Lessening coke formation and boosting gasoline yield by incorporating scrap tire pyrolysis oil in the cracking conditions of an FCC unit
Graphical abstract
Introduction
End of life tire (ELT) management is becoming one of the most important environmental issues worldwide. Almost 1.3–1.5 billion tires are produced every year, giving way to approximately 17 Mt of used tires [1], [2]. According to the European Tyre & Rubber Manufacturers' Association (ETRMA) [3], around 95% of ELTs (accounting for more or less 2 million metric tons of scrap tires) are recycled or recovered for material (40%), energy (38%), reconstruction (7%), and reuse/export (10%) purposes [4], [5], [6]. However, in many countries the remaining waste tires are still dumped, although its disposal in landfills is being banned (i.e. EU) [7], as it contributes to serious environmental and human health problems (e.g. risk of spontaneous fires or diseases caused by mosquitoes and rodents) [1]. The environmental impact derived from chemical leaching, waste tire combustion and incineration, particulate pollution, as well as the presence of micro-plastics (MPs) in the oceans is also attracting great concern [8], [9], [10].
In this sense, several alternative valorization routes (i.e. gasification [11], [12], [13], [14], [15] and pyrolysis [7], [16], [17], [18], [19]) are emerging. Particularly, the flash pyrolysis of waste tires gives way to three main product fractions, which are highly dependent on the type of tires, the operating conditions, and the reaction technology used [1], [20], [21], [22]: a gas phase, with a high energy content; a solid product (adulterated carbon black), to be valorized via gasification or applied as adsorbent or catalyst support (after sulfur removal); and, a liquid phase, so-called scrap tire pyrolysis oil (STPO), with promising prospects for fuel blending [6], [7], [23], [24] and for the production of added-value chemicals, such as aromatics BTX (benzene, toluene and xylenes) [25], [26], [27] and limonene [28], among others. However, the STPO shows several drawbacks for being directly used as automotive fuel, due to its high content in heteroatoms and aromatics. Furthermore, several pretreatments are needed to comply with emission legislations [29]. In this context, the use of available and depreciated refinery units (mainly fluid catalytic cracking, FCC, and hydroprocessing units) could contribute to intensify high-quality fuel production from these non-biodegradable wastes (Waste Refinery) [5], [30]. FCC units and catalysts have shown an outstanding versatility to treat non-alternative feedstock, usually blended with vacuum gas oil (VGO) [31], [32], such as, secondary refinery streams (naphtha, waxes, residues) [33], [34], [35], [36], [37], biomass-derived feeds (bio-oil, pyrolysis oil, vegetable oils and animal fats) [38], [39], [40], [41], [42], [43], [44], [45], [46], [47], and plastic wastes [35], [48], [49]. The possibility of upgrading STPO alone and/or blended with VGO has been reported by several authors [29], [50], [51].
A conventional FCC unit is provided with a fluidized bed reactor, with vertical transportation of both the catalyst and feed/products (riser), a stripper, and a fluidized bed regenerator [36]. FCC catalysts have greatly evolved over the years, from natural clays and silica and alumina catalysts to zeolite Y based catalysts. The main challenges for catalyst design are aimed to improve [37]: (i) activity, selectivity, and accessibility; (ii) resistance to attrition; (iii) hydrothermal stability; (iv) metal tolerance, in the case of feeding heavier feedstocks; (v) hydrodynamics related to fluidization; and (v) catalyst stability, by minimizing coke selectivity. A typical FCC catalyst is composed of a stabilized form of zeolite Y as an active phase, clay as filler, and alumina and/or silica sources to provide a meso- and macroporous matrix. The components are usually mixed in an aqueous slurry, and then spray-dried to obtain almost uniform spherical particles to facilitate the fluidization in the regenerator. Several initiatives to enhance zeolite Y catalytic performance include adding rare earth metals and providing hierarchical porous structure, among others [31], [37], [52].
Together with the catalyst properties, the operating conditions and feed composition also have a great influence on product distribution and coke deactivation, due to the complex reaction network, where several acid catalyzed reactions are involved [31], [37], [52]: β-scission, protolytic cracking, isomerization, hydrogen transfer, cyclization, and aromatization. Coke formation also has a crucial impact on catalyst performance and product distribution, as well as on the heat balance of the FCC unit [53], [54]. It can be formed from the condensation and dehydrogenation of hydrocarbons (catalytic coke), but also from thermal cracking (thermal coke), from heavy molecules present in the feed (Conradson coke), dehydrogenation reactions catalyzed by metal poisoning or from entrained catalytic products in the small pores, not having been removed by stripping [31], [54]. Hence, coke deactivation entails active site deactivation, thermal aging and pore blockage, among others. Successive reaction-regeneration cycles can also damage the crystalline structure of the catalyst and affect the morphology of the coke [55].
Special efforts are made in the recent literature to contribute to a better understanding of the deactivation phenomena on FCC catalysts. Cerqueira et al. [54] and more recently, Bai et al. [31] have reviewed the latest findings on this issue, by focusing on: (i) the origin of coke and its location on the catalyst surface [56]; (ii) multi-technique catalyst characterization [57]; (iii) deactivation mechanisms for different contaminants; as well as, (iv) technical approaches to controlling coke deactivation. Moreover, recent studies on the deactivation of FCC catalysts at the single particle level are positively contributing to this field as well [58], [59], [60], [61], [62].
In this work, we have studied the upgrading of STPO as feed or co-feed together with VGO in an FCC unit: cracking VGO or STPO individually and a blend of 20 wt% of STPO and 80 wt% of VGO. The net outcomes of STPO have been analyzed on the grounds of synergetic/proportional/uncooperative effect in product distribution (mainly gasoline), and, especially in coke deactivation. Based on the extensive previous work of Rodriguez et al. [29], [30], [51] we have selected a set of conditions in a riser simulator where the effect of deactivation is significant. In order to correlate the coke features (coke content, nature, and its location) with those of the feeds, we have developed an exhaustive coke characterization study by the following techniques: N2 adsorption–desorption; temperature programmed oxidation (TPO); Fourier transform infrared (FTIR) spectroscopy; TPO-MS (mass spectrometry) coupled with FTIR spectroscopy; Raman spectroscopy, laser desorption ionization time-of-flight mass spectroscopy (LDI TOF-MS), and soluble coke characterization by GC–MS. This study contributes to a better understanding of coke deactivation from non-conventional feedstock to be treated in FCC units in order to approach the future waste-refinery concept, where refinery put its experience and infrastructure to work in scrap tire valorization.
Section snippets
Feeds
The vacuum gas oil (VGO) as the standard feed of the FCC unit was provided by Petronor S.A (Muskiz, Spain) and scrap tire pyrolysis oil (STPO) was produced by flash pyrolysis of waste tires in a conical spouted bed reactor operating at 500 °C. 3 g min−1 of scrap tyre were brought into contact with 9.5 L min−1 of previously preheated N2 (1.2 times the minimum for fluidization) in a reactor where a bed of 35 g of sand (0.63–1 mm in diameter) was used to guarantee isothermicity during the
Results
Properties of the catalyst used were previously mentioned in Table 1 of the experimental section. These features correspond to a typical equilibrated FCC catalyst. The elemental analysis and the physical properties of the used feeds are shown in Table 2. As can be seen, the VGO is a slightly more hydrogenated sample, whereas STPO contains larger amounts of nitrogen, albeit nearly equal amounts of sulfur. Regardless, STPO has a more volatile nature, as proved in the simulated distillation curves
Discussion
The results of the previous sections show, first of all, the difficulty in interpreting the results of the analysis of coke, since its nature is mainly conditioned by the severity of the reaction conditions in the FCC, especially by high reaction temperatures, as well as for the absence of hydrogen in the reaction medium, which are conditions that favor the condensation of the coke components (aging) towards polycyclic aromatic structures. Furthermore, the feeds studied (VGO and STPO) have a
Conclusions
In this work we have proved the benefits of using scrap tire pyrolysis oil (STPO) as feed or co-feed in the fluid catalytic cracking unit. We have compared the cracking results of a benchmark feed (vacuum gas oil, VGO) in standard refinery unit conditions using a riser simulator. The products as well as the coked catalyst have been analyzed in detail using a set of different techniques.
Our results demonstrate that the cracking of STPO produces more gasoline and this effect is kept
CRediT authorship contribution statement
Elena Rodríguez: Conceptualization, Methodology, Formal analysis, Investigation, Visualization, Writing - original draft. Sepideh Izaddoust: Methodology, Formal analysis, Investigation, Visualization, Writing - original draft. José Valecillos: Methodology, Formal analysis, Investigation. Javier Bilbao: Methodology, Writing - review & editing. José M. Arandes: Validation, Software, Writing - review & editing. Pedro Castaño: Supervision, Conceptualization, Project administration, Resources,
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This research was funded by the Ministry of Economy and Competitiveness (MINECO) of the Spanish Government (CTQ2016-79646-P); the Ministry of Science, Innovation and Universities (MICINN) of the Spanish Governement (RTI2018-096981-B-100); the European Commission (HORIZON H2020-MSCA RISE-2018. Contract No. 823745); the European Regional Development Funds (ERDF) and the Basque Government (IT1218-19). Dr. Rodriguez is thankful to the University of the Basque Country UPV/EHU and Petronor S.A.
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2022, Journal of Cleaner ProductionCitation Excerpt :The produced aromatics can be the primary nuclei for intensifying the Diels-Alder reactions at high temperatures, increasing the number of aromatic rings in the pyrolysis product (di and poly aromatics), and ultimately leading to coke formation (Chao et al., 2020; Zhou et al., 2019). However, monitoring coke formation is complicated due to the high amount of CB and other mineral additives present in used tires (Rodríguez et al., 2020; Roy et al., 1999). Even CB can be involved as the initial nucleus of Diels-Alder reactions that leads to the formation of a new structure on the CB surface.
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2022, FuelCitation Excerpt :However, the resulting values are relatively similar, and these interpretations are preliminary. To gain further insight into the structural and compositional properties of the formed coke species, we applied an HF disaggregation and organic solvent extraction on the catalyst samples deactivated at three different temperatures to isolate the soluble and insoluble coke fractions [52,58]. Fig. 7a shows the LDI FT-ICR MS spectra corresponding to the analyses of the insoluble cokes.
Revisiting a large-scale FCC riser reactor with a particle-scale model
2022, Chemical Engineering ScienceCitation Excerpt :These technologies share common characteristics of harsh operating conditions and/or multiple reaction zones. However, the former (e.g., high reaction temperature and long residence time of oil vapor in the riser reactors) always leads to high yield of undesirable dry gas and coke (Wang et al., 2013; Rodríguez et al., 2020a). Regarding the latter, the design, scale-up and optimization of riser reactors with multiple reaction zones, also known as multi-regime reactors, are still primarily depending on experience (Lan et al., 2009; Gan et al., 2011).
Product composition and coke deposition in the hydrocracking of polystyrene blended with vacuum gasoil
2021, Fuel Processing TechnologyCitation Excerpt :The procedure consists on the addition of 0.12 mL of hydrofluoric acid to a sample of 15 mg of spent catalyst to destroy the crystal structure of the zeolite. This way, the coke trapped within the micropores of the zeolite is liberated [22]. In order to ensure the degradation of the framework structure of the zeolite, the acid attack has been performed for 1 h. Then, 3 mL of dichloromethane have been added to extract the soluble coke from the mixture.