Residence Time Characteristics

The residence time characteristics in an extruder have a great influence on product quality, for instance when processing heat-sensitive products, in reactive processes, and in the case of dispersion and melting. As with all continuous processes, there is no precisely defined residence time in co-rotating twin screw extruders, but rather a residence time distribution. 

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. 

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]. 

The minimum residence times in compounding twin screw extruders for an average production run range from 5-15 seconds. 

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Bucky Badger has been investigating the residence time characteristics of a reactor that he plans to use for pilot plant work. He suggests... 

The residence time characteristics of a reaction vessel are investigated under steady-flow conditions. A pulse of tracer is injected upstream and samples are taken at both the inlet and outlet of the vessel with the following results ... 

If we assume that the current in the annulus is not back-mixed, then it has a residence time characteristic of an ideal plug flow. Due to the fact that flotation is a depletion process which obeys the 1st order time law, the flotation kinetics is given by the correlation... 

Dry, R. J. Radial concentration profiles in a fast fluidized bed, Powder Technol. 49,37-44(1987). Dry. R. J., and White, C. C. Gas residence time characteristics in a high-velocity circulating fluidized bed of FCC catalyst, Powder Techn. 58, 17 (1989). 

To account for these different time scales, different activity functions are used for the formation of coke and for the formation of the other products the activity for coke formation is described by an exponentially decaying function of the residence time (characteristic time of about 0.02 s) the conversion and formation of the other products are described by one activity function that decreases exponentially as function of the catalyst coke content. This model describes the conversion obtained with the regenerated catalyst according to the curve that was drawn in Figure 2. 

For heat transfer applications the static mixer has an advantage over the empty tube in that it has a much more uniform residence time distribution characteristic, further reducing the possibility of damage to heat sensitive materials. The more uniform residence time characteristic has other advantages, particularly for the design of continuous flow tubular reactors. 

Damkohler number (characteristic residence time/characteristic reaction time) end group phase equilibrium formaldehyde... 

The distributor -type membrane reactor possesses different residence-time characteristics and different local concentration profiles compared to the conventional FBR. The additional degrees of freedom allow in complex networks of consecutive and parallel reactions, the selectivity and the yield to be enhanced with respect to a certain target product. The concept can be considered as an interesting option in the current attempts to improve and intensify reaction processes. 

Dry RJ, White CC. Gas residence-time characteristics in a high-velocity circulating fluidized bed of FCC catalyst. Powder Technol 58 17-23, 1989. 

Based on the examples presented, it is clear that intensity function representation of residence tiaie variability is a valuable tool for understanding and discerning fluid mixing characteristics. Effective utilization of this tool requires good experimental technique. Determination of residence time characteristics and use of intensity function representation/ interpretation should be a critical step in the sequential development of exploratory, pilot unit and plant scale reactors. 

Damkoehler number—system residence time/characteristic reaction time (dimensionless) diameter of microcarrier beads (p,m) energy dissipation/circulation fnnetion P/(kD total force imparted by the impeller, radial and axial (eq. 18-64) (Nm-2)... 

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