From applications using a clean high temperature flame for welding and cutting operations, acetylene is a gas encompassing a wide range of applications. Its also a gas of presenting immense transport and storage complexities, the intricacies of which are examined here in Part 7 of our Gas Cylinders series of features.
In the beginning
In the industrial gases industry, acetylene (C2H2) was considered the wonder gas in the early 20th century. However acetylene is a highly flammable compound with a wide flammability limit when mixed in air, ranging from 2.5% to 80%. This range is wider than any other commonly found flammable gas. Also, the ignition energy for acetylene is low and the burning velocity and flame temperatures are exceedingly high, with the minimum ignition energy for acetylene in air among the lowest measured for a flammable gas.
Hence it was not surprising that the initial disastrous experience of transporting acetylene as a liquid in steel cylinders prompted other developments for its safe storage. In 1897 Claude and Hess established that the gas was very soluble in acetone, which by 1900 led Berthelot to show that such a solution was safe up to 10 bar. Finally Le Chatelier proposed storing this solution in a porous mass (now referred to as porous material) inside a gas cylinder.
In the UK the Home Office launched this dissolved acetylene industry in 1901 and soon international developments followed in France, and then in 1904 in the US as an illuminating gas in car headlights. But the career of acetylene as an illuminant was short-lived, and huge applications in which its high flame temperatures are exploited were developed. In the metal working industries acetylene was used for welding, cutting and extensions of these uses such as hard facing, flame gouging, scarfing and flame hardening to name but a few, also sprang up.
In its simplest form acetylene is produced by the reaction of calcium carbide and water, with the generation of a lot of heat:-
About 1kg of acetylene is liberated from every 3kg of calcium carbide. The calcium carbide is formed by reacting calcium oxide (lime) with carbon in an electric arc furnace. In turn the calcium oxide is initially made using calcium carbonate – the purity of which is very important if impurities such as phosphine and hydrogen sulphide in the final acetylene are to be avoided.
Thus by adding these two equations, the overall reaction in the ideal case is:
As can be seen this is a very energy intensive process requiring temperatures in excess of 2000˚C.
Unlike most gas cylinders which are hollow, an acetylene cylinder contains a ‘porous material’, within the cylindrical shell, into which a solution of acetylene is stored under pressure. The most widely used solvent for the acetylene is acetone, though when a low volatility is required such as for acetylene bundles, which are expensive to dismantle and re-charge with solvent, the heavier (by 30% compared to acetone) and expensive dimethylformamide (DMF) is employed. For each bar of pressure, acetone absorbs 25 times its own volume of acetylene.
a) The Shell
The cylinder shell is either of seamless steel or of a welded steel construction, the latter being capable of storing the same quantity of acetylene as in a seamless shell of a comparable volume, but weighing much less. But being of a thinner construction, the welded shell is more prone to external damage. Another potential safety concern is if the welded joint is of a joggle geometry (as commonly used for LPG cylinders), as there is then a suspicion that it may contribute to unnecessary cracking of the porous material, thus creating a safety hazard. Butt welded joints are much preferred, though their manufacture is more expensive than the joggle counterpart.
There are a few aluminium alloy shells in acetylene service, but their use needs utmost care, especially during the massing stage of the porous material, where the cylinder is heated in an oven.
b) The Porous Material Until a couple of years ago, the substance used to store the acetylene in solution under pressure was known as the ‘porous mass’. Now however, in order to use a harmonised definition internationally, the ‘porous mass’ is referred to as the ‘porous material’.
The porous material resembles a honeycomb with a density of about 8-20% of the volume in the cylinder, depending on the type of materials used. It is intended to keep acetylene molecules separated as far as possible to prevent any detonation reactions and to suppress the spread of an acetylene decomposition, if the cylinder is accidentally impacted or locally heated.
The earliest porous materials were based on fibrous materials such as kapok, slag wool, animal hair, flax and mixtures thereof. These were developed in the early 20th century, while in the 1920’s came the generation of granular materials which used charcoal, wooden chips, kieselguhr, or pumice to pack the shell.
Whilst charcoal plus kieselguhr was a common combination in the Commonwealth countries, balsa wood and kieselguhr was preferred in the US. The porosity of these fibrous and granular materials seldom exceeded 80%. In order to transport more acetylene a higher porosity was sought and the answer was the monolithic material. At first a calcium silicate/carbon/asbestos mixture was employed whose asbestos component was substituted, in the 1990’s with glass fibre, from a health and safety viewpoint. The resulting porous material is up to 92% porous.
An aqueous slurry of the various components is forced into the cylinder which sets into a solid mass within the cylinder. The water is then driven off by setting the material under pressure in the cylinder at 175-190˚C, resulting in the required high porosity. In order to completely fill the cylinder without leaving any air gaps either within the porous material or at the top of the cylinder, a specific procedure for the particular plant is developed. This procedure involves a careful blend of vibrating the cylinder, whilst filling with the slurry, to eliminate all air-bubbles and some black art. Personnel who assemble acetylene cylinders develop their skillset over many years of careful trial and error.
Unplanned minor changes in the recipe have led to disastrous results with huge cavities being present in the finished cylinder.
Once the whole lump of this concrete-like structure has petrified, a specified core-hole of around 10mm diameter x 50 mm deep is drilled into the top face of the porous material. This cavity is then packed with felts and finally topped off with a metal gauze. The latter combination facilitates ease of filling and filters out any fine dust particles from the gas stream when extracting the acetylene gas during use. Some newer types of valves which are fitted to cylinders already, have a built-in gauze in the valve stem, in which case the gauze on top of the felts is omitted. After all these operations, a maximum air gap of 2-5mm may be present at the top of the cylinder, depending on the manufacturer’s specification. Additionally, the porous material must have the specified crush strength.
c) The Safety Relief Devices
This is really the subject of an entire article itself, since the world is undecided upon what the best solution can be. Suffice to say that there are two international standards, ISO 3807-1 and -2 dealing with acetylene cylinders with and without safety relief devices respectively.
Acetylene cylinders are filled to pressures between 15-20 bar depending on local regulations and type of cylinder. The heat of solution of acetylene in acetone is removed during the charging process by sprinkling water over the cylinders. Charging times vary from 6-12 hours for most rigs, though some fast-filling rigs may only take 3-5 hours.
The Twilight Era
The industrial usefulness of acetylene in the 21st century is limited by its unpredictable behaviour. Even though we have learnt many lessons from using the gas for over 100 years, it still needs a lot of care and attention.
It is also facing competition from lower cost substitutes such as propane (though they do not always provide the same degree of quality) and sophisticated electric welding technology. Nevertheless, the need for acetylene is there in numerous different spheres and the gas will be here to stay for many years