Reaction vinyl chloride monomer process

Telescope the Process by Combining Stages. This has been done successfully in the conversion of propylene to acrylonitrile by direct ammoxidation rather than oxidation to acrolein followed by reaction with ammonia in a separate stage, as was described in the earlier patent literature. The oxychlorination of ethylene and HC1 directly to vinyl chloride monomer is another good example of the telescoping of stages to yield an economic process. 

This process is shown schematically in Figure 7. The ethylene part of the feed reacts with chlorine in the liquid phase to produce 1,2-di-chloroethane (EDC) by a simple addition reaction, in the presence of a ferric chloride catalyst (9). Thermal dehydrochlorination, or cracking, of the intermediate EDC then produces the vinyl chloride monomer and by-product HC1 (1). Acetylene is still needed as the other part of the over-all feed, to react with this by-product HC1 and produce VCM as in the all-acetylene route. 

The production of vinyl chloride monomer is only a part of PVC production. Polymerization of the monomer completes the process. Commercially, it is a batch operation by one of three methods suspension, emulsion, or bulk. In all three methods, the chemical reaction is a free radical-initiated chain reaction. Peroxides or redox systems generally are used to provide the initial free radicals. 

A polymer is a large molecule built up by a repetition of small simple chemical units. These large molecules are formed by the reaction of a monomer.72 For example, the monomer for the plastic polyvinyl chloride (PVC) is vinyl chloride. When the vinyl chloride monomer is subjected to heat and pressure it undergoes a process called polymerization (Table 1.3) the joining together of many small molecules in repeat units to make a very large molecule. Structural representations of the monomer repeat unit and polymer are shown below. 

Presence of impurities in excipients can have a dramatic influence on the safety, efficacy or stability of the drug product. Monomers or metal catalysts used during a polymerization process are toxic and can also destabilize the drug product if present in trace amounts. Due to safety concerns, the limit of vinyl chloride (monomer) in polyvinyl pyrrolidone is nmt 10 ppm, and for hydrazine (a side product of polymerization reaction) nmt 1 ppm. Monomeric ethylene oxide is highly toxic and can be present in ethoxylated excipients such as PEGs, ethoxylated fatty acids, etc. 

Table 22.8 compares the performance of the SR process for cleaning a 1 MM SCFD (miUion standard cubic feet per day) (0.0283 x lO m /day) air stream containing 260 ppm vinyl chloride monomer (VCM) to a level of 1 ppm with that of a conventional plug-flow reactor using a standard oxidation catalyst at a reaction temperature of 600 K [18]. The adsorbent in the SR process was RB... 

Figure 22.8 Sorption-reaction (SR) process for removal of trace organic contaminants (a) schematic drawing of a two column SR process, (b) shell and tube reactor configuration for the process, (c) isotherms for adsorption of trace vinyl chloride monomer (VCM) on an activated carbon.

Figure 2 shows typical steps in the manufacture of an emulsion resin. Basically, the vinyl chloride monomer is added to a pressure vessel (reactor) where in contact with the emulsifier and the initiator the polymerization is carried out under conditions of controlled pressure and temperature. Since it is uneconomical to carry out the reaction to 100% conversion, unreacted monomer is removed in a stripping vessel. If the polymer is to be applied via the latex (water-based coating) technique, the manufacturing process can now be considered complete. If a dispersion resin is to be produced, then latex from the blend tank is transferred to a spray dryer where water is removed. These very small polymer particles... 

By far the greatest part of PVC production across the world is now made by the suspension process. Vinyl chloride monomer (derived from a reaction between ethylene (derived from oil) and chlorine (derived from common salt) is dispersed in deionised water with the help of small quantities of chemical dispersants and polymerisation initiators (typically peroxide compounds). At moderately raised temperature (50 C) and pressure (0.7 MPa) polymerisation proceeds and the polymer can be removed from the resulting slurry by de-watering and steam stripping the unconverted vinyl chloride monomer. 

Most vinyl chloride monomer today is made via a three-step process using ethylene oxyhydro-chlorination. A small amount is made by the reaction of acetylene and hydrogen chloride, either as liquids or gases, with a copper chloride catalyst in the liquid process and a mercury catalyst in the gas process. Vinyl chloride is also made by the heating of ethylene chloride with alcoholic alkali. 

Ethylene dicldoride (EDC) is made by the direct chlorination of ethylene [8] in the presence of a catalyst such as iron (III) chloride or copper (II) chloride. EDC is also made by oxychlorination, a process where ethylene reacts with HCl and oxygen in the presence of a catalyst, often copper (II) chloride [9,10]. The EDC is then converted to vinyl chloride, commonly called vinyl chloride monomer (VCM) by a dehydrohalogenation reaction. The VCM is then polymerized to polyvinyl chloride (PVC). 

Vinyl chloride monomer is synthesized from the reactions shown in Figure 4.22. However, Process 2 is more commonly used. 

In the polymerization of polyvinyl chloride, unwanted gas is generated by side reactions, and some small amounts of air leak into the reactors. These inerts must be vented from the process. Because vinyl chloride monomer (VCM) is extremely volatile, the purge gas, although it is typically at 4-5 bars pressure, can contain as much as 50vol% monomer. As a consequence, the vented gas stream, although small, may contain several hundred thousand dollars worth of monomer values. A typical process flow scheme to recover VCM is shown in Figure 21.10 (Lahierc et al., 1993). 

An example of a commercial semibatch polymerization process is the early Union Carbide process for Dynel, one of the first flame-retardant modacryhc fibers (23,24). Dynel, a staple fiber that was wet spun from acetone, was introduced in 1951. The polymer is made up of 40% acrylonitrile and 60% vinyl chloride. The reactivity ratios for this monomer pair are 3.7 and 0.074 for acrylonitrile and vinyl chloride in solution at 60°C. Thus acrylonitrile is much more reactive than vinyl chloride in this copolymerization. In addition, vinyl chloride is a strong chain-transfer agent. To make the Dynel composition of 60% vinyl chloride, the monomer composition must be maintained at 82% vinyl chloride. Since acrylonitrile is consumed much more rapidly than vinyl chloride, if no control is exercised over the monomer composition, the acrylonitrile content of the monomer decreases to approximately 1% after only 25% conversion. The low acrylonitrile content of the monomer required for this process introduces yet another problem. That is, with an acrylonitrile weight fraction of only 0.18 in the unreacted monomer mixture, the low concentration of acrylonitrile becomes a rate-limiting reaction step. Therefore, the overall rate of chain growth is low and under normal conditions, with chain transfer and radical recombination, the molecular weight of the polymer is very low. 

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