Nitrogen Industry

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Chemical Process Technology

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  • Chemical Process technology

    CHE C322

    Ammonia

    Nitric Acid

    Urea

    Ammonium Nitrate

    Nitrogen Industry

    30-Jan-13

  • Nitrogen Industry

    30-Jan-13

    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

    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.

    End uses

    Direct application as fertilizer

    Urea, Ammonium phosphates,

    nitrate, sulfate

    Production of nitric acid, amines,

    nitriles.

    Environmental application:

    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

    30-Jan-13

    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

    T=400-500 C

    Catalyst : Promoted iron oxide catalyst

  • Process Selection

    30-Jan-13

    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)

    30-Jan-13

    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

    condensate

  • 30-Jan-13

    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.

    Reforming

    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.

  • 30-Jan-13

    Shift conversion

    High Temperature Shift Converter- CO is converted to CO2 in presence of chromium promoted iron catalyst

    (FeO+Cr2O3)

    CO + H2O CO2 + H2(-9.8 kcal)

    Low Temperature Shift Converter- The low-temperature

    shift converter is filled with a copper oxide/zinc oxide

    catalyst.

    Carbon dioxide can yield carbonates and carbamates

    which are undesirable because they can deposit in

    piping.

    In addition oxides of carbon poisons ammonia catalyst.

  • 30-Jan-13

    CO2 Removal

    Carbon dioxide is removed by scrubbing by

    monoethanolamine and hot potassium carbonate.

    Regeneration of solvent is done by pressure letdown

    plus some air stripping.

    Methanation

    Residual CO2 in the synthesis gas is removed by

    catalytic methanation over a nickel catalyst

    CO + 3H2 CH4 + H2O

    CO2 + 4H2 CH4 +2 H2O

  • 30-Jan-13

    Ammonia Synthesis

    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.

  • 30-Jan-13

    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.

    30-Jan-13

  • Catalyst development

    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

    30-Jan-13

  • NITRIC ACID (HNO3)

    Properties:

    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

    Uses :

    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

    conditions.

    NO oxidation is famous reaction(one of the 3rd order reactions known).

    Peculiarly rate constant increases with decreasing temperature.

  • 30-Jan-13

    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

    gas phase).

    Operates best at high pressure (7- 12 bar).

  • PROCESS DESCRIPTION

    Description:

    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