Polymer isolation as important step in synthetic rubber production processes
May 10-15, 2015
The Fraunhofer Society is with more the 20.000 employees organized in 66 institutes the largest research organization for applied research in Europe. The Fraunhofer Polymer Pilot Plant Center, a joint initiative of the two Fraunhofer Institutes for Applied Polymer Research IAP in Potsdam/Golm and Mechanics of Materials IWM in Halle, is situated in Schkopau, approx. 180 km south of Berlin and is active in contract research in the area of polymer synthesis and polymer processing, in particular in scale-up and process development. One important research area in the pilot plant center is synthetic rubber. With about 12 Mio tons of global annual production, synthetic rubber is an important class of polymeric materials, its demand is increasing with the increasing mobility demands in the world. Modern high-performance rubbers with controlled microstructure are produced either by anionic or coordinative solution polymerization. In such solution polymerization processes, the maximum polymer content in the solution is limited by viscosity to in between 10 and 20 wt.%. Downstream the polymerization section, the rubber is isolated from the solvent, the solvent is purified and fed back to the polymerization section, while the rubber is degassed and dried. It is important to realize, that -due to the limited polymer content in the polymer solution-, the mass flow of the recycled solvent exceeds the rubber production capacity by 5 to 10 times. This illustrates the importance of the rubber isolation and solvent purification steps for the overall process efficiency and process economics. In the contribution, three different rubber isolation technologies available in the Fraunhofer Polymer Pilot Plant Center will be discussed. In the traditional coagulation/stripping process, polymer solution is contacted with steam and water in a nozzle; the solvent is evaporated and the rubber solidifies in the form of sticky rubber crumbs. Due to the evaporation with steam, wet solvent is recovered and needs to be purified extensively, since in anionic or coordinative polymerization only very low water contents of a few ppm in the solvent can be allowed. Also the rubber crumbs are obtained wet and need to be dewatered and dried without damaging the temperature-sensitive polymer. Roughly 80% of the overall energy consumption of conventional rubber production does account for polymer isolation and solvent recovery. Operation and operability on pilot scale is tricky. One alternative polymer isolation process is the direct devolatization in kneader reactors, a process developed and owned by LIST company/Switzerland. In a first stage, >99% of solvent is evaporated in a high-viscosity kneader by input of thermal- and mechanical energy. In a second kneader, the rubber is further degassed down to solvent-residues in the ppm range. Since majority of the solvent does not get into contact with water, one key-advantage of the process is significant energy saving due to much reduced effort for solvent purification. Furthermore, milder drying for thermal sensitive grades is possible and the overall process setup is much simpler with lower space-requirements compared to traditional coagulation/stripping. A number of pilot runs have been carried out already for LIST in the Fraunhofer Polymer Pilot Plant Center and excellent product qualities have been acknowledged by several rubber producers. The 3rd process to be discussed is a new, proprietary development by Fraunhofer, a stripping process with short residence time. Key idea is to generate small rubber particles, which degas in a highly turbulent flow in much shorter residence time compared to the conventional coagulation/stripping. The process consists of a nozzle, where rubber-solution is contacted with steam and circulating water, a residence-time section, and two phase separators. In first a gas-liquid separator, the solvent vapors are separated from the liquid phase containing rubber crumbs, in a following solid-liquid separator, the rubber crumbs are separated from the circulating liquid phase. The process has already been applied successfully in a number of projects with different products. Key advantages are reduced hold-up and fast transitions due to the reduced residence time and -at least in our pilot-scale experience- a much better operability compared to the traditional coagulation/stripping process.
Michael Bartke, Marcus Vater, Ulrich Wendler, and Boyd Safrit, "Polymer isolation as important step in synthetic rubber production processes" in "Polymer Reaction Engineering IX (2015)", E. Vivaldo-Lima, UNAM; J. Debling, BASF; F. Zaldo-Garcia, CP-COMEX; J. Tsavalas, Univ. of New Hampshire Eds, ECI Symposium Series, (2015). https://dc.engconfintl.org/polymer_rx_eng_IX/41