Agglomerator medium pressure

To render materials suitable for pelleting or extrusion, they must have inherent binding characteristics or contain binders and feature a certain lubricity. Therefore, most medium pressure agglomeration techniques use moist mixtures which are prepared in a mixing step prior to pelleting. 

An important advantage of medium pressure agglomeration is that, in comparison with tumble or low pressure granulation, only one half to one third of the agglomeration liquid is required. Therefore, drying takes place more quickly and with less energy. 

For mechanical reasons it is not easily possible to equip the dies with bores of less than 1 mm diameter. This is why agglomerates formed by medium pressure (extrusion) are normally dried and then crumbled by crushing. Fines may be screened out and recycled to the mixer for renewed agglomeration. 

Medium-pressure agglomeration Pelleting, extrusion through perforates die plates (Section 8.4.2)... 

In medium-pressure agglomeration (see Section 8.4.2), the extrusion through perforated die plates (II.2), higher densification can be obtained by two effects. First, the feed, that must have binding characteristics and be somewhat plastic, is predensified prior to extrusion by the screw(s) or between the press roUer(s) and the die. Secondly, the material is passed through holes in the die where pressure builds up due to wall friction. The length over diameter ratio of the hole (= extrusion channel) and its shape as well as the properties of the feed determine final densification. Because of the wall friction, the surface of the extrudate is always denser than the center (see also Section... 

Fig. 6.4 Schematic representation of equipment for (a) low- and (b) medium-pressure agglomeration, (a.l) Screen,...

Medium-pressure a lomeration or "pelleting comprises processes in which sufficiently plastic and lubricated particle mixtures are extruded through perforated dies (see Chapter 6, Fig. 6.4b). In contrast to the dies used in low-pressure agglomeration, the openings feature significant length and the densification pressure is caused by the frictional resistance in the orifice during extrusion (see Section... 

Therefore, while in tumble/growth a lomeration at least one post-treatment step is almost always a necessary part of any system, the same is true for all low-pressure agglomeration but only for some medium-pressure agglomeration and very seldom for high-pressure a lomeration processes. Increasing with the applied force during the process, post-treatment methods in pressure agglomeration are directed towards modifications of product size, shape, and/or structure. 

Fig. 8.37 Sketches of six typical extrusion channel designs for medium-pressure agglomeration. 

Fig. 8.39 Replaceable inserts for medium-pressure agglomeration by extrusion with (a) short and (b) long bores (courtesy HOSOKAWA BEPEX/Hutt, Leingarten, Germany).

Fig. 8.40 Sketches explaining the basic principle of medium-pressure agglomeration by extrusion [B.42]. For explanations see text.

In medium-pressure agglomeration equipment (Fig. 8.42), the perforated support (die) can be either flat (a), concave (b), or convex (c). For all three designs, intermeshing, toothed executions have been proposed in the patent literature [B.42] to avoid slippage and improve extrusion as well as extrudate quality, but only the so called gear pelleter , in which the convex dies (Fig. 8.42c) are hollow gears with extrusion channels between the teeth, has reached commercial importance (see below). 

In spite of the problems that are associated with this design, it is often applied, particularly in clean environments and for easily deformable materials which require small extrusion pressure. Tab. 8.2 lists the standard models offered by one manufacturer and summarizes their most important technical details. Among the medium-pressure agglomerators or pellet mills, the Alexanderwerk moist granulator represents equipment that operates with the lowest forces. In Fig. 8.48 three examples of products are shown. As can be easily seen, granules or pellets can be well formed (a and c) or somewhat crumbly (b). If the latter is not acceptable, the extrusion characteristics of the feed may be adjusted by changing (in this case increasing) the moisture, binder, and/or lubricant contents. 

As in all medium-pressure agglomerators that use rollers for the densification and extrusion of moist and/or plastic materials, the primary cause for roller rotation is friction between the roller surface and the material to be processed. The roller is mounted on a shaft with sealed antifriction bearings and is brought to close proximity... 

Medium Pressure Axial Screw Extruders Axial screw extruders normalty operate with low (see Section 8.4.1) or high (see Section 8.4.3) pressure. The basic principle of an axial screw extruder was shown in Fig. 6.4b.1 (Chapter 6) and Fig. 8.24 (Section 8.4.1). The pressure that is developed by the screw(s) depends on the power of the drive and the frictional resistance in the extrusion channel or other discharge device. Axial screw extruders that rely solely on the pressure developed by the rotating screw(s) employ hydrostatic pressure as the driving mechanism for extrusion. Such machines generally use high pressure. However, under certain conditions and for specific applications, some can be classified as medium-pressure agglomerators. 

Other medium-pressure agglomerators that are offered by several manufacturers and are mostly used in the food and animal feed industries are pressure cooker extruders. They apply medium pressure because the mostly grain and/or vegetable based starchy, organic feeds are conditioned by pressure and heat into easily deformable and extrudable masses. 

After doing this, not much change in ranking occurs in this example. The only remarkable modification is, that spray drying moves up in rank and looks now as good as or better than the mixer. Also, pelleting, a medium-pressure agglomeration technique, becomes a feasible alternative. 

Medium-pressure agglomeration in pellet mills can be easily simulated because even a single bore with the correct diameter to length ratio and featuring all other details of the orifice (e.g. inlet chamfer, discharge cone, relief bore, etc., see Section... 

On first sight, the block diagram for pressure agglomeration looks almost identical to that of tumble/growth agglomeration, particularly when post-treatment is required. Typically, this is the case if low- and medium-pressure agglomeration methods are used. These methods require liquid binders to guarantee easy formability (see Sections... 

In a much smaller scale, the conditions in the die holes during medium pressure agglomeration or pelleting (Chapter 5, Fig. 5-10bl-b6) realize the same densification process as developed in the (long) channel of ram or screw extrusion presses. Therefore, medium pressure agglomerators can be also used for the production of fuel pellets from, for example, shredded paper, saw dust, plastic foil, and similar waste materials [B.97]. Fig. 6.10-27 shows two examples of lose raw materials and the resulting pelleted product. 

Fig. 6.10-27 Two waste materials and the resulting products after medium pressure agglomeration in a flat die pelleting machine (courtesy Amandus Kahl, Reinbek/Hamburg, Germany)...

Low- and medium-pressure agglomeration methods apply extrusion through holes in die plates with different thicknesses. 

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