The lower-pressure synthesis loop also allowed the use of centrifugal compressors instead of reciprocating compressors. Another improvement was recovering heat to generate high-pressure steam for steam turbine drives.
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. Competition between technology suppliers is quite fierce. Ammonia Casale, which offers an axial-radial catalyst bed design, is a market leader in revamps of existing plants. Modern ammonia plants designed by KBR employ its proprietary Purifier design.
Most of the ammonia plants recently designed by KBR utilize its Purifier process Figure 4 , which combines low-severity reforming in the primary reformer, a liquid N 2 wash purifier downstream of the methanator to remove impurities and adjust the H 2 :N 2 ratio, a proprietary waste-heat boiler design, a unitized chiller, and a horizontal ammonia synthesis converter.
Because the secondary reformer uses excess air, the primary reformer can be smaller than in conventional designs. KBR also offers a low-pressure ammonia loop that employs a combination of magnetite catalyst and its proprietary ruthenium catalyst. More recent developments include the S and S converter designs. The S converter is a three-bed radial-flow configuration with internal heat exchangers, while the S design combines an S converter with an S single-bed design with waste-heat recovery between converters to maximize ammonia conversion.
ThyssenKrupp offers a conventional plant Figure 6 with a unique secondary reformer design, a proprietary waste-heat boiler, radial-flow converters, and a dual-pressure ammonia synthesis loop. Today, a production rate of 3, m. The Linde Ammonia Concept LAC features a pressure-swing adsorption unit for high-purity hydrogen production and an air separation unit for high-purity nitrogen production. The Linde Ammonia Concept LAC is an established technology process scheme with over 25 years of operating experience in plants with capacities from m.
The LAC process scheme Figure 7 replaces the costly and complex front end of a conventional ammonia plant with two well-proven, reliable process units:. One of the key features of this design is axial-radial technology in the catalyst bed Figure 8.
In an axial-radial 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.
Some technology suppliers have offered gas-heated reformers GHRs for the production of ammonia in small-capacity 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 CO 2 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. China produces most of its 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 coal-based ammonia plant are the ASU for the separation of O 2 and N 2 from air, the gasifier, the sour gas shift SGS unit, the acid gas removal unit AGRU , and the ammonia synthesis unit. 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. After gasification, any particulate matter in the synthesis gas is removed and steam is added to the SGS unit.
The SGS process typically utilizes a cobalt and molybdenum CoMo catalyst specially designed for operation in a sulfur environment. The CO 2 overhead is either vented or fed to a urea plant. The sulfur outlet stream is fed to a sulfur recover unit SRU. During the past 60 years, ammonia process technology has improved drastically. Plant layouts evolved from multi-train designs, often with different numbers of trains in the front end and synthesis loop, to single-train designs. Focusing on the Haber—Bosch process, many efforts to reduce its extreme conditions have been carried out.
They include the introduction of an extra component in order to inhibit the catalysis and the alteration of geometry and electronic nature of the reacting components in order to optimize the energetics of catalysis [ 13 ]. However, Ru-based catalysts are expensive and suffer from hydrogen poisoning [ 14 ] [ 15 ]. Alkaline earth metal oxides and hydroxides have been identified as promoters to improve the catalytic performance of Ru-based catalysts [ 16 ].
Transition metals TM can also improve synthesis performance, including lowering the pressure and temperature. This is due to the existence of scaling relations between the transition-state energy required for the dissociation of nitrogen and the adsorption energy of all the intermediates [ 18 ] [ 19 ]. Furthermore, Kawamura and Taniguchi [ 20 ] have tested sodium melt as a catalyst for ammonia synthesis.
However, further analysis and experimentation are required to bring this method to the level of being applicable. Although electrochemical processing is significantly under-developed compared to the Haber—Bosch process, it is expected to realize higher energy performance. Figure 2 shows the schematic flow diagram of electrochemical ammonia synthesis. The process is considered simple; therefore, its application is considered to potentially reduce system configuration and control complexity.
In addition, the investment cost can be lower compared to currently adopted ammonia synthesis systems. The reactions at both cathode and anode in proton conducting cells are shown in reactions 2 and 3 , respectively. The reactions at each cathode and anode are basically reversible. Four different types of electrolytes are currently available: a liquid electrolytes, b molten salt, c composite membranes and d solid state electrolytes.
Liquid electrolytes can operate under atmospheric temperature and pressure [ 22 ]. There are some potential liquid electrolytes, including LiClO 4 0. Ammonia production of 3. However, the research related to this issue is still limited to lab experiments, in small dimensions of cells and limited operation times [ 2 ]. The reported ammonia production rate is 3. The electrolytes comprise the main ionic conducting phase and an additional phase that is attached to the main phase to improve the electrical, mechanical and thermal properties [ 26 ].
As the representative of composite electrolytes, alkali metal carbonate such as LiCO 3 and oxide such as LiAlO 2 and CeO 2 doped with Sm 2 O 3 have shown the expected properties, including oxygen ion, carbonate ion and proton conductivity [ 27 ].
In addition, Amar et al. They obtained an ammonia production rate of 2. There are different materials which can be included in this type of electrolyte.
These include perovskites such as cerate and zirconate [ 28 ] , fluorites such as doped zirconia, ceria and thoria , pyrochlores such as calcium doped lanthanum zirconate and other materials including brownmillerite, eulytite and monazite [ 26 ].
By adopting this kind of solid state electrolyte, the ammonia production rate of 3. As an alternative process for ammonia production, a process employing the thermochemical cycle has been developed [ 30 ].
The system consists of two circulated processes: reduction nitrogen activation and steam-hydrolysis ammonia formation. Both reactions are summarized as follows:. Figure 3 shows the schematic diagram of the thermochemical cycle of ammonia production. The primary energy sources are pre-treated and converted to carbon before being fed to the thermochemical cycle process. In the first reduction process reaction 4 , the AlN is produced through the carbothermal reduction of Al 2 O 3 and nitrogen.
Moreover, in the second reaction, which is steam-hydrolysis reaction 5 , the AlN produced in the first reduction process is reacted with steam H 2 O producing Al 2 O 3. The produced Al 2 O 3 from this second reaction is then circulated to the first reduction process. Detailed reaction kinetics have been analyzed in detail in [ 31 ]. Unlike the Haber—Bosch process, this thermochemical cycle can be carried out at atmospheric pressure and without a catalyst. The process allows independent reaction control for nitrogen activation reaction 4 and ammonia formation reaction 5.
Furthermore, as could be observed from reaction 4 , the system can produce ammonia directly from carbonized material, instead of pure hydrogen. Therefore, this system is expected to be able to reduce the energy consumption during ammonia production. However, this system has the biggest challenge related to its very high operating temperature, leading to limited heat sources and materials.
Various ideas have been suggested for the heat supply, including the utilization of concentrated solar heat. Juangsa and Aziz [ 32 ] have developed an integrated system, consisting of nitrogen production, ammonia production employing the thermochemical cycle and power generation. In their system, the heat required for reduction is basically covered by heat generated by the combustion of fuel gases produced during ammonia production. To speed it up, chemists raise the temperature. So chemists raise the pressure to bring the yields back up.
One of the other possible advantages of the electrochemical approach is that the reaction system can be small. Meanwhile, other researchers are looking to nature to understand how to efficiently reduce nitrogen to ammonia. Some bacteria use large protein complexes called nitrogenases to grab nitrogen out of the air and make ammonia. Minteer and her team have been studying this system to connect these bacterial enzymes to electrodes to create new electrocatalysts.
But they still have a long way to go, Minteer says. Their systems do more proton reduction than ammonia production. Scientists throughout the field face this problem with catalyst yield and selectivity. As a result, the ammonia coming out of these non-Haber-Bosch systems is a trickle, not a torrent. Once the bond breaks, the catalyst needs to form the three nitrogen-hydrogen bonds, all at ambient conditions without high temperatures to accelerate the kinetics.
Scientists have been intensely studying hydrogen-evolution catalysts for about the past 20 years. Greenlee points out that the solutions have to go beyond catalyst design. Scientists need to figure out how to control, reduce, or eliminate the hydrogen-evolution reaction. For a new ammonia production system to be practical, such as in an electrochemical device like the one his group is working on, catalysts will need to remain active and viable for years, even if the system could be taken apart and refurbished, he says.
Related: Tackling sustainable fertilizer production with an alternative electrolyte. The road to Haber-Bosch-free ammonia is long, Minteer says. Searching for alternatives to Haber-Bosch is also risky, Manthiram says, because what scientists are pursuing now may not pan out.
But with ammonia production touching so many things that we use every day, including our food and pharmaceuticals, scientists need to find a way to make these lab-scale systems work on larger scales, he says. Errors from the air: The trials and tribulations of developing ammonia catalysts. When Shelley Minteer at the University of Utah first got started studying how bacterial enzymes called nitrogenases produce ammonia, she noticed something funny.
The culprit? The cleaning lady. Nitrogen and ammonia are all around us. These molecules can stick to tubing, gloves, and glassware. Contaminants also include other nitrogen-containing compounds, such as nitrites and nitrates, which can easily react to make ammonia.
The field of new ammonia-producing catalysts is still young, says Lauren Greenlee, a chemical engineer at the University of Arkansas. If part of that ammonia is coming from the background, scientists might think that their catalyst is working well when it may not be.
Whether the journals should require those controls is a matter of debate in the community. Maybe, these members of the field argue, the catalyst community will move forward, and catalysts will get more efficient so that the difference between what the catalyst is producing and the amount of ambient ammonia will become larger.
While that may happen, that wait-and-see approach has issues, Greenlee says. Greenlee thinks that researchers should run controls and take background measurements for every catalyst on every day they run experiments.
Such controls would include running experiments with isotopically labeled molecules as a final evaluation of successful catalysts so scientists know where the nitrogen in ammonia came from. Papers should also report the results from these control experiments. Contact us to opt out anytime. Contact the reporter. Submit a Letter to the Editor for publication. In this particular instance, it will increase their chances of hitting and sticking to the surface of the catalyst where they can react.
The higher the pressure the better in terms of the rate of a gas reaction. You have to build extremely strong pipes and containment vessels to withstand the very high pressure. That increases your capital costs when the plant is built. High pressures cost a lot to produce and maintain.
That means that the running costs of your plant are very high. If the pressure used is too high, the cost of generating it exceeds the price you can get for the extra ammonia produced. The catalyst has no effect whatsoever on the position of the equilibrium.
Adding a catalyst doesn't produce any greater percentage of ammonia in the equilibrium mixture. Its only function is to speed up the reaction. In the absence of a catalyst the reaction is so slow that virtually no reaction happens in any sensible time. The catalyst ensures that the reaction is fast enough for a dynamic equilibrium to be set up within the very short time that the gases are actually in the reactor. When the gases leave the reactor they are hot and at a very high pressure.
Ammonia is easily liquefied under pressure as long as it isn't too hot, and so the temperature of the mixture is lowered enough for the ammonia to turn to a liquid. The nitrogen and hydrogen remain as gases even under these high pressures, and can be recycled.
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