May-2023

Reduce the carbon footprint of tail gas treating with reactivated catalysts

Times have changed since catalytic processes were established.

Brian Visioli
Evonik Catalysts

Article Summary

Between long-term sustainability goals and global economic instability, there is mounting pressure on the chemical manufacturing industry to stay competitive and thrive in today’s challenging climate.

The question is, how can refineries reduce operating costs and carbon emissions while maintaining levels of performance?
The answer: through catalyst reuse.

Tried and tested
For more than 40 years, the global petroleum refining industry has successfully recycled hydroprocessing catalysts through ex-situ reactivation to benefit from the economic and environmental benefits these technologies deliver. The process provides more cost-effective catalyst configurations whilst continuing to deliver performance levels equivalent to fresh catalysts.

Of the 150,000 tons of hydroprocessing catalysts replaced in refineries annually, more than 20% have been reactivated via ex-situ processes, resulting in the installation of more than 30,000 tons of reactivated hydroprocessing catalysts per year. At the end of an operational cycle, a hydroprocessing catalyst is taken from the reactor, classified as hazardous and removed via one of three methods: waste landfill disposal, processing for metal reclamation and catalyst reuse.

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 considered. Processing for metal reclamation is a more favourable approach to the above, but it is energy intensive and still leaves a potentially environmentally harmful waste stream that must be disposed of. The third option – catalyst reuse – is the most preferred, as refineries can take full advantage of all useful materials in their spent catalyst by reactivating them to use them again.

Given the clear economic and environmental benefits this tried-and-tested technology offers, Evonik decided to explore how to apply hydroprocessing catalysts in Claus tail gas treating processes. The result has been the development of a lower cost catalyst that offers equivalent performance levels, has a lesser environmental impact and contributes to refiners’ circularity and sustainability objectives.

Hydroprocessing catalysts vs tail gas treating catalysts
So, how do the two types of catalysts compare? There are several similarities between hydroprocessing catalysts and tail gas treating unit catalysts. Both applications make use of metals such as cobalt and molybdenum, supported by an alumina substrate and to achieve the desired reaction, each catalyst needs to be converted to a metal-sulphide state.

However, there are also important differences to note. For example, the quantity of active metal applied to hydroprocessing catalysts is commonly much greater than that on tail gas treating catalysts. A key factor to note is the operating pressure of a hydroprocessing catalyst is considerably higher (up to 140+ barg) compared to a tail gas treating catalyst, which is less than 1 barg. These key differences influence which types of deactivation occur during its operation, which in turn has an impact on whether the catalyst can be reused.

To offset the differences and fully maximise the potential of a spent catalyst, Evonik has developed and patented a method for treating a spent catalyst from a refinery hydroprocessing unit to remove contaminants and optimise it for use in a tail gas treating hydrogenation reactor. The catalyst – EcoMax TG – offers significant cost savings, between 20-40%, compared to typical tail gas catalysts, as well as very high activity and a lower environmental footprint.
Sustainability gains

To evaluate the ecological effects of using EcoMax TG compared to a freshly manufactured tail gas catalyst, Evonik utilised a Life Cycle Assessment (LCA). The LCA compared the carbon footprint between the two products using a ‘cradle to gate’ methodology.

The results showed a 75% lower total carbon footprint for EcoMax TG than a fresh tail gas catalyst. This is largely down to EcoMax TG not requiring any new metal raw material mining or additional processing and transportation, unlike new catalysts.

This demonstrates the substantial ecological and financial benefits catalyst recycling can offer, with catalyst reuse also reducing waste that would otherwise be disposed of in landfill.
Moreover, this method is less energy intensive than the process of forming particles to make a fresh catalyst. The numerous ‘Scope 3’ emissions (indirect emissions associated with ‘upstream’ processes that are tied up in making a fresh catalyst) are, therefore, effectively avoided with the more sustainable EcoMax TG catalyst.

Performance comparison
Catalyst performance is measured through rigorous testing of catalyst conversion performance under identical controlled conditions. The performance of a traditional benchmark catalyst, EcoMax TG, and Maxcel TGE-01 is shown below.

The economics of tail gas catalysts are a function of the cost of the catalyst components itself, manufacturing cost, transportation cost, and catalyst density.
To standardise the effects of different types and locations of manufacturing, tail gas catalyst costs may be computed per unit volume. The comparison in the table below is based on actual fill cost, thereby removing the density variable.

The carbon intensity of manufacturing each type of catalyst has been studied extensively by Evonik.

For example, a typical tail gas treating unit may require 30 MT of tail gas catalyst and a resulting carbon footprint of 660 tons CO₂e. However, by replacing the traditional catalyst with EcoMax TG, the total carbon footprint would be reduced significantly, by nearly 500 tons CO₂e.