Essay on Fixed and Fluidized Bed

In the article “Effect of uniformity of gas distribution on fluidization characteristics in conical gas fluidized beds,” Son, Lee, and Kim (2006) conduct an experiment on uniform gas distributors to determine their impact on fluidization features of fluidized beds. This paper provides a summary of their study. It presents an overview of the research background, procedure, findings, and conclusion.

Fluidized bed reactors are primarily preferred for their effectiveness in mass and heat transfer. Consequently, the stability of their operations is affected by particle segregation, whose result is poor fluidization. Past studies have been inadequate in addressing the hydrodynamic properties of conical fluidized beds. Gas distributors ensure stable fluidization through uniform particle distribution. Their slight pressure drop may lead to partial defluidization. However, a substantial decrease could lead to higher power consumption. Son, Lee, and Kim (2006) fill the gap present in research by investigating the minimum velocity of full and partial fluidization, bubble frequency, minimum bubbling velocity, and bed pressure reduction fluidization features. The study evaluates and compares the variations in fluidization properties with varied opening fractions to the trends of non-uniform distributors.

Son, Lee, and Kim (2006, p.557) used a conical-shaped gas fluidized bed with a 20° apex angle and transparent acryl column parts. They also utilized 1 mm glass beads with 2500 kg/m³ density and a flow meter to measure air velocity. The study designed four uniform distributors with opening fractions of between 0.92 to 3.87% and holes similar to the non-uniform ones used in past research. The fluidized bed pressure was measured using a differential transducer. At the same time, bubble properties, such as velocity and size, were estimated by three optical probes placed at 0.05mm intervals above the distributor.

The study established that pressure decrease of the distributor raised with reduced opening fraction and increased gas velocity (U_g). Moreover, it found that the initial minimum bubble velocity is greater with reducing than raising gas velocity. Decreasing minimum bubble velocities (U_mb) is different in the case of non-uniform and uniform distribution but increasing U_mb is similar. Moreover, the researchers established that the impact of the opening fraction on minimum bubble velocity is more significant with reducing than raising U_g in cases of non-uniformity since aggregation force is required to move joined particles. With increasing U_g, the opening friction and minimum velocity of full fluidization are similar. At the same time, in the case of falling U_g, there are disparities in maximum velocity at full defluidization and U_mb.

Furthermore, Son, Lee, and Kim (2006) established that bed pressure drop in the case of uniform distributors remains constant until the full fluidization minimum velocity is reached, then it reduces with falling gas velocity. Also, the standard deviation of bubble size and bed pressure decrease raise with increasing fully fluidized area’s gas velocity (U_g). Nevertheless, there are no changes in bubble frequency with different U_g levels. An increase in U_g causes a dramatic increment in bubble size. Therefore, raising the standard deviation of bed pressure drop is linked to the bubble size.

Son, Lee, and Kim (2006) conclude that the minimum bubbling velocity in bed characterized by non-uniformity and uniformity of distributors with raising U_g is comparable but somehow dissimilar with falling U_g. Also, bubble size expands with increasing air velocity of fully fluidized area. The standard deviation of pressure drop shows the trend in the results. However, air velocity does not affect bubble frequency.

References

Son, S. Y., Lee, D. H., & Kim, S. D. (2006). Effect of uniformity of gas distribution on fluidization characteristics in conical gas fluidized beds. Studies in surface science and catalysis, 557-560.