3 IntroductionMany primary particles (asbestos and heavy metals) are more toxic.Many primary particles are respirable – health concern.Wall collection devices: driving the particles to a solid wall where they form agglomerates – gravity settler, cyclones, and ESP.Dividing collection devices: divide the flow into small parts where they can collect the particles – surface and depth filters, and scrubbers.
4 Wall Collection Devices – Gravity settlers A long chamber through which the contaminated gas passes slowly, allowing time for particles to settle by gravity.Unsophisticated, easy to construct, little maintenance, treating very dirty gases (smelters and metallurgical processes), easy math.
5 Wall Collection Devices – Gravity settlers Cross-sectional area (WH) > duct much lower velocity.Baffles spread the inflow evenly.Two ideal (limiting cases)Plug (block) flow model: unmixed.Mixed model
6 Gravity Settlers – Plug model Particle removal efficiency related toresidence time in chamberterminal settling velocity (Stokes’ law)distance to travel before hitting wall
7 Gravity Settlers – Mixed model Totally mixed in z-direction lead to decrease in (as gas move away from the inlet, C in a cross-section is homogeneous, so some particles still stay on the top, while the plug model particles will be more concentrated toward the lower sections).
8 Gravity Settlers – Ex. 9.1Find -D relationship for a gravity settler (H = 2 m, L = 10 m, Vavg = 1 m/s).For 1- particle (how about 50-?)
9 Gravity SettlersGravity settling is effective for large particles (>100), in reasonably sized chambers.To increase : making L larger (expensive), H smaller (hard to clean), Vavg smaller (expensive), increasing g.Increasing g: centrifugal.Horizontal elutriators: small gravity settlers used for particle sampling.
10 Centrifugal Separators Ex. 9.2: A particle travels with a gas stream with velocity of 60 fps (18.3 m/s) and r = 1 ft.Ex 9.3: Find the terminal velocity of 1- particleCyclone (cyclone separator): most widely used particle collection device in the world.
11 Centrifugal Separators Rectangular gas inlet (2x as high as wide) tangentially to the vertical cylindrical body.The gas spirals around the outer part of the cylindrical body with downward component, then turns and spirals upward.The particles are driven to the wall by the centrifugal force.Dimensions are based on Do.
12 Centrifugal Separators Inlet stream has a “height” Wi in the radial direction – the max. distance the particle needs to the wall.Length of flow path = NDo. (N = number of turns that gas makes traversing the outer helix = 5 typical).
13 Centrifugal Separators Ex. 9.4: Compute -diameter relation for a cyclone separator with Wi = 0.5’, Vc = 60 fps and N =5.For 1- (how about 10-?)Cut diameter: diameter of a particle for which efficiency curve has the value of 50%.
14 Centrifugal Separators For a typical cyclone, Dcut ~ 5.If gas contains few particles <5 cyclone is the first choice (low cost and easy maintenance). Not good for sticky particles such as tar droplets.Efficiency increases (Dcut decreases) with increasing Vcircular. But, P~ V2circular.Reduce inlet duct Width (and diameter in proportion)Split flow into multiple cyclones to keep Vcircular constantIf Wi = 0.125’ Dcut = 2.3.
15 Centrifugal Separators Eq is not a good predictor for (9.19 is a little better one).An empirical data-fitting equation Is more satisfactory.
16 Cyclone Collection Efficiency with Particle Size Distribution CE Control of Primary ParticulatesCyclone Collection Efficiency with Particle Size DistributionCollection efficiency varies with particle terminal velocity, which in turn varies with particle diameter D and densityEx 9.6: Performance computation for a cyclone separator of Dcut = 5 m with log normally distributed particle size: Dmass mean = 20 m, = 1.25.Divide the distribution into 10 fractions.Find z (= number of standard deviation).p = penetrationJeff Kuo, Ph.D., P.E. (CSUF)6
17 Overall ~ 81%Mass mean diameter that passes thru the cyclone?The diameter corresponds to half of ~ of the mean diameter ~ 4.
19 Cyclone – Pressure drop CE Control of Primary ParticulatesCyclone – Pressure dropVi = velocity at the inlet to the cyclone (~1.5x the V in the duct approaching the cyclone).K ~8 for most cyclones.Ex. 9-8: A cyclone has a reported pressure loss of 8 velocity head and Vi = 60 fps.Blower before cyclone: particles get into bearings and collect on blades; after cyclone: air may be sucked in and re-entrain particles due to vacuum.Jeff Kuo, Ph.D., P.E. (CSUF)8
20 Advantages Disadvantages General Cyclone Thoughts Mechanism= streamlinesBrownianMotion(diffusion)impactioninterceptionGeneral Cyclone ThoughtsMechanism=IMPACTION!AdvantagesCheapNo moving parts (low maintenance)Removes solid or liquid particles (non-corrosive particles)Harsh conditions (high temperatures)Time-proven technology (1940s)DisadvantagesLow efficiency for small particles (Dp<10 um)High pressure drops High operating costsCan’t do sticky particles
21 Electrostatic Precipitators (ESP) CE Control of Primary ParticulatesElectrostatic Precipitators (ESP)ESPs are effective on much smaller particles.Viscous resistance (Stokes’ law) ~ D.For gravity settlers or cyclones: driving force ~D3.For ESPs: electrostatic force ~D2.It’ hard for ESPs to collect smaller particles (~ 1/D), but still easier than cyclones and settlers.Give the particles an electrostatic charge and put them in an electrostatic field.Rows of wires held at –40,000 V and plates are electrically grounded.On the plates, particles lose their charge and form a cake – removed by rappers, or a film of water.Jeff Kuo, Ph.D., P.E. (CSUF)8
22 CE 583 - Control of Primary Particulates How Do ESPs Work?Jeff Kuo, Ph.D., P.E. (CSUF)8
23 How Do ESPs Work? www.state.ia.us One stage esp (www.zet.com) Two stage esp (www.airwater.com)
24 ESPs (Cottrell precipitators) CE Control of Primary ParticulatesESPs (Cottrell precipitators)In a typical ESP, the distance between wire and plate is 4 – 6”. The field strength near the wire would be much higher because much small surface area.Jeff Kuo, Ph.D., P.E. (CSUF)
25 ESPs (Cottrell precipitators) CE Control of Primary ParticulatesESPs (Cottrell precipitators)H : the height through which particles must travel, at right angles to gas flow, before hitting wallL : distance traveled by gas in the collection device.The H will be small in ESP, the velocity of particles much higher because of the electrostatic force.Jeff Kuo, Ph.D., P.E. (CSUF)
26 ESPs (Cottrell precipitators) CE Control of Primary ParticulatesESPs (Cottrell precipitators)Corona discharge at the wire: electrons collide with gas molecules, knock out electrons (ionizing the gas) knock more electrons loose to form a steady corona discharge.Field charging away from the wire: as electrons fly towards wall, they collide with particles and captured by particles, negatively charged particles attracted to wall and discharged there.Diffusion charging: for particles smaller than ~0.15 , the interaction with electrons is mainly due to their random motion as a result of electron-gas molecule collisions (not due to electric field).Jeff Kuo, Ph.D., P.E. (CSUF)
27 ESPs – Maximum charge on a particle CE Control of Primary ParticulatesESPs – Maximum charge on a particleEx. 9-9: How many electronic charges on 1- ( = 6 and Eo = 300 kV/m)? How about 1/3-particles?Jeff Kuo, Ph.D., P.E. (CSUF)
28 CE 583 - Control of Primary Particulates ESPs – Drift velocity (terminal settling velocity under electrostatic force)Force on particle: F = qEP (EP, local electric field strength)Resulting terminal settling velocity (with Stokes law for drag force)Ex. 9-10:Jeff Kuo, Ph.D., P.E. (CSUF)
29 CE 583 - Control of Primary Particulates ESPs – Drift velocityw ~ E2 (E ~ wire voltage/wire-to-plate distance). One can raise the voltage or reduce distance, but limitation is sparking (most set for ~50 – 100 sparks/minute).The drift velocity is only ~5x as fast of Vc of cyclone. Why ESPs are much more effective?The drift velocity ~D for ESPs and ~D2 for cyclones.To achieve high V in cyclones, one must have high gas V. Typical gas V ~ 3-5 fps for ESPs ( ~ 3 to 10 s), while V ~ 60 fps ( ~ 0.5 s) for cyclones.Jeff Kuo, Ph.D., P.E. (CSUF)
30 ESPs – Collection Efficiency CE Control of Primary ParticulatesESPs – Collection EfficiencyBlock (plug) flow:Mixed flow:Jeff Kuo, Ph.D., P.E. (CSUF)
31 ESPs – Collection Efficiency CE Control of Primary ParticulatesESPs – Collection EfficiencyEx. 9.11: Compute -diameter relation for an ESP that has particles with =6 and A/Q = 0.2 min/ft. For 1- particle:Efficiency =99.8% for D = 5 Drift velocity is a function of DThe cut-diameter ~ 0.5 .Log(p) vs. A/Q is linear.Jeff Kuo, Ph.D., P.E. (CSUF)
32 ESPs – Collection Efficiency CE Control of Primary ParticulatesESPs – Collection EfficiencyEx. 9.12: Estimate w for coal containing 1% S.From the figure at = 99.5% A/Q = 0.31 min/ftJeff Kuo, Ph.D., P.E. (CSUF)
33 ESPs – Performance & Cake Resistivity CE Control of Primary ParticulatesESPs – Performance & Cake ResistivityHigh resistivity ash (elemental S): large Vcake , small Vwire, poor charging, low - electron flow within cake, back coronaLow resistivity ash (carbon black): small Vcake , weak attraction to collection plate, re-entrainmentBack corona is a conversion of electrostatic energy to thermal energy that will cause minor gas explosion blow the cake off the plate.The practical resistivity range: > 107 and < 2 x 1010 ohm-cm.Jeff Kuo, Ph.D., P.E. (CSUF)
34 ESP – Performance and Cake Resistivity CE Control of Primary ParticulatesESP – Performance and Cake ResistivityLittle can be done on low resistivity ash.Remedies for high resistivity ash:- Higher T, hot ESP (improves conduction of some materials in the ash under high T)- Gas conditioning, add hygroscopic components to gas to improve surface conductivity. Some S in coal is converted to SO3 (absorbs water). Coal ash is basic needs acidic conditioner. NH3 works for acidic Portland cement ash.Jeff Kuo, Ph.D., P.E. (CSUF)
35 CE 583 - Control of Primary Particulates ESPs – PerformanceEx 9.13: If of an ESP = 90%. How much must we increase the surface area to have = 99%?From 90% to 99.9% triple the area. However, w is ~ diameter (harder to remove the fines).Ex. 9-14: Use the modified D-A equation with k =2, the area needs to be quadrupled (not 2x).Jeff Kuo, Ph.D., P.E. (CSUF)
36 CE 583 - Control of Primary Particulates ESPs – PerformanceEx 9.15: An ESP has two identical sections in parallel, each receive ½ of gas flow and = 95%. If the flow is mal-distributed into 1/3 and 2/3, = ?It shows the importance of flow distribution.Jeff Kuo, Ph.D., P.E. (CSUF)
37 CE 583 - Control of Primary Particulates ESPs – PerformanceThe typical linear V inside an ESP ~ 3 to 5 fps and pressure drop is 0.1 – 0.5” water.The technology is established with up to 99.5%+.Wet-ESP can have higher w, more complex and the collected aren’t dry powder (but it seems worthwhile)Jeff Kuo, Ph.D., P.E. (CSUF)
38 Advantages Disadvantages General ESP Thoughts High for even small particlesLow P even with high flowDry or wet collectionWide range of temperatureLow operating costsPower plantsCement plantsPaper millsSteel foundriesIndoor air qualityDisadvantagesTake up lots of spaceHigh capital costNot flexible to changeMay need a pre-cleaner at high concentrations…cyclone?
39 ESP - Costs Capital Costs depend on total plate area ‘A’ Purchase price = aAba=962, b=0.628 for 10,000 ft2 < A < 50,000 ft2Total delivered equipment cost (DEC)=1.18*(purchase price)Installation cost : ~2.2*DECOperating Costs - depend on power consumptionFan pulling the air through the plates
40 Dividing Collection Devices Divide the flow into small parts and bring it in contact with large surface areaSurface filtersDepth filtersScrubbersSurface filters: fine particles are caught on the sides of holes of a filter (a membrane – sheet steel, cloth, wire mesh or paper) and a cake is formed (the actual filter)
41 Dividing Collection Devices – Surface filters Surface velocity (face velocity, approach velocity, superficial velocity, air to cloth ratio).Vs = Q/APressure drop for flow through porous mediaPtotal = Pfilter + Pcake
42 Filters - What Happens to the Collected Particles? ShakerPulse-jetSonic hornReverse airDifferent types of cleaningMain way to identify bag housesDifferent bag materials (woven vs. ‘felted’)Different cleaning frequency
45 CE 583 - Control of Primary Particulates Surface FiltersAs the cake builds up, the outlet C declines and stabilizing at a value about 0.001x the inlet C.The falls with increasing Vs (Figure 9.15).At low Vs, they will also have high on fine particles (ESPs have difficulties to collect particles of 0.1 to 0.5).Jeff Kuo, Ph.D., P.E. (CSUF)6
46 Shaker and reverse air use woven materials Pulse jet use felted materialsWoven:Stronger tensile strengthLonger time between cleaning (1/2 hr- several hours)Hold more filter cakeFelted:Less tensile strengthShort time between cleaning (every few minutes)Abrasive particles, smaller particles always
47 CE 583 - Control of Primary Particulates Depth FiltersDepth filters collect particles throughout the entire filter body.Mechanisms that contribute to particle capture: impaction, interception, and diffusion (Table 9-3).High-efficiency, particle-arresting (HEPA) filters – thrown-away type (no cleaning).streamlinesBrownianMotion(diffusion)impactioninterceptionJeff Kuo, Ph.D., P.E. (CSUF)
49 CE 583 - Control of Primary Particulates Depth FiltersEx to 9-20: A cylindrical fiber 10 is placed perpendicular to a gas stream (V = 1 m/s) with C = 1 mg/m3 and d = 1. Find . Find for a row of parallel fibers with center-to-center spacing of 5 fibers. How about 100 rows?Jeff Kuo, Ph.D., P.E. (CSUF)6
50 Advantages Disadvantages General Fabric Filter Thoughts High efficiency for even small particlesWide variety of solid particle typesModular flexible design, flexible conditionsLow pressure dropssolid waste.dpwt.comDisadvantagesMining plantTake up lots of spaceBad for high T and corrosivityBad for moist conditionsPotential for fire/explosionNeed frequent cleaningNeed bag replacement
51 When Would I Use a Fabric Filter? Size classification is not desiredHigh efficiency is requiredValuable dry material needs to be recoveredRelatively low volumesRelatively low temperaturesFibreboard plantPower plantsFertilizerFood processingPaper millsOre processing
52 ScrubbersBring the flow of gas in contact with a large number of liquid droplets representing a large surface areaNatural occurrence: rainfall
53 Scrubbers - Removal of particles from a volume of air during a rainstorm Ex 9-22: Q/A = 0.1”/hr with Ddrop = 1 mm. Air contains dparticle = 3 m, C0 = 100 g/m3. C1-hr =?Find Vt = 14 ft/s (4.2 m/s) for 1 mm raindropCalculate Ns (=0.23)Find t ~ 0.23 (Fig. 9-18)C/C0 = 0.43 C = 43 g/m3
54 Removal of Particles in a Cross-flow Scrubber Make Ddrop small, and/or z largeBoth measures would result in some liquid droplets being carried out of the scrubber.
55 Removal of Particles in a Counter-flow Scrubber As Vt VG , C 0But, this means droplets are nearly stationary with respect to the container flooding
56 Removal of Particles in a Co-flow Scrubber Need high relative velocity between gas and droplets without loosing the droplets or equipment flooding.IDEA: Introduce water droplets at right angles to gas but let them go out with the gas, then separate them in a cyclone.This is a modification of the way a cross-flow scrubber is operated.
57 Removal of Particles in a Co-flow Scrubber Idea is to increase velocity difference between particles and droplets and thus improve impaction.Venturi design is widely used because it saves fan power.
58 Removal of Particles in a Co-flow Scrubber Integration difficult because VG, Vrel, t all change with xDdrop is non-uniform, and not constant with x
59 CE 583 - Control of Primary Particulates ScrubbersEx. 9-23: In a venturi scrubber the throat V = 122 m/s. Particles to be removed = 1 and drop D = 100. QL/QG = At a point Vrel = 0.9 VG, what is the rate of decrease in C in the gas phase?Jeff Kuo, Ph.D., P.E. (CSUF)6
60 Scrubbers – Pressure drop CE Control of Primary ParticulatesScrubbers – Pressure dropEx. 9-25: A venturi scrubber has a throat area of 0.5 m2, a throat velocity of 100 m/s, and P = 100 cm water (9806 N/m2). Assuming motor&blower = 100%, find the power required.Ex. 9-26: For a scrubber using water as the scrubbing fluid, estimate the pressure drop: VG = QG/xy = 100 m/s and QL/QG = 0.001Jeff Kuo, Ph.D., P.E. (CSUF)6
61 CE 583 - Control of Primary Particulates Ex. 9-27: Dcut = 0.5 , QL/QG = 0.001, & C = 1.24, find gas velocity at the throat and P.Daerodynamic cut diameter =(0.5)(2*1.24)0.5 = 0.79V = 90 m/s (Fig. 9.27)P =~ 80 cm of water (Fig. 9.27)Jeff Kuo, Ph.D., P.E. (CSUF)6
62 Advantages Disadvantages General Scrubber Thoughts Flammable and explosive dusts are OKGas adsorption and particle collectionCan do mistsCools hot gases (can feed to fabric filter if dried)FlexibleChemicals may become less nasty through reactionDisadvantagesCorrosion issues - water may increase corrosivityCreates wet waste stream- water pollution + $$$Need to remove collected particles from water before recirculatingHigh power input to generate well-dispersed droplets
64 When Would I Use a Scrubber??? Wet particles that are in hot gas streamCorrosive particlesVery fine particles requiring high efficiencyParticles are with gases that also need to be removedCombustible gasesCooling is desirable and added moisture is not badPower plantsPaper millsFood industryCosmeticsSteel/metal industry
65 Choosing a collectorSmall or occasional flow throwaway device (also a good final cleanup device).Sticky particles throwaway or into liquid.Particles that adhere well to each other but not to solid surfaces are easy to collect.Electrical properties of particles are of paramount importance in ESPs.For non-sticky particles >5 cyclones.For particles <5 ESPs, filters, and scrubbers.For large flows, pumping cost makes scrubbers $$$.Corrosion resistance and acid dew point must always be considered.
66 SummaryGravity settlers, cyclones, ESPs drive particles to wall, similar design equations.Filters and scrubbers divide the flow. Different design equations.Surface filters for heavy laden gas streams; depth filters for final clean-up of air, or clean gas, or for fine liquid drops that coalesce on them and then drop off.To collect small particles, a scrubber must have a very large relative velocity between gas and liquid. co-flow scrubbers venturi scrubbers.