1. Production of Acetylene from Calcium Carbide

Calcium carbide (CaC₂) is a chemical compound produced by heating lime (calcium oxide, CaO) and coke (carbon, C) in an electric arc furnace at high temperatures (around 2,200°C). The reaction is as follows:

CaO+3C→CaC2+COCaO+3C→CaC2+CO

Calcium carbide is then reacted with water to produce acetylene gas (C₂H₂):

CaC2+2H2O→C2H2+Ca(OH)2CaC2+2H2O→C2H2+Ca(OH)2

Acetylene is a highly flammable gas and serves as the starting material for PVC synthesis.

2. Conversion of Acetylene to Vinyl Chloride Monomer (VCM)

The next step involves converting acetylene into vinyl chloride monomer (VCM), the precursor to PVC. This is achieved through a hydrochlorination reaction, where acetylene reacts with hydrogen chloride (HCl) in the presence of a catalyst (typically mercury chloride or other metal chlorides):

C2H2+HCl→CH2=CHCl(Vinyl Chloride Monomer)C2H2+HCl→CH2=CHCl(Vinyl Chloride Monomer)

This reaction is highly exothermic and requires careful temperature control to ensure optimal yield and safety.

3. Polymerization of Vinyl Chloride Monomer (VCM) to PVC

The final step is the polymerization of VCM into polyvinyl chloride (PVC). This is typically done through a free-radical polymerization process, which can be carried out in suspension, emulsion, or bulk polymerization methods. The most common method is suspension polymerization, where VCM is dispersed in water with the help of suspending agents and initiators. The reaction is as follows:

 CH2=CHCl→[−CH2−CHCl−](PVC)nCH2=CHCl→[−CH2−CHCl−]n(PVC)

During polymerization, the double bond in VCM breaks, and the monomers link together to form long polymer chains. The resulting PVC is a white powder, which can then be processed into various products by adding plasticizers, stabilizers, and other additives.

4. Environmental and Safety Considerations

While the acetylene route for PVC synthesis is well-established, it has some environmental and safety concerns. The use of mercury-based catalysts in the hydrochlorination step has raised issues due to mercury's toxicity. As a result, many manufacturers are transitioning to mercury-free catalysts or alternative processes, such as the ethylene-based route, which is more environmentally friendly.

Conclusion

The synthesis of PVC from calcium carbide via the acetylene route is a classic example of industrial chemistry. Despite its historical significance, the process is gradually being replaced by more sustainable methods. However, it remains an important part of the PVC production landscape, particularly in regions where calcium carbide is readily available. Innovations in catalyst technology and process optimization continue to improve the efficiency and environmental impact of this method.

This process highlights the interconnectedness of chemical engineering and material science in creating one of the world's most versatile plastics.