Lecture 16- Manufacture of nitric acid from chile saltpetre or nitrate HNO3.pdf

Manufacture of nitric acid from chile saltpetre or nitrate

1. Module 4
Lecture: 16 Nitric acid
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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% HNO 3, while the
solution containing more than 86% HNO 3, 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 s ynthesized 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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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


Block diagram of manufacturing process

Diagram with process equipment

Animation Cast Iron
Retort Furnace NaNO3 + H2SO4 HNO3 Conc. HNO3 Cooled Silica
Pipes Water or Dil. HNO3 Stoneware
Balls Dil. HNO3 Figure: Manufacture of nitric acid from chile saltpetre or nitrate Water
out Water in

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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 200
0
C 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 ΔH = + 43.2 kcals
2NO + O2 2NO2 ΔH = - 26.92 kcals
4NO2 + 2H2O + O2 4HNO3

Manufacture

Block diagram of manufacturing process

Diagram with process equipment Na2CO3
Solution Water 50 % Nitric
Acid Absorption
Towers Figure: Manufacturing of Nitric Acid by Arc Process Soda
Tower Na2CO3 Tower Air Cooling
water Electric Arc
Furnace Oxidation
Tower Boiler

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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 2000
0
C temperature
to form nitric oxide. The hot exit gases (1000
0
C) leaving the chamber is passed
through tube fire boiler for steam generation. The temperature of gases leaving the
boiler is significantly reduced up to 150
0
C. 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 1
st
tower are leave at the top.
Continuous counter current flow of gases in each tower is maintained by centrifugal
fan. The 3
rd
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 1
st
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 HNO 3 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 = 3000Nm
3

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 ΔH = - 216.6kcals ---- (1)
2NO+O2 2NO2 ΔH = - 27.1kcals ---- (2)

Side reactions
4NH3 + 3O2 2N2 + 6H2O ΔH = - 302.7kcals ---- (3)
2NH3 N2 + 3H2 ΔH = + 26.7 kcals ---- (4)
2NH3 + 2O2 N2O + 3H2O ΔH = - 65.9kcals ---- (5)
4NH3 + 6NO 5N2 + 6H2O + 432.25kcal ΔH = - 431.9kcals ---- (6)
Nitrous oxide oxidation and absorption
2NO+O2 2NO2 ΔH = - 27.1kcals ---- (7)
3NO2 + H2O 2HNO3+ NO ΔH = - 32.2kcals ---- (8)
2NO2 N2O4 ΔH = - 13.9kcals ---- (9)
2NO2 + H2O HNO3 + HNO2 ΔH = - 27.7kcals ---- (10)
HNO2 H2O + NO + NO2 ---- (11)

Manufacture
Nitric acid is made by the oxidation of ammonia, using platinum or platinum-
10% 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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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 g as passes downward in the catalyst zone
2.5 X 10
-4
sec and are heated at 800
0
C.

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 oxidizer-
absorber system. Air is added to convert NO to NO 2 at the more favourable
temperature (40-50
0
C) environment of the absorption system. The equipment in the Vaporizer Compresed
preheated air Water cooling Figure: Manufacturing of Nitric acid from by oxidation of ammonia Convertor Compressor Catalyst
Recovery
Filter 57-60 % HNO3 solution for use or
Concentrated to 95% HNO3 Air NH3
storage Heat
Recovery
Boiler Super
heater Converter
800
0
C Tail gas
heater Steam
Economiser Absorption
Tower Oxidation
Tower Water
cooling Water Condense Eapander Turbine Process Steam Exhaust
Gas Make up
Water Air Air Tail Gas

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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 57-
60% HNO3 solution which can be sold as or concentrated as follows

Concentration by H2SO4
Rectification with 93% H2SO4 (66
0
Be) 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 100
0
C 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)
2NO N2 + O2 ΔH = - 43kcal ---- (14)

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 600
0
C 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 500
0
C, while the reaction (14) are not favoured if
the process is carried out under 1200
0
C

 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 40-50
0
C. All the nitrogen oxide liberated on
absorption of NO2 must be reoxidized in absorption tower

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Absorption of nitrogen oxides into water
Following design criteria should be considered
 Rate of abortion depends on concentration of NO 2 in gas phase. In absorber
where concentration of NO 2 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/cm
2
. 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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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 : 121
0
C (68% HNO3 solution)
 Melting point : -42
0
C
 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.

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.