VAM-PVAc Production Process Safety

The process units for the production of VAM–PVAc should consider the following two exothermic reactions that occur during operation because of their catalysts [17 (link), 18 (link)]. VAM has a significant level of water-solubility solvent, which contributes to its wide polymerization application. Free-radical polymerization of VAM is illustrated as follows: $${\text{C}}_2{\text{H}}_4+{\text{CH}}_3\text{COOH}+\frac{1}{2}{\text{O}}_2\stackrel{\text{metal}\text{catalyst}}{\to }{\text{CH}}_2={\text{CHOCOCH}}_3+{\text{H}}_2\text{O}$$ $$({\mathrm{CH}}_3{)}_2\mathrm{C}(\mathrm{CN})\mathrm{N}=\mathrm{C}({\mathrm{CH}}_3{)}_2\mathrm{CN}\stackrel{\mathrm{\Delta }}{\to }2({\mathrm{CH}}_3{)}_2\dot{\mathrm{C}}(\mathrm{CN})+{\mathrm{N}}_2$$ $${\text{nCH}}_2={\text{CHOCOCH}}_3\underset{60{}^{\circ }C}{\overset{{({\text{CH}}_3)}_2\text{C}\left(\text{CN}\right)\text{N}}{\to }}{[\begin{array}{c}\begin{array}{c}\\ \end{array}\\ -{\text{CH}}_2-\\ \begin{array}{c}\\ \end{array}\end{array}\begin{array}{c}\begin{array}{c}\\ \end{array}\\ \text{CH}-\\ \begin{array}{c}|\\ {\text{COOCH}}_3\end{array}\end{array}]}_n$$
Free-radical-induced polymerization is the predominant reaction mechanism of VAM. The catalyzed initiation of VAM polymerization can be induced by peroxides, azo compounds, light, and high-energy radiation. To rapidly produce the free radicals, initiators are usually poured into the reactor to supply a large amount of free radicals and reduce the activation energy needed for the initiation procedure. Peroxidation of VAM occurs in the presence of oxygen at ambient temperature, and two structures of VAM polyperoxide are shown in Fig. 1, with structure 2 being more unstable than structure 1.Fig. 1

Two structures of VAM peroxidation that occur in the presence of oxygen

The decomposition of the initiator can be induced by thermolysis or photolysis. Equation (3) shows that initiator (I) receives the external energy breaking into two radicals ( $I·$ and $I^{\prime }·$ ) and attracts the neighboring monomer (M) to form initiating radicals ( $\mathrm{I}-\mathrm{M}\bullet$ ). Naturally, the initiating radicals grow into propagating radicals ( $\mathrm{I}-\mathrm{M}-\mathrm{M}\bullet$ ), combining with a new monomer. The polymer chain can grow interminably before the chain transfer or termination reaction occurs. Unstable VAM proceeds to thermally initiated polymerization on the effect of oxygen at 50 °C–120 °C [19 (link)]. An unpredictable bulk polymerization of VAM involving the monomer and solvent is dangerous because the exothermic reaction possibly occurred in the process vessels. VAM is recognized as an NFPA class IB flammable liquid due to a flash point of ca.−8 °C and boiling point of 72.7 °C (which is close to the process operation temperatures of 60 °C–70 °C). A higher vapor density of 2.97 (air is 1.0) allows the vaporizing VAM to form the vapor cloud in case of release or crack from the vessel or pipeline and then, to stay near the ground, which may ignite to cause a vapor cloud explosion (VCE). The workers should be aware of the flammable vapor mixed with air at ambient temperatures and take precautions against the ignition source in the processing or storing units [20 ]. The polymerization might be initiated under ambient temperatures, including a chemical acceleration trend related to the free-radical nature of the VAM chain reaction [21 (link)]. Both industrial accidents relating to VAM–PVAc processes occurred in a petrochemical plant in Taiwan [22 , 23 ]. The first case caused severe fires in the plant because of failure management of change (MOC) in a residual vapors/gasses recycle system during a VAM polymerizing process in 2011. Due to the flammable VAM and methanol released during alkalization, both vaporizable reactants ran into heat sources from an alkaline machine, causing the unit to burst into flames. Another one resulted in an explosion of two refined vessels containing 76% PVAc and 24% methanol due to a hot work error of maintenance in a feeding pipeline in 2012. The main reasons from the official investigation were tanks not completely empty and purged before using an electric welding machine in hot operations, as well as failures of MOC, preliminary hazard assessment (PHA), and contractors’ management. Both accidents resulted in one death and eight injuries of the employees during two vessels rupture and severe blast effects. Figure 2 illustrates an overview of this accident.Fig. 2

Scheme of an explosion accident that occurred in two PVAc refined tanks due to a lack of proper hot work management