## 4 Discussion
Therefore, a different diesel fraction quantity registration in both cases must be ascribed to a separation of an additional amount of diesel fraction from the H-Oil VGO blend processed in the stabilization column during Case 2. The higher fuel gas consumption in the heater, heating the stabilization column feed during Case 2 suggests a higher extent of evaporation occurring in the stabilization column feed during Case 2, which could be due to a higher amount of low boiling material in the column feed in Case 2. If one calculates the total fluid flow rate through the six passes of the heater from Figure 6, it can be seen that for Case 1, it is 562 m\({}^{3}\)/h, versus 552 m\({}^{3}\)/h for Case 2. Irrespective of the lower total fluid flow rate in the heater in Case 2, the higher fuel gas consumption confirms the suggestion that the stabilization column feed should contain a higher amount of lighter material. The higher total fluid flow rate in Case 1, irrespective of the lower throughput (160 t/h feed rate for the FCCPT in Case 1) than in Case 2 (160 t/h feed rate for the FCCPT reaction section plus 39 t/h H-Oil VGO blend in the stabilization section), can be explained by the higher percent of recycling of stabilized HTVGO through the heater. This is evident from the higher percent of opening the valve providing recycling during the heater for Case 1. The data in Tables 3 and 4 indicate much better agreement between the separated diesel fraction from H-Oil VGO blend (8.7 wt.%) in the FCCPT stabilization column and that measured in the H-Oil VGO blend by ASTM D 7169 (9.6 wt.%) than the measured by ASTM D 1160 diesel fraction content in the H-Oil VGO blend (1.7 vol.%). Having in mind that the density of the diesel fraction (see the data in Table S2) is lower than that of the H-Oil VGO blend, the amount of diesel fraction in H-Oil VGO blend becomes even lower; 1.6 wt.% based on the ASTM D 1160 distillation data. These results support the opinion of [PERSON] et al. [26; 27; 28] that the gas chromatographic method ASTM D 7169 better characterizes heavy oils than the ASTM D 1160 method. The separation efficiency of the FCCPT stabilization column seems to be higher than that of the ASTM D 1160 apparatus, and it is better matched to the ASTM D 7169 method. The data in Table 5 exhibit that the removal of the diesel fraction from the H-Oil VGO has a positive effect on the FCC unit performance leading to increased conversion by 0.53% that along with the increased diesel production in the FCCPT section resulted in improved refinery performance expressed by increased margin with 7.2 MM USD/year. The results shown in Figure S2 (Simulation Case 1) indicate a very good match between the operating data and the simulated data of the stabilization column. This suggests that the characterization of the streams of the stabilization column using ASTM D 86 distillation data for wild naphtha, and FCCP diesel, and the simulated distillation ASTM D 2887 data for the stabilized hydrotreated vacuum gas oil and the applied approach the FCCPT stabilization column operation simulation are correct. The results shown in Figure S4 (Simulation Case 2) show that agreement between the operating data and the simulated data of the stabilization column is not very good, especially for the draw-off of diesel fraction. The draw-off of diesel fraction is simulated to be 17,900 kg/h, while the actual draw-off of the diesel fraction was 20,150 kg/h. This is an indicator for underestimation of diesel fraction content in the feed for the stabilization column, which should be attributed to the lower content of diesel fraction in the heavy oils measured by the ASTM D 1160 method for the H-Oil VGO blend, and the stabilized hydrotreated vacuum gas oil. All attempts to increase the draw-off of diesel fraction like modifying operating conditions of the simulated stabilization column failed to give any positive results. This clearly implies that the column feed characterization is not sufficiently well defined when ASTM D 1160 is used.