Extrusion processes solid feeding

If the actual solids conveying performance is as sensitive to the coefficient of friction as the theory indicates, small changes in the actual coefficient of friction can have a substantial effect on the entire extrusion process. This is particularly true for low values of the coefficient of friction against the barrel. The high sensitivity to the coefficient of friction will tend to result in unstable extruder performance. At high values of the barrel coefficient of friction the extruder performance will be less affected by changes in friction and the extruder will be more stable under these conditions. This explains why grooved feed extruders tend to be more stable than smooth bore extruders. 

Heat migration using an integral barrel-feed port barrel configuration will aid in solids conveying and increased output in the extrusion process, because of the increase in coefficient of friction between the pellet and the barrel wall in the feed section of the screw. 

In summary, fixed-bed processes have advantages in ease of scaleup and operation. The reactors operate in a downflow mode, with liquid feed trickling downward over the solid catalyst concurrent with the hydrogen gas. The usual catalyst is cobalt/molybdenum (Co/Mo) or nickel/molybdenum (Ni/Mo) on alumina (A1203) and contain 11-14% molybdenum and 2-3% of the promoter nickel or cobalt. The alumina typically has a pore volume of 0.5 ml/g. The catalyst is formed into pellets by extrusion, in shapes such as cylinders (ca. 2 mm diameter), lobed cylinders, or rings. 

The sohds remaining after fat removal are generally rich in protein and find a ready market in animal feeds. Some oilseed solids, especially soybean, go into human foods as flours, concentrates, textured particles, or protein isolates. Some oilseed sohds contain toxins or allergens that make them unfit for animal feeds tung nut and castor bean, for example. Unless treated, these solid residues go into fertilizers. Various processes have been developed to remove or chemically destroy undesirable compounds (10). One process developed at Texas A M University for UNIDO (11, 12) uses a chemical additive and extrusion to detoxify and deallergenate castor meal making it suitable for animal feed. 

Another modular system is often called laboratory equipment it can be used for small scale production and for the laboratory evaluation of small samples. Fig. 11.26 depicts the design and some of the accessories. In the most simple execution a hopper feeds a pair of rollers which are driven by a hand crank. The rollers can be solid and may be equipped with compacting or briquetting surfaces (see Section 8.4.3) or two perforated, geared, intermeshing pelleting rolls (see Section 8.4.2) are installed to accomplish medium pressure extrusion. In a modular fashion the rollers can be motorized, screw feeders can be added, and the rolls may be oriented vertical or horizontal or in any other direction. As shown in the photographs of Fig. 11.27 the roller frame can be totally enclosed for dust control if toxic or hazardous materials are processed. A panel includes controls and instrumentation for data display and collection. 

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