Cleaning and liquid contact with solids – Processes – Combined
Reexamination Certificate
2001-03-30
2003-06-17
El-Arini, Zeinab (Department: 1746)
Cleaning and liquid contact with solids
Processes
Combined
C134S022100, C134S024000, C134S026000, C134S16700R, C134S16800C
Reexamination Certificate
active
06579379
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an improved method of washing the media bed of a gas pollution abatement reactor, such as a regenerative thermal oxidizer or a selective catalytic reduction system, and a bed washer apparatus.
Regenerative thermal oxidizers or RTOs are now widely used for oxidizing gaseous pollutants including volatile organic compounds, such as hydrocarbons, in waste or exhaust gas streams. A typical regenerative thermal oxidizer includes at least two heat exchange chambers each having a heat exchange media bed therein and a combustion chamber located above the heat exchange media communicating with the heat exchange chambers. The waste gas stream is directed alternatively or periodically into one of the heat exchange chambers which has been previously heated and wherein the gaseous pollutants are oxidized. The gas then flows into the combustion chamber, wherein any remaining gaseous pollutants are oxidized. The combustion chamber is also used to preheat the gas flowing through the regenerative thermal oxidizer during start-up and to oxidize any remaining pollutants in the waste gas stream. The cleansed heated gas then flows into the second heat exchange chamber, heating the media bed in the second heat exchange chamber and the cleansed gas from the second media bed is vented to atmosphere. The gas flow through the regenerative thermal oxidizer is then reversed, such that the waste gas stream flows into the heat exchange media in the second heat exchange chamber, oxidizing the pollutants, etc. A regenerative thermal oxidizer thereby conserves heat resulting in a more efficient gas pollution abatement system.
A regenerative thermal oxidizer of the type described herein may include two or three heat exchange chambers, wherein the third chamber serves as a purge chamber. A series of control valves then directs the gas through the heat exchange chambers as described above. By alternating the flow through the regenerative thermal oxidizer through the heat exchange chambers, the pollutants in the process or exhaust gas is removed and oxidized without exhausting pollutants to the atmosphere and the heat exchange media is periodically cleaned.
The heat exchange media in the media bed may comprise relatively small ceramic elements, generally saddle shaped ceramic elements, or the media bed may be formed of stacked ceramic blocks each having small continuous passages therethrough. The ceramic media may also include or be coated with a catalyst resulting in a catalytic reaction within the chamber to remove gaseous pollutants. Where the media includes a catalyst, the gaseous pollutant abatement system is generally referred to as a selective catalytic reduction apparatus or SCR system used primarily to treat NOx, including NO and NO
2
. The ceramic media may be coated with a suitable catalyst or the catalyst may be mixed with the ceramic matrix prior to firing. Typical catalysts include noble metal catalysts, such as platinum, and base metal catalysts, such as vanadium or manganese oxide or Zeolite. A typical SCR system includes only one reaction chamber filled with a catalytic media bed as described. The gas to be treated flows through the bed of catalytic media in the reaction chamber where the NOx is reduced to nitrogen gas and non-polluting oxides.
As used herein, the term “gas pollution abatement reactor” is intended to include both RTOs and SCRs and other gas pollution abatement apparatus having a media bed, wherein the media bed is heated to oxidize or react with the gaseous pollutants, thereby removing the pollutants prior to venting the gas to atmosphere. However, in many real world applications of these systems, the industrial process gas emissions further contain solid particulate material in addition to the gas phase pollutants the abatement reactor is intended to destroy. These particulates can accumulate in the media bed in sufficient quantities such that the accumulated particulate material will cause an increase in the airflow resistance through the media bed, increasing the pressure drop across the media bed, thereby restricting the airflow capacity of the system and preventing the process equipment from operating properly. Because these particulates are endemic to many real world applications and they can cause the gas pollution abatement reactor to become inoperative, techniques have been developed to clean these particulates from the media bed.
The presently preferred method of cleaning particulates from the media bed is water washing. Water washing is used primarily to clean non-burnable particulate accumulations from RTOs and non-reactive particulates from SCRs. Burnable particulates are typically cleaned from the media bed of an RTO using a “bake out” technique. Conventional water washing is accomplished by the following procedure. First, the gas pollution abatement reactor is taken “offline,” shut down and cooled to ambient temperature. The access door located above the media bed is then opened and the atmosphere is checked as required for personnel entry. The media bed is then washed, typically using a fire hose connected to a supply of wash water. The wash water is then sprayed over the media bed by personnel standing on the media bed who manually distribute the wash water over the media bed by moving the hose from place to place. After the washing is completed, the media bed is dried and reheated prior to placing the gas pollution abatement reactor back in service.
There are several important disadvantages of this conventional washing technique. First is worker safety. If there are any “hot spots” in the media bed when the washing is started and the water is directed onto these hot spots, steam will be generated and released. This can cause hazardous temperatures. In addition, the steam can fog the workers' eyewear, making it difficult for them to exit. Depending on how the reaction media is cooled, these hot spots may be well below the upper surface of the media and not apparent upon inspection. Another disadvantage of this water washing technique is the potential for the workers to wet or damage the ceramic fiber insulation inside the combustion chamber of an RTO. If this ceramic fiber insulation is wetted, it can sag or shrink, creating gaps which will lead to hot spots on the outside skin of the reaction chamber. These hot spots can lead to unnecessary heat loss, cosmetic damage to the exterior finish of the housing, and possible corrosion of the outer shell. Another disadvantage of this method is the potential for poor distribution of the wash water over the surface of the media bed. Because the workers must manually move the hose from place to place, there is the potential for some places being missed and other places washed more than necessary. Thus, this method is wasteful of wash water and can lead to incomplete washing.
There is, therefore, a long felt need to improve the method of washing the media bed of a gas pollution reactor such as an RTO or SCR which assures personnel safety, reduces damage to the ceramic fiber lining of the housing, and assures even distribution of wash water over the media bed. The method of washing the media bed of a gas pollution abatement reactor and bed washer apparatus of this invention solves these problems by eliminating the need of a worker being located within the reactor housing during washing, and by utilizing a bed washer which assures even distribution of the wash water over the media bed without spraying the ceramic fiber installation inside the housing.
SUMMARY OF THE INVENTION
As set forth above, the present invention relates to a method of washing the reaction media bed of a gas pollution abatement reactor and an apparatus or bed washer for washing the media bed which eliminates the requirement for personnel to be located within the housing during washing and which assures even distribution of the wash water over the surface of the media bed without spraying the insulation during washing. A typical gas pollution abatement reactor of the type described above inc
Durr Environmental, Inc.
El-Arini Zeinab
Howard & Howard
LandOfFree
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