## 3 Results and Discussion
### Hydrocracking Experiments  
Before co-processing (i.e., hydrocracking with VGO), the heavy fraction of the distilled FTS waxes (H-FTS) was mixed with VGO in different ratios to analyse sulphur content and viscosity. This point is a key part of the experiment to ensure that the product can be adequately pumped through the unit and has enough sulphur to maintain the catalyst activity [42; 43]. Table 3 shows a basic characterisation of VGO and its mixtures with H-FTS. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Analysis** & **Fossil** & **Mix 1** & **Mix 2** & **Mix 3** & **Mix 4** & **Diesel Fuel Standards EN590** \\  & **Diesel** & **D-FTS:FD** & **D-FTS:FD** & **D-FTS:FD** & **D-FTS:FD** & **Min.** & **Max.** \\ \hline Water content, wt.\% & \textless{}0.01 & \textless{}0.01 & \textless{}0.01 & \textless{}0.01 & \textless{}0.01 & N/A & 0.02 \\ \hline Impurities, mg/kg & 0.81 & 1.19 & 0.62 & 0.83 & 0.84 & N/A & 24 \\ \hline Oxidity stabilisation, h & 28.6 & 29.3 & 23.3 & 23.7 & 29.4 & 20 & N/A \\ \hline Colour ASTM D 1500 & 0.9 & 0.9 & 0.9 & 0.8 & 0.7 & N/A & 2.0 \\ \hline CPFF & \(-12\) & \(-10\) & \(-8\) & \(-1\) & 2 & N/A & \({}^{*}\) \\ \hline \hline \end{tabular} \({}^{*}\)Summer diesel (max. 0 \({}^{\circ}\)C), transition diesel (max. \(-10\) \({}^{\circ}\)C), winter diesel (max. \(-20\) \({}^{\circ}\)C).  
\end{table} 
Table 2: _Cont._  
Figure 3: Properties of fossil diesel and diesel fractions of Fischer-Tropsch waxes (D-FTS) and their linear correlation: (**a**) D-FTS addition (wt.%) vs. density at 15 \({}^{\circ}\)C (kg/m\({}^{3}\)); (**b**) D-FTS addition (wt.%) vs. CFPP (\({}^{\circ}\)C). Table 3 reveals the importance of the VGO:H-FTS ratio in co-processing. A 50:50 wt.% ratio was found to meet the viscosity (-5.5 mm\({}^{2}\)/s) and sulphur content (>0.5 wt.%) requirements while also maximising the H-FTS percentage in the feed. This ratio was then co-processed at industrial hydrocracking conditions for over 36 days at two different WHSV's (i.e., 0.5-1.0 h\({}^{-1}\)). Figure 4 provides a comprehensive view of the density, Simdis, sulphur content, and nitrogen content of the hydrocracking products obtained during the VGO:H-FTS co-processing. There is a direct correlation between WHSV and catalyst activity. During the initial 500 h of the experiment (T = 420 \({}^{\circ}\)C, WHSV = 1.0 h\({}^{-1}\), P = 10.0 MPa), the catalyst activity steadily declined. This led to hydrocracking products with increased density, sulphur content, nitrogen content, and a lower yield of diesel fraction compounds. To restore catalyst activity, WHSV was reduced from 1.0 to 0.5 h\({}^{-1}\). This operational change significantly enhanced the catalyst activity from TOS 504 h, resulting in a hydrocracking product with lower product density (Figure 4a), lower sulphur and nitrogen content (S: <10 mg/kg; N: <5 mg/kg--Figure 4b), and a higher diesel fraction (BP: 180-360 \({}^{\circ}\)C = 67.1 wt.%--Figure 4c). A mass balance of products, along with product distillation, was performed to evaluate the quality of the diesel fraction of the hydrocracked product. Figure 5 presents the product distribution for each operational condition. The decrease in WHSV throughout the experiment led to a significant increase in the percentage of diesel boiling point products, from 51.4 to 67.1 wt.%. This point was accompanied by a notable decrease in the undesired heavy fraction (Figure 5). Thus, this finding can be attributed to the longer contact time between the feedstock (VGO:H-FTS) and the hydrocracking catalysts, which in turn seems to enhance the catalyst activity. Therefore, it can be concluded that a lower WHSV is conducive to the production of transportation fuels in the diesel range, aligning with the research's objective. The gases from the reactor outlet had a similar composition for both WHSVs. Hydrogen (72.5 wt%) was the main compound, followed by methane (8.1 wt%), propane (3.4 wt%), n-butane (3.1 wt%), iso-pentane (2.3 wt%), and ethane (1.4 wt%).