Drying Sawdust in a Pulsed Fluidized Bed

Abstract

The objective of this work was the experimental and theoretical study of sawdust drying, in batch and continuous experiences, using a pulsed fluidized bed dryer.

In the batch experiences, a 2³ factorial design was used to determine the kinetics of drying, the critical moisture content, and the effective coefficients of both diffusivity and heat transfer, all of them as a function of the velocity and temperature of the air, the speed of turning of the slotted plate that generates the air pulses in the dryer, using sawdust with 65% moisture in each run.

In the continuous operation, a 2³ factorial design was used to study the effect of the solid flow and the velocity and temperature of the air on both the product moisture and the distribution of residence times. In order to determine these last ones, digital image processing was used, utilizing sawdust colored by a solution of methylene blue as tracer.

The statistically significant factors were the velocity and the temperature of the heating air, for both the continuous and batch operations. Although the speed of turn of the slotted plate was not significant, it was observed that the air pulses increased the movement of particles, facilitating its fluidization, especially at the beginning of drying.

The heat transfer coefficients were adjusted according to the equation Nu = 0.0014 Re _p ^1.52, whose standard deviation of fit is 0.145.

The period of decreasing rate was adjusted to several diffusivity models, giving the best fit the simplified variable diffusivity model (SVDM). The curve of distribution of residence times was adjusted using the model of tanks in series, with values between 2.6 and 5 tanks.

Keywords:

• Pulsed fluidized bed
• Residence time distribution
• Sawdust

ACKNOWLEDGMENT

The authors are grateful for the financial support of FONDECYT project No. 1040090.

Notes

^a V _ic  = Air velocity at the inlet of the drying chamber (D _c  = 0.25 m).

^a Time at which each sample reached a moisture content of X/X _o  = 0.2 (dwb).

^b Coded level for V _ic  = 2.5 m/s.

^a Statistically significant effect at a level of P = 0.05.

^b Based on 4 d.f.

^a Initial effective diffusivity (D_o ) calculated with the SVDM model.

Air velocity, referred to inlet diameter of the drying chamber (D _c  = 0.25 m).

^a Statistically significant effect according to the Normal Probability Graph.