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1 CE 583 – Control of Primary Particulates Jeff Kuo, Ph.D., P.E.

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Presentation on theme: "1 CE 583 – Control of Primary Particulates Jeff Kuo, Ph.D., P.E."— Presentation transcript:

1 1 CE 583 – Control of Primary Particulates Jeff Kuo, Ph.D., P.E.

2 2 Content Wall Collection Devices Gravity Settlers Centrifugal Separators Electrostatic Precipitators (ESP) Dividing Collection Devices Surface Filters Depth Filters Filter Media Scrubber for Particulate Control Choosing a Collector

3 3 Introduction Many 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 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 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 6 Gravity Settlers – Plug model Particle removal efficiency related to residence time in chamber terminal settling velocity (Stokes’ law) distance to travel before hitting wall

7 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 8 Gravity Settlers – Ex. 9.1 Find  -D relationship for a gravity settler (H = 2 m, L = 10 m, V avg = 1 m/s). For 1-  particle (how about 50-  ?)

9 9 Gravity Settlers Gravity settling is effective for large particles (>100  ), in reasonably sized chambers. To increase  : making L larger (expensive), H smaller (hard to clean), V avg smaller (expensive), increasing g. Increasing g: centrifugal. Horizontal elutriators: small gravity settlers used for particle sampling.

10 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-  particle Cyclone (cyclone separator): most widely used particle collection device in the world.

11 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 D o.

12 12 Centrifugal Separators Inlet stream has a “height” W i in the radial direction – the max. distance the particle needs to the wall. Length of flow path = N  D o. (N = number of turns that gas makes traversing the outer helix = 5 typical).

13 13 Centrifugal Separators Ex. 9.4: Compute  -diameter relation for a cyclone separator with W i = 0.5’, V c = 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 14 Centrifugal Separators For a typical cyclone, D cut ~ 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 (D cut decreases) with increasing V circular. But,  P~ V 2 circular. Reduce inlet duct W idth (and diameter in proportion) Split flow into multiple cyclones to keep V circular constant If W i = 0.125’  D cut = 2.3 .

15 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 16 Cyclone Collection Efficiency with Particle Size Distribution Collection efficiency varies with particle terminal velocity, which in turn varies with particle diameter D and density Ex 9.6: Performance computation for a cyclone separator of D cut = 5  m with log normally distributed particle size: D mass mean = 20  m,  = Divide the distribution into 10 fractions. Find z (= number of standard deviation). p = penetration

17 17 Overall  ~ 81% Mass mean diameter that passes thru the cyclone? The diameter corresponds to half of  ~ of the mean diameter  ~ 4 .

18 18

19 19 Cyclone – Pressure drop V i = 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 V i = 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.

20 20 Cheap No moving parts (low maintenance) Removes solid or liquid particles (non- corrosive particles) Harsh conditions (high temperatures) Time-proven technology (1940s) Low efficiency for small particles (D p <10 um) High pressure drops  High operating costs Can’t do sticky particles General Cyclone Thoughts IMPACTION!Mechanism= streamlines Brownian Motion (diffusion) impaction interception

21 21 Electrostatic Precipitators (ESP) ESPs are effective on much smaller particles. Viscous resistance (Stokes’ law) ~ D. For gravity settlers or cyclones: driving force ~D 3. For ESPs: electrostatic force ~D 2. 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.

22 22 How Do ESPs Work?

23 23 How Do ESPs Work? Two stage esp (www.airwater.com) One stage esp (www.zet.com)

24 24 ESPs (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.

25 25 ESPs (Cottrell precipitators) H : the height through which particles must travel, at right angles to gas flow, before hitting wall L : 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.

26 26 ESPs (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).

27 27 ESPs – Maximum charge on a particle Ex. 9-9: How many electronic charges on 1-  (  = 6 and E o = 300 kV/m)? How about 1/3-  particles?

28 28 ESPs – Drift velocity (terminal settling velocity under electrostatic force) Force on particle: F = qE P (E P, local electric field strength) Resulting terminal settling velocity (with Stokes law for drag force) Ex. 9-10:

29 29 ESPs – Drift velocity w ~ E 2 (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 V c of cyclone. Why ESPs are much more effective? The drift velocity ~D for ESPs and ~D 2 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.

30 30 ESPs – Collection Efficiency Block (plug) flow: Mixed flow:

31 31 ESPs – Collection Efficiency Ex. 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 D The cut-diameter ~ 0.5 . Log(p) vs. A/Q is linear.

32 32 ESPs – Collection Efficiency Ex. 9.12: Estimate w for coal containing 1% S. From the figure at  = 99.5%  A/Q = 0.31 min/ft

33 33 ESPs – Performance & Cake Resistivity High resistivity ash (elemental S): large  V cake, small  V wire, poor charging, low  - electron flow within cake, back corona Low resistivity ash (carbon black): small  V cake, weak attraction to collection plate, re- entrainment Back 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: > 10 7 and < 2 x ohm-cm.

34 34 ESP – Performance and Cake Resistivity Little 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 SO 3 (absorbs water). Coal ash is basic needs acidic conditioner. NH 3 works for acidic Portland cement ash.

35 35 ESPs – Performance Ex 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).

36 36 ESPs – Performance Ex 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.

37 37 ESPs – Performance The 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)

38 38 High  for even small particles Low  P even with high flow Dry or wet collection Wide range of temperature Low operating costs Take up lots of space High capital cost Not flexible to change May need a pre-cleaner at high concentrations…cyclone? General ESP Thoughts  Power plants  Cement plants  Paper mills  Steel foundries  Indoor air quality

39 39 Capital Costs depend on total plate area ‘A’ Purchase price = aA b a=962, b=0.628 for 10,000 ft 2 < A < 50,000 ft 2 Installation cost : ~2.2*DEC Operating Costs - depend on power consumption Fan pulling the air through the plates Total delivered equipment cost (DEC)=1.18*(purchase price) ESP - Costs

40 40 Dividing Collection Devices Divide the flow into small parts and bring it in contact with large surface area Surface filters Depth filters Scrubbers Surface 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 41 Dividing Collection Devices – Surface filters Surface velocity (face velocity, approach velocity, superficial velocity, air to cloth ratio). V s = Q/A Pressure drop for flow through porous media  P total =  P filter +  P cake

42 42 Filters - What Happens to the Collected Particles? Shaker Pulse-jet Sonic horn Different types of cleaning Main way to identify bag houses Different bag materials (woven vs. ‘felted’) Different cleaning frequency Reverse air

43 43 interior exterior

44 44

45 45 Surface Filters As the cake builds up, the outlet C declines and stabilizing at a value about 0.001x the inlet C. The  falls with increasing V s (Figure 9.15). At low V s, they will also have high  on fine particles (ESPs have difficulties to collect particles of 0.1 to 0.5  ).

46 46 Woven: Stronger tensile strength Longer time between cleaning (1/2 hr- several hours) Hold more filter cake Shaker and reverse air use woven materials Pulse jet use felted materials Felted: Less tensile strength Short time between cleaning (every few minutes) Abrasive particles, smaller particles always

47 47 Depth Filters Depth 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). streamlines Brownian Motion (diffusion) impaction interception

48 48 Depth Filters Impaction parameter (separation number):

49 49 Depth Filters Ex to 9-20: A cylindrical fiber 10  is placed perpendicular to a gas stream (V = 1 m/s) with C = 1 mg/m 3 and d = 1 . Find . Find  for a row of parallel fibers with center-to-center spacing of 5 fibers. How about 100 rows?

50 50 High efficiency for even small particles Wide variety of solid particle types Modular  flexible design, flexible conditions Low pressure drops Take up lots of space Bad for high T and corrosivity Bad for moist conditions Potential for fire/explosion Need frequent cleaning Need bag replacement General Fabric Filter Thoughts solid waste.dpwt.com Mining plant

51 51 When Would I Use a Fabric Filter? Size classification is not desired High efficiency is required Valuable dry material needs to be recovered Relatively low volumes Relatively low temperatures  Power plants  Fertilizer  Food processing  Paper mills  Ore processing Fibreboard plant

52 52 Scrubbers Bring the flow of gas in contact with a large number of liquid droplets representing a large surface area Natural occurrence: rainfall

53 53 Scrubbers - Removal of particles from a volume of air during a rainstorm Ex 9-22: Q/A = 0.1”/hr with D drop = 1 mm. Air contains d particle = 3  m, C 0 = 100  g/m 3. C 1-hr =? Find V t = 14 ft/s (4.2 m/s) for 1 mm raindrop Calculate N s (=0.23) Find  t ~ 0.23 (Fig. 9-18) C/C 0 = 0.43  C = 43  g/m 3

54 54 Removal of Particles in a Cross- flow Scrubber Make D drop small, and/or  z large Both measures would result in some liquid droplets being carried out of the scrubber.

55 55 Removal of Particles in a Counter-flow Scrubber As V t  V G, C  0 But, this means droplets are nearly stationary with respect to the container  flooding

56 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 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 58 Removal of Particles in a Co- flow Scrubber Integration difficult because V G, V rel,  t all change with x D drop is non- uniform, and not constant with x

59 59 Scrubbers Ex. 9-23: In a venturi scrubber the throat V = 122 m/s. Particles to be removed = 1  and drop D = 100 . Q L /Q G = At a point V rel = 0.9 V G, what is the rate of decrease in C in the gas phase?

60 60 Scrubbers – Pressure drop Ex. 9-25: A venturi scrubber has a throat area of 0.5 m 2, a throat velocity of 100 m/s, and  P = 100 cm water (9806 N/m 2 ). Assuming  motor&blower = 100%, find the power required. Ex. 9-26: For a scrubber using water as the scrubbing fluid, estimate the pressure drop: V G = Q G /  x  y = 100 m/s and Q L /Q G = 0.001

61 61 Ex. 9-27: D cut = 0.5 , Q L /Q G = 0.001, & C = 1.24, find gas velocity at the throat and  P. D aerodynamic cut diameter = (0.5  )(2*1.24) 0.5 = 0.79 V = 90 m/s (Fig. 9.27)  P =~ 80 cm of water (Fig. 9.27)

62 62 Flammable and explosive dusts are OK Gas adsorption and particle collection Can do mists Cools hot gases (can feed to fabric filter if dried) Flexible Chemicals may become less nasty through reaction Corrosion issues - water may increase corrosivity Creates wet waste stream- water pollution + $$$ Need to remove collected particles from water before recirculating High power input to generate well-dispersed droplets General Scrubber Thoughts

63 63 What happens to the collected particles?

64 64 When Would I Use a Scrubber??? Wet particles that are in hot gas stream Corrosive particles Very fine particles requiring high efficiency Particles are with gases that also need to be removed Combustible gases Cooling is desirable and added moisture is not bad  Power plants  Paper mills  Food industry  Cosmetics  Steel/metal industry

65 65 Choosing a collector Small 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 66 Summary Gravity 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.


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