(423j) Development of a Small Scale Fluidized Bed Dryer

Fluidization is a unit operation largely used in process plants, where a gas stream flows upwards through a solid particle bed. Examples of fluidization include fluidized bed reactors, which have been extensively used in the reform of hydrocarbons and gasification of coal and biomass. Moreover, there are also examples of fluidized bed dryers that can be used in areas such as food, pharmaceutical and chemical industries. In virtue of mixing and of good contact between the gas phase and the solid, fluidization leads to high heat and mass transfer rates. The aim of this work was the development and building of a laboratory scale batch fluidized bed dryer, to be used in undergraduate Chemical Engineering courses.

There follows a brief description of the didactic fluidized bed dryer constructed. The base of an acrylic tube, with 0.045 m of internal diameter, where compressed air is fed and the differential pressure and temperature are measured, constitutes the bottom of the bed. The fluidized bed of particles to be dried is placed inside the acrylic tube, over a 325 mesh screen, and a total height of 0.500 m. Air flow is measured in a rotameter and inlet air temperature and speed are set on a control panel. An electrical resistance heats the air stream, in temperatures ranging from 300 K to 350 K.

Experimental tests involving flaxseed drying were carried out in the prototype. The minimum velocity at which fluidization begins is the minimum fluidization velocity and it can be predicted by basic fluidization equations. The experimental minimum fluidization velocity observed for a 0.150 kg bed of flaxseed was 0.71 m/s whereas theoretical correlations predicted 0.67 m/s, in close agreement. The predicted pressure drop at the onset of fluidization was equal to 934 Pa, which was a good estimative when compared to the experimental value of 883 Pa. The same holds for the minimum height of the bed at beginning of the fluidization, with 0.170 m for experimental results and 0.155 m for the predicted ones.

In addition to hydraulic considerations, it is important to point out the results concerning drying. The initial free moisture of the flaxseed bed, written in kilograms of water per kilograms of dry material, was equal to 0.11. The inlet temperature of the air was 313 K with a relative humidity of 40%. After 30 minutes of drying, equilibrium was attained and the moisture in the bed decreased to 0.07. The plot of the curve of the moisture versus time and the drying-rate curve for the process showed a constant-rate period and a falling-rate one, as it is typical for many solids.