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AmmoniaAmmonia

Background informationBackground information

Ammonia is one of the main products of chemical industry (especially for fertilizers and explosives)(especially for fertilizers and explosives)

In the past ammonia was produced as a by-product during the distillation of coal in coke ovens and gas works insufficient already at the turn of the century

Ammonia production using nitrogen from the air : Haber-Bosch process

The discovery of nitrogen fixation from air was suggested to be the most important scientific advancement of the 20th century.

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Ammonia production in numbersAmmonia production in numbers

Ammonia usesAmmonia uses

85 % of ammonia is used for nitrogen fertilizers-urea is the most important accounting for 40% of ammonia usage

Other industrial uses include the production of nitric acid, amines, nitriles, nylon and organic nitrogen compounds

Environmental applications removal of NOx from flue gases of pilot plants

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Energy data for nitrogenEnergy data for nitrogen

Value(kJ/mol)

Compare with(kJ/mol)(kJ/mol) (kJ/mol)

Bond dissociation energy 945 C-H in CH4: 439Ionization energy 1503 O2: 1165Electron affinity 34900 O2: 43

H = -91 44 kJ/molH = -91.44 kJ/mol

Ammonia production from airAmmonia production from air

Problems: Bond dissociation energy and electron affinity very high

ti ti b i i ti l i iblactivation by ionization nearly impossible

Fraction of NH3 in a 3:1 mixture of H2 and N2 at 1290 K: 0,01%

Industrial production only possible if good catalysts are found

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Haber Haber Bosch Bosch process developmentprocess development

Haber and Bosch developed a process for the production of ammonia from nitrogen in 1908 1913

Testing 6500 catalysts they discovered the high activity of iron based catalysts.

Modern catalysts are still closely related to original ones

ThermodynamicsThermodynamics

High pressure and low temperature are needed At temperatures lower than 670 K the rate of the reaction is very

llow minimum temperature required to reach the equilibrium sufficiently fast

Typical reaction conditions: 675 K at inlet; 720 770 K at outlet 100 250 bar100 250 bar

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ThermodynamicsThermodynamics

Catalysts for ammonia synthesis

The iron catalyst consists of magnetite (Fe3O4) which is enriched(promoted) most frequently with Al- and K- (or Ca, Mg, Si)-oxides. The promoters are dissolved in the magnetite or form metal ferrites

Composition: 89-95% Fe3O4, 2-4% Al2O3, 0.5-1% K2O, 2-4% CaO Catalyst is produced by fusing (melting) magnetite ore with the other

promoters at 1700oC and pouring melt into water to form fine particles

Iron being a transition metal with partially occupied d-bands represents asurface suitable for adsorption and dissociation of N2 molecules

Not only iron based catalysts are active for ammonia synthesis Os, Ru are equally active with Fe Ru is most active in ammonia decomposition Mo, U and Mn show quite high activity in ammonia synthesis

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Role of catalyst promoters Role of catalyst promoters -- Poisons for ammonia synthesisPoisons for ammonia synthesis

The addition of Al and K promoters drastically increases the activity K acts as an electron donor that increases the electron density and thus

provides a higher number of active sites on the surface

Water, hydrogen sulfide and halogens are strong poisons as they adsorb strongly on the surface

H O causes site blocking but it is reversible

g Al influences the morphology of the catalyst, increasing its porosity and free

surface area

H2O causes site blocking but it is reversible 2S also blocks the surface by S* irreversibly. Large partial pressures

of H2S may cause the formation of FeS

Oxygen concentration should be kept low < 50 ppm as it is adsorbed strongly

Reduction of the catalystsReduction of the catalysts

Before use, the catalyst is reduced with H2 at 500C for up to 100h to be converted to the active structure to provide metallic Fe removal of oxygen makes the material porous specific surface area increases

The promoters are in still oxidic phase after the reduction The surface are of the reduced catalyst is around 290 m2/g

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General mechanism of ammonia synthesis

The reaction follows a Langmuir-Hinshelwood (LH) mechanism both reactants are dissociatively adsorbed on the surface before reaction

The dissociative adsorption of N2 is the rate determining step

Calculated reaction pathways for ammonia synthesis over

Ru catalyst

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Reactor designReactor designRequirements to reactors: Gas phase reactor with a solid catalyst Temperature control as the reaction is exothermic Two methods applied Two methods applied

Quench reactor in which cold gas is inserted at different heights of the reactor

Reactor with heat exchangers between the catalyst beds

The ICI The ICI quench reactorquench reactor

Part of the cold feed flows down on the outer shell ofthe reactor to cool it (feature used in most converters).

It is then heated in the effluent heat exchanger before entering the catalyst bed

One single catalyst bed containing of three zones separated by gas distributors.

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Kellogg vertical quench reactorKellogg vertical quench reactor

Feed enters at the bottom and cools theshell while flowing upwards

Four separated catalyst beds

Inlet of quench gas between the beds

Haldor TopsHaldor Topse radicale radical--flow reactorflow reactor

Main difference compared to other reactors:

di l fl th h t l t b dradial flow through catalyst bed reduces pressure drop allows smaller particles more catalytic area per unit volume

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Integrated ammonia plantIntegrated ammonia plant

Pressure to improve the economy is very highPressure to improve the economy is very high

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Ammonia synthesis loop

The yield of ammonia per pass ranges between 20 and 30% Unconverted synthesis gas leave the reactor together with

ammonia

Trends in catalysts for ammonia synthesisTrends in catalysts for ammonia synthesis

TEM image of the Ba-Ru/MgO catalyst after reduction at 783 K

Catalytic activity of a)the bari m promoted catal st (Ba R /MgO)a)the barium-promoted catalyst (Ba-Ru/MgO),b) the cesium-promoted catalyst (Cs-Ru/MgO),c) the industrial iron catalyst, d, e) unpromoted Ru/MgO catalysts (Ru/MgO), f) ammonia mole fractions at equilibrium

M. Muhler, O. Hinrichsen Angew. Chem. 2001