Compacted zone

The TF-Mini and TF-156 compactors (Vector Corporation, Marion, Iowa, U.S.A.) were equipped with concavo-convex rolls and single flight screws. The roll speed of the TF-156 was scaled to achieve the same linear velocity 74.2 in./min as the TF-Mini model. This setting maintained a comparable dwell time for material in the compaction zone. The TF-156 roll force was scaled to 5.6 ton, which equaled a force per linear-inch approximately equal to the TF-Mini 3.1 ton/in. roll width. The authors established a feed screw speed to roll speed ratio of 1.3 1 for the trials. Table 7 gives the compactor equipment settings. 

Figure 11.1. Sedimentation behavior of a slurry, showing loose and compacted zones (Osborne, 1981).

The dry granulation throughput (mass per unit time) is defined by the following two equations, assuming a no-slip condition at the interface between the roller and the ribbon at the compaction zone, and negligible compaction zone bypass. 

We expand on the separation power exhibited along the flow axis. We noted earlier that flow is a powerful transport mechanism, capable of carrying components rapidly over considerable distances. Flow is also capable of keeping the components in fairly compact zones by virtue of its power to evacuate component material from one region as it carries it into another. These capabilities, as noted in Section 7.7, stem from the flow transport term, -vdc/dx, in the basic transport equation, Eq. 3.30. When acting alone, this term gives... 

Pietsch noted that the peripheral roll speed and particulate powder speed are not equivalent in the entire compaction zone. Throughput does not increase proportionally with roll speed. There are two effects that hinder throughput starved conditions in the feed zone, and secondly, too much squeezed air from the particle mass flows upward and against the powder flow, reducing fhe supply of maferial fo fhe nip... 

Froude Number has been described for powder blending and was suggested as a criterion for dynamic similarity and a scale-up parameter in wet granulation. The mechanics of the phenomenon was described as interplay of the centrifugal force (pushing the particles against the mixer wall) and the centripetal force produced by the wall, creating a compaction zone. ... 

Choose a layer and mobile-phase combination that will separate the analyte as a compact zone with an Rf value in the range 0.2-0.8. 

In older roller compactors, the feed screw did not always adequately deliver the powder to the gripping and compaction zone due to stationary side seals which resist the flow. Factors affecting uniform distribution of compaction pressures were investigated by Funakoshi etal. (2S). The distribution of compaction pressures across the entire ribbon was estimated by determining the force needed to drill the ribbon at various locations. 

The process occurring during compaction of particulate matter in roller presses is described and interpreted by different authors in a rather similar way. The feed mechanism is characterized by the pressure caused by gravity or a force feeder system and the friction between the material and roller surface. Compaction between two rolls may be explained by dividing the nip area into two zones the feed zone and the compaction zone. 

One of the problems of counter-currently rotating double roller presses results from the quick change of cross section of the nip, i.e. a fast increase in densification, particularly between rollers of small diameter. Especially between essentially smooth rollers establishment of a compaction zone is difficult if no positive feed pressure is applied. 

Roll speed For most considerations and approximations it is assumed that the peripheral speed of the rollers and the speed of the particulate matter are identical in the entire compaction zone. In reality this is not true throughput does not increase proportionately with roll speed. The maximum speed is determined by two effects starved conditions in the compaction zone develop if ... 

A more detailed study of material motion in the actual compaction zone has been made in the roll press simulator using marker beads, 16 mm cinematographic photography, and stereoanalysis. The flow pattern of particles in the cups of the roller press was recorded which enabled determination of strain distribution. Figure 251 shows examples of bead positions before and after partial compaction. 

Design of the roller surfaces In designing a roller press surface, some fundamental considerations do influence the possible shape, size, and distribution of the cups or indentations.Limitations include the relationships of circumferential pocket dimension/roller circumference and pocket depth/roller diameter as well as the pocket shape. Associated with these are problems of release, clam-shelling (splitting), extrusion, removal of air from the compaction zone, and uneven distribution of feed as well as compaction pressure. In spite of those limitations, a large variety of shapes can be produced. 

Figure 226(a) shows a straight screw used for metering material into the compaction zone between the rolls, to maintain a certain pressure in the nip, or to transport sticky, not freely flowing, material. Particularly in the last case. 

If a force feeder is used, the power drawn by the motor driving this device determines the pressure in the material above the compaction zone. 

Campanelli et al (2004) discuss a model of rubber mixing in an internal mixer based on kinetic, thermodynamic and rheological equations that is used to determine the extent of dispersion, batch temperature and relative batch viscosity over time. The chemoviscosity model is complex in that a model must be developed for the compaction zone, the incorporation zone and the size-reduction zone, as defined by the mixing-time zones in Figure 6.21. 

Seep or Blowout. Compacted zones may cause entrapment of solutions, resulting in the formation of a perched water table within the dump. The buildup of hydrostatic pressure can cause surface seepage and even expulsion of solid material from the dump wall,... 

Fig. 8.123 describes the compaction of a particulate solid in the nip between two gravity fed counter rotating rolls. For clarity, the roller diameter D and the distance between the rolls are not to scale. In reality the roll gap is much smaller as compared with the roller diameter (e.g. D/Iia 100/2 to 100/5). Compaction between two smooth rolls may be explained by dividing the nip area into three zones The feed zone, the compaction zone, and the extrusion zone. 

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