Study of Residence Time Distribution in a Pilot-Scale Screw Conveyor Dryer

Abstract

The screw conveyor dryer (SCD) finds varied applications in the process industries either as a pre-dryer or for post-drying operation. In certain cases, it can be used as the main dryer. The full potential of SCD has not been exploited for the lack of its understanding. The conveying of paste or particles through a screw conveyor may follow a near plug flow behavior; however, it is essential to ascertain a desired level of inter-particle mixing during the course of drying/conveying so as to obtain the desired drying conditions. The effects of feed rate (15–176 kg/h) and screw speed (10.8–28 rpm) on the RTD were studied for the conveying of sand in a pilot-scale SCD using pulse input of a tracer (dye-coated sand). Two new parameters defining the relative stagnancy and screw effectiveness were introduced to study the flow performance of SCD. The flow in the SCD approaches plug flow with an increase in feed rate or by decrease in the screw speed. A low value of the holdback, a parameter of relative stagnancy, indicates the flow in SCD to be near plug flow. Mean residence time was found to be 3 to 4 times longer than the linear residence time due to lower values of screw effectiveness (0.24–0.32).

Keywords:

• Axial dispersion
• Backflow
• Fraction of plug flow
• Mean residence time
• Pilot scale
• Relative stagnancy
• RTD
• Screw effectiveness
• Variance

ACKNOWLEDGEMENT

The authors gratefully acknowledge the support from Shri. B. R. Sastry, M/s. Mathesis Engineering, Hyderabad, India.

Notes

*Tapped and untapped bulk densities as per ISO 5311:1992.

**Tracer consisted of sand coated with water-soluble red dye.