Various uses for nitric acid are given in Problem \(6.43,\) along with
information about how this important chemical is synthesized industrially. The
key reactions are oxidations of ammonia to nitric oxide and of nitric oxide to
nitrogen dioxide, followed by dissolution of \(\mathrm{NO}_{2}\) in water:
$$\begin{aligned}
4 \mathrm{NH}_{3}(\mathrm{g})+5 \mathrm{O}_{2}(\mathrm{g}) & \rightarrow 4
\mathrm{NO}(\mathrm{g})+6 \mathrm{H}_{2} \mathrm{O}(\mathrm{v}) \\
2 \mathrm{NO}(\mathrm{g})+\mathrm{O}_{2}(\mathrm{g}) & \rightarrow 2
\mathrm{NO}_{2}(\mathrm{g}) \\
3 \mathrm{NO}_{2}(\mathrm{g})+\mathrm{H}_{2} \mathrm{O}(1) & \rightarrow 2
\mathrm{HNO}_{3}(\mathrm{aq})+\mathrm{NO}(\mathrm{g})
\end{aligned}$$
Nitric oxide generated on dissolution of \(\mathrm{NO}_{2}\) in water is
oxidized to produce additional \(\mathrm{NO}_{2},\) which is then combined with
water to form more \(\mathrm{HNO}_{3}\). In this problem we neglect side
reactions that would lower the product yield.
Ammonia vapor at \(275^{\circ} \mathrm{C}\) and 8 atm is mixed with air, also at
\(275^{\circ} \mathrm{C}\) and 8 atm, and the combined stream is fed to a
converter. Fresh air entering the system at \(30^{\circ} \mathrm{C}\) and 1 atm
with a relative humidity of \(50 \%\) is compressed to \(100^{\circ} \mathrm{C}\)
and 8 atm, and the compressed air then exchanges heat with the product gas
leaving the converter. The quantity of oxygen in the feed to the converter is
\(20 \%\) in excess of the amount theoretically required to convert all of the
ammonia to \(\mathrm{HNO}_{3}\). The entire process after the compressor may be
taken to operate at a constant pressure of 8 atm.
In the converter, the ammonia is completely oxidized, with a negligible amount
of \(\mathrm{NO}_{2}\) formed. The product gas leaves the converter at
\(850^{\circ} \mathrm{C}\), and, as described in the preceding paragraph,
exchanges heat with the air entering the system. The product gas then is fed
to a waste-heat boiler that produces superheated steam at \(200^{\circ}
\mathrm{C}\) and 10 bar from liquid water at \(35^{\circ} \mathrm{C}\). The
product gas leaving the wasteheat boiler is cooled further to \(35^{\circ}
\mathrm{C}\) and fed to an absorption column in which the NO is completely
oxidized to \(\mathrm{NO}_{2},\) which in turn combines with water (some of
which is present in the product gas). Water is fed to the absorber at
\(25^{\circ} \mathrm{C},\) at a rate sufficient to form a 55 wt\% aqueous nitric
acid solution. The NO formed in the reaction of \(\mathrm{NO}_{2}\) to produce
\(\mathrm{HNO}_{3}\) is oxidized, and the NO \(_{2}\) produced is hydrated
to form still more \(\mathrm{HNO}_{3}\). The off-gas from the process may be
taken to contain only \(\mathrm{N}_{2}\) and \(\mathrm{O}_{2}\)
(a) Construct a flowchart showing all process streams, including input and
output from the process and the following equipment: converter, air
compressor, exchanger recovering heat from the converter product, waste-heat
boiler producing superheated steam, exchanger cooling the product gas before
it is fed to the absorber, and absorber.
(b) Taking a basis of \(100 \mathrm{kmol}\) of ammonia fed to the process,
develop spreadsheets (preferably incorporating the use of APEx) to determine
the following:
(i) Molar amounts (kmol) of oxygen, nitrogen, and water vapor in the air fed
to the process, cubic meters of air fed to the process, and kmol of water fed
to the absorber.
(ii) Molar amounts, molar composition, and volume of the off-gas leaving the
absorber.
(iii) Mass (kg) of product nitric acid solution.
(iv) Molar amounts and composition of the gas leaving the converter.
(v) Heat removed from or added to (state which) the converter.
(vi) Temperature of the product gas after it has exchanged heat with the air,
assuming no heat is transferred between the heat exchanger and the
surroundings.
(vii) Production rate of superheated steam if the gas temperature leaving the
boiler is \(205^{\circ} \mathrm{C}\). Before performing this calculation,
determine if condensation of water occurs when the gas is cooled to
\(205^{\circ} \mathrm{C}\). Since the superheated steam temperature is
\(200^{\circ} \mathrm{C}\), explain why the selected temperature of the product
gas is reasonable.
(viii) Heat removed from the product gas before it is fed to the absorber
(Hint: Check the condition of the gas at \(35^{\circ} \mathrm{C}\) ) and mass of
cooling water required to remove that heat if the water temperature can only
be increased by \(5^{\circ} \mathrm{C}\). Assume no heat is transferred between
the heat exchanger and the surroundings.
(ix) Heat removed from or added to the absorber. Assume the heat capacity of
the nitric acid solution is approximately the same as that of liquid water and
the outlet temperatures of the off-gas and product streams are \(25^{\circ}
\mathrm{C}\) and \(35^{\circ} \mathrm{C}\), respectively.
(c) Scale up the results calculated in Part (b) to determine all stream flow
rates and heat transfer rates
for a production rate of \(5.0 \times 10^{3} \mathrm{kg} / \mathrm{h}\) of the
product solution.
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