Screw self-wiping

Other methods also exist for handling the reaction mixture. For instance, self-wiping, twin-screw extmder using an intermeshing has been proposed (271). The advantages claimed include higher saponification temperature (less catalyst), increased water solubiUty (271), and fewer impurities (272). 

The constraint that the tip of one screw element wipe the flank of its mate in self-wiping, corotating twin-screw extruders leads to a unique relationship for the shape of the screw channel (Booy, 1978,1980). Figure 13 is an isometric view of this channel and Fig. 14 is a cross section of the channel in a plane that is perpendicular to the plane which defines the helix angle. Figure 14 shows the actual shape of the channel, which is described by the following expressions ... 

White, J. L., Kim, E. K., Keum, ]. M., and Jung, H. C., Modeling Heat Transfer in Screw Extrusion with Special Application to Modular Self-Wiping Co-Rotating Twin-Screw Extrusion, Polym. Eng. Set, 41, 1448 (2001)... 

S. Bawiskar and J. L. White, Melting Model for Modular Self Wiping Co-rotating Twin Screw Extruders, Polym. Eng. Sci., 38, 727 (1998). 

C. D. Denson and B. K. Hwang, The Influence of the Axial Pressure Gradient on Flow Rate for Newtonian Liquids in a Self Wiping, Corotating Twin Screw Extruder, Polym. Eng. Sci., 20, 965 (1980). 

The SSE is an important and practical LCFR. We discussed the flow fields in SSEs in Section 6.3 and showed that the helical shape of the screw channel induces a cross-channel velocity profile that leads to a rather narrow residence time distribution (RTD) with crosschannel mixing such that a small axial increment that moves down-channel can be viewed as a reasonably mixed differential batch reactor. In addition, this configuration provides self-wiping between barrel and screw flight surfaces, which reduces material holdback to an acceptable minimum, thus rendering it an almost ideal TFR. 

Fig. 13.1. Different types of extruders, a) single screw b) co-kneader c) non-intermeshing, mixing mode d) nonintermeshing, transport mode e) counter-rotating, closely intermeshing f) co-rotating, closely intermeshing g) conical counter-rotating h) self-wiping, co-rotating.

Self-wiping screw elements with the maximum possible number of threads give the largest free cross-section (see Fig. 2.5, Chapter 2). Thus, triple-flighted elements should be used in the intake zone of twin screw extruders with Dc/Dj < 1.3, and double-flighted elements in the case of D0/D, > 1.3. An intake zone length of 4 to 6 D is generally adequate for effective product feed. 

The enormous flexibility and variability of the system was possible thanks to the constant development of the process related components, such as the barrels and screw elements and new geometry variants being developed continuously. The initial aim of the design, to maintain the self-wiping function of the screw elements, is sometimes abandoned, in part or in whole, to permit innovative geometries. This enabled the co-rotating, twin-screw extruder to carry out the most diverse processes and handle the most diverse products with this single machine system. 

A cut perpendicular to the axle of a self-wiping screw element shows the same free cross-section at any point independent of the screw pitch or pitch direction. If this needs to be enlarged to improve product intake, the contour of the intermeshing profile may have to be abandoned in some cases. These undercut or double undercut elements (Fig. 12.12) offer 15 to 30% increase in the product volume, but increase the risk for product building up on the un-cleaned flanks of the elements. 

Although the cross section of the self- wiping profile is not affected by the pitch, this is not true for the free volume of a screw channel. The larger the screw pitch, the greater the free volume in the screw channel. The screw pitch determines the fill of the screw channel (Fig. 12.14). 

Conventional machine systems try to meet these conflicting demands by the separate optimization of screw geometries in the individual functional zones. However, the shear rate is similar in every zone, because the screw speed is always the same, and the self-wiping... 

Fig. 4 Self-wiping in the two and three lobed co-rotating twin-screw extruders. (Courtesy of Coperion Corporation.) (View this art in color at www.dekker.com.)...

Material transport in intermeshing, co-rotating twin-screw extruders is generally dependent on drag flow. The screws pick up the material as they rotate and, where the two screws meet, a complete transfer of the material from one screw to the other takes place, see Fig, 4. The tip of one screw wipes the flanks and roots of the other screw, resulting in a self-wiping action. As the material is transferred from one screw to the other, the direction of material flow is changed and new material surfaces are created with each screw revolution. 

Twin-screw extruder Dimensionless calculations of throughput and pressure gradients for flow of non-Newtonian fluids in screw and kneading disk elements of a modular intermeshing self-wiping corotating twin-screw extruder, as well as the temperature rise. Chen and White, 1992, 1993... 

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