Single-screw extruders devolatilization

Single Screw Extruder Devolatilization Using Latinen s Model Review the paper by Biesenberger and Kessidis and discuss (a) the experimental method used... 

A specific expression for the mass transfer rate in Eq. (11) was first developed by Latinen (1962) in a classic paper that showed how penetration theory can be applied to the analysis of devolatilization processes in single-screw extruders. The derivation presented here parallels that by Latinen but differs in some respects for reasons of clarity. 

J.A. Biesenberger and G. Kessidis, Devolatilization of Polymer Melts in Single Screw Extruders, Polym. Eng. Sci., 22, 832 (1982). 

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 pressme of volatiles. Styrene-acrylonitrile copolymers devolatilized in flash devolatilizer had a final concentration of ethylbenzene of 0.04-0.06. Devolatilization of LLDPE in a single-screw 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... 

Because recycled plastics have had a first use and have been environmentally exposed, required conversion processes are often more stringent. For example, compounding may have to be done on a twin rather than a single-screw extruder for dispersive mixing. In addition, compounding purification steps such as devolatilization and melt filtration may be required for some recycled feedstock. Consequently, the compounding process can have a significant impact on the overall conversion cost. Another contributor to conversion cost is the compatibilizer additive. The most effective of these additives cost about 2.00 per pound. This adds 10 cents per pound of product at a 5% level of compatibilizer additive. 

There are many different designs of devolatilizing single screw extruders with widely differing devolatilization capacity. Some of the more common ones will be described and discussed next. 

This equation corresponds to Eq. 7.434 for devolatilization in single screw extruders. In Eq. 10.119, the variables are ... 

DESIGN PROBLEM VII DESIGN OF A DEVOLATILIZATION SECTION FOR A SINGLE-SCREW EXTRUDER... 

The few models available concerned with devolatilization (DV) in single-screw extruders are based on diffusion theory (Biesenberger and Sebastian, 1983). Experimental data available suggest that the process is more rapid than can be accounted for by diffusion alone. However, a summary of the model is useful, because at least the most important variables are brought out. 

Single-screw and double-screw extruders are normally used for polymer melts to accomplish the deaeration or devolatilization of residual volatiles. Devolatilization in an extruder is effected through formation of the venting zone inside the chamber by carefully designed upstream and downstream screw sections. 

Devolatilizing Screw Extruder A 150-mm-diameter, square-pitched, single-flighted screw extruder, with screw channel depth of 25 mm and 20-mm flight width is used to devolatilize a 1000-kg/h stream with 0.78-g/cm3 density at 200° C and 125 torr. (a) At what frequency of screw rotation will the channel be 30% or less full (b) With water injection, if density is halved by formation of 1-mm bubbles, how much surface area (per meter length) is created (c) How does... 

If a linear rubber is used as a feedstock for the mass process (85), the rubber becomes insoluble in the mixture of monomers and SAN polymer which is formed in the reactors, and discrete rubber particles are formed. This is referred to as phase inversion since the continuous phase shifts from rubber to SAN. Grafting of some of the SAN onto the rubber particles occurs as in the emulsion process. Typically, the mass-produced rubber particles are larger (0.5 to 5 JJ.ni) than those of emulsion-based ABS (0.1 to 1 Jim) and contain much larger internal occlusions of SAN polymer. The reaction recipe can include polymerization initiators, chain-transfer agents, and other additives. Diluents are sometimes used to reduce the viscosity of the monomer and polymer mixture to facilitate processing at high conversion. The product from the reactor system is devolatilized to remove the unreacted monomers and is then pelletized. Equipment used for devolatilization includes single- and twin-screw extruders, and flash and thin film evaporators. Unreacted monomers are recovered for recycle to the reactors to improve the process yield. 

Polymer devolatilization can also be accomplished by the use of single and twin screw extruders [61]. Extruders are well suited for high viscosity materials and, when equipped with appropriate vents, allow the discharge of volatiles. 

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