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CEP September 2016 wwwaicheorgcep 69 Back to Basics M ost people associate the pungent smell of ammo nia NH3 with cleaners or smelling salts How ever the use of ammonia in these two products represents only a small fraction of the total global ammonia production which was around 176 million metric tons in 2014 1 To appreciate where the industry and technology are today lets first take a look at how we got here Ammonia has been known for more than 200 years Joseph Priestley an English chemist first isolated gaseous ammonia in 1774 Its composition was ascertained by French chemist Claude Louis Berthollet in 1785 In 1898 Adolph Frank and Nikodem Caro found that N2 could be fixed by calcium carbide to form calcium cyanamide which could then be hydrolyzed with water to form ammonia 2 CaO 3C CaC2 CO CaC2 N2 CaCN2 C CaCN2 3H2O CaCO3 2NH3 The production of significant quantities of ammonia using the cyanamide process did not occur until the early 20th century Because this process required large amounts of energy scientists focused their efforts on reducing energy requirements German chemist Fritz Haber performed some of the most important work in the development of the modern ammonia industry Working with a student at the Univ of Karlsruhe he synthesized ammonia in the laboratory from N2 and H2 Meanwhile Walther Nernst a professor of physical chemistry at the Univ of Berlin developed a process to make ammonia by passing a mixture of N2 and H2 across an iron catalyst at 1000C and 75 barg pressure He was able to produce larger quantities of ammonia at this pressure than earlier experiments by Haber and others at atmospheric pressure However Nernst concluded that the process was not feasible because it was difficult or almost impossible at that time to produce large equipment capable of operat ing at that pressure Nonetheless both Haber and Nernst pursued the high pressure route to produce ammonia over a catalyst Haber finally developed a process for producing commercial quan tities of ammonia and in 1906 he was able to achieve 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 realized that the amount of ammonia formed in a single pass through a converter was far too low to be of com mercial interest To produce more ammonia from the makeup gas he proposed a recycle system and received a patent for the concept Habers recycle idea changed the perception of process engineering as static in favor of a more dynamic approach In addition to the chemical reaction equilibrium Haber recognized that reaction rate was a determining factor Instead of simple yield in a oncethrough process he concen trated on spacetime yield in a system with recycle BASF purchased Habers patents and started develop Ammonia is critical in the manufacturing of fertilizers and is one of the largestvolume synthetic chemicals produced in the world This article explores the evolution of ammonia production and describes the current manufacturing technologies Venkat Pattabathula Incitec Pivot Ltd Jim Richardson Catalysts and Chemicals LLC Introduction to Ammonia Production Copyright 2016 American Institute of Chemical Engineers AIChE 70 wwwaicheorgcep September 2016 CEP Back to Basics ment of a commercial process After testing more than 2500 different catalysts Carl Bosch Alvin Mittasch and other BASF chemists developed a promoted iron catalyst for the production of ammonia in 1910 Developing equipment that could withstand the necessary high temperatures and pressure was an even more difficult task An early mild steel reactor lasted only 80 hours before failure due to decarbonization Lining mild steel reactors with soft iron which was not vul nerable 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 included designing a heat exchanger to bring the inlet gas to reaction temperatures and cool the exit gas and devising a method to bring the catalyst to reaction temperature The first commercial ammonia plant based on the Haber Bosch process was built by BASF at Oppau Germany The plant went onstream on Sept 9 1913 with a production capacity of 30 mtday Figure 1 is a flowsheet of the first commercial ammonia plant The reactor contained an internal heat exchanger in addition to those shown on the schematic Global production rates Ammonia production has become one of the most important industries in the world Without the crop yield made possible by ammoniabased fertilizers and chemi cals the global population would be at least two to three billion less than it is today 3 Ammonia production has increased steadily since 1946 Figure 2 and it is estimated that the annual production of ammonia is worth more than 100 billion with some plants producing more than 3000 mtday of NH3 In 1983 on the occasion of the 75th anniversary of AIChEs founding a blue ribbon panel of distinguished chemical engineers named what they believed to be the worlds ten greatest chemical engineering achievements 4 Embracing such feats as wonder drugs synthetic fibers and atomic energy the citation also included the breakthrough that permitted the production of large quantities of ammonia in compact singleunit plants Within the past decades chemical engineers have suc ceeded in creating processes that make vast amounts of ammonia at relatively low costs As recently as 80 years ago the total annual production of synthesized ammonia was just over 300000 mt Thanks to chemical engineering break throughs one modern ammonia plant can produce more than 750000 mtyr Approximately 88 of ammonia made annually is con sumed in the manufacturing of fertilizer Most of the remain der goes into the production of formaldehyde China pro duced about 326 of the global production in 2014 while Russia India and the US produced 81 76 and 64 respectively 1 While most of the global production of ammonia is based on steam reforming of natural gas signifi cant quantities are produced by coal gasification most of the gasification plants are located in China Modern production processes The tremendous increase in ammonia demand from 1950 to 1980 necessitated larger moreenergyefficient plants Those decades also saw a change in design philosophy Until that time an ammonia plant was regarded as an assembly of unrelated units such as gas preparation gas purification gas compression and ammonia synthesis New innovations and an integral design tied process units together in the most effective and efficient ways In the mid1960s the American Oil Co installed a singleconverter ammonia plant engineered by MW Kellogg MWK at Texas City TX with a capacity of 544 mtday The singletrain design concept Figure 3 was so revolution ary that it received the Kirkpatrick Chemical Engineering Achievement Award in 1967 The plant used a fourcase centrifugal compressor to compress the syngas to a pressure of 152 bar and final p Figure 1 This is a simplified flowsheet of the first commercial ammonia plant by BASF p Figure 2 Worldwide ammonia production has steadily increased from 1946 to 2014 H2N2 Compressor Filter Reactor Heat Exchangers Condenser Ammonia Separator Circulator Absorber 22 NH3 aq Water Water Gas Purge 1946 0 20 40 60 80 100 120 140 160 Annual Ammonia Production Million mt 180 200 1954 1962 1970 1978 1986 1994 2002 2010 2014 Copyright 2016 American Institute of Chemical Engineers AIChE CEP September 2016 wwwaicheorgcep 71 compression to an operating pressure of 324 bar occurred in a reciprocating compressor Centrifugal compressors for the synthesis loop and refrigeration services were also imple mented which provided significant cost savings The key differences between the MWK process and the processes used in previous ammonia plants included using a centrifugal compressor as part of the synthesis gas compression maximizing the recovery of waste heat from the process generating steam from the waste heat for use in steam turbine drivers using the refrigeration compressor for rundown and atmospheric refrigeration An integrated scheme that balanced energy consumption energy production equipment size and catalyst volumes was incorporated throughout the plant Most plants built between 1963 and 1993 had large singletrain designs with synthesis gas production at 2535 bar and ammonia synthesis at 150200 bar Another variation by Braun now KBR offered slight modifica tions to the basic design The Braun Purifier process plants utilized a primary or tubular reformer with a low outlet temperature and high methane leakage to reduce the size and cost of the reformer Excess air was added to the second ary reformer to reduce the methane content of the primary reformer exit stream to 12 Excess nitrogen and other impurities were removed downstream of the methanator Because the synthesis gas was essentially free of impurities two axialflow ammonia converters were used to achieve a high ammonia conversion Some recently built plants have a synthesis gas genera tion system with only one reformer no secondary reformer a pressureswing adsorption PSA system for H2 recovery and an air separation plant as the source of N2 Improve ments in converter design such as radial and horizontal catalyst beds internal heat exchangers and synthesis gas treatment helped increase ammonia concentrations exiting the synthesis converter from about 12 to 1921 A higher conversion per pass along with moreefficient turbines and compressors further reduced energy consumption More efficient CO2 removal solutions such as potassium carbon ate and methyldiethanolamine MDEA have contributed to improved energy efficiency Most modern plants can p Figure 3 KBR designed one of the first singletrain largecapacity ammonia plants Fuel Natural Gas Boiler Feedwater BFW Steam Steam Drum Steam BFW Heat Recovery CO Converter BFW Steam Water First Stage HighTemperature Synthesis Second Stage LowTemperature Synthesis Condensate Gas to Purification Cooler Compressor Heat Recovery Section Primary Reformer Secondary Reformer Power Air Air Raw Synthesis Gas Rich Solution Lean Solution Heat Water CO2 CO2 Absorber CO2 Stripper NH3 Converter Water Water Cooler Compression Heat Exchanger Purge Flash Gas Ammonia NH3 Cooled Condenser Cooler Heat Methanator Separator Copyright 2016 American Institute of Chemical Engineers AIChE 72 wwwaicheorgcep September 2016 CEP Back to Basics produce ammonia with an energy consumption of 28 GJmt In addition to the design mechanical and metallurgical improvements made during this time the operating pressure of the synthesis loop was significantly reduced When the first singletrain plant was built in the 1960s it contained a highpressure synthesis loop In 1962 MWK received an inquiry from Imperial Chemical Industries ICI for a proposal to build a 544mtday plant at their Severnside site MWK proposed a 152bar synthesis loop instead of a 324bar loop Because 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 contacted Haldor Topsøe to support their plans Topsøe had data covering the entire pressure range of interest to MWK In addition they had a computer program for calculating the quantity of catalyst that was required at the lower operat ing pressure Even though ICI chose Bechtel to design the plant MWK was able to develop a flowsheet for a 544mtday design with centrifugal compressors and a lowpressure synthesis loop which some people consider the single most important event in the development of the singletrain ammonia plant Approximately twice as much catalyst was required at 152 bar as at 324 bar an increase that seemed econom ically feasible Although the converter would need twice the volume the lower operating pressure would reduce the required thickness of the pressure shell As a result the weight of metal required for the converter plus the catalyst remained about the same The lowerpressure synthesis loop also allowed the use of centrifugal compressors instead of reciprocating compressors Another improvement was recovering heat to generate highpressure steam for steam turbine drives Plant designs in the 21st century During the first few years of the 21st century many improvements were made in ammonia plant technology that allow existing plants to increase production rates and new plants to be built with larger and larger capacities Compe tition between technology suppliers is quite fierce Three technology licensors KBR Kellogg Brown and Root Haldor Topsøe and ThyssenKrupp Industrial Solutions TKIS currently dominate the market Ammonia Casale which offers an axialradial catalyst bed design is a market leader in revamps of existing plants p Figure 4 Modern ammonia plants designed by KBR employ its proprietary Purifier design Methanator CO2 Absorber Synthesis Gas Compressor Dryer CO2 Stripper To BFW System To Process Stream Condensate Stripper HTS LTS Refrigeration Compressor Ammonia Waste Gas to Fuel FeedEffluent Exchanger Expander Condenser Rectifier Column MP Stream Heat Recovery Heat Recovery Heat Recovery Heat Recovery Horizontal Magnetite Converter CO2 Unitized Chiller Feed Gas Compressor Heat Recovery Primary Reformer Secondary Reformer Process Stream Natural Gas Feed Excess Air Air Compressor Sulfur Removal Cooling Purifer Copyright 2016 American Institute of Chemical Engineers AIChE CEP September 2016 wwwaicheorgcep 73 Most of the ammonia plants recently designed by KBR utilize its Purifier process Figure 4 which combines lowseverity reforming in the primary reformer a liquid N2 wash puri fier downstream of the metha nator to remove impurities and adjust the H2N2 ratio a propri etary wasteheat boiler design a unitized chiller and a horizontal ammonia synthesis converter Depending on the configura tion of the plant energy consump tion can be as low as 28 GJmt Because the secondary reformer uses excess air the primary reformer can be smaller than in conventional designs The cryo genic purifier shown in Figure 4 in light green with a light orange background which consists of an expander condenser feedeffluent exchanger and rectifier column removes impurities such as CO CH4 and argon from the synthesis gas while adjusting the H2N2 ratio of the makeup gas in the ammonia loop to the optimum level The ammonia concentration exiting the lowpressure drop horizontal converter is 2021 which reduces energy requirements for the recycle compressor KBR also offers a lowpressure ammonia loop that employs a combination of magnetite catalyst and its proprietary ruthenium catalyst The syngas generation section or front end of a Haldor Topsøedesigned plant Figure 5 is quite traditional with the exception of its proprietary sidefired reformer which uses radiant burners to supply heat for the reforming reac tion Haldor Topsøe also offers a proprietary ironbased synthesis catalyst radialflow converters consisting of one two or three beds and a proprietary bayonettube waste heat boiler More recent developments include the S300 and S350 converter designs The S300 converter is a threebed radialflow configuration with internal heat exchangers while the S350 design combines an S300 converter with an S50 singlebed design with wasteheat recovery between converters to maximize ammonia conversion ThyssenKrupp offers a conventional plant Figure 6 with a unique secondary reformer design a proprietary wasteheat boiler radialflow converters and a dualpressure ammonia synthesis loop Today a production rate of 3300 mtday can be achieved using the TKIS dualpressure process The Linde Ammonia Concept LAC is an established technology process scheme with over 25 years of operating experience in plants with capacities from 200 mtday to over 1750 mtday The LAC process scheme Figure 7 next page replaces the costly and com plex front end of a conventional ammonia plant with two wellproven reliable process units production of ultrahighpurity hydro gen from a steammethane reformer with PSA purification production of ultrahighpurity nitrogen by a cryogenic nitrogen generation unit also known as an air separation unit ASU Ammonia Casales plant design has a produc tion rate of 2000 mtday One of the key features p Figure 6 ThyssenKrupps dualpressure synthesis loop design features a oncethrough reactor between syngas compressors p Figure 5 Haldor Topsøe offers an ammonia plant design that has a proprietary sidefired reformer in which radiant burners supply heat for the reforming reaction Ammonia Purge Gas Fluegas Process Air Natural Gas CO2 Removal Process Stream Desulfurization Reforming WaterGas Shift Ammonia Synthesis Methanation Once through Synthesis Oncethrough Reactor HighPressure Steam Purge Gas Ammonia Ammonia NH3 Synthesis Loop Separator Water Chiller Water Chiller Synthesis Gas Makeup Water Chillers 1 2 3 Recycle Stage Copyright 2016 American Institute of Chemical Engineers AIChE 74 wwwaicheorgcep September 2016 CEP Back to Basics of this design is axialradial technology in the catalyst bed Figure 8 In an axialradial catalyst bed most of the synthesis gas passes through the catalyst bed in a radial direction creating a very low pressure drop The rest of the gas passes down through a top layer of catalyst in an axial direction eliminating the need for a top cover on the catalyst bed Casales axialradial catalyst bed technology is used in both hightemperature and lowtemperature shift converters as well as in the synthesis converter Other technologies Some technology suppliers have offered gasheated reformers GHRs for the production of ammonia in smallcapacity plants or for capacity increases Unlike conventionally designed plants that use a primary reformer and secondary reformer operating in series plants with GHRs use the hot process gas from the secondary reformer to supply heat to the primary reformer This reduces the size of the primary reformer and eliminates CO2 emissions from the primary reformer stack making the process more environmentally friendly Even though some ammonia producers advocate for distributed production of ammonia in small ammonia plants most companies prefer to build large facilities near cheap raw material sources and transport the product by ship rail or pipeline to the consumers Ammonia from coal China produces more ammonia than any other country and produces the majority of its ammonia from coal Figure 9 The basic processing units in a coalbased ammonia plant are the ASU for the separation of O2 and N2 from air the p Figure 7 The Linde Ammonia Concept LAC features a pressureswing adsorption unit for highpurity hydrogen production and an air separation unit for highpurity nitrogen production p Figure 9 China produces most of its ammonia from coal u Figure 8 Ammonia Casales process employs a catalyst bed that harnesses axialradial technology which has a lower pressure drop and higher efficiency than standard catalyst beds Desulfurization Desulfurization Methanation Ammonia Synthesis Air Air Feed Feed Purge Gas Separation Primary Reformer Primary Reformer Nitrogen Unit Isothermal Shift Pressure Swing Adsorption CO2 NH3 NH3 Ammonia Synthesis Secondary Reformer High Temperature Shift Low Temperature Shift CO2 Removal Conventional Ammonia Production Process Linde Ammonia Concept LAC Production Process Coal Preparation Gasification Syngas Saturation and Sour Shift HeatRecovery Trace Metal Removal Air Separation Unit ASU Coal Air O2 Ash Boiler Feedwater Sulfur Recovery Unit Acid Gas Removal Unit N2 Wash Unit Purge Gas N2 to N2 Wash Unit Ammonia Acid Gas Tail Gas to Boiler O2 Sulfur CO2 to Urea Ammonia Synthesis Copyright 2016 American Institute of Chemical Engineers AIChE CEP September 2016 wwwaicheorgcep 75 gasifier the sour gas shift SGS unit the acid gas removal unit AGRU and the ammonia synthesis unit Oxygen from the ASU is fed to the gasifier to convert coal into synthe sis gas H2 CO CO2 and CH4 There are many gasifier designs but most modern gasifiers are based on fluidized beds that operate above atmospheric pressure and have the ability to utilize different coal feeds Depending on the design CO levels of 3060 by volume may be produced After gasification any particulate matter in the synthe sis gas is removed and steam is added to the SGS unit The SGS process typically utilizes a cobalt and molybde num CoMo catalyst specially designed for operation in a sulfur environment After reducing the CO concentration in the synthesis gas to less than 1 vol the syngas is fed to an AGRU where a chilled methanol scrubbing solution eg Rectisol removes CO2 and sulfur from the synthesis gas The CO2 overhead is either vented or fed to a urea plant The sulfur outlet stream is fed to a sulfur recover unit SRU Syngas that passes through the AGRU is typically puri fied by one of two methods a nitrogen wash unit to remove residual CO and CH4 from the syngas before it is fed to the synthesis loop a PSA system for CO and CH4 removal Closing thoughts During the past 60 years ammonia process technol ogy has improved drastically Plant layouts evolved from multitrain designs often with different numbers of trains in the front end and synthesis loop to singletrain designs Synthesis gas preparation in the front end of the plant increased from atmospheric pressure to 3050 barg pres sure Capacities increased from 100 mtday to as much as 3300 mtday in a single train Energy efficiencies have improved as well from con sumptions well above 60 GJmt of ammonia in cokebased plants to 4050 GJmt in the first naturalgasbased plants to 3040 GJmt in the first singletrain plants Modern plants have added heat recovery by steam production at pressures as high as 125 barg in both the syngas preparation section and the synthesis loop In terms of process equipment there has been a shift from reciprocating compressors to centrifugal compres sors An internal heat exchanger has been implemented in the synthesis converter to increase conversion of H2 and N2 to NH3 Designers have tapped into hydrogen recovery from purge gas in units such as PSA systems to enhance production or reduce the plant energy consumption Design ers have also implemented hot feed gas desulfurization systems There have been significant improvements in the catalysts used in reforming shift conversion methanation and ammonia synthesis To improve process control and safety distributed con trol systems DCSs for advanced process control as well as safetyinstrumented systems SISs are now standard in ammonia plants Before any process goes online hazard and operability HAZOP studies and layer of protection analy ses LOPAs are performed Advances in training simulators and education practices ensure that operators and engineers can perform their duties safely and effectively These are just a few of the thousands of improvements in technology and safety that have been implemented to make the ammonia industry one of the most productive and safe industries in the world Literature Cited 1 US Geological Survey Nitrogen Fixed Ammonia Statis tics mineralsusgsgovmineralspubshistoricalstatisticsds140 nitroxlsx Last Modified Jan 28 2016 2 Slack A V and G R James eds Ammonia Parts I II and III Marcel Dekker New York NY 1974 3 Smil V Enriching the Earth Fritz Haber Carl Bosch and the Transformation of World Food Production The MIT Press Cambridge MA Dec 2000 4 Williams G and V Pattabathula One Hundred Years of Ammonia Production A Recap of Significant Contributions to Feeding the World 58th Annual Safety in Ammonia Plants and Related Facilities Symposium AIChE Aug 2529 2013 VENKAT PATTABATHULA is the Global Ammonia Technology Manager for Incitec Pivot at its facility at Gibson Island Brisbane Australia Email venkatpattabathulaincitecpivotcomau He supports manufac turing facilities in Australia and North America His role involves highlevel technical support to all ammonia plants of Incitec Pivot and its subsidiary Dyno Nobel Americas His specialties include process design project development commissioning plant operation process safety management manufacturing excellence programs and networking with the global ammonia industry He has a masters degree MTech in chemical engineering from the Indian Institute of Technology IIT and has worked in many nitrogen fertilizer manufac turing companies in India the Middle East and North America over the past 25 years Pattabathula has been an elected member of the Ammonia Safety Committee of AIChE since 2005 and has been a member of AIChE since 1989 He is a chartered professional engineer of Engineers Australia JIM RICHARDSON has worked in RD sales and technical service in the field of catalysis related to chemical petrochemical and refining industries at SüdChemieClariant for 42 years before retiring in March 2015 He is now the owner of Catalysts and Chemicals Email jimrichardsoncatchemcom which provides consulting and startup services for syngas plants He has a BS degree in chemical engineering from Tennessee Technological Univ He was a member of the AIChE Ammonia Safety Committee from 1990 to 2015 and in that role he gained extensive knowledge in the operation of H2 NH3 and methanol plants Acknowledgments The authors acknowledge the assistance of KBR ThyssenKrupp Industrial Solutions Haldor Topsøe Linde and Casale for providing technical literature on their respective process technologies CEP Copyright 2016 American Institute of Chemical Engineers AIChE
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CEP September 2016 wwwaicheorgcep 69 Back to Basics M ost people associate the pungent smell of ammo nia NH3 with cleaners or smelling salts How ever the use of ammonia in these two products represents only a small fraction of the total global ammonia production which was around 176 million metric tons in 2014 1 To appreciate where the industry and technology are today lets first take a look at how we got here Ammonia has been known for more than 200 years Joseph Priestley an English chemist first isolated gaseous ammonia in 1774 Its composition was ascertained by French chemist Claude Louis Berthollet in 1785 In 1898 Adolph Frank and Nikodem Caro found that N2 could be fixed by calcium carbide to form calcium cyanamide which could then be hydrolyzed with water to form ammonia 2 CaO 3C CaC2 CO CaC2 N2 CaCN2 C CaCN2 3H2O CaCO3 2NH3 The production of significant quantities of ammonia using the cyanamide process did not occur until the early 20th century Because this process required large amounts of energy scientists focused their efforts on reducing energy requirements German chemist Fritz Haber performed some of the most important work in the development of the modern ammonia industry Working with a student at the Univ of Karlsruhe he synthesized ammonia in the laboratory from N2 and H2 Meanwhile Walther Nernst a professor of physical chemistry at the Univ of Berlin developed a process to make ammonia by passing a mixture of N2 and H2 across an iron catalyst at 1000C and 75 barg pressure He was able to produce larger quantities of ammonia at this pressure than earlier experiments by Haber and others at atmospheric pressure However Nernst concluded that the process was not feasible because it was difficult or almost impossible at that time to produce large equipment capable of operat ing at that pressure Nonetheless both Haber and Nernst pursued the high pressure route to produce ammonia over a catalyst Haber finally developed a process for producing commercial quan tities of ammonia and in 1906 he was able to achieve 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 realized that the amount of ammonia formed in a single pass through a converter was far too low to be of com mercial interest To produce more ammonia from the makeup gas he proposed a recycle system and received a patent for the concept Habers recycle idea changed the perception of process engineering as static in favor of a more dynamic approach In addition to the chemical reaction equilibrium Haber recognized that reaction rate was a determining factor Instead of simple yield in a oncethrough process he concen trated on spacetime yield in a system with recycle BASF purchased Habers patents and started develop Ammonia is critical in the manufacturing of fertilizers and is one of the largestvolume synthetic chemicals produced in the world This article explores the evolution of ammonia production and describes the current manufacturing technologies Venkat Pattabathula Incitec Pivot Ltd Jim Richardson Catalysts and Chemicals LLC Introduction to Ammonia Production Copyright 2016 American Institute of Chemical Engineers AIChE 70 wwwaicheorgcep September 2016 CEP Back to Basics ment of a commercial process After testing more than 2500 different catalysts Carl Bosch Alvin Mittasch and other BASF chemists developed a promoted iron catalyst for the production of ammonia in 1910 Developing equipment that could withstand the necessary high temperatures and pressure was an even more difficult task An early mild steel reactor lasted only 80 hours before failure due to decarbonization Lining mild steel reactors with soft iron which was not vul nerable 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 included designing a heat exchanger to bring the inlet gas to reaction temperatures and cool the exit gas and devising a method to bring the catalyst to reaction temperature The first commercial ammonia plant based on the Haber Bosch process was built by BASF at Oppau Germany The plant went onstream on Sept 9 1913 with a production capacity of 30 mtday Figure 1 is a flowsheet of the first commercial ammonia plant The reactor contained an internal heat exchanger in addition to those shown on the schematic Global production rates Ammonia production has become one of the most important industries in the world Without the crop yield made possible by ammoniabased fertilizers and chemi cals the global population would be at least two to three billion less than it is today 3 Ammonia production has increased steadily since 1946 Figure 2 and it is estimated that the annual production of ammonia is worth more than 100 billion with some plants producing more than 3000 mtday of NH3 In 1983 on the occasion of the 75th anniversary of AIChEs founding a blue ribbon panel of distinguished chemical engineers named what they believed to be the worlds ten greatest chemical engineering achievements 4 Embracing such feats as wonder drugs synthetic fibers and atomic energy the citation also included the breakthrough that permitted the production of large quantities of ammonia in compact singleunit plants Within the past decades chemical engineers have suc ceeded in creating processes that make vast amounts of ammonia at relatively low costs As recently as 80 years ago the total annual production of synthesized ammonia was just over 300000 mt Thanks to chemical engineering break throughs one modern ammonia plant can produce more than 750000 mtyr Approximately 88 of ammonia made annually is con sumed in the manufacturing of fertilizer Most of the remain der goes into the production of formaldehyde China pro duced about 326 of the global production in 2014 while Russia India and the US produced 81 76 and 64 respectively 1 While most of the global production of ammonia is based on steam reforming of natural gas signifi cant quantities are produced by coal gasification most of the gasification plants are located in China Modern production processes The tremendous increase in ammonia demand from 1950 to 1980 necessitated larger moreenergyefficient plants Those decades also saw a change in design philosophy Until that time an ammonia plant was regarded as an assembly of unrelated units such as gas preparation gas purification gas compression and ammonia synthesis New innovations and an integral design tied process units together in the most effective and efficient ways In the mid1960s the American Oil Co installed a singleconverter ammonia plant engineered by MW Kellogg MWK at Texas City TX with a capacity of 544 mtday The singletrain design concept Figure 3 was so revolution ary that it received the Kirkpatrick Chemical Engineering Achievement Award in 1967 The plant used a fourcase centrifugal compressor to compress the syngas to a pressure of 152 bar and final p Figure 1 This is a simplified flowsheet of the first commercial ammonia plant by BASF p Figure 2 Worldwide ammonia production has steadily increased from 1946 to 2014 H2N2 Compressor Filter Reactor Heat Exchangers Condenser Ammonia Separator Circulator Absorber 22 NH3 aq Water Water Gas Purge 1946 0 20 40 60 80 100 120 140 160 Annual Ammonia Production Million mt 180 200 1954 1962 1970 1978 1986 1994 2002 2010 2014 Copyright 2016 American Institute of Chemical Engineers AIChE CEP September 2016 wwwaicheorgcep 71 compression to an operating pressure of 324 bar occurred in a reciprocating compressor Centrifugal compressors for the synthesis loop and refrigeration services were also imple mented which provided significant cost savings The key differences between the MWK process and the processes used in previous ammonia plants included using a centrifugal compressor as part of the synthesis gas compression maximizing the recovery of waste heat from the process generating steam from the waste heat for use in steam turbine drivers using the refrigeration compressor for rundown and atmospheric refrigeration An integrated scheme that balanced energy consumption energy production equipment size and catalyst volumes was incorporated throughout the plant Most plants built between 1963 and 1993 had large singletrain designs with synthesis gas production at 2535 bar and ammonia synthesis at 150200 bar Another variation by Braun now KBR offered slight modifica tions to the basic design The Braun Purifier process plants utilized a primary or tubular reformer with a low outlet temperature and high methane leakage to reduce the size and cost of the reformer Excess air was added to the second ary reformer to reduce the methane content of the primary reformer exit stream to 12 Excess nitrogen and other impurities were removed downstream of the methanator Because the synthesis gas was essentially free of impurities two axialflow ammonia converters were used to achieve a high ammonia conversion Some recently built plants have a synthesis gas genera tion system with only one reformer no secondary reformer a pressureswing adsorption PSA system for H2 recovery and an air separation plant as the source of N2 Improve ments in converter design such as radial and horizontal catalyst beds internal heat exchangers and synthesis gas treatment helped increase ammonia concentrations exiting the synthesis converter from about 12 to 1921 A higher conversion per pass along with moreefficient turbines and compressors further reduced energy consumption More efficient CO2 removal solutions such as potassium carbon ate and methyldiethanolamine MDEA have contributed to improved energy efficiency Most modern plants can p Figure 3 KBR designed one of the first singletrain largecapacity ammonia plants Fuel Natural Gas Boiler Feedwater BFW Steam Steam Drum Steam BFW Heat Recovery CO Converter BFW Steam Water First Stage HighTemperature Synthesis Second Stage LowTemperature Synthesis Condensate Gas to Purification Cooler Compressor Heat Recovery Section Primary Reformer Secondary Reformer Power Air Air Raw Synthesis Gas Rich Solution Lean Solution Heat Water CO2 CO2 Absorber CO2 Stripper NH3 Converter Water Water Cooler Compression Heat Exchanger Purge Flash Gas Ammonia NH3 Cooled Condenser Cooler Heat Methanator Separator Copyright 2016 American Institute of Chemical Engineers AIChE 72 wwwaicheorgcep September 2016 CEP Back to Basics produce ammonia with an energy consumption of 28 GJmt In addition to the design mechanical and metallurgical improvements made during this time the operating pressure of the synthesis loop was significantly reduced When the first singletrain plant was built in the 1960s it contained a highpressure synthesis loop In 1962 MWK received an inquiry from Imperial Chemical Industries ICI for a proposal to build a 544mtday plant at their Severnside site MWK proposed a 152bar synthesis loop instead of a 324bar loop Because 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 contacted Haldor Topsøe to support their plans Topsøe had data covering the entire pressure range of interest to MWK In addition they had a computer program for calculating the quantity of catalyst that was required at the lower operat ing pressure Even though ICI chose Bechtel to design the plant MWK was able to develop a flowsheet for a 544mtday design with centrifugal compressors and a lowpressure synthesis loop which some people consider the single most important event in the development of the singletrain ammonia plant Approximately twice as much catalyst was required at 152 bar as at 324 bar an increase that seemed econom ically feasible Although the converter would need twice the volume the lower operating pressure would reduce the required thickness of the pressure shell As a result the weight of metal required for the converter plus the catalyst remained about the same The lowerpressure synthesis loop also allowed the use of centrifugal compressors instead of reciprocating compressors Another improvement was recovering heat to generate highpressure steam for steam turbine drives Plant designs in the 21st century During the first few years of the 21st century many improvements were made in ammonia plant technology that allow existing plants to increase production rates and new plants to be built with larger and larger capacities Compe tition between technology suppliers is quite fierce Three technology licensors KBR Kellogg Brown and Root Haldor Topsøe and ThyssenKrupp Industrial Solutions TKIS currently dominate the market Ammonia Casale which offers an axialradial catalyst bed design is a market leader in revamps of existing plants p Figure 4 Modern ammonia plants designed by KBR employ its proprietary Purifier design Methanator CO2 Absorber Synthesis Gas Compressor Dryer CO2 Stripper To BFW System To Process Stream Condensate Stripper HTS LTS Refrigeration Compressor Ammonia Waste Gas to Fuel FeedEffluent Exchanger Expander Condenser Rectifier Column MP Stream Heat Recovery Heat Recovery Heat Recovery Heat Recovery Horizontal Magnetite Converter CO2 Unitized Chiller Feed Gas Compressor Heat Recovery Primary Reformer Secondary Reformer Process Stream Natural Gas Feed Excess Air Air Compressor Sulfur Removal Cooling Purifer Copyright 2016 American Institute of Chemical Engineers AIChE CEP September 2016 wwwaicheorgcep 73 Most of the ammonia plants recently designed by KBR utilize its Purifier process Figure 4 which combines lowseverity reforming in the primary reformer a liquid N2 wash puri fier downstream of the metha nator to remove impurities and adjust the H2N2 ratio a propri etary wasteheat boiler design a unitized chiller and a horizontal ammonia synthesis converter Depending on the configura tion of the plant energy consump tion can be as low as 28 GJmt Because the secondary reformer uses excess air the primary reformer can be smaller than in conventional designs The cryo genic purifier shown in Figure 4 in light green with a light orange background which consists of an expander condenser feedeffluent exchanger and rectifier column removes impurities such as CO CH4 and argon from the synthesis gas while adjusting the H2N2 ratio of the makeup gas in the ammonia loop to the optimum level The ammonia concentration exiting the lowpressure drop horizontal converter is 2021 which reduces energy requirements for the recycle compressor KBR also offers a lowpressure ammonia loop that employs a combination of magnetite catalyst and its proprietary ruthenium catalyst The syngas generation section or front end of a Haldor Topsøedesigned plant Figure 5 is quite traditional with the exception of its proprietary sidefired reformer which uses radiant burners to supply heat for the reforming reac tion Haldor Topsøe also offers a proprietary ironbased synthesis catalyst radialflow converters consisting of one two or three beds and a proprietary bayonettube waste heat boiler More recent developments include the S300 and S350 converter designs The S300 converter is a threebed radialflow configuration with internal heat exchangers while the S350 design combines an S300 converter with an S50 singlebed design with wasteheat recovery between converters to maximize ammonia conversion ThyssenKrupp offers a conventional plant Figure 6 with a unique secondary reformer design a proprietary wasteheat boiler radialflow converters and a dualpressure ammonia synthesis loop Today a production rate of 3300 mtday can be achieved using the TKIS dualpressure process The Linde Ammonia Concept LAC is an established technology process scheme with over 25 years of operating experience in plants with capacities from 200 mtday to over 1750 mtday The LAC process scheme Figure 7 next page replaces the costly and com plex front end of a conventional ammonia plant with two wellproven reliable process units production of ultrahighpurity hydro gen from a steammethane reformer with PSA purification production of ultrahighpurity nitrogen by a cryogenic nitrogen generation unit also known as an air separation unit ASU Ammonia Casales plant design has a produc tion rate of 2000 mtday One of the key features p Figure 6 ThyssenKrupps dualpressure synthesis loop design features a oncethrough reactor between syngas compressors p Figure 5 Haldor Topsøe offers an ammonia plant design that has a proprietary sidefired reformer in which radiant burners supply heat for the reforming reaction Ammonia Purge Gas Fluegas Process Air Natural Gas CO2 Removal Process Stream Desulfurization Reforming WaterGas Shift Ammonia Synthesis Methanation Once through Synthesis Oncethrough Reactor HighPressure Steam Purge Gas Ammonia Ammonia NH3 Synthesis Loop Separator Water Chiller Water Chiller Synthesis Gas Makeup Water Chillers 1 2 3 Recycle Stage Copyright 2016 American Institute of Chemical Engineers AIChE 74 wwwaicheorgcep September 2016 CEP Back to Basics of this design is axialradial technology in the catalyst bed Figure 8 In an axialradial catalyst bed most of the synthesis gas passes through the catalyst bed in a radial direction creating a very low pressure drop The rest of the gas passes down through a top layer of catalyst in an axial direction eliminating the need for a top cover on the catalyst bed Casales axialradial catalyst bed technology is used in both hightemperature and lowtemperature shift converters as well as in the synthesis converter Other technologies Some technology suppliers have offered gasheated reformers GHRs for the production of ammonia in smallcapacity plants or for capacity increases Unlike conventionally designed plants that use a primary reformer and secondary reformer operating in series plants with GHRs use the hot process gas from the secondary reformer to supply heat to the primary reformer This reduces the size of the primary reformer and eliminates CO2 emissions from the primary reformer stack making the process more environmentally friendly Even though some ammonia producers advocate for distributed production of ammonia in small ammonia plants most companies prefer to build large facilities near cheap raw material sources and transport the product by ship rail or pipeline to the consumers Ammonia from coal China produces more ammonia than any other country and produces the majority of its ammonia from coal Figure 9 The basic processing units in a coalbased ammonia plant are the ASU for the separation of O2 and N2 from air the p Figure 7 The Linde Ammonia Concept LAC features a pressureswing adsorption unit for highpurity hydrogen production and an air separation unit for highpurity nitrogen production p Figure 9 China produces most of its ammonia from coal u Figure 8 Ammonia Casales process employs a catalyst bed that harnesses axialradial technology which has a lower pressure drop and higher efficiency than standard catalyst beds Desulfurization Desulfurization Methanation Ammonia Synthesis Air Air Feed Feed Purge Gas Separation Primary Reformer Primary Reformer Nitrogen Unit Isothermal Shift Pressure Swing Adsorption CO2 NH3 NH3 Ammonia Synthesis Secondary Reformer High Temperature Shift Low Temperature Shift CO2 Removal Conventional Ammonia Production Process Linde Ammonia Concept LAC Production Process Coal Preparation Gasification Syngas Saturation and Sour Shift HeatRecovery Trace Metal Removal Air Separation Unit ASU Coal Air O2 Ash Boiler Feedwater Sulfur Recovery Unit Acid Gas Removal Unit N2 Wash Unit Purge Gas N2 to N2 Wash Unit Ammonia Acid Gas Tail Gas to Boiler O2 Sulfur CO2 to Urea Ammonia Synthesis Copyright 2016 American Institute of Chemical Engineers AIChE CEP September 2016 wwwaicheorgcep 75 gasifier the sour gas shift SGS unit the acid gas removal unit AGRU and the ammonia synthesis unit Oxygen from the ASU is fed to the gasifier to convert coal into synthe sis gas H2 CO CO2 and CH4 There are many gasifier designs but most modern gasifiers are based on fluidized beds that operate above atmospheric pressure and have the ability to utilize different coal feeds Depending on the design CO levels of 3060 by volume may be produced After gasification any particulate matter in the synthe sis gas is removed and steam is added to the SGS unit The SGS process typically utilizes a cobalt and molybde num CoMo catalyst specially designed for operation in a sulfur environment After reducing the CO concentration in the synthesis gas to less than 1 vol the syngas is fed to an AGRU where a chilled methanol scrubbing solution eg Rectisol removes CO2 and sulfur from the synthesis gas The CO2 overhead is either vented or fed to a urea plant The sulfur outlet stream is fed to a sulfur recover unit SRU Syngas that passes through the AGRU is typically puri fied by one of two methods a nitrogen wash unit to remove residual CO and CH4 from the syngas before it is fed to the synthesis loop a PSA system for CO and CH4 removal Closing thoughts During the past 60 years ammonia process technol ogy has improved drastically Plant layouts evolved from multitrain designs often with different numbers of trains in the front end and synthesis loop to singletrain designs Synthesis gas preparation in the front end of the plant increased from atmospheric pressure to 3050 barg pres sure Capacities increased from 100 mtday to as much as 3300 mtday in a single train Energy efficiencies have improved as well from con sumptions well above 60 GJmt of ammonia in cokebased plants to 4050 GJmt in the first naturalgasbased plants to 3040 GJmt in the first singletrain plants Modern plants have added heat recovery by steam production at pressures as high as 125 barg in both the syngas preparation section and the synthesis loop In terms of process equipment there has been a shift from reciprocating compressors to centrifugal compres sors An internal heat exchanger has been implemented in the synthesis converter to increase conversion of H2 and N2 to NH3 Designers have tapped into hydrogen recovery from purge gas in units such as PSA systems to enhance production or reduce the plant energy consumption Design ers have also implemented hot feed gas desulfurization systems There have been significant improvements in the catalysts used in reforming shift conversion methanation and ammonia synthesis To improve process control and safety distributed con trol systems DCSs for advanced process control as well as safetyinstrumented systems SISs are now standard in ammonia plants Before any process goes online hazard and operability HAZOP studies and layer of protection analy ses LOPAs are performed Advances in training simulators and education practices ensure that operators and engineers can perform their duties safely and effectively These are just a few of the thousands of improvements in technology and safety that have been implemented to make the ammonia industry one of the most productive and safe industries in the world Literature Cited 1 US Geological Survey Nitrogen Fixed Ammonia Statis tics mineralsusgsgovmineralspubshistoricalstatisticsds140 nitroxlsx Last Modified Jan 28 2016 2 Slack A V and G R James eds Ammonia Parts I II and III Marcel Dekker New York NY 1974 3 Smil V Enriching the Earth Fritz Haber Carl Bosch and the Transformation of World Food Production The MIT Press Cambridge MA Dec 2000 4 Williams G and V Pattabathula One Hundred Years of Ammonia Production A Recap of Significant Contributions to Feeding the World 58th Annual Safety in Ammonia Plants and Related Facilities Symposium AIChE Aug 2529 2013 VENKAT PATTABATHULA is the Global Ammonia Technology Manager for Incitec Pivot at its facility at Gibson Island Brisbane Australia Email venkatpattabathulaincitecpivotcomau He supports manufac turing facilities in Australia and North America His role involves highlevel technical support to all ammonia plants of Incitec Pivot and its subsidiary Dyno Nobel Americas His specialties include process design project development commissioning plant operation process safety management manufacturing excellence programs and networking with the global ammonia industry He has a masters degree MTech in chemical engineering from the Indian Institute of Technology IIT and has worked in many nitrogen fertilizer manufac turing companies in India the Middle East and North America over the past 25 years Pattabathula has been an elected member of the Ammonia Safety Committee of AIChE since 2005 and has been a member of AIChE since 1989 He is a chartered professional engineer of Engineers Australia JIM RICHARDSON has worked in RD sales and technical service in the field of catalysis related to chemical petrochemical and refining industries at SüdChemieClariant for 42 years before retiring in March 2015 He is now the owner of Catalysts and Chemicals Email jimrichardsoncatchemcom which provides consulting and startup services for syngas plants He has a BS degree in chemical engineering from Tennessee Technological Univ He was a member of the AIChE Ammonia Safety Committee from 1990 to 2015 and in that role he gained extensive knowledge in the operation of H2 NH3 and methanol plants Acknowledgments The authors acknowledge the assistance of KBR ThyssenKrupp Industrial Solutions Haldor Topsøe Linde and Casale for providing technical literature on their respective process technologies CEP Copyright 2016 American Institute of Chemical Engineers AIChE