Fluidization - Fundamentals and Applications - A Tutorial

Transcription

1 Fluidization - Fundamentals and Applications - A Tutorial Joachim Werther Institute of Solids Engineering Hamburg University of Technology D Hamburg, Germany 5th World Congress on Particle Technology, April 23-27, 2006 Orlando, Florida, USA

2 Contents 2 1. Introduction 1.1 Definitions 1.2 Forms of fluidized beds 1.3 Advantages and disadvantages of the fluidized bed as a reactor 1.4 Comparison of the fluidized bed reactor with other types of gassolid reactors 2. Typical fluidized bed applications 2.1 Historical development of the fluidization technique 2.2. Technical applications of the fluidized bed 2.21 Physical processes Mechanical processes Processes with heat and mass transfer 2.22 Chemical processes Heterogeneous catalytic reactions Polymerizations reactions Solids as heat carriers Processes with reacting solids

3 3 3. Fluid-mechanical principles 3.1 Minimum fluidization velocity 3.11 Experimental determination of the minimum fluidization velocity 3.12 Prediction of the minimum fluidization velocity 3.2 Fluidization properties of typical solids (Geldart s classification) 3.3. The state diagram of fluidized beds according to Reh 3.4 Gas distribution Devices for gas distribution Minimum pressure drop Design of perforated plates 4. Local fluid mechanics of gas-solid fluidization 4.1 Isolated bubbles in fluidized beds 4.2 Bubble coalescence and splitting 5. Circulating fluidized beds 5.1 Fluid mechanical characteristics 5.2 Design characteristics

4 4 6. Entrainment 6.1 Mechanisms 6.2 Definitions and correlations 7. Solids mixing in fluidized beds 7.1 Mechanisms 7.2 Solids dispersion coefficients 8. Literature

5 What is fluidization? 5 Definition: A fixed bed may be brought into a liquidlike (fluidized) state by an upward flowing fluid once the flow exceeds a minimum value. In the fluidized state the fluid experiences a pressure drop which is equal to the weight of the particle bed minus its buoyancy divided by the bed s cross-sectional area. V. Δp Δ p = fb A H (1- ε) ( ρ ρ ) g t s f A t Δp fb A t = cross-sectional area of column ε = voidage of the bed Δp Packed bed V. Fluidized bed

6 Forms of gas-solid fluidized beds 6 state of minimum fluidization bubbling fluidized bed slugging fluidized bed turbulent fluidization circulating fluidized bed

7 Some properties of the fluidized bed 7 specific lighter objects are floating on the bed surface upon tilting a horizontal adjustment of the bed surface occurs through a hole in the wall the bed will flow out like a liquid

8 8 Advantages and disadvantages of gas-solid fluidized beds Advantages: - intense solids mixing by rising bubbles causes uniform temperature distribution, even with highly exothermal reactions no hot spots - large transfer area between gas and solids - excellent heat transfer between fluidized bed and walls or internals - liquid-like behavior of fluidized bed makes solids handling easy Disadvantages: - existence of bubbles causes bypass of reactant gas - intense solids mixing causes backmixing of reactant gas - intense movement of particles is responsible for particle attrition ( catalyst costs) and erosion of walls and bed internals - scale-up of fluidized bed processes is more difficult than for fixed beds

9 Comparison of gas-solid reaction systems 9 Characteristics Fixed bed Moving bed Fluidized bed Entrained flow Suitability for heterogeneous catalytic gasphase reactions only for catalyst that is deactivated very slowly Catalyst attrition negligible can also be used with catalyst that is rapidly deactivated Catalyst attrition may be critical, depending on operating conditions Plug flow gas ensures high gas conversion Backmixing of gas due to mixing motion of solids and bubble-gas bypass lead to lower conversion Gas in virtually plug flow; high conversion possible Suitability for gas-solid reactions Unsuitable for continuous processes; batchwise operation yields nonuniform product For uniform feed particle size with low fines content; large reactor capacities possible No special requirements for feed particle-size distribution; high fines content also possible; continuous operation yields uniform product Possible for fast reactions; recycle of unreacted fines often difficult Temperature distribution Danger of hot spots with exothermic reactions Temperature gradients can be held within limits by virtue of high solids circulation and high gas throughput High solids mixing ensures uniform temperature distribution in bed; temperature control by heat exchangers immersed in bed or by admission and removal of solids Axial temperature gradients can be held within limits by high solids circulation Heat supply and removal, heat exchange Poor heat exchange; heat transport limits scale-up Poor heat exchange; due to high heat capacity of solids transport of large quantities of heat by way of circulating solids Very efficient exchange, good heat transport by solids Properties intermediate between fluidized bed and moving bed Large pellets (ca. 8- Particle Institute size of Solids Process 20mm), Engineering as uniform as and Particle possible; Technology no fines Medium size (ca. 2-6mm) and uniform; no fines Broad particle-size distribution Fine ( mm), with (ca mm); high fines narrow particle-size content acceptable Technische Universität distribution Hamburg-Harburg

10 Fluidized bed - applications 10 The first patent was issued in 1922 to BASF in Germany for a fluidizedbed gasifier for lignite. Inventor: Fritz Winkler

11 Winkler s gasifier 11 air oxygen

12 12 FCC Fluid Catalytic Cracking - the most successful fluid bed process The problem: carbon deposition from cracking deactivates catalyst H H C C H H H H C C H H H H C C H H H C H The solution: cycling a fluidized catalyst between reactor and regenerator, use hot regenerated catalyst as heat carrier for supplying heat to endothermal cracking reaction development work by Esso and MIT ,000 barrels/day plant in Baton Rouge

13 FCC process: Kellogg-Orthoflow system 13 Reactor: oil vapors react in presence of catalyst Regenerator: coke is burned off to regenerate the catalyst

14 The riser cracking process: the UOP system 14 reactor stripper regenerator riser air grid slide valve

15 Fluidization technology: physical processes 15 air slide conveyor for the transport of solids solid-liquid suspension fines fluidized bed elutriator fines are elutriated from coarse particle fluidized bed classification water coarse

16 Fluidization technology: processes with heat and mass transfer 16 fluidized bed cooler for alumina particles

17 Fluidization technology: processes with heat and mass transfer 17 fluidized-bed drying fluidized-bed spray granulation Sprühflüssigkeit solution product gas product gas solution bottom spray top spray

18 Fluidization technology: processes with heat and mass transfer 18 Coating of glass beads with paraffin in the supercritical fluidized bed fluidizing medium: CO 2 fluidized bed 80 bar, 40 C supercritical solution before expansion: 160bar, 70 C 20 µm

19 Fluidized bed chemical processes solid is a catalyst 19 Example 1: Phtalic anhydride from naphtalene (Badger/Sherwin- Williams process) naphtalene product gas filter steam Dowtherm air problem: highly exothermal reaction, explosion risk limits inlet concentration with fixed bed reactors solution: -naphtalin is injected in the liquid form mixing occurs in the fluidized bed, no explosion possible, no separate evaporator -temperature homogeneity avoids hot spots -in-bed heat exchanger extracts heat of reaction

20 Fluidized bed chemical processes solid is a catalyst 20 Example 2: Synthesis of acrylonitrile (Sohio process) Ammoxidation of propylene C3H 6 + NH O 2 CH2 = CH CN + 3H2O 2 - precise adjustment of reaction temperature leads to optimum yield - mixing of reactants inside the fluidized bed avoids risk of explosion - steam raising via in-bed heat exchanger tubes

21 21 Fluidized-bed chemical processes solid is the product in a catalytic process compressor reactor Gas-phase polymerization of ethylene (Unipol process) cooler separator catalyst

22 Fluidized-bed chemical processes: solid particles act as heat carriers 22 Fluid Coking Process for the thermal cracking of heavy residues, cracking leads to coke deposition on bed particles (petroleum coke) coke is partially burned and heated in the heater hot, particles supply heat to the reactor a) Slurry recycle; b) Stripper; c) Scrubber; d) Reactor; e) Heater; f) Quench elutriator

23 Fluidized-bed chemical processes: solid particles are reactants 23 Calcination of aluminium hydroxide (Lurgi process) - endothermal reaction - countercurrent flow of gas and solids through the process saves energy a) Venturi fluidized bed b) Cyclone c) Fluidized-bed furnace d) Fluidized-bed cooler e) Recycle cyclone f) Electrostatic precipitator

24 Coal combustion in the (circulating) fluidized bed a) Circulating fluidizedbed reactor b) Recycle cyclone c) Siphon d) Fluidized-bed heat exchanger e) Convective pass f) Dust filter g) Turbine h) Stack - in-situ desulphurization with limestone dosing: CaCO 3 CaO + CO 2 CaO + SO 2 + 1/2O 2 CaSO 4 -low NO x by staged combustion

25 Fluid-mechanical principles the minimum fluidization velocity 25 Measurement in the laboratory: A t : cross-sectional area of column

26 Minimum fluidization velocity evaluation of measurement Δp Δp fb u mf bed + distributor bed distributor Δp fb - segregation occurs around u mf avoid measuring here! - just take measurements in the fully fluidized state and in the fixed bed state - u mf is then determined by extrapolation - a reproducible fixed bed is obtained by shutting the gas supply suddenly off in the fully fluidized state u

27 Minimum fluidizing velocity - calculation 27 fluidized bed pressure drop: Δp fb (u u mf ) = (1-ε)(ρ s -ρ f )gh fixed bed pressure drop: Δp fix (u u mf ) = function of u, d p, ε, gas conditions (e.g. Ergun s equation) u mf from Δp fb = Δp fix (u=u mf ) Good approximation: Re mf =33.7 {( Ar) 0.5-1} (Wen + Yu, 1966) If a sample of the bed solids is available, the following procedure is recommended: 1. Measure u mf with air under ambient conditions in the lab 2. Calculate the Sauter diameter of the bed solids from Ergun s equation 3. Convert u mf (air, ambient conditions) to u mf (gas at process conditions) by using Ergun s equation (Werther, Chem.-Ing.-Techn., 1976)

28 28 Minimum fluidization velocity Calculation of u mf under process conditions Measured and calculated minimum fluidization velocities as function of pressure and temperature. (Measurements by Knowlton, 1974 with nitrogen (T=293K) and by Janssen, 1973 with air (p= 1bar)). Comparison between measured and calculated minimum fluidization velocities for different gases (Measurements by Singh, Rigby and Callcott, 1973).

29 29 Geldart (1973): The existence of types of powders with characteristic behaviors Group C: fine, cohesive materials, difficult to fluidize, particles are sticking to each other, rat holes are formed, mechanically stirring of the bed may be needed Group A: typical is FCC catalyst, good fluidization, above u mf first homogeneous fluidization which breaks down at u mb, upon shutting off the gas supply the bed is slowly collapsing. Group B: typical is sand of mm, bubbling occurs immediately above u mf, upon shutting off the gas supply the bed is rapidly collapsing. Group D: large particles, typical are wheat grains, formation of very large bubbles.

30 Fluid-mechanical principles - Reh s status diagram 30 purpose: to characterize the state of fluidization for a given system by an (average) voidage ε abscissa: particle Reynolds number Re = ud p ν ordinate: 3 ρ u 4 gd 2 f Fr with Fr = ρs ρf p auxiliary grid with gd Ar =, M = ν ρ g ν ρ ρ 3 3 p ρs ρf u ρf 2 f s f

31 Reh sstatusdiagram 31 - its backbone is the force balance on a single particle (ε o) - the lines ε=const for gas-solid (bubbling, aggregative ) fluidization are based on experiments

32 Reh s status diagram 32 may be used to locate different fluidized bed (and even fixed bed) processes a) Circulating fluidized bed b) Fluidized-Bed roaster c) Bubbling fluidized bed d) Shaft furnace e) Moving bed

33 Reh s status diagram 33 can answer a number of practical questions: S 1 S 4 S 3 S S 2 - which voidage is expected for given solids (d p,ρ s ), gas (ν,ρ,g) and gas velocity u? calculate Ar, Re status S - particles of which size will be elutriated? use M = const S 1 - if particle agglomeration occurs: for which size fluidization will break down? use M = const S 2 - find the minimum fluidization velocity use Ar = const S 3 - where is a (theoretical) upper limit of fluidization? use Ar = const S 4

34 The role of the gas distributor in the fluidized bed 34 The distributor shall - ensure uniform fluidization over the entire cross-section of the bed - provide complete fluidization of the bed without dead spots (where, for example, deposits can form) - maintain a constant pressure drop over long operation periods (outlet holes must not become clogged) - prevent solids from raining through the grid both during operation and after the bed has been shut off Distributor types: - porous plates in the laboratory - perforated plates, nozzles, bubble caps, spargers in technical units

35 Gas distribution devices in large-scale fluidized bed combustors 35 1 Nozzle 7 bubble cap 2-6 and 8 combined types with mixed characteristics of bubble caps and nozzles, 10 sparger 9,11,12, special designs (after VGB_Merkblatt M218 H)

36 Gas distributor design 36 Basic requirement: Δp distributor Δp bed Design procedure: ρo 2 - Δ pd = CD u0 (index o relates to conditions in orifice, 2 drag coefficient C D from measurement) - with u o calculate number n o of orifices from continuity Problems with gas distributor: - open jets will cause attrition of bed solids - pressure fluctuations may cause backflow of solids into the windbox

37 Local fluid mechanics: Bubble formation 37 Gas-solid fluidized beds are characterized by the presence of bubbles bubbles are responsible for the temperature homogeneity of fluidized-bed reactors (bubbles are stirring the bed) and for the excellent heat transfer between bed and walls or intervals (bubbles account for surface renewal at the heat transfer surfaces) but: bubbles are also responsible for drawbacks of the fluidized-bed reactor: - bubbles cause a bypass of reaction gas which limits the conversion of a catalytic gas-phase reaction - bubble-induced solids movement leads to attrition of the bed particles and erosion of walls and internals the ultimate cause of bubble formation is the universal tendency of gas-solid flows to segregate. Stability theories (Jackson, Molerus etc.) indicate that disturbances induced in an initially homogenous gas-solid suspension do not decay but always lead to the formation of macroscopic voids

38 38 Local fluid mechanics: Gas flow in and around a rising bubble Davidsons s bubble model: p dp dh ( ρ ρ )( ε ) = 1 g s f mf pressure inside bubble is constant h pressure outside bubble is higher than inside gas will flow into the bubble streamlines of fluid (broken lines) and particles (solid lines) around a spherical bubble

39 39 Visualization of bubble fluid dynamics X-ray photo of a 3 D bubble (Rowc, 1971) Injection of an NO 2 bubble into an incipiently fluidized 2 D bed Davidson and Harrison, 1971)

40 Coalescence and splitting of bubbles 40 Coalescence of bubbles from Toei et al. (1965) (X-ray photo and dimensionless correlation) Splitting of a single bubble (X-ray sequence, Rowe, 1971)

41 Calculation of bubble growth 41 for Geldart group A and B solids (Hilligardt and Werther, 1987) dd dh v = 2ε 9π coalescence b 1/ 3 λ = mean bubble lifetime d v 3λu b splitting d v = diameter of volume-equivalent sphere h = height above distributor ε b = bubble volume fraction u b = bubble rise velocity at h=h 0 : ε porous plate 1/ 3 b dv,o = V& 0 industrial gas distributor with m 1.3 g V & 0 = volumetric gas flow through a single orifice λ = u mf 280 (typically s) g

42 Gas jets in fluidized beds 42 (after Karri and Werther, 2003) correlations suggested by Merry (1974): L up ρfd o u o = d o ρsd p gd p hor ρou d 0 ρf p L = d o ( 1 ε) ρsgd p ρs do

43 Circulating fluidized beds 43 most important applications: catalytic cracking (FCC process) fluidized-bed combustion

44 Operating characteristics of FCC risers and CFB combustors 44 geometry: cross-section of riser riser diameter height-to-diameter ratio walls of riser bed particle size distribution: Sauter diameter d ps operating characteristics: superficial gas velocity (external) solids circulation rate G s average solids residence time per single pass mean solids volume concentration in upper dilute zone of riser FCC riser circular m >20 flat approx mm between m/s (min. velocity at bottom) and m/s (at riser exit) >300 kg/m 2 s approx. 4s >1 % (1-10 %) CFB combustor mostly rectangular or square 4-8m (hydraulic diameter) <5(10) membrane walls (vertical tubes/fins) approx. 0.2 mm broad size distribution 5-8 m/s 5-8 m/s approx s <1% ( %)

45 CFB fluid mechanics 45 Q 3 = cumulative mass distribution, u t = single particle terminal velocity circulating fluidized beds are operated well above the single particles terminal velocities!

46 CFB fluid mechanics 46 pneumatic conveying CFB bubbling (stationary) fluidized bed u sl = slip velocity G s = solids circulation rate kg/m 2 s c v,mf CFB is characterized by very high slip velocity!

47 CFB local fluid mechanics (combustion systems) 47 Flensburg combustor 105 MWth, 100 % load, u = 6.3 m/s s = thickness of hydrodynamical boundary layer

48 Pressure distribution in the CFB system 48 a = fluidized bed, b = return leg

49 CFB: Design options for the pressure seal 49 Siphon L-valve

50 Entrainment from fluidized beds 50 - bubble eruptions shed particles into the freeboard - entrained particles disengage in the freeboard: coarser particles sink back into the bed, finer particles are elutriated - the disengagement process is finished after TDH (= transport disengaging height)

51 Calculation of entrainment 51 the specific mass flow rate of solids leaving the CFB at the trop (above TDH) is G = s x i i χ i x i = mass fraction of the (entrainable) particle size fraction in the bed material χ i* = elutriation rate constant for this size fraction, kg/m 2 s obtainable from various empirical correlations

52 10 Estimation of TDH for FCC catalyst type particles 52 transport disengaging height TDH, m 1 0,1 3 m 1.5 m 0.6 m 7.5 m 0.3 m 0.15 m m D = m (after Zenz and Othmer,1960; the parameter is the bed diameter) 0,1 1 U - U mf, m/s

53 Solids mixing in fluidized beds 53 The mechanism: Solids are displaced by rising bubbles particle drift effect causes particle mixing dispersion process Solids are carried upward in the wakes of rising bubbles convective transport The consequence: mixing in the vertical direction is much better than in the horizontal direction!

54 Solids circulation in fluidized beds 54 radial distribution of the visible bubble flow bubble-induced solids circulation pattern (Werther, 1974)

55 55 Lateral solids mixing in a bubbling fluidized bed Measurements by Bellgardt and Werther (1986) Solid CO 2 (dry ice) was injected through the side wall of the bed. Sublimation cooling led to a steady-state temperature distribution in the bed with a distinct temperature gradient in the horizontal direction

56 Vertical dispersion of solids in fine-particle fluidized beds 56 better mixing with increasing fluidizing velocity and in beds of larger diameters horizontal dispersion coefficients are two orders of magnitude lower!

57 Heat transfer to internals / walls in fluidized beds 57

58 Bed-to-wall heat transfer depends on particle size 58

59 Maximum heat transfer coefficient as a function of particle size 59 α max decreases because heat capacity of small particles is rapidly exhausted heat conduction in the gas-filled gap between particle and wall is limiting gas convection is increasingly contributing

60 Literature: 60 [1] D. Kunii and O. Levenspiel: Fluidization Engineering Second Edition. Butterworth-Heinemann, Boston 1991 [2] J.R. Grace, A.A. Avidan, T.M. Knowlton (Eds): Circulating fluidized beds Blackie Academic and Professional, London 1997 [3] W.C. Yang (Ed.): Handbook of Fluidization and Fluid-Particle Systems. Marcel Dekker, New York The current state of the art is documented in the proceedings of three conference series: Fluidization (Engineering Foundation Conference, 11th was 2004) International Conference on Circulating Fluidized Beds (8th was 2005) Fluidized Bed Combustion Conference (18th was 2005)