Falling strands

Based on some physically reasoned assumptions, Wirth (1990) obtained the following relations for estimating Reynolds number of the falling strands ... 

Many types of equipment have been developed to improve the evaporation of solvent in order to provide energy savings. The most widely used techniques for devolatilization are the falling strand devolatilizer (FSD), the thin-film evaporator and the vented extruder [113]. 

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. 

R. H. M. Simon, Flash Evaporators and Falling Strand Devolatilizers in Devolatilization of Polymers, J. A. Biesenberger, Ed., Hanser, Munich, 1983. 

Another technique to expedite the transport of the volatile components from the molten polymer is to increase the number and rate of bubbles formed [14], Techniques that have been used to increase the number of bubbles and their rate of formation (nucleation) are the addition of chemical nucleating agents [15] and ultrasound [16]. Nucleation of bubbles in the molten polymer can help expedite the achievement of equilibrium in conventional falling strand devolatilizers. However, this facilitation mechanism cannot get below equilibrium and thus has minimal value. 

Many standard methods are used to devolatilize materials. Flash devolatilizer or falling strand devolatilizer are synonyms of equipment in which the falling melt is kept below the saturation pressure of volatiles. Styrene-acrylonitrile copolymers devolatilized in flash devolatilizer had a final eoncentration of ethylbenzene of 0.04-0.06. Devolatilization of LLDPE in a single-serew extruder leaves 100 ppm of hydrocarbon solvent. 500 ppm chlorobenzene remains in similarly extruded polycarbonate. It is estimated that if the polymer contains initially 1-2% solvent, 50-70% of that solvent will be removed through the vacuum port of an extruder. These data seem to corroborate the information included in the above... 

Roving is fed into a cutter on top of plenum chamber. Chopped strands are directed onto a spinning fiber distributor to separate chopped strands and distribute strands uniformly in plenum chamber. Falling strands are sucked onto preform screen. Resinous binder is sprayed on. Preform is positioned in a curing oven. New screen is indexed in plenum chamber for repeat cycle. 

Fig. 5.89 Polished thin sections of a low orienta- " tion Gxtrudate observed in polarized light. An unoriented free fall strand shows banding normal to the strand axis (arrow) and away from the slightly oriented skin.

Falling-film or falling-strand devolatilizators receive increasing attention as they require no heavy and expensive machinery Polymer is simply pumped through small slits or holes. Efficient surface renewal is achieved by shear thinning during the fall [59] strands and films become very thin and may be completely depleted of... 

Numerous variations of the interfacial process have been published. The reactions can be carried out in batch in stirred tank reactors or continuously in series of CSTRs and tubular reactors. Intensive mixing with dispersion and redispersion is required throughout the reaction stages. After the reaction is complete, the brine phase is separated and the polymer solution washed to remove residual amine and base. Several processes for devolatilization are in use, including solventless precipitation, steam precipitation, spray drying, falling-strand devolatilization, and vacuum extrusion in devolatilizing extruders. 

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