Process for removing NOx from nitrosylsulphuric acid

Details Process for removing NOx from nitrosylsulphuric acid Process for removing NOx from nitrosylsulphuric acid

1999-05-14

2001-05-29

Langel, Wayne (Department: 1754)

Chemistry of inorganic compounds

Nitrogen or compound thereof

Oxygen containing

C423S531000

Reexamination Certificate

active

06238639

ABSTRACT:

This invention relates to a process of removing NO
x
from nitrosyl hydrogensulfate by mixing nitrosyl hydrogensulfate in a mixing reactor with sulfuric acid, which is saturated with SO
2
, where a sulfuric acid containing N
2
O
3
is withdrawn from the mixing reactor, and a stripping gas is passed through the withdrawn sulfuric acid.
Such process is known from GB-A-0,348,866. For the expulsion of residual nitrogen oxides from the sulfuric acid flue gas or an inert gas is passed through the sulfuric acid, with the temperature lying in the range from 100 to 200° C.
The object underlying the invention is to remove NO
x
from nitrosyl hydrogensulfate in a simple and inexpensive way. In accordance with the invention this is accomplished in the above-stated process in that the N
2
O
3
-containing sulfuric acid withdrawn from the mixing reactor is added to a saturation reactor, where in the lower portion of the saturation reactor an SO
2
-containing gas is introduced at the same time, which at least in part flows upwards through the N
2
O
3
-containing sulfuric acid, that water is introduced into the saturation reactor, and from the saturation reactor a virtually NO
x
-free sulfuric acid saturated with SO
2
is withdrawn, which has a H
2
SO
4
concentration of 5 to 60 wt-%, and a partial stream of which is passed into the mixing reactor, where SO
2
is supplied to the mixing reactor in a stoichiometric surplus of at least 2 wt-% with reference to the NO
x
content of nitrosyl hydrogensulfate.
In accordance with the invention, the term NO
x
refers to a mixture of NO and NO
2
. NO
x
is present in sulfuric acid as dissolved nitrosyl hydrogensulfate. Nitrosyl hydrogensulfate is formed from NO, which is contained in SO
2
-containing gas. The SO
2
-containing gas originates for instance from a roasting, sulfur combustion, sulfate separation or metallurgical process. NO is oxidized at the oxidation catalyst of a sulfuric acid plant for up to 50% to form NO
2
. The mixture of NO and NO
2
is reacted with sulfuric acid to form nitrosyl hydrogensulfate as follows:
NO+NO
2
+2H
2
SO
4
→2HNOSO
4
+H
2
O
It is known that SO
2
reacts with nitrosyl hydrogensulfate by forming sulfuric acid and nitrogen oxides:
SO
2
+2HNOSO
4
+2H
2
O→3H
2
SO
4
+2NO
In the production of sulfuric acid nitrosyl hydrogensulfate, usually with a NO
x
content of more than 2.5 wt-%, is separated as a condensate. The NO
x
content is present in the condensate as nitrosyl hydrogensulfate in addition to sulfuric acid.
In formal terms, nitrosyl hydrogensulfate can be reacted with SO
2
and water to form sulfuric acid and N
2
. The redox reaction takes place between the dissolved SO
2
in the form of SO
3
2−
with the N
2
O
3
in the form of NO
2
−
, which was produced in the hydrolysis of nitrosyl hydrogensulfate. The reduction of the nitrogen oxides NO and NO
2
to nitrogen is effected in the hydrolysis of nitrosyl hydrogensulfate with dilute, SO
2
-saturated sulfuric acid in that in the diluting solution the reducing agent is provided in the form of SO
3
2−
in an overstoichiometric amount, so that the equilibrium is shifted towards the formation of N
2
.
It is an advantage of the present invention that the amount sulfuric acid present in a condensate containing nitrosyl hydrogensulfate can be recirculated to a sulfuric acid production without any NO
x
. In accordance with the conventional processes the condensate is withdrawn from the process and must be subjected to a treatment or be disposed of. A particular advantage of the inventive process consists in that nitrogen is formed from the noxious substance NO
x
and is discharged together with the exhaust gas.
Preferably, SO
2
is supplied to the mixing reactor in a stoichiometric surplus of at least 5 wt-% with reference to the NO
x
content of nitrosyl hydrogensulfate. With this surplus of SO
2
a good result is achieved in the reduction of the NO
x
content. SO
2
is not lost as a result of the reduction of NO
x
to N
2
, but is oxidized to form SO
4
2−
. Excess SO
2
can be supplied to a plant for the production of sulfuric acid, so that an extensive treatment is not necessary.
Advantageously, the sulfuric acid concentration after the addition of SO
2
is 5 to 30 wt-%. In this range of the sulfuric acid concentration particularly good results are achieved for the reduction of NO
x
to N
2
.
In accordance with an advantageous aspect of the invention gaseous SO
2
is brought into aqueous solution in a saturation reactor designed as packed column, and is combined with the NO
x
-containing sulfuric acid as an aqueous solution. The SO
2
saturation is promoted by operating the saturator under an increased gas pressure.
In accordance with the invention, the NO
x
content of a NO
x
-containing sulfuric acid (nitrosyl hydrogensulfate) or a NO
x
-containing sulfuric acid mixture is reduced. The process in accordance with the invention can advantageously not only be applied to NO
x
-containing sulfuric acid, but with very good results also to mixtures containing NO
x
and sulfuric acid, such as nitrating acid or sulfuric acid contaminated with other compounds.


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Database WPI, Week 9115, Derwent Publications Ltd., London, GB; AN 108779, XP002027953, P.P. Kim et al., “Denitration of spent sulphuric acid.” & SU 1 586 996 A (Gorki Poly) Aug. 23, 1990; & Chemical Abstracts, vol. 113, No. 26, Dec. 24, 1990, Columbus, Ohio, US; Abstract No. 234134; P.P. Kim et al., “Method o denitration of spent sulfuric acid”.

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