New ATR Based Ammonia Process - IFFCO : KANDLA

New ATR Based Ammonia Process Until now, large-scale ammonia production in single line configuration has suffered from a lack of

demonstrated plant concepts with proven feasibility and competitiveness and traditionally, the industry has perceived 3,500 MTPD as the limit in single line capacity. However, the

commercialization of the catalyst, SK-501 FlexTM, has enabled that the benefits of low S/C autothermal reforming technology (ATR) is now available to the ammonia industry. Consequently, ammonia plants with single line capacities above 6,000 MTPD solely based on industrially proven

equipment sizes and catalysts could become plant owners’ preferred choice due to resulting benefits from economy of scale.

Per Juul Dahl, Christian Speth, Annette E. Krøll Jensen, Marcus Symreng, Merethe Kjul Hoffmann, Pat A. Han, Svend Erik Nielsen

Haldor Topsøe A/S

Introduction his paper presents Haldor Topsoe’s new ammonia process based on advanced au-tothermal reforming.

Ammonia plant owners will benefit from an in-novative and very competitive technology ap-plication defying existing single line plant ca-pacity limits while staying within referenced applications of technology elements. The pro-cess is equally suitable for stand-alone ammonia plants and for integrated urea complexes. It is also an inherently safer process than the conven-tional. Haldor Topsoe pioneered advanced autothermal reforming throughout the 1990ies and commer-cialized the low steam-to-carbon (S/C) ATR technology in 2002. The first industry to take advantage of this unique technology was the

H2/CO industry closely followed by the gas-to-liquids (GTL) industry. The technology break-through came in the early stages of the develop-ing GTL industry, which suffered from the lack of a cost effective solution that could meet the H2/CO ratio needed for the Fischer-Tropsch process and produce syngas in large single line capacity at the same time. The ATR technology became a real game changer removing the limitations that other technologies had in reaching the optimal syngas composition. The advanced ATR technology provides plant owners with a huge leap towards economy of scale in combination with signifi-cant operational expenditure (OPEX) improve-ments through lower specific net energy con-sumption (SNEC), high reliability and uptime, lower requirements for operators and reduced maintenance. Today, the combined industrial operation exceeds 70 years and both industries regard the ATR technology as highly successful. The technology is continuously developed and

T

1612017 AMMONIA TECHNICAL MANUAL

improvements are made in very close collabora-tion with customers. Large-scale GTL plants producing syngas equivalent to more than 6,000 MTPD (6,613 STPD) of ammonia have already been in opera-tion for 10 years. A few years back, the ATR technology was introduced to the methanol in-dustry and consequently, Topsoe’s first 5,000 MTPD (5,511 STPD) ATR based methanol plant is under construction with expected start-up in 2018. It has been Haldor Topsoe’s focus to also make the benefits of the ATR technology available to the ammonia industry and to utilize its operation at low S/C ratio to enable much larger single train ammonia plants. Topsoe has enabled this through the development and commercialization of its high temperature shift catalyst, SK-501 FlexTM, which has been in successful industrial operation for more than 3 years. Consequently, large-scale ATR based ammonia plant technology solely based on industrially proven equipment sizes and catalysts is now available to the market.

Capacity One of the biggest trends in the chemical indus-try for years has been a pull for plants with larg-er single line capacity The availability of this well-proven technology to ammonia producers is a breakthrough in the industry as it accommo-dates such a need. The trend has been driven by wishes to take advantage of economy of scale and to reduce operating cost in a quest to reduce total product cost, similar to what has been the case in the H2/CO and GTL industries. The market for large scale ammonia plants with ca-pacities above 3,500 MTPD (3,858 STPD) has not really picked up speed, to a certain extent due to lack of demonstrated plant concepts with proven feasibility and competitiveness above this capacity, which is traditionally perceived as the limit in single line capacity.

Today’s large-scale ammonia plant technologies all use some degree of tubular steam reforming. This is a very mature technology, which has significant drawbacks. When scaling the capaci-ty, the scaling exponent is very close to one re-sulting in an almost linear function. Additional-ly, although it is technically possible to build large tubular steam reformers, they become in-creasingly difficult to operate in terms of con-trol, safety and maintenance regardless of the specific type of tubular reformer. According to market feedback, the upper practical capacity limit for this technology is around 3,500 MTPD (3,858 STPD) of ammonia, and the way forward for the tubular reforming technology should be a division into more trains. However, that would jeopardize the benefit from economy of scale in this very important plant section, which typical-ly corresponds to ~25% of total plant cost. Fur-thermore, some of the synthesis loops currently proposed and in operation increase complexity and overall capex cost due to multiple pressure levels and reactors.

Figure 1. Comparison between steam methane reformer (SMR) vs. ATR In comparison, the ATR technology scales with a lower exponent. Figure 1 illustrates the differ-ence in scaling between tubular steam reforming and ATR. The ATR based plant is referenced within the full capacity range to above 6,000 MTPD (6,613 STPD). Based on its new applica-tion continuous developments and improve-ments to technology as well as the catalysts

Cost

Capacity

Front end CAPEX cost

SMR based

ATR based2017

SMR basedBeyond reference

Developmentof ATR based

162 2017AMMONIA TECHNICAL MANUAL

could reduce cost even further. The dotted line indicates where the SMR technology is beyond reference, which is above 3,500 MTPD (3,858 STPD) ammonia. With SMR being a very ma-ture technology further cost reduction is not to be expected. From a capital expenditure (CAPEX) perspec-tive, both plant types can be considered for low-er capacities. The ATR technology is competi-tive from well inside the conventional SMR capacities and clearly, it becomes the preferred choice at large capacities because of its refer-enced single line capacity above 3,500 MTPD (3,858 STPD) and the resulting benefits from economy of scale to lower the CAPEX. Where oxygen is available over the fence, the ATR technology is even more attractive from a CAPEX point of view. Detailed studies show the following additional advantages of Topsoe’s ATR based reforming units: 1. 3% lower SNEC, which is a very significant OPEX benefit. 2. Up to 50% make-up water savings, which is especially important in areas where water is a scare resource. 3. An average availability above 99% of the ATR reforming unit.

Picture 1. ATR with an equivalent capacity of more than 6,000 MTPD (6,613 STPD) of am-monia

When scaling up plant capacity, size matters. The ATR has a small physical footprint provid-ing extremely high reforming intensity. Picture 1 shows how small the reactor is, even though its capacity corresponds to 6,000 MTPD (6,613 STPD) of ammonia. Picture 2 shows the large footprint of a tubular reformer and a secondary reformer with a capacity of 1,500 MTPD (1,653 STPD). In comparison, the plot sizes of the ATR and the secondary reformer are almost similar and cor-responds to less than 5% of the plot size of the SMR.

Picture 2. SMR based reforming section with secondary reformer, 1,500 MTPD (1,653 STPD) ammonia plant It is critically important to stay within the com-mercially available standard sizes for equipment and piping. Exceeding these can be expensive because this will limit the number of possible vendors and increase cost with a scaling factor exceeding one. Picture 3 illustrates how large the piping sizes can be. Haldor Topsoe’s ATR based plant offers unique benefits which make the plant live up to such prerequisites. The most significant differences between a traditional SMR based plant and an ATR based plant is the S/C ratios, where the traditional SMR based plants operate at S/C ra-


tio around 3 and an ATR based plant operates at S/C ratio around 0.6. Consequently, steam throughput decreases by 80%. The ATR based plant also benefits from an inert free ammonia synthesis with the re-quired nitrogen admitted just upstream of the ammonia synthesis section, whereas the conven-tional plant introduces the nitrogen in the re-forming section. These features enable significantly reduced pipe and equipment sizes not only in the frontend (re-forming, shift and CO2 removal sections), but also in the backend (ammonia synthesis section) including a smaller synthesis gas compres-sor/recirculator, ammonia converter and high-pressure heat exchangers. A further advantage of the inert free synthesis gas is that a purge gas ammonia wash and hydrogen recovery unit is not required.

Picture 3. Large sizes of piping and valves can make a considerable impact on CAPEX. The design of the inert free ammonia synthesis loop provides another huge advantage. Where other large-scale designs require multiple pres-sure levels and multiple reactors in the ammonia

synthesis section, the new ATR layout uses a single S-300 ammonia converter in a standard, well-proven Haldor Topsoe ammonia synthesis loop with single pressure level. More important-ly, today the required converter size is already well referenced with ammonia converters hav-ing a catalyst volume above 150 m3 (5,300 ft3). For information, an inert free 4,000 MTPD (4,409 STPD) ammonia synthesis loop in an ATR based plant will require only 105 m3 (3,700 ft3) of catalyst.

Technology Haldor Topsoe’s ATR technology operating at S/C ratio of 0.6, has already been described in details, please refer to “References”, during the last 15 years. Without any further changes, this technology can be used as a synthesis gas gen-erator in an ammonia plant. In fact, the ATR technology is very well suited as a synthesis gas hub supplying synthesis gas to ammonia, meth-anol, GTL, CO and other synthesis gas consum-ing processes. The ATR reactor design consists of a burner, a combustion chamber, target tiles, a fixed catalyst bed, a catalyst bed support, a re-fractory lining, and a reactor pressure shell as illustrated in Figure 2.

Figure 2. Topsoe’s ATR reformer


Figure 3. Simplified process sheet of Topsoe’s ATR based ammonia plant The ATR based ammonia technology is not only about a change in reforming technology. Reduc-ing the reforming S/C ratio from 3 to 0.6 has a huge impact on the entire process scheme. This calls for innovative re-design of various plant sections, the main challenge being a workable shift section. With the ATR based ammonia technology the use of two high temperature shift reactors in se-ries, a nitrogen wash to remove the CO, and re-cycling of shift by-products has resulted in nu-merous benefits such as by-product formation being reduced close to zero. Several conven-tional process steps such as methanation, purge gas recovery, ammonia absorption and hydrogen recovery become obsolete, thus resulting in less need of compressor/recycle power and signifi-cantly reduced sizes of high-pressure equipment and piping. Figure 3 shows the main process steps for the new ATR based ammonia plant, and Table 1 provides a comparison of the main differences between a conventional ammonia plant and the ATR based ammonia plant.

Shift section - by-product formation close to zero

The conventional plant based on tubular reform-ing has a shift section containing a high temper-ature shift step followed by a low temperature shift step. A standard high temperature shift us-es a Fe/Cr based catalyst that cannot operate at S/C ratio below 2.6. To overcome this limitation, Haldor Topsoe installed the first charge of SK-501 FlexTM in an industrial plant in 2014. The temperature profiles from the HTS reactor at the industrial reference (Figure 4) show that SK-501 Flex™ is successfully resistant to poi-sons, at start-of-run and after 28 months of op-eration.

Figure 4. HTS reactor: Temperature profiles


The SK-501 FlexTM in itself is a game changer based on promoted zinc aluminum spinel, which can operate at very low S/C ratios at typical high temperature shift conditions, but without risk of mechanical integrity or by-products as-sociated with a Fe/Cr catalyst. This catalyst en-ables a shift section that perfectly matches the S/C ratio of 0.6 in the ATR based design. In conventional plants, the iron based high tem-perature shift catalyst sets the minimum allowa-ble S/C ratio for the shift section. When the S/C ratio is lowered to 0.6, three factors limit the shift section, i.e. the required water content to perform the shift reaction, the acceptable CO slip, and the formation of by-products. An efficient solution to these limitations is the introduction of a second shift operated at medi-um to high temperature in combination with re-circulation of steam from the process conden-sate stripper. Depending on the specific requirement, the catalyst in this second shift can either be SK-501 FlexTM or a Cu based catalyst. In conventional plant designs, the slip of CO is converted to methane in a downstream methana-tor. This methane goes to the ammonia synthesis

loop where it acts as an inert and it builds up if not purged out. A high content of inert requires a high rate of purge gas. When ammonia has been washed out, the purge gas is used as fuel in the tubular reformer. Hydrogen is partly recovered and recycled to the synthesis loop. In the ATR based process, a nitrogen wash re-places the conventional steps for methanation, ammonia wash and hydrogen recovery. The ni-trogen wash removes both the slip of CO from the shift section and the CH4 slip from the re-forming section. The off gas from the nitrogen wash can be used as fuel without any further treatment. This design generates an inert free synthesis gas, which provides benefits in terms of less need of compressor/recycle power and significantly reduced sizes of high-pressure equipment and piping. The well-known issue of undesired shift by-product formation links closely to S/C ratio and temperature. The amount of by-products in-creases at low S/C operation, and especially the formation of the main by-product, methanol in-creases at low temperature.

*) An equivalent production of 4,000 MTPD

Table 1. Comparison of main differences between a conventional and an SMR based ammonia plant


The ATR operates at low S/C ratio, but a high temperature is accepted as long as the CO slip is kept at an acceptable level. This is part of the reason for selecting only high temperature shifts. Topsoe has successfully designed the new pro-cess to reduce by-product formation to practical-ly zero. An innovative and simple solution that recycles the by-products achieves this. After the shift section, by-products will be part-ly condensed out together with the process con-densate. Further amounts of by-products can be washed out of the synthesis gas to the level re-quired. The solution also eliminates the well-known problem of especially methanol entering the CO2 removal section in conventional process layouts. The process condensate and washing water, which contains the by-products from the shift, flows to a process condensate stripper, where practically all shift by-products are stripped off. The resulting stripper steam, which now con-tains the by-products from the shift, is recycled to the synthesis gas inlet at the high temperature shift section. This has several advantages: • The main by-product formation is by equi-

librium reactions. Adding an equilibrium by-product component to the feed of an equilib-rium byproduct generator, such as a shift re-actor, will stop further formation of that component. The main shift by-product, methanol, is formed by an equilibrium reac-tion.

• Dissolved synthesis gas in the process con-densate returns to the process.

• The stripper steam will increase the S/C in the shift section. Even if this is not strictly necessary, it reduces the CO slip, making the net effect of the recycling positive.

The process concept based on two high tem-perature shift reactors in series, a nitrogen wash to remove the CO, and recycling of shift by-products has solved all the challenges in design-ing a workable shift section of a low S/C ATR based ammonia plant. The involved solutions

are cost efficient and the process technology well proven.

CO2 removal and nitrogen wash – inert free synthesis gas

The CO2 removal unit in an ATR based ammo-nia plant can be a standard commercial solution. However, it is an advantage to remove CO2 to a lower level than in a conventional plant, as re-maining CO2 will have to be removed by a downstream drier unit. The CO2 absorber is relatively smaller than for conventional design because no nitrogen is added to the synthesis gas. After the CO2 removal section, the synthesis gas contains mainly H2 with CH4, Ar, CO, CO2 and H2O. At first, CO2 and H2O is removed in a syn-thesis gas drier unit and then CH4, Ar and CO are removed in a nitrogen wash, in which N2 is admitted to the synthesis gas to adjust the hy-drogen to nitrogen ratio to the level required by the ammonia synthesis. This results in an inert free ammonia synthesis gas, and that makes a purge gas ammonia wash and hydrogen recov-ery unit obsolete.

Plant reliability and availability – excellent track record demonstrated

To meet very competitive industrial require-ments in large-scale plants, where each added day of operation can be worth more than one million euros, plant reliability and availability are of utmost importance. Topsoe’s ATR based reforming units have demonstrated availability factors greater than 99% as an average over op-erating periods exceeding 5 years.

Environmental impact – potential CO2 reductions

The environmental impact of a process is an in-direct safety element. Release of greenhouse gases and other contaminants has an impact on people’s health and safety. The overall energy consumption figures for an ATR based design is


up to 3% lower than for the conventional design and a considerable part of the energy does not need to come from fossil fuels. The power for the air separation unit can come from sustaina-ble resources, thus reducing the CO2 re-lease/MTPD of product considerably. See table 2.

Table 2. Potential reduction in CO2 emission

Proven design A main criterion for the development of the ATR based process has been to utilize known and industrially proven process steps and equipment. • The low S/C ATR reforming concept has

been in industrial operation since 2002 and it is referenced up to an equivalent ammonia capacity above 6,000 MTPD (6,613 STPD).

• The high temperature shift reactors are with-in referenced size at a capacity of 4,000 MTPD.

• Topsoe’s SK-501 FlexTM catalyst has been in industrial operation since 2014.

• The CO2 removal section is within refer-enced size at an equivalent ammonia capaci-ty above 6,000 MTPD (6,613 STPD).

• The synthesis gas drier and wash section is industrially proven and available from a number of suppliers.

• The synthesis gas compressor/turbine is within referenced size at an ammonia capac-ity of 4,000 MTPD (4,409 STPD). Larger compressors/turbines are in industrial opera-tion in other industries.

• The ammonia converter is within referenced size up to a capacity above 4,000 MTPD (4,409 STPD).

• The refrigeration compressor is within refer-enced size up to a capacity above 4,000 MTPD

• The ASU is within referenced size up to a capacity above 6,000 MTPD (6,613 STPD).

A review of the above list shows that the pro-cess equipment of the ATR based ammonia plant is referenced up to an equivalent capacity of 4,000 MTPD (4,409 STPD) and that a capaci-ty above 6,000 MTPD (6,613 STPD) is possible by accepting proven references from other in-dustrial processes.

Safety The most important parameter when designing and operating an ammonia plant is safety, both in terms of personal safety, environmental im-pact and process reliability. All new design work needs to address these key issues. The ATR based plant significantly improves all three parameters. It offers several benefits that make it inherently safer than conventional plants and it has the potential to bring down the num-ber of days with lost production significantly.

Personal safety and fieldwork

The main difference between the conventional and the ATR based process design is the reform-ing section. It is well known that tubular reform-ing involves a great deal of manual operation in the field, and that it can be difficult to imple-ment and maintain a comprehensive Safety In-tegrated System (SIS) for this technology for the same reason. Scaling up the conventional tubu-lar reforming capacity does not change this situ-ation. The amount of work in the field will in-crease and more field operators will be required.


Picture 4. Manual inspection of tube tempera-ture in a tubular reformer A large part of the manual fieldwork relating to tubular reforming, regardless of its type, is re-quired to avoid unplanned shutdowns, tube rup-tures and to secure continuous performance. The replacement of a tubular reformer with an ATR reactor has many additional installation and operating advantages: - Tube ruptures no longer exist, and the risk of unplanned shutdowns decreases. - The required amount of fieldwork significantly decreases, since field measurements are no longer required. - Human interaction with the reformer no long-er exists. - The required skill sets will be different and the dependence on the procedures, tools and fre-quency for tube measurements is no longer pre-sent. - The needed amount of surveillance data is much less for one reactor, and data quality im-proves significantly due to a simpler automated process. The ATR based design incorporates a complete integrated SIS, which is in place during startup, normal operation and shut down. The control system will guide plant operators, thus ensuring safe operation in all operating modes, and a permissive system ensures that no mistakes are made. The permissive system has a set of condi-

tional requirements to be satisfied before start of an activity in order to safeguard the plant. The ATR itself requires no fieldwork during op-eration. Typically, in every work shift only a plant walk-through is necessary to perform sur-face monitoring and alternatively, camera sur-veillance can replace the human surface moni-toring of the ATR shell. An ATR based plant still requires some field-work, but much less than a conventional plant with tubular reforming and more importantly, it does not increase in a similar way when scaling up capacity. The difference in fieldwork be-tween large scale tubular reforming and ATR could be as much as 2 to 3 persons in favor of ATR. An ATR based plant therefore reduces the num-ber of Lost Time Incidents, simply because less people are prone to accidents. Fieldwork can turn into control room work with more time spent proactively to optimize performance. See Picture 5.

Picture 5. Safer working environment with time to optimize performance

Process reliability

A higher degree of automation has a hugely pos-itive impact on safety and reliability. It reduces the risk of human error considerably simply be-cause of the reduced number of needed operator interactions, and when interactions are required by setting up strict permissives. This is a win-


win situation since a reduced amount of human errors result in less lost time accidents, increases the availability of the plant and thereby reduces the environmental impact by reducing flaring caused by erroneous upset situations. Automation of the ATR plant operation also provides the possibility to establish remote op-eration which can provide the benefit of less er-rors and more efficient operation. The net result is a higher general safety level and a better bot-tom line.

Environmental impact

The SK-501 FlexTM catalyst provides the plant with other benefits due to its complete absence of chromium, most notably the highly toxic hexavalent chromium found in iron-based HTS catalysts in the market. With SK-501 Flex™, plants avoid the potential risk that hexavalent chromium poses to personnel safety and to the environment during product handling and dur-ing operation. By avoiding this risk, plants also reduce the possibility of unplanned and costly downtimes as well as long-term liability issues.

Conclusion The introduction of the ATR based ammonia process will allow plant owners to benefit from an innovative and very competitive technology application defying existing single line plant ca-pacity limits. Conventional ammonia process technology still has a merit for many years to come. It is a fully optimized and refined technology, but it loses competitiveness for large-scale capacities due to its scaling exponent and at the same time be-comes increasingly difficult to operate in terms of control, safety and maintenance regardless of the specific type of tubular reformer The ATR based process enables ammonia plants with single line capacities above 6,000 MTPD (6,613 STPD) solely based on industrially prov-

en equipment sizes and catalysts. Its process steps are referenced to this capacity when ac-cepting references from other similar industrial processes. The key benefits for plant owners utilizing the ATR based ammonia process include a huge leap towards economy of scale in combination with significant OPEX improvements through lower specific net energy consumption, high re-liability and uptime, lower requirements for op-erators and reduced maintenance. Because of the resulting benefits from economy of scale to lower the CAPEX, the ATR based ammonia process clearly becomes the preferred choice at large capacities. Adding to this, de-tailed studies show 3% lower specific net ener-gy consumption, a very significant OPEX bene-fit. A demonstrated average availability above 99% of Topsoe’s ATR based reforming units sup-ports its competitiveness. The process is an inherently safer process than the conventional. Using Haldor Topsoe’s ATR technology requires much less fieldwork than a conventional plant with tubular reforming and more importantly, it does not increase in a simi-lar way when scaling up capacity. It is a more automated process safeguarded by the control and safety system, reducing the number of pos-sible human errors. Combined with less environmental impact, the above is an answer to required improvements on general safety issues and the reason for the ATR based ammonia process technology becoming plant owners’ preferred choice in future plants. References; [1] T.S. Christensen, I. I. Primdahl, Hydcar

Proc., 73 (1994) 39 [2] W.S. Ernst, S.C. Venables, P.S. Christensen,

A.C. Berthelsen, Hydcar Proc. 79(3), 2000, 100C.


[3] K. Aasberg-Petersen, T.S. Christensen, I. Dybkjaer, J. Sehested, M. Østberg, R.M. Coertzen, M.J. Keyser and A.P. Steynberg. . “Synthesis gas production for FT synthesis”, Stud. Surf. Sci.Cat. 152, (2004), 258

[4] P.J.Dahl, T.S. Christensen, S. Winter-Madsen, S.M. King, “Proven autothermal reforming technology for modern large-scale methanol plants“, Proc. Nitrogen+Syngas 2014 conference, Paris, February 2014.

[5] “Process burners for syngas production”, Ni-trogen+Syngas, 305, May-June 2010, 37

[6] T.S. Christensen, J.R. Larsen, L.J.Shah, M. Stenseng, “Market-leading developments for oxygen-blown secondary and autothermal reformers”, Proc. Nitrogen+Syngas 2017 conference, London, March 2017

[7] ATR movie: www.topsoe.com



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New ATR Based Ammonia Process - IFFCO : KANDLA