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