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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 3 + 5 2 4 + 6 2, = 907 /

b) oxidation of NO to NO2; 2 + 2 2 2, = 113.8 /

c) Absorption of NO2 in water to form HNO3. 3 2 + 2 2 3 + ,

= 1261 /

A side reaction in which NH3 is oxidized to N2 is also possible:

4 3 + 3 2 4 2 + 6 2, = 1261 /

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. high temperature

B. low temperature

C. high residence time

D. low residence time

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in

out

out

in in

out

Shallow

catalyst

bed

Steam

out

Steam

in

Select an appropriate reactor for NH3 oxidation to NO

a) Fixed bed adiabatic reactor b) Multi Tubular Reactor c) Gauge reactor

What could be the primary concern in deciding composition of NH3air 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

oC to improve the rate of reaction and conversion without significant increase in Pt loss. His

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suggestion was followed but, the conversion did not rise. (Hint: Think about various steps

involved in the reaction)

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.

0

10

20

30

40

50

60

70

80

90

100

600 700 800 900 1000 1100 1200

% P

t lo

ss

Temperature, oC

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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.

02 = 2 02

The reaction mechanism proposed for oxidation of NO is as follows:

2 ()2 ()

()2 + ()2 () 22 ()

()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, , for ()2 formation: =

Derive equation for rate of 2 formation. Your equation should contain only .

Speculate why 02 is lower at higher temperatures.

Based on the kinetics of 2 formation, what is done to the gases exiting the ammonia

oxidation reactor?

When the effluent gases are cooled to about 30-40 o

C, under pressure, what is (are) expected

to form?

(A) 2 (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.

4 + 43 + 2 4 2 + 6 2 = 408 /

22 + 43 + 2 3 2 + 6 2 = 669 /

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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Feed Product Feed Product

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.

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.

catalyst

catalyst

Inert beads

First half of cycle Second half of cycle

Valve open

Valve closed