Mar-2023

Reactivated catalysts can offer sustainability benefits in TGTUs

Refiners can reduce operating costs and carbon emissions while retaining performance through conversion of reactivated hydroprocessing catalysts as tail gas catalysts for TGTUs.

Brian Visioli
Evonik Catalysts

Article Summary

It is no secret that the world relies on catalysts in many aspects of life. They are proven to improve the quality and standard of living thanks to their wide range of applications, including the manufacture of fine chemicals, agriculture, health care, and food products. However, the world in which these processes were established has changed, with many industries – not least the chemical manufacturing industry – facing a host of new challenges, most notably around sustainability.

With surging energy prices and pressure to reduce environmental footprints, the spotlight is on the industry to reform and revitalise its operations to reduce waste, cut pollution, and minimise impact on the environment. Alongside the transition to greener, more sustainable manufacturing processes, refiners are having to contend with seemingly ever-rising costs, with events such as the Ukraine-Russia conflict putting a strain on the global economy. On top of this, experts are predicting a mild global recession in 2023.

These external factors have placed further pressure on refineries to find cost-effective methods to complete their operations without compromising performance or safety levels. The million-dollar question is, how can refineries reduce their operating costs and carbon emissions while maintaining levels of performance? The answer: through catalyst reuse. Here we explore how replacing fresh tail gas catalysts with reactivated hydroprocessing catalysts can yield significant environmental benefits and cost savings for refiners while ensuring equivalent performance and consistency of production methods.

Reactivating hydroprocessing catalysts
Reusing hydroprocessing catalysts via ex-situ regeneration and/or rejuvenation treatment has long been common practice in the global petroleum industry to provide cost-advantaged catalyst configurations with similar or equivalent performance to fresh catalysts. Catalyst recycling not only extends the useful life of catalysts but also substantially reduces the quantity of hazardous spent (used) catalyst waste sent to landfill and reduces the carbon footprint of catalysis on industrial scales. Due to these benefits, it is easy to see why catalyst reuse schemes have been implemented in the refining industry for more than 40 years.

To put this in context, 154,000 MT of hydroprocessing catalysts are replaced in refineries each year, more than 20% of which are catalysts that have been regenerated or rejuvenated via ex-situ processes. This translates to the installation of more than 30,000 tons of regenerated and rejuvenated hydroprocessing catalysts per year. At the end of an operational cycle, a hydroprocessing catalyst is removed from the reactor, classified as hazardous, and disposed of via one of three methods.

Disposal in a hazardous waste landfill is the least desirable option as the refiner loses the value within the spent catalyst, a fee must be paid, and the environmental impact must be considered. Processing for metal reclamation is a more favourable approach. However, while refiners can be credited a fraction of the value of certain components reclaimed from the spent catalyst, this method is energy intensive and still leaves a potentially environmentally harmful waste stream that must be disposed of.

Moreover, both options involve processing costs, and neither leverages the value of the technology inherent in the catalyst particle for the refiner’s benefit, which is why the third option – catalyst reuse – is the most preferred. This is where refineries can take full advantage of all useful materials in their spent catalyst by reactivating them in order to use them again.

The process involves oxidation under controlled conditions to remove carbon and sulphur compounds but preserve catalyst qualities for certain applications (an additional chemical treatment, commonly referred to as ‘rejuvenation’, may also be employed). Ultimately, it allows the refiner to leverage the maximum value of their spent catalyst, avoids the economic and environmental impacts of landfill disposal, and gives the active metal components, which comprise a hydroprocessing catalyst, a ‘fresh start’. And equally important, regenerating and reusing the catalyst in this way reduces the refining industry’s dependence on freshly mined metals.

Reusing tail gas treating catalysts
Given the clear economic and environmental benefits of this tried-and-tested technology, what if it could be applied beyond hydroprocessing to Claus tail gas treating? That was the question posed against a backdrop of decades of experience in both hydroprocessing catalyst recovery and sulphur recovery catalysis. The result: a lower-cost catalyst offering the same level of performance has a lesser environmental impact and contributes to circularity and sustainability for refiners.

Once a hydroprocessing catalyst has been successfully reactivated, the catalyst may be utilised in tail gas treating processes as there are several similarities between the catalysts. Both applications generally make use of metals such as cobalt and molybdenum, supported by an alumina substrate. Moreover, in each case, the catalyst needs to be converted to a metal-sulphide state for activity toward the desired reactions, and both consume hydrogen as a reactant in the desired reactions.

However, there are also important differences to note between the two types of catalysts and their applications. For example, the quantity of active metal applied to hydroprocessing catalysts is commonly much greater than that on tail gas treating catalysts. Also, hydroprocessing catalysts do not typically encounter species containing oxygen atoms when processing fossil fuels, and the operating pressure is significantly higher in hydroprocessing (up to 2000+ psi/140+ bar) compared to tail gas treating. These differences influence which types of deactivation typically occur during operation, which in turn has an impact on whether the catalyst can be reused.

Treating spent catalyst
To counteract the differences and fully maximise the spent catalyst potential, Evonik has developed and patented a method for treating a spent catalyst from a refinery hydroprocessing unit to remove contaminants from the catalyst and optimise the catalyst for use in a tail gas treating hydrogenation reactor. This catalyst, EcoMax TG, has significantly lower costs – 20-40% – than typical tail gas catalysts but provides very high activity combined with minimal environmental impact.

The ecological effects of using EcoMax TG compared to a freshly manufactured tail gas catalyst using virgin raw materials were evaluated using a Life Cycle Assessment (LCA) based on Evonik-internal manufacturing data as well as data from peer-reviewed publications. The LCA compared the carbon footprint between the two products using a ‘cradle to gate’ methodology.

As shown in Figure 1, the total carbon footprint associated with manufacturing a fresh tail gas catalyst using virgin raw materials is approximately 22.1 kg per kg of catalyst. The total carbon footprint for EcoMax TG is estimated to be a significant 75% lower impact: approximately 5.5 kg per kg catalyst. This is largely because no new metal raw material (aluminum, cobalt, or molybdenum) mining is required, as is the case for a freshly manufactured catalyst, and consequently no additional processing or transportation.

Furthermore, as previously outlined, reusing the catalyst reduces waste that would otherwise be disposed of in landfill. The catalyst reuse method is also less energy intensive than the process of forming particles to make a fresh catalyst. So, taken altogether, numerous ‘Scope 3’ emissions – indirect emissions associated with ‘upstream’ processes tied up in making a fresh catalyst – are effectively avoided with the more sustainable EcoMax TG catalyst.