Acetylene (\(\text{C}_2\text{H}_2\)) is a colorless hydrocarbon gas known for its high energy content and chemically reactive triple carbon-carbon bond. It is used across several industrial sectors, primarily for producing a very hot flame and for chemical synthesis. Because of its inherent instability, it is not stored as a simple compressed gas; instead, “dissolved acetylene” (DA) is the standard, safe commercial form used globally.
Why Acetylene Must Be Dissolved
Acetylene is highly unstable when compressed above relatively low pressures, becoming prone to explosive decomposition above 15 pounds per square inch (psi). This decomposition is highly exothermic and can initiate a chain reaction within the cylinder. To prevent this hazard, acetylene is stored in specially designed steel cylinders containing a porous filler material saturated with a solvent, usually acetone or dimethylformamide (DMF). The gas is dissolved into this solvent under pressure, which stabilizes the gas and prevents it from spontaneously decomposing. This method allows for the safe storage and transportation of a much greater quantity of the gas.
The Dominant Role in Oxy-Fuel Operations
The most recognized application of dissolved acetylene is in combination with pure oxygen for oxy-fuel processes. This pairing creates the highest flame temperature of any common fuel gas, reaching up to 3,500 degrees Celsius (6,330 degrees Fahrenheit) through a two-stage combustion process. This intense heat is indispensable for various thermal metalworking tasks.
Welding and Cutting
The high-temperature flame is used extensively for welding, particularly fusion welding of steel. In cutting operations, the oxy-acetylene flame preheats steel to its ignition temperature (typically 700 to 900 degrees Celsius) before a jet of pure oxygen oxidizes and removes the metal.
Localized Heating
Acetylene’s flame is also used for localized heating applications, such as brazing, soldering, flame hardening, and metal straightening. For welding, the neutral flame’s inner cone can reach around 3,320 degrees Celsius (6,000 degrees Fahrenheit). This concentrated heat allows for rapid piercing and melting of heavy-gauge materials that other fuel gases cannot handle efficiently, making it the preferred choice for heavy fabrication and repair work.
Use as a Chemical Feedstock
Beyond its role as a fuel gas, acetylene is a fundamental building block in organic synthesis, utilized for its highly reactive triple bond. This characteristic makes it a versatile intermediate for manufacturing various high-value chemicals and polymers.
Polymer Production
About one-third of global vinyl chloride monomer (VCM) production still relies on acetylene as a starting material. In this process, acetylene reacts with hydrogen chloride to form the monomer used to create polyvinyl chloride (PVC) plastic, which is widely used in piping, window frames, and wire insulation. Historically, acetylene was also instrumental in the synthesis of acrylic acid derivatives.
Specialty Chemicals
The chemical reactivity of acetylene extends to the production of synthetic rubber and solvents. It is used in the manufacturing of neoprene, a synthetic rubber known for its resistance to oil and weathering. Furthermore, acetylene is a precursor in the production of 1,4-butanediol (BDO), an organic compound used in the creation of various plastics, elastic fibers, and polyurethane materials.
Less Common Industrial Applications
Acetylene’s unique properties lend themselves to several specialized industrial and scientific uses. It serves as a carbon source for manufacturing specialized forms of carbon black, which is used as a filler in tires and as a pigment in plastics and inks.
In analytical chemistry, acetylene is the fuel gas of choice for atomic absorption spectroscopy (AAS) instruments. When combined with air or oxygen, the acetylene flame in AAS atomizes a sample, allowing for the quantitative analysis of trace metal elements. Additionally, acetylene is sometimes used in the glass and materials processing industries to apply carbon coatings, providing a lubricated surface.