simulation and optimization of emissions from nitric acid plant of ...

SIMULATION AND OPTIMIZATION OF EMISSIONS FROM NITRIC
ACID PLANT OF SHIRAZ PETROCHEMICAL COMPLEX
Mohammad Ali Farhadinia
Research Institiue of Petrolum Industry
ABSTRACT
Nitric acid, HNO3 , became a major industrial chemical with the development of the
explosives and dyestuffs industries at the end of the nineteenth century. Nowadays, nitric acid
production methods are based on the catalytic oxidation of ammonia and finally absorbtion
of nitric oxides. Basically absorbtion occurs in the absorbtion tower.
In general not only noxious effects of nitrogen oxides and the problems that Cause acid
rains but also the emissions that is visible from stacks, result in prevention of emissions below
the amount that is economical for many industries.
In Application America and Japan the upper limit concentration of exit NOx, (NO+NO2), is
about 200 ppmv. This limit is only possible in a high pressure absorber tower. Other methods
such as, chemical scrubing, molecular sieve absorbtion of NOx and catalytic reduction of exit
NOx are available but improve the production cost.
The best way to overcome this problem is to design an absorbtion tower that operates with
suitable cost.
In this paper the absorbtion tower of nitric acid plant of Shiraz petrochemical is simulated
with PRO/II package, then the effects of parameters such as: number of trays, operation
pressure and temperature, temperature and cooling water on the NOx emissions is inspected.
INTRODUCTION
In spite of the worldwide growth in the popularity of urea, nitric acid is still used in
substantial tonages for producing ammonium nitrate based fertilizers, and it has other
significant non-fertilizer uses, particulary in manufacturing explosives and a variety of
organic chemicals.
Today essentially thereis only one route for its commericial manufacture:
The catalytic oxidation of ammonia to nitrogen oxides followed by further oxidation of the
nitric oxide to nitrogen dioxide, which is absorbed in water to form nitric acid.
The three main reactions taking place in the production of nitric acid can be represented as
follows:
4 NH3 + 5O2 = 4NO + 6H2O (1)
2NO + O2 = 2NO2 (2)
3NO2 + H2O = 2HNO3 + NO (3)
In addition to these three basic reactions, some of the nitrogen dioxide dimerizes to
nitrogen tetroxide, (N2O4), and this is absorbed into water and reacts with it to form nitric
acid [1,2,3].
Absorbtion tower of Shiraz nitric acid plant consists of 33 sieve trays, process wateris
entered from first and third trays and process gas from bottom are intered. The weak acid that
is produced in cooler-condenser section also is interes themiddle trays. Because the reactions
are exothermic, cooling coils are installed to absorb excess heat [4].
INDUSTRIAL ABSORBTION OF NITROGEN OXIDES
In general two kinds of processes between gases and liquds can be considered: gas treating
processes such as CO2, H2S and HCl removal with absorbing solutions, and oxidation
processes such as chlorination in liquid phase. Absorbtion of nitrogen oxides takes place in
two steps. In nitric acid production, nitrogen oxides that result from ammonia oxidation are
absorbed in aqueous solution, this occurs in nonhomogeneus reactors that produce nitric acid,
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The next section of absorbtion tower is the gas treating section, the production of nitric acid
in this stage is very low.
Because of complicated phenomena such as electrolytic solutions, absorbtion with chemical
reactions and transfer phenomena, an emprical examination to optimize absorbtion tower
from NOx emissions and material consumption point of view[6].is needed.
OXIDATION AND ABSORBTION OF NITROGEN OXIDES IN HIGH PRESSURE
Nitric acid is produced from absorbtion and reaction of nitrogen dioxide (NO2), with water.
Absorbtion stage happens at ahigher pressure than oxidation of ammonia. Higher pressure and
lower temperature enhance the reaction. In absorbtion tower the following reactions occur:
NO + 1/2O2 NO2 (4)
2NO2 N2O4 (5)
N2O4 + H2O HNO2 + HNO3 (6)
3HNO2 HNO3 + 2NO + H2O (7)
By addition of the two last reactions, the production of nitric acid formula is gained:
3NO2 + H2O 2HNO3 + NO (8)
The equilibrium constants of this reaction are: [7]
K= (PNO * a 2 HNO3) / (P 3 NO2 * aH2O) = K1*K2
K1=F1 (gas composition in equilibrium) K2= F2 (concentration of acid in liquid phase)
P NO = partial pressure of NO in gas P NO2 = Partial pressure of NO2 in gas
a HNO = activity of acid in liquid phase
3 a H2O = activity of water in liquid phase
SIMULATION OF ABSORBTION TOWER
Absorbtion tower is simulated based on the following reactions:
NO + 1/2O2 NO2 (gas phase) (9)
2NO2 N2O4 (gas phase) (10)
3NO2 + H2O 2HNO3 + NO (gas and liquid phase) (11)
3/2N2O4 + H2O 2HNO3 + NO (gas and liquid phase) (12)
In this model Ten theoretical trays for production of acid and five for treating of the exit gas
are considered. Also theare considered in the seven lower trays, cooling coil for absorbing
excess heat are considered.
RESULTS OF SIMULATION AND OPTIMIZATION
EFFECT OF PRESSURE ON ABSORBTION AND NOx EMISSION
As mentioned, the important factor is the control of NOx emission. After absorbtion of
process gas, exit gas from the top of absorbtion tower has some quantity of nitrogen oxide and
nitrogen dioxide. According to the international standards the upper limit tar Nox emission is
200ppm that is equal to 0.02 percent.,In this condition the absortion efficiency is almost 100
percent. If we consider the stoichiometric equation of nitric acid production with respect to 50
percent excess air, we have 20 mole emission gas for every mole of acid produced,. Since we
have 400 kilomol acid per hour we will have 8000 kilomole emission gas per hour. Figure 1
shows that operating pressure above 9 kg/cm2 provides the desired limit.
EFFECT OF NUMBER OF TRAYS ON NOx EMISSION
Figure 2 shows the concentration of NOx versus number of theoretical trays. According to
this figure the concentration of NOx is reduced with increasing number of trays and after tray
number ten on wards is actually negligible.
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Figure.1 Effect of pressure NOx emission
Figure 2 Effect of number of Trays on NOx emission
EFFECT OF TEMPERATURE AND COOLING ON ABSORBTION
Oxidation rate of nitrogen monoxide is seriously dependeut on temperature,When the
temperature increases the rate of NO oxidation decreases (dv to enegative temperature
coefficient).
The design of tower is in such a manner that in different seasons according to the temperature
of process water temperature the emissiond NOx and produced acid Will be constant.
However Figure 3 shows that with respect to limitations of operation and temperature of
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process water, for every 1 MKCa l/hr cooling the tower, the production of acid will increase
about 3 kmol/hr. This means that the NOx emission will decrease by 3kmol/her.
Figure.3 Effect of cooling on HNO3 produced
CONCLUSION:
Simulation packages have their own advantages. With help of PRO/II package and aids of
simple assumptions and different sovrces, the absorbtion tower of nitric acid plant is
simulated. After that the effects of different operating parameters on NOx emission is
investigated. It is concluded that the most important parameter in controling the NOx
emission is the operating pressure.
REFERENCES:
1. BRADLY J.K. AND DRAKE G.,CHEMICAL AGE OF INDIA, JAN. 1982, VOL.33,
NO.1, PP.9-25.
2. . CUTIERREZ C., etal., COMPUTER AND CHEMICAL ENGINEERING, SEPT.1989,
VOL.13, NO.9, PP.985-1002.
3. . NITROGEN NO.188, NOVEMBER-DECEMBER 1990. (LIBRARY OF
PETROCHEMICAL COMPANY)
4. . OPERATIONAL MANUAL OF PLANT.
5. . PRO/II KEYWORD INPUT MANUAL.
6. . RIGGEL , S HANDBOOK OF INDUSTRIAL CHEMISTRY.
7. . ULLMAN , S ENCYCLOPEDIA OF INDUSTRIAL CHEMISTRY, 1991, A17 PP.293-
339.
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