Extruder static mixer

In the discussion on residence time distribution it was mentioned that the RTD of a static mixer tends to be narrower than the RTD of a narrow pipe. However, in addition to the RTD the actual residence times are important as well. Static mixers always add extra volume between the extruder and the die. Therefore, when a static mixer is used, the mean residence time will always increase when the entire extrusion system is considered, i.e., extruder, static mixer, and die. If the static mixer section is long, which it should be to give good mixing, then the increase in mean residence time can be considerable. This should be a concern in thermally less stable polymers. 

Elucidation of degradation kinetics for the reactive extrusion of polypropylene is constrained by the lack of kinetic data at times less than the minimum residence time in the extruder. The objectives of this work were to develop an experimental technique which could provide samples for short reaction times and to further develop a previously published kinetic model. Two experimental methods were examined the classical "ampoule technique" used for polymerization kinetics and a new method based upon reaction in a static mixer attached to a single screw extruder. The "ampoule technique was found to have too many practical limitations. The "static mixer method" also has some difficult aspects but did provide samples at a reaction time of 18.6 s and is potentially capable of supplying samples at lower times with high reproducibility. Kinetic model improvements were implemented to remove an artificial high molecular weight tail which appeared at high initiator concentrations and to reduce step size sensitivity. 

A new approach for obtaining kinetic data at short reaction times utilizing a static mixer coupled to the end of an extruder was introduced. This approach yielded kinetic data for an estimated 18.6 s reaction time and is potentially capable of much smaller times. 

Static mixers offer an alternative route to enhancing mixture uniformity, through repeated division and combination of polymer passing through a tube containing motionless profiled elements (Fig. 23g). Mixers of this type closely control the applied shear strain developed and are positioned between the extruder and die [96]. 

Fig. 23. Screw sections for enhancing distributive mixing in single-screw extruders (a) Dul-madge mixer, (b) Saxton mixer, (c) pin mixer, (d) pineapple mixer, (e) cavity transfer mixer, (f) slotted screw flight, (g) Kenics and Ross ISG static mixers [148]...

A number of innovative polymerization reactors using loop reactors, plug-flow and static mixer reactors, and continuous stirred-tank reactors have been reported. For example, Wilkinson and Geddes (15) describe a 50-liter reactor that has the same capacity as a 5000-gallon batch reactor. Extruders, thin-film evaporators, and other devices designed to provide intense mixing for viscous or unstable materials have also been used as reactors. 

The extruder is used since it mixes, melts, and degases the polymer. However, as said in the previous paragraph, from an exergetic point of view, it is not an efficient apparatus, since it dissipates mechanical work into heat by frictional forces. A possible alternative to the extruder could therefore be a separate degasser, a static mixer, or a gear pump to push the polymer melt through the mixer and perforated plate (Figure 11.6). 

The extruder is substituted by a deep flash vessel (which operates at 150 mbar), a gear pump, and a static mixer. The only exergy input is the energy requirement of the pump and the compressor, which removes the gas. The pump only needs to increase the pressure of the polymer such that it can... 

By computing the pressure drop in the static mixer and the pressure drop in the granulating head and adding this to the exergy requirements of the compressor, it is possible to compute the exergy input of the alternative scheme, and compare this with the regular extruder (Figure 11.7). 

Scaled exergy input of the alternative extruder and regular extruder plotted against static mixer diameter. 

Exergy requirement of alternative and existing extruder as a function of the static mixer diameter. 

The change from extruder to gear pump/static mixer could potentially increase the gel count. As LDPE and LLDPE primarily find their uses in foils, laboratory scale tests are recommended to assess the impact of the proposed setup on the gel count. Extruders usually decrease the gel count by kneading the polymer. It is unclear whether the proposed setup can accomplish the same. 

This study serves to illustrate the methodology of the lamellar model approach and indicate its complexity and limitations. Extension to other mixer-reactors with complicated flow patterns, e.g., static mixers or twin screw extruders is possible with very little conceptual modification. 

Mixing operations can be performed in a number of static and dynamic devices, e. g., in static mixers, kneaders, or twin screw extruders [1]- Extruders belong to the group of continuous dynamic mixers. 

Machine Type single screw extruder, twin screw extruder (co-rotating), twin screw extruder (counter-rotating), multi screw extruder, buss co-kneader, internal mixer, static mixer. 

PP was blended with ECP 2 extruders with a static mixer, to stretch LCP into microfibrils Sukhadiaela/., 1991, 1992... 

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