Comparative Evaluation of Synthesis Catalyst Features

This article examines the development of the ammonia synthesis loop, from the inception of com- mercial ammonia production to today. Opportunities are explored for cost-effective, commercially viable improvements in the ammonia synthesis process through new catalysts and new loop configurations,

Claus J. H. Jacobsen and S vend Erik Nielsen Haldor Tops0e A/S, DK-2800 Lyngby, Denmark

Introduction mercially viable process was developed. From 1909-1912, Bosch was responsible for the develop- oday, catalytic ammonia synthesis is consid- ment of both the large-scale high-pressure ammonia ered a mature and highly optimized technolo- synthesis converters and the processes for supplying Tgy. Almost 100 years have passed since Fritz sufficient amounts of a sufficiently pure synthesis gas. Haber initiated his pioneering studies of the equilibri- During that same period, Mittasch discovered the dou- um between dihydrogen, dinitrogen, and ammonia bly promoted kon catalyst and many other active com- (Haber and van Oordt, 1904). These studies were the positions, including ruthenium (Mittasch, 1950), in an platform from which Haber developed his process for impressive research program involving catalytic activ- the continuous production of ammonia from the ele- ity measurements of more than 2,000 catalysts in ments (Haber and Elek, 1910). This process, originally almost 6,500 experiments. By 1922, Mittasch and his patented by BASF (DRP, 1908, 1909), revolutionized team had conducted more than 20,000 experiments. the chemical industry and it remains the cornerstone of When the decision to construct a full-scale ammonia catalytic ammonia synthesis. Haber realized that sever- synthesis plant was made in the summer of 1911, the al elements (kon, osmium, uranium, cerium, man- whole technology was completely new and many diffi- ganese, molybdenum, tungsten) were active catalysts culties still remained to be overcome. In view of this, it for the synthesis and decomposition reactions (Haber, is impressive that in September 1913, less than two 1920-1923). However, it was only through the impres- years after the construction was initiated, the fkst sive technical developments by (Bosch, ammonia synthesis plant was put into operation in 1933) and Alwin Mittasch (Mittasch, 1950) that a com- Oppau, . Soon after, a new plant was con-

AMMONIA TECHNICAL MANUAL 212 2002 structed within eleven months by BASF in Leuna near stability was developed (Jacobsen, 2001). It is dis- . In 1937, these two BASF plants still account- cussed if new catalysts can be utilized in further ed for more than 70% of the annual world production improved ammonia synthesis processes. We provide capacity (Slack and James, 1973). cost estimates and consumption figures for different Since these earliest developments, the importance of process schemes utilizing different catalysts. From our catalytic ammonia synthesis has steadily increased. In current understanding of the ammonia synthesis reac- 2000, the annual world capacity for ammonia produc- tion, it is possible to accurately predict the maximally tion exceeded 150-106 metric tons (t) of ammonia and achievable activity of an ammonia synthesis catalyst the ammonia production required around 1.5% of the under specified reaction conditions. Based on this, we total global energy consumption (Appl, 2000). Today, speculate about possible future developments of the about 85% of all ammonia is used for production of ammonia synthesis loop. nitrogen-containing fertilizers and thereby ammonia plays a central role in sustaining the growing popula- Developments of the Ammonia Synthesis tion of the world. It has been estimated that at least two Loop billion of the current global population can only be nourished through provision of proteins available via Refer to Slack and James (1973), Appl (2000), dinitrogen fixation by the Haber-Bosch process (Smil, Topham (1985), Hooper (1991), Vancini (1970), and 1997). Consequently, a strong driving force for contin- Dybkjaer (1995). The developments by BASF stimu- uously improving the technology exists. In the scientif- lated other researchers to develop alternative synthesis ic community, a massive effort has been focused on the schemes, partly to circumvent the elaborate worldwide understanding of the fundamental details of this cat- patent protection of the German developments and alytic reaction. Thus, ammonia synthesis has been the partly to search for improvements. The first successful cradle and testing ground for chemical concepts since plant outside Germany was started in Terni, Italy in the beginning of the 20th century (Somorjai and 1920. Some of the earliest synthesis loops employed Materer, 1994). Despite these efforts, few fundamen- significantly higher pressures (about 750-1,000 bar) tally new catalysts have been discovered and many than the BASF plants, thereby allowing increased con- controversies regarding the detailed energetics of the version per pass and a simplified ammonia separation. elementary reaction steps remain, although important In the U.S., after the less successful "US Nitrate Plant insight has resulted from single crystal studies No. 1" (using a sodium promoted cobalt catalyst) com- (Somorojai and Materer, 1994; Ertl, 1991; Somorjai, missioned in 1918 in Alabama, a new plant was put 1991; Ertl, 1980). However, during the last few years, into operation in Syracuse, NY in 1921. Early develop- our understanding of the ammonia synthesis reaction ments in Europe included the technology by Fauser, has significantly improved. used in a plant operated with coke oven gas from the Bosch already realized that generation of the pure company Mont Cenis. This process operated at a very synthesis gas would be the largest single contributor to low pressure (100 bar) and featured a catalyst based on the total production cost of ammonia (Bosch, 1933). iron cyanide. The ammonia synthesis process in the Although most of the significant improvements of the form originally developed by Haber and Bosch deliv- ammonia synthesis process during the 20th century ered ammonia with an energy consumption of 80-90 have been in the synthesis gas generation (front end), GJ/t. After this pioneering work, the modern history of we focus this article on the improvements of the ammo- industrial ammonia synthesis can largely be catego- nia synthesis loop that have occurred since the first rized in three eras based on the available synthesis cat- plants were constructed. Some of the recent process alysts, synthesis loops, energy consumption figures, developments rely on new catalysts (US, 1987, 1979) and available production capacities. In the classifica- with significantly higher activities than those of the tra- tion outlined below, the trends are illustrated with some ditional, promoted iron catalysts. Recently, a new of the common catalysts and synthesis loop configura- ruthenium catalyst with an unprecedented activity and tions.

AMMONIA TECHNICAL MANUAL 213 2002 1965-1985: Integrated plant design utilizing 1930-1965: Small units utilizing internal cooling quench or indirect cooling and iron catalysts and iron catalysts 500-1,200 MTPD, multibed ammonia synthesis 100-500 metric ton per day (MTPD), single bed loops, quench or indirect cooling, Mittasch-type cata- ammonia synthesis loops, internal cooling, Mittasch- lysts, energy consumption between 45 and 32 GJ/t. type catalyst, energy consumption between 80 and 45 During this period, the concept of integrated plant GJ/t. The first ammonia synthesis loops all used axial design was pioneered. This was achieved through con- flow converters. The first ammonia synthesis converter struction of large single-train plants with high degrees built in Haber's laboratory in Karlsruhe already relied of energy integration. Reciprocating compressors were on an internal feed-effluent heat exchanger. However, replaced by centrifugal compressors and most new electrical preheat of the synthesis gas was necessary. plants were based on steam or naphtha reforming at Soon after, Bosch constructed the first autothermal pressures of 15-30 bar. Instead of internally cooled ammonia synthesis converter (pilot scale) by improved converters, quench cooling or indirect cooling was pri- insulation and heat recovery. These features were gen- marily employed in synthesis loops containing more erally maintained in the industrial ammonia synthesis catalyst beds operating around 150-220 bar. Although converters of this period. Industrially, the heat of reac- most converters of this period had several catalyst tion was removed using internal cooling in either coun- beds, this was not a new development since the early tercurrent (such as TVA-type converters) or cocurrent Fauser-Montecatini reactor used more beds with indi- (such as NEC-type converters) flow in cooling tubes. rect cooling. The promoted iron catalyst was still Throughout the period, only Mittasch-type promoted exclusively used for industrial application. However, iron catalysts were employed. Due to the axial flow use of the inherently more active, smaller catalyst par- configuration, larger catalyst particles were used (typi- ticles (down to 1.5-3 mm) was made possible through cally 6-12 mm) resulting in pressure drops of 15-20 the invention of radial flow and horizontal converters. bar around the synthesis loop and significant rate limi- This decreased the pressure drop hi the synthesis loop tations due to mass transport restrictions. In those days, to around 9-10 bar and at the same time essentially the synthesis gas production was exclusively based on removed the significant mass-transport limitations of coke-oven gas. Even after scrubbing with water for the reaction rate. Introduction of the low-temperature CO2 removal, the synthesis feed gas still contained shift process and methanation reduced impurity levels around 10-15 ppm of oxygen containing impurities. In of oxygen compounds to about 2-3 ppm. the 1950s, steam-reforming at pressures of 0.5 to 15 bar (originally developed by BASF in the 1920s and 1985-today: Large plants based on new catalysts extended to naphtha feedstocks by ICI in the early or improved conventional schemes 1960s) was introduced in U.S., thereby significantly reducing the capital cost of the plants. Until around 1,200-2,000 MTPD, < 32 GJ/t. During this latest 1950, plant capacities were expanded by installation of period, the primary industrial developments have been parallel lines of about 70-120 MTPD units. With a few continuations of the trends of the previous period. exceptions (such as Claude and Casale units), the syn- Larger plants have been constructed with some thesis process ran at pressures of about 300 to 350 bar. improvements. This has led to energy consumptions Reciprocating compressors, with as much as seven down to approximately 28 GJ/ton. However, some stages in linear arrangement with intermediate cooling, noteworthy deviations from the general trends have were used and usually there were two parallel lines of also emerged. In the ICI LCA process, a promoted iron compressors to minimize downtime. cobalt catalyst was introduced after being tested in a Canadian plant with Id's AMV process. The synthesis loop of the LCA process operates at a pressure of 80 bar. In 1992, the first non-iron ammonia synthesis cat-

AMMONIA TECHNICAL MANUAL 214 2002 alyst was introduced in the KAAP process of M. W. illustrates the activity of such a catalyst compared to Kellogg. The promoted ruthenium catalyst supported KM1R. on a special graphitized carbon support is claimed to be Detailed insight into unpromoted (Dahl et al., 1999, 10-20 times more active than the traditional, promoted 2000 a,b) and promoted carbon-supported ruthenium iron catalyst. To fully utilize the different kinetic fea- catalysts has been obtained (Kowalczyk et al., 1996, tures of this catalyst, it is operated at an H/N ratio 1998; Forni et al., 1999), but more importantly, new lower than stoichiometric and at a pressure about 90 significantly unproved ruthenium catalysts have also bar. been reported. Ba-Ru/MgO was claimed by Muhler and co-workers to be more active than the commercial Development of Ammonia Synthesis Ru catalyst (Bielawa et al., 2001; Muhler et al., 2000) Catalysts and it exhibited stable activity for 1,000 h at 750°C at 50 bar. During the past decades, we have developed Several reviews give detailed descriptions of known ruthenium-containing ammonia synthesis catalysts uti- ammonia synthesis catalysts (Nielsen, 1968; Stoltze, lizing a wide range of support materials, electronic and 1995; Ozaki and Aika, 1981; Tennison, 1991; Aika and structural promoters. From these studies, several Ru Tamaru, 1995). From these, it is clear that despite the catalysts with high activities have been achieved. extensive efforts of the scientific community, only few Particularly, ruthenium catalysts supported on magne- fundamentally new catalysts with high activities have sium aluminum spinels (Fastrup, 1997; Dohrup et al., been discovered since the original work of Haber 1999) and on high surface area graphite (see Figure 2) (Haber, 1920-1923) and Mittasch (Mittasch, 1950). have shown promising activity. However, they have not This situation is also reflected by the almost exclusive shown the desired stabililty at the interesting process use of promoted iron catalysts in industrial ammonia conditions. Recently, a Ba-Ru/BN with unprecedented synthesis loops, as discussed above. Among the pri- activity and stability was developed (Jacobsen, 2001) mary virtues of the promoted iron catalyst is the high through insight into both the optimal ruthenium crystal intrinsic activity (that is, high activity per active site), size and the influence of the support on the catalytic the exceptionally long lifetime, the high density (result- activity (Jacobsen et al., 2000). Boron nitride (occa- ing in a high volume-based activity), and not the least sionally known as "white graphite") is a very attractive the fact that this catalyst is significantly less expensive support material for ruthenium-based ammonia synthe- than all the known alternatives. Throughout the 20th sis catalyst. It has almost the same structure as graphite century, efforts have been made to improve the pro- (except for a different stacking of the individual lay- moted catalyst and to identify new catalysts. ers), but it is completely stable towards hydrogénation Particularly during the 1970s and 1980s, Japanese under all conditions relevant to industrial ammonia researchers actively pursued the developments of suit- synthesis. At the same tune, boron nitride is known for able ruthenium catalysts (Ozaki and Aika, 1981; Aika its high temperature resistance (Greenwood and and Tamaru, 1995). Within the last few years, there has Earnshaw, 1984). Contrary to graphite, boron nitride is been a renewed interest in non-iron catalysts. an insulator. This demonstrates that the effect of the Obviously, the commercialization of the promoted electronic promoter (such as K, Ba, Cs) does not occur iron-cobalt catalyst in the ICI AMW and ICI LCA via the support as it has been claimed (Aika and process (Pinto et al., 1989), and, more recently, the pro- Tamaru, 1995), but through a direct contact between moted carbon-supported ruthenium catalyst in the the electronic promoter and the metal as recently KAAP process (LeBlanc et al, 1985; Czuppon and demonstrated (Hansen et al., 2001) by transmission Knez, 1996) has pushed this development. Completely electron microscopy. The Ba-Ru/BN catalyst has new non-iron, non-ruthenium ammonia synthesis cata- proved completely stable during 5,000 h operation at lysts such as Cs/Co3Mo3N with activities higher than 100 bar and 550°C in an equilibrated 3:1 dihydro- the promoted iron catalyst have been discovered gen/dinitrogen mixture. In Figure 2, the activity and (Jacobsen, 2000; Kojima and Aika, 2000). Figure 1 stability of this catalyst is compared to a similar cata-

AMMONIA TECHNICAL MANUAL 215 2002 1000 50bar,400°C

^ Ba-Ru/C 100 CO ) "5 KM1

o 3:1 H2/N2 a 1:1 H2/N2

10 Ammonia exit concentration %

Figure 1. Comparison of ammonia synthesis activity of 5% Cs/Co3Mo3N with KM1R at 400°C, SO bar, in 3:1 and 1:1 dihydrogen/dinitrogen mixtures. It is seen that the kinetics of the Cs/Co3Mo3N catalyst is intermediate between those of promoted iron and 6% Ba, 6.7% Ru/C catalysts. Cs/Co3Mo3N can be easily regenerated into the oxidic precursor by calcination in air at 600° C.

100 400°C, 100 bar Ba-Ru/BN aged at 550°C

Ba-Ru/C aged at 450°C

500 1000 1500 2000 2500 Run hours Figure 2. Activity and stability of promoted Ru/BN catalyst (5.6% Ba, 4.1% Ru, 81 m2/g BN from H. C. Starck GmbH) and promoted Ru/graphite catalyst (6% Ba, 6.7% Ru, 300 m2/g HSAG from Timcal). The boron nitride-supported catalyst is stable also at significantly higher pressures and temperatures. The activities of the Ru/BN and Ru/C catalysts are measured by lowering the temperature to 400°C (4.5% NH3 in inlet; 12.0% NH3 out- let, 100 bar, 3:1 H2/N2>. Ru/BN is aged at 550°C and Ru/graphite at 450°C.

AMMONIA TECHNICAL MANUAL 216 2002 lyst supported on high surface area graphite. the only option. Energy consumption was high for As might be expected, Ba-Ru/BN exhibits the same makeup gas compression, but on the other hand, the reaction kinetics as Ba-Ru/C. Compared to promoted ammonia product could be condensed almost solely by iron catalysts, this means less inhibition by ammonia, water cooling. Consequently, the ammonia refrigera- lower dihydrogen reaction order, and higher activation tion unit was very modest. An integration with the energy (Jacobsen et al., 2000). steam system and waste heat recovery was not an issue The catalytic activity of promoted Ru/BN catalysts those days. can be tuned to attain the optimal performance at spe- Later on in the mid-1960s, plant capacities increased cific reaction conditions (temperature, pressure, H/N due to the availability of centrifugal compressors, ratio, NH3 concentration) by proper choice of boron allowing handling of larger makeup gas flows. nitride surface area, ruthenium concentration, promot- Furthermore, the front end was based on steam reform- er(s), promoter(s) concentrations, and pellet size and ing technology at somewhat higher pressure than used density. Furthermore, it is possible to regenerate useful in the earlier period. Due to the requirement for larger Ba, Ru, and B compounds from the Ba-Ru/BN catalyst capacities, the radial flow converter was developed by in high yields using a procedure similar to that report- Haldor Tops0e A/S in the mid-1960s to assure a low- ed for Ba-Ru/MgO (Mulder et al., 2000). pressure drop and to minimise the compression work required. As seen from Table 1, the loop pressure was Comparisons of Ammonia Synthesis typically 220 bar and due to the lower pressure in the Loops with Traditional and New Catalysts loop, the demand increased on the refrigeration unit to condense the ammonia product. Today, the radial flow From the historic development of the ammonia syn- converter is the preferred converter choice, and ammo- thesis process during the 20th century, it is clear that nia synthesis loops with 2, 3 and even 4 catalyst beds the choice of front-end, ammonia synthesis converter exist based on this principle. In the table, two different, and loop configuration has been changed to allow modern loop configurations are described, that is, a 140 reduced specific investments and to lower the energy bar loop using a 3-bed converter system based on the consumption. This has been achieved by increasing Fe catalyst, and a 90 bar loop based on a 4-bed con- plant capacities and improved waste heat recovery con- verter system using a promoted iron catalyst in the first cepts. Although it has been claimed that this trend bed and a promoted ruthenium catalyst in the rest. The might not continue (Hooper, 1991) due to the increased layout for the iron catalyst based loop is shown in difficulties in financing such projects, hi finding the Figure 4. appropriate feedstock supplies and in selling the prod- From the table, it is seen that for the two modern ucts, there are clear indications that still larger plants loops, the H2/N2 module at the converter inlet differs. will be constructed. With the recent developments on Whereas the Fe-based loop has a module of 3.0, the ammonia synthesis catalysts, it is interesting to investi- loop using Ru catalyst operates optimally at a some- gate potential benefits on the overall consumption and what lower module of 2.1. It is especially interesting to investment figures by use of new or improved catalysts note that the more active Ru catalyst will enable design in known ammonia synthesis loop configurations. of a converter with less catalyst volume compared to a Table 1 gives such a comparison between various converter based on promoted Fe catalyst only. ammonia loops. The table is made to show the histori- However, the energy efficiency of the loop is more or cal development hi the loop technology according to less unaffected by this modification. When operating a the above classification. loop at a pressure of 90 bar, the synthesis gas compres- In the early days from 1930-1965, the loop configu- sor can be a single-casing compressor, which is an ration shown in Figure 3 was generally used, and axial advantage. On the other hand, the load on the refriger- flow TVA-type converters (with cooling tubes) were ation compressor is significantly increased due to the often seen. The loop capacity was low, usually the loop difficulties in condensing the product at the resulting pressure was high, and reciprocating compressors were lower ammonia partial pressure. Overall, looking at the

AMMONIA TECHNICAL MANUAL 217 2002 Table 1. Ammonia Synthesis Loop Comparison 1930-1965 1965-1985 1985-Today 1985-Today Vintage Vintage Vintage Vintage

Capacity, MTPD 100 1,000 2,000 2,000 Synthesis Catalyst Fe Fe Fe/Ru Fe Converter Type TVA 2-bed radial 4-bed radial 3-bed radial Synthesis gas H/N ratio 3.0 3.0 2.1 3.0 Loop Pressure, bar 330 220 90 140 Syngas compressor power, kWh 2,700 15,400 14,300 19,500 Refrigeration compressor power, kWh 350 3,000 11,500 7,700 Cooling water consumption, m3/h 850 3,200 8,200 6,400 Makeup gas pressure, bar 1 26 31 31

Purge gas_

Figure 3. Flowsheet for ammonia synthesis plant typical for the period 1930-1965.

AMMONIA TECHNICAL MANUAL 218 2002 Converter

Product Ammonia

Figure 4. Flowsheet for ammonia synthesis loop based on promoted iron catalyst typical of the period 1985-today.

200 Ruthenium

150

N S 100 w =>

50

l l 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 Year

Figure 5. Price of ruthenium since 1992.

AMMONIA TECHNICAL MANUAL 219 2002 shaft power consumption of the synthesis gas compres- Figure 5. sor, as well as the refrigeration compressor, the advan- tage of an Ru catalyst is rather limited with respect to energy consumption. At the same time, the investment Possibilities for New Ammonia Synthesis cost of the Ru-based loop is significantly higher than Catalysts that of the Fe-based loop. Since the requirements for increased plant capacities are continuing, it is impor- Within the last year, it has become possible to tant to be able to build larger plants. Most likely, this describe the catalytic activities of transition metal- will require an increased pressure in the synthesis loop, based ammonia synthesis catalysts exclusively by use such as 200 bar simply to reduce the size of equipment of theoretical modeling. It has been shown that the and pipes. The newly developed Ru catalyst on the BN binding energy of nitrogen to the catalyst is decisive for support is perfectly suited for these conditions since it the catalyst performance (Logadottir et al., 2001). This is completely stable towards hydrogénation which new model accurately accounts for the catalytic activi- could be a severe problem with carbon supported cata- ties of known unpromoted (Logadottir et al., 2001), lysts at the higher dihydrogen pressures. However, with promoted (Dahl et al., 2001), and bimetallic catalysts today's very high Ru prices, the payback time will still (Jacobsen et al., 2001). Furthermore, the influence of be too long for the Ru loop to be any competition to Fe the operating conditions (temperature, pressure, and catalyst based loops. Particularly, this is the case with gas composition) on the activity can be easily assessed. the current trend in the industry of locating new large Consequently, it is possible to predict the optimal prop- plants in areas where the gas is cheap. Lately, the price erties (that is, the optimal nitrogen binding energy) of a of ruthenium has increased due to new applications of given catalyst under specific reaction conditions ruthenium in the electronics industry as shown in (Jacobsen et al., 2001). As an example, Figure 6 illus-

0.000001 -150 -100 -50 0 50 100 Relative nitrogen binding energy kJ/mol

Figure 6. Turnover frequencies of promoted ammonia synthesis catalyst at 450°C, 100 bar, 3:1 H2/N2, and

0.01% NH3,0.1% NH3, and 10% NH3. The abscissa is the binding energy of nitrogen to the catalyst surface. Some catalysts are shown explicitly in the igure, the position of others can be estimated by interpolation or extrapolation in the Periodic Table (Greenwood et al., 1984; Hansen et al., 2001; Logadottir et al., 2001).

AMMONIA TECHNICAL MANUAL 220 2002 trates the activity trends of promoted transition metal Literature Cited catalysts at different ammonia partial pressures. In accordance with experimental observations, the Aika, K., and K. Tamaru, Ammonia: Catalysis and activity of a promoted ruthenium catalyst is 10-20 Manufacture, A. Nielsen, ed., Springer, p. 103 times higher than that of a promoted iron catalyst at (1995). high ammonia concentrations (Appl, 2000). As dis- Appl, M., Ullman's Encyclopedia of Industrial cussed above, application of this catalyst has lowered Chemistry, 6th ed., electronic release (2000). the energy consumption for ammonia production only Bielawa, H., O. Hinrichsen, A. Birkner, and M. Muhler, marginally and at the cost of a higher capital invest- Angew. Chem., 113, 1093 (2001). ment. It is apparent from Figure 6 that a maximally Bosch, C, Nobel Prize Lecture, Die Chem. Fabrik, 12, achievable ammonia synthesis activity exists. 127 (1933). Dependent on the specific conditions, the maximal Czuppon, T. A., S. A. Knez, and R. B. Strait, activity is about 2-3 times higher than that of the pro- "Commercial Experience with KAAP and KRES," moted ruthenium catalyst at the relevant ammonia con- Ammonia Plant Safety & Related Facilities, Vol. centrations. However, realization of such a catalyst 37, AIChE, New York (1997). requires that a stable composition with the desired Dahl, S., A. Logadottir, R. C. Egeberg, J. H. Larsen, I. nitrogen binding energy can be designed (such as Chorkendorff, E. Törnqvist, and J. K. N0rskov, Phys Fe/Ru), and also that the preparation route can be opti- Rev. Lett., 83, 1814 (1999). mized to the same extent as that of, for example, Ba- Dahl, S., A. Logadottir, C. J. H. Jacobsen, B. S. Ru/BN in terms of support stability, crystal-size distri- Clausen, and J. K. N0rskov, Appl. CataL, in press bution, promoters, promoter contents, and regenerabil- (2001). ity. So far, the promoted Cs/CoßMx^N catalyst has Dahl, S., J. Sehested, C. J. H. Jacobsen, E. Törnqvist, some of the desired properties. It is, however, not yet and I. Chorkendorff, J. CataL, 192, 391 (2000). available with the desired surface area. However, even Dahl, S., E. Törnqvist, and I. Chorkendorff, J. CataL, with such a catalyst, it is doubtful whether it could 192, 381 (2000). compete with the traditional promoted iron catalyst. Dohrup, J., C. J. H. Jacobsen, and C. Olsen, US-A Therefore, it appears that significant improvements of 9/467,242 (1999). the ammonia synthesis process are dependent on new Dybkjaer, I., Ammonia: Catalysis and Manufacture, A. loop configurations, new separation processes, further Nielsen, ed., Springer, p. 199 (1995). improvements of the front end or possibly by new cat- Ertl, G., CataL Reb. Sei. Eng., 21, 201 (1980). alysts that perform by a completely new reaction mech- Ertl, G., Catalytic Ammonia Synthesis, J. R. Jennings, anism or relies on new electronic promoters. To the ed., Plenum Press, New York, p. 109 (1991). extent that ammonia production will be further opti- Fastrup, B., CataL Lett., 48, 111 (1997). mized along the current trends, it seems likely that new Fomi, L., D. Molinari, I. Rossetti, and N. Pernicone, plants will be constructed at locations close to large Appl. CataL, 185, 269 (1999). feedstock resources and the required infrastructure. If Greenwood, N. N., and A. Earnshaw, Chemistry of the new plants with higher production capacities (> 2,500 Elements, Pergamon Press, p. 235 (1984). MTPD) are constructed to lower the specific invest- Haber, F., Fünf Vorträge aus den Jahren 1920-1923 ment, they will most likely operate at pressures above (Nobel Prize Lecture) (1920-1923). 150 bar to keep piping and equipment at manageable Haber, F., Z Elek., 16, 244 (1910). sizes. Haber, F., and G. van Oordt, Z. Anorg. Chem., 43, 111 (1904). Acknowledgment Hansen, T. W., P. L. Hansen, S. Dahl, and C. J. H. Jacobsen, submitted (2001). We thank S0ren Dahl for helpful discussions. Hooper, C. W., Catalytic Ammonia Synthesis, J. R. Jennings, ed., Plenum Press, New York, p. 253

AMMONIA TECHNICAL MANUAL 221 2002 (1991). Ozaki, A., and K. Aika, Catalysis Science and Jacobsen, C. J. H., Chem. Commun., 1057 (2000). Technology, J. R. Anderson and M. Boudait, eds., Jacobsen, C. J. H., J. Catal, 200, l (2001). Springer, p. 87 (1981). Jacobsen, C. J. H., S. Dahl, B. S. Clausen, S. Bahn, A. Pinto, A., J. M. S. Moss, and T. C. Hicks, Ammonia Logadottir, and J. K. N0rskov, J. Am. Chem. Soc. Plant Safety and Related Facilities, Vol.30, AIChE, in press (2001). New York (1990). Jacobsen, C. J. H., S. Dahl, B. S. Clausen, and J. K. Slack, A. V, and G. Russell James, eds., Ammonia, Vol. N0rskov, J. Catal, in press (2001). 1, Marcel Dekker, New York (1973). Jacobsen, C. J. H., S. Dahl, P. L. Hansen, E. Törnqvist, Smil, V., Scientific American, p. 58 (July 1997). L. Jensen, H. Tops0e, D. V. Prip, P. B. M0enshaug, Somorjai, G., Catalytic Ammonia Synthesis, J. R. and I. Chorkendorff, J. Mol. Cat. A., 163,19 (2000). Jennings, ed., Plenum Press, New York, p. 109 Kojima, R., and K. Aika, Chem. Lett., 514 (2000). (1991). Kowalczyk, Z., S. Jodzis, W. Rarog, J. Zielinski, and J. Somorjai, G. A., and N. Materer, Top. in Catal. 1, Pielaszek, Appl. Catal, A 173, 153 (1998). 215 (1994). Kowalczyk, Z., J. Sentek, S. Jodzis, R. Diduszko, A. Stoltze, P., Ammonia: Catalysis and Manufacture, Presz, A. Terzyk, Z. Kucharski, and J. Suwalski, A. Nielsen, ed., Springer, p. 17 (1995). Carbon, 34, 403 (1996). Tennison, S. R., Catalytic Ammonia Synthesis, J. R. LeBlanc, J. R., R. Schneider, and K. W. Wright, Jennings, ed., Plenum Press, New York, p. 303 Ammonia Plant Safety & Related Facilities, Vol. 26, (1991). AIChE, New York (1986). Topham, S. A., Catalysis Science and Technology, Vol. Logadottir, A., T. H. Rod, B. Hammer, J. K. N0rskov, 7, J. A. Anderson and M. Boudart, eds., Springer- S. Dahl, and C. J. H. Jacobsen, J. Catal, 197, 229 Verlag, p. 1 (1985). (2001). Vancini, C. A., Synthesis of Ammonia, Macmillan, Mittasch, A., Adv. Catal, 2, 81 (1950). London (1970). Muhler, M., O. Hinrichsen, H. Bielawa, and C. J. H. DRP 235421 (1908). Jacobsen, DK Patent A 3157237003 (2000). DRP 238450 (1909). Nielsen, A., An Investigation on Promoted Iron U.S. 4,668,657 (1987). Catalysts for the Synthesis of Ammonia, 3rd ed., U.S. 4,163,775 (1979). Gjellerup, Copenhagen (1968).

QUESTIONS AND ANSWERS

Ian Welch, PCS Nitrogen: On the barium-promoted boron wide range of H:N ratios. However, from our ongoing nitride catalyst, did you investigate the sensitivity of the investigations, it appears that the kinetics of ammonia syn- reaction to H:N ratio? thesis is identical to that observed on similarly promoted Claus J. H. Jacobsen, Tops0e: We have not yet studied a graphite-supported catalysts. So far, we have limited our interest to H:N ratios between 3.0 and 2.1.

AMMONIA TECHNICAL MANUAL 222 2002