C. Delhomme-NEUDECKER, N. SCHÖDEL, Á. TÓTA and G. SCHMIDT, Linde Engineering, Munich, Germany
Plastic waste is a worldwide challenge. To reduce its carbon dioxide (CO2) footprint, the plastics industry is striving to increase circularity by using plastic pyrolysis oils (pyoils) from waste plastics as steam cracker feedstock. Nevertheless, the quality of these plastic pyoils can vary widely.
The authors’ company has developed a holistic approach to support the decision-making process concerning the use of new feed materials, and to strengthen design and revamp activities. The first two pillars—detailed analytics know-how for untypical feedstocks, along with knowledge regarding pilot cracking—were presented in detail in Part 1 of this article. These two pillars give essential information (e.g., boiling range, hydrocarbon matrix, impurity distribution, cracking behavior) to support design activities. The importance of deep and wide-ranging design expertise is presented here.
Process design (purification, furnace, separation section). Plastic pyoils are untypical feedstocks that require special considerations for the process design. It is important to combine deep knowledge in furnace design with a detailed understanding of impurities distribution throughout the plant and extensive knowledge of purification/upgrade options. An ideal design offers tailor-made impurities removal while ensuring safe, reliable and cost-efficient processing. A design should ensure the required vaporization of the feedstock in the cracking furnaces to avoid reliability and maintenance issues.
Main process design considerations. Processing plastic pyoils in existing plants has already been performed by various ethylene producers. Small quantities of pyoil in blends often enable a high dilution of contaminants. This results in a relatively small variation of cracked gas yields and shows low criticality for insufficient feed vaporization. However, when the availability of plastic pyoils increases, ethylene producers will want to increase the feedstock blending ratio. Three main domains of expertise should be considered for the evaluation of the blending ratio:
Pretreatment and purification considerations. Today, high-dilution cases have significantly lower purification needs than future scenarios, where processed amounts of pyoils are set to increase. Thus, in low concentrations, waste plastic pyoils might be directly fed into the cracking furnace. However, increasing pyoil feed volumes possibly requires an increase in pretreatment efforts before cracking to limit the impact of contaminants on plant operations. The testing of current pyoils revealed impurities above accepted levels. Perhaps in the future, more stringent specifications for pyoils from suppliers can be expected.
Suitable pretreatment and purification sequences must be evaluated on a case-by-case basis since the level of contamination and purification requirements might differ considerably. Information from detailed analytics and knowledge of contaminant behavior in the furnace (gleaned from a large database of pilot tests—see Part 1) are key to defining a suitable and tailor-made purification/upgrade concept.
FIG. 6 provides an overview of principal options from which a combination of several steps is typically selected on a case-by-case basis. Some of these options may also be in the scope of the pyoil producer—for example, water wash to extract inorganic contaminants like halogenides and oxygenates, or distillation to adjust the desired boiling range. Other measures—such as hydroprocessing—are more reasonably performed in large-scale centralized units or by utilizing existing refinery capacities for pyoil upgrades to fulfill more stringent steam cracker feedstock specifications. Finally, removal steps for trace components that do not impact cracking furnace integrity or performance might be more economically included in the cracker site scope. Here, guard beds or additional removal systems downstream of the furnace section could be used.
Today, the pyrolysis gas fraction is mainly thermally utilized in waste plastic pyrolysis units. The recovery of light olefinic components, along with hydrogen and some aromatics present in these streams, represents a further possible level of integration into the steam cracker units. With respect to contaminants, these streams show similarities with those from fluid catalytic cracking unit (FCCU) off-gases or other olefinic purge streams from refineries. Consequently, processing schemes rely on technologies that have been proven in refinery off-gas (ROG) treatment for many years.
Impact on furnace design. To evaluate the impact of plastic pyoil feed on an existing furnace operation, plastic pyoil characteristics must be addressed regarding the several aspects listed below. Here, information gathered from the detailed analysis and pilot testing support evaluation:
When plastic pyoil is available in sufficient quantities to supply a complete steam cracker facility, design needs must be adjusted accordingly. Regarding the furnaces, there is ample knowledge and experience on how to deal with heavy, high-boiling feedstocks. The challenges are concentrated more on the downstream side as discussed below.
Impact on plant operation and design. A safe, stable and largely trouble-free operation of the plant is the priority, regardless of whether it is about processing conventional feedstock or waste plastic pyoils.
The type and concentration of impurities in pyoils can vary greatly. The level of contamination is typically significantly higher than that of conventional naphthas or distillates, except for sulfur compounds. The complete removal of all interference components from the feed is associated with significant technical effort, which strongly impacts profitability. Therefore, it is necessary to identify and prioritize the removal of key components under consideration of such aspects as process reliability, expected deterioration form, and the rate of progression and economic impact.
Distinct knowledge of the behavior (e.g., conversion rates and side products) and the distribution of these components in the cracking furnace and separation section is necessary (FIG. 8). As discussed earlier, these data can preferably be determined in pilot tests.
The expected effects of the distribution of impurities in the respective plant units are diverse. TABLE 1—presented in Part 1 of this work—showed the main issues related to the contaminants. Furthermore, FIG. 8 depicts the units that are impacted.
In addition to increased corrosion and fouling rates in the cracking furnaces and hot sections, operating fluctuations caused by the typically changing quality of pyoils are a challenge. While the presence of increased ammonia (NH3) and/or inorganic/organic acids in the cracked gas requires robust pH control in the process water system, fluctuations in the carbon monoxide (CO) concentration can destabilize adiabatic front-end (FE) acetylene converters. Shorter catalyst runtime and service life—for example, in the pygas section—are additional much-feared deterioration patterns that must be counteracted by suitable measures (including guard beds and de-metallization).
Valuable support for decision-making. The use of plastic pyoil in existing or new plants can be evaluated based on detailed pyoil analysis, cracking pilot tests and deep design know-how. Given the interests of ethylene producers, this knowledge basis can be used to evaluate possible impacts of impurities on a plant, identify purification requirements, establish tailored removal or upgrade strategies, evaluate the impact of pyoils on furnace performance, define acceptable plastic pyoil addition limits, and derive revamp possibilities to increase pyoil content.
Takeaways. The reduction of greenhouse gas emissions and circularity are two major trends facing the ethylene industry. Using plastic pyoil from waste plastics is one of the possibilities to make the industry more sustainable. However, it should be noted that currently produced plastic pyoils present a broad spectrum of characteristics, depending on the type of plastic waste available, the waste sorting process, the pyrolysis technology, the pyrolysis conditions and the pre-/post-treatment applied to the oils (e.g., distillation, adsorption, absorption). Ethylene producers are therefore faced with the difficult task of selecting suitable pyrolysis partners and evaluating the possibility of using such oils in their plants. Furthermore, the design or revamp of a plant processing such oils must take many aspects of these untypical feedstocks into account.
To support the decision-making process, the authors’ company has derived a holistic approach based on three major pillars. First, plastic pyoils are analyzed in detail to identify the impurities, the hydrocarbon matrix, the boiling range and other characteristics. Second, pilot testing facilities allow for detailed and comparative characterization of these feedstocks under commercial operating conditions. In turn, this leads to a better understanding of the behavior of impurities and of the cracked gas yields. Third, deep design knowledge of furnaces, separation, and the distribution of contaminants and their impact on plant operations, as well as possible purification sequences, are required to develop a cost-effective solution while avoiding operational risks.
Against this background, the authors’ company can support ethylene producers for diverse tasks (FIG. 9), such as screening plastic pyoils from different producers, evaluating possible impacts of impurities on a plant, designing purification and upgrade sequences, evaluating the impact of pyoils on furnace performance, determining acceptable blending limits of plastic pyoils in existing plants, predicting cracked gas yields, and designing plant modifications or new plants for circular feedstocks. With these support activities, special care is taken to balance operational and capital expenditures and to maximize plant revenues.
The authors’ company’s holistic approach makes it possible to find solutions for various issues and to master challenges associated with the new feed materials, thereby driving the transition to more sustainable olefin production and a more circular economy. HP
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.).