June 2024

Special Focus: Process Optimization

A promising new FCC feedstock: Heavy-cut pyrolysis oil from post-consumer plastic waste

Plastics are an essential material in modern civilization. Their light weight, durability and property flexibility provide benefits in food preservation, medical applications, building and vehicle energy efficiency, and many other areas.

IP: 52.14.148.153

Plastics are an essential material in modern civilization. Their light weight, durability and property flexibility provide benefits in food preservation, medical applications, building and vehicle energy efficiency, and many other areas. The environmental pollution from plastic waste is one of the pressing challenges to be addressed by the global community. The circularity of plastics is a fundamental method to relieve the environmental burden and convert the valuable molecules in waste plastics to feedstocks for new plastics.

Mechanical and advanced chemical recycling are rapidly finding global interest. In the U.S., more than $8.7 B in investments have been announced to improve the circularity of waste plastics and convert them to alternative products, thus reducing the carbon intensity.¹ Thermal pyrolysis is widely applied in advanced chemical recycling to valorize mixed plastic waste and convert it to fuels and petrochemical building blocks.

The primary product from thermal pyrolysis is a liquid that can be distilled into a series of boiling point fractions for further use. The heaviest cut from the distillation process is a logical feedstock for fluid catalytic cracking (FCC) where it can be cracked and upgraded into transportation fuels. In industrial practice, the waste plastic pyrolysis oil (WPPO) stream would be a fraction of the total charge to an FCC unit (FCCU) since there are significantly lower quantities of commercially available waste plastic pyrolysis oil than those of fossil crudes. To help refiners understand the yields and potential challenges of using pyrolysis oil from post-consumer plastic waste as a feedstock, this article presents pilot plant results when co-processing vacuum gasoil (VGO) with 10 wt% and 20 wt% of the heavy cut of a commercially produced pyrolysis oil made from mixed post-consumer plastic waste.

Production of pyrolysis oil from post-consumer plastic waste

Following more than a decade of process development in Yonkers, New York, the co-authors’ company operates a learning center for the advanced recycling of plastic in Zebulon, North Carolina (U.S.). The center produces International Sustainability and Carbon Certification (ISCC) PLUS certified, sustainable pyrolysis oil and char from mixed-waste plastics while solidifying the design basis of future company facilities. The typical plastic used in the process consists of post-consumer types #2, 4, 5 and 6 [low-density polyethylene (LDPE), polypropylene (PP), high-density polyethylene (HDPE) and polystyrene (PS)].

The learning center consists of a material handling section, a system for conveying pre-processed feedstock to the reactor, a pyrolysis section consisting of one full-scale reactor train^a inclusive of char handling, a product recovery section as well as product storage and loading equipment. Nominally, during operation at the learning center, the co-authors’ company’s reactor train^a processes between 1,000 lb/hr and 1,500 lb/hr of plastic, depending on feedstock and desired product yields (future commercial facilities intend to run at higher processing rates per reactor train).

The material handling section takes in post-consumer, mixed waste plastic, sorts and shreds it (when necessary) and pneumatically conveys it to the reactor train^a, the heart of the company’s proprietary advanced recycling process (FIG. 1). The incoming waste plastic feeds the reactor train^a extruder section, which in turn feeds plastic to the conversion reactor with negligible oxygen contamination. The pyrolysis reactor consists of high-grade stainless-steel tubes/augers and multiple zoned heaters providing the energy that thermally cracks the plastics into pyrolysis oil^b, energy-rich vapor (mostly non-condensable light gases) and char. The multiple heating zones allow the company to adjust operating conditions as needed to optimize product yield from the wide range of mixed-waste plastics. The bulk of the char is removed from a draw in the final reactor section.

FIG. 1. A simplified flowsheet of the co-authors’ company’s process.

 

The reactor effluent (mainly the company’s proprietary vapor^c which refers to the combination of pyrolysis oil^b and the energy-rich vapor) feeds a simple solid vapor section that reduces the amount of entrained char, ash, tar and atomized oil droplets being carried over to the recovery section (knocking these materials to the back to the last reaction section for removal with the bulk char). After passing through the solid-vapor separation section, the vapor^c is fed to the product recovery section, which separates the energy-rich vapor from the pyrolysis oils^b. The energy-rich vapor is recovered and used to heat the reactor^a and drive thermal cracking. The recovery section also separates the pyrolysis oils^b into fractions that can be integrated into the company’s customers’ petrochemical processes. One of the fractions that the learning center can produce is the pyrolysis oil^b heavy cut, which is predominantly material that boils above 600°F (boiling curve details are provided below). The company’s pyrolysis oil^b heavy cut is a waste plastic-derived pyrolysis oil (WPPO).

The feedstock used to produce the pyrolysis oil^b heavy cut meets the ISCC “definition of waste or residue, i.e., it was not intentionally produced and not intentionally modified, or contaminated, or discarded, to meet the definition of waste or residue.” The reactor train^a design emphasizes maximizing the range of waste plastics it can convert. The waste material feedstock used to produce the pyrolysis oil^b heavy cut used in this study was primarily polyolefin based. Conditions in the reactor train^a were optimized to yield premium lighter range pyrolysis oils^b, such that the pyrolysis oil^b heavy cut in this study reflects the anticipated product quality in the company’s future multi-train facilities.

Properties of heavy cut pyrolysis oil from post-consumer plastic waste

Detailed physical and chemical characterizations were conducted on the heavy cut of pyrolysis oil produced from mixed post-consumer plastic waste (WPPO). Properties are compared to those of a typical Mid-Continent VGO in TABLE 1. Compared to the VGO, the WPPO has a lower specific gravity, higher calculated hydrogen (H) content and a higher Watson K-factor, reflecting a higher paraffinicity of the WPPO. The calculated H content of 13.7 wt% for the WPPO is high compared to the H content of FCC feedstocks, where 12.7 wt% H is typical and 13.5 wt% is considered a deeply hydrotreated paraffinic feed.² Nitrogen and sulfur contents of the WPPO are both low at 0.02 wt%. The WPPO has a wider boiling point distribution than the VGO and a higher percent boiling point above 1,000°F (538°C). It also has a slightly higher Conradson carbon level (1 wt% vs. 0.4 wt%).

 

The WPPO has a higher level of metals impurities than the VGO. A wide range of heteroatoms are typically found in heavy-cut pyrolysis oil including metals (chromium, copper, iron, magnesium, nickel, titanium and vanadium), alkali-earth metals (magnesium, calcium), alkali metals (sodium, potassium), as well as phosphorus and sodium. These trace contaminants come from the contaminants and additives in the starting post-consumer plastic waste used to manufacture the pyrolysis oil.

Typical contaminant concentrations are in the range of 5 mg/kg–35 mg/kg, but certain elements, most commonly calcium, iron and silicon, can spike higher in individual batches depending on the starting post-consumer plastic waste processed. To manage contaminant spikes in individual batches in commercial practice, specifications can be set on metals levels and blending of different heavy-cut pyrolysis oil batches to achieve the specifications. For the heavy-cut WPPO batch used in this study, there was no vanadium, and aluminum, antimony, barium, chromium, copper, manganese and zinc levels are within ranges normally seen for FCC feedstocks.

Calcium, iron, magnesium, phosphorus and sodium levels are at the high end of typical FCC feedstock ranges. The calcium contribution is likely from calcium carbonate used as a filler in plastics. For a unit with a catalyst addition rate of 0.12 lb/bbl feed, at steady state, 1 wt% of this batch of WPPO would result in 0.18 wt% added calcium oxide (CaO) on the equilibrium catalyst (Ecat), 5 wt% WPPO would result in 0.89 wt% added CaO on Ecat, and 10 wt% WPPO would result in 1.78 wt% added CaO on Ecat. High levels of calcium added to Ecat are likely to result in pore blockage and reduce conversion. Depending on the blending levels of WPPO, a catalyst strategy for metals management may be needed. Possible management methods include:

• Reviews of optimum insertion points for WPPO in the refinery and opportunities for contaminant removal
• A catalyst management strategy to keep contaminant metals at target levels
• The use of improved metals tolerance FCC catalyst technologies.

Since feed contaminants can vary between different WPPOs, a metals assay and careful planning for potential metals increases are needed when beginning a WPPO trial. Based on feed analysis, WPPO would be considered a high H-content, paraffinic feedstock that should have high crackability.

FCC of waste plastic pyrolysis oil

To understand the yields of WPPO, blends of 10 wt% and 20 wt% WPPO were co-processed with Mid-Continent VGO in a continuous circulating riser FCC pilot plant^d. As a base case, 100% Mid-Continent VGO was cracked. The WPPO blend ratios might overstate actual realistic co-processing levels given the supply situation. However, they have been selected to result in yield changes sufficiently significant for the corresponding analysis methods and minimize the uncertainty in the determination of incremental yields that can ultimately be used to interpolate low co-processing level impacts.

The riser outlet temperature was 970°F (521°C). The unit operated in adiabatic mode, where changing feed preheat or regenerator temperatures will result in a change in catalyst circulation to maintain reactor outlet temperature, the same process control strategy used in commercial FCCUs. The catalyst-to-oil (C/O) ratio was varied by changing feed preheat in increments between 300°F (149°C) and 700°F (371°C). The feed rate was 1 kg/hr. The catalyst used for the experiments was a low-metals commercial Ecat that did not contain ZSM-5 or sulfur oxides (SO_x) reduction additives to avoid any bias in yields from additive usage^e. The catalyst properties are listed in TABLE 2. It should be noted that in WPPO commercial application, steady-state metals levels on the Ecat will be higher than those on the sample used in this experiment due to the contaminant metals in the WPPO.

 

No miscibility issues or processing issues were encountered when running the WPPO blended in VGO. One feed blend was run per day, and the typical time on feed was approximately 9 hr. The inspection of feed tanks after co-processing runs did not note any deposits or build-up. There was no difference in feed nozzle pressure between the 100% VGO case and the blends with 10 wt% and 20 wt% WPPO. In all cases, the nozzle pressure was between 31 psig and 32 psig (2.1 barg and 2.2 barg) and was steady throughout the experiments, suggesting no deposit formation on the nozzle.

Plots of conversion, propylene, total C₄ olefins, gasoline, light cycle oil (LCO), and coke yields vs. C/O are presented in FIG. 2. As expected, based on the paraffinic nature of WPPO, the co-processing of WPPO resulted in increased yields of propylene, butylene and gasoline and decreased yields of LCO. The coke yield was unchanged vs. the 100% Mid-Continent VGO case. The gasoline composition and properties were analyzed via a proprietary octane calculation model^f based on detailed hydrocarbon analysis.^3,4

FIG. 2. Product yields as a function of C/O ratio and feed.

 

FIG. 3 presents the gasoline properties as a function of the C/O ratio. When WPPO is blended with VGO, the gasoline produced has a higher concentration of paraffins and iso-paraffins and a lower concentration of aromatics, resulting in lower gasoline octane [about 0.4 G-Con research octane number (RON) and 0.3 G-Con motor octane number (MON) for the 10 wt% WPPO blend]. The higher concentration of paraffins is as expected, based on the more paraffinic nature of the WPPO feedstock. TABLE 3 presents interpolated yields at C/O = 7 for the three feeds. As seen in the table, increasing amounts of WPPO resulted in the increased production of propylene, butylenes and gasoline. Dry gas production was unchanged when WPPO was blended with VGO. The WPPO blends produced C₃ and C₄ streams with higher olefinicity ratios than the 100% VGO case.

 

FIG. 3. Gasoline properties as a function of C/O ratio and feed.

 

The concept of incremental yield can be used to estimate the yields of the WPPO within the environment of the base feedstock. Incremental yield is defined as (Eq. 1):

Y ' = [Y_blend – (1 – X )Y_base ] / X  (1)

where:

Y_blend is the yield of the blended feedstock

Y_base is the yield of the base feedstock

X is the weight fraction of the blend component.⁵

TABLE 4 presents the incremental yields of the WPPO for the 10 wt% and 20 wt% cases, along with yields for the 100% VGO case for comparison. As seen in the table, WPPO cracks to high yields of desirable products like propylene, butylene and gasoline. The yields of LCO and bottoms are low, while the yields of coke are comparable to those of VGO. It is believed that the heaviest boiling tail of the WPPO contributes to the coke yield.

 

In adiabatic pilot plant operation, the energy for cracking must be supplied by the catalyst coming from the regenerator. Changes in the heat of cracking will change the catalyst circulation and, consequently, the C/O ratio at the same feed preheat and riser outlet temperatures. Within the range of the measurements, no difference to the 100% VGO case was noted in the C/O ratio required to achieve a given riser outlet temperature at a given feed preheat when the blends of the WPPO were run. Therefore, the heat of cracking of the WPPO is similar to that of VGO. This is expected, since both feeds are of similar nature comprising large long-chain hydrocarbons. In contrast, feedstocks containing heteroatoms like oxygen have been observed to have significantly different heats of cracking compared to VGO.⁶

As expected, based on the lower sulfur content of WPPO vs. VGO, the sulfur content in products drops with the increasing amounts of WPPO in the feed.

The total liquid product (TLP) from the study was analyzed by X-ray fluorescence (XRF) for metal contaminants. Calcium (Ca), chromium (Cr), copper (Cu), iron (Fe), potassium (K), manganese (Mn), nickel (Ni) and vanadium (V) were below detection limits of ~1 mg/kg for all samples. This confirms that calcium from WPPO feed is not transferred to liquid product. The elemental composition of Ecat was measured by ICP-OES before and after runs to determine if any feed impurities built up on the Ecat. The time on the WPPO feed each day was ~9 hr, and a fresh Ecat sample was used for each feed blend to prevent any bias from a potential feed impact on catalyst inventory.

Within the limited time onstream for the catalyst, the only element that had a measurable increase on the Ecat was calcium. FIG. 4 shows CaO on the Ecat after 9 hr on feed as a function of percentage WPPO in the feed blend. The CaO level on the virgin Ecat was 477 mg/kg. The amount of calcium deposited on the catalyst was close to that expected based on the calcium concentration in the WPPO and assuming complete catalyst deposition.

FIG. 4. Concentration of CaO on Ecat after 9 hr on feed.

Takeaway

Compared to most FCC feedstocks, the heavy cut of pyrolysis oil produced from mixed post-consumer plastic waste (WPPO) is a high-H content, paraffinic feed. The co-processing of such a waste-derived stream in a circulating FCC pilot plant confirmed the high crackability of the material, increasing the yields of saleable FCC product. No processing issues were encountered when running the WPPO blends.

Compared to the 100% VGO case, blends with WPPO cracked to higher conversion, producing more propylene, C₄ olefins and gasoline. LCO and bottoms decreased. Coke and dry gas were unchanged. The heat of cracking of the pyrolysis oil was similar to that of VGO. The yield shifts observed when co-processing the heavy-cut pyrolysis oil derived from post-consumer plastic waste were consistent with expected changes based on the feed properties. Levels of WPPO co-processed will depend on commercial availability and logistics considerations. Depending on the blend level, the higher level of unconventional FCC feed contaminants in WPPO might require considerations on catalyst management strategy, reviews on optimum insertion points for WPPO in the refinery, and/or use of improved metals tolerance of FCC catalyst technologies.

This work shows that heavy-cut pyrolysis oil derived from mixed post-consumer plastic waste is a promising feedstock for FCC. FCC processing of such waste plastics-derived feed streams results in lower carbon intensity FCC products, valorizes the hydrocarbon building blocks in plastic waste, redirects waste plastics from landfills to a more circular approach, and contributes to the reduction of the globally growing plastic waste problem. HP

ACKNOWLEDGEMENTS

The authors would like to thank George Allen and the Braven operations team at the company’s Zebulon plant for producing the material, drawing representative samples from its operations, and coordinating the transport to W. R. Grace. The hard work and dedication of the technicians, chemists and engineers at W. R. Grace who conducted the feed analysis, pilot plant testing and product analysis is gratefully acknowledged.

NOTES

1. Braven Reactor Train™ (BRT™)
2. Braven’s PyChem™
3. Braven’s PyVapor™
4. Grace Davison Circulating Riser (DCR™) Pilot Plant
5. The equilibrium catalyst was from a commercial FCC operation 100% supplied by W.R. Grace & Co. that did not use purchased Ecat or additives.
6. Grace G-Con^® Octane Calculation Software

LITERATURE CITED

1. American Chemistry Council, “The potential economic impact of advanced recycling and recovery facilities in the United States,” April 2022.
2. Grace, “Guide to fluid catalytic cracking—Unlocking FCC value,” 2nd Ed., Ch. 5, 2021.
3. Haas, A., G. McElhiney, W. Ginzel and A. Buchsbaum, “Gasoline quality—The measurement of compositions and calculation of octanes,” Petrochem/Hydrocarbon Technology, 1990.
4. Cotterman, R. L. and K. W. Plumlee, “Effects of gasoline composition on octane number,” ACS Meeting, Miami Beach, Florida, 1989.
5. Harding, R. H., X. Zhao, K. Qian, K. Rajagopalan and W. C. Cheng, “Fluid catalytic cracking selectivities of gas oil boiling point and hydrocarbon fractions,” Industrial & Engineering Chemistry Research, August 1996.
6. Bryden, K., G. Weatherbee and E. T. Habib, “Flexible pilot plant technology for evaluation of unconventional feedstocks and processes,” AM-13-04, 2013 AFPM Annual Meeting, San Antonio, Texas.
7. Winn, F. W., “Physical properties by nomogram,” Petroleum Refiner, 1957.

The Authors

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