Ammonia Synthesis - American Chemical Society

Chapter 2

Ammonia Synthesis: Catalyst and Technologies Svend E r i k Nielsen

Downloaded by OHIO STATE UNIV LIBRARIES on June 29, 2012 | http://pubs.acs.org Publication Date: December 31, 2008 | doi: 10.1021/bk-2009-1000.ch002

Haldor Topsoe A/SNymoelleves 55, DK-2800 Lyngby, Denmark

The following chapter gives a historical perspective of the ammonia synthesis technology including developments of the ammonia synthesis catalyst. Operating parameters in the ammonia synthesis are discussed and the various configurations with respect to selection of converter types, purge gas recovery systems, steam production, etc. are convered in detail for the technology currently in industrial use. Also, the most widely used commercial processes are described in this chapter, and finally prospectives and ideas for the future are mentioned.

Introduction The development of the synthesis of ammonia was a landmark for the entire world and for the chemical industry in particular, since it not only solved the problem of securing food supply to the ever increasing world population, but it also had a significant impact on the industrial chemistry and laid the foundation for the theory and further developments in the industrial practice of heterogeneous catalysis. The ammonia industry has contributed significantly to improvements within chemistry and chemical engineering, and the very high R & D activity with ammonia catalysts had a major effect on other catalyst based chemical industries. The ammonia synthesis reaction is probably the best fundamentally understood and documented catalytic process. Since the initial development of its synthesis, the industrial production of ammonia has been steadily increasing as can be seen from the curve (Figure 1), and in 2005 the world production has reached 147 million metric tonnes. Today 1.2% of the total world energy consumption is used for production of ammonia, © 2009 American Chemical Society

In Innovations in Industrial and Engineering Chemistry; Flank, W., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2008.

15

16


1940

1950

1960

1970

1980

1990

2000

2010

Year

Figure L World Ammonia Production (Source Indexmundi, see (J))

and ammonia has become one of the largest chemical products since the production really started to increase rapidly in the beginning of the 1960's. The present chapter describes ammonia synthesis technology. The first part gives a brief historical overview of the ammonia synthesis process including some early developments to better understand the current standing of the technology. The second part gives a description of the current state of ammonia synthesis technology, and the third part will deal with some ideas that may be addressed in the future in order to improve the technology even further. For more comprehensive details and surveys, please see (2, 3).

Historical Perspective At the end of the 19th century, it became evident that the known resources of nitrogen, which were primarily ammonium sulphate (as a by-product from coke and town gas production from coal) and natural deposits of saltpetre from Chile, were not sufficient to cover the demand for fertilizers required to increase the agricultural yield to satisfy the world food production. From here, the history of modern ammonia synthesis started in Germany just after year 1900, when Fritz Haber and his assistants developed and patented the process concept that forms the basis for all ammonia production today. Fritz Haber together with Carl Bosch from B A S F jointly developed the Haber-Bosch process, and Alvin Mittasch discovered the promoted iron catalyst. They all played a significant role in the developments, and the splendid work of these gentlemen resulted in the first commercial plant at Oppau/Ludwigshafen near Mannheim in Germany. The plant was commissioned in 1913 and the capacity was 30 M T P D (Metric Tons Per Day). The capacity of this plant was rapidly increased to 250 M T P D in 1916. In 1917 a second plant was started at Leuna near Leipzig, which after further expansion produced 240,000 tonnes/year at the end of the First World War.



17 In parallel, independent development work took place in the USA. In 1918 a plant called " U S Nitrate Plant No. 1" was constructed in Alabama. The plant was, however, not very successful, but the developments continued in the 1920's with the construction of several successful plants. In 1928 an American company (NEC) was commissioned to construct a plant in Europe, thereby entering worldwide competition. During the 1930's and the Second World War the worldwide production capacity was increasing rapidly, and in 1945 about 125 ammonia plants existed with a total capacity of 4.5 million tonnes/year. The most important processes at that time were Haber-Bosch, Casale, Claude, Fauser, N E C and Mont Cenis. Already at that time it was the impression that the ammonia technology was mature, and that no significant further developments could be expected. This was to a certain extent true, since most of the features that characterise modern synthesis technology - including catalyst type and most major converter designs - were already well proven in industrial applications. Since that time the developments in the ammonia technology have primarily been in the synthesis gas generation section and not the ammonia synthesis itself. Also the type of feedstock to the plants has changed, and natural gas is today the predominant feedstock for ammonia production. In the early days the synthesis gas was produced at atmospheric pressure, and the synthesis gas was compressed in reciprocating compressors to pressures as high as 100 M P a in some cases. Capacities were limited to around 300 - 400 M T P D due to limitations in reciprocating compressors. However, with the development of steam reformer based front-ends and the introduction of centrifugal compressors, the ammonia plant capacities suddenly increased to 1000 M T P D with ammonia synthesis loop pressures typically around 15 MPa. Since the I960's new developments have been in the ammonia converter designs, such as introduction of radial flow converters and introduction of converters with multiple catalyst beds to increase ammonia conversion. Efforts to develop more active ammonia synthesis catalysts have been ongoing. As an example, a Ru-based ammonia synthesis catalyst has been introduced to the market. This catalyst is more active than the iron-based catalyst, but the cost is also much higher due to the high Ru price, and the use of the catalyst is very limited. In spite of the fact that the same process scheme, with a few radical developments, has been used for decades, there has been a tremendous development in the scale of operation, and the largest plant today produces as much as 3,300 M T P D of ammonia. At the same time the energy consumption has been reduced from 50 - 63 GJ/MT N H in the 1960'ies to below 29 GJ/MT N H today, close to the theoretical minimum of 20 GJ/MT N H . 3

3

3

Ammonia Synthesis Catalyst The formation of ammonia from hydrogen and nitrogen is a strongly exothermic reaction


18 N + 3 H τ± 2 N H 2

2

3

ΔΗ

298

= -92.44 k J m o l

1

(1)


The reaction is favoured by low temperatures and high pressures, in industrial practice ranging between 8-20 M P a and 350-480°C to yield a conversion per pass of reactants of 25-35% depending on reactor design and configuration. The reaction is reversible and under all practical conditions limited by equilibrium. Recycle of unconverted reactants is required to obtain high overall conversion.

Figure 2. Ammonia catalyst precursor manufacture

Traditional industrial ammonia catalysts are iron-based. They are prepared by fusion of iron ore (Figure 2) adjusted with various promoters, predominantly potassium, calcium and aluminium, which may be added to the melt as oxides, hydroxides, carbonates or nitrates. Upon solidification the product is crushed and sieved to the desired particle size, typically 1.5-3 mm irregular shaped particles (Figure 3). A small particle size is essential to reduce diffusional limitation. The unreduced catalyst consists predominantly of magnetite, F e 0 containing minor amounts of wustite, FeO, or in some cases hematite, F e 0 . In the boundaries between the iron domains are crystalline and amorphous phases containing mainly oxides of potassium and calcium, both of which are too large to enter the magnetite structure. The unreduced catalyst is virtually non-porous with a typical 3

2

4

3



19

Figure 3. Ammonia synthesis catalyst

2

1

surface area of less than 1 m g" . Wustite was reported (4, 5) as an alternative precursor leading to more active catalysts compared to those based on magnetite. The addition of modest amounts of cobalt, 3-6 wt%, increases the activity, and cobalt-containing catalysts have been commercialised (6). Cobalt is incorporated into the crystal lattice and upon reduction forms an alloy with iron, from which partial segregation of Co takes place (7). The catalyst is supplied in either the as-prepared, unreduced form or in the prereduced, metallic state. In the prereduced form the catalyst is highly pyrophoric and must be passivated by controlled oxidation of the iron surface in order to enable safe handling upon exposure to air. Metallic iron is the active and main phase in ammonia catalysts. In spite of this, pure iron exhibits only poor synthesis activity unless it is promoted by various amounts of oxides, primarily those of potassium, aluminium, calcium and magnesium, but numerous combinations involving additional elements, e.g., Si, Cr, T i and Zr have been claimed. Thus, promoters are crucial to catalytic performance. Promoters may be classified in two categories. Those which enhance the catalytic activity by inducing and preserving a high catalyst surface area are referred to as structural promoters, while those increasing the catalytic activity per unit surface area of iron are termed electronic promoters. To the first category belong the oxides of aluminium, calcium and magnesium, which during reduction segregate to form spacers that inhibit the growth of iron crystallites, and thereby assist in developing and maintaining a high surface area. In the absence of structural promoters rapid sintering of the pure iron phase under normal operating conditions would occur, whereas a small amount effectively inhibits crystal growth by forming a physical boundary between the iron crystallites. The second category, the electronic promoters, is represented by alkali compounds like potassium that strongly enhance the specific activity



20 of the iron surface. Contrary to the structural promoters, potassium is highly dispersed in the reduced catalyst, and a uniform distribution over the iron surface is observed. The promoting effect of alkali is rationalised by electrostatic interactions and the effect applies to non-iron based catalysts as well. Alternatives to the iron-based catalysts have been thoroughly researched. Although the high activity of Ru-based catalysts were early recognised (8), it was the work by Japanese researchers (9, 10) on alkali promoted, carbon supported ruthenium catalysts that triggered renewed interest and immense research efforts in identifying and researching alternative catalysts. Using Cs or Ba-promoted ruthenium catalysts, activities about an order of magnitude higher than seen for the iron catalyst may be achieved especially at high ammonia partial pressure, since the inhibition by ammonia is significantly less severe than for the traditional catalyst. Several other supports have been reported including M g O , M g A l 0 , boron nitride and zeolites. Although promoted ruthenium catalysts have been introduced in industry (11), it is still an open question whether the shorter lifetime and higher cost justify the use of Ru-based catalysts. Other alternatives, including non-iron, non-ruthenium catalysts, have been researched. Among these a novel class of high-active bimetallic nitrides have been discovered, e.g., Cs-promoted C o M o N (12, 13) and Ba-promoted cobalt catalysts (14). However, common to any alternative to the classical catalyst is that they are competing against a class of traditional low cost iron-based catalysts characterised by high activity and with a durability that is unsurpassed in industrial catalysis. The reduction of the magnetite catalyst precursor is endothermic: 2

4

3

3

F e 0 + 4 H τ± 3Fe + 4 H 0 3

4

2

2

ΔΗ

298

= 149.9 kJmof

1

(2)

It involves complete removal of oxygen from the magnetite lattice and a redistribution of iron atoms transforming the bulk non-porous magnetite into a porous metallic structure. Also, redistribution and segregation of promoters take place. The reduction does not cause any changes in overall particle dimensions. The reduction process is complex and numerous reduction studies have been published (15, 16, 17), and various models have been proposed. A more elaborate treatment of the reduction process is given by Schlôgl (18). Activation of the catalyst is carried out in a H / N mixture at moderate pressure. The process must be carefully controlled in order to ensure the development of maximum surface area. The rate of reduction is controlled by adjusting the temperature; the lower the reduction rate, the better the development of the micro porous structure (r = 100 to 150 Â). Exposure to water/steam of the reduced part of the catalyst will cause sintering and loss of activity and counter-diffusion of water should be minimised by operating at highest possible gas velocities. Water is also reported to inhibit the magnetite reduction rate (15). It is removed by condensation before the exit gas is 2

2



21 recirculated. As the catalyst becomes reduced ammonia formation begins, and recirculation rate to be increased. As the reduction accelerates, the loop pressure and recirculation rate may be gradually increased, providing a smooth transition from reduction to normal operation. Different reduction procedures apply i f the catalyst is prereduced or when a combination of prereduced and unreduced catalyst is used. Whereas reduction of the bulk magnetite catalyst goes on over days, the reduction of the superficial oxidic layer of the prereduced catalyst is facile and may be accomplished within approximately one day i f solely prereduced catalyst is charged. Often the first bed is charged with prereduced catalyst to enable fast reduction and onset of the ammonia synthesis reaction, which thereby liberates heat to support the endothermic reduction in the remaining part of the bed. Upon reduction, the crystalline density increases from about 5 to almost 8 g cm" . However, the reduction is not accompanied by any changes in particle dimension, hence settling of the catalyst bed does not occur thereby making radial flow converters the preferred choice in consideration of pressure drop issues. Under normal operating conditions sintering is negligible and catalyst lifetimes exceeding ten years are far from unusual. However, the combination of high temperature and the presence of water and other oxygen-containing compounds may lead to accelerated crystal growth. Under mild conditions oxygen-containing gaseous components such as water and carbon oxides are classified as temporary poisons because the decrease in activity caused by oxygenates is regained when the oxygen species are no longer present in the feed (19). Kinetic rate expressions accounting for the effect of partial poisoning by water have been established (20, 21). Under more severe conditions or at prolonged exposure, high concentrations and excessive temperatures do lead to permanent loss in activity. Sulphur compounds, halides, phosphorus and arsenic are permanent poisons to ammonia catalysts (22). However, in most plants the upstream lowtemperature shift catalyst and the Ni-based methanation catalyst both serve as efficient guards by irreversibly adsorbing traces of such compounds. Thus, permanent poisons are normally not a severe problem. It was early recognised that the rate limiting step in the ammonia synthesis is the dissociative adsorption of nitrogen (23) and that hydrogénation proceeds at a much faster rate (24). Temkin and Pyzhev (25) proposed a rate expression, 3

2

a

2

3

1

a

r = k P {(P )V(P 3) } -k. {(PNH3) /(PH2) } " ; (0.5

Ammonia Synthesis - American Chemical Society

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