Catalytic Advancements since Haber Bosch

Authors:

Venkat Pattabathula akh.deocamdata.ro and Jim Richardson Clariant (Singapore) Pte. Ltd.

It has been 100 years since ammonia was first produced on an industrial scale by BASF in Germany. Since that time, significant contributions by technology providers have substantially improved the operating rate and safety of modern ammonia production facilities. Catalysts have also been instrumental in improving the performance and reliability of ammonia plants. This paper discusses many of the improvements and establishes a timeline showing how each catalyst development led to greater efficiency and reliability.

INTRODUCTION

CLARIANT INTERNATIONAL LTD.

Since catalysts have been instrumental in development of many of the features found in modern ammonia plants, a brief introduction to the Catalyst business of Clariant should be made. Clariant was formed in 1995 as a spin off from the chemical company Sandoz which was established in Basel Switzerland in 1886. Clariant’s Catalyst BU was created by the acquisition of Süd-Chemie AG in 2011. Süd-Chemie was a specialty chemical company based in Munich, Germany that was established in 1857. Süd-Chemie started making catalyst in Germany during the 1960’s through a JV with Girdler Corporation, a Louisville, Kentucky USA based company that had started manufacturing catalyst for the syngas industry during the early 1940’s. Süd-Chemie AG acquired Catalysts and Chemicals Inc. (CCI) in the 1970’s. CCI, a company formed by former Girdler employees started producing syngas catalysts in Louisville in 1957. After gaining control of CCI, Süd-Chemie merged Girdler and CCI to form United Catalysts Inc. To establish the Süd-Chemie brand around the world, most of Süd-Chemie’s catalyst manufacturing sites took the Süd-Chemie name in 2000 and 2001. A timeline for catalyst manufacturing by Clariant is shown in Figure 1.

Fig 1. Clariant Catalyst Production History

AMMONIA’S SIGNIFICANCE

From the discovery that ammonia could be formed from calcium carbide more than 100 years ago, ammonia production has developed into one of the most important industries in the world. Since the first plant based on the Haber Bosch process was built by BASF, the industry has grown significantly and made high crop yields possible to feed billions of people across the globe. Without this growth in crop yields, the population of the world would be at least two to three billion less than it is today (Ref. 1). Today, it is estimated that the annual production of ammonia is worth more than US$100 billion with some plants producing more than 3000 MTPD of NH3.

HISTORY OF THE AMMONIA INDUSTRY

During 1898, Frank, Caro and Tothe found that N2 could be fixed by calcium carbide to form calcium cyanamide which could then be hydrolysed with water to form ammonia.

CaO + 3C ↔ CaC2 + CO
CaC2 + N2 ↔ CaCN2 + C
CaCN2 + 3H2O ↔ CaCO3 + 2NH3

Following this discovery, many people started working on commercialization. The high electrical energy consumption of the cyanamide process led to work on processes with lower energy requirements. The most important toward the development of the modern ammonia industry was the work done by Fritz Haber. He first became interested in nitrogen fixation for the production of ammonia and nitrogen oxides while visiting the Niagara Falls, New York cyanamide based ammonia plant in 1902. At that time, he was a Chemistry professor at the Karlsruhe Engineering College in Germany. The Margulies brothers of Vienna contracted Haber to do research on the production of ammonia from the elements. One of his students, Gabriel van Oordt, worked with him to synthesize ammonia in the laboratory from N2 and H2. Walther Nernst, professor of physical chemistry at the University of Berlin, competed with Haber to develop a process to make ammonia from N2 and H2. In 1901, Le Chatelier in France developed a high-pressure synthesis route to ammonia. He received a patent for his work. Unfortunately, his work eventually led to an explosion that ended his work on the process.

Both Haber and Nernst adopted the high-pressure route to produce ammonia over a catalyst. Haber, working with research assistant, Robert Le Rossignol, and a mechanic named Kirchenbauer – whose work was invaluable in the difficult task of designing and building the equipment required to withstand high temperature and pressure – finally developed a process for producing commercial quantities of ammonia. In 1906, Haber was able to produce a 6% ammonia concentration in a reactor loaded with an osmium catalyst. This is generally recognized as the turning point in the development of a practical process for the production of ammonia in commercial quantities.

Haber also realized that the amount of ammonia formed in a single pass through a converter was far too low to be of commercial interest. To produce more ammonia from the make-up-gas, he proposed a recycle system, and received a patent for the concept. Haber’s recycle idea changed the static conception of process engineering in favor of a more dynamic approach. For the first time, reaction kinetics was considered as well as the thermodynamics of the system. In addition to the chemical engineering equilibrium, Haber recognized that for the technical realization, reaction rate was a determining factor. Instead of simple yield in a once through process, he concentrated on space time yield in a recycle system. BASF purchased Haber’s patents and started development of a commercial process. Carl Bosch and Alvin Mittasch along with BASF chemists developed a promoted iron catalyst for the production of ammonia in 1910. The next problem, development of equipment, was an extremely difficult one. Ordinary steel did not last very long at the high temperature and pressure needed for production of ammonia across an iron catalyst. An early mild steel reactor only lasted 80 hours before failure due to decarbonization. Lining mild steel reactors with soft iron, which was not subject to decarbonization and adding grooves between the two liners to release hydrogen that had diffused through the soft iron liner solved this problem. Other major challenges were the design of a heat exchanger to bring the inlet gas to reaction temperatures and cool the exit gas, and a method to bring the catalyst to reaction temperature.

These problems were finally solved, and the first commercial ammonia plant based on the Haber-Bosch process was built by BASF at Oppau, Germany which is located just 3 km north of BASF’s Ludwigshafen plant complex. The plant went onstream on September 9, 1913 with a production capacity of 30 tons/day. Figure 2 is a painting of the Oppau Plant.

Fig 2. BASF Oppau Ammonia Plant – world’s first ammonia plant 1920 Painting by Otto Bollhagen Courtesy of BASF Corp., Ludwigshafen, Germany

Another German plant was built at Leuna and started up during April 1917 producing 36,000 tons of ammonia per year. By the end of World War I, it had been expanded to produce 240,000 tons per year. A flow sheet of the first commercial ammonia plant is shown in Figure 3 (Ref. 3). The reactor contained an internal heat exchanger in addition to those shown on the schematic.

Fig 3. First Commercial Plant built by BASF Corporation

While BASF was developing the Haber-Bosch process in Europe, interest was also growing in the United States for production of ammonia from the elements. Companies and organizations such as the General Chemical Company and the U.S. Department of Agriculture began research on ammonia synthesis from H2 and N2 with one objective to develop a method that would not infringe BASF patents. The de Jahn process was patented and developed and commercialized during the 1920’s. Due to the urgent need for nitrates during World War I, the U.S. government contracted with General Chemical during 1917 to build a plant in Muscle Shoals, Alabama. The plant was constructed and completed in record time. The first steel was set on June 4, 1918 with the first ammonia produced on September 16, 1918. The plant was called

U.S. Nitrate Plant No. 1. It ran until the end of the war.

During the 1920’s, research and development was continued by the Atmospheric Nitrogen Corporation and the Fixed Nitrogen Research Laboratory in the United States. A better ammonia synthesis catalyst was developed and a plant built in Syracuse, New York in 1921. This successful plant was followed by a very large installation built by Atmospheric Nitrogen in Hopewell, Virginia. The American process developed by the Fixed Nitrogen Research Laboratory was represented by a plant built in Niagara Falls by the Mathieson Alkali Works during 1922. The Niagara Ammonia Company built a plant in Niagara Falls, New York in 1924 using European technology. This was followed by a DuPont plant in Belle, West Virginia in 1926, and Shell Chemical at Long Beach, California using the same technology. Various other plants were built with the ammonia industry thriving in the U.S. by 1930.

The American ammonia technology was also exported to Europe. The Nitrogen Engineering Corporation was commissioned by Kuhlmann in France to build an ammonia plant near Paris in 1928. Thereafter, NEC built plants all over the world, including Russia and the Far East. NEC was later taken over by the Chemical Construction Corporation who built many plants around the world up to the late 1970’s.

BASF continued process development and plant expansions in Europe. Other organizations such as Casale, Fauser, Claude and Mont Cenis also entered the field of design and construction of ammonia plants. Their plants differed from the Haber-Bosch process in various ways. For example, Casale built a plant in Terni, Italy during 1920 with a synthesis loop operating at almost 775 bar.

In the decades after 1930, further improvements were made in ammonia production technology. However, the synthesis section design remained essentially the same. Reactor capacity increased; however, few were built with capacities higher than 100 tons/day. Various “named” processes were used in new plants without much change from older ones.

Although the technology changed very little during this period, production capacity increased significantly driven by nitrogen products demand during World War II. In 1932, there were only ten plants in the United States. By the early 1940’s, 10 more plants had been built in the US with a total capacity more than twice that in 1932.

World production was also increasing rapidly. By 1945, about 125 plants were reported to be in operation with a capacity of over 4.5 million tonnes of nitrogen per year. Production had increased fourfold, from about 900,000 tonnes of nitrogen in 1930 to 3,650,000 tonnes in 1950. This compares to a production of about 4,000 tonnes per year in 1914, 100,000 in 1920 and 400,000 in 1925.

MODERN TECHNOLOGY

The growing need for nitrogen fertilizers brought about a rapid expansion of ammonia production between 1950 and 1980. One of the developments that helped make this possible was the development of methanation catalyst to remove carbon oxides from synthesis gas.

Early ammonia plants utilized the copper liquor process for purification of the synthesis gas to the ammonia loop. The copper liquor process can be described as follows:

Cu(NH3)2A + CO + NH4OH Cu(NH3)3A . CO + H2O
2NH4OH + CO2 (NH4)2CO3 + H2O

The scrubbing system contained both cupric and cuprous ammoniacal salts of acids such as formic, acetic or carbonic plus an excess of ammonia. In operation, these salts form complexes with CO and hold it loosely. Adsorption is carried out at high pressure, typically 120 bar, and low temperature, typically 0ºC. The copper liquor process is no longer used in the ammonia industry due to the difficulty to control and environmental unfriendliness. The last plant in North America utilizing copper liquor for CO removal shutdown during the 1970’s.

Girdler Corporation located in Louisville, Kentucky USA built H2 Plants for industry starting in the 1930’s. During WW II, Girdler began producing catalyst for these plants due to lack of supply from other sources. When the company started building ammonia plants during the 1940’s they recognized that they needed a process to remove carbon oxides to replace the copper liquor process. Work started in their R&D lab where they developed and commercialized high nickel, high activity methanation catalyst with the first charge produced in 1948. The first ammonia plant built with a methanator instead of copper liquor was built by Girdler Engineering for Mississippi Chemical Corporation in Yazoo City, Mississippi during 1949. Other plants soon followed. Girdler’s G-65 series of methanation catalysts allowed more stable operation of the plant, leading to better onstream factors and better energy efficiency. Girdler produced high nickel methanation catalysts until the early 1960’s. At that time, Catalysts and Chemicals, commonly known as CCI, introduced C13 a low nickel, high activity methanation catalyst in the form of spheres. Production as spheres allowed optimization of the pore structure of the particle, making lower nickel levels possible. This spherical methanation catalyst (Figure 4) has been the preferred catalyst of the ammonia industry for more than 40 years. Many charges that have been onstream for 15-20 years continue to perform well after surviving severe upset conditions with no impact on performance.

Fig 4. C13 Spherical Methanation Catalyst

Due to Girdler’s success in producing catalysts for the plants that they engineered and built, the company started selling catalyst used to produce NH3 during 1947. This was essentially the beginning of the commercial syngas catalyst business in North America.

The tremendous increase in demand from 1950 to 1980 led to the building of larger more energy-efficient plants. The new technology of these plants began during the early 1950’s. Developments included the use of centrifugal compressors to replace reciprocating compressors and increased recovery of process energy that was used directly to supply a portion of the plant energy requirement. Additionally, there was a change in design philosophy. Until this time, an ammonia plant was regarded as an assembly of unrelated units such as gas preparation, gas purification, gas compression and ammonia synthesis. New developments were based on an integral design that would tie the units together in the most effective and efficient way.

Development continued on single converter capacity until the mid-1960’s when the first new generation of ammonia plant was built. The American Oil Company installed a single converter, integrated energy ammonia plant engineered by M.W. Kellogg at Texas City, Texas with a capacity of 544 MTPD during 1963. Even though the plant used a four-case centrifugal compressor to compress the syngas to a pressure of 152 bar, final compression to an operating pressure of 324 bar was done using a reciprocating compressor. The synthesis loop also used a centrifugal recycle compressor (Ref. 2). Centrifugal compressors for the air and refrigeration services were also used which provided significant cost savings.

Other major changes were made relative to the concept of integrated design.

Waste heat recovery was increased throughout the system.
High-pressure steam was used to drive compressors and other equipment.
Higher pressure was used in the gas preparation section; thereby, reducing overall energy requirement.

Throughout the plant, an integrated scheme was applied; i.e., balancing energy consumption, energy production, equipment size and catalyst volumes. A typical flow sheet for a 544 MTPD NH3 plant built during the mid-1960’s is shown in Figure 5.

Fig 5. Flowsheet for KBR Design NH3 Plant

Although this flowsheet is fairly typical, many minor variations are encountered. The heat recovery system is especially subject to variation with different combinations of waste heat boilers and heat exchangers. However, all designs are aimed at high-energy efficiency. Most plants built between 1963 and 1993 were basically based on large single train designs with synthesis gas production at 25-35 bar and ammonia synthesis at 150-200 bar. Other variations such as Braun’s Purifier design (now KBR) offered slight modifications of the basic design. Recently, some plants have been built based on a synthesis gas generation system with no secondary reformer, a PSA H2 recovery system and an air separation plant for a source of N2. Improvements in converter design such as radial and horizontal catalyst beds, internal heat exchangers, and synthesis gas treatment led to an increase in ammonia concentrations exiting the synthesis converter from about 12% to 19-21%. This increased conversion per pass along with more efficient turbines and compressors led to further reductions in energy consumption. More efficient CO2 removal solutions such as potassium carbonate and aMDEA have contributed to improved energy efficiency. Most modern plants can produce ammonia with an energy consumption of 28 GJ/t. Recently introduced technologies involve gas-heated reformers for synthesis gas production and low-pressure synthesis loops, to reduce energy consumption even further.

In addition to design, mechanical and metallurgical improvements during this time, the operating pressure of the synthesis loop was significantly reduced. When the first single train plant was built in the 1960’s, it contained a high-pressure synthesis loop. In 1962 MWK received an inquiry from ICI for a proposal to build a 544 MTPD plant at their Severnside site. MWK decided to propose a 152 bar synthesis loop instead of a 324 bar loop. Since the development of kinetic data for the ammonia reaction at 152 bar would take more time than MWK had to respond to the ICI inquiry, they decided to contact Haldor Topsoe to support their plans. Topsoe had data covering the entire pressure range of interest to Kellogg. In addition, they had a computer program for calculating the quantity of catalyst that was required at the lower operating pressure. Even though ICI chose Bechtel to design the plant, MWK was able to develop a flowsheet for a 544 MTPD design with centrifugal compressors and a low-pressure synthesis loop which some people consider the single most important event in the development of the single train ammonia plant.

The catalyst requirement at 152 bar compared to 324 bar essentially doubled, an increase that seemed economically feasible. Although the converter would need twice the volume, the operating pressure was cut by more than half, thus reducing the thickness of the pressure shell. As a result, the weight of metal required for the converter and hence the cost remained about the same. Using a lower pressure loop also allowed the use of centrifugal compressors instead of reciprocating compressors. Other improvements included recovering heat to generate high-pressure steam for steam turbines drives. Process steam was also generated by the expansion of high-pressure steam through steam turbine drivers. These improvements and the resultant design may be considered some of the most important which led to modern plant designs.

CATALYST DEVELOPMENTS

The 1960’s

Reforming catalysts based on calcium aluminate cement operated very well for many years until the introduction of high flux, high pressure reformers by the M.W. Kellogg Company during the middle 1960’s. These reformers utilized tubes with internal diameters of 72 mm, operating pressures as high as 35 bar and flux rates as high as 100,000 Watts/m2. With cocurrent flow of process gas and flue gas, localized flux rates in the top of these furnaces could exceed 120,000 Watts/m2. This led to excessive tube metal temperatures with existing catalysts. To solve this problem, CCI introduced alpha alumina based reforming catalyst during the late 1960’s. CCI’s C11-9 alpha alumina reforming catalyst with its excellent physical properties could be made in sizes as small as 16 x 6 x 6 mm rings (Figure 6). The higher activity of this smaller size resulted in much cooler tube metal temperatures and longer catalyst life.

Fig 6. C11-9-02 Alpha Alumina Reforming Catalyst

Another catalyst development that significantly improved the energy consumption of NH3 plants was commercialized during the 1960’s. It was called low temperature shift catalyst because it operated at temperatures lower than high temperature shift catalyst to take advantage of better equilibrium. The first patents for LTS catalyst were issued during 1928. However, the first charge did not go onstream until 1962. Producers of LTS catalysts during this time period used them in their own plants so they were not available commercially. To improve the overall energy consumption of ammonia and hydrogen plants, CCI started making and selling copper/zinc based low temperature shift catalyst in 1964. Initial charges were placed in plants without LTS catalyst or into plants that utilized two HTS reactors with two CO2 removal systems. A reduction in the CO concentration inlet the methanator from 0.5-1.5% (HTS) to 0.18-0.20% in plants switching to C18 LTS catalyst produced by CCI increased production by 5-15%. Because of the benefits of low temperature CO conversion, most plants that have been built since 1964 have utilized LTS catalyst.

CCI also started producing ammonia synthesis catalyst through a license from Norsk Hydro in 1965. The catalyst which was produced, C73, enjoyed great success in the NH3 industry and was installed in many plants around the world through the mid-1980’s. Due to its activity and robustness, there are a number of charges of C73 still onstream today even after operating for more than 25 years.

The 1970’s

Small raschig ring alpha alumina based catalysts were utilized in most reforming applications throughout the early and mid-1970’s. However, as plants continued to upgrade equipment and introduce new technology to achieve higher operating rates, the need for an even more active reforming catalyst developed. In 1978, United Catalysts (formed by the merger of CCI and Girdler) introduced C11-9-09 HGS (later EW), which was the first “shaped” reforming catalyst in the synthesis gas industry. This catalyst was alpha alumina based and very quickly became known as “Wagon Wheels” due to its distinctive shape (Figure 7). During the next seven years, United Catalysts’ C11-9-09 HGS became the most widely used reforming catalyst in the Western Hemisphere. Most plants were able to increase their run length by 50-100% with no loss in efficiency due to either excessive tube metal temperatures or methane approaches to equilibrium. Other manufacturers soon introduced their own version of multi-hole rings that are in use today.

Fig 7. UCI’s “Wagon Wheel” Shaped Catalysts

To prevent premature plant shutdowns due to high CO leakage from the LTS converter, many plants added a low temperature shift guard reactor ahead of their existing LTS reactor. LTS guard beds contained 25-50% of the volume of catalyst contained in the LTS main bed. Because LTS catalyst is poisoned by trace concentrations of sulfur and chlorides in the feed, the concept was to trap poisons in the guard bed to extend the life of the main bed. Since the guard bed catalyst needed to be changed while the plant was still running, block valves were used to isolate the bed during catalyst replacement and the subsequent reduction. United Catalysts’ C18-HC was widely used in guard beds due to it ability to efficiently trap poisons; thereby, protecting the main bed so that it could be run for up to 10 years between replacement.

During the 1970’s, tube metallurgy changed. Instead of just supplying high alloy HK-40 type tubes, manufacturers began to custom formulate tubes to balance rupture and creep strength, carburization resistance, ductility, toughness and weldability to suit varying operating conditions. New alloys like HP Modified were introduced which allowed plants to reduce the tube wall thickness of reformer tubes. This led to better heat transfer and lower tube wall temperatures. Today, new micro alloy materials with even better properties are available to improve the performance of tubular reformers.

The 1980’s

Prior to the mid-1980’s, most of the natural gas in North America used for synthesis gas production was rich in methane with only small concentrations of heavier hydrocarbons. Natural gas “Stripper” plants cryogenically removed most C3+ hydrocarbons since they were more valuable as refinery and chemical plant feedstocks. This was not as common in other parts of the world where heavier natural gas streams were commonly used. With high methane content natural gas, coking or “hot banding” in top fired high flux reformers was not a problem. However, as natural gas streams became heavier and plants decreased their S/C ratio to save energy, “hot banding” which had not been a significant problem since the introduction of high geometric surface area reforming catalysts, began to appear once again. The good physical properties of C11-9 EW enabled the periodic steam removal of carbon deposition which caused “hot banding”. This however, required a plant outage, and resulted in production losses.

To eliminate unscheduled outages for steaming, alkalized catalysts were introduced for steam/natural gas reforming (Ref. 4). Heavily alkalized catalysts for naphtha reforming had been introduced years earlier, but the inherent problems with potassium and silica carryover to waste heat boilers and shift converters made them unsuitable for NG based plants. The initial catalysts that were used were raschig rings derived from naphtha reforming catalyst with lower potassium contents. While they solved “hot banding” problems in high flux reformers, they were less active than non-promoted catalysts resulting in higher tube metal temperatures and methane approaches to equilibrium. To solve this problem, shaped alkalized reforming catalysts such as United Catalysts’ G-91 were introduced during the 1980’s. The higher geometric surface area and activity of G-91 resulted in low tube metal temperatures and methane approaches to equilibrium. The G-91 formulation is still used in the synthesis gas industry for reforming of natural gases and heavier hydrocarbon streams.

During the mid-1980’s, many plants decided to reduce their reformer S/C ratio to reduce pressure drop and improve energy efficiency. As a consequence of this, over reduction of the iron/chrome HTS catalyst leading to Fischer-Tropsch synthesis occurred. Fischer-Tropsch synthesis across the HTS catalyst reduced the efficiency of the plant and eventually resulted in replacement of the catalyst due to a loss in physical strength.

By 1985, Süd-Chemie had recognized this problem (Ref. 5) and had a replacement catalyst available, which would eliminate or delay the onset of Fischer-Tropsch synthesis. The first charges of G-3C and C12-4 copper promoted HTS catalysts went onstream during the late 1980’s. Both catalysts essentially eliminated Fischer-Tropsch synthesis in most plants even at reformer S/C ratios less than 3.0. Another positive benefit of the addition of copper was an improvement in the activity of the catalyst so that plants could operate with inlet temperatures in the range of 300-320oC. This reduced the CO leakage due to more favorable equilibrium leading to lower CO leakages from the LTS converter. Copper promoted HTS catalyst such as Clariant’s ShiftMax 120 is still used today in synthesis gas plants operating with reformer S/C ratios as low as 2.5.

Another improvement in plant design that is indirectly related to performance of the reforming section of the plant was introduction of purge gas recovery systems. The concept was to recover H2 from the purge gas and re-inject it into the MUG going to the loop. This allowed plants to reduce firing on the primary reformer and increase the air/gas ratio in the secondary reformer. Since the secondary was required to do more work, the primary reformer catalyst had to continue operating at equilibrium at lower exit temperatures while the secondary reforming catalyst had to operate at higher temperatures in the top of the reactor. This led to a reduction in the volume of secondary catalyst to allow more mixing space in the top of the reactor. Since the secondary volume was reduced, smaller size alpha alumina catalyst had to be used to achieve equilibrium conversion. Up until that time, large calcium aluminate cement based catalyst was commonly used.

The use of mole sieve dryers to remove moisture and trace CO2 concentrations from MUG also became widely practiced during the 1980’s as plant rates increased. In many plants built before 1980, the main NH3 separator (sometimes referred to as the secondary separator) was located downstream of the recycle compressor. Moisture in the fresh MUG and NH3 in the recycle gas were removed in the separator before the process gas went back to the converter. Adding mole sieve dryers downstream of the methanator allowed plants to condense and separate NH3 from the process gas prior to circulating back to the recycle compressor. In addition to saving energy, mole sieve dryers prevented CO2 from passing into the loop, thus preventing shutdowns due to carbamate formation in NH3 chillers. Mole sieve dryers also replaced many of the NH3 scrubbing systems that were utilized in some plants for feedgas purification.

As energy costs around the world increased during the 1980’s, existing ammonia plants continued searching for ways to lower energy costs. In the synthesis loop, individual beds in axial flow reactors utilizing 6-10 mm size catalyst were converted to radial flow or axial/radial flow beds which could utilize smaller size catalyst due to less pressure drop. By replacing 6-10 mm size catalyst with 1.5-3.0 mm size catalyst, most plants were able to increase the NH3 concentration exit the reactor from 12% up to 14.5- 15.0%. This increased conversion allowed plants to increase the MUG rate to the loop; thereby increasing NH3 production. Some 900 MTPD plants were able to increase their production rate to 1360 MTPD after converting their axial flow converter beds to radial flow beds. This concept was further enhanced during the 1990’s with the advent of internal heat exchangers and variations of the radial flow concept such as horizontal converters. An example of conversion of a 4-Bed quench converter to a 3-Bed with internal heat exchangers by Ammonia Casale is shown in Figure 8.

Today, most plants operate with 1.5-3.0 mm size catalyst with an NH3 concentration of at least 16% exit the converter and some with as much as 21% NH3 in the converter outlet stream.

Process air preheat also became widespread during the 1980’s as a retrofit for older plants and as standard equipment for new plants. Heat from flue gas exit the reformer was used to preheat process air to decrease the amount of fuel needed in the reformer. Some newer plants utilized gas turbine exhaust rich in oxygen as process air for the same reason.

The 1990’s

Energy consumption and environmental regulations continued to drive improvements in NH3 plant design and operation during the 1990’s. As 900 MTPD plants increased capacity to 1400 MTPD or higher, pressure drop across the reformer became a bottleneck. Even after new developments in tube metallurgy that allowed plants to increase tube ID’s fro 72 mm up to 100 mm or larger, pressure drop across the reformer remained a major energy consumer. Operators either had to compress feedgas to a higher level or consume more power to compress MUG going to the synthesis loop.

Süd-Chemie introduced 10-Hole LDP reforming catalyst in 1998 (Figure 9) to reduce pressure drop across the reformer while maintaining sufficient activity and heat transfer to keep tube wall temperatures and methane leakages at acceptable levels. This shape had the same activity properties of the earlier EW shape, however, pressure drop is about 40% lower than the earlier EW material. A lower pressure drop across the reformer results in less energy needed to compress the reformer feed or allows plants to increase the feedrate to produce more ammonia. Another advantage of the 10-Hole shape is enhanced physical properties. Since it is stronger than previously produced catalysts, it can withstand the forces caused by expansion and contraction of the reformer tubes much better which leads to longer lives.

Figure 9 ReforMax® LDP Shape

As governments around the world enacted new environmental regulations on plant operators during the 1990’s, by-product make across the LTS converter became an issue. Compounds such as methanol and amines which can be made across HTS and LTS catalysts, eventually end up in the process condensate or overhead CO2. Methanol in process condensate that goes to a low pressure condensate stripper usually ends up in the atmosphere with the steam exiting the stripper. Methanol is not as troublesome in plants with a high pressure stripper since the methanol in the overhead steam can be recycled back to the reformer. Methanol in the condensate knock-out overhead usually ends up with the CO2 exiting the CO2 removal system. This contamination reduces the value of the CO2 if it is sold as a feedstock for other processes. In addition, methanol make reduces the efficiency of a synthesis gas plant. For example, in an NH3 plant, each tonne of methanol that is produced reduces NH3 production by 1.1 tonnes. This is equivalent to hundreds of thousands of dollars in lost production every year for a modern NH3 plant.

Since installation of a high-pressure condensate stripper is quite expensive, the Süd-Chemie Group developed a catalyst designated C18-HALM (high activity, low methanol) and introduced this to the industry during the mid-1990’s (Ref. 6) Whereas conventional catalysts produced about 90% of the equilibrium methanol across an LTS converter, C18-HALM produced only 10% of the equilibrium amount.

This allowed plants with low pressure condensate strippers to meet environmental regulations by simply changing their LTS catalyst. Less methanol make also improved the economics for most plants since more H2 ended up in ammonia instead of undesirable products.

Another catalyst development that was first used during the 1990’s in new plants was ruthenium-promoted ammonia synthesis catalyst. Ruthenium promoted catalyst was developed by KBR and installed in two KAAP plants during the 1990’s (Ref. 7). The catalyst is much more active than magnetite which allowed KBR to design the NH3 loop with an operating pressure of less than 100 bar. Operating at a lower temperature, the catalyst was able to achieve NH3 concentrations of more than 20% exit the converter even at this low pressure. Lower operating pressure also reduced the capital cost of the plant since piping, vessels and other equipment in the loop could be fabricated from thinner wall materials.

Gas heated reformers such as KBR’s KRES technology and ICI’s LCA were also introduced during the 1990’s. Gas heated reformers reduced the cost of the reformer by utilizing secondary reformer effluent as the source of heat for the primary reforming reaction. They also reduced energy consumption and CO2 emissions to the atmosphere. Because of the unique reforming conditions in GHR’s, United Catalyst developed special catalyst and catalyst sizes to provide the same type of performance and reliability of expected with fired reformers.

The 2000’s

During the first decade of the new century, many improvements were made in ammonia plant technology that allowed existing plants to increase rates even further and new plants to be built with higher and higher capacity. Uhde designed a plant for SAFCO using their dual pressure ammonia process that produces 3300 MTPD of NH3. This plant started up in 2006. Plants with capacities as high as 5000 MTPD have been proposed (Ref. 8, 9), but none have been built.

The most significant catalyst development of the 2000’s was the introduction of AmoMax-10 Wustite NH3 synthesis (Ref. 10). This product is made by Süd-Chemie, now Clariant. Wustite is a non-stoichiometric iron oxide with properties that produce a catalyst with the following benefits:

Table 1 AmoMax-10 Features

20% higher activity compared to Magnetite
Fe3O4 Low temperature low pressure activity
Extremely good thermal stability for long life
Easy to activate + quick reduction
High poison resistance
Very high crushing strength
Available in oxide and pre-reduced form

The first charge of AmoMax-10 in a plant with 1000 MTPD capacity or higher went onstream at Liaohe Chemical Fertilizer during December 2003. This charge is currently operating at near start-of-run (SOR) conditions and has led to more than 85 other charges onstream since 2005. As can be seen in Figure 10, AmoMax is being used in NH3 plants all around the world. It has been used as a drop-in replacement as well as part of a converter revamp.

CONCLUSIONS

Since BASF started producing 30 MTPD of NH3 using the Haber Bosch process 100 years ago, advancements in all aspects of the NH3 production process have been realized so that plants producing more than 3000 MTPD are operating today. Advances in process design, equipment, safety and catalysts have all contributed to the current state of NH3 production. Technology to produce 5000 MTPD is available and will surely be commercialized within a few years to produce fertilizer for a growing world population. Such technical progress made during the last 100 years, enables the production of enough food to feed the current and future population on planet earth.

References

“Enriching the Earth – Fritz Haber, Carl Bosch, and the Transformation of World Food Production” – Vaclav Smil, The MIT Press.
“One Hundred Years of Ammonia Production – A Recap of Significant Contributions to Feeding the World” – Gerald Williams, Plant Surveys International and Venkat Pattabathula, Incitec Pivot Ltd. Australia, Ammonia Technical Manual 2013, AIChE NH3 Safety
“Ammonia Part 1, 2 and 3” – A.V. Slack and G.R.
“Catalyst Development for Improved Reformer Performance” – Dr. Yeping Cai and Richard P. Zoldak, Süd-Chemie Inc., Louisville, KY, Ammonia Technical Manual 2006, AIChE NH3 Safety Symposium.
“Effect of Low Steam on Water Gas Shift Operation” – R.W. Hartye, United Catalysts, Inc., Louisville, KY, Ammonia Technical Manual 1985, AIChE NH3 Safety
“LTS Catalyst Design: Maximizing Shift Converter Performance” – Jim Richardson, Dr. Jayanta Bandyopadhyay, and Dr. Yeping Cai, Süd-Chemie Inc. Louisville, KY, Ammonia Technical Manual 2002, AIChE NH3 Safety
“First Two 1,850 MTPD Grassroots Ammonia Plants” – R.B. Strait, Ian Welch, Kellogg Brown & Root, Inc., Houston, TX, Ammonia Technical Manual 2000, AIChE NH3 Safety Symposium
“MEGAMMONIA® – the Mega-Ammonia Process for the New Century” – William L.E. Davey, Dr. Thomas Wurzel, Lurgi AG, Frankfurt, Germany and Dr. Ermanno Filippi, Ammonia Casale SA, Lugano, Switzerland, Ammonia Technical Manual 2004, AIChE NH3 Safety Symposium.
“Single Train 4000-5000 MTPD KBR Ammonia Plant” – John Abughazaleh, Jim Gosnell and R.B. Strait, Kellogg Brown & Root, Houston, TX, Ammonia Technical Manual 2002, AIChE NH3 Safety Symposium.
“AmoMax-10 – A novel Ammonia Synthesis Catalyst” – Norbert Ringer, Dr. Marcus Michel, Süd- Chemie AG Germany, and Rob Stockwell, Süd-Chemie Inc., Louisville, KY, Ammonia Technical Manual 2005, AIChE NH3 Safety