Process for separation of NOx from N2O in a mixture

Details Process for separation of NOx from N2O in a mixture Process for separation of NOx from N2O in a mixture

1997-02-26

2003-02-11

Langel, Wayne A. (Department: 1754)

Chemistry of inorganic compounds

Nitrogen or compound thereof

Oxygen containing

C095S129000, C423S239100, C423S402000

Reexamination Certificate

active

06517797

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a reactive adsorption process for separating N
2
O from a mixture that includes other nitrogen oxides (“NO
x
”) including NO and NO
2
/N
2
O
4
and, in particular to a process for selectively converting NO in a mixture by an oxidant to NO
2
/N
2
O
4
, which is selectively adsorbed from the mixture by an adsorbent and recovering a substantially reduced NO
x
stream of N
2
O stream. The adsorbed NO
x
can be recovered from the adsorbent by a number of procedures such as elevating the temperature, reducing the pressure, inert gas purging, water washing or any combination thereof.
BACKGROUND OF INVENTION
Nitric acid oxidation of hydrocarbons is a well known oxidation reaction. This reaction has been applied in chemical industry to produce important chemicals and chemical intermediates such as adipic acid for many years. Unfortunately, this reaction can produce a significant amount of N
2
O by-product (~1 mole of N
2
O produced per mole of adipic acid produced). NO and NO
2
/N
2
O
4
is also produced in this nitric acid oxidation reaction. The co-existence of NO and NO
2
/N
2
O
4
with N
2
O prevents the off-gas from being used in other medical or industrial applications. Furthermore, all of these nitrogen oxides have a significant negative impact on the environment. Presently, all nitric acid oxidation reaction users are required to spend significant amounts of money as capital investment and operating costs to abate their off-gas before the off-gas can be released to the environment.
The status of various off-gas abatement processes in the adipic acid industry and potential new abatement technologies were reviewed by Reimer et al. “Abatement of N
2
O Emissions Produced in the Adipic Acid Industry” R. A. Reimer, et al, Environmental Progress, vol. 13, no.2 p.134 (May 1994). Tretijak et al., European Patent, EP 625369; Fetzer et al., German Patents DE 4301470, DE 4224881, DE 4128629 also described various processes and catalysts for decomposing nitrous oxide in nitric acid oxidation off gases by catalytic reactions. EPA technical report EPA-450/3-91-026 provides a review of various NO
x
control techniques for nitric acid and adipic acid processes. For use in nitric acid plants, extended absorption, non-selective catalytic reduction and selective catalytic reduction are used most often. For adipic acid plants extended absorption and thermal reduction are used most often.
Badische's FR 1531266, described a process to selectively remove NO
2
and O
2
from a mixture of nitrogen oxides, and O
2
by catalytic reduction with H
2
using an Ag
2
O—MnO
2
—Al
2
O
3
catalyst. Using this process, only NO
2
and O
2
can be removed. NO still co-exists, with N
2
O; therefore, this stream needs further treatment. Nakai et al., JP 661021 describes a process for refining nitrous oxide from a mixture of other nitrogen oxides and CO
2
. This process consists of passing the mixture gas through an NaOH absorption tower first and then an KMnO
4
—NaOH absorption tower in order to remove NO and NO
2
from the N
2
O stream. However, in this process, due to the interference from CO
2
, large amounts of NaOH are consumed to remove NO
x
as well as CO
2
. Furthermore, a liquid waste stream is generated which must be further treated.
The prior art does not recognize that an adsorbent can be used to selectively adsorb NO
2
/N
2
O
4
from a mixture of NO, NO
2
/N
2
O
4
and N
2
O. Nor does the art recognize that a reactive adsorption process can be used to first selectively convert NO in the mixture with an oxidant to NO
2
/N
2
O
4
and then NO
2
/N
2
O
4
can be selectively adsorbed from N
2
O in the mixture. Therefore separate NO
2
O from other nitrogen oxides in a mixture. By using this reactive adsorption process, both N
2
O and the remaining nitrogen oxides can be recovered as useful products and zero waste is generated.
Accordingly, it is an object of the present invention to provide a method for effectively removing a substantial portion of NO
2
/N
2
O
4
from a mixture containing N
2
O in a gaseous mixture stream. It is also an object of the invention to provide a reactive adsorption process that can be used to first selectively convert NO in a mixture with an oxidant to produce NO
2
/N
2
O
4
and then to selectively adsorb produced NO
2
/N
2
O
4
from the N
2
O in the mixture stream, thus separating the N
2
O from other nitrogen oxides. A further object of the invention is to provide a N
2
O stream having a significantly reduced NO
x
that is suitable for use as a feedstock for other medical or industrial applications and to provide for the recovery of the adsorbed NO
x
which can be reconverted to nitric acid. It is also an object of the invention to provide a method which can reduce abatement costs and generate two useful products.
SUMMARY OF INVENTION
Generally, the present invention is a reactive adsorption process for separating NO
x
from N
2
O in a mixture. The process comprises (A) selectively converting with an oxidant NO in a mixture to NO
2
/N
2
O
4
; (B) selectively adsorbing NO
2
/N
2
O
4
from the mixture by contacting same with an adsorbent; and (C) recovering a stream of N
2
O having a substantially reduced NO
x
concentration. The adsorbed NO
x
can be recovered from the adsorbent by elevated temperature, reduced pressure, inert gas purge, water wash or any combination thereof. The advantages of the invention will be better understood with reference to the following detailed description of presently preferred embodiments of the process taken in connection with illustrative examples.
DESCRIPTION OF INVENTION
In a preferred embodiment of the invention N
2
O is present as one of the major components of a mixture and other nitrogen oxides and an oxidant are present in minor amounts. The mixture may also comprise minor amounts of such other compounds as CO, CO
2
, N
2
, other inert gases or hydrocarbons.
A suitable oxidant for use in the present invention is preferably selected from the group consisting of O
2
, O
3
, H
2
O
2
and KMnO
4
. An especially preferred oxidant is O
2
.
The oxidant concentration in the mixture is not critical and can generally be in the range of 0.01% to 40%. A preferred oxidant level is within a range of 0.5% to 21%.
The NO oxidation reaction can be conducted with or without the presence of a catalyst. However, it is preferable to utilize a catalyst for the NO oxidation reaction selected from the group comprising activated carbons, carbon molecular sieves, carbonaceous chars, zeolites, silica gel, activated alumina, manganese oxides, copper oxides, aluminum silicate, carbo alumina gel, iron oxide, nickel oxide, cobalt oxide, various platinum group metals and any combination of thereof. A preferred catalyst is an activated carbon.
The mixture comprising the nitrogen oxides, including oxidant and N
2
O can be contacted with the adsorbent by standard methods. A suitable adsorbent for this process is selected from the group comprising activated carbons, carbon molecular sieves, carbonaceous chars, zeolites, silica gel, activated alumina, silicates, clay(s), adsorbent polymers, and any combination thereof. A preferred adsorbent is an activated carbon.
The NO oxidation reaction and the NO
x
adsorption can be conducted in two separated steps or in one combined oxidation/adsorption step. The presently preferred embodiment of the process is to conduct the selective oxidation reaction and adsorption in a single step. A suitable material (an oxidation catalyst/adsorbent) for this combined oxidation/reaction step is selected from the group comprising an activated carbon, carbon molecular sieve, carbonaceous char, zeolite, silica gel, activated alumina, silicate, clay(s) or any combination of the above mentioned catalysts with adsorbents. A preferred material to be used in the combined oxidation/adsorption step is an activated carbon.
The process of the invention can be carried out as a batch process, a semi-batch process or a continuous process. In a preferred embodiment, the mixture is contacted with one or mor

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