Catalysts for Renewable Fuels Production

Catalysts play an essential role in renewable fuels production and can help to overcome the challenges associated with processing renewable feedstocks, writes Henrik Rasmussen, Topsoe.

This article was first published in Decarbonisation Technology and is reprinted with their kind permission. 

The global energy landscape is undergoing constant change as energy security and diversification shape the future of fuel.

Among the most promising avenues in this transition is the production of renewable fuels, which offer a viable alternative and supplement to traditional fossil fuels.

Hydrotreating is a ubiquitous technology present in every refinery in the world. It is commonplace for a refinery to have several different hydrotreating units, each designed and optimised to treat different fractions from the initial distillation of crude oil, including naphtha hydrotreaters, diesel hydrotreaters, and FCC pretreat units. The process conditions and catalyst systems are tailored to the specific requirements of the different feed streams while meeting the required specifications for each product stream.

At one level, adapting hydrotreating technology to process renewable feedstocks to produce hydrotreated esters and fatty acids (HEFAs) represents one of the lowest-cost, easiest-to-implement options in the transition from fossil to renewable-based products.

However, the production of renewable fuels for both existing refiners and new producers is not without its challenges when it comes to the catalytic processes involved. The catalyst system must be designed to manage the specific characteristics of renewable feedstocks and produce high-quality fuels. This article explores the critical role of catalysts in renewable fuels production, the challenges associated with processing renewable feedstocks, and the innovative solutions that are driving the industry forwards.

Complexities of Renewable Feedstock Processing

Hydrotreating catalyst systems have evolved over the years such that most units today are loaded in layers of catalysts, each with different catalytic functions. The top layer comprises guard bed catalysts specifically designed to remove contaminants such as nickel and vanadium metals from fossil feed streams while minimising the build-up of pressure drop to ensure this function does not limit the operating cycle of the unit.

The next layers comprise catalyst developed for maximum hydrogenation efficiency to remove sulphur and, if needed, also saturate aromatics. The system may include catalysts to remove nitrogen if this is required (for example, for hydrocracking units). At the bottom of the hydrogenation unit is a catalyst layer designed to minimise sulphur recombination.

Renewable feedstocks, such as vegetable oils, animal fats, and waste oils, pose unique challenges compared to traditional fossil-based feedstocks. The complex reaction mechanisms involved in hydrotreating renewable materials differ significantly from those of conventional petroleum refining. If catalysts are not specifically designed to handle these complexities, the consequences can be severe: poor product yield, byproduct formation, accelerated catalyst deactivation, and pressure drop build-up.

Additionally, the hydrotreated product has to meet the required product specifications, namely ASTM D-975 for diesel and D-7566 for SAF, including the cold flow properties, which are critical for fuel performance in colder climates (diesel) or at altitude sustainable aviation fuel (SAF).

To address these challenges, catalyst systems must be tailor-made for renewable feedstock processing. This involves developing catalysts that can effectively manage the hydrodeoxygenation (HDO) of renewable feeds, improve cold flow properties through isomerisation, and maintain stable operation under varying conditions.

To meet these complex requirements, Topsoe has developed specialised catalysts for these purposes, enabling the production of on-spec renewable diesel and SAF without operational issues.

Catalyst Innovations for Renewable Fuels

Central to this has been the introduction of a range of dedicated catalysts designed specifically for the hydroprocessing of renewable feedstocks. Recently launched to the market are third-generation Topsoe HDO catalysts selective for the hydrogenation route designated TK-3001, TK-3002, and TK-3003. These will provide producers with better HDO selectivity via the hydrogenation route, higher activity, and high metals, resulting in a significantly longer cycle length. The higher HDO selectivity throughout the cycle length of the catalyst will result in a significantly higher average product yield compared to the previous generation of HDO catalysts, which will increase the profitability of the plant.

The catalyst loading also includes an isomerising dewaxing catalyst to ensure that the renewable diesel and SAF meet the required cloud point and freeze point of the final product. The specialised guard beds catalysts are designed to pick up the contaminants in the feedstock, such as P, Na, K, Ca, Mg, Fe, Ni, and V. The most significant poison is typically phosphorus (P). To address this, Topsoe has developed a more efficient P guard, TK-3000 PhosTrap, which picks up four to six times as much P on a volume basis as its previous P guard in renewable service.

Reduced Complexity and Lower Costs

In addition to picking up contaminants, the guard bed catalysts are designed to use the hydrogenation route like the main bed catalyst to ensure that the oxygen in the triglyceride is converted to water instead of CO₂. The hydrogenation route improves the product yield and will not make CO₂ as opposed to the decarboxylation route, which produced significant amounts of CO₂.

The fact that Topsoe’s specialised HDO main bed catalysts and guard bed catalysts produced almost no CO₂ is the reason why its licensed process technology called HydroFlex does not include an amine unit in the high-pressure loop. The elimination of the amine unit from the process layout has many advantages, such as reduced Capex and Opex, lower carbon intensity (CI score), and decreased operating complexity.

The effectiveness of these HDO and dewaxing catalysts has been demonstrated in more than 25 industrial units, and many of Topsoe-licensed units have used loads of its HDO and dewaxing catalysts.

Choosing the Right Catalyst for Co-Processing

Introducing even minor amounts of biomaterial into a fossil diesel or kerosene hydrotreater requires a thorough understanding of the implications and strategies to mitigate potential risks. Co-processing renewable and fossil feedstocks introduces additional complexities. The hydrotreater must still convert all organic sulphur and nitrogen in the fossil portion of the feed to meet product specifications, while the HDO catalysts must convert all of the renewable feed.

To achieve these multiple objectives, the HDO-graded bed catalysts are typically loaded on top of a NiMo fossil catalyst. The trick is to load the right amount of each type of catalyst so that the cycle length of the HDO catalyst matches that of the fossil NiMo catalyst while maintaining a stable pressure drop across all the catalysts. The ratio of the different types of catalysts is, of course, governed by the percentage of co-processing.

It is also important to determine if a dewaxing catalyst is needed in the bottom of the reactor (below the fossil catalyst) to ensure that the mixture of fossil and renewable diesel still meets the required cloud point. Topsoe is experienced in designing the loading of a co-processing unit to meet all product and cycle length objective and has more than 20 hydroprocessing units currently in operation in co-processing mode.

Conclusion

The production of renewable fuels represents a significant step forwards in the diversification of the global energy landscape. However, success depends on the development and deployment of advanced catalysts that can handle the unique challenges of renewable feedstock processing. Topsoe’s third-generation HDO catalyst, selective towards the hydrogenation route, and its specialised graded bed HDO catalysts are effective in both co-processing and standalone units.

The catalyst and process technology not only ensure the production of high-quality renewable fuels but also enhance the profitability of the unit by increasing the product yield and extending the cycle length, thus improving the onstream performance. The enhanced capacity for picking up contaminants, including phosphorus, helps the refiner process lower-quality renewable feedstocks to further enhance the profitability of the operation.

As the industry continues to evolve, the role of catalysts in enabling the energy future cannot be overstated.