(619a) Catalytic Cracking of Crude Waste Plastic Pyrolysis Oil to Maximize Light Olefin Production in a Pilot-Scale Circulating Fluidized Bed Reactor

The invention and widespread adoption of plastic have been considered as one of the most significant advancements of the 20th century, exerting a profound impact on human life. However, alongside the numerous benefits offered by plastic products, the indiscriminate use of plastic has emerged as a considerable challenge that necessitates prompt attention. For instance, out of the 353 million metric tons (MMT) of plastic waste generated in 2019, only 9% underwent recycling, while the majority ended up in landfills (50%), incineration (19%), and mismanagement (22%) [1]. In addition to mismanaged plastic waste, the handling of plastic waste through landfilling and incineration has led to considerable environmental pollution and ecological hazards. Moreover, enormous amounts of greenhouse gases are generated during the production and disposal of plastics [2]. Therefore, recycling of waste plastics with high efficiency is indispensable for preventing environmental pollution as well as low carbon emission, leading to a climate-neutral future.

Among various waste plastic recycling methods, mechanical recycling and thermochemical recycling have been highlighted as the most promising technologies [2]. Mechanical recycling stands out as one of the most energy-efficient and sustainable options. However, it demands highly sorted and clean waste plastics; otherwise, it frequently yields lower-quality products compared to those derived from virgin polymers due to the inherent impurities in plastic waste streams. On the other hand, thermochemical recycling converts plastic waste into its monomer, and low-quality or mixed plastic waste can be used. From a different perspective, among the various types of plastic, polyolefinic plastics, such as polyethylene (PE) and polypropylene (PP), which are widely used as packaging materials, account for more than 60% of the global waste plastic production owing to their short product lifetime [3]. Therefore, there is an upmost emphasis on developing new thermochemical recycling technology to produce light olefins (e.g. ethylene, propylene, butenes) from contaminated and mixed waste polyolefins, suitable for the their remanufacturing.

In this context, ongoing endeavors are being pursued to advance the commercialization of this recycling pathway [4], as illustrated in Figure. This approach involves subjecting waste plastics to pyrolysis in an oxygen-free environment, resulting in the generation of gas, liquid oil, and residue. The Crude waste plastic pyrolysis oil (WPPO) is subjected to post-treatment techniques, such as hydroprocessing, to remove contaminants and impurities, leading to production of t-WPPO (hydrotreated WPPO). Subsequently, the naphtha range of t-WPPO is converted into light olefins utilizing well-established naphtha-steam-cracking units.

However, the state-of-the-art commercial scheme is thought to have several drawbacks, mostly derived from the notable differences in properties between conventional naphtha and WPPO. First, crude WPPO typically comprises a broader range of carbon numbers (from C₅ to C₄₄) than conventional naphtha, which contains paraffinic straight-chain hydrocarbons from C₅ to C₉ [5]. Owing to this difference, only a fraction of the pyrolysis oil is utilized, or the hydrocarbon chain length requires further reduction to produce a steam-cracker-suitable feedstock. Additionally, crude WPPO typically contains a substantial amount of olefins (30–50 wt%). Industries typically limit the olefin content of the feedstocks for thermal naphtha cracking to a maximum of 2 wt% owing to the tendency of high olefin contents to cause coke formation [5]. This imbalance could be resolved by moderately diluting crude WPPO with fossil naphtha or subjecting it to a post-saturation process to convert olefinic compounds into single-bond ones for meeting the given specifications. Finally, WPPO contains various kind of contaminants such as heteroatomic compounds (nitrogen, sulfur, oxygen and chlorine) and metallic compounds, some of which exceed typical specification of naphtha steam cracker [5]. In addition to the partial dilution of WPPO with conventional naphtha, catalytic hyprotreating such as hydrodesulfurization (HDS), hydrodenitrogenation (HDN), hydrodeoxygenation (HDO), hydrodechlorination (HDCl), Hydrodemetalation (HDM), which are widely used in commercial application, are thought to be useful for removing heteroatomic and metallic compounds in WPPO to meet the current contaminants specifications set for industrial steam cracker feedstocks. However, diluting WPPO with naphtha restricts its use, whereas post-saturation requires an appreciable amount of hydrogen, which can be detrimental to economic feasibility.

In our previous work, a new naphtha catalytic cracking process based on a circulating fluidized bed reactor (CFBR) was developed collaboratively by our group, SK Corporation, and Kellogg Brown & Root (KBR) to replace the traditional naphtha thermal cracking process, which was successfully commercialized under names ACO^TM (Advanced Catalytic Olefins) and later K-COT^TM (KBR catalytic olefins Technology) [6]. This system presents several advantages over conventional non-catalytic naphtha thermal cracking, including enhanced feedstock flexibility (the utilization of heavier feedstocks, olefin-rich feed streams, and oxygenates), higher overall yields of light olefins (approximately 10–25% more total olefins than a typical steam cracker), a higher propylene to ethylene ratio (1:1 propylene-to-ethylene ratio compared to 1:2 ratio delivered by steam crackers), and reduced energy consumption due to relatively lower cracking temperature (reduced from 850 °C to below 700 °C).

Given the distinctive properties of WPPO and the advantageous nature of catalytic cracking in CFBR systems, particularly in processing feeds with such properties, catalytic cracking of WPPO within a CFBR system emerges as a highly promising alternative to conventional thermal cracking, akin to the case of naphtha. This system could not only address the inherent limitations associated with using WPPO but also offers notable advantages such as reduced energy consumption and increased yields of light olefins.

In this regard, we investigated the feasibility of continuous catalytic cracking of crude WPPO in a microactivity test (MAT) unit [7]. Interestingly, the catalytic WPPO cracking led to a remarkable increase in C₂–C₄ olefins yield, surpassing those obtained using catalytic naphtha cracking as well as thermal WPPO cracking and thermal naphtha cracking. This superiority of WPPO as a feedstock over naphtha was attributed to its unique properties, particularly its high content of long-chain paraffins and olefins, which facilitated enhanced crackability on the catalyst compared to the predominantly short-chain paraffins found in conventional naphtha. Additionally, the feasibility of continuous operation was examined by monitoring the tendencies of the catalyst to deactivate rapidly via coke formation and to recover its activity through regeneration. Also, the continuous catalytic cracking of crude WPPO for light olefin production was assessed in a pilot-scale CFBR (1,000 g of WPPO input/hour scale). A comprehensive investigation was carried out to examine the effect of operating parameters on the product distribution, and a 24-hour durability test was employed to scrutinize the stability of the CFBR system throughout continuous catalytic cracking of crude WPPO.

The findings of this study can be exploited to implement a new approach to directly produce light olefins by integrating the catalytic cracking scheme for WPPO in a circulating fluidized bed reactor system, as shown in Figure. This strategy offers several advantages over most commercially developed approaches, such as permitting the utilization of the entire crude WPPO range without requiring any post-treatment processes. This can lead to high light-olefin yields, continuous coke removal, and reductions in the energy consumption. Moreover, this approach eliminates the need for hydrogen, which is typically required during the post-hydroprocessing stage of commercial schemes.

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