Patent Application
20180318845
The present invention relates generally to control and containment of gases and more specifically to an electrostatic precipitator in a gas scrubbing apparatus.
A variety of industrial processes create gas streams that must be scrubbed of contaminants before being released to the outside world. The manufacture of electronics, solar cells, display devices, communications devices, metals, ceramics, and polymers, as well as the processing of chemicals, drugs, and other materials, often requires the use of exhaust gas scrubbers. Scrubbers typically receive a substantially gaseous exhaust stream (sometimes containing fine particles or fine mists) and remove contaminants from the stream before the stream is released to the environment.
Exhaust streams from electronic fabrication processes may include a variety of contaminants, including but not limited to perfluorocarbon (PFC) etch gases such as SF6, NF3, CF4, C2F6, C4F8, COF2, and C4F6. Exhaust streams may also include toxic hydrides such as AsH3, PH3, P2H4, or B2H6, pyrophoric or flammable gases such as SiH4, H2, Si2H6, GeH4, and/or gases such as WF6, SiF4, HCl, BCl3, Cl2, TiCl4, F2, HF, and various chlorosilanes. Other industrial processes may also create toxic or polluting exhaust streams particular to a specific material or manufacturing process.
In such processes, a proportion of the gas supplied to the chamber may be exhausted from the chamber, together with solid and gaseous by-products from the process occurring within the chamber. Further, a process tool may have a plurality of process chambers, each of which may be at respective different stage in a deposition, etching or cleaning process. Therefore, during processing a waste stream may be formed from a combination of the gases exhausted from the chambers that may have various different chemical or particulate compositions.
Thus, before the waste stream is vented into the atmosphere, it is typically treated to remove selected gases and solid particles therefrom. Acid gases such as HF and HCl are often soluble in water, and are commonly removed from a gas stream using a wet scrubber, for example, a packed tower scrubber, in which the acid gases are taken into solution by a scrubbing liquid flowing through the scrubber. Some contaminants are water-reactive, and may or may not dissolve in water, depending upon various conditions. These contaminants may also react with water to form solid reaction products.
Some contaminants are often abated by using heat to break down or combust the contaminant to form water-soluble reaction products. Sometimes, this requires high temperatures. For example, NF3 may be combusted at temperatures above 900 degrees Celsius; CF4 may be broken down at temperatures over 1200 degrees Celsius. Other contaminants such as SiH4 may sometimes be combusted simply by exposing the contaminant to an oxygen source.
The water-insoluble, thermally decomposed contaminants may form reaction products (e.g., HF) that may then be removed by wet scrubbing the reacted gas stream. Other water-insoluble contaminants (e.g., SiH4) may form reaction products that include solid species (e.g., SiO2), when thermally reacted.
Generally, such solid species in a waste stream may be present as fine particles in a liquid phase (e.g., water associated with a scrubber), in the gas phase, deposited on a solid surface, or in other ways. These solid species may also nucleate directly on various surfaces. While the formation of solid reaction products may enable certain removal methods (e.g., filtration), these species may also deposit on and clog various lines, inlets, passages, surfaces, and other aspects of the system, reducing the system's efficiency or stopping its operation.
For gas streams including a variety of contaminants, effective scrubbing may require multiple systems, such as a wet scrubber to remove water-soluble contaminants combined with a combustion chamber to combust water-insoluble contaminants. Even such a combination may not be able to remove all of the particles from a gas stream, particular those under a certain size.
In view of this, it is known, to provide an electrostatic precipitator downstream from the wet scrubber and/or combustion chamber to remove these smaller particles from the waste stream. An electrostatic precipitator typically involves injecting a gas from which particulates are to be removed and water mist into a space between two electrodes (the second electrode is sometimes referred to as a collector). However, in some prior art electrostatic precipitators, some electrode configurations can result in water mist collecting on an electrode such that there is undesirable arcing between the electrodes.
Accordingly, it would be useful to have an improved electrode configuration that prevents water from collecting and causing arcing in an electrostatic precipitator.
An improved electrode for use in an electrostatic precipitator is disclosed.
One embodiment discloses an electrode assembly for use in an electrostatic precipitator, comprising: a first electrode including a generally tubular conductive portion; and a second electrode, comprising: a rod-shaped conductive central portion located along a longitudinal central axis of the first electrode and having a top end and a bottom end; and a plurality of conductive disc-shaped elements, each disc-shaped element having sharp points spaced around its circumference and a plurality of openings near its center, the central portion of the second electrode passing through the centers of each of the plurality of disc-shaped elements such that the disc-shaped elements are located parallel to one another along the central portion of the second electrode.
Another embodiment discloses an apparatus for treating gas, comprising an electrostatic precipitator section having: a casing having an upper end and a lower end; a gas inlet for receiving gas located toward the lower end of the casing; a gas outlet for exhausting gas located near the upper end of the casing; a first electrode including a generally tubular conductive portion; a second electrode, comprising: a rod-shaped conductive central portion located along a longitudinal central axis of the first electrode; and a plurality of conductive disc-shaped elements, each disc-shaped element having sharp points spaced around its circumference and a plurality of openings near its center, the central portion of the second electrode passing through the centers of each of the plurality of disc-shaped elements such that the disc-shaped elements are located parallel to one another along the central portion of the second electrode; a power supply having a positive terminal connected to the first electrode and a negative terminal connected to the second electrode; a liquid inlet located toward the upper end of the casing for receiving a water spray; and a liquid outlet located toward the lower end of the casing.
An improved electrode for use in an electrostatic precipitator is disclosed. In one embodiment, the electrode comprises a generally rod-shaped conductive central portion, to which are attached a plurality of conductive disc-shaped elements. Each disc-shaped element has a number of sharp points spaced around its circumference and a plurality of openings near its center. The central portion of the electrode passes through the centers of each of the plurality of disc-shaped elements such that the disc-shaped elements are located parallel to one another along the central portion, and may be equally spaced along the central portion. The disc-shaped elements are conical or convex in shape, and oriented with their rims raised above their centers so that any water that collects on them runs toward the center of the disc-shaped elements and out through the openings and down the central portion of the electrode.
As noted above, in some prior art electrostatic precipitators, some electrode configurations have disc shaped elements or other shapes that may result in water mist collecting on the electrode. Such collection of water mist can result in arcing between the electrode and the collector (i.e., the second electrode). The increased current that accompanies arcing may cause damage to the high-voltage power supply or other components, and also momentarily reduces operating voltage and thus the particle removal efficiency. It is thus preferable to prevent such arcing if possible.
The present configuration significantly ameliorates this problem since any water mist that collects on a disc shaped element will collect at the bottom of the disc shaped element and run out of one or more of the openings at the bottom of the disc shaped element and down the central portion of the electrode, where it can be drained off without arcing. In the absence of any collected water mist at the edges of disc shaped elements, the chance of arcing between the electrode and the collector is eliminated or at least greatly reduced.
As above, one of skill in the art will appreciate that an electrostatic precipitator may be only one part of an abatement system for treating a gas stream containing various contaminants.
Generally, exhaust gas streams may flow through abatement system 100 from left to right as shown. An exhaust gas stream may first be scrubbed of water reactive and/or water-soluble contaminants in a burner system and/or wet scrubber 102. It may be advantageous to use a wet scrubber that removes as many water reactive and/or water-soluble contaminants from the gas stream as possible, as their removal prior to subsequent reaction systems may improve performance of those systems.
The scrubbed gas stream may then be treated in electrostatic precipitation system 104. Generally, electrostatic precipitation system 104 may be used to remove many of the remaining contaminants from the scrubbed gas stream as further described below. The reacted gas stream may then pass to wet scrubber 106. In this example, there may be separate wet scrubbers 102 and 106, although the system may be designed such that a single wet scrubber is used.
For embodiments in which abatement system 100 includes such elements as electrostatic precipitation system 104 and one or more wet scrubbers, substantial amounts of liquid may be needed. In such cases, it may be advantageous to include a separate liquid handling system 108. Liquid handling system 108 may provide liquids to any of wet scrubbers 102 and/or 106 and electrostatic precipitation system 104, as well as handle liquids received from these systems.
As an example, U.S. Pat. No. 8,888,900 shows one embodiment of an abatement system in which a wet scrubber is followed by an electrostatic precipitator.
One end of a high-voltage power supply 308 is connected to the first electrode 302, while the other end of power supply 308 is connected to a second electrode (or collector) 310 (with ground return pathways to the power supply if desired), such that an electric field is created in the space between the two electrodes 302 and 310. One of skill in the art will appreciate that acceptable electrode voltages depend upon the spacing between the electrodes, i.e., between the first electrode and the collector. A range of 5,000 to 15,000 volts per centimeter of distance between the electrodes may result in good particle abatement without arcing. As illustrated in
As shown in
Some of the electrons 312 passing from the disc shaped elements 306 on first electrode 302 to second electrode 310 will strike particles in the gas stream, such as particle 314 as shown in
As illustrated in
In a wet electrostatic precipitator, the second electrode, or collector, is typically continuously washed with a flow of water. Once the charged particle 314 reaches second electrode 310, the particle 314 is thus washed down to the bottom of second electrode 310 and can then flow out of the electrostatic precipitator.
As further shown in
As also shown in
Central hole 404 also has a plurality of openings 410 around the circular main portion of central hole 404, so that when the disc shaped electrode is mounted on a central portion of an electrode as in
When attached to the central portion of an electrode, the rim of electrode element 402 should be toward the top of the electrostatic precipitator and the center of electrode element 402 toward the bottom of the electrostatic precipitator. This allows fluid from the water mist that collects on electrode element 402 to run to the center of electrode element 402 where it can drain through one or more of the openings 410, and down the central portion of the electrode. As above, this prevents collected water from collecting on or dripping off the rim of electrode element 402, greatly reducing or eliminating arcing, and also allows the collected water to actively rinse the electrode, slowing deposition of any particles on the electrode and removing any previously accumulated matter.
In some embodiments, as is known in the art the electrostatic precipitator may be contained in a vertically oriented generally tubular container, in which first electrode 302 extends down the longitudinal axis of the container, and second electrode 310 is the inner wall of the container. As above, second electrode 310 will be continuously washed with a flow of water.
As shown herein, the tubular container is cylindrical, and thus has a circular cross-section. In other embodiments, the tubular shape may have an oval, ovoid, or rectilinear cross-section, or even an irregularly shaped cross-section. Those of skill in the art, in light of the teachings herein, will appreciate the issues that may arise with such other shapes and the implementation variations required for such other shapes.
When a different treatment method is used before an electrostatic precipitator, the prior treatment method and resulting products may cause certain design decisions to be more desirable. For example, in some embodiments water from a wet scrubber may be used as the flow of water over the second electrode. In some cases, this fluid may contain acids such as HF or HCl, having a pH value less than 1 and thus being corrosive. In such cases the body of the electrostatic precipitator may be made of a corrosion resistant plastic such as PVC, and the water used to flush the collection surface used as the conductor of the second electrode.
In other cases, water from the wet scrubber may contain dissolved CO2 from a prior thermal process. When this water is used to flush the collection surface, it may be desirable to supply deionized water with a very low conductivity to be used as the water mist in the electrostatic precipitator. The dissolved CO2 will partially convert to carbonic acid, which increases the conductivity of the collected water and mist and allows the electrostatic precipitator to remain effective.
As above, it is known to treat a gas stream with a wet scrubber before further treatment with an electrostatic precipitator. In a different embodiment described herein, the sequence is reversed, with treatment of a gas by the electrostatic precipitator prior to treatment by the wet scrubber. The electrostatic precipitator may use the improved electrode described above.
As shown by arrow 512, gas, for example, from a burn chamber (not shown) moves upwards through a sump-supplied, recirculating water spray in the central collection tube of electrostatic precipitator 504 and into a sump-supplied, recirculating water feed ring 508 at the top of the apparatus 500. Here, as shown by arrow 514, the gas reverses direction and flows downwards through fresh water wash packing in the wet scrubber 502 that encircles or surrounds the core assembly of electrostatic precipitator 504. The gas then passes through a packing stop 510 at the bottom of the packed column and, as shown by arrow 516, exits through a removable flanged exhaust to a facility exhaust system.
In some embodiments, the electrostatic precipitator 504 of apparatus 500 may only receive sump-supplied, recirculating water while the wet scrubber 502 may only receive a supply of fresh water. These two different water sources may be physically separate such that the respective liquids are never combined in apparatus 500. As above, the electrostatic precipitator 504 is located concentric to the wet scrubber 502 in apparatus 500, with neither being physically located above the other.
In some embodiments, the connection from the high voltage power supply 518 to the electrostatic precipitator electrode 506 may pass through a chamber 520 which is filled with an inert gas, such as nitrogen, or clean dry air. The gas or clean dry air in the chamber 520 is at a higher pressure than gas in the electrostatic precipitator. A small opening is provided between chamber 520 and the top of the electrostatic precipitator, and the pressure difference between the gas or clean dry air in chamber 520 and gas in the electrostatic precipitator allows the gas or clean dry air to flow only from chamber 520 into the electrostatic precipitator, thus preventing the accumulation of water and/or particles on the connection from high voltage power supply 518.
As above, it is expected that many or most particles will acquire a negative charge and be pulled away from the first electrode of the electrostatic precipitator, such as electrode 506 in
To accomplish this, the electrostatic precipitator may be provided with a built in rinsing system which projects a liquid onto first electrode 506 so as to rinse away any material that has been so deposited. In one embodiment a single hole is used to direct a water stream to the top of first electrode 506 that rinses the accumulated material off of the central portion of first electrode 506 and then cascades down, sequentially rinsing the individual disc shaped elements 306.
In other embodiments, a conduit with multiple outlet holes may be used; the holes may be aligned parallel to the longitudinal axis of first electrode 506, so as to rinse the entire electrode at once from multiple directions, or alternatively may rinse various sections of first electrode 506 sequentially from one or more directions. In still further embodiments, a spiral manifold with a plurality of water nozzles may be used; in some cases the spacing of the water nozzles may approximate the pitch of the disc shaped elements 306. Alternatively, a plurality of cylindrical or toroid shaped manifolds may be used. In light of the teachings herein, those of skill in the art will be able to determine which configuration will provide the best cleaning results in a given case.
The amount of water used to rinse first electrode 506 should be adequate to wash all of disc shaped elements 306, thus providing cleaning of all of first electrode 506. The water may be any water; for example, it could be fresh city water, scrubber sump water, or sump water or city water that has been treated with a cleaner. The cleaner may be acid based, alkaline, or may include a plurality of like or different chemicals or compounds or mixtures that speed the removal of deposits.
In various embodiments, the rinsing operation may be automated to occur at predetermined time intervals. In other embodiments, measured operating parameters of the system such as voltage, current, remaining particles after precipitation or other parameters, may indicate that performance of the electrostatic precipitator has fallen below some predetermined level, causing rinsing to occur. Alternatively, rinsing may be manually commenced. Those of skill in the art will appreciate many other control methods and techniques that may be used to control a rinse cycle.
It will be appreciated that the water used in rinsing can cause both shorting and/or an arc since the water stream provides a pathway to ground. This may be mitigated by decreasing the voltage to first electrode 506 during the rinsing operation, although this may also cause the particle scrubbing action of the electrostatic precipitator to decrease. Since the rinsing will preferably be of relatively short duration, the decrease in particle scrubbing action should not be excessive. Alternatively, the electrode power supply may be shut off allowing the rinse operation to proceed without a charge being supplied to the electrode.
In some embodiments, the rinse system will include a pressurized water supply and an automated shutoff valve. The water flow may be either variable or predetermined; depending upon the process type, a flow of 0.1 to 0.5 gallons per minute may be used, but higher flows may be necessary for certain types of deposited minerals. Rinsing type may be as short as 2 seconds or as long as several minutes, depending upon the chemistry of the rinsing water and the characteristics of the deposited material.
Once a rinse is completed, time may be provided to allow first electrode 506 to dry. Where first electrode 506 is on at reduced voltage during the rinse cycle, the voltage may be gradually increased as rinse water drips off first electrode 506. In higher voltage systems where power to first electrode 506 is turned off during rinsing, power may be turned on in as little as one second or as long as several minutes after rinsing is complete. In other embodiments, power may be pulsed during the drying time to cause droplets to be pushed off first electrode 506, although possibly with some resulting arcing. In still other embodiments, a hot nitrogen gas stream may be passed over first electrode 506 after rinsing to accelerate the drying time.
Lab results show such an electrostatic precipitator can reduce the particle matter coming from the abatement system by more than 99.9% of the original particle content of the gas. Current abatement systems produce particle output loads of 30 grams per hour or more depending on the makeup of the incoming gases. A reduction of the particle output load to less than 0.1 grams per hour has been measured by using this process.
The disclosed system and method has been explained above with reference to several embodiments. Other embodiments will be apparent to those skilled in the art in light of this disclosure. Certain aspects of the described method and apparatus may readily be implemented using configurations or steps other than those described in the embodiments above, or in conjunction with elements other than or in addition to those described above. It will also be apparent that in some instances the order of the processes described herein may be altered without changing the overall result of the performance of all of the described processes, as well as the possible use of different types of air scrubbing systems.
For example, one of skill in the art will appreciate that wet scrubbers before or after an electrostatic precipitator may or may not have packing, may use different sources of water, such as water from the electrostatic precipitator, clean municipal water, etc., and may be irrigated in different ways, such as by a continuous stream of water, spray nozzles only, etc.
It should also be appreciated that the described method and apparatus can be implemented in numerous ways, including as a process, an apparatus, or a system. The methods described herein may be implemented by program instructions for instructing a processor to perform such methods, and such instructions recorded on a computer readable storage medium such as a hard disk drive, floppy disk, optical disc such as a compact disc (CD) or digital versatile disc (DVD), flash memory, etc. It may be possible to incorporate some methods into hard-wired logic if desired. It should be noted that the order of the steps of the methods described herein may be altered and still be within the scope of the disclosure.
It is to be understood that the examples given are for illustrative purposes only and may be extended to other implementations and embodiments with different conventions and techniques. While a number of embodiments are described, there is no intent to limit the disclosure to the embodiment(s) disclosed herein. On the contrary, the intent is to cover all alternatives, modifications, and equivalents apparent to those familiar with the art.
In the foregoing specification, the invention is described with reference to specific embodiments thereof, but those skilled in the art will recognize that the invention is not limited thereto. Various features and aspects of the above-described invention may be used individually or jointly. Further, the invention can be utilized in any number of environments and applications beyond those described herein without departing from the broader spirit and scope of the specification. The specification and drawings are, accordingly, to be regarded as illustrative rather than restrictive. It will be recognized that the terms “comprising,” “including,” and “having,” as used herein, are specifically intended to be read as open-ended terms of art.
This application claims priority to Provisional Application No. 62/500,964, filed May 3, 2017, which is incorporated by reference herein in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 62500964 | May 2017 | US |
