NOx-separation         back to index       to range of washers 

    Products- Washer programme
  NOx separation from waste air  
   Method of recovery of raw materials from waste gas without any origin of waste
1. Process description
3. Regeneration process DGE
4. Example of the use
5. Literature
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
Velocity constant
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         back to index       to range of washers