Hydrogen Pressure as a Key Parameter to Control the Quality of the Naphtha Produced in the Hydrocracking of an HDPE/VGO Blend

Citation: Vela, F.J.; Palos, R.; Bilbao, J.; Arandes, J.M.; Gutiérrez, A. Hydrogen Pressure as a Key Parameter to Control the Quality of the Naphtha Produced in the Hydrocracking of an HDPE/VGO Blend. Catalysts 2022,12, 543. https://doi.org/10.3390/ catal12050543 Academic Editors: Hugo de Lasa and Mohammad Mozahar Hossain Received: 22 April 2022 Accepted: 13 May 2022 Published: 16 May 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). catalysts Article Hydrogen Pressure as a Key Parameter to Control the Quality of the Naphtha Produced in the Hydrocracking of an HDPE/ VGO Blend Francisco J. Vela 1, Roberto Palos 1,2 , Javier Bilbao 1, JoséM. Arandes 1and Alazne Gutiérrez 1,* 1Department of Chemical Engineering, University of the Basque Country UPV/EHU, P.O. Box 644, 48080 Bilbao, Spain; franciscojavier[email protected] (F.J.V.); [email protected] (R.P.); javier[email protected] (J.B.); [email protected] (J.M.A.) 2Department of Chemical and Environmental Engineering, University of the Basque Country UPV/EHU, Plaza Ingeniero Torres Quevedo 1, 48013 Bilbao, Spain *Correspondence: alazne.gutierr[email protected] Abstract: The hydrocracking of high-density polyethylene (HDPE) blended with vacuum gas oil (VGO) has been studied to assess the effect of H 2 pressure on the yield and composition of the products and with the aim of obtaining commercial fuels, mainly naphtha. The experiments have been performed using a PtPd/HY catalyst in a semibatch reactor under the following conditions: H 2 pressure, 20–110 bar; 440 ◦ C; catalyst to feed ratio, 0.1 g cat (g feed ) −1 ; HDPE to total feed ratio, 0.2 g HDPE (g feed ) −1 ; and reaction time, 2 h. The composition of the main fractions produced (gas, naphtha, and light cycle oil) reveals the interest in carrying out the process at 110 bar. Thus, conversions of 96 and 79% for the removal of heavy hydrocarbons and the removal of HDPE molecules have been obtained, respectively, together with a yield of naphtha of 53.4 wt%. This naphtha is mainly paraffinic, and it has a RON of 91.5 (within the commercial standards). Furthermore, three fractions have been observed in the analysis (temperature-programmed oxidation) of the coke. This analysis reveals that at 110 bar, the coke retained in the HY zeolite cages is less developed and burns at a moderate temperature. Keywords: hydrocracking; HDPE; VGO; pressure; hydrogen; fuel; naphtha; medium distillates; coke 1. Introduction Hydrocracking is a key process in refineries for the conversion of heavy oil fractions into low-molecular-weight streams, at the same time undesirable molecules are removed, in particular S, N, and metals. Thus, after subsequent reforming and conditioning stages, hydrocracked streams are suitable for being used in the blending of commercial naphtha and medium distillates [ 1 , 2 ]. In addition, the versatility of the hydrocracking process places it at the forefront of the intensification processes that aim for the direct production of fuels with the composition established by legal requirements [ 3 ]. This way, hydrocracking processes have been widely used in the valorization of different secondary refinery streams, such as vacuum residue [ 4 ], light cycle oil [ 5 ], and vacuum gas oil [ 6 ]. In addition, its use has also been proposed for treating bio-oil [ 7 ], which is a liquid product obtained in the fast pyrolysis of lignocellulosic biomass, as well as wastes from the consumers’ society (mainly waste plastics and end-of-life tires) and the oil obtained in the pyrolysis of these wastes [ 8 , 9 ]. The co-feeding of these wastes to refinery units (Waste-Refinery) is an innovative management strategy that is receiving increasing attention in the literature [ 10 ]. The Waste-Refinery proposal has the attraction of bringing the Circular Economy strategy closer to the oil industry, which will entail important savings in raw materials and financing the inventory required for the valorization of wastes. In parallel, it will contribute to solving the environmental issues derived from landfilling [ 11 ] or incineration [ 12 ] of these wastes. Catalysts 2022,12, 543. https://doi.org/10.3390/catal12050543 https://www.mdpi.com/journal/catalysts Catalysts 2022,12, 543 2 of 17 However, the industrial scale-up of this strategy requires establishing the appropriate operating conditions and adapting the catalysts for this specific goal. The hydrocracking of waste plastic into fuels has received less attention than other thermochemical processes, such as thermal and catalytic cracking. Thermal cracking is particularly interesting for the recovery of monomers from styrene [ 13 ], polymethyl methacrylate (PMMA) [ 14 ], and polyolefins [ 15 ]. In contrast, catalytic cracking over zeolite-based catalysts (HZSM-5 being the most studied one) is interesting for the selective production of light olefins [ 16 ]. Nonetheless, under the conditions required for the production of fuels (high temperatures and high contents of strong acid sites in the zeolite), the composition of the liquid product obtained presents high contents of olefins and aromatics that impede the direct use of this liquid as automotive fuel [ 17 ], requiring a subsequent adaptation of its composition by hydroprocessing. The hydrocracking of polyolefins (LDPE, HDPE, and PP) over a Pt/Beta catalyst at 330 ◦ C and 20 bar H 2 resulted in an excessive production of gases (C 3 –C 4 ) [ 18 ], which is attributed to the diffusional limitations and overcracking of the macromolecules. Nevertheless, the use of Pt catalysts supported on USY zeolites increased remarkably the yield of naphtha (C5–C12) and medium distillates (C13–C20) [19]. The co-feeding of plastic together with a refinery stream, such as VGO (benchmark feed of hydrocracking units), shows important advantages [ 9 ]. Firstly, it facilitates the extent of the hydrocracking reactions, increasing the level of conversion attained. Secondly, the presence of HDPE in the reaction medium leads to the production of a less aromatic and more paraffinic naphtha fraction. Moreover, a previous work [ 20 ] investigated the effect of the temperature on the performance of a PtPd/HY catalyst (activity and selectivity) in the hydrocracking of HDPE blended with VGO (20/80 in mass) and found that this catalyst was appropriate for maximizing the yield of naphtha at 420 ◦ C. In addition, this temperature was appropriate for achieving high rates in the reactions that lead to the formation of olefins from the HDPE (by means of a free radical mechanism) and to their subsequent protonation. The synergistic effects between the hydrocracking mechanisms entailed an increase in the conversion achieved with respect to that obtained in the hydrocracking of neat VGO. The composition of the naphtha lump obtained in the hydrocracking of the HDPE/VGO blend was also more favourable for being added to the lump of commercial gasoline since it had an important concentration of paraffins and a low concentration of aromatics. Equally, in the LCO fraction obtained in the hydrocracking of the blend at 420 ◦ C, the concentration of paraffins is higher than that obtained in the hydrocracking of VGO, whereas that of aromatics is lower. It means a higher interest in being valorized within the diesel pool. In this context, this work assesses the effect of the H 2 pressure used in the hydrocracking of a blend of HDPE and VGO since this parameter will undoubtedly condition the economy of the process. Even though the relevant effect of this variable on the yields and product distribution has been widely demonstrated [ 21 ], few authors have tackled its direct study in the hydrocracking of waste plastics. The aim of the work is to obtain detailed information about the effect of H 2 pressure on the yields, selectivities, and composition of the fractions with commercial interest obtained in the hydrocracking of the HDPE/VGO blend. For this purpose, the products have been lumped into different fractions as is customary in the oil industry: heavy cycle oil (HCO), light cycle oil (LCO), naphtha, liquefied petroleum gas (LPG), dry gas, and coke. Furthermore, in view of the importance of catalyst deactivation by coke deposition, the effect of H 2 pressure on the content and nature of the coke has also been studied. 2. Results and Discussion 2.1. Catalyst Properties The main characterization results have been summed up in Table 1. A more detailed description of the techniques, equipment, and experimental procedures used in catalyst characterization can be found in previous work [ 20 ]. The channel structure of the Y zeolite Catalysts 2022,12, 543 3 of 17 can be seen in the TEM image at 100 nm (Figure 1a), while in the image at 50 nm (Figure 1b), the good dispersion of Pt and Pd particles (dark spots) on the support can be observed. Table 1. Textural properties, chemical composition, and acidic properties of the PtPd/HY catalyst. Property PtPd/HY SBET (m2g−1)620 Smicropore (m2g−1)543 Smesopore (m2g−1)77 Vpore (cm3g−1)0.39 dpore (nm) 8.44 Pt (wt%) 1.19 Pd (wt%) 0.53 AT(mmolt-BA g−1)11.69 AS(kJ molt-BA−1)2135 B/L ratio 1.53 1Total acidity; 2average acidic strength. Catalysts 2022, 12, 543 3 of 18 2. Results and Discussion 2.1. Catalyst Properties The main characterization results have been summed up in Table 1. A more detailed description of the techniques, equipment, and experimental procedures used in catalyst characterization can be found in previous work [20]. The channel structure of the Y zeolite can be seen in the TEM image at 100 nm (Figure 1a), while in the image at 50 nm (Figure 1b), the good dispersion of Pt and Pd particles (dark spots) on the support can be observed. Table 1. Textural properties, chemical composition, and acidic properties of the PtPd/HY catalyst. Property PtPd/HY SBET (m2 g−1) 620 Smicropore (m2 g−1) 543 Smesopore (m2 g−1) 77 Vpore (cm3 g−1) 0.39 dpore (nm) 8.44 Pt (wt%) 1.19 Pd (wt%) 0.53 AT (mmolt-BA g−1) 1 1.69 AS (kJ molt-BA−1) 2 135 B/L ratio 1.53 1 Total acidity; 2 average acidic strength. (a) (b) Figure 1. Transmission electron microscopy (TEM) at 100 nm (a), and at 50 nm (b) of fresh PtPd/HY catalyst. 2.2. Hydrocracking Yields and Conversion The effect of H2 pressure on the yields of products and the extent of conversion attained is depicted in Figure 2. Overall, an increase in the H2 pressure favours the conversion of both HDPE and HCO, as well as the formation of light products, mainly naphtha and gases. In addition, an important reduction in the yield of coke should be highlighted that passes from 2.4 wt% at 20 bar to a yield of 1.1 wt% at 110 bar. Figure 1. Transmission electron microscopy (TEM) at 100 nm ( a ), and at 50 nm ( b ) of fresh PtPd/ HY catalyst. 2.2. Hydrocracking Yields and Conversion The effect of H 2 pressure on the yields of products and the extent of conversion attained is depicted in Figure 2. Overall, an increase in the H 2 pressure favours the conversion of both HDPE and HCO, as well as the formation of light products, mainly naphtha and gases. In addition, an important reduction in the yield of coke should be highlighted that passes from 2.4 wt% at 20 bar to a yield of 1.1 wt% at 110 bar. The yield obtained at 20 bar for the HCO fraction was 25.2 wt%, which corresponds to a conversion level of 67%. With regard to the yield of products, a balanced distribution has been obtained for the yield of gas, naphtha, and LCO fractions (16, 18, and 23 wt%, respectively). An increase in the H 2 pressure up to 50 bar has not affected the HCO conversion, but it has affected product distribution. The yields of naphtha and gas fractions have increased (up to 31 and 17 wt%, respectively) to the detriment of LCO (12 wt%). A further increase in the H 2 pressure to 80 bar has revealed the important role of this parameter. This way, the HCO conversion has increased up to 73%, and the yields of both HCO and LCO fractions have decreased down to 9 and 21 wt%, respectively. Consequently, Catalysts 2022,12, 543 4 of 17 the yields of the light fractions have continued increasing, reaching a value of 37 wt% for the naphtha and 25 wt% for the gas. Finally, at the highest working H 2 pressure (110 bar), the lowest yields of LCO and HCO fractions have been obtained, which correspond to almost total HCO conversion (94%). Attending to the yields of products, naphtha is by far the main one (53 wt%), even though an important production of gases has also been obtained (30 wt%). Catalysts 2022, 12, 543 4 of 18 Figure 2. Effect of H2 pressure on product yields and conversions. The yield obtained at 20 bar for the HCO fraction was 25.2 wt%, which corresponds to a conversion level of 67%. With regard to the yield of products, a balanced distribution has been obtained for the yield of gas, naphtha, and LCO fractions (16, 18, and 23 wt%, respectively). An increase in the H2 pressure up to 50 bar has not affected the HCO conversion, but it has affected product distribution. The yields of naphtha and gas fractions have increased (up to 31 and 17 wt%, respectively) to the detriment of LCO (12 wt%). A further increase in the H2 pressure to 80 bar has revealed the important role of this parameter. This way, the HCO conversion has increased up to 73%, and the yields of both HCO and LCO fractions have decreased down to 9 and 21 wt%, respectively. Consequently, the yields of the light fractions have continued increasing, reaching a value of 37 wt% for the naphtha and 25 wt% for the gas. Finally, at the highest working H2 pressure (110 bar), the lowest yields of LCO and HCO fractions have been obtained, which correspond to almost total HCO conversion (94%). Attending to the yields of products, naphtha is by far the main one (53 wt%), even though an important production of gases has also been obtained (30 wt%). Additionally, a remarkable positive effect of the H2 pressure on the HDPE conversion attained can be observed in Figure 2. Thus, an increase in the H2 pressure entails a linear increase in the HDPE conversion. This way, it has gone from a relatively moderate value of 24% for 20 bar to a very attractive conversion level of 79% at 110 bar. The product distribution obtained is reflected in the evolution of the selectivity to fuel index (Figure 3). Therefore, an increase in the H2 pressure from 20 to 80 bar brings a stepwise increase in this index from 0.7 to 0.8 because of the increase in the yield of naphtha and the decrease in the yields of HCO and wax. In addition, the abrupt change that has taken place in product distribution when increasing the H2 pressure from 80 to 110 bar (Figure 2) has caused a sharp increase in the selectivity to fuel index reaching its maximum value of 1.5. 20 50 80 110 0 20 40 60 80 100 0 20 40 60 80 100 Conversion (%) Yield (wt%) Pressure (bar) Gas Naphtha LCO X HCO HCO Wax Coke X HDPE Figure 2. Effect of H2pressure on product yields and conversions. Additionally, a remarkable positive effect of the H 2 pressure on the HDPE conversion attained can be observed in Figure 2. Thus, an increase in the H 2 pressure entails a linear increase in the HDPE conversion. This way, it has gone from a relatively moderate value of 24% for 20 bar to a very attractive conversion level of 79% at 110 bar. The product distribution obtained is reflected in the evolution of the selectivity to fuel index (Figure 3). Therefore, an increase in the H 2 pressure from 20 to 80 bar brings a stepwise increase in this index from 0.7 to 0.8 because of the increase in the yield of naphtha and the decrease in the yields of HCO and wax. In addition, the abrupt change that has taken place in product distribution when increasing the H 2 pressure from 80 to 110 bar (Figure 2) has caused a sharp increase in the selectivity to fuel index reaching its maximum value of 1.5. To explain these results, the effect of the H 2 pressure on the hydrocracking mechanism must be analysed in detail. It is well established that in the hydrocracking of hydrocarbons and polyolefins, the cracking takes place via carbenium ions. Moreover, under the reaction conditions employed (420 ◦ C), the extent of thermal cracking reactions, with free radicals as intermediates, also has to be considered [ 21 ]. Afterwards, the hydrocarbons are dehydrogenated over the metal sites producing olefins, which are subsequently protonated and isomerised over the acid sites. In addition, if the catalyst has a sufficient density of strong acid sites (as occurs with HY zeolites), the cracking takes place by β -scission of the protonated and branched olefins, forming unsaturated species that are, in turn, saturated over the hydrogenation sites [ 22 , 23 ]. In the hydrocracking of polyolefins, the presence of strong acidic sites favours the formation of tertiary carbenium ions that are cracked by β -scission reactions resulting in olefins and carbenium ions of lower molecular weight, which are also cracked in subsequent stages. The olefins formed are protonated to paraffins Catalysts 2022,12, 543 5 of 17 by metathesis reactions in the presence of H 2 in the reaction medium [ 24 , 25 ]. There is also evidence of the existence of synergistic effects between the thermal and catalytic hydrocracking mechanisms because the former leads to the formation of molecules that are more reactive in the latter [26,27]. Catalysts 2022, 12, 543 5 of 18 Figure 3. Effect of H2 pressure on selectivity to fuel index. To explain these results, the effect of the H2 pressure on the hydrocracking mechanism must be analysed in detail. It is well established that in the hydrocracking of hydrocarbons and polyolefins, the cracking takes place via carbenium ions. Moreover, under the reaction conditions employed (420 °C), the extent of thermal cracking reactions, with free radicals as intermediates, also has to be considered [21]. Afterwards, the hydrocarbons are dehydrogenated over the metal sites producing olefins, which are subsequently protonated and isomerised over the acid sites. In addition, if the catalyst has a sufficient density of strong acid sites (as occurs with HY zeolites), the cracking takes place by β-scission of the protonated and branched olefins, forming unsaturated species that are, in turn, saturated over the hydrogenation sites [22,23]. In the hydrocracking of polyolefins, the presence of strong acidic sites favours the formation of tertiary carbenium ions that are cracked by β-scission reactions resulting in olefins and carbenium ions of lower molecular weight, which are also cracked in subsequent stages. The olefins formed are protonated to paraffins by metathesis reactions in the presence of H2 in the reaction medium [24,25]. There is also evidence of the existence of synergistic effects between the thermal and catalytic hydrocracking mechanisms because the former leads to the formation of molecules that are more reactive in the latter [26,27]. The aforementioned characteristics of the mechanisms highlight the role of the H2 pressure, which attenuates the formation of free radicals and favours the extent of the hydrocracking of hydrocarbons and polyolefins. Studies of the effect of H2 pressure on the hydrocracking of different polyolefins have shown that, in general, increasing the pressure of H2 increases the conversion and the yield of liquid products. However, this effect is progressively smaller as the pressure of H2 increases [24,28–30]. The effect on the gas yield is uneven, although it decreases with pressure. The explanation is that an increase in the H2 pressure favours the catalytic hydrocracking mechanism with respect to the thermal one, to which the attenuation of the formation of free radicals in the presence of H2 contributes. In addition, the rates of the olefins cyclization and polyaromatics condensation reactions are also attenuated at high H2 pressures, leading to a lesser coke formation [31]. A similar result was obtained by Akah et al. [30] in the hydrocracking of postconsumer plastics. Consequently, the lower catalyst deactivation may explain some of the results displayed in Figure 2 that can be considered unexpected. Some of them would be the constant gas production observed when increasing the H2 pressure from 20 to 50 bar [28], as well as the product distribution that has remained unaltered [32]. Apart from the 20 50 80 110 0.6 0.8 1.0 1.2 1.4 1.6 Pressure (bar) Selectivity to fuel index Figure 3. Effect of H2pressure on selectivity to fuel index. The aforementioned characteristics of the mechanisms highlight the role of the H 2 pressure, which attenuates the formation of free radicals and favours the extent of the hydrocracking of hydrocarbons and polyolefins. Studies of the effect of H 2 pressure on the hydrocracking of different polyolefins have shown that, in general, increasing the pressure of H 2 increases the conversion and the yield of liquid products. However, this effect is progressively smaller as the pressure of H 2 increases [ 24 , 28 – 30 ]. The effect on the gas yield is uneven, although it decreases with pressure. The explanation is that an increase in the H 2 pressure favours the catalytic hydrocracking mechanism with respect to the thermal one, to which the attenuation of the formation of free radicals in the presence of H2contributes. In addition, the rates of the olefins cyclization and polyaromatics condensation reactions are also attenuated at high H 2 pressures, leading to a lesser coke formation [ 31 ]. A similar result was obtained by Akah et al. [ 30 ] in the hydrocracking of postconsumer plastics. Consequently, the lower catalyst deactivation may explain some of the results displayed in Figure 2that can be considered unexpected. Some of them would be the constant gas production observed when increasing the H 2 pressure from 20 to 50 bar [ 28 ], as well as the product distribution that has remained unaltered [ 32 ]. Apart from the attenuation of the deactivation, the side effects of the partial pressure of H 2 that strongly affect the composition of the products will contribute to the difficult interpretation of the results. Therefore, their reactivity in cracking and hydrogenation reactions will be different. Other factors that will condition the results depicted in Figure 2will be (i) the different diffusivity of the reaction mixture in the different hydrocracking stages, which will be crucial for the HDPE-derived macromolecules and the heavy molecules within the HCO fraction of the VGO, and (ii) the synergistic effects in the hydrocracking mechanisms between the VGO and the HDPE-derived molecules. Catalysts 2022,12, 543 6 of 17 2.3. Composition of the Gas Fraction The effect of the H 2 pressure on the composition of the gas fraction is depicted in Figure 4. The gas fraction is composed of hydrocarbons between one and four carbon atoms (CH 4 , C 2 H 4 , C 2 H 6 , C 3 H 6 , C 3 H 8 , i-C 4 H 10 , C 4 H 8 , and n-C 4 H 10 ). One should note that an H 2 pressure of 20 bar is not high enough for saturating all the light olefins. Indeed, hydrogenation is an exothermic reaction favoured at low temperatures and high H 2 pressures. Therefore, small concentrations of ethylene, propylene and butylenes (0.4, 6.9 and 4.7 wt%, respectively) have been detected in the gas fraction. Splitting the gas fraction into dry gas (C 1 –C 2 ) and LPG (C 3 –C 4 ), the concentration of the latter is by far higher as it accounts for 68.9 wt%, its main compounds being propane and n-butane (25.5 and 23.6 wt%, respectively). Regarding the composition of the dry gas fraction, it is almost totally composed of methane and ethane (13.5 and 17.1 wt%, respectively). Catalysts 2022, 12, 543 6 of 18 attenuation of the deactivation, the side effects of the partial pressure of H2 that strongly affect the composition of the products will contribute to the difficult interpretation of the results. Therefore, their reactivity in cracking and hydrogenation reactions will be different. Other factors that will condition the results depicted in Figure 2 will be (i) the different diffusivity of the reaction mixture in the different hydrocracking stages, which will be crucial for the HDPE-derived macromolecules and the heavy molecules within the HCO fraction of the VGO, and (ii) the synergistic effects in the hydrocracking mechanisms between the VGO and the HDPE-derived molecules. 2.3. Composition of the Gas Fraction The effect of the H2 pressure on the composition of the gas fraction is depicted in Figure 4. The gas fraction is composed of hydrocarbons between one and four carbon atoms (CH4, C2H4, C2H6, C3H6, C3H8, i-C4H10, C4H8, and n-C4H10). One should note that an H2 pressure of 20 bar is not high enough for saturating all the light olefins. Indeed, hydrogenation is an exothermic reaction favoured at low temperatures and high H2 pressures. Therefore, small concentrations of ethylene, propylene and butylenes (0.4, 6.9 and 4.7 wt%, respectively) have been detected in the gas fraction. Splitting the gas fraction into dry gas (C1–C2) and LPG (C3–C4), the concentration of the latter is by far higher as it accounts for 68.9 wt%, its main compounds being propane and n-butane (25.5 and 23.6 wt%, respectively). Regarding the composition of the dry gas fraction, it is almost totally composed of methane and ethane (13.5 and 17.1 wt%, respectively). Figure 4. Effect of H2 pressure on the composition of the gas fraction. An increase in the pressure up to 50 bar brings a marked change in the composition of the gas fraction, led by an almost total olefins disappearance and a boosting of the isomerization reactions. Furthermore, the yield of the dry gas fraction is also reduced at the same time as C3 and C4 products became the main ones. Indeed, the lowest concentration of the dry gas fraction has been attained at this pressure reaching just a value of 20.8 wt%. Regarding LPG fraction, propane continues to be the main product (34.5 wt%), followed by iso-butane (27.3 wt%), whereas the concentration of n-butane has been reduced down to 16.9 wt%. Morawski and Mosio-Mosiewski [33] also observed the same trends for the dry gas and LPG fractions when increasing the H2 pressure from 20 to 50 bar in the hydrocracking of vacuum residue over a NiMo/Al2O3 catalyst. 20 50 80 110 0 20 40 60 80 100 Concentration (wt%) Pressure (bar) CH 4 C 2 H 4 C 2 H 6 C 3 H 6 C 3 H 8 C 4 H 8 i-C 4 H 10 n-C 4 H 10 Figure 4. Effect of H2pressure on the composition of the gas fraction. An increase in the pressure up to 50 bar brings a marked change in the composition of the gas fraction, led by an almost total olefins disappearance and a boosting of the isomerization reactions. Furthermore, the yield of the dry gas fraction is also reduced at the same time as C 3 and C 4 products became the main ones. Indeed, the lowest concentration of the dry gas fraction has been attained at this pressure reaching just a value of 20.8 wt%. Regarding LPG fraction, propane continues to be the main product (34.5 wt%), followed by iso-butane (27.3 wt%), whereas the concentration of n-butane has been reduced down to 16.9 wt%. Morawski and Mosio-Mosiewski [ 33 ] also observed the same trends for the dry gas and LPG fractions when increasing the H 2 pressure from 20 to 50 bar in the hydrocracking of vacuum residue over a NiMo/Al2O3catalyst. However, a further increase in the H 2 pressure to 80 bar has caused a moderate increase in the concentration of the dry gas fraction, principally to the detriment of the C 4 compounds and, to a lesser extent, the C 3 compounds. Indeed, propane is still the main compound (35.9 wt%). In addition, no olefins have been detected at this pressure because the hydrogenation of the components within the LGP fraction has been greatly favoured. At the highest H 2 pressure (110 bar), the concentration of dry gas has increased up to 30.9 wt%—the concentrations of ethane and methane at 16.2 and 14.7 wt%, respectively. Concerning the LPG, the largest concentration corresponds to propane (35.9 wt%), followed by isoand n-butane (21.4 and 11.8 wt%, respectively). The increase observed in the Catalysts 2022,12, 543 7 of 17 concentration of dry gas at high H 2 pressures results from a high level of thermal cracking being maintained. 2.4. Composition and RON of the Naphtha Fraction The composition of the naphtha fraction, together with its research octane number (RON), is collected in Figure 5. At low H 2 pressure (20 bar), the naphtha is mainly aromatic because of the aromatics already present in the VGO (Table S1) and the limited extent of the hydrogenation reactions that can be achieved under these conditions. Aromatics are the predominant compounds (59.3 wt%), the alkylbenzenes (A 1 ) being the most abundant compounds (56.2 wt%). The concentration of 2-ring aromatics (A 2 ) is 3.1 wt%, and naphthenic compounds are in a relatively low amount (10.5 wt%). A small quantity of olefins, 0.7 wt%, has also been detected. Linear paraffins account for almost 3 wt% of naphtha, whereas ramified paraffins account for 21.9 wt%. Catalysts 2022, 12, 543 7 of 18 However, a further increase in the H2 pressure to 80 bar has caused a moderate increase in the concentration of the dry gas fraction, principally to the detriment of the C4 compounds and, to a lesser extent, the C3 compounds. Indeed, propane is still the main compound (35.9 wt%). In addition, no olefins have been detected at this pressure because the hydrogenation of the components within the LGP fraction has been greatly favoured. At the highest H2 pressure (110 bar), the concentration of dry gas has increased up to 30.9 wt%—the concentrations of ethane and methane at 16.2 and 14.7 wt%, respectively. Concerning the LPG, the largest concentration corresponds to propane (35.9 wt%), followed by isoand n-butane (21.4 and 11.8 wt%, respectively). The increase observed in the concentration of dry gas at high H2 pressures results from a high level of thermal cracking being maintained. 2.4. Composition and RON of the Naphtha Fraction The composition of the naphtha fraction, together with its research octane number (RON), is collected in Figure 5. At low H2 pressure (20 bar), the naphtha is mainly aromatic because of the aromatics already present in the VGO (Table S1) and the limited extent of the hydrogenation reactions that can be achieved under these conditions. Aromatics are the predominant compounds (59.3 wt%), the alkylbenzenes (A1) being the most abundant compounds (56.2 wt%). The concentration of 2-ring aromatics (A2) is 3.1 wt%, and naphthenic compounds are in a relatively low amount (10.5 wt%). A small quantity of olefins, 0.7 wt%, has also been detected. Linear paraffins account for almost 3 wt% of naphtha, whereas ramified paraffins account for 21.9 wt%. Figure 5. Effect of H2 pressure on the composition of the naphtha fraction. Key: n-P—n-paraffins; i-P—isoparaffins; N—naphthenes; O—olefins; A1—1-ring aromatics; A2—2-ring aromatics. An increase in the H2 pressure promotes the extent of the hydrogenation reactions reducing the concentration of aromatics and increasing that of naphthenes [21]. However, the boosting of the rate of hydrogenation reactions is not directly proportional to the increase in pressure. This way, an increase in the H2 pressure from 20 to 50 bar has a subtle effect on the composition of the naphtha fraction. The total concentration of aromatics has barely decreased to 57.9 wt%, causing a slight increase in the aliphatic compounds (up to 41.9 wt%). It should also be highlighted that the concentration of olefins has been notably reduced (0.12 wt%), exposing that an H2 pressure of 50 bar is not high 20 50 80 110 0 20 40 60 80 100 Concentration (wt%) Pressure (bar) n-P i-P N O A 1 A 2 RON 89 90 91 92 93 RON Figure 5. Effect of H 2 pressure on the composition of the naphtha fraction. Key: n-P—n-paraffins; i-P—isoparaffins; N—naphthenes; O—olefins; A1—1-ring aromatics; A2—2-ring aromatics. An increase in the H 2 pressure promotes the extent of the hydrogenation reactions reducing the concentration of aromatics and increasing that of naphthenes [21]. However, the boosting of the rate of hydrogenation reactions is not directly proportional to the increase in pressure. This way, an increase in the H 2 pressure from 20 to 50 bar has a subtle effect on the composition of the naphtha fraction. The total concentration of aromatics has barely decreased to 57.9 wt%, causing a slight increase in the aliphatic compounds (up to 41.9 wt%). It should also be highlighted that the concentration of olefins has been notably reduced (0.12 wt%), exposing that an H 2 pressure of 50 bar is not high enough to saturate all the olefins, as aforementioned for the gas fraction (Figure 4). Nevertheless, an increase in the H 2 pressure to 80 bar significantly modifies the composition of the naphtha fraction. The concentration of all the saturated compounds has increased, reaching concentrations of 9.8, 27.8, and 14.0 wt% for the n-paraffins, i-paraffins, and naphthenes, respectively. Consequently, the concentration of the aromatic compounds has been reduced to 48.4 wt%, the mono-aromatics being the clear predominant ones (46.6 wt%). Note that olefins are no longer detected in the naphtha fraction. An additional increase in the H 2 pressure to 110 bar has exposed the key role of this parameter in the hydrocracking of the HDPE/VGO blend. Thus, the concentration of both ramified and linear paraffins has increased significantly (43.8 and 22.6 wt%, respectively). Indeed, isoparaffins have clearly Catalysts 2022,12, 543 8 of 17 become the predominant compounds. In the same line, the concentration of naphthenes has increased, reaching a value of 20.4 wt%, which is the maximum concentration obtained for this family of compounds in the whole range of H 2 pressure studied. With regard to aromatic compounds, the concentration of 2-ring aromatics has been significantly reduced to an almost symbolic value (0.3 wt%), and that of 1-ring aromatics has reached its minimum value (13.0 wt%). A similar trend has been reported in the literature in the hydrocracking of different feeds [ 33 , 34 ], in which a substantial H 2 pressure is required to totally remove olefins and achieve remarkable concentrations of aliphatic compounds. As the RON strongly depends on the composition of the naphtha fraction, the value obtained for this parameter has remained almost similar for H 2 pressures of 20 and 50 bar (89.9 and 90.2, respectively). However, when the pressure is raised to 80 bar, there is a considerable increase in the RON, reaching a value of 92.5. This result lies in the increase in the concentrations of isoparaffins since this family of compounds has a markedly positive effect on this index. Nonetheless, an increase in the pressure to values above 80 bar decreases the value obtained for the RON, as the concentration of aromatic compounds has been importantly reduced. Thus, at the highest pressure (110 bar), the RON decreases slightly to 91.5. A positive effect of the H 2 pressure has also been reported in the hydrocracking of heavy oils and petroleum residues regarding RON of the naphtha fraction operating at pressures below 75 bar [35]. 2.5. Composition and Cetane Index of the LCO Fraction The composition of the LCO fraction for the different H 2 pressures studied is displayed in Figure 6. In addition, the cetane index (CI) has been calculated in order to compare the effect of the pressure on the quality of the LCO fraction. Overall, a clear influence of the H 2 pressure on the composition of the LCO fraction can be observed. This way, high pressures increase the concentration of saturated compounds, mainly that of isoparaffins, whereas the concentration of 1-, 2-, and 3+-ring aromatics decreases. Catalysts 2022, 12, 543 9 of 18 Figure 6. Effect of H2 pressure on the composition of the LCO fraction. Key: n-P—n-paraffins; i-P—isoparaffins; N—naphthenes; A1—1—ring aromatics; A2—2—ring aromatics; A3+—3+—ring aromatics. At a pressure of 20 bar, a low concentration of total paraffins has been obtained (30.5 wt%), where isoparaffins are in a greater amount than normal paraffins (24.2 vs. 6.2 wt%, respectively). Naphthenes are the less important compounds accounting for just a 1.9 wt%. Therefore, aromatics are the predominant compounds with a total concentration of 67.7 wt%, the di-aromatics being the main ones (51.9 wt%), followed by polyand mono-aromatics (10.1 and 5.6 wt%, respectively). An increase in the H2 pressure modifies the composition of the LCO fraction in the same way that has modified the composition of the naphtha fraction (Figure 5). Looking more closely at the concentration of the different families, it can be seen that the concentration of isoparaffins has increased exponentially with H2 pressure, whereas that of n-paraffins has risen more linearly. Furthermore, naphthenes have decreased for pressures below 80 bar, but at 110 bar, they have achieved their maximum concentration (4.4 wt%). Regarding the trends of the different aromatic compounds, the concentration of 2-ring and 3+-ring aromatics has decreased with pressure, causing an increase in the mono-aromatics when increasing H2 pressure. This result shows that an increase in the H2 pressure promotes the hydrodearomatization reactions, but it also shows that 2-ring and 3+-rings aromatics are partially hydrogenated, leading to the formation of 1-ring aromatics. Subsequently, these mono-aromatics will be hydrogenated, forming totally saturated naphthenes, which are easily cracked into ramified paraffins. This mechanism was also observed by Palos et al. [36] in the hydrotreating of a highly aromatic stream obtained as a by-product of the FCC unit. Moreover, the significantly high acidity of the catalyst (Table 1) promotes the isomerization and skeletal rearrangement reactions between the paraffins and to a larger extent as the H2 pressure is increased, leading to the formation of isoparaffins. In addition, the acidic properties of the catalyst will also boost the hydrogenolysis reactions as the pressure is increased, obtaining even a higher dehydroaromatization degree [37]. According to the described trend, at an H2 pressure of 110 bar, the concentration of all the families of aliphatic compounds achieves its maximum value. Thus, the concentration of normal paraffins and naphthenes has increased up to 12.3 and 4.4 wt%, whereas isoparaffins have become by far the predominant compounds (53.3 wt%) in the 20 50 80 110 0 20 40 60 80 100 Concentration (wt%) Pressure (bar) n-P i-P N A 1 A 2 A 3+ CI 30 35 40 45 50 55 Cetane Index Figure 6. Effect ofH 2 pressure on the composition of the LCO fraction. Key: n-P—n-paraffins; i-P—isoparaffins; N—naphthenes; A1—1—ring aromatics; A2—2—ring aromatics; A3+—3+—ring aromatics. At a pressure of 20 bar, a low concentration of total paraffins has been obtained (30.5 wt%), where isoparaffins are in a greater amount than normal paraffins (24.2 vs. 6.2 wt%, respectively). Naphthenes are the less important compounds accounting for just a 1.9 wt%. Therefore, aromatics are the predominant compounds with a total concentration Catalysts 2022,12, 543 9 of 17 of 67.7 wt%, the di-aromatics being the main ones (51.9 wt%), followed by polyand mono-aromatics (10.1 and 5.6 wt%, respectively). An increase in the H 2 pressure modifies the composition of the LCO fraction in the same way that has modified the composition of the naphtha fraction (Figure 5). Looking more closely at the concentration of the different families, it can be seen that the concentration of isoparaffins has increased exponentially with H 2 pressure, whereas that of n-paraffins has risen more linearly. Furthermore, naphthenes have decreased for pressures below 80 bar, but at 110 bar, they have achieved their maximum concentration (4.4 wt%). Regarding the trends of the different aromatic compounds, the concentration of 2-ring and 3 + -ring aromatics has decreased with pressure, causing an increase in the mono-aromatics when increasing H 2 pressure. This result shows that an increase in the H 2 pressure promotes the hydrodearomatization reactions, but it also shows that 2-ring and 3 + -rings aromatics are partially hydrogenated, leading to the formation of 1-ring aromatics. Subsequently, these mono-aromatics will be hydrogenated, forming totally saturated naphthenes, which are easily cracked into ramified paraffins. This mechanism was also observed by Palos et al. [ 36 ] in the hydrotreating of a highly aromatic stream obtained as a by-product of the FCC unit. Moreover, the significantly high acidity of the catalyst (Table 1) promotes the isomerization and skeletal rearrangement reactions between the paraffins and to a larger extent as the H 2 pressure is increased, leading to the formation of isoparaffins. In addition, the acidic properties of the catalyst will also boost the hydrogenolysis reactions as the pressure is increased, obtaining even a higher dehydroaromatization degree [37]. According to the described trend, at an H 2 pressure of 110 bar, the concentration of all the families of aliphatic compounds achieves its maximum value. Thus, the concentration of normal paraffins and naphthenes has increased up to 12.3 and 4.4 wt%, whereas isoparaffins have become by far the predominant compounds (53.3 wt%) in the LCO fraction. At the same time, the concentration of total aromatics has been decreased down to 30.1 wt%, with specific values of 11.4, 17.8, and 0.87 wt% for mono-, di-, and poly-aromatics, respectively. Regarding the cetane index, there is a clear effect of H 2 pressure on it because of the compositional changes obtained when raising the pressure. At the lowest H 2 pressure (20 bar), the LCO fraction has the poorest cetane index (31.6). However, when pressure rises, the cetane index also increases, achieving its maximum value of 53.1 at 110 bar. This result can be correlated with the contents of paraffins and aromatics. Thus, the greater the paraffin concentration, the higher the cetane index. However, attending to the LCO fraction operating at such high pressures is not required. The quality of the LCO fraction obtained at both 50 and 80 bar is high enough to enable the use of this fraction in the blending of commercial diesel in refineries. 2.6. Coke Deposition The effect of H 2 pressure on the deposition of coke on the catalyst has been studied by temperature-programmed oxidation (TPO) analysis, and obtained results are depicted in Figure 7. This analysis is suitable for quantifying the amount of coke deposited but also for identifying its nature and location in the catalyst particle [ 38 ]. Therefore, the temperature corresponding to the maximum combustion rate (T Max ) is a parameter that can be used as an index for assessing the ease of combustion. It depends on [ 39 ] (i) the nature of the coke because combustion takes place at a lower temperature in a poorly structured coke (with a high H/C ratio), and (ii) the location of the coke, since the coke located on the outside of the catalyst particle and on the outside of the zeolite micropores will be burnt faster (combustion takes place without diffusional limitations). 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