C. Delhomme-NEUDECKER, N. SCHÖDEL, Á. TÓTA and G. SCHMIDT, Linde Engineering,
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).
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.
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.
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.
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.
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
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
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.
analyses and main findings. Below are some of the primary findings from several analyses.
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
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
FIG. 4 shows the difference in 2D-GC analyses of plastic pyoils derived from PE-rich
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.
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.
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
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
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
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
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
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
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
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.).