takeaways

The water-gas shift reaction adjusts syngas's H2:CO ratio to whatever the downstream process needs; the contact process for sulfuric acid absorbs SO3 into concentrated acid, never directly into water.
Ammonia synthesis operates at a deliberate compromise temperature and pressure — not the equilibrium-optimal low temperature — because reaction rate over the catalyst would be impractically slow there; urea then consumes ammonia and the ammonia plant's own byproduct CO2.
A kraft pulp mill recovers and regenerates its cooking chemicals through an evaporation-and-combustion cycle, which is what makes chemical pulping economically viable at scale.
Ordinary distillation cannot separate ethanol beyond its azeotrope with water (about 95-96%); fuel-grade anhydrous ethanol needs a separate dehydration step such as molecular sieves.
LDPE and HDPE are made from the same ethylene monomer; it is the polymerisation route and catalyst (free-radical/high-pressure versus Ziegler-Natta/lower-pressure), not the monomer, that sets the resulting polymer's density and properties.

Practice questions (9)

Attempt each one before opening the answer. Every explanation names the tempting wrong option as well as the right one, because that is where marks are lost.

The water-gas shift reaction, CO + H2O -> CO2 + H2, is used in syngas processing chiefly to
1. remove all carbon from the gas stream
2. adjust the H2:CO ratio to what the downstream process needs
3. produce sulfuric acid directly
4. cool the syngas to ambient temperature
Show answer
Answer: B — adjust the H2:CO ratio to what the downstream process needs
Different downstream syntheses need different H2:CO ratios (ammonia synthesis needs pure hydrogen with nitrogen, methanol synthesis needs roughly 2:1 H2:CO, Fischer-Tropsch its own ratio), and the water-gas shift reaction is exactly the tool used to convert some CO to CO2 (easily removed) plus extra H2, tuning the ratio to the target process.
In the contact process for sulfuric acid, SO3 is absorbed into
1. pure water directly
2. concentrated sulfuric acid, forming oleum, which is then diluted
3. liquid ammonia
4. molten sulfur
Show answer
Answer: B — concentrated sulfuric acid, forming oleum, which is then diluted
Absorbing SO3 directly into water is avoided because the reaction is so exothermic and produces such a fine, hard-to-capture acid mist; absorbing it into already-concentrated acid instead forms oleum (fuming sulfuric acid), which is then diluted to the desired final concentration in a controlled way.
Ammonia synthesis (Haber-Bosch) operates at high temperature (400-500 C) despite the exothermic reaction being favoured by low temperature at equilibrium, because
1. high temperature increases the equilibrium conversion further
2. the reaction rate over the catalyst would be impractically slow at low temperature
3. high pressure is only achievable at high temperature
4. urea cannot be made from low-temperature ammonia
Show answer
Answer: B — the reaction rate over the catalyst would be impractically slow at low temperature
Low temperature does favour higher equilibrium conversion for this exothermic reaction, but the iron catalyst's activity is too low at low temperature to give a commercially useful rate — the actual operating point is a deliberate rate-versus-equilibrium compromise, with unconverted gas recycled to compensate for the resulting lower per-pass conversion.
Which of the following are true of urea manufacture?
1. It proceeds via an ammonium carbamate intermediate.
2. The carbamate-to-urea dehydration step is the slower, equilibrium-limiting step.
3. Its CO2 feedstock is very often supplied by an adjacent ammonia plant's own byproduct.
4. It requires no ammonia at all as feedstock.
Show answer
Answer: A — It proceeds via an ammonium carbamate intermediate.; B — The carbamate-to-urea dehydration step is the slower, equilibrium-limiting step.; C — Its CO2 feedstock is very often supplied by an adjacent ammonia plant's own byproduct.
The fourth statement is false: ammonia is one of urea's two feedstocks (along with CO2), reacting first to form carbamate and again in excess to push the slower dehydration step toward urea. The first three correctly describe the two-step mechanism and the reason ammonia and urea plants are almost always co-located.
The kraft process for chemical pulping is economically viable at scale chiefly because
1. it uses no chemicals at all
2. its cooking chemicals are recovered and regenerated through an evaporation-and-combustion cycle
3. mechanical pulping gives a stronger fibre at the same cost
4. it consumes no wood
Show answer
Answer: B — its cooking chemicals are recovered and regenerated through an evaporation-and-combustion cycle
The kraft process's sodium hydroxide/sodium sulfide cooking liquor is expensive, so the mill recovers spent liquor, evaporates and burns it to regenerate the active chemicals and generate steam and power — without this recovery cycle, one-pass chemical consumption would make the process uneconomical.
Ordinary distillation of a dilute ethanol-water fermentation broth can concentrate ethanol up to approximately
1. 100% (pure anhydrous ethanol)
2. the ethanol-water azeotrope, about 95-96% by volume, and no further
3. only up to 50% by volume
4. 0%, since distillation cannot separate ethanol from water at all
Show answer
Answer: B — the ethanol-water azeotrope, about 95-96% by volume, and no further
Ethanol and water form a minimum-boiling azeotrope at about 95-96% ethanol by volume, at which vapour and liquid compositions become identical, so ordinary distillation cannot cross that composition regardless of how many stages are added — a separate step (molecular sieves, or azeotropic/extractive distillation with a third component) is needed to reach fuel-grade anhydrous ethanol.
Catalytic reforming of naphtha is primarily used to
1. break heavy molecules into much lighter ones
2. rearrange its molecular structure to raise octane number without much change in carbon count
3. remove all sulfur from the feedstock
4. produce sulfuric acid as a byproduct
Show answer
Answer: B — rearrange its molecular structure to raise octane number without much change in carbon count
Reforming isomerises and aromatises naphtha's straight-chain and naphthenic molecules into higher-octane branched and aromatic ones with roughly the same carbon number — it is cracking, not reforming, that breaks heavy molecules into lighter ones, and hydrotreating, not reforming, that is the dedicated sulfur-removal step.
LDPE and HDPE, both made from ethylene, differ mainly in density and mechanical properties because of
1. using entirely different monomers
2. the polymerisation route and catalyst used (high-pressure free-radical versus Ziegler-Natta/metallocene)
3. one being made from styrene and the other from ethylene
4. differences in reactor colour
Show answer
Answer: B — the polymerisation route and catalyst used (high-pressure free-radical versus Ziegler-Natta/metallocene)
Both LDPE and HDPE start from the same ethylene monomer; it is the choice of polymerisation mechanism and catalyst — high-pressure free-radical polymerisation producing a branched chain (LDPE) versus lower-pressure Ziegler-Natta or metallocene catalysis producing a linear chain (HDPE) — that sets the resulting polymer's density, crystallinity and mechanical behaviour.
Styrene, the monomer for polystyrene, is manufactured commercially by dehydrogenating which precursor made from benzene and ethylene?
Numerical answer — type the value.
Show answer
Answer: ethylbenzene
Benzene is alkylated with ethylene to give ethylbenzene, which is then catalytically dehydrogenated to styrene — this two-step ethylbenzene route is how nearly all commercial styrene is made worldwide.