Optimization of time-proven catalyst boosts conversion and efficiency

The Haber-Bosch process revolutionized ammonia production by introducing promoted magnetite catalysts. Despite numerous alternatives, magnetite continues to dominate due to continuous advancements in optimizing iron crystal morphology and promoter dispersion, resulting in the most active magnetite catalyst ever developed.

Magnetite-based solutions offer exceptional longevity, with service lives so extensive that selecting a catalyst is often a once-in-a-career decision. The robustness of the catalyst is even more important when producing green ammonia, as these plants operate under fluctuating conditions depending on electricity availability.

The catalyst chosen will significantly impact the plant's operating economics, lasting 15-20 years before replacement is needed. A poor choice can have costly consequences, as replacing a catalyst involves significant downtime and lost production.

→ Read the original article that appeared in Fertilizer Focus, Vol. 42, Issue 1 in pdf form.
    The article is republished on Topsoe Knowledge & Insights with their kind permission. 

The Margins Matter

The Haber-Bosch process is widely regarded as one of the most transformative industrial chemistry innovations, making ammonia fertilizer readily available and fueling dramatic increases in agricultural yields. Beyond agriculture, ammonia has become a key ingredient in pharmaceuticals, plastics, textiles, and countless other chemicals.

Even minor enhancements in this mature technology can have outsized impacts, underscoring the importance of operating margins in determining both profitability and market share.

Under the Net Zero Emissions by 2050 scenario, global ammonia demand is projected to rise to 470 million tons, driven by slight increases in fertilizer use and a 29.5% growth in industrial applications. However, the most significant demand surge will likely come from new applications, particularly ammonia's use as a marine fuel, which could account for 43% of direct ammonia use by 2050.

Additionally, ammonia's role as a hydrogen carrier is set to grow due to its high energy density, ease of transport, and existing infrastructure, making it vital for the energy transition.

Developments and Parameters

The development of various alternative formulations and technological approaches for ammonia synthesis catalysts has been ongoing for years.

About 25 years ago, a catalyst featuring ruthenium on a carbon carrier system, combined with a unique process design, generated significant buzz. However, only a few installations were realized due to challenges such as methanation side-reactions; high sensitivity to poisoning; scarcity of ruthenium; and complex production process.

These factors led to prohibitively high costs, making the process commercially unviable.

In 2005, a new iron catalyst, based on a promoted wustite phase, was introduced commercially, driven by a Chinese development initiative. This catalyst has since gained market share in China.

To evaluate the characteristics and advantages of magnetite-based versus wustite-based catalysts, it is essential to understand the key features that contribute to a long-lasting ammonia synthesis catalyst, including:

• High and stable activity
• Low deactivation rate
• Effective stabilization of pre-reduced versions

Activity and Balance

Achieving high catalyst activity requires:

• The optimal amount and distribution of promoters on the iron surface
• A substantial presence of highly active Fe(111) sites

These Fe(111) sites have an open iron surface, allowing easier access for gas reactants. This results in significantly higher ammonia synthesis activity compared to the Fe(100) and Fe(110) sites.

Magnetite itself does not inherently favor any of these sites, so their formation is controlled and optimized during catalyst fabrication by selecting the appropriate manufacturing conditions.

While suitable promoters can partially compensate for the absence of Fe(111) sites, they can never fully achieve the same level of activity.

The Importance of Promoter Distribution

The use of structural promoters such as Al, Ca, Si, and Mg is crucial for:

• Reducing sintering of active iron sites during operation
• Lowering deactivation rates
• Maintaining stable production rates in industrial units

For optimal promotion effects, promoters must be evenly distributed across the iron surface. On magnetite catalysts, this uniform distribution can be achieved through proper fabrication techniques.

Researchers highlight the challenges of achieving even promoter distribution on wustite-based catalysts, often leading to higher deactivation rates.

To explore this issue further, the Topsoe Research & Development department conducted aging experiments on both magnetite and wustite-based catalysts under the following conditions:

Temperature: 500°C; Gas Composition: Hydrogen/Nitrogen ratio of 3; Pressure: 20 MPa.

These tests revealed that magnetite-based materials lose only 10% of their activity, while wustite-based materials experience a 30% loss of Start-of-Run (SOR) activity.

Stability of Pre-Reduced Ammonia Synthesis Catalysts

In a typical ammonia synthesis reactor, catalyst loading consists of a pre-reduced layer in the first bed and oxidic catalysts in the lower beds.

The oxidic catalyst in the lower beds must be reduced in situ over several days during start-up. Some plants use pre-reduced catalyst in all beds to save start-up time and reduce ammonia-containing water production during catalyst reduction.

This approach can save 2-3 days of reduction time, significantly increasing ammonia production.

The pre-reduced catalyst is manufactured in a separate step after the oxidic catalyst is produced. While most catalysts are pre-reduced at the same facility as the oxidic catalyst, some are handled by third parties.

It is crucial that this process is carefully controlled to ensure maximum activity, as the catalyst's pore structure is formed during this reduction step. Key factors that must be meticulously monitored include heating rates and water content.

A detailed investigation revealed:

Fast reduction results in a narrow reaction zone progressing from the surface to the dense center.

High H₂O concentrations inhibit the reaction, leading to uneven reduction across the iron particles.

The Importance of Proper Passivation

After achieving full reduction, a separate passivation step is necessary. Without proper passivation, the catalyst may:

• Heat up upon air exposure, causing significant delays during loading.
• Require the reactor to be blanketed with nitrogen in most cases.
• Lose expected activity levels if passivation is inadequate.

In severe cases, if the catalyst's activity is too low, it should be discarded to avoid performance issues.

From Research Investigations to Market Feedback

How Does Magnetite Perform in Industrial Settings? After 13 years of successful operation with the KM magnetite catalyst, one plant switched to a wustite-based catalyst. However, this wustite-based catalyst exhibited much faster deactivation than the previous KM charge.

After 10 years, the plant decided to revert to the Topsoe KM magnetite catalyst. The first 12 months of operation with the new Topsoe KM catalyst confirmed its high activity level.

Industry Experience and Market Adoption

Years of Topsoe's research into magnetite phases and suitable promoters, combined with industrial feedback, led to the launch of the KM 111 Catalyst and Pre-Reduced KMR 111 Catalyst in 2014.

Since their introduction, these catalysts have been installed in over 70 ammonia plants worldwide, representing 25% of the plants for which Topsoe provides catalyst solutions.

A recent example includes a U.S. ammonia plant that replaced a wustite catalyst with KM111 in a three-bed reactor configuration, with a pre-reduced catalyst in the first bed and KM111 in the second and third beds.

The wustite-based catalyst was replaced after just four years due to mechanical issues in the ammonia converter. The plant opted for the magnetite-based KM111 due to lower deactivation rates, and higher activity, as clearly demonstrated in real-world applications.