381416 A Method for Determining the Practical Limits of Polymer Pre-Concentration in Flash Evaporators: Modeling and Simulation
The polymerization of monomers is a process that produces commercially valuable materials that in their concentrated state typically have very high viscosities. These polymerization reactions are often exothermic and require large amounts of heat to be removed during the polymerization process. Due to the high viscosity of the concentrated polymer and the exothermic nature of the reactions, these polymerization reactions are often conducted using a large volume of solvents or a large excess of monomers. The solvents or excess monomers act as a heat sink to absorb the heat of reaction while also reducing the viscosity of the reaction mixture to a level that can be processed. The monomers and solvents are relatively volatile materials so they are referred to as volatiles. These volatiles must be reduced to very low levels to meet the final requirements of the polymer. The preferred method for removing these volatile materials from the polymer mixture is through several direct devolatilization steps. The direct devolatilization steps can be classified by the relative amount of volatile material removed during the step. Pre-Concentration refers to the first step where the largest amount of volatile material is removed, Final Devolatilization refers to the final step where the volatile level is reduced to the meet the final requirements, and Main Evaporation refers to the step(s) between the Pre-concentration step and the Final Evaporation step.
There are several commercially available systems used for the direct devolatilization steps, and in the very broadest terms these systems can be divided into two categories: Mechanically Agitated systems and Non-Mechanically Agitated systems. The most common Mechanically Agitated systems include High Volume Kneaders, Vented Extruders, and Thin-Film Evaporators. The Non-Mechanically Agitated systems include Flash Evaporators, Forced Circulation Evaporators and Falling Strand Evaporators.
Flash Evaporators have been used for over 80 years for the devolatilization of polymers. This technology is very robust and has been used for Pre-Concentration, Main Evaporation, and even for Final Devolatilization. However, this technology does not mechanically agitate the mixture so it is not suitable for all polymers / volatile mixtures. Using model transport, equilibrium and rheology equations, as well as available data for the main components of Flash Evaporators, a study was performed that shows the practical limits of the Flash Evaporator technology for use in Pre-Concentration systems.