Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Nitrogen or nitrogenous component
Reexamination Certificate
1999-10-07
2001-12-18
Griffin, Steven P. (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Nitrogen or nitrogenous component
C422S171000, C422S173000, C422S177000, C423S239100
Reexamination Certificate
active
06331281
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a process and an apparatus or cleaning ammonia-containing exhaust gas. More particularly, the present invention pertains to a process and an apparatus for cleaning ammonia-containing exhaust gas which is exhausted from a semiconductor manufacturing process or a chemical treatment process.
2. Description of the Related Arts
Ammonia has widely been used in recent years in each of manufacturing processes with continuous development of a semiconductor manufacturing industry, an optoelectronics industry, a precision machinery and instrument manufacturing industry, a cemented carbide material industry, an ornament manufacturing industry and the like. Among the foregoing industries, a large amount of ammonia is used in the manufacturing process of a nitride film of a compound semiconductor.
Although ammonia is an indispensable substance in the manufacturing process of a compound semiconductory, it is highly toxic, and thus has a threshold limit value in time weighted average (TLV-TWA) of 25 ppm. Therefore, in the case where an ammonia-containing exhaust gas is exhausted in the atmosphere, it exerts evil influence upon human bodies and environment. Accordingly, it is necessary to clean, prior to exhaust in the atmosphere, an ammonia-containing exhaust gas which is exhausted after ammonia is used in a semiconductor manufacturing process or the like.
In addition, a large amount of ammonia is used in some chemical industries. In this case, an ammonia-containing exhaust gas is exhausted as the case may be, and needs to be cleaned prior to exhaust into the atmosphere.
There have heretofore been known as a process for cleaning ammonia-containing exhaust gas, {circle around (1)} a process in which an ammonia-containing exhaust gas is brought into contact with an acidic aqueous solution of sulfuric acid or the like so as to clean the gas by capturing the ammonia contained therein as a salt such as ammonium sulfate; {circle around (2)} a process in which an ammonia-containing exhaust gas is introduced in a combustion furnace so as to clean the gas by converting the ammonia into water and nitrogen; {circle around (3)} a process in which an ammonia-containing exhaust gas is brought into contact with a dry system ammonia harm-removing agent to clean the gas; {circle around (4)} a process in which an ammonia-containing exhaust gas is brought into contact with an ammonia decomposition catalyst under heating so as to clean the gas by converting the ammonia into nitrogen and hydrogen ; {circle around (5)} a process for cleaning an ammonia-containing exhaust gas by the combination of the ammonia decomposition catalyst and the dry system ammonia harm-removing agent; and the like process.
However, the above-mentioned processes involves such problems as described hereunder.
The cleaning process {circle around (1)} of bringing into contact with an acidic aqueous solution of sulfuric acid or the like suffers from the defect of by-producing a large amount of an ammonium salt at the time of cleaning treatment. The cleaning process {circle around (2)} by combustion in a combustion furnace suffers from not only the disadvantage that the process is possibly accompanied with incomplete combustion, abnormal combustion or the danger of explosion because of the variations in the gas flow rate, combustible components and the like, since the exhaust gas is not exhausted under stationary state in many cases, but also the drawback that harmful nitrogen oxides are generated by the combustion treatment. The cleaning process {circle around (3)} by bringing the gas into contact with a dry system ammonia harm-removing agent suffers from the shortcomings that in the case of treating a large amount of ammonia contained in the gas, a large amount of the harm-removing agent is needed, whereby a great expense is incurred and further, a large sized treatment unit is required in cleaning treatment. The cleaning process {circle around (4)} by bringing the gas into contact with an ammonia decomposition catalyst under heating suffers from the disadvantage that a part of ammonia remains undecomposed in a concentration of several tens to several hundreds ppm in the cleaned exhaust gas, since the ammonia decomposition ratio is specified by the chemical equilibrium among the reactants. In addition, the cleaning process {circle around (5)} by the combination of the decomposition catalyst and the harm-removing agent suffers from the drawback that a large amount of the harm-removing agent is needed owing to a high ammonia concentration of several tens to several hundreds ppm in the treated exhaust gas.
Under such circumstances, it has been eagerly desired to develop and provide an efficient process and a compact apparatus which exhibit high cleaning performance, facilitate the treatment after the cleaning, minimize ammonia effluence in the exhaust gas after cleaning and are capable of treating an ammonia-containing gas at a low cost.
SUMMARY OF THE INVENTION
Under such circumstances, intensive extensive research and investigation were accumulated by the present inventors in order to solve the above-mentioned problems involved in the prior arts. As a result, it has been found that ammonia-containing gas can be almost completely cleaned by a process which comprises the steps of bringing an ammonia-containing exhaust gas into contact with an ammonia decomposition catalyst under heating to decompose the ammonia into nitrogen and hydrogen, thereafter bringing undecomposed ammonia remaining in the gas after the ammonia decomposition treatment into contact with an adsorbent to separate ammonia through adsorption, and subsequently again bringing the exhaust gas used for heating regenerating the adsorbent into contact with said ammonia decomposition catalyst. It has been found that the treated gas can be exhausted under the condition of a minimized concentration of ammonia by bringing said exhaust gas used for heating regenerating the adsorbent into contact with another ammonia decomposition catalyst (a second ammonia decomposition catalyst) which has otherwise been provided. The present invention has been accomplished by the foregoing findings and information.
Specifically, the present invention relates to a process for cleaning ammonia-containing exhaust gas which comprises the steps of bringing an ammonia-containing exhaust gas into contact with at least one train of an ammonia decomposition catalyst under heating to decompose the ammonia into nitrogen and hydrogen, thereafter bringing the resultant mixed gas into contact with at least one train of an ammonia adsorbent which is installed adsorb undecomposed ammonia remaining in the exhaust gas so as to clean the exhaust gas, and subsequently heating the adsorbent to regenerate the same, while bringing reproduced exhaust gas containing the ammonia desorbed from the adsorbent into contact under heating, with either said ammonia decomposition catalyst or another ammonia decomposition catalyst so as to clean the exhaust gas containing the desorbed ammonia. The present invention also pertains to an apparatus for cleaning ammonia-containing exhaust gas which comprises an ammonia decomposition column packed inside with an ammonia decomposition catalyst and equipped with a heater; an ammonia adsorption column packed inside with an ammonia adsorbent and equipped with a heater in the subsequent stage; and such a constitution that reproduced exhaust gas generated at the time of heating regenerating the adsorbent is fed to either said ammonia decomposition catalyst or another ammonia decomposition catalyst installed separately therefrom.
A first embodiment of the present invention relates to a process for cleaning ammonia-containing exhaust gas which comprises the steps of bringing an ammonia-containing exhaust gas into contact with at least one train of an ammonia decomposition catalyst under heating to decompose the ammonia into nitrogen and hydrogen, thereafter bringing the resultant mixed gas into contact with either
Akita Noboru
Miyano Yasusada
Otsuka Kenji
Shimada Takashi
Teru Suehachi
Griffin Steven P.
Japan Pionics Co., Ltd.
Medina Maribel
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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