Chemical Process technology
The production of nitrogen is a major branch of the fertilizer industry and it opens up a most important segment of the chemical industry.
Pure nitrogen may be obtain by separation from air by liquid air distillation.
By consumption of the oxygen of air by burning of fuel, which leaves nitrogen residue.
Nitrogen, however, is a rather inert element; it is difficult to get it to combine with any other element.
Haber succeeded in getting nitrogen to combine with hydrogen by the use of high pressure, moderately high temperatures, and a catalyst.
Ammonia; Nitric acid, Ammonium nitrate/chloride, Urea
Ammonia or azane is a compound of nitrogen and hydrogen with the formula NH3.
Colorless gas with a characteristic pungent smell
Soluble in water (aq. Solution : weak acid)
Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers.
Direct application as fertilizer
Urea, Ammonium phosphates,
Production of nitric acid, amines,
removal of Nox from flue gases
of power plants.
Nitrogen consumption in fertilizer
(80% of NH3production)
Mixed fertilizers (NPK)
Chemical nitrogen fertilizer 30-Jan-13
Principle of Ammonia Synthesis The Haber or Haber-Bosch Process
The ammonia synthesis reaction is represented by
N2 +3H2 NH3 H = -22.0 kcal
Increase in the pressure on this system increases the equilibrium ammonia concentration.
While raising the synthesis temperature gives a faster rate, it also displaces the equilibrium to the left, giving smaller potential conversion.
Condition used for most synthesis ammonia process
P = 100-300 atm
Catalyst : Promoted iron oxide catalyst
Feedstocks for Ammonia synthesis by air distillation
Cryogenic low temperature technology
Feasible only for small ammonia plant (100 tonne/day)
or where abundant hydrogen is available.
Ammonia feedstocks by reforming and secondary
reforming (from coal, petroleum and natural gas)
Coal as a source via water gas (CO+H2) reaction :
higher capital investment, environmental problem
Natural gas reforming: can use any kind of petroleum
feed stock, easier to clean prior to use, less emission
Process flow sheet (Text book)
Synthesis gas production is taken separately in textbook. So we will not refer this flow sheet.
Integrated Ammonia Plant
Process steam Pre-reforming Sec reforming HT shift LT shift
Ammonia synthesis Methanation Process
Natural Gas Desulfurization
The sulphur content in natural gas is reduced to below 280 g/m3 to prevent poisoning of the nickel catalyst in the primary reformer.
Desulfurization can be accomplished by using either activated carbon or zinc oxide.
Primary Steam Reforming- This reaction requires continued external heat (combustion of methane).
Catalyst : Ni
CH4 + H2O 3H2 + CO H = +49.7 kcal
Secondary Air Reforming-Sufficient air is added to provide nitrogen required for synthesis
CH4 + yair 2H2 + CO +xN2 H = +8.5 kcal
The gas leaving the secondary reformer is then cooled in a waste heat boiler.
High Temperature Shift Converter- CO is converted to CO2 in presence of chromium promoted iron catalyst
CO + H2O CO2 + H2(-9.8 kcal)
Low Temperature Shift Converter- The low-temperature
shift converter is filled with a copper oxide/zinc oxide
Carbon dioxide can yield carbonates and carbamates
which are undesirable because they can deposit in
In addition oxides of carbon poisons ammonia catalyst.
Carbon dioxide is removed by scrubbing by
monoethanolamine and hot potassium carbonate.
Regeneration of solvent is done by pressure letdown
plus some air stripping.
Residual CO2 in the synthesis gas is removed by
catalytic methanation over a nickel catalyst
CO + 3H2 CH4 + H2O
CO2 + 4H2 CH4 +2 H2O
Nitrogen and hydrogen (obtain from any route) are required in mole ratio 1:3, raised to very high pressure 100-900 atm range (centrifugal compressor).
Material of construction: Steel (hydrogen embrittlement)
Commercial Synthesis Reactor
Pressure vessel with sections for catalyst beds and heat exchangers
Cold feed gas is added in quench reactor
The heat produced is removed between the catalyst beds by heat exchanger
Product recovery (Condensation)
Ammonia is condensed from this gas mixture by cooling the gases
At high pressure (converter exit pressure) ammonia condenses easily
It can be absorbed in water if solution to be marketed.
Ammonia synthesis reactor design
Major Engineering Problems
Thermodynamic and Kinetic considerations
Optimization of space velocity
Fraction of NH3 (x) = f V-n
High space velocity Increase cost of
NH3 recovery and pumping cost
Space volumetric feed rate
velocity volume of reactor/catalyst
Space velocity is inversely proportional to contact time.
To date catalysts are based on Iron oxide
promoted by alkali K2O (1-2%) and metal oxide (Al2O3)
Al2O3 support to prevent sintering
Potassium reduces the activation energy of dissociation
Kellogg : Commercialized the Kellogg advanced Ammonia process using ruthenium on a graphite support
Process design modifications
Modern trend toward: lower pressure & increased flow rates
Large single-train plant
NITRIC ACID (HNO3)
Appearance colorless to yellowish liquid
Mol. Wt- 63.03
M.P - -42.5 oC
B.P 86 oC with decomposition
Completely miscible with water, forms a constant boiling mixture
For production of ammonium nitrate, Adipic acid, dinitrotoulene,
nitrobenzene, sodium, potassium and calcium nitrates, nitro
compound for explosives etc.
Methods of Production
1. From saltpeter (NaNo3 + H2SO4 process)
Old process, practiced in middle age
2. Ammonia oxidation process
Modern nitric acid production process
3. N2 fixation from air ( Wisconsin process)
Production of NO and NO2 by high temperature reaction using air
4. Nitrogen fixation by nuclear fission fragments
Air exposed to radiation in a nuclear reactor to form NO.
Present economics gives too high a plant investment.
AMMONIA OXIDATION PROCESS
( Oswald process)
Modern nitric acid production uses
Catalytic oxidation of ammonia in air
Catalyst : Platinum Rhodium alloy gauze (Pt/Rh)
Followed by absorption of the oxidation product in
water to yield nitric acid.
Overall reaction reads:
NH3+2O2HNO3+H2O H =-78.9 kcal
Many reactions are involved in the overall process.
1) Ammonia oxidation : (Mixture of ammonia 9-11 % in air)
a. NH3 + 5/4O2 NO +3/2 H2O H =-78.9 kcal
b. 2 NO + O2 2 NO2
2) Ammonia oxidation (side reaction ):
a) NH3 +3/4O2 1/2N2 +3/2 H2O These reactions can be
b) NH3 1/2 N2 + 3/2 H2 overcome by using a selective
c) NH3 + O2 1/2 N2O + 3/2H2O catalyst and short residence
d) NH3 +3/2 NO 5/4 N2 + 3/2H2O time at high temperature900C
3) Nitric oxide oxidation and :
a) 2NO + O2 2NO2 Non catalyzed reaction. NO2 is
b) 2NO2 N2 O4 in equilibrium with its dimer.
Thermodynamic data show low temperature and high pressure as favorable
NO oxidation is famous reaction(one of the 3rd order reactions known).
Peculiarly rate constant increases with decreasing temperature.
4) Absorption of nitrogen dioxide in water
a) 3NO2 + H2O 2 HNO3 + NO
b) 2NO2 + H2O HNO3 + HNO2
c) 2 HNO2 H2O + NO + NO2
The absorption of NO2 in water is quite complex, because
several reactions can be occur (both in liquid phase and
Operates best at high pressure (7- 12 bar).
Compressed air is mixed with anhydrous ammonia, fed to shell and tube converter designed so that the preheater and a steam heat recovery boiler-super heater are within the same reactor shell.
In the converter section the gas passes downward with