Module 4
Lecture: 16 Nitric acid
Dr. N. K. Patel
Module: 4
Lecture: 16
NITRIC ACID
INTRODUCTION
Nitric acid (HNO3), also known as aqua fortis (strong water) and spirit of niter,
is a highly corrosive strong mineral acid. The pure compound is colourless, but older
samples are yellowish in colour due to the accumulation of oxides of nitrogen.
Commercially available nitric acid having concentration of 68% HNO3, while the
solution containing more than 86% HNO3, is referred to as fuming nitric acid.
Depending on the amount of nitrogen dioxide present, fuming nitric acid is further
characterized as white fuming nitric acid or red fuming nitric acid, at concentrations
above 95%.
First Nitric acid was mentioned in Pseudo-Geber's De Inventione Veritatis
which is prepared by calcining a mixture of saltpetre (Niter KNO3), alum and sulfuric
acid. Also, described by Albertus in the 13th century and by Ramon Lull, who
prepared it by heating niter and clay and called as "eau forte" (aqua fortis).
Glauber invent the process to obtain HNO3 by heating niter with strong sulfuric
acid. In 1776 Lavoisier showed that it contained oxygen, and in 1785 Henry
Cavendish determined its precise composition and synthesized it by passing a
stream of electric sparks through moist air.
MANUFACTURE
Nitric acid is manufactured by three methods.
1. From Chile saltpetre or nitrate
2. Arc process or Birkeland and eyde process
3. Ostwald's process or Ammonia oxidation process
1. From Chile saltpeter or nitrate
It is the first commercial process of manufacture of nitric acid from sodium
nitrate extracted from Chile saltpeter. The process is now become obsolete since
second decade of nineteenth century.
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Dr. N. K. Patel
Raw materials
Basis: 1000kg Nitric acid (95% yields)
Sodium Nitrate
= 1420kg
Sulfuric acid
= 1638kg
Sources of raw material
Sulfuric acid can be obtained by contact process as described in Module: 4,
Lecture: 18.
Sodium nitrate can be obtained from caliche ore. Also, it is manufactured by
neutralization of soda ash with nitric acid as well by reaction of ammonium nitrate
and sodium hydroxide.
Reaction
NaNO3 + H2SO4
NaHSO4 + HNO3
Manufacture
Cooled Silica
Pipes
Cast Iron
Retort
Water
out
Water in
NaNO3 + H2SO4
Furnace
Water or Dil. HNO3
HNO3
Stoneware
Balls
Conc. HNO3
Dil. HNO3
Figure: Manufacture of nitric acid from chile saltpetre or nitrate
Block diagram of manufacturing process
Diagram with process equipment
Animation
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Dr. N. K. Patel
Equal weight of sodium nitrate (or potassium nitrate) and sulfuric acid is
charged to cast iron retort having outlet provided at bottom to take out solution of
sodium bisulfate. The reactants are heated to about 2000C by the hot furnace
gases. The furnace gases are produced by combustion of coal in the furnace. Then
the vapour of nitric acid are cooled and condensed in water cooled silica pipes.
The cooled acid is collected in stoneware receiver. The un-condensed vapours are
scrubbed with water in absorption tower which is packed with stone ware balls and
cooled by cold water. The dilute acid is re-circulated till it becomes concentrated.
The residual sodium bisulfate is removed by outlet provided at the bottom of retort.
2. Arc process or Birkeland and eyde process
Raw materials
Basis: 1000kg Nitric acid (98% yield)
Air
= 198kg
Water = 145kg
Reaction
N2 + O2
2NO
2NO + O2
2NO2
4NO2 + 2H2O + O2
ΔH = + 43.2 kcals
ΔH = - 26.92 kcals
4HNO3
Manufacture
Na2CO3
Solution
Water
Oxidation
Tower
Cooling
water
Air
Boiler
Electric Arc
Furnace
Absorption
Towers
Soda
Tower
Na2CO3 Tower
50 % Nitric
Acid
Figure: Manufacturing of Nitric Acid by Arc Process
Block diagram of manufacturing process
Diagram with process equipment
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Lecture: 16 Nitric acid
Dr. N. K. Patel
Animation
Air freed from CO2 and moisture is passed through electric arc chamber
having two copper electrodes which are continuously circulated by cold water and
are connected with AC dynamo. A powerful electromagnet placed at right angles
to the electrodes spreads the arc in the form of a disc. The chamber is also provided
with inside suction pumps for rapid circulation of air across the flame through holes
of refractory fire work. Nitrogen and oxygen of air combines at 20000C temperature
to form nitric oxide. The hot exit gases (10000C) leaving the chamber is passed
through tube fire boiler for steam generation. The temperature of gases leaving the
boiler is significantly reduced up to 1500C. The gases are allowed to pass through
oxidation chambers made of iron and lined inside with acid proof stone. Here, nitric
oxide is further oxidizing to nitrogen peroxide in presence of air. The exit gases from
oxidation towers are passed through series of absorption tower filled with broken
quartz through which cold water or dilute nitric acid is continuously sprayed from
top. The gases which enter from the base of 1st tower are leave at the top.
Continuous counter current flow of gases in each tower is maintained by centrifugal
fan. The 3rd tower is fed with cold water and the dilute nitric acid is collected at the
base is re-circulated to the top of the preceding tower. 50% HNO3 is obtained at the
base of 1st tower. The gases leaving the last absorption tower contains traces of
nitrogen oxides. The gases are allowed to pass through two wooden towers which
are sprayed down by dilute solution of soda ash. The solution at the base of sodium
carbonate tower is evaporated to collect crystal of sodium nitrate.
Engineering aspects
The conversion of NO to HNO3 was carried out by means of oxidation and
hydration processes which is same as product obtained from oxidation of ammonia
Reason for obsolesce
High electrical energy consumed. There were enormous amounts of gas in
circulation compared to low concentration of NO which was formed (about 2%) on
account of the fact that high temperature also promote the reverse dissociation
reaction.
3. Ostwald's process or Ammonia oxidation process
Raw Materials
Basis: 1000kg nitric acid (100%)
Ammonia
= 290kg
Air
= 3000Nm3
Platinum
= 0.001kg
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Water
= 120000kg
Steam credit = 1000kg @ 200psig
Power
= 10-30KWH
Sources of raw material
Ammonia can be synthesized by Haber – Bosch or Modern process as
described in Module: 2, Lecture: 6.
Reaction
Major reactions
4NH3 + 5O2
4NO + 6H2O
2NO+O2
2NO2
ΔH = - 216.6kcals
---- (1)
ΔH = - 27.1kcals
---- (2)
ΔH = - 302.7kcals
---- (3)
ΔH = + 26.7 kcals
---- (4)
ΔH = - 65.9kcals
---- (5)
ΔH = - 431.9kcals
---- (6)
ΔH = - 27.1kcals
---- (7)
ΔH = - 32.2kcals
---- (8)
ΔH = - 13.9kcals
---- (9)
ΔH = - 27.7kcals
---- (10)
Side reactions
4NH3 + 3O2
2NH3
2N2 + 6H2O
N2 + 3H2
2NH3 + 2O2
N2O + 3H2O
4NH3 + 6NO
5N2 + 6H2O + 432.25kcal
Nitrous oxide oxidation and absorption
2NO+O2
2NO2
3NO2 + H2O
2NO2
N2O4
2NO2 + H2O
HNO2
2HNO3+ NO
HNO3 + HNO2
H2O + NO + NO2
---- (11)
Manufacture
Nitric acid is made by the oxidation of ammonia, using platinum or platinum10% rhodium as catalyst, followed by the reaction of the resulting nitrogen oxides
with water.
Block diagram of manufacturing process
Diagram with process equipment
Animation
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Dr. N. K. Patel
Process Steam
Vaporizer
Tail Gas
NH3
storage
Water cooling
Converter
800 0C
Heat
Recovery
Boiler
Convertor
Turbine
Tail gas
heater
Catalyst
Recovery
Filter
Steam
Economiser
Air
Air
Compressor
Eapander
Air
Exhaust
Gas
Absorption
Tower
Super
heater
Oxidation
Tower
Compresed
preheated air
Water
cooling
Make up
Water
57-60 % HNO3 solution for use or
Concentrated to 95% HNO3
Water Condense
Figure: Manufacturing of Nitric acid from by oxidation of ammonia
The process involves four steps
1. Catalytic oxidation of ammonia with atmospheric oxygen to yield nitrogen
monoxide
2. Oxidation of the nitrogen monoxide product to nitrogen dioxide or dinitrogen
tetroxide
3. Absorption of the nitrogen oxides to yield nitric acid
4. Concentration of nitric acid
Compressed air is mixed with anhydrous ammonia, fed to a shell and tube
convertor designed so that the preheater and steam heat recovery boiler-super
heater are within the same reactor shell. The convertor section consists of 10-30
sheets of Pt-Rh alloy in the form of 60-80 mesh wire gauge packed in layers inside the
tube. Contact time and of the gas passes downward in the catalyst zone
2.5 X 10-4sec and are heated at 8000C.
Product gases from the reactor which contain 10-12% NO, are sent through
heat recovery units consisting of heat recovery boiler, super heater and quenching
unit for rapid cooling to remove large fraction of product heat, and into the oxidizerabsorber system. Air is added to convert NO to NO2 at the more favourable
temperature (40-500C) environment of the absorption system. The equipment in the
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Dr. N. K. Patel
absorption train may be series of packed or sieve tray vertical towers or a series of
horizontal cascade absorbers. The product from this water absorption system is 5760% HNO3 solution which can be sold as or concentrated as follows
Concentration by H2SO4
Rectification with 93% H2SO4 (660Be) in silicon-iron or stoneware tower
produces concentrated nitric acid and 70% H2SO4 which can be re-evaporated to
93% H2SO4 or used as it is.
Concentration by Mg(NO3)2
Magnesium nitrate solution containing 70-75% Mg(NO3)2 is fed to dehydrating
tray along with dilute HNO3 from the absorption tower. The salt solution acts as an
extractive distillation agent, removing water at 1000C or higher, thus allowing
rectification with azeotropic formation. The dilute Mg(NO3)2 solution re-concentrated
by evaporation
Advantages
 Operating cost is half compare to H2SO4 process
 Acid quality and yield improved
Disadvantage
 Increase in 70% capital expenditure
Engineering aspects
Thermodynamics and kinetics
4NH3 + 3O2
2N2 + 6H2O
ΔH = - 302.64kcal
---- (12)
4NH3 + 6NO
5N2 + 6H2O
ΔH = - 432.25kcal
---- (13)
ΔH = - 43kcal
---- (14)
2NO
N2 + O2
All the above exothermic reaction takes place in more or less extent.
Reaction 12 and 13 occurs with decrease in enthalpy with increase in number of
moles followed by increase in entropy.
4NH3 + 5O2
4NO + 6H2O
Ammonia oxidation reaction has an extremely favourable equilibrium
constant so that one step, high temperature converter design may be used.
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Further, ammonia air mixture exhibit explosion limits. At STP it is 15.6%
ammonia, while temperature above 6000C and 1atm pressure, the limit is lowered to
10.5%
The following condition should be fulfilled to convert NH3 into NO
 Explosion limit
The explosion limits are avoided by employing quantity of air such that the
amount of ammonia mixed with it is less than 10.5vol% of total volume.
 Thermodynamics
The thermodynamics of competing reactions (12) and (13) are rendered
unfavourable by working above 5000C, while the reaction (14) are not favoured if
the process is carried out under 12000C
 Kinetics
Kinetics of reaction (1) is speeded up by use of catalyst. This is also done by
preventing any reduction in the velocity of the reaction brought about by presence
of inert gas nitrogen in the reaction zone.
Reaction kinetics in ammonia oxidation stage
 Rate of reaction is directly proportional to system pressure
 Alloying of platinum with rhodium improves yield at given set of conditions
 Reaction to form NO is favoured by increasing temperature until an optimum
is reached which increases with higher velocities. This results from the
prevention of back diffusion of NO into higher NH3 concentration region. If this
occurs the following reaction is quite probable and should be avoided for
high NO yield.
4NH3 + 6NO
5N2 + 6H2O
 Rate of NO formation very nearly corresponds to diffusional transport of
ammonia molecules to the catalyst surface
There is slight equilibrium advantage to operation at atmospheric pressure.
This is more than offset by increased capacity in a given reactor volume with
subsequent catalyst and reactor savings when operating high pressures (3-8atm.)
Oxidation of nitrogen oxide does not have as large equilibrium constant.
There so, the reaction predominates in water and absorption portions of the process,
which operates at low temperature at 40-500C. All the nitrogen oxide liberated on
absorption of NO2 must be reoxidized in absorption tower
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Dr. N. K. Patel
Absorption of nitrogen oxides into water
Following design criteria should be considered
 Rate of abortion depends on concentration of NO2 in gas phase. In absorber
where concentration of NO2 is greater than 5%, the controlling reaction is
solution of N2O4 accompanied by hydrolysis of HNO3 and HNO2.
 Low temperature is beneficial for absorber operation efficiency
 Increasing pressure favours physical absorption rate and shift chemical
equilibrium to produce higher acid strength
Process design modification
Most plants operate at higher pressure (3-8atm) rather than complete
atmospheric pressure. Some operates at a combination of 1atm pressure oxidation
and high pressure absorption. Very high pressure is limited due to cost of pressure
vessel.
Advantages and disadvantages of elevated pressure are as follows
Advantages
 Higher acid strength
 Lower investment cost
 Higher reaction rate and lower volume in both oxidation and absorption
equipment
Disadvantages
 Lower oxidation yield
 Higher power require if recovery units are not specified
 Higher catalyst loss unless good catalyst recovery procedure are not used
Catalyst for oxidation of ammonia
Platinum/rhodium alloy containing 10% rhodium is the only industrially viable
catalyst. Rhodium not only improves the catalytic properties of platinum but also
improves mechanical and anti-abrasive properties of material under the operating
condition such as to counter the severe corrosion and oxidation atmosphere. 4–10 %
of rhodium used in Pt/Rh supported catalyst. Higher efficiencies and smaller platinum
losses can be achieved by knitted gauzes.
The metallic alloy catalyst is prepared into very fine threads of diameter
0.05mm which are woven into meshes with more than 1000stiches/cm2. Two to four
or even more of these meshes are placed on top of one another inside the reactors
when these are put into operation.
Catalyst threads are smooth, bright and less active at initial stage, as the time
progresses they becomes dull and wrinkled whereupon their activity rises to the
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Dr. N. K. Patel
maximum. Finally they become spongy with activity falling off. When it is in most
active state, ammonia oxidation yields up to 98% of NO are obtained.
Ammonia conversion efficiency is a function of pressure and temperature. As
the pressure increases, higher temperatures are needed to obtain the high
conversion efficiency. An increased flow rate and the presence of several layers of
the catalyst help to minimize undesirable side reactions. However, high flow rates
increase the catalyst loss which leads to search for non-platinum catalysts for
ammonia oxidation. The most prospective non-platinum catalysts are based on
oxides of Co, Fe or Cr.
Catalyst poison
Sulfates, H2S, chlorides, Arsenic and its oxide, Si, P, Pb, Sn and Bi are
permanently poisoning the catalyst. These elements lead to the formation of
inactive compounds in the wires resulting in decreasing of the catalytic activity.
Traces of acetylene, ethylene, Cr, Ni and Fe temporarily reduce the conversion
efficiency which can be restored by treatment with HCl. There so air should be freed
from all above impurities along with suspended particles of lubricants, fats, fine dust
and abrasive powder. Also, suspension of Fe2O3 from ammonia is removed. For that
efficient filtration system along with magnetic separators are provided.
PROPERTIES
Physical Properties
Molecular formula : HNO3
Molecular weight : 63.013gm/mole
Appearance
: Colourless liquid
Odour
: Pungent
Boiling point
: 1210C (68% HNO3 solution)
Melting point
: -420C
Density
: 1.5129gm/mL (liquid)
Solubility
: Miscible with water in all proportions
The impure nitric acid is yellow due to dissolved oxides of nitrogen, mainly
NO2.
 It has a corrosive action on skin and causes painful blisters.
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Chemical Properties
 Acidic properties: It is a strong monobasic acid and ionization in aqueous
solution.
 Oxidizing properties: It acts as a powerful oxidizing agent, due to the
formation of nascent oxygen.
 Action on metals: It reacts with almost all the metals, except noble metals, like
Pt and Au. The metals are oxidized to their corresponding positive metal ions
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while HNO3 is reduced to NO, NO2. N2O, NH2OH or NH3, depending upon the
conditions such as temperature, nature of metal and concentration of the
acid.
 Nitric acid has ability to separate gold and silver.
USES
 As a starting material in the manufacture of nitrogen fertilizers such as
ammonium nitrate, ammonium phosphate and nitrophosphate. Large
amounts are reacted with ammonia to yield ammonium nitrate.
 Weak acid are used to digest crude phosphates.
 As a nitrating agent in the preparation of explosives such as TNT,
nitroglycerine, cellulose polynitrate, ammonium picrate
 In manufacture of organic intermediates such as nitroalkanes and
nitroaromatics.
 Used in the production of adipic acid.
 Used in fibers, plastics and dyestuffs industries
 Used in metallurgy and in rocket fuel production
 As the replacement of sulfuric acid in acidulation of phosphate rock.
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