Particulate solids channel

The unit can be fed polymer in the particulate solids form or as strips, as in the case of rubber extrusion. The solids (usually in pellet or powder form) in the hopper flow by gravity into the screw channel, where they are conveyed through the solids conveying section. They are compressed by a drag-induced mechanism in the transition section. In other words, melting is accomplished by heat transfer from the heated barrel surface and by mechanical shear heating. 

Fig. 4.11 A column of particulate solids compressed between two pistons in a channel with a constant cross section. A force F0 is applied at x = 0, which is balanced by a force FL at x = L. The column is either moving at constant velocity or is stationary. The upper plate is either stationary or is moving with constant velocity.

Press in which a fly-wheel powered reciprocating ram densifies and extrudes particulate solids through a long extrusion channel. Particularly suitable for elastic materials (such as peat, lignite, biomass, etc.). Also ram press, exter press. 

Once the particulate solids have reached the feed port of the extruder, the material will flow down until it is situated in the screw channel. At this point, the gravity induced flow mechanism will essentially cease. In most extruders, the screw and barrel are placed in a horizontal direction and the role of gravity becomes a very minor one. In fact, in most analyses of solids conveying in single screw extruders. 

It has been found experimentally that, in most cases, polymeric particulate solids compact readily in the early portion of the screw channel. As a result, the solids form into a solid bed and the solids move down the screw channel in plug flow thus, at any cross-section of the solid bed all elements move at the same velocity. In other words, there is no internal deformation taking place inside the solid bed. This compaction of the particulate solids into a solid bed can occur only if there is a sufficient amount of pressure generation in the screw channel. 

To facilitate an understanding of how particulate solids are transported through a single-screw extruder we start with a model for drag-induced flow in straight channels. We then summarize the equations for flow of particulate solids in the single-screw extruder. We next add heat transfer to the transport of the particulate solids. 

The small particles are reported to be very harmful for human health [98]. To remove particulate emissions from diesel engines, diesel particulate filters (DPF) are used. Filter systems can be metallic and ceramic with a large number of parallel channels. In applications to passenger cars, only ceramic filters are used. The channels in the filter are alternatively open and closed. Consequently, the exhaust gas is forced to flow through the porous walls of the honeycomb structure. The solid particles are deposited in the pores. Depending on the porosity of the filter material, these filters can attain filtration efficiencies up to 97%. The soot deposits in the particulate filter induce a steady rise in flow resistance. For this reason, the particulate filter must be regenerated at certain intervals, which can be achieved in the passive or active process [46]. 

Several solids conveying models were developed by Campbell and his students at Clarkson University [19, 20]. These models will be referred to as either the Clarkson University models or the Campbell models. They proposed that the movement of the screw flight was pushing the polymer bed as the screw turns rather than the frictional force at the barrel moving the polymer pellets down the screw. For these models, they assumed that the solid bed behaved more like an elastic fluid rather than a solid and removed the torque balance constraint. Campbell and Dontula [20] reasoned that because the solid polymer pellets more closely resemble an elastic particulate fluid, no torque balance in the bed would be necessary. They further assumed that the force normal to the pushing flight was due to a combination of the force due to the pressure in the channel and a force proportional to the frictional force exerted at the barrel by the solid bed. The Campbell-Dontula model was first published as ... 

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