Ammonia syntheses

Ammonia is the second largest synthetic commodity manufactured by the chemical industry, with the global production capacity exceeding 140 million metric tons. Haber, in 1909, demonstrated that ammonia can be produced at a high pressure by the reaction 

This process is favored by a high pressure and a low temperature. 

The promoted iron catalyst to accelerate this reaction was discovered by Bosch, Mittasch, and coworkers, in 1909. Consequently, the industrial process for the production of ammonia is named the Haber-Bosch process. In this process, ammonia is formed by the reaction between N2 and H2 using a Fe304 (magnetite) catalyst promoted with A1203, CaO, K20, and other oxides. 

The active Fe is formed from the magnetite through a reduction produced by the reactant mixture both A1203 and CaO are structural promoters which preserve the high surface area of the active iron catalyst [5], The K influences the activity per unit area of the Fe by enhancing of the velocity of dissociative nitrogen chemisorption by increments of the adsorption energy [129], 

It has been established that the molecules implicated in the ammonia synthesis reaction undergo the following adsorption-desorption steps [127,129]  

Ammonia (mp —77 C, bp. ois —33J C. 4 0.6650y recovered during the water scrubbing of raw coke oven gases accounts for only a very small firaction in comparison with ammonia manufactured from its elements. 

Ammonia, or alkaline air, was isolated by Priestley in 1724, who found that it could be decomposed by electric sparks to give an increased volume of an inflammable gas. Later, it was shown that the decomposition product was a ttux-ture of hydrogen and lutrogen and that the reaction was reversible because 100% decomposition was not achieved at the elevated temperature required.  

Aimnonia could be formed when a nuxture of nitrogen and hydrogen was exposed to electric sparks. Ramsay and Young also found that traces of atmno-nia formed when hydrogen and rutrogen were passed over a heated plati- 

Le Chatelier began to work on the high-pressure formation of ammonia in 1901, but discontinued his experiments following a serious explosion. By 1900 it was thought that it should be possible to synthesize ammoiua from its elements, but it was not yet known whether a suitable industrial process using catalysts could be developed. 

The synthesis of ammonia from nitrogen and hydrogen is one of the most important processes in the chemical industry over 100 x 10 t/a of ammonia is produced worldwide. The Haber-Bosch process, introduced in 1913, was the first high-pressure industrial process. Ammonia synthesis is carried out at ca. 300 bar and 500 °C on iron catalysts with small amounts of the promoters AI2O3, K2O, and CaO. 

The elucidation of the reaction mechanism has occupied catalysis researchers up to the present day [11]. Over 20000 catalysts have been tested, but none has been 

Other metals are more active in cleaving the N=N bond (e.g., Li) but the resulting metal nitrides are too stable to take part in a catalytic cycle. 

For economic reasons, industrial catalysts consist of smelted iron oxides (60-70 % Fe) mixed with oxides of Al, Ca, Mg, and L, ground to 6-20 mm. During the activation of the catalyst by reduction, iron crystallites are formed with an interconnected pore system and an inner surface area of 10-20 m /g. The surface is partially covered by promoter oxides. 

The thermodynamic energy requirement is 2 x lO kJ/t NH3, which represents the theoretical minimum for all conceivable processes. Modern processes for the production of ammonia from natural gas have energy consumptions of around 3x10 kJ/t NH3, i.e., only 1.5 times the theoretical minimum energy consumption. Today much of the energy requirement can be covered by means of heat recovery. Modern ammonia plants produce up to 2000 t/d. 

The direct gas-phase synthesis of ammonia from nitrogen and hydrogen (the Haber process, 1908) is presently the cornerstone of the fertilizer industry  

It is implicit in reaction 9.4 that the equilibrium yield of ammonia is favored by high pressures and low temperatures (Table 9.1). However, compromises must be made, as the capital cost of high pressure equipment is high and the rate of reaction at low temperatures is slow, even when a catalyst is used. In practice, Haber plants are usually operated at 80 to 350 bars and at 400 to 540 °C, and several passes are made through the converter. The catalyst (Section 6.2) is typically finely divided iron (supplied as magnetite, Fe304 which is reduced by the H2) with a KOH promoter on a support of refractory metallic oxide. The upper temperature limit is set by the tendency of the catalyst to sinter above 540 °C. To increase the yield, the gases may be cooled as they approach equilibrium. 

The nitrogen required is obtained by fractional distillation of liquid air. The hydrogen used to be obtained by electrolysis of liquid water if inexpensive surplus electrical capacity becomes available in the future, this method may well be reintroduced. Catalytic photolysis of water using sunlight is another possible future source of H2. The Haher-Bosch process of 1916 used water-gas, which is a mixture of H2, CO, and CO2 made by alternating blasts of steam and air over coke at red heat  

The periodic air blasts to reheat the coke are necessary because of the marked endothermicity of reactions 9.5 and 9.6. The CO content is used to generate further H2 in the water-gas shift reaction-. 

The exothermicity of reaction 9.7 dictates a low temperature (450 °C or less) and use of a catalyst (Fe203/Al203, usually) if good yields are to be obtained in a reasonable time. Usually, the reaction is carried out in two steps. First temperatures of 450 to 500 °C are used and a substantial degree of conversion quickly results. Then the temperature is dropped to about 200 °C to optimize the yield. The CO2 is removed by scrubbing the gas with water, in which CO2 is much more soluble than H2. 

The early development of catalysts for ammonia synthesis was based on iron catalysts prepared by fusion of magnetite with small amounts of promoters. However, Ozaki et al. [52] showed several years ago that carbon-supported alkali metal-promoted ruthenium catalysts exhibited a 10-fold increase in catalytic activity over conventional iron catalysts under the same conditions. In this way, great effort has been devoted during recent years to the development of a commercially suitable ruthenium-based catalyst, for which carbon support seems to be most promising. The characteristics of the carbon surface, the type of carbon material, and the presence of promoters are the variables that have been studied most extensively. 

Thermal treatment of the support at much lower temperatures has also resulted in positive effects on the catalytic performance of Ru in ammonia synthesis. Zhong and Aika [56,57] treated three commercial activated carbons, with different ash contents, at temperatures ranging from 1073 to 1188 K under hydrogen. 

Catalysts prepared with raw activated carbons showed different activities, depending on their ash content, but heat-treated carbons showed similar activities. It was conclnded that the heat treatment was able to remove acidic imparities which were able to remove electron density from ruthenium and, in this way, decrease its catalytic activity. 

It has been claimed that carbon-supported ruthenium-based catalysts for ammonia synthesis show some important drawbacks, such as high catalyst cost and methanation of the carbon snpport under industrial reaction conditions. This has stimnlated the research for alternative catalysts, although the use of carbon snpports is a common feature. One example of these new catalysts is provided by the work of Hagen et al. [61], who reported very high levels of activity with barinm-promoted cobalt catalysts snpported on Vulcan XC-72. It was demonstrated that althongh cobalt had received little attention as a catalyst for ammonia synthesis, promotion with barium and the nse of a carbon support resulted in very active catalysts with very low NH3 inhibition. 

In reviewing thermodynamics, the nature of the heat of reaction and whether the reaction is reversible can be investigated. In the synthesis of aimnonia, reactors operate at about 200 atm and 350°C and approach the equilibrium conversion of about 70% in each pass. 

The ammonia is separated from unreacted H2 and N2, and these are recycled back to the reactor. The overall process of a tubular reactor plus separation and recycle produces essentially 100% NH3 conversion. 

In ammonia synthesis, high temperatures eonespond to small reaetor volumes. For exothermie reaetions, the equilibrium eonversion deereases as the temperature inereases. Therefore, these reaetions are often earried out in a series of adiabatie beds with either intermediate heat exehangers to eool the gases or bypass the eold feed to deerease the temperatures between the beds. Some eompromise ean be aehieved between high temperatures involving small reaetor volumes and high equilibrium eonversions. 

The equilibrium eonstant for ammonia synthesis is expressed as a funetion of the partial pressure as 

Since the partial pressure is the mole fraction in the vapor phase multiplied by the total pressure, (i.e., Pi = Yi P), the equilibrium 

More than half of the hydrogen used in the chemical industries or 40 % of the world production, approx. 200 10 Nm, is consumed in armnonia synthesis 

Feed gas for the steam reformer is methane or gasoline. High dilution with steam is chosen to keep the methane contents on a low level. Adding air in a secondary reformer leads to partial oxidation of the residual methane and of the CO. After separation of the CO2, the product gas is a mixture of nitrogen and hydrogen whose ratio (desired 1 3) is adjusted by the operating conditions. The system pressure is about 5 MPa, the synthesis temperature is 400 °C [27]. 

The production of one ton of ammonia requires about 2(XX) Nm hydrogen as well as approx. 800 kWh for compression of the synthesis gas and provision of the nitrogen (by air liquefaction). 

The world production of methanol is currently estimated to be about 27 million (metric) tons per year (1995). The methanol synthesis consumes about 5 % of the world hydrogen production. Methanol is basically used in the chemical industry as an intermediate product ( Cl chemistry ). It is gaining further attention as a secondary energy carrier with less CO2 emission, e.g., as a direct vehicle fuel or as a basis for the production of hydrogen-rich gas to feed fuel cells. 

The formation of methanol from synthesis gas can be described by the two independent reverse reactions that were given in section 5.1.1.6. for the methanol splitting process by high temperatures or steam reforming. In the conventional method, a mixture of CO, CO2, and H2 is compressed to about 10 MPa and introduced into a fixed-bed catalytic reactor at temperatures of 220 - 280 °C and pressures of 5 - 20 MPa [27]. 

The observed rate law depends on the type of catalyst used with promoted iron catalysts a rather complex dependence on nitrogen, hydrogen, and ammonia pressures is observed, and it has been difficult to obtain any definitive form from experimental data (although note Eq. XVIII-20). A useful alternative approach 

Ammonia is an important intermediate in the production of fertilizer and explosives. As explained in Chapter 1, the development of the ammonia synthesis represented a formidable technological achievement and a significant stimulus for the development of catalysis as a science. A plant for the synthesis of ammonia contains several catalytic processes. It starts with the production of hydrogen, by the nickel-catalyzed steam-reforming of natural gas 

The latter is carried out in two steps 1) with a cheap iron oxide catalyst and 2) with a more expensive supported copper-zinc oxide catalyst. After removal of the CO2 by pressure washing, residual traces of CO are hydrogenated in the methanation reaction, the reverse of the steam reforming (3.3), on a supported nickel catalyst. Nitrogen is obtained from the air. 

As high pressures and low temperatures shift the equilibrium to the right, the process calls for an active catalyst capable of driving the reaction to equilibrium at relatively low temperatures. 

Iron forms sufficiently strong bonds with nitrogen atoms to overcome the high bond energy of the N2 molecule. Ammonia is the end product of a chain of surface association reactions of adsorbed nitrogen and hydrogen atoms that proceeds via the formation of adsorbed NH intermediates. This is expressed by the following simplified scheme of reactions 

These reactions lead to a rate of reaction of the form 

Since the partial pressure is the mole fraction in the vapor phase multiplied by the total pressure, (i.e., p, = y, P), the equilibrium constant Keq is expressed as Keq = Ky PAn, where An = (2 - 1 - 3), the difference between the gaseous moles of the products and the reactants in the ammonia synthesis reaction. 

Initially, the source of ammonia was coke oven gas and Chile saltpetre. In Germany, in particular, it was recognized as early as the turn of the 20th century that insufficient ammonia was available for agricultural needs. Moreover, the use of ammonia for the manufacture of explosives increased dramatically due to the beginning of the First World War. Extensive efforts were made by teams in many countries, but particularly in Germany, to synthesise NH3 directly from N2. Non-catalyzed routes were discovered and were commercialised but they were very inefficient. The breakthrough was the development of a catalytic process. 

In 1905 Haber reported a successful experiment in which he succeeded in producing NH3 catalytically. However, under the conditions he used (1293 K) he only found minor amounts of NH3. He extrapolated his value to lower temperatures (at 1 bar) and concluded that a temperature of 520 K was the maximum temperature for a commercial process. This was the first application of chemical thermodynamics to catalysis, and precise thermodynamic data were not then known. At that time Haber regarded the development of a commercial process for ammonia synthesis as hopeless and he stopped his work. Meanwhile, Nernst had also investigated the ammonia synthesis reaction and concluded that the thermodynamic data Haber used were not correct. He arrived at different values and this led Haber to continue his work at higher pressures. Haber tried many catalysts and found that a particular sample of osmium was the most active one. This osmium was a very fine amorphous powder. He approached BASF and they decided to start a large program in which Bosch also became involved. 

The process development studies were carried out in a systematic way. A good catalyst had to be formulated the reactor was to be scaled up and an integrated process had to be designed, including the production of sufficiently pure synthesis gas. Haber envisaged the process scheme given in Fig. 1.1. 

Process scheme for the production of ammonia according to Haber. 

Systematic studies were carried out in order to discover a suitable catalyst. Iron catalysts were especially tried, because it was known that iron catalyzes the decomposition of ammonia, which is the reverse of the reaction being studied. It 

In most processes the reaction takes place on an iron catalyst. The reaction pressure is normally in the range of 150 to 250 bar, and temperatures are in the range of 350°C to 550°C. At the usual commercial converter operating conditions, the conversion achieved per pass is only 20% to 30%53. In most commercial ammonia plants, the Haber recycle loop process is still used to give substantially complete conversion of the synthesis gas. In the Haber process the ammonia is separated from the recycle gas by cooling and condensation. Next the unconverted synthesis gas is supplemented with fresh makeup gas, and returned as feed to the ammonia synthesis converter74. 

C) Product Recovery Before Recycle Compression (Four-Nozzle Compressor Design) 

The loop purge should be taken out after ammonia condensation and before make-up gas addition. This configuration depends on the make-up gas being treated in a drying step before it enters the loop. If the make-up gas contains traces of H20 or C02, it must be added before ammonia condensation. However this addition point will have negative effects on both ammonia condensation and energy efficiency53. 

Conventional reforming with methanation as the final purification step produces a synthesis gas that contains inerts (CH4 and argon) in quantities that do not dissolve in the condensed ammonia. Most of the inerts are removed by taking a purge stream out of the synthesis loop. The size of this purge stream controls the level of inerts in the loop at about 10% to 15%. The purge gas is scrubbed with water to remove ammonia and then it can be used as fuel or sent to hydrogen recovery. 

The competition for adsorption sites is very important for the kinetics of a heterogeneous catalytic reaction. For this reason sites,, are included as a reactant in the kinetic model. 

As a site must be either free or occupied by one of the surface intermediates, there is a conservation law for the coverages 

In writing this equation we have implicitely defined 0x =1 to be saturation. With this convention, coverages may be interpreted as probabilities. 

When the mechanism has only one slow step, the system of equilibrium equations and the rate equation may be solved with respect to 0  

The coverages by intermediates may be expressed by 0 and the partial pressures of the reactants and products. 

A linear relationship was found between the catalyst activity and the electron work function cp. A heterogeneous ionic mechanism (Sect. 4.1.1.5)waspropo  

Possibly, for a larger p the Nj ion expends a greater part of its energy on the extraction of an electron from the lattice and displacement of another (which migrates to the plasma), the energy being otherwise consumed in the raising of temperature and desorption of atoms. 

In another series of experiments palladized Pd and platinized Pt wires obtained by electrolyzing in PdCl2 and H2PtCl6 solutions were found to be more active than 

Pd and Pt wires, respectively (Fig. 20), perhaps due to their large surface areas and to vigorous recombination of H atoms on these metals. On clean glass, the maximum concentration of NH3 was reached for the stoichiometric N2 + 3 Hj mixture on palladized Pd for N2 H2 = 2 3 (Fig. 21). The displacement of the maximum concentration of NH3 towards N2 has been explained on the basis of an increase of the concentration of ions which are discharged cm the surface of the metal and dissociate in the adsorbed state forming the N atoms necessary for NH3 formation. 

The rate of reaction is then fonnd by taking the rate of the rate-determining step (since it has the only net rate that has not been assumed to be equal to zero) by insert- 

The fact that steps (2) to (6) are equilibrated means that 

The first of these equations give the hydrogen coverage as  

Combining with the site conservation rule (Equation (5.38), we can solve analytically to get the coverage of free sites as 

This allows us to write the rate of ammonia synthesis as 

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The technique of low-energy electron diffraction, LEED (Section VIII-2D), has provided a considerable amount of information about the manner in which a chemisorbed layer rearranges itself. Somotjai [13] has summarized LEED results for a number of systems. Some examples are collected in Fig. XVlII-1. Figure XVIII-la shows how N atoms are arranged on a Fe(KX)) surface [14] (relevant to ammonia synthesis) even H atoms may be located, as in Fig. XVIII-Ih [15]. Figure XVIII-Ic illustrates how the structure of the adsorbed layer, or adlayer, can vary wiA exposure [16].f There may be a series of structures, as with NO on Ru(lOTO) [17] and HCl on Cu(llO) [18]. Surface structures of... 

The addition of potassium to Fe single crystals also enliances the activity for ammonia synthesis. Figure A3.10.19 shows the effect of surface potassium concentration on the N2 sticking coefficient. There is nearly a 300-fold increase in the sticking coefficient as the potassium concentration reaches -1.5 x 10 K atoms cm ... 

Strongin D R, Carrazza J, Bare S R and Somoqai G A 1987 The importance of Cj sites and surface roughness in the ammonia synthesis reaction over iron J. Catal. 103 213... 

ErtI G 1991 Catalytic Ammonia Synthesis Fundamentals and Practice, Fundamentals and Applied Catalysis ed J R Jennings (New York Plenum)... 

Bare S R, Strongin D R and Somoqai G A 1986 Ammonia synthesis over iron single crystal catalysts—the effects of alumina and potassium J. Phys. Chem. 90 4726... 

Urea is produced from liquid NH and gaseous CO2 at high, pressure and temperature both reactants are obtained from an ammonia-synthesis plant. The latter is a by-product stream, vented from the CO2 removal section of the ammonia-synthesis plant. The two feed components are deUvered to the high pressure urea reactor, usually at a mol ratio >2.5 1. Depending on the feed mol ratio, more or less carbamate is converted to urea and water per pass through the reactor. 

The hydrogen can be used for organic hydrogenation, catalytic reductions, and ammonia synthesis. It can also be burned with chlorine to produce high quaHty HCl and used to provide a reducing atmosphere in some appHcations. In many cases, however, it is used as a fuel. 

Ammonia from coal gasification has been used for fertilizer production at Sasol since the beginning of operations in 1955. In 1964 a dedicated coal-based ammonia synthesis plant was brought on stream. This plant has now been deactivated, and is being replaced with a new faciUty with three times the production capacity. Nitric acid is produced by oxidation and is converted with additional ammonia into ammonium nitrate fertilizers. The products are marketed either as a Hquid or in a soHd form known as Limestone Ammonium Nitrate. Also, two types of explosives are produced from ammonium nitrate. The first is a mixture of fuel oil and porous ammonium nitrate granules. The second type is produced by emulsifying small droplets of ammonium nitrate solution in oil. 

Nearly all commercial nitrogen fertilizer is derived from synthetic ammonia. However, prior to the introduction of ammonia synthesis processes in the early 1900s dependence was entirely on other sources. These sources are stdl utilized, but their relative importance has diminished. 

Resources for Nitrogen Fertilizers. The production of more than 95% of all nitrogen fertilizer begins with the synthesis of ammonia, thus it is the raw materials for ammonia synthesis that are of prime interest. Required feed to the synthesis process (synthesis gas) consists of an approximately 3 1 mixture (by volume) of hydrogen and nitrogen. 

Hydrogen is used mainly in ammonia synthesis, methanol synthesis, and petroleum refining. 

There has been an increasing interest in utilising off-gas technology to produce ammonia. A number of ammonia plants have been built that use methanol plant purge gas, which consists typically of 80% hydrogen. A 1250 t/d methanol plant can supply a sufficient amount of purge gas to produce 544 t/d of ammonia. The purge gas is first subjected to a number of purification steps prior to the ammonia synthesis. 

The Texaco process was first utilized for the production of ammonia synthesis gas from natural gas and oxygen. It was later (1957) appHed to the partial oxidation of heavy fuel oils. This appHcation has had the widest use because it has made possible the production of ammonia and methanol synthesis gases, as well as pure hydrogen, at locations where the lighter hydrocarbons have been unavailable or expensive such as in Maine, Puerto Rico, Brazil, Norway, and Japan. 

High temperature steam reforming of natural gas accounts for 97% of the hydrogen used for ammonia synthesis in the United States. Hydrogen requirement for ammonia synthesis is about 336 m /t of ammonia produced for a typical 1000 t/d ammonia plant. The near-term demand for ammonia remains stagnant. Methanol production requires 560 m of hydrogen for each ton produced, based on a 2500-t/d methanol plant. Methanol demand is expected to increase in response to an increased use of the fuel—oxygenate methyl /-butyl ether (MTBE). 

In 1974 a 1000 t/d ammonia plant went into operation near Johaimesburg, South Africa. The lignitic (subbituminous) coal used there contains about 14% ash, 36% volatile matter, and 1% sulfur. The plant has six Koppers-Totzek low pressure, high temperature gasifiers. Refrigerated methanol (—38° C, 3.0 MPa (30 atm)) is used to remove H2S. A 58% CO mixture reacts with steam over an iron catalyst to produce H2. The carbon dioxide is removed with methanol (at —58° C and 5.2 MPa (51 atm)). Ammonia synthesis is carried out at ca 22 MPa (220 atm) (53) (see Ammonia). 

Reforming is completed in a secondary reformer, where air is added both to elevate the temperature by partial combustion of the gas stream and to produce the 3 1 H2 N2 ratio downstream of the shift converter as is required for ammonia synthesis. The water gas shift converter then produces more H2 from carbon monoxide and water. A low temperature shift process using a zinc—chromium—copper oxide catalyst has replaced the earlier iron oxide-catalyzed high temperature system. The majority of the CO2 is then removed. 

Because the ammonia synthesis reaction is an equiUbrium, the quantity of ammonia depends on temperature, pressure, and the H2 to-N2 ratio. At 500°C and 20.3 MPa (200 atm), the equiUbrium mixture contains 17.6% ammonia. The ammonia formed is removed from the exit gases by condensation at about —20° C, and the gases are recirculated with fresh synthesis gas into the reactor. The ammonia must be removed continually as its presence decreases both the equiUbrium yield and the reaction rate by reducing the partial pressure of the N2—H2 mixture. 

Recent commercialization efforts have focused on improved activity synthesis catalysts, which allow ammonia synthesis to be conducted at significantly lower pressures and temperatures. Catalyst manufacturers have focused on enhancing the activity of the iron-based catalyst through the use of promoters (23). 

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