Traditional processing of elastomers uses a high concentration of solvent to keep the rubber solution viscosity low in order to create better mixing and heat transfer during the reaction. Once polymerisation is complete, the resulting prod-uct can contain up to 93 % solvent. Removal, or devolatilisation, of the solvent represents a significant cost for processors.
During conventional devolatilisation, superheated steam is injected into a hot water bath containing the rubber solution. As the solvent is removed, the polymer concentrates and coagulates. The solvent and off-gas are then separated through condensation and distillation. Finally, the mixture of water and coagulated rubber crumb is extruded through a dewatering screw and output to a belt dryer. Although steam stripping represents the status quo within the rubber industry, it is not ideal. The time, cost and water required affect profitability and create environmental issues. In addition, new generation rubbers are increasingly difficult to be devolatilised with steam stripping, direct devolatilisation on the other hand shows no difficulties with low Mooney viscosity and sticky grades, which are characteristic for new generation rubbers.
Direct devolatilisation
List engineers developed the direct devolatilisation process. It reduces multi-phase steam stripping to two steps and enables processors to eliminate a majority of the water, energy and waste disposal costs associated with traditional devolatilisation. Direct devolatilisation begins inside the main evaporator where up to 98 % of the solvent is removed using mechan-ical and thermal energy. The kneading action of the rotating shafts supplies the mechanical energy, which is the majority of the required evaporation duty. In the second step, material with 2 to 5 % solvent is transferred to a separate finisher. Continuous mixing and kneading creates constant surface renewal, allowing the solvent to quickly evaporate. The residual solvent level is 10 to 200 ppm. In addition to streamlining the devolatilisation process, Lists’s direct devolatilisation provides improvement in terms of environmental impact. It enables processors to reduce the amount of water and resources consumed, as well as the costs and risks associated with disposing of the contaminated waste-water and solvent-rich off-gas. By eliminating the drying process, direct devolatilisation also allows processors to avoid exposing workers to one of the more dangerous operations in the production process.
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