Apr-2025

Impact of hydrocracker catalyst changes on crude oil selection

Produce high-quality, on-spec base oils from a greater variety of crude oils, including those previously considered the lower threshold for crude qualBase oil production requires crude oil with a high paraffinic content, which restricts the choice of crude oil types. Crude oil rich in paraffins is more expensive compared to other varieties.

Bilge Karahan, Merve Çinbar and Nilay Aktaş Tupraş
Peter Nymann Topsoe

Article Summary

Therefore, oil refineries must gain flexibility in crude oil selection to increase the profit margin. Several methods can be adopted to address this limitation, and co-processing of unconverted oil (UCO) from the hydrocracker is one of the best practices.

Topsoe has supplied hydrocracking catalysts involving several cycles for Tüpraş. In the most recent cycle, the installed catalyst system was modified to provide a higher viscosity index (VI) of the UCO to potentially enable use in the refinery base oil plant. Using high VI UCO from the hydrocracker provides better quality base oil production and enables the use of a wider range of crudes.

When processing crude oil with low paraffin content at the refinery, the base oil fails to meet the minimum VI specification. To improve VI of the base oil, UCO co-processing has proven to be an effective solution. Co-processing of a higher VI UCO enables greater flexibility in crude oil selection. Besides improving VI values, co-processing UCO also boosts the yield of base oil products. Base oil production includes several modes, namely spindle oil, light neutral, heavy neutral, and bright stock modes. In the heavy neutral mode, UCO co-processing is required to achieve the desired VI specifications. For the light neutral mode, co-processing with UCO enhances yield improvements. Conversely, UCO cannot be co-processed in spindle and bright stock modes due to the significant difference in viscosities (at 100°C) between UCO and these products.

Base oil process configuration
The base oil complex in İzmir Refinery was designed for the production of Group I base oils consisting of four different grades: spindle oil, light neutral oil, heavy neutral oil, and bright stock oil. The complex (see Figure 1) consists of a vacuum distillation unit (VDU), propane deasphalting unit (PDU), furfural extraction unit (FEU), MEK-toluene dewaxing unit (MDU), and a hydrofinishing unit (HFU). VDU and PDU are running in continuous mode. FEU, MDU, and HFU operate in batch mode for different products.  
The production of Group I base oils begins with vacuum gasoil (VGO), a heavy product derived from the crude unit. Impurities, ranging from 50-80%, are removed using solvents. However, the treated base oil still contains paraffins, which need to be eliminated. This is achieved through a dewaxing process, where solvents cool the oil to low temperatures, causing the wax to solidify. The key refining steps in Group I base oil production are vacuum distillation to remove metals and heavy asphaltic compounds, separating the oil into different viscosity fractions. Deasphalting eliminates asphaltenes, which can form carbon deposits when the oil is exposed to heat. The deasphalting unit is used when the lube plant produces bright stock.

The furfural extraction unit removes aromatic compounds, particularly multi-ring aromatics, to enhance thermal stability and VI. The MEK-toluene dewaxing unit removes wax to improve cold flow properties such as pour point. The hydrofinishing unit improves the colour and stability of the final base oil.
It is always better to decrease reliance on crude oil quality in base oil production, which is why UCO from the hydrocracker (Figure 1) is injected before the furfural extraction unit when the base oil plant operates in light and heavy neutral mode.

Hydrocracker process configuration
The hydrocracking unit converts VGO into middle distillate products (diesel fuel, jet fuel, and kerosene), naphtha, and LPG while simultaneously performing treating, hydrogenation, and cracking reactions. Hydrocracking is carried out at elevated temperatures and pressures in a hydrogen-rich atmosphere. The study was conducted on the refinery’s hydrocracking unit with a single-stage reactor plus recycle system. The recycle line from the fractionator bottom is normally not in operation. Consequently, UCO is not directed back to feed inlet or a secondary cracking reactor for further processing.

Performance comparison
Topsoe has been supplying catalyst systems for the hydrocracker for some of the cycles over the last decade of operation. Production of UCO for lube base oil has been evaluated in previous cycles, but catalyst selection has focused more on other unit objectives. UCO has, therefore, not been as suitable for this purpose in previous cycles. Previous cycles in the hydrocracker have provided UCO with VI in the range of 100-110, which is not significantly higher than what is achieved from the crudes normally selected for direct lube production.

Proposed catalyst solution
Objectives for the hydrocracking unit have changed from cycle to cycle, and the catalyst selections made by the refinery have reflected these changing objectives. For the 2024 cycle, a catalyst system with enhanced hydrogenation activity was proposed to specifically target hydrogenation of the high boiling range of the feedstock. High hydrogenation activity results in higher aromatic saturation and opening naphthenic rings, making product fractions more paraffinic. The high UCO paraffin content results in higher VI, making it a more valuable addition to the base oil pool.

Figure 2 shows the VI obtained in the 2024 cycle together with the VI obtained in previous cycles. As can be seen, the VI is significantly higher than obtained in the previous cycles using Topsoe catalyst systems. The high VI makes it a very suitable blending component in the lube base oil production.

In one of the past cycles, a catalyst system with less focus on the UCO viscosity properties was installed. However, in 2024, one of the main objectives was obtaining UCO with high VI in order to use it as a component in the lube base oil production, as mentioned earlier. Comparison of UCO viscosities obtained in the two different cycles, therefore, serves as a good illustration of how the catalyst loading can be tailored to achieve certain properties.

During normal operation, the feed quality to the unit changes as the crude supply to the refinery changes. Furthermore, operating conditions such as feedstock quality and fractionator cut points change. These changes cause variations in UCO product quality. Heavier feedstocks with higher viscosity lead to higher viscosity UCO at the same constant conversion. Higher cut points between diesel and UCO also lead to higher viscosities of the products. Higher conversion in the unit, however, leads to lower viscosity.
VI of the UCO behaves in a somewhat similar manner. Some feed properties and some operating conditions improve the VI. Therefore, it can be a challenge to compare the UCO quality from cycle to cycle and even from run day to run day. A good way of comparing data is to plot the viscosity at two different temperatures against each other. On the same plot, the corresponding viscosity at each temperature resulting in a VI of, for example, 115, 125, and 135 may be plotted as ‘iso-VI’ lines. This way, it can be clearly illustrated how different lube base oil qualities can be obtained from different catalyst systems. Alternatively, this type of plot may be used to identify the effect of different feedstocks or operating scenarios.

Figure 3 shows the previously mentioned viscosity plot from the two different cycles with Topsoe catalyst systems, including the ‘iso-VI’ curves. It is obvious from the plot that the catalyst system loaded in the 2024 cycle provides a significant improvement to the UCO VI compared to the Topsoe catalyst system loaded in one of the previous cycles.

The refinery produces a wide range of base oils for different applications characterised by having different viscosity ranges to fit the many applications. Each base oil fraction is made up of different components that, when combined, obtain the desired properties. Target components for each base oil product, therefore, have viscosities within the specification or just outside.