Paste Residence Time in a Spouted Bed Dryer. II: Effect of Spout Operational Conditions

Abstract

Residence time distributions for an aqueous solution of 10% sodium chloride in a spouted bed dryer of inert particles were determined using the stimulus-response technique. Glass and polyethylene beads with diameters 2.6 and 3.4 mm were used as inert bodies in a cylindrical column of 14.0 cm diameter and 60° conical base. The effects of inert bodies load, air, and paste flow rates on the mean residence times and RTD were determined following 2³ factorial designs. The RTD could be correlated to the perfect mixing cell model with R² varying from 0.8684 to 0.9815. The mean residence times in CSBD varied from 10.8 to 13.9 and 10.7 to 13.3 min for glass and polyethylene beads, respectively. For both inert particles, mean residence times increased with bed height and decreased with paste feed rates. Also, terms of interaction among the factors were significant in some cases, showing a complex behavior of paste residence times. Equations obtained by response surface regression could predict mean residence times on glass and polyethylene beads with deviations lower than ±10%.

Keywords:

• Air velocity
• Bed height
• Inert particles
• NaCl solution
• Paste feed rate

ACKNOWLEDGEMENTS

Financial support from FAPESP (03/09447-6) and the PQ/CNPq grant to Prof. Freitas are gratefully acknowledged.

Notes

Glass beads: dp = 2.6 mm, ρ = 2.50 g/cm³, ρ _b = 1.59 g/cm³, ε _o = 0.39.

Polyethylene beads: dp = 3.4 mm, ρ = 0.79 g/cm³, ρ _b = 0.49 g/cm³, ε _o = 0.38.

∗T _i  = 130°C.

∗Replicates of exp. 2 for glass beads; ∗∗replicates of exp. 4 and 3 for polyethylene beads.

Replicates in case of: ∗glass beads; ∗∗polyethylene beads (see Table 3).

Significant at: ^a 1%

^b 5%

^c 10%

^d 15%

Significant at: ^a 1%

^b 5%