Lessening coke formation and boosting gasoline yield by incorporating scrap tire pyrolysis oil in the cracking conditions of an FCC unit

Highlights

• Effect of co-cracking scrap tire pyrolysis oil with vacuum gasoil in an FCC riser.

• Addition of STPO shows positive synergistic effects on gasoline yield and quality.

• STPO yields less coke, being more aliphatic, lighter, and located in the micropores.

• Two types of coke: soluble in the micropores and insoluble-structured in the matrix.

• Co-cracking of scrap tire pyrolysis oil may enhance the performance of the FCC unit.

Abstract

We have studied the effect of adding scrap tire pyrolysis oil (STPO) as feed or co-feed in the cracking of vacuum gasoil (VGO) using a commercial equilibrated catalyst. The cracking experiments were performed in a laboratory scale fluid catalytic cracking (FCC) simulator using VGO, STPO, or a blend of the two (20 wt% of STPO), contact time = 6 s, catalyst/feed ratio = 5, and 530 °C. The composition of the different feeds has been correlated with the yield of products and the amount-location-nature of the deactivating species (coke). Our results indicate that adding STPO increases proportionally the gasoline yield, synergistically increase the yield of light cycle oil while uncooperatively decrease the yields of heavy cycle oil and coke. We further investigated the effect on coke formation, characterizing deeply the coked catalyst and coke. In fact, the coke deposited under the cracking of STPO is more aliphatic, lighter, and located in the micropores of the catalyst. The complete analysis of the coke fractions (soluble and insoluble) have lighted the peculiar chemistry of these species as a function of the type of feed used. The results point to a viable and economically attractive valorization route for discarded tires.

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.