Commercial production of VCM started in the 1920s based on the catalytic hydrochlorination of acetylene but this route suffered from high energy costs and has become obsolete except for China.
Nearly all production outside of China is now based on ethylene, which is first reacted with chlorine to make ethylene dichloride (EDC). There are two routes commonly used to make EDC: direct chlorination using pure chlorine and ethylene, and oxychlorination in which the ethylene reacts with chlorine in hydrogen chloride. The EDC is then converted to VCM by thermal cracking and the hydrogen chloride by-product can be recycled to an oxychlorination plant to make more EDC.
The thermal cracking of EDC is carried out at a temperature of 450-550oC and 1-40 bar pressures with pumice or charcoal used as a catalyst. The exit gases are cooled rapidly in a quench tower to limit formation of byproducts. Hydrogen chloride is removed by distillation in the first column while the VCM obtained from the top of the second distillation column is washed with dilute sodium hydroxide and then purified by extractive distillation with acetonitrile.
For economic reasons, many EDC/VCM complexes use an integrated chlorination-oxychlorination process which proceeds in three stages: the chlorination of ethylene in the liquid or vapour phase to make EDC; the thermal cracking of EDC to form VCM and hydrogen chloride; and the oxychlorination of ethylene with recycled hydrogen chloride to make more EDC.
The oxychlorination reaction takes place in either a fixed bed or fluid bed reactor, the latter being preferred as it is easier to control the temperature. The oxychlorination unit can use air or pure oxygen but the oxygen route is favoured on environmental and efficiency grounds.
In China, the dominant process to make VCM is based on acetylene produced from calcium carbide. Coke made from coal and calcium oxide from limestone are heated to over 2000oC to make calcium carbide and carbon monoxide. Water is then mixed with the calcium carbide to make acetylene which reacts with anhydrous hydrogen chloride to form VCM.
The attraction of this technology in China is that it does not need ethylene and all the raw materials are domestically available. Other advantages include lower capital costs to build an integrated vinyls complex and a simpler technology. However, the process is highly polluting to the environment and has a high energy requirement.
A number of attempts had been made to develop ethane-based processes to make VCM, including by Geon, Lummus and OxyChem, but without success. However, EVC (now Ineos Vinyls) appears to have made the breakthrough with its new catalytic process for the generation of VCM directly from ethane. The EVC process has been tested in a pilot plant at Wilhelmshaven, Germany. EVC also claims a 20-30% reduction in production costs across the PVC chain while the process decouples VCM/PVC production from the ethylene cracker.
Vinyl chloride
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Vinyl chloride
Uses and Outlook
Vinyl chloride monomer (VCM) is used almost exclusively in polyvinyl chloride (PVC) manufacture. The remainder is consumed in polyvinylidene chloride and chlorinated solvents. With over 98% of VCM used in PVC, VCM demand is very dependent on the fortunes of the PVC market.
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