Ammonia synthesis
Production
Ammonia is the second largest synthetic chemical product; more than 90% of world consumption is manufactured from the elements nitrogen and hydrogen in a catalytic process originally developed by Fritz Haber and Carl Bosch using a promoted iron catalyst discovered by Alwin Mittasch. The way to prepare it has been the same from the beginning until now.A hydrogen – nitrogen mixture reacts over the iron catalyst ) at elevated temperature in the range of 400 – 500 ◦ C ) and pressures above 100 bar with recycle of the unconverted part of the synthesis gas and separation of the ammonia product under high pressure. End of the 1990s a ruthenium-based catalyst was introduced which allows slightly lower synthesis pressure. In contrast with this, in the dramatic changes happenedover the years in the technology of synthesis-gas generation, and technical ammonia processes
The catalytic synthesis of ammonia from its elements is one of the greatest achievements of industrial chemistry, This process not only solved a fundamental problem in securing our food supply by production of fertilizers but also opened a new phase of industrial chemistry by laying the foundations for subsequent high-pressure processes like methanol synthesis, oxo synthesis, Fischer – Tropsch Process, coal liquefaction, and Reppe reactions. Ammonia is also important in the production of nitric acid.
Synthesis reaction
To form ammonia from hydrogen and nitrogen molecules, significant energy input is required for the nitrogen molecule to achieve the activated state. This is because of its high dissociation energy of 941 kJ/mol,which is considerably higher than that of hydrogen.Initiation of ammonia synthesis homogeneously in the gas phase requires an activation energy of 230 – 420 kJ/mol.
At pressures above 200 MPa (2000 bar), the synthesis of ammonia proceeds even in the absence of specific catalysts. At such extreme pressures the vessel walls appear to catalyze the formation of ammonia.
In the catalytic combination of nitrogen and hydrogen, the molecules lose their translational
degrees of freedom by fixation on the catalyst surface. This drastically reduces the required energy of activation, for example, to 103 kJ/mol on iron. The reaction may then proceed
in the temperature range 250 – 400 ◦C. In 1972, it was discovered that electron donor – acceptor
(EDA) complexes permit making ammonia with measurable reaction rate at room temperature.
If we focus on the catalytic part, iron is the most used catalyst. Iron catalysts which are generally used until today in commercial production units are composed in unreduced form of iron oxides
(mainly magnetite) and a few percent of Al, Ca, and K; other elements such as Mg and Si may
also be present in small amounts. Activation is usually accomplished in situ by reduction with
synthesis gas. Prereduced catalysts are also commercially available.
As with every catalytic gas-phase reaction, the course of ammonia synthesis by the Haber – Bosch process can be divided into the following steps:
1) Transport of the reactants by diffusion and convection out of the bulk gas stream, through a laminar boundary layer, to the outer surface of the catalyst particles, and further through the pore system to the inner surface (pore walls)
2) Adsorption of the reactants (and catalyst poisons) on the inner surface
3) Reaction of the adsorbed species, if need be with participation by hydrogen from the gas phase, to form activated intermediate compounds
4) Desorption of the ammonia formed into the gas phase
5) Transport of the ammonia through the pore system and the laminar boundary layer into
the bulk gas stream
Only the portion of the sequence that occurs on the catalyst surface is significant for the intrinsic catalytic reaction. Of special importance is the adsorption of nitrogen. This assumption is decisive in representing the synthesis reaction kinetics. The transport processes occurring in the pores of the catalyst in accordance with the classical laws of diffusion are of importance
in industrial synthesis.
Talking about catalyst surface and reaction mechanism, on iron catalysts nitrogen adsorption and dissociation can be regarded as the rate-determining step for the intrinsic reaction. The activation energy for dissociative nitrogen adsorption is not constant; it increases as the surface becomes increasingly covered with adsorbed nitrogen species.
At lower temperatures nitrogen becomes adsorbed only in the molecular state, but subsequently dissociates when the temperature is raised. Isotopic experiments with 30N2 and 29N2 showed that
the surface species resulting from low-temperature adsorption was molecular, whereas that from high-temperature adsorption was atomic. Besides, ammonia synthesis is highly sensitive to the orientation of the different crystal planes of iron in the catalyst ( Fe(111) is the most active
surface, as we can see in the next picture)
A possible explanation for the high activity of faces (111) and (211) is that these are the only surfaces which expose C7 sites (iron with seven nearest neighbors) to the reactant gases. Highly coordinated surface atoms should show increased catalytic activity due to low-energy charge fluctuations in the d-bands of these highly coordinated atoms. This argument might probably be
the key for the special role of C7 sites. Other reasons discussed are based on charge transfer and
interaction of iron d-bands with anti-bonding 2 π* orbitals of nitrogen.
Promotion with potassium of single iron crystals enhances the sticking probability for nitrogen dissociation much more on the Fe(100) and (110) than on the Fe(111) to the effect that the differences in surface orientation disappear. Other experiments show that even the least active
face Fe(110) becomes as active for the synthesis as Fe(111) after addition of alumina with subsequent annealing with oxygen and water vapor. The mechanisms seems to be that first alumina
forms an iron aluminate FeAl2O4 on the surface.
This new surface then may serve as a template on which iron grows with (111) and (211) orientation upon exposure to the synthesis-gas mixture in the reaction.
Based on these experimental results a reaction scheme for the ammonia synthesis may be
formulated comprising the following sequence of individual steps:
Industrial ammonia synthesis in the homogeneous gas phase is not feasible because of the
high dissociation energies for the initial steps. The reaction over a catalyst avoids this problem since the energy gain associated with the surface atom bonds overcompensates these dissociation energies and the first steps have actually become exothermic.
Dissociative nitrogen adsorption remains nevertheless the rate-determinating step, because of the very unfavorable preexponential factor in its rate constant. The subsequent hydrogenation steps are energetically uphill, but the energy differences involved can easily be overcome at the temperatures
used in industrial ammonia synthesis. It is, however, quite apparent that the rate-controlling step
would switch from dissociative nitrogen adsorption to hydrogenation of adsorbed atomic nitrogen species if the temperature were lowered sufficiently because of these differences in activation energy.
For industrial catalysts made by careful reduction of magnetite fused with nonreducible
oxide promoters the important role of the (111) face seems to be confirmed. However, the
question whether the active industrial catalyst exposes mostly (111) faces remains unresolved.
If not, further improvements of the catalyst are at least theoretically possible.
This was indeed the case with the new AmoMax-10 catalyst [169, 170] of Sudchemie. This catalyst in its oxidic form is based on wustite instead of magnetite and has a significantly better activity at lower temperature than magnetite.Wustite is a nonstoichiometric iron oxide(Fe1−xO) with a cubic crystal structure, x ranging from 0.03 to 0.15.With still (111) faces present in the reduced state the catalyst has a higher specific surface area and an improved pore structure.
Nielsen investigated ammonia synthesis on a commercial Topsøe catalyst, KM IR, over
a wide temperature range. They found evidence that a different reaction mechanism predominates below and above 330 ◦C. Also, at low temperatures, chemisorbed hydrogen blocks the catalyst surface.
Reaction mechanisms on non-iron catalysts
Non-iron systems which exhibit some potential to catalyze ammonia synthesis can be divided into the following groups:
– Platinum group metals such as Ru, Os Ir, Pt
(no nitrides)
– Mn, Co, Ni, Tc, Rh and their alloys (no nitride formation under synthesis conditions)
– Mn, Mo, V (present as nitrides under the reaction conditions)
In the non-iron systems the rate-determining step is also dissociative adsorption of nitrogen and the catalyst effectivity depends on the activation energy of the dissociation reaction. The
factor common with the iron catalyst is the structure sensitivity.
The only system which seems to be promising for industrial application is ruthenium promoted
with rubidium on graphite as carrier.
Kinetics
The catalyst used in ammonia synthesis have an influence also in the kinetics of the process. The applicability of a particular equation depends on the state of reduction of the catalyst and the type of promoter
Transport phenomena
The high velocity of the gas passing through the converter creates sufficient turbulence to keep the film thickness rather small in relation to the catalyst grain size.
For this reason the largest concentration gradient (with respect to the concentration in the bulk gas stream) is within the catalyst particles. Since the thermal conductivity of the iron catalysts is much higher than that of the synthesis gas, the major temperature difference is in the external gas film, while the catalyst particles themselves operate under approximately isothermal conditions.
For the particle sizes used in industrial reactors (≤ 1.5 mm), intra particle transport of the
reactants and ammonia to and from the active inner catalyst surface may be slower than the
intrinsic reaction rate and therefore cannot be neglected.
The overall reaction can in this way be considerably limited by ammonia diffusion through the pores within the catalysts. Besides, the apparent activation energy and reaction order, as well as the ammonia production per unit volume of catalyst, decrease with increasing catalyst particle size.
Catalysts
This is the main topic of the course, so it needs to be explained in the correct way. Moreover, catalysts is viewed as the most important part in an ammonia plant. For a given operating pressure and desired production, it determines the operating temperature range, recycle gas flow, and refrigeration requirement. As a result, it directly fixes vessel and exchanger design in the synthesis loop. It also indirectly influences the make-up gas purity requirement, and so the operating pressure, and capital cost, and energy consumption for synthesis gas production and purification. Although the proportionate cost of catalysts compared to the total cost of a modern ammonia synthesis plant is negligible, the economics of the total process are determined considerably by the performance of the ammonia catalyst.
Industrial catalysts for ammonia synthesis must satisfy the following requirements:
1) High catalyst activity at the lowest possible reaction temperatures in order to take advantage of the favorable thermodynamic equilibrium situation at low temperatures. Average commercial catalysts yield about 25 vol% ammonia when operating at 40 Mpa (400 bar) and 480º C catalyst end temperature, which corresponds to a 535º C equilibrium temperature
2) The highest possible insensitivity to oxygen and chlorine-containing catalyst poisons,
which may be present in even the very effectively purified synthesis gas of a modern process. In assessing the newly developed catalyst systems recommended for operation at very low temperatures, it must be kept in mind that the effect of poisons, for example, oxygen compounds, may become more severe as temperature declines.
3) Long life, which is determined essentially by resistance to thermal degradation and to irreversible poisoning. In older high-pressure plants (60 – 100 Mpa), catalyst life was a big issue; because the catalysts in these plants showed a markedly reduced life owing to the severe operating conditions, the necessary downtime for removing, replacing, and reducing the catalyst had a considerable effect on the ammonia manufacturing cost. In modern single-train ammonia plants, conventional iron catalysts achieve service lifetimes up to 14 years.
4) Mechanical Strength. Insufficient pressure and abrasion resistance may lead to an excessive increase in converter pressure drop, and so to a premature plant shutdown.
5) Because of the high and increasing world demand for ammonia, a reliable primary
raw material source. For example, osmium, which was planned as the first industrial catalyst, is so scarce that, in 1910, as a precautionary measure for this option, BASF had secured almost the total world supply.
The ammonia synthesis catalyst problem has been more intensively studied than the catalysis of any other industrial reaction. At BASF A. Mittasch started a tremendous program, in which up to 1911 more than 2500 different formulations were testet in more than 6500 runs.
The experiments were finally brought to an end in 1922 after a total of 22 000 tests. They tested almost all elements of the periodic table for their suitability as ammonia catalysts. From these experiments came a series of technical findings, for example, concerning the relationships between catalytic effectiveness and the strength of the nitrogen bond and reducibility, or relating to the mechanism of opposing activation or inactivation in doubly promoted systems. In principle, metals or metal alloys are suitable as ammonia catalysts, above all those from the transition-metal group.
Metals or metal compounds for which the chemisorption energy of nitrogen is neither too
high nor too low show the greatest effectiveness, so the magnetite based catalyst proved suitable for industrial use. In the next picture we can see the rate constants of ammonia decomposition (A) on an ammonia synthesis capacities (B) of metals as a function of the enthalpy.
Classical iron catalysts
The only catalysts that have been used have been iron catalysts promoted with nonreducible oxides. Recently, a ruthenium based catalyst promoted with rubidium has found industrial application. The basic composition of iron catalysts is still very similar to that of the first catalyst developed by BASF.
The catalytic activity of iron was already known well before the advent of industrial ammonia synthesis. Ramsay and Young used metallic iron for decomposing ammonia. Pure iron
showed noticeable initial activity which, however, could be maintained for longer operating
periods only with extremely pure synthesis gas.
The ammonia synthesis catalyst problem could be considered solved when the catalytic effectiveness of iron in conversion and its onstream life were successfully and substantially improved by adding reduction-resistant metal oxides. The iron catalysts promoted with aluminum and potassium oxides. proved to be most serviceable. Later, calcium was added as the third activator. Modern catalysts additionally contain other promoters that were present in the older catalysts only as natural impurities from the raw materials.
The high purity gas of modern processes and the trend to lower synthesis pressures especially favor the development of more active and easily reducible types of catalysts, at some sacrifice in temperature stability and resistance to poisons. Thus, individual catalyst manufacturers now offer several catalyst types in various particle size distributions, in oxidic and pre-reduced states.
Composition
The principal component of oxidic catalysts is more or less stoichiometric magnetite, Fe3O4 ,which transforms after reduction into the catalytically active form of α-iron. The degree of oxidation of industrial catalysts has a considerable influence on their catalytic properties. For industrial catalysts, the highest ammonia yields are observed with an Fe(II) – Fe(III) ratio of 0.5 – 0.6.
Freshly reduced commercial iron catalysts that contain aluminum, potassium, and calcium oxides as basic promoters consist of approximately 30-nm primary crystallites; the spaces between them form an interconnecting system of pores. Besides a maximum at a pore of about 10 nm that originates on reduction of the Fe3O4 (magnetite) phase of the nonporous oxidic catalyst, the pore distribution curve (Fig. 15) generally shows a peak at 25 – 50 nm that is formed on reduction of the wustite
phase.
After reduction, about 1 wt% of the alumina also remains statistically distributed in the form of FeAl2O4 molecular groups built into the α-iron lattice of the reduced catalyst. A correlation exists between the distribution of the potassium and that of aluminum and/or silicon. Calcium oxide segregates, essentially at the grain boundaries, into separate regions, probably as a mixture of the silicate and ferrite. Apparently homogeneous regions that have originated from reduction of Fe3O4 crystallites alternate with nonhomogeneous regions that are formed by the reduction of FeO crystals or consist of amorphous phases.
About the structure, macroscopic particles in the reduced catalyst are confined by fracture lines running through a system of blocks consisting of stacks of slabs in parallel orientation. This texture is stabilized by structural promoters, which act as spacers and “glue,” separating neighboring platelets and thus providing voids for the interconnection of the pore system. There is also evidence that the basal plane of many platelets has the Fe(111) orientation.
There is an influence of the promoters, which have specific action of the metal oxides.
· Structural stabilizers, such as Al2O3, produce a high inner surface during reduction and stabilize it under thermal stress by restraining iron crystallite growth.
· Electronic promoters, such as the alkali oxides, enhance the specific activity (based on a unit surface) of iron – alumina catalysts. However, they reduce the inner surface or lower the temperature stability and the resistance to oxygen-containing catalyst poisons. In the alkali metal series, the promoter effect increases with increasing atomic radius, and the destructive effect with decreasing atomic radius.
· Promoter oxides that are reduced to the metal during the activation process and form an alloy with the iron are a special group.
For ammonia plants operating at pressures up to 35 MPa (350 bar), catalyst end temperatures of 520 – 530º C maximum, and with highly purified synthesis gas, the preferred catalysts contain 2.5-4.0% Al2O3 , 0.5 – 1.2% K2O, 2.0 – 3.5% CaO, 0 – 1.0% MgO, and a natural content of about 0.2 –0.5% SiO2.
The effect of the promoters on the rate of reduction and the temperature required for reducing the iron oxide phase is also significant in industrial practice.
Mechanism of the Promoter Effect.
The action of the so-called structural promoters (stabilizers), such as Al2O3, is closely associated
with their solubilities in the iron oxide matrix of the unreduced catalyst or with the capability of the regular crystallizing magnetite to form solid solutions with iron – aluminum spinels. During reduction part of the alumina precipitates with other promoters into the surface of the iron crystallite in a molecularly dispersed distribution or in small islands. Small crystals of nonreducible oxides dispersed on the internal interfaces of the basic structural units (platelets) will stabilize the active catalyst surface Fe(111). The primary function of alumina is to prevent sintering by acting as a spacer, and in part it may also contribute to stabilizing the Fe(111) faces.
We can also find calcium oxide. In the reduced catalysts, it segregates between the iron crystallites and so possibly prevents sintering together at high operating temperatures.
In the reduced catalysts, the potassium exists as a K + O adsorption layer that covers about 20 – 50% of the iron surface. The enhancement of the catalysts specific activity by potassium oxide is accompanied by a decrease in the electron work function. The promoting effect of potassium seems
to be based on two factors which probably act simultaneously. One mechanism is the lowering of the activation energy for the dissociative adsorption of nitrogen (A nitrogen molecule adsorbed near such a site will experience a more pronounced backbonding effect from the metal to its antibonding
π orbitals). . The other effect consists of lowering the adsorption energy of ammonia, which avoids hindering of nitrogen adsorption by blocking (poisoning) of the catalyst surface by adsorbed ammonia molecules. With increasing potassium concentration we obtain the decreasing specific surface in the reduced catalyst.
Particle size and shape
The choice depends on two main factors:
· Catalyst performance.
· Pressure drop
From the standpoint of space – time yield, it is desirable to use the finest possible particle,
which, practically speaking, is about 1–2mm; however,with decreasing particle size, the pressure drop and the risk of destructive fluidization of the catalyst increase.
In catalyst zones in which the ammonia formation rate is so high that the allowable temperature limits are exceeded, it may be advantageous to use coarse particles for suppressing the reaction.
Two effects cause the low production capacity of coarse-grained catalyst: first, large grain
size retards transport of the ammonia from the particle interior into the bulk gas stream, because this proceeds only by slow diffusion through the pore system. Slow ammonia diffusion inhibits
the rate of reaction. At the high reaction rate typical for the converter inlet layer, only a surface layer of the catalyst grains, about 1–2mm thick, participates in the reaction.
The second effect is a consequence of the fact that a single catalyst grain in the oxidic state is
reduced from the outside to the interior of the particle: the water vapor produced in the
grain interior by reduction comes into contact with already reduced catalyst on its way to the
particle outer surface; this induces a severe recrystallization.
An irregular grain shape has a more favorable effective activity for the individual particle and for radial intermixing of mass and heat in an industrial converter, but regular shapes have the advantages of greater abrasion resistance and lower pressure drop.
The application of macroporous catalysts ought to be especially useful for very low synthesis pressures and in plants in which large catalyst particles must be used for reasons of low pressure drop.
The term “ammonia catalyst” commonly refers to the oxidic form consisting of magnetite and
oxidic promoters. In fact this is only the catalyst precursor which is transformed into the active catalyst composed of α-iron and promoters by reduction with synthesis gas, usually in situ. The reduction step is very important for catalyst performance.
In 1996 the prices of commercial ammonia catalysts were about 2 $/lb (3.58 €/kg) for oxidic and about 5.5 $/lb (10.23 €/kg) for prereduced. The manufacture process is showed in the next picture.
Catalyst reduction
The reduction of oxidic catalyst is generally effected with synthesis gas. The magnetite is converted into a highly porous, high surface area, highly catalytically active form of α-iron. The promoters, with the exception of cobalt, are not reduced. It is important to hold the partial pressure of the resulting water vapor as low as possible and to insure that the water vapor does not come into contact with regions that have already been reduced. High temperature and high water vapor partial pressure markedly accelerate premature catalyst aging by recrystallization. Therefore, the reduction should be carried out at high gas velocities [about 5000 – 15 000 m3 m−3 h−1
(STP)], at the lowest temperatures sufficient for complete reduction, and at not too high pressures (7 – 12 MPa in low pressure, 25 – 30 MPa in high-pressure plants) to hold the exothermic formation of ammonia under better control during the reduction. The reducibility of industrial catalysts is dependent on both the combination of promoters and the degree of oxidation. The FeO (wustite) phase is reduced faster and at lower temperatures than the Fe3O4 (magnetite) phase.
On the atomic scale the reaction is controlled by
two processes:
1) Metallic iron is formed from wustite by direct chemical reaction controlled in the initial
phase by the reaction rate (activation energy ca. 65 kJ/mol) and in the final stage by diffusion processes involving hydrogen and water on the reaction site.
2) The chemical reaction creates an iron(II) ion
concentration gradient in the solid. This gradient leads to a rapid diffusion of iron(II) ions
from magnetite through wustite to the chemical reaction interface, where they are reduced and precipitated as iron nuclei.
Prereduced catalysts have the full pore structure of active catalysts, although the pore surface has been oxidized to a depth of a few atomic layers to make these catalysts nonpyrophoric.
Catalyst poisons
The activity of an ammonia synthesis catalyst may be lowered by certain substances, commonly referred to as poisons. These substances can be minor gaseous constituents of the synthesis gas or solids introduced into the catalysts during the manufacturing procedure, derived from impurities in the natural magnetite from which the catalyst is made. General measures to avoid this sort of contamination include selecting a rather pure magnetite, the application of pretreatment processes, and the use of high-purity promoters. The melting process itself may also contribute to minimizing the content of some minor impurities.
Oxygen compounds have a reversible effect on iron catalysts at not too high temperatures. That is, the activity of a damaged catalyst may be practically completely restored by reduction with clean synthesis gas. With continuing exposure oxygen compounds also cause irreversible damage to the catalyst activity that is causally linked with growth of the iron primary crystallite.
Sulfur,Phosphorus, and Arsenic Compounds. Sulfur, occasionally present in synthesis gases
from coal or heavy fuel oil, is more tightly bound on iron catalysts than oxygen. For example, catalysts partially poisoned with hydrogen sulfide cannot be regenerated under the conditions of industrial ammonia synthesis. Compounds of phosphorus and arsenic are poisons but are not generally present in industrial syngas.
As a little summary, The catalyst has no effect on the position of chemical equilibrium; rather, it provides an alternative pathway with lower activation energy and hence increases the reaction rate, while remaining chemically unchanged at the end of the reaction. The first Haber–Bosch reaction chambers used osmium and ruthenium as catalysts. However, under Bosch's direction in 1909, the BASF researcher Mittasch discovered a much less expensive iron-based catalyst that is still used today. Part of the industrial production now takes place with a ruthenium rather than an iron catalyst (the KAAP process), because this more active catalyst allows reduced operating pressures.
In industrial practice, the iron catalyst is prepared by exposing a mass of magnetite, an iron oxide, to the hot hydrogen feedstock. This reduces some of the magnetite to metallic iron, removing oxygenin the process. However, the catalyst maintains most of its bulk volume during the reduction, and so the result is a highly porous material whose large surface area aids its effectiveness as a catalyst. Other minor components of the catalyst include calcium and aluminium oxides, which support the porous iron catalyst and help it maintain its surface area over time, and potassium, which increases the electron density of the catalyst and so improves its activity.
Rafael Giner Tovar
References:
Wiley library online
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