Guide
How much acetylene dissolves in water and in carbide sludge and how it comes back out in the pit
Acetylene dissolves in water at 1.97 g/kg (1.68 volumes per volume) at 0 °C and 1 atm, falling to 1.229 g/kg at 20 °C. Carbide sludge behaves like water, holding 0.685 g/l at 60 °C and 0.438 g/l at 80 °C, and releases that gas by convection in open pits.
- Solubility in water at 1 atm
- 1.97 g/kg
- Bunsen coefficient series
- 1.73 at 0 °C
- Solubility in carbide sludge
- 0.685 g/l at 60 °C and 0.438 g/l at 80 °C; saturated lime water dissolves acetylene as pure water does over 10
- Desorption rate
- about 3.3 cm³ NTP per minute per cm² of surface from a 1 g/l solution stirred at 200 rpm
- Heat of solution in water
- 3.96 kcal/mol over 1
Water dissolves a little acetylene, and carbide sludge dissolves about the same amount as water. At 0 °C and 1 atm the figure is 1.97 g per kg, or 1.68 volumes of gas per volume of water; at the 60–70 °C of a working carbide-to-water generator it is nearer 0.7 g per litre. Those are small numbers beside the 300-odd litres of gas a kilogram of carbide produces, but a generator passes six to eight litres of water per kilogram of carbide, the sludge leaves hot and saturated at generator pressure, and the gas it carries comes back out, by convection, into whatever pit or tanker receives it. This page gives the solubility figures, the desorption kinetics and the plant rules that follow.
Solubility in water
The reference data are Winkler's (Z. phys. Chem. 1900), carried in Landolt-Börnstein and in the 1965 compilation; the C.P.I. handbook (Paris, 1968) prints Vogel's Bunsen coefficients, which agree. Both are at 1 atm.
| Temperature | g acetylene per kg water | Volumes per volume (Winkler) | Bunsen coefficient α (Vogel, C.P.I.) |
|---|---|---|---|
| 0 °C | 1.97 | 1.68 | 1.73 |
| 10 °C | 1.561 | 1.33 | 1.31 |
| 20 °C | 1.229 | 1.05 | 1.03 |
| 30 °C | 1.006 | 0.86 | 0.84 |
IS 14814:2023 cl 4.3.17 gives 1.2 g/l at 20 °C, consistent with both. A single Winkler value at 50 °C, about 0.7 g/kg, is the origin of the 0.64 vol/vol figure that sometimes appears on datasheets; it belongs at 50 °C, not at 75 °C, and the compiled data contain no measured value at 75 °C. The mean heat of solution over 1–30 °C is 3.96 kcal/mol, which is why solubility falls by a third between 0 °C and 30 °C.
Solubility rises roughly in proportion with pressure at low pressures, but the proportionality is cut short by hydrate formation: above about 7 atm at 0 °C and above about 20 atm at 10 °C a solid clathrate, C₂H₂·5.75H₂O, separates. That is a compressor and drier hazard rather than a generator one, and it has its own page, acetylene hydrate — the plugging hazard.
Two other water figures are worth having. The diffusivity of acetylene into still water at 20 °C is 1.56 cm²/day (Tammann and Jessen, 1929), slow enough that diffusion alone would take weeks to clear a pit; and in stirred absorption the liquid-film coefficient for acetylene at 25 °C is about twice that of hydrogen or oxygen (Hutchinson and Sherwood, 1937), so agitated water takes acetylene up quickly and gives it up just as quickly.
Rules that matter
The generator does not run on clean water. After the first charge the liquid is a suspension of calcium hydroxide, saturated lime water above a settling bed of slaked lime, and it is reasonable to ask whether the lime changes the picture. The measurements say it does not, and that any change is downwards.
Solubility in lime water and carbide sludge
- Acetylene dissolves in saturated calcium hydroxide solution over 10–50 °C virtually as it does in distilled water (Strizhevskii and Strebulaeva, 1955).
- In lime slurry, that is carbide sludge as it leaves a generator, the same workers found 0.685 g/l at 60 °C and 0.438 g/l at 80 °C.
- The C.P.I. (1968) records that in milk of lime the solubility varies with lime concentration and is always less than in the purest water.
- Strong alkali reduces solubility markedly: 3 molar sodium hydroxide holds about 0.35 g/kg at 15 °C against 1.37 g/kg for water (a 1902 series in Z. phys. Chem.). Lime water is far too dilute for that effect to matter.
So there is no lime scavenging. Sludge drawn hot from a generator carries about 0.4–0.7 g of dissolved acetylene per litre, which is 0.4–0.6 litres of gas per litre of sludge, and the sludge from one kilogram of carbide at a water ratio of 6–8 litres per kilogram carries several litres of gas out of the plant. On top of that, the sludge contains lime-coated lumps of unreacted carbide that keep reacting with the water around them for hours, which is why the C.P.I. requires settling basins to hold residues long enough for coated lumps to decompose completely.
How much gas the sludge takes with it
Loss accounting as compiled by S. A. Miller (1965) puts dissolved acetylene in the sludge at about 1.0 % of the make with the generator at 60 °C and 0.5–0.6 % at 75 °C; the warmer generator makes wetter gas but loses less in the liquor. The wet gasholder, for comparison, loses about 0.007 %. The C.P.I. states the same two principles in its generator design rules: acetylene is soluble in water, less so the warmer, so a hot generator gives smaller losses in the drainage water but wetter gas; and solubility rises with pressure, so losses in the drainage water increase with generator pressure. The water ratio itself, and the 18 % solids fluidity limit that sets its floor, are covered in generator water-to-carbide ratio.
Half a per cent of make is a small loss against the 13 % spread that carbide grading introduces, and nobody runs a generator hot to recover it. The reason the figure matters is where the gas goes.
Desorption: why a sludge pit keeps breathing acetylene
Hölemann, Hasselmann and Strootmann (Forschungsberichte des Landes Nordrhein-Westfalen No. 569, 1958) measured the rate at which acetylene leaves water, and their findings explain field experience of sludge pits exactly.
| Finding | Figure |
|---|---|
| Evasion rate, 1 g/l solution at 20 °C, copper vessel stirred at 200 rpm | about 3.3 cm³ NTP per minute per cm² of surface |
| Comparative rates under the same conditions | CO₂ 3.84; H₂S 3.76; NH₃ 0.09 cm³/min·cm² |
| Thickness of the controlling surface layer | 0.5 mm |
| Half-life of a solution at rest at 20 °C | 5H hours, H = depth in cm |
| Fraction remaining after 24 h at rest | 0.43 at 1 cm depth; 0.77 at 2 cm; 0.92 at 3 cm; 0.98 at 4 cm |
| Desorption rate vs stirring, 0.16 g/kg solution | 0.166 r + 3.2 g per m² per hour, r in rpm over 50–150 |
The central point is mechanism. Desorption is far faster than diffusion through the bulk liquid could explain, because even a solution at rest convects: as the surface layer loses gas it becomes denser and sinks, and fresh solution replaces it. The rate therefore depends on surface area and agitation, not on depth. A still 20 cm deep layer of saturated liquor takes about four days to lose half its gas; the same liquor pumped, raked or tipped releases most of it within the hour. Emptying a pit or loading a tanker is a gas-release event, and ammonia and hydrogen sulphide (the latter at nearly the same rate as acetylene) come out with it.
Rules that matter
The C.P.I. handbook derives its sludge rules from this physics, and IS 8471:2003 and the PESO licensing conditions carry the Indian requirements for generator design and siting; the layout rules are in plant layout and safety distances.
Plant rules that follow
- Drainage pipes for milk of lime, at least over a sufficient part of their length, and decanting trenches are to be exposed to the open air, to avoid an explosive atmosphere forming in a closed container. Open trenches, not buried pipes.
- Residual lime discharge must allow for the acetylene liberated by outgassing of the lime solution and by late decomposition of coated carbide.
- Solution drained from high-pressure driers must not go to a sewer; it is taken to a place where the gas cannot ignite or form an explosive mixture. The same reasoning covers any closed drain that receives sludge.
- Any container that has held acetylene, or in which acetylene may be liberated later, is treated a priori as capable of holding an explosive mixture. Sludge pits, settling tanks and tankers are entered only as confined spaces, after a gas test, with ignition sources excluded. IS 14814:2023 cl 7.3.4 separately says containers, pipes, valves and fittings of copper or copper alloys should not be used in acetylene service.
- Pits should be sited and shaped so that the gas they release cannot drift into buildings; acetylene is only 10 % lighter than air and does not reliably rise, as is acetylene lighter than air? explains.
- Never put water spray or water-type extinguishers on a fire in a carbide godown or generator hopper (IS 14814:2023 cl 9.2).
The residue itself, 1.166 kg of solid per kilogram of carbide or about 1.27 tonnes of carbide lime per tonne, its pH and its reuse are the subject of carbide lime sludge handling and disposal. The process upstream is described in how acetylene is made from calcium carbide, and the generators KIGL builds on the acetylene gas plant page.
Rules that matter
Rules to apply
- Budget about 0.5–1 % of make as dissolved loss in the sludge, more at low generator temperature and higher generator pressure.
- Treat sludge as a gas carrier: it leaves saturated at generator pressure and temperature and releases its gas on cooling, depressurising and agitation.
- Keep sludge handling in the open air; never pipe it to a closed sump or sewer.
- Expect the release to continue for days in a still pit and to peak whenever the pit is disturbed.
- Keep ignition sources, including non-flameproof lighting and steel tools, away from pits and tankers, and test before entry.
How much acetylene dissolves — frequently asked questions
How soluble is acetylene in water?
Modestly. At 1 atm and 0 °C water dissolves 1.97 g of acetylene per kg, which is 1.68 volumes of gas per volume of water; at 20 °C it is 1.229 g/kg or 1.05 vol/vol, and at 30 °C 1.006 g/kg. IS 14814:2023 gives 1.2 g/l at 20 °C. Solubility rises roughly in proportion with pressure until a hydrate forms, above about 7 atm at 0 °C.
Does carbide sludge dissolve more or less acetylene than water?
About the same, or a little less. Saturated lime water was found to dissolve acetylene exactly as distilled water does over 10–50 °C, and lime slurry held 0.685 g/l at 60 °C and 0.438 g/l at 80 °C (Strizhevskii and Strebulaeva, 1955). The C.P.I. (1968) states that solubility in milk of lime is always less than in the purest water. There is no lime scavenging effect.
Why does a sludge pit smell of acetylene?
Because hot sludge leaves the generator saturated with acetylene at generator pressure and releases it as it cools and depressurises, and because lime-coated carbide lumps keep reacting for hours. The desorption is driven by density convection, not diffusion, so a still pit keeps releasing gas from its whole depth. That is why pits are open to the air.
How much acetylene is lost in the sludge?
Roughly 1 % of the make at a generator temperature of 60 °C, falling to about 0.5–0.6 % at 75 °C, because solubility falls with temperature. The wet gasholder by comparison loses only about 0.007 %. Running the generator warm costs wetter gas but saves about half a per cent of output; both figures are as compiled by S. A. Miller (1965).
How long does dissolved acetylene take to leave a sludge pit?
For a solution at rest at 20 °C the time to fall to half its initial concentration is about 5H hours, where H is the depth in centimetres, so a 20 cm deep layer takes about four days. Stirring shortens that sharply; the surface layer controlling desorption is only 0.5 mm thick. Agitation during emptying is therefore a gas-release event.
Can I send sludge or condensate to a sewer?
No. The C.P.I. (1968) forbids discharging solution drained from high-pressure driers into a sewer because of the risk of an explosive acetylene–air mixture forming in it, and requires drainage pipes and decanting trenches for residual lime to be exposed to the open air for the same reason. The same logic applies to any closed drain carrying sludge.
Is carbide sludge an explosive atmosphere hazard?
Any tank or container that has held acetylene, or in which acetylene may be liberated later, must a priori be considered capable of containing an explosive mixture, in the words of the C.P.I. handbook. A sludge pit or tanker meets that definition. Treat entry as confined-space entry with a gas test, and keep every ignition source out.
References
- Bureau of Indian Standards. (2003). Acetylene Generators — Requirements (with Amendment 1, 2024) (IS 8471:2003, First Revision).
- Bureau of Indian Standards. (2023). Acetylene — Code of Safety (IS 14814:2023, First Revision).
- C.P.I. (1968). Safety in the production and use of acetylene. Paris.
- Linstrom, P. J., & Mallard, W. G. (Eds.). (n.d.). NIST Chemistry WebBook (NIST Standard Reference Database 69). National Institute of Standards and Technology. [acetylene and water data] https://webbook.nist.gov/chemistry/
- Winkler, L. W., Z. phys. Chem. 1900, 55, 344 — solubility of acetylene in water
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