Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Nitrogen or nitrogenous component
Reexamination Certificate
2000-05-18
2002-12-31
Silverman, Stanley S. (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Nitrogen or nitrogenous component
C422S171000, C422S172000, C422S173000, C422S177000, C423S239100
Reexamination Certificate
active
06500398
ABSTRACT:
TECHNICAL FIELD
This invention relates to a method for treating a gas causing global warming and an apparatus therefor. More particularly, it relates to a method for preventing global warming by exothermally decomposing nitrous oxide (N
2
O), which causes global warming, into nitrogen (N
2
), oxygen (O
2
) and optionally nitrogen oxides (NO, NO
2
, etc.) and an apparatus therefor.
BACKGROUND ART
In the process of producing adipic acid, nitric acid is used as an oxidizing agent. It has been an accepted practice to release into the atmosphere nitrous oxide formed as a by-product in the formation of adipic acid from the oxidation of cyclohexanone and/or cyclohexanol with nitric acid.
However, nitrous oxide has recently attracted public attention as one of the gases causing global warming, though it is not as well known as carbon dioxide which is a typical gas causing global warming.
Nitrous oxide evolves mostly from natural soil or farmlands. Thus, the chemical industry causes only a small part of the nitrous oxide evolving on the earth. However, it is considered that the amount of nitrous oxide formed by chemical processes such as the adipic acid production process, which are artificial N
2
O sources, can be controlled. Therefore, attempts have been made in recent years to reduce, first of all, nitrous oxide generated from these chemical processes.
There have been proposed various methods for reducing nitrous oxide generated from chemical processes. Many of these proposals relate to methods for decomposing nitrous oxide (N
2
O) into nitrogen (N
2
) and oxygen (O
2
) and optionally nitrogen oxides (NO, NO
2
). These methods involve two main types for decomposing N
2
O, namely, thermal decomposition methods wherein decomposition is carried out by heating without using any catalyst and catalytic decomposition methods wherein decomposition is carried out by using a catalyst. Now, each type of these methods will be described.
Known examples of the thermal decomposition methods without using any catalyst include those proposed in, for example, U.S. Pat. No. 2,974,019, JP-A-61-257940, JP-A-5-339003 and JP-W-A-9-508346 (the term “JP-A” as used herein means an “unexamined published Japanese patent application” and the term “JP-W-A” as used herein means an “international patent application published in the Japanese national proceeding”). However, these proposals each suffers from unsolved problems as will be described hereinafter. That is, no satisfactory method for thermally decomposing nitrous oxide has been proposed hitherto.
That is, U.S. Pat. No. 2,974,019 proposes an apparatus by which N
2
O is thermally decomposed at a high temperature under elevated pressure (to 1692° C., to 25.5 atm) to give NO
2
. However, a highly reliable material for this apparatus used to resist the high temperature and elevated pressure is not readily available and makes the apparatus highly expensive. Therefore, this method is used very little in practice.
JP-A-61-257940, which has been applied by the same applicant as in the present invention, discloses that when a discharged gas containing N
2
O is preheated and then heated, the thermal decomposition of N
2
O starts at about 900° C. and N
2
O can be thermally decomposed at 1000° C. or above. In the method of the thermal decomposition of N
2
O proposed in this document, it is necessary to control the total content of NO and NO
2
to be 10% or less in the N
2
O-containing gas to be treated. Thus, there arises a problem that an additional step is needed for controlling the discharged gas composition.
JP-A-5-339003 proposes an improved method over the above-mentioned JP-A-61-257940 for thermochemically decomposing N
2
O with a flame treatment in the thermal decomposition method. In this method, N
2
O is thermochemically decomposed in the presence of flame by the combustion heat of the flame. Therefore, it is feared that the thermochemical reaction in this method should be performed at a considerably higher temperature due to the combustion heat of the flame coupled with the decomposition heat of N
2
O. In this method wherein the flame is continuously employed in the thermochemical decomposition of N
2
O, it is unavoidable to use a considerably large amount of fuel for the generation of the flame. As a result, a large amount of a combustion gas is formed and, therefore, the NO and NO
2
concentrations in the thermochemically decomposed gas are lowered, which brings about another fear that a large-scaled device (for example, an absorption tower) should be used to recover the NO and NO
2
.
JP-W-A-9-508346 proposes a method wherein the method of the thermal decomposition of N
2
O as disclosed in the above-described JP-A-61-257940 is improved in the preheating portion to thereby produce NO from N
2
O. That is, this document proposes a method for producing NO from N
2
O by heating an N
2
O-containing gas to about 400 to 700° C. by using a heat exchanger, then heating the gas to about 850° C. without a heat exchanger and using the combustion heat of a combustible gas, etc., thermally decomposing N
2
O in the gas at 1000° C. or above, and then quickly cooling the gas thus formed to thereby recover NO. In the case of this method, however, it is needed to heat the whole N
2
O-containing gas to be treated to 850° C. by using the combustion of a combustible gas, etc. It is therefore unavoidable to use a large amount of the combustible gas. Accordingly, this method suffers from the same problem as in the method proposed by JP-A-5-339003 described above. In this method, furthermore, it is feared that the temperature in the reaction chamber is elevated to a considerably high level, since a large amount of decomposition heat is generated in the reaction chamber from the N
2
O holding the combustion heat as described above. Regarding this point, it is described in the specification of this application that the temperature in the reaction chamber might be elevated to 1500° C.
In the thermal decomposition reaction of N
2
O, the reaction by which N
2
O is decomposed into N
2
and O
2
is an exothermic reaction. Accordingly, there arises a problem that the temperature in the reaction system, where N
2
O is thermally decomposed, is remarkably elevated due to the decomposition heat generated in a large amount. As the temperature in the reaction system is elevated, more expensive heat-resistant materials should be employed in the reactor and various devices for treating the gas discharged from the reactor (for example, a heat exchanger, a device for absorbing the thus formed gas, pipes connecting these devices, etc.). In addition, it is also feared that the maintenance of the equipment becomes more difficult thereby. In the conventional proposals as described above, however, no consideration has been given with respect to these problems accompanying the decomposition heat of N
2
O.
That is to say, no satisfactory method for thermally decomposing nitrous oxide has been proposed hitherto.
As examples of the catalytic decomposition methods with the use of a catalyst, proposals have been made by JPA-5-4027, JP-A-6-277453, etc. However, these proposals each suffer from unsolved problems as will be described hereinafter. That is, no satisfactory method for catalytically decomposing nitrous oxide has been proposed hitherto similar to the case of the thermal decomposition methods.
For example, JP-A-5-4027, which has been applied by the same applicant as in the present invention, proposes a method for catalytically decomposing an N
2
O-containing discharged gas into N
2
and O
2
in the presence of a copper(II) oxide catalyst. This document discloses that the reaction temperature preferably ranges from 400 to 600° C.; that it is desirable in case of an adiabatic reaction to supply the gas while diluted with air, etc. into the reactor, since the temperature in the outlet side of the reactor is elevated due to the large reaction heat of the catalytic decomposition; and that the reaction heat of the catalytic decomposition is recovered from the gas as steam after the completion of the catalytic
Kodama Souhei
Miura Koji
Shimizu Atsushi
Tagawa Katsushi
Tanaka Katsutoshi
Asahi Kasei Kabushiki Kaisha
Birch & Stewart Kolasch & Birch, LLP
Medina Maribel
Silverman Stanley S.
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