Vibratory Fluidized Bed (VFB) is a type of fluidized bed where the mechanical vibration enhances the performance of fluidization process. Since the first discovery of vibratory fluidized bed, its vibration properties proves to be more efficient in dealing with fine particles which appears to be very difficult to achieve with normal fluidized bed. Even though numerous publications and its popularity in industrial applications, the knowledge about vibratory dynamics and properties are very limited. Future research and development are needed to further improve this technology to bring it to another level.
Introduction Vibratory fluidized bed technology has been around since its first discovery in 1984 by Geldart, where he conducted an experiment to observe the behaviour of different types of particle groups behave when vibration mechanism are introduced to further fluidized the particles. Fluidization of cohesive powders, Geldart, D., Harnby, N., Wong, A.C., ‘Fluidization of cohesive powders’, Powder Technology, January 1984, 37(1), pp.25-37 Although it has been around for the past 20 years, only a few research has been done to further improve this technology. Recently, the world is focusing on environmental friendly machinery for the sustainability of the earth. Therefore, more research has been conducted to study the effect of vibration in fluidisation because not only vibratory fluidized bed is environmental friendly, it is also cheaper compare to other fluidized bed.
Basic fundamental Improvement over conventional fluidized bed technology has led to discovery of vibratory fluidized bed where the bed is design by combining vibration and gas flowing vertically towards the conveyor bed. It offers some advantages of fluidized bed, however the feed will move along the vibrating conveyor until they have dried adequately to break up and this will cause lower chance of agglomerates build-up in the feed; hence it is useful for processing group C particles which have small size of fine particles, into smaller agglomerates. Vibrofluidized bed drying of citrus processing residue for byproduct recovery , Roe, E.A., (2003). ‘Vibrofluidized bed drying of citrus processing residue for byproduct recovery’ Fluidized Bed Dryers – Recent Advance , Daud, W.R.W., (2008). ‘Fluidized Bed Dryers – Recent Advance’, Advance Powder Technology, 19, 403–418
Range of application Vibratory fluidized beds are mainly used in several industries such as pharmaceutical, agricultural, catalyst, plastics, minerals, food processes. Fluidised-Bed Technology for Product Finishing , ALLGAIER, ‘Fluidised-Bed Technology for Product Finishing’ Simulation of a vibrated fluidised bed dryer for solids containing a multicomponent moisture , Picado, A., & Martinez, J., (2006). ‘Simulation of a vibrated fluidised bed dryer for solids containing a multicomponent moisture’, Interamerican Confederation of Chemical Engineering Typical applications for vibratory fluidized beds are drying products in the form of grains and crystals, cooling the dried products, agglomeration and granulation of coarse particles, and sterilising. Drying & Cooling Carrier Fluid Bed , Carrier, ‘Drying & Cooling Carrier Fluid Bed’
Design available As mention above vibratory fluidized bed are mainly used in numerous industries where certain particle size are needed to be consistent without any defect to produce better product for their consumer. Most common process operations used in vibratory fluidized bed technology are dryers and coolers.
Vibratory fluidized dryers Standard type of vibratory fluidized dryer consist of vibrating tray conveyor where hot gases from the chamber will flow through the holes within the tray and come in contact with the materials to be dried. The tray area is big enough to tolerate constant flow of material through the bed and passed along the deck with a low depth on the tray. The vibrations to the deck are directed in vertical component to assist in fluidization of the material whereas the horizontal component of the vibration, support in transporting materials along the tray.
Vibratory fluidized cooler Vibratory fluidized coolers operate in the same manner but instead of hot gases being feed from the chamber, it have recirculating air flowing through the chamber and equipped with atomizing nozzle to generate water mist as cooling medium. Other alternate designs include the use of cold water coils with the inlet air passing over them and this option are used when the incoming air have a large temperature difference compare to the material being cooled. Transferring Heat with a Vibrating Fluidized Bed , Kinergy Corporation, ‘Transferring Heat with a Vibrating Fluidized Bed’
Advantages and limitations of vibratory fluidized bed
Some of the advantages of vibratory fluidized beds include: Homogeneous Fluidization Characteristics of Vibrating Fluidized Beds , Jin, H., Tong, Z., Zhang, J. and Zhang, B. (2004), ‘Homogeneous Fluidization Characteristics of Vibrating Fluidized Beds’. Can. J. Chem. Eng., 82: 1048–1053 Continuous drying throughout the unit. Handle products with a wide range of particle size and shape. Minimal fluidization velocity and pressure drop due to the vibration energy being transferred along the bed. Increase efficiency of gas to solid contact. Mechanical vibration enhances homogeneity and stability of fluidized bed layers. Easier to control the residence time distribution of processed material by manipulating the intensity of amplitude and frequency of vibration.
Limitations of vibratory fluidized bed are as follow: Hydrodynamics Of Vibro-Fluidized Beds , S. Satija & I.L. Zucker (1986), ‘Hydrodynamics Of Vibro-Fluidized Beds’, Drying Technology: An International Journal, 4:1, 19-43 Analysis of Aerodynamics and Heat Transfer in Vibro-Fluidized Beds, D. U. Ringer & A. S. Mujumdar (1983), ‘Analysis of Aerodynamics and Heat Transfer in Vibro-Fluidized Beds’, Drying Technology: An International Journal, 2:4, 449-470
The inlet air temperature to the dryer process is limited. Climate condition can affect the unit's thermal efficiency. Build-up of local expansion region lead to unstable behaviour to the bed structure.
Main process characteristics To give a more detail insight into the vibrating fluidized bed, several characteristics have been stated below to show the relationship amongst the characteristics as well as the operating conditions to how they could possibly affect some process conducted using a vibrating fluidized bed.
Voidage behaviour on particle size The term voidage refers to the spacing between the materials. It is critical to know how the voidage behaviour of certain particle sizes affects the process in a vibrating fluidized bed as they are one of the key factors to be considered when designing and scaling up the vibrating fluidized bed from lab scale to industrial scale. From several experiments conducted, it was shown that vibration helps in the fluidization of particles as the axial and radial voidage distribution become more homogeneous. This is especially true for vibrating fluidized beds with large vibration amplitudes. It was also found that with increasing bed height, the layers of particles in the bed could be damped out by the vibration energy. Analysis of the wave propagation showed that its parameters are affected by the fluidization behaviour. The effect of vibration on bed voidage behaviors in fluidized beds with large particles , Jin, H., Zhang, J., Zhang, B. (2007). ‘The effect of vibration on bed voidage behaviors in fluidized beds with large particles’, Brazilian Journal Chemical Engineering, vol.24, n.3, pp. 389-397.
Energy transfer In a vibrating fluidized bed, energy is transferred when the vibrating wall comes into contact with the particles. These particles collide with other particles in the bed which passes kinetic energy in the form of wave propagation throughout the vibrating fluidized bed. The magnitude of the energy transferred is relative to the amplitude. This is because of the oscillations caused by the wave reflection of the medium boundary in the vibrating fluidized bed. Energy transfer mechanism in a vibrating fluidized bed, Wang, T.J., Jin, Y., Tsutsumi, A., Wang, Z., Cui, Z., (2000). ‘Energy transfer mechanism in a vibrating fluidized bed’, Chemical Engineering Journal, Volume 78, Issues 2–3, Pages 115-123,
Bubbling behaviour To assess the bubbling behaviour of the vibrating fluidized bed, factors such as the size of the bubble and its velocity were also taken into account. For various vibration amplitudes and frequencies, numerical simulations of the vibrating fluidized bed was conducted to better understand the behaviour of the bubbles under the vibrating conditions. The results showed that due to the oscillatory displacement of the vibrating fluidized bed causes the mean bubble diameter to increase but lowers the acceleration rate of the bubbles. Thus, it was concluded that bubbling behaviour in a vibrating fluidized bed is dependent on the vibrations. Characterization of the Bubble Behavior in Vibrated Fluidized Beds by means of Two-Fluid CFD Simulations Coupled with Accelerometry Data , E. Cano-Pleite, J. Gómez-Hernández, J. Sánchez-Prieto, and A. Acosta-Iborra,. ‘Characterization of the Bubble Behavior in Vibrated Fluidized Beds by means of Two-Fluid CFD Simulations Coupled with Accelerometry Data’ in "The 14th International Conference on Fluidization – From Fundamentals to Products", Eds, ECI Symposium Series, Volume (2013)
Multicomponent moisture To consider multicomponent moisture solid in a vibrating fluidized bed drier, a model was used to assess the characteristics of drying a thin layer of particle which was wetted with a multicomponent mixture. This was done to gain a better understanding of the complex treatment of multicomponent drying which is tedious and time-consuming process. Based on the model using a plug flow of solids, the selectivity and best drying conditions to achieve the ideal final moisture composition were determined. For a component mixture which is highly volatile, the composition of the liquid which was left in the product from the vibrating fluidized bed can be controlled using small amount of the other components to the solid feed.
Pressure drop Knowing that one of the advantages of the vibrating fluidized bed is its small pressure drop, several studies has been made to show that for a given operating condition range, the pressure drop of the vibrating bed when compared to a conventional one is much smaller. This is also the case when comparing the minimum fluidization pressure drop as the vibration decreases due to the increase in amplitude and decrease in frequency. Characterization of the hydrodynamic of a vibrating fluid bed dryer by a modular neutral network model , Alvarez, P.I., Cubillos, F.A., Blasco, R. ‘Characterization of the hydrodynamic of a vibrating fluid bed dryer by a modular neutral network model’ The presence of this pressure drop across the vibrating fluidized bed has a large impact on the heat and mass transfer in the process. There is an increase in bed porosity which corresponds to the pressure loss reduction. This change in pressure loss is dependent on the frequency and amplitude of the vibration of the surface.Some properties of a vibrating fluidized bed , Chlenov, V.A., Mikhailov N.V., (1967). Journal of Engineering Physics, 9(2), pp. 137-139.
Effects of Bed Height