CE 583 – Control of Primary Particulates

CE 583 – Control of Primary Particulates
Jeff Kuo, Ph.D., P.E. Jeff Kuo, Ph.D., P.E. (CSUF)

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

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.

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.

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

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

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).

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

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), Vavg smaller (expensive), increasing g. Increasing g: centrifugal. Horizontal elutriators: small gravity settlers used for particle sampling.

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.

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.

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

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%.

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 constant If Wi = 0.125’  Dcut = 2.3.

Eq is not a good predictor for  (9.19 is a little better one). An empirical data-fitting equation Is more satisfactory.

Cyclone Collection Efficiency with Particle Size Distribution
CE Control of Primary Particulates 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 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 = penetration Jeff Kuo, Ph.D., P.E. (CSUF) 6

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

Cyclone – Pressure drop
CE Control of Primary Particulates Cyclone – Pressure drop Vi = 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

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

Electrostatic Precipitators (ESP)
CE Control of Primary Particulates Electrostatic 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

CE 583 - Control of Primary Particulates
How Do ESPs Work? Jeff Kuo, Ph.D., P.E. (CSUF) 8

How Do ESPs Work? www.state.ia.us One stage esp (www.zet.com)
Two stage esp (

ESPs (Cottrell precipitators)
CE Control of Primary Particulates 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. Jeff Kuo, Ph.D., P.E. (CSUF)

CE Control of Primary Particulates 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. Jeff Kuo, Ph.D., P.E. (CSUF)

CE Control of Primary Particulates 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). Jeff Kuo, Ph.D., P.E. (CSUF)

ESPs – Maximum charge on a particle
CE Control of Primary Particulates ESPs – Maximum charge on a particle Ex. 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)

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)

ESPs – Drift velocity w ~ 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)

ESPs – Collection Efficiency
CE Control of Primary Particulates ESPs – Collection Efficiency Block (plug) flow: Mixed flow: Jeff Kuo, Ph.D., P.E. (CSUF)

CE Control of Primary Particulates 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. Jeff Kuo, Ph.D., P.E. (CSUF)

CE Control of Primary Particulates 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 Jeff Kuo, Ph.D., P.E. (CSUF)

ESPs – Performance & Cake Resistivity
CE Control of Primary Particulates ESPs – Performance & Cake Resistivity High resistivity ash (elemental S): large Vcake , small Vwire, poor charging, low  - electron flow within cake, back corona Low resistivity ash (carbon black): small Vcake , 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: > 107 and < 2 x 1010 ohm-cm. Jeff Kuo, Ph.D., P.E. (CSUF)

ESP – Performance and Cake Resistivity
CE Control of Primary Particulates 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 SO3 (absorbs water). Coal ash is basic needs acidic conditioner. NH3 works for acidic Portland cement ash. Jeff Kuo, Ph.D., P.E. (CSUF)

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). Jeff Kuo, Ph.D., P.E. (CSUF)

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. Jeff Kuo, Ph.D., P.E. (CSUF)

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) Jeff Kuo, Ph.D., P.E. (CSUF)

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

ESP - Costs Capital Costs depend on total plate area ‘A’
Purchase price = aAb a=962, b=0.628 for 10,000 ft2 < A < 50,000 ft2 Total delivered equipment cost (DEC)=1.18*(purchase price) Installation cost : ~2.2*DEC Operating Costs - depend on power consumption Fan pulling the air through the plates

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)

Dividing Collection Devices – Surface filters
Surface velocity (face velocity, approach velocity, superficial velocity, air to cloth ratio). Vs = Q/A Pressure drop for flow through porous media Ptotal = Pfilter + Pcake

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

exterior interior

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 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

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

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 Jeff Kuo, Ph.D., P.E. (CSUF)

Depth Filters Impaction parameter (separation number):

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/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

Advantages Disadvantages General Fabric Filter Thoughts
High efficiency for even small particles Wide variety of solid particle types Modular  flexible design, flexible conditions Low pressure drops solid waste.dpwt.com Disadvantages Mining plant 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

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 Fibreboard plant Power plants Fertilizer Food processing Paper mills Ore processing

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

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 raindrop Calculate Ns (=0.23) Find t ~ 0.23 (Fig. 9-18) C/C0 = 0.43 C = 43 g/m3

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

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

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.

Idea is to increase velocity difference between particles and droplets and thus improve impaction. Venturi design is widely used because it saves fan power.

Integration difficult because VG, Vrel, t all change with x Ddrop is non-uniform, and not constant with x

Scrubbers Ex. 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

Scrubbers – Pressure drop
CE Control of Primary Particulates Scrubbers – Pressure drop Ex. 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/xy = 100 m/s and QL/QG = 0.001 Jeff Kuo, Ph.D., P.E. (CSUF) 6

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 = 0.79 V = 90 m/s (Fig. 9.27) P =~ 80 cm of water (Fig. 9.27) Jeff Kuo, Ph.D., P.E. (CSUF) 6

Advantages Disadvantages General Scrubber Thoughts
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 Disadvantages 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

What happens to the collected particles?

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

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.

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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