Why acetylene is dissolved in acetone and what the porous mass does

The problem acetylene sets

Almost every industrial gas is stored the same way: squeeze it into a steel shell and let pressure do the work. Acetylene will not accept that treatment.

Acetylene is an endothermic compound. The molecule holds energy rather than needing to find it, and under pressure it can break down into carbon and hydrogen on its own — releasing heat, with no oxygen involved at all. Once that begins inside a vessel it can propagate, and the result is an explosion in a cylinder that never met an oxidant.

That is why the line-pressure limit is 1.5 bar g, and why a cylinder simply pumped full of acetylene is not a design that can be made safe.

The solution: dissolve it

Acetylene is very soluble in acetone — roughly 25 volumes of acetylene per volume of acetone at 12 bar, and more as pressure rises. In solution the acetylene is no longer free gas under pressure. It behaves like the carbon dioxide in a soft drink: held in solution, released as the pressure falls.

This is why the cylinder is filled to 20 bar g while the line may never exceed 1.5. The high pressure drives gas into solution; it is not what you draw against.

The porous mass

Acetone alone is not enough. A cylinder part-full of liquid solvent would slosh, would collect at the valve when laid down, and would leave open volumes where a decomposition could still run.

So the cylinder is filled with a solid porous mass, formed in place, doing two jobs at once:

• it holds the acetone distributed through the cylinder rather than pooled at the bottom
• it divides the interior into cells small enough that a decomposition starting in one place cannot build into a propagating reaction

The mass is part of the cylinder. It cannot be poured in, taken out, repaired or replaced. When it is damaged, settled or contaminated, the cylinder is withdrawn from service.

What follows from this

Nearly every acetylene rule traces back to the acetone and the mass:

Rule	Because
Keep the cylinder upright	Lying down lets liquid acetone reach the valve
Draw no more than one-seventh per hour	Faster draw-off pulls acetone out with the gas
Weigh to check contents	The gauge reads acetone vapour pressure, not gas remaining
Stand a horizontal cylinder for an hour	The acetone needs to drain back into the mass
Retest every 2 years, not 5	The mass and solvent must be assessed, not just the steel
The cylinder is heavy	Steel, mass and solvent all travel with the gas

The handling rules themselves are set out on handling acetylene cylinders.

Where DMF comes in

Acetone is the standard solvent, and it is what an industrial dissolved-acetylene cylinder to IS 308 contains.

For ultra-high-purity acetylene, KIGL uses dimethylformamide (DMF) instead. Acetone carries over into the gas stream in small quantities — irrelevant to a cutting torch, ruinous to an instrument. DMF has a much lower vapour pressure, so far less solvent reaches the analyser. That is the entire reason a separate UHP acetylene product exists.

Why this matters when you buy

A supplier who understands the mass is a supplier who fills correctly. Over-fast filling, incorrect acetone top-up and neglected mass inspection all produce cylinders that look normal, weigh roughly right, and then underperform or fail in service.

KIGL builds the plants that generate acetylene as well as filling the gas, so the person answering a question about your cylinder has run the generator behind it.

Sources

• IS 7312 — Dissolved acetylene cylinders
• IS 14814 — Acetylene, Code of Safety