PRESENTATION SLIDES: Measuring the Gas-Solids Distribution in Fluidized Beds - A Review

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1 Refereed Proceedings The 12th International Conference on Fluidization - New Horizons in Fluidization Engineering Engineering Conferences International Year 2007 PRESENTATION SLIDES: Measuring the Gas-Solids Distribution in Fluidized Beds - A Review J. Ruud van Ommen Delft University of Technology, j.r.vanommen@tudelft.nl This paper is posted at ECI Digital Archives. xii/130

van Ommen: Measuring the Gas-Solids Distribution in Fluidized Beds Measuring the Gas-Solids Distribution in Fluidized Beds a Review J. Ruud van Ommen and Robert F.

2 van Ommen: Measuring the Gas-Solids Distribution in Fluidized Beds Measuring the Gas-Solids Distribution in Fluidized Beds a Review J. Ruud van Ommen and Robert F. Mudde Delft Univ. of Technology, The Netherlands The 12th International Conference on Fluidization, May 2007 j.r.vanommen@tnw.tudelft.nl Published by ECI Digital Archives,

3 The 12th International Conference on Fluidization - New Horizons in Fluidization Engineering, Art. 130 [2007] Measurement techniques in fluid beds Werther, Powder Technol. 102 (1999) 15 Industrial routine measurements Pressure Temperature Occasional industrial measurements Heat transfer probes Solids flow measurements Solids volume concentration (e.g. capacitance probes) Particle size measurements γ-ray transmission tomography New developments / measurement techniques in academicia 2

4 van Ommen: Measuring the Gas-Solids Distribution in Fluidized Beds This presentation Tomography Optical probes Capacitance probes Pressure measurements Published by ECI Digital Archives,

5 The 12th International Conference on Fluidization - New Horizons in Fluidization Engineering, Art. 130 [2007] Tomography Soft field Voidage distribution influences shape of field lines non-linear field lines non-local response Fluidization research: electric capacitance tomography Hard field Field lines only attenuated linear field lines local response Fluidization research: X-ray and γ-ray tomography 4

6 van Ommen: Measuring the Gas-Solids Distribution in Fluidized Beds Electric Capacitance Tomography: principle φ=0 φ=0 7 φ=v C 12 C 13 2 φ=0 φ=0 6 5 φ=0 4 3 φ=0 φ=0 Picture adapted from Dyakowski et al., Powder Technol. 112 (2000) 174 N electrodes N(N 1)/2 independent measurements C ij N=8 28 meas. N= meas. Published by ECI Digital Archives,

7 The 12th International Conference on Fluidization - New Horizons in Fluidization Engineering, Art. 130 [2007] Electric Capacitance Tomography: reconstruction φ=0 φ=0 7 φ=v C 12 C 13 2 φ=0 Varying permittivity, no charge: [ ε( xy, ) φ( xy, ] ) = 0 φ= φ=0 permittivity distribution ε is directly connected to voidage φ=0 φ=0 For every electrode pair: C ij Qi = = V ij ε( x, y) φ( x, y) ndl ˆ V ij 6

8 van Ommen: Measuring the Gas-Solids Distribution in Fluidized Beds Electric Capacitance Tomography: challenges Spatial resolution typically ~ 5-10% of bed diameter. Resolution deteriorates towards the bed center of the fluidized bed. Electrostatic charge can distort the picture. Published by ECI Digital Archives,

9 The 12th International Conference on Fluidization - New Horizons in Fluidization Engineering, Art. 130 [2007] X-ray and γ-ray tomography photons Röntgen (X) excited electrons gamma s (γ) excited nuclei Energy: kev Energy: 10 10,000 kev overlap: difference in generation electrons V X-rays high voltage high energy: high penetration depth dangerous 8

10 scanning van Ommen: Measuring the Gas-Solids Distribution in Fluidized Beds X/γ-ray tomography: principle principle: absorption/scattering of original radiation I(x) I(x+dx) x x+dx intensity = photons/m 2 s di dx = µ I linear absorption coeff. (m -1 ) = material property = function of photon energy of given energy transmitted I = I 0 exp (-µx) initial Lambert-Beer holds only for fixed energy Published by ECI Digital Archives,

11 The 12th International Conference on Fluidization - New Horizons in Fluidization Engineering, Art. 130 [2007] X/γ-ray tomography: principle collimator BGO-detector ½d w L transmitted photons of original energy fluidized powder absorption by particles absorption by wall air bubble absorption by air I = I 0 exp (-µx) = I 0 exp [-{µ w d w +µ s (1-α)L+µ air αl}] distance through wall ( wall thickness) count rate: R(E) = I(E) η(e) (line) fraction of bubbles counting efficiency of detector: energy dependent! 10

12 nuclear decay: statistical process van Ommen: Measuring the Gas-Solids Distribution in Fluidized Beds X/γ-ray tomography: accuracy if N = average counts in time t count rate: R = N/ t Poison statistics st.dev. σ N = N (assuming decay is dominating error-source) σ R = σ N / t = N/ t = (R/ t) Measuring longer decreases the error, provided the flow field is frozen!!! 1. Optimize measurement time count rate: R = R 0 exp(-µx) error analysis µx = 2 minimizes the error x is the path length attenuation coeff. µ=µ(e) the photon energy is a design variable 2. Optimize photon energy Published by ECI Digital Archives,

13 The 12th International Conference on Fluidization - New Horizons in Fluidization Engineering, Art. 130 [2007] X/γ-ray tomography: detector limitations Detector: incoming photon electric pulse pulse height photon energy V time no AD-conversion (too slow), but two-level test Problem: pile-up (summing up) of pulses no pile-up pile-up: two counts lost pile-up: one count lost pile-up: one false count 12

Projection data is obtained by rotating the source and

14 van Ommen: Measuring the Gas-Solids Distribution in Fluidized Beds Computed tomography Detector array Tube X-ray beams Projection data is obtained by rotating the source and detectorarray around the object Static images Published by ECI Digital Archives,

130 [2007] Time-resolved computed tomography Bubble diameter 10-2 m Velocity 10-1 m/s τ ~100

15 The 12th International Conference on Fluidization - New Horizons in Fluidization Engineering, Art. 130 [2007] Time-resolved computed tomography Bubble diameter 10-2 m Velocity 10-1 m/s τ ~100 ms Required temporal resolution: <10 ms Multi-source setup Alternative image reconstruction methods Improved spatial resolution 14

16 van Ommen: Measuring the Gas-Solids Distribution in Fluidized Beds Iterative image reconstruction p i w11 w12 w1 N µ 1 p1 w21 µ 2 p2 = wm1 wm N µ N pm weighting coefficient matrix image vector projection data vector µ 1 µ k 1 µ 2 w ik µ N iterative solution, via algebraic reconstruction technique number of pixels >number of detectors ill-posed Published by ECI Digital Archives,

130 [2007] Column diameter 23 cm 3 artificial bubbles Reconstruction 61x61 pixels 5x30

17 The 12th International Conference on Fluidization - New Horizons in Fluidization Engineering, Art. 130 [2007] Column diameter 23 cm 3 artificial bubbles Reconstruction 61x61 pixels 5x30 detectors Data-aqcuisition time: Example 400 µs Remove pepper & salt noise: simultaneous reconstruction median filter 16

18 van Ommen: Measuring the Gas-Solids Distribution in Fluidized Beds Optical probes purpose: local measurement of gas fraction bubble size & velocity characteristics: point measurement thin but intrusive fast low noise level typical material: glass fiber different principles: light scattering/reflection light transmission Published by ECI Digital Archives,

19 The 12th International Conference on Fluidization - New Horizons in Fluidization Engineering, Art. 130 [2007] Optical probes: reflection type poorly defined measuring volume well-defined measuring measuring volume volume Blind zone glass window Light in Light out Liu et al., AIChE J. 49 (2003) 1405 receiving fiber sending fiber Rundqvist et al., Exp Fluids 35 (2003)

20 van Ommen: Measuring the Gas-Solids Distribution in Fluidized Beds Optical probes: probe size vs particle size probe large, particles small: swarm measured probe small, particles large: individual particles measured Liu et al., AIChE J., 49 (2003) 1405 Published by ECI Digital Archives,

21 Double horse shoe probe The 12th International Conference on Fluidization - New Horizons in Fluidization Engineering, Art. 130 [2007] Optical probes: transmission type Groen et al., AIChE Symp Ser 93,

22 van Ommen: Measuring the Gas-Solids Distribution in Fluidized Beds Capacitance measurements Electric capacitance tomography Capacitance probe φ=0 φ=0 φ=v 0 φ=0 C 12 C 13 φ=0 φ=v 0 C φ=0 φ=0 φ=0 φ=0 Published by ECI Digital Archives,

23 The 12th International Conference on Fluidization - New Horizons in Fluidization Engineering, Art. 130 [2007] Capacitance probes Plate probe Needle probe Geldart & Kelsey, Powder Technol. 6 (1972) 45 Werther & Molerus, Int. J. Multiphase Flow 1 (1973)

24 van Ommen: Measuring the Gas-Solids Distribution in Fluidized Beds Capacitance probes: guard electrode Guard electrode at same voltage as sensor: absorbs disturbances from outside sources more accurate signal M. Louge, Experimental techniques, in: J.R. Grace, T. Knowlton, A.A. Avidan (Eds.), Circulating Fluidized Beds, Chap. 9, Chapman & Hall, London, Published by ECI Digital Archives,

130 [2007] Example of capacitance probe results Chalmers CFB

25 The 12th International Conference on Fluidization - New Horizons in Fluidization Engineering, Art. 130 [2007] Example of capacitance probe results Chalmers CFB boiler, Wiesendorf & Werther, Powder Technol. 110 (2000)

26 van Ommen: Measuring the Gas-Solids Distribution in Fluidized Beds Pressure measurements Pressure drop versus local pressure Pressure drop (time-aveaged): Good estimate of the average density. ρ avg P = g h Deviations due to acceleration effects possible! P P Info about dynamics: Local pressure measurements (interpretation is less difficult). Sample frequency: ~200Hz Published by ECI Digital Archives,

27 The 12th International Conference on Fluidization - New Horizons in Fluidization Engineering, Art. 130 [2007] Pros and cons pressure fluctuation measurements Applicable both in lab and in full-scale units Cheap Virtually non-intrusive High frequency info characterization of dynamics Large measurement volume Ill-defined measurement volume Pressure signal difficult to interpret 26

28 van Ommen: Measuring the Gas-Solids Distribution in Fluidized Beds Measurement volume of pressure probe In the horizontal plane, pressure probes can detect phenomena up to about 50 cm away. 50 cm 80 cm Van Ommen et al., Powder Technol. 139 (2004) 264 Published by ECI Digital Archives,

29 The 12th International Conference on Fluidization - New Horizons in Fluidization Engineering, Art. 130 [2007] Absolute pressure versus relative pressure pressure 0 time Absolute pressure: Poor resolution of the fluctuations pressure avg. pressure time Relative pressure: Optimal use of measurement range 28

30 van Ommen: Measuring the Gas-Solids Distribution in Fluidized Beds Filtering High-pass filter (cut-off frequency ~ 0.1 Hz): remove baseline and trend Low-pass filter (cut-off freq.: 0.5 * sample freq. or lower): avoid aliasing (Nyquist-Shannon sampling theorem) Pressure Time Pressure Time Published by ECI Digital Archives,

31 The 12th International Conference on Fluidization - New Horizons in Fluidization Engineering, Art. 130 [2007] Two ways of mounting transducer pressure port sensing line or probe remote transducer fluidized bed flush-mounted transducer: easily damaged transducer volume 30

32 van Ommen: Measuring the Gas-Solids Distribution in Fluidized Beds Effect of probe dimensions on measurements original l=2.5m d=5mm l=2.5m d=1mm van Ommen et al., Powder Technol. 106 (1999) 199 (Erratum: 113 (2000) 217) Optimal length: as short as possible Optimal internal diameter: ~ 4mm Published by ECI Digital Archives,

33 The 12th International Conference on Fluidization - New Horizons in Fluidization Engineering, Art. 130 [2007] Wire gauze versus purge flow Two common ways of preventing particles from intruding the probe: Wire gauze at probe tip Easy, but not suited for very small particles. Can give problems at high temp.: sintering. Purge flow Better protection: against fines, aggressive gases, sintering. Flow should be very constant! Beware of effect of creating bubbles. 32

34 van Ommen: Measuring the Gas-Solids Distribution in Fluidized Beds Several analysis methods available (1) Time domain methods ( standard statistic) standard deviation probability density function (2) Frequency domain methods (spectral analysis) power spectrum coherence function (3) State space methods (non-linear or chaos analysis) Kolmogorov entropy attractor comparison method Overview: Johnsson et al., IJMF 26 (2000) 663 Published by ECI Digital Archives,

35 The 12th International Conference on Fluidization - New Horizons in Fluidization Engineering, Art. 130 [2007] Applications of pressure signals Identification of various phenomena formation, coalescence, and eruption of bubbles Characterisation discrimination of regimes Validation results obtained by computations fluid dynamics Monitoring early detection of agglomeration 34

36 van Ommen: Measuring the Gas-Solids Distribution in Fluidized Beds Different phenomena in pressure signal Theoretical pressure signal of a spherical bubble P Pressure Time Davidson, Trans. Inst. Chem. Eng. 39 (1961) 230 The results on the following slides are all obtained for Geldart B particles (particles ~ 400 µm). Published by ECI Digital Archives,

37 The 12th International Conference on Fluidization - New Horizons in Fluidization Engineering, Art. 130 [2007] Different phenomena in pressure signal Single bubble injection bubble formation rising bubble bubble eruption P Pressure Time A B C Van der Schaaf et al., Powder Technol. 95 (1998)

38 van Ommen: Measuring the Gas-Solids Distribution in Fluidized Beds Decomposition of pressure signal P Pressure [Pa] time [s] P Pressure [Pa] time [s] 3 Published by ECI Digital Archives,

39 +1000 Press. [Pa] Press. [Pa] Decomposition of pressure signal time [s] time [s] The 12th International Conference on Fluidization - New Horizons in Fluidization Engineering, Art. 130 [2007] Coherence [-] using spectral analysis 0 0 Frequency [Hz] 20 PSD [Pa /Hz] PSD [Pa /Hz] Frequency [Hz] Frequency [Hz] PSD = Power Spectral Density Van der Schaaf et al., IJMF 28 (2002)

40 van Ommen: Measuring the Gas-Solids Distribution in Fluidized Beds Decomposition of pressure signal PSD σ 2 Frequency Coherent Output Power fast-travelling waves σ 2 c A,B,D Coh. variance Frequency passing bubbles Coherence Frequency Incoherent Output Power σ 2 i C Incoh. variance Frequency ~D b 2 Incoherent Output Power = (1-coherene) PSD Published by ECI Digital Archives,

41 The 12th International Conference on Fluidization - New Horizons in Fluidization Engineering, Art. 130 [2007] Decomposition of pressure signal Is the obtained bubble size info realistic? αd b from σ i [m] sand D bed =40cm In this particular case: α=1 (coincidence!!). This is normally not the case: Kleijn van Willigen et al., Int. J. Chem. Reactor Eng., 1 (2003) A21 D b from Darton [m] Darton et al., Trans. Inst. Chem. Eng. 55 (1977)

42 van Ommen: Measuring the Gas-Solids Distribution in Fluidized Beds Applications of pressure signals Identification of various phenomena formation, coalescence, and eruption of bubbles Characterisation discrimination of regimes Validation results obtained by computations fluid dynamics Monitoring early detection of agglomeration Published by ECI Digital Archives,

43 The 12th International Conference on Fluidization - New Horizons in Fluidization Engineering, Art. 130 [2007] Example: monitoring Early detection of agglomeration during lab-scale straw gasification defluidization S [-] 20 threshold for S = 3 stationarity 0 early warning for agglomeration average pressure drop [kpa] reference time [min] Van Ommen et al., Proc. 16th Int. Conf. on Fluid. Bed Comb. (2001) paper

44 van Ommen: Measuring the Gas-Solids Distribution in Fluidized Beds Concluding remarks Electric capacitance tomography: fast, but spatial resolution is troublesome. X- and γ-ray tomography: quite good spatial resolution, but temporal resolution needs improvement Optical probes and capacitance probes determine ε(t) in a small measurement volume. They are reasonably well-developed. Time-averaged pressure measurements are commonly used to determine the average bed density and bed height. Obtaining quantitative voidage data from pressure fluctuations (or acoustic measurements) is not straight-forward, but they are very useful to determine changes in the voidage dynamics and distribution. Published by ECI Digital Archives,

45 The 12th International Conference on Fluidization - New Horizons in Fluidization Engineering, Art. 130 [2007] 44

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