Modeling and measurement of bubble size in a rotating fluidized bed

Nakamura, Hideya; Iwasaki, Takashi; Watano, Satoru

doi:10.1002/aic.11260

Cited by 9 publications

(4 citation statements)

References 26 publications

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“…The centrifugal force can be increased by several fold of gravity to increase fluidization and gas-solid slip velocities, to improve inter-phase mass and heat transfer through good contact efficiency, and to prevent agglomeration and entrainment of particles [76][77][78]. Recently, models of rotating fluidized beds were proposed for the bubble size [79] as well as for the whole rotating fluidized bed [80]. A novel rotating fluidized bed formed by injecting gas tangentially in a static chamber was recently invented [81].…”

Section: Centrifugal and Rotating Fluidized Bedmentioning

confidence: 99%

Fluidized Bed Dryers — Recent Advances

Daud¹

2008

adv powder technol

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Although industrial fluidized bed dryers have been used successfully for the drying of wet solid particles for many years, the development of industrial fluidized bed dryers for any particular application is fraught with difficulties such as scaling-up, poor fluidization and non-uniform product quality. Scaling-up is the major problem and there are very few good, reliable theoretical models that can replace the expensive laboratory work and pilot-plant trials. This problem is mainly due to the different behavior of bubbles and mixing regimes in fluidized bed dryers of different size. Simple transformation of laboratory batch drying data to continuous back-mixed dryers using the residence time distribution of the solids is insufficient to account for the complex flow and heat and mass transfer phenomena occurring in the bed. Although time scaling using temperature driving forces and solids mass flux for the same change in moisture content in the batch and continuous dryers has been successful in predicting moisture content profiles in the continuous dryer at the constant rate period, it does not take into account solid mixing. Two-phase Davidson-Harrison models have been used in modeling of the continuous back-mixed dryer with various degrees of success. On the other hand, the three-phase Kunii-Levenspiel model is seldom used in modeling fluidized bed dryers because it is too complex to handle. A combination of multi-phase models and residence time distribution could improve predicting power for back-mixed dryers because this combination takes into account both the bubbles and solid mixing phenomena. Incremental models were widely used to model continuous plug flow fluidized bed dryers, but the cross-flow of drying medium has not been sufficiently modeled except by the author. In some incremental models, axial dispersion is modeled using the Peclet number, Pe. A combination of an incremental model with an axial dispersion and cross-flow model of drying medium would improve predicting power. Poor fluidization of Geldart group C particles could be improved by the assistance of external means such as vibration, agitation, rotation and centrifugation. Both vibrated and agitated fluidized bed dryers have been successfully used in industry, but rotating or centrifugal fluidized bed dryers are still not available for industrial use. Nomenclature A bed area (m 2 ) E(t) RTD curve (-) ε bed porosity (-) F dry basis solid flow rate (kg/h) f falling rate drying factor ( • C) G dry basis gas flow rate (kg/h) h bed height (m) L total length of dryer (m) m B mass of bed (kg) N V local solids dying rate (kg/h/m 2 ) NTU number of transfer units (-) Nu nusselt number (-) Pe Peclet number (-)Pr Prandtl number (-) Re p particle Reynolds number (-) T G local gas temperature ( • C) T GI initial gas temperature ( • C) T i inlet gas tenmperature ( • C) T o exhaust gas temperature ( • C) T S local solids temperature ( • C) T wb wet bulb temperature ( • C) t time (s) τ residence time (s) X dry basis solids moisture content (kg/kg) X i inle...

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Section: Centrifugal and Rotating Fluidized Bedmentioning

confidence: 99%

Fluidized Bed Dryers — Recent Advances

Daud¹

2008

adv powder technol

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“…Various researchers have realized the significance of bubbles in gas–solid fluidized beds and used various internals, e.g., baffles, tubes, packings, and inserted bodies, and other configurations to generate evenly distributed smaller bubbles to improve the quality of fluidization. Their main features and inherited limitations have been thoroughly discussed by Jin et al,22 the core issues of which are briefly summarized here, along with more recent findings 23–28. Most transverse baffles in fluidized beds effectively increase gas residence time and reduce solids entrainment.…”

Section: Introductionmentioning

confidence: 96%

“…However, when compared with internals‐free fluidized beds, their utilization is limited by excessive pressure drop, dead spots, channeling–prompting environment, segregation of particles, and lower effective thermal conductivity 22. Other innovative designs have recently emerged to reduce bubble sizes and to enhance gas–solid contacting, such as the rotating fluidized bed and its static variant with tangential gas injection,24, 25 and electrically stimulated fluidized beds applying electrical fields on uncharged polarizable particles 30…”

Section: Introductionmentioning

confidence: 99%

Bubble behavior in corrugated‐wall bubbling fluidized beds—Experiments and CFD simulations

Wardag¹,

Larachi²

2011

AIChE Journal

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A new concept to harness bubble dynamics in bubbling fluidization of Geldart D particles was proposed. Various geometrical declinations of a cold‐prototype corrugated‐wall bubbling fluidized bed were compared at different flow rates (Ug) to conventional flat‐wall fluidized bed using high‐speed digital image analysis. Hydrodynamic studies were carried out to appraise the effect of triangular‐shaped wall corrugation on incipient fluidization, bubble coalescence (size and frequency), bubble rise velocity, and pressure drop. Bubble size and rise velocity in corrugated‐wall beds were appreciably lower, at given Ug/Umb, than in flat‐wall beds with equal flow cross‐sectional areas and initial bed heights. The decrease (increase) in size (frequency) of bubbles during their rise was sustained by their periodic breakups while protruding through the necks between corrugated plates. Euler‐Euler transient full three‐dimensional computational fluid dynamic simulations helped shape an understanding of the impact of corrugation geometry on lowering the minimum bubbling fluidization and improving gas distribution. © 2011 American Institute of Chemical Engineers AIChE J, 2012

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“…In spite of the advantages, the applications have been limited to small-scale set-ups (Saunders, 1986;Wong et al, 2000) due to the rotating nature of the reactor causing difficulties in scale up because of mechanical vibrations due to the rotating geometry, the use of rotating seals, etc. The current lack of proper understanding of a fundamental fluidization model which can well describe the process is also a major hindrance to the industrial scale-up of rotating fluidized bed units (Nakamura et al, 2007).…”

Section: Introductionmentioning

confidence: 99%