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O.N. Bliznjuk, N.F.Kleshchev, A.N. Ogurtsov,
National Technical University
"Kharkiv Polytechnic Institute"
MODELING THE CATALYTIC
REDUCING OF N2O AMOUNT
IN NITRIC ACID PRODUCTION
Currently, the only
industrial process for producing nitric acid is the catalytic oxidation of
ammonia with oxygen from the air, followed by absorption of nitrogen oxides
produced by water. The stage of NH3 change ratio largely determines
the performance of all nitric acid production. The conditions and the quality
of its implementation determine the consumption indices of NH3,
input and losses of platinum group metals (PGM) and energetic capabilities of
process conditions.
The studies conducted
[1–5] justify the assumption about the mechanism of the ammonia catalytic
oxidation on platinoid gauzes, give a kinetic model of the reaction, and allow
us to determine the conditions of maximum NO output. As the analysis of the
bibliography and our experimental research has shown, to ensure the maximum NO
selectivity and high intensity of the process it is necessary to maintain
external diffusion conditions. At low NH3concentrations of the
catalyst surface, which is typical for external diffusion conditions, the rates
of its oxidation to N2O and N2 decrease dramatically and
NH3 selective oxidation to NO takes place.
One of the shortcomings
of previous studies and previously-developed models is the lack of data on the
nitrogen oxide formation developed as a byproduct by NH3 catalytic
oxidation in the HNO3 manufacture process. Nitrogen oxide passes
through the remaining stages of the HNO3 production technology
unchanged and then is released into the atmosphere. The facilities for the
nitric acid manufacture in the CIS countries currently emit about 6.0–7.5 kg of
N2O per year per ton of HNO3. According to the latest
research N2O is a strong greenhouse gas, whose global warming
potential is 310 times greater than the one of CO2.To create a
mathematical model of the ammonia oxidation to NO with due consideration of N2-
and N2O-synthesis, a complex of physicochemical, kinetic and
technological research has been carried out.
Taking to account the
formation of nitrogen (I) oxide for a PGM-catalyst, the stoichiometric basis of
ammonia oxidation reaction routes, has the following form:
|
4NH3 + 5O2 |
(1) |
|
4NH3 + 4O2 |
(2) |
|
4NH3 + 3O2 |
(3) |
|
2NO |
(4) |
Modelling of the process
is reduced to a mathematical description of the process in the catalyst bed
considering the assumed reaction mechanism. At a high temperature oxygen on the
platinum surface is mainly in the dissociated state. The ammonia adsorption, as
well as its dissociation and formation of intermediates interacting with each
other, leads to the formation of NO, N2O, N2, Í2Î
and desorption from the free catalyst surface at 973–1193 K. In the process of
catalysis the ammonia acts as an electron-donor, and oxygen – as an
electron-acceptor. Adsorbed Î2 molecules attach electrons from the
catalyst surface and turn into oxygen surface atoms. NH3 molecule
donates electrons from the catalyst surface which is not covered with oxygen,
N-H bonds are thus weakening and the imido-particles are being formed. On the
catalyst surface coated with oxygen, the ammonia forms particles of the
imide-type, which are then recombining and form the nitrogen. Further addition
of oxygen to the imide leads to the formation of NO. Other steps are connected
with N2O, N2 and Í2Î formation.
The proposed detailed
mechanism of the process has been used for deriving kinetic equations, which
describe the reactions on the catalyst surface and relate to the formation of
O, N2O, N2.
The process in the catalyst bed for
external diffusion condition is characterized by a large temperature gradient.
At a small distance - a few centimeters in front of the catalyst – the gas is
heated by the amount of the adiabatic heating of the reaction mixture from 473
to 1173 K. This can lead to significant longitudinal heat and mass transfer.
However, the effective coefficients of longitudinal diffusion and thermal
conductivity of the gas phase are small and longitudinal heat transfer is
carried out mainly along the solid skeleton of the catalyst bed. When
describing the process the influence of the flows arising in non-isothermal
boundary layer of multi-component mixture - Stefan flow, thermal diffusion, as
well as diffusion thermal conductivity can be neglected, since the volume of
the reaction mixture does not vary by more than 10%, and it is highly diluted
with inert gas N2; molecular weights of components and, thus, their
diffusion coefficients differ insignificantly.The temperature of the catalyst
in the bed of PGM-gauzes is assumed to be constant and is determined by the
adiabatic heating of the coming reaction mixture. On this basis, to describe
the process, a model of plug-flow for the gas phase is accepted with the use of
material balance equations for each component.
The degree of NH3 oxidation on
PGM-catalysts was calculated according to the developed mathematical model. For
calculating this process it is necessary to know the coefficients of heat and
mass transfer from the gas flow to the catalyst, as well as the physical and
chemical properties of the mixture, depending on the temperature, pressure and
composition of the mixture.
By solving the inverse problem of chemical kinetics,
using the experimental data, we determined reactions
rates constants connected with the formation of NO (k1), N2O
(k2), N2 (k 3) and N2, resulting
from the nitrogen (²²) oxide
decomposition (k4), and the concentrations of the substances on
the catalyst surface. The comparison of calculated and experimental conversion
coefficients shows good agreement, the difference is not more than 1–3%.
A mathematical model of the oxidation process of NH3,
considering the physical and chemical properties of the ammonia-air mixture and
nitrous gas has been developed. The rate constants and their temperature
dependence have been determined. This will allow to determine the optimal process
conditions of NH3 oxidation on the PGM-catalyst at different
pressures in a wide range of process parameters, as well as to determine the
amount of nitrogen oxides N2O formed. On the basis of the developed
model, a program for calculating the NH3 oxidation reactor using
PGM-catalysts of different geometric profile has been created. During the
operation process, the volume and the surface of the catalyst are changed. Thus
the mathematical model for the rates of ammonia oxidation steps per unit mass
of PGM-catalyst has been developed.
REFERENCES
[1] Savenkov À.S.:
Catalyticheskiesvoistvapoverhnostiplatiny v
reakciiîkisleniaamiakaimatematicheskoemodelirovanieprocessa [in:] A. Loboiko
(Ed.), Catalyticheskieimassoobmenyeprocessy pod davleniem v
technologiineorganicheskihveshchestv, Kharkov, Osnova 1993, 46-63.
[2] Savenkov À.S., Beskov V.S.:
Êineticaprocessaokisleniaammiakanaplatinovomkatalizatore pod davleniem [in:]
V.I.Atroshchenko (Ed.), Êineticageterogenno-kataliticheskihprocessov pod
davleniem, Kharkov, Vishchashkola 1974, 67–83.
[3] VyatkinJu.L., Savenkov À.S., Beskov V.S.:
KhimicheskajaPromishlennost, 1979, 10,
618.
[4] Bliznjuk Î.N., Savenkov À.S., MasalitinaN.Yu.,
Ogurtsov A.N.: KhimichnaPromyshlovist Ukraine, 2010, 3(98), 3.
[5] Bliznjuk Î.N., Savenkov À.S., Ogurtsov A.N.,
Chlopitskii A.A.: VoprosyKhimiiiKhimichnoitechnologii, 2012, 6, 92.