Industrial gases and coal-based chemicals
Cryogenic, PSA and membrane air separation, hydrogen and CO2, coal carbonisation, gasification reactions and gasifier types, and syngas to methanol and liquid fuels.
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Why it matters
Oxygen, nitrogen, argon, hydrogen and carbon dioxide are consumed in huge volumes by steel plants, refineries, fertiliser plants, hospitals and electronics. India also has large coal reserves and a national push for coal gasification to make methanol, ammonia and fuels, so coal carbonisation and gasification are again live industrial topics. Both areas rest on thermodynamics: low-temperature distillation and adsorption for gases, and endothermic–exothermic heat balancing for gasification.
Key ideas
Air separation. Dry air is about 78.08 % N₂, 20.95 % O₂ and 0.93 % Ar by volume.
- Cryogenic distillation (large plants): air is filtered, compressed to about 5–6 bar, cooled, and purified of H₂O and CO₂ in molecular-sieve adsorbers (they would freeze). It is cooled to near its dew point in brazed aluminium exchangers against product streams, and refrigeration is supplied by expansion in a turbo-expander. The Linde double column has a high-pressure column (about 5–6 bar) and a low-pressure column (about 1.2–1.5 bar) thermally linked by a condenser–reboiler: N₂ condensing at high pressure boils O₂ at low pressure. Normal boiling points: N₂ 77.4 K, Ar 87.3 K, O₂ 90.2 K, so N₂ goes overhead, O₂ collects at the bottom, and a side-arm column recovers crude argon from the argon-rich middle of the LP column. Products can be liquid or gas, high purity (O₂ above 99.5 %).
- Pressure swing adsorption (PSA/VPSA): zeolites (13X, LiX) adsorb N₂ more strongly than O₂, giving 90–95 % O₂ (medical oxygen concentrators); carbon molecular sieves adsorb O₂ faster (kinetic selectivity), giving N₂. Beds alternate between adsorption at high pressure and regeneration at low pressure.
- Membranes: polymer hollow fibres in which O₂ permeates faster give 95–99 % N₂ for blanketing.
Hydrogen. Mainly from steam methane reforming (CH₄ + H₂O ⇌ CO + 3H₂, then shift), also from coal or residue gasification, refinery off-gases and water electrolysis ("green" hydrogen). PSA purifies it to 99.9 %+.
Carbon dioxide is recovered from ammonia plant CO₂ removal, fermentation and flue gas; purified, liquefied and used for urea, beverages, dry ice and welding. Acetylene is made by CaC₂ + 2H₂O → C₂H₂ + Ca(OH)₂ (calcium carbide made from lime and coke in an electric furnace) or by hydrocarbon pyrolysis. Argon shields welds; helium is from natural gas.
Coal. Characterised by proximate analysis (moisture, volatile matter, ash, fixed carbon) and ultimate analysis (C, H, N, S, O). Indian coals are typically high in ash (often 30–45 %) and low in sulphur, which affects gasifier choice.
Carbonisation.
- High-temperature carbonisation in by-product coke ovens at about 1000–1100 °C for 15–20 h gives metallurgical coke for blast furnaces, plus coke-oven gas (rich in H₂ and CH₄, used as fuel), coal tar (source of naphthalene, anthracene, phenols, creosote, pitch), ammonia (recovered as ammonium sulphate) and crude benzol (BTX).
- Low-temperature carbonisation at about 500–700 °C gives a reactive smokeless fuel (semi-coke) and more tar.
Gasification converts carbon to a combustible gas with steam, oxygen or air at about 800–1500 °C:
- C + H₂O → CO + H₂ (water-gas reaction, strongly endothermic)
- C + ½O₂ → CO and C + O₂ → CO₂ (exothermic; supply the heat)
- C + CO₂ → 2CO (Boudouard, endothermic)
- CO + H₂O ⇌ CO₂ + H₂ (shift, mildly exothermic)
- C + 2H₂ → CH₄ (methanation, exothermic; favoured at high pressure and lower temperature) Gas types: producer gas (air + steam; about 50 % N₂, low calorific value), water gas (steam on hot coke in alternating air "blow" and steam "run" cycles; mostly CO + H₂), and synthesis gas from oxygen-blown gasifiers. Gasifier types: moving (fixed) bed (Lurgi; handles high-ash coal, makes some CH₄ and tar), fluidised bed (Winkler; suits reactive, high-ash coals) and entrained flow (GE/Texaco, Shell; above 1300 °C, slagging, tar-free gas).
Syngas to chemicals and fuels. After shift and acid-gas removal (Rectisol or Selexol): methanol (CO + 2H₂ → CH₃OH over Cu/ZnO/Al₂O₃ at about 50–100 bar and 220–280 °C), ammonia (shift to H₂ + N₂), and Fischer–Tropsch liquids (Fe or Co catalysts; SASOL in South Africa). Direct liquefaction (Bergius) hydrogenates coal with a solvent at high pressure.
Formulas
C + H₂O → CO + H₂, ΔH ≈ +131 kJ/mol
C + O₂ → CO₂, ΔH ≈ −394 kJ/mol
C + ½O₂ → CO, ΔH ≈ −111 kJ/mol
C + CO₂ → 2CO, ΔH ≈ +172 kJ/mol
CO + H₂O ⇌ CO₂ + H₂, ΔH ≈ −41 kJ/mol
(heats at 25 °C; use data-book values at operating temperature for design)
Thermoneutral ratio (carbon burnt to CO₂ per carbon gasified by steam) = 131 / 394
Gas volume at 0 °C, 1 atm = n × 22.414 m³/kmol
Product recovery = component in product / component in feed
Worked examples
Example 1 (standard): oxygen from an air separation unit. 1000 kmol/h of dry air (20.95 % O₂) is processed; 95 % of the oxygen is recovered as 99.5 % pure O₂. Find the product flow in kmol/h and Nm³/h.
- O₂ in feed = 0.2095 × 1000 = 209.5 kmol/h.
- O₂ recovered = 0.95 × 209.5 = 199.0 kmol/h.
- Product flow = 199.0 / 0.995 = 200.0 kmol/h.
- Volume = 200.0 × 22.414 = 4483 Nm³/h.
Answer: about 200 kmol/h, i.e. about 4480 Nm³/h of 99.5 % O₂.
Example 2 (GATE level): thermoneutral water-gas generation. Coke (pure carbon) is gasified with steam; the heat for the endothermic water-gas reaction comes from burning part of the carbon completely to CO₂ with oxygen. Neglecting heat losses and sensible heats, find the fraction of carbon gasified by steam and the volume of (CO + H₂) produced per kg of carbon.
- Carbon burnt per mol gasified = 131 / 394 = 0.3325 mol.
- Fraction gasified = 1 / (1 + 0.3325) = 0.7505.
- Total carbon = 1 / 12.011 = 0.08326 kmol; gasified = 0.7505 × 0.08326 = 0.06248 kmol.
- CO + H₂ = 2 × 0.06248 = 0.1250 kmol.
- Volume = 0.1250 × 22.414 = 2.80 Nm³.
Answer: about 75 % of the carbon is gasified; about 2.8 Nm³ of CO + H₂ per kg carbon (real gasifiers get less because of heat losses).
Common mistakes
- Putting oxygen at the top of the air-separation column; it is the least volatile of the three and collects at the bottom.
- Forgetting to remove H₂O and CO₂ before the cold box; they freeze and block exchangers.
- Treating the water-gas reaction as exothermic; it needs a heat source.
- Counting only 1 mol H₂ per mol C when the CO is also shifted; full shift gives 2 mol H₂ per mol C.
- Confusing producer gas (N₂-rich, low CV) with water gas (CO + H₂, higher CV).
For GATE CH
Expect questions on boiling points and the Linde double column, PSA adsorbents, coke-oven by-products, gasification reactions and their heat effects, gasifier types, and syngas uses (methanol, ammonia, Fischer–Tropsch). Numericals: air-separation recoveries, gasification stoichiometry and heat balances, hydrogen yield after shift, and normal-volume conversions.
Quick check
- Which component of air is the most volatile?
- What are the main by-products of a coke oven?
- Why is oxygen rather than air used in syngas gasifiers?
- What catalyst is used for methanol synthesis?
Answers: 1. nitrogen (77.4 K); 2. coke-oven gas, tar, ammonia and crude benzol; 3. to avoid diluting the syngas with nitrogen; 4. Cu/ZnO/Al₂O₃.
Interview questions
All Chemical Technology interview questionsTry answering each one aloud before you open it.
1.What are industrial gases, and why are they important in chemical engineering?Concept
Industrial gases are gaseous materials that are manufactured for use in various industrial processes. They include gases like oxygen, nitrogen, hydrogen, carbon dioxide, and argon. These gases are crucial in chemical engineering because they are used in processes such as combustion, oxidation, and as protective atmospheres in manufacturing. They also play a role in refrigeration, water treatment, and as raw materials for chemical synthesis.
2.Explain the process of coal gasification and its significance.Concept
Coal gasification is a process that converts coal into syngas, a mixture of hydrogen, carbon monoxide, and carbon dioxide. This is achieved by reacting coal with oxygen and steam at high temperatures. The significance of coal gasification lies in its ability to produce cleaner energy from coal, as it allows for the removal of impurities before combustion. It also provides a versatile feedstock for producing chemicals, fertilizers, and fuels.
3.How is hydrogen produced from coal, and what are its applications?Concept
Hydrogen can be produced from coal through a process called coal gasification, where coal is reacted with steam and oxygen to produce syngas. The syngas is then subjected to a water-gas shift reaction to increase the hydrogen content. Hydrogen produced from coal is used in refining processes, ammonia production, and as a clean fuel in fuel cells and hydrogen-powered vehicles.
4.Why is nitrogen used in the food packaging industry?Application
Nitrogen is used in the food packaging industry because it is an inert gas that helps to displace oxygen, which can cause spoilage and oxidation of food products. By replacing oxygen with nitrogen, the shelf life of food products is extended, and the growth of aerobic bacteria and fungi is inhibited. This helps in maintaining the quality and freshness of packaged foods.
5.What happens if carbon dioxide is not removed from syngas during the production of chemicals?Application
It depends on the product. For ammonia, CO2 (and CO) must be removed almost completely: oxygen compounds poison the iron synthesis catalyst and CO2 would react with ammonia to form carbamate that blocks the loop, so plants use CO2 absorption followed by methanation. For methanol some CO2 is actually wanted as a reactant, but too much lowers the stoichiometric number and adds inert load. In all cases excess CO2 dilutes the gas, raising compression and recycle costs.
6.Explain the role of oxygen in steel manufacturing.Application
Oxygen plays a critical role in steel manufacturing, particularly in the basic oxygen steelmaking (BOS) process. In this process, oxygen is blown into molten iron to oxidize impurities such as carbon, silicon, and phosphorus. This oxidation process generates heat, which helps to maintain the temperature of the molten metal and facilitates the removal of impurities as gases or slag. The result is a purer form of steel with desired properties.
7.Calculate the volume of hydrogen gas produced at STP from 1 kg of coal, assuming complete gasification and a yield of 0.04 kg of hydrogen per kg of coal.Numerical
To calculate the volume of hydrogen gas produced at STP, we first determine the mass of hydrogen produced: 1 kg of coal × 0.04 kg H₂/kg coal = 0.04 kg H₂. The molar mass of hydrogen is approximately 2 g/mol, so the number of moles of hydrogen is 0.04 kg × 1000 g/kg / 2 g/mol = 20 mol. At STP, 1 mole of gas occupies 22.4 L, so the volume of hydrogen gas is 20 mol × 22.4 L/mol = 448 L.
8.What are the environmental impacts of coal-based chemical production, and how can they be mitigated?Application
Coal-based chemical production can have several environmental impacts, including air pollution from the release of CO2, SO2, and NOx, water pollution from effluents, and land degradation from mining activities. These impacts can be mitigated by implementing cleaner technologies such as carbon capture and storage (CCS), using scrubbers to remove pollutants from emissions, treating wastewater before discharge, and adopting sustainable mining practices.
9.Why is argon used in the welding industry?Application
Argon is used in the welding industry as a shielding gas to protect the weld area from atmospheric gases such as oxygen, nitrogen, and water vapor. These gases can cause defects in the weld, such as porosity and oxidation. Argon, being inert, provides a stable and non-reactive environment, ensuring a clean and high-quality weld. It is particularly useful in welding non-ferrous metals and stainless steel.
10.Determine the amount of CO2 produced from the complete combustion of 1 kg of coal, assuming coal is pure carbon.Numerical
C + O2 -> CO2, so 1 mol C gives 1 mol CO2. Moles of carbon = 1000 g / 12.01 g/mol = 83.3 mol, and CO2 = 83.3 mol x 44.01 g/mol = 3664 g, i.e. about 3.66 kg (the ratio 44/12 = 3.67). Real coal contains ash, moisture, H and O, so CO2 per kg of coal is lower, roughly in proportion to its carbon content.
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