Acrylonitrile (ACN) is a foundational organic compound in the petrochemical industry, bearing the chemical formula C3H3N. This molecule is highly reactive, allowing it to easily link together with other molecules to form long polymer chains. It functions as a primary monomer for a wide array of synthetic materials manufactured on an industrial scale. The production of this compound is fundamental to the creation of many common plastics and synthetic fibers used globally.
The Essential Starting Materials
The industrial manufacturing of virtually all acrylonitrile relies on the catalytic ammonoxidation process, often called the Sohio process. This method produces over 90% of the world’s supply because it utilizes readily available and inexpensive raw materials. The process requires three main chemical inputs: propylene, ammonia, and air.
Propylene (C3H6) serves as the hydrocarbon backbone, derived from petroleum refining or natural gas processing. Ammonia (NH3) provides the nitrogen atom necessary to form the nitrile group. Air is used as the source of oxygen required to drive the oxidation part of the reaction.
The Catalytic Ammonoxidation Reaction
The chemical transformation takes place within a specialized fluidized bed reactor. Propylene, ammonia, and air are introduced, encountering a proprietary solid catalyst, typically a mixed metal oxide based on bismuth and molybdenum. The reactor is maintained under extreme conditions, with temperatures usually ranging between 400 and 510°C. The catalyst is suspended within the gaseous reactants by the flow of incoming materials, creating a fluid-like bed that ensures excellent mixing and consistent temperature control.
The reaction is a single-step, highly exothermic process where propylene reacts with ammonia and oxygen to form acrylonitrile and water vapor. The high temperatures and reactive environment also lead to the formation of byproducts. The two most significant byproducts are hydrogen cyanide (HCN) and acetonitrile (CH3CN), which must be separated later in the manufacturing chain.
Isolating the Final Product
Once the ammonoxidation reaction is complete, the hot gaseous stream exits the reactor and is directed into a quench tower. The quench tower rapidly cools the stream by bringing it into direct contact with a circulating aqueous solution. Sulfuric acid is often added to the quench water to neutralize unreacted ammonia, forming ammonium sulfate. The cooling condenses much of the acrylonitrile and water into a rich solution, which then moves into a series of distillation and separation columns for purification.
The purification section is complex. Acrylonitrile is first separated from the large volume of water and the byproducts, hydrogen cyanide and acetonitrile, often through extractive distillation. The final stage involves further distillation to achieve a high purity level, typically above 99%, ready for commercial use.
Why Acrylonitrile Production Matters
Acrylonitrile production is foundational to several major industries. A significant portion of manufactured ACN is polymerized to create acrylic fibers, used extensively in textiles such as clothing, carpets, and upholstery.
Key Applications
ACN is an essential component in several engineering plastics. The plastic ABS (Acrylonitrile Butadiene Styrene) is valued for its impact resistance and rigidity, making it suitable for automotive parts, electronic casings, and toys. SAN (Styrene-Acrylonitrile) resin is prized for its clarity and stiffness. ACN is also used to produce nitrile rubber, which is used in seals and hoses due to its resistance to oils and solvents.