Nitric acid (HNO3) HNO3 is also one of the largest bulk chemicals produced throughout the world. Major uses of nitric acid are in the production of fertilizers, explosives, specialty chemicals and as a precursor in nylon manufacture. HNO3 is manufactured as 60-62 wt% HNO3 in water. The three major steps in the commercial production of HNO3 are: a) oxidation NH3 to NO; 4 𝑁𝐻3 + 5 𝑂2 → 4 𝑁𝑂 + 6 𝐻2 𝑂, ∆𝐻 = −907 𝑘𝐽/𝑚𝑜𝑙 b) oxidation of NO to NO2; 2 𝑁𝑂 + 𝑂2 → 2 𝑁𝑂2 , ∆𝐻 = −113.8 𝐾𝐽/𝑚𝑜𝑙 c) Absorption of NO2 in water to form HNO3. 3 𝑁𝑂2 + 𝐻2 𝑂 ↔ 2 𝐻𝑁𝑂3 + 𝑁𝑂, ∆𝐻 = −1261 𝐾𝐽/𝑚𝑜𝑙 A side reaction in which NH3 is oxidized to N2 is also possible: 4 𝑁𝐻3 + 3 𝑂2 → 4 𝑁2 + 6 𝐻2 𝑂,
∆𝐻 = −1261 𝑘𝐽/𝑚𝑜𝑙
Oxidation of NH3 To improve selectivity toward NO, a catalyst, mixture of 90% Pt – 10% Rh, is chosen. This catalyst, as you can guess, is expensive. It is known that in the presence of catalyst, the side reaction proceeds slowly than the NO formation reaction. Choose the operating conditions for the above reactions. Think what conditions achieve high rates and less side products. A. B. C. D.
high temperature low temperature high residence time low residence time
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Select an appropriate reactor for NH3 oxidation to NO a) Fixed bed adiabatic reactor
b) Multi Tubular Reactor
c) Gauge reactor
in
in
in
Steam out
out
Shallow catalyst bed
Steam in out
out
What could be the primary concern in deciding composition of NH3–air mixture input into the reactor? A. Rate of main reaction B. Safety C. Side product formation UFL = 25 % NH3, LFL = 16 % NH3 for the feed mixtures. Beyond the limits, that is mixtures having >25% or < 16% do not explode. Between the limits, the mixtures are highly inflammable. Typical feed is approximately 10% NH3.
SAFETY IS UTMOST IMPORTANT IN CHEMICAL PLANTS
As an engineer, is distribution of feed mixture over catalyst bed a great concern to you? Why?
A small fraction of Pt vaporizes and is lost during the reaction (see next page). A fellow engineer suggests that the reactor operating temperature could be raised to 900 oC from 800 o C to improve the rate of reaction and conversion without significant increase in Pt loss. His 14
suggestion was followed but, the conversion did not rise. (Hint: Think about various steps involved in the reaction) 100 90 80
% Pt loss
70 60 50 40 30 20 10 0 600
700
800
900
1000
1100
1200
Temperature, oC
Compare the reactor configuration for H2SO4 oxidation and NH3 oxidation. List and explain similarities and differences.
Could we operate the reactor at > 25% NH3 in the feed mixture? What are the implications on rate of reaction, residence time, gas distribution etc.
The gases exiting the shallow catalyst bed are cooled. The heat recovered from the gases converts water to steam.
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Oxidation of NO The second main reaction in nitric production is oxidation of NO to NO2. This is one of the five known third-order-homogeneous-reactions. 2 −𝑟𝑁02 = 𝑘 𝑝𝑁𝑂 𝑝02
The reaction mechanism proposed for oxidation of NO is as follows: 2𝑁𝑂 𝑔 ↔ (𝑁𝑂)2 (𝑔) (𝑁𝑂)2 𝑔 + (𝑂)2 (𝑔) → 2𝑁𝑂2 (𝑔)
(𝑁𝑂)2 is an intermediate compound. It is assumed that the first reaction is fast and attains equilibrium quickly. The second reaction is slow and thus it is the rate controlling step. Write equation for equilibrium constant, 𝐾𝑐 , for (𝑁𝑂)2 formation: 𝐾𝑐 =
Derive equation for rate of 𝑁𝑂
2
𝑔 formation. Your equation should contain only 𝑁𝑂 .
Speculate why −𝑟𝑁02 is lower at higher temperatures.
Based on the kinetics of 𝑁𝑂2 formation, what is done to the gases exiting the ammonia oxidation reactor?
When the effluent gases are cooled to about 30-40 oC, under pressure, what is (are) expected to form? (A) 𝑁𝑂2
(B) HNO3
(C) Both
Is the produced HNO3 is dilute or concentrated?
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The steam exiting the heat exchanger (condenser) is a gas-liquid mixture; gas phase consists of predominantly NO2, little NO, and a little HNO3; liquid phase consists of HNO3 mostly. These two phases are separated. How do we handle the two separated phases? Look at the reactions listed at the beginning of this topic. Draw block diagram for nitric acid production
Absorption is favored at high pressures. Oxidation of NH3 and NO do not need high pressure; however, experiments indicate that high pressures give high selectivity toward NO. Should we adopt high pressure for the entire process or low pressure for oxidation and high pressure for adsorption? Both processes are used commercially as shown in next couple of pages.
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On the trays shown in absorbed what are the reactions happening in gas phase and liquid phase? Gas phase reaction: Liquid phase reaction:
What is the purpose of cooling the gas above trays? These trays are to be cooled by a refrigerant to achieve low temperatures. Suggest a coolant. Hint: In what state is NH3 supplied to the plant?
What is purpose of adding stripping air?
In the dual pressure process, vent gas is expanded while gas fed to the absorber is pressurized. Is there scope for energy conservation?
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Pollution abatement Even after cooling, some NOx escapes the absorption tower. NOx levels are brought down by installing a single, long absorption tower OR by adding a second tower in series with the first. This is called extended absorption. Alternatively, NOx levels are lowered below the threshold limit of ~200 ppm by catalyst reduction to N2. 4𝑁𝑂 + 4𝑁𝐻3 + 𝑂2 −→ 4 𝑁2 + 6 𝐻2 𝑂 ∆𝐻 = −408 𝑘𝐽/𝑚𝑜𝑙 2𝑁𝑂2 + 4𝑁𝐻3 + 𝑂2 −→ 3 𝑁2 + 6 𝐻2 𝑂 ∆𝐻 = −669 𝑘𝐽/𝑚𝑜𝑙
What could be one reason for which this reduction is attractive in nitric acid plants?
The catalyst for NOx reduction is expensive. A research article developed an adsorption system, using molecular sieves, to adsorb NOx from tail gas (i.e gas leaving from absorption tower) and recycling the adsorbed gases. Would you do to convince management about the system? A) concentration of NOx leaving the system B) cost of the system C) pay back period
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It turns out that tail gas (i.e gas leaving the absorption tower) cannot be heated to the desired temperature in the second heat exchanger after the ammonia oxidation reactor. Installing another heat exchanger achieves this temperature. An ingenious way to combine the heat exchange and reaction is proposed below. Think for yourself first and then discuss with your neighbor working principle and benefits of the proposal.
Feed
catalyst
Product
First half of cycle
Product
Feed
Second half of cycle
catalyst
Valve open
Inert beads
Valve closed
This portion is not included for the exams Absorption column in nitric acid plant is probably the most complex unit of its kind in chemical industry. The column achieves HNO3 production and NOx abatement simultaneously. NOx gases consist of NO, NO2, N2O3, N2O4, HNO2 and HNO3. The liquid phase contains two acids HNO2 and HNO3. NO has low solubility in water but it reacts with NO2 to form N2O3 which dissolves in water to form HNO2. Hence there are reactions occurring in gas phase, absorption accompanied by reactions happening in liquid phase. High temperature favors reactions in liquid phase but retard NO oxidation. High pressure promotes absorption and oxidation of NO but adds to the cost. It is possible to simulate nitric acid plant with all these complexities in APSEN or by writing a computer code. Simulations aid in optimizing the performance of the plant defined to minimize cost, emissions of NO etc.
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