Mist elimination hood

CA 02532640 2006-01-16

WO 2005/007295 PCT/CA2004/001037

TITLE OF INVENTION

MIST ELIMINATION HOOD

FIELD OF INVENTION

[0001] The present invention is concerned with a mist elimination hood for a wet electrostatic precipitator system to remove moisture from a gas stream.

BACKGROUND OF THE INVENTION

[0002] Wet electrostatic precipitators (WESP) have been used for many years to remove dust, acid mist and other particulates from water-saturated air and other gases by electrostatic means. In a WESP, particulates and/or mist laden water-saturated air flows in a region of the precipitator between discharge and collecting electrodes, where the particulates and/or mist is electrically charged by corona emitted from the high voltage discharge electrodes. As the water-saturated gas flows further within the WESP, the charged particulates matter and/or mist is electrostatically attracted to grounded collecting plates or electrodes where it is collected. The accumulated materials are continuously washed off by both an irrigating film of water and periodic flushing.

[0003] This type of system is used to remove pollutants from the gas streams exhausting from various industrial sources, such as incinerators, coke ovens, glass fixmaces, non-ferrous metallurgical plants, coal-fired generation plants, forest product facilities, food drying plants and petrochemical plants.

[0004] The elimination of free moisture (mist) from the gas stream discharging from the WESP is often provided by mesh pads or chevrons located at the outlet from the WESP. Both have problems associated with them. Mesh pads are best suited for the removal of entrained droplets from particulate-free stream, with high removals being achieved at low micron sizes. However, pads often suffer plugging problems where fiber, particulates and/or VOCs (tars and sublimates) are present in the gas stream.. Chevrons provide a high efficiency entrainment separation with limit drop sizes of 15 to 25 microns, depending on gas velocity and blade spacing, but can also suffer from

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WO 2005/007295 PCT/CA2004/001037 plugging problems. These problems, in turn, lead to regions of excessive velocity causing droplet re-entrainment and carry-over.

SUMMARY OF INVENTION

[0005] The present invention provides a novel mist elimination device which replaces conventional mesh pads and chevrons.

[0006] In the present invention, the gas exiting the WESP passes downwardly into an open-topped housing and over an outer hood surface within the housing and then upwardly to an outlet communicating with an upper region of the hood. Liquid droplets accumulate and are drained from a lower end of the housing. Such hood structures have previously been used to attempt to remove the liquid droplets but exhibit considerable moisture carry-over into the exhaust gas stream, especially under flushing conditions.

[0007] In accordance with the present invention, the problems of the prior art hood arrangement are decreased and exhaust gas stream can be provided with minimal or non-detectable carry-over by providing on the outer hood surface a series of gutters which trap water droplets and direct the moisture to the channels to drain into the lower region of the housing. In addition, gutters are also provided on the interior surface of the hood to catch liquid droplets carried by the gas stream into the interior surface of the hood to drip down into lower region of the housing, to the moisture outlet from the housing.

[0008] The present invention, in addition to overcoming the problems associated with prior hood structures, overcomes the inherent limitations and drawbacks of mesh pads and chevrons. The mist elimination device of the invention prevents liquid droplet carryover, both during normal operation and during flushing of the WESP. No additional mist elimination device is required. The mist elimination device of the invention requires no maintenance, a considerable advantage over the mesh pads and chevrons.

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BRIEF DESCRIPTION OF DRAWINGS

[0009] Figure 1 is a perspective view of WESP incorporating a mist eliminator according to one embodiment of the invention;

[0010] Figure 2 is a close-up view of the mist eliminator of Figure 1 ;

[0011] Figure 3 is an elevational view of the hood structure;

[0012] Figure 4 is a perspective view from below of the mist eliminator of

Figure 1;

[0013] Figure 5 is a detail view of a ring channel;

[0014] Figure 6 is a perspective view of a WESP incorporating a mist eliminator according to another embodiment of the invention;

[0015] Figure 7 is an elevational view of the mist eliminator hood of Figure 6;

and

[0016] Figure 8 is a view from above of the mist eliminator hood of Figure 6.

DESCRIPTION OF PREFERRED EMBODIMENT

[0017] In the drawings, preferred embodiments of the invention are illustrated by way of Example. It is to be expressly understood that the description and drawings are only for the purpose of illustration and as an aid to understanding, and are not intended to be a definition of the limits of the invention.

[0018] Referring first to Figures 1 to 5 of the drawings, Figure 1 shows a WESP installation 10 modified to include a mist eliminator 12 constructed according to one embodiment of the invention. The WESP installation is of conventional construction comprising vertically arranged comprising discharge electrodes and collecting electrodes. Any desired arrangement of such elements may be employed, including square tube, round tube, hexagonal tube or plate. A moisture-laden gas stream to be

CA 02532640 2006-01-16

WO 2005/007295 PCT/CA2004/001037 treated is fed through an inlet header 14 to the upper inlet to the WESP down through the tubes containing the electrodes to the lower outlet 16.

[0019] Connected to the lower outlet 16 is the mist eliminator device 12 provided in accordance with one embodiment of the present invention. The mist eliminator device 12 includes a chamber 18 having a sloped bottom wall 20 to a moisture outlet 22. Inside the chamber 18 is a hood 24 comprising upwardly sloping walls 26.

[0020] The mist eliminating device 12 accelerates the gas flow entering the device, causing droplets of free moisture to accelerate downward and then break free of the gas stream as the gas stream turns approximately 180 degrees into the interior of the hood 24 and from there to the gas outlet 28 from the chamber.

[0021] A problem of moisture moving down the outer wall 26 of the hood 24 and dripping off the edge of the hood 24 to be swept into the exiting gas stream, is eliminated by providing a series of drip rings or guides or gutters 30 on the external walls 26 of the hood 24. The gutters 30 are downwardly sloping from an apex to guide the moisture on the hood to the sides of the walls and then to drains 32 from which the accumulated moisture drops into the lower portion of the chamber 18 to the moisture outlet 22.

[0022] To capture any moisture which may be swept into the gas stream, further gutters 34 are provided on the interior wall of the hood 24. In order to capture any moisture which may remain or accumulate on the inner walls of the gas outlet 28 and which is swept along by the gas stream, a ring channel 36 may be provided on the inner wall of the outlet 28 with any collected moisture draining through drain 38 to the moisture outlet.

[0023] The gutters 30 may also provide structural support to the hood 28.

[0024] Turning now to Figures 6 to 8, there is shown thereon an embodiment of the invention in which the WESP is of circular cross-section. The same reference numerals are utilized to identify the equivalent parts.

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WO 2005/007295 PCT/CA2004/001037

EXAMPLES

Example 1:

[0025] This Example illustrates the results obtained using a laboratory scale

WESP structure.

[0026] A 1/8 scale model of a commercial rectangular cross-section WESP structure was set up as illustrated in Figures 1 to 5. The model extended from the WESP inlet, through the collection tubes and into the outlet hood and duct. The humidification spray nozzle was simulated using a single multi-orifice air atomized spray nozzle operated at 50 to 60 psi to ensure good atomization and fine droplet size.

[0027] In the absence of the gutters provided in the hood, there was strong visual evidence of a significant amount of water being carried into the outlet duct, particularly during a tube wash operation. However, with the addition of the water collection gutters to the outside of the hood, the amount of liquid observed to be carried over was significantly reduced.

[0028] The liquid carryover was determined by droplet counters and the average carryover was 0.000764 US gpm/fit. With the wash spray on, the carryover was increased to only 0.00542 US gpm/fit².

[0029] After several minutes of operation, liquid began to accumulate on the duct walls and ran along the surface, not being measured by the droplet counters in the outlet duct. A ring channel was added to the interior of the hood to capture the accumulated liquid from the duct walls.

Example 2:

[0030] This Example illustrates the results obtained using a further laboratory scale WESP structure.

[0031] A 1/6 scale model of a circular cross-section WESP structure was set up as illustrated in Figures 6 to 8, with the gutters in place. The model extended from the horizontal inlet duct through the WESP vessel to the outlet duct. The humidification

CA 02532640 2006-01-16

WO 2005/007295 PCT/CA2004/001037 spray nozzle was a single multiple-orifice air-atomized spray nozzle operated at 50 psig to ensure good atomization and fine droplet size with a mean of 27 microns.

[0032] The WESP flush sprays were simulated by a 8 Bete WL-1/4 60 degree hydraulic nozzles mounted on a ring header at equal spaces. During wash simulation, the header was operated at a flow rate of 6 usgpm at approximately 10 psig.

[0033] The liquid carryover to the outlet duct was determined by droplet

2 counters. With only the inlet spray operating, the carryover was 4.43 x 10' usgpm/ft with an average droplet size in the outlet duct of 3.2 microns. With only the flush sprays operating, the carryover was 6.06 x 10 usgpm/ft with an average droplet size of 20.6 microns. With both sprays operating, the carryover was 9.07 x 10’ usgpm/ft with an average droplet size of 12.1 microns.

[0034] There was little airborne water observed entering the outlet ductwork, as seen from these measurements. Any liquid running along the wall of the outlet duct, resulting from droplet impingement on the duct walls and condensation, was captured by an additional gutter or collection channel at the outlet.

Example 3:

[0035] This Example illustrates the results obtained using a plant scale WESP structure.

[0036] A rectangular full scale mist eliminator hood structure as illustrated in

Figures 1 to 5 was installed to replace an existing mesh pad arrangement which was causing operational problems due to plugging and free moisture carryover from the WESP.

[0037] The installation reduced moisture carryover to values below detection as measured using EPA Method 5 and has been operating for more than eight months without maintenance or interfering with production.

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PCT/CA2004/001037

SUMMARY OF DISCLOSURE

[0038] In summary of this disclosure, the present invention provides a novel mist elimination hood for a WESP system that removes specific amount of moisture from the gas stream while avoiding the problems of plugging and maintenance associated with the most commonly-employed mist elimination systems. Modifications are possible within the scope of the invention.