Residence time distribution screw speed

Along with the screw configuration, the residence time distribution in twin screw extruders is also influenced by the process parameters. As shown in Fig. 4.24, throughput has a greater influence on the average residence time than screw speed and pitch [7]. 

Fig. 9.14 Values of F(y) for a 6-in-diameter, 20 1 IVD extruder at constant flow rate (500 lb/h) with screw speed as a parameter. Simulation was made for a square pitched screw with a constant channel depth of 0.6 in. [Reprinted by permission from G. Lidor and Z. Tadmor, Theoretical Analysis of Residence Time Distribution Functions and Strain Distribution Functions in Plasticating Extruders, Polym. Eng. Sci., 16, 450-462 (1976).]...

While the minimum residence time is significant for dispersive mixing, the residence time distribution in the extruder influences distributive mixing as a measure for longitudinal mixing performance [20]. The average residence time depends only on screw pitch and screw speed in partially-filled screw sections, and only on throughput in fully-filled screw sections. 

Figure 4.23 Residence time distribution for different screw configurations in a co-rotating twin screw extruder (constant screw speed and throughput)...

Another scale-up variable that can be easily controlled is the length to diameter ratio L/D). Scale-up with an equivalent L/D ratio is beneficial in avoiding non-linear scale-up issues, especially at production scale. Residence time distribution (RTD) is a useful term for understanding scale-up of hot-melt extrusion processing. The RTD is used to attempt to quantify the average amount of time a material spends in the processor. The RTD depends on screw speed, screw element design, and material characteristics. The preference during scale-up is to maintain an equivalent ... 

The residence time distribution depends on the screw configuration, throughput (or the degree of fill) and the screw speed. All three factors need to be optimized considering the rate of reaction, degradability of materials and productivity. 

The extrusion products typically contain 8%-10% of flavor oil, but the final washed surface makes the product very stable (Rish and Reineccius, 1988, Qi and Xu, 1999). Carbohydrate matrices in the glassy state have very good barrier properties (i.e., atmosphere gases). Dripping and jet breakup are the methods to form microcapsules (Whelehan and Marison, 2011). Parameters are screw temperatures and speed, and residence time distribution (Yuliani et al., 2006). 

Residence time distribution, RTD, too wide] [degree of fill too low] /feedrate too small/screws speed too fast. 

It appeared that an increasing screw speed decreases the average residence time. However, changes in screw speed had a negligible influence on the residence time distribution in dimensionless form. 

The biomass is fed overbed through multiple feed chutes using air jets to help distribute the fuel over the surface of the bed. Variable-speed screw conveyors are usually used to meter the fuel feed rate and control steam output. Feedstocks such as bark and waste wood are chipped to a top size of 25 mm (1 in) to ensure complete combustion. The bed usually consists of sand around 1 m (3 ft) deep. This serves to retain the fuel in the furnace, extending its in-furnace residence time and increasing combustion efficiency. It also provides a heat sink to help maintain bed temperature during periods of fluctuating fuel moisture content. 

Reactive extruders and extrusion dies of different designs can be easily included in standard technological scheme of polymer production plants, such as those for polycaproamide synthesis, as shown in Fig. 4.39. In this case, a reactive material premixed in a tank 1 is fed into a static device 2 for prepolymerization, where part of the polymerization process takes place. Then the reactive mixture enters the extruder-reactor 3. The necessary temperature distribution is maintained along the extruder. Transfer of the reactive mass proceeds by a system of two coaxial screws mounted in series in a common barrel. Controlling the relative rotation speed of both screws provides the necessary residence time for the reactive mass in the extrader, so that the material reaching the outlet section of the die is a finished polymer. 

Screw configuration has a pronounced effect on the RTD. The effects of different screw configurations on the broadness of mixing compared their subtle differences. Specific throughput is defined as the ratio between throughput and screw speed. The residence time of a material element is the time it spends in the extruder. All material elements do not necessarily spend the same period of time in the extruder. Residence times have a distribution called RTD. 

Also, the starting point of the reaction is affected by the screw speed. Due to thermal effects, the reaction will start at a later point in the extruder at increasing rotation rates. The shorter reaction length has also a negative effect on the conversion due to a loss of mixing and residence time. Nevertheless, the influence of this effect is small and the conversion of the reaction is mainly determined by the two competitive effects of mean residence time and distributive mixing. 

The stress magnitude s history of three maker-particles which have different residence time is described in Fig. 7. The curve oscillates periodically due to the screw rotation. We focus on the highest value of the stress level as circled in Fig. 7 and its distribution is estimated by Eq. 4. The bimodal stress distribution is obtained in the case of the rotor at low rotational speed in Fig. 8. The height of the distribution curve corresponds to the flow rate and about half of the polymer melt does not overpass the wingtip at lOOrpm. On the contrary, the KB and rotor at high rotational speed shows uni-modal distribution and uniform stress induced mixing is expected. 

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