Understanding Acetylene Gas Storage

Acetylene (\(\text{C}_2\text{H}_2\)) is a fuel gas widely used in various industrial applications, most notably for welding and cutting (oxy-acetylene process). However, storing and transporting acetylene gas can be challenging and dangerous, especially under pressure.

Acetylene is highly unstable in its free gaseous state, particularly when pressurized above about 15 pounds per square inch gauge (psig) or 103 kilopascals (kPa). At higher pressures, it can decompose explosively, even without the presence of air or oxygen. This decomposition reaction generates significant heat and can lead to severe accidents.

The Problem with Pressurized Acetylene

Because of its instability under pressure, acetylene cannot be simply compressed and stored in standard cylinders like many other gases (e.g., oxygen, nitrogen, argon). A safe method is needed to store a substantial amount of acetylene in a manageable volume for distribution and use.

The Solution: Dissolving Acetylene in a Solvent

To overcome the safety risks associated with storing acetylene under pressure, a special method is used. The gas is dissolved in a suitable solvent. This allows a much larger quantity of acetylene to be stored safely within a cylinder at moderate pressures compared to storing it as a free gas.

The cylinder used for acetylene storage is not empty. It is filled with a porous material (like calcium silicate, charcoal, or diatomaceous earth) and saturated with a solvent. Acetylene gas is then pumped into the cylinder, where it dissolves in the solvent absorbed by the porous filler material.

Why Acetone is the Preferred Solvent

Among various potential solvents, acetone (\(\text{CH}_3\text{COCH}_3\)) is the most commonly used and effective solvent for dissolving acetylene for storage. Acetone has a remarkable ability to dissolve large volumes of acetylene under pressure. At room temperature and a pressure of around 15 psig, one volume of acetone can dissolve about 25 volumes of acetylene. This solubility increases significantly with pressure.

The combination of the porous filler material and the dissolved state in acetone prevents the acetylene gas from accumulating in large free pockets and significantly reduces the risk of explosive decomposition.

Considering the options provided:

• Water: Acetylene has limited solubility in water, making it unsuitable for storing large volumes under pressure.
• Mercury: Mercury is a toxic heavy metal and does not effectively dissolve acetylene for safe storage; in fact, acetylene can react with mercury salts to form shock-sensitive acetylides.
• Acetone: Acetone is the standard solvent used because of its high solubility for acetylene and its stability under cylinder conditions.
• Carbon dioxide: Carbon dioxide is a gas itself and does not function as a suitable solvent to safely store pressurized acetylene.

Therefore, for safe storage and usage, acetylene gas is dissolved in acetone.

Revision Table: Key Facts about Acetylene Storage

Additional Information on Acetylene and Solvents

Beyond safe storage, the solubility of acetylene in acetone is crucial for its practical use. As acetylene is drawn from the cylinder, the pressure decreases, and the dissolved acetylene comes out of the solution as a gas, ready for use. The porous filler ensures even distribution of the acetone and provides a stable matrix.

Acetylene cylinders are designed with fusible plugs that melt at a relatively low temperature (around 212°F or 100°C) to release pressure in case of a fire, further enhancing safety by preventing cylinder rupture.

While acetone is the primary solvent, other solvents like Dimethylformamide (DMF) are also used in some cases, especially for higher pressures or specific applications, as DMF can dissolve even larger quantities of acetylene than acetone.

In summary, dissolving acetylene in acetone is a critical safety measure that allows this important industrial gas to be safely stored, transported, and used under pressure.