Drying acetylene for cylinder filling: calcium chloride driers condensate drains and the empty-space rule

Acetylene for dissolved-acetylene cylinders leaves the generator saturated with water, and it is kept that way through the scrubber because wet gas is safer to compress. It must then be dried before it reaches the cylinders. The driers that do it are simple vessels of calcium chloride, but the high pressure ones hold compressed acetylene at up to 24 kg/cm²g and are treated with the same respect as any other part of the high pressure system.

For fuel gas used straight from a generator, drying is usually unnecessary: cooled to ambient, the gas carries too little moisture to affect the flame. For cylinder filling it matters because water fed into a cylinder accumulates in the acetone. As compiled by S. A. Miller (1965), with rack gas at an average of about 11 atm gauge and 15 °C, the water vapour after compression would be about 0.15 % of the free gas, and if that were the only source it would take over fifty years for the solvent to reach the water content at which the discharged gas carried as much water as went in. The effect is slow, but it is a continuous, irreversible loss of dissolving power and a source of internal rust. IS 308:2024 limits moisture in dissolved acetylene to 0.25 % by mass, and acetone itself is water-sensitive: a few per cent of water measurably reduces its capacity, which the solvent guide quantifies.

Dissolved-acetylene stations dry in three steps, an arrangement the 1965 literature, as compiled by S. A. Miller, described as virtually universal:

1. A low pressure calcium chloride drier, with 75 to 85 % CaCl₂, removes the bulk of the water before the purifier.
2. More water separates in the compressor, removed from the traps and crankcase as an emulsion with the oil.
3. The high pressure gas passes through 95 to 99 % CaCl₂, capable of taking moisture below about 0.5 mm vapour pressure, an average of under 0.07 % on the free gas.

EIGA Doc 241 sets the modern target: high pressure drier outlet at or below 100 ppmv, with the desiccant void no more than 10 % or 3 litres.

The gas passes upward. Most of the moisture is absorbed at the bottom of the bed, where the solid gradually dissolves into a liquor that is drained off from time to time while fresh solid is added at the top. The gas leaves after its last contact with dry solid, so drying efficiency is unaffected by hydrated solid or even liquor at the bottom. A bed with liquor below is a bed working correctly; operators who replace the whole charge whenever liquor appears waste calcium chloride, and operators who never drain it flood the bed.

Two practical points come from audits of plants of several makes. Fine calcium chloride smaller than the support mesh falls through and chokes the drain, so fused anhydrous lump of the correct grade is used. And a bed fed with gas that is too wet, or contaminated with ammonia, can crust over into a layer that is mechanically intact but chemically non-functional, so the medium is inspected, not only weighed.

The purifier sits before the high pressure drier for a reason. A ferric chloride purifier re-wets the gas, so drying ahead of it would be undone.

KIGL's high pressure train is an oil separator packed with compacted chain followed by calcium chloride driers, with a back-pressure valve holding 14 kg/cm²g during the early part of charging so that water is squeezed out more effectively at the higher pressure. The manual's warning is unambiguous: the high pressure driers must be kept filled with anhydrous calcium chloride, because a void could result in an explosion, and moisture carried to the manifolds is deposited in cylinders and slowly destroys their absorption capacity. They are refilled daily before start-up.

The 1968 C.P.I. code gave the same rule its reason: driers must be designed and operated to reduce the empty space to a minimum. An empty volume of compressed acetylene is the space a decomposition has to run in.

Wet acetylene under pressure forms a solid hydrate, a clathrate of acetylene and water first reported by Villard in 1895 and 1897 and given its structure by von Stackelberg. It forms above about 7 atm at 0 °C and above about 20 atm at 10 °C, so it can plug a cold discharge line, intercooler or neglected drier on a winter night at a temperature well above freezing. The hydrate guide covers it in detail; on the drier side the protection is to keep the low pressure drier and compressor traps working so that wet gas never reaches high pressure.

Refrigeration is limited by liquefaction: acetylene at 20 atm gauge boils at about −8 °C, and cooling it to 0 °C leaves about 0.23 mg/l of water in the free gas. Alumina and silica gel dry more deeply than calcium chloride, to about 0.002 mg/l (as compiled by S. A. Miller, 1965), but adsorb acetylene, 1.8 % and 3.7 % respectively, and the C.P.I. code required special precautions so that silica gel in a high pressure circuit could never rise in temperature or hold an air-acetylene mixture. The same compilation judges the extra dryness of little value because the cylinder solvent mass is not that dry either. Calcium carbide as a drier fails because the lumps break down into fine lime that blocks the gas.