May 2024

Special Focus: Biofuels, Alternative/Renewable Fuels

Tackle operational challenges with FCC coprocessing applications

Refiners are actively developing new solutions to decarbonize their finished products—such as transportation fuels or commodity chemicals—as part of the energy transition. As a result, refiners are now exploring the co-feeding of alternative feedstocks, including renewable and recyclable crude oils, as a means of lowering the carbon footprint of their products.

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Refiners are actively developing new solutions to decarbonize their finished products—such as transportation fuels or commodity chemicals—as part of the energy transition. As a result, refiners are now exploring the co-feeding of alternative feedstocks, including renewable and recyclable crude oils, as a means of lowering the carbon footprint of their products. Given its inherent flexibility, the fluid catalytic cracking (FCC) process has potential to lower the carbon intensity of its products. However, these new feeds still present challenges, such as additional contaminants, instability and miscibility issues, and elevated acidity that can lead to a variety of operational challenges in an FCC unit (FCCU). This article provides an overview of the different sustainable feedstocks and possible solutions to overcome these challenges, including feed pretreatment, operational adjustments and hardware design options.

In Europe, the decarbonization of the transportation sector is driven in large part by the Renewable Energy Directive III (RED III), which was officially adopted on October 18, 2023.¹ RED III has established two objectives, requiring European member states to adopt one of these targets by 2030: (1) a reduction of greenhouse gas (GHG) intensity by 14.5% in transport, or (2) a 29% share of renewables in the total energy consumption of transportation (FIG. 1). There are also minimum requirements for renewable fuels of non-biological origin (RFNBO), established, at minimum, to be a goal of 1% by 2030.

 

Non-EU countries also have systems encouraging renewable fuels use, such as the Renewable Fuel Standard (RFS) in the U.S., which incentivizes U.S. refiners² to blend renewables into transportation fuels or to obtain renewable identification number (RIN) credits. Another example is the RenovaBio policy in Brazil (adopted in December 2017) that aims to reduce the carbon intensity of transportation fuels by the production, commercialization and use of biofuels.

Potential refinery insertion points for sustainable feedstocks 

Typical locations to add sustainable feeds in refineries are: (1) before the crude distillation column without pretreatment, (2) in some of the processing units with and without pretreatment, or (3) with the finished fuels after high-severity upgrading. Refiners likely will see the most benefit if the alternative feed is introduced into processing units such as the FCCU, hydrotreater or hydrocracker. The insertion of sustainable feeds, such as biocrude, at the distillation stage contains risk, as contaminants—mainly oxygenates—could be distributed throughout the refinery. Coprocessing through an FCCU offers the benefit of requiring no additional hydrogen (H₂) and minimal modifications to the refinery. The FCCU’s catalysts are continuously regenerated by burning off coke in the regenerator before recirculating the catalyst to the riser for further cracking reactions, and by breaking down large molecules to more valuable products, such as gasoline and liquefied petroleum gas (LPG) olefins. Hydroprocessing applications can pose higher risk compared to FCC because reactors are loaded with fixed catalytic beds that can result in metal deactivation and coking during the cycle length. Furthermore, FCCU catalysts are highly tunable and can be designed to adjust yields, or to direct deoxygenation pathways toward water (H₂O) production to preserve renewable carbon or toward carbon oxides to preserve H₂ in products.

Sustainable feedstock options and their associated challenges 

Coprocessing through FCCUs offers a refinery flexibility to introduce a variety of different sustainable feedstocks (FIG. 2) to create lower-carbon-intensity fuels or chemicals, such as:

• Plastic wastes, including pyrolysis oils (pyoils) and Fischer-Tropsch waxes
• Plant- and livestock-derived oils and fats
• Biomass wastes, including pyoils and hydrothermal liquefaction (HTL) oils.

 

Pyoils from waste plastics might contain significant amounts of chlorine [e.g., polyvinyl chloride (PVC)], metals and oxygenates [e.g., polyethylene terephthalate (PET) and polycarbonates]. The waxy/heavy fractions of plastic pyoils are typically soluble in conventional vacuum gasoil (VGO) or resid feeds and are mainly composed with aliphatic compounds that can be easily further cracked through FCCUs.

Plant- and livestock-derived oils and fats—such as vegetable oils, animal fats and used cooking oils—contain triglycerides and are soluble in conventional FCCU feeds. They are typically paraffinic and can be easily cracked through FCCUs. While they do not contain free H₂O and have lower oxygen content compared to other bio-oils, they might contain elevated levels of free fatty acids and metal impurities, such as alkali and earth alkaline metals.

Biomass wastes, like fast pyrolysis and HTL oils, have already demonstrated crackability in FCCUs, but they have also introduced operational challenges in FCC co-processing applications, including:

• Miscibility issues with fossil feedstocks due to high-polarity molecules and free H₂O, requiring dedicated storage, pumping and piping metallurgy
• Instability of bio-oils during transportation and at feed injection temperatures, requiring specific vessels and dedicated injection line delivery systems, respectively—and, if a dedicated injection nozzle is required, its location must be optimized within the FCCU riser
• High variability in alkali, earth alkaline metals, acidity and oxygen contents.

A summary of these impurities and their impacts is detailed in TABLE 1.

The upgradability of bio-oils also induces another challenge due to their oxygenate content. Huber and Corma³ described that the conversion of oxygenates from biomass-derived feedstocks in the FCCU mainly occurs through five different reactions: (1) dehydration reactions, (2) the cracking of large oxygenated molecules to smaller molecules, (3) H₂-producing reactions, (4) H₂-consuming reactions (H₂ transfer), and (5) the production of larger molecules by C–C bond-forming reactions (aldol condensation or Diels-Alder reactions).

An example of this concept is the crackability of glycerol toward propylene. According to Eq. 1, the yield of propylene from glycerol in FCC can be up to 77% (based on carbon), the balance being carbon dioxide (CO₂) and H₂O. However, oxygen could be rejected as carbon monoxide (CO) and H₂O (Eq. 2), resulting in a maximum propylene yield of 66%. Oxygen could also be rejected as H₂O by dehydration reactions (Eq. 3), giving an even lower maximum propylene yield of 33%:

9/7 C₃H₈O₃ → C₃H₆ + 6/7 CO₂ + 15/7 H₂O          Carbon yield to propylene = 77%          (1)

1.5 C₃H₈O₃ → C₃H₆ + 1.5 CO + 3 H₂O                 Carbon yield to propylene = 66%          (2)

3 C₃H₈O₃ → C₃H₆ + 6 C + 9 H₂O                          Carbon yield to propylene = 33%          (3)

Therefore, to maximize propylene yield from glycerol, the dehydration, H₂ forming and H₂ transfer reactions (H₂ consuming) should be minimized by choosing proper catalyst designs and operating conditions to maximize the reaction in Eq. 1.

Pretreatment of sustainable feedstocks 

Sustainable feedstocks—including both biogenic feedstocks and waste plastic-derived oils—differ from crude oils due to the presence of oxygen and elevated levels of alkali metals (e.g., Na, K), earth alkaline metals (e.g., Ca, Mg), chlorides and phosphorus. Since these contaminants can cause catalyst deactivation and operational issues, such as fouling or corrosion issues, it is recommended to reduce their concentration prior to co-processing. At commercial scale, several pretreatment processes exist to remove contaminants,⁴ such as:

• Filtration
• Hydrocyclonic removal of particles
• Desalting
• H₂O or acid washing (degumming)
• Hydrotreating applications (hydrodeoxygenation, fixed-guard beds)
• Purification adsorbents.

Particles and other solids in these sustainable feeds can lead to instability. Filtration has been shown to remove particulates such as char and alkali metals. Degumming is another technique that has demonstrated the ability to remove phospholipids and trace metal ions from crude vegetable oils. H₂O degumming is effective for phospholipid removal, while alkali salts require acid degumming. FIG. 3 shows an example of methane sulfonic acid (MSA) vs. citric acid for the degumming of crude soybean oil, with MSA reducing the concentration of Ca, Mg and phosphorus.

 

Sustainable feedstocks might also contain elevated chloride levels that should be minimized prior to FCC introduction. Chemically, chlorides can result in the reactivation of contaminant Ni deposited on equilibrium catalysts, leading to unwanted dehydrogenation reactions (higher H₂ and coke yields). Operationally, since there is often an excess of ammonia (NH₃) from feed cracking, any additional chlorides can lead to the formation of incremental ammonium chloride (NH₄Cl) deposits at the top of the main fractionator. As such, the introduction of chlorides should be limited by feed management and careful catalyst selection to minimize chlorides in FCCU catalysts, such as in-situ catalyst technology. Feed chlorides can be reduced with adsorbents. Conventional alkali-promoted alumina will not be efficient for dechlorinating waste-plastic pyoils. A chloride guard oriented toward the removal of organic chlorides is preferred to maximize the dechlorination process.

Crude oils typically contain < 2% oxygen, while biogenic feedstocks can contain up to 60% oxygen. The oxygen exists in a variety of chemical functional groups, including carboxylic acids, alcohols, aldehydes, esters, sugars, furans, ethers and hydroxyl groups. As noted previously, oxygen is undesirable in an FCCU, as it can limit conversion of the intended reaction. Additionally, oxygen-containing functional groups in bio-based feedstocks are hydrophilic and the presence of H₂O can cause significant problems, including the corrosion of metallurgy in refinery processing units and piping. Finally, oxygen-containing groups are also very reactive and can cause polymerization between molecules, forming gums, acids and other impurities during storage.⁴ Mild hydrotreatment, performed at low temperatures, is a treatment method to reduce the oxygen-containing molecules in biocrudes. During such a mild hydrotreatment step, deoxygenation reactions will take place (Eqs. 4–6):

R–CH₂–CH₂–COOH → R–CH=CH₂ + CO + H₂O                          (Decarbonylation)            (4)

R–CH₂–CH₂–COOH → R–CH₂–CH₃ + CO₂                                  (Decarboxylation)             (5)

R–CH₂–CH₂–COOH + 3H₂ → R–CH₂–CH₂–CH₃ + 2H₂O          (Hydrodeoxygenation)         (6)

Catalytic pyrolysis has also been used to stabilize the bio-oil before coprocessing through the FCCU. In catalytic pyrolysis, oxygen is removed as H₂O and carbon oxides over a zeolite-based catalyst.

The importance of miscibility 

Coprocessing mixtures of fossil and biogenic feeds might result in blending issues. Lipids, such as triglycerides and fatty acids, are miscible with fossil feeds and form stable emulsions. However, for bio-oils, the presence of many oxygen-containing molecules results in a polar phase immiscible with fossil feedstocks.⁴ Other factors—such as the density, viscosity, surface tension, heteroatom distribution, refractive index and boiling point ranges—might limit feed miscibility, as well.

The mild hydrotreatment of biocrudes could improve miscibility through oxygen removal. However, the oxygen content at which miscibility is no longer an issue could vary. Another solution is to use separate injection nozzles at the bottom of the FCCU’s riser.⁵ This allows using lower temperatures for the bio-oil (< 50°C) and higher temperatures for the fossil oil (220°C–280°C) to reduce its viscosity and achieve good atomization and dispersion through the feeding nozzle. The preferred location for injection of the bio-oil is at the reactor bottom close to the feed injection area, where thermal cracking of bio-oil can occur at high temperatures and high catalyst/oil ratios. This thermal cracking transfers the large bio-oil molecules into smaller ones, which can penetrate and react in the catalyst pores to lead to cracking reactions. Careful consideration with the FCC technology licensor should be taken to determine the type of oil, the ratio of the coprocessing oil to the primary feedstock and the location of injection, since operating conditions, feed zone configuration and coprocessing objectives will vary among FCCUs.

The licensor and testing arms of the FCC Alliance^a, in collaboration with a spray nozzle manufacturer^b, have developed a two-fluid atomizing nozzle specifically for injecting fast pyrolysis bio-oil (FPBO) into the FCCU’s riser. Understanding the issues of immiscibility with petroleum crude, along with the unique mixture of components of bio-oil that can polymerize and cause plugging, and the role that both time and temperature can play in increasing bio-oil viscosity, enables several features to be incorporated in the injector’s design to ensure that refiners can co-process bio-oils while also minimizing potential plugging or corrosion. A schematic of the bio-oil injector is shown in FIG. 4.

 

To limit exposure of bio-oil to hot metal surfaces inside the injector, the inner bio-oil lance is insulated. This layer of insulation helps maintain bio-oil temperatures between 40°C and 70°C, which is low enough to prevent bio-oil polymerization while providing a suitable viscosity for good atomization. Dispersion gas—which can be steam or fuel gas—flows in the annulus between the outside diameter of the insulation casing and the inner diameter of the injector barrel before mixing with the bio-oil near the injector tip. The mixing chamber is designed to minimize the contact time between dispersion gas and the bio-oil, and does not require bio-oil to flow through very small orifices to minimize the tendency for plugging. The mixture of bio-oil and dispersion gas exits through a slotted tip at the end of the injector where the spray pattern is formed. While specifically designed for injecting fast-pyrolysis-type bio-oils, the injector is flexible to atomize a wide variety of other hydrocarbon feedstocks. Operation of the injector is like typical petroleum injectors, although several additional steps must be taken to minimize plugging when bringing the injector into or out of service.

Since FPBOs tend to have elevated concentrations of organic acids and chlorides, the injector is fabricated with 317L stainless-steel material. For feedstocks with high chloride content, Hastelloy may be used for additional protection against pitting and corrosion within the injector. Bio-oil storage and feed lines should also be a stainless-steel material like 316L or 317L, as these have proven resistant to corrosion over long-duration exposures to bio-oils.

Potential corrosion impacts 

As bio-oils and plastic pyrolysis oils present higher chlorides, total acid numbers (TANs), H₂O and oxygen, they are more prone to cause corrosion. For bio-feedstocks, organic acid [carboxylic acid (RCOOH), measured by TAN] and chlorides are the main corrosion-causing pollutants. The long-chain organic acids that are usually present in these feedstocks are weak acids that only slightly acidify free H₂O in contact with the feedstock (e.g., storage system) at ambient temperature. The corrosion rate of carbon steel may increase to moderate values if there is a continuous wetting of carbon-steel surfaces with free H₂O that dissolves organic acids. Conversely, if H₂O content is minimized by free H₂O separation, continuous wetting and moderate corrosion by acidic H₂O are no longer expected, and carbon steel-like metallurgy is adapted for and operated (from the storage system) at ambient temperature. Finally, stainless steel is preferred for bio-oil storage and feed lines for an extended design life, and to avoid injector fouling by corrosion products of carbon steel.

Organic acids can also promote high-temperature corrosion similar to naphthenic acids in crude distillation towers. Carbon steels and low-alloy steel materials are the first to be impacted, but acids at high temperatures might also attack non-molybdenum (Mo) stainless steels. The selection of SS 317L is a standard countermeasure to reduce high-temperature corrosion by organic acids. It is recommended to select this grade for severe conditions with high temperatures and high TANs.

After reaching very high temperatures > 400°C in the FCCU’s reaction section, organic acids are degraded to CO₂ and do not promote high-temperature corrosion by organic acids anymore. However, CO₂ is a weak acid that promotes further corrosion of carbon steel and low-alloy steel materials in the presence of a humid or a free H₂O phase (“wet CO₂”) in downstream equipment and piping. This corrosion is mainly influenced by temperature, the presence of hydrogen sulfide (H₂S), H₂O wetting of metallic surfaces and CO₂ partial pressure, and it may require the selection of a high-chromium corrosion-resistant alloy (e.g., SS 316L) or the injection of corrosion inhibitors for the neutralization of carbonic acid.

Chloride-containing compounds may quickly form hydrogen chloride (HCl) in reaction sections, which promotes the formation of hydrochloric acid (a strong acid) in the presence of free H₂O. Free H₂O acidified by HCl dissolution and without any neutralizing agent will be very corrosive to carbon steel and to low-alloy steel materials (generalized corrosion), but also to stainless steels (localized corrosion: pitting and stress corrosion cracking). The selection of corrosion-resistant materials with significant chromium (resistance to generalized corrosion) and significant nickel (resistance to localized corrosion) is a standard countermeasure to mitigating corrosion by wet HCl. In the main fractionation overhead system, corrosion by wet HCl is linked to an acidic pH. If the pH remains near neutral or alkaline, then the impact of corrosion on the materials in place (carbon steel) should remain limited, and the co-authors’ company recommends an inhibitor injection package to be implemented in the H₂O recycle line to the main fractionator air condenser. There are different types of corrosion inhibitors: anodic, cathodic, neutralizing and film forming, among others. The design of the inhibitor solution depends on many factors, including the metallurgy and operating conditions, and it should be performed in partnership with experienced suppliers of inhibitor solutions.

NH₄Cl salt deposition represents another phenomenon that negatively impacts FCCUs, specifically in the mid-sections to upper sections of the main fractionator column and overhead condensing system. The formation of solid NH₄Cl occurs when the salts “precipitate” from the vapor phase, and the deposition takes place when the temperature at any point drops below the salt’s dewpoint (at cold spots). Salts are deposited in different zones (FIG. 5). The main consequences of salt fouling are the plugging of trays, distributors and products draws; the loss of fractionation efficiency between gasoline and light-cycle oil (LCO); an impact on the pressure balance of the unit; and increased corrosion, among others.

 

NH₄Cl corrosion occurs primarily on cold spots (like the pumparound return) inside the column. Once formed, the NH₄Cl salts are highly hygroscopic, and the result can be a very aggressive under-deposit corrosion of the tower internals. For column internals, H₂O can be injected at the column top (using the reflux line at reduced capacity), or salt dispersant can be injected in the reflux or the pumparound return. To prevent salt deposition in the overhead system, washwater is recycled continuously upstream of the fractionator air condenser. In addition, a chloride-free catalyst is recommended to minimize the introduction of chlorides from the catalyst through the FCCU, such as in-situ technology.

Takeaways 

Coprocessing renewable or recycled oils offers refiners the possibility to lower the carbon footprint of the transportation fuels or chemicals they produce, and the FCCU stands out as an ideal process within a refinery for upgrading these alternative feeds. However, these alternative feedstocks introduce significant challenges in both catalyst performance and unit operations. For instance, elevated metal levels in the feeds can lead to catalyst deactivation by active site neutralization, zeolite destruction or through pore mouth plugging. Operationally, some alternative feeds demonstrate miscibility issues with fossil-based feedstocks. Additionally, they are unstable at standard temperatures and can cause corrosion of process equipment.

This article introduces practical solutions to each of these challenges. A drop-in catalyst activity or changes in product selectivity can be mitigated by careful selection of optimal catalyst technology. The immiscibility and instability of biogenic feedstocks can be addressed through the correct feed injection technology. The selection of the correct metallurgy and corrosion inhibitors is important to guard against possible corrosion introduced by some alternative feeds. In this article, several pretreatment solutions—such as mild hydrotreatment, degumming, adsorption, catalytic pyoil and others—have also been highlighted to further minimize these operational challenges. The process of increasing the share of renewables in transportation fuels to reach 29% by 2030—as per RED III—can be achieved by taking advantage of the flexibility of the FCCU to co-process a variety of alternative feedstocks. Today, the industry is collectively taking a step into the future of refining as we learn how to implement these changes together more fully. HP

NOTE

1. Technip Energies, Axens and IFP Energies nouvelles (IFPEN)
2. Spraying Systems Co.

LITERATURE CITED

1. European Union, “Directive (EU) 2023/2413 of the European Parliament and of the Council,” October 18, 2023, online: https://eur-lex.europa.eu/legal-content/ES/TXT/HTML/?uri=OJ:L_202302413
2. U.S. Environmental Protection Agency (EPA), “Overview for renewable fuel standard,” January 2024, online: https://www.epa.gov/renewable-fuel-standard-program/overview-renewable-fuel-standard
3. Huber, G. W. and A. Corma, “Synergies between bio- and oil refineries for the production of fuels from biomass,” Angewandte Chemie, September 24, 2007.
4. Dyk, S. V., J. Su, M. Ebadian and J. Saddler, "Production of lower carbon-intensity fuels by coprocessing biogenic feedstocks: Potential and challenges for refineries,” Fuel, September 2022.
5. Pinho, A. R., M. B. B. Almedia, F. L. Mendes, V. L. Ximenes and L. C. Casavechia, “Coprocessing raw bio-oil and gasoil in an FCC unit,” Fuel Processing Technology, March 2015.

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