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What could be done for de-aromatization of uncoverted oil (UCO or off test) from hydrocraker unit? Could hydrotreating be effective or not? In the other words, is it possible to saturate PNA's (which they are in the UCO) by hydrotreating?

Jul-2024

Answers

Jake Gotham, InSite Technical Services, jake.gotham@insitetechnical.com

In principle, as Mr de Silva says, PNAs can be significantly reduced by hydrotreating assuming sufficiently aggressive conditions are achieved. The hydrocracker itself is a very severe hydrotreater, with high operating pressure, low space velocity and high activity catalyst. Hence to further reduce the PNAs in a downstream hydrotreater will not be easy, but it can be done. A large capital investment is required to achieve this, and there is rarely an economic case to do this in a fuels refinery.

Further hydroprocessing of hydrocracker UCO is a standard feature of modern lubes production. The UCO is fed first to a dewaxing reactor then to a hydrofinishing reactor. The hydrofinishing reactor achieves almost complete saturation of the PNAs, but this requires a large, high pressure reactor operating with noble metal catalyst.
Alternatively, technology exists to remove PNAs from UCO by carbon absorption.

Jul-2024
Marcio Wagner da Silva, Petrobras, marciows@petrobras.com.br

Polynuclear aromatics compounds (PNA) are refractory to hydrotreating reactions due to their molecular arrangement and chemical stability, demanding severe and specific design and operating conditions to reach good saturation of these molecules. It's totally possible to reach a good performance in the PNA saturation in hydroprocessing units, but as previously mentioned it's necessary to supply adequate conditions for this.

Normally, the hydrotreating technology licensors of processing units dedicated to process high PNA content recommends the operation under high hydrogen partial pressure (higher than 55 kgf/cm2) and high activity catalyst like Ni-Mo which presents a high hydrogenation potential that will lead to a good performance to aromatics saturation as well as high WABT (Weighted Average Bed Temperature). It's important to taking into account that the aromatics saturation is limited thermodynamically and there is a temperature limit where the aromatics saturation performance will achieve the equilibrium and the performance will be not improved simply raising the WABT, at this point raise the temperature will only cause a severe deactivation of the catalyst. Under this scenario, if possible the refiner can try to raise the hydrogen partial pressure to supply better thermodynamic conditions and displace the aromatics saturation equilibrium.

In summary, it's totally possible to reach Polynuclear Aromatics Saturation in hydrotreaters, as long as adequate operating conditions in the processing unit.

Jul-2024