Vinyl chloride (VC), also known as vinyl chloride monomer (VCM) or chloroethene, is a colorless, flammable gas at room temperature and pressure. VC serves as the fundamental building block for the ubiquitous plastic, polyvinyl chloride (PVC). As a chemical intermediate, VC is almost exclusively transformed into this polymer counterpart. The entire manufacturing process is geared toward the efficient, high-volume synthesis of this monomer to supply the global plastics market.
Required Starting Materials
The industrial synthesis of vinyl chloride relies on two readily available, high-volume chemical feedstocks: ethylene and chlorine. Ethylene is a hydrocarbon gas primarily sourced from the cracking of petroleum or natural gas liquids, tying its availability directly to the petrochemical industry. High purity ethylene is required for the production process to maintain efficiency and product quality.
Chlorine is the second necessary component, and it is manufactured through the energy-intensive chlor-alkali process, which involves the electrolysis of a saltwater brine. This process yields both chlorine and sodium hydroxide, linking the supply and cost of chlorine to the overall chlor-alkali market. The combination of these two core chemicals dictates the economics of modern vinyl chloride production.
Hydrogen chloride (HCl) is also important, though it is not a primary feed but a co-product of one reaction step. The modern synthesis method is designed specifically to recycle this HCl stream back into the process. This internal recycling mechanism minimizes waste and maximizes the conversion of the initial feedstocks.
The Balanced Oxychlorination Process
The dominant method for producing vinyl chloride is the “balanced process,” which integrates two distinct reactions to convert ethylene and chlorine into the final product. This system centers on the intermediate compound 1,2-dichloroethane (EDC), the precursor to the vinyl chloride monomer. The balancing act ensures nearly all the chlorine introduced is utilized without generating excess hydrogen chloride waste.
Direct Chlorination
The first step, direct chlorination, is a highly exothermic reaction where liquid ethylene reacts with chlorine to form EDC. This reaction is conducted at relatively low temperatures, often in the presence of a catalyst like ferric chloride (\(\text{FeCl}_3\)). This direct route is efficient, producing high-purity EDC with few by-products.
Oxychlorination
The second step, oxychlorination, provides the mechanism for recycling the hydrogen chloride. In this process, ethylene reacts with hydrogen chloride and oxygen (or air) to produce more EDC and water. This reaction is carried out in a separate, highly exothermic reactor, using a copper chloride (\(\text{CuCl}_2\)) catalyst at elevated temperatures.
Thermal Cracking
The EDC generated from both the direct chlorination and the oxychlorination steps is then combined and purified before the final conversion. The purified EDC is heated to high temperatures in a thermal cracking furnace. This endothermic process, known as pyrolysis, breaks the EDC molecule down into the desired vinyl chloride monomer (VCM) and hydrogen chloride (HCl).
The hydrogen chloride produced during the thermal cracking step is routed directly back to the oxychlorination reactor after separation and purification. There, it is consumed to create more EDC. This closed-loop recycling of HCl is the defining feature of the balanced process, allowing for maximum utilization of the raw chlorine input.
Transformation into Polyvinyl Chloride
The purpose of synthesizing vinyl chloride monomer is its transformation into the polymer, polyvinyl chloride (PVC). Once VCM is produced and purified, it is ready for polymerization, which links the small VCM units into long molecular chains. This is an addition polymerization reaction, meaning the entire VCM molecule is incorporated into the growing polymer chain with no by-products released.
The process begins by introducing liquefied VCM into a reactor containing water, along with chemical initiators and suspending agents. The most common commercial method is suspension polymerization. In this technique, the VCM is dispersed as fine droplets in the water, which acts as a medium for heat removal, controlling the temperature of the exothermic reaction.
The initiators, often organic peroxides, decompose under heat to create highly reactive free radicals that attack the carbon-carbon double bond in the VCM molecule. This action opens the bond, allowing the monomer to link with other VCM molecules in a chain reaction. The resulting PVC precipitates out of the droplets as a fine, porous powder, commonly known as PVC resin.
After the desired conversion is reached, unreacted VCM is recovered and recycled back to the reactor. The PVC resin particles are then separated from the water, washed, and dried to form the final white powder product. This versatile resin is then ready to be compounded with additives and processed into the rigid or flexible plastic products used in construction, automotive, and consumer goods industries.