3.1. Investigation of H-Oil Vacuum Residue Hydrocracking
The main problems in the ebullated bed vacuum residue hydrocracking operation are sediment formation and the related fouling [
23,
27,
36,
37,
38,
39,
40,
41,
42,
43]. The increase in reaction temperature typically leads to the exponential enhancement of the sediment formation rate and fouling [
42,
43].
Figure 5 presents a graph of the H-Oil atmospheric tower bottom (ATB) sediment content variation (TSE) and that of the partially blended fuel oil (PBFO) after thermal aging (TSP) along with the weighted average bed temperature (WABT) of both H-Oil hydrocracking reactors.
The data in
Figure 5 indicate that irrespective of the WABT enhancement from 414.5 to 430 °C, the ATB TSE did not show any trend toward augmentation. Instead, the ATB TSE remained under the safe limit of about 0.3 wt.%. The reduction in WABT from 430 to 405 °C also showed no effect on the ATB TSE at the end of the study period. This supports the correctness of the decision to replace the cascade mode of fresh catalyst addition with the parallel mode and the optimization of the fresh catalyst addition rate, as discussed in our recent research [
31]. The data shown in
Figure 6 confirm the very low fouling rate registered during this study.
The unusually high PBFO TSP observed on 18 April 2023, 24 April 2023, and 28 April 2023 can be explained by the shutdown of the FCC unit for repairs during the period of 14 April 2023–30 April 2023. The lack of highly aromatic FCC gas oils (LCO, HCO, and SLO) due to the FCC unit shutdown negatively affected the sediment content after the thermal aging of the PBFO (
Figure 5). This was reported in our earlier studies [
23,
27].
Figure 7 shows the graphs of variation of TSE and the TSP of blends of H-Oil VTB obtained at very low severities (WABT of 405 °C); high aromatic FCC LCO, HCO, and SLO; and low aromatic FCCPT diesel with the alteration of diluent concentration. The properties of these four gas oils are shown in
Table S2.
It is obvious from the data in
Figure 7 that the sediment content of the blend of H-Oil VTB–diluent is specific in relationship with the diluent, a finding that was also observed in our recent research [
44]. The FCC HCO turned out to be the most effective diluent related to sediment content reduction, while the low aromatic FCCPT diesel exhibited the highest TSP, confirming that the shutdown of the FCC unit results in PBFO TSP enlargement. It deserves mentioning here that at a higher reaction severity (WABT of 430 °C), the TSE of the blend of H-Oil VTB–FCCPT diesel was about 1.0 wt.% [
44], while at a WABT of 405 °C, as obvious from the data in
Figure 7d, it was about 0.2 wt.%. This suggests that at higher a reaction severity, the H-Oil VTB asphaltenes become less soluble.
The operating variables affecting vacuum residue conversion, hydrodeasphaltization (HDAs) extent, and hydrodemetallization (HDM), such as the reaction temperature and reaction time (
), are summarized in
Table S3.
Using the data from
Table S3 and considering that the conversion depends on the reaction temperature (WABT, °C) and reaction time (
), two models, as mentioned in
Section 2 (Materials and Methods), were developed to relate the vacuum residue conversion to the reaction temperature and reaction time.
The regression equation that predicts the conversion takes the form of Equation (9):
where:
LHSV = liquid hourly space velocity = ; h−1; feed flow rate in m3/h; EBR = ebullated bed reactor volume in m3;
WABT = weighted average bed temperature, °C.
For the dataset in
Table S3, the following kinetic parameters for the CSTR model (Equation (2)) were found:
;
kJ/mol; n = 1.82.
For the dataset in
Table S3, the following kinetic parameters for the plug flow model (Equation (8)) were found:
;
kJ/mol; n = 1.59, following the iteratively minimization of Equation (8) using the differential evolution algorithm (diffevol) in Maple’s Global Optimization Package.
Figure 8 shows the parity graphs for the measured versus the predicted conversion using Equation (2) with the determined kinetic parameters (
;
kJ/mol; n = 1.82) by the use of the regression model (Equation (9)) and Equation (8) (
;
kJ/mol; n = 1.59).
In
Figure 8, the training set is related to development of Equations (2), (8) and (9) with the data in
Table S3. The test set was used to verify Equations (2), (8) and (9) with data not included in the process of equation development.
The statistical parameters used to evaluate the precision of conversion prediction are presented in
Table 4. A total of 121 datasets were used to calculate them. Equation (8) (the plug flow model) seemed to outperform the regression (Equation (9)) and Equation (2) (CSTR model).
The data in
Table S3 along with the data in
Tables S4–S9, which encompass the H-Oil operating conditions, conversions, yields as well as the characteristics of the H-Oil feeds and H-Oil VTBs were evaluated by the use of ICrA. The results of ICrA in terms of the μ- and υ-values with statistically meaningful relationships between the variables (HDAs, HDM, conversion, reactor temperatures, ATB TSE, PBFO TSP, product yields, and H-Oil VTB properties) are summarized in
Tables S10 and S11.
The data in
Tables S10 and S11 indicate that both de-asphaltization degrees (HDAs (C
5) and C
7), hydrodemetallization (HDM), and 540 °C+ conversion had a statistically significant positive association. This implies that the factors contributing to 540 °C+ conversion enhancement contribute to HDAs and HDM enlargement. The data in
Tables S10 and S11 also indicate that the higher the molecular weight of the H-Oil feed, the lower the HDAs and HDM because they have a statistically meaningful negative association. This suggests that higher-molecular-weight H-Oil feeds may impede HDAs and HDM. It is evident from the data in
Tables S10 and S11 that the 540 °C+ conversion and VTB density and the VTB C
5 and C
7 asphaltene contents had a statistically significant positive association, along with a statistically meaningful negative association with VTB molecular weight and T
50%. This means that the magnification of conversion is accompanied by the enhancement of VTB density and VTB C
5 and C
7 asphaltene contents as well as a reduction in VTB molecular weight. This can be explained by the cracking of the H-Oil 540 °C+ feed material, which leads to the production of lower-molecular-weight products like gas, naphtha, diesel, and VGO and leaves a lower-molecular-weight, higher aromaticity unconverted 540 °C+ material, denoted as VTB.
Figure 9 illustrates the relationship between the VTB molecular weight and density and the 540 °C+ conversion.
It is worth mentioning that along with the VTB density augmentation as a consequence of the increase in H-Oil conversion, the gas oils from the H-Oil, which make up the FCC feed, also followed the trend of VTB density enhancement, as illustrated in
Figure 10. The data in
Figure 10 indicate the same slope of the variation of the density of HVGO and LVGO with the VTB density alteration, whereas the HAGO density slope was substantially lower, suggesting that this H-Oil gas oil is less vulnerable to secondary cracking than HVGO and LVGO. Thus, it can be expected that the HAGO conversion observed in the FCC plant is less dependent on the severity of the H-Oil hydrocracker reaction than LVGO and HVGO.
Another interesting fact observed in the data in
Tables S10 and S11 is the relatively strong positive association (μ = 0.89; υ = 0.00) between the VTB sulfur content and the sulfur content of the H-Oil feed. The regression that relates the VTB sulfur content to the H-Oil feed sulfur content for this data set is given in Equation (10):
where:
HOil VTB Sul = Sulfur content in the H-Oil VTB, wt.%;
HOil Feed Sul = Sulfur content in the H-Oil feed, wt.%.
A statistically meaningful weak association between the VTB vanadium content and the VTB sulfur content can be seen in the data in
Tables S10 and S11, suggesting that the higher sulfur content of the H-Oil feeds are more difficult to demetallize.
Figure 11 presents the graphs of the function of the yields of H-Oil gas, naphtha, diesel, VGO, and VTB yields of conversion. It can be observed in the data in
Figure 11 that the yields of gas, naphtha, and diesel, which are primary and secondary stable products [
10], increased, while the VTB yield decreased as conversion increased. The yield of VGO, which is a primary unstable product, should exhibit a decrease as conversion increases beyond 70 wt.%, as shown in our recent investigation [
23]. The presence of a trend toward augmentation with conversion enhancement can be ascribed to the processing of VGO material in the H-Oil feed during the study.
Using the data in
Table 4 and
Tables S4–S9 and employing statistical analyses, the following regression equations for the yields of H-Oil gas and diesel products were established:
Recycle = Recycled PBFO, % of H-Oil feed;
FCC SLO in Feed (%) = Share of FCC SLO in H-Oil feed, % of H-Oil feed;
HOil Feed (360 °C) = Content of fraction boiling below 360 °C in H-Oil feed, % of H-Oil feed;
VGO in H-Oil Feed (%) = Content of VGO material in H-Oil feed, % of H-Oil feed;
Conv = Conversion as calculated by Equation (1).
The remaining yields of naphtha and VTB depended only on conversion by the regression equations embedded in
Figure 11b,e.
It should be noted here that none of the H-Oil catalyst properties not shown in this work due to confidential reasons exhibited any statistically meaningful relationships with sediment formation or fouling rates or with the conversion level or product yields.
3.2. Investigation of Fluid Catalytic Cracking (FCC)
The operating conditions and calculated parameters from the heat balance of the FCC unit for the cases studied are summarized in
Table S12, and the conversion and product yields obtained fromt the FCC unit for cases studied are presented in
Table S13.
Figure 12 plots the variation of the FCC product yields as conversion changed.
Figure 12a indicates that yield of dry gas did not change with the conversion variation, supporting the thermal cracking nature of this product [
45]. The PPF, BBF, and gasoline yields show an increasing trend with increasing conversion, while the LCO and SLO yields exhibited a decreasing trend with conversion augmentation. The HCO yield demonstrated a difficult-to-distinguish trend of reduction as conversion increased. The reason for the relatively large dispersion in the data of the yields of FCC gas oils may be attributed to the various separation efficiencies of the FCC main fractionator, the FCC gasoline stabilizer, and the section of absorption and gas fractionation in this study. This is related to the production of different grades of finished automotive gasoline and diesel fuels.
Tables S14 and S15 present the μ and υ values of ICrA evaluation of the relationships between the FCC performance variables and the H-Oil performance variables. The data in these tables indicate that the H-Oil conversion level showed a statistically meaningful positive association with the density of H-Oil VGO that was processed in the FCC unit; with coke yield, Δ coke; and with FCC HCO yield, as well as a statistically meaningful negative association with the FCC LCO yield.
This implies that increasing the H-Oil conversion is associated with an enhancement of the coke yield, Δ coke, and the FCC HCO yield, along with a decrease in the FCC LCO yield.
The TSP of PBFO showed a statistically meaningful negative association with the FCC HCO yield, meaning that a decrease in the HCO yield is accompanied by an increase of PBFO TSP.
The content of FCC SLO in the H-Oil feed had a statistically meaningful negative association with FCC conversion and a statistically meaningful positive association with the FCC SLO yield. This suggests that an increase in FCC SLO content in the H-Oil feed is accompanied by a decrease in the FCC conversion and an increase inthe FCC SLO yield. This may also imply that the FCC SLO does not convert in the H-Oil unit, and the recycling of this material between the H-Oil and FCC unit occurs. This finding supports Marques et al.’s [
46] observation that the FCC SLO material was refractory to hydroconversion under ebullated bed hydroconversion conditions.
The content of the H-Oil VGO in the FCC feed had a statistically meaningful negative association with FCC conversion and FCC gasoline and a statistically meaningful positive association with the FCC SLO yield. This means that the enhancement of the H-Oil VGO share in the FCC feed is accompanied by FCC conversion reduction, a drop in the gasoline yield, and an increase in the FCC SLO yield.
The FCC conversion was found to depend on only two variables for the studied cases, and these were the content of H-Oil VGO in the FCC feed and the content of FCC SLO in the H-Oil feed. The dependence of FCC conversion on content of H-Oil VGO in the FCC feed and content of FCC SLO in the H-Oil feed is shown in Equation (14).
where:
FCC conv = FCC conversion, wt.%;
FCC SLO in HOil feed (%) = content of FCC SLO in H-Oil feed, wt.% of H-Oil feed;
HOil VGO in FCC feed (%) = content of H-Oil VGO in FCC feed, wt.% of FCC feed.