Acetone in Acetylene Cylinders: Loss, Top-Up and Capacity

The short answer

A dissolved-acetylene cylinder is filled with a solid porous mass saturated with acetone. Acetylene is not compressed into the free space — it dissolves into the acetone, which is what allows the cylinder to hold roughly 16 atmospheres of gas safely when free acetylene above about 1.5 bar can decompose on its own.

Acetone leaves with the gas. A cylinder discharged completely at 20 °C gives up acetylene carrying about 2% acetone by volume, and that solvent does not come back. Over repeated cycles the charge falls, and it must be restored by top-up — a controlled operation carried out at a licensed filling station by weight, never by eye.

How much acetylene the acetone actually holds

The capacity of the cylinder is a property of the solvent, not of the steel. At 20 °C and a full charge of about 15 atmospheres, one litre of the dilated acetone solution carries roughly 222 g of acetylene that can be released as the pressure lets down to atmospheric.

The solution swells as it takes up gas — that dilation is why a cylinder is never charged with acetone to the point where a full gas charge would leave it liquid-full. The permitted solvent charge and the maximum filling pressure are set together, and neither can be varied on its own.

How much acetone leaves with the gas

Acetone has a real vapour pressure, so gas drawn off a cylinder always carries solvent vapour with it. How much depends on temperature and on how fast the cylinder is discharged.

Discharge rate matters as much as temperature. Measured on a 110 ft³ cylinder at 20 °C, a slow draw of 0.5 ft³/h gave a mean of about 1.7% acetone, while a fast draw of 16 ft³/h gave about 2.4% — because rapid withdrawal cools the cylinder and drives harder gas-phase stripping of the solvent.

This is the operational reason behind the familiar withdrawal limit of one-seventh of cylinder contents per hour. It is usually taught as a way to avoid drawing solvent into the torch, and it is — but it is also what protects the cylinder’s own solvent charge over its service life.

Why low acetone is a safety problem, not just a capacity problem

A cylinder that has lost solvent will not accept its rated charge, which is the visible symptom. The hazard is the invisible one: acetylene that cannot dissolve stays as free gas in the pore space, and free acetylene under pressure is precisely what the dissolved-acetylene design exists to prevent. Acetylene can decompose without any oxygen present, and confined decomposition raises pressure by ten to eleven times.

This is why solvent content is a filling-station control, not a customer-side one, and why cylinders are checked by weight before filling. It is also why a cylinder that will not take its normal charge must be taken out of service and investigated rather than simply filled to whatever it accepts.

Checking and topping up the solvent charge

1. 1
   1 · Weigh against the stamped tare

   Every DA cylinder carries a stamped tare weight that includes the porous mass and the acetone charge. The check is arithmetic: actual empty weight against stamped tare tells you how much solvent is missing.
2. 2
   2 · Confirm the cylinder is genuinely empty

   Residual acetylene falsifies the weight. Weigh at the defined residual pressure the station works to, not at whatever pressure the cylinder came back at.
3. 3
   3 · Top up by measured weight

   Restore the deficit with acetone of the specified grade, measured by weight into the cylinder. Volume dosing is not equivalent — temperature changes the volume and the mass is what matters.
4. 4
   4 · Allow the solvent to distribute

   Let the cylinder stand valve-up so the acetone soaks evenly through the porous mass before charging. Filling a cylinder whose solvent has not distributed leaves dry regions of mass.
5. 5
   5 · Re-weigh and record

   Verify the corrected weight and record it against the cylinder number, so the rate of solvent loss per cylinder is visible over time.
6. 6
   6 · Investigate the outliers

   A cylinder losing solvent much faster than the fleet is telling you something — a passing valve, an over-fast discharge in service, or a degraded porous mass. Track it rather than topping it up indefinitely.

Solvent top-up is a licensed filling-station operation governed by cylinder regulations. It is not a field task, and it is not something a gas user should attempt on a cylinder in service.

Water is the other way capacity disappears

Solvent loss is the obvious mechanism. The quieter one is dilution, and it does not announce itself.

Acetylene is markedly less soluble in wet acetone than in dry. Around 6.8% water in the acetone at 30 °C reduces acetylene solubility by about 26%. Water arrives with the gas, so any moisture that survives the plant’s drying train is deposited into the cylinder solvent at every fill and stays there.

The arithmetic is slow but relentless. Gas charged at around 11 atm and 13 °C without drying carries roughly 0.15% water by volume — enough that, over something like fourteen years of typical service, the cylinder liquid drifts toward a quarter water. No single fill causes a problem; the fleet simply gets worse every year.

This is the real reason a plant’s high-pressure drier is not optional, and why a purity certificate on the gas says nothing about whether the cylinders are being looked after. See our guide on purification and drying for how the three-stage calcium-chloride train prevents it.

Acetone or DMF?

Dimethylformamide is the one genuine alternative to acetone, and the trade-off is worth understanding before anyone proposes switching a fleet.

DMF earns its place where solvent carry-over is the binding constraint — notably in acetylene recovery on hydrocarbon-route plants. For general dissolved-acetylene cylinder service, acetone’s cost and long-established handling keep it the standard, and the industry has not found a reason to move. Of the alternatives that appear attractive on paper, only acetonitrile comes close on capacity at cylinder pressure, and only if it ever became price-competitive. A cylinder is charged for the solvent it was designed and stamped for; the solvent is not a field-substitutable item.