NOx-separation
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| Products-
Washer programme |
|
NOx separation from waste
air
Method of recovery of raw materials from waste gas without any
origin of waste |
|
|
| 2. Purification
process: |
| In the following
points they are appreciated exclusively methods of alkaline washing and
acid washing for recovery of nitric acid. |
|
| 2.1 Alkaline washing
: |
| Waste gases containing
NOx are often purified by alkaline washing with the use of caustic soda
lye, caustic potash lye or a mixture with hydrogen peroxide. The use of
this methods results in a consequent high salt concentration in liquidated
waste water. In alkaline washing with the use of NaOH the following reactions
occur for example: |
| (4) |
2 NO2 |
+ 2 NaOH |
« |
NaNO3 + NaNO2
+ H2O |
+ 55.400 cal |
| (5) |
NO + NO2 |
+ 2 NaOH |
« |
2 NaNO2 + H2O |
+ 45.100 cal |
| (6) |
2 N2O3 |
+ 2 NaOH |
« |
2 NaNO2 + H2O |
+ 35.500 cal |
|
|
| In addition to
it, in these reactions it is necessary to take into consideration consequently
the formation of nitric acid or eventually of nitrous acid and also neutralization
heat liberation of these acid caused by alkali. |
| (7) |
NaOH . 100 H2O |
+ HNO3 . 100 H2O |
|
+ 13.840 cal |
| (8) |
NaOHl |
+ HNO2,l |
|
+ 11.100 cal |
|
|
| 2.2 Nitric acid
recovery : |
| At nitric acid recovery
very complex, mutually overlapping reactions take place. It does not make
it possible to interpret and determine the process. For single components
it is possible to assume the following reaction scheme: |
| (9) |
3 NO2 |
+ |
H2O |
« |
2 HNO3 |
+ |
NO |
|
|
D HR
= + 32.530 cal |
| (10) |
N2O4 |
+ |
H2O |
« |
HNO3 |
+ |
NHO2 |
|
|
D HR
= + 14.130 cal |
| (10a) |
3 N2O4 |
+ |
2 H2O |
« |
4 HNO3 |
+ |
2 NO |
|
|
|
| (11) |
N2O3 |
+ |
H2O |
« |
2 NHO2 |
|
|
|
|
D HR
= + 13.300 cal |
| (12) |
3 HNO2 |
|
|
« |
HNO3 |
+ |
2 NO |
+ |
H2O |
D HR
= - 18.130 cal |
|
|
|
If the process conditions are adjusted in such a way
that amount of N2O3 is negligible and it is not
necessary that be taken into consideration the computation is made often
according to equation (9). It is clear that at nitric
acid formation an amount of oxygen of 1.5 times higher shall oxidize
than it is necessary for one cycle.
It is the case when it is necessary to assess nitric acid formation
in accordance with equations (10) a (10a).
It is clear that at nitric acid recovery the equation (1)
with NO formed represents a stage determining for velocity. It is important
particularly at realization and maintaining low residual NOx concentrations. |
|
| 2.2.1 Methods
of acceleration of NO oxidation reaction |
(13)
2NO
+ 3H2O2
®
2 HNO3 + 2H2O |
| It shows that using
this reaction it is possible to accelerate NO oxidation time. |
|
| The acceleration
of NO oxidation reaction with the help of hydrogen peroxide has been investigated
extensively /5,6/ When compared to reactions
in NO-NO2-H2O system markedly faster reaction mechanisms have been proved.
/7/ have developed a model at which a
catalytic acceleration of the reaction takes place caused by elimination
of radicals in liquid phase. Also /8/
already have proven the catalytic acceleration of these reactions in liquid
phase. |
|
| /3/
in addition have proven successfully the catalytic acceleration in comparison
with homogeneous reaction in gas phase according to equation (1).
As appropriate catalysts are for example: special activated charcoal,
silica gel, molecular sieves, vanadium and chrome – zinc catalysts.
The following results have been obtained for example: |
| Process
|
Temperature
°C |
Velocity
constant
Minutes |
| Homogeneous
reaction in gas phase |
25 |
0,66 x 106 |
|
Catalytic reaction on activated carbon
|
25 |
0,6 x 109 |
|
| It corresponds to
a faster conversion, with a factor of about 1000. A disadvantage of the
use of activated carbon and silica gel consists in the fact that as a
consequence of a high portion of water vapour at nitric acid production
poisoning of the catalyst occurs. |
|
|
|
NOx-separation
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