Feedstock pellets

Conventional transition sections are constructed by simply decreasing the depth of the channel in the down-channel direction. The amount and rate of the depth change sets the performance of the melting process and the removal of entrained air that resides between the feedstock pellets or powders. The compression ratio sets the amount of compression while the compression rate sets the rate of the compression. The compression ratio and compression rate are calculated as follows for conventional-flighted transition sections ... 

U.S. Pat. No. 5,938,994 [112] describes a WPC material produced as feedstock pellets in a twin screw extruder, and comprising wood flour (about 20-80%) and polyethylene (80-20%) as a preferred plastic. 

U.S. Pat. No. 6,632,863 [115] (by Crane Plastics Company, Timber-Tech) discloses a wood-plastic composition manufactured as feedstock pellets comprising 55-90% cellulosic material such as wood flour and wood fiber, 10-40% of polyolefin such as HDPE, LDPE, and polypropylene, and 0-35% total of additive(s), such as lubricants and inorganic fillers, such as talc and mica. 

If a linear mbber is used as a feedstock for the mass process (85), the mbber becomes insoluble in the mixture of monomers and SAN polymer which is formed in the reactors, and discrete mbber particles are formed. This is referred to as phase inversion since the continuous phase shifts from mbber to SAN. Grafting of some of the SAN onto the mbber particles occurs as in the emulsion process. Typically, the mass-produced mbber particles are larger (0.5 to 5 llm) than those of emulsion-based ABS (0.1 to 1 llm) and contain much larger internal occlusions of SAN polymer. The reaction recipe can include polymerization initiators, chain-transfer agents, and other additives. Diluents are sometimes used to reduce the viscosity of the monomer and polymer mixture to faciUtate processing at high conversion. The product from the reactor system is devolatilized to remove the unreacted monomers and is then pelletized. Equipment used for devolatilization includes single- and twin-screw extmders, and flash and thin film evaporators. Unreacted monomers are recovered for recycle to the reactors to improve the process yield. 

Particle Size Reduction. Changes in the physical characteristics of a biomass feedstock often are requited before it can be used as a fuel. Particle size reduction (qv) is performed to prepare the material for direct fuel use, for fabrication into fuel pellets, or for a conversion process. Particle size of the biomass also is reduced to reduce its storage volume, to transport the material as a slurry or pneumatically, or to faciHtate separation of the components. 

The catalyst is employed in bead, pellet, or microspherical form and can be used as a fixed bed, moving bed, or fluid bed. The fixed-bed process was the first process used commercially and employs a static bed of catalyst in several reactors, which allows a continuous flow of feedstock to be maintained. The cycle of operations consists of (/) the flow of feedstock through the catalyst bed (2) the discontinuance of feedstock flow and removal of coke from the catalyst by burning and (J) the insertion of the reactor back on-stream. The moving-bed process uses a reaction vessel, in which cracking takes place, and a kiln, in which the spent catalyst is regenerated and catalyst movement between the vessels is provided by various means. 

Bismuth pellets range from 4.5 to 60 g in size and are used for metallurgical additives. Thek convenient size and specific weights make them particularly useful as feedstock when a given quantity of bismuth must be added regularly to a melt. 

All single-screw extruders have several common characteristics, as shown in Figs. 1.1 and 1.2. The main sections of the extruder include the barrel, a screw that fits inside the barrel, a motor-drive system for rotating the screw, and a control system for the barrel heaters and motor speed. Many innovations on the construction of these components have been developed by machine suppliers over the years. A hopper is attached to the barrel at the entrance end of the screw and the resin is either gravity-fed (flood-fed) into the feed section of the screw or metered (starve-fed) through the hopper to the screw flights. The resin can be in either a solid particle form or molten. If the resin feedstock is in the solid form, typically pellets (or powders), the extruder screw must first convey the pellets away from the feed opening, melt the resin, and then pump and pressurize it for a down-... 

As shown in Fig. 4.1, resin feedstocks have a considerable level of interparticle space that is occupied by air. This level of space and thus the bulk density of the feedstock depend on the temperature, pressure, pellet (or powder) shape, resin type, and the level and shape of the recycle material. For a specific resin feedstock, the bulk density Increases with both temperature and the applied pressure. Understanding the compaction behavior of a resin feedstock is essential for both screw design and numerical simulation of the solids-conveying and melting processes. Screw channels must be able to accommodate the change in the bulk density to mitigate the entrainment of air and the decomposition of resin at the root of the screw. Typically, screw channels are set by using an acceptable compression ratio and compression rate for the resin. These parameters will be discussed in Section 6.1. 

Figure 4.6 Bulk density as a function of temperature and pressure for a feedstock containing 10% HIPS pellets and 90% of an in-plant HIPS recycle stream...

The compression ratio for pellet feedstocks typically ranges from about 1.8 for PMMA resins [2] to 4.5 for LDPE resins [3], although many exceptions are practiced. Compression ratios for several common resins are reported by Giles, Wagner, and Mount [2]. If high levels of a low-density recycle stream are added back into the feed hopper, then the compression ratio and compression rate will need to be increased as follows ... 

Compression rates typically vary between 0.0025 and 0.0055 for pellet feedstocks. If the compression rate is too low, then the compaction rate of the solid feedstock may not be high enough to force the entrained gas out through the hopper. If the compression rate is too high, poor melting performance and solid bed breakup can occur. For example, LDPE resins can be melted very easily using a screw with a compression rate of 0.0055, while LLDPE resins perform best with a compression rate near 0.0030 [3, 4]. If an LLDPE is extruded using a screw with a compression rate of 0.0055, then solid bed breakup and solid polymer particles in the extrudate are likely to occur. 

Cf compression ratio that should be used for a lower density feedstock resin Cpeiiets compression ratio used for a pellet feedstock specific heat of the solid resin diameter at the barrel wall local diameter at the screw core... 

The experiments were repeated with a mixture of 60% small-diameter pellets and 40% low-density GPPS recycle material. The bulk density for this feedstock was measured at 0.10 g/cm a bulk density that was about 40% less than that for the commercial pellet-low-density recycle blend. This relatively large difference in density was attributed to the variability of the recycle material density. As indicated by the data in Table 12.11, the rate with no ledge was 20 kg/h, a rate that was about 30% less than that for the commercial pellet blend. Like before, the rate difference is primarily due to the differences between the feedstock bulk densities. When the ledge plates were positioned in the equipment, the solids-conveying rate was about 75% of the original rate. This rate decrease is very similar to the rate decrease that was experienced with the 114 mm diameter commercial extruder. Recall that the commercial extruder was operating at the maximum screw speed and at a rate that was only about 60% of the expected rate. 

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