Guide
How acetylene is made: carbide water, and the eight stages between the generator and the cylinder
Acetylene is made by reacting calcium carbide with water: CaC₂ + 2H₂O → C₂H₂ + Ca(OH)₂. In a carbide-to-water generator, carbide is screw-fed into water held below 70 °C at 6 to 8 litres per kg; the gas is cooled, dried, purified, compressed and dissolved into acetone in cylinders. Good carbide yields about 300 L/kg.
- Reaction
- CaC₂ + 2H₂O → C₂H₂ + Ca
- Theoretical yield
- About 347 litres of acetylene per kg of pure calcium carbide at 0 °C and 1 atm
- Commercial carbide yield
- 300 L/kg for 25
- Water-to-carbide ratio
- 6 to 8 litres per kg in KIGL's continuous-feed generators; never below 6
- Generator temperature ceiling
- 70 °C; set point starts at 58 °C
The reaction
Calcium carbide is a grey-black solid made in an arc furnace from lime and coke. Dropped into water it reacts at once:
CaC₂ + 2 H₂O → C₂H₂ + Ca(OH)₂
One mole of carbide gives one mole of acetylene and one of calcium hydroxide, and a great deal of heat. On the molar masses, a kilogram of pure carbide would give about 347 litres of gas at 0 °C and atmospheric pressure. Nobody buys pure carbide. The graded product an acetylene plant uses is specified by its yield, and that figure is what the plant is designed and sized around.
| Carbide lump size | Minimum average yield |
|---|---|
| 25–50 mm and 50–80 mm | 300 litres per kg |
| 15–25 mm | 280 litres per kg |
| 4–7 mm and 7–15 mm | 260 litres per kg |
Individual samples may not fall more than 3 % below these figures. KIGL's DA-25 is rated for 4–25 mm carbide and the larger plants for 4–50 mm; fines reduce yield and demand longer agitation of the slurry, and a lump over 80 mm will jam the hopper neck.
Why the carbide goes into the water
The heat is the whole design problem. A carbide-to-water generator — the type KIGL builds and runs — holds a large volume of water and screw-feeds carbide into it, so each piece meets an excess of water that absorbs the heat of reaction. The alternative, water dripped onto a charge of carbide, lets the temperature in the charge run away.
The ratio is controlled, not left to chance. KIGL's generators admit a minimum of 6 litres of water per kilogram of carbide and run correctly between 6 and 8; the weekly water meter total should sit within 7.5 times the weekly carbide weight. IS 8471 cl 8.2.1 requires not less than 8 litres per kg for a generator unless water and carbide are introduced continuously in a predetermined ratio, which is the continuous-feed case these plants are built for. EIGA's guidance for the same type of plant is "approximately 8:1, but may vary between 7:1 and 10:1".
Temperature is the other control. The water-inlet controller is set initially at 58 °C and adjusted a degree at a time against the measured water-per-kg; in no case is the setting allowed above 70 °C. The generator's high-temperature trip stops the carbide feed, and the water-inlet valve is air-operated so it closes on loss of instrument air.
Pressure: why an acetylene generator runs at a fraction of a bar
Acetylene can decompose explosively on its own, without any air, once pressure and temperature are high enough. So a generator makes gas at low pressure and the cylinder, with its acetone and porous mass, is where the gas is finally stored at pressure. KIGL's generators are settable between 0.2 and 0.92 kg/cm²g; the three safety valves — one on the tank outlet and one on each wet flashback arrestor — relieve just above the working maximum, and IS 8471 cl 11.5 caps the water-seal relief at 152 kPa, about 1.55 kg/cm²g. Two pressure switches start and stop the carbide screw feed to hold the set pressure, so generation follows the compressor's demand rather than the operator's guesswork.
The eight stages from generator to cylinder
| Stage | What it does | What the operator watches |
|---|---|---|
| 1. Hoppers and screw feed | Two carbide hoppers used alternately, each with a vent valve interlocked to its lid, feeding carbide by screw into the generator | Hopper vented to atmosphere before its lid is opened; only one hopper on line at a time |
| 2. Generator | The reaction tank: paddle agitator at about 20 rpm, water level and temperature controlled, slurry drained automatically on level | Water-to-carbide ratio; temperature below 70 °C; pressure below 0.92 kg/cm²g; low- and high-level trips on the water level |
| 3. Wet flashback arrestors | Two water-sealed arrestors in parallel on the outlet, with non-return valves, so a flame cannot travel back into the generator | Water level checked at every hopper fill; a 0.21 kg/cm²g drop in pressure across them means sludge in the pipe |
| 4. Cooler-condenser | Shell-and-tube exchanger that cools the gas, condenses water vapour and ammonia, and drops lime particles | Gas leaving as near room temperature as the cooling water allows; condensate drained hourly |
| 5. Low-pressure drier | Bed of anhydrous calcium chloride holding the gas at a constant moisture content before purification | Refilled weekly; never taken out of service with the plant running |
| 6. Chemical purifier | Layered bed of purifying mass that removes phosphine and hydrogen sulphide to inside the IS 308 limits (0.01 % PH₃, 0.02 % H₂S by volume) | Silver nitrate test on the outlet gas every shift; regenerated at least every three weeks |
| 7. Scrubber | Water column that catches carried-over purifier dust and saturates the gas for safe compression | Level checked daily, drained and refilled weekly |
| 8. Compressor, HP driers and manifold | Multi-stage water-immersed reciprocating compressor raising the gas to the cylinder-charging pressure; high-pressure driers (an oil separator and calcium chloride beds); banks of cylinders on a charging manifold under water spray | Suction pressure held inside the generator's working band; gas dry before it reaches the manifold; filling pressure checked against cylinder temperature |
Everything between the generator and the compressor runs below one bar; everything after it is where pressure is allowed, and only once the gas is dry, pure and on its way into acetone.
Filling the cylinder
A dissolved acetylene cylinder is not an empty shell. It holds a monolithic porous mass saturated with acetone, and the gas is compressed into solution in that acetone — about eight times what the same container could hold as free gas at the same pressure. Each cylinder is weighed in, checked for the correct acetone charge against its stencilled weight, filled on the manifold under water spray, and weighed out, because weight — not pressure — is how acetylene content is measured. The settled pressure of a full cylinder is temperature-dependent, with a maximum of 15.495 atm at 15 °C under IS 14814 : 2023 cl 7.2.2, so the filling pressure is read against the cylinder temperature. A normal charge takes about ten hours at 20 °C, usually one shift and a top-off the next morning after the cylinders have cooled and settled.
What leaves the plant besides gas
The lime. Every kilogram of carbide leaves roughly a kilogram of calcium hydroxide as slurry. It drains from the generator into a vented outdoor pit — never a sewer — and settles for at least ten hours in at least four times its volume of water before any liquid is released. Acetylene-rich water recovered from the pit after settling can be recirculated to the generator, which improves yield. Carbide lime has uses in construction and soil treatment, but its classification under India's hazardous-waste rules turns on the pH of the material as disposed, so a laboratory pH test on the actual slurry is the first step before any disposal route is chosen.
Yield is the number to watch
KIGL's shift log computes cubic metres of acetylene per kilogram of carbide at the end of every shift. It is the best early-warning figure on the sheet: a falling yield means wet or dusty carbide, a flooded purifier, a leak, or a generator running outside its ratio, and it moves weeks before an alarm does. A plant that is not logging yield is not really being operated; it is being watched.
How acetylene is made — frequently asked questions
What is acetylene made from?
Calcium carbide and water. Calcium carbide is itself made in an electric arc furnace from lime and coke; the acetylene plant buys it as graded lump and reacts it with water in a generator. Each kilogram of good carbide gives about 300 litres of acetylene and leaves calcium hydroxide — carbide lime — as a slurry.
How much acetylene does 1 kg of calcium carbide produce?
Pure calcium carbide would give about 347 litres at 0 °C and atmospheric pressure. Commercial carbide is not pure, so the grade specification sets a minimum average of 300 litres per kg for 25–50 mm lump, 280 for 15–25 mm and 260 for the 4–15 mm sizes. A plant that logs its yield every shift sees it fall weeks before anything else goes wrong.
Why is the carbide fed into the water, and not water onto the carbide?
Because the reaction releases a great deal of heat and the water is the heat sink. In a carbide-to-water generator each piece of carbide meets a large excess of water, so the gas and the slurry stay cool; adding water to a mass of carbide lets the temperature run away. IS 8471 requires at least 8 litres of water per kg of carbide for a batch generator unless carbide and water are fed continuously in a predetermined ratio, which is how KIGL's plants run.
What temperature does an acetylene generator run at?
KIGL generators start with the water-inlet controller set at 58 °C and are never set above 70 °C. Above that, acetylene begins to polymerise and the risk of decomposition rises; the high-temperature trip stops the carbide feed, and the water-inlet valve fails closed on loss of instrument air.
Why does the gas need purifying?
Commercial carbide carries phosphorus and sulphur as impurities, which come off as phosphine and hydrogen sulphide, with ammonia and water vapour. IS 308 limits phosphorus compounds to 0.01 % and sulphur compounds to 0.02 % by volume, so the gas passes a cooler-condenser, a low-pressure drier, a chemical purifier and a scrubber before it is compressed.
What happens to the lime?
Calcium hydroxide leaves the generator as a slurry through the residue drain into a vented outdoor pit, where it settles for at least ten hours in not less than four times its bulk of water before any liquid goes to drain. Its disposal classification under the hazardous waste rules depends on the pH of the material as disposed, which a laboratory test should settle.
References
- Bureau of Indian Standards. (1987). Calcium Carbide, Technical — Specification (IS 1040:1987, Third Revision).
- Bureau of Indian Standards. (2003). Acetylene Generators — Requirements (IS 8471:2003, First Revision).
Enquiries
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