Haber process

The Haber -Bosch process is a chemical process for the production of ammonia from the elements nitrogen and hydrogen. It is named after its developers Fritz Haber and Carl Bosch. It is the most important industrial process for the conversion of atmospheric nitrogen into a usable non-reactive nitrogen compound. Ammonia is needed for the production of fertilizer, which was the basis for the diet of most of the world's population and is. The Haber -Bosch process also made it possible to get along in the production of explosives without natural saltpetre.

In connection with the Haber -Bosch process several Nobel Prizes in chemistry were awarded: Fritz Haber, Carl Bosch in 1918 and 1931 ( together with Friedrich Bergius ) and Gerhard Ertl in 2007, among other things, for the complete theoretical explanation of the mechanism.

• 3.1 fission gas generation
• 3.2 secondary reformer
• 3.3 Converting ( oxidation ) of carbon monoxide to carbon dioxide
• 3.4 Absorption of carbon dioxide ( gas scrubber )
• 3.5 Implementation of the synthesis gas to ammonia gas

• 4.1 reduction of the catalyst
• 4.2 catalyst poisons
• 4.3 Other catalysts than iron

• 6.1 Partial Oxidation
• 6.2 Electrolysis of water
• 6.3 Water gas production

History and Significance

Before the industrial use of the Haber- Bosch process arable lands of the necessary nitrogen was supplied to the plants in the form of manure, compost or certain crop rotations. As agriculture intensified, guano was - the droppings of countless generations of seabirds on islands off the South American Pacific coast - and mined for profitable export business. However, the growth of the world population in the 19th century created a huge demand that could not be satisfied. In June 1898, the noted British chemist William Crookes stopped in front of the British Association for the Advancement of Science in Bristol a sensational speech in which he came to the conclusion that in an estimated 20 years, the nitrogen demand will exceed supply, and then the Western world a daunting famine threatens. The only solution he saw in the chemical fixation of atmospheric nitrogen.

The possible through the Haber -Bosch process large-scale industrial production of artificial fertilizers made ​​the immense growth of the world population in the 20th century and thus modern society possible.

The main scientific achievements to implement this method were firstly to investigate the underlying chemical reaction by Fritz Haber and Walther Nernst, on the other hand, the systematic search for suitable catalysts by Alwin Mittasch. For the technical realization of large-scale completely new solutions have been developed in many areas of chemical engineering and reactor construction by Carl Bosch and Fritz Haber.

The Haber -Bosch process was filed by BASF in 1910 a patent for a previously filed, faulty patent Haber on the same subject was withdrawn simultaneously. Large-scale systems were operated among others Oppau in Ludwigshafen, Leuna and Bitterfeld by BASF and after merger in the German conglomerate IG Farben. The first used to produce high-pressure reactor is still preserved in the original and can be visited on the BASF in Ludwigshafen casino in the public park.

Ammonia can for instance be processed in the Ostwald process for nitric acid, or by reaction with carbon dioxide ( CO2) to urea, which is used as a fertilizer.

Of military importance ammonium nitrate is ( saltpeter ), a product of ammonia and nitric acid, for the manufacture of explosives. Therefore, the development of the process was accelerated up to large industrial applicability in 1914 under pressure from the German Chief of Staff Erich von Falkenhayn and it came to Salpeterversprechen. When the German Empire was cut off during World War II by the Allied naval blockade of natural nitrogen sources ( Chile saltpetre ), succeeded only with the help of the Haber - Bosch process to avert the otherwise impending end of 1914, collapse of the German munitions production and also to maintain the production of fertilizers.

The availability of large amounts of nitrogen fertilizer gave rise to extensive agricultural research, in which the amounts of fertilizer used were optimized depending on soil and plant type (Minimum- ton). Through large-scale use of fertilizers global agricultural production could be increased significantly.

According to this method, the majority of the ammonia produced per year is produced 100 million tonnes. Due to the high demand for energy in the production of the required purity hydrogen accounts for about 1.4% of world energy consumption on the Haber -Bosch process.

Urea is also for the importance of environmental protection: The harmful nitrogen oxides contained in the flue gases can be personalized with urea via Selective non-catalytic reduction and selective catalytic reduction reduced to form harmless nitrogen.

Nowadays, at least in the population of developed countries, about 40 % of the nitrogen present in the human body participated in the Haber- Bosch synthesis before.

Synthesis conditions

In the Haber- Bosch process, a gas mixture of hydrogen and nitrogen on an iron oxide mixed catalyst of iron (II / III) oxide ( Fe3O4), K2O, CaO, Al2O3 and SiO2 at about 300 bar pressure and 450 ° C is reacted:

The actual catalyst is α -Fe ( ferrite ) is formed only in the reactor by the reduction of Fe3O4 with hydrogen.

The ammonia synthesis will take place today under the following reaction parameters:

• Ratio of nitrogen to hydrogen = 1: 3
• Pressure: 250 bar to 350 bar
• Temperature: 450-550 ° C ( the decomposition of ammonia and the effectiveness of the catalyst increase with the temperature, here is the optimum )
• Use of α - iron as a catalyst obtained by reduction of iron (II / III) oxide ( Fe3O4), K2O, CaO, Al2O3 and SiO2 is obtained as promoters.

Through the catalyst, the activation barrier is indeed greatly reduced for the cleavage of the triple bond of the nitrogen molecule, yet high temperatures for a reasonable rate of reaction are still required. Because the overall reaction is exothermic ( form 2 NH3: ΔH0 = -92.5 kJ / mol), the high temperature to a shift in the equilibrium to the reactants ( starting materials ) and therefore reduces the yield ( Le Chatelier's principle ). Since the reaction proceeds with reduction in volume, the high operating pressure results in an increase in the yield. In addition, you remove the ammonia that is formed continuously from the reaction system.

The volume fraction of NH3 in the gas mixture is approximately 17.6%.

The necessary for the reaction, hydrogen is now mostly developed by the partial oxidation of natural gas, the nitrogen is taken directly from the air as in the original procedure. The disturbing air oxygen was previously converted by reduction with hydrogen to form water, thus separated. Meanwhile, the air separation by fractionation of liquid air is more economical.

Modern ammonia plants produce more than 3,000 tons per day in a production line.

Mechanism of the reaction

The reaction can, as each heterogeneously catalyzed reaction, are classified into the following five steps:

At IR studies NH and NH2 species were found on the surface. Allowed to adsorb nitrogen at low temperatures on a surface of the nitrogen physisorption first and dissociated ( chemisorbed ) only when the temperature is increased. As well as the nitrogen passes through at high temperatures and then only the physisorption, chemisorption. Other studies it is known that the rate-determining step of the reaction is the dissociation of nitrogen.

Based on these experimental results, a reaction scheme can be created, which consists of the following individual steps:

Nitrogen forms on the iron surface by dissociative adsorption of a surface nitride ( Nads ); the nitride surface may be understood as a complex of iron and the surface nitrogen atom. This is followed quickly add hydrogen atoms ( Hads ); these are on the surface very mobile. It formed Oberflächenimide ( Nhad ) Oberflächenamide (NH2, ad) and surface ammoniates (NH3, ad), the latter decay (desorption) under NH3 emission.

With the knowledge of the reaction enthalpy of the individual steps of an energy diagram can be created. With the aid of the energy diagram homogeneous and heterogeneous reaction can be compared: Because of the high activation energy of the dissociation of nitrogen, the homogeneous gas phase reaction is not feasible. The catalyst circumvents this problem because the energy gain resulting from the binding of nitrogen atoms on the catalyst surface over compensates for the necessary dissociation energy so that the reaction is exothermic, eventually even. Nevertheless, there remains the dissociative adsorption of nitrogen, the rate-determining step: not because of the activation energy, but mainly due to the unfavorable pre-exponential factor of the rate constant. Although the hydrogenation is endothermic, however, this energy can be slightly different from the reaction temperature (about 700 K) to be applied.

The adsorption of N2 is similar to the chemisorption of CO. Since it is isoelectronic to CO, he adsorbed in the on -end configuration MNN. Ab initio MO calculations have shown that there is in addition to the σ - Hinbindung to a metal π back-donation from the d orbitals of the metal into the π * orbitals of the nitrogen. Wherein the resulting bond weakening could be shown experimentally by a reduction in the wave number in comparison with the gas phase of the N2 stretching.

The dissociative adsorption of the nitrogen on the surface can be considered in greater detail:

N2 → S * N2 ( γ - state) → S * S * N2 - ( α - state) → 2 S * -N ( β - state)

The molecular adsorption ( physisorption ) of N2 is the dissociative adsorption ahead first. The individual molecules have been identified and associated with X- ray photoelectron spectroscopy (XPS ), high-resolution electron energy -loss spectroscopy ( HREELS ) and IR spectroscopy. Fe on a (111 ) surface, the adsorption of N2 initially leads to an adsorbed γ species with an adsorption energy of 24 kJ · mol -1 and a stretching vibration of NN- 2100 cm -1. Together with results from photoelectron spectroscopy resulting in a picture of N2 molecules adsorbed to the surface perpendicular. The nitrogen in the α - state is more strongly bound with 31 kJ · mol -1. The NN stretching vibration decreases to 1490 cm - 1, indicating a reduction in the bond order. A comparison with the vibration spectra of the complex compounds can be concluded that the N2 molecule " side-on " is bonded to an N atom in contact with a C7 -site. The extension or weakening of the NN bond is based on a π - back-bonding, which introduces electron density into the antibonding orbitals of the nitrogen. A further heating of the Fe ( 111) surface, which is covered with α -N2, leads to both desorption and the appearance of a new band at 450 cm -1. This represents a metal -N vibration, the β - state. This structure is (again, above) often referred to as " Oberflächennitrid ". The Oberflächennitrid is very strongly bound to the surface, it desorbs only ( in the context of recombination) at about 700 K.

Iron also has various crystal faces. The reactivity of the crystal faces is highly variable. The (111 ) - and ( 211) surfaces have by far the highest activity. One explanation for this is that only these two surfaces C7 sites have - iron atoms with seven nearest neighbors. There are also theoretical reasons why the C7 - sites have a particular activity.

Industrial production of ammonia in five steps

Ammonia is formed in an equilibrium reaction of the elements hydrogen and nitrogen. For this you need:

• From the air
• Of methane gas ( ) and water vapor

Fission gas generation

In the first step, hydrogen is separated from the carbon. This allowed to methane with water vapor with the aid of a catalyst ( nickel -alumina catalyst ), to react carbon monoxide and hydrogen ( primary reformer ).

Secondary reformer

Since the above reaction converting the methane gas incompletely, it can be in a second step the remaining methane with oxygen react to form carbon monoxide and hydrogen (secondary reforming ). The secondary reformer is supplied with air for this purpose, whereby the space required for the subsequent ammonia synthesis, nitrogen comes automatically into the gas mixture.

Convert ( oxidation ) of carbon monoxide to carbon dioxide

In a third step, the carbon monoxide has to be oxidized to carbon dioxide, which is known as conversion, or water-gas shift reaction.

Carbon monoxide and carbon dioxide form with ammonia solids ( carbamates), which would clog pipes and appliances in a short time. The following process step, therefore, the carbon dioxide must be removed from the gas mixture.

Absorption of carbon dioxide ( gas scrubber )

In contrast to carbon monoxide carbon dioxide can be easily removed by a gas scrubbing of the gas mixture. Carbon dioxide is removed by washing with triethanolamine from the mixture.

Reacting the synthesis gas to ammonia gas

In the last critical step of the so- produced hydrogen-nitrogen mixture at high pressure and about 450 ° C is catalytically converted to ammonia gas:

Catalyst

The catalyst precursor is composed of magnetite ( Fe3O4) and oxidic promoters. This is (mostly by synthesis gas ) is reduced to the actual catalyst of α - iron. The precursor is prepared in a melt process (magnetite with promoters ). The advantage of fusion over alternative methods ( oxidation of pure iron ) results from the fact that magnetite above 100 ° C is the thermodynamically most stable iron oxide phase. This is advantageous as are formed from precursor magnetite as particularly effective catalysts. The raw materials should be free of catalyst poisons. The promoters should be homogeneously distributed in the molten magnetite, this is achieved by initially heating the mixture at about 3500 ° C. , Another factor in the production of the cooling rate of the melt. Rapid cooling results in better quality catalysts which are less abrasion - resistant but; it is preferred in practice.

Reduction of the catalyst

The reduction is carried out with synthesis gas obtained from the magnetite, a highly porous, catalytically very active form of alpha- iron. The promoters are not reduced (except Cobalt ). For optimum quality the reduction catalyst in a certain way has to be performed. The vapor pressure of the resulting water in the reduction needs to be kept as low as possible in the gas mixture ( less than 3 g · m -3). Water vapor is in contact with already reduced catalyst, which leads, particularly in connection with high temperatures to premature aging of the catalyst by recrystallization. The reduction is therefore carried out at a high gas exchange, low pressure and lowest possible temperatures. In practice, the reduction of the catalyst is carried out directly on the production line. The exothermic reaction of the ammonia is used for a gradual increase in temperature.

The reduction of fresh, fully oxidized catalyst (or precursor ) to reach the full capacity then takes 4-10 days. Phase the FeO ( wustite ) is reduced more quickly and at lower temperatures than the Fe2O3 phase ( magnetite). For detailed kinetic microscopic and röntgenspektroskopischen investigations, the following model was developed for the reduction:

This process then results in a core-shell structure.

In practice, prereduced, stabilized catalyst types gained a significant market share. These are catalysts that have already received the full pore structure of the normal catalyst by reduction, as an active catalyst, but were then again superficially oxidized. By this oxidation are no longer pyrophoric, as are the reduced catalysts otherwise. The Reactivate such prereduced catalysts requires only 30-40 hours. In addition to the low start-up time such catalysts have other advantages, such as less risk of damaging the catalyst by water and lower weight.

Catalyst poisons

Catalyst poisons to reduce the activity of the catalyst. These substances are either part of the synthesis gas or from impurities from the catalyst itself does not play a larger role second option. H2O, CO, CO2 and O2 are temporary catalyst poisons. Sulfur, phosphorus, arsenic and chlorine compounds are permanent catalyst poisons.

Other catalysts than iron

Most of the efforts to improve the industrial process for the synthesis of ammonia included in the synthesis gas production. There, significant progress has been actually achieved. In the improvement of the catalyst for ammonia synthesis, however, there has been no significant progress more since the 1920s. The iron catalyst still needs a high pressure ( at least 130 bar ), high temperatures ( 400-500 ° C ) and large volume (reactor of at least 100 m3 for a production capacity of 1,800 t / d). According to theoretical and practical studies of the scope for further improvement of the iron catalyst is limited. The theoretical possibilities for energy savings, however, are enormous; when the pressure to the level of the synthesis gas production may be reduced about 1 could GJ / t of ammonia can be saved.

In the search for alternative catalysts most metals have been extensively tested. A great improvement was a modification of the iron catalyst with cobalt to 1984. The only other promising element ruthenium is today. Iron was chosen because of price, availability, ease of processing, the lifetime of the catalyst and the activity. Haber himself initially wanted to use osmium and uranium catalysts. The Vulcono plot shows a good activity for both metals. However, uranium reacts during catalysis to nitride oxide, osmium oxide is volatile and highly toxic, which is a problem in the catalyst preparation. It can be seen in Vulcano plot an optimum for metals of group 8 (iron group). A catalytically active element must be nitrogen on one side dissociatively adsorb, bind on the other side but not too much, as this will cause self- poisoning. The metals of the iron group left to show a strong bond to nitrogen and poisoning themselves through a form of volume or Oberflächennitriden. Right metals of the iron group do not form a bond to nitrogen and therefore do not facilitate the dissociation. After the adsorption, the binding energy of the adsorbed atom DM (570 kJ mol- 1) and the dissociation energy of nitrogen yields 200 kJ mol -1. Iron has a good balance between the two thermodynamic quantities.

Ruthenium catalysts are known as catalysts of the second generation, as they show a higher activity with comparable pressures and lower temperatures. The activity of the ruthenium catalyst is strongly dependent on the support and the promoters. As promoters of a variety of substances in question, in addition to carbon, these are MgO, Al2O3, zeolites, MgAl2O4 and BN. Ru / C is already used industrially since 1992 in the " Kellog Brown & Rood advanced ammonia process" ( KBRAAP ). The problem here is that the carbon support will responds to methane and thereby reduced, which reduces the life span of the catalyst. To alleviate the problem of methanation, the carbon is previously treated at 1500 ° C. In addition, proceeds from the finely dispersed carbon of an explosion from. So far, magnesium oxide has proven to be the best alternative, especially because of the low acidity. In general, ruthenium is possible on basic support most reactive. Due to the low structural stability MgO is not used in the industry. A support having acidic properties is generally not used, since it deprives the ruthenium available electrons. The search for a suitable support will therefore be continued.

As with any catalyst, the N2 dissociation and the rate determining step for ruthenium. The active center is noted in a ruthenium B5 site consisting of two atoms of an edge ( step edge) and three on the lower deck. The number of sites depends on the B5 size and the form of the ruthenium particles, the ruthenium precursor and the amount of ruthenium used. The enhancing effect of the basic support has the same effect as the promoter effect of alkali metals, which comes here as well as the iron catalyst to bear.

Large-scale application

The Haber -Bosch process is used industrially in large systems, to cheap to win ammonia. The figure shows the layout of such a facility and should be read from left to right:

• Top left methane and water vapor is introduced into the so-called primary reformer, there already created a part of the hydrogen.
• Left center, air is introduced, which consists of 21 percent oxygen and 79 percent of parts from parts of nitrogen. Due to the atmospheric oxygen produced more hydrogen.
• The carbon monoxide contained in the resulting mixture is reacted in the first gray - blue lined reactor with steam in the presence of a catalyst to form carbon dioxide.
• In the scrubbing tower, the carbon dioxide reacts with calcium hydroxide under pressure in aqueous solution and is separated from the desired gaseous reaction educts, hydrogen and nitrogen, and it is formed of calcium carbonate ( another possible washing liquid is, for example, triethanolamine).
• Subsequently, the reaction educts are prepared for the conditions in the actual reaction reactor where they are heated to 450 ° C and compressed to 300 bar.
• In the middle subscribed reaction reactor, the actual production of ammonia takes place.
• The reaction products are continuously removed to a maximum yield. They are cooled down from 450 ° C, and the produced ammonia is condensed out. The not yet reacted products nitrogen and hydrogen are added to the fresh gas, returned to the reactor circuit.

Alternative methods

The above-described steam reforming process for the production of synthesis gas is the most commonly applied. Other possible methods are:

Partial Oxidation

In this case, coal or hydrocarbons, with oxygen and water vapor in an open reactor without a catalyst at approximately 1100 ° C to be gasified, and the synthesis gas is further processed as in the steam reforming. Nitrogen is added prior to entry into the ammonia synthesis in a stoichiometric amount.

Electrolysis of water

This is broken down with a high amount of electrical energy of water into H2 and O2. Nitrogen is fed to the hydrogen thus obtained in a stoichiometric amount. This process is only economical when cheap electricity from hydropower, for example, is available, for which there is no other use.

Water gas production

Hydrogen is produced via the reaction of steam with hot coke (see water gas). Air is supplied, but only so much that the oxygen is completely consumed, whereby carbon monoxide is produced. The space required for the subsequent synthesis of ammonia nitrogen remains in the water gas. Subsequently, the carbon monoxide is as already described above by means of conversion into easily removable carbon dioxide converted. This method is only of historical importance.