Ammonia synthesis

Ammonia is used in various applications, such as textile processing, water purification, and manufacturing explosives. The main part, however, is used as fertilizer. Ammonia production consumes about half the hydrogen produced today and is the primary chemical industry use of hydrogen. Ammonia is currently made where there is inexpensive natural gas that provides economical hydrogen and shipped to the costumer. The low cost of shipping ammonia favors ver large ammonia production plants with very large demands for hydrogen.

At the beginning of the 20^th century, the use of nitrogenous fertilizers was already well established. Haber and Bosch development the direct synthesis of ammonia form hydrogen and nitrogen.

The synthesis of ammonia from nitrogen and hydrogen is a clean reaction, in that it is not complicated by the formation of byproducts, such as hydrazine, and the thermodynamics are seemingly straightforward.The reaction is exothermic and is accompanied by a decrease in volume at constant pressure. The value of the equilibrium constant therefore increases as the temperature is lowered, and the equilibrium ammonia concentration increases with increasing pressure.

The formation of ammonia is favoured by operation at high pressure and low temperature. The optimum pressure for economic operation with the available catalysts has been in the range 150-350 bar. Normally, the advantages of the higher equilibrium concentration of ammonia at very high pressure are more than offset by the higher costs of both gas compression and additional plant capital. The temperature at which the synthesis process is operated is determined by the activity characteristics of the catalyst. Thermodynamically, low temperature is advantageous, but for kinetic reasons high temperatures have to be used. The most effective catalyst is clearly the one that will give the highest rate of conversion of ammonia at the lowest temperature.

As the synthesis reaction proceed, the heat of reaction causes the temperature to rise down the bed, o making the specific rate of reaction faster. Since the equilibrium becomes less favorable at higher temperature, the rate of the reverse reaction is progressively increased and the overall conversion becomes equilibrium-controlled. Careful control of the temperature profile is therefore necessary for the equilibrium balance to be obtained between the limit set by the forward (synthesis) and reverse (ammonia decomposition) directions.

Reaction Mechanism

The ammonia synthesis reaction from N₂ and H₂ is composed of serveral steps:

dissociation of N₂, dissociation of H₂, and formation of N-H bonds.

On the best elementary metal catalyst(Ru, Fe) the branchless NH₃ synthesis proceeds along a well-defined Langmuir-Hinshelwood reaction path. Hence, the stepwise addition of adsorbed H to the N-compounds can be written as:

Where* and X* correspond to an empty site and an adsorbed X chemical compound, respectively.

The main role of the ammonia catalyst id to dissociate the N₂ bond. Under industrial conditions, this is the rate-determining step for NH₃ synthesis on Ru, due to the high bond energy (Fig1.1). The dissociate takes place at defects and steps, rendering the NH₃ synthesis extremely structure sensitive.

Ammonia Synthesis Catalyst

All commercial ammonia synthesis catalysts are currently based on metallic iron promoted with alkali(K), and various metal oxides, such as those of aluminum, calcium or magnesium. The principal material used to make these catalysts is usually magnetite(Fe₃O₄), with some of the components in the catalyst originating as impurities in the magnetite. A typical catalyst contains approximately 0.8% K₂O,2.0% CaO, 0.3MgO, 2.5% Al₂O₃ and 0.4% SiO₂, as well as traces of TiO₂, ZrO₂, and V₂O₅. in developing the process to manufacture catalyst of this sort, it was recognized that these minor components could have a large effect on the performance of the final catalyst, since they may also interact with each other, giving rise to both harmful and beneficial effects. In modern catalysts, these factors have been taken onto account, resulting in optimized performance in terms of high activity and long life.

The main component of the catalyst, iron, has remained unchanged since catalyst was first introduced in 1913, despite a large amount of research the into alternative formulations. Magnetite has a spinet structure (similar to that of MgA1₂O₄) consisting of a cubic packing of oxygen ions, in the interstices of which Fe²⁺ and Fe³⁺ ions are distributed. During reduction, oxygen is removed from the crystal  lattice without  shrinkage, so the metallic iron is obtained as a pseudomorph of the original magnetite. Metallic iron produced in this way is therefore extremely porous, and the way in which porosity is developed is an important factor affecting the activity of the final catalyst. Another major factor is the size of the individual crystals of iron produced during reduction. This is largely determined by the nature and amounts of promoters present and of course of the actual conditions during reduction.

The most important promoters are alumina and potash, which generate the so-called "doubly promoted" catalyst, but several other oxides may also be added. Promoters as calcium oxide, silica, magnesia and the oxide of manganese, chromium, zirconium and vanadium, are conveniently classified as structural or electronic, depending on their accepted mode of action.

The production and preservation of a porous structure during the reduction of the catalyst is of fundamental importance in obtaining high activity, and the prime role of structural promoters (A1₂O₃, MgO and Cr₂O₃) is to facilitate the formation of porous, high area, metallic iron.

Silica and other acidic components are common impurities in natural magnetite used for catalysts production, and consequently they become incorporated into the structure of the final catalyst. They have the effect of neutralizing the electronic promotional effect of basic components such as K₂O and CaO, and their presence in excess can result in lower catalyst activities. However, silica also has a stabilizing effect like alumina, and high-silica catalysts tend to be more resistant to both water poisoning and sintering.

The  presence of alkali-metal species in the ammonia synthesis catalyst is essential to obtain high activity. Although all of the alkali metals are effective to some extent, the best are potassium, rubidium and cesium, with potassium being the most cost effective. On reduction of the catalyst, much of the potash remains associated with the somewhat acidic support phase, though some interacts with the iron particles and greatly increases their activity.

In the early 1970s, an alkali-metal-promoted carbon-supported Ru catalyst was introduced, it exhibited a 10-fold increase in activity over the conventional iron-based catalyst under similar conditions. In 1992 using this kind of graphite-supported ruthenium catalyst, a 600 tonn NH₃ per day plant has begun to produce ammonia under milder condition compared with the plants using iron-based catalysts. Thus, it is believed that Ru一based catalyst could become the second-generation catalyst for ammonia synthesis.  However, the ruthenium-based  catalysts  are not widely employed in industrial ammonia synthesis because they are expensives.

Because of the advance in the ruthenium-based catalyst for ammonia synthesis, a number of research groups have investigated the roles of support and promoter of the catalyst played in the catalytic reaction. Various supports, such as active carbon and graphite, alumina, carbon-coated alumina, magnesia, a series of zeolites and even rare-earth metal oxides were used in Ru-based catalysts for the ammonia synthesis. The promoters used are mainly alkali metal, alkaline earth metal, rare earth metal and their oxides or hydroxides. However, it seems that the ruthenium supported on carbon material with promoters is the most promising catalyst for commercial applications. This is due to the many attributes linked with carbon materials, such as specific electronic properties, variable surface functional groups, and easy metal recovery by the burning of the support.

For the above-mentioned reason, Li and coworkers carried out studies to investigate in detail the ammonia synthesis on the ruthenium catalysts supported on different carbon materials  promoted with K and Ba compounds under moderate pressure. They found that the Ba and K promoters change Ru dispersion, modify electron properties on Ru surface, and neutralize the surface groups of carbon supports; as a result, these promoters significantly enhance the ammonia synthesis activity of Ru/C catalysts. The Ba-promoted Ru/AC (active carbon) catalyst gives the highest ammonia concentration, because Ru metal is well dispersed on the support, while Ru-Ba/ACF (active carbon fiber) gives the highest TOF value (turnover frequency). This is due to the high purity and electronic conductivity of ACF. The low activity on the CMS (carbon molecular sieve) support can be attributed to the low surface area and the lower graphitization. Therefore, the carbon supports with high purity, high electronic conductivity and high surface area favor the high activity of Ru-based catalysts for ammonia synthesis. The ammonia synthesis on Ru/C catalyst is structure sensitive and exhibits higher activity, but the TOF becomes constant when Ru particle size is beyond about 5nm.

To avoid methanation and the easy loss of the carbon support, Liao and coworkers studied bimetallic catalysts (Ru-M, M=Fe, Co, Ni, Mo) with low ruthenium loadings and without a carbon support, but MgA1₂O₄, MgO and a Mg-AI complex oxide obtained by calcining hydrotalcite type precursors at 800℃. It was shown that the bimetallic Ru-Co/MgO exhibited good ammonia synthesis activitys.

Ru-based catalysts are known to be active for NH₃ synthesis at atmospheric pressure and low temperature'0,". The most stable and commercially available Ru precursor used for ammonia synthesis is RuCl₃. The main advantage of this metal precursor is its low cost in comparison to other metal precursors, but the main disadvantage of using RuCl₃ is the strong binding of chlorine atom with the metal surface even after reduction. Alkali metal nitrate used as a promoter is thought to have two effects: one is to remove chlorine ions from the catalyst and the other is to donate electrons to ruthenium. The promoting effect of alkali metal to Ru on NH₃ synthesis activity is found to be inversely proportional to the electronegativity of the alkali metal in the order of Cs>Ba>K>Na. It has been observed that the high activity of the active carbon supported promoted ruthenium catalysts are attributed to the electron deficient graphite lattice of active carbon. The Mg0 support shows less activity because of strong interaction with chlorine in RuCl₃. The strong chemisorption of product ammonia on the acid centers of alumina makes it a less attractive support for ruthenium. Hydrotalcite like compounds are a class of precursors useful for the preparation of catalytically active oxides showing basic properties. Rama Rao et al" shows in a recent work the comparison of activity studies of ammonia synthesis over Ru supported on mixed oxide support obtained from Mg-AI HT precursor (with Mg/AI ratio of 2) with the Ru catalysts supported on simple oxides Mg0 and A1₂O₃ under atmospheric pressure. They found that calcined hydrotalcite precursor is the promising support for the ammonia synthesis,  because it is highly basic, thermally stable, and is resistant to Cl^- when compared to MgO support. Ruthenium with cesium promoter over novel Mg-AI HT support is found to be very efficient for the synthesis of ammonia at atmospheric pressure compared to Cs-Ru catalysts on MgO and Al₂O₃ supports. The higher activity of the Cs- Ru/HT catalyst has been attributed to the presence of easily reducible Ru species and nano-particles of Ru in highly dispersed form over calcined Mg-AI HT support".

In a recent work, Liao et al studied new supported catalysts to develop catalysts with high stabilization (MgO) and high activity (CNTs=carbon nanotubes). They studied K-Ru/Mg0 and K-Ru/CNTs separately and then combined at different weight ratio to obtain a combination-type  catalyst. For the as-prepared combination-type ruthenium catalysts, ruthenium particles well disperse on the surfaces of MgO and CNTs relatively. The optimal weight ratio of K-Ru/MgO to K- Ru/CNTs for the preparation of the combination-type ruthenium catalysts is 1/1，that is because the combination between MgO and CNTs is strong and the interactional effects are prominent  at this weight ratio. A complementary interaction between two supports is suggested, which promotes electron transfer from alkali metallic atoms to the B5-sites of ruthenium more easily. Due to the high catalytic activity and high thermal stability under operating conditions, the combination-type ruthenium catalyst of K-Ru/MgO and K-Ru/CNTs may be a good catalyst for ammonia synthesis.

Ammonia Plant

The principle of circulating gases over the catalyst, which was first appreciated by Haber in 1908, is still an important feature of modern ammonia plants. Synthesis gas of the appropriate composition passes through the catalyst beds, and the ammonia produced is condensed and recovered. Unreacted gas, to which fresh make-up gas is added, is then recirculated through the catalyst. Using heater exchangers the temperature of the recirculating gas is raised in two stages to a reaction temperature of about 400℃ and, at the same time, the temperature of the converter effluent gas is reduced. To prevent accumulation of inert gases generally present in the synthesis gas, part of the circulating gas is purged. The residual ammonia in the purge gas is usually recovered, and the hydrogen content  is  either used  as  fuel  in  the  primary  reformer or recovered  and recirculated'.

In modern ammonia plants, it takes 28-30 GJ/te to produce ammonia from natural gas by the overall reaction: 

Natural gas+water+air → 3H₂+N₂+CO₂ →ammonia+CO₂

The exergy consumption for ammonia production depends strongly on the ammonia synthesis loop design. In general, even at high pressure up to 300 bar, not more than the 20-25% of the synthesis gas is converted to ammonia per pass. After the removal of ammonia by its condensation at low temperatures, the unreacted hydrogen一nitrogen mixture is returned to the reactor. Therefore, since its first development in 1913 by Haber and Bosch, industrial ammonia synthesis always has been implemented as a recycle process. Thus, to produce 1Kg ammonia, 4-6kg synthesis gas must be recycled through the reactor.

The energy efficiency of an industrial ammonia synthesis process depends on two types of parameters:

.Parameters defined by external systems: pressure and inerts (CH₄+Ar) content of the make-up gas, refrigeration plant and purge recovery.

.Internal parameters defined by the loop design: degree of conversion (or recycle ratio), reaction approach to equilibrium, inerts (CH₄+Ar) content in the circulating gas and reaction heat- and cold recovery system parameters.

The exergy efficiency of an ammonia synthesis loop depends strongly on the degree of conversion, respectively, on the recycle ratio. The higher the conversion is (respectively the lower the recycled/fresh gas ratio), the better the heat utilization and the less the exergy consumption for gas recycling and for ammonia condensation. However, exergy efficiency increases when the reaction approaches equilibrium, because exergy consumption for ammonia condensation decreases.

The equilibrium concentrations of components in the ammonia synthesis reaction depend on the pressure and the temperature, and, to a lesser extent, on the concentration of inerts. Since catalysts are active over the narrow temperature range of 380-500℃ and unable to approach more than 80%of the equilibrium, the maximum ammonia concentration in the recycling gas at the reactor outlet depends in fact only on the pressure.

Actually, outlet ammonia concentration is being chosen somewhat lower in order to reduce the catalyst volume, and the reactor costs. In most cases, the inlet and outlet ammonia concentrations are connected depending on the reactor design. The oldest "quench" and "cooling tube" reactors afford a degree of conversion of no more than  10-11%.Modern  reactor designs with intercoolers between catalyst beds provide a degree of conversion as high as 15%.With maximum outlet ammonia concentration of 20%，a degree of conversion of more than 16% may be reached only in the case of full removal of ammonia from recycling gas mixture, e.g. by water absorption. However, water absorption is not used in industrial ammonia plants and ammonia condensation is still the only way to separate ammonia from the unreacted gas mixture.

The inlet ammonia concentration depends on the vapor-liquid equilibrium (VLE) in the system NH₃-N₂-H₃-CH₄-Ar. Ammonia is the only component of this system, which might be condensed at non-cryogenic temperatures. However, the VLE position of ammonia in the presence of non-condensable gases at high pressures (80-300 bar) is very unfavorable. The real vapor pressure of ammonia in this system is up to twice as high as the equilibrium pressure of pure ammonia at the same temperature. Thus, the partial condensation of ammonia from the outlet gas mixture, containing 20%ammonia at 300 bar, begins at relatively low temperature (65-70℃). To obtain ammonia content as low as 2%, the gas mixture must be cooled down to temperatures such as-20℃. Therefore, the first stage of condensation is carried out in water-cooled or  air-cooled condensers. But the ammonia content at temperatures 25-30℃ is still high (7-8%)and in most modern ammonia plants an ammonia refrigeration unit is used to cool the recycling gas down to-20/+10℃ in order to decrease the converter inlet ammonia concentration down to 2一5%.

1.4 direct reduction of iron ore

Direct reduction of iron ore is today's major process for generating metallic iron, necessary in the iron and steel industry. World production of direct reduce iron (DRI) has grown from near zero in 1970 to 45.1 Mt in 20023,'x. In the production processes for converting iron ores into iron and steel, carbon, primarily in the form of coke, has been traditionally used to reduce the iron oxides

to iron metal. However, in the last several decades, there has been increasing production of iron using the direct reduction iron (DRI) process. In 1998, about 4%of the primary iron in the world was produced by the DRI process with rapid growth in iron production. In the DRI process, syngas (a mixture of hydrogen and carbon monoxide) made from natural gas is used to reduce iron ores to iron. The major chemical reactions are as follows:

\

The DRI process has lower capital costs than alternative methods used to produce iron, but requires a low-cost source of hydrogen. The primary market for DRI is to provide a purified iron feed for electric arc furnaces (EAFs) that produce various steel products. EAFs have lower capital costs than traditional steel mills and are environmentally cleaner operations than blast furnaces. Over a third of the world's steel production uses this process. It is predicted that by 2010 up to

45%of the world's steel may be made with EAFs. Historically, scrap metal has been the traditional feed for EAFs. However, there are two constraints: the availability of scrap metal and the various difficult-to-remove impurities (copper, nickel, chrome, molybdenum, etc.) that are present in the lower-grade scrap metal. Blending clean DRI一process iron with scrap metal dilutes the impurities below the level that affect product quality. Traditional steel-making processes using coke result in iron with a high carbon content and various other impurities from the coke.

Iron production is potentially a significant existing market for hydrogen. If low-cost hydrogen is available, the DRI process would replace other methods of iron production. The economics of DRI relative to other processes (and the potential demand for hydrogen) depend upon three factors.

.Technological developments: The continuing improvements in EAF technology in terms of reduced production costs and increased capabilities to produce higher-quality steel have expanded the market share of this technology. That, in turn, creates the demand for more high purity iron by the DRI  process as traditional sources of scrap metal are exhausted.

.Environmental protection: Traditional steel processes use coal and generate large quantities of  pollutants. Clean air requirements strongly affect the economics of these competing processes.

.Hydrogen costs: The process is used where there is low-cost natural gas for hydrogen production near iron deposits.