Vinyl Chloride Monomer (VCM)
Vinyl Chloride Monomer is a colorless, flammable gas. It is of very great importance as feedstock in the production of polyvinyl chloride (PVC), one of the world´s most versatile thermoplastics. Originally, acetylene has been used as feedstock for VCM production but was replaced stepwise by the inexpensive ethylene. Complete changeover to almost exclusive use of ethylene became possible in 1955, when the large-scale oxychlorination of ethylene to 1, 2 dichlorethane became possible. Today, more than 90 % of the world wide VCM production is based on ethylene.
Modern VCM plants use integrated processes combining the highly exothermic reactions of ethylene chlorination and oxychlorination with the endothermic cracking process, which results in an almost energy balanced plant operation.
Modern VCM plants are optimized with regard to the reuse of by-products. A mixture of high-boiling and low-boiling compounds is formed during the process steps in liquid or gaseous form. They are almost completely converted into reusable compounds (which are returned into the process), energy and water. This principle is the basis of the VCM production flow diagram as shown in fig.1.
Vinyl chloride monomer (VCM) is used as feedstock in the production of Polyvinyl chloride (PVC). Although modern VCM plants are optimized to recycle almost all by-products, some residues (waste gases) must be disposed from the process through incineration. Because of their composition, waste gases from VCM plants are potentially explosive and must be continuously and reliably monitored for their oxygen content.
The In-situ Laser Gas Analyzer offers the best possible capabilities for this application. It is installed right in the process gas flow and delivers fast and accurate O2 concentration data in real-time. This case study presents details of this application.
VCM production at a glance (fig. 1)
In the 'Direct Chlorination' route, ethylene and chlorine form 1,2-dichlorethane (EDC), in a catalytic reaction, together with heat, water and HCl-rich waste gas. The EDC is stored and the exhaust gas fed as a reactant to the oxychlorination process.
In the 'Oxychlorination' route, ethylene, oxygen and hydrochloric acid react in a fluidized-bed reactor. Raw EDC is formed, removed by condensation and fed to the EDC distillation and from there to the storage tank. Exhaust gas and effluent are fed to further treatment units.
Cracking of EDC to form vinyl chloride monomer (VCM) and HCl is performed in a cracking furnace at 500 °C and above. Some EDC remains unconverted and is recycled. HCl is fed back into the oxychlorination unit and is reused. The VCM is used for the production of PVC.
Cryogenic VCM recovery
Waste gases containing VCM, oxygen and other components are generated during VCM production at different sections of the plant (fig. 1). For environmental and economical reasons the gases must be further treated and recycled as far as possible.
To recover VCM, the waste gases containing unreacted vinyl chloride are first transferred into a gas holding tank (fig.2).
From there, the gas is compressed and fed to a single or multi-stage VCM recovery unit where the VCM is separated from the gas by condensation. VCM and water leave the condensation plant while the rest of the waste gas is forwarded for further treatment, typically by incineration.
Waste gas incineration
Waste gas from VCM production generally contains oxygen because oxygen is an important reactant in the process. Oxygen, when present in a gas in a certain concentration, generally imposes the risk of explosion. Therefore it is crucial for personnel and plant protection to monitor the oxygen concentration of the waste gas continuously up-stream the inlet of the incinerator (fig 2). As the oxygen concentration comes close to the critical level, a limit switch is activated causing nitrogen to be automatically fed to the stream to dilute the gas and to keep it in a safe concentration range.
Measuring task is to determine continuously, with high accuracy and reliability the oxygen concentration level in the waste gas upstream the incineration plant and to provide limit value signals when critical limits values are reached. A short response time, of course, is the most crucial request on the measuring device for this application in order to detect fast critical O2 contents.