ERTC 2024
Plan for corrosion when co-processing renewable feedstocks
Rene Gonzalez Editor, PTQ
oxidative degradation by neutralising free radicals or decomposing peroxides. Different feedstocks used for biofu- els have varying susceptibility to oxidative stress. For example, biodiesel produced from polyunsaturated fats (such as vegeta- ble oils) tends to be more prone to oxidation compared to biodiesel produced from sat- urated fats (such as animal fats). Oxidative stress presents challenges in both the pro- duction and use of biofuels, accelerating equipment corrosion and leading to insta- bility in the final fuel products, potentially decreasing shelf life and performance. Managing oxidative stability through anti- oxidants 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, poly- meric 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 acceler- ate 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 forma- tion of unwanted byproducts such as alde- hydes, 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 reac- tions in refinery feedstocks, particularly in unsaturated hydrocarbons like olefins. These reactions can lead to the formation of unstable free radicals, which may fur- ther react to form polymers or other com- plex organic species. Polymer formation is problematic because it can lead to process equipment fouling, pipeline plugging, and product quality degradation. This is particu- larly a concern in fuel storage and transpor- tation, 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 pos- sible. 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 pre- vent 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 una-
Renewable co-processing through refinery hydrotreaters and FCC units benefits from subsidies and cred- its 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. 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. feedstock To begin with, additional corrosion con- cerns are not present in traditional fossil feed hydrotreating. Nevertheless, building the business case for co-processing favours debottlenecking or increasing the capac- ity of existing refineries rather than building new facilities for the production of renew- able diesel (RD) that meets EN 590 speci- fications 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 discus- sion 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 require- ments, the following discussion will elabo- rate 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 affect- ing connected rotating equipment, fraction- ators, heat exchangers, and other linked assets. Key factors influencing corrosion include: ○ Acid formation ○ Metals oxidation
Renewable Naphtha
Eco ning ™ * & UOP Renewable Jet Fuel Process
Inedible
Sustainable Aviation Fuel (SAF) Renewable Diesel (RD)
UOP Distillate Unionfining™ Process
Partial SAF Partial RD
Vegetable oils
Animal fats
Greases
Algal oil
Partial renewable LPG Partial renewable gasoline
UOP FCC Coprocessing
Petroleum
VGO Inedible FOGs
Partial RD
RFO for Heating/Power
Envergent RTP® (Pyrolysis)
Biomass
Renewable Naphtha RD and renewable Marine SAF
Hydrotreating/ Upgrading
Gasi cation
Fisher - Tropsch + UOP FT-Unicracking™
SAF eSAF (when green H used)
H
CO
Fisher - Tropsch + UOP FT-Unicracking™
+
UOP Ethanol to Jet Process
Ethanol
SAF
Figure 1 Honeywell UOP route to SAF production
Courtesy: Honeywell UOP
corrode refinery equipment. Oxidation reactions in refinery operations can lead to undesirable compound formation, impact- ing both product quality and the integ- rity 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 com- monly found in refinery processes. Oxidative stress in biofuels Biofuels or other renewable feedstocks with naturally higher oxygen than hydrocar- bon-based feedstocks are prone to oxida- tive degradation. Biodiesel and bioethanol are susceptible to oxidative degradation, which occurs when oxygen reacts with the unsaturated fatty acids present in biofu- els. 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 envi- ronmental factors such as elevated tem- peratures accelerating oxidation reactions, exposure to sunlight or UV light that can cat- alyse oxidative reactions, and trace metals such as copper and iron present in biofuels or storage containers that can act as oxida- tive 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 crit- ical 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 compo- nents. This is why antioxidant compounds are often added to biofuels to slow the oxi- dation process. These compounds inhibit
Oxygen can react directly with metal sur- faces, especially in high-temperature envi- ronments such as furnaces, heaters, and reactors. Metal oxidation leads to the for- mation 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 trans- fer efficiency and increases the risk of fail- ure due to overheating. In processes like catalytic reforming, hydrotreating, and hydrocracking, oxygen can cause catalyst deactivation by pro- moting the formation of carbon deposits (coking) or reacting with metal catalysts, altering their surface properties. Catalyst deactivation reduces conversion unit pro- cess efficiencies, requiring more frequent regeneration or replacement of catalysts, leading to increased operational costs and potential downtime. Increasing O 2 content Renewable feedstocks typically contain higher O2 levels compared to fossil fuels. This oxygen can increase the risk of oxi- dation reactions, resulting in higher rates of corrosion, especially in the presence of water and heat. By managing oxidation reactions, refineries can maintain opera- tional efficiency, reduce corrosion-related maintenance, and ensure the production of high-quality fuel products. For example, renewable feedstocks, such as triglycer- ides (C₁₇ to C₂ 0 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 primar- ily present in the form of carboxyl, hydroxyl, and ester groups. Higher oxygen content can result in the formation of acidic com- pounds, such as organic acids, which can
○ 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 fos- sil 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 distil- lation towers or heat exchanger units. This acid corrosion is particularly damaging to carbon steel and other common refin- ery materials, predicating a metallurgical upgrade to more corrosion-resistant (and more expensive) trays such as 316 S.S., Monel or titanium.
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