Process intensified ammonia plants based on advanced compressor technology for distributed production

Christophe Pochari, Pochari Technologies, Bodega Bay, California, USA.

Small scale ammonia synthesis: An extensive techno-economic assessment.

The ammonia industry is in need of disruption. Highly consolidated industries such as chemical production are notably resistant to change and unwilling to alter their business models. Consumers often suffer as the major players will price-gouge the buyers with little bargaining power. Ammonia is a prime example, there is no substitute for anhydrous ammonia, thus farmers are at the mercy of large producers generating high returns. Large producers have monopolized the market and charge far above what the price of natural gas would predict. For example, as of November 2021, the price of anhydrous is over $1200/ton, reaching as much as $1350 per ton in Illinois. While we expect this price to stabilize to around $800 per ton, these elevated prices make electrolysis based production more competitive than ever. While the price of natural gas is only $5/thousand-cubic feet, or $0.26/kg. The price of producing hydrogen from natural gas is only $1-1.5/kg, so the price of ammonia should only be $213/ton, with a reasonable estimate for plant CAPEX of $1500/TPY amortized over 15 years yielding less than $100/ton. For example, Kapsom industrial solutions in Nanjing built a 600,000 TPY plant for $650,000,000, this includes hydrogen gas production and purification, which is a CAPEX of $1083/TPY, amortized over 20 years equates to $54/ton, including 5% annual maintenance, adds another $54/ton, but this number is too conservative, ammonia plants require very little maintenance, the annualized plan CAPEX per ton is well below $100, as much of the maintenance cost is related to the steam methane reforming systems. A fair estimate is $75/ton CAPEX, less than 6% the cost of the current anhydrous spot price. Thanks to the use of process intensification, commercial off the shelf equipment procurement, minimal customization, improved use of modularity, off-site manufacturing enabled by containerization, and improvements in compressor technology, Pochari Technologies CAPEX for our miniature ammonia plant is $50,000 for a 100 tons per year system excluding solar farm and electrolyzer. Including a solar farm at $300/kW and our COTS electrolyzers at $50/kW, our total CAPEX is $1500/TPY. This means after the solar farm needs replacement (20 years, similar to the plant systems), the levelized cost of ammonia is $75 for CAPEX and $117 for OPEX, mainly hydrogen production electrical consumption. This allows ammonia to be produced anywhere in the world where solar energy and water is present for under $300 per ton. If natural prices remain over $5/mbtu, solar-based production will completely dominate and displace stream methane reforming as the method of choice. With production from inexhaustible water and air, distributed production becomes feasible, overturning the current business model away from centralized production.

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Average ammonia prices for the month of November 2021 in North America. This statistic from the USDA illustrates the high cost of ammonia relative to natural gas. This incongruity can be explained by Porter’s 5 forces, producers of a value-added chemical have more bargaining power than their constituent customers since the demand elasticity is very low. Note that the 2020-2021 North American lumber price crisis illustrates this exact phenomenon. For high barrier to entry production sectors, producers retain nearly all bargaining power. Only distributed production can break this monopoly. Another important factor to note is that most commodities have a fixed production/extraction cost in addition to the prevailing market value. Value and cost can diverge significantly. For example, in 2008, polysilicon sold for $400/kg, while in 2020, it was less than $20/kg, a factor of 20x difference without any underlying change in the technological or material basis of the process. To summarize, just because a synthetic chemical sells for X price does not in any indicate it cost X price to produce or manufacture, many other variables are at play, and often, market failures are to blame. Technology can often help rectify these market failures caused by a lack of competition.

With the availability of low-cost photovoltaic energy without the need for balancing grid demand fluctuation, the need for storage is negated, this opens up the possibility of producing ultra-low cost hydrogen even below the price of methane reforming. Until ammonia begins its use as a low-emission fuel in the near future, the fertilizer market is the biggest opportunity for disruption. The size of the ammonia industry is nearly $100 billion annually, with production volumes as high as 200 million tons, nearly 20% of the total production of crude oil for comparison. Rather than purchasing overpriced ammonia from major producers who charge high margins, farmers can produce it themselves at cost, savings considerable sums of money and paying for the capital expenditure of the plant in a short time provided they have access to low-cost electricity. Low-cost photovoltaics have completely transformed the energy dynamic. Ready to install PV-farm kits including invertors, panel frames, wiring, and all ancillary components sell for as little as 30 cents per watt on commercial marketplaces such as Alibaba. See Hefei Yangtze Solar Power Co., Ltd‘s 1-megawatt solar farm for $300,000. Assuming this module was installed in a geography featuring high annual irradiance, for example, 1700-1800 kWh/kWp, once could expect nearly 1,800,000 kWh per year amortized over 15 years minus 0.8%/anum degradation yielding a price per kWh-DC of less than 1.2 cents, congruous with findings of major solar projects around the world under 2 cents. Most hydrogen experts agree that below or at this price, green hydrogen becomes very attractive provided electrolyzer CAPEX can be further reduced, *see Pochari Technologies’ COTS electrolyzer technology with CAPEX below $50/kW. Assuming a price of 1.5 cents and electrolyzer power consumption of 4 kWh/m3, the cost to produce 1 kg of hydrogen is only 72 cents, or $128 per ton of ammonia. Note that photovolatic prices along with electrolyzer prices will continue to fall since their raw material feedstock is not in short supply, polysilicon is still trading far above the theoretical production costs, and alkaline water electrolyzers are 100x more expensive than bare material costs due to low production volumes and uncompetitive markets.

Aside from Pochari Technologies, only one other expert has realized the potential of small-scale ammonia production using advanced technology. Richard Strait of Rice University has given a talk on his concept for the miniaturization of ammonia production.

“The small self-contained ammonia manufacturing box (AMB) came out of the need for farmers in remote locations containing modest amounts of natural gas and water to produce enough fertilizer for their local needs. Presently ammonia plants are getting larger and larger. The future however, is to make small highly automated plants that are portable, use readily available inputs and produce the products where they are needed. To this end the new small-scale plant will include all the latest leading edge technologies that apply such as Micro Processing Technology (MPT), Process Intensification (PI) and automated safety systems. The ammonia plant of the future is contained on one rail car, shop fabricated and contains all the latest technology. To date the “economy of scale” has been the way to reduce the cost of an ammonia or methanol plant but we are at or near the limit of what is possible. Moreover very few companies need such large plants with their large price tags. The future is to make small highly automated plants that are portable, use readily available inputs and produce the products where they are needed. To this end the new small scale plant will include all the latest leading edge technologies that apply such as Micro Processing Technology (MPT), Process Intensification (PI) and automated safety systems. The ammonia plant of the future is contained on one rail car, shop fabricated and containing all the latest technology. The methanol plant of the future could also be a standalone railcar. The DME plant of the future could also be a standalone railcar”

See: Ammonia Manufacturing Box: Small Ammonia Plant Design Using Micro Processing Technology and Process Intensification Technology

Pochari Technologies views the present situation in a similar manner. We differ in that we are confident that rather than methane reforming, photovoltaic-based hydrogen production will actually surpass methane in cost-competitiveness.

The important question is why downscale?

A number of reasons arise:

#1 Ability to integrate with intermittent renewables that are often sparsely scattered across diverse and rural geographies. Additionally, if containerized ammonia plants are built at wind or solar farms, significant cost savings can be realized by dispensing with grid connections and long-distance power lines.

#2 Potential to reduce CAPEX with the use of ubiquitous components. Large is not better, often larger components are more difficult and technically demanding to fabricate, leaving few suppliers to choose from. A small scale plant be easily assembled using parts purchased online from Alibaba.com

#3 Breaking the monopoly of centralized production based on natural gas and coal. This enables farmers to produce ammonia for less than the price of commercially acquired anhydrous from large suppliers.

#4 Portability: Small scale containerized plants can be moved easily by truck, increasing their long-term value, versatility, and asset recoverability, since the plant can be sold quickly since geography is no longer a constraint. If economic conditions change for the worse in a local region, the plant can be moved where conditions are more propitious.

More detail on why scale is not better:

“Sometimes individual components bigger actually make them more expensive per unit of output. Example: A fixed-tube reactor, when the shell diameter is under 12 ft, can be built in numerous shell and tube heat exchanger shop around the world. We have identified almost 100 shops in North America alone that are capable of building up to 12 ft diameter reactors. When the diameter gets bigger than 18 feet, as is the case with Shell’s reactor, the number of shops that can handle the fabrication (and the weight) drops to a handful and the cost per tube (a measure of unit productivity) goes up, not down. We have received quotes for reactors under 12 ft in diameter and compared them to quotes for reactors in the 18+ ft range. The cost per unit of reactor volume actually increased with the larger reactor (18 ft+ diameter). Additional shipping costs and erection costs also follow extremely large and heavy components”

http://www.fischer-tropsch.org/

The above is a comment by a small company that manufactures Fischer-Tropsch synthetic fuel reactors. Interestingly, they came to the same conclusion we did. An ammonia converter 3 meters in diameter is markedly more difficult to manufacture than one that is only 200 millimeters in diameter. In fact, a 200 millimeter reactor can be fabricated by just about anyone. One important fact to highlight is a modular plant based on small scale ubiquitous commercial off the shelf components can be built in a factor using lower cost labor. A large ammonia plant has to be built on site with much higher labor costs.

One of the major advantages of electrolysis-based hydrogen production is the high purity and lack of permanent catalyst poisons present in the gas stream. During the production of hydrogen from natural feedstocks such as coal or natural gas, significant quantities of sulfur are present and must be removed otherwise the permanent de-activation of the catalyst will occur. Additionally, carbon monoxide is also classified as a permanent catalyst poison. Synthesis gas from methane reforming contains trace amounts of carbon monoxide even after passing through the water-gas shift reactor, requiring an additional mechanization reactor to remove these residual impurities. In China where coal is a major hydrogen feedstock for ammonia synthesis due to the lack of natural gas, short catalyst lifespan has been a common feature for many medium-size ammonia plants. If subjected to even trace concentrations of sulfur, the iron catalyst will rapidly deactivate. The beautify of electrolysis-based hydrogen production is that only one impurity is required for removal: oxygen, while oxygen is a severe catalyst poison, it is classified as only a temporary catalyst poison since it can be reduced by passing hydrogen through the catalyst bed. Additionally, oxygen is a much larger molecule permitting easy separation using carbon-based molecular sieves.

Synthesis gas compression, not kinetics or thermodynamics, limit the down-scaling of conventional high-pressure ammonia synthesis. Ammonia synthesis stands out from other similar temperature catalytic processes insofar the operating pressure is many times greater than what is typically found in corresponding chemical production operations. This obviously means compressor design, maintenance, repair, and operation dominate the overall design criteria and parameters of an ammonia plant.

Therefore, a significant improvement in compressor technology has great implications for improving the 100-year-old Haber-Bosch process. The reason for this is that larger plants make use of more compact and thus lower cost and longer-lasting centrifugal multi-stage syngas compressors, while for smaller plants, these centrifugal compressors become too inefficient due to tip losses, turbulence-induced parasitic losses due to high Mach numbers, and the boundary layer effect. Additionally, centrifugal compressors have a very low-pressure rise per stage, this is exacerbated especially due to the low density of H2/N2 mixtures at a 3:1 ratio, yielding a gas with a mean density of 0.2 kg/m3. Since the operating principle of a centrifugal compressor is based on imparting kinetic energy into the gas medium rather than squeezing it in a fixed volume chamber, this makes centrifugal compressors very sensitive to gas density so there is a clear limit to how light a gas can become before it can no longer be compressed. For example, pure hydrogen cannot be compressed without the addition of higher molar mass dilutant gases.

Despite having lower footprints compared to reciprocating systems, centrifugal compressors still have relatively large footprints due to the multi-staging required for to achieve pressures over 150 bar. This, unfortunately, means that for small plants of 100 TPY, which is the size of Pochari Technologie’s CP100 plant, a radial compressor is not suitable. Thus, the only suitable compression medium is based on conventional oil-lubricated reciprocating compressor. Traditional reciprocating compressors, either based on conventional or diaphragm architecture, suffer from high capital costs, bulkiness, poor time between overhauls, and excessive failure rates. Diaphragm compressors, the backbone of high pressure hydrogen compression are not suitable for small scale ammonia production either due to a different technical issue: there lack of reliability and longevity for for cyclical operations. Since distributed green ammonia uses solar or wind energy, the plant must be able to operate at partial load capacity, with frequent starts ups and shutdowns. According to Klaus Hoff of NEUMAN & ESSER GROUP, diaphragm compressors suffer from premature diaphragm failure attributable to sharp thermal gradients experienced when reaching operating temperature from static temperature. Diaphragm compressors operated with frequent startup and shutdown can last as little as 6 months before replacement of the diaphragm. During continuous use, a life of 5-10 years is common. Since photovoltaic is intermittent, compressors will be subject to daily cycling since hydrogen is only produced from the electrolyzer for 12 hours at a time, requiring the compressor to cycle twice a day. Replacing the diaphragm is a time-consuming and laborious process requiring personnel to visit the plant multiple times a year. A small scale ammonia production requires highly reliable near 24-7 continuous operation with minimal human supervision, downtime must be kept to a minimum. Since the reactor, purification system, and electrolyzer have effectively no moving parts, the compressor is the weakest link in the chain.

Clearly, a better solution is required or small scale ammonia will fail and continue to be ignored by the industry.

In order to permit high-pressure ammonia synthesis plants to compete in a decentralized production model where containerized plants feed off solar and wind farms, a new and improved low-friction compression technology is needed. Electrochemical hydrogen compression is one obvious option, but the energy consumption of electrochemical hydrogen compression is somewhat too high to offset mechanical compressor CAPEX savings. The state-of-the-art electrochemical compression, while featuring outstanding compactness, uses about 6.6 kWh/kg-H2 to compress to 400 bar. In addition, the CAPEX of these units is too high since they make use of expensive membrane-electrode assemblies (MEAs) and proton-exchange-membranes (PEMs), made of expensive and specialized materials that do not adhere to our COTS methodology.

We believe the solution is liquid-piston gas compression, using either ionic liquids or hydraulic fluid, preferably ionic liquids. One of the major advantages of using an ionic liquid is twofold, eliminating oil contamination of the synthesis gas and achieving superb tribological performance, permitting nearly friction-free compression dramatically increasing TBO and the attendant Levelized CAPEX. Ionic liquids also feature very low compressibility, reducing mechanical losses. In addition, most ionic liquids have high viscosity, enabling leak-free design using standard seals, they also have low corrosivity against stainless steel. One of the central advantages of this compression technology is its drastically higher compactness. Ionic liquid piston compressors thanks to the heat capacity of the working fluid could achieve near isothermal compression, reaching close to 100% efficiency. Ionic compressors also enjoy much longer operating intervals before scheduled maintenance, in the order of 500 days, a factor of 10x over mechanical piston compressors in large part due to their reduced number of moving parts, only 8 vs over 500 in mechanical compressors. The ideal ionic liquid is immiscible with the gas, meaning that no gas will become trapped in the ionic liquid, nor does the ionic liquid possess a significant vapor pressure, in fact, the chosen ionic liquid has effectively zero vapor pressure.

Linde Kryotechnik AG first invented the ionic compressor in 2001, with the originally patent already expired, it offers tremendous potential for commercializing in the ammonia production industry, the timing could not be perfect. One of the reasons this technology has enjoyed little in the way of commercialization in surrounding industries is due to the bulk of the initial intellectual property being held by Linde, making it difficult for competitors in the West to offer this technology. This is quickly changing, however, since not only are the patents close to expiration, but there is increasing latitude for foreign manufactures to offer slightly different designs while still using the principle of the ionic separator/piston. China has recently begun developing its own ionic liquid compressors for hydrogen refueling by circumventing Linde’s original patent.

The Linde ionic liquid compressor designed for hydrogen refueling stations is 25x more compact than a standard reciprocating compressor. A 900 bar 370 Nm3/hr ionic liquid hydrogen compressor has a height of 1.1 meters, while a 100 Nm3/hr 440 bar reciprocating piston compressor has a height of 3.5 meters, adjusting for pressure and mass flow, the improvement is nearly 30x.

The working principle of an ionic compressor is simple. One can visualize the ionic liquid as a separation medium, substituting the metallic piston assembly. In the Linde Adler design, the mechanical force needed to push the ionic liquid to compress the gas derives from conventional hydraulic fluid, the ionic liquid is not compressed with a hydraulic a pump. Rather, a hydraulic piston beneath a layer of ionic liquid above it forces the entire assembly to move upward. The ionic liquid itself is not a working fluid, but rather a separation medium to avoid contamination of the product gases, reduce friction, and provide near-isothermal operation by absorbing heat from the compressed gases. The ionic liquid can be recirculated to maintain a constant temperature.

Pochari Technologies’ has chosen an alternative design. Two separate chambers are present, one for a metal piston to displace the ionic liquid into the gas compression chamber. The metal piston in the ionic section is lubricated by the ionic liquid itself, hydraulic fluid is not present to avoid contamination and mechanical losses from the hydraulic pump. It has been demonstrated that ionic liquid, especially those that are hydrophobic (the ones intended for use), possess extraordinary lubricant properties, exceeding that of conventional mineral and synthetic oils.

“For lubricating well-known systems, such as steel on steel, as well as on difficult to lubricate systems, such as steel on aluminium, ionic liquid lubricants have been shown to outperform commercially available lubricants, such as fully formulated engine oil”

“Even without any additives, ILs have shown remarkably better lubrication and anti-wear properties than normally used synthetic oils and are promising alternatives with considerably lower development costs. Furthermore, ionic liquids have superior durability as lubricants”. “Long-term experiments have shown that ionic liquids even during long-term operation do not lose or change their properties. Therefore from today’s perspective, a change of ionic liquid must at the earliest after about 10,000 hours of operation, while conventional lubricating oils an oil change after about 3,000 hours is required”. As previously mentioned, ionic liquid-based liquid piston compressors have been successfully implemented for hydrogen fueling stations, this technology has the potential to spill over into the domain of Haber-Bosch technology to further process-intensify, enabling the desired down-scaling. “A suitable ionic liquid is selected as the most reliable replacement for the solid piston in the conventional reciprocating compressors. Ionic liquids are room temperature salts that have very low vapor pressures. The ability to tune the physicochemical properties of ionic liquids by varying the cation-anion combinations is the feature of these liquids that make them as promising candidates to replace the solid piston. The ionic liquid: 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide is recommended as the best candidate that can be safely used as a replacement for the solid piston in the conventional reciprocating compressors for compressing hydrogen in the hydrogen refueling stations. In addition, the corrosion behavior of various commercially available stainless steels and nickel-based alloys as possible construction materials for the components which are in direct contact with the selected ionic liquids is evaluated. The results show a very high corrosion resistance and high stability for all of the alloys tested in any of the five selected ionic liquids. The stainless steel alloy, AISI 316L, with high corrosion resistance and the lowest cost is selected as a material for all the components in direct contact with the ionic liquid, in the designed ionic liquid hydrogen compressor.”

Regardless of whether a mechanical or liquid piston is used, hydrogen compression work input is logarithmic as a function of pressure, this is also the case for highly non-ideal gases like hydrogen, meaning that compression energy demand is non-linear, with decreasing amounts of energy required at higher pressures, this makes the use of medium pressure alkaline electrolyzers very attractive. For example, only 1.2 times more power is required to compress to 900-1000 bar vs 200 bar. If the hydrogen is already compressed to 50 bar in the electrolyzer, only 0.48 kWh/kg is required to reach 300 bar, this increases slightly to 0.65 kWh/kg if the electrolyzer pressure is reduced to 30 bar, a number that is very reasonable with the current maturity of alkaline technology. Thus, what can be inferred is that it is not the energy consumption of compression per se that is the limiting factor of downscaling, rather it’s the footprint, and attendant CAPEX of low-speed reciprocating compressors that disincentives decentralization of production. Therefore, Pochari Technologies logically concludes the ideal avenue to approach down-scaling is improving compression technology. The availability of much-improved compression technology opens up another avenue of process intensification. The equilibrium yield of ammonia at 400 C is close to 70% at 900 bar. The concept of ultra-high pressure ammonia synthesis was originally developed by Georges Claude in 1917. The principle was sound, the equilibrium favored very high pressures at moderate temperatures, this was evident as Claude’s reactor was far more compact, measured in TPD (tons per day) per cubic meter of volume, the Claude reactor operating at 1000 bar achieved 120 TPD/m3 vs only 25 for the Haber-Bosch reactors of the time. The major disadvantage of Claude’s reactor was significantly shorter catalyst life, while Haber-Bosch catalysts lasted 2 years, Claude’s lasted only 3 months, hence the abandonment of the Claude and paving the way for the domination of the Haber-Bosch design. With newer more stable and durable catalysts it may be possible to get close to these pressures while maintaining acceptable catalyst life. The trade-off that must be made is between catalyst life, reactor compactness, and conversion per pass. When Claude was trying to achieve ultra-high pressure ammonia synthesis, compressor technology was much capable than it is today, ionic liquid compressors may enable the resurrection of the Claude process to enable miniature ammonia plants of even smaller footprints. Considering the combination of logarithmic compression energy, the development of compact and high-pressure ionic compressors, and longer-lasting catalysts, the Claude design warrants serious consideration.

Since we have covered the key requirements of synthesis gas compression that makes the Haber-Bosch process unique among chemical production methods, we will cover the topics of reaction kinetics as determined by scale, and its design implication.

The kinetics of the actual reaction, of which the most important is the rate-limiting step of nitrogen dissociation, is not affected by the gross volume of the catalyst, it is however strongly affected by the pore size of the active catalyst the packing density.

The second variable is often mistaken for kinetics. While heat transfer may strongly affect the reaction rate, for the most part in ammonia synthesis, heat transfer mainly determines how much leftover waste heat is produced along with the equilibrium temperature within the reactor. Heat transfer could be argued to be the only reaction variable affected by scale, smaller reactors have higher surface-to-volume ratios, thus will produce less useable heat since they would radiate more heat into their surroundings. Approximately 1.5 kWh of thermal energy is released upon the formation of 1 kilogram of anhydrous ammonia while only 0.45 kWh of heat is needed to heat and maintain 1 kg of synthesis gas to 500 degrees Celsius, this means the reaction is moderately exothermic and more heat is produced than is needed to maintain desired reaction temperature. In industrial-scale plants, this surplus of heat is harnessed to generate steam to cover most of the compression energy requirements. Per kg of hydrogen, 8.5 kWh of thermal energy is released, representing 18% of the total energy for electrolysis. If an organic Rankine cycle can be employed to harness this waste heat source at an efficiency of 15%, about 1.3 kWh per kg of hydrogen is recovered. Obviously, unless insulated to compensate for the difference in the surface to volume ratio, smaller-scale reactors would purge more heat, therefore leaving less heat left over to generate the steam to cover compression energy. For a miniature plant, the added hassle of installing a boiler and steam turbine is unattractive, and as with all turbomachinery, small scale results in abysmal efficiency, so a steam turbine would be useless, therefore additional heat loss is not a design consideration. Additionally, since the small-scale plant uses much more efficient ionic liquid piston compressors rather than less efficient centrifugal compressors, compression energy has less importance to the overall design criteria.

Another important design variable is found in the design and optimization of gas-separation heat exchangers. At 300 bar, ammonia must be cooled to 130 degrees C in order to bring the gas phase into the liquid state to permit separation, once liquefaction, the ammonia is removed and the unreacted gases are reheated to the operating temperature. Smaller high surface area high heat transfer exchangers would be more compact and efficient than on larger scale plants since they would perform their heat exchanging function with greater rapidity. This is a major advantage of a small-scale plant.

The second variable is material usage, all else equal, a smaller reactor vessel will require more material per unit mass/volume of catalyst than a larger reactor. This difference in material usage is minimal and does not greatly affect plant economics. The state-of-the-art ammonia converters, such as the S-200 Topsoe use the SA-542 Grade-B Class-4 vanadium/molybdenum steel for the converter wall construction, while this is a very high-strength alloy, it is not made out of a high-cost nickel/chromium alloy such as Inconel, thus, the cost of the converter construction material is very small compared to the total annual output of ammonia, and in some cases, the catalyst cost exceeding the converter costs. For large-scale plants, the volume may be less important than the conversion rate per loop, hence the choice to operate at lower space velocities. For small-scale plants, what is desired is maximal footprint reduction, hence the choice to increase space velocity at the expense of conversion. The higher converter material loading is tolerated as a necessary design compromise that is unavoidable by down-scaling.

The additional material loading from down-scaling pales in comparison to the total system CAPEX, principally dominated by the prime-mover, such as solar farm or wind farm.

For industrial-scale plants, space velocity is largely a function of the pressure, the common space velocity of a low-pressure loop set at 5000–10000 hrs, a medium-pressure loop at 15000–30000hrs, and a high-pressure loop at as high as 60000 hrs. For our smaller-scale plant, we have chosen higher pressure, thus higher space velocity, to increase power density, hence to “process intensify” the operation. Since compressor CAPEX, footprint, and OPEX is a major limiting factor, by using ionic-liquid compressors, we allow the plant to raise pressure while keeping compressor footprint low, enabling a raising of the reactor output. Hence, there exists a compromise between compressor footprint and reactor footprint, ionic liquid compression thus plays a central role in permitting the process intensification and consequent miniaturization of the ammonia plant. For example, a 40 kg per hour ammonia plant uses only 6.9 liters of catalyst at a high space velocity of 40,000 hrs.

What can be concluded in the attempt to down-scale ammonia synthesis technology is that reaction kinetics remain unaffected while increased thermal transfer does not impose major design considerations other than minor reactor insulation. What is highlighted is that it is not kinetics, but rather compression, that remains on the obstacle and explains the preference to build ever-larger plants. It is therefore paramount to focus much of the research and develop efforts on compression technology, rather than catalyst technology. Over 100,000 different material combinations have been evaluated as ammonia synthesis catalysts, none have proven sufficiently superior to iron to pose as a possible contender to replace iron. Only cobalt and molybdenum bi-metallic nitrides could be possible contenders to replace iron. In 2001, it was predicted that cobalt/molybdenum bi-metallic catalysts could attain high activity for ammonia synthesis due to their excessively high and low nitrogen binding energies. Molybdenum binds nitrogen very strongly, while cobalt binds very weakly, combining the two proves to yield a highly effective catalyst. The reason for this is that we want a high absorption energy for performing nitrogen disassociation when N2 is bound to the catalyst surface, it breaks the bond, forming atomic nitrogen, but when ammonia is formed, it is desired to have a certain percentage of catalyst sites producing only weak nitrogen absorption, with a low surface coverage for atomic nitrogen. A cobalt/molybdenum bi-metallic alloy could perhaps achieve activities as high as ruthenium, with is exactly 4.21x higher, the issue is the cost of such a catalyst could possibly offset the reduced reactor volume and catalyst mass. For decomposition, (the reverse of synthesis), its been demonstrated that Cantor alloys (high entropy alloys), based on Co/Mo nanoparticles achieve decomposition turnover frequency 20x higher than straight metallic ruthenium. One cannot compare and extrapolate synthesis and dissociation in a simplistic framework of reversing the catalysts in order. For example, ruthenium is 10x more active than the second most active catalyst decomposition: Ni, while iron has very low activity, but ruthenium is only 4x more active than its second-best: Iron, while nickel has a very low activity for synthesis. The mass reaction rate (rm) of ruthenium catalyst is higher by 3.9 times and the TOF is higher by about 3.4 times than that of wüstite.

For ammonia decomposition, the catalytic activity is as follows:

Ru > Ni > Rh > Co > Ir > Fe.

For ammonia synthesis, it looks very different.

Fe > Os > U > Mo > Ru > Mn > Ce. Ni has the lowest activity, followed by Co.

What can be observed is that many of the most actives elements for decomposition do not show any activity for synthesis, and vice versa.

Iron is twenty cents per kilogram, while molybdenum is $30/kg, and cobalt is $79/kg, with an average of $54/kg, yielding a 272x price difference for the raw material alone, not including catalyst processing and deposition costs. In conclusion, alternative catalysts will require much higher activities than currently found to justify their higher costs.

Early ammonia synthesis catalysts were based on magnetite, reduced with hydrogen after melting with alkali promoters to produce the desired iron crystal structure. The later catalyst made use of the wustite crystal structure.

What’s remarkable is the ammonia synthesis catalyst is considered the most durable catalyst among all industrial catalysts, showing virtually no structural and morphological change after 20 years of extreme conditions of pressure and temperature. As long as the catalyst is not exposed to oxygen, a severe poison (less than 0.5 PPM O2 can reduce activity by 25%), due to the formation of iron oxide, the Wüstite or Magnetite-based catalyst is highly durable, active, and cheap, making its substitution and replacement a very tall order indeed. Nitrogen feed-gas is purified using carbon molecular sieves to 10 PPM. Oxygen is absorbed using iron-powder which is periodically regenerated by reduction with carbon. Iron has a very high oxygen capacity by weight allowing the oxygen absorbent to last for years before requiring regeneration.

In 1994, Huazhang Liu and other Chinese researchers invented a new high-activity ammonia synthesis catalyst based on wustite as precursor in place of magnetite, the oxidic promoters being substantially the same. It was shown that the reaction rate of the new catalyst is 30- 90% higher than that of the traditional one and that such an activity difference increases when temperature decreases. Resistance to sintering and mechanical strength are reported to be the same of the traditional catalyst, while resistance to CO impurities is higher. It was also reported that the new catalyst is being used successfully in Chinese ammonia plants. The overall ammonia productivity increased significantly, for example, the figure quoted above from older magnetite catalysts from the 1960s shows 19.6% conversion at a GHSV of 29,000, while the Chinese wustite catalyst demonstrates the conversion of up to 33% at a GHSV of 30,000. The wüstite-based catalysts exhibit about 60% higher catalytic activity, some data show 70% higher than magnetite. Wustite is considered the most advanced commercial iron catalyst with the highest activity and low production cost in the world, and it is competitive with ruthenium-based ammonia synthesis catalysts. The Chinese catalyst was licensed to German and Swiss companies and is marketed in the West as AmoMax® 10 by Clariant. China continues to dominate wustite based ammonia synthesis catalyst manufacturing, Shaoxing Shangyu Catalyst Co., Ltd is a leading manufacture of the A-301, A110-2, and ZA-5 catalyst. At 15 MPa and 425°C, the Al2O3, K2O, CaO promoted Wustite catalysts exhibits a peak turnover frequency or reaction rate of 95 millimoles NH3 per gram catalyst per hour, translating into 1.62 kg NH3 per kg catalyst per hour, and that is at only 150 bar permitting low-cost fittings and reactor components. At this rate of activity, a reactor vessel of only 8 liters in volume is required for a 77 ton per year plant!

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Activity of a Wustite ammonia synthesis catalyst from Huazhang Liu and Wenfeng Han,
Institute of Industrial Catalysis, Zhejiang University of Technology, Hangzhou 310014, China.

The use of the higher activity wustite usually encourages industrial operators to reduce operating pressure, in fact some plants using the wustite based catalyst in China run at only 15 MPa. In our case, we want footprint reduction, so pressure the same while GHSV increases commensurately with the higher TOF.

Pressure drop is an important design consideration due to the phenomenon of intraparticle diffusion. Pressure drop is a design concern due to the requirement to repressurize the feed gases during each loop, consuming additional power. Pressure drop is a direct function of the void volume of the catalyst particle, large particles feature higher void volumes permitting higher diffusion, especially at higher pressures, thus, smaller particles dramatically reduce gas diffusion and hence pressure drop. It was found that particles less than 0.9mm effectively eliminate intraparticle diffusion. Spherical-shaped particles are ideal, they minimize both pressure drop as well as maximize activity. Smaller particles also possess much higher activities than larger particles. For example, for particles 0.6-1.2mm in diameter, the relative TOF is 300, whereas it drops to 61 for a particle size of 6-9mm. The durability of the wustite catalyst was found to be significantly improved over the magnetite-based catalyst. The attrition rate for magnetite was 1.5% over 6.5 years while only 0.5% for wustite. If the traditional magnetite has a TOF of 4.8 kg-NH3/liter-catalyst at a GHSV 40,000 with 19% conversion, we can assume this can be increased to 5.76 kg/liter-catalyst per hour assuming a 20% increase. This means a 40 kg-hr plant uses only 7 liters of volume, the size of 14 small handheld water bottles.

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Catalyst productivity as a function of GHSV: Catalyst density is 2.7 g/cm3

Financial viability, specifications and rough cost breakdown for 77 TPY NH3 plant:

Solar array: 300 kWp $0.30/watt monocrystalline photovoltaic panels at 2030 kWh/kWp (Lancaster, CA): $90,000

DC-DC step up converter: $50/kw: $3,500

Osmosis water distillation: 2 kWh/m3 15 LPH, ($1,500 for 500 LPH).

300 kW carbon-steel electrode low-cost COTs alkaline water electrolyzer: $30/kW: $9,000

Electrolyzer sized for 100% kWp capacity at peak, can increase power density by 25% during peak hours, equaling 168 kW, oversized by 140% to eliminate the need for battery storage. 

Compression energy: 8 kWh heat energy per kg of hydrogen, waste heat insufficient to cover 3.5 kWh/kg compression.

300 kg carbon-molecular sieve, 98 Nm3 N2/hr/ton at 99.999% purity, $9.5/kg: $3600

100 kg Fe2O3/K2O/CaO Wustite catalyst-hr, 2700 kg/m3 bulk density, 20 year year lifetime, 30,000 GHSV, $45/kg: $4,500

Low-speed 350 bar 8 Nm3/hr (10 bar pre-compressed H2) Ionic liquid reciprocating compressor (20,000 hr MTBO): $15,000

Reactor materials, Inconel tubing, insulation, valves, fittings, flowmeters, and miscellaneous items: $10,000 (current wholesale market prices from Alibaba quotes, assuming steel prices of ¥6000/ton and non-custom components only)

Reactor volume: 37L

Reactor weight:

Total: $131,300

Pay-off time: 2.49 years

Total gross revenue @$800/ton: $61,600

Net revenue: $58,600

1st year return on capital (ROI): 44.63%

Annual maintenance cost, primarily compressor maintenance, principally compressor piston ring replacement: $3000

One thought on “Process intensified ammonia plants based on advanced compressor technology for distributed production

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