Devolatilization description

As already indicated, a physical description in terms of a process with a single time constant is fair but an assessment on the basis of two time-scales gives even improved results. Therefore, more research has to be carried out to determine the characteristics of the most important additional phenomena. As an example a two time-scale model is applied to the previously reported measurements of Fig. 8.3 and displayed in Fig. 8.4. Clearly, the release is governed by two rates, typically a smaller and a larger time scale appear compared with the single rate case. However, the single rate results are still very valuable because they describe the apparent rate very well and this would be the only thing that can be described in coarse scale models of devolatilization, e.g., in CFD of biomass conversion. 

Large diameter, melt-fed extruders are commonly used for the final devolatilization and pelletization of LDPE and PE copolymers in resin manufacturing plants. A full description of this type of extruder and process is provided in Section 15.3. Simulation of these processes is complicated by the multiple flights used in the design and the high H/W aspect ratios of the channels. The processes can be simulated from the feed hopper to discharge, however, since they are not required to convey solids and melt resin. This section will show the requirements and difficulties for simulating these processes. 

Theoretical description of devolatilization of particulate polymer can generally be achieved with a relatively high degree of accuracy. In most cases, the process will be diffusion controlled. The diffusion coefficients in solid polymers are very low, ranging from about 10" mys to 10 mys. The temperature in the polymeric particle can usually be taken as constant since the thermal diffusivity (a 10" mys) is many orders of magnitude higher than the diffusion coefficient. 

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