A Simplified Model for the Melting of Polymer Pellets under Compression in a Twin-Screw Extruder
Plastic energy dissipation (PED) is an important mechanism involving the melting of polymer in a twin-screw extruder. In the present study, the melting behavior of polymer pellets under compression is studied by a simplified model with the finite element method, based on the fact that mechanical energy from the deformation of a single pellet is converted into heat energy. The model shows that the heating of the pellet is not uniform. Under compression, the contact area of the pellet yields first, and the deformation in the pellet depends on the compression ratio and properties of the pellet. The simulated results show that the temperature in the volume around the line passing through the contacting point and the center of the pellet is higher than the temperature further from the contact area.
Experimental investigation of polymer pellets melting mechanisms in corotating twin‐screw extrusion
With the visualization corotating twin-screw extruder, the melting of HDPE pellets under different screw configurations and operational conditions was investigated on-line. It is shown that the melting progression of polymer pellets in corotating twin-screw extruders is different from that in single-screw extruders. Screw configuration, screw speed, and feed rate are three decisive factors in polymer melting. Based on the melting phenomena, a new melting mechanism, island-sea melting model, was defined. It was found that island-sea melting model is an important mechanism to realize high efficiency melting progression. The state of polymer, the degree of fill, and the structure of kneading blocks are factors in determining the energy consumption of kneading blocks in melting zone. The conveying of polymer pellets in kneading blocks, in which pellets were compressed and deformed, consumed much more energy than the conveying of polymer melt. This indicates that the control of the deformation of polymer pellets in corotating twin-screw extrusion is a useful concept in developing new screw elements for high efficiency polymer melting. © 2002 Wiley Periodicals, Inc. Adv Polym Techn 21: 188–200, 2002; Published online in Wiley Interscience (www.interscience.wiley.com). DOI 10.1002/adv.10021
Increasing pulp yield in kraft cooking of softwoods by high initial effective alkali concentration (HIEAC) during impregnation leading to decreasing secondary peeling of cellulose
Abstract Pulp yield can be improved by a more homogeneous delignification of the chips, achieved by improved impregnation prior to the cooking stage. Complete and efficient impregnation is obtained by increasing the diffusion rate by means of an impregnation liquor with a high initial effective alkali concentration (HIEAC). In the present study, the effect of HIEAC in the impregnation was evaluated and compared to a reference impregnation procedure and a prolonged impregnation. After the various impregnation scenarios, the alkali concentration was always adjusted to the same level in the beginning of the cooking stage. Impregnation with a HIEAC resulted in yield improvements by 1–1.5% units, due to a higher cellulose yield and possibly also to higher yield of glucomannan. The HIEAC with an even alkali distribution within the chips prior to the cooking stage resulted in a more uniform delignification carbohydrate degradation. Yield increase obtained by uniform delignification is due to both decreased shives content as well as less secondary peeling.