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• How are catalyst suppliers further enhancing catalyst formulations for refiners focused on processing a wider array of feedstocks (such as renewables, plastic waste, and heavy crudes)?

  Mar-2024

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Answers

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• Andrea Battiston, Ketjen, andrea.battiston@ketjen.com

  The energy transition is compelling refiners to process more renewable and recyclable feedstocks like vegetable oil and waste plastic oils (WPOs) for production of transportation fuels and chemicals. These feedstocks present new challenges for the hydrotreating catalyst systems, necessitating enhanced formulations and new ways to apply them in commercial practice. These challenges can be summarised into three main types, each demanding a distinct approach and solution.

  Inorganic impurities
  Firstly, the new feedstocks can contain inorganic impurities not present in fossil feedstocks or in different concentration and molecular forms. Removal of the impurities by means of reaction and deposition in the guard catalyst section is required to prevent contamination and deactivation of the main catalyst. The case of phosphorus (P) and metals trapping is the most common challenge and illustrates how catalyst systems are being improved. P-containing molecules present in fossil-spent lube streams are generally highly reactive.

  In contrast, the phospholipids prominent in animal fats, for instance, are highly reactive and bulky. As a result, the guard bed catalyst must provide the right balance between its active sites’ accessibility, pore volume storage capacity, and active phase activity. In this way, the maximum amount of phosphorous and metals can be reacted and trapped in the whole catalyst pore volume and not just in the proximity of its external surface.

  In WPOs phosphorous is present as the remnants of P-containing flame retardants alongside a broad range of, sometimes exotic, elements and metals that one would not find in any other feedstock. The guard catalyst, in this case, needs to be tailored to trap all these elements. Note that for waste plastics hydrotreating, there are large differences in the pretreatment and the trapping strategy depending on the source of the plastics, be it olefins or aromatics.

  Oxygenates
  A second challenge arises from the presence of oxygenates in non-fossil feedstocks, necessitating their removal to meet final product specifications. Once again, the challenges related to removing oxygen depend on the type of feedstock. Triglycerides contained in vegetable oil and animal fats are readily converted over hydroprocessing catalysts, but the pathway for their decomposition into paraffins can significantly affect the process’s effectiveness.

  Depending on reaction conditions and catalyst composition, oxygen can be removed via hydrodeoxygenation (HDO), producing water, or via decarbonylation and/or decarboxylation, releasing CO and CO₂, respectively. A high selectivity towards the HDO pathway is generally desired as it maximises the hydrocarbon product yield and, where applicable, prevents downstream catalyst poisoning by CO. For example, when renewable feedstock is co-processed with fossil fuel, CO inhibits the hydrogenolysis reaction pathway to remove suphur, impacting the performance of hydrotreaters loaded with CoMo catalyst, which are typically those operating at low hydrogen pressure.

  For HDO, selection of the active metals in hydrotreating catalyst formulation is key to balancing the hydrogenolysis and hydrogenation functions. In addition, HDO catalysts need to be accessible to large molecules (triglycerides) and capable of tolerating metal slip from the metal trapping layers above the reaction zone, which can occur later in the operating cycle, so excellent pore accessibility remains a key property throughout the cycle.

  This explains why HDO catalysts also require an open pore structure. Note that in bio-oils obtained by liquefaction of biomass sources such as lignocellulose, oxygen concentrations are very significant. The extremely high reactivity of some of the oxygenates can result in stability and handling issues, so a stabilisation step at low temperature with a catalyst with a specific composition is applied prior to regular hydrotreating.
  Nitrogen-containing compounds

  Thirdly, as is the case of fossil feedstock hydroprocessing, and with renewable and recycled feedstocks, the presence of nitrogen-containing compounds can negatively affect overall hydroprocessing catalyst system performance; hence, a catalyst load with proper hydrodenitrogenation (HDN) activity is required. To produce renewable diesel or sustainable aviation fuel (SAF) via the hydroprocessed esters and fatty acids (HEFA) route, nitrogen must be removed to prevent deactivation of the downstream hydroisomerisation catalyst. Especially when animal fats are processed, the feedstock is rich in nitrogen, which is difficult to convert, as in the case of tertiary amides.

  To handle these large refractory molecules, a specific catalyst is required with high HDN and hydrogenation activity, and excellent pore accessibility. On the other hand, in WPOs, the total nitrogen content can occasionally be high, typically consisting of easy, neutral species, with only negligible amounts of refractory compounds like carbazoles (see Figure 1 above).

  In summary, effective hydroprocessing of renewable and recycled feedstocks demands tailored catalyst formulations and loading configurations, informed by a comprehensive understanding of feedstock molecular composition and reactivity, as well as catalyst functionalities. This requires extensive specific work in the lab and on the commercial units. Collaboration between process operators and catalyst suppliers, leveraging decades of experience, is essential. The proprietary ReNewFine catalyst solutions developed through a decade-long partnership between Ketjen and Neste, and applied using Ketjen’s proprietary ReNewSTAX catalyst loading strategy exemplify the success of this collaborative approach in producing renewable diesel and sustainable aviation fuel.

   

  Apr-2024