Heavy inorganics: salt, sulphur, nitrogen and phosphorus: Industrial gases, and the heavy inorganics side by side
Understanding Chemicals / Module 4: Heavy inorganics: salt, sulphur, nitrogen and phosphorus Part 5 of 5Industrial gases, and the heavy inorganics side by side
Industrial gases are made from air or hydrocarbons and sold by pipeline, as liquid or in cylinders. Their cost is mostly electricity and capital, and plants sit beside their customers. Hydrogen and carbon dioxide tie this industry back to ammonia. The part ends by comparing every product on tonnage, energy and use.
5.1 Air separation is distillation far below zero, paid for almost entirely in electricity
Air is 78% nitrogen, 21% oxygen and 0.93% argon. Cryogenic air separation, commercial since 1902, compresses air, removes water and CO2, cools it against the outgoing products and distils it (Kent). Shreve describes air compressed to about 520 kPa feeding a double column. The lower column runs at about 580-650 kPa and the upper at 120-180 kPa, linked by a condenser-reboiler. Nitrogen condensing at the higher pressure is warmer than oxygen boiling at the lower one. So one exchanger condenses the lower column and boils the upper. An expansion turbine supplies the refrigeration.
Argon boils between nitrogen and oxygen, so a side stream of about 10% argon is drawn into a crude argon column and raised to 95% (Kent). Argon is the most valuable air gas, used to shield welds, refine stainless steel and grow silicon crystals.
Energy is the main running cost. Dryden gives 450-480 kWh of electricity, 3,600 Nm3 of air and 1.75 t of steam per tonne of 95% oxygen in a 300 t/day plant. Nitrogen gas comes off the same columns; it became the largest-volume US industrial gas in 1981 (Shreve). Cryogenic plants range from under 1 t/day to over 1,800 t/day (Shreve). For smaller needs, pressure swing adsorption (PSA), which traps one gas on a solid adsorbent under pressure and releases it when the pressure drops, makes 90-95% oxygen at 1-60 t/day.
Transport is so costly that big plants are built on the customer's land or feed pipelines, and oxygen behaves like a utility (Shreve). Steel is the anchor customer. In the US 55% of oxygen went to primary metals (Dryden), and an oxygen steel converter uses 0.07-0.08 t of oxygen per tonne of steel. Dryden reports Praxair building the world's largest air separation plant for Jindal's Karnataka steelworks in the mid-1990s.
to about 520 kPa
2 Front-end clean-upreversing exchangers or molecular sieves remove H2O and CO2; adsorbers catch hydrocarbons
3 Main heat exchangerair cooled against outgoing products
4 Expansion turbinepart of the air expanded to supply refrigeration
5 High-pressure columnabout 580-650 kPa; nitrogen at top, oxygen-rich liquid at bottom
6 Low-pressure column120-180 kPa; 99.5% oxygen at bottom, nitrogen at top
7 Crude argon columnside draw of about 10% Ar raised to 95%
Outputs Oxygen1 t (gas or liquid) Nitrogenproduct gas and liquid Crude argonfor purification Waste nitrogenvented Source: Dryden (1997), Ch IIC Cryogenics; Shreve (1984), Ch 7 Industrial Gases; Kent (2012), Ch 27 Industrial Gases, Dryden pdf pp. 113-117; Shreve pdf pp. 135-138; Kent pdf pp. 1043, 1048-1049- Kent (2012), Ch 27 Industrial Gases · pdf pp. 1043-1048
- Shreve (1984), Ch 7 Industrial Gases · pdf pp. 133-138
- Dryden (1997), Ch IIC Cryogenics · pdf pp. 112-118
5.2 Hydrogen and carbon dioxide are mostly made and used inside the ammonia and refining chain
World hydrogen use was about 42 million tonnes a year in the figures Kent cites (from 2003). About 60% went to ammonia, 23% to oil refining and 9% to methanol. By origin, 48% came from natural gas, 30% from oil, 18% from coal and 4% from electrolysis. In the US, of more than 9 Mt a year, 7.5 Mt was used where it was made and 1.5 Mt sold as merchant gas, increasingly to refiners meeting the 15 ppm sulphur limit for diesel (Kent).
Merchant hydrogen plants use the ammonia front end without the air. PSA has become the standard final step, giving 99.9% or purer hydrogen (Kent). Gas is the main cost. Kent cites estimates that hydrogen costs, before capital charges, about 150% of the natural gas price per unit of energy.
Electrolysis gives very pure hydrogen but needs a lot of electricity. Kent puts the theoretical energy at about 32.9 kWh per kg at 1.23 V. Commercial cells at about 1.75 V and 1 A/cm2 use about 46.8 kWh per kg, an efficiency of 70%. A tonne of hydrogen therefore needs about 46,800 kWh, twenty times the electricity in a tonne of chlorine. Kent, writing in 2012, saw electrolysis as sensible mainly where power is cheap, with wind the leading candidate.
Carbon dioxide is mostly a by-product. The main commercial sources are the CO2 scrubbed from ammonia and hydrogen plants, fermentation and natural wells (Shreve, Kent). In the US about 25 Mt a year of CO2 gas went mainly to enhanced oil recovery and urea, and 7.5 Mt as liquid and solid to food and drink (Kent). Shreve's 1980s split of US CO2 use was 40% urea and 35% oil wells.
| Item | Share or value | Note |
|---|---|---|
| From natural gas | 48% | 240 billion Nm3 a year |
| From oil | 30% | 150 billion Nm3 |
| From coal | 18% | 90 billion Nm3 |
| From electrolysis | 4% | 20 billion Nm3 |
| Used for ammonia | about 60% | of about 42 Mt a year |
| Used in oil refining | about 23% | desulphurisation, hydrocracking |
| Used for methanol | about 9% | |
| Electrolysis, theoretical | 32.9 kWh per kg | at 1.23 V |
| Electrolysis, commercial | 46.8 kWh per kg | at 1.75 V; 70% efficiency |
- Kent (2012), Ch 27 Industrial Gases · pdf pp. 1044-1055
- Kent (2012), Ch 22 Synthetic Nitrogen Products · pdf pp. 892, 898
- Shreve (1984), Ch 7 Industrial Gases · pdf pp. 120-121
5.3 Side by side: what makes each heavy inorganic cheap or dear
The table gathers the module's numbers with their years. Read it for patterns, not as a snapshot.
First, energy runs in both directions. Sulphuric acid and nitric acid plants are net exporters of energy, because their reactions release more heat than the process needs. Chlorine, caustic soda, oxygen and electrolytic hydrogen are bought with electricity, so the price and reliability of power decide where they are made, as the Indian caustic case shows. Ammonia sits in between: very large energy use, almost all of it as natural gas feed and fuel, so the gas price decides where it is made.
Second, raw materials and transport often matter more than process efficiency. Dryden notes that a tonne of Solvay soda ash needs about eight tonnes of brine, limestone and coal, so plants are built at the salt, and trona beats them with less than half the energy. Phosphate rock is cheap to mine but costly to ship relative to its nutrient content, so SSP was made near farms while TSP and DAP are made near mines or ports. Sulphur travels well and acid does not, so acid is made where it is used.
Third, by-products set limits. Each tonne of chlorine brings 1.1 t of caustic soda. Each tonne of P2O5 brings about 5 t of gypsum. Each tonne of Solvay ash brings about a tonne of calcium chloride. A plant's economics often depend on what it can do with its second product.
For India the sources keep returning to the same pressures: imported sulphur and rock, dear and unreliable power for chlor-alkali, and ammonia from naphtha, oil and coal where gas was lacking.
| Product | Tonnage (year) | Energy per tonne (year) | Main uses |
|---|---|---|---|
| Sulphuric acid | 191.3 Mt world (2008) | net exporter: about 5.4 GJ heat released, over 1.3 t steam exported (1984-2012) | phosphate fertiliser (74% of US use, 2008), refining, leaching, chemicals |
| Chlorine | 42.3 Mt world (2008) | about 2,900 kWh electricity (1997); about 2,300 kWh in a modern membrane cell (arithmetic) | EDC and PVC (43% of US use), other organics, water treatment |
| Caustic soda | 45.0 Mt world (2008) | shares the cell power; diaphragm caustic needs 2,600 kg water evaporated per t of 50% (1984) | chemicals, pulp and paper, alumina, soaps, textiles |
| Soda ash | 34.3 Mt synthetic + 11.7 Mt natural (2010) | Solvay 18.4 GJ; natural 8.4 GJ (1984) | glass (46% of US use, 2010), chemicals, detergents |
| Ammonia | 135.8 Mt N capacity, about 165 Mt NH3 (2005) | best plants about 27 GJ, world average 40-45 GJ (about 2010); 55.9 GJ (1950) | about 85% fertiliser; nitric acid, urea, acrylonitrile |
| Nitric acid | 53 Mt world (1999) | net steam exporter: 0.51-0.60 t HP steam; 0.28-0.29 t NH3 (2012) | ammonium nitrate (about 65%), adipic acid, nitrobenzene |
| Urea | 23.0 Mt N, about 49 Mt urea (1983/84) | about 145 kWh plus 2.4 t steam (1997); cash cost 1.4-1.6 x ammonia price (2012) | fertiliser, resins, melamine, feed |
| Ammonium nitrate | 7.1 Mt in the US (2001) | reaction heat evaporates water; 0.22 t NH3 per t (1997) | fertiliser (CAN), mining explosives |
| Phosphoric acid | 32.6 Mt P2O5 world (2008) | wet process mainly evaporation steam (hemihydrate saves 2.3-3.5 GJ); furnace acid 18.8 GJ electricity (1984) | about 85% fertiliser; detergents, feed, food |
| DAP and MAP | 16.7 Mt P2O5 (1999) | 525 MJ fuel + 200 MJ electricity per t DAP (1984) | two-thirds of world phosphate fertiliser |
| Oxygen | 11.75 billion m3 in the US (1981) | 450-480 kWh per t of 95% O2 (1997) | steel (55% of US use), chemicals, partial oxidation |
| Hydrogen | about 42 Mt world (about 2003) | electrolysis 46.8 kWh per kg (2012); reforming cost about 1.5 x gas cost per unit energy | ammonia 60%, refining 23%, methanol 9% |
Ammonia and urea tonnages converted with 17/14 and urea's 46.65% N. Units differ by product; read each figure with its year.
Source: Kent (2012) Chs 22-27; Shreve (1984) Chs 7, 13, 16, 18, 19; Dryden (1997) Chs IIA-IIH, see section sources- Kent (2012), Ch 22 Synthetic Nitrogen Products · pdf pp. 907, 917, 919
- Kent (2012), Ch 25 Sulfur and Sulfuric Acid · pdf pp. 1011, 1018
- Kent (2012), Ch 26 Salt, Chlor-Alkali, and Related Heavy Chemicals · pdf pp. 1024, 1031
- Dryden (1997), Ch IIH Chlor-Alkali Industry · pdf pp. 204, 216
- Dryden (1997), Ch IIA Sulfur and Sulfuric Acid · pdf p. 77
See it in the table
H₂O WaterGas Natural gasP Phosphate rockSt SteelSi SiliconNaCl SaltSda Soda ashGyp GypsumH2SO4 Sulphuric acidHNO3 Nitric acidNaOH Caustic sodaCl2 ChlorineH2 HydrogenO2 OxygenCheck your understanding
Why does the double column in an air separation plant run its two columns at different pressures?
At higher pressure nitrogen condenses at a temperature above the boiling point of oxygen at lower pressure, so one condenser-reboiler links the two columns without an outside heat source.About how much electricity does electrolysis need per tonne of hydrogen at Kent's commercial figure, compared with a tonne of chlorine?
46.8 kWh per kg is 46,800 kWh per tonne, roughly twenty times the 2,300-2,900 kWh per tonne of chlorine.