Particulate Control2 Fabric Filters Particulate Scrubbers Lecture notes
Particulate Control-2 Fabric Filters Particulate Scrubbers Lecture notes adapted from Prof. Dr. Dentel Notes and Prof. Dr. Chang-Yu Wu
Fabric Filters • Well known and accepted method for separating dry particles from a gas stream • Many different types of fabrics, different ways of configuring bags in a baghouse and different ways of flowing the air through the bags. • There are 3 common types of baghouse based on cleaning method – Reverse-air – Shaker – Pulse-jet
Fabric Filters
Fabric Filters A shaker baghouse Filter compartements
Fabric Filters
Fabric Filters
Filtration Theory
Filtration Theory
Filtration Theory Figure 6. 2 pp 186
Filtration Theory
Filtration Theory
Design Considerations
Cleaning Cycles • tf: time interval between two cleanings of the same compartment • tr: time interval between cleanings of any two compartment
Variation of pressure drop with time DPm DP tr tc Time
Cleaning Cycles
Table 6. 4 Total Number of Compartments N uj/u. N-1 f. N 3 0. 87 4 0. 80 5 0. 76 7 0. 71 10 0. 67 12 0. 65 15 0. 64 20 0. 62 Ratio of actual filtering velocity uj to average filtering velocity u. N-1 in a multicompartament baghouse
Maximum Filtering Velocities in Shaker or Reverse Air Baghouses Dusts Max. Filtering V (ft/min) Activated charcoal, carbon black, detergents, metal fumes 1. 5 Aluminum oxide, carbon, fertilizer, graphite, iron ore, lime, 2 paint pigments, fly ash, dyes Aluminum, clay, coke, charcoal, cocoa, lead oide, mica soap, sugar, talc 2. 25 Bauxite, ceramics, chorme ore, feldsapr, blour, flint, glass, gypsum, plastics, cement 2. 5 Asbestos, limestone, quartz, silica 2. 75 Cork, feeds and grain, marble, oyster shell, salt 3 -3. 25 Leather, paper, tobacco, wood 3. 5 • Table 6. 1
Fabric Selection Fabric Max Temp, C Acid resistance Base resistance Dynel 71 Good Cotton 82 Poor Good Wool 93 Good Poor Nylon 93 Poor Good Polypropylene 93 Excellent Orlon 127 Good Fair Dacron 135 Good Fair Teflon 204 Excellent Glass 288 Good Table 6. 2
Pulse Jet Filters • Introduced 45 years ago captured one-half of the industrial air filtration market • Air is filtered through the bags from outside to the inside, a cage inside each bag prevents the bag from collapsing • The bags are cleaned by short blast of high pressure air (90 -100 psi) • Each bag is pulsed every few minutes • On stream use
Pulse Jet Filters • There are no compartments and thus no extra bags which reduces size and cost (for a large coal -fired power plant, the baghouse is so large that it is designed with separate compartments) • Since bags are placed from the top, no need to provide walkways between rows of bags (reducing the size) • Felted fabrics can be used at much higher air to cloth ratio (higher filtering velocities)
Pulse Jet Filters • Table 6. 5. Maximum Filtering Velocities for Various Dust or Fumes Dusts or Fumes Maximum Filtering Velocity (ft/min) Carbon, Graphite, Metallurgical Fumes, Soap, Detergents; Zinc oxide 5 -6 Cement (Raw), Clay (Green), Plastics, paitn Pigments, Starch, Sugar, Wood, Gypsum, Zinc 7 -8 Aluminum oxide, cement (finished), Clay (vitrifies), Limestone, Mica, Quartz, soybean, Talc 9 -11 Cocoa, Cholocate, Flour, Grains, Leather Dust, Sawdust, tobacco 12 -14
Advantages • -High collection efficiency on both coarse and fine PM • -Insensitive to fluctuations in gas stream • -Filtered outlet air is very clean and may be recirculated within the plant • -Collected particles are dry for subsuquent processing or disposal • -Operation is relatively simple • -Unlike ESP, do not require the use of high voltages, maintenance is simplified • -The use of selected fibrous or granular filter aids (precoating) permits the high efficiency collection of PM 1 smokes and gaseous contaminants • -Can be modular
Disadvantages • Over 290 C expensive special refractory mineral or metallic fabrics are required • Concentrations of some dusts in the collector, may represent a fire of explosion if a spark or flame is accidentally admitted. • Fabrics can burn if readily oxidizable dust is being collected • Fabric filters have relatively high maintenance requirements (periodic bag replacement)
Disadvantages • Fabric life may be shortened at elevated temperatures and in the presence of acid or alkaline PM or gas constituents • Cant be operated in the presence of hygroscopic materials, condensation of moisture or tarry adhesive components may cause plugging of the farbri or reqire sepcial additives • Pesprotory protection for maintenance personel may be required when replacing fabric • Medium pressure drop is required (4 to 10 inches of water column)
Example
Example
Example
Other Considerations • Temperature and Humidity : Fabrics have different maximum allowable teperatures. Low T can cause condensation of acid and/or blinding of the fabric with wet dust • Chemical nature of gas: Different fabrics have different resistance to acids or alkalies • Fire/explosion: Some fabric are flammable; Some dust are explosive • Dust Handling: dust removal rate, conveyor system, and hopper slope should all be considered
Educational Videos • http: //www. youtube. com/watch? v=bb. XZCzg. Zh 4 w Animation showing the pulse jet filtration • http: //www. youtube. com/watch? v=9 BXEcb 5 RXG 4 A presentation from Baghouse and Wet Scrubber Production Firm, Mikro. Pul
Wet Scrubbers
Particulate Scrubbers Reading: Chap. 7 • Types of scrubbers: spray chamber and venturi scrubber • Theory and design consideration • Pressure drop • Contacting power 2020/11/26 Aerosol & Particulate Research Lab 32
Collecting medium: · Liquid drops · Wetted surface Recirculated water Spray Chamber Water to settling basin and recycle pump Vertical spray chamber (countercurrent flow) 2020/11/26 Aerosol & Particulate Research Lab 33
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Cyclone Spray Chamber & Impingement Scrubber Flagan & Seinfeld, Fundamental of Air Pollution Engineering, 1988 2020/11/26 Aerosol & Particulate Research Lab 35
Venturi Scrubber High efficiency even for small particles QL/QG: 0. 001 - 0. 003 VG: 60 - 120 m/s Handbook of Air Pollution Control Engineering & Technology, Mycock, Mc. Kenna & Theodore, CRC Inc. , 1995. 2020/11/26 Aerosol & Particulate Research Lab 36
Theory: Spray Chamber Volume of each droplet Total number of droplets that pass the chamber per second QL: volumetric liquid flow rate Droplet concentration in the chamber VG Vd Vtd Vd: droplet falling velocity relative to a fixed coordinate Vtd: droplet terminal settling velocity in still air (i. e. relative to the gas flow) 2020/11/26 Aerosol & Particulate Research Lab 37
At a given time dt, the distance a droplet falls is Volume of air that flows through the cross-section area of a single droplet during the time dt Total effective volume of gas swept clean per second by all droplets in dz Total number of particles swept clean per second by all droplets in dz 2020/11/26 Aerosol & Particulate Research Lab 38
Total number of particles removed per second over dz QL Penetration can be defined as the fraction of particles of a specified diameter (Pd) that are not capture. hd=1 -Pd Particle penetration in a countercurrent vertical spray chamber: Cross-sectional area of all the droplets Overall penetration then will be sum of Pd 2020/11/26 Aerosol & Particulate Research Lab QG 39
If QL in gal/min and QG in cfm, z in ft and dd in mm Particle penetration in a cross-flow spray chamber Q: How do we have higher collection efficiency? Q: What are the collection mechanisms (we need it for hd)? 2020/11/26 Aerosol & Particulate Research Lab 40
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Deposition of Particles on a Spherical Collector Particle Reynolds # Particle Schmidt # Diameter ratio Particle Stokes # Viscosity ratio Single droplet collection efficiency d (diffusion) (interception) 2020/11/26 Aerosol & Particulate Research Lab (impaction) 42
Impaction only (Impaction parameter Kp is used in textbook; Kp = 2 St) rp = 2 g/cm 3
Venturi Scrubber • Use intertial impaction of suspended particles on water droplet formed by gas atomization
Venturi Scrubbers: Calvert Design Particle penetration through a venturi scrubber Kpo=2 St (aerodynamic diameter) using throat velocity f = 0. 5 for hydrophilic materials, 0. 25 for hydrophobic materials Atomization produces a wide distribution of droplet size. However using the Sauter mean droplet diameter (dd) equation can be solved with satisfactory results. k 1 = 58600 if VG is in cm/s = 1920 if VG is in ft/s 2020/11/26 s in dyne/cm, r. L in g/cm 3 and m should be in poise QL and QG should be of the same unit 45
Pressure Drop Venturi Scrubber lt: venturi throat length X: dimensionless throat length Ex: 10” water, 2 mm, h = ? 2020/11/26 Aerosol & Particulate Research Lab 46
Contacting Power Approach When compared at the same power consumption, all scrubbers give the same degree of collection of a given dispersed dust, regardless of the mechanisms involved and regardless of whether the pressure drop is obtained by high gas flow rate or high water flow rate Nt: Number of transfer unit (unitless) (PT : contacting power in hp / 1000 cfm) a and b: coefficient and exponent of PT PT should be determined from the friciton loss across the wetted portion of the scrubber. 47
Contacting Power Approach Venturi scrubber collecting a metallurgical fume Contacting power, hp/cfm
Example Q: Tests of a venturi scrubber show the results listed on the right. Estimate the contacting power required to attain 97% efficiency. Friction loss (in H 2 O) h (%) 12. 7 56 38. 1 89 Nt: Number of transfer unit (unitless) (PT contacting power in hp / 1000 acfm) (1 inch of water = 0. 1575 hp/1000 cfm)
Example Convert friction loss to contacting power (hp/1000 cfm): 1 in H 20 =0. 1575 hp/1000 cfm Friction loss (in H 2 O) PT hp/1000 cfm 12. 7 2 38. 1 6 h (%) Nt 56 0. 821 89 2. 207 97 3. 506
Example Substractin Eq A from Eq B: A B
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Problem 7. 1
Solution Impaction parameter Kp is used in textbook Determine the density of water and the viscosity of the air at 80 °F from Appendix B
Solution
Solution
Elimination of Liquid Entrainment To avoid recontamination of the gas by the entrainment of liquid in the exiting gas stream, it is neccessary to remove at least 95% of the liquid carryover In spray scrubbers, louver-type mist elimanators are frequently used. ADD
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