C. Delhomme-NEUDECKER, N. SCHÖDEL, Á. TÓTA and G. SCHMIDT, Linde Engineering, Munich, Germany
Plastic waste is a global challenge. Today, only a minimal amount of plastic waste is recycled, mostly mechanically. However, as public awareness and regulations are changing, greenhouse gas reduction measures and plastic circularity technologies are being implemented. A strong increase in demand for plastics recycling is therefore expected in the future. Only by improving the rate of circularity can the plastics industry minimize its Scope 3 emissions (from upstream and downstream activities).
Among the strategies for plastics recycling, plastic pyrolysis can process waste that cannot be recycled mechanically or recycled by the recovery of monomers (e.g., solvolysis). In the future, this route may significantly contribute to the annual polyethylene (PE) and polypropylene (PP) production if the loop via steam crackers can be closed in a way that is economically reasonable.
Nevertheless, numerous qualities of plastic pyrolysis oil (pyoil) are currently produced, depending on the plastic feedstock, the pyrolysis technology, the reaction conditions, and the pre- and post-treatments applied (e.g., distillation, absorption, adsorption). With all these variations, ethylene producers are facing the challenge of determining if these oils can be used as a blend or pure feedstock in their units, and if upgrading or purification steps are required. The design of new plants or the revamp of older ones must consider the complexity of these oils—from large boiling ranges to a broad range of impurities and other cracking behavior. Finally, many options for purification and upgrading must be considered from a technical and economical point of view.
Three main pillars of knowledge can support the evaluation of these oils as feedstocks for steam crackers, and strengthen design and revamp activities: detailed analytics know-how for untypical feedstocks; pilot cracking know-how that allows for flexible tests under commercially relevant conditions; and well-established and wide-ranging design know-how regarding the furnace, separation behaviors, the distribution of contaminants throughout the plant, and the required purifications for conventional and unconventional feedstocks (FIG. 1).
These three areas will be covered in two articles. The first, Part 1, will explore analytics and pilot testing. The second article—Part 2, to be published in the February issue of Hydrocarbon Processing—will look at process design considerations.
Overall, the authors want to emphasize the importance of a detailed and holistic evaluation of these feedstocks to allow a thorough decision-making process and to ensure safe, reliable and cost-effective plant operations. There is a trade-off among meeting minimum requirements for safe and reliable operations, minimizing efforts for purification, maximizing flexibility and optimizing overall economics.
Commercial adaptation is expected to be a stepwise process, first with integration into existing steam crackers—starting with liquid crackers that do not have a dedicated furnace. Ramping up will strongly depend on the availability of sufficient volumes of suitable pyoil at an acceptable cost (including possible pre-treatment).
Detailed analytics of plastic pyoils. Detailed analytics expertise in unconventional feedstocks is the first step for the decision-making process when it comes to using plastic pyoils in a steam cracker. The quality of plastic pyoils can vary substantially. Historically, feedstocks for steam crackers have been defined by a limited number of parameters, such as the hydrogen-to-carbon (H/C) ratio, density, refraction index and final boiling point. These characteristics of plastic pyoils are not far off from the traditional crude-based cracker feedstocks: the H/C ratio, density, refraction index and final boiling point are within ranges and below requested limits for cracker feedstocks.
However, only looking at these parameters to rank the feedstocks could be misleading in the case of plastic pyoils. Two plastic pyoils showing similar characteristics (such as boiling curve and density) might contain very different components based on the plastic’s source. A detailed feed analysis can shed light on the differences in the feeds, which can then result in distinct product yields. Furthermore, the impurities that deviate significantly from conventional feedstocks must be better understood to evaluate purification requirements.
Detailed analyses and main findings. Below are some of the primary findings from several analyses.
Boiling curve. The boiling range of pyoil samples is typically much broader than conventional feedstocks, as presented in FIG. 2. The majority of plastic pyoil components (> 90%) fall in a boiling range that can be cracked and operated with a suitable furnace design. However, in practice, the ability to take a certain amount of these pyoils without cut is strongly case dependent. A furnace designed for light naphtha may quickly reach its limits. Therefore, an appropriate overall process concept evaluation is required—this concept will be explored in Part 2.
Furthermore, a similar boiling range should not be understood as having similar chemical properties. Plastic pyoils prepared from various plastic sources (e.g., PE-rich, PP-rich, PE-PP mixes) can present similar boiling curves and contain very different chemical components, leading to distinct cracking yields. Therefore, the boiling curve and density should not solely be used to differentiate oils, especially when it comes to determining high-value chemical yields.
U.S. Bureau of Mines Correlation Index (BMCI). Another widely used parameter to describe standard oil-based feedstocks is the U.S. BMCI, which provides an indication of the aromaticity level of conventional feedstocks, and also offers a good first indication of steam cracking yields and the propylene-to-ethylene (P/E) ratio (FIG. 3) for these feedstocks.
However, using the BMCI is unsuitable for predicting light olefin yields from plastic pyoils due to their uncommon compositions. Even with similar BMCI parameters, PE-rich and PP-rich pyoils show very different behaviors in terms of ethylene and propylene yields, as shown in FIG. 3. Therefore, deeper insight into plastic pyoil feed characteristics is necessary to better understand the cracking behavior of these oils.
Hydrocarbon matrix. A detailed paraffin, isoparaffin, olefin, naphthene, aromatic (PIONA) analysis of the plastic pyoil and the distribution of the components by carbon number deliver crucial information for accurately rating the plastic pyoils. Such an analysis can be performed using two-dimensional gas chromatography (2D-GC) with the specific identification of olefins. Furthermore, the 2D-GC analysis can identify key components present in the plastic pyoils, which serve as evidence for the type of waste plastic used for pyrolysis. From this information, it is possible to get an indication of the PE-to-PP ratio of the plastic mix used initially in the pyrolysis. This ratio is then related to specific product yields in the steam cracker.
FIG. 4 shows the difference in 2D-GC analyses of plastic pyoils derived from PE-rich (FIG. 4A) and PP-rich (FIG. 4B) plastic feedstocks. The different component categories can be well identified, especially the different types of long-chain olefins, which are the dominating species in non-hydrogenated pyoils (30 wt%–80 wt%, depending on the origin).
Based on a broad database, correlations on pyrolysis feedstock [e.g., PE, PP, polystyrene (PS)] and cracking behavior can be established within a defined confidence level.
Impurities. The range of impurities found in plastic pyoils is much broader than in conventional feedstocks. Furthermore, it differs widely from one oil to another. It should be noted that the type of plastics recycled, the sorting process, the plastic conversion process and any related pyoil conditioning processes strongly impact the pyoil properties and contaminant levels. Additionally, regulations governing these businesses might greatly differ throughout the world.
Key challenges of plastic pyoils include the content of heteroatomic compounds [e.g., oxygen (O), nitrogen (N), chlorine (Cl)] and the metal content [e.g., sodium (Na), silicon (Si), heavy metals] in the plastic pyoils. These contaminants are linked to the plastic processed in pyrolysis (TABLE 1).
Cl and other halogenides [e.g., fluorine (F) and bromine (Br)] typically exceed the limit that can be accepted for a cracker feedstock. Only a minority of plastic pyoil samples tested in the authors’ company’s lab contained halogenides in quantities close to the accepted limits. Because of several serious issues related to halogenide compounds (TABLE 1), a hard limit is applied.
By order of magnitude, the level of oxygenates is typically higher than the preferred values. For oxygen content, the hard limit is not as high as for halogenides. The maximum-allowed concentration depends on the type of oxygen component and cracker setup. However, for levels typically measured in plastic pyoils, a significant impact on the operability of the cracker is expected (see related issues in TABLE 1). A similar, but somehow lower, impact is also expected for nitrogen compounds. In contrast, sulfur compounds are typically of minor concern.
Other elements of concern are Na—which has a hard limit due to the hot corrosion of coils—and Si (limitations are due to fouling and catalyst poisoning), both of which often exceed limits by a factor of 2 to 10.
Based on the authors’ company’s in-house experience, the limiting factor for first commercial trials is often the halogenide content (especially Cl), which often leads to high dilution requirements. Similar concentration ranges have also been reported.1–3 If plastic pyoils with a high halogenide content are to be diluted, a specific evaluation should be performed regarding the allowed halogenide concentration in the diluted feedstock, as well as in the complete feedstock slate, considering the impact on the furnace and separation section.
Valuable support for decision-making. Conducting 2D-GC analysis, along with analyzing a broad range of impurities and properties, can help ethylene producers obtain an initial feedstock ranking based on impurities, boiling range and hydrocarbon matrix. This is a valuable tool for screening potential pyrolysis partners and pyoil sources, and also for evaluating purification/upgrade requirements. These findings can also be used to provide feedback to pyrolysis companies on how to further improve the quality of their oils to meet steam cracker requirements. Additional information on the behavior of these impurities within a steam cracker is required and can be obtained with pilot cracking testing.
Pilot cracking testing for unconventional feedstocks. Pyoil analytics require special efforts, and even the best available analytics are unable to identify all individual components, especially regarding impurities remaining in the pyoil. Therefore, it is important to gather additional information from direct cracking tests of these plastic pyoils. Through these tests, the reactions of nitrogen, oxygen, chloride and sulfur components in the cracking reactor can be investigated.
Plastic pyoils based on PE and PP present different unconventional chemical components and demonstrate different cracking behavior. By performing cracking tests, the influence of different parameters under commercially relevant conditions can be better understood.
All gathered data are crucial for de-risking the application of pyoils at commercial scale. The results serve as a basis for commercial furnace design or for evaluating existing furnaces planned for pyoil take-in. Furthermore, they provide valuable information for evaluating the impact of these oils on the product slate of the steam cracker.
Cracking tests and main findings. The following are the primary findings from cracker tests.
Pilot plant. A pilot cracking unit located in Pullach, Germany (FIG. 5) has been used extensively to test conventional and unconventional feedstocks. As early as the 1980s, the authors’ company tested the introduction of plastic material into the steam cracker. This setup makes it possible to explore conditions that cannot be directly tested in commercial units. For example, even undiluted pyoil can be processed. In addition, the pilot plant can be operated for a broad range of parameters. Extensive feed and product analyses can also be performed. The pilot plant is operated at the same steam cracking process conditions as commercial cracking furnaces and enables a direct transfer of information to commercial units.
The pilot unit is composed of three main sections: a feed supply section for steam and different gaseous, liquid or even wax-like hydrocarbon feedstocks; the furnaces where different coil types are available; and the cooling, separation and sampling sections.
All streams (feeds and gaseous, liquid and water products) can be balanced and analyzed related to typical cracker product components and trace impurities [e.g., hydrochloric acid (HCl), organic Cl compounds, ammonia, hydrogen cyanide, nitric oxide, nitriles, amines, oxygenates, sulfur compounds and siloxanes].
Yields from plastic pyoils. Based on the pilot tests, cracked gas compositions from diverse feedstocks under commercially relevant reaction conditions can be compared. This is a useful tool for ranking the pyoils from diverse producers/technologies. TABLE 2 presents a comparison of typical cracked gas compositions for conventional feedstocks and plastic pyoils.
Plastic pyoils can yield reasonable amounts of high-value-added chemicals, such as ethylene and propylene. However, big differences can be seen among oils prepared from different plastic mixes. If the plastic mix used for pyrolysis contains a high portion of PE, the cracked gas from the derived pyoil will reach combined propylene and ethylene (P+E) yields comparable to conventional liquid feedstock in the same boiling range.
Conversely, if the oil has been produced from a PP-rich plastic mix, the cracked gas will show overall lower yields and a very high P/E ratio. Therefore, this oil can be used to boost propylene production. Furthermore, the proportion of C4 olefins in the cracked gas of the PP-based pyoil is also increased and should be considered for the overall operation scenario. Finally, it should be noted that the C5+ fraction of the cracked gases from all plastic pyoils is at the higher range of the values measured for oil-based feedstocks. Nevertheless, these C5+ fractions show similar components to the ones found in C5+ fractions from conventional feedstocks, such as naphtha, atmospheric gasoil (AGO) and hydrocarbon residue (HCR).
Impurities distribution. Aside from the main hydrocarbon product yield, the pilot cracking results provided insights into trace impurities, which can impact cracker operation significantly. As already stated, the level and type of impurities may vary among different plastic pyoils. Therefore, understanding the fate of contaminants entering the furnace is vital. Downstream from the furnace, the unconverted impurities and converted products are finally distributed to the gas, liquid hydrocarbon and/or water phase. Risks and consequences must be judged on a case-by-case basis. Nevertheless, several key learnings have been summarized in TABLE 3.
Valuable support for decision-making. Based on cracked gas composition and contaminants in the cracked gas, pilot cracking tests create a comprehensive data basis that can be used to realize a detailed ranking of selected plastic pyoils. The pilot tests can also be used to optimize reaction conditions or to evaluate co-feeding scenarios. Finally, tests produce a strong knowledge basis for process design.
Takeaway. The first two pillars to support the decision-making process regarding the use of plastic pyoils in a steam cracker have been presented in this article. First, plastic pyoils are analyzed in detail to identify impurities, the hydrocarbon matrix, the boiling range and other characteristics. Second, pilot testing facilities allow for detailed and comparative characterizations of these feedstocks under commercial operating conditions, which, in turn, leads to a better understanding of the behavior of impurities and of the cracked gas yields. These first two pillars are powerful tools to create a database to facilitate future evaluations of these oils and design activities.
Part 2 of this article will concentrate on the third pillar, which consists of well-established and wide-ranging design know-how regarding the furnace, separation, contaminant distribution and required purification steps. HP
LITERATURE CITED
Clara Delhomme-Neudecker is a Process Engineer for cracking furnaces at Linde Engineering in Munich, Germany. Since joining Linde Engineering in 2012, Dr. Delhomme-Neudecker has worked in R&D and in the company’s process design department for sustainable hydrocarbons, supporting the development of sustainable technologies (e.g., unconventional feedstocks and the electrification of processes) and working on cracking furnace design. She earned a PhD in chemical engineering from the Technical University of Munich.
Nicole Schödel is the Director of Chemical Technology Service at Linde Engineering in Munich. Dr. Schödel has worked for Linde Engineering for more than 30 yr in various functions, particularly in the development and optimization of processes, with a focus on hydrogen, syngas and petrochemical processes, gas purification and downstream processes. In 2012, she was named a fellow at Linde Engineering. Dr. Schödel earned a PhD in technical chemistry from the Technical University of Munich.
Ákos Tóta is a Senior Process Design Engineer for Linde Engineering in Munich. Since joining Linde Engineering in 2007, Dr. Tóta has worked in several functions relating to adsorption and membrane technologies and process design for sustainable hydrocarbons. His focus has been particularly on downstream separation, purification and upgrading technologies. He earned a PhD in chemical engineering from Otto von Guericke University of Magdeburg, Germany.
Gunther Schmidt is the Group Lead of Process Design for cracking furnaces at Linde Engineering in Munich. Since joining Linde Engineering in 1985, he has been involved in process and mechanical design, as well as in the operational concerns of cracking furnaces. After gathering experience in development, sales, field work and project management, Schmidt took over responsibility for furnace design in 2006. He studied chemical engineering at the University of Erlangen–Nuremberg, where he graduated with a Dipl.-Ing. (M.Sc.).