In a 2001 comprehensive investigation report on reactive chemicals, the United States Chemical Safety and Hazard Investigation Board (CSB) reported that 22% of reactive chemicals incidents occurred in storage equipment and 25% occurred in reactors. 167 incidents were considered between 1980 and 2001. Although not specific to polymer systems, the storage equipment category includes monomer storage tanks and the reactors category includes polymerization reactors. Free-radical polymerization reactions are the best studied reactions in all of chemistry. In 1998, the chemical and petrochemical industries produced 87 billion lbs of polymers including thermoplastics, thermosets, synthetic fibers, and synthetic rubber. Acrylonitrile-Butadiene-Styrene (ABS) accounted for three billion lbs of thermoplastics. Many chemical operators have developed and tuned polymerization models (anionic, free-radical, etc.) that can easily and readily be coupled with thermally-initiated polymerization kinetics for pressure relief systems design under runaway reaction conditions for storage and process vessels.
Common polymerization reactions where emergency relief may be necessary include bulk, solution, suspension, and emulsion polymerizations. Bulk addition polymerization is the simplest of the four polymerization types. Bulk polymerization is a homogeneous system where the reaction occurs ”in bulk” and most often an initiator is used. If the polymer is soluble in the monomer, then the monomer concentration will decrease as more polymer is formed and as the mixture viscosity changes. If the polymer is not soluble in the monomer, initiation, propagation, and termination of polymerizationmight occur only in the monomer phase. Bulk polymerizations are inherently more difficult to control under runaway conditions because the increase in viscosity drastically reduces convection heat transfer/cooling.
Solution polymerization reactions are bulk polymerization reactions but use a solvent or diluent for better heat control. The amount and rate of heat removed due to the presence of the solvent depend on whether the boiling point of the solvent is above or below the onset of the polymerization reaction, and whether the monomer, polymer, or both are soluble in the solvent.
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