Process steps, vinyl chloride monomer

Vinyl chloride monomer (VCM) manufacture Maximization of VCM production and minimization of environmental burden, environmental impact and operating cost simultaneously. e-constraint method A design methodology consisting of 4 steps was proposed and applied to VCM plant The steps are (1) life cycle analysis of the process, (2) formulation of the design problem, (3) MOO, and (4) multi-criteria decision-making to find best compromise solutions. Khan et al. (2001)... 

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... 

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. 

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. 

In addition polymerization, monomers react to form a polymer chain without net loss of atoms. The most common type of addition polymerization involves the free-radical chain reaction of molecules that have C = C bonds. As in the chain reactions considered in Section 18.4, the overall process consists of three steps initiation, propagation (repeated many times to build up a long chain), and termination. As an example, consider the polymerization of vinyl chloride (chloro-ethene, CH2 = CHC1) to polyvinyl chloride (Fig. 23.1). This process can be initiated by a small concentration of molecules that have bonds weak enough to be broken by the action of light or heat, giving radicals. An example of such an initiator is a peroxide, which can be represented as R—O—O—R, where R and R represent alkyl groups. The weak 0—0 bonds break... 

Addition polymerization takes place for unsaturated monomers. In the presence of a catalyst, such as a free radical, a pi bond in the monomer is disturbed, and the resulting molecule is. itself, a chemically active free radical. This first step of the process is called initiation. The process may then continue, with the new molecule bonding with additional monomers in the same manner, thus forming a chain. Following this propagation step, free radicals may combine, thus forming a more stable polymer chain. This final step is called termination. Peroxides, such as benzoyl peroxide, are common agents that, when heat is applied, form free radicals that can initiate the polymerization process. An example of addition polymerization is shown below for the monomer vinyl chloride, which forms polyvinyl chloride. 

There are two fundamental polymerization mechanisms. Classically, they have been differentiated as addition polymerization and condensation polymerization. In the addition process, no by-product is evolved, as in the polymerization of vinyl chloride (see below) whereas in the condensation process, just as in various condensation reactions (e.g., esterification, etherification, amidation, etc.) of organic chemistry, a low-molecular-weight by-product (e.g., H2O, HCl, etc.) is evolved. Polymers formed by addition polymerization do so by the successive addition of unsaturated monomer units in a chain reaction promoted by the active center. Therefore, addition polymerization is called chain polymerization. Similarly, condensation polymerization is referred to as step polymerization since the polymers in this case are formed by stepwise, intermolecular condensation of reactive groups. (The terms condensation and step are commonly used synonymously, as we shall do in this book, and so are the terms addition and chain. However, as it will be shown later in this section, these terms cannot always be used synonymously. In fact, the condensation-addition classification is primarily applicable to the composition or structure of polymers, whereas the step-chain classification applies to the mechanism of polymerization reactions.)... 

The earliest polymerization processes were either batch mode or semibatch. The semibatch method was used for products, where the two monomers differed greatly in reactivity, as in Union Carbide s early Dynel, acrylonitrile-vinyl chloride, process. Bulk, solution, and emulsion polymerization processes have also been developed for acrylonitrile and its copolymers. However, in recent years nearly every major acrylic fiber producer has used a continuous aqueous suspension process, employing a redox catalyst, followed by a series of steps, which includes slurry filtration and polymer drying. 

Although flash devolatilization has been applied for high volatility monomers removal, such as vinyl chloride from poly(vinyl chloride) [103] and butadiene from polybutadiene [104], devolatihzation of aqueous polymer dispersions is usually carried out using a stripping agent (steam and nitrogen are the most commonly used air can also be used, but explosive vapor mixtures can be produced). Devolatilization of aqueous phase dispersions is a mass-transfer process, which involves the following steps in series ... 

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