Free-bubble devolatilization

The interfacial area between the polymer and the gas phase is one of the key parameters in the devolatilization processes, because the rate of VOC removal is proportional to it. The devolatilizers are designed to maximize this interface area. For free-bubble devolatilization, the area of the polymer film that is exposed to devolatilization can be calculated, taking into account the geometry of the equipment. Based on those principles, several models to describe the devolatilization in singlescrew extruders [11-14] and twin-screw extruders [15, 16] have been presented. 

As viscosity increases with decreasing volatile content, the flash tank becomes inefficient as bubbles are entrapped and redissolved upon discharge. The falling-strand devolatilizer, shown schematically in Fig. 8.2, was developed to answer this problem, and represents an improvement over the ordinary flash tank. Here the polymer solution is pumped at high superheat into thin strands that fall gravitationally into the vacuum tank. Free of hydrostatic or shear-induced pressure fields, the bubbles nucleate, grow, coalesce, and rupture so that the volatiles are released before they get trapped in the melt of the cachepot. 

As mentioned earlier, entrained free air cannot survive indefinitely in a liquid, but it can still play a role in the devolatilization process in rotary machinery, where the moving surface can drag free air into the melt, forming small bubbles that can serve as nuclei for... 

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