Effect of Air Distributor Types and Distribution Techniques on Fluidized Bed Drying of Sunflower Seeds

Abstract

The objectives of this research were to investigate the effect of distributor types on the hydrodynamic behavior of a fluidized bed containing sunflower seeds for selecting an optimal distributor plate, to investigate operating parameters effecting on drying kinetics and salted sunflower seed quality utilizing a fluidized bed, to develop the drying kinetics models and to investigate the effect of distribution techniques in fluidized bed on the drying performance for salted sunflower seed.

Firstly, nine different distributors were designed with various hole diameters (2.5 mm, 3.5 mm and 4.5 mm) and various the open area ratios (25%, 30% and 35%). The experiments were carried out in a square bed column with four different static bed heights (5, 10, 15 and 20 cm). The parameters of the hydrodynamics of fluidization were the pressure drop of the distributor, minimum fluidization velocity, pressure drop of the bed and the volumetric fraction of bed. It is suggested that the C-3 perforated distributor type of a diameter size 4.5 mm and open area ratio about 35% was the most appropriate for the fluidization application because the pressure drop of the distributor was low and the volumetric fraction of bed was higher than other distributor.

Secondly, drying salted sunflower seeds using a fluidized bed was studied to investigate the effects of various operating parameters on the drying kinetics. These parameters were the drying air temperature, the static bed height and the superficial air velocity. Their effect on sunflower seed quality was investigated. The quality was determined from the kernel color, rupture force of the seed and consumer acceptance test scores. Additionally, the drying kinetics model was developed using an exponential relationship. The results show that an increasing drying temperature caused a decrease in the rupture force of dry seed. It also results in decreasing the redness and yellowness. However, the drying temperature had no effect on lightness. The static bed height and the superficial air velocity had no effect on the quality. The appropriate drying conditions for drying salted sunflower seeds using a fluidized bed should be the following: An air temperature of 170oC, static bed height of 5 cm and a superficial air velocity of 1.5Umf. This drying condition gave the highest drying rate, shortest drying time and had high product quality. Comparison between the predicted results of the exponential relationship model and the experimental data gave good agreement.

Thirdly, four models from the literature, a diffusion model and a model proposed by the authors were selected and fitted with experimental data to model the drying kinetics of sunflower seeds. The experiments were conducted utilizing a fluidized bed dryer with drying air temperatures in the range of 110-170oC at a constant air velocity 3 m/s and a constant static bed height 5 cm. The samples of sunflower seeds were dried from their initial moisture content of 90.1% dry basis to a final moisture content of 4.5% dry basis. The values of the constants in the models were found by fitting the experimental data to the models using non-linear regression. The criteria for evaluating the models were the values of a coefficient of determination, a root mean square error and a reduced chi-square. It was found that the drying kinetics model for sunflower seeds which gave the best fit was the diffusion approach model proposed by the authors.

Finally, a normal air distribution technique, a pulsed distribution technique and air distribution technique by constant fluidized bed height were investigated to determine the drying performance. The drying experiments were performed under drying air temperatures in the range of 110-170oC with a constant air velocity 1.5Umf and pulsed frequencies in the range of 0.2-3.33 Hz. It was found that the pulsed distribution technique with various frequencies and no pulsation had no affect on the specific drying rate. The pulsed distribution technique had clearly an effect on the specific energy consumption compared with no pulsed air. The drying temperature of 170°C, superficial air velocities 3m/s and pulsed frequency of 0.2 Hz was the best condition test which it gave the lowest specific energy consumption. The technique of controlling fluidization height was performed under drying air temperatures in the range of 110-170oC with constant fluidization height. It was found that the variable speed of blower led to lower specific energy consumption about 7 to 9% when compared with the normal air distribution technique.