Nov-2024

Plan for corrosion when co-processing renewable feedstocks (ERTC 2024)

Renewable feedstock co-processing through refinery hydrotreaters and FCC units benefits from subsidies and credits such as the US government’s Inflation Reduction Act (IRA), while in Europe the Fit for 55 initiative from 2021 mandates a sustainable aviation fuel (SAF) content of 5% in jet fuel by 2030, and up to 63% in 2050.

Rene Gonzalez
Editor, PTQ

Article Summary

While corrosion problems at 5% SAF production may be manageable, there is concern that treating contaminant levels expected at the 63% SAF target may be overwhelming.

To begin with, additional corrosion concerns are not present in traditional fossil feed hydrotreating. Nevertheless, building the business case for co-processing favours debottlenecking or increasing the capacity of existing refineries rather than building new facilities for the production of renewable diesel (RD) that meets EN 590 specifications and SAF, estimated at 230 kbpd by mid-2025.

Recent publications in the trade press and scientific journals point out several issues affecting the economic and operational viability of co-processing, not the least of which is corrosion. While a detailed discussion on all the challenges affecting efficient co-processing is beyond the scope of this discussion, such as the need to upgrade a hydrotreating unit’s recycle gas compressor to mitigate increased quench gas requirements, the following discussion will elaborate on the wide range of corrosion factors.

Corrosion pathways
Introducing renewable feedstocks alters the chemical environment inside reactors (such as hydrotreaters and FCCs), leading to new types of corrosion pathways affecting connected rotating equipment, fractionators, heat exchangers, and other linked assets. Key factors influencing corrosion include:
○ Acid formation
○ Metals oxidation
○ Catalyst deactivation
○ Higher oxygen content
○ Oxidative stress in biofuels
○ Gum and varnish formation
○ Light hydrocarbons oxidation
○ Auto-oxidation and polymerisation.

Oxygen can react with combined fossil and renewable feedstocks, particularly during processing stages like combustion or gasification, leading to the formation of sulphur oxides (SOx) and sulphuric acid. Similarly, nitrogen compounds can form nitrogen oxides (NOx) and nitric acid. These acids are highly corrosive, especially when they condense on equipment surfaces in cooler parts of the refinery, such as distillation towers or heat exchanger units. This acid corrosion is particularly damaging to carbon steel and other common refinery materials, predicating a metallurgical upgrade to more corrosion-resistant (and more expensive) trays such as 316 S.S., Monel or titanium.

Oxygen can react directly with metal surfaces, especially in high-temperature environments such as furnaces, heaters, and reactors. Metal oxidation leads to the formation of metal oxides, such as iron oxide or rust, which can weaken metal structures, degrade equipment, and contribute to scale formation. If scale builds up on heat exchanger surfaces, it reduces heat transfer efficiency and increases the risk of failure due to overheating.

In processes like catalytic reforming, hydrotreating, and hydrocracking, oxygen can cause catalyst deactivation by promoting the formation of carbon deposits (coking) or reacting with metal catalysts, altering their surface properties. Catalyst deactivation reduces conversion unit process efficiencies, requiring more frequent regeneration or replacement of catalysts, leading to increased operational costs and potential downtime.

Increasing O2 content
Renewable feedstocks typically contain higher O2 levels compared to fossil fuels. This oxygen can increase the risk of oxidation reactions, resulting in higher rates of corrosion, especially in the presence of water and heat. By managing oxidation reactions, refineries can maintain operational efficiency, reduce corrosion-related maintenance, and ensure the production of high-quality fuel products. For example, renewable feedstocks, such as triglycerides  (C₁₇ to C₂0 chains of paraffins attached to a glycerol backbone) in vegetable oils and fatty acids, contain higher oxygen content compared to hydrocarbon feedstocks, such as VGO, naphtha, and coker gasoils.

Renewable feedstock oxygen is primarily present in the form of carboxyl, hydroxyl, and ester groups. Higher oxygen content can result in the formation of acidic compounds, such as organic acids, which can corrode refinery equipment. Oxidation reactions in refinery operations can lead to undesirable compound formation, impacting both product quality and the integrity of a wide range of process equipment. These reactions occur when hydrocarbons or other components within the feedstocks come into contact with oxygen, particularly at high temperatures and pressures commonly found in refinery processes.

Oxidative stress in biofuels
Biofuels or other renewable feedstocks with naturally higher oxygen than hydrocarbon-based feedstocks are prone to oxidative degradation. Biodiesel and bioethanol are susceptible to oxidative degradation, which occurs when oxygen reacts with the unsaturated fatty acids present in biofuels. This eventually leads to the formation of peroxides, aldehydes, ketones, and other degradation products, reducing biofuels quality, decreasing their energy content, causing gum formation, and clogging filters and engines.

Oxidative stress in biofuels arises from exposure to oxygen in the air, as well as environmental factors such as elevated temperatures accelerating oxidation reactions, exposure to sunlight or UV light that can catalyse oxidative reactions, and trace metals such as copper and iron present in biofuels or storage containers that can act as oxidative stress in biofuels. All these factors weigh heavily on targeted diesel quality, especially when the final objective is RD or SAF.

The oxidative stability of biofuels is a critical factor in determining their shelf life and usability. Poor oxidative stability can lead to significant degradation, resulting in lower engine performance, increased emissions, and potential damage to engine components. This is why antioxidant compounds are often added to biofuels to slow the oxidation process. These compounds inhibit oxidative degradation by neutralising free radicals or decomposing peroxides.

Different feedstocks used for biofuels have varying susceptibility to oxidative stress. For example, biodiesel produced from polyunsaturated fats (such as vegetable oils) tends to be more prone to oxidation compared to biodiesel produced from saturated fats (such as animal fats). Oxidative stress presents challenges in both the production and use of biofuels, accelerating equipment corrosion and leading to instability in the final fuel products, potentially decreasing shelf life and performance. Managing oxidative stability through antioxidants and careful handling can mitigate degradation and improve the longevity and performance of biofuels, making them more viable as sustainable energy sources.

Additional concerns
When oxygen reacts with hydrocarbons, especially unsaturated compounds (olefins and aromatics), it can lead to the formation of gums and varnishes. These sticky, polymeric materials can deposit on surfaces, particularly in pipelines, heat exchangers, and reactors. This leads to fouling, reduced heat transfer efficiency, and potential flow restrictions. The deposits can also accelerate under-deposit corrosion.

Light hydrocarbons (such as methane, C₂, and C₃) are susceptible to oxidation, particularly during processes like catalytic cracking. Oxygen can lead to the formation of unwanted byproducts such as aldehydes, ketones, and alcohols. The presence of these oxygenated compounds in refinery products can reduce fuel quality, such as octane rating in gasoline. In extreme cases, it can cause off-spec products that need additional treatment or reprocessing.

Oxygen can initiate auto-oxidation reactions in refinery feedstocks, particularly in unsaturated hydrocarbons like olefins. These reactions can lead to the formation of unstable free radicals, which may further react to form polymers or other complex organic species. Polymer formation is problematic because it can lead to process equipment fouling, pipeline plugging, and product quality degradation. This is particularly a concern in fuel storage and transportation, where exposure to air can initiate auto-oxidation.

Mitigation strategies
To mitigate oxidation reactions and their detrimental effects, refineries use several strategies such as excluding oxygen from processing environments wherever possible. This can involve purging vessels and pipelines with inert gases like nitrogen to prevent oxidation. In addition, the use of antioxidant additives is often added to prevent the formation of free radicals in fuels, minimising gum and varnish formation. In biofuel processing, stabilisers are used to inhibit oxidative degradation.

In critical areas where oxidation is unavoidable, corrosion-resistant materials like stainless steel, high-nickel alloys, or ceramic coatings are used to protect equipment from oxidation-related damage. These materials also ensure that catalysts are properly maintained and regenerated to try and reduce coke build-up and other oxygen-related degradation products.

Process control, AI, and predictive maintenance-focused articles have previously been published in PTQ, highlighting the aim  to maintain optimal co-processing operating conditions (temperature, pressure, flow rates), helping to minimise oxidative reactions that can lead to undesirable byproducts and equipment corrosion.

Corrosion at higher bio-oils rates
Many renewable feedstocks, such as bio-oils or fats, can have higher levels of organic acids, particularly carboxylic acids. These acids can exacerbate corrosion, particularly in distillation columns and heat exchangers. Increased acidity can accelerate general and localised corrosion, especially in areas where acidic species concentrate, such as heat transfer zones.

Renewable feedstocks may also contain sulphur and nitrogen compounds, though often in lower concentrations than fossil feedstocks. However, interactions between sulphur from fossil fuels and oxygen from renewables can lead to the formation of highly corrosive compounds like sulphuric acid or nitric acid. These acids can cause significant damage to equipment, especially if the process operates under high temperatures and pressures.

Renewable feedstocks can introduce metals such as sodium, potassium, calcium, and magnesium. These metals can form corrosive deposits, particularly on catalyst surfaces that can foul and corrode process units. Sodium and potassium salts can cause hot corrosion in high-temperature environments, particularly in furnaces and reactors, leading to accelerated material degradation. Some renewable feedstocks are thermally and chemically unstable at the high temperatures typical of refinery processes, leading to the formation of polymers, gums, and coke, which can coat equipment surfaces, interfere with heat transfer, and increase the risk of under-deposit corrosion, leading to equipment failure.

During co-processing, renewable feedstocks may cause phase separation, particularly if the renewable fraction contains water or other polar compounds. Water can combine with acidic or sulphurous compounds to form highly corrosive environments. Water and acidic species, especially under high temperatures, can lead to aqueous phase corrosion, typically affecting distillation units and piping.

With expected annual increases in co-processing capacity comes a higher risk of hydrogen embrittlement, especially in high-pressure hydrogen environments like hydrotreaters or hydrocrackers. The interaction between hydrogen and renewable feedstock components may facilitate the absorption of hydrogen into metal surfaces. Hydrogen embrittlement can lead to catastrophic failure of metal components by making them brittle and prone to cracking.

Solutions
From chemical inhibitors to AI-based predictive corrosion monitoring, a variety of solutions are available to deal with co-processing challenges, including the pretreatment of renewable feedstocks to remove contaminants (such as water, acids, and metals) that can promote corrosion. For example, chemical inhibitors to neutralise acids or passivate metal surfaces reduce corrosion rates. Implementation of corrosion monitoring techniques, like ultrasonic testing, coupons, or electrical resistance probes, is necessary to detect early signs of corrosion and perform regular maintenance.

A recent co-processing example in PTQ Q4 2024 from G. Vincent at BASF Refinery Catalysts discussed overcoming challenges during FCC fast pyrolysis bio-oils (FPBO) co-processing with vacuum gasoil (VGO). Vincent noted that FPBO, most often called bio-oils, having already demonstrated crackability in FCC units, can introduce operational challenges such as:
○ Miscibility issues with fossil feedstocks due to high polarity molecules and free water, 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. If a dedicated injection nozzle is required, its location needs to be optimised within the FCC riser.
○ High variability in alkali, earth alkaline metals, acidity, and oxygen contents.

Bio-oils differ from crude oils due to the presence of oxygen and elevated levels of alkali metals (such as Na, K), earth alkaline metals (such as Ca, Mg), chlorides, and phosphorus. Since these contaminants can cause catalyst deactivation and operational issues, like 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, including:
○ Filtration
○ Desalting
○ Degumming
○ Hydrotreating applications
○ Purification adsorbents.

Understanding and mitigating these challenges is critical for the success of co-processing operations, ensuring long-term equipment reliability and minimising costly downtime due to corrosion-related failures.

This short article originally appeared in the 2024 ERTC Newspaper, which you can VIEW HERE