Introduction to Fluidized Bed Technology J. Ruud van Ommen Reactor & Catalysis Engineering Delft University of Technology E-mail:
[email protected] Glatt Seminar 18 March 2003 Introduction Fluidized bed: particles suspended in an upward gas stream drag force equals gravitational force Outline Classification: Particle size Hydrodynamic regime (~ gas velocity) Lay-out Design Applications Typical properties: Low pressure drop Heat and mass transfer Erosion / measurements Segregation and agglomeration Modelling Summary References Geldart’s powder classification Geldart’s powder classification C Cohesive 0-30 µm flour Drag Attraction A Aeratable 30-100 µm milk powder Gravity B Bubbling 100-1000 µm sand D Spoutable >1000 µm coffee beans Geldart’s powder classification A powder (30-100 µm) maximum bubble size B powder (100-1000 µm) bubbles keep on growing From CD-ROM: Laboratory Demonstrations in Particle Technology, M. Rhodes Geldart’s powder classification C powder (1000 µm) spouted fluidization From CD-ROM: Laboratory Demonstrations in Particle Technology, M. Rhodes Influence of particle size distribution 20 mass% 15 10 0 Conversion [-] 5 0 50 100 150 particle diameter [micron] 1.0 0.5 wide size distribution narrow size distribution 20 mass% 15 10 5 0 Adapted from Sun & Grace (1990) 0.0 0 4 8 Dimensionless kinetic rate constant [-] Fluidization Regimes fixed bed homogeneous bubbling slugging solids returns turbulent fast fluidization solids returns pneumatic transport solids returns gas gas only A powders at low gas velocity gas gas only narrow beds gas gas gas gas velocity Fluidized bed lay-outs laterally staged bed circulating bed turbulent bed bubbling bed vertically staged bed spouted bed riser downer floating bed twin bed Fluidized bed design product gas cyclones freeboard heat exchange tubes reactant gas windbox/plenum Applications Physical processes: heat exchange drying coating granulation gas purification via adsorption Chemical processes: Fluid Catalytic Cracking (FCC) Synthesis reactions (e.g., vinyl acetate, phtalic anhydride, acrylonitrile) Polymerization of olefines (ethylene, propylene) Silicon production Fischer-Tropsch synthesis of gasoline Fluid Coking and Flexi-Coking Coal / biomass / waste combustion Coal / biomass / waste gasification Low pressure drop Packed bed pressure drop gas velocity pressure drop Fluidized bed Lower pressure drop lower power costs onset of fluidization gas velocity Heat and mass transfer Heat transfer: particle to wall or internal Mass transfer: gas to particle Fluidized beds show an excellent heat transfer Mixing of solids by (large) bubbles almost constant temperature throughout the reactor However, large bubbles decrease the mass transfer Research decrease bubble size Bubble: shortcut of gas Interstitial gas: effective Ways to decrease the bubble size Vibration Magnetic field Electric field rel. bubble size 0.6 Mori et al., Nagoya Inst of Techn. Rosensweig, Exxon 1 0 field strength [kV/cm] 5 Kleijn van Willigen et al., TU Delft Fractal injector Pulsed gas injection Optimizing particle properties Coppens and Lems, TU Delft Coppens et al., TU Delft Van Ommen et al., TU Delft Erosion / measurements Fluidized bed: • often high temperature • often chemically aggressive • large mechanical stress Erosion Cross-sectional picture of thermocouple (Sethi et al., Kentucky Energy Cabinet Lab.) High erosion rate Opaque nature Only few measurement techniques are available! Industrial fluidized beds: only pressure and temperature measurements on a routinely base. Segregation and agglomeration Difference in size and/or density can lead to segregation of the particles. Agglomeration problems occur in various fluidized bed processes Hoomans, Kuipers, et al., Twente University More information presentation ‘Agglomeration detection’ this afternoon Modelling Simple engineering models Computational Fluid Dynamics (CFD) ‘Two fluid’ model Two-region model Discrete particle model Levenspiel, Oregon State Univ. Van Wachem, Van den Bleek, et al., Delft Univ. of Techn. Hoomans, Kuipers, et al., Twente Univ. Models still show shortcomings scaling-up remains troublesome Summary Fluidized bed: particles suspended in a gas stream Particle size and gas velocity strongly influence the fluidized bed behaviour Large range of application and many different lay-out + Low pressure drop + Heat transfer +/- Mass transfer - Erosion - Segregation & agglomeration Improvement of models is still continuing More information Books: Fluidization Engineering, Kunii & Levenspiel, ISBN 0409902330 Gas Fluidization, Mell Pell, ISBN 0444883355 Circulating Fluidized Beds, Grace, ISBN 0751402710 Articles: Review turbulent fluidization, Bi et al., Chem.Eng.Sci. (2000) 55, pp. 4789 Measurement techniques, Werther, Powder Technol., 102 (1999) pp. 15 Web-sites: Tutorials: www.erpt.org/technoar/fluidbed.htm This presentation: www.dct.tudelft.nl/~vanommen