Heat vinyl acetate monomer process

We first review in Part 1 the basics of plantwide control. We illustrate its importance by highlighting the unique characteristics that arise when operating and controlling complex integrated processes. The steps of our design procedure are described. In Part 2, we examine how the control of individual unit operations fits within the context of a plantwide perspective. Reactors, heat exchangers, distillation columns, and other unit operations are discussed. Then, the application of the procedure is illustrated in Part 3 with four industrial process examples the Eastman plantwide control process, the butane isomerization process, the HDA process, and the vinyl acetate monomer process. 

Addition polymerization is exothermic, and one of the major constraints to high production rates is the problems associated with heat removal. In processes using ethylene, the pressure of the gas determines solubility in the liquid phases (i.e., water and vinyl acetate monomer droplets) and in the polymer particles. This concentration of ethylene at the point of polymerization determines the ethylene content of the final polymer. Use of high pressures in such systems eliminates refluxing of the vinyl acetate, losing a very eflective heat removal mechanism available to simple batch-process PVA production. Refluxing,... 

Process Make a presolution of the poly(vinyl alcohol). Add to the polymerization kettle, agitate, and heat to 65°C, meanwhile adding the other ingredients of the initial reactor charge. At 65°C, add the initial vinyl acetate monomer and the first initiator. Heat cautiously to 80°C, during which time the initial vinyl acetate will polymerize (shown by the development of a blue color, a reduction or cessation of reflux, and a slight exotherm). 

Polymerization of a monomer in a solvent overcomes many of the disadvantages of the bulk process. The solvent acts as diluent and aids in the transfer of the heat of polymerization. The solvent also allows easier stirring, since the viscosity of the reaction mixture is decreased. Thermal control is much easier in solution polymerization compared to bulk polymerization. On the other hand, the presence of solvent may present new difficulties. Unless the solvent is chosen with appropriate consideration, chain transfer to solvent can become a problem. Further, the purity of the polymer may be affected if there are difficulties in removal of the solvent. Vinyl acetate, acrylonitrile, and esters of acrylic acid are polymerized in solution. 

The final product of emulsion polymerization is an emulsion —a stable, heterogeneous mixture of fine polymer beads in an aqueous solution, sometime called a latex emulsion. Water-based paints, for example, can be formed from the emulsion polymerization of vinyl acetate. In this process, I m of water containing 3% poly(vinyl alcohol) and 1% surfactant are heated to 60°C in a reaction vessel (see Figure 3.27) The temperature rises to around 80°C over a 4 to 5 hour period as monomer and an aqueous persulfate solution are added. The rate at which heat can be removed limits the rate at which monomer can be added. 

Copolymerization. Vinyl chloride can be copolymerized with a variety of monomers. Vinyl acetate, the most important commercial comonomer, is used to reduce crystallinity, winch aids fusion and allows lower processing temperatures. Copolymers are used in flooring and coatings. This copolymer sometimes contains maleic add or vinyl alcohol (hydrolyzed from the poly(vinyl acetate ) to improve the coating s adhesion to other materials, including metals, Copolymers with vinylidene chloride are used as barrier films and coatings. Copolymers of vinyl chlonde with acrylic esters in latex from are used as film formers in paint, nonwoven fabric binders, adhesives, and coatings. Copolymers with olefins improve thermal stability and melt flow, but at some loss of heat-deflection temperature,... 

The bulk polymerization of vinyl acetate is primarily of interest for laboratory studies, although a few large-scale procedures have been reported. Since the heat of polymerization is quite high (21 kcal/mole) and the boiling point of the monomer is relatively low (72.7°C) (Table I), not only must the reaction temperature be monitored closely, but the reaction temperature must be kept low, unless pressure equipment is used. The low temperatures mean that the usual initiators of free-radical polymerization will act rather slowly. To further complicate bulk polymerizations, the polymerization process is strongly auto-catalytic [17, 68]. 

In a complex apparatus, Gimesch and Schneider [30, 119] studied the suspension polymerization of vinyl acetate. Their procedure involved equipment which automatically added tempered water to the reacting system as heat was evolved as a result of the polymerization process. Thus they maintained isothermal reaction conditions. The rate of reaction could be followed by recording the water uptake of the equipment with time. The heat of polymerization was also determined (found to be 23 kcal/mole which was considered a satisfactory check of the literature value which is scattered around 21.4 kcal/mole). From this work, a somewhat different mechanism of the suspension polymerization process emerges than the widely accepted concept of the water-cooled bulk polymerization of small particles. It was noted that with an increase in the initiator concentration, there was the expected increase in polymerization rate. With increasing stirring rate, the rate of polymerization decreased. Along with the suspension polymerization, there was always a certain amoimt of imdesirable emulsion polymerization. It was postulated that in the process, free radicals, formed in a monomer drop may be extracted into the aqueous phase where they may act on dissolved vinyl acetate by kinetic processes unique to this system and different from the conventional mechanism of suspension polymerization. 

The process represented in Figure 5.82(d) is similar to that in Fignre 5.82(c), except that polymerization is initiated in the water-immiscible solvent phase. A vinyl monomer, e.g., styrene, methyl methacrylate, or vinyl acetate, is dissolved in a water-immiscible solvent together with an initiator. The solution is emulsified in water using an emulsifier and heated to initiate free-radical polymerization. The resulting polymer deposits at the solvent-water interface forming the capsule wall. 

There are four kinds of polymerization processes bulk, solution, emulsion, and suspension polymerization. As Table 4.7 shows [24], the heat of polymerization of vinyl acetate is high compared to other monomers hence, the control of temperature is difficult in bulk polymerization. In the case of emulsion and suspension polymerization, it is somewhat troublesome to separate dispersed polyvinyl acetate particles from the aqueous medium, and it is necessary to remove the emulsifier and stabilizer completely because these substances induce problems in the process of fiber-making. 

Polyvinyl alcohol is typically produced by the hydrolysis of polyvinyl acetate in a continuous process [52, 53], Although specialty grades of polyvinyl acetate may be formed using a batch process, the majority of commercial polyvinyl acetate is formed by the free radical polymerization of a vinyl acetate solution in methanol using a continuous process. Polymerization reactions are typically conducted at 55-85 °C, where heat is removed from the reaction mixture by condensing the monomer. At the end of the process, residual vinyl acetate and methanol are stripped from the reactor and recycled. Polyvinyl acetate formed by this process has long and short chain... 

The concept of unsaturated polyesters (international abbreviation UP) which can be cured and hardened by heating with sources of radicals was developed by C. Ellis in the 1930s [1-3], and the first patent application was submitted in 1936 Ellis also discovered that dilution of the unsaturated polyesters with vinyl monomers, such as styrene (most widely used) or vinyl acetate is favorable for most applications. The dilution reduces the viscosity. Eases the dissolution of additives and catalysts and lowers the costs. The most frequently used catalysts for the curing process are peroxides. The variation of their structure allows for optimization of the cure. The most important application of UPs are ... 

The continuous bulk polymerization of methyl methacrylate was used as an example in Section 5.2. A stirred bulk polymerization like that used for styrene (Section 5.4) could be adapted for methyl methacrylate. A suspension process for poly(methyl methacrylate) was described in Section 5.4. The polymerization of ethyl acrylate most often is carried out in emulsion. A process such as that used for vinyl acetate is suitable (Section 16.4). Like vinyl acetate, the monomer is slightly water soluble, so true emulsion polymerization kinetics are not followed. That is, there is initiation of monomer dissolved in water in addition to that dissolved in growing polymer particles. Ethyl acrylate is distinguished by its rapid rate of propagation. Initiation of a 20% monomer emulsion at room temperature by the redox couple persulfate-metabisulflte can result in over 95% conversion in less than a minute. As with vinyl acetate polymerization, a continuous addition of monomer at a rate commensurate with the heat transfer capacity of the reactor is necessary in order to control the temperature. 

---