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

Filtration Theory

Figure 6.2 pp 186

Filtration Theory

Design Considerations

Cleaning Cycles t f : time interval between two cleanings of the same compartment t r : time interval between cleanings of any two compartment

Variation of pressure drop with time Time PP PmPm trtr tctc

Cleaning Cycles

Maximum Filtering Velocities in Shaker or Reverse Air Baghouses Table 6.1 DustsMax. Filtering V (ft/min) Activated charcoal, carbon black, detergents, metal fumes 1.5 Aluminum oxide, carbon, fertilizer, graphite, iron ore, lime, paint pigments, fly ash, dyes 2 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, silica2.75 Cork, feeds and grain, marble, oyster shell, salt Leather, paper, tobacco, wood3.5

Fabric Selection FabricMax Temp, CAcid resistanceBase resistance Dynel71Good Cotton82PoorGood Wool93GoodPoor Nylon93PoorGood Polypropylene93Excellent Orlon127GoodFair Dacron135GoodFair Teflon204Excellent Glass288Good 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 ( 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 FumesMaximum 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), Lime, Limestone, Mica,Quartz, soybean, Talc 9-11 Cocoa, Cholocate,Flour,Grains, Leather Dust, Sawdust,tobacco 12-14

Advantages

Disadvantages

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 hav different resistance to acisd 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

Wet Scrubbers

2015/5/17Aerosol & Particulate Research Lab29 Particulate Scrubbers Types of scrubbers: spray chamber and venturi scrubber Theory and design consideration Pressure drop Contacting power Reading: Chap. 7

2015/5/17Aerosol & Particulate Research Lab30 Spray Chamber Recirculated water Water to settling basin and recycle pump Vertical spray chamber (countercurrent flow) Collecting medium:  Liquid drops  Wetted surface

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2015/5/17Aerosol & Particulate Research Lab32 Cyclone Spray Chamber & Impingement Scrubber Flagan & Seinfeld, Fundamental of Air Pollution Engineering, 1988

2015/5/17Aerosol & Particulate Research Lab33 Venturi Scrubber Handbook of Air Pollution Control Engineering & Technology, Mycock, McKenna & Theodore, CRC Inc., High efficiency even for small particles Q L /Q G : V G : m/s

2015/5/17Aerosol & Particulate Research Lab34 Theory: Spray Chamber Droplet concentration in the chamber V d : droplet falling velocity relative to a fixed coordinate V td : droplet terminal settling velocity in still air (i.e. relative to the gas flow) Volume of each droplet Total number of droplets that pass the chamber per second V td VGVG VdVd Q L : volumetric liquid flow rate

2015/5/17Aerosol & Particulate Research Lab35 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 At a given time dt, the distance a droplet falls is Total number of particles swept clean per second by all droplets in dz

2015/5/17Aerosol & Particulate Research Lab36 QLQL QGQG Total number of particles removed per second over dx Particle penetration in a countercurrent vertical spray chamber Cross-sectional area of all the droplets

2015/5/17Aerosol & Particulate Research Lab37 If Q L in gal/min and Q G in cfm, z in ft and d d in  m 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  d )?

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2015/5/17Aerosol & Particulate Research Lab39 Single droplet collection efficiency Diameter ratio Viscosity ratio d Particle Reynolds # Particle Schmidt # Particle Stokes # Deposition of Particles on a Spherical Collector (diffusion) (interception) (impaction)

Impaction only  p = 2 g/cm 3 (Impaction parameter K p is used in textbook; K p = 2 St)

Venturi Scrubber Use intertial impaction of suspended particles on water droplet formed by gas atomization

2015/5/1742 Venturi Scrubbers: Calvert Design Particle penetration through a venturi scrubber K po =2St (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 (d d ) equation can be solved with satisfactory results. k 1 = if V G is in cm/s = 1920 if V G is in ft/s  in dyne/cm,  L in g/cm 3 and  should be in poise Q L and Q G should be of the same unit

2015/5/17Aerosol & Particulate Research Lab43 Pressure Drop Venturi Scrubber l t : venturi throat length X: dimensionless throat length Ex: 10” water, 2  m,  = ?

44 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 (P T : contacting power in hp / 1000 cfm)  and  : coefficient and exponent of P T N t : Number of transfer unit (unitless) P T should be determined from the friciton loss across the wetted portion of the scrubber.

Contacting Power Approach Venturi scrubber collecting a metallurgical fume Contacting power, hp/cfm

Example (P T contacting power in hp / 1000 acfm) N t : Number of transfer unit (unitless) (1 inch of water = hp/1000 cfm) Q: Tests of a venturi scrubber show the results listed on the right. Estimate the contacting power required to attain 9 7 % efficiency. Friction loss (in H 2 O)  (%)

Example Convert friction loss to contacting power (hp/1000 cfm): 1 in H20 = hp/1000cfm Friction loss (in H 2 O) P T hp/1000cfm  (%) NtNt

Example Substractin Eq A from Eq B: ABAB

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Problem 7.1

Solution Impaction parameter K p is used in textbook Determine the density of water and the viscosity of the air at 80 °F from Appendix B

Solution