van Ommen, J. Ruud

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Engineering Conferences International

Refereed ProceedingsThe 12th International Conference onFluidization - New Horizons in FluidizationEngineeringEngineering Conferences International Year 2007PRESENTATION SLIDES: Measuringthe Gas-Solids Distribution in FluidizedBeds - A ReviewJ. Ruud van OmmenDelft University of Technology, j.r.vanommen@tudelft.nlThis paper is posted at ECI Digital Archives.http://dc.engconfintl.org/fluidization xii/130Measuring the Gas-Solids Distributionin Fluidized Beds – a ReviewJ. Ruud van Ommen and Robert F. MuddeDelft Univ. of Technology, The NetherlandsThe 12th International Conference on Fluidization, 13-18 May 2007j.r.vanommen@tnw.tudelft.nl1van Ommen: Measuring the Gas-Solids Distribution in Fluidized BedsPublished by ECI Digital Archives, 2007Measurement techniques in fluid bedsWerther, Powder Technol. 102 (1999) 15Industrial routine measurements• Pressure• TemperatureOccasional industrial measurements• Heat transfer probes• Solids flow measurements• Solids volume concentration (e.g. capacitance probes)• Particle size measurements• γ-ray transmission tomographyNew developments / measurement techniques in academicia2The 12th International Conference on Fluidization - New Horizons in Fluidization Engineering, Art. 130 [2007]http://dc.engconfintl.org/fluidization_xii/130This presentation• Tomography• Optical probes• Capacitance probes• Pressure measurements3van Ommen: Measuring the Gas-Solids Distribution in Fluidized BedsPublished by ECI Digital Archives, 2007TomographySoft fieldVoidage distribution influences shape of field linesnon-linear field linesnon-local responseFluidization research:electric capacitance tomographyHard fieldField lines only attenuatedlinear field lineslocal responseFluidization research:X-ray and γ-ray tomography4The 12th International Conference on Fluidization - New Horizons in Fluidization Engineering, Art. 130 [2007]http://dc.engconfintl.org/fluidization_xii/130Electric Capacitance Tomography: principleN electrodesN(N−1)/2 independent measurements CijN=8 28 meas.N=16 120 meas.φ=0φ=0φ=0 φ=V0φ=0φ=0φ=0φ=012345678 C12 C13Picture adapted fromDyakowski et al., Powder Technol. 112 (2000) 1745van Ommen: Measuring the Gas-Solids Distribution in Fluidized BedsPublished by ECI Digital Archives, 2007[ ]ε φ∇ ⋅ ∇ =) 0( ) (, ,x yx yε φ∇ ⋅= =∆ ∆∫ ˆ( , ) ( , )iijij ijx y x y ndlQCV VVarying permittivity, no charge:For every electrode pair:permittivity distribution ε isdirectly connected to voidageφ=0φ=0φ=0 φ=V0φ=0φ=0φ=0φ=012345678 C12 C13Electric Capacitance Tomography: reconstruction6The 12th International Conference on Fluidization - New Horizons in Fluidization Engineering, Art. 130 [2007]http://dc.engconfintl.org/fluidization_xii/130Electric 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.7van Ommen: Measuring the Gas-Solids Distribution in Fluidized BedsPublished by ECI Digital Archives, 2007X-ray and γ-ray tomographyphotonsgamma’s (γ)Röntgen (X)excited nucleiexcited electronsEnergy: 10 – 10,000 keVEnergy: 1 – 200 keVoverlap: difference in generationhigh energy: high penetration depthdangerous∆V high voltageelectronsX-rays8The 12th International Conference on Fluidization - New Horizons in Fluidization Engineering, Art. 130 [2007]http://dc.engconfintl.org/fluidization_xii/130principle: absorption/scattering of original radiationI(x) I(x+dx)x x+dxintensity = photons/m2sIdxdI µ−=linear absorption coeff. (m-1)= material property= function of photon energyI = I0 exp (-µx)initialtransmittedof given energyLambert-Beerholds only for fixed energyscanning X/γ-ray tomography: principle9van Ommen: Measuring the Gas-Solids Distribution in Fluidized BedsPublished by ECI Digital Archives, 2007½dwLair bubbleI = I0 exp (-µx) = I0 exp [-{µwdw+µs(1-α)L+µairαL}]transmitted photonsof original energy absorption by walldistance through wall (≠wall thickness)absorption by particles absorptionby air(line) fractionof bubblesfluidized powdercollimatorBGO-detectorcount rate: R(E) = I(E) η(E)counting efficiency of detector:  energy dependent!X/γ-ray tomography: principle10The 12th International Conference on Fluidization - New Horizons in Fluidization Engineering, Art. 130 [2007]http://dc.engconfintl.org/fluidization_xii/130nuclear decay: statistical processPoison statisticsif N = average counts in time ∆t st.dev. σN = √N(assuming decay is dominating error-source)count rate: R = N/∆t σR = σN/∆t = √N/∆t = √(R/∆t)Measuring longer decreases the error, provided the flow field is ‘frozen’!!!count rate: R = R0 exp(-µx)1. Optimize measurement timex is the path lengthattenuation coeff. µ=µ(E) the photon energy is a ‘design variable’µx = 2 minimizes the errorerror analysisX/γ-ray tomography: accuracy2. Optimize photon energy11van Ommen: Measuring the Gas-Solids Distribution in Fluidized BedsPublished by ECI Digital Archives, 2007Detector: incoming photon → … → … → electric pulsepulse height ∝ photon energy no AD-conversion(too slow), buttwo-level testtimeVProblem: pile-up (summing up) of pulsesno pile-uppile-up:one countlostpile-up:two countslostpile-up:one falsecountX/γ-ray tomography: detector limitations12The 12th International Conference on Fluidization - New Horizons in Fluidization Engineering, Art. 130 [2007]http://dc.engconfintl.org/fluidization_xii/130Computed tomographyProjection data is obtained by rotating the source and detector-array around the object Static imagesTubeX-ray beamsDetector array13van Ommen: Measuring the Gas-Solids Distribution in Fluidized BedsPublished by ECI Digital Archives, 2007Time-resolved computed tomography• Bubble diameter ∼ 10-2 m• Velocity ∼ 10-1 m/sRequired temporal resolution:   <10 msMulti-source setup• Alternative image reconstruction methods• Improved spatial resolutionτ ~ 100 ms14The 12th International Conference on Fluidization - New Horizons in Fluidization Engineering, Art. 130 [2007]http://dc.engconfintl.org/fluidization_xii/130Iterative image reconstructionipNµikw1µ1kµ −2µ11 12 1 11 21122NM M NN Mw w w pw pw w pµµµ⎛ ⎞⎜ ⎟⎜ ⎟⎜ ⎟⎜ ⎟⎝ ⎠⎛ ⎞ ⎛ ⎞⎜ ⎟ ⎜ ⎟⎜ ⎟ ⎜ ⎟=⎜ ⎟ ⎜ ⎟⎜ ⎟ ⎜ ⎟⎝ ⎠⎝ ⎠…number of pixels >number of detectors ⇒ ill-posediterative solution, via algebraic reconstruction techniqueimagevectorprojectiondatavectorweightingcoefficientmatrix15van Ommen: Measuring the Gas-Solids Distribution in Fluidized BedsPublished by ECI Digital Archives, 2007Example• Column diameter 23 cm• 3 artificial ‘bubbles’• Reconstruction 61x61 pixels• 5x30 detectors• Data-aqcuisition time:400 µsRemove pepper & salt noise:  • simultaneous reconstruction• median filter16The 12th International Conference on Fluidization - New Horizons in Fluidization Engineering, Art. 130 [2007]http://dc.engconfintl.org/fluidization_xii/130Optical probespurpose:local measurement of gas fractionbubble size & velocitycharacteristics:• point measurement• thin but intrusive• fast• low noise leveldifferent principles:• light scattering/reflection• light transmissiontypical material: glass fiber17van Ommen: Measuring the Gas-Solids Distribution in Fluidized BedsPublished by ECI Digital Archives, 2007measuringvolumesendingfiberreceivingfiberglasswindowRundqvist et al., Exp Fluids35 (2003) 572Optical probes: reflection typepoorly defined measuring volumeLiu et al., AIChE J. 49 (2003) 1405Blind zoneLightinLightoutwell-defined measuring volume18The 12th International Conference on Fluidization - New Horizons in Fluidization Engineering, Art. 130 [2007]http://dc.engconfintl.org/fluidization_xii/130probe large,particles small:swarm measuredprobe small,particles large:individual particlesmeasuredLiu et al., AIChE J., 49 (2003) 1405Optical probes: probe size vs particle size19van Ommen: Measuring the Gas-Solids Distribution in Fluidized BedsPublished by ECI Digital Archives, 2007Double horse shoe probeGroen et al., AIChESymp Ser 93, 1997Optical probes: transmission type20The 12th International Conference on Fluidization - New Horizons in Fluidization Engineering, Art. 130 [2007]http://dc.engconfintl.org/fluidization_xii/130Capacitance measurementsElectric capacitance Capacitance probetomographyφ=0φ=0φ=0 φ=V0φ=0φ=0φ=0φ=0C12 C13φ=0 φ=V0C21van Ommen: Measuring the Gas-Solids Distribution in Fluidized BedsPublished by ECI Digital Archives, 2007Capacitance probesWerther & Molerus, Int. J. MultiphaseFlow 1 (1973) 103Geldart & Kelsey, Powder Technol. 6 (1972) 45Plate probe Needle probe22The 12th International Conference on Fluidization - New Horizons in Fluidization Engineering, Art. 130 [2007]http://dc.engconfintl.org/fluidization_xii/130Capacitance probes: guard electrodeM. Louge, Experimental techniques, in: J.R. Grace, T. Knowlton, A.A. Avidan (Eds.), Circulating Fluidized Beds, Chap. 9, Chapman & Hall, London, 1996. Guard electrode at same voltage as sensor:absorbs disturbances from outside sourcesmore accurate signal23van Ommen: Measuring the Gas-Solids Distribution in Fluidized BedsPublished by ECI Digital Archives, 2007Example of capacitance probe results Chalmers CFB boiler, Wiesendorf & Werther, Powder Technol. 110 (2000) 143.24The 12th International Conference on Fluidization - New Horizons in Fluidization Engineering, Art. 130 [2007]http://dc.engconfintl.org/fluidization_xii/130Pressure measurements∆P PavgPg hρ ∆=⋅∆Info about dynamics:Local pressure measurements (interpretation is less difficult).Sample frequency: ~200HzPressure drop(time-aveaged):Good estimate of the average density.Deviations due to acceleration effects possible!Pressure drop versus local pressure25van Ommen: Measuring the Gas-Solids Distribution in Fluidized BedsPublished by ECI Digital Archives, 2007Pros and cons pressure fluctuation measurementsApplicable both in lab and in full-scale unitsCheapVirtually non-intrusiveHigh frequency info characterization of dynamicsLarge measurement volume– Ill-defined measurement volume– Pressure signal difficult to interpret26The 12th International Conference on Fluidization - New Horizons in Fluidization Engineering, Art. 130 [2007]http://dc.engconfintl.org/fluidization_xii/130‘Measurement volume’ of pressure probe80 cm50 cmIn the horizontal plane, pressure probes can detect phenomena up to about 50 cm away.Van Ommen et al., Powder Technol. 139 (2004) 264 27van Ommen: Measuring the Gas-Solids Distribution in Fluidized BedsPublished by ECI Digital Archives, 2007Absolute pressure versus relative pressuretimepressure0avg. pressure timepressureAbsolute pressure:Poor resolution of the fluctuationsRelative pressure:Optimal use of measurement range28The 12th International Conference on Fluidization - New Horizons in Fluidization Engineering, Art. 130 [2007]http://dc.engconfintl.org/fluidization_xii/130FilteringHigh-pass filter (cut-off frequency ~ 0.1 Hz):remove baseline and trendLow-pass filter (cut-off freq.: 0.5 * sample freq. or lower):avoid aliasing (Nyquist-Shannon sampling theorem)PressurePressureTimeTime29van Ommen: Measuring the Gas-Solids Distribution in Fluidized BedsPublished by ECI Digital Archives, 2007Two ways of mounting transducerflush-mountedtransducer:easily damagedremotetransducersensingline orprobefluidized bedpressureporttransducervolume30The 12th International Conference on Fluidization - New Horizons in Fluidization Engineering, Art. 130 [2007]http://dc.engconfintl.org/fluidization_xii/130Effect of probe dimensions on measurementsOptimal length: as short as possibleOptimal internal diameter: ~ 4mmoriginall=2.5md=5mml=2.5md=1mmvan Ommen et al., PowderTechnol. 106 (1999) 199  (Erratum: 113 (2000) 217)31van Ommen: Measuring the Gas-Solids Distribution in Fluidized BedsPublished by ECI Digital Archives, 2007Wire gauze versus purge flow Two common ways of preventing particles from intruding the probe:• Wire gauze at probe tipEasy, 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.32The 12th International Conference on Fluidization - New Horizons in Fluidization Engineering, Art. 130 [2007]http://dc.engconfintl.org/fluidization_xii/130Several 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 methodOverview: Johnsson et al., IJMF 26 (2000) 66333van Ommen: Measuring the Gas-Solids Distribution in Fluidized BedsPublished by ECI Digital Archives, 2007Applications of pressure signals• Identification of various phenomenaformation, coalescence, and eruption of bubbles• Characterisationdiscrimination of regimes• Validationresults obtained by computations fluid dynamics• Monitoringearly detection of agglomeration34The 12th International Conference on Fluidization - New Horizons in Fluidization Engineering, Art. 130 [2007]http://dc.engconfintl.org/fluidization_xii/130Different phenomena in pressure signalThe results on the following slides are all obtained for Geldart B particles (particles ~ 400 µm).Davidson, Trans. Inst. Chem. Eng. 39 (1961) 230PTimePressureTheoretical pressure signal of a spherical bubble35van Ommen: Measuring the Gas-Solids Distribution in Fluidized BedsPublished by ECI Digital Archives, 2007bubbleeruptionrisingbubblebubbleformationDifferent phenomena in pressure signalA B CVan der Schaaf et al., Powder Technol. 95 (1998) 220Single bubble injectionTimePressureP36The 12th International Conference on Fluidization - New Horizons in Fluidization Engineering, Art. 130 [2007]http://dc.engconfintl.org/fluidization_xii/130Decomposition of pressure signalPP+1000+1000-1000-10000033Pressure[Pa]Pressure[Pa]time [s]time [s]37van Ommen: Measuring the Gas-Solids Distribution in Fluidized BedsPublished by ECI Digital Archives, 2007Decomposition of pressure signal+1000+1000-1000-10000033Press.[Pa]Press.[Pa]time [s]time [s]10 310 3108108002020Frequency [Hz]Frequency [Hz]PSD [Pa  /Hz]2PSD [Pa  /Hz]20 20Frequency [Hz]01Coherence[-]Van der Schaaf et al., IJMF 28 (2002) 865PSD = Power Spectral Densityusing spectral analysis38The 12th International Conference on Fluidization - New Horizons in Fluidization Engineering, Art. 130 [2007]http://dc.engconfintl.org/fluidization_xii/130Decomposition of pressure signalA,B,DCIncoherent Output Power = (1-coherene) PSDFrequencyFrequencyFrequencyFrequencyPSD CoherentOutputPowerCoherencepassing bubblesfast-travelling wavesσσσci222Coh. varianceIncoh. varianceIncoherentOutputPower~Db239van Ommen: Measuring the Gas-Solids Distribution in Fluidized BedsPublished by ECI Digital Archives, 2007Decomposition of pressure signalIs the obtained bubble size info realistic?Db fromDarton [m]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) A21Darton et al., Trans. Inst. Chem. Eng. 55 (1977) 274αDbfromσi[m]sandDbed=40cm40The 12th International Conference on Fluidization - New Horizons in Fluidization Engineering, Art. 130 [2007]http://dc.engconfintl.org/fluidization_xii/130Applications of pressure signals• Identification of various phenomenaformation, coalescence, and eruption of bubbles• Characterisationdiscrimination of regimes• Validationresults obtained by computations fluid dynamics• Monitoringearly detection of agglomeration41van Ommen: Measuring the Gas-Solids Distribution in Fluidized BedsPublished by ECI Digital Archives, 2007Example: monitoringreferenceS [-]02040      early warning foragglomerationdefluidizationS = 3 0.60.81.0time [min]0 20 40averagepressuredrop[kPa]Early detection of agglomeration during lab-scale straw gasificationVan Ommen et al., Proc. 16th Int. Conf. on Fluid. Bed Comb. (2001) paper 131threshold for stationarity42The 12th International Conference on Fluidization - New Horizons in Fluidization Engineering, Art. 130 [2007]http://dc.engconfintl.org/fluidization_xii/130Concluding 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.43van Ommen: Measuring the Gas-Solids Distribution in Fluidized BedsPublished by ECI Digital Archives, 200744The 12th International Conference on Fluidization - New Horizons in Fluidization Engineering, Art. 130 [2007]http://dc.engconfintl.org/fluidization_xii/130

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

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

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