Jun-2022

Rejuvenated catalysts optimise refinery margins in high severity ULSD applications

Rejuvenated catalysts compare well with fresh catalysts for performance in high pressure ULSD applications with added cost savings.

Ioan-Teodor Trotus and Jean-Claude Adelbrecht, hte
Michael Martinez and Guillaume Vincent, Evonik

Article Summary

Across the world, slowed growth in fuel demand, weak profit margins, strict environmental regulations on CO2 emissions, and legislation on renewable fuels are significantly  impacting the refining industry. As a result, there is a greater need to optimise refining costs and maximise profitability.

The cost of replacing a catalyst at the end of its life is substantial, so it is important to choose an appropriate catalyst. Evonik’s Excel rejuvenated hydrotreating catalyst for ultra-low sulphur diesel (ULSD) applications can help refiners reduce operating costs and maximise profitability while remaining environmentally conscious with their hydrotreating applications.1 In this article, we compare the performance of this catalyst with its fresh counterpart in a parallel test at hte.

What are rejuvenated catalysts?
In a diesel hydrotreatment reactor (ULSD application), catalysts are typically replaced every two to four years, depending on the severity of the unit. ULSD catalysts deactivate mainly due to coke deposition over their lifetime. To recover the activity of the catalyst, coke is removed by carefully burning it under mild oxidative conditions. Referred to as regeneration, the active sites over the catalyst may sinter or agglomerate due to exotherms during this process. Rejuvenation allows for the selective removal of metal contaminant deposits, restoring the spent catalyst’s activity to near fresh.1-2 Excel rejuvenation enables the redispersion of agglomerates on the regenerated catalyst to restore its activity to fresh by utilising a proprietary chemical treatment.

Evonik’s hydroprocessing solutions can achieve the following benefits:
- Reduction of the catalyst refill cost by about 50% compared to fresh catalyst
- Faster catalyst supply compared to long lead times for fresh catalyst
- Better environmental footprint, since these hydroprocessing solutions decrease CO2 emissions, preserve natural resources, and avoid catalyst waste being sent to landfill
- Similar performance compared to fresh catalyst in terms of activity and, more importantly, in terms of stability 

Using Excel rejuvenated catalysts, compared with fresh catalyst production, CO2 emissions are reduced by approximately 6000kg CO2 per ton of fresh catalyst replaced, thereby significantly contributing to the circular economy. Over the last five years, Evonik has successfully supplied more than 8000 tons of Excel rejuvenated catalyst to refineries worldwide, resulting in 48000 tons of CO2 not being emitted to the atmosphere.

To demonstrate the robustness of these rejuvenated catalysts, independent catalyst testing and comparison was performed at hte, the high throughput experimentation company. In this study, different Excel rejuvenated NiMo catalyst configurations were compared with their parent fresh material. The commercial NiMo catalyst is a well-proven, high activity catalyst for producing ULSD at moderate to high pressure when sufficient hydrogen is available to maximise its hydrogenation activity. It is designed to maximise nitrogen and sulphur removal with increased hydrogenation for polyaromatic conversion and volume gain. In addition, the same Excel rejuvenated NiMo catalyst was loaded as a standalone in a commercial ULSD unit. This article also presents commercial data comparing Excel rejuvenated NiMo with a fresh alternative NiMo catalyst to illustrate the stability of the Excel catalysts.

Experimental
The test was performed in an X4500 trickle-bed high throughput test unit at hte’s laboratories in Heidelberg. This state-of-the-art reactor system has consistently proven to be an excellent tool for comparing different catalyst systems head-to-head under identical conditions. In addition, the test unit was equipped with individually heated reactors, allowing for the simultaneous testing of different reactor temperatures.3 Catalyst testing was performed at multiple temperatures while having the same pressure, hydrogen-to-oil ratio, liquid hourly space velocity (LHSV), and hydrogen purity.

The performance of Excel rejuvenated NiMo was compared with its parent fresh catalyst by loading the rejuvenated NiMo catalyst as a standalone or as stacked beds, with either fresh NiMo or Excel rejuvenated CoMo catalyst, as per the loadings shown in Figure 1 and Table 1.

The catalyst volume employed was 2 mL per reactor, with catalyst stacks as small as 0.6 mL used for reactors loaded with 30% rejuvenated and 70% fresh catalyst. The catalysts were tested as full-bodied extrudates, which were sorted by length to select only those with a length shorter than 4mm. The inner diameter of the reactors was 4.8mm.
The feed, a blend of SRGO (50 wt%), LCO (25 wt%), and CGO (25 wt%) from a European refinery diesel hydrotreater, was used to carry out catalyst testing (see Table 2). The experiments were conducted at a hydrogen partial pressure of 75 barg, LHSV = 1.00 h-1 and H2/oil = 609 Nm3/m3 using four different temperatures (see Table 3).

After a dry-out step at 115°C for four hours, a common wetting and sulphiding procedure was implemented, where dimethyl disulphide (DMDS) was added (2.5 wt%) to straight-run gasoil for catalyst activation. This was followed by a line-out period and start-of-run (SOR) temperature conditions. The experiments were designed in such a way that sulphur effluents at different conditions ranged from 10 to 50 ppmwt. The feed for this test was chosen to test the catalysts with very high concentrations of nitrogen and aromatics. For this reason, the gas-to-oil ratio (GTO) was kept at a relatively high level to ensure no more than 30% of the hydrogen introduced was consumed. This precaution was taken to mitigate the concerns of catalyst deactivation due to operating in a hydrogen starved regime.

Excel rejuvenation enables the recovery of equivalent activity to fresh. To detect activity differences of less than 10% in a laboratory test, every aspect of the test — from reactor loading, temperature control, equal feed distribution between reactors to product sample preparation and analyses — has to be carried out with utmost care to minimise all possible errors. For accuracy, it is crucial when comparing catalyst activities to have a good mass balance throughout the experiments. In this case, mass balances for all catalysts compared were in the range of 99.5 ± 1%.

Results and discussion
The fresh and rejuvenated catalysts were tested in parallel at various process conditions while focusing on the parameters below after the hydrotreating reaction:
- Hydrodesulphurisation (HDS) activity
- Hydrodenitrogenation (HDN) activity
- Aromatic saturation and volume swell
- Hydrogen consumption
- C5+ yield