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OAR Box 1313 

Prepped by LaTeia Shed 

Document Number: 

84) ll-B-58 

Docket Number: 

A-88-02 

Printed 6/12/2008 9:03:31 AM Sheet 84 of 99 
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MRI 1^1 

A-88-02 

MIDWES JWESTT RESEAR( RESEARCH INSTITUTE 

I E - fe-S8 Suite 350 

401 Harrison Oaks Boulevard 

Cary. N.C. 27513 

Date: October 27, 1988 

(Finalized March 22, 1989) 

Subject: Research Study of Mesh-Pad and Fiber-Bed M1st Eliminators 

Chromium Electroplating NESHAP 

EPA Contract 68-02-3817; ESD Project 85/02a 

MRI Project 7717-K 

From: Robin Barker 

To: Andrew M. Smith 

ESD/ISB/SSS (MD-13) (r-i |RR, _ . wwr !**.. 

U. S. Environmental Protection Agency 87

 IDUI o I wvi : • 

Research Triangle Park, NC 27711 • •'••-

 !^ 

I. Introduction 

; • . i. . . .. ...„ .-.,•, ' -iU_ :x_£j 

There are four types of mist eliminators commonly used to control 

acid mist generated from industrial processes. The four types are: 

(1) fiber-bed; (2) mesh-pad; (3) packed-bed; and (4) chevron-blade mist 

eliminators. M1st eliminators have been used predominantly as a secondary 

control device, called an entrainment separator, after wet scrubbers and 

absorbers. 

Mist eliminators control mist by a combination of mechanisms 

including inertial impaction, direct interception, and Brownian 

diffusion. The control mechanisms for chevron-blade and packed-bed mist 

eliminators are inertial impaction and direct interception of the mist. 

The control mechanisms for mesh-pad and fiber-bed mist eliminators include 

inertial impaction, direct interception, and Brownian diffusion. However, 

Brownian diffusion is a principal control mechanism only for fiber-bed 

mist eliminators. Perry's Chemical Engineering Handbook states that a 

well-designed mist eliminator can achieve approximately 100 percent 

removal of particles above 5 microns (ym) (0.20 thousandth^ of an inch 

[mils]) in size as a result of impaction and interception. For particles 

less than 3 um (0.12 mils) in size, Brownian diffusion is required to 

achieve high removal efficiencies (95 to 99 percent) of these smaller 

particles. 

Collection efficiencies for mist eliminators of any type are 

dependent upon the gas velocity through the unit. As velocity increases, 

collection of particles through inertial impaction increases. 

Consequently, low gas velocities will result in less than optimum 

efficiency. However, high gas velocities can cause collected particles 

to be reentrained in the gas stream, which may result in lower overall 

control efficiency. Therefore, gas velocities should be maintained high 

enough to optimize collection through inertial impaction yet not cause 

reentrainment. 

A 
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The following sections present a more thorough discussion of meshpad and fiber-bed mist eliminators, including operating parameters and 

practices and the applicability of these devices for controlling chromic 

acid mist. A thorough discussion of chevron-blade mist eliminators is 

presented in Chapter 4 of the Chromium Electroplating NESHAP Background 

Information Document. 

II. Mesh-Pad Mist Eliminators 

Mesh-pad mist eliminators consist of layers of interlocked wire or 

plastic filaments densely packed between two supporting grids. Although 

their thickness and density may vary, mesh pads commonly used are 

15 centimeters (cm) (6 inches [in.]) thick with a mesh density of 

144 kijog^ams per cubic meter (kg/m ) (9 pounds per cubic foot 

[lb/ft ]. The pressure drop across mesh-pad units typically ranges 

between 0.25 and 0.62 kilopascal (kPa) (1 and 2.5 inches of water column 

[in. w.c.]) depending on the mesh density. Figure 1 presents a schematic 

of a mesh-pad unit. The most common application for mesh-pad mist 

eliminators in industry is as an entrainment separator to remove droplets 

entrained in exhaust gases from scrubbers, absorbers, distillation 

columns, refinery towers, and evaporators. 

Mesh-pad material is available through several manufacturers such as 

Kimre, Inc. and Otto H. York Company. Factory-assembled units can be 

purchased from a variety of air pollution control equipment vendors. 

A. Control Mechanism 

The principal control mechanisms for mesh-pad units are inertial 

impaction and direcl: interception. These control mechanisms are depicted 

in Figures 2 and 3. Collection by impaction is simply the result of 

large particles (above 3 ym [0.12 mils] in diameter) with sufficient 

velocity colliding with fibers and adhering to their surface. In direct 

interception, particles attempt to follow the streamline around the 

fibers, but due to their size and relative velocity, are intercepted by 

the fluid layer that surrounds the fiber surface. Collected liquid 

droplets flow along the fibers to a point where adjacent fibers cross. 

These crossover points become rapidly loaded with liquid, and droplets 

drain to the bottom of the mist eliminator due to gravity. 

Some particles are also removed by Brownian diffusion (or Brownian 

motion); however, this mechanism is not a major factor for particles 

larger than 3 ym (0.12 mils). Brownian motion is the random movement of 

particles due to collisions on a molecular or atomic level.1* The 

collection of small particles is a result of random motion which places 

the particles on a collision path with the fibers (see Figure 4) . 

Because the motion is random, the mesh pad must have a high density 

(surface area-to-volume ratio) to collect a high percentage of these small 

particles. Typically, mesh densities are reported in terms of bulk 

density, (mass per volume units) because it is easier to measure than the 

surface area-to-volume ratio. As the bulk density increases, the surface 

area-to-volume ratio will also increase because more fibers are compacted 

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stream lines 

IMPACTION 

Figure 2. Intertial impaction."* 

stream lines 

particle^ 

DIRECT INTERCEPTION 

Figure 3. Direct interception. 

stream lines 

partie 

DIFFUSION 

Figure 4. Brownian diffusion. 

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into the same volume. Available literature suggests that Brownian 

diffusion can account for the collection of some particles below 3 ym 

(0.12 mils) in mesh-pad mist eliminators. 

B. Operating Parameters and Practices 

The fiber size in mesh-pad units can vary from about 0.015 cm 

(0.006 in.) to 0.38 cm (0.15 in.), depending upon the application. Pad 

thickness typically ranges from 10 to 15 cm (4 to 6 in.) with pressure 

drops between 0.25 and 0.62 kPa (1 and 2.5 in. w.c) . Typical bulk mesh 

densities range from 48 to 529 kg/m (3 to 33 lb/ft ). Collection 

efficiency is close to 100 percent for particles larger than 5 ym 

(0.20 mils) at gas velocities between Jl and 300 meters per minute (m/min) 

(36 and 984 feet per minute [ft/min]). Efficiency of these units drops 

off rapidly for particles less than 5 ym (0.20 mils) in diameter. 

Dr. Seymour Calvert performed tests on mesh-pad units to evaluate 

reentrainment on units that were installed vertically in horizontal ducts 

(see Figure 5) and units that were installed horizontally in vertical 

ducts (see Figure 6). Dr. Calvert found that vertically installed units 

had: (1) higher reentrainment velocities than horizontally installed 

units because vertically installed units provide better liquid drainage; 

and (2) less reentrainment at the same gas velocity than with horizontally 

installed units. 

To offset the drop in efficiency when particles below 5 ym 

(0.20 mils) are encountered, two mesh pads in series can be used. Studies 

performed at Consolidated Chemical Industries, Division of Stauffer 

Chemical in Houston, Texas, on a sulfuric acid process showed that when 

two mesh pads in series are used, the overall performance of the unit 

improved. The mesh pads were installed downstream of an absorber to 

collect any sulfuric acid mist formed in the stack due to the release of 

unabsorbed sulfur trioxide. The first mesh pad is made up of fine 

filaments and is operated beyond the flood point to enhance the scrubbing 

of the gas stream, increase its velocity, and hence increase impaction of 

fine particles. (Flood point is a term used to describe the situation 

where the gas flow rate is high enough to hamper liquid flow through the 

pad and produce a buildup of liquid in the pad.) The second pad is made 

up of standard filaments and operated below the flood point to catch any 

reentrained particles from the first pad. Flooding in the first pad can 

be assisted by water sprays and irrigation. Optimum gas velocities at the 

Houston facility were found to be between 274 and 329 m/min (900 and 

1,080 ft/min). 

C. Applicability to Chromic Acid Mist Control 

Control equipment vendors typically do not recommend the use of 

mesh-pad mist eliminators for controlling chromic acid mist because these 

units tend to plug up when operated under high inlet loadings. Most meshpad units that have been installed on chromium plating tanks do not have 

internal water spray systems that clean the mesh pad and prevent buildup 

of chromium that may eventually cause plugging of the pad. Units that 

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PROCESS 

TANK 

MESH-PAD MIST 

ELIMINATOR 

VENTILATION 

HOOD 

Figure 5. Mesh-pad mist eliminator installed 

vertically in horizontal section of duct. 
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/ 

MESH-PAD MIST 

ELIMINATOR 

VENTILATION 

HOOD 

Figure 6. Mesh-pad mist eliminator installed 

horizontally in vertical section of duct. 

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8 

control chromic acid mist typically use a pad that is approximately 15 cm 

(6 in.) thick, have a pressure drop of 0.3 kPa (1 in. w.c) , and are 

installed horizontally in a vertical section of duct. 

Test results are available from two faci1i|i|s that use mesh-pad 

mist eliminators on hard chromium plating tanks. » Table 1 presents a 

comparison between the two units tested. Unit 1 achieved 94 percent 

control and Unit 2 achieved 99+ percent control. The unit that achieved 

99+ percent control has three technical advantages over the unit that 

achieved 94 percent control: (1) the unit is installed vertically in a 

horizontal section of duct as Calvert recommends for better drainage and 

less reentrainment; (2) two pads are used in series; and (3) the unit is 

equipped with an internal spray system and removable cover to allow for 

the cleaning of the mesh pads. 

These test results show that mesh-pad mist eliminators can be used 

to efficiently control chromic acid mist from chromium electroplating 

operations. 

III. Fiber-Bed Mist Eliminators 

There are three types of fiber-bed mist eliminators used to control 

fine mis*£s: (1) high efficiency; (2) high velocity; and (3) spray 

catcher. Fiber-bed mist eliminators are either packed- or wound-beds, 

fabricated with glass, ceramic, plasty, or metal fibers in bulk (loose) 

or roving (rope) form (see Figure 7). The individual fibers are usually 

less than 25 ym (1 mil) in diameter. Figure 7^resents a crosssectional view of the high-efficiency type unit. Fiber-bed mist 

eliminators have been predominantly used to control acid mists from 

sulfuric, phosphoric, and nitric acid plants. The two major suppliers of 

fiber-bed mist eliminators are Otto H. York Company and Monsanto EnviroChem. 

A. Control Mechanism 

The predominant control mechanism for the high-velocity and SDj**aycatcher type fiber-bed mist eliminator units is inertial impaction. 

Monsanto Enviro-Chem performed pilot scale testing of fiber-bed mist 

eliminators in 1968. Table 2 presents the control efficiencies that 

these type units achieved during testing when used to control sulfuric 

acid mist.1 Fiber-bed mist eliminators are able to remove smaller 

particles by inertial impaction than mesh-pad mist eliminators because of 

the higher surface area-to-volume ratios (30 to 150 times higher) of 

fiber-bed mist eliminators. The predominant control mechanism for the 

high-efficiency units is Brownian diffusion which, combined with inertial 

impaction, has been demonstrated to result in 100 percent control of 

sulfuric acid mist particles above 3 ym (0.1^ mjls^ and 95 to 99 percent 

control of particles below 3 ym (0.12 mils). • » 

? 
TABLE 1. TEST RESULTS FOR MESH-PAD MIST ELIMINATORS 

Unit 1 Unit 2 

Plant 

Plant name 

Location 

Description of mesh-pad unit 

Manufacturer 

No. of pads: 

Spacing between pads, cm (in.) 

Thickness of pad, cm (1n.)7 

Mesh density, kg/m (lb/ftJ) 

Design pressure drop, kPa (in. w.c ) 

Design gas velocity, m/m1n (ft/min) 

Spray washdown system 

Installation in duct 

Test results 

Total current, ampere-hours 

Inlet concentration, mg/dscm 

(gr/dscf) 

Inlet emission rate, kg/h (lb/h) 

Outlet concentration, mg/dscm 

(gr/dscf) 

Outlet emission rate, kg/h (mg/h) 

Collection efficiency, percent 

Norfolk Naval Shipyard 

Portsmouth, Virginia 

Otto York Company 

1 

NAa 

15.(6.0) 

NAD 

0.3 (1.0) 

180 (600) 

None 

Horizontal 

3,440c 

0.609 (0.00026)c e 

0.0087 (0.019)c f 

0.0257 (1.12x10 ) c e 

0.00045 (0.001) 

9 4c e 

c e 

Precision Machine and Hydraulic, Inc 

Worthington, W. Virginia 

ChromeTech, Inc 

2 

10 (4) 

7.6 (3.0), 3.8 (1.5) 

64.0 (4.0) 

0.62 (2.5) 

150 to 200 (500 to 700) 

Internal system 

Vertical 

12,160° 

11.4 (0.00499)d r 

0.0829 (0.183)d /. , 

0.0326 (1.0x10 ) d f 

0.00023 (0.00051)d f 

99+d 

•£> 

rHot applicable. 

^Not available. 

^Based on average of three test runs. 

Based on average of five test runs. 

^Based on total chromium analysis. 

Based on hexavalent chromium analysis, 

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FIBER PACKING 

MIST-LADEN 

GAS 

CLEAN GAS 

CONCENTRIC SCREENS 

LIQUID OUT 

HALF COUPLING 

LIQUID SEAL POT 

Figure 7. Cross-sectional view of high-efficiency 

type fiber-bed mist eliminator. 

./& 
TABLE 2. OPERATING CHARACTERISTICS OF VARIOUS TYPES OF FIBER-BED MIST ELIMINATORS AS USED ON 

SULFURIC ACID PLANTS » a 

High efficiency High velocity Spray catcher 

Controlling mechanism for mist collection 

diffusion 

Superficial velocity, m/m1n (ft/min) 4.5 to 12 

(15 to 40) 

Efficiency on sulfuric acid mist particles larger 

than 3 ym (0.12 mil), percent 100 

Efficiency on sulfuric acid mist particles smaller 

than 3 ym (0.12 mil), percent 

Pressure drop, kPa (in. w.c.) 1.2 to 3.7 

(4.7 to 15) 

Brownian 

120 to 150 

(390 to 490) 

Essentially 

100 

95-99+ 

1.5 to 2.0 

(6 to 8) 

Impaction 

120 to 150 

(390 to 490) 

Essentially 

100 

90-98 

0.1 to 0.25 

(0.4 to 1.0) 

Impaction 

Essentially 

15-30 

aResults were obtained during pilot-scale testing by Monsanto Enviro-Chem. 

^ 

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12 

B. Operating Parameters and Practices 

A typical fiber-bed cylinder is constructed of two concentric 

screens attached to a top flange and a bottom drain plate, each of which 

typically has 1-in. openings. Fiber is packed into the annular space 

between these screens to form a bed with a radial thickness of 5 to 8 cm 

(2 to 3 in.). The screen framework that contains the fiber, along with 

the top flange and bottom plate attachments, is typically referred to as a 

fiber-bed cage. 

The cages are typically fabricated from either metal or fiberglassreinforced plastic. The fiber beds may be formed into either impaction 

or Brownian-diffusion ^p e beds, depending on the size of the fiber with 

which they are packed. A typical impaction cylinder has an outside 

screen diameter of 66 cm (26 in.),xa bed thickness of 5 cm (2 in.), and an 

overall length of 180 cm (72 in.). Typical Brownian-diffusion cylinders 

have an outside screen diameter of 61 cm (24 in.), a.bed thickness of 8 cm 

(3 in.), and an overall length of 305 cm (120 in.). Pressure drops for 

impaction units range from 0.15 to 2.0 kPa (0.5 to 8 in. of w.c ) and 

pressure drops for Brownian-diffusion units range from 1.2 to 3.7 kPa (5 

to 15 in. of w.c). 

Fiber-bed mist eliminators are also equipped with two spray 

nozzles. One spray nozzle is located at the top of the unit and is used 

to wash down any large particulate that may clog the mist eliminator, and 

the other spray nozzle is located at the bottom of the unit and is used as 

an aerosol spray to remove any contaminants that can crystallize in the 

mist eliminator and cause plugging or improper drainage. 

Fiber-bed units are not operated with vertical upflow of gas; a 

large amount of contacting equipment (e.g. fiber-bed size, transition 

zones, etc.) would be required to slow the gas velocity down below the 

flooding velocity because the flooding velocity decreases with 

bed-specific surface area. (Flooding velocity is the velocity of the gas 

stream at which reentrainment occurs.). Fiber-bed units are designed 

for horizontal, concurrent gas-liquid flow through the bed. The gas and 

liquid flow to the downstream face of the bed where the liquid drains down 

the outer face to the sump and the cleaned gas flows up and out the top of 

the unit. 

As with mesh-pad mist eliminators, gas flow rates with impactiontype fiber-bed units are limited on the low end by the decrease in 

efficiency of inertial impaction. ° The Brownian-diffusion type units 

have no lower limit^n gas flow rate because mist collection increases as 

gas flow decreases. This effect will continue until the gas flow rate 

reaches zero. The maximum gas flow rate in fiber-bed mist eliminators is 

limited by either: (1) decrease in efficiency withincreasing gas flow 

rate; or (2) a gas-phase pressure drop limitation. 

Fiber-bed mist eliminators have no limitations on inlet loadings 

because flooding is not a problem; they have operated with inlet loadings 

up to 106 grams per cubic meter (46 grains per cubic foot) with no 

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observed reentrainment. ° Mr. Prafel Malkan of Monsanto Enviro-Chem 

indicated that in operations where potential plugging of the fiber-bed 

mist eliminator exists, the units are operated with the water spray 

nozzles in the unit activated to aid in removal of particulate buildup.12 

In addition, a facility operating a fiber-bed mist eliminator may choose 

to purchase an additional fiber cage so that if the monitored pressure 

drop exceeds the design range, the fiber-bed cage can be removed and 

replaced by the spare cage. The plugged cage can then be sent back to the 

manufacturer to be rewound with new fiber material. 

C. Applicability to Chromic Acid Mist Control 

Fiber-bed mist eliminators were first fabricated by Monsanto to aid 

in sulfuric acid mist control. These devices have been very successful 

for this type application. The State of California, in conjunction with 

the Metal Finishing Association of Southern California, currently plans to 

test the control efficiency of fiber-bed mist eliminators for controlling 

chromic acid mist from electroplating operations in the first quarter of 

1989. 

IV. References 

1. Fair, J. Liquid-Gas Systems. In: Perry's Chemical Engineers 

Handbook. 6th Edition. Perry, R. and Green, D., eds. New York, 

McGraw Hill Book Company. 1984. p.18-78 

2. Reference 1. pp. 18-83 and 18-84. 

3. Holmes, T. L., and Chen, G. K. Design and Selection of Spray/M1st 

Elimination Equipment. Chemical Engineering. October 15, 1984. 

p. 86. 

4. Bethea, R. M. Air Pollution Control Technology: An Engineering 

Analysis Point of View. Van Nostrand Reinhold Company. New York, 

New York. 1978. pp. 146-148. 

5. York, 0. H. Performance of the Wire Mesh Demister. Reprinted from 

Chemical Engineering Progress. Bulletin 635. Parsippany, New Jersey. 

1963. p. 2. 

6. Calvert, S., Yung, S., and Leung, J. Entrainment Separators for 

Scrubbers: Final Report. U. S. Environmental Protection Agency, 

Office of Research and Development, Washington, D.C. August 1975. 

p. 71. 

7. Massey, 0. D. Demisters For Sulfuric Acid Plant Stacks. Chemical 

Engineering Progress. May 1959. p. 117. 

8. Norfolk Naval Shipyard, Norfolk, Virginia. Source Emission Testing 

of the Building 195 Plating Shop at Norfolk Naval Shipyard, 

Portsmouth, Virginia. March 11-18, 1985. Naval Energy and 

Environmental Support Activity, Port Hueneme, California. 

NEESA 2-124. May 1985. 

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14 

9. Draft Emission Test Report: Precision Machine and Hydraulic, Inc, 

Worthington, West Virginia. Peer Consultants, Inc., Dayton, Ohio. 

Prepared for U. S. Environmental Protection Agency, Research Triangle 

Park, North Carolina. September 1988. 

10. Reference 3, p. 87. 

11. Brink, J. A., Burgrabbe, W.F., and Greenwell, L.E. Mist Eliminators 

for Sulfuric Acid Plants. Chemical Engineering Progress. November 

1968. p. 82-85. 

12. Contact Report. Barker, R., MRI, with Malkan, P., Monsanto EnviroChem. January 25, 1989. Information on design and control of fiberbed mist eliminators. 

/¥