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The Haber Process

By: Brandon Hao

What is the Haber Process?

  • Combines nitrogen with hydrogen to produce ammonia
  • Used to produce fertilizer and, historically, explosives
  • Nitrogen harvested from air
  • Hydrogen derived from methane
  • Reversible and exothermic reaction
  • Gas is typically recycled
  • Each production pass is only 15% efficient

Conditions

Three main conditions affecting the reaction:

  • Catalyst
  • Pressure
  • Temperature

Catalyst

  • Catalyst used to increase speed of reaction
  • Iron based catalysts used
  • Reduced magnetite (Fe3O4) encased in wüstite (FeO)
  • Promoters such as K2O, CaO, SiO2, and Al2O3 used
  • Increases efficiency

Pressure

  • Reaction occurs at high pressures
  • 15 - 25 MPa (150-250 bar, 2,200-3,600 psi)
  • Pressure affects equilibrium
  • Increased pressure favors forward reaction
  • 4 moles of reactant to 2 moles of product

Temperature

  • Reaction occurs at a moderate temperature
  • Typically 400-450°C
  • Affects equilibrium
  • Exothermic reaction
  • Lower temperature makes reaction forward favored

Catalysts

  • Originally osmium was used
  • Haber noted that uranium was nearly as effective and easier to obtain
  • Later a cheaper iron-based catalyst was discovered
  • Most common catalyst used today
  • Ruthenium-based catalysts also used
  • Do not affect equilibrium according to Le Chatelier's Principle

Iron-based

  • Obtained from finely ground iron powder
  • Reduction of high purity magnetite (Fe3O4)
  • Particles are partially reduced
  • Some oxygen is removed
  • Core made of magnetite
  • Surrounded by wüstite (FeO, ferrous oxide)
  • Wüstite surrounded by shell of iron metal
  • High surface area
  • Contains promoters such as calcium and aluminum oxides
  • Increases effciency of catalyst and maintains surface area

Pressure

  • Reaction occurs at high pressure
  • 25 MPa (150-250 bar, 2,200-3600 psi)
  • 4 moles of reactant vs 2 moles of product
  • High pressure causes reaction to be forward favored according to Le Châtelier's Principle
  • High pressure increases production but increases cost of equipment
  • Equipment needs to withstand high pressure
  • Compromise at ~20MPa

Temperature

  • Exothermic reaction
  • According to Le Chatelier's Principle the reaction will be forward favored at lower temperatures
  • Reaction will produce more heat to counteract lower temperatures
  • Ideal temperature at 150-200°C
  • High temperature is required for catalyst to be effective
  • At least 400°C
  • Benefit from catalyst outweighs benefit from temperature below 400°C
  • Reaction typically occurs at 400-450°C

Gas Recycling

  • The process is only 15% effecient each pass
  • Unreacted gas is recycled through the reactor again
  • Through continous recycling efficiency reaches 98%

References:

BBC - GCSE Bitesize: The Haber process. (2018). Bbc.co.uk. Retrieved 13 April 2018, from http://www.bbc.co.uk/schools/gcsebitesize/science/ocr_gateway/chemical_resources/ammoniarev2.shtml

Bohnet, M. (1999). Ullmann's encyclopedia of industrial chemistry (7th ed.). [Hoboken, NJ?]: Wiley Interscience.

Clark, J. (2013). The Haber Process for the manufacture of ammonia. Chemguide.co.uk. Retrieved 13 April 2018, from https://www.chemguide.co.uk/physical/equilibria/haber.html

Haber-Bosch process chemistry. (2018). Encyclopedia Britannica. Retrieved 13 April 2018, from https://www.britannica.com/technology/Haber-Bosch-process

May, P. (1999). The Haber Process. Chm.bris.ac.uk. Retrieved 13 April 2018, from http://www.chm.bris.ac.uk/~paulmay/haber/haber.htm

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